Lecture 10: Addressing

Transcription

1 Lectre 10: Addressing CSE 123: Compter Networks Alex C. Snoeren HW 2 de NEXT FRIDAY

2 Lectre 10 Overview ICMP The other network-layer protocol IP Addresses Class-based addressing Sbnetting Classless addressing CSE 123 Lectre 10: Addressing 2

3 Aside: ICMP What happens when things go wrong? Need a way to test/debg a large, widely distribted system ICMP = Internet Control Message Protocol (RFC792) Companion to IP reqired fnctionality Used for error and information reporting: Errors that occr dring IP forwarding Qeries abot the stats of the network CSE 123 Lectre 10: Addressing 3

4 ICMP Error Message Generation IP packet sorce Error dring forwarding! dest ICMP IP packet CSE 123 Lectre 10: Addressing 4

5 Common ICMP Messages Destination nreachable Redirect Destination can be host, network, port, or protocol To shortct circitos roting TTL Expired Used by the tracerote program» tracerote traces packet rotes throgh Internet Echo reqest/reply Used by the ping program» ping jst tests for host liveness ICMP messages inclde portion of IP packet that triggered the error (if applicable) in their payload CSE 123 Lectre 10: Addressing 5

6 ICMP Restrictions The generation of error messages is limited to avoid cascades error cases error that cases error Don t generate ICMP error in response to: An ICMP error Broadcast/mlticast messages (link or IP level) IP header that is corrpt or has bogs sorce address Fragments, except the first ICMP messages are often rate-limited too Don t waste valable bandwidth sending tons of ICMP messages CSE 123 Lectre 10: Addressing 6

7 Addressing Considerations Fixed length or variable length addresses? Isses: Flexibility Processing costs Header size Engineering choice: IP ses fixed length addresses CSE 123 Lectre 10: Addressing 7

8 IP Addresses 32-bits in an IPv4 address Dotted decimal format a.b.c.d Each represent 8 bits of address Hierarchical: Network part and host part E.g. IP address refers to the UCSD camps network refers to the host ieng6.csd.ed Which part is network vs. host? CSE 123 Lectre 10: Addressing 8

9 Class-based Addressing Most significant bits determines class of address Class A 0 Network Host 127 nets, 16M hosts Class B Network 16 Host 16K nets, 64K hosts Class C Network 8 Host 2M nets, 254 hosts Special addresses Class D (1110) for mlticast, Class E (1111) experimental : local host (a.k.a. the loopback address) Host bits all set to 0: network address Host bits all set to 1: broadcast address CSE 123 Lectre 10: Addressing 9

10 IP Forwarding Tables Roter needs to know where to forward a packet Forwarding table contains: List of network names and next hop roters Local networks have entries specifying which interface» Link-local hosts can be delivered with Layer-2 forwarding E.g. address is Class B address class + network is Lookp in forwarding table Prefix part of address that really matters for roting CSE 123 Lectre 10: Addressing 10

11 Sbnetting Individal networks may be composed of several LANs Only want traffic destined to local hosts on physical network Roters need a way to know which hosts on which LAN Networks can be arbitrarily decomposed into sbnets Each sbnet is simply a prefix of the host address portion Sbnet prefix can be of any length, specified with netmask Network Sbnet Host Prefix CSE 123 Lectre 10: Addressing 11

12 Sbnet Addresses Every (sb)network has an address and a netmask Netmask tells which bits of the network address is important Convention sggests it be a proper prefix Netmask written as an all-ones IP address E.g., Class B netmask is Sometimes expressed in terms of nmber of 1s, e.g., /16 Need to size sbnet appropriately for each LAN Only have remaining bits to specify host addresses CSE 123 Lectre 10: Addressing 12

13 IP Address Problem (1991) Address space depletion In danger of rnning ot of classes A and B Why? Class C too small for most organizations (only ~250 addresses) Very few class A very carefl abot giving them ot (who has 16M hosts anyway?) Class B greatest problem CSE 123 Lectre 10: Addressing 13

14 For Next Time Read Trn in Homework 2 at beginning of class Friday CSE 123 Lectre 10: Addressing 14