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Versions: 00 01 02 03 04 05 06 07 08 09 10 RFC 4728

IETF MANET Working Group               David B. Johnson, Rice University
INTERNET-DRAFT                              David A. Maltz, AON Networks
21 February 2002                            Yih-Chun Hu, Rice University
                         Jorjeta G. Jetcheva, Carnegie Mellon University



                  The Dynamic Source Routing Protocol
                    for Mobile Ad Hoc Networks (DSR)

                     <draft-ietf-manet-dsr-07.txt>


Status of This Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 10 of RFC 2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note
   that other groups may also distribute working documents as
   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at
   any time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress".

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft is a submission to the IETF Mobile Ad Hoc
   Networks (MANET) Working Group.  Comments on this draft may be sent
   to the Working Group at manet@itd.nrl.navy.mil, or may be sent
   directly to the authors.


















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Abstract

   The Dynamic Source Routing protocol (DSR) is a simple and efficient
   routing protocol designed specifically for use in multi-hop wireless
   ad hoc networks of mobile nodes.  DSR allows the network to be
   completely self-organizing and self-configuring, without the need
   for any existing network infrastructure or administration.  The
   protocol is composed of the two main mechanisms of "Route Discovery"
   and "Route Maintenance", which work together to allow nodes to
   discover and maintain source routes to arbitrary destinations in the
   ad hoc network.  The use of source routing allows packet routing
   to be trivially loop-free, avoids the need for up-to-date routing
   information in the intermediate nodes through which packets are
   forwarded, and allows nodes forwarding or overhearing packets to
   cache the routing information in them for their own future use.  All
   aspects of the protocol operate entirely on-demand, allowing the
   routing packet overhead of DSR to scale automatically to only that
   needed to react to changes in the routes currently in use.  This
   document specifies the operation of the DSR protocol for routing
   unicast IPv4 packets in multi-hop wireless ad hoc networks.

   The DSR protocol is designed for mobile ad hoc networks with up to
   around two hundred nodes, and is designed to cope with relatively
   high rates of mobility.



























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                                Contents



Status of This Memo                                                    i

Abstract                                                              ii


 1. Introduction                                                       1

 2. Assumptions                                                        3

 3. DSR Protocol Overview                                              5

     3.1. Basic DSR Route Discovery . . . . . . . . . . . . . . . .    5
     3.2. Basic DSR Route Maintenance . . . . . . . . . . . . . . .    7
     3.3. Additional Route Discovery Features . . . . . . . . . . .    9
           3.3.1. Caching Overheard Routing Information . . . . . .    9
           3.3.2. Replying to Route Requests using Cached Routes  .   10
           3.3.3. Preventing Route Reply Storms . . . . . . . . . .   11
           3.3.4. Route Request Hop Limits  . . . . . . . . . . . .   13
     3.4. Additional Route Maintenance Features . . . . . . . . . .   14
           3.4.1. Packet Salvaging  . . . . . . . . . . . . . . . .   14
           3.4.2. Queued Packets Destined over a Broken Link  . . .   14
           3.4.3. Automatic Route Shortening  . . . . . . . . . . .   15
           3.4.4. Increased Spreading of Route Error Messages . . .   16

 4. Conceptual Data Structures                                        17

     4.1. Route Cache . . . . . . . . . . . . . . . . . . . . . . .   17
     4.2. Send Buffer . . . . . . . . . . . . . . . . . . . . . . .   20
     4.3. Route Request Table . . . . . . . . . . . . . . . . . . .   21
     4.4. Gratuitous Route Reply Table  . . . . . . . . . . . . . .   22
     4.5. Network Interface Queue and Maintenance Buffer  . . . . .   23
     4.6. Blacklist . . . . . . . . . . . . . . . . . . . . . . . .   24

 5. DSR Header Format                                                 25

     5.1. Fixed Portion of DSR Header . . . . . . . . . . . . . . .   26
     5.2. Route Request Option  . . . . . . . . . . . . . . . . . .   28
     5.3. Route Reply Option  . . . . . . . . . . . . . . . . . . .   30
     5.4. Route Error Option  . . . . . . . . . . . . . . . . . . .   32
     5.5. Acknowledgment Request Option . . . . . . . . . . . . . .   35
     5.6. Acknowledgment Option . . . . . . . . . . . . . . . . . .   36
     5.7. DSR Source Route Option . . . . . . . . . . . . . . . . .   37
     5.8. Pad1 Option . . . . . . . . . . . . . . . . . . . . . . .   39
     5.9. PadN Option . . . . . . . . . . . . . . . . . . . . . . .   40



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 6. Detailed Operation                                                41

     6.1. General Packet Processing . . . . . . . . . . . . . . . .   41
           6.1.1. Originating a Packet  . . . . . . . . . . . . . .   41
           6.1.2. Adding a DSR Header to a Packet . . . . . . . . .   41
           6.1.3. Adding a DSR Source Route Option to a Packet  . .   42
           6.1.4. Processing a Received Packet  . . . . . . . . . .   43
           6.1.5. Processing a Received DSR Source Route Option . .   45
     6.2. Route Discovery Processing  . . . . . . . . . . . . . . .   48
           6.2.1. Originating a Route Request . . . . . . . . . . .   48
           6.2.2. Processing a Received Route Request Option  . . .   50
           6.2.3. Generating a Route Reply using the Route Cache  .   52
           6.2.4. Originating a Route Reply . . . . . . . . . . . .   54
           6.2.5. Processing a Received Route Reply Option  . . . .   56
     6.3. Route Maintenance Processing  . . . . . . . . . . . . . .   57
           6.3.1. Using Link-Layer Acknowledgments  . . . . . . . .   57
           6.3.2. Using Passive Acknowledgments . . . . . . . . . .   58
           6.3.3. Using Network-Layer Acknowledgments . . . . . . .   59
           6.3.4. Originating a Route Error . . . . . . . . . . . .   62
           6.3.5. Processing a Received Route Error Option  . . . .   63
           6.3.6. Salvaging a Packet  . . . . . . . . . . . . . . .   64

 7. Multiple Interface Support                                        66

 8. Fragmentation and Reassembly                                      67

 9. Protocol Constants and Configuration Variables                    68

10. IANA Considerations                                               69

11. Security Considerations                                           70


Appendix A. Link-MaxLife Cache Description                            71

Appendix B. Location of DSR in the ISO Network Reference Model        73

Appendix C. Implementation and Evaluation Status                      74


Changes from Previous Version of the Draft                            75

Acknowledgements                                                      76

References                                                            77

Chair's Address                                                       80

Authors' Addresses                                                    81




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1. Introduction

   The Dynamic Source Routing protocol (DSR) [13, 14] is a simple and
   efficient routing protocol designed specifically for use in multi-hop
   wireless ad hoc networks of mobile nodes.  Using DSR, the network
   is completely self-organizing and self-configuring, requiring no
   existing network infrastructure or administration.  Network nodes
   cooperate to forward packets for each other to allow communication
   over multiple "hops" between nodes not directly within wireless
   transmission range of one another.  As nodes in the network move
   about or join or leave the network, and as wireless transmission
   conditions such as sources of interference change, all routing is
   automatically determined and maintained by the DSR routing protocol.
   Since the number or sequence of intermediate hops needed to reach any
   destination may change at any time, the resulting network topology
   may be quite rich and rapidly changing.

   The DSR protocol allows nodes to dynamically discover a source
   route across multiple network hops to any destination in the ad hoc
   network.  Each data packet sent then carries in its header the
   complete, ordered list of nodes through which the packet will pass,
   allowing packet routing to be trivially loop-free and avoiding the
   need for up-to-date routing information in the intermediate nodes
   through which the packet is forwarded.  By including this source
   route in the header of each data packet, other nodes forwarding or
   overhearing any of these packets can also easily cache this routing
   information for future use.

   In designing DSR, we sought to create a routing protocol that had
   very low overhead yet was able to react very quickly to changes in
   the network.  The DSR protocol provides highly reactive service in
   order to help ensure successful delivery of data packets in spite of
   node movement or other changes in network conditions.

   The DSR protocol is composed of two main mechanisms that work
   together to allow the discovery and maintenance of source routes in
   the ad hoc network:

    -  Route Discovery is the mechanism by which a node S wishing to
       send a packet to a destination node D obtains a source route
       to D.  Route Discovery is used only when S attempts to send a
       packet to D and does not already know a route to D.

    -  Route Maintenance is the mechanism by which node S is able
       to detect, while using a source route to D, if the network
       topology has changed such that it can no longer use its route
       to D because a link along the route no longer works.  When Route
       Maintenance indicates a source route is broken, S can attempt to
       use any other route it happens to know to D, or can invoke Route
       Discovery again to find a new route for subsequent packets to D.



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       Route Maintenance for this route is used only when S is actually
       sending packets to D.

   In DSR, Route Discovery and Route Maintenance each operate entirely
   "on demand".  In particular, unlike other protocols, DSR requires no
   periodic packets of any kind at any layer within the network.  For
   example, DSR does not use any periodic routing advertisement, link
   status sensing, or neighbor detection packets, and does not rely on
   these functions from any underlying protocols in the network.  This
   entirely on-demand behavior and lack of periodic activity allows
   the number of overhead packets caused by DSR to scale all the way
   down to zero, when all nodes are approximately stationary with
   respect to each other and all routes needed for current communication
   have already been discovered.  As nodes begin to move more or
   as communication patterns change, the routing packet overhead of
   DSR automatically scales to only that needed to track the routes
   currently in use.  Network topology changes not affecting routes
   currently in use are ignored and do not cause reaction from the
   protocol.

   In response to a single Route Discovery (as well as through routing
   information from other packets overheard), a node may learn and cache
   multiple routes to any destination.  This allows the reaction to
   routing changes to be much more rapid, since a node with multiple
   routes to a destination can try another cached route if the one it
   has been using should fail.  This caching of multiple routes also
   avoids the overhead of needing to perform a new Route Discovery each
   time a route in use breaks.

   The operation of both Route Discovery and Route Maintenance in DSR
   are designed to allow unidirectional links and asymmetric routes
   to be easily supported.  In particular, as noted in Section 2, in
   wireless networks, it is possible that a link between two nodes may
   not work equally well in both directions, due to differing antenna
   or propagation patterns or sources of interference.  DSR allows such
   unidirectional links to be used when necessary, improving overall
   performance and network connectivity in the system.

   This document specifies the operation of the DSR protocol for
   routing unicast IPv4 packets in multi-hop wireless ad hoc networks.
   Advanced, optional features, such as Quality of Service (QoS) support
   and efficient multicast routing, and operation of DSR with IPv6 [6],
   are covered in other documents.  The specification of DSR in this
   document provides a compatible base on which such features can be
   added, either independently or by integration with the DSR operation
   specified here.

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [4].



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2. Assumptions

   We assume in this document that all nodes wishing to communicate with
   other nodes within the ad hoc network are willing to participate
   fully in the protocols of the network.  In particular, each node
   participating in the ad hoc network SHOULD also be willing to forward
   packets for other nodes in the network.

   The diameter of an ad hoc network is the minimum number of hops
   necessary for a packet to reach from any node located at one extreme
   edge of the ad hoc network to another node located at the opposite
   extreme.  We assume that this diameter will often be small (e.g.,
   perhaps 5 or 10 hops), but may often be greater than 1.

   Packets may be lost or corrupted in transmission on the wireless
   network.  We assume that a node receiving a corrupted packet can
   detect the error and discard the packet.

   Nodes within the ad hoc network MAY move at any time without notice,
   and MAY even move continuously, but we assume that the speed with
   which nodes move is moderate with respect to the packet transmission
   latency and wireless transmission range of the particular underlying
   network hardware in use.  In particular, DSR can support very
   rapid rates of arbitrary node mobility, but we assume that nodes do
   not continuously move so rapidly as to make the flooding of every
   individual data packet the only possible routing protocol.

   A common feature of many network interfaces, including most current
   LAN hardware for broadcast media such as wireless, is the ability
   to operate the network interface in "promiscuous" receive mode.
   This mode causes the hardware to deliver every received packet to
   the network driver software without filtering based on link-layer
   destination address.  Although we do not require this facility, some
   of our optimizations can take advantage of its availability.  Use
   of promiscuous mode does increase the software overhead on the CPU,
   but we believe that wireless network speeds are more the inherent
   limiting factor to performance in current and future systems; we also
   believe that portions of the protocol are suitable for implementation
   directly within a programmable network interface unit to avoid this
   overhead on the CPU [14].  Use of promiscuous mode may also increase
   the power consumption of the network interface hardware, depending
   on the design of the receiver hardware, and in such cases, DSR can
   easily be used without the optimizations that depend on promiscuous
   receive mode, or can be programmed to only periodically switch the
   interface into promiscuous mode.  Use of promiscuous receive mode is
   entirely optional.

   Wireless communication ability between any pair of nodes may at
   times not work equally well in both directions, due for example to
   differing antenna or propagation patterns or sources of interference



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   around the two nodes [1, 18].  That is, wireless communications
   between each pair of nodes will in many cases be able to operate
   bidirectionally, but at times the wireless link between two nodes
   may be only unidirectional, allowing one node to successfully send
   packets to the other while no communication is possible in the
   reverse direction.  Although many routing protocols operate correctly
   only over bidirectional links, DSR can successfully discover and
   forward packets over paths that contain unidirectional links.  Some
   MAC protocols, however, such as MACA [17], MACAW [2], or IEEE
   802.11 [11], limit unicast data packet transmission to bidirectional
   links, due to the required bidirectional exchange of RTS and CTS
   packets in these protocols and due to the link-layer acknowledgment
   feature in IEEE 802.11; when used on top of MAC protocols such as
   these, DSR can take advantage of additional optimizations, such as
   the ability to reverse a source route to obtain a route back to the
   origin of the original route.

   The IP address used by a node using the DSR protocol MAY be assigned
   by any mechanism (e.g., static assignment or use of DHCP for dynamic
   assignment [7]), although the method of such assignment is outside
   the scope of this specification.
































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3. DSR Protocol Overview

3.1. Basic DSR Route Discovery

   When some source node originates a new packet addressed to some
   destination node, the source node places in the header of the packet
   a source route giving the sequence of hops that the packet is to
   follow on its way to the destination.  Normally, the sender will
   obtain a suitable source route by searching its "Route Cache" of
   routes previously learned; if no route is found in its cache, it will
   initiate the Route Discovery protocol to dynamically find a new route
   to this destination node.  In this case, we call the source node
   the "initiator" and the destination node the "target" of the Route
   Discovery.

   For example, suppose a node A is attempting to discover a route to
   node E.  The Route Discovery initiated by node A in this example
   would proceed as follows:

            ^    "A"    ^   "A,B"   ^  "A,B,C"  ^ "A,B,C,D"
            |   id=2    |   id=2    |   id=2    |   id=2
         +-----+     +-----+     +-----+     +-----+     +-----+
         |  A  |---->|  B  |---->|  C  |---->|  D  |---->|  E  |
         +-----+     +-----+     +-----+     +-----+     +-----+
            |           |           |           |
            v           v           v           v

   To initiate the Route Discovery, node A transmits a "Route
   Request" as a single local broadcast packet, which is received by
   (approximately) all nodes currently within wireless transmission
   range of A, including node B in this example.  Each Route Request
   identifies the initiator and target of the Route Discovery, and
   also contains a unique request identification (2, in this example),
   determined by the initiator of the Request.  Each Route Request also
   contains a record listing the address of each intermediate node
   through which this particular copy of the Route Request has been
   forwarded.  This route record is initialized to an empty list by the
   initiator of the Route Discovery.  In this example, the route record
   initially lists only node A.

   When another node receives this Route Request (such as node B in this
   example), if it is the target of the Route Discovery, it returns
   a "Route Reply" to the initiator of the Route Discovery, giving
   a copy of the accumulated route record from the Route Request;
   when the initiator receives this Route Reply, it caches this route
   in its Route Cache for use in sending subsequent packets to this
   destination.

   Otherwise, if this node receiving the Route Request has recently seen
   another Route Request message from this initiator bearing this same



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   request identification and target address, or if this node's own
   address is already listed in the route record in the Route Request,
   this node discards the Request.  Otherwise, this node appends its
   own address to the route record in the Route Request and propagates
   it by transmitting it as a local broadcast packet (with the same
   request identification).  In this example, node B broadcast the Route
   Request, which is received by node C; nodes C and D each also, in
   turn, broadcast the Request, resulting in a copy of the Request being
   received by node E.

   In returning the Route Reply to the initiator of the Route Discovery,
   such as in this example, node E replying back to node A, node E will
   typically examine its own Route Cache for a route back to A, and if
   found, will use it for the source route for delivery of the packet
   containing the Route Reply.  Otherwise, E SHOULD perform its own
   Route Discovery for target node A, but to avoid possible infinite
   recursion of Route Discoveries, it MUST piggyback this Route Reply
   on the packet containing its own Route Request for A.  It is also
   possible to piggyback other small data packets, such as a TCP SYN
   packet [28], on a Route Request using this same mechanism.

   Node E could instead simply reverse the sequence of hops in the route
   record that it is trying to send in the Route Reply, and use this as
   the source route on the packet carrying the Route Reply itself.  For
   MAC protocols such as IEEE 802.11 that require a bidirectional frame
   exchange as part of the MAC protocol [11], the discovered source
   route MUST be reversed in this way to return the Route Reply since it
   tests the discovered route to ensure it is bidirectional before the
   Route Discovery initiator begins using the route; this route reversal
   also avoids the overhead of a possible second Route Discovery.
   However, this route reversal technique will prevent the discovery of
   routes using unidirectional links, and in wireless environments where
   the use of unidirectional links is permitted, such routes may in some
   cases be more efficient than those with only bidirectional links, or
   they may be the only way to achieve connectivity to the target node.

   When initiating a Route Discovery, the sending node saves a copy of
   the original packet (that triggered the Discovery) in a local buffer
   called the "Send Buffer".  The Send Buffer contains a copy of each
   packet that cannot be transmitted by this node because it does not
   yet have a source route to the packet's destination.  Each packet in
   the Send Buffer is logically associated with the time that it was
   placed into the Send Buffer and is discarded after residing in the
   Send Buffer for some timeout period; if necessary for preventing the
   Send Buffer from overflowing, a FIFO or other replacement strategy
   MAY also be used to evict packets even before they expire.

   While a packet remains in the Send Buffer, the node SHOULD
   occasionally initiate a new Route Discovery for the packet's
   destination address.  However, the node MUST limit the rate at which



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   such new Route Discoveries for the same address are initiated, since
   it is possible that the destination node is not currently reachable.
   In particular, due to the limited wireless transmission range and the
   movement of the nodes in the network, the network may at times become
   partitioned, meaning that there is currently no sequence of nodes
   through which a packet could be forwarded to reach the destination.
   Depending on the movement pattern and the density of nodes in the
   network, such network partitions may be rare or may be common.

   If a new Route Discovery was initiated for each packet sent by a
   node in such a partitioned network, a large number of unproductive
   Route Request packets would be propagated throughout the subset
   of the ad hoc network reachable from this node.  In order to
   reduce the overhead from such Route Discoveries, a node SHOULD use
   an exponential back-off algorithm to limit the rate at which it
   initiates new Route Discoveries for the same target, doubling the
   timeout between each successive Discovery initiated for the same
   target.  If the node attempts to send additional data packets to this
   same destination node more frequently than this limit, the subsequent
   packets SHOULD be buffered in the Send Buffer until a Route Reply is
   received giving a route to this destination, but the node MUST NOT
   initiate a new Route Discovery until the minimum allowable interval
   between new Route Discoveries for this target has been reached.  This
   limitation on the maximum rate of Route Discoveries for the same
   target is similar to the mechanism required by Internet nodes to
   limit the rate at which ARP Requests are sent for any single target
   IP address [3].


3.2. Basic DSR Route Maintenance

   When originating or forwarding a packet using a source route, each
   node transmitting the packet is responsible for confirming that data
   can flow over the link from that node to the next hop.  For example,
   in the situation shown below, node A has originated a packet for
   node E using a source route through intermediate nodes B, C, and D:

         +-----+     +-----+     +-----+     +-----+     +-----+
         |  A  |---->|  B  |---->|  C  |-->? |  D  |     |  E  |
         +-----+     +-----+     +-----+     +-----+     +-----+

   In this case, node A is responsible for the link from A to B, node B
   is responsible for the link from B to C, node C is responsible for
   the link from C to D, node D is responsible for the link from D to E.

   An acknowledgment can provide confirmation that a link is capable of
   carrying data, and in wireless networks, acknowledgments are often
   provided at no cost, either as an existing standard part of the MAC
   protocol in use (such as the link-layer acknowledgment frame defined
   by IEEE 802.11 [11]), or by a "passive acknowledgment" [16] (in



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   which, for example, B confirms receipt at C by overhearing C transmit
   the packet when forwarding it on to D).

   If a built-in acknowledgment mechanism is not available, the node
   transmitting the packet can explicitly request a DSR-specific
   software acknowledgment be returned by the next node along the route;
   this software acknowledgment will normally be transmitted directly
   to the sending node, but if the link between these two nodes is
   unidirectional, this software acknowledgment could travel over a
   different, multi-hop path.

   After an acknowledgment has been received from some neighbor, a node
   MAY choose to not require acknowledgments from that neighbor for a
   brief period of time, unless the network interface connecting a node
   to that neighbor always receives an acknowledgment in response to
   unicast traffic.

   When a software acknowledgment is used, the acknowledgment request
   SHOULD be retransmitted up to a maximum number of times.  A
   retransmission of the acknowledgment request can be sent as a
   separate packet, piggybacked on a retransmission of the original
   data packet, or piggybacked on any packet with the same next-hop
   destination that does not also contain a software acknowledgment.

   After the acknowledgment request has been retransmitted the maximum
   number of times, if no acknowledgment has been received, then the
   sender treats the link to this next-hop destination as currently
   "broken".  It SHOULD remove this link from its Route Cache and
   SHOULD return a "Route Error" to each node that has sent a packet
   routed over that link since an acknowledgment was last received.
   For example, in the situation shown above, if C does not receive
   an acknowledgment from D after some number of requests, it would
   return a Route Error to A, as well as any other node that may have
   used the link from C to D since C last received an acknowledgment
   from D. Node A then removes this broken link from its cache; any
   retransmission of the original packet can be performed by upper
   layer protocols such as TCP, if necessary.  For sending such a
   retransmission or other packets to this same destination E, if A has
   in its Route Cache another route to E (for example, from additional
   Route Replies from its earlier Route Discovery, or from having
   overheard sufficient routing information from other packets), it
   can send the packet using the new route immediately.  Otherwise, it
   SHOULD perform a new Route Discovery for this target (subject to the
   back-off described in Section 3.1).









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3.3. Additional Route Discovery Features

3.3.1. Caching Overheard Routing Information

   A node forwarding or otherwise overhearing any packet SHOULD add all
   usable routing information from that packet to its own Route Cache.
   The usefulness of routing information in a packet depends on the
   directionality characteristics of the physical medium (Section 2), as
   well as the MAC protocol being used.  Specifically, three distinct
   cases are possible:

    -  Links in the network frequently are capable of operating only
       unidirectionally (not bidirectionally), and the MAC protocol in
       use in the network is capable of transmitting unicast packets
       over unidirectional links.

    -  Links in the network occasionally are capable of operating only
       unidirectionally (not bidirectionally), but this unidirectional
       restriction on any link is not persistent, almost all links
       are physically bidirectional, and the MAC protocol in use in
       the network is capable of transmitting unicast packets over
       unidirectional links.

    -  The MAC protocol in use in the network is not capable of
       transmitting unicast packets over unidirectional links;
       only bidirectional links can be used by the MAC protocol for
       transmitting unicast packets.  For example, the IEEE 802.11
       Distributed Coordination Function (DCF) MAC protocol [11]
       is capable of transmitting a unicast packet only over a
       bidirectional link, since the MAC protocol requires the return
       of a link-level acknowledgment packet from the receiver and also
       optionally requires the bidirectional exchange of an RTS and CTS
       packet between the transmitter and receiver nodes.

   In the first case above, for example, the source route used in a data
   packet, the accumulated route record in a Route Request, or the route
   being returned in a Route Reply SHOULD all be cached by any node in
   the "forward" direction; any node SHOULD cache this information from
   any such packet received, whether the packet was addressed to this
   node, sent to a broadcast (or multicast) MAC address, or overheard
   while the node's network interface is in promiscuous mode.  However,
   the "reverse" direction of the links identified in such packet
   headers SHOULD NOT be cached.










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   For example, in the situation shown below, node A is using a source
   route to communicate with node E:

         +-----+     +-----+     +-----+     +-----+     +-----+
         |  A  |---->|  B  |---->|  C  |---->|  D  |---->|  E  |
         +-----+     +-----+     +-----+     +-----+     +-----+

   As node C forwards a data packet along the route from A to E, it
   SHOULD add to its cache the presence of the "forward" direction
   links that it learns from the headers of these packets, from itself
   to D and from D to E.  Node C SHOULD NOT, in this case, cache the
   "reverse" direction of the links identified in these packet headers,
   from itself back to B and from B to A, since these links might be
   unidirectional.

   In the second case above, in which links may occasionally operate
   unidirectionally, the links described above SHOULD be cached in both
   directions.  Furthermore, in this case, if node X overhears (e.g.,
   through promiscuous mode) a packet transmitted by node C that is
   using a source route from node A to E, node X SHOULD cache all of
   these links as well, also including the link from C to X over which
   it overheard the packet.

   In the final case, in which the MAC protocol requires physical
   bidirectionality for unicast operation, links from a source route
   SHOULD be cached in both directions, except when the packet also
   contains a Route Reply, in which case only the links already
   traversed in this source route SHOULD be cached, but the links not
   yet traversed in this route SHOULD NOT be cached.


3.3.2. Replying to Route Requests using Cached Routes

   A node receiving a Route Request for which it is not the target,
   searches its own Route Cache for a route to the target of the
   Request.  If found, the node generally returns a Route Reply to the
   initiator itself rather than forwarding the Route Request.  In the
   Route Reply, this node sets the route record to list the sequence of
   hops over which this copy of the Route Request was forwarded to it,
   concatenated with the source route to this target obtained from its
   own Route Cache.

   However, before transmitting a Route Reply packet that was generated
   using information from its Route Cache in this way, a node MUST
   verify that the resulting route being returned in the Route Reply,
   after this concatenation, contains no duplicate nodes listed in the
   route record.  For example, the figure below illustrates a case in






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   which a Route Request for target E has been received by node F, and
   node F already has in its Route Cache a route from itself to E:

         +-----+     +-----+                 +-----+     +-----+
         |  A  |---->|  B  |-               >|  D  |---->|  E  |
         +-----+     +-----+ \             / +-----+     +-----+
                              \           /
                               \ +-----+ /
                                >|  C  |-
                                 +-----+
                                   | ^
                                   v |
           Route Request         +-----+
           Route: A - B - C - F  |  F  |  Cache: C - D - E
                                 +-----+

   The concatenation of the accumulated route record from the Route
   Request and the cached route from F's Route Cache would include a
   duplicate node in passing from C to F and back to C.

   Node F in this case could attempt to edit the route to eliminate the
   duplication, resulting in a route from A to B to C to D and on to E,
   but in this case, node F would not be on the route that it returned
   in its own Route Reply.  DSR Route Discovery prohibits node F
   from returning such a Route Reply from its cache; this prohibition
   increases the probability that the resulting route is valid, since
   node F in this case should have received a Route Error if the route
   had previously stopped working.  Furthermore, this prohibition
   means that a future Route Error traversing the route is very likely
   to pass through any node that sent the Route Reply for the route
   (including node F), which helps to ensure that stale data is removed
   from caches (such as at F) in a timely manner; otherwise, the next
   Route Discovery initiated by A might also be contaminated by a Route
   Reply from F containing the same stale route.  If node F, due to this
   restriction on returning a Route Reply based on information from its
   Route Cache, does not return such a Route Reply, node F propagates
   the Route Request normally.


3.3.3. Preventing Route Reply Storms

   The ability for nodes to reply to a Route Request based on
   information in their Route Caches, as described in Section 3.3.2,
   could result in a possible Route Reply "storm" in some cases.  In
   particular, if a node broadcasts a Route Request for a target node
   for which the node's neighbors have a route in their Route Caches,
   each neighbor may attempt to send a Route Reply, thereby wasting
   bandwidth and possibly increasing the number of network collisions in
   the area.




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   For example, the figure below shows a situation in which nodes B, C,
   D, E, and F all receive A's Route Request for target G, and each has
   the indicated route cached for this target:

                +-----+                 +-----+
                |  D  |<               >|  C  |
                +-----+ \             / +-----+
      Cache: C - B - G   \           /  Cache: B - G
                          \ +-----+ /
                           -|  A  |-
                            +-----+\     +-----+     +-----+
                             |   |  \--->|  B  |     |  G  |
                            /     \      +-----+     +-----+
                           /       \     Cache: G
                          v         v
                    +-----+         +-----+
                    |  E  |         |  F  |
                    +-----+         +-----+
               Cache: F - B - G     Cache: B - G

   Normally, each of these nodes would attempt to reply from its own
   Route Cache, and they would thus all send their Route Replies at
   about the same time, since they all received the broadcast Route
   Request at about the same time.  Such simultaneous Route Replies
   from different nodes all receiving the Route Request may cause local
   congestion in the wireless network and may create packet collisions
   among some or all of these Replies if the MAC protocol in use does
   not provide sufficient collision avoidance for these packets.  In
   addition, it will often be the case that the different replies will
   indicate routes of different lengths, as shown in this example.

   In order to reduce these effects, if a node can put its network
   interface into promiscuous receive mode, it MAY delay sending its
   own Route Reply for a short period, while listening to see if the
   initiating node begins using a shorter route first.  Specifically,
   this node MAY delay sending its own Route Reply for a random period

      d = H * (h - 1 + r)

   where h is the length in number of network hops for the route to be
   returned in this node's Route Reply, r is a random floating point
   number between 0 and 1, and H is a small constant delay (at least
   twice the maximum wireless link propagation delay) to be introduced
   per hop.  This delay effectively randomizes the time at which each
   node sends its Route Reply, with all nodes sending Route Replies
   giving routes of length less than h sending their Replies before this
   node, and all nodes sending Route Replies giving routes of length
   greater than h sending their Replies after this node.





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   Within the delay period, this node promiscuously receives all
   packets, looking for data packets from the initiator of this Route
   Discovery destined for the target of the Discovery.  If such a data
   packet received by this node during the delay period uses a source
   route of length less than or equal to h, this node may infer that the
   initiator of the Route Discovery has already received a Route Reply
   giving an equally good or better route.  In this case, this node
   SHOULD cancel its delay timer and SHOULD NOT send its Route Reply for
   this Route Discovery.


3.3.4. Route Request Hop Limits

   Each Route Request message contains a "hop limit" that may be used
   to limit the number of intermediate nodes allowed to forward that
   copy of the Route Request.  This hop limit is implemented using the
   Time-to-Live (TTL) field in the IP header of the packet carrying
   the Route Request.  As the Request is forwarded, this limit is
   decremented, and the Request packet is discarded if the limit reaches
   zero before finding the target.  This Route Request hop limit can be
   used to implement a variety of algorithms for controlling the spread
   of a Route Request during a Route Discovery attempt.

   For example, a node MAY use this hop limit to implement a
   "non-propagating" Route Request as an initial phase of a Route
   Discovery.  A node using this technique sends its first Route Request
   attempt for some target node using a hop limit of 1, such that any
   node receiving the initial transmission of the Route Request will
   not forward the Request to other nodes by re-broadcasting it.  This
   form of Route Request is called a "non-propagating" Route Request;
   it provides an inexpensive method for determining if the target is
   currently a neighbor of the initiator or if a neighbor node has a
   route to the target cached (effectively using the neighbors' Route
   Caches as an extension of the initiator's own Route Cache).  If no
   Route Reply is received after a short timeout, then the node sends a
   "propagating" Route Request (i.e., with no hop limit) for the target
   node.

   As another example, a node MAY use this hop limit to implement an
   "expanding ring" search for the target [14].  A node using this
   technique sends an initial non-propagating Route Request as described
   above; if no Route Reply is received for it, the node originates
   another Route Request with a hop limit of 2.  For each Route Request
   originated, if no Route Reply is received for it, the node doubles
   the hop limit used on the previous attempt, to progressively explore
   for the target node without allowing the Route Request to propagate
   over the entire network.  However, this expanding ring search
   approach could have the effect of increasing the average latency of
   Route Discovery, since multiple Discovery attempts and timeouts may
   be needed before discovering a route to the target node.



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3.4. Additional Route Maintenance Features

3.4.1. Packet Salvaging

   When an intermediate node forwarding a packet detects through Route
   Maintenance that the next hop along the route for that packet is
   broken, if the node has another route to the packet's destination in
   its Route Cache, the node SHOULD "salvage" the packet rather than
   discarding it.  To salvage a packet, the node replaces the original
   source route on the packet with the route from its Route Cache.  The
   node then forwards the packet to the next node indicated along this
   source route.  For example, in the situation shown in the example of
   Section 3.2, if node C has another route cached to node E, it can
   salvage the packet by replacing the original route in the packet with
   this new route from its own Route Cache, rather than discarding the
   packet.

   When salvaging a packet, a count is maintained in the packet of the
   number of times that it has been salvaged, to prevent a single packet
   from being salvaged endlessly.  Otherwise, it could be possible for
   the packet to enter a routing loop, as different nodes repeatedly
   salvage the packet and replace the source route on the packet with
   routes to each other.

   As described in Section 3.2, an intermediate node, such as in this
   case, that detects through Route Maintenance that the next hop along
   the route for a packet that it is forwarding is broken, the node also
   SHOULD return a Route Error to the original sender of the packet,
   identifying the link over which the packet could not be forwarded.
   If the node sends this Route Error, it SHOULD originate the Route
   Error before salvaging the packet.


3.4.2. Queued Packets Destined over a Broken Link

   When an intermediate node forwarding a packet detects through Route
   Maintenance that the next-hop link along the route for that packet
   is broken, in addition to handling that packet as defined for Route
   Maintenance, the node SHOULD also handle in a similar way any pending
   packets that it has queued that are destined over this new broken
   link.  Specifically, the node SHOULD search its Network Interface
   Queue and Maintenance Buffer (Section 4.5) for packets for which
   the next-hop link is this new broken link.  For each such packet
   currently queued at this node, the node SHOULD process that packet as
   follows:

    -  Remove the packet from the node's Network Interface Queue and
       Maintenance Buffer.





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    -  Originate a Route Error for this packet to the original sender of
       the packet, using the procedure described in Section 6.3.4, as if
       the node had already reached the maximum number of retransmission
       attempts for that packet for Route Maintenance.  However, in
       sending such Route Errors for queued packets in response to a
       single new broken link detected, the node SHOULD send no more
       than one Route Error to each original sender of any of these
       packets.

    -  If the node has another route to the packet's IP
       Destination Address in its Route Cache, the node SHOULD
       salvage the packet as described in Section 6.3.6.  Otherwise, the
       node SHOULD discard the packet.


3.4.3. Automatic Route Shortening

   Source routes in use MAY be automatically shortened if one or more
   intermediate nodes in the route become no longer necessary.  This
   mechanism of automatically shortening routes in use is somewhat
   similar to the use of passive acknowledgments [16].  In particular,
   if a node is able to overhear a packet carrying a source route (e.g.,
   by operating its network interface in promiscuous receive mode), then
   this node examines the unexpended portion of that source route.  If
   this node is not the intended next-hop destination for the packet
   but is named in the later unexpended portion of the packet's source
   route, then it can infer that the intermediate nodes before itself in
   the source route are no longer needed in the route.  For example, the
   figure below illustrates an example in which node D has overheard a
   data packet being transmitted from B to C, for later forwarding to D
   and to E:

         +-----+     +-----+     +-----+     +-----+     +-----+
         |  A  |---->|  B  |---->|  C  |     |  D  |     |  E  |
         +-----+     +-----+     +-----+     +-----+     +-----+
                        \                       ^
                         \                     /
                          ---------------------

   In this case, this node (node D) SHOULD return a "gratuitous" Route
   Reply to the original sender of the packet (node A).  The Route
   Reply gives the shorter route as the concatenation of the portion of
   the original source route up through the node that transmitted the
   overheard packet (node B), plus the suffix of the original source
   route beginning with the node returning the gratuitous Route Reply
   (node D). In this example, the route returned in the gratuitous Route
   Reply message sent from D to A gives the new route as the sequence of
   hops from A to B to D to E.





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   When deciding whether to return a gratuitous Route Reply in this way,
   a node MAY factor in additional information beyond the fact that it
   was able to overhear the packet.  For example, the node MAY decide to
   return the gratuitous Route Reply only when the overheard packet is
   received with a signal strenth or signal-to-noise ratio above some
   specific threshold.  In addition, each node maintains a Gratuitous
   Route Reply Table, as described in Section 4.4, to limit the rate at
   which it originates gratuitous Route Replies for the same returned
   route.


3.4.4. Increased Spreading of Route Error Messages

   When a source node receives a Route Error for a data packet that
   it originated, this source node propagates this Route Error to its
   neighbors by piggybacking it on its next Route Request.  In this way,
   stale information in the caches of nodes around this source node will
   not generate Route Replies that contain the same invalid link for
   which this source node received the Route Error.

   For example, in the situation shown in the example of Section 3.2,
   node A learns from the Route Error message from C, that the link
   from C to D is currently broken.  It thus removes this link from
   its own Route Cache and initiates a new Route Discovery (if it has
   no other route to E in its Route Cache).  On the Route Request
   packet initiating this Route Discovery, node A piggybacks a copy
   of this Route Error, ensuring that the Route Error spreads well to
   other nodes, and guaranteeing that any Route Reply that it receives
   (including those from other node's Route Caches) in response to this
   Route Request does not contain a route that assumes the existence of
   this broken link.






















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4. Conceptual Data Structures

   This document describes the operation of the DSR protocol in terms
   of a number of conceptual data structures.  This section describes
   each of these data structures and provides an overview of its use
   in the protocol.  In an implementation of the protocol, these data
   structures MAY be implemented in any manner consistent with the
   external behavior described in this document.


4.1. Route Cache

   All ad hoc network routing information needed by a node implementing
   DSR is stored in that node's Route Cache.  Each node in the network
   maintains its own Route Cache.  A node adds information to its
   Route Cache as it learns of new links between nodes in the ad hoc
   network; for example, a node may learn of new links when it receives
   a packet carrying a Route Request, Route Reply, or DSR source route.
   Likewise, a node removes information from its Route Cache as it
   learns that existing links in the ad hoc network have broken; for
   example, a node may learn of a broken link when it receives a packet
   carrying a Route Error or through the link-layer retransmission
   mechanism reporting a failure in forwarding a packet to its next-hop
   destination.

   Anytime a node adds new information to its Route Cache, the node
   SHOULD check each packet in its own Send Buffer (Section 4.2) to
   determine whether a route to that packet's IP Destination Address
   now exists in the node's Route Cache (including the information just
   added to the Cache).  If so, the packet SHOULD then be sent using
   that route and removed from the Send Buffer.

   It is possible to interface a DSR network with other networks,
   external to this DSR network.  Such external networks may, for
   example, be the Internet, or may be other ad hoc networks routed
   with a routing protocol other than DSR.  Such external networks may
   also be other DSR networks that are treated as external networks
   in order to improve scalability.  The complete handling of such
   external networks is beyond the scope of this document.  However,
   this document specifies a minimal set of requirements and features
   necessary to allow nodes only implementing this specification to
   interoperate correctly with nodes implementing interfaces to such
   external networks.  This minimal set of requirements and features
   involve the First Hop External (F) and Last Hop External (L)
   bits in a DSR Source Route option (Section 5.7) and a Route Reply
   option (Section 5.3) in a packet's DSR header (Section 5).  These
   requirements also include the addition of an External flag bit
   tagging each link in the Route Cache, copied from the First Hop
   External (F) and Last Hop External (L) bits in the DSR Source Route
   option or Route Reply option from which this link was learned.



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   The Route Cache SHOULD support storing more than one route to each
   destination.  In searching the Route Cache for a route to some
   destination node, the Route Cache is indexed by destination node
   address.  The following properties describe this searching function
   on a Route Cache:

    -  Each implementation of DSR at any node MAY choose any appropriate
       strategy and algorithm for searching its Route Cache and
       selecting a "best" route to the destination from among those
       found.  For example, a node MAY choose to select the shortest
       route to the destination (the shortest sequence of hops), or it
       MAY use an alternate metric to select the route from the Cache.

    -  However, if there are multiple cached routes to a destination,
       the selection of routes when searching the Route Cache MUST
       prefer routes that do not have the External flag set on any link.
       This preference will select routes that lead directly to the
       target node over routes that attempt to reach the target via any
       external networks connected to the DSR ad hoc network.

    -  In addition, any route selected when searching the Route Cache
       MUST NOT have the External bit set for any links other than
       possibly the first link, the last link, or both; the External bit
       MUST NOT be set for any intermediate hops in the route selected.

   An implementation of a Route Cache MAY provide a fixed capacity
   for the cache, or the cache size MAY be variable.  The following
   properties describe the management of available space within a node's
   Route Cache:

    -  Each implementation of DSR at each node MAY choose any
       appropriate policy for managing the entries in its Route Cache,
       such as when limited cache capacity requires a choice of which
       entries to retain in the Cache.  For example, a node MAY chose a
       "least recently used" (LRU) cache replacement policy, in which
       the entry last used longest ago is discarded from the cache if a
       decision needs to be made to allow space in the cache for some
       new entry being added.

    -  However, the Route Cache replacement policy SHOULD allow routes
       to be categorized based upon "preference", where routes with a
       higher preferences are less likely to be removed from the cache.
       For example, a node could prefer routes for which it initiated
       a Route Discovery over routes that it learned as the result of
       promiscuous snooping on other packets.  In particular, a node
       SHOULD prefer routes that it is presently using over those that
       it is not.






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   Any suitable data structure organization, consistent with this
   specification, MAY be used to implement the Route Cache in any node.
   For example, the following two types of organization are possible:

    -  In DSR, the route returned in each Route Reply that is received
       by the initiator of a Route Discovery (or that is learned from
       the header of overhead packets, as described in Section 6.1.4)
       represents a complete path (a sequence of links) leading to the
       destination node.  By caching each of these paths separately,
       a "path cache" organization for the Route Cache can be formed.
       A path cache is very simple to implement and easily guarantees
       that all routes are loop-free, since each individual route from
       a Route Reply or Route Request or used in a packet is loop-free.
       To search for a route in a path cache data structure, the sending
       node can simply search its Route Cache for any path (or prefix of
       a path) that leads to the intended destination node.

       This type of organization for the Route Cache in DSR has been
       extensively studied through simulation [5, 9, 12, 19] and
       through implementation of DSR in a mobile outdoor testbed under
       significant workload [20, 21, 22].

    -  Alternatively, a "link cache" organization could be used for the
       Route Cache, in which each individual link (hop) in the routes
       returned in Route Reply packets (or otherwise learned from the
       header of overhead packets) is added to a unified graph data
       structure of this node's current view of the network topology.
       To search for a route in link cache, the sending node must use
       a more complex graph search algorithm, such as the well-known
       Dijkstra's shortest-path algorithm, to find the current best path
       through the graph to the destination node.  Such an algorithm is
       more difficult to implement and may require significantly more
       CPU time to execute.

       However, a link cache organization is more powerful than a path
       cache organization, in its ability to effectively utilize all of
       the potential information that a node might learn about the state
       of the network.  In particular, links learned from different
       Route Discoveries or from the header of any overheard packets can
       be merged together to form new routes in the network, but this
       is not possible in a path cache due to the separation of each
       individual path in the cache.

       This type of organization for the Route Cache in DSR, including
       the effect of a range of implementation choices, has been studied
       through detailed simulation [9].

   The choice of data structure organization to use for the Route Cache
   in any DSR implementation is a local matter for each node and affects




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   only performance; any reasonable choice of organization for the Route
   Cache does not affect either correctness or interoperability.

   Each entry in the Route Cache SHOULD have a timeout associated
   with it, to allow that entry to be deleted if not used within some
   time.  The particular choice of algorithm and data structure used
   to implement the Route Cache SHOULD be considered in choosing the
   timeout for entries in the Route Cache.  The configuration variable
   RouteCacheTimeout defined in Section 9 specifies the timeout to be
   applied to entries in the Route Cache, although it is also possible
   to instead use an adaptive policy in choosing timeout values rather
   than using a single timeout setting for all entries; for example, the
   Link-MaxLife cache design (below) uses an adaptive timeout algorithm
   and does not use the RouteCacheTimeout configuration variable.

   As guidance to implementors, Appendix A describes a type of link
   cache known as "Link-MaxLife" that has been shown to outperform
   other types of link caches and path caches studied in detailed
   simulation [9].  Link-MaxLife is an adaptive link cache in which each
   link in the cache has a timeout that is determined dynamically by the
   caching node according to its observed past behavior of the two nodes
   at the ends of the link; in addition, when selecting a route for a
   packet being sent to some destination, among cached routes of equal
   length (number of hops) to that destination, Link-MaxLife selects the
   route with the longest expected lifetime (highest minimum timeout of
   any link in the route).  Use of the Link-MaxLife design for the Route
   Cache is recommended in implementations of DSR.


4.2. Send Buffer

   The Send Buffer of a node implementing DSR is a queue of packets that
   cannot be sent by that node because it does not yet have a source
   route to each such packet's destination.  Each packet in the Send
   Buffer is logically associated with the time that it was placed into
   the Buffer, and SHOULD be removed from the Send Buffer and silently
   discarded after a period of SendBufferTimeout after initially being
   placed in the Buffer.  If necessary, a FIFO strategy SHOULD be used
   to evict packets before they timeout to prevent the buffer from
   overflowing.

   Subject to the rate limiting defined in Section 6.2, a Route
   Discovery SHOULD be initiated as often as possible for the
   destination address of any packets residing in the Send Buffer.









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4.3. Route Request Table

   The Route Request Table of a node implementing DSR records
   information about Route Requests that have been recently originated
   or forwarded by this node.  The table is indexed by IP address.

   The Route Request Table on a node records the following information
   about nodes to which this node has initiated a Route Request:

    -  The Time-to-Live (TTL) field used in the IP header of the Route
       Request for the last Route Discovery initiated by this node for
       that target node.  This value allows the node to implement a
       variety of algorithms for controlling the spread of its Route
       Request on each Route Discovery initiated for a target.  As
       examples, two possible algorithms for this use of the TTL field
       are described in Section 3.3.4.

    -  The time that this node last originated a Route Request for that
       target node.

    -  The number of consecutive Route Discoveries initiated for this
       target since receiving a valid Route Reply giving a route to that
       target node.

    -  The remaining amount of time before which this node MAY next
       attempt at a Route Discovery for that target node.  When the
       node initiates a new Route Discovery for this target node, this
       field in the Route Request Table entry for that target node is
       initialized to the timeout for that Route Discovery, after which
       the node MAY initiate a new Discovery for that target.  Until
       a valid Route Reply is received for this target node address,
       a node MUST implement a back-off algorithm in determining this
       timeout value for each successive Route Discovery initiated
       for this target using the same Time-to-Live (TTL) value in the
       IP header of the Route Request packet.  The timeout between
       such consecutive Route Discovery initiations SHOULD increase by
       doubling the timeout value on each new initiation.

   In addition, the Route Request Table on a node also records the
   following information about initiator nodes from which this node has
   received a Route Request:

    -  A FIFO cache of size RequestTableIds entries containing the
       Identification value and target address from the most recent
       Route Requests received by this node from that initiator node.

   Nodes SHOULD use an LRU policy to manage the entries in their Route
   Request Table.





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   The number of Identification values to retain in each Route
   Request Table entry, RequestTableIds, MUST NOT be unlimited, since,
   in the worst case, when a node crashes and reboots, the first
   RequestTableIds Route Discoveries it initiates after rebooting
   could appear to be duplicates to the other nodes in the network.
   In addition, a node SHOULD base its initial Identification value,
   used for Route Discoveries after rebooting, on a battery backed-up
   clock or other persistent memory device, in order to help avoid
   any possible such delay in successfully discovering new routes
   after rebooting; if no such source of initial Identification
   value is available, a node after rebooting SHOULD base its initial
   Identification value on a random number.


4.4. Gratuitous Route Reply Table

   The Gratuitous Route Reply Table of a node implementing DSR records
   information about "gratuitous" Route Replies sent by this node as
   part of automatic route shortening.  As described in Section 3.4.3,
   a node returns a gratuitous Route Reply when it overhears a packet
   transmitted by some node, for which the node overhearing the
   packet was not the intended next-hop node but was named later in
   the unexpended hops of the source route in that packet; the node
   overhearing the packet returns a gratuitous Route Reply to the
   original sender of the packet, listing the shorter route (not
   including the hops of the source route "skipped over" by this
   packet).  A node uses its Gratuitous Route Reply Table to limit the
   rate at which it originates gratuitous Route Replies to the same
   original sender for the same node from which it overheard a packet to
   trigger the gratuitous Route Reply.

   Each entry in the Gratuitous Route Reply Table of a node contains the
   following fields:

    -  The address of the node to which this node originated a
       gratuitous Route Reply.

    -  The address of the node from which this node overheard the packet
       triggering that gratuitous Route Reply.

    -  The remaining time before which this entry in the Gratuitous
       Route Reply Table expires and SHOULD be deleted by the node.
       When a node creates a new entry in its Gratuitous Route Reply
       Table, the timeout value for that entry should be initialized to
       the value GratReplyHoldoff.

   When a node overhears a packet that would trigger a gratuitous
   Route Reply, if a corresponding entry already exists in the node's
   Gratuitous Route Reply Table, then the node SHOULD NOT send a
   gratuitous Route Reply for that packet.  Otherwise (no corresponding



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   entry already exists), the node SHOULD create a new entry in its
   Gratuitous Route Reply Table to record that gratuitous Route Reply,
   with a timeout value of GratReplyHoldoff.


4.5. Network Interface Queue and Maintenance Buffer

   Depending on factors such as the structure and organization of
   the operating system, protocol stack implementation, network
   interface device driver, and network interface hardware, a packet
   being transmitted could be queued in a variety of ways.  For
   example, outgoing packets from the network protocol stack might be
   queued at the operating system or link layer, before transmission
   by the network interface.  The network interface might also
   provide a retransmission mechanism for packets, such as occurs in
   IEEE 802.11 [11]; the DSR protocol, as part of Route Maintenance,
   requires limited buffering of packets already transmitted for
   which the reachability of the next-hop destination has not yet been
   determined.  The operation of DSR is defined here in terms of two
   conceptual data structures that together incorporate this queueing
   behavior.

   The Network Interface Queue of a node implementing DSR is an output
   queue of packets from the network protocol stack waiting to be
   transmitted by the network interface; for example, in the 4.4BSD
   Unix network protocol stack implementation, this queue for a network
   interface is represented as a "struct ifqueue" [33].  This queue is
   used to hold packets while the network interface is in the process of
   transmitting another packet.

   The Maintenance Buffer of a node implementing DSR is a queue of
   packets sent by this node that are awaiting next-hop reachability
   confirmation as part of Route Maintenance.  For each packet in
   the Maintenance Buffer, a node maintains a count of the number
   of retransmissions and the time of the last retransmission.  The
   Maintenance Buffer MAY be of limited size; when adding a new packet
   to the Maintenance Buffer, if the buffer size is insufficient to hold
   the new packet, the new packet SHOULD be silently discarded.  If,
   after MaxMaintRexmt attempts to confirm next-hop reachability of
   some node, no confirmation is received, all packets in this node's
   Maintenance Buffer with this next-hop destination SHOULD be removed
   from the Maintenance Buffer; in this case, the node also SHOULD
   originate a Route Error for this packet to each original source of
   a packet removed in this way (Section 6.3) and SHOULD salvage each
   packet removed in this way (Section 6.3.6) if it has another route
   to that packet's IP Destination Address in its Route Cache.  The
   definition of MaxMaintRexmt conceptually includes any retransmissions
   that might be attempted for a packet at the link layer or within
   the network interface hardware.  The timeout value to use for each
   transmission attempt for an acknowledgment request depends on the



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   type of acknowledgment mechanism used for Route Maintenance for that
   attempt, as described in Section 6.3.


4.6. Blacklist

   When a node using the DSR protocol is connected through an
   interface that requires physically bidirectional links for unicast
   transmission, it MUST keep a blacklist.  A Blacklist is a table,
   indexed by neighbor address, that indicates that the link between
   this node and the specified neighbor may not be bidirectional.  A
   node places another node's address in this list when it believes that
   broadcast packets from that other node reach this node, but that
   unicast transmission between the two nodes is not possible.  For
   example, if a node forwarding a Route Reply discovers that the next
   hop is unreachable, it places that next hop in the node's blacklist.

   Once a node discovers that it can communicate bidirectionally with
   one of the nodes listed in the blacklist, it SHOULD remove that node
   from the blacklist.  For example, if A has B in its blacklist, but
   A hears B forward a Route Request with a hop list indicating that
   the broadcast from A to B was successful, A SHOULD remove B from its
   blacklist.

   A node MUST associate a state with each node in the blacklist,
   specifying whether the unidirectionality is "questionable" or
   "probable." Each time the unreachability is positively determined,
   the node SHOULD set the state to "probable." After the unreachability
   has not been positively determined for some amount of time, the state
   should revert to "questionable." A node MAY expire nodes from its
   blacklist after a reasonable amount of time.






















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5. DSR Header Format

   The Dynamic Source Routing protocol makes use of a special header
   carrying control information that can be included in any existing IP
   packet.  This DSR header in a packet contains a small fixed-sized,
   4-octet portion, followed by a sequence of zero or more DSR options
   carrying optional information.  The end of the sequence of DSR
   options in the DSR header is implied by total length of the DSR
   header.

   For IPv4, the DSR header MUST immediately follow the IP header in the
   packet.  (If a Hop-by-Hop Options extension header, as defined in
   IPv6 [6], becomes defined for IPv4, the DSR header MUST immediately
   follow the Hop-by-Hop Options extension header, if one is present in
   the packet, and MUST otherwise immediately follow the IP header.)

   To add a DSR header to a packet, the DSR header is inserted following
   the packet's IP header, before any following header such as a
   traditional (e.g., TCP or UDP) transport layer header.  Specifically,
   the Protocol field in the IP header is used to indicate that a DSR
   header follows the IP header, and the Next Header field in the DSR
   header is used to indicate the type of protocol header (such as a
   transport layer header) following the DSR header.

   If any headers follow the DSR header in a packet, the total length
   of the DSR header (and thus the total, combined length of all DSR
   options present) MUST be a multiple of 4 octets.  This requirement
   preserves the alignment of these following headers in the packet.

























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5.1. Fixed Portion of DSR Header

   The fixed portion of the DSR header is used to carry information that
   must be present in any DSR header.  This fixed portion of the DSR
   header has the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  |    Reserved   |        Payload Length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                            Options                            .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Next Header

         8-bit selector.  Identifies the type of header immediately
         following the DSR header.  Uses the same values as the IPv4
         Protocol field [29].

      Reserved

         MUST be sent as 0 and ignored on reception.

      Payload Length

         The length of the DSR header, excluding the 4-octet fixed
         portion.  The value of the Payload Length field defines the
         total length of all options carried in the DSR header.

      Options

         Variable-length field; the length of the Options field is
         specified by the Payload Length field in this DSR header.
         Contains one or more pieces of optional information (DSR
         options), encoded in type-length-value (TLV) format (with the
         exception of the Pad1 option, described in Section 5.8).

   The placement of DSR options following the fixed portion of the DSR
   header MAY be padded for alignment.  However, due to the typically
   limited available wireless bandwidth in ad hoc networks, this padding
   is not required, and receiving nodes MUST NOT expect options within a
   DSR header to be aligned.








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   The following types of DSR options are defined in this document for
   use within a DSR header:

    -  Route Request option (Section 5.2)

    -  Route Reply option (Section 5.3)

    -  Route Error option (Section 5.4)

    -  Acknowledgment Request option (Section 5.5)

    -  Acknowledgment option (Section 5.6)

    -  DSR Source Route option (Section 5.7)

    -  Pad1 option (Section 5.8)

    -  PadN option (Section 5.9)



































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5.2. Route Request Option

   The Route Request option in a DSR header is encoded as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Option Type  |  Opt Data Len |         Identification        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Target Address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Address[1]                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Address[2]                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Address[n]                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IP fields:

      Source Address

         MUST be set to the address of the node originating this packet.
         Intermediate nodes that retransmit the packet to propagate the
         Route Request MUST NOT change this field.

      Destination Address

         MUST be set to the IP limited broadcast address
         (255.255.255.255).

      Hop Limit (TTL)

         MAY be varied from 1 to 255, for example to implement
         non-propagating Route Requests and Route Request expanding-ring
         searches (Section 3.3.4).

   Route Request fields:

      Option Type

         2

      Opt Data Len

         8-bit unsigned integer.  Length of the option, in octets,
         excluding the Option Type and Opt Data Len fields.




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      Identification

         A unique value generated by the initiator (original sender) of
         the Route Request.  Nodes initiating a Route Request generate
         a new Identification value for each Route Request, for example
         based on a sequence number counter of all Route Requests
         initiated by the node.

         This value allows a receiving node to determine whether it
         has recently seen a copy of this Route Request:  if this
         Identification value is found by this receiving node in its
         Route Request Table (in the cache of Identification values
         in the entry there for this initiating node), this receiving
         node MUST discard the Route Request.  When propagating a Route
         Request, this field MUST be copied from the received copy of
         the Route Request being propagated.

      Target Address

         The address of the node that is the target of the Route
         Request.

      Address[1..n]

         Address[i] is the address of the i-th node recorded in the
         Route Request option.  The address given in the Source Address
         field in the IP header is the address of the initiator of
         the Route Discovery and MUST NOT be listed in the Address[i]
         fields; the address given in Address[1] is thus the address
         of the first node on the path after the initiator.  The
         number of addresses present in this field is indicated by the
         Opt Data Len field in the option (n = (Opt Data Len - 6) / 4).
         Each node propagating the Route Request adds its own address to
         this list, increasing the Opt Data Len value by 4 octets.

   The Route Request option MUST NOT appear more than once within a DSR
   header.
















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5.3. Route Reply Option

   The Route Reply option in a DSR header is encoded as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   |  Option Type  |  Opt Data Len |L|   Reserved  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Address[1]                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Address[2]                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Address[n]                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IP fields:

      Source Address

         Set to the address of the node sending the Route Reply.
         In the case of a node sending a reply from its Route
         Cache (Section 3.3.2) or sending a gratuitous Route Reply
         (Section 3.4.3), this address can differ from the address that
         was the target of the Route Discovery.

      Destination Address

         MUST be set to the address of the source node of the route
         being returned.  Copied from the Source Address field of the
         Route Request generating the Route Reply, or in the case of a
         gratuitous Route Reply, copied from the Source Address field of
         the data packet triggering the gratuitous Reply.

   Route Reply fields:

      Option Type

         3

      Opt Data Len

         8-bit unsigned integer.  Length of the option, in octets,
         excluding the Option Type and Opt Data Len fields.







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      Last Hop External (L)

         Set to indicate that the last hop given by the Route Reply
         (the link from Address[n-1] to Address[n]) is actually an
         arbitrary path in a network external to the DSR network; the
         exact route outside the DSR network is not represented in the
         Route Reply.  Nodes caching this hop in their Route Cache MUST
         flag the cached hop with the External flag.  Such hops MUST NOT
         be returned in a cached Route Reply generated from this Route
         Cache entry, and selection of routes from the Route Cache to
         route a packet being sent MUST prefer routes that contain no
         hops flagged as External.

      Reserved

         MUST be sent as 0 and ignored on reception.

      Address[1..n]

         The source route being returned by the Route Reply.  The route
         indicates a sequence of hops, originating at the source node
         specified in the Destination Address field of the IP header
         of the packet carrying the Route Reply, through each of the
         Address[i] nodes in the order listed in the Route Reply,
         ending with the destination node indicated by Address[n].
         The number of addresses present in the Address[1..n]
         field is indicated by the Opt Data Len field in the option
         (n = (Opt Data Len - 1) / 4).

   A Route Reply option MAY appear one or more times within a DSR
   header.






















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5.4. Route Error Option

   The Route Error option in a DSR header is encoded as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Option Type  |  Opt Data Len |   Error Type  |Reservd|Salvage|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Error Source Address                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Error Destination Address                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                   Type-Specific Information                   .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Option Type

         4

      Opt Data Len

         8-bit unsigned integer.  Length of the option, in octets,
         excluding the Option Type and Opt Data Len fields.

         For the current definition of the Route Error option,
         this field MUST be set to 10, plus the size of any
         Type-Specific Information present in the Route Error.  Further
         extensions to the Route Error option format may also be
         included after the Type-Specific Information portion of the
         Route Error option specified above.  The presence of such
         extensions will be indicated by the Opt Data Len field.
         When the Opt Data Len is greater than that required for
         the fixed portion of the Route Error plus the necessary
         Type-Specific Information as indicated by the Option Type
         value in the option, the remaining octets are interpreted as
         extensions.  Currently, no such further extensions have been
         defined.

      Error Type

         The type of error encountered.  Currently, the following type
         value is defined:

             1 = NODE_UNREACHABLE

         Other values of the Error Type field are reserved for future
         use.



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      Reservd

         Reserved.  MUST be sent as 0 and ignored on reception.

      Salvage

         A 4-bit unsigned integer.  Copied from the Salvage field in
         the DSR Source Route option of the packet triggering the Route
         Error.

         The "total salvage count" of the Route Error option is derived
         from the value in the Salvage field of this Route Error option
         and all preceding Route Error options in the packet as follows:
         the total salvage count is the sum of, for each such Route
         Error option, one plus the value in the Salvage field of that
         Route Error option.

      Error Source Address

         The address of the node originating the Route Error (e.g., the
         node that attempted to forward a packet and discovered the link
         failure).

      Error Destination Address

         The address of the node to which the Route Error must be
         delivered For example, when the Error Type field is set to
         NODE_UNREACHABLE, this field will be set to the address of the
         node that generated the routing information claiming that the
         hop from the Error Source Address to Unreachable Node Address
         (specified in the Type-Specific Information) was a valid hop.

      Type-Specific Information

         Information specific to the Error Type of this Route Error
         message.

   Currently, the Type-Specific Information field is defined only for
   Route Error messages of type NODE_UNREACHABLE.  In this case, the
   Type-Specific Information field is defined as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Unreachable Node Address                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+







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      Unreachable Node Address

         The address of the node that was found to be unreachable
         (the next-hop neighbor to which the node with address
         Error Source Address was attempting to transmit the packet).

   A Route Error option MAY appear one or more times within a DSR
   header.













































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5.5. Acknowledgment Request Option

   The Acknowledgment Request option in a DSR header is encoded as
   follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Option Type  |  Opt Data Len |         Identification        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Option Type

         5

      Opt Data Len

         8-bit unsigned integer.  Length of the option, in octets,
         excluding the Option Type and Opt Data Len fields.

      Identification

         The Identification field is set to a unique value and is copied
         into the Identification field of the Acknowledgment option when
         returned by the node receiving the packet over this hop.

   An Acknowledgment Request option MUST NOT appear more than once
   within a DSR header.

























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5.6. Acknowledgment Option

   The Acknowledgment option in a DSR header is encoded as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Option Type  |  Opt Data Len |         Identification        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       ACK Source Address                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     ACK Destination Address                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Option Type

         6

      Opt Data Len

         8-bit unsigned integer.  Length of the option, in octets,
         excluding the Option Type and Opt Data Len fields.

      Identification

         Copied from the Identification field of the Acknowledgment
         Request option of the packet being acknowledged.

      ACK Source Address

         The address of the node originating the acknowledgment.

      ACK Destination Address

         The address of the node to which the acknowledgment is to be
         delivered.

   An Acknowledgment option MAY appear one or more times within a DSR
   header.














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5.7. DSR Source Route Option

   The DSR Source Route option in a DSR header is encoded as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Option Type  |  Opt Data Len |F|L|Reservd|Salvage| Segs Left |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Address[1]                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Address[2]                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Address[n]                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Option Type

         7

      Opt Data Len

         8-bit unsigned integer.  Length of the option, in octets,
         excluding the Option Type and Opt Data Len fields.  For the
         format of the DSR Source Route option defined here, this field
         MUST be set to the value (n * 4) + 2, where n is the number of
         addresses present in the Address[i] fields.

      First Hop External (F)

         Set to indicate that the first hop indicated by the DSR
         Source Route option is actually an arbitrary path in a network
         external to the DSR network; the exact route outside the DSR
         network is not represented in the DSR Source Route option.
         Nodes caching this hop in their Route Cache MUST flag the
         cached hop with the External flag.  Such hops MUST NOT be
         returned in a Route Reply generated from this Route Cache
         entry, and selection of routes from the Route Cache to route
         a packet being sent MUST prefer routes that contain no hops
         flagged as External.

      Last Hop External (L)

         Set to indicate that the last hop indicated by the DSR Source
         Route option is actually an arbitrary path in a network
         external to the DSR network; the exact route outside the DSR
         network is not represented in the DSR Source Route option.
         Nodes caching this hop in their Route Cache MUST flag the



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         cached hop with the External flag.  Such hops MUST NOT be
         returned in a Route Reply generated from this Route Cache
         entry, and selection of routes from the Route Cache to route
         a packet being sent MUST prefer routes that contain no hops
         flagged as External.

      Reserved

         MUST be sent as 0 and ignored on reception.

      Salvage

         A 4-bit unsigned integer.  Count of number of times that
         this packet has been salvaged as a part of DSR routing
         (Section 3.4.1).

      Segments Left (Segs Left)

         Number of route segments remaining, i.e., number of explicitly
         listed intermediate nodes still to be visited before reaching
         the final destination.

      Address[1..n]

         The sequence of addresses of the source route.  In routing
         and forwarding the packet, the source route is processed as
         described in Sections 6.1.3 and 6.1.5.  The number of addresses
         present in the Address[1..n] field is indicated by the
         Opt Data Len field in the option (n = (Opt Data Len - 2) / 4).

   When forwarding a packet along a DSR source route using a DSR Source
   Route option in the packet's DSR header, the Destination Address
   field in the packet's IP header is always set to the address of the
   packet's ultimate destination.  A node receiving a packet containing
   a DSR header with a DSR Source Route option MUST examine the
   indicated source route to determine if it is the intended next-hop
   node for the packet and determine how to forward the packet, as
   defined in Sections 6.1.4 and 6.1.5.















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5.8. Pad1 Option

   The Pad1 option in a DSR header is encoded as follows:

   +-+-+-+-+-+-+-+-+
   |  Option Type  |
   +-+-+-+-+-+-+-+-+

      Option Type

         0

   A Pad1 option MAY be included in the Options field of a DSR header
   in order to align subsequent DSR options, but such alignment is
   not required and MUST NOT be expected by a node receiving a packet
   containing a DSR header.

   If any headers follow the DSR header in a packet, the total length of
   a DSR header, indicated by the Payload Length field in the DSR header
   MUST be a multiple of 4 octets.  In this case, when building a DSR
   header in a packet, sufficient Pad1 or PadN options MUST be included
   in the Options field of the DSR header to make the total length a
   multiple of 4 octets.

   If more than one consecutive octet of padding is being inserted in
   the Options field of a DSR header, the PadN option, described next,
   SHOULD be used, rather than multiple Pad1 options.

   Note that the format of the Pad1 option is a special case; it does
   not have an Opt Data Len or Option Data field.























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5.9. PadN Option

   The PadN option in a DSR header is encoded as follows:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
   |  Option Type  |  Opt Data Len |   Option Data
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -

      Option Type

         1

      Opt Data Len

         8-bit unsigned integer.  Length of the option, in octets,
         excluding the Option Type and Opt Data Len fields.

      Option Data

         A number of zero-valued octets equal to the Opt Data Len.

   A PadN option MAY be included in the Options field of a DSR header
   in order to align subsequent DSR options, but such alignment is
   not required and MUST NOT be expected by a node receiving a packet
   containing a DSR header.

   If any headers follow the DSR header in a packet, the total length of
   a DSR header, indicated by the Payload Length field in the DSR header
   MUST be a multiple of 4 octets.  In this case, when building a DSR
   header in a packet, sufficient Pad1 or PadN options MUST be included
   in the Options field of the DSR header to make the total length a
   multiple of 4 octets.





















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6. Detailed Operation

6.1. General Packet Processing

6.1.1. Originating a Packet

   When originating any packet, a node using DSR routing MUST perform
   the following sequence of steps:

    -  Search the node's Route Cache for a route to the address given in
       the IP Destination Address field in the packet's header.

    -  If no such route is found in the Route Cache, then perform
       Route Discovery for the Destination Address, as described in
       Section 6.2.  Initiating a Route Discovery for this target node
       address results in the node adding a Route Request option in
       a DSR header in this existing packet, or saving this existing
       packet to its Send Buffer and initiating the Route Discovery
       by sending a separate packet containing such a Route Request
       option.  If the node chooses to initiate the Route Discovery
       by adding the Route Request option to this existing packet,
       it will replace the IP Destination Address field with the IP
       "limited broadcast" address (255.255.255.255) [3], copying the
       original IP Destination Address to the Target Address field of
       the new Route Request option added to the packet, as described in
       Section 6.2.1.

    -  If the packet now does not contain a Route Request option,
       then this node must have a route to the Destination Address
       of the packet; if the node has more than one route to this
       Destination Address, the node selects one to use for this packet.
       If the length of this route is greater than 1 hop, or if the
       node determines to request a DSR network-layer acknowledgment
       from the first-hop node in that route, then insert a DSR header
       into the packet, as described in Section 6.1.2, and insert a DSR
       Source Route option, as described in Section 6.1.3.  The source
       route in the packet is initialized from the selected route to the
       Destination Address of the packet.

    -  Transmit the packet to the first-hop node address given in
       selected source route, using Route Maintenance to determine the
       reachability of the next hop, as described in Section 6.3.


6.1.2. Adding a DSR Header to a Packet

   A node originating a packet adds a DSR header to the packet, if
   necessary, to carry information needed by the routing protocol.  A
   packet MUST NOT contain more than one DSR header.  A DSR header is
   added to a packet by performing the following sequence of steps



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   (these steps assume that the packet contains no other headers that
   MUST be located in the packet before the DSR header):

    -  Insert a DSR header after the IP header but before any other
       header that may be present.

    -  Set the Next Header field of the DSR header to the Protocol
       number field of the packet's IP header.

    -  Set the Protocol field of the packet's IP header to the Protocol
       number assigned for a DSR header (TBA???).


6.1.3. Adding a DSR Source Route Option to a Packet

   A node originating a packet adds a DSR Source Route option to the
   packet, if necessary, in order to carry the source route from this
   originating node to the final destination address of the packet.
   Specifically, the node adding the DSR Source Route option constructs
   the DSR Source Route option and modifies the IP packet according to
   the following sequence of steps:

    -  The node creates a DSR Source Route option, as described in
       Section 5.7, and appends it to the DSR header in the packet.
       (A DSR header is added, as described in Section 6.1.2, if not
       already present.)

    -  The number of Address[i] fields to include in the DSR Source
       Route option (n) is the number of intermediate nodes in the
       source route for the packet (i.e., excluding address of the
       originating node and the final destination address of the
       packet).  The Segments Left field in the DSR Source Route option
       is initialized equal to n.

    -  The addresses within the source route for the packet are copied
       into sequential Address[i] fields in the DSR Source Route option,
       for i = 1, 2, ..., n.

    -  The First Hop External (F) bit in the DSR Source Route option is
       copied from the External bit flagging the first hop in the source
       route for the packet, as indicated in the Route Cache.

    -  The Last Hop External (L) bit in the DSR Source Route option is
       copied from the External bit flagging the last hop in the source
       route for the packet, as indicated in the Route Cache.

    -  The Salvage field in the DSR Source Route option is
       initialized to 0.





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6.1.4. Processing a Received Packet

   When a node receives any packet (whether for forwarding, overheard,
   or as the final destination of the packet), if that packet contains a
   DSR header, then that node MUST process any options contained in that
   DSR header, in the order contained there.  Specifically:

    -  If the DSR header contains a Route Request option, the node
       SHOULD extract the source route from the Route Request and add
       this routing information to its Route Cache, subject to the
       conditions identified in Section 3.3.1.  The routing information
       from the Route Request is the sequence of hop addresses

          initiator, Address[1], Address[2], ..., Address[n]

       where initiator is the value of the Source Address field in
       the IP header of the packet carrying the Route Request (the
       address of the initiator of the Route Discovery), and each
       Address[i] is a node through which this Route Request has passed,
       in turn, during this Route Discovery.  The value n here is the
       number of addresses recorded in the Route Request option, or
       (Opt Data Len - 6) / 4.

       After possibly updating the node's Route Cache in response to
       the routing information in the Route Request option, the node
       MUST then process the Route Request option as described in
       Section 6.2.2.

    -  If the DSR header contains a Route Reply option, the node SHOULD
       extract the source route from the Route Reply and add this
       routing information to its Route Cache, subject to the conditions
       identified in Section 3.3.1.  The source route from the Route
       Reply is the sequence of hop addresses

          initiator, Address[1], Address[2], ..., Address[n]

       where initiator is the value of the Destination Address field in
       the IP header of the packet carrying the Route Reply (the address
       of the initiator of the Route Discovery), and each Address[i]
       is a node through which the source route passes, in turn, on
       the route to the target of the Route Discovery.  Address[n] is
       the address of the target.  If the Last Hop External (L) bit is
       set in the Route Reply, the node MUST flag the last hop from
       the Route Reply (the link from Address[n-1] to Address[n]) in
       its Route Cache as External.  The value n here is the number of
       addresses in the source route being returned in the Route Reply
       option, or (Opt Data Len - 1) / 4.

       After possibly updating the node's Route Cache in response to
       the routing information in the Route Reply option, then if the



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       packet's IP Destination Address matches one of this node's IP
       addresses, the node MUST then process the Route Reply option as
       described in Section 6.2.5.

    -  If the DSR header contains a Route Error option, the node MUST
       process the Route Error option as described in Section 6.3.5.

    -  If the DSR header contains an Acknowledgment Request option, the
       node MUST process the Acknowledgment Request option as described
       in Section 6.3.3.

    -  If the DSR header contains an Acknowledgment option, then subject
       to the conditions identified in Section 3.3.1, the node SHOULD
       add to its Route Cache the single link from the node identified
       by the ACK Source Address field to the node identified by the
       ACK Destination Address field.

       After possibly updating the node's Route Cache in response to
       the routing information in the Acknowledgment option, the node
       MUST then process the Acknowledgment option as described in
       Section 6.3.3.

    -  If the DSR header contains a DSR Source Route option, the node
       SHOULD extract the source route from the DSR Source Route and
       add this routing information to its Route Cache, subject to the
       conditions identified in Section 3.3.1.  If the value of the
       Salvage field in the DSR Source Route option is zero, then the
       routing information from the DSR Source Route is the sequence of
       hop addresses

          source, Address[1], Address[2], ..., Address[n], destination

       and otherwise (Salvage is nonzero), the routing information from
       the DSR Source Route is the sequence of hop addresses

          Address[1], Address[2], ..., Address[n], destination

       where source is the value of the Source Address field in the IP
       header of the packet carrying the DSR Source Route option (the
       original sender of the packet), each Address[i] is the value in
       the Address[i] field in the DSR Source Route, and destination is
       the value of the Destination Address field in the packet's IP
       header (the last-hop address of the source route).  The value n
       here is the number of addresses in source route in the DSR Source
       Route option, or (Opt Data Len - 2) / 4.

       After possibly updating the node's Route Cache in response to
       the routing information in the DSR Source Route option, the node
       MUST then process the DSR Source Route option as described in
       Section 6.1.5.



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    -  Any Pad1 or PadN options in the DSR header are ignored.

   Finally, if the Destination Address in the packet's IP header matches
   one of this receiving node's own IP address(es), remove the DSR
   header and all the included DSR options in the header, and pass the
   rest of the packet to the network layer.


6.1.5. Processing a Received DSR Source Route Option

   When a node receives a packet containing a DSR Source Route option
   (whether for forwarding, overheard, or as the final destination of
   the packet), that node SHOULD examine the packet to determine if
   the receipt of that packet indicates an opportunity for automatic
   route shortening, as described in Section 3.4.3.  Specifically, if
   this node is not the intended next-hop destination for the packet
   but is named in the later unexpended portion of the source route in
   the packet's DSR Source Route option, then this packet indicates an
   opportunity for automatic route shortening:  the intermediate nodes
   after the node from which this node overheard the packet and before
   this node itself, are no longer necessary in the source route.  In
   this case, this node SHOULD perform the following sequence of steps
   as part of automatic route shortening:

    -  The node searches its Gratuitous Route Reply Table for an entry
       describing a gratuitous Route Reply earlier sent by this node,
       for which the original sender of the packet triggering the
       gratuitous Route Reply and the transmitting node from which this
       node overheard that packet in order to trigger the gratuitous
       Route Reply, both match the respective node addresses for this
       new received packet.  If such an entry is found in the node's
       Gratuitous Route Reply Table, the node SHOULD NOT perform
       automatic route shortening in response to this receipt of this
       packet.

    -  Otherwise, the node creates an entry for this overheard packet in
       its Gratuitous Route Reply Table.  The timeout value for this new
       entry SHOULD be initialized to the value GratReplyHoldoff.  After
       this timeout has expired, the node SHOULD delete this entry from
       its Gratuitous Route Reply Table.

    -  After creating the new Gratuitous Route Reply Table entry
       above, the node originates a gratuitous Route Reply to the
       IP Source Address of this overheard packet, as described in
       Section 3.4.3.

       If the MAC protocol in use in the network is not capable of
       transmitting unicast packets over unidirectional links, as
       discussed in Section 3.3.1, then in originating this Route Reply,
       the node MUST use a source route for routing the Route Reply



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       packet that is obtained by reversing the sequence of hops over
       which the packet triggering the gratuitous Route Reply was routed
       in reaching and being overheard by this node; this reversing of
       the route uses the gratuitous Route Reply to test this sequence
       of hops for bidirectionality, preventing the gratuitous Route
       Reply from being received by the initiator of the Route Discovery
       unless each of the hops over which the gratuitous Route Reply is
       returned is bidirectional.

    -  Discard the overheard packet, since the packet has been received
       before its normal traversal of the packet's source route would
       have caused it to reach this receiving node.  Another copy of
       the packet will normally arrive at this node as indicated in
       the packet's source route; discarding this initial copy of the
       packet, which triggered the gratuitous Route Reply, will prevent
       the duplication of this packet that would otherwise occur.

   If the packet is not discarded as part of automatic route shortening
   above, then the node MUST process the option according to the
   following sequence of steps:

    -  If the value of the Segments Left field in the DSR Source Route
       option equals 0, then remove the DSR Source Route option from the
       DSR header.

    -  Else, let n equal (Opt Data Len - 2) / 4.  This is the number of
       addresses in the DSR Source Route option.

    -  If the value of the Segments Left field is greater than n, then
       send an ICMP Parameter Problem, Code 0, message [26] to the IP
       Source Address, pointing to the Segments Left field, and discard
       the packet.  Do not process the DSR Source Route option further.

    -  Else, decrement the value of the Segments Left field by 1.  Let i
       equal n minus Segments Left.  This is the index of the next
       address to be visited in the Address vector.

    -  If Address[i] or the IP Destination Address is a multicast
       address, then discard the packet.  Do not process the DSR Source
       Route option further.

    -  If the MTU of the link over which this node would transmit
       the packet to forward it to the node Address[i] is less than
       the size of the packet, the node MUST either discard the
       packet and send an ICMP Packet Too Big message to the packet's
       Source Address [26] or fragment it as specified in Section 8.

    -  Forward the packet to the IP address specified in the Address[i]
       field of the IP header, following normal IP forwarding
       procedures, including checking and decrementing the Time-to-Live



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       (TTL) field in the packet's IP header [27, 3].  In this
       forwarding of the packet, the next-hop node (identified by
       Address[i]) MUST be treated as a direct neighbor node:  the
       transmission to that next node MUST be done in a single IP
       forwarding hop, without Route Discovery and without searching the
       Route Cache.

    -  In forwarding the packet, perform Route Maintenance for the
       next hop of the packet, by verifying that the next-hop node is
       reachable, as described in Section 6.3.

   Multicast addresses MUST NOT appear in a DSR Source Route option or
   in the IP Destination Address field of a packet carrying a DSR Source
   Route option in a DSR header.







































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6.2. Route Discovery Processing

   Route Discovery is the mechanism by which a node S wishing to send a
   packet to a destination node D obtains a source route to D.  Route
   Discovery is used only when S attempts to send a packet to D and
   does not already know a route to D.  The node initiating a Route
   Discovery is known as the "initiator" of the Route Discovery, and the
   destination node for which the Route Discovery is initiated is known
   as the "target" of the Route Discovery.

   Route Discovery operates entirely on demand, with a node initiating
   Route Discovery based on its own origination of new packets for
   some destination address to which it does not currently know a
   route.  Route Discovery does not depend on any periodic or background
   exchange of routing information or neighbor node detection at any
   layer in the network protocol stack at any node.

   The Route Discovery procedure utilizes two types of messages, a Route
   Request (Section 5.2) and a Route Reply (Section 5.3), to actively
   search the ad hoc network for a route to the desired destination.
   These DSR messages MAY be carried in any type of IP packet, through
   use of the DSR header as described in Section 5.

   Except as discussed in Section 6.3.5, a Route Discovery for a
   destination address SHOULD NOT be initiated unless the initiating
   node has a packet in its Send Buffer requiring delivery to that
   destination.  A Route Discovery for a given target node MUST NOT be
   initiated unless permitted by the rate-limiting information contained
   in the Route Request Table.  After each Route Discovery attempt, the
   interval between successive Route Discoveries for this target SHOULD
   be doubled, up to a maximum of MaxRequestPeriod, until a valid Route
   Reply is received for this target.


6.2.1. Originating a Route Request

   A node initiating a Route Discovery for some target creates and
   initializes a Route Request option in a DSR header in some IP packet.
   This MAY be a separate IP packet, used only to carry this Route
   Request option, or the node MAY include the Route Request option
   in some existing packet that it needs to send to the target node
   (e.g., the IP packet originated by this node, that caused the node to
   attempt Route Discovery for the destination address of the packet).
   The Route Request option MUST be included in a DSR header in the
   packet.  To initialize the Route Request option, the node performs
   the following sequence of steps:

    -  The Option Type in the option MUST be set to the value 2.





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    -  The Opt Data Len field in the option MUST be set to the value 6.
       The total size of the Route Request option when initiated
       is 8 octets; the Opt Data Len field excludes the size of the
       Option Type and Opt Data Len fields themselves.

    -  The Identification field in the option MUST be set to a new
       value, different from that used for other Route Requests recently
       initiated by this node for this same target address.  For
       example, each node MAY maintain a single counter value for
       generating a new Identification value for each Route Request it
       initiates.

    -  The Target Address field in the option MUST be set to the IP
       address that is the target of this Route Discovery.

   The Source Address in the IP header of this packet MUST be the node's
   own IP address.  The Destination Address in the IP header of this
   packet MUST be the IP "limited broadcast" address (255.255.255.255).

   A node MUST maintain in its Route Request Table, information about
   Route Requests that it initiates.  When initiating a new Route
   Request, the node MUST use the information recorded in the Route
   Request Table entry for the target of that Route Request, and it MUST
   update that information in the table entry for use in the next Route
   Request initiated for this target.  In particular:

    -  The Route Request Table entry for a target node records the
       Time-to-Live (TTL) field used in the IP header of the Route
       Request for the last Route Discovery initiated by this node for
       that target node.  This value allows the node to implement a
       variety of algorithms for controlling the spread of its Route
       Request on each Route Discovery initiated for a target.  As
       examples, two possible algorithms for this use of the TTL field
       are described in Section 3.3.4.

    -  The Route Request Table entry for a target node records the
       number of consecutive Route Requests initiated for this target
       since receiving a valid Route Reply giving a route to that target
       node, and the remaining amount of time before which this node MAY
       next attempt at a Route Discovery for that target node.

       A node MUST use these values to implement a back-off algorithm to
       limit the rate at which this node initiates new Route Discoveries
       for the same target address.  In particular, until a valid Route
       Reply is received for this target node address, the timeout
       between consecutive Route Discovery initiations for this target
       node with the same hop limit SHOULD increase by doubling the
       timeout value on each new initiation.





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   The behavior of a node processing a packet containing DSR header
   with both a DSR Source Route option and a Route Request option is
   unspecified.  Packets SHOULD NOT contain both a DSR Source Route
   option and a Route Request option.

   Packets containing a Route Request option SHOULD NOT include
   an Acknowledgment Request option, SHOULD NOT expect link-layer
   acknowledgment or passive acknowledgment, and SHOULD NOT be
   retransmitted.  The retransmission of packets containing a Route
   Request option is controlled solely by the logic described in this
   section.


6.2.2. Processing a Received Route Request Option

   When a node receives a packet containing a Route Request option, that
   node MUST process the option according to the following sequence of
   steps:

    -  If the Target Address field in the Route Request matches this
       node's own IP address, then the node SHOULD return a Route Reply
       to the initiator of this Route Request (the Source Address in the
       IP header of the packet), as described in Section 6.2.4.  The
       source route for this Reply is the sequence of hop addresses

          initiator, Address[1], Address[2], ..., Address[n], target

       where initiator is the address of the initiator of this
       Route Request, each Address[i] is an address from the Route
       Request, and target is the target of the Route Request (the
       Target Address field in the Route Request).  The value n here
       is the number of addresses recorded in the Route Request, or
       (Opt Data Len - 6) / 4.

       The node then MUST replace the Destination Address field in
       the Route Request packet's IP header with the value in the
       Target Address field in the Route Request option, and continue
       processing the rest of the Route Request packet normally.  The
       node MUST NOT process the Route Request option further and MUST
       NOT retransmit the Route Request to propagate it to other nodes
       as part of the Route Discovery.

    -  Else, the node MUST examine the route recorded in the Route
       Request option (the IP Source Address field and the sequence of
       Address[i] fields) to determine if this node's own IP address
       already appears in this list of addresses.  If so, the node MUST
       discard the entire packet carrying the Route Request option.

    -  Else, if the Route Request through a network interface that
       requires physically bidirectional links for unicast transmission,



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       the node MUST check if the Request was last forwarded by a node
       on its blacklist.  If such an entry is found, and the state of
       the unidirectional link is "probable," then the Request MUST be
       silently discarded.

    -  Else, if the Route Request through a network interface that
       requires physically bidirectional links for unicast transmission,
       the node MUST check if the Request was last forwarded by a node
       on its blacklist.  If such an entry is found, and the state of
       the unidirectional link is "questionable," then the node MUST
       create and unicast a Route Request packet to that previous node,
       setting the IP Time-To-Live (TTL) to 1 to prevent the Request
       from being propagated.  If the node receives a Route Reply in
       response to the new Request, it MUST remove the blacklist entry
       for that node, and SHOULD continue processing.  If the node does
       not receive a Reply within some reasonable amount of time, MUST
       silently discard the Route Request packet.

    -  Else, the node MUST search its Route Request Table for an entry
       for the initiator of this Route Request (the IP Source Address
       field).  If such an entry is found in the table, the node MUST
       search the cache of Identification values of recently received
       Route Requests in that table entry, to determine if an entry
       is present in the cache matching the Identification value
       and target node address in this Route Request.  If such an
       (Identification, target address) entry is found in this cache in
       this entry in the Route Request Table, then the node MUST discard
       the entire packet carrying the Route Request option.

    -  Else, this node SHOULD further process the Route Request
       according to the following sequence of steps:

        o  Add an entry for this Route Request in its cache of
           (Identification, target address) values of recently received
           Route Requests.

        o  Conceptually create a copy of this entire packet and perform
           the following steps on the copy of the packet.

        o  Append this node's own IP address to the list of Address[i]
           values in the Route Request, and increase the value of the
           Opt Data Len field in the Route Request by 4 (the size of an
           IP address).

        o  This node SHOULD search its own Route Cache for a route
           (from itself, as if it were the source of a packet) to the
           target of this Route Request.  If such a route is found in
           its Route Cache, then this node SHOULD follow the procedure
           outlined in Section 6.2.3 to return a "cached Route Reply"




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           to the initiator of this Route Request, if permitted by the
           restrictions specified there.

        o  If the node does not return a cached Route Reply, then this
           node SHOULD link-layer re-broadcast this copy of the packet,
           with a short jitter delay before the broadcast is sent.  The
           jitter period SHOULD be chosen as a random period, uniformly
           distributed between 0 and BroadcastJitter.


6.2.3. Generating a Route Reply using the Route Cache

   As described in Section 3.3.2, it is possible for a node processing a
   received Route Request to avoid propagating the Route Request further
   toward the target of the Request, if this node has in its Route Cache
   a route from itself to this target.  Such a Route Reply generated by
   a node from its own cached route to the target of a Route Request is
   called a "cached Route Reply", and this mechanism can greatly reduce
   the overall overhead of Route Discovery on the network by reducing
   the flood of Route Requests.  The general processing of a received
   Route Request is described in Section 6.2.2; this section specifies
   the additional requirements that MUST be met before a cached Route
   Reply may be generated and returned and specifies the procedure for
   returning such a cached Route Reply.

   While processing a received Route Request, for a node to possibly
   return a cached Route Reply, it MUST have in its Route Cache a route
   from itself to the target of this Route Request.  However, before
   generating a cached Route Reply for this Route Request, the node MUST
   verify that there are no duplicate addresses listed in the route
   accumulated in the Route Request together with the route from this
   node's Route Cache.  Specifically, there MUST be no duplicates among
   the following addresses:

    -  The IP Source Address of the packet containing the Route Request,

    -  The Address[i] fields in the Route Request, and

    -  The nodes listed in the route obtained from this node's Route
       Cache, excluding the address of this node itself (this node
       itself is the common point between the route accumulated in the
       Route Request and the route obtained from the Route Cache).

   If any duplicates exist among these addresses, then the node MUST NOT
   send a cached Route Reply.  The node SHOULD continue to process the
   Route Request as described in Section 6.2.2.

   If the Route Request and the route from the Route Cache meet the
   restriction above, then the node SHOULD construct and return a cached
   Route Reply as follows:



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    -  The source route for this reply is the sequence of hop addresses

          initiator, Address[1], Address[2], ..., Address[n], c-route

       where initiator is the address of the initiator of this Route
       Request, each Address[i] is an address from the Route Request,
       and c-route is the sequence of hop addresses in the source route
       to this target node, obtained from the node's Route Cache.  In
       appending this cached route to the source route for the reply,
       the address of this node itself MUST be excluded, since it is
       already listed as Address[n].

    -  Send a Route Reply to the initiator of the Route Request, using
       the procedure defined in Section 6.2.4.  The initiator of the
       Route Request is indicated in the Source Address field in the
       packet's IP header.

   If the node returns a cached Route Reply as described above, then
   the node MUST NOT propagate the Route Request further (i.e., the
   node MUST NOT rebroadcast the Route Request).  In this case, instead,
   if the packet contains no other DSR options and contains no payload
   after the DSR header (e.g., the Route Request is not piggybacked
   on a TCP or UDP packet), then the node SHOULD simply discard the
   packet.  Otherwise (if the packet contains other DSR options or
   contains any payload after the DSR header), the node SHOULD forward
   the packet along the cached route to the target of the Route Request.
   Specifically, if the node does so, it MUST use the following
   steps:

    -  Copy the Target Address from the Route Request option in the
       DSR header to the Destination Address field in the packet's IP
       header.

    -  Remove the Route Request option from the DSR header in the
       packet, and add a DSR Source Route option to the packet's DSR
       header.

    -  In the DSR Source Route option, set the Address[i] fields
       to represent the source route found in this node's Route
       Cache to the original target of the Route Discovery (the
       new IP Destination Address of the packet).  Specifically,
       the node copies the hop addresses of the source route into
       sequential Address[i] fields in the DSR Source Route option,
       for i = 1, 2, ..., n.  Address[1] here is the address of this
       node itself (the first address in the source route found from
       this node to the original target of the Route Discovery).  The
       value n here is the number of hop addresses in this source route,
       excluding the destination of the packet (which is instead already
       represented in the Destination Address field in the packet's IP
       header).



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    -  Initialize the Segments Left field in the DSR Source Route option
       to n as defined above.

    -  The First Hop External (F) bit in the DSR Source Route option is
       copied from the External bit flagging the first hop in the source
       route for the packet, as indicated in the Route Cache.

    -  The Last Hop External (L) bit in the DSR Source Route option is
       copied from the External bit flagging the last hop in the source
       route for the packet, as indicated in the Route Cache.

    -  The Salvage field in the DSR Source Route option MUST be
       initialized to some nonzero value; the particular nonzero value
       used SHOULD be MAX_SALVAGE_COUNT.  By initializing this field to
       a nonzero value, nodes forwarding or overhearing this packet will
       not consider a link to exist between the IP Source Address of the
       packet and the Address[1] address in the DSR Source Route option
       (e.g., they will not attempt to add this to their Route Cache as
       a link).  By choosing MAX_SALVAGE_COUNT as the nonzero value to
       which the node initializes this field, nodes furthermore will not
       attempt to salvage this packet.

    -  Transmit the packet to the next-hop node on the new source route
       in the packet, using the forwarding procedure described in
       Section 6.1.5.


6.2.4. Originating a Route Reply

   A node originates a Route Reply in order to reply to a received and
   processed Route Request, according to the procedures described in
   Sections 6.2.2 and 6.2.3.  The Route Reply is returned in a Route
   Reply option (Section 5.3).  The Route Reply option MAY be returned
   to the initiator of the Route Request in a separate IP packet, used
   only to carry this Route Reply option, or it MAY be included in any
   other IP packet being sent to this address.

   The Route Reply option MUST be included in a DSR header in the packet
   returned to the initiator.  To initialize the Route Reply option, the
   node performs the following sequence of steps:

    -  The Option Type in the option MUST be set to the value 3.

    -  The Opt Data Len field in the option MUST be set to the value
       (n * 4) + 3, where n is the number of addresses in the source
       route being returned (excluding the Route Discovery initiator
       node's address).

    -  The Last Hop External (L) bit in the option MUST be
       initialized to 0.



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    -  The Reserved field in the option MUST be initialized to 0.

    -  The Route Request Identifier MUST be initialized to the
       Identifier field of the Route Request that this reply is sent in
       response to.

    -  The sequence of hop addresses in the source route are copied into
       the Address[i] fields of the option.  Address[1] MUST be set to
       the first-hop address of the route after the initiator of the
       Route Discovery, Address[n] MUST be set to the last-hop address
       of the source route (the address of the target node), and each
       other Address[i] MUST be set to the next address in sequence in
       the source route being returned.

   The Destination Address field in the IP header of the packet carrying
   the Route Reply option MUST be set to the address of the initiator
   of the Route Discovery (i.e., for a Route Reply being returned in
   response to some Route Request, the IP Source Address of the Route
   Request).

   After creating and initializing the Route Reply option and the IP
   packet containing it, send the Route Reply.  In sending the Route
   Reply from this node (but not from nodes forwarding the Route Reply),
   this node SHOULD delay the Reply by a small jitter period chosen
   randomly between 0 and BroadcastJitter.

   When returning any Route Reply in the case in which the MAC protocol
   in use in the network is not capable of transmitting unicast packets
   over unidirectional links, the source route used for routing the
   Route Reply packet MUST be obtained by reversing the sequence
   of hops in the Route Request packet (the source route that is
   then returned in the Route Reply).  This restriction on returning
   a Route Reply enables the Route Reply to test this sequence of
   hops for bidirectionality, preventing the Route Reply from being
   received by the initiator of the Route Discovery unless each of
   the hops over which the Route Reply is returned (and thus each
   of the hops in the source route being returned in the Reply) is
   bidirectional.

   If sending a Route Reply to the initiator of the Route Request
   requires performing a Route Discovery, the Route Reply Option MUST
   be piggybacked on the packet that contains the Route Request.  This
   piggybacking prevents a loop wherein the target of the new Route
   Request (which was itself the initiator of the original Route
   Request) must do another Route Request in order to return its
   Route Reply.

   If sending the Route Reply to the initiator of the Route Request
   does not require performing a Route Discovery, a node SHOULD send a




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   unicast Route Reply in response to every Route Request it receives
   for which it is the target node.


6.2.5. Processing a Received Route Reply Option

   Section 6.1.4 describes the general processing for a received packet,
   including the addition of routing information from options in the
   packet's DSR header to the receiving node's Route Cache.

   If the received packet contains a Route Reply, no additional special
   processing of the Route Reply option is required beyond what is
   described there.  As described in Section 4.1 anytime a node adds
   new information to its Route Cache (including the information added
   from this Route Reply option), the node SHOULD check each packet in
   its own Send Buffer (Section 4.2) to determine whether a route to
   that packet's IP Destination Address now exists in the node's Route
   Cache (including the information just added to the Cache).  If so,
   the packet SHOULD then be sent using that route and removed from the
   Send Buffer.  This general procedure handles all processing required
   for a received Route Reply option.

   When a MAC protocol requires bidirectional links for unicast
   transmission, a unidirectional link may be discovered by the
   propagation of the Route Request.  When the Route Reply is sent over
   the reverse path, a forwarding node may discover that the next-hop is
   unreachable.  In this case, it MUST add the next-hop address to its
   blacklist.

























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6.3. Route Maintenance Processing

   Route Maintenance is the mechanism by which a source node S is able
   to detect, while using a source route to some destination node D,
   if the network topology has changed such that it can no longer use
   its route to D because a link along the route no longer works.  When
   Route Maintenance indicates that a source route is broken, S can
   attempt to use any other route it happens to know to D, or can invoke
   Route Discovery again to find a new route for subsequent packets
   to D.  Route Maintenance for this route is used only when S is
   actually sending packets to D.

   Specifically, when forwarding a packet, a node MUST attempt
   to confirm the reachability of the next-hop node, unless such
   confirmation had been received in the last MaintHoldoffTime.
   Individual implementations MAY choose to bypass such confirmation
   for some limited number of packets, as long as those packets
   all fall within MaintHoldoffTime within the last confirmation.
   If no confirmation is received after the retransmission of
   MaxMaintRexmt acknowledgment requests, after the initial transmission
   of the packet, and conceptually including all retransmissions
   provided by the MAC layer, the node determines that the link
   for this next-hop node of the source route is "broken".  This
   confirmation from the next-hop node for Route Maintenance can be
   implemented using a link-layer acknowledgment (Section 6.3.1),
   using a "passive acknowledgment" (Section 6.3.2), or using a
   network-layer acknowledgment (Section 6.3.3); the particular strategy
   for retransmission timing depends on the type of acknowledgment
   mechanism used.  When passive acknowledgments are being used, each
   retransmitted acknowledgment request SHOULD be explicit software
   acknowledgment requests.  If no acknowledgment is received after
   MaxMaintRexmt retransmissions (if necessary), the node SHOULD
   originate a Route Error to the original sender of the packet, as
   described in Section 6.3.4.

   In deciding whether or not to send a Route Error in response to
   attempting to forward a packet from some sender over a broken link,
   a node MUST limit the number of consecutive packets from a single
   sender that the node attempts to forward over this same broken
   link for which the node chooses not to return a Route Error; this
   requirement MAY be satisfied by returning a Route Error for each
   packet that the node attempts to forward over a broken link.


6.3.1. Using Link-Layer Acknowledgments

   If the MAC protocol in use provides feedback as to the successful
   delivery of a data packet (such as is provided by the link-layer
   acknowledgment frame defined by IEEE 802.11 [11]), then the use
   of the DSR Acknowledgment Request and Acknowledgment options



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   is not necessary.  If such link-layer feedback is available, it
   SHOULD be used instead of any other acknowledgment mechanism for
   Route Maintenance, and the node SHOULD NOT use either passive
   acknowledgments or network-layer acknowledgments for Route
   Maintenance.

   When using link-layer acknowledgments for Route Maintenance, the
   retransmission timing and the timing at which retransmission attempts
   are scheduled are generally controlled by the particular link layer
   implementation in use in the network.  For example, in IEEE 802.11,
   the link-layer acknowledgment is returned after the data packet as
   a part of the basic access method of of the IEEE 802.11 Distributed
   Coordination Function (DCF) MAC protocol; the time at which the
   acknowledgment is expected to arrive and the time at which the next
   retransmission attempt (if necessary) will occur are controlled by
   the MAC protocol implementation.

   When a node receives a link-layer acknowledgment for any packet in
   its Maintenance Buffer, that node SHOULD remove that packet, as well
   as any other packets in its Maintenance Buffer with the same next-hop
   destination, from its Maintenance Buffer.


6.3.2. Using Passive Acknowledgments

   When link-layer acknowledgments are not available, but passive
   acknowledgments [16] are available, passive acknowledgments SHOULD be
   used for Route Maintenance when originating or forwarding a packet
   along any hop other than the last hop (the hop leading to the IP
   Destination Address node of the packet).  In particular, passive
   acknowledgments SHOULD be used for Route Maintenance in such cases if
   the node can place its network interface into "promiscuous" receive
   mode, and network links used for data packets generally operate
   bidirectionally.

   A node MUST NOT attempt to use passive acknowledgments for Route
   Maintenance for a packet originated or forwarded over its last hop
   (the hop leading to the IP Destination Address node of the packet),
   since the receiving node will not be forwarding the packet and thus
   no passive acknowledgment will be available to be heard by this node.
   Beyond this restriction, a node MAY utilize a variety of strategies
   in using passive acknowledgments for Route Maintenance of a packet
   that it originates or forwards.  For example, the following two
   strategies are possible:

    -  Each time a node receives a packet to be forwarded to a node
       other than the final destination (the IP Destination Address of
       the packet), that node sends the original transmission of that
       packet without requesting a network-layer acknowledgment for it.
       If no passive acknowledgment is received within PassiveAckTimeout



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       after this transmission, the node retransmits the packet, again
       without requesting a network-layer acknowledgment for it; the
       same PassiveAckTimeout timeout value is used for each such
       attempt.  If no acknowledgment has been received after a total
       of TryPassiveAcks retransmissions of the packet, network-layer
       acknowledgments (as described in Section 6.3.3) are used for all
       remaining attempts for that packet.

    -  Each node keeps a table of possible next-hop destination nodes,
       noting whether or not passive acknowledgments can typically
       be expected from transmission to that node, and the expected
       latency and jitter of a passive acknowledgment from that node.
       Each time a node receives a packet to be forwarded to a node
       other than the IP Destination Address, the node checks its table
       of next-hop destination nodes to determine whether to use a
       passive acknowledgment or a network-layer acknowledgment for
       that transmission to that node.  The timeout for this packet
       can also be derived from this table.  A node using this method
       SHOULD prefer using passive acknowledgments to network-layer
       acknowledgments.

   In using passive acknowledgments for a packet that it originates or
   forwards, a node considers the later receipt of a new packet (e.g.,
   with promiscuous receive mode enabled on its network interface) to be
   an acknowledgment of this first packet if both of the following two
   tests succeed:

    -  The Source Address, Destination Address, Protocol,
       Identification, and Fragment Offset fields in the IP header
       of the two packets MUST match [27], and

    -  If either packet contains a DSR Source Route header, both packets
       MUST contain one, and the value in the Segments Left field in the
       DSR Source Route header of the new packet MUST be less than that
       in the first packet.

   When a node hears such a passive acknowledgment for any packet in its
   Maintenance Buffer, that node SHOULD remove that packet, as well as
   any other packets in its Maintenance Buffer with the same next-hop
   destination, from its Maintenance Buffer.


6.3.3. Using Network-Layer Acknowledgments

   When a node originates or forwards a packet and has no other
   mechanism of acknowledgment available to determine reachability of
   the next-hop node in the source route for Route Maintenance, that
   node SHOULD request a network-layer acknowledgment from that next-hop
   node.  To do so, the node inserts an Acknowledgment Request option
   in the DSR header in the packet.  The Identification field in that



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   Acknowledgment Request option MUST be set to a value unique over all
   packets transmitted by this node to the same next-hop node that are
   either unacknowledged or recently acknowledged.

   When a node receives a packet containing an Acknowledgment Request
   option, then that node performs the following tests on the packet:

    -  If the indicated next-hop node address for this packet does not
       match any of this node's own IP addresses, then this node MUST
       NOT process the Acknowledgment Request option.  The indicated
       next-hop node address is the next Address[i] field in the DSR
       Source Route option in the DSR header in the packet, or is the IP
       Destination Address in the packet if the packet does not contain
       a DSR Source Route option or the Segments Left there is zero.

    -  If the packet contains an Acknowledgment option, then this node
       MUST NOT process the Acknowledgment Request option.

   If neither of the tests above fails, then this node MUST process the
   Acknowledgment Request option by sending an Acknowledgment option
   to the previous-hop node; to do so, the node performs the following
   sequence of steps:

    -  Create a packet and set the IP Protocol field to the protocol
       number assigned for a DSR header (TBA???).

    -  Set the IP Source Address field in this packet to the IP address
       of this node, copied from the source route in the DSR Source
       Route option in that packet (or from the IP Destination Address
       field of the packet, if the packet does not contain a DSR Source
       Route option).

    -  Set the IP Destination Address field in this packet to the IP
       address of the previous-hop node, copied from the source route
       in the DSR Source Route option in that packet (or from the IP
       Source Address field of the packet, if the packet does not
       contain a DSR Source Route option).

    -  Add a DSR header to the packet, and set the DSR header's
       Next Header field to the "No Next Header" value.

    -  Add an Acknowledgment option to the DSR header in the packet;
       set the Acknowledgment option's Option Type field to 6 and the
       Opt Data Len field to 10.

    -  Copy the Identification field from the received Acknowledgment
       Request option into the Identification field in the
       Acknowledgment option.





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    -  Set the ACK Source Address field in the Acknowledgment option to
       be the IP Source Address of this new packet (set above to be the
       IP address of this node).

    -  Set the ACK Destination Address field in the Acknowledgment
       option to be the IP Destination Address of this new packet (set
       above to be the IP address of the previous-hop node).

    -  Send the packet as described in Section 6.1.1.

   Packets containing an Acknowledgment option SHOULD NOT be placed in
   the Maintenance Buffer.

   When a node receives a packet with both an Acknowledgment option and
   an Acknowledgment Request option, if that node is not the destination
   of the Acknowledgment option (the IP Destination Address of the
   packet), then the Acknowledgment Request option MUST be ignored.
   Otherwise (that node is the destination of the Acknowledgment
   option), that node MUST process the Acknowledgment Request option
   by returning an Acknowledgment option according to the following
   sequence of steps:

    -  Create a packet and set the IP Protocol field to the protocol
       number assigned for a DSR header (TBA???).

    -  Set the IP Source Address field in this packet to the IP address
       of this node, copied from the source route in the DSR Source
       Route option in that packet (or from the IP Destination Address
       field of the packet, if the packet does not contain a DSR Source
       Route option).

    -  Set the IP Destination Address field in this packet to the IP
       address of the node originating the Acknowledgment option.

    -  Add a DSR header to the packet, and set the DSR header's
       Next Header field to the "No Next Header" value.

    -  Add an Acknowledgment option to the DSR header in this packet;
       set the Acknowledgment option's Option Type field to 6 and the
       Opt Data Len field to 10.

    -  Copy the Identification field from the received Acknowledgment
       Request option into the Identification field in the
       Acknowledgment option.

    -  Set the ACK Source Address field in the option to be the IP
       Source Address of this new packet (set above to be the IP address
       of this node).





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    -  Set the ACK Destination Address field in the option to be the IP
       Destination Address of this new packet (set above to be the IP
       address of the node originating the Acknowledgment option.)

    -  Send the packet directly to the destination.  The IP
       Destination Address MUST be treated as a direct neighbor node:
       the transmission to that node MUST be done in a single IP
       forwarding hop, without Route Discovery and without searching
       the Route Cache.  In addition, this packet MUST NOT contain a
       DSR Acknowledgment Request, MUST NOT be retransmitted for Route
       Maintenance, and MUST NOT expect a link-layer acknowledgment or
       passive acknowledgment.

   When using network-layer acknowledgments for Route Maintenance,
   a node SHOULD use an adaptive algorithm in determining the
   retransmission timeout for each transmission attempt of an
   acknowledgment request.  For example, a node SHOULD maintain a
   separate round-trip time (RTT) estimate for each to which it has
   recently attempted to transmit packets, and it SHOULD use this RTT
   estimate in setting the timeout for each retransmission attempt
   for Route Maintenance.  The TCP RTT estimation algorithm has been
   shown to work well for this purpose in implementation and testbed
   experiments with DSR [20, 22].


6.3.4. Originating a Route Error

   When a node is unable to verify reachability of a next-hop node after
   reaching a maximum number of retransmission attempts, a node SHOULD
   send a Route Error to the IP Source Address of the packet.  When
   sending a Route Error for a packet containing either a Route Error
   option or an Acknowledgment option, a node SHOULD add these existing
   options to its Route Error, subject to the limit described below.

   A node transmitting a Route Error MUST perform the following steps:

    -  Create an IP packet and set the Source Address field in this
       packet's IP header to the address of this node.

    -  If the Salvage field in the DSR Source Route option in the
       packet triggering the Route Error is zero, then copy the
       Source Address field of the packet triggering the Route Error
       into the Destination Address field in the new packet's IP
       header; otherwise, copy the Address[1] field from the DSR Source
       Route option of the packet triggering the Route Error into the
       Destination Address field in the new packet's IP header

    -  Insert a DSR header into the new packet.





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    -  Add a Route Error Option to the new packet, setting the Error
       Type to NODE_UNREACHABLE, the Salvage value to the Salvage
       value from the DSR Source Route option of the packet triggering
       the Route Error, and the Unreachable Node Address field to
       the address of the next-hop node from the original source
       route.  Set the Error Source Address field to this node's IP
       address, and the Error Destination field to the new packet's IP
       Destination Address.

    -  If the packet triggering the Route Error contains any Route Error
       or Acknowledgment options, the node MAY append to its Route Error
       each of these options, with the following constraints:

        o  The node MUST NOT include any Route Error option from the
           packet triggering the new Route Error, for which the total
           salvage count (Section 5.4) of that included Route Error
           would be greater than MAX_SALVAGE_COUNT in the new packet.

        o  If any Route Error option from the packet triggering the new
           Route Error is not included in the packet, the node MUST NOT
           include any following Route Error or Acknowledgment options
           from the packet triggering the new Route Error.

        o  Any appended options from the packet triggering the Route
           Error MUST follow the new Route Error in the packet.

        o  In appending these options to the new Route Error, the order
           of these options from the packet triggering the Route Error
           MUST be preserved.

    -  Send the packet as described in Section 6.1.1.


6.3.5. Processing a Received Route Error Option

   When a node receives a packet containing a Route Error option, that
   node MUST process the Route Error option according to the following
   sequence of steps:

    -  The node MUST remove from its Route Cache the link from the
       node identified by the Error Source Address field to the node
       identified by the Unreachable Node Address field (if this link is
       present in its Route Cache).  If the node implements its Route
       Cache as a link cache, as described in Section 4.1, only this
       single link is removed; if the node implements its Route Cache as
       a path cache, however, all routes (paths) that use this link are
       removed.

    -  If the option following the Route Error is an Acknowledgment
       or Route Error option sent by this node (that is, with



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       Acknowledgment or Error Source Address equal to this node's
       address), copy the DSR options following the current Route Error
       into a new packet with IP Source Address equal to this node's own
       IP address and IP Destination Address equal to the Acknowledgment
       or Error Destination Address.  Transmit this packet as described
       in Section 6.1.1, with the salvage count in the DSR Source Route
       option set to the Salvage value of the Route Error.

   In addition, after processing the Route Error as described above,
   the node MAY initiate a new Route Discovery for any destination node
   for which it then has no route in its Route Cache as a result of
   processing this Route Error, if the node has indication that a route
   to that destination is needed.  For example, if the node has an open
   TCP connection to some destination node, then if the processing of
   this Route Error removed the only route to that destination from this
   node's Route Cache, then this node MAY initiate a new Route Discovery
   for that destination node.  Any node, however, MUST limit the rate at
   which it initiates new Route Discoveries for any single destination
   address, and any new Route Discovery initiated in this way as part of
   processing this Route Error MUST conform to this limit.


6.3.6. Salvaging a Packet

   When an intermediate node forwarding a packet detects through Route
   Maintenance that the next-hop link along the route for that packet is
   broken (Section 6.3), if the node has another route to the packet's
   IP Destination Address in its Route Cache, the node SHOULD "salvage"
   the packet rather than discarding it.  To do so using the route found
   in its Route Cache, this node processes the packet as follows:

    -  If the MAC protocol in use in the network is not capable of
       transmitting unicast packets over unidirectional links, as
       discussed in Section 3.3.1, then if this packet contains a Route
       Reply option, remove and discard the Route Reply option in the
       packet; if the DSR header in the packet then contains no DSR
       options, remove the DSR header from the packet.  If the resulting
       packet then contains only an IP header, the node SHOULD NOT
       salvage the packet and instead SHOULD discard the entire packet.

       When returning any Route Reply in the case in which the MAC
       protocol in use in the network is not capable of transmitting
       unicast packets over unidirectional links, the source route
       used for routing the Route Reply packet MUST be obtained by
       reversing the sequence of hops in the Route Request packet (the
       source route that is then returned in the Route Reply).  This
       restriction on returning a Route Reply and on salvaging a packet
       that contains a Route Reply option enables the Route Reply to
       test this sequence of hops for bidirectionality, preventing the
       Route Reply from being received by the initiator of the Route



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       Discovery unless each of the hops over which the Route Reply is
       returned (and thus each of the hops in the source route being
       returned in the Reply) is bidirectional.

    -  Modify the existing DSR Source Route option in the packet so
       that the Address[i] fields represent the source route found in
       this node's Route Cache to this packet's IP Destination Address.
       Specifically, the node copies the hop addresses of the source
       route into sequential Address[i] fields in the DSR Source Route
       option, for i = 1, 2, ..., n.  Address[1] here is the address
       of the salvaging node itself (the first address in the source
       route found from this node to the IP Destination Address of the
       packet).  The value n here is the number of hop addresses in this
       source route, excluding the destination of the packet (which is
       instead already represented in the Destination Address field in
       the packet's IP header).

    -  Initialize the Segments Left field in the DSR Source Route option
       to n as defined above.

    -  The First Hop External (F) bit in the DSR Source Route option is
       copied from the External bit flagging the first hop in the source
       route for the packet, as indicated in the Route Cache.

    -  The Last Hop External (L) bit in the DSR Source Route option is
       copied from the External bit flagging the last hop in the source
       route for the packet, as indicated in the Route Cache.

    -  The Salvage field in the DSR Source Route option is set to 1 plus
       the value of the Salvage field in the DSR Source Route option of
       the packet that caused the error.

    -  Transmit the packet to the next-hop node on the new source route
       in the packet, using the forwarding procedure described in
       Section 6.1.5.

   As described in Section 6.3.4, the node in this case also SHOULD
   return a Route Error to the original sender of the packet.  If the
   node chooses to salvage the packet, it SHOULD do so after originating
   the Route Error.













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7. Multiple Interface Support

   A node in DSR MAY have multiple network interfaces that support
   ad hoc network routing.  This section describes special packet
   processing at such nodes.

   A node with multiple network interfaces MUST have some policy for
   determining which Request packets are forwarded out which network
   interfaces.  For example, a node MAY choose to forward all Requests
   out all network interfaces.

   When a node with multiple network interfaces propagates a Route
   Request on an network interface other than the one it received the
   Request on, it MUST modify the address list between receipt and
   re-propagation as follows:

    -  Append the address of the incoming interface

    -  If the incoming interface and outgoing interface differ in
       whether or not they require bidirectionality for unicast
       transmission, append the address 127.0.0.1

    -  If the incoming interface and outgoing interface differ in
       whether or not unidirectional links are common, append the
       address 127.0.0.2

    -  Append the address of the outgoing interface

   When a node forwards a packet containing a source route, it MUST
   assume that the next hop is reachable on the incoming interface,
   unless the next hop is the address of one of this node's interfaces,
   in which case this node MUST process the packet in the same way as if
   the node had just received it from that interface.

   If a node which previously had multiple network interfaces receives a
   packet sent with a source route specifying an interface change to an
   interface that is no longer available, it MAY send a Route Error to
   the source of the packet without attempting to forward the packet on
   the incoming interface, unless the network uses an autoconfiguration
   mechanism that may have allowed another node to acquire the now
   unused address of the unavailable interface.

   Source routes MUST never contain the special addresses 127.0.0.1 and
   127.0.0.2.









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8. Fragmentation and Reassembly

   When a node using DSR wishes to fragment a packet that contains a DSR
   header not containing a Route Request option, it MUST perform the
   following sequence of steps:

    -  Remove the DSR header from the packet.

    -  Fragment the packet.

    -  IP-in-IP encapsulate each fragment.

    -  Add the DSR header to each fragment.  If a Source Route header is
       present in the DSR header, increment the Salvage field.

   When a node using the DSR protocol receives an IP-in-IP encapsulated
   packet destined to itself, it SHOULD decapsulate the packet and
   reassemble any fragments contained inside, in accordance with
   RFC 791 [27].


































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9. Protocol Constants and Configuration Variables

   Any DSR implementation MUST support the following configuration
   variables and MUST support a mechanism enabling the value of these
   variables to be modified by system management.  The specific variable
   names are used for demonstration purposes only, and an implementation
   is not required to use these names for the configuration variables,
   so long as the external behavior of the implementation is consistent
   with that described in this document.

   For each configuration variable below, the default value is specified
   to simplify configuration.  In particular, the default values given
   below are chosen for a DSR network running over 2 Mbps IEEE 802.11
   network interfaces using the Distributed Coordination Function (DCF)
   MAC with RTS and CTS [11, 5].

       BroadcastJitter                     10   milliseconds

       RouteCacheTimeout                  300   seconds

       SendBufferTimeout                   30   seconds

       RequestTableSize                    64   nodes
       RequestTableIds                     16   identifiers
       MaxRequestRexmt                     16   retransmissions
       MaxRequestPeriod                    10   seconds
       RequestPeriod                      500   milliseconds
       NonpropRequestTimeout               30   milliseconds

       RexmtBufferSize                     50   packets

       MaintHoldoffTime                   250   milliseconds
       MaxMaintRexmt                        2   retransmissions

       TryPassiveAcks                       1   attempt
       PassiveAckTimeout                  100   milliseconds

       GratReplyHoldoff                     1   second

   In addition, the following protocol constant MUST be supported by any
   implementation of the DSR protocol:

       MAX_SALVAGE_COUNT                   15   salvages










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10. IANA Considerations

   This document proposes the use of a DSR header, which requires an IP
   Protocol number.

   In addition, this document proposes use of the value "No Next Header"
   (originally defined for use in IPv6) within an IPv4 packet, to
   indicate that no further header follows a DSR header.













































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11. Security Considerations

   This document does not specifically address security concerns.  This
   document does assume that all nodes participating in the DSR protocol
   do so in good faith and without malicious intent to corrupt the
   routing ability of the network.  In mission-oriented environments
   where all the nodes participating in the DSR protocol share a
   common goal that motivates their participation in the protocol, the
   communications between the nodes can be encrypted at the physical
   channel or link layer to prevent attack by outsiders.











































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Appendix A. Link-MaxLife Cache Description

   As guidance to implementors of DSR, the description below outlines
   the operation of a possible implementation of a Route Cache for DSR
   that has been shown to outperform other other caches studied in
   detailed simulations.  Use of this design for the Route Cache is
   recommended in implementations of DSR.

   This cache, called "Link-MaxLife" [9], is a link cache, in that each
   individual link (hop) in the routes returned in Route Reply packets
   (or otherwise learned from the header of overhead packets) is added
   to a unified graph data structure of this node's current view of the
   network topology, as described in Section 4.1.  To search for a route
   in this cache to some destination node, the sending node uses a graph
   search algorithm, such as the well-known Dijkstra's shortest-path
   algorithm, to find the current best path through the graph to the
   destination node.

   The Link-MaxLife form of link cache is adaptive in that each link in
   the cache has a timeout that is determined dynamically by the caching
   node according to its observed past behavior of the two nodes at the
   ends of the link; in addition, when selecting a route for a packet
   being sent to some destination, among cached routes of equal length
   (number of hops) to that destination, Link-MaxLife selects the route
   with the longest expected lifetime (highest minimum timeout of any
   link in the route).

   Specifically, in Link-MaxLife, a link's timeout in the Route Cache
   is chosen according to a "Stability Table" maintained by the caching
   node.  Each entry in a node's Stability Table records the address of
   another node and a factor representing the perceived "stability" of
   this node.  The stability of each other node in a node's Stability
   Table is initialized to InitStability.  When a link from the Route
   Cache is used in routing a packet originated or salvaged by that
   node, the stability metric for each of the two endpoint nodes of that
   link is incremented by the amount of time since that link was last
   used, multiplied by StabilityIncrFactor (StabilityIncrFactor >= 1);
   when a link is observed to break and the link is thus removed
   from the Route Cache, the stability metric for each of the two
   endpoint nodes of that link is multiplied by StabilityDecrFactor
   (StabilityDecrFactor < 1).

   When a node adds a new link to its Route Cache, the node assigns a
   lifetime for that link in the Cache equal to the stability of the
   less "stable" of the two endpoint nodes for the link, except that a
   link is not allowed to be given a lifetime less than MinLifetime.
   When a link is used in a route chosen for a packet originated or
   salvaged by this node, the link's lifetime is set to be at least
   UseExtends into the future; if the lifetime of that link in the




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   Route Cache is already further into the future, the lifetime remains
   unchanged.

   When a node using Link-MaxLife selects a route from its Route Cache
   for a packet being originated or salvaged by this node, it selects
   the shortest-length route that has the longest expected lifetime
   (highest minimum timeout of any link in the route), as opposed to
   simply selecting an arbitrary route of shortest length.

   The following configuration variables are used in the description
   of Link-MaxLife above.  The specific variable names are used for
   demonstration purposes only, and an implementation is not required
   to use these names for these configuration variables.  For each
   configuration variable below, the default value is specified to
   simplify configuration.  In particular, the default values given
   below are chosen for a DSR network where nodes move at relative
   velocities between 12 and 25 seconds per transmission radius.

       InitStability                       25   seconds
       StabilityIncrFactor                  4
       StabilityDecrFactor                  2

       MinLifetime                          1   second
       UseExtends                         120   seconds





























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Appendix B. Location of DSR in the ISO Network Reference Model

   When designing DSR, we had to determine at what layer within
   the protocol hierarchy to implement ad hoc network routing.  We
   considered two different options:  routing at the link layer (ISO
   layer 2) and routing at the network layer (ISO layer 3).  Originally,
   we opted to route at the link layer for several reasons:

    -  Pragmatically, running the DSR protocol at the link layer
       maximizes the number of mobile nodes that can participate in
       ad hoc networks.  For example, the protocol can route equally
       well between IPv4 [27], IPv6 [6], and IPX [32] nodes.

    -  Historically [13, 14], DSR grew from our contemplation of
       a multi-hop propagating version of the Internet's Address
       Resolution Protocol (ARP) [25], as well as from the routing
       mechanism used in IEEE 802 source routing bridges [24].  These
       are layer 2 protocols.

    -  Technically, we designed DSR to be simple enough that it could
       be implemented directly in the firmware inside wireless network
       interface cards [13, 14], well below the layer 3 software within
       a mobile node.  We see great potential in this for DSR running
       inside a cloud of mobile nodes around a fixed base station,
       where DSR would act to transparently extend the coverage range
       to these nodes.  Mobile nodes that would otherwise be unable
       to communicate with the base station due to factors such as
       distance, fading, or local interference sources could then reach
       the base station through their peers.

   Ultimately, however, we decided to specify and to implement [20]
   DSR as a layer 3 protocol, since this is the only layer at which we
   could realistically support nodes with multiple network interfaces of
   different types forming an ad hoc network.



















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Appendix C. Implementation and Evaluation Status

   The initial design of the DSR protocol, including DSR's basic Route
   Discovery and Route Maintenance mechanisms, was first published in
   December 1994 [13], with significant additional design details and
   initial simulation results published in early 1996 [14].

   The DSR protocol has been extensively studied since then through
   additional detailed simulations.  In particular, we have implemented
   DSR in the ns-2 network simulator [23, 5] and performed extensive
   simulations of DSR using ns-2 (e.g., [5, 19]).  We have also
   conducted evaluations of different caching strategies documented in
   this draft [9].

   We have also implemented the DSR protocol under the FreeBSD 2.2.7
   operating system running on Intel x86 platforms.  FreeBSD [8] is
   based on a variety of free software, including 4.4 BSD Lite from the
   University of California, Berkeley.  For the environments in which
   we used it, this implementation is functionally equivalent to the
   version of the DSR protocol specified in this draft.

   During the 7 months from August 1998 to February 1999, we designed
   and implemented a full-scale physical testbed to enable the
   evaluation of ad hoc network performance in the field, in an actively
   mobile ad hoc network under realistic communication workloads.  The
   last week of February and the first week of March of 1999 included
   demonstrations of this testbed to a number of our sponsors and
   partners, including Lucent Technologies, Bell Atlantic, and DARPA.
   A complete description of the testbed is available as a Technical
   Report [20].

   We have since ported this implementation of DSR to FreeBSD 3.3, and
   we have also added a preliminary version of Quality of Service (QoS)
   support for DSR. A demonstration of this modified version of DSR was
   presented in July 2000.  These QoS features are not included in this
   draft, and will be added later in a separate draft on top of the base
   protocol specified here.

   DSR has also been implemented under Linux by Alex Song at the
   University of Queensland, Australia [31].  This implementation
   supports the Intel x86 PC platform and the Compaq iPAQ.

   The Network and Telecommunications Research Group at Trinity College
   Dublin have implemented a version of DSR on Windows CE.

   Several other independent groups have also used DSR as a platform for
   their own research, or and as a basis of comparison between ad hoc
   network routing protocols.





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Changes from Previous Version of the Draft

   This appendix briefly lists some of the major changes in this
   draft relative to the previous version of this same draft,
   draft-ietf-manet-dsr-06.txt:

    -  Added a blacklist mechanism for handling unidirectional links
       when the network interface requires bidirectionality.

    -  Added language describing multiple interface support.

    -  Described fragmentation and reassembly processing.

    -  Updated the implementation and evaluation list.







































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Acknowledgements

   The protocol described in this draft has been designed and developed
   within the Monarch Project, a research project at Rice University
   (previously at Carnegie Mellon University) that is developing
   adaptive networking protocols and protocol interfaces to allow truly
   seamless wireless and mobile node networking [15, 30].

   The authors would like to acknowledge the substantial contributions
   of Josh Broch in helping to design, simulate, and implement the DSR
   protocol.  Josh is currently on leave of absence from Carnegie Mellon
   University at AON Networks.  We thank him for his contributions to
   earlier versions of this draft.

   We would also like to acknowledge the assistance of Robert V. Barron
   at Carnegie Mellon University.  Bob ported our DSR implementation
   from FreeBSD 2.2.7 into FreeBSD 3.3.

   Many valuable suggestions came from participants in the IETF process.
   We would like to acknowledge Fred Baker, who provided extensive
   feedback on our previous draft, as well as the working group chairs,
   for their suggestions of previous versions of the draft.































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References

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        Alternatives.  IEEE Network, 8(2):43--53, March/April 1994.

    [2] Vaduvur Bharghavan, Alan Demers, Scott Shenker, and Lixia
        Zhang.  MACAW: A Media Access Protocol for Wireless LAN's.  In
        Proceedings of the ACM SIGCOMM '94 Conference, pages 212--225,
        August 1994.

    [3] Robert T. Braden, editor.  Requirements for Internet
        Hosts---Communication Layers.  RFC 1122, October 1989.

    [4] Scott Bradner.  Key words for use in RFCs to Indicate
        Requirement Levels.  RFC 2119, March 1997.

    [5] Josh Broch, David A. Maltz, David B. Johnson, Yih-Chun Hu,
        and Jorjeta Jetcheva.  A Performance Comparison of Multi-Hop
        Wireless Ad Hoc Network Routing Protocols.  In Proceedings of
        the Fourth Annual ACM/IEEE International Conference on Mobile
        Computing and Networking, pages 85--97, October 1998.

    [6] Stephen E. Deering and Robert M. Hinden.  Internet Protocol
        Version 6 (IPv6) Specification.  RFC 2460, December 1998.

    [7] Ralph Droms.  Dynamic Host Configuration Protocol.  RFC 2131,
        March 1997.

    [8] The FreeBSD Project.  Project web page available at
        http://www.freebsd.org/.

    [9] Yih-Chun Hu and David B. Johnson.  Caching Strategies in
        On-Demand Routing Protocols for Wireless Ad Hoc Networks.  In
        Proceedings of the Sixth Annual ACM International Conference on
        Mobile Computing and Networking, August 2000.

   [10] Yih-Chun Hu, David B. Johnson, and David A. Maltz.  Flow
        State in the Dynamic Source Routing Protocol for Mobile Ad Hoc
        Networks.  Internet-Draft, draft-ietf-manet-dsrflow-00.txt,
        February 2001.  Work in progress.

   [11] IEEE Computer Society LAN MAN Standards Committee.  Wireless
        LAN Medium Access Control (MAC) and Physical Layer (PHY)
        Specifications, IEEE Std 802.11-1997.  The Institute of
        Electrical and Electronics Engineers, New York, New York, 1997.

   [12] Per Johansson, Tony Larsson, Nicklas Hedman, Bartosz Mielczarek,
        and Mikael Degermark.  Scenario-based Performance Analysis of
        Routing Protocols for Mobile Ad-hoc Networks.  In Proceedings




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        of the Fifth Annual ACM/IEEE International Conference on Mobile
        Computing and Networking, pages 195--206, August 1999.

   [13] David B. Johnson.  Routing in Ad Hoc Networks of Mobile Hosts.
        In Proceedings of the IEEE Workshop on Mobile Computing Systems
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   [14] David B. Johnson and David A. Maltz.  Dynamic Source Routing in
        Ad Hoc Wireless Networks.  In Mobile Computing, edited by Tomasz
        Imielinski and Hank Korth, chapter 5, pages 153--181. Kluwer
        Academic Publishers, 1996.

   [15] David B. Johnson and David A. Maltz.  Protocols for Adaptive
        Wireless and Mobile Networking.  IEEE Personal Communications,
        3(1):34--42, February 1996.

   [16] John Jubin and Janet D. Tornow.  The DARPA Packet Radio Network
        Protocols.  Proceedings of the IEEE, 75(1):21--32, January 1987.

   [17] Phil Karn.  MACA---A New Channel Access Method for Packet Radio.
        In ARRL/CRRL Amateur Radio 9th Computer Networking Conference,
        pages 134--140, September 1990.

   [18] Gregory S. Lauer.  Packet-Radio Routing.  In Routing in
        Communications Networks, edited by Martha E. Steenstrup,
        chapter 11, pages 351--396. Prentice-Hall, Englewood Cliffs,
        New Jersey, 1995.

   [19] David A. Maltz, Josh Broch, Jorjeta Jetcheva, and David B.
        Johnson.  The Effects of On-Demand Behavior in Routing Protocols
        for Multi-Hop Wireless Ad Hoc Networks.  IEEE Journal on
        Selected Areas of Communications, 17(8):1439--1453, August 1999.

   [20] David A. Maltz, Josh Broch, and David B. Johnson.  Experiences
        Designing and Building a Multi-Hop Wireless Ad Hoc Network
        Testbed.  Technical Report CMU-CS-99-116, School of Computer
        Science, Carnegie Mellon University, Pittsburgh, Pennsylvania,
        March 1999.

   [21] David A. Maltz, Josh Broch, and David B. Johnson.  Quantitative
        Lessons From a Full-Scale Multi-Hop Wireless Ad Hoc Network
        Testbed.  In Proceedings of the IEEE Wireless Communications and
        Networking Conference, September 2000.

   [22] David A. Maltz, Josh Broch, and David B. Johnson.  Lessons From
        a Full-Scale MultiHop Wireless Ad Hoc Network Testbed.  IEEE
        Personal Communications, 8(1):8--15, February 2001.

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   [24] Radia Perlman.  Interconnections:  Bridges and Routers.
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   [25] David C. Plummer.  An Ethernet Address Resolution Protocol:
        Or Converting Network Protocol Addresses to 48.bit Ethernet
        Addresses for Transmission on Ethernet Hardware.  RFC 826,
        November 1982.

   [26] J. B. Postel, editor.  Internet Control Message Protocol.
        RFC 792, September 1981.

   [27] J. B. Postel, editor.  Internet Protocol.  RFC 791, September
        1981.

   [28] J. B. Postel, editor.  Transmission Control Protocol.  RFC 793,
        September 1981.

   [29] Joyce K. Reynolds and Jon Postel.  Assigned Numbers.  RFC 1700,
        October 1994.  See also http://www.iana.org/numbers.html.

   [30] Rice University Monarch Project.  Monarch Project Home Page.
        Available at http://www.monarch.cs.rice.edu/.

   [31] Alex Song.  picoNet II: A Wireless Ad Hoc Network for Mobile
        Handheld Devices.  Submitted for the degree of Bachelor of
        Engineering (Honours) in the division of Electrical Engineering,
        Department of Information Technology and Electrical Engineering,
        University of Queensland, Australia, October 2001.  Available at
        http://student.uq.edu.au/~s369677/main.html.

   [32] Paul Turner.  NetWare Communications Processes.  NetWare
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        Massachusetts, 1995.
















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Chair's Address

   The MANET Working Group can be contacted via its current chairs:


   M. Scott Corson                        Phone: +1 908 947-7033
   Flarion Technologies, Inc.             Email: corson@flarion.com
   Bedminster One
   135 Route 202/206 South
   Bedminster, NJ  07921
   USA


   Joseph Macker                          Phone: +1 202 767-2001
   Information Technology Division        Email: macker@itd.nrl.navy.mil
   Naval Research Laboratory
   Washington, DC  20375
   USA



































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Authors' Addresses

   Questions about this document can also be directed to the authors:


   David B. Johnson                       Phone: +1 713 348-3063
   Rice University                        Fax:   +1 713 348-5930
   Computer Science Department, MS 132    Email: dbj@cs.rice.edu
   6100 Main Street
   Houston, TX 77005-1892
   USA


   David A. Maltz                         Phone: +1 650 688-3128
   AON Networks                           Fax:   +1 650 688-3119
   3045 Park Blvd.                        Email: dmaltz@cs.cmu.edu
   Palo Alto, CA 94306
   USA


   Yih-Chun Hu                            Phone: +1 412 268-3075
   Rice University                        Fax:   +1 412 268-5576
   Computer Science Department, MS 132    Email: yihchun@cs.cmu.edu
   6100 Main Street
   Houston, TX 77005-1892
   USA


   Jorjeta G. Jetcheva                    Phone: +1 412 268-3053
   Carnegie Mellon University             Fax:   +1 412 268-5576
   Computer Science Department            Email: jorjeta@cs.cmu.edu
   5000 Forbes Avenue
   Pittsburgh, PA  15213-3891
   USA



















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