3rd Edition: Chapter 4 - Clark U

3rd Edition: Chapter 4 - Clark U

CS 280: Link Layer and LANs John Magee 28 November 2016 Most slides adapted from Kurose and Ross, Computer Networking 7/e Source material copyright 1996-2016 J.F Kurose and K.W. Ross 1 Chapter 6: Link layer and LANs 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 and LANs 6-2 Link layer, LANs: outline

6.1 introduction, services 6.2 error detection, correction 6.3 multiple access protocols 6.4 LANs 6.5 link virtualization: MPLS 6.6 data center networking 6.7 a day in the life of a web request addressing, ARP Ethernet switches VLANS Link Layer and LANs 6-3 Link layer: introduction terminology:

hosts and routers: nodes communication channels that connect adjacent nodes along communication path: links wired links wireless links LANs layer-2 packet: frame, data-link layer has responsibility of encapsulates datagram transferring datagram from one node to physically adjacent node over a link Link Layer and LANs 6-4 Link layer: context datagram transferred by different link protocols over different links: e.g., Ethernet on first link, frame relay on intermediate links, 802.11 on last

link each link protocol provides different services e.g., may or may not provide rdt over link (rdt = reliable data transfer) 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 Link Layer and LANs 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 source, destination different from IP address! Optional 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 and LANs 6-6 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 and LANs 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 hosts system buses combination of hardware, software, firmware

application transport network link CPU memory (OS/software) controller link physical host bus (e.g., PCI) physical transmission network adapter card Link Layer and LANs 6-8 Adaptors communicating datagram datagram

controller controller sending host receiving host datagram frame 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 Link Layer and LANs 6-9 Link layer, LANs: outline

6.1 introduction, services 6.2 error detection, correction 6.3 multiple access protocols 6.4 LANs 6.5 link virtualization: MPLS 6.6 data center networking 6.7 a day in the life of a web request addressing, ARP Ethernet switches VLANS Link Layer and LANs 6-10 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 and LANs 6-11 Parity checking single bit parity: two-dimensional bit parity: detect single bit errors detect and correct single bit errors 0 * Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive/ 0 Link Layer and LANs 6-12 Internet checksum (review)

goal: detect errors (e.g., flipped bits) in transmitted packet (note: used at transport layer only) receiver: sender: compute checksum of treat segment received segment contents as check if computed sequence of 16-bit integers checksum equals checksum: addition checksum field value: (1s complement NO - error detected sum) of segment YES - no error contents detected. But sender puts maybe errors checksum value nonetheless? into UDP checksum field Link Layer and LANs 6-13

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 and LANs 6-14 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: R = remainder[ D.2r ] G * Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive/ Link Layer and LANs 6-15 Link layer, LANs: outline 6.1 introduction, services 6.2 error detection, correction 6.3 multiple access protocols 6.4 LANs 6.5 link virtualization: MPLS 6.6 data center

networking 6.7 a day in the life of a web request addressing, ARP Ethernet switches VLANS Link Layer and LANs 6-16 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., cabled Ethernet) shared RF (e.g., 802.11 WiFi)

shared RF (satellite) humans at a Clark Keg cocktail party (shared air, acoustical) Link Layer and LANs 6-17 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 and LANs 6-18 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 and LANs 6-19 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 and LANs 6-20 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 packets to send, slots 2,5,6 idle 6-slot frame 6-slot frame 1 3 4 1 3 4 Link Layer and LANs 6-21 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 packet to send, frequency bands 2,5,6 idle FDM cable frequency bands time Link Layer and LANs 6-22 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 and LANs 6-23 Slotted ALOHA assumptions: operation: all frames same size when node obtains fresh time divided into frame, transmits in next slot equal size slots (time to transmit 1 frame) if no collision: node nodes start to can send new frame transmit only slot in next slot beginning if collision: node nodes are

retransmits frame in synchronized each subsequent slot if 2 or more nodes with prob. p until transmit in slot, all success nodes detect collision Link Layer and LANs 6-24 Slotted ALOHA node 1 1 1 node 2 2 2 node 3 3 C

1 1 2 3 E C S E C 3 E S S Pros: Cons: single active node

can continuously transmit at full rate of channel highly decentralized: only slots in nodes need to be in sync simple collisions, wasting slots idle slots nodes may be able to detect collision in less than time to transmit packet clock Link Layer and LANs 6-25 Slotted ALOHA: efficiency efficiency: long-run fraction of successful slots (many nodes, all with many frames to send) suppose: N nodes with many frames to send, each transmits

in slot with probability p prob that given node has success in a slot = p(1-p)N-1 prob that any node has a success = Np(1-p)N-1 max efficiency: find p* that maximizes Np(1-p)N-1 for many nodes, take limit of Np*(1-p*)N-1 as N goes to infinity, gives: max efficiency = 1/e = .37 at best: channel used for useful transmissions 37% of time! !

Link Layer and LANs 6-26 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 and LANs 6-27 Pure ALOHA efficiency P(success by given node) = P(node transmits) . P(no other node transmits in [t01,t0] . P(no other node transmits in [t01,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 and LANs 6-28 CSMA (carrier sense multiple access)

CSMA: listen before transmit: if channel sensed idle: transmit entire frame if channel sensed busy, defer transmission human analogy: dont interrupt others! Youre at a Clark Keg Party polite cocktail party. You join a group discussing politics. You wait for aLink Layer and LANs 6-29 CSMA collisions spatial layout of nodes collisions can still occur: propagation delay means two nodes may not hear each others transmission collision: entire packet transmission time wasted distance & propagation delay play role in in determining collision

probability Link Layer and LANs 6-30 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 Clark Keg Party Polite cocktail party: After waiting for a break in the conversation Link Layer and LANs 6-31 CSMA/CD (collision detection) spatial layout of nodes Link Layer and LANs 6-32

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 4. If NIC detects another transmission while transmitting, aborts and sends jam signal 5. After aborting, NIC enters binary (exponential) backoff:

after mth collision, NIC chooses K at random from {0,1,2, , 2m-1}. NIC waits Link K512 bit Layer and LANs 6-33 CSMA/CD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame 1 efficiency 1 5t prop /ttrans efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity better performance than ALOHA: and simple, cheap, decentralized! Link Layer and LANs 6-34 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 and LANs 6-35 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 poll master data slaves Link Layer and LANs 6-36 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) Like the spirit

stick in a T (nothing to send) T data Link Layer and LANs 6-37 Cable access network Internet frames, TV channels, control transmitted downstream at different frequencies cable headend CMTS cable modem termination system ISP

splitter cable modem 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) Link Layer and LANs 6-38 Cable access network cable headend MAP frame for Interval [t1, t2] Downstream channel i CMTS Upstream channel j

t1 Minislots containing minislots request frames t2 Residences with cable modems 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 Link Layer and LANs 6-39 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 (wifi) taking turns polling from central site, token passing Bluetooth, FDDI, token ring Link Layer and LANs 6-40 Link layer, LANs: outline 6.1 introduction, services 6.2 error detection, correction 6.3 multiple access protocols 6.4 LANs 6.5 link virtualization: MPLS 6.6 data center networking 6.7 a day in the life of a web request

addressing, ARP Ethernet switches VLANS Link Layer and LANs 6-41 MAC addresses and ARP 32-bit IPv4 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. Like a serial number for your NIC. hexadecimal (base 16) notation e.g.: 1A-2F-BB-76-09-AD (each numeral represents 4 bits) Link Layer and LANs 6-42

LAN addresses and ARP each adapter on LAN has unique LAN address 1A-2F-BB-76-09-AD LAN (wired or wireless) 71-65-F7-2B-08-53 adapter 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 Link Layer and LANs 6-43 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 and LANs 6-44 ARP: address resolution protocol Question: how to determine interfaces MAC address, knowing its IP address? 137.196.7.78 1A-2F-BB-76-09-AD 137.196.7.23 137.196.7.14 LAN 71-65-F7-2B-08-53 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 137.196.7.88 ARP table: each IP node (host, router) on LAN has table IP/MAC address

mappings for some LAN nodes: < IP address; MAC address; TTL> TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) Link Layer and LANs 6-45 ARP protocol: same LAN A wants to send datagram to B Bs MAC address not in As ARP table. A broadcasts ARP query packet, containing B's IP address destination MAC address = FF-FF-FF-FFFF-FF all nodes on LAN receive ARP query B receives ARP packet, replies to A

with its (B's) MAC address frame sent to As MAC A caches (saves) IPto-MAC address pair in its ARP table until information becomes old (times out) soft state: information that times out (goes away) unless refreshed ARP is plug-andplay: nodes create their ARP tables without intervention from net administrator Link Layer and LANs 6-46 Data Link Layer 5-47 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 Bs IP address assume A knows IP address of first hop router, R (how?) assume A knows Rs MAC address (how?) A 111.111.111.111 74-29-9C-E8-FF-55 R B 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.220 1A-23-F9-CD-06-9B 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 Link Layer and LANs 6-48 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 destination address, 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 111.111.111.111 74-29-9C-E8-FF-55 R B 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.220 1A-23-F9-CD-06-9B 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 Link Layer and LANs 6-49 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 IP Eth Phy

R B 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.220 1A-23-F9-CD-06-9B 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 Link Layer and LANs 6-50 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 destination address, 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 IP Eth Phy B 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.220 1A-23-F9-CD-06-9B 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

Link Layer and LANs 6-51 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 destination address, 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 IP Eth Phy B 222.222.222.222

49-BD-D2-C7-56-2A 222.222.222.220 1A-23-F9-CD-06-9B 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 Link Layer and LANs 6-52 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 B 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.220 1A-23-F9-CD-06-9B 111.111.111.112 CC-49-DE-D0-AB-7D 111.111.111.110 E6-E9-00-17-BB-4B * Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive/ 222.222.222.221 88-B2-2F-54-1A-0F Link Layer and LANs 6-53 Link layer, LANs: outline 6.1 introduction, services

6.2 error detection, correction 6.3 multiple access protocols 6.4 LANs 6.5 link virtualization: MPLS 6.6 data center networking 6.7 a day in the life of a web request addressing, ARP Ethernet switches VLANS Link Layer and LANs 6-54 Ethernet dominant wired LAN technology: single chip, multiple speeds (e.g., Broadcom BCM5761) first widely used LAN technology simpler, cheap

kept up with speed race: 10 Mbps 10 Gbps Bob Metcalfes Ethernet sketch on a napkin! Link Layer and LANs 6-55 Ethernet: physical topology bus: popular through mid 90s all nodes in same collision domain (can collide with each other) star: prevails today Early dumb hub in center, today active switch in center each spoke runs a (separate) Ethernet protocol (nodes do not collide with each other) switch bus: coaxial cable star Link Layer and LANs 6-56 Ethernet frame structure sending adapter encapsulates IP datagram (or other network layer

protocol packet) type in Ethernet frame dest. source preamble address address data (payload) CRC preamble: 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 used to synchronize receiver, sender clock rates Link Layer and LANs 6-57 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 This happens in hardware usually. Some

hardware can be set to promiscuous mode. type: indicates higher layer protocol (mostly IP but others possible, e.g., Novell IPX, AppleTalk) type CRC: cyclic redundancy check at receiver dest. source data preamble address address (payload) error detected: frame is dropped CRC Link Layer and LANs 6-58 Ethernet: unreliable, connectionless connectionless: no handshaking between sending and receiving NICs unreliable: receiving NIC doesn't 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 Ethernets MAC protocol: unslotted CSMA/CD with binary backoff Link Layer and LANs 6-59 802.3 Ethernet standards: link & physical layers many different Ethernet standards common MAC protocol and frame format different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1Gbps, 10 Gbps, 40 Gbps different physical layer media: fiber, cable application transport network link physical MAC protocol and frame format 100BASE-TX 100BASE-T2 100BASE-FX 100BASE-T4

100BASE-SX 100BASE-BX copper (twister pair) physical layer fiber physical layer Link Layer and LANs 6-60 Link layer, LANs: outline 6.1 introduction, services 6.2 error detection, correction 6.3 multiple access protocols 6.4 LANs 6.5 link virtualization: MPLS 6.6 data center networking 6.7 a day in the life

of a web request addressing, ARP Ethernet switches VLANS Link Layer and LANs 6-61 Ethernet switch (vs. a dumb hub) link-layer device: takes an active role store, forward Ethernet frames examine incoming frames 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 and LANs 6-62 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 Bto-B can transmit simultaneously, without collisions A B C 6 1 2 4 5 B 3 C A

switch with six interfaces (1,2,3,4,5,6) Link Layer and LANs 6-63 Switch forwarding table Q: how does switch know A reachable via interface 4, B reachable via A: each5? interface switch has a switch table, each entry: A B (MAC address of host, interface to reach host, time stamp) looksare likeentries a routing table! Q:how created, maintained in switch

table? something like a routing protocol? B C 6 1 2 4 5 3 C A switch with six interfaces (1,2,3,4,5,6) Link Layer and LANs 6-64 Switch: self-learning switch learns which hosts can be reached

through which interfaces when frame received, switch learns location of sender: incoming LAN segment records sender/location pair in switch table MAC addr A Source: A Dest: A A A A B C 1 6 2 4 5

B 3 C A interface TTL 1 60 Switch table (initially empty) Link Layer and LANs 6-65 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 /* forward on all interfaces except Link Layer and LANs 6-66 Self-learning, forwarding: example frame destination, A, location flood unknown: destination A locationselectively known: send on just one link Source: A Dest: A A A A B

C 1 6 2 A A 4 5 B 3 C A A A MAC addr interface A A 1 4 TTL 60 60

switch table (initially empty) Link Layer and LANs 6-67 Interconnecting switches self-learning switches can be connected together: S4 S1 S3 S2 A B C F D E I G

H Q: sending from A to G - how does S1 know to forward frame destined to G via S4 and S3? A: self learning! (works exactly the same as in single-switch case!) Link Layer and LANs 6-68 Self-learning multi-switch example Suppose C sends frame to I, I responds to C S4 S1 S3 S2 A B C F D E

I G H Q: show switch tables and packet forwarding in S1, S2, S3, S4 Link Layer and LANs 6-69 Institutional network mail server to external network router web server IP subnet Link Layer and LANs 6-70 Switches vs. routers both are store-andforward: routers: network-layer devices (examine

network-layer headers) switches: link-layer devices (examine linklayer headers) both have forwarding tables: routers: compute tables using routing algorithms, IP addresses switches: learn application transport datagram network frame link physical frame link physical switch router network datagram link frame

physical application transport network link physical Link Layer and LANs 6-71 VLANs: motivation consider: Computer Science Electrical Engineering Computer Engineering CS user moves office to EE, but wants connect to CS switch? single broadcast domain: all layer-2 broadcast traffic (ARP, DHCP,

unknown location of destination MAC address) must cross entire LAN security/privacy, efficiency issues Link Layer and LANs 6-72 VLANs Virtual Local Area Network switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure. port-based VLAN: switch ports grouped (by switch management software) so that single physical switch 1 7

9 15 2 8 10 16 Electrical Engineering (VLAN ports 1-8) Computer Science (VLAN ports 9-15) operates as multiple virtual switches 1 7 9

15 2 8 10 16 Electrical Engineering (VLAN ports 1-8) Computer Science (VLAN ports 9-16) Link Layer and LANs 6-73 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 Electrical Engineering (VLAN ports 1-8) among VLANs forwarding between VLANS: done via routing (just as with separate switches)

Computer Science (VLAN ports 9-15) in practice vendors sell combined switches plus routers Link Layer and LANs 6-74 VLANS spanning multiple switches 1 7 9 15 1 3 5 7 2 8

10 16 2 4 6 8 Electrical Engineering (VLAN ports 1-8) Computer Science (VLAN ports 9-15) Ports 2,3,5 belong to EE VLAN 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 cant 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 and LANs 6-75 802.1Q VLAN frame format type preamble dest. address source address data (payload) CRC 802.1 frame type preamble dest. address source address

data (payload) 2-byte Tag Protocol Identifier (value: 81-00) CRC 802.1Q frame Recomputed CRC Tag Control Information (12 bit VLAN ID field, 3 bit priority field like IP TOS) Link Layer and LANs 6-76 Link layer, LANs: outline 6.1 introduction, services 6.2 error detection, correction 6.3 multiple access protocols 6.4 LANs

6.5 link virtualization: MPLS 6.6 data center networking 6.7 a day in the life of a web request addressing, ARP Ethernet switches VLANS Link Layer and LANs 6-77 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 header MPLS header

label 20 IP header remainder of link-layer frame Exp S TTL 3 1 5 Link Layer and LANs 6-78 MPLS capable routers a.k.a. label-switched router forward packets to outgoing interface based only on label value (dont 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 and LANs 6-79 MPLS versus IP paths R6 D R4 R3 R5 A R2 IP routing: path to destination determined by destination address alone IP router Link Layer and LANs 6-80 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 by destination address alone path to MPLS routing: destination can be based on source and destination address fast reroute: precompute backup routes in case of link failure IP-only router MPLS and IP router Link Layer and LANs 6-81 MPLS signaling modify OSPF, IS-IS link-state flooding

protocols to carry info used by MPLS routing, e.g.,MPLS link bandwidth, amount of reserved link entry router uses RSVP-TE signaling bandwidth protocol to set up MPLS forwarding at downstream routers RSVP-TE R6 D R4 R5 modified link state flooding A

Link Layer and LANs 6-82 MPLS forwarding tables in out label label dest interface 10 12 8 out A D A R6 0 0 1 in out label label dest interface

0 R4 R5 10 6 A 1 12 9 D 0 0 1 R3 out

D 1 0 0 R2 in out label label dest interface 8 6 A out in outR1 label label dest interface 6

- A A out 0 0 Link Layer and LANs 6-83 Link layer, LANs: outline 6.1 introduction, services 6.2 error detection, correction 6.3 multiple access protocols 6.4 LANs 6.5 link virtualization: MPLS 6.6 data center

networking 6.7 a day in the life of a web request addressing, ARP Ethernet switches VLANS Link Layer and LANs 6-84 Data center networks 10s to 100s 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 and LANs 6-85 Data center networks load balancer: application-layer routing receives external client requests directs workload within data center returns results to external client (hiding data center internals from Border router client) Internet Load balancer Access router Tier-1 switches B A Load balancer Tier-2 switches

C TOR switches (Top of Rack Switches) Server racks 1 2 3 4 5 6 7 8 Link Layer and LANs 6-86 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 and LANs 6-87 Chapter 6: 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 and LANs 6-88 Chapter 6: lets 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 Link Layer and LANs 6-89

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