Chapter 6: The Data Link Layer

Introduction to Computer Networking Guy Leduc Chapter 6 Link Layer and LANs Computer Networking: A Top Down Approach, 7 th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2016. DataLink Layer 6-1 Chapter 6: The Data Link Layer Our goals: understand principles behind data link layer services: error detection sharing a broadcast channel: multiple access link layer addressing local area networks: Ethernet instantiation and implementation of various link layer technologies DataLink Layer 6-2 1

Link Layer 6.1 Introduction and services 6.2 Error detection 6.3 Multiple access protocols 6.4 LANs Addressing, ARP Ethernet Switches 6.5 Data center networking 6.6 A day in the life of a web request DataLink Layer 6-3 Link Layer: Introduction Some terminology: hosts and routers are nodes = devices with a network layer (L3) communication channels that connect adjacent nodes along communication path are links wired links wireless links LANs layer-2 packet is a frame, encapsulates datagram global ISP data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link DataLink Layer 6-4 2

Link layer: context datagram transferred by different link protocols over different links: e.g., Ethernet on first link, MPLS on intermediate links, WiFi on last link each link protocol provides different services e.g., may or may not provide reliable data transfer (rdt) over link transportation analogy trip from Princeton to Lausanne limo: Princeton to JFK plane: JFK to Geneva train: Geneva to Lausanne tourist = datagram transport segment = communication link transportation mode = link layer protocol travel agent = routing algorithm DataLink Layer 6-5 Link Layer Services framing, link access: encapsulate datagram into frame, adding header, trailer channel access if shared medium MAC addresses used in frame headers to identify sending, receiving node interfaces different from IP addresses! (which identify source, dest host interfaces) 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? DataLink Layer 6-6 3

Link Layer Services (more) flow control: pacing between adjacent sending and receiving nodes error detection: errors caused by signal attenuation, noise receiver detects presence of errors: signals sender for retransmission or drops frame error correction: receiver identifies and corrects bit error(s) without resorting to retransmission half-duplex and full-duplex with half duplex, nodes at both ends of link can transmit, but not at same time DataLink Layer 6-7 Where is the link layer implemented? in each and every host link layer implemented in adaptor (aka network interface card NIC) or on a chip Ethernet card, 802.11 card; Ethernet chipset implements link, physical layer attaches into host s system buses combination of hardware, software, firmware application transport network link link physical cpu controller physical transmission memory host bus (e.g., PCI) network adapter card DataLink Layer 6-8 4

Adaptors communicating datagram datagram controller controller sending host frame datagram receiving host sending side: encapsulates datagram in frame adds error checking bits (rdt, flow control, etc.) receiving side looks for errors (rdt, flow control, etc) extracts datagram, passes to upper layer at receiving side DataLink Layer 6-9 Link Layer 6.1 Introduction and services 6.2 Error detection 6.3 Multiple access protocols 6.4 LANs addressing, ARP Ethernet switches 6.5 Data center networking 6.8 A day in the life of a web request DataLink Layer 6-10 5

Error Detection EDC= Error Detection (and sometimes 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 Is EDC = f (D )? EDC = f (D) for some function f DataLink Layer 6-11 Parity Checking Single Bit Parity: Detect single bit errors Two Dimensional Bit Parity: Detect and correct single bit errors 0 0 DataLink Layer 6-12 6

Internet checksum (review) Goal: detect errors (e.g., flipped bits) in transmitted packet (note: used at transport layer only) Sender: treat segment contents as sequence of 16-bit integers checksum: addition (1 s complement sum) of segment contents sender puts checksum value into UDP/TCP checksum field Receiver: compute checksum of received segment check if computed checksum equals checksum field value: NO - error detected YES - no error detected. But maybe errors nonetheless DataLink Layer 6-13 Cyclic Redundancy Check (CRC) more powerful error-detection coding view data bits, D, as a binary number choose r+1 bit pattern (generator), G goal: choose r CRC bits, R, such that <D,R> exactly divisible by G (in base-2 arithmetic) receiver knows G, divides <D,R> by G. If non-zero remainder: error detected! can detect any single error burst not longer than r bits (see later) widely used in practice (Ethernet, 802.11 WiFi, ATM) <D,R> = = DataLink Layer 6-14 7

CRC Example Want: D. 2 r XOR R = ng equivalently: D. 2 r = ng XOR R equivalently: if we divide D. 2 r by G or (in base-2 arithmetic), want remainder R D 2r R = remainder G R = D. 2 r mod G Divisor In base-2 arithmetic: no carries no borrows 1 + 1 = 0 0 1 = 1 +, -, XOR: all equivalent Quotient G r = 3 D 101011 1001 101110000 1001 Remainder 101 000 1010 1001 110 000 1100 1001 1010 1001 011 Dividend DataLink Layer 6-15 CRC Example: the polynomial view D(x) = x 5 + x 3 + x 2 + x r=3 G(x) = x 3 + 1 R(x) = remainder D(x) x r G(x) Transmitted frame: T(x) = D(x). x r - R(x) Is divisible by: G(x) R(x) = x + 1 DataLink Layer 6-16 8

Why does it work? We have T(x) = D(x). x r - R(x) By construction T(x) is divisible by G(x) T(x) mod G(x) = 0 This is easy to check at the receiver, provided that the sender and the receiver agree on a certain G(x) Suppose some errors occur during transmission The received frame is T(x) + E(x) The receiver will then calculate the remainder of (T(x) + E(x)) / G(x) This remainder is equal to the remainder of E(x) / G(x) (T(x) + E(x)) mod G(x) = E(x) mod G(x) If the error is an E(x) that is not divisible by G(x), it will be detected! The choice of G(x) is thus very important! But impossible to detect all errors! Why? From From Computer Networks, Networking, by Tanenbaum by Kurose&Ross Prentice Hall DataLink Layer 6-17 Example of CRC, with properties (1) Example of Generator: G(x) = x 16 + x 12 + x 5 + 1 Property 1: G(x) detects every error consisting of an odd number of error bits Proof: G(x) is divisible by (x + 1), in other words G(x) = (x + 1) H(x) in base-2 arithmetic, G(1) = 1+1+1+1 = 0 An odd number of bit errors is modelled by a polynomial E(x) with an odd number of terms Such E(x) cannot be divisible by (x + 1) in base-2 arithmetic, E(1) = 1 for such E(x) Therefore E(x) is not divisible by G(x) More generally, a G(x) composed of an even number of terms detects every error consisting of an odd number of error bits At least as good as a parity bit! Could a parity bit be seen as a trivial CRC? Any G(x) to suggest? Property 2: G(x) detects every 2-bit error (in any place in the frame) From From Computer Networks, Networking, by Tanenbaum by Kurose&Ross Prentice Hall DataLink Layer 6-18 9

Example of CRC, with properties (2) G(x) = x 16 + x 12 + x 5 + 1 Property 3: G(x) detects every single error burst of length 16 bits An error burst of length n ( 2) is 1 error bit, followed by n-2 bits (correct or not), followed by 1 error bit Proof: An error burst of length 16 bits can be modelled by E(x) = H(x). x k, with H(x) of degree 15, and k being the number of bits after the last error bit H(x) is not divisible by G(x), because H(x) is of degree 15, and G(x) of degree 16 x k is not divisible by G(x) either (same reasoning as in property 1) Therefore, E(x) is not divisible by G(x) More generally, a G(x) of degree r detects every single r-bit error burst If errors are random, G(x) detects 99,997% of the 17-bit error bursts (Any undetectable error burst to suggest?) 99,998% of the 18-bit error bursts From From Computer Networks, Networking, by Tanenbaum by Kurose&Ross Prentice Hall DataLink Layer 6-19 Link Layer 6.1 Introduction and services 6.2 Error detection 6.3 Multiple access protocols 6.4 LANs Addressing, ARP Ethernet Switches 6.5 Data center networking 6.6 A day in the life of a web request DataLink Layer 6-20 10

Multiple Access Links and Protocols Two types of links : point-to-point PPP for dial-up access point-to-point link between Ethernet switch and host broadcast (shared wire or medium) old-fashioned Ethernet upstream Hybrid Fiber Coax (HFC) 802.11 wireless LAN shared wire (e.g., cabled Ethernet) shared RF (e.g., 802.11 WiFi) shared RF (satellite) humans at a cocktail party (shared air, acoustical) DataLink Layer 6-21 Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes: interference collision if node receives two or more signals at the same time multiple access protocol distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit communication about channel sharing must use channel itself! no out-of-band channel for coordination DataLink Layer 6-22 11

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 (fairness) 3. fully decentralized: no special node to coordinate transmissions no synchronization of clocks, slots 4. simple DataLink Layer 6-23 MAC Protocols: a taxonomy Three broad classes: Channel Partitioning divide channel into smaller pieces (time slots, frequency, code) allocate piece to node for exclusive use Random Access channel not divided, allow collisions recover from collisions Taking turns nodes take turns, but nodes with more to send can take longer turns DataLink Layer 6-24 12

Channel Partitioning MAC protocols: TDMA TDMA: time division multiple access access to channel in "rounds" each station gets fixed length slot (length = packet transmission time) in each round unused slots go idle example: 6-station LAN, 1,3,4 have packet, slots 2,5,6 idle 6-slot frame 1 3 4 1 3 4 DataLink Layer 6-25 Channel Partitioning MAC protocols: FDMA FDMA: frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example: 6-station LAN, 1,3,4 have packet, frequency bands 2,5,6 idle FDM cable frequency bands time DataLink Layer 6-26 13

Random Access Protocols When node has packet to send transmit at full channel data rate R no a priori coordination among nodes two or more transmitting nodes collision, random access MAC protocol specifies: how to detect collisions how to recover from collisions (e.g., via delayed retransmissions) Examples of random access MAC protocols: slotted ALOHA ALOHA CSMA, CSMA/CD, CSMA/CA DataLink Layer 6-27 Slotted ALOHA Assumptions: all frames have same size time is divided into equal size slots (time to transmit 1 frame) nodes start to transmit only at slot beginning nodes are synchronized if 2 or more nodes transmit in slot, all nodes detect collision Operation: when node obtains fresh frame, transmits in next slot if no collision: node can send new frame in next slot if collision: node retransmits frame in each subsequent slot with probability p until success DataLink Layer 6-28 14

Slotted ALOHA node 1 1 1 1 1 node 2 2 2 2 node 3 3 3 3 Pros single active node can continuously transmit at full rate of channel highly decentralized: only slots in nodes need to be in sync simple C E C S E C E S S Cons collisions, wasting slots idle slots nodes may be able to detect collision in less than time to transmit packet clock synchronization DataLink Layer 6-29 Slotted Aloha efficiency Efficiency is the long-run fraction of successful slots when there are many nodes, each with many frames to send Suppose N nodes with many frames to send, each transmits in slot with probability p Note: not exactly slotted ALOHA! prob that node 1 has success in a slot = p(1-p) N-1 prob that any node has a success = Np(1-p) N-1 For max efficiency with N nodes, find p* that maximizes Np(1-p) N-1 p* = 1/N For many nodes, take limit of Np*(1-p*) N-1 = (1-1/N) N-1 as N goes to infinity, it gives a max efficiency of 1/e = 0.37 N lim 1 G N N At best: channel used for useful transmissions 37% of time! = e G! DataLink Layer 6-30 15

Pure (unslotted) ALOHA unslotted Aloha: simpler, no synchronization when frame first arrives transmit immediately collision probability increases: frame sent at t 0 collides with other frames sent in [t 0-1,t 0 +1] In frame time units DataLink Layer 6-31 Pure Aloha efficiency P(success by given node) = P(node transmits). P(no other node transmits in [t 0-1,t 0 ]. P(no other node transmits in [t 0,t 0 +1] = p. (1-p) N-1. (1-p) N-1 = p. (1-p) 2(N-1) P (success by any node) = Np. (1-p) 2(N-1) choosing optimum p and then letting n go to infinity... = 1/(2e) = 0.18 Even worse than slotted Aloha! DataLink Layer 6-32 16

Efficiency wrt average traffic load Let G = pn be the average aggregated traffic load (or demand) per frame time = nr. of transmission attempts per frame time N stations sending one frame with probability p in every frame time Efficiency Slotted ALOHA: Np(1-p) N-1 = G. (1-G/N) N-1 ALOHA: Np(1-p) 2(N-1) = G. (1-G/N) 2(N-1) Efficiency (for a given G) when N >> Slotted ALOHA: G. e -G ALOHA: G. e -2G If G << 1: efficiency G, perfect lim 1 G N N N = e G DataLink Layer 6-33 ALOHA versus Slotted ALOHA Efficiency Ideal: S = G Optimum: out of 100 frame times, - 37 are used effectively - 26 are collisions - 37 are just empty frame From From Computer Computer Networks, Networking, by Tanenbaum by Kurose&Ross Prentice Hall DataLink Layer 6-34 17

CSMA (Carrier Sense Multiple Access) Improving pure ALOHA with carrier sensing CSMA: listen before transmit: If channel sensed idle: transmit entire frame If channel sensed busy: defer transmission LBT: Listen Before Talking (and deference) human analogy: don t interrupt others! DataLink Layer 6-35 CSMA collisions spatial layout of nodes collisions can still occur: propagation delay means two nodes may not hear each other s transmission collision: entire packet transmission time wasted distance & propagation delay play role in determining collision probability DataLink Layer 6-36 18

p-persistent CSMA persistent CSMA (if channel is busy, listen until it is freed) non persistent CSMA (if channel is busy, program a new attempt later) p-persistent CSMA: While true do if channel is free then { with probability p: immediate transmission; or with probability 1-p: stay idle during at least propagation time (τ) } else listen until the channel is freed. Trade-off between efficiency and delay This introduces a useless delay at low loads But the efficiency of the channel is better at high loads DataLink Layer 6-37 Efficiency versus load for various random access protocols Efficiency Ideal frame From From Computer Computer Networks, Networking, by Tanenbaum by Kurose&Ross Prentice Hall DataLink Layer 6-38 19

Engineering a CSMA network B = Channel Data Rate (so-called Bandwidth) (bps) F = (Maximum) Frame size (bits) L = Length of the channel (m) c = Propagation speed (m/s) τ = Propagation delay = L / c (s) T = Transmission delay = F / B (s) τ : contention period (risk of collision) After τ: channel implicitly reserved during T-τ Let a = τ / T = BL / cf Need small a! Let a = 1% F = 100 BL / c = ± 5 10-7 BL (with c = ± 200,000 km/s) Let B = 10 Mbps, L = 2.5 km F = 12,500 bits (= 1,562.5 bytes) This is roughly the Ethernet frame size DataLink Layer 6-39 CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA collisions detected within short time colliding transmissions aborted, reducing channel wastage 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 human analogy: the polite conversationalist DataLink Layer 6-40 20

CSMA/CD collision detection CSMA/CD CSMA Lost time 2 τ (worst case: 2 max propagation times when the 2 stations are at the 2 ends) + detect/abort time No abort, so loss is T DataLink Layer 6-41 Minimal frame size with CSMA/CD A Packet starts at time 0 B A Packet almost at B at τ-ε B (a) (b) A B A Noise burst gets back to A at 2τ B (c) Collision at time τ (d) To detect collision, the sender must still be transmitting when the collision propagates back to it. So the condition is: T > 2τ, which means F/B > 2 L/c, which leads to a minimal F min = 2BL/c = ± BL 10-8 bits Let B = 10 Mbps, L = 2.5 km F min = ± 250 bits (= ± 32 bytes) Ethernet has chosen 64 bytes = 512 bits (extra margin due to other delays) From From Computer Computer Networks, Networking, by Tanenbaum by Kurose&Ross Prentice Hall DataLink Layer 6-42 21

Ethernet CSMA/CD algorithm 1. NIC receives datagram from network layer, creates frame 2. If NIC senses channel idle, starts frame transmission If NIC senses channel busy, waits until channel idle, then transmits 3. If NIC transmits entire frame without detecting another transmission, NIC is done with frame! 4. If NIC detects another transmission while transmitting (collision), aborts and sends jam signal 5. After aborting, NIC enters binary (exponential) backoff: after mth collision (m 10) for this frame, NIC chooses K at random from {0,1,2,,2 m -1}. NIC waits K 512 bit times, returns to Step 2 (for m > 10, take K in {0..1023}) So, longer backoff interval when more collisions for a given frame With this backoff algorithm, Ethernet is sort of p-persistent CSMA with an adaptive p after the 1st collision DataLink Layer 6-43 Channel efficiency S 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Channel efficiency of an adaptive p-persistent CSMA/CD Let α = the average number of slots (2 τ) before any successful transmission Increasing frame size 1024 byte frames 512 byte frames 256 byte frames 128 byte frames 64 byte frames S = T / (T + α2τ) = 1 / (1 + α2τ/t) = 1 / (1 + α2bl/cf ) For large N, α converges towards e (±2.7) for a p-persistent CSMA with p* = 1/N (ideal case) So, for N >>, S cannot be better than 1 / (1 + e2τ/t) = 1 / (1 + 5.4 τ/t) 0 1 2 4 8 16 32 64 128 256 From Number of active stations = N From Computer Computer Networks, Networking, by Tanenbaum by Kurose&Ross Prentice Hall DataLink Layer 6-44 22

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! DataLink Layer 6-45 Taking Turns MAC protocols Polling: master node invites slave nodes to transmit in turn typically used with dumb slave devices concerns: polling overhead latency single point of failure (master) data slaves data poll master DataLink Layer 6-46 23

Taking Turns MAC protocols Token passing: control token passed from one node to next sequentially token message concerns: token overhead latency single point of failure (token)! known upper bound on access time to channel! o CSMA (nothing to send) T T data DataLink Layer 6-47 Cable access network Internet frames,tv channels, control transmitted downstream at different frequencies cable headend CMTS cable modem termination system splitter cable modem ISP upstream Internet frames, TV control, transmitted upstream at different frequencies in time slots " multiple 40Mbps downstream (broadcast) channels # single CMTS transmits into channels " multiple 30 Mbps upstream channels # multiple access: all users contend for certain upstream channel time slots (others assigned) DataLink Layer 6-48 24

Cable access network cable headend CMTS MAP frame for Interval [t1, t2] Downstream channel i Upstream channel j t 1 t 2 Residences with cable modems Minislots containing minislots request frames Assigned minislots containing cable modem upstream data frames DOCSIS: Data Over Cable Service Interface Spec " FDM over upstream, downstream frequency channels " 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 DataLink Layer 6-49 Summary of MAC protocols channel partitioning, by time, frequency (or code) Time Division, Frequency Division 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 taking turns polling from central site, token passing Bluetooth, FDDI, Token Ring Cable access networks use a combination of them DataLink Layer 6-50 25

Link Layer 6.1 Introduction and services 6.2 Error detection 6.3 Multiple access protocols 6.4 LAN Addressing, ARP Ethernet Switches 6.5 Data center networking 6.6 A day in the life of a web request DataLink Layer 6-51 MAC Addresses and ARP 32-bit IP address: network-layer address for interface used for layer 3 (network layer) forwarding 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) DataLink Layer 6-52 26

LAN addresses and ARP Each adapter on LAN has unique LAN address 1A-2F-BB-76-09-AD 71-66-F7-2B-08-53 LAN (wired or wireless) 58-23-D7-FA-20-B0 adapter 0C-C4-11-6F-E3-98 DataLink Layer 6-53 LAN addresses (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space (to assure uniqueness) analogy: MAC address: like Social Security Number IP address: like postal address MAC flat address portability can move LAN card from one LAN to another IP hierarchical address NOT portable address depends on IP subnet to which node is attached DataLink Layer 6-54 27

ARP: Address Resolution Protocol Question: how to determine interface s MAC address knowing its IP address? 137.196.7.23 71-66-F7-2B-08-53 137.196.7.88 LAN 137.196.7.78 1A-2F-BB-76-09-AD 137.196.7.14 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 ARP table: each IP node (host, router) on LAN has table IP/MAC address mappings for some LAN nodes < IP addr; MAC addr; TTL> TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) DataLink Layer 6-55 ARP protocol: same LAN A wants to send datagram to B B s MAC address not in A s ARP table A broadcasts ARP query packet, containing B's IP address dest MAC address = FF-FF-FF-FF-FF-FF all nodes on LAN receive ARP query B receives ARP packet, replies to A with its (B's) MAC address frame sent to A s MAC address (unicast) A caches (saves) B s IP-to- MAC address pair in its ARP table until information becomes old (times out) soft state: information that times out (goes away) unless refreshed All other stations had also cached A s IP-to-MAC pair! ARP is plug-and-play : nodes create their ARP tables without intervention from net administrator DataLink Layer 6-56 28

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?) This is a really important example make sure you understand! A 111.111.111.111 74-29-9C-E8-FF-55 R 222.222.222.220 1A-23-F9-CD-06-9B B 222.222.222.222 49-BD-D2-C7-56-2A 111.111.111.112 CC-49-DE-D0-AB-7D 111.111.111.110 E6-E9-00-17-BB-4B 222.222.222.221 88-B2-2F-54-1A-0F DataLink Layer 6-57 Addressing: routing to another LAN " A creates IP datagram with IP source A, destination B " A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram IP Eth Phy MAC src: 74-29-9C-E8-FF-55 MAC dest: E6-E9-00-17-BB-4B IP src: 111.111.111.111 IP dest: 222.222.222.222 A 111.111.111.111 74-29-9C-E8-FF-55 R 222.222.222.220 1A-23-F9-CD-06-9B B 222.222.222.222 49-BD-D2-C7-56-2A 111.111.111.112 CC-49-DE-D0-AB-7D 111.111.111.110 E6-E9-00-17-BB-4B 222.222.222.221 88-B2-2F-54-1A-0F DataLink Layer 6-58 29

Addressing: routing to another LAN " frame sent from A to R " frame received at R, datagram removed, passed up to IP MAC src: 74-29-9C-E8-FF-55 MAC dest: E6-E9-00-17-BB-4B IP src: 111.111.111.111 IP dest: 222.222.222.222 IP src: 111.111.111.111 IP dest: 222.222.222.222 IP Eth Phy A 111.111.111.111 74-29-9C-E8-FF-55 111.111.111.112 CC-49-DE-D0-AB-7D IP Eth Phy R 111.111.111.110 E6-E9-00-17-BB-4B 222.222.222.220 1A-23-F9-CD-06-9B B 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.221 88-B2-2F-54-1A-0F DataLink Layer 6-59 Addressing: routing to another LAN " R forwards datagram with IP source A, destination B " R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagram A 111.111.111.111 74-29-9C-E8-FF-55 IP Eth Phy R 222.222.222.220 1A-23-F9-CD-06-9B MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A IP src: 111.111.111.111 IP dest: 222.222.222.222 IP Eth Phy B 222.222.222.222 49-BD-D2-C7-56-2A 111.111.111.112 CC-49-DE-D0-AB-7D 111.111.111.110 E6-E9-00-17-BB-4B 222.222.222.221 88-B2-2F-54-1A-0F DataLink Layer 6-60 30

Addressing: routing to another LAN " R forwards datagram with IP source A, destination B " R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagram A 111.111.111.111 74-29-9C-E8-FF-55 IP Eth Phy R 222.222.222.220 1A-23-F9-CD-06-9B MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A IP src: 111.111.111.111 IP dest: 222.222.222.222 IP Eth Phy B 222.222.222.222 49-BD-D2-C7-56-2A 111.111.111.112 CC-49-DE-D0-AB-7D 111.111.111.110 E6-E9-00-17-BB-4B 222.222.222.221 88-B2-2F-54-1A-0F DataLink Layer 6-61 Addressing: routing to another LAN " R forwards datagram with IP source A, destination B " R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagram MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A IP src: 111.111.111.111 IP dest: 222.222.222.222 IP Eth Phy A 111.111.111.111 74-29-9C-E8-FF-55 R 222.222.222.220 1A-23-F9-CD-06-9B B 222.222.222.222 49-BD-D2-C7-56-2A 111.111.111.112 CC-49-DE-D0-AB-7D 111.111.111.110 E6-E9-00-17-BB-4B 222.222.222.221 88-B2-2F-54-1A-0F DataLink Layer 6-62 31

Link Layer 6.1 Introduction and services 6.2 Error detection 6.3 Multiple access protocols 6.4 LANs Addressing, ARP Ethernet Switches 6.5 Data center networking 6.6 A day in the life of a web request DataLink Layer 6-63 Ethernet dominant wired LAN technology: first widely used LAN technology simple, cheap single chip, multiple speeds (e.g., Broadcom BCM5761) kept up with speed race: 10 Mbps 10 Gbps Metcalfe s Ethernet sketch DataLink Layer 6-64 32

Ethernet: physical topology bus: popular through mid 90s all nodes in same collision domain (can collide with each other) star: prevails today bus was replaced by central device (initially hubs, now switches) star bus: coaxial cable hub or switch DataLink Layer 6-65 Hubs physical-layer ( dumb ) repeaters: bits coming in one link go out all other links at same rate all nodes connected to hub can collide with one another no frame buffering at hub no CSMA/CD at hub: host NICs detect collisions hub DataLink Layer 6-66 33

Switches switches prevail today active switch in center each spoke runs a (separate) Ethernet protocol (nodes do not collide with each other) more later switch DataLink Layer 6-67 Ethernet Frame Structure Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame type preamble dest. address source address data (payload) CRC Preamble: 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 used to synchronize receiver, sender clock rates DataLink Layer 6-68 34

Ethernet Frame Structure (more) Addresses: 6 byte source, destination MAC addresses if adapter receives frame with matching destination address, or with broadcast address (e.g., ARP packet), it passes data in frame to network layer protocol otherwise, adapter discards frame Type: indicates higher layer protocol (mostly IP but others possible, e.g., Novell IPX, AppleTalk) CRC: cyclic redundancy check at receiver error detected, frame is dropped type preamble dest. address source address data (payload) CRC DataLink Layer 6-69 Ethernet: Unreliable, connectionless connectionless: No handshaking between sending and receiving NICs unreliable: receiving NIC doesn t send acks nor nacks to sending NIC data in dropped frames recovered only if initial sender uses higher layer rdt (e.g., TCP), otherwise dropped data lost Ethernet s MAC protocol: unslotted CSMA/CD with binary backoff DataLink Layer 6-70 35

802.3 Ethernet Standards: Link & Physical Layers many different Ethernet standards common MAC protocol and frame format different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1 Gbps, 10 Gbps different physical layer media: fiber, cable application transport network link physical 100BASE-TX 100BASE-T4 MAC protocol and frame format 100BASE-T2 100BASE-SX 100BASE-FX 100BASE-BX copper (twisted pair) physical layer fiber physical layer DataLink Layer 6-71 Link Layer 6.1 Introduction and services 6.2 Error detection 6.3 Multiple access protocols 6.4 LANs Addressing, ARP Ethernet Switches 6.5 Data center networking 6.6 A day in the life of a web request DataLink Layer 6-72 36

Ethernet Switch link-layer device: smarter than hubs, take active role store, forward Ethernet frames examine incoming frame s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/ CD to access segment transparent hosts are unaware of presence of switches plug-and-play, self-learning switches do not need to be configured DataLink Layer 6-73 Switch: multiple simultaneous transmissions hosts have dedicated, direct connection to switch switches buffer packets Ethernet protocol used on each incoming link, but no collisions; full duplex each link is its own collision domain switching: A-to-A and B-to-B can transmit simultaneously, without collisions not possible with dumb hub C 6 5 A A 1 2 B C 4 3 switch with six interfaces (1,2,3,4,5,6) B DataLink Layer 6-74 37

Switch forwarding table Q: how does switch know A reachable via interface 4, B reachable via interface 5? " A: each switch has a switch table, each entry: # (MAC address of host/router, interface to reach host/router, time stamp) # looks like a forwarding table! Q: how are entries created, maintained in switch table? # something like a routing protocol? C 6 5 A A 1 2 B C 4 3 switch with six interfaces (1,2,3,4,5,6) B DataLink Layer 6-75 Switch: self-learning Source: A Dest: A switch learns which hosts/ routers can be reached through which interfaces when frame received, switch learns location of sender: incoming LAN segment records sender/location pair in switch table A A A C B 6 1 2 5 4 3 B C A MAC addr interface TTL A 1 60 Switch table (initially empty) DataLink Layer 6-76 38

Switch: frame filtering/forwarding When frame received at switch: 1. record incoming link, MAC address of sending host/router 2. index switch table using MAC destination address 3. if entry found for destination then { if dest on segment from which frame arrived then drop the frame else forward the frame on interface indicated by entry } else flood forward on all but the interface on which the frame arrived DataLink Layer 6-77 Self-learning, forwarding: example Source: A Dest: A frame destination, A, location unknown: flood " destination A location known: selectively send on just one link A A A C B 6 1 2 A A 5 4 3 B C A A A MAC addr interface TTL A 1 60 A 4 60 switch table (initially empty) DataLink Layer 6-78 39

Interconnecting switches switches can be connected together S 4 A B S 1 C S 2 D E F G S 3 H I Q: sending from A to F - how does S 1 know how to forward frame destined for F via S 4 and S 2? A: self learning! (works exactly the same as in single-switch case!) DataLink Layer 6-79 Self-learning multi-switch example Suppose C sends frame to I, I responds to C S 4 A B S 1 C S 2 D E F G S 3 H I Q: show switch tables and packet forwarding in S 1, S 2, S 3, S 4 DataLink Layer 6-80 40

Institutional network to external network router mail server web server IP subnet DataLink Layer 6-81 More than one switch in a subnet S 4 A B S 1 C S 2 D E F G S 3 H I All principles seen so far are applicable to a subnet with several switches, if there is no cycle in the topology No cycle = no redundancy in case of failure Part of subnet can be disconnected From From Computer Networks, Networking, by Tanenbaum by Kurose&Ross Prentice Hall DataLink Layer 6-82 41

Problem with cycles F S 2 F forwarded by S 2 and by S 3 F Initial frame S 1 F S 3 Assume frame F has a destination address that is unknown to all switches (not in their forwarding tables) Problem: frame F is flooded by S 1 and then by all other switches, and will loop forever Solution: build a logical spanning tree topology over the real topology DataLink Layer 6-83 1 LAN Spanning tree (1) Root switch A sends <A,A,0> on LAN1 and LAN2 A H B sends <B,A,1> on LAN3 5 2 From From Computer Networks, Networking, by Tanenbaum by Kurose&Ross Prentice Hall B G I 3 C D E F 8 9 4 6 7 J Switch Build a logical tree reaching all LAN segments (here simply called LAN s) 1. Determine the root switch (has the smallest switch id) Switch id = (priority, MAC addr) Switches regularly flood control messages (BPDUs) on all their output ports: BPDU = <Source switch id, root as assumed, distance to root> All switches will soon discover the root id Note: A «LAN segment» is sometimes called a «collision domain», materialized by a hub, but in practice a LAN segment is most often just a cable DataLink Layer 6-84 42

Spanning tree (2) Root switch A r B Root port 1 2 3 4 C r r r D E F r 2. Build the tree By continuously receiving these BPDUs (possibly on several ports), a switch knows its distance to the root and which port leads to the root by that shortest distance (root port, r) H r 5 r r G I 8 9 6 7 r J Similar to a DV routing protocol with a distance vector limited to a unique component: the root switch In practice, distances are inversely proportional to link capacities Remember InvCap From From Computer Networks, Networking, by Tanenbaum by Kurose&Ross Prentice Hall DataLink Layer 6-85 Spanning tree (3) f A B 1 2 3 4 H C f r f r f r 5 f r D E F r r G I 8 9 Forwarding port r f f f f 6 7 r J r Blocking port 3. Decide if non-root ports are (data) forwarding or (data) blocking A port is "forwarding (f)" on a given LAN iff the BPDUs this switch sends on this LAN are "smaller" than those other switches (would) send. Smaller = shorter distance, or equal distance and smaller switch id Example: on LAN6, E sends <E,A,2>, G would send <G,A,2>, and J would send <J,A,3> So E is elected to be the only one to forward frames on LAN6: J is too far away from the root, and G > E From From Computer Networks, Networking, by Tanenbaum by Kurose&Ross Prentice Hall DataLink Layer 6-86 43

The resulting spanning tree A B C 1 2 3 4 Routing on a spanning tree is not optimal! LAN Switch that is part of the spanning tree D E F 5 6 7 H I 8 9 Switch that is not part of the spanning tree Some switches (e.g., G and J) are not part of the tree Same spanning tree for all source-destination pair! Compare to layer-3 routing Another case could be that some ports of some switches are blocking They could become part of the tree if another switch or port would fail (leading to no refresh of BPDUs) So, switches have to listen to BPDUs on blocking ports to detect failures From From Computer Networks, Networking, by Tanenbaum by Kurose&Ross Prentice Hall DataLink Layer 6-87 Switches vs. routers both are store-and-forward: # routers: network-layer devices (examine networklayer headers) # switches: link-layer devices (examine link-layer headers) both have forwarding tables: # routers: compute tables using routing algorithms, IP addresses # switches: learn forwarding table using flooding, learning, MAC addresses datagram frame application transport network link physical switch application transport network link physical link physical network link physical frame datagram frame DataLink Layer 6-88 44

Summary comparison hubs routers switches traffic isolation no yes yes plug & play yes no yes optimal routing cut through no yes no yes no yes Some do DataLink Layer 6-89 Link Layer 6.1 Introduction and services 6.2 Error detection 6.3Multiple access protocols 6.4 LANs Addressing, ARP Ethernet Switches 6.5 Data center networking 6.6 A day in the life of a web request DataLink Layer 6-90 45

Data center networks 10 s to 100 s of thousands of hosts, often closely coupled, in close proximity: e-business (e.g. Amazon) content-servers (e.g., YouTube, Akamai, Apple, Microsoft) search engines, data mining (e.g., Google)! challenges: o multiple applications, each serving massive numbers of clients o managing/balancing load, avoiding processing, networking, data bottlenecks Inside a 40-ft Microsoft container, Chicago data center DataLink Layer 6-91 Data center networks Internet Access router Border router load balancer: application-layer routing # receives external client requests # directs workload within data center # returns results to external client (hiding data center internals from client) Load balancer B Tier-1 switches Load balancer A C Tier-2 switches 1 2 3 4 5 6 7 8 TOR switches (Top Of Rack) Server racks DataLink Layer 6-92 46

Data center networks! rich interconnection among switches, racks: o increased throughput between racks (multiple routing paths possible) o increased reliability via redundancy Tier-1 switches Tier-2 switches TOR switches Server racks 1 2 3 4 5 6 7 8 DataLink Layer 6-93 Link Layer 6.1 Introduction and services 6.2 Error detection 6.3 Multiple access protocols 6.4 LANs Addressing, ARP Ethernet Switches 6.5. Data center networking 6.6 A day in the life of a web request DataLink Layer 6-94 47

Synthesis: a day in the life of a web request journey down protocol stack complete! application, transport, network, link putting-it-all-together: synthesis! goal: identify, review, understand protocols (at all layers) involved in seemingly simple scenario: requesting www page scenario: student attaches laptop to campus network, requests/receives www.google.com DataLink Layer 6-95 A day in the life: scenario browser DNS server Comcast network 68.80.0.0/13 school network 68.80.2.0/24 web page web server 64.233.169.105 Google s network 64.233.160.0/19 DataLink Layer 6-96 48

A day in the life connecting to the Internet DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP UDP IP Eth Phy DHCP DHCP UDP IP Eth Phy router (runs DHCP) connecting laptop needs to get its own IP address, addr of first-hop router, addr of DNS server: use DHCP! DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in 802.3 Ethernet! Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server (+ switch learning)! Ethernet demuxed to IP demuxed, UDP demuxed to DHCP DataLink Layer 6-97 A day in the life connecting to the Internet DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP UDP IP Eth Phy DHCP UDP IP Eth Phy router (runs DHCP) DHCP server formulates DHCP ACK containing client s IP address, IP address of first-hop router for client, name & IP address of DNS server! encapsulation at DHCP server, frame forwarded (switch learning) through LAN, demultiplexing at client! DHCP client receives DHCP ACK reply Client now has IP address, knows name & addr of DNS server, IP address of its first-hop router DataLink Layer 6-98 49

A day in the life ARP (before DNS, before HTTP) DNS DNS DNS ARP query DNS UDP IP Eth Phy ARP ARP reply ARP Eth Phy router (runs DHCP) before sending HTTP request, need IP address of www.google.com: DNS! DNS query created, encapsulated in UDP, encapsulated in IP, encapsulated in Eth. To send frame to router, need MAC address of router interface: ARP! ARP query broadcast, received by router, which replies with ARP reply giving MAC address of router interface! client now knows MAC address of first hop router, so can now send frame containing DNS query DataLink Layer 6-99 A day in the life using DNS DNS DNS DNS DNS DNS DNS UDP IP Eth Phy DNS DNS DNS DNS DNS UDP IP Eth Phy Comcast network 68.80.0.0/13 DNS server router (runs DHCP)! IP datagram containing DNS query forwarded via LAN switch from client to 1 st hop router! IP datagram forwarded from campus network into comcast network, routed (tables created by RIP, OSPF, IS-IS and/or BGP routing protocols) to DNS server! demux ed to DNS server! DNS server replies to client with IP address of www.google.com (perhaps after querying other DNS servers) DataLink Layer 6-100 50

A day in the life TCP connection carrying HTTP HTTP SYNACK SYNACK SYNACK HTTP TCP IP Eth Phy SYNACK SYNACK SYNACK TCP IP Eth Phy web server 64.233.169.105 router (runs DHCP)! to send HTTP request, client first opens TCP socket to web server! TCP SYN segment (step 1 in 3-way handshake) interdomain routed to web server! web server responds with TCP SYNACK (step 2 in 3- way handshake)! TCP connection established! DataLink Layer 6-101 A day in the life HTTP request/reply HTTP HTTP HTTP HTTP HTTP HTTP TCP IP Eth Phy " web page finally (!!!) displayed HTTP HTTP HTTP HTTP HTTP TCP IP Eth Phy web server 64.233.169.105 router (runs DHCP)! HTTP request sent into TCP socket! IP datagram containing HTTP request routed to www.google.com! web server responds with HTTP reply (containing web page)! IP datagram containing HTTP reply routed back to client DataLink Layer 6-102 51

Chapter 5: Summary principles behind data link layer services: error detection sharing a broadcast channel: multiple access link layer addressing instantiation and implementation of various link layer technologies Ethernet switched LANs learning, building spanning tree evolution of the term link 1. physical wire connecting two communicating nodes 2. physical channel point-to-point or shared wire, wire could also be radio spectrum 3. complex switch infrastructure all these links still viewed by IP as a layer 2 channel DataLink Layer 6-103 Chapter 5: let s take a breath journey down protocol stack complete (except PHY) solid understanding of networking principles, practice.. could stop here. but lots of interesting topics! wireless networks and mobility content distribution and multimedia applications engineering networks to provide better quality to apps advanced routing and traffic engineering securing networks managing networks DataLink Layer 6-104 52