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DesignXMPP im » History » Version 3

Anonymous, 02/29/2012 03:42 PM

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= Gateway for Instant Messaging=
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[[TOC(DesignXMPP_im, depth=2)]]





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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.





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Message types:





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 * Normal: the default message type. A reply is not expected from the recipient.





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 * Chat: This message type implies both parties have engaged a conversation.





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 * 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.





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In SIP there are currently 2 ways of doing Instant Messaging:





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 * SIP MESSAGE (RFC 3428)





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 * MSRP (RFC 4975)





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The first one is session-less and the latter is session based.





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The mechanisms described here follow the currently available specifications for SIP-XMPP interoperability:





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 * http://xmpp.org/internet-drafts/draft-saintandre-sip-xmpp-im-01.html





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 * http://xmpp.org/internet-drafts/draft-saintandre-sip-xmpp-chat-03.html





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== XMPP single message <-> SIP MESSAGE ==





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XMPP single messages are mapped directly to SIP MESSAGE requests and ''vice versa''.





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[[Image(xmppgq_im_normal.png)]]





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=== Overview ===





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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.





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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".





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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.