Subject: [Simple] UDP - what if response is bigger than 1500 bytes (ethernet MTU)?

From: <[email-masked]>
Date: Wed, 19 Jul 2006 14:48:57 +0200

Hello, 

Could someone help me with following problem? 

I am not sure what should be done when the response to SIP request sent
over UDP is bigger than 1500 bytes? It can happen e.g. when PUBBLISH
request is responded by 415 Unsupported Media Type and Accept,
Accept-Encoding and Accept-Language headers are added.

Thank you, 
Silvestr Peknik

Extract from RFC3261:
18.1.1 Sending requests:
...
If a request is within 200 bytes of the path MTU, or if it is larger
   than 1300 bytes and the path MTU is unknown, the request MUST be sent
   using an RFC 2914 [43] congestion controlled transport protocol, such
   as TCP. If this causes a change in the transport protocol from the
   one indicated in the top Via, the value in the top Via MUST be
   changed.  This prevents fragmentation of messages over UDP and
   provides congestion control for larger messages.  However,
   implementations MUST be able to handle messages up to the maximum
   datagram packet size.  For UDP, this size is 65,535 bytes, including
   IP and UDP headers.

      The 200 byte "buffer" between the message size and the MTU
      accommodates the fact that the response in SIP can be larger than
      the request.  This happens due to the addition of Record-Route
      header field values to the responses to INVITE, for example.  With
      the extra buffer, the response can be about 170 bytes larger than
      the request, and still not be fragmented on IPv4 (about 30 bytes
      is consumed by IP/UDP, assuming no IPSec).  1300 is chosen when
      path MTU is not known, based on the assumption of a 1500 byte
      Ethernet MTU.
... 

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From: Thomas Froment <[email-masked]>
Date: Wed, 19 Jul 2006 15:31:18 +0200

> Accept-Encoding and Accept-Language headers are added.

This is a good question: I think it was mentionned in last SIP WG IETF 
meeting that having responses greater than path MTU  (-200 bytes)
in SIP was not currently well specified. It was an open issue on 
draft-ietf-sip-hop-limit-diagnostics-03.txt which raises this problem
since it leads naturally to send big SIP responses.
SIP WG people may probably answer you if something is planned to handle 
this problem...
Thomas

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From: "Vijay K. Gurbani" <[email-masked]>
Date: Wed, 19 Jul 2006 17:46:10 -0500

> SIP WG people may probably answer you if something is planned to handle 

This remains a problem; I have had a quick chat with Scott
Lawrence on it and we'll be submitting a draft latter part
of next week on one way to deal with this.

Thanks.

- vijay
-- 
Vijay K. Gurbani  vkg@{lucent.com,research.bell-labs.com,acm.org}
Bell Laboratories, Lucent Technologies, Inc.
2701 Lucent Lane, Rm. 9F-546, Lisle, Illinois 60532 (USA)