is parsing OBS-FOLD mandatory?

[mnot@…]

2.5 Conformance and Error Handling says "...recipient MUST be able to parse any value that would match the ABNF rules..." yet 3.2.2 only make parsing obs-fold a SHOULD. Which is it?

[mnot@…]

In Atlanta:

rf: if you can find no interoperable folding implementations, then we can remove it from the grammar [[scribe: may not have got this right, action more important to capture]] ACTION: mnot to run some tests on folding to understand its interoperability

[fielding@…]

From [2039]:

Parsing obs-fold is necessary for backwards compat; addresses #409

[mnot@…]

I thought the approach we were going to take here was to modify the overall requirement regarding syntax conformance to exempt the obs-* constructs, rather than un-deprecate obs-fold...

[fielding@…]

We could still do that, but I needed to make the existing text consistent first.

Note that we still need to discuss such a change on list.

[mnot@…]

Deprecating line-folding was an explicit decision of the WG in #77; see [395].

[2039] effectively reverts that decision. Taking to list.

[mnot@…]

Proposal:

""" Historically, HTTP header field values could be extended over multiple lines by preceding each extra line with at least one space or horizontal tab (obs-fold). This specification deprecates such line folding except within the message/http media type (Section 7.3.1). Senders MUST NOT generate messages that include line folding (i.e., that contain any field-value that matches the obs-fold rule) unless the message is intended for packaging within the message/http media type. Recipients MUST either:

• accept line folding and replace any embedded obs-fold whitespace with either a single SP or a matching number of SP octets (to avoid buffer copying) prior to interpreting the field value or forwarding the message downstream, or

• reject a message with line folding present. Servers can do for requests by responding with 400 Bad Request and a representation explaining the condition; clients can only discard the message.

In particular, recipients who choose not to implement obs-fold processing (as described above) MUST NOT accept messages containing headers with leading whitespace, as this can expose them to attacks that exploit this difference in processing. """

[fielding@…]

It isn't possible for clients to simply discard a message, since responses are queued. I will commit a slight variation of the above.

[fielding@…]

From [2146]:

Allow recipients to reject obsolete line folding; addresses #409

[mnot@…]

WFM.