Network Working Group D. Johnson Request for Comments: 4728 Rice University Category: Experimental Y. Hu UIUC D. Maltz Microsoft Research February 2007 The Dynamic Source Routing Protocol (DSR) for Mobile Ad Hoc Networks for IPv4 Status of This Memo This memo defines an Experimental Protocol for the Internet community. It does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract The Dynamic Source Routing protocol (DSR) is a simple and efficient routing protocol designed specifically for use in multi-hop wireless ad hoc networks of mobile nodes. DSR allows the network to be completely self-organizing and self-configuring, without the need for any existing network infrastructure or administration. The protocol is composed of the two main mechanisms of "Route Discovery" and "Route Maintenance", which work together to allow nodes to discover and maintain routes to arbitrary destinations in the ad hoc network. All aspects of the protocol operate entirely on demand, allowing the routing packet overhead of DSR to scale automatically to only what is needed to react to changes in the routes currently in use. The protocol allows multiple routes to any destination and allows each sender to select and control the routes used in routing its packets, for example, for use in load balancing or for increased robustness. Other advantages of the DSR protocol include easily guaranteed loop- free routing, operation in networks containing unidirectional links, use of only "soft state" in routing, and very rapid recovery when routes in the network change. The DSR protocol is designed mainly for mobile ad hoc networks of up to about two hundred nodes and is designed to work well even with very high rates of mobility. This document specifies the operation of the DSR protocol for routing unicast IPv4 packets. Johnson, et al. Experimental [Page 1] RFC 4728 The Dynamic Source Routing Protocol February 2007 Table of Contents 1. Introduction ....................................................5 2. Assumptions .....................................................7 3. DSR Protocol Overview ...........................................9 3.1. Basic DSR Route Discovery .................................10 3.2. Basic DSR Route Maintenance ...............................12 3.3. Additional Route Discovery Features .......................14 3.3.1. Caching Overheard Routing Information ..............14 3.3.2. Replying to Route Requests Using Cached Routes .....15 3.3.3. Route Request Hop Limits ...........................16 3.4. Additional Route Maintenance Features .....................17 3.4.1. Packet Salvaging ...................................17 3.4.2. Queued Packets Destined over a Broken Link .........18 3.4.3. Automatic Route Shortening .........................19 3.4.4. Increased Spreading of Route Error Messages ........20 3.5. Optional DSR Flow State Extension .........................20 3.5.1. Flow Establishment .................................21 3.5.2. Receiving and Forwarding Establishment Packets .....22 3.5.3. Sending Packets along Established Flows ............22 3.5.4. Receiving and Forwarding Packets Sent along Established Flows ..................................23 3.5.5. Processing Route Errors ............................24 3.5.6. Interaction with Automatic Route Shortening ........24 3.5.7. Loop Detection .....................................25 3.5.8. Acknowledgement Destination ........................25 3.5.9. Crash Recovery .....................................25 3.5.10. Rate Limiting .....................................25 3.5.11. Interaction with Packet Salvaging .................26 4. Conceptual Data Structures .....................................26 4.1. Route Cache ...............................................26 4.2. Send Buffer ...............................................30 4.3. Route Request Table .......................................30 4.4. Gratuitous Route Reply Table ..............................31 4.5. Network Interface Queue and Maintenance Buffer ............32 4.6. Blacklist .................................................33 5. Additional Conceptual Data Structures for Flow State Extension ......................................................34 5.1. Flow Table ................................................34 5.2. Automatic Route Shortening Table ..........................35 5.3. Default Flow ID Table .....................................36 6. DSR Options Header Format ......................................36 6.1. Fixed Portion of DSR Options Header .......................37 6.2. Route Request Option ......................................40 6.3. Route Reply Option ........................................42 Johnson, et al. Experimental [Page 2] RFC 4728 The Dynamic Source Routing Protocol February 2007 6.4. Route Error Option ........................................44 6.4.1. Node Unreachable Type-Specific Information .........46 6.4.2. Flow State Not Supported Type-Specific Information ........................................46 6.4.3. Option Not Supported Type-Specific Information .....46 6.5. Acknowledgement Request Option ............................46 6.6. Acknowledgement Option ....................................47 6.7. DSR Source Route Option ...................................48 6.8. Pad1 Option ...............................................50 6.9. PadN Option ...............................................50 7. Additional Header Formats and Options for Flow State Extension ......................................................51 7.1. DSR Flow State Header .....................................52 7.2. New Options and Extensions in DSR Options Header ..........52 7.2.1. Timeout Option .....................................52 7.2.2. Destination and Flow ID Option .....................53 7.3. New Error Types for Route Error Option ....................54 7.3.1. Unknown Flow Type-Specific Information .............54 7.3.2. Default Flow Unknown Type-Specific Information .....55 7.4. New Acknowledgement Request Option Extension ..............55 7.4.1. Previous Hop Address Extension .....................55 8. Detailed Operation .............................................56 8.1. General Packet Processing .................................56 8.1.1. Originating a Packet ...............................56 8.1.2. Adding a DSR Options Header to a Packet ............57 8.1.3. Adding a DSR Source Route Option to a Packet .......57 8.1.4. Processing a Received Packet .......................58 8.1.5. Processing a Received DSR Source Route Option ......60 8.1.6. Handling an Unknown DSR Option .....................63 8.2. Route Discovery Processing ................................64 8.2.1. Originating a Route Request ........................65 8.2.2. Processing a Received Route Request Option .........66 8.2.3. Generating a Route Reply Using the Route Cache .....68 8.2.4. Originating a Route Reply ..........................71 8.2.5. Preventing Route Reply Storms ......................72 8.2.6. Processing a Received Route Reply Option ...........74 8.3. Route Maintenance Processing ..............................74 8.3.1. Using Link-Layer Acknowledgements ..................75 8.3.2. Using Passive Acknowledgements .....................76 8.3.3. Using Network-Layer Acknowledgements ...............77 8.3.4. Originating a Route Error ..........................80 8.3.5. Processing a Received Route Error Option ...........81 8.3.6. Salvaging a Packet .................................82 8.4. Multiple Network Interface Support ........................84 8.5. IP Fragmentation and Reassembly ...........................84 8.6. Flow State Processing .....................................85 8.6.1. Originating a Packet ...............................85 8.6.2. Inserting a DSR Flow State Header ..................88 Johnson, et al. Experimental [Page 3] RFC 4728 The Dynamic Source Routing Protocol February 2007 8.6.3. Receiving a Packet .................................88 8.6.4. Forwarding a Packet Using Flow IDs .................93 8.6.5. Promiscuously Receiving a Packet ...................93 8.6.6. Operation Where the Layer below DSR Decreases the IP TTL ...............................94 8.6.7. Salvage Interactions with DSR ......................94 9. Protocol Constants and Configuration Variables .................95 10. IANA Considerations ...........................................96 11. Security Considerations .......................................96 Appendix A. Link-MaxLife Cache Description ........................97 Appendix B. Location of DSR in the ISO Network Reference Model ....99 Appendix C. Implementation and Evaluation Status .................100 Acknowledgements .................................................101 Normative References .............................................102 Informative References ...........................................102 Johnson, et al. Experimental [Page 4] RFC 4728 The Dynamic Source Routing Protocol February 2007 1. Introduction The Dynamic Source Routing protocol (DSR) [JOHNSON94, JOHNSON96a] is a simple and efficient routing protocol designed specifically for use in multi-hop wireless ad hoc networks of mobile nodes. Using DSR, the network is completely self-organizing and self-configuring, requiring no existing network infrastructure or administration. Network nodes cooperate to forward packets for each other to allow communication over multiple "hops" between nodes not directly within wireless transmission range of one another. As nodes in the network move about or join or leave the network, and as wireless transmission conditions such as sources of interference change, all routing is automatically determined and maintained by the DSR routing protocol. Since the number or sequence of intermediate hops needed to reach any destination may change at any time, the resulting network topology may be quite rich and rapidly changing. In designing DSR, we sought to create a routing protocol that had very low overhead yet was able to react very quickly to changes in the network. The DSR protocol provides highly reactive service in order to help ensure successful delivery of data packets in spite of node movement or other changes in network conditions. The DSR protocol is composed of two main mechanisms that work together to allow the discovery and maintenance of source routes in the ad hoc network: - Route Discovery is the mechanism by which a node S wishing to send a packet to a destination node D obtains a source route to D. Route Discovery is used only when S attempts to send a packet to D and does not already know a route to D. - Route Maintenance is the mechanism by which node S is able to detect, while using a source route to D, if the network topology has changed such that it can no longer use its route to D because a link along the route no longer works. When Route Maintenance indicates a source route is broken, S can attempt to use any other route it happens to know to D, or it can invoke Route Discovery again to find a new route for subsequent packets to D. Route Maintenance for this route is used only when S is actually sending packets to D. In DSR, Route Discovery and Route Maintenance each operate entirely "on demand". In particular, unlike other protocols, DSR requires no periodic packets of any kind at any layer within the network. For example, DSR does not use any periodic routing advertisement, link status sensing, or neighbor detection packets and does not rely on these functions from any underlying protocols in the network. This Johnson, et al. Experimental [Page 5] RFC 4728 The Dynamic Source Routing Protocol February 2007 entirely on-demand behavior and lack of periodic activity allows the number of overhead packets caused by DSR to scale all the way down to zero, when all nodes are approximately stationary with respect to each other and all routes needed for current communication have already been discovered. As nodes begin to move more or as communication patterns change, the routing packet overhead of DSR automatically scales to only what is needed to track the routes currently in use. Network topology changes not affecting routes currently in use are ignored and do not cause reaction from the protocol. All state maintained by DSR is "soft state" [CLARK88], in that the loss of any state will not interfere with the correct operation of the protocol; all state is discovered as needed and can easily and quickly be rediscovered if needed after a failure without significant impact on the protocol. This use of only soft state allows the routing protocol to be very robust to problems such as dropped or delayed routing packets or node failures. In particular, a node in DSR that fails and reboots can easily rejoin the network immediately after rebooting; if the failed node was involved in forwarding packets for other nodes as an intermediate hop along one or more routes, it can also resume this forwarding quickly after rebooting, with no or minimal interruption to the routing protocol. In response to a single Route Discovery (as well as through routing information from other packets overheard), a node may learn and cache multiple routes to any destination. This support for multiple routes allows the reaction to routing changes to be much more rapid, since a node with multiple routes to a destination can try another cached route if the one it has been using should fail. This caching of multiple routes also avoids the overhead of needing to perform a new Route Discovery each time a route in use breaks. The sender of a packet selects and controls the route used for its own packets, which, together with support for multiple routes, also allows features such as load balancing to be defined. In addition, all routes used are easily guaranteed to be loop-free, since the sender can avoid duplicate hops in the routes selected. The operation of both Route Discovery and Route Maintenance in DSR are designed to allow unidirectional links and asymmetric routes to be supported. In particular, as noted in Section 2, in wireless networks, it is possible that a link between two nodes may not work equally well in both directions, due to differing transmit power levels or sources of interference. It is possible to interface a DSR network with other networks, external to this DSR network. Such external networks may, for example, be the Internet or may be other ad hoc networks routed with Johnson, et al. Experimental [Page 6] RFC 4728 The Dynamic Source Routing Protocol February 2007 a routing protocol other than DSR. Such external networks may also be other DSR networks that are treated as external networks in order to improve scalability. The complete handling of such external networks is beyond the scope of this document. However, this document specifies a minimal set of requirements and features necessary to allow nodes only implementing this specification to interoperate correctly with nodes implementing interfaces to such external networks. This document specifies the operation of the DSR protocol for routing unicast IPv4 packets in multi-hop wireless ad hoc networks. Advanced, optional features, such as Quality of Service (QoS) support and efficient multicast routing, and operation of DSR with IPv6 [RFC2460], will be covered in other documents. The specification of DSR in this document provides a compatible base on which such features can be added, either independently or by integration with the DSR operation specified here. As described in Appendix C, the design of DSR has been extensively studied through detailed simulations and testbed implementation and demonstration; this document encourages additional implementation and experimentation with the protocol. The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. 2. Assumptions As described here, the DSR protocol is designed mainly for mobile ad hoc networks of up to about two hundred nodes and is designed to work well even with very high rates of mobility. Other protocol features and enhancements that may allow DSR to scale to larger networks are outside the scope of this document. We assume in this document that all nodes wishing to communicate with other nodes within the ad hoc network are willing to participate fully in the protocols of the network. In particular, each node participating in the ad hoc network SHOULD also be willing to forward packets for other nodes in the network. The diameter of an ad hoc network is the minimum number of hops necessary for a packet to reach from any node located at one extreme edge of the ad hoc network to another node located at the opposite extreme. We assume that this diameter will often be small (e.g., perhaps 5 or 10 hops), but it may often be greater than 1. Johnson, et al. Experimental [Page 7] RFC 4728 The Dynamic Source Routing Protocol February 2007 Packets may be lost or corrupted in transmission on the wireless network. We assume that a node receiving a corrupted packet can detect the error, such as through a standard link-layer checksum or Cyclic Redundancy Check (CRC), and discard the packet. Nodes within the ad hoc network MAY move at any time without notice and MAY even move continuously, but we assume that the speed with which nodes move is moderate with respect to the packet transmission latency and wireless transmission range of the particular underlying network hardware in use. In particular, DSR can support very rapid rates of arbitrary node mobility, but we assume that nodes do not continuously move so rapidly as to make the flooding of every individual data packet the only possible routing protocol. A common feature of many network interfaces, including most current LAN hardware for broadcast media such as wireless, is the ability to operate the network interface in "promiscuous" receive mode. This mode causes the hardware to deliver every received packet to the network driver software without filtering based on link-layer destination address. Although we do not require this facility, some of our optimizations can take advantage of its availability. Use of promiscuous mode does increase the software overhead on the CPU, but we believe that wireless network speeds and capacity are more the inherent limiting factors to performance in current and future systems; we also believe that portions of the protocol are suitable for implementation directly within a programmable network interface unit to avoid this overhead on the CPU [JOHNSON96a]. Use of promiscuous mode may also increase the power consumption of the network interface hardware, depending on the design of the receiver hardware, and in such cases, DSR can easily be used without the optimizations that depend on promiscuous receive mode or can be programmed to only periodically switch the interface into promiscuous mode. Use of promiscuous receive mode is entirely optional. Wireless communication ability between any pair of nodes may at times not work equally well in both directions, due, for example, to transmit power levels or sources of interference around the two nodes [BANTZ94, LAUER95]. That is, wireless communications between each pair of nodes will in many cases be able to operate bidirectionally, but at times the wireless link between two nodes may be only unidirectional, allowing one node to successfully send packets to the other while no communication is possible in the reverse direction. Some Medium Access Control (MAC) protocols, however, such as MACA [KARN90], MACAW [BHARGHAVAN94], or IEEE 802.11 [IEEE80211], limit unicast data packet transmission to bidirectional links, due to the required bidirectional exchange of request to send (RTS) and clear to send (CTS) packets in these protocols and to the link-layer acknowledgement feature in IEEE 802.11. When used on top of MAC Johnson, et al. Experimental [Page 8] RFC 4728 The Dynamic Source Routing Protocol February 2007 protocols such as these, DSR can take advantage of additional optimizations, such as the ability to reverse a source route to obtain a route back to the origin of the original route. The IP address used by a node using the DSR protocol MAY be assigned by any mechanism (e.g., static assignment or use of Dynamic Host Configuration Protocol (DHCP) for dynamic assignment [RFC2131]), although the method of such assignment is outside the scope of this specification. A routing protocol such as DSR chooses a next-hop for each packet and provides the IP address of that next-hop. When the packet is transmitted, however, the lower-layer protocol often has a separate, MAC-layer address for the next-hop node. DSR uses the Address Resolution Protocol (ARP) [RFC826] to translate from next-hop IP addresses to next-hop MAC addresses. In addition, a node MAY add an entry to its ARP cache based on any received packet, when the IP address and MAC address of the transmitting node are available in the packet; for example, the IP address of the transmitting node is present in a Route Request option (in the Address list being accumulated) and any packets containing a source route. Adding entries to the ARP cache in this way avoids the overhead of ARP in most cases. 3. DSR Protocol Overview This section provides an overview of the operation of the DSR protocol. The basic version of DSR uses explicit "source routing", in which each data packet sent carries in its header the complete, ordered list of nodes through which the packet will pass. This use of explicit source routing allows the sender to select and control the routes used for its own packets, supports the use of multiple routes to any destination (for example, for load balancing), and allows a simple guarantee that the routes used are loop-free. By including this source route in the header of each data packet, other nodes forwarding or overhearing any of these packets can also easily cache this routing information for future use. Section 3.1 describes this basic operation of Route Discovery, Section 3.2 describes basic Route Maintenance, and Sections 3.3 and 3.4 describe additional features of these two parts of DSR's operation. Section 3.5 then describes an optional, compatible extension to DSR, known as "flow state", that allows the routing of most packets without an explicit source route header in the packet, while the fundamental properties of DSR's operation are preserved. Johnson, et al. Experimental [Page 9] RFC 4728 The Dynamic Source Routing Protocol February 2007 3.1. Basic DSR Route Discovery When some source node originates a new packet addressed to some destination node, the source node places in the header of the packet a "source route" giving the sequence of hops that the packet is to follow on its way to the destination. Normally, the sender will obtain a suitable source route by searching its "Route Cache" of routes previously learned; if no route is found in its cache, it will initiate the Route Discovery protocol to dynamically find a new route to this destination node. In this case, we call the source node the "initiator" and the destination node the "target" of the Route Discovery. For example, suppose a node A is attempting to discover a route to node E. The Route Discovery initiated by node A in this example would proceed as follows: ^ "A" ^ "A,B" ^ "A,B,C" ^ "A,B,C,D" | id=2 | id=2 | id=2 | id=2 +-----+ +-----+ +-----+ +-----+ +-----+ | A |---->| B |---->| C |---->| D |---->| E | +-----+ +-----+ +-----+ +-----+ +-----+ | | | | v v v v To initiate the Route Discovery, node A transmits a "Route Request" as a single local broadcast packet, which is received by (approximately) all nodes currently within wireless transmission range of A, including node B in this example. Each Route Request identifies the initiator and target of the Route Discovery, and also contains a unique request identification (2, in this example), determined by the initiator of the Request. Each Route Request also contains a record listing the address of each intermediate node through which this particular copy of the Route Request has been forwarded. This route record is initialized to an empty list by the initiator of the Route Discovery. In this example, the route record initially lists only node A. When another node receives this Route Request (such as node B in this example), if it is the target of the Route Discovery, it returns a "Route Reply" to the initiator of the Route Discovery, giving a copy of the accumulated route record from the Route Request; when the initiator receives this Route Reply, it caches this route in its Route Cache for use in sending subsequent packets to this destination. Johnson, et al. Experimental [Page 10] RFC 4728 The Dynamic Source Routing Protocol February 2007 Otherwise, if this node receiving the Route Request has recently seen another Route Request message from this initiator bearing this same request identification and target address, or if this node's own address is already listed in the route record in the Route Request, this node discards the Request. (A node considers a Request recently seen if it still has information about that Request in its Route Request Table, which is described in Section 4.3.) Otherwise, this node appends its own address to the route record in the Route Request and propagates it by transmitting it as a local broadcast packet (with the same request identification). In this example, node B broadcast the Route Request, which is received by node C; nodes C and D each also, in turn, broadcast the Request, resulting in receipt of a copy of the Request by node E. In returning the Route Reply to the initiator of the Route Discovery, such as in this example, node E replying back to node A, node E will typically examine its own Route Cache for a route back to A and, if one is found, will use it for the source route for delivery of the packet containing the Route Reply. Otherwise, E SHOULD perform its own Route Discovery for target node A, but to avoid possible infinite recursion of Route Discoveries, it MUST in this case piggyback this Route Reply on the packet containing its own Route Request for A. It is also possible to piggyback other small data packets, such as a TCP SYN packet [RFC793], on a Route Request using this same mechanism. Node E could instead simply reverse the sequence of hops in the route record that it is trying to send in the Route Reply and use this as the source route on the packet carrying the Route Reply itself. For MAC protocols, such as IEEE 802.11, that require a bidirectional frame exchange for unicast packets as part of the MAC protocol [IEEE80211], the discovered source route MUST be reversed in this way to return the Route Reply, since this route reversal tests the discovered route to ensure that it is bidirectional before the Route Discovery initiator begins using the route. This route reversal also avoids the overhead of a possible second Route Discovery. When initiating a Route Discovery, the sending node saves a copy of the original packet (that triggered the discovery) in a local buffer called the "Send Buffer". The Send Buffer contains a copy of each packet that cannot be transmitted by this node because it does not yet have a source route to the packet's destination. Each packet in the Send Buffer is logically associated with the time that it was placed into the Send Buffer and is discarded after residing in the Send Buffer for some timeout period SendBufferTimeout; if necessary for preventing the Send Buffer from overflowing, a FIFO or other replacement strategy MAY also be used to evict packets even before they expire. Johnson, et al. Experimental [Page 11] RFC 4728 The Dynamic Source Routing Protocol February 2007 While a packet remains in the Send Buffer, the node SHOULD occasionally initiate a new Route Discovery for the packet's destination address. However, the node MUST limit the rate at which such new Route Discoveries for the same address are initiated (as described in Section 4.3), since it is possible that the destination node is not currently reachable. In particular, due to the limited wireless transmission range and the movement of the nodes in the network, the network may at times become partitioned, meaning that there is currently no sequence of nodes through which a packet could be forwarded to reach the destination. Depending on the movement pattern and the density of nodes in the network, such network partitions may be rare or common. If a new Route Discovery was initiated for each packet sent by a node in such a partitioned network, a large number of unproductive Route Request packets would be propagated throughout the subset of the ad hoc network reachable from this node. In order to reduce the overhead from such Route Discoveries, a node SHOULD use an exponential back-off algorithm to limit the rate at which it initiates new Route Discoveries for the same target, doubling the timeout between each successive discovery initiated for the same target. If the node attempts to send additional data packets to this same destination node more frequently than this limit, the subsequent packets SHOULD be buffered in the Send Buffer until a Route Reply is received giving a route to this destination, but the node MUST NOT initiate a new Route Discovery until the minimum allowable interval between new Route Discoveries for this target has been reached. This limitation on the maximum rate of Route Discoveries for the same target is similar to the mechanism required by Internet nodes to limit the rate at which ARP Requests are sent for any single target IP address [RFC1122]. 3.2. Basic DSR Route Maintenance When originating or forwarding a packet using a source route, each node transmitting the packet is responsible for confirming that data can flow over the link from that node to the next hop. For example, in the situation shown below, node A has originated a packet for node E using a source route through intermediate nodes B, C, and D: +-----+ +-----+ +-----+ +-----+ +-----+ | A |---->| B |---->| C |-->? | D | | E | +-----+ +-----+ +-----+ +-----+ +-----+ In this case, node A is responsible for the link from A to B, node B is responsible for the link from B to C, node C is responsible for the link from C to D, and node D is responsible for the link from D to E. Johnson, et al. Experimental [Page 12] RFC 4728 The Dynamic Source Routing Protocol February 2007 An acknowledgement can provide confirmation that a link is capable of carrying data, and in wireless networks, acknowledgements are often provided at no cost, either as an existing standard part of the MAC protocol in use (such as the link-layer acknowledgement frame defined by IEEE 802.11 [IEEE80211]), or by a "passive acknowledgement" [JUBIN87] (in which, for example, B confirms receipt at C by overhearing C transmit the packet when forwarding it on to D). If a built-in acknowledgement mechanism is not available, the node transmitting the packet can explicitly request that a DSR-specific software acknowledgement be returned by the next node along the route; this software acknowledgement will normally be transmitted directly to the sending node, but if the link between these two nodes is unidirectional (Section 4.6), this software acknowledgement could travel over a different, multi-hop path. After an acknowledgement has been received from some neighbor, a node MAY choose not to require acknowledgements from that neighbor for a brief period of time, unless the network interface connecting a node to that neighbor always receives an acknowledgement in response to unicast traffic. When a software acknowledgement is used, the acknowledgement request SHOULD be retransmitted up to a maximum number of times. A retransmission of the acknowledgement request can be sent as a separate packet, piggybacked on a retransmission of the original data packet, or piggybacked on any packet with the same next-hop destination that does not also contain a software acknowledgement. After the acknowledgement request has been retransmitted the maximum number of times, if no acknowledgement has been received, then the sender treats the link to this next-hop destination as currently "broken". It SHOULD remove this link from its Route Cache and SHOULD return a "Route Error" to each node that has sent a packet routed over that link since an acknowledgement was last received. For example, in the situation shown above, if C does not receive an acknowledgement from D after some number of requests, it would return a Route Error to A, as well as any other node that may have used the link from C to D since C last received an acknowledgement from D. Node A then removes this broken link from its cache; any retransmission of the original packet can be performed by upper layer protocols such as TCP, if necessary. For sending such a retransmission or other packets to this same destination E, if A has in its Route Cache another route to E (for example, from additional Route Replies from its earlier Route Discovery, or from having overheard sufficient routing information from other packets), it can Johnson, et al. Experimental [Page 13] RFC 4728 The Dynamic Source Routing Protocol February 2007 send the packet using the new route immediately. Otherwise, it SHOULD perform a new Route Discovery for this target (subject to the back-off described in Section 3.1). 3.3. Additional Route Discovery Features 3.3.1. Caching Overheard Routing Information A node forwarding or otherwise overhearing any packet SHOULD add all usable routing information from that packet to its own Route Cache. The usefulness of routing information in a packet depends on the directionality characteristics of the physical medium (Section 2), as well as on the MAC protocol being used. Specifically, three distinct cases are possible: - Links in the network frequently are capable of operating only unidirectionally (not bidirectionally), and the MAC protocol in use in the network is capable of transmitting unicast packets over unidirectional links. - Links in the network occasionally are capable of operating only unidirectionally (not bidirectionally), but this unidirectional restriction on any link is not persistent; almost all links are physically bidirectional, and the MAC protocol in use in the network is capable of transmitting unicast packets over unidirectional links. - The MAC protocol in use in the network is not capable of transmitting unicast packets over unidirectional links; only bidirectional links can be used by the MAC protocol for transmitting unicast packets. For example, the IEEE 802.11 Distributed Coordination Function (DCF) MAC protocol [IEEE80211] is capable of transmitting a unicast packet only over a bidirectional link, since the MAC protocol requires the return of a link-level acknowledgement packet from the receiver and also optionally requires the bidirectional exchange of an RTS and CTS packet between the transmitter and receiver nodes. In the first case above, for example, the source route used in a data packet, the accumulated route record in a Route Request, or the route being returned in a Route Reply SHOULD all be cached by any node in the "forward" direction. Any node SHOULD cache this information from any such packet received, whether the packet was addressed to this node, sent to a broadcast (or multicast) MAC address, or overheard while the node's network interface is in promiscuous mode. However, the "reverse" direction of the links identified in such packet headers SHOULD NOT be cached. Johnson, et al. Experimental [Page 14] RFC 4728 The Dynamic Source Routing Protocol February 2007 For example, in the situation shown below, node A is using a source route to communicate with node E: +-----+ +-----+ +-----+ +-----+ +-----+ | A |---->| B |---->| C |---->| D |---->| E | +-----+ +-----+ +-----+ +-----+ +-----+ As node C forwards a data packet along the route from A to E, it SHOULD add to its cache the presence of the "forward" direction links that it learns from the headers of these packets, from itself to D and from D to E. Node C SHOULD NOT, in this case, cache the "reverse" direction of the links identified in these packet headers, from itself back to B and from B to A, since these links might be unidirectional. In the second case above, in which links may occasionally operate unidirectionally, the links described above SHOULD be cached in both directions. Furthermore, in this case, if node X overhears (e.g., through promiscuous mode) a packet transmitted by node C that is using a source route from node A to E, node X SHOULD cache all of these links as well, also including the link from C to X over which it overheard the packet. In the final case, in which the MAC protocol requires physical bidirectionality for unicast operation, links from a source route SHOULD be cached in both directions, except when the packet also contains a Route Reply, in which case only the links already traversed in this source route SHOULD be cached. However, the links not yet traversed in this route SHOULD NOT be cached. 3.3.2. Replying to Route Requests Using Cached Routes A node receiving a Route Request for which it is not the target searches its own Route Cache for a route to the target of the Request. If it is found, the node generally returns a Route Reply to the initiator itself rather than forward the Route Request. In the Route Reply, this node sets the route record to list the sequence of hops over which this copy of the Route Request was forwarded to it, concatenated with the source route to this target obtained from its own Route Cache. However, before transmitting a Route Reply packet that was generated using information from its Route Cache in this way, a node MUST verify that the resulting route being returned in the Route Reply, after this concatenation, contains no duplicate nodes listed in the route record. For example, the figure below illustrates a case in which a Route Request for target E has been received by node F, and node F already has in its Route Cache a route from itself to E: Johnson, et al. Experimental [Page 15] RFC 4728 The Dynamic Source Routing Protocol February 2007 +-----+ +-----+ +-----+ +-----+ | A |---->| B |- >| D |---->| E | +-----+ +-----+ \ / +-----+ +-----+ \ / \ +-----+ / >| C |- +-----+ | ^ v | Route Request +-----+ Route: A - B - C - F | F | Cache: C - D - E +-----+ The concatenation of the accumulated route record from the Route Request and the cached route from F's Route Cache would include a duplicate node in passing from C to F and back to C. Node F in this case could attempt to edit the route to eliminate the duplication, resulting in a route from A to B to C to D and on to E, but in this case, node F would not be on the route that it returned in its own Route Reply. DSR Route Discovery prohibits node F from returning such a Route Reply from its cache; this prohibition increases the probability that the resulting route is valid, since node F in this case should have received a Route Error if the route had previously stopped working. Furthermore, this prohibition means that a future Route Error traversing the route is very likely to pass through any node that sent the Route Reply for the route (including node F), which helps to ensure that stale data is removed from caches (such as at F) in a timely manner; otherwise, the next Route Discovery initiated by A might also be contaminated by a Route Reply from F containing the same stale route. If, due to this restriction on returning a Route Reply based on information from its Route Cache, node F does not return such a Route Reply, it propagates the Route Request normally. 3.3.3. Route Request Hop Limits Each Route Request message contains a "hop limit" that may be used to limit the number of intermediate nodes allowed to forward that copy of the Route Request. This hop limit is implemented using the Time- to-Live (TTL) field in the IP header of the packet carrying the Route Request. As the Request is forwarded, this limit is decremented, and the Request packet is discarded if the limit reaches zero before finding the target. This Route Request hop limit can be used to implement a variety of algorithms for controlling the spread of a Route Request during a Route Discovery attempt. Johnson, et al. Experimental [Page 16] RFC 4728 The Dynamic Source Routing Protocol February 2007 For example, a node MAY use this hop limit to implement a "non- propagating" Route Request as an initial phase of a Route Discovery. A node using this technique sends its first Route Request attempt for some target node using a hop limit of 1, such that any node receiving the initial transmission of the Route Request will not forward the Request to other nodes by re-broadcasting it. This form of Route Request is called a "non-propagating" Route Request; it provides an inexpensive method for determining if the target is currently a neighbor of the initiator or if a neighbor node has a route to the target cached (effectively using the neighbors' Route Caches as an extension of the initiator's own Route Cache). If no Route Reply is received after a short timeout, then the node sends a "propagating" Route Request for the target node (i.e., with hop limit as defined by the value of the DiscoveryHopLimit configuration variable). As another example, a node MAY use this hop limit to implement an "expanding ring" search for the target [JOHNSON96a]. A node using this technique sends an initial non-propagating Route Request as described above; if no Route Reply is received for it, the node originates another Route Request with a hop limit of 2. For each Route Request originated, if no Route Reply is received for it, the node doubles the hop limit used on the previous attempt, to progressively explore for the target node without allowing the Route Request to propagate over the entire network. However, this expanding ring search approach could increase the average latency of Route Discovery, since multiple Discovery attempts and timeouts may be needed before discovering a route to the target node. 3.4. Additional Route Maintenance Features 3.4.1. Packet Salvaging When an intermediate node forwarding a packet detects through Route Maintenance that the next hop along the route for that packet is broken, if the node has another route to the packet's destination in its Route Cache, the node SHOULD "salvage" the packet rather than discard it. To salvage a packet, the node replaces the original source route on the packet with a route from its Route Cache. The node then forwards the packet to the next node indicated along this source route. For example, in the situation shown in the example of Section 3.2, if node C has another route cached to node E, it can salvage the packet by replacing the original route in the packet with this new route from its own Route Cache rather than discarding the packet. When salvaging a packet, a count is maintained in the packet of the number of times that it has been salvaged, to prevent a single packet from being salvaged endlessly. Otherwise, since the TTL is Johnson, et al. Experimental [Page 17] RFC 4728 The Dynamic Source Routing Protocol February 2007 decremented only once by each node, a single node could salvage a packet an unbounded number of times. Even if we chose to require the TTL to be decremented on each salvage attempt, packet salvaging is an expensive operation, so it is desirable to bound the maximum number of times a packet can be salvaged independently of the maximum number of hops a packet can traverse. As described in Section 3.2, an intermediate node, such as in this case, that detects through Route Maintenance that the next hop along the route for a packet that it is forwarding is broken, the node also SHOULD return a Route Error to the original sender of the packet, identifying the link over which the packet could not be forwarded. If the node sends this Route Error, it SHOULD originate the Route Error before salvaging the packet. 3.4.2. Queued Packets Destined over a Broken Link When an intermediate node forwarding a packet detects through Route Maintenance that the next-hop link along the route for that packet is broken, in addition to handling that packet as defined for Route Maintenance, the node SHOULD also handle in a similar way any pending packets that it has queued that are destined over this new broken link. Specifically, the node SHOULD search its Network Interface Queue and Maintenance Buffer (Section 4.5) for packets for which the next-hop link is this new broken link. For each such packet currently queued at this node, the node SHOULD process that packet as follows: - Remove the packet from the node's Network Interface Queue and Maintenance Buffer. - Originate a Route Error for this packet to the original sender of the packet, using the procedure described in Section 8.3.4, as if the node had already reached the maximum number of retransmission attempts for that packet for Route Maintenance. However, in sending such Route Errors for queued packets in response to detection of a single, new broken link, the node SHOULD send no more than one Route Error to each original sender of any of these packets. - If the node has another route to the packet's IP Destination Address in its Route Cache, the node SHOULD salvage the packet as described in Section 8.3.6. Otherwise, the node SHOULD discard the packet. Johnson, et al. Experimental [Page 18] RFC 4728 The Dynamic Source Routing Protocol February 2007 3.4.3. Automatic Route Shortening Source routes in use MAY be automatically shortened if one or more intermediate nodes in the route become no longer necessary. This mechanism of automatically shortening routes in use is somewhat similar to the use of passive acknowledgements [JUBIN87]. In particular, if a node is able to overhear a packet carrying a source route (e.g., by operating its network interface in promiscuous receive mode), then this node examines the unexpended portion of that source route. If this node is not the intended next-hop destination for the packet but is named in the later unexpended portion of the packet's source route, then it can infer that the intermediate nodes before itself in the source route are no longer needed in the route. For example, the figure below illustrates an example in which node D has overheard a data packet being transmitted from B to C, for later forwarding to D and to E: +-----+ +-----+ +-----+ +-----+ +-----+ | A |---->| B |---->| C | | D | | E | +-----+ +-----+ +-----+ +-----+ +-----+ \ ^ \ / --------------------- In this case, this node (node D) SHOULD return a "gratuitous" Route Reply to the original sender of the packet (node A). The Route Reply gives the shorter route as the concatenation of the portion of the original source route up through the node that transmitted the overheard packet (node B), plus the suffix of the original source route beginning with the node returning the gratuitous Route Reply (node D). In this example, the route returned in the gratuitous Route Reply message sent from D to A gives the new route as the sequence of hops from A to B to D to E. When deciding whether to return a gratuitous Route Reply in this way, a node MAY factor in additional information beyond the fact that it was able to overhear the packet. For example, the node MAY decide to return the gratuitous Route Reply only when the overheard packet is received with a signal strength or signal-to-noise ratio above some specific threshold. In addition, each node maintains a Gratuitous Route Reply Table, as described in Section 4.4, to limit the rate at which it originates gratuitous Route Replies for the same returned route. Johnson, et al. Experimental [Page 19] RFC 4728 The Dynamic Source Routing Protocol February 2007 3.4.4. Increased Spreading of Route Error Messages When a source node receives a Route Error for a data packet that it originated, this source node propagates this Route Error to its neighbors by piggybacking it on its next Route Request. In this way, stale information in the caches of nodes around this source node will not generate Route Replies that contain the same invalid link for which this source node received the Route Error. For example, in the situation shown in the example of Section 3.2, node A learns from the Route Error message from C that the link from C to D is currently broken. It thus removes this link from its own Route Cache and initiates a new Route Discovery (if it has no other route to E in its Route Cache). On the Route Request packet initiating this Route Discovery, node A piggybacks a copy of this Route Error, ensuring that the Route Error spreads well to other nodes, and guaranteeing that any Route Reply that it receives (including those from other node's Route Caches) in response to this Route Request does not contain a route that assumes the existence of this broken link. 3.5. Optional DSR Flow State Extension This section describes an optional, compatible extension to the DSR protocol, known as "flow state", that allows the routing of most packets without an explicit source route header in the packet. The DSR flow state extension further reduces the overhead of the protocol yet still preserves the fundamental properties of DSR's operation. Once a sending node has discovered a source route such as through DSR's Route Discovery mechanism, the flow state mechanism allows the sending node to establish hop-by-hop forwarding state within the network, based on this source route, to enable each node along the route to forward the packet to the next hop based on the node's own local knowledge of the flow along which this packet is being routed. Flow state is dynamically initialized by the first packet using a source route and is then able to route subsequent packets along the same flow without use of a source route header in the packet. The state established at each hop along a flow is "soft state" and thus automatically expires when no longer needed and can be quickly recreated as necessary. Extending DSR's basic operation based on an explicit source route in the header of each packet routed, the flow state extension operates as a form of "implicit source routing" by preserving DSR's basic operation but removing the explicit source route from packets. Johnson, et al. Experimental [Page 20] RFC 4728 The Dynamic Source Routing Protocol February 2007 3.5.1. Flow Establishment A source node sending packets to some destination node MAY use the DSR flow state extension described here to establish a route to that destination as a flow. A "flow" is a route from the source to the destination represented by hop-by-hop forwarding state within the nodes along the route. Each flow is uniquely identified by a combination of the source node address, the destination node address, and a flow identifier (flow ID) chosen by the source node. Each flow ID is a 16-bit unsigned integer. Comparison between different flow IDs MUST be performed modulo 2**16. For example, using an implementation in the C programming language, a flow ID value (a) is greater than another flow ID value (b) if ((short)((a) - (b)) > 0), if a C language "short" data type is implemented as a 16-bit signed integer. A DSR Flow State header in a packet identifies the flow ID to be followed in forwarding that packet. From a given source to some destination, any number of different flows MAY exist and be in use, for example, following different sequences of hops to reach the destination. One of these flows MAY be considered the "default" flow from that source to that destination. If a node receives a packet with neither a DSR Options header specifying the route to be taken (with a Source Route option in the DSR Options header) nor a DSR Flow State header specifying the flow ID to be followed, it is forwarded along the default flow for the source and destination addresses specified in the packet's IP header. In establishing a new flow, the source node generates a nonzero 16-bit flow ID greater than any unexpired flow IDs for this (source, destination) pair. If the source wishes for this flow to become the default flow, the low bit of the flow ID MUST be set (the flow ID is an odd number); otherwise, the low bit MUST NOT be set (the flow ID is an even number). The source node establishing the new flow then transmits a packet containing a DSR Options header with a Source Route option. To establish the flow, the source node also MUST include in the packet a DSR Flow State header, with the Flow ID field set to the chosen flow ID for the new flow, and MUST include a Timeout option in the DSR Options header, giving the lifetime after which state information about this flow is to expire. This packet will generally be a normal data packet being sent from this sender to the destination (for example, the first packet sent after discovering the new route) but is also treated as a "flow establishment" packet. Johnson, et al. Experimental [Page 21] RFC 4728 The Dynamic Source Routing Protocol February 2007 The source node records this flow in its Flow Table for future use, setting the TTL in this Flow Table entry to the value used in the TTL field in the packet's IP header and setting the Lifetime in this entry to the lifetime specified in the Timeout option in the DSR Options header. The TTL field is used for Default Flow Forwarding, as described in Sections 3.5.3 and 3.5.4. Any further packets sent with this flow ID before the timeout that also contain a DSR Options header with a Source Route option MUST use this same source route in the Source Route option. 3.5.2. Receiving and Forwarding Establishment Packets Packets intended to establish a flow, as described in Section 3.5.1, contain a DSR Options header with a Source Route option and are forwarded along the indicated route. A node implementing the DSR flow state extension, when receiving and forwarding such a DSR packet, also keeps some state in its own Flow Table to enable it to forward future packets that are sent along this flow with only the flow ID specified. Specifically, if the packet also contains a DSR Flow State header, this packet SHOULD cause an entry to be established for this flow in the Flow Table of each node along the packet's route. The Hop Count field of the DSR Flow State header is also stored in the Flow Table, as is the lifetime specified in the Timeout option specified in the DSR Options header. If the Flow ID is odd and there is no flow in the Flow Table with Flow ID greater than the received Flow ID, set the default Flow ID for this (IP Source Address, IP Destination Address) pair to the received Flow ID, and the TTL of the packet is recorded. The Flow ID option is removed before final delivery of the packet. 3.5.3. Sending Packets along Established Flows When a flow is established as described in Section 3.5.1, a packet is sent that establishes state in each node along the route. This state is soft; that is, the protocol contains mechanisms for recovering from the loss of this state. However, the use of these mechanisms may result in reduced performance for packets sent along flows with forgotten state. As a result, it is desirable to differentiate behavior based on whether or not the sender is reasonably certain that the flow state exists on each node along the route. We define a flow's state to be "established end-to-end" if the Flow Tables of all nodes on the route contains forwarding information for that flow. While it is impossible to detect whether or not a flow's state has Johnson, et al. Experimental [Page 22] RFC 4728 The Dynamic Source Routing Protocol February 2007 been established end-to-end without sending packets, implementations may make reasonable assumptions about the retention of flow state and the probability that an establishment packet has been seen by all nodes on the route. A source wishing to send a packet along an established flow determines if the flow state has been established end-to-end. If it has not, a DSR Options header with Source Route option with this flow's route is added to the packet. The source SHOULD set the Flow ID field of the DSR Flow State header either to the flow ID previously associated with this flow's route or to zero. If it sets the Flow ID field to any other value, it MUST follow the processing steps in Section 3.5.1 for establishing a new flow ID. If it sets the Flow ID field to a nonzero value, it MUST include a Timeout option with a value not greater than the timeout remaining in the node's Flow Table, and if its TTL is not equal to that specified in the Flow Table, the flow MUST NOT be used as a default flow in the future. Once flow state has been established end-to-end for non-default flows, a source adds a DSR Flow State header to each packet it wishes to send along that flow, setting the Flow ID field to the flow ID of that flow. A Source Route option SHOULD NOT be added to the packet, though if one is, then the steps for processing flows that have not been established end-to-end MUST be followed. Once flow state has been established end-to-end for default flows, sources sending packets with IP TTL equal to the TTL value in the local Flow Table entry for this flow then transmit the packet to the next hop. In this case, a DSR Flow State header SHOULD NOT be added to the packet and a DSR Options header likewise SHOULD NOT be added to the packet; though if one is, the steps for sending packets along non-default flows MUST be followed. If the IP TTL is not equal to the TTL value in the local Flow Table, then the steps for processing a non-default flow MUST be followed. 3.5.4. Receiving and Forwarding Packets Sent along Established Flows The handling of packets containing a DSR Options header with both a nonzero Flow ID and a Source Route option is described in Section 3.5.2. The Flow ID is ignored when it is equal to zero. This section only describes handling of packets without a Source Route option. If a node receives a packet with a Flow ID in the DSR Options header that indicates an unexpired flow in the node's Flow Table, it increments the Hop Count in the DSR Options header and forwards the packet to the next hop indicated in the Flow Table. Johnson, et al. Experimental [Page 23] RFC 4728 The Dynamic Source Routing Protocol February 2007 If a node receives a packet with a Flow ID that indicates a flow not currently in the node's Flow Table, it returns a Route Error of type UNKNOWN_FLOW with Error Destination and IP Destination addresses copied from the IP Source of the packet triggering the error. This error packet SHOULD be MAC-destined to the node from which the packet was received; if it cannot confirm reachability of the previous node using Route Maintenance, it MUST send the error as described in Section 8.1.1. The node sending the error SHOULD attempt to salvage the packet triggering the Route Error. If it does salvage the packet, it MUST zero the Flow ID in the packet. If a node receives a packet with no DSR Options header and no DSR Flow State header, it checks the Default Flow Table. If there is a matching entry, it forwards to the next hop indicated in the Flow Table for the default flow. Otherwise, it returns a Route Error of type DEFAULT_FLOW_UNKNOWN with Error Destination and IP Destination addresses copied from the IP Source Address of the packet triggering the error. This error packet SHOULD be MAC-destined to the node from which it was received; if this node cannot confirm reachability of the previous node using Route Maintenance, it MUST send the error as described in Section 8.1.1. The node sending the error SHOULD attempt to salvage the packet triggering the Route Error. If it does salvage the packet, it MUST zero the Flow ID in the packet. 3.5.5. Processing Route Errors When a node receives a Route Error of type UNKNOWN_FLOW, it marks the flow to indicate that it has not been established end-to-end. When a node receives a Route Error of type DEFAULT_FLOW_UNKNOWN, it marks the default flow to indicate that it has not been established end- to-end. 3.5.6. Interaction with Automatic Route Shortening Because a full source route is not carried in every packet, an alternative method for performing automatic route shortening is necessary for packets using the flow state extension. Instead, nodes promiscuously listen to packets, and if a node receives a packet with (IP Source, IP Destination, Flow ID) found in the Flow Table but the MAC-layer (next hop) destination address of the packet is not this node, the node determines whether the packet was sent by an upstream or downstream node by examining the Hop Count field in the DSR Flow State header. If the Hop Count field is less than the expected Hop Count at this node (that is, the expected Hop Count field in the Flow Table described in Section 5.1), the node assumes that the packet was sent by an upstream node and adds an entry for the packet to its Automatic Route Shortening Table, possibly evicting an earlier entry added to this table. When the packet is then sent to that node for Johnson, et al. Experimental [Page 24] RFC 4728 The Dynamic Source Routing Protocol February 2007 forwarding, the node finds that it has previously received the packet by checking its Automatic Route Shortening Table and returns a gratuitous Route Reply to the source of the packet. 3.5.7. Loop Detection If a node receives a packet for forwarding with TTL lower than expected and default flow forwarding is being used, it sends a Route Error of type DEFAULT_FLOW_UNKNOWN back to the IP source. It can attempt delivery of the packet by normal salvaging (subject to constraints described in Section 8.6.7). 3.5.8. Acknowledgement Destination In packets sent using Flow State, the previous hop is not necessarily known. In order to allow nodes that have lost flow state to determine the previous hop, the address of the previous hop can optionally be stored in the Acknowledgement Request. This extension SHOULD NOT be used when a Source Route option is present, MAY be used when flow state routing is used without a Source Route option, and SHOULD be used before Route Maintenance determines that the next-hop destination is unreachable. 3.5.9. Crash Recovery Each node has a maximum Timeout value that it can possibly generate. This can be based on the largest number that can be set in a timeout option (2**16 - 1 seconds) or may be less than this, set in system software. When a node crashes, it does not establish new flows for a period equal to this maximum Timeout value, in order to avoid colliding with its old Flow IDs. 3.5.10. Rate Limiting Flow IDs can be assigned with a counter. More specifically, the "Current Flow ID" is kept. When a new default Flow ID needs to be assigned, if the Current Flow ID is odd, the Current Flow ID is assigned as the Flow ID and the Current Flow ID is incremented by one; if the Current Flow ID is even, one plus the Current Flow ID is assigned as the Flow ID and the Current Flow ID is incremented by two. If Flow IDs are assigned in this way, one algorithm for avoiding duplicate, unexpired Flow IDs is to rate limit new Flow IDs to an average rate of n assignments per second, where n is 2**15 divided by the maximum Timeout value. This can be averaged over any period not exceeding the maximum Timeout value. Johnson, et al. Experimental [Page 25] RFC 4728 The Dynamic Source Routing Protocol February 2007 3.5.11. Interaction with Packet Salvaging Salvaging is modified to zero the Flow ID field in the packet. Also, anytime this document refers to the Salvage field in the Source Route option in a DSR Options header, packets without a Source Route option are considered to have the value zero in the Salvage field. 4. Conceptual Data Structures This document describes the operation of the DSR protocol in terms of a number of conceptual data structures. This section describes each of these data structures and provides an overview of its use in the protocol. In an implementation of the protocol, these data structures MUST be implemented in a manner consistent with the external behavior described in this document, but the choice of implementation used is otherwise unconstrained. Additional conceptual data structures are required for the optional flow state extensions to DSR; these data structures are described in Section 5. 4.1. Route Cache Each node implementing DSR MUST maintain a Route Cache, containing routing information needed by the node. A node adds information to its Route Cache as it learns of new links between nodes in the ad hoc network; for example, a node may learn of new links when it receives a packet carrying a Route Request, Route Reply, or DSR source route. Likewise, a node removes information from its Route Cache as it learns that existing links in the ad hoc network have broken. For example, a node may learn of a broken link when it receives a packet carrying a Route Error or through the link-layer retransmission mechanism reporting a failure in forwarding a packet to its next-hop destination. Anytime a node adds new information to its Route Cache, the node SHOULD check each packet in its own Send Buffer (Section 4.2) to determine whether a route to that packet's IP Destination Address now exists in the node's Route Cache (including the information just added to the Cache). If so, the packet SHOULD then be sent using that route and removed from the Send Buffer. It is possible to interface a DSR network with other networks, external to this DSR network. Such external networks may, for example, be the Internet or may be other ad hoc networks routed with a routing protocol other than DSR. Such external networks may also be other DSR networks that are treated as external networks in order to improve scalability. The complete handling of such external networks is beyond the scope of this document. However, this document specifies a minimal set of requirements and features Johnson, et al. Experimental [Page 26] RFC 4728 The Dynamic Source Routing Protocol February 2007 necessary to allow nodes only implementing this specification to interoperate correctly with nodes implementing interfaces to such external networks. This minimal set of requirements and features involve the First Hop External (F) and Last Hop External (L) bits in a DSR Source Route option (Section 6.7) and a Route Reply option (Section 6.3) in a packet's DSR Options header (Section 6). These requirements also include the addition of an External flag bit tagging each link in the Route Cache, copied from the First Hop External (F) and Last Hop External (L) bits in the DSR Source Route option or Route Reply option from which this link was learned. The Route Cache SHOULD support storing more than one route to each destination. In searching the Route Cache for a route to some destination node, the Route Cache is searched by destination node address. The following properties describe this searching function on a Route Cache: - Each implementation of DSR at any node MAY choose any appropriate strategy and algorithm for searching its Route Cache and selecting a "best" route to the destination from among those found. For example, a node MAY choose to select the shortest route to the destination (the shortest sequence of hops), or it MAY use an alternate metric to select the route from the Cache. - However, if there are multiple cached routes to a destination, the selection of routes when searching the Route Cache SHOULD prefer routes that do not have the External flag set on any link. This preference will select routes that lead directly to the target node over routes that attempt to reach the target via any external networks connected to the DSR ad hoc network. - In addition, any route selected when searching the Route Cache MUST NOT have the External bit set for any links other than possibly the first link, the last link, or both; the External bit MUST NOT be set for any intermediate hops in the route selected. An implementation of a Route Cache MAY provide a fixed capacity for the cache, or the cache size MAY be variable. The following properties describe the management of available space within a node's Route Cache: - Each implementation of DSR at each node MAY choose any appropriate policy for managing the entries in its Route Cache, such as when limited cache capacity requires a choice of which entries to retain in the Cache. For example, a node MAY chose a "least recently used" (LRU) cache replacement policy, in which the entry Johnson, et al. Experimental [Page 27] RFC 4728 The Dynamic Source Routing Protocol February 2007 last used longest ago is discarded from the cache if a decision needs to be made to allow space in the cache for some new entry being added. - However, the Route Cache replacement policy SHOULD allow routes to be categorized based upon "preference", where routes with a higher preferences are less likely to be removed from the cache. For example, a node could prefer routes for which it initiated a Route Discovery over routes that it learned as the result of promiscuous snooping on other packets. In particular, a node SHOULD prefer routes that it is presently using over those that it is not. Any suitable data structure organization, consistent with this specification, MAY be used to implement the Route Cache in any node. For example, the following two types of organization are possible: - In DSR, the route returned in each Route Reply that is received by the initiator of a Route Discovery (or that is learned from the header of overhead packets, as described in Section 8.1.4) represents a complete path (a sequence of links) leading to the destination node. By caching each of these paths separately, a "path cache" organization for the Route Cache can be formed. A path cache is very simple to implement and easily guarantees that all routes are loop-free, since each individual route from a Route Reply or Route Request or used in a packet is loop-free. To search for a route in a path cache data structure, the sending node can simply search its Route Cache for any path (or prefix of a path) that leads to the intended destination node. This type of organization for the Route Cache in DSR has been extensively studied through simulation [BROCH98, HU00, JOHANSSON99, MALTZ99a] and through implementation of DSR in a mobile outdoor testbed under significant workload [MALTZ99b, MALTZ00, MALTZ01]. - Alternatively, a "link cache" organization could be used for the Route Cache, in which each individual link (hop) in the routes returned in Route Reply packets (or otherwise learned from the header of overhead packets) is added to a unified graph data structure of this node's current view of the network topology. To search for a route in link cache, the sending node must use a more complex graph search algorithm, such as the well-known Dijkstra's shortest-path algorithm, to find the current best path through the graph to the destination node. Such an algorithm is more difficult to implement and may require significantly more CPU time to execute. Johnson, et al. Experimental [Page 28] RFC 4728 The Dynamic Source Routing Protocol February 2007 However, a link cache organization is more powerful than a path cache organization, in its ability to effectively utilize all of the potential information that a node might learn about the state of the network. In particular, links learned from different Route Discoveries or from the header of any overheard packets can be merged together to form new routes in the network, but this is not possible in a path cache due to the separation of each individual path in the cache. This type of organization for the Route Cache in DSR, including the effect of a range of implementation choices, has been studied through detailed simulation [HU00]. The choice of data structure organization to use for the Route Cache in any DSR implementation is a local matter for each node and affects only performance; any reasonable choice of organization for the Route Cache does not affect either correctness or interoperability. Each entry in the Route Cache SHOULD have a timeout associated with it, to allow that entry to be deleted if not used within some time. The particular choice of algorithm and data structure used to implement the Route Cache SHOULD be considered in choosing the timeout for entries in the Route Cache. The configuration variable RouteCacheTimeout defined in Section 9 specifies the timeout to be applied to entries in the Route Cache, although it is also possible to instead use an adaptive policy in choosing timeout values rather than using a single timeout setting for all entries. For example, the Link-MaxLife cache design (below) uses an adaptive timeout algorithm and does not use the RouteCacheTimeout configuration variable. As guidance to implementers, Appendix A describes a type of link cache known as "Link-MaxLife" that has been shown to outperform other types of link caches and path caches studied in detailed simulation [HU00]. Link-MaxLife is an adaptive link cache in which each link in the cache has a timeout that is determined dynamically by the caching node according to its observed past behavior of the two nodes at the ends of the link. In addition, when selecting a route for a packet being sent to some destination, among cached routes of equal length (number of hops) to that destination, Link-MaxLife selects the route with the longest expected lifetime (highest minimum timeout of any link in the route). Use of the Link-MaxLife design for the Route Cache is recommended in implementations of DSR. Johnson, et al. Experimental [Page 29] RFC 4728 The Dynamic Source Routing Protocol February 2007 4.2. Send Buffer The Send Buffer of a node implementing DSR is a queue of packets that cannot be sent by that node because it does not yet have a source route to each such packet's destination. Each packet in the Send Buffer is logically associated with the time that it was placed into the buffer and SHOULD be removed from the Send Buffer and silently discarded after a period of SendBufferTimeout after initially being placed in the buffer. If necessary, a FIFO strategy SHOULD be used to evict packets before they time out to prevent the buffer from overflowing. Subject to the rate limiting defined in Section 4.3, a Route Discovery SHOULD be initiated as often as allowed for the destination address of any packets residing in the Send Buffer. 4.3. Route Request Table The Route Request Table of a node implementing DSR records information about Route Requests that have been recently originated or forwarded by this node. The table is indexed by IP address. The Route Request Table on a node records the following information about nodes to which this node has initiated a Route Request: - The Time-to-Live (TTL) field used in the IP header of the Route Request for the last Route Discovery initiated by this node for that target node. This value allows the node to implement a variety of algorithms for controlling the spread of its Route Request on each Route Discovery initiated for a target. As examples, two possible algorithms for this use of the TTL field are described in Section 3.3.3. - The time that this node last originated a Route Request for that target node. - The number of consecutive Route Discoveries initiated for this target since receiving a valid Route Reply giving a route to that target node. - The remaining amount of time before which this node MAY next attempt at a Route Discovery for that target node. When the node initiates a new Route Discovery for this target node, this field in the Route Request Table entry for that target node is initialized to the timeout for that Route Discovery, after which the node MAY initiate a new Discovery for that target. Until a valid Route Reply is received for this target node address, a node MUST implement a back-off algorithm in determining this timeout Johnson, et al. Experimental [Page 30] RFC 4728 The Dynamic Source Routing Protocol February 2007 value for each successive Route Discovery initiated for this target using the same Time-to-Live (TTL) value in the IP header of the Route Request packet. The timeout between such consecutive Route Discovery initiations SHOULD increase by doubling the timeout value on each new initiation. In addition, the Route Request Table on a node also records the following information about initiator nodes from which this node has received a Route Request: - A FIFO cache of size RequestTableIds entries containing the Identification value and target address from the most recent Route Requests received by this node from that initiator node. Nodes SHOULD use an LRU policy to manage the entries in their Route Request Table. The number of Identification values to retain in each Route Request Table entry, RequestTableIds, MUST NOT be unlimited, since, in the worst case, when a node crashes and reboots, the first RequestTableIds Route Discoveries it initiates after rebooting could appear to be duplicates to the other nodes in the network. In addition, a node SHOULD base its initial Identification value, used for Route Discoveries after rebooting, on a battery backed-up clock or other persistent memory device, if available, in order to help avoid any possible such delay in successfully discovering new routes after rebooting; if no such source of initial Identification value is available, a node after rebooting SHOULD base its initial Identification value on a random number. 4.4. Gratuitous Route Reply Table The Gratuitous Route Reply Table of a node implementing DSR records information about "gratuitous" Route Replies sent by this node as part of automatic route shortening. As described in Section 3.4.3, a node returns a gratuitous Route Reply when it overhears a packet transmitted by some node, for which the node overhearing the packet was not the intended next-hop node but was named later in the unexpended hops of the source route in that packet; the node overhearing the packet returns a gratuitous Route Reply to the original sender of the packet, listing the shorter route (not including the hops of the source route "skipped over" by this packet). A node uses its Gratuitous Route Reply Table to limit the rate at which it originates gratuitous Route Replies to the same original sender for the same node from which it overheard a packet to trigger the gratuitous Route Reply. Johnson, et al. Experimental [Page 31] RFC 4728 The Dynamic Source Routing Protocol February 2007 Each entry in the Gratuitous Route Reply Table of a node contains the following fields: - The address of the node to which this node originated a gratuitous Route Reply. - The address of the node from which this node overheard the packet triggering that gratuitous Route Reply. - The remaining time before which this entry in the Gratuitous Route Reply Table expires and SHOULD be deleted by the node. When a node creates a new entry in its Gratuitous Route Reply Table, the timeout value for that entry SHOULD be initialized to the value GratReplyHoldoff. When a node overhears a packet that would trigger a gratuitous Route Reply, if a corresponding entry already exists in the node's Gratuitous Route Reply Table, then the node SHOULD NOT send a gratuitous Route Reply for that packet. Otherwise (i.e., if no corresponding entry already exists), the node SHOULD create a new entry in its Gratuitous Route Reply Table to record that gratuitous Route Reply, with a timeout value of GratReplyHoldoff. 4.5. Network Interface Queue and Maintenance Buffer Depending on factors such as the structure and organization of the operating system, protocol stack implementation, network interface device driver, and network interface hardware, a packet being transmitted could be queued in a variety of ways. For example, outgoing packets from the network protocol stack might be queued at the operating system or link layer, before transmission by the network interface. The network interface might also provide a retransmission mechanism for packets, such as occurs in IEEE 802.11 [IEEE80211]; the DSR protocol, as part of Route Maintenance, requires limited buffering of packets already transmitted for which the reachability of the next-hop destination has not yet been determined. The operation of DSR is defined here in terms of two conceptual data structures that, together, incorporate this queuing behavior. The Network Interface Queue of a node implementing DSR is an output queue of packets from the network protocol stack waiting to be transmitted by the network interface; for example, in the 4.4BSD Unix network protocol stack implementation, this queue for a network interface is represented as a "struct ifqueue" [WRIGHT95]. This queue is used to hold packets while the network interface is in the process of transmitting another packet. Johnson, et al. Experimental [Page 32] RFC 4728 The Dynamic Source Routing Protocol February 2007 The Maintenance Buffer of a node implementing DSR is a queue of packets sent by this node that are awaiting next-hop reachability confirmation as part of Route Maintenance. For each packet in the Maintenance Buffer, a node maintains a count of the number of retransmissions and the time of the last retransmission. Packets are added to the Maintenance buffer after the first transmission attempt is made. The Maintenance Buffer MAY be of limited size; when adding a new packet to the Maintenance Buffer, if the buffer size is insufficient to hold the new packet, the new packet SHOULD be silently discarded. If, after MaxMaintRexmt attempts to confirm next-hop reachability of some node, no confirmation is received, all packets in this node's Maintenance Buffer with this next-hop destination SHOULD be removed from the Maintenance Buffer. In this case, the node also SHOULD originate a Route Error for this packet to each original source of a packet removed in this way (Section 8.3) and SHOULD salvage each packet removed in this way (Section 8.3.6) if it has another route to that packet's IP Destination Address in its Route Cache. The definition of MaxMaintRexmt conceptually includes any retransmissions that might be attempted for a packet at the link layer or within the network interface hardware. The timeout value to use for each transmission attempt for an acknowledgement request depends on the type of acknowledgement mechanism used by Route Maintenance for that attempt, as described in Section 8.3. 4.6. Blacklist When a node using the DSR protocol is connected through a network interface that requires physically bidirectional links for unicast transmission, the node MUST maintain a blacklist. The blacklist is a table, indexed by neighbor node address, that indicates that the link between this node and the specified neighbor node may not be bidirectional. A node places another node's address in this list when it believes that broadcast packets from that other node reach this node, but that unicast transmission between the two nodes is not possible. For example, if a node forwarding a Route Reply discovers that the next hop is unreachable, it places that next hop in the node's blacklist. Once a node discovers that it can communicate bidirectionally with one of the nodes listed in the blacklist, it SHOULD remove that node from the blacklist. For example, if node A has node B listed in its blacklist, but after transmitting a Route Request, node A hears B forward the Route Request with a route record indicating that the broadcast from A to B was successful, then A SHOULD remove the entry for node B from its blacklist. Johnson, et al. Experimental [Page 33] RFC 4728 The Dynamic Source Routing Protocol February 2007 A node MUST associate a state with each node listed in its blacklist, specifying whether the unidirectionality of the link to that node is "questionable" or "probable". Each time the unreachability is positively determined, the node SHOULD set the state to "probable". After the unreachability has not been positively determined for some amount of time, the state SHOULD revert to "questionable". A node MAY expire entries for nodes from its blacklist after a reasonable amount of time. 5. Additional Conceptual Data Structures for Flow State Extension This section defines additional conceptual data structures used by the optional "flow state" extension to DSR. In an implementation of the protocol, these data structures MUST be implemented in a manner consistent with the external behavior described in this document, but the choice of implementation used is otherwise unconstrained. 5.1. Flow Table A node implementing the flow state extension MUST implement a Flow Table or other data structure consistent with the external behavior described in this section. A node not implementing the flow state extension SHOULD NOT implement a Flow Table. The Flow Table records information about flows from which packets recently have been sent or forwarded by this node. The table is indexed by a triple (IP Source Address, IP Destination Address, Flow ID), where Flow ID is a 16-bit number assigned by the source as described in Section 3.5.1. Each entry in the Flow Table contains the following fields: - The MAC address of the next-hop node along this flow. - An indication of the outgoing network interface on this node to be used in transmitting packets along this flow. - The MAC address of the previous-hop node along this flow. - An indication of the network interface on this node from which packets from that previous-hop node are received. - A timeout after which this entry in the Flow Table MUST be deleted. - The expected value of the Hop Count field in the DSR Flow State header for packets received for forwarding along this field (for use with packets containing a DSR Flow State header). Johnson, et al. Experimental [Page 34] RFC 4728 The Dynamic Source Routing Protocol February 2007 - An indication of whether or not this flow can be used as a default flow for packets originated by this node (the Flow ID of a default flow MUST be odd). - The entry SHOULD record the complete source route for the flow. (Nodes not recording the complete source route cannot participate in Automatic Route Shortening.) - The entry MAY contain a field recording the time this entry was last used. The entry MUST be deleted when its timeout expires. 5.2. Automatic Route Shortening Table A node implementing the flow state extension SHOULD implement an Automatic Route Shortening Table or other data structure consistent with the external behavior described in this section. A node not implementing the flow state extension SHOULD NOT implement an Automatic Route Shortening Table. The Automatic Route Shortening Table records information about received packets for which Automatic Route Shortening may be possible. The table is indexed by a triple (IP Source Address, IP Destination Address, Flow ID). Each entry in the Automatic Route Shortening Table contains a list of (packet identifier, Hop Count) pairs for that flow. The packet identifier in the list may be any unique identifier for the received packet; for example, for IPv4 packets, the combination of the following fields from the packet's IP header MAY be used as a unique identifier for the packet: Source Address, Destination Address, Identification, Protocol, Fragment Offset, and Total Length. The Hop Count in the list in the entry is copied from the Hop Count field in the DSR Flow State header of the received packet for which this table entry was created. Any packet identifier SHOULD appear at most once in an entry's list, and this list item SHOULD record the minimum Hop Count value received for that packet (if the wireless signal strength or signal-to-noise ratio at which a packet is received is available to the DSR implementation in a node, the node MAY, for example, remember instead in this list the minimum Hop Count value for which the received packet's signal strength or signal-to-noise ratio exceeded some threshold). Space in the Automatic Route Shortening Table of a node MAY be dynamically managed by any local algorithm at the node. For example, in order to limit the amount of memory used to store the table, any existing entry MAY be deleted at any time, and the number of packets listed in each entry MAY be limited. However, when reclaiming space in the table, nodes SHOULD favor retaining information about more Johnson, et al. Experimental [Page 35] RFC 4728 The Dynamic Source Routing Protocol February 2007 flows in the table rather than about more packets listed in each entry in the table, as long as at least the listing of some small number of packets (e.g., 3) can be retained in each entry. 5.3. Default Flow ID Table A node implementing the flow state extension MUST implement a Default Flow Table or other data structure consistent with the external behavior described in this section. A node not implementing the flow state extension SHOULD NOT implement a Default Flow Table. For each (IP Source Address, IP Destination Address) pair for which a node forwards packets, the node MUST record: - The largest odd Flow ID value seen. - The time at which all the corresponding flows that are forwarded by this node expire. - The current default Flow ID. - A flag indicating whether or not the current default Flow ID is valid. If a node deletes this record for an (IP Source Address, IP Destination Address) pair, it MUST also delete all Flow Table entries for that pair. Nodes MUST delete table entries if all of this (IP Source Address, IP Destination Address) pair's flows that are forwarded by this node expire. 6. DSR Options Header Format The Dynamic Source Routing protocol makes use of a special header carrying control information that can be included in any existing IP packet. This DSR Options header in a packet contains a small fixed- sized, 4-octet portion, followed by a sequence of zero or more DSR options carrying optional information. The end of the sequence of DSR options in the DSR Options header is implied by the total length of the DSR Options header. For IPv4, the DSR Options header MUST immediately follow the IP header in the packet. (If a Hop-by-Hop Options extension header, as defined in IPv6 [RFC2460], becomes defined for IPv4, the DSR Options header MUST immediately follow the Hop-by-Hop Options extension header, if one is present in the packet, and MUST otherwise immediately follow the IP header.) Johnson, et al. Experimental [Page 36] RFC 4728 The Dynamic Source Routing Protocol February 2007 To add a DSR Options header to a packet, the DSR Options header is inserted following the packet's IP header, before any following header such as a traditional (e.g., TCP or UDP) transport layer header. Specifically, the Protocol field in the IP header is used to indicate that a DSR Options header follows the IP header, and the Next Header field in the DSR Options header is used to indicate the type of protocol header (such as a transport layer header) following the DSR Options header. If any headers follow the DSR Options header in a packet, the total length of the DSR Options header (and thus the total, combined length of all DSR options present) MUST be a multiple of 4 octets. This requirement preserves the alignment of these following headers in the packet. 6.1. Fixed Portion of DSR Options Header The fixed portion of the DSR Options header is used to carry information that must be present in any DSR Options header. This fixed portion of the DSR Options header has the following format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Header |F| Reserved | Payload Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . Options . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Next Header 8-bit selector. Identifies the type of header immediately following the DSR Options header. Uses the same values as the IPv4 Protocol field [RFC1700]. If no header follows, then Next Header MUST have the value 59, "No Next Header" [RFC2460]. Flow State Header (F) Flag bit. MUST be set to 0. This bit is set in a DSR Flow State header (Section 7.1) and clear in a DSR Options header. Reserved MUST be sent as 0 and ignored on reception. Johnson, et al. Experimental [Page 37] RFC 4728 The Dynamic Source Routing Protocol February 2007 Payload Length The length of the DSR Options header, excluding the 4-octet fixed portion. The value of the Payload Length field defines the total length of all options carried in the DSR Options header. Options Variable-length field; the length of the Options field is specified by the Payload Length field in this DSR Options header. Contains one or more pieces of optional information (DSR options), encoded in type-length-value (TLV) format (with the exception of the Pad1 option described in Section 6.8). The placement of DSR options following the fixed portion of the DSR Options header MAY be padded for alignment. However, due to the typically limited available wireless bandwidth in ad hoc networks, this padding is not required, and receiving nodes MUST NOT expect options within a DSR Options header to be aligned. Each DSR option is assigned a unique Option Type code. The most significant 3 bits (that is, Option Type & 0xE0) allow a node not implementing processing for this Option Type value to behave in the manner closest to correct for that type: - The most significant bit in the Option Type value (that is, Option Type & 0x80) represents whether or not a node receiving this Option Type (when the node does not implement processing for this Option Type) SHOULD respond to such a DSR option with a Route Error of type OPTION_NOT_SUPPORTED, except that such a Route Error SHOULD never be sent in response to a packet containing a Route Request option. - The two following bits in the Option Type value (that is, Option Type & 0x60) are a two-bit field indicating how such a node that does not support this Option Type MUST process the packet: 00 = Ignore Option 01 = Remove Option 10 = Mark Option 11 = Drop Packet When these 2 bits are 00 (that is, Option Type & 0x60 == 0), a node not implementing processing for that Option Type MUST use the Opt Data Len field to skip over the option and continue processing. When these 2 bits are 01 (that is, Option Type & 0x60 == 0x20), a node not implementing processing for that Option Type Johnson, et al. Experimental [Page 38] RFC 4728 The Dynamic Source Routing Protocol February 2007 MUST use the Opt Data Len field to remove the option from the packet and continue processing as if the option had not been included in the received packet. When these 2 bits are 10 (that is, Option Type & 0x60 == 0x40), a node not implementing processing for that Option Type MUST set the most significant bit following the Opt Data Len field, MUST ignore the contents of the option using the Opt Data Len field, and MUST continue processing the packet. Finally, when these 2 bits are 11 (that is, Option Type & 0x60 == 0x60), a node not implementing processing for that Option Type MUST drop the packet. The following types of DSR options are defined in this document for use within a DSR Options header: - Route Request option (Section 6.2) - Route Reply option (Section 6.3) - Route Error option (Section 6.4) - Acknowledgement Request option (Section 6.5) - Acknowledgement option (Section 6.6) - DSR Source Route option (Section 6.7) - Pad1 option (Section 6.8) - PadN option (Section 6.9) In addition, Section 7 specifies further DSR options for use with the optional DSR flow state extension. Johnson, et al. Experimental [Page 39] RFC 4728 The Dynamic Source Routing Protocol February 2007 6.2. Route Request Option The Route Request option in a DSR Options header is encoded as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type | Opt Data Len | Identification | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Target Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address[1] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address[2] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address[n] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ IP fields: Source Address MUST be set to the address of the node originating this packet. Intermediate nodes that retransmit the packet to propagate the Route Request MUST NOT change this field. Destination Address MUST be set to the IP limited broadcast address (255.255.255.255). Hop Limit (TTL) MAY be varied from 1 to 255, for example, to implement non- propagating Route Requests and Route Request expanding-ring searches (Section 3.3.3). Route Request fields: Option Type 1. Nodes not understanding this option will ignore this option. Johnson, et al. Experimental [Page 40] RFC 4728 The Dynamic Source Routing Protocol February 2007 Opt Data Len 8-bit unsigned integer. Length of the option, in octets, excluding the Option Type and Opt Data Len fields. MUST be set equal to (4 * n) + 6, where n is the number of addresses in the Route Request Option. Identification A unique value generated by the initiator (original sender) of the Route Request. Nodes initiating a Route Request generate a new Identification value for each Route Request, for example based on a sequence number counter of all Route Requests initiated by the node. This value allows a receiving node to determine whether it has recently seen a copy of this Route Request. If this Identification value (for this IP Source address and Target Address) is found by this receiving node in its Route Request Table (in the cache of Identification values in the entry there for this initiating node), this receiving node MUST discard the Route Request. When a Route Request is propagated, this field MUST be copied from the received copy of the Route Request being propagated. Target Address The address of the node that is the target of the Route Request. Address[1..n] Address[i] is the IPv4 address of the i-th node recorded in the Route Request option. The address given in the Source Address field in the IP header is the address of the initiator of the Route Discovery and MUST NOT be listed in the Address[i] fields; the address given in Address[1] is thus the IPv4 address of the first node on the path after the initiator. The number of addresses present in this field is indicated by the Opt Data Len field in the option (n = (Opt Data Len - 6) / 4). Each node propagating the Route Request adds its own address to this list, increasing the Opt Data Len value by 4 octets. The Route Request option MUST NOT appear more than once within a DSR Options header. Johnson, et al. Experimental [Page 41] RFC 4728 The Dynamic Source Routing Protocol February 2007 6.3. Route Reply Option The Route Reply option in a DSR Options header is encoded as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type | Opt Data Len |L| Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address[1] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address[2] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address[n] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ IP fields: Source Address Set to the address of the node sending the Route Reply. In the case of a node sending a reply from its Route Cache (Section 3.3.2) or sending a gratuitous Route Reply (Section 3.4.3), this address can differ from the address that was the target of the Route Discovery. Destination Address MUST be set to the address of the source node of the route being returned. Copied from the Source Address field of the Route Request generating the Route Reply or, in the case of a gratuitous Route Reply, copied from the Source Address field of the data packet triggering the gratuitous Reply. Route Reply fields: Option Type 2. Nodes not understanding this option will ignore this option. Johnson, et al. Experimental [Page 42] RFC 4728 The Dynamic Source Routing Protocol February 2007 Opt Data Len 8-bit unsigned integer. Length of the option, in octets, excluding the Option Type and Opt Data Len fields. MUST be set equal to (4 * n) + 1, where n is the number of addresses in the Route Reply Option. Last Hop External (L) Set to indicate that the last hop given by the Route Reply (the link from Address[n-1] to Address[n]) is actually an arbitrary path in a network external to the DSR network; the exact route outside the DSR network is not represented in the Route Reply. Nodes caching this hop in their Route Cache MUST flag the cached hop with the External flag. Such hops MUST NOT be returned in a cached Route Reply generated from this Route Cache entry, and selection of routes from the Route Cache to route a packet being sent SHOULD prefer routes that contain no hops flagged as External. Reserved MUST be sent as 0 and ignored on reception. Address[1..n] The source route being returned by the Route Reply. The route indicates a sequence of hops, originating at the source node specified in the Destination Address field of the IP header of the packet carrying the Route Reply, through each of the Address[i] nodes in the order listed in the Route Reply, ending at the node indicated by Address[n]. The number of addresses present in the Address[1..n] field is indicated by the Opt Data Len field in the option (n = (Opt Data Len - 1) / 4). A Route Reply option MAY appear one or more times within a DSR Options header. Johnson, et al. Experimental [Page 43] RFC 4728 The Dynamic Source Routing Protocol February 2007 6.4. Route Error Option The Route Error option in a DSR Options header is encoded as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type | Opt Data Len | Error Type |Reservd|Salvage| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Error Source Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Error Destination Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . Type-Specific Information . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Option Type 3. Nodes not understanding this option will ignore this option. Opt Data Len 8-bit unsigned integer. Length of the option, in octets, excluding the Option Type and Opt Data Len fields. For the current definition of the Route Error option, this field MUST be set to 10, plus the size of any Type-Specific Information present in the Route Error. Further extensions to the Route Error option format may also be included after the Type-Specific Information portion of the Route Error option specified above. The presence of such extensions will be indicated by the Opt Data Len field. When the Opt Data Len is greater than that required for the fixed portion of the Route Error plus the necessary Type-Specific Information as indicated by the Option Type value in the option, the remaining octets are interpreted as extensions. Currently, no such further extensions have been defined. Error Type The type of error encountered. Currently, the following type values are defined: Johnson, et al. Experimental [Page 44] RFC 4728 The Dynamic Source Routing Protocol February 2007 1 = NODE_UNREACHABLE 2 = FLOW_STATE_NOT_SUPPORTED 3 = OPTION_NOT_SUPPORTED Other values of the Error Type field are reserved for future use. Reservd Reserved. MUST be sent as 0 and ignored on reception. Salvage A 4-bit unsigned integer. Copied from the Salvage field in the DSR Source Route option of the packet triggering the Route Error. The "total salvage count" of the Route Error option is derived from the value in the Salvage field of this Route Error option and all preceding Route Error options in the packet as follows: the total salvage count is the sum of, for each such Route Error option, one plus the value in the Salvage field of that Route Error option. Error Source Address The address of the node originating the Route Error (e.g., the node that attempted to forward a packet and discovered the link failure). Error Destination Address The address of the node to which the Route Error must be delivered. For example, when the Error Type field is set to NODE_UNREACHABLE, this field will be set to the address of the node that generated the routing information claiming that the hop from the Error Source Address to Unreachable Node Address (specified in the Type-Specific Information) was a valid hop. Type-Specific Information Information specific to the Error Type of this Route Error message. A Route Error option MAY appear one or more times within a DSR Options header. Johnson, et al. Experimental [Page 45] RFC 4728 The Dynamic Source Routing Protocol February 2007 6.4.1. Node Unreachable Type-Specific Information When the Route Error is of type NODE_UNREACHABLE, the Type-Specific Information field is defined as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Unreachable Node Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Unreachable Node Address The IP address of the node that was found to be unreachable (the next-hop neighbor to which the node with address Error Source Address was attempting to transmit the packet). 6.4.2. Flow State Not Supported Type-Specific Information When the Route Error is of type FLOW_STATE_NOT_SUPPORTED, the Type-Specific Information field is empty. 6.4.3. Option Not Supported Type-Specific Information When the Route Error is of type OPTION_NOT_SUPPORTED, the Type-Specific Information field is defined as follows: 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |Unsupported Opt| +-+-+-+-+-+-+-+-+ Unsupported Opt The Option Type of option triggering the Route Error. 6.5. Acknowledgement Request Option The Acknowledgement Request option in a DSR Options header is encoded as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type | Opt Data Len | Identification | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Johnson, et al. Experimental [Page 46] RFC 4728 The Dynamic Source Routing Protocol February 2007 Option Type 160. Nodes not understanding this option will remove the option and return a Route Error. Opt Data Len 8-bit unsigned integer. Length of the option, in octets, excluding the Option Type and Opt Data Len fields. Identification The Identification field is set to a unique value and is copied into the Identification field of the Acknowledgement option when returned by the node receiving the packet over this hop. An Acknowledgement Request option MUST NOT appear more than once within a DSR Options header. 6.6. Acknowledgement Option The Acknowledgement option in a DSR Options header is encoded as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type | Opt Data Len | Identification | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ACK Source Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ACK Destination Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Option Type 32. Nodes not understanding this option will remove the option. Opt Data Len 8-bit unsigned integer. Length of the option, in octets, excluding the Option Type and Opt Data Len fields. Identification Copied from the Identification field of the Acknowledgement Request option of the packet being acknowledged. Johnson, et al. Experimental [Page 47] RFC 4728 The Dynamic Source Routing Protocol February 2007 ACK Source Address The address of the node originating the acknowledgement. ACK Destination Address The address of the node to which the acknowledgement is to be delivered. An Acknowledgement option MAY appear one or more times within a DSR Options header. 6.7. DSR Source Route Option The DSR Source Route option in a DSR Options header is encoded as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type | Opt Data Len |F|L|Reservd|Salvage| Segs Left | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address[1] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address[2] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address[n] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Option Type 96. Nodes not understanding this option will drop the packet. Opt Data Len 8-bit unsigned integer. Length of the option, in octets, excluding the Option Type and Opt Data Len fields. For the format of the DSR Source Route option defined here, this field MUST be set to the value (n * 4) + 2, where n is the number of addresses present in the Address[i] fields. First Hop External (F) Set to indicate that the first hop indicated by the DSR Source Route option is actually an arbitrary path in a network external to the DSR network; the exact route outside the DSR Johnson, et al. Experimental [Page 48] RFC 4728 The Dynamic Source Routing Protocol February 2007 network is not represented in the DSR Source Route option. Nodes caching this hop in their Route Cache MUST flag the cached hop with the External flag. Such hops MUST NOT be returned in a Route Reply generated from this Route Cache entry, and selection of routes from the Route Cache to route a packet being sent SHOULD prefer routes that contain no hops flagged as External. Last Hop External (L) Set to indicate that the last hop indicated by the DSR Source Route option is actually an arbitrary path in a network external to the DSR network; the exact route outside the DSR network is not represented in the DSR Source Route option. Nodes caching this hop in their Route Cache MUST flag the cached hop with the External flag. Such hops MUST NOT be returned in a Route Reply generated from this Route Cache entry, and selection of routes from the Route Cache to route a packet being sent SHOULD prefer routes that contain no hops flagged as External. Reserved MUST be sent as 0 and ignored on reception. Salvage A 4-bit unsigned integer. Count of number of times that this packet has been salvaged as a part of DSR routing (Section 3.4.1). Segments Left (Segs Left) Number of route segments remaining, i.e., number of explicitly listed intermediate nodes still to be visited before reaching the final destination. Address[1..n] The sequence of addresses of the source route. In routing and forwarding the packet, the source route is processed as described in Sections 8.1.3 and 8.1.5. The number of addresses present in the Address[1..n] field is indicated by the Opt Data Len field in the option (n = (Opt Data Len - 2) / 4). When forwarding a packet along a DSR source route using a DSR Source Route option in the packet's DSR Options header, the Destination Address field in the packet's IP header is always set to the address Johnson, et al. Experimental [Page 49] RFC 4728 The Dynamic Source Routing Protocol February 2007 of the packet's ultimate destination. A node receiving a packet containing a DSR Options header with a DSR Source Route option MUST examine the indicated source route to determine if it is the intended next-hop node for the packet and how to forward the packet, as defined in Sections 8.1.4 and 8.1.5. 6.8. Pad1 Option The Pad1 option in a DSR Options header is encoded as follows: +-+-+-+-+-+-+-+-+ | Option Type | +-+-+-+-+-+-+-+-+ Option Type 224. Nodes not understanding this option will drop the packet and return a Route Error. A Pad1 option MAY be included in the Options field of a DSR Options header in order to align subsequent DSR options, but such alignment is not required and MUST NOT be expected by a node receiving a packet containing a DSR Options header. If any headers follow the DSR Options header in a packet, the total length of a DSR Options header, indicated by the Payload Length field in the DSR Options header MUST be a multiple of 4 octets. In this case, when building a DSR Options header in a packet, sufficient Pad1 or PadN options MUST be included in the Options field of the DSR Options header to make the total length a multiple of 4 octets. If more than one consecutive octet of padding is being inserted in the Options field of a DSR Options header, the PadN option described next, SHOULD be used, rather than multiple Pad1 options. Note that the format of the Pad1 option is a special case; it does not have an Opt Data Len or Option Data field. 6.9. PadN Option The PadN option in a DSR Options header is encoded as follows: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - | Option Type | Opt Data Len | Option Data +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - Johnson, et al. Experimental [Page 50] RFC 4728 The Dynamic Source Routing Protocol February 2007 Option Type 0. Nodes not understanding this option will ignore this option. Opt Data Len 8-bit unsigned integer. Length of the option, in octets, excluding the Option Type and Opt Data Len fields. The size of the Option Data field. Option Data A number of zero-valued octets equal to the Opt Data Len. A PadN option MAY be included in the Options field of a DSR Options header in order to align subsequent DSR options, but such alignment is not required and MUST NOT be expected by a node receiving a packet containing a DSR Options header. If any headers follow the DSR Options header in a packet, the total length of a DSR Options header, indicated by the Payload Length field in the DSR Options header, MUST be a multiple of 4 octets. In this case, when building a DSR Options header in a packet, sufficient Pad1 or PadN options MUST be included in the Options field of the DSR Options header to make the total length a multiple of 4 octets. 7. Additional Header Formats and Options for Flow State Extension The optional DSR flow state extension requires a new header type, the DSR Flow State header. In addition, the DSR flow state extension adds the following options for the DSR Options header defined in Section 6: - Timeout option (Section 7.2.1) - Destination and Flow ID option (Section 7.2.2) Two new Error Type values are also defined for use in the Route Error option in a DSR Options header: - UNKNOWN_FLOW - DEFAULT_FLOW_UNKNOWN Finally, an extension to the Acknowledgement Request option in a DSR Options header is also defined: Johnson, et al. Experimental [Page 51] RFC 4728 The Dynamic Source Routing Protocol February 2007 - Previous Hop Address This section defines each of these new header, option, or extension formats. 7.1. DSR Flow State Header The DSR Flow State header is a small 4-byte header optionally used to carry the flow ID and hop count for a packet be