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IETF MANET Working Group                                         V. Park
INTERNET-DRAFT                                 Naval Research Laboratory
draft-ietf-manet-tora-spec-02.txt                              S. Corson
                                                  University of Maryland
                                                         22 October 1999


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

Status of this Memo

   This document is an Internet-Draft and is NOT offered in accordance
   with Section 10 of RFC2026, and the author does not provide the IETF
   with any rights other than to publish as an Internet-Draft.

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Abstract

   This document provides a detailed specification of the Temporally-
   Ordered Routing Algorithm (TORA)--a distributed routing protocol for
   multihop networks. A key concept in the protocol's design is an
   attempt to de-couple (to the greatest extent possible) the generation
   of far-reaching control message propagation from the dynamics of the
   network topology. The basic, underlying algorithm is neither
   traditionally distance-vector nor link-state; it is one of a family
   of algorithms referred to as "link reversal" algorithms. In
   particular, the protocol's reaction to certain link failures is
   structured as a temporally-ordered sequence of diffusing
   computations, each computation consisting of a sequence of directed
   link reversals. The protocol can operate in either a reactive,
   proactive or mixed mode.





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1 Introduction

   The Temporally-Ordered Routing Algorithm (TORA) [1] is an adaptive
   routing protocol for multihop networks. It possesses the following
   attributes:
     *  Distributed execution,
     *  Loop-free routing,
     *  Multipath routing,
     *  Reactive or proactive route establishment and maintenance, and
     *  Minimization of communication overhead via localization of
        algorithmic reaction to topological changes when possible.
   Its operation can be biased towards high reactivity (i.e., low time
   complexity) and bandwidth conservation (i.e., low communication
   complexity) rather than routing optimality (i.e., continuous
   shortest-path computation). Its design and flexability make it
   potentially well-suited for use mobile ad hoc networks (MANETs).

   TORA is based, in part, on the work presented in [2] and [3]. A key
   concept in the protocol's design is an attempt to de-couple (to the
   greatest extent possible) the generation of far-reaching control
   message propagation from the dynamics of the network topology. The
   scope of TORA's control messaging is typically localized to a very
   small set of nodes near a topological change. TORA includes a
   secondary mechanism that is independent of network topology dynamics,
   which allows far-reaching control message propagation as a means of
   route optimization or soft-state route verification.

   TORA is distributed, in that nodes need only maintain information
   about adjacent nodes (i.e., one-hop knowledge). Like a distance-
   vector routing approaches, TORA maintains state on a per-destination
   basis. Its design allows reactive operation, in which sources
   initiate the establishment of routes to a given destination on-
   demand, since it may not be necessary (nor desirable) to maintain
   routes between every source/destination pair at all times. At the
   same time, selected destinations can initiate proactive operation,
   resembling traditional table-driven routing approaches. TORA
   maintains loop-free routing, and typically provides multiple routes
   for any source/destination pair that requires a route. In the event
   of a network partition, the protocol detects the partition and erases
   invalid routes.

2 Terminology

   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



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      physical layer communication device. These devices may employ
      wireless or hardwired communications, and a router may
      simultaneously 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
      MANET router and a device utilizing 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.



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   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 Protocol Functional Description

   TORA has been designed to work on top of lower layer mechanisms or
   protocols that provide the following basic services between
   neighboring routers:
     *  Link status sensing and neighbor discovery
     *  Reliable, in-order control packet delivery
     *  Link and network layer address resolution and mapping
     *  Security authentication
   Events such as the reception of control messages and changes in
   connectivity with neighboring routers trigger TORA's algorithmic
   reactions.

   A logically separate version of TORA is run for each "destination" to
   which routing is required. The following discussion focuses on a
   single version of TORA running for a given destination. The term
   destination is used herein to refer to a traditional IP routing
   destination, which is identified by an IP address and mask. Thus, the
   route to a destination may correspond to the individual address of an
   interface on a specific machine (i.e., a host route) or an
   aggregation of addresses (i.e., a network route). TORA assigns
   directions to the links between routers to form a routing structure
   that is used to forward datagrams to the destination. A router
   assigns a direction ("upstream" or "downstream") to the link with a
   neighboring router based on the relative values of a metric
   associated with each router. The metric maintained by a router can
   conceptually be thought of as the router's "height" (i.e., links are
   directed from the higher router to the lower router). The
   significance of the heights and the link directional assignments is
   that a router may only forward datagrams downstream. Links from a
   router to any neighboring routers with an unknown or "null" height
   are considered undirected and cannot be used for forwarding.
   Collectively, the heights of the routers and the link directional
   assignments form a multipath routing structure, in which all directed
   paths lead downstream to the destination.




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   TORA can be separated into four basic functions: creating routes,
   maintaining routes, erasing routes, and optimizing routes. Creating
   routes corresponds to the selection of heights to form a directed
   sequence of links leading to the destination in a previously
   undirected network or portion of the network. Maintaining routes
   refers to the adapting the routing structure in response to network
   topological changes. For example, following the loss of some router's
   last downstream link, some directed paths may temporarily no longer
   lead to the destination--resulting a sequence of directed link
   reversals (caused by the re-selection of router heights) to re-orient
   the routing structure such that all directed paths again lead to the
   destination. In cases where the network becomes partitioned, links in
   the portion of the network that has become partitioned from the
   destination must be marked as undirected to erase invalid routes.
   During this erasing routes process, routers set their heights to null
   and their adjacent links become undirected. Finally, TORA includes a
   secondary mechanism for route optimization, in which routers re-
   select their heights in order to improve the routing structure. TORA
   accomplishes these four functions through the use of four distinct
   control packets: query (QRY), update (UPD), clear (CLR), and
   optimization (OPT).

3.1 Protocol State

   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 an 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



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   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
   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.

3.2 Creating Routes

   Creating routes requires use of the QRY and UPD packets. A QRY packet



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   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
   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



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

3.3 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.



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      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.

         (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



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         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
   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.

3.4 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.

3.5 Optimizing Routes

   TBD.

4 Protocol Specification




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   The subsequent specification is intended to be of sufficient detail
   to serve as a template for implementations.

4.1 Configuration

   For each interface "i" of a router, the following configuration
   parameters are maintained.

   IP_ADDR[i]     IP address of interface.
   ADDR_MASK[i]   Address mask of interface.
   PRO_MODE[i]    Indicates reactive/proactive mode of operation.
   OPT_MODE[i]    Indicates optimization mode of operation.
   OPT_PERIOD[i]  Period for optimization mechanism.

   For each interface, a network route corresponding to the address and
   mask of the interface may be added to the routing table.
   Additionally, TORA may respond to requests (i.e., QRY packets) for
   routes to destination addresses that match the set of addresses
   identified by the interface configurations. PRO_MODE[i] (0=OFF, 1=ON)
   indicates if routes to the destination identified by the
   corresponding interface address and mask should be created
   proactively. OPT_MODE[i] (00=OFF, 01=PARTIAL, 10=FULL, 11=reserved
   for future use) indicates the type (if any) of optimizations that
   should be used for the destination identified by the corresponding
   interface address and mask, while the OPT_PERIOD[i] sets the
   frequency at which the optimizations will occur.

   A router is also configured with a router ID (RID), which must be
   unique among the set of routers collectively running TORA.

4.2 State Variables

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

   HEIGHT[j]    This router's height metric for routing to "j".
   PRO_MODE[j]  Indicates reactive/proactive mode of operation for "j".
   OPT_MODE[j]  Indicates optimization mode of operation for "j".
   MODE_SEQ[j]  Sequence number of most recent mode change regarding "j".
   RT_REQ[j]    Indicates whether a route to "j" is required.
   TIME_UPD[j]  Time last UPD packet regarding "j" sent by this router.

   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."



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   TIME_ACT[j][k]  Time the link to neighbor "k" became active.

4.3 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.

4.4 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 which the height metric refers.

   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.

4.5 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;



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

4.6 TORA Packet Formats

   TBD.

4.7 Event Processing

4.7.1 Initialization

   TBD

4.7.2 Connection Status Change

   The TORA process receives notification of link status changes. It is
   anticipated that the TORA process will have access to all the
   information 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 (4.7.3 or 4.7.4). 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.

4.7.3 Link with a New Neighbor "k" Established

   For each destination "j":

   Set TIME_ACT[j][k] to the current time and increment num_active[j].

   If the neighbor "k" is the destination "j", then set
   HT_NEIGH[j][k]=ZERO, LNK_STAT[j][k]=DN and increment num_down[j],
   else set HT_NEIGH[j][k]=NULL and LNK_STAT[j][k]=UN.

   If the RT_REQ[j] flag is set && neighbor "k" is the destination "j"
   then I) else II).

      I) Set HEIGHT[j]=HT_NEIGH[j][k].  Increment HEIGHT[j].delta.  Set
      HEIGHT[j].id to the unique id of this node.  Update LNK_STAT[j][n]
      for all n.  Unset the RT_REQ[j] flag.  Set TIME_UPD[j] 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 PRO_MODE==1 and HEIGHT[j]!=NULL then A) else B).

         A) Set TIME_UPD[j] to the current time.  Create an UPD packet



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         and place it in the queue to be sent to all neighbors.  If the
         RT_REQ[j] flag is set, create a QRY packet and place it in the
         queue to be sent to all neighbors.  Event Processing Complete.

         B) If the RT_REQ[j] flag is set, create a QRY packet and place
         it in the queue to be sent to all neighbors.  Event Processing
         Complete.

4.7.4  Link with Prior Neighbor "k" Severed

   For each destination "j":

   Decrement num_active[j].  If LNK_STAT[j][k]==DN, decrement
   num_down[j].  If LNK_STAT[j][k]==UP, decrement num_up[j].

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

      I) If num_active[j]==0 then A) else B).

         A) Set HEIGHT[j]=NULL.  Unset the RT_REQ[j] flag.  Event
         Processing Complete.

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

      II) Event Processing Complete.

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

   If the RT_REQ[j] flag is set then I) else II).

      I) Event Processing Complete.




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      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[j] 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[j] 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[j] flag. If num_active[j]>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.

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

   If MODE_SEQ field of received packet is greater than MODE_SEQ[j],
   update entries PRO_MODE[j], OPT_MODE[j], and MODE_SEQ[j].

   Update the entries HT_NEIGH[j][k], and LNK_STAT[j][k].  If the
   RT_REQ[j] 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[j] flag.  Set TIME_UPD[j] 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).

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

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




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               a) Event Processing Complete.

               b) Set HEIGHT[j]=NULL.  Set TIME_UPD[j] 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[j] 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][n].oid==id, where n is such that
                  HT_NEIGH[j][n] is non-NULL and 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[j]=0.  Set
                     num_up[j]=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[j]
                     flag.  Set TIME_UPD[j] 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
               non-NULL height entries.  Find m such that HT_NEIGH[j][m]
               is the minimum of the non-NULL height entries with the



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               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[j] to the current
               time.  Create an UPD packet and place it in the queue to
               be sent to all neighbors.  Event Processing Complete.

         B) IF PRO_MODE changed from OFF to ON as a result of this UPD
         packet reception and HEIGHT[j]==NULL then 1) else 2)

            1) Find m such that HT_NEIGH[j][m] is the minimum of all
            non-NULL height entries.  Set HEIGHT[j]=HT_NEIGH[j][m].
            Increment HEIGHT[j].delta.  Set HEIGHT[j].id to the unique
            id of this node.  Update LNK_STAT[j][n] for all n.  Set
            TIME_UPD[j] 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.

4.7.7 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[j]=0.  Set
      num_up[j]=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[j]>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[j]==0 then
      A) else B).

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

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




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               a) Event Processing Complete.

               b) Set HEIGHT[j]=NULL.  Set TIME_UPD[j] 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 node.  Update LNK_STAT[j][n] for all n.  Unset the
            RT_REQ[j] flag.  Set TIME_UPD[j] 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.

4.7.8 OPT Packet Regarding Destination "j" Received from Neighbor "k"

   If MODE_SEQ field of received packet is greater than MODE_SEQ[j] then
   I) else II).

      I) Update entries PRO_MODE[j], OPT_MODE[j], and MODE_SEQ[j].  If
      PRO_MODE[j] changed as a result of this OPT packet reception ||
      (OPT_MODE[j]==PARTIAL && HEIGHT[j]!=NULL) || OPT_MODE[j]==FULL
      then A) else B).

         A) Set HEIGHT[j]=ZERO.  Set HEIGHT[j].delta to the value of the
         DELTA field in the received OPT packet + 1.  Set HEIGHT[j].id
         to the unique id of this node.  Update LNK_STAT[j][n] for all
         n.  Unset the RT_REQ[j] flag.  Set TIME_UPD[j] to the current
         time.  Create an OPT packet and place it in the queue to be
         sent to all neighbors.  Event Processing Complete.

         B) Event Processing Complete.

      II) Event Processing Complete.

4.7.9 Mode Configuration Change or Optimization Timer Event for local
interface "i"
   Increment MODE_SEQ[i]. Create an OPT packet and place it in the queue
   to be sent to all neighbors. If OPT_MODE[i]==PARTIAL ||
   OPT_MODE[i]==FULL, schedule a local optimization timer event for
   interface "i" to occur at a time randomly selected between
   0.5*OPT_PERIOD[i] and 1.5*OPT_PERIOD[i] seconds based on a uniform
   distribution.  Event Processing Complete.


   5 Security Considerations



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   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-
   [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).

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

















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