HyBi Working Group J. Tamplin
Internet-Draft T. Yoshino
Intended status: Standards Track Google, Inc.
Expires: May 16, 2013 November 12, 2012
A Multiplexing Extension for WebSockets
draft-ietf-hybi-websocket-multiplexing-09
Abstract
The WebSocket Protocol [RFC6455] requires a new transport connection
for every WebSocket connection. This presents a scalability problem
when many clients connect to the same server, and is made worse by
having multiple clients running in different tabs of the same user
agent. This extension provides a way for separate logical WebSocket
connections to share an underlying transport connection.
Please send feedback to the hybi@ietf.org mailing list.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current.
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."
This Internet-Draft will expire on May 16, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Physical Connection and Logical Channels . . . . . . . . . 3
2. Conformance Requirements . . . . . . . . . . . . . . . . . . . 4
3. Extension Negotiation . . . . . . . . . . . . . . . . . . . . 5
4. Interaction with other Extensions / Framing Mechanisms . . . . 6
4.1. Ordering Extensions . . . . . . . . . . . . . . . . . . . 6
4.1.1. Efficiency . . . . . . . . . . . . . . . . . . . . . . 6
4.1.2. Security . . . . . . . . . . . . . . . . . . . . . . . 7
5. Multiplexed Connections . . . . . . . . . . . . . . . . . . . 8
6. Flow Control . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. New Channel Slot . . . . . . . . . . . . . . . . . . . . . 9
6.2. Send Quota . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Framing . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8. Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . 13
9. Multiplex Control Blocks . . . . . . . . . . . . . . . . . . . 15
9.1. Number Encoding in Multiplex Control Blocks . . . . . . . 16
9.2. AddChannelRequest . . . . . . . . . . . . . . . . . . . . 17
9.3. AddChannelResponse . . . . . . . . . . . . . . . . . . . . 20
9.4. FlowControl . . . . . . . . . . . . . . . . . . . . . . . 22
9.5. DropChannel . . . . . . . . . . . . . . . . . . . . . . . 23
9.5.1. Drop Reason Codes . . . . . . . . . . . . . . . . . . 24
9.6. NewChannelSlot . . . . . . . . . . . . . . . . . . . . . . 27
10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
11. Client Behavior . . . . . . . . . . . . . . . . . . . . . . . 31
12. Buffering . . . . . . . . . . . . . . . . . . . . . . . . . . 32
13. Fairness . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
14. Proxies . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
15. Timeout . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
16. Close the Logical Channel . . . . . . . . . . . . . . . . . . 36
17. Fail the Logical Channel . . . . . . . . . . . . . . . . . . . 37
18. Fail the Physical Connection . . . . . . . . . . . . . . . . . 38
19. Operations and Events on Multiplexed Connection . . . . . . . 39
20. Security Considerations . . . . . . . . . . . . . . . . . . . 40
21. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 41
22. References . . . . . . . . . . . . . . . . . . . . . . . . . . 42
22.1. Normative References . . . . . . . . . . . . . . . . . . . 42
22.2. Informative References . . . . . . . . . . . . . . . . . . 42
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 43
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1. Overview
This document describes a multiplexing extension for the WebSocket
Protocol. With this extension, one TCP connection can provide
multiple virtual WebSocket connections by encapsulating frames tagged
with a channel ID. A client that supports this extension will
advertise support for it in the client's opening handshake using the
"Sec-WebSocket-Extensions" header. If the server supports this
extension and supports parameters compatible with the client's
request, it accepts the use of this extension by the
"Sec-WebSocket-Extensions" header in the server's opening handshake.
1.1. Physical Connection and Logical Channels
Under use of this extension, one transport connection is shared by
multiple application-level instances. The WebSocket connection which
lies directly on the TCP connection and negotiated this multiplexing
extension is called "physical connection". Virtual WebSocket
connections established for each application-level instance are
called "multiplexed connections". Data channels virtually
established by ID tagging are called "logical channels". This
extension assigns a non-zero ID for each multiplexed connection.
Each logical channel with a non-zero ID exchanges frames of the
multiplexed connection with the same ID. The logical channel with ID
of 0 exchanges data to control multiplexing.
Data for different logical channels are distinguished by the logical
channel ID tag field placed at the head of the message that
encapsulates the original frame of a multiplex connection.
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2. Conformance Requirements
All diagrams, examples, and notes in this specification are non-
normative, as are all sections explicitly marked non-normative.
Everything else in this specification is normative.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [RFC2119].
Requirements phrased in the imperative as part of algorithms (such as
"strip any leading space characters" or "return false and abort these
steps") are to be interpreted with the meaning of the key word
("must", "should", "may", etc) used in introducing the algorithm.
Conformance requirements phrased as algorithms or specific steps MAY
be implemented in any manner, so long as the end result is
equivalent. (In particular, the algorithms defined in this
specification are intended to be easy to follow, and not intended to
be performant.)
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3. Extension Negotiation
The registered extension token for this extension is "mux".
To request use of the WebSocket Multiplexing Extension, a client MUST
include an element with the "mux" extension token as its extension
identifier in the "Sec-WebSocket-Extensions" header in its opening
handshake. The element MAY contain an extension parameter named
"quota". The value of the "quota" extension parameter specifies the
server's send quota for the "Implicitly Opened Connection".
To accept use of the WebSocket Multiplexing Extension, a server MUST
include an element with the "mux" extension token in the
"Sec-WebSocket-Extensions" header in its opening handshake. The
element MUST NOT contain any extension parameter.
A server MAY choose to reject use of the WebSocket Multiplexing
Extension by not including the element for the extension in the
"Sec-WebSocket-Extensions" header in its opening handshake. If any
elements were listed after the element for the WebSocket Multiplexing
Extension in the "Sec-WebSocket-Extensions" from the client, they
MUST also be rejected.
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4. Interaction with other Extensions / Framing Mechanisms
If any extension (e.g. compression) is placed before this extension
in the "Sec-WebSocket-Extensions" header of the physical connection,
that extension is applied to multiplexed connections unless otherwise
noted in the extension's spec.
If any extension is placed after this extension in the
"Sec-WebSocket-Extensions" header of the physical connection, that
extension is applied to frames after multiplexing on the sender side,
and before demultiplexing on the receiver side unless otherwise noted
in the extension's spec.
A client MAY request such an extension for both the physical
connection and multiplexed connections by placing extension entries
before and after this multiplexing extension. In this case, the
server SHOULD reject at least either of them if it's useless to apply
the same extension twice.
For example, if we have a compression extension called foo-compress,
the client sends
Sec-WebSocket-Extensions: foobar, mux, foobar
in the client's opening handshake of the physical connection to
request use of the foobar extension for both physical and multiplexed
connections. Then, the server would send back
Sec-WebSocket-Extensions: mux, foobar
to apply the foobar extension after multiplexing, or
Sec-WebSocket-Extensions: foobar, mux
to apply the foobar extension to multiplexed connections.
4.1. Ordering Extensions
4.1.1. Efficiency
Where to apply a compression extension makes difference to resource
consumption and flexibility. Compression algorithms often use some
memory to keep its context. Some of compression extensions may keep
using the same context for all the messages on the same connection.
If such a compression extension is applied to the physical
connection, intermediaries that want to demultiplex or multiplex the
connection need to decompress (before demultiplexing) and recompress
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(before multiplexing again) all the frames.
If such an compression extension is applied to each multiplexed
connection, we can control to which multiplexed connection we apply
the compression, so we can avoid applying compression to multiplexed
connections transferring incompressible data. Intermediaries that
want to demultiplex can forward payload leaving it untouched.
However, compressing each multiplexed connection is expensive in
terms of memory consumption.
4.1.2. Security
If any history-based compression extension such as DEFLATE is applied
to the physical connection that is tunneled over Transport Layer
Security (TLS) [RFC2818], it may spoil TLS's confidentiality [CRIME].
If the client may run malicious script such as a web browser, it MUST
NOT request use of the multiplexing extension and such a compression
extension in the order by which the compression extension is applied
to the physical connection side.
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5. Multiplexed Connections
The multiplexing extension maintains separate logical channels, each
of which provides fully the logical equivalent of an independent
WebSocket connection, including separate handshake headers. If the
multiplexing extension is successfully negotiated, one multiplexed
connection is automatically established, and the headers on the
opening handshake of the physical connection are automatically taken
to mean ones for the multiplexed connection. It's called "Implicitly
Opened Connection". It's served by the logical channel with ID of 1
which is also implicitly opened on completion of the opening
handshake. New logical channels are added by the client issuing the
AddChannelRequest multiplex control block (note that only the client
may initiate new WebSocket connections), including any handshake
headers which do not have the same value as the client's opening
handshake of the physical connection. The server's
AddChannelResponse likewise includes any handshake headers which are
different from the server's opening handshake of the physical
connection Channel 0 (control channel) is reserved for multiplex
control blocks and does not contain Payload Data from any multiplexed
connection. A client which attempts to add a channel to an existing
connection that is not accepted by the server SHOULD attempt to open
a new underlying connection and open a new WebSocket connection on
it.
Once the multiplexing extension is negotiated on a connection, all
frames of multiplexed connection MUST be prefixed with a channel ID
number and encapsulated into binary messages. The channel ID is
assigned by the client on issuing an AddChannelRequest.
A receiver MAY process frames for different non-control channels in
parallel. A receiver MUST process frames for the control channel
exclusively.
A receiver MUST _Fail the Physical Connection_ if any of these rules
are violated by the sender.
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6. Flow Control
6.1. New Channel Slot
A client has a pool of slots called "new channel slots". It's
initialized to be empty on establishment of the physical connection.
A NewChannelSlot multiplex control block sent by the server adds
slots to the pool.
Each slot has a non-negative integer value called "initial send
quota". Its function is explained in the later subsection.
When sending an AddChannelRequest, the client MUST pick the oldest
new channel slot from the pool and remove it from the pool. If there
are no slots in the pool, the client MUST NOT issue an
AddChannelRequest. An endpoint MUST _Fail the Logical Channel_ with
drop reason code of 2007 when it's clear that the other peer violates
this.
A server can regulate the rate of AddChannelRequests by not
replenishing the pool.
6.2. Send Quota
For each logical channel with non-zero ID, server and client are
respectively given a non-negative integer value called "send quota".
For the logical channel created for the "Implicitly Opened
Connection", the client's "send quota" is initialized to 0 on
establishment of the physical connection. The server's "send quota"
for the channel is initialized on sending its opening handshake for
the physical connection. The "quota" extension parameter attached to
the extension token for this multiplexing extension in the client's
opening handshake for the physical connection specifies the initial
value. If the "quota" extension parameter is not specified, the
initial value is set to 0. The extension parameter has the initial
value on its parameter value side as a non-negative integer in
decimal.
For a logical channel added by issuing an AddChannelRequest, a client
gets "send quota" equal to the "initial send quota" value on the "new
channel slot" picked for the AddChannelRequest on sending it.
For a logical channel added by accepting an AddChannelRequest, a
server gets "send quota" of 0 on sending the corresponding
AddChannelResponse.
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When an endpoint receives a FlowControl for a logical channel, its
"send quota" for the channel gets replenished.
An endpoint MUST NOT send a frame on a logical channel with non-zero
ID while the "send quota" of the endpoint for the channelis less than
the cost of the frame. The cost of a frame is sum of the following
two values:
o The length of the "Payload data" of the frame.
o Per-message extra cost. It's 1 if the frame is the first fragment
of a message. Otherwise, it's 0.
An endpoint MUST _Fail the Logical Channel_ with drop reason code of
3005 when it's clear that the other peer violates this.
When a frame is sent on a logical channel with non-zero ID, the
length of the "Payload data" of the frame is subtracted from the
"send quota" of the endpoint for the channel.
An endpoint SHOULD NOT delay replenishment of the other peer's "send
quota" for a logical channel when it has more room for accepting new
data for the channel.
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7. Framing
The multiplexing extension uses binary messages to transfer both data
for controlling multiplexing and data of multiplexed connections.
Binary messages have the logical channel ID tag field at the head of
them. Logical channel ID of 0 is designated for control channel
where multiplex control blocks are exchanged. Non-zero logical
channel IDs are used for non-control channels transferring data for
multiplexed connections.
The ID in the logical channel ID tag field is encoded as variable
number of bytes (1, 2, 3 or 4 octets), as follows:
0 1 2 3 4 5 6 7
+-+-------------+
|0|Channel ID(7)|
+-+-------------+
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+---------------------------+
|1|0| Channel ID (14) |
+-+-+---------------------------+
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-----------------------------------------+
|1|1|0| Channel ID (21) |
+-+-+-+-----------------------------------------+
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+---------------------------------------------------------+
|1|1|1| Channel ID (29) |
+-+-+-+---------------------------------------------------------+
This encoding is also used by multiplex control blocks where they
need to specify the ID of the objective channel.
A field using this encoding is considered to be invalid when more
than the minimal number of bytes necessary to represent the integer
is used.
Unless any other negotiated extension defines a meaning for them,
endpoints MUST NOT send any data message other than "binary". When
an endpoint received such a frame, it MUST _Fail the Physical
Connection_ with drop reason code of 2001.
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When an endpoint received a binary message with an incomplete or
invalid logical channel ID tag field at the head of the message,
_Fail the Physical Connection_ with drop reason code of 2002.
See Section 8 (non-control channel) and Section 9 (control channel)
for more details on fields that follow the logical channel ID tag
field.
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8. Encapsulation
This extension encapsulates each frame of a multiplexed connection
into a binary message with Payload Data obtained by concatenating the
following data in the order they are listed:
1. The logical channel ID tag field representing the ID of the
logical channel for the multiplexed connection.
2. FIN, RSV1, RSV2, RSV3 and opcode of the original frame.
3. Unmasked "Payload Data" of the original frame.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+
| Logical channel ID tag |
| (8/16/24/32) |
+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +
| Logical channel ID tag continued |
+-+-+-+-+-------+-----------------------------------------------+
|F|R|R|R| opcode| Payload Data of original frame |
|I|S|S|S| (4) | |
|N|V|V|V| | |
| |1|2|3| | |
+-+-+-+-+-------+ - - - - - - - - - - - - - - - - - - - - - - - +
: Payload Data of original frame continued ... :
+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +
: Payload Data of original frame continued ... :
+---------------------------------------------------------------+
These messages are called encapsulating messages.
A receiver MUST restore the original frame from the data and deliver
to the corresponding multiplexed connection based on the ID in the
logical channel ID tag field in the order they are received.
This extension MAY change the fragmentation of the original message
before encapsulation in order to insert multiplex control blocks or
adjust the amount of data to flush along with flow control.
When received an encapsulated frame with non continuation data opcode
though the last data message of that logical channel has not yet been
terminated by a fragment with the FIN bit set, the endpoint MUST
_Fail the Logical Channel_ with drop reason code of 3009.
When received an encapsulated frame with the continuation opcode
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though there's no preceding message that has not yet been terminated
on that logical channel, the endpoint MUST _Fail the Logical Channel_
with drop reason code of 3009.
On logical channels, control messages MAY also be fragmented.
Fragmented control messages MUST be delivered to the corresponding
multiplexed connection after receiving all fragments and
defragmenting them. For non-first fragments of a control message,
the continuation opcode (%x0) MUST be used for the opcode field as
well as data messages. Fragments for any other message MUST NOT be
injected between fragments of a control message on the same logical
channel. When a demultiplexer received an encapsulated frame with a
control opcode and the FIN bit unset, it MUST process encapsulated
frames on the same logical channel with the continuation opcode
following the encapsulated frame as fragments of the control message
until it encounters one with the FIN bit set. When a demultiplexer
encountered any encapsulated frame whose opcode is not continuation
injected between fragments of a control message on the same logical
channel, the endpoint MUST _Fail the Logical Channel_ with drop
reason code of 3009.
To allow for adjustment of fragmentation, this multiplexing extension
MUST NOT be used after any extension that does any of the followings:
o Require frame boundary on its output to be preserved.
o Use the "Extension data" field or any of the reserved bits on the
WebSocket header as per-frame attribute.
Intermediaries that doesn't understand the WebSocket Multiplexing
Extension MAY fragment the encapsulating messages.
When received a binary message with a non-zero logical channel ID of
an inactive channel (e.g. no channel has been opened for the logical
channel ID, or the channel has been closed (by a DropChannel or an
AddChannelResponse with the failure bit set) and not yet reopened),
the endpoint MUST ignore the message.
When received a binary message with a non-zero logical channel ID
which contains no octets in its payload after octets for the logical
channel ID tag field, the endpoint _Fail the Physical Connection_
with drop reason code of 2003.
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9. Multiplex Control Blocks
A binary message with the logical channel ID of 0 contains zero or
more multiplex control blocks in "Payload data".
0 1 2 3 4 5 6 7
+---------------+
|Channel ID of 0|
+---------------+
|1st multiplex |
:control block :
| |
+---------------+
|2nd multiplex |
:control block :
| |
+---------------+
|... |
: :
| |
+---------------+
Putting multiple control blocks into one WebSocket message saves
framing overhead.
Each multiplex control block has fields as follows:
0 1 2 3 4 5 6 7
+-----+---------+
| Opc | |
+-----+ :
| Opc specific :
: data :
| |
+---------------+
Opc
A multiplex control opcode as defined in the following
subsections. Opc of 5-7 are reserved for future use.
Opc specific data
Data interpreted according to that opcode.
Each of the following subsections describes one multiplex control
opcode and how to interpret opc specific data for that opcode.
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If any reserved opcode is set to opc, the endpoint MUST _Fail the
Physical Connection_ with drop reason code of 2004.
If any truncated multiplex control block is found, the endpoint MUST
_Fail the Physical Connection_ with drop reason code of 2005 unless
_Fail the Physical Connection_ is already done for any other error.
RSVs in the field diagrams of multiplex control blocks in this
section means reserved bits. If any multiplex control block with any
of the reserved bits set is found, the endpoint MUST _Fail the
Physical Connection_ with drop reason code of 2005 unless _Fail the
Physical Connection_ is already done for any other error.
An endpoint MAY process one or more multiplex control blocks without
any error that are in the same message together with one with an
error. This means that an endpoint doesn't have to inspect all the
blocks in the received message before starting processing blocks. An
endpoint MAY also skip processing multiplex control blocks without
any error that are in the same message together with one with an
error.
9.1. Number Encoding in Multiplex Control Blocks
In addition to the logical channel ID encoding defined in the
Section 7, we reuse the number encoding defined for payload length in
the Section 5.2 of [RFC6455] for multiplex control blocks with a
little modification. We call this number encoding "1/3/9 number
encoding". Integers up to 0x7D MUST be encoded into 1 octet field
containing the integer as is. Integers from 0x7E to 0xFFFF MUST be
encoded into an octet of 0x7E followed by two octets containing the
integer in network byte order. Integers from 0x10000 to
0x7FFFFFFFFFFFFFFF MUST be encoded into an octet of 0x7F followed by
eight octets containing the integer in network byte order. A field
using the 1/3/9 number encoding is considered to be invalid when any
of the following conditions is violated.
o The most significant bit of the first octet MUST be 0.
o The minimal number of bytes necessary to represent the integer
MUST be used.
o If the first byte is 0x7F, the most significant bit of the next
octet MUST be 0.
When received a multiplex control block with an invalid field using
the 1/3/9 number encoding, the endpoint MUST _Fail the Physical
Connection_ with drop reason code of 2005 unless _Fail the Physical
Connection_ is already done for any other error.
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9.2. AddChannelRequest
AddChannelRequest is sent only by clients to create a new logical
channel, as if a new WebSocket connection were received on a separate
transport connection.
When a client received an AddChannelRequest, it MUST _Fail the
Physical Connection_ with drop reason code of 2005 unless _Fail the
Physical Connection_ is already done for any other error.
Multiplex control opcode of AddChannelRequest is 0.
AddChannelRequest has fields as follows:
0 1 2 3 4 5 6 7
+-+-+-+-----+---+
|0|0|0| RSV |Enc|
+-+-+-+-----+---+
|Objective |
:channel ID :
|(1-4 octet) |
+---------------+
|Handshake size |
:(1-9 octet) :
| |
+---------------+
|Handshake |
: :
| |
+---------------+
Enc
Handshake encoding type:
0 - identity
The client's handshake data in the handshake field are sent
as-is without any special encoding or compression applied, and
constitute the complete set of a Request-Line, headers and the
CRLF after the last header that would have been sent on opening
handshake.
1 - delta-encoded
The client's handshake data in the handshake field are delta-
encoded, where any header that is not given is assumed to have
the same value as that given on the current delta base.
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The delta base is initialized to the client's opening handshake
of the physical connection but after modifying some headers as
follows:
+ Remove the token for this extension and following ones from
"Sec-WebSocket-Extensions". After this step,
"Sec-WebSocket-Extensions" contains only extensions applied
to the multiplexed connection.
+ Remove Upgrade, Sec-WebSocket-Key and Sec-WebSocket-Version
header. They are not meaningful under multiplexing.
Every time, an AddChannelRequest where Enc field is identity is
received, the delta base is updated to its handshake after this
modification. The Request-Line MUST be sent regardless if it's
the same as one in the delta base or not.
For multiple headers with the same header name, multiplexer and
demultiplexer MUST preserve the order in which they appeared.
If any header which had multiple header lines in the delta-base
is inherited, a demultiplexer MUST recover them in the same
order they appeared in the delta-base.
A header with an empty value means that the header is not
inherited from the delta base. If there were multiple headers
with the specified name, none of them is inherited. When to
send valueless headers, identity encoding MUST be used.
2-3 - reserved
Reserved for future use.
An endpoint MUST _Fail the Physical Connection_ with drop reason
code of 2010 if any of reserved type is specified.
Objective channel ID
The ID of the logical channel objective to this operation.
Encoding is the same as one used for the logical channel ID tag
field. An endpoint MUST _Fail the Physical Connection_ with drop
reason code of 2005 if this field is invalid.
Handshake size
The size of the handshake field encoded by the 1/3/9 number
encoding.
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Handshake
The client's opening handshake as defined in Section 4 of RFC 6455
[RFC6455] for the new multiplexed connection. Upgrade, Sec-
WebSocket-Key and Sec-WebSocket-Version header are excluded. This
field is encoded using the encoding specified by the Enc field.
An endpoint MUST _Fail the Physical Connection_ with drop reason
code of 2009 if any problem is found in parsing this field.
If the logical channel ID specified by an AddChannelRequest is in use
(including 0 for the control channel), it MUST _Fail the Physical
Connection_ with drop reason code of 2006.
To accept an AddChannelRequest, the endpoint MUST send an
AddChannelResponse with the failure bit unset and the objective
channel ID field set to the objective channel ID specified in the
AddChannelRequest. In this case, the channel becomes active.
To respond to an AddChannelRequest with status meaning handshake
failure, the endpoint MUST send an AddChannelResponse with the
failure bit set and its objective channel ID field set to the
objective channel ID specified in the AddChannelRequest. In this
case, the channel stays inactive.
An endpoint MAY reject an AddChannelRequest also by doing _Fail the
Logical Channel_ with drop reason code of 3000. In this case, the
channel stays inactive.
A server MAY delay responding to an AddChannelRequest and proceed to
process subsequent multiplex control blocks or frames for multiplexed
connections.
Channel ID assignment is done by client side. A client MAY use any
algorithm to choose logical channel IDs for new channels. Note that
logical channel ID assignment might be changed by intermediaries, so
it's not guaranteed that the value of logical channel ID is the same
on the other peer.
Different from non-multiplexed WebSocket connection, a client MAY
send frames of multiplexed connections except for "Implicitly Opened
Connection" before receiving AddChannelResponse as far as there's
sufficient send quota. In case the AddChannelRequest fails, those
frames are discarded by the other peer. This doesn't mean that users
of this protocol such as the WebSocket API are required to allow
their users to send frames before receiving the server's opening
handshake.
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9.3. AddChannelResponse
AddChannelResponse is sent only by servers in response to the
AddChannelRequest.
When a server received an AddChannelResponse, it MUST _Fail the
Physical Connection_ with drop reason code of 2005 unless _Fail the
Physical Connection_ is already done for any other error.
Multiplex control opcode of the AddChannelResponse is 1.
AddChannelResponse has fields as follows:
0 1 2 3 4 5 6 7
+-+-+-+-+---+---+
|0|0|1|F|RSV|Enc|
+-+-+-+-+---+---+
|Objective |
:channel ID :
|(1-4 octet) |
+---------------+
|Handshake size |
:(1-9 octet) :
| |
+---------------+
|Handshake |
: :
| |
+---------------+
F
Failure bit.
If the failure bit is not set, then the server has accepted the
AddChannelRequest. The handshake field MUST contain a response to
the request made by the AddChannelRequest, In this case, the
channel becomes active.
If the failure bit is set, then the server has rejected the
AddChannelRequest and this SHOULD be treated exactly the same as
if a separate connection was attempted and the connection was
closed after receiving the server's handshake. Enc MUST be set to
identity in this case. The handshake field MUST contain a
response to the request made by the AddChannelRequest. In this
case, the channel stays inactive. The sender of the
AddChannelResponse with the failure bit set doesn't have to send a
DropChannel following the AddChannelResponse.
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Enc
Encoding type.
Types are the same as ones defined for the AddChannelRequest
opcode (but replacing "AddChannelRequest" with
"AddChannelResponse", and "Request-Line" with "Response-Line").
The delta base is initialized to the server's opening handshake of
the physical connection but after modifying some headers as
follows:
* Remove the token for this extension and following ones from
"Sec-WebSocket-Extensions". After this step,
"Sec-WebSocket-Extensions" contains only extensions applied to
the multiplexed connection.
* Remove Upgrade and Sec-WebSocket-Accept header. They are not
meaningful under multiplexing.
An endpoint MUST _Fail the Physical Connection_ with drop reason
code of 2012 if any of reserved type is specified.
Objective channel ID
Same as one in the AddChannelRequest. If an inactive channel is
specified, the endpoint MUST ignore this AddChannelResponse.
An endpoint MUST _Fail the Physical Connection_ with drop reason
code of 2005 if this field is invalid.
Handshake size
The size of the handshake field encoded by the 1/3/9 number
encoding.
Handshake
The server's opening handshake as defined in Section 4 of RFC 6455
[RFC6455] for this multiplexed connection. Upgrade and Sec-
WebSocket-Accept header are excluded because they are not
meaningful under multiplexing. This field is encoded using the
encoding specified by the Enc field.
An endpoint MUST _Fail the Physical Connection_ with drop reason
code of 2011 if any problem is found in parsing this field.
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If the server's opening handshake is validated, the client MUST take
this as _The WebSocket Connection is Established_.
9.4. FlowControl
FlowControl is used to replenish the other peer's send quota for the
specified logical channel.
Multiplex control opcode of FlowControl is 2.
FlowControl has fields as follows.
0 1 2 3 4 5 6 7
+-+-+-+---------+
|0|1|0| RSV |
+-+-+-+---------+
|Objective |
:channel ID :
|(8-32 bit) |
+---------------+
|Replenished |
:send quota :
|(1-9 octet) |
+---------------+
Objective channel ID
Same as one in the AddChannelRequest. If an inactive channel is
specified, the endpoint MUST ignore this FlowControl. An endpoint
MUST _Fail the Physical Connection_ with drop reason code of 2005
if this field is invalid.
Replenished quota
The number of bytes the receiver can have outstanding towards the
sender of the FlowControl message. It's encoded by the 1/3/9
number encoding.
An endpoint MUST _Fail the Logical Channel_ with drop reason code of
3006 if its send quota for the channel exceeds 0x7FFFFFFFFFFFFFFF
when the replenished quota is added. The endpoint MAY delay this
_Fail the Logical Channel_ operation to process following multiplex
control blocks and encapsulating messages that don't affect the
logical channel. When received a FlowControl with an invalid value
in the replenished quota field, the endpoint MUST _Fail the Physical
Connection_ as specified above rather than taking it as overflow.
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9.5. DropChannel
DropChannel is used to close a logical channel.
Multiplex control opcode of DropChannel is 3.
DropChannel has fields as follows:
0 1 2 3 4 5 6 7
+-+-+-+---------+
|0|1|1| RSV |
+-+-+-+---------+
|Objective |
:channel ID :
|(1-4 octet) |
+---------------+
|Reason size |
:(1-9 octet) :
| |
+---------------+
|Reason |
: :
| |
+---------------+
Objective channel ID
Same as one in the AddChannelRequest. An endpoint MUST _Fail the
Physical Connection_ with drop reason code of 2005 if this field
is invalid.
Reason size
The size of the reason field encoded by the 1/3/9 number encoding.
A DropChannel block with 1-octet reason field MUST be considered
as a truncated multiplex control block.
Reason
The reason of closure. Reason MAY be empty. If reason is not
empty, the first two bytes MUST be a 2-byte unsigned integer (in
network byte order) representing a drop reason code. Following
the 2-byte integer, reason MAY contain UTF-8-encoded human
readable drop reason phrase.
When an endpoint received a DropChannel for an active non-control
channel, the endpoint MUST tear down the logical channel, and the
application instance that used the logical channel MUST treat this as
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closure of underlying transport.
When an endpoint received a DropChannel in response to an
AddChannelRequest, the endpoint MUST abort creation of the logical
channel, and the application instance that requested creation of the
logical channel MUST treat this as closure of underlying transport
without receiving reply for the creation request.
When an endpoint sent or received a DropChannel for an active non-
control channel, the endpoint MUST mark the channel as inactive. If
the endpoint is server and it has not already sent a DropChannel for
the channel, it MUST send a DropChannel with drop reason code of 3008
so that the client can mark the ID of the channel available for a new
AddChannelRequest.
Once received a DropChannel for a non-control channel, the ID of the
logical channel becomes available again for a new AddChannelRequest.
9.5.1. Drop Reason Codes
Drop reason codes are 4 digit unsigned integers.
1000-1999 are for normal closure on a logical channel without any
multiplexing level error. These codes are used for dropping non-
control channels.
1000 Normal closure
DropChannel with this drop reason code is commonly sent when
_Close the WebSocket Connection_ is made on the multiplexed
connection.
2000-2999 are for errors that _Fail the Physical Connection_. These
codes are used for dropping the control channel.
2000 Physical connection failed
Used if a more specific error is not available.
2001 Invalid encapsulating message
Received a data message with non binary opcode.
2002 Channel ID is truncated or invalid
Received an encapsulating message with a logical channel ID which
is truncated or invalid.
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2003 Encapsulated frame is truncated
Received an encapsulating message that contains only the logical
channel ID tag field with non-zero value.
2004 Unknown multiplex control opcode
Encountered a multiplex control block with unknown multiplex
opcode.
2005 Invalid multiplex control block
Encountered an invalid multiplex control block. E.g. objective
channel ID is truncated, reserved bit is raised.
2006 Channel already exists
Received an AddChannelRequest for an active logical channel.
2007 New channel slot violation
Received an AddChannelRequest though the other peer has no new
channel slot.
2008 New channel slot overflow
Received a NewChannelSlot that overflows the number of new channel
slots.
2009 Bad request
Received an AddChannelRequest with a malformed handshake.
2010 Unknown request encoding
Received an AddChannelRequest with an unknown encoding type.
2011 Bad response
Received an AddChannelResponse with a malformed handshake.
2012 Unknown response encoding
Received an AddChannelResponse with an unknown encoding type.
3000-3999 are for errors that _Fail the Logical channel_. These
codes are used for dropping non-control channels.
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3000 Logical channel failed
Used if a more specific error is not available.
3005 Send quota violation
Received an encapsulating message exceeding send quota.
3006 Send quota overflow
Received a FlowControl that overflows send quota.
3007 Idle timeout
Terminating an idle logical channel.
3008 DropChannel acknowledged
Used for a DropChannel sent in response to received DropChannel.
When a server received a DropChannel and it hasn't sent any
DropChannel for that logical channel, the server MUST send a
DropChannel with this reason code so that the client can release
the channel ID and reuse it for a new AddChannelRequest safely.
3009 Bad fragmentation
Received an encapsulating message with bad fragmentation that
cannot be delivered to the corresponding multiplexed connection.
4000-4999 are for requesting the other peer to take some actions.
These codes are used for dropping non-control channels.
4001 Use another physical connection
The server is requesting the client to open a new physical
connection and use it than adding any more logical channel until
receiving a NewChannelSlot. A client received this reason code
SHOULD NOT issue an AddChannelRequest on this physical connection
until receiving a NewChannelSlot.
4002 Busy
The server is requesting the client to stop issuing an
AddChannelRequest until receiving a NewChannelSlot. A client
received this reason code SHOULD NOT issue an AddChannelRequest on
this physical connection until receiving a NewChannelSlot.
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9.6. NewChannelSlot
NewChannelSlot is sent only by servers to add new slots to the
client's new channel pool.
When a server received an NewChannelSlot, it MUST _Fail the Physical
Connection_ with drop reason code of 2005 unless _Fail the Physical
Connection_ is already done for any other error.
Multiplex control opcode of NewChannelSlot is 4.
NewChannelSlot has fields as follows:
0 1 2 3 4 5 6 7
+-+-+-+-------+-+
|1|0|0| RSV |F|
+-+-+-+-------+-+
|Number of slots|
:(1-9 octet) :
| |
+---------------+
|Initial send |
:quota :
|(1-9 octet) |
+---------------+
F
Fallback bit.
If the fallback bit is false, normal slot is added.
If the fallback bit is true, fallback suggestion slot is added.
Number of slots field and initial quota field MUST be 0 for
fallback suggestion slot. When a client encounters a fallback
suggestion slot, it MUST open a new physical connection and use it
than adding any more logical channel on this physical connection
until any normal slot is available.
When received a NewChannelSlot block with the fallback bit set and
any of the number of slots field or the initial quota field is not
zero, the endpoint MUST _Fail the Physical Connection_ with drop
reason code of 2005 unless _Fail the Physical Connection_ is
already done for any other error.
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Number of slots
The number of slots to add. It's encoded by the 1/3/9 number
encoding. This value MAY be 0 when it makes sense.
Initial quota
The initial quota each of slots added by this NewChannelSlot gets.
It's encoded by the 1/3/9 number encoding.
When a client received a NewChannelSlot, the client MUST add new
slots of the specified number. Each of new slots gets the specified
initial send quota.
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10. Examples
_This section is non-normative._
The examples below assume the handshake has already completed and the
multiplexing extension was negotiated. Quotes are for clarity.
Frames of encapsulating messages from client to server MUST be
masked. The examples below are not masked for simplicity.
0x82 0x0d 0x01 0x81 "Hello world"
This is a non-fragmented text message of "Hello world" on logical
channel 1 encapsulated into a non-fragmented encapsulating
message.
0x82 0x07 0x01 0x01 "Hello" 0x82 0x08 0x01 0x80 " world"
This is a text message of "Hello world" fragmented into two frames
of "Hello" and " world" on logical channel 1 encapsulated into two
non-fragmented encapsulating messages. A multiplexer may change
fragmentation of a message before encapsulation like this so that
frames of other logical channels (including the control channel)
can be injected in the middle of the message.
0x82 0x07 0x01 0x01 "Hello" 0x82 0x05 0x02 0x81 "bye" 0x82 0x08 0x01
0x80 " world"
This example shows how data for two logical channels are
interleaved. There're three non-fragmented encapsulating
messages. As explained in the previous example, the text message
of "Hello world" is split into two frames before encapsulation.
The first and third frame in this example contain each of the two
fragments of the text message of "Hello world" on logical channel
1. The second frame contains a non-fragmented text message of
"bye" on logical channel 2.
0x82 0x04 0x01 0x01 "Te" 0x82 0x04 0x01 0x09 "Pi" 0x82 0x04 0x01 0x80
"ng" 0x82 0x04 0x01 0x80 "xt"
A ping message "Ping" is injected in the middle of a text message
"Text" on the original connection. The multiplexer fragmented the
ping message due to some reason into two fragments.
0x02 0x07 0x01 0x81 "Hello" 0x80 0x06 " world"
Encapsulating messages output from the multiplexer can be
fragmented by intermediaries without knowledge of the Multiplexing
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Extension. This is an example of a fragmented encapsulating
message. It's equivalent to the first example as a message.
0x82 0x16 0x00 0x01 0x02 0x12 "GET / HTTP/1.1" 0x0d 0x0a 0x0d 0x0a
This is a message on the control channel carrying one
AddChannelRequest. The first two octets are the WebSocket
headers. The 3rd octet is logical channel ID field of 0. The 4th
octet has opcode, RSV and Enc field. Objective channel ID is 2.
The handshake is delta-encoded.
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11. Client Behavior
When a client is asked to _Establish a WebSocket Connection_ by some
WebSocket application instance, it MAY choose to share an existing
WebSocket connection if all of the following are true:
o the multiplexing extension was successfully negotiated on that
connection
o the scheme portions of the URIs match exactly
o the host portions of the URIs either match exactly or resolve to
the same IP address (TBD: consider DNS rebind attacks)
o the port portions of the URIs (either explicit or implied by the
scheme) match exactly
o the connection has an availablle logical channel ID
If the client chooses to share the existing WebSocket connection with
multiplexing, it sends an AddChannelRequest as described above. If
the AddChannelRequest is accepted, WebSocket frames may be sent over
that logical channel as normal. If the server rejects the
AddChannelRequest, the client SHOULD attempt to open a new physical
WebSocket connection (for example, in a shared hosting environment a
server may not be prepared to multiplex connections from different
customers despite having a single IP address for them).
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12. Buffering
There will be lots of small frames sent in this protocol
(particularly replenishing send quotas), so a sender SHOULD attempt
to aggregate multiplex control blocks into larger WebSocket frames.
For data frames, a sender also SHOULD attempt to aggregate fragments
into one packet of the underlying transport. However, care must be
taken to avoid introducing excessive latency - the exact heuristics
for delaying in order to aggregate blocks is TBD.
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13. Fairness
A multiplexing implementation MUST ensure reasonable fairness among
the logical channels. This is accomplished in several ways:
Receiver side
o The receiver MAY limit the send quota of a logical channel by not
replenishing it to make sure that any logical channel doesn't
dominate the connection.
o Send quota for one logical channel SHOULD be determined
considering the processing capacity (buffer size, processing
power, throughput, etc.) of that logical channel. For example,
when a logical channel with excess load cannot drain data from the
connection smoothly, the other logical channels get stuck even
when they have room of processing capacity. Unless there's
special need to give such a big quota for the channel, such
condition just makes overall performance low.
Sender side
o The sender MUST use a fair mechanism for selecting which logical
channel's data to send in the next WebSocket message. Simple
implementations may choose a round-robin scheduler, while more
advanced implementations may adjust priority based on the amount
or frequency of data sent by each logical channel.
o The sender MUST fragment a large message into smaller frames to
prevent a large message in a logical channel occupying the
physical connection and thus delaying messages in other logical
channels.
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14. Proxies
Proxies which do not multiplex/demultiplex are not affected by the
presence of this extension -- they simply process WebSocket frames as
usual. Proxies which filter or monitor WebSocket traffic will need
to understand the multiplexing extension in order to extract the data
from logical connections or to terminate individual logical
connections when policy is violated. Proxies which actively
multiplex connections or demultiplex them (for example, a mobile
network might have a proxy which aggregates WebSocket connections at
a single cell to conserve bandwidth to the main gateway) will require
additional configuration (perhaps including the client) that is
outside the scope of this document.
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15. Timeout
When all the logical channels are closed, each endpoint MAY _Start
the WebSocket Closing Handshake_ on the physical connection. Such
_Start the WebSocket Closing Handshake_ operation SHOULD be delayed
assuming the physical connection may be reused after some idle
period.
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16. Close the Logical Channel
To _Close the Logical Channel_, an endpoint MUST send a DropChannel
multiplex control block with drop reason code of 1000.
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17. Fail the Logical Channel
To _Fail the Logical Channel_, an endpoint MUST send a DropChannel
multiplex control block with drop reason code in the range of 3000-
3999, tear down the logical channel, and the application instance
that used the logical channel MUST treat this as closure of
underlying transport.
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18. Fail the Physical Connection
To _Fail the Physical Connection_, an endpoint MUST send a
DropChannel multiplex control block with objective channel ID of 0
and drop reason code in the range of 2000-2999, and then _Fail the
WebSocket Connection_ on the physical connection with status code of
1011.
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19. Operations and Events on Multiplexed Connection
When an endpoint is asked to perform any operation defined in the
WebSocket Protocol except for _Close the WebSocket Connection_ by
some application instance, the endpoint MUST perform the operation on
the corresponding logical channel.
Any event on a logical channel except for _The WebSocket Connection
is Closed_, MUST be taken as one for the corresponding application
instance.
When an endpoint is asked to do _Close the WebSocket Connection_ by
some application instance, it MUST perform _Close the Logical
Channel_ on the corresponding logical channel.
When a DropChannel is received, or the physical connection is closed,
it MUST be taken as _The WebSocket Connection is Closed_ event for
the corresponding application instance(s).
What to set to _Extension In Use_ for each multiplexed connection is
TBD.
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20. Security Considerations
A client MUST be prepared to receive a NewChannelSlot with huge value
on the number of slots field.
As noted in the Section 4.1.2, be careful in using combination of any
compression extensions and this extension.
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21. IANA Considerations
This specification is registering a value of the Sec-WebSocket-
Extension header field in accordance with Section 11.4 of the
WebSocket protocol [RFC6455] as follows:
Extension Identifier
mux
Extension Common Name
Multiplexing Extension for WebSockets
Extension Definition
This document
Known Incompatible Extensions
None
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22. References
22.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol",
RFC 6455, December 2011.
22.2. Informative References
[CRIME] Rizzo, J. and T. Duong, "The CRIME attack", Ekoparty 2012,
September 2012.
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Authors' Addresses
John A. Tamplin
Google, Inc.
Email: jat@jaet.org
Takeshi Yoshino
Google, Inc.
Email: tyoshino@google.com
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