RFC 1220 (rfc1220) - Page 2 of 18

Point-to-Point Protocol extensions for bridging

RFC 1220            Bridging Point-to-Point Protocol          April 1991


        reliability, but message sequence issues are solved by the
        transmitting end.

3.  General Considerations

3.1.  Link Quality Monitoring

   It is strongly recommended that Point-to-Point Bridge Protocol
   implementations utilize Magic Number Loopback Detection and Link-
   Quality-Monitoring.  This is because the 802.1 Spanning Tree
   protocol, which is integral to both Transparent Bridging and Source
   Routing (as standardized), is unidirectional during normal operation,
   with HELLO PDUs emanating from the Root System in the general
   direction of the leaves, without any reverse traffic except in
   response to network events.

3.2.  Message Sequence

   The multiple link case requires consideration of message
   sequentiality.  The transmitting station must determine either that
   the protocol being bridged requires transmissions to arrive in the
   order of their original transmission, and enqueue all transmissions
   on a given conversation onto the same link to force order
   preservation, or that the protocol does NOT require transmissions to
   arrive in the order of their original transmission, and use that
   knowledge to optimize the utilization of the several links, enqueuing
   traffic to links to minimize delay.

   In the absence of such a determination, the transmitting station must
   act as though all protocols require order preservation; many
   protocols designed primarily for use on a single LAN in fact do.  A
   protocol could be described to maintain message sequentiality across
   multiple links, either by sequence numbering or by fragmentation and
   re-assembly, but this is neither elegant nor absolutely necessary.

3.3.  Maximum Receive Unit Considerations

   Please note that the negotiated MRU must be large enough to support
   the MAC Types that are negotiated for support, there being no
   fragmentation and re-assembly.  Even Ethernet frames are larger than
   the default MRU of 1500 octets.

3.4.  Separation of Spanning Tree Domains

   It is conceivable that a network manager might wish to inhibit the
   exchange of BPDUs on a link in order to logically divide two regions
   into separate Spanning Trees with different Roots (and potentially
   different Spanning Tree implementations or algorithms).  In order to



Point-to-Point Protocol Extensions Working Group