HTTP                                                       M. Nottingham
Internet-Draft                                            March 18, 2020
Intended status: Informational
Expires: September 19, 2020

                    Advisory Content-Length for HTTP


   The HTTP Content-Length header field is overloaded with (at least)
   two duties: message delimitation in HTTP/1, and metadata about the
   length of an incoming request body to the software handling it.

   This causes confusion, and sometimes problems.  This document
   proposes a new header to untangle these semantics (at least

Note to Readers

   _RFC EDITOR: please remove this section before publication_

   The issues list for this draft can be found at [1].

   The most recent (often, unpublished) draft is at [2].

   Recent changes are listed at
   pages/bikeshed-length [3].

   See also the draft's current status in the IETF datatracker, at

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any

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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 19, 2020.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   4
   2.  The Bikeshed-Length HTTP Header Field . . . . . . . . . . . .   4
     2.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   6
     5.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   The HTTP Content-Length header field ([RFC7230]) is overloaded with
   (at least) two duties: message delimitation in HTTP/1, and metadata
   about the length of an incoming request body to the software handling

   Message delimitation is a core feature of the protocol; it allows
   more than one message to be sent in a given direction on a
   connection.  It is also security-critical; if it is under attacker
   control, it's possible to confuse a recipient about how requests and
   responses are associated in HTTP/1.1 (as "smuggling" attacks).

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   As such, it has been treated progressively more strictly in HTTP
   specifications.  HTTP/1.1 introduced chunked transfer encoding, and
   forbade sending Content-Length when it is in use.  From [RFC2616]:

      Messages MUST NOT include both a Content-Length header field and a
      non-identity transfer-coding.  If the message does include a non-
      identity transfer-coding, the Content-Length MUST be ignored.

      If a message is received with both a Transfer-Encoding header
      field and a Content-Length header field, the latter MUST be

   [RFC7230] strengthened that to:

      A sender MUST NOT send a Content-Length header field in any
      message that contains a Transfer-Encoding header field.

      If a message is received with both a Transfer-Encoding and a
      Content-Length header field, the Transfer-Encoding overrides the
      Content-Length.  Such a message might indicate an attempt to
      perform request smuggling (Section 9.5) or response splitting
      (Section 9.4) and ought to be handled as an error.  A sender MUST
      remove the received Content-Length field prior to forwarding such
      a message downstream.

   HTTP/2 ([RFC7540]) does not use Content-Length for message
   delimitation, but still requires it to match the number of bytes that
   its framing mechanism sends:

      A request or response that includes a payload body can include a
      content-length header field.  A request or response is also
      malformed if the value of a content-length header field does not
      equal the sum of the DATA frame payload lengths that form the

   It further requires such malformed responses to generate a "hard"
   error, so that a downstream recipient that implements HTTP/1 can't be

      Intermediaries that process HTTP requests or responses (i.e., any
      intermediary not acting as a tunnel) MUST NOT forward a malformed
      request or response.  Malformed requests or responses that are
      detected MUST be treated as a stream error (Section 5.4.2) of type

      For malformed requests, a server MAY send an HTTP response prior
      to closing or resetting the stream.  Clients MUST NOT accept a
      malformed response.  Note that these requirements are intended to

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      protect against several types of common attacks against HTTP; they
      are deliberately strict because being permissive can expose
      implementations to these vulnerabilities.

   The currently proposed HTTP/3 specification [I-D.ietf-quic-http] has
   language similar to that in HTTP/2.

   Unfortunately, this makes _other_ uses of Content-Length more
   difficult to implement.

   In particular, many servers will reject a request without an explicit
   Content-Length using 411 (Length Required), because they want to know
   how many bytes are being sent before deciding to devote resources to
   serving the request.  However, depending on the protocol version(s)
   between the user agent and the origin server, a Content-Length header
   might not make it all the way, or the request might be rejected.

   Likewise, some applications would like to use Content-Length to
   indicate progress of a large download, but its successful traversal
   cannot be relied upon.

   While it's questionable whether all of the requirements above
   regarding Content-Length are honoured by implementations uniformly,
   there is enough diversity in implementation (particularly on the
   server side and in intermediaries) to make deployment of these uses

   Therefore, this specification proposes a new HTTP header field to
   carry _advisory_ content length information.  It is intended only for
   these uses, and _not_ message delimitation.

1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  The Bikeshed-Length HTTP Header Field

   NOTE: The final name of this header field will be selected using a
   to-be-defined process.  Warm up your paintbrushes...

   The Bikeshed-Length HTTP header field is a HTTP Structured Field
   [I-D.ietf-httpbis-header-structure] that conveys an advisory content
   length for the message body:

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   Bikeshed-Length = sh-item

   Its value MUST be an Integer (Section x.x of
   [I-D.ietf-httpbis-header-structure]), indicating a decimal number of
   octets for a potential payload body.

   Note that it is specifically a header field; it is not allowed to
   occur in trailer sections, and SHOULD be ignored if encountered

2.1.  Example

   A resource might allow requests with bodies up to a given size.  If
   an incoming request omits both Content-Length and Bikeshed-Length,
   they can respond with 411 (Length Required).  If either request
   header field is present, and the value given is not acceptable, they
   can respond with 413 (Payload Too Large).  If Bikeshed-Length is used
   and deemed to be acceptable, the resource still ought to monitor the
   number of incoming bytes to assure that they do not exceed the
   anticipated value.

3.  IANA Considerations


4.  Security Considerations

   The Value of Bikeshed-Length is advisory only; software that uses it
   will need to monitor the actual number of octets received to assure
   that it is not exceeded, and take appropriate action if it is.

5.  References

5.1.  Normative References

              Nottingham, M. and P. Kamp, "Structured Field Values for
              HTTP", draft-ietf-httpbis-header-structure-17 (work in
              progress), March 2020.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

5.2.  Informative References

              Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", draft-ietf-quic-http-27 (work in progress),
              February 2020.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616,
              DOI 10.17487/RFC2616, June 1999,

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

5.3.  URIs





Author's Address

   Mark Nottingham


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