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Network Working Group                                           F. BakerRequest For Comments: 1638                                           ACCCategory: Standards Track                                       R. Bowen                                                                     IBM                                                                 Editors                                                               June 1994PPP 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 Contents1.     Historical Perspective ................................22.     Methods of Bridging ...................................32.1       Transparent Bridging ............................32.2       Remote Transparent Bridging .....................32.3       Source Routing ..................................42.4       Remote Source Route Bridging ....................52.5       SR-TB Translational Bridging ....................63.     Traffic Services ......................................63.1       LAN Frame Checksum Preservation .................63.2       Traffic having no LAN Frame Checksum ............63.3       Tinygram Compression ............................73.4       LAN Identification ..............................74.     A PPP Network Control Protocol for Bridging ...........94.1       Sending Bridge Frames ...........................104.1.1  Maximum Receive Unit Considerations .............104.1.2  Loopback and Link Quality Monitoring ............114.1.3  Message Sequence ................................11Baker & Bowen                                                   [Page 1]

RFC 1638                      PPP Bridging                     June 19944.1.4  Separation of Spanning Tree Domains .............114.2       Bridged LAN Traffic .............................124.3       Spanning Tree Bridge PDU ........................165.     BCP Configuration Options .............................175.1       Bridge-Identification ...........................175.2       Line-Identification .............................195.3       MAC-Support .....................................205.4       Tinygram-Compression ............................215.5       LAN-Identification ..............................225.6       MAC-Address .....................................235.7       Spanning-Tree-Protocol ..........................24        APPENDICES ................................................26A.     Tinygram-Compression Pseudo-Code ...................26        SECURITY CONSIDERATIONS ...................................27        REFERENCES ................................................27     ACKNOWLEDGEMENTS .............................................28     CHAIR'S ADDRESS ..............................................28     AUTHOR'S ADDRESS .............................................281.  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.Baker & Bowen                                                   [Page 2]

RFC 1638                      PPP Bridging                     June 19942.  Methods of Bridging2.1.  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 areBaker & Bowen                                                   [Page 3]

RFC 1638                      PPP Bridging                     June 1994   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.Baker & Bowen                                                   [Page 4]

RFC 1638                      PPP Bridging                     June 1994   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 theBaker & Bowen                                                   [Page 5]

RFC 1638                      PPP Bridging                     June 1994   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?".Baker & Bowen                                                   [Page 6]

RFC 1638                      PPP Bridging                     June 1994   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.Baker & Bowen                                                   [Page 7]

RFC 1638                      PPP Bridging                     June 1994   Each bridge port may be considered to have two additional variables   associated with it: "domain" and "checkDomain".   The variable "domain" (a 32-bit unsigned integer) is assigned a value   that uniquely labels a set of bridge ports in an extended network,   with a default value of 1, and the values of 0 and 0xffffffff being   reserved.   The variable "checkDomain" (a boolean) controls whether this value is   used to filter output to a bridge port.  The variable "checkDomain"   is generally set to the boolean value True for LAN bridge ports, and   set to the boolean value False for WAN bridge ports.   The action of the bridge is then as modified as expressed in the   following C code fragments:      On a packet being received from a bridge port:      if (domainNotPresentWithPacket) {          packetInformation.domain = portInformation[inputPort].domain;      } else {          packetInformation.domain = domainPresentWithPacket;      }      On a packet being transmitted from a bridge port:      if (portInformation[outputPort].checkDomain &&          portInformation[outputPort] != packetInformation.domain) {          discardPacket();          return;      }   For example, suppose you have the following configuration:           E1     +--+            +--+     E3      ------------|  |            |  |------------                  |  |     W1     |  |                  |B1|------------|B2|           E2     |  |            |  |     E4      ------------|  |            |  |------------                  +--+            +--+   E1, E2, E3, and E4 are Ethernet LANs (or Token Ring, FDDI, etc.).  W1   is a WAN (PPP over T1).  B1 and B2 are MAC level bridges.   You want End Stations on E1 and E3 to communicate, and you want End   Stations on E2 and E4 to communicate, but you do not want End   Stations on E1 and E3 to communicate with End Stations on E2 and E4.Baker & Bowen                                                   [Page 8]

RFC 1638                      PPP Bridging                     June 1994   This is true for Unicast, Multicast, and Broadcast traffic.  If a   broadcast datagram originates on E1, you want it only to be   propagated to E3, and not on E2 or E4.   Another way of looking at it is that E1 and E3 form a Virtual LAN,   and E2 and E4 form a Virtual LAN, as if the following configuration   were actually being used:           E1     +--+     W2     +--+     E3      ------------|B3|------------|B4|------------                  +--+            +--+           E2     +--+     W3     +--+     E4      ------------|B5|------------|B6|------------                  +--+            +--+   To accomplish this (using the LAN Identification option), B1 and B2   negotiate this option on, and send datagrams with bit 6 set to 1,   with the LAN ID field inserted in the frame.  Traffic on E1 and E3   would be assigned LAN ID 1, and traffic on E2 and E4 would be   assigned LAN ID 2.  Thus B1 and B2 can separate traffic going over   W1.   Note that execution of the spanning tree algorithm may result in the   subdivision of a domain.  The administrator of LAN domains must   ensure, through spanning tree configuration and topology design, that   such subdivision does not occur.4.  A PPP Network Control Protocol for Bridging   The Bridging Control Protocol (BCP) is responsible for configuring,   enabling and disabling the bridge protocol modules on both ends of   the point-to-point link.  BCP uses the same packet exchange mechanism   as the Link Control Protocol.  BCP packets may not be exchanged until   PPP has reached the Network-Layer Protocol phase.  BCP packets   received before this phase is reached SHOULD be silently discarded.   The Bridging Control Protocol is exactly the same as the Link Control   Protocol [6] with the following exceptions:   Frame Modifications      The packet may utilize any modifications to the basic frame format      which have been negotiated during the Link Establishment phase.      Implementations SHOULD NOT negotiate Address-and-Control-Field-      Compression or Protocol-Field-Compression on other than low speed      links.Baker & Bowen                                                   [Page 9]

RFC 1638                      PPP Bridging                     June 1994   Data Link Layer Protocol Field      Exactly one BCP packet is encapsulated in the PPP Information      field, where the PPP Protocol field indicates type hex 8031 (BCP).   Code field      Only Codes 1 through 7 (Configure-Request, Configure-Ack,      Configure-Nak, Configure-Reject, Terminate-Request, Terminate-Ack      and Code-Reject) are used.  Other Codes SHOULD be treated as      unrecognized and SHOULD result in Code-Rejects.   Timeouts      BCP packets may not be exchanged until PPP has reached the      Network-Layer Protocol phase.  An implementation SHOULD be      prepared to wait for Authentication and Link Quality Determination      to finish before timing out waiting for a Configure-Ack or other      response.  It is suggested that an implementation give up only      after user intervention or a configurable amount of time.   Configuration Option Types      BCP has a distinct set of Configuration Options, which are defined      in this document.4.1.  Sending Bridge Frames   Before any Bridged LAN Traffic or BPDUs may be communicated, PPP MUST   reach the Network-Layer Protocol phase, and the Bridging Control   Protocol MUST reach the Opened state.   Exactly one Bridged LAN Traffic or BPDU is encapsulated in the PPP   Information field, where the PPP Protocol field indicates type hex   0031 (Bridged PDU).4.1.1.  Maximum Receive Unit Considerations   The maximum length of a Bridged datagram transmitted over a PPP link   is the same as the maximum length of the Information field of a PPP   encapsulated packet.  Since there is no standard method for   fragmenting and reassembling Bridged PDUs, PPP links supporting   Bridging MUST negotiate an MRU large enough to support the MAC Types   that are later negotiated for Bridging support.  Because they include   the MAC headers, even bridged Ethernet frames are larger than the   default PPP MRU of 1500 octets.Baker & Bowen                                                  [Page 10]

RFC 1638                      PPP Bridging                     June 19944.1.2.  Loopback and Link Quality Monitoring   It is strongly recommended that PPP Bridge Protocol implementations   utilize Magic Number Loopback Detection and Link-Quality-Monitoring.   The 802.1 Spanning Tree protocol, which is integral to both   Transparent Bridging and Source Routing (as standardized), is   unidirectional during normal operation.  Configuration BPDUs emanate   from the Root system in the general direction of the leaves, without   any reverse traffic except in response to network events.4.1.3.  Message Sequence   The multiple link case requires consideration of message   sequentiality.  The transmitting system may 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 several links, enqueuing traffic to   multiple links to minimize delay.   In the absence of such a determination, the transmitting system MUST   act as though all protocols require order preservation.  Many   protocols designed primarily for use on a single LAN require order   preservation.   Work is currently in progress on a protocol to allow use of multiple   PPP links [7].  If approved, this protocol will allow use of multiple   links while maintaining message sequentiality for Bridged LAN Traffic   and BPDU frames.4.1.4.  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   do that, he should configure both ends to not exchange BPDUs on a   link.  An implementation that does not support any spanning tree   protocol MUST silently discard any received IEEE 802.1D BPDU packets,   and MUST either silently discard or respond to other received BPDU   packets with an LCP Protocol-Reject packet.Baker & Bowen                                                  [Page 11]

RFC 1638                      PPP Bridging                     June 19944.2.  Bridged LAN Traffic   For Bridging LAN traffic, the format of the frame on the line is   shown below.  The fields are transmitted from left to right.   802.3 Frame format    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+   |   HDLC FLAG   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |      Address and Control      |      0x00     |      0x31     |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |F|I|Z|0| Pads  |    MAC Type   |  LAN ID high word (optional)  |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |   LAN ID low word (optional)  |      Destination MAC Address  |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                       Destination MAC Address                 |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                       Source MAC Address                      |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Source MAC Address        |      Length/Type              |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |               LLC data       ...   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                   LAN FCS (optional)                          |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                potential line protocol pad                    |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |          Frame FCS            |   HDLC FLAG   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Baker & Bowen                                                  [Page 12]

RFC 1638                      PPP Bridging                     June 1994   802.4/802.5/FDDI Frame format    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+   |   HDLC FLAG   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |      Address and Control      |      0x00     |      0x31     |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |F|I|Z|0| Pads  |    MAC Type   |  LAN ID high word (optional)  |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |   LAN ID low word (optional)  |   Pad Byte    | Frame Control |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                       Destination MAC Address                 |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Destination MAC Address   |  Source MAC Address           |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                       Source MAC Address                      |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |               LLC data       ...   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                   LAN FCS (optional)                          |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |              optional Data Link Layer padding                 |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |          Frame FCS            |   HDLC FLAG   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Address and Control      As defined by the framing in use.   PPP Protocol      0x0031 for PPP Bridging   Flags      bit F:  Set if the LAN FCS Field is present      bit I:  Set if the LAN ID Field is present      bit Z:  Set if IEEE 802.3 Pad must be zero filled to minimum size      bit 0:  reserved, must be zero   Pads      Any PPP frame may have padding inserted in the "Optional Data Link      Layer Padding" field.  This number tells the receiving system how      many pad octets to strip off.Baker & Bowen                                                  [Page 13]

RFC 1638                      PPP Bridging                     June 1994   MAC Type      Up-to-date values of the MAC Type field are specified in the most      recent "Assigned Numbers" RFC [4].  Current values are assigned as      follows:          0: reserved          1: IEEE 802.3/Ethernet  with canonical addresses          2: IEEE 802.4           with canonical addresses          3: IEEE 802.5           with non-canonical addresses          4: FDDI                 with non-canonical addresses       5-10: reserved         11: IEEE 802.5           with canonical addresses         12: FDDI                 with canonical addresses      "Canonical" is the address format defined as standard address      representation by the IEEE.  In this format, the bit within each      byte that is to be transmitted first on a LAN is represented as      the least significant bit.  In contrast, in non-canonical form,      the bit within each byte that is to be transmitted first is      represented as the most-significant bit.  Many LAN interface      implementations use non-canonical form.  In both formats, bytes      are represented in the order of transmission.      If an implementation supports a MAC Type that is the higher-      numbered format of that MAC Type, then it MUST also support the      lower-numbered format of that MAC Type.  For example, if an      implementation supports FDDI with canonical address format, then      it MUST also support FDDI with non-canonical address format.  The      purpose of this requirement is to provide backward compatibility      with earlier versions of this specification.      A system MUST NOT transmit a MAC Type numbered higher than 4      unless it has received from its peer a MAC-Support Configuration      Option indicating that the peer is willing to receive frames of      that MAC Type.   LAN ID      This optional 32-bit field identifies the Community of LANs which      may be interested to receive this frame.  If the LAN ID flag is      not set, then this field is not present, and the PDU is four      octets shorter.   Frame Control      On 802.4, 802.5, and FDDI LANs, there are a few octets preceding      the Destination MAC Address, one of which is protected by the FCS.Baker & Bowen                                                  [Page 14]

RFC 1638                      PPP Bridging                     June 1994      The MAC Type of the frame determines the contents of the Frame      Control field.  A pad octet is present to provide 32-bit packet      alignment.   Destination MAC Address      As defined by the IEEE.  The MAC Type field defines the bit      ordering.   Source MAC Address      As defined by the IEEE.  The MAC Type field defines the bit      ordering.   LLC data      This is the remainder of the MAC frame which is (or would be were      it present) protected by the LAN FCS.      For example, the 802.5 Access Control field, and Status Trailer      are not meaningful to transmit to another ring, and are omitted.   LAN FCS      If present, this is the LAN FCS which was calculated by (or which      appears to have been calculated by) the originating station.  If      the LAN FCS flag is not set, then this field is not present, and      the PDU is four octets shorter.   Optional Data Link Layer Padding      Any PPP frame may have padding inserted between the Information      field and the Frame FCS.  The Pads field contains the length of      this padding, which may not exceed 15 octets.      The PPP LCP Extensions [5] specify a self-describing pad.      Implementations are encouraged to set the Pads field to zero, and      use the self-describing pad instead.   Frame FCS      Mentioned primarily for clarity.  The FCS used on the PPP link is      separate from and unrelated to the LAN FCS.Baker & Bowen                                                  [Page 15]

RFC 1638                      PPP Bridging                     June 19944.3.  Spanning Tree Bridge PDU   This is the Spanning Tree BPDU, without any MAC or 802.2 LLC header   (these being functionally equivalent to the Address, Control, and PPP   Protocol Fields).  The LAN Pad and Frame Checksum fields are likewise   superfluous and absent.   The Address and Control Fields are subject to LCP Address-and-   Control-Field-Compression negotiation.   A PPP system which is configured to participate in a particular   spanning tree protocol and receives a BPDU of a different spanning   tree protocol SHOULD reject it with the LCP Protocol-Reject.  A   system which is configured not to participate in any spanning tree   protocol MUST silently discard all BPDUs.   Spanning Tree Bridge PDU    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+   |   HDLC FLAG   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |      Address and Control      |     Spanning Tree Protocol    |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |              BPDU data       ...                              |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |          Frame FCS            |   HDLC FLAG   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Address and Control      As defined by the framing in use.   Spanning Tree Protocol      Up-to-date values of the Spanning-Tree-Protocol field are      specified in the most recent "Assigned Numbers" RFC [4].  Current      values are assigned as follows:         Value (in hex)  Protocol         0201            IEEE 802.1 (either 802.1D or 802.1G)         0203            IBM Source Route Bridge         0205            DEC LANbridge 100      The two versions of the IEEE 802.1 spanning tree protocol frames      can be distinguished by fields within the BPDU data.Baker & Bowen                                                  [Page 16]

RFC 1638                      PPP Bridging                     June 1994   BPDU data      As defined by the specified Spanning Tree Protocol.5.  BCP Configuration Options   BCP Configuration Options allow modifications to the standard   characteristics of the network-layer protocol to be negotiated.  If a   Configuration Option is not included in a Configure-Request packet,   the default value for that Configuration Option is assumed.   BCP uses the same Configuration Option format defined for LCP [6],   with a separate set of Options.   Up-to-date values of the BCP Option Type field are specified in the   most recent "Assigned Numbers" RFC [4].  Current values are assigned   as follows:         1       Bridge-Identification         2       Line-Identification         3       MAC-Support         4       Tinygram-Compression         5       LAN-Identification         6       MAC-Address         7       Spanning-Tree-Protocol5.1.  Bridge-Identification   Description      The Bridge-Identification Configuration Option is designed for use      when the line is an interface between half bridges connecting      virtual or physical LAN segments.  Since these remote bridges are      modeled as a single bridge with a strange internal interface, each      remote bridge needs to know the LAN segment and bridge numbers of      the adjacent remote bridge.  This option MUST NOT be included in      the same Configure-Request as the Line-Identification option.      The Source Routing Route Descriptor and its use are specified by      the IEEE 802.1D Appendix on Source Routing.  It identifies the      segment to which the interface is attached by its configured      segment number, and itself by bridge number on the segment.      The two half bridges MUST agree on the bridge number.  If a bridge      number is not agreed upon, the Bridging Control Protocol MUST NOT      enter the Opened state.Baker & Bowen                                                  [Page 17]

RFC 1638                      PPP Bridging                     June 1994      Since mismatched bridge numbers are indicative of a configuration      error, it is strongly recommended that a system not change its      bridge number for the purpose of resolving a mismatch.  However,      to allow two systems to proceed to the Opened state despite a      mismatch, a system MAY change its bridge number to the higher of      the two numbers.  A higher-numbered system MUST NOT change its      bridge number to a lower number.      By default, a system that does not negotiate this option is      assumed to be configured not to use the model of the two systems      as two halves of a single source-route bridge.  It is instead      assumed to be configured to use the model of the two systems as      two independent bridges.   Example      If System A announces LAN Segment AAA, Bridge #1, and System B      announces LAN Segment BBB, Bridge #1, then the resulting Source      Routing configuration (read in the appropriate direction) is then      AAA,1,BBB.   A summary of the Bridge-Identification Option format is shown below.   The fields are transmitted from left to right.    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |    Length     | LAN Segment Number    |Bridge#|   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Type      1   Length      4   LAN Segment Number      A 12-bit number identifying the LAN segment, as defined in the      IEEE 802.1D Source Routing Specification.   Bridge Number      A 4-bit number identifying the bridge on the LAN segment, as      defined in the IEEE 802.1D Source Routing Specification.Baker & Bowen                                                  [Page 18]

RFC 1638                      PPP Bridging                     June 19945.2.  Line-Identification   Description      The Line-Identification Configuration Option is designed for use      when the line is assigned a LAN segment number as though it were a      two system LAN segment in accordance with the Source Routing      algorithm.  This option MUST NOT be included in the same      Configure-Request as the Bridge-Identification option.      The Source Routing Route Descriptor and its use are specified by      the IEEE 802.1D Appendix on Source Routing.  It identifies the      segment to which the interface is attached by its configured      segment number, and itself by bridge number on the segment.      The two bridges MUST agree on the LAN segment number.  If a LAN      segment number is not agreed upon, the Bridging Control Protocol      MUST NOT enter the Opened state.      Since mismatched LAN segment numbers are indicative of a      configuration error, it is strongly recommended that a system not      change its LAN segment number for the purpose of resolving a      mismatch.  However, to allow two systems to proceed to the Opened      state despite a mismatch, a system MAY change its LAN segment      number to the higher of the two numbers.  A higher-numbered system      MUST NOT change its LAN segment number to a lower number.      By default, a system that does not negotiate this option is      assumed to have its LAN segment number correctly configured by the      user.   A summary of the Line-Identification Option format is shown below.   The fields are transmitted from left to right.    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |    Length     | LAN Segment Number    |Bridge#|   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Type      2   Length      4Baker & Bowen                                                  [Page 19]

RFC 1638                      PPP Bridging                     June 1994   LAN Segment Number      A 12-bit number identifying the LAN segment, as defined in the      IEEE 802.1D Source Routing Specification.   Bridge Number      A 4-bit number identifying the bridge on the LAN segment, as      defined in the IEEE 802.1D Source Routing Specification.5.3.  MAC-Support   Description      The MAC-Support Configuration Option is provided to permit      implementations to indicate the sort of traffic they are prepared      to receive.  Negotiation of this option is strongly recommended.      By default, when an implementation does not announce the MAC Types      that it supports, all MAC Types are sent by the peer which are      capable of being transported given other configuration parameters.      The receiver will discard those MAC Types that it does not      support.      A device supporting a 1600 octet MRU might not be willing to      support 802.5, 802.4 or FDDI, which each support frames larger      than 1600 octets.      By announcing the MAC Types it will support, an implementation is      advising its peer that all unspecified MAC Types will be      discarded.  The peer MAY then reduce bandwidth usage by not      sending the unsupported MAC Types.      Announcement of support for multiple MAC Types is accomplished by      placing multiple options in the Configure-Request.      The nature of this option is advisory only.  This option MUST NOT      be included in a Configure-Nak.   A summary of the MAC-Support Option format is shown below.  The   fields are transmitted from left to right.    0                   1                   2    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |    Length     |    MAC Type   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Baker & Bowen                                                  [Page 20]

RFC 1638                      PPP Bridging                     June 1994   Type      3   Length      3   MAC Type      One of the values of the PDU MAC Type field (previously described      in the "Bridged LAN Traffic" section) that this system is prepared      to receive and service.5.4.  Tinygram-Compression      Description      This Configuration Option permits the implementation to indicate      support for Tinygram compression.      Not all systems are prepared to make modifications to messages in      transit.  On high speed lines, it is probably not worth the      effort.      This option MUST NOT be included in a Configure-Nak if it has been      received in a Configure-Request.  This option MAY be included in a      Configure-Nak in order to prompt the peer to send the option in      its next Configure-Request.      By default, no compression is allowed.  A system which does not      negotiate, or negotiates this option to be disabled, should never      receive a compressed packet.   A summary of the Tinygram-Compression Option format is shown below.   The fields are transmitted from left to right.    0                   1                   2    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |    Length     | Enable/Disable|   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Type      4Baker & Bowen                                                  [Page 21]

RFC 1638                      PPP Bridging                     June 1994   Length      3   Enable/Disable      If the value is 1, Tinygram-Compression is enabled.  If the value      is 2, Tinygram-Compression is disabled, and no decompression will      occur.      The implementations need not agree on the setting of this      parameter.  One may be willing to decompress and the other not.5.5.  LAN-Identification   Description      This Configuration Option permits the implementation to indicate      support for the LAN Identification field, and that the system is      prepared to service traffic to any labeled LANs beyond the system.      A Configure-NAK MUST NOT be sent in response to a Configure-      Request that includes this option.      By default, LAN-Identification is disabled.  All Bridge LAN      Traffic and BPDUs that contain the LAN ID field will be discarded.      The peer may then reduce bandwidth usage by not sending the      unsupported traffic.   A summary of the LAN-Identification Option format is shown below.   The fields are transmitted from left to right.    0                   1                   2    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |    Length     | Enable/Disable|   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Type      5   Length      3Baker & Bowen                                                  [Page 22]

RFC 1638                      PPP Bridging                     June 1994   Enable/Disable      If the value is 1, LAN Identification is enabled.  If the value is      2, LAN Identification is disabled.      The implementations need not agree on the setting of this      parameter.  One may be willing to accept LAN Identification and      the other not.5.6.  MAC-Address   Description      The MAC-Address Configuration Option enables the implementation to      announce its MAC address or have one assigned.  The MAC address is      represented in IEEE 802.1 Canonical format, which is to say that      the multicast bit is the least significant bit of the first octet      of the address.      If the system wishes to announce its MAC address, it sends the      option with its MAC address specified.  When specifying a non-zero      MAC address in a Configure-Request, any inclusion of this option      in a Configure-Nak MUST be ignored.      If the implementation wishes to have a MAC address assigned, it      sends the option with a MAC address of 00-00-00-00-00-00.  Systems      that have no mechanism for address assignment will Configure-      Reject the option.      A Configure-Nak MUST specify a valid IEEE 802.1 format physical      address; the multicast bit MUST be zero.  It is strongly      recommended (although not mandatory) that the "locally assigned      address" bit (the second least significant bit in the first octet)      be set, indicating a locally assigned address.   A summary of the MAC-Address Option format is shown below.  The   fields are transmitted from left to right.    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |    Length     |MAC byte 1 |L|M|  MAC byte 2   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |  MAC byte 3   |  MAC byte 4   |  MAC byte 5   |  MAC byte 6   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Baker & Bowen                                                  [Page 23]

RFC 1638                      PPP Bridging                     June 1994   Type      6   Length      8   MAC Byte      Six octets of MAC address in 802.1 Canonical order.  For clarity,      the position of the Local Assignment (L) and Multicast (M) bits      are shown in the diagram.5.7.  Spanning-Tree-Protocol   Description      The Spanning-Tree-Protocol Configuration Option enables the      Bridges to negotiate the version of the spanning tree protocol in      which they will participate.      If both bridges support a spanning tree protocol, they MUST agree      on the protocol to be supported.  When the two disagree, the      lower-numbered of the two spanning tree protocols should be used.      To resolve the conflict, the system with the lower-numbered      protocol SHOULD Configure-Nak the option, suggesting its own      protocol for use.  If a spanning tree protocol is not agreed upon,      except for the case in which one system does not support any      spanning tree protocol, the Bridging Control Protocol MUST NOT      enter the Opened state.      Most systems will only participate in a single spanning tree      protocol.  If a system wishes to participate simultaneously in      more than one spanning tree protocol, it MAY include all of the      appropriate protocol types in a single Spanning-Tree-Protocol      Configuration Option.  The protocol types MUST be specified in      increasing numerical order.  For the purpose of comparison during      negotiation, the protocol numbers MUST be considered to be a      single number.  For instance, if System A includes protocols 01      and 03 and System B indicates protocol 03, System B should      Configure-Nak and indicate a protocol type of 03 since 0103 is      greater than 03.      By default, an implementation MUST either support the IEEE 802.1D      spanning tree or support no spanning tree protocol.  An      implementation that does not support any spanning tree protocol      MUST silently discard any received IEEE 802.1D BPDU packets, andBaker & Bowen                                                  [Page 24]

RFC 1638                      PPP Bridging                     June 1994      MUST either silently discard or respond to other received BPDU      packets with an LCP Protocol-Reject packet.   A summary of the Spanning-Tree-Protocol Option format is shown below.   The fields are transmitted from left to right.    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |    Length     |  Protocol 1   |  Protocol 2   | ...   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Type      7   Length      2 octets plus 1 additional octet for each protocol that will be      actively supported.  Most systems will only support a single      spanning tree protocol, resulting in a length of 3.   Protocol n      Each Protocol field is one octet and indicates a desired spanning      tree protocol.  Up-to-date values of the Protocol field are      specified in the most recent "Assigned Numbers" RFC [4].  Current      values are assigned as follows:           Value     Protocol             0       Null (no Spanning Tree protocol supported)             1       IEEE 802.1D spanning tree             2       IEEE 802.1G extended spanning tree protocol             3       IBM Source Route Spanning tree protocol             4       DEC LANbridge 100 Spanning tree protocolBaker & Bowen                                                  [Page 25]

RFC 1638                      PPP Bridging                     June 1994A.  Tinygram-Compression Pseudo-Code    PPP Transmitter:    if (ZeroPadCompressionEnabled &&        BridgedProtocolHeaderFormat == IEEE8023 &&        PacketLength == Minimum8023PacketLength) {     /*      * Remove any continuous run of zero octets preceding,      * but not including, the LAN FCS, but not extending      * into the MAC header.      */        Set (ZeroCompressionFlag);            /* Signal receiver */        if (is_Set (LAN_FCS_Present)) {            FCS = TrailingOctets (PDU, 4);    /* Store FCS */            RemoveTrailingOctets (PDU, 4);    /* Remove FCS */            while (PacketLength > 14 &&       /* Stop at MAC header or */                   TrailingOctet (PDU) == 0)  /*  last non-zero octet */                RemoveTrailingOctets (PDU, 1);/* Remove zero octet */            Appendbuf (PDU, 4, FCS);          /* Restore FCS */        }        else {            while (PacketLength > 14 &&       /* Stop at MAC header */                   TrailingOctet (PDU) == 0)  /*  or last zero octet */                RemoveTrailingOctets (PDU, 1);/* Remove zero octet */        }    }    PPP Receiver:    if (ZeroCompressionFlag) {                /* Flag set in header? */     /* Restoring packet to minimum 802.3 length */        Clear (ZeroCompressionFlag);        if (is_Set (LAN_FCS_Present)) {            FCS = TrailingOctets (PDU, 4);   /* Store FCS */            RemoveTrailingOctets (PDU, 4);   /* Remove FCS */            Appendbuf (PDU, 60 - PacketLength, zeroes);/* Add zeroes */            Appendbuf (PDU, 4, FCS);         /* Restore FCS */        }        else {            Appendbuf (PDU, 60 - PacketLength, zeroes);/* Add zeroes */        }    }Baker & Bowen                                                  [Page 26]

RFC 1638                      PPP Bridging                     June 1994Security Considerations   Security issues are not discussed in this memo.References   [1] IBM, "Token-Ring Network Architecture Reference", 3rd edition,       September 1989.   [2] IEEE 802.1, "Draft Standard 802.1G: Remote MAC Bridging",       P802.1G/D7, December 30, 1992.   [3] IEEE 802.1, "Media Access Control (MAC) Bridges", ISO/IEC 15802-       3:1993 ANSI/IEEE Std 802.1D, 1993 edition., July 1993.   [4] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,RFC 1340,       USC/Information Sciences Institute, July 1992.   [5] Simpson, W., "PPP LCP Extensions",RFC 1570, Daydreamer, January       1994.   [6] Simpson, W., "The Point-to-Point Protocol (PPP)",RFC 1548,       Daydreamer, December 1993.   [7] Sklower, K., "A Multilink Protocol for Synchronizing the       Transmission of Multi-protocol Datagrams", Work in Progress,       August 1993.Baker & Bowen                                                  [Page 27]

RFC 1638                      PPP Bridging                     June 1994Acknowledgments   This document is a product of the Point-to-Point Protocol Extensions   Working Group.   Special thanks go to Steve Senum of Network Systems, Dino Farinacci   of 3COM, Rick Szmauz of Digital Equipment Corporation, and Andrew   Fuqua of IBM.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: fbaker@acc.comAuthor's Address   Questions about this memo can also be directed to:   Rich Bowen   International Business Machines Corporation   P. O. Box 12195   Research Triangle Park, NC  27709   Phone: (919) 543-9851   EMail: Rich_Bowen@vnet.ibm.comBaker & Bowen                                                  [Page 28]