Mobile Ad Hoc Networking Working Group Charles Perkins INTERNET DRAFT Sun Microsystems 10 August 1998 Elizabeth M. Royer University of California, Santa Barbara Ad Hoc On Demand Distance Vector (AODV) Routing draft-ietf-manet-aodv-01.txt Status of This Memo This document is a submission by the Mobile Ad Hoc Networking Working Group of the Internet Engineering Task Force (IETF). Comments should be submitted to the manet@itd.nrl.navy.mil mailing list. Distribution of this memo is unlimited. This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. 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.'' To view the entire list of current Internet-Drafts, please check the ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). Abstract The Ad Hoc On-Demand Distance Vector (AODV) routing protocol is intended for use by mobile nodes in an ad hoc network characterized by frequent changes in link connectivity to each other caused by relative movement. It offers quick adaptation to dynamic link conditions, low processing and memory overhead, low network utilization, and establishment of both unicast and multicast routes between sources and destinations which are loop free at all times. It makes use of destination sequence numbers, which are a novel means of ensuring loop freedom even in the face of anomalous delivery of routing control messages, and which solve classical problems associated with distance vector protocols, including the problem of "counting to infinity". Perkins, Royer Expires 10 February 1999 [Page i]
Internet Draft AODV 10 August 1998 Contents Status of This Memo i Abstract i 1. Introduction 2 2. Overview 2 3. AODV Terminology 4 4. Route Request Message Format 6 5. Route Reply Message Format 8 6. Multicast Route Invalidation Message Format 10 7. Node Operation - Unicast 11 7.1. Maintaining Route Utilization Records . . . . . . . . . . 11 7.2. Generating Route Requests . . . . . . . . . . . . . . . . 11 7.3. Forwarding Route Requests . . . . . . . . . . . . . . . . 12 7.4. Generating Route Replies . . . . . . . . . . . . . . . . 13 7.5. Generating Hello Messages . . . . . . . . . . . . . . . . 13 7.6. Initiating Triggered Route Replies . . . . . . . . . . . 14 7.7. Detecting Link Breakage . . . . . . . . . . . . . . . . . 15 8. Node Operation - Multicast 15 8.1. Maintaining Multicast Tree Utilization Records . . . . . 15 8.2. Generating Multicast Route Requests . . . . . . . . . . . 15 8.3. Forwarding Multicast Route Requests . . . . . . . . . . . 17 8.4. Generating Multicast Route Replies . . . . . . . . . . . 17 8.5. Route Deletion and Multicast Tree Pruning . . . . . . . . 18 8.6. Repairing Link Breakages . . . . . . . . . . . . . . . . 20 8.7. Initiating Triggered Route Replies . . . . . . . . . . . 22 9. Configuration Parameters 22 10. Extensions 24 11. Security Considerations 24 Perkins, Royer Expires 10 February 1999 [Page 1]
Internet Draft AODV 10 August 1998 1. Introduction The Ad-Hoc On-Demand Distance Vector (AODV) algorithm enables dynamic, self-starting, multihop routing between participating mobile nodes wishing to establish and maintain an ad-hoc network. AODV allows mobile nodes to obtain routes quickly for new destinations, and does not require nodes to maintain routes to destinations that are not in active communication. Additionally, AODV allows for the formation of multicast groups whose membership is free to change during the lifetime of the network. AODV also defines timely responses to link breakages and changes in network topology. The operation of AODV is loop free, and by avoiding the Bellman-Ford "counting to infinity" problem offers quick convergence when the ad-hoc network topology changes (typically, when a node moves in the network). One distinguishing feature of AODV is its use of a destination sequence number for each route entry. The destination sequence number is created by the destination or the multicast grouphead for any usable 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 always selects the one with the greatest sequence number. Another feature of AODV is that link breakages cause immediate notifications to be sent to the affected set of nodes, but only that set of nodes. 2. Overview Route Requests (RREQs), Route Replies (RREPs), and Multicast Route Invalidations (MINVs) are the three message types defined by AODV. These message types are handled by UDP, and normal IP header processing applies. So, for instance, the requesting node is expected to use its IP address as the source IP address for the messages. The range of dissemination of broadcast RREQs can be indicated by the TTL in the IP header. Fragmentation is typically not required. As long as the endpoints of a communication connection have valid routes to each other, AODV does not play any role. When a route to a new destination (either a single node or a multicast group) is needed, the node uses a broadcast RREQ to find a route to the destination. A route can be determined when the request reaches either the destination itself, or an intermediate node with a fresh enough route to the destination. The route is made available by unicasting a RREP back to the source of the RREQ. Since each node Perkins, Royer Expires 10 February 1999 [Page 2]
Internet Draft AODV 10 August 1998 receiving the request keeps track of a route back to the source of the request, the RREP can be unicast back from the destination to the source, or from any intermediate node that is able to satisfy the request back to the source. In the multicast scenario, RREQs are also used when a node wishes to join a multicast group. A special join flag in the RREQ lets nodes know that when they receive the RREP, they are not just setting route pointers but are actually grafting a branch on to the multicast tree. If a RREP is broadcast to the limited broadcast address (255.255.255.255), and has a TTL of one, and a destination address of the node itself with metric 0, then it is received by all the node's neighbors, and treated by them as a "hello" message. This hello message is a local advertisement for the continued presence of the node. Neighbors that are using routes through the broadcasting node will continue to mark the routes as valid. If hello messages from a particular node stop coming, the neighbor can assume that the node has moved away. When that happens, the neighbor will mark the link to the node as broken, and may trigger a notification to some of its other neighbors that the link has broken. Hello messages also carry multicast group and corresponding grouphead IP addresses. This information is used for repairing multicast trees after a previously disconnected portion of the network containing part of the multicast tree becomes reachable once again. Since AODV is a routing protocol, it deals with route table management. AODV assumes the following fields exist in each route table entry: - Destination IP Address - Destination Sequence Number - Hop Count - Next Hop - Lifetime This information must be kept even for ephemeral routes, such as are created to temporarily keep track of reverse paths towards nodes originating RREQs. For multicast tree routes, the Next Hop field is likely to contain more than one entry. For multicast tree routes, the following information is stored in each entry of the multicast route table: - Multicast Group IP Address - Multicast Grouphead IP Address - Hop Count - Next Hops - Lifetime Perkins, Royer Expires 10 February 1999 [Page 3]
Internet Draft AODV 10 August 1998 Here the Hop Count corresponds to the distance in hops to the multicast grouphead. Also, the Next Hops field is a linked list of structures, each of which contain the fields: - Node IP Address - Active Flag The Active Flag indicates whether the link has actually been set, or whether an MINV messages is still pending (see Section 8.5). 3. AODV Terminology This section defines terminology used with AODV that is not already defined in [2]. multicast grouphead A node which is a member of the given multicast group and which is the first such group member in the connected portion of the network. This node is responsible for initializing the multicast group destination sequence number. multicast tree The tree containing all nodes which are members of the multicast group and all nodes which are needed to connect the multicast group members. multicast route table The table were ad-hoc nodes keep routing (including next hops) information for various multicast groups. request table The table where ad-hoc nodes keep information concerning the first node to request to join a multicast group. There is one entry in the table for each multicast group for which the node has received a RREQ with the J flag set (see Section 8.2. route table The table where ad-hoc nodes keep routing (including next hop) information for various destinations. For IPv6, this can be associated with the Destination Cache. Perkins, Royer Expires 10 February 1999 [Page 4]
Internet Draft AODV 10 August 1998 triggered update An unsolicited route update transmitted by an intermediate node along the path to the destination. This protocol specification uses conventional meanings [1] for capitalized words such as MUST, SHOULD, etc., to indicate requirement levels for various protocol features. Perkins, Royer Expires 10 February 1999 [Page 5]
Internet Draft AODV 10 August 1998 4. Route Request Message 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type |J|R| Reserved | Hop Count | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Broadcast ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination IP address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source IP address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The format of the Route Request message is illustrated above, and contains the following fields: Type xx J Join flag; set when source node wants to join multicast group. R Repair flag; set when a node wants to initiate a repair to connect two previously disconnected portions of the multicast tree. Reserved Sent as 0; ignored on reception. Hop Count The number of hops from the Source IP Address to the node handling the request. Broadcast ID A sequence number identifying the particular RREQ uniquely when taken in conjunction with the source node's IP address. Destination IP Address The IP address of the destination for which a route is desired. Destination Sequence Number The last sequence number received in the past by the source for any route towards the destination. Perkins, Royer Expires 10 February 1999 [Page 6]
Internet Draft AODV 10 August 1998 Source IP Address The IP address of the node which originated the Route Request. Source Sequence Number The current sequence number for route information generated by the source of the route request. Extension: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Multicast Grouphead IP Addr... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...Multicast Grouphead IP Addr | Multicast Group Hop Count | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type xx Length The length of the extension field. Multicast Grouphead IP Address The IP Address of the Multicast Grouphead. This extension is only used when a route to the Multicast Grouphead is known. Multicast Group Hop Count The distance in hops of the node sending the RREQ from the Multicast Grouphead. This extension is only used for route rebuilding. This extension is included only when a route to the multicast grouphead is known. Perkins, Royer Expires 10 February 1999 [Page 7]
Internet Draft AODV 10 August 1998 5. Route Reply Message 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type |L| Reserved | Hop Count | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination IP address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The format of the Route Reply message is illustrated above, and contains the following fields: Type xx Reserved Sent as 0; ignored on reception. Hop Count The number of hops from the Source IP Address to the Destination IP Address. For multicast route requests this indicates the number of hops to the multicast grouphead. L If the 'L' bit is set, the message is a "hello" message and contains a list of the node's neighbors. Destination IP Address The IP address of the destination for which a route is supplied. Destination Sequence Number The destination sequence number associated to the route. Lifetime The time for which nodes receiving the RREP consider the route to be valid. Perkins, Royer Expires 10 February 1999 [Page 8]
Internet Draft AODV 10 August 1998 Extension: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Multicast Group IP Address ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... Multicast Group IP Address | Multicast Grouphead IP Addr ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... Multicast Grouphead IP Addr | Multicast Group Seq Number ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... Multicast Group Seq Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type xx Length The length of the extension field. Multicast Group IP Address The IP Address of the Multicast Group. Multicast Grouphead IP Address The IP Address of the Multicast Grouphead. Multicast Group Sequence Number The current sequence number of the Multicast Group. This extension is included when responding to a multicast group route request. Perkins, Royer Expires 10 February 1999 [Page 9]
Internet Draft AODV 10 August 1998 6. Multicast Route Invalidation Message 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Reserved | Hop Count | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination IP address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source IP address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The format of the Multicast Route Invalidation message is illustrated above, and contains the following fields: Type xx Reserved Sent as 0; ignored on reception. Hop Count The number of hops from the Source IP Address to the Destination IP Address. Destination IP Address The IP address of the Multicast Group for which a route is supplied. Destination Sequence Number The destination sequence number associated to the Multicast Group. Source IP Address The IP address of the node which originated the Route Request. Source Sequence Number The current sequence number for route information generated by the source of the route request. Perkins, Royer Expires 10 February 1999 [Page 10]
Internet Draft AODV 10 August 1998 Extensions: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Next Hop IP Address... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...Next Hop IP Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type xx Length The length of the extension field Next Hop IP Address The IP address of the node chosen to be the next hop for the multicast tree. This extension is included when a source node wishes to invalidate all but one of the routes set up by RREPs. It is not included when a multicast tree member is pruning itself from the tree. 7. Node Operation - Unicast This section describes the scenarios under which nodes generate RREQs and RREPs for unicast communication, and how the fields in the message are handled. 7.1. Maintaining Route Utilization Records For each valid route maintained by a node (containing a finite metric), the node also maintains a list of those neighbors that are actively using the route. This active-list of neighbors will receive notifications from the node in the event of detection of a link breakage. 7.2. Generating Route Requests A node broadcasts a RREQ when it determines that it needs a route to a destination and does not have one available. This can happen if the destination is previously unknown to the node, or if a previously valid route to the destination expires. Routes can become invalid if they time out (the Lifetime associated with the route expires), or else if a link breakage results in an infinite metric being associated with the route. When a route table entry is marked Perkins, Royer Expires 10 February 1999 [Page 11]
Internet Draft AODV 10 August 1998 with an infinite metric, its expiration time is also updated to be the current time plus BAD_LINK_LIFETIME milliseconds. After the expiration time, the route MAY be expunged from the node's route table. After broadcasting a RREQ a node waits for a RREP, and if the reply is not received within RREP_WAIT_TIME seconds, the node may rebroadcast the RREQ. The RREQ may be rebroadcast up to a maximum of RREQ_RETRIES times. Each rebroadcast has to increment the Broadcast ID field. The node MAY choose to use larger TTL values in the IP header field, or wait for longer times for the RREP to arrive. 7.3. Forwarding Route Requests When a node receives a broadcast RREQ, it first checks to see whether it has received a RREQ with the same Source IP Address and broadcast ID fields within the last BCAST_ID_SAVE milliseconds. If such a RREQ has been received, the node silently discards the newly received RREQ. Otherwise, the node checks to see whether it has a route to the destination. If the node does not have a route, it rebroadcasts the RREQ from its interface(s) with the same field values, but using its own IP address in the IP header of the outgoing RREQ. The TTL or hop limit field in the outgoing IP header is decreased by one. The Hop Count field in the broadcast RREQ message is incremented by one, to account to the new hop through the intermediate node. In this case, the node also creates a reverse route to the Source IP Address in its routing table with next hop equal to the IP address of the neighboring node that sent the broadcast RREQ (often not equal to the Source IP Address field in the RREQ message). This reverse route might be used for an eventual RREP back to the original node making the RREQ (identified by the Source IP Address). The reverse route is put into the route table with lifetime REV_ROUTE_LIFE milliseconds. If, on the other hand, the node does have a route for the destination, it compares the destination sequence number (dest-seqno) for that route with the Destination Sequence Number field of the incoming RREQ. If the node's existing dest-seqno is smaller than the Destination Sequence Number field of the RREQ, the node again rebroadcasts the RREQ just as if it did not have a route to the destination at all. If the node has a route to the destination, and the node's existing dest-seqno is greater than or equal to the Destination Sequence Number of the RREQ, then the node generates a RREP as discussed further in section 7.4. Perkins, Royer Expires 10 February 1999 [Page 12]
Internet Draft AODV 10 August 1998 7.4. Generating Route Replies If a node receives a route request for a destination, and has a fresh enough route to satisfy the request, the node generates a RREP message and unicasts it back to the node indicated by the Source IP Address field of the received RREQ. First, the node copies over its destination sequence number from the entry in its route table, or if the generating node is the node itself, it uses a destination sequence number at least equal to a sequence number generated after the last detected change in its neighbor set. If the node has not detected any change in its set of neighbors since it last incremented its destination sequence number, it may use the same destination sequence number. As part of the process of generating the RREP, the generating node creates or updates an entry in its routing table for the Source IP Address, if necessary as described in section 7.3. The Source Sequence Number is put into the route entry, along with the Hop Count from the RREQ. The expiration time for the route table entry is set to the current time plus ACTIVE_ROUTE_TIMEOUT seconds. If the generating node is not the destination node, then the generating node calculates the Hop Count between the Source IP Address and the Destination IP Address by adding together the Hop Count from the RREQ and the hop count stored in the route table entry for the destination node. If, on the other hand, the generating node is the destination node itself, the Hop Count field in the RREP is simply equal to the Hop Count received in the RREQ. If the node is not the destination node, it calculates the Lifetime field of the RREP by subtracting the current time from the expiration time in its route table entry. Otherwise, if the generating node is also the destination node, it copies the value MY_ROUTE_TIMEOUT into the Lifetime field of the RREP. If the generating node is not the node indicated by the Destination IP Address, then it puts the next hop towards the destination in the active-list for the reverse path route entry. 7.5. Generating Hello Messages Every node generates a "hello" message once every HELLO_INTERVAL milliseconds. This hello message is a broadcast RREP with TTL = 1, and the message fields set as follows: Destination IP Address the node's IP address, Perkins, Royer Expires 10 February 1999 [Page 13]
Internet Draft AODV 10 August 1998 Destination Sequence Number the latest sequence number Hop Count 0 Lifetime (1 + ALLOWED_HELLO_LOSS) * HELLO_INTERVAL The Hello Messages MAY also contain extensions denoting the multicast groups which are known to the node, along with the groups' corresponding groupheads. These extensions can be used by nodes which have just joined the network to fill in their Request Table up-to-date request information. The information is also used for route rebuilding, as is described later. The extensions have the following format: Multicast Group IP Address IP address of known multicast group Multicast Grouphead IP Address IP Address of corresponding multicast grouphead 7.6. Initiating Triggered Route Replies A node can trigger an unsolicited RREP if either it detects a link breakage for a next hop along an active route in its route table, or if it receives a RREP from a neighbor with an infinite metric for an active route (i.e., containing a Destination IP Address for which there is a route table entry with a nonempty active-list) The unsolicited RREP is unicast to each neighbor in the nonempty active-list for the route to that destination. The contents of the RREP fields are set as follows: L 0 Hop Count 65,535 Destination IP Address The destination in the broken route Destination Sequence Number One plus the destination sequence number recorded in the route. Perkins, Royer Expires 10 February 1999 [Page 14]
Internet Draft AODV 10 August 1998 7.7. Detecting Link Breakage A node can detect a link breakage by listening for "hello" messages from its set of neighbors. If it has received hello messages from a particular neighbor, but misses more than ALLOWED_HELLO_LOSS consecutive hello messages from that neighbor, the node can presume that the particular neighbor is no longer able to maintain a direct link with the mobile node. When this happens, the node should assume that its link with the former neighbor has been broken, and proceed as in Section 7.6. A node should assume that a hello message has been missed if it is not received within 1.5 times the duration of the HELLO_INTERVAL. Alternatively, the node can use any physical-layer or link-layer methods to detect link breakages with nodes it has considered as neighbors. 8. Node Operation - Multicast This section describes the scenarios under which nodes generate RREQs, RREPs, and MINVs for multicast communication, and how the fields in the messages are handled. 8.1. Maintaining Multicast Tree Utilization Records For each valid multicast group (containing a finite metric) of which a node is a part, either because it is a member of the group or because it is a router for the multicast tree, the node also maintains a list of those neighbors that are likewise a part of the multicast tree. This active-list of neighbors is used for forwarding messages received for the multicast group. A node will forward such a message to every neighbor listed as a part of the multicast tree, except that neighbor from which the message arrived. 8.2. Generating Multicast Route Requests A node sends a route request (RREQ) either when it determines that it should be a part of a multicast group, and it is not already a member of that group, or when it has a message to send to the multicast group but does not have a route to that group. If the node wishes to join the multicast group, it sets the flag J in the RREQ; otherwise, it leaves the flag unset. The destination address of the RREQ is always set to the multicast group address. If the node has a record of another node (the multicast grouphead) requesting to be a member of that multicast group, it has two options. If the node has a known route to the grouphead, it will place the address of that node in Perkins, Royer Expires 10 February 1999 [Page 15]
Internet Draft AODV 10 August 1998 the extension field and will unicast the RREQ to the corresponding next hop for that destination. Otherwise, if the node does not have a route to the grouphead, or if it does not know who the multicast grouphead is, it will broadcast the RREQ with destination IP address set to the IP address of the multicast group, and it will not include the extension field. These scenarios can occur during initialization of a node, when a node discovers it should be a member of a multicast group, or when a previously valid branch of the multicast tree expires. Branches of the multicast tree become invalid if they time out (the Lifetime associated with the route expires), or if a link breakage results in an infinite metric being associated with the route. The process of waiting for a RREP to a RREQ with a multicast destination address is the same as that described in Section 7.2. The node may resend the RREQ up to RREQ_RETRIES times if a RREP is not received. If the original RREQ was unicast to a specific node and a RREP is not received within RREP_WAIT_TIME seconds, the node will broadcast the next RREQ (and all subsequent RREQs for that multicast group) across the network. The destination IP address of the rebroadcast is set to the address of the multicast group, and the extension field containing the multicast grouphead address is not included. If a RREP is not received after RREQ_RETRIES total requests, the node may assume that there are no other members of that particular group within the network. If it wanted to join the multicast group, it will then become the multicast grouphead for that multicast group and initialize the destination sequence number of the multicast group. Otherwise, if it only wanted to send packets to that group without actually joining the group, it will drop the packets it had for that group. Each node in the network receiving a RREQ message with the J flag set, i.e. every member of the network, checks their Request Table to see whether there is already an entry for this multicast group. If there is no entry for the group, the node records the IP Address of the node which sent the RREQ, together with the IP address of the group for which it requested to be a member, in the Request Table. Because the first node to request to be in a group becomes the multicast grouphead, entries in the Request Table represent multicast groupheads. If a node wishes to join or send a message to a multicast group in the future, it will first consult its Request Table to see if another node had previously requested to join that group. Based on the existence or nonexistence of an entry for the multicast group in the Request Table, the node will then send the RREQ as described at the beginning of the section. Perkins, Royer Expires 10 February 1999 [Page 16]
Internet Draft AODV 10 August 1998 8.3. Forwarding Multicast Route Requests The operation of nodes forwarding RREQs for multicast is similar to that for the reception and forwarding of RREQs as described in Section 7.3, with the following exceptions. If the RREQ is a join request, when the node creates a reverse route to the Source IP Address, it places a route pointer in its multicast routing table, in addition to its (unicast) routing table. Further, a node can only respond to a join RREQ if it is a member of the multicast tree. The generation of the route reply (RREP) message is discussed in the following section. 8.4. Generating Multicast Route Replies If a node receives a multicast join route request for a multicast group, and it is already a member of the multicast tree for that group, the node updates its route and multicast route tables and then generates a RREP message. It unicasts the RREP back to the node indicated by the Source IP Address field of the received RREQ. The RREP contains the current destination sequence number for the multicast group, as well as the IP address of the multicast grouphead. If a node receives a multicast join route request for a multicast group and it is not already a member of the multicast tree for that group, it will rebroadcast the RREQ to its neighbors. If a node receives a multicast route request that is not a join message, it can reply if it has a route to the multicast tree. Otherwise it will continue forwarding the message. In the event that a node receives a unicasted multicast route request that specifies its own IP address as the destination address (i.e. the source node believes this destination node to be the multicast grouphead), but the node is in fact not the grouphead, it can simply ignore the RREQ. The source node will time out after RREP_WAIT_TIME seconds and will broadcast a new RREQ without the grouphead address specified. Every time the Multicast Grouphead sends an RREP in response to a RREQ, it increments the multicast group sequence number by one and attaches the new value of the sequence number to the RREP. Regardless of whether the multicast grouphead or an intermediate node generates the RREP, the RREP fields are set as follows: Hop Count The distance in hops the node initiating the RREP is from the multicast grouphead. This field is Perkins, Royer Expires 10 February 1999 [Page 17]
Internet Draft AODV 10 August 1998 incremented by each node that forwards the RREP along the route to the source. Destination IP Address The IP address of the destination for which a route is supplied, in this case the multicast grouphead. Destination Sequence Number The destination sequence number associated with the route to the grouphead. Lifetime The time for which nodes receiving the RREP consider the route to be valid. Multicast Group IP Address The IP Address of the Multicast Group. Multicast Grouphead IP Address The IP Address of the Multicast Grouphead. Multicast Group Sequence Number The current sequence number of the Multicast Group. 8.5. Route Deletion and Multicast Tree Pruning When a node broadcasts an RREQ message, it is likely to receive more than one reply since any node in the multicast tree can respond. If the RREQ was a join request, the RREP message traveling back to the node which originated the request sets up route pointers, effectively grafting a branch onto the multicast tree. If multiple branches to the same destination are created in such a manner, a loop will be formed. Hence, in order to prevent the formation of any such loops, it is necessary to delete all but one of the routes created by the RREP messages. The RREP containing the largest destination sequence number is chosen to be the added branch to the multicast tree. In the event that a node receives more than one RREP with the same (largest) sequence number, it selects the first one with the smallest hop count, i.e. the shortest distance to the multicast grouphead. After waiting for RREP_WAIT_TIME seconds, the node must then deactivate all routes created by other RREPs. This is accomplished by broadcasting a multicast-invalidate (MINV) message. The Destination IP Address of the MINV packet is set to the IP address of the multicast group, and the IP address of the next hop along the branch which was added to the multicast tree is included in an extension field. The Hop Count field of the MINV is set to 1. All nodes receiving this message whose address does not match that listed in the extension field of the packet will delete the multicast tree pointer to the node from which the packet came. The node which Perkins, Royer Expires 10 February 1999 [Page 18]
Internet Draft AODV 10 August 1998 was chosen as the next hop sets the 'active' flag for the sending node to true, thereby finalizing the creation of the tree branch. Various scenarios exist for the nodes receiving the MINV message. If the node receiving this message is a member of the multicast group, it will not forward the MINV any further. If it is not a member of the multicast group and no other nodes use it as a router for the multicast group, it will propagate the MINV further up the tree, effectively removing (pruning) itself from the multicast tree. The Destination IP Address of the propagated MINV message is set to the IP address of the multicast group, and the extension field indicating the next hop is not included. The lack of the next hop extension field indicates to all nodes receiving the packet that their multicast tree route pointer to this source node (if such a route pointer exists) should be deleted. If the next hop selected by the source node's MINV message was not previously a multicast tree member, it will have propagated the original RREQ further up the network in search of nodes which are tree members. Thus it is possible that this node also received more than one RREP. When the node receives more than one RREP for the same RREQ, it operates in a manner similar to the source node by saving the route information with the greatest sequence number, and beyond that the lowest hop count; it discards all other RREPs. This node forwards the first RREP towards the source of the RREQ, and then forwards later RREPs only if they have a greater sequence number or smaller metric. When the node receives an MINV announcing it as the next hop, it will send its own MINV announcing the node it has chosen as its next hop, and so on up the tree, until a node which was already a part of the multicast tree is reached. If a node receives an MINV and discovers it was not chosen as the next hop and is not otherwise a part of the multicast tree, it will delete the tree pointers and send an MINV without the next hop extension field to prune itself from the tree. When a source node sends an MINV selecting a next hop, it sets the 'active' flag for this next hop to true. If the next hop also needs to send an MINV message specifying which node it has chosen as its next hop, it lists the IP address of this next hop in the next hop extension of the MINV. Upon receiving this MINV message, the source node will not delete the tree pointer to this node (even though its IP address is not listed in the next hop extension) because the 'active' flag has already been set. To prevent the possibility of multicast group data messages being delivered to the source node from multiple neighboring nodes before the MINV messages is broadcast, no node is allowed to forward a data packet to this source node before the reception of the MINV message. The nodes know they have not yet received the MINV message because the 'active' flag for that tree branch remains unset. Only after receiving the MINV and setting the 'active' flag can the node to Perkins, Royer Expires 10 February 1999 [Page 19]
Internet Draft AODV 10 August 1998 which the MINV is addressed forward any multicast group data packets to the node. If a multicast group member revokes its member status and wishes to remove itself from the multicast tree, it can do so if it is not a multicast router for any other nodes in the multicast group. If this is the case, it may broadcast an MINV message without the next hop extension and with the Destination IP Address set to the IP address of the multicast group to prune itself from the tree. Similarly, if the node receiving this message is not a member of the multicast group and does not have any other nodes routing through it, it may send its own MINV message up the tree. 8.6. Repairing Link Breakages When a link breakage is detected between two nodes on the multicast tree, the node upstream of the break (i.e. the node which is further from the multicast grouphead) is responsible for initiating the repair of the broken link. In order to build the route back up, this node will broadcast a RREQ with destination IP address set to the IP address of the grouphead and with the J flag set. The destination sequence number of the RREQ is the last known sequence number of the multicast group. The Multicast Group Hop Count field is set to the distance of the source node from the multicast grouphead. Only a node which has a hop count for the multicast group smaller than the indicated value can respond. This hop count requirement is included to prevent nodes on the same side of the break as the node initiating the repair from replying to the RREQ. The RREQ is broadcast using an expanding rings search. Because of the high probability that other nearby nodes can be used to rebuild the route to the grouphead, the original RREQ is broadcast with a TTL (time to live) field value equal to the Multicast Group Hop Count. In this way, the effects of the link breakage may be localized. If no reply is received within RREP_WAIT_TIME seconds, the RREQ will be rebroadcast with a larger TTL value, and so on until the message is broadcast across the entire network or until the route is rebuilt. Any node that is a part of the multicast tree and which had a multicast group hop count smaller than that contained in the RREQ can return an RREP. If there is more than one RREP received at the originating node, route deletions occur as described in the previous section. If no response is received after RREQ_RETRIES broadcasts, it can be assumed that the network has become partitioned and the multicast tree cannot be repaired at this time. In this situation, the node which had initiated the route rebuilding becomes the new multicast grouphead for its part of the multicast tree partition. It broadcasts a RREP with an infinity metric and with the multicast group address extension field containing the corresponding multicast Perkins, Royer Expires 10 February 1999 [Page 20]
Internet Draft AODV 10 August 1998 group IP address included. All nodes receiving this RREP update their Request Tables to indicate the new grouphead information. Nodes which are a part of the multicast group also update the grouphead information for that group in their Multicast Route Table to indicate the new grouphead. All nodes will change the information in their hello messages to reflect this update. In the event that the link break could not be repaired, the multicast tree will remain partitioned until the two parts of the network become connected once again. A node from one partition of the network will know that it has come into contact with a node from the other side of the network by noting the difference in the hello message multicast group information. The node who is a part of the network partition with the lower grouphead IP address will initiate the tree repair. It will unicast a RREQ message with the R flag set back to the multicast grouphead of its partition in order to get permission to rebuild the tree. The node must seek permission to rebuild the tree in order to prevent multiple nodes from attempting to rebuild the tree if contact between the two partitions is re-established in more than one place. Multiple repairs would create loops within the multicast tree. Additionally, since the node initiating the repair is not necessarily a multicast tree member, it may itself have become disconnected from the multicast grouphead on its side of the partition, and so the lack of reply will prevent it from attempting to repair the tree. The grouphead is the only node which can respond to an RREQ with the R flag set. It will respond to the request by sending an RREP granting permission to one and only one node to rebuild the tree. Any nodes which requested permission and which do not receive an RREP will time out and not attempt the repair. As the RREP travels back to the node, it will establish a multicast tree branch if one did not already exist. After receiving the RREP, the node which sent the repair request will unicast a RREQ to the grouphead of the other network partition, using the node it had received the hello message from as the next hop. This RREQ will contain the current value of the partitions multicast group sequence number. Upon receiving the RREQ, the multicast grouphead will take the larger of its and the received multicast group sequence number, increment this value by one, and respond with a RREP. As the RREP is propagated back to the source node, a branch on to the multicast tree is added. When the initiating node receives the RREP, it will broadcast across the network an RREP with an infinity metric and the multicast group address extension field containing the corresponding multicast group IP address, and with the multicast grouphead IP address and multicast group sequence number fields set to show the updated information. All nodes receiving this RREP (i.e. the entire connected portion of the network), will have the updated multicast group information for that group. The node which was the grouphead of the other partition will also note this message and update its Perkins, Royer Expires 10 February 1999 [Page 21]
Internet Draft AODV 10 August 1998 tables to indicate that the other grouphead is now the multicast grouphead for the entire network. 8.7. Initiating Triggered Route Replies A node can trigger an unsolicited RREP if it has an entry in its Request Table for a multicast group, sends a RREQ to join the multicast group, and after RREQ_RETRIES times does not receives a response. The node will then become the new multicast grouphead, and it will broadcast a RREP with infinity metric and with the multicast group / grouphead extension information set to reflect that it is now the grouphead for the multicast group. In addition, in order to ensure nodes maintain consistent and up-to-date information about who the multicast groupheads are, any node which is a grouphead for a multicast group will broadcast an unsolicited RREP containing its IP Address and the multicast group IP address for which it is the grouphead across the network every RREP_UPDATE seconds. The contents of the RREP fields are set as follows: L 0 Hop Count 65,535 Destination IP Address The IP Address of the node sending the RREP. Destination Sequence Number One plus the destination sequence number recorded in the route. Multicast Group IP Address The IP Address of the Multicast Group of which the node just became the grouphead. Multicast Grouphead IP Address The IP Address of the new Multicast Grouphead, i.e. the node sending the RREP. Multicast Group Sequence Number The Sequence Number of the multicast group, as set by the new multicast grouphead. 9. Configuration Parameters This section gives default values for some important values associated with AODV protocol operations. Perkins, Royer Expires 10 February 1999 [Page 22]
Internet Draft AODV 10 August 1998 ACTIVE_ROUTE_TIMEOUT 3000 ALLOWED_HELLO_LOSS 2 BAD_LINK_LIFETIME 2 * RREP_WAIT_TIME BCAST_ID_SAVE 3000 HELLO_INTERVAL 1000 NET_DIAMETER 35 NODE_TRAVERSAL_TIME 40 MY_ROUTE_TIMEOUT 6000 REV_ROUTE_LIFE RREP_WAIT_TIME RREP_UPDATE 5000 RREP_WAIT_TIME 3 * NODE_TRAVERSAL_TIME * NET_DIAMETER / 2 RREQ_RETRIES 3 Note that the network may contain more than NET_DIAMETER ** 2 nodes. NET_DIAMETER measures the number of "cells" (typically wireless) that would have to be placed end to end in order to cover the area of the network. Perkins, Royer Expires 10 February 1999 [Page 23]
Internet Draft AODV 10 August 1998 10. Extensions RREQ, RREP, and MINV messages may have further extensions defined in future versions of the protocol. These extensions will have 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | type-specific data ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where: Type xx Length The length of the type-specific data, not including the Type and Length fields of the extension. Extensions with types between 128 and 255 may NOT be skipped. The rules for extensions will be spelled out more fully, and conform with the rules for handling IPv6 options. 11. Security Considerations Currently, AODV does not specify any special security measures. Route protocols, however, are prime targets for impersonation attacks, and must be protected by use of authentication techniques involving generation of unforgeable and cryptographically strong message digests or digital signatures. It is expected that, in environments where security is an issue, that IPSec authentication headers will be deployed along with the necessary key management to distribute keys to the members of the ad hoc network using AODV. Perkins, Royer Expires 10 February 1999 [Page 24]
Internet Draft AODV 10 August 1998 References [1] S. Bradner. Key Words for Use in RFCs to Indicate Requirement Levels. RFC 2119, March 1997. [2] Charles E. Perkins. Terminology for Ad-Hoc Networking. draft-ietf-manet-terms-00.txt, November 1997. (work in progress). Author's Address Questions about this memo can be directed to: Charles E. Perkins Sun Microsystems 901 San Antonio Rd. Palo Alto, CA 94303 USA 1 650 786 6464 1 650 786 6445 (fax) cperkins@eng.sun.com Elizabeth M. Royer Dept of Electrical and Computer Engineering University of California, Santa Barbara Santa Barbara, CA 93106 1 805 893 7788 1 805 893 3262 (fax) eroyer@alpha.ece.ucsb.edu Perkins, Royer Expires 10 February 1999 [Page 25]
