Internetworking

1. Internetworking CMSC 150

2. Assumptions • Data pipe from every machine to every other machine. • Need not be single link (and in most cases will involve several links and several networks). • Pipe can lose or corrupt data (think postal system analogy – vast majority of time it arrives, but not always). • We transmit data in discrete chunks called “packets” • So what info do we need to build a single “logical” network (either reliable or unreliable)? CMSC 150

3. Issues • Getting various technologies to work with one another (I.e. creating a single “network” from many heterogeneous systems). • Problem magnified since packet may need to traverse several different networks (and network technologies), each with their own addressing schemes, service models, media access protocols, etc. • Scale: It’s the big issue • How can you find an efficient path through a network with millions (and perhaps billions eventually) of nodes? • How do you provide addressing for a network with this many nodes? CMSC 150

4. Internetwork: • Arbitrary collection of possibly heterogeneous networks interconnected to provide host-to-host packet delivery service. • Network: Directly connected or switched network that uses a single technology (i.e. ATM, 802.5, Ethernet). • Could be many physical networks creating a single logical network. • E.g. several Ethernet networks connected on a university campus CMSC 150

5. Internetwork • Internet—THE internetwork. • Runs the Internet Protocol (Kahn-Cerf) • Interesting because it has faced the problems of scale • internet—abstract internetwork CMSC 150

6. IP is a big deal • Vint Cerf and Bob Kahn with Pres. Bush at 2006 ceremony where they received the Presidential Medal of Freedom for their work on IP. White House News & Policies photo CMSC 150

7. IP Internet Note Hn denotes host, Rn denotes router. • Concatenation of Networks CMSC 150

8. H1 H8 TCP TCP R1 R2 R3 IP IP IP IP IP FDDI PPP ETH ETH ETH FDDI PPP ETH IP Internet • Protocol Stack CMSC 150

9. The Internet Outline Best Effort Service Model Global Addressing Scheme CMSC 150

10. Service Model • Connectionless (datagram-based) • So each packet must be “self-contained” • 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 CMSC 150

11. Why?! • Best Effort service model is as simple a model as you can design, and this is a strong point! • If you provide best effort service over a network technology that provides reliable delivery, you’re fine • If you provide reliable delivery over a network technology that is unreliable, then you’ve got a problem: you need lots of extra functionality in the routers to handle the network deficiencies, and keeping the routers as simple as possible was an IP design goal. (Why?) • Note: IP today runs over many technologies that were not in existence when IP was invented! CMSC 150

12. 0 4 8 16 19 31 In 32 bit words TOS Length V ersion HLen In bytes Ident Flags Offset TTL Protocol Checksum SourceAddr DestinationAddr Pad Options (variable) (variable) Data IP Datagram Format Note: fields aligned on 32 bit boundaries CMSC 150

13. Fields • Version: note placement at front of packet (why?) • Header Length: in 32 bit words (20 bytes when no options) • Type of service: later • Length: of entire packet in bytes (note max of 65,535 bytes because of 16 bit length field) • Ident, flags, offset all deal with fragmentation • Time to live: first seconds, but evolved to be hop count CMSC 150

14. Fields • Protocol: demux key specifying higher level protocol that gets datagram • Checksum: take IP header as sequence of 16 bit words, add them using ones complement, take ones complement of result. • Relatively easy to calculate in software • Not as strong error detection as CRC • Bad packets discarded • Src, dest address: pretty clear (and these are unique!) • Options: rare, but complete IP implementation must handle them all! Presence determined by header length field CMSC 150

15. Fragmentation and Reassembly • Each network has some MTU (why?) • Why not some uniform standard? • What is a reasonable choice for a given host? • Strategy • fragment when necessary (MTU < Datagram length) • try to avoid fragmentation at source host • re-fragmentation is possible • fragments are self-contained datagrams • delay reassembly until destination host • do not recover from lost fragments CMSC 150

16. Fragmentation and Reassembly • Ident field: chosen by sending host, intended to be unique among all datagrams that might be received at this dest from this source over reasonable time period. • All fragments keep this same ident value • Offset: specifies 8 bytes chunk of data (why?) • Flags: M is “more” flag CMSC 150

17. Start of header Ident = x Offset = 0 0 Rest of header MTU 532 bytes 1400 data bytes Start of header Ident = x 1 Offset = 0 Rest of header 512 data bytes Start of header Ident = x 1 Offset = 512 Rest of header 512 data bytes Start of header Ident = x 0 Offset = 1024 Rest of header 376 data bytes Example Note: fragmentation can occur at multiple hops! CMSC 150

18. 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 (don’t want anyone with my phone #) • Why not just use Ethernet address?! • hierarchical: network + host (really interface) • Dot Notation • 10.3.2.4 • 128.96.33.81 • 192.12.69.77 CMSC 150

19. IP Internet Note Hn denotes host, Rn denotes router. Routers need two IP addresses. All hosts on same network have same network part of IP address CMSC 150

20. Terminology • Routing Mechanism: How a router selects the link over which to forward a packet • Routing Protocol: Policies that determine what is placed in the routing tables. These are not the same thing! CMSC 150

21. Datagram Forwarding • Strategy • every datagram contains destination’s address • if directly connected to destination network, then forward to host • if not directly connected to destination network, 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 CMSC 150

22. 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 Recall: CMSC 150

23. Pseudocode if (networknum dest = networknum my interface) deliver packet over that interface else if (networknum in my routing table) deliver packet to next hop router else deliver packet to default router CMSC 150