July 2007


Isode's M-Link product provides presence and real time messaging services using XMPP. XMPP is the Internet Standard eXtensible Messaging and Presence Protocol, sometimes referred to as Jabber. This paper sets out:

  1. An introduction to XMPP.
  2. Why Presence and Real Time Messaging are important to Isode's customers and markets.
  3. Why XMPP, and not another technology.
  4. Broad goals of Isode's use of XMPP.

Feedback on this strategy and input on product requirements are welcome.

Creative Commons License

How does XMPP Work?

XMPP defines protocols for communicating between a client and a server, and between two servers, as illustrated below. An XMPP Client will talk to the server with which it is registered.

XMPP Communications

XMPP Clients are named in the same way as email addresses (e.g., Juliet@capulet.com) and XMPP Servers are named as domains (e.g., capulet.com is juliet@capulet.com's XMPP server).

XMPP Messages are extensible, and can be used to carry messages of different types. For example, an XMPP Message can carry Instant Messaging (IM) type traffic. To send a message, an XMPP Client will simply send it to its local server. If the recipient XMPP client uses the same XMPP server, the XMPP server will send it directly to the recipient. If the recipient uses a different XMPP server, the XMPP server will send the message to the recipient's XMPP server. There are some key differences between this scenario and traditional store and forward messaging (email):

  1. XMPP messages have a single recipient only.
  2. Messages delivery is optimized for clients that are online (i.e., to clients that are connected to their XMPP server).
  3. Messages do not have to be stored reliably (e.g., on a disk), so may be dropped if the XMPP server fails.
  4. XMPP servers are generally optimized for short messages (1 kbyte or less).

This scenario is familiar to many IM users, although many of the well known IM services are single server and do not communicate with other servers in the way that XMPP does. Although IM is an important and well known use of the messaging part of XMPP, it is important to understand that the messaging part of XMPP can be used to carry data other than IM. XMPP is much more than just IM.

XMPP Clients can be interactive GUIs or applications. The basic messaging part of XMPP can be used as a building block for more complex applications, and in particular to transfer XML data defined in support of other applications. Chat rooms are an extension to the core XMPP service, providing a mechanism for multiple users to exchange and share information.

In order for this messaging to work usefully, a mechanism is needed to determine if and where a client is online. This is handled by the Presence part of XMPP. When an XMPP Client connects to its server, its basic presence is registered, which can be qualified by additional information provided by the XMPP user, such as "in a meeting" . Thus the XMPP server has presence information for all local clients. Each XMPP Client maintains on the XMPP server a list of users, called a Roster

The XMPP server determines presence status for each member of the roster, and keeps this information up to date. This requires XMPP servers to share presence status of their clients, to enable information on each client's roster to be kept up to date. The functionality to efficiently manage presence in a distributed environment is the hard part of XMPP.

XMPP Client to Client Communications

XMPP provides a mechanism for XMPP clients to exchange messages through the XMPP servers. This message exchange can be used "as is", for example to support IM traffic. This message exchange can also be used to enable two applications using XMPP clients to establish a direct connection. A good example of this is the JINGLE mechanism based on XMPP that enables a direct connection to be established for voice traffic (which has too high a data volume to be sent indirectly through the XMPP servers). The presence part of XMPP is critical, as in general the clients would not have a fixed location. XMPP can play a key role

XMPP has been widely adopted, including by Google as part of Google Talk, and there are many XMPP clients and servers available. Technical information on XMPP can be found on the XMPP Standards Foundation (XSF) Web site.

Why Real Time Messaging?

IM, based on real time messaging, has become mainstream. Users increasingly expect to have IM provided alongside email (store and forward messaging). Real time messaging is an important building block for distributed systems that can be used in conjunction with store and forward messaging. Services that can be provided include:

  • Multi-User Chat.
  • White Boarding.
  • Publishing and sharing data between applications, using the PubSub publish and subscribe mechanism.
  • Setup of direct communication such as:
    • File Transfer
    • Voice
    • Streaming Video

Why Presence?

Presence is not such an obvious service to the end user, but it is critical both to enable services such as IM and to enable direct client to client communication. It is a key building block for advanced communication applications. Extended Presence allows sharing of extended information, and provides support for capabilities such as geo-location.


Isode believes that XMPP will become the single standard for Real Time Messaging and Presence services.

Presence and IM are on their way to becoming universal services. In support of this, users will want to have a single IM/Presence address that can be used with everyone. For many users, it would be convenient to have this address the same as the user's email address. XMPP enables this.

Today, many users advertise multiple IM addresses associated with different centralized IM service providers such AIM, MSN and Yahoo!. This is reminiscent of messaging in the 1980s, when users would list a set of email providers, such as AOL and CompuServe. Isode believes that convergence will happen for IM, in an analogous way to the way that email convergence happened.

Convergence requires an open standard supported by a recognized standards body, that defines both client/server and server/server operation. Instant Message and Presence Service (IMPS) from the Open Mobile Alliance meets two of these criteria, but does not define a server to server protocol, which is key for an integrated global service.

SIMPLE (Session Initiation Protocol for Instant Messaging and Presence Leveraging Extensions) is an Internet Standard, and the only serious alternative to XMPP. Isode believes that XMPP has already won out over SIMPLE for a number of reasons:

  • XMPP has a much larger deployed base, and is actively deployed in a distributed manner using server/server communication.
  • The core SIMPLE specification is significantly more complex than XMPP.
  • Scaling analysis shows that XMPP scales to large deployments in a much better manner than SIMPLE.
  • There are many more XMPP client and server products.
  • XMPP has a very active community centred on the XMPP Standards Foundation that is working on a wide family of standards based around XMPP.

In summary, we believe that XMPP is the only credible open standards choice.

XMPP and Isode

XMPP is important technology in its own right. The rest of this paper looks at how XMPP fits with other Isode products and technologies.

XMPP and Directory

Directory and XMPP presence deal with complementary information on a user:

  • Directory holds static information, such as email address and telephone number.
  • An XMPP server holds information that may not always be available and may change rapidly, such as location and activity.

There is immediate benefit in providing integration of this information in both directions:

  1. The directory can be used to hold static information for the XMPP server, including user information, and rosters. This approach is consistent with Isode's general approach of using directory to hold configuration information for Isode applications.
  2. Presence information stored in the XMPP server can be made available to directory clients, so that when information is looked up, this information is available to the directory use along with other information. For a white pages application, this would enable presence information such as “User in meeting until 11:30” to be displayed along with email address and other directory information. It would also enable an application making information delivery decisions to make use of presence information in the calculation.

XMPP and Store & Forward Messaging

Isode's store and forward messaging solutions are designed for high reliability environment. A key goal of many such environments is timely delivery, and an XMPP service can help here.

Isode provides monitoring capabilities, so that an operator can determine when messages have not been read by the intended recipient. (see here for more details). An XMPP service will help the operator in this situation, by providing information on availability of the intended recipient, and a mechanism to communicate with the intended recipient using IM to determine if the message can be handled soon. In the event that the operator determines that the message needs to be sent on to an alternate recipient, the XMPP capabilities can be used to help select an alternate recipient who is ready to handle the message immediately.

It can also make sense to automate the previous operator assisted scenario. Where a message may be processed by one of several recipients, presence information can be used by the store and forward messaging system to help automatically determine the best place to deliver the message.

XMPP and Events

Isode has a model of events and faults described in the Isode white paper Operational Monitoring and Control of Systems using Isode Servers. When an event or fault occurs that (may) require operator attention, it is important to provide a flexible approach to get this information to an operator. XMPP alerting of events can add value to this approach, either by sending information to an individual user or to a chat room.

This approach can be particularly helpful where the number of operator events is very low, and operators are handling many non-operational tasks. Use of a chat room can also be helpful where a number of people are providing cover. That chat room can then also be used to work out who is dealing with a specific event, and avoid duplication of effort.

XMPP Server and User Configuration

Isode provides Web based configuration of its Internet messaging servers using Internet Messaging Administrator (IMA). This includes integrated configurion of XMPP services, and can also be used to configure XMPP only configurations.

When a user is added, XMPP support is selectable as an option, leading to integrated support with email address and XMPP address being the same. This integrated provisioning is provided using a directory back end, and so can be easily be integrated with a third party provisioning system to give the same result.

Configuration of the M-Link server is also be done using directory and with GUI configuration provided by IMA.


This paper has set Isode’s high level XMPP Strategy, and how this fits with.other Isode products and technologies.