Computer Networking: A Top-Down Approach 6th Edition PDF

Chapter 5 Link Layer A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). Computer They’re in PowerPoint form so you see the animations; and can add, modify, and delete slides (including this one) and slide content to suit your needs. Networking: A Top They obviously represent a lot of work on our part. In return for use, we only ask the following: Down Approach  If you use these slides (e.g., in a class) that you mention their source (after all, we’d like people to use our book!) 6th edition  If you post any slides on a www site, that you note that they are adapted Jim Kurose, Keith Ross from (or perhaps identical to) our slides, and note our copyright of this material. Addison-Wesley March 2012 Thanks and enjoy! JFK/KWR All material copyright 1996-2012 J.F Kurose and K.W. Ross, All Rights Reserved Link Layer 5-1 Chapter 5: Link layer our goals:  understand principles behind link layer services:  error detection, correction  sharing a broadcast channel: multiple access  link layer addressing  local area networks: Ethernet, VLANs  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:  hosts and routers: nodes  communication channels that global ISP connect adjacent nodes along communication path: links  wired links  wireless links  LANs  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  datagram transferred by transportation analogy: different link protocols over  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  tourist = datagram on last link  transport segment =  each link protocol provides communication link different services  transportation mode = link  e.g., may or may not layer protocol provide rdt over link  travel agent = routing algorithm Link Layer 5-5 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 source, dest different from IP address!  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)  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 Link Layer 5-7 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 application  Ethernet card, 802.11 transport network cpu memory card; Ethernet chipset link  implements link, physical host layer controller bus (e.g., PCI) attaches into host’s system link  physical buses physical transmission  combination of hardware, software, firmware network adapter card Link Layer 5-8 Adaptors communicating datagram datagram controller controller sending host receiving host datagram frame  sending side:  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 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 5-10 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:  detect single bit  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:  treat segment contents  compute checksum of as sequence of 16-bit received segment integers  check if computed  checksum: addition (1’s checksum equals checksum complement sum) of field value: segment contents  NO - error detected  sender puts checksum  YES - no error detected. value into UDP But maybe errors checksum field nonetheless? Link Layer 5-13 Cyclic redundancy check  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  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  widely used in practice (Ethernet, 802.11 WiFi, ATM) Link Layer 5-14 CRC example want: D.2r XOR R = nG equivalently: D.2r = nG XOR R equivalently: if we divide D.2r by G, want remainder R to satisfy: D.2r R = remainder[ ] G Link Layer 5-15 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 5-16 Multiple access links, protocols two types of “links”:  point-to-point  PPP for dial-up access  point-to-point link between Ethernet switch, host  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  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 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:  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 Link Layer 5-20 Channel partitioning MAC protocols: TDMA TDMA: time division multiple access  access to channel in "rounds"  each station gets fixed length slot (length = pkt trans time) in each round  unused slots go idle  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  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 pkt, frequency bands 2,5,6 idle frequency bands FDM cable Link Layer 5-22 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 Link Layer 5-23 Slotted ALOHA assumptions: operation:  all frames same size  when node obtains fresh  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  nodes start to transmit  if collision: node retransmits only slot beginning frame in each subsequent  nodes are synchronized slot with prob. p until  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:  single active node can  collisions, wasting slots continuously transmit at  idle slots full rate of channel  nodes may be able to  highly decentralized: only detect collision in less slots in nodes need to be in sync than time to transmit packet  simple  clock synchronization Link Layer 5-25 Slotted ALOHA: efficiency efficiency: long-run  max efficiency: find p* that fraction of successful slots maximizes (many nodes, all with many Np(1-p)N-1 frames to send)  for many nodes, take limit of Np*(1-p*)N-1 as N goes  suppose: N nodes with to infinity, gives: many frames to send, each max efficiency = 1/e =.37 transmits in slot with probability p !  prob that given node has at best: channel success in a slot = p(1- used for useful p)N-1 transmissions 37%  prob that any node has a of time! success = Np(1-p)N-1 Link Layer 5-26 Pure (unslotted) ALOHA  unslotted Aloha: simpler, no synchronization  when frame first arrives  transmit immediately  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  if channel sensed busy, defer transmission  human analogy: don’t interrupt others! Link Layer 5-29 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 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  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 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 sends jam signal idle, starts frame 5. After aborting, NIC transmission. If NIC enters binary (exponential) senses channel busy, backoff: waits until channel idle,  after mth collision, NIC then transmits. chooses K at random 3. If NIC transmits entire from {0,1,2, …, 2m-1}. frame without detecting NIC waits K·512 bit another transmission, times, returns to Step 2 NIC is done with frame !  longer backoff interval with more collisions Link Layer 5-33 CSMA/CD efficiency  Tprop = max prop delay between 2 nodes in LAN  ttrans = time to transmit max-size frame 1 efficiency  1  5t prop /t trans  efficiency goes to 1  as tprop goes to 0  as ttrans goes to infinity  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:  master node “invites” slave nodes to transmit data in turn poll  typically used with “dumb” slave devices master data  concerns:  polling overhead  latency  single point of slaves failure (master) Link Layer 5-36 “Taking turns” MAC protocols token passing: T  control token passed from one node to next sequentially.  token message (nothing  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 … cable cable modem … splitter modem termination system 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) Cable access network cable headend MAP frame for Interval [t1, t2] Downstream channel i CMTS 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  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 Link Layer 5-39 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 Link Layer 5-40 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 5-41 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) 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)  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 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 forgotten (typically 20 58-23-D7-FA-20-B0 min) 0C-C4-11-6F-E3-98 137.196.7.88 Link Layer 5-45 ARP protocol: same LAN  A wants to send datagram to B  B’s MAC address not in  A caches (saves) IP-to- A’s ARP table. MAC address pair in its  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  ARP is “plug-and-play”:  B receives ARP packet,  nodes create their ARP replies to A with its (B's) tables without intervention from net administrator MAC address  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 111.111.111.112 111.111.111.110 222.222.222.221 CC-49-DE-D0-AB-7D E6-E9-00-17-BB-4B 88-B2-2F-54-1A-0F Link Layer 5-47 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 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 Eth Phy 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 111.111.111.112 111.111.111.110 222.222.222.221 CC-49-DE-D0-AB-7D E6-E9-00-17-BB-4B 88-B2-2F-54-1A-0F Link Layer 5-48 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 IP Eth Eth Phy Phy 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 111.111.111.112 111.111.111.110 222.222.222.221 CC-49-DE-D0-AB-7D E6-E9-00-17-BB-4B 88-B2-2F-54-1A-0F Link Layer 5-49 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 IP Eth Eth Phy Phy 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 111.111.111.112 111.111.111.110 222.222.222.221 CC-49-DE-D0-AB-7D E6-E9-00-17-BB-4B 88-B2-2F-54-1A-0F Link Layer 5-50 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 IP Eth Eth Phy Phy 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 111.111.111.112 111.111.111.110 222.222.222.221 CC-49-DE-D0-AB-7D E6-E9-00-17-BB-4B 88-B2-2F-54-1A-0F Link Layer 5-51 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 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 111.111.111.112 111.111.111.110 222.222.222.221 CC-49-DE-D0-AB-7D E6-E9-00-17-BB-4B 88-B2-2F-54-1A-0F Link Layer 5-52 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 5-53 Ethernet “dominant” wired LAN technology:  cheap $20 for NIC  first widely used LAN technology  simpler, cheaper than token LANs and ATM  kept up with speed race: 10 Mbps – 10 Gbps Metcalfe’s Ethernet sketch Link Layer 5-54 Ethernet: physical topology  bus: popular through mid 90s  all nodes in same collision domain (can collide with each other)  star: prevails today  active switch in center  each “spoke” runs a (separate) Ethernet protocol (nodes do not collide with each other) switch star bus: coaxial cable Link Layer 5-55 Ethernet frame structure sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame type dest. source data preamble address address (payload) CRC preamble:  7 bytes with pattern 10101010 followed by one byte with pattern 10101011  used to synchronize receiver, sender clock rates Link Layer 5-56 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 dest. source data preamble address address (payload) CRC Link Layer 5-57 Ethernet: unreliable, connectionless  connectionless: no handshaking between sending and receiving NICs  unreliable: receiving NIC doesnt send acks or 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 wth binary backoff Link Layer 5-58 802.3 Ethernet standards: link & physical layers  many different Ethernet standards  common MAC protocol and frame format  different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1Gbps, 10G bps  different physical layer media: fiber, cable MAC protocol application and frame format transport network 100BASE-TX 100BASE-T2 100BASE-FX link 100BASE-T4 100BASE-SX 100BASE-BX physical copper (twister fiber physical layer pair) physical layer Link Layer 5-59 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 5-60 Ethernet switch  link-layer device: takes an 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 Link Layer 5-61 Switch: multiple simultaneous transmissions  hosts have dedicated, direct A connection to switch B  switches buffer packets C’  Ethernet protocol used on each 6 1 2 incoming link, but no collisions; full duplex 5 4 3  each link is its own collision C domain B’  switching: A-to-A’ and B-to-B’ can transmit simultaneously, A’ without collisions switch with six interfaces (1,2,3,4,5,6) Link Layer 5-62 Switch forwarding table Q: how does switch know A’ A reachable via interface 4, B’ B reachable via interface 5? C’  A: each switch has a switch 6 1 2 table, each entry: 5 4 3  (MAC address of host, interface to reach host, time stamp) B’ C  looks like a routing table! A’ Q: how are entries created, switch with six interfaces maintained in switch table? (1,2,3,4,5,6)  something like a routing protocol? Link Layer 5-63 Switch: self-learning Source: A Dest: A’ A A A’  switch learns which hosts can be reached through B which interfaces C’  when frame received, 6 1 2 switch “learns” location of sender: 5 4 3 incoming LAN segment  records sender/location B’ C pair in switch table A’ MAC addr interface TTL A 1 60 Switch table (initially empty) Link Layer 5-64 Switch: frame filtering/forwarding when frame received at switch: 1. record incoming link, MAC address of sending host 2. index switch table using MAC destination address 3. if entry found for destination then { if destination on segment from which frame arrived then drop frame else forward frame on interface indicated by entry } else flood Link Layer 5-65 Self-learning, forwarding: example Source: A Dest: A’ A A A’  frame destination, A’, locaton unknown: flood C’ B  destination A location 6 1 2 known: selectively send A A’ 5 4 3 on just one link B’ C A’ A A’ MAC addr interface TTL A 1 60 switch table A’ 4 60 (initially empty) Link Layer 5-66 Interconnecting switches  switches can be connected together S4 S1 S3 A S2 F D I B C G H E Q: sending from A to G - how does S1 know to forward frame destined to F via S4 and S3?  A: self learning! (works exactly the same as in single-switch case!) Link Layer 5-67 Self-learning multi-switch example Suppose C sends frame to I, I responds to C S4 S1 S3 A S2 F D I B C G H E  Q: show switch tables and packet forwarding in S1, S2, S3, S4 Link Layer 5-68 Institutional network mail server to external network router web server IP subnet Link Layer 5-69 Switches vs. routers application transport both are store-and-forward: datagram network  routers: network-layer frame link devices (examine network- physical link frame layer headers) physical  switches: link-layer devices (examine link-layer switch headers) network datagram both have forwarding tables: link frame physical  routers: compute tables using routing algorithms, IP application addresses transport  switches: learn forwarding network table using flooding, link learning, MAC addresses physical Link Layer 5-70 VLANs: motivation consider:  CS user moves office to EE, but wants connect to CS switch?  single broadcast domain:  all layer-2 broadcast traffic (ARP, DHCP, Computer unknown location of Science Electrical Computer Engineering destination MAC Engineering address) must cross entire LAN  security/privacy, efficiency issues Link Layer 5-71 port-based VLAN: switch ports VLANs grouped (by switch management software) so that single physical switch …… Virtual Local 1 7 9 15 Area Network 2 8 10 16 switch(es) supporting VLAN capabilities can … … be configured to Electrical Engineering Computer Science (VLAN ports 9-15) define multiple virtual (VLAN ports 1-8) LANS over single … operates as multiple virtual switches physical LAN infrastructure. 1 7 9 15 2 8 10 16 … … Electrical Engineering Computer Science (VLAN ports 1-8) (VLAN ports 9-16) Link Layer 5-72 Port-based VLAN router  traffic isolation: frames to/from ports 1-8 can only reach ports 1-8  can also define VLAN based on MAC addresses of endpoints, rather than switch port 1 7 9 15 2 8 10 16  dynamic membership: ports can be dynamically assigned … … among VLANs Electrical Engineering Computer Science (VLAN ports 1-8) (VLAN ports 9-15)  forwarding between VLANS: done via routing (just as with separate switches)  in practice vendors sell combined switches plus routers Link Layer 5-73 VLANS spanning multiple switches 1 7 9 15 1 3 5 7 2 8 10 16 2 4 6 8 … … Electrical Engineering Computer Science Ports 2,3,5 belong to EE VLAN (VLAN ports 1-8) (VLAN ports 9-15) Ports 4,6,7,8 belong to CS VLAN  trunk port: carries frames between VLANS defined over multiple physical switches  frames forwarded within VLAN between switches can’t be vanilla 802.1 frames (must carry VLAN ID info)  802.1q protocol adds/removed additional header fields for frames forwarded between trunk ports Link Layer 5-74 802.1Q VLAN frame format type preamble dest. source data (payload) CRC address address 802.1 frame type dest. source preamble address address data (payload) CRC 802.1Q frame 2-byte Tag Protocol Identifier Recomputed (value: 81-00) CRC Tag Control Information (12 bit VLAN ID field, 3 bit priority field like IP TOS) Link Layer 5-75 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 5-76 Multiprotocol label switching (MPLS)  initial goal: high-speed IP forwarding using fixed length label (instead of IP address)  fast lookup using fixed length identifier (rather than shortest prefix matching)  borrowing ideas from Virtual Circuit (VC) approach  but IP datagram still keeps IP address! PPP or Ethernet MPLS header IP header remainder of link-layer frame header label Exp S TTL 20 3 1 5 Link Layer 5-77 MPLS capable routers  a.k.a. label-switched router  forward packets to outgoing interface based only on label value (don’t inspect IP address)  MPLS forwarding table distinct from IP forwarding tables  flexibility: MPLS forwarding decisions can differ from those of IP  use destination and source addresses to route flows to same destination differently (traffic engineering)  re-route flows quickly if link fails: pre-computed backup paths (useful for VoIP) Link Layer 5-78 MPLS versus IP paths R6 D R4 R3 R5 A R2  IP routing: path to destination determined IP router by destination address alone Link Layer 5-79 MPLS versus IP paths entry router (R4) can use different MPLS routes to A based, e.g., on source address R6 D R4 R3 R5 A R2  IP routing: path to destination determined IP-only by destination address alone router  MPLS routing: path to destination can be MPLS and IP router based on source and dest. address  fast reroute: precompute backup routes in case of link failure Link Layer 5-80 MPLS signaling  modify OSPF, IS-IS link-state flooding protocols to carry info used by MPLS routing,  e.g., link bandwidth, amount of “reserved” link bandwidth  entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers RSVP-TE R6 D R4 R5 modified link state A flooding Link Layer 5-81 MPLS forwarding tables in out out label label dest interface 10 A 0 in out out 12 D 0 label label dest interface 8 A 1 10 6 A 1 12 9 D 0 R6 0 0 D 1 1 R4 R3 R5 0 0 A R2 in outR1 out label label dest interface in out out label label dest interface 6 - A 0 8 6 A 0 Link Layer 5-82 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 5-83 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:  multiple applications, each serving massive numbers of clients  managing/balancing load, avoiding processing, networking, data bottlenecks Inside a 40-ft Microsoft container, Chicago data center Link Layer 5-84 Data center networks load balancer: application-layer routing  receives external client requests  directs workload within data center  returns results to external client (hiding data Internet center internals from client) Border router Load Load balancer Access router balancer Tier-1 switches B A C Tier-2 switches TOR switches Server racks 1 2 3 4 5 6 7 8 Link Layer 5-85 Data center networks  rich interconnection among switches, racks:  increased throughput between racks (multiple routing paths possible)  increased reliability via redundancy Tier-1 switches Tier-2 switches TOR switches Server racks 1 2 3 4 5 6 7 8 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 5-87 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 Link Layer 5-88 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 Google’s network 64.233.169.105 64.233.160.0/19 Link Layer 5-89 A day in the life… connecting to the Internet DHCP DHCP  connecting laptop needs to DHCP UDP IP get its own IP address, addr of first-hop router, addr of DHCP DHCP Eth Phy DNS server: use DHCP DHCP  DHCP request encapsulated in UDP, encapsulated in IP, DHCP DHCP DHCP UDP encapsulated in 802.3 DHCP IP Ethernet DHCP Eth router Phy (runs DHCP)  Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server  Ethernet demuxed to IP demuxed, UDP demuxed to DHCP Link Layer 5-90 A day in the life… connecting to the Internet DHCP DHCP  DHCP server formulates DHCP UDP DHCP ACK containing DHCP IP client’s IP address, IP DHCP Eth address of first-hop router Phy for client, name & IP address of DNS server  encapsulation at DHCP DHCP DHCP server, frame forwarded DHCP UDP (switch learning) through DHCP IP LAN, demultiplexing at DHCP Eth router client (runs DHCP) DHCP Phy  DHCP client receives DHCP ACK reply Client now has IP address, knows name & addr of DNS server, IP address of its first-hop router Link Layer 5-91 A day in the life… ARP (before DNS, before HTTP) DNS DNS  before sending HTTP request, need DNS UDP IP address of www.google.com: DNS ARP IP DNS ARP query Eth Phy  DNS query created, encapsulated in UDP, encapsulated in IP, encapsulated in Eth. To send frame ARP to router, need MAC address of ARP reply Eth Phy router interface: ARP router  ARP query broadcast, received by (runs DHCP) 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 Link Layer 5-92 A day in the life… using DNS DNS DNS UDP DNS server DNS IP DNS DNS DNS Eth DNS UDP DNS Phy DNS IP DNS Eth Phy DNS Comcast network 68.80.0.0/13 router  IP datagram forwarded from (runs DHCP) campus network into comcast  IP datagram containing DNS network, routed (tables created query forwarded via LAN by RIP, OSPF, IS-IS and/or BGP switch from client to 1st hop routing protocols) to DNS server router  demux’ed to DNS server  DNS server replies to client with IP address of www.google.com Link Layer 5-93 A day in the life…TCP connection carrying HTTP HTTP HTTP SYNACK SYN TCP SYNACK SYN IP SYNACK SYN Eth Phy  to send HTTP request, client first opens TCP socket to web server router  TCP SYN segment (step 1 in 3- (runs DHCP) SYNACK SYN TCP way handshake) inter-domain SYNACK SYN IP routed to web server SYNACK SYN Eth Phy  web server responds with TCP SYNACK (step 2 in 3-way web server handshake) 64.233.169.105  TCP connection established! Link Layer 5-94 A day in the life… HTTP request/reply HTTP HTTP HTTP  web page finally (!!!) displayed HTTP HTTP TCP HTTP HTTP IP HTTP HTTP Eth Phy  HTTP request sent into TCP socket router  IP datagram containing HTTP HTTP HTTP HTTP TCP (runs DHCP) request routed to HTTP IP www.google.com HTTP Eth  web server responds with Phy HTTP reply (containing web page) web server 64.233.169.105  IP datagram containing HTTP reply routed back to client Link Layer 5-95 Chapter 5: Summary  principles behind data link layer services:  error detection, correction  sharing a broadcast channel: multiple access  link layer addressing  instantiation and implementation of various link layer technologies  Ethernet  switched LANS, VLANs  virtualized networks as a link layer: MPLS  synthesis: a day in the life of a web request Link Layer 5-96 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  multimedia  security  network management Link Layer 5-97

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