IEEE - Aerospace and Electronic Systems - May 2020 - 8

Study on Checkpoint Timer Optimal Setup of Licklider Transmission Protocol

Figure 2.
An enlarged view of the TSG in Figure 1 for an interactive transmission of two LTP blocks sent in the middle of the file, illustrating the bundle
aggregation of LTP and the acknowledging mechanism of one RS per block with the receipt of RS prior to the expiration of CP timer.

Goodput of a data transport protocol is commonly adopted as a good measure of its application data
transmission efficiency. Figure 3 illustrates the corresponding goodput variation trend measured for the TSG
presented in Figure 1. Corresponding to the linear-type of
increase of the data block line in Figure 1, a smooth and
consistent increase in goodput is achieved. It is observed
that the goodput keeps increasing until the channel capacity is reached around 2 s. After that, the data segments are
transmitted in a more controlled manner so that the channel capacity is fully utilized but not overflowed, leading to
consistently high goodput. This attributes to the rate-based

transmission mechanism of LTP. The transmission continues until the end of the entire file. It turns out that the
averaged goodput performance for the transmission
approaches to 220 Kbytes/s.
In summary, for the LTP transmission over symmetric
channel, because of no delay in RS transmission, the configured short CP timer length of 4 s is longer than the estimated
RTT of 2.8 s for each block. As a result, the RS is received
by the block sender prior to the expiration of the CP timeout
timer, and thus no need to retransmit the CP segment. In
other words, only the original transmission of the blocks is
able to ensure the successful delivery of the entire file.

Figure 3.
A goodput graph illustrating the transmission performance at the sending node for delivery of a file of 1 Mbyte over an emulated symmetric
cislunar communication channel with a CP timer of 4 s.

8

IEEE A&E SYSTEMS MAGAZINE

MAY 2020



IEEE - Aerospace and Electronic Systems - May 2020

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