&xmppcore; specifies the use of Transport Layer Security (TLS; see &rfc5246;) for encryption of XML streams, and TLS includes the ability to compress encrypted traffic (see &rfc3749;). 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:

• 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 is not used.

For detailed recommendations regarding the order of stream feature negotiation, refer to &xep0170;.

Support for the ZLIB compression method as specified in &rfc1950; 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:

• &xep0229;

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.

Due to attacks like &CRIME; that apply equally to the zlib method defined here, this method is deemed insecure.

This document requires no interaction with &IANA;.