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 nontransparent 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 27 0 1 A B C D Number of first fragment Packet number 27 0 0 A B Last fragment EbitF G H I 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
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