23.1 Implementation Details

DelayBox maintains two tables: a rule table and a flow table. Entries in the rule table are added by the user in the OTcl simulation script and give an outline of how flows from a source to a destination should be treated. The fields are source, destination, delay Random Variable (in ms), loss rate Random Variable (in fraction of packets dropped), and bottleneck link speed Random Variable (in Mbps). The bottleneck link speed field is optional. Entries in the flow table are created internally and specify exactly how each flow should be handled. Its values are obtained by sampling from the distributions given in the rule table. The fields are source, destination, flow ID, delay, loss, and bottleneck link speed (if applicable). Full-TCP flows are defined as beginning at the receipt of the first SYN of a new flow ID and ending after the sending of the first FIN. Packets after the first FIN are forwarded immediately (<i>i.e.</i>, they are neither delayed nor dropped by DelayBox). For TcpAgent, flows are defined as beginning at the receipt of the first 40 byte packet of a new flow ID. Since there are no FIN packets in TcpAgent, TcpAgent flows are never considered finished nor are they removed from the flow table.

DelayBox also maintains a set of queues to handle delaying packets. There is one queue per entry in the flow table. These queues are implemented as delta queues, in that the time to transfer the packet is kept only for the head packet. All other packets are stored with the difference between the time they should be transferred and the time the previous packet should be transferred. The actual time the previous packet should be transferred is stored in the variable deltasum_, named so because it is the sum of all delta values in the queue (including the head packet's transfer time). If the bottleneck link speed has been specified for the flow, a processing delay is computed for each packet by dividing the size of the packet by the flow's bottleneck link speed.

When a packet is received, its transfer time (current time + delay) is calculated. (This transfer time is the time that the first bit of the packet will begin transfer. Packets that wait in the queue behind this packet must be delayed by the amount of time to transfer all bits of the packet over the bottleneck link.) There are two scenarios to consider in deciding how to set the packet's delta:

1. If the packet is due to be transferred before the last bit of the last packet in the queue, its delta (the time between transferring the previous packet and transferring this packet) is set to the previous packet's processing delay. This packet has to queue behind the previous packet, but will be ready to be transmitted as soon as the previous packet has completed its transfer.
2. If the packet is due to be transferred after the last bit of the last packet in the queue, its delta is difference between this packet's transfer time and the previous packet's transfer time.

If the current packet is the only packet in the queue, DelayBox schedules a timer for the receipt of the packet. When this timer expires, DelayBox will pass the packet on to the standard packet forwarder for processing. Once a packet has been passed up, DelayBox will look for the next packet in the queue to be processed and schedule a timer for its transfer. All packets, both data and ACKs, are delayed in this manner.

Packets that should be dropped are neither queued nor passed on. All packets in a queue are from the same connection and are delayed the same amount (except for delays due to packet size) and are dropped with the same probability. Note: Drops at DelayBox are not recorded in the trace-queue file.