a=sctp-init

attribute definition

where the attribute is allowed

The attribute is defined only for an m= line whose protocol identifier is UDP/DTLS/SCTP (defined in RFC 8841) with an <fmt> value of webrtc-datachannel (defined in RFC 8832). The draft notes it also covers TCP/DTLS/SCTP, and that usage with any other proto value “needs to be defined in a separate specification”.

m=application 9 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 0.0.0.0
a=ice-ufrag:UgEn
a=ice-pwd:f/+ugRILrIUlAkSmkStnZb/h
a=ice-options:trickle
a=fingerprint:sha-256 6A:15:F0:08:...:2D:7A
a=setup:actpass
a=mid:0
a=sctp-port:5000
a=max-message-size:262144
a=sctp-init:AQAAHols3R0AUAAA/////+B5ZR3AAAAEgAgABoLA

worked decode

The example value above decodes to this SCTP INIT chunk (draft §4.1; chunk layout per RFC 9260 §3.3.2):

generation, validation, renegotiation

• Generation (offer §5.2 / answer §5.4): the endpoint MUST have its SCTP implementation generate the serialized INIT chunk it would normally send over the network, base64-encode those bytes, and add the line to the data m= section. The offerer does this when it has created a WebRTC data channel; the answerer does it when it negotiated a data m= section and detected a valid a=sctp-init in the offer.
• Validation (§5.3, §5.5): improperly base64-encoded data results in an error; the SCTP implementation then validates the chunk format and a byte string that is not a valid SCTP INIT chunk also results in an error.
• Renegotiation (§5.6): subsequent offers and answers MUST include the attribute with the same value as in the initial negotiation. A remote offer MAY carry a new value only in conjunction with a new SCTP association (RFC 8841 §9.3) or a new DTLS association (RFC 8842 §5.5). Attempting to add sctp-init to an existing SCTP association results in an error.
• Fallback: an endpoint that does not support the extension MUST NOT include the attribute in its answer (§5.4) — negotiation silently degrades to the classic SCTP handshake.

IANA status

Draft §9.1 defines sctp-init as a new SDP media-level attribute (details per §4.2). As of 2026-07-28 the draft is an expired, unadopted individual submission and no IANA registration has been made — the IANA SDP Parameters registry carries no sctp-init entry (checked 2026-07-28). Treat the attribute name as provisional until a working-group draft registers it.

security notes

Exposing the SCTP INIT chunk — in particular the Initiate Tag — in SDP introduces no new security concern per draft §8, because SCTP-over-DTLS protects against the off-path attacks of RFC 9260 §5.3.1. Application-layer code (JavaScript, signaling) that can read or rewrite SDP can add or remove the chunk’s variable-length parameters; per §8, removing one is equivalent to the peer not supporting that parameter, and adding an unsupported one is equivalent to receiving a packet for an unnegotiated feature. See the overview security section for the amplification analysis.

compatibility

see also

• Feature overview — WebRTC Data Channel: SCTP Negotiation Acceleration Protocol
• SNAP offer/answer procedures
• IETF datatracker — draft-hancke-tsvwg-snap (source + issue tracker)
• ChromeStatus — 5137946677215232
• RFC 9260 §3.3.2 — the INIT chunk