1 / 36

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:

boyce
Télécharger la présentation

CMPE 150 Fall 2005 Lecture 24

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related