RFC 4728
The Dynamic Source Routing Protocol (DSR) for Mobile Ad Hoc Networks for IPv4
3.5. Optional DSR Flow State Extension
This section describes an optional, compatible extension to the DSR protocol, known as "flow state", that allows the routing of most packets without an explicit source route header in the packet. The DSR flow state extension further reduces the overhead of the protocol yet still preserves the fundamental properties of DSR's operation. Once a sending node has discovered a source route such as through DSR's Route Discovery mechanism, the flow state mechanism allows the sending node to establish hop-by-hop forwarding state within the network, based on this source route, to enable each node along the route to forward the packet to the next hop based on the node's own local knowledge of the flow along which this packet is being routed. Flow state is dynamically initialized by the first packet using a source route and is then able to route subsequent packets along the same flow without use of a source route header in the packet. The state established at each hop along a flow is "soft state" and thus automatically expires when no longer needed and can be quickly recreated as necessary. Extending DSR's basic operation based on an explicit source route in the header of each packet routed, the flow state extension operates as a form of "implicit source routing" by preserving DSR's basic operation but removing the explicit source route from packets.
3.5.1. Flow Establishment
A source node sending packets to some destination node MAY use the DSR flow state extension described here to establish a route to that destination as a flow. A "flow" is a route from the source to the destination represented by hop-by-hop forwarding state within the nodes along the route. Each flow is uniquely identified by a combination of the source node address, the destination node address, and a flow identifier (flow ID) chosen by the source node. Each flow ID is a 16-bit unsigned integer. Comparison between different flow IDs MUST be performed modulo 2**16. For example, using an implementation in the C programming language, a flow ID value (a) is greater than another flow ID value (b) if ((short)((a) - (b)) > 0), if a C language "short" data type is implemented as a 16-bit signed integer. A DSR Flow State header in a packet identifies the flow ID to be followed in forwarding that packet. From a given source to some destination, any number of different flows MAY exist and be in use, for example, following different sequences of hops to reach the destination. One of these flows MAY be considered the "default" flow from that source to that destination. If a node receives a packet with neither a DSR Options header specifying the route to be taken (with a Source Route option in the DSR Options header) nor a DSR Flow State header specifying the flow ID to be followed, it is forwarded along the default flow for the source and destination addresses specified in the packet's IP header. In establishing a new flow, the source node generates a nonzero 16-bit flow ID greater than any unexpired flow IDs for this (source, destination) pair. If the source wishes for this flow to become the default flow, the low bit of the flow ID MUST be set (the flow ID is an odd number); otherwise, the low bit MUST NOT be set (the flow ID is an even number). The source node establishing the new flow then transmits a packet containing a DSR Options header with a Source Route option. To establish the flow, the source node also MUST include in the packet a DSR Flow State header, with the Flow ID field set to the chosen flow ID for the new flow, and MUST include a Timeout option in the DSR Options header, giving the lifetime after which state information about this flow is to expire. This packet will generally be a normal data packet being sent from this sender to the destination (for example, the first packet sent after discovering the new route) but is also treated as a "flow establishment" packet.
The source node records this flow in its Flow Table for future use, setting the TTL in this Flow Table entry to the value used in the TTL field in the packet's IP header and setting the Lifetime in this entry to the lifetime specified in the Timeout option in the DSR Options header. The TTL field is used for Default Flow Forwarding, as described in Sections 3.5.3 and 3.5.4. Any further packets sent with this flow ID before the timeout that also contain a DSR Options header with a Source Route option MUST use this same source route in the Source Route option.3.5.2. Receiving and Forwarding Establishment Packets
Packets intended to establish a flow, as described in Section 3.5.1, contain a DSR Options header with a Source Route option and are forwarded along the indicated route. A node implementing the DSR flow state extension, when receiving and forwarding such a DSR packet, also keeps some state in its own Flow Table to enable it to forward future packets that are sent along this flow with only the flow ID specified. Specifically, if the packet also contains a DSR Flow State header, this packet SHOULD cause an entry to be established for this flow in the Flow Table of each node along the packet's route. The Hop Count field of the DSR Flow State header is also stored in the Flow Table, as is the lifetime specified in the Timeout option specified in the DSR Options header. If the Flow ID is odd and there is no flow in the Flow Table with Flow ID greater than the received Flow ID, set the default Flow ID for this (IP Source Address, IP Destination Address) pair to the received Flow ID, and the TTL of the packet is recorded. The Flow ID option is removed before final delivery of the packet.3.5.3. Sending Packets along Established Flows
When a flow is established as described in Section 3.5.1, a packet is sent that establishes state in each node along the route. This state is soft; that is, the protocol contains mechanisms for recovering from the loss of this state. However, the use of these mechanisms may result in reduced performance for packets sent along flows with forgotten state. As a result, it is desirable to differentiate behavior based on whether or not the sender is reasonably certain that the flow state exists on each node along the route. We define a flow's state to be "established end-to-end" if the Flow Tables of all nodes on the route contains forwarding information for that flow. While it is impossible to detect whether or not a flow's state has
been established end-to-end without sending packets, implementations may make reasonable assumptions about the retention of flow state and the probability that an establishment packet has been seen by all nodes on the route. A source wishing to send a packet along an established flow determines if the flow state has been established end-to-end. If it has not, a DSR Options header with Source Route option with this flow's route is added to the packet. The source SHOULD set the Flow ID field of the DSR Flow State header either to the flow ID previously associated with this flow's route or to zero. If it sets the Flow ID field to any other value, it MUST follow the processing steps in Section 3.5.1 for establishing a new flow ID. If it sets the Flow ID field to a nonzero value, it MUST include a Timeout option with a value not greater than the timeout remaining in the node's Flow Table, and if its TTL is not equal to that specified in the Flow Table, the flow MUST NOT be used as a default flow in the future. Once flow state has been established end-to-end for non-default flows, a source adds a DSR Flow State header to each packet it wishes to send along that flow, setting the Flow ID field to the flow ID of that flow. A Source Route option SHOULD NOT be added to the packet, though if one is, then the steps for processing flows that have not been established end-to-end MUST be followed. Once flow state has been established end-to-end for default flows, sources sending packets with IP TTL equal to the TTL value in the local Flow Table entry for this flow then transmit the packet to the next hop. In this case, a DSR Flow State header SHOULD NOT be added to the packet and a DSR Options header likewise SHOULD NOT be added to the packet; though if one is, the steps for sending packets along non-default flows MUST be followed. If the IP TTL is not equal to the TTL value in the local Flow Table, then the steps for processing a non-default flow MUST be followed.3.5.4. Receiving and Forwarding Packets Sent along Established Flows
The handling of packets containing a DSR Options header with both a nonzero Flow ID and a Source Route option is described in Section 3.5.2. The Flow ID is ignored when it is equal to zero. This section only describes handling of packets without a Source Route option. If a node receives a packet with a Flow ID in the DSR Options header that indicates an unexpired flow in the node's Flow Table, it increments the Hop Count in the DSR Options header and forwards the packet to the next hop indicated in the Flow Table.
If a node receives a packet with a Flow ID that indicates a flow not currently in the node's Flow Table, it returns a Route Error of type UNKNOWN_FLOW with Error Destination and IP Destination addresses copied from the IP Source of the packet triggering the error. This error packet SHOULD be MAC-destined to the node from which the packet was received; if it cannot confirm reachability of the previous node using Route Maintenance, it MUST send the error as described in Section 8.1.1. The node sending the error SHOULD attempt to salvage the packet triggering the Route Error. If it does salvage the packet, it MUST zero the Flow ID in the packet. If a node receives a packet with no DSR Options header and no DSR Flow State header, it checks the Default Flow Table. If there is a matching entry, it forwards to the next hop indicated in the Flow Table for the default flow. Otherwise, it returns a Route Error of type DEFAULT_FLOW_UNKNOWN with Error Destination and IP Destination addresses copied from the IP Source Address of the packet triggering the error. This error packet SHOULD be MAC-destined to the node from which it was received; if this node cannot confirm reachability of the previous node using Route Maintenance, it MUST send the error as described in Section 8.1.1. The node sending the error SHOULD attempt to salvage the packet triggering the Route Error. If it does salvage the packet, it MUST zero the Flow ID in the packet.3.5.5. Processing Route Errors
When a node receives a Route Error of type UNKNOWN_FLOW, it marks the flow to indicate that it has not been established end-to-end. When a node receives a Route Error of type DEFAULT_FLOW_UNKNOWN, it marks the default flow to indicate that it has not been established end- to-end.3.5.6. Interaction with Automatic Route Shortening
Because a full source route is not carried in every packet, an alternative method for performing automatic route shortening is necessary for packets using the flow state extension. Instead, nodes promiscuously listen to packets, and if a node receives a packet with (IP Source, IP Destination, Flow ID) found in the Flow Table but the MAC-layer (next hop) destination address of the packet is not this node, the node determines whether the packet was sent by an upstream or downstream node by examining the Hop Count field in the DSR Flow State header. If the Hop Count field is less than the expected Hop Count at this node (that is, the expected Hop Count field in the Flow Table described in Section 5.1), the node assumes that the packet was sent by an upstream node and adds an entry for the packet to its Automatic Route Shortening Table, possibly evicting an earlier entry added to this table. When the packet is then sent to that node for
forwarding, the node finds that it has previously received the packet by checking its Automatic Route Shortening Table and returns a gratuitous Route Reply to the source of the packet.3.5.7. Loop Detection
If a node receives a packet for forwarding with TTL lower than expected and default flow forwarding is being used, it sends a Route Error of type DEFAULT_FLOW_UNKNOWN back to the IP source. It can attempt delivery of the packet by normal salvaging (subject to constraints described in Section 8.6.7).3.5.8. Acknowledgement Destination
In packets sent using Flow State, the previous hop is not necessarily known. In order to allow nodes that have lost flow state to determine the previous hop, the address of the previous hop can optionally be stored in the Acknowledgement Request. This extension SHOULD NOT be used when a Source Route option is present, MAY be used when flow state routing is used without a Source Route option, and SHOULD be used before Route Maintenance determines that the next-hop destination is unreachable.3.5.9. Crash Recovery
Each node has a maximum Timeout value that it can possibly generate. This can be based on the largest number that can be set in a timeout option (2**16 - 1 seconds) or may be less than this, set in system software. When a node crashes, it does not establish new flows for a period equal to this maximum Timeout value, in order to avoid colliding with its old Flow IDs.3.5.10. Rate Limiting
Flow IDs can be assigned with a counter. More specifically, the "Current Flow ID" is kept. When a new default Flow ID needs to be assigned, if the Current Flow ID is odd, the Current Flow ID is assigned as the Flow ID and the Current Flow ID is incremented by one; if the Current Flow ID is even, one plus the Current Flow ID is assigned as the Flow ID and the Current Flow ID is incremented by two. If Flow IDs are assigned in this way, one algorithm for avoiding duplicate, unexpired Flow IDs is to rate limit new Flow IDs to an average rate of n assignments per second, where n is 2**15 divided by the maximum Timeout value. This can be averaged over any period not exceeding the maximum Timeout value.
3.5.11. Interaction with Packet Salvaging
Salvaging is modified to zero the Flow ID field in the packet. Also, anytime this document refers to the Salvage field in the Source Route option in a DSR Options header, packets without a Source Route option are considered to have the value zero in the Salvage field.4. Conceptual Data Structures
This document describes the operation of the DSR protocol in terms of a number of conceptual data structures. This section describes each of these data structures and provides an overview of its use in the protocol. In an implementation of the protocol, these data structures MUST be implemented in a manner consistent with the external behavior described in this document, but the choice of implementation used is otherwise unconstrained. Additional conceptual data structures are required for the optional flow state extensions to DSR; these data structures are described in Section 5.4.1. Route Cache
Each node implementing DSR MUST maintain a Route Cache, containing routing information needed by the node. A node adds information to its Route Cache as it learns of new links between nodes in the ad hoc network; for example, a node may learn of new links when it receives a packet carrying a Route Request, Route Reply, or DSR source route. Likewise, a node removes information from its Route Cache as it learns that existing links in the ad hoc network have broken. For example, a node may learn of a broken link when it receives a packet carrying a Route Error or through the link-layer retransmission mechanism reporting a failure in forwarding a packet to its next-hop destination. Anytime a node adds new information to its Route Cache, the node SHOULD check each packet in its own Send Buffer (Section 4.2) to determine whether a route to that packet's IP Destination Address now exists in the node's Route Cache (including the information just added to the Cache). If so, the packet SHOULD then be sent using that route and removed from the Send Buffer. It is possible to interface a DSR network with other networks, external to this DSR network. Such external networks may, for example, be the Internet or may be other ad hoc networks routed with a routing protocol other than DSR. Such external networks may also be other DSR networks that are treated as external networks in order to improve scalability. The complete handling of such external networks is beyond the scope of this document. However, this document specifies a minimal set of requirements and features
necessary to allow nodes only implementing this specification to interoperate correctly with nodes implementing interfaces to such external networks. This minimal set of requirements and features involve the First Hop External (F) and Last Hop External (L) bits in a DSR Source Route option (Section 6.7) and a Route Reply option (Section 6.3) in a packet's DSR Options header (Section 6). These requirements also include the addition of an External flag bit tagging each link in the Route Cache, copied from the First Hop External (F) and Last Hop External (L) bits in the DSR Source Route option or Route Reply option from which this link was learned. The Route Cache SHOULD support storing more than one route to each destination. In searching the Route Cache for a route to some destination node, the Route Cache is searched by destination node address. The following properties describe this searching function on a Route Cache: - Each implementation of DSR at any node MAY choose any appropriate strategy and algorithm for searching its Route Cache and selecting a "best" route to the destination from among those found. For example, a node MAY choose to select the shortest route to the destination (the shortest sequence of hops), or it MAY use an alternate metric to select the route from the Cache. - However, if there are multiple cached routes to a destination, the selection of routes when searching the Route Cache SHOULD prefer routes that do not have the External flag set on any link. This preference will select routes that lead directly to the target node over routes that attempt to reach the target via any external networks connected to the DSR ad hoc network. - In addition, any route selected when searching the Route Cache MUST NOT have the External bit set for any links other than possibly the first link, the last link, or both; the External bit MUST NOT be set for any intermediate hops in the route selected. An implementation of a Route Cache MAY provide a fixed capacity for the cache, or the cache size MAY be variable. The following properties describe the management of available space within a node's Route Cache: - Each implementation of DSR at each node MAY choose any appropriate policy for managing the entries in its Route Cache, such as when limited cache capacity requires a choice of which entries to retain in the Cache. For example, a node MAY chose a "least recently used" (LRU) cache replacement policy, in which the entry
last used longest ago is discarded from the cache if a decision
needs to be made to allow space in the cache for some new entry
being added.
- However, the Route Cache replacement policy SHOULD allow routes to
be categorized based upon "preference", where routes with a higher
preferences are less likely to be removed from the cache. For
example, a node could prefer routes for which it initiated a Route
Discovery over routes that it learned as the result of promiscuous
snooping on other packets. In particular, a node SHOULD prefer
routes that it is presently using over those that it is not.
Any suitable data structure organization, consistent with this
specification, MAY be used to implement the Route Cache in any node.
For example, the following two types of organization are possible:
- In DSR, the route returned in each Route Reply that is received by
the initiator of a Route Discovery (or that is learned from the
header of overhead packets, as described in Section 8.1.4)
represents a complete path (a sequence of links) leading to the
destination node. By caching each of these paths separately, a
"path cache" organization for the Route Cache can be formed. A
path cache is very simple to implement and easily guarantees that
all routes are loop-free, since each individual route from a Route
Reply or Route Request or used in a packet is loop-free. To
search for a route in a path cache data structure, the sending
node can simply search its Route Cache for any path (or prefix of
a path) that leads to the intended destination node.
This type of organization for the Route Cache in DSR has been
extensively studied through simulation [BROCH98, HU00,
JOHANSSON99, MALTZ99a] and through implementation of DSR in a
mobile outdoor testbed under significant workload [MALTZ99b,
MALTZ00, MALTZ01].
- Alternatively, a "link cache" organization could be used for the
Route Cache, in which each individual link (hop) in the routes
returned in Route Reply packets (or otherwise learned from the
header of overhead packets) is added to a unified graph data
structure of this node's current view of the network topology. To
search for a route in link cache, the sending node must use a more
complex graph search algorithm, such as the well-known Dijkstra's
shortest-path algorithm, to find the current best path through the
graph to the destination node. Such an algorithm is more
difficult to implement and may require significantly more CPU time
to execute.
However, a link cache organization is more powerful than a path
cache organization, in its ability to effectively utilize all of
the potential information that a node might learn about the state
of the network. In particular, links learned from different Route
Discoveries or from the header of any overheard packets can be
merged together to form new routes in the network, but this is not
possible in a path cache due to the separation of each individual
path in the cache.
This type of organization for the Route Cache in DSR, including
the effect of a range of implementation choices, has been studied
through detailed simulation [HU00].
The choice of data structure organization to use for the Route Cache
in any DSR implementation is a local matter for each node and affects
only performance; any reasonable choice of organization for the Route
Cache does not affect either correctness or interoperability.
Each entry in the Route Cache SHOULD have a timeout associated with
it, to allow that entry to be deleted if not used within some time.
The particular choice of algorithm and data structure used to
implement the Route Cache SHOULD be considered in choosing the
timeout for entries in the Route Cache. The configuration variable
RouteCacheTimeout defined in Section 9 specifies the timeout to be
applied to entries in the Route Cache, although it is also possible
to instead use an adaptive policy in choosing timeout values rather
than using a single timeout setting for all entries. For example,
the Link-MaxLife cache design (below) uses an adaptive timeout
algorithm and does not use the RouteCacheTimeout configuration
variable.
As guidance to implementers, Appendix A describes a type of link
cache known as "Link-MaxLife" that has been shown to outperform other
types of link caches and path caches studied in detailed simulation
[HU00]. Link-MaxLife is an adaptive link cache in which each link in
the cache has a timeout that is determined dynamically by the caching
node according to its observed past behavior of the two nodes at the
ends of the link. In addition, when selecting a route for a packet
being sent to some destination, among cached routes of equal length
(number of hops) to that destination, Link-MaxLife selects the route
with the longest expected lifetime (highest minimum timeout of any
link in the route). Use of the Link-MaxLife design for the Route
Cache is recommended in implementations of DSR.
4.2. Send Buffer
The Send Buffer of a node implementing DSR is a queue of packets that cannot be sent by that node because it does not yet have a source route to each such packet's destination. Each packet in the Send Buffer is logically associated with the time that it was placed into the buffer and SHOULD be removed from the Send Buffer and silently discarded after a period of SendBufferTimeout after initially being placed in the buffer. If necessary, a FIFO strategy SHOULD be used to evict packets before they time out to prevent the buffer from overflowing. Subject to the rate limiting defined in Section 4.3, a Route Discovery SHOULD be initiated as often as allowed for the destination address of any packets residing in the Send Buffer.4.3. Route Request Table
The Route Request Table of a node implementing DSR records information about Route Requests that have been recently originated or forwarded by this node. The table is indexed by IP address. The Route Request Table on a node records the following information about nodes to which this node has initiated a Route Request: - The Time-to-Live (TTL) field used in the IP header of the Route Request for the last Route Discovery initiated by this node for that target node. This value allows the node to implement a variety of algorithms for controlling the spread of its Route Request on each Route Discovery initiated for a target. As examples, two possible algorithms for this use of the TTL field are described in Section 3.3.3. - The time that this node last originated a Route Request for that target node. - The number of consecutive Route Discoveries initiated for this target since receiving a valid Route Reply giving a route to that target node. - The remaining amount of time before which this node MAY next attempt at a Route Discovery for that target node. When the node initiates a new Route Discovery for this target node, this field in the Route Request Table entry for that target node is initialized to the timeout for that Route Discovery, after which the node MAY initiate a new Discovery for that target. Until a valid Route Reply is received for this target node address, a node MUST implement a back-off algorithm in determining this timeout
value for each successive Route Discovery initiated for this
target using the same Time-to-Live (TTL) value in the IP header of
the Route Request packet. The timeout between such consecutive
Route Discovery initiations SHOULD increase by doubling the
timeout value on each new initiation.
In addition, the Route Request Table on a node also records the
following information about initiator nodes from which this node has
received a Route Request:
- A FIFO cache of size RequestTableIds entries containing the
Identification value and target address from the most recent Route
Requests received by this node from that initiator node.
Nodes SHOULD use an LRU policy to manage the entries in their Route
Request Table.
The number of Identification values to retain in each Route Request
Table entry, RequestTableIds, MUST NOT be unlimited, since, in the
worst case, when a node crashes and reboots, the first
RequestTableIds Route Discoveries it initiates after rebooting could
appear to be duplicates to the other nodes in the network. In
addition, a node SHOULD base its initial Identification value, used
for Route Discoveries after rebooting, on a battery backed-up clock
or other persistent memory device, if available, in order to help
avoid any possible such delay in successfully discovering new routes
after rebooting; if no such source of initial Identification value is
available, a node after rebooting SHOULD base its initial
Identification value on a random number.
4.4. Gratuitous Route Reply Table
The Gratuitous Route Reply Table of a node implementing DSR records
information about "gratuitous" Route Replies sent by this node as
part of automatic route shortening. As described in Section 3.4.3, a
node returns a gratuitous Route Reply when it overhears a packet
transmitted by some node, for which the node overhearing the packet
was not the intended next-hop node but was named later in the
unexpended hops of the source route in that packet; the node
overhearing the packet returns a gratuitous Route Reply to the
original sender of the packet, listing the shorter route (not
including the hops of the source route "skipped over" by this
packet). A node uses its Gratuitous Route Reply Table to limit the
rate at which it originates gratuitous Route Replies to the same
original sender for the same node from which it overheard a packet to
trigger the gratuitous Route Reply.
Each entry in the Gratuitous Route Reply Table of a node contains the
following fields:
- The address of the node to which this node originated a gratuitous
Route Reply.
- The address of the node from which this node overheard the packet
triggering that gratuitous Route Reply.
- The remaining time before which this entry in the Gratuitous Route
Reply Table expires and SHOULD be deleted by the node. When a
node creates a new entry in its Gratuitous Route Reply Table, the
timeout value for that entry SHOULD be initialized to the value
GratReplyHoldoff.
When a node overhears a packet that would trigger a gratuitous Route
Reply, if a corresponding entry already exists in the node's
Gratuitous Route Reply Table, then the node SHOULD NOT send a
gratuitous Route Reply for that packet. Otherwise (i.e., if no
corresponding entry already exists), the node SHOULD create a new
entry in its Gratuitous Route Reply Table to record that gratuitous
Route Reply, with a timeout value of GratReplyHoldoff.
4.5. Network Interface Queue and Maintenance Buffer
Depending on factors such as the structure and organization of the
operating system, protocol stack implementation, network interface
device driver, and network interface hardware, a packet being
transmitted could be queued in a variety of ways. For example,
outgoing packets from the network protocol stack might be queued at
the operating system or link layer, before transmission by the
network interface. The network interface might also provide a
retransmission mechanism for packets, such as occurs in IEEE 802.11
[IEEE80211]; the DSR protocol, as part of Route Maintenance, requires
limited buffering of packets already transmitted for which the
reachability of the next-hop destination has not yet been determined.
The operation of DSR is defined here in terms of two conceptual data
structures that, together, incorporate this queuing behavior.
The Network Interface Queue of a node implementing DSR is an output
queue of packets from the network protocol stack waiting to be
transmitted by the network interface; for example, in the 4.4BSD Unix
network protocol stack implementation, this queue for a network
interface is represented as a "struct ifqueue" [WRIGHT95]. This
queue is used to hold packets while the network interface is in the
process of transmitting another packet.
The Maintenance Buffer of a node implementing DSR is a queue of packets sent by this node that are awaiting next-hop reachability confirmation as part of Route Maintenance. For each packet in the Maintenance Buffer, a node maintains a count of the number of retransmissions and the time of the last retransmission. Packets are added to the Maintenance buffer after the first transmission attempt is made. The Maintenance Buffer MAY be of limited size; when adding a new packet to the Maintenance Buffer, if the buffer size is insufficient to hold the new packet, the new packet SHOULD be silently discarded. If, after MaxMaintRexmt attempts to confirm next-hop reachability of some node, no confirmation is received, all packets in this node's Maintenance Buffer with this next-hop destination SHOULD be removed from the Maintenance Buffer. In this case, the node also SHOULD originate a Route Error for this packet to each original source of a packet removed in this way (Section 8.3) and SHOULD salvage each packet removed in this way (Section 8.3.6) if it has another route to that packet's IP Destination Address in its Route Cache. The definition of MaxMaintRexmt conceptually includes any retransmissions that might be attempted for a packet at the link layer or within the network interface hardware. The timeout value to use for each transmission attempt for an acknowledgement request depends on the type of acknowledgement mechanism used by Route Maintenance for that attempt, as described in Section 8.3.4.6. Blacklist
When a node using the DSR protocol is connected through a network interface that requires physically bidirectional links for unicast transmission, the node MUST maintain a blacklist. The blacklist is a table, indexed by neighbor node address, that indicates that the link between this node and the specified neighbor node may not be bidirectional. A node places another node's address in this list when it believes that broadcast packets from that other node reach this node, but that unicast transmission between the two nodes is not possible. For example, if a node forwarding a Route Reply discovers that the next hop is unreachable, it places that next hop in the node's blacklist. Once a node discovers that it can communicate bidirectionally with one of the nodes listed in the blacklist, it SHOULD remove that node from the blacklist. For example, if node A has node B listed in its blacklist, but after transmitting a Route Request, node A hears B forward the Route Request with a route record indicating that the broadcast from A to B was successful, then A SHOULD remove the entry for node B from its blacklist.
A node MUST associate a state with each node listed in its blacklist, specifying whether the unidirectionality of the link to that node is "questionable" or "probable". Each time the unreachability is positively determined, the node SHOULD set the state to "probable". After the unreachability has not been positively determined for some amount of time, the state SHOULD revert to "questionable". A node MAY expire entries for nodes from its blacklist after a reasonable amount of time.5. Additional Conceptual Data Structures for Flow State Extension
This section defines additional conceptual data structures used by the optional "flow state" extension to DSR. In an implementation of the protocol, these data structures MUST be implemented in a manner consistent with the external behavior described in this document, but the choice of implementation used is otherwise unconstrained.5.1. Flow Table
A node implementing the flow state extension MUST implement a Flow Table or other data structure consistent with the external behavior described in this section. A node not implementing the flow state extension SHOULD NOT implement a Flow Table. The Flow Table records information about flows from which packets recently have been sent or forwarded by this node. The table is indexed by a triple (IP Source Address, IP Destination Address, Flow ID), where Flow ID is a 16-bit number assigned by the source as described in Section 3.5.1. Each entry in the Flow Table contains the following fields: - The MAC address of the next-hop node along this flow. - An indication of the outgoing network interface on this node to be used in transmitting packets along this flow. - The MAC address of the previous-hop node along this flow. - An indication of the network interface on this node from which packets from that previous-hop node are received. - A timeout after which this entry in the Flow Table MUST be deleted. - The expected value of the Hop Count field in the DSR Flow State header for packets received for forwarding along this field (for use with packets containing a DSR Flow State header).
- An indication of whether or not this flow can be used as a default
flow for packets originated by this node (the Flow ID of a default
flow MUST be odd).
- The entry SHOULD record the complete source route for the flow.
(Nodes not recording the complete source route cannot participate
in Automatic Route Shortening.)
- The entry MAY contain a field recording the time this entry was
last used.
The entry MUST be deleted when its timeout expires.
5.2. Automatic Route Shortening Table
A node implementing the flow state extension SHOULD implement an
Automatic Route Shortening Table or other data structure consistent
with the external behavior described in this section. A node not
implementing the flow state extension SHOULD NOT implement an
Automatic Route Shortening Table.
The Automatic Route Shortening Table records information about
received packets for which Automatic Route Shortening may be
possible. The table is indexed by a triple (IP Source Address, IP
Destination Address, Flow ID). Each entry in the Automatic Route
Shortening Table contains a list of (packet identifier, Hop Count)
pairs for that flow. The packet identifier in the list may be any
unique identifier for the received packet; for example, for IPv4
packets, the combination of the following fields from the packet's IP
header MAY be used as a unique identifier for the packet: Source
Address, Destination Address, Identification, Protocol, Fragment
Offset, and Total Length. The Hop Count in the list in the entry is
copied from the Hop Count field in the DSR Flow State header of the
received packet for which this table entry was created. Any packet
identifier SHOULD appear at most once in an entry's list, and this
list item SHOULD record the minimum Hop Count value received for that
packet (if the wireless signal strength or signal-to-noise ratio at
which a packet is received is available to the DSR implementation in
a node, the node MAY, for example, remember instead in this list the
minimum Hop Count value for which the received packet's signal
strength or signal-to-noise ratio exceeded some threshold).
Space in the Automatic Route Shortening Table of a node MAY be
dynamically managed by any local algorithm at the node. For example,
in order to limit the amount of memory used to store the table, any
existing entry MAY be deleted at any time, and the number of packets
listed in each entry MAY be limited. However, when reclaiming space
in the table, nodes SHOULD favor retaining information about more
flows in the table rather than about more packets listed in each entry in the table, as long as at least the listing of some small number of packets (e.g., 3) can be retained in each entry.5.3. Default Flow ID Table
A node implementing the flow state extension MUST implement a Default Flow Table or other data structure consistent with the external behavior described in this section. A node not implementing the flow state extension SHOULD NOT implement a Default Flow Table. For each (IP Source Address, IP Destination Address) pair for which a node forwards packets, the node MUST record: - The largest odd Flow ID value seen. - The time at which all the corresponding flows that are forwarded by this node expire. - The current default Flow ID. - A flag indicating whether or not the current default Flow ID is valid. If a node deletes this record for an (IP Source Address, IP Destination Address) pair, it MUST also delete all Flow Table entries for that pair. Nodes MUST delete table entries if all of this (IP Source Address, IP Destination Address) pair's flows that are forwarded by this node expire.6. DSR Options Header Format
The Dynamic Source Routing protocol makes use of a special header carrying control information that can be included in any existing IP packet. This DSR Options header in a packet contains a small fixed- sized, 4-octet portion, followed by a sequence of zero or more DSR options carrying optional information. The end of the sequence of DSR options in the DSR Options header is implied by the total length of the DSR Options header. For IPv4, the DSR Options header MUST immediately follow the IP header in the packet. (If a Hop-by-Hop Options extension header, as defined in IPv6 [RFC2460], becomes defined for IPv4, the DSR Options header MUST immediately follow the Hop-by-Hop Options extension header, if one is present in the packet, and MUST otherwise immediately follow the IP header.)
To add a DSR Options header to a packet, the DSR Options header is inserted following the packet's IP header, before any following header such as a traditional (e.g., TCP or UDP) transport layer header. Specifically, the Protocol field in the IP header is used to indicate that a DSR Options header follows the IP header, and the Next Header field in the DSR Options header is used to indicate the type of protocol header (such as a transport layer header) following the DSR Options header. If any headers follow the DSR Options header in a packet, the total length of the DSR Options header (and thus the total, combined length of all DSR options present) MUST be a multiple of 4 octets. This requirement preserves the alignment of these following headers in the packet.6.1. Fixed Portion of DSR Options Header
The fixed portion of the DSR Options header is used to carry information that must be present in any DSR Options header. This fixed portion of the DSR Options header has the following format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Header |F| Reserved | Payload Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . Options . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Next Header 8-bit selector. Identifies the type of header immediately following the DSR Options header. Uses the same values as the IPv4 Protocol field [RFC1700]. If no header follows, then Next Header MUST have the value 59, "No Next Header" [RFC2460]. Flow State Header (F) Flag bit. MUST be set to 0. This bit is set in a DSR Flow State header (Section 7.1) and clear in a DSR Options header. Reserved MUST be sent as 0 and ignored on reception.
Payload Length
The length of the DSR Options header, excluding the 4-octet
fixed portion. The value of the Payload Length field defines
the total length of all options carried in the DSR Options
header.
Options
Variable-length field; the length of the Options field is
specified by the Payload Length field in this DSR Options
header. Contains one or more pieces of optional information
(DSR options), encoded in type-length-value (TLV) format (with
the exception of the Pad1 option described in Section 6.8).
The placement of DSR options following the fixed portion of the DSR
Options header MAY be padded for alignment. However, due to the
typically limited available wireless bandwidth in ad hoc networks,
this padding is not required, and receiving nodes MUST NOT expect
options within a DSR Options header to be aligned.
Each DSR option is assigned a unique Option Type code. The most
significant 3 bits (that is, Option Type & 0xE0) allow a node not
implementing processing for this Option Type value to behave in the
manner closest to correct for that type:
- The most significant bit in the Option Type value (that is, Option
Type & 0x80) represents whether or not a node receiving this
Option Type (when the node does not implement processing for this
Option Type) SHOULD respond to such a DSR option with a Route
Error of type OPTION_NOT_SUPPORTED, except that such a Route Error
SHOULD never be sent in response to a packet containing a Route
Request option.
- The two following bits in the Option Type value (that is, Option
Type & 0x60) are a two-bit field indicating how such a node that
does not support this Option Type MUST process the packet:
00 = Ignore Option
01 = Remove Option
10 = Mark Option
11 = Drop Packet
When these 2 bits are 00 (that is, Option Type & 0x60 == 0), a
node not implementing processing for that Option Type MUST use the
Opt Data Len field to skip over the option and continue
processing. When these 2 bits are 01 (that is, Option Type & 0x60
== 0x20), a node not implementing processing for that Option Type
MUST use the Opt Data Len field to remove the option from the
packet and continue processing as if the option had not been
included in the received packet. When these 2 bits are 10 (that
is, Option Type & 0x60 == 0x40), a node not implementing
processing for that Option Type MUST set the most significant bit
following the Opt Data Len field, MUST ignore the contents of the
option using the Opt Data Len field, and MUST continue processing
the packet. Finally, when these 2 bits are 11 (that is, Option
Type & 0x60 == 0x60), a node not implementing processing for that
Option Type MUST drop the packet.
The following types of DSR options are defined in this document for
use within a DSR Options header:
- Route Request option (Section 6.2)
- Route Reply option (Section 6.3)
- Route Error option (Section 6.4)
- Acknowledgement Request option (Section 6.5)
- Acknowledgement option (Section 6.6)
- DSR Source Route option (Section 6.7)
- Pad1 option (Section 6.8)
- PadN option (Section 6.9)
In addition, Section 7 specifies further DSR options for use with the
optional DSR flow state extension.
6.2. Route Request Option
The Route Request option in a DSR Options header is encoded as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type | Opt Data Len | Identification | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Target Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address[1] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address[2] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address[n] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ IP fields: Source Address MUST be set to the address of the node originating this packet. Intermediate nodes that retransmit the packet to propagate the Route Request MUST NOT change this field. Destination Address MUST be set to the IP limited broadcast address (255.255.255.255). Hop Limit (TTL) MAY be varied from 1 to 255, for example, to implement non- propagating Route Requests and Route Request expanding-ring searches (Section 3.3.3). Route Request fields: Option Type 1. Nodes not understanding this option will ignore this option.
Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len fields. MUST be set
equal to (4 * n) + 6, where n is the number of addresses in the
Route Request Option.
Identification
A unique value generated by the initiator (original sender) of
the Route Request. Nodes initiating a Route Request generate a
new Identification value for each Route Request, for example
based on a sequence number counter of all Route Requests
initiated by the node.
This value allows a receiving node to determine whether it has
recently seen a copy of this Route Request. If this
Identification value (for this IP Source address and Target
Address) is found by this receiving node in its Route Request
Table (in the cache of Identification values in the entry there
for this initiating node), this receiving node MUST discard the
Route Request. When a Route Request is propagated, this field
MUST be copied from the received copy of the Route Request
being propagated.
Target Address
The address of the node that is the target of the Route
Request.
Address[1..n]
Address[i] is the IPv4 address of the i-th node recorded in the
Route Request option. The address given in the Source Address
field in the IP header is the address of the initiator of the
Route Discovery and MUST NOT be listed in the Address[i]
fields; the address given in Address[1] is thus the IPv4
address of the first node on the path after the initiator. The
number of addresses present in this field is indicated by the
Opt Data Len field in the option (n = (Opt Data Len - 6) / 4).
Each node propagating the Route Request adds its own address to
this list, increasing the Opt Data Len value by 4 octets.
The Route Request option MUST NOT appear more than once within a DSR
Options header.
6.3. Route Reply Option
The Route Reply option in a DSR Options header is encoded as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type | Opt Data Len |L| Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address[1] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address[2] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address[n] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ IP fields: Source Address Set to the address of the node sending the Route Reply. In the case of a node sending a reply from its Route Cache (Section 3.3.2) or sending a gratuitous Route Reply (Section 3.4.3), this address can differ from the address that was the target of the Route Discovery. Destination Address MUST be set to the address of the source node of the route being returned. Copied from the Source Address field of the Route Request generating the Route Reply or, in the case of a gratuitous Route Reply, copied from the Source Address field of the data packet triggering the gratuitous Reply. Route Reply fields: Option Type 2. Nodes not understanding this option will ignore this option.
Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len fields. MUST be set
equal to (4 * n) + 1, where n is the number of addresses in the
Route Reply Option.
Last Hop External (L)
Set to indicate that the last hop given by the Route Reply (the
link from Address[n-1] to Address[n]) is actually an arbitrary
path in a network external to the DSR network; the exact route
outside the DSR network is not represented in the Route Reply.
Nodes caching this hop in their Route Cache MUST flag the
cached hop with the External flag. Such hops MUST NOT be
returned in a cached Route Reply generated from this Route
Cache entry, and selection of routes from the Route Cache to
route a packet being sent SHOULD prefer routes that contain no
hops flagged as External.
Reserved
MUST be sent as 0 and ignored on reception.
Address[1..n]
The source route being returned by the Route Reply. The route
indicates a sequence of hops, originating at the source node
specified in the Destination Address field of the IP header of
the packet carrying the Route Reply, through each of the
Address[i] nodes in the order listed in the Route Reply, ending
at the node indicated by Address[n]. The number of addresses
present in the Address[1..n] field is indicated by the Opt Data
Len field in the option (n = (Opt Data Len - 1) / 4).
A Route Reply option MAY appear one or more times within a DSR
Options header.
6.4. Route Error Option
The Route Error option in a DSR Options header is encoded as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type | Opt Data Len | Error Type |Reservd|Salvage| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Error Source Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Error Destination Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . Type-Specific Information . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Option Type 3. Nodes not understanding this option will ignore this option. Opt Data Len 8-bit unsigned integer. Length of the option, in octets, excluding the Option Type and Opt Data Len fields. For the current definition of the Route Error option, this field MUST be set to 10, plus the size of any Type-Specific Information present in the Route Error. Further extensions to the Route Error option format may also be included after the Type-Specific Information portion of the Route Error option specified above. The presence of such extensions will be indicated by the Opt Data Len field. When the Opt Data Len is greater than that required for the fixed portion of the Route Error plus the necessary Type-Specific Information as indicated by the Option Type value in the option, the remaining octets are interpreted as extensions. Currently, no such further extensions have been defined. Error Type The type of error encountered. Currently, the following type values are defined:
1 = NODE_UNREACHABLE
2 = FLOW_STATE_NOT_SUPPORTED
3 = OPTION_NOT_SUPPORTED
Other values of the Error Type field are reserved for future
use.
Reservd
Reserved. MUST be sent as 0 and ignored on reception.
Salvage
A 4-bit unsigned integer. Copied from the Salvage field in the
DSR Source Route option of the packet triggering the Route
Error.
The "total salvage count" of the Route Error option is derived
from the value in the Salvage field of this Route Error option
and all preceding Route Error options in the packet as follows:
the total salvage count is the sum of, for each such Route
Error option, one plus the value in the Salvage field of that
Route Error option.
Error Source Address
The address of the node originating the Route Error (e.g., the
node that attempted to forward a packet and discovered the link
failure).
Error Destination Address
The address of the node to which the Route Error must be
delivered. For example, when the Error Type field is set to
NODE_UNREACHABLE, this field will be set to the address of the
node that generated the routing information claiming that the
hop from the Error Source Address to Unreachable Node Address
(specified in the Type-Specific Information) was a valid hop.
Type-Specific Information
Information specific to the Error Type of this Route Error
message.
A Route Error option MAY appear one or more times within a DSR
Options header.
6.4.1. Node Unreachable Type-Specific Information
When the Route Error is of type NODE_UNREACHABLE, the Type-Specific Information field is defined as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Unreachable Node Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Unreachable Node Address The IP address of the node that was found to be unreachable (the next-hop neighbor to which the node with address Error Source Address was attempting to transmit the packet).6.4.2. Flow State Not Supported Type-Specific Information
When the Route Error is of type FLOW_STATE_NOT_SUPPORTED, the Type-Specific Information field is empty.6.4.3. Option Not Supported Type-Specific Information
When the Route Error is of type OPTION_NOT_SUPPORTED, the Type-Specific Information field is defined as follows: 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |Unsupported Opt| +-+-+-+-+-+-+-+-+ Unsupported Opt The Option Type of option triggering the Route Error.6.5. Acknowledgement Request Option
The Acknowledgement Request option in a DSR Options header is encoded as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type | Opt Data Len | Identification | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
160. Nodes not understanding this option will remove the
option and return a Route Error.
Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len fields.
Identification
The Identification field is set to a unique value and is copied
into the Identification field of the Acknowledgement option
when returned by the node receiving the packet over this hop.
An Acknowledgement Request option MUST NOT appear more than once
within a DSR Options header.
6.6. Acknowledgement Option
The Acknowledgement option in a DSR Options header is encoded as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Opt Data Len | Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACK Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACK Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
32. Nodes not understanding this option will remove the
option.
Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len fields.
Identification
Copied from the Identification field of the Acknowledgement
Request option of the packet being acknowledged.
ACK Source Address
The address of the node originating the acknowledgement.
ACK Destination Address
The address of the node to which the acknowledgement is to be
delivered.
An Acknowledgement option MAY appear one or more times within a DSR
Options header.
6.7. DSR Source Route Option
The DSR Source Route option in a DSR Options header is encoded as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Opt Data Len |F|L|Reservd|Salvage| Segs Left |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[2] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[n] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
96. Nodes not understanding this option will drop the packet.
Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len fields. For the
format of the DSR Source Route option defined here, this field
MUST be set to the value (n * 4) + 2, where n is the number of
addresses present in the Address[i] fields.
First Hop External (F)
Set to indicate that the first hop indicated by the DSR Source
Route option is actually an arbitrary path in a network
external to the DSR network; the exact route outside the DSR
network is not represented in the DSR Source Route option.
Nodes caching this hop in their Route Cache MUST flag the
cached hop with the External flag. Such hops MUST NOT be
returned in a Route Reply generated from this Route Cache
entry, and selection of routes from the Route Cache to route a
packet being sent SHOULD prefer routes that contain no hops
flagged as External.
Last Hop External (L)
Set to indicate that the last hop indicated by the DSR Source
Route option is actually an arbitrary path in a network
external to the DSR network; the exact route outside the DSR
network is not represented in the DSR Source Route option.
Nodes caching this hop in their Route Cache MUST flag the
cached hop with the External flag. Such hops MUST NOT be
returned in a Route Reply generated from this Route Cache
entry, and selection of routes from the Route Cache to route a
packet being sent SHOULD prefer routes that contain no hops
flagged as External.
Reserved
MUST be sent as 0 and ignored on reception.
Salvage
A 4-bit unsigned integer. Count of number of times that this
packet has been salvaged as a part of DSR routing (Section
3.4.1).
Segments Left (Segs Left)
Number of route segments remaining, i.e., number of explicitly
listed intermediate nodes still to be visited before reaching
the final destination.
Address[1..n]
The sequence of addresses of the source route. In routing and
forwarding the packet, the source route is processed as
described in Sections 8.1.3 and 8.1.5. The number of addresses
present in the Address[1..n] field is indicated by the Opt Data
Len field in the option (n = (Opt Data Len - 2) / 4).
When forwarding a packet along a DSR source route using a DSR Source
Route option in the packet's DSR Options header, the Destination
Address field in the packet's IP header is always set to the address
of the packet's ultimate destination. A node receiving a packet containing a DSR Options header with a DSR Source Route option MUST examine the indicated source route to determine if it is the intended next-hop node for the packet and how to forward the packet, as defined in Sections 8.1.4 and 8.1.5.6.8. Pad1 Option
The Pad1 option in a DSR Options header is encoded as follows: +-+-+-+-+-+-+-+-+ | Option Type | +-+-+-+-+-+-+-+-+ Option Type 224. Nodes not understanding this option will drop the packet and return a Route Error. A Pad1 option MAY be included in the Options field of a DSR Options header in order to align subsequent DSR options, but such alignment is not required and MUST NOT be expected by a node receiving a packet containing a DSR Options header. If any headers follow the DSR Options header in a packet, the total length of a DSR Options header, indicated by the Payload Length field in the DSR Options header MUST be a multiple of 4 octets. In this case, when building a DSR Options header in a packet, sufficient Pad1 or PadN options MUST be included in the Options field of the DSR Options header to make the total length a multiple of 4 octets. If more than one consecutive octet of padding is being inserted in the Options field of a DSR Options header, the PadN option described next, SHOULD be used, rather than multiple Pad1 options. Note that the format of the Pad1 option is a special case; it does not have an Opt Data Len or Option Data field.6.9. PadN Option
The PadN option in a DSR Options header is encoded as follows: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - | Option Type | Opt Data Len | Option Data +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
Option Type
0. Nodes not understanding this option will ignore this
option.
Opt Data Len
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Opt Data Len fields. The size of
the Option Data field.
Option Data
A number of zero-valued octets equal to the Opt Data Len.
A PadN option MAY be included in the Options field of a DSR Options
header in order to align subsequent DSR options, but such alignment
is not required and MUST NOT be expected by a node receiving a packet
containing a DSR Options header.
If any headers follow the DSR Options header in a packet, the total
length of a DSR Options header, indicated by the Payload Length field
in the DSR Options header, MUST be a multiple of 4 octets. In this
case, when building a DSR Options header in a packet, sufficient Pad1
or PadN options MUST be included in the Options field of the DSR
Options header to make the total length a multiple of 4 octets.
(page 51 continued on part 3)
