RFC 9110, "HTTP Semantics", June 2022

Errata ID: 7870
Status: Reported
Type: Technical
Publication Format(s) : TEXT
Reported By: Ben Kallus
Date Reported: 2024-03-24

Section 8.6 says:

   Likewise, a sender MUST NOT forward a message with a Content-Length
   header field value that does not match the ABNF above, with one
   exception: a recipient of a Content-Length header field value
   consisting of the same decimal value repeated as a comma-separated
   list (e.g, "Content-Length: 42, 42") MAY either reject the message as
   invalid or replace that invalid field value with a single instance of
   the decimal value, since this likely indicates that a duplicate was
   generated or combined by an upstream message processor.

It should say:

   Likewise, a sender MUST NOT send a message with a Content-Length
   header field value that does not match the ABNF above. A
   recipient of a Content-Length header field value consisting of
   the same decimal value repeated as a comma-separated list (e.g,
   "Content-Length: 42, 42") MAY either reject the message as invalid
   or replace that invalid field value with a single instance of the
   decimal value, since this likely indicates that a duplicate was
   generated or combined by an upstream message processor.

Notes:

This change aims to fix 2 issues with the text:

Issue #1
Recall the following from section 8.6:
> Likewise, a sender MUST NOT forward a message with a Content-Length header field value that does not match the ABNF above, ...

It wasn't immediately clear to me which of these was the intended meaning:
1. Upon receipt of a message with an invalid Content-Length value, senders MUST NOT forward the message.
2. Upon receipt of a message with an invalid Content-Length value, senders MUST NOT forward the message with the invalid value intact.

Mark Nottingham confirmed on GitHub that the intended meaning is option 2:
https://github.com/httpwg/http-core/issues/1113#issuecomment-1937914210

I propose that the word "forward" be changed to "send" to clear up the ambiguity.

Issue #2
We've just established that the intended meaning of the first half of the sentence in question is that malformed CL header values MUST NOT be forwarded intact.
An exception to this rule is (by definition) a situation in which invalid CL header values *are* permitted to be forwarded intact.
The "exception" described in the text does not allow for invalid header values to be forwarded intact, so it is a misuse of the word "exception."

To clear this up, I propose that the sentence be split in two, and that the word "exception" be removed.

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