IETF MANET Working Group Mario Gerla INTERNET-DRAFT Xiaoyan Hong Expiration: November 17, 2001 Li Ma University of California, Los Angeles Guangyu Pei Rockwell Science Center May 17, 2001 Landmark Routing Protocol (LANMAR) for Large Scale Ad Hoc Networks Status of This Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026 except that the right to produce derivative works is not granted. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft is a submission to the IETF Mobile Ad Hoc Networks (MANET) Working Group. Comments on this draft may be sent to the Working Group at manet@itd.nrl.navy.mil, or may be sent directly to the authors. Abstract The Landmark Routing Protocol (LANMAR) utilizes the concept of "landmark" for scalable routing in large, mobile ad hoc networks. It relies on the notion of group mobility: i.e., a logical group (for example a team of coworkers at a convention) moves in a Gerla, Hong, Ma and Pei [Page 1] INTERNET-DRAFT Landmark Routing Protocol May 17, 2001 coordinated fashion. The existence of such logical group can be efficiently reflected in the addressing scheme. It assumes that an IP like address is used consisting of a group ID (or subnet ID) and a host ID, i.e. . A landmark is dynamically elected in each group. The route to a landmark is propagated throughout the network using a Distance Vector mechanism. Separately, each node in the network uses a "scoped" routing algorithm (e.g., FSR) to learn about routes within a given (max number of hops) scope. To route a packet to a destination outside its scope, a node will direct the packet to the landmark corresponding to the group ID of such destination. Once the packet is within the scope of the landmark, it will typically be routed directly to the destination. Remote groups of nodes are "summarized" by the corresponding landmarks. The solution to drifters (i.e., nodes outside of the scope of their landmark) is also handled by LANMAR. Landmark dynamic election enables LANMAR to cope with mobile environments. Thus, by using the truncated local routing table and the "summarized" landmark distance vector, LANMAR dramatically reduces routing table size and update overhead in large nets. LANMAR is well suited to provide an efficient and scalable routing solution in large, mobile, ad hoc environments in which group behavior applies and high mobility renders traditional routing schemes inefficient. Contents Status of This Memo . . . . . . . . . . . . . . . . . . . . . . . 1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Changes ....................................................... 5 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1. General Terms . . . . . . . . . . . . . . . . . . . . . 5 3.2. Specification Language . . . . . . . . . . . . . . . . . 5 4. Protocol Applicability . . . . . . . . . . . . . . . . . . . . 6 4.1. Networking Context . . . . . . . . . . . . . . . . . . . 6 4.2. Protocol Characteristics and Mechanisms . . . . . . . . 6 5. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 8 5.1. Protocol Descriptions . . . . . . . . . . . . . . . . . 8 5.2. Landmark Election . . . . . . . . . . . . . . . . . . . 9 5.3. Drifters . . . . . . . . . . . . . . . . . . . . . . . . 10 Gerla, Hong, Ma and Pei [Page 2] INTERNET-DRAFT Landmark Routing Protocol May 17, 2001 6. Protocol Specifications . . . . . . . . . . . . . . . . . . . 10 6.1. Data Structures . . . . . . . . . . . . . . . . . . . 11 6.1.1 Landmark Status tuple . . . . . . . . . . . . . . 11 6.1.2 Landmark Distance Vector . . . . . . . . . . . . 11 6.1.3 Drifter List . . . . . . . . . . . . . . . . . . 11 6.1.4 Neighbor List . . . . . . . . . . . . . . . . . . 12 6.2. LANMAR Update Message Format . . . . . . . . . . . . 12 6.2.1 Description of the fields . . . . . . . . . . . . 12 6.3. Processing Landmark Updates . . . . . . . . . . . . . . 13 6.3.1 Originating a Landmark in a Subnet . . . . . . . 13 6.3.2 Receiving Landmark Updates . . . . . . . . . . . 14 6.3.3 Winner Competition . . . . . . . . . . . . . . . 14 6.4. Processing Drifter Updates . . . . . . . . . . . . . . . 14 6.4.1 Originating a Drifter Entry . . . . . . . . . . . 15 6.4.2 Receiving Drifter Updates . . . . . . . . . . . . 15 6.4.3 Removing a Drifter Entry . . . . . . . . . . . . 15 6.5. Processing Neighbor List . . . . . . . . . . . . . . . . 15 7. Data Packet Forwarding . . . . . . . . . . . . . . . . . . . . 15 8. Discussion about Storage and Processing Overhead for Drifters 16 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 16 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Chair's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 1. Introduction This document describes the Landmark Routing Protocol (LANMAR) [1,2] developed by the Wireless Adaptive Mobility (WAM) Laboratory [4] at Computer Science Department, University of California, Los Angeles. The concept of landmark routing was first introduced in wired area networks [6]. The original scheme required predefined multi-level hierarchical addressing. The hierarchical address of each node reflects its position within the hierarchy and helps to find a route to it. Each node knows the routes to all the nodes within its hierarchical partition. Moreover, each node knows the routes to various "landmarks" at different hierarchical levels. Packet forwarding is consistent with the landmark hierarchy and the path is gradually refined from the top level hierarchy to lower levels as a packet approaches its destination. Gerla, Hong, Ma and Pei [Page 3] INTERNET-DRAFT Landmark Routing Protocol May 17, 2001 LANMAR borrows the concept of landmark and extends it to the wireless ad hoc environment. LANMAR scheme does not require predefined hierarchical address, but it uses the notion of landmarks to keep track of logical subnets in which the members have a commonality of interests and are likely to move as a "group" (e.g., brigade in the battlefield, a group of students from same class and a team of co-workers at a convention). Each such logical group has an elected landmark. For each group, underlying "scoped" routing algorithm will provide a one-level scope. The routing update packets are restricted only within the scope. Accurate routing information for nodes within scope is maintained. The routing information to remote nodes (nodes outside the node's scope) is "summarized" by the corresponding landmarks. Thus, the LANMAR scheme largely reduces the routing table size and the routing update traffic overhead. It greatly improves scalability. In addition, in order to recover from landmark failures, a "landmark" node is elected in each subnet. Landmark election provides a flexible way for the LANMAR protocol to cope with a dynamic and mobile network. The protocol also provides a solution for drifters (Nodes that are outside the fisheye scopes of the landmarks of their logical groups). Extra storage, processing and line overhead will be incurred for landmark election and drifter bookkeeping. However, the design of the algorithms provides solutions without compromising scalability. For example, the routing overhead for handling drifters is typically small if the fraction of drifting nodes is small. More analysis is given in Section 8. The LANMAR runs on top of a proactive routing protocol. It requires that the underlying routing protocol support the scoped subnetworking. Fisheye State Routing Protocol (FSR) [7,8] is such a protocol that supports LANMAR. In FSR, the link state protocol is used within the scope. The fisheye scope technique can also be applied to a distance vector type protocol, such as DSDV [3], in which the hop distance can be used to bind the scope for routing message updating. The main advantage of LANMAR is that the routing table includes only the nodes within the scope and the landmark nodes. This feature greatly improves scalability by reducing routing table size and update traffic O/H. Thus the Landmark Routing Protocol provides an efficient and scalable routing solution for a mobile, ad hoc environment while keeping line and storage overhead (O/H) low. Moreover, the election provides a much needed recovery from landmark failures. Gerla, Hong, Ma and Pei [Page 4] INTERNET-DRAFT Landmark Routing Protocol May 17, 2001 2. Changes Major changes from version 00 to version 01: - A destination sequence number for each landmark is used to ensure loop-free updates for a particular landmark. - Landmark updates are propagated in separate messages, instead of being piggybacked on local routing updates. This modification decouples landmark routing from the underlying proactive routing protocol. 3. Terminology 3.1. General Terms This section defines terminology used in LANMAR. node A MANET router that implements Landmark Routing Protocol. neighbor Nodes that are within the radio transmission range. scope A zone that is bounded by hop distances. host protocol A proactive protocol underlies the Landmark Routing Protocol. subnet Logical groups of nodes that present similar motion behavior. group This term is used interchangeably with subnet. 3.2. Specification Language The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [5]. Gerla, Hong, Ma and Pei [Page 5] INTERNET-DRAFT Landmark Routing Protocol May 17, 2001 4. Protocol Applicability 4.1. Networking Context LANMAR is best suited for large scale mobile ad hoc wireless networks. The landmark scheme on top of "scoped" routing algorithm has large advantages in reducing routing update packet size and keeping progressive accurate routes to remote nodes. It achieves high data packet delivery ratio. Moreover, the fact that the route error is blurred by distance obviously reduces the sensitivity to network size. LANMAR is also suited for high mobility ad hoc wireless networks. This is because in a mobility environment, a change on a link far away from the source does not necessarily cause a change in the routing table at the source and that all the information about remote nodes is summarized by landmarks. 4.2. Protocol Characteristics and Mechanisms * Does the protocol provide support for unidirectional links?(if so, how?) No. * Does the protocol require the use of tunneling? (if so, how?) No. * Does the protocol require using some form of source routing? (if so, how?) No. * Does the protocol require the use of periodic messaging? (if so, how?) Yes. The LANMAR periodically broadcasts landmark information to its neighbors. * Does the protocol require the use of reliable or sequenced packet delivery? (if so, how?) No. As the packets are sent periodically, they need not be sent reliably. * Does the protocol provide support for routing through a multi- Gerla, Hong, Ma and Pei [Page 6] INTERNET-DRAFT Landmark Routing Protocol May 17, 2001 technology routing fabric? (if so, how?) Yes. It is assumed that each node's network interface is assigned a unique IP address. * Does the protocol provide support for multiple hosts per router? (if so, how?) Yes. The router that has multiple hosts can use network ID of these hosts as the address to participate LANMAR. * Does the protocol support the IP addressing architecture? (if so, how?) Yes. Each node is assumed to have a unique IP address (or set of unique IP addresses in the case of multiple interfaces). The LANMAR references all nodes/interfaces by their IP address. This version of the LANMAR also supports IP network addressing (network prefixes) for routers that provide access to a network of non-router hosts. * Does the protocol require link or neighbor status sensing (if so, how?) No. * Does the protocol have dependence on a central entity? (if so, how?) No. * Does the protocol function reactively? (if so, how?) No. * Does the protocol function proactively? (if so, how?) Yes. The LANMAR proactively maintains landmark information at each node. * Does the protocol provide loop-free routing? (if so, how?) Yes. For in-scope destinations, the protocol uses routing paths learned from the host protocol. If the host protocol provides loop-free routing, e.g., FSR and DSDV, so does LANMAR. For out-scope destinations, only routes to landmarks are used. Because these routes are DSDV, it is loop free. When a packet approaches the destination, in-scope routes are used again. Gerla, Hong, Ma and Pei [Page 7] INTERNET-DRAFT Landmark Routing Protocol May 17, 2001 * Does the protocol provide for sleep period operation?(if so, how?) Yes. However, this requires TDMA MAC layer support. The router can be scheduled to sleep during idle periods. * Does the protocol provide some form of security? (if so, how?) Yes. When a node broadcasts routing update message, only entries of in-scope nodes and landmarks are included. This will prevent other remote nodes from being heard. * Does the protocol provide support for utilizing multi-channel, link-layer technologies? (if so, how?) Yes. In fact, the multi-channel can be used to separate routing messages from user data packets. 5. Protocol Overview 5.1. Protocol Descriptions As mentioned in Section 1, the landmark concept we adopt here uses the notion of logical subnets in which the members have a commonality of interests and are likely to move as a "group". Each logical subnet has one node serving as a "landmark" of that subnet. The protocol requires that the landmark of each subnet have the knowledge of all the members in its group. The LANMAR protocol also uses a scope at each node. The size of the scope is a parameter measured in hop distance. It is chosen in such a way that if a node is at the center of a subnet, the scope will cover the majority of the subnet members. If the shape of a subnet is likely to be a cycle, the center node's scope will cover all the members of the subnet. If this center node is elected as a landmark, it fulfills the requirement of the protocol. The elected landmark uses a destination sequence number to ensure its routing entry update is loop-free. The landmarks are propagated in a distance vector mechanism. Each node maintains a distance vector for landmarks of all the subnets. The size of the landmark distance vector equals to the number of logical subnets and thus landmark nodes. If a landmark does not locate at the center, there will be some members drifting off its scope. The landmark will keep a distance vector for drifters of its group. The distance vectors for landmarks and drifters are exchanged among neighbors in periodical routing update packets. The LANMAR relies on an underlying proactive protocol with the Gerla, Hong, Ma and Pei [Page 8] INTERNET-DRAFT Landmark Routing Protocol May 17, 2001 ability of providing "scoped" routing. In the scoped routing, each node broadcasts routing information periodically to its immediate neighbors. In each update packet, the node sends routing table entries within its scope. The host routing protocol uses sequence numbers for routing entries. Each node advances its sequence number before sending an update packet. Through the exchange process, the table entries with larger sequence numbers replace the ones with smaller sequence numbers. Let's take, for example, the FSR as our host protocol. For nodes within the scope, the updating is in a certain frequency. But for nodes beyond the scope, the update frequency is reduced to zero; Only the update frequency of the landmark nodes remains unaltered. As a result, each node maintains accurate routing information for in-scope nodes and keep routing directions to the landmark nodes for out-scope nodes, or say, for remote groups. A packet directed to a remote destination initially aims at the landmark of that remote group; as it gets closer to the landmark, it may eventually switch to the accurate route to the destination provided by in-scope nodes of the destination. 5.2. Landmark Election Dynamic election/re-election of landmark node is essential for LANMAR to work in a wireless mobile environment. Basically, each node tracks other nodes of its group in its scope and computes "weight", e.g. the number of the nodes it has found. At the beginning of the LANMAR, no landmark exists. Protocol LANMAR only uses the host protocol functionality. As host routing computation progresses, one of the nodes will learn (from the host protocol's routing table) that more than a certain number of group members (say, T) are in its scope. It then proclaims itself as a landmark for this group and adds itself to the landmark distance vector. Landmarks broadcast the election weights to the neighbors jointly with the landmark update packets. When more than one node declares itself as a landmark in the same group, as the landmark information floods out, each node will perform a winner competition procedure. Only one landmark for each group will survive and it will be elected. To avoid flapping between landmarks (very possible in a mobile situation), we use hysteresis in the replacement of an existing landmark. I.e., the old Landmark is replaced by the new one only if its weight is, say less than 1/2 of the weight of the current election winner. Once ousted, the old leader needs the full weight superiority to be reinstated. This procedure is carried out periodically in the background (low Gerla, Hong, Ma and Pei [Page 9] INTERNET-DRAFT Landmark Routing Protocol May 17, 2001 overhead, anyway). At steady state, a landmark propagates its presence to all other nodes like a sink in DSDV. It is extremely simple and it converges (by definition). In a mobile environment, an elected landmark may eventually lose its role. The role shifting is a frequent event. In a transient period, there exist several landmarks in a single group. The transient period may be actually the norm at high mobility. This transient behavior can be drastically reduced by using hysteresis. When a landmark dies, its neighbors will detect the silence after a given timeout. The neighbors of the same group will then take the responsibilities as landmarks and broadcast new landmark information. A new round of landmark election then floods over the entire network. 5.3. Drifters Typically, all members in a logical subnet are within the scope of the landmark, thus the landmark has a route to all members. It may happen, however, that some of the members "drift off" the scope, for example, a tank in a battalion may become stranded or lost. To keep track of such "drifters", i.e., to make the route to them known to the landmark, the following modification to the routing table exchange is necessary. Each node, say i, on the shortest path between a landmark L and a drifter l associated with that landmark keeps a distance vector entry to l. Note that if l is within the scope of i, this entry is already included in the routing table of node i. When i transmits its distance vector to neighbor, say j, then j will retain the entry to member l only if d(j,l) is smaller than the scope or d(j,L) is smaller than d(i,L). The latter condition occurs if j is on the shortest path from i (and therefore from l) to L. This way, a path is maintained from the landmark to each of its members, including drifters. The procedure starts from l, at the time when a node finds it becomes a drifter. It informs the landmark hop by hop about its presence. The occurrences of drifters are dynamic in a mobile network. In order to timely remove the staled drifter information, the time when a node hears a drifter is recorded. 6. Protocol Specifications This section discusses the operation of LANMAR routing protocol. The sending and receiving of landmark updates are in the proactive nature. The routing packets are processed separately from ordinary data packets. Gerla, Hong, Ma and Pei [Page 10] INTERNET-DRAFT Landmark Routing Protocol May 17, 2001 6.1. Data Structures Each node has a unique "logical" identifier defined by a subnet field and a host field. The host field is unique in the subnet and might in fact coincide with the physical address. The "logical" identifier can also be an IP address when the subnet address can logically group the nodes. Moreover, each node keeps a landmark status tuple. As LANMAR runs on top of a host routing protocol, it shares the underlying routing table structures. LANMAR maintains a neighbor list and shares it with the host protocol. In addition, LANMAR keeps a drifter list and a landmark distance vector. 6.1.1. Landmark Status Tuple Each node has not only a "logical" identifier, which basically is its address, but also a landmark status tuple. The tuple includes a flag which indicates whether the node is a landmark or not, a election weight (the number of group members the node detects within its scope) and a sequence number. When a node is elected, the status tuple will be copied to its landmark distance vector. The sequence number is advanced. There are three fields for a tuple: - Landmark flag - Number of group members in its scope - Sequence number 6.1.2. Landmark Distance Vector Landmark distance vector (LMDV) gives the next hop information to all landmarks in the network. Every subnet has an entry in LMDV. The latest route information to the landmark of each subnet is learned when a landmark update message is received. LMDV functions as a part of the routing table. It has the following fields: - Landmark status tuple - Next hop address - Distance 6.1.3. Drifter List The drifter list (DFDV) of LANMAR provides the next hop information of the drifters known to the current node. The entries are updated with landmark update message. The latest time a drifter is heard is recorded in DFDV. The DFDV works as a part of routing table. It has the following fields: - Destination drifter address - Next hop address - Distance - Last heard time Gerla, Hong, Ma and Pei [Page 11] INTERNET-DRAFT Landmark Routing Protocol May 17, 2001 6.1.4. Neighbor List The neighbor list of LANMAR keeps current neighbor information for a node. The latest time a neighbor is heard is recorded. The neighbor list has the following fields: - Neighbor address - Neighbor landmark flag - Last heard time 6.2. LANMAR Update Message Format There is only one message type of LANMAR protocol. The messages are periodically exchanged with neighbors. They update the landmark distance vector LMDV and the drifter list DFDV. The processing of the LMDV and DFDV will be describe separately. The following format does not include the node's identifier because it can be obtained from IP Header. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Landmark Flag | N_landmarks | N_drifters | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Landmark Address 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Hop Address 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Distance 1 | N_members 1 | Sequence Number 1 : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : . . . : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Drifter Address 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Hop Address 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Distance 1 | ... : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : . . . | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 6.2.1. Description of the fields Landmark Flag The landmark flag of the sending node. Gerla, Hong, Ma and Pei [Page 12] INTERNET-DRAFT Landmark Routing Protocol May 17, 2001 N_landmarks The number of entries of the landmark distance vector. N_drifters The number of entries of the drifter list. Reserved The bits are set to '0' and are ignored on reception. Landmark Address 1, Next Hop Address 1, Distance 1, N_members 1 and Sequence Number 1 The first entry in the landmark distance vector. Landmark Address 1, N_members 1 and Sequence Number 1 are the status tuple of the destination landmark. Next Hop Address 1 and Distance 1 is the next hop and distance to the landmark. These fields are repeated N_landmarks times for each entry in landmark distance vector. Drifter Address 1, Next Hop Address 1 and Distance 1 The first entry in the drifter list. Next Hop Address 1 and Distance 1 are the next hop and distance to the Drifter Address 1. These fields are repeated N_drifters times for each entry in the drifter list. The length of the message is limited to the maximum IP packet size. In that case, multiple packets may be required to broadcast all the entries. 6.3. Processing Landmark Updates Landmark update information is a part of the LANMAR periodic routing update message. The update information includes sender's LMDV. Landmark update message is used for landmark election and building paths to landmarks. 6.3.1. Originating a Landmark in a Subnet Every time a node detects a neighbor change, it recalculates the number of group members in its scope. The new number of group Gerla, Hong, Ma and Pei [Page 13] INTERNET-DRAFT Landmark Routing Protocol May 17, 2001 members is recorded in its election weight field. If this number is greater than a threshold T, the node qualifies as a landmark only when it is the only landmark for the group so far, or it wins the election when competing with the existing landmark. When it becomes a landmark, it increases its sequence number by 2. The old landmark entry is replaced with the new winner. The landmark entry will be broadcast to neighbors with the next update packet. Every time before a landmark sends updates, it increases its sequence number and copies it to LMDV. 6.3.2. Receiving Landmark Updates When a node receives a landmark update message, it compares its LMDV entries with the incoming LMDV updates for each subnet. A landmark update corresponding to a new subnet will be copied. An update having the same landmark as already given (in node's LMDV) will be accepted only if it contains a larger sequence number. If an update contains a different landmark for the same subnet as recorded in LMDV, only one landmark will be elected through a winner competition algorithm. LMDV will be updated according to the outcomes of the competition. 6.3.3. Winner Competition When more than one node declares itself as a landmark in the same group, a simple solution is to let the node with the largest number of group members win the election and in case of tie, lowest ID breaks the tie. The other competing nodes defer. However, this method is likely to cause the oscillation of landmark roles between nodes. To use hysteresis in replacing an existing landmark, let us assume the competing node's number of members is M, the existing landmark's number of members is N and a factor value S. When M is greater than N*S, then the competing node replaces the existing landmark. Or, when N reduces to a value smaller than a threshold T, then it gives up the landmark role. A tie occurs when M falls within an interval [N*1/S, N*S], then the node with larger member number wins the election. If a tie occurs again with equal member number, i.e., M equals to N, it is broken using lowest ID. A tie can always be broken using lowest ID as the address is used as ID and it is unique. 6.4. Processing Drifter Updates Drifter update information is a part of the LANMAR periodical routing update message. The update information is the drifter list (DFDV) of the sender. The computation of the DFDV at each node Gerla, Hong, Ma and Pei [Page 14] INTERNET-DRAFT Landmark Routing Protocol May 17, 2001 includes checking the node itself to see whether it is a drifter and recording paths to other drifters. 6.4.1. Originating a Drifter Entry By checking the distance to the landmark of its group, each node easily knows whether it has become a drifter. If the distance is larger than the scope, the node will put itself into its drifter list. This drifter information will be sent back to the landmark hop by hop along the shortest path to it which can be learned from the LMDV. For each drifter, only the node on its shortest path to the landmark needs to receive its information, so before the entry is broadcast, the next hop to landmark is attached with its entry. The DFDV will be propagated with the next update packet. 6.4.2. Receiving Drifter Updates Upon receiving an update packet, the DFDV part is retrieved and processed. If an entry of incoming DFDV indicates that the current node is its next hop to the landmark, i.e., the current node is on the drifter's shortest path to the landmark, the current node will insert or update its drifter list. The receiving time is stamped in the DFDV. The node sending the update packet is recorded as the next hop to the drifter. The reverse path to the drifter is thus built up. The procedure ends when the landmark receives the drifter entry. The updated DFDV will be propagated with the next update packet. 6.4.3. Removing a Drifter Entry Each entry in DFDV is time stamped of its last receiving time. Every time before the DFDV is sent or routing by DFDV is needed, the table is checked for staled entries. If such an entry is found, it is removed. 6.5. Processing Neighbor List When a node receives either a landmark update or a host protocol routing update, the neighbor list is inserted if the update comes from a new neighbor, or the corresponding neighbor entry is updated. Current time is recorded with the entry. The neighbor list is also search at this time for possible lost neighbors according to the time stamps. If such a neighbor is found, it is removed from the list. 7. Data Packet Forwarding Data packets are relayed hop by hop. The host protocol routing Gerla, Hong, Ma and Pei [Page 15] INTERNET-DRAFT Landmark Routing Protocol May 17, 2001 table, drifter list and landmark distance vector are looked up sequentially for the destination entry. If the destination is within a node's scope, the entry can be found directly in the routing table and the packet is forwarded to the next hop node. Otherwise, the drifter list DFDV is searched for the destination. If the entry is found, the packet is forwarded using the next hop address from DFDV. If not, the logical subnet field of the destination is retrieved and the LMDV entry of the landmark corresponding to the destination's logical subnet is searched. The data packet is then routed towards the landmark using the next hop address from LMDV. The packet, however, is not necessary to pass through the landmark. Rather, once the packet gets within the scope of the destination on its way towards the landmark, it is routed to the destination directly. 8. Discussion about Storage and Processing Overhead for Drifters The routing storage and processing overhead introduced by the distance vector extension to handle drifters is typically small if the fraction of drifting nodes is small. Consider a network with N nodes and L landmarks, and assume that a fraction F of the members of each logical subnet have drifted. In the worst case, the path length from landmark to drifter is the square root of N (assuming a grid topology). Thus, sqrt(N) is the bound on the number of extra routing entries required at the nodes along the path to the drifter. The total number of extra routing entries is sqrt(N)*L*(F*N/L) where N/L is the average logical group size. Thus, the extra storage per node is F*sqrt(N). Now, let us assume that the number of nodes in the scope = # of landmarks = logical group size = sqrt(N). Then, the basic routing table overhead per node (excluding drifters) is 3*sqrt(N). Thus, the extra overhead caused by drifters is F/3. If 20% of the nodes in a group are outside of the landmark scope, i.e., have drifted, the extra routing O/H required to keep track of them is only 7%. Acknowledgments This work was supported in part by NSF under contract ANI-9814675, by DARPA under contract DAAB07-97-C-D321, and by ONR under contract N00014-01-C-0016. References [1] G. Pei, M. Gerla and X. Hong, "LANMAR: Landmark Routing for Large Scale Wireless Ad Hoc Networks with Group Mobility", Proceedings of IEEE/ACM MobiHOC 2000, Boston, MA, Aug. 2000. Gerla, Hong, Ma and Pei [Page 16] INTERNET-DRAFT Landmark Routing Protocol May 17, 2001 [2] M. Gerla, X. Hong, G. Pei, "Landmark Routing for Large Ad Hoc Wireless Networks", Proceedings of IEEE GLOBECOM 2000, San Francisco, CA, Nov. 2000. [3] C.E. Perkins and P. Bhagwat, "Highly Dynamic Destination- Sequenced Distance-Vector Routing (DSDV) for Mobile Computers," In Proceedings of ACM SIGCOMM'94, London, UK, Sep. 1994, pp. 234-244. [4] UCLA Wireless Adaptive Mobility (WAM) Laboratory. http://www.cs.ucla.edu/NRL/wireless [5] S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. RFC 2119, March 1997. [6] P. F. Tsuchiya, "The Landmark Hierarchy: a new hierarchy for routing in very large networks", Computer Communication Review, vol.18, no.4, Aug. 1988, pp. 35-42. [7] G. Pei, M. Gerla, and T.-W. Chen, "Fisheye State Routing: A Routing Scheme for Ad Hoc Wireless Networks", Proceedings of ICC 2000, New Orleans, LA, Jun. 2000. [8] G. Pei, M. Gerla, and T.-W. Chen, "Fisheye State Routing in Mobile Ad Hoc Networks", Proceedings of Workshop on Wireless Networks and Mobile Computing, Taipei, Taiwan, Apr. 2000. Chair's Address The MANET Working Group can be contacted via its current chairs: M. Scott Corson Institute for Systems Research University of Maryland College Park, MD 20742 USA Phone: +1 301 405-6630 Email: corson@isr.umd.edu Joseph Macker Information Technology Division Naval Research Laboratory Washington, DC 20375 USA Gerla, Hong, Ma and Pei [Page 17] INTERNET-DRAFT Landmark Routing Protocol May 17, 2001 Phone: +1 202 767-2001 Email: macker@itd.nrl.navy.mil Authors' Addresses Questions about this document can also be directed to the authors: Mario Gerla 3732F Boelter Hall Computer Science Department University of California Los Angeles, CA 90095-1596 USA Phone: +1 310 825-4367 Fax: +1 310 825-7578 Email: gerla@cs.ucla.edu Xiaoyan Hong 3803F Boelter Hall Computer Science Department University of California Los Angeles, CA 90095-1596 USA Phone: +1 310 825-4623 Fax: +1 310 825-7578 Email: hxy@cs.ucla.edu Li Ma 3803D Boelter Hall Computer Science Department University of California Los Angeles, CA 90095-1596 USA Phone: +1 310 825-1888 Fax: +1 310 825-7578 Email: mary@cs.ucla.edu Guangyu Pei Rockwell Science Center 1049 Camino Dos Rios Gerla, Hong, Ma and Pei [Page 18] INTERNET-DRAFT Landmark Routing Protocol May 17, 2001 P.O. Box 1085 Thousand Oaks, CA 91358-0085 USA Phone: +1 805 373-4639 Fax: +1 805 373-4383 Email: gpei@rsc.rockwell.com Gerla, Hong, Ma and Pei [Page 19]