Internetworking COS 461 Spring 2003 CS 461 1

Internetworking COS 461 Spring 2003 CS 461 1

Mechanics • Today: finish switches, cover IP – Should have read 4. 1 • Wednesday: Routing and TCP – Read 4. 2 -4. 4, 5. 1 -5. 2 • Projects: begin Project 2 – Can be done in pairs – Speak up soon if you want alternate projects Spring 2003 CS 461 2

Outline Best Effort Service Model Global Addressing Scheme Spring 2003 CS 461 3

IP Internet • Concatenation of Networks Network 1 (Ethernet) H 7 H 2 H 1 R 3 H 8 H 3 Network 4 (point-to-point) Network 2 (Ethernet) R 1 R 2 H 4 • Protocol Stack Network 3 (FDDI) H 5 H 6 H 1 H 8 TCP R 1 IP IP ETH Spring 2003 R 2 ETH R 3 IP FDDI IP PPP CS 461 PPP TCP IP ETH 4

Service Model • Connectionless (datagram-based) • Best-effort delivery (unreliable service) – – packets are lost packets are delivered out of order duplicate copies of a packet are delivered packets can be delayed for a long time • Datagram format 0 4 Version 8 HLen 16 TOS 31 Length Ident TTL 19 Flags Protocol Offset Checksum Source. Addr Destination. Addr Options (variable) Pad (variable) Data Spring 2003 CS 461 5

Fragmentation and Reassembly • Each network has some MTU • Design decisions – – – – fragment when necessary (MTU < Datagram) try to avoid fragmentation at source host re-fragmentation is possible fragments are self-contained datagrams use CS-PDU (not cells) for ATM delay reassembly until destination host do not recover from lost fragments Spring 2003 CS 461 6

Example Start of header Ident= x 0 Offset= 0 Rest of header 1400 data bytes Start of header H 1 R 2 R 3 H 8 Ident= x 1 Offset= 0 Rest of header 512 data bytes Start of header ETH IP (1400) FDDI IP (1400) PPP IP (512) ETH IP (512) Rest of header PPP IP (376) ETH IP (376) 512 data bytes Ident= x 1 Offset= 512 Start of header Ident= x 0 Offset= 1024 Rest of header 376 data bytes Spring 2003 CS 461 7

Global Addresses • Properties – globally unique – hierarchical: network + host • Dot Notation – 10. 3. 2. 4 – 128. 96. 33. 81 – 192. 12. 69. 77 A: B: C: Spring 2003 0 7 24 Network Host 1 0 1 1 0 CS 461 14 16 Network Host 21 8 Network Host 8

Datagram Forwarding • Strategy – – – every datagram contains destination’s address if connected to destination network, then forward to host if not directly connected, then forward to some router forwarding table maps network number into next hop each host has a default router each router maintains a forwarding table • Example (R 2) Spring 2003 Network Number 1 2 3 4 CS 461 Next Hop R 3 R 1 interface 0 9

Address Translation • Map IP addresses into physical addresses – destination host – next hop router • Techniques – encode physical address in host part of IP address – table-based • ARP – – table of IP to physical address bindings broadcast request if IP address not in table target machine responds with its physical address table entries are discarded if not refreshed Spring 2003 CS 461 10

ARP Details • Request Format – – – Hardware. Type: type of physical network (e. g. , Ethernet) Protocol. Type: type of higher layer protocol (e. g. , IP) HLEN & PLEN: length of physical and protocol addresses Operation: request or response Source/Target-Physical/Protocol addresses • Notes – – table entries timeout in about 10 minutes update table with source when you are the target update table if already have an entry do not refresh table entries upon reference Spring 2003 CS 461 11

ARP Packet Format 0 8 16 Hardware type = 1 HLen = 48 31 Protocol. T ype = 0 x 0800 PLen = 32 Operation Source. Hardware. Addr (bytes 0 – 3) Source. Hardware. Addr (bytes 4 – 5) Source. Protocol. Addr (bytes 0 – 1) Source. Protocol. Addr (bytes 2 – 3) Target. Hardware. Addr (bytes 0 – 1) Target. Hardware. Addr (bytes 2 – 5) Target. Protocol. Addr (bytes 0 – 3) Spring 2003 CS 461 12

Internet Control Message Protocol (ICMP) • • Echo (ping) Redirect (from router to source host) Destination unreachable (protocol, port, or host) TTL exceeded (so datagrams don’t cycle forever) Checksum failed Reassembly failed Cannot fragment Spring 2003 CS 461 13