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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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.
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].
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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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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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.
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.
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.
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.
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.
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.
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.
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.
[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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