Internet DRAFT - draft-tamplin-hybi-google-mux
draft-tamplin-hybi-google-mux
HyBi Working Group J. Tamplin
Internet-Draft T. Yoshino
Intended status: Standards Track Google, Inc.
Expires: August 31, 2012 February 28, 2012
A Multiplexing Extension for WebSockets
draft-tamplin-hybi-google-mux-03
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 August 31, 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
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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 and Logical Channel . . . . . . . . . . . . . . . 3
2. Conformance Requirements . . . . . . . . . . . . . . . . . . . 4
3. Interaction with other Extensions / Framing Mechanisms . . . . 5
3.1. Choosing the point to apply an extension . . . . . . . . . 6
4. Logical Channels . . . . . . . . . . . . . . . . . . . . . . . 7
5. Flow Control . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Framing . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Multiplex Control Frames . . . . . . . . . . . . . . . . . . . 11
7.1. Multiplex Control Opcodes . . . . . . . . . . . . . . . . 12
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
9. Client Behavior . . . . . . . . . . . . . . . . . . . . . . . 17
10. Buffering . . . . . . . . . . . . . . . . . . . . . . . . . . 18
11. Fairness . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
12. Proxies . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
13. Nesting . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
14. Timeout . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
15. Close the Logical Channel . . . . . . . . . . . . . . . . . . 23
16. Fail the Logical Channel . . . . . . . . . . . . . . . . . . . 24
17. Fail the Physical Channel . . . . . . . . . . . . . . . . . . 25
18. Handling Operations On Logical Channel . . . . . . . . . . . . 26
19. Security Considerations . . . . . . . . . . . . . . . . . . . 27
20. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
21. Normative References . . . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30
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1. Overview
This document describes a multiplexing extension for the WebSocket
Protocol. 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 and Logical Channel
Under this extension, one transport connection is shared by multiple
application-level instances. The WebSocket connection which lies
directly on the transport connection and negotiated this multiplexing
extension is called "physical channel". Virtually established
WebSocket connections for each WebSocket appplication instances are
called "logical channels".
Data for different logical channels are distinguished by the channel
ID allocated in the "Extension data" portion of each frame.
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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 WebSocket payload data is masked by a per-frame key, such masking
is applied to frames for each logical channel separately.
If any extension (e.g. compression) is placed before this extension
in the "Sec-WebSocket-Extensions" header of the physical channel,
that extension is applied to logical channels unless otherwise noted
in the extension's spec.
If such an extension define 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 channel, 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 channel
and the logical channels 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 channel to request
use of the compression for both physical and logical channels. 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 logical channels.
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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.
If such an extension is applied to the physical channel,
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 logical channel, 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 logical
channel is expensive in terms of memory consumption.
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4. Logical Channels
The multiplexing extension maintains separate logical channels, each
of which is fully the logical equivalent of an independent WebSocket
connection, including separate handshake headers. If the
multiplexing extension is successfully negotiated, the headers on the
opening handshake of the physical channel are automatically taken to
mean one for the logical channel 1, which is implicitly opened by
completing the handshake. New channels are added by the client
issuing the AddChannel request (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 channel. The server's AddChannel response likewise
includes any handshake headers which are different from the server's
opening handshake of the physical channel (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 frames
and does not contain payload data from any logical channel. In
interpreting "Sec-WebSocket-Extensions" header for a logical channel,
the entry for this multiplexing extension is ignored but is used to
adjust parameters for the logical channel. 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 WebSocket connection.
If any inconsistency is found between the "Sec-WebSocket-Extensions"
header for the physical channel and one for a logical channel (after
decoding header compression), the server MUST reject the AddChannel
request.
Once the multiplexing extension is negotiated on a connection, all
frames must be prefixed with a channel ID number in the "Extension
data". Control frames with a channel ID 0 refer to the physical
channel, other control frames MUST be delivered on the logical
channel in order with data frames for that logical channel. Control
frames SHOULD be sent only on channel 0 where possible, though
control frames for other extensions in particular may need to apply
to individual logical channels.
A receiver MUST _Fail the Physical Channel_ if any of these rules are
violated by the sender.
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5. Flow Control
Each logical channel, including the implicitly created channel 1, is
initially given a quota of bytes that may be transmitted in each
direction without acknowledgement. It is illegal to send more bytes
than the remaining send quota, and the receiver MUST _Fail the
Logical Channel_ for any sender that does so. This send quota is
replenished via control frames as the receiver processes the data.
The initial send quota is specified with the "quota" extension
parameter, and defaults to 64k (TBD) if it is not specified. The
client and server each may specify a "quota" parameter and these are
unrelated -- each specifies how many bytes the other side may send
without acknowledgement. The quota values in the opening handshakes
of the physical channel apply to the implicitly opened channel 1.
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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
specified logical channel 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.
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 channel will close all logical channels
as well (details TBD).
If a frame doesn't contain valid channel ID, _Fail the Physical
Channel_. The cases where it's considered that the channel ID is
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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 Frames
Binary frames with a channel ID of 0 are multiplex control frames.
Unless another negotiated extension defines a meaning for them, any
data frames on channel 0 with an opcode other than "binary frame"
MUST _Fail the Physical Channel_ "Payload data" of a multiplex
control frames consists of a zero or more multiplex control blocks,
each defined as follows:
0 1 2 3 4 5 6 7
+---------------+
| Objective |
+ - - - - - - - +
: channel ID :
+ - - - - - - - +
| (8-32) |
+-----+---------+
| Opc | Opcdata |
+-----+---------+
| Additional |
+ - - - - - - - +
: data :
+ - - - - - - - +
| |
+---------------+
Objective channel ID
The channel ID of the logical channel objective to this operation.
Encoding is the same as that in the extension data (designated as
control channel)
opc
A multiplex control opcode as defined in Section 7.1.
opcdata
Data interpreted according to that opcode
Additional data
Zero or more bytes defined by that opcode
If any incomplete multiplex control block is found, _Fail the
Physical Channel_.
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7.1. Multiplex Control Opcodes
0 - AddChannel request (only from client)
Create a new logical channel, exactly as if a new connection were
received on a separate transport connection, except for the
encoding of the headers. opcdata is interpreted as follows:
3 4 5 6 7
+-+---+---+
|R|Enc|Len|
+-+---+---+
R is reserved for future use.
Len is the number of bytes used to represent the length of
following handshake data minus 1.
Enc is an encoding scheme type:
0 - uncompressed
The handshake data that follow are uncompressed, and constitute
the complete set of a Request-Line and headers that would have
been sent on a WebSocket opening handshake
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 client's opening handshake of the physical
connection. 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 client's opening
handshake for the physical channel. If the
"Sec-WebSocket-Extensions" header is not given, its value is
assumed to be the extension entry for this multiplexing
extension and ones following that in the client's opening
handshake of the physical channel. A header with an empty
value means that header is not inherited from the initial
connection. (TBD: this means that valueless headers cannot be
encoded with this scheme).
2-3 - reserved
Reserved for future use
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The following n bytes, where n is the value of len inside opcdata
plus 1, are an 8-32 bit length of the client's opening handshake
for the new logical channel that follows, in network byte order.
It's encoded as defined by the enc value in opcdata.
The initial quota for the new logical channel is 0, so the client
may not send any data for this connection until the AddChannel
response is received.
The server always responds with an AddChannel response, described
below.
1 - AddChannel response (only from server)
opcdata is defined as follows:
3 4 5 6 7
+-----+---+
|F|Enc|Len|
+-----+---+
F is true if this response indicates a rejection of AddChannel
request.
Len is the number of bytes used to represent the length of
following handshake data minus 1.
Enc is an encoding scheme type defined as in the AddChannel
request (but replacing Request-Line with Response-Line).
If F is set, then the server has rejected the AddChannel request
and this SHOULD be treated exactly the same as if a separate
connection was attempted and the opening handshake failed. Enc is
ignored in this case, and the following n bytes, where n is the
value of len inside opcdata plus 1, are an 8-32 bit length of the
server's opening handshake for this logical channel that follows,
in network byte order. It SHOULD be treated as the response to an
HTTP Upgrade request for the request made by the AddChannel
request, For example:
HTTP/1.1 404 Not found
404 message body...
If F is not set, then the server has accepted the AddChannel
request.
The following n bytes, where n is the value of len inside opcdata
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plus 1, are an 8-32 bit length of the server's opening handshake
for this logical channel that follows, in network byte order.
It's encoded according to enc as defined in the AddChannel
request, and 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_.
2 - FlowControl
opcdata is defined as follows:
3 4 5 6 7
+-----+---+
| RSV |Len|
+-----+---+
RSV is reserved for future use.
Len is the number of bytes used to represent the number of bytes
to be added to the quota minus 1.
The following n bytes, treated as an unsigned integer in network
byte order, is 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?)
3 - DropChannel
DropChannel is used to close a logical channel for both error
cases and normal cases.
3 4 5 6 7
+-+---+---+
|R|RSV|Len|
+-+---+---+
If R is set, it means that this DropChannel control block was sent
due to _Fail the Logical Channel_. If R is unset, it means that
this DropChannel control block was sent due to _Close the Logical
Channel_.
RSV is reserved for future use.
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Len is the number of bytes used to represent the length of
following reason data minus 1.
The following n bytes, where n is the value of len inside opcdata
plus 1, are an 8-32 bit length of the DropChannel reason string in
network byte order.
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.
4-7 - reserved
Reserved for future use (TBD: do we need some support for
quiescence?)
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8. Examples
_This section is non-normative._
The examples below assume the handshake has already completed and the
x-google-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 AddChannel request as described above. If the AddChannel
request 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 other peer's send quota of a logical
channel by not replenishing the send quota to make sure that any
logical channel cannot dominate its buffer space on the sender.
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 message into smaller frames when it's
too big so that that logical channel will occupy the connection
and the other logical channels get stuck for long time.
o Logical channel frames that are sent SHOULD be limited in size
(such as by refragmenting) when there is contention for the
physical channel to minimize head-of-line blocking
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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 channel 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 R bit unset. The endpoint MAY provide
the reason of closure 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 R bit set. The endpoint MAY provide the
reason of failure in the DropChannel block.
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17. Fail the Physical Channel
To _Fail the Physical Channel_, an endpoint MUST send a DropChannel
multiplex control block with objective channel ID of 0, and then
_Fail the WebSocket Connection_ on the physical channel with status
code of 1002 (TBD).
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18. Handling Operations On Logical Channel
When an endpoint is asked to perform any operation defined in the
WebSocket Protocol except for _Close the WebSocket Connection_ by
some application instance, it MUST perform it 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 and the logical channel hasn't yet
received DropChannel before that, it MUST be taken as _The WebSocket
Connection is Closed_ event for the corresponding application
instance.
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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
Mulplexing Extension for WebSockets
Extension Definition
This document [draft-tamplin-hybi-google-mux] defines the mux
extension.
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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