HyBi Working Group J. Tamplin
Internet-Draft T. Yoshino
Intended status: Standards Track Google, Inc.
Expires: December 27, 2012 June 25, 2012
A Multiplexing Extension for WebSockets
draft-ietf-hybi-websocket-multiplexing-02
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 December 27, 2012.
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. Interaction with other Extensions / Framing Mechanisms . . . . 5
3.1. Choosing the point to apply an extension . . . . . . . . . 5
4. Multiplexed Connections . . . . . . . . . . . . . . . . . . . 7
5. Flow Control . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. New Channel Slot . . . . . . . . . . . . . . . . . . . . . 9
5.2. Send Quota . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Framing . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7. Multiplex Control Blocks . . . . . . . . . . . . . . . . . . . 13
7.1. AddChannelRequest . . . . . . . . . . . . . . . . . . . . 14
7.2. AddChannelResponse . . . . . . . . . . . . . . . . . . . . 16
7.3. FlowControl . . . . . . . . . . . . . . . . . . . . . . . 17
7.4. DropChannel . . . . . . . . . . . . . . . . . . . . . . . 18
7.5. EncapsulatedControlFrame . . . . . . . . . . . . . . . . . 19
7.6. NewChannelSlot . . . . . . . . . . . . . . . . . . . . . . 20
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
9. Client Behavior . . . . . . . . . . . . . . . . . . . . . . . 22
10. Buffering . . . . . . . . . . . . . . . . . . . . . . . . . . 23
11. Fairness . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
12. Proxies . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
13. Nesting . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
14. Timeout . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
15. Close the Logical Channel . . . . . . . . . . . . . . . . . . 28
16. Fail the Logical Channel . . . . . . . . . . . . . . . . . . . 29
17. Fail the Physical Connection . . . . . . . . . . . . . . . . . 30
18. Operations and Events on Multiplexed Connection . . . . . . . 31
19. Security Considerations . . . . . . . . . . . . . . . . . . . 32
20. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
21. Normative References . . . . . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35
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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 tagging frames using
"Extension data". 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". Data channels virtually
established by ID tagging are called "logical channels". Control
information and data for each WebSocket appplication instances are
sent over corresponding logical channels.
Data for different logical channels are distinguished by the channel
ID allocated in the "Extension data" portion of each frame.
The opcode field in the base protocol's framing is used as-is to
indicate the type of a frame in transferring data frames of
multiplexed connections. Not to confuse intermediaries that don't
understand multiplexing extension, multiplex control blocks are used
to transfer control frames of multiplexed connections. For example,
it's possible that such intermediaries disconnect underlying
transport when they see a frame with opcode of close.
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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. 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 such an extension defines fields in the "Extension data", they
come after this multiplexing extension's field.
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.
If such an extension define fields in the "Extension data", they come
before this multiplexing extension's field.
A client MAY request such an extension for both the physical
connection and the 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: foo-compress, mux, foo-compress
in the client's opening handshake of the physical connection to
request use of the compression for both physical and multiplexed
connections. Then, the server would send back
Sec-WebSocket-Extensions: mux, foo-compress
to apply compression after multiplexing, or
Sec-WebSocket-Extensions: foo-compress, mux
to apply compression to multiplexed connections.
3.1. Choosing the point to apply an extension
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 frames on the same connection.
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If such an extension is applied to the physical connection,
intermediaries that want to demultiplex or multiplex the connection
need to decompress (before demultiplexing) and recompress (before
multiplexing again) all the frames.
If such an extension is applied to each multiplexed connection, we
can control to which channel we apply the compression, so we can
avoid applying compression to channels transferring incompressible
data. Intermediaries that want to demultiplex can forward
Application data field leaving it untouched. However, compressing
each multiplexed connection is expensive in terms of memory
consumption.
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4. 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 channel
ID of 1 which is also implicitly opened on completion of the opening
handshake. New 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 (the details of this are TBD, but a simple suggestion for
a delta encoding is given below). Channel 0 (control channel) is
reserved for multiplex control blocks and does not contain payload
data from any multiplexed connection. In interpreting
"Sec-WebSocket-Extensions" header for a multiplexed connection, the
entry for this multiplexing extension is ignored but is used to
adjust parameters for the 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.
If any inconsistency is found between the "Sec-WebSocket-Extensions"
header for the physical connection and one for a multiplexed
connection (after decoding header compression), a server MUST reject
the AddChannelRequest, and a client MUST _Fail the Physical
Connection_.
Once the multiplexing extension is negotiated on a connection, all
frames must be prefixed with a channel ID number in the "Extension
data". Channel ID is assigned by client on issuing
AddChannelRequest. Control frames with a channel ID 0 refer to the
physical connection. Control frames of multiplexed connection MUST
be delivered by EncapsulatedControlFrame multiplex control block.
Control frames with non-zero channel ID MUST NOT be sent. Control
frames SHOULD be sent only on channel 0 where
A receiver MAY process frames for different non-control logical
channels in parallel. A receiver MUST process frames for the control
channel exclusively.
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A receiver MUST _Fail the Physical Connection_ if any of these rules
are violated by the sender.
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5. Flow Control
5.1. 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.
A server can regulate the rate of AddChannelRequest by not
replenishing the pool.
5.2. 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. 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.
When an endpoint receives a FlowControl for a logical channel, its
"send quota" for the channel gets replenished.
When sending a frame on a logical channel with non-zero ID, the
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length of the "Payload data" of the frame MUST NOT be greater than
the "send quota" of the endpoint for the channel. An endpoint MUST
_Fail the Logical Channel_ 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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6. Framing
If the extension is successfully negotiated during the opening
handshake, all frames have a channel ID in the "Extension data". The
channel ID is encoded as a variable number of bytes, 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) |
+-+-+-+---------------------------------------------------------+
The base spec requires that a sequence of frames on the wire be a
sequence of valid fragments (or one of valid unfragmented frames).
The multiplexing extension relaxes this requirement to be for just
frames of one logical channel, and that frames of other logical
channels may be interleaved arbitrarily.
All frames with a non-zero channel ID MUST be delivered to the
corresponding multiplexed connection in the order they are received,
though fragmentation may be changed if appropriate. Control frames
with a non-zero channel ID may also trigger additional processing by
the multiplexing extension.
Intermediaries MAY fragment or defragment frames with the same
channel ID as far as they maintain resulting frames prefixed with one
valid channel ID. When multiple frames are defragmented into one
frame, channel ID field for second and later frames MUST be removed.
Frames MUST NOT be defragmented if there's any frame with channel ID
of 0 between them. When a frames is fragmented into multiple frames,
all the resulting fragments MUST prefixed with the channel ID.
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Control frames with a channel ID of 0 refer to the physical
connection, and may also trigger additional processing - for example,
a close frame on the physical connection will close all logical
channels as well (details TBD).
If a frame doesn't contain valid channel ID, _Fail the Physical
Connection_. The cases where it's considered that the channel ID is
invalid are:
o The "Payload data" portion doesn't contain a complete channel ID.
o No channel has been opened for the channel ID.
o The channel has been closed and not reopened.
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7. Multiplex Control Blocks
Binary frames with a channel ID of 0 contain zero or more multiplex
control blocks in "Payload data". If possible, putting multiple
control blocks into one WebSocket message saves framing overhead.
Unless another negotiated extension defines a meaning for them,
endpoints MUST NOT send any data frame with an opcode other than
"binary frame" on the control channel. If an endpoint received such
frame, it MUST _Fail the Physical Connection_.
Multiplex control blocks are sent in data frames, so they can be
fragmented and interleaved with other logical channels. Processing
of multiplex control blocks MUST be done when the entire message
containing them is received (and the original data become available
after decoding if any encoding is applied to the "Payload data"
portion).
Control blocks has fields as follows:
0 1 2 3 4 5 6 7
+---------------+
| Objective |
: channel ID :
| (8-32 bit) |
+-----+---------+
| Opc | |
+-----+ :
| Opc specific :
: data :
| |
+---------------+
Objective channel ID
The channel ID of the logical channel objective to this operation.
Encoding is the same as one used in the extension data (designated
as control channel).
Opc
A multiplex control opcode as defined in the following
subsections. Opc of 5-7 are Reserved for future use (TBD: do we
need some support for quiescence?).
Opc specific data
Data interpreted according to that opcode.
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Each of the following subsections describes one multiplex control
opcode and how to interpret opc specific data for that opcode.
If any incomplete multiplex control block is found, the endpoint MUST
_Fail the Physical Connection_.
7.1. AddChannelRequest
AddChannelRequest is used to create the objective channel, as if a
new connection were received on a separate transport connection,
except for the encoding of the headers. Multiplex control opcode of
AddChannelRequest is 0. AddChannelRequest is sent only from clients.
AddChannelRequest has fields as follows:
0 1 2 3 4 5 6 7
+-+-+-+-+---+---+
|0|0|0|R|Enc|Len|
+-+-+-+-+---+---+
|Size of encoded|
:handshake :
|(8-32 bit) |
+---------------+
|Encoded |
:handshake :
| |
+---------------+
R
Reserved for future use.
Len
The size of the size of encoded handshake field in bytes minus 1.
Enc
Encoding scheme type:
0 - identity
The handshake data that follow are sent as-is without any
special encoding or compression applied, and constitute the
complete set of a Request-Line and headers that would have been
sent on a WebSocket opening handshake.
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1 - delta-encoded
The handshake data that follow 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 handshake data. The delta
base is initially the client's opening handshake of the
physical connection. Every time, an AddChannelRequest with its
Enc equals to identity is received, the delta base is updated
to the handshake in the AddChannelRequest. The only exceptions
are the Request-Line and the "Sec-WebSocket-Extensions" header.
The Request-Line MUST be sent even if it's the same as one in
the delta base. If the "Sec-WebSocket-Extensions" header is
not given, its value is assumed to be the extension entry for
this multiplexing extension and ones follow it in in the delta
base. A header with an empty value means that header is not
inherited from the delta base. When to send valueless headers,
identity encoding MUST be used.
2-3 - reserved
Reserved for future use.
Size of encoded handshake
The size of the following encoded handshake field in bytes in
network byte order.
Encoded handshake
The client's opening handshake as defined in Section 4 of RFC 6455
[RFC6455] for the new multiplexed connection. This field is
encoded as defined in the description for the Enc field. The
complete set of a Request-Line and headers after decoding is
treated exactly as if it was sent on a separate connection.
If there's already a logical channel with the same channel ID as one
specified in this AddChannelRequest, it MUST _Fail the Physical
Connection_. Once the server receives an AddChannelRequest, it MUST
decide whether to accept or reject the request. To accept the
request, it MUST create a new logical channel with the channel ID set
to the objective channel ID field of the AddChannelRequest.
Channel ID assignment is done by the client. Client MAY use any
algorithm to choose channel IDs for new channels. Note that channel
ID assignment might be changed by intermediaries, so it's not
guaranteed that the value of channel ID is the same on the other
peer.
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The server MUST respond to the request with an AddChannelResponse,
described below.
The same limit on simultaneous opening handshake as specified in
Section 4.1 of [RFC6455] is applied to AddChannelRequests for
multiplexed channels.
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.
7.2. AddChannelResponse
AddChannelResponse is sent in response to the AddChannelRequest.
Multiplex control opcode of the AddChannelResponse is 1.
AddChannelResponse is sent only from servers. AddChannelResponse has
fields as follows:
0 1 2 3 4 5 6 7
+-+-+-+-+---+---+
|0|0|1|F|Enc|Len|
+-+-+-+-+---+---+
|Size of encoded|
:handshake :
|(8-32 bit) |
+---------------+
|Encoded |
:handshake :
| |
+---------------+
F
If F 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 opening handshake failed. Enc
MUST be set to identity in this case, and encoded handshake field
MUST contain the response to an HTTP Upgrade request for the
request made by the AddChannelRequest, For example:
HTTP/1.1 404 Not found
404 message body...
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If F is not set, then the server has accepted the
AddChannelRequest.
Len
The size of the size of encoded handshake field in bytes minus 1.
Enc
Encoding scheme type the same as defined for the AddChannelRequest
opcode (but replacing "AddChannelRequest" with
"AddChannelResponse", and "Request-Line" with "Response-Line").
Size of encoded handshake
The size of the following encoded handshake field in bytes in
network byte order.
Encoded handshake
The server's opening handshake as defined in Section 4 of RFC 6455
[RFC6455] for this multiplexed connection. This field is encoded
according to Enc. The complete set of a Response-Line and headers
after decoding is treated exactly as if it was received in
response to a client's opening handshake on a separate connection.
If the server's opening handshake is validated, the client MUST take
this as _The WebSocket Connection is Established_.
7.3. FlowControl
FlowControl is used to replenish the other peer's send quota for each
logical channel or new channel slot. Multiplex control opcode of
FlowControl is 2. FlowControl has fields as follows.
0 1 2 3 4 5 6 7
+-+-+-+-----+---+
|0|1|0| RSV |Len|
+-+-+-+-----+---+
|Replenished |
:quota :
|(8-32 bit) |
+---------------+
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RSV
Reserved for future use.
Len
The size of replenished quota field in bytes minus 1.
Replenished quota
An unsigned integer in network byte order added to the quota of
the number of bytes the receiver can have outstanding towards the
sender of the FlowControl message. (TBD: is it worth having some
non-linear encoding to reduce the average bits required to
represent these values?)
7.4. 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|F|RSV|Len|
+-+-+-+-+---+---+
|Size of reason |
:(8-32 bit) :
| |
+---------------+
|Reason |
: :
| |
+---------------+
F
F is set when this DropChannel is due to multiplexing level error.
F is unset when this DropChannel is sent because the multiplexed
connection is asked to _Close the WebSocket Connection_, and the
reason field MUST be empty for this case.
RSV
Reserved for future use.
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Len
The size of the size of reason field in bytes minus 1.
Size of reason
The size of the reason field in bytes in network byte order.
Reason
The reason of closure.
When an endpoint received DropChannel, the endpoint MUST remove the
logical channel and the application instance that used the logical
channel MUST treat this as closure of underlying transport. Once a
logical channel is removed by the DropChannel request, the channel ID
of the logical channel becomes available again for AddChannelRequest.
7.5. EncapsulatedControlFrame
EncapsulatedControlFrame is used to transfer control frames of
multiplexed connections. Multiplex control opcode of
EncapsulatedControlFrame is 4.
0 1 2 3 4 5 6 7
+-+-+-+---------+
|1|0|0| RSV |
+-+-+-+---------+
|Encapsulated |
:frame :
| |
+---------------+
RSV
Reserved for future use.
Encapsulated frame
One encapsulated control frame including the original WebSocket
header. MASK bit in the encapsulated control frame MUST be unset
and the "Payload data" in it MUST be unmasked.
When an endpoint received EncapsulatedControlFrame, the endpoint MUST
treat the control frame encapsulated in it as one received for the
corresponding multiplexed channel.
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7.6. NewChannelSlot
NewChannelSlot is sent only by the server-side. The objective
channel ID field for this multiplex control block MUST be 0.
NewChannelSlot adds new slots to the client's new channel pool.
Multiplex control opcode of NewChannelSlot is 5.
0 1 2 3 4 5 6 7
+-+-+-+-+---+---+
|1|0|1|R|NL |QL |
+-+-+-+-+---+---+
|Number of slots|
:(8-32 bit) :
| |
+---------------+
|Initial |
:send quota :
|(8-32 bit) |
+---------------+
R
Reserved for future use.
NL
The size of the number of slots field in bytes minus 1.
QL
The size of the initial quota field in bytes minus 1.
Number of slots
The number of slots to add in network byte order.
Initial quota
The initial quota each of slots added by this NewChannelSlot gets
in bytes in network byte order.
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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8. Examples
_This section is non-normative._
The examples below assume the handshake has already completed and the
mux extension was negotiated.
01 06 01 "Hello" 81 04 02 "bye" 80 07 01 " world"
This is a fragmented text message of "Hello world" on channel 1
interleaved with a text message of "bye" on channel 2. Note that
the sequence of opcodes/FIN bits cannot be understood without
considering the channel ID of each frame.
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9. Client Behavior
When a client is asked to _Establish a WebSocket Connection_ by some
WebSocket application instance, it MAY choose to reuse 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 reuse an existing multiplexed connection, it
sends an AddChannelRequest as described above. If the
AddChannelRequest is accepted, WebSocket frames may be sent over that
channel as normal. If the server rejects the AddChannel, 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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10. 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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11. 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 frame. 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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12. 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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13. Nesting
TBD: Should we allow nesting of multiplexed channels, or should we
require that an intermediary multiplexing channels flatten it? The
advantage of nesting is it is conceptually cleaner and less work for
an intermediary, while the disadvantage is that flow control messages
will get amplified by nesting and the ultimate server's job is a bit
more complicated to keep a tree of channel mappings.
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14. 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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15. Close the Logical Channel
To _Close the Logical Channel_, an endpoint MUST send a DropChannel
multiplex control block with F bit unset. The endpoint MAY provide
the reason of failure in the DropChannel block.
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16. Fail the Logical Channel
To _Fail the Logical Channel_, an endpoint MUST send a DropChannel
multiplex control block with F bit set. The endpoint MAY provide the
reason of failure in the DropChannel block.
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17. 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 F bit set, and then _Fail the WebSocket Connection_ on the
physical connection with status code of 1002 (TBD).
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18. 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. If the operation involves control
frames, they MUST be encapsulated in EncapsulatedControlFrame
multiplex control blocks.
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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19. Security Considerations
To protect a server from denial-of-service attack, implementation
SHOULD have a way to limit the number of concurrent logical channels.
TBD
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20. 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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21. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol",
RFC 6455, December 2011.
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Authors' Addresses
John A. Tamplin
Google, Inc.
Email: jat@google.com
Takeshi Yoshino
Google, Inc.
Email: tyoshino@google.com
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