rfc1663.txt

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RFC 1663               PPP Reliable Transmission               July 1994


      If magic numbers have been negotiated on the link, the system with
      the numerically smaller magic number SHOULD specify the smaller
      address.

5.  Numbered-Mode Operation

   When using the Numbered-Mode, each link is established in the usual
   manner for the type of link.  The Numbered-Mode Configuration Option
   is negotiated, the Magic-Number Configuration Option MUST also be
   negotiated, and the Address-and-Control-Field-Compression
   Configuration Option MUST NOT be negotiated.

   Following the successful negotiation of the Numbered-Mode
   Configuration Option during LCP Link Establishment phase, the system
   with the numerically smaller Magic-Number will send a SABM or
   SABM(E), and the other will respond with a UA.  In the event that
   either the SABM or UA is lost, this exchange may be repeated
   according to the same parameters as the configuration exchange
   itself, using the Restart Timer and counter values.  Authentication,
   Link Quality Determination, and NCP Configuration follow this step.

   Once the link has been established with Numbered-Mode, when re-
   negotiation of link configuration occurs, the entire re-negotiation
   MUST be conducted in Numbered-Mode.  If the Numbered-Mode
   Configuration Option is not successfully re-negotiated, the link
   reverts to Unnumbered-Information operation prior to Authentication,
   Link Quality Determination, and NCP Configuration.

   When an implementation which is capable of Numbered-Mode, and is not
   currently configured for Numbered-Mode operation, detects a frame
   which has a correct FCS but does not have a UI Control octet, the
   implementation MUST send a DM message, immediately followed by a LCP
   Configure-Request.

   When an implementation which is currently configured for Numbered-
   Mode operation receives a DM message, it MUST revert to Unnumbered-
   Information operation, and immediately send a LCP Configure-Request.

5.1.  Single Link

   When Network-Layer packets are sent over a single link, the packets
   are encapsulated in the following order:

    +----------+   +----------+   +----------+
    |          |   |          |   | Numbered |
    | Header   |-->| Data     |-->| Mode     |--> link
    | Compress |   | Compress |   | Header   |
    +----------+   +----------+   +----------+



Rand                                                            [Page 5]

RFC 1663               PPP Reliable Transmission               July 1994


5.2.  Inverse Multiplexing

   Since sending several connections over a single link is often called
   "multiplexing", sending packets from a single connection over
   multiple parallel links is sometimes called "inverse-multiplexing".
   By default, PPP performs no special processing for such links.  Each
   link is established and terminated independently, negotiates its own
   configuration options, and may have different combinations of such
   options as ACCM, Protocol Field Compression and IP-Address.  This
   facilitates using the links simultaneously over dissimilar media,
   such as 56K sync with async backup.

   Every link in a single machine MUST have different Magic Numbers, and
   each end of every link between two peers SHOULD have Magic Numbers
   which are unique to those peers.  This protects against patch-panel
   errors in addition to looped-back links.

   The distribution to each link is controlled by higher level routing
   mechanisms.  When Network-Layer specific compression techniques (such
   as Van Jacobsen Compression) rely on sequential delivery, without
   Multi-Link Procedure support such compression MUST be applied on a
   link by link basis.

                    +----------+   +----------+   +----------+
                    |          |   |          |   | Numbered |
               +--->| Header   |-->| Data     |-->| Mode     |--> link 1
               |    | Compress |   | Compress |   | Header   |
  +--------------+  +----------+   +----------+   +----------+
  | Distribution |
  +--------------+  +----------+   +----------+   +----------+
               |    |          |   |          |   | Numbered |
               +--->| Header   |-->| Data     |-->| Mode     |--> link 2
                    | Compress |   | Compress |   | Header   |
                    +----------+   +----------+   +----------+

5.3.  Using Multi-Link Procedure

   This document does not offer a standard for ISO Multi-Link, but does
   offer a method for agreeing on the addressing scheme usable with
   Multi-Link.  A sample implementation is shown below.  Implementation
   of Multi-Link is not required.

   When using the ISO 7776 Multi-Link Procedure, each link is
   established as described above.  In addition, the Numbered-Mode
   Configuration Option is negotiated with appropriate addresses for the
   Multi-Link Procedure.  The distribution to each link is controlled by
   the Multi-Link Procedure, as is the recovery of sequence in the
   receiving system.



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RFC 1663               PPP Reliable Transmission               July 1994


                                                            +---> link 1
  +----------+   +----------+   +----------+                |
  |          |   |          |   | Multi    |   +--------------+
  | Header   |-->| Data     |-->| Link     |-->| Distribution |
  | Compress |   | Compress |   | Procedure|   +--------------+
  +----------+   +----------+   +----------+                |
                                                            +---> link 2

5.4.  LAPB Parameter defaults

   The following guidelines specify the default values of LAPB
   configurable parameters.

      Timer T1

         Timer T1 is the maximum time permitted before a retransmission
         is started, as a result of no response to a transmitted I
         frame.  This value must be greater than the time required for a
         maximum sized frame to be received by the other side of the
         link, and for a response to be generated for the frame.  This
         SHOULD be determined dynamically, based on the measured round
         trip time delay of the link at the LAPB level.  In the event
         that the system cannot determine the round trip time of the
         link, this value SHOULD be set to twice the bit rate of the
         link, divided by the maximum number of bits per frame, plus 100
         milliseconds processing time.  For example, on a 14,400 bps
         link, with a maximum frame size of 8000 bits (1000 octects),
         the T1 value would be set to 3.7 seconds.

      Timer T3

         Timer T3 gives an indication of the idle state of the link.
         Its value must be greater than the T1 value.

      Maximum number of attempts to complete a transmission, N2

         Parameter N2 gives the maximum number of retransmission
         attempts for a given frame.  If this value is exceeded, the
         link SHOULD be terminated.  The default value for parameter N2
         SHOULD be 3.

Security Considerations

   Security issues are not discussed in this memo.







Rand                                                            [Page 7]

RFC 1663               PPP Reliable Transmission               July 1994


References

   [1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51,
       RFC 1661, Daydreamer, July 1994.

   [2] ISO 7776, Information Processing Systems - Data Communication -
       High Level Data Link Control Procedures - Description of the X.25
       LAPB-Compatible DTE Data Link Procedures

   [3] Simpson, W., Editor, "PPP in HDLC Framing", STD 51, RFC 1662,
       Daydreamer, July 1994.

   [4] Sklower, K., "PPP MultiLink Procedure", Work in Progress.

Acknowledgments

   Fred Baker was the original author of this document.

   Bill Simpson contributed materially to the document.

Chair's Address

   The working group can be contacted via the current chair:

   Fred Baker
   Advanced Computer Communications
   315 Bollay Drive
   Santa Barbara, California  93117

   EMail: [e-mail]

Author's Address

   Questions about this memo can also be directed to:

   Dave Rand
   2180 Fortune Drive
   San Jose, CA  95131

   Phone: [phone]
   EMail: [e-mail]










Rand                                                            [Page 8]