tech-invite   World Map     

3GPP     Specs     Glossaries     Architecture     IMS     UICC       IETF     RFCs     Groups     SIP     ABNFs       Search

RFC 8054

Proposed STD
Pages: 23
Top     in Index     Prev     Next
in Group Index     Prev in Group     No Next: Highest Number in Group     Group: ~proto

Network News Transfer Protocol (NNTP) Extension for Compression

 


Top       ToC       Page 1 
Internet Engineering Task Force (IETF)                      K. Murchison
Request for Comments: 8054                    Carnegie Mellon University
Category: Standards Track                                        J. Elie
ISSN: 2070-1721                                             January 2017


                 Network News Transfer Protocol (NNTP)
                       Extension for Compression

Abstract

   This document defines an extension to the Network News Transport
   Protocol (NNTP) that allows a connection to be effectively and
   efficiently compressed between an NNTP client and server.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc8054.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Top       Page 2 
Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  About TLS-Level Compression . . . . . . . . . . . . . . .   3
     1.2.  Conventions Used in This Document . . . . . . . . . . . .   4
   2.  The COMPRESS Extension  . . . . . . . . . . . . . . . . . . .   4
     2.1.  Advertising the COMPRESS Extension  . . . . . . . . . . .   4
     2.2.  COMPRESS Command  . . . . . . . . . . . . . . . . . . . .   5
       2.2.1.  Usage . . . . . . . . . . . . . . . . . . . . . . . .   5
       2.2.2.  Description . . . . . . . . . . . . . . . . . . . . .   6
       2.2.3.  Examples  . . . . . . . . . . . . . . . . . . . . . .   8
   3.  Compression Efficiency  . . . . . . . . . . . . . . . . . . .  11
   4.  DEFLATE Specificities . . . . . . . . . . . . . . . . . . . .  12
   5.  Augmented BNF Syntax for the COMPRESS Extension . . . . . . .  13
     5.1.  Commands  . . . . . . . . . . . . . . . . . . . . . . . .  13
     5.2.  Capability Entries  . . . . . . . . . . . . . . . . . . .  13
     5.3.  General Non-terminals . . . . . . . . . . . . . . . . . .  13
   6.  Summary of Response Codes . . . . . . . . . . . . . . . . . .  13
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
     8.1.  "NNTP Compression Algorithms" Registry  . . . . . . . . .  15
       8.1.1.  Algorithm Name Registration Procedure . . . . . . . .  16
       8.1.2.  Comments on Algorithm Registrations . . . . . . . . .  17
       8.1.3.  Change Control  . . . . . . . . . . . . . . . . . . .  17
     8.2.  Registration of the DEFLATE Compression Algorithm . . . .  18
     8.3.  Registration of the NNTP COMPRESS Extension . . . . . . .  18
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  20
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  20
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

Top      ToC       Page 3 
1.  Introduction

   The goal of COMPRESS is to reduce the bandwidth usage of NNTP.

   Compared to PPP compression [RFC1962] and modem-based compression
   ([MNP] and [V42bis]), COMPRESS offers greater compression efficiency.
   COMPRESS can be used together with Transport Layer Security (TLS)
   [RFC5246], Simple Authentication and Security Layer (SASL) encryption
   [RFC4422], Virtual Private Networks (VPNs), etc.

   The point of COMPRESS as an NNTP extension is to act as a compression
   layer, similar to a security layer like the one negotiated by
   STARTTLS [RFC4642].  Therefore, compression can be beneficial to all
   NNTP commands sent or received after the use of COMPRESS.  This
   facility responds to a long-standing need for NNTP to compress data.
   It is currently addressed only partially by unstandardized commands
   like XZVER, XZHDR, XFEATURE COMPRESS, or MODE COMPRESS.  Yet, these
   commands are not wholly satisfactory because they enable compression
   only for the responses sent by the news server.  In comparison, the
   COMPRESS command permits the compression of data sent by both the
   client and the server, and removes the constraint of having to
   implement compression separately in each NNTP command.  Besides, the
   compression level can be dynamically adjusted and optimized at any
   time during the connection, which even allows disabling compression
   for certain commands, if needed.  If the news client wants to stop
   compression on a particular connection, it can simply use QUIT
   ([RFC3977], Section 5.4) and establish a new connection.  For these
   reasons, using other NNTP commands than COMPRESS to enable
   compression is discouraged once COMPRESS is supported.

   In order to increase interoperability, it is desirable to have as few
   different compression algorithms as possible, so this document
   specifies only one.  The DEFLATE algorithm (defined in [RFC1951])
   MUST be implemented as part of this extension.  This compression
   algorithm is standard, widely available, and fairly efficient.

   This specification should be read in conjunction with the NNTP base
   specification [RFC3977].  In the case of a conflict between these two
   documents, [RFC3977] takes precedence.

1.1.  About TLS-Level Compression

   Though lossless data compression is already possible via the use of
   TLS with NNTP [RFC4642], the best current practice is to disable TLS-
   level compression as explained in Section 3.3 of [RFC7525].  The
   COMPRESS command will permit keeping the compression facility in
   NNTP, and control when it is available during a connection.

Top      ToC       Page 4 
   Compared to TLS-level compression [RFC3749], NNTP COMPRESS has the
   following advantages:

   o  COMPRESS can be implemented easily both by NNTP servers and
      clients.

   o  COMPRESS benefits from an intimate knowledge of the NNTP
      protocol's state machine, allowing for dynamic and aggressive
      optimization of the underlying compression algorithm's parameters.

   o  COMPRESS can be activated after authentication has completed, thus
      reducing the chances that authentication credentials can be leaked
      via, for instance, a CRIME attack ([RFC7457], Section 2.6).

1.2.  Conventions Used in This Document

   The notational conventions used in this document are the same as
   those in [RFC3977], and any term not defined in this document has the
   same meaning as it does in that one.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

   In the examples, commands from the client are indicated with [C], and
   responses from the server are indicated with [S].  The client is the
   initiator of the NNTP connection; the server is the other endpoint.

2.  The COMPRESS Extension

   The COMPRESS extension is used to enable lossless data compression on
   an NNTP connection.

   This extension provides a new COMPRESS command and has the capability
   label COMPRESS.

2.1.  Advertising the COMPRESS Extension

   A server supporting the COMPRESS command as defined in this document
   will advertise the "COMPRESS" capability label in response to the
   CAPABILITIES command ([RFC3977], Section 5.2).  However, this
   capability MUST NOT be advertised once a compression layer is active
   (see Section 2.2.2).  This capability MAY be advertised both before
   and after any use of the MODE READER command ([RFC3977],
   Section 5.3), with the same semantics.

Top      ToC       Page 5 
   The COMPRESS capability label contains a whitespace-separated list of
   available compression algorithms.  This document defines one
   compression algorithm: DEFLATE.  This algorithm is mandatory to
   implement; it MUST be supported and listed in the advertisement of
   the COMPRESS extension.

   Future extensions may add additional compression algorithms to this
   capability.  Unrecognized algorithms MUST be ignored by the client.

   Example:

      [C] CAPABILITIES
      [S] 101 Capability list:
      [S] VERSION 2
      [S] READER
      [S] IHAVE
      [S] COMPRESS DEFLATE SHRINK
      [S] LIST ACTIVE NEWSGROUPS
      [S] .

   As the COMPRESS command is related to security because it can weaken
   encryption, cached results of CAPABILITIES from a previous session
   MUST NOT be relied on, as per Section 12.6 of [RFC3977].

2.2.  COMPRESS Command

2.2.1.  Usage

   This command MUST NOT be pipelined.

   Syntax
     COMPRESS algorithm

   Responses
     206 Compression active
     403 Unable to activate compression
     502 Command unavailable [1]

   [1] If a compression layer is already active, COMPRESS is not a valid
       command (see Section 2.2.2).

   Parameters
     algorithm = Name of compression algorithm (e.g., "DEFLATE")

Top      ToC       Page 6 
2.2.2.  Description

   The COMPRESS command instructs the server to use the named
   compression algorithm ("DEFLATE" is the only one defined in this
   document) for all commands and responses after COMPRESS.

   The client MUST NOT send any further commands until it has seen the
   result of COMPRESS.

   If the requested compression algorithm is syntactically incorrect,
   the server MUST reject the COMPRESS command with a 501 response code
   ([RFC3977], Section 3.2.1).  If the requested compression algorithm
   is invalid (e.g., is not supported), the server MUST reject the
   COMPRESS command with a 503 response code ([RFC3977], Section 3.2.1).
   If the server is unable to activate compression for any reason (e.g.,
   a server configuration or resource problem), the server MUST reject
   the COMPRESS command with a 403 response code ([RFC3977],
   Section 3.2.1).  Otherwise, in case no other generic response code
   representing the situation applies, the server issues a 206 response
   code and the compression layer takes effect for both client and
   server immediately following the CRLF of the success reply.

   Additionally, the client MUST NOT issue a MODE READER command after
   activating a compression layer, and a server MUST NOT advertise the
   MODE-READER capability.

   Both the client and the server MUST know if there is a compression
   layer active (for instance, via the previous use of the COMPRESS
   command or the negotiation of a TLS-level compression method
   [RFC3749]).  A client MUST NOT attempt to activate compression (for
   instance, via the COMPRESS command) or negotiate a TLS security layer
   (because STARTTLS [RFC4642] may activate TLS-level compression) if a
   compression layer is already active.  A server MUST NOT return the
   COMPRESS or STARTTLS capability labels in response to a CAPABILITIES
   command received after a compression layer is active, and a server
   MUST reply with a 502 response code if a syntactically valid COMPRESS
   or STARTTLS command is received while a compression layer is already
   active.

   In order to help mitigate leaking authentication credentials via, for
   instance, a CRIME attack [CRIME], authentication MUST NOT be
   attempted after a successful use of the COMPRESS command.
   Consequently, a server MUST either list the AUTHINFO capability with
   no arguments or not advertise it at all, in response to a
   CAPABILITIES command received from an unauthenticated client after a
   successful use of the COMPRESS command, and such a client MUST NOT
   attempt to utilize any AUTHINFO [RFC4643] commands.  This implies
   that a server MUST reply with a 502 response code if a syntactically

Top      ToC       Page 7 
   valid AUTHINFO command is received after a successful use of the
   COMPRESS command.  (Note that this specification does not change the
   behavior of AUTHINFO as described in [RFC4643] independently of TLS-
   level compression.  Authentication is therefore still allowed, even
   though TLS-level compression is active.)

   For DEFLATE [RFC1951] (as for many other compression algorithms), the
   sending compressor can trade speed against compression ratio.  The
   receiving decompressor MUST automatically adjust to the parameters
   selected by the sender.  Consequently, the client and server are both
   free to pick the best reasonable rate of compression for the data
   they send.  Besides, all data that was submitted for compression MUST
   be included in the compressed output, and appropriately flushed so as
   to ensure that the receiving decompressor can completely decompress
   it.

   When COMPRESS is combined with TLS [RFC5246] or SASL [RFC4422]
   security layers, the processing order of the three layers MUST be
   first COMPRESS, then SASL, and finally TLS.  That is, before data is
   transmitted, it is first compressed.  Second, if a SASL security
   layer has been negotiated, the compressed data is then signed and/or
   encrypted accordingly.  Third, if a TLS security layer has been
   negotiated, the data from the previous step is signed and/or
   encrypted accordingly (with a possible additional TLS-level
   compression).  When receiving data, the processing order MUST be
   reversed.  This ensures that before sending, data is compressed
   before it is encrypted.

   When compression is active and either the client or the server
   receives invalid or corrupted compressed data, the receiving end
   immediately closes the connection, in response to which the sending
   end will do the same.

Top      ToC       Page 8 
2.2.3.  Examples

   Example of layering a TLS security layer and NNTP compression:

      [C] CAPABILITIES
      [S] 101 Capability list:
      [S] VERSION 2
      [S] READER
      [S] STARTTLS
      [S] AUTHINFO
      [S] COMPRESS DEFLATE
      [S] LIST ACTIVE NEWSGROUPS
      [S] .
      [C] STARTTLS
      [S] 382 Continue with TLS negotiation
      [TLS negotiation without compression occurs here]
      [Following successful negotiation, all traffic is encrypted]
      [C] CAPABILITIES
      [S] 101 Capability list:
      [S] VERSION 2
      [S] READER
      [S] AUTHINFO USER
      [S] COMPRESS DEFLATE
      [S] LIST ACTIVE NEWSGROUPS
      [S] .
      [C] AUTHINFO USER fred
      [S] 381 Enter passphrase
      [C] AUTHINFO PASS flintstone
      [S] 281 Authentication accepted
      [C] CAPABILITIES
      [S] 101 Capability list:
      [S] VERSION 2
      [S] READER
      [S] POST
      [S] COMPRESS DEFLATE
      [S] LIST ACTIVE NEWSGROUPS
      [S] .
      [C] COMPRESS DEFLATE
      [S] 206 Compression active
      [Henceforth, all traffic is compressed before being encrypted]
      [C] CAPABILITIES
      [S] 101 Capability list:
      [S] VERSION 2
      [S] READER
      [S] POST
      [S] LIST ACTIVE NEWSGROUPS
      [S] .

Top      ToC       Page 9 
   Example of a server failing to activate compression:

      [C] CAPABILITIES
      [S] 101 Capability list:
      [S] VERSION 2
      [S] IHAVE
      [S] COMPRESS DEFLATE
      [S] .
      [C] COMPRESS DEFLATE
      [S] 403 Unable to activate compression

   Example of attempting to use an unsupported compression algorithm:

      [C] CAPABILITIES
      [S] 101 Capability list:
      [S] VERSION 2
      [S] IHAVE
      [S] COMPRESS DEFLATE
      [S] .
      [C] COMPRESS SHRINK
      [S] 503 Compression algorithm not supported

   Example of a server refusing to compress twice:

      [C] CAPABILITIES
      [S] 101 Capability list:
      [S] VERSION 2
      [S] IHAVE
      [S] STARTTLS
      [S] COMPRESS DEFLATE
      [S] .
      [C] STARTTLS
      [S] 382 Continue with TLS negotiation
      [TLS negotiation with compression occurs here]
      [Following successful negotiation, all traffic is encrypted]
      [C] CAPABILITIES
      [S] 101 Capability list:
      [S] VERSION 2
      [S] IHAVE
      [S] .
      [C] COMPRESS DEFLATE
      [S] 502 Compression already active via TLS

Top      ToC       Page 10 
   Example of a server refusing to negotiate a TLS security layer after
   compression has been activated:

      [C] CAPABILITIES
      [S] 101 Capability list:
      [S] VERSION 2
      [S] IHAVE
      [S] STARTTLS
      [S] COMPRESS DEFLATE
      [S] .
      [C] COMPRESS DEFLATE
      [S] 206 Compression active
      [Henceforth, all traffic is compressed]
      [C] CAPABILITIES
      [S] 101 Capability list:
      [S] VERSION 2
      [S] IHAVE
      [S] .
      [C] STARTTLS
      [S] 502 DEFLATE compression already active

   Example of a server not advertising AUTHINFO arguments after
   compression has been activated:

      [C] CAPABILITIES
      [S] 101 Capability list:
      [S] VERSION 2
      [S] READER
      [S] AUTHINFO USER
      [S] COMPRESS DEFLATE
      [S] LIST ACTIVE NEWSGROUPS
      [S] .
      [C] COMPRESS DEFLATE
      [S] 206 Compression active
      [Henceforth, all traffic is compressed]
      [C] CAPABILITIES
      [S] 101 Capability list:
      [S] VERSION 2
      [S] READER
      [S] AUTHINFO
      [S] LIST ACTIVE NEWSGROUPS
      [S] .
      [C] AUTHINFO USER fred
      [S] 502 DEFLATE compression already active

Top      ToC       Page 11 
3.  Compression Efficiency

   This section is informative, not normative.

   NNTP poses some unusual problems for a compression layer.

   Upstream traffic is fairly simple.  Most NNTP clients send the same
   few commands again and again, so any compression algorithm that can
   exploit repetition works efficiently.  The article posting and
   transfer commands (e.g., POST, IHAVE, and TAKETHIS [RFC4644]) are
   exceptions; clients that send many article posting or transfer
   commands may want to surround large multi-line data blocks with a
   dictionary flush and/or, depending on the compression algorithm, a
   change of compression level in the same way as is recommended for
   servers later in this document (Section 4).

   Downstream traffic has the unusual property that several kinds of
   data are sent, possibly confusing a dictionary-based compression
   algorithm.

   NNTP responses that are not related to article header/body retrieval
   are one type.  Compressing NNTP simple responses (e.g., in answer to
   CHECK [RFC4644], DATE, GROUP, LAST, NEXT, STAT, etc.) generally does
   not save many bytes, unless repeated several times in the same NNTP
   session.  On the contrary, most of the NNTP multi-line responses
   (e.g., in answer to LIST, LISTGROUP, NEWGROUPS, NEWNEWS, etc.) are
   highly compressible; using its least CPU-intensive setting, zlib
   compresses typical responses to 25-40% of their original size.

   Article headers (as retrieved, for instance, via the HEAD, HDR, OVER,
   or ARTICLE commands) are another type.  These are equally
   compressible, and benefit from using the same dictionary as the NNTP
   responses.

   A third type is article body text (as retrieved, for instance, via
   the BODY or ARTICLE commands).  Text is usually fairly short and
   includes much ASCII, so the same compression dictionary will do a
   good job here, too.  When multiple messages in the same thread are
   read at the same time, quoted lines, etc., can often be compressed
   almost to zero.

   Finally, non-text article bodies or attachments (as retrieved, for
   instance, via the BODY or ARTICLE commands) are transmitted in
   encoded form, usually Base64 [RFC4648], UUencode [IEEE.1003.1-2008],
   or yEnc [yEnc].

Top      ToC       Page 12 
   When such non-text article bodies or attachments are retrieved, a
   compression algorithm may be able to compress them, but the format of
   their encoding is usually not NNTP-like, so the dictionary built
   while compressing NNTP does not help much.  The compressor has to
   adapt its dictionary from NNTP to the attachment's encoding format,
   and then back.

   When attachments are retrieved in Base64 or UUencode form, the
   Huffman coding usually compresses those to approximately only 75% of
   their encoding size.  8-bit compression algorithms such as DEFLATE
   work well on 8-bit file formats; however, both Base64 and UUencode
   transform a file into something resembling 6-bit bytes, hiding most
   of the 8-bit file format from the compressor.

   On the other end, attachments encoded using a compression algorithm
   that retains the full 8-bit spectrum, like yEnc, are much more likely
   to be incompressible.

4.  DEFLATE Specificities

   When using the zlib library (see [RFC1951]), the functions
   deflateInit2(), deflate(), inflateInit2(), and inflate() suffice to
   implement this extension.

   The windowBits value MUST be in the range -8 to -15 for
   deflateInit2(), or else it will use the wrong format.  The windowBits
   value SHOULD be -15 for inflateInit2(), or else it will not be able
   to decompress a stream with a larger window size, thus reducing
   interoperability.  deflateParams() can be used to improve compression
   rate and resource use.  Regarding flush operations, the Z_FULL_FLUSH
   argument to deflate() permits to clear the dictionary, which
   generally results in compression that is less effective than
   performing a Z_PARTIAL_FLUSH.  As a matter of fact, keeping the 32 KB
   dictionary from previous data, no matter how unrelated, can be of
   help (if there are no matching strings in there, then it is simply
   not referenced).

   A server can improve downstream compression and the CPU efficiency of
   both the server and the client if it adjusts the compression level
   (e.g., using the deflateParams() function in zlib) at the start and
   end of large non-text multi-line data blocks (before and after
   'content-lines' in the definition of 'multi-line-data-block' in
   [RFC3977], Section 9.8).  This mechanism prevents the server from
   trying to compress incompressible attachments.

Top      ToC       Page 13 
   A very simple strategy is to change the compression level to 0 at the
   start of an incompressible multi-line data block, for instance when
   encoded using yEnc [yEnc], and to keep it at 1-5 the rest of the
   time.  More complex strategies are, of course, possible and
   encouraged.

5.  Augmented BNF Syntax for the COMPRESS Extension

   This section describes the formal syntax of the COMPRESS extension
   using ABNF [RFC7405] and [RFC5234].  It extends the syntax in
   Section 9 of [RFC3977], and non-terminals not defined in this
   document are defined there.  The NNTP ABNF [RFC3977] should be
   imported first, before attempting to validate these rules.

5.1.  Commands

   This syntax extends the non-terminal <command>, which represents an
   NNTP command.

     command =/ compress-command

     compress-command = "COMPRESS" WS algorithm

5.2.  Capability Entries

   This syntax extends the non-terminal <capability-entry>, which
   represents a capability that may be advertised by the server.

     capability-entry =/ compress-capability

     compress-capability = "COMPRESS" 1*(WS algorithm)

5.3.  General Non-terminals

     algorithm = %s"DEFLATE" / 1*20alg-char  ; case-sensitive
     alg-char = UPPER / DIGIT / "-" / "_"

6.  Summary of Response Codes

   This section defines the following new response code.  It is not
   multi-line and has no arguments.

   Response code 206
      Generated by: COMPRESS
      Meaning: compression layer activated

Top      ToC       Page 14 
7.  Security Considerations

   Security issues are discussed throughout this document.

   In general, the security considerations of the NNTP core
   specification ([RFC3977], Section 12) and the DEFLATE compressed data
   format specification ([RFC1951], Section 6) are applicable here.

   Implementers should be aware that combining compression with
   encryption like TLS can sometimes reveal information that would not
   have been revealed without compression, as explained in Section 6 of
   [RFC3749].  As a matter of fact, adversaries that observe the length
   of the compressed data might be able to derive information about the
   corresponding uncompressed data.  The CRIME and the BREACH attacks
   ([RFC7457], Section 2.6) are examples of such case.

   In order to help mitigate leaking authentication credentials, this
   document states in Section 2.2.2 that authentication MUST NOT be
   attempted after a successful use of COMPRESS.  Therefore, when a
   client wants to authenticate, compress data, and negotiate a TLS
   security layer (without TLS-level compression) in the same NNTP
   connection, it MUST use the STARTTLS, AUTHINFO, and COMPRESS commands
   in that order.  Of course, instead of using the STARTTLS command, a
   client can also use implicit TLS, that is to say it begins the TLS
   negotiation immediately upon connection on a separate port dedicated
   to NNTP over TLS.

   NNTP commands other than AUTHINFO are not believed to divulge
   confidential information as long as only public Netnews newsgroups
   and articles are accessed.  That is why this specification only
   prohibits the use of AUTHINFO after COMPRESS.  In case confidential
   articles are accessed in private newsgroups, special care is needed:
   implementations SHOULD NOT compress confidential data together with
   public data when a TLS [RFC5246] or SASL [RFC4422] security layer is
   active.  As a matter of fact, adversaries that observe the length of
   the compressed data might be able to derive information about it,
   when public data (that adversaries know is read) and confidential
   data are compressed in the same compression session.

   Additionally, it is preferable not to compress the contents of two
   distinct confidential articles together if it can be avoided, as
   adversaries might be able to derive information about them (for
   instance, if they have a few header fields or body lines in common).
   This can be achieved, for instance, with DEFLATE by clearing the
   compression dictionary each time a confidential article is sent.
   More complex implementations are, of course, possible and encouraged.

Top      ToC       Page 15 
   Implementations are encouraged to unconditionally allow compression
   when no security layer is active, and to support an option to enable
   or disable compression when a security layer is active.  Such an
   option could, for instance, have global scope or be server/
   connection-based.  Besides, as compression may in general weaken the
   confidentiality of a security layer, implementations SHOULD NOT
   automatically enable compression when a security layer is active
   unless the user explicitly enabled it with this knowledge.

   Future extensions to NNTP that define commands conveying confidential
   data SHOULD be sure to state that these confidential data SHOULD NOT
   be compressed together with public data when a security layer is
   active.

   Last but not least, careful consideration should be given to
   protections against implementation errors that introduce security
   risks with regards to compression algorithms.  See, for instance, the
   part of Section 6 of [RFC3749] about compression algorithms that can
   occasionally expand, rather than compress, input data.

8.  IANA Considerations

8.1.  "NNTP Compression Algorithms" Registry

   The "NNTP Compression Algorithms" registry is maintained by IANA.
   The registry is available at
   <http://www.iana.org/assignments/nntp-parameters>.

   The purpose of this registry is not only to ensure uniqueness of
   values used to name NNTP compression algorithms, but also to provide
   a definitive reference to technical specifications detailing each
   NNTP compression algorithm available for use on the Internet.

   An NNTP compression algorithm is either a private algorithm, or its
   name is included in the IANA "NNTP Compression Algorithms" registry
   (in which case it is a "registered NNTP compression algorithm").
   Different entries in the registry MUST use different names.

   Private algorithms with unregistered names are allowed, but SHOULD
   NOT be used because it is difficult to achieve interoperability with
   them.

   The 206, 403, and 502 response codes that a news server answers to
   the COMPRESS command using a private compression algorithm MUST have
   the same meaning as the one documented in Section 2.2 of this
   document.

Top      ToC       Page 16 
   The procedure detailed in Section 8.1.1 is to be used for
   registration of a value naming a specific individual compression
   algorithm.

   Any name that conforms to the syntax of an NNTP compression algorithm
   name (Section 5.3) can be used.  Especially, NNTP compression
   algorithms are named by strings, from 1 to 20 characters in length,
   consisting of uppercase letters, digits, hyphens, and/or underscores.

   Comments may be included in the registry as discussed in
   Section 8.1.2 and may be changed as discussed in Section 8.1.3.

8.1.1.  Algorithm Name Registration Procedure

   IANA will register new NNTP compression algorithm names on a First
   Come First Served basis, as defined in BCP 26 [RFC5226].  IANA has
   the right to reject obviously bogus registration requests, but will
   not perform a review of claims made in the registration form.

   Registration of an NNTP compression algorithm is requested by filling
   in the following template and sending it via electronic mail to IANA
   at <iana@iana.org>:

      Subject: Registration of NNTP compression algorithm Z

      NNTP compression algorithm name:

      Security considerations:

      Published specification (recommended):

      Contact for further information:

      Intended usage: (One of COMMON, LIMITED USE, or OBSOLETE)

      Owner/Change controller:

      Note: (Any other information that the author deems relevant may be
             added here.)

   While this registration procedure does not require expert review,
   authors of NNTP compression algorithms are encouraged to seek
   community review and comment whenever that is feasible.  Authors may
   seek community review by posting a specification of their proposed
   algorithm as an Internet-Draft.  NNTP compression algorithms intended
   for widespread use should be standardized through the normal IETF
   process, when appropriate.

Top      ToC       Page 17 
8.1.2.  Comments on Algorithm Registrations

   Comments on a registered NNTP compression algorithm should first be
   sent to the "owner" of the algorithm and/or to the mailing list for
   the now concluded NNTPEXT working group (<ietf-nntp@lists.eyrie.org>)
   of the IETF.

   Submitters of comments may, after a reasonable attempt to contact the
   owner and/or the above mailing list, request IANA to attach their
   comment to the NNTP compression algorithm registration itself by
   sending mail to <iana@iana.org>.  At IANA's sole discretion, IANA may
   attach the comment to the NNTP compression algorithm's registration.

8.1.3.  Change Control

   Once an NNTP compression algorithm registration has been published by
   IANA, the owner may request a change to its definition.  The change
   request follows the same procedure as the initial registration
   request.

   The owner of an NNTP compression algorithm may pass responsibility
   for the algorithm to another person or agency by informing IANA; this
   can be done without discussion or review.

   The IESG may reassign responsibility for an NNTP compression
   algorithm.  The most common case of this will be to enable changes to
   be made to algorithms where the owner of the registration has died,
   has moved out of contact, or is otherwise unable to make changes that
   are important to the community.

   NNTP compression algorithm registrations MUST NOT be deleted;
   algorithms that are no longer believed appropriate for use can be
   declared OBSOLETE by a change to their "intended usage" field; such
   algorithms will be clearly marked in the registry published by IANA.

   The IESG is considered to be the owner of all NNTP compression
   algorithms that are on the IETF Standards Track.

Top      ToC       Page 18 
8.2.  Registration of the DEFLATE Compression Algorithm

   This section gives a formal definition of the DEFLATE compression
   algorithm as required by Section 8.1.1 for the IANA registry.

      NNTP compression algorithm name: DEFLATE

      Security considerations: See Section 7 of this document

      Published specification: This document

      Contact for further information: Authors of this document

      Intended usage: COMMON

      Owner/Change controller: IESG <iesg@ietf.org>

      Note: This algorithm is mandatory to implement

   This registration appears as follows in the "NNTP Compression
   Algorithms" registry:

   +------------+------------+--------------+--------------+-----------+
   | Algorithm  | Intended   | Comment      | Change       | Reference |
   | Name       | Usage      |              | Controller   |           |
   +------------+------------+--------------+--------------+-----------+
   | DEFLATE    | COMMON     | Mandatory to | IESG         | RFC 8054  |
   |            |            | implement    |              |           |
   +------------+------------+--------------+--------------+-----------+

8.3.  Registration of the NNTP COMPRESS Extension

   This section gives a formal definition of the COMPRESS extension as
   required by Section 3.3.3 of [RFC3977] for the IANA registry.

   o  The COMPRESS extension allows an NNTP connection to be effectively
      and efficiently compressed.

   o  The capability label for this extension is "COMPRESS", whose
      arguments list the available compression algorithms.

   o  This extension defines one new command, COMPRESS, whose behavior,
      arguments, and responses are defined in Section 2.2.

   o  This extension does not associate any new responses with
      pre-existing NNTP commands.

Top      ToC       Page 19 
   o  This extension does affect the overall behavior of both server and
      client, in that after successful use of the COMPRESS command, all
      communication is transmitted in a compressed format.

   o  This extension does not affect the maximum length of commands or
      initial response lines.

   o  This extension does not alter pipelining, but the COMPRESS command
      cannot be pipelined.

   o  Use of this extension does alter the capabilities list; once the
      COMPRESS command has been used successfully, the COMPRESS
      capability can no longer be advertised by CAPABILITIES.
      Additionally, the STARTTLS and MODE-READER capabilities MUST NOT
      be advertised, and the AUTHINFO capability label MUST either be
      listed with no arguments or not advertised at all after a
      successful execution of the COMPRESS command.

   o  This extension does not cause any pre-existing command to produce
      a 401, 480, or 483 response code.

   o  This extension is unaffected by any use of the MODE READER
      command; however, the MODE READER command MUST NOT be used in the
      same session following a successful execution of the COMPRESS
      command.

   o  The STARTTLS and AUTHINFO commands MUST NOT be used in the same
      session following a successful execution of the COMPRESS command.

   o  Published Specification: This document.

   o  Contact for Further Information: Authors of this document.

   o  Change Controller: IESG <iesg@ietf.org>

   This registration will appear as follows in the "NNTP Capability
   Labels" registry contained in the "Network News Transfer Protocol
   (NNTP) Parameters" registry:

        +----------+----------------------------------+-----------+
        | Label    | Meaning                          | Reference |
        +----------+----------------------------------+-----------+
        | COMPRESS | Supported compression algorithms | RFC 8054  |
        +----------+----------------------------------+-----------+

Top      ToC       Page 20 
9.  References

9.1.  Normative References

   [RFC1951]  Deutsch, P., "DEFLATE Compressed Data Format Specification
              version 1.3", RFC 1951, DOI 10.17487/RFC1951, May 1996,
              <http://www.rfc-editor.org/info/rfc1951>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3977]  Feather, C., "Network News Transfer Protocol (NNTP)",
              RFC 3977, DOI 10.17487/RFC3977, October 2006,
              <http://www.rfc-editor.org/info/rfc3977>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,
              <http://www.rfc-editor.org/info/rfc7405>.

9.2.  Informative References

   [CRIME]    Rizzo, J. and T. Duong, "The CRIME Attack", Ekoparty
              Security Conference, 2012.

   [IEEE.1003.1-2008]
              IEEE, "Information Technology - Portable Operating System
              Interface (POSIX(R))", IEEE Standard 1003.1-2008,
              DOI 10.1109/IEEESTD.2016.7582338, 2008,
              <https://standards.ieee.org/findstds/
              standard/1003.1-2008.html>.

   [MNP]      Held, G., "The Complete Modem Reference", Second
              Edition, John Wiley & Sons, Inc., May 1994.

Top      ToC       Page 21 
   [RFC1962]  Rand, D., "The PPP Compression Control Protocol (CCP)",
              RFC 1962, DOI 10.17487/RFC1962, June 1996,
              <http://www.rfc-editor.org/info/rfc1962>.

   [RFC3749]  Hollenbeck, S., "Transport Layer Security Protocol
              Compression Methods", RFC 3749, DOI 10.17487/RFC3749, May
              2004, <http://www.rfc-editor.org/info/rfc3749>.

   [RFC4422]  Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
              Authentication and Security Layer (SASL)", RFC 4422,
              DOI 10.17487/RFC4422, June 2006,
              <http://www.rfc-editor.org/info/rfc4422>.

   [RFC4642]  Murchison, K., Vinocur, J., and C. Newman, "Using
              Transport Layer Security (TLS) with Network News Transfer
              Protocol (NNTP)", RFC 4642, DOI 10.17487/RFC4642, October
              2006, <http://www.rfc-editor.org/info/rfc4642>.

   [RFC4643]  Vinocur, J. and K. Murchison, "Network News Transfer
              Protocol (NNTP) Extension for Authentication", RFC 4643,
              DOI 10.17487/RFC4643, October 2006,
              <http://www.rfc-editor.org/info/rfc4643>.

   [RFC4644]  Vinocur, J. and K. Murchison, "Network News Transfer
              Protocol (NNTP) Extension for Streaming Feeds", RFC 4644,
              DOI 10.17487/RFC4644, October 2006,
              <http://www.rfc-editor.org/info/rfc4644>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <http://www.rfc-editor.org/info/rfc4648>.

   [RFC4978]  Gulbrandsen, A., "The IMAP COMPRESS Extension", RFC 4978,
              DOI 10.17487/RFC4978, August 2007,
              <http://www.rfc-editor.org/info/rfc4978>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC7457]  Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
              Known Attacks on Transport Layer Security (TLS) and
              Datagram TLS (DTLS)", RFC 7457, DOI 10.17487/RFC7457,
              February 2015, <http://www.rfc-editor.org/info/rfc7457>.

Top      ToC       Page 22 
   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <http://www.rfc-editor.org/info/rfc7525>.

   [V42bis]   International Telecommunications Union, "Data compression
              procedures for data circuit-terminating equipment (DCE)
              using error correction procedures", ITU-T
              Recommendation V.42bis, January 1990,
              <http://www.itu.int/rec/T-REC-V.42bis>.

   [yEnc]     Helbing, J., "yEnc - Efficient encoding for Usenet and
              eMail", March 2002, <http://www.yenc.org/>.

Acknowledgments

   This document draws heavily on ideas in [RFC4978] by Arnt
   Gulbrandsen; a large portion of this text was borrowed from that
   specification.

   The authors would like to thank the following individuals for
   contributing their ideas and reviewing this specification: Mark
   Adler, Russ Allbery, Stephane Bortzmeyer, Francis Dupont, Angel
   Gonzalez, Barry Leiba, John Levine, and Brian Peterson.

   Special thanks to our Document Shepherd, Michael Baeuerle, who
   significantly helped to increase the quality of this specification,
   and to Stephen Farrell for his encouragement to pursue the efforts in
   standardizing this NNTP extension.

   Many thanks to the Responsible Area Director, Alexey Melnikov, for
   reviewing and sponsoring this document.

Top      ToC       Page 23 
Authors' Addresses

   Kenneth Murchison
   Carnegie Mellon University
   5000 Forbes Avenue
   Pittsburgh, PA  15213
   United States of America

   Phone: +1 412 268 1982
   Email: murch@andrew.cmu.edu


   Julien Elie
   10 allee Clovis
   Noisy-le-Grand  93160
   France

   Email: julien@trigofacile.com
   URI:   http://www.trigofacile.com/