Chapter 7 Packet-Switching Networks Network Services and Internal Network Operation Packet Network Topology Datagrams and Virtual Circuits
Chapter 7 Packet-Switching Networks Network Services and Internal Network Operation
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? Efficiency: how to forward the sheer volume of traffic? 3
Packet Switching t 0 t 1 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 4
Network Service Network layer can offer a variety of services to transport layer Connection-oriented service or connectionless service 5 Best-effort or delay/loss guarantees
Interworking/Internetworking Host H1 WAN Host H8 TCP Router WAN R2 Router R3 TCP IP IP IP IP ETH ETH PPP /SONET PPP /SONET ETH ETH LAN LAN 16-Oct-12 SYSC4602: Intr. to Computer Communications 6
IP = Network Layer Routing Table IP Address port Router 0 1 2 3 4 Packet in Packet out A network Router Host Control plan vs. data plan: 1. How to build routing table? 2. What information to carry in the packet header? 3. How to use this information together with Routing table to forward packets? 16-Oct-12 SYSC4602: Intr. to Computer Communications 7
Router Generic Architecture Cross connect Line card L1 & L2 L3 Ingress L3 Egress L1 & L2 L3 L3 Interface (port) Ethernet 100BaseT Line card OC3 line card L3 L3 L1 & L2 T1 line card 8
Network Service vs. Operation Network Service Connectionless Datagram Transfer Connection-Oriented Reliable and possibly constant bit rate transfer Network Operation Connectionless IP Connection-Oriented Telephone connection ATM Various combinations are possible Connection-oriented service over Connectionless operation Connectionless service over Connection-Oriented operation Context & requirements determine what makes sense 9
Complexity at the Edge or in the Core? C 1 2 3 2 1 1 2 End system α 4 3 2 1 1 2 3 2 1 1 2 Medium 3 2 1 1 2 3 4 2 1 Physical layer entity A Network Data link layer entity 3 Network layer entity B End system β 3 Network layer entity 4 Transport layer entity 10
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 reestablishment 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 11
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 Others (examples): Signaling, traffic engineering Protection and restoration Virtual private networks 12
Chapter 7 Packet-Switching Networks Packet Network Topology
End-to-End Packet Network Packet networks (packet switching) very different than conventional telephone networks (circuit switching) 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 14
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 15
Oversubscription and Statistical Multiplexing r r r Nr Nc nc Access Multiplexer 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 16
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) 17
Network Address Translation NAT is used for ISPs to assign a single global network address to the subscriber in order to conserve address space. NAT converts a private address (only defined in a home network or enterprise network) to a global network address when a packet leaves the home (enterprise) network and vice versa when a packet arrives at the home (enterprise) network. 18
DHCP Dynamic Host Configuration Protocol Centralized repository of configuration data for all clients (hosts) on network Host Configuration options: Can be manually configured (hardware address vs. configuration), but could be error prone Dynamic Host Configuration Protocol (DHCP) makes life easy: Host DHCP server discovery DHCPDISCOVER broadcast message on boot up (Plug & Play) DHCP server (or relay agent) responds Dynamic address assignment from an address pool Address reuse Private address - networks 10. And 192.168 DHCP packet is carried over UDP 19
LAN Concentration Switch / Router LAN Hubs and switches in the access network also aggregate packet streams that flows into switches and routers 20
Interworking/Internetworking Host H1 WAN Host H8 TCP Router WAN R2 Router R3 TCP IP IP IP IP ETH ETH PPP /SONET PPP /SONET ETH ETH LAN LAN 16-Oct-12 SYSC4602: Intr. to Computer Communications 21
IP = Network Layer Routing Table IP Address port Router 0 1 2 3 4 Packet in Packet out A network, e.g., campus network Router Host Control plan vs. data plan: 1. How to build routing table? 2. What information to carry in the packet header? 3. How to use this information together with Routing table to forward packets? 16-Oct-12 SYSC4602: Intr. to Computer Communications 22
Campus Network Departmental Server To Internet or wide area network Only High-speed outgoing packets campus leave LAN backbone through net router connects dept routers Backbone s s s Organization Servers R Gateway R S S s R Servers have redundant connectivity to backbone 23 s R s s S R s R s s s
Connecting to Internet Service Provider Internet service provider Border routers Campus Network Autonomous system or domain LAN s s s Border routers Intradomain level Interdomain level network administered by single organization 24
Internet Backbone National Service Provider A National Service Provider B NAP National Service Provider C Private peering NAP Network Access Points: set up during original commercialization of Internet to facilitate exchange of traffic Private Peering Points: two-party inter-isp agreements to Prof. Chung-Horng Lung exchange traffic Fall 2012 25
(a) National Service Provider A National Service Provider B NAP National Service Provider C Private peering NAP (b) NAP R A Route Server LAN R B R C 26
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 27
Chapter 7 Packet-Switching Networks Datagrams and Virtual Circuits
The Switching Function Dynamic interconnection of inputs to outputs Enables dynamic sharing of transmission resource Two fundamental approaches: Connectionless Connection-Oriented: Call setup control, Connection control Backbone Network Switch Access Network 29
Message Switching Message Source Message Switches Message Message Destination Message switching invented for telegraphy Entire messages multiplexed onto shared lines, stored & forwarded Headers for source & destination addresses Loss of messages may occur when a switch has insufficient buffering to store the message 30
Packet Switching Network User Network Transmission line 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 31
Message Switching Delay Source T t Switch 1 Switch 2 τ t t Destination Delay t Minimum delay = 3τ + 3T τ: propagation delay T: transmission delay Additional queueing delays possible at each link 32
Long Messages vs. Packets 1 Mbit message Approach 1: send 1 Mbit message Probability message arrives correctly On average it takes about 3 transmissions/hop Total # bits transmitted 6 Mbits source BER=p=10-6 BER=10-6 dest How many bits need to be transmitted to deliver message? 6 6 6 Approach 2: send 10 100-kbit packets Probability packet arrives correctly P c 5 5 6 6 10 10 10 1 6 10 10 10 0.1 P c = (1 10 ) e = e 1/ 3 = (1 10 ) e = e 0. 9 On average it takes about 1.1 transmissions/hop Total # bits transmitted 2.2 Mbits 33
Connectionless 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 2 Packet 1 Packet 2 Packet 1 Packet 2 34
Packet Switching Delay Assume three packets corresponding to one message traverse same path τ 1 T/3 T 2 3 1 2 3τ+2(T/3) Delay 3 1 2 Minimum Delay = 3τ + 5(T/3) (single path assumed) Additional queueing delays possible at each link Packet pipelining enables message to arrive sooner 3 3τ+5(T/3) t t t t 35
Delay for k-packet Message over L Hops Source Switch 1 Switch 2 Destination τ 1 2 3 1 2 3τ + 2(T/3) first bit received 3τ + 3(T/3) first bit released 3 1 2 3 hops L hops 3 Lτ + (L-1)P first bit received P: transmission delay / packet Lτ + LP first bit released t t t t 3τ + 5 (T/3) last bit released Lτ + LP + (k-1)p last bit released where T = k P 36
Routing Tables in Datagram Networks Destination address Output port 0785 7 1345 12 1566 2458 6 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 37
Example: Internet Routing Internet protocol uses datagram packet switching across networks A packet arrives at a router Router will do a table lookup of the packet destination address if address is within its network packet will be forwarded to the appropriate output link if address is not in the given network router will forward packet to a router of another network (next hop network) after performing suitable encapsulation i.e. 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 To be discussed later 38
Packet Switching Virtual Circuit Packet Packet Packet Packet Virtual circuit Call set-up phase sets up pointers in fixed path along network All packets for a connection follow the same path Abbreviated header identifies connection on each link Packets queue for transmission Variable bit rates possible, negotiated during call set-up Delays variable, cannot be less than circuit switching 39
Connection Setup Connect request Connect confirm SW 1 Connect request Connect confirm SW 2 SW n Connect request Connect confirm Signaling messages propagate as route is selected Signaling messages identify connection and set up 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 set, packets can flow along path 40
Connection Setup Delay Connect request CR CR CC CC Connect confirm 1 2 3 1 2 3 1 2 3 Release t t t 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 Fall 2012 packets are being transferred Prof. Chung-Horng Lung 41
Virtual Circuit Forwarding Tables Input VCI Output port 15 15 58 Output VCI 12 13 44 27 13 7 23 16 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 42
Cut-Through switching Source Switch 1 Switch 2 Destination 1 T 2 3 1 2 3 1 2 3 Minimum delay = 3τ + T t t t 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 43
Message vs. Packet Minimum Delay Message: L τ + L T = L τ + (L 1) T + T Packet L τ + L P + (k 1) P = L τ + (L 1) P + T Cut-Through Packet (Immediate forwarding after header) = L τ + T Above neglect header processing delays 44
Example: ATM Networks All information mapped into short fixed-length packets called cells Connections set up across network Virtual circuits established across networks Tables setup at ATM switches Several types of network services offered Constant bit rate connections Variable bit rate connections 45
Chapter 7 Packet-Switching Networks Datagrams and Virtual Circuits Structure of a Packet Switch
Packet Switch: Intersection where Traffic Flows Meet 1 1 2 2 N N Inputs contain multiplexed flows from access muxs & other packet switches Flows demultiplexed at input, routed and/or forwarded to output ports Packets buffered, prioritized, and multiplexed on output lines 47
1 2 3 N Generic Packet Switch Line card Line card Line card Input ports Line card Controller Data path Control path Interconnection fabric (a) Line card Line card Line card Line card 1 2 3 N Output ports Unfolded View of Switch Ingress Line Cards Header processing Demultiplexing Routing in large switches Controller Routing protocols Signalling & resource allocation Interconnection Fabric Transfer packets between line cards Egress Line Cards Scheduling & priority Multiplexing 48
Line Cards Transceiver Framer Network processor Backplane transceivers Transceiver Framer To physical ports Folded View To switch fabric To other line cards Interconnection fabric 1 circuit board is ingress/egress line card Physical layer processing Data link layer processing Network header processing Fall 2012 Physical layer across fabric Prof. Chung-Horng + framing Lung 49
Shared Memory Packet Switch Ingress Processing 1 2 Connection Control Queue Control Output Buffering 1 2 3 Shared Memory 3 N N Fall 2012 Small switches can be built Prof. by Chung-Horng reading/writing Lung into shared memory 50
Crossbar Switches Inputs 1 (a) Input buffering Inputs 1 (b) Output buffering 2 3 8 3 2 3 3 N N 1 2 3 N Outputs 1 2 3 N Outputs Large switches built from crossbar & multistage space switches Requires centralized controller/scheduler (who sends to whom when) 51 Can buffer at input, output, or both (performance vs complexity)
Self-Routing Switches Inputs 0 1 Outputs 0 1 2 3 4 5 6 7 Stage 1 Stage 2 Stage 3 2 3 4 5 6 7 Self-routing switches do not require controller Output port number determines route Fall 2012 101 (1) lower port, Prof. (2) Chung-Horng upper Lung port, (3) lower port 52