rfc1329.txt

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Network Working Group                                           P. Kuehn
Request for Comments: 1329                                      May 1992


       Thoughts on Address Resolution for Dual MAC FDDI Networks

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard.  Distribution of this memo is
   unlimited.

1. Abstract

   In this document an idea is submitted how IP and ARP can be used on
   inhomogeneous FDDI networks (FDDI networks with single MAC and dual
   MAC stations) by introducing a new protocol layer in the protocol
   suite of the dual MAC stations.  Thus two dual MAC stations are able
   to do a load splitting across the two rings and use the double
   bandwidth of 200 Mbits/s as single MAC stations.  The new layer is an
   extension of layer 3.  For the user, the higher layer protocols, IP
   and ARP the property "dual MAC" is transparent.  No modification is
   required in the protocol suite of single MAC stations and transparent
   bridges.

2. Acknowledgements

   This paper is a result of a diploma thesis prepared at the Technical
   University of Munich, Lehrstuhl fuer Kommunikationsnetze, in co-
   operation with the Siemens Nixdorf AG.  The author would like to
   thank Jrg Eberspher and Bernhard Edmaier from the university, Andreas
   Thimmel and Jens Horstmeier from the SNI AG at Augsburg for the
   helpful comments and discussions.

3. Conventions

   Primary MAC, P-MAC           MAC, placed on the primary ring
   Secondary MAC, S-MAC         MAC, placed on the secondary ring
   Inhomogeneous ring           configuration of a dual FDDI ring with
                                single MAC and dual MAC stations

   DMARP                        Dual MAC Address Resolution Protocol

4. Assumptions

   When a dual FDDI ring wraps, both MACs in a dual MAC station are
   assumed to remain connected to the ring.  ANSI is just investigating
   whether the Configuration Management in the Station Management of a



Kuehn                                                           [Page 1]

RFC 1329     Address Resolution for Dual MAC FDDI Networks      May 1992


   FDDI station can be modified to allow this.  According to the FDDI
   SMT standard [1], different addresses are required for all MACs on
   the primary and the secondary ring.

   In this paper, the MAC in a single MAC station is assumed to reside
   on the primary ring.  The application of single MAC stations which
   have their MAC attached to the secondary ring is not precluded, but
   therefor additional connectivity between the two rings is required.
   These configurations are beyond the scope of this document.

5. The Application of Transparent Bridges

   Transparent bridges can provide links to other 802 LANs or further
   inhomogeneous FDDI rings.  The connection between two inhomogeneous
   FDDI rings can be realized by one or two transparent bridges. When
   two transparent bridges are used, one transparent bridge links the
   primary rings, the other the secondary rings.  If two secondary rings
   are connected by a transparent bridge, a path of transparent bridges
   must exist between the two primary rings.  No transparent bridges are
   allowed between the primary and the secondary ring.

6. Protocol Layers in Single MAC Stations

   The new protocol layer, named load sharing layer, is drafted to be
   introduced only in dual MAC stations.  In single MAC stations, IP and
   ARP are working on top of the Subnetwork Access Protocol (SNAP) 04]
   and the Logical Link Control protocol (802.2 LLC) [3].  LLC type 1 is
   used because connectionless services are investigated only.























Kuehn                                                           [Page 2]

RFC 1329     Address Resolution for Dual MAC FDDI Networks      May 1992


      +--------------------------+
      |   IP                     |
      +--------------------------+
      +--------------------------+
      |   ARP                    |
      +--------------------------+
       |             |
       | ARP frames  | IP frames
       |             |
      +--------------------------+
      |   SNAP                   |
      +--------------------------+
      +--------------------------+
      |   LLC                    |
      +--------------------------+
      +--------------------------++-------+
      |   FDDI-MAC               || F     |
      +--------------------------+| D  S  |
      +--------------------------+| D  M  |
      |   FDDI PHY and PMD       || I  T  |
      +--------------------------++-------+

   For the ARP layer, the following model is assumed:
   +-------------------------------------------------------X-----------+
   |  - ARP entity -                                       |           |
   |                                                       | IP frames |
   | +----------------+   +----------------+ read          |           |
   | | Cache          |   |                | entries +-------------+   |
   | | Administration |->-|  Address Cache |------>--| Address     |   |
   | +----------------+   |                |         | Conversion  |   |
   |     |                +----------------+         | Unit        |   |
   |     | ARP frames                                +-------------+   |
   |     |                                               / |           |
   |     | ___________ <- ARP requests _________________/  | IP frames |
   |     |/                                                |           |
   +-----X-------------------------------------------------X-----------+

   The Address Conversion Unit handles the actual conversion of IP
   addresses to hardware addresses.  For this purpose, it uses the
   information in the ARP cache.  The cache administration communicates
   with other ARP entities by ARP and creates, deletes and renews the
   entries in the cache.

7. Protocol Layers in Dual MAC Stations

   The load sharing layer provides the same interface to ARP as SNAP
   does.  To exchange information about addresses and reachability, the
   load sharing entities in dual MAC stations communicate with the Dual



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RFC 1329     Address Resolution for Dual MAC FDDI Networks      May 1992


   MAC Address Resolution Protocol (DMARP).  For the transmission of
   DMARP frames the SNAP SAP of LLC is used, as for IP and ARP, too.
   The Organizationally Unique Identifier (OUI) in the SNAP header is
   set to zero (24 bit), the EtherType field (16 bit) contains a new
   number indicating DMARP, which is not defined yet.

   +---------------------------------------------------------+
   |                         IP                              |
   +---------------------------------------------------------+
   +---------------------------------------------------------+
   |                         ARP                             |
   +---------------------------------------------------------+
             | ARP frames                 | IP frames
   +---------------------------------------------------------+
   |                 Load Sharing Layer                      |
   +---------------------------------------------------------+
    |        |        |          |        |        |
    | ARP    | DMARP  | IP       | ARP    | DMARP  | IP
    | frames | frames | frames   | frames | frames | frames
    |        |        |          |        |        |
   +-------------------------+  +----------------------------+
   |   SNAP 1                |  |    SNAP 2                  |
   +-------------------------+  +----------------------------+
   +-------------------------+  +----------------------------+
   |   LLC 1                 |  |    LLC 2                   |
   +-------------------------+  +----------------------------+
   +-------------------------+  +----------------------------++-------+
   |   Primary MAC           |  |    Secondary MAC           || F     |
   +-------------------------+  +----------------------------+| D  S  |
   +---------------------------------------------------------+| D  M  |
   |                  FDDI PHY and PMD                       || I  T  |
   +---------------------------------------------------------++-------+

8. Running Inhomogeneous FDDI Rings

8.1. Exchange of Primary MAC Addresses between Stations

   IP and higher layer protocols only use the network independent IP
   addresses.  The ARP entity takes upon the conversion of an IP address
   to the appropriate hardware address.  To make the property dual MAC"
   transparent, ARP may only know the addresses of MACs on the primary
   ring. Therefore, the load sharing entity always delivers ARP frames
   to SNAP 1 for transmission.  By this way, communication with ARP is
   done over the primary ring in normal state.  A secondary MAC can
   receive an ARP frame when the dual ring is wrapped and the
   destination hardware address is a multicast or broadcast address.
   These frames will be discarded because they were received twice.




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RFC 1329     Address Resolution for Dual MAC FDDI Networks      May 1992


   By this way, the associations of IP addresses to primary MAC
   addresses for the single MAC and dual MAC stations are stored in the
   ARP cache.  The ARP cache contains no secondary MAC addresses.

8.2. Exchange of Secondary MAC Addresses between Dual MAC Stations

   The load sharing layer needs to know the secondary MAC addresses of
   the other dual MAC stations.  The DMARP is used to get these
   addresses.  Whenever the load sharing entity delivers an ARP frame to
   SNAP 1, a DMARP reply frame will be sent on the secondary ring,
   containing the stations primary and secondary MAC address. The
   destination hardware address in this DMARP frame is the broadcast MAC
   address, the EtherType field in the SNAP header identifies DMARP.
   The IP destination address is copied from the ARP frame.  If the ARP
   frame that was transmitted parallel to the DMARP reply was a request,
   an ARP reply frame will be sent back to the sending station by the
   ARP entity in the receiving station. When the load sharing layer in
   the receiving station delivers this ARP reply frame to SNAP 1, it
   sends a DMARP reply frame on the secondary ring.

   By this way, DMARP exchanges the additionally required secondary MAC
   addresses between the dual MAC stations.  This is done parallel to
   the exchange of the ARP frames.

8.3. Communication of Dual MAC Stations on Different Dual FDDI Rings

   If two inhomogeneous dual FDDI rings are connected by one transparent
   bridge, dual MAC stations placed on different dual FDDI rings cannot
   perform a load sharing.  If both dual FDDI rings remain in normal
   state, no DMARP reply frames get from one secondary ring to the other
   secondary ring.  A dual MAC station realizes another dual MAC station
   placed on the other dual ring as a single MAC station, because it
   only receives ARP frames from it.  If one of the dual rings is
   wrapped, a DMARP reply frame can get on the primary ring of the other
   dual ring.  A target station on the unwrapped ring receives this
   DMARP frame by the primary MAC and the load sharing entity stores the
   contained addresses in an entry in the address cache.  This entry is
   marked with a control bit, named the OR-bit Other ring bit").  No
   load sharing will be done with a station related to an entry with the
   OR-bit set.

   If both dual FDDI rings are wrapped, the MACs of all stations reside
   on one ring.  Now, dual MAC stations placed on different dual rings
   can communicate with DMARP.  If a DMARP reply frame is received by
   the primary MAC and no entry exists for the sending station, a new
   entry with OR-Bit set will be created.  Otherwise, the OR-bit will be
   set in the existing entry.  If a DMARP reply frame is received by the
   secondary MAC and an entry with OR-bit set already exists for the



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RFC 1329     Address Resolution for Dual MAC FDDI Networks      May 1992


   sending station, the bit will not be reset.

   This mechanism provides that no load sharing will be done between
   Dual MAC stations on different dual rings if the dual rings are
   linked with one transparent bridge.  An additional DMARP error frame
   is used to provide against errors when a DMARP reply frame gets lost
   on the ring.

8.4. Timeout of Entries Marked with OR-Bit Set

   If a FDDI ring is wrapped, the DMARP reply frames are received by the
   primary and secondary MACs of the target dual MAC stations.  In that
   case, the entries for dual MAC stations on the same dual ring are
   also marked with the OR-bit, although the load sharing is possible
   between these stations.

   When an OR-bit in an entry is set for the first time, a timer entity
   is started. If the timer entity runs out, a DMARP request frame is
   sent over SNAP 2 to the secondary MAC of the associated target)
   station.  Then the entry will be discarded.

   If the request cannot be received by the target station because the
   network configuration has changed, there is no entry in the address
   cache for this station any more and no load sharing is computed.  If
   the target station receives the DMARP request frame, it sends back a
   DMARP reply frame.

8.5. Problems with the Application of Large FDDI Networks

   With an increasing number of dual FDDI rings, each one linked
   together by two transparent bridges, the probability increases, that
   one of these inhomogeneous dual FDDI rings is wrapped in the moment
   when two dual MAC stations exchange ARP frames and DMARP replies.

   If two dual MAC stations are communicating for the first time, the
   probability decreases that a load sharing is really computed after
   the exchange of DMARP replies, although this would be possible
   according to the network configuration.  It relies upon the fact,
   that DMARP replies get to the primary ring over the wrapped dual ring
   and only entries marked with the OR-bit set are created. To solve
   this problem further expedients are invented:

   At first, entries in the address cache can be marked read-only by the
   setting of the R-bit.  In dual MAC stations, entries can be written
   manually for other dual MAC stations that are frequently talked to or
   that have a special importance.  The control bits of these entries
   cannot be changed by DMARP.




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RFC 1329     Address Resolution for Dual MAC FDDI Networks      May 1992


   Next, additional control bits are introduced.  One of these bits is
   the Hold-bit (H-bit). When two dual MAC stations exchange ARP frames
   and DMARP replies to create entries in their address caches, one
   station starts sending a DMARP reply, first.  According to the
   network state, it sends an additional DMARP error frame, a moment
   later.  Within a maximum period of time (see "Configuring the Timer
   Parameters"), all frames arrive at the neighbour station and are
   received by the primary and/or secondary MAC.  If the OR-bit was not
   set for an entry within this period of time, it is clear, that no
   further DMARP frames will be received, which result in setting the
   OR-bit.  For such an entry the H-bit is set.  As the reception of
   reply and error frames is not sufficient for setting the OR-bit when
   the H-bit is set, the load sharing is assumed to be sure.  The
   correctness of the H-bit will be verified in relatively long time
   periods by queries (query and hold frames) at the station associated.

   For two communicating stations there exists a possibility to get
   information from a third station.  Always, when the OR-bit is set for
   an entry in a dual MAC station, a search frame is transmitted by the
   secondary MAC, containing the own primary MAC address and the primary
   MAC address of the counter station.  If a third station can compute a
   sure load sharing with both stations (the H-bit is set for the
   associated entries), the stations can perform a load sharing between
   them, too.  The third station informs these stations by sending found
   frames to them.

8.6. Multicast and Broadcast Addresses in IP Frames

   If the destination hardware address of an IP frame is a multicast or
   broadcast hardware address, the frame is always delivered to SNAP 1
   and sent on the primary ring, because one of the addressed stations
   could be a single MAC station.  IP frames which are delivered to the
   load sharing entity by SNAP 2 are discarded by the load sharing
   entity.  Thus, the duplication of these frames can be prevented.

9. Internal Structure

   One load Sharing entity exists in the load sharing layer.  This load
   sharing entity consists of the address cache, the cache
   administration and the multiplexer.