Eugene Grosbein

You could find useful ktrace(1) kernel facility to record system calls and sent/received data of mpd5 process: ktrace -i mpd5 ... kdump

Make sure that your AT-commands script for the modem initializes it exactly same way as one for user-ppp.

Your logs already have it, for example: Sep 16 12:08:38 box-174 mpd[6329]: [B-Link] rec'd 27 bytes frame from link proto=0xc021 Sep 16 12:08:38 box-174 mpd[6329]: c0 21 01 06 00 19 02 06 00 00 00 00 03 05 c2 23 .!.............# Sep 16 12:08:38 box-174 mpd[6329]: 05 05 06 3a 14 eb db 07 02 08 02 ...:.......

Perhaps, your ISP somehow detects you are "sharing" the connection among multiple devices while it does not permit that? Try filtering all IP traffic running over ng0 interface for some time to see if termination delays, too?

OTOH, maybe other side wants to serve only "known" PPP clients. Try to pretend this is user-ppp: set link ident "user-ppp 3.4.2"

OTOH, maybe other side wants to serve only "known" PPP clients. Try to pretend ths is user-ppp: set link ident "user-ppp 3.4.2"

The log shows the following: Sep 16 12:08:38 box-174 mpd[6329]: [B-Link] LCP: SendIdent #1 Sep 16 12:08:38 box-174 mpd[6329]: [B-Link] MESG: synciot Sep 16 12:08:38 box-174 mpd[6329]: [B-Link] xmit frame to link proto=0xc021 Sep 16 12:08:38 box-174 mpd[6329]: ff 03 c0 21 0c 01 00 10 21 10 f1 31 73 79 6e 63 ...!....!..1sync Sep 16 12:08:38 box-174 mpd[6329]: 69 6f 74 00 The documentation https://mpd.sourceforge.net/doc5/mpd20.html#20 tells: set link ident string This enables the sending of an identification...

The log shows the following: Sep 16 12:08:38 box-174 mpd[6329]: [B-Link] LCP: SendIdent #1 Sep 16 12:08:38 box-174 mpd[6329]: [B-Link] MESG: synciot Sep 16 12:08:38 box-174 mpd[6329]: [B-Link] xmit frame to link proto=0xc021 Sep 16 12:08:38 box-174 mpd[6329]: ff 03 c0 21 0c 01 00 10 21 10 f1 31 73 79 6e 63 ...!....!..1sync Sep 16 12:08:38 box-174 mpd[6329]: 69 6f 74 00 The documentation https://mpd.sourceforge.net/doc5/mpd20.html#20 tells: set link ident string This enables the sending of an identification...