Cisco IOS XRおよびBGPでのパスMTUディスカバリの確認

はじめに

このドキュメントでは、Cisco IOS® XRデバイスのTransmission Control Protocol(TCP)パス最大伝送ユニット(MTU)ディスカバリ(PMTUD)について説明します。

背景説明

PMTUDメカニズムは、2つのホスト間のパスのどこでもフラグメンテーションを必要としない最大のインターネットプロトコル(IP)パケットサイズを決定しようとします。確立された値は指定パスMTUであり、各ホップのMTU値の最小値に等しくなります。情報を送信するときにパスMTUを考慮すると、ネットワーク容量を最大限に活用し、フラグメンテーションと伝送効率を回避できます。PMTUDのメカニズムと実装は、PMTUDの動作を徐々に明らかにするクライアントプロトコルとしてボーダーゲートウェイプロトコル(BGP)を使用して、さまざまなシナリオにわたって導入されます。

TCP PMTUDおよびTCP MSS

TCPはPMTUDの結果を利用して、ローカルの最大セグメントサイズ(MSS)に影響を与えます。つまり、検出されたパスMTUに動的に適応します。したがって、PMTUDに進む前に、TCPの最大セグメントサイズ(MSS)を簡単に確認し、その意味と目的を理解できます。

MSSによるRFC879での当初の定義によると、MSSオプションの定義は次のように表記できます。TCPヘッダーオプションのないTCPセグメントでは、このTCPオプションの送信者が受信できるデータオクテットの最大数は、IPヘッダーオプションのないIPデータグラムで送信されます。

いくつかの側面を明確にし、実装者にアドバイスを提供するため、 RFC6691 mss値の計算方法を示します。 

TCP MSSオプションに含める値を計算する際には、MTU値を固定IPおよびTCPヘッダーのサイズ分だけ小さくし、IPまたはTCPオプションを考慮して小さくしてはなりません。逆に、送信側は、送信するパケットに含まれるIPまたはTCPオプションを考慮して、TCPデータ長を小さくする必要があります。

MSSの詳細な定義については、『Routing Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 7.9.x』を参照してください。 

MSSは、単一のフラグメント化されていないTCPセグメントでコンピュータまたは通信デバイスが受信できるデータの最大量です。すべてのTCPセッションは、1つのパケットで転送できるバイト数の制限によって制限されます。この制限はMSSです。TCPは、パケットを送信キュー内のチャンクに分割してから、IP層に渡します。

TCP MSS値は、インターフェイスのMTUに依存します。これは、プロトコルによって1つのインスタンスで送信できるデータの最大長です。最大TCPパケット長は、送信元デバイスの発信インターフェイスのMTUと、TCPセットアッププロセス中に宛先デバイスによって通知されるMSSの両方によって決まります。MSSがMTUに近いほど、BGPメッセージの転送が効率的になります。データフローの各方向は、異なるMSS値を使用できます。

TCPが特定のTCPセッションのMSSについて考慮する必要がある値はどれか？どのように計算しますか。

RFC879に準拠したデフォルト値では、宛先ホストが大きいデータグラムを受け入れる準備ができているという特別な情報がない限り、ホストは576オクテットを超えるデータグラムを送信できません。 TCPの最大セグメントサイズは、IPの最大データグラムサイズから40を引いた値です。

IPの最大データグラムサイズのデフォルトは576です。

デフォルトのTCP最大セグメントサイズは536です。

これは、576バイトのIP MTU値を考慮に入れます。ただし、実際のIP MTU値を無視した場合、TCP MSSの計算は次のように要約できます。

• アクティブピア：SYNパケットを使用して初期MSSを計算し、送信します。
  
  

MSS = IPMTU - sizeof(minimum TCPHDR) - sizeof(minimum IPHDR)
Where,
sizeof(minimum TCPHDR) = 20 bytes.
sizeof(minimum IPHDR) = 20 bytes.

• パッシブピア：最初のMSSを計算し、アクティブピアから受信したMSSと比較して、これらのMSS値の低い方でSYN、ACKを送信します。
  
  

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.

MSSオプション値に関するネゴシエーションはありません。各ノードはそれ自体の値を決定し、TCPセッションの確立時に同じ値をアナウンスします。MSSの計算で考慮されるIP MTU値がPMTUDから導出できる場合、MSS値を、特定のパスMTUに対して最も効果的な値に適合させることが明らかになります。Cisco IOS XRの動作には、MSSの計算とPMTUDの役割に関する詳細が次にまとめられています。

Cisco IOS XRでは、PMTUDはデフォルトで無効になっています。

• ローカルの初期MSS計算では、IP MTUは次のように考慮されます。
  • 直接接続されたピアの場合：出力インターフェイスのIP MTUを検討します。
  • 直接接続されていないピアの場合：1280バイトのIP MTUを考慮します。
  • MSS値は、設定されているTCPオプションに影響されます。

Cisco IOS XRでPMTUDが有効な場合：

• ローカルの初期MSS計算では、IP MTUは次のように考慮されます。
  • 直接接続されたピアか非直接接続されたピアかに関係なく、出力インターフェイスのIP MTUを考慮します。
  • MSS値は、設定されているTCPオプションに影響されます。

PMTUDのメカニズムと実装については、考慮する必要があるその他の詳細があります。このドキュメントでは、次の表にまとめた実例を使用して、これらの詳細を紹介します。この表には、アクティブおよびパッシブTCPピアのIP MTUと、考慮する各シナリオで選択されているMSS値も示されています。

シナリオ：TCP PMTUDが無効

デフォルトのMTU値を使用

図 2.1.デフォルトのMTU値

図2.1に示すeBGPピアの場合、R6はTCP接続を管理します。これは、アクティブな役割を果たし、宛先ポート179でR5とのTCPセッションを開始することを意味します。ピアは直接接続され、両方とも各インターフェイスでデフォルトのIP MTU値を使用します。このドキュメントの冒頭で共有した情報に基づき、このシナリオでのMSSの計算は次のように要約できます。  

• 両方のノードは、1500バイトのデフォルトIP MTUを使用します
• TCPパスMTUディスカバリはデフォルトで無効になっています
• TCPピアは直接接続されます
  • R6はBGP接続を管理します
  • R6は1460バイトのMSSでSYNを送信します
    • 1500（インターフェイスIP MTU） – 20(minTCP_H)- 20(minIP_H)
  • R5は1460バイトのMSSでSYN、ACKを送信します
    • [Received MSS; Local initial MSS]の下限を送信
    • 受信MSS 1460バイト、ローカル初期MSS 1460バイト
  • 両方のピアで最小のMSS値が使用される

R6に表示されるTCPセッションの詳細：アクティブ：

! – 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

R5に表示されるTCPセッションの詳細：パッシブ：

! – 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#

デフォルト以外のMTU値の使用：アクティブなTCPピア

図2.2：アクティブピアがデフォルト以外のMTU値を使用している

このシナリオは基本的に前のシナリオと同じですが、アクティブなTCPピアR6がデフォルト以外のIP MTU値を使用する点が異なっています。MSS値の最初の計算と決定がパッシブTCPピアR5によってどのように行われるかに注意してください。このシナリオでのTCP MSSの計算は、次のように要約できます。  

• R6はデフォルト以外のIP MTU 4460バイトを使用します
• TCPパスMTUディスカバリはデフォルトで無効になっています
• TCPピアは直接接続されます
  • R6はBGP接続を管理します
  • R6は4420バイトのMSSでSYNを送信します
    • 4460（インターフェイスIP MTU） – 20(minTCP_H)- 20(minIP_H)
  • R5は1460バイトのMSSでSYN、ACKを送信します
    • [Received MSS; Local initial MSS]の下位を送信
    • 受信MSS 4420バイト、ローカル初期MSS 1460バイト
  • 両方のピアで最小のMSS値が使用される

R6から送信されたTCP SYN:

! – 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)

R5から送信されたTCP SYN、ACK:

! – 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)

R6に表示されるTCPセッションの詳細：アクティブ：

! - 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#

R5に表示されるTCPセッションの詳細：パッシブ：

! - 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#

デフォルト以外のMTU値の使用：パッシブTCPピア

図2.3：パッシブピアがデフォルト以外のMTU値を使用している

依然として同じeBGPシナリオを使用しますが、ここではデフォルト以外のIP MTUで設定されたパッシブTCPピアR5と、デフォルトのIP MTU値で設定されたアクティブTCPピアR6を使用します。前のシナリオと同様に、MSS値がパッシブピアR5によってどのように選択されるかに注意してください。このシナリオでのTCP MSSの計算は、次のように要約できます。  

• R5はデフォルト以外のIP MTU 4460バイトを使用します
• TCPパスMTUディスカバリはデフォルトで無効になっています
• TCPピアは直接接続されます
  • R6はBGP接続を管理します
  • R6は1460バイトのMSSでSYNを送信します
    • 1500（インターフェイスIP MTU） – 20(minTCP_H)- 20(minIP_H)
  • R5は1460バイトのMSSでSYN、ACKを送信します
    • [Received MSS; Local initial MSS]の下位を送信
    • 受信MSS 1460バイト、ローカル初期MSS 4420バイト
  • 両方のピアで最小のMSS値が使用される

R6から送信されたTCP SYN:

! – 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)

R5から送信されたTCP SYN、ACK:

! – 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)

R6に表示されるTCPセッションの詳細：アクティブ：

! - 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#

R5に表示されるTCPセッションの詳細：パッシブ：

! - 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#

TCPオプションの使用 – XRアクティブ

このドキュメントで前述したように、TCPオプション(TCP MD5、TCP selective-ack、TCP timestampsなど)を使用すると、MSS計算で追加のバイトが必要になるため、MSS計算はこれらのオプションの使用によって影響を受けます。

このセクションと次の目的は、TCPオプションが存在する場合にピアによって行われるMSS計算を説明することです。例としてTCP MD5認証オプションを使用します。図2.4の参照シナリオを参照してください。

図2.4 - TCPオプションの使用(MD5) - XRアクティブ

このシナリオでは、両方のピアがデフォルトのIP MTU値を使用し、直接接続され、ピアR6がTCPのアクティブな役割を果たします。すでに、追加のオーバーヘッドのためにTCP MD5認証アカウントの設定と使用を共有しています。この特定のシナリオでのTCP MSSの計算は、次のように要約できます。  

• 両方のノードは、1500バイトのデフォルトIP MTUを使用します
• TCPパスMTUディスカバリはデフォルトで無効になっています
• TCPピアは直接接続されます
• 両方のノードでTCP MD5認証が有効
  • R6はBGP接続を管理します
  • R6は1436バイトのMSSでSYNを送信します
    • 1500（インターフェイスIP MTU） – 20(minTCP_H)- 20(minIP_H)- 24バイト（IOS XR TCPオプションオーバーヘッド）
  • R5は1436バイトのMSSでSYN、ACKを送信します
    • [Received MSS; Local initial MSS]の下位を送信
    • 受信MSS 1436バイト、ローカル初期MSS 1460バイト
  • 両方のピアで最小のMSS値が使用される

要約からわかるように、Cisco IOS XRの動作は厳密にはRFC 879およびRFC 6691に従っていません。これらの規定には、TCPオプションをMSS計算では考慮してはならないことが記載されています。

tcpヘッダー長の余分な要素に関するCisco IOS XRアカウントについては、Cisco Bug ID CSCvf20166で詳しく説明されています。

(...)XRがBGP接続を開始する際、BGPは最初にソケットを作成し、次にMD5を含むソケットオプションを設定します。 これにより、tcpオプションヘッダー長= 24になります。 したがって、最初のMSSは1500 - 40 - 24 = 1436になります。これはピアに送信され、ピアはmin(1436, 1460) = 1436.(..)を使用します。

R6から送信されたTCP SYN:

! – 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)

R5から送信されたTCP SYN、ACK:

! – 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)

R6に表示されるTCPセッションの詳細：アクティブ：

! - 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#

R5に表示されるTCPセッションの詳細：パッシブ：

! - 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#

同様の動作は、他のTCPオプションでも確認できます。このオプションを設定すると、追加のオーバーヘッドが考慮され、Cisco IOS XRのMSS計算に影響を与えます。TCPタイムスタンプとTCP selective-ackオプションが設定されている場合のMSSの計算について説明した、同じシナリオおよびこれらの例を検討してください。

R6 - ACTIVEに表示されるTCPセッションの詳細（TCPオプションtimestampおよびselective-ackオプションが設定されている場合）:

! - 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>

TCPオプションの使用 – XRパッシブ

初期MSS計算に関してパッシブロールの場合のCisco IOS XRノードの動作は、以前のシナリオとは異なることにお気づきになったと思います。ノードではtcpオプションのヘッダー長が考慮されません。 このシナリオの目的は、Cisco Bug ID（登録ユーザ専用）にも記述されている動作に注目することです。

「(...)：ピアが接続を開始すると、初期MSSを1460として送信します。XR TCPはソケットやpcbなどを作成し、次に指定された順序で次の2つのアクションを実行します。

– 最初に、tcpオプションヘッダー長を減算した後の初期MSSを計算します。 MD5オプションはlistenソケットからこのソケットにまだ継承されていないため、これは'0'です。

– 次に、「MD5」およびその他のオプションを継承し、「option header bytes length」を24にします。

したがって、この場合、XR TCPは最初のMSSを1460として送信するため、このアドレスは両方で使用されます。(…)“

このシナリオでは、アクティブなTCPピアR8はCisco IOSノードですが、この事実は、このシナリオで強調する内容の違いや詳細を説明するものではありません。それにもかかわらず、興味深いことに、前のセクションのシナリオで示されたCisco IOS XRとは異なり、アクティブなTCPピアR8は初期MSS計算でTCPオプションを考慮しません。

図2.5 - TCPオプション(MD5)の使用 – XRパッシブ

両方のピアはデフォルトのIP MTU値を使用し、直接接続されます。Cisco IOSピアR8がアクティブな役割を果たします。このシナリオでのTCP MSSの計算は、次のように要約できます。  

• 両方のノードは、1500バイトのデフォルトIP MTUを使用します
• TCPパスMTUディスカバリは、Cisco IOS XR R7ではデフォルトで無効になっています
• Cisco IOS R8では、TCPパスMTUディスカバリがデフォルトで有効になっています
• TCPピアは直接接続されています
• 両方のノードでTCP MD5認証が有効
  • IOS R8はBGP接続を管理します
  • IOS R8は1460バイトのMSSでSYNを送信します
    • 1500（インターフェイスIP MTU） – 20(minTCP_H)- 20(minIP_H)
  • IOS XR R7は1460バイトのMSSでSYN、ACKを送信します
    • [Received MSS; Local initial MSS]の下位を送信
    • 受信MSS 1460バイト、ローカル初期MSS 1460バイト
  • 両方のピアで最小のMSS値が使用される

R8から送信されたTCP SYN - 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)

R7から送信されたTCP SYN、ACK - 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)

R8に表示されるTCPセッションの詳細 – Cisco IOS – アクティブ：

! – 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#

R7（Cisco IOS XR – パッシブ）に表示されるTCPセッションの詳細：

! – 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#

直接接続されていないTCPピア

ピアが直接接続されていない場合、TCP MSSの初期計算方法は、このドキュメントの最初のセクションで説明したように変更されます。すべてのピアがデフォルトのIP MTU値で設定されているiBGPセッションのシナリオを使用して、MSSの計算を実行します。

図2.6：直接接続されていないTCPピア：iBGP

注意すべき重要な点は、TCPパスMTUディスカバリが無効で、ピアが直接接続されていない場合、Cisco IOS XRでは1280バイトの固定IP MTU値が使用される点です。

上の図では、R4がアクティブな役割を果たし、TCP接続を管理します。R4は、宛先ポート179でR1とのTCPセッションを開きます。両方のノードがそれぞれのインターフェイスでデフォルトのIP MTU値を使用します。このシナリオでのMSSの計算は、次のように要約できます。  

• すべてのノードは1500バイトのデフォルトIP MTUを使用します
• TCPパスMTUディスカバリはデフォルトで無効になっています
• TCPピアが直接接続されていない
  • R4はBGP接続を管理します
  • R4は1240バイトのMSSでSYNを送信します
    • ピアが直接接続されておらず、TCPパスMTUディスカバリが無効な場合、インターフェイスMTUは考慮されません
    • Cisco IOS XRの設計では、1280バイトはTCP_DEFAULT_MTUと見なされます
    • 1280(TCP_DEFAULT_MTU)- 20(minTCP_H)- 20(minIP_H)
  • R1は1240バイトのMSSでSYN、ACKを送信します
    • [Received MSS; Local initial MSS]の下限を送信
    • 受信MSS 1240バイト、ローカル初期MSS 1240バイト
  • 両方のピアで最小のMSS値が使用される

R4から送信されたTCP SYN:

! – 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)

R1から送信されたTCP SYN、ACK:

! – 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)

R4に表示されるTCPセッションの詳細：アクティブ：

! – 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#

R1に表示されるTCPセッションの詳細：パッシブ：

! – 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#

直接接続されていないTCPピア – TCPオプションの使用(MD5)

直接接続されていないピアシナリオでTCP MD5認証を使用する場合、すでに説明した以前のテストケースやシナリオと基本的な違いはありません。TCP MD5認証で前述したように、Cisco IOS XRでは追加のオーバーヘッドが考慮され、初期MSS値には同じ値が反映されます。TCPオプションがTCP MSS計算に与える影響についての詳細は、前のセクション「TCPオプションの使用 – XRアクティブ」および「TCPオプションの使用 – XRパッシブ」を参照してください。

図2.7：直接接続されていないTCPピア：iBGP + TCP MD5

このシナリオでのTCP MSSの計算は、次のように要約できます。  

• すべてのノードは1500バイトのデフォルトIP MTUを使用します
• TCPパスMTUディスカバリはデフォルトで無効になっています
• TCPピアが直接接続されていない
  • R4はBGP接続を管理します
  • 宛先R1は直接接続されていません
  • R4は1216バイトのMSSでSYNを送信します
    • ピアが直接接続されておらず、TCPパスMTUディスカバリが無効な場合、インターフェイスMTUは考慮されません
    • 設計では、1280バイトはTCP_DEFAULT_MTUと見なされます
    • 1280(TCP_DEFAULT_MTU)- 20(minTCP_H)- 20(minIP_H)- 24バイト（IOS XR TCPオプションオーバーヘッド）
  • R1は1216バイトのMSSでSYN、ACKを送信します
    • [Received MSS; Local initial MSS]の下限を送信
    • 受信MSS 1216バイト、ローカル初期MSS 1240バイト
  • 両方のピアで最小のMSS値が使用される

R4から送信されたTCP SYN:

! – 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)

R1から送信されたTCP SYN、ACK:

! – 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)

R4に表示されるTCPセッションの詳細：アクティブ：

! – 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#

R1に表示されるTCPセッションの詳細：パッシブ：

! – 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#

TCPピアが直接接続されていない：パスセグメントのIP MTUが小さい

次のシナリオの目標は、デフォルト状態ではより低いIP MTUの中間パスセグメントがある場合、つまりTCP PMTUDが無効である場合にどうなるかを観察して結論を出すことです。この画像を参照してください。

図2.8:R2/R3パスセグメントのIP MTUが小さい

初期シナリオとして、BGP情報が最小限であると考えてください。つまり、BGPピア間で交換する必要があるものは、最小パスMTUの512バイトに収まるIPパケットで実現できます。この想定では、「直接接続されていないTCPピア」セクションで説明されているように、MSSの計算が行われます。R1とR4はどちらも1240バイトのMSS値を選択します。

R4に表示されるTCPセッションの詳細：アクティブ：

! – 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>

R1に表示されるTCPセッションの詳細：パッシブ：

! – 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>

ここでBGPセッションを確立すると、最小パスMTUの512バイトよりも大きいサイズのBGPアップデートメッセージがトリガーされると考えられます。出力からわかるように、Cisco IOS XRではBGP更新メッセージでdfビットを設定しません。これは、中継ノードでのパケットフラグメンテーションが原因でBGP情報が送信されることを意味します。

R1から発信されたBGPアップデート：パッシブ：

! – 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

ノードR1から送信されたBGPアップデートメッセージのフラグメンテーションは、R2インターフェイスGE0/0/0/1で実行されたトラフィックキャプチャから確認できるように、ノードR2で発生します。

ノードR2でのIPフラグメンテーション：

! – 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>

シナリオ：TCP PMTUDの有効化

PMTUDの有効化

PMTUDを有効にすると、ピアが直接接続されているかどうかに関係なく、MSSの初期計算では出力インターフェイスのIP MTUが常に考慮されます。

このシナリオでは、PMTUDが有効な場合に予想される動作について詳しく説明します。ここでは、Cisco IOS XRノードR4がアクティブロールを果たし、TCP接続を管理し、宛先ポート179でCisco IOS XRノードR1とのTCPセッションを開きます。両方のノードがそれぞれのインターフェイスでデフォルトのIP MTU値を使用します。

図3.1 - TCP PMTUDが有効

このシナリオでのMSSの計算は、次のように要約できます。  

• すべてのノードは1500バイトのデフォルトIP MTUを使用します
• TCPパスMTUディスカバリが有効である
• TCPピアが直接接続されていない
  • R4はBGP接続を管理します
  • R4は1460バイトのMSSでSYNを送信します
    • 1500（インターフェイスIP MTU） – 20(minTCP_H)- 20(minIP_H)
  • R1は1460バイトのMSSでSYN、ACKを送信します
    • [Received MSS; Local initial MSS]の下限を送信
    • 受信MSS 1460バイト、ローカル初期MSS 1460バイト
  • 両方のピアで最小のMSS値が使用される

enable PMTUDによって導入された動作の変更を強調するために、次の出力はイベントのシーケンスを示しています。

1. PMTUDが無効になっているデフォルトシナリオでの、確立されたTCPセッションの初期状態。
2. PMTUDは、TCPピアR4とR1の両方で設定され、有効になっています。
3. TCPセッションが再起動し、MSSの計算が行われ、TCP PMTUDの影響を受けます。

R4 - ACTIVE - TCP PMTUD無効（デフォルト）で確認できます。

! – 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#

R1 - PASSIVE - TCP PMTUD無効（デフォルト）で確認できます。

! – 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#

R4 - ACTIVE - TCP PMTUD有効で確認できます。

! – '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#

R1 - PASSIVE - TCP PMTUD有効で確認できます。

! – '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#

PMTUエージャタイマーの動作に注意してください。

! – 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

R4 - ACTIVE - BGP Session restart - TCP SYNで確認できます。

! – 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)

R1 - PASSIVE - BGP Session restart - TCP SYN、ACKでわかります。

! – 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)

R4 - ACTIVEに表示されるTCPセッションの詳細：TCP PMTUDが有効になり、BGPセッションがクリアされた後

! – 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#

R1 - PASSIVEに表示されるTCPセッションの詳細：TCP PMTUDが有効になり、BGPセッションがクリアされた後。

! – 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：パスセグメントのIP MTUが小さい

前のシナリオは、PMTUDが有効な状態で最初にTCPセッションを確立した際に何が起こるかを理解するのに役立ちました。このシナリオは上に構築されており、TCP PMTUDの動作の仕組みと、確立されたTCPセッションに与える影響の理解に役立ちます。

図3.2:PMTUDが有効で、パスセグメントのIP MTUが小さい

前の図を参照として考えてみます。BGPセッションが確立され、512バイトを超えるサイズのIPパケットによって伝送されたBGPアップデートメッセージがR1から送信されると仮定します。PMTUDを有効にすると、DFビット(Don't Fragment)が設定されます。したがって、ノードR2はIPパケットをドロップし、ICMP(Internet Control Message Protocol)メッセージ（宛先到達不能 – タイプ3；必要なフラグメンテーション – コード4）をR1に返信します。ICMPメッセージを受信した後、ノードR1でPMTUDがトリガーされ、パスの最小IP MTUの確立を試みます。これは、明確に定義された一連のプラトー値レベルの次に低い値を使用して行われます。つまり、新しいTCPセッションMSS値と見なされます。次にTCPは新しいMSS値を使用して元のBGPアップデートを再送信し、ICMPメッセージ(Destination Unreachable - type 3; Fragmentation needed - Code 4)を受信しなくなるまで、このプロセスが必要な回数だけ繰り返されます。つまり、使用されているMSS値が、送信されたすべてのパケットが最も低いパスセグメントIP MTUに入る値になるまで使用されます。時間が経過すると、PmtuAgerタイマーによって制御されるPMTUDは、プラトーのレベルを逆方向に走査し、MSSを最大値に戻します。任意の時点で、ICMPメッセージ(Destination Unreachable - type 3 ; Fragmentation needed - Code 4)が再度受信されると、PMTUDは前述のように動作します。

次の出力は、上記のPMTUDの動作を順を追って説明し、確立されたTCPセッションのシナリオから開始します。ここでは、Cisco IOS XRノードR4がアクティブな役割を果たしているため、TCP接続を管理し、宛先ポート179でR1とのTCPセッションを開きます。両方のノードがそれぞれのインターフェイスでデフォルトのIP MTU値を使用します。このシナリオでのMSSの初期計算は、次のように要約できます。  

• R2ノードとR3ノード間の中間セグメントは、デフォルト以外のIP MTU 512バイトを使用します。
• R1とR4は、インターフェイスでデフォルトのMTU値を使用します。
• TCPパスMTUディスカバリが有効になっている。
• TCPピアは直接接続されません。
  • R4はBGP接続を管理します。
  • R4は1460バイトのMSSでSYNを送信します。
    • 1500（インターフェイスIP MTU） – 20(minTCP_H)- 20(minIP_H)。
  • R1は1460バイトのMSSでSYN、ACKを送信します。
    • [Received MSS ; Local initial MSS]の下限を送信します。
    • 受信MSS 1460バイト、ローカル初期MSS 1460バイト。
  • 最小のMSS値が両方のピアで使用されます。

R4から送信されたTCP SYN:

! – 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)

R1から送信されたTCP SYN、ACK:

! – 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)

BGPセッションが確立されると、ノードR1はBGP Updateメッセージを送信し、ノードR2から返されたICMPメッセージ(Destination Unreachable - type 3 ; Fragmentation needed - Code 4)を受信します。

これは、BGPアップデートメッセージを伝送するIPパケットにDFビットが設定されており、R2/R3セグメントで使用される512バイトのIP MTUが1116バイトのIPパケットサイズよりも小さいために発生します。すでに説明したように、ICMPメッセージを受信すると、PMTUDがトリガーされます。

R1 ICMPで、タイプ3/コード4メッセージが受信されます。

! – 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]

ノードR1では、ICMPメッセージによってトリガーされ、TCP PMTUDは、明確に定義された一連のプラトー(IP MTU)レベルからのさらに低い値を使用して、エンドツーエンドの最も低いIP MTUの確立を試みます。これらのプラトーのレベルは、RFC1191 - Path MTU discoveryに記載されています。

MTU plateaus from RFC 1191
- values include both TCP and IP headers
65535
32000
17914
8166
4352
2002
1492
1006
508
296
68

しかし、ノードR1によって受信されたICMP(Destination Unreachable - type 3; Fragmentation needed - Code 4)メッセージはネクストホップのMTUを伝送し、次に示すように、ノードR1はこの値（この例では512バイト）を使用して、TCPセッションのMSS値を調整します。元のTCPセグメントの長さは1076バイトであることに注意してください。したがって、元のTCPセグメントを再送信するには3つのパケットが必要です。

R1 - PASSIVE - PMTUDの動作で確認できます。

! – 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

先に述べたように、すべてのパケットが時間の経過とともに送信されると、PMTUDはPmtuAgerタイマーによって決定される逆方向でプラトー準位を通過し、設定されているシナリオに従ってMSSを最大値に上げることを試みます。

R1で見られるように、定義された複数のエリアにまたがるPMTUDは次のようになります。

! – 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

最終状態は、これらの出力で確認できます。特に、ノードR1によって示される最小と最大のMSS値に注意してください。この値は、PMTUDがトリガーされたことを強調表示および信号します。

R4に表示されるTCPセッションの詳細：アクティブ：

! – 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#

R1に表示されるTCPセッションの詳細：パッシブ：

! – 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#

最後に、任意の時点でICMP(Destination Unreachable - type 3 ; Fragmentation needed - Code 4)メッセージを再度受信した場合、PMTUDは前述のように動作します。

R1 - PASSIVE - PMTUDが再びトリガーしたことがわかります。

! – 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#

Cisco Bug ID CSCvf10395 の影響を受けるCisco IOS XRリリースでは、ICMPエラーメッセージに含まれるネクストホップは無視され、ノードは、前述であり、RFC1191 - Path MTU discoveryに記載されている明確に定義された一連のプラトー(IP MTU)レベルから1つ下の値を使用して、エンドツーエンドの最小IP MTUの確立を試みます。これらの試みは送信が成功するまで行われ、ICMP(Destination Unreachable - type 3 ; Fragmentation needed - Code 4)メッセージが受信されなくなることを意味します。

Cisco Bug ID CSCvf10395の影響を受けるCisco IOS XRリリースのノードから確認できます。

! – 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

次のステップとして、すべてのインターフェイスで同じシナリオでラベル配布プロトコル(LDP)を使用することを検討します。ここでの目的は、MPLSを有効にした環境で以前のシナリオとどのような違いが観察できるかを理解することです。

図3.3:PMTUDが有効で、パスセグメントのIP MTUが小さい：MPLSのシナリオ

まず、次に示すように、PMTUDトリガーの前に確立されたBGPセッションの初期段階を検討します。

TCP(BGP)の初期状態（R4 - ACTIVE - MPLS対応シナリオで表示）:

! – 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#

R1 - PASSIVE - MPLS対応シナリオで表示されるTCP(BGP)の初期状態：

! – 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#

このMPLS対応シナリオでは、TCP(LDP)セッションの詳細が確立されていることが確認できます。TCP(BGP)セッションのMSS計算に関して前述したすべての説明は、TCP(LDP)セッションにも適用されます。例として、ノードR3およびR2のTCP(LDP)セッションMSSの計算を次にまとめます。

• R2とR3はどちらも512バイトの非デフォルトIP MTUを使用します。
• パスMTUディスカバリがイネーブルになっている。
• TCPピアが直接接続されていない（TCPセッションがループバックインターフェイス間で確立されている）。
  • R3はLDP接続を管理します。
  • R3は472バイトのMSSでSYNを送信します。
    • 512（インターフェイスIP MTU） – 20(minTCP_H)- 20(minIP_H)。
  • R2は472バイトのMSSでSYN、ACKを送信します。
    • [Received MSS; Local initial MSS]の下位を送信します。
    • 受信MSS 472バイト、ローカル初期MSS 472バイト。
  • 最小のMSS値が両方のピアで使用されます。

R3 - ACTIVE - MPLS対応シナリオに表示されるTCP(LDP)セッションの詳細：

! – 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#

R2 - PASSIVE - MPLS対応シナリオに表示されるTCP(LDP)セッションの詳細：

! – 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#

BGPセッションが確立されると、R1はBGP Updateメッセージを送信し、ICMPメッセージ(Destination Unreachable - type 3; Fragmentation needed - Code 4)を返として、ノードR2から受信します。これによりノードR1でTCP PMTUDがトリガーされます。これは、BGPアップデートメッセージを伝送するIPパケットにDFビットが設定されており、R2/R3セグメントで使用される512バイトのIP MTUが1116バイトのIPパケットサイズよりも小さいために発生します。前と同様に、このICMPメッセージを受信すると、PMTUDがトリガーされます。以前の非MPLSシナリオと比較した場合のMPLS対応シナリオの違いは、ノードR2 ICMPメッセージ（宛先到達不能 – タイプ3、必要なフラグメンテーション – コード4）に含まれるネクストホップのMTU値に関するものです。 このMPLS対応シナリオでは、ネクストホップのMTU値が4バイトの追加MPLSオーバーヘッドを占め、つまり、次の出力に示すように、R2での出力MPLSラベルスタックを占めることを意味します。

R1 - PASSIVE - MPLS対応シナリオで確認される稼働中のTCPパスMTUディスカバリ：

! – 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

R1 - PASSIVE - TCP PMTUD triggered - MPLS対応シナリオからわかります。

! – 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#

MPLS対応シナリオでは、ノードR2のICMPメッセージに含まれているネクストホップのMTUの値が、出力MPLSラベルスタックを示すことに注意してください。この側面をさらに強化するために、次の例を検討します。R2でフィルタリングされたIPパケットがL3VPNサービスに関連付けられている場合は、イーサネットフレームが2つのラベル（IGPラベルとVPNラベル）を搬送していることを意味します。 その後、ネクストホップのMTUは、必要なラベルスタックサイズを反映します。次の出力を参照してください。

R1 - PASSIVE - L3 VPNサービスパケットに次のように表示されます。

! – 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

R1 - PASSIVE - L3 VPN service - ICMP Type 3/Code 4に示されているように、

! – 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 - TCPオプション(MD5)

図3.4:PMTUDが有効になっていて、TCP MD5認証

TCP MD5認証が有効になっている場合、前述のシナリオで説明したものとPMTUDの動作が区別されません。以前に使用されているTCP MD5認証で共有したように、Cisco IOS XRでは追加のオーバーヘッドが考慮され、アクティブなTCPピアの初期MSS値は同じ値を反映します。TCPオプションの使用による影響についての詳細は、前のセクション「TCPオプションの使用：XRアクティブ」および「TCPオプションの使用：XRパッシブ」を参照してください。このシナリオでのTCP MSSの計算は、次のように要約できます。  

• すべてのノードが1500バイトのデフォルトIP MTUを使用します。
• TCPパスMTUディスカバリが有効になっている。
• TCPピアは直接接続されません。
• R1とR4の両方でTCP MD5認証が有効になっている。
  • R4はBGP接続を管理します。
  • R4は1436バイトのMSSでSYNを送信します。
    • 1500（インターフェイスIP MTU） – 20(minTCP_H)- 20(minIP_H)- 24バイト（IOS XR TCPオプションオーバーヘッド）
  • R1は1436バイトのMSSでSYN、ACKを送信します。
    • [Received MSS ; Local initial MSS]の下限を送信します。
    • 受信MSS 1436バイト、ローカル初期MSS 1460バイト。
  • 最も低いMSS値が両方のピアで使用されます。

R4から送信されたTCP SYN:

! – 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)

R1から送信されたTCP SYN、ACK:

! – 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)

R4に表示されるTCPセッションの詳細：アクティブ：

! – 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#

R1に表示されるTCPセッションの詳細：パッシブ：

! – 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：ブラックホール検出

前述の「PMTUD：パスセグメントのIP MTUが小さい」の項で説明したように、TCP PMTUDが有効な場合は、ICMP(Destination Unreachable - Type 3; Fragmentation Needed - Code 4)メッセージの受信によってトリガーされます。これらのメッセージが何らかの理由で受信されず、その結果PMTUDがトリガーされない場合があります。この場合、TCPピア間のパスの最小のIP MTUは学習されません。このようなシナリオでは、IPパケットにDFビットが設定されており、それらのサイズが最も小さいIP MTUパスセグメントよりも大きい場合に、ブラックホールが発生する可能性があります。これらのパケットは通知なしに廃棄されます。

このセクションでは、このような潜在的なブラックホールのシナリオをCisco IOS XRがどのように検出して対処するかを説明します。この目的のために、次の図とCLI出力に示すように、R2インターフェイスGE0/0/0/0でIPv4アンリーチャブル機能が無効になっています。

図3.5:R1/R4でPMTUDが有効で、R2でIPv4到達不能が無効になっている。

R2で無効にされたIPv4到達不能：

!- 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

Cisco IOS XRがこのブラックホールのシナリオに対処する方法は、同じパケットを2回再送信し、それでも失敗する場合、つまり予期されるTCP ACKが受信されない場合は、再試行した後、『RFC1191 - Path MTU discovery』で説明されている、次に低い適切に定義されたプラトー値を使用することです(プラトーのリストについては、「PMTUD パスセグメントのIP MTUがより小い」を参照)。 要約すると、Cisco IOS XRでは、パケットのサイズとパケット再送信による回避策により、パケットが宛先へのパス内のどこかでドロップされる可能性があると想定しています。この動作は、ノードR1インターフェイスで取得されたパケットキャプチャの次の例とdebug tcp pmtudの出力で確認できます。

R1でのIOS-XRブラックホール検出：

! – 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