Gaurav Kheterpal <[EMAIL PROTECTED]> asks:
>
> I would like to know if there is a recommended behaviour of SIP ALGs
> with respect to fragmentation of (SIP) UDP packets which are
> larger than
> 1500 bytes. Some NAT routers explicity disable the treatment of
> fragmented UDP packets.
>
> - Does this hold true in case of SIP ALGs and STUN servers as well ?
> - Is it common to have INVITEs whose size exceeds maximum size of a
> single UDP packet (> 1500 bytes) ? What is the maximum size of a SIP
> packet (if any) ?
No SIP agent should ever receive a UDP packet larger than 1500
bytes. RFC 3261, section 18.1.1 says:
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.
frank
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