12. DSR Routing

Dynamic Source Routing (DSR) protocol is a reactive routing protocol designed specifically for use in multi-hop wireless ad hoc networks of mobile nodes.

This model was developed by the ResiliNets research group at the University of Kansas.

12.1. DSR Routing Overview

This model implements the base specification of the Dynamic Source Routing (DSR) protocol. Implementation is based on RFC 4728, with some extensions and modifications to the RFC specifications.

DSR operates on a on-demand behavior. Therefore, our DSR model buffers all packets while a route request packet (RREQ) is disseminated. We implement a packet buffer in dsr-rsendbuff.cc. The packet queue implements garbage collection of old packets and a queue size limit. When the packet is sent out from the send buffer, it will be queued in maintenance buffer for next hop acknowledgment.

The maintenance buffer then buffers the already sent out packets and waits for the notification of packet delivery. Protocol operation strongly depends on broken link detection mechanism. We implement the three heuristics recommended based the RFC as follows:

First, we use link layer feedback when possible, which is also the fastest mechanism of these three to detect link errors. A link is considered to be broken if frame transmission results in a transmission failure for all retries. This mechanism is meant for active links and works much faster than in its absence. DSR is able to detect the link layer transmission failure and notify that as broken. Recalculation of routes will be triggered when needed. If user does not want to use link layer acknowledgment, it can be tuned by setting “LinkAcknowledgment” attribute to false in “dsr-routing.cc”.

Second, passive acknowledgment should be used whenever possible. The node turns on “promiscuous” receive mode, in which it can receive packets not destined for itself, and when the node assures the delivery of that data packet to its destination, it cancels the passive acknowledgment timer.

Last, we use a network layer acknowledge scheme to notify the receipt of a packet. Route request packet will not be acknowledged or retransmitted.

The Route Cache implementation support garbage collection of old entries and state machine, as defined in the standard. It implements as a STL map container. The key is the destination IP address.

DSR operates with direct access to IP header, and operates between network and transport layer. When packet is sent out from transport layer, it passes itself to DSR and DSR header is appended.

We have two caching mechanisms: path cache and link cache. The path cache saves the whole path in the cache. The paths are sorted based on the hop count, and whenever one path is not able to be used, we change to the next path. The link cache is a slightly better design in the sense that it uses different subpaths and uses Implemented Link Cache using Dijkstra algorithm, and this part is implemented by Song Luan <lsuper@mail.ustc.edu.cn>.

The following optional protocol optimizations aren’t implemented:

  • Flow state

  • First Hop External (F), Last Hop External (L) flags

  • Handling unknown DSR options

  • Two types of error headers:
    1. flow state not supported option

    2. unsupported option (not going to happen in simulation)

12.1.1. DSR update in ns-3.17

We originally used “TxErrHeader” in Ptr<WifiMac> to indicate the transmission error of a specific packet in link layer, however, it was not working quite correctly since even when the packet was dropped, this header was not recorded in the trace file. We used to a different path on implementing the link layer notification mechanism. We look into the trace file by finding packet receive event. If we find one receive event for the data packet, we count that as the indicator for successful data delivery.

12.1.2. Useful parameters

+------------------------- +------------------------------------+-------------+
| Parameter                | Description                        | Default     |
| MaxSendBuffLen           | Maximum number of packets that can | 64          |
|                          | be stored in send buffer           |             |
+------------------------- +------------------------------------+-------------+
| MaxSendBuffTime          | Maximum time packets can be queued | Seconds(30) |
|                          | in the send buffer                 |             |
+------------------------- +------------------------------------+-------------+
| MaxMaintLen              | Maximum number of packets that can | 50          |
|                          | be stored in maintenance buffer    |             |
+------------------------- +------------------------------------+-------------+
| MaxMaintTime             | Maximum time packets can be queued | Seconds(30) |
|                          | in maintenance buffer              |             |
+------------------------- +------------------------------------+-------------+
| MaxCacheLen              | Maximum number of route entries    | 64          |
|                          | that can be stored in route cache  |             |
+------------------------- +------------------------------------+-------------+
| RouteCacheTimeout        | Maximum time the route cache can   | Seconds(300)|
|                          | be queued in route cache           |             |
+------------------------- +------------------------------------+-------------+
| RreqRetries              | Maximum number of retransmissions  | 16          |
|                          | for request discovery of a route   |             |
+------------------------- +------------------------------------+-------------+
| CacheType                | Use Link Cache or use Path Cache   | "LinkCache" |
|                          |                                    |             |
+------------------------- +------------------------------------+-------------+
| LinkAcknowledgment       | Enable Link layer acknowledgment   | True        |
|                          | mechanism                          |             |
+------------------------- +------------------------------------+-------------+

12.1.3. Implementation modification

  • The DsrFsHeader has added 3 fields: message type, source id, destination id, and these changes only for post-processing 1. Message type is used to identify the data packet from control packet 2. source id is used to identify the real source of the data packet since we have to deliver the packet hop-by-hop and the Ipv4Header is not carrying the real source and destination ip address as needed 3. destination id is for same reason of above

  • Route Reply header is not word-aligned in DSR RFC, change it to word-aligned in implementation

  • DSR works as a shim header between transport and network protocol, it needs its own forwarding mechanism, we are changing the packet transmission to hop-by-hop delivery, so we added two fields in dsr fixed header to notify packet delivery

12.1.4. Current Route Cache implementation

This implementation used “path cache”, which is simple to implement and ensures loop-free paths:

  • the path cache has automatic expire policy

  • the cache saves multiple route entries for a certain destination and sort the entries based on hop counts

  • the MaxEntriesEachDst can be tuned to change the maximum entries saved for a single destination

  • when adding multiple routes for one destination, the route is compared based on hop-count and expire time, the one with less hop count or relatively new route is favored

  • Future implementation may include “link cache” as another possibility

12.2. DSR Instructions

The following should be kept in mind when running DSR as routing protocol:

  • NodeTraversalTime is the time it takes to traverse two neighboring nodes and should be chosen to fit the transmission range

  • PassiveAckTimeout is the time a packet in maintenance buffer wait for passive acknowledgment, normally set as two times of NodeTraversalTime

  • RouteCacheTimeout should be set smaller value when the nodes’ velocity become higher. The default value is 300s.

12.3. Helper

To have a node run DSR, the easiest way would be to use the DsrHelper and DsrMainHelpers in your simulation script. For instance:

DsrHelper dsr;
DsrMainHelper dsrMain;
dsrMain.Install(dsr, adhocNodes);

The example scripts inside src/dsr/examples/ demonstrate the use of DSR based nodes in different scenarios. The helper source can be found inside src/dsr/helper/dsr-main-helper.{h,cc} and src/dsr/helper/dsr-helper.{h,cc}

12.4. Examples

The example can be found in src/dsr/examples/:

  • dsr.cc use DSR as routing protocol within a traditional MANETs environment[3].

DSR is also built in the routing comparison case in examples/routing/:

  • manet-routing-compare.cc is a comparison case with built in MANET routing protocols and can generate its own results.

12.5. Validation

This model has been tested as follows:

  • Unit tests have been written to verify the internals of DSR. This can be found in src/dsr/test/dsr-test-suite.cc. These tests verify whether the methods inside DSR module which deal with packet buffer, headers work correctly.

  • Simulation cases similar to [3] have been tested and have comparable results.

  • manet-routing-compare.cc has been used to compare DSR with three of other routing protocols.

A paper was presented on these results at the Workshop on ns-3 in 2011.

12.6. Limitations

The model is not fully compliant with RFC 4728. As an example, Dsr fixed size header has been extended and it is four octets longer then the RFC specification. As a consequence, the DSR headers can not be correctly decoded by Wireshark.

The model full compliance with the RFC is planned for the future.