Vérifier la découverte MTU du chemin sur Cisco IOS XR et BGP
Contenu
Introduction
Ce document décrit la découverte PMTUD (Transmission Control Protocol) TCP (Transmission Control Protocol) sur les périphériques Cisco IOS® XR.
Informations générales
Le mécanisme PMTUD tente de déterminer la plus grande taille de paquet IP (Internet Protocol) qui ne nécessite pas de fragmentation n'importe où sur le chemin entre deux hôtes. La valeur établie est désignée Chemin MTU et est égale à un minimum des valeurs MTU pour chaque saut. Si vous considérez le MTU du chemin lorsque vous transmettez des informations, il vous permet de tirer le meilleur parti de la capacité du réseau et d'éviter la fragmentation et l'efficacité de la transmission. La mécanique et la mise en oeuvre de la PMTUD sont introduites dans un ensemble varié de scénarios avec l'utilisation du protocole BGP (Border Gateway Protocol) comme protocole client qui révèle progressivement le comportement de la PMTUD.
TCP PMTUD et TCP MSS
TCP exploite le résultat PMTUD afin d'influencer la taille maximale de segment (MSS) locale, ce qui signifie qu'il s'adapte dynamiquement à la MTU de chemin découverte. Par conséquent, avant de passer à PMTUD, vous pouvez rapidement passer en revue la taille maximale de segment TCP (MSS) et comprendre ce qu'elle signifie et son objectif.
Selon la définition originale de MSS de RFC879 : La définition de l'option MSS peut être précisée : Nombre maximal d'octets de données pouvant être reçus par l'expéditeur de cette option TCP dans les segments TCP sans option d'en-tête TCP transmise dans les datagrammes IP sans option d'en-tête IP.
Préciser certains aspects et donner des conseils aux agents d'exécution, RFC6691 souligne comment la valeur MSS doit être calculée :
Lorsque vous calculez la valeur à mettre dans l'option TCP MSS, la valeur MTU doit être réduite de la taille des en-têtes IP et TCP fixes uniquement et ne doit pas être diminuée pour tenir compte d'options IP ou TCP possibles ; à l’inverse, l’expéditeur DOIT réduire la longueur des données TCP pour tenir compte des options IP ou TCP qu’il inclut dans les paquets qu’il envoie.
Une définition plus détaillée de MSS peut être extraite du Guide de configuration du routage pour les routeurs de la gamme Cisco ASR 9000, IOS XR version 6.7.x :
MSS est la plus grande quantité de données qu'un ordinateur ou un périphérique de communication peut recevoir dans un segment TCP unique et non fragmenté. Toutes les sessions TCP sont limitées par une limite du nombre d'octets pouvant être transportés dans un seul paquet ; cette limite est MSS. Le protocole TCP divise les paquets en segments dans une file d’attente de transmission avant de les transmettre à la couche IP.
La valeur MSS TCP dépend du MTU d'une interface, qui est la longueur maximale de données pouvant être transmises par un protocole à une instance. La longueur maximale du paquet TCP est déterminée à la fois par le MTU de l'interface de sortie sur le périphérique source et par le MSS annoncé par le périphérique de destination au cours du processus de configuration TCP. Plus le MSS est proche du MTU, plus le transfert des messages BGP est efficace. Chaque direction du flux de données peut utiliser une valeur MSS différente.
Quelle serait alors la valeur que TCP devrait prendre en compte pour MSS sur une session TCP donnée ? Et comment est-ce calculé ?
Pour les valeurs par défaut selon RFC879, vous avez : Les hôtes ne doivent pas envoyer de datagrammes de plus de 576 octets, à moins d’avoir une connaissance spécifique que l’hôte de destination est prêt à accepter des datagrammes plus volumineux. LA TAILLE MAXIMALE DE SEGMENT TCP CORRESPOND À LA TAILLE MAXIMALE DE DATAGRAMME IP MINUS QUARANTE.
La taille maximale du datagramme IP par défaut est 576.
La taille maximale de segment TCP par défaut est 536.
Cela prend en compte une valeur MTU IP de 576 octets. Mais si vous ignorez la valeur MTU IP réelle, le calcul MSS TCP peut être résumé comme suit :
- Peer actif : calcule et envoie le MSS initial avec le paquet SYN.
MSS = IPMTU - sizeof(minimum TCPHDR) - sizeof(minimum IPHDR)
Where,
sizeof(minimum TCPHDR) = 20 bytes.
sizeof(minimum IPHDR) = 20 bytes.
- Passif Peer : calcule le MSS initial, le compare au MSS reçu de Active Peer et envoie SYN, ACK avec la valeur la plus basse de ces MSS.
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.
Il n'y a aucune négociation concernant la valeur de l'option MSS. Chaque noeud détermine sa propre valeur et annonce la même valeur lors de l’établissement de la session TCP. Il devient clair que si la valeur MTU IP prise en compte pour le calcul MSS peut être dérivée de PMTUD, alors la valeur MSS peut être adaptée à la valeur la plus efficace pour une MTU de chemin donnée. Le comportement de Cisco IOS XR présente quelques détails concernant le calcul MSS et le rôle PMTUD résumés ici.
PMTUD est désactivé par défaut sur Cisco IOS XR :
- Le calcul MSS initial local prend en compte le MTU IP comme suit :
- Si des homologues sont directement connectés, considérez l'interface de sortie MTU IP.
- Si les homologues ne sont pas directement connectés, considérez la valeur MTU IP de 1280 octets.
- La valeur MSS est influencée par les options TCP configurées.
Lorsque PMTUD est activé sur Cisco IOS XR :
- Le calcul MSS initial local prend en compte le MTU IP comme suit :
- Indépendamment des homologues connectés directement/non directement - considérez l'interface de sortie MTU IP.
- La valeur MSS est influencée par les options TCP configurées.
Il convient de tenir compte d'autres détails sur la mécanique et la mise en oeuvre de la PMTUD et que le présent document présente au moyen d'exemples concrets résumés dans le tableau suivant. Ce tableau présente également les MTU IP des homologues TCP actifs et passifs ainsi que les valeurs MSS sélectionnées pour chaque scénario pris en compte.
Scénarios - TCP PMTUD désactivé
Utiliser les valeurs MTU par défaut
Image 2.1. Avec les valeurs MTU par défaut
Dans le cas des homologues eBGP illustrés dans l'image 2.1, R6 gère la connexion TCP, ce qui signifie qu'il joue le rôle actif et démarre la session TCP avec R5 sur le port de destination 179. Les homologues sont directement connectés et utilisent les valeurs MTU IP par défaut sur les interfaces respectives. Sur la base des informations partagées au début de ce document, le calcul MSS dans ce scénario peut être résumé comme suit :
- Les deux noeuds utilisent une MTU IP par défaut de 1 500 octets
- La découverte MTU du chemin TCP est désactivée par défaut
- Les homologues TCP sont directement connectés
- R6 gère la connexion BGP
- R6 envoie SYN avec MSS de 1 460 octets
- 1500 (MTU IP d'interface) - 20 (minTCP_H) - 20 (minIP_H)
- R5 envoie SYN, ACK avec MSS de 1 460 octets
- Envoie le plus faible de [MSS reçu ; MSS initial local]
- MSS reçu 1 460 octets ; MSS initial local 1 460 octets
- La valeur MSS la plus faible est utilisée sur les deux homologues
Détails de la session TCP tels qu'ils apparaissent sur R6 - ACTIVE :
! – 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
Détails de la session TCP tels qu'ils apparaissent sur 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#
Utiliser une valeur MTU non par défaut - homologue TCP actif
Image 2.2 - L'homologue ACTIVE utilise une valeur MTU autre que la valeur par défaut
Ce scénario est essentiellement le même que le précédent, avec la seule différence que l'homologue TCP actif R6 utilise maintenant une valeur de MTU IP autre que celle par défaut. Notez comment le calcul initial et la décision sur la valeur MSS sont effectués par l'homologue TCP passif R5. Le calcul MSS TCP dans ce scénario peut être résumé comme suit :
- R6 utilise une MTU IP non par défaut de 4 460 octets
- La découverte MTU du chemin TCP est désactivée par défaut
- Les homologues TCP sont directement connectés
- R6 gère la connexion BGP
- R6 envoie SYN avec MSS de 4 420 octets
- 4460 (MTU IP d'interface) - 20 (minTCP_H) - 20 (minIP_H)
- R5 envoie SYN, ACK avec MSS de 1 460 octets
- envoie le plus faible de [Received MSS ; MSS initial local]
- MSS reçu 4420 octets ; MSS initial local 1 460 octets
- La valeur MSS la plus faible est utilisée sur les deux homologues
SYN TCP provenant de 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 provenant de 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)
Détails de la session TCP tels qu'ils apparaissent sur R6 - ACTIVE :
! - 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#
Détails de la session TCP tels qu'ils apparaissent sur 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#
Utiliser une valeur MTU non par défaut - homologue TCP passif
Image 2.3 - L'homologue PASSIVE utilise une valeur MTU autre que la valeur par défaut.
Avec toujours le même scénario eBGP, mais maintenant avec l'homologue TCP passif R5 configuré avec une MTU IP non par défaut et l'homologue TCP actif R6 avec une valeur MTU IP par défaut. Comme dans le scénario précédent, notez comment la valeur MSS est sélectionnée par l'homologue passif R5. Le calcul MSS TCP dans ce scénario peut être résumé comme suit :
- R5 utilise une MTU IP non par défaut de 4 460 octets
- La découverte MTU du chemin TCP est désactivée par défaut
- Les homologues TCP sont directement connectés
- R6 gère la connexion BGP
- R6 envoie SYN avec MSS de 1 460 octets
- 1500 (MTU IP d'interface) - 20 (minTCP_H) - 20 (minIP_H)
- R5 envoie SYN, ACK avec MSS de 1 460 octets
- envoie le plus faible de [Received MSS ; MSS initial local]
- MSS reçu 1 460 octets ; MSS initial local 4 420 octets
- La valeur MSS la plus faible est utilisée sur les deux homologues
SYN TCP provenant de 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 provenant de 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)
Détails de la session TCP tels qu'ils apparaissent sur R6 - ACTIVE :
! - 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#
Détails de la session TCP tels qu'ils apparaissent sur 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#
Utiliser les options TCP - XR actif
Comme mentionné précédemment dans ce document, l'utilisation d'options TCP (telles que TCP MD5, TCP sélectif-ack ou TCP timestamps) influence le calcul MSS, car ces options entraînent la prise en compte d'octets supplémentaires dans le calcul MSS.
Cette section ainsi que la suivante ont pour but d'illustrer le calcul MSS effectué par des homologues en présence d'options TCP. L'option d'authentification MD5 TCP est utilisée comme exemple. Reportez-vous au scénario de référence dans Images 2.4, tel qu'illustré dans l'image.
Image 2.4 - Utiliser les options TCP (MD5) - XR actif.
Dans ce scénario, les deux homologues utilisent des valeurs MTU IP par défaut, sont directement connectés et l'homologue R6 joue un rôle actif TCP. Comme déjà partagé la configuration et l'utilisation du compte d'authentification MD5 TCP pour une surcharge supplémentaire. Le calcul MSS TCP dans ce scénario particulier peut être résumé comme suit :
- Les deux noeuds utilisent une MTU IP par défaut de 1 500 octets
- La découverte MTU du chemin TCP est désactivée par défaut
- Les homologues TCP sont directement connectés
- Authentification MD5 TCP activée sur les deux noeuds
- R6 gère la connexion BGP
- R6 envoie SYN avec MSS de 1 436 octets
- 1500 (interface IP MTU) - 20 (minTCP_H) - 20 (minIP_H) - 24 octets (surcharge des options TCP IOS XR)
- R5 envoie SYN, ACK avec MSS de 1 436 octets
- envoie le plus faible de [Received MSS ; MSS initial local]
- MSS reçu 1 436 octets ; MSS initial local 1 460 octets
- La valeur MSS la plus faible est utilisée sur les deux homologues
Comme le montre le résumé, le comportement de Cisco IOS XR n'est pas strictement conforme aux normes RFC 879 et RFC 6691, qui stipulent que les options TCP ne doivent pas être prises en compte dans le calcul MSS.
Le compte Cisco IOS XR d'un facteur supplémentaire sur la longueur de l'en-tête tcp est documenté plus en détail sur l'ID de bogue Cisco CSCvf20166 :
“(..) Lorsque XR démarre la connexion BGP, BGP crée d'abord le socket, puis définit les options de socket incluant MD5. Cela fait que la longueur d'en-tête de l'option tcp = 24. Et donc le MSS initial devient 1500 - 40 - 24 = 1436. Ceci est envoyé à homologue et homologue utilise min(1436, 1460) = 1436.(..)
SYN TCP provenant de 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 provenant de 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)
Détails de la session TCP tels qu'ils apparaissent sur R6 - ACTIVE :
! - 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#
Détails de la session TCP tels qu'ils apparaissent sur 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 comportement similaire peut être observé avec d'autres options TCP qui, lorsqu'elles sont configurées, prennent en compte les frais généraux supplémentaires et influencent le calcul MSS dans Cisco IOS XR. Considérez le même scénario et ces exemples qui documentent le calcul MSS lorsque les horodatages TCP et les options de serveur sélectif TCP sont configurés.
Détails de la session TCP tels qu'ils apparaissent sur R6 - ACTIVE - avec l'horodatage des options TCP et les options d'ack sélectif configurées :
! - 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>
Utiliser les options TCP - XR Passive
Dans le scénario précédent, vous avez probablement remarqué le comportement distinct du noeud XR de Cisco IOS lorsqu'il est dans un rôle passif en ce qui concerne le calcul MSS initial. Le noeud ne prend pas en compte la longueur d'en-tête de l'option tcp. Ce scénario vise à mettre en évidence ce comportement distinct qui est également décrit par l'ID de bogue Cisco :
“(...) : lorsque l'homologue initie la connexion, il envoie le MSS initial sous la forme 1460. XR TCP crée socket, pcb, etc, puis il prend moins de deux actions dans l'ordre donné :
- Tout d'abord, il calcule le MSS initial après avoir soustrait la longueur d'en-tête de l'option tcp. Il s'agit de '0' car l'option MD5 n'est pas encore héritée de cette socket à partir de la socket d'écoute.
- Ensuite, il hérite de la 'MD5' et d'autres options et cela fait de 'option header bytes length' une longueur d'en-tête de 24.
Donc, dans ce cas, XR TCP envoie le MSS initial sous la forme 1460 et donc il est utilisé par les deux. (…)“
Dans ce scénario, bien que l'homologue TCP actif R8 soit un noeud Cisco IOS, ce fait n'introduit aucune différence ou spécification sur ce que le scénario vise à mettre en évidence. Néanmoins, et c'est intéressant, notez que différemment de Cisco IOS XR comme indiqué dans le scénario de section précédente, ici l'homologue TCP actif R8 ne prend pas en compte les options TCP lors du calcul MSS initial.
Image 2.5 - Utiliser les options TCP (MD5) - XR Passive.
Les deux homologues utilisent des valeurs MTU IP par défaut et sont directement connectés. L'homologue Cisco IOS R8 joue un rôle actif. Le calcul MSS TCP dans ce scénario peut être résumé comme suit :
- Les deux noeuds utilisent une MTU IP par défaut de 1 500 octets
- La découverte MTU du chemin TCP est désactivée par défaut sur Cisco IOS XR R7
- La détection MTU du chemin TCP est activée par défaut sur Cisco IOS R8
- Les homologues TCP sont directement connectés
- Authentification MD5 TCP activée sur les deux noeuds
- IOS R8 gère la connexion BGP
- IOS R8 envoie SYN avec MSS de 1 460 octets
- 1500 (MTU IP d'interface) - 20 (minTCP_H) - 20 (minIP_H)
- IOS XR R7 envoie SYN, ACK avec MSS de 1 460 octets
- envoie le plus faible de [Received MSS ; MSS initial local]
- MSS reçu 1 460 octets ; MSS initial local 1 460 octets
- La valeur MSS la plus faible est utilisée sur les deux homologues
TCP SYN provenant de 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 provenant de 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)
Détails de la session TCP tels qu'ils apparaissent sur R8 - Cisco IOS - ACTIF :
! – 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#
Détails de la session TCP tels qu'ils apparaissent sur 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#
Homologues TCP non connectés directement
Lorsque les homologues ne sont pas directement connectés, la façon dont le calcul initial de TCP MSS est effectué change comme décrit précédemment dans la section d'introduction de ce document. Le scénario d'une session iBGP avec tous les homologues configurés avec des valeurs MTU IP par défaut est utilisé pour parcourir le calcul MSS.
Image 2.6 - homologues TCP non directement connectés - iBGP.
L'aspect important à noter est que lorsque la découverte MTU du chemin TCP est désactivée et que les homologues ne sont pas directement connectés, par conception, Cisco IOS XR utilise une valeur MTU IP fixe de 1280 octets.
Dans l’image précédente, R4 joue un rôle actif et gère la connexion TCP, R4 ouvre la session TCP avec R1 sur le port de destination 179. Les deux noeuds utilisent la valeur MTU IP par défaut sur leurs interfaces. Le calcul MSS dans ce scénario peut être résumé comme suit :
- Tous les noeuds utilisent une MTU IP par défaut de 1 500 octets
- La découverte MTU du chemin TCP est désactivée par défaut
- Les homologues TCP ne sont pas directement connectés
- R4 gère la connexion BGP
- R4 envoie SYN avec MSS de 1 240 octets
- L'interface MTU n'est pas prise en compte lorsque les homologues ne sont pas directement connectés et que la découverte MTU du chemin TCP est désactivée
- Selon la conception Cisco IOS XR, 1 280 octets sont considérés comme TCP_DEFAULT_MTU
- 1280 (TCP_DEFAULT_MTU) - 20 (minTCP_H) - 20 (minIP_H)
- R1 envoie SYN, ACK avec MSS de 1 240 octets
- Envoie le plus faible de [MSS reçu ; MSS initial local]
- MSS reçu 1 240 octets ; MSS initial local 1 240 octets
- La valeur MSS la plus faible est utilisée sur les deux homologues
SYN TCP provenant de 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 provenant de 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)
Détails de la session TCP tels qu'ils apparaissent sur R4 - ACTIF :
! – 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#
Détails de la session TCP tels qu'ils apparaissent sur 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#
Homologues TCP non connectés directement - Utiliser les options TCP (MD5)
Pour les scénarios homologues non directement connectés et l'utilisation de l'authentification MD5 TCP, il n'y a aucune différence fondamentale par rapport aux scénarios ou scénarios de test précédents déjà décrits. Comme nous l'avons vu précédemment avec l'authentification MD5 TCP, Cisco IOS XR considère que la surcharge supplémentaire et la valeur MSS initiale reflètent la même chose. Reportez-vous aux sections précédentes Utilisation des options TCP - XR actif et Utilisation des options TCP - XR passif pour plus de détails sur les options TCP influant sur le calcul TCP MSS.
Image 2.7 - homologues TCP non directement connectés - iBGP + TCP MD5.
Le calcul MSS TCP dans ce scénario peut être résumé comme suit :
- Tous les noeuds utilisent une MTU IP par défaut de 1 500 octets
- La découverte MTU du chemin TCP est désactivée par défaut
- Les homologues TCP ne sont pas directement connectés
- R4 gère la connexion BGP
- La destination R1 n’est pas directement connectée
- R4 envoie SYN avec MSS de 1 216 octets
- L'interface MTU n'est pas prise en compte lorsque les homologues ne sont pas directement connectés et que la découverte MTU du chemin TCP est désactivée
- Selon la conception, 1280 octets sont considérés comme TCP_DEFAULT_MTU
- 1280 (TCP_DEFAULT_MTU) - 20 (minTCP_H) - 20 (minIP_H) - 24 octets (surcharge des options TCP IOS XR)
- R1 envoie SYN, ACK avec MSS de 1 216 octets
- Envoie le plus faible de [MSS reçu ; MSS initial local]
- MSS reçu 1216 octets ; MSS initial local 1 240 octets
- La valeur MSS la plus faible est utilisée sur les deux homologues
SYN TCP provenant de 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 provenant de 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)
Détails de la session TCP tels qu'ils apparaissent sur R4 - ACTIF :
! – 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#
Détails de la session TCP tels qu'ils apparaissent sur 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#
Homologues TCP non connectés directement - Le segment de chemin a une MTU IP inférieure
Dans le scénario suivant, l'objectif est d'observer et de conclure ce qui se passe s'il y a un segment de chemin intermédiaire avec une MTU IP inférieure alors qu'il est en état par défaut, ce qui signifie que TCP PMTUD est désactivé. Reportez-vous à cette image.
Image 2.8 - Le segment de chemin R2/R3 a une MTU IP inférieure.
Dans un scénario initial, considérez que les informations BGP sont minimales, c'est-à-dire que tout ce qui est nécessaire à l'échange entre homologues BGP peut être accompli avec des paquets IP qui s'ajustent sous le chemin minimal MTU de 512 octets. Avec cette hypothèse, le calcul MSS se fait comme décrit dans la section Homologues TCP non connectés directement. R1 et R4 sélectionnent une valeur MSS de 1 240 octets.
Détails de la session TCP tels qu'ils apparaissent sur R4 - ACTIF :
! – 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>
Détails de la session TCP tels qu'ils apparaissent sur 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>
Maintenant que la session BGP est établie, considérez qu'un message de mise à jour BGP d'une taille supérieure à la MTU du chemin minimal de 512 octets est déclenché. Comme on peut le constater à partir des sorties, Cisco IOS XR ne définit pas le df-bit avec le message de mise à jour BGP, ce qui signifie que les informations BGP sont transmises au détriment de la fragmentation des paquets sur les noeuds intermédiaires.
Mise à jour BGP provenant de R1 - PASSIVE :
! – 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 fragmentation du message de mise à jour BGP provenant du noeud R1 se produit au niveau du noeud R2, comme le montre la capture du trafic effectuée à l'interface GE0/0/0/1 de R2.
Fragmentation IP au niveau du noeud 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>
Scénarios - TCP PMTUD activé
Activer PMTUD
Une fois PMTUD activé, indépendamment du fait que les homologues soient connectés directement ou non directement, le calcul initial MSS prend toujours en compte le MTU IP de l'interface de sortie.
Ce scénario fournit des informations sur le comportement attendu lorsque la PMTUD est activée. Ici, le noeud Cisco IOS XR R4 joue le rôle actif, gère la connexion TCP et ouvre la session TCP avec le noeud Cisco IOS XR R1 sur le port de destination 179. Les deux noeuds utilisent les valeurs MTU IP par défaut sur leurs interfaces.
Image 3.1 - TCP PMTUD activé.
Le calcul MSS dans ce scénario peut être résumé comme suit :
- Tous les noeuds utilisent une MTU IP par défaut de 1 500 octets
- La découverte MTU du chemin TCP est activée
- Les homologues TCP ne sont pas directement connectés
- R4 gère la connexion BGP
- R4 envoie SYN avec MSS de 1 460 octets
- 1500 (MTU IP d'interface) - 20 (minTCP_H) - 20 (minIP_H)
- R1 envoie SYN, ACK avec MSS de 1 460 octets
- Envoie le plus faible de [MSS reçu ; MSS initial local]
- MSS reçu 1 460 octets ; MSS initial local 1 460 octets
- La valeur MSS la plus faible est utilisée sur les deux homologues
Afin de mettre en évidence le changement de comportement introduit par l'activation de PMTUD, les sorties suivantes illustrent la séquence d'événements :
- État initial de la session TCP établie dans le scénario par défaut de PMTUD désactivée ;
- PMTUD est configuré et activé sur les homologues TCP R4 et R1 ;
- La session TCP est redémarrée, le calcul MSS a lieu et est influencé par TCP PMTUD.
Comme indiqué sur R4 - ACTIVE - TCP PMTUD désactivé (par défaut) :
! – 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#
Comme indiqué sur R1 - PASSIVE - TCP PMTUD désactivé (par défaut) :
! – 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#
Comme indiqué sur R4 - ACTIVE - TCP PMTUD activé :
! – '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#
Comme indiqué sur R1 - PASSIVE - TCP PMTUD activé :
! – '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#
Notez le comportement du minuteur 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
Comme indiqué sur R4 - ACTIVE - Redémarrage de la session 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)
Comme indiqué sur R1 - PASSIVE - Redémarrage de la session BGP - 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)
Détails de la session TCP tels qu'ils apparaissent sur R4 - ACTIVE - après l'activation de TCP PMTUD et l'effacement de la session 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#
Détails de la session TCP tels qu'ils apparaissent sur R1 - PASSIVE - après l'activation de TCP PMTUD et l'effacement de la session 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 - Le segment de chemin a une MTU IP inférieure
Le scénario précédent a permis de comprendre ce qui se passe lors de l'établissement initial de la session TCP avec PMTUD activée. Ce scénario s'appuie sur le haut et aide à comprendre le fonctionnement de TCP PMTUD et son influence sur les sessions TCP établies.
Image 3.2 - PMTUD activée, et le segment de chemin a une MTU IP inférieure.
Considérez l'image précédente comme référence, supposez que la session BGP est établie et R1 envoie le message de mise à jour BGP transmis par un paquet IP d'une taille supérieure à 512 octets. Lorsque PMTUD est activé, le bit DF (Don’t Fragment) est maintenant défini. Par conséquent, le noeud R2 supprime le paquet IP et envoie un message ICMP (Internet Control Message Protocol) (Destination Unreachable - type 3 ; Fragmentation requise - code 4) vers R1. Au niveau du noeud R1 après la réception du message ICMP, PMTUD est déclenché et tente d’établir le chemin de MTU IP le plus bas. Il le fait en utilisant la valeur inférieure suivante d'un ensemble de niveaux de plateau bien définis, c'est-à-dire une nouvelle valeur MSS de session TCP. TCP retransmet ensuite la mise à jour BGP d'origine avec la nouvelle valeur MSS et ce processus est répété autant de fois que nécessaire jusqu'au message ICMP (Destination Unreachable - type 3); Fragmentation requise - Code 4) n'est plus reçu. Cela signifie que jusqu'à ce que la valeur MSS utilisée soit telle que chaque paquet envoyé tombe sous la MTU IP du segment de chemin le plus bas. Au fil du temps, la PMTUD dirigée par le compteur PmtuAger traverse les niveaux de plateau dans la direction inverse et ramène le MSS à sa valeur maximale. À tout moment donné si un message ICMP (Destination inaccessible - type 3 ; Fragmentation nécessaire - Code 4) est de nouveau reçu, puis PMTUD agit comme décrit précédemment.
Les sorties suivantes passent en revue le comportement PMTUD décrit et démarrent à partir du scénario d'une session TCP établie. Ici, le noeud Cisco IOS XR R4 joue un rôle actif, gérant ainsi la connexion TCP et ouvrant la session TCP avec R1 sur le port de destination 179. Les deux noeuds utilisent les valeurs MTU IP par défaut sur leurs interfaces. Le calcul MSS initial dans ce scénario peut être résumé comme suit :
- Le segment intermédiaire entre les noeuds R2 et R3 utilise un MTU IP non par défaut de 512 octets.
- R1 et R4 utilisent des valeurs MTU par défaut sur leurs interfaces.
- La découverte MTU du chemin TCP est activée.
- Les homologues TCP ne sont pas directement connectés.
- R4 gère la connexion BGP.
- R4 envoie SYN avec MSS de 1 460 octets.
- 1500 (interface IP MTU) - 20 (minTCP_H) - 20 (minIP_H).
- R1 envoie SYN, ACK avec MSS de 1 460 octets.
- Envoie la valeur inférieure de [MSS reçu ; MSS initial local].
- MSS reçu 1 460 octets ; Local initial MSS 1460 octets.
- La valeur MSS la plus faible est utilisée sur les deux homologues.
SYN TCP provenant de 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 provenant de 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)
Une fois la session BGP établie, le noeud R1 envoie le message de mise à jour BGP et reçoit le message ICMP (Destination Unreachable - type 3 ; Fragmentation requise - Code 4) en retour provenant du noeud R2.
Cela se produit parce que le paquet IP qui transporte le message de mise à jour BGP a le bit DF défini et la MTU IP de 512 octets utilisée au niveau du segment R2/R3 est inférieure à la taille de paquet IP de 1116 octets. Comme expliqué précédemment, la réception du message ICMP déclenche PMTUD.
À R1 ICMP, un message de type 3/Code 4 est reçu :
! – 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]
Au niveau du noeud R1, déclenché par un message ICMP, TCP PMTUD tente d’établir la MTU IP la plus basse de bout en bout en utilisant la valeur inférieure suivante d’un ensemble de niveaux de plateau bien définis (MTU IP). Ces niveaux de plateau sont documentés sur RFC1191 - Découverte de MTU de chemin.
MTU plateaus from RFC 1191
- values include both TCP and IP headers
65535
32000
17914
8166
4352
2002
1492
1006
508
296
68
Mais depuis ICMP (Destination inaccessible - type 3); Fragmentation requise - Code 4) Le message reçu par le noeud R1 transmet le MTU du saut suivant puis, comme indiqué ci-après, le noeud R1 utilise cette valeur, qui dans notre exemple est de 512 octets, et règle la valeur MSS de session TCP. Notez que la longueur du segment TCP d’origine était de 1 076 octets. Trois paquets sont donc nécessaires pour retransmettre le segment TCP d’origine.
Comme indiqué sur R1 - PASSIVE - Fonctionnement de la 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
Comme indiqué précédemment, une fois que tous les paquets ont été transmis au fil du temps, PMTUD traverse les niveaux de plateau dans la direction inverse dirigée par le compteur PmtuAger et tente d'élever le MSS à sa valeur maximale selon le scénario en place.
Comme indiqué sur R1 - PMTUD sur des plateaux définis :
! – 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
L'état final de ces résultats peut être observé. Notez en particulier les valeurs MSS min et max exposées par le noeud R1, qui met en évidence et signale que PMTUD a été déclenché.
Détails de la session TCP tels qu'ils apparaissent sur R4 - ACTIF :
! – 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#
Détails de la session TCP tels qu'ils apparaissent sur 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#
Enfin, si à un moment donné un ICMP (Destination Inaccessible - type 3 ; Fragmentation requise - Code 4) message de nouveau reçu puis PMTUD agit de nouveau comme décrit précédemment.
Comme le montre R1 - PASSIVE - PMTUD s'est à nouveau déclenché :
! – 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#
Sur les versions de Cisco IOS XR affectées par l'ID de bogue Cisco CSCvf10395, le saut suivant contenu dans le message d'erreur ICMP est ignoré et le noeud tente d'établir le MTU IP le plus bas de bout en bout en utilisant la valeur inférieure suivante de l'ensemble des niveaux de plateau (MTU IP) bien définis mentionnés précédemment et documentés par RFC119 1 - Découverte MTU du chemin. Ces tentatives se produisent jusqu’à la transmission réussie, ce qui signifie jusqu’à ICMP (Destination inaccessible - type 3 ; Fragmentation requise - Code 4) Les messages ne sont plus reçus.
Comme le montre un noeud avec la version de Cisco IOS XR affectée par l'ID de bogue 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
À l’étape suivante, envisagez le même scénario, mais avec le protocole LDP (Label Distribution Protocol) sur toutes les interfaces. L'objectif est ici de comprendre les différences qui peuvent être observées par rapport aux scénarios précédents dans un environnement compatible MPLS.
Image 3.3 - PMTUD activée, et le segment de chemin a un MTU IP plus faible - scénario MPLS.
Tout d'abord, considérez l'étape initiale de la session BGP établie avant le déclencheur PMTUD comme indiqué ici.
État initial du protocole TCP (BGP) tel qu'indiqué sur R4 - ACTIVE - scénario compatible 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#
État initial du protocole TCP (BGP) tel qu'indiqué sur R1 - PASSIVE - scénario compatible 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#
Dans ce scénario prenant en charge MPLS, il est observé que les détails des sessions TCP (LDP) ont été établis. Notez que tous les éléments décrits précédemment en ce qui concerne le calcul MSS pour les sessions TCP (BGP) s'appliquent également aux sessions TCP (LDP). Par exemple, le calcul MSS des noeuds R3 et R2 TCP (LDP) de session peut être résumé comme suit :
- R2 et R3 utilisent une MTU IP non par défaut de 512 octets.
- La découverte MTU du chemin est activée.
- Les homologues TCP ne sont pas directement connectés (la session TCP est établie entre les interfaces de bouclage).
- R3 gère la connexion LDP.
- R3 envoie SYN avec MSS de 472 octets.
- 512 (interface IP MTU) - 20 (minTCP_H) - 20 (minIP_H).
- R2 envoie SYN, ACK avec un MSS de 472 octets.
- Envoie le plus faible de [MSS reçu ; MSS initial local].
- MSS reçu 472 octets ; MSS initial local de 472 octets.
- La valeur MSS la plus faible est utilisée sur les deux homologues.
Détails de la session TCP (LDP) tels qu'ils apparaissent sur R3 - ACTIVE - scénario compatible 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#
Détails de la session TCP (LDP) tels qu'ils apparaissent sur R2 - PASSIVE - scénario compatible 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#
Une fois la session BGP établie, R1 envoie le message de mise à jour BGP et reçoit le message ICMP (Destination Unreachable - type 3 ; Fragmentation requise - Code 4) en retour provenant du noeud R2 qui déclenche TCP PMTUD au niveau du noeud R1. Cela se produit parce que le paquet IP qui transporte le message de mise à jour BGP a le bit DF défini et la MTU IP de 512 octets utilisée au niveau du segment R2/R3 est inférieure à la taille de paquet IP de 1116 octets. Comme précédemment, la réception de ce message ICMP déclenche PMTUD. La différence dans le scénario MPLS par rapport aux scénarios précédents non MPLS concerne la valeur MTU du saut suivant incluse dans le message ICMP du noeud R2 (Destination inaccessible - type 3); Fragmentation nécessaire - Code 4). Dans ce scénario compatible MPLS, la valeur MTU du saut suivant représente la surcharge MPLS supplémentaire de 4 octets, ce qui signifie qu'elle tient compte de la pile d'étiquettes MPLS de sortie sur R2, comme le montrent ces sorties.
Découverte MTU du chemin TCP en action comme vu sur R1 - PASSIVE - scénario compatible 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
Comme le montre R1 - PASSIVE - TCP PMTUD déclenché - scénario 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#
Notez que dans le scénario MPLS, la valeur de la valeur MTU du saut suivant incluse sur le noeud R2 ICMP compte pour la pile d'étiquettes MPLS de sortie. Pour renforcer cet aspect, prenons l'exemple suivant. Si le paquet IP filtré sur R2 est associé à un service L3VPN, cela signifie que la trame Ethernet porte désormais deux étiquettes (étiquette IGP et étiquette VPN). Ensuite, la MTU du saut suivant reflète la taille de pile d'étiquette requise. Reportez-vous à ces résultats.
Comme indiqué sur R1 - PASSIVE - Paquet de service VPN L3 :
! – 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
Comme indiqué sur R1 - PASSIVE - Service VPN L3 - ICMP Type 3/Code 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 - Options TCP (MD5)
Image 3.4 - PMTUD activée et TCP MD5 Authentication.
Aucune distinction n'est introduite en ce qui concerne le comportement PMTUD par rapport à ce qui a déjà été décrit dans les scénarios précédents avec l'authentification MD5 TCP activée. Comme précédemment partagé avec l'authentification MD5 TCP utilisée, Cisco IOS XR considère une surcharge supplémentaire et la valeur MSS initiale de l'homologue TCP actif reflète la même valeur. Reportez-vous aux sections précédentes Utiliser les options TCP - XR actif et Utiliser les options TCP - XR passif pour plus de détails sur l'impact des options TCP utilisées. Le calcul MSS TCP dans ce scénario peut être résumé comme suit :
- Tous les noeuds utilisent une MTU IP par défaut de 1 500 octets.
- La découverte MTU du chemin TCP est activée.
- Les homologues TCP ne sont pas directement connectés.
- Authentification MD5 TCP activée sur R1 et R4.
- R4 gère la connexion BGP.
- R4 envoie SYN avec MSS de 1 436 octets.
- 1500 (interface IP MTU) - 20 (minTCP_H) - 20 (minIP_H) - 24 octets (IOS XR TCP Options Overhead).
- R1 envoie SYN, ACK avec MSS de 1 436 octets.
- envoie le plus faible de [MSS reçu ; MSS initial local].
- MSS reçu 1 436 octets ; Local initial MSS 1460 octets.
- La valeur MSS la plus faible est utilisée sur les deux homologues.
SYN TCP provenant de 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 provenant de 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)
Détails de la session TCP tels qu'ils apparaissent sur R4 - ACTIF :
! – 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#
Détails de la session TCP tels qu'ils apparaissent sur 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 - Détection des trous noirs
Comme expliqué précédemment dans la section PMTUD - Le segment de chemin a une MTU IP inférieure, la PMTUD TCP lorsqu'elle est activée est déclenchée par la réception d'un ICMP (Destination Unreachable - type 3); Fragmentation requise - Code 4). Il peut arriver que ces messages ne soient pas reçus pour une raison quelconque et que les résultats de PMTUD ne soient pas déclenchés. Dans ce cas, la MTU IP la plus basse du chemin entre les homologues TCP n'est pas apprise. Un tel scénario introduirait un trou noir potentiel si les paquets IP ont le bit DF défini et s'ils ont une taille supérieure au segment de chemin MTU IP le plus bas. Ces paquets seraient abandonnés en silence.
Cette section vise à mettre en évidence la manière dont Cisco IOS XR détecte et agit sur un tel scénario de trou noir potentiel. À cette fin, la fonctionnalité IPv4 unreachables est désactivée sur l'interface GE0/0/0 de R2, comme indiqué dans l'image suivante et dans la sortie CLI.
Image 3.5 - PMTUD activée sur R1/R4 et R2 IPv4 inaccessible désactivée.
IPv4 inaccessible désactivé sur 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
La manière dont Cisco IOS XR gère ce scénario de trou noir consiste à retransmettre le même paquet deux fois et si cela échoue encore, c'est-à-dire que l'ACK TCP attendu n'est pas reçu, puis recommencez mais utilisez la valeur supérieure de plateau bien définie comme indiqué dans RFC1191 - Découverte de MTU de chemin (voir la section PMTUD - Le segment de chemin a une MTU de MTU de MTU de plus faible IP pour la liste des plateaux IP) aus). En résumé, Cisco IOS XR suppose que les paquets peuvent être abandonnés quelque part dans le chemin vers leur destination en raison de leur taille et des tentatives de contournement par retransmission de paquets. Ce comportement peut être observé avec l'exemple suivant d'une capture de paquets prise au niveau de l'interface du noeud R1 et le résultat de la commande debug tcp pmtud.
Détection de trou noir IOS-XR sur 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
Historique de révision
| Révision | Date de publication | Commentaires |
|---|---|---|
1.0 |
16-Aug-2021 |
Première publication |
Commentaires