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PROPOSED STANDARD
Errata Exist
Network Working Group J. Vinocur
Request for Comments: 4644 Cornell University
Updates: 2980 K. Murchison
Category: Standards Track Carnegie Mellon University
October 2006
Network News Transfer Protocol (NNTP) Extension for Streaming Feeds
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This memo defines an extension to the Network News Transfer Protocol
(NNTP) to provide asynchronous (otherwise known as "streaming")
transfer of articles. This allows servers to transfer articles to
other servers with much greater efficiency.
This document updates and formalizes the CHECK and TAKETHIS commands
specified in RFC 2980 and deprecates the MODE STREAM command.
Table of Contents
1. Introduction ....................................................2
1.1. Conventions Used in this Document ..........................2
2. The STREAMING Extension .........................................3
2.1. Streaming Article Transfer .................................3
2.2. Advertising the STREAMING Extension ........................4
2.3. MODE STREAM Command ........................................5
2.3.1. Usage ...............................................5
2.3.2. Description .........................................5
2.3.3. Examples ............................................5
2.4. CHECK Command ..............................................6
2.4.1. Usage ...............................................6
2.4.2. Description .........................................6
2.4.3. Examples ............................................6
2.5. TAKETHIS Command ...........................................7
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2.5.1. Usage ...............................................7
2.5.2. Description .........................................7
2.5.3. Examples ............................................8
3. Augmented BNF Syntax for the STREAMING Extension ................9
3.1. Commands ...................................................9
3.2. Command Datastream .........................................9
3.3. Responses .................................................10
3.4. Capability Entries ........................................10
4. Summary of Response Codes ......................................10
5. Security Considerations ........................................11
6. IANA Considerations ............................................11
7. Acknowledgements ...............................................12
8. References .....................................................12
8.1. Normative References ......................................12
8.2. Informative References ....................................12
1. Introduction
According to the NNTP specification [NNTP], a peer uses the IHAVE
command to query whether a server wants a particular article.
Because the IHAVE command cannot be pipelined, the need to stop and
wait for the remote end's response greatly restricts the throughput
that can be achieved.
The ad-hoc CHECK and TAKETHIS commands, originally documented in
[NNTP-COMMON], provide an alternative method of peer-to-peer article
transfer that permits a more effective use of network bandwidth. Due
to their proven usefulness and wide deployment, they are formalized
in this specification.
The ad-hoc MODE STREAM command, also documented in [NNTP-COMMON], is
deprecated by this specification, but due to its ubiquity is
documented here for backwards compatibility.
1.1. Conventions Used in this Document
The notational conventions used in this document are the same as
those in [NNTP] and any term not defined in this document has the
same meaning as in that one.
The key words "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD NOT",
"MAY", and "OPTIONAL" in this document are to be interpreted as
described in "Key words for use in RFCs to Indicate Requirement
Levels" [KEYWORDS].
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This document assumes you familiarity with NNTP [NNTP]. In general,
the connections described below are from one peer to another, but we
will continue to use "client" to mean the initiator of the NNTP
connection, and "server" to mean the other endpoint.
In the examples, commands from the client are indicated with [C], and
responses from the server are indicated with [S].
2. The STREAMING Extension
This extension provides three new commands: MODE STREAM, CHECK, and
TAKETHIS. The capability label for this extension is STREAMING.
2.1. Streaming Article Transfer
The STREAMING extension provides the same functionality as the IHAVE
command ([NNTP] section 6.3.2) but splits the query and transfer
functionality into the CHECK and TAKETHIS commands respectively.
This allows the CHECK and TAKETHIS commands to be pipelined ([NNTP]
section 3.5) and provides for "streaming" article transfer.
A streaming client will often pipeline many CHECK commands and use
the responses to construct a list of articles to be sent by a
pipelined sequence of TAKETHIS commands, thus increasing the fraction
of time spent transferring articles. The CHECK and TAKETHIS commands
utilize distinct response codes so that these commands can be
intermingled in a pipeline and the response to any single command can
be definitively identified by the client.
The client MAY send articles via TAKETHIS without first querying the
server with CHECK. The client SHOULD NOT send every article in this
fashion unless explicitly configured to do so by the site
administrator based on out-of-band information. However, the client
MAY use an adaptive strategy where it initially sends CHECK commands
for all articles, but switches to using TAKETHIS without CHECK if
most articles are being accepted (over 95% acceptance might be a
reasonable metric in some configurations). If the client uses such a
strategy, it SHOULD also switch back to using CHECK on all articles
if the acceptance rate ever falls much below the threshold.
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2.2. Advertising the STREAMING Extension
A server supporting the streaming commands described in this document
will advertise the "STREAMING" capability label in response to the
CAPABILITIES command ([NNTP] section 5.2). The server MUST continue
to advertise this capability after a client has issued the MODE
STREAM command. This capability MAY be advertised both before and
after any use of the MODE READER command ([NNTP] section 5.3), with
the same semantics.
Example of a client using CAPABILITIES and MODE STREAM on a mode-
switching server:
[C] CAPABILITIES
[S] 101 Capability list:
[S] VERSION 2
[S] MODE-READER
[S] IHAVE
[S] LIST ACTIVE
[S] STREAMING
[S] .
[C] MODE STREAM
[S] 203 Streaming permitted
[C] CAPABILITIES
[S] 101 Capability list:
[S] VERSION 2
[S] MODE-READER
[S] IHAVE
[S] LIST ACTIVE
[S] STREAMING
[S] .
[C] MODE READER
[S] 200 Posting allowed
[C] CAPABILITIES
[S] 101 Capability list:
[S] VERSION 2
[S] READER
[S] POST
[S] LIST ACTIVE NEWSGROUPS HEADERS
[S] HDR
[S] .
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2.3. MODE STREAM Command
Historically this command was used by a client to discover if a
server supported the CHECK and TAKETHIS commands. This command is
deprecated in favor of the CAPABILITIES discovery command and is only
provided here for compatibility with legacy implementations
[NNTP-COMMON] of these transport commands.
New clients SHOULD use the CAPABILITIES command to check a server for
support of the STREAMING extension but MAY use the MODE STREAM
command for backwards compatibility with legacy servers that don't
support the CAPABILITIES discovery command. Servers MUST accept the
MODE STREAM command for backwards compatibility with legacy clients
that don't use the CAPABILITIES discovery command.
NOTE: This command may be removed from a future version of this
specification, therefore clients are urged to transition to the
CAPABILITIES command wherever possible.
2.3.1. Usage
Syntax
MODE STREAM
Responses
203 Streaming permitted
2.3.2. Description
If a server supports this extension, it MUST return a 203 response to
the MODE STREAM command (or 501 if an argument is given). The MODE
STREAM command MUST NOT affect the server state in any way (that is,
it is not a mode change despite the name), therefore this command MAY
be pipelined. A server MUST NOT require that the MODE STREAM command
be issued by the client before accepting the CHECK or TAKETHIS
commands.
2.3.3. Examples
Example of a client checking the ability to stream articles on a
server which does not support this extension:
[C] MODE STREAM
[S] 501 Unknown MODE variant
Example of a client checking the ability to stream articles on a
server which supports this extension:
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[C] MODE STREAM
[S] 203 Streaming permitted
2.4. CHECK Command
2.4.1. Usage
Syntax
CHECK message-id
Responses
238 message-id Send article to be transferred
431 message-id Transfer not possible; try again later
438 message-id Article not wanted
Parameters
message-id = Article message-id
The first parameter of the 238, 431, and 438 responses MUST be the
message-id provided by the client as the parameter to CHECK.
2.4.2. Description
The CHECK command informs the server that the client has an article
with the specified message-id. If the server desires a copy of that
article, a 238 response MUST be returned, indicating that the client
may send the article using the TAKETHIS command. If the server does
not want the article (if, for example, the server already has a copy
of it), a 438 response MUST be returned, indicating that the article
is not wanted. Finally, if the article isn't wanted immediately but
the client should retry later if possible (if, for example, another
client has offered to send the same article to the server), a 431
response MUST be returned.
NOTE: The responses to CHECK are advisory; the server MUST NOT rely
on the client to behave as requested by these responses.
2.4.3. Examples
Example of a client checking whether the server would like a set of
articles and getting a mixture of responses:
[C] CHECK <i.am.an.article.you.will.want@example.com>
[S] 238 <i.am.an.article.you.will.want@example.com>
[C] CHECK <i.am.an.article.you.have@example.com>
[S] 438 <i.am.an.article.you.have@example.com>
[C] CHECK <i.am.an.article.you.defer@example.com>
[S] 431 <i.am.an.article.you.defer@example.com>
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Example of pipelining the CHECK commands in the previous example:
[C] CHECK <i.am.an.article.you.will.want@example.com>
[C] CHECK <i.am.an.article.you.have@example.com>
[C] CHECK <i.am.an.article.you.defer@example.com>
[S] 238 <i.am.an.article.you.will.want@example.com>
[S] 438 <i.am.an.article.you.have@example.com>
[S] 431 <i.am.an.article.you.defer@example.com>
2.5. TAKETHIS Command
2.5.1. Usage
A client MUST NOT use this command unless the server advertises the
STREAMING capability or returns a 203 response to the MODE STREAM
command.
Syntax
TAKETHIS message-id
Responses
239 message-id Article transferred OK
439 message-id Transfer rejected; do not retry
Parameters
message-id = Article message-id
The first parameter of the 239 and 439 responses MUST be the
message-id provided by the client as the parameter to TAKETHIS.
2.5.2. Description
The TAKETHIS command is used to send an article with the specified
message-id to the server. The article is sent immediately following
the CRLF at the end of the TAKETHIS command line. The client MUST
send the entire article, including headers and body, to the server as
a multi-line data block ([NNTP] section 3.1.1). Thus, a single dot
(".") on a line indicates the end of the text, and lines starting
with a dot in the original text have that dot doubled during
transmission. The server MUST return either a 239 response,
indicating that the article was successfully transferred, or a 439
response, indicating that the article was rejected. If the server
encounters a temporary error that prevents it from processing the
article but does not want to reject the article, it MUST reply with a
400 response to the client and close the connection.
This function differs from the POST command in that it is intended
for use in transferring already-posted articles between hosts. It
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SHOULD NOT be used when the client is a personal news-reading
program, since use of this command indicates that the article has
already been posted at another site and is simply being forwarded
from another host. However, despite this, the server MAY elect not
to post or forward the article if, after further examination of the
article, it deems it inappropriate to do so. Reasons for such
subsequent rejection of an article may include problems such as
inappropriate newsgroups or distributions, disk space limitations,
article lengths, garbled headers, and the like. These are typically
restrictions enforced by the server host's news software and not
necessarily by the NNTP server itself.
The client SHOULD NOT assume that the article has been successfully
transferred unless it receives an affirmative response from the
server. A lack of response (such as a dropped network connection or
a network timeout) or a 400 response SHOULD be treated as a temporary
failure and cause the transfer to be tried again later if possible.
Because some news server software may not immediately be able to
determine whether an article is suitable for posting or forwarding,
an NNTP server MAY acknowledge the successful transfer of the article
(with a 239 response) but later silently discard it.
2.5.3. Examples
Example of streaming two articles to another site (the first article
is accepted and the second is rejected):
[C] TAKETHIS <i.am.an.article.you.will.want@example.com>
[C] Path: pathost!demo!somewhere!not-for-mail
[C] From: "Demo User" <nobody@example.com>
[C] Newsgroups: misc.test
[C] Subject: I am just a test article
[C] Date: 6 Oct 1998 04:38:40 -0500
[C] Organization: An Example Com, San Jose, CA
[C] Message-ID: <i.am.an.article.you.will.want@example.com>
[C]
[C] This is just a test article.
[C] .
[C] TAKETHIS <i.am.an.article.you.have@example.com>
[C] Path: pathost!demo!somewhere!not-for-mail
[C] From: "Demo User" <nobody@example.com>
[C] Newsgroups: misc.test
[C] Subject: I am just a test article
[C] Date: 6 Oct 1998 04:38:40 -0500
[C] Organization: An Example Com, San Jose, CA
[C] Message-ID: <i.am.an.article.you.have@example.com>
[C]
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[C] This is just a test article.
[C] .
[S] 239 <i.am.an.article.you.will.want@example.com>
[S] 439 <i.am.an.article.you.have@example.com>
Example of sending an article to a site where the transfer fails:
[C] TAKETHIS <i.am.an.article.you.will.want@example.com>
[C] Path: pathost!demo!somewhere!not-for-mail
[C] From: "Demo User" <nobody@example.com>
[C] Newsgroups: misc.test
[C] Subject: I am just a test article
[C] Date: 6 Oct 1998 04:38:40 -0500
[C] Organization: An Example Com, San Jose, CA
[C] Message-ID: <i.am.an.article.you.will.want@example.com>
[C]
[C] This is just a test article.
[C] .
[S] 400 Service temporarily unavailable
[Server closes connection.]
3. Augmented BNF Syntax for the STREAMING Extension
This section describes the formal syntax of the STREAMING extension
using ABNF [ABNF]. It extends the syntax in section 9 of [NNTP], and
non-terminals not defined in this document are defined there. The
[NNTP] ABNF should be imported first before attempting to validate
these rules.
3.1. Commands
This syntax extends the non-terminal "command", which represents an
NNTP command.
command =/ check-command /
mode-stream-command /
takethis-command
check-command = "CHECK" WS message-id
mode-stream-command = "MODE" WS "STREAM"
takethis-command = "TAKETHIS" WS message-id
3.2. Command Datastream
This syntax extends the non-terminal "command-datastream", which
represents the further material sent by the client in the case of
streaming commands.
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command-datastream =/ takethis-datastream
takethis-datastream = encoded-article
3.3. Responses
This syntax extends the non-terminal "initial-response-content",
which represents an initial response line sent by the server.
initial-response-content =/ response-238-content /
response-239-content /
response-431-content /
response-438-content /
response-439-content
response-238-content = "238" SP message-id
response-239-content = "239" SP message-id
response-431-content = "431" SP message-id
response-438-content = "438" SP message-id
response-439-content = "439" SP message-id
3.4. Capability Entries
This syntax extends the non-terminal "capability-entry", which
represents a capability that may be advertised by the server.
capability-entry =/ streaming-capability
streaming-capability = "STREAMING"
4. Summary of Response Codes
This section contains a list of each new response code defined in
this document and indicates whether it is multi-line, which commands
can generate it, what arguments it has, and what its meaning is.
Response code 203
Generated by: MODE STREAM
Meaning: streaming permitted.
Response code 238
Generated by: CHECK
1 argument: message-id
Meaning: send article to be transferred.
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Response code 239
Generated by: TAKETHIS
1 argument: message-id
Meaning: article transferred OK.
Response code 431
Generated by: CHECK
1 argument: message-id
Meaning: transfer not possible; try again later.
Response code 438
Generated by: CHECK
1 argument: message-id
Meaning: article not wanted.
Response code 439
Generated by: TAKETHIS
1 argument: message-id
Meaning: transfer rejected; do not retry.
5. Security Considerations
No new security considerations are introduced by this extension,
beyond those already described in the core specification [NNTP].
6. IANA Considerations
This section gives a formal definition of the STREAMING extension as
required by Section 3.3.3 of [NNTP] for the IANA registry.
o The STREAMING extension provides for streaming transfer of
articles.
o The capability label for this extension is "STREAMING".
o The capability label has no arguments.
o The extension defines three new commands, MODE STREAM, CHECK, and
TAKETHIS, whose behavior, arguments, and responses are defined in
Sections 2.3, 2.4, and 2.5 respectively.
o The extension does not associate any new responses with pre-
existing NNTP commands.
o The extension does not affect the behavior of a server or client
other than via the new commands.
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o The extension does not affect the maximum length of commands or
initial response lines.
o The extension does not alter pipelining, and the MODE STREAM,
CHECK, and TAKETHIS commands can be pipelined.
o Use of this extension does not alter the capabilities list.
o The extension does not cause any pre-existing command to produce a
401, 480, or 483 response.
o Use of the MODE READER command on a mode-switching server may
disable this extension.
o Published Specification: This document.
o Contact for Further Information: Authors of this document.
o Change Controller: IESG <iesg@ietf.org>.
7. Acknowledgements
This document is based heavily on the relevant sections of RFC 2980
[NNTP-COMMON], by Stan Barber.
Special acknowledgement also goes to Russ Allbery, Clive Feather,
Andrew Gierth, and others who commented privately on intermediate
revisions of this document, as well as the members of the IETF NNTP
Working Group for continual (yet sporadic) insight in discussion.
8. References
8.1. Normative References
[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", RFC 4234, October 2005.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[NNTP] Feather, C., "Network News Transfer Protocol (NNTP)",
RFC 3977, October 2006.
8.2. Informative References
[NNTP-COMMON] Barber, S., "Common NNTP Extensions", RFC 2980, October
2000.
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Authors' Addresses
Jeffrey M. Vinocur
Department of Computer Science
Upson Hall
Cornell University
Ithaca, NY 14853
EMail: vinocur@cs.cornell.edu
Kenneth Murchison
Carnegie Mellon University
5000 Forbes Avenue
Cyert Hall 285
Pittsburgh, PA 15213 USA
EMail: murch@andrew.cmu.edu
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