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RFC 4728 - The Dynamic Source Routing Protocol (DSR) for Mobile Ad Hoc Networks for IPv4
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RFC4728 - The Dynamic Source Routing Protocol (DSR) for Mobile
Network Working Group D. Johnson
Request for Comments: 4728 Rice University
Category: Experimental Y. Hu
UIUC
D. Maltz
Microsoft Research
February 2007
The Dynamic Source Routing Protocol (DSR)
for Mobile Ad Hoc Networks for IPv4
Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
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 what is
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 even with very high rates of mobility. This
document specifies the operation of the DSR protocol for routing
unicast IPv4 packets.
Table of Contents
1. Introduction ....................................................5
2. Assumptions .....................................................7
3. DSR Protocol Overview ...........................................9
3.1. Basic DSR Route Discovery .................................10
3.2. Basic DSR Route Maintenance ...............................12
3.3. Additional Route Discovery Features .......................14
3.3.1. Caching Overheard Routing Information ..............14
3.3.2. Replying to Route Requests Using Cached Routes .....15
3.3.3. Route Request Hop Limits ...........................16
3.4. Additional Route Maintenance Features .....................17
3.4.1. Packet Salvaging ...................................17
3.4.2. Queued Packets Destined over a Broken Link .........18
3.4.3. Automatic Route Shortening .........................19
3.4.4. Increased Spreading of Route Error Messages ........20
3.5. Optional DSR Flow State Extension .........................20
3.5.1. Flow Establishment .................................21
3.5.2. Receiving and Forwarding Establishment Packets .....22
3.5.3. Sending Packets along Established Flows ............22
3.5.4. Receiving and Forwarding Packets Sent along
Established Flows ..................................23
3.5.5. Processing Route Errors ............................24
3.5.6. Interaction with Automatic Route Shortening ........24
3.5.7. Loop Detection .....................................25
3.5.8. Acknowledgement Destination ........................25
3.5.9. Crash Recovery .....................................25
3.5.10. Rate Limiting .....................................25
3.5.11. Interaction with Packet Salvaging .................26
4. Conceptual Data Structures .....................................26
4.1. Route Cache ...............................................26
4.2. Send Buffer ...............................................30
4.3. Route Request Table .......................................30
4.4. Gratuitous Route Reply Table ..............................31
4.5. Network Interface Queue and Maintenance Buffer ............32
4.6. Blacklist .................................................33
5. Additional Conceptual Data Structures for Flow State
Extension ......................................................34
5.1. Flow Table ................................................34
5.2. Automatic Route Shortening Table ..........................35
5.3. Default Flow ID Table .....................................36
6. DSR Options Header Format ......................................36
6.1. Fixed Portion of DSR Options Header .......................37
6.2. Route Request Option ......................................40
6.3. Route Reply Option ........................................42
6.4. Route Error Option ........................................44
6.4.1. Node Unreachable Type-Specific Information .........46
6.4.2. Flow State Not Supported Type-Specific
Information ........................................46
6.4.3. Option Not Supported Type-Specific Information .....46
6.5. Acknowledgement Request Option ............................46
6.6. Acknowledgement Option ....................................47
6.7. DSR Source Route Option ...................................48
6.8. Pad1 Option ...............................................50
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 ..........52
7.2.1. Timeout Option .....................................52
7.2.2. Destination and Flow ID Option .....................53
7.3. New Error Types for Route Error Option ....................54
7.3.1. Unknown Flow Type-Specific Information .............54
7.3.2. Default Flow Unknown Type-Specific Information .....55
7.4. New Acknowledgement Request Option Extension ..............55
7.4.1. Previous Hop Address Extension .....................55
8. Detailed Operation .............................................56
8.1. General Packet Processing .................................56
8.1.1. Originating a Packet ...............................56
8.1.2. Adding a DSR Options Header to a Packet ............57
8.1.3. Adding a DSR Source Route Option to a Packet .......57
8.1.4. Processing a Received Packet .......................58
8.1.5. Processing a Received DSR Source Route Option ......60
8.1.6. Handling an Unknown DSR Option .....................63
8.2. Route Discovery Processing ................................64
8.2.1. Originating a Route Request ........................65
8.2.2. Processing a Received Route Request Option .........66
8.2.3. Generating a Route Reply Using the Route Cache .....68
8.2.4. Originating a Route Reply ..........................71
8.2.5. Preventing Route Reply Storms ......................72
8.2.6. Processing a Received Route Reply Option ...........74
8.3. Route Maintenance Processing ..............................74
8.3.1. Using Link-Layer Acknowledgements ..................75
8.3.2. Using Passive Acknowledgements .....................76
8.3.3. Using Network-Layer Acknowledgements ...............77
8.3.4. Originating a Route Error ..........................80
8.3.5. Processing a Received Route Error Option ...........81
8.3.6. Salvaging a Packet .................................82
8.4. Multiple Network Interface Support ........................84
8.5. IP Fragmentation and Reassembly ...........................84
8.6. Flow State Processing .....................................85
8.6.1. Originating a Packet ...............................85
8.6.2. Inserting a DSR Flow State Header ..................88
8.6.3. Receiving a Packet .................................88
8.6.4. Forwarding a Packet Using Flow IDs .................93
8.6.5. Promiscuously Receiving a Packet ...................93
8.6.6. Operation Where the Layer below DSR
Decreases the IP TTL ...............................94
8.6.7. Salvage Interactions with DSR ......................94
9. Protocol Constants and Configuration Variables .................95
10. IANA Considerations ...........................................96
11. Security Considerations .......................................96
Appendix A. Link-MaxLife Cache Description ........................97
Appendix B. Location of DSR in the ISO Network Reference Model ....99
Appendix C. Implementation and Evaluation Status .................100
Acknowledgements .................................................101
Normative References .............................................102
Informative References ...........................................102
1. Introduction
The Dynamic Source Routing protocol (DSR) [JOHNSON94, JOHNSON96a] 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 it 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 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 what is 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" [CLARK88], 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 transmit power
levels or sources of interference.
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 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
[RFC2460], will be covered in other documents. The specification of
DSR in this document provides a compatible base on which such
features can be added, either independently or by integration with
the DSR operation specified here. As described in Appendix C, the
design of DSR has been extensively studied through detailed
simulations and testbed implementation and demonstration; this
document encourages additional implementation and experimentation
with the protocol.
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 [RFC2119].
2. Assumptions
As described here, the DSR protocol is designed mainly for mobile ad
hoc networks of up to about two hundred nodes and is designed to work
well even with 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 it 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, such as through a standard link-layer checksum or
Cyclic Redundancy Check (CRC), 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 and capacity are more the
inherent limiting factors 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 [JOHNSON96a]. Use of
promiscuous mode may also increase the power consumption of the
network interface hardware, depending on the design of the receiver
hardware, and in such cases, DSR can easily be used without the
optimizations that depend on promiscuous receive mode or can be
programmed to only periodically switch the interface into promiscuous
mode. Use of promiscuous receive mode is entirely optional.
Wireless communication ability between any pair of nodes may at times
not work equally well in both directions, due, for example, to
transmit power levels or sources of interference around the two nodes
[BANTZ94, LAUER95]. 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 Medium Access Control (MAC) protocols, however, such as MACA
[KARN90], MACAW [BHARGHAVAN94], or IEEE 802.11 [IEEE80211], limit
unicast data packet transmission to bidirectional links, due to the
required bidirectional exchange of request to send (RTS) and clear to
send (CTS) packets in these protocols and 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 Dynamic Host
Configuration Protocol (DHCP) for dynamic assignment [RFC2131]),
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) [RFC826] 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.
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 the fundamental properties
of DSR's operation are preserved.
3.1. Basic DSR Route Discovery
When some source node originates a new packet addressed to some
destination node, the source node places in the header of the packet
a "source route" giving the sequence of hops that the packet is to
follow on its way to the destination. Normally, the sender will
obtain a suitable source route by searching its "Route Cache" of
routes previously learned; if no route is found in its cache, it will
initiate the Route Discovery protocol to dynamically find a new route
to this destination node. In this case, we call the source node the
"initiator" and the destination node the "target" of the Route
Discovery.
For example, suppose a node A is attempting to discover a route to
node E. The Route Discovery initiated by node A in this example
would proceed as follows:
^ "A" ^ "A,B" ^ "A,B,C" ^ "A,B,C,D"
| id=2 | id=2 | id=2 | id=2
+-----+ +-----+ +-----+ +-----+ +-----+
| A |---->| B |---->| C |---->| D |---->| E |
+-----+ +-----+ +-----+ +-----+ +-----+
| | | |
v v v v
To initiate the Route Discovery, node A transmits a "Route Request"
as a single local broadcast packet, which is received by
(approximately) all nodes currently within wireless transmission
range of A, including node B in this example. Each Route Request
identifies the initiator and target of the Route Discovery, and also
contains a unique request identification (2, in this example),
determined by the initiator of the Request. Each Route Request also
contains a record listing the address of each intermediate node
through which this particular copy of the Route Request has been
forwarded. This route record is initialized to an empty list by the
initiator of the Route Discovery. In this example, the route record
initially lists only node A.
When another node receives this Route Request (such as node B in this
example), if it is the target of the Route Discovery, it returns a
"Route Reply" to the initiator of the Route Discovery, giving a copy
of the accumulated route record from the Route Request; when the
initiator receives this Route Reply, it caches this route in its
Route Cache for use in sending subsequent packets to this
destination.
Otherwise, if this node receiving the Route Request has recently seen
another Route Request message from this initiator bearing this same
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 receipt of
a copy of the Request 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
one is 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 in this case 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 [RFC793], 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 for unicast packets as part of the MAC protocol
[IEEE80211], the discovered source route MUST be reversed in this way
to return the Route Reply, since this route reversal tests the
discovered route to ensure that it is bidirectional before the 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 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 [RFC1122].
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, and 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 [IEEE80211]), or by a "passive acknowledgement"
[JUBIN87] (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 that 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 not to 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 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).
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 on 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 [IEEE80211]
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.
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. However, 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 it is found, the node generally returns a Route Reply to
the initiator itself rather than forward 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
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, due to this restriction
on returning a Route Reply based on information from its Route Cache,
node F does not return such a Route Reply, it 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.
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 [JOHNSON96a]. 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 increase 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
discard it. To salvage a packet, the node replaces the original
source route on the packet with a 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 the TTL is
decremented only once by each node, a single node could salvage a
packet an unbounded number of times. Even if we chose to require the
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
detection of a single, new broken link, 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.
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 [JUBIN87]. 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,
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
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 the "default" flow
from that source to that destination. If a node receives 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, it 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 destination (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 the value used in the TTL
field in the packet's IP header and setting the Lifetime in this
entry to 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.
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 the lifetime specified in the Timeout 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 that 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
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 the packet
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 in the packet.
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 a
matching 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 Address of the packet triggering
the error. This error packet SHOULD be MAC-destined to the node from
which it was received; if this node 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 in the packet.
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).
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 may be less than this, 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 in the packet. Also,
anytime 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.
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 MUST be implemented in a manner consistent with the
external behavior described in this document, but the choice of
implementation used is otherwise unconstrained. 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
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 searched 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 SHOULD 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.
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 [BROCH98, HU00,
JOHANSSON99, MALTZ99a] and through implementation of DSR in a
mobile outdoor testbed under significant workload [MALTZ99b,
MALTZ00, MALTZ01].
- 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 [HU00].
The choice of data structure organization to use for the Route Cache
in any DSR implementation is a local matter for each node and affects
only performance; any reasonable choice of organization for the Route
Cache does not affect either correctness or interoperability.
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 implementers, 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
[HU00]. 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 time out 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 allowed for the destination
address of any packets residing in the Send Buffer.
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.
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, if available, 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 (i.e., if no
corresponding 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
[IEEE80211]; 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" [WRIGHT95]. 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. Packets are
added to the Maintenance buffer after the first transmission attempt
is made. 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 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 a network
interface that requires physically bidirectional links for unicast
transmission, the node 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 route record 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.
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 MUST be implemented in a manner
consistent with the external behavior described in this document, but
the choice of implementation used is otherwise unconstrained.
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 number 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 ID of a default
flow 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.
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
Offset, 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 an entry's list, 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 about 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.
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
behavior described in this section. A node not implementing the flow
state extension SHOULD NOT implement a Default Flow Table.
For each (IP Source Address, IP Destination Address) pair for which a
node forwards packets, the node MUST record:
- The largest odd Flow ID value seen.
- The time at which all the corresponding 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 an (IP Source Address, IP
Destination Address) pair, it MUST also delete all Flow Table entries
for that pair. Nodes MUST delete table entries if all of this (IP
Source Address, IP Destination Address) pair's flows that are
forwarded by this node expire.
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 the 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 [RFC2460], 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.
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 [RFC1700]. If no header follows, then Next
Header MUST have the value 59, "No Next Header" [RFC2460].
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,
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 (when the node does not implement processing for 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 following 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 2 bits are 00 (that is, Option Type & 0x60 == 0), a
node not implementing processing for that Option Type MUST use the
Opt Data Len field to skip over the option and continue
processing. When these 2 bits are 01 (that is, Option Type & 0x60
== 0x20), a node not implementing processing for that Option Type
MUST use the Opt Data Len field to remove the option from the
packet and continue processing as if the option had not been
included in the received packet. When these 2 bits are 10 (that
is, Option Type & 0x60 == 0x40), a node not implementing
processing for that Option Type MUST set the most significant bit
following the Opt Data Len field, MUST ignore the contents of the
option using the Opt Data Len field, and MUST continue processing
the packet. Finally, when these 2 bits are 11 (that is, Option
Type & 0x60 == 0x60), a node not implementing processing for that
Option Type MUST drop the packet.
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.
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.
Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len fields. MUST be set
equal to (4 * n) + 6, where n is the number of addresses in the
Route Request Option.
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 (for this IP Source address and Target
Address) 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 a Route Request is propagated, 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 IPv4 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 IPv4
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.
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. MUST be set
equal to (4 * n) + 1, where n is the number of addresses in the
Route Reply Option.
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 SHOULD 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
at the 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.
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
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.
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 IP 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 Option Type of option triggering the Route Error.
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.
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.
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 SHOULD prefer routes that contain no hops
flagged as External.
Last Hop External (L)
Set to indicate that the last hop indicated by the DSR Source
Route option is actually an arbitrary path in a network
external to the DSR network; the exact route outside the DSR
network is not represented in the DSR Source Route option.
Nodes caching this hop in their Route Cache MUST flag the
cached hop with the External flag. Such hops MUST NOT be
returned in a Route Reply generated from this Route Cache
entry, and selection of routes from the Route Cache to route a
packet being sent SHOULD prefer routes that contain no hops
flagged as External.
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 how to forward the packet, as
defined in Sections 8.1.4 and 8.1.5.
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.
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. The size of
the Option Data field.
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.
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, option, or extension
formats.
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 [RFC1700].
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.
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 | Opt Data Len | 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.
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 | Opt Data Len | 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