[lwip-users] PPPoS: How to config proper PPP_MAXIDLEFLAG?

[Axel Lin]

Hi,
While reading the code and the comment, I don't quite understanding
how to config proper PPP_MAXIDLEFLAG.
The comment says "If the link has been idle, we'll send a fresh flag
character to flush any noise".

I'm curious in what kind of situation a user needs to change PPP_MAXIDLEFLAG
and how to judge a good PPP_MAXIDLEFLAG setting?
What happened if PPP_MAXIDLEFLAG is too small or too large?

I google and found some users set PPP_MAXIDLEFLAG to 0:
(But I don't know if below comment is correct or not.)
e.g.
https://github.com/espressif/esp-idf/blob/master/components/lwip/include/lwip/port/lwipopts.h

/**
 * PPP_MAXIDLEFLAG: Max Xmit idle time (in ms) before resend flag char.
 * TODO: If PPP_MAXIDLEFLAG > 0 and next package is send during
PPP_MAXIDLEFLAG time,
 *       then 0x7E is not added at the begining of PPP package but
0x7E termination
 *       is always at the end. This behaviour brokes PPP dial with GSM (PPPoS).
 *       The PPP package should always start and end with 0x7E.
 */

Regards,
Axel