IP Datagrams And Datagram Forwarding 1 Motivation For

Motivation For IP Packets • Because it can connect heterogeneous networks, a router cannot

Internet Packets • • • Abstraction Created and understood only by software Contains sender

Packet • Because it can connect heterogeneous networks, a router cannot transmit a copy

The Two Parts Of An IP Datagram • Header – Contains destination address –

Datagram Header • Three key fields – Source IP address – Destination IP address

IP Datagram Forwarding • Performed by routers • Similar to WAN forwarding – Table-driven

Example Of An IP Routing Table • Table (b) is for center router in

Routing Table Size • Because each destination in a routing table corresponds to a

Datagram Forwarding • • • Given a datagram Extract destination address field, D Look

Mask Field and Datagram Forwarding • The destination address in a datagram header always

Key Concept • The destination address in a datagram header always refers to the

IP Semantics • IP is connectionless – Datagram contains identity of destination – Each

IP Semantics (continued) • IP allows datagrams to be – Delayed – Duplicated –

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IP Datagrams And Datagram Forwarding 1

Motivation For IP Packets • Because it can connect heterogeneous networks, a router cannot transmit a copy of a frame that arrives on one network across another. To accommodate heterogeneity, an internet must define a hardware-independent packet format. 2

Internet Packets • • • Abstraction Created and understood only by software Contains sender and destination addresses Size depends on data being carried Called IP datagram 3

Packet • Because it can connect heterogeneous networks, a router cannot transmit a copy of a frame that arrives on one network across another. To accommodate heterogeneity, an internet must define a hardware-independent packet format 4

The Two Parts Of An IP Datagram • Header – Contains destination address – Fixed-size fields • Payload – Variable size up to 64 K – No minimum size 5

Datagram Header • Three key fields – Source IP address – Destination IP address – Type (contents) 6

IPV 4 Header 1. From Tanenbaum’s book 7

IP Datagram Forwarding • Performed by routers • Similar to WAN forwarding – Table-driven – Entry specifies next hop • Unlike WAN forwarding – Uses IP addresses – Next-hop is router or destination 8

An Example Internet 9

Example Of An IP Routing Table • Table (b) is for center router in part (a) 10

Routing Table Size • Because each destination in a routing table corresponds to a network, the number of entries in a routing table is proportional to the number of networks in an internet. 11

Datagram Forwarding • • • Given a datagram Extract destination address field, D Look up D in routing table Find next-hop address, N Send datagram to N 12

Mask Field and Datagram Forwarding • The destination address in a datagram header always refers to the ultimate destination. When a router forwards the datagram to another router, the address of the next hop does not appear in the datagram header. • If (Mask[i] & D) == Destination[i]) forward to Next. Hop[i]; 13

Key Concept • The destination address in a datagram header always refers to the ultimate destination. When a router forwards the datagram to another router, the address of the next hop does not appear in the datagram header. 14

IP Semantics • IP is connectionless – Datagram contains identity of destination – Each datagram sent/handled independently • Routes can change at any time 15

IP Semantics (continued) • IP allows datagrams to be – Delayed – Duplicated – Delivered out of order – Lost • Called best effort delivery • Motivation: accommodate all possible networks 16