IETF MANET Working Group Josh Broch
INTERNET-DRAFT David B. Johnson
David A. Maltz
Carnegie Mellon University
22 October 1999
The Dynamic Source Routing Protocol for Mobile Ad Hoc Networks
<draft-ietf-manet-dsr-03.txt>
Status of This Memo
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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 DSR's Route Discovery, completely eliminates the
need for periodic router advertisements and link status packets,
significantly 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. Changes 1
3. Assumptions 1
4. Terminology 2
4.1. General Terms . . . . . . . . . . . . . . . . . . . . . . 2
4.2. Specification Language . . . . . . . . . . . . . . . . . 4
5. Protocol Overview 5
5.1. Route Discovery and Route Maintenance . . . . . . . . . . 5
5.2. Packet Forwarding . . . . . . . . . . . . . . . . . . . . 6
5.3. Multicast Routing . . . . . . . . . . . . . . . . . . . . 7
6. Conceptual Data Structures 7
6.1. Route Cache . . . . . . . . . . . . . . . . . . . . . . . 7
6.2. Route Request Table . . . . . . . . . . . . . . . . . . . 9
6.3. Send Buffer . . . . . . . . . . . . . . . . . . . . . . . 9
6.4. Retransmission Buffer . . . . . . . . . . . . . . . . . . 9
7. Packet Formats 11
7.1. Destination Options Headers . . . . . . . . . . . . . . . 11
7.1.1. DSR Route Request Option . . . . . . . . . . . . 12
7.2. Hop-by-Hop Options Headers . . . . . . . . . . . . . . . 14
7.2.1. DSR Route Reply Option . . . . . . . . . . . . . 15
7.2.2. DSR Route Error Option . . . . . . . . . . . . . 17
7.2.3. DSR Acknowledgment Option . . . . . . . . . . . . 18
7.3. DSR Routing Header . . . . . . . . . . . . . . . . . . . 20
8. Detailed Operation 23
8.1. Originating a Data Packet . . . . . . . . . . . . . . . . 23
8.2. Originating a Packet with a DSR Routing Header . . . . . 23
8.3. Processing a Routing Header . . . . . . . . . . . . . . . 24
8.4. Route Discovery . . . . . . . . . . . . . . . . . . . . . 25
8.4.1. Originating a Route Request . . . . . . . . . . . 25
8.4.2. Processing a Route Request Option . . . . . . . . 26
8.4.3. Generating Route Replies using the Route Cache . 27
8.4.4. Originating a Route Reply . . . . . . . . . . . . 28
8.4.5. Processing a Route Reply Option . . . . . . . . . 29
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8.5. Route Maintenance . . . . . . . . . . . . . . . . . . . . 30
8.5.1. Using Network-Layer Acknowledgments . . . . . . . 30
8.5.2. Using Link Layer Acknowledgments . . . . . . . . 32
8.5.3. Originating a Route Error . . . . . . . . . . . . 32
8.5.4. Processing a Route Error Option . . . . . . . . . 33
8.5.5. Salvaging a Packet . . . . . . . . . . . . . . . 33
9. Optimizations 35
9.1. Leveraging the Route Cache . . . . . . . . . . . . . . . 35
9.1.1. Promiscuous Learning of Source Routes . . . . . . 35
9.2. Preventing Route Reply Storms . . . . . . . . . . . . . . 36
9.3. Piggybacking on Route Discoveries . . . . . . . . . . . . 37
9.4. Discovering Shorter Routes . . . . . . . . . . . . . . . 37
9.5. Rate Limiting the Route Discovery Process . . . . . . . . 38
9.6. Improved Handling of Route Errors . . . . . . . . . . . . 39
9.7. Increasing Scalability . . . . . . . . . . . . . . . . . 39
10. Path-State and Flow-State Mechanisms 40
10.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 40
10.2. Path-State and Flow-State Identifiers . . . . . . . . . . 41
10.3. Path-State Creation, Use, and Maintenance . . . . . . . . 42
10.3.1. Creating Path-State for Routing . . . . . . . . . 42
10.3.2. Monitoring Characteristics of the Path . . . . . 43
10.3.3. Candidate Metrics . . . . . . . . . . . . . . . . 45
10.4. Flow-State Creation, Use, and Maintenance . . . . . . . . 46
10.4.1. Requesting Promises along Existing Paths . . . . 46
10.4.2. Requesting Promises as Part of Route Discovery . 48
10.4.3. Providing Notifications of Changing Path Metrics 49
10.5. Expiration of State from Intermediate Nodes . . . . . . . 50
10.6. Packet Formats . . . . . . . . . . . . . . . . . . . . . 51
10.6.1. Identifier Option . . . . . . . . . . . . . . . . 51
10.6.2. Path-Metrics Option . . . . . . . . . . . . . . . 52
10.6.3. Flow Request Option . . . . . . . . . . . . . . . 54
10.6.4. Encoding Path-Metrics . . . . . . . . . . . . . . 55
11. Constants 58
12. IANA Considerations 59
13. Security Considerations 60
Location of DSR Functions in the ISO Model 61
Implementation Status 62
Acknowledgments 63
References 64
Chair's Address 66
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Authors' Addresses 67
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1. Introduction
This document describes Dynamic Source Routing (DSR) [8, 9], a
protocol developed by the Monarch Project [10, 19] at Carnegie Mellon
University for routing packets in a mobile ad hoc network [5].
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
node.
DSR offers a number of potential advantages over other routing
protocols for mobile ad hoc networks. First, DSR uses no periodic
routing messages of any kind (e.g., no router advertisements and no
link-level neighbor status messages), thereby significantly reducing
network bandwidth overhead, conserving battery power, reducing the
probability of packet collision, 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 node 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 always find a correct route even in the
presence of asymmetric links.
2. Changes
Changes from version 02 to version 03 (10/1999)
- Added description of path-state and flow-state maintenance
(Section 10). These extensions remove the need for every
data packet to carry a source route, thereby decreasing
the byte-overhead of DSR. They also provide a framework for
supporting QoS inside DSR networks.
3. Assumptions
We assume that all nodes wishing to communicate with other nodes
within the ad hoc network are willing to participate fully in the
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protocols of the network. In particular, each node participating in
the network should also be willing to forward packets for other nodes
in the network.
We refer to the minimum number of hops necessary for a packet to
reach from any node located at one extreme edge of the network to
another node 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.
Packets may be lost or corrupted in transmission on the wireless
network. A node receiving a corrupted packet can detect the error
and discard the packet.
We assume that nodes can enable 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 its availability. Use of promiscuous
mode does increase the software overhead on the CPU, but we believe
that wireless network speeds are more the inherent limiting factor
to performance in current and future systems. We also believe
that portions of the protocol are also suitable for implementation
directly within a programmable network interface unit to avoid this
overhead on the CPU.
4. Terminology
4.1. General Terms
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 7-bit quantity which uniquely identifies an interface among
a given node's interfaces. Each node can assign interface
indices to its interfaces using any scheme it wishes.
The index IF_INDEX_MA is reserved for use by Mobile IP [14]
mobility agents (home or foreign agents) to indicate that they
believe they can reach a destination via a connected internet
infrastructure. The index IF_INDEX_ROUTER is reserved for
use by routers not acting as Mobile IP mobility agents to
indicate that they believe they can reach the destination via a
connected internet infrastructure.
The distinction between the index for mobility agents and
the index for routers, allows mobility agents to advertise
their existence ``for free''. A node that processes a routing
header listing the interface index IF_INDEX_MA, can then send
a unicast Agent Solicitation to the corresponding address in
the routing header to obtain complete information about the
mobility services being provided.
link-layer address
A link-layer identifier for an interface, such as IEEE 802
addresses on Ethernet links.
packet
An IP header plus payload.
piggybacking
Including two or more conceptually different types of data in
the same packet so that all data elements move through the
network together.
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 [14]. 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 a node S to some node D is an ordered list
of home addresses and interface indexes that contains all the
information that would be needed to forward a packet through
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the ad hoc network. For each node that will transmit the
packet, the source route provides the index of the interface
over which the packet should be transmitted, and the address of
the node which is intended to receive the packet.
DSR Routing Headers as described in Section 7.3 use a more
compact encoding of the source route and do not explicitly list
address S in the Routing Header`, since it is carried as the IP
Source Address of the packet.
A source route is described as ``broken'' when the specific
path it describes through the network is not actually viable.
Route Discovery
The method in DSR by which a node S dynamically obtains a
source route to some node D that will be used by S to route
packets through the network to D. 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 link removed from use.
4.2. 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 [3].
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5. Protocol Overview
5.1. 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 link along the
route no longer works. When Route Maintenance indicates a source
route is broken, S can attempt to use any other route it happens to
know to D, or can invoke Route Discovery again to find a new route.
To perform Route Discovery, the source node S link-layer broadcasts
a Route Request packet. Here, node S is termed the initiator of the
Route Discovery, and the node to which S is attempting to discover a
source route, say D, is termed the target of the Discovery.
Each node that hears the Route Request packet forwards a copy of the
Request, if appropriate, by adding its own address to a source route
being recorded in the Request packet and then rebroadcasting the
Route Request.
The forwarding of Route 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, (2) is not already
listed in the recorded source route in this copy of the Request, and
(3) has not recently seen another Route Request packet belonging to
this same Route Discovery. A node can determine if it has recently
seen such a Route Request, since each Route Request packet contains
a unique identifier for this Route Discovery, generated by the
initiator of the Discovery. Each node maintains an LRU cache of the
unique identifier from each recently received Route Request. By not
propagating any copies of a Request after the first, the overhead of
forwarding additional copies that reach this node along different
paths is avoided.
In addition, 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 [17, 2]. Additional optimizations on the handling and
forwarding of Route Requests are also used to further reduce the
Route Discovery overhead.
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When the target of the Request (e.g., node D) receives the Route
Request, the recorded source route in the Request identifies the
sequence of hops over which this copy of the Request reached D.
Node D copies this recorded source route into a Route Reply packet
and sends this Route 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.
Further, when some intermediate node B receives a Route Request from
S for some target node D, B not equal D, B searches its own Route
Cache for a route to D. If B finds such a route, it might not have
to propagate the Route Request, but instead return 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 9.1.
As a node overhears routes being used by others, either on data
packets or on control packets used by Route Discovery or Route
Maintenance, the node MAY insert those routes into its Route Cache,
leveraging the Route Discovery operations of the other nodes in
the network. Such route information MAY be learned either by
promiscuously snooping on packets or when forwarding packets.
5.2. Packet Forwarding
To represent a source route within a packet's header, DSR uses a
Routing Header similar to the Routing Header format specified for
IPv6, adapted to the needs of DSR and to the use of 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 [6].
To forward a packet, a receiving node N simply processes the Routing
Header as specified in Section 8.3 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 this packet informing S that this link is "broken".
If node N's Route Cache contains a different route to the destination
of the original packet, then the packet is salvaged using the new
source route (Section 8.5.5). Otherwise, the packet is dropped.
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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.
5.3. Multicast Routing
At this time DSR does not support true multicasting. However, it
does support the controlled flooding of a data packet to all nodes in
the network that are within some number of hops of the originator.
While this mechanism does not support pruning of the broadcast
tree to conserve network resources, it can be used to distribute
information to nodes in the network.
When an application on a DSR node sends a packet to a multicast
address, DSR piggybacks the data from the packet inside a Route
Request packet targeted at the multicast address. The normal Route
Request distribution scheme described in Sections 5.1 and 8.4.2
will result in this packet being efficiently distributed to all
nodes in the network within the specified TTL of the originator.
The receiving nodes can then do destination address filtering on
the packet, discarding it if they do not wish to receive multicast
packets destined to this multicast address.
6. Conceptual Data Structures
In order to participate in the Dynamic Source Routing Protocol, a
node needs four conceptual data structures: a Route Cache, a Route
Request Table, 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.
6.1. Route Cache
All routing information needed by a node participating in an ad hoc
network using DSR is stored in a Route Cache. Each node in the
network maintains its own Route Cache. The node adds information
to the 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 address, and supports the following
operations:
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void Insert(Route RT)
Inserts information extracted from source route RT into the
Route Cache.
Route Get(Node DEST)
Returns a source route from this node to DEST (if one is
known).
void Delete(Node FROM, Interface INDEX, Node TO)
Removes from the route cache any routes which assume that a
packet transmitted by node FROM over its interface with the
given INDEX will be received by node TO.
Each implementation MAY choose the cache replacement and cache search
strategies for its Route Cache that are 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 there are multiple cached routes to a destination, the Route Get()
operation SHOULD prefer routes that do not traverse a hop with an
interface index of IF_INDEX_MA or IF_INDEX_ROUTER. This will prefer
routes that lead directly to the target node over routes that attempt
to reach the target via any internet infrastructure connected to the
ad hoc network.
If a node S is using a source route to some destination D that
includes intermediate node N, S SHOULD shorten the route to
destination D when it learns of a shorter route to node N than the
one that is listed as the prefix of its current route to D.
A node S using a source route to destination D through intermediate
node N, MAY shorten the source route if it learns of a shorter path
from node N 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 on other packets. In particular, a node SHOULD prefer
routes that it is presently using over those that it is not.
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6.2. Route Request Table
The Route Request Table is a collection of records about Route
Request packets that were recently originated or forwarded by this
node. The table is indexed by the home address of the target of the
route discovery. A record maintained on node S for node D contains
the following:
- The time that S last originated a Route Discovery for D.
- The remaining amount of time that S must wait before the next
attempt at a Route Discovery for D.
- The Time-to-live (TTL) field in the IP header of last Route
Request originated by S for D.
- A FIFO cache of the last ID_FIFO_SIZE Identification values from
Route Request packets targeted at node D that were forwarded by
this node.
Nodes SHOULD use an LRU policy to manage the entries of in their
Route Request Table.
ID_FIFO_SIZE MUST NOT be set to an unlimited value, since, in the
worst case, when a node crashes and reboots the first ID_FIFO_SIZE
Route Request packets it sends may appear to be duplicates to the
other nodes in the network.
6.3. Send Buffer
The Send Buffer of some node is a queue of packets that cannot be
transmitted by that node because it does not yet have a source
route to each respective packet's destination. Each packet in the
Send Buffer is stamped with the time that it is placed into the
Buffer, and SHOULD be removed from the Send Buffer and discarded
SEND_BUFFER_TIMEOUT seconds after initially being placed in the
Buffer. If necessary, a FIFO strategy SHOULD be used to evict
packets before they timeout to prevent the buffer from overflowing.
Subject to the rate limiting defined in Section 8.4, a Route
Discovery SHOULD be initiated as often as possible for the
destination address of any packets residing in the Send Buffer.
6.4. Retransmission Buffer
The Retransmission Buffer of a node is a queue of packets sent by
this node that are awaiting the receipt of an acknowledgment from the
next hop in the source route (Section 7.3).
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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
DSR_MAXRXTSHIFT. In the later case, the removal of the packet from
the Retransmission Buffer SHOULD result in a Route Error being
returned to the initial source of the packet (Section 8.5).
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7. Packet Formats
Dynamic Source Routing makes use of four options carrying control
information that can be piggybacked in any existing IP packet.
The mechanism used for these options is based on the design of the
Hop-by-Hop and Destination Options mechanisms in IPv6 [6]. The
ability to generate and process such options must be added to an IPv4
protocol stack. Specifically, the Protocol field in the IP header
is used to indicate that a Hop-by-Hop Options or Destination Options
extension 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 each extension header will then indicate the
type of header that follows it in a packet.
7.1. Destination Options Headers
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 (or
Protocol) value of 60 in the immediately preceding header, and has
the following format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
. .
. Options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header
8-bit selector. Identifies the type of header immediately
following the Destination Options header. Uses the same values
as the IPv4 Protocol field [20].
Hdr Ext Len
8-bit unsigned integer. Length of the Destination Options
header in 4-octet units, not including the first 8 octets.
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Options
Variable-length field, of length such that the complete
Destination Options header is an integer multiple of 4 octets
long. Contains one or more TLV-encoded options.
The following destination option is used by the Dynamic Source
Routing protocol:
- DSR Route Request option (Section 7.1.1)
This destination option MUST NOT appear multiple times within a
single Destination Options header.
7.1.1. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| IN Index[1] |C| IN Index[2] |C| IN Index[3] |C| IN Index[4] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|OUT Index[1] |C|OUT Index[2] |C|OUT Index[3] |C|OUT Index[4] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[2] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[3] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[4] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| IN Index[5] |C| IN Index[6] |C| IN Index[7] |C| IN Index[8]|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|OUT Index[5] |C|OUT Index[6] |C| OUT Index[7] |C|OUT Index[8]|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[5] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP fields:
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Source Address
MUST be the home address of the node originating this packet.
Intermediate nodes that repropagate the request do not change
this field.
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 and the Option Data may change en-route (the top
three bits are 011).
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.
Change Interface (C) bit[1..n]
A flag associated with each interface index that indicates
whether or not the corresponding node repropagated the Request
over a different physical interface type than over which it
received the Request.
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IN Index[1..n]
IN Index[i] is the index of the interface over which the node
indicated by Address[i] received the Route Request option.
These are used to record a reverse route from the target of
the request to the originator, over which a Route Reply MAY be
sent.
OUT Index[1..n]
OUT Index[i] is the interface index that the node indicated by
Address[i-1] used when rebroadcasting the Route Request option.
Address[1..n]
Address[i] is the home address of the ith hop recorded in the
Route Request option.
7.2. Hop-by-Hop Options Headers
The Hop-by-Hop Options header is used to carry optional information
that must be examined by every node along a packet's delivery path.
The Hop-by-Hop Options header is identified by a Next Header (or
Protocol) value of ??? in the IP header, and has the following
format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
. .
. Options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header
8-bit selector. Identifies the type of header immediately
following the Hop-by-Hop Options header. Uses the same values
as the IPv4 Protocol field [20].
Hdr Ext Len
8-bit unsigned integer. Length of the Hop-by-Hop Options
header in 4-octet units, not including the first 8 octets.
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Options
Variable-length field, of length such that the complete
Hop-by-Hop Options header is an integer multiple of 4 octets
long. Contains one or more TLV-encoded options.
The following hop-by-hop options are used by the Dynamic Source
Routing protocol:
- DSR Route Reply option (Section 7.2.1)
- DSR Route Error option (Section 7.2.2)
- DSR Acknowledgment option (Section 7.2.3)
All of these destination options MAY appear one or more times within
a single Hop-by-Hop Options header.
7.2.1. DSR Route Reply Option
The DSR Route Reply hop-by-hop option is encoded in type-length-value
(TLV) format as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Target Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|OUT Index[1] |C|OUT Index[2] |C|OUT Index[3] |C|OUT Index[4] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[2] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[3] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[4] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|OUT Index[5] |C|OUT Index[6] |C|OUT Index[7] |C|OUT Index[8] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[5] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Option Type
???. A node that does not understand this option should ignore
this option and continue processing the packet, and the Option
Data does not change en-route (the top three bits are 000).
Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields.
Reserved
Sent as 0; ignored on reception.
Target Address
The home address of the node to which the Route Reply must be
delivered.
Change Interface (C) bit[1..n]
If the C bit associated with a node N is set, it implies N will
be forwarding the packet out a different interface than the one
over which it was received (i.e., the node sending the packet
to N should not expect a passive acknowledgment).
OUT Index[1..n]
OUT Index[i] is the interface index of the ith hop listed in
the Route Reply option. It denotes the interface that should
be used by Address[i-1] to reach Address[i] when using the
specified source route.
Address[1..n]
Address[i] is the home address of the ith hop listed in the
Route Reply option.
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7.2.2. DSR Route Error Option
The DSR Route Error hop-by-hop option is encoded in type-length-value
(TLV) format as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length | Index | Error Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unreachable Node Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
???. A node that does not understand this option should ignore
the option and continue processing the packet, and the Option
Data does not change en-route (the top three bits are 000).
Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields.
Index
The interface index of the network interface over which the
node designated by Error Source Address tried to forward a
packet to the node designated by Unreachable Node Address.
Error Type
The type of error encountered. Values between 0 and 127
inclusive indicate a hard failure of connectivity between the
Error Source Address and the Unreachable Node Address. Values
between 128 and 255 inclusive indicate a soft failure.
NODE_UNREACHABLE 1
PATH_STATE_NOT_FOUND 129
Error Source Address
The home address of the node originating the Route Error (e.g.,
the node that attempted to forward a packet and discovered the
link failure).
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Error Destination Address
The home address of the node to which the Route Error must be
delivered (e.g, the node that generated the routing information
claiming that the hop Error Source Address to Unreachable Node
Address was a valid hop).
Unreachable Node Address
The home address of the node that was found to be unreachable
(the next hop neighbor to which the node at ``Error Source
Address'' was attempting to transmit the packet).
7.2.3. DSR Acknowledgment Option
The DSR Acknowledgment hop-by-hop option is encoded in
type-length-value (TLV) format as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACK Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACK Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
???. A node that does not understand this option should ignore
the option and continue processing the packet, and the Option
Data does not change en-route (the top three bits are 000).
Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields.
Identification
A 32-bit value that when taken in conjunction with Data Source
Address, uniquely identifies the packet being acknowledged.
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The Identification value is computed as ((ip_id << 16) | ip_off)
where ip_id is the value of the 16-bit Identification field in
the IP header of the packet being acknowledged, and ip_off is
the value of the 13-bit Fragment Offset field in the IP header
of the packet being acknowledged.
When constructing the Identification, ip_id and ip_off MUST be
in host byte-order. The entire Identification value MUST then
be converted to network byte-order before being placed in the
Acknowledgment option.
ACK Source Address
The home address of the node originating the Acknowledgment.
ACK Destination Address
The home address of the node to which the Acknowledgment must
be delivered.
Data Source Address
The IP Source Address of the packet being acknowledged.
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7.3. DSR Routing Header
As specified for IPv6 [6], 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|S| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|OUT Index[1] |C|OUT Index[2] |C|OUT Index[3] |C|OUT Index[4] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[2] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[3] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[4] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|OUT Index[5] |C|OUT Index[6] |C|OUT Index[7] |C|OUT Index[8] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[5] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Routing Header Fields:
Next Header
8-bit selector. Identifies the type of header immediately
following the Routing header.
Hdr Ext Len
8-bit unsigned integer. Length of the Routing header in
4-octet units, not including the first 8 octets.
Routing Type
???
Segments Left
Number of route segments remaining, i.e., number of explicitly
listed intermediate nodes still to be visited before reaching
the final destination.
Type Specific Fields:
Acknowledgment Request (R)
The Acknowledgment Request (R) bit is set to request an
explicit acknowledgment from the next hop. After processing
the Routing Header, The IP Destination Address lists the
address of the next hop.
Salvaged Packet (S)
The Salvaged Packet (S) bit indicates that this packet has been
salvaged by an intermediate node, and thus that this Routing
Header was generated by Address[1] and not the IP Source
Address (Section 8.5.5).
Reserved
Sent as 0; ignored on reception.
Change Interface (C) bit[1..n]
If the C bit associated with a node N is set, it implies N will
be forwarding the packet out a different interface than the one
over which it was received (i.e., the node sending the packet
to N should not expect a passive acknowledgment and MAY wish to
set the R bit).
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OUT Index[1..n]
Index[i] is the interface index that the node indicated
by Address[i-1] must use when transmitting the packet to
Address[i]. Index[1] indicates which interface the node
indicated by the IP Source Address uses to transmit the packet.
Address[1..n]
Address[i] is the home address of the ith hop in the Routing
header.
Note that Address[1] is the first intermediate hop along the route.
The address of the originating node is the IP Source Address. The
only exception to this rule is for packets that are salvaged, as
described in Section 8.5.5. A packet that has been salvaged has an
alternate route placed on it by an intermediate node in the network,
and in this case, the address of the originating node (the salvaging
node) is Address[1]. Salvaged packets are indicated by setting the S
bit in the DSR Routing header.
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8. Detailed Operation
8.1. Originating a Data Packet
When node A originates a packet, the following steps MUST be taken
before transmitting the packet:
1. If the destination address is a multicast address, piggyback the
data packet on a Route Request targeting the multicast address.
The following fields MUST be initialized as specified:
IP.Source_Address = Home address of node A
IP.Destination_Address = 255.255.255.255
Request.Target_Address = Multicast destination address
DONE.
2. Otherwise, call Route_Cache.Get() to determine if there is a
cached source route to the destination.
3. If the cached route indicates that the destination is directly
reachable over one hop, no Routing Header should be added to the
packet. Initialize the following fields:
IP.Source_Address = Home address of node A
IP.Destination_Address = Home address of the Destination
DONE.
4. Otherwise, if the cached route indicates that multiple hops are
required to reach the destination, insert a Routing Header into
the packet as described in Section 8.2. DONE.
5. Otherwise, if no cached route to the destination is found, insert
the packet into the Send Buffer and initiate Route Discovery as
described in Section 8.4.
8.2. Originating a Packet with a DSR Routing Header
When a node originates a packet with a Routing Header, the address
of the first hop in the source route MUST be listed as the IP
Destination Address as well as Address[1] in the Routing Header.
The final destination of the packet is listed as the last hop
in the Routing Header (Address[n]). At each intermediate hop i,
Address[i] is copied into the IP Destination Address and the packet
is retransmitted.
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For example, suppose node A originates a packet destined for node D
that should pass through intermediate hops B and C. The packet MUST
be initialized as follows:
IP.Source_Address = Home address of node A
IP.Destination_Address = Home address of node B
RT.Segments_Left = 2
RT.Out_Index[1] = Interface index used by A to reach B
RT.Out_Index[2] = Interface index used by B to reach C
RT.Out_Index[3] = Interface index used by C to reach D
RT.Address[1] = Home address of node B
RT.Address[2] = Home address of node C
RT.Address[3] = Home address of node D
8.3. Processing a Routing Header
Excluding the exceptions listed here, a DSR Routing Header is
processed using the same rules as outlined for Type 0 Routing Headers
in IPv6 [6]. The Routing Header is only processed by the node whose
address appears as the IP destination of the packet. The following
additional rules apply to processing the type specific data of a DSR
Source Route:
Let
SegLft = the value of Segments Left when the packet was received
NumAddrs = the total number of addresses in the Routing Header
1. The address of the next hop, Address[NumAddrs - SegLft + 1],
is copied into the IP.Destination_Address of the packet. The
existing IP.Destination_Address is NOT copied back into the
Address list of the Routing Header.
2. The interface used to transmit the packet to its next hop from
this node MUST be the interface denoted by Index[NumAddrs -
SegLft + 1].
3. If the Acknowledgment Request (R) bit is set, the node MUST
transmit a packet containing the DSR Acknowledgment option to
the previous hop, Address[NumAddrs - SegLft - 1], performing
Route Discovery if necessary. (Address[0] is taken as the
IP.Source_Address)
4. Perform Route Maintenance by verifying that the packet was
received by the next hop as described in Section 8.5.
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8.4. Route Discovery
Route Discovery is the on-demand 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 is known as the "target" of the Route
Discovery. A Route Discovery for a destination SHOULD NOT be
initiated unless the initiating node has a packet in the Send Buffer
requiring delivery to that destination. A Route Discovery for a
given target node MUST NOT be initiated unless permitted by the
rate-limiting information contained in the Route Request Table.
After each Route Discovery attempt, the interval between successive
Route Discoveries for this target must be doubled, up to a maximum of
MAX_REQUEST_PERIOD.
Route Discoveries for a multicast address SHOULD NOT be rate limited,
and SHOULD always be permitted.
8.4.1. Originating a Route Request
The basic Route Discovery algorithm for a unicast destination is as
follows:
1. Originate a Route Request packet with the IP header Time-to-Live
field initialized to 1. This type of Route Request is called a
non-propagating Route Request and allows the originator of the
Request to inexpensively query the route caches of each of its
neighbors for a route to the destination.
2. If a Route Reply is received in response to the non-propagating
Request, use the returned source route to transmit all packets
for the destination that are in the Send Buffer. DONE.
3. Otherwise, if no Route Reply is received within
RING0_REQUEST_TIMEOUT seconds, transmit a Route Request
with the IP header Time-to-Live field initialized to
MAX_ROUTE_LEN. This type of Route Request is called a propagating
Route Request. Update the information in the Route Request
Table, to double the amount of time before any subsequent Route
Discovery attempt to this target.
4. If no Route Reply is received within the time interval indicated
by the Route Request Table, GOTO step 1.
The Route Request option SHOULD be initialized as follows:
IP.Source_Address = This node's home address
IP.Destination_Address = 255.255.255.255
Request.Target = Home address of intended destination
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Request.OUT_Index[1] = Index of interface used to transmit the Request
The behavior of a node processing a packet containing both a Routing
Header and a Route Request Destination option is unspecified.
Packets SHOULD NOT contain both a Routing Header and a Route Request
Destination option. [This is not exactly true: A Route Request
option appearing in the second Destination Options header that IPv6
allows after the Routing Header would probably do-what-you-mean,
though we have not triple-checked it yet. Namely, it would allow the
originator of a route discovery to unicast the request to some other
node, where it would be released and begin the flood fill. We call
this a Route Request Blossom since the unicast portion of the path
looks like a stem on the blossoming flood-fill of the request.]
Packets containing a Route Request Destination option SHOULD NOT be
retransmitted, SHOULD NOT request an explicit DSR Acknowledgment by
setting the R bit, SHOULD NOT expect a passive acknowledgment, and
SHOULD NOT be placed in the Retransmission Buffer. The repeated
transmission of packets containing a Route Request Destination option
is controlled solely by the logic described in this section.
8.4.2. Processing a Route Request Option
When a node A receives a packet containing a Route Request option,
the Route Request option is processed as follows:
1. If Request.Target_Address matches the home address of this node,
then the Route Request option 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 8.4.4.
(b) Continue processing the packet in accordance with the Next
Header value contained in the Destination Option extension
header. DONE.
2. Otherwise, if the combination (IP.Source_Address,
Request.Identification) is found in the Route Request
Table, then discard the packet, since this is a copy of a
recently seen Route Request. DONE.
3. Otherwise, if Request.Target_Address is a multicast address then:
(a) If node A is a member of the multicast group indicated by
Request.Target_Address, then create a copy of the packet,
setting IP.Destination_Address = REQUEST.Target_Address, and
continue processing the copy of the packet in accordance with
the Next Header field of the Destination option.
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(b) If IP.TTL is non-zero, decrement IP.TTL, and retransmit the
packet. DONE.
(c) Otherwise, discard the packet. DONE.
4. Otherwise, if the home address of node A is already listed in
the Route Request (IP.Source_Address or Request.Address[]), then
discard the packet. DONE.
5. Let
m = number of addresses currently in the Route Request option
n = m + 1
6. Otherwise, append the home address of node A to the Route Request
option (Request.Address[n]).
7. Set Request.IN_Index[n] = index of interface packet was received
on.
8. If a source route to Request.Target_Address is found in our Route
Cache and the rules of Section 8.4.3 permit it, return a Cached
Route Reply as described in Section 8.4.3. DONE.
9. Otherwise, for each interface on which the node is configured to
participate in a DSR ad hoc network:
(a) Make a copy of the packet containing the Route Request.
(b) Set Request.OUT_Index[n+1] = index of the interface.
(c) If the outgoing interface is different from the incoming
interface, then set the C bit on both Request.OUT_Index[n+1]
and Request.IN_Index[n]
(d) Link-layer re-broadcast the packet containing the Route
Request on the interface jittered by T milliseconds, where
T is a uniformly distributed, random number between 0 and
BROADCAST_JITTER. DONE.
8.4.3. Generating Route Replies 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 based on the logic of Section 8.4.2.
If the node has a Route Cache entry for the target of the Request,
it SHOULD append this cached route to the accumulated route record
in the packet and return this route in a Route Reply packet to
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the initiator without propagating (re-broadcasting) the Route
Request. Thus, for example, if node F in the example network shown
in Figure 8.4.3 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 node A, node 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 contains no loops.
2. The node issuing the Route Reply is listed in the route that it
specifies in its Reply. 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. Additionally, this
requirement means that a Route Error traversing the route will
pass through the node that issued the Reply based on stale cache
data, which is critical for ensuring stale data is removed from
caches in a timely manner. Without this requirement, the next
Route Discovery initiated by the original requester might also be
contaminated by a Route Reply from this node containing the same
stale route.
8.4.4. Originating a Route Reply
Let REQPacket denote a packet received by node A that
contains a Route Request option which lists node A as the
REQPacket.Request.Target_Address. Let REPPacket be a packet
transmitted by node A that contains a corresponding Route Reply. The
Route Reply option transmitted in response to a Route Request MUST be
initialized as follows:
B->C->D
+---+ +---+ +---+ +---+
| A |---->| B |---->| C |---->| D |
+---+ +---+ +---+ +---+
+---+
| F | +---+
+---+ | E |
+---+
Figure 1: An example network where A knows a
route to D via B and C.
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1. If REQPacket.Request.Address[] does not contain any hops, then
node A is only a single hop from the originator of the Route
Request. Build a Route Reply packet as follows:
REPPacket.IP.Source_Address = REQPacket.Request.Target_Address
REPPacket.Reply.Target = REQPacket.IP.Source_Address
REPPacket.Reply.OUT_Index[1] = REQPacket.Request.OUT_index[1]
REPPacket.Reply.OUT_C_bit[1] = REQPacket.Request.OUT_C_bit[1]
REPPacket.Reply.Address[1] = The home address of node A
GOTO step 3.
2. Otherwise, build a Route Reply packet as follows:
REPPacket.IP.Source_Address = The home address of node A
REPPacket.Reply.Target = REQPacket.IP.Source_Address
REPPacket.Reply.OUT_Index[1..n]= REQPacket.Request.OUT_index[1..n]
REPPacket.Reply.OUT_C_bit[1..n]= REQPacket.Request.OUT_C_bit[1..n]
REPPacket.Reply.Address[1..n] = REQPacket.Request.Address[1..n]
3. Send 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
hop-by-hop option MUST be piggybacked on the packet that contains the
Route Request. This prevents a loop wherein the target of the new
Route Request (which was itself the originator of the original Route
Request) must do another Route Request in order to return its Route
Reply.
If sending the Route Reply to the originator of the Route Request
does not require performing Route Discovery, a node SHOULD send a
unicast Route Reply in response to every Route Request targeted at
it.
8.4.5. Processing a Route Reply Option
Upon receipt of a Route Reply, a node should extract the source route
(Target_Address, OUT_Index[1]:Address[1], .. OUT_Index[n]:Address[n]
) and insert this route into its Route Cache. All the packets in the
Send Buffer SHOULD be checked to see whether the information in the
Reply allows them to be sent immediately.
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8.5. Route Maintenance
Route Maintenance requires that whenever a node transmits a data
packet, a Route Reply, or a Route Error, it must verify that the next
hop (indicated by the Destination IP Address) correctly receives the
packet.
If the sender cannot verify that the next hop received the packet, it
MUST decide that its link to the next hop is broken and MUST send a
Route Error to the node responsible for generating the Routing Header
that contains the broken link (Section 8.5.3).
The following ways may be used to verify that the next hop correctly
received a packet:
- The receipt of a passive acknowledgment (Section 8.5.1).
- The receipt of an explicitly requested acknowledgment
(Section 8.5.1).
- By the presence of positive feedback from the link layer
indicating that the packet was acknowledged by the next hop
(Section 8.5.2).
- By the absence of explicit failure notification from the link
layer that provides reliable hop-by-hop delivery such as MACAW or
802.11 (Section 8.5.2).
Nodes MUST NOT perform Route Maintenance for packets containing a
Route Request option or packets containing only an Acknowledgment
option. Sending Acknowledgments for packets containing only
an Acknowledgment option would create an infinite loop whereby
acknowledgments would be sent for acknowledgments. Acknowledgments
should be always sent for packets containing a Routing Header with
the R bit set (e.g., packets which contain only an Acknowledgment
and a Routing Header for which the last forwarding hop requires an
explicit acknowledgment of receipt by the final destination).
8.5.1. Using Network-Layer Acknowledgments
For link layers that do not provide explicit failure notification,
the following steps SHOULD be used by a node A to perform Route
Maintenance.
When receiving a packet:
- If the packet contains a Routing Header with the R bit set, send
an explicit acknowledgment as described in Section 8.3.
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- If the packet does not contain a Routing Header, the node MUST
transmit a packet containing the DSR Acknowledgment option
to the previous hop as indicated by the IP.Source_Address.
Since the receiving node is the final destination, there
will be no opportunity for the originator to obtain a
passive acknowledgment, and the receiving node must infer the
originator's request for an explicit acknowledgment.
When sending a packet:
1. Before sending a packet, insert a copy of the packet into the
Retransmission Buffer and update the information maintained about
this packet in the Retransmission Buffer.
2. If after processing the Routing Header, RH.Segments_Left is equal
to 0, then node A MUST set the Acknowledgment Request (R) bit in
the Routing Header before transmitting the packet over its final
hop.
3. If after processing the Routing Header and copying
RH.Address[n] to IP.Destination_Address, node A determines that
RH.OUT_C_bit[n+1] is set, then node A MUST set the Acknowledgment
Request (R) bit in the Routing Header before transmitting the
packet (since the C bit was set during Route Discovery by the
node now listed as the IP.Destination_Address to indicate that
it will propagate the packet out a different interface, and that
node A will not receive a passive acknowledgment).
4. Set the retransmission timer for the packet in the Retransmission
Buffer.
5. Transmit the packet.
6. If a passive or explicit acknowledgment is received before the
retransmission timer expires, then remove the packet from the
Retransmission Buffer and disable the retransmission timer.
DONE.
7. Otherwise, when the Retransmission Timer expires, remove the
packet from the Retransmission Buffer.
8. If DSR_MAXRXTSHIFT transmissions have been done, then attempt
to salvage the packet (Section 8.5.5). Also, generate a Route
Error. DONE.
9. GOTO step 1.
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8.5.2. Using Link Layer Acknowledgments
If explicit failure notifications are provided by the link layer,
then all packets are assumed to be correctly received by the next hop
and a Route Error is sent only when a explicit failure notification
is made from the link layer.
Nodes receiving a packet without a Routing Header do not need to send
an explicit Acknowledgment to the packet's originator, since the
link layer will notify the originator if the packet was not received
properly.
8.5.3. Originating a Route Error
If the next hop of a packet is found to be unreachable as described
in Section 8.5, a Route Error packet (Section 7.2.2) MUST be returned
to the node whose cache generated the information used to route the
packet.
When a node A generates a Route Error for packet P, it MUST
initialize the fields in the Route Error as follows:
Error.Source_Address = Home address of node A
Error.Unreachable_Address = Home address of the unreachable node
- If the packet contains a DSR Routing Header and the S bit is NOT
set, the packet has been forwarded without the need for salvaging
up to this point.
Error.Destination_Address = P.IP.Source_Address
- Otherwise, if the packet contains a DSR Routing Header and the S
bit IS set, the packet has been salvaged by an intermediate node,
and thus this Routing Header was placed there by the salvaging
node.
Error.Destination_Address = P.RoutingHeader.Address[1]
- Otherwise, if the packet does not contain a DSR Routing Header,
the packet must have been originated by this node A.
Error.Destination_Address = Home address of node A
Send the packet containing the Route Error to Error.Destination_Address,
performing Route Discovery if necessary.
As an optimization, Route Errors that are discovered by the
packet's originator (such that Error.Source_Address is equal to
Error.Destination_Address) SHOULD be processed internally. Such
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processing should invoke all the steps that would be taken if a Route
Error option was created, transmitted, received, and processed,
but an actual packet containing a Route Error option SHOULD NOT be
transmitted.
8.5.4. 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
(Error.Source_Address, Error.Index to Error.Unreachable_Address).
This includes all Route Errors overheard, and those processed
internally as described in Section 8.5.3.
When the node identified by Error.Destination_Address receives
the Route Error, it SHOULD verify that the source route
responsible for delivering the Route Error includes the same
hops as the working prefix of the original packet's source route
(Error.Destination_Address to Error.Source_Address). If any
hop listed in the working prefix is not included in the Route
Error's source route, then the originator SHOULD forward the Route
Error back along the working prefix (Error.Destination_Address to
Error.Source_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 is Error.Destination_Address and the packet contains
additional Route Error option(s) later on the inside of the Hop
by Hop options header, we call the additional Route Errors nested
Route Errors. The node MUST deliver the first nested Route Error
to Nested_Error.Destination_Address, performing Route Discovery if
needed. It does this by removing the Route Error option listing
itself as the Error.Destination_Address, finding the first nested
Route Error option, and originating the remaining packet to
Nested_Error.Destination_Address. This mechanism allows for the
proper handling of Route Errors that are discovered while delivering
a Route Error.
8.5.5. Salvaging a Packet
When node A attempts to salvage a packet originated at node S and
destined for node D, it MUST perform the following steps:
1. Generate and send a Route Error to S as explained in
Section 8.5.3.
2. Call Route_Cache.Get() to determine if it has a cached source
route to the packet's ultimate destination D (which is the last
Address listed in the Routing Header).
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3. If node A does not have a cached route for node D, it MUST
discard the packet. DONE.
4. Otherwise, let Salvage_Address[1] through Salvage_Address[m] be
the sequence of hops returned from the Route Cache. Initialize
the following fields in the packet's header:
RT.Segments_Left = m - 2;
RT.S = 1
RT.Address[1] = Home address of Node A
RT.Address[2] = Salvage.Address[1]
...
RT.Address[n] = Salvage.Address[m]
The IP Source Address of the packet MUST remain unchanged. When the
Routing Header in the outgoing packet is processed, RT.Address[2],
will be copied to the IP Destination Address field.
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9. Optimizations
A number of optimizations can be added to the basic operation of
Route Discovery and Route Maintenance as described in Sections 8.4
and 8.5 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.
9.1. 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.
9.1.1. 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 any intermediate node B forwards packets from A to D, B SHOULD
add the source route information from that packet's Routing Header
to its own Route Cache. If a node forwards a Route Reply packet, it
SHOULD also add the source route information from the route record
being returned in the Route Reply, to its own Route Cache.
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In addition, since all wireless network transmissions at the physical
layer are inherently broadcast, it may be possible for a node to
configure its network interface into promiscuous receive mode, such
that the node is able to receive all packets without link layer
address filtering. In this case, the node MAY add to its Route Cache
the route information from any packet it can overhear.
9.2. Preventing Route Reply Storms
The ability for nodes to reply to a Route Request not targeted at
them by 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 9.1, if both node A and node 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 Route 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 Route 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.
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
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timer 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 equally good or better
route.
9.3. Piggybacking on Route Discoveries
As described in Section 5.1, 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, buffering the
original packet until the Route Reply is returned. 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. Small data
packets such as the initial SYN packet opening a TCP connection [18]
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 9.1), 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 MUST 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.
9.4. 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 all traffic from the source to the destination as long as
it continues to work, 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.
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Furthermore, when a data packet is received as the result of
operating in promiscuous receive mode, the node checks if the Routing
Header packet contains its address in the unprocessed portion of the
source route (Address[NumAddrs - SegLft] to Address[NumAddrs]). If
so, the node knows that packet could bypass the unprocessed hops
preceding it in the source route. The node then sends what is called
a gratuitous Route Reply message to the packet's source, giving it
the shorter route without these hops.
The following algorithm describes how a node A should process packets
with an IP.Destination_Address not addressed to A or the IP broadcast
address or a multicast address that are received as a result of A
being in promiscuous receive mode:
1. If the packet is not a data packet containing a Routing Header,
drop the packet. DONE.
2. 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.
3. Otherwise, the node B that just transmitted the packet (indicated
by Address[NumAddrs - SegLft - 1]) can communicate directly with
this node A. Create a Route Reply. The Route Reply MUST list
the entire source route contained in the received packet with the
exception of the intermediate nodes between node B and node A.
4. Send this gratuitous Route Reply to the node listed as the
IP.Source_Address of the received packet. If Route Discovery
is required it MAY be initiated, or the gratuitous Route Reply
packet MAY be dropped.
9.5. 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
back-off 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
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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 [2].
9.6. 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 effectively
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.
9.7. Increasing Scalability
We recently designed and began experimenting with ways to integrate
ad hoc networks with the Internet and with Mobile IP [14]. In
addition to this, we are also exploring ways to increase the
scalability of ad hoc networks by taking advantage of their
cooperative nature and the fact that some hierarchy can be imposed
on an ad hoc network, just be assigning addresses to the nodes in a
reasonable way. These ideas are described in a workshop paper [4].
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10. Path-State and Flow-State Mechanisms
This section describes the current design of our framework for
supporting better-than-best-effort Quality of Service in DSR
networks. The framework dovetails into DSR's existing mechanisms,
and, like DSR itself, is completely on-demand in nature --- no
packets are sent unless there is user data to transfer. The
framework is based on the introduction of two kinds of soft-state,
called path-state and flow-state, at the intermediate nodes along the
path between senders and destinations.
Taken together, the path-state and flow-state extensions extend the
Quality of Service provided by DSR networks in the following ways:
- They eliminate the need for all data packets to carry a source
route, increasing the efficiency of the protocol in general.
- They provide accurate measurements of the state of the network to
higher layers protocols for use in adaptation.
- They enable senders to explicitly manage the consumption of
resources across the network.
- They enable the network to provide better-than-best-effort
service via admission control and per-flow resource management.
10.1. Overview
Path-state allows intermediate nodes to forward packets according to
a predetermined source route, even when most packets do not include
the full source route. Conceptually, the originator of each data
packet initially includes both a source route and a unique path
identifier in each packet it sends. As intermediate nodes forward
the packet, they cache the source route from the packet, indexed by
the path identifier. The source can then send subsequent packets
carrying only the path identifier, since intermediate nodes will be
able to forward the packet based on the source route for the path
that they have cached.
While path-state allows the elimination of the source route from each
packet, thereby reducing the overhead of the DSR protocol, it also
provides a way for sources to monitor the state of each path through
the network. When a source wishes to know the characteristics of
a path through the network, it piggybacks a path-metrics header
onto any data or control packet traversing the path. As the
packet propagates through the network, each intermediate node
updates the set of path-metrics carried by the packet to reflect
the local network conditions seen at the node. These metrics are
reflected back to the sender by the destination, along with the path
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identifier, and enable the sender to track the value of these metrics
for each of the source routes it is currently using.
We are currently experimenting with metrics that are easy for nodes
to measure, that require constant size to represent regardless of
source route length, and that would enable the sender's network
layer to provide useful feedback to higher layers on the state of
the network. For example, by including ``available bandwidth''
or ``battery level'' as a metric, senders can load balance
the consumption of resources across the network. We have also
considered the possibility of replacing the TCP congestion control
algorithm with a ``leaky-bucket'' system controlled by the reflected
path-metrics --- our measured performance results show this could
dramatically improve TCP throughput in environments where many
packets are lost due to packet corruption. The feedback could also
be used as inputs to other researcher's systems for improving the
transport layer, such as Liu and Singh's ATCP [11], or for adaptation
at higher layers, as in Odyssey [13].
Flow-state allows a source to differentiate its traffic into
flows, and then request better-than-best-effort handling for these
flows. With the additional information provided by the flow-state,
the network can provide admission control and promise a specific
Quality of Service (QoS) to each flow. Since the ad hoc network may
frequently change topology, the flow-state mechanisms are directly
integrated into the routing protocol to minimize their reaction time
and provide notifications to a flow when the network must break its
promise for a specific level of QoS.
10.2. Path-State and Flow-State Identifiers
The metadata that intermediate nodes in the network must process
is divided into path-state and flow-state, where path-state is
the fundamental unit of routing information and flow-state is the
fundamental unit of Quality of Service information.
Path-state is associated with a particular set of hops through the
network from some source S to a destination D (i.e., a particular
source route in the network). It consists of the information needed
to route packets along the path, and information about the carrying
capacity of the path, such as the unused bandwidth along the path or
the minimum latency of the path.
Flow-state is specific to a particular class of packets flowing
between S and D that is routed over a given path. Flow-state is
used to record Quality of Service promises that have been made for a
particular flow, and allows packets from S to D that take the same
path through the network to be treated differently by intermediate
nodes. For example, all the TCP connections between S and D that
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take the same path will share the same path-state, but may have
independent flow-state. At any point in time, S may use multiple
paths for its traffic to D and each of these paths may be comprised
of many flows. However, a single network layer flow may not be
multiplexed over different paths.
To represent paths and flows inside the network, we use a scheme
inspired by the Virtual Path Index and Virtual Circuit Index of
ATM networks [23, p. 451]. Paths are identified by the logical
concatenation of the source node address and a 16-bit path identifier
where the low 5 bits are 0. Flows are identified by a path
identifier where the low 5 bits are used to distinguish between the
different flows that use the same path.
This scheme has two main advantages. First, each source node can
independently generate globally unique path- and flow-identifiers.
Second, the hierarchical relation of flow-identifiers to
path-identifiers means that many flows from the same source node can
share the same path-state, which reduces the overhead of maintaining
the routing information. The drawback is that if a flow must be
rerouted, its flow identifier will change. However, when a flow is
rerouted the QoS metadata must be renegotiated anyway, so changing
flow identifiers will not create needless additional work in the
network.
10.3. Path-State Creation, Use, and Maintenance
The path-state portion of the protocol has two major goals. The
first goal is to ensure sufficient state exists at the nodes along a
path from a source S to a destination D so that packets from S to D
do not need to carry the complete source route. The second goal is
to allow S to discover the characteristics of a particular path to D
so that it can adapt its sending pattern to the capabilities of the
path, or even choose a different path entirely.
The next sections describe how the path-state is created, how the
characteristics of the path are discovered, and what metrics can be
used to characterize the path.
10.3.1. Creating Path-State for Routing
To create the path-state, we assume that Route Discovery proceeds as
normal in DSR. Once the source node S has obtained a source route to
the destination D, it begins sending data packets to D as normally
done in DSR, with each packet carrying a full source route header.
Internally, S assigns a path-identifier to that particular source
route and stores the path-identifier in its route cache along with
the source route. S then includes the path-identifier as part of the
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A -----------------> B -----------------> C -----------------> D
+-------------+ +-------------+ +-------------+
|src: A | |src: A | |src: A |
|dst: D | |dst: D | |dst: D |
|path-id: 15 | |path-id: 15 | |path-id: 15 |
|rt: A,(B),C,D| |rt: A,B,(C),D| |rt: A,B,C,(D)|
+-------------+ +-------------+ +-------------+
| payload | | payload | | payload |
(a) Packet with path identifier and source route.
A -----------------> B -----------------> C -----------------> D
+-------------+ +-------------+ +-------------+
|src: A | |src: A | |src: A |
|dst: D | |dst: D | |dst: D |
|path-id: 15 | |path-id: 15 | |path-id: 15 |
+-------------+ +-------------+ +-------------+
| payload | | payload | | payload |
(b) Packet with path identifier only.
Figure 2: Path identifiers assigned to a source route by the
originating node A enable later packets to omit the source route.
source route header as shown in Figure 2(a). As each intermediate
node processes the source route to forward the packet, it also stores
the source route in its route cache, indexed by the source and
path-identifier.
After sending a packet containing both the source route and the
path-identifier into the network, S can begin sending subsequent
packets to D without a full source route --- carrying only the
path-identifier as shown in Figure 2(b). Each intermediate node
receiving such a packet queries its route cache to find the route
the packet is supposed to take, and determines its next hop. As
explained in Section 10.5, if the cached source route is not
available at some intermediate node, S will receive a Route Error and
can then correct the situation.
10.3.2. Monitoring Characteristics of the Path
In order to support network layer services such as balancing the
traffic load across the network, end-systems must have a method for
determining the characteristics of the paths through the network that
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they could use. While many schemes have been proposed by which the
end-systems themselves can measure the characteristics of a path
(e.g., TCP congestion window and RTT calculations [1, 22, 24] and
SPAND [21]), we hypothesize that, particularly in the in the dynamic
environment of an ad hoc network, more useful, more accurate, and
more timely information can be developed by enlisting the aid of the
nodes along the path to measure the path characteristics.
We propose that each node can measure the activity around itself,
and thereby determine information such as: the mean latency it adds
to the packets it forwards and the latency variation (jitter); the
number of additional packets per second it believes it can process;
or the unused amount of wireless media capacity in the air around
the node. Experimentation will be required to discover exactly
which metrics will prove to be accurately measurable and useful,
though Section 10.3.3 provides several proposals. If the metrics
kept by each node on a path are combined, the result should be a
characterization of the path that the packet sender can use to
organize or adapt its offered load.
To implement this scheme, we first define a new type of extension
header for DSR than can be piggybacked onto a packet in the same way
as the existing DSR headers. This new header is called the path
metrics header (written as Measure) and conceptually consists of the
path-identifier of the path along which the metrics are measured,
the type of the Measure, and the metrics themselves encoded in a TLV
format (Section 10.6.2).
Whenever a sender S wishes to measure the characteristics of a path
it is using, it includes the Measure header in any packet it sends
along that path, setting the type of the header to record. As each
node along the path forwards the packet, it updates the variables
inside the Measure header with the metrics it has measured locally.
When the header reaches the final destination D, D sets the type
of the Measure header to return and piggybacks the header into any
packet headed back to S. Since the path metrics header includes
the path-identifier of the path along which it was measured, S can
include the data into its route cache for future use, and can treat
the receipt of the path metrics header as a positive acknowledgment
that the path-state between S and D for the given path-identifier
is correctly set up. This could lead S to cease including source
routes in the packets it sends along the path, as described in
Section 10.3.1.
If we find that it is valuable to immediately provide S with the path
metrics of every discovered route, we could alter Route Discovery
slightly to generate this information. Currently, if an intermediate
node has a cached route that it can use to answer a Route Request,
it generates a Route Reply itself. Instead, we could require it to
place its proposed route on the Route Request (turning it from a
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flood-fill broadcast into a unicast packet) and send the packet to
the destination so it will measure the metrics of the complete path.
The destination will then return the metrics to the source along with
the Route Reply as described above.
We have been intending to experiment with this alteration to
Route Discovery for some time, since it offers two benefits,
even without path-state metrics. It should decrease the
number of broken routes returned by Route Discovery since
each cached route is tested before being returned, and
it should save us from jeopardizing one data packet for
every bad route in someone's cache. The cost is some extra
latency on Route Discovery.
10.3.3. Candidate Metrics
In order to limit the additional overhead that collecting and
distributing path-state metrics will place on the network, all the
metrics must have the property that the amount of space required to
express the metric does not increase as the number of hops on the
path increases. Experimentation will be required to determine which
metrics are most accurately measured and most useful, but our initial
set of candidates includes the following:
- Interface queue length --- Our previous work [12] has shown that
this is a good estimator of local congestion.
- Rate of interface queue draining --- When an interface is
backlogged, the rate at which packets leave the queue directly
measures the usable capacity of that interface.
- Quiet time fraction --- When an interface is not backlogged,
the usable capacity of the interface can be estimated by
promiscuously listening to the media and measuring the fraction
of time during which it is not in use (though this will
overestimate the capacity).
- Fraction Free Air Time --- The fraction of time our interface
would be able to send a packet. That is, the fraction of time
the interface does not sense carrier, is not deferring, and is
not backed off. Current experiments show this is an excellent
predictor of congestion and available capacity.
- Forwarding latency and variation --- This can be measured
as the time between when a packet is received and when it is
acknowledged by the next hop.
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- Unidirectional links --- Paths containing unidirectional links
are usable, but undesirable as they increase the overhead of
Route Maintenance.
- Packet loss rate --- Signal quality information from the
interface itself, or the frequency of hop-by-hop retransmission,
can be used to estimate the loss rate of each link.
- Likelihood of path breakage --- Intermediate nodes may know ahead
of time that they intend to shutdown or move such that paths
through them will no longer work.
These metrics all have the property that they can be expressed in
a single value that each node can measure locally. As a packet
with a path metrics header passes through a node, the metrics in
the header can be updated to reflect the node's metrics using a
combination function like minimum, maximum, sum, or weighted average
that produces another single value to replace the one already in
the header. This updating will be done at the last possible moment
before the packet is forwarded, in order to assure the packet has the
most current metrics on it when it leaves.
10.4. Flow-State Creation, Use, and Maintenance
The flow-state portion of the protocol enables a sender to obtain
promises from all nodes along a path to a destination that a
certain set of resources are available along the path, and that
the intermediate nodes are committed to making these resources
available for the particular flow. This allows a sender to obtain
better-than-best-effort Quality of Service for a flow by obtaining
promises from the intermediate nodes to reserve the resources needed
to provide that QoS.
Unlike prior QoS work in wired networks, at this point we cannot
formally characterize or bound exactly what type of services the
flow-state protocol will be able to offer. The goal is to provide
CBR and TCP streams with the ability to specify and obtain a
minimum bandwidth and delay/jitter bound. If the environment is
particularly harsh, it is possible that only best-effort service will
be offerable. It is this intuition that leads us to the system of
promises and notifications. Experimentally, we hope to determine
how stable and effective this system will be in a multi-hop ad hoc
network environment.
10.4.1. Requesting Promises along Existing Paths
Similar to the use of the path metrics header, at any time a promise
can be requested or changed along any path an originator is currently
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using. Once an originating node has created a path-identifier
for a route through the network, it can request a promise of
network resources along that route by first generating a new
flow-identifier to identify the promise. The originator then fills
out a flow-request header (written as Flow Request) and inserts it
into any packet sent along that path.
Figure 3 shows the conceptual layout of a Flow Request, which
contains the new path-identifier assigned by the originator, the
flow-identifier of the promise that this request supersedes (if any),
the requested lifetime of the promise, and the QoS parameters that
describe the requested promise itself. Section 10.6.3 provides the
detailed packet format. The use of the minimum and requested fields
for the QoS parameters differs depending on whether the Flow Request
is piggybacked on a Route Request or not, as described below.
When a Flow Request piggybacked on a unicast packet is received by a
node, the node performs the following steps:
- If the node is the destination of the packet, it converts the
Flow Request into a Measure with type return and uses the current
values in the desired fields of the Flow Request to populate the
fields of the Measure. It then piggybacks the Measure onto any
packet being returned to the originator.
- Else if the intermediate node has available enough resources to
meet the minimum requested promise in the Flow Request, it:
* Sets aside the maximum of its available resources and the
desired resources. The set aside resources are held in a
tentative promise pool until the promise is confirmed, or a
relatively short timeout expires.
* Nodes can recycle resources from listed old flow-id
+--------------------------------------+
| flow-id | old flow-id |
+--------------------------------------+
| lifetime |
+--------------------------------------+
| capacity | min | desired |
| latency | min | desired |
|variation | min | desired |
| loss | min | desired |
+--------------------------------------+
Figure 3: Conceptual layout of the Flow Request header.
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* Updates the desired fields of the Flow Request to reflect
the resources set aside (there is questionable value in a
down stream node allocating more resources to a flow than an
upstream node can currently handle).
* Forward the packet and piggybacked Flow Request to the next
node on the path.
- Else, the node does not have enough resources to meet the
minimum requested promise, so it sends the originator a Route
Error piggybacked with a Measure reflecting the minimum of the
current values of the desired fields in the Flow Request and the
available resources. The type field is set to refused. Such a
Measure enables the originator to learn three things: that its
requested cannot be satisfied along the given path; the identity
of the bottleneck node; and the available resources up to and
through the bottleneck node.
When the originating node receives a Measure header of type return
for a flow on which it has an outstanding Flow Request, it accepts
the promised level of service by changing the type of the Measure
header to confirm and piggybacking the header on any packet going
along the flow. This informs the intermediate nodes to move the set
aside resources from the tentative promise pool to the allocated
pool, and enables upstream nodes to free any set aside resources in
excess of the capacity of a bottleneck downstream node.
The use of the old flow-id to recycle resources is important for two
reasons. First, it enables an originator to attempt to increase or
decrease the amount of a current promise without losing the resources
it already has promised. Second, both packet loss and the expanding
ring search of Route Discovery may result in several Flow Requests
being sent for the same flow. If subsequent Flow Requests for a
flow were not able to notify intermediate nodes that they can reuse
resources set aside while processing earlier Flow Requests, the
network could quickly reach a state where admissible flows are being
needlessly rejected.
10.4.2. Requesting Promises as Part of Route Discovery
The scheme for requesting promises described in the previous section
has the advantage that it enables an originator to request or update
a promise for a flow along any route currently in its route cache,
regardless of how it obtained the route. For the common case in
which a node wishes to obtain a resource promise for a new flow to
a previously unknown destination, we can integrate the flow request
with the Route Discovery for the destination.
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Integrating the flow request with Route Discovery enables us to avoid
the inefficiency of discovering routes that will not be usable by the
flow due to insufficient resources. The integration of flow requests
with Route Discovery also allows us to avoid a common pitfall of
QoS schemes that layer a reservation signaling protocol on top of
a unicast routing algorithm --- schemes without tight integration
will refuse admissible flows whenever the unicast routing algorithm
directs the request packets into a congested area of the network,
unless the signaling protocol also provides a method to backtrack
the request and route around the congested area. Utilizing the same
mechanisms currently used in Route Discovery, we can avoid the need
for backtracking.
We call the combination of flow requests with Route Discovery
QoS-guided Route Discovery, which originating nodes can invoke simply
by piggybacking a Flow Request on the Route Request. Each node
receiving the Flow Request uses the same algorithm described in
Section 10.4.1, with two exceptions:
- Nodes silently discard the Route Request if they can not meet
minimum requirements
- Unless the Route Request indicates that replying from cache is
forbidden, nodes with a cached route to destination unicast the
Route Request along the cached route.
A node requiring a route with a QoS promise uses the following
algorithm. First, it sends a Route Request that permits intermediate
nodes to reply from cache. If the network is uncongested, this
should frequently and quickly succeed in returning both a Route Reply
and a Measure describing the available QoS along the discovered
path. If after a timeout, the originating node has not received a
Route Reply, it begins another Route Discovery, this time forbidding
replies from cache, which will force an exploration of all feasible
paths to the destination.
This scheme does risk an implosion of unicast Requests at the target
of the Route Discovery (e.g., if target is a popular server to which
many nodes have cached routes). At the cost of additional complexity
and soft-state, it would be possible to add hold-downs at the nodes
surrounding the target so that only the first few Requests are
forwarded towards the target.
10.4.3. Providing Notifications of Changing Path Metrics
When a node detects that it must break a promise, it must notify the
node to which it made that promise. It is an open question how the
now reduced resources should be distributed among the flows. We
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currently pick the minimum set of promises to break that leave the
other promises unchanged.
The difficulty in providing notification of a changed path metric is
getting this information back to the source. When promise must be
broken at a node B, it sends a Measure to the originator indicating
what resources are now available. The use of Measure headers to
determine the currently available resources along a path is more
problematic, however, as for every Measure sent by the originator,
the destination must send a response containing the measured metrics.
If the traffic is TCP, the overhead of the responses are low, as
they can be piggybacked on the ACK stream. For one-way CBR traffic
though, introducing the overhead of a reverse stream to carry the
changing metrics could be severe.
If the overhead of the responses becomes a problem, it may be
possible to implement a enhanced piggyback mechanism. The approach
is based on the fact that although no work has been exerted to create
hop-by-hop routing information at each node, chances are good that
each node can determine a next-hop for packets headed to any known
destination by simply examining its route cache. By piggybacking
the Measure header for one hop onto any packet that is headed to
that next-hop, we can cheaply create a reverse flow of information
that will eventually reach the originator of the Measure. Each
node who receives a Measure with a type of return simply piggybacks
the Measure for one-hop on packets that seem to be flowing the
right direction back to the source. To insure the timeliness of
the information, each Measure being returned to an originator could
include a deadline by which the information is supposed to reach the
originator. If it appears that hop-by-hop propagation will result
in missing the deadline, the Measure can be unicast as a first-class
packet to the originator.
10.5. Expiration of State from Intermediate Nodes
Since there is no guarantee that either the source or destination of
a packet flow will be able to communicate with all of the nodes that
carried the flow when they wish to terminate the flow, there must
be time-based expiration mechanism by which intermediate nodes can
purge the path-state and flow-state from their caches and reclaim the
resources set aside to maintain it. However, if intermediate nodes
were to purge the state of an active flow, the intermediate nodes
would find themselves with packets to forward that do not contain
a source route, but only contain a flow-identifier that references
state they no longer hold. Since intermediate nodes do not
necessarily know the timing with which the sender originates packets,
an inactivity timer alone would have to be set very conservatively to
prevent purging the path-state of low bit-rate connections.
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To solve the expiration problem, we take advantage of the relatively
``soft'' nature of the path-state and flow-state. When the state
is created, the source node specifies a time after which it should
be discarded (This time will typically be on the order of a hundred
seconds). The source node can thereby estimate how often it must
refresh the state, for example, by sending packets that contain a
full source route on them. Should the state have somehow expired
at an intermediate node when a packet labeled with a flow or path
identifier arrives, the intermediate node can return a Route Error to
the source node specifying ``missing state information'' as the cause
of the Error and elicit the sender to refresh the missing state.
Since all path-state information is guaranteed to have expired from
the network after a bounded amount of time, nodes can safely and
unambiguously reuse path- and flow-identifiers after that period.
10.6. Packet Formats
10.6.1. Identifier Option
Path and flow identifiers are carried as an option inside the
Hop-by-Hop options header. This option MAY NOT appear more than once
in a single Hop-by-Hop 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length | Path-ID | Flow-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
???. A node that does not understand this option should ignore
this option and continue processing the packet, and the Option
Data does not change en-route (the top three bits are 000).
Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields.
Path-ID
The identifier assigned to this path by the node listed as the
IP Source Address (Section 10.2).
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Flow-ID
The identifier assigned by the node listed as the IP Source
Address to a particular flow along the path identified by the
Path-ID. If this portion is 0, the option names a path, but not
a particular flow.
Discussion: This encoding of the path and flow identifiers will cost
8 bytes of additional header overhead in a data packet with no other
extensions or options (4 bytes for the Hop-by-Hop options header, and
4 bytes for the identifier option). A more compact encoding would be
to define that, in a DSR network, an IP destination address with a
first octet of 127 actually encodes the path and flow identifiers 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 1 1 1 1 1 1| reserved | Path-ID | Flow-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The DSR module of the final destination would replace the IP
destination address with its actual value before passing the packet
up the stack for further processing.
This encoding has the advantage that it requires no additional
overhead in a data packet. The disadvantage is that if the packet
was somehow received by a DSR-unaware node without first being
processed by a DSR gateway node [4], the DSR-unaware node will either
drop the packet or will attempt to receive it locally (since the IP
destination address belongs to the loopback subnet).
10.6.2. Path-Metrics Option
Path-metrics are carried as an option inside the Hop-by-Hop 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length | Path-ID | Flow-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Metrics...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Option Type
???. A node that does not understand this option should ignore
this option and continue processing the packet, and the Option
Data does change en-route (the top three bits are 001).
Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields.
Path-ID and Flow-ID
The path identifier of the path that the metrics correspond
to. If the Path-Metrics Option Type equals Measure, then the
Path-ID and Flow-ID fields MUST equal those in any Identifier
Option carried in the Hop-by-Hop Options Header.
Type
One of
Measure
Each node processing the option should update the metrics
to reflect the conditions at that node.
Reply
The metrics in this option SHOULD NOT be modified by any
intermediate node. They represent the metrics measured
along the identified path.
Confirm
The metrics in this option MUST NOT be modified by any
intermediate node. They represent a confirmation by
the sender that will transmit traffic conforming to the
listed Quality of Service metrics along the identified
flow.
Metrics
The individual path-metrics, encoded as described in
Section 10.6.4. Unknown metrics SHOULD be ignored. If a
single value is provide for the metric, it MUST be interpreted
as the metrics value. If two values are provided for the
metric, they MUST be interpreted as the range of values taken
by the metric (low value first). It is undefined for there to
be more than two values for the metric.
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10.6.3. Flow Request Option
Flow-requests are carried as an option inside the Hop-by-Hop options
header. They allow a sender to request that intermediate nodes
reserve sufficient resources for a flow to provide that flow with the
QoS characteristics described by the metrics.
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 | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| old | old | new | new |
| Path-ID | Flow-ID | Path-ID | Flow-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metrics ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
???. A node that does not understand this option should ignore
this option and continue processing the packet, and the Option
Data does change en-route (the top three bits are 001).
Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields.
old Path-ID and old Flow-ID
The flow identifier provide in a previous request which this
request supersedes.
new Path-ID and new Flow-ID
The flow identifier that will be used with to identify the
packets that should receive the QoS described by the included
metrics.
Metrics
The metrics that characterize the desired QoS, encoded as
described in Section 10.6.4. Unknown metrics SHOULD be
ignored. If a range of values are provided for a metric, they
MUST be interpreted as the minimum acceptable value and the
desired value.
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10.6.4. Encoding Path-Metrics
Each path-metric is encoded in a modified Type-Length-Value form as
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type |R| Length | Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
The type of metric
R bit
If 0, the data is a list of discrete values the metric can
or did take. If 1, the data represent a range of values
the metric can or did take. If a single metric value is
supplied, the range is assumed to be 0 <= metric <= value. If
two metric values are supplied, the range is assumed to be
value1 <= metric <= value2.
Option Length
8-bit unsigned integer. Length of the metric, in octets,
excluding the Type and Length fields.
The currently defined metric types follow:
Padding
Type: 0
The padding metric is special in that it contains no length field and
no data.
Available Capacity
Type: 1
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Data encoded as
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mantissa | Shift |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where the value is (Mantissa << Shift) bits per second.
Delay and Delay Variation
Data encoded as
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 2 - Delay
The value is Delay milliseconds.
Type: 3 - Delay Variation
The value is the standard deviation of Delay, in milliseconds.
Link Bidirectionality
Type: 16 - Link Bidirectionality
Data encoded as
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| # Uni-links | #Explicit ACK |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where # Uni-links is the number of uni-directional links on the path,
and # Explicit ACK is the number of hops which require explicit
acknowledgments.
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Packet Loss Rate
Data encoded as
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| # Packets Lost |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where the loss rate is (# Packets Lost / 2 ** 16).
Type: 17 - Path Packet Loss Rate
The value is the expected packet loss rate of the entire path
Type: 18 - Worst Loss Rate
The value is the expected packet loss rate of the single worst link
in the path.
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11. Constants
BROADCAST_JITTER 10 milliseconds
MAX_ROUTE_LEN 15 nodes
Interface Indexes
IF_INDEX_INVALID 0x7F
IF_INDEX_MA 0x7E
IF_INDEX_ROUTER 0x7D
Route Cache
ROUTE_CACHE_TIMEOUT 300 seconds
Send Buffer
SEND_BUFFER_TIMEOUT 30 seconds
Request Table
MAX_REQUEST_ENTRIES 32 nodes
MAX_REQUEST_IDS 8 identifiers
MAX_REQUEST_REXMT 16 retransmissions
MAX_REQUEST_PERIOD 10 seconds
REQUEST_PERIOD 500 milliseconds
RING0_REQUEST_TIMEOUT 30 milliseconds
Retransmission Buffer
DSR_RXMT_BUFFER_SIZE 50 packets
Retransmission Timer
DSR_MAXRXTSHIFT 2
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12. IANA Considerations
This document proposes the use of the Destination Options header and
the Hop-by-Hop Options header, originally defined for IPv6, in IPv4.
The Next Header values indicating these two extension headers thus
must be reserved within the IPv4 Protocol number space.
Furthermore, this document defines four new types of destination
options, each of which must be assigned an Option Type value:
- The DSR Route Request option, described in Section 7.1.1
- The DSR Route Reply option, described in Section 7.2.1
- The DSR Route Error option, described in Section 7.2.2
- The DSR Acknowledgment option, described in Section 7.2.3
DSR also requires a routing header Routing Type be allocated for the
DSR Source Route defined in Section 7.3.
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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13. 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 Reference 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 IPv4 [17], IPv6 [6], and IPX [7] nodes.
- Historically, DSR grew from our contemplation of a multi-hop ARP
protocol [8, 9] and source routing bridges [15]. ARP [16] is a
layer 2 protocol.
- 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 specify 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 2.2.7 operating system running on Intel x86 platforms.
FreeBSD is based on a variety of free software, including 4.4 BSD
Lite from the University of California, Berkeley.
During the 7 months from August 1998 to February 1999, we designed
and implemented a full-scale physical testbed to enable the
evaluation of ad hoc network performance in the field. The last
week of February and the first week of March included demonstrations
of this testbed to a number of our sponsors and partners, including
Lucent Technologies, Bell Atlantic, and DARPA. A complete description
of the testbed is available as a Technical Report [12].
The software is currently being ported to FreeBSD 3.3.
Implementors notes:
- Added field to Route Error
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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 node
networking [10, 19]. The current members of the CMU Monarch Project
include:
- Robert V. Barron
- Josh Broch
- Yih-Chun Hu
- Jorjeta Jetcheva
- David B. Johnson
- Qifa Ke
- David A. Maltz
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References
[1] M. Allman, V. Paxson, and W. Stevens. Tcp congestion control.
Internet Request For Comments RFC 2581, April 1999.
[2] R. Braden, editor. Requirements for Internet Hosts --
Communication Layers. RFC 1122, October 1989.
[3] Scott Bradner. Key words for use in RFCs to Indicate
Requirement Levels. RFC 2119, March 1997.
[4] Josh Broch, David A. Maltz, and David B. Johnson. Supporting
Hierarchy and Heterogeneous Interfaces in Multi-Hop Wireless
Ad Hoc Networks. In Proceedings of the Workshop on Mobile
Computing held in conjunction with the International Symposium
on Parallel Architectures, Algorithms, and Networks, pages
370--375, Perth, Australia, June 1999.
[5] M. Scott Corson and Joe Macker. Mobile Ad hoc Networking
(MANET): Routing Protocol Performance Issues and Evaluation
Considerations howpublished = RFC 2501, month = jan, year =
1999.
[6] Stephen E. Deering and Robert M. Hinden. Internet Protocol,
version 6 (IPv6) Specification. RFC 2460, December 1998.
[7] IPX Router Specification. Novell Part Number 107-000029-001,
Document Version 1.30, March 1996.
[8] 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.
[9] 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.
[10] David B. Johnson and David A. Maltz. Protocols for Adaptive
Wireless and Mobile Networking. IEEE Personal Communications,
3(1):34--42, February 1996.
[11] Jian Liu and Suresh Singh. Atcp: Tcp for mobile ad hoc
networks. Available from web page??? Personal Communication,
June 1999.
[12] David A. Maltz, Josh Broch, and David B. Johnson. Experiences
Designing and Building a Multi-Hop Wireless Ad Hoc Network
Testbed. Technical Report 99-116, School of Computer Science,
Carnegie Mellon University, March 1999.
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[13] Brian D. Noble, M. Satyanarayanan, Dushyanth Narayanan,
Eric J. Tilton, Jason Flinn, and Kevin R. Walker. Agile
Application-Aware Adaptation for Mobility. In Proceedings of
the 16th ACM Symposium on Operating System Principles, pages
276--287, St. Malo, France, October 1997.
[14] Charles Perkins, editor. IP Mobility Support. RFC 2002,
October 1996.
[15] Radia Perlman. Interconnections: Bridges and Routers.
Addison-Wesley, Reading, Massachusetts, 1992.
[16] David C. Plummer. An Ethernet address resolution protocol:
Or converting network protocol addresses to 48.bit Ethernet
addresses for transmission on Ethernet hardware. RFC 826,
November 1982.
[17] J. Postel. Internet Protocol. RFC 791, September 1981.
[18] J. Postel. Transmission Control Protocol. RFC 793, September
1981.
[19] The CMU Monarch Project. http://www.monarch.cs.cmu.edu/.
Computer Science Department, Carnegie Mellon University.
[20] J. Reynolds and J. Postel. Assigned Numbers. RFC 1700, October
1994.
[21] Srinivasan Seshan, Mark Stemm, and Randy H. Katz. Spand:
Shared passive network performance discovery. In Proceedings of
the USENIX Symposium on Internet Technologies and Systems, pages
135--146, dec 1997.
[22] W. Richard Stevens. TCP/IP IIlustrated, The Protocols,
volume 1. Addison-Welsley, 1994.
[23] Andrew S. Tannenbaum. Computer Networks. Prentice Hall, third
edition, 1996.
[24] Gary R. Wright and W. Richard Stevens. TCP/IP IIlustrated, The
Implementation, volume 2. Addison-Welsley, 1995.
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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
5000 Forbes Avenue
Pittsburgh, PA 15213-3890
USA
Phone: +1 412 268-3056
Fax: +1 412 268-7196
Email: broch@cs.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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