[reSIProcate] Max Size of an INVITE message using UDP

[Mehul Jain]

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. 
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