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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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Full Copyright Statement

Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
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Acknowledgement

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).







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