Telephony Routing over IP (TRIP) is a policy driven inter-administrative domain protocol for advertising the reachability of telephony destinations between location servers, and for advertising attributes of the routes to those destinations. TRIP's operation is independent of any signaling protocol, hence TRIP can serve as the 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 information includes the reachability of telephony destinations, the routes towards these destinations, and information about gateways towards those telephony destinations residing in the PSTN. LSs exchange sufficient routing information to construct a graph of ITAD connectivity so that routing loops may be prevented. In addition, TRIP can be used to exchange attributes necessary to enforce policies and to select routes based on path or gateway characteristics. This specification defines TRIP's transport and synchronization mechanisms, its finite state machine, and the TRIP data. This specification defines the basic attributes of TRIP. The TRIP attribute set is extendible, so additional attributes may be defined in future documents.

TRIP, used to distribute telephony routing information between telephony administrative domains, is modeled after the Border Gateway Protocol 4, which

is used to distribute routing information between administrative domains. TRIP is enhanced with some link state features, as in the Open Shortest Path First (OSPF) protocol, IS-IS, and the Server Cache Synchronization Protocol (SCSP). TRIP uses BGP's inter-domain transport mechanism, BGP's peer communication, BGP's finite state machine, and similar formats and attributes as BGP. Unlike BGP however, TRIP permits generic intra-domain LS topologies, which simplifies configuration and increases scalability in contrast to BGP's full mesh requirement of internal BGP speakers. TRIP uses an intra-domain flooding mechanism similar to that used in OSPF, IS-IS, and SCSP.

TRIP runs over a reliable transport protocol. This eliminates the need to implement explicit fragmentation, retransmission, acknowledgment, and sequencing. The error notification mechanism used in TRIP assumes that the transport protocol supports a graceful close, i.e., that all outstanding data will be delivered before the connection is closed.

TRIP's operation is independent of any particular telephony signaling protocol. Therefore, TRIP can be used as the routing protocol for any of these protocols, e.g., H.323 and SIP.

The LS peering topology is independent of the physical topology of the network. In addition, the boundaries of an ITAD are independent of the boundaries of the layer 3 routing autonomous systems. Neither internal nor external TRIP peers need to be physically adjacent.

Protocol Structure

Each TRIP message has a fixed-size header. There may or may not be a data portion following the header, depending on the message type

Length - unsigned integer indicates the total length of 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 the Length field must always be at least 3 and no greater than 4096, and may be further constrained depending on the message type. No padding of extra data after the message is allowed, so the Length field must have the smallest value possible given the rest of the message.

Type - unsigned integer indicates the type code of the message. The following type codes are defined:

• 1 - OPEN
• 2 - UPDATE
• 3 - NOTIFICATION
• 4 - KEEPALIVE

Related protocols: SIP, Megaco/H.248, H.323, BGP

Sponsor Source: TRIP is defined by IETF (http://www.ietf.org ) standard.

Reference: http://www.javvin.com/protocol/rfc3219.pdf: Telephony Routing over IP (TRIP)