Hi James, James Carlson wrote: >Christian writes: > > >>i'm trying to understand how ppp transports frames in the "sync" mode. I >>want to do the following scenario: >> >> > >In sync mode, each write from pppd represents a complete PPP frame, >including the HDLC-like Address and Control fields, but not including >the trailing FCS or any 0-bit stuffing. > > > This is not totally true. I found out that the ppp_synch module makes copy_to_users of 1502 bytes (default mtu), interestingly pppd reads 1504 bytes (MRU+PPP_HDR), which leads to problems in synch mode because 2 bytes of the next frame are inside of the current frame, i don't know why this does not create issues in other constelllations, but in mine it does. So i just changed the maximum read to MRU+PPP_HDR-2 in the tty.c file and indeed it worked for me immediately. >Each read returns a complete PPP frame, again including the HDLC-like >Address and Control fields, but not including the FCS or 0-bit >stuffing. > > > >>I've hacked pppd to use a simple framing method for stdin/stdout, to >>guarantee the transport of complete ppp frames. >> >> > >I'm confused. At what point are stdin/stdout connected to a sync >device that preserves frame boundaries? > > > I have hacked the tty.c to prepend each write with a length field, so i added boundaries, my programm connects to stdin/stdout and reads the length field and therefore knows what a frame means. >>But unfortunately i >>found out, that in the tty.c i get mixed up frames from the >>master_pty_f, like i read a buffer and in the buffer i get : >> >> > >If you're talking about ptys, then you're not talking about sync. > > > well the notty option leads to a Master/Slave pty which is connected to the stdin/stdout pipes in the charshunt() as far as i understood. It seems to me pppd needs terminal like fds internally, but i don't really understand why this is so. But anyway my fix solved my problems now. Thanks for your time! Cheers, Christian - To unsubscribe from this list: send the line "unsubscribe linux-ppp" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
