[SR-Users] packets exceeding MTU size

[Alex Balashov]

> On Aug 18, 2022, at 7:15 AM, Greg Troxel <gdt at lexort.com> wrote:
> 
> 
> Alex Balashov <abalashov at evaristesys.com> writes:
> 
>> Hi Ali,
>> 
>> Kamailio reassembles fragmented UDP just fine. 
> 
> Do you really mean that, or "operating systems reassemble fragmented UDP
> packets and hand the full packet to Kamailio"?

No, it was shorthand for the latter. The intent was to emphasise that there’s nothing about Kamailio which is inherently confounded by fragmentation.
> 
>> This should not prevent the INVITE from being parsed; typically in
>> real-world scenarios with a 1500 byte MTU, the first fragment captures
>> all SIP headers, and fragmentation slices up the SDP
>> payload. Fragmentation won’t adulterate the Request Line (first line),
>> which contains the “INVITE” method verb. I suppose it is conceivable
>> that a fragmentation boundary could occur in the middle of a SIP
>> header and/or header value, causing the entire message to be
>> discarded.
> 
> I would expect that if there  is say a 1500-byte fragment and a 700-byte
> fragment (to make things up) and a firewall drops the 700-byte one, then
> reassembly at the OS would fail and Kamailio would never see anything.

In principle, that’s right. Practically, this depends on the behaviour of various intermediaries. I have seen both behaviours. In the scenarios I have troubleshot, receiving only the first fragment on the other side of—for example—a NAT gateway is fairly common.

> Or does Kamailio do some sort of raw socket listening and actually do
> fragment reassembly in user space?  That would be very surprising to me.

No, it doesn’t. Sloppy phraseology on my part.

— Alex

-- 
Alex Balashov | Principal | Evariste Systems LLC

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