rfc5573
Network Working Group M. Thomson
Request for Comments: 5573 Andrew
Category: Experimental June 2009
Asynchronous Channels for the Blocks Extensible Exchange Protocol (BEEP)
Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
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document authors. All rights reserved.
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Abstract
The Blocks Extensible Exchange Protocol (BEEP) provides a protocol
framework for the development of application protocols. This
document describes a BEEP feature that enables asynchrony for
individual channels.
Table of Contents
1. Introduction ....................................................2
2. Asynchronous BEEP Channels ......................................3
2.1. Asynchronous Feature .......................................3
2.2. Starting an Asynchronous Channel ...........................4
2.3. Asynchronous Channel Behavior ..............................5
2.4. Error Handling .............................................6
3. Alternatives ....................................................6
3.1. Increasing Throughput ......................................6
3.2. Asynchrony in the Application Protocol .....................7
4. Security Considerations .........................................7
5. IANA Considerations .............................................8
6. References ......................................................8
6.1. Normative References .......................................8
6.2. Informative References .....................................8
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1. Introduction
The Blocks Extensible Exchange Protocol (BEEP) provides a protocol
framework that manages many of the aspects necessary in developing an
application protocol: framing, encoding, privacy, authentication, and
asynchrony. However, the asynchrony provided by BEEP is limited to
asynchrony between channels; replies to messages sent on any channel
are strictly ordered.
Serial processing behavior is desirable for a range of applications.
However, serial processing is less suitable for applications that
rely more heavily on asynchrony. In particular, if a response takes
a significant amount of time to create, the channel is effectively
blocked until the request has been processed and the response sent.
Pipelining only ensures that network latency does not add to this
time; subsequent requests cannot be processed until a response is
made to the first request.
Asynchronous applications require a protocol that is able to support
a large number of concurrent outstanding requests. The analogy of a
channel as a thread does not scale to the large number of threads
used in modern systems. Modern applications regularly have large
numbers of concurrent processing threads. Thus, a better way of
multiplexing large numbers of concurrent requests is required.
This document describes a BEEP feature, an extension to BEEP, that
enables the creation of an asynchronous channel. An asynchronous
channel is a channel where response ordering is not fixed to the
order of the requests sent by the client peer. An asynchronous
channel is identical to other channels, using unmodified framing;
except that requests may be processed in parallel and responses may
be sent in any order.
An asynchronous channel enables the efficient use of a single channel
for multiple concurrent requests. There is no impact on requests
arising from the timing of responses to other requests. The
requesting peer can process responses to the requests it sends as
they come available; similarly, the serving peer can take advantage
of parallel processing without artificial constraints on the order of
responses.
Asynchronous channels allow for greater throughput where the serving
peer requires any time to process requests. This is particularly
relevant where the serving peer needs to perform lengthy computations
or make network-based requests as a part of servicing the request.
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BEEP feature negotiation is used to ensure that both peers are
mutually willing to create asynchronous channels. A means for
establishing an asynchronous channel is described.
This document is published as an Experimental RFC in order to find
out whether the extension is going to be deployed for use in a
variety of use cases and applications.
2. Asynchronous BEEP Channels
This document defines a BEEP feature that enables the use of
asynchronous channels. An asynchronous channel is a BEEP channel
that is not subject to the restrictions of Section 2.6.1 of [RFC3080]
regarding ordering of responses; requests can be processed and
responded to in any order by the serving peer.
Asynchronous channels use the "msgno" element of the BEEP frame
header to correlate request and response. Regular BEEP channels do
not use "msgno" for request/response correlation, contrary to what
might be inferred by the presence of the parameter. In a regular
BEEP channel, the "msgno" only serves as a means of checking for
protocol errors.
Asynchronous channels are not suitable for applications where state
established by requests is relied upon in subsequent requests or the
ordering of messages is significant.
2.1. Asynchronous Feature
The "feature" attribute in the BEEP greeting contains a whitespace-
separated list of features supported by each peer. If both lists
contain the same feature, that feature may be used by either peer.
This document registers the feature "async". If either peer does not
include this feature in the greeting message, neither peer is able to
create an asynchronous channel.
Figure 1 shows an example exchange where both peers declare
willingness to use this feature.
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L: <wait for incoming connection>
I: <open connection>
L: RPY 0 0 . 0 133
L: Content-Type: application/beep+xml
L:
L: <greeting features="async x-foo">
L: <profile uri="http://example.com/beep/APP" />
L: </greeting>
L: END
I: RPY 0 0 . 0 69
I: Content-Type: application/beep+xml
I:
I: <greeting features="async" />
I: END
Figure 1: BEEP Greetings with Asynchronous Feature
The registration template for BEEP features is included in Section 5.
2.2. Starting an Asynchronous Channel
To create an asynchronous channel, an "async" parameter set to "true"
is included in the "start" request. If omitted, or set to "false",
the channel is not asynchronous.
Figure 2 shows how the "async" attribute can be used to start an
asynchronous channel.
C: MSG 0 1 . 52 130
C: Content-Type: application/beep+xml
C:
C: <start number="1" async="true">
C: <profile uri="http://example.org/protocol"/>
C: </start>
C: END
S: RPY 0 1 . 221 102
S: Content-Type: application/beep+xml
S:
S: <profile uri="http://example.org/protocol"/>
S: END
Figure 2: Asynchronous Channel Start
If the serving peer is unable to create an asynchronous channel for
any reason, the channel start is rejected. This could occur if the
selected profile is not suitable for an asynchronous channel. The
response can include the "553" response code (parameter invalid) and
an appropriate message, as shown in Figure 3.
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C: MSG 0 1 . 52 128
C: Content-Type: application/beep+xml
C:
C: <start number="1" async="true">
C: <profile uri="http://example.org/serial"/>
C: </start>
C: END
S: ERR 0 1 . 221 152
S: Content-Type: application/beep+xml
S:
S: <error code="553">Profile <http://example.org/serial>
S: cannot be used for asynchronous channels.</error>
S: END
Figure 3: Asynchronous Channel Start Error
2.3. Asynchronous Channel Behavior
Asynchronous channels differ from normal BEEP channels in one way
only: an asynchronous channel is not subject to the restrictions in
Section 2.6.1 of [RFC3080] regarding the processing and response
ordering. A peer in the serving role may process and respond to
requests in any order it chooses.
In an asynchronous channel, the "msgno" element of the frame header
is used to correlate request and response. A BEEP peer receiving
responses in a different order than the requests that triggered them
must not regard this as a protocol error.
"MSG" messages sent on an asynchronous channel may be processed in
parallel by the serving peer. Responses ("RPY", "ANS", "NUL", or
"ERR" messages) can be sent in any order. Different "ANS" messages
that are sent in a one-to-many exchange may be interleaved with
responses to other "MSG" messages.
An asynchronous channel must still observe the rules in [RFC3080]
regarding segmented messages. Each message must be completed before
any other message can be sent on that same channel.
Note: An exception to this rule is made in [RFC3080] for interleaved
"ANS" segments sent in response to the same "MSG". It is
recommended that BEEP peers do not generate interleaved ANS
segments.
The BEEP management channel (channel 0) is never asynchronous.
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2.4. Error Handling
BEEP does not provide any mechanism for managing a peer that does not
respond to a request. Synchronous channels cannot be used or even
closed if a peer does not provide a response to a request. The only
remedy available is closing the underlying transport. While an
asynchronous channel cannot be closed, it can still be used for
further requests. However, any outstanding request still consumes
state resources. Client peers may dispose of such state after a
configured interval, but must be prepared to discard unrecognized
responses if they do so.
3. Alternatives
The option presented in this document provides for asynchronous
communication. Asynchronous channels might not be applicable in
every circumstance, particularly where ordering of requests is
significant. Depending on application protocol requirements, the
alternatives discussed in this section could be more applicable.
3.1. Increasing Throughput
In some cases, asynchronous channels can be used to remove
limitations on message processing throughput. Alternatively,
pipelining of requests can increase throughput significantly where
network latency is the limiting factor. Spreading requests over
several channels increases overall throughput, if throughput is the
only consideration.
Note: Be wary of false optimizations that rely on the pipelining of
requests. If later requests in a series of pipelined requests
rely on state established by earlier requests, errors in earlier
requests could invalidate later requests.
The flow control window used in the TCP mapping [RFC3081] can
introduce a limiting factor in throughput for individual channels.
Choice of TCP window size similarly limits throughput, see [RFC1323].
To avoid limitations introduced by flow control, receiving peers can
increase the window size used; sending peers can open additional
channels with the same profile. Correctly managing flow control also
applies to asynchronous channels.
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3.2. Asynchrony in the Application Protocol
With changes to the application protocol, serial channels can be used
for asynchronous exchanges. Asynchrony can be provided at a protocol
layer above BEEP by separating request and response. This requires
the addition of proprietary MIME headers or modifications to the
application protocol.
The serving peer provides an immediate "RPY" (or "NUL") response to
requests. This frees the channel for further requests. The actual
response is sent as a separate "MSG" using a special identifier
included in the original request to correlate the response with the
request. This second "MSG" can be sent on the same channel (since
these are full duplex) or on a channel specifically created for this
purpose.
This method is not favored since it requires that the application
protocol solve the problem of correlating request with response.
BEEP aims to provide a general framework for the creation of an
application protocol, and for it to not provide request/response
correlation would limit its usefulness. Using a MIME header is also
possible, but using "msgno" is the most elegant solution.
4. Security Considerations
Enabling asynchronous messaging for a channel potentially requires
the maintenance of additional state information. A peer in the
server role that does not reply to messages can cause the
accumulation of state at the client peer. If this state information
were not limited, this mode could be used to perform denial of
service. This problem, while already present in BEEP, is potentially
more significant due to the nature of the processing on the serving
peer that might occur for requests received on an asynchronous
channel. The extent to which denial is possible is limited by what a
serving peer accepts; the number of outstanding requests can be
restricted to protect against excessive accumulation of state.
A client peer maintains state for each request that it sends. A
client peer should enforce a configurable limit on the number of
requests that it will allow to be outstanding at any time. This
limit could be enforced at channel, connection, or application scope.
Once this limit is reached, the client peer might prevent or block
further requests from been generated.
Peers that serve requests on asynchronous channels also accumulate
state when a request is accepted for processing. Peers in the
serving role may similarly limit to the number of requests that are
processed concurrently. Once this limit is reached the receiving
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peer can either stop reading new requests, or might start rejecting
such requests by generating error responses. Alternatively, the flow
control [RFC3081] can be used; "SEQ" frames can be suppressed,
allowing the flow control window to close and preventing the receipt
of further requests.
5. IANA Considerations
This section registers the BEEP "async" feature in the BEEP
parameters registry, following the template from Section 5.2 of
[RFC3080].
Feature Identification: async
Feature Semantics: This feature enables the creation of asynchronous
channels, see Section 2 of RFC 5573.
Contact Information: Martin Thomson <martin.thomson@andrew.com>
6. References
6.1. Normative References
[RFC3080] Rose, M., "The Blocks Extensible Exchange Protocol Core",
RFC 3080, March 2001.
6.2. Informative References
[RFC3081] Rose, M., "Mapping the BEEP Core onto TCP", RFC 3081,
March 2001.
[RFC1323] Jacobson, V., Braden, B., and D. Borman, "TCP Extensions
for High Performance", RFC 1323, May 1992.
Author's Address
Martin Thomson
Andrew
PO Box U40
Wollongong University Campus, NSW 2500
AU
Phone: +61 2 4221 2915
EMail: martin.thomson@andrew.com
URI: http://www.andrew.com/
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ERRATA
