The MessageBroker WebSocket Subprotocol
draft-hapner-hybi-messagebroker-subprotocol-01
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| Last updated | 2012-03-27 | ||
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draft-hapner-hybi-messagebroker-subprotocol-01
Network Working Group M. Hapner, Ed.
Internet-Draft Huawei
Intended status: Standards Track C. Suconic
Expires: September 28, 2012 redhat
March 27, 2012
The MessageBroker WebSocket Subprotocol
draft-hapner-hybi-messagebroker-subprotocol-01
Abstract
The WebSocket protocol [I-D.ietf-hybi-thewebsocketprotocol] provides
a subprotocol extension facility. The MessageBroker WebSocket
Subprotocol (MBWS) is a WebSocket Subprotocol used by messaging
clients to send messages to, and receive messages from an internet
message broker (herein called a message broker). A message broker is
a messaging intermediary that queues messages sent by its clients for
asynchronous delivery to its clients.
Messages are addressed to message-broker-specific address names.
Clients send messages to addresses and consume messages from
addresses. Clients do not send messages directly to other clients.
Message brokers provide a range of functionality that is outside the
scope of MBWS. Typically an internet message broker provides a REST
API for working with this functionality; such as configuring client
credentials; setting client access controls; configuring address
routing; etc.
MBWS limits its scope to the definition of a WebSocket subprotocol
that provides a full duplex, reliable message transport protocol
between message brokers and their clients; and, between message
brokers.
Since reliable message transport is often independent of a broker's
particular features, MBWS can be used as the message transport
protocol for a wide range of message brokers.
The MBWS subprotocol defines a binary message frame and a text
message frame. Both types of frame carry the same protocol; however,
the protocol bindings differ slightly. The binary frame is a
WebSocket binary message that contains an MBWS binary header followed
by a binary message body. The text frame is a WebSocket UTF-8 text
message that contains an MBWS text header followed by a text message
body.
Status of this Memo
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This Internet-Draft will expire on September 28, 2012.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. MBWS Functionality . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Connection Recovery . . . . . . . . . . . . . . . . . . . 5
2.1.1. MBWS Connections . . . . . . . . . . . . . . . . . . . 5
2.1.2. MBWS Connection Name and Connection Recovery . . . . . 5
2.1.3. Message Synchronization of a Recovered MBWS
Connection . . . . . . . . . . . . . . . . . . . . . . 6
2.1.3.1. Broker-message-delivery-resync . . . . . . . . . . 7
2.1.3.2. Client-message-delivery-resync . . . . . . . . . . 7
2.1.4. MBLWS Connections . . . . . . . . . . . . . . . . . . 7
2.1.5. Message Metadata . . . . . . . . . . . . . . . . . . . 7
2.1.5.1. Address List . . . . . . . . . . . . . . . . . . . 8
2.1.5.1.1. Undeliverable Messages . . . . . . . . . . . . 8
2.1.5.2. Content-Type . . . . . . . . . . . . . . . . . . . 8
2.1.5.3. Property List . . . . . . . . . . . . . . . . . . 8
3. Additional Issues . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Sec-WebSocket-Protocol Field . . . . . . . . . . . . . . . 9
3.2. Client Identity . . . . . . . . . . . . . . . . . . . . . 9
3.3. Message Security . . . . . . . . . . . . . . . . . . . . . 9
3.4. Empty Protocol Values . . . . . . . . . . . . . . . . . . 9
4. MBWS/MBLWS Protocol ABNF . . . . . . . . . . . . . . . . . . . 9
5. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1. MBWS Connection Recovery Scenario . . . . . . . . . . . . 11
5.2. MBLWS Session Scenario . . . . . . . . . . . . . . . . . . 12
6. Issues Outside the Scope of this Document . . . . . . . . . . 12
6.1. Messaging Scope . . . . . . . . . . . . . . . . . . . . . 12
6.2. Message Acknowledgement Interval . . . . . . . . . . . . . 12
6.3. Synchronous Messaging . . . . . . . . . . . . . . . . . . 12
6.4. End-to-End Reliability . . . . . . . . . . . . . . . . . . 13
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
The WebSocket protocol [I-D.ietf-hybi-thewebsocketprotocol] provides
a subprotocol extension facility. The MessageBroker WebSocket
Subprotocol (MBWS) is a WebSocket Subprotocol used by messaging
clients to send messages to, and receive messages from an internet
message broker (herein called a message broker). A message broker is
a messaging intermediary that queues messages sent by its clients for
asynchronous delivery to its clients.
Messages are addressed to message-broker-specific address names.
Clients send messages to addresses and consume messages from
addresses. Clients do not send messages directly to other clients.
Message brokers provide a range of functionality that is outside the
scope of MBWS. Typically an internet message broker provides a REST
API for working with this functionality; such as configuring client
credentials; setting client access controls; configuring address
routing; etc.
MBWS limits its scope to the definition of a WebSocket subprotocol
that provides a full duplex, reliable message transport protocol
between message brokers and their clients; and, between message
brokers.
Since reliable message transport is often independent of a broker's
particular features, MBWS can be used as the message transport
protocol for a wide range of message brokers.
The MBWS subprotocol defines a binary message frame and a text
message frame. Both types of frame carry the same protocol; however,
the protocol bindings differ slightly. The binary frame is a
WebSocket binary message that contains an MBWS binary header followed
by a binary message body. The text frame is a WebSocket UTF-8 text
message that contains an MBWS text header followed by a text message
body.
2. MBWS Functionality
MBWS subprotocol defines two capabilities:
o Connection Recovery - the ability to support a logical, reliable
connection that spans a sequence of WebSocket sessions (herein,
such a connection is called a 'connection')
o Message Metadata - the ability to annotate a WebSocket message
with metadata to support the functionality of a message broker
This document defines two subprotocols - MessageBroker WebSocket
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Subprotocol (MBWS) and MessageBrokerLight WebSocket Subprotocol
(MBLWS). MBWS supports both Connection Recovery and Message
Metadata. MBLWS supports only Message Metadata.
The protocol description defines the logical MBWS and MBLWS
subprotocols. The protocol ABNF [RFC5234] defines the binding of
these protocols to MBWS binary frames and text frames. MBLWS uses
the same frames as MBWS.
2.1. Connection Recovery
When a WebSocket is normally closed, both client and server can
assume the messages they sent/delivered have been received by the
other party.
The reliability of the WebSocket's underlying TCP connection,
combined with the WebSocket close protocol, insures that both parties
have implicitly acknowledged the receipt of the all messages they
have been sent.
If a WebSocket session fails, the protocol does not define how the
parties resolve what messages have been received and what messages
have been lost. In many cases, this is not an issue; however,
message brokers typically provide once-and-only-once QoS and
WebSocket alone is not sufficient to support this.
MBWS defines a Connection Recovery subprotocol that allows a message
broker client whose connection's session has failed to create a new
WebSocket session that extends the connection and reliably
resynchronizes its full duplex message transport such that no
messages are lost or duplicated.
2.1.1. MBWS Connections
MBWS defines a connection that spans a sequence of one or more
WebSocket sessions. During the time period between the failure of
one of its sessions and the creation of its next session, its parties
must maintain the state required to recover the connection. Since
messages may be lost when a session fails, this state must contain a
window of recently sent messages. MBWS provides support for
identifying connections; maintaining recently sent message windows;
recovering a connection on a new session; and, resynchronizing a
recovered connection's message transport.
2.1.2. MBWS Connection Name and Connection Recovery
When a client requests a new connection it sends a Connect frame with
an empty connection name. The server must respond with a Connect
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frame containing the name of a new connection. The MBWS client must
retain this connection name so that it can be used later to recovery
this connection if this connection's current WebSocket session were
to fail. If a connection's WebSocket session is closed, as defined
by the WebSocket close protocol, it also closes the connection. It
is recommended but not required that connection name be a URN.
When a client requests the recovery of a connection, it sends a
Connect frame containing the name of the connection to be recovered.
The message broker must then respond with a Connect frame containing
a connection name. If this connection name matches the value sent by
the client, the server has accepted the recovery request. If the
name does not match, the server has rejected the recovery request and
has opened a new connection.
Connection's are identified by a combination of client origin and
connection name. Only the client origin that opened the connection
can recover the connection.
2.1.3. Message Synchronization of a Recovered MBWS Connection
MBWS requires clients and message brokers to use an implicit sequence
numbering protocol for the messages transported by a connection.
Each direction of transport defines a separate sequence. The first
message sent by each party is sequence number 1, the next is 2, etc.
Since both parties are guaranteed to see the messages in the order
sent, no explicit exchange of sequence numbers is required.
Both parties must acknowledge receipt of messages they receive. This
is done by sending an Acknowledge frame with the sequence number of
the last message reliably received. When a sending party receives an
Acknowledge frame from its receiving party, the sending party can
delete from its message recovery window all messages with sequence
numbers less than or equal to the Acknowledge sequence number.
If a session abnormally terminates and a message broker accepts a
client's request to recover the connection, both client and message
broker must verify that they can resume sending messages with the
message sequence number required by each. Connection recovery
message resynchronization is a serial two phase process. First, the
client provides the message broker with the information required for
the broker to restart message delivery. This phase is named broker-
message-delivery-resync. Second, the message broker provides the
client with information required for the client to restart message
delivery. This phase is named client-message-delivery-resync.
If both broker-message-delivery-resync and client-message-delivery-
resync succeed, the connection has been recovered. If either fails,
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a new connection is opened.
2.1.3.1. Broker-message-delivery-resync
The client sends an Acknowledge frame containing the sequence number
of the last message it has received. The message broker validates it
can resume sending with the next message in sequence.
If so, the message broker must reply with a Connect frame containing
the connection name being recovered. Connection recovery then
proceeds with the client-message-deliveryresync phase.
If the message broker cannot restart with this message, it must reply
with a Connect frame with a new connection name. Message transport
then begins on this new connection.
2.1.3.2. Client-message-delivery-resync
The message broker sends the client an Acknowledge frame containing
the sequence number of the last message it has received. The client
validates it can resume sending with the next message in sequence.
If so, the client must reply with a Connect frame containing the
connection name being recovered. This completes a successful
connection recovery and normal full duplex message transport resumes.
If the client cannot restart with this message, it must reply with a
Connect frame containing an empty connection name. The message
broker must then respond with a Connect frame containing a new
connection name. Message transport then begins on this new
connection.
2.1.4. MBLWS Connections
An MBLWS client opens a new connection with the same Connect frame
protocol as used by an MBWS client. MBLWS does not support
connection recovery. MBLWS connections do not span WebSocket
sessions. If an MBLWS client sends a Connect frame containing a
connection name, the connection name must be ignored and a new
connection must be opened. MBLWS connections do not use
Acknowledgement frames. If an MBLWS client sends an Acknowledgement
frame, it must be ignored. If connection's WebSocket session fails
or is closed, the connection is closed.
2.1.5. Message Metadata
MBWS and MBLWS define a message header containing three metadata
elements. In order, these are Address List, Content-Type and
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Property List.
2.1.5.1. Address List
For messages sent by a client to a broker, the Address List contains
the list of destination Addresses to which to send the message.
Empty Addresses are ignored. For messages delivered by a message
broker to a client, Address List contains the addresses from which
the message originated.
It is recommended but not required that address value be a URN.
The format and semantics of Address is message broker dependent and
is outside the scope of MBWS. For instance, some brokers may treat
Address as a strictly local name; other brokers may support a more
global form of addressing. Broker-specific message routing semantics
determine how a destination Address's messages are to be routed and
how message's origination Addresses are determined. This includes
defining the meaning of an empty destination Address List and an
empty origination Address List.
2.1.5.1.1. Undeliverable Messages
An messages's Address may not be known to a broker. MBWS does not
define how such dead-letters are handled once they are received by a
message broker. MBWS requires a message broker to acknowledge every
message sent to it, whether or not it can deliver it.
2.1.5.2. Content-Type
Immediately following Address List, a message header contains a
Content-Type. Its value is a UTF-8 string containing the MIME
discrete type [RFC2045] that describes the message's content.
Content-Type may be empty.
2.1.5.3. Property List
Immediately following Content-Type, a message header contains a
Property List. This list contains zero or more Properties. Each
Property is a Name/Value pair with each being a UTF-8 string. MBWS
does not define the semantics of Properties.
3. Additional Issues
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3.1. Sec-WebSocket-Protocol Field
Sec-WebSocket-Protocol Field Values
+------------------+
| Value |
+------------------+
| MBWS.huawei.com |
| MBLWS.huawei.com |
+------------------+
WebSocket defines the subprotocol negotiation process. This starts
with a client including the Sec-WebSocket-Protocol Field with one or
more subprotocol names in its WebSocket upgrade request. The table
above specifies the values for the two subprotocols defined in this
document.
3.2. Client Identity
WebSocket uses the HTTP origin model to identify clients. MBWS uses
the same client identity model.
3.3. Message Security
WebSocket supports TLS and MBWS/MBLWS recommends, but does not
require, its use. In addition to providing better security the use
of TLS and port 443 insures that MBWS connections avoid the overhead
and latency of having to traverse web proxies.
3.4. Empty Protocol Values
In several places, the protocol refers to an 'empty' UTF-8 string
element. In MBWS, UTF-8 string protocol elements are length-
delimited. An 'empty' element is one with a zero valued length
delimiter.
4. MBWS/MBLWS Protocol ABNF
mbws-frame = binary-frame / text-frame
;the frame used with a WS binary message
binary-frame =
binary-connect-frame / binary-acknowledge-frame / binary-message-frame
binary-connect-frame = binary-connect-frame-id binary-connection-name
binary-connect-frame-id = %x01
binary-connection-name = binary-string
binary-acknowledge-frame =
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binary-acknowledge-frame-id binary-message-sequence-number
binary-acknowledge-frame-id = %x02
binary-message-sequence-number = varint
binary-message-frame =
binary-message-frame-id binary-message-header binary-message-body
binary-message-frame-id = %x03
binary-message-header =
binary-address-list binary-content-type binary-property-list
binary-address-list = binary-list-length *binary-address
binary-address = binary-string
binary-content-type = binary-string
binary-property-list = binary-list-length *binary-property
binary-property = binary-property-name binary-property-value
binary-property-name = binary-string
binary-property-value = binary-string
binary-message-body = *OCTET
;the frame used with a WS text message
text-frame =
text-connect-frame / text-acknowledge-frame / text-message-frame
text-connect-frame = text-connect-frame-id text-connection-name
text-connect-frame-id = %x31 SP
text-connection-name = text-string
text-acknowledge-frame =
text-acknowledge-frame-id text-message-sequence-number
text-acknowledge-frame-id = %x32 SP
text-message-sequence-number = text-int
text-message-frame =
text-message-frame-id text-message-header text-message-body
text-message-frame-id = %x33 SP
text-message-header =
text-address-list text-content-type text-property-list
text-address-list = text-list-length *text-address
text-address = text-string
text-content-type = text-string
text-property-list = text-list-length *text-property
text-property = text-property-name text-property-value
text-property-name = text-string
text-property-value = text-string
text-message-body = UTF8-string
;UTF8 encoded character string
UTF8-string = *(OCTET)
;Google Protocol Buffers base 128 varint
varint = 1*8(OCTET)
;the number of characters in a UTF8 string
binary-string-length = varint
binary-string = binary-string-length UTF8-string
;the number of entries in a list
binary-list-length = varint
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text-int = DIGIT *DIGIT SP
;the number of characters in a UTF8 string
text-string-length = text-int
text-string = text-string-length UTF8-string
;the number of entries in a list
text-list-length = text-int ;the number of entries in a list
Figure 1
5. Scenarios
5.1. MBWS Connection Recovery Scenario
1. Broker provides 'ws:' and/or 'wss:' URIs for accepting MBWS
connections.
2. Client establishes an HTTP session with Broker; identifies
itself using HTTP client origin; and, authenticates itself using
HTTP authentication.
3. If successful, Client requests HTTP upgrade to MBWS Subprotocol.
4. If upgrade successful, Client sends Connect frame with empty
connection name.
5. Broker responds with Connect frame containing a new connection
name.
6. Broker starts streaming messages to client; and, Client starts
streaming messages to Broker.
7. Client and Broker periodically acknowledge receipt of each
other's messages using Acknowledge frames.
8. Client or Broker may initiate session close as defined by
WebSocket.
9. If session abnormally terminates, client recovers connection by
executing (1) through (3) and then continues with (10)
10. Client sends Connect frame containing connection name it wishes
to recover
11. Broker responds with Connect frame. If Connect frame contains a
new connection name, broker has rejected recovery and opened a
new connection, processing continues with (6). If Connect frame
contains recovery connection name, broker has accepted recovery.
12. Client sends Acknowledge frame containing the sequence number of
the last message it has received.
13. Broker responds with Connect frame. If Connect frame contains a
new connection name, broker has rejected recovery and opened a
new connection, processing continues with (6). If Connect frame
contains recovery connection name, broker has accepted recovery.
14. Broker sends Acknowledge frame containing the sequence number of
the last message it has received.
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15. Client responds with Connect frame. If Connect frame contains
an empty connection name, client has rejected recovery and
processing continues with (5). If the connection name is the
recovery connection name, processing continues at (6)
5.2. MBLWS Session Scenario
1. Broker provides 'ws:' and/or 'wss:' URIs for accepting MBLWS
sessions.
2. Client establishes an HTTP session with Broker; identifies itself
using HTTP client origin; and, authenticates itself using HTTP
authentication.
3. Broker starts streaming available messages to client; and, Client
starts streaming messages to Broker.
4. Client or Broker may initiate session close as defined by
WebSocket.
6. Issues Outside the Scope of this Document
_This section is non-normative._
6.1. Messaging Scope
Message brokers provide message-broker-specific functionality for
routing, queueing, forwarding, filtering, transporting, etc.
messages. This results in the broker delivering specific messages to
specific clients. This document defines how a message broker uses
the subprotocols defined here to transport messages to/from a client.
All other message broker functionality is outside the scope of this
document.
6.2. Message Acknowledgement Interval
The parties of an MBWS connection decide when to send Acknowledge
frames. Typically these are sent after some number of messages have
been received or some time interval has elapsed within which at least
one message has been received. The choice of acknowledgement
interval is outside the scope of this document.
6.3. Synchronous Messaging
Message brokers have a history of supporting synchronous messaging
where clients make blocking calls to send and to receive messages.
WebSocket and MBWS are natively asynchronous messaging protocols.
MBWS is optimized for asynchronous, full duplex message transport.
It has not been designed for synchronous messaging.
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6.4. End-to-End Reliability
The responsibility for reliable message delivery over a MBWS
connection is not the responsibility of the message broker alone - it
is only achieved when both clients and brokers implement recovery of
connections. The degree to which clients and message brokers are
able to recover from failure is outside the scope of this document.
7. References
[I-D.ietf-hybi-thewebsocketprotocol]
Fette, I. and A. Melnikov, "The WebSocket protocol",
draft-ietf-hybi-thewebsocketprotocol-17 (work in
progress), September 2011.
[GPBE] "Google Protocol Buffers Encoding <http://code.google.com/
apis/protocolbuffers/docs/encoding.html>".
[RFC2045] Freed, N. and Borenstein, N., "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", November 1966.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", January 2008.
Authors' Addresses
Mark Hapner (editor)
Huawei
Email: mhapner@huawei.com
Clebert Suconic
redhat
Email: csuconic@redhat.com
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