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Internet Draft                                                   V. Park
draft-ietf-manet-tora-spec-00.txt              Naval Research Laboratory
                                                               S. Corson
                                                  University of Maryland
                                                 Submitted: Nov 26, 1997
                                                 Expires:   May 26, 1997


         Temporally-Ordered Routing Algorithm (TORA) Version 1
                        Functional Specification

Status of this Memo

   This document is an Internet-Draft. 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
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   ftp.isi.edu (US West Coast).

Abstract

   This document provides a detailed specification of Version 1 of the
   Temporally-Ordered Routing Algorithm (TORA)--a distributed routing
   protocol for mobile, multihop, wireless networks. Its intended use is
   for routing of Internet Protocol (IP) datagrams within an autonmous
   system. The basic, underlying algorithm is neither a distance-vector
   nor a link-state; it is one of a family of algorithms referred to as
   ''link reversal'' algorithms. The protocol's reaction is structured as
   a temporally-ordered sequence of diffusing computations, each
   computation consisting of a sequence of directed link reversals. The
   protocol is highly adaptive, efficient and scalable; and is well-
   suited for use in large, dense, mobile networks. In these networks,
   the protocol's reaction to link failures typically involves only a
   localized ''single pass'' of the distributed algorithm. This desirable
   behavior is achieved through the use of a physical or logical clock
   to establish the ''temporal order'' of topological change events. The
   established temporal ordering is subsequently used to structure (or
   order) the algorithm's reaction to topological changes.



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1 Introduction

   The Temporally-Ordered Routing Algorithm (TORA) [1] is a highly
   adaptive routing protocol, which has been tailored for operation in a
   Mobile Ad hoc Network (MANET). Such a network can be envisioned as a
   collection of routers (equipped with wireless receiver/transmitters),
   which are free to move about arbitrarily. The status of the
   communication links between the routers, at any given time, is a
   function of their positions, transmission power levels, antenna
   patterns, cochannel interference levels, etc. The mobility of the
   routers and the variability of other connectivity factors result in a
   network with a potentially rapid and unpredictably changing topology.
   Congested links are also an expected characteristic of such a network
   as wireless links inherently have significantly lower capacity than
   hardwired links and are therefore more prone to congestion. TORA's
   design is predicated on the notion that a routing algorithm that is
   well-suited for operation in this environment should possess the
   following properties:
   *  Executes distributedly
   *  Provides loop-free routes
   *  Provides multiple routes (i.e., to reduce the frequency of
      reactions to topological changes and potentially to alleviate
      congestion)
   *  Establishes routes quickly (i.e., so they may be used before
      the topology changes)
   *  Minimizes communication overhead by localizing algorithmic
      reaction to topological changes when possible (i.e., to conserve
      available bandwidth and increase scalability)
   Routing optimality (i.e., determination of the shortest-path) is of
   less importance. It is also not necessary (nor desirable) to maintain
   routes between every source/destination pair at all times. The
   overhead expended to establish a route between a given
   source/destination pair will be wasted if the source does not require
   the route prior to its invalidation due to topological changes.

   TORA is based, in part, on the work presented in [2] and [3]. TORA is
   designed to minimize reaction to topological changes. A key concept
   in its design is that it decouples the generation of potentially
   far-reaching control message propagation from the rate of topological
   changes. Control messaging is typically localized to a very small set
   of nodes near the change without having to resort to a dynamic,
   hierarchical routing solution with its attendant complexity. TORA
   includes a secondary mechanism, which allows far-reaching control
   message propagation as a means of infrequent route optimization and
   soft-state route verification. This propogation occurs periodically
   at a very low rate and is independent of the network topology
   dynamics.




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   TORA is distributed in that nodes need only maintain information
   about adjacent nodes (i.e., one hop knowledge). It guarantees all
   routes are loop-free, and typically provides multiple routes for any
   source/destination pair that requires a route. TORA is "source
   initiated" and quickly creates a set of routes to a given destination
   only when desired. Since multiple routes are typically established
   and having a single route is sufficient, many topological changes
   require no reaction at all. Following topological changes that do
   require reaction, the protocol quickly re-establishes valid routes.
   This ability to initiate and react infrequently serves to minimize
   communication overhead. Finally, in the event of a network partition,
   the protocol detects the partition and erases all invalid routes.

2 MANET Routing Functional Description

   A protocol for routing packets in a MANET can be divided into two
   functional distinct components--a link status sensing mechanism and a
   routing mechanism. Although these two components are somewhat
   orthogonal, they can be designed to work together in a synergistic
   fashion. Originally, these two mechanisms were being designed
   together as a part of the TORA protocol specification. However, since
   the basic functionality provided by a link status sensing mechanism
   is required by a variety of different routing mechanisms, it seemed
   appropriate that a more generic link status sensing mechanism should
   be designed and specified as a separate, underlying  protocol. This
   underlying protocol--the Internet MANET Encapsulation Protocol (IMEP)
   [4]--is being designed to provide several other basic functions that
   are commonly required by a routing mechanism. Thus, TORA provides
   only the routing mechanism and depends on IMEP for other underlying
   functions. Should it later be decided that the two protocols should
   be combined, IMEP's functionality will be incorporated back into the
   TORA specification.

2.1 Internet MANET Encapsulation Protocol

   IMEP and TORA have been designed to work together synergistically.
   TORA relies on IMEP for the following underlying functions and
   services:
   *  Link status sensing (i.e., monitoring and maintaining the
      status of connectivity with the set of neighboring routers)
   *  Control packet delivery (i.e., reliable, in-order control packet
      delivery to the set of neighbors)
   *  Network-layer address resolution and mapping
   *  Security authentication
   IMEP provides a rich interface for use by a variety of different
   mobile wireless routing protocols, which may have varied needs for
   underlying services.




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2.2 Temporally-Ordered Routing Algorithm

   This section provides a functional description of TORA. A detailed
   specification of TORA is provided in a subsequent section.

2.2.1 Notation

   A network can be modeled as a graph with a finite set of nodes
   connected by a set of initially undirected links--wherea node
   represents a router and a link represents communication connectivity
   between two routers.  Each node i in the network is assumed to have a
   unique node identifier (ID), and each link (i, k) is assumed to allow
   two-way communication (i.e., nodes connected by a link can
   communicate with each other in either direction). Due to the mobility
   of the nodes, the set of links in the network is changing with time
   (i.e., new links can be established and existing links can be
   severed). Each initially undirected link (i, k) may subsequently be
   assigned one of three states; (1) undirected, (2) directed from node
   i to node k, or (3) directed from node k to node i. If a link (i, k)
   is directed from node i to node k, node i is said to be "upstream"
   from node k, while node k is said to be "downstream" from node i. For
   each node i, we define the "neighbors" of i, to be the set of nodes k
   such that there exists a link between nodes i and k. The following
   assumptions account for the functionality provided by the IMEP. It is
   assumed that each node i is always aware of its set of neighbors.
   Additionally, it is assumed that when a node i transmits a packet, it
   is broadcast to all of its neighbors and that all transmitted packets
   are received correctly and in order of transmission.

2.2.2 Foundation and Basic Structure

   A logically separate version of TORA is run for each destination to
   which routing is required. The following discussion focuses on a
   single version running for a given destination, j.

   TORA can be separated into three basic functions: creating routes,
   maintaining routes, and erasing routes. Creating a route from a given
   node to the destination requires establishment of a sequence of
   directed links leading from the node to the destination. This
   function is only initiated when a node with no directed links
   requires a route to the destination. Thus, creating routes
   essentially corresponds to assigning directions to links in an
   undirected network or portion of the network. The method used to
   accomplish this is an adaptation of the query/reply process described
   in [2], which builds a directed acyclic graph (DAG) rooted at the
   destination (i.e., the destination is the only node with no
   downstream links). Such a DAG will be referred to as a "destination-
   oriented" DAG. Maintaining routes refers to reacting to topological



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   changes in the network in a manner such that routes to the
   destination are re-established within a finite time--meaning that its
   directed portions return to a destination-oriented DAG within a
   finite time. Gafni and Bertsekas (GB) described two algorithms [3],
   which are members of a general class of algorithms designed to
   accomplish this task. However, the GB algorithms are designed for
   operation in connected networks. Due to instability exhibited by
   these algorithms in portions of the network that become partitioned
   from the destination, they are deemed unacceptable for the current
   task. TORA incorporates a new algorithm, in the same general class,
   that is more efficient in reacting to topological changes and capable
   of detecting a network partition. This leads to the third function--
   erasing routes. Upon detection of a network partition, all links (in
   the portion of the network that has become partitioned from the
   destination) must be marked as undirected to erase invalid routes.

   TORA accomplishes these three functions through the use of three
   distinct control packets: query (QRY), update (UPD), and clear (CLR).
   QRY packets are used for creating routes; UPD packets are used for
   both creating and maintaining routes; and CLR packets are used for
   erasing routes.

2.2.3 General Class of Algorithms

   It is beneficial at this point to briefly review the earlier GB
   algorithms. Consider a connected DAG with at least one node (in
   addition to the destination) that has no downstream links. Such a DAG
   will be referred to as "destination-disoriented." The following
   excerpts (punctuation added for clarity) from [3] loosely describe
   the two algorithms designed to transform a destination-disoriented
   DAG into a destination-oriented DAG.

   Full Reversal Method: At each iteration, each node (other than the
   destination) that has no outgoing links reverses the direction of all
   its incoming links.

   Partial Reversal Method: Every node i (other than the destination)
   keeps a list of its neighboring nodes k that have reversed the
   direction of the corresponding links (i, k). At each iteration, each
   node i that has no outgoing links reverses the directions of the
   links (i, k), for all k which do not appear on its list, and empties
   the list. If no such k exists (i.e., the list is full), node i
   reverses the directions of all incoming links and empties the list.

   These two algorithms are subsequently re-stated in the context of a
   generalized numbering scheme that will be summarize here; however,
   much detail will be left out. For a thorough understanding, one
   should review the original paper. Essentially, a value is associated



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   with each node at all times, and the values are such that they can be
   totally ordered. For example, in the full reversal method, a pair
   (alpha[i], i) is associated with each node where i is the unique ID
   of the node and alpha[i] is an integer. The pairs can then be totally
   ordered lexicographically (e.g. (alpha[i], i) > (alpha[k], k) if
   alpha[i] > alpha[k] ,or if alpha[i] = alpha[k] and i > k). The value
   associated with each node i will be referred to as its "height" and
   denoted h[i]. Now, assume that we assign an initial height to each
   node in the destination-disoriented DAG such that node i is upstream
   from node k if and only if h[i] > h[k]. Then it is clear that node i
   has no downstream links when, measured by its height, it is a local
   minimum with respect to its neighbors, h[i] < h[k] for all neighbors
   k. To achieve the desired behavior in the full reversal method, node
   i must select a new height such that it becomes a local maximum with
   respect to its neighbors (i.e., h[i] > h[k] for all neighbors k).
   Node i simply selects a new value alpha[i] = max {alpha[k] such that
   k is a neighbor of i}+1 and broadcasts the value to all of its
   neighbors. The partial reversal method can neither be viewed
   conceptually nor explained as easily. Again, a node selects a new
   height only when it is a local minimum, but it does not always become
   a local maximum. To reverse only some of its links (i.e., partial
   reversal), a node selects a new height that is higher than its own
   previous height and the height of some of its neighbors, but not
   higher than the height of all of its neighbors.

   The algorithms that belong to this general class are shown to be
   loop-free, and terminate in a finite number of iterations to a
   destination-oriented DAG [3]. Furthermore, only nodes that have lost
   all downstream paths to the destination react to a given failure. The
   new algorithm incorporated into TORA for the maintaining routes
   process is a member of this class, and thus inherits many of these
   properties. The new algorithm is similar to the partial reversal
   method in that it often reverses only some of its links. However, in
   order to provide a partition detection capability, the rules for the
   selection of a new height are significantly more complex. These rules
   are discussed in detail in section 2.2.4.2.

   The basic idea is as follows. When a node loses its last downstream
   link (i.e., becomes a local minimum) as a result of a link failure,
   the node selects a new height such that it becomes a global maximum
   by defining a new "reference level". By design, when a new reference
   level is defined, it is higher than any previously defined reference
   levels. This action results in link reversals that may cause other
   nodes to lose their last downstream link. Any such node executes a
   partial reversal with respect to its neighbors that have heights
   already associated with the newest (highest) reference level. In this
   manner, the new reference level is propagated outward from the point
   of the original failure (re-directing links in order to re-establish



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   routes to the destination). This propagation will only extend through
   nodes that (as a result of the initial link failure) have lost all
   routes to the destination. Some nodes may experience link reversals
   from all neighbors (as a result of the same initial link failure).
   Any such node must select a new height such that it becomes a local
   maximum. This is accomplished by defining a higher sub-level
   associated with the new reference level, which will be referred to as
   the "reflected reference level". This node essentially "reflects"
   this higher sub-level back toward the node that originally defined
   the new reference level. Should this reflected reference level be
   propagated back to the originating node from all of its neighbors,
   then it is determined that no route to the destination exists. The
   originating node has then detected a partition and can begin the
   process of erasing the invalid routes.

2.2.4 Detailed Description

   At any given time, an ordered quintuple, HEIGHT = (tau[i], oid[i],
   r[i], delta[i], i), is associated with each node i, where i is the
   unique ID of the node. Conceptually, the quintuple associated with
   each node represents the height of the node as defined by two
   parameters: a reference level and a offset with respect to the
   reference level. The reference level is represented by the first
   three values in the quintuple, while the offset is represented by the
   last two values. A new reference level is defined each time a node
   loses its last downstream link due to a link failure. The first value
   representing the reference level, tau[i], is a time tag set to the
   "time" of the link failure. For now, it is assumed that all nodes
   have synchronized clocks. This could be accomplished via interface
   with an external time source such as the Global Positioning System
   (GPS) [5] or through use of an algorithm such as the Network Time
   Protocol [6]. This time tag need not actually indicate or be "time,"
   nor will relaxation of the synchronization requirement invalidate the
   protocol. The second value, oid[i], is the originator-ID (i.e., the
   unique ID of the node that defined the new reference level). This
   ensures that the reference levels can be totally ordered
   lexicographically, even if multiple nodes define reference levels due
   to failures that occur simultaneously (i.e., with equal time tags).
   The third value, r[i], is a single bit used to divide each of the
   unique reference levels into two unique sub-levels. This bit is used
   to distinguish between the original reference level and its
   corresponding, higher, reflected reference level. When a distinction
   is not required, both the original and reflected reference levels
   will simply be referred to as "reference levels." The first value
   representing the offset, delta[i], is an integer used to order nodes
   with respect to a common reference level. This value is instrumental
   in the propagation of a reference level. How delta is selected will
   be clarified in a subsequent section. Finally, the second value



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   representing the offset, i, is the unique ID of the node itself. This
   ensures that nodes with a common reference level and equal values of
   delta (and in fact all nodes) can be totally ordered
   lexicographically at all times.

   Each node i (other than the destination) maintains its height,
   HEIGHT. Initially the height of each node in the network (other than
   the destination) is set to NULL, HEIGHT = (-, -, -, -, i), where i is
   the unique ID of the node. Subsequently, the height of each node i
   can be modified in accordance with the rules of the protocol. The
   height of the destination j is always ZERO, HEIGHT = (0, 0, 0, 0, j),
   where j is the unique ID of the destination for which the algorithm
   is running). In addition to its own height, each node i maintains a
   height table with an entry HT_NEIGH[k] for each neighbor k. Initially
   the height of each neighbor is set to NULL, HT_NEIGH[k] = (-, -, -,
   -, k). If the destination j is a neighbor of node i, node i sets the
   corresponding height entry to ZERO, HT_NEIGH[j] = (0, 0, 0, 0, j).

   Each node i (other than the destination) also maintains a link-status
   table with an entry LNK_STAT[k] for each link (i, k), where node k is
   a neighbor of node i. The status of the links is determined by the
   height of the node, HEIGHT, and its height entry for the neighbor,
   HT_NEIGH[k]. The link is directed from the higher node to the lower
   node. If a neighbor k is higher than node i, the link is marked
   upstream (UP). If a neighbor k is lower than node i, the link is
   marked downstream (DN). If the neighbor's height entry, HT_NEIGH[k],
   is NULL, the link is marked undirected (UN). Finally, if the height
   of node i is NULL, then any neighbor's height that is not NULL is
   considered lower, and the corresponding link is marked downstream
   (DN). When a new link (i, k) is established (i.e., node i has a new
   neighbor k), node i adds entries for the new neighbor to the height
   and link-status tables. If the new neighbor is the destination j, the
   corresponding height entry is set to ZERO, HT_NEIGH[j] = (0, 0, 0, 0,
   j); otherwise it is set to NULL, HT_NEIGH[k] = (-, -, -, -, k). The
   corresponding link-status entry, LNK_STAT[k], is set as outlined
   above. Nodes need not communicate any routing information upon link
   activation.

2.2.4.1 Creating Routes

   Creating routes requires use of the QRY and UPD packets. A QRY packet
   consists of the destination-ID, j, which identifies the destination
   for which the algorithm is running. An UPD packet consists of the
   destination-ID, j, and the height of the node i that is broadcasting
   the packet, HEIGHT.

   Each node i (other than the destination) maintains a route-required
   flag, which is initially un-set. Each node i (other than the



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   destination) also maintains the time at which the last UPD packet was
   broadcast and the time at which each link (i, k), where node k is
   neighbor of node i, became active.

   When a node with no directed links and an un-set route-required flag
   requires a route to the destination, it broadcasts a QRY packet and
   sets its route-required flag. When a node i receives a QRY it reacts
   as follows:

      a) If the receiving node i has no downstream links and its route-
      required flag is un-set, it re-broadcasts the QRY packet and sets
      its route-required flag.

      b) If the receiving node i has no downstream links and the route-
      required flag is set, it discards the QRY packet.

      c) If the receiving node i has at least one downstream link and
      its height is NULL, it sets its height to HEIGHT = (tau[k],
      oid[k], r[k], delta[k] + 1, i), where HT_NEIGH[k] = (tau[k],
      oid[k], r[k], delta[k], k) is the minimum height of its non-NULL
      neighbors, and broadcasts an UPD packet.

      d) If the receiving node i has at least one downstream link and
      its height is non-NULL, it first compares the time the last UPD
      packet was broadcast to the time the link over which the QRY
      packet was received became active. If an UPD packet has been
      broadcast since the link became active, it discards the QRY
      packet; otherwise, it broadcasts an UPD packet.

   If a node has the route-required flag set when a new link is
   established, it must broadcast a QRY packet.

   When a node i receives an UPD packet from a neighbor k, node i first
   updates the entry HT_NEIGH[k] in its height table with the height
   contained in the received UPD packet. Node i then updates the entry
   LNK_STAT[k] in its link-status table and reacts as follows:

      a) If the route-required flag is set (which implies that the
      height of node i is NULL), node i sets its height to HEIGHT =
      (tau[k], oid[k], r[k], delta[k] + 1, i)--where HT_NEIGH[k] =
      (tau[k], oid[k], r[k], delta[k], k) is the minimum height of its
      non-NULL neighbors, updates all the entries in its link-status
      table, un-sets the route-required flag and then broadcasts an UPD
      packet that contains its new height.

      b) If the route-required flag is not set, node i need only react
      if it has lost its last downstream link. The section on
      maintaining routes discusses the reaction that occurs if reception



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      of the UPD packet resulted in loss of the last downstream link.

2.2.4.2 Maintaining Routes

   Maintaining routes is only performed for nodes that have a height
   other than NULL. Furthermore, any neighbor's height that is NULL is
   not used for the computations. A node i is said to have no downstream
   links if HEIGHT < HT_NEIGH[k] for all non-NULL neighbors k. This will
   result in one of five possible reactions depending on the state of
   the node and the preceding event. Each node (other than the
   destination) that has no downstream links modifies its height, HEIGHT
   = (tau[i], oid[i], r[i], delta[i], i), as follows:

      Case 1 (Generate):

         Node i has no downstream links (due to a link failure).

         (tau[i], oid[i], r[i])=(t, i, 0), where t is the time of the
         failure.

         (delta[i],i)=(0, i)

         In essence, node i defines a new reference level. The above
         assumes node i has at least one upstream neighbor. If node i
         has no upstream neighbors it simply sets its height to NULL.

      Case 2 (Propagate):

         Node i has no downstream links (due to a link reversal
         following reception of an UPD packet) and the ordered sets
         (tau[k], oid[k], r[k]) are not equal for all neighbors k.

         (tau[i], oid[i], r[i])=max{(t[k], oid[k], r[k]) of all
         neighbors k}

         (delta[i],i)=(delta[m]-1, i), where m is the lowest neighbor
         with the maximum reference level defined above.

         In essence, node i propagates the reference level of its
         highest neighbor and selects a height that is lower than all
         neighbors with that reference level.

      Case 3 (Reflect):

         Node i has no downstream links (due to a link reversal
         following reception of an UPD packet) and the ordered sets
         (tau[k], oid[k], r[k]) are equal with r[k] = 0 for all
         neighbors k.



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         (tau[i], oid[i], r[i])=(tau[k], oid[k], 1)

         (delta[i],i)=(0, i)

         In essence, the same level (which has not been "reflected") has
         propagated to node i from all of its neighbors. Node i
         "reflects" back a higher sub-level by setting the bit r.

      Case 4 (Detect):

         Node i has no downstream links (due to a link reversal
         following reception of an UPD packet), the ordered sets
         (tau[k], oid[k], r[k]) are equal with r[k] = 1 for all
         neighbors k, and oid[k] = i (i.e., node i defined the level).

         (tau[i], oid[i], r[i])=(-, -, -)

         (delta[i],i)=(-, i)

         In essence, the last reference level defined by node i has been
         reflected and propagated back as a higher sub-level from all of
         its neighbors. This corresponds to detection of a partition.
         Node i must initiate the process of erasing invalid routes as
         discussed in the next section.

      Case 5 (Generate):

         Node i has no downstream links (due to a link reversal
         following reception of an UPD packet), the ordered sets
         (tau[k], oid[k], r[k]) are equal with r[k] = 1 for all
         neighbors k, and oid[k] != i (i.e., node i did not define the
         level).

         (tau[i], oid[i], r[i])=(t, i, 0), where t is the time of the
         failure

         (delta[i],i)=(0, i)

         In essence, node i experienced a link failure (which did not
         require reaction) between the time it propagated a reference
         level and the reflected higher sub-level returned from all
         neighbors. This is not necessarily an indication of a
         partition. Node i defines a new reference level.

   Following determination of its new height in cases 1, 2, 3, and 5,
   node i updates all the entries in its link-status table; and
   broadcasts an UPD packet to all neighbors k. The UPD packet consists
   of the destination-ID, j, and the new height of the node i that is



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   broadcasting the packet, HEIGHT. When a node i receives an UPD packet
   from a neighbor k, node i reacts as described in the creating routes
   section and in accordance with the cases outlined above. In the event
   of the failure a link (i, k) that is not its last downstream link,
   node i simply removes the entries HT_NEIGH[k] and LNK_STAT[k] in its
   height and link-status tables.

2.2.4.3 Erasing Routes

   Following detection of a partition (case 4), node i sets its height
   and the height entry for each neighbor k to NULL (unless the
   destination j is a neighbor, in which case the corresponding height
   entry is set to ZERO), updates all the entries in its link-status
   table, and broadcast a CLR packet. The CLR packet consists of the
   destination-ID, j, and the reflected reference level of node i,
   (tau[i], oid[i], 1). In actuality the value r[i] = 1 need not be
   included since it is always 1 for a reflected reference level. When a
   node i receives a CLR packet from a neighbor k it reacts as follows:

      a) If the reference level in the CLR packet matches the reference
      level of node i; it sets its height and the height entry for each
      neighbor k to NULL (unless the destination j is a neighbor, in
      which case the corresponding height entry is set to ZERO), updates
      all the entries in its link-status table and broadcasts a CLR
      packet.

      b) If the reference level in the CLR packet does not match the
      reference level of node i; it sets the height entry for each
      neighbor k (with the same reference level as the CLR packet) to
      NULL and updates the corresponding link-status table entries.
      Thus, the height of each node in the portion of the network that
      was partitioned is set to NULL and all invalid routes are erased.
      If (b) causes node i to lose its last downstream link, it reacts
      as in case 1 of maintaining routes.

3 Protocol Specification

   In the previous description of TORA, some simplifications were made
   for clarity. In particular, j was used to represent the unique ID of
   the destination for which the algorithm was running.  However, when
   forwarding IP datagrams in an internetwork, "destinations" to which
   routing is required are usually identified by an IP address and mask.
   Together, these two values may correspond to an individual interface
   on a specific machine, or an aggregation of addresses (e.g., a
   network address).  Thus, in the subsequent discussion a destination
   "j" refers to a typical IP destination.  Another significant
   simplification pertains to the link between to nodes. In the most
   general case, a MANET router may have multiple wireless and hardwired



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   interfaces with differing communication technologies.  Therefore, it
   is necessary to make a distiction between a physical communication
   "connection" between two routers and a logical communication "link"
   between two routers.  The previous description also ommited any
   discussion about how the next-hop forwarding decision is made.  It is
   assumed that the IP packet forwarding performed by the kernel in
   accordance with a standard IP routing table maintained in kernel
   space.  The TORA process must have access to the information in the
   table and be able to manipulate the table entries.  The details
   regarding how the routing table manipulations made by TORA will be
   described in detail in the subsequent sections. Since the subsequent
   description is intended to be in sufficient detail to serve as a
   template for implementations, some additional terminology is defined
   first.

3.1 Terminology

   The following definitions are identical to the definitions used in
   the IMEP specification.  Many of these definitions may be replaced by
   or merged with those of the MANET working group's terminology draft
   [7] now under development.

   MANET router or router:
      A device--identified by a "unique Router ID" (RID)--that executes
      a MANET routing protocol and, under the direction of which,
      forwards IP packets.  It may have multiple interfaces, each
      identified by an IP address.  Associated with each interface is a
      physical layer communication device.  These devices may employ
      wireless or hardwired communications, and a router may
      simultaneousl employ devices of differing technologies.  For
      example, a MANET router may have four interfaces with differing
      communications technologies: two hardwired (Ethernet and FDDI) and
      two wireless (spread spectrum and impulse radio).

   adjacency:
      The name given to an "interface on a neighboring router".

   medium:
      A communication channel such as free space, cable or fiber through
      which connections are established.

   communications technology:
      The means employed by two devices to transfer information between
      them.

   connection:
      A physical-layer connection--which may be through a wired or
      wireless medium--between a device attached to an interface of one



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      MANET router and a device utilizating the same communications
      technology attached to an interface on another MANET router.  From
      the perspective of a given router, a connection is a (interface,
      adjacency) pair.

   link:
      A "logical connection" consisting of the logical *union* of one or
      more connections between two MANET routers.  Thus, a link may
      consist of a heterogeneous combination of connections through
      differing media using different communications technologies.

   neighbor:
      From the perspective of a given MANET router, a "neighbor" is any
      other router to which it is connected by a link.

   topology:
      A network can be viewed abstractly as a "graph" whose "topology"
      at any point in time is defined by set of "points" connected by
      "edges."  This term comes from the branch of mathematics bearing
      the same name that is concerned with those properties of geometric
      configurations (such as point sets) which are unaltered by elastic
      deformations (such as stretching) that are homeomorphisms.

   physical-layer topology:
      A topology consisting of connections (the edges) through the
      *same* communications medium between devices (the points)
      communicating using the *same* communications technology.

   network-layer topology:
      A topology consisting of links (the edges) between MANET routers
      (the points) which is used as the basis for MANET routing.  Since
      "links" are the logical union of physical-layer "connections," it
      follows that the "network-layer topology" is the logical union of
      the various "physical-layer topologies."

   IP routing fabric:
      The heterogeneous mixture of communications media and technologies
      through which IP packets are forwarded whose topology is defined
      by the network-layer topology.

3.2 State Variables

      For each destination "j" to which routing is required, a router
      maintains the following state variables.

      HEIGHT[j]    The height metric of this router.
      RT_REQ[j]    Flag indicating route to the destination is required.
      TIME_UPD[j]  Time an UPD packet was last sent by this router.



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      For each destination "j" to which routing is required, a router
      maintains a separate instance of the following state variables for
      each neighbor "k".

      HT_NEIGH[j][k]  The height metric of neighbor "k."
      LNK_STAT[j][k]  The assigned status of the link to neighbor "k."
      TIME_ACT[j][k]  Time the link to neighbor "k" became active.

3.3 Auxiliary Variables

      For each destination "j" to which routing is required, a router
      may maintain the following auxiliary variables. Although each of
      the variables can be computed based on the entries in the LNK_STAT
      table, maintaining the values continuously may facilitate
      implementation of the protocol.

      num_active[j]  Number of neighbors (i.e., active links).
      num_down[j]    Number of links marked DN in the LNK_STAT table.
      num_up[j]      Number of links marked UP in the LNK_STAT table.

3.4 Height Data Structure

      Each HEIGHT[j] and HT_NEIGH[j][k] entry requires a data structure
      that comprises five components. The first three components of the
      Height data structure represent the reference level of the height
      entry, while the last two components represent an offset with
      respect to the reference level. The five components of the Height
      data structure are as follows.

      Height.tau   Time the reference level was created.
      Height.oid   Unique id of the router that created the reference level.
      Height.r     Flag indicating if it is a reflected reference level.
      Height.delta Value used in propagation of a reference level.
      Height.id    Unique id of the router.

      To simplify notation in this specification, a height may be
      written as an ordered quintuple--e.g.,
      HEIGHT[j]=(tau,oid,r,delta,id). The following two predefined
      values for a height are used throughout the specification of the
      protocol.

      NULL=(-,-,-,-,id)  An unknown or undefined height. Conceptually,
                         this can be thought of as an infinite height.

      ZERO=(0,0,0,0,id)  The assumed height of a given destination. Note
                         that here "id" is the unique id of the given
                         destination.




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3.5 Determination of Link Status

      Each entry in the LNK_STAT table is maintained in accordance with
      the following rule.

      if        HT_NEIGH[k]==NULL    then   LNK_STAT[k]=UN;
      else if   HEIGHT==NULL         then   LNK_STAT[k]=DN;
      else if   HT_NEIGH[k]<HEIGHT   then   LNK_STAT[k]=DN;
      else if   HT_NEIGH[k]>HEIGHT   then   LNK_STAT[k]=UP;

3.6 TORA Packet Formats

      TORA packets are encapsulated in IMEP messages, which are sent as
      "raw" IP datagrams with protocol number ?. The bit level format of
      the TORA packets has yet to be defined.

3.7 Event Processing

3.7.1 Initialization

   TBD.

3.7.2 Connection Status Change

   The TORA process receives notification of connection status changes
   from the the IMEP process. The interface between these two processes
   has yet to be defined. However, it is anticipated that the TORA
   process will have access to all the information maintained by the
   IMEP process about the connections. Thus, upon notification, TORA
   will have sufficient information to determine if any new links have
   been established or any existing links have been severed. If either
   is the case, then TORA must proceed as outlined in appropriate
   subsequent section (3.7.3 or 3.7.4).  In addition, it is also
   possible for a connection that was used in the routing table to be
   severed without resulting in the corresponding link being severed. In
   this case TORA must modify the appropriate routing table entries as
   outlined in section 3.7.5.

3.7.3 Link with a New Neighbor "k" Established

   TBD.

3.7.4  Link with Prior Neighbor "k" Severed

   TBD.

3.7.5  Connection Used in Routing Table Severed




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   TBD.

3.7.6 QRY Packet Regarding Destination "j" Received from Neighbor "k"

   If the RTE_REQ flag set then I) else II).  nk with Prior Neighbor "k"
   Severed

      I) Event Processing Complete.

      II) If HEIGHT[j].r==0 then A) else B).

         A) If TIME_ACT[j][k]>TIME_UPD[j] then 1) else 2).

            1) Set TIME_UPD to the current time. Create an UPD packet
            and place it in the queue to be sent to all neighbors. Event
            Processing Complete.

            2) Event Processing Complete.

         B) If HT_NEIGH[j][n].r==0 for any n then 1) else 2).

            1) Find m such that HT_NEIGH[j][m] is the minimum of all
            height entries with HT_NEIGH[j][n].r==0. Set
            HEIGHT[j]=HT_NEIGH[j][m]. Increment HEIGHT.delta. Set
            HEIGHT[j].id to the unique id of this node. Update
            LNK_STAT[j][n] for all n. Set TIME_UPD to the current time.
            Create an UPD packet and place it in the queue to be sent to
            all neighbors. Event Processing Complete.

            2) Set the RT_REQ flag. If num_active>1 then a) else b).

               a) Create a QRY packet and place it in the queue to be
               sent to all neighbors. Event Processing Complete.

               b) Event Processing Complete.

3.7.7 UPD Packet Regarding Destination "j" Received from Neighbor "k"

   Update the entries HT_NEIGH[j][k] and LNK_STAT[j][k]. If the RT_REQ
   flag is set and HT_NEIGH[j][k].r==0 then I) else II).

      I) Set HEIGHT[j]=HT_NEIGH[j][k]. Increment HEIGHT.delta. Set
      HEIGHT[j].id to the unique id of this node. Update LNK_STAT[j][n]
      for all n. Unset the RT_REQ flag. Set TIME_UPD to the current
      time. Create an UPD packet and place it in the queue to be sent to
      all neighbors. Event Processing Complete.

      II) If num_down[j]==0 then A) else B).



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         A) If num_up[j]==0 then 1) else 2).

            1) If HEIGHT[j]==NULL then a) else b).

               a) Event Processing Complete.

               b) Set HEIGHT[j]=NULL. Set TIME_UPD to the current time.
               Create an UPD packet and place it in the queue to be sent
               to all neighbors. Event Processing Complete.

            2) If all HT_NEIGH[j][n], for all n such that HT_NEIGH[j][n]
            is non-NULL, have the same reference level then a) else b).

               a) If HT_NEIGH[j][n].r==0, for any n such that
               HT_NEIGH[j][n] is non-NULL, then i) else ii).

                  i) Set HEIGHT[j]=HT_NEIGH[j][n], where n is such that
                  HT_NEIGH[j][n] is non-NULL. Set HEIGHT[j].r=1. Set
                  HEIGHT[j].delta=0. Set HEIGHT[j].id to the unique id
                  of this node. Update LNK_STAT[j][n] for all n. Set
                  TIME_UPD to the current time. Create an UPD packet and
                  place it in the queue to be sent to all neighbors.
                  Event Processing Complete.

                  ii) If HT_NEIGH[j].oid==id, where id is the unique id
                  of this node, then x) else y).

                     x) Save the current values of HEIGHT[j].tau and
                     HEIGHT[j].oid in temporary variables. Set
                     HEIGHT[j]=NULL. Set num_down=0. Set num_up=0. For
                     every active link n, if the neighbor connected via
                     link n is the destination j, set
                     HT_NEIGH[j][n]=ZERO and LNK_STAT[j][n]=DN else set
                     HT_NEIGH[j][n]=NULL and LNK_STAT[j][n]=UN. Create a
                     CLR packet, with the previously saved values of tau
                     and oid, and place it in the queue to be sent to
                     all neighbors. Event Processing Complete.

                     y) Set HEIGHT[j].tau to the current time. Set
                     HEIGHT[j].oid to the unique id of this node. Set
                     HEIGHT[j].r=0. Set HEIGHT[j].delta=0. Set
                     HEIGHT[j].id to the unique id of this node. Update
                     LNK_STAT[j][n] for all n. Unset the RT_REQ flag.
                     Set TIME_UPD to the current time. Create an UPD
                     packet and place it in the queue to be sent to all
                     neighbors. Event Processing Complete.

               b) Find n such that HT_NEIGH[j][n] is the maximum of all



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               non-NULL height entries. Find m such that HT_NEIGH[j][m]
               is the minimum of the non-NULL height entries with the
               same reference level as HT_NEIGH[j][n]. Set
               HEIGHT[j]=HT_NEIGH[j][m]. Decrement HEIGHT.delta. Set
               HEIGHT[j].id to the unique id of this node. Update
               LNK_STAT[j][n] for all n. Set TIME_UPD to the current
               time. Create an UPD packet and place it in the queue to
               be sent to all neighbors. Event Processing Complete.

         B) Event Processing Complete.

3.7.8 CLR Packet Regarding Destination "j" Received from Neighbor "k"

   If HEIGHT[j].tau and HEIGHT[j].oid match the values of tau and oid
   from the CLR packet and HEIGHT[j].r==1 then I) else II).

      I) Save the current values of HEIGHT[j].tau and HEIGHT[j].oid in
      temporary variables. Set Height[j]=NULL. Set num_down=0. Set
      num_up=0. For every active link n, if the neighbor connected via
      link n is the destination j, set HT_NEIGH[j][n]=ZERO and
      LNK_STAT[j][n]=DN else set HT_NEIGH[j][n]=NULL and
      LNK_STAT[j][n]=UN. If num_active>1 then A) else B).

         A) Create a CLR packet, with the previously saved values of tau
         and oid, and place it in the queue to be sent to all neighbors.
         Event Processing Complete.

         B) Event Processing Complete.

      II) Set HT_NEIGH[j][k]=NULL and LNK_STAT[j][k]=UN. For all n such
      that HT_NEIGH[j][n].tau and HT_NEIGH[j][n].oid match the values of
      tau and oid from the CLR packet and HT_NEIGH[j][n].r==1, set
      HT_NEIGH[j][n]=NULL and LNK_STAT[j][n]=UN. If num_down==0 then A)
      else B).

         A) If num_up==0 then 1) else 2).

            1) If HEIGHT[j]==NULL then a) else b).

               a) Event Processing Complete.

               b) Set HEIGHT[j]=NULL. Set TIME_UPD to the current time.
               Create an UPD packet and place it in the queue to be sent
               to all neighbors. Event Processing Complete.

            2) Set HEIGHT[j].tau to the current time. Set HEIGHT[j].oid
            to the unique id of this node. Set HEIGHT[j].r=0. Set
            HEIGHT[j].delta=0. Set HEIGHT[j].id to the unique id of this



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            node. Update LNK_STAT[j][n] for all n. Unset the RT_REQ
            flag. Set TIME_UPD to the current time. Create an UPD packet
            and place it in the queue to be sent to all neighbors. Event
            Processing Complete.

         B) Event Processing Complete.

4 Security Considerations

   TBD.

References

   [1] V. Park and M. S. Corson, A Highly Adaptive Distributed Routing
   Algorithm for Mobile Wireless Networks, Proc. IEEE INFOCOM '97, Kobe,
   Japan (1997).
   [2] M.S. Corson and A. Ephremides, A distributed routing algorithm
   for mobile wireless networks, Wireless Networks 1 (1995).
   [3] E. Gafni and D. Bertsekas, Distributed algorithms for generating
   loop-free routes in networks with frequently changing topology, IEEE
   Trans. Commun. (January 1981).
   [4] M.S. Corson and V. Park, An Internet MANET Encapsulation Protocol
   (IMEP), draft-ietf-manet-imep-spec-00.txt
   [5] NAVSTAR GPS user equipment introduction, MZ10298.001 (February
   1991).
   [6] D. Mills, Network time protocol, specification, implementation
   and analysis, Internet RFC-1119 (September 1989).
   [7] C. Perkins, Mobile Ad Hoc Networking Terminology, draft-ietf-
   manet-term-00.txt, (October 1997).

Author's Addresses

   Vincent D. Park
   Information Technology Division
   Naval Research Laboratory
   Washington, DC 20375
   (202) 767-5098
   vpark@itd.nrl.navy.mil

   M. Scott Corson
   Institute for Systems Research
   University of Maryland
   College Park, MD 20742
   (301) 405-6630
   corson@isr.umd.edu






Park, Corson                                                 [Page 20]


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