Asterisk 16.28 setting udp DF flag and not reacting to ICMP max-size?


Is there a reason why asterisk is setting DF when using PJSIP (updated after hin from Joshua)? Is there a way to configure asterisk not to set the DF (don’t fragment) flag on udp sip packets?

Issue: Asterisk behind a VDSL Line (MTU 1472). First invite without credentials goes through.

Second invite, containing crypto, ice, credentials and about 3 codecs, is just shy under 1500 bytes, but above the VDSL 1472 MTU.
Because of the set DF bit, the VDSL router is sending back an ICMP instructing the sender to reduce the size, but it looks like this is ignored on the asterisk side.

When testing same scenario with another SIP Client behind that same VDSL which does not set DF, the VDSL router is fragmenting the udp packet and the receiving OS is correctly reassembling the packet before passing it on to the application.


There are two different SIP channel drivers, you’d need to specify which one. For PJSIP we don’t do any of the transport stuff for UDP, that’s completely implemented within PJSIP itself. It may not have such a thing implemented, in which case it would need to be implemented in the PJSIP stack itself and then enabled by Asterisk.

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