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Re: PPP Question

  • From: William Allen Simpson
  • Date: Tue Dec 16 15:58:09 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
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 did not allow sending a "join",
"request", "subscribe" or other subject to the mailing list directly.
(heavy sigh)

> From: "Chris MacFarlane" <>
> 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 <>
> >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.
    Key fingerprint =  17 40 5E 67 15 6F 31 26  DD 0D B9 9B 6A 15 2C 32

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