Defining TCP Maximum Segment Size

The ip tcp adjust-mss command enables you to modify the TCP maximum segment size (MSS) for TCP sessions.

When defined, the router modifies the maximum segment size (MSS) for TCP SYN packets traveling through the interface. The modification occurs only for packets that contain values smaller than the adjusted MSS value. When the packet does not contain an MSS field value, the router assumes an MSS value of 536 and makes any modifications accordingly.

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.

ip tcp adjust-mss

• Use to modify the maximum segment size (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: The purpose behind using MSS is to alleviate problems with Path 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.)

• Example

  host1(config-if)#ip tcp adjust-mss 1000

• Use the no version to remove the MSS assignment from the profile.
• See ip tcp adjust-mss

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