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Comet Delay |
Overview
"Comet Delay" provides configurable delayed packet transmission with bridging (L2) between networks connectable to a Comet Network Adapter (Comet). This tool is made to observe the effect of delay on the networks to Internet communication. In the real working wide area network, communication is obstructed by the routers and other packets behaviors as well as transmission delay caused by long distance. For the previous step to experiment on wide area network, it is very important to comprehend pure delay effect without those complex parameters.
Figure 1. Comet Delay
Table 1. Specifications
Hardware
This tool is consisted by host PC and a Comet (i-NIC-1 or i-NIC-2) on the host PCI BUS. Because delayed transmission function is handled on the Comet, the host PC is not required high speed PCI. The host PC and Comet are communicate only for initializing, sending commands, and uploading statistic data. To bridging two networks, Comet requires two network interfaces. Currently, we tested "Lamix 1000-SX 2 port" and "Lamix 1000-T 2port" boards for it. Because i-NIC-1 and i-NIC-2 are equipped one PMC slot, we use 2 port NIC. But if we use former model of Comet which quipped two PMC, we could use heterogeneous networks (i.e. ATM and Gigabit Ethernet...) because of the Comet Delay firmware structure.
Firmware
Delay function:
Comet delay stores arriving packets into own delay queue with attaching current time information. And it checks periodically the head of the queue with the difference of current time and packet queued time. If the difference is bigger than given delay time, Comet output the packet.
Commands:
This system provides those commands: (re)set delay time, dump and clear statistic data. (Currently, the command shell scripts (on linux system) are supported for each i-NIC-1 and i-NIC-2 separately.)
Statistic data:
Comet Delay provides those statistic data for each port: Out going total bytes, Out going packet count, current packet number in delay queue. You can get those data by shell scripts and MRTG (via snmp).
Delay Experiment Example:
We observed delay effect to TCP/IP transmission as in figure 2. The communication loads are netperf and ftp on Linux Red hat 7.2J. We changed TCP window size and improving transmission performance. The window size are descript in the parentheses on the side of each label.
Figure 2. Delay and Performance of TCP/IP transmission