Sat (Satellite Technology) HOWTO

8. Appendix A - Notes

8.1 Translation Dynamic IP -> MAC Address

The translation used by some ISPs to calculate MAC address (which need to have your DVB card to receive their packets) is:

00 : 01 : IP[0] : IP[1] : IP[2] : IP[3]

where

IP[0].IP[1].IP[2].IP[3] is your dynamic IP address.

This feature is used, for example, by EON.

8.2 TCP Window

Satellite connections are an interesting example of very high RTT (round trip time, access time): another example is the Mars - Earth communication or also the Moon - Earth one.

These connections have a very bad feature: very low interactively.

Typical network (or digital, generally) connections use the so called transmission "window", which represents the data buffer can be sent before waiting for an answer. In TCP/IP protocol stack this is the TCP Window.

                  ----------------------
  | - - - - - - - > can continue |-|-|-|---->
  |               ----------------------
  |             Buffer sendable before confirm
  |
  | - - - - - - - <----------------------
                   Confirmation Answering

Now, if our communication has an high access time and if we had a little TCP Window we would lose very much time only waiting for the data confirmation (ACK), so the real bandwidth would decrease (for example if you have a 16KB TCP window, typical of Windows systems and a RTT of 400 ms, you cannot overcome 16KB/0.4 = 40 KB/s)

Solution is to use a very high TCP Window (such as 256 KB or some MB).

Unfortunately, under many systems, is not so simple to have a great TCP Window, so, in latest years, it starts to appear new applications ("download accelerators" described in next section) that split in many pieces a file and download all them in the main time: this is just equivalent to download only one file with a single piece size, avoiding the RTT problem.

8.3 Download accelerator

We report here some useful link to so called "download accelerator" which is an application that does 2 things:

1. resuming session management, it allows stopping downloading and restarting later (this can be done thanks to "resuming feature" introduced in Http and Ftp server, which allows you to specify at what byte number start to download).
2. splitting, divide the file in many pieces and let them start in a multithreaded fashion.

As we saw in the previous section, a download accelerator allows us to increase satellite bandwidth.

Under Linux

• Aria

Under Windows

• FlashGet
• GetRight
• Mass Downloader