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rfc:rfc3422

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.

Okamoto, et. al. Informational [Page 1] RFC 3422 Forwarding MAC Frames over MAPOS November 2002

 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.

Okamoto, et. al. Informational [Page 2] RFC 3422 Forwarding MAC Frames over MAPOS November 2002

                              +------------+
                              |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.

Okamoto, et. al. Informational [Page 3] RFC 3422 Forwarding MAC Frames over MAPOS November 2002

    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)

Okamoto, et. al. Informational [Page 4] RFC 3422 Forwarding MAC Frames over MAPOS November 2002

    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)

Okamoto, et. al. Informational [Page 5] RFC 3422 Forwarding MAC Frames over MAPOS November 2002

    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)

Okamoto, et. al. Informational [Page 6] RFC 3422 Forwarding MAC Frames over MAPOS November 2002

    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.

Okamoto, et. al. Informational [Page 7] RFC 3422 Forwarding MAC Frames over MAPOS November 2002

 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.

Okamoto, et. al. Informational [Page 8] RFC 3422 Forwarding MAC Frames over MAPOS November 2002

                              +------------+
      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.

Okamoto, et. al. Informational [Page 9] RFC 3422 Forwarding MAC Frames over MAPOS November 2002

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.

Okamoto, et. al. Informational [Page 10] RFC 3422 Forwarding MAC Frames over MAPOS November 2002

                            +-----------+
      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.

Okamoto, et. al. Informational [Page 11] RFC 3422 Forwarding MAC Frames over MAPOS November 2002

 (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.

Okamoto, et. al. Informational [Page 12] RFC 3422 Forwarding MAC Frames over MAPOS November 2002

                                   +----+
   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.
  1. 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.
  1. A NA should never import discovered address table entries with a

MAPOS address that is not the address of one of its VLAN peers.

  1. 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.

Okamoto, et. al. Informational [Page 13] RFC 3422 Forwarding MAC Frames over MAPOS November 2002

 By default, frame filtering by MAPOS switches is optional.  It is
 desirable for a MAPOS switch to implement the following filtering
 features.
  1. 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.
  1. 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.
  1. 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.
  1. 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.

Okamoto, et. al. Informational [Page 14] RFC 3422 Forwarding MAC Frames over MAPOS November 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.

Okamoto, et. al. Informational [Page 15] RFC 3422 Forwarding MAC Frames over MAPOS November 2002

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:
  1. MAC frames are correctly forwarded by MAPOS network, and
  1. 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
  1. Enable spanning tree on S1 and S2.
  1. Connect S1 and S2 via path (P2) for redundancy.

Okamoto, et. al. Informational [Page 16] RFC 3422 Forwarding MAC Frames over MAPOS November 2002

                             +--------------+
                      +------+ 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
  1. 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).

Okamoto, et. al. Informational [Page 17] RFC 3422 Forwarding MAC Frames over MAPOS November 2002

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

Okamoto, et. al. Informational [Page 18] RFC 3422 Forwarding MAC Frames over MAPOS November 2002

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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Okamoto, et. al. Informational [Page 19]

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