动态源路由协议(Dynamic Source Routing, DSR)

动态源路由协议(Dynamic Source Routing, DSR)是在移动自组网(MANET)中使用的一种路由协议。它工作在TCP/IP协议族的网际层。

　　

它的首部采用扩展性良好的TLV格式。除固定部分外，不同类型的选项(option)以TLV格式附加在固定部分之后。

　　

选项的种类包括：

　　路由请求(Route Request)

　　路由应答(Route Reply)

　　确认请求(ACK Request)

　　确认(ACK)

　　源路由(Source Route)

 

 

Dynamic source routing protocol (DSR) is an on-demand protocol designed to restrict the bandwidth consumed by control packets in ad hoc wireless networks by eliminating the periodic table-update messages required in the table-driven approach.

 

The major difference between this and the other on-demand routing protocols is that it is beacon-less and hence does not require periodic hello packet (beacon) transmissions, which are used by a node to inform its neighbors of its presence.

 

The basic approach of this protocol (and all other on-demand routing protocols) during the route construction phase is to establish a route by flooding RouteRequest packets in the network. The destination node, on receiving a RouteRequest packet, responds by sending a RouteReply packet back to the source, which carries the route traversed by the RouteRequest packet received.

 

当一个节点发包到一个目标节点而且没有到这个点的路径时， 它发出一个RouteRequest包。 RouteRequest涌出到整个网络。每个节点收到RouteRequest后继续广播到它们的相邻的节点，直到发到目标节点为止（在没有超过TTL的情况下）。 每个RouteRequest都含有一个序号sequence number， 这个number由源节点和RouteRequest穿过的路径生成。 当一个几点收到RouteRequest时，它在转发之前先检查这个序号。只有不是重复的RouteRequest（序号不同）才会转发。 当目标节点收到RouteRequest后，通过反转RouteRequest中保存的路径发送回复到源节点。

 

建立连接还可以通过使用route cache， 当一个中间节点收到RouteRequest而且这个节点的route cache中有到目标节点的路径， 它就直接回复一个包含从源节点到目标节点完整路径信息的RouteReply到源节点

 

Consider a source node that does not have a route to the destination. When it has data packets to be sent to that destination, it initiates a RouteRequest packet. This RouteRequest is flooded throughout the network. Each node, upon receiving a RouteRequest packet, rebroadcasts the packet to its neighbors if it has not forwarded already or if the node is not the destination node, provided the packet’s time to live (TTL) counter has not exceeded. Each RouteRequest carries a sequence number generated by the source node and the path it has traversed. A node, upon receiving a RouteRequest packet, checks the sequence number on the packet before forwarding it. The packet is forwarded only if it is not a duplicate RouteRequest. The sequence number on the packet is used to prevent loop formations and to avoid multiple transmissions of the same RouteRequest by an intermediate node that receives it through multiple paths. Thus, all nodes except the destination forward a RouteRequest packet during the route construction phase. A destination node, after receiving the first RouteRequest packet, replies to the source node through the reverse path the RouteRequest packet had traversed.

 

Nodes can also learn about the neighboring routes traversed by data packets if operated in the promiscuous mode (the mode of operation in which a node can receive the packets that are neither broadcast nor addressed to itself). This route cache is also used during the route construction phase. If an intermediate node receiving a RouteRequest has a route to the destination node in its route cache, then it replies to the source node by sending a RouteReply with the entire route information from the source node to the destination node.

 

 

 

Advantages and Disadvantages

This protocol uses a reactive approach which eliminates the need to periodically flood the network with table update messages which are required in a table-driven approach.

 

In a reactive (on-demand) approach such as this, a route is established only when it is required and hence the need to find routes to all other nodes in the network as required by the table-driven approach is eliminated.

 

The intermediate nodes also utilize the route cache information efficiently to reduce the control overhead.

 

The disadvantage

 

• this protocol is that the route maintenance mechanism does not locally repair a broken link.
• Stale route cache information could also result in inconsistencies during the route reconstruction phase.
• The connection setup delay is higher than in table-driven protocols.
• Even though the protocol performs well in static and low-mobility environments, the performance degrades rapidly with increasing mobility.
• Also, considerable routing overhead is involved due to the source-routing mechanism employed in DSR. This routing overhead is directly proportional to the path length.

 

与AODV对比

 

The AODV Routing protocol uses an on-demand approach for finding routes, that is, a route is established only when it is required by a source node for transmitting data packets. It employs destination sequence numbers to identify the most recent path.

 

The major difference between AODV and Dynamic Source Routing (DSR) stems out from the fact that DSR uses source routing in which a data packet carries the complete path to be traversed. However, in AODV, the source node and the intermediate nodes store the next-hop information corresponding to each flow for data packet transmission.

 

In an on-demand routing protocol, the source node floods the RouteRequest packet in the network when a route is not available for the desired destination. It may obtain multiple routes to different destinations from a single RouteRequest. The major difference between AODV and other on-demand routing protocols is that it uses a destination sequence number (DestSeqNum) to determine an up-to-date path to the destination. A node updates its path information only if the DestSeqNum of the current packet received is greater than the last DestSeqNum stored at the node.

 

 

 

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