Transparent Interconnection of Lots of Links (TRILL): RBridge Channel Support

Internet Engineering Task Force (IETF)                   D. Eastlake 3rd
Request for Comments: 7178                                        Huawei
Category: Standards Track                                      V. Manral
ISSN: 2070-1721                                              Ionos Corp.
                                                                   Y. Li
                                                               S. Aldrin
                                                                  Huawei
                                                                 D. Ward
                                                                   Cisco
                                                                May 2014


         Transparent Interconnection of Lots of Links (TRILL):
                        RBridge Channel Support

Abstract

   This document specifies a general channel mechanism for sending
   messages, such as Bidirectional Forwarding Detection (BFD) messages,
   between Routing Bridges (RBridges) and between RBridges and end
   stations in an RBridge campus through extensions to the Transparent
   Interconnection of Lots of Links (TRILL) protocol.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7178.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
      1.1. RBridge Channel Requirements ...............................3
      1.2. Relation to the MPLS Generic Associated Channel ............4
      1.3. Terminology ................................................4
   2. Inter-RBridge Channel Messages ..................................4
      2.1. RBridge Channel Message Inner Frame ........................6
           2.1.1. RBridge Channel Header ..............................6
           2.1.2. Inner Ethernet Header ...............................8
           2.1.3. Inner.VLAN Tag ......................................8
      2.2. TRILL Header for RBridge Channel Messages ..................9
      2.3. Ethernet Link Header and Trailer ..........................10
      2.4. Special Transmission and Rate Considerations ..............10
   3. Processing RBridge Channel TRILL Data Messages .................11
      3.1. Processing the RBridge Channel Header .....................12
      3.2. RBridge Channel Errors ....................................13
   4. Native RBridge Channel Frames ..................................14
   5. Indicating Support for RBridge Channel Protocols ...............16
   6. Congestion Considerations ......................................16
   7. Allocation Considerations ......................................17
      7.1. IANA Considerations .......................................17
      7.2. IEEE Registration Authority Considerations ................18
   8. Security Considerations ........................................18
   9. References .....................................................19
      9.1. Normative References ......................................19
      9.2. Informative References ....................................20
   10. Acknowledgments ...............................................20

1.  Introduction

   RBridge campuses provide transparent least-cost forwarding using the
   Transparent Interconnection of Lots of Links (TRILL) protocol that
   builds on Intermediate System to Intermediate System (IS-IS) routing
   [IS-IS] [RFC1195] [RFC7176].  Devices that implement TRILL are called
   Routing Bridges (RBridges) or TRILL Switches.  However, the TRILL
   base protocol standard [RFC6325] provides only for TRILL Data
   messages and TRILL IS-IS messages.

   This document specifies a general channel mechanism for the
   transmission of other messages within an RBridge campus, such as BFD
   [RFC5880] messages, (1) between RBridges and end stations that are
   directly connected on the same link and (2) between RBridges.  This
   mechanism supports a requirement to be able to operate with minimal
   configuration.

1.1.  RBridge Channel Requirements

   It is anticipated that various protocols operating at the TRILL layer
   will be desired in RBridge campuses.  For example, there is a need
   for rapid-response continuity checking with a protocol such as BFD
   [RFC5880] [RFC5882] and for a variety of optional reporting.

   To avoid the requirement to design and specify a way to carry each
   such protocol, this document specifies a general channel for sending
   messages between RBridges in a campus at the TRILL level by extending
   the TRILL protocol.  To accommodate a wide variety of protocols, this
   RBridge Channel facility accommodates all the regular modes of TRILL
   Data transmission including single- and multiple-hop unicast as well
   as VLAN-scoped multi-destination distribution.

   To minimize any unnecessary burden on transit RBridges and to provide
   a more realistic test of network continuity and the like, RBridge
   Channel messages are designed to look like TRILL Data frames and, in
   the case of multi-hop messages, can normally be handled by transit
   RBridges as if they were TRILL Data frames; however, to enable
   processing at transit RBridges when required by particular messages,
   they may optionally use the RBridge Channel Alert TRILL extended
   header flags [RFC7179] that causes a transit RBridge implementing the
   flag to more closely examine a flagged frame.

   This document also specifies a format for sending RBridge Channel
   messages between RBridges and end stations that are directly
   connected over a link, in either direction, when provided for by the
   protocol involved.  For the most part, this format is the same as the
   format that is encapsulated by TRILL Data for inter-RBridge Channel
   messages.

   Each particular protocol using the RBridge Channel facility will
   likely use only a subset of the facilities specified herein.

1.2.  Relation to the MPLS Generic Associated Channel

   The RBridge Channel is similar to the MPLS Generic Associated Channel
   specified in [RFC5586].  Instead of using a special MPLS label to
   indicate a special channel message, an RBridge Channel message is
   indicated by a special multicast Inner.MacDA and inner Ethertype (see
   Section 2.1).

1.3.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   The terminology and acronyms of [RFC6325] are used in this document
   with the additions listed below.

      BFD - Bidirectional Forwarding Detection

      CHV - Channel Header Version

      MH - Multi-Hop

      NA - Native

      SL - Silent

2.  Inter-RBridge Channel Messages

   Channel messages between RBridges are transmitted as TRILL Data
   frames.  (For information on channel messages that can be transmitted
   between RBridges and end stations that are directly connected by a
   link, see Section 4.)  Inter-RBridge Channel messages are identified
   as such by their Inner.MacDA, which is the All-Egress-RBridges
   multicast address, together with their inner Ethertype, which is the
   RBridge-Channel Ethertype.  This Ethertype is part of and starts the
   RBridge Channel Header.

   The diagram below shows the overall structure of an RBridge Channel
   Message frame on a link between two RBridges:

              Frame Structure             Section of This Document
                                          ------------------------
     +--------------------------------+
     |          Link Header           |   Section 2.3 if Ethernet link
     +--------------------------------+
     |          TRILL Header          |   Section 2.2
     +--------------------------------+
     |     Inner Ethernet Header      |   Section 2.1.2
     +--------------------------------+
     |     RBridge Channel Header     |   Section 2.1.1
     +--------------------------------+
     |   Protocol-Specific Payload    |   See specific channel protocol
     +--------------------------------+
     | Link Trailer (FCS if Ethernet) |
     +--------------------------------+

                Figure 1: RBridge Channel Frame Structure

   Optionally, some channel messages may require examination of the
   frame by transit RBridges that support the RBridge Channel feature,
   to determine if they need to take any action.  To indicate this, such
   messages use an RBridge Channel Alert extended TRILL Header flag as
   further described in Section 3 below.

   Sections 2.1 and 2.2 describe the inner frame and the TRILL Header
   for frames sent in an RBridge Channel.  As always, the outer Link
   Header and Link Trailer are whatever is needed to get a TRILL Data
   frame to the next-hop RBridge, depending on the technology of the
   link, and can change with each hop for multi-hop messages.  Section
   2.3 describes the outer Link Header for Ethernet links, and Section
   2.4 discusses some special considerations for the first hop
   transmission of RBridge Channel messages.

   Section 3 describes some details of RBridge Channel message
   processing.  Section 4 provides the specifications for native RBridge
   Channel frames between RBridges and end stations that are directly
   connected over a link.  Section 5 describes how support for RBridge
   Channel protocols is indicated.  And Sections 6, 7, and 8 give
   congestion, allocation (IANA and IEEE), and security considerations
   respectively.

2.1.  RBridge Channel Message Inner Frame

   The encapsulated inner frame within an RBridge Channel message frame
   is as shown below.

       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
    Inner Ethernet Header:
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Special Inner.MacDA = All-Egress-RBridges             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Special Inner.MacDA cont.   |         Inner.MacSA           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Inner.MacSA cont.                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       VLAN Tag Ethertype      |  Priority, DEI, VLAN ID       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    RBridge Channel Header:
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    RBridge-Channel Ethertype  |  CHV  |   Channel Protocol    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Flags        |  ERR  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Information specific to the RBridge Channel Protocol:
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                 Channel-Protocol-Specific Data
      |  ...

            Figure 2: RBridge Channel Inner Frame Header Fields

   The Channel-Protocol-Specific Data contains the information related
   to the specific channel protocol used in the channel message.
   Details of that data are outside the scope of this document, except
   in the case of the RBridge Channel Error protocol specified in
   Section 3.2.

2.1.1.  RBridge Channel Header

   As shown in Figure 2, the RBridge Channel Header starts with the
   RBridge-Channel Ethertype (see Section 7.2).  Following that is a
   four-byte quantity with four sub-fields as follows:

      CHV: A 4-bit field that gives the RBridge Channel Header Version.
         This document specifies version zero.

      Channel Protocol: A 12-bit unsigned integer that specifies the
         particular RBridge Channel protocol to which the message
         applies.

      Flags: Provides 12 bits of flags as described below.

      ERR: A 4-bit unsigned integer used in connection with error
         reporting at the RBridge Channel level as described in Section
         3.

   The flag bits are numbered from 0 to 11 as shown below.

                | 0  1  2  3  4  5  6  7  8  9 10 11|
                +--+--+--+--+--+--+--+--+--+--+--+--+
                |SL|MH|NA|        Reserved          |
                +--+--+--+--+--+--+--+--+--+--+--+--+

                 Figure 3: Channel Header Flag Bits

      Bit 0: The SL or Silent bit, the high-order bit in network order.
         If it is a one, it suppresses RBridge Channel Error messages
         (see Section 3).

      Bit 1: The MH or Multi-Hop bit.  It is used to inform the
         destination RBridge protocol that the message may be multi-hop
         (MH=1) or was intended to be one-hop only (MH=0).

      Bit 2: The NA or Native bit.  It is used as described in Section
         4.

      Reserved: Bits reserved for future specification that MUST be sent
         as zero and ignored on receipt.

   The RBridge Channel Protocol field specifies the protocol that the
   channel message relates to.  The initial defined value is listed
   below.

         Protocol  Name - Section of This Document
         --------  -------------------------------
          0x001    RBridge Channel Error - Section 3

   IANA Considerations for RBridge Channel protocol numbers are provided
   in Section 7.  These include provisions for Private Use protocol
   numbers.  Because different uses of Private Use RBridge Channel
   protocol numbers may conflict, such use MUST be within a private
   network.  It is the responsibility of the private network manager to
   avoid conflicting use of these code points and unacceptable burdens
   within the private network from their use.

2.1.2.  Inner Ethernet Header

   The special Inner.MacDA is the All-Egress-RBridges multicast Media
   Access Control (MAC) address to signal that the frame is intended for
   the egress (decapsulating) RBridge itself (or the egress RBridges
   themselves if the frame is multi-destination).  (This address is
   called the All-ESADI-RBridges address in [RFC6325].)  The RBridge-
   Channel Ethertype indicates that the frame is an RBridge Channel
   message.  The only other Ethertype currently specified for use with
   the All-Egress-RBridges Inner.MacDA is L2-IS-IS to indicate an ESADI
   frame [RFC6325].  In the future, additional Ethertypes may be
   specified for use with the All-Egress-RBridges multicast address.

   The RBridge originating the channel message selects the Inner.MacSA.
   The Inner.MacSA MUST be set by the originating RBridge to a MAC
   address unique within the campus owned by the originating RBridge.
   This MAC address can be considered, in effect, the MAC address of a
   virtual internal end station that handles the RBridge Channel frames
   originated by or destined for that RBridge.  It MAY be the same as
   the Inner.MacSA used by the RBridge when it originates ESADI frames
   [RFC6325].

2.1.3.  Inner.VLAN Tag

   As with all frames formatted to be processed as a TRILL Data frame,
   an Inner.VLAN tag is present.  Use of a VLAN tag Ethertype other than
   0x8100 or stacked tags is beyond the scope of this document but is an
   obvious extension.

   Multi-destination RBridge Channel messages are, like all multi-
   destination TRILL Data messages, VLAN scoped so the Inner.VLAN ID
   MUST be set to the VLAN of interest.  To the extent that distribution
   tree pruning is in effect in the campus, such channel messages may
   only reach RBridges advertising that they have connectivity to that
   VLAN.

   For channel messages sent as known unicast TRILL Data frames, the
   default value for the Inner.VLAN ID is VLAN 1, but particular RBridge
   Channel protocols MAY specify other values.

   The Inner.VLAN also specifies a three-bit frame priority for which
   the following recommendations apply:

   1.  For one-hop channel messages critical to network connectivity,
       such as one-hop BFD for rapid link-failure detection in support
       of TRILL IS-IS, the RECOMMENDED priority is 7.

   2.  For single and multi-hop unicast channel messages important to
       network operation but not critical for connectivity, the
       RECOMMENDED priority is 6.

   3.  For other unicast channel messages and all multi-destination
       channel messages, it is RECOMMENDED that the default priority
       zero be used.  In any case, priorities higher than 5 SHOULD NOT
       be used for such frames.

   There is one additional bit in a VLAN tag value between the 12-bit
   VLAN ID and 3-bit priority, the Drop Eligibility Indicator (DEI)
   [RFC7180].  It is RECOMMENDED that this bit be zero for the first two
   categories of channel messages listed immediately above.  The setting
   of this bit for channel messages in the third category may be
   dependent on the channel protocol and no general recommendation is
   made for that case.

2.2.  TRILL Header for RBridge Channel Messages

   After the outer Link Header (that, for an Ethernet link, ends with
   the TRILL Ethertype) and before the encapsulated frame, the channel
   message's TRILL Header initially appears as follows:

      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
                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     |V=0| R |M| Op-Len  | Hop Count |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       Egress Nickname         |       Ingress Nickname        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 4: RBridge Channel TRILL Header Fields

   The TRILL Header version (V) MUST be zero; the R bits are reserved;
   the M bit is set appropriately as the channel message is to be
   forwarded as known destination unicast (M=0) or multi-destination
   (M=1), regardless of the fact that the Inner.MacDA is always the All-
   Egress-RBridges multicast address; and Op-Len is set appropriately
   for the length of the TRILL Header extensions area, if any, all as
   specified in [RFC6325].

   When an RBridge Channel message is originated, the Hop Count field
   defaults to the maximum value, 0x3F, but particular RBridge Channel
   protocols MAY specify other values.  For messages sent a known number
   of hops, such as one-hop messages or a two-hop self-addressed message
   intended to loop back through an immediate neighbor RBridge, setting
   the Hop Count field in the TRILL Header to the maximum value and
   checking its value on receipt provides an additional validity check
   as discussed in [RFC5082], where this type of field is referred to as
   "TTL" or "Hop Limit".

   The RBridge originating a channel message places a nickname that it
   holds in the Ingress Nickname field.

   There are several cases for the Egress Nickname field.  If the
   channel message is multi-destination, then the Egress Nickname
   designates the distribution tree to use.  If the channel message is a
   multi-hop unicast message, then the Egress Nickname is a nickname of
   the target RBridge; this includes the special case of a message
   intended to loop back from an immediate neighbor where the originator
   places one of its own nicknames in both the Ingress Nickname and
   Egress Nickname fields.  If the channel message is a one-hop unicast
   message, there are two possibilities for the Egress Nickname.

   o  The Egress Nickname can be set to a nickname of the target
      neighbor RBridge.

   o  The special nickname Any-RBridge may be used.  RBridges supporting
      the RBridge Channel facility MUST recognize the Any-RBridge
      special nickname and accept TRILL Data frames having that value in
      the Egress Nickname field as being sent to them as the egress.
      Thus, for such RBridges, using this egress nickname guarantees
      processing by an immediate neighbor regardless of the state of
      nicknames.

2.3.  Ethernet Link Header and Trailer

   An RBridge Channel frame has the usual Link Header and Link Trailer
   for a TRILL Data frame depending on the type of link on which it is
   sent.

   For an Ethernet link [RFC6325], the Outer.MacSA is the MAC address of
   the port from which the frame is sent.  The Outer.MacDA is the MAC
   address of the next-hop RBridge port for unicast RBridge Channel
   messages or the All-RBridges multicast address for multi-destination
   RBridge Channel messages.  The Outer.VLAN tag specifies the
   designated VLAN for that hop, and the priority should be the same as
   in the Inner.VLAN tag; however, the output port may have been
   configured to strip VLAN tags, in which case no Outer.VLAN tag
   appears on the wire.  And the Link Trailer is the Ethernet FCS.

2.4.  Special Transmission and Rate Considerations

   If a multi-hop RBridge Channel message is received by an RBridge, the
   criteria and method of forwarding it are the same as for any TRILL
   Data frame.  If it is so forwarded, it will be on a link that was
   included in the routing topology because it was in the Report state
   as specified in [RFC7177].

   However, special considerations apply to single-hop messages because,
   for some RBridge Channel protocols, it may be desirable to send
   RBridge Channel messages over a link that is not yet fully up.  In
   particular, it is permissible, if specified by the particular channel
   protocol, for the source RBridge that has created an RBridge Channel
   message to attempt to transmit it to a next-hop RBridge when the link
   is in the Detect or 2-Way state, as specified in [RFC7177], as well
   as when it is in the Report state.  Such messages can also be sent on
   point-to-point links that are not in the Up state.

   RBridge Channel messages represent a burden on the RBridges, and
   links in a campus and should be rate limited, especially if they are
   sent as high priority, multi-destination, or multi-hop frames or have
   an RBridge Channel Alert extended header flag set.  See Section 6,
   "Congestion Considerations".

3.  Processing RBridge Channel TRILL Data Messages

   RBridge Channel TRILL Data messages are designed to look like and, to
   the extent practical, be forwarded as regular TRILL Data frames.  On
   receiving a channel message, an RBridge performs the usual initial
   tests on the frame and makes the same forwarding and/or decapsulation
   decisions as for a regular TRILL Data frame [RFC6325] with the
   following exceptions for RBridges implementing the RBridge Channel
   facility:

   1.  An RBridge implementing the RBridge Channel facility MUST
       recognize the Any-RBridge egress nickname in TRILL Data frames,
       decapsulating such frames if they meet other checks.  (Such a
       frame cannot be a valid multi-destination frame because the Any-
       RBridge nickname is not a valid distribution tree root.)

   2.  If an RBridge Channel Alert extended header flag is set, then the
       RBridge MUST process the RBridge Channel message as described
       below even if it is not egressing the frame.  If it is egressing
       the frame, then no additional processing beyond egress processing
       is needed even if an RBridge Channel Alert flag is set.

   3.  On decapsulation, the special Inner.MacDA value of All-Egress-
       RBridges MUST be recognized to trigger checking the
       Inner.Ethertype and processing as an RBridge Channel message if
       that Ethertype is RBridge-Channel.

   RBridge Channel messages SHOULD only be sent to RBridges that
   advertise support for the channel protocol involved as described in
   Section 5.

   All RBridges supporting the RBridge Channel facility MUST recognize
   the RBridge-Channel inner Ethertype.

3.1.  Processing the RBridge Channel Header

   Knowing that it has an RBridge Channel message, the egress RBridge,
   and any transit RBridge if an RBridge Channel Alert bit is set in the
   TRILL Header, looks at the CHV (RBridge Channel Header Version) and
   Channel Protocol fields.

   If any of the following conditions occur at an egress RBridge, the
   frame is not processed, an error may be generated as specified in
   Section 3.2, and the frame is discarded.  The behavior is the same if
   the frame is being processed at a transit RBridge because the RBridge
   Critical Channel Alert flag is set [RFC7179].  However, if these
   conditions are detected at a transit RBridge examining the message
   because the RBridge Non-critical Channel Alert flag is set [RFC7179]
   but the RBridge Critical Channel Alert flag is not set, no error is
   generated, and the frame is still forwarded normally.

   Error Conditions:

   1.  The Ethertype is not RBridge-Channel and not any other Ethertype
       known to the RBridge as usable with the All-Egress-RBridges
       Inner.MacDA, or the frame is so short that the Ethertype is
       truncated.

   2.  The CHV field is non-zero, or the frame is so short that the
       version zero Channel Header is truncated.

   3.  The Channel Protocol field is a reserved value or a value unknown
       to the processing RBridge.

   4.  The ERR field is non-zero, and Channel Protocol is a value other
       than 0x001.

   5.  The RBridge Channel Header NA flag is set to one, indicating that
       the frame should have been received as a native frame rather than
       a TRILL Data frame.

   If the CHV field and NA flag are zero and the processing RBridge
   recognizes the Channel Protocol value, it processes the message in
   accordance with that channel protocol.  The processing model is as if
   the received frame starting with and including the TRILL Header is
   delivered to the Channel protocol along with a flag indicating
   whether this is (a) transit RBridge processing due to an RBridge
   Channel Alert flag being set or (b) egress processing.

   Errors within a recognized Channel Protocol are handled by that
   channel protocol itself and do not produce RBridge Channel Error
   frames.

3.2.   RBridge Channel Errors

   A variety of problems at the RBridge Channel level cause the return
   of an RBridge Channel Error frame unless one of the following apply:
   (a) the "SL" (Silent) flag is a one in the channel message for which
   the problem was detected, (b) the processing is due to the RBridge
   Non-critical Channel Alert flag being set, (c) the frame in error
   appears, itself, to be an RBridge Channel Error frame (has a non-zero
   ERR field or a Channel Protocol of 0x001), or (d) the error is
   suppressed due to rate limiting.

   An RBridge Channel Error frame is a multi-hop unicast RBridge Channel
   message with the Ingress Nickname set to a nickname of the RBridge
   detecting the error and the Egress Nickname set to the value of the
   Ingress Nickname in the channel message for which the error was
   detected.  No per-hop transit processing is specified for such error
   frames, so the RBridge Channel Alert extended header flags SHOULD, if
   an extension is present, be set to zero.  The SL and MH flags SHOULD
   be set to one; the NA flag MUST be zero; and the ERR field MUST be
   non-zero as described below.  For the protocol-specific data area, an
   RBridge Channel Message Error frame has at least the first 256 bytes
   (or less if less are available) of the erroneous decapsulated channel
   message starting with the TRILL Header.  (Note: The TRILL Header does
   not include the TRILL Ethertype that is part of the Link Header on
   Ethernet links.)

   The following values for ERR are specified:

      ERR   RBridge Channel Error Code Meaning
      ---   ----------------------------------
       0    No error
       1    Frame too short (truncated Ethertype or Channel Header)
       2    Unrecognized Ethertype
       3    Unimplemented value of CHV
       4    Wrong value of NA flag
       5    Channel Protocol is reserved or unimplemented
      6-14  Unassigned (see Section 7)
      15    Reserved (see Note)
      Note:  Intended to be allocated by Standards Action for an error
             code expansion feature when it appears likely that all
             other available error codes are being allocated.

   All RBridges implementing the RBridge Channel feature MUST recognize
   the RBridge Channel Error protocol value (0x001).  They MUST NOT
   generate an RBridge Channel Error message in response to an RBridge
   Channel Error message, that is, a channel message with a protocol
   value of 0x001 or with a non-zero ERR field.

4.  Native RBridge Channel Frames

   Other sections of this document specify non-native RBridge Channel
   messages and their processing, that is, RBridge Channel messages
   formatted as TRILL Data frames and sent between RBridges.  This
   section specifies the differences for native RBridge Channel
   messages.

   If provided for by the channel protocol involved, native RBridge
   Channel messages may be sent between end stations and RBridges that
   are directly connected over a link, in either direction.  On an
   Ethernet link, such native frames have the RBridge-Channel Ethertype
   and are like the encapsulated frame inside an RBridge Channel message
   except as follows:

   1.  TRILL does not require the presence of a VLAN tag on such native
       RBridge Channel frames.  However, port configuration, link
       characteristics, or the channel protocol involved may require
       such tagging.

   2.  If the frame is unicast, the destination MAC address is the
       unicast MAC address of the RBridge or end-station port that is
       its intended destination.  If the frame is multicast by an end
       station to all the RBridges on a link that support an RBridge
       Channel protocol using this transport, the destination MAC
       address is the All-Edge-RBridges multicast address (see Section
       7).  A native RBridge Channel frame received at an ingress
       RBridge is discarded if its destination MAC address is neither
       the unicast address of the port nor the multicast address All-
       Edge-RBridges.  If the frame is multicast by an RBridge to all
       the devices that TRILL considers to be end stations on a link and
       that support an RBridge Channel protocol using this transport,
       the destination MAC address is the TRILL-End-Stations multicast
       address (see Section 7).  A native RBridge Channel frame received
       at an end station is discarded if its destination MAC address is
       neither the unicast address of the port nor the multicast address
       TRILL-End-Stations.

   3.  The RBridge-Channel outer Ethertype must be present.  In the
       future, there may be other protocols using the All-Edge-RBridges
       and/or TRILL-End-Stations multicast addresses on native frames
       distinguished by different Ethertypes.

   4.  The NA or Native bit in the RBridge Channel Header flags MUST be
       a one.

   5.  There might be additional tags present between the Outer.MacDA,
       Outer.MacSA pair, and the RBridge-Channel Ethertype.

   The RBridge Channel protocol number space for native RBridge Channel
   messages and TRILL Data formatted RBridge Channel messages is the
   same.  If provided for by the channel protocol involved, the receipt
   of a native RBridge Channel frame MAY lead to the generation and
   transmission of one or more Inter-RBridge Channel frames.  The
   decapsulation and processing of a TRILL Data RBridge Channel frame
   MAY, if provided for by the channel protocol involved, result in the
   sending of one or more native RBridge Channel frames to one or more
   end stations.  Thus, there could be an RBridge Channel protocol that
   involved an RBridge Channel message sent (1) from an origin RBridge
   where the message is created, (2) through one or more transit
   RBridges, and (3) from a final RBridge as a native RBridge Channel
   message to an end station (or the reverse of such a three-part path);
   however, to do this, the RBridge Channel protocol involved must be
   implemented at the RBridge where the channel message is changed
   between a native frame and a TRILL Data format frame, and that
   RBridge must change the channel message itself, at a minimum
   complementing the NA flag in the Channel Header and making
   appropriate MAC address changes.

   An erroneous native channel message results in a native RBridge
   Channel Error message under the same conditions for which a TRILL
   Data RBridge Channel message would result in a TRILL Data RBridge
   Channel Error message.  However, in a native RBridge Channel Error
   message, the NA flag MUST be one.  Also, since there is no TRILL
   Header in native RBridge Channel protocol frames, the beginning part
   of the frame in which the error was detected that is included in
   native RBridge Channel Error frames starts with the RBridge Channel
   Header (including the RBridge-Channel Ethertype).  The destination
   MAC address of such error messages is set to the source MAC address
   of the native RBridge Channel message that was in error.

   There is no mechanism to stop end stations from directly exchanging
   native RBridge Channel messages, but such usage is beyond the scope
   of this document.

5.  Indicating Support for RBridge Channel Protocols

   Support for RBridge Channel protocols is indicated by the presence of
   one or more TLVs and/or sub-TLVs in an RBridge's Link State PDU (LSP)
   as documented in [RFC7176].

   RBridge Channel protocols 0 and 0xFFF are reserved, and protocol 1,
   the RBridge Channel Error protocol, MUST be implemented as part of
   the RBridge Channel feature.  Thus, if an RBridge supports the
   RBridge Channel feature, it should be advertising support for
   protocol 1 and not advertising support for protocols 0 or 0xFFF in
   its LSP.  However, indication of support or non-support for RBridge
   Channel protocol 1 is ignored on receipt, and support for it is
   always assumed if support for any RBridge Channel is indicated in the
   RBridge's LSP.

6.  Congestion Considerations

   The bandwidth resources used by RBridge Channel protocols are
   recommended to be small compared to the total bandwidth of the links
   they traverse.  When doing network planning, the bandwidth
   requirements for TRILL Data, TRILL IS-IS, TRILL ESADI, RBridge
   Channel protocol traffic, and any other link-local traffic need to be
   taken into account.

   Specifications for particular RBridge Channel protocols MUST consider
   congestion and bandwidth usage implications and provide guidance on
   bandwidth or packet-frequency management.  RBridge Channel protocols
   can have built-in bandwidth management in their protocols.  Outgoing
   channel messages SHOULD be rate-limited, by configuring the
   underlying protocols or otherwise, to prevent aggressive connectivity
   verification or other applications consuming excessive bandwidth,
   causing congestion, or becoming denial-of-service attacks.

   If these conditions cannot be followed, an adaptive loss-based scheme
   SHOULD be applied to congestion-control outgoing RBridge Channel
   traffic, so that it competes fairly, taking into account packet
   priorities and drop eligibility as indicated in the Inner.VLAN, with
   TCP or similar traffic within an order of magnitude.  One method of
   determining an acceptable bandwidth for RBridge Channel traffic is
   described in [RFC5348]; other methods exist.  For example, bandwidth
   or packet-frequency management can include any of the following: a
   negotiation of transmission interval/rate such as that provided in
   BFD [RFC5880], a throttled transmission rate on "congestion detected"
   situations, a gradual ramp-up after shutdown due to congestion and
   until basic connectivity is verified, and other mechanisms.

   Connectivity-checking applications such as BFD [RFC5880] SHOULD be
   rate-limited to below 5% of the bitrate of the associated link or
   links.  For this purpose, the mean or sustained bitrate of the link
   or links is used.

   Incoming RBridge Channel messages MAY be rate-limited as a protection
   against denial-of-service attacks.  This throttling of incoming
   messages SHOULD honor packet priorities and drop eligibility
   indications as indicated in the Inner.VLAN, preferentially discarding
   drop-eligible and lower-priority packets.

7.  Allocation Considerations

   The following subsections give IANA and IEEE allocation
   considerations.  In this document, the allocation procedure
   specifications are as defined in [RFC5226].

7.1.  IANA Considerations

   IANA has allocated a previously unassigned TRILL Nickname as follows:

         Any-RBridge           0xFFC0

   IANA has added "All-Egress-RBridges" to the TRILL Parameter Registry
   as an alternative name for the "All-ESADI-RBridges" multicast
   address.

   IANA has allocated two previously unassigned TRILL multicast
   addresses as follows:

         TRILL-End-Stations    01-80-C2-00-00-45
         All-Edge-RBridges     01-80-C2-00-00-46

   IANA has created an additional sub-registry in the TRILL Parameter
   Registry for RBridge Channel Protocols, with initial contents as
   follows:

      Protocol      Description                     Reference
      --------      -----------                     ---------

      0x000         Reserved; not to be allocated   (This document)
      0x001         RBridge Channel Error           (This document)
      0x002-0x0FF   Unassigned (1)
      0x100-0xFF7   Unassigned (2)
      0xFF8-0xFFE   Reserved for Private Use
      0xFFF         Reserved; not to be allocated   (This document)
   (1) RBridge Channel protocol code points from 0x002 to 0x0FF require
       a Standards Action, as modified by [RFC7120], for allocation.

   (2) RBridge Channel protocol code points from 0x100 to 0xFF7 are RFC
       Required to allocate a single value or IESG Approval to allocate
       multiple values.

   IANA has created an additional sub-registry in the TRILL Parameter
   Registry for RBridge Channel Header Flags with initial contents as
   follows:

         Flag Bit  Mnemonic  Allocation
         --------  --------  ----------

            0         SL     Silent
            1         MH     Multi-hop
            2         NA     Native
           3-11       -      Unassigned

   Allocation of an RBridge Channel Header Flag is based on IETF Review.

   IANA has created an additional sub-registry in the TRILL Parameter
   Registry for RBridge Channel Error Codes with initial contents as
   listed in Section 3.2 above and with available values allocated by
   Standards Action as modified by [RFC7120].

7.2.  IEEE Registration Authority Considerations

   The IEEE Registration Authority has assigned the Ethertype 0x8946 for
   TRILL RBridge Channel.

8.  Security Considerations

   No general integrity, authentication, or encryption mechanisms are
   provided herein for RBridge Channel messages.  If these services are
   required for a particular RBridge Channel protocol, they MUST be
   supplied by that channel protocol.  See, for example, the BFD
   Authentication mechanism [RFC5880].

   See [RFC6325] for general TRILL security considerations.  As stated
   therein, no protection is provided by TRILL against forging of the
   Ingress Nickname in a TRILL Data formatted channel message or the
   Outer.MacSA in a native RBridge Channel frame on an Ethernet link.
   This may result in misdirected return responses or error messages.
   However, link-level security protocols may be used to authenticate
   the origin station on a link and protect against attacks on links.
   See also Section 6 concerning congestion.

   If indications of RBridge Channel Protocol support are improperly
   absent from an RBridge's LSP, it could deny all RBridge Channel
   services, for example, some BFD services, for the RBridge in
   question.  If a particular RBridge Channel protocol is incorrectly
   not advertised as supported, it could deny the service of that
   channel protocol to the RBridge in question.

   Incorrect indication of RBridge Channel Protocol support or incorrect
   assertion of support for a channel protocol could encourage RBridge
   Channel messages to be sent to an RBridge that does not support the
   channel feature or the particular channel protocol used.  The inner
   frame of such messages could be decapsulated and that inner frame
   could be sent out all ports that are Appointed Forwarders for the
   frame's Inner.VLAN.  However, this is unlikely to cause much harm; in
   particular, there are two possibilities as follows: (a) if end
   stations do not recognize the RBridge-Channel Ethertype of the frame,
   they will drop it, and (b) if end stations do recognize the RBridge-
   Channel Ethertype and the channel protocol indicated in the frame,
   they should refuse to process the frame due to an incorrect value of
   the RBridge Channel Header NA flag.

9.  References

9.1.  Normative References

   [IS-IS]    International Organization for Standardization,
              "Intermediate System to Intermediate System intra-domain
              routeing information exchange protocol for use in
              conjunction with the protocol for providing the
              connectionless-mode network service (ISO 8473)", Second
              Edition, November 2002.

   [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
              dual environments", RFC 1195, December 1990.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5348]  Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification", RFC
              5348, September 2008.

   [RFC6325]  Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
              Ghanwani, "Routing Bridges (RBridges): Base Protocol
              Specification", RFC 6325, July 2011.

   [RFC7120]  Cotton, M., "Early IANA Allocation of Standards Track Code
              Points", BCP 100, RFC 7120, January 2014.

   [RFC7176]  Eastlake 3rd, D., Senevirathne, T., Ghanwani, A., Dutt,
              D., and A. Banerjee, "Transparent Interconnection of Lots
              of Links (TRILL) Use of IS-IS", RFC 7176, May 2014.

   [RFC7177]  Eastlake 3rd, D., Perlman, R., Ghanwani, A., Yang, H., and
              V. Manral, "Transparent Interconnection of Lots of Links
              (TRILL): Adjacency", RFC 7177, May 2014.

   [RFC7179]  Eastlake 3rd, D., Ghanwani, A., Manral, V., Li, Y., and C.
              Bestler, "Transparent Interconnection of Lots of Links
              (TRILL): Header Extension", RFC 7179, May 2014.

9.2.  Informative References

   [RFC5082]  Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C.
              Pignataro, "The Generalized TTL Security Mechanism
              (GTSM)", RFC 5082, October 2007.

   [RFC5586]  Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
              "MPLS Generic Associated Channel", RFC 5586, June 2009.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, June 2010.

   [RFC5882]  Katz, D. and D. Ward, "Generic Application of
              Bidirectional Forwarding Detection (BFD)", RFC 5882, June
              2010.

   [RFC7180]  Eastlake 3rd, D., Zhang, M., Ghanwani, A., Manral, V., and
              A. Banerjee, "Transparent Interconnection of Lots of Links
              (TRILL): Clarifications, Corrections, and Updates", RFC
              7180, May 2014.

10.  Acknowledgments

   The authors gratefully acknowledge the comments and contributions of
   the follows, listed is alphabetic order: Stewart Bryant, Somnath
   Chatterjee, Adrian Farrel, Stephen Farrell, Miguel A. Garcia, Anoop
   Ghanwani, Brian Haberman, Rakesh Kumar, Barry Leiba, and Tissa
   Senevirathne.

Authors' Addresses

   Donald Eastlake 3rd
   Huawei R&D USA
   155 Beaver Street
   Milford, MA 01757
   USA
   Phone: +1-508-333-2270
   EMail: d3e3e3@gmail.com


   Vishwas Manral
   Ionos Corp.
   4100 Moorpark Ave.
   San Jose, CA  95117
   USA
   EMail: vishwas@ionosnetworks.com


   Yizhou Li
   Huawei Technologies
   101 Software Avenue,
   Nanjing 210012
   China
   Phone: +86-25-56622310
   EMail: liyizhou@huawei.com


   Sam Aldrin
   Huawei Technologies
   2330 Central Expressway
   Santa Clara, CA 95050
   USA
   Phone: +1-408-330-5000
   EMail: sam.aldrin@huawei.com


   Dave Ward
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA 95134
   USA

   EMail: dward@cisco.com