Chapter Packet-Switching Networks Network Services and Internal Network Operation Packet Network Topology Datagrams and Virtual Circuits Routing in Packet Networks Shortest Path Routing ATM Networks Traffic Management Chapter Packet-Switching Networks Network Services and Internal Network Operation Network Layer z Network Layer: the most complex layer z z Requires the coordinated actions of multiple, geographically distributed network elements (switches & routers) Must be able to deal with very large scales z z Billions of users (people & communicating devices) Biggest Challenges z z Addressing: where should information be directed to? Routing: what path should be used to get information there? Packet Switching t1 t0 Network z z z Transfer of information as payload in data packets Packets undergo random delays & possible loss Different applications impose differing requirements on the transfer of information Network Service Messages Messages Segments Transport layer Transport layer Network service Network service Network layer Network layer Network layer Network layer Data link layer Data link layer Data link layer Data link layer layer Physical layer Physical layer Physical layer End system Physical α z z z 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 Network Service vs Operation Network Service z Connectionless z z Datagram Transfer Internal Network Operation z Connectionless Connection-Oriented z Reliable and possibly constant bit rate transfer z z IP Connection-Oriented z z Telephone connection ATM Various combinations are possible z Connection-oriented service over Connectionless operation z Connectionless service over Connection-Oriented operation z Context & requirements determine what makes sense Complexity at the Edge or in the Core? C 12 End system α 21 21 1 12 21 Medium A Physical layer entity Data link layer entity 12 B Network Network layer entity 21 End system β 123 Network layer entity Transport layer entity The End-to-End Argument for System Design z An end-to-end function is best implemented at a higher level than at a lower level z z z End-to-end service requires all intermediate components to work properly Higher-level better positioned to ensure correct operation Example: stream transfer service z z 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 not deal with each explicit connection and hence are much simpler in design Network Layer Functions Essential z Routing: mechanisms for determining the set of best paths for routing packets requires the collaboration of network elements z Forwarding: transfer of packets from NE inputs to outputs z Priority & Scheduling: determining order of packet transmission in each NE Optional: congestion control, segmentation & reassembly, security Chapter Packet-Switching Networks Packet Network Topology Leaky Bucket Example I=4 L=6 Nonconforming Packet arrival Time L+I Bucket content I * * * * * * * * * Non-conforming packets not allowed into bucket & hence not included in calculations Time Policing Parameters T = / peak rate MBS = maximum burst size I = nominal interarrival time = / sustainable rate ⎡ L ⎤ MBS = + ⎢ ⎣ I − T ⎥⎦ MBS T L I Time Dual Leaky Bucket Dual leaky bucket to police PCR, SCR, and MBS: Incoming traffic Leaky bucket SCR and MBS Tagged or dropped Untagged traffic Leaky bucket PCR and CDVT Untagged traffic Tagged or dropped PCR = peak cell rate CDVT = cell delay variation tolerance SCR = sustainable cell rate MBS = maximum burst size Traffic Shaping Traffic shaping Network A z z z Policing Traffic shaping Network B Policing Network C Networks police the incoming traffic flow Traffic shaping is used to ensure that a packet stream conforms to specific parameters Networks can shape their traffic prior to passing it to another network Leaky Bucket Traffic Shaper Incoming traffic Size N Shaped traffic Server Packet z z z z z Buffer incoming packets Play out periodically to conform to parameters Surges in arrivals are buffered & smoothed out Possible packet loss due to buffer overflow Too restrictive, since conforming traffic does not need to be completely smooth Token Bucket Traffic Shaper Tokens arrive periodically An incoming packet must have sufficient tokens before admission into the network Size K Token Incoming traffic Size N Shaped traffic Server Packet z z z Token rate regulates transfer of packets If sufficient tokens available, packets enter network without delay K determines how much burstiness allowed into the network Token Bucket Shaping Effect The token bucket constrains the traffic from a source to be limited to b + r t bits in an interval of length t b bytes instantly b+rt r bytes/second t Packet transfer with Delay Guarantees (a) Bit rate > R > r e.g., using WFQ A(t) = b+rt Token Shaper No backlog of packets R(t) (b) Buffer occupancy at Buffer occupancy at z z z b R b R-r Empty t Assume fluid flow for information Token bucket allows burst of b bytes & then r bytes/second z Since R>r, buffer content @ never greater than b byte z Thus delay @ mux < b/R Rate into second mux is rr H hop path m is maximum packet size for the given flow M maximum packet size in the network Rj transmission rate in jth hop Maximum end-to-end delay that can be experienced by a packet from flow i is: b ( H − 1)m H M D≤ + +∑ R R j =1 R j Scheduling for Guaranteed Service z z z z Suppose guaranteed bounds on end-to-end delay across the network are to be provided A call admission control procedure is required to allocate resources & set schedulers Traffic flows from sources must be shaped/regulated so that they not exceed their allocated resources Strict delay bounds can be met Current View of Router Function Routing Agent Reservation Agent Mgmt Agent Admission Control [Routing database] [Traffic control database] Classifier Input driver Internet forwarder Pkt scheduler Output driver Closed-Loop Flow Control z Congestion control z z z z End-to-end vs Hop-by-hop z z feedback information to regulate flow from sources into network Based on buffer content, link utilization, etc Examples: TCP at transport layer; congestion control at ATM level Delay in effecting control Implicit vs Explicit Feedback z z Source deduces congestion from observed behavior Routers/switches generate messages alerting to congestion End-to-End vs Hop-by-Hop Congestion Control Source Packet flow Destination (a) Source Destination (b) Feedback information Traffic Engineering z z z Management exerted at flow aggregate level Distribution of flows in network to achieve efficient utilization of resources (bandwidth) Shortest path algorithm to route a given flow not enough z z z Does not take into account requirements of a flow, e.g bandwidth requirement Does not take account interplay between different flows Must take into account aggregate demand from all flows (a) Shortest path routing congests link to 8 (b) Better flow allocation distributes flows more uniformly