[SR-Users] SIP Fragments

[Alex Balashov]

On 12/17/2014 05:14 PM, Marc Soda wrote:

> I'm having a problem reassembling UDP packets on my Asterisk servers
> after passing through Kamailio (it appears to me an OS level issue,
> nothing to do with Kamailio).  Is there a way, with Kamailio, to limit
> the size of a SIP message?  I know I can just start removing headers,
> but that doesn't seem like a realistic solution.  I see that Kamailio
> can compress the message body, but can Asterisk handle that?  How do
> other people handle this?

1. Any SIP-compliant endpoint should be able to handle compact headers. 
Per RFC 3261 7.3.3 ("Compact Form"):

    Implementations MUST accept both the long and short forms of
    each header name.

2. Some headers are critical should not be removed. Others really are 
mostly useless bloat commonly added by verbose UACs, and, practically 
speaking, the other peer will be neither colder nor warmer if they are 
removed, unless there is a specific use for them.

Good candidates are:

a) The "Date" header.
b) Accept: headers listing every MIME type in the known universe.

3. If one or more of your endpoints offer every codec in the known 
universe in the SDP, you can restrict the codecs offered to reduce the 
SDP size.

4. You could use TCP. In fact, RFC 3261 actually mandates this. Per RFC 
3261 Section 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.

Of course, in reality, nobody cares or follows this, and many SIP 
endpoints don't even support TCP (also mandated by RFC 3261).

5. In some situations, header bloat comes from requests passing through 
numerous proxies, each of which add a stackable Via header and, if 
applicable, a Route/Record-Route set.

Reducing the number of intermediate proxies can help with this.

6. You could run the traffic through a lightweight, signalling-only 
B2BUA, such as SEMS, which deals with fragmented UDP in incoming 
requests just fine, but does not reoriginate on leg B all the bloated 
headers that came in on leg A.

7. Other than these things, there are no real solutions.

-- Alex

-- 
Alex Balashov - Principal
Evariste Systems LLC
235 E Ponce de Leon Ave
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Tel: +1-678-954-0670
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