Chapter 5 Link Layer

Home , Broadcast address, Broadcast domain, Datagram, Link layer, MAC address

Computer Networking: A Top Down Approach th 6 edition Jim Kurose, Keith Ross

Addison-Wesley

March 2012

Link Layer 5-1 Chapter 5: Link layer our goals: v understand principles behind link layer services: § error detection, correction § sharing a broadcast channel: multiple access § link layer addressing § local area networks: Ethernet, VLANs v instantiation, implementation of various link layer technologies

Link Layer 5-2 Link layer, LANs: outline

5.1 introduction, services 5.5 link virtualization: 5.2 error detection, MPLS correction 5.6 data center 5.3 multiple access networking protocols 5.7 a day in the life of a 5.4 LANs web request § addressing, ARP § Ethernet § switches § VLANS

Link Layer 5-3 Link layer: introduction terminology: v hosts and routers: nodes global ISP v communication channels that connect adjacent nodes along communication path: links § wired links § wireless links § LANs v layer-2 packet: frame, encapsulates datagram data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link

Link Layer 5-4 Link layer: context v datagram transferred by transportation analogy: different link protocols over v trip from Princeton to Lausanne different links: § limo: Princeton to JFK § e.g., Ethernet on first link, § plane: JFK to Geneva frame relay on § train: Geneva to Lausanne intermediate links, 802.11 v tourist = datagram on last link v transport segment = v each link protocol provides communication link different services v transportation mode = link § e.g., may or may not layer protocol provide rdt over link v travel agent = routing algorithm

Link Layer 5-5 Link layer services v framing, link access: § encapsulate datagram into frame, adding header, trailer § channel access if shared medium § “MAC” addresses used in frame headers to identify source, dest • different from IP address! v reliable delivery between adjacent nodes § we learned how to do this already (chapter 3)! § seldom used on low bit-error link (fiber, some twisted pair) § wireless links: high error rates • Q: why both link-level and end-end reliability?

Link Layer 5-6 Link layer services (more) v flow control: § pacing between adjacent sending and receiving nodes v error detection: § errors caused by signal attenuation, noise. § receiver detects presence of errors: • signals sender for retransmission or drops frame v error correction: § receiver identifies and corrects bit error(s) without resorting to retransmission v half-duplex and full-duplex § with half duplex, nodes at both ends of link can transmit, but not at same time

Link Layer 5-7 Where is the link layer implemented? v in each and every host v link layer implemented in “adaptor” (aka network interface card NIC) or on a

chip application transport § Ethernet card, 802.11 network cpu memory card; Ethernet chipset link

§ implements link, physical host bus layer controller (e.g., PCI) link v attaches into host’s system physical physical buses transmission v combination of hardware, network adapter software, firmware card

Link Layer 5-8 Adaptors communicating

datagram datagram

controller controller

sending host receiving host datagram frame v sending side: v receiving side § encapsulates datagram in § looks for errors, rdt, frame flow control, etc § adds error checking bits, § extracts datagram, passes rdt, flow control, etc. to upper layer at receiving side

Link Layer 5-9 Link layer, LANs: outline

Link Layer 5-10 Error detection

EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields

• Error detection not 100% reliable! • protocol may miss some errors, but rarely • larger EDC field yields better detection and correction

otherwise

Link Layer 5-11 Parity checking

single bit parity: two-dimensional bit parity: v detect single bit v detect and correct single bit errors errors

0 0

Link Layer 5-12 Internet checksum (review)

goal: detect “errors” (e.g., flipped bits) in transmitted packet (note: used at transport layer only)

sender: receiver: v treat segment contents v compute checksum of as sequence of 16-bit received segment integers v check if computed v checksum: addition (1’s checksum equals checksum complement sum) of field value: segment contents § NO - error detected v sender puts checksum § YES - no error detected. value into UDP But maybe errors checksum field nonetheless?

Link Layer 5-13 Cyclic redundancy check v more powerful error-detection coding v view data bits, D, as a binary number v choose r+1 bit pattern (generator), G v goal: choose r CRC bits, R, such that § exactly divisible by G (modulo 2) § receiver knows G, divides by G. If non-zero remainder: error detected! § can detect all burst errors less than r+1 bits v widely used in practice (Ethernet, 802.11 WiFi, ATM)

Link Layer 5-14 CRC example G D r = 3 101000! ! want: 1001!101110000! D.2r XOR R = nG 1001! equivalently: 101! . r 000! D 2 = nG XOR R 1010! equivalently: 1001! if we divide D.2r by 110! 000! G, want remainder R 1100! to satisfy: 1001! 0101! D.2r 000! R = remainder[ ] R G 1010! 1001! 0011!

Link Layer 5-15 Link layer, LANs: outline

Link Layer 5-16 Multiple access links, protocols two types of “links”: v point-to-point § PPP for dial-up access § point-to-point link between Ethernet switch, host v broadcast (shared wire or medium) § old-fashioned Ethernet § upstream HFC § 802.11 wireless LAN

shared wire (e.g., shared RF shared RF humans at a cabled Ethernet) (e.g., 802.11 WiFi) (satellite) cocktail party (shared air, acoustical)

Link Layer 5-17 Multiple access protocols v single shared broadcast channel v two or more simultaneous transmissions by nodes: interference § collision if node receives two or more signals at the same time multiple access protocol v distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit v communication about channel sharing must use channel itself! § no out-of-band channel for coordination

Link Layer 5-18 An ideal multiple access protocol given: broadcast channel of rate R bps desiderata: 1. when one node wants to transmit, it can send at rate R. 2. when M nodes want to transmit, each can send at average rate R/M 3. fully decentralized: • no special node to coordinate transmissions • no synchronization of clocks, slots 4. simple

Link Layer 5-19 MAC protocols: taxonomy three broad classes: v channel partitioning § divide channel into smaller “pieces” (time slots, frequency, code) § allocate piece to node for exclusive use v random access § channel not divided, allow collisions § “recover” from collisions v “taking turns” § nodes take turns, but nodes with more to send can take longer turns

Link Layer 5-20 Channel partitioning MAC protocols: TDMA TDMA: time division multiple access v access to channel in "rounds" v each station gets fixed length slot (length = pkt trans time) in each round v unused slots go idle v example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle

6-slot 6-slot frame frame 1 3 4 1 3 4

Link Layer 5-21 Channel partitioning MAC protocols: FDMA

FDMA: frequency division multiple access v channel spectrum divided into frequency bands v each station assigned fixed frequency band v unused transmission time in frequency bands go idle v example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle

time

FDM cable frequencybands

Link Layer 5-22 Random access protocols v when node has packet to send § transmit at full channel data rate R. § no a priori coordination among nodes v two or more transmitting nodes ➜ “collision”, v random access MAC protocol specifies: § how to detect collisions § how to recover from collisions (e.g., via delayed retransmissions) v examples of random access MAC protocols: § slotted ALOHA § ALOHA § CSMA, CSMA/CD, CSMA/CA

Link Layer 5-23 Slotted ALOHA assumptions: operation: v all frames same size v when node obtains fresh v time divided into equal size frame, transmits in next slot slots (time to transmit 1 § if no collision: node can send frame) new frame in next slot v nodes start to transmit only § if collision: node retransmits slot beginning frame in each subsequent v nodes are synchronized slot with prob. p until v if 2 or more nodes transmit success in slot, all nodes detect collision

Link Layer 5-24 Slotted ALOHA

node 1 1 1 1 1

node 2 2 2 2

node 3 3 3 3

C E C S E C E S S Pros: Cons: v single active node can v collisions, wasting slots continuously transmit at v idle slots full rate of channel v nodes may be able to v highly decentralized: only detect collision in less slots in nodes need to be in sync than time to transmit packet v simple v clock synchronization

Link Layer 5-25 Slotted ALOHA: efficiency efficiency: long-run v max efficiency: find p* that maximizes fraction of successful slots N-1 (many nodes, all with many Np(1-p) frames to send) v for many nodes, take limit of Np*(1-p*)N-1 as N goes v suppose: N nodes with to infinity, gives: many frames to send, each max efficiency = 1/e = .37 transmits in slot with probability p v prob that given node has at best: channel success in a slot = p(1- used for useful p)N-1 transmissions 37% of time! ! v prob that any node has a success = Np(1-p)N-1

Link Layer 5-26 Pure (unslotted) ALOHA v unslotted Aloha: simpler, no synchronization v when frame first arrives § transmit immediately v collision probability increases:

§ frame sent at t0 collides with other frames sent in [t0-1,t0+1]

Link Layer 5-27 Pure ALOHA efficiency

P(success by given node) = P(node transmits) . . P(no other node transmits in [t0-1,t0] P(no other node transmits in [t0-1,t0]

= p . (1-p)N-1 . (1-p)N-1 = p . (1-p)2(N-1)

… choosing optimum p and then letting n = 1/(2e) = .18

even worse than slotted Aloha!

Link Layer 5-28 CSMA (carrier sense multiple access)

CSMA: listen before transmit: if channel sensed idle: transmit entire frame v if channel sensed busy, defer transmission

v human analogy: don’t interrupt others!

Link Layer 5-29 CSMA collisions spatial layout of nodes v collisions can still occur: propagation delay means two nodes may not hear each other’s transmission v collision: entire packet transmission time wasted § distance & propagation delay play role in in determining collision probability

Link Layer 5-30 CSMA/CD (collision detection)

CSMA/CD: carrier sensing, deferral as in CSMA § collisions detected within short time § colliding transmissions aborted, reducing channel wastage v collision detection: § easy in wired LANs: measure signal strengths, compare transmitted, received signals § difficult in wireless LANs: received signal strength overwhelmed by local transmission strength v human analogy: the polite conversationalist

Link Layer 5-31 CSMA/CD (collision detection)

spatial layout of nodes

Link Layer 5-32 Ethernet CSMA/CD algorithm

1. NIC receives datagram 4. If NIC detects another from network layer, transmission while creates frame transmitting, aborts and 2. If NIC senses channel idle, sends jam signal starts frame transmission. 5. After aborting, NIC If NIC senses channel enters binary (exponential) busy, waits until channel backoff: idle, then transmits. § after mth collision, NIC 3. If NIC transmits entire chooses K at random frame without detecting from {0,1,2, …, 2m-1}. another transmission, NIC waits K·512 bit NIC is done with frame ! times, returns to Step 2 § longer backoff interval with more collisions Link Layer 5-33 CSMA/CD efficiency

v Tprop = max prop delay between 2 nodes in LAN v ttrans = time to transmit max-size frame

1 efficiency = 1+ 5t prop /ttrans v efficiency goes to 1

§ as tprop goes to 0 § as ttrans goes to infinity v better performance than ALOHA: and simple, cheap, decentralized!

Link Layer 5-34 “Taking turns” MAC protocols

channel partitioning MAC protocols: § share channel efficiently and fairly at high load § inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node! random access MAC protocols § efficient at low load: single node can fully utilize channel § high load: collision overhead “taking turns” protocols look for best of both worlds!

Link Layer 5-35 “Taking turns” MAC protocols

polling: v master node “invites” slave nodes to transmit data in turn poll v typically used with “dumb” slave devices master data v concerns: § polling overhead § latency § single point of slaves failure (master)

Link Layer 5-36 “Taking turns” MAC protocols token passing: T v control token passed from one node to next sequentially. v token message (nothing v concerns: to send) § token overhead T § latency § single point of failure (token)

data

Link Layer 5-37 Cable access network

Internet frames,TV channels, control transmitted downstream at different frequencies

cable headend

CMTS …

splitter cable modem cable modem … termination system ISP upstream Internet frames, TV control, transmitted upstream at different frequencies in time slots

v multiple 40Mbps downstream (broadcast) channels § single CMTS transmits into channels v multiple 30 Mbps upstream channels § multiple access: all users contend for certain upstream channel time slots (others assigned) Cable access network

cable headend MAP frame for Interval [t1, t2] CMTS Downstream channel i

Upstream channel j

t1 t2 Residences with cable modems

Minislots containing Assigned minislots containing cable modem minislots request frames upstream data frames DOCSIS: data over cable service interface spec v FDM over upstream, downstream frequency channels v TDM upstream: some slots assigned, some have contention § downstream MAP frame: assigns upstream slots § request for upstream slots (and data) transmitted random access (binary backoff) in selected slots Link Layer 5-39 Summary of MAC protocols v channel partitioning, by time, frequency or code § Time Division, Frequency Division v random access (dynamic), § ALOHA, S-ALOHA, CSMA, CSMA/CD § carrier sensing: easy in some technologies (wire), hard in others (wireless) § CSMA/CD used in Ethernet § CSMA/CA used in 802.11 v taking turns § polling from central site, token passing § bluetooth, FDDI, token ring

Link Layer 5-40 Link layer, LANs: outline

Link Layer 5-41 MAC addresses and ARP v 32-bit IP address: § network-layer address for interface § used for layer 3 (network layer) forwarding v MAC (or LAN or physical or Ethernet) address: § function: used ‘locally” to get frame from one interface to another physically-connected interface (same network, in IP- addressing sense) § 48 bit MAC address (for most LANs) burned in NIC ROM, also sometimes software settable § e.g.: 1A-2F-BB-76-09-AD

hexadecimal (base 16) notation (each “number” represents 4 bits)

Link Layer 5-42 LAN addresses and ARP each adapter on LAN has unique LAN address

1A-2F-BB-76-09-AD

LAN (wired or adapter wireless) 71-65-F7-2B-08-53 58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

Link Layer 5-43 LAN addresses (more) v MAC address allocation administered by IEEE v manufacturer buys portion of MAC address space (to assure uniqueness) v analogy: § MAC address: like Social Security Number § IP address: like postal address v MAC flat address ➜ portability § can move LAN card from one LAN to another v IP hierarchical address not portable § address depends on IP subnet to which node is attached

Link Layer 5-44 ARP: address resolution protocol

Question: how to determine interface’s MAC address, knowing its IP address? ARP table: each IP node (host, router) on LAN has table

137.196.7.78 § IP/MAC address mappings for some LAN 1A-2F-BB-76-09-AD nodes: 137.196.7.23 137.196.7.14 < IP address; MAC address; TTL> § TTL (Time To Live): LAN time after which address 71-65-F7-2B-08-53 mapping will be 58-23-D7-FA-20-B0 forgotten (typically 20 min) 0C-C4-11-6F-E3-98 137.196.7.88

Link Layer 5-45 ARP protocol: same LAN v A wants to send datagram to B § B’s MAC address not in v A caches (saves) IP-to- A’s ARP table. MAC address pair in its v A broadcasts ARP query ARP table until packet, containing B's IP information becomes old address (times out) § dest MAC address = FF-FF- § soft state: information that FF-FF-FF-FF times out (goes away) § all nodes on LAN receive unless refreshed ARP query v ARP is “plug-and-play”: v B receives ARP packet, § nodes create their ARP replies to A with its (B's) tables without intervention MAC address from net administrator § frame sent to A’s MAC address (unicast)

Link Layer 5-46 Addressing: routing to another LAN walkthrough: send datagram from A to B via R § focus on addressing – at IP (datagram) and MAC layer (frame) § assume A knows B’s IP address § assume A knows IP address of first hop router, R (how?) § assume A knows R’s MAC address (how?)

A B R 111.111.111.111 222.222.222.222 74-29-9C-E8-FF-55 49-BD-D2-C7-56-2A 222.222.222.220 1A-23-F9-CD-06-9B