Verifica del rilevamento dell'MTU del percorso su Cisco IOS XR e BGP
Sommario
Introduzione
In questo documento viene descritto il rilevamento della MTU (Path Maximum Transmission Unit) del protocollo TCP (Transmission Control Protocol) (PMTUD) sui dispositivi Cisco IOS® XR.
Premesse
Il meccanismo PMTUD cerca di determinare le dimensioni massime del pacchetto IP (Internet Protocol) che non devono essere frammentate in alcun punto del percorso tra due host. Il valore stabilito è MTU del percorso ed è uguale al minimo dei valori MTU di ciascun hop. Se si considera l'MTU del percorso quando si trasmettono le informazioni, è possibile sfruttare al massimo la capacità della rete ed evitare la frammentazione e l'efficienza della trasmissione. La meccanica e l'implementazione della funzionalità PMTUD vengono introdotte in una serie di scenari diversi utilizzando il protocollo Border Gateway Protocol (BGP) come protocollo client che rivela gradualmente il comportamento della funzionalità PMTUD.
TCP PMTUD e TCP MSS
Il protocollo TCP sfrutta i risultati del PMTUD per influenzare il valore MSS (Maximum Segment Size) locale, ossia si adatta dinamicamente alla MTU del percorso rilevata. Pertanto, prima di procedere alla funzionalità PMTUD, è possibile rivedere rapidamente il parametro TCP Maximum Segment Size (MSS) e comprenderne il significato e lo scopo.
Come da definizione originale MSS di RFC879: È possibile definire l'opzione MSS: Il numero massimo di ottetti di dati che possono essere ricevuti dal mittente di questa opzione TCP nei segmenti TCP senza opzioni dell'intestazione TCP trasmesse nei datagrammi IP senza opzioni dell'intestazione IP.
chiarire alcuni aspetti e fornire consulenza agli esecutori, RFC 6691 evidenzia la modalità di calcolo del valore MSS:
Quando si calcola il valore da inserire nell'opzione TCP MSS, il valore MTU deve essere diminuito solo della dimensione delle intestazioni IP e TCP fisse e non deve essere diminuito per tenere conto di eventuali opzioni IP o TCP; al contrario, il mittente DEVE ridurre la lunghezza dei dati TCP in modo da tenere conto di tutte le opzioni IP o TCP che include nei pacchetti che invia.
Per una definizione più elaborata di MSS, consultare la guida alla configurazione del routing per i router Cisco ASR serie 9000, IOS XR release 6.7.x:
Il valore MSS è la maggiore quantità di dati che un computer o un dispositivo di comunicazione può ricevere in un singolo segmento TCP non frammentato. Tutte le sessioni TCP sono limitate da un limite al numero di byte che possono essere trasportati in un singolo pacchetto; questo limite è MSS. Il protocollo TCP suddivide i pacchetti in blocchi in una coda di trasmissione prima di passarli al livello IP.
Il valore TCP MSS dipende dalla MTU di un'interfaccia, che è la lunghezza massima dei dati che possono essere trasmessi da un protocollo in un'istanza. La lunghezza massima del pacchetto TCP è determinata sia dalla MTU dell'interfaccia in uscita sul dispositivo di origine sia dal valore MSS annunciato dal dispositivo di destinazione durante il processo di configurazione TCP. Più il valore MSS si avvicina all'MTU, più efficiente è il trasferimento dei messaggi BGP. Ogni direzione del flusso di dati può utilizzare un valore MSS diverso.
Quale valore dovrebbe quindi essere preso in considerazione dal protocollo TCP per il valore MSS in una determinata sessione TCP? E come viene calcolato?
Per i valori predefiniti in base a RFC879, sono disponibili: Gli host non devono inviare datagrammi più grandi di 576 ottetti a meno che non siano a conoscenza del fatto che l'host di destinazione è pronto ad accettare datagrammi più grandi. LA DIMENSIONE MASSIMA DEL SEGMENTO TCP È LA DIMENSIONE MASSIMA DEL DATAGRAMMA IP MENO 40.
Le dimensioni massime predefinite del datagramma IP sono 576.
La dimensione massima predefinita del segmento TCP è 536.
In questo modo si prende in considerazione un valore MTU IP di 576 byte. Tuttavia, se si ignora il valore IP MTU effettivo, il calcolo del valore TCP MSS può essere riepilogato come segue:
- Active Peer - calcola e invia il valore MSS iniziale con il pacchetto SYN.
MSS = IPMTU - sizeof(minimum TCPHDR) - sizeof(minimum IPHDR)
Where,
sizeof(minimum TCPHDR) = 20 bytes.
sizeof(minimum IPHDR) = 20 bytes.
- Peer passivo: calcola il valore MSS iniziale, lo confronta con il valore MSS ricevuto dal peer attivo e invia SYN, ACK con il valore MSS inferiore.
MIN[IPMTU - sizeof(minimum TCPHDR) - sizeof(minimum IPHDR) , Received MSS value]
Where,
sizeof(minimum TCPHDR) = 20 bytes.
sizeof(minimum IPHDR) = 20 bytes.
Received MSS value = MSS value received with Active Peer TCP SYN.
Non sono previste negoziazioni per quanto riguarda il valore dell'opzione MSS. Ogni nodo determina il proprio valore e lo annuncia al momento dell'istituzione della sessione TCP. È chiaro che se il valore MTU dell'IP preso in considerazione per il calcolo del valore MSS può essere derivato dal PMTUD, il valore MSS può essere adattato al valore più efficace per una data MTU del percorso. Il comportamento Cisco IOS XR contiene alcune specifiche relative al calcolo del valore MSS e al ruolo PMTUD, che vengono riepilogate di seguito.
La funzionalità PMTUD è disabilitata per impostazione predefinita su Cisco IOS XR:
- Il calcolo MSS iniziale locale considera l'MTU IP come segue:
- Se i peer sono collegati direttamente, prendere in considerazione l'MTU IP dell'interfaccia in uscita.
- Se i peer non sono connessi direttamente, considerare un'MTU IP di 1280 byte.
- Il valore MSS è influenzato dalle opzioni TCP configurate.
Quando la funzionalità PMTUD è abilitata su Cisco IOS XR:
- Il calcolo MSS iniziale locale considera l'MTU IP come segue:
- Indipendentemente dai peer connessi direttamente/non direttamente: prendere in considerazione l'MTU IP dell'interfaccia in uscita.
- Il valore MSS è influenzato dalle opzioni TCP configurate.
È necessario prendere in considerazione ulteriori dettagli sulla meccanica e sull'implementazione della PMTUD, descritti nella tabella seguente. Questa tabella presenta anche l'MTU IP dei peer TCP attivi e passivi e i valori MSS selezionati per ciascuno scenario considerato.
Scenari - PMTUD TCP disabilitato
Usa valori MTU predefiniti
Immagine 2.1. Con valori MTU predefiniti
Nel caso dei peer eBGP mostrati nell'immagine 2.1 R6 gestisce la connessione TCP, ossia svolge il ruolo attivo e avvia la sessione TCP con R5 sulla porta di destinazione 179. I peer sono connessi direttamente ed entrambi utilizzano i valori MTU IP predefiniti sulle rispettive interfacce. In base alle informazioni condivise all'inizio di questo documento, il calcolo del valore MSS in questo scenario può essere riassunto come segue:
- Entrambi i nodi utilizzano una MTU IP predefinita di 1500 byte
- Il rilevamento dell'MTU del percorso TCP è disabilitato per impostazione predefinita
- I peer TCP sono connessi direttamente
- R6 gestisce la connessione BGP
- R6 invia SYN con MSS di 1460 byte
- 1500 (MTU IP interfaccia) - 20 (minTCP_H) - 20 (minIP_H)
- R5 invia SYN, ACK con MSS di 1460 byte
- Invia il valore più basso di [Received MSS; Local initial MSS]
- 1460 byte MSS ricevuti; Local initial MSS 1460 byte
- Il valore MSS più basso viene utilizzato su entrambi i peer
Dettagli sessione TCP come visualizzati in R6 - ATTIVO:
! – As seen on R6 – ACTIVE
RP/0/0/CPU0:R6#show interfaces gigabitEthernet 0/0/0/0
Fri Jan 8 09:35:48.553 UTC
GigabitEthernet0/0/0/0 is up, line protocol is up
Interface state transitions: 1
Hardware is GigabitEthernet, address is fa16.3e85.3dc2 (bia fa16.3e85.3dc2)
Internet address is 10.5.6.6/30
MTU 1514 bytes, BW 1000000 Kbit (Max: 1000000 Kbit)
<snip>
RP/0/0/CPU0:R6#show tcp brief
Fri Jan 8 09:36:22.491 UTC
PCB VRF-ID Recv-Q Send-Q Local Address Foreign Address State
<snip>
0x121649fc 0x60000000 0 0 10.5.6.6:24454 10.5.6.5:179 ESTAB
<snip>
RP/0/0/CPU0:R6#show tcp detail pcb 0x121649fc
Fri Jan 8 09:37:00.888 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 09:28:28 2021
PCB 0x121649fc, SO 0x121561b8, TCPCB 0x12156f64, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 1, Hash index: 78
Local host: 10.5.6.6, Local port: 24454 (Local App PID: 1011918)
Foreign host: 10.5.6.5, Foreign port: 179
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 13 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 10 2 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 3757770712 snduna: 3757770960 sndnxt: 3757770960
sndmax: 3757770960 sndwnd: 32574 sndcwnd: 4380
irs: 1072103647 rcvnxt: 1072103895 rcvwnd: 32593 rcvadv: 1072136488
SRTT: 155 ms, RTTO: 540 ms, RTV: 385 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 229 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 30, connect retry interval: 50 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R6
Dettagli della sessione TCP come visualizzati in R5 - PASSIVE:
! – As seen on R5 – PASSIVE
RP/0/0/CPU0:R5#show interfaces gigabitEthernet 0/0/0/0
Fri Jan 8 09:33:04.564 UTC
GigabitEthernet0/0/0/0 is up, line protocol is up
Interface state transitions: 1
Hardware is GigabitEthernet, address is fa16.3ead.518f (bia fa16.3ead.518f)
Internet address is 10.5.6.5/30
MTU 1514 bytes, BW 1000000 Kbit (Max: 1000000 Kbit)
<snip>
RP/0/0/CPU0:R5#show tcp brief
Fri Jan 8 09:33:53.221 UTC
PCB VRF-ID Recv-Q Send-Q Local Address Foreign Address State
<snip>
0x12155884 0x60000000 0 0 10.5.6.5:179 10.5.6.6:24454 ESTAB
<snip>
RP/0/0/CPU0:R5#show tcp detail pcb 0x12155884
Fri Jan 8 09:34:47.317 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 09:28:29 2021
PCB 0x12155884, SO 0x1215568c, TCPCB 0x12155a54, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 1, Hash index: 78
Local host: 10.5.6.5, Local port: 179 (Local App PID: 1044686)
Foreign host: 10.5.6.6, Foreign port: 24454
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 9 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 9 7 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 1072103647 snduna: 1072103857 sndnxt: 1072103857
sndmax: 1072103857 sndwnd: 32631 sndcwnd: 4380
irs: 3757770712 rcvnxt: 3757770922 rcvwnd: 32612 rcvadv: 3757803534
SRTT: 47 ms, RTTO: 300 ms, RTV: 170 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 219 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R5#
Usa valore MTU non predefinito - Peer TCP attivo
Immagine 2.2 - Il peer ATTIVO utilizza un valore MTU non predefinito
Questo scenario è essenzialmente lo stesso del precedente, con l'unica differenza che il peer TCP attivo R6 ora utilizza un valore MTU IP non predefinito. Si noti come il calcolo iniziale e la decisione sul valore MSS vengano eseguiti dal peer TCP passivo R5. In questo scenario, il calcolo del valore TCP MSS può essere riepilogato come segue:
- R6 utilizza MTU IP non predefinita di 4460 byte
- Il rilevamento dell'MTU del percorso TCP è disabilitato per impostazione predefinita
- I peer TCP sono connessi direttamente
- R6 gestisce la connessione BGP
- R6 invia SYN con MSS di 4420 byte
- 4460 (MTU IP interfaccia) - 20 (minTCP_H) - 20 (minIP_H)
- R5 invia SYN, ACK con MSS di 1460 byte
- invia il valore più basso di [Received MSS; Local initial MSS]
- Byte MSS 4420 ricevuti; Local initial MSS 1460 byte
- Il valore MSS più basso viene utilizzato su entrambi i peer
TCP SYN originato da R6:
! – TCP SYN sourced from R6
140 1598.150521 10.5.6.6 10.5.6.5 TCP 62 35502 → 179 [SYN] Seq=0 Win=16384 Len=0 MSS=4420 WS=1
Frame 140: 62 bytes on wire (496 bits), 62 bytes captured (496 bits) on interface 0
Ethernet II, Src: fa:16:3e:85:3d:c2 (fa:16:3e:85:3d:c2), Dst: fa:16:3e:ad:51:8f (fa:16:3e:ad:51:8f)
Internet Protocol Version 4, Src: 10.5.6.6, Dst: 10.5.6.5
Transmission Control Protocol, Src Port: 35502, Dst Port: 179, Seq: 0, Len: 0
Source Port: 35502
Destination Port: 179
[Stream index: 6]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 0
Header Length: 28 bytes
Flags: 0x002 (SYN)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0x219d [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (8 bytes), Maximum segment size, Window scale, End of Option List (EOL)
Maximum segment size: 4420 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 4420
Window scale: 0 (multiply by 1)
End of Option List (EOL)
TCP SYN, ACK originato da R5:
! – TCP SYN, ACK sourced from R5
141 1598.154866 10.5.6.5 10.5.6.6 TCP 62 179 → 35502 [SYN, ACK] Seq=0 Ack=1 Win=16384 Len=0 MSS=1460 WS=1
Frame 141: 62 bytes on wire (496 bits), 62 bytes captured (496 bits) on interface 0
Ethernet II, Src: fa:16:3e:ad:51:8f (fa:16:3e:ad:51:8f), Dst: fa:16:3e:85:3d:c2 (fa:16:3e:85:3d:c2)
Internet Protocol Version 4, Src: 10.5.6.5, Dst: 10.5.6.6
Transmission Control Protocol, Src Port: 179, Dst Port: 35502, Seq: 0, Ack: 1, Len: 0
Source Port: 179
Destination Port: 35502
[Stream index: 6]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 1 (relative ack number)
Header Length: 28 bytes
Flags: 0x012 (SYN, ACK)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0xe2b4 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (8 bytes), Maximum segment size, Window scale, End of Option List (EOL)
Maximum segment size: 1460 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1460
Window scale: 0 (multiply by 1)
End of Option List (EOL)
Dettagli sessione TCP come visualizzati in R6 - ATTIVO:
! - as seen on R6 – Active
RP/0/0/CPU0:R6#show interfaces gigabitEthernet 0/0/0/0
Fri Jan 8 09:46:54.138 UTC
GigabitEthernet0/0/0/0 is up, line protocol is up
Interface state transitions: 1
Hardware is GigabitEthernet, address is fa16.3e85.3dc2 (bia fa16.3e85.3dc2)
Internet address is 10.5.6.6/30
MTU 4474 bytes, BW 1000000 Kbit (Max: 1000000 Kbit)
<snip>
RP/0/0/CPU0:R6#show tcp detail pcb 0x1215761c
Fri Jan 8 09:56:25.819 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 09:51:46 2021
PCB 0x1215761c, SO 0x12156f64, TCPCB 0x1216419c, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 1, Hash index: 886
Local host: 10.5.6.6, Local port: 35502 (Local App PID: 1011918)
Foreign host: 10.5.6.5, Foreign port: 179
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 9 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 6 5 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 764231407 snduna: 764231579 sndnxt: 764231579
sndmax: 764231579 sndwnd: 32650 sndcwnd: 4380
irs: 2712512697 rcvnxt: 2712512869 rcvwnd: 32669 rcvadv: 2712545538
SRTT: 31 ms, RTTO: 300 ms, RTV: 130 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 239 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 30, connect retry interval: 50 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 4420, max MSS 4420
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R6#
Dettagli della sessione TCP come visualizzati in R5 - PASSIVE:
! - as seen on R5 – Passive
RP/0/0/CPU0:R5#show tcp detail pcb 0x12155a98
Fri Jan 8 09:55:18.193 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 09:51:47 2021
PCB 0x12155a98, SO 0x12153ea0, TCPCB 0x12154e18, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 1, Hash index: 886
Local host: 10.5.6.5, Local port: 179 (Local App PID: 1044686)
Foreign host: 10.5.6.6, Foreign port: 35502
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 6 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 6 1 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 2712512697 snduna: 2712512850 sndnxt: 2712512850
sndmax: 2712512850 sndwnd: 32688 sndcwnd: 4380
irs: 764231407 rcvnxt: 764231560 rcvwnd: 32669 rcvadv: 764264229
SRTT: 107 ms, RTTO: 538 ms, RTV: 431 ms, KRTT: 0 ms
minRTT: 29 ms, maxRTT: 219 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 4420, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R5#
Usa valore MTU non predefinito - Peer TCP passivo
Immagine 2.3 - Il peer PASSIVO utilizza un valore MTU non predefinito.
Con lo stesso scenario eBGP, ma ora con TCP peer R5 passivo configurato con MTU IP non predefinita e TCP peer R6 attivo con valore MTU IP predefinito. Come per lo scenario precedente, prendere nota di come il valore MSS viene selezionato dal peer passivo R5. Il calcolo TCP MSS in questo scenario può essere riepilogato come segue:
- R5 utilizza una MTU IP non predefinita di 4460 byte
- Il rilevamento dell'MTU del percorso TCP è disabilitato per impostazione predefinita
- I peer TCP sono connessi direttamente
- R6 gestisce la connessione BGP
- R6 invia SYN con MSS di 1460 byte
- 1500 (MTU IP interfaccia) - 20 (minTCP_H) - 20 (minIP_H)
- R5 invia SYN, ACK con MSS di 1460 byte
- invia il valore più basso di [Received MSS; Local initial MSS]
- 1460 byte MSS ricevuti; Local initial MSS 4420 byte
- Il valore MSS più basso viene utilizzato su entrambi i peer
TCP SYN originato da R6:
! – TCP SYN sourced from R6
237 2696.666481 10.5.6.6 10.5.6.5 TCP 62 47007 → 179 [SYN] Seq=0 Win=16384 Len=0 MSS=1460 WS=1
Frame 237: 62 bytes on wire (496 bits), 62 bytes captured (496 bits) on interface 0
Ethernet II, Src: fa:16:3e:85:3d:c2 (fa:16:3e:85:3d:c2), Dst: fa:16:3e:ad:51:8f (fa:16:3e:ad:51:8f)
Internet Protocol Version 4, Src: 10.5.6.6, Dst: 10.5.6.5
Transmission Control Protocol, Src Port: 47007, Dst Port: 179, Seq: 0, Len: 0
Source Port: 47007
Destination Port: 179
[Stream index: 10]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 0
Header Length: 28 bytes
Flags: 0x002 (SYN)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0x2025 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (8 bytes), Maximum segment size, Window scale, End of Option List (EOL)
Maximum segment size: 1460 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1460
Window scale: 0 (multiply by 1)
End of Option List (EOL)
TCP SYN, ACK originato da R5:
! – TCP SYN, ACK sourced from R5
238 2696.702792 10.5.6.5 10.5.6.6 TCP 62 179 → 47007 [SYN, ACK] Seq=0 Ack=1 Win=16384 Len=0 MSS=1460 WS=1
Frame 238: 62 bytes on wire (496 bits), 62 bytes captured (496 bits) on interface 0
Ethernet II, Src: fa:16:3e:ad:51:8f (fa:16:3e:ad:51:8f), Dst: fa:16:3e:85:3d:c2 (fa:16:3e:85:3d:c2)
Internet Protocol Version 4, Src: 10.5.6.5, Dst: 10.5.6.6
Transmission Control Protocol, Src Port: 179, Dst Port: 47007, Seq: 0, Ack: 1, Len: 0
Source Port: 179
Destination Port: 47007
[Stream index: 10]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 1 (relative ack number)
Header Length: 28 bytes
Flags: 0x012 (SYN, ACK)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0x7078 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (8 bytes), Maximum segment size, Window scale, End of Option List (EOL)
Maximum segment size: 1460 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1460
Window scale: 0 (multiply by 1)
End of Option List (EOL)
Dettagli sessione TCP come visualizzati in R6 - ATTIVO:
! - as seen on R6 - Active
RP/0/0/CPU0:R6#show tcp detail pcb 0x1215761c
Fri Jan 8 10:15:20.351 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 10:10:04 2021
PCB 0x1215761c, SO 0x12162aac, TCPCB 0x12156f64, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 1, Hash index: 103
Local host: 10.5.6.6, Local port: 47007 (Local App PID: 1011918)
Foreign host: 10.5.6.5, Foreign port: 179
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 10 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 7 5 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 3949093168 snduna: 3949093359 sndnxt: 3949093359
sndmax: 3949093359 sndwnd: 32631 sndcwnd: 4380
irs: 54439005 rcvnxt: 54439196 rcvwnd: 32650 rcvadv: 54471846
SRTT: 75 ms, RTTO: 459 ms, RTV: 384 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 239 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 30, connect retry interval: 50 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R6#
Dettagli della sessione TCP come visualizzati in R5 - PASSIVE:
! - as seen on R5 – Passive
RP/0/0/CPU0:R5#show interfaces gigabitEthernet 0/0/0/0
Fri Jan 8 10:10:39.110 UTC
GigabitEthernet0/0/0/0 is up, line protocol is up
Interface state transitions: 1
Hardware is GigabitEthernet, address is fa16.3ead.518f (bia fa16.3ead.518f)
Internet address is 10.5.6.5/30
MTU 4474 bytes, BW 1000000 Kbit (Max: 1000000 Kbit)
<snip>
RP/0/0/CPU0:R5#show tcp detail pcb 0x121550fc
Fri Jan 8 10:14:20.105 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 10:10:05 2021
PCB 0x121550fc, SO 0x12154e18, TCPCB 0x12154304, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 1, Hash index: 103
Local host: 10.5.6.5, Local port: 179 (Local App PID: 1044686)
Foreign host: 10.5.6.6, Foreign port: 47007
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 7 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 7 2 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 54439005 snduna: 54439177 sndnxt: 54439177
sndmax: 54439177 sndwnd: 32669 sndcwnd: 4380
irs: 3949093168 rcvnxt: 3949093340 rcvwnd: 32650 rcvadv: 3949125990
SRTT: 117 ms, RTTO: 570 ms, RTV: 453 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 229 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 4420, max MSS 4420
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R5#
Usa opzioni TCP - XR attivo
Come accennato in precedenza in questo documento, l'uso delle opzioni TCP (ad esempio TCP MD5, TCP selective-ack o timestamp TCP) influenza il calcolo del valore MSS, in quanto queste opzioni portano a byte aggiuntivi di cui tenere conto nel calcolo del valore MSS.
In questa sezione e nelle sezioni successive viene illustrato il calcolo del valore MSS eseguito dai peer in presenza delle opzioni TCP. Ad esempio, viene usata l'opzione di autenticazione TCP MD5. Fare riferimento allo scenario di riferimento nelle immagini 2.4 come mostrato nell'immagine.
Immagine 2.4 - Utilizzo delle opzioni TCP (MD5) - XR attivo.
In questo scenario entrambi i peer utilizzano valori MTU IP predefiniti, sono connessi direttamente e il peer R6 svolge il ruolo attivo di TCP. Come già condiviso, la configurazione e l'uso dell'account di autenticazione TCP MD5 per un ulteriore sovraccarico. Il calcolo del valore TCP MSS in questo scenario specifico può essere riepilogato come segue:
- Entrambi i nodi utilizzano una MTU IP predefinita di 1500 byte
- Il rilevamento dell'MTU del percorso TCP è disabilitato per impostazione predefinita
- I peer TCP sono connessi direttamente
- Autenticazione TCP MD5 abilitata su entrambi i nodi
- R6 gestisce la connessione BGP
- R6 invia SYN con MSS di 1436 byte
- 1500 (MTU IP interfaccia) - 20 (minTCP_H) - 20 (minIP_H) - 24 byte (sovraccarico opzioni TCP IOS XR)
- R5 invia SYN, ACK con MSS di 1436 byte
- invia il valore più basso di [Received MSS; Local initial MSS]
- Byte MSS ricevuti: 1436; Local initial MSS 1460 byte
- Il valore MSS più basso viene utilizzato su entrambi i peer
Come si evince dal riepilogo, il comportamento di Cisco IOS XR non è strettamente conforme alle RFC 879 e RFC 691, in cui si afferma che le opzioni TCP non devono essere prese in considerazione nel calcolo del valore MSS.
L'account Cisco IOS XR di un fattore extra sulla lunghezza dell'intestazione tcp è ulteriormente documentato sull'ID bug Cisco CSCvf20166:
"(...)Quando XR avvia la connessione BGP, BGP crea prima il socket, quindi imposta le opzioni del socket, incluso MD5. In questo modo, la lunghezza dell'intestazione dell'opzione tcp è = 24. E quindi il valore MSS iniziale diventa 1500 - 40 - 24 = 1436. Questo viene inviato agli usi peer e peer min(1436, 1460) = 1436.(...)
TCP SYN originato da R6:
! – TCP SYN sourced from R6
430 5775.839420 10.5.6.6 10.5.6.5 TCP 82 24785 → 179 [SYN] Seq=0 Win=16384 Len=0 MSS=1436 WS=1
Frame 430: 82 bytes on wire (656 bits), 82 bytes captured (656 bits) on interface 0
Ethernet II, Src: fa:16:3e:85:3d:c2 (fa:16:3e:85:3d:c2), Dst: fa:16:3e:ad:51:8f (fa:16:3e:ad:51:8f)
Internet Protocol Version 4, Src: 10.5.6.6, Dst: 10.5.6.5
Transmission Control Protocol, Src Port: 24785, Dst Port: 179, Seq: 0, Len: 0
Source Port: 24785
Destination Port: 179
[Stream index: 14]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 0
Header Length: 48 bytes
Flags: 0x002 (SYN)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0xd62b [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (28 bytes), Maximum segment size, Window scale, No-Operation (NOP), TCP MD5 signature, End of Option List (EOL)
Maximum segment size: 1436 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1436
Window scale: 0 (multiply by 1)
No-Operation (NOP)
TCP MD5 signature
End of Option List (EOL)
TCP SYN, ACK originato da R5:
! – TCP SYN, ACK sourced from R5
431 5775.845744 10.5.6.5 10.5.6.6 TCP 82 179 → 24785 [SYN, ACK] Seq=0 Ack=1 Win=16384 Len=0 MSS=1436 WS=1
Frame 431: 82 bytes on wire (656 bits), 82 bytes captured (656 bits) on interface 0
Ethernet II, Src: fa:16:3e:ad:51:8f (fa:16:3e:ad:51:8f), Dst: fa:16:3e:85:3d:c2 (fa:16:3e:85:3d:c2)
Internet Protocol Version 4, Src: 10.5.6.5, Dst: 10.5.6.6
Transmission Control Protocol, Src Port: 179, Dst Port: 24785, Seq: 0, Ack: 1, Len: 0
Source Port: 179
Destination Port: 24785
[Stream index: 14]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 1 (relative ack number)
Header Length: 48 bytes
Flags: 0x012 (SYN, ACK)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0xe83d [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (28 bytes), Maximum segment size, Window scale, No-Operation (NOP), TCP MD5 signature, End of Option List (EOL)
Maximum segment size: 1436 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1436
Window scale: 0 (multiply by 1)
No-Operation (NOP)
TCP MD5 signature
End of Option List (EOL)
Dettagli sessione TCP come visualizzati in R6 - ATTIVO:
! - as seen on R6 – Active
RP/0/0/CPU0:R6#show tcp detail pcb 0x1215761c
Fri Jan 8 11:14:13.599 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 11:01:21 2021
PCB 0x1215761c, SO 0x1216419c, TCPCB 0x121649fc, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 1, Hash index: 409
Local host: 10.5.6.6, Local port: 24785 (Local App PID: 1011918)
Foreign host: 10.5.6.5, Foreign port: 179
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 17 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 14 13 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 1379482495 snduna: 1379482819 sndnxt: 1379482819
sndmax: 1379482819 sndwnd: 32498 sndcwnd: 4308
irs: 3750694052 rcvnxt: 3750694376 rcvwnd: 32517 rcvadv: 3750726893
SRTT: 55 ms, RTTO: 300 ms, RTV: 176 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 259 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 30, connect retry interval: 50 secs
State flags: none
Feature flags: MD5, Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1436, peer MSS 1436, min MSS 1436, max MSS 1436
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R6#
Dettagli della sessione TCP come visualizzati in R5 - PASSIVE:
! - as seen on R5 – Passive
RP/0/0/CPU0:R5#show tcp detail pcb 0x12155d04
Fri Jan 8 11:12:51.984 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 11:01:22 2021
PCB 0x12155d04, SO 0x12154e18, TCPCB 0x12154304, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 1, Hash index: 409
Local host: 10.5.6.5, Local port: 179 (Local App PID: 1044686)
Foreign host: 10.5.6.6, Foreign port: 24785
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 14 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 14 3 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 3750694052 snduna: 3750694357 sndnxt: 3750694357
sndmax: 3750694357 sndwnd: 32536 sndcwnd: 4308
irs: 1379482495 rcvnxt: 1379482800 rcvwnd: 32517 rcvadv: 1379515317
SRTT: 181 ms, RTTO: 443 ms, RTV: 262 ms, KRTT: 0 ms
minRTT: 29 ms, maxRTT: 219 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: MD5, Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1436, peer MSS 1436, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R5#
Un comportamento simile può essere osservato con altre opzioni TCP che, quando configurate, tengono conto del sovraccarico aggiuntivo e influenzano il calcolo del valore MSS in Cisco IOS XR. Si consideri lo stesso scenario e gli esempi seguenti che documentano il calcolo MSS quando sono configurati i timestamp TCP e le opzioni TCP selective-ack.
Dettagli della sessione TCP come visualizzati in R6 - ATTIVO - con le opzioni TCP timestamp e selective-ack configurate:
! - as seen on R6 – Active
! -- tcp timestamp configured
! -- 12 bytes of additional overhead
RP/0/0/CPU0:R6#show tcp detail pcb 0x1539c844
<snip>
Feature flags: Timestamp, Win Scale, Nagle
Request flags: Timestamp, Win Scale
Datagrams (in bytes): MSS 1448, peer MSS 1448, min MSS 1448, max MSS 1448
<snip>
! - as seen on R6 – Active
! -- tcp selective-ack configured
! -- 36 bytes of additional overhead
RP/0/0/CPU0:R6#show tcp detail pcb 0x1539df38
<snip>
Feature flags: Sack, Win Scale, Nagle
Request flags: Sack, Win Scale
Datagrams (in bytes): MSS 1424, peer MSS 1424, min MSS 1424, max MSS 1424
<snip>
! - as seen on R6 – Active
! -- tcp selective-ack and tcp timestamp configured
! -- 40 bytes of additional overhead
RP/0/0/CPU0:R6#show tcp detail pcb 0x1539e130
<snip>
State flags: none
Feature flags: Sack, Timestamp, Win Scale, Nagle
Request flags: Sack, Timestamp, Win Scale
Datagrams (in bytes): MSS 1420, peer MSS 1420, min MSS 1420, max MSS 1420
<snip>
! - as seen on R6 – Active
! -- MD5 and tcp selective-ack configured
! -- 36 bytes of additional overhead
RP/0/0/CPU0:R6#show tcp detail pcb 0x1539b3cc
<snip>
Feature flags: Sack, MD5, Win Scale, Nagle
Request flags: Sack, Win Scale
Datagrams (in bytes): MSS 1424, peer MSS 1424, min MSS 1424, max MSS 1424
<snip>
! - as seen on R6 – Active
! -- MD5 and tcp timestamp configured
! -- 36 bytes of additional overhead
RP/0/0/CPU0:R6#show tcp detail pcb 0x15397b4c
<snip>
Feature flags: MD5, Timestamp, Win Scale, Nagle
Request flags: Timestamp, Win Scale
Datagrams (in bytes): MSS 1424, peer MSS 1424, min MSS 1424, max MSS 1424
<snip>
! - as seen on R6 – Active
! -- MD5, tcp timestamp, and tcp selective-ack configured
! -- 40 bytes of additional overhead
RP/0/0/CPU0:R6#show tcp detail pcb 0x1539a4cc
<snip>
State flags: none
Feature flags: MD5, Timestamp, Win Scale, Nagle
Request flags: Timestamp, Win Scale
Datagrams (in bytes): MSS 1420, peer MSS 1420, min MSS 1420, max MSS 1420
<snip>
Usa opzioni TCP - XR Passive
Dallo scenario precedente è probabilmente stato notato il comportamento distinto del nodo Cisco IOS XR quando si trova in un ruolo passivo per quanto riguarda il calcolo iniziale del valore MSS. Il nodo non tiene conto della lunghezza dell'intestazione dell'opzione tcp. Questo scenario intende evidenziare questo comportamento distinto, descritto anche dall'ID bug Cisco:
"(...) - Quando il peer avvia la connessione, invia il valore MSS iniziale come 1460. Il protocollo TCP XR crea socket, pcb e così via e quindi esegue due azioni nell'ordine indicato:
- In primo luogo, calcola il valore MSS iniziale dopo aver sottratto la lunghezza dell'intestazione dell'opzione tcp. Questo valore è '0' in quanto l'opzione MD5 non è ancora ereditata da questo socket dal socket di ascolto.
- Quindi, eredita 'MD5' e altre opzioni e diventa 'lunghezza byte intestazione opzione' a 24.
Pertanto, in questo caso il protocollo XR TCP invia il valore MSS iniziale come 1460, quindi questo valore viene utilizzato da entrambi. (...)"
In questo scenario, anche se il peer TCP R8 attivo è un nodo Cisco IOS, questo fatto non introduce differenze o specifiche su ciò che lo scenario intende evidenziare. Tuttavia, è interessante notare che, diversamente da Cisco IOS XR come mostrato nello scenario della sezione precedente, in questo caso il peer TCP attivo R8 non considera le opzioni TCP nel calcolo iniziale del valore MSS.
Image 2.5 - Use TCP Options (MD5) - XR Passive.
Entrambi i peer utilizzano i valori MTU IP predefiniti e sono connessi direttamente. Il peer Cisco IOS R8 svolge un ruolo attivo. Il calcolo del valore TCP MSS in questo scenario può essere riepilogato come segue:
- Entrambi i nodi utilizzano una MTU IP predefinita di 1500 byte
- Il rilevamento dell'MTU del percorso TCP è disabilitato per impostazione predefinita in Cisco IOS XR R7
- Il rilevamento dell'MTU del percorso TCP è abilitato per impostazione predefinita su Cisco IOS R8
- I peer TCP sono connessi direttamente
- Autenticazione TCP MD5 abilitata su entrambi i nodi
- IOS R8 gestisce la connessione BGP
- IOS R8 invia SYN con MSS di 1460 byte
- 1500 (MTU IP interfaccia) - 20 (minTCP_H) - 20 (minIP_H)
- IOS XR R7 invia SYN, ACK con MSS di 1460 byte
- invia il valore più basso di [Received MSS; Local initial MSS]
- 1460 byte MSS ricevuti; Local initial MSS 1460 byte
- Il valore MSS più basso viene utilizzato su entrambi i peer
TCP SYN da R8 - Cisco IOS:
! – TCP SYN sourced from R8
96 5.907127 10.7.8.2 10.7.8.1 TCP 78 52975 → 179 [SYN] Seq=0 Win=16384 Len=0 MSS=1460
Frame 96: 78 bytes on wire (624 bits), 78 bytes captured (624 bits) on interface 0
Ethernet II, Src: fa:16:3e:58:21:ba (fa:16:3e:58:21:ba), Dst: fa:16:3e:68:d9:e5 (fa:16:3e:68:d9:e5)
Internet Protocol Version 4, Src: 10.7.8.2, Dst: 10.7.8.1
Transmission Control Protocol, Src Port: 52975, Dst Port: 179, Seq: 0, Len: 0
Source Port: 52975
Destination Port: 179
[Stream index: 3]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 0
Header Length: 44 bytes
Flags: 0x002 (SYN)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0xb612 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (24 bytes), Maximum segment size, TCP MD5 signature, End of Option List (EOL)
Maximum segment size: 1460 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1460
TCP MD5 signature
End of Option List (EOL)
TCP SYN, ACK originato da R7 - Cisco IOS XR:
! – TCP SYN,ACK sourced from R7
97 0.003446 10.7.8.1 10.7.8.2 TCP 78 179 → 52975 [SYN, ACK] Seq=0 Ack=1 Win=16384 Len=0 MSS=1460
Frame 97: 78 bytes on wire (624 bits), 78 bytes captured (624 bits) on interface 0
Ethernet II, Src: fa:16:3e:68:d9:e5 (fa:16:3e:68:d9:e5), Dst: fa:16:3e:58:21:ba (fa:16:3e:58:21:ba)
Internet Protocol Version 4, Src: 10.7.8.1, Dst: 10.7.8.2
Transmission Control Protocol, Src Port: 179, Dst Port: 52975, Seq: 0, Ack: 1, Len: 0
Source Port: 179
Destination Port: 52975
[Stream index: 3]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 1 (relative ack number)
Header Length: 44 bytes
Flags: 0x012 (SYN, ACK)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0xfb47 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (24 bytes), Maximum segment size, TCP MD5 signature, End of Option List (EOL)
Maximum segment size: 1460 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1460
TCP MD5 signature
End of Option List (EOL)
Dettagli della sessione TCP come visualizzati in R8 - Cisco IOS - ACTIVE:
! – as seen from R8 - Cisco IOS
R8#show ip bgp neighbors
BGP neighbor is 10.7.8.1, remote AS 65007, external link
BGP version 4, remote router ID 192.168.0.7
BGP state = Established, up for 00:06:12
Last read 00:00:16, last write 00:00:16, hold time is 180, keepalive interval is 60 seconds
Neighbor sessions:
1 active, is not multisession capable (disabled)
Neighbor capabilities:
Route refresh: advertised and received(new)
Four-octets ASN Capability: advertised and received
Address family IPv4 Unicast: advertised and received
Enhanced Refresh Capability: advertised
Multisession Capability:
Stateful switchover support enabled: NO for session 1
Message statistics:
InQ depth is 0
OutQ depth is 0
Sent Rcvd
Opens: 1 1
Notifications: 0 0
Updates: 1 1
Keepalives: 7 7
Route Refresh: 0 0
Total: 9 9
Do log neighbor state changes (via global configuration)
Default minimum time between advertisement runs is 30 seconds
For address family: IPv4 Unicast
Session: 10.7.8.1
BGP table version 1, neighbor version 1/0
Output queue size : 0
Index 6, Advertise bit 0
6 update-group member
Slow-peer detection is disabled
Slow-peer split-update-group dynamic is disabled
Sent Rcvd
Prefix activity: ---- ----
Prefixes Current: 0 0
Prefixes Total: 0 0
Implicit Withdraw: 0 0
Explicit Withdraw: 0 0
Used as bestpath: n/a 0
Used as multipath: n/a 0
Used as secondary: n/a 0
Outbound Inbound
Local Policy Denied Prefixes: -------- -------
Total: 0 0
Number of NLRIs in the update sent: max 0, min 0
Last detected as dynamic slow peer: never
Dynamic slow peer recovered: never
Refresh Epoch: 1
Last Sent Refresh Start-of-rib: never
Last Sent Refresh End-of-rib: never
Last Received Refresh Start-of-rib: never
Last Received Refresh End-of-rib: never
Sent Rcvd
Refresh activity: ---- ----
Refresh Start-of-RIB 0 0
Refresh End-of-RIB 0 0
Address tracking is enabled, the RIB does have a route to 10.7.8.1
Connections established 6; dropped 5
Last reset 00:06:18, due to BGP Notification received of session 1, Administrative Reset
External BGP neighbor configured for connected checks (single-hop no-disable-connected-check)
Interface associated: GigabitEthernet0/1 (peering address in same link)
Transport(tcp) path-mtu-discovery is enabled
Graceful-Restart is disabled
SSO is disabled
Connection state is ESTAB, I/O status: 1, unread input bytes: 0
Connection is ECN Disabled, Mininum incoming TTL 0, Outgoing TTL 1
Local host: 10.7.8.2, Local port: 52975
Foreign host: 10.7.8.1, Foreign port: 179
Connection tableid (VRF): 0
Maximum output segment queue size: 50
Enqueued packets for retransmit: 0, input: 0 mis-ordered: 0 (0 bytes)
Event Timers (current time is 0x15DD97):
Timer Starts Wakeups Next
Retrans 10 0 0x0
TimeWait 0 0 0x0
AckHold 9 5 0x0
SendWnd 0 0 0x0
KeepAlive 0 0 0x0
GiveUp 0 0 0x0
PmtuAger 1 0 0x195465
DeadWait 0 0 0x0
Linger 0 0 0x0
ProcessQ 0 0 0x0
iss: 1154289541 snduna: 1154289755 sndnxt: 1154289755
irs: 2149897425 rcvnxt: 2149897635
sndwnd: 32612 scale: 0 maxrcvwnd: 16384
rcvwnd: 16175 scale: 0 delrcvwnd: 209
SRTT: 737 ms, RTTO: 2506 ms, RTV: 1769 ms, KRTT: 0 ms
minRTT: 7 ms, maxRTT: 1000 ms, ACK hold: 200 ms
uptime: 372981 ms, Sent idletime: 16648 ms, Receive idletime: 16431 ms
Status Flags: active open
Option Flags: nagle, path mtu capable, md5
IP Precedence value : 6
Datagrams (max data segment is 1460 bytes):
Rcvd: 18 (out of order: 0), with data: 8, total data bytes: 209
Sent: 16 (retransmit: 0, fastretransmit: 0, partialack: 0, Second Congestion: 0), with data: 9, total data bytes: 213
Packets received in fast path: 0, fast processed: 0, slow path: 0
fast lock acquisition failures: 0, slow path: 0
TCP Semaphore 0x0FBFA8A4 FREE
R8#
Dettagli della sessione TCP come visualizzati su R7 - Cisco IOS XR - PASSIVE:
! – as seen from R7 – Cisco IOS XR
RP/0/0/CPU0:R7#show tcp detail pcb 0x12152e48
Wed Jan 13 13:03:43.363 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Wed Jan 13 12:58:16 2021
PCB 0x12152e48, SO 0x1213c130, TCPCB 0x12156060, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 1, Hash index: 947
Local host: 10.7.8.1, Local port: 179 (Local App PID: 983244)
Foreign host: 10.7.8.2, Foreign port: 52975
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 8 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 8 7 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 2149897425 snduna: 2149897616 sndnxt: 2149897616
sndmax: 2149897616 sndwnd: 16194 sndcwnd: 4380
irs: 1154289541 rcvnxt: 1154289736 rcvwnd: 32631 rcvadv: 1154322367
SRTT: 125 ms, RTTO: 552 ms, RTV: 427 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 229 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: MD5, Nagle
Request flags: none
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R7#
Peer TCP non collegati direttamente
Quando i peer non sono connessi direttamente, la modalità di calcolo iniziale del valore TCP MSS cambia come descritto in precedenza nella sezione introduttiva di questo documento. Lo scenario di una sessione iBGP con tutti i peer configurati con i valori MTU IP predefiniti viene utilizzato per eseguire in modo dettagliato il calcolo del valore MSS.
Immagine 2.6 - Peer TCP non collegati direttamente - iBGP.
L'aspetto importante da notare è che quando il rilevamento dell'MTU del percorso TCP è disabilitato e i peer non sono connessi direttamente, in base alla progettazione, Cisco IOS XR utilizza un valore MTU IP fisso di 1280 byte.
Nell'immagine precedente, R4 svolge un ruolo attivo e gestisce la connessione TCP, R4 apre la sessione TCP con R1 sulla porta di destinazione 179. Entrambi i nodi utilizzano il valore MTU IP predefinito sulle rispettive interfacce. Il calcolo del valore MSS in questo scenario può essere riepilogato come segue:
- Tutti i nodi utilizzano una MTU IP predefinita di 1500 byte
- Il rilevamento dell'MTU del percorso TCP è disabilitato per impostazione predefinita
- I peer TCP non sono connessi direttamente
- R4 gestisce la connessione BGP
- R4 invia SYN con MSS di 1240 byte
- L'MTU dell'interfaccia non viene considerata quando i peer non sono connessi direttamente e il rilevamento dell'MTU del percorso TCP è disabilitato
- In base alla progettazione Cisco IOS XR, 1280 byte sono considerati TCP_DEFAULT_MTU
- 1280 (TCP_DEFAULT_MTU) - 20 (minTCP_H) - 20 (minIP_H)
- R1 invia SYN, ACK con MSS di 1240 byte
- Invia il valore più basso di [Received MSS; Local initial MSS]
- 1240 byte MSS ricevuti; Local initial MSS 1240 byte
- Il valore MSS più basso viene utilizzato su entrambi i peer
TCP SYN originato da R4:
! – TCP SYN sourced from R4
194 434.274181 192.168.0.4 192.168.0.1 TCP 62 37740 → 179 [SYN] Seq=0 Win=16384 Len=0 MSS=1240 WS=1
Frame 194: 62 bytes on wire (496 bits), 62 bytes captured (496 bits) on interface 0
Ethernet II, Src: fa:16:3e:d7:7e:f6 (fa:16:3e:d7:7e:f6), Dst: fa:16:3e:8f:8f:54 (fa:16:3e:8f:8f:54)
Internet Protocol Version 4, Src: 192.168.0.4, Dst: 192.168.0.1
Transmission Control Protocol, Src Port: 37740, Dst Port: 179, Seq: 0, Len: 0
Source Port: 37740
Destination Port: 179
[Stream index: 7]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 0
Header Length: 28 bytes
Flags: 0x002 (SYN)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0x8643 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (8 bytes), Maximum segment size, Window scale, End of Option List (EOL)
Maximum segment size: 1240 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1240
Window scale: 0 (multiply by 1)
End of Option List (EOL)
TCP SYN, ACK originato da R1:
! – TCP SYN,ACK sourced from R1
195 434.277985 192.168.0.1 192.168.0.4 TCP 62 179 → 37740 [SYN, ACK] Seq=0 Ack=1 Win=16384 Len=0 MSS=1240 WS=1
Frame 195: 62 bytes on wire (496 bits), 62 bytes captured (496 bits) on interface 0
Ethernet II, Src: fa:16:3e:8f:8f:54 (fa:16:3e:8f:8f:54), Dst: fa:16:3e:d7:7e:f6 (fa:16:3e:d7:7e:f6)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
Transmission Control Protocol, Src Port: 179, Dst Port: 37740, Seq: 0, Ack: 1, Len: 0
Source Port: 179
Destination Port: 37740
[Stream index: 7]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 1 (relative ack number)
Header Length: 28 bytes
Flags: 0x012 (SYN, ACK)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0xd8f7 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (8 bytes), Maximum segment size, Window scale, End of Option List (EOL)
Maximum segment size: 1240 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1240
Window scale: 0 (multiply by 1)
End of Option List (EOL)
Dettagli della sessione TCP rilevati su R4 - ATTIVO:
! – as seen on R4 - Active
RP/0/0/CPU0:R4#show tcp detail pcb 0x12154d3c
Fri Jan 8 12:32:41.096 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 12:17:46 2021
PCB 0x12154d3c, SO 0x12154460, TCPCB 0x1215486c, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 1577
Local host: 192.168.0.4, Local port: 37740 (Local App PID: 1052958)
Foreign host: 192.168.0.1, Foreign port: 179
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 19 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 16 15 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 2075436506 snduna: 2075436868 sndnxt: 2075436868
sndmax: 2075436868 sndwnd: 32460 sndcwnd: 3720
irs: 4238127261 rcvnxt: 4238127623 rcvwnd: 32479 rcvadv: 4238160102
SRTT: 65 ms, RTTO: 300 ms, RTV: 40 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 229 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 30, connect retry interval: 30 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1240, peer MSS 1240, min MSS 1240, max MSS 1240
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R4#
Dettagli della sessione TCP come visualizzati su R1 - PASSIVE:
! – as seen on R1 – Passive
RP/0/0/CPU0:R1#show tcp detail pcb 0x12155390
Fri Jan 8 12:23:52.041 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 12:17:43 2021
PCB 0x12155390, SO 0x121573e4, TCPCB 0x12156948, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 1577
Local host: 192.168.0.1, Local port: 179 (Local App PID: 983326)
Foreign host: 192.168.0.4, Foreign port: 37740
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 9 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 9 1 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 4238127261 snduna: 4238127471 sndnxt: 4238127471
sndmax: 4238127471 sndwnd: 32631 sndcwnd: 3720
irs: 2075436506 rcvnxt: 2075436716 rcvwnd: 32612 rcvadv: 2075469328
SRTT: 144 ms, RTTO: 578 ms, RTV: 434 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 239 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1240, peer MSS 1240, min MSS 1240, max MSS 1240
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R1#
Peer TCP non collegati direttamente - Usa opzioni TCP (MD5)
Per gli scenari peer non connessi direttamente e in uso dell'autenticazione TCP MD5, non esiste alcuna differenza sostanziale rispetto ai test case o agli scenari precedenti già descritti. Come mostrato in precedenza con l'autenticazione TCP MD5, Cisco IOS XR considera un sovraccarico aggiuntivo e il valore MSS iniziale riflette lo stesso. Per ulteriori informazioni sull'influenza delle opzioni TCP sul calcolo del valore MSS per TCP, consultare le sezioni precedenti Use TCP Options - XR Active e Use TCP Options - XR Passive.
Immagine 2.7 - Peer TCP non collegati direttamente - iBGP + TCP MD5.
Il calcolo del valore TCP MSS in questo scenario può essere riepilogato come segue:
- Tutti i nodi utilizzano una MTU IP predefinita di 1500 byte
- Il rilevamento dell'MTU del percorso TCP è disabilitato per impostazione predefinita
- I peer TCP non sono connessi direttamente
- R4 gestisce la connessione BGP
- La destinazione R1 non è collegata direttamente
- R4 invia SYN con MSS di 1216 byte
- L'MTU dell'interfaccia non viene considerata quando i peer non sono connessi direttamente e il rilevamento dell'MTU del percorso TCP è disabilitato
- Come da progettazione, 1280 byte sono considerati TCP_DEFAULT_MTU
- 1280 (TCP_DEFAULT_MTU) - 20 (minTCP_H) - 20 (minIP_H) - 24 byte (sovraccarico opzioni TCP IOS XR)
- R1 invia SYN, ACK con MSS di 1216 byte
- Invia il valore più basso di [Received MSS; Local initial MSS]
- 1216 byte MSS ricevuti; Local initial MSS 1240 byte
- Il valore MSS più basso viene utilizzato su entrambi i peer
TCP SYN originato da R4:
! – TCP SYN sourced from R4
3425 3.691042 192.168.0.4 192.168.0.1 TCP 82 42135 → 179 [SYN] Seq=0 Win=16384 Len=0 MSS=1216 WS=1
Frame 3425: 82 bytes on wire (656 bits), 82 bytes captured (656 bits) on interface 0
Ethernet II, Src: fa:16:3e:d7:7e:f6 (fa:16:3e:d7:7e:f6), Dst: fa:16:3e:8f:8f:54 (fa:16:3e:8f:8f:54)
Internet Protocol Version 4, Src: 192.168.0.4, Dst: 192.168.0.1
Transmission Control Protocol, Src Port: 42135, Dst Port: 179, Seq: 0, Len: 0
Source Port: 42135
Destination Port: 179
[Stream index: 10]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 0
Header Length: 48 bytes
Flags: 0x002 (SYN)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0xc503 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (28 bytes), Maximum segment size, Window scale, No-Operation (NOP), TCP MD5 signature, End of Option List (EOL)
Maximum segment size: 1216 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1216
Window scale: 0 (multiply by 1)
No-Operation (NOP)
TCP MD5 signature
End of Option List (EOL)
TCP SYN, ACK originato da R1:
! – TCP SYN,ACK sourced from R1
3426 0.004186 192.168.0.1 192.168.0.4 TCP 82 179 → 42135 [SYN, ACK] Seq=0 Ack=1 Win=16384 Len=0 MSS=1216 WS=1
Frame 3426: 82 bytes on wire (656 bits), 82 bytes captured (656 bits) on interface 0
Ethernet II, Src: fa:16:3e:8f:8f:54 (fa:16:3e:8f:8f:54), Dst: fa:16:3e:d7:7e:f6 (fa:16:3e:d7:7e:f6)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
Transmission Control Protocol, Src Port: 179, Dst Port: 42135, Seq: 0, Ack: 1, Len: 0
Source Port: 179
Destination Port: 42135
[Stream index: 10]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 1 (relative ack number)
Header Length: 48 bytes
Flags: 0x012 (SYN, ACK)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0xbb05 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (28 bytes), Maximum segment size, Window scale, No-Operation (NOP), TCP MD5 signature, End of Option List (EOL)
Maximum segment size: 1216 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1216
Window scale: 0 (multiply by 1)
No-Operation (NOP)
TCP MD5 signature
End of Option List (EOL)
Dettagli della sessione TCP rilevati su R4 - ATTIVO:
! – as seen from R4 – Active
RP/0/0/CPU0:R4#show tcp detail pcb 0x12154490
Tue Jan 12 14:37:32.097 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Tue Jan 12 14:27:42 2021
PCB 0x12154490, SO 0x12155014, TCPCB 0x12155a84, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 1876
Local host: 192.168.0.4, Local port: 42135 (Local App PID: 1052958)
Foreign host: 192.168.0.1, Foreign port: 179
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 14 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 11 9 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 3124761989 snduna: 3124763317 sndnxt: 3124763317
sndmax: 3124763317 sndwnd: 32711 sndcwnd: 3648
irs: 1090344992 rcvnxt: 1090346320 rcvwnd: 32730 rcvadv: 1090379050
SRTT: 28 ms, RTTO: 300 ms, RTV: 57 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 229 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 30, connect retry interval: 30 secs
State flags: none
Feature flags: MD5, Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1216, peer MSS 1216, min MSS 1216, max MSS 1216
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R4#
Dettagli della sessione TCP come visualizzati su R1 - PASSIVE:
! – as seen from R1 – Passive
RP/0/0/CPU0:R1#show tcp detail pcb 0x12168df4
Tue Jan 12 14:36:38.860 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Tue Jan 12 14:27:32 2021
PCB 0x12168df4, SO 0x12156bf8, TCPCB 0x12157a44, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 1876
Local host: 192.168.0.1, Local port: 179 (Local App PID: 983326)
Foreign host: 192.168.0.4, Foreign port: 42135
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 12 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 12 1 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 1090344992 snduna: 1090346320 sndnxt: 1090346320
sndmax: 1090346320 sndwnd: 32730 sndcwnd: 3648
irs: 3124761989 rcvnxt: 3124763317 rcvwnd: 32711 rcvadv: 3124796028
SRTT: 150 ms, RTTO: 558 ms, RTV: 408 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 239 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: MD5, Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1216, peer MSS 1216, min MSS 1240, max MSS 1240
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R1#
Peer TCP non collegati direttamente - il segmento del percorso ha una MTU IP inferiore
Nello scenario successivo, l'obiettivo è quello di osservare e concludere cosa succede se esiste un segmento di percorso intermedio con una MTU IP inferiore mentre è attiva la condizione predefinita, ossia il PMTUD TCP è disabilitato. Fare riferimento a questa immagine.
Immagine 2.8 - Il segmento del percorso R2/R3 ha una MTU IP inferiore.
Come scenario iniziale, le informazioni BGP sono minime, ossia tutto ciò che deve essere scambiato tra i peer BGP può essere realizzato con pacchetti IP che rientrano nella MTU del percorso minimo di 512 byte. In questo caso, il calcolo del valore MSS viene eseguito come descritto nella sezione Peer TCP non connessi direttamente. Sia R1 che R4 selezionano un valore MSS di 1240 byte.
Dettagli della sessione TCP rilevati su R4 - ATTIVO:
! – as seen from R4 – Active
RP/0/0/CPU0:R4#show tcp detail pcb 0x15390fe8
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Wed May 12 12:09:48 2021
PCB 0x15390fe8, SO 0x15391a7c, TCPCB 0x15391368, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 835
Local host: 192.168.0.4, Local port: 39046 (Local App PID: 1196319)
Foreign host: 192.168.0.1, Foreign port: 179
(Local App PID/instance/SPL_APP_ID: 1196319/1/0)
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 1267 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 1280 1235 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 1991226354 snduna: 1991250450 sndnxt: 1991250450
sndmax: 1991250450 sndwnd: 32578 sndcwnd: 2480
irs: 4276699304 rcvnxt: 4276746737 rcvwnd: 31568 rcvadv: 4276778305
SRTT: 213 ms, RTTO: 300 ms, RTV: 54 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 269 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 10, connect retry interval: 30 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1240, peer MSS 1240, min MSS 1240, max MSS 1240
<snip>
Dettagli della sessione TCP come visualizzati su R1 - PASSIVE:
! – as seen from R1 – Passive
RP/0/0/CPU0:R1#show tcp detail pcb 0x15393770
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Wed May 12 12:09:46 2021
PCB 0x15393770, SO 0x15392224, TCPCB 0x153928cc, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 835
Local host: 192.168.0.1, Local port: 179 (Local App PID: 1192224)
Foreign host: 192.168.0.4, Foreign port: 39046
(Local App PID/instance/SPL_APP_ID: 1192224/1/0)
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 1280 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 1264 1213 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 4276699304 snduna: 4276746718 sndnxt: 4276746718
sndmax: 4276746718 sndwnd: 31587 sndcwnd: 3720
irs: 1991226354 rcvnxt: 1991250431 rcvwnd: 32597 rcvadv: 1991283028
SRTT: 202 ms, RTTO: 355 ms, RTV: 153 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 309 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1240, peer MSS 1240, min MSS 1240, max MSS 1240
<snip>
Ora che la sessione BGP è stata stabilita, tenere presente che viene attivato un messaggio di aggiornamento BGP di dimensioni superiori alla MTU minima del percorso di 512 byte. Come si può osservare dagli output, Cisco IOS XR non imposta il bit DF con il messaggio di aggiornamento BGP, ossia le informazioni BGP vengono trasmesse ai nodi intermedi a spese della frammentazione dei pacchetti.
Aggiornamento BGP originato da R1 - PASSIVO:
! – as seen from R1 – Passive – BGP UPDATE
! – Note Total Length of 1097 bytes higher than the IP MTU value of 512 bytes at R2-R3 path segment
23 3.450878 192.168.0.1 192.168.0.4 BGP 1111 UPDATE Message
Frame 23: 1111 bytes on wire (8888 bits), 1111 bytes captured (8888 bits) on interface 0
Ethernet II, Src: fa:16:3e:42:18:05 (fa:16:3e:42:18:05), Dst: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0xc0 (DSCP: CS6, ECN: Not-ECT)
Total Length: 1097
Identification: 0x5841 (22593)
Flags: 0x00
0... .... = Reserved bit: Not set
.0.. .... = Don't fragment: Not set
..0. .... = More fragments: Not set
Fragment offset: 0
Time to live: 255
Protocol: TCP (6)
Header checksum: 0x54a4 [validation disabled]
[Header checksum status: Unverified]
Source: 192.168.0.1
Destination: 192.168.0.4
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Transmission Control Protocol, Src Port: 179, Dst Port: 39046, Seq: 20, Ack: 20, Len: 1057
Border Gateway Protocol - UPDATE Message
Marker: ffffffffffffffffffffffffffffffff
Length: 1057
Type: UPDATE Message (2)
Withdrawn Routes Length: 0
Total Path Attribute Length: 1034
Path attributes
Path Attribute - MP_REACH_NLRI
Path Attribute - ORIGIN: INCOMPLETE
Path Attribute - AS_PATH: empty
Path Attribute - MULTI_EXIT_DISC: 0
Path Attribute - LOCAL_PREF: 100
La frammentazione del messaggio di aggiornamento BGP originato dal nodo R1 avviene sul nodo R2, come può essere osservato dall'acquisizione del traffico effettuata sull'interfaccia R2 GE0/0/0/1.
Frammentazione IP sul nodo R2:
! – as seen from R2 – GE0/0/0/1
! – Node R2 fragments original packet in three distinct packets
4 1.334852 192.168.0.1 192.168.0.4 BGP 522 UPDATE Message
5 0.000289 192.168.0.1 192.168.0.4 IPv4 522 Fragmented IP protocol (proto=TCP 6, off=488, ID=7b41)
6 0.000122 192.168.0.1 192.168.0.4 IPv4 135 Fragmented IP protocol (proto=TCP 6, off=976, ID=7b41)
! – Captured frame details
Frame 4: 522 bytes on wire (4176 bits), 522 bytes captured (4176 bits) on interface 0
Ethernet II, Src: fa:16:3e:61:25:f0 (fa:16:3e:61:25:f0), Dst: fa:16:3e:23:ab:27 (fa:16:3e:23:ab:27)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0xc0 (DSCP: CS6, ECN: Not-ECT)
Total Length: 508
Identification: 0x7b41 (31553)
Flags: 0x01 (More Fragments)
0... .... = Reserved bit: Not set
.0.. .... = Don't fragment: Not set
..1. .... = More fragments: Set
Fragment offset: 0
Time to live: 254
Protocol: TCP (6)
Header checksum: 0x14f1 [validation disabled]
[Header checksum status: Unverified]
Source: 192.168.0.1
Destination: 192.168.0.4
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Transmission Control Protocol, Src Port: 179, Dst Port: 39046, Seq: 4276759681, Ack: 1991250830
Border Gateway Protocol - UPDATE Message
<snip>
Frame 5: 522 bytes on wire (4176 bits), 522 bytes captured (4176 bits) on interface 0
Ethernet II, Src: fa:16:3e:61:25:f0 (fa:16:3e:61:25:f0), Dst: fa:16:3e:23:ab:27 (fa:16:3e:23:ab:27)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0xc0 (DSCP: CS6, ECN: Not-ECT)
Total Length: 508
Identification: 0x7b41 (31553)
Flags: 0x01 (More Fragments)
0... .... = Reserved bit: Not set
.0.. .... = Don't fragment: Not set
..1. .... = More fragments: Set
Fragment offset: 488
Time to live: 254
Protocol: TCP (6)
Header checksum: 0x14b4 [validation disabled]
[Header checksum status: Unverified]
Source: 192.168.0.1
Destination: 192.168.0.4
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Data (488 bytes)
<snip>
Frame 6: 135 bytes on wire (1080 bits), 135 bytes captured (1080 bits) on interface 0
Ethernet II, Src: fa:16:3e:61:25:f0 (fa:16:3e:61:25:f0), Dst: fa:16:3e:23:ab:27 (fa:16:3e:23:ab:27)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0xc0 (DSCP: CS6, ECN: Not-ECT)
Total Length: 121
Identification: 0x7b41 (31553)
Flags: 0x00
0... .... = Reserved bit: Not set
.0.. .... = Don't fragment: Not set
..0. .... = More fragments: Not set
Fragment offset: 976
Time to live: 254
Protocol: TCP (6)
Header checksum: 0x35fa [validation disabled]
[Header checksum status: Unverified]
Source: 192.168.0.1
Destination: 192.168.0.4
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Data (101 bytes)
<snip>
Scenari - PMTUD TCP abilitato
Abilitazione della funzionalità PMTUD
Dopo aver abilitato il PMTUD, a prescindere dal fatto che i peer siano connessi direttamente o non direttamente, nel calcolo iniziale del valore MSS viene sempre presa in considerazione l'MTU IP dell'interfaccia in uscita.
In questo scenario viene illustrato il comportamento previsto quando la funzionalità PMTUD è abilitata. In questo caso, il nodo R4 di Cisco IOS XR svolge il ruolo attivo, gestisce la connessione TCP e apre la sessione TCP con il nodo R1 di Cisco IOS XR sulla porta di destinazione 179. Entrambi i nodi utilizzano i valori MTU IP predefiniti sulle rispettive interfacce.
Immagine 3.1 - TCP e PMTUD attivati.
Il calcolo del valore MSS in questo scenario può essere riepilogato come segue:
- Tutti i nodi utilizzano una MTU IP predefinita di 1500 byte
- Rilevamento MTU percorso TCP abilitato
- I peer TCP non sono connessi direttamente
- R4 gestisce la connessione BGP
- R4 invia SYN con MSS di 1460 byte
- 1500 (MTU IP interfaccia) - 20 (minTCP_H) - 20 (minIP_H)
- R1 invia SYN, ACK con MSS di 1460 byte
- Invia il valore più basso di [Received MSS; Local initial MSS]
- 1460 byte MSS ricevuti; Local initial MSS 1460 byte
- Il valore MSS più basso viene utilizzato su entrambi i peer
Per evidenziare i cambiamenti di comportamento introdotti dall'abilitazione del PMTUD, gli output successivi mostrano la sequenza di eventi:
- lo stato iniziale della sessione TCP stabilita nello scenario predefinito della funzionalità PMTUD disabilitata;
- la funzionalità PMTUD è configurata e abilitata su entrambi i peer TCP R4 e R1;
- La sessione TCP viene riavviata, viene eseguito il calcolo del valore MSS ed è influenzata dalla funzionalità PMTUD di TCP.
Come mostrato su R4 - Attivo - PMTUD TCP disabilitato (impostazione predefinita):
! – as seen on R4 – Active
! – TCP path mtu discovery disabled (default)
! – TCP session initial state
RP/0/0/CPU0:R4#show tcp detail pcb 0x121536c8
Fri Jan 8 16:06:30.237 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 16:05:15 2021
PCB 0x121536c8, SO 0x12155370, TCPCB 0x12154f64, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 376
Local host: 192.168.0.4, Local port: 20155 (Local App PID: 1052958)
Foreign host: 192.168.0.1, Foreign port: 179
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 6 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 3 2 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 357400981 snduna: 357401257 sndnxt: 357401257
sndmax: 357401257 sndwnd: 32546 sndcwnd: 3720
irs: 524019443 rcvnxt: 524019719 rcvwnd: 32565 rcvadv: 524052284
SRTT: 72 ms, RTTO: 416 ms, RTV: 344 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 229 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 30, connect retry interval: 30 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1240, peer MSS 1240, min MSS 1240, max MSS 1240
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R4#
Come mostrato sulla scheda R1 - PASSIVE - TCP PMTUD disabilitata (impostazione predefinita):
! – as seen on R1 - Passive
! – TCP path mtu discovery disabled (default)
! – TCP session initial state
RP/0/0/CPU0:R1#show tcp detail pcb 0x12157020
Fri Jan 8 16:05:52.868 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 16:05:12 2021
PCB 0x12157020, SO 0x121565ac, TCPCB 0x121560ec, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 376
Local host: 192.168.0.1, Local port: 179 (Local App PID: 983326)
Foreign host: 192.168.0.4, Foreign port: 20155
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 3 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 3 1 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 524019443 snduna: 524019700 sndnxt: 524019700
sndmax: 524019700 sndwnd: 32584 sndcwnd: 3720
irs: 357400981 rcvnxt: 357401238 rcvwnd: 32565 rcvadv: 357433803
SRTT: 46 ms, RTTO: 300 ms, RTV: 249 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 239 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: Win Scale, Nagle
Request flags: Win Scale
Datagrams (in bytes): MSS 1240, peer MSS 1240, min MSS 1240, max MSS 1240
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R1#
Come mostrato su R4 - Attivo - PMTUD TCP abilitato:
! – 'debug tcp pmtud' output on R4
! - tcp path mtu discovery enabled and uses default Path MTU aging timer (10 min / 600000 msec)
RP/0/0/CPU0:Jan 8 16:09:28.285 : tcp[399]: [t21] Try to enable path MTU discovery(neww age timer: 10 min)
RP/0/0/CPU0:Jan 8 16:09:28.285 : tcp[399]: [t21] Path mtu is ON (age-timer: 10)
! – as seen on R4 – Active
! – TCP PMTUD is enabled
RP/0/0/CPU0:R4#show tcp detail pcb 0x121536c8
Fri Jan 8 16:11:00.138 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 16:05:15 2021
PCB 0x121536c8, SO 0x12155370, TCPCB 0x12154f64, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 376
Local host: 192.168.0.4, Local port: 20155 (Local App PID: 1052958)
Foreign host: 192.168.0.1, Foreign port: 179
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 10 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 7 4 0
KeepAlive 1 0 0
PmtuAger 1 0 508096
GiveUp 0 0 0
Throttle 0 0 0
iss: 357400981 snduna: 357401333 sndnxt: 357401333
sndmax: 357401333 sndwnd: 32470 sndcwnd: 3720
irs: 524019443 rcvnxt: 524019795 rcvwnd: 32489 rcvadv: 524052284
SRTT: 116 ms, RTTO: 578 ms, RTV: 462 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 229 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 30, connect retry interval: 30 secs
State flags: PMTU ager
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 1240, peer MSS 1240, min MSS 1240, max MSS 1240
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R4#
Come mostrato sulla scheda R1 - PASSIVE - TCP PMTUD abilitata:
! – 'debug tcp pmtud' output on R1
! - tcp path mtu discovery is enabled and uses default Path MTU aging timer (10 min / 600000 msec)
RP/0/0/CPU0:Jan 8 16:09:25.214 : tcp[399]: [t21] Try to enable path MTU discovery(neww age timer: 10 min)
RP/0/0/CPU0:Jan 8 16:09:25.214 : tcp[399]: [t21] Path mtu is ON (age-timer: 10)
! – as seen on R1 - Passive
! – TCP PMTUD is enabled
RP/0/0/CPU0:R1#show tcp detail pcb 0x12157020
Fri Jan 8 16:10:03.101 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 16:05:12 2021
PCB 0x12157020, SO 0x121565ac, TCPCB 0x121560ec, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 376
Local host: 192.168.0.1, Local port: 179 (Local App PID: 983326)
Foreign host: 192.168.0.4, Foreign port: 20155
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 7 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 7 4 0
KeepAlive 1 0 0
PmtuAger 1 0 562042
GiveUp 0 0 0
Throttle 0 0 0
iss: 524019443 snduna: 524019776 sndnxt: 524019776
sndmax: 524019776 sndwnd: 32508 sndcwnd: 3720
irs: 357400981 rcvnxt: 357401314 rcvwnd: 32489 rcvadv: 357433803
SRTT: 95 ms, RTTO: 528 ms, RTV: 433 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 239 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: PMTU ager
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 1240, peer MSS 1240, min MSS 1240, max MSS 1240
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R1#
Notare il comportamento del timer PMTU:
! – Note PmtuAger timer initial value is 10min
! – but after initial interval expires then it expires every 2min
! – As seen from 'debug tcp pmtud' output
! – TCP PMTUD is enabled
RP/0/0/CPU0:Jan 8 16:09:25.214 : tcp[399]: [t21] Try to enable path MTU discovery(neww age timer: 10 min)
RP/0/0/CPU0:Jan 8 16:09:25.214 : tcp[399]: [t21] Path mtu is ON (age-timer: 10)
RP/0/0/CPU0:Jan 8 16:19:25.233 : tcp[399]: [t21] PCB 0x12157020: Trying next higher MTU: 1240
RP/0/0/CPU0:Jan 8 16:21:25.245 : tcp[399]: [t21] PCB 0x12157020: Trying next higher MTU: 1240
RP/0/0/CPU0:Jan 8 16:23:25.256 : tcp[399]: [t21] PCB 0x12157020: Trying next higher MTU: 1240
Come mostrato su R4 - ATTIVO - riavvio sessione BGP - TCP SYN:
! – Once BGP session is cleared
! – TCP SYN sourced from R4 – Active
! – MSS calculation takes place and is influenced by TCP PMTUD
2734 4.810311 192.168.0.4 192.168.0.1 TCP 62 32077 → 179 [SYN] Seq=0 Win=16384 Len=0 MSS=1460 WS=1
Frame 2734: 62 bytes on wire (496 bits), 62 bytes captured (496 bits) on interface 0
Ethernet II, Src: fa:16:3e:d7:7e:f6 (fa:16:3e:d7:7e:f6), Dst: fa:16:3e:8f:8f:54 (fa:16:3e:8f:8f:54)
Internet Protocol Version 4, Src: 192.168.0.4, Dst: 192.168.0.1
Transmission Control Protocol, Src Port: 32077, Dst Port: 179, Seq: 0, Len: 0
Source Port: 32077
Destination Port: 179
[Stream index: 25]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 0
Header Length: 28 bytes
Flags: 0x002 (SYN)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0x6398 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (8 bytes), Maximum segment size, Window scale, End of Option List (EOL)
Maximum segment size: 1460 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1460
Window scale: 0 (multiply by 1)
End of Option List (EOL)
Come mostrato su R1 - PASSIVE - BGP Session restart - TCP SYN, ACK.
! – Once BGP session is cleared
! – TCP SYN,ACK sourced from R1 - Passive
! – MSS calculation takes place and is influenced by TCP PMTUD
2735 0.003879 192.168.0.1 192.168.0.4 TCP 62 179 → 32077 [SYN, ACK] Seq=0 Ack=1 Win=16384 Len=0 MSS=1460 WS=1
Frame 2735: 62 bytes on wire (496 bits), 62 bytes captured (496 bits) on interface 0
Ethernet II, Src: fa:16:3e:8f:8f:54 (fa:16:3e:8f:8f:54), Dst: fa:16:3e:d7:7e:f6 (fa:16:3e:d7:7e:f6)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
Transmission Control Protocol, Src Port: 179, Dst Port: 32077, Seq: 0, Ack: 1, Len: 0
Source Port: 179
Destination Port: 32077
[Stream index: 25]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 1 (relative ack number)
Header Length: 28 bytes
Flags: 0x012 (SYN, ACK)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0xbf77 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (8 bytes), Maximum segment size, Window scale, End of Option List (EOL)
Maximum segment size: 1460 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1460
Window scale: 0 (multiply by 1)
End of Option List (EOL)
Dettagli della sessione TCP come mostrato su R4 - ATTIVO - dopo l'abilitazione del PMTUD TCP e la cancellazione della sessione BGP:
! – BGP session re-established
! – as seen on R4 – Active
RP/0/0/CPU0:R4#show tcp detail pcb 0x121567f4
Fri Jan 8 16:45:13.928 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 16:41:49 2021
PCB 0x121567f4, SO 0x12154460, TCPCB 0x12156190, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 10
Local host: 192.168.0.4, Local port: 32077 (Local App PID: 1052958)
Foreign host: 192.168.0.1, Foreign port: 179
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 8 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 5 3 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 1254100669 snduna: 1254100983 sndnxt: 1254100983
sndmax: 1254100983 sndwnd: 32508 sndcwnd: 4380
irs: 839938559 rcvnxt: 839938873 rcvwnd: 32527 rcvadv: 839971400
SRTT: 79 ms, RTTO: 485 ms, RTV: 406 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 229 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 30, connect retry interval: 30 secs
State flags: none
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R4#
Dettagli della sessione TCP come mostrato su R1 - PASSIVE - dopo l'abilitazione del PMTUD TCP e la cancellazione della sessione BGP.
! – BGP session re-established
! – as seen on R1 – Passive
RP/0/0/CPU0:R1#show tcp detail pcb 0x121558cc
Fri Jan 8 16:44:59.448 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Fri Jan 8 16:41:46 2021
PCB 0x121558cc, SO 0x121556d4, TCPCB 0x121575bc, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 10
Local host: 192.168.0.1, Local port: 179 (Local App PID: 983326)
Foreign host: 192.168.0.4, Foreign port: 32077
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 6 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 6 3 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 839938559 snduna: 839938873 sndnxt: 839938873
sndmax: 839938873 sndwnd: 32527 sndcwnd: 4380
irs: 1254100669 rcvnxt: 1254100983 rcvwnd: 32508 rcvadv: 1254133491
SRTT: 76 ms, RTTO: 454 ms, RTV: 378 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 219 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R1#
PMTUD - Il segmento del percorso ha una MTU IP inferiore
Lo scenario precedente ha aiutato a capire cosa succede quando viene stabilita una sessione TCP iniziale con la funzionalità PMTUD abilitata. Questo scenario si basa sull'esempio precedente e aiuta a comprendere il funzionamento della funzionalità PMTUD del protocollo TCP e l'influenza che questo ha sulle sessioni TCP stabilite.
Immagine 3.2 - PMTUD abilitato e segmento del percorso con MTU IP inferiore.
Si consideri l'immagine precedente come riferimento, si supponga che la sessione BGP sia stata stabilita e R1 invii il messaggio di aggiornamento BGP trasportato da un pacchetto IP di dimensioni superiori a 512 byte. Con la funzionalità PMTUD abilitata, il bit DF (Don't Fragment) è ora impostato. Pertanto, il nodo R2 scarta il pacchetto IP e invia un messaggio ICMP (Internet Control Message Protocol) (destinazione irraggiungibile - tipo 3; Frammentazione richiesta (codice 4) per tornare alla R1. Al nodo R1, dopo la ricezione del messaggio ICMP, viene attivato il PMTUD e viene effettuato un tentativo di stabilire l'MTU IP più bassa del percorso. A tal fine, viene usato il valore più basso successivo di un insieme di livelli di soglia ben definiti, ossia il valore MSS di una nuova sessione TCP. Infine, il protocollo TCP ritrasmette l'aggiornamento BGP originale con il nuovo valore MSS e questo processo viene ripetuto il numero di volte necessario finché non viene visualizzato il messaggio ICMP (Destination Unreachable - type 3; Frammentazione richiesta (codice 4) non più ricevuta. Ciò significa che il valore MSS in uso è tale che ogni pacchetto inviato rientra nell'MTU IP del segmento di percorso più basso. Con il passare del tempo, il PMTUD controllato dal timer PmtuAger attraversa i livelli di soglia in direzione inversa e riporta il valore MSS al valore massimo. In qualsiasi momento, se un messaggio ICMP (Destination Unreachable - type 3 ; Frammentazione richiesta (codice 4) ricevuta di nuovo, quindi il PMTUD funziona come descritto in precedenza.
Gli output successivi esaminano il comportamento PMTUD appena descritto e iniziano dallo scenario di una sessione TCP stabilita. In questo caso, il nodo R4 di Cisco IOS XR svolge un ruolo attivo, pertanto gestisce la connessione TCP e apre la sessione TCP con R1 sulla porta di destinazione 179. Entrambi i nodi utilizzano i valori MTU IP predefiniti sulle rispettive interfacce. Il calcolo MSS iniziale in questo scenario può essere riassunto come segue:
- Il segmento intermedio tra i nodi R2 e R3 utilizza 512 byte di MTU IP non predefinita.
- R1 e R4 utilizzano i valori MTU predefiniti sulle loro interfacce.
- Rilevamento dell'MTU del percorso TCP abilitato.
- I peer TCP non sono connessi direttamente.
- R4 gestisce la connessione BGP.
- R4 invia SYN con MSS di 1460 byte.
- 1500 (MTU IP dell'interfaccia) - 20 (minTCP_H) - 20 (minIP_H).
- R1 invia SYN, ACK con MSS di 1460 byte.
- Invia il valore più basso di [Received MSS ; Local initial MSS].
- 1460 byte MSS ricevuti; Local initial MSS 1460 byte.
- Il valore MSS più basso viene utilizzato su entrambi i peer.
TCP SYN originato da R4:
! – Initial TCP session establishment
! – TCP SYN sourced from R4
392 6.752774 192.168.0.4 192.168.0.1 TCP 62 32449 → 179 [SYN] Seq=0 Win=16384 Len=0 MSS=1460 WS=1
Frame 392: 62 bytes on wire (496 bits), 62 bytes captured (496 bits) on interface 0
Ethernet II, Src: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80), Dst: fa:16:3e:42:18:05 (fa:16:3e:42:18:05)
Internet Protocol Version 4, Src: 192.168.0.4, Dst: 192.168.0.1
Transmission Control Protocol, Src Port: 32449, Dst Port: 179, Seq: 0, Len: 0
Source Port: 32449
Destination Port: 179
[Stream index: 10]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 0
Header Length: 28 bytes
Flags: 0x002 (SYN)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0x6858 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (8 bytes), Maximum segment size, Window scale, End of Option List (EOL)
Maximum segment size: 1460 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1460
Window scale: 0 (multiply by 1)
End of Option List (EOL)
TCP SYN, ACK originato da R1:
! – Initial TCP session establishment
! – TCP SYN,ACK sourced from R1
393 0.003628 192.168.0.1 192.168.0.4 TCP 62 179 → 32449 [SYN, ACK] Seq=0 Ack=1 Win=16384 Len=0 MSS=1460 WS=1
Frame 393: 62 bytes on wire (496 bits), 62 bytes captured (496 bits) on interface 0
Ethernet II, Src: fa:16:3e:42:18:05 (fa:16:3e:42:18:05), Dst: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
Transmission Control Protocol, Src Port: 179, Dst Port: 32449, Seq: 0, Ack: 1, Len: 0
Source Port: 179
Destination Port: 32449
[Stream index: 10]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 1 (relative ack number)
Header Length: 28 bytes
Flags: 0x012 (SYN, ACK)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0x509e [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (8 bytes), Maximum segment size, Window scale, End of Option List (EOL)
Maximum segment size: 1460 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1460
Window scale: 0 (multiply by 1)
End of Option List (EOL)
Una volta stabilita la sessione BGP, il nodo R1 invia il messaggio di aggiornamento BGP e riceve il messaggio ICMP (Destination Unreachable - type 3 ; Frammentazione richiesta - Codice 4) restituita originata dal nodo R2.
Questo si verifica perché il pacchetto IP che invia il messaggio di aggiornamento BGP ha il bit DF impostato e l'MTU IP di 512 byte usata sul segmento R2/R3 è inferiore alle dimensioni del pacchetto IP di 1116 byte. Come spiegato in precedenza, la ricezione del messaggio ICMP attiva il meccanismo PMTUD.
In R1 ICMP, viene ricevuto il messaggio Type 3/Code 4:
! – as seen from R1 – Passive
! – After session is established R1 sends BGP Update message with IP length of 1116 Bytes
! – note IP Header Flags shows DF bit set
528 5.893055 192.168.0.1 192.168.0.4 BGP 1130 UPDATE Message, KEEPALIVE Message
Frame 528: 1130 bytes on wire (9040 bits), 1130 bytes captured (9040 bits) on interface 0
Ethernet II, Src: fa:16:3e:42:18:05 (fa:16:3e:42:18:05), Dst: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0xc0 (DSCP: CS6, ECN: Not-ECT)
Total Length: 1116
Identification: 0x8c37 (35895)
Flags: 0x02 (Don't Fragment)
Fragment offset: 0
Time to live: 255
Protocol: TCP (6)
Header checksum: 0xe09a [validation disabled]
[Header checksum status: Unverified]
Source: 192.168.0.1
Destination: 192.168.0.4
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Transmission Control Protocol, Src Port: 179, Dst Port: 32449, Seq: 318, Ack: 251, Len: 1076
Border Gateway Protocol - UPDATE Message
Border Gateway Protocol - KEEPALIVE Message
<snip>
! – as seen from R1 – Passive
! – IP MTU on R2/R3 is lower than IP packet length and DF bit is set
! – R1 receives ICMP error message from R2
! – note R2 ICMP error message carries Next-Hop MTU
! – “The size in octets of the largest datagram that could be forwarded, along the path of
! the original datagram, without being fragmented at this router. The size includes the
! IP header and IP data, and does not include any lower-level headers.”
529 0.002423 10.2.3.1 192.168.0.1 ICMP 110 Destination unreachable (Fragmentation needed)
Frame 529: 110 bytes on wire (880 bits), 110 bytes captured (880 bits) on interface 0
Ethernet II, Src: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80), Dst: fa:16:3e:42:18:05 (fa:16:3e:42:18:05)
Internet Protocol Version 4, Src: 10.2.3.1, Dst: 192.168.0.1
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
Total Length: 96
Identification: 0x0001 (1)
Flags: 0x00
Fragment offset: 0
Time to live: 255
Protocol: ICMP (1)
Header checksum: 0xac97 [validation disabled]
[Header checksum status: Unverified]
Source: 10.2.3.1
Destination: 192.168.0.1
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Internet Control Message Protocol
Type: 3 (Destination unreachable)
Code: 4 (Fragmentation needed)
Checksum: 0x2d52 [correct]
[Checksum Status: Good]
Length: 17
[Length of original datagram: 68]
Unused: 0011
MTU of next hop: 512
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0xc0 (DSCP: CS6, ECN: Not-ECT)
Total Length: 1116
Identification: 0x8c37 (35895)
Flags: 0x02 (Don't Fragment)
Fragment offset: 0
Time to live: 254
Protocol: TCP (6)
Header checksum: 0xe19a [validation disabled]
[Header checksum status: Unverified]
Source: 192.168.0.1
Destination: 192.168.0.4
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Transmission Control Protocol, Src Port: 179, Dst Port: 32449, Seq: 2847698730, Ack: 2130367817
Border Gateway Protocol - UPDATE Message
[Packet size limited during capture: IPv4 truncated]
Sul nodo R1, attivato dal messaggio ICMP, il PMTUD TCP tenta di stabilire la MTU IP inferiore end-to-end utilizzando il valore immediatamente inferiore di un set di livelli di soglia ben definiti (IP MTU). Questi livelli di soglia sono documentati nella RFC 1191 - Path MTU discovery.
MTU plateaus from RFC 1191
- values include both TCP and IP headers
65535
32000
17914
8166
4352
2002
1492
1006
508
296
68
Tuttavia, poiché ICMP (Destination Unreachable - type 3; Frammentazione richiesta - Codice 4) Il messaggio ricevuto dal nodo R1 trasmette l'MTU dell'hop successivo, come mostrato di seguito, il nodo R1 utilizza questo valore, che nell'esempio è di 512 byte, e regola il valore MSS della sessione TCP. Notare che la lunghezza originale del segmento TCP era di 1076 byte, quindi sono necessari tre pacchetti per ritrasmettere il segmento TCP originale.
Come mostrato sul funzionamento R1 - PASSIVE - PMTUD:
! – As seen from R1 – Passive
! – Hint is provided by ICMP unreachable message MTU of next-hop field: 512 bytes
! – R1 then considers this value and retransmits BGP Update split in three distinct packets
! – Sum of TCP length = 472 + 472 + 132 = 1076 bytes
530 0.007497 192.168.0.1 192.168.0.4 TCP 526 [TCP Out-Of-Order] 179 → 32449 [ACK] Seq=318 Ack=251 Win=32593 Len=472
532 0.015374 192.168.0.1 192.168.0.4 TCP 526 [TCP Retransmission] 179 → 32449 [ACK] Seq=790 Ack=251 Win=32593 Len=472
533 0.004129 192.168.0.1 192.168.0.4 TCP 186 [TCP Retransmission] 179 → 32449 [PSH, ACK] Seq=1262 Ack=251 Win=32593 Len=132
Come accennato in precedenza, dopo aver trasmesso tutti i pacchetti nel tempo, il PMTUD attraversa i livelli di soglia nella direzione inversa controllata dal timer PmtuAger e cerca di aumentare il valore MSS al valore massimo secondo lo scenario in uso.
Come mostrato nella funzionalità R1 - PMTUD su piattaforme definite:
! – As seen from R1 – Passive – 'debug tcp pmtud' and 'debug icmp' active
! – TCP PMTUD is triggered once ICMP unreachable received
RP/0/0/CPU0:May 12 09:09:22.763 UTC: ipv4_io[266]: IPv4 ICMP: Received ICMP too big from 192.168.0.1 about 192.168.0.4, MTU=512
RP/0/0/CPU0:May 12 09:09:22.763 UTC: ipv4_io[266]: ipv4_icmp_unreachable_rcvd ICMP unreach recvd: sending pak(0xb0c07d8f) to transport: 6, tid: 5
RP/0/0/CPU0:May 12 09:09:22.763 UTC: ipv4_io[266]: ip_icmp_lib_ipv4_receive: sending pak(0xb0c07d8f) to transport: 1, tid: 5
RP/0/0/CPU0:May 12 09:09:22.763 UTC: tcp[399]: [t4] PCB 0x15393770: Process ICMP Dest-unreach (next hop mtu: 512)
! – attempt new MSS 472 = MTU of next-hop(512) - TCP_H(20) - IP_H(20)
RP/0/0/CPU0:May 12 09:09:22.763 UTC: tcp[399]: [t4] PCB 0x15393770: Process ICMP Dest-unreach (next hop mtu: 512)
RP/0/0/CPU0:May 12 09:09:22.763 UTC: tcp[399]: [t4] PCB 0x15393770: Try to use new MSS: 472
RP/0/0/CPU0:May 12 09:09:22.763 UTC: tcp[399]: [t4] PCB 0x15393770, New path MTU decided to use: 472 configured tp_user_mss 0
! – over time PMTUD attempts to raise MSS as per egress interface configured MTU
RP/0/0/CPU0:May 12 09:19:22.782 UTC: tcp[399]: [t23] PCB 0x15393770: Trying next higher MTU: 966
RP/0/0/CPU0:May 12 09:21:22.793 UTC: tcp[399]: [t23] PCB 0x15393770: Trying next higher MTU: 1452
RP/0/0/CPU0:May 12 09:23:22.805 UTC: tcp[399]: [t23] PCB 0x15393770: Trying next higher MTU: 1460
Lo stato finale può essere osservato in questi output. Notare in particolare i valori MSS minimo e massimo mostrati dal nodo R1, che evidenzia e segnala l'attivazione del PMTUD.
Dettagli della sessione TCP rilevati su R4 - ATTIVO:
! – Final stage as seen from R4 – Active
RP/0/0/CPU0:R4#show tcp detail pcb 0x153913b8
Wed May 12 10:09:43.246 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Wed May 12 09:02:07 2021
PCB 0x153913b8, SO 0x153917f0, TCPCB 0x1538fb58, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 382
Local host: 192.168.0.4, Local port: 32449 (Local App PID: 1196319)
Foreign host: 192.168.0.1, Foreign port: 179
(Local App PID/instance/SPL_APP_ID: 1196319/1/0)
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 72 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 71 69 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 2130367566 snduna: 2130368957 sndnxt: 2130368957
sndmax: 2130368957 sndwnd: 31453 sndcwnd: 2920
irs: 2847698412 rcvnxt: 2847700946 rcvwnd: 31799 rcvadv: 2847732745
SRTT: 220 ms, RTTO: 300 ms, RTV: 12 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 239 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 10, connect retry interval: 30 secs
State flags: none
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
Socket misc info : Rcv data size (sb_cc) 0, so_qlen 0,
so_q0len 0, so_qlimit 0, so_error 0
so_auto_rearm 1
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
Num of peers with authentication info: 0
RP/0/0/CPU0:R4#
Dettagli della sessione TCP come visualizzati su R1 - PASSIVE:
! – Final stage as seen from R1 – Passive
RP/0/0/CPU0:R1#show tcp detail pcb 0x15393770
Wed May 12 10:12:41.432 UTC
==============================================================
Connection state is ESTAB, I/O status: 240, socket status: 0
Established at Wed May 12 09:02:05 2021
PCB 0x15393770, SO 0x15394ea0, TCPCB 0x15391c0c, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 382
Local host: 192.168.0.1, Local port: 179 (Local App PID: 1192224)
Foreign host: 192.168.0.4, Foreign port: 32449
(Local App PID/instance/SPL_APP_ID: 1192224/1/0)
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 75 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 73 71 0
KeepAlive 1 0 0
PmtuAger 28 27 41595
GiveUp 0 0 0
Throttle 0 0 0
iss: 2847698412 snduna: 2847701003 sndnxt: 2847701003
sndmax: 2847701003 sndwnd: 31742 sndcwnd: 4380
irs: 2130367566 rcvnxt: 2130369014 rcvwnd: 31396 rcvadv: 2130400410
SRTT: 224 ms, RTTO: 300 ms, RTV: 23 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 259 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: PMTU ager
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 472, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
Socket misc info : Rcv data size (sb_cc) 0, so_qlen 0,
so_q0len 0, so_qlimit 0, so_error 0
so_auto_rearm 1
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x20 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
Num of peers with authentication info: 0
RP/0/0/CPU0:R1#
Infine, se in un dato momento un ICMP (destinazione irraggiungibile - tipo 3; Frammentazione richiesta (codice 4): il messaggio viene ricevuto di nuovo, quindi il PMTUD agisce come descritto in precedenza.
Come mostrato dalla modalità R1 - PASSIVO - il PMTUD è stato attivato nuovamente:
! – As seen from R1 – Passive
! – TCP PMTUD is again triggered upon new ICMP unreachable received
! – Behavior can be triggered via clearing redistributed, network and aggregate routes originated
RP/0/0/CPU0:R1#clear bgp ipv4 all self-originated
Wed May 12 10:19:06.836 UTC
RP/0/0/CPU0:R1#
! – New BGP update message is sourced from R1 after clear bgp command
1707 1.712657 192.168.0.1 192.168.0.4 BGP 1121 UPDATE Message
Frame 1707: 1121 bytes on wire (8968 bits), 1121 bytes captured (8968 bits) on interface 0
Ethernet II, Src: fa:16:3e:42:18:05 (fa:16:3e:42:18:05), Dst: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0xc0 (DSCP: CS6, ECN: Not-ECT)
Total Length: 1107
Identification: 0x1a38 (6712)
Flags: 0x02 (Don't Fragment)
Fragment offset: 0
Time to live: 255
Protocol: TCP (6)
Header checksum: 0x52a3 [validation disabled]
[Header checksum status: Unverified]
Source: 192.168.0.1
Destination: 192.168.0.4
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Transmission Control Protocol, Src Port: 179, Dst Port: 32449, Seq: 2705, Ack: 1562, Len: 1067
Border Gateway Protocol - UPDATE Message
! – ICMP Destination Unreachable / Fragmentation needed is received and triggers PMTUD
1708 0.001614 10.2.3.1 192.168.0.1 ICMP 110 Destination unreachable (Fragmentation needed)
Frame 1708: 110 bytes on wire (880 bits), 110 bytes captured (880 bits) on interface 0
Ethernet II, Src: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80), Dst: fa:16:3e:42:18:05 (fa:16:3e:42:18:05)
Internet Protocol Version 4, Src: 10.2.3.1, Dst: 192.168.0.1
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
Total Length: 96
Identification: 0x0002 (2)
Flags: 0x00
Fragment offset: 0
Time to live: 255
Protocol: ICMP (1)
Header checksum: 0xac96 [validation disabled]
[Header checksum status: Unverified]
Source: 10.2.3.1
Destination: 192.168.0.1
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Internet Control Message Protocol
Type: 3 (Destination unreachable)
Code: 4 (Fragmentation needed)
Checksum: 0x3b73 [correct]
[Checksum Status: Good]
Length: 17
[Length of original datagram: 68]
Unused: 0011
MTU of next hop: 512
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0xc0 (DSCP: CS6, ECN: Not-ECT)
Total Length: 1107
Identification: 0x1a38 (6712)
Flags: 0x02 (Don't Fragment)
Fragment offset: 0
Time to live: 254
Protocol: TCP (6)
Header checksum: 0x53a3 [validation disabled]
[Header checksum status: Unverified]
Source: 192.168.0.1
Destination: 192.168.0.4
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Transmission Control Protocol, Src Port: 179, Dst Port: 32449, Seq: 2847701117, Ack: 2130369128
Border Gateway Protocol - UPDATE Message
! – Note new/updated MSS value and PmtuAger
! - MSS 472 ; Aligned with “MTU of next hop” value contained in ICMP message
RP/0/0/CPU0:R1#show tcp detail pcb 0x15393770
Wed May 12 10:19:31.494 UTC
==============================================================
Connection state is ESTAB, I/O status: 240, socket status: 0
Established at Wed May 12 09:02:05 2021
PCB 0x15393770, SO 0x15394ea0, TCPCB 0x15391c0c, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 382
Local host: 192.168.0.1, Local port: 179 (Local App PID: 1192224)
Foreign host: 192.168.0.4, Foreign port: 32449
(Local App PID/instance/SPL_APP_ID: 1192224/1/0)
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 83 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 80 77 0
KeepAlive 1 0 0
PmtuAger 32 30 575401
GiveUp 0 0 0
Throttle 0 0 0
iss: 2847698412 snduna: 2847702184 sndnxt: 2847702184
sndmax: 2847702184 sndwnd: 32173 sndcwnd: 944
irs: 2130367566 rcvnxt: 2130369147 rcvwnd: 32730 rcvadv: 2130401877
SRTT: 221 ms, RTTO: 300 ms, RTV: 16 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 259 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: PMTU ager
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 472, peer MSS 1460, min MSS 472, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
Socket misc info : Rcv data size (sb_cc) 0, so_qlen 0,
so_q0len 0, so_qlimit 0, so_error 0
so_auto_rearm 1
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x20 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
Num of peers with authentication info: 0
RP/0/0/CPU0:R1#
Nelle versioni Cisco IOS XR interessate dall'ID bug Cisco CSCvf10395, l'hop successivo contenuto nel messaggio di errore ICMP viene ignorato e il nodo cerca di stabilire l'MTU IP inferiore end-to-end utilizzando il valore più basso successivo del set di livelli di MTU IP (IP Plateau) ben definiti menzionato in precedenza e documentato dalla RFC1191 - Rilevamento dell'MTU del percorso. Questi tentativi continuano a verificarsi fino alla trasmissione corretta, ossia fino a quando la destinazione non è raggiungibile (ICMP) - tipo 3 ; Frammentazione richiesta - Codice 4) i messaggi non vengono più ricevuti.
Come mostrato da un nodo con versione Cisco IOS XR interessata dall'ID bug Cisco CSCvf10395:
! – As seen from IOX XR node with a release impacted by Cisco bug ID CSCvf10395
! – Node ignores “MTU of next hop” and tries next lower plateau
! – This is observed till ICMP error messages are no longer received
! – Practical consequence is extra retransmissions occurrence
RP/0/0/CPU0:Feb 23 17:05:32.929 : tcp[399]: [t4] PCB 0x12152adc: Process ICMP Dest-unreach (next hop mtu: 33554432)
RP/0/0/CPU0:Feb 23 17:05:32.929 : tcp[399]: [t4] PCB 0x12152adc: Invalid next hop mtu (33554432), ignore it
RP/0/0/CPU0:Feb 23 17:05:34.649 : tcp[399]: [t27] PCB 0x12152adc: Trying next lower MTU: 1452 <<<<<<<< HERE: Plateau 1492
RP/0/0/CPU0:Feb 23 17:05:35.519 : tcp[399]: [t4] PCB 0x12152adc: Process ICMP Dest-unreach (next hop mtu: 33554432)
RP/0/0/CPU0:Feb 23 17:05:35.519 : tcp[399]: [t4] PCB 0x12152adc: Invalid next hop mtu (33554432), ignore it
RP/0/0/CPU0:Feb 23 17:05:37.239 : tcp[399]: [t27] PCB 0x12152adc: Trying next lower MTU: 966 <<<<<<<< HERE: Plateau 1006
RP/0/0/CPU0:Feb 23 17:05:38.109 : tcp[399]: [t4] PCB 0x12152adc: Process ICMP Dest-unreach (next hop mtu: 33554432)
RP/0/0/CPU0:Feb 23 17:05:38.109 : tcp[399]: [t4] PCB 0x12152adc: Invalid next hop mtu (33554432), ignore it
RP/0/0/CPU0:Feb 23 17:05:39.829 : tcp[399]: [t27] PCB 0x12152adc: Trying next lower MTU: 468 <<<<<<<< HERE: Plateau 508
Nella fase successiva, considerare lo stesso scenario ma con il protocollo LDP (Label Distribution Protocol) su tutte le interfacce. L'obiettivo è comprendere quali differenze possono essere osservate rispetto agli scenari precedenti in un ambiente abilitato per MPLS.
Immagine 3.3 - PMTUD abilitato, e segmento del percorso con MTU IP inferiore - scenario MPLS.
In primo luogo, considerare la fase iniziale della sessione BGP stabilita prima del trigger PMTUD, come mostrato di seguito.
Stato iniziale TCP (BGP) come in R4 - ATTIVO - scenario abilitato per MPLS:
! – as seen on R4 – Active
! – TCP path MTU discovery enabled
! – MPLS LDP enabled
! – TCP session initial state
RP/0/0/CPU0:R4#show tcp detail pcb 0x153bdaf0
Mon May 17 08:32:16.673 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Mon May 17 08:31:57 2021
PCB 0x153bdaf0, SO 0x153acc80, TCPCB 0x153acea8, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 757
Local host: 192.168.0.4, Local port: 57400 (Local App PID: 1196319)
Foreign host: 192.168.0.1, Foreign port: 179
(Local App PID/instance/SPL_APP_ID: 1196319/1/0)
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 5 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 2 1 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 1386459919 snduna: 1386460037 sndnxt: 1386460037
sndmax: 1386460037 sndwnd: 32726 sndcwnd: 4380
irs: 3874414679 rcvnxt: 3874414864 rcvwnd: 32678 rcvadv: 3874447542
SRTT: 48 ms, RTTO: 300 ms, RTV: 228 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 229 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 10, connect retry interval: 30 secs
State flags: none
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
Socket misc info : Rcv data size (sb_cc) 0, so_qlen 0,
so_q0len 0, so_qlimit 0, so_error 0
so_auto_rearm 1
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 1 Label Stack: 0x5dc2
Num of peers with authentication info: 0
RP/0/0/CPU0:R4#
Stato iniziale TCP (BGP) come in R1 - PASSIVO - scenario abilitato per MPLS:
! – as seen on R1 – Passive
! – TCP path MTU discovery enabled
! – MPLS LDP enabled
! – TCP session initial state
RP/0/0/CPU0:R1#show tcp detail pcb 0x153acc8c
Mon May 17 08:32:56.618 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Mon May 17 08:31:55 2021
PCB 0x153acc8c, SO 0x153adad4, TCPCB 0x153adcfc, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 757
Local host: 192.168.0.1, Local port: 179 (Local App PID: 1192224)
Foreign host: 192.168.0.4, Foreign port: 57400
(Local App PID/instance/SPL_APP_ID: 1192224/1/0)
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 3 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 3 1 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 3874414679 snduna: 3874414864 sndnxt: 3874414864
sndmax: 3874414864 sndwnd: 32678 sndcwnd: 4380
irs: 1386459919 rcvnxt: 1386460037 rcvwnd: 32726 rcvadv: 1386492763
SRTT: 45 ms, RTTO: 300 ms, RTV: 239 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 229 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
Socket misc info : Rcv data size (sb_cc) 0, so_qlen 0,
so_q0len 0, so_qlimit 0, so_error 0
so_auto_rearm 1
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x20 PD ctx: size: 0 data:
Num Labels: 1 Label Stack: 0x5dc3
Num of peers with authentication info: 0
RP/0/0/CPU0:R1#
In questo scenario abilitato per MPLS, si noti che sono stati stabiliti i dettagli per le sessioni TCP (LDP). Tenere presente che quanto descritto in precedenza per quanto riguarda il calcolo del valore MSS per le sessioni TCP (BGP), si applica anche alle sessioni TCP (LDP). Ad esempio, il calcolo del valore MSS della sessione TCP (LDP) dei nodi R3 e R2 può essere riepilogato come segue:
- Sia R2 che R3 utilizzano una MTU IP non predefinita di 512 byte.
- Rilevamento dell'MTU del percorso abilitato.
- I peer TCP non sono connessi direttamente (la sessione TCP viene stabilita tra le interfacce di loopback).
- R3 gestisce la connessione LDP.
- R3 invia SYN con MSS di 472 byte.
- 512 (MTU IP interfaccia) - 20 (minTCP_H) - 20 (minIP_H).
- R2 invia SYN, ACK con un MSS di 472 byte.
- Invia il valore più basso di [Received MSS; Local initial MSS].
- Byte MSS ricevuti: 472; Local initial MSS 472 byte.
- Il valore MSS più basso viene utilizzato su entrambi i peer.
Dettagli della sessione TCP (LDP) come visualizzati in R3 - ATTIVO - scenario abilitato per MPLS:
! – as seen on R3 – Active
! – TCP path MTU discovery enabled
! – MPLS LDP enabled
! – TCP session initial state
RP/0/0/CPU0:R3#show tcp detail pcb 0x15393fbc
Mon May 17 08:33:30.627 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Mon May 17 08:30:04 2021
PCB 0x15393fbc, SO 0x15393d94, TCPCB 0x153941b4, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 970
Local host: 192.168.0.3, Local port: 57146 (Local App PID: 1151216)
Foreign host: 192.168.0.2, Foreign port: 646
(Local App PID/instance/SPL_APP_ID: 1151216/0/0)
Current send queue size in bytes: 0 (max 16384)
Current receive queue size in bytes: 0 (max 16384) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 60)
Timer Starts Wakeups Next(msec)
Retrans 8 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 6 4 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 2917752466 snduna: 2917752838 sndnxt: 2917752838
sndmax: 2917752838 sndwnd: 16013 sndcwnd: 944
irs: 228184383 rcvnxt: 228184763 rcvwnd: 16005 rcvadv: 228200768
SRTT: 103 ms, RTTO: 580 ms, RTV: 477 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 279 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 1, connect retry interval: 3 secs
State flags: none
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 472, peer MSS 472, min MSS 472, max MSS 472
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_SEL, SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/16384
Socket send buffer : Low/High watermark 2048/16384, Notify threshold 0
Socket misc info : Rcv data size (sb_cc) 0, so_qlen 0,
so_q0len 0, so_qlimit 0, so_error 0
so_auto_rearm 1
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 1 Label Stack: 0x5dc2
Num of peers with authentication info: 0
RP/0/0/CPU0:R3#
Dettagli della sessione TCP (LDP) come visualizzati in R2 - PASSIVO - scenario abilitato per MPLS:
! – as seen on R2 – Passive
! – TCP path MTU discovery enabled
! – MPLS LDP enabled
! – TCP session initial state
RP/0/0/CPU0:R2#show tcp detail pcb 0x153a1f44
Mon May 17 08:34:28.843 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Mon May 17 08:30:31 2021
PCB 0x153a1f44, SO 0x153a1d1c, TCPCB 0x153a213c, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 970
Local host: 192.168.0.2, Local port: 646 (Local App PID: 1151216)
Foreign host: 192.168.0.3, Foreign port: 57146
(Local App PID/instance/SPL_APP_ID: 1151216/0/0)
Current send queue size in bytes: 0 (max 16384)
Current receive queue size in bytes: 0 (max 16384) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 60)
Timer Starts Wakeups Next(msec)
Retrans 7 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 7 5 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 228184383 snduna: 228184763 sndnxt: 228184763
sndmax: 228184763 sndwnd: 16005 sndcwnd: 944
irs: 2917752466 rcvnxt: 2917752856 rcvwnd: 15995 rcvadv: 2917768851
SRTT: 95 ms, RTTO: 561 ms, RTV: 466 ms, KRTT: 0 ms
minRTT: 0 ms, maxRTT: 219 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 472, peer MSS 472, min MSS 472, max MSS 472
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_SEL, SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/16384
Socket send buffer : Low/High watermark 2048/16384, Notify threshold 0
Socket misc info : Rcv data size (sb_cc) 0, so_qlen 0,
so_q0len 0, so_qlimit 0, so_error 0
so_auto_rearm 1
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x60 PD ctx: size: 0 data:
Num Labels: 1 Label Stack: 0x5dc1
Num of peers with authentication info: 0
RP/0/0/CPU0:R2#
Dopo aver stabilito la sessione BGP, R1 invia il messaggio di aggiornamento BGP e riceve il messaggio ICMP (Destination Unreachable - type 3; Frammentazione richiesta (codice 4) restituita dal nodo R2 che attiva il PMTUD TCP sul nodo R1. Questo si verifica perché il pacchetto IP che trasmette il messaggio di aggiornamento BGP ha il bit DF impostato e la MTU IP di 512 byte usata sul segmento R2/R3 è inferiore alle dimensioni del pacchetto IP di 1116 byte. Come in precedenza, la ricezione di questo messaggio ICMP attiva il meccanismo PMTUD. La differenza tra lo scenario abilitato per MPLS e gli scenari non MPLS precedenti è relativa all'MTU del valore dell'hop successivo incluso nel messaggio ICMP del nodo R2 (destinazione irraggiungibile - tipo 3; Frammentazione richiesta - Codice 4). In questo scenario abilitato per MPLS, l'MTU del valore dell'hop successivo tiene conto del sovraccarico MPLS aggiuntivo di 4 byte, ossia tiene conto dello stack di etichette MPLS in uscita in R2, come mostrato in questi output.
Rilevamento MTU del percorso TCP in azione come mostrato in R1 - PASSIVO - scenario abilitato per MPLS:
! – as seen from R1 – Passive
! – R1 sends BGP Update message with IP length of 1116 Bytes
! – Note MPLS Header as packet is to be label-switched (single label ; IGP label)
! – note IP Header Flags shows DF bit set
455 0.044859 192.168.0.1 192.168.0.4 BGP 1134 UPDATE Message, KEEPALIVE Message
Frame 455: 1134 bytes on wire (9072 bits), 1134 bytes captured (9072 bits) on interface 0
Ethernet II, Src: fa:16:3e:42:18:05 (fa:16:3e:42:18:05), Dst: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80)
MultiProtocol Label Switching Header, Label: 24002, Exp: 6, S: 1, TTL: 255
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0xc0 (DSCP: CS6, ECN: Not-ECT)
Total Length: 1116
Identification: 0xc6dd (50909)
Flags: 0x02 (Don't Fragment)
0... .... = Reserved bit: Not set
.1.. .... = Don't fragment: Set
..0. .... = More fragments: Not set
Fragment offset: 0
Time to live: 255
Protocol: TCP (6)
Header checksum: 0xa5f4 [validation disabled]
[Header checksum status: Unverified]
Source: 192.168.0.1
Destination: 192.168.0.4
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Transmission Control Protocol, Src Port: 179, Dst Port: 57400, Seq: 242, Ack: 175, Len: 1076
Border Gateway Protocol - UPDATE Message
Border Gateway Protocol - KEEPALIVE Message
<snip>
! – as seen from R1 – Passive
! – IP MTU on R2/R3 of 512 bytes is lower than IP packet length and DF bit is set
! – R1 receives ICMP error message from R2
! – note R2 ICMP error message carries Next-Hop MTU
! – “The size in octets of the largest datagram that could be forwarded, along the path of
! the original datagram, without being fragmented at this router. The size includes the
! IP header and IP data, and does not include any lower-level headers.”
! – In present MPLS-enabled scenario Next-Hop MTU value is 508 bytes
! – In previous non-MPLS scenario Next-Hop MTU value was 512 bytes
456 0.014117 10.2.3.1 192.168.0.1 ICMP 182 Destination unreachable (Fragmentation needed)
Frame 456: 182 bytes on wire (1456 bits), 182 bytes captured (1456 bits) on interface 0
Ethernet II, Src: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80), Dst: fa:16:3e:42:18:05 (fa:16:3e:42:18:05)
Internet Protocol Version 4, Src: 10.2.3.1, Dst: 192.168.0.1
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
Total Length: 168
Identification: 0x001f (31)
Flags: 0x00
0... .... = Reserved bit: Not set
.0.. .... = Don't fragment: Not set
..0. .... = More fragments: Not se
Fragment offset: 0
Time to live: 251
Protocol: ICMP (1)
Header checksum: 0xb031 [validation disabled]
[Header checksum status: Unverified]
Source: 10.2.3.1
Destination: 192.168.0.1
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Internet Control Message Protocol
Type: 3 (Destination unreachable)
Code: 4 (Fragmentation needed)
Checksum: 0x5199 [correct]
[Checksum Status: Good]
Length: 17
[Length of original datagram: 68]
Unused: 0011
MTU of next hop: 508
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
Transmission Control Protocol, Src Port: 179, Dst Port: 57400, Seq: 3874414921, Ack: 1386460094
Border Gateway Protocol - UPDATE Message
! – As seen from R1 – Passive
! – Hint is provided by ICMP unreachable message MTU of next-hop field: 508 bytes
! – R1 then considers this value and retransmits BGP Update split in three distinct packets
! – Sum of TCP length = 468 + 468 + 140 = 1076 bytes
457 0.006689 192.168.0.1 192.168.0.4 TCP 526 [TCP Retransmission] 179 → 57400 [ACK] Seq=242 Ack=175 Win=32669 Len=468
460 0.004001 192.168.0.1 192.168.0.4 TCP 526 [TCP Retransmission] 179 → 57400 [ACK] Seq=710 Ack=175 Win=32669 Len=468
461 0.001788 192.168.0.1 192.168.0.4 TCP 198 [TCP Retransmission] 179 → 57400 [PSH, ACK] Seq=1178 Ack=175 Win=32669 Len=140
463 0.056695 192.168.0.4 192.168.0.1 TCP 54 57400 → 179 [ACK] Seq=175 Ack=1318 Win=31545 Len=0
! – As seen from R1 – Passive – 'debug tcp pmtud' and 'debug icmp' active
! – TCP PMTUD is triggered once ICMP unreachable received
RP/0/0/CPU0:May 17 08:29:56.131 UTC: tcp[399]: [t1] Try to enable path MTU discovery(neww age timer: 10 min)
RP/0/0/CPU0:May 17 08:29:56.131 UTC: tcp[399]: [t1] Path mtu is ON (age-timer: 10)
RP/0/0/CPU0:May 17 08:35:51.726 UTC: ipv4_io[266]: ip_icmp_lib_ipv4_receive: Receiving pak(0xb0c07d8f) tid: 5
RP/0/0/CPU0:May 17 08:35:51.726 UTC: ipv4_io[266]: Entering ipv4_mtu_update_cb
RP/0/0/CPU0:May 17 08:35:51.726 UTC: ipv4_io[266]: IPv4 ICMP: Received ICMP too big from 192.168.0.1 about 192.168.0.4, MTU=508
RP/0/0/CPU0:May 17 08:35:51.726 UTC: ipv4_io[266]: ipv4_icmp_unreachable_rcvd ICMP unreach recvd: sending pak(0xb0c07d8f) to transport: 6, tid: 5
RP/0/0/CPU0:May 17 08:35:51.726 UTC: ipv4_io[266]: ip_icmp_lib_ipv4_receive: sending pak(0xb0c07d8f) to transport: 1, tid: 5
RP/0/0/CPU0:May 17 08:35:51.726 UTC: tcp[399]: [t4] PCB 0x153acc8c: Process ICMP Dest-unreach (next hop mtu: 508)
! – attempt new MSS 468 = MTU of next-hop(508) - TCP_H(20) - IP_H(20)
RP/0/0/CPU0:May 17 08:35:51.726 UTC: tcp[399]: [t4] PCB 0x153acc8c: Try to use new MSS: 468
RP/0/0/CPU0:May 17 08:35:51.726 UTC: tcp[399]: [t4] PCB 0x153acc8c, New path MTU decided to use: 468 configured tp_user_mss 0
! – over time PMTUD attempts to raise MSS as per egress interface configured MTU
RP/0/0/CPU0:May 17 08:45:51.745 UTC: tcp[399]: [t29] PCB 0x153acc8c: Trying next higher MTU: 966
RP/0/0/CPU0:May 17 08:47:51.757 UTC: tcp[399]: [t29] PCB 0x153acc8c: Trying next higher MTU: 1452
RP/0/0/CPU0:May 17 08:49:51.769 UTC: tcp[399]: [t29] PCB 0x153acc8c: Trying next higher MTU: 1460
Come mostrato in R1 - PASSIVO - TCP PMTUD attivato - scenario abilitato per MPLS:
! – as seen on R1 – Passive
! – R1 session details after TCP PMTUD trigger
RP/0/0/CPU0:R1#show tcp detail pcb 0x153acc8c
Mon May 17 08:43:07.077 UTC
==============================================================
Connection state is ESTAB, I/O status: 240, socket status: 0
Established at Mon May 17 08:31:55 2021
PCB 0x153acc8c, SO 0x153adad4, TCPCB 0x153adcfc, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 757
Local host: 192.168.0.1, Local port: 179 (Local App PID: 1192224)
Foreign host: 192.168.0.4, Foreign port: 57400
(Local App PID/instance/SPL_APP_ID: 1192224/1/0)
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 15 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 14 9 0
KeepAlive 1 0 0
PmtuAger 1 0 164599
GiveUp 0 0 0
Throttle 0 0 0
iss: 3874414679 snduna: 3874416130 sndnxt: 3874416130
sndmax: 3874416130 sndwnd: 31412 sndcwnd: 936
irs: 1386459919 rcvnxt: 1386460246 rcvwnd: 32517 rcvadv: 1386492763
SRTT: 180 ms, RTTO: 509 ms, RTV: 329 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 239 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: PMTU ager
Feature flags: Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 468, peer MSS 1460, min MSS 468, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
Socket misc info : Rcv data size (sb_cc) 0, so_qlen 0,
so_q0len 0, so_qlimit 0, so_error 0
so_auto_rearm 1
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x20 PD ctx: size: 0 data:
Num Labels: 1 Label Stack: 0x5dc3
Num of peers with authentication info: 0
RP/0/0/CPU0:R1#
Notare che nello scenario abilitato per MPLS il valore dell'MTU dell'hop successivo incluso nei messaggi ICMP del nodo R2 per lo stack di etichette MPLS in uscita. Per rafforzare ulteriormente questo aspetto, si consideri il prossimo esempio. Se il pacchetto IP filtrato in R2 è associato a un servizio L3VPN, il frame Ethernet avrà ora due etichette (etichetta IGP e etichetta VPN). Quindi, l'MTU dell'hop successivo riflette le dimensioni dello stack di etichette richieste. Fare riferimento a questi output.
Come mostrato nel pacchetto di servizio VPN L3 - R1 - PASSIVE:
! – as seen from R1 – Passive
! – L3 VPN service packet is sourced by node R1 and destined to node R4
! – Note presence of MPLS label stack – both IGP and VPN label are present
! – Note IP Total Length of 610 bytes higher than the IP MTU on R2/R3 segment
! – note IP Header Flags shows DF bit set
2024 0.302370 10.1.14.1 10.1.14.14 TELNET 632 Telnet Data ...
Frame 2024: 632 bytes on wire (5056 bits), 632 bytes captured (5056 bits) on interface 0
Ethernet II, Src: fa:16:3e:42:18:05 (fa:16:3e:42:18:05), Dst: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80)
MultiProtocol Label Switching Header, Label: 24002, Exp: 0, S: 0, TTL: 255
0000 0101 1101 1100 0010 .... .... .... = MPLS Label: 24002
.... .... .... .... .... 000. .... .... = MPLS Experimental Bits: 0
.... .... .... .... .... ...0 .... .... = MPLS Bottom Of Label Stack: 0
.... .... .... .... .... .... 1111 1111 = MPLS TTL: 255
MultiProtocol Label Switching Header, Label: 24005, Exp: 0, S: 1, TTL: 255
0000 0101 1101 1100 0101 .... .... .... = MPLS Label: 24005
.... .... .... .... .... 000. .... .... = MPLS Experimental Bits: 0
.... .... .... .... .... ...1 .... .... = MPLS Bottom Of Label Stack: 1
.... .... .... .... .... .... 1111 1111 = MPLS TTL: 255
Internet Protocol Version 4, Src: 10.1.14.1, Dst: 10.1.14.14
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
Total Length: 610
Identification: 0x7c9f (31903)
Flags: 0x02 (Don't Fragment)
0... .... = Reserved bit: Not set
.1.. .... = Don't fragment: Set
..0. .... = More fragments: Not set
Fragment offset: 0
Time to live: 255
Protocol: TCP (6)
Header checksum: 0xcce5 [validation disabled]
[Header checksum status: Unverified]
Source: 10.1.14.1
Destination: 10.1.14.14
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Transmission Control Protocol, Src Port: 22008, Dst Port: 23, Seq: 34755, Ack: 93250, Len: 570
Come mostrato su R1 - PASSIVE - L3 VPN Service - ICMP Tipo 3/Codice 4:
! – as seen from R1 – Passive
! – IP MTU on R2/R3 of 512 bytes is lower than IP packet length and DF bit is set
! – R1 receives ICMP error message from R2
! – note R2 ICMP error message carries Next-Hop MTU
! – “The size in octets of the largest datagram that could be forwarded, along the path of
! the original datagram, without being fragmented at this router. The size includes the
! IP header and IP data, and does not include any lower-level headers.”
! – In present L3VPN MPLS-enabled scenario (dual-label) Next-Hop MTU value is 504 bytes
! – In previous MPLS scenario (single-label) Next-Hop MTU value was 508 bytes
2030 0.020299 10.2.3.1 10.1.14.1 ICMP 190 Destination unreachable (Fragmentation needed)
Frame 2030: 190 bytes on wire (1520 bits), 190 bytes captured (1520 bits) on interface 0
Ethernet II, Src: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80), Dst: fa:16:3e:42:18:05 (fa:16:3e:42:18:05)
MultiProtocol Label Switching Header, Label: 24005, Exp: 0, S: 1, TTL: 251
0000 0101 1101 1100 0101 .... .... .... = MPLS Label: 24005
.... .... .... .... .... 000. .... .... = MPLS Experimental Bits: 0
.... .... .... .... .... ...1 .... .... = MPLS Bottom Of Label Stack: 1
.... .... .... .... .... .... 1111 1011 = MPLS TTL: 251
Internet Protocol Version 4, Src: 10.2.3.1, Dst: 10.1.14.1
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
Total Length: 172
Identification: 0x002b (43)
Flags: 0x00
0... .... = Reserved bit: Not set
.0.. .... = Don't fragment: Not set
..0. .... = More fragments: Not set
Fragment offset: 0
Time to live: 253
Protocol: ICMP (1)
Header checksum: 0x9821 [validation disabled]
[Header checksum status: Unverified]
Source: 10.2.3.1
Destination: 10.1.14.1
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Internet Control Message Protocol
Type: 3 (Destination unreachable)
Code: 4 (Fragmentation needed)
Checksum: 0xbbac [correct]
[Checksum Status: Good]
Length: 17
[Length of original datagram: 68]
Unused: 0011
MTU of next hop: 504
Internet Protocol Version 4, Src: 10.1.14.1, Dst: 10.1.14.14
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
Total Length: 610
Identification: 0x7c9f (31903)
Flags: 0x02 (Don't Fragment)
0... .... = Reserved bit: Not set
.1.. .... = Don't fragment: Set
..0. .... = More fragments: Not set
Fragment offset: 0
Time to live: 255
Protocol: TCP (6)
Header checksum: 0xcce5 [validation disabled]
[Header checksum status: Unverified]
Source: 10.1.14.1
Destination: 10.1.14.14
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Transmission Control Protocol, Src Port: 22008, Dst Port: 23, Seq: 586828435, Ack: 754580617
PMTUD - Opzioni TCP (MD5)
Immagine 3.4 - Abilitazione della funzionalità PMTUD e autenticazione TCP MD5.
Non viene introdotta alcuna distinzione rispetto al comportamento PMTUD rispetto a quanto già descritto negli scenari precedenti con l'autenticazione TCP MD5 abilitata. Come condiviso in precedenza con l'autenticazione TCP MD5 in uso, Cisco IOS XR considera l'ulteriore sovraccarico e il valore MSS iniziale del peer TCP attivo riflette lo stesso. Per ulteriori informazioni sull'impatto delle opzioni TCP utilizzate, consultare le sezioni precedenti Uso delle opzioni TCP - XR Attivo e Uso delle opzioni TCP - XR Passivo. Il calcolo del valore TCP MSS in questo scenario può essere riepilogato come segue:
- Tutti i nodi utilizzano un'MTU IP predefinita di 1500 byte.
- Rilevamento dell'MTU del percorso TCP abilitato.
- I peer TCP non sono connessi direttamente.
- Autenticazione TCP MD5 abilitata su R1 e R4.
- R4 gestisce la connessione BGP.
- R4 invia SYN con MSS di 1436 byte.
- 1500 (MTU IP interfaccia) - 20 (minTCP_H) - 20 (minIP_H) - 24 byte (sovraccarico opzioni TCP IOS XR).
- R1 invia SYN, ACK con MSS di 1436 byte.
- invia il valore più basso di [Received MSS ; Local initial MSS].
- Byte MSS ricevuti: 1436; Local initial MSS 1460 byte.
- Il valore MSS più basso viene utilizzato su entrambi i peer.
TCP SYN originato da R4:
! – TCP SYN sourced from R4
2408 5.695076 192.168.0.4 192.168.0.1 TCP 82 59050 → 179 [SYN] Seq=0 Win=16384 Len=0 MSS=1436 WS=1
Frame 2408: 82 bytes on wire (656 bits), 82 bytes captured (656 bits) on interface 0
Ethernet II, Src: fa:16:3e:d7:7e:f6 (fa:16:3e:d7:7e:f6), Dst: fa:16:3e:8f:8f:54 (fa:16:3e:8f:8f:54)
Internet Protocol Version 4, Src: 192.168.0.4, Dst: 192.168.0.1
Transmission Control Protocol, Src Port: 59050, Dst Port: 179, Seq: 0, Len: 0
Source Port: 59050
Destination Port: 179
[Stream index: 8]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 0
Header Length: 48 bytes
Flags: 0x002 (SYN)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0x20d7 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (28 bytes), Maximum segment size, Window scale, No-Operation (NOP), TCP MD5 signature, End of Option List (EOL)
Maximum segment size: 1436 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1436
Window scale: 0 (multiply by 1)
No-Operation (NOP)
TCP MD5 signature
End of Option List (EOL)
TCP SYN, ACK originato da R1:
! – TCP SYN,ACK sourced from R1
2409 0.004352 192.168.0.1 192.168.0.4 TCP 82 179 → 59050 [SYN, ACK] Seq=0 Ack=1 Win=16384 Len=0 MSS=1436 WS=1
Frame 2409: 82 bytes on wire (656 bits), 82 bytes captured (656 bits) on interface 0
Ethernet II, Src: fa:16:3e:8f:8f:54 (fa:16:3e:8f:8f:54), Dst: fa:16:3e:d7:7e:f6 (fa:16:3e:d7:7e:f6)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
Transmission Control Protocol, Src Port: 179, Dst Port: 59050, Seq: 0, Ack: 1, Len: 0
Source Port: 179
Destination Port: 59050
[Stream index: 8]
[TCP Segment Len: 0]
Sequence number: 0 (relative sequence number)
Acknowledgment number: 1 (relative ack number)
Header Length: 48 bytes
Flags: 0x012 (SYN, ACK)
Window size value: 16384
[Calculated window size: 16384]
Checksum: 0xcbf8 [unverified]
[Checksum Status: Unverified]
Urgent pointer: 0
Options: (28 bytes), Maximum segment size, Window scale, No-Operation (NOP), TCP MD5 signature, End of Option List (EOL)
Maximum segment size: 1436 bytes
Kind: Maximum Segment Size (2)
Length: 4
MSS Value: 1436
Window scale: 0 (multiply by 1)
No-Operation (NOP)
TCP MD5 signature
End of Option List (EOL)
Dettagli della sessione TCP rilevati su R4 - ATTIVO:
! – as seen from R4 - Active
RP/0/0/CPU0:R4#show tcp detail pcb 0x121542c0
Tue Jan 12 13:27:23.526 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Tue Jan 12 13:25:41 2021
PCB 0x121542c0, SO 0x1213c0e4, TCPCB 0x12156010, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 359
Local host: 192.168.0.4, Local port: 59050 (Local App PID: 1052958)
Foreign host: 192.168.0.1, Foreign port: 179
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 6 1 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 3 2 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 3299472269 snduna: 3299473445 sndnxt: 3299473445
sndmax: 3299473445 sndwnd: 31646 sndcwnd: 4308
irs: 3225544359 rcvnxt: 3225545535 rcvwnd: 31665 rcvadv: 3225577200
SRTT: 89 ms, RTTO: 530 ms, RTV: 441 ms, KRTT: 0 ms
minRTT: 19 ms, maxRTT: 239 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 30, connect retry interval: 30 secs
State flags: none
Feature flags: MD5, Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 1436, peer MSS 1436, min MSS 1436, max MSS 1436
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R4#
Dettagli della sessione TCP come visualizzati su R1 - PASSIVE:
! – as seen from R1 – Passive
RP/0/0/CPU0:R1#show tcp detail pcb 0x121560ec
Tue Jan 12 13:25:59.310 UTC
==============================================================
Connection state is ESTAB, I/O status: 0, socket status: 0
Established at Tue Jan 12 13:25:31 2021
PCB 0x121560ec, SO 0x121556d4, TCPCB 0x121575bc, vrfid 0x60000000,
Pak Prio: Medium, TOS: 192, TTL: 255, Hash index: 359
Local host: 192.168.0.1, Local port: 179 (Local App PID: 983326)
Foreign host: 192.168.0.4, Foreign port: 59050
Current send queue size in bytes: 0 (max 24576)
Current receive queue size in bytes: 0 (max 32768) mis-ordered: 0 bytes
Current receive queue size in packets: 0 (max 0)
Timer Starts Wakeups Next(msec)
Retrans 3 0 0
SendWnd 0 0 0
TimeWait 0 0 0
AckHold 3 2 0
KeepAlive 1 0 0
PmtuAger 0 0 0
GiveUp 0 0 0
Throttle 0 0 0
iss: 3225544359 snduna: 3225545516 sndnxt: 3225545516
sndmax: 3225545516 sndwnd: 31684 sndcwnd: 4308
irs: 3299472269 rcvnxt: 3299473426 rcvwnd: 31665 rcvadv: 3299505091
SRTT: 37 ms, RTTO: 300 ms, RTV: 244 ms, KRTT: 0 ms
minRTT: 9 ms, maxRTT: 239 ms
ACK hold time: 200 ms, Keepalive time: 0 sec, SYN waittime: 30 sec
Giveup time: 0 ms, Retransmission retries: 0, Retransmit forever: FALSE
Connect retries remaining: 0, connect retry interval: 0 secs
State flags: none
Feature flags: MD5, Win Scale, Nagle, Path MTU
Request flags: Win Scale
Datagrams (in bytes): MSS 1436, peer MSS 1436, min MSS 1460, max MSS 1460
Window scales: rcv 0, snd 0, request rcv 0, request snd 0
Timestamp option: recent 0, recent age 0, last ACK sent 0
Sack blocks {start, end}: none
Sack holes {start, end, dups, rxmit}: none
Socket options: SO_REUSEADDR, SO_REUSEPORT, SO_NBIO
Socket states: SS_ISCONNECTED, SS_PRIV
Socket receive buffer states: SB_DEL_WAKEUP
Socket send buffer states: SB_DEL_WAKEUP
Socket receive buffer: Low/High watermark 1/32768
Socket send buffer : Low/High watermark 2048/24576, Notify threshold 0
PDU information:
#PDU's in buffer: 0
FIB Lookup Cache: IFH: 0x40 PD ctx: size: 0 data:
Num Labels: 0 Label Stack:
RP/0/0/CPU0:R1#
PMTUD - Rilevamento buchi neri
Come spiegato in precedenza nella sezione PMTUD - Path Segment has Lower IP MTU, il rilevamento dell'MTU del percorso (PMTUD) quando abilitato è attivato viene attivato dalla ricezione di un messaggio ICMP (Destination Unreachable - type 3; Frammentazione richiesta - Codice 4) messaggio. In alcuni casi, i messaggi non vengono ricevuti e quindi la funzionalità PMTUD non viene attivata. In questo caso, non viene rilevata l'MTU IP più bassa del percorso tra i peer TCP. In uno scenario di questo tipo, potrebbe crearsi un blackhole se i pacchetti IP hanno il bit DF impostato e se le dimensioni sono superiori al segmento del percorso della MTU IP più basso. Quei pacchetti verrebbero scartati in silenzio.
In questa sezione viene illustrato come Cisco IOS XR rileva e interviene in questo potenziale scenario di blackhole. A questo scopo, la funzione IPv4 unreachables è disabilitata sull'interfaccia R2 GE0/0/0/0, come mostrato nella prossima immagine e nell'output della CLI.
Immagine 3.5 - PMTUD abilitato su R1/R4 e R2 IPv4 non raggiungibile disabilitato.
Impossibile raggiungere IPv4 disabilitato in R2:
!- R2 - IP unreachables is disabled
RP/0/0/CPU0:R2#show run interface gigabitEthernet 0/0/0/0
Thu May 13 12:09:45.483 UTC
interface GigabitEthernet0/0/0/0
ipv4 address 10.1.2.2 255.255.255.252
ipv4 unreachables disable
!
RP/0/0/CPU0:R2#show ipv4 interface gigabitEthernet 0/0/0/0
Thu May 13 12:10:04.112 UTC
GigabitEthernet0/0/0/0 is Up, ipv4 protocol is Up
Vrf is default (vrfid 0x60000000)
Internet address is 10.1.2.2/30
MTU is 1514 (1500 is available to IP)
Helper address is not set
Multicast reserved groups joined: 224.0.0.2 224.0.0.1 224.0.0.5
224.0.0.6
Directed broadcast forwarding is disabled
Outgoing access list is not set
Inbound common access list is not set, access list is not set
Proxy ARP is disabled
ICMP redirects are never sent
ICMP unreachables are never sent
ICMP mask replies are never sent
Table Id is 0xe0000000
Per risolvere questo problema, Cisco IOS XR trasmette lo stesso pacchetto due volte e, se il problema persiste, il pacchetto TCP ACK previsto non viene ricevuto. Quindi, riprovare e usare il valore di soglia immediatamente inferiore, come documentato nella RFC1191 - Rilevamento dell'MTU del percorso (per l'elenco degli slot, fare riferimento alla sezione PMTUD - Il segmento del percorso ha una MTU IP inferiore). In sintesi, Cisco IOS XR presume che i pacchetti vengano scartati all'interno del percorso verso la destinazione a causa delle dimensioni e cerca di aggirarli tramite ritrasmissione dei pacchetti. Questo comportamento può essere osservato con l'esempio successivo da un'acquisizione di pacchetto effettuata all'interfaccia del nodo R1 e con l'output del comando debug tcp pmtud.
Rilevamento blackhole IOS-XR su R1:
! – at R1
! – Original BGP Update message is sent
! – Note IP Total Length of 1116 bytes and TCP Segment Length of 1076 bytes
! – R2 filters such packet and send and ICMP error message towards R1 which triggers PMTUD
! – But because IPv4 unreachables are disabled at R2 GE0/0/0/0 ICMP message is not sent
! – Hence BGP message is silently filtered at R2
562 7.638774 192.168.0.1 192.168.0.4 BGP 1130 UPDATE Message, KEEPALIVE Message
Frame 562: 1130 bytes on wire (9040 bits), 1130 bytes captured (9040 bits) on interface 0
Ethernet II, Src: fa:16:3e:42:18:05 (fa:16:3e:42:18:05), Dst: fa:16:3e:5c:f1:80 (fa:16:3e:5c:f1:80)
Internet Protocol Version 4, Src: 192.168.0.1, Dst: 192.168.0.4
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0xc0 (DSCP: CS6, ECN: Not-ECT)
Total Length: 1116
Identification: 0x4a37 (18999)
Flags: 0x02 (Don't Fragment)
0... .... = Reserved bit: Not set
.1.. .... = Don't fragment: Set
..0. .... = More fragments: Not set
Fragment offset: 0
Time to live: 255
Protocol: TCP (6)
Header checksum: 0x229b [validation disabled]
[Header checksum status: Unverified]
Source: 192.168.0.1
Destination: 192.168.0.4
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
Transmission Control Protocol, Src Port: 179, Dst Port: 57082, Seq: 318, Ack: 251, Len: 1076
Border Gateway Protocol - UPDATE Message
Border Gateway Protocol - KEEPALIVE Message
<snip>
! – at R1
! – No TCP ACK is received
! – Packet retransmission is attempted (2 attempts)
! – Note initial MSS value is of 1460 bytes
563 0.560058 192.168.0.1 192.168.0.4 TCP 1130 [TCP Retransmission] 179 → 57082 [PSH, ACK] Seq=318 Ack=251 Win=32593 Len=1076
564 1.101367 192.168.0.1 192.168.0.4 TCP 1130 [TCP Retransmission] 179 → 57082 [PSH, ACK] Seq=318 Ack=251 Win=32593 Len=1076
! – at R1
! – Still no TCP ACK received; previous retransmissions failed
! – Next lower plateau value is attempted – 1492 bytes
! – Packet retransmission is attempted (2 attempts)
RP/0/0/CPU0:May 13 10:20:44.251 UTC: tcp[399]: [t1] PCB 0x15392224: Trying next lower MTU: 1452
567 1.850294 192.168.0.1 192.168.0.4 TCP 1130 [TCP Retransmission] 179 → 57082 [PSH, ACK] Seq=318 Ack=251 Win=32593 Len=1076
568 1.111361 192.168.0.1 192.168.0.4 TCP 1130 [TCP Retransmission] 179 → 57082 [PSH, ACK] Seq=318 Ack=251 Win=32593 Len=1076
! – at R1
! – Still no TCP ACK received; previous retransmissions failed
! – Next lower plateau value is attempted – 1006 bytes
! – Packet retransmission is attempted (2 attempts)
RP/0/0/CPU0:May 13 10:20:47.560 UTC: tcp[399]: [t1] PCB 0x15392224: Trying next lower MTU: 966
569 2.198327 192.168.0.1 192.168.0.4 TCP 1020 [TCP Retransmission] 179 → 57082 [ACK] Seq=318 Ack=251 Win=32593 Len=966
570 1.109602 192.168.0.1 192.168.0.4 TCP 1020 [TCP Retransmission] 179 → 57082 [ACK] Seq=318 Ack=251 Win=32593 Len=966
! – at R1
! – Still no TCP ACK received; previous retransmissions failed
! – Next lower plateau value is attempted – 508 bytes
! – Original information (TCP Length of 1076 bytes) is split in three distinct packets
! – TCP Segment Lengths 468 + 468 + 140 = 1076
! – TCP ACK is received from peer R4
RP/0/0/CPU0:May 13 10:20:50.870 UTC: tcp[399]: [t1] PCB 0x15392224: Trying next lower MTU: 468
571 2.205552 192.168.0.1 192.168.0.4 TCP 522 [TCP Retransmission] 179 → 57082 [ACK] Seq=318 Ack=251 Win=32593 Len=468
573 0.004254 192.168.0.1 192.168.0.4 TCP 522 [TCP Retransmission] 179 → 57082 [ACK] Seq=786 Ack=251 Win=32593 Len=468
574 0.002724 192.168.0.1 192.168.0.4 TCP 194 [TCP Retransmission] 179 → 57082 [PSH, ACK] Seq=1254 Ack=251 Win=32593 Len=140
! – Peer R4 TCP ACK is received
575 0.223172 192.168.0.4 192.168.0.1 TCP 54 57082 → 179 [ACK] Seq=251 Ack=1394 Win=31469 Len=0
Cronologia delle revisioni
| Revisione | Data di pubblicazione | Commenti |
|---|---|---|
1.0 |
16-Aug-2021 |
Versione iniziale |
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