IETF MANET Working Group David B. Johnson, Rice University
INTERNET-DRAFT David A. Maltz, AON Networks
2 March 2001 Yih-Chun Hu, Rice University
Jorjeta G. Jetcheva, Carnegie Mellon University
The Dynamic Source Routing Protocol for Mobile Ad Hoc Networks
<draft-ietf-manet-dsr-05.txt>
Status of This Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026 except that the right to
produce derivative works is not granted.
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 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 IP 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 . . . . . . . . . . . 8
3.3.1. Caching Overheard Routing Information . . . . . . 8
3.3.2. Replying to Route Requests using Cached Routes . 9
3.3.3. Preventing Route Reply Storms . . . . . . . . . . 10
3.3.4. Route Request Hop Limits . . . . . . . . . . . . 12
3.4. Additional Route Maintenance Features . . . . . . . . . . 13
3.4.1. Packet Salvaging . . . . . . . . . . . . . . . . 13
3.4.2. Automatic Route Shortening . . . . . . . . . . . 13
3.4.3. Increased Spreading of Route Error Messages . . . 14
4. Conceptual Data Structures 15
4.1. Route Cache . . . . . . . . . . . . . . . . . . . . . . . 15
4.2. Route Request Table . . . . . . . . . . . . . . . . . . . 17
4.3. Send Buffer . . . . . . . . . . . . . . . . . . . . . . . 18
4.4. Retransmission Buffer . . . . . . . . . . . . . . . . . . 19
5. DSR Header Format 20
5.1. Fixed Portion of DSR Header . . . . . . . . . . . . . . . 21
5.2. Route Request Option . . . . . . . . . . . . . . . . . . 23
5.3. Route Reply Option . . . . . . . . . . . . . . . . . . . 25
5.4. Route Error Option . . . . . . . . . . . . . . . . . . . 27
5.5. Acknowledgment Request Option . . . . . . . . . . . . . . 29
5.6. Acknowledgment Option . . . . . . . . . . . . . . . . . . 30
5.7. Source Route Option . . . . . . . . . . . . . . . . . . . 31
5.8. Pad1 Option . . . . . . . . . . . . . . . . . . . . . . . 33
5.9. PadN Option . . . . . . . . . . . . . . . . . . . . . . . 34
6. Detailed Operation 35
6.1. General Packet Processing . . . . . . . . . . . . . . . . 35
6.1.1. Originating a Packet . . . . . . . . . . . . . . 35
6.1.2. Adding a DSR Header to a Packet . . . . . . . . . 35
6.1.3. Adding a Source Route Option to a Packet . . . . 36
6.1.4. Receiving a Packet . . . . . . . . . . . . . . . 36
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6.1.5. Processing a Received Source Route Option . . . . 38
6.2. Route Discovery Processing . . . . . . . . . . . . . . . 40
6.2.1. Originating a Route Request . . . . . . . . . . . 40
6.2.2. Processing a Received Route Request Option . . . 42
6.2.3. Generating Route Replies using the Route Cache . 43
6.2.4. Originating a Route Reply . . . . . . . . . . . . 44
6.2.5. Processing a Route Reply Option . . . . . . . . . 46
6.3. Route Maintenance Processing . . . . . . . . . . . . . . 47
6.3.1. Using Network-Layer Acknowledgments . . . . . . . 47
6.3.2. Using Link Layer Acknowledgments . . . . . . . . 48
6.3.3. Originating a Route Error . . . . . . . . . . . . 48
6.3.4. Processing a Route Error Option . . . . . . . . . 49
6.3.5. Salvaging a Packet . . . . . . . . . . . . . . . 49
7. Constants 50
8. IANA Considerations 51
9. Security Considerations 52
Appendix A. Location of DSR in the ISO Network Reference Model 53
Appendix B. Implementation and Evaluation Status 54
Acknowledgements 55
References 56
Chair's Address 59
Authors' Addresses 60
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1. Introduction
The Dynamic Source Routing protocol (DSR) [12, 13] 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 may 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 quickly to changes in the
network. The DSR protocol provides highly reactive service 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 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 level 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 IP packets in multi-hop wireless ad hoc networks. Advanced,
optional features, such as Quality of Service (QoS) support and
efficient multicast routing, 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 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 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 [13]. 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, 17]. 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 [16], MACAW [2], or IEEE
802.11 [10], 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-level 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 easy 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 [8]), 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, but 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 [25], 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 [10], this route reversal
is preferred, as it avoids the overhead of a possible second
Route Discovery, and it tests the discovered route to ensure it is
bi-directional before the Route Discovery initiator begins using the
route. However, this technique will prevent the discovery of routes
using uni-directional links. 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
destination address. However, the node MUST limit the rate at which
such new Route Discoveries for the same address are initiated, since
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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 MUST use an exponential
back-off algorithm to limit the rate at which it initiates new Route
Discoveries 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 hop 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 |--x | 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.
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-level acknowledgement frame defined by IEEE
802.11 [10]), or by a "passive acknowledgement" [15] (in which,
for example, B confirms receipt at C by overhearing C transmit the
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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 hop; 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 may
travel over a different, multi-hop path.
If no receipt confirmation is received after the packet has been
retransmitted the maximum number of attempts by some hop, 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 example shown above, if C is unable to deliver
the packet to the next hop 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 exponential back-off
described in Section 3.1).
3.3. Additional Route Discovery Features
3.3.1. Caching Overheard Routing Information
A node forwarding or otherwise overhearing any packet MAY add the
routing information from that packet to its own Route Cache. In
particular, the source route used in a data packet, the accumulated
route record in a Route Request, or the route being returned in a
Route Reply MAY all be cached by any node. Routing information from
any of these packets received can be cached, whether the packet
was addressed to this node, sent to a broadcast (or multicast)
MAC address, or received while the node's network interface is in
promiscuous mode.
One limitation, however, on caching of such overheard routing
information is the possible presence of uni-directional links in the
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ad hoc network (Section 2). For example, in the situation shown
below, node A is using a source route to communicate with node E:
+-----+ +-----+ +-----+ +-----+ +-----+
| A |---->| B |---->| C |---->| D |---->| E |
+-----+ +-----+ +-----+ +-----+ +-----+
^
|
+-----+ +-----+ +-----+ +-----+ +-----+
| V |---->| W |---->| X |---->| Y |---->| Z |
+-----+ +-----+ +-----+ +-----+ +-----+
As node C forwards a data packet along the route from A to E, it
MAY add to its cache the presence of the "forward" direction links
that it learns from the headers of these packets, from itself to D
and from D to E. However, the "reverse" direction of the links
identified in the packet headers, from itself back to B and from B
to A, may not work for it since these links might be uni-directional.
If C knows that the links are in fact bi-directional, for example due
to the MAC protocol in use, it could cache them but otherwise SHOULD
not.
Likewise, node V in the example above is using a different source
route to communicate with node Z. If node C overhears node X
transmitting a data packet to forward it to Y (from V), node C SHOULD
consider whether the links involved can be known to be bi-directional
or not before caching them. If the link from X to C (over which this
data packet was received) can be known to be bi-directional, then C
MAY cache the link from itself to X, the link from X to Y, and the
link from Y to Z. If all links can be assumed to be bi-directional,
C MAY also cache the links from X to W and from W to V. Similar
considerations apply to the routing information that might be learned
from forwarded or otherwise overheard Route Request or Route Reply
packets.
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,
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after this concatenation, contains no duplicate nodes listed in the
route record. For example, the figure below illustrates a case in
which a Route Request for target E has been received by node F, and
node F already has in its Route Cache a route from itself to E:
+-----+ +-----+ +-----+ +-----+
| 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 for two reasons. First,
this limitation 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. Second, this
limitation means that a Route Error traversing the route is very
likely to pass through any node that sent the Route Reply for the
route (including 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 the Route
Request does not meet these restrictions, the node (node F in this
example) discards the Route Request rather than replying to it or
propagating it.
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
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bandwidth and possibly increasing the number of network collisions in
the area.
For example, the figure below shows a situation in which nodes B, C,
D, E, and F all receive A's Route Request for target G, and each has
the indicated route cached for this target:
+-----+ +-----+
| D |< >| C |
+-----+ \ / +-----+
Cache: C - B - G \ / Cache: B - G
\ +-----+ /
-| A |-
+-----+\ +-----+ +-----+
| | \--->| B | | G |
/ \ +-----+ +-----+
/ \ Cache: G
v v
+-----+ +-----+
| E | | F |
+-----+ +-----+
Cache: F - B - G Cache: B - G
Normally, these nodes would all attempt to reply from their own
Route Caches, and would 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 replies from different nodes
all receiving the Route Request may create packet collisions among
some or all of these Replies and may cause local congestion in the
wireless network. In addition, it will often be the case that the
different replies will indicate routes of different lengths, as shown
in this example.
If a node can put its network interface into promiscuous receive
mode, it SHOULD delay sending its own Route Reply for a short period,
while listening to see if the initiating node begins using a shorter
route first. That is, this node SHOULD delay sending 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 send 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 rebroadcasting
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 a
"propagating" Route Request (i.e., with no hop limit) MAY be sent.
Another possible use of the hop limit in a Route Request is to
implement an "expanding ring" search for the target [13]. For
example, a node could send an initial non-propagating Route Request
as described above; if no Route Reply is received for it, the node
could initiate another Route Request with a hop limit of 2. For
each Route Request initiated, if no Route Reply is received for it,
the node could double 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
After sending a Route Error message as part of Route Maintenance
as described in Section 3.2, a node MAY attempt to "salvage" the
data packet that caused the Route Error rather than discarding the
packet. To attempt to salvage a packet, the node sending a Route
Error searches its own Route Cache for a route from itself to the
destination of the packet causing the Error. If such a route is
found, the node MAY salvage the packet after returning the Route
Error by replacing 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 in this way, 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.
3.4.2. Automatic Route Shortening
Source routes in use MAY be automatically shortened if one or more
intermediate hops in the route become no longer necessary. This
mechanism of automatically shortening routes in use is somewhat
similar to the use of passive acknowledgements [15]. 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 unused portion of that source route. If this
node is not the intended next hop for the packet but is named in
the later unused 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
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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) returns 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.
3.4.3. 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 routing information needed by a node participating in an ad hoc
network using 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 either a Route Reply or a DSR Routing
header. 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.
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 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 node in the Route Cache, copied from the First Hop
External (F) and Last Hop External (L) bits in the Source Route
option or Route Reply option from which the link to this node was
learned.
The Route Cache SHOULD support storing more than one route to each
destination. In searching the Route Cache for a route to some
destination node, the Route Cache is indexed by destination node
address. The following properties describe this searching function
on a Route Cache:
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- 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 SHOULD
prefer routes that do not have the External flag set on any node.
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 nodes other than
possibly the first node, the last node, 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.
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
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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, 11, 18] and
through implementation of DSR in a mobile outdoor testbed under
significant workload [19, 20, 20].
- 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: 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
only performance; any reasonable choice of organization for the Route
Cache does not affect either correctness or interoperability.
4.2. Route Request Table
The Route Request Table records information about Route Requests that
have been recently originated or forwarded by this node. The table
is indexed by IP address.
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The Route Request Table on a node records the following information
about nodes to which this node has initiated a Route Request:
- The time that this node last originated a Route Request for that
target node.
- The number of consecutive Route Requests 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.
- The Time-to-Live (TTL) field used in the IP header of last Route
Request initiated by this node for that target node.
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 REQUEST_TABLE_IDS entries containing the
Identification value and target address from the most recent
Route Requests received by this node from that initiator node.
Nodes SHOULD use an LRU policy to manage the entries in their Route
Request Table.
The number of Identification values to retain in each Route Request
Table entry, REQUEST_TABLE_IDS, MUST NOT be unlimited, since,
in the worst case, when a node crashes and reboots, the first
REQUEST_TABLE_IDS 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 SHOULD base its initial Identification value after
rebooting on a random number.
4.3. 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 SEND_BUFFER_TIMEOUT seconds after initially being placed in
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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.
4.4. Retransmission Buffer
The Retransmission Buffer of a node implementing DSR is a queue
of packets sent by this node that are awaiting the receipt of an
acknowledgment from the next hop in the source route (Section 5.7).
For each packet in the Retransmission Buffer, a node maintains (1) a
count of the number of retransmissions and (2) the time of the last
retransmission.
Packets are removed from the Retransmission Buffer when an
acknowledgment is received or when the number of retransmissions
exceeds DSR_MAXRXTSHIFT. In the later case, the removal of the
packet from the Retransmission Buffer SHOULD result in a Route Error
being returned to the original source of the packet (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.
The DSR header is inserted in the packet 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.
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 any 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 [26].
Reserved
Sent as 0; 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.
The following types of DSR options are defined in this document for
use within a DSR header:
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- 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)
- 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 DSR option 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 hop 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 - 2) / 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 DSR option 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 | Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 3.4.2), 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 node indicated by the Route
Reply (Address[n]) is actually in a network external to the
DSR network; the exact sequence of hops leading to it outside
the DSR network is not represented in the Route Reply. Nodes
caching this hop in their Route Cache MUST flag the cached hop
with the External flag. Such hops MUST NOT be returned in a
cached Route Reply generated from this Route Cache entry, and
selection of routes from the Route Cache to route a packet
being sent SHOULD prefer routes that contain no hops flagged as
External.
Reserved
Sent as 0; ignored on reception.
Identification
Copied from the Identification field of the Route Request for
which this Reply is sent in response. Sent as 0 if the Route
Reply is not sent in response to a Route Request (a gratuitous
Route Reply).
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 - 3) / 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 DSR option 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. Sent as 0; ignored on reception.
Salvage
A 4-bit unsigned integer. Copied from the Salvage field in the
Source Route option of the packet triggering the Route Error,
incremented by the node returning the Route Error.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 DSR option 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 Request Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 nonzero value and
is copied into the Identification field of the Acknowledgement
option when returned by the node receiving the packet over this
hop.
ACK Request Source Address
The address of the node requesting the acknowledgment.
An Acknowledgement Request option MUST NOT appear more than once
within a DSR header.
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5.6. Acknowledgment Option
The Acknowledgment DSR option 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. Source Route Option
The Source Route DSR option 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 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 node indicated by the Source
Route option is actually in a network external to the DSR
network; the exact sequence of hops leading from it outside the
DSR network are not represented in the Source Route option.
Nodes caching this hop in their Route Cache MUST flag the
cached hop with the External flag. Such hops MUST NOT be
returned in a Route Reply generated from this Route Cache
entry, and selection of routes from the Route Cache to route
a packet being sent SHOULD prefer routes that contain no hops
flagged as External.
Last Hop External (L)
Set to indicate that the last hop indicated by the Source Route
option is actually in a network external to the DSR network;
the exact sequence of hops leading to it outside the DSR
network are not represented in the Source Route option. Nodes
caching this hop in their Route Cache MUST flag the cached
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hop with the External flag. Such hops MUST NOT be returned
in a Route Reply generated from this Route Cache entry, and
selection of routes from the Route Cache to route a packet
being sent SHOULD prefer routes that contain no hops flagged as
External.
Reserved
Sent as 0; 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.
When forwarding a packet along a DSR source route using a Source
Route option in the packet's DSR header, the Source 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 Source Route option MUST examine the indicated source
route to determine if it is the intended next hop 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 DSR option 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 nodes receiving packets
containing a DSR header.
The total length of a DSR header, indicated by the Payload Length
field in the DSR header MUST be a multiple of 4 octets. 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 DSR option 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 nodes receiving packets
containing a DSR header.
The total length of a DSR header, indicated by the Payload Length
field in the DSR header MUST be a multiple of 4 octets. 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.
- If the packet contains a Route Request option, then replace the
IP Destination Address field with the IP "limited broadcast"
address (255.255.255.255) [3].
- Else, this node must have a route to the Destination Address
of the packet (since otherwise a Route Request would have
been added to the 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 of the route,
then insert a DSR header as described in Section 6.1.2, and
insert a Source Route option, as described in Section 6.1.3. The
source route in the packet is initialized from the route to the
Destination Address found in the Route Cache.
- Transmit the packet to the address given in the next hop, 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
(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.
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- Set the Protocol field of the packet's IP header to the Protocol
number assigned for a DSR header (???).
6.1.3. Adding a Source Route Option to a Packet
A node originating a packet adds a Source Route option to the packet,
if necessary, in order to carry the source route of hops from this
originating node to the final destination address of the packet.
Specifically, the node adding the Source Route option constructs
the Source Route option and modifies the IP packet according to the
following sequence of steps:
- A Source Route option, as described in Section 5.7, is created
and appended 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 Destination Address from the IP header is copied into
Address[n] in the DSR Source Route option.
- The first hop of the source route for the packet is copied into
the Destination Address field in the IP header.
- The remaining hops of the source route for the packet are copied
into sequential Address[i] fields in the Source Route option,
for i = 1, 2, ..., n-1.
- The First Hop External (F) bit in the Source Route option is
copied from the External bit flagging the first hop node in the
source route for the packet, as indicated in the Route Cache.
- The Last Hop External (L) bit in the Source Route option is
copied from the External bit flagging the last hop node in the
source route for the packet, as indicated in the Route Cache.
6.1.4. Receiving a Packet
When a node receives any packet containing a DSR header, it MUST
process the packet according to the following sequence of steps:
- If the Destination Address in the packet's IP header matches
one of this receiving node's own IP address(es), remove the DSR
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header and all the included DSR options in the header, and pass
the rest of the packet to the network layer.
- Examine and process each of the options (if any) in the DSR
header in the order in which they occur in the packet, skipping
over any Pad1 or PadN options.
Any DSR routing information carried in a packet SHOULD be examined
and reflected in the node's Route Cache, even if the options in
the packet are not otherwise processed as described above. In
particular, the following routing information SHOULD be handled in
this way:
- In a Route Request option, the accumulated route record,
represented by the IP Source Address of the packet and by the
sequence of Address[i] entries in the Route Request option SHOULD
be added to the node's Route Cache.
- In a Route Reply option, the route record being returned,
represented by the sequence of Address[i] entries in the Route
Request option and by the Destination Address in the packet's IP
header SHOULD be added to the node's Route Cache.
- In an Acknowledgement option, the single link from the
ACK Source Address to the ACK Destination Address SHOULD be added
to the node's Route Cache.
- In a Route Error option, the single link from the
Error Source Address to the Unreachable Node Address MUST
be removed from the node's Route Cache.
- In a Source Route option, the indicated source route SHOULD
be added to the node's Route Cache, subject to the conditions
identified in Section 3.3.1. The full sequence of hops in the
DSR Source Route option is as follows:
* The Source Address in the packet's IP header is the first hop
(the sender of the packet).
* The sequence of hops
Address[1], Address[2], ..., Address[n]
follow immediately after the IP Source Address in the source
route, where n is the number of addresses in the packet, or
(Opt Data Len - 2) / 4.
* The Destination Address in the packet's IP header is the
final destination of the packet and is the last hop of the
source route.
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In addition to the processing of received packets described above, a
node SHOULD examine the packet to determine if the receipt of this
packet indicates an opportunity for automatic route shortening, as
described in Section 3.4.2. If the received packet satisfies the
tests described there, then this node SHOULD perform the following
sequence of steps:
- Return a gratuitous Route Reply to the IP Source Address of the
packet, as described in Section 3.4.2.
- Discard the received 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.
6.1.5. Processing a Received Source Route Option
If a received packet contains a DSR header with a DSR Source Route
option, the Source Route option MUST be examined and processed (even
though this node is not indicated in the Destination Address field of
the packet's IP header).
If, after processing a Source Route option in a received packet, an
intermediate node determines that the packet is to be forwarded onto
a link whose link MTU 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 [23].
A Source Route option in a DSR header for IPv4 is processed according
to the following sequence of steps:
- If the value of the Segments Left field in the Source Route
option equals 0, then remove the Source Route option from the DSR
header.
- Else, let n equal (Opt Data Len - 2) / 4. This is the number of
addresses in the Source Route option.
- If the value of the Segments Left field is greater than n, then
send an ICMP Parameter Problem, Code 0, message [23] to the IP
Source Address, pointing to the Segments Left field, and discard
the packet. Do not process the 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.
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- If Address[i] or the IP Destination Address is a multicast
address, then discard the packet. Do not process the Source
Route option further.
- 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 [24, 3]. In this
forwarding of the packet, the next hop node (identified by
Address[i]) MUST be treated as a direct neighbor node; the
transmission to that next node MUST be done in a single IP
forwarding hop, without Route Discovery and without searching the
Route Cache.
- In forwarding the packet, perform Route Maintenance for the next
hop of the packet, by verifying that the packet was received by
that next hop, as described in Section 6.3.
Multicast addresses MUST NOT appear in a Source Route option or in
the IP Destination Address field of a packet carrying a 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.
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 MUST be doubled, up to a maximum of
MAX_REQUEST_PERIOD, 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 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 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 last Route
Request 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.
These values MUST be used to implement an exponential back-off
algorithm to limit the rate at which this node initiates new
Route Discoveries for the same target address. Until a valid
Route Reply is received for this target node address, the timeout
between consecutive Route Discovery initiations for this target
node SHOULD increase by doubling the timeout value on each new
initiation.
The behavior of a node processing a packet containing DSR header with
both a Source Route option and a Route Request option is unspecified.
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Packets SHOULD NOT contain both a Source Route option and a Route
Request option.
Packets containing a Route Request option SHOULD NOT be
retransmitted, SHOULD NOT request a DSR acknowledgment by including
an Acknowledgement Request option, SHOULD NOT expect a passive
acknowledgment, and SHOULD NOT be placed in the Retransmission
Buffer. The repeated transmission 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, the
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 hops
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 node MUST then continue processing the rest of the packet
normally. The node in this case MUST NOT retransmit the Route
Request to propagate it to other nodes. Do not process the Route
Request option further.
- 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
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
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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:
* Add an entry for this Route Request in its cache of
(Identification, target address) values of recently received
Route Requests.
* Create a copy of this entire packet and perform the following
steps on the copy of the packet.
* 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).
* 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.
* 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 BROADCAST_JITTER.
6.2.3. Generating Route Replies using the Route Cache
As described in Section 3.3.2, it is possible for a node processing a
received Route Request to avoid propagating the Route Request further
toward the target of the Request, if this node has in its Route Cache
a route from itself to this target. Such a Route Reply generated by
a node from its own cached route to the target of a Route Request is
called a "cached Route Reply", and this mechanism can greatly reduce
the overall overhead of Route Discovery on the network by reducing
the flood of Route Requests. The general processing of a received
Route Request is described in Section 6.2.2; this section specifies
the additional requirements that MUST be met before a cached Route
Reply may be generated and returned and specifies the procedure for
returning such a cached Route Reply.
While processing a received Route Request, for a node to possibly
return a cached Route Reply, it MUST have in its Route Cache a route
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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 hops
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 hops 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.
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
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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 addresses of the source route are copied into
the Address[i] fields of the option. Address[1] MUST be set
to the first hop of the route after the initiator of the Route
Discovery, Address[n] MUST be set to the last hop of the source
route (the address of the target node), and each other Address[i]
MUST be set to the next address in sequence in the source route
being returned.
The Destination Address field in the IP header of the packet carrying
the Route Reply option MUST be set to the address of the initiator
of the Route Discovery (i.e., for a Route Reply being returned in
response to some Route Request, the IP Source Address of the Route
Request).
After creating and initializing the Route Reply option and the IP
packet containing it, send the Route Reply. In sending the Route
Reply from this node (but not from nodes forwarding the Route Reply),
this node SHOULD delay the rely by a small jitter period chosen
randomly between 0 and BROADCAST_JITTER milliseconds.
If the MAC layer above which DSR is operating requires
bidirectionality for unidirectional transmissions, the Route
Reply MUST be sent by reversing the sequence of hops that are stored
in it.
If sending a Route Reply to the originator of the Route Request
requires performing a Route Discovery, the Route Reply Option MUST
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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 originator of the original Route
Request) must do another Route Request in order to return its Route
Reply.
If sending the Route Reply to the originator of the Route Request
does not require performing Route Discovery, a node SHOULD send a
unicast Route Reply in response to every received Route Request
targeted at it.
6.2.5. Processing a Route Reply Option
Upon receiving a Route Reply, a node SHOULD extract the source route
from the Route Reply and add this routing information to its Route
Cache. The source route from the Route Reply is the sequence of hops
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 hop Address[n] in its Route Cache as External.
Each packet in the Send Buffer SHOULD then be checked to see whether
the information in the Route Reply and now in the Route Cache allows
it to be sent immediately.
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6.3. Route Maintenance Processing
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. Route Maintenance for this route is
used only when S is actually sending packets to D.
When forwarding a packet, a node MUST attempt to receive an
acknowledgement for the packet from the next hop. If no
acknowledgement is received, the node SHOULD return a Route Error to
the IP Source Address of the packet, as described in Section 6.3.3.
A node's algorithm for deciding whether or not to return a Route
Error MUST NOT allow any node to attempt to send an unbounded number
of packets along a broken link without receiving a Route Error.
6.3.1. Using Network-Layer Acknowledgments
When a node retransmits a packet or has no other way to ensure
successful delivery of a packet to the next hop, it MUST request a
network-layer acknowledgement by placing inserting an Acknowledgement
Request the DSR header. The Identification value contained in that
header MUST be unique over all packets delivered to the same next hop
which are either unacknowledged or recently acknowledged.
A node receiving an Acknowledgement Request MUST send an
acknowledgement to the previous hop by performing the following
sequence of steps:
- Create a packet and set the IP Source Address to the address
of this node, the IP Destination Address to the address of the
previous hop, and the IP Protocol field to the protocol number
reserved for DSR headers.
- Set the DSR header's Next Header field to be the "No Next Header"
value.
- Set the Acknowledgement option's Option Type field to 6, and the
Opt Data Len field to 10.
- Copy the Identification field from the 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 and the ACK Destination Address field to the IP
Destination Address.
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- Send the packet as described in Section 6.1.1.
6.3.2. Using Link Layer Acknowledgments
If explicit failure notifications are provided by the link layer,
then all packets are assumed to be correctly received by the
next hop, and a Route Error is sent only when an explicit failure
notification is made from the link layer.
Nodes receiving a packet without an Acknowledgement Request Option
do not need to send an explicit Acknowledgment to the packet's
originator, since the link layer will notify the originator if the
packet was not received properly.
6.3.3. Originating a Route Error
When a node is unable to verify successful delivery of a packet to
the next hop after a maximum number of retransmission attempts,
a node SHOULD send a Route Error to the IP Source Address of the
packet. In addition, a node's algorithm for deciding whether or not
to return a Route Error MUST NOT allow any node to attempt to send
an unbounded number of packets along a broken link without receiving
a Route Error. When sending a Route Error for a packet containing
either a Route Error option or an Acknowledgement option, a node
SHOULD add these options to its Route Error, subject to some limit on
lifetime. Specifically, we define the "salvage count" of an option
to be the sum of one plus the salvage count recorded in the Source
Route option plus the sum of the salvage counts of any Route Errors
preceding that option.
A node transmitting a Route Error MUST follow the following steps:
- Create a packet and set the IP Source Address to the address of
this node, the IP Destination Address to the address IP Source
Address of the packet experiencing the error.
- Insert a DSR header into the packet.
- Add a Route Error Option, setting the Error Type to
NODE_UNREACHABLE, the Reserved bits to 0, the Salvage value to
one plus the Salvage value from the DSR Source Route option, and
the Unreachable Node Address to the address of the next hop. Set
the Error Source Address to the IP Source Address and the Error
Destination to the IP Destination Address.
- The node MAY append each Route Error and Acknowledgement
option, in order, from the packet experiencing the error,
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though it MUST exclude options with salvage counts greater
than MAX_SALVAGE_TIMES.
- Send the packet as described in Section 6.1.1.
6.3.4. Processing a Route Error Option
A node receiving a Route Error MUST process it as follows:
- Delete all routes from the Route Cache that have a link from the
Route Error Source Address to the Unreachable Node Address.
- If the option following the Route Error is an Acknowledgement
or Route Error option sent by this node (that is, with
Acknowledgement or Error Source Address equal to this node's
address), copy the DSR options following the current Route
Error into a new packet with IP Source Address equal to this
node's own IP address and IP Destination Address equal to the
Acknowledgement or Error Destination Address. Transmit this
packet as described in Section 6.1.1, with the salvage count in
the Source Route option set to the Salvage value of the Route
Error.
6.3.5. Salvaging a Packet
When a node is unable to verify successful delivery of a packet
to the next hop after a maximum number of retransmission attempts
and has transmitted a Route Error to the sender, it MAY attempt to
salvage the packet by examining its route cache. If the node can
find a route to the packet's IP Destination Address in its own Route
Cache, then this node replaces the packet's Source Route option
with a new Source Route option in the same way as described in
Section 6.1.3, except that Address[1] MUST be set to the address of
this node and the Salvage field MUST be set to 1 plus the value of
the Salvage field in the Source Route option that caused the error.
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7. Constants
BROADCAST_JITTER 10 milliseconds
MAX_ROUTE_LEN 15 nodes
MAX_SALVAGE_TIMES 15 salvages
Route Cache
ROUTE_CACHE_TIMEOUT 300 seconds
Send Buffer
SEND_BUFFER_TIMEOUT 30 seconds
Route Request Table
REQUEST_TABLE_SIZE 64 nodes
REQUEST_TABLE_IDS 16 identifiers
MAX_REQUEST_REXMT 16 retransmissions
MAX_REQUEST_PERIOD 10 seconds
REQUEST_PERIOD 500 milliseconds
NONPROP_REQUEST_TIMEOUT 30 milliseconds
Retransmission Buffer
DSR_RXMT_BUFFER_SIZE 50 packets
Retransmission Timer
DSR_MAXRXTSHIFT 2
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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. 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 [24], IPv6 [7], and IPX [27] nodes.
- Historically [12, 13], DSR grew from our contemplation of
a multi-hop propagating version of the Internet's Address
Resolution Protocol (ARP) [22], as well as from the routing
mechanism used in IEEE 802 source routing bridges [21]. 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 [12, 13], 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 [19]
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 B. Implementation and Evaluation Status
The DSR protocol has been implemented under the FreeBSD 2.2.7
operating system running on Intel x86 platforms. FreeBSD is based
on a variety of free software, including 4.4 BSD Lite from the
University of California, Berkeley. For the environments in which
we used it, this implementation is functionally equivalent to the
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 a actively
mobile ad hoc network under realistic communication workloads.
The last week of February and the first week of March 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 [19].
The software was ported to FreeBSD 3.3, and a preliminary version
of Quality of Service (QoS) support was added. A demonstration of
this modified version of DSR was presented in July 2000. Those 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.
The DSR protocol has been extensively studied using simulation; we
have implemented DSR in the ns-2 simulator [5, 18] and conducted
evaluations of different caching strategies documented in this
draft [9].
Several 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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Acknowledgements
The protocol described in this draft has been designed and developed
within the Monarch Project, a research project at Rice University and
Carnegie Mellon University which is developing adaptive networking
protocols and protocol interfaces to allow truly seamless wireless
and mobile node networking [14, 6].
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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References
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[3] Robert T. Braden, editor. Requirements for Internet
hosts---communication layers. RFC 1122, October 1989.
[4] Scott Bradner. Key words for use in RFCs to indicate
requirement levels. RFC 2119, March 1997.
[5] Josh Broch, David A. Maltz, David B. Johnson, Yih-Chun Hu,
and Jorjeta Jetcheva. A performance comparison of multi-hop
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version 6 (IPv6) specification. RFC 2460, December 1998.
[8] Ralph Droms. Dynamic Host Configuration Protocol. RFC 2131,
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[25] J. B. Postel, editor. Transmission Control Protocol. RFC 793,
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Chair's Address
The MANET Working Group can be contacted via its current chairs:
M. Scott Corson Phone: +1 301 405-6630
Institute for Systems Research Email: corson@isr.umd.edu
University of Maryland
College Park, MD 20742
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
Johnson, et al Expires 2 September 2001 [Page 60]
