WebRTC datachannel ignores SCTP packet with multiple data chunks

(NeedInfo from)



WebRTC: Networking
2 years ago
a month ago


(Reporter: xmo, Unassigned, NeedInfo)


42 Branch
Windows 7

Firefox Tracking Flags

(Not tracked)




2 years ago
User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/46.0.2490.86 Safari/537.36

Steps to reproduce:

1. My app opens a WebRTC data channel (SCTP/DTLS/UDP) towards Firefox
2. My app performs file transfer over this data channel.  File size is a few 10MBs.

Actual results:

Whenever an SCTP packet contains multiple data chunks, it's like Firefox ignores it:
 - No SACK is sent back (related to this packet)
 - When subsequent packets are sent, the SACK's TSN is the one of the previous packet.

I can provide SCTP traces offline.

Expected results:

Transfer should complete.


2 years ago
Component: Untriaged → WebRTC: Signaling
OS: Unspecified → Windows 7
Product: Firefox → Core
Hardware: Unspecified → x86_64


2 years ago
Component: WebRTC: Signaling → WebRTC
Is this a dup?
Flags: needinfo?(rjesup)
Is this a dup of bug 1214269?
Flags: needinfo?(rjesup) → needinfo?(tuexen)

Comment 3

2 years ago
(In reply to Randell Jesup [:jesup] from comment #2)
> Is this a dup of bug 1214269?

No, this one was related to DTLS.
The new one is kind of similar, but at SCTP layer.

I will send SCTP traces to you and Niels.

Please send the traces to myself and tuexen (cc'd here)
Flags: needinfo?(xmo)

Comment 5

2 years ago
(sorry for Nils, cc'd too ;)
Flags: needinfo?(xmo)

Comment 6

2 years ago
OK, I looked at the tracefile. Everything seems more or less fine until frame 2272. This contains the TSNs 74 and 75 (I'm only using the last two digits of the TSN). The DATA chunks with TSNs 76, ..., 79 are also missing at the receiver. The receiver SACKs up to 73 and from 80 onwards. The sender correctly does fast retransmits. The problem is that the bundled frames in frame 2272 don't get accepted.
The reason could be that the SCTP doesn't handle bundled DATA chunks correctly. But we tested it and it works at least in general. But what if the SCTP stack doesn't get the packet. The receiver is missing 5 packets. Could it be that the packet in frame 2272 is too long and screws up the receiver for a short while. That would explain, why frame 2272 and its retransmissions are dropped.

Is it possible to provide a tracefile from Firefox as described in
Flags: needinfo?(tuexen)
I think Michael meant frame 2277, not 2272. But yeah that is the first frame which is bigger then 1294 bytes, and it appears to be the one where the trouble starts. I'll try to have quick look if I can find a similar buffer problem as with the DTLS layer.
This looks a little bit suspicious to me:
There we properly parse the length of the first SCTP header, but then:
and some following calls keep using the length information from the wire. My guess is we would need to put a loop around that code to be able to handle multiple SCTP packets coming in at once.

Should be fairly easy to test once bug 1214269 lands as the sctp_unittest uses the same transportlayerloopback code as the DTLS tests. So the extensions from bug 1214269 should allow us to add tests to the sctp_unittest for multiple SCTP packets as well.

Comment 9

2 years ago
usrsctp_conninput() only expects a single SCTP packet containing of a common header and a number of chunks.
That looks good to me.
I'm on PTO, but IIRC we disabled PMTU because of problems getting errors back when we tried sending too-large packets through ICE/DTLS;check back on bugs (open and closed) that Michael is cc'd on or look at hg logs from DataChannel.cpp
See bug 1194817 where we disabled PMTU

I suspect a similar issue, where the packets aren't passing through the ICE stack due to limits in the ICE implementation.
Flags: needinfo?(drno)
Confirming, -> Networking
We may well end up an mtransport/ICE bug for large-packet support
Component: WebRTC → WebRTC: Networking
Ever confirmed: true
backlog: --- → webrtc/webaudio+
Rank: 25
Priority: -- → P2
Mass change P2->P3 to align with new Mozilla triage process.
Priority: P2 → P3
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