XEP-0138: Stream Compression

NOTICE: The protocol defined herein is a Draft Standard of the XMPP Standards Foundation. Implementations are encouraged and the protocol is appropriate for deployment in production systems, but some changes to the protocol are possible before it becomes a Final Standard.

Table of Contents

1. Introduction

XMPP Core [1] specifies the use of Transport Layer Security (TLS; see RFC 4346 [2]) for encryption of XML streams, and TLS includes the ability to compress encrypted traffic (see RFC 3749 [3]). However, not all computing platforms are able to implement TLS, and traffic compression may be desirable for communication by applications on such computing platforms. This document defines a mechanism for negotiating the compression of XML streams outside the context of TLS.

2. Use Case

The protocol flow is as follows:

Example 1. Receiving Entity Offers Stream Compression Feature

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
  <compression xmlns='http://jabber.org/features/compress'>
    <method>zlib</method>
    <method>lzw</method>
  </compression>
</stream:features>
    

Note: The <compression/> element MUST contain at least one <method/> child element. Each <method/> element MUST contain XML character data that specifies the name of a compression method, and such method names SHOULD be registered as described in the Compression Methods Registry section of this document. The methods SHOULD be provided in order of preference.

The initiating entity then MAY request compression by specifying one of the methods advertised by the receiving entity:

Example 2. Initiating Entity Requests Stream Compression

<compress xmlns='http://jabber.org/protocol/compress'>
  <method>zlib</method>
</compress>
    

Note: If the initiating entity did not understand any of the advertised compression methods, it SHOULD ignore the compression option and proceed as if no compression methods were advertised.

If the initiating entity requests a stream compression method that is not supported by the receiving entity, the receiving entity MUST return an <unsupported-method/> error:

Example 3. Receiving Entity Reports That Method is Unsupported

<failure xmlns='http://jabber.org/protocol/compress'>
  <unsupported-method/>
</failure>
    

If the receiving entity finds the requested method unacceptable or unworkable for any other reason, it MUST return a <setup-failed/> error:

Example 4. Receiving Entity Reports That Negotiation of Stream Compression Failed

<failure xmlns='http://jabber.org/protocol/compress'>
  <setup-failed/>
</failure>
    

Note: Failure of the negotiation SHOULD NOT be treated as an unrecoverable error and therefore SHOULD NOT result in a stream error. In particular, the initiating entity is free to retry the compression negotiation if it fails.

If no error occurs, the receiving entity MUST inform the initiating entity that compression has been successfully negotiated:

Example 5. Receiving Entity Acknowledges Negotiation of Stream Compression

<compressed xmlns='http://jabber.org/protocol/compress'/>
    

Both entities MUST now consider the previous (uncompressed) stream to be null and void, just as with TLS negotiation and SASL negotiation (as specified in RFC 3920) and MUST begin compressed communications with a new (compressed) stream. Therefore the initiating entity MUST initiate a new stream to the receiving entity:

Example 6. Initiating Entity Initiates New (Compressed) Stream

<stream:stream
    xmlns='jabber:client'
    xmlns:stream='http://etherx.jabber.org/streams'
    to='shakespeare.lit'>
    

If compression processing fails after the new (compressed) stream has been established, the entity that detects the error SHOULD generate a stream error and close the stream:

Example 7. Entity Closes Stream Because of a Processing Error

<stream:error>
  <undefined-condition xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
  <failure xmlns='http://jabber.org/protocol/compress'/>
    <processing-failed/>
  </failure>
</stream:error>
</stream:stream>
    

3. Business Rules

The following business rules apply:

• If stream compression is negotiated, it MUST be used in both directions.
• TLS compression and stream compression SHOULD NOT be used simultaneously.
• Stream compression MAY be offered after TLS negotiation if TLS compression fails.
• Because negotiation of stream compression should not be completed after application of any encryption layers and because SASL negotiation (see RFC 3920) may involve application of an encryption layer, stream compression SHOULD be negotiated after SASL negotiation. For detailed recommendations regarding the order of stream feature negotiation, refer to Recommended Order of Stream Feature Negotiation [4].

4. Mandatory-to-Implement Technologies

Support for the ZLIB compression method as specified in RFC 1950 [5] is REQUIRED.

All other methods are OPTIONAL; such methods may be defined in future specifications or by registration as described in the Compression Methods Registry section of this document.

5. Optional Technologies

Implementations MAY support the following methods in addition to ZLIB:

• Stream Compression with LZW [6]

6. Implementation Notes

When using ZLIB for compression, the sending application SHOULD complete a partial flush of ZLIB when its current send is complete. Note that this statement is deliberately somewhat vague: the sending application may end up performing this partial flush after sending every XML stanza, but on the other hand may perform the partial flush only after sending a group of stanzas that have been queued up for delivery. When to flush the state of the compression application is up to the sending application.

7. Security Considerations

Stream encryption via TLS (as defined in RFC 3920) and stream compression (as defined herein) are not mutually exclusive, but stream encryption via TLS MUST be negotiated before negotiation of stream compression in order to secure the stream.

8. IANA Considerations

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [7].

9. XMPP Registrar Considerations

<method>
  <name>the XML character data of the method element</name>
  <desc>a natural-language description of the compression method</desc>
  <doc>the document that specifies or registers the compression method</doc>
</method>
        

<method>
  <name>zlib</name>
  <desc>the ZLIB compression method</desc>
  <doc>RFC 1950</doc>
</method>
        

10. XML Schemas

<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='http://jabber.org/features/compress'
    xmlns='http://jabber.org/features/compress'
    elementFormDefault='qualified'>

  <xs:annotation>
    <xs:documentation>
      The protocol documented by this schema is defined in
      XEP-0138: http://www.xmpp.org/extensions/xep-0138.html
    </xs:documentation>
  </xs:annotation>

  <xs:element name='compression'>
    <xs:complexType>
      <xs:sequence>
        <xs:element name='method' type='xs:NCName' maxOccurs='unbounded'/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

</xs:schema>
      

<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='http://jabber.org/protocol/compress'
    xmlns='http://jabber.org/protocol/compress'
    elementFormDefault='qualified'>

  <xs:import namespace='urn:ietf:params:xml:ns:xmpp-stanzas'/>

  <xs:annotation>
    <xs:documentation>
      The protocol documented by this schema is defined in
      XEP-0138: http://www.xmpp.org/extensions/xep-0138.html
    </xs:documentation>
  </xs:annotation>

  <xs:element name='compress'>
    <xs:complexType>
      <xs:sequence>
        <xs:element name='method' type='xs:NCName' minOccurs='1' maxOccurs='unbounded'/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

  <xs:element name='compressed' type='empty'/>

  <xs:element name='failure'>
    <xs:complexType>
      <xs:choice>
        <xs:element name='setup-failed' type='empty'/>
        <xs:element name='processing-failed' type='empty'/>
        <xs:element name='unsupported-method' type='empty'/>
        <xs:sequence xmlns:err='urn:ietf:params:xml:ns:xmpp-stanzas'>
          <xs:group ref='err:stanzaErrorGroup'/>
          <xs:element ref='err:text' minOccurs='0'/>
        </xs:sequence>
      </xs:choice>
    </xs:complexType>
  </xs:element>

  <xs:simpleType name='empty'>
    <xs:restriction base='xs:string'>
      <xs:enumeration value=''/>
    </xs:restriction>
  </xs:simpleType>

</xs:schema>
      

Appendices

Appendix A: Document Information

Series: XEP
Number: 0138
Publisher: XMPP Standards Foundation
Status: Draft
Type: Standards Track
Version: 1.3
Last Updated: 2007-09-26
Approving Body: XMPP Council
Dependencies: XMPP Core
Supersedes: None
Superseded By: None
Short Name: compress
XML Schema for compress namespace: <http://www.xmpp.org/schemas/compress.xsd>
XML Schema for feature namespace: <http://www.xmpp.org/schemas/compress-feature.xsd>
Registry: <http://xmpp.org/registrar/compress.html>
Source Control: HTML  RSS

Appendix B: Author Information

Appendix C: Legal Notices

Appendix D: Relation to XMPP

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 3920) and XMPP IM (RFC 3921) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.

Appendix E: Discussion Venue

The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.

Discussion on other xmpp.org discussion lists might also be appropriate; see <http://xmpp.org/about/discuss.shtml> for a complete list.

Errata can be sent to <editor@xmpp.org>.

Appendix F: Requirements Conformance

The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".

Appendix G: Notes

Appendix H: Revision History

END