This specification arose from work on deploying XMPP in high latency environments, with round trips of several second. Even with data transfer rates as low as 2400 bit per second, XMPP works well once connections are established as compressed messages are small and the protocols are fully asynchronous. However the combination of low data rate and high latency led to connection establishment times of several minutes. This was unworkable, particularly when connections were prone to failure.
The solution set out here is to eliminate all the intial handshaking and to start the S2S communication as if the handshaking had been correctly completed. This cannot be used for communication between an arbitrary pair of servers, as in general the negotiation associated with the handshaking is vital for correctly determining a variety of parameters for use in the connection. However, a pair of servers may operate by locally configuring information that would have been negotiated. This enables the pair of servers to eliminate initial handshaking and data exchange.
This specification can be considered as a profile for server to server XMPP communication, to enable XMPP deployment over high latency links. This profile MUST only be used where its use has been pre-agreed and configured for both participating servers.
An example scenario where this protocol is important is to support use of XMPP communication on an aircraft which only has slow high latency air ground communication, with round trips of several seconds (e.g., UHF or Satcom). Use of stanard XMPP protocols (server to server or client to server) leads to long setup times. This protocol can be deployed between an XMPP server on the aircraft and one on the ground. This will be operated over a closed private network, where security considerations can be addressed primarily at the network level.
In simple terms, this can be considered as operation of RFC 6121 communication between a pair of XMPP servers without the preliminary negotiation done in RFC 6120. It might be considered that the start point is the DONE box in Figure 3 of RFC 6121. The TCP connection is opened and messages start to flow, as if the preceding RFC 6120 exchanges had taken place. All configuration informaition, including choice of port is handled by the a priori configuration.
Note that stream open is not sent. The interaction takes place as if this had completed. All XML elements received over the stream are treated as if they were inside a normal S2S stream that has been opened with the following stream opening tag, defining the default namespace of jabber:server and the stream namespace of <stream:stream xmlns:stream='...' xmlns='...'>. When a stream is closed, a stream close SHOULD be exchanged.
A connection between a pair of servers will be by TCP, with or without TLS. The pair of servers need to identify each other at the connection level. Three mechanisms are noted:
The server will then associate one or more XMPP domains with this connection level identity.
Connections may be opened by one server only or by either server. The choice is part of the a priori configured agreement. It is generally recommended to allow connections to be opened by either server. However policy or network constraints may require that the connection is initiated by one server only. When a server initiates a connection it will generally use this connection to send messages to the other server. The server opening a connection is responsible for closing it at the end of its use.
Consider a scenario with two servers: server A and server B. When a connection is opened by server A to server B, the server B MAY use this connection to send messages to server A or MAY open a connection to server A. It is recommended that only a single connection is used in this scenario and so in many cases this protocol will proceed with a single bidirectional TCP connection and messages flowing in both directions. In the event of both servers opening connections at the same time, both TCP connections SHOULD be used unidirectional with messages sent on the connection opened by the message sender only.
Typically a pair of XMPP servers connecting using this protocl will communcicate with multiple domains (e.g., a base domain and a MUC domain). It is generally desirable to configure things so that all communications will share the same link, rather than establishing separate links for each domain, essentially piggy-backing multiple logical connections onto a single TCP connection. Two or more connections MAY be initiated from one server to the other but this is NOT RECOMMENDED.
An XMPP server receiving data over such a link should appropriately validate to and from elements of stream child elements. The rules for this SHOULD be controlled by an priori agreement. An inbound connection will generally be associated with several peer domains. A RECOMMENDED approach is to consider each of these peers in turn and validate in the manner of a peer XMPP server connected using RFC 6020 for that domain. In the event that an inbound message is not considered to be valid, it should be handled in a manner that this invalid message would be handled if it arrived over standard S2S.
This protocol MAY be deployed directly over TCP. This will often be appropriate for environments where network security is handled at IP or lower layers or where the system is operated in closed network environment.
This protocol may be deployed over TLS operating over TCP. If this is done, TLS client and or server X.509 based authentication may be used, with certificate validation achieved by PKI or simply pinning (configuring) a trusted certificate. This configuration and authentication is a part of the a priori configuration.
This protocol operates without the standard XMPP security negotiation. It is imperative that consideration is given to link security whenever this protocol is set up. The identity verification facilities of Server Dialback and SASL EXTERNAL are not available in this protocol so other mechanisms are needed. Use of TLS with mutual authentication is the approach that provides best security with this protocol.
When TLS is not used, the only option available is for the responder to identify the initiator based on source IP address and port. This mechanism is prone to attacks, and so should be used with care. Where source IP address is checked, this may be done directly by match of IP address or by use of reverse DNS lookup to identify the connecting server. If reverse DNS Lookup is used, it is RECOMMENDED to use DNS SEC to mitigate against DNS attacks.
Dave Cridland, Curtis King, Kevin Smith and Kurt Zeilenga worked out and validated the approach documented in this XEP.
Kevin Smith and Kurt Zeilenga provided review and input to this document.
This document in other formats: XML PDF
This XMPP Extension Protocol is copyright © 1999 – 2018 by the XMPP Standards Foundation (XSF).
Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation.
## 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. ##
In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages.
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 <firstname.lastname@example.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.
Given that this XMPP Extension Protocol normatively references IETF technologies, discussion on the <email@example.com> list might also be appropriate.
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".
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Assume an open <stream:stream>, implicitly parenting all XML elements received and sent.
Initial published version approved by the XMPP Council.