Last 3 Lectures: Summary Chapter 5: The Data Link Layer Goals: Overview: Our goals: network layer services understand understand principles behind data link layer IP addressing principles behind services: network layer routing principle: path selection o error detection, correction services: IP o sharing a broadcast channel: multiple access o routing (path hierarchical routing selection) o link layer addressing Internet routing protocols o dealing with scale o reliable data transfer, flow control: done! reliable transfer o how a router works instantiation and implementation of various intra-domain o o advanced topics: link layer technologies inter-domain o IPv6, multicast what’s inside a router? instantiation and IPv6 implementation in multicast routing the Internet 7/10 Datakommunikation & Internet, Anders Broberg, UmU Link layer 1 7/10 Datakommunikation & Internet, Anders Broberg, UmU Link layer 2 Chapter 5 outline Link Layer: Introduction “link” Some terminology: hosts and routers are nodes Self studies (extensive) 5.1 Introduction and (bridges and switches too) services 5.2 Error detection and communication channels that 5.4 LAN addresses and connect adjacent nodes along correction ARP communication path are links 5.8 PPP wired links 5.5 Ethernet o wireless links o 5.9 ATM 5.6 Hubs, bridges, and LANs o switches Level 2-PDU is a frame, 5.10 Frame Relay encapsulates datagram 5.7 Wireless links and LANs 5.3Multiple access protocols data-link layer has responsibility of Intro o transferring datagram from one node (CSMA/CD) o to adjacent node over a link 7/10 Datakommunikation & Internet, Anders Broberg, UmU Link layer 3 7/10 Datakommunikation & Internet, Anders Broberg, UmU Link layer 4 Link layer: context Link Layer Services transportation analogy Datagram transferred Framing, link access: trip from Princeton to Lausanne by different link limo: Princeton to JFK encapsulate datagram into frame, adding header, trailer o o protocols over different Media Access Control Protocol (MAC) plane: JFK to Geneva o o links: ‘channel access if shared medium train: Geneva to Lausanne o o o e.g., Ethernet on first tourist = datagram ‘physical addresses’ used in frame headers to identify source, o link, frame relay on dest transport segment = intermediate links, 802.11 different from IP address! communication link on last link Reliable delivery between adjacent nodes transportation mode = link Each link protocol we learned how to do this already (chapter 3)! layer protocol o provides different seldom used on low bit error link (fiber, some twisted pair) o travel agent = routing services wireless links: high error rates algorithm o Q: why both link-level and end-end reliability? o e.g., may or may not provide rdt over link 7/10 Datakommunikation & Internet, Anders Broberg, UmU Link layer 5 7/10 Datakommunikation & Internet, Anders Broberg, UmU Link layer 6 1
Link Layer Services (more) Adaptors Communicating datagram rcving link layer protocol Flow Control: sending node pacing between adjacent sending and receiving nodes o node frame frame Error Detection : errors caused by signal attenuation, noise. o adapter adapter receiver detects presence of errors: o link layer implemented receiving side signals sender for retransmission or drops frame in “adaptor” (aka NIC) o looks for errors, rdt, flow Error Correction: o Ethernet card, PCMCI control, etc receiver identifies and corrects bit error(s) without resorting o card, 802.11 card o extracts datagram, to retransmission sending side: passes to rcving node Half-duplex and full-duplex o encapsulates datagram in adapter is semi- with half duplex, nodes at both ends of link can transmit, but o a frame not at same time autonomous o adds error checking bits, rdt, flow control, etc. link & physical layers 7/10 Datakommunikation & Internet, Anders Broberg, UmU Link layer 7 7/10 Datakommunikation & Internet, Anders Broberg, UmU Link layer 8 Chapter 5 outline LAN Addresses and ARP Self studies (extensive) 32-bit IP address: 5.1 Introduction and services 5.4 LAN addresses and ARP 5.2 Error detection network-layer address 5.5 Ethernet and correction used to get datagram to destination IP network (recall IP 5.6 Hubs, bridges, and network definition) 5.8 PPP switches LAN (or MAC or physical or Ethernet) address: 5.7 Wireless links and LANs 5.9 ATM used to get datagram from one interface to another physically- 5.3Multiple access protocols 5.10 Frame Relay connected interface (same network) Intro o (CSMA/CD) 48 bit MAC address (for most LANs) o burned in the adapter ROM 7/10 Datakommunikation & Internet, Anders Broberg, UmU Link layer 9 7/10 Datakommunikation & Internet, Anders Broberg, UmU Link layer 10 LAN Addresses and ARP LAN Address (more) Each adapter on LAN has unique LAN address 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 o IP hierarchical address NOT portable depends on IP network to which node is attached o 7/10 Datakommunikation & Internet, Anders Broberg, UmU Link layer 11 7/10 Datakommunikation & Internet, Anders Broberg, UmU Link layer 12 2
ARP: Address Resolution Protocol Recall earlier routing discussion Starting at A, given IP Each IP node (Host, Router) A Question: how to determine datagram addressed to B: 223.1.1.1 on LAN has ARP table MAC address of B look up net. address of B, find 223.1.2.1 ARP Table: IP/MAC address 223.1.1.2 B on same net. as A mappings for some LAN knowing B’s IP address? 223.1.2.9 223.1.1.4 nodes link layer send datagram to B B 223.1.2.2 < IP address; MAC address; TTL> inside link-layer frame E 223.1.1.3 223.1.3.27 TTL (Time To Live): time o after which address 223.1.3.2 223.1.3.1 mapping will be forgotten frame source, datagram source, (typically 20 min) dest address dest address A’s IP B’s MAC A’s MAC B’s IP IP payload addr addr addr addr datagram frame 7/10 Datakommunikation & Internet, Anders Broberg, UmU Link layer 13 7/10 Datakommunikation & Internet, Anders Broberg, UmU Link layer 14 Routing to another LAN ARP protocol walkthrough: send datagram from A to B via R assume A know’s B IP address A wants to send datagram to A caches (saves) IP-to- B, and A knows B’s IP address. MAC address pair in its ARP table until Suppose B’s MAC address is not in A’s ARP table. information becomes old (times out) A broadcasts ARP query packet, containing B's IP o soft state: information address that times out (goes A all machines on LAN o away) unless refreshed receive ARP query ARP is “plug-and-play”: B receives ARP packet, replies to A with its (B's) o nodes create their ARP MAC address R tables without B frame sent to A’s MAC intervention from net o address (unicast) administrator Two ARP tables in router R, one for each IP network (LAN) In routing table at source Host, find router 111.111.111.110 In ARP table at source, find MAC address E6-E9-00-17-BB-4B, etc 7/10 Datakommunikation & Internet, Anders Broberg, UmU Link layer 15 7/10 Datakommunikation & Internet, Anders Broberg, UmU Link layer 16 A creates datagram with source A, destination B Chapter 5 outline A uses ARP to get R’s MAC address for 111.111.111.110 A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram A’s data link layer sends frame 5.1 Introduction and Self studies (extensive) R’s data link layer receives frame services 5.2 Error detection and R removes IP datagram from Ethernet frame, sees its destined to 5.4 LAN addresses and correction B ARP 5.8 PPP R uses ARP to get B’s physical layer address 5.5 Ethernet 5.9 ATM R creates frame containing A-to-B IP datagram sends to B 5.6 Hubs, bridges, and 5.10 Frame Relay switches 5.7 Wireless links and LANs A 5.3Multiple access protocols o Intro R o (CSMA/CD) B 7/10 Datakommunikation & Internet, Anders Broberg, UmU Link layer 17 7/10 Datakommunikation & Internet, Anders Broberg, UmU Link layer 18 3
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