《计算机网络与通讯》课程教学资源（PPT课件讲稿，英文版）DataLink Layer

Summary of mac protocols o What do you do with a shared media? o Channel Partitioning by time, frequency or code Time Division, Code Division, Frequency Division o Random partitioning dynamic) ALOHA, S-ALOHA, CSMA, CSMA/CD carrier sensing: easy in some technoligies(wire), hard in others(wireless) CSMA/CD used in Ethernet o Taking Turns polling from a central cite, token passing 5: DataLink Layer 5a-1

5: DataLink Layer 5a-1 Summary of MAC protocols  What do you do with a shared media?  Channel Partitioning, by time, frequency or code • Time Division,Code Division, Frequency Division  Random partitioning (dynamic), • ALOHA, S-ALOHA, CSMA, CSMA/CD • carrier sensing: easy in some technoligies (wire), hard in others (wireless) • CSMA/CD used in Ethernet  Taking Turns • polling from a central cite, token passing

LAN technologies Data link layer so far: o services, error detection/correction, multiple access Next: LAN technologies o addressing o Ethernet o hubs, bridges, switches 802.11 user hosts O PPP Server O ATM Interne user hosts 5: DataLink Layer 5a-2

5: DataLink Layer 5a-2 LAN technologies Data link layer so far:  services, error detection/correction, multiple access Next: LAN technologies  addressing  Ethernet  hubs, bridges, switches  802.11  PPP  ATM

LAN Addresses and aRp 32-bit IP address: 口 network- ayer address O used to get datagram to destination network (recall IP network definition) LAN (or MAC or physical)address O used to get datagram from one interface to another physically-connected interface(same network) 0 48 bit MaC address for most LaNs) burned in the adapter rom 5: DataLink Layer 5a-3

5: DataLink Layer 5a-3 LAN Addresses and ARP 32-bit IP address:  network-layer address  used to get datagram to destination network (recall IP network definition) LAN (or MAC or physical) address:  used to get datagram from one interface to another physically-connected interface (same network)  48 bit MAC address (for most LANs) burned in the adapter ROM

LAN Addresses and ARP Each adapter on Lan has unique LAN address 1A-23F9-CD-06-9日 dapter node LAN 5C-66AB90-75-B1 88-日22F-54-1A0F 49-BD-D2C7-56-2A node 5: DataLink Lo 5a-4

5: DataLink Layer 5a-4 LAN Addresses and ARP Each adapter on LAN has unique LAN address

LAN Address(more) O MAC address allocation administered by ieee o manufacturer buys portion of mac address space (to assure uniqueness) 口 analogy: (a)MAC address: like Social Security Number (b)IP address: like postal address o MaC flat address = portability o can move lan card from one lan to another o IP hierarchical address not portable o depends on network to which one attaches 5: DataLink Layer 5a-5

5: DataLink Layer 5a-5 LAN Address (more)  MAC address allocation administered by IEEE  manufacturer buys portion of MAC address space (to assure uniqueness)  Analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address  MAC flat address => portability  can move LAN card from one LAN to another  IP hierarchical address NOT portable  depends on network to which one attaches

Recall earlier routing discussion Starting at A, given IP 恩 223.1.1.1 datagram addressed to b: 22312.1 o look up net address of b find B 2 23.1.1.2 on same net, as a 223.1.2.9 g link layer send datagram to B 223.1 inside link-layer frame 223.1.13223.1.327 2 ED 223.1.3 223.1.3.2 frame source datagram source, dest address dest address Bs MAC A's MAC As IP addr Bs IP IP po payload adar addr adar datagram frame 5: DataLink Layer 5a-6

5: DataLink Layer 5a-6 Recall earlier routing discussion 223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.1 223.1.3.2 223.1.3.27 A B E Starting at A, given IP datagram addressed to B:  look up net. address of B, find B on same net. as A  link layer send datagram to B inside link-layer frame B’s MAC addr A’s MAC addr A’s IP addr B’s IP addr IP payload datagram frame frame source, dest address datagram source, dest address

ARP: Address resolution protocol Question: how to determine D Each IP node(host, MAC address of B Router on lan has given B's IP address? ARP module table 口 arP Table:IP/MAC 2222222220 address mappings for 1A-23-F9cD06-9日 some LAN nodes adapter IP address, MAC address, TTL> 222222.222223 nod nodel-222.222.222.221 LAN o TTL (Time To Live): time after which address 5C-66-AB-90-75日 88-日22F54-1A0F mapping will be forgotten (typically 20 min) 49-RD-D2-C7-56-2A nde-222222 5: DataLink Layer 5a-7

5: DataLink Layer 5a-7 ARP: Address Resolution Protocol  Each IP node (Host, Router) on LAN has ARP module, table  ARP Table: IP/MAC address mappings for some LAN nodes  TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) Question: how to determine MAC address of B given B’s IP address?

ARP protoco O A knows bs IP address, wants to learn physical address of b O A broadcasts arP query pkt, containing B's IP address o all machines on LAN receive ARP query o B receives ARP packet, replies to A with its(Bs) physical layer address O A caches(saves)IP-to-physical address pairs until information becomes old (times out) o soft state: information that times out (goes away) unless refreshed 5: DataLink Layer 5a-8

5: DataLink Layer 5a-8 ARP protocol  A knows B's IP address, wants to learn physical address of B  A broadcasts ARP query pkt, containing B's IP address  all machines on LAN receive ARP query  B receives ARP packet, replies to A with its (B's) physical layer address  A caches (saves) IP-to-physical address pairs until information becomes old (times out)  soft state: information that times out (goes away) unless refreshed

Routing to another LAN walkthrough: routing from a to B via R 88-日2-2F-54-1A0F 74-29-9c-E8-FF-55 E6-E9-00-17BB-4日 hos A hoe 111.111.111.1 1A-23-F9-CD-06-9g 222.222.222.221 LAN 1 ROUTER AN 2 R 222.222.222.222 111.111.111.1 22222222220 111.111.111.110 Hoed hoe 49-日D-D2-C7-56-2A C-49-DE-DO-A8-7D 5: DataLink Layer 5a-9

5: DataLink Layer 5a-9 Routing to another LAN walkthrough: routing from A to B via R A R B

D A creates IP packet with source A, destination B o A uses aRP to get R's physical layer address for 111.111.111.110 o A creates Ethernet frame with Rs physical address as dest Ethernet frame contains A-to-b IP datagram D As data link layer sends Ethernet frame o R's data link layer receives ethernet frame o R removes ip datagram from ethernet frame sees its destined to b o R uses ARP to get B's physical layer address o R creates frame containing a-to-b iP datagram sends to B 88-日2-2F-54-1A-0F 74-29-9c-E8-FF-55 alhe E6-E9-00-17-BB-4日 11111111 1A-23-F9cD-06-9g 222.222.222.221 LAN 1 ROUTER LAN 2 R 222.222,222.222 222222222.230 111.111.111 host 111.111.111.110 B 49-BD-D2-C7-56-2A CC-49-DE-D0-AB-7D yer 5a-10

5: DataLink Layer 5a-10  A creates IP packet with source A, destination B  A uses ARP to get R’s physical layer address for 111.111.111.110  A creates Ethernet frame with R's physical address as dest, Ethernet frame contains A-to-B IP datagram  A’s data link layer sends Ethernet frame  R’s data link layer receives Ethernet frame  R removes IP datagram from Ethernet frame, sees its destined to B  R uses ARP to get B’s physical layer address  R creates frame containing A-to-B IP datagram sends to B A R B