DesignXMPP im » History » Revision 3
Revision 2 (Anonymous, 02/29/2012 03:32 PM) → Revision 3/29 (Anonymous, 02/29/2012 03:42 PM)
= Gateway for Instant Messaging= [[TOC(DesignXMPP_im, depth=2)]] In XMPP there are several types of messages which lead to different semantics when exchanging XMPP ''message stanzas'' between 2 endpoints. This section focuses only on message types that allow 2 endpoints to send instant messages to each other. Message types: * Normal: the default message type. A reply is not expected from the recipient. * Chat: This message type implies both parties have engaged a conversation. * Headline: An endpoint receiving this type of message should never reply, since it's meant to be used by servers or other entities to deliver announcements. In SIP there are currently 2 ways of doing Instant Messaging: * SIP MESSAGE (RFC 3428) * MSRP (RFC 4975) The first one is session-less and the latter is session based. The mechanisms described here follow the currently available specifications for SIP-XMPP interoperability: * http://xmpp.org/internet-drafts/draft-saintandre-sip-xmpp-im-01.html * http://xmpp.org/internet-drafts/draft-saintandre-sip-xmpp-chat-03.html == XMPP single message <-> SIP MESSAGE == XMPP single messages are mapped directly to SIP MESSAGE requests and ''vice versa''. [[Image(xmppgq_im_normal.png)]] TODO (image) === Overview === The mechanism for translating XMPP normal message stanzas and SIP MESSAGE requests is straightforward, they map one to one. However, since SIP is used mainly with UDP as a transport, if a XMPP stanza is bigger than 1500 bytes it will be chunked into smaller pieces to avoid ethernet fragmentation related issues. Since SIP MESSAGE is a non INVITE transaction, it has to be replied immediately, because there is no way to avoid retransmissions. This means that the SIP-XMPP gateway will reply on the SIP side before knowing if the message was actually delivered to the XMPP side. In order to express this a "202 Accepted" reply will be sent to the SIP request instead of a "200 OK". On the other hand, when an XMPP stanza is translated into a SIP MESSAGE request the SIP-XMPP gateway is able to report back the result (in case of error) by using a message stanza of type ''error''. This is possible because of the asynchronous nature of stanza processing in the XMPP protocol.