IETF MANET Working Group David B. Johnson, Rice University
INTERNET-DRAFT David A. Maltz, Carnegie Mellon University
19 July 2004 Yih-Chun Hu, Rice University
The Dynamic Source Routing Protocol
for Mobile Ad Hoc Networks (DSR)
<draft-ietf-manet-dsr-10.txt>
Status of This Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC 2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note
that other groups may also distribute working documents as
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at
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material or to cite them other than as "work in progress".
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft is a submission to the IETF Mobile Ad Hoc
Networks (MANET) Working Group. Comments on this draft may be sent
to the Working Group at manet@itd.nrl.navy.mil, or may be sent
directly to the authors.
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Abstract
The Dynamic Source Routing protocol (DSR) is a simple and efficient
routing protocol designed specifically for use in multi-hop wireless
ad hoc networks of mobile nodes. DSR allows the network to be
completely self-organizing and self-configuring, without the need for
any existing network infrastructure or administration. The protocol
is composed of the two main mechanisms of "Route Discovery" and
"Route Maintenance", which work together to allow nodes to discover
and maintain routes to arbitrary destinations in the ad hoc network.
All aspects of the protocol operate entirely on-demand, allowing
the routing packet overhead of DSR to scale automatically to only
that needed to react to changes in the routes currently in use. The
protocol allows multiple routes to any destination and allows each
sender to select and control the routes used in routing its packets,
for example for use in load balancing or for increased robustness.
Other advantages of the DSR protocol include easily guaranteed
loop-free routing, operation in networks containing unidirectional
links, use of only "soft state" in routing, and very rapid recovery
when routes in the network change. The DSR protocol is designed
mainly for mobile ad hoc networks of up to about two hundred nodes,
and is designed to work well with even very high rates of mobility.
This document specifies the operation of the DSR protocol for routing
unicast IPv4 packets.
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Contents
Status of This Memo i
Abstract ii
1. Introduction 1
2. Assumptions 4
3. DSR Protocol Overview 6
3.1. Basic DSR Route Discovery . . . . . . . . . . . . . . . . 6
3.2. Basic DSR Route Maintenance . . . . . . . . . . . . . . . 9
3.3. Additional Route Discovery Features . . . . . . . . . . . 11
3.3.1. Caching Overheard Routing Information . . . . . . 11
3.3.2. Replying to Route Requests using Cached Routes . 12
3.3.3. Route Request Hop Limits . . . . . . . . . . . . 13
3.4. Additional Route Maintenance Features . . . . . . . . . . 14
3.4.1. Packet Salvaging . . . . . . . . . . . . . . . . 14
3.4.2. Queued Packets Destined over a Broken Link . . . 15
3.4.3. Automatic Route Shortening . . . . . . . . . . . 16
3.4.4. Increased Spreading of Route Error Messages . . . 16
3.5. Optional DSR Flow State Extension . . . . . . . . . . . . 17
3.5.1. Flow Establishment . . . . . . . . . . . . . . . 17
3.5.2. Receiving and Forwarding Establishment Packets . 19
3.5.3. Sending Packets Along Established Flows . . . . . 19
3.5.4. Receiving and Forwarding Packets Sent Along
Established Flows . . . . . . . . . . . . 20
3.5.5. Processing Route Errors . . . . . . . . . . . . . 21
3.5.6. Interaction with Automatic Route Shortening . . . 21
3.5.7. Loop Detection . . . . . . . . . . . . . . . . . 21
3.5.8. Acknowledgement Destination . . . . . . . . . . . 22
3.5.9. Crash Recovery . . . . . . . . . . . . . . . . . 22
3.5.10. Rate Limiting . . . . . . . . . . . . . . . . . . 22
3.5.11. Interaction with Packet Salvaging . . . . . . . . 22
4. Conceptual Data Structures 23
4.1. Route Cache . . . . . . . . . . . . . . . . . . . . . . . 23
4.2. Send Buffer . . . . . . . . . . . . . . . . . . . . . . . 26
4.3. Route Request Table . . . . . . . . . . . . . . . . . . . 27
4.4. Gratuitous Route Reply Table . . . . . . . . . . . . . . 28
4.5. Network Interface Queue and Maintenance Buffer . . . . . 29
4.6. Blacklist . . . . . . . . . . . . . . . . . . . . . . . . 30
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5. Additional Conceptual Data Structures for Flow State Extension 31
5.1. Flow Table . . . . . . . . . . . . . . . . . . . . . . . 31
5.2. Automatic Route Shortening Table . . . . . . . . . . . . 32
5.3. Default Flow ID Table . . . . . . . . . . . . . . . . . . 32
6. DSR Options Header Format 34
6.1. Fixed Portion of DSR Options Header . . . . . . . . . . . 35
6.2. Route Request Option . . . . . . . . . . . . . . . . . . 38
6.3. Route Reply Option . . . . . . . . . . . . . . . . . . . 40
6.4. Route Error Option . . . . . . . . . . . . . . . . . . . 42
6.4.1. Node Unreachable Type-Specific Information . . . 44
6.4.2. Flow State Not Supported Type-Specific Information 44
6.4.3. Option Not Supported Type-Specific Information . 44
6.5. Acknowledgement Request Option . . . . . . . . . . . . . 45
6.6. Acknowledgement Option . . . . . . . . . . . . . . . . . 46
6.7. DSR Source Route Option . . . . . . . . . . . . . . . . . 47
6.8. Pad1 Option . . . . . . . . . . . . . . . . . . . . . . . 49
6.9. PadN Option . . . . . . . . . . . . . . . . . . . . . . . 50
7. Additional Header Formats and Options for Flow State Extension 51
7.1. DSR Flow State Header . . . . . . . . . . . . . . . . . . 52
7.2. New Options and Extensions in DSR Options Header . . . . 53
7.2.1. Timeout Option . . . . . . . . . . . . . . . . . 53
7.2.2. Destination and Flow ID Option . . . . . . . . . 54
7.3. New Error Types for Route Error Option . . . . . . . . . 55
7.3.1. Unknown Flow Type-Specific Information . . . . . 55
7.3.2. Default Flow Unknown Type-Specific Information . 56
7.4. New Acknowledgement Request Option Extension . . . . . . 57
7.4.1. Previous Hop Address Extension . . . . . . . . . 57
8. Detailed Operation 58
8.1. General Packet Processing . . . . . . . . . . . . . . . . 58
8.1.1. Originating a Packet . . . . . . . . . . . . . . 58
8.1.2. Adding a DSR Options Header to a Packet . . . . . 58
8.1.3. Adding a DSR Source Route Option to a Packet . . 59
8.1.4. Processing a Received Packet . . . . . . . . . . 60
8.1.5. Processing a Received DSR Source Route Option . . 62
8.1.6. Handling an Unknown DSR Option . . . . . . . . . 64
8.2. Route Discovery Processing . . . . . . . . . . . . . . . 66
8.2.1. Originating a Route Request . . . . . . . . . . . 66
8.2.2. Processing a Received Route Request Option . . . 68
8.2.3. Generating a Route Reply using the Route Cache . 70
8.2.4. Originating a Route Reply . . . . . . . . . . . . 72
8.2.5. Preventing Route Reply Storms . . . . . . . . . . 74
8.2.6. Processing a Received Route Reply Option . . . . 75
8.3. Route Maintenance Processing . . . . . . . . . . . . . . 77
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8.3.1. Using Link-Layer Acknowledgements . . . . . . . . 77
8.3.2. Using Passive Acknowledgements . . . . . . . . . 78
8.3.3. Using Network-Layer Acknowledgements . . . . . . 79
8.3.4. Originating a Route Error . . . . . . . . . . . . 82
8.3.5. Processing a Received Route Error Option . . . . 83
8.3.6. Salvaging a Packet . . . . . . . . . . . . . . . 84
8.4. Multiple Network Interface Support . . . . . . . . . . . 86
8.5. IP Fragmentation and Reassembly . . . . . . . . . . . . . 87
8.6. Flow State Processing . . . . . . . . . . . . . . . . . . 88
8.6.1. Originating a Packet . . . . . . . . . . . . . . 88
8.6.2. Inserting a DSR Flow State Header . . . . . . . . 90
8.6.3. Receiving a Packet . . . . . . . . . . . . . . . 90
8.6.4. Forwarding a Packet Using Flow IDs . . . . . . . 95
8.6.5. Promiscuously Receiving a Packet . . . . . . . . 95
8.6.6. Operation where the Layer below DSR Decreases
the IP TTL Non-Uniformly . . . . . . . . . 96
8.6.7. Salvage Interactions with DSR . . . . . . . . . . 96
9. Protocol Constants and Configuration Variables 97
10. IANA Considerations 98
11. Security Considerations 99
Appendix A. Link-MaxLife Cache Description 100
Appendix B. Location of DSR in the ISO Network Reference Model 102
Appendix C. Implementation and Evaluation Status 103
Changes from Previous Version of the Draft 105
Acknowledgements 106
References 107
Chair's Address 111
Authors' Addresses 112
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1. Introduction
The Dynamic Source Routing protocol (DSR) [15, 16] is a simple and
efficient routing protocol designed specifically for use in multi-hop
wireless ad hoc networks of mobile nodes. Using DSR, the network
is completely self-organizing and self-configuring, requiring no
existing network infrastructure or administration. Network nodes
cooperate to forward packets for each other to allow communication
over multiple "hops" between nodes not directly within wireless
transmission range of one another. As nodes in the network move
about or join or leave the network, and as wireless transmission
conditions such as sources of interference change, all routing is
automatically determined and maintained by the DSR routing protocol.
Since the number or sequence of intermediate hops needed to reach any
destination may change at any time, the resulting network topology
may be quite rich and rapidly changing.
In designing DSR, we sought to create a routing protocol that had
very low overhead yet was able to react very quickly to changes in
the network. The DSR protocol provides highly reactive service in
order to help ensure successful delivery of data packets in spite of
node movement or other changes in network conditions.
The DSR protocol is composed of two main mechanisms that work
together to allow the discovery and maintenance of source routes in
the ad hoc network:
- 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.
Route Maintenance for this route is used only when S is actually
sending packets to D.
In DSR, Route Discovery and Route Maintenance each operate entirely
"on demand". In particular, unlike other protocols, DSR requires no
periodic packets of any kind at any layer within the network. For
example, DSR does not use any periodic routing advertisement, link
status sensing, or neighbor detection packets, and does not rely on
these functions from any underlying protocols in the network. This
entirely on-demand behavior and lack of periodic activity allows
the number of overhead packets caused by DSR to scale all the way
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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.
All state maintained by DSR is "soft state" [6], in that the loss
of any state will not interfere with the correct operation of the
protocol; all state is discovered as needed and can easily and
quickly be rediscovered if needed after a failure without significant
impact on the protocol. This use of only soft state allows the
routing protocol to be very robust to problems such as dropped or
delayed routing packets or node failures. In particular, a node in
DSR that fails and reboots can easily rejoin the network immediately
after rebooting; if the failed node was involved in forwarding
packets for other nodes as an intermediate hop along one or more
routes, it can also resume this forwarding quickly after rebooting,
with no or minimal interruption to the routing protocol.
In response to a single Route Discovery (as well as through routing
information from other packets overheard), a node may learn and
cache multiple routes to any destination. This support for multiple
routes allows the reaction to routing changes to be much more rapid,
since a node with multiple routes to a destination can try another
cached route if the one it has been using should fail. This caching
of multiple routes also avoids the overhead of needing to perform a
new Route Discovery each time a route in use breaks. The sender of
a packet selects and controls the route used for its own packets,
which together with support for multiple routes also allows features
such as load balancing to be defined. In addition, all routes used
are easily guaranteed to be loop-free, since the sender can avoid
duplicate hops in the routes selected.
The operation of both Route Discovery and Route Maintenance in DSR
are designed to allow unidirectional links and asymmetric routes to
be supported. In particular, as noted in Section 2, in wireless
networks, it is possible that a link between two nodes may not
work equally well in both directions, due to differing antenna or
propagation patterns or sources of interference.
This document specifies the operation of the DSR protocol for
routing unicast IPv4 packets in multi-hop wireless ad hoc networks.
Advanced, optional features, such as Quality of Service (QoS) support
and efficient multicast routing, and operation of DSR with IPv6 [7],
are covered in other documents. The specification of DSR in this
document provides a compatible base on which such features can be
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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
The DSR protocol as described here is designed mainly for mobile
ad hoc networks of up to about two hundred nodes, and is designed
to work well with even very high rates of mobility. Other protocol
features and enhancements that may allow DSR to scale to larger
networks are outside the scope of this document.
We assume in this document that all nodes wishing to communicate with
other nodes within the ad hoc network are willing to participate
fully in the protocols of the network. In particular, each node
participating in the ad hoc network SHOULD also be willing to forward
packets for other nodes in the network.
The diameter of an ad hoc network is the minimum number of hops
necessary for a packet to reach from any node located at one extreme
edge of the ad hoc network to another node located at the opposite
extreme. We assume that this diameter will often be small (e.g.,
perhaps 5 or 10 hops), but 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 [16]. 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
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interface into promiscuous mode. Use of promiscuous receive mode is
entirely optional.
Wireless communication ability between any pair of nodes may at
times not work equally well in both directions, due for example to
differing antenna or propagation patterns or sources of interference
around the two nodes [1, 20]. That is, wireless communications
between each pair of nodes will in many cases be able to operate
bidirectionally, but at times the wireless link between two nodes
may be only unidirectional, allowing one node to successfully
send packets to the other while no communication is possible
in the reverse direction. Some MAC protocols, however, such as
MACA [19], MACAW [2], or IEEE 802.11 [13], limit unicast data
packet transmission to bidirectional links, due to the required
bidirectional exchange of RTS and CTS packets in these protocols and
due to the link-layer acknowledgement feature in IEEE 802.11; when
used on top of MAC protocols such as these, DSR can take advantage
of additional optimizations, such as the ability to reverse a source
route to obtain a route back to the origin of the original route.
The IP address used by a node using the DSR protocol MAY be assigned
by any mechanism (e.g., static assignment or use of DHCP for dynamic
assignment [8]), although the method of such assignment is outside
the scope of this specification.
A routing protocol such as DSR chooses a next-hop for each packet
and provides the IP address of that next-hop. When the packet
is transmitted, however, the lower-layer protocol often has a
separate, MAC-layer address for the next-hop node. DSR uses the
Address Resolution Protocol (ARP) [30] to translate from next-hop IP
addresses to next-hop MAC addresses. In addition, a node MAY add
an entry to its ARP cache based on any received packet, when the IP
address and MAC address of the transmitting node are available in
the packet; for example, the IP address of the transmitting node
is present in a Route Request option (in the Address list being
accumulated) and any packets containing a source route. Adding
entries to the ARP cache in this way avoids the overhead of ARP in
most cases.
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3. DSR Protocol Overview
This section provides an overview of the operation of the DSR
protocol. The basic version of DSR uses explicit "source routing",
in which each data packet sent carries in its header the complete,
ordered list of nodes through which the packet will pass. This use
of explicit source routing allows the sender to select and control
the routes used for its own packets, supports the use of multiple
routes to any destination (for example, for load balancing), and
allows a simple guarantee that the routes used are loop-free; by
including this source route in the header of each data packet, other
nodes forwarding or overhearing any of these packets can also easily
cache this routing information for future use. Section 3.1 describes
this basic operation of Route Discovery, Section 3.2 describes basic
Route Maintenance, and Sections 3.3 and 3.4 describe additional
features of these two parts of DSR's operation. Section 3.5 then
describes an optional, compatible extension to DSR, known as "flow
state", that allows the routing of most packets without an explicit
source route header in the packet, while still preserves the
fundamental properties of DSR's operation.
3.1. Basic DSR Route Discovery
When some source node originates a new packet addressed to some
destination node, the source node places in the header of the packet
a "source route" giving the sequence of hops that the packet is to
follow on its way to the destination. Normally, the sender will
obtain a suitable source route by searching its "Route Cache" of
routes previously learned; if no route is found in its cache, it will
initiate the Route Discovery protocol to dynamically find a new route
to this destination node. In this case, we call the source node
the "initiator" and the destination node the "target" of the Route
Discovery.
For example, suppose a node A is attempting to discover a route to
node E. The Route Discovery initiated by node A in this example
would proceed as follows:
^ "A" ^ "A,B" ^ "A,B,C" ^ "A,B,C,D"
| id=2 | id=2 | id=2 | id=2
+-----+ +-----+ +-----+ +-----+ +-----+
| A |---->| B |---->| C |---->| D |---->| E |
+-----+ +-----+ +-----+ +-----+ +-----+
| | | |
v v v v
To initiate the Route Discovery, node A transmits a "Route
Request" as a single local broadcast packet, which is received by
(approximately) all nodes currently within wireless transmission
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range of A, including node B in this example. Each Route Request
identifies the initiator and target of the Route Discovery, and
also contains a unique request identification (2, in this example),
determined by the initiator of the Request. Each Route Request also
contains a record listing the address of each intermediate node
through which this particular copy of the Route Request has been
forwarded. This route record is initialized to an empty list by the
initiator of the Route Discovery. In this example, the route record
initially lists only node A.
When another node receives this Route Request (such as node B in this
example), if it is the target of the Route Discovery, it returns
a "Route Reply" to the initiator of the Route Discovery, giving
a copy of the accumulated route record from the Route Request;
when the initiator receives this Route Reply, it caches this route
in its Route Cache for use in sending subsequent packets to this
destination.
Otherwise, if this node receiving the Route Request has recently seen
another Route Request message from this initiator bearing this same
request identification and target address, or if this node's own
address is already listed in the route record in the Route Request,
this node discards the Request. (A node considers a Request recently
seen if it still has information about that Request in its Route
Request Table, which is described in Section 4.3.) Otherwise, this
node appends its own address to the route record in the Route Request
and propagates it by transmitting it as a local broadcast packet
(with the same request identification). In this example, node B
broadcast the Route Request, which is received by node C; nodes C
and D each also, in turn, broadcast the Request, resulting in a copy
of the Request being received by node E.
In returning the Route Reply to the initiator of the Route Discovery,
such as in this example, node E replying back to node A, node E will
typically examine its own Route Cache for a route back to A, and if
found, will use it for the source route for delivery of the packet
containing the Route Reply. Otherwise, E SHOULD perform its own
Route Discovery for target node A, but to avoid possible infinite
recursion of Route Discoveries, it MUST piggyback this Route Reply
on the packet containing its own Route Request for A. It is also
possible to piggyback other small data packets, such as a TCP SYN
packet [33], on a Route Request using this same mechanism.
Node E could instead simply reverse the sequence of hops in the route
record that it is trying to send in the Route Reply, and use this as
the source route on the packet carrying the Route Reply itself. For
MAC protocols such as IEEE 802.11 that require a bidirectional frame
exchange as part of the MAC protocol [13], the discovered source
route MUST be reversed in this way to return the Route Reply since it
tests the discovered route to ensure it is bidirectional before the
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Route Discovery initiator begins using the route; this route reversal
also avoids the overhead of a possible second Route Discovery.
When initiating a Route Discovery, the sending node saves a copy of
the original packet (that triggered the Discovery) in a local buffer
called the "Send Buffer". The Send Buffer contains a copy of each
packet that cannot be transmitted by this node because it does not
yet have a source route to the packet's destination. Each packet in
the Send Buffer is logically associated with the time that it was
placed into the Send Buffer and is discarded after residing in the
Send Buffer for some timeout period SendBufferTimeout; if necessary
for preventing the Send Buffer from overflowing, a FIFO or other
replacement strategy MAY also be used to evict packets even before
they expire.
While a packet remains in the Send Buffer, the node SHOULD
occasionally initiate a new Route Discovery for the packet's
destination address. However, the node MUST limit the rate at which
such new Route Discoveries for the same address are initiated (as
described in Section 4.3), since it is possible that the destination
node is not currently reachable. In particular, due to the limited
wireless transmission range and the movement of the nodes in the
network, the network may at times become partitioned, meaning that
there is currently no sequence of nodes through which a packet could
be forwarded to reach the destination. Depending on the movement
pattern and the density of nodes in the network, such network
partitions may be rare or may be common.
If a new Route Discovery was initiated for each packet sent by a
node in such a partitioned network, a large number of unproductive
Route Request packets would be propagated throughout the subset
of the ad hoc network reachable from this node. In order to
reduce the overhead from such Route Discoveries, a node SHOULD use
an exponential back-off algorithm to limit the rate at which it
initiates new Route Discoveries for the same target, doubling the
timeout between each successive Discovery initiated for the same
target. If the node attempts to send additional data packets to this
same destination node more frequently than this limit, the subsequent
packets SHOULD be buffered in the Send Buffer until a Route Reply is
received giving a route to this destination, but the node MUST NOT
initiate a new Route Discovery until the minimum allowable interval
between new Route Discoveries for this target has been reached. This
limitation on the maximum rate of Route Discoveries for the same
target is similar to the mechanism required by Internet nodes to
limit the rate at which ARP Requests are sent for any single target
IP address [3].
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3.2. Basic DSR Route Maintenance
When originating or forwarding a packet using a source route, each
node transmitting the packet is responsible for confirming that data
can flow over the link from that node to the next hop. For example,
in the situation shown below, node A has originated a packet for
node E using a source route through intermediate nodes B, C, and D:
+-----+ +-----+ +-----+ +-----+ +-----+
| A |---->| B |---->| C |-->? | D | | E |
+-----+ +-----+ +-----+ +-----+ +-----+
In this case, node A is responsible for the link from A to B, node B
is responsible for the link from B to C, node C is responsible for
the link from C to D, node D is responsible for the link from D to E.
An acknowledgement can provide confirmation that a link is capable of
carrying data, and in wireless networks, acknowledgements are often
provided at no cost, either as an existing standard part of the MAC
protocol in use (such as the link-layer acknowledgement frame defined
by IEEE 802.11 [13]), or by a "passive acknowledgement" [18] (in
which, for example, B confirms receipt at C by overhearing C transmit
the packet when forwarding it on to D).
If a built-in acknowledgement mechanism is not available, the
node transmitting the packet can explicitly request a DSR-specific
software acknowledgement be returned by the next node along the
route; this software acknowledgement will normally be transmitted
directly to the sending node, but if the link between these two nodes
is unidirectional (Section 4.6), this software acknowledgement could
travel over a different, multi-hop path.
After an acknowledgement has been received from some neighbor, a node
MAY choose to not require acknowledgements from that neighbor for a
brief period of time, unless the network interface connecting a node
to that neighbor always receives an acknowledgement in response to
unicast traffic.
When a software acknowledgement is used, the acknowledgement
request SHOULD be retransmitted up to a maximum number of times.
A retransmission of the acknowledgement request can be sent as a
separate packet, piggybacked on a retransmission of the original
data packet, or piggybacked on any packet with the same next-hop
destination that does not also contain a software acknowledgement.
After the acknowledgement request has been retransmitted the maximum
number of times, if no acknowledgement has been received, then the
sender treats the link to this next-hop destination as currently
"broken". It SHOULD remove this link from its Route Cache and
SHOULD return a "Route Error" to each node that has sent a packet
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routed over that link since an acknowledgement was last received.
For example, in the situation shown above, if C does not receive
an acknowledgement from D after some number of requests, it would
return a Route Error to A, as well as any other node that may have
used the link from C to D since C last received an acknowledgement
from D. Node A then removes this broken link from its cache; any
retransmission of the original packet can be performed by upper
layer protocols such as TCP, if necessary. For sending such a
retransmission or other packets to this same destination E, if A has
in its Route Cache another route to E (for example, from additional
Route Replies from its earlier Route Discovery, or from having
overheard sufficient routing information from other packets), it
can send the packet using the new route immediately. Otherwise, it
SHOULD perform a new Route Discovery for this target (subject to the
back-off described in Section 3.1).
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3.3. Additional Route Discovery Features
3.3.1. Caching Overheard Routing Information
A node forwarding or otherwise overhearing any packet SHOULD add all
usable routing information from that packet to its own Route Cache.
The usefulness of routing information in a packet depends on the
directionality characteristics of the physical medium (Section 2), as
well as the MAC protocol being used. Specifically, three distinct
cases are possible:
- Links in the network frequently are capable of operating only
unidirectionally (not bidirectionally), and the MAC protocol in
use in the network is capable of transmitting unicast packets
over unidirectional links.
- Links in the network occasionally are capable of operating only
unidirectionally (not bidirectionally), but this unidirectional
restriction on any link is not persistent, almost all links
are physically bidirectional, and the MAC protocol in use in
the network is capable of transmitting unicast packets over
unidirectional links.
- The MAC protocol in use in the network is not capable of
transmitting unicast packets over unidirectional links;
only bidirectional links can be used by the MAC protocol for
transmitting unicast packets. For example, the IEEE 802.11
Distributed Coordination Function (DCF) MAC protocol [13]
is capable of transmitting a unicast packet only over a
bidirectional link, since the MAC protocol requires the return of
a link-level acknowledgement packet from the receiver and also
optionally requires the bidirectional exchange of an RTS and CTS
packet between the transmitter and receiver nodes.
In the first case above, for example, the source route used in a data
packet, the accumulated route record in a Route Request, or the route
being returned in a Route Reply SHOULD all be cached by any node in
the "forward" direction; any node SHOULD cache this information from
any such packet received, whether the packet was addressed to this
node, sent to a broadcast (or multicast) MAC address, or overheard
while the node's network interface is in promiscuous mode. However,
the "reverse" direction of the links identified in such packet
headers SHOULD NOT be cached.
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For example, in the situation shown below, node A is using a source
route to communicate with node E:
+-----+ +-----+ +-----+ +-----+ +-----+
| A |---->| B |---->| C |---->| D |---->| E |
+-----+ +-----+ +-----+ +-----+ +-----+
As node C forwards a data packet along the route from A to E, it
SHOULD add to its cache the presence of the "forward" direction
links that it learns from the headers of these packets, from itself
to D and from D to E. Node C SHOULD NOT, in this case, cache the
"reverse" direction of the links identified in these packet headers,
from itself back to B and from B to A, since these links might be
unidirectional.
In the second case above, in which links may occasionally operate
unidirectionally, the links described above SHOULD be cached in both
directions. Furthermore, in this case, if node X overhears (e.g.,
through promiscuous mode) a packet transmitted by node C that is
using a source route from node A to E, node X SHOULD cache all of
these links as well, also including the link from C to X over which
it overheard the packet.
In the final case, in which the MAC protocol requires physical
bidirectionality for unicast operation, links from a source route
SHOULD be cached in both directions, except when the packet also
contains a Route Reply, in which case only the links already
traversed in this source route SHOULD be cached, but the links not
yet traversed in this route SHOULD NOT be cached.
3.3.2. Replying to Route Requests using Cached Routes
A node receiving a Route Request for which it is not the target,
searches its own Route Cache for a route to the target of the
Request. If found, the node generally returns a Route Reply to the
initiator itself rather than forwarding the Route Request. In the
Route Reply, this node sets the route record to list the sequence of
hops over which this copy of the Route Request was forwarded to it,
concatenated with the source route to this target obtained from its
own Route Cache.
However, before transmitting a Route Reply packet that was generated
using information from its Route Cache in this way, a node MUST
verify that the resulting route being returned in the Route Reply,
after this concatenation, contains no duplicate nodes listed in the
route record. For example, the figure below illustrates a case in
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which a Route Request for target E has been received by node F, and
node F already has in its Route Cache a route from itself to E:
+-----+ +-----+ +-----+ +-----+
| A |---->| B |- >| D |---->| E |
+-----+ +-----+ \ / +-----+ +-----+
\ /
\ +-----+ /
>| C |-
+-----+
| ^
v |
Route Request +-----+
Route: A - B - C - F | F | Cache: C - D - E
+-----+
The concatenation of the accumulated route record from the Route
Request and the cached route from F's Route Cache would include a
duplicate node in passing from C to F and back to C.
Node F in this case could attempt to edit the route to eliminate the
duplication, resulting in a route from A to B to C to D and on to E,
but in this case, node F would not be on the route that it returned
in its own Route Reply. DSR Route Discovery prohibits node F
from returning such a Route Reply from its cache; this prohibition
increases the probability that the resulting route is valid, since
node F in this case should have received a Route Error if the route
had previously stopped working. Furthermore, this prohibition
means that a future Route Error traversing the route is very likely
to pass through any node that sent the Route Reply for the route
(including node F), which helps to ensure that stale data is removed
from caches (such as at F) in a timely manner; otherwise, the next
Route Discovery initiated by A might also be contaminated by a Route
Reply from F containing the same stale route. If node F, due to this
restriction on returning a Route Reply based on information from its
Route Cache, does not return such a Route Reply, node F propagates
the Route Request normally.
3.3.3. 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.
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For example, a node MAY use this hop limit to implement a
"non-propagating" Route Request as an initial phase of a Route
Discovery. A node using this technique sends its first Route Request
attempt for some target node using a hop limit of 1, such that any
node receiving the initial transmission of the Route Request will
not forward the Request to other nodes by re-broadcasting it. This
form of Route Request is called a "non-propagating" Route Request;
it provides an inexpensive method for determining if the target is
currently a neighbor of the initiator or if a neighbor node has a
route to the target cached (effectively using the neighbors' Route
Caches as an extension of the initiator's own Route Cache). If no
Route Reply is received after a short timeout, then the node sends
a "propagating" Route Request for the target node (i.e., with hop
limit as defined by the value of the DiscoveryHopLimit configuration
variable).
As another example, a node MAY use this hop limit to implement an
"expanding ring" search for the target [16]. A node using this
technique sends an initial non-propagating Route Request as described
above; if no Route Reply is received for it, the node originates
another Route Request with a hop limit of 2. For each Route Request
originated, if no Route Reply is received for it, the node doubles
the hop limit used on the previous attempt, to progressively explore
for the target node without allowing the Route Request to propagate
over the entire network. However, this expanding ring search
approach could have the effect of increasing the average latency of
Route Discovery, since multiple Discovery attempts and timeouts may
be needed before discovering a route to the target node.
3.4. Additional Route Maintenance Features
3.4.1. Packet Salvaging
When an intermediate node forwarding a packet detects through Route
Maintenance that the next hop along the route for that packet is
broken, if the node has another route to the packet's destination in
its Route Cache, the node SHOULD "salvage" the packet rather than
discarding it. To salvage a packet, the node replaces the original
source route on the packet with the route from its Route Cache. The
node then forwards the packet to the next node indicated along this
source route. For example, in the situation shown in the example of
Section 3.2, if node C has another route cached to node E, it can
salvage the packet by replacing the original route in the packet with
this new route from its own Route Cache, rather than discarding the
packet.
When salvaging a packet, a count is maintained in the packet of the
number of times that it has been salvaged, to prevent a single packet
from being salvaged endlessly. Otherwise, since TTL is decremented
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only once by each node, a single node could salvage a packet an
unbounded number of times. Even if we chose to require TTL to be
decremented on each salvage attempt, packet salvaging is an expensive
operation, so it is desirable to bound the maximum number of times a
packet can be salvaged independently of the maximum number of hops a
packet can traverse.
As described in Section 3.2, an intermediate node, such as in this
case, that detects through Route Maintenance that the next hop along
the route for a packet that it is forwarding is broken, the node also
SHOULD return a Route Error to the original sender of the packet,
identifying the link over which the packet could not be forwarded.
If the node sends this Route Error, it SHOULD originate the Route
Error before salvaging the packet.
3.4.2. Queued Packets Destined over a Broken Link
When an intermediate node forwarding a packet detects through Route
Maintenance that the next-hop link along the route for that packet
is broken, in addition to handling that packet as defined for Route
Maintenance, the node SHOULD also handle in a similar way any pending
packets that it has queued that are destined over this new broken
link. Specifically, the node SHOULD search its Network Interface
Queue and Maintenance Buffer (Section 4.5) for packets for which
the next-hop link is this new broken link. For each such packet
currently queued at this node, the node SHOULD process that packet as
follows:
- Remove the packet from the node's Network Interface Queue and
Maintenance Buffer.
- Originate a Route Error for this packet to the original sender of
the packet, using the procedure described in Section 8.3.4, as if
the node had already reached the maximum number of retransmission
attempts for that packet for Route Maintenance. However, in
sending such Route Errors for queued packets in response to a
single new broken link detected, the node SHOULD send no more
than one Route Error to each original sender of any of these
packets.
- If the node has another route to the packet's IP
Destination Address in its Route Cache, the node SHOULD
salvage the packet as described in Section 8.3.6. Otherwise, the
node SHOULD discard the packet.
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3.4.3. Automatic Route Shortening
Source routes in use MAY be automatically shortened if one or more
intermediate nodes in the route become no longer necessary. This
mechanism of automatically shortening routes in use is somewhat
similar to the use of passive acknowledgements [18]. In particular,
if a node is able to overhear a packet carrying a source route (e.g.,
by operating its network interface in promiscuous receive mode), then
this node examines the unexpended portion of that source route. If
this node is not the intended next-hop destination for the packet
but is named in the later unexpended portion of the packet's source
route, then it can infer that the intermediate nodes before itself in
the source route are no longer needed in the route. For example, the
figure below illustrates an example in which node D has overheard a
data packet being transmitted from B to C, for later forwarding to D
and to E:
+-----+ +-----+ +-----+ +-----+ +-----+
| A |---->| B |---->| C | | D | | E |
+-----+ +-----+ +-----+ +-----+ +-----+
\ ^
\ /
---------------------
In this case, this node (node D) SHOULD return a "gratuitous" Route
Reply to the original sender of the packet (node A). The Route
Reply gives the shorter route as the concatenation of the portion of
the original source route up through the node that transmitted the
overheard packet (node B), plus the suffix of the original source
route beginning with the node returning the gratuitous Route Reply
(node D). In this example, the route returned in the gratuitous Route
Reply message sent from D to A gives the new route as the sequence of
hops from A to B to D to E.
When deciding whether to return a gratuitous Route Reply in this way,
a node MAY factor in additional information beyond the fact that it
was able to overhear the packet. For example, the node MAY decide to
return the gratuitous Route Reply only when the overheard packet is
received with a signal strength or signal-to-noise ratio above some
specific threshold. In addition, each node maintains a Gratuitous
Route Reply Table, as described in Section 4.4, to limit the rate at
which it originates gratuitous Route Replies for the same returned
route.
3.4.4. Increased Spreading of Route Error Messages
When a source node receives a Route Error for a data packet that
it originated, this source node propagates this Route Error to its
neighbors by piggybacking it on its next Route Request. In this way,
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stale information in the caches of nodes around this source node will
not generate Route Replies that contain the same invalid link for
which this source node received the Route Error.
For example, in the situation shown in the example of Section 3.2,
node A learns from the Route Error message from C, that the link
from C to D is currently broken. It thus removes this link from
its own Route Cache and initiates a new Route Discovery (if it has
no other route to E in its Route Cache). On the Route Request
packet initiating this Route Discovery, node A piggybacks a copy
of this Route Error, ensuring that the Route Error spreads well to
other nodes, and guaranteeing that any Route Reply that it receives
(including those from other node's Route Caches) in response to this
Route Request does not contain a route that assumes the existence of
this broken link.
3.5. Optional DSR Flow State Extension
This section describes an optional, compatible extension to the DSR
protocol, known as "flow state", that allows the routing of most
packets without an explicit source route header in the packet. The
DSR flow state extension further reduces the overhead of the protocol
yet still preserves the fundamental properties of DSR's operation.
Once a sending node has discovered a source route such as through
DSR's Route Discovery mechanism, the flow state mechanism allows the
sending node to establish hop-by-hop forwarding state within the
network, based on this source route, to enable each node along the
route to forward the packet to the next hop based on the node's own
local knowledge of the flow along which this packet is being routed.
Flow state is dynamically initialized by the first packet using a
source route and is then able to route subsequent packets along
the same flow without use of a source route header in the packet.
The state established at each hop along a flow is "soft state" and
thus automatically expires when no longer needed and can be quickly
recreated as necessary. Extending DSR's basic operation based on an
explicit source route in the header of each packet routed, the flow
state extension operates as a form of "implicit source routing" by
preserving DSR's basic operation but removing the explicit source
route from packets.
3.5.1. Flow Establishment
A source node sending packets to some destination node MAY use the
DSR flow state extension described here to establish a route to
that destination as a flow. A "flow" is a route from the source to
the destination represented by hop-by-hop forwarding state within
the nodes along the route. Each flow is uniquely identified by a
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combination of the source node address, the destination node address,
and a flow identifier (flow ID) chosen by the source node.
Each flow ID is a 16-bit unsigned integer. Comparison between
different flow IDs MUST be performed modulo 2**16. For example,
using an implementation in the C programming language, a
flow ID value (a) is greater than another flow ID value (b) if
((short)((a) - (b)) > 0), if a C language "short" data type is
implemented as a 16-bit signed integer.
A DSR Flow State header in a packet identifies the flow ID to
be followed in forwarding that packet. From a given source to
some destination, any number of different flows MAY exist and
be in use, for example following different sequences of hops to
reach the destination. One of these flows may be considered to be
the "default" flow from that source to that destination. A node
receiving a packet with neither a DSR Options header specifying the
route to be taken (with a Source Route option in the DSR Options
header) nor a DSR Flow State header specifying the flow ID to be
followed, is forwarded along the default flow for the source and
destination addresses specified in the packet's IP header.
In establishing a new flow, the source node generates a nonzero
16-bit flow ID greater than any unexpired flow IDs for this
(source, destination) pair. If the source wishes for this flow to
become the default flow, the low bit of the flow ID MUST be set (the
flow ID is an odd number); otherwise, the low bit MUST NOT be set
(the flow ID is an even number).
The source node establishing the new flow then transmits a packet
containing a DSR Options header with a Source Route option; to
establish the flow, the source node also MUST include in the packet
a DSR Flow State header, with the Flow ID field set to the chosen
flow ID for the new flow, and MUST include a Timeout option in the
DSR Options header, giving the lifetime after which state information
about this flow is to expire. This packet will generally be a normal
data packet being sent from this sender to the receiver (for example,
the first packet sent after discovering the new route) but is also
treated as a "flow establishment" packet.
The source node records this flow in its Flow Table for future use,
setting the TTL in this Flow Table entry to be the value used in the
TTL field in the packet's IP header and setting the Lifetime in this
entry to be the lifetime specified in the Timeout option in the DSR
Options header. The TTL field is used for Default Flow Forwarding,
as described in Sections 3.5.3 and 3.5.4.
Any further packets sent with this flow ID before the timeout that
also contain a DSR Options header with a Source Route option MUST use
this same source route in the Source Route option.
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3.5.2. Receiving and Forwarding Establishment Packets
Packets intended to establish a flow, as described in Section 3.5.1,
contain a DSR Options header with a Source Route option, and are
forwarded along the indicated route. A node implementing the DSR
flow state extension, when receiving and forwarding such a DSR
packet, also keeps some state in its own Flow Table to enable it
to forward future packets that are sent along this flow with only
the flow ID specified. Specifically, if the packet also contains
a DSR Flow State header, this packet SHOULD cause an entry to be
established for this flow in the Flow Table of each node along the
packet's route.
The Hop Count field of the DSR Flow State header is also stored in
the Flow Table, as is Lifetime option specified in the DSR Options
header.
If the Flow ID is odd and there is no flow in the Flow Table with
Flow ID greater than the received Flow ID, set the default Flow ID
for this (IP Source Address, IP Destination Address) pair to the
received Flow ID, and the TTL of the packet is recorded.
The Flow ID option is removed before final delivery of the packet.
3.5.3. Sending Packets Along Established Flows
When a flow is established as described in Section 3.5.1, a packet
is sent which establishes state in each node along the route.
This state is soft; that is, the protocol contains mechanisms for
recovering from the loss of this state. However, the use of these
mechanisms may result in reduced performance for packets sent
along flows with forgotten state. As a result, it is desirable
to differentiate behavior based on whether or not the sender is
reasonably certain that the flow state exists on each node along
the route. We define a flow's state to be "established end-to-end"
if the Flow Tables of all nodes on the route contains forwarding
information for that flow. While it is impossible to detect whether
or not a flow's state has been established end-to-end without sending
packets, implementations may make reasonable assumptions about the
retention of flow state and the probability that an establishment
packet has been seen by all nodes on the route.
A source wishing to send a packet along an established flow
determines if the flow state has been established end-to-end. If
it has not, a DSR Options header with Source Route option with this
flow's route is added to the packet. The source SHOULD set the
Flow ID field of the DSR Flow State header either to the flow ID
previously associated with this flow's route or to zero. If it sets
the Flow ID field to any other value, it MUST follow the processing
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steps in Section 3.5.1 for establishing a new flow ID. If it sets the
Flow ID field to a nonzero value, it MUST include a Timeout option
with a value not greater than the timeout remaining in the node's
Flow Table, and if its TTL is not equal to that specified in the Flow
Table, the flow MUST NOT be used as a default flow in the future.
Once flow state has been established end-to-end for non-default
flows, a source adds a DSR Flow State header to each packet it wishes
to send along that flow, setting the Flow ID field to the flow ID of
that flow. A Source Route option SHOULD NOT be added to the packet,
though if one is, then the steps for processing flows that have not
been established end to end MUST be followed.
Once flow state has been established end-to-end for default flows,
sources sending packets with IP TTL equal to the TTL value in the
local Flow Table entry for this flow then transmit the packet to the
next hop. In this case, a DSR Flow State header SHOULD NOT be added
to the packet and a DSR Options header likewise SHOULD NOT be added
to the packet; though if one is, the steps for sending packets along
non-default flows MUST be followed. If the IP TTL is not equal to
the TTL value in the local Flow Table, then the steps for processing
a non-default flow MUST be followed.
3.5.4. Receiving and Forwarding Packets Sent Along Established Flows
The handling of packets containing a DSR Options header with
both a nonzero Flow ID and a Source Route option is described in
Section 3.5.2. The Flow ID is ignored when it is equal to zero.
This section only describes handling of packets without a Source
Route option.
If a node receives a packet with a Flow ID in the DSR Options
header that indicates an unexpired flow in the node's Flow Table, it
increments the Hop Count in the DSR Options header and forwards the
packet to the next hop indicated in the Flow Table.
If a node receives a packet with a Flow ID that indicates a flow not
currently in the node's Flow Table, it returns a Route Error of type
UNKNOWN_FLOW with Error Destination and IP Destination addresses
copied from the IP Source of the packet triggering the error. This
error packet SHOULD be MAC-destined to the node from which it was
received; if it cannot confirm reachability of the previous node
using Route Maintenance, it MUST send the error as described in
Section 8.1.1. The node sending the error SHOULD attempt to salvage
the packet triggering the Route Error. If it does salvage the
packet, it MUST zero the Flow ID.
If a node receives a packet with no DSR Options header and no DSR
Flow State header, it checks the Default Flow Table. If there is
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an entry, it forwards to the next hop indicated in the Flow Table
for the default flow. Otherwise, it returns a Route Error of type
DEFAULT_FLOW_UNKNOWN with Error Destination and IP Destination
addresses copied from the IP Source of the packet triggering the
error. This error packet SHOULD be MAC-destined to the node from
which it was received; if it cannot confirm reachability of the
previous node using Route Maintenance, it MUST send the error as
described in Section 8.1.1. The node sending the error SHOULD
attempt to salvage the packet triggering the Route Error. If it does
salvage the packet, it MUST zero the Flow ID.
3.5.5. Processing Route Errors
When a node receives a Route Error of type Unknown Flow, it marks
the flow to indicate that it has not been established end-to-end.
When a node receives a Route Error of type Default Flow Unknown, it
marks the default flow to indicate that it has not been established
end-to-end.
3.5.6. Interaction with Automatic Route Shortening
Because a full source route is not carried in every packet, an
alternative method for performing automatic route shortening is
necessary for packets using the flow state extension. Instead, nodes
promiscuously listen to packets, and if a node receives a packet with
(IP Source, IP Destination, Flow ID) found in the Flow Table but the
MAC-layer (next hop) destination address of the packet is not this
node, the node determines whether the packet was sent by an upstream
or downstream node by examining the Hop Count field in the DSR Flow
State header. If the Hop Count field is less than the expected
Hop Count at this node (that is, the expected Hop Count field in
the Flow Table described in Section 5.1), the node assumes that the
packet was sent by an upstream node, and adds an entry for the packet
to its Automatic Route Shortening Table, possibly evicting an earlier
entry added to this table. When the packet is then sent to that node
for forwarding, the node finds that it has previously received the
packet by checking its Automatic Route Shortening Table, and returns
a gratuitous Route Reply to the source of the packet.
3.5.7. Loop Detection
If a node receives a packet for forwarding with TTL lower than
expected and default flow forwarding is being used, it sends a
Route Error of type Default Flow Unknown back to the IP source. It
can attempt delivery of the packet by normal salvaging (subject
to constraints described in Section 8.6.7) or by inserting a
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Flow ID option with Special TTL extension based on what that node's
understanding of the default Flow ID and TTL.
3.5.8. Acknowledgement Destination
In packets sent using Flow State, the previous hop is not necessarily
known. In order to allow nodes that have lost flow state to
determine the previous hop, the address of the previous hop can
optionally be stored in the Acknowledgement Request. This extension
SHOULD NOT be used when a Source Route option is present, MAY be used
when flow state routing is used without a Source Route option, and
SHOULD be used before Route Maintenance determines that the next-hop
destination is unreachable.
3.5.9. Crash Recovery
Each node has a maximum Timeout value that it can possibly generate.
This can be based on the largest number that can be set in a timeout
option (2**16 - 1 seconds) or set in system software. When a node
crashes, it does not establish new flows for a period equal to this
maximum Timeout value, in order to avoid colliding with its old
Flow IDs.
3.5.10. Rate Limiting
Flow IDs can be assigned with a counter. More specifically, the
"Current Flow ID" is kept. When a new default Flow ID needs to be
assigned, if the Current Flow ID is odd, the Current Flow ID is
assigned as the Flow ID and the Current Flow ID is incremented by
one; if the Current Flow ID is even, one plus the Current Flow ID is
assigned as the Flow ID and the Current Flow ID is incremented by
two.
If Flow IDs are assigned in this way, one algorithm for avoiding
duplicate, unexpired Flow IDs is to rate limit new Flow IDs to an
average rate of n assignments per second, where n is 2**15 divided by
the maximum Timeout value. This can be averaged over any period not
exceeding the maximum Timeout value.
3.5.11. Interaction with Packet Salvaging
Salvaging is modified to zero the Flow ID field. Also, any time the
this document refers to the Salvage field in the Source Route option
in a DSR Options header, packets without a Source Route option are
considered to have the value zero in the Salvage field.
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4. Conceptual Data Structures
This document describes the operation of the DSR protocol in terms
of a number of conceptual data structures. This section describes
each of these data structures and provides an overview of its use
in the protocol. In an implementation of the protocol, these data
structures MAY be implemented in any manner consistent with the
external behavior described in this document. Additional conceptual
data structures are required for the optional flow state extensions
to DSR; these data structures are described in Section 5.
4.1. Route Cache
Each node implementing DSR MUST maintain a Route Cache, containing
routing information needed by the node. A node adds information to
its Route Cache as it learns of new links between nodes in the ad hoc
network; for example, a node may learn of new links when it receives
a packet carrying a Route Request, Route Reply, or DSR source route.
Likewise, a node removes information from its Route Cache as it
learns that existing links in the ad hoc network have broken; for
example, a node may learn of a broken link when it receives a packet
carrying a Route Error or through the link-layer retransmission
mechanism reporting a failure in forwarding a packet to its next-hop
destination.
Anytime a node adds new information to its Route Cache, the node
SHOULD check each packet in its own Send Buffer (Section 4.2) to
determine whether a route to that packet's IP Destination Address
now exists in the node's Route Cache (including the information just
added to the Cache). If so, the packet SHOULD then be sent using
that route and removed from the Send Buffer.
It is possible to interface a DSR network with other networks,
external to this DSR network. Such external networks may, for
example, be the Internet, or may be other ad hoc networks routed
with a routing protocol other than DSR. Such external networks may
also be other DSR networks that are treated as external networks
in order to improve scalability. The complete handling of such
external networks is beyond the scope of this document. However,
this document specifies a minimal set of requirements and features
necessary to allow nodes only implementing this specification to
interoperate correctly with nodes implementing interfaces to such
external networks. This minimal set of requirements and features
involve the First Hop External (F) and Last Hop External (L) bits
in a DSR Source Route option (Section 6.7) and a Route Reply option
(Section 6.3) in a packet's DSR Options header (Section 6). These
requirements also include the addition of an External flag bit
tagging each link in the Route Cache, copied from the First Hop
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External (F) and Last Hop External (L) bits in the DSR Source Route
option or Route Reply option from which this link was learned.
The Route Cache SHOULD support storing more than one route to each
destination. In searching the Route Cache for a route to some
destination node, the Route Cache is indexed by destination node
address. The following properties describe this searching function
on a Route Cache:
- Each implementation of DSR at any node MAY choose any appropriate
strategy and algorithm for searching its Route Cache and
selecting a "best" route to the destination from among those
found. For example, a node MAY choose to select the shortest
route to the destination (the shortest sequence of hops), or it
MAY use an alternate metric to select the route from the Cache.
- However, if there are multiple cached routes to a destination,
the selection of routes when searching the Route Cache MUST
prefer routes that do not have the External flag set on any link.
This preference will select routes that lead directly to the
target node over routes that attempt to reach the target via any
external networks connected to the DSR ad hoc network.
- In addition, any route selected when searching the Route Cache
MUST NOT have the External bit set for any links other than
possibly the first link, the last link, or both; the External bit
MUST NOT be set for any intermediate hops in the route selected.
An implementation of a Route Cache MAY provide a fixed capacity
for the cache, or the cache size MAY be variable. The following
properties describe the management of available space within a node's
Route Cache:
- Each implementation of DSR at each node MAY choose any
appropriate policy for managing the entries in its Route Cache,
such as when limited cache capacity requires a choice of which
entries to retain in the Cache. For example, a node MAY chose a
"least recently used" (LRU) cache replacement policy, in which
the entry last used longest ago is discarded from the cache if a
decision needs to be made to allow space in the cache for some
new entry being added.
- However, the Route Cache replacement policy SHOULD allow routes
to be categorized based upon "preference", where routes with a
higher preferences are less likely to be removed from the cache.
For example, a node could prefer routes for which it initiated
a Route Discovery over routes that it learned as the result of
promiscuous snooping on other packets. In particular, a node
SHOULD prefer routes that it is presently using over those that
it is not.
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Any suitable data structure organization, consistent with this
specification, MAY be used to implement the Route Cache in any node.
For example, the following two types of organization are possible:
- In DSR, the route returned in each Route Reply that is received
by the initiator of a Route Discovery (or that is learned from
the header of overhead packets, as described in Section 8.1.4)
represents a complete path (a sequence of links) leading to the
destination node. By caching each of these paths separately,
a "path cache" organization for the Route Cache can be formed.
A path cache is very simple to implement and easily guarantees
that all routes are loop-free, since each individual route from
a Route Reply or Route Request or used in a packet is loop-free.
To search for a route in a path cache data structure, the sending
node can simply search its Route Cache for any path (or prefix of
a path) that leads to the intended destination node.
This type of organization for the Route Cache in DSR has been
extensively studied through simulation [5, 10, 14, 21] and
through implementation of DSR in a mobile outdoor testbed under
significant workload [22, 23, 24].
- Alternatively, a "link cache" organization could be used for the
Route Cache, in which each individual link (hop) in the routes
returned in Route Reply packets (or otherwise learned from the
header of overhead packets) is added to a unified graph data
structure of this node's current view of the network topology.
To search for a route in link cache, the sending node must use
a more complex graph search algorithm, such as the well-known
Dijkstra's shortest-path algorithm, to find the current best path
through the graph to the destination node. Such an algorithm is
more difficult to implement and may require significantly more
CPU time to execute.
However, a link cache organization is more powerful than a path
cache organization, in its ability to effectively utilize all of
the potential information that a node might learn about the state
of the network. In particular, links learned from different
Route Discoveries or from the header of any overheard packets can
be merged together to form new routes in the network, but this
is not possible in a path cache due to the separation of each
individual path in the cache.
This type of organization for the Route Cache in DSR, including
the effect of a range of implementation choices, has been studied
through detailed simulation [10].
The choice of data structure organization to use for the Route Cache
in any DSR implementation is a local matter for each node and affects
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only performance; any reasonable choice of organization for the Route
Cache does not affect either correctness or interoperability.
Each entry in the Route Cache SHOULD have a timeout associated
with it, to allow that entry to be deleted if not used within some
time. The particular choice of algorithm and data structure used
to implement the Route Cache SHOULD be considered in choosing the
timeout for entries in the Route Cache. The configuration variable
RouteCacheTimeout defined in Section 9 specifies the timeout to be
applied to entries in the Route Cache, although it is also possible
to instead use an adaptive policy in choosing timeout values rather
than using a single timeout setting for all entries; for example, the
Link-MaxLife cache design (below) uses an adaptive timeout algorithm
and does not use the RouteCacheTimeout configuration variable.
As guidance to implementors, Appendix A describes a type of link
cache known as "Link-MaxLife" that has been shown to outperform
other types of link caches and path caches studied in detailed
simulation [10]. Link-MaxLife is an adaptive link cache in which
each link in the cache has a timeout that is determined dynamically
by the caching node according to its observed past behavior of the
two nodes at the ends of the link; in addition, when selecting a
route for a packet being sent to some destination, among cached
routes of equal length (number of hops) to that destination,
Link-MaxLife selects the route with the longest expected lifetime
(highest minimum timeout of any link in the route). Use of
the Link-MaxLife design for the Route Cache is recommended in
implementations of DSR.
4.2. Send Buffer
The Send Buffer of a node implementing DSR is a queue of packets that
cannot be sent by that node because it does not yet have a source
route to each such packet's destination. Each packet in the Send
Buffer is logically associated with the time that it was placed into
the Buffer, and SHOULD be removed from the Send Buffer and silently
discarded after a period of SendBufferTimeout after initially being
placed in the Buffer. If necessary, a FIFO strategy SHOULD be used
to evict packets before they timeout to prevent the buffer from
overflowing.
Subject to the rate limiting defined in Section 4.3, a Route
Discovery SHOULD be initiated as often as possible for the
destination address of any packets residing in the Send Buffer.
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4.3. Route Request Table
The Route Request Table of a node implementing DSR records
information about Route Requests that have been recently originated
or forwarded by this node. The table is indexed by IP address.
The Route Request Table on a node records the following information
about nodes to which this node has initiated a Route Request:
- The Time-to-Live (TTL) field used in the IP header of the Route
Request for the last Route Discovery initiated by this node for
that target node. This value allows the node to implement a
variety of algorithms for controlling the spread of its Route
Request on each Route Discovery initiated for a target. As
examples, two possible algorithms for this use of the TTL field
are described in Section 3.3.3.
- The time that this node last originated a Route Request for that
target node.
- The number of consecutive Route Discoveries initiated for this
target since receiving a valid Route Reply giving a route to that
target node.
- The remaining amount of time before which this node MAY next
attempt at a Route Discovery for that target node. When the
node initiates a new Route Discovery for this target node, this
field in the Route Request Table entry for that target node is
initialized to the timeout for that Route Discovery, after which
the node MAY initiate a new Discovery for that target. Until
a valid Route Reply is received for this target node address,
a node MUST implement a back-off algorithm in determining this
timeout value for each successive Route Discovery initiated
for this target using the same Time-to-Live (TTL) value in the
IP header of the Route Request packet. The timeout between
such consecutive Route Discovery initiations SHOULD increase by
doubling the timeout value on each new initiation.
In addition, the Route Request Table on a node also records the
following information about initiator nodes from which this node has
received a Route Request:
- A FIFO cache of size RequestTableIds entries containing the
Identification value and target address from the most recent
Route Requests received by this node from that initiator node.
Nodes SHOULD use an LRU policy to manage the entries in their Route
Request Table.
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The number of Identification values to retain in each Route
Request Table entry, RequestTableIds, MUST NOT be unlimited, since,
in the worst case, when a node crashes and reboots, the first
RequestTableIds Route Discoveries it initiates after rebooting
could appear to be duplicates to the other nodes in the network.
In addition, a node SHOULD base its initial Identification value,
used for Route Discoveries after rebooting, on a battery backed-up
clock or other persistent memory device, in order to help avoid
any possible such delay in successfully discovering new routes
after rebooting; if no such source of initial Identification
value is available, a node after rebooting SHOULD base its initial
Identification value on a random number.
4.4. Gratuitous Route Reply Table
The Gratuitous Route Reply Table of a node implementing DSR records
information about "gratuitous" Route Replies sent by this node as
part of automatic route shortening. As described in Section 3.4.3,
a node returns a gratuitous Route Reply when it overhears a packet
transmitted by some node, for which the node overhearing the
packet was not the intended next-hop node but was named later in
the unexpended hops of the source route in that packet; the node
overhearing the packet returns a gratuitous Route Reply to the
original sender of the packet, listing the shorter route (not
including the hops of the source route "skipped over" by this
packet). A node uses its Gratuitous Route Reply Table to limit the
rate at which it originates gratuitous Route Replies to the same
original sender for the same node from which it overheard a packet to
trigger the gratuitous Route Reply.
Each entry in the Gratuitous Route Reply Table of a node contains the
following fields:
- The address of the node to which this node originated a
gratuitous Route Reply.
- The address of the node from which this node overheard the packet
triggering that gratuitous Route Reply.
- The remaining time before which this entry in the Gratuitous
Route Reply Table expires and SHOULD be deleted by the node.
When a node creates a new entry in its Gratuitous Route Reply
Table, the timeout value for that entry should be initialized to
the value GratReplyHoldoff.
When a node overhears a packet that would trigger a gratuitous
Route Reply, if a corresponding entry already exists in the node's
Gratuitous Route Reply Table, then the node SHOULD NOT send a
gratuitous Route Reply for that packet. Otherwise (no corresponding
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entry already exists), the node SHOULD create a new entry in its
Gratuitous Route Reply Table to record that gratuitous Route Reply,
with a timeout value of GratReplyHoldoff.
4.5. Network Interface Queue and Maintenance Buffer
Depending on factors such as the structure and organization of
the operating system, protocol stack implementation, network
interface device driver, and network interface hardware, a packet
being transmitted could be queued in a variety of ways. For
example, outgoing packets from the network protocol stack might be
queued at the operating system or link layer, before transmission
by the network interface. The network interface might also
provide a retransmission mechanism for packets, such as occurs in
IEEE 802.11 [13]; the DSR protocol, as part of Route Maintenance,
requires limited buffering of packets already transmitted for
which the reachability of the next-hop destination has not yet been
determined. The operation of DSR is defined here in terms of two
conceptual data structures that together incorporate this queuing
behavior.
The Network Interface Queue of a node implementing DSR is an output
queue of packets from the network protocol stack waiting to be
transmitted by the network interface; for example, in the 4.4BSD
Unix network protocol stack implementation, this queue for a network
interface is represented as a "struct ifqueue" [38]. This queue is
used to hold packets while the network interface is in the process of
transmitting another packet.
The Maintenance Buffer of a node implementing DSR is a queue of
packets sent by this node that are awaiting next-hop reachability
confirmation as part of Route Maintenance. For each packet in
the Maintenance Buffer, a node maintains a count of the number
of retransmissions and the time of the last retransmission. The
Maintenance Buffer MAY be of limited size; when adding a new packet
to the Maintenance Buffer, if the buffer size is insufficient to hold
the new packet, the new packet SHOULD be silently discarded. If,
after MaxMaintRexmt attempts to confirm next-hop reachability of
some node, no confirmation is received, all packets in this node's
Maintenance Buffer with this next-hop destination SHOULD be removed
from the Maintenance Buffer; in this case, the node also SHOULD
originate a Route Error for this packet to each original source of
a packet removed in this way (Section 8.3) and SHOULD salvage each
packet removed in this way (Section 8.3.6) if it has another route
to that packet's IP Destination Address in its Route Cache. The
definition of MaxMaintRexmt conceptually includes any retransmissions
that might be attempted for a packet at the link layer or within
the network interface hardware. The timeout value to use for each
transmission attempt for an acknowledgement request depends on the
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type of acknowledgement mechanism used by Route Maintenance for that
attempt, as described in Section 8.3.
4.6. Blacklist
When a node using the DSR protocol is connected through an
interface that requires physically bidirectional links for unicast
transmission, it MUST maintain a Blacklist. The Blacklist is a
table, indexed by neighbor node address, that indicates that the
link between this node and the specified neighbor node may not be
bidirectional. A node places another node's address in this list
when it believes that broadcast packets from that other node reach
this node, but that unicast transmission between the two nodes is not
possible. For example, if a node forwarding a Route Reply discovers
that the next hop is unreachable, it places that next hop in the
node's Blacklist.
Once a node discovers that it can communicate bidirectionally with
one of the nodes listed in the Blacklist, it SHOULD remove that
node from the Blacklist. For example, if node A has node B listed
in its Blacklist, but after transmitting a Route Request, node A
hears B forward the Route Request with a hop list indicating that the
broadcast from A to B was successful, then A SHOULD remove the entry
for node B from its Blacklist.
A node MUST associate a state with each node listed in its Blacklist,
specifying whether the unidirectionality of the link to that node
is "questionable" or "probable". Each time the unreachability is
positively determined, the node SHOULD set the state to "probable".
After the unreachability has not been positively determined for some
amount of time, the state SHOULD revert to "questionable". A node
MAY expire entries for nodes from its Blacklist after a reasonable
amount of time.
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5. Additional Conceptual Data Structures for Flow State Extension
This section defines additional conceptual data structures used by
the optional "flow state" extension to DSR. In an implementation of
the protocol, these data structures MAY be implemented in any manner
consistent with the external behavior described in this document.
5.1. Flow Table
A node implementing the flow state extension MUST implement a Flow
Table or other data structure consistent with the external behavior
described in this section. A node not implementing the flow state
extension SHOULD NOT implement a Flow Table.
The Flow Table records information about flows from which packets
recently have been sent or forwarded by this node. The table is
indexed by a triple (IP Source Address, IP Destination Address,
Flow ID), where Flow ID is a 16-bit token assigned by the source as
described in Section 3.5.1. Each entry in the Flow Table contains
the following fields:
- The MAC address of the next-hop node along this flow.
- An indication of the outgoing network interface on this node to
be used in transmitting packets along this flow.
- The MAC address of the previous-hop node along this flow.
- An indication of the network interface on this node from which
packets from that previous-hop node are received.
- A timeout after which this entry in the Flow Table MUST be
deleted.
- The expected value of the Hop Count field in the DSR Flow State
header for packets received for forwarding along this field (for
use with packets containing a DSR Flow State header).
- An indication of whether or not this flow can be used as a
default flow for packets originated by this node (the flow IP
MUST be odd).
- The entry SHOULD record the complete source route for the flow.
(Nodes not recording the complete source route cannot participate
in Automatic Route Shortening.)
- The entry MAY contain a field recording the time this entry was
last used.
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The entry MUST be deleted when its timeout expires.
5.2. Automatic Route Shortening Table
A node implementing the flow state extension SHOULD implement an
Automatic Route Shortening Table or other data structure consistent
with the external behavior described in this section. A node
not implementing the flow state extension SHOULD NOT implement an
Automatic Route Shortening Table.
The Automatic Route Shortening Table records information about
received packets for which Automatic Route Shortening may be
possible. The table is indexed by a triple (IP Source Address, IP
Destination Address, Flow ID). Each entry in the Automatic Route
Shortening Table contains a list of (packet identifier, Hop Count)
pairs for that flow. The packet identifier in the list may be any
unique identifier for the received packet; for example, for IPv4
packets, the combination of the following fields from the packet's
IP header MAY be used as a unique identifier for the packet: Source
Address, Destination Address, Identification, Protocol, Fragment,
and Total Length. The Hop Count in the list in the entry is copied
from the Hop Count field in the DSR Flow State header of the received
packet for which this table entry was created. Any packet identifier
SHOULD appear at most once in the list in an entry, and this list
item SHOULD record the minimum Hop Count value received for that
packet (if the wireless signal strength or signal-to-noise ratio at
which a packet is received is available to the DSR implementation
in a node, the node MAY, for example, remember instead in this list
the minimum Hop Count value for which the received packet's signal
strength or signal-to-noise ratio exceeded some threshold).
Space in the Automatic Route Shortening Table of a node MAY be
dynamically managed by any local algorithm at the node. For example,
in order to limit the amount of memory used to store the table, any
existing entry MAY be deleted at any time, and the number of packets
listed in each entry MAY be limited. However, when reclaiming space
in the table, nodes SHOULD favor retaining information about more
flows in the table rather than more packets listed in each entry
in the table, as long as at least the listing of some small number
of packets (e.g., 3) can be retained in each entry. In addition,
subject to any implementation limit on the number of packets listed
in each entry in the table, information about a packet listed in an
entry SHOULD be retained until the expiration of the packet's IP TTL.
5.3. Default Flow ID Table
A node implementing the flow state extension MUST implement a Default
Flow Table or other data structure consistent with the external
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behavior described in this section. A node not implementing the flow
state extension SHOULD NOT implement a Default Flow Table.
For each (source, destination) pair for which a node forwards
packets, the node MUST record:
- the largest odd Flow ID value seen
- the time at which all of this (source, destination) pair's flows
that are forwarded by this node expire
- the current default Flow ID
- a flag indicating whether or not the current default Flow ID is
valid
If a node deletes this record for a (source, destination) pair,
it MUST also delete all Flow Table entries for that (source,
destination) pair. Nodes MUST delete table entries if all of this
(source, destination) pair's flows that are forwarded by this node
expire.
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6. DSR Options Header Format
The Dynamic Source Routing protocol makes use of a special header
carrying control information that can be included in any existing
IP packet. This DSR Options header in a packet contains a small
fixed-sized, 4-octet portion, followed by a sequence of zero or more
DSR options carrying optional information. The end of the sequence
of DSR options in the DSR Options header is implied by total length
of the DSR Options header.
For IPv4, the DSR Options header MUST immediately follow the IP
header in the packet. (If a Hop-by-Hop Options extension header, as
defined in IPv6 [7], becomes defined for IPv4, the DSR Options header
MUST immediately follow the Hop-by-Hop Options extension header, if
one is present in the packet, and MUST otherwise immediately follow
the IP header.)
To add a DSR Options header to a packet, the DSR Options header is
inserted following the packet's IP header, before any following
header such as a traditional (e.g., TCP or UDP) transport layer
header. Specifically, the Protocol field in the IP header is used
to indicate that a DSR Options header follows the IP header, and the
Next Header field in the DSR Options header is used to indicate the
type of protocol header (such as a transport layer header) following
the DSR Options header.
If any headers follow the DSR Options header in a packet, the total
length of the DSR Options header (and thus the total, combined length
of all DSR options present) MUST be a multiple of 4 octets. This
requirement preserves the alignment of these following headers in the
packet.
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6.1. Fixed Portion of DSR Options Header
The fixed portion of the DSR Options header is used to carry
information that must be present in any DSR Options header. This
fixed portion of the DSR Options header has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header |F| Reserved | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Options .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header
8-bit selector. Identifies the type of header immediately
following the DSR Options header. Uses the same values as the
IPv4 Protocol field [34].
Flow State Header (F)
Flag bit. MUST be set to 0. This bit is set in a DSR Flow
State header (Section 7.1) and clear in a DSR Options header.
Reserved
MUST be sent as 0 and ignored on reception.
Payload Length
The length of the DSR Options header, excluding the 4-octet
fixed portion. The value of the Payload Length field defines
the total length of all options carried in the DSR Options
header.
Options
Variable-length field; the length of the Options field is
specified by the Payload Length field in this DSR Options
header. Contains one or more pieces of optional information
(DSR options), encoded in type-length-value (TLV) format (with
the exception of the Pad1 option, described in Section 6.8).
The placement of DSR options following the fixed portion of the DSR
Options header MAY be padded for alignment. However, due to the
typically limited available wireless bandwidth in ad hoc networks,
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this padding is not required, and receiving nodes MUST NOT expect
options within a DSR Options header to be aligned.
Each DSR option is assigned a unique Option Type code. The most
significant 3 bits (that is, Option Type & 0xE0) allow a node not
implementing processing for this Option Type value to behave in the
manner closest to correct for that type:
- The most significant bit in the Option Type value (that is,
Option Type & 0x80) represents whether or not a node receiving
this Option Type SHOULD respond to such a DSR option with a Route
Error of type OPTION_NOT_SUPPORTED, except that such a Route
Error SHOULD never be sent in response to a packet containing a
Route Request option.
- The two follow bits in the Option Type value (that is,
Option Type & 0x60) are a two-bit field indicating how such a
node that does not support this Option Type MUST process the
packet:
00 = Ignore Option
01 = Remove Option
10 = Mark Option
11 = Drop Packet
When these two bits are zero (that is, Option Type & 0x60 == 0),
a node not implementing processing for that Option Type
MUST use the Opt Data Len field to skip over the option and
continue processing. When these two bits are 01 (that is,
Option Type & 0x60 == 0x20), a node not implementing processing
for that Option Type MUST use the Opt Data Len field to remove
the option from the packet and continue processing as if the
option had not been included in the received packet. When these
two bits are 10 (that is, Option Type & 0x60 == 0x40), a node not
implementing processing for that Option Type MUST set the most
significant bit following the Opt Data Len field, MUST ignore the
contents of the option using the Opt Data Len field, and MUST
continue processing the packet. Finally, when these two bits are
11 (that is, Option Type & 0x60 == 0x60), a node not implementing
processing for that Option Type MUST drop the packet.
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The following types of DSR options are defined in this document for
use within a DSR Options header:
- Route Request option (Section 6.2)
- Route Reply option (Section 6.3)
- Route Error option (Section 6.4)
- Acknowledgement Request option (Section 6.5)
- Acknowledgement option (Section 6.6)
- DSR Source Route option (Section 6.7)
- Pad1 option (Section 6.8)
- PadN option (Section 6.9)
In addition, Section 7 specifies further DSR options for use with the
optional DSR flow state extension.
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6.2. Route Request Option
The Route Request option in a DSR Options header is encoded as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Opt Data Len | Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Target Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[2] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[n] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP fields:
Source Address
MUST be set to the address of the node originating this packet.
Intermediate nodes that retransmit the packet to propagate the
Route Request MUST NOT change this field.
Destination Address
MUST be set to the IP limited broadcast address
(255.255.255.255).
Hop Limit (TTL)
MAY be varied from 1 to 255, for example to implement
non-propagating Route Requests and Route Request expanding-ring
searches (Section 3.3.3).
Route Request fields:
Option Type
1. Nodes not understanding this option will ignore this
option.
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Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len 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 propagated.
Target Address
The address of the node that is the target of the Route
Request.
Address[1..n]
Address[i] is the address of the i-th node recorded in the
Route Request option. The address given in the Source Address
field in the IP header is the address of the initiator of
the Route Discovery and MUST NOT be listed in the Address[i]
fields; the address given in Address[1] is thus the address
of the first node on the path after the initiator. The
number of addresses present in this field is indicated by the
Opt Data Len field in the option (n = (Opt Data Len - 6) / 4).
Each node propagating the Route Request adds its own address to
this list, increasing the Opt Data Len value by 4 octets.
The Route Request option MUST NOT appear more than once within a DSR
Options header.
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6.3. Route Reply Option
The Route Reply option in a DSR Options header is encoded as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Opt Data Len |L| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[2] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[n] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP fields:
Source Address
Set to the address of the node sending the Route Reply.
In the case of a node sending a reply from its Route
Cache (Section 3.3.2) or sending a gratuitous Route Reply
(Section 3.4.3), this address can differ from the address that
was the target of the Route Discovery.
Destination Address
MUST be set to the address of the source node of the route
being returned. Copied from the Source Address field of the
Route Request generating the Route Reply, or in the case of a
gratuitous Route Reply, copied from the Source Address field of
the data packet triggering the gratuitous Reply.
Route Reply fields:
Option Type
2. Nodes not understanding this option will ignore this
option.
Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len fields.
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Last Hop External (L)
Set to indicate that the last hop given by the Route Reply
(the link from Address[n-1] to Address[n]) is actually an
arbitrary path in a network external to the DSR network; the
exact route outside the DSR network is not represented in the
Route Reply. Nodes caching this hop in their Route Cache MUST
flag the cached hop with the External flag. Such hops MUST NOT
be returned in a cached Route Reply generated from this Route
Cache entry, and selection of routes from the Route Cache to
route a packet being sent MUST prefer routes that contain no
hops flagged as External.
Reserved
MUST be sent as 0 and ignored on reception.
Address[1..n]
The source route being returned by the Route Reply. The route
indicates a sequence of hops, originating at the source node
specified in the Destination Address field of the IP header
of the packet carrying the Route Reply, through each of the
Address[i] nodes in the order listed in the Route Reply,
ending with the destination node indicated by Address[n].
The number of addresses present in the Address[1..n]
field is indicated by the Opt Data Len field in the option
(n = (Opt Data Len - 1) / 4).
A Route Reply option MAY appear one or more times within a DSR
Options header.
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6.4. Route Error Option
The Route Error option in a DSR Options header is encoded as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Opt Data Len | Error Type |Reservd|Salvage|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Type-Specific Information .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
3. Nodes not understanding this option will ignore this
option.
Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len fields.
For the current definition of the Route Error option,
this field MUST be set to 10, plus the size of any
Type-Specific Information present in the Route Error. Further
extensions to the Route Error option format may also be
included after the Type-Specific Information portion of the
Route Error option specified above. The presence of such
extensions will be indicated by the Opt Data Len field.
When the Opt Data Len is greater than that required for
the fixed portion of the Route Error plus the necessary
Type-Specific Information as indicated by the Option Type
value in the option, the remaining octets are interpreted as
extensions. Currently, no such further extensions have been
defined.
Error Type
The type of error encountered. Currently, the following type
values are defined:
1 = NODE_UNREACHABLE
2 = FLOW_STATE_NOT_SUPPORTED
3 = OPTION_NOT_SUPPORTED
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Other values of the Error Type field are reserved for future
use.
Reservd
Reserved. MUST be sent as 0 and ignored on reception.
Salvage
A 4-bit unsigned integer. Copied from the Salvage field in
the DSR Source Route option of the packet triggering the Route
Error.
The "total salvage count" of the Route Error option is derived
from the value in the Salvage field of this Route Error option
and all preceding Route Error options in the packet as follows:
the total salvage count is the sum of, for each such Route
Error option, one plus the value in the Salvage field of that
Route Error option.
Error Source Address
The address of the node originating the Route Error (e.g., the
node that attempted to forward a packet and discovered the link
failure).
Error Destination Address
The address of the node to which the Route Error must be
delivered For example, when the Error Type field is set to
NODE_UNREACHABLE, this field will be set to the address of the
node that generated the routing information claiming that the
hop from the Error Source Address to Unreachable Node Address
(specified in the Type-Specific Information) was a valid hop.
Type-Specific Information
Information specific to the Error Type of this Route Error
message.
A Route Error option MAY appear one or more times within a DSR
Options header.
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6.4.1. Node Unreachable Type-Specific Information
When the Route Error is of type NODE_UNREACHABLE, the
Type-Specific Information field is defined as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unreachable Node Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Unreachable Node Address
The address of the node that was found to be unreachable
(the next-hop neighbor to which the node with address
Error Source Address was attempting to transmit the packet).
6.4.2. Flow State Not Supported Type-Specific Information
When the Route Error is of type FLOW_STATE_NOT_SUPPORTED, the
Type-Specific Information field is empty.
6.4.3. Option Not Supported Type-Specific Information
When the Route Error is of type OPTION_NOT_SUPPORTED, the
Type-Specific Information field is defined as follows:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|Unsupported Opt|
+-+-+-+-+-+-+-+-+
Unsupported Opt
The type of option triggering the Route Error.
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6.5. Acknowledgement Request Option
The Acknowledgement Request option in a DSR Options header is encoded
as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Opt Data Len | Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
160. Nodes not understanding this option will remove the
option and return a Route Error.
Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len fields.
Identification
The Identification field is set to a unique value and is copied
into the Identification field of the Acknowledgement option
when returned by the node receiving the packet over this hop.
An Acknowledgement Request option MUST NOT appear more than once
within a DSR Options header.
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6.6. Acknowledgement Option
The Acknowledgement option in a DSR Options header is encoded as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Opt Data Len | Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACK Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACK Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
32. Nodes not understanding this option will remove the
option.
Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len fields.
Identification
Copied from the Identification field of the Acknowledgement
Request option of the packet being acknowledged.
ACK Source Address
The address of the node originating the acknowledgement.
ACK Destination Address
The address of the node to which the acknowledgement is to be
delivered.
An Acknowledgement option MAY appear one or more times within a DSR
Options header.
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6.7. DSR Source Route Option
The DSR Source Route option in a DSR Options header is encoded as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Opt Data Len |F|L|Reservd|Salvage| Segs Left |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[2] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[n] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
96. Nodes not understanding this option will drop the packet.
Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len fields. For the
format of the DSR Source Route option defined here, this field
MUST be set to the value (n * 4) + 2, where n is the number of
addresses present in the Address[i] fields.
First Hop External (F)
Set to indicate that the first hop indicated by the DSR
Source Route option is actually an arbitrary path in a network
external to the DSR network; the exact route outside the DSR
network is not represented in the DSR Source Route option.
Nodes caching this hop in their Route Cache MUST flag the
cached hop with the External flag. Such hops MUST NOT be
returned in a Route Reply generated from this Route Cache
entry, and selection of routes from the Route Cache to route
a packet being sent MUST prefer routes that contain no hops
flagged as External.
Last Hop External (L)
Set to indicate that the last hop indicated by the DSR Source
Route option is actually an arbitrary path in a network
external to the DSR network; the exact route outside the DSR
network is not represented in the DSR Source Route option.
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Nodes caching this hop in their Route Cache MUST flag the
cached hop with the External flag. Such hops MUST NOT be
returned in a Route Reply generated from this Route Cache
entry, and selection of routes from the Route Cache to route
a packet being sent MUST prefer routes that contain no hops
flagged as External.
Reserved
MUST be sent as 0 and ignored on reception.
Salvage
A 4-bit unsigned integer. Count of number of times that
this packet has been salvaged as a part of DSR routing
(Section 3.4.1).
Segments Left (Segs Left)
Number of route segments remaining, i.e., number of explicitly
listed intermediate nodes still to be visited before reaching
the final destination.
Address[1..n]
The sequence of addresses of the source route. In routing
and forwarding the packet, the source route is processed as
described in Sections 8.1.3 and 8.1.5. The number of addresses
present in the Address[1..n] field is indicated by the
Opt Data Len field in the option (n = (Opt Data Len - 2) / 4).
When forwarding a packet along a DSR source route using a DSR Source
Route option in the packet's DSR Options header, the Destination
Address field in the packet's IP header is always set to the address
of the packet's ultimate destination. A node receiving a packet
containing a DSR Options header with a DSR Source Route option MUST
examine the indicated source route to determine if it is the intended
next-hop node for the packet and determine how to forward the packet,
as defined in Sections 8.1.4 and 8.1.5.
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6.8. Pad1 Option
The Pad1 option in a DSR Options header is encoded as follows:
+-+-+-+-+-+-+-+-+
| Option Type |
+-+-+-+-+-+-+-+-+
Option Type
224. Nodes not understanding this option will drop the packet
and return a Route Error.
A Pad1 option MAY be included in the Options field of a DSR Options
header in order to align subsequent DSR options, but such alignment
is not required and MUST NOT be expected by a node receiving a packet
containing a DSR Options header.
If any headers follow the DSR Options header in a packet, the total
length of a DSR Options header, indicated by the Payload Length field
in the DSR Options header MUST be a multiple of 4 octets. In this
case, when building a DSR Options header in a packet, sufficient Pad1
or PadN options MUST be included in the Options field of the DSR
Options header to make the total length a multiple of 4 octets.
If more than one consecutive octet of padding is being inserted in
the Options field of a DSR Options header, the PadN option, described
next, SHOULD be used, rather than multiple Pad1 options.
Note that the format of the Pad1 option is a special case; it does
not have an Opt Data Len or Option Data field.
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6.9. PadN Option
The PadN option in a DSR Options header is encoded as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
| Option Type | Opt Data Len | Option Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
Option Type
0. Nodes not understanding this option will ignore this
option.
Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len fields.
Option Data
A number of zero-valued octets equal to the Opt Data Len.
A PadN option MAY be included in the Options field of a DSR Options
header in order to align subsequent DSR options, but such alignment
is not required and MUST NOT be expected by a node receiving a packet
containing a DSR Options header.
If any headers follow the DSR Options header in a packet, the total
length of a DSR Options header, indicated by the Payload Length field
in the DSR Options header MUST be a multiple of 4 octets. In this
case, when building a DSR Options header in a packet, sufficient Pad1
or PadN options MUST be included in the Options field of the DSR
Options header to make the total length a multiple of 4 octets.
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7. Additional Header Formats and Options for Flow State Extension
The optional DSR flow state extension requires a new header type, the
DSR Flow State header.
In addition, the DSR flow state extension adds the following options
for the DSR Options header defined in Section 6:
- Timeout option (Section 7.2.1
- Destination and Flow ID option (Section 7.2.2
Two new Error Type values are also defined for use in the Route Error
option in a DSR Options header:
- Unknown Flow
- Default Flow Unknown
Finally, an extension to the Acknowledgement Request option in a DSR
Options header is also defined:
- Previous Hop Address
This section defines each of these new header or option formats.
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7.1. DSR Flow State Header
The DSR Flow State header is a small 4-byte header optionally used
to carry the flow ID and hop count for a packet being sent along a
DSR flow. It is distinguished from the fixed DSR Options header
(Section 6.1) in that the Flow State Header (F) bit is set in the DSR
Flow State header and is clear in the fixed DSR Options header.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header |F| Hop Count | Flow Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header
8-bit selector. Identifies the type of header immediately
following the DSR Flow State header. Uses the same values as
the IPv4 Protocol field [34].
Flow State Header (F)
Flag bit. MUST be set to 1. This bit is set in a DSR Flow
State header and clear in a DSR Options header (Section 6.1).
Hop Count
7-bit unsigned integer. The number of hops through which this
packet has been forwarded.
Flow Identification
The flow ID for this flow, as described in Section 3.5.1.
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7.2. New Options and Extensions in DSR Options Header
7.2.1. Timeout Option
The Timeout option is defined for use in a DSR Options header to
indicate the amount of time before the expiration of the flow ID
along which the packet is being sent.
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 | Timeout |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
128. Nodes not understanding this option will ignore the
option and return a Route Error.
Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len fields.
When no extensions are present, the Opt Data Len of a Timeout
option is 2. Further extensions to DSR may include additional
data in a Timeout option. The presence of such extensions is
indicated by an Opt Data Len greater than 2. Currently, no
such extensions have been defined.
Timeout
The number of seconds for which this flow remains valid.
The Timeout option MUST NOT appear more than once within a DSR
Options header.
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7.2.2. Destination and Flow ID Option
The Destination and Flow ID option is defined for use in a DSR
Options header to send a packet to an intermediate host along one
flow, for eventual forwarding to the final destination along a
different flow. This option enables the aggregation of the state of
multiple flows.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length | New Flow Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| New IP Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
129. Nodes not understanding this option will ignore the
option and return a Route Error.
Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len fields.
When no extensions are present, the Opt Data Len of a
Destination and Flow ID option is 6. Further extensions to
DSR may include additional data in a Destination and Flow ID
option. The presence of such extensions is indicated by an
Opt Data Len greater than 6. Currently, no such extensions
have been defined.
New Flow Identifier
Indicates the next identifier to store in the Flow ID field of
the DSR Options header.
New IP Destination Address
Indicates the next address to store in the Destination Address
field of the IP header.
The Destination and Flow ID option MAY appear one or more times
within a DSR Options header.
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7.3. New Error Types for Route Error Option
7.3.1. Unknown Flow Type-Specific Information
A new Error Type value of 129 (Unknown Flow) is defined for use in
a Route Error option in a DSR Options header. The Type-Specific
Information for errors of this type is encoded as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Original IP Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flow ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Original IP Destination Address
The IP Destination Address of the packet that caused the error.
Flow ID
The Flow ID contained in the DSR Flow ID option that caused the
error.
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7.3.2. Default Flow Unknown Type-Specific Information
A new Error Type value of 130 (Default Flow Unknown) is defined
for use in a Route Error option in a DSR Options header. The
Type-Specific Information for errors of this type is encoded as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Original IP Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Original IP Destination Address
The IP Destination Address of the packet that caused the error.
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7.4. New Acknowledgement Request Option Extension
7.4.1. Previous Hop Address Extension
When the Option Length field of an Acknowledgement Request option
in a DSR Options header is greater than or equal to 6, a Previous
Hop Address Extension is present. The option is then formatted as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length | Packet Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACK Request Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
5
Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields.
When no extensions are presents, the Option Length of a
Acknowledgement Request option is 2. Further extensions to
DSR may include additional data in a Acknowledgement Request
option. The presence of such extensions is indicated by an
Opt Data Len greater than 2.
Currently, one such extension has been defined. If the
Option Length is at least 6, then a ACK Request Source Address
is present.
Packet Identifier
The Packet Identifier field is set to a unique number and is
copied into the Identification field of the DSR Acknowledgement
option when returned by the node receiving the packet over this
hop.
ACK Request Source Address
The address of the node requesting the DSR Acknowledgement.
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8. Detailed Operation
8.1. General Packet Processing
8.1.1. Originating a Packet
When originating any packet, a node using DSR routing MUST perform
the following sequence of steps:
- 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 8.2. Initiating a Route Discovery for this target node
address results in the node adding a Route Request option in
a DSR Options header in this existing packet, or saving this
existing packet to its Send Buffer and initiating the Route
Discovery by sending a separate packet containing such a Route
Request option. If the node chooses to initiate the Route
Discovery by adding the Route Request option to this existing
packet, it will replace the IP Destination Address field with the
IP "limited broadcast" address (255.255.255.255) [3], copying the
original IP Destination Address to the Target Address field of
the new Route Request option added to the packet, as described in
Section 8.2.1.
- If the packet now does not contain a Route Request option,
then this node must have a route to the Destination Address
of the packet; if the node has more than one route to this
Destination Address, the node selects one to use for this packet.
If the length of this route is greater than 1 hop, or if the
node determines to request a DSR network-layer acknowledgement
from the first-hop node in that route, then insert a DSR Options
header into the packet, as described in Section 8.1.2, and insert
a DSR Source Route option, as described in Section 8.1.3. The
source route in the packet is initialized from the selected route
to the Destination Address of the packet.
- Transmit the packet to the first-hop node address given in
selected source route, using Route Maintenance to determine the
reachability of the next hop, as described in Section 8.3.
8.1.2. Adding a DSR Options Header to a Packet
A node originating a packet adds a DSR Options header to the packet,
if necessary, to carry information needed by the routing protocol.
A packet MUST NOT contain more than one DSR Options header. A DSR
Options header is added to a packet by performing the following
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sequence of steps (these steps assume that the packet contains no
other headers that MUST be located in the packet before the DSR
Options header):
- Insert a DSR Options header after the IP header but before any
other header that may be present.
- Set the Next Header field of the DSR Options header to the
Protocol number field of the packet's IP header.
- Set the Protocol field of the packet's IP header to the Protocol
number assigned for DSR (TBA???).
8.1.3. Adding a DSR Source Route Option to a Packet
A node originating a packet adds a DSR Source Route option to the
packet, if necessary, in order to carry the source route from this
originating node to the final destination address of the packet.
Specifically, the node adding the DSR Source Route option constructs
the DSR Source Route option and modifies the IP packet according to
the following sequence of steps:
- The node creates a DSR Source Route option, as described
in Section 6.7, and appends it to the DSR Options header in
the packet. (A DSR Options header is added, as described in
Section 8.1.2, if not already present.)
- The number of Address[i] fields to include in the DSR Source
Route option (n) is the number of intermediate nodes in the
source route for the packet (i.e., excluding address of the
originating node and the final destination address of the
packet). The Segments Left field in the DSR Source Route option
is initialized equal to n.
- The addresses within the source route for the packet are copied
into sequential Address[i] fields in the DSR Source Route option,
for i = 1, 2, ..., n.
- The First Hop External (F) bit in the DSR Source Route option is
copied from the External bit flagging the first hop in the source
route for the packet, as indicated in the Route Cache.
- The Last Hop External (L) bit in the DSR Source Route option is
copied from the External bit flagging the last hop in the source
route for the packet, as indicated in the Route Cache.
- The Salvage field in the DSR Source Route option is
initialized to 0.
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8.1.4. Processing a Received Packet
When a node receives any packet (whether for forwarding, overheard,
or as the final destination of the packet), if that packet contains
a DSR Options header, then that node MUST process any options
contained in that DSR Options header, in the order contained there.
Specifically:
- If the DSR Options header contains a Route Request option, the
node SHOULD extract the source route from the Route Request and
add this routing information to its Route Cache, subject to the
conditions identified in Section 3.3.1. The routing information
from the Route Request is the sequence of hop addresses
initiator, Address[1], Address[2], ..., Address[n]
where initiator is the value of the Source Address field in
the IP header of the packet carrying the Route Request (the
address of the initiator of the Route Discovery), and each
Address[i] is a node through which this Route Request has passed,
in turn, during this Route Discovery. The value n here is the
number of addresses recorded in the Route Request option, or
(Opt Data Len - 6) / 4.
After possibly updating the node's Route Cache in response to
the routing information in the Route Request option, the node
MUST then process the Route Request option as described in
Section 8.2.2.
- If the DSR Options header contains a Route Reply option, the node
SHOULD extract the source route from the Route Reply and add this
routing information to its Route Cache, subject to the conditions
identified in Section 3.3.1. The source route from the Route
Reply is the sequence of hop addresses
initiator, Address[1], Address[2], ..., Address[n]
where initiator is the value of the Destination Address field in
the IP header of the packet carrying the Route Reply (the address
of the initiator of the Route Discovery), and each Address[i]
is a node through which the source route passes, in turn, on
the route to the target of the Route Discovery. Address[n] is
the address of the target. If the Last Hop External (L) bit is
set in the Route Reply, the node MUST flag the last hop from
the Route Reply (the link from Address[n-1] to Address[n]) in
its Route Cache as External. The value n here is the number of
addresses in the source route being returned in the Route Reply
option, or (Opt Data Len - 1) / 4.
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After possibly updating the node's Route Cache in response to
the routing information in the Route Reply option, then if the
packet's IP Destination Address matches one of this node's IP
addresses, the node MUST then process the Route Reply option as
described in Section 8.2.6.
- If the DSR Options header contains a Route Error option,
the node MUST process the Route Error option as described in
Section 8.3.5.
- If the DSR Options header contains an Acknowledgement Request
option, the node MUST process the Acknowledgement Request option
as described in Section 8.3.3.
- If the DSR Options header contains an Acknowledgement option,
then subject to the conditions identified in Section 3.3.1, the
node SHOULD add to its Route Cache the single link from the node
identified by the ACK Source Address field to the node identified
by the ACK Destination Address field.
After possibly updating the node's Route Cache in response to
the routing information in the Acknowledgement option, the node
MUST then process the Acknowledgement option as described in
Section 8.3.3.
- If the DSR Options header contains a DSR Source Route option, the
node SHOULD extract the source route from the DSR Source Route
and add this routing information to its Route Cache, subject to
the conditions identified in Section 3.3.1. If the value of the
Salvage field in the DSR Source Route option is zero, then the
routing information from the DSR Source Route is the sequence of
hop addresses
source, Address[1], Address[2], ..., Address[n], destination
and otherwise (Salvage is nonzero), the routing information from
the DSR Source Route is the sequence of hop addresses
Address[1], Address[2], ..., Address[n], destination
where source is the value of the Source Address field in the IP
header of the packet carrying the DSR Source Route option (the
original sender of the packet), each Address[i] is the value in
the Address[i] field in the DSR Source Route, and destination is
the value of the Destination Address field in the packet's IP
header (the last-hop address of the source route). The value n
here is the number of addresses in source route in the DSR Source
Route option, or (Opt Data Len - 2) / 4.
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After possibly updating the node's Route Cache in response to
the routing information in the DSR Source Route option, the node
MUST then process the DSR Source Route option as described in
Section 8.1.5.
- Any Pad1 or PadN options in the DSR Options header are ignored.
Finally, if the Destination Address in the packet's IP header matches
one of this receiving node's own IP address(es), remove the DSR
Options header and all the included DSR options in the header, and
pass the rest of the packet to the network layer.
8.1.5. Processing a Received DSR Source Route Option
When a node receives a packet containing a DSR Source Route option
(whether for forwarding, overheard, or as the final destination of
the packet), that node SHOULD examine the packet to determine if
the receipt of that packet indicates an opportunity for automatic
route shortening, as described in Section 3.4.3. Specifically, if
this node is not the intended next-hop destination for the packet
but is named in the later unexpended portion of the source route in
the packet's DSR Source Route option, then this packet indicates an
opportunity for automatic route shortening: the intermediate nodes
after the node from which this node overheard the packet and before
this node itself, are no longer necessary in the source route. In
this case, this node SHOULD perform the following sequence of steps
as part of automatic route shortening:
- The node searches its Gratuitous Route Reply Table for an entry
describing a gratuitous Route Reply earlier sent by this node,
for which the original sender of the packet triggering the
gratuitous Route Reply and the transmitting node from which this
node overheard that packet in order to trigger the gratuitous
Route Reply, both match the respective node addresses for this
new received packet. If such an entry is found in the node's
Gratuitous Route Reply Table, the node SHOULD NOT perform
automatic route shortening in response to this receipt of this
packet.
- Otherwise, the node creates an entry for this overheard packet in
its Gratuitous Route Reply Table. The timeout value for this new
entry SHOULD be initialized to the value GratReplyHoldoff. After
this timeout has expired, the node SHOULD delete this entry from
its Gratuitous Route Reply Table.
- After creating the new Gratuitous Route Reply Table entry
above, the node originates a gratuitous Route Reply to the
IP Source Address of this overheard packet, as described in
Section 3.4.3.
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If the MAC protocol in use in the network is not capable of
transmitting unicast packets over unidirectional links, as
discussed in Section 3.3.1, then in originating this Route Reply,
the node MUST use a source route for routing the Route Reply
packet that is obtained by reversing the sequence of hops over
which the packet triggering the gratuitous Route Reply was routed
in reaching and being overheard by this node; this reversing of
the route uses the gratuitous Route Reply to test this sequence
of hops for bidirectionality, preventing the gratuitous Route
Reply from being received by the initiator of the Route Discovery
unless each of the hops over which the gratuitous Route Reply is
returned is bidirectional.
- Discard the overheard packet, since the packet has been received
before its normal traversal of the packet's source route would
have caused it to reach this receiving node. Another copy of
the packet will normally arrive at this node as indicated in
the packet's source route; discarding this initial copy of the
packet, which triggered the gratuitous Route Reply, will prevent
the duplication of this packet that would otherwise occur.
If the packet is not discarded as part of automatic route shortening
above, then the node MUST process the Source Route option according
to the following sequence of steps:
- If the value of the Segments Left field in the DSR Source Route
option equals 0, then remove the DSR Source Route option from the
DSR Options header.
- Else, let n equal (Opt Data Len - 2) / 4. This is the number of
addresses in the DSR Source Route option.
- If the value of the Segments Left field is greater than n, then
send an ICMP Parameter Problem, Code 0, message [31] to the IP
Source Address, pointing to the Segments Left field, and discard
the packet. Do not process the DSR Source Route option further.
- Else, decrement the value of the Segments Left field by 1. Let i
equal n minus Segments Left. This is the index of the next
address to be visited in the Address vector.
- If Address[i] or the IP Destination Address is a multicast
address, then discard the packet. Do not process the DSR Source
Route option further.
- If this node has more than one network interface and if
Address[i] is the address of one this node's network interfaces,
then this indicates a change in the network interface to use in
forwarding the packet, as described in Section 8.4. In this
case, decrement the value of the Segments Left field by 1 to
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skip over this address (that indicated the change of network
interface) and go to the first step above (checking the value of
the Segments Left field) to continue processing this Source Route
option; in further processing of this Source Route option, the
indicated new network interface MUST be used in forwarding the
packet.
- If the MTU of the link over which this node would transmit
the packet to forward it to the node Address[i] is less than
the size of the packet, the node MUST either discard the
packet and send an ICMP Packet Too Big message to the packet's
Source Address [31] or fragment it as specified in Section 8.5.
- Forward the packet to the IP address specified in the Address[i]
field of the IP header, following normal IP forwarding
procedures, including checking and decrementing the Time-to-Live
(TTL) field in the packet's IP header [32, 3]. In this
forwarding of the packet, the next-hop node (identified by
Address[i]) MUST be treated as a direct neighbor node: the
transmission to that next node MUST be done in a single IP
forwarding hop, without Route Discovery and without searching the
Route Cache.
- In forwarding the packet, perform Route Maintenance for the
next hop of the packet, by verifying that the next-hop node is
reachable, as described in Section 8.3.
Multicast addresses MUST NOT appear in a DSR Source Route option or
in the IP Destination Address field of a packet carrying a DSR Source
Route option in a DSR Options header.
8.1.6. Handling an Unknown DSR Option
Nodes implementing DSR MUST handle all options specified in this
document, except those options pertaining to the optional flow
state extension (Section 7). However, further extensions to
DSR may include other option types that may not be understood by
implementations conforming to this version of the DSR specification.
In DSR, Option Type codes encode required behavior for nodes not
implementing that type of option. These behaviors are included in
the most significant three bits of the Option Type.
If the most significant bit of the Option Type is set (that is,
Option Type & 0x80 is nonzero), and this packet does not contain
a Route Request option, a node SHOULD return a Route Error to the
IP Source Address, following the steps described in Section 8.3.4,
except that the Error Type MUST be set to OPTION_NOT_SUPPORTED and
the Unsupported Opt field MUST be set to the Option Type triggering
the Route Error.
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Whether or not a Route Error is sent in response to this DSR option,
as described above, the node also MUST examine the next two most
significant bits (that is, Option Type & 0x60):
- When these two bits are zero (that is, Option Type & 0x60 == 0),
a node not implementing processing for that Option Type MUST
use the Opt Data Len field to skip over the option and continue
processing.
- When these two bits are 01 (that is, Option Type & 0x60 == 0x20),
a node not implementing processing for that Option Type MUST use
the Opt Data Len field to remove the option from the packet and
continue processing as if the option had not been included in the
received packet.
- When these two bits are 10 (that is, Option Type & 0x60 == 0x40),
a node not implementing processing for that Option Type MUST set
the most significant bit following the Opt Data Len field; in
addition, the node MUST then ignore the contents of the option
using the Opt Data Len field, and MUST continue processing the
packet.
- Finally, when these two bits are 11 (that is,
Option Type & 0x60 == 0x60), a node not implementing processing
for that Option Type MUST drop the packet.
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8.2. Route Discovery Processing
Route Discovery is the mechanism by which a node S wishing to send a
packet to a destination node D obtains a source route to D. Route
Discovery is used only when S attempts to send a packet to D and
does not already know a route to D. The node initiating a Route
Discovery is known as the "initiator" of the Route Discovery, and the
destination node for which the Route Discovery is initiated is known
as the "target" of the Route Discovery.
Route Discovery operates entirely on demand, with a node initiating
Route Discovery based on its own origination of new packets for
some destination address to which it does not currently know a
route. Route Discovery does not depend on any periodic or background
exchange of routing information or neighbor node detection at any
layer in the network protocol stack at any node.
The Route Discovery procedure utilizes two types of messages, a Route
Request (Section 6.2) and a Route Reply (Section 6.3), to actively
search the ad hoc network for a route to the desired destination.
These DSR messages MAY be carried in any type of IP packet, through
use of the DSR Options header as described in Section 6.
Except as discussed in Section 8.3.5, a Route Discovery for a
destination address SHOULD NOT be initiated unless the initiating
node has a packet in its Send Buffer requiring delivery to that
destination. A Route Discovery for a given target node MUST NOT be
initiated unless permitted by the rate-limiting information contained
in the Route Request Table. After each Route Discovery attempt, the
interval between successive Route Discoveries for this target SHOULD
be doubled, up to a maximum of MaxRequestPeriod, until a valid Route
Reply is received for this target.
8.2.1. Originating a Route Request
A node initiating a Route Discovery for some target creates and
initializes a Route Request option in a DSR Options header in some
IP packet. This MAY be a separate IP packet, used only to carry
this Route Request option, or the node MAY include the Route Request
option in some existing packet that it needs to send to the target
node (e.g., the IP packet originated by this node, that caused the
node to attempt Route Discovery for the destination address of the
packet). The Route Request option MUST be included in a DSR Options
header in the packet. To initialize the Route Request option, the
node performs the following sequence of steps:
- The Option Type in the option MUST be set to the value 2.
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- The Opt Data Len field in the option MUST be set to the value 6.
The total size of the Route Request option when initiated
is 8 octets; the Opt Data Len field excludes the size of the
Option Type and Opt Data Len fields themselves.
- The Identification field in the option MUST be set to a new
value, different from that used for other Route Requests recently
initiated by this node for this same target address. For
example, each node MAY maintain a single counter value for
generating a new Identification value for each Route Request it
initiates.
- The Target Address field in the option MUST be set to the IP
address that is the target of this Route Discovery.
The Source Address in the IP header of this packet MUST be the node's
own IP address. The Destination Address in the IP header of this
packet MUST be the IP "limited broadcast" address (255.255.255.255).
A node MUST maintain in its Route Request Table, information about
Route Requests that it initiates. When initiating a new Route
Request, the node MUST use the information recorded in the Route
Request Table entry for the target of that Route Request, and it MUST
update that information in the table entry for use in the next Route
Request initiated for this target. In particular:
- The Route Request Table entry for a target node records the
Time-to-Live (TTL) field used in the IP header of the Route
Request for the last Route Discovery initiated by this node for
that target node. This value allows the node to implement a
variety of algorithms for controlling the spread of its Route
Request on each Route Discovery initiated for a target. As
examples, two possible algorithms for this use of the TTL field
are described in Section 3.3.3.
- The Route Request Table entry for a target node records the
number of consecutive Route Requests initiated for this target
since receiving a valid Route Reply giving a route to that target
node, and the remaining amount of time before which this node MAY
next attempt at a Route Discovery for that target node.
A node MUST use these values to implement a back-off algorithm to
limit the rate at which this node initiates new Route Discoveries
for the same target address. In particular, until a valid Route
Reply is received for this target node address, the timeout
between consecutive Route Discovery initiations for this target
node with the same hop limit SHOULD increase by doubling the
timeout value on each new initiation.
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The behavior of a node processing a packet containing DSR Options
header with both a DSR Source Route option and a Route Request option
is unspecified. Packets SHOULD NOT contain both a DSR Source Route
option and a Route Request option.
Packets containing a Route Request option SHOULD NOT include
an Acknowledgement Request option, SHOULD NOT expect link-layer
acknowledgement or passive acknowledgement, and SHOULD NOT be
retransmitted. The retransmission of packets containing a Route
Request option is controlled solely by the logic described in this
section.
8.2.2. Processing a Received Route Request Option
When a node receives a packet containing a Route Request option, that
node MUST process the option according to the following sequence of
steps:
- If the Target Address field in the Route Request matches this
node's own IP address, then the node SHOULD return a Route Reply
to the initiator of this Route Request (the Source Address in the
IP header of the packet), as described in Section 8.2.4. The
source route for this Reply is the sequence of hop addresses
initiator, Address[1], Address[2], ..., Address[n], target
where initiator is the address of the initiator of this
Route Request, each Address[i] is an address from the Route
Request, and target is the target of the Route Request (the
Target Address field in the Route Request). The value n here
is the number of addresses recorded in the Route Request, or
(Opt Data Len - 6) / 4.
The node then MUST replace the Destination Address field in
the Route Request packet's IP header with the value in the
Target Address field in the Route Request option, and continue
processing the rest of the Route Request packet normally. The
node MUST NOT process the Route Request option further and MUST
NOT retransmit the Route Request to propagate it to other nodes
as part of the Route Discovery.
- Else, the node MUST examine the route recorded in the Route
Request option (the IP Source Address field and the sequence of
Address[i] fields) to determine if this node's own IP address
already appears in this list of addresses. If so, the node MUST
discard the entire packet carrying the Route Request option.
- Else, if the Route Request was received through a network
interface that requires physically bidirectional links for
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unicast transmission, the node MUST check if the Route Request
was last forwarded by a node on its Blacklist (Section 4.6).
If such an entry is found in the Blacklist, and the state of
the unidirectional link is "probable", then the Request MUST be
silently discarded.
- Else, if the Route Request was received through a network
interface that requires physically bidirectional links for
unicast transmission, the node MUST check if the Route Request
was last forwarded by a node on its Blacklist. If such an entry
is found in the Blacklist, and the state of the unidirectional
link is "questionable", then the node MUST create and unicast
a Route Request packet to that previous node, setting the
IP Time-To-Live (TTL) to 1 to prevent the Request from being
propagated. If the node receives a Route Reply in response to
the new Request, it MUST remove the Blacklist entry for that
node, and SHOULD continue processing. If the node does not
receive a Route Reply within some reasonable amount of time, the
node MUST silently discard the Route Request packet.
- 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 further process the Route Request
according to the following sequence of steps:
o Add an entry for this Route Request in its cache of
(Identification, target address) values of recently received
Route Requests.
o Conceptually create a copy of this entire packet and perform
the following steps on the copy of the packet.
o Append this node's own IP address to the list of Address[i]
values in the Route Request, and increase the value of the
Opt Data Len field in the Route Request by 4 (the size of
an IP address). However, if the node has multiple network
interfaces, this step MUST be modified by the special
processing specified in Section sec:multiple.
o This node SHOULD search its own Route Cache for a route
(from itself, as if it were the source of a packet) to the
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target of this Route Request. If such a route is found in
its Route Cache, then this node SHOULD follow the procedure
outlined in Section 8.2.3 to return a "cached Route Reply"
to the initiator of this Route Request, if permitted by the
restrictions specified there.
o If the node does not return a cached Route Reply, then this
node SHOULD transmit this copy of the packet as a link-layer
broadcast, with a short jitter delay before the broadcast is
sent. The jitter period SHOULD be chosen as a random period,
uniformly distributed between 0 and BroadcastJitter.
8.2.3. Generating a Route Reply using the Route Cache
As described in Section 3.3.2, it is possible for a node processing a
received Route Request to avoid propagating the Route Request further
toward the target of the Request, if this node has in its Route Cache
a route from itself to this target. Such a Route Reply generated by
a node from its own cached route to the target of a Route Request is
called a "cached Route Reply", and this mechanism can greatly reduce
the overall overhead of Route Discovery on the network by reducing
the flood of Route Requests. The general processing of a received
Route Request is described in Section 8.2.2; this section specifies
the additional requirements that MUST be met before a cached Route
Reply may be generated and returned and specifies the procedure for
returning such a cached Route Reply.
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 8.2.2.
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If the Route Request and the route from the Route Cache meet the
restriction above, then the node SHOULD construct and return a cached
Route Reply as follows:
- The source route for this reply is the sequence of hop addresses
initiator, Address[1], Address[2], ..., Address[n], c-route
where initiator is the address of the initiator of this Route
Request, each Address[i] is an address from the Route Request,
and c-route is the sequence of hop addresses in the source route
to this target node, obtained from the node's Route Cache. In
appending this cached route to the source route for the reply,
the address of this node itself MUST be excluded, since it is
already listed as Address[n].
- Send a Route Reply to the initiator of the Route Request, using
the procedure defined in Section 8.2.4. The initiator of the
Route Request is indicated in the Source Address field in the
packet's IP header.
Before sending the cached Route Reply, however, the node MAY delay
the Reply in order to help prevent a possible Route Reply "storm", as
described in Section 8.2.5.
If the node returns a cached Route Reply as described above,
then the node MUST NOT propagate the Route Request further (i.e.,
the node MUST NOT rebroadcast the Route Request). In this case,
instead, if the packet contains no other DSR options and contains
no payload after the DSR Options header (e.g., the Route Request is
not piggybacked on a TCP or UDP packet), then the node SHOULD simply
discard the packet. Otherwise (if the packet contains other DSR
options or contains any payload after the DSR Options header), the
node SHOULD forward the packet along the cached route to the target
of the Route Request. Specifically, if the node does so, it MUST use
the following steps:
- Copy the Target Address from the Route Request option in the DSR
Options header to the Destination Address field in the packet's
IP header.
- Remove the Route Request option from the DSR Options header in
the packet, and add a DSR Source Route option to the packet's DSR
Options header.
- In the DSR Source Route option, set the Address[i] fields
to represent the source route found in this node's Route
Cache to the original target of the Route Discovery (the
new IP Destination Address of the packet). Specifically,
the node copies the hop addresses of the source route into
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sequential Address[i] fields in the DSR Source Route option,
for i = 1, 2, ..., n. Address[1] here is the address of this
node itself (the first address in the source route found from
this node to the original target of the Route Discovery). The
value n here is the number of hop addresses in this source route,
excluding the destination of the packet (which is instead already
represented in the Destination Address field in the packet's IP
header).
- Initialize the Segments Left field in the DSR Source Route option
to n as defined above.
- The First Hop External (F) bit in the DSR Source Route option is
copied from the External bit flagging the first hop in the source
route for the packet, as indicated in the Route Cache.
- The Last Hop External (L) bit in the DSR Source Route option is
copied from the External bit flagging the last hop in the source
route for the packet, as indicated in the Route Cache.
- The Salvage field in the DSR Source Route option MUST be
initialized to some nonzero value; the particular nonzero value
used SHOULD be MAX_SALVAGE_COUNT. By initializing this field to
a nonzero value, nodes forwarding or overhearing this packet will
not consider a link to exist between the IP Source Address of the
packet and the Address[1] address in the DSR Source Route option
(e.g., they will not attempt to add this to their Route Cache as
a link). By choosing MAX_SALVAGE_COUNT as the nonzero value to
which the node initializes this field, nodes furthermore will not
attempt to salvage this packet.
- Transmit the packet to the next-hop node on the new source route
in the packet, using the forwarding procedure described in
Section 8.1.5.
8.2.4. Originating a Route Reply
A node originates a Route Reply in order to reply to a received and
processed Route Request, according to the procedures described in
Sections 8.2.2 and 8.2.3. The Route Reply is returned in a Route
Reply option (Section 6.3). The Route Reply option MAY be returned
to the initiator of the Route Request in a separate IP packet, used
only to carry this Route Reply option, or it MAY be included in any
other IP packet being sent to this address.
The Route Reply option MUST be included in a DSR Options header in
the packet returned to the initiator. To initialize the Route Reply
option, the node performs the following sequence of steps:
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- The Option Type in the option MUST be set to the value 3.
- The Opt Data Len field in the option MUST be set to the value
(n * 4) + 3, where n is the number of addresses in the source
route being returned (excluding the Route Discovery initiator
node's address).
- The Last Hop External (L) bit in the option MUST be
initialized to 0.
- The Reserved field in the option MUST be initialized to 0.
- The Route Request Identifier MUST be initialized to the
Identifier field of the Route Request that this reply is sent in
response to.
- The sequence of hop addresses in the source route are copied into
the Address[i] fields of the option. Address[1] MUST be set to
the first-hop address of the route after the initiator of the
Route Discovery, Address[n] MUST be set to the last-hop address
of the source route (the address of the target node), and each
other Address[i] MUST be set to the next address in sequence in
the source route being returned.
The Destination Address field in the IP header of the packet carrying
the Route Reply option MUST be set to the address of the initiator
of the Route Discovery (i.e., for a Route Reply being returned in
response to some Route Request, the IP Source Address of the Route
Request).
After creating and initializing the Route Reply option and the IP
packet containing it, send the Route Reply. In sending the Route
Reply from this node (but not from nodes forwarding the Route Reply),
this node SHOULD delay the Reply by a small jitter period chosen
randomly between 0 and BroadcastJitter.
When returning any Route Reply in the case in which the MAC protocol
in use in the network is not capable of transmitting unicast packets
over unidirectional links, the source route used for routing the
Route Reply packet MUST be obtained by reversing the sequence of
hops in the Route Request packet (the source route that is then
returned in the Route Reply). This restriction on returning a Route
Reply enables the Route Reply to test this sequence of hops for
bidirectionality, preventing the Route Reply from being received by
the initiator of the Route Discovery unless each of the hops over
which the Route Reply is returned (and thus each of the hops in the
source route being returned in the Reply) is bidirectional.
If sending a Route Reply to the initiator of the Route Request
requires performing a Route Discovery, the Route Reply option MUST
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be piggybacked on the packet that contains the Route Request. This
piggybacking prevents a loop wherein the target of the new Route
Request (which was itself the initiator of the original Route
Request) must do another Route Request in order to return its
Route Reply.
If sending the Route Reply to the initiator of the Route Request
does not require performing a Route Discovery, a node SHOULD send a
unicast Route Reply in response to every Route Request it receives
for which it is the target node.
8.2.5. Preventing Route Reply Storms
The ability for nodes to reply to a Route Request based on
information in their Route Caches, as described in Sections 3.3.2
and 8.2.3, could result in a possible Route Reply "storm" in some
cases. In particular, if a node broadcasts a Route Request for a
target node for which the node's neighbors have a route in their
Route Caches, each neighbor may attempt to send a Route Reply,
thereby wasting bandwidth and possibly increasing the number of
network collisions in the area.
For example, the figure below shows a situation in which nodes B, C,
D, E, and F all receive A's Route Request for target G, and each has
the indicated route cached for this target:
+-----+ +-----+
| D |< >| C |
+-----+ \ / +-----+
Cache: C - B - G \ / Cache: B - G
\ +-----+ /
-| A |-
+-----+\ +-----+ +-----+
| | \--->| B | | G |
/ \ +-----+ +-----+
/ \ Cache: G
v v
+-----+ +-----+
| E | | F |
+-----+ +-----+
Cache: F - B - G Cache: B - G
Normally, each of these nodes would attempt to reply from its own
Route Cache, and they would thus all send their Route Replies at
about the same time, since they all received the broadcast Route
Request at about the same time. Such simultaneous Route Replies
from different nodes all receiving the Route Request may cause local
congestion in the wireless network and may create packet collisions
among some or all of these Replies if the MAC protocol in use does
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not provide sufficient collision avoidance for these packets. In
addition, it will often be the case that the different replies will
indicate routes of different lengths, as shown in this example.
In order to reduce these effects, if a node can put its network
interface into promiscuous receive mode, it MAY delay sending its
own Route Reply for a short period, while listening to see if the
initiating node begins using a shorter route first. Specifically,
this node MAY delay sending its own Route Reply for a random period
d = H * (h - 1 + r)
where h is the length in number of network hops for the route to be
returned in this node's Route Reply, r is a random floating point
number between 0 and 1, and H is a small constant delay (at least
twice the maximum wireless link propagation delay) to be introduced
per hop. This delay effectively randomizes the time at which each
node sends its Route Reply, with all nodes sending Route Replies
giving routes of length less than h sending their Replies before this
node, and all nodes sending Route Replies giving routes of length
greater than h sending their Replies after this node.
Within the delay period, this node promiscuously receives all
packets, looking for data packets from the initiator of this Route
Discovery destined for the target of the Discovery. If such a data
packet received by this node during the delay period uses a source
route of length less than or equal to h, this node may infer that the
initiator of the Route Discovery has already received a Route Reply
giving an equally good or better route. In this case, this node
SHOULD cancel its delay timer and SHOULD NOT send its Route Reply for
this Route Discovery.
8.2.6. Processing a Received Route Reply Option
Section 8.1.4 describes the general processing for a received packet,
including the addition of routing information from options in the
packet's DSR Options header to the receiving node's Route Cache.
If the received packet contains a Route Reply, no additional special
processing of the Route Reply option is required beyond what is
described there. As described in Section 4.1 anytime a node adds
new information to its Route Cache (including the information added
from this Route Reply option), the node SHOULD check each packet in
its own Send Buffer (Section 4.2) to determine whether a route to
that packet's IP Destination Address now exists in the node's Route
Cache (including the information just added to the Cache). If so,
the packet SHOULD then be sent using that route and removed from the
Send Buffer. This general procedure handles all processing required
for a received Route Reply option.
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When using a MAC protocol that requires bidirectional links for
unicast transmission, a unidirectional link may be discovered by the
propagation of the Route Request. When the Route Reply is sent over
the reverse path, a forwarding node may discover that the next-hop is
unreachable. In this case, it MUST add the next-hop address to its
Blacklist (Section 4.6).
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8.3. Route Maintenance Processing
Route Maintenance is the mechanism by which a source node S is able
to detect, while using a source route to some destination node D,
if the network topology has changed such that it can no longer use
its route to D because a link along the route no longer works. When
Route Maintenance indicates that a source route is broken, S can
attempt to use any other route it happens to know to D, or can invoke
Route Discovery again to find a new route for subsequent packets
to D. Route Maintenance for this route is used only when S is
actually sending packets to D.
Specifically, when forwarding a packet, a node MUST attempt
to confirm the reachability of the next-hop node, unless such
confirmation had been received in the last MaintHoldoffTime.
Individual implementations MAY choose to bypass such confirmation
for some limited number of packets, as long as those packets all
fall within MaintHoldoffTime within the last confirmation. If no
confirmation is received after the retransmission of MaxMaintRexmt
acknowledgement requests, after the initial transmission of the
packet, and conceptually including all retransmissions provided
by the MAC layer, the node determines that the link for this
next-hop node of the source route is "broken". This confirmation
from the next-hop node for Route Maintenance can be implemented
using a link-layer acknowledgement (Section 8.3.1), using a
"passive acknowledgement" (Section 8.3.2), or using a network-layer
acknowledgement (Section 8.3.3); the particular strategy for
retransmission timing depends on the type of acknowledgement
mechanism used. When passive acknowledgements are being used, each
retransmitted acknowledgement request SHOULD be explicit software
acknowledgement requests. If no acknowledgement is received after
MaxMaintRexmt retransmissions (if necessary), the node SHOULD
originate a Route Error to the original sender of the packet, as
described in Section 8.3.4.
In deciding whether or not to send a Route Error in response to
attempting to forward a packet from some sender over a broken link,
a node MUST limit the number of consecutive packets from a single
sender that the node attempts to forward over this same broken
link for which the node chooses not to return a Route Error; this
requirement MAY be satisfied by returning a Route Error for each
packet that the node attempts to forward over a broken link.
8.3.1. Using Link-Layer Acknowledgements
If the MAC protocol in use provides feedback as to the successful
delivery of a data packet (such as is provided by the link-layer
acknowledgement frame defined by IEEE 802.11 [13]), then the use
of the DSR Acknowledgement Request and Acknowledgement options
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is not necessary. If such link-layer feedback is available, it
SHOULD be used instead of any other acknowledgement mechanism
for Route Maintenance, and the node SHOULD NOT use either passive
acknowledgements or network-layer acknowledgements for Route
Maintenance.
When using link-layer acknowledgements for Route Maintenance, the
retransmission timing and the timing at which retransmission attempts
are scheduled are generally controlled by the particular link layer
implementation in use in the network. For example, in IEEE 802.11,
the link-layer acknowledgement is returned after the data packet as
a part of the basic access method of of the IEEE 802.11 Distributed
Coordination Function (DCF) MAC protocol; the time at which the
acknowledgement is expected to arrive and the time at which the next
retransmission attempt (if necessary) will occur are controlled by
the MAC protocol implementation.
When a node receives a link-layer acknowledgement for any packet in
its Maintenance Buffer, that node SHOULD remove that packet, as well
as any other packets in its Maintenance Buffer with the same next-hop
destination, from its Maintenance Buffer.
8.3.2. Using Passive Acknowledgements
When link-layer acknowledgements are not available, but passive
acknowledgements [18] are available, passive acknowledgements SHOULD
be used for Route Maintenance when originating or forwarding a packet
along any hop other than the last hop (the hop leading to the IP
Destination Address node of the packet). In particular, passive
acknowledgements SHOULD be used for Route Maintenance in such cases
if the node can place its network interface into "promiscuous"
receive mode, and network links used for data packets generally
operate bidirectionally.
A node MUST NOT attempt to use passive acknowledgements for Route
Maintenance for a packet originated or forwarded over its last hop
(the hop leading to the IP Destination Address node of the packet),
since the receiving node will not be forwarding the packet and thus
no passive acknowledgement will be available to be heard by this
node. Beyond this restriction, a node MAY utilize a variety of
strategies in using passive acknowledgements for Route Maintenance of
a packet that it originates or forwards. For example, the following
two strategies are possible:
- Each time a node receives a packet to be forwarded to a node
other than the final destination (the IP Destination Address
of the packet), that node sends the original transmission of
that packet without requesting a network-layer acknowledgement
for it. If no passive acknowledgement is received within
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PassiveAckTimeout after this transmission, the node retransmits
the packet, again without requesting a network-layer
acknowledgement for it; the same PassiveAckTimeout timeout value
is used for each such attempt. If no acknowledgement has been
received after a total of TryPassiveAcks retransmissions of
the packet, network-layer acknowledgements (as described in
Section 8.3.3) are used for all remaining attempts for that
packet.
- Each node maintains a table of possible next-hop destination
nodes, noting whether or not passive acknowledgements can
typically be expected from transmission to that node, and the
expected latency and jitter of a passive acknowledgement from
that node. Each time a node receives a packet to be forwarded
to a node other than the IP Destination Address, the node checks
its table of next-hop destination nodes to determine whether to
use a passive acknowledgement or a network-layer acknowledgement
for that transmission to that node. The timeout for this packet
can also be derived from this table. A node using this method
SHOULD prefer using passive acknowledgements to network-layer
acknowledgements.
In using passive acknowledgements for a packet that it originates or
forwards, a node considers the later receipt of a new packet (e.g.,
with promiscuous receive mode enabled on its network interface) to be
an acknowledgement of this first packet if both of the following two
tests succeed:
- The Source Address, Destination Address, Protocol,
Identification, and Fragment Offset fields in the IP header
of the two packets MUST match [32], and
- If either packet contains a DSR Source Route header, both packets
MUST contain one, and the value in the Segments Left field in the
DSR Source Route header of the new packet MUST be less than that
in the first packet.
When a node hears such a passive acknowledgement for any packet in
its Maintenance Buffer, that node SHOULD remove that packet, as well
as any other packets in its Maintenance Buffer with the same next-hop
destination, from its Maintenance Buffer.
8.3.3. Using Network-Layer Acknowledgements
When a node originates or forwards a packet and has no other
mechanism of acknowledgement available to determine reachability
of the next-hop node in the source route for Route Maintenance,
that node SHOULD request a network-layer acknowledgement from that
next-hop node. To do so, the node inserts an Acknowledgement Request
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option in the DSR Options header in the packet. The Identification
field in that Acknowledgement Request option MUST be set to a value
unique over all packets transmitted by this node to the same next-hop
node that are either unacknowledged or recently acknowledged.
When a node receives a packet containing an Acknowledgement Request
option, then that node performs the following tests on the packet:
- If the indicated next-hop node address for this packet does not
match any of this node's own IP addresses, then this node MUST
NOT process the Acknowledgement Request option. The indicated
next-hop node address is the next Address[i] field in the DSR
Source Route option in the DSR Options header in the packet, or
is the IP Destination Address in the packet if the packet does
not contain a DSR Source Route option or the Segments Left there
is zero.
- If the packet contains an Acknowledgement option, then this node
MUST NOT process the Acknowledgement Request option.
If neither of the tests above fails, then this node MUST process the
Acknowledgement Request option by sending an Acknowledgement option
to the previous-hop node; to do so, the node performs the following
sequence of steps:
- Create a packet and set the IP Protocol field to the protocol
number assigned for DSR (TBA???).
- Set the IP Source Address field in this packet to the IP address
of this node, copied from the source route in the DSR Source
Route option in that packet (or from the IP Destination Address
field of the packet, if the packet does not contain a DSR Source
Route option).
- Set the IP Destination Address field in this packet to the IP
address of the previous-hop node, copied from the source route
in the DSR Source Route option in that packet (or from the IP
Source Address field of the packet, if the packet does not
contain a DSR Source Route option).
- Add a DSR Options header to the packet, and set the DSR Options
header's Next Header field to the "No Next Header" value.
- Add an Acknowledgement option to the DSR Options header in the
packet; set the Acknowledgement option's Option Type field to 6
and the Opt Data Len field to 10.
- Copy the Identification field from the received Acknowledgement
Request option into the Identification field in the
Acknowledgement option.
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- Set the ACK Source Address field in the Acknowledgement option to
be the IP Source Address of this new packet (set above to be the
IP address of this node).
- Set the ACK Destination Address field in the Acknowledgement
option to be the IP Destination Address of this new packet (set
above to be the IP address of the previous-hop node).
- Send the packet as described in Section 8.1.1.
Packets containing an Acknowledgement option SHOULD NOT be placed in
the Maintenance Buffer.
When a node receives a packet with both an Acknowledgement option
and an Acknowledgement Request option, if that node is not the
destination of the Acknowledgement option (the IP Destination Address
of the packet), then the Acknowledgement Request option MUST
be ignored. Otherwise (that node is the destination of the
Acknowledgement option), that node MUST process the Acknowledgement
Request option by returning an Acknowledgement option according to
the following sequence of steps:
- Create a packet and set the IP Protocol field to the protocol
number assigned for DSR (TBA???).
- Set the IP Source Address field in this packet to the IP address
of this node, copied from the source route in the DSR Source
Route option in that packet (or from the IP Destination Address
field of the packet, if the packet does not contain a DSR Source
Route option).
- Set the IP Destination Address field in this packet to the IP
address of the node originating the Acknowledgement option.
- Add a DSR Options header to the packet, and set the DSR Options
header's Next Header field to the "No Next Header" value.
- Add an Acknowledgement option to the DSR Options header in this
packet; set the Acknowledgement option's Option Type field to 6
and the Opt Data Len field to 10.
- Copy the Identification field from the received Acknowledgement
Request option into the Identification field in the
Acknowledgement option.
- Set the ACK Source Address field in the option to be the IP
Source Address of this new packet (set above to be the IP address
of this node).
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- Set the ACK Destination Address field in the option to be the IP
Destination Address of this new packet (set above to be the IP
address of the node originating the Acknowledgement option.)
- Send the packet directly to the destination. The IP
Destination Address MUST be treated as a direct neighbor node:
the transmission to that node MUST be done in a single IP
forwarding hop, without Route Discovery and without searching
the Route Cache. In addition, this packet MUST NOT contain a
DSR Acknowledgement Request, MUST NOT be retransmitted for Route
Maintenance, and MUST NOT expect a link-layer acknowledgement or
passive acknowledgement.
When using network-layer acknowledgements for Route Maintenance,
a node SHOULD use an adaptive algorithm in determining the
retransmission timeout for each transmission attempt of an
acknowledgement request. For example, a node SHOULD maintain a
separate round-trip time (RTT) estimate for each to which it has
recently attempted to transmit packets, and it SHOULD use this RTT
estimate in setting the timeout for each retransmission attempt
for Route Maintenance. The TCP RTT estimation algorithm has been
shown to work well for this purpose in implementation and testbed
experiments with DSR [22, 24].
8.3.4. Originating a Route Error
When a node is unable to verify reachability of a next-hop node after
reaching a maximum number of retransmission attempts, a node SHOULD
send a Route Error to the IP Source Address of the packet. When
sending a Route Error for a packet containing either a Route Error
option or an Acknowledgement option, a node SHOULD add these existing
options to its Route Error, subject to the limit described below.
A node transmitting a Route Error MUST perform the following steps:
- Create an IP packet and set the Source Address field in this
packet's IP header to the address of this node.
- If the Salvage field in the DSR Source Route option in the
packet triggering the Route Error is zero, then copy the
Source Address field of the packet triggering the Route Error
into the Destination Address field in the new packet's IP
header; otherwise, copy the Address[1] field from the DSR Source
Route option of the packet triggering the Route Error into the
Destination Address field in the new packet's IP header
- Insert a DSR Options header into the new packet.
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- Add a Route Error Option to the new packet, setting the Error
Type to NODE_UNREACHABLE, the Salvage value to the Salvage
value from the DSR Source Route option of the packet triggering
the Route Error, and the Unreachable Node Address field to
the address of the next-hop node from the original source
route. Set the Error Source Address field to this node's IP
address, and the Error Destination field to the new packet's IP
Destination Address.
- If the packet triggering the Route Error contains any Route Error
or Acknowledgement options, the node MAY append to its Route
Error each of these options, with the following constraints:
o The node MUST NOT include any Route Error option from the
packet triggering the new Route Error, for which the total
salvage count (Section 6.4) of that included Route Error
would be greater than MAX_SALVAGE_COUNT in the new packet.
o If any Route Error option from the packet triggering the new
Route Error is not included in the packet, the node MUST NOT
include any following Route Error or Acknowledgement options
from the packet triggering the new Route Error.
o Any appended options from the packet triggering the Route
Error MUST follow the new Route Error in the packet.
o In appending these options to the new Route Error, the order
of these options from the packet triggering the Route Error
MUST be preserved.
- Send the packet as described in Section 8.1.1.
8.3.5. Processing a Received Route Error Option
When a node receives a packet containing a Route Error option, that
node MUST process the Route Error option according to the following
sequence of steps:
- The node MUST remove from its Route Cache the link from the
node identified by the Error Source Address field to the node
identified by the Unreachable Node Address field (if this link is
present in its Route Cache). If the node implements its Route
Cache as a link cache, as described in Section 4.1, only this
single link is removed; if the node implements its Route Cache as
a path cache, however, all routes (paths) that use this link are
removed.
- If the option following the Route Error is an Acknowledgement
or Route Error option sent by this node (that is, with
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Acknowledgement or Error Source Address equal to this node's
address), copy the DSR options following the current Route
Error into a new packet with IP Source Address equal to this
node's own IP address and IP Destination Address equal to the
Acknowledgement or Error Destination Address. Transmit this
packet as described in Section 8.1.1, with the salvage count
in the DSR Source Route option set to the Salvage value of the
Route Error.
In addition, after processing the Route Error as described above,
the node MAY initiate a new Route Discovery for any destination node
for which it then has no route in its Route Cache as a result of
processing this Route Error, if the node has indication that a route
to that destination is needed. For example, if the node has an open
TCP connection to some destination node, then if the processing of
this Route Error removed the only route to that destination from this
node's Route Cache, then this node MAY initiate a new Route Discovery
for that destination node. Any node, however, MUST limit the rate at
which it initiates new Route Discoveries for any single destination
address, and any new Route Discovery initiated in this way as part of
processing this Route Error MUST conform to this limit.
8.3.6. Salvaging a Packet
When an intermediate node forwarding a packet detects through Route
Maintenance that the next-hop link along the route for that packet is
broken (Section 8.3), if the node has another route to the packet's
IP Destination Address in its Route Cache, the node SHOULD "salvage"
the packet rather than discarding it. To do so using the route found
in its Route Cache, this node processes the packet as follows:
- If the MAC protocol in use in the network is not capable of
transmitting unicast packets over unidirectional links, as
discussed in Section 3.3.1, then if this packet contains a Route
Reply option, remove and discard the Route Reply option in the
packet; if the DSR Options header in the packet then contains no
DSR options, remove the DSR Options header from the packet. If
the resulting packet then contains only an IP header, the node
SHOULD NOT salvage the packet and instead SHOULD discard the
entire packet.
When returning any Route Reply in the case in which the MAC
protocol in use in the network is not capable of transmitting
unicast packets over unidirectional links, the source route
used for routing the Route Reply packet MUST be obtained by
reversing the sequence of hops in the Route Request packet (the
source route that is then returned in the Route Reply). This
restriction on returning a Route Reply and on salvaging a packet
that contains a Route Reply option enables the Route Reply to
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test this sequence of hops for bidirectionality, preventing the
Route Reply from being received by the initiator of the Route
Discovery unless each of the hops over which the Route Reply is
returned (and thus each of the hops in the source route being
returned in the Reply) is bidirectional.
- Modify the existing DSR Source Route option in the packet so
that the Address[i] fields represent the source route found in
this node's Route Cache to this packet's IP Destination Address.
Specifically, the node copies the hop addresses of the source
route into sequential Address[i] fields in the DSR Source Route
option, for i = 1, 2, ..., n. Address[1] here is the address
of the salvaging node itself (the first address in the source
route found from this node to the IP Destination Address of the
packet). The value n here is the number of hop addresses in this
source route, excluding the destination of the packet (which is
instead already represented in the Destination Address field in
the packet's IP header).
- Initialize the Segments Left field in the DSR Source Route option
to n as defined above.
- The First Hop External (F) bit in the DSR Source Route option is
copied from the External bit flagging the first hop in the source
route for the packet, as indicated in the Route Cache.
- The Last Hop External (L) bit in the DSR Source Route option is
copied from the External bit flagging the last hop in the source
route for the packet, as indicated in the Route Cache.
- The Salvage field in the DSR Source Route option is set to 1 plus
the value of the Salvage field in the DSR Source Route option of
the packet that caused the error.
- Transmit the packet to the next-hop node on the new source route
in the packet, using the forwarding procedure described in
Section 8.1.5.
As described in Section 8.3.4, the node in this case also SHOULD
return a Route Error to the original sender of the packet. If the
node chooses to salvage the packet, it SHOULD do so after originating
the Route Error.
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8.4. Multiple Network Interface Support
A node using DSR MAY have multiple network interfaces that support
ad hoc network routing. This section describes special packet
processing at such nodes.
A node with multiple network interfaces MUST have some policy for
determining which Route Request packets are forwarded out which
network interfaces. For example, a node MAY choose to forward all
Route Requests out all network interfaces.
When a node with multiple network interfaces propagates a Route
Request on an network interface other than the one one which it
received the Route Request, it MUST modify the address list between
receipt and propagation as follows:
- Append the address of the incoming network interface.
- Append the address of the outgoing network interface.
When a node forwards a packet containing a source route, it MUST
assume that the next-hop node is reachable on the incoming network
interface, unless the next hop is the address of one of this node's
network interfaces, in which case this node MUST skip over this
address in the source route and process the packet in the same way as
if it had just received it from that network interface, as described
in section 8.1.5.
If a node that previously had multiple network interfaces receives
a packet sent with a source route specifying a change to a network
interface that is no longer available, it MAY send a Route Error to
the source of the packet without attempting to forward the packet
on the incoming network interface, unless the network uses an
autoconfiguration mechanism that may have allowed another node to
acquire the now unused address of the unavailable network interface.
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8.5. IP Fragmentation and Reassembly
When a node using DSR wishes to fragment a packet that contains a DSR
header not containing a Route Request option, it MUST perform the
following sequence of steps:
- Remove the DSR Options header from the packet.
- Fragment the packet. When determining the size of each fragment
to create from the original packet, the fragment size MUST be
reduced by the size of the DSR Options header from the original
packet.
- IP-in-IP encapsulate each fragment [28]. The IP Destination
address of the outer (encapsulating) packet MUST be set equal to
the IP Destination address of the original packet.
- Add the DSR Options header from the original packet to each
resulting encapsulating packet. If a Source Route header is
present in the DSR Options header, increment the Salvage field.
When a node using the DSR protocol receives an IP-in-IP encapsulated
packet destined to itself, it SHOULD decapsulate the packet [28] and
then process the inner packet according to standard IP reassembly
processing [32].
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8.6. Flow State Processing
A node implementing the optional DSR flow state extension MUST follow
these additional processing steps.
8.6.1. Originating a Packet
When originating any packet to be routed using flow state, a node
using DSR flow state MUST:
- If the route to be used for this packet has never had a DSR
flow state established along it (or the existing flow state has
expired):
o Generate a 16-bit Flow ID larger than any unexpired Flow IDs
used for this destination. Odd Flow IDs MUST be chosen for
"default" flows; even Flow IDs MUST be chosen for non-default
flows.
o Add a DSR Options header, as described in Section 8.1.2.
o Add a DSR Flow State header, as described in Section 8.6.2.
o Initialize the Hop Count field in the DSR Flow State header
to 0.
o Set the Flow ID field in the DSR Flow State header to the
Flow ID generated in the first step.
o Add a Timeout option to the DSR Options header.
o Add a Source Route option after the Timeout option. with the
route to be used, as described in Section 8.1.3.
o The source SHOULD record this flow in its Flow Table.
o If this flow is recorded in the Flow Table, the TTL MUST be
set to be the TTL of the flow establishment packet.
o If this flow is recorded in the Flow Table, the timeout MUST
be set to a value no less than the value specified in the
Timeout option.
- If the route to be used for this packet has had DSR flow state
established along it, but has not been established end-to-end:
o Add a DSR Options header, as described in Section 8.1.2.
o Add a DSR Flow State header, as described in Section 8.6.2.
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o Initialize the Hop Count field in the DSR Flow State header
to 0.
o The Flow ID field of the DSR Flow State header SHOULD be the
Flow ID previously used for this route. If it is not, the
steps for sending packets along never before established
routes MUST be followed in place of these.
o Add a Timeout option to the DSR Options header, setting the
Timeout to a value not greater than the timeout remaining for
this flow in the Flow Table.
o Add a Source Route option after the Timeout option with the
route to be used, as described in Section 8.1.3
o If the IP TTL is not equal to the TTL specified in the Flow
Table, the source MUST set a flag to indicate that this flow
cannot be used as default.
- If the route the node wishes to use for this packet has been
established end-to-end and is not the default flow:
o Add a DSR Flow State header, as described in Section 8.6.2.
o Initialize the Hop Count field in the DSR Flow State header
to 0.
o The Flow ID field of the DSR Flow State header SHOULD be the
Flow ID previously used for this route. If it is not, the
steps for sending packets along never before established
routes MUST be followed in place of these.
o If the next hop requires a Hop-by-Hop acknowledgement,
add a DSR Options header, as described in Section 8.1.2,
and an Acknowledgement Request option, as described in
Section 8.3.3.
o A DSR Options header SHOULD NOT be added to a packet, unless
it is added to carry an Acknowledgement Request option, in
which case:
+ A Source Route option in the DSR Options header SHOULD
NOT be added.
+ If a Source Route option in the DSR Options header is
added, the steps for sending packets along routes not
yet established end-to-end MUST be followed in place of
these.
+ A Timeout option SHOULD NOT be added.
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+ If a Timeout option is added, it MUST specify a timeout
not greater than the timeout remaining for this flow in
the Flow Table.
- If the route the node wishes to use for this packet has been
established end-to-end and is the current default flow:
o If the IP TTL is not equal to the TTL specified in the Flow
Table, the source MUST follow the steps for sending a packet
along a non-default flow that has been established end-to-end
in place of these steps.
o If the next hop requires a Hop-by-Hop acknowledgement,
the sending node MUST add a DSR Options header and
an Acknowledgement Request option, as described in
Section 8.3.3. The sending node MUST NOT add any additional
options to this header.
o A DSR Options header SHOULD NOT be added, except as specified
in the previous step. If one is added in a way inconsistent
with the previous step, the source MUST follow the steps
for sending a packet along a non-default flow that has been
established end-to-end in place of these steps.
8.6.2. Inserting a DSR Flow State Header
A node originating a packet adds a DSR Flow State header to the
packet, if necessary, to carry information needed by the routing
protocol. Only one DSR Flow State header may be in any packet.
A DSR Flow State header is added to a packet by performing the
following sequence of steps:
- Insert a DSR Flow State header after the IP header and any
Hop-by-Hop Options header that may already be in the packet, but
before any other header that may be present.
- Set the Next Header field of the DSR Flow State header to the
Next Header field of the previous header (either an IP header or
a Hop-by-Hop Options header).
- Set the Next Header field of the previous header to the Protocol
number assigned for DSR (TBA???).
8.6.3. Receiving a Packet
This section describes processing only for packets that are sent to
the processing node as the next-hop node; that is, when the MAC-layer
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destination address is the MAC address of this node. Otherwise, the
process described in Sections 8.6.5 should be followed.
The flow along which a packet is being sent is considered to be in
the Flow Table if the triple (IP Source Address, IP Destination
Address, Flow ID) has an unexpired entry in the Flow Table.
When a node using DSR flow state receives a packet, it MUST follow
the following steps for processing:
- If a DSR Flow State header is present, increment the Hop Count
field.
- In addition, if a DSR Flow State header is present, then if the
triple (IP Source Address, IP Destination Address, Flow ID) is
in this node's Automatic Route Shortening Table and the packet
is listed in the entry, then the node MAY send a gratuitous
Route Reply as described in Section 4.4, subject to the rate
limiting specified in Section 4.4. This gratuitous Route Reply
gives the route by which the packet originally reached this
node. Specifically, the node sending the gratuitous Route Reply
constructs the route to return in the Route Reply as follows:
o Let k = (packet Hop Count) - (table Hop Count), where
packet Hop Count is the value of the Hop Count field in this
received packet, and table Hop Count is the Hop Count value
stored for this packet in the corresponding entry in this
node's Automatic Route Shortening Table.
o Copy the complete source route for this flow from the
corresponding entry in the node's Flow Table.
o Remove from this route the k hops immediately preceding this
node in the route, since these are the hops "skipped over"
by the packet as recorded in the Automatic Route Shortening
Table entry.
- Process each of the DSR options within the DSR Options header in
order:
o On receiving a Pad1 or PadN option, skip over the option
o On receiving a Route Request for which this node is the
destination, remove the option and return a Route Reply as
specified in Section 8.2.2.
o On receiving a broadcast Route Request that this node has not
previously seen for which this node is not the destination,
append this node's incoming interface address to the Route
Request, continue propagating the Route Request as specified
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in Section 8.2.2, send the payload, if any, to the network
layer, and stop processing.
o On receiving a Route Request that this node has not
previously seen for which this node is not the destination,
discard the packet and stop processing.
o On receiving any Route Request, add appropriate links to the
cache, as specified in Section 8.2.2.
o On receiving a Route Reply that this node is the Requester
for, remove the Route Reply from the packet and process it as
specified in Section 8.2.6.
o On receiving any Route Reply, add appropriate links to the
cache, as specified in Section 8.2.6.
o On receiving any Route Error of type NODE_UNREACHABLE,
remove appropriate links to the cache, as specified in
Section 8.3.5.
o On receiving a Route Error of type NODE_UNREACHABLE that
this node is the Error Destination Address of, remove the
Route Error from the packet and process it as specified
in Section 8.3.5. It also MUST stop originating packets
along any flows using the link from Error Source Address to
Unreachable Node, and it MAY remove from its Flow Table any
flows using the link from Error Source Address to Unreachable
Node.
o On receiving a Route Error of type UNKNOWN_FLOW that this
node is not the Error Destination Address of, the node checks
if the Route Error corresponds to a flow in its Flow Table.
If it does not, the node silently discards the Route Error;
otherwise, it forwards the packet to the expected previous
hop of the corresponding flow. If Route Maintenance cannot
confirm the reachability of the previous hop, the node checks
if the network interface requires bidirectional links for
operation. If it does, the node silently discards the Error;
otherwise, it sends the Error as if it were originating it,
as described in Section 8.1.1.
o On receiving a Route Error of type UNKNOWN_FLOW that this
node is the Error Destination Address of, remove the Route
Error from the packet and mark the flow specified by the
triple (Error Destination Address, Original IP Destination
Address, Flow ID) as not having been established end-to-end.
o On receiving a Route Error of type DEFAULT_FLOW_UNKNOWN
that this node is not the Error Destination Address of, the
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node checks if the Route Error corresponds to a flow in
its Default Flow Table. If it does not, the node silently
discards the Route Error; otherwise, it forwards the packet
to the expected previous hop of the corresponding flow.
If Route Maintenance cannot confirm the reachability of
the previous hop, the node checks if the network interface
requires bidirectional links for operation. If it does,
the node silently discards the Error; otherwise, it sends
the Error as if it were originating it, as described in
Section 8.1.1.
o On receiving a Route Error of type DEFAULT_FLOW_UNKNOWN that
this node is the Error Destination Address of, remove the
Route Error from the packet and mark the default flow between
the Error Destination Address and the Original IP Destination
Address as not having been established end-to-end.
o On receiving a Acknowledgement Request option, the receiving
node removes the Acknowledgement Request option and replies
to the previous hop with a Acknowledgement option. If the
previous hop cannot be determined, the Acknowledgement
Request option is discarded, and processing continues.
o On receiving a Acknowledgement option, the receiving node
removes the Acknowledgement option and processes it.
o On receiving any Acknowledgement option, add the appropriate
link to the cache, as specified in Section 8.1.4
o On receiving any Source Route option, add appropriate links
to the cache, as specified in Section 8.1.4.
o On receiving a Source Route option and either no DSR Flow
State header is present, the flow this packet is being sent
along is in the Flow Table, or no Timeout option preceded the
Source Route option in this DSR Options header, process it
as specified in Section 8.1.4. Stop processing this packet
unless the last address in the Source Route option is an
address of this node.
o On receiving a Source Route option in a packet with a DSR
Flow State header, and the Flow ID specified in the DSR Flow
State header is not in the Flow Table, add the flow to the
Flow Table, setting the Timeout value to a value not greater
than the Timeout field of the Timeout option in this header.
If no Timeout option preceded the Source Route option in this
header, the flow MUST NOT be added to the Flow Table.
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If the Flow ID is odd and larger than any unexpired, odd
Flow IDs, it is set to be default in the Default Flow ID
Table.
Then process the Route option as specified in Section 8.1.4.
Stop processing this packet unless the last address in the
Source Route option is an address of this node.
o On receiving a Timeout option, check if this packet contains
a DSR Flow State header. If this packet does not contain a
DSR Flow State header, discard the DSR option. Otherwise,
record the Timeout value in the option for future reference.
The value recorded SHOULD be discarded when the node has
finished processing this DSR Options header. If the flow
that this packet is being sent along is in the Flow Table, it
MAY set the flow to time out no more than Timeout seconds in
the future.
o On receiving a Destination and Flow ID option, if the
IP Destination Address is not an address of this node,
forward the packet according to the Flow ID, as described in
Section 8.6.4, and stop processing this packet.
o On receiving a Destination and Flow ID option, if the IP
Destination Address is an address of this node, set the
IP Destination Address to the New IP Destination Address
specified in the option, and set the Flow ID to the New
Flow Identifier. Then remove the DSR option from the packet
and continue processing.
- If the IP Destination Address is an address of this node, remove
the DSR Options header, if any, and pass the packet up the
network stack and stop processing.
- If there is still a DSR Options header containing no options,
remove the DSR Options header.
- If there is still a DSR Flow State header, forward the packet
according to the Flow ID, as described in Section 8.6.4.
- If there is neither a DSR Options header nor a DSR Flow State
header, but there is an entry in the Default Flow Table for the
(IP Source Address, IP Destination Address) pair:
o If the IP TTL is not equal to the TTL expected in the Flow
Table, insert a DSR Flow State header, setting Hop Count
equal to the Hop Count of this node, and the Flow ID equal
to the default Flow ID found in the table, and forward
this packet according to the Flow ID, as described in
Section 8.6.4.
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o Otherwise, follow the steps for forwarding the packet using
Flow IDs described in Section 8.6.4, but taking the Flow ID
to be the default Flow ID found in the table.
- If there is no DSR Options header, no DSR Flow State header, and
no default flow can be found, the node returns a Route Error of
type Default Flow Unknown to the IP Source Address, specifying
the IP Destination Address as the Original IP Destination in the
type-specific field.
8.6.4. Forwarding a Packet Using Flow IDs
To forward a packet using Flow IDs, a node MUST follow the following
sequence of steps:
- If the triple (IP Source Address, IP Destination Address,
Flow ID) is not in the Flow Table, return a Route Error of type
Unknown Flow.
- If a hop-by-hop acknowledgement is required for the next hop, the
node MUST include an Acknowledegment Request option as specified
in Section 8.3.3. If no DSR Options header is in the packet for
the Acknowledgement Request option to be attached to, it MUST be
included, as described in Section 8.1.2, except that it MUST be
added after the DSR Flow State header, if one is present.
- Attempt to transmit this packet to the next hop as specified in
the Flow Table, performing Route Maintenance to detect broken
routes.
8.6.5. Promiscuously Receiving a Packet
This section describes processing only for packets that have MAC
destinations other than the processing node. Otherwise, the process
described in Section 8.6.3 should be followed.
When a node using DSR flow state promiscuously overhears a packet, it
SHOULD follow the following steps for processing:
- If the packet contains a DSR Flow State header, and if the triple
(IP Source Address, IP Destination Address, Flow ID) is in the
Flow Table and the Hop Count is less than the Hop Count in the
flow's entry, the node MAY retain the packet in the Automatic
Route Shortening Table. If it can be determined that this
Flow ID has been recently used, it SHOULD retain the packet in
the Automatic Route Shortening Table.
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- If the packet contains neither a DSR Flow State header nor a
Source Route option, and a Default Flow ID can be found in
the Default Flow Table for (IP Source Address, IP Destination
Address), and the IP TTL is greater than the TTL in the table
for the default flow, the node MAY retain the packet in the
Automatic Route Shortening Table. If it can be determined that
this Flow ID has been used recently, the node SHOULD retain the
packet in the Automatic Route Shortening Table.
8.6.6. Operation where the Layer below DSR Decreases
the IP TTL Non-Uniformly
Some nodes may use an IP tunnel as a DSR hop. If different packets
sent along this IP tunnel can take different routes, the reduction
in IP TTL across this link may be different for different packets.
This prevents the Automatic Route Shortening and Loop Detection
functionality from working properly when used in conjunction with
default routes.
Nodes forwarding packets without a Source Route option onto a link
with unpredictable TTL changes MUST ensure that a DSR Flow State
header is present, indicating the correct Hop Count and Flow ID.
8.6.7. Salvage Interactions with DSR
Nodes salvaging packets MUST remove the DSR Flow State header, if
present.
Any time this document refers to the Salvage field in the Source
Route option, packets without a Source Route option are considered to
have the value zero in the Salvage field.
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9. Protocol Constants and Configuration Variables
Any DSR implementation MUST support the following configuration
variables and MUST support a mechanism enabling the value of these
variables to be modified by system management. The specific variable
names are used for demonstration purposes only, and an implementation
is not required to use these names for the configuration variables,
so long as the external behavior of the implementation is consistent
with that described in this document.
For each configuration variable below, the default value is specified
to simplify configuration. In particular, the default values given
below are chosen for a DSR network running over 2 Mbps IEEE 802.11
network interfaces using the Distributed Coordination Function (DCF)
MAC with RTS and CTS [13, 5].
DiscoveryHopLimit 255 hops
BroadcastJitter 10 milliseconds
RouteCacheTimeout 300 seconds
SendBufferTimeout 30 seconds
RequestTableSize 64 nodes
RequestTableIds 16 identifiers
MaxRequestRexmt 16 retransmissions
MaxRequestPeriod 10 seconds
RequestPeriod 500 milliseconds
NonpropRequestTimeout 30 milliseconds
RexmtBufferSize 50 packets
MaintHoldoffTime 250 milliseconds
MaxMaintRexmt 2 retransmissions
TryPassiveAcks 1 attempt
PassiveAckTimeout 100 milliseconds
GratReplyHoldoff 1 second
In addition, the following protocol constant MUST be supported by any
implementation of the DSR protocol:
MAX_SALVAGE_COUNT 15 salvages
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10. IANA Considerations
This document specifies the DSR Options header and DSR Flow State
header, which require an IP Protocol number. A single IP protocol
number can be used for both header types, since they can be
distinguished by the Flow State Header (F) bit in each header.
In addition, this document proposes use of the value "No Next Header"
(originally defined for use in IPv6) within an IPv4 packet, to
indicate that no further header follows a DSR Options header.
Finally, this document introduces a number of DSR options for use in
the DSR Options header, and additional new DSR options may be defined
in the future. Each of these options requires a unique Option Type
value, with the most significant 3 bits (that is, Option Type & 0xE0)
encoded as defined in Section 6.1. It is necessary only that each
Option Type value be unique, not that they be unique in the remaining
5 bits of the value after these 3 most significant bits. Assignment
of new values for DSR options will be by Expert Review [25], with the
authors of this document serving as the Designated Experts.
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11. Security Considerations
This document does not specifically address security concerns. This
document does assume that all nodes participating in the DSR protocol
do so in good faith and without malicious intent to corrupt the
routing ability of the network.
Depending on the threat model, a number of different mechanisms can
be used to secure DSR. For example, in an environment where node
compromise is unrealistic and where where all the nodes participating
in the DSR protocol share a common goal that motivates their
participation in the protocol, the communications between the nodes
can be encrypted at the physical channel or link layer to prevent
attack by outsiders. Cryptographic approaches, such as that provided
by Ariadne [12] or SRP [27], can resist stronger attacks.
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Appendix A. Link-MaxLife Cache Description
As guidance to implementors of DSR, the description below outlines
the operation of a possible implementation of a Route Cache for DSR
that has been shown to outperform other other caches studied in
detailed simulations. Use of this design for the Route Cache is
recommended in implementations of DSR.
This cache, called "Link-MaxLife" [10], is a link cache, in that each
individual link (hop) in the routes returned in Route Reply packets
(or otherwise learned from the header of overhead packets) is added
to a unified graph data structure of this node's current view of the
network topology, as described in Section 4.1. To search for a route
in this cache to some destination node, the sending node uses a graph
search algorithm, such as the well-known Dijkstra's shortest-path
algorithm, to find the current best path through the graph to the
destination node.
The Link-MaxLife form of link cache is adaptive in that each link in
the cache has a timeout that is determined dynamically by the caching
node according to its observed past behavior of the two nodes at the
ends of the link; in addition, when selecting a route for a packet
being sent to some destination, among cached routes of equal length
(number of hops) to that destination, Link-MaxLife selects the route
with the longest expected lifetime (highest minimum timeout of any
link in the route).
Specifically, in Link-MaxLife, a link's timeout in the Route Cache
is chosen according to a "Stability Table" maintained by the caching
node. Each entry in a node's Stability Table records the address of
another node and a factor representing the perceived "stability" of
this node. The stability of each other node in a node's Stability
Table is initialized to InitStability. When a link from the Route
Cache is used in routing a packet originated or salvaged by that
node, the stability metric for each of the two endpoint nodes of that
link is incremented by the amount of time since that link was last
used, multiplied by StabilityIncrFactor (StabilityIncrFactor >= 1);
when a link is observed to break and the link is thus removed
from the Route Cache, the stability metric for each of the two
endpoint nodes of that link is multiplied by StabilityDecrFactor
(StabilityDecrFactor < 1).
When a node adds a new link to its Route Cache, the node assigns a
lifetime for that link in the Cache equal to the stability of the
less "stable" of the two endpoint nodes for the link, except that a
link is not allowed to be given a lifetime less than MinLifetime.
When a link is used in a route chosen for a packet originated or
salvaged by this node, the link's lifetime is set to be at least
UseExtends into the future; if the lifetime of that link in the
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Route Cache is already further into the future, the lifetime remains
unchanged.
When a node using Link-MaxLife selects a route from its Route Cache
for a packet being originated or salvaged by this node, it selects
the shortest-length route that has the longest expected lifetime
(highest minimum timeout of any link in the route), as opposed to
simply selecting an arbitrary route of shortest length.
The following configuration variables are used in the description
of Link-MaxLife above. The specific variable names are used for
demonstration purposes only, and an implementation is not required
to use these names for these configuration variables. For each
configuration variable below, the default value is specified to
simplify configuration. In particular, the default values given
below are chosen for a DSR network where nodes move at relative
velocities between 12 and 25 seconds per transmission radius.
InitStability 25 seconds
StabilityIncrFactor 4
StabilityDecrFactor 1/2
MinLifetime 1 second
UseExtends 120 seconds
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Appendix B. Location of DSR in the ISO Network Reference Model
When designing DSR, we had to determine at what layer within
the protocol hierarchy to implement ad hoc network routing. We
considered two different options: routing at the link layer (ISO
layer 2) and routing at the network layer (ISO layer 3). Originally,
we opted to route at the link layer for several reasons:
- Pragmatically, running the DSR protocol at the link layer
maximizes the number of mobile nodes that can participate in
ad hoc networks. For example, the protocol can route equally
well between IPv4 [32], IPv6 [7], and IPX [37] nodes.
- Historically [15, 16], DSR grew from our contemplation of
a multi-hop propagating version of the Internet's Address
Resolution Protocol (ARP) [30], as well as from the routing
mechanism used in IEEE 802 source routing bridges [29]. 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 [15, 16], 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 [22]
DSR as a layer 3 protocol, since this is the only layer at which we
could realistically support nodes with multiple network interfaces of
different types forming an ad hoc network.
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Appendix C. Implementation and Evaluation Status
The initial design of the DSR protocol, including DSR's basic Route
Discovery and Route Maintenance mechanisms, was first published in
December 1994 [15], with significant additional design details and
initial simulation results published in early 1996 [16].
The DSR protocol has been extensively studied since then through
additional detailed simulations. In particular, we have implemented
DSR in the ns-2 network simulator [26, 5] and performed extensive
simulations of DSR using ns-2 (e.g., [5, 21]). We have also
conducted evaluations of the different caching strategies in this
document [10].
We have also implemented the DSR protocol under the FreeBSD 2.2.7
operating system running on Intel x86 platforms. FreeBSD [9] is
based on a variety of free software, including 4.4 BSD Lite from the
University of California, Berkeley. For the environments in which
we used it, this implementation is functionally equivalent to the
version of the DSR protocol specified in this document.
During the 7 months from August 1998 to February 1999, we designed
and implemented a full-scale physical testbed to enable the
evaluation of ad hoc network performance in the field, in an actively
mobile ad hoc network under realistic communication workloads. The
last week of February and the first week of March of 1999 included
demonstrations of this testbed to a number of our sponsors and
partners, including Lucent Technologies, Bell Atlantic, and DARPA.
A complete description of the testbed is available as a Technical
Report [22].
We have since ported this implementation of DSR to FreeBSD 3.3, and
we have also added a preliminary version of Quality of Service (QoS)
support for DSR. A demonstration of this modified version of DSR was
presented in July 2000. These QoS features are not included in this
document, and will be added later in a separate document on top of
the base protocol specified here.
DSR has also been implemented under Linux by Alex Song at the
University of Queensland, Australia [36]. This implementation
supports the Intel x86 PC platform and the Compaq iPAQ.
The Network and Telecommunications Research Group at Trinity College
Dublin have implemented a version of DSR on Windows CE.
Microsoft Research has implemented a version of DSR on Windows XP,
and has used it in testbeds of over 15 nodes. Several machines use
this implementation as their primary means of accessing the Internet.
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Several other independent groups have also used DSR as a platform for
their own research, or and as a basis of comparison between ad hoc
network routing protocols.
A preliminary version of the optional DSR flow state extension was
implemented in FreeBSD 3.3. A demonstration of this modified version
of DSR was presented in July 2000. The DSR flow state extension has
also been extensively evaluated using simulation [11].
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Changes from Previous Version of the Draft
This appendix briefly lists some of the major changes in this
draft relative to the previous version of this same draft,
draft-ietf-manet-dsr-09.txt:
- Changed the values used for the Route Request and Route Reply
options so that they are assigned in a more logical order (Route
Request is now 1 and Route Reply is 2, rather than the other way
around).
- Specification of interaction of DSR with ARP.
- Better integration of multiple network interfaces into the main
packet processing specification in Section 8.
- Removal of optimizations for unidirectional links, based on
special 127.0.0.1 and 127.0.0.2 flags in a Route Request and
Route Reply. These optimizations were not fully specified in
the draft and will be included in future versions of the DSR
specification.
- Clarification of rules for IP fragmentation in Section 8.5.
- Revisions to the IANA Considerations section to state that the
DSR Options header and DSR Flow State header can share a single
IP protocol number assignment, and to add of a policy for DSR
options assignments.
- Other general clarification of the specification, based on
feedback received in Area Director review comments.
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Acknowledgements
The protocol described in this document has been designed and
developed within the Monarch Project, a research project at Rice
University (previously at Carnegie Mellon University) that is
developing adaptive networking protocols and protocol interfaces to
allow truly seamless wireless and mobile node networking [17, 35].
The authors would like to acknowledge the substantial contributions
of Josh Broch in helping to design, simulate, and implement the DSR
protocol. We thank him for his contributions to earlier versions of
this document.
We would also like to acknowledge the assistance of Robert V. Barron
at Carnegie Mellon University. Bob ported our DSR implementation
from FreeBSD 2.2.7 into FreeBSD 3.3.
Many valuable suggestions came from participants in the IETF process.
We would particularly like to acknowledge Fred Baker, who provided
extensive feedback on a previous version of this document, as well as
the working group chairs, for their suggestions of previous versions
of the document.
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Chair's Address
The MANET Working Group can be contacted via its current chairs:
M. Scott Corson Phone: +1 908 947-7033
Flarion Technologies, Inc. Email: corson@flarion.com
Bedminster One
135 Route 202/206 South
Bedminster, NJ 07921
USA
Joseph Macker Phone: +1 202 767-2001
Information Technology Division Email: macker@itd.nrl.navy.mil
Naval Research Laboratory
Washington, DC 20375
USA
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Authors' Addresses
Questions about this document can also be directed to the authors:
David B. Johnson Phone: +1 713 348-3063
Rice University Fax: +1 713 348-5930
Computer Science Department, MS 132 Email: dbj@cs.rice.edu
6100 Main Street
Houston, TX 77005-1892
USA
David A. Maltz Phone: +1 412 268-5329
Carnegie Mellon University Fax: +1 412 268-5576
Computer Science Department Email: dmaltz@cs.cmu.edu
5000 Forbes Avenue
Pittsburgh, PA 15213
USA
Yih-Chun Hu Phone: +1 412 268-3075
Rice University Fax: +1 412 268-5576
Computer Science Department, MS 132 Email: yihchun@cs.cmu.edu
6100 Main Street
Houston, TX 77005-1892
USA
Johnson, et al Expires 19 January 2005 [Page 112]
