[SR-Users] packets exceeding MTU size

[Alex Balashov]

Hi Ali,

Kamailio reassembles fragmented UDP just fine. 

However, additional UDP fragments beyond the first packet don’t always get to Kamailio, since they don’t have a UDP header. UDP fragments can be dropped by stateful firewalls and/or NAT gateways for this reason.

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. 

Your best bet for finding out what’s really going on is to look at Kamailio logs. An event like a parsing failure due to fragmentation should be logged.

— Alex

> On Aug 18, 2022, at 4:20 AM, Ali Taher <ataher at vanrise.com> wrote:
> 
> Hello all,
>  
> Lately I faced an issue that Kamailio is not responding to some invite packets coming from SBC. When investigating I found that these packets exceeds MTU size.
> Could this be the issue? Knowing that in Kamailio configuration I have a condition on is_method("INVITE"). Could be that in case of packet exceeding MTU size it’s not considered as invite packet by Kamailio?
>  
> I have Kamailio 5.2.5.
>  
> Regards,
> Ali Taher
>  
>  
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-- 
Alex Balashov | Principal | Evariste Systems LLC

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