Lectures The Network Layer

Transcription

1 Computer Communications Lectures The Network Layer Required Reading: Tanenbaum (chapter 5 and section 2.5.5) Circuit Switching (a) Circuit switching. (b) Packet switching. 2 1

2 Circuit vs. Message vs. Packet Switching (a) Circuit switching (b) Message switching (c) Packet switching 3 Circuit vs. Packet Switching A comparison of circuit switched and packet-switched networks. 4 2

3 Context The environment of the network layer protocols. 5 Implementation of Connectionless Service Datagrams and datagram subnet Routing within a diagram subnet. 6 3

4 Implementation of Connection-Oriented Service Virtual-Circuits (VCs) and virtual circuit subnet Routing within a virtual-circuit subnet. 7 Comparison of Datagram and Virtual-Circuit Subnets 8 4

5 The Network Layer in the Internet The IP Protocol IP Addresses Internet Control Protocols OSPF The Interior Gateway Routing Protocol BGP The Exterior Gateway Routing Protocol Internet Multicasting Mobile IP IPv6 9 Design Principles for Internet 1.Make sure it works. 2.Keep it simple. 3.Make clear choices. 4.Exploit modularity. 5.Expect heterogeneity. 6.Avoid static options and parameters. 7.Look for a good design; it need not be perfect. 8.Be strict when sending and tolerant when receiving. 9.Think about scalability. 10.Consider performance and cost. 10 5

6 Collection of Subnetworks The Internet is an interconnected collection of many networks. 11 IP (Internet Protocol) Best-effort, datagram delivery service Datagram size <= 64KB Supports internetworking The IPv4 (Internet Protocol) header. 12 6

7 IP Options Some of the IP options. 13 IP Addresses An IP address refers to a network interface, not a host Managed by ICANN (Internet Corporation for Assigned Names and Numbers) Dotted decimal notation 128/16 x /64K 2 x 10 6 /256 IP address formats. 14 7

8 IP Addresses (2) Special IP addresses. 15 Subnets Splitting a network into several parts for internal use but still act like a single network to the outside world A campus network consisting of LANs for various departments. 16 8

9 Subnetting Example Subnet mask indicates the split between network + subnet number, and host For the example below: Subnet mask: Alternative notation: /22 A class B network subnetted into 64 subnets. 17 IP Routing IP routing table without subnetting A set of <(Network, 0) IP address, interface, > to reach distant networks A set of <(this-network, host) IP address, interface, > entries for local hosts A default router entry IP routing table with subnetting A set of <(this-network, subnet, 0) IP address, interface, > to reach other subnets on this network A set of <(this-network, this-subnet, host) IP address, interface, > entries for local hosts on this subnet A default router entry Subnetting reduces router table space by creating a three-level hierarchy consisting of network, subnet and host 18 9

10 CIDR Classless InterDomain Routing Allocate remaining IP addresses in variable-sized blocks, without regard to the classes Each routing table entry: <IP address, subnet mask, outgoing line> triple Longest mask matching for forwarding Use of aggregation for reducing routing table sizes A set of IP address assignments. 19 Routing Algorithms Main function of network layer: routing packets from source to destination Routing is an issue even for broadcast networks if source and destination are not on same network Routing algorithm Part of network layer software that updates the routing table Decides the output line on which an incoming packet should be transmitted Datagram subnet routing vs. virtual-circuit subnet routing (session routing) Contrast with forwarding which refers to routing table lookup operation Desirable properties Correctness Simplicity Robustness Stability (convergence to an equilibrium) Optimality Fairness 20 10

11 Routing Algorithms: Optimality and Fairness Optimality Min (mean packet delay) Max (total network throughput) Min (number of hops for a packet) Fairness Contradictory goals Conflict between fairness and optimality. 21 Routing Algorithms: Classification Non-adaptive (static) algorithms Route choices computed in advance and downloaded to routers at network boot time Adaptive (dynamic) algorithms Adapt routing decisions in response to topology and traffic changes Various algorithms differ in terms of:» Where they get their information (e.g., locally, from adjacent routers, or from all routers)» When they change the routes (e.g., periodically, when load changes, or when topology changes) periodic vs. event-driven» What metric they use for optimization (e.g., distance, number of hops, or estimated transit time) 22 11

12 The Optimality Principle General statement about optimal routes without regard to topology or traffic: If router J is on the optimal path from router I to router K, then the optimal path from J to K also falls along the same route Sink tree: Set of optimal routes from all sources to a given destination form a tree rooted at the destination; not necessarily unique; no loops Benchmark against which other routing algorithms can be measured (a) A subnet. (b) A sink tree for router B. 23 Resource Reservation (Section 5.4, page 405) Effective quality-of-service (QoS) provisioning requires reserving resources for a flow along its assigned route (virtual-circuit like approach) Resources Bandwidth not oversubscribing any output line Buffer space reserve buffers for each QoS flow up to some maximum CPU cycles: reserving this resource a bit more tricky! Consider:» A router taking 1µsec to process a packet doesn t mean it can process 1 million packets/sec because of idle periods from statistical fluctuations in the load» Even with a load slightly below the theoretically capacity, queues can build up and delays can occur Random packet arrivals with mean rate λ packets/sec Random CPU processing time with mean capacity µ packets/sec Assume poisson arrival and service distributions Using queuing theory:» mean delay experienced by a packet (including queueing and service time), T = (1/ µ) x (1/(1- λ/ µ)) = (1/ µ) x (1/(1- ρ)), where CPU utilization, ρ = λ/ µ 24 12

13 Distance Vector Routing (1) An example of dynamic and distributed routing algorithm Sometimes also called the distributed Bellman-Ford algorithm Original ARPANET routing algorithm Also was used in the Internet under the name RIP Each router maintains a table (e.g., a vector) giving the best known distance to each destination and which outgoing line to use to reach that destination distance or metric can be number of hops, time delay in milliseconds, total number of packets queued along the path, etc. Assumption: each router knows the distance to each of its neighbors Tables updated via information exchange with neighbors 25 Distance Vector Routing (2) Note that old routing table is not used in the calculation (a) A subnet. (b) Input from A, I, H, K, and the new routing table for J

14 Distance Vector Routing (3) Works in theory, but convergence is an issue Count-to-infinity problem: reacts rapidly to good news, but bad news travels slowly X tells Y that it has a path somewhere, but Y has no way of knowing whether it itself is on the path Some solutions (e.g., split horizon with poisoned reverse in RFC 1058) but none work in general The count-to-infinity problem. 27 Link State Routing Replaced distance vector based routing algorithm in ARPANET in 1979 Used in Internet OSPF protocol and IS-IS (a precursor to OSPF) Assume globally unique router addresses. Each router must do the following: Discover its neighbors, learn their network address (using HELLO packets). Measure the delay or cost to each of its neighbors (e.g., using ECHO packets). Construct a packet telling all it has just learned ( link state packet ) Send this packet to all other routers (via flooding). Compute the shortest path to every other router (using Dijkstra s algorithm)

15 Adapting to Load and Route Oscillations A subnet in which the East and West parts are connected by two lines. 29 Building Link State Packets Periodically or driven by events (e.g., line down) (a) A subnet. (b) The link state packets for this subnet

16 Flooding Send every incoming packet on every outgoing line except the one it arrived on many duplicates Measures to reduce duplicates Hop counter in each packet, decremented at each hop, when zero packet discarded Keep track of packets flooded earlier using source sequence numbers and avoid sending them again Selective flooding: propagate incoming packets only on output lines going in the right direction Uses of flooding Robustness for military applications Concurrent updates in distributed database applications Wireless multicast advantage Benchmark for routing algorithms 31 Distributing the Link State Packets Basic distribution algorithm is flooding with source sequence numbers Issues: sequence number wrap around, router crashes, sequence number corruption Refinements:» Include a short waiting time before propagating link state packets to suppress stale and duplicate packets» Use acknowledgements The packet buffer for router B in the previous slide

17 Shortest Path Routing Represent network as a graph and find shortest path on the graph Dijkstra s algorithm The first 5 steps used in computing the shortest path from A to D. The arrows indicate the working node. 33 Hierarchical Routing To deal with problems arising from increase in network size Regions, clusters, zones, groups, Can increase path length How many levels should the hierarchy have?» Kamoun and Kleinrock (1979): ln N optimal levels with e ln N for size N net Hierarchical routing

18 Internetworking Interconnecting two or more different networks to form an internet A collection of interconnected networks. 35 How Networks Differ Some of the many ways networks can differ (in network layer). Also differences in physical and data link layers, e.g., different modulation techniques and frame formats 36 18

19 How Networks Can Be Connected Physical layer repeaters and hubs Data link layer switches and bridges Network layer (multiprotocol) routers Transport layer gateways Application layer gateways (a) Two Ethernets connected by a switch. (b) Two Ethernets connected by routers. 37 Concatenated Virtual Circuits vs. Connectionless Internetworking Internetworking using concatenated virtual circuits. A connectionless internet

20 Tunneling To handle the special case where source and destination hosts are on same type of network, but intermediate network is different Tunneling a packet from Paris to London. Tunneling a car from France to England. 39 Internetwork Routing Interior gateway protocol for intranetwork routing or routing within a network (autonomous system) Exterior gateway protocol for internetwork routing or routing between networks (autonomous systems) Issues like crossing international boundaries and cost need to be considered (a) An internetwork. (b) A graph of the internetwork

21 Routing in the Internet OSPF: The Interior Gateway Routing Protocol Design goals Published in open literature Support a variety of distance metrics Dynamic: adapt to topology changes automatically and quickly Support routing based on type of service Do load balanced routing, i.e., splitting the load over multiple lines Support for hierarchical systems Security Support routers connected to Internet via a tunnel Supports three kinds of connections and networks Point-to-point lines between exactly two routers Multiaccess networks with broadcasting (e.g., most LANs) Multiaccess networks without broadcasting (e.g., most packet-switched WANs) 42 21

22 OSPF (2) Operates by abstracting the collection of actual networks, routers and lines into a directed graph with a cost for each arc. (a) An autonomous system. (b) A graph representation of (a). 43 OSPF (3) Many ASes in the Internet large, so OSPF supports hierarchical routing by allowing ASes to be divided into numbered areas Backbone area (area 0) connects to all other areas Three types of routes needed: intra-area, inter-area and inter-as The five types of OSPF messages. The relation between ASes, backbones, and areas in OSPF

23 BGP: The Exterior Gateway Routing Protocol Designed to allow many kinds of routing policies to be enforced in the interas traffic Policies themselves not part of the protocol, they are manually configured into each BGP router Stub networks, transit networks and multiconnected networks Communication between each pair of BGP routers via TCP connections Fundamentally distance vector, but maintains complete path info (a) A set of BGP routers. (b) Information sent to F. 45 Fragmentation (a) Transparent fragmentation. (b) Nontransparent fragmentation

24 Fragmentation (2) Fragmentation when the elementary data size is 1 byte. (a) Original packet, containing 10 data bytes. (b) Fragments after passing through a network with maximum packet size of 8 payload bytes plus header. (c) Fragments after passing through a size 5 gateway. 47 Other Topics Internet control protocols: ICMP, ARP, RARP, BOOTP, DHCP & NAT. Broadcast Routing, Multicast Routing & IP Multicasting Routing for mobile hosts & Mobile IP IPv6 Congestion control and QoS 48 24