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Quiz #5 What is the difference between DCF and PCF? 1 Note 7: Local Area Networks LAN Bridges 2 Hubs, Bridges & Routers • Hub: Active central element in a star topology – – – – Twisted Pair: inexpensive, easy to insall Simple repeater in Ethernet LANs “Intelligent hub”: fault isolation, net configuration, statistics Requirements that arise: User community grows, need to interconnect hubs Hubs are for different types of LANs ? Hub Two Twisted Pairs Two Twisted Pairs Station Hub Station Station Station Station Station 3 Hubs, Bridges, Routers & Gateways • Interconnecting Hubs – At the physical layer Repeater – At the MAC or data link layer Higher Scalability Bridges – At the network layer Router – At even higher layers Gateway ? Hub Hub Two Twisted Pairs Station Two Twisted Pairs Station Station Station Station Station 4 General Bridge Issues Network Network LLC LLC MAC 802.3 802.3 802.5 802.5 MAC PHY 802.3 802.3 802.5 802.5 PHY 802.3 CSMA/CD 802.5 Token Ring • Operation at data link level implies capability to work with multiple network layers • However, must deal with – Difference in MAC formats – Difference in data rates; buffering; timers – Difference in maximum frame length 5 Bridges of Same Type Network Network Bridge LLC LLC MAC MAC MAC MAC Physical Physical Physical Physical • Common case involves LANs of same type • Bridging is done at MAC level 6 Transparent Bridges • Interconnection of IEEE LANs with complete transparency • Use table lookup, and – discard frame, if source & destination in same LAN – forward frame, if source & destination in different LAN – use flooding, if destination unknown • Use backward learning to build table – observe source address of arriving frames – handle topology changes by removing old entries S1 S2 S3 LAN1 Bridge LAN2 S4 S6 S5 7 S1 S2 S3 LAN1 LAN2 LAN3 B1 Port 1 S5 S4 B2 Port 2 Address Port Port 1 Port 2 Address Port 8 S1→S5 S1 S2 S3 S1 to S5 S1 to S5 S1 to S5 LAN1 S1 to S5 LAN2 LAN3 B1 B2 Port 1 Port 2 Address Port S1 S5 S4 1 Port 1 Port 2 Address Port S1 1 9 S3→S2 S1 S2 S3 S3S2 S3S2 S3S2 S3S2 S3S2 LAN1 LAN2 LAN3 B1 B2 Port 1 Port 2 Address Port S1 S3 S5 S4 1 2 Port 1 Port 2 Address Port S1 S3 1 1 10 S4S3 S1 S2 S3 S4 B1 Port 1 S4S3 Port 2 Address Port S1 S3 S4 1 2 2 S3 S4S3 S4S3 LAN1 S5 S4 LAN2 LAN3 B2 Port 1 Port 2 Address Port S1 S3 S4 1 1 2 11 S2S1 S1 S2 S3 S5 S4 S2S1 LAN1 LAN2 S2S1 LAN3 B1 B2 Port 1 Port 2 Address Port S1 S3 S4 S2 1 2 2 1 Port 1 Port 2 Address Port S1 S3 S4 1 1 2 12 Adaptive Learning • In a static network, tables eventually store all addresses & learning stops • In practice, stations are added & moved all the time – Introduce timer (minutes) to age each entry & force it to be relearned periodically – If frame arrives on port that differs from frame address & port in table, update immediately 13 Avoiding Loops LAN1 (1) (1) B1 B2 (2) B3 LAN2 B4 LAN3 B5 LAN4 14 Spanning Tree Algorithm 1. Select a root bridge among all the bridges. • root bridge = the lowest bridge ID. 2. Determine the root port for each bridge except the root bridge • root port = port with the least-cost path to the root bridge 3. Select a designated bridge for each LAN • designated bridge = bridge has least-cost path from the LAN to the root bridge. • designated port connects the LAN and the designated bridge 4. All root ports and all designated ports are placed into a “forwarding” state. These are the only ports that are allowed to forward frames. The other ports are placed into a “blocking” state. 15 LAN1 (1) (1) B1 B2 (1) (2) (2) LAN2 B3 (3) (2) (1) B4 (2) LAN3 (1) B5 (2) LAN4 16 LAN1 (1) (1) B1 Bridge 1 selected as root bridge B2 (1) (2) (2) LAN2 B3 (3) (2) (1) B4 (2) LAN3 (1) B5 (2) LAN4 17 LAN1 (1) R (1) B1 B2 (2) (2) LAN2 R (1) B3 R (1) Root port selected for every bridge except root bridge (3) (2) B4 (2) LAN3 R (1) B5 (2) LAN4 18 LAN1 D (1) R (1) B1 B2 (2) D (2) LAN2 R (1) B3 R (1) Select designated bridge for each LAN D (2) (3) D B4 (2) LAN3 R (1) B5 (2) LAN4 19 LAN1 D (1) R (1) B1 B2 (2) D (2) LAN2 R (1) B3 R (1) All root ports & designated ports put in forwarding state D (2) (3) D B4 (2) LAN3 R (1) B5 (2) LAN4 20 Source Routing Bridges • To interconnect IEEE 802.5 token rings • Each source station determines route to destination • Routing information inserted in frame Routing control 2 bytes Route 1 Route 2 designator designator 2 bytes 2 bytes Destination Source Routing address address information Route m designator 2 bytes Data FCS 21 Route Discovery • To discover route to a destination each station broadcasts a single-route broadcast frame • Frame visits every LAN once & eventually reaches destination • Destination sends all-routes broadcast frame which generates all routes back to source • Source collects routes & picks best 22 Detailed Route Discovery • • • • • Bridges must be configured to form a spanning tree Source sends single-route frame without route designator field Bridges in first LAN add incoming LAN #, its bridge #, outgoing LAN # into frame & forwards frame Each subsequent bridge attaches its bridge # and outgoing LAN # Eventually, one single-route frame arrives at destination • • • • • When destination receives singleroute broadcast frame it responds with all-routes broadcast frame with no route designator field Bridge at first hop inserts incoming LAN #, its bridge #, and outgoing LAN # and forwards to outgoing LAN Subsequent bridges insert their bridge # and outgoing LAN # and forward Before forwarding bridge checks to see if outgoing LAN already in designator field Source eventually receives all routes to destination station 23 Find routes from S1 to S3 LAN 2 S1 B4 LAN 4 B1 LAN 1 S2 B5 B3 B7 B2 S3 B6 LAN 3 LAN1 B1 B3 LAN3 B4 LAN4 LAN 5 B6 LAN5 LAN2 24 LAN 2 S1 LAN 4 B4 B1 S2 LAN 1 B3 B5 LAN 3 B6 B7 B2 B6 LAN3 S3 B2 LAN1 B1 LAN2 B3 LAN2 B1 B4 LAN1 LAN4 LAN4 B4 LAN2 B5 LAN5 B7 B1 B4 B7 LAN 5 B4 B2 B5 B7 B1 B3 LAN4 B5 B7 LAN1 B2 B2 LAN3 B2 B5 B6 B1 B1 B4 LAN1 B3 B5 B6 B1 LAN2 LAN1 B3 B4 B2 LAN2 LAN4 B5 LAN1 B3 LAN3 B3 LAN3 B2 B3 B6 LAN1 LAN2 25 Virtual LAN VLAN 1 S3 VLAN 2 S6 VLAN 3 S9 Floor n + 1 Physical S2 S5 S8 partition Floor n 1 2 3 4 5 6 or 7 8 switch 9 Bridge S1 S4 S7 Floor n – 1 Logical partition 26 Per-Port VLANs VLAN 1 S3 VLAN 2 S6 VLAN 3 S9 Floor n + 1 S2 S5 S8 Floor n 1 2 3 4 5 6 Bridge 7 or 8 switch 9 S1 S4 S7 Floor n – 1 Logical partition Bridge only forwards frames to outgoing ports associated with same VLAN 27 Tagged VLANs • More flexible than Port-based VLANs • Insert VLAN tag after source MAC address in each frame – VLAN protocol ID + tag • VLAN-aware bridge forwards frames to outgoing ports according to VLAN ID • VLAN ID can be associated with a port statically through configuration or dynamically through bridge learning • IEEE 802.1q 28 ECE 683 Computer Network Design & Analysis Note 8: Packet Switching Networks (Network Layer Protocols) 29 Outline • Network Services and Internal Network Operation • Packet Network Topology • Datagrams and Virtual Circuits • Routing in Packet Networks • Shortest Path Routing 30 Network Layer • Network Layer: the most complex layer – Requires the coordinated actions of multiple, geographically distributed network elements (switches & routers) – Must be able to deal with very large scales Billions of users (people & communicating devices) – Biggest Challenges Addressing: where should information be directed to? Routing: what path should be used to get information there? 31 Packet Switching t1 t0 Network • Transfer of information as payload in data packets • Packets undergo random delays & possible loss • Different applications impose differing requirements on the transfer of information 32 Network Service Messages Messages Segments Transport layer Transport layer Network service Network service End system α Network layer Network layer Network layer Network layer Data link layer Data link layer Data link layer Data link layer Physical layer Physical layer Physical layer Physical layer End system β • Network layer can offer a variety of services to transport layer • Connection-oriented service or connectionless service • Best-effort or delay/loss guarantees 33 Network Service vs. Operation Network Service • Connectionless – Datagram Transfer • Connection-Oriented – Reliable and possibly constant bit rate transfer Internal Network Operation • Connectionless – IP • Connection-Oriented – ATM Various combinations are possible • Connection-oriented service over Connectionless operation • Connectionless service over Connection-Oriented operation • Context & requirements determine what makes sense 34 Complexity at the Edge or in the Core? C 12 3 21 End system α 4 3 21 End system β 12 3 21 Medium A 12 3 B 2 1 Network 1 Physical layer entity 2 Data link layer entity 3 Network layer entity 21 123 4 3 Network layer entity 4 Transport layer entity 35 The End-to-End Argument for System Design • An end-to-end function is best implemented at a higher level than at a lower level – End-to-end service requires all intermediate components to work properly – Higher-level better positioned to ensure correct operation • Example: stream transfer service – Establishing an explicit connection for each stream across network requires all network elements (NEs) to be aware of connection; All NEs have to be involved in re-establishment of connections in case of network fault – In connectionless network operation, NEs do not deal with each explicit connection and hence are much simpler in design 36 Network Layer Functions Essential • Routing: mechanisms for determining the set of best paths for routing packets requires the collaboration of network elements • Forwarding: transfer of packets from NE inputs to outputs • Priority & Scheduling: determining order of packet transmission in each NE Optional: congestion control, segmentation & reassembly, security 37 Note 8: Packet Switching Networks Packet Network Topology 38 End-to-End Packet Network • Packet networks very different from telephone networks • Individual packet streams are highly bursty – Statistical multiplexing is used to concentrate streams • User demand can undergo dramatic change – Peer-to-peer applications stimulated huge growth in traffic volumes • Internet structure highly decentralized – Paths traversed by packets can go through many networks controlled by different organizations – No single entity responsible for end-to-end service 39 Access Multiplexing Access MUX To packet network • Packet traffic from users multiplexed at access to network into aggregated streams • DSL traffic multiplexed at DSL Access Mux • Cable modem traffic multiplexed at Cable Modem Termination System 40 Oversubscription • Access Multiplexer ••• ••• r r r Nr – – – – – nc Nc N subscribers connected @ c bps to mux Each subscriber active r/c of time Mux has C=nc bps to network Oversubscription rate: N/n Find n so that at most 1% overflow probability Feasible oversubscription rate increases with size N r/c n N/n 10 0.01 1 10 10 extremely lightly loaded users 10 0.05 3 3.3 10 very lightly loaded user 10 0.1 4 2.5 10 lightly loaded users 20 0.1 6 3.3 20 lightly loaded users 40 0.1 9 4.4 40 lightly loaded users 100 0.1 18 5.5 100 lightly loaded users 41 Home LANs WiFi Ethernet Home Router To packet network • Home Router – LAN Access using Ethernet or WiFi (IEEE 802.11) – Private IP addresses in Home (192.168.0.x) using Network Address Translation (NAT) – Single global IP address from ISP issued using Dynamic Host Configuration Protocol (DHCP) 42 LAN Concentration Switch / Router • LAN Hubs and switches in the access network also aggregate packet streams that flows into switches and routers 43 Campus Network Organization Servers To Internet or wide area network s Servers have redundant connectivity to backbone s Gateway Backbone R R R S R Departmental Server S S R R s s High-speed Only outgoing campus leave packets backbone net LAN through connects dept router routers s s s s s s s 44 Connecting to Internet Service Provider Internet service provider Border routers Campus Network Border routers Interdomain level Autonomous system or domain Intradomain level s LAN s s network administered by single organization Internet Backbone National Service Provider A National Service Provider B NAP NAP National Service Provider C Private peering • Network Access Points: set up during original commercialization of Internet to facilitate exchange of traffic • Private Peering Points: two-party inter-ISP agreements to exchange traffic National Service Provider A (a) National Service Provider B NAP NAP National Service Provider C (b) NAP Private peering RA Route RB Server LAN RC Key Role of Routing How to get packet from here to there? • Decentralized nature of Internet makes routing a major challenge – Interior gateway protocols (IGPs) are used to determine routes within a domain – Exterior gateway protocols (EGPs) are used to determine routes across domains – Routes must be consistent & produce stable flows • Scalability required to accommodate growth – Hierarchical structure of IP addresses essential to keeping size of routing tables manageable 48 Note 8: Packet Switching Networks Datagrams and Virtual Circuits 49 Packet Switching Network User Transmission line Network Packet switch Packet switching network • Transfers packets between users • Transmission lines + packet switches (routers) • Origin in message switching Two modes of operation: • Connectionless • Virtual Circuit 50 Message Switching Message Message Message Source Message Switches • Message switching invented for telegraphy • Entire messages multiplexed onto shared lines, stored & forwarded • Headers for source & destination addresses • Routing at message switches • Connectionless Destination 51 Message Switching Delay Source T t Switch 1 t Switch 2 t t Destination Delay Minimum delay = 3 + 3T over a path with two intermediate switches Additional queueing delays possible at each link 52 Long Messages vs. Packets 1 Mbit message source dest BER=p=10-6 BER=10-6 How many bits need to be transmitted to deliver message? • Approach 1: send 1 Mbit message • Probability message arrives correctly 6 10 6 Pc (1 10 ) e 10 610 6 e 1 1 / 3 • On average it takes about 3 transmissions each hop • Total # bits transmitted ≈ 6 Mbits (3 transmissions x 2 hops) • Approach 2: send ten 100kbit packets • Probability packet arrives correctly 6 Pc (1 10 6 )10 e 10 10 e 0.1 0.9 5 5 • On average it takes about 1/0.9=1.1 transmissions/hop • Total # bits transmitted ≈ 2.2 Mbits (1.1 x 2) 53 Packet Switching - Datagram • Messages broken into smaller units (packets) • Source & destination addresses in packet header • Connectionless, packets routed independently (datagram) • Packet may arrive out of order • Pipelining of packets across network can reduce delay, increase throughput • Lower delay than message switching, suitable for interactive traffic Packet 1 Packet 1 Packet 2 Packet 2 Packet 2 54 Packet Switching Delay Assume three packets corresponding to one message traverse the same path with two switches t 1 2 3 t 1 2 3 t 1 2 3 t Delay Minimum Delay = 3τ + 5(T/3) (single path assumed) Additional queueing delays possible at each link Packet pipelining enables message to arrive sooner 55 Delay for k-Packet Message over L Hops Source Switch 1 Switch 2 t 1 2 3 t 1 2 3 t 1 Destination 2 3 t L hops 3 hops 3 + 2(T/3) first bit received L + (L-1)P first bit received 3 + 3(T/3) first bit released L + LP first bit released 3 + 5 (T/3) last bit released L + LP + (k-1)P last bit released where T = k P 56 Routing Tables in Datagram Networks Destination address Output port 0785 7 1345 12 1566 6 2458 12 • Route determined by table lookup • Routing decision involves finding next hop in route to given destination • Routing table has an entry for each destination specifying output port that leads to next hop • Size of table becomes impractical for very large number of destinations 57 Example: Internet Routing • Internet protocol uses datagram packet switching across networks – Networks are treated as data links • Hosts have two-part IP address: – Network address + Host address • Routers do table lookup on network address – This reduces size of routing table • In addition, network addresses are assigned so that they can also be aggregated – Discussed as CIDR (classless interdomain routing) in Chapter 8 58 Packet Switching – Virtual Circuit Packet Packet Packet Packet Virtual circuit • Call set-up phase sets ups pointers in fixed path along network • All packets for a connection follow the same path • Abbreviated header identifies connection on each link • Packets queued for transmission • Variable bit rates possible, negotiated during call set-up • Delays variable, cannot be less than circuit switching 59 Connection Setup Connect request Connect confirm SW 1 Connect request Connect confirm SW 2 … SW n Connect request Connect confirm • Signaling messages propagate as a route is selected • Signaling messages identify connection and setup tables in switches • Typically a connection is identified by a local tag, Virtual Circuit Identifier (VCI) • Each switch only needs to know how to relate an incoming tag in one input to an outgoing tag in the corresponding output • Once tables are setup, packets can flow along path 60 Connection Setup Delay t Connect request CC CR CC CR Connect confirm 1 2 3 1 2 Release 3 t t 1 2 3 t • Connection setup delay is incurred before any packet can be transferred • Delay is acceptable for sustained transfer of large number of packets • This delay may be unacceptably high if only a few packets are being transferred 61 Virtual Circuit Forwarding Tables Input VCI Output port Output VCI 12 13 44 15 15 23 27 13 16 58 7 34 • Each input port of packet switch has a forwarding table • Lookup entry for VCI of incoming packet • Determine output port (next hop) and insert VCI for next link • Very high speeds are possible • Table can also include priority or other information about how packet should be treated 62 Cut-Through switching Source t Switch 1 2 1 3 t Switch 2 2 1 3 t 1 Destination 2 3 t Minimum delay = 3 + T • Some networks perform error checking on header only, so packet can be forwarded as soon as header is received & processed • Delays reduced further with cut-through switching 63 Message vs. Packet Minimum Delay • Message: L + LT = L + (L – 1) T + T • Packet L + L P + (k – 1) P = L + (L – 1) P + T • Cut-Through Packet (Immediate forwarding after header) = L+ T Above neglect header processing delays L-# of hops, Propagation time, T - a message’s transmission time, P=T/k, each packet’s transmission time 64