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DYNAMIC SOURCE ROUTING PROTOCOL
Submitted in partial fulfillment of the requirement of University of MumbaiFor the Degree of
BachelorofEngineering(Computer Engineering )
By
PranavBhujle
Ashish Mathew
Aditya Prasad
Pankajkumar Thakur
under the guidance of
Prof. Payel Gupta
DEPARTMENT OF COMPUTER ENGINEERING
PILLAIS INSTITUTE OF INFORMATION TECHNOLOGY,
ENGINEERING, MEDIA STUDIES & RESEARCH
NEW PANVEL 410 206
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1. INTRODUCTION
Mobile hosts such as notebook computers, featuring powerfulCPUs, large main memories,
hundreds of megabytesof disk space, multimedia sound capabilities, and colour displays,arenow easily affordable and are becoming quitecommon in everyday business and personal life.
At thesame time, network connectivity options for use with mobilehosts have increased
dramatically, including support fora growing number of wireless networking products
basedon radio and infrared.With this type of mobile computing equipment, there isa natural
desire and ability to share information betweenmobile users. Often, mobile users will meet
under circumstancesthat are not explicitly planned for and in whichno connection to a
standard wide-area network such as is available. For example, employees may
findthemselves together in a meeting room; friends or businessassociates may run into each
other in an airport terminal; ora collection of computer science researchers may gather ina
hotel ballroom for a workshop or conference. Requiringeach user to connect to a wide-area
network in such situations,only to communicate with each other, may not bepossible due to
lack of facilities, or may be inconvenientor impractical due to the time or expense required
for suchconnection.
These kinds of networks of mobile hosts have becomeknown as ad hoc networks. An ad
hoc network is a collectionof wireless mobile hosts forming a temporary networkwithout the
aid of any centralized administration or standardsupport services regularly available on the
wide-areanetwork to which the hosts may normally be connected.
Some form of routing protocol is in general necessaryin such an environment, since two
hosts that may wish toexchange packets might not be able to communicate directly.
For example, Figure 1 illustrates a simple ad hocnetwork of three mobile hosts using
wireless network interfaces.Host C is not within the range of host As wirelesstransmitter
(indicated by the circle around A) and hostA is not within the range of host Cs wireless
transmitter.IfA andC wish to exchange packets, they may inthis case enlist the services of
host B to forward packetsfor them, since B is within the overlap betweenAsrange and Cs
range. The maximum number of networkhops needed to reach another mobile host in any
practicalad hoc network is likely to be small, but may oftenbe greater than one as shown here.
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Figure 1:An ad hoc network of threewireless mobile hosts
The routing problemin a real ad hoc network may be even more complicatedthan this
example suggests, due to the inherent nonuniformpropagation characteristics of wireless
transmissionsand since any or all of the hosts involved may move atany time.
The Dynamic Source Routing protocol (DSR) [3][1] 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 (computers) cooperate to
forward packets for each other to allow communication over multiple hops between nodes
not directly within wireless transmission range of one another. As nodes in the network move
about or join or leave the network, and as wireless transmission conditions such as sources of
interference change, all routing is automatically determined and maintained by the DSR
routing protocol. Since the number or sequence of intermediate hops needed to reach any
destination may change at any time, the resulting network topology may be quite rich and
rapidly changing.
The DSR protocol allows nodes to dynamically discover a source route across multiple
network hopsto any destination in the ad hoc network. Each data packet sent then carries in
its header the completeordered list of nodes through which the packet must pass, allowing
packet routing to be trivially loop-free and avoiding the need for up-to-date routing
information in the intermediate nodes through which the packet is forwarded. By including
this source route in the header of each data packet, other nodes forwarding or overhearing any
of these packets may also easily cache this routing information for future use. This DSR
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protocol is a part of the Monarch Project at Carnegie Mellon University [3], a long-term
research project that is developing networking protocols and protocol interfaces to allow truly
seamless wireless and mobile networking. The Monarch Project is named in reference to the
migratory behaviour of the monarch butterfly, and can also be considered as an acronym for
Mobile Networking Architectures. The scope of the research includes protocol design,
implementation, performance evaluation, and usage-based validation, spanning areas ranging
roughly from portions of the ISO Data Link layer (layer 2) through the Presentation layer
(layer 6).
Comparing DSR protocol to other ad-hoc network protocols, it is a routing protocol that
had very low overhead yet was able to react quickly to changes in the network, providing
highly reactive service to help ensure successful delivery of data packets in spite of node
movement or other changes in network conditions. The protocol specification for DSR has
also been submitted to the Internet Engineering Task Force (IETF), the principal protocol
standards development body for the Internet, and is currently one of the protocols under
consideration in the IETF Mobile Ad Hoc Networks (MANET) Working Group for adoption
as an Internet Standard for IP routing in ad hoc networks.
This paper describes the design of the DSR protocol. Literature Survey and related work
on the DSR protocol is described in Section 2. Section 3 of this paper discusses our
assumptions in the design of DSR. In Section 4, we present the design of the DSR protocol
and describe the resulting important properties of this design. In particular, we describe here
the design of the two mechanisms that make up the operation of DSR, Route Discoveryand
Route Maintenance; we also discuss the use of DSR in supporting heterogeneous networks
and interconnecting to the Internet, and describe the current support present in DSR for
routing of multicast packets in ad hoc networks. We also discuss the header format of the
DSR protocol.
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2. LITERATURE SURVEY AND RELATED WORK
Research in the area of routing in multi-hop wireless ad hoc networks dates back at least to
1973, whenthe U.S. Defence Advanced Research Projects Agency (DARPA) began the
Packet Radio Network (PRNET) project.PRNET and its successor, the Survivable Adaptive
Networks (SURAN)project, generated a substantial number of fundamental results in this
area. With the increasingcapabilities and decreasing costs of small, portable computers such
as laptops and PDAs (PersonalDigital Assistants), and with the increasing availability of
inexpensive wireless network interface devicessuch as wireless LAN interfaces packaged as
PCMCIA PC Cards, a growing number of other researchprojects in ad hoc networking have
developed.
In our literature survey here, we concentrate on research specifically related to the DSR
protocol.The initial design of the DSR protocol, including our basic Route Maintenance andRoute Discoverymechanisms, was first published in December 1994, with significant
additional design details and initialsimulation results published in early 1996 [3]. As noted in
Section 1, the designspecification for DSR has also been submitted to the MANET (Mobile
Ad Hoc Networks) Working Groupof the IETF (Internet Engineering Task Force) in their
efforts to standardize a protocol for routing of IPpackets in an ad hoc network [1].The
originalmotivation in the design of DSR came fromthe operation of theAddressResolution
Protocol (ARP) used in the TCP/IP suite of protocols in the Internet. ARP is used on
Ethernets andother types of networks to find the link-layer MAC address of a node on the
same subnet as the sender.
A node sending a packet to a local IP address for which it does not yet have the MAC
address cached,broadcasts an ARP REQUEST packet on the local subnet link, giving the IP
address of the node it is lookingfor; that node responds with an ARP REPLY packet, giving
its MAC address, and all other nodes ignore theREQUEST. If all nodes in an ad hoc network
are within wireless transmission range of each other, this is theonly routing protocol needed
for the ad hoc network. DSR extends this basic behaviour of ARP by allowingthe REQUEST
packet (the ROUTE REQUEST rather than an ARP REQUEST) to be propagated multiple
hopsaway by being forwarded by neighbour nodes, with the ultimate ROUTE REPLY being
returned over multiplehops back to the initiator of the REQUEST.
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DSRs nonpropagating ROUTE REQUEST packets are indeed quite similar to the basic
ARP REQUESTbehaviour, except that a mobile node may answer the ROUTEREQUEST
fromits cache,whereasARP REQUESTSare normally only answered by the target node itself.
The original implementation of DSR in 1997 also was structured as an extension of
ARP,integrated into the existing ARP implementation in the FreeBSD Unix kernel
[FreeBSD], using an extensionof the ARP REQUEST and ARP REPLY packet formats; as
described in Sections 4.8, however, it wasultimately decided to operate DSR at the network
layer rather than at the link layer, to allow routing betweendifferent heterogeneous networks
all forming a single ad hoc network.
DSR is also similar in approach to the source routing discovery mechanism used in the
IEEE 802 SRTbridge standard, and related mechanisms have also been used in other systems
includingFLIP and SDRP. The amateur radio community has also worked extensively with
routing in wireless networks of (sometimes) mobile hosts, holding an annual packet radio
computer networking conference sponsoredby the American Radio Relay League (ARRL)
since 1981. Amateur packet radio networking originallyused only source routing, with
explicit source routes constructed by the user, although some had consideredthe possibility of
a more dynamic source routing scheme. A system known as NET/ROMwas also developed to
allow the routing decisions to be automated, using a form of distance vector routingprotocol
rather than source routing. NET/ROMalso allows updating of its routingtable based on the
source address information in the headers of packets that it receives [6].
Recently, a number of other protocols have been structured around mechanisms similar
to the RouteDiscovery and Route Maintenance mechanisms in DSR. For example, the Signal
Stability-Based Adaptiverouting protocol (SSA) and the Associativity Based Routing
protocol (ABR) eachdiscover routes on demand in a way similar to Route Discovery in DSR,
but each attempts to select onlylong-lived links between nodes where possible; favouring
long-lived links helps avoid routes breaking soonafter discovering them but may result in use
of routes over a greater number of hops than the shortest routesavailable. ABR also adds
overhead for periodic beacon packets required to monitor link stability. TheAd Hoc On-
Demand Distance Vector routing protocol (AODV) [4] uses mechanisms similar toDSRs
Route Discovery and Route Maintenance, but it uses them to create hop-by-hop routes rather
thansource routes as is done in DSR; this use of hop-by-hop routes avoids the source routing
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header overheadof DSR but prevents or makes difficult many of the route caching and other
Route Discovery optimizationspresent in DSR and prevents AODV from supporting uni-
directional links between nodes. The ZoneRouting Protocol (ZRP) defines a routing zone
around each individual node,with a periodic (proactive) protocol such as distance vector or
link state for routing within a zone and anon-demand protocol such as DSR for routing
between zones; the use of routing zones reduces some aspectsof the overhead of the Route
Discovery procedure as in DSR but adds the overhead of maintaining zonemembership and
routing information within each zone. ZRP may also fail at times to successfully
deliverpackets with highly mobile nodes, since the routing protocol within a zone does not
utilize on-demandoperation.
Finally, DSR has recently been used as a basis for further work by other researchers,
including suggestedimprovements to the Route Discovery mechanism. For example, Ko and
Vaidya [6] have proposedan optimization to Route Discovery, known as Location-Aided
Routing (LAR), that uses knowledge of thephysical (geographical) location of the target node
of the Route Discovery (e.g., from GPS, the GlobalPositioning System) to narrow the area of
the network over which the ROUTE REQUEST packets must bepropagated. Castneda and
Das [2] have proposed a similar Route Discovery optimizationthat uses only logical
(topological) location information, not physical location information, and thus doesnot
require access to GPS. Above the routing layer, Holland and Vaidya [6] have recentlystudied
the behaviour of TCP in ad hoc networks, using DSR as a routing protocol; their work added
explicitinteraction between TCP and the Route Discovery and Route Maintenance
mechanisms to allow TCP tocorrectly react to a route failure rather than treating it as network
congestion, and to allow TCP to restartsending as soon as a new route to the destination is
discovered.
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3. ASSUMPTIONS
We assume that all nodes wishing to communicate with other nodes within the ad hoc
network are willingto participate fully in the protocols of the network. In particular, each
node participating in the networkshould also be willing to forward packets for other nodes in
the network.
We refer to the minimum number of hops necessary for a packet to reach from any node
located at oneextreme edge of the ad hoc network to another node located at the opposite
extreme, as the diameterof thead hoc network. We assume that the diameter of an ad hoc
network will often be small (e.g., perhaps 5 or 10 hops), but may often be greater than 1.
Packets may be lost or corrupted in transmission on the wireless network. A node
receiving a corruptedpacket can detect the error and discard the packet.
Nodeswithin the ad hoc networkmay move at any time without notice, andmayevenmove continuously,but we assume that the speed with which nodes move is moderate
with respect to the packet transmissionlatency 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 continuouslymove so rapidly as
to make the flooding of every individual data packet the only possible routing protocol.
We assume that nodes may be able to enablepromiscuous receive mode on their wireless
networkinterface hardware, causing the hardware to deliver every received packet to the
network driver softwarewithout filtering based on link-layer destination address. Although
we do not require this facility, it is,for example, common in current LAN hardware for
broadcast media including wireless, and some of our optimizations can take advantage of its
availability. Use of promiscuous mode does increase the softwareoverhead on the CPU, but
we believe that wireless network speeds are more the inherent limiting factorto performance
in current and future systems; we also believe that portions of the protocol are suitablefor
implementation directly within a programmable network interface unit to avoid this overhead
on theCPU [3]. Use of promiscuous mode may also increase the power consumption of the
networkinterface hardware, depending on the design of the receiver hardware, and in such
cases, DSR can easily beused without the optimizations that depend on promiscuous receive
mode, or can be programmed to onlyperiodically switch the interface into promiscuous mode.
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Wireless communication ability between any pair of nodes may at times not work
equally well in bothdirections, due for example to differing antenna or propagation patterns
or sources of interference aroundthe two nodes. That is, wireless communications between
each pair of nodes willin many cases be able to operate bi-directionally, but at times the
wireless link between two nodes may beonly uni-directional, allowing one node to
successfully send packets to the other while no communicationis possible in the reverse
direction. Although many routing protocols operate correctly only over bidirectionallinks,
DSR can successfully discover and forward packets over paths that contain uni-
directionallinks. Some MAC protocols, however, such as MACA, MACAW, or IEEE802.11
[5], limit unicast data packet transmission to bi-directional links, due to the required
bidirectionalexchange of RTS and CTS packets in these protocols and due to the link-level
acknowledgementfeature in IEEE 802.11; when used on top of MAC protocols such as these,
DSR can take advantage ofadditional optimizations, such as the route reversal optimization
described below.
Each node selects asingle IP address by which it will be known in the ad hoc network.
Although asingle node may have many different physical network interfaces, which in a
typical IP network wouldeach have a different IP address, we require each node to select one
of these and to use only that addresswhen participating in the DSR protocol. This allows each
node to be recognized by all other nodes in thead hoc network as a single entity regardless of
which network interface they use to communicate with it. Inkeeping with the terminology
used by Mobile IP [3], we refer to the address bywhich each mobile node is known in the ad
hoc network as the nodes home address, as this address wouldtypically be the address that
the node uses while connected to its home network (rather than while away,being a member
of the ad hoc network). Each nodes home address may be assigned by any mechanism (e.g.,
static assignment or use of DHCP for dynamic assignment), although the method ofsuch
assignment is outside the scope of the DSR protocol.
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4. DETAILED DESCRIPTIONOF THE DSR PROTOCOL
4.1 Overviewand Important Properties ofthe Protocol
TheDSR protocol is composed of twomechanisms thatwork together to allowthe discovery
and maintenanceof source routes in the ad hoc network:
Route Discovery is the mechanismby which a node S wishing to send a packet to a
destination node Dobtains a source route to D. Route Discovery is used only when S attempts
to send a packet to D anddoes not already know a route to D
Route Maintenance is the mechanism by which node S is able to detect, while using a
source routeto D, if the network topology has changed such that it can no longer use its route
to D because a linkalong the route no longer works. When Route Maintenance indicates a
source route is broken, S canattempt to use any other route it happens to know to D, or can
invoke Route Discovery again to find anew route. Route Maintenance is used only when S isactually sending packets to D.
Route Discovery and Route Maintenance each operate entirely on demand. In particular,
unlike otherprotocols, DSR requires no periodic packets ofany kindat any levelwithin the
network. For example, DSRdoes not use any periodic routing advertisement, link status
sensing, or neighbour detection packets, and doesnot rely on these functions from any
underlying protocols in the network. This entirely on-demand behaviourand lack of periodic
activity allows the number of overhead packets caused by DSR to scale all the way downto
zero, when all nodes are approximately stationary with respect to each other and all routes
needed forcurrent communication have already been discovered. As nodes begin to move
more or as communicationpatterns change, the routing packet overhead of DSRautomatically
scales to only that needed to track theroutes currently in use.
In response to a single Route Discovery (as well as through routing information from
other packetsoverheard), a node may learn and cache multiple routes to any destination. This
allows the reaction torouting changes to be much more rapid, since a node with multiple
routes to a destination can try anothercached route if the one it has been using should fail.
This caching ofmultiple routes also avoids the overheadof needing to perform a new Route
Discovery each time a route in use breaks.
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The operation of Route Discovery and Route Maintenance in DSR are designed to allow
uni-directionallinks and asymmetric routes to be easily supported. In particular, as noted in
Section 3, in wireless networks,it is possible that a link between two nodes may not work
equally well in both directions, due to differingantenna or propagation patterns or sources of
interference. DSR allows such uni-directional links to be usedwhen necessary, improving
overall performance and network connectivity in the system.
DSR also supports internetworking between different types of wireless networks,
allowing a sourceroute to be composed of hops over a combination of any types of networks
available [1][6]. Forexample, some nodes in the ad hoc network may have only short-range
radios, while other nodes have bothshort-range and long-range radios; the combination of
these nodes together can be considered by DSR as asingle ad hoc network. In addition, the
routing of DSR has been integrated into standard Internet routing,where a gateway node
connected to the Internet also participates in the ad hoc network routing protocols;and has
been integrated intoMobile IP routing, where such a gateway node also serves the role of
aMobile IPforeign agent [3][6].
4.2 Basic DSR Route Discovery
When some node S originates a new packet destined to some other node D, it places in the
header of thepacket asource route giving the sequence of hops that the packet should follow
on its way to D. Normally,S will obtain a suitable source route by searching itsRoute Cache
of routes previously learned, but if noroute is found in its cache, it will initiate the Route
Discovery protocol to dynamically find a new route to D.
In this case, we call S the initiatorand D the targetof the Route Discovery.
For example, Figure 2 illustrates an example Route Discovery, in which a node A is
attempting todiscover a route to node E. To initiate the Route Discovery, A transmits a
ROUTE REQUEST message asa single local broadcast packet, which is received by
(approximately) all nodes currently within wirelesstransmission range ofA. Each ROUTE
REQUEST message identifies the initiator and target of the RouteDiscovery, and also
contains a unique request id, determined by the initiator of the REQUEST. Each
ROUTEREQUEST also contains a record listing the address of each intermediate node
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Figure 2: Route Discovery example: Node A is the initiator, and node E is the target.
through which this particularcopy of the ROUTE REQUEST message has been forwarded.
This route record is initialized to an empty listby the initiator of the Route Discovery.
When another node receives a ROUTE REQUEST, if it is the target of the Route
Discovery, it returnsa ROUTE REPLY message to the initiator of the Route Discovery,
giving a copy of the accumulated routerecord from the ROUTE REQUEST; when the
initiator receives this ROUTE REPLY, it caches this route in itsRoute Cache for use in
sending subsequent packets to this destination. Otherwise, if this node receiving theROUTE
REQUEST has recently seen another ROUTE REQUEST message from this initiator bearing
this samerequest id, or if it finds that its own address is already listed in the route record in
the ROUTE REQUESTmessage, it discards the REQUEST. Otherwise, this node appends its
own address to the route record in theROUTE REQUEST message and propagates it by
transmitting it as a local broadcast packet (with the samerequest id).
In returning the ROUTE REPLY to the initiator of the Route Discovery, such as node E
replying back to Ain Figure 1, node E will typically examine its own Route Cache for a route
back to A, and if found, will useit for the source route for delivery of the packet containing
the ROUTE REPLY. Otherwise, Emay perform itsown Route Discovery for target node A,
but to avoid possible infinite recursion of Route Discoveries, it mustpiggyback this ROUTE
REPLY on its own ROUTE REQUEST message for A. It is also possible to piggybackother
small data packets, such as a TCP SYN packet, on a ROUTE REQUEST using this
samemechanism. Node E could also simply reverse the sequence of hops in the route record
that it trying tosend 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
bi-directional frame exchange as part of the MACprotocol [5], this route reversal is preferred
as it avoids the overhead of a possible second RouteDiscovery, and it tests the discovered
route to ensure it is bi-directional before the Route Discovery initiatorbegins using the route.
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However, this technique will prevent the discovery of routes using uni-directionallinks. In
wireless environments where the use of uni-directional links is permitted, such routes may in
somecases be more efficient than those with only bi-directional links, or they may be the only
way to achieveconnectivity to the target node.
When initiating a Route Discovery, the sending node saves a copy of the original packet
in a local buffercalled the SendBuffer. The Send Buffer contains a copy of each packet that
cannot be transmitted by thisnode because it does not yet have a source route to the packets
destination. Each packet in the Send Bufferis stamped with the time that it was placed into
the Buffer and is discarded after residing in the Send Bufferfor some timeout period; if
necessary for preventing the Send Buffer from overflowing, a FIFO or otherreplacement
strategy can also be used to evict packets before they expire.
While a packet remains in the Send Buffer, the node should occasionally initiate a new
Route Discoveryfor the packets destination address. However, the node must limit the rate at
which such new RouteDiscoveries for the same address are initiated, since it is possible that
the destination node is not currentlyreachable. In particular, due to the limited wireless
transmission range and the movement of the nodes inthe network, the network may at times
become partitioned, meaning that there is currently no sequence ofnodes through which a
packet could be forwarded to reach the destination. Depending on the movementpattern and
the density of nodes in the network, such network partitions may be rare or may be common.
If a new Route Discovery was initiated for each packet sent by a node in such a situation,
a large numberof unproductive ROUTE REQUEST packets would be propagated throughout
the subset of the adhoc networkreachable from this node. Inorder to reduce the overhead from
such Route Discoveries, we use exponential back-off to limit the rate at which new Route
Discoveries may be initiated by any node for the same target. If the node attempts to send
additional data packets to this same node more frequently than this limit, thesubsequent
packets should be buffered in the Send Buffer until a ROUTE REPLY is received, but the
node mustnot initiate a new Route Discovery until the minimum allowable interval between
new Route Discoveriesfor this target has been reached. This limitation on the maximum rate
of Route Discoveries for the sametarget is similar to the mechanism required by Internet
nodes to limit the rate at which ARP REQUESTs aresent for any single target IP address.
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Figure 3: Route Maintenance example: Node C is unable to forward a packet from A to E
over its link to next hop D.
4.3Basic DSR Route Maintenance
When originating or forwarding a packet using a source route,each node transmitting the
packet is responsiblefor confirming that the packet has been received by the next hop along
the source route; the packet isretransmitted (up to a maximum number of attempts) until this
confirmation of receipt is received. Forexample, in the situation illustrated in Figure 2, node
A has originated a packet forE using a source routethrough intermediate nodes B, C, and D.
In this case, node A is responsible for receipt of the packet at B,node B is responsible forreceipt at C, node C is responsible for receipt at D, and node D is responsible forreceipt
finally at the destination E. This confirmation of receipt in many cases may be provided at no
cost toDSR, either as an existing standard part of theMAC protocol in use (such as the link-
level acknowledgementframe defined by IEEE 802.11 [5]), or by a passive acknowledgement
(in which, for example, B confirms receipt at C by overhearing C transmit the packet to
forward it on to D). If neither ofthese confirmation mechanisms are available, the node
transmitting the packet may set a bit in the packetsheader to request a DSR-specific software
acknowledgement be returned by the next hop; this softwareacknowledgement will normally
be transmitted directly to the sending node, but if the link between thesetwo nodes is uni-
directional, this software acknowledgement may travel over a different, multi-hop path.
If the packet is retransmitted by some hop the maximum number of times and no receipt
confirmationis received, this node returns a ROUTE ERROR message to the original sender
of the packet, identifying thelink over which the packet could not be forwarded. For example,
in Figure 3, ifC is unable to deliver thepacket to the next hop D, then C returns a ROUTE
ERROR to A, stating that the link from C to Dis currentlybroken. Node A then removes
this broken link from its cache; any retransmission of the original packetis a function for
upper layer protocols such as TCP. For sending such a retransmission or other packets tothis
same destination E, if A has in its Route Cache another route to E (for example, from
additional ROUTEREPLYs from its earlier Route Discovery, or from having overheard
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sufficient routing information from otherpackets), it can send the packet using the new route
immediately. Otherwise, it may perform a new RouteDiscovery for this target (subject to the
exponential backoff described in Section 4.2).
4.4 Additional Route Discovery Features
4.4.1 Caching Overheard Routing Information
A node forwarding or otherwise overhearing any packet may add the routing information
from that packet to its own Route Cache. In particular, the source route used in a data packet,
the accumulated route record in overhears packets from X. a ROUTE REQUEST, or the route
being returned in a ROUTE REPLY may all be cached by any node. Routing information
from any of these packets received may be cached, whether the packet was addressed to this
node, sent to a broadcast (or multicast) MAC address, or received while the nodes network
interface is in promiscuous mode.
One limitation, however, on caching of such overheard routing information is the
possible presence ofuni-directional links in the ad hoc network (Section 2). For example,
Figure 4 illustrates a situation in which node A is using a source route to communicate with
node E. As node C forwards a data packet along the route from A to E, it can always 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. However, the reverse direction of the links
identified in the packet headers, from itself back to Band from Bto A, may not work for it
since these links might be uni-directional. IfC knows that the links are in fact bi-directional,
for example due to the MAC protocol in use, it could cache them but otherwise should not.
Likewise, node V in Figure 4 is using a different source route to communicate with node
Z. If node C overhears node X transmitting a data packet to forward it to Y (from V), node C
should consider whether the links involved can be known to be bi-directional or not before
caching them. If the link from X to C (over which this data packet was received) can be
known to be bi-directional, then C could cache the link from itself to X, the link from X to Y,
and the link from Y to Z. If all links can be assumed to be bi-directional, C could also cache
the links from X to W and from W to V. Similar considerations apply to the routing
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Figure 4:Limitations on caching overheard routing information: Node C is forwarding
packets to E andoverhears packets from X.
information thatmight be learned from forwarded or otherwise overheard ROUTE REQUEST
or ROUTE REPLY packets.
4.4.2 Replying to ROUTE REQUESTs using Cached Routes
A node receiving a ROUTE REQUEST for which it is not the target, searches its own Route
Cache for a route to the target of the REQUEST. If found, the node generally returns a
ROUTE REPLY to the initiator itself rather than forwarding the ROUTE REQUEST. In the
ROUTE REPLY, it sets the route record to list the sequence of hops over which this copy of
the ROUTE REQUEST was forwarded to it, concatenated with its own idea of the route from
itself to the target from its 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, Figure 5 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. The concatenation of the accumulated route from the ROUTE REQUEST and the cached
route from Fs Route Cache would include a duplicate node in passing from C to F and back
to C.
Node F in this case couldattempt to edit the route to eliminate the duplication, resulting in a
route from A to Bto 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 for two reasons. First, this limitation
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Figure 5: A possible duplication of route hops avoided by the Route Discovery limitation on
replying to ROUTE REQUESTs from the Route Cache.
increases the probability that the resulting route is valid, since F in this case should have
received a ROUTE ERROR if the route had previously stopped working. Second, this
limitation means that a ROUTE ERROR traversing the route is very likely to pass through
any node that sent the ROUTE REPLY for the route (including 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 Amight also be contaminated by a ROUTE REPLY from F
containing the same stale route. If the ROUTE REQUEST does not meet these restrictions,
the node (node F in this example) discards the ROUTE REQUEST rather than replying to it
or propagating it.
4.4.3 Preventing ROUTE REPLY Storms
The ability for nodes to reply to a ROUTE REQUEST based on information in their Route
Caches, as described in Section 4.4.2, could result in a possible ROUTE REPLY storm in
some cases. In particular, if a nodebroadcasts a ROUTE REQUEST for a target node for
which the nodes neighbours have a route in their RouteCaches, each neighbour may attempt
to send a ROUTE REPLY, thereby wasting bandwidth and possiblyincreasing the number of
network collisions in the area.
For example, in the situation shown in Figure 6, nodes B, C, D, E, and F all receive
As ROUTE REQUESTfor target G, and each have the indicated route cached for this target.
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Figure 6: A ROUTE REPLY storm could result if many nodes all reply to the same ROUTE
REQUEST fromtheir own Route Caches. The route listed next to each node shows the route
to destination G currently listedin that nodes Route Cache.
Normally, they would all attempt toreply from their own Route Caches, and would all send
their REPLYs at about the same time since they allreceived the broadcast ROUTE
REQUEST at about the same time. Such simultaneous replies from differentnodes all
receiving the ROUTE REQUEST may create packet collisions among some or all of these
REPLIESand may cause local congestion in the wireless network. In addition, it will often be
the case that thedifferent replies will indicate routes of different lengths, as shown in this
example.If a node can put its network interface into promiscuous receive mode, it should
delay sending its ownROUTE REPLY for a short period, while listening to see if the
initiating node begins using a shorter route first.That is, this node should delay sending itsown 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 nodes ROUTE REPLY,r is a
random number between 0 and 1, andH is a small constant delay (at least twice the maximum
wirelesslink propagation delay) to be introduced per hop. This delay effectively randomizes
the time at which each node sends its ROUTE REPLY, with all nodes sending ROUTE
REPLYs giving routes of length less than h sending their REPLYs before this node, and all
nodes sending ROUTE REPLYs giving routes of length greaterthan h sending their REPLYs
after this node. Within the delay period, this node promiscuously receives allpackets, looking
for data packets from the initiator of this Route Discovery destined for the target of
theDiscovery. If such a data packet received by this node during the delay period uses a
source route of lengthless than or equal to h, this node may infer that the initiator of the Route
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Discovery has already received aROUTE REPLY giving an equally good or better route. In
this case, this node cancels its delay timer and doesnot send its ROUTE REPLY for this
Route Discovery.
4.4.4 ROUTE REQUEST Hop Limits
Each ROUTE REQUEST message contains a hop limit that may be used to limit the
number of intermediatenodes allowed to forward that copy of the ROUTE REQUEST. As the
REQUEST is forwarded, this limit isdecremented, and the REQUEST packet is discarded if
the limit reaches zero before finding the target. Wecurrently use this mechanism to send a
nonpropagatingROUTE REQUEST (i.e., with hop limit 0) as aninexpensive method of
determining if the target is currently a neighbour of the initiator or if a neighbournodehas a
route to the target cached (effectively using the neighbours caches as anextension of the
initiatorsown cache). If no ROUTE REPLY is received after a short timeout, then a
propagatingROUTE REQUEST (i.e.,with no hop limit) is sent.
This mechanism can be used to implement an expanding ringsearch for the target [3].
For example, a node could send an initial nonpropagating ROUTE REQUEST asdescribed
above; if no ROUTE REPLY is received for it, the node could initiate another ROUTE
REQUESTwith a hop limit of 1. For each ROUTE REQUEST initiated, if no ROUTE
REPLY is received for it, thenode could double the hop limit used on the previous attempt, to
progressively explore for the target nodewithout allowing the ROUTE REQUEST to
propagate over the entire network. However, this expanding ringsearch approach could have
the effect of increasing the average latency of Route Discovery, since multipleDiscovery
attempts and timeouts may be needed before discovering a route to the target node.
4.5 Additional Route Maintenance Features
4.5.1 Packet Salvaging
After sending a ROUTE ERROR message as part of Route Maintenance as described in
Section 4.3, a nodemay attempt to salvage the data packet that caused the ROUTE ERROR
rather than discarding it. To attemptto salvage a packet, the node sending a ROUTE ERROR
searches its own Route Cache for a route from itselfto the destination of the packet causing
the ERROR. If such a route is found, the node may salvage the packetafter returning the
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Figure 7: Node C notices that the source route to D can be shortened, since it overheard a
packet from Aintended first forB.
ROUTE ERROR by replacing the original source route on the packet with the route fromits
Route Cache. The node then forwards the packet to the next node indicated along this source
route. Forexample, in Figure 3, if node C has another route cached to node E, it can salvage
the packet by applyingthis route to the packet rather than discarding the packet.When
salvaging a packet in this way, the packet is also marked as having been salvaged, to prevent
asingle packet being salvagedmultiple times. Otherwise, it could be possible for the packet to
enter a routingloop, as different nodes repeatedly salvage the packet and replace the source
route on the packet with routesto each other. An alternative mechanism of salvaging that we
have considered would be to replace onlythe unused suffix of the original route (the portion
in advance of this node) with the new route from thisnodes Route Cache, forming a new
route whose prefix is the original route and whose suffix is the routefrom the Cache. In this
case, the normal rules for avoiding duplicated nodes being listed in a source routeare
sufficient to avoid routing loops. However, this mechanism of salvaging would prevent the
new routefrom backtracking from this node to an earlier node already traversed by this
packet, to then be forwardedalong a different remaining sequence of hops to the destination.
The current salvaging mechanism allowsbacktracking but prevents a packet from being
salvaged more than once.
4.5.2 Automatic Route Shortening
Source routes in use may be automatically shortened if one or more intermediate hops in theroute becomeno longer necessary. This mechanism of automatically shortening routes in use
is somewhat similar to theuse of passive acknowledgements. In particular, if a node is able to
overhear a packet carrying a sourceroute (e.g., by operating its network interface in
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promiscuous receive mode), then this node examines theunused portion of that source route.
If this node is not the intended next hop for the packet but is namedin the later unused portion
of the packets source route, then it can infer that the intermediate nodes beforeitself in the
source route are no longer needed in the route. For example, Figure 7 illustrates an examplein
which node C has overheard a data packet being transmitted from A to B, for later
forwarding to C; thearrow pointing to one node in the source route in each packet indicates
the intended next receiver of thepacket along the route.
In this case, this node (node C) returns agratuitous ROUTE REPLY message to the
original sender of thepacket (nodeA). The ROUTE REPLY gives the shorter route as the
concatenation of the portion of the originalsource route up through the node that transmitted
the overheard packet, plus the suffix of the original sourceroute beginning with the node
returning the gratuitous ROUTE REPLY. In this example, the route returned inthe gratuitous
ROUTE REPLY message sent from C to A gives the new route as the sequence of hops from
Ato C to D.
4.5.3 Increased Spreading ofROUTE ERROR Messages
When a source node receives a ROUTE ERROR for a data packet that it originated, this
source node propagatesthis ROUTE ERROR to its neighbours 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 REPLYs that contain thesame invalid link for which this
source node received the ROUTE ERROR.
For example, in the situation shown in Figure 3, node A learns from the ROUTE
ERROR message fromC, that the link from C to Dis currently broken. It thus removes this
link from its own Route Cache andinitiates a new Route Discovery (if it doesnt have another
route to E in its Route Cache). On the ROUTEREQUEST packet initiating this Route
Discovery, node A piggybacks a copy of this ROUTE ERROR message,ensuring that the
ROUTE ERROR message spreads well to other nodes, and guaranteeing that any
ROUTEREPLY that it receives (including those from other nodes Route Caches) in response
to this ROUTE REQUESTdoes not contain a route that assumes the existence of this broken
link.
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A further improvement to Route Maintenance can be considered, in which anode, such as
Ain Figure 5, that receives a ROUTE ERROR will forward the ERROR along the same
sourceroute that resulted in the ERROR. This will almost guarantee that the ROUTE ERROR
reaches the node thatgenerated the ROUTE REPLY containing the broken link, which will
prevent that node from contaminating afuture Route Discovery with the same broken link.
4.5.4 Caching Negative Information
In some cases, DSR could potentially benefit from nodes caching negative information in
their RouteCaches. For example, in Figure 3, if node A caches the fact that the link from C to
Dis currently broken(rather than simply removing this hop from its Route Cache), it can
guarantee that no ROUTE REPLY thatit receives in response to its new Route Discovery will
be accepted that utilizes this broken link. A shortexpiration period must be placed on this
negative cached information, since while this entry is in its RouteCache, A will otherwise
refuse to allow this link in its cache, even if this link begins working again.
Another case in which caching negative information in a nodes Route Cache might be
useful is the casein which a link is providing highly variable service, sometimes working
correctly but often not working.This situation could occur, for example, in the case in which
the link is near the limit of the sendingnodes wireless transmission range and there are
significant sources of interference (e.g., multipath) nearthe receiving node on this link. In this
case, by caching the negative information that this link is broken, anode could avoid adding
this problematic link back to its Route Cache during the brief periods in which itis working
correctly.
4.6. Supportfor Heterogeneous Networks and Mobile IP
In configuring and deploying an ad hoc network, in many cases, all nodes will be equipped
with the same typeof wireless network interfaces, allowing simple routing between nodes
over arbitrary sequences of networkhops. However, a more flexible configuration might be to
also equip a subset of the nodes with a secondnetwork interface consisting of a longer-range
(and thus generally lower speed) wireless network interface.For example, in a military
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Figure 8: An ad hoc network consisting of nodes communicating via short-range radios, with
nodes A, B,andC also having long-range radios.
setting, a group of soldiers might all use short-range radios to communicateamong
themselves, while relaying through truck-mounted higher power radios to communicate with
othergroups.
This general type of network configuration is the ad hoc networking equivalent of
wireless overlaynetworks. Due to the high degree of locality likely to be present among
directly cooperatingnodes communicatingwith each other, such a network
configurationwould allowhigh speed communicationamong such cooperating nodes, while at
the same time allowing communication with other nodes furtheraway without requiring verylarge numbers of network hops. The longer-range radios might also allow gapsbetween
different groups of nodes to be spanned, reducing the probability of network partition. A
simpleexample of such an ad hoc network configuration is shown in Figure 8. Nodes A, B,
and C, here, each haveboth short-range and long-range radio interfaces, all other nodes in the
ad hoc network have only short-rangeradio network interfaces. Node X is using a source
route to node Y that uses a sequence of both short-rangeand long-range hops.
4.6.1 UseofInterface Indices in DSR
DSR supports automatic, seamless routing in these and other heterogeneous configurations,
through itslogical addressing model [1]. Using conventional IP addressing, each ad hoc
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Figure 9: An ad hoc network consisting of nodes with heterogeneous network interfaces.
network nodewould configure a different IP address for each of its possibly many network
interfaces, but as noted inSection 3, each node using DSR chooses one of these as its home
addressto use for all communication whilein the ad hoc network. This use of a single IP
address per node gives DSR the ability to treat the overallnetwork as single routing domain.
To then distinguish between the different network interfaces on a node,each node
independently assigns a locally unique interface index to each of its own network
interfaces.The interface index for any network interface on a node is an opaque value
assigned by the node itself.
The particular value chosen must be unique among the network interfaces on that
individual node butneed have no other significance and need not be coordinated with any
other nodes in choosing their owninterface indices. On many operating systems, a unique
value to identify each network interface is alreadyavailable and can be used for this purpose;
for example, the if index field in the ifnet structure for anetwork interface in BSD Unix-based
networking stacks can be used directly by a node forthe interface index for that network
interface.
For example, Figure 9 illustrates a simple ad hoc network of four nodes, in which node Ais using onetype of network interface (represented by the triangles), node C and node D are
using an different type ofphysical network interface (represented by the circles), and node B
is configured with both types of networkinterfaces and can forward packets between the two
different types of radio technologies. The numberlabelling each network interface indicates
the interface index chosen by the corresponding node for thatinterface. Since the interface
indices are chosen independently by each node, it is possible, for example,that nodes Band D
each chose index 1 for their circle network interfaces, but node C chooses index 4.The
interface index is used as part of each hop in each source route discovered and used by
DSR.Specifically, a path through the ad hoc network from a source node N0 to a destination
node Nm is fullyrepresented as a series of hops N0/i0 _N1/i1 _N2/i2 __Nm, where
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is also possible for a node to use a single network interface both for participationin the ad hoc
network and also for connection to the Internet through standard IP routing; in this case,the
reserved interface index identifies the logically separate functionality of this interface for its
Internetconnection, and the node uses another (locally assigned) interface index value to
identify this interface inits separate logical function of participation in the ad hoc network.
If the gateway node is acting as a Mobile IP home agent or foreign agent (termed a
mobility agent)on this network interface, it uses the reserved interface index value
IF_INDEX_MA. Otherwise, thegateway node uses the reserved value IF_INDEX_ROUTER.
The distinction between the reserved indexvalues for mobility agents and for routers allows
mobility agents to advertise their existence (as needed forMobile IP) at no cost. A node in the
ad hoc network that processes a routing header listing the interface indexIF_INDEX_MA can
then send a unicastMobile IPAGENT SOLICITATION to the correspondingaddress in the
routing header to obtain complete information about the Mobile IP services being provided.
In processing a received ROUTE REQUEST, a gateway node generates a ROUTE
REPLY, giving its reservedinterface index value, if it believes it may be able to reach the
target node through its Internet connection. Thus, the originator of the Route Discovery may
receive REPLYs both from the gateway and from the nodeitself, if the node is really present
in the ad hoc network. When later sending packets to this destination,the sender should prefer
cached routes that do not traverse a hop with an interface index of IF_INDEX_MAor
IF_INDEX_ROUTER, since this will prefer routes that lead directly to the destination node
within thead hoc network.
4.7 Multicast Routing with DSR
DSR does not currently support true multicast routing, but does support an approximation of
this thatis sufficient in many network contexts. Through an extension of the Route Discovery
mechanism, DSRsupports the controlled flooding of a data packet to all nodes in the ad hoc
network that are within somespecified number of hops of the originator; these nodes may
then apply destination address filtering (e.g., insoftware) to limit the packet to those nodes
subscribed to the packets indicated multicast destination address.
While this mechanism does not support pruning of the broadcast tree to conserve
network resources, it canbe used to distribute information to all nodes in the ad hoc network
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subscribed to the destination multicastaddress. This mechanism may also be useful for
sending application level packets to all nodes in a limitedrange around the sender.
To utilize this form of multicasting, when an application on a DSR node sends a packet
to a multicastdestination address, DSR piggybacks the data from the packet inside a ROUTE
REQUEST targeted at themulticast address. The normal ROUTE REQUEST propagation
scheme described in Section 4.2 will resultin this packet being efficiently distributed to all
nodes in the network within the specified hop count (TTL)of the originator. After forwarding
the packet as defined for Route Discovery, each receiving node thenindividually examines
the destination address of the packet and discards the packet if it is destined to amulticast
address to which this node is not subscribed.
4.8 Location ofDSR Functions in the ISO Network Reference Model
When designing DSR, two different options were considered, regarding where should DSR
be placed in the ISO model: routing at the link layer(ISO layer 2) and routing atthenetwork
layer(ISO layer 3). Originally, it was opted to route at the link layer for several reasons:
Pragmatically, running the DSR protocol at the link layer maximizes the number of
mobile nodesthat can participate in ad hoc networks. For example, the protocol can route
equally well betweenIPv4,IPv6, and IPX nodes.
Historically [3], as mentioned in Section 2, DSR grew fromour contemplation of a multi-
hop propagating version of the Internets Address Resolution Protocol(ARP), as well as from
the routing mechanism used in IEEE 802 source routingbridges. These are layer 2 protocols.
Technically, DSR was designed to be simple enough that that it could be implemented
directly inthe firmware inside wireless network interface cards [3], well below thelayer 3
software within amobile node. There is great potential in this for DSR running inside a cloud
ofmobile nodes around a fixed base station, where DSR would act to transparently extend the
coveragerange to these nodes. Mobile nodes that would otherwise be unable to communicate
with the basestation due to factors such as distance, fading, or local interference sources
could then reach the basestation through their peers.
Ultimately, however, it was decided to specify [1] and to implement DSR as a layer
3protocol, since this is the only layer at which DSR could realistically support nodes with
multiple networkinterfaces of different types, as described in Section 4.6.
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4.9 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 thesame 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.
4.9.1 Flow Establishment
A source node sending packets to some destination node MAY use the DSR flow stateextension 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.
4.9.2 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.
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4.10 DSRon IPv4
The Dynamic Source Routing protocol makes use of special header carrying control
information that can be included in any existing IP packet. The reason that DSR on IPv4 is
specified here is that it requires much less bandwidth than DSR on IPv6.
DSR header is an IP-level protocol just as TCP or UDP are; it is inserted directly after IP
header. The protocol number in the IP header is changed to DSR header (to be assigned by
IANA), and next header field in the DSR header is updated to point to the original protocol
number of the terminal header.
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 ofDSR options in the DSR Options header is implied by the total length of the DSR
Options header.
4.10.1 Fixed Portion ofDSR Header
DSR header has a fixed portion of 4 bytes (Next header, reserved, Flow State Header,
payload length) and any number of options encoded in Type-Length-Value (TLV) notation.
The Next Header is a 8-bit selector and identifies the type of header that immediately
follows the DSR Header. The reserved bits are to be ignored, as they are reserved for further
use. The payload length is a 16-bit field which mentions the length of the DSR Options
Header excluding the 4-octet fixed portion. It defines the total length of all options included
in the DSR Options Header.
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
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Fi : Format
Fi re 11:Fi ed Portion of DSROptions Header
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.
The two following bits in the Option Type value (thatis, 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
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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 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.
4.10.2DSR Options
The following options have been defined: Route Request,Route Reply, Route Error,
Acknowledgment Request,Acknowledgment, DSR Source Route, Pad1, and PadN[7]. These
are only quickly introduced here; all thegory details are omitted.
Route Request contains Identification, TargetAddress and Address[1..n] fields.
Route requestswill be sent to the limited broadcast address255.255.255.255, and the
destination address wherethe source wants to find the route for is placed in Targetaddress
field. Identification is a sequence number,used to distinguish between already seen and old
messages.Address [1..n] are used to store the addressesalong the the path of Route
Discovery.
Route Reply contains L-bit, Reserved and Address[1..n] fields. Last Hop External
bit indicates thatthe last address represents the last node in the DSR network, and the actual
destination is outside of MANET.Address[1..n] contains the source route gathered within
the route request. Route Replies are usually sent backto the originator of Route Request by
either the nodethat was the Target of the original Route Request or by some intermediate
node, as a Reply from Cache or as agratuitous Reply.
Route Error contains Error Type, Reserved, Salvage,Error Source Address,
Error Destination Address,and error type -specific information. Currently,only one error
type, Node Unreachable, is specified.The contents of Salvage field are derived from
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theDSR Source Route option triggering the error. ErrorSource Address is the address of the
node which encounteredan error; Error Destination Address is theoriginal source of the
failed packet. In the case of NodeUnreachable message, type-specific information containsthe
address of the unreachable node. Route Errorsare sent to inform the source (and intermediate
nodes)of failed source routes.
Acknowledgment Request contains Identificationfield. It is a unique value that will be
used in Acknowledgmentresponse to create a link between the two. Acknowledgmentsare
used for ensuring the reliable deliveryof Route Maintenance packets if no other form
(e.g.link-layer acknowledgments, passive acknowledgmentsby promiscuous mode) is
available.Acknowledgment contains Identification, ACKSource Address, and ACK
Destination Addressfields. Identification is copied from the Request, ACK
Source Address is the address of the node originatingthe acknowledgment, and ACK
Destination Address isthe address to which the acknowledgment will be deliveredto.
DSR Source Route contains F and L -bits,Reserved, Salvage, Segments Left,
and Address [1..n] fields. First and Last Hop External bitsindicate whether the route leads
to or from outside ofthe DSR network; such paths must not be returnedfrom the cache of
intermediate nodes. Salvage indicatesthe number of times the packet has been salvaged
(see section 4.5.1), that is, rewritten by an intermediatenode to ensure delivery. Segments
Left indicateshow many addresses in the option must still be traverseduntil reaching the
final destination. Address[1..n] liststhese intermediate nodes which the packet has been,
andwill be, through. DSR Source Route option is presentin almost every packet.
Pad1 and PadN options include requested amount ofpadding, to ensure that the total DSR
Header will beproperly aligned to a multiple of 4 bytes.
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5. FUTURE WORK
The scope of applicability of DSR needs to be moreprecisely defined. Interactions with
IPSEC, and routingprotocol security in general should be studied. Interactionswith packet
filters should be explored. Thespecification text should be clarified on some points,
especiallyregarding retransmissions. Especially a mixednetwork of uni- and bidirectional
links should be simulated.How destructive a DSR node (or a set of nodes)can be with old
cache entries should be studied. Eventhough DSR is not meant for many hundreds of nodes,
comparisons with other MANET protocols with theseamounts of nodes might be interesting.
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6. CONCLUSION
DSR applies rather well to a smallish, less than a hundred, network of nodes that trust each
other. In such networks, DSR may be especially useful if it is important to be able to deal
with both high and low node mobility with reasonably small overhead without any manual
changes; DSR is highly adaptive.
There are some areas that need clarifications or more study; some of these are applicable to
some otherMANET protocols too, though:
Study on the more generic problem of trusting routing updates in ad-hoc networking
Clarifications and tests on cache management, especially ensuring cache freshness and
how far wrong data could spread
Being able to use IPSEC to verify either the payload or DSR header data, so that link-
layer security would not be required
Clarifications on retransmission mechanisms and how that applies to unreliable IP
Clarifications on interactions with firewalls and packet filters
Clarifications and tests with bi- and uni-directionality of links
Some other clarifications on the specification text
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7. REFERENCES
[1] Josh Broch, David B. Johnson, and David A. Maltz.The Dynamic Source RoutingProtocol for Mobile Ad Hoc Networks. Internet-Draft, draft-ietf-manet-dsr-03.txt,
October 1999. Work in progress. Earlier revisions published June 1999, December1998, and March 1998.
[2] Robert Castaneda and Samir R. Das. Query Localization Techniques for On-demand
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