| Network Working Group | C. Perkins |
| Internet-Draft | Futurewei |
| Intended status: Standards Track | S. Ratliff |
| Expires: September 1, 2019 | Idirect |
| J. Dowdell | |
| Airbus Defence and Space | |
| L. Steenbrink | |
| Freie Universitaet Berlin | |
| V. Pritchard | |
| Airbus Defence and Space | |
| February 28, 2019 |
Ad Hoc On-demand Distance Vector Version 2 (AODVv2) Routing
draft-perkins-manet-aodvv2-03
The Ad Hoc On-demand Distance Vector Version 2 (AODVv2) routing protocol is intended for use by mobile routers in wireless, multihop networks. AODVv2 determines unicast routes among AODVv2 routers within the network in an on-demand fashion.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 1, 2019.
Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
The Ad hoc On-Demand Distance Vector Version 2 (AODVv2) protocol enables dynamic, multihop routing between participating mobile routers wishing to establish and maintain an ad hoc network. The basic operations of the AODVv2 protocol are route discovery and route maintenance. AODVv2 does not require nodes to maintain routes to destinations that are not in active communication. AODVv2 allows mobile nodes to respond to link breakages and changes in network connectivity.
AODVv2 is a distance-vector routing protocol, which means that routes are stored with information about the next hop (vector for forwarding) and the metric or cost of using the route (a "distance" to the destination along that route). Typically, if multiple routes to a particular destination are available to be selected, the route with the least cost (e.g., shortest distance) is chosen for the purpose of forwarding packets to that destination. Distance-vector (Bellman-Ford) routing protocols were historically vulnerable to what became known as the "counting to infinity" problem. That problem causes routes to grow quickly worse over time, and results from a router's inability to detect out-of-sequence routing updates and subsequent retransmission of stale information. AODVv2 uses sequence numbers to assure detection of stale routing information, as was done previously in DSDV and as described in [Perkins94]. The operation of AODVv2 is consequently loop-free and offers quick convergence when the ad hoc network topology changes (typically, when a node moves in the network). When links break, AODVv2 causes the affected set of nodes to be notified so that they are able to invalidate the routes using the broken link.
AODVv2 stores a destination sequence number for each route entry. The destination sequence number is created by the destination to be included along with any route information it sends to requesting nodes. Using destination sequence numbers ensures loop freedom and is simple to program. Given the choice between two routes to a destination, a requesting node is required to select the one with the greatest sequence number.
The basic protocol mechanisms are as follows. AODVv2 is a reactive protocol, meaning that route discovery is initiated only when a route to the target is needed (i.e. when a router's client has data to send (see Section 4.2)). For this purpose, AODVv2 uses Route Request (RREQ) and Route Reply (RREP) messages as follows: an RREQ is distributed across the network. Routers (other than the target) receiving the RREQ retransmit it and also store a reverse route back to the originator of the RREQ.
: / \ :
| / \| \
- S ----- A ---... \ M-----D
| / \ /-----J---... /
: / \ / / /|\ /
: -B-------/ / : \ /
| K-----L--....
: | /|
Figure 1: Flooding a RREQ through the network
In figure Figure 1, node 'S' needs to discover a route to a desired destination node 'D'. S generates a RREQ to be flooded throughout the network. The RREQ traverses nodes A, B, J, K, L, and M before finally arriving at the target node D. The RREQ will also most likely be received and retransmitted by many other nodes in the network. Each intermediate node that receives the RREQ stores a reverse route back to node S so that communication between S and D can be established in case that intermediate node receives a RREP in response to the RREQ.
When the target receives the RREQ, it answers with an RREP, which is then relayed back to the originator along the path stored by the intermediate routers. A metric value is included within the messages to indicate the cost of the route.
S <--- A <--- B <--- J <--- K <--- L <--- M <--- D
Figure 2: D sends a RREP back to the node S
In figure Figure 2, node 'D' transmits a RREP back towards S. Node M has stored a route back to S by way of node L. Node L has stored a route back to S by way of node K, and so on. Also, each node that receives the RREP uses the information in the RREP to store a route to node 'D'. For instance, the RREP contains the metric describing the cost of the route from an intermediate node X to node D, as well as information about the next hop towards D -- namely, the node which transmitted the RREP to X.
AODVv2 uses sequence numbers as described above to identify stale routing information, and compares route metric values to determine if advertised routes could form loops. Route maintenance includes confirming bidirectionality of links to next-hop AODVv2 routers, managing route timeouts, using Route Error (RERR) messages to inform other routers of broken links, and reacting to received Route Error messages.
AODVv2 requires indications to be exchanged between AODVv2 and the forwarding subsystem for the following conditions:
Security for authentication of AODVv2 routers and encryption of control messages is accomplished using the TIMESTAMP and ICV TLVs defined in [RFC7182].
AODVv2 operates in a fashion very similar to AODV[RFC3561]. The mechanism for route discovery is basically the same, so that similar performance results can be expected. Compared to AODV, AODVv2 has moved some features out of the scope of the document, notably intermediate route replies, and expanding ring search. However, this document has been designed to allow specification of those features in a separate document.
AODVv2 control messages are defined as sets of data. As described in Section 8, these sets of data can be mapped to message elements using the Generalized MANET Packet/Message Format defined in [RFC5444] and sent using the parameters in [RFC5498]. Additional refinements have been made for route timeouts and state management. Other mappings for the sets of data for the control messages are possible.
Compared to AODV[RFC3561], the following protocol mechanisms have changed:
AODVv2 has not been designed to be interoperable with AODV. However, it would be straightforward to allow both protocols to be used in the same ad hoc network as long as compatible metrics were used.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. The names of the protocol messages in AODVv2 are chosen to conform to the names of the similar protocol messages in [RFC3561]. In addition, this document uses terminology from [RFC5444], and defines the following terms:
This document uses the notational conventions in Table 1 to simplify the text.
| Notation | Meaning |
|---|---|
| Route[Address] | A route toward Address |
| Route[Address].Field | A field in a route toward Address |
| McMsg.Field | A field in a Multicast Message entry |
| RteMsg.Field | A field in either RREQ or RREP |
| RerrMsg.Field | A field in a RERR |
The AODVv2 routing protocol is a reactive routing protocol designed for use in mobile ad hoc wireless networks, and may also be useful in networks where the nodes are not mobile but economical route maintenance is still required. A reactive protocol only sends messages to discover a route when there is data to send on that route. This requires an interaction with the forwarding plane, to indicate when a packet is to be forwarded, in case reactive route discovery is needed. The set of signals exchanged between AODVv2 and the forwarding plane are discussed in Section 6.4.
AODVv2 is designed for stub or disconnected mobile ad hoc networks, i.e., non-transit networks or those not connected to the internet. AODVv2 routers can, however, be configured to perform gateway functions when attached to external networks, as discussed in Section 9.
AODVv2 handles a wide variety of mobility and traffic patterns by determining routes on-demand. In networks with a large number of routers, AODVv2 is best suited for relatively sparse traffic scenarios where each router forwards IP packets to a small percentage of destination addresses in the network. In such cases fewer routes are needed, and far less control traffic is produced. In large networks with dense traffic patterns, AODVv2 control messages may cause a broadcast storm, overwhelming the network with control messages. The transmission priorities described in Section 6.5 prioritize route maintenance traffic over route discovery traffic.
Data packets may be buffered until a route to their destination is available, as described in Section 6.6.
AODVv2 is well suited to reactive scenarios such as emergency and disaster relief, where the ability to communicate might be more important than being assured of secure operations. For many other ad hoc networking applications, in which insecure operation could negate the value of establishing communication paths, it is important for neighboring AODVv2 routers to establish security associations with one another.
AODVv2 provides for message integrity and security against replay attacks by using integrity check values, timestamps and sequence numbers, as described in Section 14. When security associations have been established, encryption can be used for AODVv2 messages to ensure that only trusted routers participate in routing operations.
The AODVv2 route discovery process aims for a route to be established in both directions along the same path. Uni-directional links are not suitable; AODVv2 will detect and exclude those links from route discovery. The route discovered is optimized for the requesting router, and the return path may not be the optimal route.
AODVv2 is applicable to memory constrained devices, since only a little routing state is maintained in each AODVv2 router. AODVv2 routes that are not needed for forwarding data do not need to be maintained.
AODVv2 supports routers with multiple interfaces and multiple IP addresses per interface. A router may also use the same IP address on multiple interfaces. AODVv2 requires only that each interface configured for AODVv2 has at least one unicast IP address (see Section 4.1). Address assignment procedures are out of scope for AODVv2.
AODVv2 supports Router Clients with multiple interfaces, as long as each Client interface is configured with its own unicast IP address.
The routing algorithm in AODVv2 has been operated at layers other than the network layer, using layer-appropriate addresses.
AODVv2 has to maintain information about all network interfaces configured for sending or receiving AODVv2 messages. Any interface with an IP address can be used. Multiple interfaces on a single router can be used. Multiple interfaces on the same router may be configured with the same IP address; in this case, sufficient information about those network interfaces has to be maintained by the AODVv2 router in order to determine which of them has received an incoming AODVv2 message. For instance, it is often possible to determine the incoming interface by inspecting the MAC address of the layer-2 frame header.
An AODVv2 router discovers routes for its own local applications and also for its Router Clients that are reachable without traversing another AODVv2 router. The addresses used by these devices, and the AODVv2 router itself, are configured in the Router Client Set. An AODVv2 router will only originate Route Request and Route Reply messages on behalf of its configured Router Client addresses.
Router Client Set entries are configured manually or by mechanisms out of scope for this document. Each client's entry contains:
A Neighbor Set is used to maintain information about neighboring AODVv2 routers. Neighbor Set entries are stored when AODVv2 messages are received. If the Neighbor is chosen as a next hop on an installed route, the link to the Neighbor is tested for bidirectionality; the result is stored in the Neighbor Set.
Neighbor Set entries MUST contain:
See Section 11.3 and Section 11.4 for more information on how Neighbor.AckSeqNum and Neighbor.HeardRERRSeqNum are used.
Sequence Numbers enable AODVv2 routers to determine the temporal order of route discovery messages that originate from a AODVv2 router, and thus to identify stale routing information so that it can be discarded. The sequence number fulfills the same roles as the "Destination Sequence Number" of DSDV [Perkins94], and the AODV Sequence Number in [RFC3561]. The sequence numbers from two different routers are not comparable; route discovery messages with sequence numbers belonging to two different routers cannot be compared to determine temporal ordering.
Each AODVv2 router in the network MUST maintain its own sequence number. All RREQ and RREP messages created by an AODVv2 router include the router's sequence number, reported as a 16-bit unsigned integer. Each AODVv2 router MUST ensure that its sequence number is strictly increasing, and that it is incremented by one (1) whenever an RREQ or RREP is created, except when the sequence number is 65,535 (the maximum value of a 16-bit unsigned integer), in which case it MUST be reset to one (1) to achieve wrap around. The value zero (0) is reserved to indicate that the router's sequence number is unknown.
An AODVv2 router MUST use its sequence number only on behalf of its configured Router Clients; route messages forwarded by other routers retain the originator's sequence number.
To determine if newly received information is stale and therefore redundant compared to other information originated by the same router, the sequence number attached to the information is compared to the sequence number of existing information about the same route. The comparison is carried out by subtracting the existing sequence number from the newly received sequence number, using unsigned arithmetic. The result of the subtraction is to be interpreted as a signed 16-bit integer.
Along with the algorithm in Section 6.7.1, maintaining temporal ordering ensures loop freedom.
An AODVv2 router SHOULD maintain its sequence number in persistent storage. On routers unable to store persistent AODVv2 state, recovery can impose a performance penalty (e.g., in case of AODVv2 router reboot), since if a router loses its sequence number, there is a delay (by default, on the order of minutes) before the router can resume full operations. If the sequence number is lost, the router MUST follow the procedure in Section 6.1 to safely resume routing operations with a new sequence number.
All AODVv2 routers MUST maintain a Local Route Set, containing information obtained from AODVv2 route messages. The Local Route Set may be considered to be stored separately from the forwarding plane's routing table (referred to as Routing Information Base (RIB)), which may be updated by other routing protocols operating on the AODVv2 router as well. The Routing Information Base is updated using information from the Local Route Set. Alternatively, if the information specified below can be added to RIB entries, implementations MAY choose to modify the Routing Information Base directly instead of maintaining a dedicated Local Route Set. In this case, since the route table entry is accessed whenever a packet uses the route, the LastUsed (see below) field can be tested to determine the state of the route, including whether or not the route has timed out. For example, if CurrentTime is less than LocalRoute.LastUsed + ACTIVE_INTERVAL + MAX_IDLETIME, a valid route can still be used to forward the packet. Otherwise, the route has timed out, and the state of the route MUST be changed to be Invalid. When this method of managing route timeouts can be used, AODVv2 does not otherwise require the implementation to maintain a timer interrupt. This may be considered an "on-demand" method for managing route timeouts.
Routes obtained from AODVv2 route messages are referred to in this document as LocalRoutes, and MUST contain the following information:
There are four possible states for a LocalRoute:
If the Local Route Set is stored separately from the RIB, then routes are added to the RIB when LocalRoute.State becomes Active, and removed from the RIB when LocalRoute.State becomes Invalid. Changes to LocalRoute state are detailed in Section 6.10.1.
Multicast RREQ messages SHOULD be tested for redundancy to avoid unnecessary processing and forwarding.
The Multicast Message Set is a conceptual set which contains information about previously received multicast messages, so that incoming messages can be compared with previously received messages to determine if the incoming information is redundant or stale, so that the router can avoid sending redundant control traffic.
Multicast Message Set entries contain the following information:
The Multicast Message Set is maintained so that no two entries have the same OrigPrefix, OrigPrefixLen, TargPrefix, and MetricType. See Section 6.8 for details about updating this set.
Each RERR message sent because no route exists for packet forwarding SHOULD be recorded in a conceptual set called the Route Error (RERR) Set. Each entry contains the following information:
See Section 6.9 for instructions on how to update the set.
Metrics measure a cost or quality associated with a route or a link, e.g., latency, delay, financial cost, energy, etc. Metric values are reported in Route Request and Route Reply messages.
In Route Request messages, the metric describes the cost of the route from OrigPrefix to the router transmitting the Route Request. For RREQ_Gen, this is the cost associated with the Router Client Set entry which includes OrigAddr. For routers which forward the RREQ, this is the cost from OrigPrefix to the forwarding router, combining the metric value from the received RREQ message with knowledge of the link cost from the sender to the receiver, i.e., the incoming link cost. This updated route cost is included when forwarding the Route Request message, and used to install a route to OrigPrefix.
Similarly, in Route Reply messages, the metric reflects the cost of the route from TargPrefix to the router transmitting the Route Reply. For RREP_Gen, this is the cost associated with the Router Client Set entry which includes TargAddr. For routers which forward the RREP, this is the cost from TargPrefix to the forwarding router, combining the metric value from the received RREP message with knowledge of the link cost from the sender to the receiver, i.e., the incoming link cost. This updated route cost is included when forwarding the Route Reply message, and used to install a route to TargPrefix.
When link metrics are symmetric, the cost of the routes installed in the Local Route Set at each router will be correct. This assumption is often inexact, but calculating incoming/outgoing metric data is outside of scope of this document. The route discovered is good for the requesting router, but the return path may not be the optimal route.
AODVv2 enables the use of multiple metric types. Each route discovery attempt indicates the metric type which is requested for the route. Multiple valid routes may exist in the Local Route Set for the same address and prefix length but for different metric types. More than one route to a particular address and prefix length MUST NOT exist in the Routing Information Base unless each packet can be inspected to determine which route in the RIB has the proper metric type as required for that packet. Otherwise, only one route at a time to a particular address and prefix length may exist in the RIB. The algorithm used to inspect the packet and make the determination about which the routes should be installed in the Routing Information Base is outside the scope of AODVv2.
For each MetricType, AODVv2 requires:
AODVv2 currently supports cost metrics where Cost(R) is strictly increasing, by defining:
Implementers MAY consider metric types that are not strictly increasing, but the definitions of Cost and LoopFree functions for such types are undefined, and interoperability issues need to be considered.
AODVv2 protocol operations include:
These processes are discussed in detail in the following sections.
When an AODVv2 router does not have information about its previous sequence number, or if its sequence number is lost at any point, the router reinitializes its sequence number to one (1). However, other AODVv2 routers may still hold sequence number information that this router previously issued. Since sequence number information is removed if there has been no update to the sequence number in MAX_SEQNUM_LIFETIME, the re-initializing router MUST wait for MAX_SEQNUM_LIFETIME before it creates any messages containing its new sequence number. Nevertheless, the re-initializing router can still participate in creating routes as an intermediate router.
During this wait period, the router is permitted to do the following:
To ensure AODVv2 routers do not establish routes over uni-directional links, AODVv2 routers MUST verify that the link to the next hop router is bidirectional before marking a route as valid in the Local Route Set.
AODVv2 provides a mechanism for testing bidirectional connectivity during route discovery, and blacklisting routers where bidirectional connectivity is not available. AODVv2 treats blacklisted routers as ineligible to be receivers of RREPs; then, a route through a different neighbor might be discovered. A route is not to be used for forwarding until the link to the next hop is confirmed to be bidirectional. AODVv2 routers do not need to monitor bidirectionality of links to neighboring routers which are not used as next hops on routes in the Local Route Set.
To assist with next hop monitoring, a Neighbor Set (Section 4.3) is maintained. When an RREQ or RREP is received, an AODVv2 router searches for an entry in the Neighbor Set where all of the following conditions are met:
If no such entry exists, a new entry is created as described in Section 6.3. While the value of Neighbor.State is HEARD, acknowledgement of RREP messages sent to that neighbor MUST be requested. If an acknowledgement is requested but not received within the timeout period, Neighbor.State for that neighbor MUST be set to BLACKLISTED. If an acknowledgement is received within the timeout period, Neighbor.State is set to CONFIRMED. When the value of Neighbor.State is CONFIRMED, the request for an acknowledgement of any other RREP message is unnecessary. AODVv2 does not require periodic or continuous recertification of bidirectionality.
There are many external mechanisms that can provide indications of connectivity. Among them are the following:
If the MAC layer, or such an external process as listed above, signals that the link to a neighbor is bidirectional, the AODVv2 router MAY update the matching Neighbor Set entry by changing the value of Neighbor.State to CONFIRMED. If an external process signals that a link is not bidirectional, then the value of Neighbor.State MAY be changed to BLACKLISTED.
On receipt of an RREQ or RREP message, the Neighbor Set MUST be checked for an entry with Neighbor.IPAddress which matches the source IP address of a packet containing the AODVv2 message. If no matching entry is found, a new entry is created.
A new Neighbor Set entry is created as follows:
When an RREP_Ack request is sent to a neighbor, the Neighbor Set entry is updated as follows:
When a received message is one of the following:
then the link to the neighbor is bidirectional and the Neighbor Set entry is updated as follows:
If the Neighbor.Timeout is reached and Neighbor.State is HEARD, then an RREP_Ack response has not been received from the neighbor within RREP_Ack_SENT_TIMEOUT of sending the RREP_Ack request. The link is considered to be uni-directional and the Neighbor Set entry is updated as follows:
When the Neighbor.Timeout is reached and Neighbor.State is BLACKLISTED, the Neighbor Set entry is updated as follows:
If an external mechanism reports a link as broken, the Neighbor Set entry MUST be removed.
Route requests (RREQs) from neighbors with Neighbor.State set to BLACKLISTED MUST be ignored, to avoid persistent IP packet loss or protocol failures. Neighbor.Timeout allows the neighbor to again be allowed to participate in route discoveries after MAX_BLACKLIST_TIME, in case the link between the routers has become bidirectional.
The signals described in the following are conceptual in nature, and can be implemented in various ways. The following descriptions of these signals and interactions are intended to assist implementers who may find them useful. Implementations of AODVv2 do not have to implement the forwarding plane separately from the control plane or data plane. By keeping track of success or failure of packet forwarding, AODV can avoid unnecessary route discovery operations, while still invalidating routes as early as possible to avoid transmitting packets over failed routes.
AODVv2 makes use of the following signals:
AODVv2 sends signals to the conceptual forwarding plane when:
AODVv2 sends [RFC5444] formatted messages using the parameters for port number and IP protocol specified in [RFC5498]. Mapping of AODVv2 data to [RFC5444] messages is detailed in Section 8. AODVv2 multicast messages are sent to the link-local multicast address LL-MANET-Routers [RFC5498]. All AODVv2 routers MUST subscribe to LL-MANET-Routers on all AODVv2 interfaces [RFC5498] to receive AODVv2 messages. Such messages MAY be transmitted via unicast. For example, this may occur for certain link-types (non-broadcast media), for manually configured router adjacencies, or in order to improve robustness.
When multiple interfaces are available, an AODVv2 router transmitting a multicast message to LL-MANET-Routers MUST send the message on all interfaces that have been configured for AODVv2 operation, (see Section 4.1).
To avoid congestion, each AODVv2 router's rate of message generation SHOULD be administratively configurable and rate-limited (CONTROL_TRAFFIC_LIMIT). Messages SHOULD NOT be sent more frequently than one message per (1 / CONTROL_TRAFFIC_LIMIT)th of a second. If this threshold is reached, messages MUST be sent based on their priority:
To implement the congestion control, a queue length is set. If the queue is full, in order to queue a new message, a message of lower priority must be removed from the queue. If this is not possible, the new message MUST be discarded. The queue should be sorted in order of message priority
AODVv2's RREQ and RREP messages are used for route discovery. RREQ messages are multicast to solicit an RREP, whereas RREP are unicast. The constants used in this section are defined in Section 12.
When an AODVv2 router needs to forward an IP packet (with source address OrigAddr and destination address TargAddr) from one of its Router Clients, it needs a route to TargAddr in its Routing Information Base. If no route exists, the AODVv2 router (RREQ_Gen) generates and multicasts a Route Request message (RREQ), on all configured interfaces, containing information about the source and destination. The procedure for this is described in Section 7.1.1. Each generated RREQ results in an increment to the router's sequence number. The AODVv2 router generating an RREQ is referred to as RREQ_Gen.
Buffering might be configured for IP packets awaiting a route for forwarding by RREQ_Gen, if sufficient memory is available. Buffering of IP packets might have both positive and negative effects. TCP connection establishment will benefit if packets are queued while route discovery is performed [Koodli01], but real-time traffic, voice, and scheduled delivery may suffer if packets are buffered and subjected to delays. Recommendations for appropriate buffer methods are out of scope for this specification. Determining which packets to discard first when the buffer is full is a matter of policy. Using different (or no) buffer methods might affect performance but does not affect interoperability.
RREQ_Gen awaits reception of a Route Reply message (RREP) containing a route toward TargAddr. This can be achieved by monitoring the entry in the Multicast Message Set that corresponds to the generated RREQ. When CurrentTime exceeds McMsg.Timestamp + RREQ_WAIT_TIME and no RREP has been received, RREQ_Gen will retry the route discovery.
To reduce congestion in a network, repeated attempts at route discovery for a particular target address utilize a binary exponential backoff: for each additional attempt, the time to wait for receipt of the RREP is multiplied by 2. If the requested route is not discovered within the wait period, another RREQ is sent, up to a total of DISCOVERY_ATTEMPTS_MAX. This is the same technique used in AODV [RFC3561].
Through the use of bidirectional link monitoring and blacklists (see Section 6.2), uni-directional links on an initially selected route will be ignored on subsequent route discovery attempts.
After DISCOVERY_ATTEMPTS_MAX and the corresponding wait time for an RREP response to the final RREQ, route discovery is considered to have failed. If an attempted route discovery has failed, RREQ_Gen SHOULD wait at least RREQ_HOLDDOWN_TIME before attempting another route discovery to the same destination, in order to avoid repeatedly generating control traffic that is unlikely to discover a route. Any IP packets buffered for TargAddr are also dropped, and a Destination Unreachable ICMP message (Type 3) with a code of 1 (Host Unreachable Error) MUST be sent to the source of the packet so that the application knows about the failure.
If RREQ_Gen does receive a route message containing a route to TargAddr within the timeout, it processes the message according to Section 7. When a valid LocalRoute entry is created in the Local Route Set, the route is also installed in the Routing Information Base, and the router will begin sending the buffered IP packets. Any retry timers for the corresponding RREQ are then cancelled.
During route discovery, all routers on the path obtain a route to both OrigPrefix and TargPrefix, so that routes are constructed in both directions. The route is optimized for the forward route.
A Route Request (RREQ) contains a route to OrigPrefix, and a Route Reply (RREP) contains a route to TargPrefix. Incoming information is checked to verify that it offers an improvement to existing information and that it would not create a routing loop, as explained in Section 6.7.1. If these checks pass, and if sufficient memory is available, refer to Section 6.7.2 for details about how to update the Local Route Set.
RteMsg denotes the received route message (RREP or RREQ), AdvRte denotes the route defined by the information within the RteMsg, and LocalRoute denotes an existing entry in the Local Route Set which matches the address, prefix length, metric type, and SeqNoRtr of the AdvRte.
AdvRte contains the following information extracted from the RteMsg:
An incoming advertised route (AdvRte) is compared to existing LocalRoutes to determine whether the advertised route is to be used to update the AODVv2 Local Route Set. The incoming route information MUST be processed as follows:
If the advertised route is to be used to update the Local Route Set, the procedure in Section 6.7.2 MUST be followed. If not, non-optimal routes will remain in the Local Route Set.
For information on how to apply these changes to the Routing Information Base, see Section 4.5.
After determining that AdvRte is to be used to update the Local Route Set (as described in Section 6.7.1), the following procedure applies.
If AdvRte is obtained from an RREQ message, the link to the next hop neighbor may not be confirmed as bidirectional (see Section 4.3). If there is no existing matching route in the Local Route Set, AdvRte MUST be installed to allow a corresponding RREP to be sent. If a matching entry already exists, and the link to the neighbor can be confirmed as bidirectional, AdvRte offers potential improvement.
The route update is applied as follows:
If this update to the Local Route Set results in two LocalRoutes to the same address, the best LocalRoute will be Unconfirmed. In order to improve the route used for forwarding, the router SHOULD try to determine if the link to the next hop of that LocalRoute is bidirectional, by using that LocalRoute to forward future RREPs and request acknowledgements (see Section 7.2.1 and Section 7.3.
When route messages are flooded in a MANET, an AODVv2 router may receive several instances of the same message. Forwarding every one of these would provide little additional benefit, while generating unnecessary signaling traffic and consequently additional interference. There have been a number of variations of AODV (e.g., [I-D.ietf-roll-aodv-rpl]), that have specified multicast for flooding RREP messages as well as RREQ messages. Since, in this document, suppression techniques are only needed for RREQ messages, multicast RREP messages are not considered. However, the technique involved is almost identical, and can be handled by substituting "RteMsg" instead of RREQ in the following text.
Each AODVv2 router stores information about recently received RREQ messages in the AODVv2 Multicast Message Set (Section 4.6).
In this section, an entry in the Multicast Message Set will be called a "multicast entry" for short. Each multicast entry SHOULD be maintained for at least RteMsg_ENTRY_TIME after the last Timestamp update in order to account for long-lived RREQs traversing the network. An entry MUST be deleted when the sequence number is no longer valid, i.e., after MAX_SEQNUM_LIFETIME. Memory-constrained devices MAY remove the entry before this time.
Received RREQs are tested against multicast entries containing information about previously received RREQs. A multicast entry is considered to be compatible with a received RREQ, or another multicast entry, if they both contain the same OrigPrefix, OrigPrefixLen, TargPrefix, and MetricType. A multicast entry is considered to be comparable with a received RREQ, or another multicast entry, if they are compatible and if, in addition, they both have the same SeqNoRtr. These terms will be used in the following algorithm determining how to process a received RREQ, and whether or not the RREQ is redundant.
If the received message is determined to be redundant, no forwarding or response to the message is needed. A message is considered to be redundant if either (a) a comparable newer (as determined by the OrigSeqNum) entry has already been received with information about the source and destination addresses of the route discovery operation or (b) it cannot be determined whether the message is newer compared to existing entries, but the received message metric value is not any better than metric values in compatible multicast entries.
To use the received RREQ to update the Multicast Message Set, and to determine whether or not the received RREQ requires additional processing as specified in Section 7, perform the following steps:
If processing for the RREQ has not been discontinued according to the above instructions, then continue processing the message as specified in Section 7.1.3.
In order to avoid flooding the network with RERR messages when a stream of IP packets to an unreachable address arrives, an AODVv2 router SHOULD avoid creating duplicate messages by determining whether an equivalent RERR has recently been sent. This is achieved with the help of the Route Error Set (see Section 4.7).
To determine if a RERR should be created:
Route maintenance involves the following operations:
During normal operation, AODVv2 does not require explicit timeouts to manage the lifetime of a valid route. At any time, any LocalRoute MAY be examined and updated according to the rules below. In case a Routing Information Base is used for forwarding, the corresponding RIB entry MUST be updated as soon as the state of a LocalRoute.State changes. Otherwise, if timers are not used to prompt updates of LocalRoute.State, the LocalRoute.State MUST be checked before IP packet forwarding and before any operation based on LocalRoute.State.
Route timeout behaviour is as follows:
LocalRoutes can become Invalid before a timeout occurs, as follows:
A LocalRoute can be confirmed by inferring connectivity to OrigAddr.
LocalRoutes are updated when Neighbor.State is updated:
Memory constrained devices MAY choose to expunge routes from the AODVv2 Local Route Set at other times, but MUST adhere to the following rules:
When LocalRoute.State changes from Active to Invalid as a result of a broken link or a received Route Error (RERR) message, other AODVv2 routers MUST be informed by sending an RERR message containing details of the invalidated route.
An RERR message MUST also be sent when an AODVv2 router receives an RREP message to forward, but the LocalRoute to the OrigAddr in the RREP is not available or is marked as Invalid.
A packet or message triggering the RERR MUST be discarded.
Generation of an RERR message is described in Section 7.4.1.
AODVv2 defines four message types: Route Request (RREQ), Route Reply (RREP), Route Reply Acknowledgement (RREP_Ack), and Route Error (RERR).
Each AODVv2 message is defined as a set of data. Rules for the generation, reception and forwarding of each message type are described in the following sections. Section 8 discusses how the data is mapped to [RFC5444] Message TLVs, Address Blocks, and Address TLVs.
Route Request messages are used in route discovery operations to request a route to a specified target address. RREQ messages have the following contents:
+-----------------------------------------------------------------+ | msg_hop_limit | +-----------------------------------------------------------------+ | AddressList | +-----------------------------------------------------------------+ | PrefixLengthList (optional) | +-----------------------------------------------------------------+ | OrigSeqNum, (optional) TargSeqNum | +-----------------------------------------------------------------+ | MetricType | +-----------------------------------------------------------------+ | OrigMetric | +-----------------------------------------------------------------+
Figure 3: RREQ message contents
An RREQ is generated to discover a route when an IP packet needs to be forwarded for a Router Client, and no valid route currently exists for the packet's destination in the Routing Information Base.
If the limit for the rate of AODVv2 control message generation has been reached, no message SHOULD be generated Section 6.5. Before creating an RREQ, the router SHOULD check the Multicast Message Set to see if a compatible RREQ has already been sent for the requested destination. If so, and the wait time for a reply has not yet been reached, the router SHOULD continue to await a response without generating a new RREQ. If the timeout has been reached, a new RREQ MAY be generated. If buffering is configured, incoming IP packets awaiting this route SHOULD be buffered until the route discovery is completed.
To generate the RREQ, the router (referred to as RREQ_Gen) follows this procedure:
This AODVv2 message is used to create a corresponding [RFC5444] message (see Section 8) which is handed to the RFC5444 multiplexer for further processing. By default, the multiplexer is instructed to multicast RREQ messages to LL-MANET-Routers on all interfaces configured for AODVv2 operation.
Upon receiving a Route Request, an AODVv2 router performs the following steps:
Forwarding or responding to a RteMsg provides up-to-date information and improved metrics to other routers. If a RteMsg is not forwarded, routes needed by applications may not be discovered.
By forwarding an RREQ, a router advertises that it will forward IP packets to the OrigPrefix contained in the RREQ according to the information enclosed. The router MAY choose not to forward the RREQ, for example if the router is heavily loaded or low on energy and therefore unwilling to advertise routing capability for more traffic. This could, however, decrease connectivity in the network or result in non-optimal paths.
The RREQ MUST NOT be forwarded if the received msg_hop_limit = 1, or if the limit for the rate of AODVv2 control message generation has been reached. Otherwise, the RREQ is updated and forwarded as follows:
This modified RREQ is handed to the [RFC5444] multiplexer for further processing. By default, the multiplexer is instructed to multicast the message to LL-MANET-Routers on all interfaces configured for AODVv2 operation.
When a Route Request message is received, requesting a route to a target address (TargAddr) which is configured as part of a Router Client Set entry, a Route Reply message is sent in response. The RREP offers a route to TargPrefix.
RREP messages have the following contents:
+-----------------------------------------------------------------+ | msg_hop_limit | +-----------------------------------------------------------------+ | AddressList | +-----------------------------------------------------------------+ | PrefixLengthList (optional) | +-----------------------------------------------------------------+ | TargSeqNum | +-----------------------------------------------------------------+ | MetricType | +-----------------------------------------------------------------+ | TargMetric | +-----------------------------------------------------------------+
Figure 4: RREP message contents
A Route Reply message is generated when a Route Request for a Router Client of the AODVv2 router arrives. This is the case when RteMsg.TargPrefix matches an entry in the Router Client Set of the AODVv2 router.
Before creating an RREP, the router SHOULD check whether CONTROL_TRAFFIC_LIMIT has been reached. If so, the RREP SHOULD NOT be created.
The RREP will traverse the path of the route to OrigPrefix. If the best route to OrigPrefix in the Local Route Set is Unconfirmed, the link to the next hop neighbor is not yet confirmed as bidirectional (see Section 6.2). In this case an RREP_Ack MUST also be sent as described in Section 7.3, in order to request an acknowledgement message from the next hop router to prove that the link is bidirectional. If the best route to OrigPrefix in the Local Route Set is valid, the link to the next hop neighbor is already confirmed as bidirectional, and no acknowledgement is required.
Implementations MAY allow a number of retries of the RREP if a requested acknowledgement is not received within RREP_Ack_SENT_TIMEOUT, doubling the timeout with each retry, up to a maximum of RREP_RETRIES, using the same exponential backoff described in Section 6.6 for RREQ retries. The acknowledgement MUST be considered to have failed after the wait time for an RREP_Ack response to the final RREP.
To generate the RREP, the router (also referred to as RREP_Gen) follows this procedure:
This AODVv2 message is used to create a corresponding [RFC5444] message (see Section 8) which is handed to the RFC5444 multiplexer for further processing. The multiplexer is instructed to unicast the RREP to LocalRoute[OrigPrefix].NextHop. The RREP MUST be sent over LocalRoute[OrigPrefix].NextHopInterface.
Upon receiving a Route Reply, an AODVv2 router performs the following steps:
then, ignore this RREP for further processing, since it does not correspond to a previously sent RREQ. Otherwise continue as follows:
A received Route Reply message is forwarded toward OrigPrefix. By forwarding the RREP, a router advertises that it has a route to TargPrefix.
The RREP MUST NOT be forwarded if the received msg_hop_limit = 1, or if CONTROL_TRAFFIC_LIMIT has been reached. Otherwise, the router MUST forward the RREP.
The procedure for RREP forwarding is as follows:
This modified message is handed to the [RFC5444] multiplexer for further processing. The multiplexer is instructed to unicast the RREP to LocalRoute[OrigPrefix].NextHop. The RREP MUST be sent over LocalRoute[OrigPrefix].NextHopInterface.
The Route Reply Acknowledgement is used as both a request and a response message to test bidirectionality of a link over which a Route Reply has also been sent. The router which forwards the RREP MUST send a Route Reply Acknowledgement message to the intended next hop, if the link to the next hop neighbor is not yet confirmed as bidirectional.
The receiving router MUST then reply with a Route Reply Acknowledgement response message.
When the Route Reply Acknowledgement response message is received by the sender of the RREP, it confirms that the link between the two routers is bidirectional (see Section 6.2).
If the Route Reply Acknowledgement is not received within RREP_Ack_SENT_TIMEOUT, the link is determined to be unidirectional.
+-----------------------------------------------------------------+ | AckReq (optional) | +-----------------------------------------------------------------+
Figure 5: RREP_Ack message contents
An RREP_Ack MUST be generated if a Route Reply is sent over a link which is not known to be bidirectional. It includes an AckReq element to indicate that it is a request for acknowledgement.
The RREP_Ack SHOULD NOT be generated if the limit for the rate of AODVv2 control message generation has been reached.
The [RFC5444] representation of the RREP_Ack is discussed in Section 8.
RREP_Ack requests MUST be unicast to LocalRoute[OrigPrefix].NextHop via LocalRoute[OrigPrefix].NextHopInterface. The multiplexer SHOULD be instructed to send the RREP_Ack in the same [RFC5444] packet as the RREP.
The Neighbor Set entry for LocalRoute[OrigPrefix].NextHop MUST also be updated to indicate that an RREP_Ack is required (see Section 6.3).
Upon receiving an RREP_Ack, an AODVv2 router performs the following steps:
An RREP_Ack response MUST be generated if a received RREP_Ack includes an AckReq, unless the limit for the rate of AODVv2 control message generation has been reached in which case the RREP_Ack response SHOULD NOT be generated.
There is no further data in an RREP_Ack response. The [RFC5444] representation is discussed in Section 8. In this case, the multiplexer is instructed to unicast the RREP_Ack to the source IP address of the RREP_Ack message that requested it, over the same interface on which the RREP_Ack was received.
A Route Error message is generated by an AODVv2 router to notify other AODVv2 routers about routes that are no longer available. An RERR message has the following contents:
+-----------------------------------------------------------------+ | PktSource (optional) | +-----------------------------------------------------------------+ | AddressList | +-----------------------------------------------------------------+ | PrefixLengthList (optional) | +-----------------------------------------------------------------+ | SeqNumList (optional) | +-----------------------------------------------------------------+ | MetricTypeList | +-----------------------------------------------------------------+
Figure 6: RERR message contents
A Route Error message is generated when an AODVv2 router (also referred to as RERR_Gen) needs to report that a destination is not reachable. There are three events that cause this response:
The RERR SHOULD NOT be generated if CONTROL_TRAFFIC_LIMIT has been reached. The RERR also SHOULD NOT be generated if it is a duplicate, as determined by Section 6.9.
Incidentally, if an AODVv2 router receives an ICMP error packet to or from the address of one of its Router Clients, it forwards the ICMP packet in the same way as any other IP packet, and will not generate any RERR message based on the contents of the ICMP packet.
To generate the RERR, the router follows this procedure:
The AODVv2 message is used to create a corresponding [RFC5444] message (see Section 8).
If the RERR is sent in response to an undeliverable IP packet or RREP message (i.e., if PktSource is included), the RERR SHOULD be sent unicast to the next hop on the route to PktSource. It MUST be sent over the same interface on which the undeliverable IP packet was received. If there is no route to PktSource, the RERR SHOULD be multicast to LL-MANET-Routers. If the RERR is sent in response to a broken link, i.e., PktSource is not included, the RERR is, by default, multicast to LL-MANET-Routers.
Section 10 describes processing steps when the optional precursor lists feature is implemented.
Upon receiving a Route Error, an AODVv2 router performs the following steps:
The Route Error message SHOULD NOT be regenerated if CONTROL_TRAFFIC_LIMIT has been reached.
The procedure for RERR regeneration is as follows:
The AODVv2 message is used to create a corresponding [RFC5444] message (see Section 8). If the RERR contains PktSource, the regenerated RERR SHOULD be sent unicast to the next hop on the LocalRoute to PktSource. It MUST be sent over the same interface on which the undeliverable IP packet was received. If there is no route to PktSource, or PktSource is a Router Client, it SHOULD be multicast to LL-MANET-Routers. If the RERR is sent in response to a broken link, the RERR is, by default, multicast to LL-MANET-Routers.
AODVv2 specifies that all control messages between routers MUST use the Generalized Mobile Ad Hoc Network Packet/Message Format [RFC5444], and therefore AODVv2's route messages comprise data which is mapped to message elements in [RFC5444].
[RFC5444] provides a multiplexed transport for multiple protocols. An [RFC5444] implementation MAY choose to optimize the content of certain elements during message creation to reduce control message overhead.
A brief summary of the [RFC5444] format:
AODVv2 does not require access to the [RFC5444] packet header.
In the message header, AODVv2 uses <msg-type>, <msg-hop-limit> and <msg-addr-length>. The <msg-addr-length> field indicates the length of any addresses in the message, using <msg-addr-length> := (address length in octets - 1), i.e. 3 for IPv4 and 15 for IPv6.
The addresses in an Address Block MAY appear in any order, and values in a TLV in the Address Block TLV Block must be associated with the correct address in the Address Block by the [RFC5444] implementation. To indicate which value is associated with each address, the AODVv2 message representation uses lists where the order of the addresses in the AODVv2 AddressList matches the order of values in other data lists, e.g., the order of SeqNums in the SeqNumList in an RERR. [RFC5444] maps this information to Address Block TLVs associated with the relevant addresses in the Address Block.
Each address included in the Address Block is identified as OrigPrefix, TargPrefix, PktSource, SeqNoRtr, or Unreachable Address by including an ADDRESS_TYPE TLV in the Address Block TLV Block.
The following sections show how AODVv2 data is represented in [RFC5444] messages. In Section 13.3, AODVv2 defines several new TLVs.
Where the extension type of a TLV is set to zero, this is the default [RFC5444] value and the extension type will not be included in the message.
| Data | Header Field | Value |
|---|---|---|
| None | <msg-type> | RREQ |
| msg_hop_limit | <msg-hop-limit> | MAX_HOPCOUNT, reduced by number of hops traversed so far by the message. |
AODVv2 does not define any Message TLVs for an RREQ message.
An RREQ contains OrigPrefix and TargPrefix, and each of these addresses has an associated prefix length. If the prefix length has not been included in the AODVv2 message, it is equal to the address length in bits.
| Data | Address Block |
|---|---|
| OrigPrefix/OrigPrefixLen | <address> + <prefix-length> |
| TargPrefix/TargPrefixLen | <address> + <prefix-length> |
| SeqNoRtr/PrefixLen | <address> + <prefix-length> |
Address Block TLVs are always associated with one or more addresses in the Address Block. The following sections show the TLVs that apply to each address.
| Data | TLV Type | Extension Type | Value |
|---|---|---|---|
| None | ADDRESS_TYPE | 0 | ORIGPREFIX |
| OrigSeqNum | SEQ_NUM | 0 | Sequence number of RREQ_Gen, the router which initiated route discovery. |
| OrigMetric /MetricType | PATH_METRIC | MetricType | Metric value for the route to OrigPrefix, using MetricType. |
| Data | TLV Type | Extension Type | Value |
|---|---|---|---|
| None | ADDRESS_TYPE | 0 | TARGPREFIX |
| TargSeqNum | SEQ_NUM | 0 | The last known TargSeqNum for TargPrefix. |
| Data | Header Field | Value |
|---|---|---|
| None | <msg-type> | RREP |
| msg_hop_limit | <msg-hop-limit> | MAX_HOPCOUNT - msg_hop_limit from the corresponding RREQ, reduced by number of hops traversed so far by the message. |
AODVv2 does not define any Message TLVs for an RREP message.
An RREP contains OrigPrefix and TargPrefix, and each of these addresses has an associated prefix length. If the prefix length has not been included in the AODVv2 message, it is equal to the address length in bits.
| Data | Address Block |
|---|---|
| OrigPrefix/OrigPrefixLen | <address> + <prefix-length> |
| TargPrefix/TargPrefixLen | <address> + <prefix-length> |
| SeqNoRtr/PrefixLen | <address> + <prefix-length> |
Address Block TLVs are always associated with one or more addresses in the Address Block. The following sections show the TLVs that apply to each address.
| Data | TLV Type | Extension Type | Value |
|---|---|---|---|
| None | ADDRESS_TYPE | 0 | ORIGPREFIX |
| Data | TLV Type | Extension Type | Value |
|---|---|---|---|
| None | ADDRESS_TYPE | 0 | TARGPREFIX |
| TargSeqNum | SEQ_NUM | 0 | Sequence number of RREP_Gen, the router which created the RREP. |
| TargMetric /MetricType | PATH_METRIC | MetricType | Metric value for the route to TargPrefix, using MetricType. |
| Data | Header Field | Value |
|---|---|---|
| None | <msg-type> | RREP_Ack |
AODVv2 defines an AckReq Message TLV, included when an acknowledgement of this message is required, in order to monitor adjacency, as described in Section 6.2.
| Data | TLV Type | Extension Type | Value |
|---|---|---|---|
| AckReq | ACK_REQ | 0 | None |
AODVv2 does not define an Address Block for an RREP_Ack message.
AODVv2 does not define any Address Block TLVs for an RREP_Ack message.
Route Error Messages MAY be split into multiple [RFC5444] messages when the desired contents would exceed the MTU. However, all of the resulting messages MUST have the same message header as described below. If PktSource is included in the AODVv2 message, it MUST be included in all of the resulting [RFC5444] messages.
| Data | Header Field | Value |
|---|---|---|
| None | <msg-type> | RERR |
AODVv2 does not define any Message TLVs for an RERR message.
The Address Block in an RERR MAY contain PktSource, the source address of the IP packet triggering RERR generation, as detailed in Section 7.4. The prefix length associated with PktSource is equal to the address length in bits.
Address Block always contains one address per route that is no longer valid, and each address has an associated prefix length. If a prefix length has not been included for this address, it is equal to the address length in bits.
| Data | Address Block |
|---|---|
| PktSource | <address> + <prefix-length> for PktSource |
| AddressList/PrefixLengthList | <address> + <prefix-length> for each unreachable address in AddressList |
Address Block TLVs are always associated with one or more addresses in the Address Block. The following sections show the TLVs that apply to each type of address in the RERR.
| Data | TLV Type | Extension Type | Value |
|---|---|---|---|
| PktSource | ADDRESS_TYPE | 0 | PKTSOURCE |
| Data | TLV Type | Extension Type | Value |
|---|---|---|---|
| None | ADDRESS_TYPE | 0 | UNREACHABLE |
| SeqNumList | SEQ_NUM | 0 | Sequence number associated with invalid route to the unreachable address. |
| MetricTypeList | PATH_METRIC | MetricType | None. Extension Type set to MetricType of the route to the unreachable address. |
Figure 7 shows a stub (i.e., non-transit) network of AODVv2 routers which is attached to an external network (i.e., a network not using AODVv2) via a single External Network Access Router (ENAR).
As in any externally-attached network, AODVv2 routers and Router Clients that wish to be reachable from the external network MUST have IP addresses within the ENAR's routable and topologically correct prefix (e.g., 191.0.2.0/24 in Figure 7). This AODVv2 network and networks attached to routers within it will be advertised to the external network using other routing protocols or procedures which are out of scope for this specification.
/-------------------------\ / +----------------+ \ / | AODVv2 Router | \ | | 191.0.2.2/32 | | | +----------------+ | Routable | +-----+--------+ Prefix | | ENAR | /191.0.2.0/24 | | AODVv2 Router| / | | 191.0.2.1 |/ /---------------\ | | serving net +------+ External \ | | 191.0.2.0/24 | \ Network / | +-----+--------+ \---------------/ | +----------------+ | | | AODVv2 Router | | | | 191.0.2.3/32 | | \ +----------------+ / \ / \-------------------------/
Figure 7: Simple External Network Attachment Example
When an AODVv2 router within the AODVv2 MANET wants to discover a route toward an address on the external network, it uses the normal AODVv2 route discovery for that IP Destination Address.
The ENAR MUST respond to RREQ on behalf of all external network destinations, that is, destinations which are not on the configured 191.0.2.0 /24 network. The ENAR MUST NOT respond with a TargPrefix and TargPrefixLen which includes any of the networks configured as part of the AODVv2 network. Sending a Route Request for a gateway is not currently supported.
If more than one gateway is configured to serve the same external network, each such gateway MUST configure that external network as a Router Client with its IP address as the value of SeqNoRtr for the RouterClient. AODVv2 messages SHOULD NOT be transmitted to routers in the External Network.
RREQs for addresses inside the AODVv2 network, e.g. destinations on the configured 191.0.2.0/24 network, are handled using the standard processes described in Section 7. Note that AODVv2 does not currently support route discovery for prefixes that do not equal address length, but RREPs do advertise the prefix on which TargAddr resides.
When an IP packet from an address on the external network destined for an address in the AODVv2 MANET reaches the ENAR, if the ENAR does not have a route toward that destination in its Routing Information Base, it will perform normal AODVv2 route discovery for that destination.
Configuring the ENAR as a default router is outside the scope of this specification.
This section specifies an interoperable, optional enhancement to AODVv2 enabling more economical Route Error notifications.
There can be several sources of traffic for a certain destination. Each source of traffic and each upstream router between the forwarding AODVv2 router and the traffic source is known as a "precursor" for the destination. For each destination, an AODVv2 router MAY choose to keep track of precursors that have provided traffic for that destination. Route Error messages about that destination can then be sent unicast to these precursors instead of multicast to all AODVv2 routers.
Since an RERR will be regenerated if it comes from a next hop on a valid LocalRoute, the RERR SHOULD ideally be sent backwards along the route that the source of the traffic uses, to ensure it is regenerated at each hop and reaches the traffic source. If the reverse path is unknown, the RERR SHOULD be sent toward the source along any available route. Therefore, the options for saving precursor information are as follows:
During normal operation, each AODVv2 router maintaining precursor lists for a LocalRoute must update the precursor list whenever it uses this route to forward traffic to the destination. Precursors are classified as Active if traffic has recently been forwarded by the precursor. The precursor is marked with a timestamp to indicate the time it last forwarded traffic on this route.
When an AODVv2 router detects that one or more LocalRoutes are broken, it MAY notify each Active precursor using a unicast Route Error message instead of creating multicast traffic. Unicast is applicable when there are few Active precursors compared to the number of neighboring AODVv2 routers. However, the default multicast behavior is still preferable when there are many precursors, since fewer message transmissions are required.
When an AODVv2 router supporting precursor lists receives an RERR message, it SHOULD identify the list of its own affected Active precursors for the routes in the RERR, and choose to send a unicast RERR to those, rather than send a multicast RERR.
When a LocalRoute is expunged, any precursor list associated with it MUST also be expunged.
Implementations of AODVv2 MUST support ICV TLVs using type-extensions 1 and 2, hash-function AES-CCM, and cryptographic function AES-CCM, both as defined in [RFC7251]. An ICV MUST be included with every message. The ICV value MAY be truncated as specified in [RFC7182].
Since the msg-hop-limit and PATH_METRIC values are mutable when included in AODVv2 messages, these values are set to zero before calculating an ICV. This means that these values are not protected end-to-end and are therefore susceptible to manipulation. This form of attack is described in Section 14.3.2.
Implementations of AODVv2 MUST support a TIMESTAMP TLV using type-extension 0. The timestamp used is a sequence number, and therefore the length of the <TIMESTAMP-value> field matches the AODVv2 sequence number defined in Section 4.4. The TIMESTAMP TLV MUST be included in RREP_Ack and RERR messages.
When more than one message is included in an RFC5444 packet, using a single ICV Packet TLV or single TIMESTAMP Packet TLV is more efficient than including ICV and TIMESTAMP Message TLVs in each message created. If the RFC5444 multiplexer is capable of adding the Packet TLVs, it SHOULD be instructed to include the Packet TLVs in packets containing AODVv2 messages. However, if the multiplexer is not capable of adding the Packet TLVs, the TLVs MUST be included as Message TLVs in each AODVv2 message in the packet.
After message generation, but before transmission, the ICV and TIMESTAMP TLVs MUST be added according to each message type as detailed in the following sections. The following steps list the procedure to be performed:
On message reception, and before message processing, verification of the received message MUST take place:
Since OrigPrefix is included in the RREQ, the ICV can be calculated and verified using the [RFC5444] contents.The ICV TLV has type extension := 1. Inclusion of an ICV TLV message integrity and endpoint authentication, because trusted routers MUST hold the shared key in order to calculate the ICV value, both to include when creating a message, and to validate the message by checking that the ICV is correct.
Since RREQ_Gen's sequence number is incremented for each new RREQ, replay protection is already afforded and no extra TIMESTAMP TLV is required.
After message generation and before message transmission:
On message reception and before message processing:
Since TargPrefix is included in the RREP, the ICV can be calculated and verified using the [RFC5444] contents. The ICV TLV has type extension 1. Inclusion of an ICV provides message integrity and endpoint authentication, because trusted routers MUST hold a valid key in order to calculate the ICV value, both to include when creating a message, and to validate the message by checking that the ICV is correct.
Since RREP_Gen's sequence number is incremented for each new RREP, replay protection is already afforded and no extra TIMESTAMP TLV is required.
After message generation and before message transmission:
On message reception and before message processing:
Since no sequence number is included in the RREP_Ack, a TIMESTAMP TLV MUST be included to protect against replay attacks. The value in the TIMESTAMP TLV is set as follows:
Since no addresses are included in the RREP_Ack, and the receiver of the RREP_Ack uses the source IP address of a received RREP_Ack to identify the sender, the ICV MUST be calculated using the message contents and the IP source address. The ICV TLV has type extension := 2 in order to accomplish this. This provides message integrity and endpoint authentication, because trusted routers MUST hold the correct key in order to calculate the ICV value.
After message generation and before message transmission:
On message reception and before message processing:
Since the sender's sequence number is not contained in the RERR, a TIMESTAMP TLV MUST be included to protect against replay attacks. The value in the TIMESTAMP TLV is set by incrementing and using RERR_Gen's sequence number.
Since the receiver of the RERR MUST use the source IP address of the RERR to identify the sender, the ICV MUST be calculated using the message contents and the IP source address. The ICV TLV has type extension := 2 in order to accomplish this. This provides message integrity and endpoint authentication, because trusted routers MUST hold the shared key in order to calculate the ICV value.
After message generation and before message transmission:
On message reception and before message processing:
AODVv2 uses various parameters which can be grouped into the following categories:
This section show the parameters along with their definitions and default values (if any).
Note that several fields have limited size (bits or bytes). These sizes and their encoding may place specific limitations on the values that can be set.
AODVv2 requires certain timing information to be associated with Local Route Set entries and message replies. The default values are as follows:
| Name | Default Value |
|---|---|
| ACTIVE_INTERVAL | 5 second |
| MAX_IDLETIME | 200 seconds |
| MAX_BLACKLIST_TIME | 200 seconds |
| MAX_SEQNUM_LIFETIME | 300 seconds |
| RERR_TIMEOUT | 3 seconds |
| RteMsg_ENTRY_TIME | 12 seconds |
| RREQ_WAIT_TIME | 2 seconds |
| RREP_Ack_SENT_TIMEOUT | 1 second |
| RREQ_HOLDDOWN_TIME | 10 seconds |
The above timing parameter values have worked well for small and medium well-connected networks with moderate topology changes. The timing parameters SHOULD be administratively configurable. Ideally, for networks with frequent topology changes the AODVv2 parameters SHOULD be adjusted using experimentally determined values or dynamic adaptation. For example, in networks with infrequent topology changes MAX_IDLETIME MAY be set to a much larger value. If the values were configured differently, the following consequences may be observed:
MAX_SEQNUM_LIFETIME MUST be configured to have the same values for all AODVv2 routers in the network.
AODVv2 protocol constants typically do not require changes. The following table lists these constants, along with their values and a reference to the section describing their use.
| Name | Default | Description |
|---|---|---|
| DISCOVERY_ATTEMPTS_MAX | 3 | Section 6.6 |
| RREP_RETRIES | 2 | Section 7.2.1 |
| MAX_METRIC[MetricType] | [see below] | Section 5 |
| MAX_METRIC[HopCount] | 255 | Section 5 and Section 7 |
| MAX_HOPCOUNT | 20 | Limit to number of hops an |
| RREQ or RREP message can traverse | ||
| INFINITY_TIME | [see below] | Maximum expressible clock |
| time (Section 6.7.2) |
MAX_HOPCOUNT cannot be larger than 255.
MAX_METRIC[MetricType] MUST always be the maximum expressible metric value of type MetricType. Field lengths associated with metric values are found in Section 13.4.
These protocol constants MUST have the same values for all AODVv2 routers in the ad hoc network. If the values were configured differently, the following consequences may be observed:
The following table lists AODVv2 parameters which SHOULD be administratively configured for each router:
| Name | Default Value | Description |
|---|---|---|
| Router Client Set | Section 4.2 | |
| BUFFER_SIZE_PACKETS | 2 | Section 6.6 |
| BUFFER_SIZE_BYTES | MAX_PACKET_SIZE [TBD] | Section 6.6 |
| CONTROL_TRAFFIC_LIMIT | [Adjust for 10% capacity] | Section 7 |
The following administrative controls MAY be used to change the operation of the network. The same settings SHOULD be used across the network. Inconsistent settings at different routers in the network will not result in protocol errors.
| Name | Default | Description |
|---|---|---|
| ENABLE_IDLE_IN_RERR | Disabled | Section 7.4.1 |
This section specifies several [RFC5444] message types and address tlv-types required for AODVv2, as well as a new Code value for ICMP Destination Unreachable.
This specification defines four Message Types, to be allocated from the 0-223 range of the "Message Types" namespace defined in [RFC5444].
| Name of Message | Type |
|---|---|
| Route Request (RREQ) | 10 (TBD) |
| Route Reply (RREP) | 11 (TBD) |
| Route Error (RERR) | 12 (TBD) |
| Route Reply Acknowledgement (RREP_Ack) | 13 (TBD) |
This specification defines one Message TLV Type, to be allocated from the Message-Type-specific "Message TLV Types" namespace defined in [RFC5444], as specified in Table 6.
| Name of TLV | Type | Length (octets) | Reference |
|---|---|---|---|
| ACK_REQ | 128 (TBD) | 0 | Section 6.2 |
This specification defines three Address Block TLV Types, to be allocated from the Message-Type-specific "Address Block TLV Types" namespace defined in [RFC5444], as specified in Table 7.
| Name of TLV | Type | Length (octets) | Reference |
|---|---|---|---|
| PATH_METRIC | 129 (TBD) | depends on MetricType | Section 7 |
| SEQ_NUM | 130 (TBD) | 2 | Section 7 |
| ADDRESS_TYPE | 131 (TBD) | 1 | Section 8 |
The metric types used by AODVv2 are identified according to Table 8. All implementations MUST use these values.
| Name of MetricType | Type | Metric Value Size |
|---|---|---|
| Unassigned | 0 | Undefined |
| Hop Count | 1 | 1 octet |
| Unallocated | 2 - 254 | TBD |
| Reserved | 255 | Undefined |
These values are used in the [RFC5444] Address Type TLV discussed in Section 8. All implementations MUST use these values.
| Address Type | Value |
|---|---|
| ORIGPREFIX | 0 |
| TARGPREFIX | 1 |
| UNREACHABLE | 2 |
| PKTSOURCE | 3 |
| UNSPECIFIED | 255 |
A new Code Value is defined for ICMP Destination Unreachable messages (see Section 7.1.2).
| Code Value | Value |
|---|---|
| Metric Type Mismatch | 8 (TBD) |
This section describes various security considerations and potential avenues to secure AODVv2 routing. The main objective of the AODVv2 protocol is for each router to communicate reachability information about addresses for which it is responsible, and for routes it has discovered from other AODVv2 routers.
Networks using AODVv2 to maintain connectivity and establish routes on demand may be vulnerable to certain well-known types of threats, which will be detailed in this section. Some of the threats described can be mitigated or eliminated. Tools to do so will be described.
With the exception of metric values, AODVv2 assures the integrity of all RteMsg data end-to-end though the use of ICVs (see Section 14.4.2. AODVv2 implementations support ICV and TIMESTAMP TLVs, unless the implementation is intended for an environment in which security is unnecessary; otherwise, AODVv2 deployments are configured to use these TLVs to secure messages.
The on-demand nature of AODVv2 route discovery automatically reduces the vulnerability to route disruption. Since control traffic for updating route tables is diminished, there is less opportunity for attack and failure.
Threats to AODVv2 which reduce availability are considered below.
Flooding attacks using RREQ amount to a (BLIND) denial of service for route discovery: By issuing RREQ messages for targets that don't exist, an attacker can flood the network, blocking resources and drowning out legitimate traffic. By triggering the generation of CONTROL_TRAFFIC_LIMIT amount of messages (for example by sending RREQs for many non-existent destinations), an attacker can prevent legitimate messages from being generated. The effect of this attack is dampened by the fact that duplicate RREQ messages are dropped (preventing the network from DDoSing itself). Processing requirements for AODVv2 messages are typically quite small, however AODVv2 routers receiving RREQs do allocate resources in the form of Neighbor Set, Local Route Set and Multicast Route Message Set entries. The attacker can maximize their impact on set growth by changing OrigPrefix or OrigPrefixLen for each RREQ. If a specific node is to be targeted, this attack may be carried out in a distributed fashion, either by compromising its direct neighbors or by specifying the target's address with TargPrefix and TargPrefixLen. Note that it might be more economical for the attacker to simply jam the medium; an attack which AODVv2 cannot defend itself against.
Mitigation:
RERR messages are designed to cause removal of installed routes. A malicious node could send an RERR message with false information to attempt to get other routers to remove a route to one or more specific destinations, therefore disrupting traffic to the advertised destinations.
Routes will be deleted if an RERR is received, withdrawing a route for which the sender is the receiver's next hop, if both of the following conditions are met:
Routes will also be deleted if a received RERR contains a PktSource address corresponding to a Router Client.
The information necessary to construct a malicious RERR could be discovered by eavesdropping, either by listening to AODVv2 messages or by watching data packet flows.
When the RERR is multicast, it can be received by many routers in the ad hoc network, and will be regenerated when processing results in an active route being removed. This threat could have serious impact on applications communicating by way of the sender of the RERR message.
Mitigation:
Links could be erroneously treated as bidirectional if malicious unsolicited or spoofed RREP messages were to be accepted. This would result in a route being installed which could not in fact be used to forward data to the destination, and may divert data packets away from the intended destination.
There is a window of RREQ_WAIT_TIME after an RREQ is sent, in which any malicious router could send an RREP in response, in order for the link to the malicious router to be deemed as bidirectional.
Mitigation:
A malicious router could decide not to forward an RREQ or RREP or RERR message. Not forwarding a RERR or RREP message would disrupt route discovery. Not regenerating a RERR message would result in the source of data packets continuing to maintain and use the route, and further RERR messages being generated by the sender of the non-regenerated RERR. A malicious router could intentionally disrupt traffic flows by not allowing the source of data traffic to re-discover a new route when one breaks.
Failing to send an RREP_Ack would also disrupt route establishment, by not allowing the reverse route to be validated. Return traffic which needs that route will prompt a new route discovery, wasting resources and incurring a slight delay but not disrupting the ability for applications to communicate.
Mitigation:
Passive inspection (eavesdropping) of AODVv2 control messages could enable unauthorized devices to gain information about the network topology, since exchanging such information is the main purpose of AODVv2.
Eavesdropping of data traffic could allow a malicious device to obtain information about how data traffic is being routed. With knowledge of source and destination addresses, malicious messages could be constructed to disrupt normal operation.
Integrity of route information can be compromised in the following types of attack:
Valid route set entries can be replaced or modified by maliciously constructed AODVv2 messages, destroying existing routes and the network's integrity. Any router may pose as another router by sending RREQ, RREP, RREP_Ack and RERR messages in its name.
Mitigation:
Any AODVv2 router can forward messages with modified data.
Mitigation:
Replaying of RREQ or RREP messages would be of less use to an attacker, since they would be dropped immediately due to their stale sequence number. RERR messages may or may not include sequence numbers and are therefore susceptible to replay attacks. RREP_Ack messages do not include sequence numbers and are therefore susceptible to replay attacks.
Mitigation:
Encryption MAY be used for AODVv2 messages. If the routers share a packet-level security association, the message data can be encrypted prior to message transmission. The establishment of such security associations is outside the scope of this specification. Encryption will not only protect against unauthorized devices obtaining information about network topology (eavesdropping) but will ensure that only trusted routers participate in routing operations.
Cryptographic Integrity Check Values (ICVs) can be used to ensure integrity of received messages, protecting against man in the middle attacks. Further, by using ICVs, only those routers with knowledge of a shared secret key are allowed to participate in routing information exchanges. [RFC7182] defines ICV TLVs for use with [RFC5444].
The data contained in AODVv2 routing protocol messages MUST be verified using Integrity Check Values, to avoid accepting tampered messages.
[RFC7182] defines a TIMESTAMP TLV for use with [RFC5444] which can be used to prevent replay attacks when sequence numbers are not already included.
The data contained in AODVv2 routing protocol messages can be protected with a TIMESTAMP value to ensure the protection against replaying of the message. Sequence numbers can be used in place of timestamps, since they are known to be strictly increasing.
The method of distribution of shared secret keys is out of the scope of this protocol. Key management is not specified for the following reasons:
Against [RFC4107], an analysis as to whether automated or manual key management should be used shows a compelling case for automated management. In particular:
On reviewing the case for manual key management against the same document, it can be seen that manual management might be advantageous in environments with limited bandwidth or high round trip times. AODVv2 lends itself to sparse ad hoc networks where transmission conditions may indeed be limited, depending on the bearers selected for use.
However, [RFC4107] assumes that the connectivity between endpoints is already available. In AODVv2, no route is available to a given destination until a router client requests that user traffic be transmitted. It is required to secure the signalling path of the routing protocol that will establish the path across which key exchange functions might subsequently be applied, which is clearly the reverse of the expected functionality. A different strategy is therefore required.
There are two possible solutions. In each case, it is assumed that a defence in depth security posture is being adopted by the system integrator, such that each function in the network as a whole is appropriately secured or defended as necessary, and that there is not complete reliance on security mechanisms built in to AODVv2. Such additional mechanisms could include a suitable wireless device security technology, so that wireless devices are authenticated and secured by their peers prior to exchanging user data, which in this case would include AODVV2 signalling traffic as a payload, and mechanisms which verify the authenticity and/or integrity of application-layer user data transported once a route has been established.
AODVv2 is a descendant of the design of previous MANET on-demand protocols, especially AODV [RFC3561] and DSR [RFC4728]. Changes to previous MANET on-demand protocols stem from research and implementation experiences. Thanks to Elizabeth Belding and Ian Chakeres for their long time authorship of AODV. Additional thanks to Derek Atkins, Emmanuel Baccelli, Abdussalam Baryun, Ramon Caceres, Justin Dean, Christopher Dearlove, Fatemeh Ghassemi, Sri Gundavelli, Ulrich Herberg, Henner Jakob, Ramtin Khosravi, Luke Klein-Berndt, Lars Kristensen, Tronje Krop, Koojana Kuladinithi, Kedar Namjoshi, Keyur Patel, Alexandru Petrescu, Alvaro Retana, Henning Rogge, Fransisco Ros, Pedro Ruiz, Christoph Sommer, Romain Thouvenin, Pascal Thubert, Richard Trefler, Jiazi Yi, Seung Yi, Behnaz Yousefi, and Cong Yuan, for their reviews of AODVv2 and DYMO, as well as numerous specification suggestions.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC3561] | Perkins, C., Belding-Royer, E. and S. Das, "Ad hoc On-Demand Distance Vector (AODV) Routing", RFC 3561, DOI 10.17487/RFC3561, July 2003. |
| [RFC4443] | Conta, A., Deering, S. and M. Gupta, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", STD 89, RFC 4443, DOI 10.17487/RFC4443, March 2006. |
| [RFC5444] | Clausen, T., Dearlove, C., Dean, J. and C. Adjih, "Generalized Mobile Ad Hoc Network (MANET) Packet/Message Format", RFC 5444, DOI 10.17487/RFC5444, February 2009. |
| [RFC5498] | Chakeres, I., "IANA Allocations for Mobile Ad Hoc Network (MANET) Protocols", RFC 5498, DOI 10.17487/RFC5498, March 2009. |
| [RFC7182] | Herberg, U., Clausen, T. and C. Dearlove, "Integrity Check Value and Timestamp TLV Definitions for Mobile Ad Hoc Networks (MANETs)", RFC 7182, DOI 10.17487/RFC7182, April 2014. |
This section lists the changes between recent AODVv2 revisions ...-02.txt and ...-03.txt.
This section lists the changes between AODVv2 revisions ...-00.txt and ...-02.txt.