Newer dialup modems (presumably the majority of those still in use) support error correction via V.42 LAPM and/or MNP. This may be sufficient to carry BTP without an intermediate reliability layer. If so, the reliability code (SLTP) could be removed, reducing the size of the codebase and making BTP connections harder to identify: they would simply be SLIP-framed random data. (SLIP-framed UDP packets full of random data would be another easily implemented option, if that would be less likely to stand out.)

AT commands for enabling error correction (on which modems?):

• AT\N2: Require error correction, report NO CARRIER if the other end doesn't support it.
• AT&Q5: Try to negotiate an error corrected link.