Sierra Wireless DART 200 CDPD Modem manual Maximum packet size transmit control

DART 200 CDPD Modem User’s Guide 8 Application Programming

Maximum packet size transmit control

This feature is not under end-user control. It causes the data buffer to be transmitted if the maximum packet size is reached and the conditions covered by either, or both, of the above features are not met, for example, if the packet is very large. It is also a backup if both of the above features are inadvertently disabled. The maximum packet size for the DART 200 is 1924 bytes (characters) and is not user adjustable. Use of this technique as the primary data forwarding method is not recommended.

The maximum physical size of an IP transmission packet is referred to as the maximum transmission unit (MTU). To avoid packet segmentation, the maximum segment size (MSS) for a data packet should be the MTU size less the packet header size. The standard header sizes are 40 bytes for TCP and 28 bytes for UDP. The following network parameters also need be considered:

∙Largest MTU size - usually at the host LAN and is defined by the Ethernet which has an MTU of 1500 bytes and an MSS of 1460 bytes

∙Smallest MTU size - usually found at an intermediate router. IP standards require that network routers support a minimum MTU of 576 bytes. In most cases, this will be the limiting factor

∙Largest MSS - determined by the smallest MTU found at the various routing points along the path taken by the packet. Usually, this is 576 minus the packet header size

Based on the above, the largest TCP packet which will avoid segmentation is 536 bytes; for UDP it is 548 bytes.

Any end-user data packets exceeding the MSS for the network are fragmented by the network software at one end and rebuilt at the other. This is transparent to the application, but adds to the transmission time. However, if any of the fragments are lost the entire MSS, not just the dropped fragment, must be retransmitted. TCP and telnet (both TCP-based) do this automatically. For UDP, however; this problem must be detected and corrected by the application.

There is no method defined for IP to determine the minimum segment size between two endpoints. Even if one could be developed, the dynamic nature of IP routing makes it very likely that this value would fluctuate and not be dependable. Therefore, to ensure the best application performance use manual or timed data forwarding, and the data packet sizes suggested above.

Sierra Wireless, Inc. 8-10