IETF MANET Working Group Josh Broch
INTERNET-DRAFT David B. Johnson
David A. Maltz
Carnegie Mellon University
13 March 1998
The Dynamic Source Routing Protocol for Mobile Ad Hoc Networks
<draft-ietf-manet-dsr-00.txt>
Status of This Memo
This document is a submission to the Mobile Ad-hoc Networks (manet)
Working Group of the Internet Engineering Task Force (IETF).
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Abstract
Dynamic Source Routing (DSR) is a routing protocol designed
specifically for use in mobile ad hoc networks. The protocol allows
nodes to dynamically discover a source route across multiple network
hops to any destination in the ad hoc network. When using source
routing, each packet to be routed carries in its header the complete,
ordered list of nodes through which the packet must pass. A key
advantage of source routing is that intermediate hops do not need
to maintain routing information in order to route the packets they
receive, since the packets themselves already contain all of the
necessary routing information. This, coupled with the dynamic,
on-demand nature of Route Discovery, completely eliminates the need
for periodic router advertisements and link status packets, reducing
the overhead of DSR, especially during periods when the network
topology is stable and these packets serve only as keep-alives.
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Contents
Status of This Memo i
Abstract i
1. Introduction 1
2. Assumptions 1
3. Terminology 2
3.1. General Terms . . . . . . . . . . . . . . . . . . . . . . 2
3.2. Specification Language . . . . . . . . . . . . . . . . . 4
4. Protocol Overview 5
4.1. Route Discovery and Route Maintenance . . . . . . . . . . 5
4.2. Packet Forwarding . . . . . . . . . . . . . . . . . . . . 6
4.3. Conceptual Data Structures . . . . . . . . . . . . . . . 6
4.3.1. Route Cache . . . . . . . . . . . . . . . . . . . 6
4.3.2. Node Information Cache . . . . . . . . . . . . . 8
4.3.3. Send Buffer . . . . . . . . . . . . . . . . . . . 8
4.3.4. Retransmission Buffer . . . . . . . . . . . . . . 8
5. Packet Formats 10
5.1. Destination Options Headers . . . . . . . . . . . . . . . 10
5.1.1. DSR Route Request Option . . . . . . . . . . . . 11
5.1.2. DSR Route Reply Option . . . . . . . . . . . . . 13
5.1.3. DSR Route Error Option . . . . . . . . . . . . . 14
5.1.4. DSR Acknowledgment Option . . . . . . . . . . . . 15
5.2. DSR Routing Header . . . . . . . . . . . . . . . . . . . 17
6. Detailed Operation 19
6.1. Route Discovery . . . . . . . . . . . . . . . . . . . . . 19
6.1.1. Originating a Route Request . . . . . . . . . . . 19
6.1.2. Processing a Route Request Option . . . . . . . . 19
6.1.3. Originating a Route Reply . . . . . . . . . . . . 20
6.1.4. Processing a Route Reply Option . . . . . . . . . 21
6.2. Route Maintenance . . . . . . . . . . . . . . . . . . . . 21
6.2.1. Originating a Route Error . . . . . . . . . . . . 21
6.2.2. Processing a Route Error Option . . . . . . . . . 21
6.2.3. Processing a DSR Acknowledgment Option . . . . . 22
6.3. Processing a Routing Header . . . . . . . . . . . . . . . 22
7. Optimizations 24
7.1. Leveraging the Route Cache . . . . . . . . . . . . . . . 24
7.1.1. Promiscuous Learning of Source Routes . . . . . . 24
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7.1.2. Answering Route Requests using the Route Cache . 25
7.2. Route Discovery Using Expanding Ring Search . . . . . . . 25
7.3. Preventing Route Reply Storms . . . . . . . . . . . . . . 26
7.4. Piggybacking on Route Discoveries . . . . . . . . . . . . 27
7.5. Discovering Shorter Routes . . . . . . . . . . . . . . . 27
7.6. Rate Limiting the Route Discovery Process . . . . . . . . 28
7.7. Improved Handling of Route Errors . . . . . . . . . . . . 29
8. Constants 30
9. IANA Considerations 31
10. Security Considerations 32
Location of DSR Functions in the ISO Model 33
Implementation Status 34
Acknowledgments 35
Areas for Refinement 36
References 37
Chair's Address 39
Authors' Addresses 40
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1. Introduction
This document describes Dynamic Source Routing (DSR) [6, 7], a
protocol developed by the Monarch Project [8, 14] at Carnegie Mellon
University for routing packets in a mobile ad hoc network [3].
Source routing is a routing technique in which the sender of a packet
determines the complete sequence of nodes through which to forward
the packet; the sender explicitly lists this route in the packet's
header, identifying each forwarding "hop" by the address of the next
node to which to transmit the packet on its way to the destination
host.
DSR offers a number of potential advantages over other routing
protocols for mobile ad hoc networks. First, DSR uses no periodic
routing messages (e.g., no router advertisements and no link-level
neighbor status messages), thereby reducing network bandwidth
overhead, conserving battery power, and avoiding the propagation of
potentially large routing updates throughout the ad hoc network. Our
Dynamic Source Routing protocol is able to adapt quickly to changes
such as host movement, yet requires no routing protocol overhead
during periods in which no such changes occur.
In addition, DSR has been designed to compute correct routes in
the presence of asymmetric (uni-directional) links. In wireless
networks, links may at times operate asymmetrically due to sources
of interference, differing radio or antenna capabilities, or the
intentional use of asymmetric communication technology such as
satellites. Due to the existence of asymmetric links, traditional
link-state or distance vector protocols may compute routes that
do not work. DSR, however, will find a correct route even in the
presence of asymmetric links.
2. Assumptions
We assume that all hosts wishing to communicate with other hosts
within the ad hoc network are willing to participate fully in the
protocols of the network. In particular, each host participating in
the network should also be willing to forward packets for other hosts
in the network.
We refer to the minimum number of hops necessary for a packet to
reach from any host located at one extreme edge of the network to
another host located at the opposite extreme, as the diameter of the
network. We assume that the diameter of an ad hoc network will be
small (e.g., perhaps 5 or 10 hops), but may often be greater than 1.
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Packets may be lost or corrupted in transmission on the wireless
network. A host receiving a corrupted packet can detect the error
and discard the packet.
We assume that hosts can enable a promiscuous receive mode on
their wireless network interface hardware, causing 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, it is for example common in current LAN
hardware for broadcast media including wireless, and some of our
optimizations take advantage of it if available. Use of promiscuous
mode does increase the software overhead on the CPU, but we believe
that wireless network speeds are more the inherent limiting factor to
performance in current and future systems. We believe that portions
of the protocol are also suitable for implementation directly within
a programmable network interface unit to avoid this overhead on the
CPU.
3. Terminology
3.1. General Terms
node
A device that implements IP.
router
A node that forwards IP packets not explicitly addressed to
itself.
host
Any node that is not a router.
link
A communication facility or medium over which nodes can
communicate at the link layer, such as an Ethernet (simple or
bridged). A link is the layer immediately below IP.
interface
A node's attachment to a link.
prefix
A bit string that consists of some number of initial bits of an
address.
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interface index
An 8-bit quantity which uniquely identifies an interface among
a given node's interfaces.
link-layer address
A link-layer identifier for an interface, such as IEEE 802
addresses on Ethernet links.
packet
An IP header plus payload.
home address
An IP address that is assigned for an extended period of time
to a mobile node. It remains unchanged regardless of where
the node is attached to the Internet [9]. If a node has more
than one home address, it SHOULD select and use a single home
address when participating in the ad hoc network.
source route
A source route from node A to node B is an ordered list of home
addresses, starting with the home address of node A and ending
with the home address of the node B. Between A and B, the
source route includes an ordered list of all the intermediate
hops between A and B, as well as the interface index of the
interface through which the packet should be transmitted to
reach the next hop. Note that the packet formats defined in
Section 5.1 encode the Target Address (node B) separately,
instead of encoding it as the last hop on the source route.
Route Discovery
The method in DSR by which a node A dynamically obtains a
source route to node B that will carry packets through the
network from A to B. Performing a route discovery involves
sending one or more Route Request packets.
Route Maintenance
The process in DSR of monitoring the status of a source route
while in use, so that any link-failures along the source route
can be detected and the broken source route removed from use.
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3.2. Specification Language
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 [2].
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4. Protocol Overview
4.1. Route Discovery and Route Maintenance
A source routing protocol must solve two challenges, which DSR terms
Route Discovery and Route Maintenance. Route Discovery is the
mechanism whereby a node S wishing to send a packet to a destination
D obtains a source route to D.
Route Maintenance is the mechanism whereby 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 hop along the
route no longer works. When Route Maintenance indicates a source
route is broken, S can attempt to use any other route it happens to
know to D, or can invoke Route Discovery again to find a new route.
To perform Route Discovery, the source node S broadcasts a Route
Request packet with a recorded source route listing only itself.
Each node that hears the Route Request forwards the Request if
appropriate, adding its own address to the recorded source route in
this copy of the Request and rebroadcasts the packet. The forwarding
of Requests is constructed so that copies of the Request propagate
hop-by-hop outward from the node initiating the Route Discovery,
until either the target of the Request is found or until another node
is found that can supply a route to the target.
The basic mechanism of forwarding Route Requests forwards the Request
if the node (1) is not the target of the Request and (2) is not
already listed in the recorded source route in this copy of the
Request. In addition, however, each node maintains an LRU cache of
recently received Route Requests and does not propagate any copies
of a Request after the first, avoiding the overhead of forwarding
additional copies that reach this node along different paths. Also,
the Time-to-Live field in the IP header of the packet carrying the
Route Request MAY be used to limit the scope over which the Request
will propagate, using the normal behavior of Time-to-Live defined by
IP [12, 1]. Additional optimizations on the handling and forwarding
of Route Requests are also used to further reduce the Route Discovery
overhead. When the target of the Request (e.g., node D) receives the
Route Request, it copies the recorded source route into a Route Reply
packet which it then sends this Reply back to the initiator of the
Route Request (e.g., node S).
All source routes learned by a node are kept in a Route Cache, which
is used to further reduce the cost of Route Discovery. When a node
wishes to send a packet, it examines its own Route Cache and performs
Route Discovery only if no suitable source route is found in its
Cache.
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Further, when a node B receives a Route Request from S for another
node D, B searches its own Route Cache for a route to D. If B finds
such a route, it does not propagate the Route Request, but instead
returns a Route Reply to node S based on the concatenation of the
recorded source route from S to B in the Route Request and the cached
route from B to D. The details of replying from a Route Cache in this
way are discussed in Section 7.1.
As a node overhears routes being used by others, either by
promiscuously snooping on them or when forwarding packets, the node
MAY insert those routes into its Route Cache, leveraging the Route
Discovery operations of the other nodes.
4.2. Packet Forwarding
To represent a source route within a packet's header, DSR uses a
Routing Header that conforms to the Routing Header format specified
for IPv6, adapted to the needs of DSR and to the use of the DSR in
IPv4 (or in IPv6 in the future). The DSR Routing Header uses a
unique Routing Type field value to distinguish it from the existing
Type 0 Routing Header defined within IPv6 [4].
To forward a packet, a receiving node N simply processes the Routing
Header as specified in the IPv6 [4] and transmits the packet to
the next hop. If a forwarding error occurs along the link to
the next hop in the route, this node N sends a Route Error back
to the originator S of the packet informing S that this link is
"broken". If node N's Route Cache contains a different route to the
destination, then the packet is retransmitted using the new source
route. Each node overhearing or forwarding a Route Error packet also
removes from its Route Cache the link indicated to be broken, thereby
cleaning the stale cache data from the network.
4.3. Conceptual Data Structures
All information a node needs for participation in an ad hoc
network using the Dynamic Source Routing Protocol can be organized
conceptually into four data structures: a Route Cache, a Node
Information Cache, a Send Buffer, and a Retransmission Buffer. These
data structures MAY be implemented in any manner consistent with the
external behavior described in this document.
4.3.1. Route Cache
All routing information needed by a node participating in an ad hoc
network is stored in a Route Cache. Each node in the network
maintains its own Route Cache. The node adds information to the
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cache as it learns of new links between nodes in the ad hoc network,
for example through packets carrying either a Route Reply or a
Routing Header. Likewise, the node removes information from the
cache as it learns existing links in the ad hoc network have broken,
for example through packets carrying a Route Error or through the
link-layer retransmission mechanism reporting a failure in forwarding
a packet to its next-hop destination. The Route Cache is indexed
logically by destination node, and supports the following operations:
void Insert(Route RT)
Information extracted from source route RT is inserted into the
Route Cache.
Route Get(Node DEST)
A source route from this node to DEST (if it exists) is
returned.
void Delete(Node FROM, Node TO)
Any routes in the cache that assume the existence of a
unidirectional link from node FROM to node TO are removed from
the cache.
Each implementation MAY choose the cache replacement and cache search
strategies most appropriate for its particular network environment.
For example, some environments may choose to return the shortest
route to a node (the shortest sequence of hops), while others may
select an alternate metric for the Get() operation.
The Route Cache SHOULD support storing more than one source route for
each destination.
If node S is using a source route to destination D that includes
intermediate node I, S SHOULD shorten the route to destination D when
it learns of a shorter route to node I. A node S using a source route
to destination D through node I, MAY shorten the source route if it
learns of a shorter path from node I to node D.
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. In particular, a node SHOULD prefer routes that it is
presently using over those that it is not.
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The Route Cache SHOULD time-stamp each route as it is inserted into
the cache. If the route is not used within ROUTE_CACHE_TIMEOUT
seconds, it SHOULD be removed from the cache.
4.3.2. Node Information Cache
The Node Information Cache is a collection of records indexed by home
address. A record maintained on node N1 for node N2 contains the
following:
- The time that N1 last began a Route Discovery for N2.
- The interval of time that N1 must wait before the next attempt at
a Route Discovery for N2.
- The Time-to-live (TTL) field in the IP header of last Route
Request transmitted by N1 for N2.
- A FIFO cache of the last ID_FIFO_SIZE Identification values
observed in Route Request packets initiated by N2.
Nodes SHOULD use an LRU policy to manage the entries of the Node
Information Cache.
4.3.3. Send Buffer
The Send Buffer is a queue of packets that cannot be transmitted
because the transmitting node does not yet have a source route
to the packets' destinations. Each packet in the Send Buffer is
stamped with the time that it is placed into the Buffer, and SHOULD
be removed from the Send Buffer and discarded SEND_BUFFER_TIMEOUT
seconds after initially being placed in the Buffer. If necessary, a
FIFO strategy SHOULD be used to evict packets before they timeout to
prevent the buffer from overflowing.
Subject to the rate limiting defined in Section 6.1, a Route
Discovery SHOULD be initiated as often as possible for any packets
residing in the Send Buffer.
4.3.4. Retransmission Buffer
The Retransmission Buffer is a queue of packets that are awaiting the
receipt of an explicit acknowledgment from the next hop in the source
route (Section 5.2).
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For each packet in the Retransmission Buffer, a node maintains (1) a
count of the number of retransmissions and (2) the time of the last
retransmission.
Packets are removed from the buffer when an acknowledgment
is received, or when the number of retransmissions exceeds
MAX_EXPLICIT_REXMIT. In the later case, the removal of the packet
from the Retransmission Buffer should result in a Route Error being
returned to the initial source of the packet (Section 6.2).
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5. Packet Formats
5.1. Destination Options Headers
Dynamic Source Routing makes use of four options carrying control
information that can be piggybacked in any existing IP packet.
The mechanism used for these options is based on the design of
the Destination Option mechanism in IPv6 [4]. This notion of
a Destination Option must be build in to a IPv4 protocol stack.
Specifically, the Protocol field in the IP header should be used to
indicate that a Destination Options header exists between the IP
header and the remaining portion of a packet's payload (such as a
transport layer header). The Next Header field in the Destination
Options header will then indicate the type of header that follows it
in a packet.
The Destination Options header is used to carry optional information
that need be examined only by a packet's destination node(s). The
Destination Options header is identified by a Next Header value of 60
in the immediately preceding header, and has the following format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
. .
. Options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following destination options are used by the Dynamic Source
Routing protocol:
- DSR Route Request option (Section 5.1.1)
- DSR Route Reply option (Section 5.1.2)
- DSR Route Error option (Section 5.1.3)
- DSR Acknowledgement option (Section 5.1.4)
All of these destination options MAY appear multiple times within a
single Destination Options header.
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5.1.1. DSR Route Request Option
The DSR Route Request destination option is encoded in
type-length-value (TLV) format as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length | Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Target Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Index[1] | Index[2] | Index[3] | Index[4] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[2] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[3] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[4] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Index[5] | Index[6] | Index[7] | Index[8] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP fields:
Source Address
MUST be the home address of the node transmitting this packet.
Destination Address
MUST be the limited broadcast address (255.255.255.255).
Hop Limit (TTL)
Can be varied from 1 to 255, for example to implement
expanding-ring searches.
Route Request fields:
Option Type
???. A node that does not understand this option MUST discard
the packet (the top two bits must be 01).
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Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields.
Identification
A unique value generated by the initiator (original sender)
of the Route Request. This value allows a recipient to
determine whether or not it has recently seen this a copy of
this Request; if it has, the packet is simply discarded. When
propagating a Route Request, this field MUST be copied from the
received copy of the Request being forwarded.
Target Address
The home address of the node that is the target of the Route
Request.
Index[1..n]
Index[i] is the interface index of the ith hop recorded in in
the Route Request option (in Address[i]).
Address[1..n]
Address[i] is the home address of the ith hop recorded in the
Route Request option.
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5.1.2. DSR Route Reply Option
The DSR Route Reply destination option is encoded in
type-length-value (TLV) format as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length |R|F| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Target Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Index[1] | Index[2] | Index[3] | Index[4] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[2] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[3] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[4] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Index[5] | Index[6] | Index[7] | Index[8] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
???. A node that does not understand this option should ignore
this option and continue processing the packet (the top two
bits should be 00).
Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields.
Router (R)
If the Router (R) bit is set, the last address recorded in this
header is the home address of a router that believes it can
reach the Target Address specified in the Route Request packet.
Foreign Agent (F)
If the Foreign Agent (F) bit is set, the last address recorded
in this header is the home address of an IETF Mobile IP [9]
Foreign Agent. The Router (R) bit and the Foreign Agent (F)
bit are mutually exclusive as (F) implies (R).
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Reserved
Sent as 0; ignored on reception.
Target Address
The home address of the node that is the ultimate destination
of the source route contained in the Route Reply.
Index[1..n]
Index[i] is the interface index of the ith hop listed in the
Route Reply option (in Address[i]).
Address[1..n]
Address[i] is the home address of the ith hop listed in the
Route Reply option.
5.1.3. DSR Route Error Option
The DSR Route Error destination option is encoded in
type-length-value (TLV) format as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length | Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Originator Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| From Hop Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
???. A node that does not understand this option should ignore
the option and continue processing the packet (the top two bits
must be 00).
Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields.
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Index
The interface index of the network interface over which the
link from the From Hop Address node to the Next Hop Node is
being reported as broken by this Route Error option. This
Index refers to an interface on the From Hop Address node.
Originating Address
The home address of the node which originated the packet that
could not be forwarded.
From Hop Address
The home address of the node that attempted to forward a packet
and discovered the link failure.
Next Hop Address
The home address of the node that was found to be unreachable
(the next hop neighbor to which the node at Originating Address
was attempting to transmit the packet).
5.1.4. DSR Acknowledgment Option
The DSR Acknowledgment destination option is encoded in
type-length-value (TLV) format as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length | Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
???. A node that does not understand this option should ignore
the option and continue processing the packet (the top two bits
must be 00).
Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields.
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Identification
A unique value assigned by the originator of the packet.
This value is used to match explicit acknowledgments to the
corresponding packet.
Address[1]
The home address of the original source of the IP packet.
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5.2. DSR Routing Header
As specified for IPv6 [4], a Routing header is used by a source to
list one or more intermediate nodes to be "visited" on the way to
a packet's destination. This function is similar to IPv4's Loose
Source and Record Route option, but the Routing header does not
record the route taken as the packet is forwarded. The specific
processing steps required to implement the Routing header must be
added to an IPv4 protocol stack. The Routing header is identified by
a Next Header value of 43 in the immediately preceding header, and
has the following format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len | Routing Type | Segments Left |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. type-specific data .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The type specific data for a Routing Header carrying a DSR Source
Route is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Reserved | Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Index[1] | Index[2] | Index[3] | Index[4] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[2] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[3] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[4] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Index[5] | Index[6] | Index[7] | Index[8] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Routing Header Fields:
Next Header
8-bit selector. Identifies the type of header immediately
following the Routing Request header.
Hdr Ext Len
8-bit unsigned integer. Length of the Routing header in
8-octet units, not including the first 8 octets.
Routing Type
???
Segments Left
Number of route segments remaining, i.e., number of explicitly
listed intermediate nodes still to be visited before reaching
the final destination.
Type Specific Fields:
Acknowledgment Request (R)
The Acknowledgment Request (R) bit is set to request an
explicit acknowledgment from the next hop.
Reserved
Sent as 0; ignored on reception.
Identification
A unique value assigned by the originator of the packet. This
value is used to match acknowledgments (passive or explicit) to
the appropriate packet.
Index[1..n]
Index[i] is the interface index of the ith hop in the Routing
header.
Address[1..n]
Address[i] is the home address of the ith hop in the Routing
header.
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6. Detailed Operation
6.1. Route Discovery
Route Discovery is the demand-driven process by which nodes actively
obtain source routes to destinations to which they are actively
attempting to send packets. The destination node for which a Route
Discovery is initiated to discover a route is known as the "target"
of the Route Discovery. A Route Discovery for a destination SHOULD
NOT be initiated unless the initiating node has an unexpired packet
to be delivered to that destination.
A Route Discovery for a given target node MUST NOT be initiated
unless the difference between the current time and the time that a
Route Discovery was last initiated for destination D is greater than
the backoff interval currently listed in the Node Information Cache
for node D. After each Route Discovery attempt, the interval between
successive Route Discoverys must be doubled, up to a maximum of
MAX_RTDISCOV_INTERVAL.
The basic Route Discovery algorithm is to originate a single
Route Request packet as described below that targets the desired
destination and has a maximum hop limit set to MAX_ROUTE_LEN.
6.1.1. Originating a Route Request
A node originates a Route Request for a particular host when it has
no route to that host. The Option Length field in the Route Request
option MUST be set to 6, the Identification field MUST be set to a
unique number, and the Target Address field MUST contain the Home
Address of the node for which a route is being requested.
6.1.2. Processing a Route Request Option
Let P1 be the received packet containing a Route Request option.
Let P2 be a packet containing a corresponding Route Reply. A Route
Request option is processed as follows:
1. Determine the originator of the Route Request.
If no addresses are presently listed in P1.REQUEST.Address[],
then P1.Source_Address identifies the originator of the Route
Request. Otherwise, P1.REQUEST.Address[1] identifies the
originator of the Route Request.
2. If the combination (Originator Address, P1.REQUEST.Identification)
is in the node's cache of recently seen (Address, Identification)
pairs, then discard the packet. DONE.
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3. If the home address of this node is already listed in
P1.REQUEST.Address[], then discard the packet. DONE.
4. If P1.REQUEST.Target_Address matches the home address of
this node, then this packet contains a complete source route
describing the path from the initiator of the Route Request to
this node.
(a) Send a Route Reply as described in Section 6.1.3.
(b) If P1.REQUEST.Next_Header indicates No Next Header, DONE.
(c) Otherwise, swap P1.REQUEST.Target_Address and
P1.Source_Address and pass the packet up the protocol
stack. DONE.
5. Set P1.REQUEST.Address[n+1] = home address of this node.
Re-broadcast the Route Request packet jittered by T milliseconds,
where T is a uniformly distributed, random number between 0 and
BROADCAST_JITTER. DONE.
6.1.3. Originating a Route Reply
Let P1 be the received packet containing a Route Request option. Let
P2 be a packet containing a corresponding Route Reply. A Route Reply
is transmitted in response to a Route Request as follows:
1. If P1.REQUEST.Address[] does not contain any hops, then this node
is only a single hop from the originator of the Route Request.
Build a Route Replay packet as follows:
P2.Destination_Address = P1.Source_Address
P2.Source_Address = P1.REQUEST.Target_Address
GOTO 3.
2. Otherwise, build a Route Reply packet as follows:
P2.Destination_Address = P1.REQUEST.Address[1]
P2.Source_Address = P1.REQUEST.Target_Address
P2.REPLY.Address[1..n] = P1.REQUEST.Address[1..n]
3. Transmit the Route Reply jittered by T milliseconds, where
T is a uniformly distributed, random number between 0 and
BROADCAST_JITTER. DONE.
If sending a Route Reply packet to the originator of the Route
Request requires performing a Route Discovery, the Route Reply
destination option MUST be piggybacked on the packet that contains
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the Route Request. This prevents a loop wherein the target of the
Route Request (which was itself the originator of the original Route
Request) must do another Route Request in order to return its Route
Reply.
If sending the Route Reply to the originator of the Route Request
does not require performing Route Discovery, nodes SHOULD send a
unicast Route Reply in response to every Route Request targeted at
them.
6.1.4. Processing a Route Reply Option
Upon receipt of a Route Reply, a node should extract the source route
(Address[1..n] + Target Address) and insert this route into its Route
Cache. Any packets in the Send Buffer that are addressed to Target
Address SHOULD be processed.
6.2. Route Maintenance
6.2.1. Originating a Route Error
If while forwarding a packet with a Routing Header, the next hop
specified in the source route is found to be unreachable, a Route
Error packet (Section 5.1.3) MUST be returned to the originator
(Address[1]) of the packet.
The forwarding node SHOULD consider the next hop to be unreachable if
any of the following conditions occurs:
- The failure to receive a passive acknowledgment when such passive
acknowledgments had been received previously.
- The failure to receive an explicitly requested acknowledgment
after MAX_EXPLICIT_REXMIT retransmissions.
- In link layers providing retransmissions and acknowledgments
(e.g., 802.11), a signal from the link layer that it is unable to
deliver the packet.
6.2.2. Processing a Route Error Option
Upon receipt of a Route Error via any mechanism, a node SHOULD remove
any route from its Route Cache that uses the hop (From Hop Address,
Next Hop Address).
When the Route Error is returned to the Originator Address, the
originator must verify that the source route in the Route Error
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packet (From Hop Address...Originator Address) includes the same
hops as the working prefix of the original packet's source route
(Originator Address...From Hop Address). If any hop listed in the
working prefix is not included in the Route Error's source route,
then the originator must transmit the Route Error back along the
working prefix (Originator Address...From Hop Address) so that each
node along the working prefix will remove the invalid route from its
Route Cache.
If the node processing a Route Error option discovers its home
address equals the Router Error's Originator Address and the packet
contains an additional nested Route Error, the node MUST perform the
following steps:
1. Remove the Route Error being processed from the packet.
2. Copy the Originator Address from the next nested Route Error to
the IP destination field of the packet.
3. Attach a source route and send the packet to the IP destination,
performing Route Discovery if needed.
6.2.3. Processing a DSR Acknowledgment Option
Upon receipt of a DSR Acknowledgment, a node should remove any packet
in its Retransmission Buffer matching the (Address, Identification)
pair found in the Acknowledgment option. If no match is found, the
Acknowledgment should be silently discarded.
[I'm supposed to say something intelligent here, but I can't remember
what... -josh]
6.3. Processing a Routing Header
A DSR Routing Header should be processed in accordance with the steps
outlined for Routing Headers in [4]. The Routing Header is only
processed by the node whose address appears as the IP destination
of the packet. A few additional rules apply to processing the type
specific data of a DSR Source Route:
1. The interface used to transmit the packet MUST be the interface
denoted by Index[n] where Address[n] is the home address of this
node.
2. If the Acknowledgment Request (R) bit is set, the node MUST
create and transmit a packet containing the DSR Acknowledgment
option to the IP Source of the packet, performing Route Discovery
if necessary.
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3. If the node chooses to set the Acknowledgment Request (R) bit in
the packet when it forwards it, it must first make a copy of the
packet and insert this copy into its Retransmission Buffer.
4. If a node finds the next hop in the Routing Header to be
unreachable, it MUST send a Route Error packet to the originator
of the packet, denoted by ROUTING.Address[1].
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7. Optimizations
A number of optimizations can be added to the basic operation of
Route Discovery and route maintenance as described in Section 4.1
that can reduce the number of overhead packets and improve the
average efficiency of the routes used on data packets. This section
discusses some of those optimizations.
7.1. Leveraging the Route Cache
The data in a node's Route Cache may be stored in any format, but
the active routes in its cache form a tree of routes, rooted at
this node, to other nodes in the ad hoc network. For example, the
illustration below shows an ad hoc network of six mobile nodes, in
which mobile node A has earlier completed a Route Discovery for
mobile node D and has cached a route to D through B and C:
B->C->D
+---+ +---+ +---+ +---+
| A |---->| B |---->| C |---->| D |
+---+ +---+ +---+ +---+
+---+
| F | +---+
+---+ | E |
+---+
Since nodes B and C are on the route to D, node A also learns the
route to both of these nodes from its Route Discovery for D. If A
later performs a Route Discovery and learns the route to E through B
and C, it can represent this in its Route Cache with the addition of
the single new hop from C to E. If A then learns it can reach C in a
single hop (without needing to go through B), A SHOULD use this new
route to C to also shorten the routes to D and E in its Route Cache.
7.1.1. Promiscuous Learning of Source Routes
A node can add entries to its Route Cache any time it learns a
new route. In particular, when a node forwards a data packet as
an intermediate hop on the route in that packet, the forwarding
node is able to observe the entire route in the packet. Thus, for
example, when node B forwards packets from A to D, B SHOULD add the
route information from that packet to its own Route Cache. If a
node forwards a Route Reply packet, it SHOULD also add the route
information from the route record being returned in the Route Reply,
to its own Route Cache.
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Finally, since all wireless network transmissions are inherently
broadcast, a node MAY configure its network interface into
promiscuous receive mode, and add to its Route Cache the route
information from any packet it can overhear.
7.1.2. Answering Route Requests using the Route Cache
A node SHOULD use its Route Cache to avoid propagating a Route
Request packet received from another node. In particular, suppose a
node receives a Route Request packet for which it is not the target
and which it does not discard on based on the logic of section 6.1.1.
If the node has a Route Cache entry for the target of the request,
it may append this cached route to the accumulated route record
in the packet, and may return this route in a Route Reply packet
to the initiator without propagating (re-broadcasting) the Route
Request. Thus, for example, if node F in the example network shown
in Section 7.1 needs to send a packet to node D, it will initiate
a Route Discovery and broadcast a Route Request packet. If this
broadcast is received by A, A can simply return a Route Reply packet
to F containing the complete route to D consisting of the sequence of
hops A, B, C, and D.
Before transmitting a Route Reply packet that was generated using
information from its Route Cache, a node MUST verify that:
1. The resulting route does not contain any loops.
2. The node issuing the Route Reply is listed in the route that it
is replying with. This increases the probability that the route
is valid, since the node in question should have received a Route
Error if this route stopped working.
7.2. Route Discovery Using Expanding Ring Search
The propagating nature of a basic Route Request packet means that
potentially every node in the ad hoc network will be disturbed
whenever one is originated. To reduce this network-wide cost, all
nodes SHOULD limit the maximum propagation of their Route Requests in
some way, and MAY use the following algorithm.
1. Whenever the backoff algorithm permits the initiation of a Route
Discovery, initially send a Route Request with a hop limit of one
(we refer to this as a non-propagating Route Request).
2. If no Route Reply is received from the non-propagating Route
Request after RING0_TIMEOUT seconds, send a new Route Request
with the hop limit set to MAX_ROUTE_LEN.
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A single attempt at Route Discovery for destination node D may
therefore involve sending two Route Request packets. Nodes MUST
not backoff between the sending a Route Request with a hop limit of
one and the subsequent sending of Route Request with a hop limit of
MAX_ROUTE_LEN. This procedure uses the hop limit on the Route Request
packet to inexpensively check if the target is currently within
wireless transmitter range of the initiator, or if another node
within range has a Route Cache entry for this target (effectively
using the caches of this node's neighbors as an extension of its own
cache). Since the initial request is limited to one network hop, the
timeout period before sending the propagating request can be quite
small.
7.3. Preventing Route Reply Storms
The ability for nodes to reply to a Route Request not targeted at
them using their Route Caches can result in a Route Reply storm. If
a node broadcasts a Route Request for a node that its neighbors have
in their Route Caches, each neighbor may attempt to send a Route
Reply thereby wasting bandwidth and increasing the rate of collisions
in the area. For example, in the network shown in Section 7.1, if
both A and B receive F's Route Request, they will both attempt to
reply from their Route Caches. Both will send their replies at
about the same time since they receive the broadcast at about the
same time. Particularly when more than the two mobile nodes in this
example are involved, these simultaneous replies from the mobile
nodes receiving the broadcast may create packet collisions among
some or all of these replies and may cause local congestion in the
wireless network. In addition, it will often be the case that the
different replies will indicate routes of different lengths. For
example, A's reply will indicate a route to D that is one hop longer
than that in B's reply.
For interfaces which can promiscuously listen to the channel, mobile
nodes SHOULD use the following algorithm to reduce the number of
simultaneous replies by slightly delaying their Route Reply:
1. Pick a delay period
d = H * (h - 1 + r)
where h is the length in number of network hops for the route to
be returned in this node's reply, r is a random number between 0
and 1, and H is a small constant delay to be introduced per hop.
2. Delay transmitting the Route Reply from this node for a period
of d.
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3. Within the delay period, promiscuously receive all packets at
this node. If a packet is received by this node during the delay
period that is addressed to the target of this Route Discovery
(the target is the final destination address for the packet,
through any sequence of intermediate hops), and if the length of
the route on this packet is less than h, then cancel the delay
and do not transmit the Route Reply from this node; this node
may infer that the initiator of this Route Discovery has already
received a Route Reply, giving an equal or better route.
7.4. Piggybacking on Route Discoveries
As described in Section 4.1, when one node needs to send a packet
to another, if the sender does not have a Route Cached to the
destination node, it must initiate a Route Discovery, either
buffering the original packet until the Route Reply is returned, or
discarding it and relying on a higher-layer protocol to retransmit
it if needed. The delay for Route Discovery and the total number
of packets transmitted can be reduced by allowing data to be
piggybacked on Route Request packets. Since some Route Requests may
be propagated widely within the ad hoc network, though, the amount
of data piggybacked must be limited. We currently use piggybacking
when sending a Route Reply or a Route Error packet, since both are
naturally small in size, and small data packets such as the initial
SYN packet opening a TCP connection [13] could easily be piggybacked.
One problem, however, arises when piggybacking on Route Request
packets. If a Route Request is received by a node that replies
to the request based on its Route Cache without propagating the
request (Section 7.1), the piggybacked data will be lost if the node
simply discards the Route Request. In this case, before discarding
the packet, the node must construct a new packet containing the
piggybacked data from the Route Request packet. The source route
in this packet MUST be constructed to appear as if the new packet
had been sent by the initiator of the Route Discovery and had been
forwarded normally to this node. Hence, the first portion of the
route is taken from the accumulated route record in the Route Request
packet and the remainder of the route is taken from this node's Route
Cache. The sender address in the packet should also be set to the
initiator of the Route Discovery. Since the replying node will be
unable to correctly recompute an Authentication header for the split
off piggybacked data, data covered by an Authentication header SHOULD
NOT be piggybacked on Route Request packets.
7.5. Discovering Shorter Routes
Once a route between a packet source and a destination has been
discovered, the basic DSR protocol MAY continue to use that route for
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all traffic from the source to the destination, even if the nodes
move such that a shorter route becomes possible. In many cases, the
basic route maintenance procedure will discover the shorter route,
since if a node moves enough to create a shorter route, it will
likely also move out of transmission range of at least one hop on the
existing route.
When operating in promiscuous receive mode, a node SHOULD use the
following algorithm to process a received packet. Whenever possible,
this algorithm shortens routes that already exist in the Route Cache.
1. If the packet is not a data packet containing a Routing Header,
drop the packet. DONE.
2. If the IP destination is the home address of this node, then
follow the normal steps to process the packet. DONE.
3. If the home address of this node does not appear in the portion
of the source route that has not yet been processed (indicated by
Segments Left), then drop the packet. DONE.
4. The node S indicated by the Source Address field in the IP header
can communicated directly with this node N. Create a Route Reply.
The Route Reply MUST list the entire source routing contained in
the received packet with the exception of the intermediate nodes
between node S and node N.
7.6. Rate Limiting the Route Discovery Process
One common error condition that must be handled in an ad hoc network
is the case in which the network effectively becomes partitioned.
That is, two nodes that wish to communicate are not within
transmission range of each other, and there are not enough other
mobile nodes between them to form a sequence of hops through which
they can forward packets. If a new Route Discovery was initiated
for each packet sent by a node in this situation, a large number of
unproductive Route Request packets would be propagated throughout the
subset of the ad hoc network reachable from this node. In order to
reduce the overhead from such route discoveries, we use exponential
backoff to limit the rate at which new route discoveries may be
initiated from any node for the same target. If the node attempts to
send additional data packets to this same node more frequently than
this limit, the subsequent packets SHOULD be buffered in the Send
Buffer until a Route Reply is received, but it 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 to any single IP address [1].
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7.7. Improved Handling of Route Errors
All nodes SHOULD process all of the Route Error messages they
receive, regardless of whether the node is the destination of
the Route Error, is forwarding the Route Error, or promiscuously
overhears the Route Error.
Since a Route Error packet names both ends of the hop that is no
longer valid, any of the nodes receiving the error packet may update
their Route Caches to reflect the fact that the two nodes indicated
in the packet can no longer directly communicate. A node receiving
a Route Error packet simply searches its Route Cache for any routes
using this hop. For each such route found, the route is truncated at
this hop. All nodes on the route before this hop are still reachable
on this route, but subsequent nodes are not.
An experimental optimization to improve the handling of errors is
to support the caching of "negative" information in a node's Route
Cache. The goal of negative information is to record that a given
route was tried and found not to work, so that if the same route
is discovered again shortly after the failure, the Route Cache can
ignore or downgrade the metric of the failed route.
We have not currently included this caching of negative information
in our simulations, since it appears to be unnecessary if nodes also
promiscuously receive Route Error packets.
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8. Constants
BROADCAST_JITTER 10 milliseconds
ID_FIFO_SIZE 8 identifiers
INVALID_INTERFACE_INDEX 0xFF
MAX_EXPLICIT_REXMIT 3 attempts
MAX_RTDISCOV_INTERVAL 120 seconds
MAX_ROUTE_LEN 15 nodes
RING0_TIMEOUT 30 milliseconds
ROUTE_CACHE_TIMEOUT 300 seconds
SEND_BUFFER_TIMEOUT 30 seconds
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9. IANA Considerations
This document defines four new types of IPv6 destination option, each
of which must be assigned an Option Type value:
- The DSR Route Request option, described in Section 5.1.1
- The DSR Route Reply option, described in Section 5.1.2
- The DSR Route Error option, described in Section 5.1.3
- The DSR Acknowledgment option, described in Section 5.1.4
DSR also requires a routing header Routing Type be allocated for the
DSR Source Route defined in section 5.2.
In IPv4, we require two new protocol numbers be issued to identify
the next header as either an IPv6-style destination option, or an
IPv6-style routing header. Other protocols can make use of these
protocol numbers as nodes that support them will processes any
included destination options or routing headers according to the
normal IPv6 semantics.
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10. Security Considerations
This document does not specifically address security concerns. This
document does assume that all nodes participating in the DSR protocol
do so in good faith and with out malicious intent to corrupt the
routing ability of the network. In mission-oriented environments
where all the nodes participating in the DSR protocol share a
common goal that motivates their participation in the protocol, the
communications between the nodes can be encrypted at the physical
channel or link layer to prevent attack by outsiders.
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Location of DSR Functions in the ISO Model
When designing DSR, we had to determine at what level within the
protocol hierarchy to implement source routing. We considered two
different options: routing at the link layer (ISO layer 2) and
routing at the network layer (ISO layer 3). Originally, we opted to
route at the link layer for the following reasons:
- Pragmatically, running the DSR protocol at the link layer
maximizes the number of mobile nodes that can participate in
ad hoc networks. For example, the protocol can route equally
well between IP [12], IPv6 [4], and IPX [5] nodes.
- Historically, DSR grew from our contemplation of a multihop ARP
protocol [6, 7] and source routing bridges [10]. ARP [11] is a
layer 2protocol.
- Technically, we designed DSR to be simple enough that that it
could be implemented directly in network interface cards, well
below the layer 3 software within a mobile node. We see great
potential for DSR running between clouds of mobile nodes around
fixed base stations. DSR would act to transparently fill in the
coverage gaps between base stations. Mobile nodes that would
otherwise be unable to communicate with the base station due to
factors such as distance, fading, or local interference sources
could then reach the base station through their peers.
Ultimately, however, we decided to design DSR as a layer 3 protocol
since this is the only layer at which we could realistically support
nodes with multiple interfaces of different types.
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Implementation Status
We have implemented Dynamic Source Routing (DSR) under the
FreeBSD 2.2.2 operating system running on Intel x86 platforms.
FreeBSD is based on a variety of free software, including 4.4 BSD
Lite from the University of California, Berkeley.
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Acknowledgments
The protocol described in this draft has been designed within
the CMU Monarch Project, a research project at Carnegie Mellon
University which is developing adaptive networking protocols and
protocol interfaces to allow truly seamless wireless and mobile host
networking [8, 14]. The current members of the CMU Monarch Project
include:
- Josh Broch
- Yih-Chun Hu
- Jorjeta Jetcheva
- David B. Johnson
- David A. Maltz
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Areas for Refinement
We are currently working to refine the DSR protocol in the following
ways:
- Improve the algorithms and data structures used by the Route
Cache. We currently represent the Route Cache as a directed
acyclic tree of paths branching out from a root that represents
the node owning the Route Cache. However, each source
route learned by the Route Cache effectively describes the
interconnectedness of all the hops listed on the route, and
can be treated as a type of partial information Link State
Packet as one would find in a Link State routing algorithm.
By generalizing the Route Cache to a graph of all known links
between all known nodes, it may be possible to better leverage
the information a node overhears.
- Support for better route selection. In order to select the
best source route to send a packet with, nodes need be able to
evaluate the costs/benefits of each of their cached routes to the
destination. If those routes involve forwarding through nodes
with more than one interface, some routes may be better suited to
the traffic type because the bandwidth/range/latency/error-rate
characteristics of of the interfaces used on those routes best
match the needs of the traffic type. The Route Request and Route
Reply option format must be extended to enable node to report
the properties of the interfaces on the route, as well as the
interface index used in basic DSR forwarding.
- Improved Route Discovery algorithms. We are investigating ways
to cancel a propagating Route Request if the target of the
request has already been found in another part of the network.
Similarly, we are studying various ring-search algorithms in case
a more sophisticated algorithm might perform better than the
2-step algorithm we currently use.
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References
[1] R. Braden, editor. Requirements for Internet Hosts --
Communication Layers. RFC 1122, October 1989.
[2] Scott Bradner. Key words for use in RFCs to Indicate
Requirement Levels. RFC 2119, March 1997.
[3] Scott Corson and Joseph Macker. Mobile Ad Hoc Networking
(MANET): Routing Protocol Performance Issues and Evaluation
Considerations. Internet-Draft, draft-ietf-manet-issues-00.txt,
September 1997. Work in progress.
[4] Stephen E. Deering and Robert M. Hinden. Internet
Protocol, Version 6 (IPv6) Specification. Internet-Draft,
draft-ietf-ipngwg-ipv6-spec-v2-01.txt, November 1997. Work in
progress.
[5] IPX Router Specification. Novell Part Number 107-000029-001,
Document Version 1.30, March 1996.
[6] David B. Johnson. Routing in ad hoc networks of mobile hosts.
In Proceedings of the IEEE Workshop on Mobile Computing Systems
and Applications, pages 158--163, December 1994.
[7] David B. Johnson and David A. Maltz. Dynamic source routing in
ad hoc wireless networks. In Mobile Computing, edited by Tomasz
Imielinski and Hank Korth, chapter 5, pages 153--181. Kluwer
Academic Publishers, 1996.
[8] David B. Johnson and David A. Maltz. Protocols for adaptive
wireless and mobile networking. IEEE Personal Communications,
3(1):34--42, February 1996.
[9] Charles Perkins, editor. IP Mobility Support. RFC 2002,
October 1996.
[10] Radia Perlman. Interconnections: Bridges and Routers.
Addison-Wesley, Reading, Massachusetts, 1992.
[11] David C. Plummer. An Ethernet Address Resolution Protocol: Or
Converting Network Protocol Address to 48.bit Ethernet Addresses
for Transmission on Ethernet Hardware. RFC 826, November 1982.
[12] J. Postel, editor. Internet Protocol. RFC 791, September 1981.
[13] J. Postel, editor. Transmission Control Protocol. RFC 793,
September 1981.
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[14] The CMU Monarch Project. http://www.monarch.cs.cmu.edu/.
Computer Science Department, Carnegie Mellon University.
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Chair's Address
The Working Group can be contacted via its current chairs:
M. Scott Corson
Institute for Systems Research
University of Maryland
College Park, MD 20742
USA
Phone: +1 301 405-6630
Email: corson@isr.umd.edu
Joseph Macker
Information Technology Division
Naval Research Laboratory
Washington, DC 20375
USA
Phone: +1 202 767-2001
Email: macker@itd.nrl.navy.mil
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Authors' Addresses
Questions about this document can also be directed to the authors:
Josh Broch
Carnegie Mellon University
Electrical and Computer Engineering Department
5000 Forbes Avenue
Pittsburgh, PA 15213-3891
USA
Phone: +1 412 268-3056
Email: broch@andrew.cmu.edu
David B. Johnson
Carnegie Mellon University
Computer Science Department
5000 Forbes Avenue
Pittsburgh, PA 15213-3891
USA
Phone: +1 412 268-7399
Fax: +1 412 268-5576
Email: dbj@cs.cmu.edu
David A. Maltz
Carnegie Mellon University
Computer Science Department
5000 Forbes Avenue
Pittsburgh, PA 15213-3891
USA
Phone: +1 412 268-3621
Fax: +1 412 268-5576
Email: dmaltz@cs.cmu.edu
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