Skip to main content

4. The ALTSVC HTTP/2 Frame

The ALTSVC HTTP/2 frame ([RFC7540], Section 4) advertises the availability of an alternative service to an HTTP/2 client.

The ALTSVC frame is a non-critical extension to HTTP/2. Endpoints that do not support this frame will ignore it (as per the extensibility rules defined in Section 4.1 of [RFC7540]).

An ALTSVC frame from a server to a client on a stream other than stream 0 indicates that the conveyed alternative service is associated with the origin of that stream.

An ALTSVC frame from a server to a client on stream 0 indicates that the conveyed alternative service is associated with the origin contained in the Origin field of the frame. An association with an origin that the client does not consider authoritative for the current connection MUST be ignored.

The ALTSVC frame type is 0xa (decimal 10).

+-------------------------------+-------------------------------+
|         Origin-Len (16)       | Origin? (*)                 ...
+-------------------------------+-------------------------------+
|                   Alt-Svc-Field-Value (*)                   ...
+---------------------------------------------------------------+

ALTSVC Frame Payload

The ALTSVC frame contains the following fields:

  • Origin-Len: An unsigned, 16-bit integer indicating the length, in octets, of the Origin field.
  • Origin: An OPTIONAL sequence of characters containing the ASCII serialization of an origin ([RFC6454], Section 6.2) to which the alternative service is applicable.
  • Alt-Svc-Field-Value: A sequence of octets (length determined by subtracting the length of all preceding fields from the frame length) containing a value identical to the Alt-Svc field value defined in Section 3 (ABNF production "Alt-Svc").

The ALTSVC frame does not define any flags.

The ALTSVC frame is intended for receipt by clients. A device acting as a server MUST ignore it.

An ALTSVC frame on stream 0 with empty (length 0) "Origin" information is invalid and MUST be ignored. An ALTSVC frame on a stream other than stream 0 containing non-empty "Origin" information is invalid and MUST be ignored.

The ALTSVC frame is processed hop-by-hop. An intermediary MUST NOT forward ALTSVC frames, though it can use the information contained in ALTSVC frames in forming new ALTSVC frames to send to its own clients.

Receiving an ALTSVC frame is semantically equivalent to receiving an Alt-Svc header field. As a result, the ALTSVC frame causes alternative services for the corresponding origin to be replaced. Note that it would be unwise to mix the use of Alt-Svc header fields with the use of ALTSVC frames, as the sequence of receipt might be hard to predict.

Last updated on