XMPP Core  specifies the use of Transport Layer Security (TLS; see RFC 5246 ) for encryption of XML streams, and TLS includes the ability to compress encrypted traffic (see RFC 3749 ). 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.
The protocol flow is as follows:
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:
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:
If the receiving entity finds the requested method unacceptable or unworkable for any other reason, it MUST return a <setup-failed/> error:
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:
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:
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:
The following business rules apply:
For detailed recommendations regarding the order of stream feature negotiation, refer to Recommended Order of Stream Feature Negotiation (XEP-0170) .
Support for the ZLIB compression method as specified in RFC 1950  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.
Implementations MAY support the following methods in addition to ZLIB:
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.
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.
Many of the security considerations related to TLS compression (see Section 6 of RFC 3749) also apply to stream compression.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
The XMPP Registrar includes 'http://jabber.org/protocol/compress' in its registry of protocol namespaces.
The XMPP Registrar maintains a registry of compression methods at <https://xmpp.org/registrar/compress.html>.
In order to submit new values to this registry, the registrant shall define an XML fragment of the following form and either include it in the relevant XMPP Extension Protocol or send it to the email address <firstname.lastname@example.org>:
The registrant may register more than one compression method at a time, each contained in a separate <method/> element.
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This XMPP Extension Protocol is copyright © 1999 – 2020 by the XMPP Standards Foundation (XSF).
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## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. ##
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This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <https://xmpp.org/about/xsf/ipr-policy> or obtained by writing to XMPP Standards Foundation, P.O. Box 787, Parker, CO 80134 USA).
The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) 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.
The primary venue for discussion of XMPP Extension Protocols is the <email@example.com> 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 <firstname.lastname@example.org>.
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".
7. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.
8. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <https://xmpp.org/registrar/>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Per a vote of the XMPP Council, advanced status to Final.
Moved specification of LZW algorithm to XEP-0229.
Clarified when compression shall be considered to start; per XEP-0170, specified that compression should be negotiated after SASL.
More completely specified error handling; mentioned LZW (DCLZ) method.
Per a vote of the Jabber Council, advanced status to Draft.
Modifications to address Council feedback: used RFC 3920 terminology; specified error conditions; specified ZLIB as mandatory to implement.
Corrected several errors in the schemas.
Specified compression methods registry.
Fixed TLS text per list discussion.