Friday, October 20, 2006

Cellular-IP Friendly Applications - Intro

In theory, host applications using TCP/IP on Ethernet should work over wide-area networks which support TCP/IP. Unfortunately, most host applications are written and tested for Ethernet, not generic IP. When you move into cellular IP or satellite, the high and variable latency introduced causes many host applications to either fail or generate an order of magnitute more traffic than than they should.

For example, here is a chart of 1000 Modbus/RTU polls over cellular TCP/IP. There is a random delay between polls of 30 seconds to 30 minutes. The patterns are rather striking: most polls complete in between 1 to 2 seconds, but there is clearly some systematic "aliasing" causing responses to complete in 2.8, 3.8, and 10.8 seconds.

chart showing Modbus times

After years of troubleshooting customers systems, I have been creating a running document and commentary on Bad things host apps do. I will be publishing these things over time in this blog. But to summarize:
  • The default OS timeout on opening TCP Sockets may be too short.
  • Attempting to open TCP sockets to unresponsive remotes must be a controlled process, since retries cost money.
  • Since all packets and retries cost money, all aspects of the implemented protocol must be controlled and adjustable.
  • OS stack calls may not return if the OS fails to detect a response or socket failure.
  • Responses from the remote could take 15 to 60 seconds.
  • TCP segment fragmentation and reassembly is exaggerated; can have many seconds of delay between fragments.
  • TCP sockets idle longer than 5 minutes often go away without error or detection.
  • Every byte your application sends (or resends) costs your customer money.

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