IETF MANET Working Group David B. Johnson, Rice University
INTERNET-DRAFT David A. Maltz, AON Networks
21 November 2001 Yih-Chun Hu, Rice University
Jorjeta G. Jetcheva, Carnegie Mellon University
The Dynamic Source Routing Protocol
for Mobile Ad Hoc Networks (DSR)
<draft-ietf-manet-dsr-06.txt>
Status of This Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC 2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note
that other groups may also distribute working documents as
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at
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The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft is a submission to the IETF Mobile Ad Hoc
Networks (MANET) Working Group. Comments on this draft may be sent
to the Working Group at manet@itd.nrl.navy.mil, or may be sent
directly to the authors.
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Abstract
The Dynamic Source Routing protocol (DSR) is a simple and efficient
routing protocol designed specifically for use in multi-hop wireless
ad hoc networks of mobile nodes. DSR allows the network to be
completely self-organizing and self-configuring, without the need
for any existing network infrastructure or administration. The
protocol is composed of the two main mechanisms of "Route Discovery"
and "Route Maintenance", which work together to allow nodes to
discover and maintain source routes to arbitrary destinations in the
ad hoc network. The use of source routing allows packet routing
to be trivially loop-free, avoids the need for up-to-date routing
information in the intermediate nodes through which packets are
forwarded, and allows nodes forwarding or overhearing packets to
cache the routing information in them for their own future use. All
aspects of the protocol operate entirely on-demand, allowing the
routing packet overhead of DSR to scale automatically to only that
needed to react to changes in the routes currently in use. This
document specifies the operation of the DSR protocol for routing
unicast IPv4 packets in multi-hop wireless ad hoc networks.
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Contents
Status of This Memo i
Abstract ii
1. Introduction 1
2. Assumptions 3
3. DSR Protocol Overview 5
3.1. Basic DSR Route Discovery . . . . . . . . . . . . . . . . 5
3.2. Basic DSR Route Maintenance . . . . . . . . . . . . . . . 7
3.3. Additional Route Discovery Features . . . . . . . . . . . 9
3.3.1. Caching Overheard Routing Information . . . . . . 9
3.3.2. Replying to Route Requests using Cached Routes . 10
3.3.3. Preventing Route Reply Storms . . . . . . . . . . 11
3.3.4. Route Request Hop Limits . . . . . . . . . . . . 13
3.4. Additional Route Maintenance Features . . . . . . . . . . 14
3.4.1. Packet Salvaging . . . . . . . . . . . . . . . . 14
3.4.2. Queued Packets Destined over a Broken Link . . . 14
3.4.3. Automatic Route Shortening . . . . . . . . . . . 15
3.4.4. Increased Spreading of Route Error Messages . . . 16
4. Conceptual Data Structures 17
4.1. Route Cache . . . . . . . . . . . . . . . . . . . . . . . 17
4.2. Send Buffer . . . . . . . . . . . . . . . . . . . . . . . 20
4.3. Route Request Table . . . . . . . . . . . . . . . . . . . 21
4.4. Gratuitous Route Reply Table . . . . . . . . . . . . . . 22
4.5. Network Interface Queue and Retransmission Buffer . . . . 23
5. DSR Header Format 25
5.1. Fixed Portion of DSR Header . . . . . . . . . . . . . . . 26
5.2. Route Request Option . . . . . . . . . . . . . . . . . . 28
5.3. Route Reply Option . . . . . . . . . . . . . . . . . . . 30
5.4. Route Error Option . . . . . . . . . . . . . . . . . . . 32
5.5. Acknowledgment Request Option . . . . . . . . . . . . . . 35
5.6. Acknowledgment Option . . . . . . . . . . . . . . . . . . 36
5.7. DSR Source Route Option . . . . . . . . . . . . . . . . . 37
5.8. Pad1 Option . . . . . . . . . . . . . . . . . . . . . . . 39
5.9. PadN Option . . . . . . . . . . . . . . . . . . . . . . . 40
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6. Detailed Operation 41
6.1. General Packet Processing . . . . . . . . . . . . . . . . 41
6.1.1. Originating a Packet . . . . . . . . . . . . . . 41
6.1.2. Adding a DSR Header to a Packet . . . . . . . . . 41
6.1.3. Adding a DSR Source Route Option to a Packet . . 42
6.1.4. Processing a Received Packet . . . . . . . . . . 43
6.1.5. Processing a Received DSR Source Route Option . . 45
6.2. Route Discovery Processing . . . . . . . . . . . . . . . 48
6.2.1. Originating a Route Request . . . . . . . . . . . 48
6.2.2. Processing a Received Route Request Option . . . 50
6.2.3. Generating a Route Reply using the Route Cache . 51
6.2.4. Originating a Route Reply . . . . . . . . . . . . 54
6.2.5. Processing a Received Route Reply Option . . . . 55
6.3. Route Maintenance Processing . . . . . . . . . . . . . . 57
6.3.1. Using Link-Layer Acknowledgments . . . . . . . . 57
6.3.2. Using Passive Acknowledgments . . . . . . . . . . 58
6.3.3. Using Network-Layer Acknowledgments . . . . . . . 59
6.3.4. Originating a Route Error . . . . . . . . . . . . 62
6.3.5. Processing a Received Route Error Option . . . . 63
6.3.6. Salvaging a Packet . . . . . . . . . . . . . . . 64
7. Protocol Constants and Configuration Variables 66
8. IANA Considerations 67
9. Security Considerations 68
Appendix A. Link-MaxLife Cache Description 69
Appendix B. Location of DSR in the ISO Network Reference Model 71
Appendix C. Implementation and Evaluation Status 72
Changes from Previous Version of the Draft 73
Acknowledgements 76
References 77
Chair's Address 80
Authors' Addresses 81
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1. Introduction
The Dynamic Source Routing protocol (DSR) [13, 14] is a simple and
efficient routing protocol designed specifically for use in multi-hop
wireless ad hoc networks of mobile nodes. Using DSR, the network
is completely self-organizing and self-configuring, requiring no
existing network infrastructure or administration. Network nodes
cooperate to forward packets for each other to allow communication
over multiple "hops" between nodes not directly within wireless
transmission range of one another. As nodes in the network move
about or join or leave the network, and as wireless transmission
conditions such as sources of interference change, all routing is
automatically determined and maintained by the DSR routing protocol.
Since the number or sequence of intermediate hops needed to reach any
destination may change at any time, the resulting network topology
may be quite rich and rapidly changing.
The DSR protocol allows nodes to dynamically discover a source
route across multiple network hops to any destination in the ad hoc
network. Each data packet sent then carries in its header the
complete, ordered list of nodes through which the packet will pass,
allowing packet routing to be trivially loop-free and avoiding the
need for up-to-date routing information in the intermediate nodes
through which the packet is forwarded. By including this source
route in the header of each data packet, other nodes forwarding or
overhearing any of these packets can also easily cache this routing
information for future use.
In designing DSR, we sought to create a routing protocol that had
very low overhead yet was able to react very quickly to changes in
the network. The DSR protocol provides highly reactive service in
order to help ensure successful delivery of data packets in spite of
node movement or other changes in network conditions.
The DSR protocol is composed of two main mechanisms that work
together to allow the discovery and maintenance of source routes in
the ad hoc network:
- Route Discovery is the mechanism by which a node S wishing to
send a packet to a destination node D obtains a source route
to D. Route Discovery is used only when S attempts to send a
packet to D and does not already know a route to D.
- Route Maintenance is the mechanism by which node S is able
to detect, while using a source route to D, if the network
topology has changed such that it can no longer use its route
to D because a link along the route no longer works. When Route
Maintenance indicates a source route is broken, S can attempt to
use any other route it happens to know to D, or can invoke Route
Discovery again to find a new route for subsequent packets to D.
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Route Maintenance for this route is used only when S is actually
sending packets to D.
In DSR, Route Discovery and Route Maintenance each operate entirely
"on demand". In particular, unlike other protocols, DSR requires no
periodic packets of any kind at any layer within the network. For
example, DSR does not use any periodic routing advertisement, link
status sensing, or neighbor detection packets, and does not rely on
these functions from any underlying protocols in the network. This
entirely on-demand behavior and lack of periodic activity allows
the number of overhead packets caused by DSR to scale all the way
down to zero, when all nodes are approximately stationary with
respect to each other and all routes needed for current communication
have already been discovered. As nodes begin to move more or
as communication patterns change, the routing packet overhead of
DSR automatically scales to only that needed to track the routes
currently in use. Network topology changes not affecting routes
currently in use are ignored and do not cause reaction from the
protocol.
In response to a single Route Discovery (as well as through routing
information from other packets overheard), a node may learn and cache
multiple routes to any destination. This allows the reaction to
routing changes to be much more rapid, since a node with multiple
routes to a destination can try another cached route if the one it
has been using should fail. This caching of multiple routes also
avoids the overhead of needing to perform a new Route Discovery each
time a route in use breaks.
The operation of both Route Discovery and Route Maintenance in DSR
are designed to allow uni-directional links and asymmetric routes
to be easily supported. In particular, as noted in Section 2, in
wireless networks, it is possible that a link between two nodes may
not work equally well in both directions, due to differing antenna
or propagation patterns or sources of interference. DSR allows such
uni-directional links to be used when necessary, improving overall
performance and network connectivity in the system.
This document specifies the operation of the DSR protocol for
routing unicast IPv4 packets in multi-hop wireless ad hoc networks.
Advanced, optional features, such as Quality of Service (QoS) support
and efficient multicast routing, and operation of DSR with IPv6 [6],
are covered in other documents. The specification of DSR in this
document provides a compatible base on which such features can be
added, either independently or by integration with the DSR operation
specified here.
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [4].
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2. Assumptions
We assume in this document that all nodes wishing to communicate with
other nodes within the ad hoc network are willing to participate
fully in the protocols of the network. In particular, each node
participating in the ad hoc network SHOULD also be willing to forward
packets for other nodes in the network.
The diameter of an ad hoc network is the minimum number of hops
necessary for a packet to reach from any node located at one extreme
edge of the ad hoc network to another node located at the opposite
extreme. We assume that this diameter will often be small (e.g.,
perhaps 5 or 10 hops), but may often be greater than 1.
Packets may be lost or corrupted in transmission on the wireless
network. We assume that a node receiving a corrupted packet can
detect the error and discard the packet.
Nodes within the ad hoc network MAY move at any time without notice,
and MAY even move continuously, but we assume that the speed with
which nodes move is moderate with respect to the packet transmission
latency and wireless transmission range of the particular underlying
network hardware in use. In particular, DSR can support very
rapid rates of arbitrary node mobility, but we assume that nodes do
not continuously move so rapidly as to make the flooding of every
individual data packet the only possible routing protocol.
A common feature of many network interfaces, including most current
LAN hardware for broadcast media such as wireless, is the ability
to operate the network interface in "promiscuous" receive mode.
This mode causes the hardware to deliver every received packet to
the network driver software without filtering based on link-layer
destination address. Although we do not require this facility, some
of our optimizations can take advantage of its availability. Use
of promiscuous mode does increase the software overhead on the CPU,
but we believe that wireless network speeds are more the inherent
limiting factor to performance in current and future systems; we also
believe that portions of the protocol are suitable for implementation
directly within a programmable network interface unit to avoid this
overhead on the CPU [14]. Use of promiscuous mode may also increase
the power consumption of the network interface hardware, depending
on the design of the receiver hardware, and in such cases, DSR can
easily be used without the optimizations that depend on promiscuous
receive mode, or can be programmed to only periodically switch the
interface into promiscuous mode. Use of promiscuous receive mode is
entirely optional.
Wireless communication ability between any pair of nodes may at
times not work equally well in both directions, due for example to
differing antenna or propagation patterns or sources of interference
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around the two nodes [1, 18]. That is, wireless communications
between each pair of nodes will in many cases be able to operate
bi-directionally, but at times the wireless link between two nodes
may be only uni-directional, allowing one node to successfully send
packets to the other while no communication is possible in the
reverse direction. Although many routing protocols operate correctly
only over bi-directional links, DSR can successfully discover and
forward packets over paths that contain uni-directional links.
Some MAC protocols, however, such as MACA [17], MACAW [2], or IEEE
802.11 [11], limit unicast data packet transmission to bi-directional
links, due to the required bi-directional exchange of RTS and CTS
packets in these protocols and due to the link-layer acknowledgement
feature in IEEE 802.11; when used on top of MAC protocols such as
these, DSR can take advantage of additional optimizations, such as
the ability to reverse a source route to obtain a route back to the
origin of the original route.
The IP address used by a node using the DSR protocol MAY be assigned
by any mechanism (e.g., static assignment or use of DHCP for dynamic
assignment [7]), although the method of such assignment is outside
the scope of this specification.
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3. DSR Protocol Overview
3.1. Basic DSR Route Discovery
When some source node originates a new packet addressed to some
destination node, the source node places in the header of the packet
a source route giving the sequence of hops that the packet is to
follow on its way to the destination. Normally, the sender will
obtain a suitable source route by searching its "Route Cache" of
routes previously learned; if no route is found in its cache, it will
initiate the Route Discovery protocol to dynamically find a new route
to this destination node. In this case, we call the source node
the "initiator" and the destination node the "target" of the Route
Discovery.
For example, suppose a node A is attempting to discover a route to
node E. The Route Discovery initiated by node A in this example
would proceed as follows:
^ "A" ^ "A,B" ^ "A,B,C" ^ "A,B,C,D"
| id=2 | id=2 | id=2 | id=2
+-----+ +-----+ +-----+ +-----+ +-----+
| A |---->| B |---->| C |---->| D |---->| E |
+-----+ +-----+ +-----+ +-----+ +-----+
| | | |
v v v v
To initiate the Route Discovery, node A transmits a "Route
Request" as a single local broadcast packet, which is received by
(approximately) all nodes currently within wireless transmission
range of A, including node B in this example. Each Route Request
identifies the initiator and target of the Route Discovery, and
also contains a unique request identification (2, in this example),
determined by the initiator of the Request. Each Route Request also
contains a record listing the address of each intermediate node
through which this particular copy of the Route Request has been
forwarded. This route record is initialized to an empty list by the
initiator of the Route Discovery. In this example, the route record
initially lists only node A.
When another node receives this Route Request (such as node B in this
example), if it is the target of the Route Discovery, it returns
a "Route Reply" to the initiator of the Route Discovery, giving
a copy of the accumulated route record from the Route Request;
when the initiator receives this Route Reply, it caches this route
in its Route Cache for use in sending subsequent packets to this
destination.
Otherwise, if this node receiving the Route Request has recently seen
another Route Request message from this initiator bearing this same
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request identification and target address, or if this node's own
address is already listed in the route record in the Route Request,
this node discards the Request. Otherwise, this node appends its
own address to the route record in the Route Request and propagates
it by transmitting it as a local broadcast packet (with the same
request identification). In this example, node B broadcast the Route
Request, which is received by node C; nodes C and D each also, in
turn, broadcast the Request, resulting in a copy of the Request being
received by node E.
In returning the Route Reply to the initiator of the Route Discovery,
such as in this example, node E replying back to node A, node E will
typically examine its own Route Cache for a route back to A, and if
found, will use it for the source route for delivery of the packet
containing the Route Reply. Otherwise, E SHOULD perform its own
Route Discovery for target node A, but to avoid possible infinite
recursion of Route Discoveries, it MUST piggyback this Route Reply
on the packet containing its own Route Request for A. It is also
possible to piggyback other small data packets, such as a TCP SYN
packet [28], on a Route Request using this same mechanism.
Node E could instead simply reverse the sequence of hops in the route
record that it is trying to send in the Route Reply, and use this as
the source route on the packet carrying the Route Reply itself. For
MAC protocols such as IEEE 802.11 that require a bi-directional frame
exchange as part of the MAC protocol [11], the discovered source
route MUST be reversed in this way to return the Route Reply since it
tests the discovered route to ensure it is bi-directional before the
Route Discovery initiator begins using the route; this route reversal
also avoids the overhead of a possible second Route Discovery.
However, this route reversal technique will prevent the discovery
of routes using uni-directional links, and in wireless environments
where the use of uni-directional links is permitted, such routes may
in some cases be more efficient than those with only bi-directional
links, or they may be the only way to achieve connectivity to the
target node.
When initiating a Route Discovery, the sending node saves a copy of
the original packet (that triggered the Discovery) in a local buffer
called the "Send Buffer". The Send Buffer contains a copy of each
packet that cannot be transmitted by this node because it does not
yet have a source route to the packet's destination. Each packet in
the Send Buffer is logically associated with the time that it was
placed into the Send Buffer and is discarded after residing in the
Send Buffer for some timeout period; if necessary for preventing the
Send Buffer from overflowing, a FIFO or other replacement strategy
MAY also be used to evict packets even before they expire.
While a packet remains in the Send Buffer, the node SHOULD
occasionally initiate a new Route Discovery for the packet's
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destination address. However, the node MUST limit the rate at which
such new Route Discoveries for the same address are initiated, since
it is possible that the destination node is not currently reachable.
In particular, due to the limited wireless transmission range and the
movement of the nodes in the network, the network may at times become
partitioned, meaning that there is currently no sequence of nodes
through which a packet could be forwarded to reach the destination.
Depending on the movement pattern and the density of nodes in the
network, such network partitions may be rare or may be common.
If a new Route Discovery was initiated for each packet sent by a
node in such a partitioned network, a large number of unproductive
Route Request packets would be propagated throughout the subset
of the ad hoc network reachable from this node. In order to
reduce the overhead from such Route Discoveries, a node SHOULD use
an exponential back-off algorithm to limit the rate at which it
initiates new Route Discoveries for the same target, doubling the
timeout between each successive Discovery initiated for the same
target. If the node attempts to send additional data packets to this
same destination node more frequently than this limit, the subsequent
packets SHOULD be buffered in the Send Buffer until a Route Reply is
received giving a route to this destination, but the node MUST NOT
initiate a new Route Discovery until the minimum allowable interval
between new Route Discoveries for this target has been reached. This
limitation on the maximum rate of Route Discoveries for the same
target is similar to the mechanism required by Internet nodes to
limit the rate at which ARP Requests are sent for any single target
IP address [3].
3.2. Basic DSR Route Maintenance
When originating or forwarding a packet using a source route, each
node transmitting the packet is responsible for confirming that the
packet has been received by the next node along the source route; the
packet SHOULD be retransmitted (up to a maximum number of attempts)
until this confirmation of receipt is received. 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 receipt of the packet at B,
node B is responsible for receipt at C, node C is responsible for
receipt at D, and node D is responsible for receipt finally at the
destination E.
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This confirmation of receipt in many cases may be provided at no cost
to DSR, either as an existing standard part of the MAC protocol in
use (such as the link-layer acknowledgement frame defined by IEEE
802.11 [11]), or by a "passive acknowledgement" [16] (in which,
for example, B confirms receipt at C by overhearing C transmit
the packet when forwarding it on to D). If neither of these
confirmation mechanisms are available, the node transmitting the
packet can explicitly request 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 uni-directional,
this software acknowledgement could travel over a different,
multi-hop path.
At the original sender of a packet if no receipt confirmation is
received after the sender has retransmitted the packet the maximum
number of attempts to the first intermediate node on the source
route, then the sender determines that this first hop of the route
is currently "broken". For example, in the situation shown above,
if the sender, node A, is unable to deliver the packet to the next
node B, then A determines that the hop from A to B is broken. In
this case, node A removes this link from its Route Cache and removes
the DSR routing information that it had previously added to the
packet. Node A then again searches its Route Cache for a route to
the destination node, and if no route is found in the cache, uses the
Route Discovery protocol again to dynamically discover a new route
for the packet.
At an intermediate node forwarding a packet, if no receipt
confirmation is received after the node has retransmitted the packet
the maximum number of attempts, this node SHOULD return a "Route
Error" to the original sender of the packet, identifying the link
over which the packet could not be forwarded. For example, in the
situation shown above, if C is unable to deliver the packet to the
next node D, then C returns a Route Error to A, stating that the link
from C to D is currently "broken". Node A then removes this broken
link from its cache; any retransmission of the original packet can
be performed by upper layer protocols such as TCP, if necessary.
For sending such a retransmission or other packets to this same
destination E, if A has in its Route Cache another route to E
(for example, from additional Route Replies from its earlier Route
Discovery, or from having overheard sufficient routing information
from other packets), it can send the packet using the new route
immediately. Otherwise, it SHOULD perform a new Route Discovery for
this target (subject to the back-off described in Section 3.1).
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3.3. Additional Route Discovery Features
3.3.1. Caching Overheard Routing Information
A node forwarding or otherwise overhearing any packet SHOULD add all
usable routing information from that packet to its own Route Cache.
The usefulness of routing information in a packet depends on the
directionality characteristics of the physical medium (Section 2), as
well as the MAC protocol being used. Specifically, three distinct
cases are possible:
- Links in the network frequently are capable of operating only
uni-directionally (not bi-directionally), and the MAC protocol
in use in the network is capable of transmitting unicast packets
over uni-directional links.
- Links in the network occasionally are capable of operating
only uni-directionally (not bi-directionally), but this
uni-directional restriction on any link is not persistent, almost
all links are physically bi-directional, and the MAC protocol in
use in the network is capable of transmitting unicast packets
over uni-directional links.
- The MAC protocol in use in the network is not capable of
transmitting unicast packets over uni-directional links;
only bi-directional links can be used by the MAC protocol for
transmitting unicast packets. For example, the IEEE 802.11
Distributed Coordination Function (DCF) MAC protocol [11]
is capable of transmitting a unicast packet only over a
bi-directional link, since the MAC protocol requires the return
of a link-level acknowledgement packet from the receiver and also
optionally requires the bi-directional exchange of an RTS and CTS
packet between the transmitter and receiver nodes.
In the first case above, for example, the source route used in a data
packet, the accumulated route record in a Route Request, or the route
being returned in a Route Reply SHOULD all be cached by any node in
the "forward" direction; any node SHOULD cache this information from
any such packet received, whether the packet was addressed to this
node, sent to a broadcast (or multicast) MAC address, or overheard
while the node's network interface is in promiscuous mode. However,
the "reverse" direction of the links identified in such packet
headers SHOULD NOT be cached.
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For example, in the situation shown below, node A is using a source
route to communicate with node E:
+-----+ +-----+ +-----+ +-----+ +-----+
| A |---->| B |---->| C |---->| D |---->| E |
+-----+ +-----+ +-----+ +-----+ +-----+
As node C forwards a data packet along the route from A to E, it
SHOULD add to its cache the presence of the "forward" direction
links that it learns from the headers of these packets, from itself
to D and from D to E. Node C SHOULD NOT, in this case, cache the
"reverse" direction of the links identified in these packet headers,
from itself back to B and from B to A, since these links might be
uni-directional.
In the second case above, in which links may occasionally operate
uni-directionally, 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
bi-directionality for unicast operation, links from a source
route SHOULD be cached in both directions, except when the packet
also contains a Route Reply, in which case only the links already
traversed in this source route SHOULD be cached, but the links not
yet traversed in this route SHOULD NOT be cached.
3.3.2. Replying to Route Requests using Cached Routes
A node receiving a Route Request for which it is not the target,
searches its own Route Cache for a route to the target of the
Request. If found, the node generally returns a Route Reply to the
initiator itself rather than forwarding the Route Request. In the
Route Reply, this node sets the route record to list the sequence of
hops over which this copy of the Route Request was forwarded to it,
concatenated with the source route to this target obtained from its
own Route Cache.
However, before transmitting a Route Reply packet that was generated
using information from its Route Cache in this way, a node MUST
verify that the resulting route being returned in the Route Reply,
after this concatenation, contains no duplicate nodes listed in the
route record. For example, the figure below illustrates a case in
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which a Route Request for target E has been received by node F, and
node F already has in its Route Cache a route from itself to E:
+-----+ +-----+ +-----+ +-----+
| A |---->| B |- >| D |---->| E |
+-----+ +-----+ \ / +-----+ +-----+
\ /
\ +-----+ /
>| C |-
+-----+
| ^
v |
Route Request +-----+
Route: A - B - C - F | F | Cache: C - D - E
+-----+
The concatenation of the accumulated route record from the Route
Request and the cached route from F's Route Cache would include a
duplicate node in passing from C to F and back to C.
Node F in this case could attempt to edit the route to eliminate the
duplication, resulting in a route from A to B to C to D and on to E,
but in this case, node F would not be on the route that it returned
in its own Route Reply. DSR Route Discovery prohibits node F
from returning such a Route Reply from its cache; this prohibition
increases the probability that the resulting route is valid, since
node F in this case should have received a Route Error if the route
had previously stopped working. Furthermore, this prohibition
means that a future Route Error traversing the route is very likely
to pass through any node that sent the Route Reply for the route
(including node F), which helps to ensure that stale data is removed
from caches (such as at F) in a timely manner; otherwise, the next
Route Discovery initiated by A might also be contaminated by a Route
Reply from F containing the same stale route. If node F, due to this
restriction on returning a Route Reply based on information from its
Route Cache, does not return such a Route Reply, node F propagates
the Route Request normally.
3.3.3. Preventing Route Reply Storms
The ability for nodes to reply to a Route Request based on
information in their Route Caches, as described in Section 3.3.2,
could result in a possible Route Reply "storm" in some cases. In
particular, if a node broadcasts a Route Request for a target node
for which the node's neighbors have a route in their Route Caches,
each neighbor may attempt to send a Route Reply, thereby wasting
bandwidth and possibly increasing the number of network collisions in
the area.
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For example, the figure below shows a situation in which nodes B, C,
D, E, and F all receive A's Route Request for target G, and each has
the indicated route cached for this target:
+-----+ +-----+
| D |< >| C |
+-----+ \ / +-----+
Cache: C - B - G \ / Cache: B - G
\ +-----+ /
-| A |-
+-----+\ +-----+ +-----+
| | \--->| B | | G |
/ \ +-----+ +-----+
/ \ Cache: G
v v
+-----+ +-----+
| E | | F |
+-----+ +-----+
Cache: F - B - G Cache: B - G
Normally, each of these nodes would attempt to reply from its own
Route Cache, and they would thus all send their Route Replies at
about the same time, since they all received the broadcast Route
Request at about the same time. Such simultaneous Route Replies
from different nodes all receiving the Route Request may cause local
congestion in the wireless network and may create packet collisions
among some or all of these Replies if the MAC protocol in use does
not provide sufficient collision avoidance for these packets. In
addition, it will often be the case that the different replies will
indicate routes of different lengths, as shown in this example.
In order to reduce these effects, if a node can put its network
interface into promiscuous receive mode, it MAY delay sending its
own Route Reply for a short period, while listening to see if the
initiating node begins using a shorter route first. Specifically,
this node MAY delay sending its own Route Reply for a random period
d = H * (h - 1 + r)
where h is the length in number of network hops for the route to be
returned in this node's Route Reply, r is a random floating point
number between 0 and 1, and H is a small constant delay (at least
twice the maximum wireless link propagation delay) to be introduced
per hop. This delay effectively randomizes the time at which each
node sends its Route Reply, with all nodes sending Route Replies
giving routes of length less than h sending their Replies before this
node, and all nodes sending Route Replies giving routes of length
greater than h sending their Replies after this node.
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Within the delay period, this node promiscuously receives all
packets, looking for data packets from the initiator of this Route
Discovery destined for the target of the Discovery. If such a data
packet received by this node during the delay period uses a source
route of length less than or equal to h, this node may infer that the
initiator of the Route Discovery has already received a Route Reply
giving an equally good or better route. In this case, this node
SHOULD cancel its delay timer and SHOULD NOT send its Route Reply for
this Route Discovery.
3.3.4. Route Request Hop Limits
Each Route Request message contains a "hop limit" that may be used
to limit the number of intermediate nodes allowed to forward that
copy of the Route Request. This hop limit is implemented using the
Time-to-Live (TTL) field in the IP header of the packet carrying
the Route Request. As the Request is forwarded, this limit is
decremented, and the Request packet is discarded if the limit reaches
zero before finding the target. This Route Request hop limit can be
used to implement a variety of algorithms for controlling the spread
of a Route Request during a Route Discovery attempt.
For example, a node MAY use this hop limit to implement a
"non-propagating" Route Request as an initial phase of a Route
Discovery. A node using this technique sends its first Route Request
attempt for some target node using a hop limit of 1, such that any
node receiving the initial transmission of the Route Request will
not forward the Request to other nodes by re-broadcasting it. This
form of Route Request is called a "non-propagating" Route Request;
it provides an inexpensive method for determining if the target is
currently a neighbor of the initiator or if a neighbor node has a
route to the target cached (effectively using the neighbors' Route
Caches as an extension of the initiator's own Route Cache). If no
Route Reply is received after a short timeout, then the node sends a
"propagating" Route Request (i.e., with no hop limit) for the target
node.
As another example, a node MAY use this hop limit to implement an
"expanding ring" search for the target [14]. A node using this
technique sends an initial non-propagating Route Request as described
above; if no Route Reply is received for it, the node originates
another Route Request with a hop limit of 2. For each Route Request
originated, if no Route Reply is received for it, the node doubles
the hop limit used on the previous attempt, to progressively explore
for the target node without allowing the Route Request to propagate
over the entire network. However, this expanding ring search
approach could have the effect of increasing the average latency of
Route Discovery, since multiple Discovery attempts and timeouts may
be needed before discovering a route to the target node.
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3.4. Additional Route Maintenance Features
3.4.1. Packet Salvaging
When an intermediate node forwarding a packet detects through Route
Maintenance that the next hop along the route for that packet is
broken, if the node has another route to the packet's destination in
its Route Cache, the node SHOULD "salvage" the packet rather than
discarding it. To salvage a packet, the node replaces the original
source route on the packet with the route from its Route Cache. The
node then forwards the packet to the next node indicated along this
source route. For example, in the situation shown in the example of
Section 3.2, if node C has another route cached to node E, it can
salvage the packet by replacing the original route in the packet with
this new route from its own Route Cache, rather than discarding the
packet.
When salvaging a packet, a count is maintained in the packet of the
number of times that it has been salvaged, to prevent a single packet
from being salvaged endlessly. Otherwise, it could be possible for
the packet to enter a routing loop, as different nodes repeatedly
salvage the packet and replace the source route on the packet with
routes to each other.
As described in Section 3.2, an intermediate node, such as in this
case, that detects through Route Maintenance that the next hop along
the route for a packet that it is forwarding is broken, the node also
SHOULD return a Route Error to the original sender of the packet,
identifying the link over which the packet could not be forwarded.
If the node sends this Route Error, it SHOULD originate the Route
Error before salvaging the packet.
3.4.2. Queued Packets Destined over a Broken Link
When an intermediate node forwarding a packet detects through Route
Maintenance that the next-hop link along the route for that packet
is broken, in addition to handling that packet as defined for Route
Maintenance, the node SHOULD also handle in a similar way any pending
packets that it has queued that are destined over this new broken
link. Specifically, the node SHOULD search its Network Interface
Queue and Retransmission Buffer (Section 4.5) for packets for which
the next-hop link is this new broken link. For each such packet
currently queued at this node, the node SHOULD process that packet as
follows:
- Remove the packet from the node's Network Interface Queue and
Retransmission Buffer and stop any retransmission activity for
the packet.
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- Originate a Route Error for this packet to the original sender of
the packet, using the procedure described in Section 6.3.4, as if
the node had already reached the maximum number of retransmission
attempts for that packet for Route Maintenance. However, in
sending such Route Errors for queued packets in response to a
single new broken link detected, the node SHOULD send no more
than one Route Error to each original sender of any of these
packets.
- If the node has another route to the packet's IP
Destination Address in its Route Cache, the node SHOULD
salvage the packet as described in Section 6.3.6. Otherwise, the
node SHOULD discard the packet.
3.4.3. Automatic Route Shortening
Source routes in use MAY be automatically shortened if one or more
intermediate nodes in the route become no longer necessary. This
mechanism of automatically shortening routes in use is somewhat
similar to the use of passive acknowledgements [16]. In particular,
if a node is able to overhear a packet carrying a source route (e.g.,
by operating its network interface in promiscuous receive mode), then
this node examines the unexpended portion of that source route. If
this node is not the intended next-hop destination for the packet
but is named in the later unexpended portion of the packet's source
route, then it can infer that the intermediate nodes before itself in
the source route are no longer needed in the route. For example, the
figure below illustrates an example in which node D has overheard a
data packet being transmitted from B to C, for later forwarding to D
and to E:
+-----+ +-----+ +-----+ +-----+ +-----+
| A |---->| B |---->| C | | D | | E |
+-----+ +-----+ +-----+ +-----+ +-----+
\ ^
\ /
---------------------
In this case, this node (node D) SHOULD return a "gratuitous" Route
Reply to the original sender of the packet (node A). The Route
Reply gives the shorter route as the concatenation of the portion of
the original source route up through the node that transmitted the
overheard packet (node B), plus the suffix of the original source
route beginning with the node returning the gratuitous Route Reply
(node D). In this example, the route returned in the gratuitous Route
Reply message sent from D to A gives the new route as the sequence of
hops from A to B to D to E.
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When deciding whether to return a gratuitous Route Reply in this way,
a node MAY factor in additional information beyond the fact that it
was able to overhear the packet. For example, the node MAY decide to
return the gratuitous Route Reply only when the overheard packet is
received with a signal strenth or signal-to-noise ratio above some
specific threshold. In addition, each node maintains a Gratuitous
Route Reply Table, as described in Section 4.4, to limit the rate at
which it originates gratuitous Route Replies for the same returned
route.
3.4.4. Increased Spreading of Route Error Messages
When a source node receives a Route Error for a data packet that
it originated, this source node propagates this Route Error to its
neighbors by piggybacking it on its next Route Request. In this way,
stale information in the caches of nodes around this source node will
not generate Route Replies that contain the same invalid link for
which this source node received the Route Error.
For example, in the situation shown in the example of Section 3.2,
node A learns from the Route Error message from C, that the link
from C to D is currently broken. It thus removes this link from
its own Route Cache and initiates a new Route Discovery (if it has
no other route to E in its Route Cache). On the Route Request
packet initiating this Route Discovery, node A piggybacks a copy
of this Route Error, ensuring that the Route Error spreads well to
other nodes, and guaranteeing that any Route Reply that it receives
(including those from other node's Route Caches) in response to this
Route Request does not contain a route that assumes the existence of
this broken link.
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4. Conceptual Data Structures
This document describes the operation of the DSR protocol in terms
of a number of conceptual data structures. This section describes
each of these data structures and provides an overview of its use
in the protocol. In an implementation of the protocol, these data
structures MAY be implemented in any manner consistent with the
external behavior described in this document.
4.1. Route Cache
All ad hoc network routing information needed by a node implementing
DSR is stored in that node's Route Cache. Each node in the network
maintains its own Route Cache. A node adds information to its
Route Cache as it learns of new links between nodes in the ad hoc
network; for example, a node may learn of new links when it receives
a packet carrying a Route Request, Route Reply, or DSR source route.
Likewise, a node removes information from its Route Cache as it
learns that existing links in the ad hoc network have broken; for
example, a node may learn of a broken link when it receives a packet
carrying a Route Error or through the link-layer retransmission
mechanism reporting a failure in forwarding a packet to its next-hop
destination.
Anytime a node adds new information to its Route Cache, the node
SHOULD check each packet in its own Send Buffer (Section 4.2) to
determine whether a route to that packet's IP Destination Address
now exists in the node's Route Cache (including the information just
added to the Cache). If so, the packet SHOULD then be sent using
that route and removed from the Send Buffer.
It is possible to interface a DSR network with other networks,
external to this DSR network. Such external networks may, for
example, be the Internet, or may be other ad hoc networks routed
with a routing protocol other than DSR. Such external networks may
also be other DSR networks that are treated as external networks
in order to improve scalability. The complete handling of such
external networks is beyond the scope of this document. However,
this document specifies a minimal set of requirements and features
necessary to allow nodes only implementing this specification to
interoperate correctly with nodes implementing interfaces to such
external networks. This minimal set of requirements and features
involve the First Hop External (F) and Last Hop External (L)
bits in a DSR Source Route option (Section 5.7) and a Route Reply
option (Section 5.3) in a packet's DSR header (Section 5). These
requirements also include the addition of an External flag bit
tagging each link in the Route Cache, copied from the First Hop
External (F) and Last Hop External (L) bits in the DSR Source Route
option or Route Reply option from which this link was learned.
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The Route Cache SHOULD support storing more than one route to each
destination. In searching the Route Cache for a route to some
destination node, the Route Cache is indexed by destination node
address. The following properties describe this searching function
on a Route Cache:
- Each implementation of DSR at any node MAY choose any appropriate
strategy and algorithm for searching its Route Cache and
selecting a "best" route to the destination from among those
found. For example, a node MAY choose to select the shortest
route to the destination (the shortest sequence of hops), or it
MAY use an alternate metric to select the route from the Cache.
- However, if there are multiple cached routes to a destination,
the selection of routes when searching the Route Cache MUST
prefer routes that do not have the External flag set on any link.
This preference will select routes that lead directly to the
target node over routes that attempt to reach the target via any
external networks connected to the DSR ad hoc network.
- In addition, any route selected when searching the Route Cache
MUST NOT have the External bit set for any links other than
possibly the first link, the last link, or both; the External bit
MUST NOT be set for any intermediate hops in the route selected.
An implementation of a Route Cache MAY provide a fixed capacity
for the cache, or the cache size MAY be variable. The following
properties describe the management of available space within a node's
Route Cache:
- Each implementation of DSR at each node MAY choose any
appropriate policy for managing the entries in its Route Cache,
such as when limited cache capacity requires a choice of which
entries to retain in the Cache. For example, a node MAY chose a
"least recently used" (LRU) cache replacement policy, in which
the entry last used longest ago is discarded from the cache if a
decision needs to be made to allow space in the cache for some
new entry being added.
- However, the Route Cache replacement policy SHOULD allow routes
to be categorized based upon "preference", where routes with a
higher preferences are less likely to be removed from the cache.
For example, a node could prefer routes for which it initiated
a Route Discovery over routes that it learned as the result of
promiscuous snooping on other packets. In particular, a node
SHOULD prefer routes that it is presently using over those that
it is not.
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Any suitable data structure organization, consistent with this
specification, MAY be used to implement the Route Cache in any node.
For example, the following two types of organization are possible:
- In DSR, the route returned in each Route Reply that is received
by the initiator of a Route Discovery (or that is learned from
the header of overhead packets, as described in Section 6.1.4)
represents a complete path (a sequence of links) leading to the
destination node. By caching each of these paths separately,
a "path cache" organization for the Route Cache can be formed.
A path cache is very simple to implement and easily guarantees
that all routes are loop-free, since each individual route from
a Route Reply or Route Request or used in a packet is loop-free.
To search for a route in a path cache data structure, the sending
node can simply search its Route Cache for any path (or prefix of
a path) that leads to the intended destination node.
This type of organization for the Route Cache in DSR has been
extensively studied through simulation [5, 9, 12, 19] and
through implementation of DSR in a mobile outdoor testbed under
significant workload [20, 21, 22].
- Alternatively, a "link cache" organization could be used for the
Route Cache, in which each individual link (hop) in the routes
returned in Route Reply packets (or otherwise learned from the
header of overhead packets) is added to a unified graph data
structure of this node's current view of the network topology.
To search for a route in link cache, the sending node must use
a more complex graph search algorithm, such as the well-known
Dijkstra's shortest-path algorithm, to find the current best path
through the graph to the destination node. Such an algorithm is
more difficult to implement and may require significantly more
CPU time to execute.
However, a link cache organization is more powerful than a path
cache organization, in its ability to effectively utilize all of
the potential information that a node might learn about the state
of the network. In particular, links learned from different
Route Discoveries or from the header of any overheard packets can
be merged together to form new routes in the network, but this
is not possible in a path cache due to the separation of each
individual path in the cache.
This type of organization for the Route Cache in DSR, including
the effect of a range of implementation choices, has been studied
through detailed simulation [9].
The choice of data structure organization to use for the Route Cache
in any DSR implementation is a local matter for each node and affects
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only performance; any reasonable choice of organization for the Route
Cache does not affect either correctness or interoperability.
Each entry in the Route Cache SHOULD have a timeout associated
with it, to allow that entry to be deleted if not used within some
time. The particular choice of algorithm and data structure used
to implement the Route Cache SHOULD be considered in choosing the
timeout for entries in the Route Cache. The configuration variable
RouteCacheTimeout defined in Section 7 specifies the timeout to be
applied to entries in the Route Cache, although it is also possible
to instead use an adaptive policy in choosing timeout values rather
than using a single timeout setting for all entries; for example, the
Link-MaxLife cache design (below) uses an adaptive timeout algorithm
and does not use the RouteCacheTimeout configuration variable.
As guidance to implementors, Appendix A describes a type of link
cache known as "Link-MaxLife" that has been shown to outperform
other types of link caches and path caches studied in detailed
simulation [9]. Link-MaxLife is an adaptive link cache in which each
link in the cache has a timeout that is determined dynamically by the
caching node according to its observed past behavior of the two nodes
at the ends of the link; in addition, when selecting a route for a
packet being sent to some destination, among cached routes of equal
length (number of hops) to that destination, Link-MaxLife selects the
route with the longest expected lifetime (highest minimum timeout of
any link in the route). Use of the Link-MaxLife design for the Route
Cache is recommended in implementations of DSR.
4.2. Send Buffer
The Send Buffer of a node implementing DSR is a queue of packets that
cannot be sent by that node because it does not yet have a source
route to each such packet's destination. Each packet in the Send
Buffer is logically associated with the time that it was placed into
the Buffer, and SHOULD be removed from the Send Buffer and silently
discarded after a period of SendBufferTimeout after initially being
placed in the Buffer. If necessary, a FIFO strategy SHOULD be used
to evict packets before they timeout to prevent the buffer from
overflowing.
Subject to the rate limiting defined in Section 6.2, a Route
Discovery SHOULD be initiated as often as possible for the
destination address of any packets residing in the Send Buffer.
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4.3. Route Request Table
The Route Request Table of a node implementing DSR records
information about Route Requests that have been recently originated
or forwarded by this node. The table is indexed by IP address.
The Route Request Table on a node records the following information
about nodes to which this node has initiated a Route Request:
- The Time-to-Live (TTL) field used in the IP header of the Route
Request for the last Route Discovery initiated by this node for
that target node. This value allows the node to implement a
variety of algorithms for controlling the spread of its Route
Request on each Route Discovery initiated for a target. As
examples, two possible algorithms for this use of the TTL field
are described in Section 3.3.4.
- The time that this node last originated a Route Request for that
target node.
- The number of consecutive Route Discoveries initiated for this
target since receiving a valid Route Reply giving a route to that
target node.
- The remaining amount of time before which this node MAY next
attempt at a Route Discovery for that target node. When the
node initiates a new Route Discovery for this target node, this
field in the Route Request Table entry for that target node is
initialized to the timeout for that Route Discovery, after which
the node MAY initiate a new Discovery for that target. Until
a valid Route Reply is received for this target node address,
a node MUST implement a back-off algorithm in determining this
timeout value for each successive Route Discovery initiated
for this target using the same Time-to-Live (TTL) value in the
IP header of the Route Request packet. The timeout between
such consecutive Route Discovery initiations SHOULD increase by
doubling the timeout value on each new initiation.
In addition, the Route Request Table on a node also records the
following information about initiator nodes from which this node has
received a Route Request:
- A FIFO cache of size RequestTableIds entries containing the
Identification value and target address from the most recent
Route Requests received by this node from that initiator node.
Nodes SHOULD use an LRU policy to manage the entries in their Route
Request Table.
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The number of Identification values to retain in each Route
Request Table entry, RequestTableIds, MUST NOT be unlimited, since,
in the worst case, when a node crashes and reboots, the first
RequestTableIds Route Discoveries it initiates after rebooting
could appear to be duplicates to the other nodes in the network.
In addition, a node SHOULD base its initial Identification value,
used for Route Discoveries after rebooting, on a battery backed-up
clock or other persistent memory device, in order to help avoid
any possible such delay in successfully discovering new routes
after rebooting; if no such source of initial Identification
value is available, a node after rebooting SHOULD base its initial
Identification value on a random number.
4.4. Gratuitous Route Reply Table
The Gratuitous Route Reply Table of a node implementing DSR records
information about "gratuitous" Route Replies sent by this node as
part of automatic route shortening. As described in Section 3.4.3,
a node returns a gratuitous Route Reply when it overhears a packet
transmitted by some node, for which the node overhearing the
packet was not the intended next-hop node but was named later in
the unexpended hops of the source route in that packet; the node
overhearing the packet returns a gratuitous Route Reply to the
original sender of the packet, listing the shorter route (not
including the hops of the source route "skipped over" by this
packet). A node uses its Gratuitous Route Reply Table to limit the
rate at which it originates gratuitous Route Replies to the same
original sender for the same node from which it overheard a packet to
trigger the gratuitous Route Reply.
Each entry in the Gratuitous Route Reply Table of a node contains the
following fields:
- The address of the node to which this node originated a
gratuitous Route Reply.
- The address of the node from which this node overheard the packet
triggering that gratuitous Route Reply.
- The remaining time before which this entry in the Gratuitous
Route Reply Table expires and SHOULD be deleted by the node.
When a node creates a new entry in its Gratuitous Route Reply
Table, the timeout value for that entry should be initialized to
the value GratReplyHoldoff.
When a node overhears a packet that would trigger a gratuitous
Route Reply, if a corresponding entry already exists in the node's
Gratuitous Route Reply Table, then the node SHOULD NOT send a
gratuitous Route Reply for that packet. Otherwise (no corresponding
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entry already exists), the node SHOULD create a new entry in its
Gratuitous Route Reply Table to record that gratuitous Route Reply,
with a timeout value of GratReplyHoldoff.
4.5. Network Interface Queue and Retransmission Buffer
Depending on factors such as the structure and organization of
the operating system, protocol stack implementation, network
interface device driver, and network interface hardware, a
packet being transmitted could be queued in a variety of ways.
For example, outgoing packets from the network protocol stack
might be queued at the operating system or link layer, before
transmission by the network interface. The network interface
might also provide a retransmission mechanism for packets, such
as occurs in IEEE 802.11 [11]; the DSR protocol also requires
limited retransmission of packets as part of Route Maintenance. The
operation of DSR is defined here in terms of two conceptual data
structures that together incorporate this queueing and retransmission
behavior.
The Network Interface Queue of a node implementing DSR is an output
queue of packets from the network protocol stack waiting to be
transmitted by the network interface; for example, in the 4.4BSD
Unix network protocol stack implementation, this queue for a network
interface is represented as a "struct ifqueue" [33]. This queue is
used to hold packets while the network interface is in the process of
transmitting another packet.
The Retransmission Buffer of a node implementing DSR is a queue of
packets sent by this node that are awaiting retransmission as part
of Route Maintenance. For each packet in the Retransmission Buffer,
a node maintains a count of the number of retransmissions and the
time of the last retransmission. The Retransmission Buffer MAY be
of limited size; when adding a new packet to the Retransmission
Buffer, if the buffer size is insufficient to hold the new packet,
the new packet SHOULD be silently discarded. The maximum number of
retransmission attempts for a packet for Route Maintenance (after the
initial transmission of the packet) is MaxMaintRexmt. After this
time, if Route Maintenance for a packet has not been satisfied, the
packet SHOULD be removed from the Retransmission Buffer, stopping
retransmissions for that packet; in this case, the node also SHOULD
originate a Route Error for this packet to the original source of the
packet (Section 6.3) and SHOULD salvage the packet (Section 6.3.6) if
it has another route to the 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 retransmission
timeout value to use for each transmission attempt for a packet
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depends on the type of acknowledgement mechanism used for Route
Maintenance for that attempt, as described in Section 6.3.
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5. DSR Header Format
The Dynamic Source Routing protocol makes use of a special header
carrying control information that can be included in any existing IP
packet. This DSR header in a packet contains a small fixed-sized,
4-octet portion, followed by a sequence of zero or more DSR options
carrying optional information. The end of the sequence of DSR
options in the DSR header is implied by total length of the DSR
header.
For IPv4, the DSR header MUST immediately follow the IP header in the
packet. (If a Hop-by-Hop Options extension header, as defined in
IPv6 [6], becomes defined for IPv4, the DSR header MUST immediately
follow the Hop-by-Hop Options extension header, if one is present in
the packet, and MUST otherwise immediately follow the IP header.)
To add a DSR header to a packet, the DSR header is inserted following
the packet's IP header, before any following header such as a
traditional (e.g., TCP or UDP) transport layer header. Specifically,
the Protocol field in the IP header is used to indicate that a DSR
header follows the IP header, and the Next Header field in the DSR
header is used to indicate the type of protocol header (such as a
transport layer header) following the DSR header.
If any headers follow the DSR header in a packet, the total length
of the DSR header (and thus the total, combined length of all DSR
options present) MUST be a multiple of 4 octets. This requirement
preserves the alignment of these following headers in the packet.
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5.1. Fixed Portion of DSR Header
The fixed portion of the DSR header is used to carry information that
must be present in any DSR header. This fixed portion of the DSR
header has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Reserved | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Options .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header
8-bit selector. Identifies the type of header immediately
following the DSR header. Uses the same values as the IPv4
Protocol field [29].
Reserved
MUST be sent as 0 and ignored on reception.
Payload Length
The length of the DSR header, excluding the 4-octet fixed
portion. The value of the Payload Length field defines the
total length of all options carried in the DSR header.
Options
Variable-length field; the length of the Options field is
specified by the Payload Length field in this DSR header.
Contains one or more pieces of optional information (DSR
options), encoded in type-length-value (TLV) format (with the
exception of the Pad1 option, described in Section 5.8).
The placement of DSR options following the fixed portion of the DSR
header MAY be padded for alignment. However, due to the typically
limited available wireless bandwidth in ad hoc networks, this padding
is not required, and receiving nodes MUST NOT expect options within a
DSR header to be aligned.
A node inserting a DSR header into a packet MUST set the Don't
Fragment (DF) bit in the packet's IP header.
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The following types of DSR options are defined in this document for
use within a DSR header:
- Route Request option (Section 5.2)
- Route Reply option (Section 5.3)
- Route Error option (Section 5.4)
- Acknowledgement Request option (Section 5.5)
- Acknowledgement option (Section 5.6)
- DSR Source Route option (Section 5.7)
- Pad1 option (Section 5.8)
- PadN option (Section 5.9)
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5.2. Route Request Option
The Route Request option in a DSR header is encoded as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Opt Data Len | Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Target Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[2] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[n] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP fields:
Source Address
MUST be set to the address of the node originating this packet.
Intermediate nodes that retransmit the packet to propagate the
Route Request MUST NOT change this field.
Destination Address
MUST be set to the IP limited broadcast address
(255.255.255.255).
Hop Limit (TTL)
MAY be varied from 1 to 255, for example to implement
non-propagating Route Requests and Route Request expanding-ring
searches (Section 3.3.4).
Route Request fields:
Option Type
2
Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len fields.
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Identification
A unique value generated by the initiator (original sender) of
the Route Request. Nodes initiating a Route Request generate
a new Identification value for each Route Request, for example
based on a sequence number counter of all Route Requests
initiated by the node.
This value allows a receiving node to determine whether it
has recently seen a copy of this Route Request: if this
Identification value is found by this receiving node in its
Route Request Table (in the cache of Identification values
in the entry there for this initiating node), this receiving
node MUST discard the Route Request. When propagating a Route
Request, this field MUST be copied from the received copy of
the Route Request being propagated.
Target Address
The address of the node that is the target of the Route
Request.
Address[1..n]
Address[i] is the address of the i-th node recorded in the
Route Request option. The address given in the Source Address
field in the IP header is the address of the initiator of
the Route Discovery and MUST NOT be listed in the Address[i]
fields; the address given in Address[1] is thus the address
of the first node on the path after the initiator. The
number of addresses present in this field is indicated by the
Opt Data Len field in the option (n = (Opt Data Len - 6) / 4).
Each node propagating the Route Request adds its own address to
this list, increasing the Opt Data Len value by 4 octets.
The Route Request option MUST NOT appear more than once within a DSR
header.
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5.3. Route Reply Option
The Route Reply option in a DSR header is encoded as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Opt Data Len |L| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[2] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[n] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP fields:
Source Address
Set to the address of the node sending the Route Reply.
In the case of a node sending a reply from its Route
Cache (Section 3.3.2) or sending a gratuitous Route Reply
(Section 3.4.3), this address can differ from the address that
was the target of the Route Discovery.
Destination Address
MUST be set to the address of the source node of the route
being returned. Copied from the Source Address field of the
Route Request generating the Route Reply, or in the case of a
gratuitous Route Reply, copied from the Source Address field of
the data packet triggering the gratuitous Reply.
Route Reply fields:
Option Type
3
Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len fields.
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Last Hop External (L)
Set to indicate that the last hop given by the Route Reply
(the link from Address[n-1] to Address[n]) is actually an
arbitrary path in a network external to the DSR network; the
exact route outside the DSR network is not represented in the
Route Reply. Nodes caching this hop in their Route Cache MUST
flag the cached hop with the External flag. Such hops MUST NOT
be returned in a cached Route Reply generated from this Route
Cache entry, and selection of routes from the Route Cache to
route a packet being sent MUST prefer routes that contain no
hops flagged as External.
Reserved
MUST be sent as 0 and ignored on reception.
Address[1..n]
The source route being returned by the Route Reply. The route
indicates a sequence of hops, originating at the source node
specified in the Destination Address field of the IP header
of the packet carrying the Route Reply, through each of the
Address[i] nodes in the order listed in the Route Reply,
ending with the destination node indicated by Address[n].
The number of addresses present in the Address[1..n]
field is indicated by the Opt Data Len field in the option
(n = (Opt Data Len - 1) / 4).
A Route Reply option MAY appear one or more times within a DSR
header.
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5.4. Route Error Option
The Route Error option in a DSR header is encoded as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Opt Data Len | Error Type |Reservd|Salvage|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Type-Specific Information .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
4
Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len fields.
For the current definition of the Route Error option,
this field MUST be set to 10, plus the size of any
Type-Specific Information present in the Route Error. Further
extensions to the Route Error option format may also be
included after the Type-Specific Information portion of the
Route Error option specified above. The presence of such
extensions will be indicated by the Opt Data Len field.
When the Opt Data Len is greater than that required for
the fixed portion of the Route Error plus the necessary
Type-Specific Information as indicated by the Option Type
value in the option, the remaining octets are interpreted as
extensions. Currently, no such further extensions have been
defined.
Error Type
The type of error encountered. Currently, the following type
value is defined:
1 = NODE_UNREACHABLE
Other values of the Error Type field are reserved for future
use.
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Reservd
Reserved. MUST be sent as 0 and ignored on reception.
Salvage
A 4-bit unsigned integer. Copied from the Salvage field in
the DSR Source Route option of the packet triggering the Route
Error.
The "total salvage count" of the Route Error option is derived
from the value in the Salvage field of this Route Error option
and all preceding Route Error options in the packet as follows:
the total salvage count is the sum of, for each such Route
Error option, one plus the value in the Salvage field of that
Route Error option.
Error Source Address
The address of the node originating the Route Error (e.g., the
node that attempted to forward a packet and discovered the link
failure).
Error Destination Address
The address of the node to which the Route Error must be
delivered For example, when the Error Type field is set to
NODE_UNREACHABLE, this field will be set to the address of the
node that generated the routing information claiming that the
hop from the Error Source Address to Unreachable Node Address
(specified in the Type-Specific Information) was a valid hop.
Type-Specific Information
Information specific to the Error Type of this Route Error
message.
Currently, the Type-Specific Information field is defined only for
Route Error messages of type NODE_UNREACHABLE. In this case, the
Type-Specific Information field is defined as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unreachable Node Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Unreachable Node Address
The address of the node that was found to be unreachable
(the next-hop neighbor to which the node with address
Error Source Address was attempting to transmit the packet).
A Route Error option MAY appear one or more times within a DSR
header.
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5.5. Acknowledgment Request Option
The Acknowledgment Request option in a DSR header is encoded as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Opt Data Len | Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
5
Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len fields.
Identification
The Identification field is set to a unique value and is copied
into the Identification field of the 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 header.
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5.6. Acknowledgment Option
The Acknowledgment option in a DSR header is encoded as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Opt Data Len | Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACK Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACK Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
6
Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len fields.
Identification
Copied from the Identification field of the Acknowledgement
Request option of the packet being acknowledged.
ACK Source Address
The address of the node originating the acknowledgment.
ACK Destination Address
The address of the node to which the acknowledgment is to be
delivered.
An Acknowledgement option MAY appear one or more times within a DSR
header.
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5.7. DSR Source Route Option
The DSR Source Route option in a DSR header is encoded as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Opt Data Len |F|L|Reservd|Salvage| Segs Left |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[2] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[n] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
7
Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len fields. For the
format of the DSR Source Route option defined here, this field
MUST be set to the value (n * 4) + 2, where n is the number of
addresses present in the Address[i] fields.
First Hop External (F)
Set to indicate that the first hop indicated by the DSR
Source Route option is actually an arbitrary path in a network
external to the DSR network; the exact route outside the DSR
network is not represented in the DSR Source Route option.
Nodes caching this hop in their Route Cache MUST flag the
cached hop with the External flag. Such hops MUST NOT be
returned in a Route Reply generated from this Route Cache
entry, and selection of routes from the Route Cache to route
a packet being sent MUST prefer routes that contain no hops
flagged as External.
Last Hop External (L)
Set to indicate that the last hop indicated by the DSR Source
Route option is actually an arbitrary path in a network
external to the DSR network; the exact route outside the DSR
network is not represented in the DSR Source Route option.
Nodes caching this hop in their Route Cache MUST flag the
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cached hop with the External flag. Such hops MUST NOT be
returned in a Route Reply generated from this Route Cache
entry, and selection of routes from the Route Cache to route
a packet being sent MUST prefer routes that contain no hops
flagged as External.
Reserved
MUST be sent as 0 and ignored on reception.
Salvage
A 4-bit unsigned integer. Count of number of times that
this packet has been salvaged as a part of DSR routing
(Section 3.4.1).
Segments Left (Segs Left)
Number of route segments remaining, i.e., number of explicitly
listed intermediate nodes still to be visited before reaching
the final destination.
Address[1..n]
The sequence of addresses of the source route. In routing
and forwarding the packet, the source route is processed as
described in Sections 6.1.3 and 6.1.5. The number of addresses
present in the Address[1..n] field is indicated by the
Opt Data Len field in the option (n = (Opt Data Len - 2) / 4).
When forwarding a packet along a DSR source route using a DSR Source
Route option in the packet's DSR header, the Destination Address
field in the packet's IP header is always set to the address of the
packet's ultimate destination. A node receiving a packet containing
a DSR header with a DSR Source Route option MUST examine the
indicated source route to determine if it is the intended next-hop
node for the packet and determine how to forward the packet, as
defined in Sections 6.1.4 and 6.1.5.
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5.8. Pad1 Option
The Pad1 option in a DSR header is encoded as follows:
+-+-+-+-+-+-+-+-+
| Option Type |
+-+-+-+-+-+-+-+-+
Option Type
0
A Pad1 option MAY be included in the Options field of a DSR header
in order to align subsequent DSR options, but such alignment is
not required and MUST NOT be expected by a node receiving a packet
containing a DSR header.
If any headers follow the DSR header in a packet, the total length of
a DSR header, indicated by the Payload Length field in the DSR header
MUST be a multiple of 4 octets. In this case, when building a DSR
header in a packet, sufficient Pad1 or PadN options MUST be included
in the Options field of the DSR header to make the total length a
multiple of 4 octets.
If more than one consecutive octet of padding is being inserted in
the Options field of a DSR header, the PadN option, described next,
SHOULD be used, rather than multiple Pad1 options.
Note that the format of the Pad1 option is a special case; it does
not have an Opt Data Len or Option Data field.
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5.9. PadN Option
The PadN option in a DSR header is encoded as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
| Option Type | Opt Data Len | Option Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
Option Type
1
Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len fields.
Option Data
A number of zero-valued octets equal to the Opt Data Len.
A PadN option MAY be included in the Options field of a DSR header
in order to align subsequent DSR options, but such alignment is
not required and MUST NOT be expected by a node receiving a packet
containing a DSR header.
If any headers follow the DSR header in a packet, the total length of
a DSR header, indicated by the Payload Length field in the DSR header
MUST be a multiple of 4 octets. In this case, when building a DSR
header in a packet, sufficient Pad1 or PadN options MUST be included
in the Options field of the DSR header to make the total length a
multiple of 4 octets.
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6. Detailed Operation
6.1. General Packet Processing
6.1.1. Originating a Packet
When originating any packet, a node using DSR routing MUST perform
the following sequence of steps:
- Search the node's Route Cache for a route to the address given in
the IP Destination Address field in the packet's header.
- If no such route is found in the Route Cache, then perform
Route Discovery for the Destination Address, as described in
Section 6.2. Initiating a Route Discovery for this target node
address results in the node adding a Route Request option in
a DSR header in this existing packet, or saving this existing
packet to its Send Buffer and initiating the Route Discovery
by sending a separate packet containing such a Route Request
option. If the node chooses to initiate the Route Discovery
by adding the Route Request option to this existing packet,
it will replace the IP Destination Address field with the IP
"limited broadcast" address (255.255.255.255) [3], copying the
original IP Destination Address to the Target Address field of
the new Route Request option added to the packet, as described in
Section 6.2.1.
- If the packet now does not contain a Route Request option,
then this node must have a route to the Destination Address
of the packet; if the node has more than one route to this
Destination Address, the node selects one to use for this packet.
If the length of this route is greater than 1 hop, or if the
node determines to request a DSR network-layer acknowledgement
from the first-hop node in that route, then insert a DSR header
into the packet, as described in Section 6.1.2, and insert a DSR
Source Route option, as described in Section 6.1.3. The source
route in the packet is initialized from the selected route to the
Destination Address of the packet.
- Transmit the packet to the first-hop node address given in
selected source route, using Route Maintenance to retransmit the
packet if necessary, as described in Section 6.3.
6.1.2. Adding a DSR Header to a Packet
A node originating a packet adds a DSR header to the packet, if
necessary, to carry information needed by the routing protocol. A
packet MUST NOT contain more than one DSR header. A DSR header is
added to a packet by performing the following sequence of steps
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(these steps assume that the packet contains no other headers that
MUST be located in the packet before the DSR header):
- Insert a DSR header after the IP header but before any other
header that may be present.
- Set the Next Header field of the DSR header to the Protocol
number field of the packet's IP header.
- Set the Protocol field of the packet's IP header to the Protocol
number assigned for a DSR header (TBA???).
6.1.3. Adding a DSR Source Route Option to a Packet
A node originating a packet adds a DSR Source Route option to the
packet, if necessary, in order to carry the source route from this
originating node to the final destination address of the packet.
Specifically, the node adding the DSR Source Route option constructs
the DSR Source Route option and modifies the IP packet according to
the following sequence of steps:
- The node creates a DSR Source Route option, as described in
Section 5.7, and appends it to the DSR header in the packet.
(A DSR header is added, as described in Section 6.1.2, if not
already present.)
- The number of Address[i] fields to include in the DSR Source
Route option (n) is the number of intermediate nodes in the
source route for the packet (i.e., excluding address of the
originating node and the final destination address of the
packet). The Segments Left field in the DSR Source Route option
is initialized equal to n.
- The addresses within the source route for the packet are copied
into sequential Address[i] fields in the DSR Source Route option,
for i = 1, 2, ..., n.
- The First Hop External (F) bit in the DSR Source Route option is
copied from the External bit flagging the first hop in the source
route for the packet, as indicated in the Route Cache.
- The Last Hop External (L) bit in the DSR Source Route option is
copied from the External bit flagging the last hop in the source
route for the packet, as indicated in the Route Cache.
- The Salvage field in the DSR Source Route option is
initialized to 0.
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6.1.4. Processing a Received Packet
When a node receives any packet (whether for forwarding, overheard,
or as the final destination of the packet), if that packet contains a
DSR header, then that node MUST process any options contained in that
DSR header, in the order contained there. Specifically:
- If the DSR header contains a Route Request option, the node
SHOULD extract the source route from the Route Request and add
this routing information to its Route Cache, subject to the
conditions identified in Section 3.3.1. The routing information
from the Route Request is the sequence of hop addresses
initiator, Address[1], Address[2], ..., Address[n]
where initiator is the value of the Source Address field in
the IP header of the packet carrying the Route Request (the
address of the initiator of the Route Discovery), and each
Address[i] is a node through which this Route Request has passed,
in turn, during this Route Discovery. The value n here is the
number of addresses recorded in the Route Request option, or
(Opt Data Len - 6) / 4.
After possibly updating the node's Route Cache in response to
the routing information in the Route Request option, the node
MUST then process the Route Request option as described in
Section 6.2.2.
- If the DSR header contains a Route Reply option, the node SHOULD
extract the source route from the Route Reply and add this
routing information to its Route Cache, subject to the conditions
identified in Section 3.3.1. The source route from the Route
Reply is the sequence of hop addresses
initiator, Address[1], Address[2], ..., Address[n]
where initiator is the value of the Destination Address field in
the IP header of the packet carrying the Route Reply (the address
of the initiator of the Route Discovery), and each Address[i]
is a node through which the source route passes, in turn, on
the route to the target of the Route Discovery. Address[n] is
the address of the target. If the Last Hop External (L) bit is
set in the Route Reply, the node MUST flag the last hop from
the Route Reply (the link from Address[n-1] to Address[n]) in
its Route Cache as External. The value n here is the number of
addresses in the source route being returned in the Route Reply
option, or (Opt Data Len - 1) / 4.
After possibly updating the node's Route Cache in response to
the routing information in the Route Reply option, then if the
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packet's IP Destination Address matches one of this node's IP
addresses, the node MUST then process the Route Reply option as
described in Section 6.2.5.
- If the DSR header contains a Route Error option, the node MUST
process the Route Error option as described in Section 6.3.5.
- If the DSR header contains an Acknowledgement Request option, the
node MUST process the Acknowledgement Request option as described
in Section 6.3.3.
- If the DSR 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.
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 6.3.3.
- If the DSR header contains a DSR Source Route option, the node
SHOULD extract the source route from the DSR Source Route and
add this routing information to its Route Cache, subject to the
conditions identified in Section 3.3.1. If the value of the
Salvage field in the DSR Source Route option is zero, then the
routing information from the DSR Source Route is the sequence of
hop addresses
source, Address[1], Address[2], ..., Address[n], destination
and otherwise (Salvage is nonzero), the routing information from
the DSR Source Route is the sequence of hop addresses
Address[1], Address[2], ..., Address[n], destination
where source is the value of the Source Address field in the IP
header of the packet carrying the DSR Source Route option (the
original sender of the packet), each Address[i] is the value in
the Address[i] field in the DSR Source Route, and destination is
the value of the Destination Address field in the packet's IP
header (the last-hop address of the source route). The value n
here is the number of addresses in source route in the DSR Source
Route option, or (Opt Data Len - 2) / 4.
After possibly updating the node's Route Cache in response to
the routing information in the DSR Source Route option, the node
MUST then process the DSR Source Route option as described in
Section 6.1.5.
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- Any Pad1 or PadN options in the DSR header are ignored.
Finally, if the Destination Address in the packet's IP header matches
one of this receiving node's own IP address(es), remove the DSR
header and all the included DSR options in the header, and pass the
rest of the packet to the network layer.
6.1.5. Processing a Received DSR Source Route Option
When a node receives a packet containing a DSR Source Route option
(whether for forwarding, overheard, or as the final destination of
the packet), that node SHOULD examine the packet to determine if
the receipt of that packet indicates an opportunity for automatic
route shortening, as described in Section 3.4.3. Specifically, if
this node is not the intended next-hop destination for the packet
but is named in the later unexpended portion of the source route in
the packet's DSR Source Route option, then this packet indicates an
opportunity for automatic route shortening: the intermediate nodes
after the node from which this node overheard the packet and before
this node itself, are no longer necessary in the source route. In
this case, this node SHOULD perform the following sequence of steps
as part of automatic route shortening:
- The node searches its Gratuitous Route Reply Table for an entry
describing a gratuitous Route Reply earlier sent by this node,
for which the original sender of the packet triggering the
gratuitous Route Reply and the transmitting node from which this
node overheard that packet in order to trigger the gratuitous
Route Reply, both match the respective node addresses for this
new received packet. If such an entry is found in the node's
Gratuitous Route Reply Table, the node SHOULD NOT perform
automatic route shortening in response to this receipt of this
packet.
- Otherwise, the node creates an entry for this overheard packet in
its Gratuitous Route Reply Table. The timeout value for this new
entry SHOULD be initialized to the value GratReplyHoldoff. After
this timeout has expired, the node SHOULD delete this entry from
its Gratuitous Route Reply Table.
- After creating the new Gratuitous Route Reply Table entry
above, the node originates a gratuitous Route Reply to the
IP Source Address of this overheard packet, as described in
Section 3.4.3.
If the MAC protocol in use in the network is not capable of
transmitting unicast packets over uni-directional links, as
discussed in Section 3.3.1, then in originating this Route Reply,
the node MUST use a source route for routing the Route Reply
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packet that is obtained by reversing the sequence of hops over
which the packet triggering the gratuitous Route Reply was routed
in reaching and being overheard by this node; this reversing of
the route uses the gratuitous Route Reply to test this sequence
of hops for bi-directionality, 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 bi-directional.
- Discard the overheard packet, since the packet has been received
before its normal traversal of the packet's source route would
have caused it to reach this receiving node. Another copy of
the packet will normally arrive at this node as indicated in
the packet's source route; discarding this initial copy of the
packet, which triggered the gratuitous Route Reply, will prevent
the duplication of this packet that would otherwise occur.
If the packet is not discarded as part of automatic route shortening
above, then the node MUST process the option according to the
following sequence of steps:
- If the value of the Segments Left field in the DSR Source Route
option equals 0, then remove the DSR Source Route option from the
DSR header.
- Else, let n equal (Opt Data Len - 2) / 4. This is the number of
addresses in the DSR Source Route option.
- If the value of the Segments Left field is greater than n, then
send an ICMP Parameter Problem, Code 0, message [26] to the IP
Source Address, pointing to the Segments Left field, and discard
the packet. Do not process the DSR Source Route option further.
- Else, decrement the value of the Segments Left field by 1. Let i
equal n minus Segments Left. This is the index of the next
address to be visited in the Address vector.
- If Address[i] or the IP Destination Address is a multicast
address, then discard the packet. Do not process the DSR Source
Route option further.
- If the MTU of the link over which this node would transmit the
packet to forward it to the node Address[i] is less than the size
of the packet, the node MUST discard the packet and send an ICMP
Packet Too Big message to the packet's Source Address [26].
- 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 in the packet's IP header [27, 3]. In this
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forwarding of the packet, the next-hop node (identified by
Address[i]) MUST be treated as a direct neighbor node: the
transmission to that next node MUST be done in a single IP
forwarding hop, without Route Discovery and without searching the
Route Cache.
- In forwarding the packet, perform Route Maintenance for the next
hop of the packet, by verifying that the packet was received by
that next-hop node, as described in Section 6.3.
Multicast addresses MUST NOT appear in a DSR Source Route option or
in the IP Destination Address field of a packet carrying a DSR Source
Route option in a DSR header.
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6.2. Route Discovery Processing
Route Discovery is the mechanism by which a node S wishing to send a
packet to a destination node D obtains a source route to D. Route
Discovery is used only when S attempts to send a packet to D and
does not already know a route to D. The node initiating a Route
Discovery is known as the "initiator" of the Route Discovery, and the
destination node for which the Route Discovery is initiated is known
as the "target" of the Route Discovery.
Route Discovery operates entirely on demand, with a node initiating
Route Discovery based on its own origination of new packets for
some destination address to which it does not currently know a
route. Route Discovery does not depend on any periodic or background
exchange of routing information or neighbor node detection at any
layer in the network protocol stack at any node.
The Route Discovery procedure utilizes two types of messages, a Route
Request (Section 5.2) and a Route Reply (Section 5.3), to actively
search the ad hoc network for a route to the desired destination.
These DSR messages MAY be carried in any type of IP packet, through
use of the DSR header as described in Section 5.
Except as discussed in Section 6.3.5, a Route Discovery for a
destination address SHOULD NOT be initiated unless the initiating
node has a packet in its Send Buffer requiring delivery to that
destination. A Route Discovery for a given target node MUST NOT be
initiated unless permitted by the rate-limiting information contained
in the Route Request Table. After each Route Discovery attempt, the
interval between successive Route Discoveries for this target SHOULD
be doubled, up to a maximum of MaxRequestPeriod, until a valid Route
Reply is received for this target.
6.2.1. Originating a Route Request
A node initiating a Route Discovery for some target creates and
initializes a Route Request option in a DSR header in some IP packet.
This MAY be a separate IP packet, used only to carry this Route
Request option, or the node MAY include the Route Request option
in some existing packet that it needs to send to the target node
(e.g., the IP packet originated by this node, that caused the node to
attempt Route Discovery for the destination address of the packet).
The Route Request option MUST be included in a DSR header in the
packet. To initialize the Route Request option, the node performs
the following sequence of steps:
- The Option Type in the option MUST be set to the value 2.
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- The Opt Data Len field in the option MUST be set to the value 6.
The total size of the Route Request option when initiated
is 8 octets; the Opt Data Len field excludes the size of the
Option Type and Opt Data Len fields themselves.
- The Identification field in the option MUST be set to a new
value, different from that used for other Route Requests recently
initiated by this node for this same target address. For
example, each node MAY maintain a single counter value for
generating a new Identification value for each Route Request it
initiates.
- The Target Address field in the option MUST be set to the IP
address that is the target of this Route Discovery.
The Source Address in the IP header of this packet MUST be the node's
own IP address. The Destination Address in the IP header of this
packet MUST be the IP "limited broadcast" address (255.255.255.255).
A node MUST maintain in its Route Request Table, information about
Route Requests that it initiates. When initiating a new Route
Request, the node MUST use the information recorded in the Route
Request Table entry for the target of that Route Request, and it MUST
update that information in the table entry for use in the next Route
Request initiated for this target. In particular:
- The Route Request Table entry for a target node records the
Time-to-Live (TTL) field used in the IP header of the Route
Request for the last Route Discovery initiated by this node for
that target node. This value allows the node to implement a
variety of algorithms for controlling the spread of its Route
Request on each Route Discovery initiated for a target. As
examples, two possible algorithms for this use of the TTL field
are described in Section 3.3.4.
- The Route Request Table entry for a target node records the
number of consecutive Route Requests initiated for this target
since receiving a valid Route Reply giving a route to that target
node, and the remaining amount of time before which this node MAY
next attempt at a Route Discovery for that target node.
A node MUST use these values to implement a back-off algorithm to
limit the rate at which this node initiates new Route Discoveries
for the same target address. In particular, until a valid Route
Reply is received for this target node address, the timeout
between consecutive Route Discovery initiations for this target
node with the same hop limit SHOULD increase by doubling the
timeout value on each new initiation.
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The behavior of a node processing a packet containing DSR header
with both a DSR Source Route option and a Route Request option is
unspecified. Packets SHOULD NOT contain both a DSR Source Route
option and a Route Request option.
Packets containing a Route Request option SHOULD NOT include
an Acknowledgement Request option, SHOULD NOT expect link-layer
acknowledgement or passive acknowledgment, and SHOULD NOT be
retransmitted. The retransmission of packets containing a Route
Request option is controlled solely by the logic described in this
section.
6.2.2. Processing a Received Route Request Option
When a node receives a packet containing a Route Request option, that
node MUST process the option according to the following sequence of
steps:
- If the Target Address field in the Route Request matches this
node's own IP address, then the node SHOULD return a Route Reply
to the initiator of this Route Request (the Source Address in the
IP header of the packet), as described in Section 6.2.4. The
source route for this Reply is the sequence of hop addresses
initiator, Address[1], Address[2], ..., Address[n], target
where initiator is the address of the initiator of this
Route Request, each Address[i] is an address from the Route
Request, and target is the target of the Route Request (the
Target Address field in the Route Request). The value n here
is the number of addresses recorded in the Route Request, or
(Opt Data Len - 6) / 4.
The node then MUST replace the Destination Address field in
the Route Request packet's IP header with the value in the
Target Address field in the Route Request option, and continue
processing the rest of the Route Request packet normally. The
node MUST NOT process the Route Request option further and MUST
NOT retransmit the Route Request to propagate it to other nodes
as part of the Route Discovery.
- Else, the node MUST examine the route recorded in the Route
Request option (the IP Source Address field and the sequence of
Address[i] fields) to determine if this node's own IP address
already appears in this list of addresses. If so, the node MUST
discard the entire packet carrying the Route Request option.
- Else, the node MUST search its Route Request Table for an entry
for the initiator of this Route Request (the IP Source Address
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field). If such an entry is found in the table, the node MUST
search the cache of Identification values of recently received
Route Requests in that table entry, to determine if an entry
is present in the cache matching the Identification value
and target node address in this Route Request. If such an
(Identification, target address) entry is found in this cache in
this entry in the Route Request Table, then the node MUST discard
the entire packet carrying the Route Request option.
- Else, this node SHOULD further process the Route Request
according to the following sequence of steps:
o Add an entry for this Route Request in its cache of
(Identification, target address) values of recently received
Route Requests.
o Conceptually create a copy of this entire packet and perform
the following steps on the copy of the packet.
o Append this node's own IP address to the list of Address[i]
values in the Route Request, and increase the value of the
Opt Data Len field in the Route Request by 4 (the size of an
IP address).
o This node SHOULD search its own Route Cache for a route
(from itself, as if it were the source of a packet) to the
target of this Route Request. If such a route is found in
its Route Cache, then this node SHOULD follow the procedure
outlined in Section 6.2.3 to return a "cached Route Reply"
to the initiator of this Route Request, if permitted by the
restrictions specified there.
o If the node does not return a cached Route Reply, then this
node SHOULD link-layer re-broadcast this copy of the packet,
with a short jitter delay before the broadcast is sent. The
jitter period SHOULD be chosen as a random period, uniformly
distributed between 0 and BroadcastJitter.
6.2.3. Generating a Route Reply using the Route Cache
As described in Section 3.3.2, it is possible for a node processing a
received Route Request to avoid propagating the Route Request further
toward the target of the Request, if this node has in its Route Cache
a route from itself to this target. Such a Route Reply generated by
a node from its own cached route to the target of a Route Request is
called a "cached Route Reply", and this mechanism can greatly reduce
the overall overhead of Route Discovery on the network by reducing
the flood of Route Requests. The general processing of a received
Route Request is described in Section 6.2.2; this section specifies
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the additional requirements that MUST be met before a cached Route
Reply may be generated and returned and specifies the procedure for
returning such a cached Route Reply.
While processing a received Route Request, for a node to possibly
return a cached Route Reply, it MUST have in its Route Cache a route
from itself to the target of this Route Request. However, before
generating a cached Route Reply for this Route Request, the node MUST
verify that there are no duplicate addresses listed in the route
accumulated in the Route Request together with the route from this
node's Route Cache. Specifically, there MUST be no duplicates among
the following addresses:
- The IP Source Address of the packet containing the Route Request,
- The Address[i] fields in the Route Request, and
- The nodes listed in the route obtained from this node's Route
Cache, excluding the address of this node itself (this node
itself is the common point between the route accumulated in the
Route Request and the route obtained from the Route Cache).
If any duplicates exist among these addresses, then the node MUST NOT
send a cached Route Reply. The node SHOULD continue to process the
Route Request as described in Section 6.2.2.
If the Route Request and the route from the Route Cache meet the
restriction above, then the node SHOULD construct and return a cached
Route Reply as follows:
- The source route for this reply is the sequence of hop addresses
initiator, Address[1], Address[2], ..., Address[n], c-route
where initiator is the address of the initiator of this Route
Request, each Address[i] is an address from the Route Request,
and c-route is the sequence of hop addresses in the source route
to this target node, obtained from the node's Route Cache. In
appending this cached route to the source route for the reply,
the address of this node itself MUST be excluded, since it is
already listed as Address[n].
- Send a Route Reply to the initiator of the Route Request, using
the procedure defined in Section 6.2.4. The initiator of the
Route Request is indicated in the Source Address field in the
packet's IP header.
If the node returns a cached Route Reply as described above, then
the node MUST NOT propagate the Route Request further (i.e., the
node MUST NOT rebroadcast the Route Request). In this case, instead,
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if the packet contains no other DSR options and contains no payload
after the DSR header (e.g., the Route Request is not piggybacked
on a TCP or UDP packet), then the node SHOULD simply discard the
packet. Otherwise (if the packet contains other DSR options or
contains any payload after the DSR header), the node SHOULD forward
the packet along the cached route to the target of the Route Request.
Specifically, if the node does so, it MUST use the following
steps:
- Copy the Target Address from the Route Request option in the
DSR header to the Destination Address field in the packet's IP
header.
- Remove the Route Request option from the DSR header in the
packet, and add a DSR Source Route option to the packet's DSR
header.
- In the DSR Source Route option, set the Address[i] fields
to represent the source route found in this node's Route
Cache to the original target of the Route Discovery (the
new IP Destination Address of the packet). Specifically,
the node copies the hop addresses of the source route into
sequential Address[i] fields in the DSR Source Route option,
for i = 1, 2, ..., n. Address[1] here is the address of this
node itself (the first address in the source route found from
this node to the original target of the Route Discovery). The
value n here is the number of hop addresses in this source route,
excluding the destination of the packet (which is instead already
represented in the Destination Address field in the packet's IP
header).
- Initialize the Segments Left field in the DSR Source Route option
to n as defined above.
- The First Hop External (F) bit in the DSR Source Route option is
copied from the External bit flagging the first hop in the source
route for the packet, as indicated in the Route Cache.
- The Last Hop External (L) bit in the DSR Source Route option is
copied from the External bit flagging the last hop in the source
route for the packet, as indicated in the Route Cache.
- The Salvage field in the DSR Source Route option MUST be
initialized to some nonzero value; the particular nonzero value
used SHOULD be MAX_SALVAGE_COUNT. By initializing this field to
a nonzero value, nodes forwarding or overhearing this packet will
not consider a link to exist between the IP Source Address of the
packet and the Address[1] address in the DSR Source Route option
(e.g., they will not attempt to add this to their Route Cache as
a link). By choosing MAX_SALVAGE_COUNT as the nonzero value to
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which the node initializes this field, nodes furthermore will not
attempt to salvage this packet.
- Transmit the packet to the next-hop node on the new source route
in the packet, using the forwarding procedure described in
Section 6.1.5.
6.2.4. Originating a Route Reply
A node originates a Route Reply in order to reply to a received and
processed Route Request, according to the procedures described in
Sections 6.2.2 and 6.2.3. The Route Reply is returned in a Route
Reply option (Section 5.3). The Route Reply option MAY be returned
to the initiator of the Route Request in a separate IP packet, used
only to carry this Route Reply option, or it MAY be included in any
other IP packet being sent to this address.
The Route Reply option MUST be included in a DSR header in the packet
returned to the initiator. To initialize the Route Reply option, the
node performs the following sequence of steps:
- The Option Type in the option MUST be set to the value 3.
- The Opt Data Len field in the option MUST be set to the value
(n * 4) + 3, where n is the number of addresses in the source
route being returned (excluding the Route Discovery initiator
node's address).
- The Last Hop External (L) bit in the option MUST be
initialized to 0.
- The Reserved field in the option MUST be initialized to 0.
- The Route Request Identifier MUST be initialized to the
Identifier field of the Route Request that this reply is sent in
response to.
- The sequence of hop addresses in the source route are copied into
the Address[i] fields of the option. Address[1] MUST be set to
the first-hop address of the route after the initiator of the
Route Discovery, Address[n] MUST be set to the last-hop address
of the source route (the address of the target node), and each
other Address[i] MUST be set to the next address in sequence in
the source route being returned.
The Destination Address field in the IP header of the packet carrying
the Route Reply option MUST be set to the address of the initiator
of the Route Discovery (i.e., for a Route Reply being returned in
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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 uni-directional 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 bi-directionality, 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
bi-directional.
If sending a Route Reply to the initiator of the Route Request
requires performing a Route Discovery, the Route Reply Option MUST
be piggybacked on the packet that contains the Route Request. This
piggybacking prevents a loop wherein the target of the new Route
Request (which was itself the initiator of the original Route
Request) must do another Route Request in order to return its
Route Reply.
If sending the Route Reply to the initiator of the Route Request
does not require performing a Route Discovery, a node SHOULD send a
unicast Route Reply in response to every Route Request it receives
for which it is the target node.
6.2.5. Processing a Received Route Reply Option
Section 6.1.4 describes the general processing for a received packet,
including the addition of routing information from options in the
packet's DSR header to the receiving node's Route Cache.
If the received packet contains a Route Reply, no additional special
processing of the Route Reply option is required beyond what is
described there. As described in Section 4.1 anytime a node adds
new information to its Route Cache (including the information added
from this Route Reply option), the node SHOULD check each packet in
its own Send Buffer (Section 4.2) to determine whether a route to
that packet's IP Destination Address now exists in the node's Route
Cache (including the information just added to the Cache). If so,
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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.
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6.3. Route Maintenance Processing
Route Maintenance is the mechanism by which a source node S is able
to detect, while using a source route to some destination node D,
if the network topology has changed such that it can no longer use
its route to D because a link along the route no longer works. When
Route Maintenance indicates that a source route is broken, S can
attempt to use any other route it happens to know to D, or can invoke
Route Discovery again to find a new route for subsequent packets
to D. Route Maintenance for this route is used only when S is
actually sending packets to D.
Specifically, when forwarding a packet, a node MUST attempt to
receive an acknowledgement for the packet from the next-hop node. If
no acknowledgement is received after MaxMaintRexmt retransmissions of
the packet (after the initial transmission of the packet), the node
determines that the link for this next-hop node of the source route
is "broken". This acknowledgement from the next-hop node for Route
Maintenance can be implemented using a link-layer acknowledgement
(Section 6.3.1), using a "passive acknowledgement" (Section 6.3.2),
or using a network-layer acknowledgement (Section 6.3.3); the
particular strategy for retransmission timing depends on the type of
acknowledgement mechanism used. If no acknowledgment is received
after MaxMaintRexmt retransmissions (if necessary), the node SHOULD
originate a Route Error to the original sender of the packet, as
described in Section 6.3.4.
In deciding whether or not to send a Route Error in response to
attempting to forward a packet from some sender over a broken link,
a node MUST limit the number of consecutive packets from a single
sender that the node attempts to forward over this same broken
link for which the node chooses not to return a Route Error; this
requirement MAY be satisfied by returning a Route Error for each
packet that the node attempts to forward over a broken link.
6.3.1. Using Link-Layer Acknowledgments
If the MAC protocol in use provides feedback as to the successful
delivery of a data packet (such as is provided by the link-layer
acknowledgement frame defined by IEEE 802.11 [11]), 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.
When using link-layer acknowledgements for Route Maintenance, the
retransmission timing and the timing at which retransmission attempts
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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 the data packet as
a part of the basic access method of of the IEEE 802.11 Distributed
Coordination Function (DCF) MAC protocol; the time at which the
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 Retransmission Buffer, that node SHOULD remove that packet from
its Retransmission Buffer, stopping Route Maintenance retransmissions
for that packet.
6.3.2. Using Passive Acknowledgments
When link-layer acknowledgements are not available, but passive
acknowledgements [16] 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 network links used for data packets generally
operate bi-directionally (such as when the MAC protocol requires
this, as with IEEE 802.11).
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:
- 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 acknowledgements (as described in
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Section 6.3.3) are used for all remaining attempts for that
packet.
- Each node keeps a table of possible next-hop destination nodes,
noting whether or not passive 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) to be
an acknowledgement of this first packet if both of the following two
tests succeed:
- The Source Address, Destination Address, Protocol,
Identification, and Fragment Offset fields in the IP header
of the two packets MUST match [27], and
- If either packet contains a DSR Source Route header, both packets
MUST contain one, and the value in the Segments Left field in the
DSR Source Route header of the new packet MUST be less than that
in the first packet.
When a node hears such a passive acknowledgement for any packet in
its Retransmission Buffer, that node SHOULD remove that packet from
its Retransmission Buffer, stopping Route Maintenance retransmissions
for that packet.
6.3.3. Using Network-Layer Acknowledgments
When a node originates or forwards a packet and has no other
mechanism of acknowledgement available to determine successful
delivery of the packet to 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 header in the packet.
The Identification field in that Acknowledgement Request option MUST
be set to a value unique over all packets transmitted by this node
to the same next-hop node that are either unacknowledged or recently
acknowledged.
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When a node receives a packet containing an Acknowledgement Request
option, then that node performs the following tests on the packet:
- If the indicated next-hop node address for this packet does not
match any of this node's own IP addresses, then this node MUST
NOT process the Acknowledgement Request option. The indicated
next-hop node address is the next Address[i] field in the DSR
Source Route option in the DSR header in the packet, or is the IP
Destination Address in the packet if the packet does not contain
a DSR Source Route option or the Segments Left there is zero.
- If the packet contains an 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:
- Create a packet and set the IP Protocol field to the protocol
number assigned for a DSR header (TBA???).
- Set the IP Source Address field in this packet to the IP address
of this node, copied from the source route in the DSR Source
Route option in that packet (or from the IP Destination Address
field of the packet, if the packet does not contain a DSR Source
Route option).
- Set the IP Destination Address field in this packet to the IP
address of the previous-hop node, copied from the source route
in the DSR Source Route option in that packet (or from the IP
Source Address field of the packet, if the packet does not
contain a DSR Source Route option).
- Add a DSR header to the packet, and set the DSR header's
Next Header field to the "No Next Header" value.
- Add an Acknowledgement option to the DSR header in the packet;
set the Acknowledgement option's Option Type field to 6 and the
Opt Data Len field to 10.
- Copy the Identification field from the received Acknowledgement
Request option into the Identification field in the
Acknowledgement option.
- 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).
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- 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).
- Send the packet as described in Section 6.1.1.
Packets containing an Acknowledgement option SHOULD NOT be
retransmitted by intermediate nodes for Route Maintenance, and SHOULD
NOT expect a link-layer acknowledgement or passive acknowledgment.
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:
- Create a packet and set the IP Protocol field to the protocol
number assigned for a DSR header (TBA???).
- Set the IP Source Address field in this packet to the IP address
of this node, copied from the source route in the DSR Source
Route option in that packet (or from the IP Destination Address
field of the packet, if the packet does not contain a DSR Source
Route option).
- Set the IP Destination Address field in this packet to the IP
address of the node originating the Acknowledgement option.
- Add a DSR header to the packet, and set the DSR header's
Next Header field to the "No Next Header" value.
- Add an Acknowledgement option to the DSR header in this packet;
set the Acknowledgement option's Option Type field to 6 and the
Opt Data Len field to 10.
- Copy the Identification field from the received Acknowledgement
Request option into the Identification field in the
Acknowledgement option.
- Set the ACK Source Address field in the option to be the IP
Source Address of this new packet (set above to be the IP address
of this node).
- Set the ACK Destination Address field in the option to be the IP
Destination Address of this new packet (set above to be the IP
address of the node originating the Acknowledgement option.)
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- 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 acknowledgment.
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 a packet.
For example, a node SHOULD maintain a separate round-trip time (RTT)
estimate for each to which it has recently attempted to transmit
packets, and it SHOULD use this RTT estimate in setting the timeout
for each retransmission attempt for Route Maintenance. The TCP RTT
estimation algorithm has been shown to work well for this purpose in
implementation and testbed experiments with DSR [20, 22].
6.3.4. Originating a Route Error
When a node is unable to verify successful delivery of a packet to
the next-hop node after reaching a maximum number of retransmission
attempts, a node SHOULD send a Route Error to the IP Source Address
of the packet. When sending a Route Error for a packet containing
either a Route Error option or an 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:
- Create an IP packet and set the Source Address field in this
packet's IP header to the address of this node.
- If the Salvage field in the DSR Source Route option in the
packet triggering the Route Error is zero, then copy the
Source Address field of the packet triggering the Route Error
into the Destination Address field in the new packet's IP
header; otherwise, copy the Address[1] field from the DSR Source
Route option of the packet triggering the Route Error into the
Destination Address field in the new packet's IP header
- Insert a DSR header into the new packet.
- 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
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route. Set the Error Source Address field to this node's IP
address, and the Error Destination field to the new packet's IP
Destination Address.
- If the packet triggering the Route Error contains any Route Error
or 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 5.4) of that included Route Error
would be greater than MAX_SALVAGE_COUNT in the new packet.
o If any Route Error option from the packet triggering the new
Route Error is not included in the packet, the node MUST NOT
include any following Route Error or 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.
- Send the packet as described in Section 6.1.1.
6.3.5. Processing a Received Route Error Option
When a node receives a packet containing a Route Error option, that
node MUST process the Route Error option according to the following
sequence of steps:
- The node MUST remove from its Route Cache the link from the
node identified by the Error Source Address field to the node
identified by the Unreachable Node Address field (if this link is
present in its Route Cache). If the node implements its Route
Cache as a link cache, as described in Section 4.1, only this
single link is removed; if the node implements its Route Cache as
a path cache, however, all routes (paths) that use this link are
removed.
- If the option following the Route Error is an 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
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packet as described in Section 6.1.1, with the salvage count
in the DSR Source Route option set to the Salvage value of the
Route Error.
In addition, after processing the Route Error as described above,
the node MAY initiate a new Route Discovery for any destination node
for which it then has no route in its Route Cache as a result of
processing this Route Error, if the node has indication that a route
to that destination is needed. For example, if the node has an open
TCP connection to some destination node, then if the processing of
this Route Error removed the only route to that destination from this
node's Route Cache, then this node MAY initiate a new Route Discovery
for that destination node. Any node, however, MUST limit the rate at
which it initiates new Route Discoveries for any single destination
address, and any new Route Discovery initiated in this way as part of
processing this Route Error MUST conform to this limit.
6.3.6. Salvaging a Packet
When an intermediate node forwarding a packet detects through Route
Maintenance that the next-hop link along the route for that packet is
broken (Section 6.3), if the node has another route to the packet's
IP Destination Address in its Route Cache, the node SHOULD "salvage"
the packet rather than discarding it. To do so using the route found
in its Route Cache, this node processes the packet as follows:
- If the MAC protocol in use in the network is not capable of
transmitting unicast packets over uni-directional links, as
discussed in Section 3.3.1, then if this packet contains a Route
Reply option, remove and discard the Route Reply option in the
packet; if the DSR header in the packet then contains no DSR
options, remove the DSR header from the packet. If the resulting
packet then contains only an IP header, the node SHOULD NOT
salvage the packet and instead SHOULD discard the entire packet.
When returning any Route Reply in the case in which the MAC
protocol in use in the network is not capable of transmitting
unicast packets over uni-directional 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 bi-directionality, 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 bi-directional.
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- Modify the existing DSR Source Route option in the packet so
that the Address[i] fields represent the source route found in
this node's Route Cache to this packet's IP Destination Address.
Specifically, the node copies the hop addresses of the source
route into sequential Address[i] fields in the DSR Source Route
option, for i = 1, 2, ..., n. Address[1] here is the address
of the salvaging node itself (the first address in the source
route found from this node to the IP Destination Address of the
packet). The value n here is the number of hop addresses in this
source route, excluding the destination of the packet (which is
instead already represented in the Destination Address field in
the packet's IP header).
- Initialize the Segments Left field in the DSR Source Route option
to n as defined above.
- The First Hop External (F) bit in the DSR Source Route option is
copied from the External bit flagging the first hop in the source
route for the packet, as indicated in the Route Cache.
- The Last Hop External (L) bit in the DSR Source Route option is
copied from the External bit flagging the last hop in the source
route for the packet, as indicated in the Route Cache.
- The Salvage field in the DSR Source Route option is set to 1 plus
the value of the Salvage field in the DSR Source Route option of
the packet that caused the error.
- Transmit the packet to the next-hop node on the new source route
in the packet, using the forwarding procedure described in
Section 6.1.5.
As described in Section 6.3.4, the node in this case also SHOULD
return a Route Error to the original sender of the packet. If the
node chooses to salvage the packet, it SHOULD do so after originating
the Route Error.
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7. 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 network interfaces using the Distributed Coordination
Function (DCF) MAC with RTS and CTS [11, 5].
BroadcastJitter 10 milliseconds
RouteCacheTimeout 300 seconds
SendBufferTimeout 30 seconds
RequestTableSize 64 nodes
RequestTableIds 16 identifiers
MaxRequestRexmt 16 retransmissions
MaxRequestPeriod 10 seconds
RequestPeriod 500 milliseconds
NonpropRequestTimeout 30 milliseconds
RexmtBufferSize 50 packets
MaxMaintRexmt 2 retransmissions
TryPassiveAcks 1 attempt
PassiveAckTimeout 100 milliseconds
GratReplyHoldoff 1 second
In addition, the following protocol constant MUST be supported by any
implementation of the DSR protocol:
MAX_SALVAGE_COUNT 15 salvages
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8. IANA Considerations
This document proposes the use of a DSR header, which requires an IP
Protocol number.
In addition, this document proposes use of the value "No Next Header"
(originally defined for use in IPv6) within an IPv4 packet, to
indicate that no further header follows a DSR header.
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9. Security Considerations
This document does not specifically address security concerns. This
document does assume that all nodes participating in the DSR protocol
do so in good faith and without malicious intent to corrupt the
routing ability of the network. In mission-oriented environments
where all the nodes participating in the DSR protocol share a
common goal that motivates their participation in the protocol, the
communications between the nodes can be encrypted at the physical
channel or link layer to prevent attack by outsiders.
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Appendix A. Link-MaxLife Cache Description
As guidance to implementors of DSR, the description below outlines
the operation of a possible implementation of a Route Cache for DSR
that has been shown to outperform other other caches studied in
detailed simulations. Use of this design for the Route Cache is
recommended in implementations of DSR.
This cache, called "Link-MaxLife" [9], is a link cache, in that each
individual link (hop) in the routes returned in Route Reply packets
(or otherwise learned from the header of overhead packets) is added
to a unified graph data structure of this node's current view of the
network topology, as described in Section 4.1. To search for a route
in this cache to some destination node, the sending node uses a graph
search algorithm, such as the well-known Dijkstra's shortest-path
algorithm, to find the current best path through the graph to the
destination node.
The Link-MaxLife form of link cache is adaptive in that each link in
the cache has a timeout that is determined dynamically by the caching
node according to its observed past behavior of the two nodes at the
ends of the link; in addition, when selecting a route for a packet
being sent to some destination, among cached routes of equal length
(number of hops) to that destination, Link-MaxLife selects the route
with the longest expected lifetime (highest minimum timeout of any
link in the route).
Specifically, in Link-MaxLife, a link's timeout in the Route Cache
is chosen according to a "Stability Table" maintained by the caching
node. Each entry in a node's Stability Table records the address of
another node and a factor representing the perceived "stability" of
this node. The stability of each other node in a node's Stability
Table is initialized to InitStability. When a link from the Route
Cache is used in routing a packet originated or salvaged by that
node, the stability metric for each of the two endpoint nodes of that
link is incremented by the amount of time since that link was last
used, multiplied by StabilityIncrFactor (StabilityIncrFactor >= 1);
when a link is observed to break and the link is thus removed
from the Route Cache (either due the receipt of a Route Error for
this link or due to exceeding the maximum number of retransmission
attempts for Route Maintenance for a packet being originated or
forwarded by this node), 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
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salvaged by this node, the link's lifetime is set to be at least
UseExtends into the future; if the lifetime of that link in the
Route Cache is already further into the future, the lifetime remains
unchanged.
When a node using Link-MaxLife selects a route from its Route Cache
for a packet being originated or salvaged by this node, it selects
the shortest-length route that has the longest expected lifetime
(highest minimum timeout of any link in the route), as opposed to
simply selecting an arbitrary route of shortest length.
The following configuration variables are used in the description
of Link-MaxLife above. The specific variable names are used for
demonstration purposes only, and an implementation is not required
to use these names for these configuration variables. For each
configuration variable below, the default value is specified to
simplify configuration. In particular, the default values given
below are chosen for a DSR network where nodes move at relative
velocities between 12 and 25 seconds per transmission radius.
InitStability 25 seconds
StabilityIncrFactor 4
StabilityDecrFactor 2
MinLifetime 1 second
UseExtends 120 seconds
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Appendix B. Location of DSR in the ISO Network Reference Model
When designing DSR, we had to determine at what layer within
the protocol hierarchy to implement ad hoc network routing. We
considered two different options: routing at the link layer (ISO
layer 2) and routing at the network layer (ISO layer 3). Originally,
we opted to route at the link layer for several reasons:
- Pragmatically, running the DSR protocol at the link layer
maximizes the number of mobile nodes that can participate in
ad hoc networks. For example, the protocol can route equally
well between IPv4 [27], IPv6 [6], and IPX [32] nodes.
- Historically [13, 14], DSR grew from our contemplation of
a multi-hop propagating version of the Internet's Address
Resolution Protocol (ARP) [25], as well as from the routing
mechanism used in IEEE 802 source routing bridges [24]. These
are layer 2 protocols.
- Technically, we designed DSR to be simple enough that it could
be implemented directly in the firmware inside wireless network
interface cards [13, 14], well below the layer 3 software within
a mobile node. We see great potential in this for DSR running
inside a cloud of mobile nodes around a fixed base station,
where DSR would act to transparently extend the coverage range
to these nodes. Mobile nodes that would otherwise be unable
to communicate with the base station due to factors such as
distance, fading, or local interference sources could then reach
the base station through their peers.
Ultimately, however, we decided to specify and to implement [20]
DSR as a layer 3 protocol, since this is the only layer at which we
could realistically support nodes with multiple network interfaces of
different types forming an ad hoc network.
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Appendix C. Implementation and Evaluation Status
The initial design of the DSR protocol, including DSR's basic Route
Discovery and Route Maintenance mechanisms, was first published in
December 1994 [13], with significant additional design details and
initial simulation results published in early 1996 [14].
The DSR protocol has been extensively studied since then through
additional detailed simulations. In particular, we have implemented
DSR in the ns-2 network simulator [23, 5] and performed extensive
simulations of DSR using ns-2 (e.g., [5, 19]). We have also
conducted evaluations of different caching strategies documented in
this draft [9].
We have also implemented the DSR protocol under the FreeBSD 2.2.7
operating system running on Intel x86 platforms. FreeBSD [8] is
based on a variety of free software, including 4.4 BSD Lite from the
University of California, Berkeley. For the environments in which
we used it, this implementation is functionally equivalent to the
version of the DSR protocol specified in this draft.
During the 7 months from August 1998 to February 1999, we designed
and implemented a full-scale physical testbed to enable the
evaluation of ad hoc network performance in the field, in an actively
mobile ad hoc network under realistic communication workloads. The
last week of February and the first week of March of 1999 included
demonstrations of this testbed to a number of our sponsors and
partners, including Lucent Technologies, Bell Atlantic, and DARPA.
A complete description of the testbed is available as a Technical
Report [20].
We have since ported this implementation of DSR to FreeBSD 3.3, and
we have also added a preliminary version of Quality of Service (QoS)
support for DSR. A demonstration of this modified version of DSR was
presented in July 2000. These QoS features are not included in this
draft, and will be added later in a separate draft on top of the base
protocol specified here.
DSR has also been implemented under Linux by Alex Song at the
University of Queensland, Australia [31]. This implementation
supports the Intel x86 PC platform and the Compaq iPAQ.
Several other independent groups have also used DSR as a platform for
their own research, or and as a basis of comparison between ad hoc
network routing protocols.
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Changes from Previous Version of the Draft
This appendix briefly lists some of the major changes in this
draft relative to the previous version of this same draft,
draft-ietf-manet-dsr-05.txt:
- Clarified how to handle Route Maintenance at the original sender
of a packet, which is slightly different than at an intermediate
node forwarding the packet.
- In the definition of the Route Cache in Section 4.1, if there
are multiple cached routes to a destination, a node MUST prefer
routes that do not have the External flag set on any link; this
restriction was previously specified as a "SHOULD". This change
does not affect the operation of DSR with respect to this draft,
since the use of external links is outside the scope of this
draft.
- Clarified that the Retransmission Buffer MAY be of limited size,
and that when adding a new packet to the Retransmission Buffer,
if the buffer size is insufficient to hold the new packet, the
new packet SHOULD be silently discarded.
- Changed the calculation of the Salvage field in a Route
Error option and the total salvage count of an option to not
explicitely increment the count when the count is copied from a
DSR Source Route option into a new Route Error option. Instead,
the increment is implicit in the value of the Salvage field
and is added in when the total salvage count of an option is
calculated.
- In Section 5.2, corrected the specification of the number of
Address[i] fields present in a Route Request option. The number
of addresses present is indicated by the Opt Data Len field in
the option as n = (Opt Data Len - 6) / 4.
- In Section 6.1.3, corrected the specification of the steps for
adding a DSR Source Route option to a packet. As described
elsewhere in the draft, the entire source route (excluding the
address of the originating node and the final destination address
of the packet) is copied into the DSR Source Route option, and
the IP Destination Address of the packet is not changed when
inserting the source route.
- Added a specific statement in the abstract and introduction
that this document specifies the operation of DSR only for
IPv4. Operation of DSR with IPv6 [6] will be covered in other
documents.
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- Removed the ACK Request Source Address field from the
Acknowledgement Request option, as this field was not used in
standard DSR; instead, the address of the node requesting a DSR
Acknowledgement is obtained as the previous-hop address of the
source route in the packet. This field is, however, used in the
"flow state" enhancement to DSR [10] and will be specified in
that draft.
- The DSR header was previously specified to always be a multiple
of 4 octet in size; this is now only required if any other
headers follow the DSR header in the packet.
- Clarified the definition of salvaging to be a "SHOULD" rather
than a "MAY".
- Added the definition of the Gratuitous Route Reply Table as a new
conceptual data structure in Section 4.4, and added corresponding
uses of it in the detailed operation. This data structure and
its use have always been a part of the DSR simulation but had not
previously been documented in the draft.
- Removed the Identification field from the definition of a Route
Reply option since it was not used in the protocol.
- Removed the restriction that the value of the Identification
field in an Acknowledgement Request option needed to be nonzero;
the value zero at one time had a special meaning in the protocol
but no longer is used for this purpose.
- Added a description of a specific possible implementation of the
Route Cache data structure, called "Link-MaxLife", in Appendix A.
The actual choice of data structure implementation to use for
the Route Cache in any DSR implementation is a local matter
for each node and affects only performance, not correctness or
interoperability; the Link-MaxLife cache, however, has been
studied extensively and been shown to outperform other types of
cache implementations studied in detailed simulation [9], and its
use in DSR implementations is recommended.
- Changed most of the protocol constants to now be configuration
variables, which MUST support a mechanism enabling the value of
these variables to be modified by system management. Also, to
be clear in the specification which values are variables now and
which are constants, changed the names of all variables to be in
MixedCase instead of ALL_CAPS.
- Changed name of the constant MAX_SALVAGE_TIMES to
MAX_SALVAGE_COUNT.
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- Changed the name of the variable DsrMaxRxtShift to now
be MaxMaintRexmt. Also changed the name of the variable
DsrRxmtBufferSize to now be RexmtBufferSize.
- Clarified the description of what to add to a node's Route Cache
in response to different options in the DSR header of a received
packet, and coalesced this description into Section 6.1.4.
- In Section 6.3.5, added a suggestion that a node, after
processing a Route Error, 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 (e.g., an
open TCP connection). Such Route Discoveries MUST conform to the
standard rate limiting for Route Discoveries.
- Clarified the retransmission timing for Route Maintenance
retransmissions, in Section 6.3.
- Updated the implementation and evaluation description in
Appendix C to include mention of the implementation of DSR under
Linux by Alex Song at the University of Queensland, Australia.
This implementation supports the Intel x86 PC platform and the
Compaq iPAQ.
- Changed the status of the document to indicate full conformance
with all provisions of Section 10 of RFC 2026.
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Acknowledgements
The protocol described in this draft has been designed and developed
within the Monarch Project, a research project at Rice University
(previously at Carnegie Mellon University) that is developing
adaptive networking protocols and protocol interfaces to allow truly
seamless wireless and mobile node networking [15, 30].
The authors would like to acknowledge the substantial contributions
of Josh Broch in helping to design, simulate, and implement the DSR
protocol. Josh is currently on leave of absence from Carnegie Mellon
University at AON Networks. We thank him for his contributions to
earlier versions of this draft.
We would also like to acknowledge the assistance of Robert V. Barron
at Carnegie Mellon University. Bob ported our DSR implementation
from FreeBSD 2.2.7 into FreeBSD 3.3.
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Chair's Address
The MANET Working Group can be contacted via its current chairs:
M. Scott Corson Phone: +1 908 947-7033
Flarion Technologies, Inc. Email: corson@flarion.com
Bedminster One
135 Route 202/206 South
Bedminster, NJ 07921
USA
Joseph Macker Phone: +1 202 767-2001
Information Technology Division Email: macker@itd.nrl.navy.mil
Naval Research Laboratory
Washington, DC 20375
USA
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Authors' Addresses
Questions about this document can also be directed to the authors:
David B. Johnson Phone: +1 713 348-3063
Rice University Fax: +1 713 348-5930
Computer Science Department, MS 132 Email: dbj@cs.rice.edu
6100 Main Street
Houston, TX 77005-1892
USA
David A. Maltz Phone: +1 650 688-3128
AON Networks Fax: +1 650 688-3119
3045 Park Blvd. Email: dmaltz@cs.cmu.edu
Palo Alto, CA 94306
USA
Yih-Chun Hu Phone: +1 412 268-3075
Rice University Fax: +1 412 268-5576
Computer Science Department, MS 132 Email: yihchun@cs.cmu.edu
6100 Main Street
Houston, TX 77005-1892
USA
Jorjeta G. Jetcheva Phone: +1 412 268-3053
Carnegie Mellon University Fax: +1 412 268-5576
Computer Science Department Email: jorjeta@cs.cmu.edu
5000 Forbes Avenue
Pittsburgh, PA 15213-3891
USA
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