ons with the SIP Toolkit Signalling Figure 7-14 Voice stream SIP bridging. ISUP/ ISUP/ switch switch IP network H.248 H.248 In SIP bridging scenarios, neither PSTN end user is willing to lose any special feature provided by the PSTN just because the call is traversing a SIP network. Therefore, SIP bridging has to provide a certain level of fea- ture transparency. The information contained in the PSTN signalling mes- sages has to be preserved when converted to SIP messages in the IP network, so that the egress gateway can regenerate the PSTN messages properly at the remote-user terminal. Multipart Message Bodies Problem: PSTN protocols carry some infor- mation that cannot be mapped to any SIP header. Thus, the obvious solu- tion of performing a simple mapping from ISUP to SIP won't work because it leads to some loss of information in the ISUP message. If the informa- tion lost happens to relate to a certain PSTN feature, the feature is lost as well. To prevent information loss, PSTN protocol messages are carried by SIP in message bodies. The MIME media types for ISUP and QSIG Objects were defined for this purpose [draft-ietf-sip-isup-mime]. QSIG is a PSTN signalling protocol used by PBXs. A particular SIP message carries a mul- tipart message body that contains an ISUP message, for instance, and a session description using Session Description Protocol (SDP). For the Chapter 7 concrete case of ISUP to SIP interworking (which is probably one of the most interesting scenarios), a set of guidelines [draft-ietf-sip-isup] help implementers do a consistent mapping between both protocols. INFO Method MIME media types solve the problems of sending infor- mation in SIP messages that does not map to any SIP header. That's not the whole problem, however. Some PSTN protocols have messages that don't map to any SIP message. For instance, if a gateway receives a PSTN signalling message in the middle of a call, no SIP message can be sent to the egress gateway in order to transfer it from the PSTN. INFO [RFC 2976] method was creat