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

Silvestr.Peknik@tietoenator.com wrote:
> 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.
>   

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
> 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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>   


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