Chapter 20 IP Datagrams and Datagram Forwarding

Connectionless vs Connection-oriented Service TCP/IP’s fundamental delivery service is connectionless Individual packets travel independently and contains information that identifies the intended recipient A reliable connection-oriented service is added on top of the underlying connectionless service

Datagram forwarding across heterogeneous networks Heterogeneous networks use different frame formats Router cannot forward a frame from one type of network to another without modification Two networks may use incompatible address formats (ie. address in a frame may make no sense on another network).

IP Datagram a universal, virtual, hardware-independent internet packet consisting of an IP header followed by data (fig 20.1)F(fig 20.1)F Source and destination addresses in the datagram header are IP addresses The size of an IP datagram(version 4) can vary from 1 byte of data to 64k bytes.

Routing/Forwarding of Datagrams process of using a routing table (fig 20.3) to select a next hop for a given datagram(fig 20.3) datagram with destination address D is masked with the i-th entry in the routing table to determine next hop address if ((Mask[i] & D ) == Destination[i] ) then forward to NextHop[i]

IP Datagram Header IP datagram header format (fig 20.4)(fig 20.4) Data header contains the ultimate destination, not the frame header When a router forwards the datagram to another router, the IP address of the next hop does not appear in the datagram header The address of the next hop is used to translate to a corresponding hardware address for transmission (ARP).

Unreliable Datagram Delivery IP makes a best-effort attempt to deliver each datagram No guarantee of datagram delivery Problems that can occur at layer 3 –datagram duplication due to excessive delay –out-of-order delivery –data corruption –datagram loss Higher layers of protocol software are needed to handle these errors.