rfc1638.txt

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Network Working Group                                           F. Baker
Request For Comments: 1638                                           ACC
Category: Standards Track                                       R. Bowen
                                                                     IBM
                                                                 Editors
                                                               June 1994


                  PPP Bridging Control Protocol (BCP)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   The Point-to-Point Protocol (PPP) [6] provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.  PPP
   defines an extensible Link Control Protocol, and proposes a family of
   Network Control Protocols for establishing and configuring different
   network-layer protocols.

   This document defines the Network Control Protocol for establishing
   and configuring Remote Bridging for PPP links.

Table of Contents

     1.     Historical Perspective ................................    2
     2.     Methods of Bridging ...................................    3
        2.1       Transparent Bridging ............................    3
        2.2       Remote Transparent Bridging .....................    3
        2.3       Source Routing ..................................    4
        2.4       Remote Source Route Bridging ....................    5
        2.5       SR-TB Translational Bridging ....................    6
     3.     Traffic Services ......................................    6
        3.1       LAN Frame Checksum Preservation .................    6
        3.2       Traffic having no LAN Frame Checksum ............    6
        3.3       Tinygram Compression ............................    7
        3.4       LAN Identification ..............................    7
     4.     A PPP Network Control Protocol for Bridging ...........    9
        4.1       Sending Bridge Frames ...........................   10
           4.1.1  Maximum Receive Unit Considerations .............   10
           4.1.2  Loopback and Link Quality Monitoring ............   11
           4.1.3  Message Sequence ................................   11



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           4.1.4  Separation of Spanning Tree Domains .............   11
        4.2       Bridged LAN Traffic .............................   12
        4.3       Spanning Tree Bridge PDU ........................   16
     5.     BCP Configuration Options .............................   17
        5.1       Bridge-Identification ...........................   17
        5.2       Line-Identification .............................   19
        5.3       MAC-Support .....................................   20
        5.4       Tinygram-Compression ............................   21
        5.5       LAN-Identification ..............................   22
        5.6       MAC-Address .....................................   23
        5.7       Spanning-Tree-Protocol ..........................   24
        APPENDICES ................................................   26
        A.     Tinygram-Compression Pseudo-Code ...................   26
        SECURITY CONSIDERATIONS ...................................   27
        REFERENCES ................................................   27
     ACKNOWLEDGEMENTS .............................................   28
     CHAIR'S ADDRESS ..............................................   28
     AUTHOR'S ADDRESS .............................................   28

1.  Historical Perspective

   Two basic algorithms are ambient in the industry for Bridging of
   Local Area Networks.  The more common algorithm is called
   "Transparent Bridging", and has been standardized for Extended LAN
   configurations by IEEE 802.1.  The other is called "Source Route
   Bridging", and is prevalent on IEEE 802.5 Token Ring LANs.

   The IEEE has combined these two methods into a device called a Source
   Routing Transparent (SRT) bridge, which concurrently provides both
   Source Route and Transparent bridging.  Transparent and SRT bridges
   are specified in IEEE standard 802.1D [3].

   Although IEEE committee 802.1G is addressing remote bridging [2],
   neither standard directly defines the mechanisms for implementing
   remote bridging.  Technically, that would be beyond the IEEE 802
   committee's charter.  However, both 802.1D and 802.1G allow for it.
   The implementor may model the line either as a component within a
   single MAC Relay Entity, or as the LAN media between two remote
   bridges.












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2.  Methods of Bridging

2.1.  Transparent Bridging

   As a favor to the uninitiated, let us first describe Transparent
   Bridging.  Essentially, the bridges in a network operate as isolated
   entities, largely unaware of each others' presence.  A Transparent
   Bridge maintains a Forwarding Database consisting of

                           {address, interface}

   records, by saving the Source Address of each LAN transmission that
   it receives, along with the interface identifier for the interface it
   was received on.  It goes on to check whether the Destination Address
   is in the database, and if so, either discards the message when the
   destination and source are located at the same interface, or forwards
   the message to the indicated interface.  A message whose Destination
   Address is not found in the table is forwarded to all interfaces
   except the one it was received on.  This behavior applies to
   Broadcast/Multicast frames as well.

   The obvious fly in the ointment is that redundant paths in the
   network cause indeterminate (nay, all too determinate) forwarding
   behavior to occur.  To prevent this, a protocol called the Spanning
   Tree Protocol is executed between the bridges to detect and logically
   remove redundant paths from the network.

   One system is elected as the "Root", which periodically emits a
   message called a Bridge Protocol Data Unit (BPDU), heard by all of
   its neighboring bridges.  Each of these modifies and passes the BPDU
   on to its neighbors, until it arrives at the leaf LAN segments in the
   network (where it dies, having no further neighbors to pass it
   along), or until the message is stopped by a bridge which has a
   superior path to the "Root".  In this latter case, the interface the
   BPDU was received on is ignored (it is placed in a Hot Standby
   status, no traffic is emitted onto it except the BPDU, and all
   traffic received from it is discarded), until a topology change
   forces a recalculation of the network.

2.2.  Remote Transparent Bridging

   There exist two basic sorts of bridges -- those that interconnect
   LANs directly, called Local Bridges, and those that interconnect LANs
   via an intermediate medium such as a leased line, called Remote
   Bridges.  PPP may be used to connect Remote Bridges.

   The IEEE 802.1G Remote MAC Bridging committee has proposed a model of
   a Remote Bridge in which a set of two or more Remote Bridges that are



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   interconnected via remote lines are termed a Remote Bridge Group.
   Within a Group, a Remote Bridge Cluster is dynamically formed through
   execution of the spanning tree as the set of bridges that may pass
   frames among each other.

   This model bestows on the remote lines the basic properties of a LAN,
   but does not require a one-to-one mapping of lines to virtual LAN
   segments.  For instance, the model of three interconnected Remote
   Bridges, A, B and C, may be that of a virtual LAN segment between A
   and B and another between B and C.  However, if a line exists between
   Remote Bridges B and C, a frame could actually be sent directly from
   B to C, as long as there was the external appearance that it had
   travelled through A.

   IEEE 802.1G thus allows for a great deal of implementation freedom
   for features such as route optimization and load balancing, as long
   as the model is maintained.

   For simplicity and because the 802.1G proposal has not been approved
   as a standard, we discuss Remote Bridging in this document in terms
   of two Remote Bridges connected by a single line.  Within the 802.1G
   framework, these two bridges would comprise a Remote Bridge Group.
   This convention is not intended to preclude the use of PPP bridging
   in larger Groups, as allowed by 802.1G.

2.3.  Source Routing

   The IEEE 802.1D Committee has standardized Source Routing for any MAC
   Type that allows its use.  Currently, MAC Types that support Source
   Routing are FDDI and IEEE 802.5 Token Ring.

   The IEEE standard defines Source Routing only as a component of an
   SRT bridge.  However, many bridges have been implemented which are
   capable of performing Source Routing alone.  These are most commonly
   implemented in accordance either with the IBM Token-Ring Network
   Architecture Reference [1] or with the Source Routing Appendix of
   IEEE 802.1D [3].

   In the Source Routing approach, the originating system has the
   responsibility of indicating the path that the message should follow.
   It does this, if the message is directed off of the local segment, by
   including a variable length MAC header extension called the Routing
   Information Field (RIF).  The RIF consists of one 16-bit word of
   flags and parameters, followed by zero or more segment-and-bridge
   identifiers.  Each bridge en route determines from this source route
   list whether it should accept the message and how to forward it.





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   In order to discover the path to a destination, the originating
   system transmits an Explorer frame.  An All-Routes Explorer (ARE)
   frame follows all possible paths to a destination.  A Spanning Tree
   Explorer (STE) frame follows only those paths defined by Bridge ports
   that the Spanning Tree Algorithm has put in Forwarding state.  Port
   states do not apply to ARE or Specifically-Routed Frames.  The
   destination system replies to each copy of an ARE frame with a
   Specifically-Routed Frame, and to an STE frame with an ARE frame.  In
   either case, the originating station may receive multiple replies,
   from which it chooses the route it will use for future Specifically-
   Routed Frames.

   The algorithm for Source Routing requires the bridge to be able to
   identify any interface by its segment-and-bridge identifier.  When a
   packet is received that has the RIF present, a boolean in the RIF is
   inspected to determine whether the segment-and-bridge identifiers are
   to be inspected in "forward" or "reverse" sense.  In its search, the
   bridge looks for the segment-and-bridge identifier of the interface
   the packet was received on, and forwards the packet toward the
   segment identified in the segment-and-bridge identifier that follows
   it.

2.4.  Remote Source Route Bridging

   There is no Remote Source Route Bridge proposal in IEEE 802.1 at this
   time, although many vendors ship remote Source Routing Bridges.

   We allow for modelling the line either as a connection residing
   between two halves of a "split" Bridge (the split-bridge model), or
   as a LAN segment between two Bridges (the independent-bridge model).
   In the latter case, the line requires a LAN Segment ID.

   By default, PPP Source Route Bridges use the independent-bridge
   model.  This requirement ensures interoperability in the absence of
   option negotiation.  In order to use the split-bridge model, a system
   MUST successfully negotiate the Bridge-Identification Configuration
   Option.

   Although no option negotiation is required for a system to use the
   independent-bridge model, it is strongly recommended that systems
   using this model negotiate the Line-Identification Configuration
   Option.  Doing so will verify correct configuration of the LAN
   Segment Id assigned to the line.

   When two PPP systems use the split-bridge model, the system that
   transmits an Explorer frame onto the PPP link MUST update the RIF on
   behalf of the two systems.  The purpose of this constraint is to
   ensure interoperability and to preserve the simplicity of the



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   bridging algorithm.  For example, if the receiving system did not
   know whether the transmitting system had updated the RIF, it would
   have to scan the RIF and decide whether to update it.  The choice of
   the transmitting system for the role of updating the RIF allows the
   system receiving the frame from the PPP link to forward the frame
   without processing the RIF.

   Given that source routing is configured on a line or set of lines,
   the specifics of the link state with respect to STE frames are
   defined by the Spanning Tree Protocol in use.  Choice of the split-
   bridge or independent-bridge model does not affect spanning tree
   operation.  In both cases, the spanning tree protocol is executed on
   the two systems independently.

2.5.  SR-TB Translational Bridging

   IEEE 802 is not currently addressing bridges that translate between
   Transparent Bridging and Source Routing.  For the purposes of this
   standard, such a device is either a Transparent or a Source Routing
   bridge, and will act on the line in one of these two ways, just as it
   does on the LAN.

3.  Traffic Services

   Several services are provided for the benefit of different system
   types and user configurations.  These include LAN Frame Checksum
   Preservation, LAN Frame Checksum Generation, Tinygram Compression,
   and the identification of closed sets of LANs.

3.1.  LAN Frame Checksum Preservation

   IEEE 802.1 stipulates that the Extended LAN must enjoy the same
   probability of undetected error that an individual LAN enjoys.
   Although there has been considerable debate concerning the algorithm,
   no other algorithm has been proposed than having the LAN Frame
   Checksum received by the ultimate receiver be the same value
   calculated by the original transmitter.  Achieving this requires, of
   course, that the line protocols preserve the LAN Frame Checksum from
   end to end.  The protocol is optimized towards this approach.

3.2.  Traffic having no LAN Frame Checksum

   The fact that the protocol is optimized towards LAN Frame Checksum
   preservation raises twin questions: "What is the approach to be used
   by systems which, for whatever reason, cannot easily support Frame
   Checksum preservation?" and "What is the approach to be used when the
   system originates a message, which therefore has no Frame Checksum
   precalculated?".



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   Surely, one approach would be to require stations to calculate the
   Frame Checksum in software if hardware support were unavailable; this
   would meet with profound dismay, and would raise serious questions of
   interpretation in a Bridge/Router.

   However, stations which implement LAN Frame Checksum preservation
   must already solve this problem, as they do originate traffic.
   Therefore, the solution adopted is that messages which have no Frame
   Checksum are tagged and carried across the line.

   When a system which does not implement LAN Frame Checksum
   preservation receives a frame having an embedded FCS, it converts it
   for its own use by removing the trailing four octets.  When any
   system forwards a frame which contains no embedded FCS to a LAN, it
   forwards it in a way which causes the FCS to be calculated.

3.3.  Tinygram Compression

   An issue in remote Ethernet bridging is that the protocols that are
   most attractive to bridge are prone to problems on low speed (64 KBPS
   and below) lines.  This can be partially alleviated by observing that
   the vendors defining these protocols often fill the PDU with octets
   of ZERO.  Thus, an Ethernet or IEEE 802.3 PDU received from a line
   that is (1) smaller than the minimum PDU size, and (2) has a LAN
   Frame Checksum present, must be padded by inserting zeroes between
   the last four octets and the rest of the PDU before transmitting it
   on a LAN.  These protocols are frequently used for interactive
   sessions, and therefore are frequently this small.

   To prevent ambiguity, PDUs requiring padding are explicitly tagged.
   Compression is at the option of the transmitting station, and is
   probably performed only on low speed lines, perhaps under
   configuration control.

   The pseudo-code in Appendix 1 describes the algorithms.

3.4.  LAN Identification

   In some applications, it is useful to tag traffic by the user
   community it is a part of, and guarantee that it will be only emitted
   onto a LAN which is of the same community.  The user community is
   defined by a LAN ID.  Systems which choose to not implement this
   feature must assume that any frame received having a LAN ID is from a
   different community than theirs, and discard it.

   It should be noted that the enabling of the LAN Identification option
   requires behavior consistent with the following additions to the
   standard bridging algorithm.