IETF MANET Working Group David B. Johnson, Rice University
INTERNET-DRAFT David A. Maltz, AON Networks
17 November 2000 Yih-Chun Hu, Carnegie Mellon University
Jorjeta G. Jetcheva, Carnegie Mellon University
The Dynamic Source Routing Protocol for Mobile Ad Hoc Networks
<draft-ietf-manet-dsr-04.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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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
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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 . . . . . . . . . . 12
3.4.1. Packet Salvaging . . . . . . . . . . . . . . . . 12
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. Packet Formats 20
5.1. Destination Options Header . . . . . . . . . . . . . . . 21
5.1.1. DSR Route Request Option . . . . . . . . . . . . 22
5.2. Hop-by-Hop Options Header . . . . . . . . . . . . . . . . 24
5.2.1. DSR Route Reply Option . . . . . . . . . . . . . 25
5.2.2. DSR Route Error Option . . . . . . . . . . . . . 27
5.2.3. DSR Acknowledgment Option . . . . . . . . . . . . 29
5.3. DSR Routing Header . . . . . . . . . . . . . . . . . . . 30
6. Detailed Operation 33
6.1. General Packet Processing . . . . . . . . . . . . . . . . 33
6.1.1. Originating a Packet . . . . . . . . . . . . . . 33
6.1.2. Adding a DSR Routing Header to a Packet . . . . . 34
6.1.3. Receiving a Packet . . . . . . . . . . . . . . . 36
6.1.4. Processing a Routing Header in a Received Packet 38
6.2. Route Discovery Processing . . . . . . . . . . . . . . . 40
6.2.1. Originating a Route Request . . . . . . . . . . . 40
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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 . . . . . . . . . . . . 45
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 can 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 node S originates a new packet destined to some other
node D, it 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
D. Normally, S 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 D. In this case, we call S the
"initiator" and D 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"
message 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
message 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 message 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 a Route Request (such as node B in this
example), if it is the target of the Route Discovery, it returns
a "Route Reply" message 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 request identification and target address, or if it
finds that its own address is already listed in the route record in
the Route Request message, it discards the Request. Otherwise, this
node appends its own address to the route record in the Route Request
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message 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 broadcast the Request in turn, 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 node E replying back to A in this example, 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
its own Route Request message for A.
It is also possible to piggyback other small data packets, such as a
TCP SYN packet [26], on a Route Request using this same mechanism.
Node E could also 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 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 stamped with
the time that it was placed into the 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 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
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
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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 situation, 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 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 |-- | 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 packet
to forward 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
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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" message 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 Replys 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
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 |
+-----+ +-----+ +-----+ +-----+ +-----+
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As node C forwards a data packet along the route from A to E, it
can always add to its cache the presence of the "forward" direction
links that it learns from the headers of these packets, from itself
to D and from D to E. However, the "reverse" direction of the links
identified in the packet headers, from itself back to 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
could cache the link from itself to X, the link from X to Y, and the
link from Y to Z. If all links can be assumed to be bi-directional,
C could also cache the links from X to W and from W to V. Similar
considerations apply to the routing 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, it sets the route record to list the sequence of
hops over which this copy of the Route Request was forwarded to it,
concatenated with its own idea of the route from itself to the target
from its Route Cache.
However, before transmitting a Route Reply packet that was generated
using information from its Route Cache in this way, a node MUST
verify that the resulting route being returned in the Route Reply,
after this concatenation, contains no duplicate nodes listed in the
route record. For example, 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:
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+-----+ +-----+ +-----+ +-----+
| A |---->| B |- >| D |---->| E |
+-----+ +-----+ \ / +-----+ +-----+
\ /
\ +-----+ /
>| C |-
+-----+
| ^
v |
Route Request +-----+
Route: A - B - C - F | F | Cache: C - D - E
+-----+
The concatenation of the accumulated route 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 F in this case should have received
a Route Error if the route had previously stopped working. Second,
this limitation means that a Route Error traversing the route is very
likely to pass through any node that sent the Route Reply for the
route (including F), which helps to ensure that stale data is removed
from caches (such as at F) in a timely manner. Otherwise, the next
Route Discovery initiated by 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
bandwidth and possibly increasing the number of network collisions in
the area.
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For example, the figure below shows a situation in which nodes B, C,
D, E, and F all receive A's Route Request for target G, and each have
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, they would all attempt to reply from their own Route
Caches, and would all send their Replys 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 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 Replys giving routes of length less than h sending
their Replys before this node, and all nodes sending Route Replys
giving routes of length greater than h sending their Replys after
this node. Within the delay period, this node promiscuously receives
all packets, looking for data packets from the initiator of this
Route Discovery destined for the target of the 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,
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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 it 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.
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 it. To
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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 applying this route to the packet 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. 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
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
message 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
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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 Replys 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 doesn't have
another 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 message, ensuring that the Route Error message 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 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 a 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 DSR Routing Header and a DSR Route Reply option, and 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
Routing header or Route Reply 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.
- Each implementation of DSR at any node MAY choose any appropriate
strategy and algorithm for searching its Route Cache and
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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 will prefer routes that lead directly to the target
node instead of 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.
- 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 6.1.3)
represents a complete path (a sequence of links) leading to the
destination node. By caching each of these paths separately,
a "path cache" organization for the Route Cache can be formed.
A path cache is very simple to implement and easily guarantees
that all routes are loop-free, since each individual route from
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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, 19] and
through implementation of DSR in a mobile outdoor testbed under
significant workload [20, 21].
- Alternatively, a "link cache" organization could be used for the
Route Cache, in which each individual link 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
were recently originated or forwarded by this node. The table is
indexed by IP address.
The Route Request Table on a node records the following information
about nodes to which this node has initiated a Route Request:
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- The time that this node last originated a Route Discovery 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 Requests it initiates could appear to be
duplicates to the other nodes in the network.
4.3. Send Buffer
The Send Buffer of some node is a queue of packets that cannot be
transmitted by that node because it does not yet have a source
route to each respective packet's destination. Each packet in the
Send Buffer is stamped with the time that it is placed into the
Buffer, and SHOULD be removed from the Send Buffer and discarded
SEND_BUFFER_TIMEOUT seconds after initially being placed in the
Buffer. If necessary, a FIFO strategy SHOULD be used to evict
packets before they timeout to prevent the buffer from overflowing.
Subject to the rate limiting defined in Section 6.2, a Route
Discovery SHOULD be initiated as often as possible for the
destination address of any packets residing in the Send Buffer.
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4.4. Retransmission Buffer
The Retransmission Buffer of a node 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 5.3).
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 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. Packet Formats
Dynamic Source Routing makes use of four options carrying control
information that can be piggybacked in any existing IP packet. The
mechanism used to represent these options in a packet is based on
the design of the Hop-by-Hop and Destination Options mechanisms in
IPv6 [7]. The ability to generate and process such options must
be added to an IPv4 protocol stack. Specifically, the Protocol
field in the IP header is used to indicate that a Hop-by-Hop Options
extension header or Destination Options extension header follows the
IP header, and the Next Header field in the extension header is used
to indicate the type of protocol header (such as a transport layer
header) following the extension header.
In addition, DSR makes use of one additional header type, to carry
the source route for a packet. This DSR Routing header is based on
the design of the Routing header defined for IPv6 [7]. DSR defines
a new value for the Routing Type field to distinguish a DSR Routing
header from other types of Routing headers.
For IPv6, all extension headers are a multiple of 8 bytes in length.
However, for use in IPv4 packets, all extension headers only MUST be
a multiple of 4 bytes long. This requirement preserves the alignment
of any following extension headers and of any additional header
(e.g., a TCP header [26]) following the last extension header.
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5.1. Destination Options Header
The Destination Options extension header is used to carry optional
information that needs to be examined only by a packet's destination
node(s). The Destination Options extension header is identified by
a Next Header (or Protocol) value of 60 in the immediately preceding
header [7], and 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 | Hdr Ext Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
. .
. Options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header
8-bit selector. Identifies the type of header immediately
following the Destination Options header. Uses the same values
as the IPv4 Protocol field [27].
Hdr Ext Len
8-bit unsigned integer. Length of the Destination Options
header in 4-octet units, not including the first 8 octets.
Options
Variable-length field, of length such that the complete
Destination Options header is an integer multiple of 4 octets
long. Contains one or more TLV-encoded options.
The following destination option type is used by the DSR protocol:
- DSR Route Request option (Section 5.1.1)
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5.1.1. DSR Route Request Option
The DSR Route Request destination option is encoded in
type-length-value (TLV) format 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 | Option Length | 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 repropagate the Route Request MUST not
change this field.
Destination Address
MUST be set to the limited broadcast address (255.255.255.255).
Hop Limit (TTL)
Can 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
???. The top three bits of this Option Type value are equal to
011, meaning that a node that does not understand this option
MUST discard the packet, and that the Option Data may change
en-route [7].
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Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields.
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 forwarded.
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 number of addresses present in this
field is indicated by the Option Length field in the option
(n = (Option Length - 6) / 4). Each node repropagating the
Route Request adds its own address to this list, increasing the
Option Length value by 4.
The DSR Route Request destination option MUST NOT appear more than
once within any single Destination Options extension header.
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5.2. Hop-by-Hop Options Header
The Hop-by-Hop Options extension header is used to carry optional
information that must be examined by every node along a packet's
delivery path. The Hop-by-Hop Options extension header is identified
by a Protocol value of 0 in the IP header [7], and 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 | Hdr Ext Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
. .
. Options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header
8-bit selector. Identifies the type of header immediately
following the Hop-by-Hop Options header. Uses the same values
as the IPv4 Protocol field [27].
Hdr Ext Len
8-bit unsigned integer. Length of the Hop-by-Hop Options
header in 4-octet units, not including the first 8 octets.
Options
Variable-length field, of length such that the complete
Hop-by-Hop Options header is an integer multiple of 4 octets
long. Contains one or more TLV-encoded options.
If present in an IP packet, the Hop-by-Hop Options extension header
MUST appear in the packet immediately following the IP header.
The following hop-by-hop option types are used by the DSR protocol:
- DSR Route Reply option (Section 5.2.1)
- DSR Route Error option (Section 5.2.2)
- DSR Acknowledgment option (Section 5.2.3)
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5.2.1. DSR Route Reply Option
The DSR Route Reply hop-by-hop option is encoded in type-length-value
(TLV) format 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 | Option Length |L| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[2] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[n] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
???. The top three bits of this Option Type value are equal to
000, meaning that a node that does not understand this option
SHOULD ignore this option and continue processing the packet,
and that the Option Data does not change en-route [7].
Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields.
Last Hop External (L)
Set to indicate that the last node indicated by the Route Reply
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 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 nodes flagged as External.
Reserved
Sent as 0; ignored on reception.
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Address[1..n]
The source route being returned by the Route Reply, indicating
a route from the node with address Address[1] to the node with
address Address[n]. The number of addresses present in this
field is indicated by the Option Length field in the option
(n = (Option Length - 1) / 4).
A DSR Route Reply destination option MAY appear one or more times
within a single Hop-by-Hop Options extension header.
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5.2.2. DSR Route Error Option
The DSR Route Error hop-by-hop option is encoded in type-length-value
(TLV) format 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 | Option Length | Error Type |Reservd|Salvage|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unreachable Node Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
???. The top three bits of this Option Type value are equal to
000, meaning that a node that does not understand this option
SHOULD ignore this option and continue processing the packet,
and that the Option Data does not change en-route [7].
Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields.
For the current definition of the DSR Route Error option, this
field MUST be set to 13. Extensions to the DSR Route Error
option format may be included after the fixed portion of the
DSR Route Error option specified above. The presence of such
extensions will be indicated by the Option Length field. When
the Option Length is greater than 13 octets, the remaining
octets are interpreted as extensions. Currently, no extensions
have been defined.
Error Type
The type of error encountered. Currently, the following type
value is defined:
NODE_UNREACHABLE 1
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
DSR Routing header 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 (e.g., the node that generated the routing
information claiming that the hop Error Source Address to
Unreachable Node Address was a valid hop).
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 DSR Route Error destination option MAY appear one or more times
within a single Hop-by-Hop Options extension header.
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5.2.3. DSR Acknowledgment Option
The DSR Acknowledgment hop-by-hop option is encoded in
type-length-value (TLV) format 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 | Option Length | Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACK Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACK Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
???. The top three bits of this Option Type value are equal to
000, meaning that a node that does not understand this option
SHOULD ignore this option and continue processing the packet,
and that the Option Data does not change en-route [7].
Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields.
Identification
Copied from the Identification field of the DSR Routing header
of the packet being acknowledged.
ACK Source Address
The address of the node originating the DSR Acknowledgment.
ACK Destination Address
The address of the node to which the DSR Acknowledgment is to
be delivered.
A DSR Acknowledgement destination option MAY appear one or more times
within a single Hop-by-Hop Options extension header.
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5.3. DSR Routing Header
As specified for IPv6 [7], a Routing header is used by a source to
list one or more intermediate nodes to be "visited" on the way to
a packet's destination. This function is similar to IPv4's Loose
Source and Record Route option, but the Routing header does not
record the route taken as the packet is forwarded. The specific
processing steps required to implement the Routing header must be
added to an IPv4 protocol stack. The Routing header is identified by
a Next Header value of 43 in the immediately preceding header, and
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 | Hdr Ext Len | Routing Type | Segments Left |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. type-specific data .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The type-specific data for a Routing Header carrying a DSR Source
Route is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F|L| Reserved |Salvage| Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[2] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[n] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Routing header fields:
Next Header
8-bit selector. Identifies the type of header immediately
following the Routing header.
Hdr Ext Len
8-bit unsigned integer. Length of the Routing header in
4-octet units, not including the first 8 octets.
Routing Type
???
Segments Left
Number of route segments remaining, i.e., number of explicitly
listed intermediate nodes still to be visited before reaching
the final destination.
Type-specific fields:
First Hop External (F)
Set to indicate that the first node indicated by the Routing
header 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 Routing header. 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 Routing
header 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 Routing header. 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.
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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).
Identification
Used to request that a DSR Acknowledgement option be returned
to this transmitting node for this hop. The special value of 0
indicates that no DSR Acknowledgement is requested. Otherwise,
the Identification field is set to a unique nonzero number
by this node transmitting the packet and is copied into the
Identification field of the DSR Acknowledgement option when
returned by the node receiving the packet over this hop.
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 6.1.2 and 6.1.4.
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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 Routing header into the packet, as described
in Section 6.1.2. 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 IP
Destination Address, using Route Maintenance to retransmit the
packet if necessary, as described in Section 6.3.
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6.1.2. Adding a DSR Routing Header to a Packet
The design of the DSR Routing header is based on the design of a
Routing header in IPv6 [7]. A node originating a packet adds a
DSR Routing header 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
DSR Routing header constructs the Routing header and modifies the IP
packet according to the following sequence of steps:
- A DSR Routing header, as described in Section 5.3, is created
and added to the packet after the IP header and any Hop-by-Hop
Options header that may already be in the packet, but before any
Destination Options header (e.g., containing a DSR Route Reply
option) that may be present.
- The number of Address fields to include in the DSR Routing
header (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 Routing header is initialized
equal to n.
- The Source Address from the IP header is copied into Address[n]
in the DSR Routing header.
- The first hop of the source route for the packet is copied into
the Source 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 DSR routing header,
for i = 1, 2, ..., n-1.
- The First Hop External (F) bit in the Routing header 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 Routing header is copied
from the External bit flagging the last hop node in the source
route for the packet, as indicated in the Route Cache.
- All other fields in the type-specific data in the DSR Routing
header are initialized to 0.
- The Routing Type field in the DSR Routing header is initialized
to ???.
- The Hdr Ext Len field in the DSR Routing header is initialized
to 4.
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- Next Header field in the DSR Routing header is set equal to the
current value in the Protocol field in the IP header (or the
Next Header field in the preceding extension header), and the
Protocol field (or preceding Next Header field) is set equal
to 43 to indicate a Routing header extension header [7].
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6.1.3. Receiving a Packet
When a node receives any packet, it MUST process the packet according
to the following sequence of steps:
- If the Destination Address in the packet's IP header does not
match any of this receiving node's own IP address(s), then the
processing of this packet depends on whether the packet contains
a DSR Routing header:
* If the packet contains a DSR Routing header, then discard the
packet.
* Else, if the packet contains a Hop-by-Hop Options extension
header (if present, this MUST immediately follow the packet's
IP header), then process the options contained in the
Hop-by-Hop Options extension header. Forward the packet
using normal IP forwarding proceedures and do not process the
packet further.
- Examine and process each of the extension headers (if any) in
the packet in the order in which they occur in the packet. By
dispatching on the Protocol field in the packet's IP header,
and subsequently dispatching on the Next Header field of each
encountered extension header, the appropriate protocol module is
executed by the receiving node for each extension header.
- If a Hop-by-Hop Options extension header or Destination Options
extension headers is encountered in processing the packet, the
receiving node MUST process any options given in this header in
the order in which they occur in the Options field within the
option.
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 DSR 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 DSR 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.
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- In a DSR 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 DSR 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 DSR Routing header, 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 Routing header is as follows:
* The Source Address in the packet's IP header is the first hop
(the sender of the packet).
* Let n equal Hdr Ext Len. This is the number of addresses in
the Routing header. Let i equal n minus Segments Left.
* The sequence of hops
Address[1], Address[2], ..., Address[i]
follow immediately after the IP Source Address in the source
route.
* The Destination Address in the packet's IP header follows
immediately next in the source route.
* The sequence of hops
Address[i+1], Address[i+2], ..., Address[n]
follow next in the source route. The address Address[n]
above is the final hop in the source route.
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
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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.
6.1.4. Processing a Routing Header in a Received Packet
A Routing header in a packet is not examined or processed until the
packet reaches the node identified in the Destination Address field
in the packet's IP header. In that node, dispatching on the Protocol
field in the packet's IP header (or the Next Header field in the
preceding extension header) causes the Routing header module in that
node's IP implementation to be invoked. The node then examines the
Routing Type field in the Routing header to determine the specific
type of processing for that type of Routing header. The processing
for a Routing header here in general follows the procedures specified
for IPv6 Routing headers, and the processing specifically for a DSR
Routing header in general follows the general procedures specified
for a Type 0 Routing header in IPv6 [7].
If, while processing a received packet, a node encounters a Routing
header with an unrecognized Routing Type value, the required behavior
of the node depends on the value of the Segments Left field, as
follows:
- If Segments Left is 0, the node MUST ignore the Routing header
and proceed to process the next header in the packet, whose type
is identified by the Next Header field in the Routing header.
- If Segments Left is non-zero, the node MUST discard the packet
and send an ICMP Parameter Problem, Code 0, message [24] to
the packet's Source Address, pointing to the unrecognized
Routing Type.
If, after processing a Routing header 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 [24].
A DSR Routing header is identified by a Routing Type value of ???
in the Routing header. A DSR Routing header for IPv4 is processed
according to the following sequence of steps:
- If the value of the Segments Left field in the Routing header
equals 0, then proceed to process the next header in the packet,
whose type is identified by the Next Header field in the Routing
header. Do not process the Routing header further.
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- Else, let n equal Hdr Ext Len. This is the number of addresses
in the Routing header.
- If the value of the Segments Left field is greater than n, then
send an ICMP Parameter Problem, Code 0, message [24] to the IP
Source Address, pointing to the Segments Left field, and discard
the packet. Do not process the Routing header further.
- Else, decrement the value of the Segments Left field by 1. Let i
equal n minus Segments Left. This is the index of the next
address to be visited in the Address vector.
- If Address[i] or the IP Destination Address is a multicast
address, then discard the packet. Do not process the Routing
header further.
- Else, swap the IP Destination Address and Address[i].
- Forward the packet to the IP address specified in the
Destination Address 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 [25, 3]. In
this forwarding of the packet, the next hop node (identified by
the Destination Address) 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 DSR Routing header or in
the IP Destination Address field of a packet carrying a DSR Routing
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 DSR
Route Request (Section 5.1.1) and a DSR Route Reply (Section 5.2.1),
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 extension headers as described in Section 5:
a Route Request is carried in a Destination options extension header,
and a Route Reply is carried in a Hop-by-Hop options extension
header.
A Route Discovery for a destination SHOULD NOT be initiated unless
the initiating node has a packet in the 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.
6.2.1. Originating a Route Request
A node initiating a Route Discovery for some target creates and
initializes a DSR Route Request option 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 Destination Options
extension header in the packet. To initialize the Route Request
option, the node performs the following sequence of steps:
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- The Option Type in the option MUST be set to the value ???.
- The Option Length 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 Option Length field excludes the size of the
Option Type and Option Length 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.
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The behavior of a node processing a packet containing both a Routing
Header and a Route Request Destination option is unspecified.
Packets SHOULD NOT contain both a Routing Header and a Route Request
Destination option. [This is not exactly true: A Route Request
option appearing in the second Destination Options header that IPv6
allows after the Routing Header would probably do-what-you-mean,
though we have not triple-checked it yet. Namely, it would allow the
originator of a route discovery to unicast the request to some other
node, where it would be released and begin the flood fill. We call
this a Route Request Blossom since the unicast portion of the path
looks like a stem on the blossoming flood-fill of the request.]
Packets containing a Route Request Destination option SHOULD NOT be
retransmitted, SHOULD NOT request an explicit DSR Acknowledgment by
setting the R bit, SHOULD NOT expect a passive acknowledgment, and
SHOULD NOT be placed in the Retransmission Buffer. The repeated
transmission of packets containing a Route Request Destination 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 packet normally,
including any following options or extension headers in the
packet. The node 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.
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- Else, the node MUST search its Route Request Table for an entry
for the initiator of this Route Request (the IP Source Address
field). If such an entry is found in the table, the node MUST
search the cache of Identification values of recently received
Route Requests in that table entry, to determine if an entry
is present in the cache matching the Identification value
and target node address in this Route Request. If such an
(Identification, target address) entry is found in this cache in
this entry in the Route Request Table, then the node MUST discard
the entire packet carrying the Route Request option.
- Else, this node SHOULD repropagate this Route Request. If it
does so, the node MUST do so 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
Option Length 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
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the overall overhead of Route Discovery on the network by reducing
the flood of Route Requests. The general processing of a received
Route Request is described in Section 6.2.2; this section specifies
the additional requirements that MUST be met before a cached Route
Reply may be generated and returned and specifies the procedure for
returning such a cached Route Reply.
While processing a received Route Request, for a node to possibly
return a cached Route Reply, it MUST have in its Route Cache a route
from itself to the target of this Route Request. However, before
generating a cached Route Reply for this Route Request, the node MUST
verify that there are no duplicate addresses listed in the route
accumulated in the Route Request together with the route from this
node's Route Cache. Specifically, there MUST be no duplicates among
the following addresses:
- The IP Source Address of the packet containing the Route Request,
- The Address[i] fields in the Route Request, and
- The nodes listed in the route obtained from this node's Route
Cache, excluding the address of this node itself (this node
itself is the common point between the route accumulated in the
Route Request and the route obtained from the Route Cache).
If any duplicates exist among these addresses, then the node MUST NOT
send a cached Route Reply. The node SHOULD continue to process the
Route Request as described in Section 6.2.2.
If the Route Request and the route from the Route Cache meet the
restriction above, then the node SHOULD construct and return a cached
Route Reply as follows:
- 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.
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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 DSR Route
Reply option (Section 5.2.1). The Route Reply option MAY be returned
to the initiator of the Route Request in a separate IP packet, used
only to carry this Route Reply option, or it MAY be included in any
other IP packet being sent to this address.
The Route Reply option MUST be included in a Hop-by-Hop Options
extension 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 ???.
- The Option Length field in the option MUST be set to the value
(n * 4) + 1, 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 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 DSR Route Reply option and
the IP packet containing it, send the Route Reply, jittered by
T milliseconds, where T is a uniformly distributed random number
between 0 and BROADCAST_JITTER.
If sending a Route Reply to the originator of the Route Request
requires performing a Route Discovery, the Route Reply hop-by-hop
option MUST be piggybacked on the packet that contains the Route
Request. This piggybacking prevents a loop wherein the target of the
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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
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 SHOULD request
a network-layer acknowledgement by placing a non-zero value in the
Identification field of the DSR Routing header. Such a value MUST
be unique over all packets delivered to the same next hop which are
either unacknowledged or recently acknowledged.
A node receiving a DSR Routing header with a non-zero value in the
Identification field 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 Hop-by-Hop Options extension headers.
- Set the Hop-by-Hop Options extension header's Next Header field
to be the "No Next Header" value. Set the Header Extension
Length to the size of a DSR Acknowledgement Option.
- Set the DSR Acknowledgement option's Option Type field to
the Option Type reserved for DSR Acknowledgements, and the
Option Length field to 10.
- Copy the Identification field from the Routing Header into
the Identification field in the DSR 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.
- Send the packet as described in Section 6.1.1.
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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 a Routing Header 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. When sending a Route Error for a packet containing either a
DSR Route Error option or a DSR Acknowledgement option, a node SHOULD
add these options to it's 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 DSR
Routing header plus the sum of the salvage counts of any DSR 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 Hop-by-Hop Options 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 Routing header, 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 DSR Route Error and DSR Acknowledgement,
in order, from the packet experiencing the error, though it MUST
exclude options with salvage counts greater than 15.
- Send the packet as described in Section 6.1.1.
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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 Hop-by-Hop option following the Route Error is a DSR
Acknowledgement or DSR Route Error option sent by this node
(that is, with Acknowledgement or Error Source Address equal to
this node's address), copy the Hop-by-Hop 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 DSR Routing header 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 Routing header with a new
Routing Header in the same way as described in Section 6.1.2, 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 Routing Header that caused the error.
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7. Constants
BROADCAST_JITTER 10 milliseconds
MAX_ROUTE_LEN 15 nodes
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 in IPv4 of the Destination Options
extension header, the Hop-by-Hop Options extension header, and
Routing header, which were originally defined for IPv6 [7]. The
Next Header values indicating these three extension header types (60,
0, and 43, respectively) must therefore be reserved within the IPv4
Protocol number space. In addition, the "No Next Header" type value
of 69, defined for IPv6, must also be defined for use in IPv4. Other
protocols in IPv4 wishing to use these IPv6-style extension headers
can also make use of these Protocol number assignments.
For use within a Destination Options extension header, this document
defines one new type of destination option, which must be assigned an
Option Type value:
- DSR Route Request option, described in Section 5.1.1. The top
three bits of this Option Type value MUST be 011.
For use within a Hop-by-Hop Options extension header, this document
defines three new types of hop-by-hop options, each of which must be
assigned an Option Type value:
- DSR Route Reply option, described in Section 5.2.1. The top
three bits of this Option Type value MUST be 000.
- DSR Route Error option, described in Section 5.2.2. The top
three bits of this Option Type value MUST be 000.
- DSR Acknowledgment option, described in Section 5.2.3. The top
three bits of this Option Type value MUST be 000.
For use within a Routing header, this document defines one new type
of routing header, which must be assigned an Routing Type value:
- DSR Routing Header, defined in Section 5.3.
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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 [25], IPv6 [7], and IPX [28] nodes.
- Historically [12, 13], DSR grew from our contemplation of
a multi-hop propagating version of the Internet's Address
Resolution Protocol (ARP) [23], as well as from the routing
mechanism used in IEEE 802 source routing bridges [22]. 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 [20]
DSR as a layer 3 protocol, since this is the only layer at which we
could realistically support nodes with multiple network interfaces of
different types forming an ad hoc network.
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Appendix 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 [20].
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
seprate 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, 19] and conducted
evaluations of different caching strategies documented in this
draft [9].
Several independant 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,
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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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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.com
Palo Alto, CA 94306
USA
Yih-Chun Hu Phone: +1 412 268-3075
Carnegie Mellon University Fax: +1 412 268-5576
Computer Science Department Email: yihchun@cs.cmu.edu
5000 Forbes Avenue
Pittsburgh, PA 15213-3891
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 17 May 2001 [Page 60]
