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PPP Question

  • From: George Janosik
  • Date: Wed Dec 10 16:16:13 1997

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.


      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 frames
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


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