[reSIProcate] Max Size of an INVITE message using UDP

[Jason Fischl]

This depends on all of the network elements including routers and
gateways along the path. Many residential gateway/routers will not
pass fragmented UDP packets through. There is no way for the user
agent to know if a downstream element will not support fragmented UDP.

On 8/11/06, Mehul Jain <Mehul at ingenio.com> wrote:
>
>
> Am not clear what is the max supported size of SIP Request (INVITE) sent
> using UDP.
> Went through the RFC 3261 (18.1.1 Sending Requests ).  MTU (1500 Bytes for
> Windows) looks to be the limiting factor for UDP, while later its written
> that implementations must support messages of the max datagram size (65,535
> bytes).
>
> Using the resiprocate stack over UDP, Im able to send and receive messages
> longer than 1500 bytes (e.g.  length 6K bytes) without any issues.
>
> Any pointers as to what is the accepted max length in the proxies, servers
> etc.
> Mehul.
>
>
> 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.
> _______________________________________________
> resiprocate-devel mailing list
> resiprocate-devel at list.sipfoundry.org
> https://list.sipfoundry.org/mailman/listinfo/resiprocate-devel
>
>