Chapter 3 Transport, Network and Link Layers

Transcription

1 Informática y Comunicaciones Chapter 3 Transport, Network and Link Layers All material copyright J.F Kurose and K.W. Ross, All Rights Reserved Transport, Network and Link Layers 3-1

2 Chapter 3: outline 3.1 Transport layer 3.2 Network layer 3.3 Link layer Transport, Network and Link Layers 3-2

3 Transport services and protocols provide logical communication between app processes running on different hosts transport protocols run in end systems send side: breaks app messages into segments, passes to network layer rcv side: reassembles segments into messages, passes to app layer more than one transport protocol available to apps Internet: TCP and UDP application transport network data link physical application transport network data link physical Transport, Network and Link Layers 3-3

4 Transport vs. network layer network layer: logical communication between hosts transport layer: logical communication between processes relies on, enhances, network layer services household analogy: 12 kids in Ann s house sending letters to 12 kids in Bill s house: hosts = houses processes = kids app messages = letters in envelopes transport protocol = Ann and Bill who demux to inhouse siblings network-layer protocol = postal service Transport, Network and Link Layers 3-4

5 Multiplexing/demultiplexing multiplexing at sender: handle data from multiple sockets, add transport header (later used for demultiplexing) demultiplexing at receiver: use header info to deliver received segments to correct socket application application P3 transport network link P1 transport network link physical P2 application P4 transport network link socket process physical physical Transport, Network and Link Layers 3-5

6 How demultiplexing works host receives IP datagrams each datagram has source IP address, destination IP address each datagram carries one transport-layer segment each segment has source, destination port number host uses IP addresses & port numbers to direct segment to appropriate socket 32 bits source port # dest port # other header fields application data (payload) TCP/UDP segment format Transport, Network and Link Layers 3-6

7 UDP: User Datagram Protocol no frills, bare bones Internet transport protocol best effort service, UDP segments may be: lost delivered out-of-order to app connectionless: no handshaking between UDP sender, receiver each UDP segment handled independently of others UDP use: streaming multimedia apps (loss tolerant, rate sensitive) DNS SNMP reliable transfer over UDP: add reliability at application layer application-specific error recovery! Transport, Network and Link Layers 3-7

8 UDP: segment header 32 bits source port # dest port # length checksum application data (payload) UDP segment format length, in bytes of UDP segment, including header why is there a UDP? no connection establishment (which can add delay) simple: no connection state at sender, receiver small header size no congestion control: UDP can blast away as fast as desired Transport, Network and Link Layers 3-8

9 UDP checksum Goal: detect errors (e.g., flipped bits) in transmitted segment sender: treat segment contents, including header fields, as sequence of 16-bit integers checksum: addition (one s complement sum) of segment contents sender puts checksum value into UDP checksum field receiver: compute checksum of received segment check if computed checksum equals checksum field value: NO - error detected YES - no error detected. But maybe errors nonetheless? Transport, Network and Link Layers 3-9

10 TCP: Overview point-to-point: one sender, one receiver reliable, in-order byte stream: no message boundaries pipelined: TCP congestion and flow control set window size full duplex data: bi-directional data flow in same connection MSS: maximum segment size connection-oriented: handshaking (exchange of control msgs) inits sender, receiver state before data exchange flow controlled: sender will not overwhelm receiver Transport, Network and Link Layers 3-10

11 TCP segment structure URG: urgent data (generally not used) ACK: ACK # valid PSH: push data now (generally not used) RST, SYN, FIN: connection estab (setup, teardown commands) Internet checksum (as in UDP) 32 bits source port # dest port # sequence number head len acknowledgement number not used UAP R S F checksum receive window Urg data pointer options (variable length) application data (variable length) counting by bytes of data (not segments!) # bytes rcvr willing to accept Transport, Network and Link Layers 3-11

12 TCP reliable data transfer TCP creates rdt service on top of IP s unreliable service pipelined segments cumulative acks single retransmission timer retransmissions triggered by: timeout events duplicate acks Transport, Network and Link Layers 3-12

13 Principles of congestion control congestion: informally: too many sources sending too much data too fast for network to handle different from flow control! manifestations: lost packets (buffer overflow at routers) long delays (queueing in router buffers) a top-10 problem! Transport, Network and Link Layers 3-13

14 Causes/costs of congestion: scenario 1 two senders, two receivers one router, infinite buffers output link capacity: R no retransmission original data: λ in Host A unlimited shared output link buffers throughput: λ out Host B R/2 λ out delay λ in R/2 λ in R/2 maximum per-connection throughput: R/2 large delays as arrival rate, λ in, approaches capacity Transport, Network and Link Layers 3-14

15 Causes/costs of congestion: scenario 2 one router, finite buffers sender retransmission of timed-out packet application-layer input = application-layer output: λ in = λ out transport-layer input includes retransmissions : λ in λ in Host A λ in : original data λ' in : original data, plus retransmitted data λ out Host B finite shared output link buffers Transport, Network and Link Layers 3-15

16 Causes/costs of congestion: scenario 2 idealization: perfect knowledge sender sends only when router buffers available R/2 λ out λ in R/2 copy A λ in : original data λ' in : original data, plus retransmitted data free buffer space! λ out Host B finite shared output link buffers Transport, Network and Link Layers 3-16

17 Causes/costs of congestion: scenario 2 Idealization: known loss packets can be lost, dropped at router due to full buffers sender only resends if packet known to be lost copy A λ in : original data λ' in : original data, plus retransmitted data no buffer space! λ out Host B Transport, Network and Link Layers 3-17

18 Causes/costs of congestion: scenario 2 Idealization: known loss packets can be lost, dropped at router due to full buffers sender only resends if packet known to be lost R/2 λ out λ in R/2 when sending at R/2, some packets are retransmissions but asymptotic goodput is still R/2 A λ in : original data λ' in : original data, plus retransmitted data free buffer space! λ out Host B Transport, Network and Link Layers 3-18

19 Causes/costs of congestion: scenario 2 Realistic: duplicates packets can be lost, dropped at router due to full buffers sender times out prematurely, sending two copies, both of which are delivered R/2 λ out λ in R/2 when sending at R/2, some packets are retransmissions including duplicated that are delivered! copy timeout λ in λ' in λ out A free buffer space! Host B Transport, Network and Link Layers 3-19

20 Causes/costs of congestion: scenario 2 Realistic: duplicates packets can be lost, dropped at router due to full buffers sender times out prematurely, sending two copies, both of which are delivered λ out R/2 λ in R/2 when sending at R/2, some packets are retransmissions including duplicated that are delivered! costs of congestion: more work (retrans) for given goodput unneeded retransmissions: link carries multiple copies of pkt decreasing goodput Transport, Network and Link Layers 3-20

21 Causes/costs of congestion: scenario 3 four senders multihop paths timeout/retransmit Host D Host A Q: what happens as λ in and λ in increase? A: as red λ in increases, all arriving blue pkts at upper queue are dropped, blue throughput 0 λ in : original data λ' in : original data, plus retransmitted data finite shared output link buffers λ out Host C Host B Transport, Network and Link Layers 3-21

22 Causes/costs of congestion: scenario 3 C/2 λ out λ in C/2 another cost of congestion: when packet dropped, any upstream transmission capacity used for that packet was wasted! Transport, Network and Link Layers 3-22

23 Approaches towards congestion control two broad approaches towards congestion control: end-end congestion control: no explicit feedback from network congestion inferred from end-system observed loss, delay approach taken by TCP network-assisted congestion control: routers provide feedback to end systems single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM) explicit rate for sender to send at Transport, Network and Link Layers 3-23

24 TCP Fairness fairness goal: if K TCP sessions share same bottleneck link of bandwidth R, each should have average rate of R/K TCP connection 1 TCP connection 2 bottleneck router capacity R Transport, Network and Link Layers 3-24

25 Chapter 3: outline 3.1 Transport layer 3.2 Network layer 3.3 Link layer Transport, Network and Link Layers 3-25

26 Network layer transport segment from sending to receiving host on sending side encapsulates segments into datagrams on receiving side, delivers segments to transport layer network layer protocols in every host, router router examines header fields in all IP datagrams passing through it application transport network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical Transport, Network and Link Layers 3-26

27 Two key network-layer functions forwarding: move packets from router s input to appropriate router output routing: determine route taken by packets from source to dest. routing algorithms analogy: routing: process of planning trip from source to dest forwarding: process of getting through single interchange Transport, Network and Link Layers 3-27

28 Interplay between routing and forwarding routing algorithm local forwarding table header value output link routing algorithm determines end-end-path through network forwarding table determines local forwarding at this router value in arriving packet s header Transport, Network and Link Layers 3-28

29 Connection setup 3 rd important function in some network architectures: ATM, frame relay, X.25 before datagrams flow, two end hosts and intervening routers establish virtual connection routers get involved network vs transport layer connection service: network: between two hosts (may also involve intervening routers in case of VCs) transport: between two processes Transport, Network and Link Layers 3-29

30 Network service model Q: What service model for channel transporting datagrams from sender to receiver? example services for individual datagrams: guaranteed delivery guaranteed delivery with less than 40 msec delay example services for a flow of datagrams: in-order datagram delivery guaranteed minimum bandwidth to flow restrictions on changes in inter-packet spacing Transport, Network and Link Layers 3-30

31 Network layer service models: Network Architecture Service Model Bandwidth Guarantees? Loss Order Timing Congestion feedback Internet best effort none no no no no (inferred ATM ATM ATM CBR VBR ABR constant rate guaranteed rate guaranteed minimum yes yes no yes yes yes yes yes no via loss) no congestion no congestion yes ATM UBR none no yes no no Transport, Network and Link Layers 3-31

32 Connection, connection-less service datagram network provides network-layer connectionless service virtual-circuit network provides network-layer connection service analogous to TCP/UDP connecton-oriented / connectionless transport-layer services, but: service: host-to-host no choice: network provides one or the other implementation: in network core Transport, Network and Link Layers 3-32

33 Virtual circuits source-to-dest path behaves much like telephone circuit performance-wise network actions along source-to-dest path call setup, teardown for each call before data can flow each packet carries VC identifier (not destination host address) every router on source-dest path maintains state for each passing connection link, router resources (bandwidth, buffers) may be allocated to VC (dedicated resources = predictable service) Transport, Network and Link Layers 3-33

34 VC implementation a VC consists of: 1. path from source to destination 2. VC numbers, one number for each link along path 3. entries in forwarding tables in routers along path packet belonging to VC carries VC number (rather than dest address) VC number can be changed on each link. new VC number comes from forwarding table Transport, Network and Link Layers 3-34

35 VC forwarding table forwarding table in northwest router: VC number interface number Incoming interface Incoming VC # Outgoing interface Outgoing VC # VC routers maintain connection state information! Transport, Network and Link Layers 3-35

36 Datagram networks no call setup at network layer routers: no state about end-to-end connections no network-level concept of connection packets forwarded using destination host address application transport network data link physical 1. send datagrams 2. receive datagrams application transport network data link physical Transport, Network and Link Layers 3-36

37 Datagram forwarding table routing algorithm local forwarding table dest address output link address-range 1 3 address-range 2 2 address-range 3 2 address-range billion IP addresses, so rather than list individual destination address list range of addresses (aggregate table entries) IP destination address in arriving packet s header Transport, Network and Link Layers 3-37

38 Datagram or VC network: why? Internet (datagram) data exchange among computers elastic service, no strict timing req. many link types different characteristics uniform service difficult smart end systems (computers) can adapt, perform control, error recovery simple inside network, complexity at edge ATM (VC) evolved from telephony human conversation: strict timing, reliability requirements need for guaranteed service dumb end systems telephones complexity inside network Transport, Network and Link Layers 3-38

39 Router architecture overview two key router functions: run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing link forwarding tables computed, pushed to input ports routing processor routing, management control plane (software) forwarding data plane (hardware) high-seed switching fabric router input ports router output ports Transport, Network and Link Layers 3-39

40 Input port functions physical layer: bit-level reception data link layer: e.g., Ethernet line termination link layer protocol (receive) lookup, forwarding queueing decentralized switching: switch fabric given datagram dest., lookup output port using forwarding table in input port memory ( match plus action ) goal: complete input port processing at line speed queuing: if datagrams arrive faster than forwarding rate into switch fabric Transport, Network and Link Layers 3-40

41 Switching fabrics transfer packet from input buffer to appropriate output buffer switching rate: rate at which packets can be transfer from inputs to outputs often measured as multiple of input/output line rate N inputs: switching rate N times line rate desirable three types of switching fabrics memory memory bus crossbar Transport, Network and Link Layers 3-41

42 Switching via memory first generation routers: traditional computers with switching under direct control of CPU packet copied to system s memory speed limited by memory bandwidth (2 bus crossings per datagram) input port (e.g., Ethernet) memory output port (e.g., Ethernet) system bus Transport, Network and Link Layers 3-42

43 Switching via a bus datagram from input port memory to output port memory via a shared bus bus contention: switching speed limited by bus bandwidth 32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers bus Transport, Network and Link Layers 3-43

44 Switching via interconnection network overcome bus bandwidth limitations banyan networks, crossbar, other interconnection nets initially developed to connect processors in multiprocessor advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric. Cisco 12000: switches 60 Gbps through the interconnection network crossbar Transport, Network and Link Layers 3-44

45 Output ports switch fabric datagram buffer link layer protocol (send) line termination queueing buffering required when datagrams arrive from fabric faster than the transmission rate scheduling discipline chooses among queued datagrams for transmission Transport, Network and Link Layers 3-45

46 Output port queueing switch fabric switch fabric at t, packets more from input to output one packet time later buffering when arrival rate via switch exceeds output line speed queueing (delay) and loss due to output port buffer overflow! Transport, Network and Link Layers 3-46

47 The Internet network layer host, router network layer functions: transport layer: TCP, UDP network layer routing protocols path selection RIP, OSPF, BGP forwarding table IP protocol addressing conventions datagram format packet handling conventions ICMP protocol error reporting router signaling link layer physical layer Transport, Network and Link Layers 3-47

48 IP datagram format IP protocol version number header length (bytes) type of data max number remaining hops (decremented at each router) upper layer protocol to deliver payload to how much overhead? 20 bytes of TCP 20 bytes of IP = 40 bytes + app layer overhead 32 bits ver head. type of length len service fragment 16-bit identifier flgs time to upper offset header live layer checksum 32 bit source IP address 32 bit destination IP address options (if any) data (variable length, typically a TCP or UDP segment) total datagram length (bytes) for fragmentation/ reassembly e.g. timestamp, record route taken, specify list of routers to visit. Transport, Network and Link Layers 3-48

49 IP addressing: introduction IP address: 32-bit identifier for host, router interface interface: connection between host/router and physical link router s typically have multiple interfaces host typically has one or two interfaces (e.g., wired Ethernet, wireless ) IP addresses associated with each interface = Transport, Network and Link Layers 3-49

50 IP addressing: introduction wired Ethernet interfaces connected by Ethernet switches wireless WiFi interfaces connected by WiFi base station Transport, Network and Link Layers 3-50

51 Subnets IP address: subnet part - high order bits host part - low order bits what s a subnet? device interfaces with same subnet part of IP address can physically reach each other without intervening router subnet network consisting of 3 subnets Transport, Network and Link Layers 3-51

52 Subnets recipe to determine the subnets, detach each interface from its host or router, creating islands of isolated networks each isolated network is called a subnet / / subnet /24 subnet mask: /24 Transport, Network and Link Layers 3-52

53 Subnets how many? Transport, Network and Link Layers 3-53

54 IP addressing: CIDR CIDR: Classless InterDomain Routing subnet portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in subnet portion of address subnet part host part /23 Transport, Network and Link Layers 3-54

55 IP addresses: how to get one? Q: How does a host get IP address? hard-coded by system admin in a file Windows: control-panel->network->configuration- >tcp/ip->properties UNIX: /etc/rc.config DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server plug-and-play Transport, Network and Link Layers 3-55

56 DHCP: Dynamic Host Configuration Protocol goal: allow host to dynamically obtain its IP address from network server when it joins network can renew its lease on address in use allows reuse of addresses (only hold address while connected/ on ) support for mobile users who want to join network (more shortly) DHCP overview: host broadcasts DHCP discover msg [optional] DHCP server responds with DHCP offer msg [optional] host requests IP address: DHCP request msg DHCP server sends address: DHCP ack msg Transport, Network and Link Layers 3-56

57 DHCP client-server scenario / DHCP server /24 arriving DHCP client needs address in this network /24 Transport, Network and Link Layers 3-57

58 DHCP: more than IP addresses DHCP can return more than just allocated IP address on subnet: address of first-hop router for client name and IP address of DNS sever network mask (indicating network versus host portion of address) Transport, Network and Link Layers 3-58

59 IP addresses: how to get one? Q: how does network get subnet part of IP addr? A: gets allocated portion of its provider ISP s address space ISP's block /20 Organization /23 Organization /23 Organization / Organization /23 Transport, Network and Link Layers 3-59

60 Hierarchical addressing: route aggregation hierarchical addressing allows efficient advertisement of routing information: Organization /23 Organization /23 Organization /23 Organization Fly-By-Night-ISP Send me anything with addresses beginning /20 Internet /23 ISPs-R-Us Send me anything with addresses beginning /16 Transport, Network and Link Layers 3-60

61 Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 Organization /23 Organization /23 Organization /23 Organization / Fly-By-Night-ISP ISPs-R-Us Send me anything with addresses beginning /20 Internet Send me anything with addresses beginning /16 or /23 Transport, Network and Link Layers 3-61

62 IP addressing: the last word... Q: how does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers allocates addresses manages DNS assigns domain names, resolves disputes Transport, Network and Link Layers 3-62

63 NAT: network address translation rest of Internet local network (e.g., home network) / all datagrams leaving local network have same single source NAT IP address: ,different source port numbers datagrams with source or destination in this network have /24 address for source, destination (as usual) Transport, Network and Link Layers 3-63

64 NAT: network address translation motivation: local network uses just one IP address as far as outside world is concerned: range of addresses not needed from ISP: just one IP address for all devices can change addresses of devices in local network without notifying outside world can change ISP without changing addresses of devices in local network devices inside local net not explicitly addressable, visible by outside world (a security plus) Transport, Network and Link Layers 3-64

65 NAT: network address translation implementation: NAT router must: outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #)... remote clients/servers will respond using (NAT IP address, new port #) as destination addr remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table Transport, Network and Link Layers 3-65

66 NAT: network address translation 2: NAT router changes datagram source addr from , 3345 to , 5001, updates table 2 NAT translation table WAN side addr LAN side addr , , 3345 S: , 5001 D: , 80 S: , 80 D: , : reply arrives dest. address: , S: , 3345 D: , S: , 80 4 D: , : host sends datagram to , : NAT router changes datagram dest addr from , 5001 to , 3345 Transport, Network and Link Layers 3-66

67 NAT: network address translation 16-bit port-number field: 60,000 simultaneous connections with a single LAN-side address! NAT is controversial: routers should only process up to layer 3 violates end-to-end argument NAT possibility must be taken into account by app designers, e.g., P2P applications address shortage should instead be solved by IPv6 Transport, Network and Link Layers 3-67

68 IPv6: motivation initial motivation: 32-bit address space soon to be completely allocated. additional motivation: header format helps speed processing/forwarding header changes to facilitate QoS IPv6 datagram format: fixed-length 40 byte header no fragmentation allowed Transport, Network and Link Layers 3-68

69 IPv6 datagram format priority: identify priority among datagrams in flow flow Label: identify datagrams in same flow. (concept of flow not well defined). next header: identify upper layer protocol for data ver pri flow label payload len next hdr hop limit source address (128 bits) destination address (128 bits) data 32 bits Transport, Network and Link Layers 3-69

70 Other changes from IPv4 checksum: removed entirely to reduce processing time at each hop options: allowed, but outside of header, indicated by Next Header field ICMPv6: new version of ICMP additional message types, e.g. Packet Too Big multicast group management functions Transport, Network and Link Layers 3-70

71 Transition from IPv4 to IPv6 not all routers can be upgraded simultaneously no flag days how will network operate with mixed IPv4 and IPv6 routers? tunneling: IPv6 datagram carried as payload in IPv4 datagram among IPv4 routers IPv4 header fields IPv4 source, dest addr IPv6 header fields IPv6 source dest addr UDP/TCP payload IPv4 payload IPv6 datagram IPv4 datagram Transport, Network and Link Layers 3-71

72 Chapter 3: outline 3.1 Transport layer 3.2 Network layer 3.3 Link layer Transport, Network and Link Layers 3-72

73 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, encapsulates datagram data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link global ISP Transport, Network and Link Layers 3-73

74 Link layer: context datagram transferred by different link protocols over different links: e.g., Ethernet on first link, frame relay on intermediate links, on last link each link protocol provides different services e.g., may or may not provide rdt over link transportation analogy: trip from Princeton to Lausanne limo: Princeton to JFK plane: JFK to Geneva train: Geneva to Lausanne tourist = datagram transport segment = communication link transportation mode = link layer protocol travel agent = routing algorithm Transport, Network and Link Layers 3-74

75 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! seldom used on low bit-error link (fiber, some twisted pair) wireless links: high error rates Transport, Network and Link Layers 3-75

76 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 Transport, Network and Link Layers 3-76

77 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, card; Ethernet chipset implements link, physical layer attaches into host s system buses combination of hardware, software, firmware application transport network link link physical cpu controller physical transmission memory host bus (e.g., PCI) network adapter card Transport, Network and Link Layers 3-77

78 Adaptors communicating datagram datagram controller controller sending host frame datagram receiving host sending side: encapsulates datagram in frame adds error checking bits, rdt, flow control, etc. receiving side looks for errors, rdt, flow control, etc extracts datagram, passes to upper layer at receiving side Transport, Network and Link Layers 3-78

79 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 Transport, Network and Link Layers 3-79

80 Parity checking single bit parity: detect single bit errors two-dimensional bit parity: detect and correct single bit errors 0 0 Transport, Network and Link Layers 3-80

81 Internet checksum (review) goal: detect errors (e.g., flipped bits) in transmitted packet (note: used at transport layer only) sender: treat segment contents as sequence of 16-bit integers checksum: addition (1 s complement sum) of segment contents sender puts checksum value into UDP checksum field receiver: compute checksum of received segment check if computed checksum equals checksum field value: NO - error detected YES - no error detected. But maybe errors nonetheless? Transport, Network and Link Layers 3-81

82 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 wireless LAN shared wire (e.g., cabled Ethernet) shared RF (e.g., WiFi) shared RF (satellite) humans at a cocktail party (shared air, acoustical) Transport, Network and Link Layers 3-82

83 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 Transport, Network and Link Layers 3-83

84 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 Transport, Network and Link Layers 3-84

85 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 Transport, Network and Link Layers 3-85

86 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 frame 6-slot frame Transport, Network and Link Layers 3-86

87 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 FDM cable frequency bands Transport, Network and Link Layers 3-87

88 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 Transport, Network and Link Layers 3-88

89 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! Transport, Network and Link Layers 3-89

90 Taking turns MAC protocols polling: master node invites slave nodes to transmit in turn typically used with dumb slave devices concerns: polling overhead latency single point of failure (master) data slaves data poll master Transport, Network and Link Layers 3-90

91 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) (nothing to send) T T data Transport, Network and Link Layers 3-91

92 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 IPaddressing sense) 48 bit MAC address (for most LANs) burned in NIC ROM, also sometimes software settable e.g.: 1A-2F-BB AD hexadecimal (base 16) notation (each number represents 4 bits) Transport, Network and Link Layers 3-92

93 LAN addresses and ARP each adapter on LAN has unique LAN address 1A-2F-BB AD F7-2B LAN (wired or wireless) D7-FA-20-B0 adapter 0C-C4-11-6F-E3-98 Transport, Network and Link Layers 3-93

94 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 Transport, Network and Link Layers 3-94

95 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) bus: coaxial cable star switch Transport, Network and Link Layers 3-95

96 Ethernet frame structure sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame preamble preamble: dest. address source address type data (payload) CRC 7 bytes with pattern followed by one byte with pattern used to synchronize receiver, sender clock rates Transport, Network and Link Layers 3-96

97 Ethernet frame structure (more) addresses: 6 byte source, destination MAC addresses if adapter receives frame with matching destination address, or with broadcast address (e.g. ARP packet), it passes data in frame to network layer protocol otherwise, adapter discards frame type: indicates higher layer protocol (mostly IP but others possible, e.g., Novell IPX, AppleTalk) CRC: cyclic redundancy check at receiver error detected: frame is dropped type preamble dest. address source address data (payload) CRC Transport, Network and Link Layers 3-97

98 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 Transport, Network and Link Layers 3-98

99 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 Transport, Network and Link Layers 3-99

100 Switch: multiple simultaneous transmissions hosts have dedicated, direct connection to switch switches buffer packets Ethernet protocol used on each incoming link, but no collisions; full duplex each link is its own collision domain switching: A-to-A and B-to-B can transmit simultaneously, without collisions A C B B C A switch with six interfaces (1,2,3,4,5,6) Transport, Network and Link Layers 3-100

101 Interconnecting switches switches can be connected together S 4 A B S 1 C S 3 S 2 F D E G H I Transport, Network and Link Layers 3-101

102 Institutional network to external network router mail server web server IP subnet Transport, Network and Link Layers 3-102

103 Switches vs. routers both are store-and-forward: routers: network-layer devices (examine networklayer headers) switches: link-layer devices (examine link-layer headers) both have forwarding tables: routers: compute tables using routing algorithms, IP addresses switches: learn forwarding table using flooding, learning, MAC addresses datagram frame application transport network link physical application transport network link physical switch link physical network link physical frame datagram frame Transport, Network and Link Layers 3-103

104 Chapter 3: summary our study of layers now complete! transport layer UDP TCP Congestion network layer Routing IP DHCP NAT link layer Error detection MAC Ethernet Transport, Network and Link Layers 3-104