AN EXPLORATION OF
COUNT-TO-INFINITY PROBLEM
IN NETWORKS
R.KIRUTHIKA M.C.A., M.Phil.,
Assistant Professor in M.C.A.,Vivekanandha College of Arts and Sciences for Women, Triuchengode – 637 205, India.
Dr. R. UMARANI M.C.A., M.Phil., Ph.D.,
Associate Professor in Computer Science, Sri Sarada College for Women, Salem,India.
Abstract :
Nowadays routing is the vital problem while forwarding information from one node to another node in networks. Routing algorithm can be classified as distance vector and link state. Among distance vector and link state routing, distance vector routing algorithm forwards the information to each and every node from and to its neighbors. The distributed Bellman-Ford algorithm is a well known example of distance-vector algorithm (RIP) which calculates the shortest path. RIP (Routing Information Protocol) is a widely-used protocol for managing router information within a local area network (LAN) or an interconnected group of LANs. RIP is an Interior Gateway Protocol. Collecting and broadcasting the routing information may initiates count-to-infinity problem and routing loops. Distance Vector Routing algorithm has very much impact on MANET. MANET (Mobile Ad Hoc Network) consists of a collection of mobile nodes communicating in a multi-hop way without any fixed infrastructure such as access points or base stations. The main objective of this paper is a review on distance vector routing.
Keywords: distance-vector routing, link state routing, count-to-infinity, loops, RIP, MANET.
1. Introduction
Routing protocols can be classified in to distance vector [4] and link-state protocols [4]. This routing protocol selects the best routing path based on a distance metric and an interface (the vector) or selects the best routing path by calculating the state of each link in a path.
This paper is organized as follows: Section II introduces routing methods such as link state routing and distance vector routing. Section III discusses on to-infinity problem. Section IV gives solutions to count-to-infinity problem in wired networks.Section V explores the solutions to count-to-infinity problem in wireless network.
2. Routing Methods
2.1. Link State Routing
2.2.Distance Vector Routing
The Distance vector uses the Bellman Ford algorithm [1] for finding the shortest paths. Every node counts all paths to every other router. The shortest path is based on a metric. Sometimes the metric is referred in the routing literature as a count or a distance. The main features of distance-vector are iterative, distributed, and asynchronous [1]. In Distance vector the process of exchanging information is done iteratively that is until no more information is exchanged between the neighborhoods, the information is distributed or received form one or more of its node directly from its neighbors and the algorithm does not require all of the nodes to operate in lock step (asynchronous) with each other.
In this paper the main emphasis is on distance vector routing. In distance vector each node maintains the distance or count from it, to its possible destination and sends a periodic routing update. For periodic routing updates the convergence time is slow. The slow convergence leads to count-to-infinity and routing loops problem.
Bellman-Ford algorithm
Bellman-Ford algorithm solves the single-source shortest-path problem in the general case in which edges of a given digraph can have negative weight as long as G contains no negative cycles. it uses d[u] as an upper bound on the distance d[u, v] from u to v. The algorithm progressively decreases an estimate d[v] on the weight of the shortest path from the source vertex s to each vertex v in V until it achieve the actual shortest-path. The algorithm returns Boolean TRUE if the given digraph contains no negative cycles that are reachable from source vertex s otherwise it returns Boolean FALSE.
BELLMAN-FORD (G, w, s)
1. INITIALIZE-SINGLE-SOURCE (G, s) 2. for each vertex i= 1 to V [G] - 1 do 3. for each edge (u, v) in E[G] do 4. RELAX (u, v, w)
5. for each edge (u, v) in E [G] do 6. if d[u] + w (u, v) < d[v] then 7. return FALSE
8. return TRUE
3. Count-to-Infinity
We begin our discussion with a simple example for count-to-infinity [3]. Consider the network shown in the Fig.1 denotes the distance form node A to every other node. All nodes have routes to all other nodes and can be counted with the algorithm. The link cost is "1".
Now the link between nodes C and D breaks Fig.3. The algorithm now tries to re-compute the route to D.
When the link breaks, C counts that its shortest path to D is through B Fig.4 and sends the data to B. C increases its cost to B’s + 1 = 3.
When node B receives the data, it counts its shortest path to node D and finds out that it is through A and C which computes the shortest paths Fig.5. B increases its cost to A’s + 1 = 4.
When node A receives the data, it counts its shortest path to node D and finds out that it is through B which computes the shortest paths Fig.6. A increases its cost to B’s + 1 = 5.
The entire node believes that one or all the neighbors have a shortest path to the node D and update their routing tables accordingly. The counting will not stop Fig.7, the result of that algorithm leads to count-to-infinity.
4. Solutions for Wired Networks[5]
4.1. Defining the Maximum Count
4.2. Split Horizon
Routing Information Protocol uses Split Horizon. In Split Horizon actual distance to a destination is not reported to the node through which it reaches the destination. For example if node A has learned a route to node C through B, then A does not send the distance vector of C to node B during a routing update.
4.3.Poisoned Reverse
Poisoned Reverse is an additional technique which is used with Split Horizon. With route poisoning when a node detects that one of its routes has failed, the node will poison the route by putting the cost of the link to infinity to it and advertising it to neighbors.
When a node advertises a poisoned route to its neighbor’s nodes, the neighbor’s nodes break the rule of split horizon and send back to the originator the same poisoned route, called a poison reverse. Split horizon with poisoned reserve prevents routing loops involving only two nodes, for loops involving more nodes on the same link split horizon with poisoned reverse will not be sufficient.
4.4.Triggered Updates
Triggered updates states that "counting to infinity is still possible", even with these mechanisms. When a node receives a update, it counts the shortest path again. If the counting results in as a change of the metric, that is, a new shortest path is found, the node sends updates to the network. The problem with this approach is that routes can change also while the triggered updates are being sent.
4.5.Hold Down Timer
The count to infinity problem can be avoided by using hold down timers. This is a clock that is set within the node to help ensure network stability. When a node receives an update from a neighbor indicating that a previously accessible network is not working and is inaccessible, the hold-down timer will start. If a new update arrives from a neighbor with a better metric than the original network entry, the hold-down is removed and data is passed. But an update is received from a neighbor node before the hold-down timer expires and it has a lower metric than the previous route, therefore the update is ignored and the hold-down timer keeps ticking. This allows more time for the network to converge.
5. Solutions for Wireless Networks
5.1 Sequence Number
The sequence numbers gives better solutions to count-to-infinity problem. The sequence number is used to decide whether the routing table is updated or not. When a routing message is received, the message is compared to the routing table, if the advertised route doesn’t exist in routing table; the message is added to routing table. If a route exists, then the sequence numbers from the routing table and the message are compared, if the sequence numbers are equal the routing message is discarded unless the metric is better, that is a smaller hop count. If the advertised sequence number is smaller than the number in the routing table, the old route is replaced with the new route. The mobile ad hoc network protocol DSDV [2] uses the concept of sequence number which gives the better solution to count-to-infinity problem.
DSDV protocol is a proactive protocol which means that it derives routing information before the routes are needed. The DSDV can be considered as an extension to the imaginary distance vector protocol.
6. Conclusions
References
[1] Alberto Leon-Garcia and Indra Widjaja. Communication Networks, Fundamental Concepts and Key Architectures.
McGraw´-Hill Higher Education, Singapore, International Editions 2000. ISBN 0-07-022839-6.
[2] Charles E. Perkins. Ad hoc networking. Addison-Wesley, 2001.ISBN 0-201-30976-9.
[3] Christian Huitema. Routing in the Internet, 2nd Edition. Prentice Hall, Upper Saddle River, 2000. ISBN 0-13-022647-5.
[4] Feamster N, Balakrishnan H, Rexford J, Shaikh A, and Van der Merwe K, “The case for separating routing from routers”, in
Proc.ACM SIGCOM Worskshop FDNA, Portland, OR, Sep.2004, pp.5-12.
[5] Hendrick C. Routing Information Protocol. RFC 1058, IETF Network Working Group, June 1988. Category:Historical.
[6] Schiller, Jochen H Mobile Communication Second Edition, Pearson-Education Ltd., ©2003. ISBN: 9780321123817
Authors Profile
Ms. R.Kiruthika received her M.Phil., degree from Vinayaka Mission University, Salem in 2007. This
author is working as Assistant Professor in M.C.A. Department, Vivekanandha College of Arts and Sciences for women, Tiruchengode, Tamil Nadu.India. She is currently guiding M.C.A, M.Phil research scholars of various universities. Here areas of interest include information security and mobile computing.
