1. CMPE 150- Introduction to Computer Networks 1 CMPE 150 Fall 2005 Lecture 23 Introduction to Computer Networks

2. CMPE 150- Introduction to Computer Networks 2 Announcements Homework 4 due on Wed.,11.23.05. No class on Friday, 11.25.05. We will have a “real” lab next week.

3. CMPE 150- Introduction to Computer Networks 3 Last Class… Routing (cont’d). Finished DV. Link State. Hierarchical routing.

4. CMPE 150- Introduction to Computer Networks 4 Today Finish routing. –Many-to-many routing. Broadcast. Multicast. Internetworking.

5. CMPE 150- Introduction to Computer Networks 5 Many-to-Many Routing Support many-to-many communication. Example applications: multi-point data distribution, multi-party teleconferencing.

6. CMPE 150- Introduction to Computer Networks 6 Broadcasting Send to ALL destinations. Several possible routing mechanisms to broadcasting. Simplistic approach: send separate packet to each destination. –Simple but expensive. –Source needs to know about all destinations. Flooding: –May generate too many duplicates (depending on node connectivity).

7. CMPE 150- Introduction to Computer Networks 7 Multidestination Routing Packet contains list of destinations. Router checks destinations and determines on which interfaces it will forward packet. –Router generates new copy of packet for each output line and includes in packet only the appropriate set of destinations. –Eventually, packets will only carry 1 destination.

8. CMPE 150- Introduction to Computer Networks 8 Spanning Tree Routing Use spanning tree (sink tree) rooted at broadcast initiator. No need for destination list. Each on spanning tree forwards packets on all lines on the spanning tree (except the one the packet arrived on). Efficient but needs to generate the spanning tree and routers must have that information.

9. CMPE 150- Introduction to Computer Networks 9 Reverse Path Forwarding Routers don’t have to know spanning tree. Router checks whether broadcast packet arrived on interface used to send packets to source of broadcast. –If so, it’s likely that it followed best route and thus not a duplicate; router forwards packet on all lines. –If not, packet discarded as likely duplicate.

10. CMPE 150- Introduction to Computer Networks 10 Broadcast Routing Reverse path forwarding. (a) A subnet. (b) a Sink tree. (c) The tree built by reverse path forwarding.

11. CMPE 150- Introduction to Computer Networks 11 Multicasting Special form of broadcasting: –Instead of sending messages to all nodes, send messages to a group of nodes. Multicast group management: –Creating, deleting, joining, leaving group. –Group management protocols communicate group membership to appropriate routers.

12. CMPE 150- Introduction to Computer Networks 12 Multicast Routing Each router computes spanning tree covering all other participating routers. –Tree is pruned by removing branches that do not contain any group members. 1,2 1 2 2 1 1 2 1 2 2 1 1 2 1 1 1 1 1 2 2 2 2 2

13. CMPE 150- Introduction to Computer Networks 13 Shared Tree Multicasting Source-rooted tree approaches don’t scale well! –1 tree per source, per group! –Routers must keep state for m*n trees, where m is number of sources in a group and n is number of groups. Core-based trees: single tree per group. –Host unicast message to core, where message is multicast along shared tree. –Routes may not be optimal for all sources. –State/storage savings in routers.

14. CMPE 150- Introduction to Computer Networks 14 Internetworking

15. CMPE 150- Introduction to Computer Networks 15 Internetworking What is it? –Connecting networks together forming a single “internet”.

16. CMPE 150- Introduction to Computer Networks 16 Connecting Networks A collection of interconnected networks.

17. CMPE 150- Introduction to Computer Networks 17 How Networks Differ 5-43

18. CMPE 150- Introduction to Computer Networks 18 How Networks Can Be Connected (a) Two Ethernets connected by a switch. (b) Two Ethernets connected by routers.

19. CMPE 150- Introduction to Computer Networks 19 How to Internet? Connection-oriented versus connectionless internetworking. Connection oriented internetworking: –Based on VC concatenation. Connectionless internetworking follows the datagram model.

20. CMPE 150- Introduction to Computer Networks 20 Concatenated Virtual Circuits. Builds VC crossing the different networks.. Use of gateways to perform necessary conversions. Gateway

21. CMPE 150- Introduction to Computer Networks 21 Connectionless Internetworking. Follows datagram model.. Packets from Host X to Host Y may follow different routes.. Gateways make routing decisions and perform translations.

22. CMPE 150- Introduction to Computer Networks 22 Translating versus “Gluing” Translation: converting between different protocols. Hard! Alternative: “gluing”. –I.e., using the same network layer protocol everywhere. –That’s what IP does!

23. CMPE 150- Introduction to Computer Networks 23 Tunneling Interconnecting source and destination on separate networks but of the same type. S D

24. CMPE 150- Introduction to Computer Networks 24 Tunneling Analogy

25. CMPE 150- Introduction to Computer Networks 25 More Tunneling …

26. CMPE 150- Introduction to Computer Networks 26 Internetwork Routing: Example (a) An internetwork. (b) A graph of the internetwork.

27. CMPE 150- Introduction to Computer Networks 27 Internetwork Routing Inherently hierarchical. –Routing within each network: interior gateway protocol (IGP). –Routing between networks: exterior gateway protocol (EGP). Within each network, different routing algorithms can be used. Each network is autonomously managed and independent of others: autonomous system (AS).

28. CMPE 150- Introduction to Computer Networks 28 Internetwork Routing (Cont’d) Typically, packet starts in its LAN. Gateway receives it (broadcast on LAN to “unknown” destination). Gateway sends packet to gateway on the destination network using its routing table. If it can use the packet’s native protocol, sends packet directly. Otherwise, tunnels it.

29. CMPE 150- Introduction to Computer Networks 29 Fragmentation Happens when internetworking. Network-specific maximum packet size. –Width of TDM slot. –OS buffer limitations. –Protocol (number of bits in packet length field). Maximum payloads range from 48 bytes (ATM cells) to 64Kbytes (IP packets).

30. CMPE 150- Introduction to Computer Networks 30 Problem What happens when large packet wants to travel through network with smaller maximum packet size? Fragmentation. Gateways break packets into fragments; each sent as separate packet. Gateway on the other side have to reassemble fragments into original packet. 2 kinds of fragmentation: transparent and non-transparent.

31. CMPE 150- Introduction to Computer Networks 31 Types of Fragmentation (a) Transparent fragmentation. (b) Nontransparent fragmentation.

32. CMPE 150- Introduction to Computer Networks 32 Transparent Fragmentation Small-packet network transparent to other subsequent networks. Fragments of a packet addressed to the same exit gateway, where packet is reassembled. –OK for concatenated VC internetworking. Subsequent networks are not aware fragmentation occurred. ATM networks (through special hardware) provide transparent fragmentation.

33. CMPE 150- Introduction to Computer Networks 33 Problems with Transparent Fragmentation Exit gateway must know when it received all the pieces. –Fragment counter or “end of packet” bit. Some performance penalty but requiring all fragments to go through same gateway. May have to repeatedly fragment and reassemble through series of small-packet networks.

34. CMPE 150- Introduction to Computer Networks 34 Non-Transparent Fragmentation Only reassemble at destination host. –Each fragment becomes a separate packet. –Thus routed independently. Problems: –Hosts must reassemble. –Every fragment must carry header until it reaches destination host.

35. CMPE 150- Introduction to Computer Networks 35 Keeping Track of Fragments Fragments must be numbered so that original data stream can be reconstructed. Tree-structured numbering scheme: –Packet 0 generates fragments 0.0, 0.1, 0.2, … –If these fragments need to be fragmented later on, then 0.0.0, 0.0.1, …, 0.1.0, 0.1.1, … –But, too much overhead in terms of number of fields needed. –Also, if fragments are lost, retransmissions can take alternate routes and get fragmented differently.

36. CMPE 150- Introduction to Computer Networks 36 Keeping Track of Fragments (Cont’d) Another way is to define elementary fragment size that can pass through every network. When packet fragmented, all pieces equal to elementary fragment size, except last one (may be smaller). Packet may contain several fragments.

37. CMPE 150- Introduction to Computer Networks 37 Fragmentation: Example Fragmentation when the elementary data size is 1 byte. (a) Original packet, containing 10 data bytes. (b) Fragments after passing through a network with maximum packet size of 8 payload bytes plus header. (c) Fragments after passing through a size 5 gateway.

38. CMPE 150- Introduction to Computer Networks 38 Keeping Track of Fragments Header contains packet number, number of first fragment in the packet, and last-fragment bit. 27 0 1 A B C D E F G H I J 27 0 0 A B C D E F G H 27 8 1 I J Packet number Number of first fragment Last-fragment bit (a) Original packet with 10 data bytes. (b) Fragments after passing through network with maximum packet size = 8 bytes. 1 byte

39. CMPE 150- Introduction to Computer Networks 39 The Internet