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Internetworking

Internetworking. COS 461 Spring 2003. 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. Outline. Best Effort Service Model

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Internetworking

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  1. Internetworking COS 461 Spring 2003 CS 461

  2. 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 CS 461

  3. Outline Best Effort Service Model Global Addressing Scheme CS 461

  4. Network 1 (Ethernet) H7 R3 H8 H2 H1 H3 Network 4 (point-to-point) Network 2 (Ethernet) R1 R2 H4 Network 3 (FDDI) H5 H6 H1 H8 TCP TCP R1 R2 R3 IP IP IP IP IP FDDI PPP ETH ETH ETH FDDI PPP ETH IP Internet • Concatenation of Networks • Protocol Stack CS 461

  5. 0 4 8 16 19 31 TOS Length V ersion HLen Ident Flags Offset TTL Protocol Checksum SourceAddr DestinationAddr Pad Options (variable) (variable) Data 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 CS 461

  6. 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 CS 461

  7. Start of header Ident = x Offset = 0 0 Rest of header 1400 data bytes Start of header Ident = x 1 Offset = 0 H1 R1 R2 R3 H8 Rest of header 512 data bytes Start of header ETH IP (1400) FDDI IP (1400) PPP IP (512) ETH IP (512) Ident = x 1 Offset = 512 PPP IP (512) ETH IP (512) Rest of header PPP IP (376) ETH IP (376) 512 data bytes Start of header Ident = x 0 Offset = 1024 Rest of header 376 data bytes Example CS 461

  8. 7 24 A: 0 Network Host 14 16 B: 1 0 Network Host 21 8 C: 1 1 0 Network Host Global Addresses • Properties • globally unique • hierarchical: network + host • Dot Notation • 10.3.2.4 • 128.96.33.81 • 192.12.69.77 CS 461

  9. 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) Network Number Next Hop 1 R3 2 R1 3 interface 1 4 interface 0 CS 461

  10. 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 CS 461

  11. 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 upon reference CS 461

  12. 0 8 16 31 Hardware type = 1 ProtocolT ype = 0x0800 HLen = 48 PLen = 32 Operation SourceHardwareAddr (bytes 0 – 3) SourceHardwareAddr (bytes 4 – 5) SourceProtocolAddr (bytes 0 – 1) SourceProtocolAddr (bytes 2 – 3) T argetHardwareAddr (bytes 0 – 1) T argetHardwareAddr (bytes 2 – 5) T argetProtocolAddr (bytes 0 – 3) ARP Packet Format CS 461

  13. 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 CS 461

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