[OpenSIPS-Users] Trouble with forked calls and rtpengine

[Robert Dyck]

I am reaching out to the users out there to help me figure out why I get 
occasional call failures when it involves rtpengine and forked calls. Calls 
involving rtpengine but not forked are solid. For instance there is no problem 
with a call between a SIPified WEBRTC phone and some end of life device. WEBRTC 
has very strict requirements. ICE, DTLS and rtcmux are mandatory. These are 
unknown to some devices.

I narrowed it down to forked calls. The documentation seems to suggest there 
are options for the offer command to deal with branches. Specifically the via-
branch= variants. The auto option is mentioned in the documentation but it 
doesn't seem to be implemented in opensips. Then there is the 1 option for 
offers and the 2 option for answers. The 1/2 option did not help. Looking a 
little closer at what it does, I can't see how it could have helped anyway. 
The branch parameter in the via header is not unique for the different 
branches. We have multiple callees but only one caller.

Diving deeper a look at the rtpengine debug logs only confirmed my doubt about 
the usefulness of via branch parameter. Here is an example of a three way 
fork.

First offer
"ICE": "remove", "direction": [ "ipv6", "ipv4-priv" ], "flags": [ "debug" ], 
"replace": [ "session-connection", "origin" ], "transport-protocol": "RTP/
AVP", "rtcp-mux": [ "demux" ], "call-id": "s25p40fpr5g0u52b96dp", "via-
branch": "z9hG4bK3119290", "received-from": [ "IP6", 
"2001:569:7EB9:A400:8A42:A64E:CE7C:F58F" ], "from-tag": "as1g4gcnjp", 
"command": "offer" }
Jan  1 10:03:54 slim rtpengine[2517903]: NOTICE: [s25p40fpr5g0u52b96dp]: 
[core] Creating new call
Jan  1 10:03:54 slim rtpengine[2517903]: DEBUG: [s25p40fpr5g0u52b96dp]: 
[internals] getting monologue for tag 'as1g4gcnjp' in call 
's25p40fpr5g0u52b96dp'
Jan  1 10:03:54 slim rtpengine[2517903]: DEBUG: [s25p40fpr5g0u52b96dp]: 
[internals] creating new monologue
Jan  1 10:03:54 slim rtpengine[2517903]: DEBUG: [s25p40fpr5g0u52b96dp]: 
[internals] tagging monologue with 'as1g4gcnjp'
Jan  1 10:03:54 slim rtpengine[2517903]: DEBUG: [s25p40fpr5g0u52b96dp]: 
[internals] create new "other side" monologue for viabranch z9hG4bK3119290
Jan  1 10:03:54 slim rtpengine[2517903]: DEBUG: [s25p40fpr5g0u52b96dp]: 
[internals] creating new monologue
Jan  1 10:03:54 slim rtpengine[2517903]: DEBUG: [s25p40fpr5g0u52b96dp]: 
[internals] tagging monologue with viabranch 'z9hG4bK3119290'
Jan  1 10:03:54 slim rtpengine[2517903]: DEBUG: [s25p40fpr5g0u52b96dp]: 
[internals] this= other=as1g4gcnjp

Second offer
"ICE": "remove", "direction": [ "ipv6", "ipv4-priv" ], "flags": [ "debug" ], 
"replace": [ "session-connection", "origin" ], "transport-protocol": "RTP/
AVP", "rtcp-mux": [ "demux" ], "call-id": "s25p40fpr5g0u52b96dp", "via-
branch": "z9hG4bK3119290", "received-from": [ "IP6", 
"2001:569:7EB9:A400:8A42:A64E:CE7C:F58F" ], "from-tag": "as1g4gcnjp", 
"command": "offer" }
Jan  1 10:03:54 slim rtpengine[2517903]: DEBUG: [s25p40fpr5g0u52b96dp]: 
[internals] getting monologue for tag 'as1g4gcnjp' in call 
's25p40fpr5g0u52b96dp'
Jan  1 10:03:54 slim rtpengine[2517903]: DEBUG: [s25p40fpr5g0u52b96dp]: 
[internals] found existing monologue
Jan  1 10:03:54 slim rtpengine[2517903]: DEBUG: [s25p40fpr5g0u52b96dp]: 
[internals] this= other=as1g4gcnjp

Third offer
 "ICE": "force", "DTLS-fingerprint": "sha-256", "direction": [ "ipv4-priv", 
"ipv4-ext" ], "flags": [ "debug", "SDES-off", "generate-mid" ], "replace": [ 
"session-connection", "origin" ], "transport-protocol": "UDP/TLS/RTP/SAVPF", 
"rtcp-mux": [ "require" ], "call-id": "s25p40fpr5g0u52b96dp", "via-branch": 
"z9hG4bK3119290", "received-from": [ "IP6", 
"2001:569:7EB9:A400:8A42:A64E:CE7C:F58F" ], "from-tag": "as1g4gcnjp", 
"command": "offer" }
Jan  1 10:03:54 slim rtpengine[2517903]: DEBUG: [s25p40fpr5g0u52b96dp]: 
[internals] getting monologue for tag 'as1g4gcnjp' in call 
's25p40fpr5g0u52b96dp'
Jan  1 10:03:54 slim rtpengine[2517903]: DEBUG: [s25p40fpr5g0u52b96dp]: 
[internals] found existing monologue
Jan  1 10:03:54 slim rtpengine[2517903]: DEBUG: [s25p40fpr5g0u52b96dp]: 
[internals] this= other=as1g4gcnjp

For the second and third offers the debug logs say  "found existing monologue".
This tells me that the offers are considered to be unique. However to 
requirements for modifying the SDP are unique. The identical  "via-branch": 
"z9hG4bK3119290"  appears in each offer.

My theory is that the requirements for the three branches are being stacked on 
top of each because rtpengine considers them all to be a single offer. The 
theory seems to fit with what I have observed. The calls may or not fail. It 
seems to be influenced by the order of the branches and also which branch is 
actually answered. I get weird failures like rtc-mux being missing from the 
sdp when clearly it was submitted in the offer.

Any ideas?
Regards, Rob