- defines basic unit of transfer (packet or
datagram ) - routing
- defines rules of operation (e.g., when to
discard packet)
- header
- version: 4
- header length: 4 (# of 32-bit words)
- service type: 8
- total length: 16
- identification: 16
- flags: 3
- fragment offset: 13 (# of
8-octets)
- time to live: 8
- protocol: 8
- header checksum: 16
- source IP address: 32
- destination IP address: 32
- IP options: 24
- padding: 8
- data: <64k
IP Datagram.
Adapted from Comer,IWTCP/IP, Figure
7.3.
- Ethernet
- datagrams
- problem: different maximum transfer units (MTUs) for constituent LANs
- router fragments datagram
- duplicates most of header
- adds fragment offset
- fragmented datagrams reassembled by
destination host
- performed (usually) by special-purpose computer, called a router (gateway)
- IP address
- form: X.X.X.X
- example: 152.2.128.184
- domain name
- unrelated to IP address
- form: host.domain, where domain is usually network.type
- domain names meant for human consumption
- domain names are mapped to IP addresses by a domain name server (DNS)
- direct routing
- performed by router or host when destination on same network
- map IP address into physical (e.g., Ethernet) address
- encapsulate datagram into network packet (e.g., Ethernet frame)
- deliver it to destination using network resources (e.g., send over Ethernet wire)
- indirect routing
- router to router transfers, concluding with a single direct routing transfer
- router uses IP routing table to
determine where to send datagram
("next hop")
- pairs of IP addresses: (destination, next hop)
- next hop connected to router's network
- default next hop, to reduce size of table
