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RFC 3422 - Forwarding Media Access Control (MAC) Frames over Mul


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Network Working Group                                         O. Okamoto
Request for Comments: 3422                                   M. Maruyama
Category: Informational                                 NTT Laboratories
                                                               T. Sajima
                                                        Sun Microsystems
                                                           November 2002

       Forwarding Media Access Control (MAC) Frames over Multiple
  Access Protocol over Synchronous Optical Network/Synchronous Digital
                           Hierarchy (MAPOS)

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

IESG Note

   This memo documents a way of tunneling Ethernet frames over MAPOS
   networks.  This document is NOT the product of an IETF working group
   nor is it a standards track document.  It has not necessarily
   benefited from the widespread and in-depth community review that
   standards track documents receive.

Abstract

   This memo describes a method for forwarding media access control
   (MAC) frames over Multiple Access Protocol over Synchronous Optical
   Network/Synchronous Digital Hierarchy (MAPOS), thus providing a way
   to unify MAPOS network environment and MAC-based Local Area Network
   (LAN) environment.

1. Network Model

   In the Network model assumed in this memo, MAC-based LAN traffic is
   forwarded by a MAPOS switched network.  This model allows distant
   LANs to be interconnected to form a single LAN segment.  Transparent
   LAN Service (TLS) is provided by encapsulating MAC frames in MAPOS
   frames and by mapping MAC addresses to MAPOS addresses.

   This network model is shown in figure 1. "MAPOS network" is composed
   of MAPOS switches, SONET/SDH leased lines and optical fiber cables.
   A LAN is connected to a MAPOS network by a Network Adapter (NA) which
   has a MAPOS interface and an ethernet interface.  A unique MAPOS
   address is assigned to each NA by NSP (Node-Switch Protocol) [2].

                                +-----------+
      MAC-based LAN N1 +---+    |   MAPOS   |    +---+ MAC-based LAN N2
        ---------------|   |----|  network  |----|   |---------------
         |             +---+    |           |    +---+             |
      +-----+         Network   |    N0     |   Network         +-----+
      |     |         adapter   +-----------+   adapter         |     |
      +-----+            B1                       B2            +-----+
      Host H1                                                   Host H2

            Figure 1. VPN network service model with LANs N1 and N2

   Host H1 in LAN N1 and host H2 in LAN N2 are connected to distinct
   MAC-based LANs.  Transparent LAN service is provided by MAPOS network
   N0 exchanging MAC frames between Host H1 and Host H2.

   Using this mechanism, a single VLAN segment can be setup from
   multiple LANs that may be geographically located far away from each
   other.

   The use of a switched technology is recommended for building a MAC-
   based LAN.  In some cases, however, this becomes a requirement.  A
   likely example is the situation where a MAC-based LAN having two
   network adapters, both attached to the same MAPOS network (for
   redundancy).  If the LAN is built using shared (non-switched)
   technology, then this loop configuration is bound to be stormed by
   incessant broadcast traffic.  This can only be circumvented by using
   switched technology with support for broadcast spanning tree [7].

2. Forwarding a MAC Frame

   This section describes the MAC frame forwarding mechanism in the
   MAPOS network.

2.1. Outline

   In figure 2, LANs N1 and N2 communicates via MAPOS network N0.  NAs
   B1 and B2 are gateways into Network N0, and they each have a MAPOS
   interface and an ethernet interface.

                                +------------+
                                |MAPOS header|
      +-----------+             +------------+             +-----------+
      | MAC header| encapsulate | MAC  header| decapsulate | MAC header|
      +-----------+ ----------> +------------+ ----------> +-----------+
      |information|             | information|             |information|
      +-----------+             +------------+             +-----------+
        MAC frame             Bridged MAPOS frame             MAC frame

                                +------------+
        LAN N1         +---+    |    MAPOS   |    +---+         LAN N2
        ---------------|   |----|   network  |----|   |---------------
         |             +---+    |            |    +---+             |
      +-----+            B1     |      N0    |      B2           +-----+
      |     |                   +------------+                   |     |
      +-----+                                                    +-----+
      Host H1                                                    Host H2

          Figure 2. Forwarding a MAC frame from H1 to H2 over the VPN

   The process of forwarding a MAC frame transparently from host H1 to
   host H2 is also shown in figure 2.  NA B1 encapsulates a MAC frame
   from host H1, and forwards it to MAPOS network N0.  NA B2
   decapsulates the MAPOS frame, then forwards the MAC frame to host H2.

2.2. MAPOS encapsulation format

   To transmit a MAC frame into MAPOS network, the NA encapsulates the
   frame as shown in the following figures.  This frame format is based
   on Bridged LAN Traffic for PPP [4]; only the fields with semantics
   specific to this document are described 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
      +-+-+-+-+-+-+-+-+
      |  HDLC Flag    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Address and Control      |      0xFE     |      0x31     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        (reserved)             |     Source MAPOS Address      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |F|0|Z|0| Pads  |   MAC Type    |    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 (16/32bits)                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 3. 802.3 Frame format (IEEE 802 Un-tagged Frame)

      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      |      0xFE     |      0x31     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         (reserved)            |     Source MAPOS Address      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |F|0|Z|0| Pads  |    MAC Type   |   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 (16/32bits)                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Figure 4. 802.4/802.5/FDDI Frame format (IEEE 802 Un-tagged Frame)

      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      |      0xFE     |      0x31     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        (reserved)             |     Source MAPOS Address      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |F|0|Z|0| Pads  |   MAC Type    |    Destination MAC address    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Destination MAC Address                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Source MAC Address                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Source MAC Address       |     0x81      |      0x00     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Pri  |C| VLAN ID               |      Length/Type              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    LLC data ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   LAN FCS (optional)                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 potential line protocol pad                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Frame FCS (16/32bits)                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 5. 802.3 Frame format (IEEE 802 Tagged Frame)

      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      |      0xFE     |      0x31     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        (reserved)             |     Source MAPOS Address      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |F|0|Z|0| Pads  |    MAC Type   |   Pad Byte    | Frame Control |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Destination MAC Address                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Destination MAC Address   |  Source MAC Address           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Source MAC Address                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   SNAP-encoded TPID                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   SNAP-encoded TPID                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Pri  |C| VLAN ID               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    LLC data ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   LAN FCS (optional)                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              optional Data Link Layer padding                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Frame FCS (16/32bits)                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 6. 802.4/802.5/FDDI Frame format (IEEE 802 Tagged Frame)

   Address and Control

      These fields contain the destination HDLC address as defined by
      MAPOS Version 1 [1] and MAPOS 16 [3].

   Protocol Field

      0xFE31 for bridged LAN traffic for MAPOS.  NA should only accept
      NSP (0xFE03) and bridged MAPOS frames (0xFE31) frames; others
      should be silently discarded.

   Source MAPOS address

      Contains the MAPOS address of the sending NA.  For MAPOS version 1
      [1] the 8-bit HDLC address is placed in the least significant
      place of the 16-bit field and the upper eight bits must be zero.

3. Determination of the Destination MAPOS Address

   The destination MAPOS address for a MAC frame to be bridged is
   determined by searching the address table composed of entries of the
   form

   {destination MAC address, destination MAPOS address}

   during the encapsulation phase.

   For example, in figure 2, when a MAC frame to be sent to host H2 is
   encapsulated, the destination MAPOS address corresponding to NA B2 is
   used.

   Determination of the destination MAPOS address for forwarding a MAC
   unicast frame is described in 3.1. The way for forwarding a MAC
   broadcast or multicast frame is described in 3.2.  Methods for
   populating the address table are explained in 3.3.

3.1. Destination MAPOS address for forwarding a MAC unicast frame

   In NA, entries of the form

   {destination MAC address, destination MAPOS address}

   are held in its address table.  When a MAC frame is received by the
   ethernet interface, the address table is searched using the
   destination MAC address as the key.  If a matching entry is found,
   the corresponding MAPOS address is used as the destination MAPOS
   address.  If no matching entry exists, MAC broadcast forwarding (3.2)
   is used.

3.2. Forwarding a MAC broadcast or multicast frame

   All MAC broadcast or multicast frames must be duplicated for
   transmission (via MAPOS unicast) to each of the peer network adapters
   in the same VLAN as the sending network adapter.

   Consider an example shown in figure 7 where six LANs N1 through N6
   are connected to the MAPOS network via network adapters B1 through
   B6.

                                +------------+
        LAN N1         +---+    |            |    +---+         LAN N2
        ---------------|   |----|            |----|   |---------------
         |             +---+    |            |    +---+             |
      +-----+         Network   |            |   Network         +-----+
      |     |         adapter   |            |   adapter         |     |
      +-----+            B1     |            |      B2           +-----+
      Host H1                   |            |                   Host H2
                                |            |
                                |            |
                                |            |
        LAN N3         +---+    |    MAPOS   |    +---+         LAN N4
        ---------------|   |----|   network  |----|   |---------------
         |             +---+    |            |    +---+             |
      +-----+         Network   |      N0    |   Network         +-----+
      |     |         Adapter   |            |   adapter         |     |
      +-----+            B3     |            |     B4            +-----+
      Host H3                   |            |                   Host H4
                                |            |
                                |            |
                                |            |
        LAN N5         +---+    |            |    +---+         LAN N6
        ---------------|   |----|            |----|   |---------------
         |             +---+    |            |    +---+             |
      +-----+         Network   |            |   Network         +-----+
      |     |         adapter   +------------+   adapter         |     |
      +-----+            B5                        B6            +-----+
      Host H5                                                    Host H6

             Figure 7. Six networks connected to the MAPOS network

   If a VLAN is configured with LANs N1, N2, and N3, a MAC broadcast or
   multicast frame originating from LAN N1 must not be forwarded to LAN
   N4, N5, or N6 but only to LANs N1, N2, and N3.  It is duplicated
   twice for encapsulation and delivery to B2 and B3 via MAPOS unicast.

   A set of network adapters that belongs to the same VLAN defines the
   broadcast scope of the VLAN.  Before a VLAN is put to use, each NA in
   the VLAN must be configured with the MAPOS addresses of its peer NAs.
   A NA should silently discard bridged MAPOS frames with a MAPOS source
   address that is not among the peers that the NA knows about.

   The use of MAPOS multicast for forwarding MAC broadcast frames is
   under further study.

3.3. Methods for configuring the address table

   This section describes two methods for setting up an address table:
   static and dynamic.  NA must implement the static method described in
   3.3.1.  The dynamic method (3.3.2) is optional, but an implementation
   must provide an option to disable this feature.

3.3.1. Static setup of address table

   The address table can be set up statically.  Before using a VLAN,
   address table entries for each NA in the VLAN must be populated
   manually.

   These entries are considered permanent until they are manually
   removed, and must not be "aged" or overwritten by the dynamic
   procedure described in 3.3.2.

3.3.2. Dynamic setup of address table

   The address table can also be set up dynamically.  A NA discovers
   entries for its address table from incoming encapsulated MAPOS
   frames.

   The NA adds the pair

   {source MAC address, source MAPOS address}

   to its address table when it receives an encapsulated MAPOS frame.

   Entries discovered this way are subject to aging timer (should be
   configurable with the default of 300 seconds).  Once the timer for an
   entry expires, the entry is removed from the address table.  The
   timer is reset each time an encapsulated MAPOS frame with the same
   source MAC address is received.

   There must be at most one entry for a source MAC address.  If a
   discovered MAPOS address for a MAC address differs from the
   previously discovered address, the new one takes precedence and the
   address table entry must be overwritten.  Under no circumstance may a
   discovered entry overwrite a statically created entry (3.3.1).

   Discovery process using ARP [6] packets between host H1 (the MAC
   address is h1) in LAN N1 and host H2 (the MAC address is h2) in LAN
   N2 is shown below.

   The MAPOS addresses of NAs B1, B2, B3 are b1, b2, b3 respectively.

                              +-----------+
        LAN N1       +---+    |           |
        -------------|   |----|           |
         |           +---+    |           |
      +-----+       Network   |           |
      |     |       adapter   |   MAPOS   |    +---+         LAN N2
      +-----+          B1     |  network  |----|   |------------
      Host H1                 |           |    +---+          |
   (ARP request)              |    N0     |   Network      +-----+
                              |           |   adapter      |     |
                              |           |      B2        +-----+
        LAN N3       +---+    |           |                Host H2
        -------------|   |----|           |              (ARP reply)
         |           +---+    |           |
      +-----+       Network   +-----------+
      |     |       adapter
      +-----+          B3
      Host H3

         Figure 8. Three networks connected to the MAPOS network

   (1)  Host H1 transmits an ARP request frame.  An ARP request frame is
        a MAC broadcast Frame.

   (2)  At NA B1, ARP request frame is received and is encapsulated.
        Because the VPN is composed of LANs N1, N2, and N3, the NA B1
        must send a MAPOS frame that has destination MAPOS address b2
        and another MAPOS frame that has destination MAPOS address b3.
        MAPOS address b1 is stored in the source MAPOS address field of
        each frame.

   (3)  The bridged MAPOS frame arrives at NAs B2 and B3 from the MAPOS
        network.

   (4)  NAs B2 and B3 receive the bridged MAPOS frame, and the pair

        {h1, b1}

        is added to their address tables.

   (5)  In NA B2, the received MAPOS frame is decapsulated, and the MAC
        frame is forwarded to LAN N2.  Similarly, in NA B3, the received
        MAPOS frame is decapsulated, and the MAC frame is forwarded to
        LAN N3.

   (6)  At host H2, which exists in LAN N2, an ARP reply frame is
        transmitted to host H1.

   (7)  Via the ethernet interface on NA B2, the ARP reply frame is
        received, and MAPOS encapsulation is done.

        Because the entry

        {h1, b1}

        is registered in the address table, b1 is determined to be the
        destination MAPOS address.  The bridged frame is forwarded to
        the MAPOS network.

   (8)  MAPOS network delivers the bridged MAPOS frame to NA B1.

   (9)  NA B1 decapsulates the bridged MAPOS frame, and forwards the MAC
        frame to LAN N1.  At the same time, the entry {h2 , b2} is
        registered into NA B1 address table.

   (10) Host H1 receives the ARP reply frame.

4. Connecting a MAPOS Host to the VLAN

   In order for a native MAPOS host to connect to a VLAN, it must have
   its own unique MAC address and implement all the features of a
   network adapter appropriate for the MAC framing that it wishes to
   use.

5. Security Considerations

   This section discusses some of the security factors that need to be
   considered when planning a transparent LAN service described in
   section 1, "Network Model."

5.1 Management boundaries

   In a large network, different parts of the network are managed by
   different organizations, and it is essential to clearly define the
   boundaries of management responsibilities.

   A probable scenario is that a common carrier provides transparent LAN
   service to a variety of customers.  Each customer is a distinct
   organization, expecting virtual private network service.  In such a
   case, the common carrier should take management responsibility for
   the MAPOS network, optical cables to customer sites, and the network
   adapters that reside in customer premises.

                                     +----+
     MAPOS Net +-------- ... --------+ NA +---- MAC-based LAN
                                     +----+
        Common Carrier Responsibility --->|<-- Customer Responsibility

   In essence, the customer is allowed to do no more than connecting the
   cable from their MAC-based LAN to the network adapters.  Common
   carrier should be very careful to monitor and protect their assets,
   including SONET/SDH connections and network adapters.  In particular,
   network adapters serve as the primary line of defense against attacks
   and should be closely guarded.

5.2 Risks

   Privacy of every customer connected to the carrier's MAPOS network
   may be compromised.

5.3 Attack against network adapters

   A network adapter should be a dedicated device.  This makes the
   device simple and easier to harden against break-in attempts.  In the
   worst case, the device may crash causing network outage that only
   affects the customer that the failed network adapter serves.  At this
   point, the privacy of other customers is still safe.

   A more meaningful attack would be to replace a network adapter with
   some other intelligent agent that knows how network adapters work.
   This is possible because network adapters are customer premise
   equipment.  Using such a device, an attacker can infiltrate the
   networks of other customers.  Filtering based on source MAPOS address
   in bridging traffic is ineffective because this field is filled-in by
   network adapters -- MAPOS networks do not forward source addresses.

5.4 Filtering at network adapters and MAPOS switches

   Network adapters should have the following frame filtering functions.

   -  Each NA in a VLAN is configured with the MAPOS addresses of its
      peer NAs that belongs to the same VLAN.  A NA should only accept
      bridged MAPOS frames with a source MAPOS address of one of its
      VLAN peers.

   -  A NA should never import discovered address table entries with a
      MAPOS address that is not the address of one of its VLAN peers.

   -  If a NA detects that the amount of broadcast traffic from a host
      on MAC-base LAN exceeds a predefined threshold, the NA should stop
      forwarding traffic from that host.

   By default, frame filtering by MAPOS switches is optional.  It is
   desirable for a MAPOS switch to implement the following filtering
   features.

   -  A line interface of a MAPOS switch is made aware of the MAPOS
      addresses in the VLAN to which the interface participates.  The
      interface discards all incoming bridged traffic (from the NA) that
      is destined to addresses outside of the VLAN's set.

   -  MAPOS switch assigns a MAPOS address to a NA using NSP.  The
      switch discards all incoming bridged traffic (from the NA) with
      the source MAPOS address different from the one that is assigned
      by NSP.

5.5 Additional protection measures

   A common carrier can implement additional protective measures such as
   the following.

   -  SONET/SDH connection is closely monitored.  Once a network adapter
      is detected to have gone down, subsequent attempts at
      re-connecting to the MAPOS network are refused until manually
      re-enabled.

   -  Above method is effective against real attacks, but it also
      hinders timely recovery from accidents such as power outages.  A
      reasonable trade-off solution is to implement an authentication
      mechanism between the MAPOS network and network adapters.  Much
      like Challenge Handshake Authentication Protocol (CHAP) [8] used
      in PPP connection.  Something similar may be implemented by
      defining additional message types to NSP.

6. References

   [1] Murakami, K. and M. Maruyama, "MAPOS - Multiple Access Protocol
       over SONET/SDH, Version 1", RFC 2171, June 1997.

   [2] Murakami, K. and M. Maruyama, "A MAPOS version 1 Extension -
       Node-Switch Protocol", RFC 2173, June 1997.

   [3] Murakami, K. and M. Maruyama, "MAPOS16 - Multiple Access Protocol
       over SONET/SDH with 16 Bit Addressing", RFC 2175, June 1997.

   [4] Higashiyama, M. and F.Baker, "PPP Bridging Control Protocol
       (BCP)", RFC 2878, July 2000.

   [5] Reynolds, J., Ed., "Assigned Numbers: RFC 1700 is Replaced by an
       On-line Database", RFC 3232, January 2002.

   [6] Plummer, D.C., "Ethernet Address Resolution Protocol: Or
       converting network protocol addresses to 48.bit Ethernet address
       for transmission on Ethernet hardware", STD 37, RFC 826, November
       1982.

   [7] IEEE 802.1D-1993, "Media Access Control (MAC) Bridges," ISO/IEC
       15802-3:1993 ANSI/IEEE Std 802.1D, 1993 edition, July 1993.

   [8] Simpson, W., "PPP Challenge Handshake Authentication Protocols",
       RFC 1994, August 1996.

7. Acknowledgements

   The authors would like to acknowledge the contributions and
   thoughtful suggestions of Naohisa Takahashi, Tetsuo Kawano and
   Tsuyoshi Ogura.

Appendix - Validation of the MAC Frame Forwarding Mechanism

   This appendix describes the configuration and procedure used to
   validate the soundness of the mechanism described in this document.
   The key points are:

   -   MAC frames are correctly forwarded by MAPOS network, and

   -   Even if a network contains loops, broadcast packets do not storm
       the network.  MAC-based networks must use broadcast spanning tree
       technology in order for this to work.

   (1) Verification of MAC frame forwarding on MAPOS network

       Hosts H1 and H2, Ethernet switches S1 and S2, network adapters B1
       and B2, and a MAPOS switch are connected as shown below.  An
       ethernet protocol analyzer is placed between S1 and B1 for
       traffic monitoring.

       In the diagrams that follow, the hosts are x86 PC running FreeBSD
       4.4-RELEASE, ethernet switches are Extreme Summit5i, network
       adapters are OKI Electric MA-1, and the MAPOS switch is CSR
       CoreSwitch80.

                               +--------------+
                        +------+ MAPOS SWITCH + ------+
                        |      +--------------+       |
                    +---+---+                     +---+---+
                    | NA B1 |                     | NA B2 |
                    +---+---+                     +---+---+
        +----------+    |                             |
        | Protocol |____|                             |
        | Analyzer |    |                             |
        +----------+    |                             |
                        | (P1)                   (P1) |
        +------+   +----+----+                   +----+----+   +------+
        | Host |___| EtherSW |                   | EtherSW |___| Host |
        |  H1  |   |    S1   |                   |    S2   |   |  H2  |
        +------+   +---------+                   +---------+   +------+

       Correct forwarding of unicast MAC frames (ping) are observed
       between H1 and H2 through path (P1).

   (2) Verification of spanning tree operation

       - Enable spanning tree on S1 and S2.

       - Connect S1 and S2 via path (P2) for redundancy.

                               +--------------+
                        +------+ MAPOS SWITCH + ------+
                        |      +--------------+       |
                    +---+---+                     +---+---+
                    | NA B1 |                     | NA B2 |
                    +---+---+                     +---+---+
        +----------+    |                             |
        | Protocol |____|                             |
        | Analyzer |    |                             |
        +----------+    |                             |
                        | (P1)                   (P1) |
        +------+   +----+----+                   +----+----+   +------+
        | Host |___| EtherSW |                   | EtherSW |___| Host |
        |  H1  |   |    S1   |                   |    S2   |   |  H2  |
        +------+   +----+----+                   +----+----+   +------+
                    (P2)|                             |(P2)
                        +-----------------------------+

       It is observed that broadcast packets are correctly exchanged
       between S1 and S2, and that broadcast forwarding loop does not
       exist.

   (3) Verification of spanning tree fail over

       - H1 and H2 communication takes place through path (P1).
         Spanning tree is configured such that Path (P2) is blocked.

       It is observed that severing the link at any point along path
       (P1) makes the spanning tree configure itself to use path (P2).

       It is also observed that restoring path (P1) makes the spanning
       tree configures itself to use path (P1).

Authors' Addresses

   Osamu Okamoto
   NTT Network Service System Laboratories
   3-9-11, Midori-cho Musashino-shi
   Tokyo 180-8585, Japan

   EMail: okamoto.osamu@lab.ntt.co.jp

   Mitsuru Maruyama
   NTT Network Innovation Laboratories
   3-9-11, Midori-cho Musashino-shi
   Tokyo 180-8585, Japan

   EMail: mitsuru@core.ecl.net

   Takahiro Sajima
   Sun Microsystems, K.K.
   4-10-1, Yoga Setagaya-ku
   Tokyo 158-8633, Japan

   EMail: tjs@sun.com

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