The ns Manual (formerly ns Notes and Documentation)1 - NM Lab at ...

36.4 Loss Detection—The Class SRMinfoA very small encapsulating class, entirely in C++, tracks a number of assorted state information. Each member of the group,n i , uses one SRMinfo block for every other member of the group.An SRMinfo object about group member n j at n i , contains information about the session messages received by n i from n j .n i can use this information to compute its distance to n j .If n j sends is active in sending data traffic, then the SRMinfo object will also contain information about the received data,including a bit vector indicating all packets received from n j .The agent keeps a list of SRMinfo objects, one per group member, in its member variable, sip_. Its method, get_state(intsender) will return the object corresponding to that sender, possibly creating that object, if it did not already exist. The classSRMinfo has two methods to access and set the bit vector, i.e.,ifReceived(int id)setReceived(int id)indicates whether the particular message from the appropriate sender, with id id was receivedat n i ,to set the bit to indicate that the particular message from the appropriate sender, with id id wasreceived at n i .The session message variables to access timing information are public; no encapsulating methods are provided. These are:int lsess_; /* # of last session msg received */int sendTime_; /* Time sess. msg. # sent */int recvTime_; /* Time sess. msg. # received */double distance_;/* Data messages */int ldata_; /* # of last data msg sent */36.5 Loss Recovery ObjectsIn the last section, we described the agent behavior when it receives a message. Timers are used to control when any particularcontrol message is to be sent. The SRM agent uses a separate class SRM to do the timer based processing. In this section,we describe the basics if the class SRM, and the loss recovery objects. The following section will describe how the classSRM is used for sending periodic session messages. An SRM agent will instantiate one object to recover from one lost datapacket. Agents that detect the loss will instantiate an object in the class SRM/request; agents that receive a request andhave the required data will instantiate an object in the class SRM/repair.Request Mechanisms SRM agents detect loss when they receive a message, and infer the loss based on the sequencenumber on the message received. Since packet reception is handled entirely by the compiled object, loss detection occurs inthe C++ methods. Loss recovery, however, is handled entirely by instance procedures of the corresponding interpreted objectin OTcl.When any of the methods detects new losses, it invokes Agent/SRM::request{} with a list of the message sequencenumbers that are missing. request{} will create a new requestFunction_ object for each message that is missing.The agent stores the object handle in its array of pending_ objects. The key to the array is the message identifier〈sender〉:〈msgid〉.316