1. CMPE 150- Introduction to Computer Networks 1 CMPE 150 Fall 2005 Lecture 24 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 this week. –Routing with RIP. –Print lab description before going to your lab session. Midterm statistics: –Average: 54.07 –Std. deviation: 18.21

3. CMPE 150- Introduction to Computer Networks 3 Last Class… Finished routing. Internetworking. –Interconnecting networks. –Heterogeneity. –Different approaches to internetworking. –Translating versus gluing. –Tunneling.

4. CMPE 150- Introduction to Computer Networks 4 Today Internetworking (cont’d). IP.

5. CMPE 150- Introduction to Computer Networks 5 Internetworking

6. CMPE 150- Introduction to Computer Networks 6 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).

7. CMPE 150- Introduction to Computer Networks 7 Internetwork Routing: Example (a) An internetwork. (b) A graph of the internetwork.

8. CMPE 150- Introduction to Computer Networks 8 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.

9. CMPE 150- Introduction to Computer Networks 9 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).

10. CMPE 150- Introduction to Computer Networks 10 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.

11. CMPE 150- Introduction to Computer Networks 11 Types of Fragmentation (a) Transparent fragmentation. (b) Nontransparent fragmentation. Transparent Fragmentation Non-Transparent Fragmentation

12. CMPE 150- Introduction to Computer Networks 12 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.

13. CMPE 150- Introduction to Computer Networks 13 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.

14. CMPE 150- Introduction to Computer Networks 14 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.

15. CMPE 150- Introduction to Computer Networks 15 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.

16. CMPE 150- Introduction to Computer Networks 16 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.

17. CMPE 150- Introduction to Computer Networks 17 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

18. CMPE 150- Introduction to Computer Networks 18 The Internet

19. CMPE 150- Introduction to Computer Networks 19 Design Principles for Internet Keep it simple. Exploit modularity. Expect heterogeneity. Think robustness. Avoid static options and parameters. Think about scalability. Consider performance and cost.

20. CMPE 150- Introduction to Computer Networks 20 Internet as Collection of Subnetworks

21. CMPE 150- Introduction to Computer Networks 21 IP (Internet Protocol) Glues Internet together. Common network-layer protocol spoken by all Internet participating networks. Best effort datagram service: –No reliability guarantees. –No ordering guarantees.

22. CMPE 150- Introduction to Computer Networks 22 IP Transport layer breaks data streams into datagrams; fragments transmitted over Internet, possibly being fragmented. When all packet fragments arrive at destination, reassembled by network layer and delivered to transport layer at destination host.

23. CMPE 150- Introduction to Computer Networks 23 IP Versions IPv4: IP version 4. –Current, predominant version. –32-bit long addresses. IPv6: IP version 6 (aka, IPng). –Evolution of IPv4. –Longer addresses (16-byte long).

24. CMPE 150- Introduction to Computer Networks 24 IP Datagram Format IP datagram consists of header and data (or payload). Header: –20-byte fixed (mandatory) part. –Variable length optional part.

25. CMPE 150- Introduction to Computer Networks 25 The IP v4 Header

26. CMPE 150- Introduction to Computer Networks 26 IP Options 5-54

27. CMPE 150- Introduction to Computer Networks 27 IP Addresses IP address formats.

28. CMPE 150- Introduction to Computer Networks 28 IP Addresses (Cont’d) Class A: 128 networks with 16M hosts each. Class B: 16,384 networks with 64K hosts each. Class C: 2M networks with 256 hosts each. More than 500K networks connected to the Internet. Network numbers centrally administered by ICANN.

29. CMPE 150- Introduction to Computer Networks 29 IP Addresses (Cont’d) Special IP addresses.

30. CMPE 150- Introduction to Computer Networks 30 Scalability of IP Addresses Problem: a single A, B, or C address refers to a single network. As organizations grow, what happens?

31. CMPE 150- Introduction to Computer Networks 31 Example: A Campus Network

32. CMPE 150- Introduction to Computer Networks 32 Solution Subnetting: divide the organization’s address space into multiple “subnets”. How? Use part of the host number bits as the “subnet number”. Example: Consider a university with 35 departments. –With a class B IP address, use 6-bit subnet number and 10-bit host number. –This allows for up to 64 subnets each with 1024 hosts.

33. CMPE 150- Introduction to Computer Networks 33 Subnets A class B network subnetted into 64 subnets.

34. CMPE 150- Introduction to Computer Networks 34 Subnet Mask Indicates the split between network and subnet number + host number. Subnet Mask: 255.255.252.0 or /22 (network + subnet part)

35. CMPE 150- Introduction to Computer Networks 35 Subnetting: Observations Subnets are not visible to the outside world. Thus, subnetting (and how) is a decision made by local network admin.

36. CMPE 150- Introduction to Computer Networks 36 Subnet: Example Subnet 1: 10000010 00110010 000001|00 00000001 –130.50.4.1 Subnet 2: 10000010 00110010 000010|00 00000001 –130.50.8.1 Subnet 3: 10000010 00110010 000011|00 00000001 –130.50.12.1