Network Working Group                                           PH. Kamp
Internet-Draft                                 The Varnish Cache Project
Intended status: Informational                          October 30, 2016
Expires: May 3, 2017

                      HTTP header common structure


   An abstract data model for HTTP headers, "Common Structure", and a
   HTTP/1 serialization of it, generalized from current HTTP headers.

Status of This Memo

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

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   This Internet-Draft will expire on May 3, 2017.

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   document authors.  All rights reserved.

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1.  Introduction

   The HTTP protocol does not impose any structure or datamodel on the
   information in HTTP headers, the HTTP/1 serialization is the
   datamodel: An ASCII string without control characters.

   HTTP header definitions specify how the string must be formatted and
   while families of similar headers exist, it still requires an
   uncomfortable large number of bespoke parser and validation routines
   to process HTTP traffic correctly.

   In order to improve performance HTTP/2 and HPACK uses naive text-
   compression, which incidentally decoupled the on-the-wire
   serialization from the data model.

   During the development of HPACK it became evident that significantly
   bigger gains were available if semantic compression could be used,
   most notably with timestamps.  However, the lack of a common data
   structure for HTTP headers would make semantic compression one long
   list of special cases.

   Parallel to this, various proposals for how to fulfill data-
   transportation needs, and to a lesser degree to impose some kind of
   order on HTTP headers, at least going forward were floated.

   All of these proposals, JSON, CBOR etc. run into the same basic
   problem: Their serialization is incompatible with [RFC7230]'s ABNF
   definition of 'field-value'.

   For binary formats, such as CBOR, a wholesale base64/85
   reserialization would be needed, with negative results for both
   debugability and bandwidth.

   For textual formats, such as JSON, the format must first be neutered
   to not violate field-value's ABNF, and then workarounds added to
   reintroduce the features just lost, for instance UNICODE strings, and
   suddenly it is no longer JSON anymore.

   This proposal starts from the other end, and builds and generalizes a
   data structure definition from existing HTTP headers, which means
   that HTTP/1 serialization and 'field-value' compatibility is built

   If all future HTTP headers are defined to fit into this Common
   Structure we have at least halted the proliferation of bespoke
   parsers and started to pave the road for semantic compression
   serializations of HTTP traffic.

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1.1.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119

2.  Definition of HTTP header Common Structure

   The data model of Common Structure is an ordered sequence of named
   dictionaries.  Please see Appendix A for how this model was derived.

   The definition of the data model is on purpose abstract, uncoupled
   from any protocol serialization or programming environment
   representation, meant as the foundation on which all such
   manifestations of the model can be built.

   Common Structure in ABNF:

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       import token from RFC7230
       import DIGIT from RFC5234

       common-structure = 1* ( identifier dictionary )

       dictionary = * ( identifier value )

       value = identifier /
               number /
               ascii_string /
               unicode_string /
               blob /
               timestamp /

       identifier = token  [ "/" token ]

       number = ["-"] 1*15 DIGIT
               # XXX: Not sure how to do this in ABNF:
               # XXX: A single "." allowed between any two digits
               # The range is limited is to ensure it can be
               # correctly represented in IEEE754 64 bit
               # binary floating point format.

       ascii_string = * %x20-7e
               # This is a "safe" string in the sense that it
               # contains no control characters or multi-byte
               # sequences.  If that is not fancy enough, use
               # unicode_string.

       unicode_string = * unicode_codepoint
               # XXX: Is there a place to import this from ?
               # Unrestricted unicode, because there is no sane
               # way to restrict or otherwise make unicode "safe".

       blob = * %0x00-ff
               # Intended for cryptographic data and as a general
               # escape mechanism for unmet requirements.

       timestamp = POSIX time_t with optional millisecond resolution
               # XXX: Is there a place to import this from ?

3.  HTTP/1 serialization of HTTP header Common Structure

   In ABNF:

       import OWS from {{RFC7230}}
       import HEXDIG, DQUOTE from {{RFC5234}}

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       import UTF8-2, UTF8-3, UTF8-4 from {{RFC3629}}

       h1_common-structure-header =
               ( field-name ":" OWS ">" h1_common_structure "<" )
                       # Self-identifying HTTP headers
               ( field-name ":" OWS h1_common_structure ) /
                       # legacy HTTP headers on white-list, see {{iana}}

       h1_common_structure = h1_element  * ("," h1_element)

       h1_element = identifier * (";" identifier ["=" h1_value])

       h1_value = identifier /
               number /
               h1_ascii_string /
               h1_unicode_string /
               h1_blob /
               h1_timestamp /

       h1_ascii_string = DQUOTE *(
                       ( "\" DQUOTE ) /
                       ( "\" "\" ) /
                       0x20-21 /
                       0x23-5B /
                       ) DQUOTE
               # This is a proper subset of h1_unicode_string
               # NB only allowed backslash escapes are \" and \\

       h1_unicode_string = DQUOTE *(
                       ( "\" DQUOTE )
                       ( "\" "\" ) /
                       ( "\" "u" 4*HEXDIG ) /
                       0x20-21 /
                       0x23-5B /
                       0x5D-7E /
                       UTF8-2 /
                       UTF8-3 /
                       ) DQUOTE
               # This is UTF8 with HTTP1 unfriendly codepoints
               # (00-1f, 7f) neutered with \uXXXX escapes.

       h1_blob = "'" base64 "'"
               # XXX: where to import base64 from ?

       h1_timestamp = number

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               # UNIX/POSIX time_t semantics.
               # fractional seconds allowed.

       h1_common_structure = ">" h1_common_structure "<"

   XXX: Allow OWS in parsers, but not in generators ?

   In programming environments which do not define a native
   representation or serialization of Common Structure, the HTTP/1
   serialization should be used.

4.  When to use Common Structure parser

   All future standardized and all private HTTP headers using Common
   Structure should self identify as such.  In the HTTP/1 serialization
   by making the first character ">" and the last "<".  (These two
   characters are deliberately "the wrong way" to not clash with
   exsisting usages.)

   Legacy HTTP headers which fit into Common Structure, are marked as
   such in the IANA Message Header Registry (see {iana}), and a snapshot
   of the registry can be used to trigger parsing according to Common
   Structure of these headers.

5.  Desired normative effects

   All new HTTP headers SHOULD use the Common Structure if at all

6.  Open/Outstanding issues to resolve

6.1.  Single/multiple headers

   Should we allow splitting common structure data over multiple headers


   Avoids size restrictions, easier on-the-fly editing


   Cannot act on any such header until all headers have been received.

   We must define where headers can be split (between identifier and
   dictionary ?, in the middle of dictionaries ?)

   Most on-the-fly editing is hackish at best.

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7.  Future work

7.1.  Redefining existing headers for better performance

   The HTTP/1 serializations self-identification mechanism makes it
   possible to extend the definition of existing Appendix C headers into
   Common Structure.

   For instance one could imagine:

       Date: >1475061449.201<

   Which would be faster to parse and validate than the current
   definition of the Date header and more precise too.

   Some kind of signal/negotiation mechanism would be required to make
   this work in practice.

7.2.  Define a validation dictionary

   A machine-readable specification of the legal contents of HTTP
   headers would go a long way to improve efficiency and security in
   HTTP implementations.

8.  IANA considerations

   The IANA Message Header Registry will be extended with an additional
   field named "Common Structure" which can have the values "True",
   "False" or "Unknown".

   The RFC723x headers listed in Appendix B will get the value "True" in
   the new field.

   The RFC723x headers listed in Appendix C will get the value "False"
   in the new field.

   All other existing entries in the registry will be set to "Unknown"
   until and if the owner of the entry requests otherwise.

9.  Normative References

   [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,

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

Appendix A.  Does HTTP headers have any common structure ?

   Several proposals have been floated in recent years to use some
   preexisting structured data serialization or other for HTTP headers,
   to impose some sanity.

   None of these proposals have gained traction and no obvious candidate
   data serializations have been left unexamined.

   This effort tries to tackle the question from the other side, by
   asking if there is a common structure in existing HTTP headers we can
   generalize for this purpose.

A.1.  Survey of HTTP header structure

   The RFC723x family of HTTP/1 standards control 49 entries in the IANA
   Message Header Registry, and they share two common motifs.

   The majority of RFC723x HTTP headers are lists.  A few of them are
   ordered, ('Content-Encoding'), some are unordered ('Connection') and
   some are ordered by 'q=%f' weight parameters ('Accept')

   In most cases, the list elements are some kind of identifier, usually
   derived from ABNF 'token' as defined by [RFC7230].

   A subgroup of headers, mostly related to MIME, uses what one could
   call a 'qualified token'::

       qualified_token = token_or_asterix [ "/" token_or_asterix ]

   The second motif is parameterized list elements.  The best known is
   the "q=0.5" weight parameter, but other parameters exist as well.

   Generalizing from these motifs, our candidate "Common Structure" data
   model becomes an ordered list of named dictionaries.

   In pidgin ABNF, ignoring white-space for the sake of clarity, the
   HTTP/1.1 serialization of Common Structure is is something like:

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       token_or_asterix = token from {{RFC7230}}, but also allowing "*"

       qualified_token = token_or_asterix [ "/" token_or_asterix ]

       field-name, see {{RFC7230}}

       Common_Structure_Header = field-name ":" 1#named_dictionary

       named_dictionary = qualified_token [ *(";" param) ]

       param = token [ "=" value ]

       value = we'll get back to this in a moment.

   Nineteen out of the RFC723x's 48 headers, almost 40%, can already be
   parsed using this definition, and none the rest have requirements
   which could not be met by this data model.  See Appendix B and
   Appendix C for the full survey details.

A.2.  Survey of values in HTTP headers

   Surveying the datatypes of HTTP headers, standardized as well as
   private, the following picture emerges:

A.2.1.  Numbers

   Integer and floating point are both used.  Range and precision is
   mostly unspecified in controlling documents.

   Scientific notation (9.192631770e9) does not seem to be used

   The ranges used seem to be minus several thousand to plus a couple of
   billions, the high end almost exclusively being POSIX time_t

A.2.2.  Timestamps

   RFC723x text format, but POSIX time_t represented as integer or
   floating point is not uncommon.  ISO8601 have also been spotted.

A.2.3.  Strings

   The vast majority are pure ASCII strings, with either no escapes, %xx
   URL-like escapes or C-style back-slash escapes, possibly with the
   addition of \uxxxx UNICODE escapes.

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   Where non-ASCII character sets are used, they are almost always
   implicit, rather than explicit.  UTF8 and ISO-8859-1[5] seem to be
   most common.

A.2.4.  Binary blobs

   Often used for cryptographic data.  Usually in base64 encoding,
   sometimes ""-quoted more often not.  base85 encoding is also seen,
   usually quoted.

A.2.5.  Identifiers

   Seems to almost always fit in the RFC723x 'token' definition.

A.3.  Is this actually a useful thing to generalize ?

   The number one wishlist item seems to be UNICODE strings, with a big
   side order of not having to write a new parser routine every time
   somebody comes up with a new header.

   Having a common parser would indeed be a good thing, and having an
   underlying data model which makes it possible define a compressed
   serialization, rather than rely on serialization to text followed by
   text compression (ie: HPACK) seems like a good idea too.

   However, when using a datamodel and a parser general enough to
   transport useful data, it will have to be followed by a validation
   step, which checks that the data also makes sense.

   Today validation, such as it is, is often done by the bespoke

   This then is probably where the next big potential for improvement

   Ideally a machine readable "data dictionary" which makes it possibly
   to copy that text out of RFCs, run it through a code generator which
   spits out validation code which operates on the output of the common

   But history has been particularly unkind to that idea.

   Most attempts studied as part of this effort, have sunk under
   complexity caused by reaching for generality, but where scope has
   been wisely limited, it seems to be possible.

   So file that idea under "future work".

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Appendix B.  RFC723x headers with "common structure"

       Accept              [RFC7231, Section 5.3.2]
       Accept-Charset      [RFC7231, Section 5.3.3]
       Accept-Encoding     [RFC7231, Section 5.3.4][RFC7694, Section 3]
       Accept-Language     [RFC7231, Section 5.3.5]
       Age                 [RFC7234, Section 5.1]
       Allow               [RFC7231, Section 7.4.1]
       Connection          [RFC7230, Section 6.1]
       Content-Encoding    [RFC7231, Section]
       Content-Language    [RFC7231, Section]
       Content-Length      [RFC7230, Section 3.3.2]
       Content-Type        [RFC7231, Section]
       Expect              [RFC7231, Section 5.1.1]
       Max-Forwards        [RFC7231, Section 5.1.2]
       MIME-Version        [RFC7231, Appendix A.1]
       TE                  [RFC7230, Section 4.3]
       Trailer             [RFC7230, Section 4.4]
       Transfer-Encoding   [RFC7230, Section 3.3.1]
       Upgrade             [RFC7230, Section 6.7]
       Vary                [RFC7231, Section 7.1.4]

Appendix C.  RFC723x headers with "uncommon structure"

   1 of the RFC723x headers is only reserved, and therefore have no
   structure at all:

       Close               [RFC7230, Section 8.1]

   5 of the RFC723x headers are HTTP dates:

       Date                [RFC7231, Section]
       Expires             [RFC7234, Section 5.3]
       If-Modified-Since   [RFC7232, Section 3.3]
       If-Unmodified-Since [RFC7232, Section 3.4]
       Last-Modified       [RFC7232, Section 2.2]

   24 of the RFC723x headers use bespoke formats which only a single or
   in rare cases two headers share:

       Accept-Ranges       [RFC7233, Section 2.3]
           bytes-unit / other-range-unit

       Authorization       [RFC7235, Section 4.2]
       Proxy-Authorization [RFC7235, Section 4.4]

       Cache-Control       [RFC7234, Section 5.2]

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       Content-Location    [RFC7231, Section]
           absolute-URI / partial-URI

       Content-Range       [RFC7233, Section 4.2]
           byte-content-range / other-content-range

       ETag                [RFC7232, Section 2.3]

       Forwarded           [RFC7239]

       From                [RFC7231, Section 5.5.1]

       If-Match            [RFC7232, Section 3.1]
       If-None-Match       [RFC7232, Section 3.2]
           "*" / 1#entity-tag

       If-Range            [RFC7233, Section 3.2]
           entity-tag / HTTP-date

       Host                [RFC7230, Section 5.4]
           uri-host [ ":" port ]

       Location            [RFC7231, Section 7.1.2]

       Pragma              [RFC7234, Section 5.4]

       Range               [RFC7233, Section 3.1]
           byte-ranges-specifier / other-ranges-specifier

       Referer             [RFC7231, Section 5.5.2]
           absolute-URI / partial-URI

       Retry-After         [RFC7231, Section 7.1.3]
           HTTP-date / delay-seconds

       Server              [RFC7231, Section 7.4.2]
       User-Agent          [RFC7231, Section 5.5.3]
           product *( RWS ( product / comment ) )

       Via                 [RFC7230, Section 5.7.1]
           1#( received-protocol RWS received-by [ RWS comment ] )

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       Warning             [RFC7234, Section 5.5]

       Proxy-Authenticate  [RFC7235, Section 4.3]
       WWW-Authenticate    [RFC7235, Section 4.1]

Author's Address

   Poul-Henning Kamp
   The Varnish Cache Project


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