List of Features
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Digipeating across ports with Hop-2-Hop acknowledgement,
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Automatically adapting channel access parameters,
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Frame resequencer for modulo-eight (standard)
sequence numbers,
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Van Jacobson TCP/IP header compression for
TCP/IP over "connected mode" AX.25,
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protocol booster for TCP/IP catches redundant
TCP retransmits,
Description of Features
Digipeating across ports with
Hop-2-Hop acknowledgements
Given you have two AX.25 ports named KC5RFB-0
and KC5RFB-1. KC5RFB-0 is your user access and KC5RFB-1 is an interlink
to one of your neighbour nodes, K7WJ-0. Cross port digipeating enables
your users to connect to K7WJ-0 in a single step by just connecting via
KC5RFB-1. No interaction with user-level daemons is required.
Given the interlink is of good quality
with minor packet loss while the user access suffers from multipath propagation
and frequent collisions due to hidden stations. Hop-2-Hop acknowledgement
guarantees that lost packets are retried only on the hop they got lost,
not over the whole digipeater chain. The interlink will not suffer from
retries that are due to bad conditions on the user access.
Automatically adapting channel
access parameters
Channel access strategies are most crucial
in shared media contention environments. With a static, user-configurable
channel access strategy the throughput and delay of the channel will be
far from optimal most of the time. Thus the kernel now senses the DCD pattern
of the channel and tries to derive an optimally balanced persistence setting.
Frame resequencer for modulo-eight
(standard) sequence numbers
A resequencer reassembles series of frames
received out-of-sequence and selectively requests lost frames for retransmission.
Compared to the usual go-back-N recovery, which requires the retransmission
of a full sequence of frames up to the last acknowledged frame, a resequencer
requires only the gaps to be actually retransmitted.
Van Jacobson TCP/IP header
compression for TCP/IP over "connected mode" AX.25
TCP/IP over low-badwidth radio links suffers
from great overhead due to the length of IP and TCP protocol headers. Combined
with the fact that the lossy environment requires small packet sizes of
256 bytes maximum, the payload of each packet is reduced to at best 216
bytes. This results in a 15% overhead for each transmitted packet. With
VJ compression, the size of the TCP/IP headers is reduced to 4 - 6 bytes.
Protocol booster for TCP/IP
catches redundant TCP retransmits
TCP has been designed to cope with a lossy
network. Retries due to packet loss are handled end-to-end between the
communication endpoints. Unfortunately its retransmit strategies don't
work accurate when the network provides error-free packet delivery but
is otherwise slow. The protocol booster implemented here tries to work
around this problem by monitoring the packet stream for retransmitted TCP/IP
segments that are then simply dropped. |