linksys ip phone spa922 disregarding MSS

[jalmendros]

I have a linksys SPA922 phone transporting SIP over TCP. Each particular SIP message is being carried in two of more TCP segments. When the TCP connection is established, the server announces MSS=1460 and despite of that, the phone is sending IP datagrams smaller than 576 octects, apparently disregarding the MSS option and acting as if it doesn't know the maximum segment size the server is able to receive.

I have stated that it is not a problem related to MTU knowledge, as the phone announces a MSS=1460 too when opening TCP connections.

Any idea about why the phone is not taking into account the maximum segment size the server can admit?

[paolo bevilacqua]

Even if a device advertise a certain MSS, it is not obligated to only send packts up to that size.

That shoyuld not cause any problem and everyt should work anyway.

[jalmendros]

That's right, but the smaller the segment, the less efficient is the communication. One would expect Cisco TCP implementation be a good one. That's why I find very strange such a behaviour.

[paolo bevilacqua]

There is no performance issue in compacting messages that are only occasionally sent.

[jalmendros]

It all depends on how many phones are sending messages. If there are many, perhaps SIP traffic would be noticeable. And for debugging reasons, having SIP messages scattered doesn't help.