one using an interface on one of the routers in an area. See Figure 12.15. Border Gateway Protocol (BGP) Between the AS the BGP is used. This is the mandatory exterior gateway protocol used between ISPs. The BGP is a path vector routing protocol. With path vector routing, the Understanding Changing Telecommunications - Building a Successful Telecom Business Area 0.0.0.2 (MGW) Router Router, (MGW) Router Router, Area 0.0.0.1 Area 0.0.0.3 "LAN" Area 0.0.0.0 Router Router (MGW) Router, Router Transit "LAN" Router (MGW) Router Figure 12.15 OSPF areas in hierarchical IP router networks routers simply add the AS number before sending route information. This enables the receivers to choose the shortest path. The path vector routing carries information about all the autonomous systems it passes on the way to the destination. Therefore the router can choose not only the shortest path, but also it can choose paths depending on the policies between two ISPs. BGP neighbours are called peers. The connection between the peers is provided by TCP. 12.9 IP QoS 12.9.1 RSVP RSVP was briefly described in Chapter 7. RSVP is not used very much in the backbone, since it does not scale very well. A special signalling protocol, RSVP-TE, is used in MPLS. TE stands for traffic engineering. 12.9.2 DIFFERENTIATED SERVICES A brief introduction to differentiated services is supplied in Chapter 7. Essentially, differ- entiated services (Diffserv) specify the transfer priority of a packet as it passes through each network device on its journey through the network. When using Diffserv a number of mappings are necessary. First, the actual types of traffic stream, such as VoIP and IP-VPN, are mapped to a number of traffic flows, called behaviour aggregates (BA). Traffic belonging to a certain BA is treated in the same way. These service classes are then related to a code point value. This value is called the Diffserv code point (DSCP) and it is carried in the IP headers. It is often referred to just as code point. Finally, the DSCP values are mappe