network protocols handbook.pdf

148Protocols GuideVoice Over IP(VOIP) - Other ProtocolsProtocol NameTRIP: Telephony Routing overIPProtocol DescriptionTelephony Routing over IP (TRIP) is a policy driven inter-administrativedomain protocol for advertising the reachability of telephonydestinations between location servers and for advertisingattributes of the routes to those destinations. TRIP’s operation isindependent of any signaling protocol; hence TRIP can serve asthe telephony routing protocol for any signaling protocol.The primary function of a TRIP speaker, called a location server(LS), is to exchange information with other LSs. This informationincludes the reachability of telephony destinations, the routestowards these destinations, and information about gateways towardsthose telephony destinations residing in the PSTN. LSsexchange sufficient routing information to construct a graph ofITAD connectivity so that routing loops may be prevented. Inaddition, TRIP can be used to exchange attributes necessaryto enforce policies and to select routes based on path or gatewaycharacteristics. This specification defines TRIP’s transportand synchronization mechanisms, its finite state machine, andthe TRIP data. This specification defines the basic attributes ofTRIP. The TRIP attribute set is extendible, so additional attributesmay be defined in future documents.TRIP, used to distribute telephony routing information betweentelephony administrative domains, is modeled after the BorderGateway Protocol 4, which is used to distribute routing informationbetween administrative domains. TRIP is enhanced withsome link state features, as in the Open Shortest Path First(OSPF) protocol, IS-IS, and the Server Cache SynchronizationProtocol (SCSP). TRIP uses BGP’s inter-domain transportmechanism, BGP’s peer communication, BGP’s finite state machine,and similar formats and attributes to BGP. Unlike BGPhowever, TRIP permits generic intra-domain LS topologies,which simplifies configuration and increases scalability in contrastto BGP’s full mesh requirement of internal BGP speakers.TRIP uses an intra-domain flooding mechanism similar to thatused in OSPF, IS-IS, and SCSP.TRIP runs over a reliable transport protocol. This eliminates theneed to implement explicit fragmentation, retransmission, acknowledgment,and sequencing. The error notification mechanismused in TRIP assumes that the transport protocol supportsa graceful close, i.e., that all outstanding data will be deliveredbefore the connection is closed.TRIP’s operation is independent of any particular telephony signalingprotocol. Therefore, TRIP can be used as the routingprotocol for any of these protocols, e.g., H.323 and SIP.The LS peering topology is independent of the physical topologyof the network. In addition, the boundaries of an ITAD are independentof the boundaries of the layer 3 routing autonomoussystems. Neither internal nor external TRIP peers need to bephysically adjacent.Protocol StructureEach TRIP message has a fixed-size header. There may or maynot be a data portion following the header, depending on themessage type.Length16 24bitType• Length - unsigned integer indicating the total lengthof the message, including the header, in octets. Thus,it allows one to locate, in the transport-level stream,the beginning of the next message. The value of theLength field must always be at least 3 and no greaterthan 4096, and may be further constrained dependingon the message type. No padding of extra dataafter the message is allowed, so the Length fieldmust have the smallest value possible given the restof the message.• Type - unsigned integer indicating the type code ofthe message. The following type codes are defined:1 - OPEN2 - UPDATE3 - NOTIFICATION4 - KEEPALIVERelated protocolsBGP-4, H.323, SIP, SCSPSponsor SourceTRIP is defined by IETF (www.ietf.org ) in RFC 3219.Referencehttp://www.javvin.com/protocol/rfc3219.pdfTelephony Routing over IP (TRIP)