3.1 TCP MSS Option

3.1 TCP MSS Option

A host doing PMTU Discovery must obey the rule that it not send IP datagrams larger than 576 octets unless it has permission from the receiver. For TCP connections, this means that a host must not send datagrams larger than 40 octets plus the Maximum Segment Size (MSS) sent by its peer.

Note: The TCP MSS is defined to be the relevant IP datagram size minus 40 [9]. The default of 576 octets for the maximum IP datagram size yields a default of 536 octets for the TCP MSS.

Section 4.2.2.6 of "Requirements for Internet Hosts -- Communication Layers" [1] says:

Some TCP implementations send an MSS option only if the destination host is on a non-connected network. However, in general the TCP layer may not have the appropriate information to make this decision, so it is preferable to leave to the IP layer the task of determining a suitable MTU for the Internet path.

Actually, many TCP implementations always send an MSS option, but set the value to 536 if the destination is non-local. This behavior was correct when the Internet was full of hosts that did not follow the rule that datagrams larger than 576 octets should not be sent to non-local destinations. Now that most hosts do follow this rule, it is unnecessary to limit the value in the TCP MSS option to 536 for non-local peers.

Moreover, doing this prevents PMTU Discovery from discovering PMTUs larger than 576, so hosts SHOULD no longer lower the value they send in the MSS option. The MSS option should be 40 octets less than the size of the largest datagram the host is able to reassemble (MMS_R, as defined in [1]); in many cases, this will be the architectural limit of 65495 (65535 - 40) octets. A host MAY send an MSS value derived from the MTU of its connected network (the maximum MTU over its connected networks, for a multi-homed host); this should not cause problems for PMTU Discovery, and may dissuade a broken peer from sending enormous datagrams.

Note: At the moment, we see no reason to send an MSS greater than the maximum MTU of the connected networks, and we recommend that hosts do not use 65495. It is quite possible that some IP implementations have sign-bit bugs that would be tickled by unnecessary use of such a large MSS.