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Sticky header perlim draft

From: Paul Leach <paulle@microsoft.com>
Date: Sat, 3 Aug 1996 01:57:44 -0700
Message-Id: <c=US%a=_%p=msft%l=RED-77-MSG-960803085744Z-29559@abash1.microsoft.com>
To: "'http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com'" <http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com>
Here is a preliminary version of an Internet-draft about sticky headers
and header name compression that I committed to write up at the Montreal

Comments are welcome, and will be incorporated before sending it in as
an official draft.

===============HTTP Working Group                              Paul J.
Leach, Microsoft
Expires January 11, 1996                                  July 11, 1996
                Sticky Headers and Header Name Compression
                           Preliminary Draft
This preliminary draft is submitted in response to a work item of the
HTTP Working Group for HTTP/1.2, but does not at this time represent its
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, and
its working groups. Note that other groups may also distribute working
documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in progress".
WARNING: The specification in this document is subject to change, and
will certainly change.  It is inappropriate AND STUPID to implement to
the proposed specification in this document.  In particular, anyone who
implements to this specification and then complains when it changes will
be properly viewed as an idiot, and any such complaints shall be
To learn the current status of any Internet-Draft, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
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Distribution of this document is unlimited.  Please send comments to the
HTTP working group at <http-wg@cuckoo.hpl.hp.com>.  Discussions of the
working group are archived at
<URL:http://www.ics.uci.edu/pub/ietf/http/>.  General discussions about
HTTP and the applications which use HTTP should take place on the <www-
talk@w3.org> mailing list.
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This draft defines two mechanisms to reduce the size of HTTP requests
and responses. This will result in fewer bytes transmitted over the net,
which is especially desirable for clients attached to the network via
slow links, although it may also help increase efficiency of congested
links, such as the intercontinental ones, as well. The mechanisms flow
from the following two observations.
The first is that a sequence of HTTP requests and responses between a
given client and server typically contains many headers whose values are
the same for each request and/or response. This draft proposed a way to
optionally omit sending repetitive headers, instead allowing the sender
to indicate to the receiver that the values from a previous message
should be used. Such headers are called "sticky". Special provisions are
made for proxies so that messages from many clients to a single server
can be multiplexed over a single connection and still take advantage of
sticky headers.
The second is that HTTP header names are usually chosen to be indicative
of their purpose, not for their short length. This draft proposes a way
to compress header names using tersely encoded abbreviations.
The intent is that this document be incorporated into the HTTP/1.2
specification [1], if the HTTP working group accepts the design.
However, the only dependencies in the design are that the client and
server support persistent connections, and that any proxies in the
communication chain correctly implement the Connection header, so that
it could be implemented in applications speaking earlier versions of
Table of Contents
1. Introduction............................................3
2. Overall Operation.......................................3
 2.1 Basic operation ......................................4
 2.2 Contexts .............................................4
 2.3 Changing the sticky-header set .......................5
3. Specification...........................................5
 3.1 Sticky connection-token ..............................6
 3.2 Sticky header ........................................6
4. Examples................................................6
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5. Header name compression.................................8
6. Security Considerations.................................9
7. Acknowledgments.........................................9
8. References.............................................10
9. Author's address.......................................10
1. Introduction
This draft introduces sticky headers and header name compression. Sticky
headers require two simple extensions to HTTP -- a new connection-token
for the Connection header, and a new header, the Sticky header. Header
name compression merely introduces short equivalents for existing header
names, and describes a methodology for creating short names for headers
added to HTTP in the future.
By default, HTTP/1.1 connections are persistent, and there is a fairly
widely implemented extension to HTTP/1.0 (the "Connection: Keep-Alive"
extension) which provides the same functionality when clients are
communicating directly with origin servers.
Sticky headers operate over persistent connections, and their must be
This draft is being proposed as a component of HTTP version 1.2. It is
based on work initially done by the persistent connection committee of
the HTTP working group of the IETF.
The organization of this draft is as follows. In section 2 we describe
the overall operation of sticky headers,  then section 3 specifies the
extension to the Connection header and the Sticky header, and section 4
provides some examples. In section 5 we specify header name compression.
2. Overall Operation
We describe the overall operation in stages:
  .  basic operation -- suitable for most user-agent to server
  .  contexts -- an optimization for proxy to server operation
  .  changing the default sticky headers
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  .  backwards compatibility considerations
2.1 Basic operation
The negotiation of the sticky headers option may take place on any
request sent over a persistent connection. The client may add the
connection-token "Sticky" to the Connection header in a request; if the
server accepts the use of sticky headers, it responds with the same
token in the Connection header of its response.
Once the use of sticky headers has been negotiated, specified message-
headers (see section 4.1 and 4.2 of [1]) are remembered from message to
message, so that they need to be transmitted in a message only if they
have changed since the last message. The use of sticky headers continues
until the connection is closed, without further need for the "Sticky"
connection-token on each request.
An examination of HTTP/1.1 header fields shows that the message-headers
with the following field-names are reasonably likely to have the same
field-values in consecutive requests from a user-agent to a particular
     Accept, Accept-Charset, Accept-Language, Accept-Encoding,
     Authorization, Proxy-Authorization, From, Host, User-Agent
     Note: Because there do not appear to be any response headers
     whose field-values are likely to be repeated on consecutive
     responses, other than the Server header, it was not deemed
     useful to support sticky headers in the server to client
By default, message-headers with these field-names are the ones that
must be remembered, and they comprise the "sticky-header set" for a
connection. If any of them are not present in a message received by a
server, they are taken from the remembered set and added to the message
before processing it. If any of them are present in a message, they
replace the instance in the remembered set. To send a message without
one of the remembered message-headers, send a message-header line
consisting of just the field-name and a colon, and no field-value; upon
reception of such a line, the message-header in the remembered set with
that field-name is deleted.
Section 2.3 provides a way to change the default sticky-header set.
2.2 Contexts
Proxies can operate just like user-agents if they want. However, they
typically act on behalf of many clients, multiplexing a single
connection to a server across messages from many clients. Such
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multiplexing will likely destroy the correlation between consecutive
messages that makes sticky headers an effective compression technique.
To better support multiplexed connections, we allow for the creation of
multiple sticky header "contexts" on a single persistent connection.
Once a context has been set up, each context operates with respect to
sticky-headers just as if it were a separate persistent connection.
I.e., there is a sticky-header set and a remembered set of message-
headers for each context.
To create a context, the Sticky header is sent by the client:
     Sticky:   NNNN
where NNNN is the context number. If the server response contains a
Sticky header with the same context number, then the context is set up,
otherwise it is refused. Once a context is created on an persistent
connection, subsequent messages that want to use the context include a
Sticky header with the context number used to create it, and it remains
in effect until the connection is closed.
The context in use when no Sticky header is present is context 0 (zero).
2.3 Changing the sticky-header set
To change the sticky-header set from the default, extra header field-
names can be listed in the Sticky header when a context is created. For
example, the following message-headers would negotiate the use of sticky
headers and add the Foo and Bar message-headers to the sticky header set
for the default context:
     Connection: Sticky
     Sticky: 0 Foo, Bar
The main purpose for this facility is to allow yet-to-be-invented
headers to be added to the sticky header set. There is no way to remove
headers from the sticky header set, or to add more headers to the set
after a context is created.
2.4 Backwards compatibility
A client may safely attempt to negotiate sticky headers with any server
to which it may legally set up a persistent connection.
If negotiation is attempted to an origin-server that does not support
sticky headers, it will fail, because the server will not include
Connection: sticky in its response.
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If negotiation is attempted to an HTTP/1.1 proxy that does not support
it, the Connection and Sticky headers will be removed by the proxy and
so the negotiation will fail because neither the proxy nor any inbound
server will include Connection: sticky in its response.
Negotiation will not be attempted to HTTP/1.0 proxies, because it is not
legal to set up persistent connections with them.
3. Specification
3.1 Sticky connection-token
The Sticky connection-token is used as part of a Connection header to
negotiate the use of sticky headers. If a request contains a Connection
header with the "sticky" connection-token, the client is requesting the
use of sticky headers. If the server's response contains a Connection
header with the "sticky" connection-token, then it accepts the use of
sticky headers.
3.2 Sticky header
The Sticky general header may be used to create a sticky header context,
to indicate that a message belongs to an existing sticky header context,
or to add headers to the sticky header set.
     Sticky    = "Sticky" ":" context-id 0#field-name
     context-id     = *DIGIT
     field-name     = token        ; see section 4.2 of [1]
The context-id either requests the creation of a new context with that
ID, or indicates that this message belongs to an existing context with
that ID.
Field-names may be present only when creating a new context. If present,
then the client requests that message-headers with the given field-names
be made part of the sticky header set for the context.
If the creation of a new context is requested, the server indicates
acceptance of the new context by returning a Sticky header with the same
context-id in its response message.
4. Examples
     (requesting first object, negotiating sticky headers)
     GET / HTTP/1.1<CRLF>
     Accept: text/html<CRLF>
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     Accept-Language: en<CRLF>
     Connection: sticky<CRLF>
     HTTP/1.1 200 OK<CRLF>
     MIME-Version: 1.0<CRLF>
     Connection: sticky<CRLF>
     Content-Type: text/html<CRLF>
     <94 bytes body data here>
     (client makes a request changing the Accept value but maintaining
     the value of the Accept-Language: field from the previous request)
     GET /myimg.jpg HTTP/1.1<CRLF>
     Accept: image/jpeg<CRLF>
     HTTP/1.1 200 OK<CRLF>
     MIME-Version: 1.0<CRLF>
     Content-Type: image/jpeg<CRLF>
     <4000 bytes body data here>
     (requests a second context)
     GET /myimg.jpg HTTP/1.1<CRLF>
     Accept: image/jpeg<CRLF>
     Accept-Language: fr<CRLF>
     Connection: sticky<CRLF>
     Sticky: 1<CRLF>
     (accepts second context)
     HTTP/1.1 200 OK<CRLF>
     MIME-Version: 1.0<CRLF>
     Content-Type: image/jpeg<CRLF>
     Connection: sticky<CRLF>
     Sticky: 1<CRLF>
     <4000 bytes body data here>
     (making a request in second context -- Accept-Language is "fr")
     GET / HTTP/1.1<CRLF>
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     Accept: text/html<CRLF>
     Connection: sticky<CRLF>
     Sticky: 1<CRLF>
     HTTP/1.1 200 OK<CRLF>
     MIME-Version: 1.0<CRLF>
     Content-Type: text/html<CRLF>
     <94 bytes body data here>
5. Header name compression
Once sticky headers are negotiated, then either client or server can
replace a header name in the left hand column with its abbreviation in
the right hand column of the following table.
Accept               #A
Accept-Charset       #B
Accept-Encoding      #C
Accept-Language      #D
Accept-Ranges        #E
Age                  #F
Allow                #G
Authorization        #H
Cache-Control        #I
Connection           #J
Content-Base         #K
Content-Encoding     #L
Content-Language     #M
Content-Length       #N
Content-Location     #O
Content-MD5          #P
Content-Range        #Q
Content-Type         #R
Date                 #S
ETag                 #T
Expires              #U
rom                 #V
Host                 #W
If-Modified-Since    #X
If-Match             #Y
If-None-Match        #Z
If-Range             #a
If-Unmodified-Since  #b
Last-Modified        #c
Location             #d
Max-Forwards         #e
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Pragma               #f
Proxy-Authenticate   #g
Proxy-Authorization  #h
Public               #I
Range                #j
Referer              #k
Retry-After          #l
Server               #m
Transfer-Encoding    #n
Upgrade              #o
User-Agent           #p
Vary                 #q
Via                  #r
Warning              #s
WWW-Authenticate     #t
The character after the "#" is a base64 "digit" as per RFC 1521. For the
headers defined in HTTP/1.1 it comes from the range 0-45 decimal. There
can be 18 more headers added to HTTP and still only require one digit;
at that point if more are added a second digit can handle up to 4096
A static analysis of the effectiveness of this compression method shows
that the lengths of the uncompressed headers total 480 bytes, whereas
compressed, they total 92 bytes -- an 81% savings. The actual savings
will depend on the relative frequency of use of the different headers.
6. Security Considerations
This HTTP extension has two indirect effects on security. Using sticky
headers can reduce the performance penalty of authentication, since a
client can send one authentication header and maintain that
authentication for a period of time. On the other hand an attacker could
theoretically intercept a previously initiated communication channel and
substitute itself, gaining the authentication attributes of the client
that initiated the communication. This latter attack could only work
with the non-secure authentication methods anyway so it is not
considered to be a serious concern.
7. Acknowledgments
An earlier version of this design resulted from discussion of the HTTP
persistent connection sub-working group, and was written up in an
Internet-Draft by Alex Hopmann.
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8. References
  [1]  Roy T. Fielding, et. al., Hypertext Transfer Protocol --
     HTTP/1.1.  Internet-Draft draft-ietf-http-v11-spec-06.txt, HTTP
     Working Group, July 4, 1996.
9. Author's address
Paul J. Leach
1 Microsoft Way
Redmond, Washington, 98052, U.S.A.
Email: paulle@microsoft.com
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Received on Saturday, 3 August 1996 02:02:17 EDT

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