CMPE 150 Fall 2005 Lecture 23

1. CMPE 150Fall 2005Lecture 23 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. Last Class… • Routing (cont’d). • Finished DV. • Link State. • Hierarchical routing.

4. Today • Finish routing. • Many-to-many routing. • Broadcast. • Multicast. • Internetworking.

5. Many-to-Many Routing • Support many-to-many communication. • Example applications: multi-point data distribution, multi-party teleconferencing.

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. 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. 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. 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. Broadcast Routing Reverse path forwarding. (a) A subnet. (b) a Sink tree. (c) The tree built by reverse path forwarding.

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. 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. 2 2 1 1 1,2 1,2 1,2 1,2 2 2 2 2 1 1 1 1 2 1 1 2 1 2 2 2 1 1

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. Internetworking

15. Internetworking • What is it? • Connecting networks together forming a single “internet”.

16. Connecting Networks • A collection of interconnected networks.

17. How Networks Differ 5-43

18. How Networks Can Be Connected • (a) Two Ethernets connected by a switch. • (b) Two Ethernets connected by routers.

19. How to Internet? • Connection-oriented versus connectionless internetworking. • Connection oriented internetworking: • Based on VC concatenation. • Connectionless internetworking follows the datagram model.

20. Concatenated Virtual Circuits Gateway . Builds VC crossing the different networks. . Use of gateways to perform necessary conversions.

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. 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. Tunneling • Interconnecting source and destination on separate networks but of the same type. S D

24. Tunneling Analogy

25. More Tunneling …

26. Internetwork Routing: Example • (a) An internetwork. (b) A graph of the internetwork.

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. 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. 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. 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. Types of Fragmentation • (a) Transparent fragmentation. (b) Nontransparent fragmentation.

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

39. The Internet