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entry_addrdemuxIP addressportdemuxdefaulttarget_255Src/SinkRTagentclass SatNode : public NodeSatPositionLinkHandoffMgrSatTraceList of pointers:node_head nodehead_linklist_head linklisthead_Channel* uplink_Channel* downlink_SatLinkHeadOther linkobjects...SatLinkHeadOther linkobjectsFigure 17.5: Structure of class SatNode.to populate this table correctly. The SatRouteAgent populates certain fields in the header and then sends the packet down tothe approprate link. To implement a distributed routing protocol, a new SatRouteAgent could be defined– this would learnabout topology by noting the interface index marked in each packet as it came up the stack– a helper function in the nodeintf_to_target() allows it to resolve an index value to a particular LinkHead.There are pointers to three additional objects in a SatNode. First, each SatNode contains a position object, discussed in theprevious section. Second, each SatNode contains a LinkHandoffMgr that monitors for opportunities to hand links off andcoordinates the handoffs. Satellite nodes and terminal nodes each have their specialized version of a LinkHandoffMgr.Finally, a number of pointers to objects are contained in a SatNode. We discussed linklisthead_ and nodehead_ inthe previous subsection. The uplink_ and downlink_ pointers are used for convenience under the assumption that, inmost simulations, a satellite or a terminal has only one uplink and downlink channel.17.3.3 Detailed look at satellite linksFigure 17.6 provides a more detailed look at how satellite links are composed. In this section, we describe how packetsmove up and down the stack, and the key things to note at each layer. The file ~ns/tcl/lib/ns-sat.tcl contains the various OTclinstprocs that assemble links according to Figure 17.6. We describe the composite structure herein as a “network stack.” Mostof the code for the various link components is in ~ns/satlink.{cc,h}.The entry point to a network stack is the SatLinkHead object. The SatLinkHead object derives from Class LinkHead;the aim of link head objects is to provide a uniform API for all network stacks. 4 The SatLinkHead object contains pointersto the LL, Queue, MAC, Error model, and both Phy objects. The SatLinkHead object can also be queried as to what type ofnetwork stack it is– e.g., GSL, interplane ISL, crossseam ISL, etc.. Valid codes for the type_ field are currently found in~ns/sat.h. Finally, the SatLinkHead stores a boolean variable linkup_ that indicates whether the link to at least one othernode on the channel is up. The C++ implementation of SatLinkHead is found in ~ns/satlink.{cc,h}.Packets leaving a node pass through the SatLinkHead transparently to the class SatLL object. The SatLL class derivesfrom LL (link layer). Link layer protocols (like ARQ protocols) can be defined here. The current SatLL assigns a MAC4 In the author’s opinion, all network stacks in ns should eventually have a LinkHead object at the front– the class SatLinkHead would then disappear.183