Implement SCTP Negotiation Acceleration Protocol (SNAP) #449

fippo · 2025-12-28T15:55:49Z

This commit adds a createAssociationWithOutOfBandTokens method that creates the SCTP association with a local and remote sctp-init token as described in draft-hancke-tsvwg-snap-00.

Tokens can be generated using the newly added GenerateOutOfBandToken method and set in the sctp.Config as LocalSctpInit and RemoteSctpInit.

This allows the peer connection to generate and negotiate the sctp-init in the SDP which skips two network round trips.

pion/WebRTC changes are in pion/webrtc#3327

codecov · 2025-12-28T15:59:39Z

Codecov Report

❌ Patch coverage is 84.12698% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.69%. Comparing base (767b9c4) to head (888d784).

Files with missing lines	Patch %	Lines
association.go	84.12%	6 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #449      +/-   ##
==========================================
- Coverage   83.79%   83.69%   -0.11%     
==========================================
  Files          52       52              
  Lines        4054     4108      +54     
==========================================
+ Hits         3397     3438      +41     
- Misses        481      488       +7     
- Partials      176      182       +6

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go	`83.69% <84.12%> (-0.11%)`	⬇️

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This commit adds a createAssociationWithOutOfBandTokens method that creates the SCTP association with a local and remote sctp-init token as described in draft-hancke-tsvwg-snap-00. Tokens can be generated using the newly added GenerateOutOfBandToken method and set in the sctp.Config as LocalSctpInit and RemoteSctpInit. This allows the peer connection to generate and negotiate the sctp-init attribute in the SDP which skips two network round trips.

JoTurk · 2025-12-28T16:01:13Z

association.go

+	LocalSctpInit  []byte
+	RemoteSctpInit []byte


This will break the compatibility checks, but we plan to deprecate the config struct this week for the favor of options pattern so this could be WithSCTPInit when we update or something!

#446

overall this feels like we should separate options (configured by the API) from negotiated parameters (maxMessageSize, remote sctpPort, sctpInit). Still a breaking change but if we need one anyway...

association.go

fippo · 2025-12-28T16:11:58Z

association.go

+	assoc.setRWND(min32(localInit.advertisedReceiverWindowCredit, remoteInit.advertisedReceiverWindowCredit))
+	assoc.peerVerificationTag = remoteInit.initiateTag
+	assoc.sourcePort = defaultSCTPSrcDstPort
+	assoc.destinationPort = defaultSCTPSrcDstPort


a=sctp-port is currently ignored even from the remote SDP. Surprisingly with SNAP I don't think it matters anymore since it only shows up in one of the acks (so SCTP can detect NATs) but that is not relevant to WebRTC

(added to SctpOptions which makes this fix-able in the future)

We're currently updating this library to use Pion's options patterns, SctpOptions will not be needed after this is merged #450 🙏

Changed to SctpParameters which is more in line with terminology like RTCIceParameters and RTCDtlsParameters

JoTurk · 2025-12-28T16:53:48Z

association.go

+	if len(config.RemoteSctpInit) != 0 && len(config.LocalSctpInit) != 0 {
+		// SNAP, aka sctp-init in the SDP.
+		remote := &chunkInit{}
+		err := remote.unmarshal(config.RemoteSctpInit)
+		if err != nil {
+			return nil, err
+		}
+		local := &chunkInit{}
+		err = local.unmarshal(config.LocalSctpInit)
+		if err != nil {
+			return nil, err
+		}
+		assoc = createAssociationWithOutOfBandTokens(config, local, remote)
+		assoc.lock.Lock()
+		assoc.setState(established)
+		defer assoc.lock.Unlock()
+
+		go assoc.readLoop()
+		go assoc.writeLoop()
+
+		return assoc, nil
+	}


I think this logic should be done in createAssociation and we prob should have one path to createAssociationWithTSN.

I didn't like that part either. Problem is that createAssociationWithOutOfBandTokens does too many tasks:

Validate the sctp-init's are ok

creating the association

setting some parameters which is usually in handleInit

updating the state (could probably move into createAssociationWithOutOfBandTokens and no lock required)

run the loop (normally done in init)
The errors handled below this are basically the handshake being interrupted which can no longer happen.

🤔 but that means createAssociationWithOutOfBandTokens does things I don't want "createAssociation" to do... brb.

Slightly better. WDYT about renaming init() to sendInit() and only do as client, pulling the loop goroutines out?

a lot better yeah.

JoTurk · 2025-12-28T16:54:49Z

association.go

+	init.initialTSN = globalMathRandomGenerator.Uint32()
+	init.numOutboundStreams = math.MaxUint16
+	init.numInboundStreams = math.MaxUint16
+	init.initiateTag = globalMathRandomGenerator.Uint32()


This can return 0.

Good catch, also happens in existing code? Yay bugs that only happens once every 2^32 - 1 times... will update in both places.

JoTurk · 2025-12-28T16:57:31Z

association.go

+		return assoc, nil
+	}
+	assoc = createAssociation(config)
 	assoc.init(true)


Related to the previous comment but we skip assoc.init. So it's better overall if we try to make a single path for initialization. all of these APIs are private so feel free to refactor them.

fippo marked this pull request as draft December 28, 2025 15:56

fippo force-pushed the snap branch from 277a65c to 7d76ca4 Compare December 28, 2025 16:02

JoTurk reviewed Dec 28, 2025

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fippo commented Dec 28, 2025

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fippo mentioned this pull request Dec 28, 2025

sctp: implement SNAP / sctp-init pion/webrtc#3327

Draft

JoTurk reviewed Dec 28, 2025

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fippo added 2 commits December 28, 2025 19:26

Avoid forbidden 0 init tag

9cd1454

Rearrange things a bit

4da988d

fippo force-pushed the snap branch from bb7e36f to 4da988d Compare December 28, 2025 18:26

Move things around

758fcf0

fippo force-pushed the snap branch 2 times, most recently from b234227 to a41c9fd Compare December 28, 2025 20:06

Breaking API change, add SctpOptions for paramters

cb1762e

fippo force-pushed the snap branch from a41c9fd to cb1762e Compare December 28, 2025 20:15

philipch07 mentioned this pull request Dec 28, 2025

Use options pattern instead of config #450

Draft

SctpOptions->SctpParamters

888d784

fippo mentioned this pull request Dec 31, 2025

WARP tracking issue pion/webrtc#3335

Open

Copilot AI mentioned this pull request Dec 31, 2025

Add WARP support with full SNAP implementation, SPED foundation, and vendor SCTP for Direct API algesten/str0m#810

Closed

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Implement SCTP Negotiation Acceleration Protocol (SNAP) #449

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Implement SCTP Negotiation Acceleration Protocol (SNAP) #449

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