Computer Networks with Internet Technology William Stallings

Computer Networks with Internet Technology William Stallings
Interior Routing Protocols Computer Networks with Internet Technology William Stallings Chapter 11 Interior Routing Protocols Chapter 11

Chapter 11 Interior Routing Protocols
Introduction Routing protocols essential to operation of an internet Routers forward IP datagrams from one router to another on path from source to destination Router must have idea of topology of internet Routing protocols provide this information Chapter 11 Interior Routing Protocols

Internet Routing Principles
Routers receive and forward datagrams Make routing decisions based on knowledge of topology and conditions on internet Decisions based on some least cost criterion (chapter 14) Chapter 11 Interior Routing Protocols

Fixed Routing Single permanent route configured for each source-destination pair Routes fixed May change when topology changes Link cost not based on dynamic data Based on estimated traffic volumes or capacity of link Chapter 11 Interior Routing Protocols

Figure 11.1 A Configuration of Routers and Networks
Chapter 11 Interior Routing Protocols

Discussion of Example 5 networks, 8 routers Link cost for output side of each router for each network Next slide shows how fixed cost routing may be implemented Each router has routing table Chapter 11 Interior Routing Protocols

Routing Table One required for each router Entry for each network Not for each destination Routing only needs network portion Once datagram reaches router attached to destination network, that router can deliver to host IP address typically has network and host portion Each entry shows next node on route Not whole route Chapter 11 Interior Routing Protocols

Routing Tables in Hosts
May also exist in hosts If attached to single network with single router then not needed All traffic must go through that router (called the gateway) If multiple routers attached to network, host needs table saying which to use Chapter 11 Interior Routing Protocols

Figure 11.2 Example Routing Tables

Interior Routing Protocols
Adaptive Routing As conditions on internet changes, routes may change Failure Can route round problems Congestion Can route round congestion Avoid, or at least not add to further congestion Chapter 11 Interior Routing Protocols Chapter 11

Drawbacks of Adaptive Routing
More complex routing decisions Router processing increases Depends on information collected in one place but used in another More information exchanged improves routing decisions but increases overhead May react two fast causing congestion through oscillation May react to slow, being irrelevant Can produce pathologies Fluttering Looping Chapter 11 Interior Routing Protocols

Fluttering Rapid oscillation in routing Due to router attempting load balancing or splitting Splitting traffic among a number of routes May result in successive packets bound for same destination taking very different routes (see next slide) Chapter 11 Interior Routing Protocols

Figure 11.3 Example of Fluttering

Problems with Fluttering
If in one direction only, route characteristics may differ in the two directions Including timing and error characteristics Confuses management and troubleshooting applications that measure these Difficulty estimating round trip times TCP packets arrive out of order Spurious retransmission Duplicate acknowledgements Chapter 11 Interior Routing Protocols

Looping Packet forwarded by router eventually returns to that router Algorithms designed to prevent looping May occur when changes in connectivity not propagated fast enough to all other routers Chapter 11 Interior Routing Protocols

Adaptive Routing Advantages
Improve performance as seen by user Can aid congestion control Benefits depend on soundness of design Adaptive routing very complex Continual evolution of protocols Chapter 11 Interior Routing Protocols

Classification of Adaptive Routing Strategies
Based on information sources Local E.g. route each datagram to network with shortest queue Balance loads on networks May not be heading in correct direction Include preferred direction Rarely used Adjacent nodes Distance vector algorithms All nodes Link-state algorithms Both need routing protocol to exchange information Chapter 11 Interior Routing Protocols

Autonomous Systems (AS)
Group of routers exchanging information via common routing protocol Set of routers and networks managed by single organization Connected Except in time of failure Chapter 11 Interior Routing Protocols

Interior Routing Protocol (IRP)
Passes routing information between routers within AS Does not need to be implemented outside AS Allows IRP to be tailored May be different algorithms and routing information in different connected AS Need minimum information from other connected AS At least one router in each AS must talk Use Exterior Routing Protocol (ERP) Chapter 11 Interior Routing Protocols

Exterior Routing Protocol (ERP)
Pass less information than IRP Router in first system determines route to target AS Routers in target AS then co-operate to deliver datagram ERP does not deal with details within target AS Chapter 11 Interior Routing Protocols

Figure 11.4 Application of Exterior and Interior Routing Protocols

Approaches to Routing – Distance-vector
Each node (router or host) exchange information with neighboring nodes Neighbors are both directly connected to same network First generation routing algorithm for ARPANET Node maintains vector of link costs for each directly attached network and distance and next-hop vectors for each destination Used by Routing Information Protocol (RIP) Requires transmission of lots of information by each router Distance vector to all neighbors Contains estimated path cost to all networks in configuration Changes take long time to propagate Chapter 11 Interior Routing Protocols

Approaches to Routing – Link-state
Designed to overcome drawbacks of distance-vector When router initialized, it determines link cost on each interface Advertises set of link costs to all other routers in topology Not just neighboring routers From then on, monitor link costs If significant change, router advertises new set of link costs Each router can construct topology of entire configuration Can calculate shortest path to each destination network Router constructs routing table, listing first hop to each destination Router does not use distributed routing algorithm Use any routing algorithm to determine shortest paths In practice, Dijkstra's algorithm Open shortest path first (OSPF) protocol uses link-state routing. Also second generation routing algorithm for ARPANET Chapter 11 Interior Routing Protocols

Exterior Router Protocols – Path-vector
Dispense with routing metrics Provide information about which networks can be reached by a given router and ASs crossed to get there Does not include distance or cost estimate Each block of information lists all ASs visited on this route Enables router to perform policy routing E.g. avoid path to avoid transiting particular AS E.g. link speed, capacity, tendency to become congested, and overall quality of operation, security E.g. minimizing number of transit Ass Chapter 11 Interior Routing Protocols

Least Cost Algorithms Least-cost criterion If minimize number of hops, link value 1 Link value may be inversely proportional to capacity, proportional to current load, or some combination May differ in different two directions E.g. if cost equaled length of queue Cost of path between two nodes as sum of costs of links traversed For each pair of nodes, find least cost path Dijkstra's algorithm Bellman-Ford algorithm Chapter 11 Interior Routing Protocols

Figure 11.5 Dijkstra’s Algorithm Applied to Figure 11.1

Figure 11.6 Bellman-Ford Algorithm Applied to Figure 11.1

Comparison of Algorithms
Bellman-Ford Link cost to all neighboring nodes to node n [i.e., w(j, n)] plus total path cost to those neighboring nodes from a particular source node s [i.e., Lh(j)] Each node can maintain set of costs and associated paths for every other node and exchange information with direct neighbors Each node can use Bellman-Ford based only on information from neighbors and knowledge of its link costs Dijkstra Each node must know link costs of all links Information must be exchanged with all other nodes Both converge under static conditions to same solution If costs change algorithm will attempt to catch up If cost depends on traffic Depends on routes chosen then feedback condition exists Instabilities may result Chapter 11 Interior Routing Protocols

Distance Vector Routing
Each node exchange information with neighbors Directly connected by same network Each node maintains three vectors Link cost Distance vector Next hop vector Every 30 seconds, exchange distance vector with neighbors Use this to update distance and next hop vector Chapter 11 Interior Routing Protocols

Figure 11.7 Distance Vector Algorithm Applied to Figure 11.1

Distributed Bellman-Ford
RIP is a distributed version of Bellman-Ford Original routing algorithm in ARPANET Each simultaneous exchange of vectors between routers is equivalent to one iteration of step 2 In fact, asynchronous exchange used At start-up, get vectors from neighbors Gives initial routing By own timer, update every 30 seconds Changes are propagated across network Routing converges within finite time Proportional to number of routers Chapter 11 Interior Routing Protocols

RIP Details – Incremental Update
Updates do not arrive from neighbors within small time window RIP packets use UDP Tables updated after receipt of individual distance vector Add any new destination network Replace existing routes with small delay ones If update from router R, update all routes using R as next hop Chapter 11 Interior Routing Protocols

RIP Details – Topology Change
If no updates received from a router within 180 seconds, mark route invalid Assumes router crash or network connection unstable Set distance value to infinity Actually 16 Chapter 11 Interior Routing Protocols

Counting to Infinity Problem (1)
Slow convergence may cause: All link costs 1 B has distance to network 5 as 2, next hop D A & C have distance 3 and next hop B Chapter 11 Interior Routing Protocols

Counting to Infinity Problem (2)
Suppose router D fails: B determines network 5 no longer reachable via D Sets distance to 4 based on report from A or C At next update, B tells A and C this A and C receive this and increment their network 5 distance to 5 4 from B plus 1 to reach B B receives distance count 5 and assumes network 5 is 6 away Repeat until reach infinity (16) Takes 8 to 16 minutes to resolve Chapter 11 Interior Routing Protocols

Figure 11.8 Counting to Infinity Problem

Split Horizon Counting to infinity problem caused by misunderstanding between B and A, and B and C Each thinks it can reach network 5 via the other Split Horizon rule says do not send information about a route back in the direction it came from Router sending information is nearer destination than you Erroneous route now eliminated within time out period (180 seconds) Chapter 11 Interior Routing Protocols

Poisoned Reverse Send updates with hop count of 16 to neighbors for route learned from those neighbors If two routers have routes pointing at each other advertising reverse route with metric 16 breaks loop immediately Chapter 11 Interior Routing Protocols

Figure 11.9 RIP Packet Format

RIP Packet Format Notes
Command: 1=request 2=reply Updates are replies whether asked for or not Initializing node broadcasts request Requests are replied to immediately Version: 1 or 2 Address family: 2 for IP IP address: non-zero network portion, zero host portion Identifies particular network Metric Path distance from this router to network Typically 1, so metric is hop count Chapter 11 Interior Routing Protocols

RIP Limitations Destinations with metric more than 15 are unreachable If larger metric allowed, convergence becomes lengthy Simple metric leads to sub-optimal routing tables Packets sent over slower links Accept RIP updates from any device Misconfigured device can disrupt entire configuration Chapter 11 Interior Routing Protocols

Open Shortest Path First (OSPF)
RIP limited in large internets OSPF preferred interior routing protocol for TCP/IP based internets Link state routing used Chapter 11 Interior Routing Protocols

Link State Routing When initialized, router determines link cost on each interface Router advertises these costs to all other routers in topology Router monitors its costs When changes occurs, costs are re-advertised Each router constructs topology and calculates shortest path to each destination network Not distributed version of routing algorithm Can use any algorithm Dijkstra Chapter 11 Interior Routing Protocols

Flooding Packet sent by source router to every neighbor Incoming packet resent to all outgoing links except source link Duplicate packets already transmitted are discarded Prevent incessant retransmission All possible routes tried so packet will get through if route exists Highly robust At least one packet follows minimum delay route Reach all routers quickly All nodes connected to source are visited All routers get information to build routing table High traffic load Chapter 11 Interior Routing Protocols

Figure 11.10 Flooding Example

OSPF Overview Router maintains descriptions of state of local links Transmits updated state information to all routers it knows about Router receiving update must acknowledge Lots of traffic generated Each router maintains database Directed graph Chapter 11 Interior Routing Protocols

Router Database Graph Vertices Router Network Transit Stub Edges Connecting two routers Connecting router to network Built using link state information from other routers Chapter 11 Interior Routing Protocols

Figure 11.11 Sample Autonomous System

Figure 11.12 Directed Graph of Autonomous System of Figure 19.7

Link Costs Cost of each hop in each direction is called routing metric OSPF provides flexible metric scheme based on type of service (TOS) Normal (TOS) 0 Minimize monetary cost (TOS 2) Maximize reliability (TOS 4) Maximize throughput (TOS 8) Minimize delay (TOS 16) Each router generates 5 spanning trees (and 5 routing tables) Chapter 11 Interior Routing Protocols

Figure 11.13 The SPF Tree for Router R6

Areas Make large internets more manageable Configure as backbone and multiple areas Area – Collection of contiguous networks and hosts plus routers connected to any included network Backbone – contiguous collection of networks not contained in any area, their attached routers and routers belonging to multiple areas Chapter 11 Interior Routing Protocols

Operation of Areas Each are runs a separate copy of the link state algorithm Topological database and graph of just that area Link state information broadcast to other routers in area Reduces traffic Intra-area routing relies solely on local link state information Chapter 11 Interior Routing Protocols

Inter-Area Routing Path consists of three legs Within source area Intra-area Through backbone Has properties of an area Uses link state routing algorithm for inter-area routing Within destination area Chapter 11 Interior Routing Protocols

Figure 11.14 OSPF Packet Header

Packet Format Notes Version number: 2 is current Type: one of 5, see next slide Packet length: in octets including header Router id: this packet’s source, 32 bit Area id: Area to which source router belongs Authentication type: null, simple password or encryption Authentication data: used by authentication procedure Chapter 11 Interior Routing Protocols

OSPF Packet Types Hello: used in neighbor discovery Database description: Defines set of link state information present in each router’s database Link state request Link state update Link state acknowledgement Chapter 11 Interior Routing Protocols

Required Reading Stallings chapter 11 Chapter 11 Interior Routing Protocols

Computer Networks with Internet Technology William Stallings

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