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Internetworking Outline Best Effort Service Model Global Addressing Scheme Spring 2002 CS 461 1 IP Internet • Concatenation of Networks H1 Netw ork 1 (Ethernet) H2 H7 H3 R3 Netw ork 4 (point-to-point) Netw ork 2 (Ethernet) R1 R2 H4 Netw ork 3 (FDDI) • Protocol Stack H6 H5 H1 H8 TCP R1 IP ETH Spring 2002 R2 IP ETH R3 IP FDDI FDDI IP PPP CS 461 PPP TCP IP ETH ETH 2 H8 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 SourceAddr DestinationAddr Options (variable) Pad (variable) Data Spring 2002 CS 461 3 Fragmentation and Reassembly • Each network has some MTU (Maximum Transmission Unit) • 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 IP does not recover from lost fragments Spring 2002 CS 461 4 Start of header Ident = x Example 0 Offset = 0 Rest of header (a) 1400 data bytes Start of header Ident = x H1 R1 R1 R2 R2 1 Offset = 0 H8 R3 R3 Rest of header 512 data bytes (b) ETH IP (1400) FDDI IP (1400) PPP IP (512) ETH IP (512) PPP IP (512) ETH IP (512) PPP IP (376) ETH IP (376) Start of header Ident = x 1 Offset = 64 Rest of header 512 data bytes Start of header Ident = x 0 Offset = 128 Rest of header 376 data bytes Spring 2002 CS 461 5 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 2002 0 7 24 Network Host 1 0 1 1 0 CS 461 14 16 Network Host 21 8 Network Host 6 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 (R2) Spring 2002 Network Number 1 2 3 4 CS 461 Next Hop R3 R1 interface 1 interface 0 7 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 2002 CS 461 8 ARP Details • Request Format – – – – – HardwareType: type of physical network (e.g., Ethernet) ProtocolType: 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 if not the target does not have an entry for the source Spring 2002 CS 461 9 ARP Packet Format 0 8 16 Hardware type = 1 HLen = 48 31 ProtocolType = 0x0800 PLen = 32 Operation SourceHardwareAddr (bytes 0― 3) SourceHardwareAddr (bytes ― 4 5) SourceProtocolAddr (bytes ― 0 1) SourceProtocolAddr (bytes 2― 3) TargetHardwareAddr (bytes ― 0 1) TargetHardwareAddr (bytes ― 2 5) TargetProtocolAddr (bytes 0― 3) Spring 2002 CS 461 10 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 2002 CS 461 11 Virtual Networks and Tunnels IP Tunnel: a virtual point-to-point link between a pair of nodes that are actually separated by an arbitrary number of networks. Spring 2002 CS 461 12