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CMPE 150 Fall 2005 Lecture 24

CMPE 150 Fall 2005 Lecture 24. Introduction to Computer Networks. 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:

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CMPE 150 Fall 2005 Lecture 24

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  1. CMPE 150Fall 2005Lecture 24 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. Last Class… • Finished routing. • Internetworking. • Interconnecting networks. • Heterogeneity. • Different approaches to internetworking. • Translating versus gluing. • Tunneling.

  4. Today • Internetworking (cont’d). • IP.

  5. Internetworking

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

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

  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. 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. 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. 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. 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. 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.

  18. The Internet

  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. Internet as Collection of Subnetworks

  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. 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. 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. IP Datagram Format • IP datagram consists of header and data (or payload). • Header: • 20-byte fixed (mandatory) part. • Variable length optional part.

  25. The IP v4 Header

  26. IP Options 5-54

  27. IP Addresses • IP address formats.

  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. IP Addresses (Cont’d) • Special IP addresses.

  30. Scalability of IP Addresses • Problem: a single A, B, or C address refers to a single network. • As organizations grow, what happens?

  31. Example: A Campus Network

  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. Subnets • A class B network subnetted into 64 subnets.

  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. Subnetting: Observations • Subnets are not visible to the outside world. • Thus, subnetting (and how) is a decision made by local network admin.

  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

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