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Transcript
Introduction to Computer Networks CMPE 150 Fall 2005 Lecture 24 CMPE 150- Introduction to Computer Networks 1 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 CMPE 150- Introduction to Computer Networks 2 Last Class… • Finished routing. • Internetworking. – – – – – Interconnecting networks. Heterogeneity. Different approaches to internetworking. Translating versus gluing. Tunneling. CMPE 150- Introduction to Computer Networks 3 Today • Internetworking (cont’d). • IP. CMPE 150- Introduction to Computer Networks 4 Internetworking CMPE 150- Introduction to Computer Networks 5 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). CMPE 150- Introduction to Computer Networks 6 Internetwork Routing: Example • (a) An internetwork. (b) A graph of the internetwork. CMPE 150- Introduction to Computer Networks 7 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. CMPE 150- Introduction to Computer Networks 8 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). CMPE 150- Introduction to Computer Networks 9 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. CMPE 150- Introduction to Computer Networks 10 Types of Fragmentation • (a) Transparent fragmentation. (b) Nontransparent fragmentation. Transparent Fragmentation Non-Transparent Fragmentation CMPE 150- Introduction to Computer Networks 11 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. CMPE 150- Introduction to Computer Networks 12 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. CMPE 150- Introduction to Computer Networks 13 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. CMPE 150- Introduction to Computer Networks 14 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. CMPE 150- Introduction to Computer Networks 15 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. CMPE 150- Introduction to Computer Networks 16 Keeping Track of Fragments • Header contains packet number, number of first fragment in the packet, and last-fragment bit. Last-fragment bit E F G H I 27 0 1 A B C D Number of first fragment Packet number 27 0 0 A B C D E F G H 1 byte J (a) Original packet with 10 data bytes. 27 8 1 I J (b) Fragments after passing through network with maximum packet size = 8 bytes. CMPE 150- Introduction to Computer Networks 17 The Internet CMPE 150- Introduction to Computer Networks 18 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. CMPE 150- Introduction to Computer Networks 19 Internet as Collection of Subnetworks CMPE 150- Introduction to Computer Networks 20 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. CMPE 150- Introduction to Computer Networks 21 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. CMPE 150- Introduction to Computer Networks 22 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). CMPE 150- Introduction to Computer Networks 23 IP Datagram Format • IP datagram consists of header and data (or payload). • Header: – 20-byte fixed (mandatory) part. – Variable length optional part. CMPE 150- Introduction to Computer Networks 24 The IP v4 Header CMPE 150- Introduction to Computer Networks 25 IP Options 5-54 CMPE 150- Introduction to Computer Networks 26 IP Addresses • IP address formats. CMPE 150- Introduction to Computer Networks 27 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. CMPE 150- Introduction to Computer Networks 28 IP Addresses (Cont’d) • Special IP addresses. CMPE 150- Introduction to Computer Networks 29 Scalability of IP Addresses • Problem: a single A, B, or C address refers to a single network. • As organizations grow, what happens? CMPE 150- Introduction to Computer Networks 30 Example: A Campus Network CMPE 150- Introduction to Computer Networks 31 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. CMPE 150- Introduction to Computer Networks 32 Subnets • A class B network subnetted into 64 subnets. CMPE 150- Introduction to Computer Networks 33 Subnet Mask • Indicates the split between network and subnet number + host number. Subnet Mask: 255.255.252.0 or /22 (network + subnet part) CMPE 150- Introduction to Computer Networks 34 Subnetting: Observations • Subnets are not visible to the outside world. • Thus, subnetting (and how) is a decision made by local network admin. CMPE 150- Introduction to Computer Networks 35 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 CMPE 150- Introduction to Computer Networks 36