relevant address of the external transport network. This is generally called address transla- tion and is performed by an address translator (AT) that is available to the IP- UDT. How this translation works depends on the type of external transport net- work. For example, the route in Fig. 3.76 is created before IP communication can start, by reserving a semipermanent VC connection between router RX and RY. On this VC, AAL3/4-UDTs are run. These elements provide a network service interface to the IP-UDTs that allow them to address each other by ATM addresses (aa1 and aa2). The translation between ATM addresses and the corresponding VC identifi- ers is done by the AAL3/4-UDTs. The address translation table basically contains two columns: the local route identifier and the ATM address, see Fig. 3.76 When routers connect hosts in a LAN, there is an alternative way to deal with address translation that is not shown in Fig. 3.76: the IP-UDT uses the Address Resolution Protocol (ARP, specified in RFC 826) to send out a broadcast message over the LAN, asking which of all hosts on the LAN has the particular IP address. The responding host delivers its LAN address. Note that this procedure only works if all hosts on LANs can handle the ARP (which is normally the case). There are two protocols included in the model in Fig. 3.76: the IP and the AAL3/4 protocol. The latter is chosen partly because it provides segmentation and reassembling. This allows the IP-UDT to send and receive arbitrarily size IP- SDUs. The AAL3/4-UDT fragments large IP-SDUs, considering the 48 octets that can be included in an ATM cell. Figure 3.77 shows the corresponding scenario for transport of an IP-IDU between host ipa3 and router PSX, interconnected over the ISDN packet access service. Note that hosts not deal with routes and address translation, unless they act both as hosts and routers, or they have connections to several routers. 3.4 Modelling Telecom Resource Systems source dest. Addr. Addr. IP-IDU IP-SDU IP-IDU IP-PDU ipa3 ipa1 userData IP p