PPP Question

Chris MacFarlane cjm at ican.net
Wed Dec 10 23:12:54 UTC 1997

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

Chris MacFarlane
Manager, Data Operations
ACC Telenterprises Canada
cmacfarlane at acc.ca
cjm at ican.net

-----Original Message-----
From: George Janosik <gjanosik at torrentnet.com>
To: nanog at merit.edu <nanog at merit.edu>
Date: Wednesday, December 10, 1997 4:09 PM
Subject: PPP Question

>A quick protocol question.
>Is a PPP frame size static or elastic?  RFC 1661 states:
>  Information Field
>      The Information field is zero or more octets.  The Information
>      field contains the datagram for the protocol specified in the
>      Protocol field.
>      The maximum length for the Information field, including Padding,
>      but not including the Protocol field, is termed the Maximum
>      Receive Unit (MRU), which defaults to 1500 octets.  By
>      negotiation, consenting PPP implementations may use other values
>      for the MRU.
>   Padding
>      On transmission, the Information field MAY be padded with an
>      arbitrary number of octets up to the MRU.  It is the
>      responsibility of each protocol to distinguish padding octets from
>      real information.
>This is somewhat ambiguous.  If I read it straight it sounds like PPP
>static and that padding is used to maintain the MRU between endpoints.
>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.
>Very truly yours,
>George J Janosik
>Senior Systems Engineer
>Torrent Networking Technologies
>gjanosik at torrentnet.com

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