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rfc:rfc4728

Network Working Group D. Johnson Request for Comments: 4728 Rice University Category: Experimental Y. Hu

                                                                  UIUC
                                                              D. Maltz
                                                    Microsoft Research
                                                         February 2007
             The Dynamic Source Routing Protocol (DSR)
                for Mobile Ad Hoc Networks for IPv4

Status of This Memo

 This memo defines an Experimental Protocol for the Internet
 community.  It does not specify an Internet standard of any kind.
 Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The IETF Trust (2007).

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 routes to arbitrary destinations in the ad hoc network.
 All aspects of the protocol operate entirely on demand, allowing the
 routing packet overhead of DSR to scale automatically to only what is
 needed to react to changes in the routes currently in use.  The
 protocol allows multiple routes to any destination and allows each
 sender to select and control the routes used in routing its packets,
 for example, for use in load balancing or for increased robustness.
 Other advantages of the DSR protocol include easily guaranteed loop-
 free routing, operation in networks containing unidirectional links,
 use of only "soft state" in routing, and very rapid recovery when
 routes in the network change.  The DSR protocol is designed mainly
 for mobile ad hoc networks of up to about two hundred nodes and is
 designed to work well even with very high rates of mobility.  This
 document specifies the operation of the DSR protocol for routing
 unicast IPv4 packets.

Johnson, et al. Experimental [Page 1] RFC 4728 The Dynamic Source Routing Protocol February 2007

Table of Contents

 1. Introduction ....................................................5
 2. Assumptions .....................................................7
 3. DSR Protocol Overview ...........................................9
    3.1. Basic DSR Route Discovery .................................10
    3.2. Basic DSR Route Maintenance ...............................12
    3.3. Additional Route Discovery Features .......................14
         3.3.1. Caching Overheard Routing Information ..............14
         3.3.2. Replying to Route Requests Using Cached Routes .....15
         3.3.3. Route Request Hop Limits ...........................16
    3.4. Additional Route Maintenance Features .....................17
         3.4.1. Packet Salvaging ...................................17
         3.4.2. Queued Packets Destined over a Broken Link .........18
         3.4.3. Automatic Route Shortening .........................19
         3.4.4. Increased Spreading of Route Error Messages ........20
    3.5. Optional DSR Flow State Extension .........................20
         3.5.1. Flow Establishment .................................21
         3.5.2. Receiving and Forwarding Establishment Packets .....22
         3.5.3. Sending Packets along Established Flows ............22
         3.5.4. Receiving and Forwarding Packets Sent along
                Established Flows ..................................23
         3.5.5. Processing Route Errors ............................24
         3.5.6. Interaction with Automatic Route Shortening ........24
         3.5.7. Loop Detection .....................................25
         3.5.8. Acknowledgement Destination ........................25
         3.5.9. Crash Recovery .....................................25
         3.5.10. Rate Limiting .....................................25
         3.5.11. Interaction with Packet Salvaging .................26
 4. Conceptual Data Structures .....................................26
    4.1. Route Cache ...............................................26
    4.2. Send Buffer ...............................................30
    4.3. Route Request Table .......................................30
    4.4. Gratuitous Route Reply Table ..............................31
    4.5. Network Interface Queue and Maintenance Buffer ............32
    4.6. Blacklist .................................................33
 5. Additional Conceptual Data Structures for Flow State
    Extension ......................................................34
    5.1. Flow Table ................................................34
    5.2. Automatic Route Shortening Table ..........................35
    5.3. Default Flow ID Table .....................................36
 6. DSR Options Header Format ......................................36
    6.1. Fixed Portion of DSR Options Header .......................37
    6.2. Route Request Option ......................................40
    6.3. Route Reply Option ........................................42

Johnson, et al. Experimental [Page 2] RFC 4728 The Dynamic Source Routing Protocol February 2007

    6.4. Route Error Option ........................................44
         6.4.1. Node Unreachable Type-Specific Information .........46
         6.4.2. Flow State Not Supported Type-Specific
                Information ........................................46
         6.4.3. Option Not Supported Type-Specific Information .....46
    6.5. Acknowledgement Request Option ............................46
    6.6. Acknowledgement Option ....................................47
    6.7. DSR Source Route Option ...................................48
    6.8. Pad1 Option ...............................................50
    6.9. PadN Option ...............................................50
 7. Additional Header Formats and Options for Flow State
    Extension ......................................................51
    7.1. DSR Flow State Header .....................................52
    7.2. New Options and Extensions in DSR Options Header ..........52
         7.2.1. Timeout Option .....................................52
         7.2.2. Destination and Flow ID Option .....................53
    7.3. New Error Types for Route Error Option ....................54
         7.3.1. Unknown Flow Type-Specific Information .............54
         7.3.2. Default Flow Unknown Type-Specific Information .....55
    7.4. New Acknowledgement Request Option Extension ..............55
         7.4.1. Previous Hop Address Extension .....................55
 8. Detailed Operation .............................................56
    8.1. General Packet Processing .................................56
         8.1.1. Originating a Packet ...............................56
         8.1.2. Adding a DSR Options Header to a Packet ............57
         8.1.3. Adding a DSR Source Route Option to a Packet .......57
         8.1.4. Processing a Received Packet .......................58
         8.1.5. Processing a Received DSR Source Route Option ......60
         8.1.6. Handling an Unknown DSR Option .....................63
    8.2. Route Discovery Processing ................................64
         8.2.1. Originating a Route Request ........................65
         8.2.2. Processing a Received Route Request Option .........66
         8.2.3. Generating a Route Reply Using the Route Cache .....68
         8.2.4. Originating a Route Reply ..........................71
         8.2.5. Preventing Route Reply Storms ......................72
         8.2.6. Processing a Received Route Reply Option ...........74
    8.3. Route Maintenance Processing ..............................74
         8.3.1. Using Link-Layer Acknowledgements ..................75
         8.3.2. Using Passive Acknowledgements .....................76
         8.3.3. Using Network-Layer Acknowledgements ...............77
         8.3.4. Originating a Route Error ..........................80
         8.3.5. Processing a Received Route Error Option ...........81
         8.3.6. Salvaging a Packet .................................82
    8.4. Multiple Network Interface Support ........................84
    8.5. IP Fragmentation and Reassembly ...........................84
    8.6. Flow State Processing .....................................85
         8.6.1. Originating a Packet ...............................85
         8.6.2. Inserting a DSR Flow State Header ..................88

Johnson, et al. Experimental [Page 3] RFC 4728 The Dynamic Source Routing Protocol February 2007

         8.6.3. Receiving a Packet .................................88
         8.6.4. Forwarding a Packet Using Flow IDs .................93
         8.6.5. Promiscuously Receiving a Packet ...................93
         8.6.6. Operation Where the Layer below DSR
                Decreases the IP TTL ...............................94
         8.6.7. Salvage Interactions with DSR ......................94
 9. Protocol Constants and Configuration Variables .................95
 10. IANA Considerations ...........................................96
 11. Security Considerations .......................................96
 Appendix A. Link-MaxLife Cache Description ........................97
 Appendix B. Location of DSR in the ISO Network Reference Model ....99
 Appendix C. Implementation and Evaluation Status .................100
 Acknowledgements .................................................101
 Normative References .............................................102
 Informative References ...........................................102

Johnson, et al. Experimental [Page 4] RFC 4728 The Dynamic Source Routing Protocol February 2007

1. Introduction

 The Dynamic Source Routing protocol (DSR) [JOHNSON94, JOHNSON96a] 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.
 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:
  1. 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.
  1. 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 it 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.
 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

Johnson, et al. Experimental [Page 5] RFC 4728 The Dynamic Source Routing Protocol February 2007

 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 what is 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.
 All state maintained by DSR is "soft state" [CLARK88], in that the
 loss of any state will not interfere with the correct operation of
 the protocol; all state is discovered as needed and can easily and
 quickly be rediscovered if needed after a failure without significant
 impact on the protocol.  This use of only soft state allows the
 routing protocol to be very robust to problems such as dropped or
 delayed routing packets or node failures.  In particular, a node in
 DSR that fails and reboots can easily rejoin the network immediately
 after rebooting; if the failed node was involved in forwarding
 packets for other nodes as an intermediate hop along one or more
 routes, it can also resume this forwarding quickly after rebooting,
 with no or minimal interruption to the routing 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 support for multiple routes
 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 sender of a
 packet selects and controls the route used for its own packets,
 which, together with support for multiple routes, also allows
 features such as load balancing to be defined.  In addition, all
 routes used are easily guaranteed to be loop-free, since the sender
 can avoid duplicate hops in the routes selected.
 The operation of both Route Discovery and Route Maintenance in DSR
 are designed to allow unidirectional links and asymmetric routes to
 be 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 transmit power
 levels or sources of interference.
 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

Johnson, et al. Experimental [Page 6] RFC 4728 The Dynamic Source Routing Protocol February 2007

 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 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
 [RFC2460], will be 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.  As described in Appendix C, the
 design of DSR has been extensively studied through detailed
 simulations and testbed implementation and demonstration; this
 document encourages additional implementation and experimentation
 with the protocol.
 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 [RFC2119].

2. Assumptions

 As described here, the DSR protocol is designed mainly for mobile ad
 hoc networks of up to about two hundred nodes and is designed to work
 well even with very high rates of mobility.  Other protocol features
 and enhancements that may allow DSR to scale to larger networks are
 outside the scope of this document.
 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 it may often be greater than 1.

Johnson, et al. Experimental [Page 7] RFC 4728 The Dynamic Source Routing Protocol February 2007

 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, such as through a standard link-layer checksum or
 Cyclic Redundancy Check (CRC), 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 and capacity are more the
 inherent limiting factors 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 [JOHNSON96a].  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
 transmit power levels or sources of interference around the two nodes
 [BANTZ94, LAUER95].  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.
 Some Medium Access Control (MAC) protocols, however, such as MACA
 [KARN90], MACAW [BHARGHAVAN94], or IEEE 802.11 [IEEE80211], limit
 unicast data packet transmission to bidirectional links, due to the
 required bidirectional exchange of request to send (RTS) and clear to
 send (CTS) packets in these protocols and to the link-layer
 acknowledgement feature in IEEE 802.11.  When used on top of MAC

Johnson, et al. Experimental [Page 8] RFC 4728 The Dynamic Source Routing Protocol February 2007

 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 Dynamic Host
 Configuration Protocol (DHCP) for dynamic assignment [RFC2131]),
 although the method of such assignment is outside the scope of this
 specification.
 A routing protocol such as DSR chooses a next-hop for each packet and
 provides the IP address of that next-hop.  When the packet is
 transmitted, however, the lower-layer protocol often has a separate,
 MAC-layer address for the next-hop node.  DSR uses the Address
 Resolution Protocol (ARP) [RFC826] to translate from next-hop IP
 addresses to next-hop MAC addresses.  In addition, a node MAY add an
 entry to its ARP cache based on any received packet, when the IP
 address and MAC address of the transmitting node are available in the
 packet; for example, the IP address of the transmitting node is
 present in a Route Request option (in the Address list being
 accumulated) and any packets containing a source route.  Adding
 entries to the ARP cache in this way avoids the overhead of ARP in
 most cases.

3. DSR Protocol Overview

 This section provides an overview of the operation of the DSR
 protocol.  The basic version of DSR uses explicit "source routing",
 in which each data packet sent carries in its header the complete,
 ordered list of nodes through which the packet will pass.  This use
 of explicit source routing allows the sender to select and control
 the routes used for its own packets, supports the use of multiple
 routes to any destination (for example, for load balancing), and
 allows a simple guarantee that the routes used are loop-free.  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.  Section 3.1 describes
 this basic operation of Route Discovery, Section 3.2 describes basic
 Route Maintenance, and Sections 3.3 and 3.4 describe additional
 features of these two parts of DSR's operation.  Section 3.5 then
 describes an optional, compatible extension to DSR, known as "flow
 state", that allows the routing of most packets without an explicit
 source route header in the packet, while the fundamental properties
 of DSR's operation are preserved.

Johnson, et al. Experimental [Page 9] RFC 4728 The Dynamic Source Routing Protocol February 2007

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.

Johnson, et al. Experimental [Page 10] RFC 4728 The Dynamic Source Routing Protocol February 2007

 Otherwise, if this node receiving the Route Request has recently seen
 another Route Request message from this initiator bearing this same
 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.  (A node considers a Request recently
 seen if it still has information about that Request in its Route
 Request Table, which is described in Section 4.3.)  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 receipt of
 a copy of the Request 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
 one is 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 in this case 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 [RFC793], 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 for unicast packets as part of the MAC protocol
 [IEEE80211], the discovered source route MUST be reversed in this way
 to return the Route Reply, since this route reversal tests the
 discovered route to ensure that 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.
 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 SendBufferTimeout; 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.

Johnson, et al. Experimental [Page 11] RFC 4728 The Dynamic Source Routing Protocol February 2007

 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
 such new Route Discoveries for the same address are initiated (as
 described in Section 4.3), 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 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 [RFC1122].

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, and node D is responsible for the link from D
 to E.

Johnson, et al. Experimental [Page 12] RFC 4728 The Dynamic Source Routing Protocol February 2007

 An acknowledgement can provide confirmation that a link is capable of
 carrying data, and in wireless networks, acknowledgements are often
 provided at no cost, either as an existing standard part of the MAC
 protocol in use (such as the link-layer acknowledgement frame defined
 by IEEE 802.11 [IEEE80211]), or by a "passive acknowledgement"
 [JUBIN87] (in which, for example, B confirms receipt at C by
 overhearing C transmit the packet when forwarding it on to D).
 If a built-in acknowledgement mechanism is not available, the node
 transmitting the packet can explicitly request that a DSR-specific
 software acknowledgement be returned by the next node along the
 route; this software acknowledgement will normally be transmitted
 directly to the sending node, but if the link between these two nodes
 is unidirectional (Section 4.6), this software acknowledgement could
 travel over a different, multi-hop path.
 After an acknowledgement has been received from some neighbor, a node
 MAY choose not to require acknowledgements from that neighbor for a
 brief period of time, unless the network interface connecting a node
 to that neighbor always receives an acknowledgement in response to
 unicast traffic.
 When a software acknowledgement is used, the acknowledgement request
 SHOULD be retransmitted up to a maximum number of times.  A
 retransmission of the acknowledgement 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 acknowledgement.
 After the acknowledgement request has been retransmitted the maximum
 number of times, if no acknowledgement 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 acknowledgement was last received.  For
 example, in the situation shown above, if C does not receive an
 acknowledgement 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 acknowledgement 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

Johnson, et al. Experimental [Page 13] RFC 4728 The Dynamic Source Routing Protocol February 2007

 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).

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 on the MAC protocol being used.  Specifically, three distinct
 cases are possible:
  1. 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.
  1. 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.
  1. 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 [IEEE80211]
    is capable of transmitting a unicast packet only over a
    bidirectional link, since the MAC protocol requires the return of
    a link-level acknowledgement 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.

Johnson, et al. Experimental [Page 14] RFC 4728 The Dynamic Source Routing Protocol February 2007

 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.  However, 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 it is found, the node generally returns a Route Reply to
 the initiator itself rather than forward 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
 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:

Johnson, et al. Experimental [Page 15] RFC 4728 The Dynamic Source Routing Protocol February 2007

       +-----+     +-----+                 +-----+     +-----+
       |  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, due to this restriction
 on returning a Route Reply based on information from its Route Cache,
 node F does not return such a Route Reply, it propagates the Route
 Request normally.

3.3.3. 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.

Johnson, et al. Experimental [Page 16] RFC 4728 The Dynamic Source Routing Protocol February 2007

 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 for the target node (i.e., with hop limit as defined by
 the value of the DiscoveryHopLimit configuration variable).
 As another example, a node MAY use this hop limit to implement an
 "expanding ring" search for the target [JOHNSON96a].  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 increase the average latency of
 Route Discovery, since multiple Discovery attempts and timeouts may
 be needed before discovering a route to the target node.

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
 discard it.  To salvage a packet, the node replaces the original
 source route on the packet with a 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, since the TTL is

Johnson, et al. Experimental [Page 17] RFC 4728 The Dynamic Source Routing Protocol February 2007

 decremented only once by each node, a single node could salvage a
 packet an unbounded number of times.  Even if we chose to require the
 TTL to be decremented on each salvage attempt, packet salvaging is an
 expensive operation, so it is desirable to bound the maximum number
 of times a packet can be salvaged independently of the maximum number
 of hops a packet can traverse.
 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:
  1. Remove the packet from the node's Network Interface Queue and

Maintenance Buffer.

  1. Originate a Route Error for this packet to the original sender of

the packet, using the procedure described in Section 8.3.4, 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
    detection of a single, new broken link, the node SHOULD send no
    more than one Route Error to each original sender of any of these
    packets.
  1. 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 8.3.6.  Otherwise, the node SHOULD discard
    the packet.

Johnson, et al. Experimental [Page 18] RFC 4728 The Dynamic Source Routing Protocol February 2007

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 acknowledgements [JUBIN87].  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.
 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 strength 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.

Johnson, et al. Experimental [Page 19] RFC 4728 The Dynamic Source Routing Protocol February 2007

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.

3.5. Optional DSR Flow State Extension

 This section describes an optional, compatible extension to the DSR
 protocol, known as "flow state", that allows the routing of most
 packets without an explicit source route header in the packet.  The
 DSR flow state extension further reduces the overhead of the protocol
 yet still preserves the fundamental properties of DSR's operation.
 Once a sending node has discovered a source route such as through
 DSR's Route Discovery mechanism, the flow state mechanism allows the
 sending node to establish hop-by-hop forwarding state within the
 network, based on this source route, to enable each node along the
 route to forward the packet to the next hop based on the node's own
 local knowledge of the flow along which this packet is being routed.
 Flow state is dynamically initialized by the first packet using a
 source route and is then able to route subsequent packets along the
 same flow without use of a source route header in the packet.  The
 state established at each hop along a flow is "soft state" and thus
 automatically expires when no longer needed and can be quickly
 recreated as necessary.  Extending DSR's basic operation based on an
 explicit source route in the header of each packet routed, the flow
 state extension operates as a form of "implicit source routing" by
 preserving DSR's basic operation but removing the explicit source
 route from packets.

Johnson, et al. Experimental [Page 20] RFC 4728 The Dynamic Source Routing Protocol February 2007

3.5.1. Flow Establishment

 A source node sending packets to some destination node MAY use the
 DSR flow state extension described here to establish a route to that
 destination as a flow.  A "flow" is a route from the source to the
 destination represented by hop-by-hop forwarding state within the
 nodes along the route.  Each flow is uniquely identified by a
 combination of the source node address, the destination node address,
 and a flow identifier (flow ID) chosen by the source node.
 Each flow ID is a 16-bit unsigned integer.  Comparison between
 different flow IDs MUST be performed modulo 2**16.  For example,
 using an implementation in the C programming language, a flow ID
 value (a) is greater than another flow ID value (b) if
 ((short)((a) - (b)) > 0), if a C language "short" data type is
 implemented as a 16-bit signed integer.
 A DSR Flow State header in a packet identifies the flow ID to be
 followed in forwarding that packet.  From a given source to some
 destination, any number of different flows MAY exist and be in use,
 for example, following different sequences of hops to reach the
 destination.  One of these flows MAY be considered the "default" flow
 from that source to that destination.  If a node receives a packet
 with neither a DSR Options header specifying the route to be taken
 (with a Source Route option in the DSR Options header) nor a DSR Flow
 State header specifying the flow ID to be followed, it is forwarded
 along the default flow for the source and destination addresses
 specified in the packet's IP header.
 In establishing a new flow, the source node generates a nonzero
 16-bit flow ID greater than any unexpired flow IDs for this (source,
 destination) pair.  If the source wishes for this flow to become the
 default flow, the low bit of the flow ID MUST be set (the flow ID is
 an odd number); otherwise, the low bit MUST NOT be set (the flow ID
 is an even number).
 The source node establishing the new flow then transmits a packet
 containing a DSR Options header with a Source Route option.  To
 establish the flow, the source node also MUST include in the packet a
 DSR Flow State header, with the Flow ID field set to the chosen flow
 ID for the new flow, and MUST include a Timeout option in the DSR
 Options header, giving the lifetime after which state information
 about this flow is to expire.  This packet will generally be a normal
 data packet being sent from this sender to the destination (for
 example, the first packet sent after discovering the new route) but
 is also treated as a "flow establishment" packet.

Johnson, et al. Experimental [Page 21] RFC 4728 The Dynamic Source Routing Protocol February 2007

 The source node records this flow in its Flow Table for future use,
 setting the TTL in this Flow Table entry to the value used in the TTL
 field in the packet's IP header and setting the Lifetime in this
 entry to the lifetime specified in the Timeout option in the DSR
 Options header.  The TTL field is used for Default Flow Forwarding,
 as described in Sections 3.5.3 and 3.5.4.
 Any further packets sent with this flow ID before the timeout that
 also contain a DSR Options header with a Source Route option MUST use
 this same source route in the Source Route option.

3.5.2. Receiving and Forwarding Establishment Packets

 Packets intended to establish a flow, as described in Section 3.5.1,
 contain a DSR Options header with a Source Route option and are
 forwarded along the indicated route.  A node implementing the DSR
 flow state extension, when receiving and forwarding such a DSR
 packet, also keeps some state in its own Flow Table to enable it to
 forward future packets that are sent along this flow with only the
 flow ID specified.  Specifically, if the packet also contains a DSR
 Flow State header, this packet SHOULD cause an entry to be
 established for this flow in the Flow Table of each node along the
 packet's route.
 The Hop Count field of the DSR Flow State header is also stored in
 the Flow Table, as is the lifetime specified in the Timeout option
 specified in the DSR Options header.
 If the Flow ID is odd and there is no flow in the Flow Table with
 Flow ID greater than the received Flow ID, set the default Flow ID
 for this (IP Source Address, IP Destination Address) pair to the
 received Flow ID, and the TTL of the packet is recorded.
 The Flow ID option is removed before final delivery of the packet.

3.5.3. Sending Packets along Established Flows

 When a flow is established as described in Section 3.5.1, a packet is
 sent that establishes state in each node along the route.  This state
 is soft; that is, the protocol contains mechanisms for recovering
 from the loss of this state.  However, the use of these mechanisms
 may result in reduced performance for packets sent along flows with
 forgotten state.  As a result, it is desirable to differentiate
 behavior based on whether or not the sender is reasonably certain
 that the flow state exists on each node along the route.  We define a
 flow's state to be "established end-to-end" if the Flow Tables of all
 nodes on the route contains forwarding information for that flow.
 While it is impossible to detect whether or not a flow's state has

Johnson, et al. Experimental [Page 22] RFC 4728 The Dynamic Source Routing Protocol February 2007

 been established end-to-end without sending packets, implementations
 may make reasonable assumptions about the retention of flow state and
 the probability that an establishment packet has been seen by all
 nodes on the route.
 A source wishing to send a packet along an established flow
 determines if the flow state has been established end-to-end.  If it
 has not, a DSR Options header with Source Route option with this
 flow's route is added to the packet.  The source SHOULD set the Flow
 ID field of the DSR Flow State header either to the flow ID
 previously associated with this flow's route or to zero.  If it sets
 the Flow ID field to any other value, it MUST follow the processing
 steps in Section 3.5.1 for establishing a new flow ID.  If it sets
 the Flow ID field to a nonzero value, it MUST include a Timeout
 option with a value not greater than the timeout remaining in the
 node's Flow Table, and if its TTL is not equal to that specified in
 the Flow Table, the flow MUST NOT be used as a default flow in the
 future.
 Once flow state has been established end-to-end for non-default
 flows, a source adds a DSR Flow State header to each packet it wishes
 to send along that flow, setting the Flow ID field to the flow ID of
 that flow.  A Source Route option SHOULD NOT be added to the packet,
 though if one is, then the steps for processing flows that have not
 been established end-to-end MUST be followed.
 Once flow state has been established end-to-end for default flows,
 sources sending packets with IP TTL equal to the TTL value in the
 local Flow Table entry for this flow then transmit the packet to the
 next hop.  In this case, a DSR Flow State header SHOULD NOT be added
 to the packet and a DSR Options header likewise SHOULD NOT be added
 to the packet; though if one is, the steps for sending packets along
 non-default flows MUST be followed.  If the IP TTL is not equal to
 the TTL value in the local Flow Table, then the steps for processing
 a non-default flow MUST be followed.

3.5.4. Receiving and Forwarding Packets Sent along Established Flows

 The handling of packets containing a DSR Options header with both a
 nonzero Flow ID and a Source Route option is described in Section
 3.5.2.  The Flow ID is ignored when it is equal to zero.  This
 section only describes handling of packets without a Source Route
 option.
 If a node receives a packet with a Flow ID in the DSR Options header
 that indicates an unexpired flow in the node's Flow Table, it
 increments the Hop Count in the DSR Options header and forwards the
 packet to the next hop indicated in the Flow Table.

Johnson, et al. Experimental [Page 23] RFC 4728 The Dynamic Source Routing Protocol February 2007

 If a node receives a packet with a Flow ID that indicates a flow not
 currently in the node's Flow Table, it returns a Route Error of type
 UNKNOWN_FLOW with Error Destination and IP Destination addresses
 copied from the IP Source of the packet triggering the error.  This
 error packet SHOULD be MAC-destined to the node from which the packet
 was received; if it cannot confirm reachability of the previous node
 using Route Maintenance, it MUST send the error as described in
 Section 8.1.1.  The node sending the error SHOULD attempt to salvage
 the packet triggering the Route Error.  If it does salvage the
 packet, it MUST zero the Flow ID in the packet.
 If a node receives a packet with no DSR Options header and no DSR
 Flow State header, it checks the Default Flow Table.  If there is a
 matching entry, it forwards to the next hop indicated in the Flow
 Table for the default flow.  Otherwise, it returns a Route Error of
 type DEFAULT_FLOW_UNKNOWN with Error Destination and IP Destination
 addresses copied from the IP Source Address of the packet triggering
 the error.  This error packet SHOULD be MAC-destined to the node from
 which it was received; if this node cannot confirm reachability of
 the previous node using Route Maintenance, it MUST send the error as
 described in Section 8.1.1.  The node sending the error SHOULD
 attempt to salvage the packet triggering the Route Error.  If it does
 salvage the packet, it MUST zero the Flow ID in the packet.

3.5.5. Processing Route Errors

 When a node receives a Route Error of type UNKNOWN_FLOW, it marks the
 flow to indicate that it has not been established end-to-end.  When a
 node receives a Route Error of type DEFAULT_FLOW_UNKNOWN, it marks
 the default flow to indicate that it has not been established end-
 to-end.

3.5.6. Interaction with Automatic Route Shortening

 Because a full source route is not carried in every packet, an
 alternative method for performing automatic route shortening is
 necessary for packets using the flow state extension.  Instead, nodes
 promiscuously listen to packets, and if a node receives a packet with
 (IP Source, IP Destination, Flow ID) found in the Flow Table but the
 MAC-layer (next hop) destination address of the packet is not this
 node, the node determines whether the packet was sent by an upstream
 or downstream node by examining the Hop Count field in the DSR Flow
 State header.  If the Hop Count field is less than the expected Hop
 Count at this node (that is, the expected Hop Count field in the Flow
 Table described in Section 5.1), the node assumes that the packet was
 sent by an upstream node and adds an entry for the packet to its
 Automatic Route Shortening Table, possibly evicting an earlier entry
 added to this table.  When the packet is then sent to that node for

Johnson, et al. Experimental [Page 24] RFC 4728 The Dynamic Source Routing Protocol February 2007

 forwarding, the node finds that it has previously received the packet
 by checking its Automatic Route Shortening Table and returns a
 gratuitous Route Reply to the source of the packet.

3.5.7. Loop Detection

 If a node receives a packet for forwarding with TTL lower than
 expected and default flow forwarding is being used, it sends a Route
 Error of type DEFAULT_FLOW_UNKNOWN back to the IP source.  It can
 attempt delivery of the packet by normal salvaging (subject to
 constraints described in Section 8.6.7).

3.5.8. Acknowledgement Destination

 In packets sent using Flow State, the previous hop is not necessarily
 known.  In order to allow nodes that have lost flow state to
 determine the previous hop, the address of the previous hop can
 optionally be stored in the Acknowledgement Request.  This extension
 SHOULD NOT be used when a Source Route option is present, MAY be used
 when flow state routing is used without a Source Route option, and
 SHOULD be used before Route Maintenance determines that the next-hop
 destination is unreachable.

3.5.9. Crash Recovery

 Each node has a maximum Timeout value that it can possibly generate.
 This can be based on the largest number that can be set in a timeout
 option (2**16 - 1 seconds) or may be less than this, set in system
 software.  When a node crashes, it does not establish new flows for a
 period equal to this maximum Timeout value, in order to avoid
 colliding with its old Flow IDs.

3.5.10. Rate Limiting

 Flow IDs can be assigned with a counter.  More specifically, the
 "Current Flow ID" is kept.  When a new default Flow ID needs to be
 assigned, if the Current Flow ID is odd, the Current Flow ID is
 assigned as the Flow ID and the Current Flow ID is incremented by
 one; if the Current Flow ID is even, one plus the Current Flow ID is
 assigned as the Flow ID and the Current Flow ID is incremented by
 two.
 If Flow IDs are assigned in this way, one algorithm for avoiding
 duplicate, unexpired Flow IDs is to rate limit new Flow IDs to an
 average rate of n assignments per second, where n is 2**15 divided by
 the maximum Timeout value.  This can be averaged over any period not
 exceeding the maximum Timeout value.

Johnson, et al. Experimental [Page 25] RFC 4728 The Dynamic Source Routing Protocol February 2007

3.5.11. Interaction with Packet Salvaging

 Salvaging is modified to zero the Flow ID field in the packet.  Also,
 anytime this document refers to the Salvage field in the Source Route
 option in a DSR Options header, packets without a Source Route option
 are considered to have the value zero in the Salvage field.

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 MUST be implemented in a manner consistent with the
 external behavior described in this document, but the choice of
 implementation used is otherwise unconstrained.  Additional
 conceptual data structures are required for the optional flow state
 extensions to DSR; these data structures are described in Section 5.

4.1. Route Cache

 Each node implementing DSR MUST maintain a Route Cache, containing
 routing information needed by the node.  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

Johnson, et al. Experimental [Page 26] RFC 4728 The Dynamic Source Routing Protocol February 2007

 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 6.7) and a Route Reply option
 (Section 6.3) in a packet's DSR Options header (Section 6).  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.
 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 searched by destination node
 address.  The following properties describe this searching function
 on a Route Cache:
  1. 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.
  1. However, if there are multiple cached routes to a destination, the

selection of routes when searching the Route Cache SHOULD 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.
  1. 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:
  1. 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

Johnson, et al. Experimental [Page 27] RFC 4728 The Dynamic Source Routing Protocol February 2007

    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.
  1. 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.
 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:
  1. 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 8.1.4)
    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 [BROCH98, HU00,
    JOHANSSON99, MALTZ99a] and through implementation of DSR in a
    mobile outdoor testbed under significant workload [MALTZ99b,
    MALTZ00, MALTZ01].
  1. 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.

Johnson, et al. Experimental [Page 28] RFC 4728 The Dynamic Source Routing Protocol February 2007

    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 [HU00].
 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
 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 implementers, 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
 [HU00].  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.

Johnson, et al. Experimental [Page 29] RFC 4728 The Dynamic Source Routing Protocol February 2007

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 time out to prevent the buffer from
 overflowing.
 Subject to the rate limiting defined in Section 4.3, a Route
 Discovery SHOULD be initiated as often as allowed for the destination
 address of any packets residing in the Send Buffer.

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:
  1. 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 3.3.3.
  1. The time that this node last originated a Route Request for that

target node.

  1. The number of consecutive Route Discoveries initiated for this

target since receiving a valid Route Reply giving a route to that

    target node.
  1. 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

Johnson, et al. Experimental [Page 30] RFC 4728 The Dynamic Source Routing Protocol February 2007

    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:
  1. 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.
 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, if available, 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.

Johnson, et al. Experimental [Page 31] RFC 4728 The Dynamic Source Routing Protocol February 2007

 Each entry in the Gratuitous Route Reply Table of a node contains the
 following fields:
  1. The address of the node to which this node originated a gratuitous

Route Reply.

  1. The address of the node from which this node overheard the packet

triggering that gratuitous Route Reply.

  1. 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 (i.e., if no
 corresponding 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
 [IEEE80211]; 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 queuing 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" [WRIGHT95].  This
 queue is used to hold packets while the network interface is in the
 process of transmitting another packet.

Johnson, et al. Experimental [Page 32] RFC 4728 The Dynamic Source Routing Protocol February 2007

 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.  Packets are
 added to the Maintenance buffer after the first transmission attempt
 is made.  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 8.3)
 and SHOULD salvage each packet removed in this way (Section 8.3.6) 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 acknowledgement request
 depends on the type of acknowledgement mechanism used by Route
 Maintenance for that attempt, as described in Section 8.3.

4.6. Blacklist

 When a node using the DSR protocol is connected through a network
 interface that requires physically bidirectional links for unicast
 transmission, the node MUST maintain a blacklist.  The blacklist is a
 table, indexed by neighbor node address, that indicates that the link
 between this node and the specified neighbor node 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 node A has node B listed in its
 blacklist, but after transmitting a Route Request, node A hears B
 forward the Route Request with a route record indicating that the
 broadcast from A to B was successful, then A SHOULD remove the entry
 for node B from its blacklist.

Johnson, et al. Experimental [Page 33] RFC 4728 The Dynamic Source Routing Protocol February 2007

 A node MUST associate a state with each node listed in its blacklist,
 specifying whether the unidirectionality of the link to that node 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 entries for nodes from its blacklist after a reasonable
 amount of time.

5. Additional Conceptual Data Structures for Flow State Extension

 This section defines additional conceptual data structures used by
 the optional "flow state" extension to DSR.  In an implementation of
 the protocol, these data structures MUST be implemented in a manner
 consistent with the external behavior described in this document, but
 the choice of implementation used is otherwise unconstrained.

5.1. Flow Table

 A node implementing the flow state extension MUST implement a Flow
 Table or other data structure consistent with the external behavior
 described in this section.  A node not implementing the flow state
 extension SHOULD NOT implement a Flow Table.
 The Flow Table records information about flows from which packets
 recently have been sent or forwarded by this node.  The table is
 indexed by a triple (IP Source Address, IP Destination Address, Flow
 ID), where Flow ID is a 16-bit number assigned by the source as
 described in Section 3.5.1.  Each entry in the Flow Table contains
 the following fields:
  1. The MAC address of the next-hop node along this flow.
  1. An indication of the outgoing network interface on this node to be

used in transmitting packets along this flow.

  1. The MAC address of the previous-hop node along this flow.
  1. An indication of the network interface on this node from which

packets from that previous-hop node are received.

  1. A timeout after which this entry in the Flow Table MUST be

deleted.

  1. The expected value of the Hop Count field in the DSR Flow State

header for packets received for forwarding along this field (for

    use with packets containing a DSR Flow State header).

Johnson, et al. Experimental [Page 34] RFC 4728 The Dynamic Source Routing Protocol February 2007

  1. An indication of whether or not this flow can be used as a default

flow for packets originated by this node (the Flow ID of a default

    flow MUST be odd).
  1. The entry SHOULD record the complete source route for the flow.

(Nodes not recording the complete source route cannot participate

    in Automatic Route Shortening.)
  1. The entry MAY contain a field recording the time this entry was

last used.

 The entry MUST be deleted when its timeout expires.

5.2. Automatic Route Shortening Table

 A node implementing the flow state extension SHOULD implement an
 Automatic Route Shortening Table or other data structure consistent
 with the external behavior described in this section.  A node not
 implementing the flow state extension SHOULD NOT implement an
 Automatic Route Shortening Table.
 The Automatic Route Shortening Table records information about
 received packets for which Automatic Route Shortening may be
 possible.  The table is indexed by a triple (IP Source Address, IP
 Destination Address, Flow ID).  Each entry in the Automatic Route
 Shortening Table contains a list of (packet identifier, Hop Count)
 pairs for that flow.  The packet identifier in the list may be any
 unique identifier for the received packet; for example, for IPv4
 packets, the combination of the following fields from the packet's IP
 header MAY be used as a unique identifier for the packet:  Source
 Address, Destination Address, Identification, Protocol, Fragment
 Offset, and Total Length.  The Hop Count in the list in the entry is
 copied from the Hop Count field in the DSR Flow State header of the
 received packet for which this table entry was created.  Any packet
 identifier SHOULD appear at most once in an entry's list, and this
 list item SHOULD record the minimum Hop Count value received for that
 packet (if the wireless signal strength or signal-to-noise ratio at
 which a packet is received is available to the DSR implementation in
 a node, the node MAY, for example, remember instead in this list the
 minimum Hop Count value for which the received packet's signal
 strength or signal-to-noise ratio exceeded some threshold).
 Space in the Automatic Route Shortening Table of a node MAY be
 dynamically managed by any local algorithm at the node.  For example,
 in order to limit the amount of memory used to store the table, any
 existing entry MAY be deleted at any time, and the number of packets
 listed in each entry MAY be limited.  However, when reclaiming space
 in the table, nodes SHOULD favor retaining information about more

Johnson, et al. Experimental [Page 35] RFC 4728 The Dynamic Source Routing Protocol February 2007

 flows in the table rather than about more packets listed in each
 entry in the table, as long as at least the listing of some small
 number of packets (e.g., 3) can be retained in each entry.

5.3. Default Flow ID Table

 A node implementing the flow state extension MUST implement a Default
 Flow Table or other data structure consistent with the external
 behavior described in this section.  A node not implementing the flow
 state extension SHOULD NOT implement a Default Flow Table.
 For each (IP Source Address, IP Destination Address) pair for which a
 node forwards packets, the node MUST record:
  1. The largest odd Flow ID value seen.
  1. The time at which all the corresponding flows that are forwarded

by this node expire.

  1. The current default Flow ID.
  1. A flag indicating whether or not the current default Flow ID is

valid.

 If a node deletes this record for an (IP Source Address, IP
 Destination Address) pair, it MUST also delete all Flow Table entries
 for that pair.  Nodes MUST delete table entries if all of this (IP
 Source Address, IP Destination Address) pair's flows that are
 forwarded by this node expire.

6. DSR Options 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 Options 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 Options header is implied by the total length
 of the DSR Options header.
 For IPv4, the DSR Options header MUST immediately follow the IP
 header in the packet.  (If a Hop-by-Hop Options extension header, as
 defined in IPv6 [RFC2460], becomes defined for IPv4, the DSR Options
 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.)

Johnson, et al. Experimental [Page 36] RFC 4728 The Dynamic Source Routing Protocol February 2007

 To add a DSR Options header to a packet, the DSR Options 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 Options header follows the IP header, and the
 Next Header field in the DSR Options header is used to indicate the
 type of protocol header (such as a transport layer header) following
 the DSR Options header.
 If any headers follow the DSR Options header in a packet, the total
 length of the DSR Options 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.

6.1. Fixed Portion of DSR Options Header

 The fixed portion of the DSR Options header is used to carry
 information that must be present in any DSR Options header.  This
 fixed portion of the DSR Options 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  |F|   Reserved  |        Payload Length         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 .                                                               .
 .                            Options                            .
 .                                                               .
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Next Header
       8-bit selector.  Identifies the type of header immediately
       following the DSR Options header.  Uses the same values as the
       IPv4 Protocol field [RFC1700].  If no header follows, then Next
       Header MUST have the value 59, "No Next Header" [RFC2460].
    Flow State Header (F)
       Flag bit.  MUST be set to 0.  This bit is set in a DSR Flow
       State header (Section 7.1) and clear in a DSR Options header.
    Reserved
       MUST be sent as 0 and ignored on reception.

Johnson, et al. Experimental [Page 37] RFC 4728 The Dynamic Source Routing Protocol February 2007

    Payload Length
       The length of the DSR Options 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 Options
       header.
    Options
       Variable-length field; the length of the Options field is
       specified by the Payload Length field in this DSR Options
       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 6.8).
 The placement of DSR options following the fixed portion of the DSR
 Options 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 Options header to be aligned.
 Each DSR option is assigned a unique Option Type code.  The most
 significant 3 bits (that is, Option Type & 0xE0) allow a node not
 implementing processing for this Option Type value to behave in the
 manner closest to correct for that type:
  1. The most significant bit in the Option Type value (that is, Option

Type & 0x80) represents whether or not a node receiving this

    Option Type (when the node does not implement processing for this
    Option Type) SHOULD respond to such a DSR option with a Route
    Error of type OPTION_NOT_SUPPORTED, except that such a Route Error
    SHOULD never be sent in response to a packet containing a Route
    Request option.
  1. The two following bits in the Option Type value (that is, Option

Type & 0x60) are a two-bit field indicating how such a node that

    does not support this Option Type MUST process the packet:
       00 = Ignore Option
       01 = Remove Option
       10 = Mark Option
       11 = Drop Packet
    When these 2 bits are 00 (that is, Option Type & 0x60 == 0), a
    node not implementing processing for that Option Type MUST use the
    Opt Data Len field to skip over the option and continue
    processing.  When these 2 bits are 01 (that is, Option Type & 0x60
    == 0x20), a node not implementing processing for that Option Type

Johnson, et al. Experimental [Page 38] RFC 4728 The Dynamic Source Routing Protocol February 2007

    MUST use the Opt Data Len field to remove the option from the
    packet and continue processing as if the option had not been
    included in the received packet.  When these 2 bits are 10 (that
    is, Option Type & 0x60 == 0x40), a node not implementing
    processing for that Option Type MUST set the most significant bit
    following the Opt Data Len field, MUST ignore the contents of the
    option using the Opt Data Len field, and MUST continue processing
    the packet.  Finally, when these 2 bits are 11 (that is, Option
    Type & 0x60 == 0x60), a node not implementing processing for that
    Option Type MUST drop the packet.
 The following types of DSR options are defined in this document for
 use within a DSR Options header:
  1. Route Request option (Section 6.2)
  1. Route Reply option (Section 6.3)
  1. Route Error option (Section 6.4)
  1. Acknowledgement Request option (Section 6.5)
  1. Acknowledgement option (Section 6.6)
  1. DSR Source Route option (Section 6.7)
  1. Pad1 option (Section 6.8)
  1. PadN option (Section 6.9)
 In addition, Section 7 specifies further DSR options for use with the
 optional DSR flow state extension.

Johnson, et al. Experimental [Page 39] RFC 4728 The Dynamic Source Routing Protocol February 2007

6.2. Route Request Option

 The Route Request option in a DSR Options 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 3.3.3).
 Route Request fields:
    Option Type
       1.  Nodes not understanding this option will ignore this
       option.

Johnson, et al. Experimental [Page 40] RFC 4728 The Dynamic Source Routing Protocol February 2007

    Opt Data Len
       8-bit unsigned integer.  Length of the option, in octets,
       excluding the Option Type and Opt Data Len fields.  MUST be set
       equal to (4 * n) + 6, where n is the number of addresses in the
       Route Request Option.
    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 (for this IP Source address and Target
       Address) 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 a Route Request is propagated, 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 IPv4 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 IPv4
       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
 Options header.

Johnson, et al. Experimental [Page 41] RFC 4728 The Dynamic Source Routing Protocol February 2007

6.3. Route Reply Option

 The Route Reply option in a DSR Options 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
       2.  Nodes not understanding this option will ignore this
       option.

Johnson, et al. Experimental [Page 42] RFC 4728 The Dynamic Source Routing Protocol February 2007

    Opt Data Len
       8-bit unsigned integer.  Length of the option, in octets,
       excluding the Option Type and Opt Data Len fields.  MUST be set
       equal to (4 * n) + 1, where n is the number of addresses in the
       Route Reply Option.
    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 SHOULD 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
       at the 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
 Options header.

Johnson, et al. Experimental [Page 43] RFC 4728 The Dynamic Source Routing Protocol February 2007

6.4. Route Error Option

 The Route Error option in a DSR Options 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
       3.  Nodes not understanding this option will ignore this
       option.
    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
       values are defined:

Johnson, et al. Experimental [Page 44] RFC 4728 The Dynamic Source Routing Protocol February 2007

          1 = NODE_UNREACHABLE
          2 = FLOW_STATE_NOT_SUPPORTED
          3 = OPTION_NOT_SUPPORTED
       Other values of the Error Type field are reserved for future
       use.
    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.
 A Route Error option MAY appear one or more times within a DSR
 Options header.

Johnson, et al. Experimental [Page 45] RFC 4728 The Dynamic Source Routing Protocol February 2007

6.4.1. Node Unreachable Type-Specific Information

 When the Route Error is of type NODE_UNREACHABLE, 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                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Unreachable Node Address
       The IP 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).

6.4.2. Flow State Not Supported Type-Specific Information

 When the Route Error is of type FLOW_STATE_NOT_SUPPORTED, the
 Type-Specific Information field is empty.

6.4.3. Option Not Supported Type-Specific Information

 When the Route Error is of type OPTION_NOT_SUPPORTED, the
 Type-Specific Information field is defined as follows:
 0 1 2 3 4 5 6 7
 +-+-+-+-+-+-+-+-+
 |Unsupported Opt|
 +-+-+-+-+-+-+-+-+
    Unsupported Opt
       The Option Type of option triggering the Route Error.

6.5. Acknowledgement Request Option

 The Acknowledgement Request option in a DSR Options 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        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Johnson, et al. Experimental [Page 46] RFC 4728 The Dynamic Source Routing Protocol February 2007

    Option Type
       160.  Nodes not understanding this option will remove the
       option and return a Route Error.
    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 Acknowledgement option
       when returned by the node receiving the packet over this hop.
 An Acknowledgement Request option MUST NOT appear more than once
 within a DSR Options header.

6.6. Acknowledgement Option

 The Acknowledgement option in a DSR Options 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
       32.  Nodes not understanding this option will remove the
       option.
    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 Acknowledgement
       Request option of the packet being acknowledged.

Johnson, et al. Experimental [Page 47] RFC 4728 The Dynamic Source Routing Protocol February 2007

    ACK Source Address
       The address of the node originating the acknowledgement.
    ACK Destination Address
       The address of the node to which the acknowledgement is to be
       delivered.
 An Acknowledgement option MAY appear one or more times within a DSR
 Options header.

6.7. DSR Source Route Option

 The DSR Source Route option in a DSR Options 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
       96.  Nodes not understanding this option will drop the packet.
    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

Johnson, et al. Experimental [Page 48] RFC 4728 The Dynamic Source Routing Protocol February 2007

       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 SHOULD 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
       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 SHOULD 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 8.1.3 and 8.1.5.  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 Options header, the Destination
 Address field in the packet's IP header is always set to the address

Johnson, et al. Experimental [Page 49] RFC 4728 The Dynamic Source Routing Protocol February 2007

 of the packet's ultimate destination.  A node receiving a packet
 containing a DSR Options 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 how to forward the packet, as
 defined in Sections 8.1.4 and 8.1.5.

6.8. Pad1 Option

 The Pad1 option in a DSR Options header is encoded as follows:
 +-+-+-+-+-+-+-+-+
 |  Option Type  |
 +-+-+-+-+-+-+-+-+
    Option Type
       224.  Nodes not understanding this option will drop the packet
       and return a Route Error.
 A Pad1 option MAY be included in the Options field of a DSR Options
 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 Options header.
 If any headers follow the DSR Options header in a packet, the total
 length of a DSR Options header, indicated by the Payload Length field
 in the DSR Options header MUST be a multiple of 4 octets.  In this
 case, when building a DSR Options header in a packet, sufficient Pad1
 or PadN options MUST be included in the Options field of the DSR
 Options 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 Options 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.

6.9. PadN Option

 The PadN option in a DSR Options header is encoded as follows:
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
 |  Option Type  |  Opt Data Len |   Option Data
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -

Johnson, et al. Experimental [Page 50] RFC 4728 The Dynamic Source Routing Protocol February 2007

    Option Type
       0.  Nodes not understanding this option will ignore this
       option.
    Opt Data Len
       8-bit unsigned integer.  Length of the option, in octets,
       excluding the Option Type and Opt Data Len fields.  The size of
       the Option Data field.
    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 Options
 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 Options header.
 If any headers follow the DSR Options header in a packet, the total
 length of a DSR Options header, indicated by the Payload Length field
 in the DSR Options header, MUST be a multiple of 4 octets.  In this
 case, when building a DSR Options header in a packet, sufficient Pad1
 or PadN options MUST be included in the Options field of the DSR
 Options header to make the total length a multiple of 4 octets.

7. Additional Header Formats and Options for Flow State Extension

 The optional DSR flow state extension requires a new header type, the
 DSR Flow State header.
 In addition, the DSR flow state extension adds the following options
 for the DSR Options header defined in Section 6:
  1. Timeout option (Section 7.2.1)
  1. Destination and Flow ID option (Section 7.2.2)
 Two new Error Type values are also defined for use in the Route Error
 option in a DSR Options header:
  1. UNKNOWN_FLOW
  1. DEFAULT_FLOW_UNKNOWN
 Finally, an extension to the Acknowledgement Request option in a DSR
 Options header is also defined:

Johnson, et al. Experimental [Page 51] RFC 4728 The Dynamic Source Routing Protocol February 2007

  1. Previous Hop Address
 This section defines each of these new header, option, or extension
 formats.

7.1. DSR Flow State Header

 The DSR Flow State header is a small 4-byte header optionally used to
 carry the flow ID and hop count for a packet being sent along a DSR
 flow.  It is distinguished from the fixed DSR Options header (Section
 6.1) in that the Flow State Header (F) bit is set in the DSR Flow
 State header and is clear in the fixed DSR Options header.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Next Header  |F|  Hop Count  |        Flow Identifier        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Next Header
       8-bit selector.  Identifies the type of header immediately
       following the DSR Flow State header.  Uses the same values as
       the IPv4 Protocol field [RFC1700].
    Flow State Header (F)
       Flag bit.  MUST be set to 1.  This bit is set in a DSR Flow
       State header and clear in a DSR Options header (Section 6.1).
    Hop Count
       7-bit unsigned integer.  The number of hops through which this
       packet has been forwarded.
    Flow Identification
       The flow ID for this flow, as described in Section 3.5.1.

7.2. New Options and Extensions in DSR Options Header

7.2.1. Timeout Option

 The Timeout option is defined for use in a DSR Options header to
 indicate the amount of time before the expiration of the flow ID
 along which the packet is being sent.

Johnson, et al. Experimental [Page 52] RFC 4728 The Dynamic Source Routing Protocol February 2007

  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  |            Timeout            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Option Type
       128.  Nodes not understanding this option will ignore the
       option and return a Route Error.
    Opt Data Len
       8-bit unsigned integer.  Length of the option, in octets,
       excluding the Option Type and Opt Data Len fields.
       When no extensions are present, the Opt Data Len of a Timeout
       option is 2.  Further extensions to DSR may include additional
       data in a Timeout option.  The presence of such extensions is
       indicated by an Opt Data Len greater than 2.  Currently, no
       such extensions have been defined.
    Timeout
       The number of seconds for which this flow remains valid.
 The Timeout option MUST NOT appear more than once within a DSR
 Options header.

7.2.2. Destination and Flow ID Option

 The Destination and Flow ID option is defined for use in a DSR
 Options header to send a packet to an intermediate host along one
 flow, for eventual forwarding to the final destination along a
 different flow.  This option enables the aggregation of the state of
 multiple flows.
  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  |      New Flow Identifier      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                   New IP Destination Address                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Johnson, et al. Experimental [Page 53] RFC 4728 The Dynamic Source Routing Protocol February 2007

    Option Type
       129.  Nodes not understanding this option will ignore the
       option and return a Route Error.
    Opt Data Len
       8-bit unsigned integer.  Length of the option, in octets,
       excluding the Option Type and Opt Data Len fields.
       When no extensions are present, the Opt Data Len of a
       Destination and Flow ID option is 6.  Further extensions to DSR
       may include additional data in a Destination and Flow ID
       option.  The presence of such extensions is indicated by an Opt
       Data Len greater than 6.  Currently, no such extensions have
       been defined.
    New Flow Identifier
       Indicates the next identifier to store in the Flow ID field of
       the DSR Options header.
    New IP Destination Address
       Indicates the next address to store in the Destination Address
       field of the IP header.
 The Destination and Flow ID option MAY appear one or more times
 within a DSR Options header.

7.3. New Error Types for Route Error Option

7.3.1. Unknown Flow Type-Specific Information

 A new Error Type value of 129 (UNKNOWN_FLOW) is defined for use in a
 Route Error option in a DSR Options header.  The Type-Specific
 Information for errors of this type 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                Original IP Destination Address                |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Flow ID            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Johnson, et al. Experimental [Page 54] RFC 4728 The Dynamic Source Routing Protocol February 2007

    Original IP Destination Address
       The IP Destination Address of the packet that caused the error.
    Flow ID
       The Flow ID contained in the DSR Flow ID option that caused the
       error.

7.3.2. Default Flow Unknown Type-Specific Information

 A new Error Type value of 130 (DEFAULT_FLOW_UNKNOWN) is defined
 for use in a Route Error option in a DSR Options header.  The
 Type-Specific Information for errors of this type 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |               Original IP Destination Address                 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Original IP Destination Address
       The IP Destination Address of the packet that caused the error.

7.4. New Acknowledgement Request Option Extension

7.4.1. Previous Hop Address Extension

 When the Opt Data Len field of an Acknowledgement Request option
 in a DSR Options header is greater than or equal to 6, the
 ACK Request Source Address field is present.  The option is then
 formatted 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  |       Packet Identifier       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                   ACK Request Source Address                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Option Type
       160.  Nodes not understanding this option will remove the
       option and return a Route Error.

Johnson, et al. Experimental [Page 55] RFC 4728 The Dynamic Source Routing Protocol February 2007

    Opt Data Len
       8-bit unsigned integer.  Length of the option, in octets,
       excluding the Option Type and Opt Data Len fields.
       When no extensions are presents, the Opt Data Len of an
       Acknowledgement Request option is 2.  Further extensions to DSR
       may include additional data in an Acknowledgement Request
       option.  The presence of such extensions is indicated by an Opt
       Data Len greater than 2.
       Currently, one such extension has been defined.  If the Opt
       Data Len is at least 6, then an ACK Request Source Address is
       present.
    Packet Identifier
       The Packet Identifier field is set to a unique number and is
       copied into the Identification field of the DSR Acknowledgement
       option when returned by the node receiving the packet over this
       hop.
    ACK Request Source Address
       The address of the node requesting the DSR Acknowledgement.

8. Detailed Operation

8.1. General Packet Processing

8.1.1. Originating a Packet

 When originating any packet, a node using DSR routing MUST perform
 the following sequence of steps:
  1. Search the node's Route Cache for a route to the address given in

the IP Destination Address field in the packet's header.

  1. If no such route is found in the Route Cache, then perform Route

Discovery for the Destination Address, as described in Section

    8.2.  Initiating a Route Discovery for this target node address
    results in the node adding a Route Request option in a DSR Options
    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

Johnson, et al. Experimental [Page 56] RFC 4728 The Dynamic Source Routing Protocol February 2007

    (255.255.255.255) [RFC1122], 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 8.2.1.
  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 acknowledgement from the
    first-hop node in that route, then insert a DSR Options header
    into the packet, as described in Section 8.1.2, and insert a DSR
    Source Route option, as described in Section 8.1.3.  The source
    route in the packet is initialized from the selected route to the
    Destination Address of the packet.
  1. 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 8.3.

8.1.2. Adding a DSR Options Header to a Packet

 A node originating a packet adds a DSR Options header to the packet,
 if necessary, to carry information needed by the routing protocol.  A
 packet MUST NOT contain more than one DSR Options header.  A DSR
 Options header is added to a packet by performing the following
 sequence of steps (these steps assume that the packet contains no
 other headers that MUST be located in the packet before the DSR
 Options header):
  1. Insert a DSR Options header after the IP header but before any

other header that may be present.

  1. Set the Next Header field of the DSR Options header to the

Protocol number field of the packet's IP header.

  1. Set the Protocol field of the packet's IP header to the protocol

number assigned for DSR (48).

8.1.3. 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:

Johnson, et al. Experimental [Page 57] RFC 4728 The Dynamic Source Routing Protocol February 2007

  1. The node creates a DSR Source Route option, as described in

Section 6.7, and appends it to the DSR Options header in the

    packet.  (A DSR Options header is added, as described in Section
    8.1.2, if not already present.)
  1. 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 the 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.
  1. 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.
  1. 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.
  1. 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.
  1. The Salvage field in the DSR Source Route option is initialized to

0.

8.1.4. Processing a Received Packet

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

Johnson, et al. Experimental [Page 58] RFC 4728 The Dynamic Source Routing Protocol February 2007

    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
    8.2.2.
  1. If the DSR Options 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
    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 8.2.6.
  1. If the DSR Options header contains a Route Error option, the node

MUST process the Route Error option as described in Section 8.3.5.

  1. If the DSR Options header contains an Acknowledgement Request

option, the node MUST process the Acknowledgement Request option

    as described in Section 8.3.3.
  1. If the DSR Options header contains an Acknowledgement 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.

Johnson, et al. Experimental [Page 59] RFC 4728 The Dynamic Source Routing Protocol February 2007

    After possibly updating the node's Route Cache in response to the
    routing information in the Acknowledgement option, the node MUST
    then process the Acknowledgement option as described in Section
    8.3.3.
  1. If the DSR Options header contains a DSR Source Route option, the

node SHOULD extract the source route from the DSR Source Route

    option 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
    Otherwise (i.e., if 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 option, 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
    8.1.5.
  1. Any Pad1 or PadN options in the DSR Options header are ignored.
  1. 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 Options header and all the included DSR options in the
    header, and pass the rest of the packet to the network layer.

8.1.5. Processing a Received DSR Source Route Option

 When a node receives a packet containing a DSR Source Route option
 (whether for forwarding, overheard, or 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

Johnson, et al. Experimental [Page 60] RFC 4728 The Dynamic Source Routing Protocol February 2007

 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:
  1. 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.
  1. 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.
  1. 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
    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.
  1. 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

Johnson, et al. Experimental [Page 61] RFC 4728 The Dynamic Source Routing Protocol February 2007

    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 Source Route option according
 to the following sequence of steps:
  1. 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 Options header.
  1. Else, let n equal (Opt Data Len - 2) / 4. This is the number of

addresses in the DSR Source Route option.

  1. If the value of the Segments Left field is greater than n, then

send an ICMP Parameter Problem, Code 0, message [RFC792] to the IP

    Source Address, pointing to the Segments Left field, and discard
    the packet.  Do not process the DSR Source Route option further.
  1. 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.
  1. 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.
  1. If this node has more than one network interface and if Address[i]

is the address of one this node's network interfaces, then this

    indicates a change in the network interface to use in forwarding
    the packet, as described in Section 8.4.  In this case, decrement
    the value of the Segments Left field by 1 to skip over this
    address (that indicated the change of network interface) and go to
    the first step above (checking the value of the Segments Left
    field) to continue processing this Source Route option; in further
    processing of this Source Route option, the indicated new network
    interface MUST be used in forwarding the packet.
  1. 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
    [RFC792] or fragment it as specified in Section 8.5.
  1. 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 (TTL) field

Johnson, et al. Experimental [Page 62] RFC 4728 The Dynamic Source Routing Protocol February 2007

    in the packet's IP header [RFC791, RFC1122].  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.
  1. 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 8.3.
 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 Options header.

8.1.6. Handling an Unknown DSR Option

 Nodes implementing DSR MUST handle all options specified in this
 document, except those options pertaining to the optional flow state
 extension (Section 7).  However, further extensions to DSR may
 include other option types that may not be understood by
 implementations conforming to this version of the DSR specification.
 In DSR, Option Type codes encode required behavior for nodes not
 implementing that type of option.  These behaviors are included in
 the most significant 3 bits of the Option Type.
 If the most significant bit of the Option Type is set (that is,
 Option Type & 0x80 is nonzero), and this packet does not contain a
 Route Request option, a node SHOULD return a Route Error to the IP
 Source Address, following the steps described in Section 8.3.4,
 except that the Error Type MUST be set to OPTION_NOT_SUPPORTED and
 the Unsupported Opt field MUST be set to the Option Type triggering
 the Route Error.
 Whether or not a Route Error is sent in response to this DSR option,
 as described above, the node also MUST examine the next 2 most
 significant bits (that is, Option Type & 0x60):
  1. When these 2 bits are 00 (that is, Option Type & 0x60 == 0), a

node not implementing processing for that Option Type MUST use the

    Opt Data Len field to skip over the option and continue
    processing.
  1. When these 2 bits are 01 (that is, Option Type & 0x60 == 0x20), a

node not implementing processing for that Option Type MUST use the

    Opt Data Len field to remove the option from the packet and
    continue processing as if the option had not been included in the
    received packet.

Johnson, et al. Experimental [Page 63] RFC 4728 The Dynamic Source Routing Protocol February 2007

  1. When these 2 bits are 10 (that is, Option Type & 0x60 == 0x40), a

node not implementing processing for that Option Type MUST set the

    most significant bit following the Opt Data Len field.  In
    addition, the node MUST then ignore and skip over the contents of
    the option using the Opt Data Len field and MUST continue
    processing the packet.
  1. Finally, when these 2 bits are 11 (that is,

Option Type & 0x60 == 0x60), a node not implementing processing

    for that Option Type MUST drop the packet.

8.2. 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 SHOULD be 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; a node initiates 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 6.2) and a Route Reply (Section 6.3), to actively
 search the ad hoc network for a route to the desired target
 destination.  These DSR messages MAY be carried in any type of IP
 packet, through use of the DSR Options header as described in Section
 6.
 Except as discussed in Section 8.3.5, 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.

Johnson, et al. Experimental [Page 64] RFC 4728 The Dynamic Source Routing Protocol February 2007

8.2.1. Originating a Route Request

 A node initiating a Route Discovery for some target creates and
 initializes a Route Request option in a DSR Options 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 Options
 header in the packet.  To initialize the Route Request option, the
 node performs the following sequence of steps:
  1. The Option Type in the option MUST be set to the value 2.
  1. 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.
  1. 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.
  1. 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:
  1. 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
    3.3.3.

Johnson, et al. Experimental [Page 65] RFC 4728 The Dynamic Source Routing Protocol February 2007

  1. 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.
 The behavior of a node processing a packet containing DSR Options
 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
 Acknowledgement Request option, SHOULD NOT expect link-layer
 acknowledgement or passive acknowledgement, and SHOULD NOT be
 retransmitted.  The retransmission of packets containing a Route
 Request option is controlled solely by the logic described in this
 section.

8.2.2. 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:
  1. 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 8.2.4.  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.

Johnson, et al. Experimental [Page 66] RFC 4728 The Dynamic Source Routing Protocol February 2007

    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.
  1. 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.
  1. Else, if the Route Request was received through a network

interface that requires physically bidirectional links for unicast

    transmission, the node MUST check if the Route Request was last
    forwarded by a node on its blacklist (Section 4.6).  If such an
    entry is found in the blacklist, and the state of the
    unidirectional link is "probable", then the Request MUST be
    silently discarded.
  1. Else, if the Route Request was received through a network

interface that requires physically bidirectional links for unicast

    transmission, the node MUST check if the Route Request was last
    forwarded by a node on its blacklist.  If such an entry is found
    in the blacklist, 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 Route Reply within
    some reasonable amount of time, the node MUST silently discard the
    Route Request packet.
  1. 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.

Johnson, et al. Experimental [Page 67] RFC 4728 The Dynamic Source Routing Protocol February 2007

  1. 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).  However, if the node has multiple network
       interfaces, this step MUST be modified by the special
       processing specified in Section 8.4.
    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 8.2.3 to return a "cached Route Reply" 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 transmit this copy of the packet as a link-layer
       broadcast, 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.

8.2.3. 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 8.2.2; 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.

Johnson, et al. Experimental [Page 68] RFC 4728 The Dynamic Source Routing Protocol February 2007

 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:
  1. The IP Source Address of the packet containing the Route Request,
  1. The Address[i] fields in the Route Request, and
  1. 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 using this route from the Route Cache (it
 is possible that this node has another route in its Route Cache for
 which the above restriction on duplicate addresses is met, allowing
 the node to send a cached Route Reply based on that cached route,
 instead).  The node SHOULD continue to process the Route Request as
 described in Section 8.2.2 if it does not send a cached Route Reply.
 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:
  1. The source route for this Route 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].
  1. Send a Route Reply to the initiator of the Route Request, using

the procedure defined in Section 8.2.4. The initiator of the

    Route Request is indicated in the Source Address field in the
    packet's IP header.

Johnson, et al. Experimental [Page 69] RFC 4728 The Dynamic Source Routing Protocol February 2007

 Before sending the cached Route Reply, however, the node MAY delay
 the Reply in order to help prevent a possible Route Reply "storm", as
 described in Section 8.2.5.
 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 Options 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 Options 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:
  1. Copy the Target Address from the Route Request option in the DSR

Options header to the Destination Address field in the packet's IP

    header.
  1. Remove the Route Request option from the DSR Options header in the

packet, and add a DSR Source Route option to the packet's DSR

    Options header.
  1. 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).
  1. Initialize the Segments Left field in the DSR Source Route option

to n as defined above.

  1. The First Hop External (F) bit in the DSR Source Route option MUST

be set to 0.

  1. 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.

Johnson, et al. Experimental [Page 70] RFC 4728 The Dynamic Source Routing Protocol February 2007

  1. 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.
  1. Transmit the packet to the next-hop node on the new source route

in the packet, using the forwarding procedure described in Section

    8.1.5.

8.2.4. 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 8.2.2 and 8.2.3.  The Route Reply is returned in a Route
 Reply option (Section 6.3).  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 Options header in
 the packet returned to the initiator.  To initialize the Route Reply
 option, the node performs the following sequence of steps:
  1. The Option Type in the option MUST be set to the value 3.
  1. 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).
  1. If this node is the target of the Route Request, the Last Hop

External (L) bit in the option MUST be initialized to 0.

  1. The Reserved field in the option MUST be initialized to 0.
  1. The Route Request Identifier MUST be initialized to the Identifier

field of the Route Request to which this Route Reply is sent in

    response.
  1. 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

Johnson, et al. Experimental [Page 71] RFC 4728 The Dynamic Source Routing Protocol February 2007

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

8.2.5. Preventing Route Reply Storms

 The ability for nodes to reply to a Route Request based on
 information in their Route Caches, as described in Sections 3.3.2 and
 8.2.3, could result in a possible Route Reply "storm" in some cases.
 In particular, if a node broadcasts a Route Request for a target node

Johnson, et al. Experimental [Page 72] RFC 4728 The Dynamic Source Routing Protocol February 2007

 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.
 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

Johnson, et al. Experimental [Page 73] RFC 4728 The Dynamic Source Routing Protocol February 2007

 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 send their Replies after this node.
 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 Route 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.

8.2.6. Processing a Received Route Reply Option

 Section 8.1.4 describes the general processing for a received packet,
 including the addition of routing information from options in the
 packet's DSR Options 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 using a MAC protocol that 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 (Section 4.6).

8.3. 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

Johnson, et al. Experimental [Page 74] RFC 4728 The Dynamic Source Routing Protocol February 2007

 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 period.
 Individual implementations MAY choose to bypass such confirmation for
 some limited number of packets, as long as those packets all fall
 within MaintHoldoffTime since the last confirmation.  If no
 confirmation is received after the retransmission of MaxMaintRexmt
 acknowledgement 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 acknowledgement (Section 8.3.1), a "passive acknowledgement"
 (Section 8.3.2), or a network-layer acknowledgement (Section 8.3.3);
 the particular strategy for retransmission timing depends on the type
 of acknowledgement mechanism used.  When not using link-layer
 acknowledgements for Route Maintenance, nodes SHOULD use passive
 acknowledgements when possible but SHOULD try requesting a network-
 layer acknowledgement one or more times before deciding that the link
 has failed and originating a Route Error to the original sender of
 the packet, as described in Section 8.3.4.
 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.

8.3.1. Using Link-Layer Acknowledgements

 If the MAC protocol in use provides feedback as to the successful
 delivery of a data packet (such as is provided for unicast packets by
 the link-layer acknowledgement frame defined by IEEE 802.11
 [IEEE80211]), then the use of the DSR Acknowledgement Request and
 Acknowledgement options is not necessary.  If such link-layer
 feedback is available, it SHOULD be used instead of any other
 acknowledgement mechanism for Route Maintenance, and the node SHOULD
 NOT use either passive acknowledgements or network-layer
 acknowledgements for Route Maintenance.

Johnson, et al. Experimental [Page 75] RFC 4728 The Dynamic Source Routing Protocol February 2007

 When using link-layer acknowledgements 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 acknowledgement is returned after a unicast packet as
 a part of the basic access method of the IEEE 802.11 Distributed
 Coordination Function (DCF) MAC protocol; the time at which the
 acknowledgement 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 acknowledgement for any packet in
 its Maintenance Buffer, that node SHOULD remove from its Maintenance
 Buffer that packet, as well as any other packets in its Maintenance
 Buffer with the same next-hop destination.

8.3.2. Using Passive Acknowledgements

 When link-layer acknowledgements are not available, but passive
 acknowledgements [JUBIN87] are available, passive acknowledgements
 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
 acknowledgements SHOULD be used for Route Maintenance in such cases
 if the node can place its network interface into "promiscuous"
 receive mode, and if network links used for data packets generally
 operate bidirectionally.
 A node MUST NOT attempt to use passive acknowledgements 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 acknowledgement will be available to be heard by this
 node.  Beyond this restriction, a node MAY utilize a variety of
 strategies in using passive acknowledgements for Route Maintenance of
 a packet that it originates or forwards.  For example, the following
 two strategies are possible:
  1. 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 acknowledgement for it.  If no
    passive acknowledgement is received within PassiveAckTimeout after
    this transmission, the node retransmits the packet, again without
    requesting a network-layer acknowledgement for it; the same
    PassiveAckTimeout timeout value is used for each such attempt.  If
    no acknowledgement has been received after a total of
    TryPassiveAcks retransmissions of the packet, network-layer

Johnson, et al. Experimental [Page 76] RFC 4728 The Dynamic Source Routing Protocol February 2007

    acknowledgements (as described in Section 8.3.3) are requested for
    all remaining attempts for that packet.
  1. Each node maintains a table of possible next-hop destination

nodes, noting whether or not passive acknowledgements can

    typically be expected from transmission to that node, and the
    expected latency and jitter of a passive acknowledgement 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 acknowledgement or a network-layer acknowledgement 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 acknowledgements to network-layer
    acknowledgements.
 In using passive acknowledgements 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) an
 acknowledgement of this first packet if both of the following two
 tests succeed:
  1. The Source Address, Destination Address, Protocol, Identification,

and Fragment Offset fields in the IP header of the two packets

    MUST match [RFC791].
  1. 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 acknowledgement for any packet in
 its Maintenance Buffer, that node SHOULD remove from its Maintenance
 Buffer that packet, as well as any other packets in its Maintenance
 Buffer with the same next-hop destination.

8.3.3. Using Network-Layer Acknowledgements

 When a node originates or forwards a packet and has no other
 mechanism of acknowledgement available to determine reachability of
 the next-hop node in the source route for Route Maintenance, that
 node SHOULD request a network-layer acknowledgement from that next-
 hop node.  To do so, the node inserts an Acknowledgement Request
 option in the DSR Options header in the packet.  The Identification
 field in that Acknowledgement Request option MUST be set to a value
 unique over all packets recently transmitted by this node to the same
 next-hop node.

Johnson, et al. Experimental [Page 77] RFC 4728 The Dynamic Source Routing Protocol February 2007

 When a node receives a packet containing an Acknowledgement Request
 option, that node performs the following tests on the packet:
  1. 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 Acknowledgement Request option.  The indicated next-
    hop node address is the next Address[i] field in the DSR Source
    Route option in the DSR Options header in the packet, or 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.
  1. If the packet contains an Acknowledgement option, then this node

MUST NOT process the Acknowledgement Request option.

 If neither of the tests above fails, then this node MUST process the
 Acknowledgement Request option by sending an Acknowledgement option
 to the previous-hop node; to do so, the node performs the following
 sequence of steps:
  1. Create a packet and set the IP Protocol field to the protocol

number assigned for DSR (48).

  1. 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).
  1. 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).
  1. Add a DSR Options header to the packet. Set the Next Header field

in the DSR Options header to the value 59, "No Next Header"

    [RFC2460].
  1. Add an Acknowledgement option to the DSR Options header in the

packet; set the Acknowledgement option's Option Type field to 6

    and the Opt Data Len field to 10.
  1. Copy the Identification field from the received Acknowledgement

Request option into the Identification field in the

    Acknowledgement option.

Johnson, et al. Experimental [Page 78] RFC 4728 The Dynamic Source Routing Protocol February 2007

  1. Set the ACK Source Address field in the Acknowledgement option to

be the IP Source Address of this new packet (set above to be the

    IP address of this node).
  1. Set the ACK Destination Address field in the Acknowledgement

option to be the IP Destination Address of this new packet (set

    above to be the IP address of the previous-hop node).
  1. Send the packet as described in Section 8.1.1.
 Packets containing an Acknowledgement option SHOULD NOT be placed in
 the Maintenance Buffer.
 When a node receives a packet with both an Acknowledgement option and
 an Acknowledgement Request option, if that node is not the
 destination of the Acknowledgement option (the IP Destination Address
 of the packet), then the Acknowledgement Request option MUST be
 ignored.  Otherwise (that node is the destination of the
 Acknowledgement option), that node MUST process the Acknowledgement
 Request option by returning an Acknowledgement option according to
 the following sequence of steps:
  1. Create a packet and set the IP Protocol field to the protocol

number assigned for DSR (48).

  1. 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).
  1. Set the IP Destination Address field in this packet to the IP

address of the node originating the Acknowledgement option.

  1. Add a DSR Options header to the packet, and set the DSR Options

header's Next Header field to the value 59, "No Next Header"

    [RFC2460].
  1. Add an Acknowledgement option to the DSR Options header in this

packet; set the Acknowledgement option's Option Type field to 6

    and the Opt Data Len field to 10.
  1. Copy the Identification field from the received Acknowledgement

Request option into the Identification field in the

    Acknowledgement option.

Johnson, et al. Experimental [Page 79] RFC 4728 The Dynamic Source Routing Protocol February 2007

  1. Set the ACK Source Address field in the option to the IP Source

Address of this new packet (set above to be the IP address of this

    node).
  1. Set the ACK Destination Address field in the option to the IP

Destination Address of this new packet (set above to be the IP

    address of the node originating the Acknowledgement option).
  1. 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
    Acknowledgement Request, MUST NOT be retransmitted for Route
    Maintenance, and MUST NOT expect a link-layer acknowledgement or
    passive acknowledgement.
 When using network-layer acknowledgements for Route Maintenance, a
 node SHOULD use an adaptive algorithm in determining the
 retransmission timeout for each transmission attempt of an
 acknowledgement request.  For example, a node SHOULD maintain a
 separate round-trip time (RTT) estimate for each node 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 [MALTZ99b, MALTZ01].

8.3.4. 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, it 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
 Acknowledgement 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:
  1. Create an IP packet and set the IP Protocol field to the protocol

number assigned for DSR (48). Set the Source Address field in

    this packet's IP header to the address of this node.
  1. 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;

Johnson, et al. Experimental [Page 80] RFC 4728 The Dynamic Source Routing Protocol February 2007

    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
  1. Insert a DSR Options header into the new packet.
  1. 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.
  1. If the packet triggering the Route Error contains any Route Error

or Acknowledgement 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 6.4) 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 Acknowledgement 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.
  1. Send the packet as described in Section 8.1.1.

8.3.5. 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:
  1. 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

Johnson, et al. Experimental [Page 81] RFC 4728 The Dynamic Source Routing Protocol February 2007

    single link is removed; if the node implements its Route Cache as
    a path cache, however, all routes (paths) that use this link are
    either truncated before the link or removed completely.
  1. If the option following the Route Error is an Acknowledgement or

Route Error option sent by this node (that is, with

    Acknowledgement 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 Acknowledgement
    or Error Destination Address.  Transmit this packet as described
    in Section 8.1.1, 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 as a part of this limit.

8.3.6. 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 8.3), 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 discard it.  To do so using the route found in
 its Route Cache, this node processes the packet as follows:
  1. 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 Options header in the packet then contains no
    DSR options or only a DSR Source Route Option, remove the DSR
    Options header from the packet.  If the resulting packet then
    contains only an IP header (e.g., no transport layer header or
    payload), the node SHOULD NOT salvage the packet and instead
    SHOULD discard the entire packet.

Johnson, et al. Experimental [Page 82] RFC 4728 The Dynamic Source Routing Protocol February 2007

  1. 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).
  1. Initialize the Segments Left field in the DSR Source Route option

to n as defined above.

  1. The First Hop External (F) bit in the DSR Source Route option MUST

be set to 0.

  1. 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.
  1. 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.
  1. Transmit the packet to the next-hop node on the new source route

in the packet, using the forwarding procedure described in Section

    8.1.5.
 As described in Section 8.3.4, 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.
 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 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.

Johnson, et al. Experimental [Page 83] RFC 4728 The Dynamic Source Routing Protocol February 2007

8.4. Multiple Network Interface Support

 A node using DSR MAY have multiple network interfaces that support
 DSR ad hoc network routing.  This section describes special packet
 processing at such nodes.
 A node with multiple network interfaces that support DSR ad hoc
 network routing MUST have some policy for determining which Route
 Request packets are forwarded using which network interfaces.  For
 example, a node MAY choose to forward all Route Requests over all
 network interfaces.
 When a node with multiple network interfaces that support DSR
 propagates a Route Request on a network interface other than the one
 on which it received the Route Request, it MUST in this special case
 modify the Address list in the Route Request as follows:
  1. Append the node's IP address for the incoming network interface.
  1. Append the node's IP address for the outgoing network interface.
 When a node forwards a packet containing a source route, it MUST
 assume that the next-hop node is reachable on the incoming network
 interface, unless the next hop is the address of one of this node's
 network interfaces, in which case this node MUST skip over this
 address in the source route and process the packet in the same way as
 if it had just received it from that network interface, as described
 in Section 8.1.5.
 If a node that previously had multiple network interfaces that
 support DSR receives a packet sent with a source route specifying a
 change to a network interface, as described above, 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 network
 interface, unless the network uses an autoconfiguration mechanism
 that may have allowed another node to acquire the now unused address
 of the unavailable network interface.

8.5. IP 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:
  1. Remove the DSR Options header from the packet.

Johnson, et al. Experimental [Page 84] RFC 4728 The Dynamic Source Routing Protocol February 2007

  1. Fragment the packet using normal IP fragmentation processing

[RFC791]. However, when determining the size of each fragment to

    create from the original packet, the fragment size MUST be reduced
    by the size of the DSR Options header from the original packet.
  1. IP-in-IP encapsulate each fragment [RFC2003]. The IP Destination

address of the outer (encapsulating) packet MUST be set equal to

    the IP Destination address of the original packet.
  1. Add the DSR Options header from the original packet to each

resulting encapsulating packet. If a Source Route header is

    present in the DSR Options 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 [RFC2003]
 and then process the inner packet according to standard IP reassembly
 processing [RFC791].

8.6. Flow State Processing

 A node implementing the optional DSR flow state extension MUST follow
 these additional processing steps.

8.6.1. Originating a Packet

 When originating any packet to be routed using flow state, a node
 using DSR flow state MUST do the following:
  1. If the route to be used for this packet has never had a DSR flow

state established along it (or the existing flow state has

    expired):
    o  Generate a 16-bit Flow ID larger than any unexpired Flow IDs
       used by this node for this destination.  Odd Flow IDs MUST be
       chosen for "default" flows; even Flow IDs MUST be chosen for
       non-default flows.
    o  Add a DSR Options header, as described in Section 8.1.2.
    o  Add a DSR Flow State header, as described in Section 8.6.2.
    o  Initialize the Hop Count field in the DSR Flow State header to
       0.
    o  Set the Flow ID field in the DSR Flow State header to the Flow
       ID generated in the first step.
    o  Add a Timeout option to the DSR Options header.

Johnson, et al. Experimental [Page 85] RFC 4728 The Dynamic Source Routing Protocol February 2007

    o  Add a Source Route option after the Timeout option with the
       route to be used, as described in Section 8.1.3.
    o  The source node SHOULD record this flow in its Flow Table.
    o  If this flow is recorded in the Flow Table, the TTL in this
       Flow Table entry MUST be set to be the TTL of this flow
       establishment packet.
    o  If this flow is recorded in the Flow Table, the timeout in this
       Flow Table entry MUST be set to a value no less than the value
       specified in the Timeout option.
  1. If the route to be used for this packet has had DSR flow state

established along it, but has not been established end-to-end:

    o  Add a DSR Options header, as described in Section 8.1.2.
    o  Add a DSR Flow State header, as described in Section 8.6.2.
    o  Initialize the Hop Count field in the DSR Flow State header to
       0.
    o  The Flow ID field of the DSR Flow State header SHOULD be the
       Flow ID previously used for this route.  If it is not, the
       steps for sending packets along never-before-established routes
       above MUST be followed in place of these.
    o  Add a Timeout option to the DSR Options header, setting the
       Timeout to a value not greater than the timeout remaining for
       this flow in the Flow Table.
    o  Add a Source Route option after the Timeout option with the
       route to be used, as described in Section 8.1.3.
    o  If the IP TTL is not equal to the TTL specified in the Flow
       Table, the source node MUST set a flag to indicate that this
       flow cannot be used as default.
  1. If the route the node wishes to use for this packet has been

established as a flow end-to-end and is not the default flow:

    o  Add a DSR Flow State header, as described in Section 8.6.2.
    o  Initialize the Hop Count field in the DSR Flow State header to
       0.

Johnson, et al. Experimental [Page 86] RFC 4728 The Dynamic Source Routing Protocol February 2007

    o  The Flow ID field of the DSR Flow State header SHOULD be set to
       the Flow ID previously used for this route.  If it is not, the
       steps for sending packets along never-before-established routes
       above MUST be followed in place of these.
    o  If the next hop requires a network-layer acknowledgement for
       Route Maintenance, add a DSR Options header, as described in
       Section 8.1.2, and an Acknowledgement Request option, as
       described in Section 8.3.3.
    o  A DSR Options header SHOULD NOT be added to a packet, unless it
       is added to carry an Acknowledgement Request option, in which
       case:
       +  A Source Route option in the DSR Options header SHOULD NOT
          be added.
       +  If a Source Route option in the DSR Options header is added,
          the steps for sending packets along flows not yet
          established end-to-end MUST be followed in place of these.
       +  A Timeout option SHOULD NOT be added.
       +  If a Timeout option is added, it MUST specify a timeout not
          greater than the timeout remaining for this flow in the Flow
          Table.
  1. If the route the node wishes to use for this packet has been

established as a flow end-to-end and is the current default flow:

    o  If the IP TTL is not equal to the TTL specified in the Flow
       Table, the source node MUST follow the steps above for sending
       a packet along a non-default flow that has been established
       end-to-end in place of these steps.
    o  If the next hop requires a network-layer acknowledgement for
       Route Maintenance, the sending node MUST add a DSR Options
       header and an Acknowledgement Request option, as described in
       Section 8.3.3.  The sending node MUST NOT add any additional
       options to this header.
    o  A DSR Options header SHOULD NOT be added, except as specified
       in the previous step.  If one is added in a way inconsistent
       with the previous step, the source node MUST follow the steps
       above for sending a packet along a non-default flow that has
       been established end-to-end in place of these steps.

Johnson, et al. Experimental [Page 87] RFC 4728 The Dynamic Source Routing Protocol February 2007

8.6.2. Inserting a DSR Flow State Header

 A node originating a packet adds a DSR Flow State header to the
 packet, if necessary, to carry information needed by the routing
 protocol.  A packet MUST NOT contain more than one DSR Flow State
 header.  A DSR Flow State header is added to a packet by performing
 the following sequence of steps:
  1. Insert a DSR Flow State header after the IP header and any Hop-

by-Hop Options header that may already be in the packet, but

    before any other header that may be present.
  1. Set the Next Header field of the DSR Flow State header to the Next

Header field of the previous header (either an IP header or a

    Hop-by-Hop Options header).
  1. Set the Flow (F) bit in the DSR Flow State header to 1.
  1. Set the Protocol field of the IP header to the protocol number

assigned for DSR (48).

8.6.3. Receiving a Packet

 This section describes processing only for packets that are sent to
 this processing node as the next-hop node; that is, when the MAC-
 layer destination address is the MAC address of this node.
 Otherwise, the process described in Sections 8.6.5 should be
 followed.
 The flow along which a packet is being sent is considered to be in
 the Flow Table if the triple (IP Source Address, IP Destination
 Address, Flow ID) has an unexpired entry in this node's Flow Table.
 When a node using DSR flow state receives a packet, it MUST follow
 the following steps for processing:
  1. If a DSR Flow State header is present, increment the Hop Count

field.

  1. In addition, if a DSR Flow State header is present, then if the

triple (IP Source Address, IP Destination Address, Flow ID) is in

    this node's Automatic Route Shortening Table and the packet is
    listed in the entry, then the node MAY send a gratuitous Route
    Reply as described in Section 4.4, subject to the rate limiting
    specified therein.  This gratuitous Route Reply gives the route by
    which the packet originally reached this node.  Specifically, the
    node sending the gratuitous Route Reply constructs the route to
    return in the Route Reply as follows:

Johnson, et al. Experimental [Page 88] RFC 4728 The Dynamic Source Routing Protocol February 2007

    o  Let k = (packet Hop Count) - (table Hop Count), where packet
       Hop Count is the value of the Hop Count field in this received
       packet, and table Hop Count is the Hop Count value stored for
       this packet in the corresponding entry in this node's Automatic
       Route Shortening Table.
    o  Copy the complete source route for this flow from the
       corresponding entry in the node's Flow Table.
    o  Remove from this route the k hops immediately preceding this
       node in the route, since these are the hops "skipped over" by
       the packet as recorded in the Automatic Route Shortening Table
       entry.
  1. Process each of the DSR options within the DSR Options header in

order:

    o  On receiving a Pad1 or PadN option, skip over the option.
    o  On receiving a Route Request for which this node is the
       destination, remove the option and return a Route Reply as
       specified in Section 8.2.2.
    o  On receiving a broadcast Route Request that this node has not
       previously seen for which this node is not the destination,
       append this node's incoming interface address to the Route
       Request, continue propagating the Route Request as specified in
       Section 8.2.2, pass the payload, if any, to the network layer,
       and stop processing.
    o  On receiving a Route Request that this node has previously seen
       for which this node is not the destination, discard the packet
       and stop processing.
    o  On receiving any Route Request, add appropriate links to the
       Route Cache, as specified in Section 8.2.2.
    o  On receiving a Route Reply for which this node is the
       initiator, remove the Route Reply from the packet and process
       it as specified in Section 8.2.6.
    o  On receiving any Route Reply, add appropriate links to the
       Route Cache, as specified in Section 8.2.6.
    o  On receiving any Route Error of type NODE_UNREACHABLE, remove
       appropriate links to the Route Cache, as specified in Section
       8.3.5.

Johnson, et al. Experimental [Page 89] RFC 4728 The Dynamic Source Routing Protocol February 2007

    o  On receiving a Route Error of type NODE_UNREACHABLE that this
       node is the Error Destination Address of, remove the Route
       Error from the packet and process it as specified in Section
       8.3.5.  It also MUST stop originating packets along any flows
       using the link from Error Source Address to Unreachable Node,
       and it MAY remove from its Flow Table any flows using the link
       from Error Source Address to Unreachable Node.
    o  On receiving a Route Error of type UNKNOWN_FLOW that this node
       is not the Error Destination Address of, the node checks if the
       Route Error corresponds to a flow in its Flow Table.  If it
       does not, the node silently discards the Route Error;
       otherwise, it forwards the packet to the expected previous hop
       of the corresponding flow.  If Route Maintenance cannot confirm
       the reachability of the previous hop, the node checks if the
       network interface requires bidirectional links for operation.
       If it does, the node silently discards the Route Error;
       otherwise, it sends the Error as if it were originating it, as
       described in Section 8.1.1.
    o  On receiving a Route Error of type UNKNOWN_FLOW that this node
       is the Error Destination Address of, remove the Route Error
       from the packet and mark the flow specified by the triple
       (Error Destination Address, Original IP Destination Address,
       Flow ID) as not having been established end-to-end.
    o  On receiving a Route Error of type DEFAULT_FLOW_UNKNOWN that
       this node is not the Error Destination Address of, the node
       checks if the Route Error corresponds to a flow in its Default
       Flow Table.  If it does not, the node silently discards the
       Route Error; otherwise, it forwards the packet to the expected
       previous hop of the corresponding flow.  If Route Maintenance
       cannot confirm the reachability of the previous hop, the node
       checks if the network interface requires bidirectional links
       for operation.  If it does, the node silently discards the
       Route Error; otherwise, it sends the Error as if it were
       originating it, as described in Section 8.1.1.
    o  On receiving a Route Error of type DEFAULT_FLOW_UNKNOWN that
       this node is the Error Destination Address of, remove the Route
       Error from the packet and mark the default flow between the
       Error Destination Address and the Original IP Destination
       Address as not having been established end-to-end.

Johnson, et al. Experimental [Page 90] RFC 4728 The Dynamic Source Routing Protocol February 2007

    o  On receiving an Acknowledgement Request option, the receiving
       node removes the Acknowledgement Request option and replies to
       the previous hop with an Acknowledgement option.  If the
       previous hop cannot be determined, the Acknowledgement Request
       option is discarded, and processing continues.
    o  On receiving an Acknowledgement option, the receiving node
       removes the Acknowledgement option and processes it.
    o  On receiving any Acknowledgement option, add the appropriate
       link to the Route Cache, as specified in Section 8.1.4.
    o  On receiving any Source Route option, add appropriate links to
       the Route Cache, as specified in Section 8.1.4.
    o  On receiving a Source Route option, if no DSR Flow State header
       is present, if the flow this packet is being sent along is in
       the Flow Table, or if no Timeout option preceded the Source
       Route option in this DSR Options header, process it as
       specified in Section 8.1.4.  Stop processing this packet unless
       the last address in the Source Route option is an address of
       this node.
    o  On receiving a Source Route option in a packet with a DSR Flow
       State header, if the Flow ID specified in the DSR Flow State
       header is not in the Flow Table, add the flow to the Flow
       Table, setting the Timeout value to a value not greater than
       the Timeout field of the Timeout option in this header.  If no
       Timeout option preceded the Source Route option in this header,
       the flow MUST NOT be added to the Flow Table.
       If the Flow ID is odd and larger than any unexpired, odd Flow
       IDs for this (IP Source Address, IP Destination Address), it is
       set to be default in the Default Flow ID Table.
       Then process the Route option as specified in Section 8.1.4.
       Stop processing this packet unless the last address in the
       Source Route option is an address of this node.
    o  On receiving a Timeout option, check if this packet contains a
       DSR Flow State header.  If this packet does not contain a DSR
       Flow State header, discard the DSR option.  Otherwise, record
       the Timeout value in the option for future reference.  The
       value recorded SHOULD be discarded when the node has finished
       processing this DSR Options header.  If the flow that this
       packet is being sent along is in the Flow Table, it MAY set the
       flow to time out no more than Timeout seconds in the future.

Johnson, et al. Experimental [Page 91] RFC 4728 The Dynamic Source Routing Protocol February 2007

    o  On receiving a Destination and Flow ID option, if the IP
       Destination Address is not an address of this node, forward the
       packet according to the Flow ID, as described in Section 8.6.4,
       and stop processing this packet.
    o  On receiving a Destination and Flow ID option, if the IP
       Destination Address is an address of this node, set the IP
       Destination Address to the New IP Destination Address specified
       in the option and set the Flow ID to the New Flow Identifier.
       Then remove the Destination and Flow ID option from the packet
       and continue processing.
  1. If the IP Destination Address is an address of this node, remove

the DSR Options header, if any, pass the packet up the network

    stack, and stop processing.
  1. If there is still a DSR Options header containing no options,

remove the DSR Options header.

  1. If there is still a DSR Flow State header, forward the packet

according to the Flow ID, as described in Section 8.6.4.

  1. If there is neither a DSR Options header nor a DSR Flow State

header, but there is an entry in the Default Flow Table for the

    (IP Source Address, IP Destination Address) pair:
    o  If the IP TTL is not equal to the TTL expected in the Flow
       Table, insert a DSR Flow State header, setting the Hop Count
       equal to the Hop Count of this node, and the Flow ID equal to
       the default Flow ID found in the Default Flow Table, and
       forward this packet according to the Flow ID, as described in
       Section 8.6.4.
    o  Otherwise, follow the steps for forwarding the packet using
       Flow IDs described in Section 8.6.4, but taking the Flow ID to
       be the default Flow ID found in the Default Flow Table.
  1. If there is no DSR Options header and no DSR Flow State header and

no default flow can be found, the node returns a Route Error of

    type DEFAULT_FLOW_UNKNOWN to the IP Source Address, specifying the
    IP Destination Address as the Original IP Destination in the
    type-specific field.

Johnson, et al. Experimental [Page 92] RFC 4728 The Dynamic Source Routing Protocol February 2007

8.6.4. Forwarding a Packet Using Flow IDs

 To forward a packet using Flow IDs, a node MUST follow the following
 sequence of steps:
  1. If the triple (IP Source Address, IP Destination Address, Flow ID)

is not in the Flow Table, return a Route Error of type

    UNKNOWN_FLOW.
  1. If a network-layer acknowledgement is required for Route

Maintenance for the next hop, the node MUST include an

    Acknowledgement Request option as specified in Section 8.3.3.  If
    no DSR Options header is in the packet in which the
    Acknowledgement Request option is to be added, it MUST be
    included, as described in Section 8.1.2, except that it MUST be
    added after the DSR Flow State header, if one is present.
  1. Attempt to transmit this packet to the next hop as specified in

the Flow Table, performing Route Maintenance to detect broken

    routes.

8.6.5. Promiscuously Receiving a Packet

 This section describes processing only for packets that have MAC
 destinations other than this processing node.  Otherwise, the process
 described in Section 8.6.3 should be followed.
 When a node using DSR flow state promiscuously overhears a packet, it
 SHOULD follow the following steps for processing:
  1. If the packet contains a DSR Flow State header, and if the triple

(IP Source Address, IP Destination Address, Flow ID) is in the

    Flow Table and the Hop Count is less than the Hop Count in the
    flow's entry, the node MAY retain the packet in the Automatic
    Route Shortening Table.  If it can be determined that this Flow ID
    has been recently used, the node SHOULD retain the packet in the
    Automatic Route Shortening Table.
  1. If the packet contains neither a DSR Flow State header nor a

Source Route option and a Default Flow ID can be found in the

    Default Flow Table for the (IP Source Address, IP Destination
    Address), and if the IP TTL is greater than the TTL in the Flow
    Table for the default flow, the node MAY retain the packet in the
    Automatic Route Shortening Table.  If it can be determined that
    this Flow ID has been used recently, the node SHOULD retain the
    packet in the Automatic Route Shortening Table.

Johnson, et al. Experimental [Page 93] RFC 4728 The Dynamic Source Routing Protocol February 2007

8.6.6. Operation Where the Layer below DSR Decreases the IP TTL

      Non-uniformly
 Some nodes may use an IP tunnel as a DSR hop.  If different packets
 sent along this IP tunnel can take different routes, the reduction in
 IP TTL across this link may be different for different packets.  This
 prevents the Automatic Route Shortening and Loop Detection
 functionality from working properly when used in conjunction with
 default routes.
 Nodes forwarding packets without a Source Route option onto a link
 with unpredictable TTL changes MUST ensure that a DSR Flow State
 header is present, indicating the correct Hop Count and Flow ID.

8.6.7. Salvage Interactions with DSR

 Nodes salvaging packets MUST remove the DSR Flow State header, if
 present.
 Anytime this document refers to the Salvage field in the Source Route
 option, packets without a Source Route option are considered to have
 the value zero in the Salvage field.

Johnson, et al. Experimental [Page 94] RFC 4728 The Dynamic Source Routing Protocol February 2007

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 protocol with RTS and CTS [IEEE80211, BROCH98].
    DiscoveryHopLimit                  255   hops
    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

Johnson, et al. Experimental [Page 95] RFC 4728 The Dynamic Source Routing Protocol February 2007

10. IANA Considerations

 This document specifies the DSR Options header and DSR Flow State
 header, for which the IP protocol number 48 has been assigned.  A
 single IP protocol number can be used for both header types, since
 they can be distinguished by the Flow State Header (F) bit in each
 header.
 In addition, this document proposes use of the value "No Next Header"
 (originally defined for use in IPv6 [RFC2460]) within an IPv4 packet,
 to indicate that no further header follows a DSR Options header.
 Finally, this document introduces a number of DSR options for use in
 the DSR Options header, and additional new DSR options may be defined
 in the future.  Each of these options requires a unique Option Type
 value, the most significant 3 bits (that is, Option Type & 0xE0)
 encoded as defined in Section 6.1.  It is necessary only that each
 Option Type value be unique, not that they be unique in the remaining
 5 bits of the value after these 3 most significant bits.
 Two registries (DSR Protocol Options and DSR Protocol Route Error
 Types) have been created and contain the initial registrations.
 Assignment of new values for DSR options will be by Expert Review
 [RFC2434], with the authors of this document serving as the
 Designated Experts.

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.
 Depending on the threat model, a number of different mechanisms can
 be used to secure DSR.  For example, in an environment where node
 compromise is unrealistic and 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.  Cryptographic approaches, such as that provided
 by Ariadne [HU02] or Secure Routing Protocol (SRP)
 [PAPADIMITRATOS02], can resist stronger attacks.

Johnson, et al. Experimental [Page 96] RFC 4728 The Dynamic Source Routing Protocol February 2007

Appendix A. Link-MaxLife Cache Description

 As guidance to implementers 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 caches studied in detailed
 simulations.  Use of this design for the Route Cache is recommended
 in implementations of DSR.
 This cache, called "Link-MaxLife" [HU00], 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

Johnson, et al. Experimental [Page 97] RFC 4728 The Dynamic Source Routing Protocol February 2007

 UseExtends into the future; if the lifetime of that link in the 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 wireless transmission
 radius.
    InitStability                       25   seconds
    StabilityIncrFactor                  4
    StabilityDecrFactor                0.5
    MinLifetime                          1   second
    UseExtends                         120   seconds

Johnson, et al. Experimental [Page 98] RFC 4728 The Dynamic Source Routing Protocol February 2007

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:
  1. 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 [RFC791], IPv6 [RFC2460], and IPX [TURNER90] nodes.
  1. Historically [JOHNSON94, JOHNSON96a], DSR grew from our

contemplation of a multi-hop propagating version of the Internet's

    Address Resolution Protocol (ARP) [RFC826], as well as from the
    routing mechanism used in IEEE 802 source routing bridges
    [PERLMAN92].  These are layer 2 protocols.
  1. Technically, we designed DSR to be simple enough that it could be

implemented directly in the firmware inside wireless network

    interface cards [JOHNSON94, JOHNSON96a], 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
 [MALTZ99b] 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.

Johnson, et al. Experimental [Page 99] RFC 4728 The Dynamic Source Routing Protocol February 2007

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 [JOHNSON94]; significant additional design details and
 initial simulation results were published in early 1996 [JOHNSON96a].
 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 [NS-2, BROCH98] and performed
 extensive simulations of DSR using ns-2 (e.g., [BROCH98, MALTZ99a]).
 We have also conducted evaluations of the different caching
 strategies in this document [HU00].
 We have also implemented the DSR protocol under the FreeBSD 2.2.7
 operating system running on Intel x86 platforms.  FreeBSD [FREEBSD]
 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 document.
 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 the
 Defense Advanced Research Projects Agency (DARPA).  A complete
 description of the testbed is available [MALTZ99b, MALTZ00, MALTZ01].
 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
 document and will be added later in a separate document on top of the
 base protocol specified here.
 DSR has also been implemented under Linux by Alex Song at the
 University of Queensland, Australia [SONG01].  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.
 Microsoft Research has implemented a version of DSR on Windows XP and
 has used it in testbeds of over 15 nodes.  Several machines use this
 implementation as their primary means of accessing the Internet.

Johnson, et al. Experimental [Page 100] RFC 4728 The Dynamic Source Routing Protocol February 2007

 Several other independent groups have also used DSR as a platform for
 their own research, or as a basis of comparison between ad hoc
 network routing protocols.
 A preliminary version of the optional DSR flow state extension was
 implemented in FreeBSD 3.3.  A demonstration of this modified version
 of DSR was presented in July 2000.  The DSR flow state extension has
 also been extensively evaluated using simulation [HU01].

Acknowledgements

 The protocol described in this document has been designed and
 developed within the Monarch Project, a long-term 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 [JOHNSON96b,
 MONARCH].
 The authors would like to acknowledge the substantial contributions
 of Josh Broch in helping to design, simulate, and implement the DSR
 protocol.  We thank him for his contributions to earlier versions of
 this document.
 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 particularly like to acknowledge Fred Baker, who provided
 extensive feedback on a previous version of this document, as well as
 the working group chairs, for their suggestions of previous versions
 of the document.

Johnson, et al. Experimental [Page 101] RFC 4728 The Dynamic Source Routing Protocol February 2007

Normative References

 [RFC791]       Postel, J., "Internet Protocol", STD 5, RFC 791,
                September 1981.
 [RFC792]       Postel, J., "Internet Control Message Protocol", STD
                5, RFC 792, September 1981.
 [RFC826]       Plummer, David C., "Ethernet Address Resolution
                Protocol: Or converting network protocol addresses to
                48.bit Ethernet address for transmission on Ethernet
                hardware", STD 37, RFC 826, November 1982.
 [RFC1122]      Braden, R., "Requirements for Internet Hosts -
                Communication Layers", STD 3, RFC 1122, October 1989.
 [RFC1700]      Reynolds, J. and J. Postel, "Assigned Numbers", STD 2,
                RFC 1700, October 1994.  See also
                http://www.iana.org/numbers.html.
 [RFC2003]      Perkins, C., "IP Encapsulation within IP", RFC 2003,
                October 1996.  RFC 2003, October 1996.
 [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2434]      Narten, T. and H. Alvestrand, "Guidelines for Writing
                an IANA Considerations Section in RFCs", BCP 26, RFC
                2434, October 1998.

Informative References

 [BANTZ94]      David F. Bantz and Frederic J. Bauchot.  Wireless LAN
                Design Alternatives.  IEEE Network, 8(2):43-53,
                March/April 1994.
 [BHARGHAVAN94] 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. ACM, August 1994.
 [BROCH98]      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.  ACM/IEEE, October 1998.

Johnson, et al. Experimental [Page 102] RFC 4728 The Dynamic Source Routing Protocol February 2007

 [CLARK88]      David D. Clark.  The Design Philosophy of the DARPA
                Internet Protocols.  In Proceedings of the ACM SIGCOMM
                '88 Conference, pages 106-114. ACM, August 1988.
 [FREEBSD]      The FreeBSD Project.  Project web page available at
                http://www.freebsd.org/.
 [HU00]         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. ACM, August 2000.
 [HU01]         Yih-Chun Hu and David B. Johnson.  Implicit Source
                Routing in On-Demand Ad Hoc Network Routing.  In
                Proceedings of the Second Symposium on Mobile Ad Hoc
                Networking and Computing (MobiHoc 2001), pages 1-10,
                October 2001.
 [HU02]         Yih-Chun Hu, Adrian Perrig, and David B. Johnson.
                Ariadne:  A Secure On-Demand Routing Protocol for Ad
                Hoc Networks.  In Proceedings of the Eighth Annual
                International Conference on Mobile Computing and
                Networking (MobiCom 2002), pages 12-23, September
                2002.
 [IEEE80211]    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.
 [JOHANSSON99]  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 of the Fifth Annual
                ACM/IEEE International Conference on Mobile Computing
                and Networking, pages 195-206. ACM/IEEE, August 1999.
 [JOHNSON94]    David B. Johnson.  Routing in Ad Hoc Networks of
                Mobile Hosts.  In Proceedings of the IEEE Workshop on
                Mobile Computing Systems and Applications, pages 158-
                163. IEEE Computer Society, December 1994.

Johnson, et al. Experimental [Page 103] RFC 4728 The Dynamic Source Routing Protocol February 2007

 [JOHNSON96a]   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.
 [JOHNSON96b]   David B. Johnson and David A. Maltz.  Protocols for
                Adaptive Wireless and Mobile Networking.  IEEE
                Personal Communications, 3(1):34-42, February 1996.
 [JUBIN87]      John Jubin and Janet D. Tornow.  The DARPA Packet
                Radio Network Protocols.  Proceedings of the IEEE,
                75(1):21-32, January 1987.
 [KARN90]       Phil Karn.  MACA---A New Channel Access Method for
                Packet Radio.  In ARRL/CRRL Amateur Radio 9th Computer
                Networking Conference, pages 134-140. American Radio
                Relay League, September 1990.
 [LAUER95]      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.
 [MALTZ99a]     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.
 [MALTZ99b]     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.
 [MALTZ00]      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. IEEE, September 2000.
 [MALTZ01]      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.

Johnson, et al. Experimental [Page 104] RFC 4728 The Dynamic Source Routing Protocol February 2007

 [MONARCH]      Rice University Monarch Project.  Monarch Project Home
                Page.  Available at http://www.monarch.cs.rice.edu/.
 [NS-2]         The Network Simulator -- ns-2.  Project web page
                available at http://www.isi.edu/nsnam/ns/.
 [PAPADIMITRATOS02]
                Panagiotis Papadimitratos and Zygmunt J. Haas.  Secure
                Routing for Mobile Ad Hoc Networks.  In SCS
                Communication Networks and Distributed Systems
                Modeling and Simulation Conference (CNDS 2002),
                January 2002.
 [PERLMAN92]    Radia Perlman.  Interconnections:  Bridges and
                Routers.  Addison-Wesley, Reading, Massachusetts,
                1992.
 [RFC793]       Postel, J., "Transmission Control Protocol", STD 7,
                RFC 793, September 1981.
 [RFC2131]      Droms, R., "Dynamic Host Configuration Protocol", RFC
                2131, March 1997.
 [RFC2460]      Deering, S. and R. Hinden, "Internet Protocol, Version
                6 (IPv6) Specification", RFC 2460, December 1998.
 [SONG01]       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://piconet.sourceforge.net/thesis/index.html.
 [TURNER90]     Paul Turner.  NetWare Communications Processes.
                NetWare Application Notes, Novell Research, pages 25-
                91, September 1990.
 [WRIGHT95]     Gary R. Wright and W. Richard Stevens.  TCP/IP
                Illustrated, Volume 2:  The Implementation.  Addison-
                Wesley, Reading, Massachusetts, 1995.

Johnson, et al. Experimental [Page 105] RFC 4728 The Dynamic Source Routing Protocol February 2007

Authors' Addresses

 David B. Johnson
 Rice University
 Computer Science Department, MS 132
 6100 Main Street
 Houston, TX 77005-1892
 USA
 Phone: +1 713 348-3063
 Fax:   +1 713 348-5930
 EMail: dbj@cs.rice.edu
 David A. Maltz
 Microsoft Research
 One Microsoft Way
 Redmond, WA 98052
 USA
 Phone: +1 425 706-7785
 Fax:   +1 425 936-7329
 EMail: dmaltz@microsoft.com
 Yih-Chun Hu
 University of Illinois at Urbana-Champaign
 Coordinated Science Lab
 1308 West Main St, MC 228
 Urbana, IL 61801
 USA
 Phone: +1 217 333-4220
 EMail: yihchun@uiuc.edu

Johnson, et al. Experimental [Page 106] RFC 4728 The Dynamic Source Routing Protocol February 2007

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Johnson, et al. Experimental [Page 107]

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