PPP Question

William Allen Simpson wsimpson at greendragon.com
Tue Dec 16 20:07:12 UTC 1997

Catching up on NANOG after returning from IETF, I found this incongruous
message, and would like to correct the misinformation.

The PPP Maximum Receive Unit is negotiated, but this is a "maximum", not
a "fixed" size.  Smaller packets can be sent.  While it would be
perfectly legal to fill the remainder of the packet with padding, I know
of no vendor that does it.  This would seriously waste bandwidth.  The
padding feature is available for old chipsets that require 16-bit and
32-bit output data alignment.

Further questions about PPP should be sent to the ietf-ppp at merit.edu
mailing list, as designated in RFC1661....  Yes, NANOG is also hosted by
Merit, but the lists have different purposes.  Really.

As usual, joining a list requires a message to <list>-request.  I was
amazed at the number of messages arising from nanog list members
complaining that pppsdh at greendragon.com did not allow sending a "join",
"request", "subscribe" or other subject to the mailing list directly.
(heavy sigh)

> From: "Chris MacFarlane" <cjm at ican.net>
> More to the point is that the frame size is negotiated while handshaking.
> Once two devices agree on the size it is fixed for the duration of the
> session.
> -----Original Message-----
> From: George Janosik <gjanosik at torrentnet.com>
> >Is a PPP frame size static or elastic?  RFC 1661 states:
> >...
> >But, it also says the the Information field "MAY" be padded up to the MRU.
> >
> >I have hard time believing that a PPP frame would be static.

WSimpson at UMich.edu
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