1. Prof. Younghee Lee 1 1 Computer Networks u Lecture 3: Data Link, LAN, ATM and MPLS Prof. Younghee Lee * Some part of this teaching materials are prepared referencing the lecture note made by F. Kurose, Keith W. Ross(U. of Massachusetts)

2. Prof. Younghee Lee 2 2 Link Layer Services u Framing, link access: –encapsulate datagram into frame, adding header, trailer –implement channel access if shared medium, –‘physical addresses’ used in frame headers to identify source, dest »different from IP address! u Reliable delivery between two physically connected devices: –we learned how to do this already –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?

3. Prof. Younghee Lee 3 3 Link Layer Services (more) u Flow Control: –pacing between sender and receivers u Error Detection: –errors caused by signal attenuation, noise. –receiver detects presence of errors: »signals sender for retransmission or drops frame u Error Correction: –receiver identifies and corrects bit error(s) without resorting to retransmission

4. Prof. Younghee Lee 4 4 The need for flow and error control u Flow control –protocol mechanism that enables a destination entity to regulate the flow of PDUs sent by a source entity »examples; different flow control characteristics u X.25 virtual circuit: network layer u LAPB: data link layer u TCP connections: transport layer

5. Prof. Younghee Lee 5 5 The need for flow and error control u Flow control –employed in various contexts »Hop Scope, Network interface, Entry to exit, End to end u Error control –used to recover from the loss or damage of PDUs in transit

6. Prof. Younghee Lee 6 6 Link control mechanism-Stop and Wait u Simplest form of flow control u CRC u ACK0: –acknowledges receipt of a frame numbered 1 –indicates that the receiver is ready for a frame numbered 0 –positive acknowledgement u ARQ: Automatic Repeat reQuest u Time out interval u rarely used because of inefficiency –Example: 1.5Mbps link x 45ms RTT = 67.5Kb (8KB). Assuming frame size of 1KB, stop-and-wait uses about one-eighth of the link's capacity. Want the sender to be able to transmit up to 8 frames before having to wait for an ACK. –Case: link with very small RTT?

7. Prof. Younghee Lee 7 7 Link control mechanism-Stop and Wait u Time to send 1 frame

8. Prof. Younghee Lee 8 8 Parity Checking Single Bit Parity: Detect single bit errors Two Dimensional Bit Parity: Detect and correct single bit errors 0 0

9. Prof. Younghee Lee 9 9 Cyclic Redundancy Check u Add k bits of redundant data to an n-bit message. u Represent n-bit message as an n-1 degree polynomial; e.g., MSG=10011010 corresponds to M(x) = x 7 + x 4 + x 3 + x 1. u Let k be the degree of some divisor polynomial C(x); e.g., C(x) = x 3 + x 2 + 1. u Transmit polynomial P(x) that is evenly divisible by C(x), and receive polynomial P(x) + E(x); E(x)=0 implies no errors. u Recipient divides (P(x) + E(x)) by C(x); the remainder will be zero in only two cases: E(x) was zero (i.e. there was no error), or E(x) is exactly divisible by C(x). Choose C(x) to make second case extremely rare.

10. Prof. Younghee Lee 10 Cyclic Redundancy Check u Sender: –multiply M(x) by xk; for our example, we get x 10 + x 7 + x 6 + x 4 (10011010000); –divide result by C(x) (1101); –Send 10011010000 - 101 = 10011010101, since this must be exactly divisible by C(x);

11. Prof. Younghee Lee 11 Cyclic Redundancy Check  Common polynomials for C(x): CRC CRC-8 CRC-10 CRC-12 CRC-16 CRC-CCITT CRC-32 C(x) x 8 +x 2 +x 1 +1 x 10 +x 9 +x 5 +x 4 +x 1 +1 x 12 +x 11 +x 3 +x 2 +x 1 +1 x 16 +x 15 +x 2 +1 x 16 +x 12 +x 5 +1 x 32 +x 26 +x 23 +x 22 +x 16 +x 12 +x 11 +x 10 +x 8 +x 7 +x 5 +x 4 +x 2 +x+1

12. Prof. Younghee Lee 12 Link control mechanism-Sliding window u Sliding-window Techniques –Receiver accept n frames, and A is allowed to send n frames without waiting for any acknowledgements –

13. Prof. Younghee Lee 13 Link control mechanism-Sliding window u Go-back-N: http://wps.aw.com/aw_kurose_network_2/0,7240,227091-,00.html –most commonly used –The form of error control based on sliding-window flow control –RR –REJ:negative acknowledgement –Damaged frame: »Case A: errored frame »Case B: lost in transit »Case C:lost in transit. No additional frame to send soon. Sender time out u Sender sends RR with a P bit of 1 as a command u Receiver acknowledges by sending a RR indicating the next frame that it expect u Sender retransmit –Damaged RR »damaged RR. Subsequent RR »damaged RR. A’s timer expires. u Sender send RR in Case C. set P-bit timer. u If receiver fails to respond, then sender’s P-bit timer will expire u Sender tries again multiple times. No ACK -> reset procedure –Damaged REJ: equivalent to Case C

14. Prof. Younghee Lee 14 Link control mechanism-Sliding window u Selective-reject ARQ –retransmit only the frame that receive a negative ACK: SREJ –much less widely used than go-back-N –more efficient than go-back-N –receiver must maintain a buffer large enough to save post- SREJ frames until the frame in error is retransmitted –transmitter, too, requires complex logic.

15. Prof. Younghee Lee 15 ARQ performance: Stop-and-wait ARQ u Error-free Stop-and-wait

16. Prof. Younghee Lee 16 ARQ performance: Stop-and-wait ARQ u Stop-and-wait ARQ with error

17. Prof. Younghee Lee 17 ARQ performance: Stop-and-wait ARQ u Stop-and-wait ARQ with error

18. Prof. Younghee Lee 18 ARQ performance: Stop-and-wait ARQ u The parameter a

19. Prof. Younghee Lee 19 ARQ performance: Stop-and-wait ARQ u Performance of stop-and-wait protocol(P=10 -3 )

20. Prof. Younghee Lee 20 ARQ performance:Sliding-window ARQ

21. Prof. Younghee Lee 21 ARQ performance:Sliding-window ARQ u W=1: stop and wait u W=7: many case u W=127: high speed WANs

22. Prof. Younghee Lee 22 ARQ performance:Sliding-window ARQ  Selective-reject ARQ

23. Prof. Younghee Lee 23 ARQ performance:Sliding-window ARQ  Go-back-N ARQ

24. Prof. Younghee Lee 24 ARQ performance:Sliding-window ARQ

25. Prof. Younghee Lee 25 ARQ performance:Sliding-window ARQ

26. Prof. Younghee Lee 26 Comparison L2, L4 u Flow control + Error control –RTT »Delay variance –Timeout value u Throughput –Error control at each L2’s + L4; Loss at L3 buffer –Error control only at L4; –TCP throughput in case of many loss due to intensive errors at L2? »Satellite link, wireless link…

27. Prof. Younghee Lee 27 HDLC u High Level Data Link Control : ISO u Service –Connectionless service »Acknowledged service »Unacknowledged service: most common –Connection-oriented service u Balanced mode, unbalanced mode

28. Prof. Younghee Lee 28 LLC u Logical Link Control u Data link layer for LAN consists of MAC & LLC u Service –Type 1: unacknowledged connectionless service »most common in LAN –Type2: reliable connection-oriented service: »End system without TCP –Type3: acknowledged connectionless service »The receiving node will send acknowledgments for individual frames. No virtual circuit

29. Prof. Younghee Lee 29 LANs Data link layer LLC sublayer MAC sublayer

30. Prof. Younghee Lee 30 Multiple Access Links and Protocols Three types of “links”: u point-to-point (single wire, e.g. PPP, SLIP) u broadcast (shared wire or medium; e.g, Ethernet, Wavelan, etc.) u switched (e.g., switched Ethernet, ATM etc)

31. Prof. Younghee Lee 31 Channel Partitioning MAC protocols: FDMA FDMA: frequency division multiple access u channel spectrum divided into frequency bands u each station assigned fixed frequency band u unused transmission time in frequency bands go idle u example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle u TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load. u FDM (Frequency Division Multiplexing): frequency subdivided. frequency bands time

32. Prof. Younghee Lee 32 Channel Partitioning (CDMA) CDMA (Code Division Multiple Access) u unique “code” assigned to each user; ie, code set partitioning u used mostly in wireless broadcast channels (cellular, satellite,etc) u all users share same frequency, but each user has own “chipping” sequence (ie, code) to encode data u encoded signal = (original data) X (chipping sequence) u decoding: inner-product of encoded signal and chipping sequence u allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

33. Prof. Younghee Lee 33 CDMA Encode/Decode

34. Prof. Younghee Lee 34 CDMA: two-sender interference

35. Prof. Younghee Lee 35 Random Access protocols u When node has packet to send –transmit at full channel data rate R. –no a priori coordination among nodes u two or more transmitting nodes -> “collision”, u random access MAC protocol specifies: –how to detect collisions –how to recover from collisions (e.g., via delayed retransmissions) u Examples of random access MAC protocols: –slotted ALOHA –ALOHA –CSMA and CSMA/CD

36. Prof. Younghee Lee 36 Slotted Aloha u time is divided into equal size slots (= pkt trans. time) u node with new arriving pkt: transmit at beginning of next slot u if collision: retransmit pkt in future slots with probability p, until successful. Success (S), Collision (C), Empty (E) slots

37. Prof. Younghee Lee 37 Slotted Aloha efficiency Q: what is max fraction slots successful? A: Suppose N stations have packets to send –each transmits in slot with probability p –prob. successful transmission S is: by single node: S= p (1-p) (N-1) by any of N nodes S = Prob (only one transmits) = N p (1-p) (N-1) … choosing optimum p as n -> infty... = 1/e =.37 as N -> infty At best: channel use for useful transmissions 37% of time!

38. Prof. Younghee Lee 38 Pure (unslotted) ALOHA u unslotted Aloha: simpler, no synchronization u pkt needs transmission: – send without awaiting for beginning of slot u collision probability increases: –pkt sent at t 0 collide with other pkts sent in [t 0 -1, t 0 +1]

39. Prof. Younghee Lee 39 Pure Aloha (cont.) P(success by given node) = P(node transmits). P(no other node transmits in [p 0-1, p 0 ]. P(no other node transmits in [p 0, p 0+1 ] = p. (1-p) (N-1). (1-p) (N-1) P(success by any of N nodes) = N p. (1-p) 2(N-1) … choosing optimum p as n -> infty... = 1/(2e) =.18 S = throughput = “goodput” (success rate) G = offered load = Np 0.51.0 1.5 2.0 0.1 0.2 0.3 0.4 Pure Aloha Slotted Aloha protocol constrains effective channel throughput!

40. Prof. Younghee Lee 40 CSMA: Carrier Sense Multiple Access) CSMA: listen before transmit: u If channel sensed idle: transmit entire pkt u If channel sensed busy, defer transmission –Persistent CSMA: retry immediately with probability p when channel becomes idle (may cause instability) –Non-persistent CSMA: retry after random interval u human analogy: don’t interrupt others!

41. Prof. Younghee Lee 41 CSMA collisions collisions can still occur: propagation delay means two nodes may not hear each other’s transmission collision: entire packet transmission time wasted spatial layout of nodes note: role of distance & propagation delay in determining collision probability

42. Prof. Younghee Lee 42 CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA –collisions detected within short time –colliding transmissions aborted, reducing channel wastage –persistent or non-persistent retransmission u collision detection: –easy in wired LANs: measure signal strengths, compare transmitted, received signals –difficult in wireless LANs: receiver shut off while transmitting u human analogy: the polite conversationalist u http://wps.aw.com/aw_kurose_network_2/0,7240,227091 -,00.html

43. Prof. Younghee Lee 43 CSMA/CD collision detection

44. Prof. Younghee Lee 44 “Taking Turns” MAC protocols channel partitioning MAC protocols: –share channel efficiently 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!

45. Prof. Younghee Lee 45 “Taking Turns” MAC protocols Polling: u master node “invites” slave nodes to transmit in turn u Request to Send, Clear to Send msgs u concerns: –polling overhead –latency –single point of failure (master) Token passing:  control token passed from one node to next sequentially.  token message  concerns: –token overhead –latency –single point of failure (token)  Good thing: –Fairness control –QoS control –Throughput –Use of Fiber optic links

46. Prof. Younghee Lee 46 Reservation-based protocols Distributed Polling: u time divided into slots u begins with N short reservation slots –reservation slot time equal to channel end-end propagation delay –station with message to send posts reservation –reservation seen by all stations u after reservation slots, message transmissions ordered by known priority

47. Prof. Younghee Lee 47 Ethernet LAN u History –Developed by Xerox PARC in mid-1970s –Roots in ALOHA packet-radio network –Standardized by Xerox, DEC, and Intel in 1978 –Similar to IEEE 802.3 standard u CSMA/CD –carrier sense –multiple access –collision detection u Bandwidth: 10Mbps and 100Mbps u Problem: Distributed algorithm that provides fair access to a shared medium u CDPD: Cellular Digital Packet Data –Mac protocol for CDPD: Digital sense Multiple access; a variation of CSMA/CD

48. Prof. Younghee Lee 48 Ethernet LAN Addresses: u Unique, 48-bit unicast address assigned to each adaptor u Example: 8:0:2b:e4:b1:2 u Broadcast: all 1 s u Multicast: first bit is 1

49. Prof. Younghee Lee 49 LAN Addresses and ARP 32-bit IP address: u network-layer address u used to get datagram to destination network (recall IP network definition) LAN (or MAC or physical) address: u used to get datagram from one interface to another physically-connected interface (same network) u 48 bit MAC address (for most LANs) burned in the adapter ROM

50. Prof. Younghee Lee 50 LAN Addresses and ARP Each adapter on LAN has unique LAN address

51. Prof. Younghee Lee 51 LAN Address (more) u MAC address allocation administered by IEEE u manufacturer buys portion of MAC address space (to assure uniqueness) u Analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address u MAC flat address => portability –can move LAN card from one LAN to another u IP hierarchical address NOT portable – depends on network to which one attaches

52. Prof. Younghee Lee 52 ARP: Address Resolution Protocol u Each IP node (Host, Router) on LAN has ARP table u ARP Table: IP/MAC address mappings for some LAN nodes – TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) Question: how to determine MAC address of B knowing B’s IP address? 1A-2F-BB-76-09-AD 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 71-65-F7-2B-08-53 LAN 237.196.7.23 237.196.7.78 237.196.7.14 237.196.7.88

53. Prof. Younghee Lee 53 ARP protocol: Same LAN (network) u A wants to send datagram to B, and B’s MAC address not in A’s ARP table. u A broadcasts ARP query packet, containing B's IP address –Dest MAC address = FF- FF-FF-FF-FF-FF –all machines on LAN receive ARP query u B receives ARP packet, replies to A with its (B's) MAC address –frame sent to A’s MAC address (unicast) u A caches (saves) 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 u ARP is “plug-and-play”: –nodes create their ARP tables without intervention from net administrator

54. Prof. Younghee Lee 54 Transmitter Algorithm If line is idle: u Send immediately u Upper bound message size of 1500 bytes u Must wait 51  s between back-to-back frames If line is busy: u Wait until idle and transmit immediately u Called 1-persistent (special case of p-persistent) If collision: u jam for 512 bits, then stop transmitting frame u minimum frame is 64 bytes (header + 46 bytes of data) u delay and try again –1st time: uniformly distributed between 0 and 51.2  s –2nd time: uniformly distributed between 0 and 102.4  s –3rd time: uniformly distributed between 0 and 204.8  s –give up after several tries (usually 16) –exponential backoff

55. Prof. Younghee Lee 55 Experiences Observe in Practice u 10-200 hosts (not 1024) u Length shorter than 1500m (RTT closer to 5  than 51  ) u Packet length is bimodal u High-level flow control and host performance limit load Recommendations u Do not overload (30% utilization is about max) u Implement controllers correctly u Use large packets u Get the rest of the system right (broadcast, retransmission)

56. Prof. Younghee Lee 56 Hub and Switch u Hub: –Shared medium bus –Shared medium hub –Switching hub »to achieve greater performance »throughput on the LAN in figure is 20Mbps »advantages u No change is required u aggregated capacity u scales easily. »Types u Store-and-forward switch: buffers input frame, and then routes to output line u Cut-through switch: repeating the incoming frame onto the output line after recognizing the destination address at the beginning of MAC frame. Highest possible throughput but at some risk of propagating bad frames(CRC) N x 10M

57. Prof. Younghee Lee 57 IEEE 802.11 Wireless LAN u wireless LANs: untethered (often mobile) networking u IEEE 802.11 standard: –MAC protocol –unlicensed frequency spectrum: 900Mhz, 2.4Ghz  Basic Service Set (BSS) (a.k.a. “ cell ” ) contains: –wireless hosts –access point (AP): base station  BSS ’ s combined to form distribution system (DS)

58. Prof. Younghee Lee 58 Characteristics of selected wireless link standards 384 Kbps 56 Kbps 54 Mbps 5-11 Mbps 1 Mbps 802.15 802.11b 802.11{a,g} IS-95 CDMA, GSM UMTS/WCDMA, CDMA2000.11 p-to-p link 2G 3G Indoor 10 – 30m Outdoor 50 – 200m Mid range outdoor 200m – 4Km Long range outdoor 5Km – 20Km

59. Prof. Younghee Lee 59 MAC u MAC Problems in wireless network: to use CSMA/CD –Collision Detection(CD) does not work –CS might not work in some case( if a terminal is “hidden”) u Hidden terminal problem –Nodes A and C cannot hear each other »Node A : currently transmitting to B »Node C : wants to transmit to B »Transmissions by nodes A and C can collide at node B Nodes A and C are hidden from each other A B C

60. Prof. Younghee Lee 60 MAC u Exposed terminal problem –Node C cannot send to D due to carrier of B sense »Node B : currently transmitting to A »Node C : wants to transmit to D »Carrier of C doesn’t interfere A’s reception, Carrier of B doesn’t interfere D’s reception u Waiting is not necessary »But C is waiting since it sense carrier of B –C is exposed to B A B C D

61. Prof. Younghee Lee 61 IEEE 802.11 Wireless LAN u 802.11b –2.4-5 GHz unlicensed radio spectrum –up to 11 Mbps –direct sequence spread spectrum (DSSS) in physical layer »all hosts use same chipping code –widely deployed, using base stations u 802.11a –5-6 GHz range –up to 54 Mbps u 802.11g –2.4-5 GHz range –up to 54 Mbps u All use CSMA/CA for multiple access u All have base-station and ad-hoc network versions

62. Prof. Younghee Lee 62 IEEE 802.11: multiple access u Like Ethernet, uses CSMA: –random access –carrier sense: don’t collide with ongoing transmission u Unlike Ethernet: –no collision detection – transmit all frames to completion –acknowledgment – because without collision detection, you don’t know if your transmission collided or not u Why no collision detection? –difficult to receive (sense collisions) when transmitting due to weak received signals (fading) –can’t sense all collisions in any case: hidden terminal, fading u Goal: avoid collisions: CSMA/C(ollision)A(voidance)

63. Prof. Younghee Lee 63 MAC u IFS: interframe space: depend on the type of frame to transmit –SIFS: Short IFS »High priority frame before contending for channel »ACK frame, CTS frame, data transfer of a segmented MSDU, frames from station that are responding to a poll from an AP, any frame from an AP during CFP –PIFS: PCF-IFS »Used by PCF to gain priority access to the medium at the start of a CFP(Contention Free Period) –DIFS: DCF-IFS »Used by the DCF to transmit data and management MPDU u DCF : CSMA/CA –Initial MAC PDU: medium idle for a period DIFS or greater -> transmit »Medium busy -> wait random backoff time to schedule a reattempt –Reattempt: decrement a counter each time an idle contention slot transpire –After successful frame transmission -> backoff procedure to transmit next frame Busy Medium SIFS PIFS DIFS Contention WIndow Next Frame

64. Prof. Younghee Lee 64 IEEE 802.11 MAC Protocol: CSMA/CA 802.11 sender 1 if sense channel idle for DIFS then - transmit entire frame (no CD) 2 if sense channel busy then - start random backoff time - timer counts down while channel idle - transmit when timer expires - if no ACK, increase random backoff interval, repeat 2 802.11 receiver if frame received OK - return ACK after SIFS (ACK needed due to hidden terminal problem) sender receiver DIFS data SIFS ACK

65. Prof. Younghee Lee 65 RTS/CTS idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames u optional; not typically used u sender first transmits small request-to-send (RTS) packets to AP using CSMA –RTSs may still collide with each other (but they’re short) u AP broadcasts clear-to-send CTS in response to RTS u CTS heard by all nodes –sender transmits data frame –other stations defer transmissions Avoid data frame collisions completely using small reservation packets!

66. Prof. Younghee Lee 66 Collision Avoidance: RTS-CTS exchange AP A B time RTS(A) RTS(B) RTS(A) CTS(A) DATA (A) ACK(A) reservation collision defer

67. Prof. Younghee Lee 67 hub or switch AP 2 AP 1 H1 BBS 2 BBS 1 802.11: mobility within same subnet router u H1 remains in same IP subnet: IP address can remain same u switch: which AP is associated with H1? –self-learning (Ch. 5): switch will see frame from H1 and “remember” which switch port can be used to reach H1

68. Prof. Younghee Lee 68 Mobile Switching Center Public telephone network, and Internet Mobile Switching Center Components of cellular network architecture  connects cells to wide area net  manages call setup (more later!)  handles mobility (more later!) MSC  covers geographical region  base station (BS) analogous to 802.11 AP  mobile users attach to network through BS  air-interface: physical and link layer protocol between mobile and BS cell wired network

69. Prof. Younghee Lee 69 Cellular standards: brief survey 2.5 G systems: voice and data channels u for those who can’t wait for 3G service: 2G extensions u general packet radio service (GPRS) –evolved from GSM –data sent on multiple channels (if available) u enhanced data rates for global evolution (EDGE) –also evolved from GSM, using enhanced modulation –Date rates up to 384K u CDMA-2000 (phase 1) –data rates up to 144K –evolved from IS-95

70. Prof. Younghee Lee 70 Cellular standards: brief survey 3G systems: voice/data u Universal Mobile Telecommunications Service (UMTS) –GSM next step, but using CDMA u CDMA-2000 ….. more (and more interesting) cellular topics due to mobility (stay tuned for details)

71. Prof. Younghee Lee 71 Ad Hoc Networks u Ad hoc network: IEEE 802.11 stations can dynamically form network without AP u Applications: –“laptop” meeting in conference room, car –interconnection of “personal” devices –battlefield u IETF MANET (Mobile Ad hoc Networks) working group

72. Prof. Younghee Lee 72 Bridges u Link Layer devices: operate on Ethernet frames, examining frame header and selectively forwarding frame based on its destination u Bridge isolates collision domains since it buffers frames u When frame is to be forwarded on segment, bridge uses CSMA/CD to access segment and transmit

73. Prof. Younghee Lee 73 Bridges (more) u Bridge advantages: –Isolates collision domains resulting in higher total max throughput, and does not limit the number of nodes nor geographical coverage –Can connect different type Ethernet since it is a store and forward device –Transparent: no need for any change to hosts LAN adapters

74. Prof. Younghee Lee 74 Bridges: frame filtering, forwarding u bridges filter packets –same-LAN -segment frames not forwarded onto other LAN segments u forwarding: –how to know which LAN segment on which to forward frame? –looks like a routing problem (more shortly!)

75. Prof. Younghee Lee 75 Backbone Bridge

76. Prof. Younghee Lee 76 Interconnection Without Backbone u Not recommended for two reasons: - single point of failure at Computer Science hub - all traffic between EE and SE must path over CS segment

77. Prof. Younghee Lee 77 Bridge Filtering u bridges learn which hosts can be reached through which interfaces: maintain filtering tables –when frame received, bridge “learns” location of sender: incoming LAN segment –records sender location in filtering table u filtering table entry: –(Node LAN Address, Bridge Interface, Time Stamp) –stale entries in Filtering Table dropped (TTL can be 60 minutes)

78. Prof. Younghee Lee 78 Bridge Learning: example Suppose C sends frame to D and D replies back with frame to C  C sends frame, bridge has no info about D, so floods to both LANs –bridge notes that C is on port 1 –frame ignored on upper LAN –frame received by D

79. Prof. Younghee Lee 79 Bridge Learning: example  D generates reply to C, sends –bridge sees frame from D –bridge notes that D is on interface 2 –bridge knows C on interface 1, so selectively forwards frame out via interface 1

80. Prof. Younghee Lee 80 Bridges vs. Routers u both store-and-forward devices –routers: network layer devices (examine network layer headers) –bridges are Link Layer devices u routers maintain routing tables, implement routing algorithms u bridges maintain filtering tables, implement filtering, learning and spanning tree algorithms

81. Prof. Younghee Lee 81 High Speed LANs u Most important high speed LANs –Fast Ethernet and Gigabit Ethernet –ATM LAN

82. Prof. Younghee Lee 82 Fast Ethernet u 100BASE-T u 100BASE-X: two physical links between nodes, one for reception and one for transmission u 100BASE-T4: the use of four twisted pair lines making use of 3 pairs in one direction at a time –category 3: voice grade UTP, standard telephone wire, –category 5: more tightly twisted for higher quality

83. Prof. Younghee Lee 83 Fast Ethernet u Mixed Configuration – readily supports a mixture of existing 10-Mbps LANs and newer 100-Mbps LANs

84. Prof. Younghee Lee 84 Gigabit Ethernet u Gigabit Ethernet –compatible with 100Base-T, 10Base-T –use of optical fiber over relatively short distance, although UTP, STP, and coaxial cable configurations are also allowed.

85. Prof. Younghee Lee 85 Gigabit Ethernet u Application of Gigabit Ethernet u Discussion: ATM LAN vs Gigabit Ethernet

86. Prof. Younghee Lee 86 Metropolitan Ethernet u Ethernet services for metro transport network u Carrier-class Ethernet-based transport technologies u Metro Ethernet Forum(MEF) –Forum to accelerate the adoption of optical Ethernet as the technology of choice in metro network world wide –Move beyond LAN origin to become a full duplex, switched technology capable of meeting the long-range transport, bandwidth, geographical and capacity requirements of metro networking »A compelling combination of speed, scalability, operational simplicity, and economics –Deliverables »Implementation agreements, Test procedure, positioning statements, technical specifications, Marketing –Services »Ethernet virtual private line service(EVPLS), Ethernet virtual private LAN services(EVPLns) and Circuit emulation services(CES)

87. Prof. Younghee Lee 87 Metropolitan Ethernet u IEEE 802.3 Ethernet in the First Mile(EFM) –The optimal subscriber access network »For faster, simpler, better and more profitable –Point to point on fiber(P2P), Point to multipoint fiber(EPON), Point to point copper(Copper) –Provide a family of physical layer specifications –OAM –To expand the application of Ethernet to include subscriber access networks in order to provide a significant increase in performance while maintaining equipment, operation, and maintenance cost

88. Prof. Younghee Lee 88 ATM LANs u Typical requirements for a third generation LAN –support multiple, guaranteed classes of services –provide scalable throughput –facilitate the internetworking between LAN and WAN technology u ATM ideally suited to these requirements u The Internet: connectionless, ATM: connection oriented u possible types of ATM LANs: –Gateway to ATM WAN –Backbone ATM switch: interconnect other LANs –Workgroup ATM: high performance MM WSs, connect directly to switch

89. Prof. Younghee Lee 89 ATM LANs u Need perform some sort of protocol conversion from the MAC protocol to the ATM cell stream

90. Prof. Younghee Lee 90 ATM LANs u ATM hub includes a number of ports that operate at different data rates and use different protocols u pure ATM LAN?

91. Prof. Younghee Lee 91 ATM LANs: LANE u Need Connectionless service for TCP/IP software implementation over a connection-oriented ATM network –connectionless server approach »Every host initially sets up a connection to this server –ATM LAN emulation »Every hosts has a ATM virtual circuit to every other host »These VC can be established and released dynamically as needed or can be permanent virtual circuit »LES(LAN Emulation Server): ATM ARP server »BUS(Broadcast/Unknown server): broadcasting, multicasting

92. Prof. Younghee Lee 92 High Speed LANs u Token Ring u MAN: –DQDB(802.6): Distributed Queue Dual Bus »high speed broadcast bus »slotted access u FDDI II: packet/circuit switched traffic u FFOL: FDDI Follow - on LAN –scalable beyond one Gbps –support cell-based ATM traffic –connect to SONET/SDH link u S-NET, Expressnet, Datakit u HIPPI: High Performance Parallel Interface u FC: Fiber Channel u Broadband LAN, Baseband LAN, data PABX u 100VG-AnyLAN: IEEE 802.12 –demand priority: »hub polls each computer in turn »computer transmit only when the hub grants »support a simple priority scheme »support frame format other than Ethernet

93. Prof. Younghee Lee 93 ATM-MPLS

94. Prof. Younghee Lee 94 ATM protocol architecture u ATM protocol architecture –ITU-T SG 13, SG 11 –ATM Forum –Connection-oriented packet-switched network –Used in both WAN and LAN settings –Signaling (connection setup) Protocol: Q.2931 –Packets are called cells: 5-byte header + 48-byte payload –Commonly transmitted over SONET (but not necessarily)

95. Prof. Younghee Lee 95 Cell Network u Why Cell? –Taking advantages of the characteristics of both packet and bit --> good for multimedia communications –Interference o interference at gigabit speed?

96. Prof. Younghee Lee 96 ATM architecture u adaptation layer: only at edge of ATM network –data segmentation/reassembly –roughly analagous to Internet transport layer u ATM layer: “network” layer –cell switching, routing u physical layer

97. Prof. Younghee Lee 97 ATM: network or link layer? Vision: end-to-end transport: “ATM from desktop to desktop” –ATM is a network technology Reality: used to connect IP backbone routers –“IP over ATM” –ATM as switched link layer, connecting IP routers

98. Prof. Younghee Lee 98 ATM Adaptation Layer u Adaptation layer: –to support information transfer protocols not based on ATM –PCM voice: need to assemble PCM bits into cells –LAPF: need to map LAPF frame into a number of cells »Frame Relay Data Link Layer (LAPF)Frame Relay Data Link Layer (LAPF) u AAL Services –Handling of transmission errors –Segmentation and reassembly –Handling of lost and misinserted cell conditions –Flow control and timing control

99. Prof. Younghee Lee 99 ATM Layer Service: transport cells across ATM network u analagous to IP network layer u very different services than IP network layer Network Architecture Internet ATM Service Model best effort CBR VBR ABR UBR Bandwidth none constant rate guaranteed rate guaranteed minimum none Loss no yes no Order no yes Timing no yes no Congestion feedback no (inferred via loss) no congestion no congestion yes no Guarantees ?

100. Prof. Younghee Lee 100 ATM Service Categories u Real Time Service –Constant bit rate(CBR): videoconferencing, interactive audio(telephony), Audio/video distribution(TV, distance learning, PPV), Audio/video retrieval(VOD, audio library.. Really needed?) –Real-time Variable Bit Rate(rt-VBR): compressed video u Non-real-time service –Non-real-time variable bit rate(nrt-VBR): data transfers that have critical response-time requirements; airline reservations, banking transactions,... –Unspecified bit rate: best effort service; text/data/image transfer, messaging, distribution, retrieval, remote terminal(telecommuting) –Available bit rate(ABR): PCR(Peak Cell Rate), MCR(Minimum Cell Rate), explicit feedback to to sources for fair allocation of capacity, unused capacity by ABR remains available for UBR traffic; LAN interconnection

101. Prof. Younghee Lee 101 ATM Logical Connections u VCC: Virtual Channel Connection u VPC: Virtual Path Connection u Advantages of the use of VP –Simplified network architecture –Increased network performance and reliability –Reduced processing and short connection setup time –Enhanced network service

102. Prof. Younghee Lee 102 ATM Layer: Virtual Circuits u VC transport: cells carried on VC from source to dest –call setup, teardown for each call before data can flow –each packet carries VC identifier (not destination ID) –every switch on source-dest path maintain “state” for each passing connection –link,switch resources (bandwidth, buffers) may be allocated to VC: to get circuit-like perf. u Permanent VCs (PVCs) –long lasting connections –typically: “permanent” route between to IP routers u Switched VCs (SVC): –dynamically set up on per-call basis

103. Prof. Younghee Lee 103 ATM VCs u Advantages of ATM VC approach: –QoS performance guarantee for connection mapped to VC (bandwidth, delay, delay jitter) u Drawbacks of ATM VC approach: –Inefficient support of datagram traffic –one PVC between each source/dest pair) does not scale (N*2 connections needed) –SVC introduces call setup latency, processing overhead for short lived connections

104. Prof. Younghee Lee 104 ABR Rate Control u Cell Flow –forward RM (FRM) cell –backward RM (BRM) cell u ways to provide rate control feedback –EFCI marking: switch set EFCI, destination end system set the CI in BRM –Relative rate marking: switch set CI or NI in FRM. Set CI or NI in BRM for raapid result –Explicit rate marking: reduce the value of the ER field of an FRM, BRM

105. Prof. Younghee Lee 105 Datagram Journey in IP-over-ATM Network u at Source Host: –IP layer finds mapping between IP, ATM dest address (using ARP) –passes datagram to AAL5 –AAL5 encapsulates data, segments to cells, passes to ATM layer u ATM network: moves cell along VC to destination u at Destination Host: –AAL5 reassembles cells into original datagram –if CRC OK, datgram is passed to IP

106. Prof. Younghee Lee 106 X.25 and Frame Relay Like ATM: u wide area network technologies u virtual circuit oriented u origins in telephony world u can be used to carry IP datagrams –can thus be viewed as Link Layers by IP protocol

107. Prof. Younghee Lee 107 X.25 u X.21(EIA-232) u LAPB(subset of HDLC) u virtual circuit –logical connection between two stations through the network (specific preplanned route through the network between two station): internal virtual circuit

108. Prof. Younghee Lee 108 IP versus X.25 u X.25: reliable in-sequence end-end delivery from end-to-end –“intelligence in the network” u IP: unreliable, out-of-sequence end-end delivery –“intelligence in the endpoints” u gigabit routers: limited processing possible u 2000: IP wins

109. Prof. Younghee Lee 109 Frame Relay u Designed in late ‘80s, widely deployed in the ‘90s u Frame relay service: –no error control –end-to-end congestion control

110. Prof. Younghee Lee 110 Frame Relay (more) u Designed to interconnect corporate customer LANs –typically permanent VC’s: “pipe” carrying aggregate traffic between two routers –switched VC’s: as in ATM u corporate customer leases FR service from public Frame Relay network (eg, Sprint, ATT)

111. Prof. Younghee Lee 111 Frame Relay Networks u Frame Relay Architecture –LAPF(Link Access Protocol for Frame Mode Bearer Services) »Frame delimiting, alignment, and transparency »Frame multiplexing/demultiplexing using the address field »Inspection of the frame »Detection of transmission errors »Congestion control function u User Data Transfer –DLCI swapping u Frame Relay Call Control

112. Prof. Younghee Lee 112 IP Forwarding Architecture u High Performance Forwarder –Pure destination based forwarding u Lookup based on IP address u Internal high speed switch instead of router’s internal backplane bus u Multiple lookup engine at each interface –Switched forwarding »Overlay model u Overlay an IP network onto an ATM network u CLIP(Classical IP over ATM), LANE, NHRP, MPOA »Peer model u Uses the existing IP address(or algorithmically derived ATM addresses) to identify end systems and uses IP routing protocols to set up ATM connections u MPLS

113. Prof. Younghee Lee 113 IP Forwarding u IP over ATM –LANE: ATMF, to make an ATM LAN appear as a set of logical shared medium LANs interconnected via router. –IPOA: IETF, group of ATM stations are divided into Logical IP Subnet(LIS), interconnected via router. Each LIS has an ATM ARP server for address resolution –NHRP(Next Hop Resolution Protocol): IETF, to locate an exit point in the ATM cloud closest to the destination and to obtain ATM address. –Multiprotocol over ATM(MPOA): ATMF, to provide internetworking service such as IP, IPX, and AppleTalk over an ATM network: an extension of LANE. Uses NHRP. u IP switching: Ipsilon u Tag switching: Cisco u Cell Switched Router: Toshiba u Aggregate Route Based IP Switching(ARIS): IBM u MPLS: IETF(~Tag switching)

114. Prof. Younghee Lee 114 IP over ATM u CLIP(Classical IP over ATM): –group of ATM stations are divided into Logical IP Subnet(LIS), interconnected via router. Each Logical IP subnetwork(LIS) has an ATM ARP server for address resolution –All members(IP end system) in the same LIS must use the same IP address prefix LIS1 LIS3 LIS4 LIS5 LIS6 LIS2 Router ATM network

115. Prof. Younghee Lee 115 IP over ATM u NHRP(Next Hop Resolution Protocol): – IETF, to locate an exit point in the ATM cloud closest to the destination and to obtain ATM address.: shortcut path through ATM network -> intermediate routers can be bypassed LIS1 LIS3 LIS4 LIS5 LIS6 LIS2 Router ATM network Sourc e Destinatio n Shortest path(NHRP) Default path(CLIP)

116. Prof. Younghee Lee 116 ATM Discussion u At one point, ATM was viewed as a replacement for IP. –Could carry both traditional telephone traffic (CBR circuits) and other traffic (data, VBR) –Better than IP, since it supports QoS u Complex technology. –Switching core is fairly simple, but –Support for different traffic classes –Signaling software is very complex –Technology did not match people’s experience with IP »deploying ATM in LAN is complex (e.g. broadcast) »supporting connection-less service model on connection-based technology –With IP over ATM, a lot of functionality is replicated

117. Prof. Younghee Lee 117 IP Switching u How to use ATM hardware without the software. –ATM switches are very fast data switches –software adds overhead, cost u The idea is to identify flows at the IP level and to create specific VCs to support these flows. –flows are identified on the fly by monitoring traffic –flow classification can use addresses, protocol types,... –can distinguish based on destination, protocol, QoS u Once established, data belonging to the flow bypasses level 3 routing. –never leaves the ATM switch u Interoperates fine with “regular” IP routers. –detects and collaborates with neighboring IP switches

118. Prof. Younghee Lee 118 IP Switching u How would a node in the network determine which packets should be assigned to flows for the purpose of switching? –(Without explicit guidance such as the IPv6 flow label..) –by flow statistics

119. Prof. Younghee Lee 119 An Alternative: Tag Switching u Instead of monitoring traffic to identify flows to optimize, use routing information to guide the creation of “switched” paths. –Switched paths are set up as a side effect of filling in forwarding tables u Generalize to other types of hardware. u Also introduced stackable tags. –Made it possible to temporarily merge flows and to demultiplex them without doing an IP route lookup –Requires variable size field for tag A B A B A B C C

120. Prof. Younghee Lee 120 Tag Switching u Label(tag) switching technology for layer 3 packet forwarding –Forwarding Component »Tags: Packets carry »Tag Information Base: A tag switch carries tag forwarding information in a database called TIB »Forwarding procedure: tag as index to lookup information in TIB. Switch replaces the tag and the link level information, and sends the packet to the outgoing interface u independent of routing functionality and of the network layer –Control Component »a binding between a tag and network layer routing. A tag could be bound to an individual application flow, a single route, group of routes or a multicast tree »to create tag bindings and distribute the tag binding information among the interconnected tag switch u QoS –assigning a tag to a class of packets once the classification has been done

121. Prof. Younghee Lee 121 Label Switching u Goal: speed up route look up. –IP addresses are long and a longest prefix match is expensive –Will get worse with IPv6 u Replace IP address with a shorter fixed sized address that can be looked up quickly. –Easy look up in hardware, e.g. ATM VCID –Set up a connection for that address –Label packets at entry to the network u Use of short address is typically optional. –Regular IP look up is a fallback –May be slower u Multi-protocol label switching (MPLS).

122. Prof. Younghee Lee 122 IP Switching versus Tag Switching u Flows versus routes. –tags explicitly cover groups of routes –tag bindings set up as part of route establishment –flows in IP switching are driven by traffic and detected by “filters” »Supports both fine grain application flows and coarser grain flow groups u Stackable tags. –provides more flexibility u Generality –IP switching focuses on ATM –not clear that this is a fundamental difference

123. Prof. Younghee Lee 123 Multi-Protocol Label Switching: MPLS u Goal of MPLS group in IETF: –To develop a standard for integration of layer 2 switching with layer 3 routing in order to improve price/performance of network layer routing, scalability of network layer and provide great flexibility in providing new routing service u Map packet onto Forward Equivalence Class (FEC) based on its header. –Simple case: longest prefix match of destination address –More complex if QoS of policy routing is used u In MPLS, a label is associated with the packet when it enters the network and forwarding is based on the label in the network core. –Label is swapped (as ATM VCIs) u Potential advantages. –Packet forwarding can be faster –Routing can be based on ingress router and port –Can use more complex routing decisions –Can force packets to followed a pinned route

124. Prof. Younghee Lee 124 MPLS Mechanisms u Implementation of the label is technology specific. –Could be ATM VCI or an extra header u Label Distribution Protocols distributes information on label/FEC bindings. –Extensions of existing protocols (routing, RSVP) or stand-alone protocols –Can be upstream or downstream u Supports stacked labels.

125. Prof. Younghee Lee 125 MPLS capable routers u a.k.a. label-switched router u forwards packets to outgoing interface based only on label value (don’t inspect IP address) –MPLS forwarding table distinct from IP forwarding tables u signaling protocol needed to set up forwarding –RSVP-TE –forwarding possible along paths that IP alone would not allow (e.g., source-specific routing) !! –use MPLS for traffic engineering u must co-exist with IP-only routers

126. Prof. Younghee Lee 126 Reading u [Degermark97] Small Forwarding Tables for Fast Routing Lookups