Defining TCP Maximum Segment Size for IP

You can modify the maximum segment size (MSS) for TCP sessions using the ip tcp adjust-mss command.

When defined, the router modifies the MSS for TCP SYN packets traveling through the interface. The router compares the MSS value of incoming or outgoing packets against the adjusted MSS setting and replaces smaller values that it detects in any packets with the larger setting. If the packet does not contain an MSS value, the router assumes a value of 536 for the packet MSS on which to base the comparison.

Note:

  • Implementation of the MSS value is identical for both ingress and egress interface traffic. That is, the router uses the same MSS value when adjusting inbound or outbound TCP traffic.
  • The purpose behind using MSS is to alleviate problems with path maximum transmission unit discovery (PMTUD) and resulting black hole detection issues. (See RFC 2923, “ TCP Problems with Path MTU Discovery,” for additional information about the black hole scenario.)
  • Before you configure IP, you must create the lower-layer interfaces over which IP traffic flows.
  • All IP configurations will be removed from the interface when you issue the no ip interface command in Interface Configuration mode.

To modify the MSS for TCP SYN packets:

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