Sierra Wireless DART 200 CDPD Modem - Manual (Page 104)

DART 200 CDPD Modem User’s Guide 8 Application Programming
Sierra Wireless, Inc. 8-10
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.