RFC 6265, "HTTP State Management Mechanism", April 2011

Errata ID: 3765
Status: Rejected
Type: Technical
Publication Format(s) : TEXT
Reported By: Johannes Knaupp
Date Reported: 2013-10-25
Rejected by: Barry Leiba
Date Rejected: 2013-10-26

Section 4.1.1 says:

Servers SHOULD NOT include more than one Set-Cookie header field in
the same response with the same cookie-name.  (See Section 5.2 for
how user agents handle this case.)

It should say:

Servers MUST NOT include more than one Set-Cookie header field in
the same response.

Notes:

The HTTP specification (RFC 2616) says in its section 4.2: "Multiple message-header fields with the same field-name MAY be present in a message if and only if the entire field-value for that header field is defined as a comma-separated list [i.e., #(values)]. [...]"

Since the mentioned condition is not fulfilled in the case of Set-Cookie headers, only one Set-Cookie header is permissible in an HTTP message.

This also applies to the third example in section 3.1, even though it is not clearly specified there whether or not the two Set-Cookies originate from the same server response.

On the internet many HTTP messages contain multiple Set-Cookie headers, and this seems to make sense in order to avoid additional roundtrips. This, however, (1) does not match the HTTP specification, see above, and therefore (2) cannot be used with implementations stating that they were HTTP compatible and consequently only allow a single Set-Cookie header per response. Clearly, this is not a defect of those implementations, but of the specifications which are at least mistakable (if not contradictory).
--VERIFIER NOTES--
Part of the point of RFC 6265 is to document how cookies are actually
used on the Internet. As is noted in the introduction, existing use
doesn't always conform to what it should.  In particular, we know that
RFC 6265 doesn't always match up with RFC 2616, because the actual usage
isn't always strictly correct.

The variation from RFC 2616 that this report notes is intentional,
documenting the existing usage, and this errata report is rejected.

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