In this chapter, we explained the methods and tools we used to obtain the 
results presented in Chapter 4 and Chapter 5. Most of our studies are based on 
measurements. To support this work, we have developed four new tools:

* rlpdump, 
* hiccup, 
* ReTracer, and 
* MultiTracer. 

Many thanks to Almudena Konrad, Bela Rathonyi, and Keith Sklower who contributed 
to the tools’ development.

The first two tools are kernel extensions of the BSD system with a corresponding 
user level process. rlpdump logs RLP and block erasure traces. hiccup emulates a 
semi-reliable link layer protocol with a configurable error recovery 
persistency, and with the in-order or out-of-order delivery function. It is used 
in a controlled, "non-wireless" environment. hiccup can also be used to 
artificially generate excessive packet delays and/or packet re-orderings dur-ing 
a measurement to trigger spurious retransmissions in TCP. 

The latter two tools are script files required for post-processing of TCP and 
RLP traces provid-ing the ability to visualize them correlated in time and at 
multiple levels of detail. Using these tools, we have post-processed and 
analyzed a large base of traces representing a variety of mobile data uses 
(e.g., stationary indoors, walking, driving, etc.).

We use rlpdump and MultiTracer in Section 4.2 to study in general inefficient 
cross-layer interactions that may occur when running TCP-based bulk data 
transfers over RLP in GSM-CSD. We use rlpdump and ReTracer in Section 4.3 to 
evaluate the benefit of link layer error recovery for reliable flows. This 
analysis addresses the problems of "underestimation of avail-able bandwidth" 
(see Section 2.5.1), "inefficiency of end-to-end error control" (see Section 
2.5.2), and also the problem of "failure of link layer differential encodings" 
(see Section 2.5.5). We use hiccup in Section 5.1 and Section 5.2 to further 
study and solve the problem of "competing error recovery" (see Section 2.5.4) 
for the case of TCP.

In addition to measurements, we use a model-based analysis approach to study 
TCP-Lite’s retransmission timer in Section 5.3, and to develop a new 
retransmission timer for TCP in Section 5.4. For that purpose we modeled on a 
spreadsheet the RTT, the RTO, and all other rel-evant sender-side connection 
state variables for the class of network-limited TCP bulk data transfers in 
steady state. In Section 5.3.5, we validate our model-based analysis through 
mea-surements in a real network that yield the same results. This analysis 
addresses the problem of "competing error recovery" (see Section 2.5.4).