IANA Registry for the First Nibble Following a Label Stack
draft-ietf-mpls-1stnibble-04

MPLS Working Group                                           K. Kompella
Internet-Draft                                          Juniper Networks
Updates: 4928 (if approved)                                    S. Bryant
Intended status: Standards Track               University of Surrey 5GIC
Expires: 1 September 2024                                       M. Bocci
                                                                   Nokia
                                                          G. Mirsky, Ed.
                                                                Ericsson
                                                            L. Andersson
                                                                 J. Dong
                                                     Huawei Technologies
                                                        29 February 2024

       IANA Registry for the First Nibble Following a Label Stack
                      draft-ietf-mpls-1stnibble-04

Abstract

   The goal of this memo is to create a new IANA registry (called the
   Post-stack First Nibble registry) for the first nibble (4-bit field)
   immediately following an MPLS label stack.  The memo offers a
   rationale for such a registry, describes how the registry should be
   managed, and provides some initial entries.  Furthermore, this memo
   sets out some documentation requirements for registering new values.
   Finally, it provides some recommendations that make processing MPLS
   packets easier and more robust.

   The relationship between the IANA IP Version Numbers (RFC 2780) and
   the Post-stack First Nibble registry is described in this document.

   This memo, if published, would update RFC 4928.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   This Internet-Draft will expire on 1 September 2024.

Copyright Notice

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and Definitions . . . . . . . . . . . . . . .   3
     1.2.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   6
   2.  Rationale . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Why Look at the First Nibble  . . . . . . . . . . . . . .   6
       2.1.1.  Load Balancing  . . . . . . . . . . . . . . . . . . .   6
     2.2.  Updates of RFC 4928 . . . . . . . . . . . . . . . . . . .   8
     2.3.  Why Create a Registry . . . . . . . . . . . . . . . . . .   9
     2.4.  The Relationship between IANA IP Version Numbers RFC2780
           and Post-stack First Nibble Registries  . . . . . . . . .  10
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     3.1.  The Post-stack First Nibble Registry  . . . . . . . . . .  10
       3.1.1.  Allocation Policy . . . . . . . . . . . . . . . . . .  11
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     5.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   An MPLS packet consists of a label stack, an optional "post-stack
   header" (PSH) and an optional embedded packet (in that order).  By
   PSH, we mean existing artifacts such as Control Words, BIER headers
   and the like, as well as new types of PSH being discussed by the MPLS
   Working Group.  However, in the data plane, there are scant clues
   regarding the PSH, and no clue as to the type of embedded packet;
   this information is communicated via other means, such as the routing
   protocols that signal the labels in the stack.  Nonetheless, in order
   to better handle an MPLS packet in the data plane, it is common

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   practice for network equipment to "guess" the type of embedded
   packet.  Such equipment may also need to process the PSH.  Both of
   these require parsing the data after the label stack.  To do this,
   the "first nibble" (the top four bits of the first octet following
   the label stack) is often used.  Although some existing network
   devices may use such a method, it needs to be stressed that the
   correct interpretation of the Post-stack First Nibble (PFN) in a PSH
   can be made only in the context of the Label Stack Element (LSE) or
   group of LSEs in the preceding label stack that characterize the type
   of the PSH, and that any attempt to rely on the value in any other
   context is unreliable.

   The semantics and usage of the first nibble are not well documented,
   nor are the assignments of values.  This memo serves four purposes:

   *  To document the assignments already made.

   *  To provide straightforward documentation of future assignments
      through the creation of a "Post-stack First Nibble registry".

   *  Provide a method for tracking usage by requiring more consistent
      documentation.

   *  To stress the importance that any MPLS packet not carrying plain
      IPv4 or IPv6 packets MUST contain a PSH.

   This memo introduces a requirement and a recommendation, the first
   building on the Section 2.1.1; the second deprecating the use of the
   heuristic in Section 2.1.1.1.  The intent of both of these is that
   legacy routers continue to operate as they have, with no new problems
   introduced as a result of this memo.  However, new implementations
   SHOULD follow these recommendations for a more robust operation.

1.1.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   MPLS packet:  one whose Layer 2 header declares the type to be MPLS.
      For Ethernet, that means the Ethertype is 0x8847 or 0x8848.

   Label Stack:  (of an MPLS packet) all labels (four-octet fields)
      after the Layer 2 header, up to and including the label with the
      BoS bit set ([RFC3032]).

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   Post-stack First Nibble (PFN):  the most significant four bits of the
      first octet following the label stack.

   MPLS Payload:  all data after the label stack, including the PFN, an
      optional post-stack header, and the embedded packet.

   Post-stack Header (PSH):  optional field of interest to the egress
      LSR (and possibly to transit LSRs).  Examples include a control
      word or an associated channel.  The PSH MUST indicate its length,
      so that a parser knows where the embedded packet starts.

   Embedded Packet:  all octets beyond the PSH (if any).  That could be
      an IPv4 or IPv6 packet , an Ethernet packet (for VPLS ([RFC4761],
      [RFC4762]) or EVPN [RFC7432]), or some other type of Layer 2 frame
      [RFC4446].

   Deprecation:  regardless of how the deprecation is understood in
      other IETF documents, the interpretation in this document is that
      if a practice has been deprecated, that practice should not be
      included in the new implementations or deployed in deployments.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+\
   X |                        Layer 2 Header                         | |
     |                                                               | |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+/
                                               TC   S       TTL
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+\
   Y |             Label-1                   | TC  |0|      TTL      | |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Label-2                   | TC  |0|      TTL      | |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               ...                     | TC  |0|      TTL      | |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Label-n                   | TC  |1|      TTL      | |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+/

            Figure 1: Example of an MPLS Packet With Label Stack

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     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+\
   A | (PFN) |                   IP packet                           | |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                             data                              | |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    ...                                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        end of IP packet                       | |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+/

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+\
   B | (PFN) |                 non-IP packet                         | |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                             data                              | |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    ...                                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      end of non-IP packet                     | |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+/

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+\
   C | (PFN) |                      PSH                              | |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              PSH                              | |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    ...                                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          end of PSH                           | |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        embedded packet                        | |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+/

                                  Figure 2

   Figure 1 shows an MPLS packet with Layer 2 header X and a label stack
   Y ending with Label-n.  Then, there are three examples of an MPLS
   payload.  The complete MPLS packet thus would consist of [X Y A], or
   [X Y B], or [X Y C].

   A.  The first payload is a bare IP packet, i.e., no PSH.  The PFN in
   this case overlaps with the IP version number.

   B.  The next payload is a bare non-IP packet; again, no PSH.  The PFN
   here is the first nibble of the payload, whatever it happens to be.

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   C.  The last example is an MPLS Payload that starts with a PSH
   followed by the embedded packet.  Here, the embedded packet could be
   IP or non-IP.

1.2.  Abbreviations

   LSR: Label Switching Router

   LSE: Label Stack Element

   PSH: Post-Stack Header

   PFN: Post-stack First Nibble

2.  Rationale

2.1.  Why Look at the First Nibble

   An MPLS packet can contain many types of embedded packets.  The most
   common types are:

   1.  An IPv4 packet (whose IP header has version number 4).

   2.  An IPv6 packet (whose IP header has version number 6).

   3.  A Layer 2 Ethernet frame (i.e., not including the Preamble or the
       Start frame delimiter), starting with the destination MAC
       address.

   Many other packet types are possible, and in principle, any Layer 2
   embedded packet is permissible; indeed, in the past, PPP, Frame Relay
   and ATM packets were reasonably common.

   In addition, there may be a PSH ahead of the embedded packet, and it
   needs to be parsed.  The PFN is currently used for both of these
   purposes.

2.1.1.  Load Balancing

   There are four common ways to load balance an MPLS packet:

   1.  One can use the top label alone.

   2.  One can do better by using all of the (non-SPL) labels in the
       stack.

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   3.  One can do even better by "divining" the type of embedded packet,
       and using fields from the guessed header.  The ramifications of
       using this load-balancing technique are discussed in detail in
       Section 2.2.

   4.  One can do best by using either an Entropy Label [RFC6790] or a
       FAT Pseudowire Label [RFC6391]; see Section 2.2.)

   Load balancing based on just the top label means that all packets
   with that top label will go the same way -- this is far from ideal.
   Load balancing based on the entire label stack (not including SPLs)
   is better, but it may still be uneven.  If, however, the embedded
   packet is an IP packet, then the combination of (<source IP address>,
   <dest IP address>, <transport protocol>, <source port>, and <dest
   port>) from the IP header of the embedded packet forms an excellent
   basis for load-balancing.  This is what is typically used for load
   balancing IP packets.

   An MPLS packet doesn't, however, carry a payload type identifier.
   There is a simple (but dangerous) heuristic that is commonly used to
   guess the type of the embedded packet.  The first nibble, i.e., the
   four most significant bits of the first octet, of an IP header
   contains the IP version number.  That, in turn, indicates where to
   find the relevant fields for load-balancing.  The heuristic goes
   roughly as follows:

2.1.1.1.  Heuristic for Load Balancing

   1.  If the PFN is 0x4 (0100b), treat the payload as an IPv4 packet,
       and find the relevant fields for load-balancing on that basis.

   2.  If the PFN is 0x6 (0110b), treat the payload as an IPv6 packet,
       and find the relevant fields for load-balancing on that basis.

   3.  If the PFN is anything else, the MPLS payload is not an IP
       packet; fall back to load-balancing using the label stack.

   This heuristic has been implemented in many (legacy) routers, and
   performs well in the case of Figure 2, A.  However, this heuristic
   can work very badly for Figure 2, B.  For example, if payload B is an
   Ethernet frame, then the PFN is the first nibble of the OUI of the
   destination MAC address, which can be 0x4 or 0x6, and if so would
   lead to very bad load-balancing.  This behavior can happen to other
   types of non-IP payloads as well.

   That, in turn, led to the idea of inserting a PSH (e.g., a pseudowire
   control word [RFC4385], a DetNet control word [RFC8964] or a BIER
   header [RFC8296]) where the PFN is not 0x4 or 0x6, to explicitly

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   prevent forwarding engines from confusing the MPLS payload with an IP
   packet.  [RFC8469] recommends the use of a control word when the
   embedded packet is an Ethernet frame.  RFC 8469 was published at the
   request of the operator community and the IEEE RAC as a result of
   operational difficulties with pseudowires that did not contain the
   control word.

   It is RECOMMENDED that where load-balancing of MPLS packets is
   desired, the load-balancing mechanism uses the value of a dedicated
   label, for example, either an Entropy Label [RFC6790] or a FAT
   Pseudowire Label [RFC6391].  Furthermore, the heuristic of guessing
   the type of the embedded packet, as discussed above, SHOULD NOT be
   used.

   A consequence of the latter approach is that, while legacy routers
   may look for an PFN of 0x4 [RFC0791] or 0x6 [RFC8200], no router will
   look for a PFN of 0x7 (or whatever the next IP version number will
   be) for load-balancing purposes.  This means that the values 0x4 and
   0x6 are used to (sometimes incorrectly) identify IPv4 and IPv6
   packets, but no other First Nibble values will be used to identify IP
   packets.

   This also expands the PFN Registry to all 16 possible values, not
   just 0x0 and 0x1.

2.2.  Updates of RFC 4928

   Paragraph 3 in Section 3 of RFC 4928 [RFC4928] states that:

   It is REQUIRED, however, that applications depend upon in-order
   packet delivery restrict the first nibble values to 0x0 and 0x1.
   This will ensure that their traffic flows will not be affected if
   some future routing equipment does similar snooping on some future
   version(s) of IP.

   The text in RFC 4928 [RFC4928] concerning the first nibble after the
   MPLS Label Stack has been updated by [I-D.ietf-mpls-1stnibble] and
   the heuristic for snooping this nibble has been deprecated.  RFC 4928
   is now updated as follows:

   Network equipment that complies with [I-D.ietf-mpls-1stnibble] MUST
   use a PSH (Post-Stack Header) with a PFN (Post-stack First Nibble)
   value that is neither 0x4 nor 0x6 in all cases when the MPLS payload
   is not an IP packet.

   The recommendation (see Section 2.1.1.1) replaces the paragraph 4 in
   Section 3 of RFC 4928 [RFC4928] as follows:

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   OLD TEXT:

   This behavior implies that if in the future an IP version is defined
   with a version number of 0x0 or 0x1, then equipment complying with
   this BCP would be unable to look past one or more MPLS headers, and
   load-split traffic from a single LSP across multiple paths based on a
   hash of specific fields in the IPv0 or IPv1 headers.  That is, IP
   traffic employing these version numbers would be safe from
   disturbances caused by inappropriate load-splitting, but would also
   not be able to get the performance benefits.

   NEW TEXT:

   [I-D.ietf-mpls-1stnibble] deprecated the practice of deducing the
   payload type to avoid inaccurate load balancing based on the PFN
   value.  This means that older implementations and deployments can
   continue to use that heuristic, while it must not be part of new
   implementations or deployments.  The deprecation also means that
   concerns about load balancing for future IP versions with a version
   number of 0x0 or 0x1 are now moot.

   A new document is to be published to obsolete MPLS encapsulations
   without PSH of non-IP payload when sufficient evidence exists that
   there are no marketed or deployed implementations using the heuristic
   practice.

   END

   Furthermore, the following text is appended to Section 1.1 of RFC
   4928 [RFC4928]:

   PSH: Post-Stack Header

   PFN: Post-stack First Nibble

2.3.  Why Create a Registry

   The MPLS WG is currently engaged in updating the MPLS architecture;
   part of this work may involve the use of PSHs.  That might be more
   challenging if PSH values are allocated on an ad hoc basis, and their
   parsing and semantics are ill-specified.  Consider that the PFN value
   of 0x0 has two different formats, depending on whether the PSH is a
   pseudowire control word or a DetNet control word; disambiguation
   requires the context of the service label.  This was a considered
   decision; documenting this would be helpful to future implementors.

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   With a registry, PSHs become easier to parse; not needing means
   outside the data plane to interpret them correctly; and their
   semantics and usage are documented.  (Thank you, IANA!)

2.4.  The Relationship between IANA IP Version Numbers [RFC2780] and
      Post-stack First Nibble Registries

   The use of the PFN stemmed from the desire to heuristically identify
   IP packets for load-balancing purposes.  It was then discovered that
   non-IP packets, misidentified as IP when the heuristic failed, were
   being badly load balanced, leading to [RFC4928].  This situation may
   confuse some as to the relationship between the Post-stack First
   Nibble Registry and the IP Version Numbers registry.  These
   registries are quite different:

   1.  The IP Version Numbers registry's explicit purpose is to track IP
       version numbers in an IP header.

   2.  The Post-stack First Nibble registry's purpose is to track PSH
       types.

   The only intersection points between the two registries is for values
   0x4 and 0x6 (for backward compatibility).  There is no need to track
   future IP version number allocations in the Post-stack First Nibble
   registry.

3.  IANA Considerations

3.1.  The Post-stack First Nibble Registry

   This memo recommends the creation of an IANA registry called "The
   Post-stack First Nibble Registry" with the following values:

+=======================================================================+
| Code points allocated for BIER                                        |
+=======================================================================+
| Value | PSH Type                        | Reference                   |
+-------+---------------------------------+-----------------------------+
| 0x5   | BIER header - Normal traffic    | RFC 8296                    |
+=======================================================================+
| Code points allocated for DETNET                                      |
+=======================================================================+
| Value | PSH Type                        | Reference                   |
+-------+---------------------------------+-----------------------------+
| 0x0   | DetNet Control Word             | RFC 8964                    |
+-------+---------------------------------+-----------------------------+
| 0x1   | DetNet Associated Channel       | draft-ietf-detnet-mpls-oam  |
+=======================================================================+

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| Code points allocated for Network Service Header (NSH)                |
+=======================================================================+
| Value | PSH Type                        | Reference                   |
+-------+---------------------------------+-----------------------------+
| 0x0   | NSH Base Header, Payload        | RFC 8300                    |
+-------+---------------------------------+-----------------------------+
| 0x2   | NSH Base Header, OAM            | RFC 8300                    |
+=======================================================================+
| Code points allocated for Pseudowires (PW)                            |
+=======================================================================+
| Value | PSH Type                        | Reference                   |
+-------+---------------------------------+-----------------------------+
| 0x0   | PW Control Word                 | RFC 4385                    |
+-------+---------------------------------+-----------------------------+
| 0x1   | PW Associated Channel           | RFC 4385                    |
+=======================================================================+
| Code points allocated for the MPLS Generic Associated Channel         |
+=======================================================================+
| Value | PSH Type                        | Reference                   |
+-------+---------------------------------+-----------------------------+
| 0x1   | MPLS G-ACh                      | RFC 5586                    |
+=======================================================================+
| Reserved Code Points, not to be allocated                             |
+=======================================================================+
| Value | Usage                           | Reference                   |
+-------+---------------------------------+-----------------------------+
| 0x4   | IPv4 Protocol Number            | RFC 791                     |
+-------+---------------------------------+-----------------------------+
| 0x6   | IPv6 Protocol Number            | RFC 8200                    |
+=======================================================================+
| Unassigned Code Points                                                |
+=======================================================================+
| Value  | PSH Type                        | Reference                  |
+--------+---------------------------------+----------------------------+
| 0x3    | -                               | -                          |
+--------+---------------------------------+----------------------------+
| 0x7-0xF| -                               | -                          |
+--------+---------------------------------+----------------------------+

             Figure 3: The Post-stack First Nibble Values

3.1.1.  Allocation Policy

   All new values registered here MUST use the Standards Action policy
   [RFC8126].

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4.  Security Considerations

   This document proposes a new IANA registry and does not raise any
   security concerns or issues in addition to ones common to networking
   and those specific to MPLS networks.

5.  References

5.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
              <https://www.rfc-editor.org/info/rfc3032>.

   [RFC4385]  Bryant, S., Swallow, G., Martini, L., and D. McPherson,
              "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
              Use over an MPLS PSN", RFC 4385, DOI 10.17487/RFC4385,
              February 2006, <https://www.rfc-editor.org/info/rfc4385>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC4928]  Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal
              Cost Multipath Treatment in MPLS Networks", BCP 128,
              RFC 4928, DOI 10.17487/RFC4928, June 2007,
              <https://www.rfc-editor.org/info/rfc4928>.

   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              RFC 6790, DOI 10.17487/RFC6790, November 2012,
              <https://www.rfc-editor.org/info/rfc6790>.

   [RFC8469]  Bryant, S., Malis, A., and I. Bagdonas, "Recommendation to
              Use the Ethernet Control Word", RFC 8469,
              DOI 10.17487/RFC8469, November 2018,
              <https://www.rfc-editor.org/info/rfc8469>.

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   [RFC8300]  Quinn, P., Ed., Elzur, U., Ed., and C. Pignataro, Ed.,
              "Network Service Header (NSH)", RFC 8300,
              DOI 10.17487/RFC8300, January 2018,
              <https://www.rfc-editor.org/info/rfc8300>.

   [RFC8296]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
              Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation
              for Bit Index Explicit Replication (BIER) in MPLS and Non-
              MPLS Networks", RFC 8296, DOI 10.17487/RFC8296, January
              2018, <https://www.rfc-editor.org/info/rfc8296>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8964]  Varga, B., Ed., Farkas, J., Berger, L., Malis, A., Bryant,
              S., and J. Korhonen, "Deterministic Networking (DetNet)
              Data Plane: MPLS", RFC 8964, DOI 10.17487/RFC8964, January
              2021, <https://www.rfc-editor.org/info/rfc8964>.

   [RFC6391]  Bryant, S., Ed., Filsfils, C., Drafz, U., Kompella, V.,
              Regan, J., and S. Amante, "Flow-Aware Transport of
              Pseudowires over an MPLS Packet Switched Network",
              RFC 6391, DOI 10.17487/RFC6391, November 2011,
              <https://www.rfc-editor.org/info/rfc6391>.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <https://www.rfc-editor.org/info/rfc791>.

   [RFC2780]  Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
              Values In the Internet Protocol and Related Headers",
              BCP 37, RFC 2780, DOI 10.17487/RFC2780, March 2000,
              <https://www.rfc-editor.org/info/rfc2780>.

   [I-D.ietf-mpls-1stnibble]
              Kompella, K., Bryant, S., Bocci, M., Mirsky, G.,
              Andersson, L., and J. Dong, "IANA Registry for the First
              Nibble Following a Label Stack", Work in Progress,
              Internet-Draft, draft-ietf-mpls-1stnibble-03, 9 February
              2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
              mpls-1stnibble-03>.

5.2.  Informative References

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   [RFC4761]  Kompella, K., Ed. and Y. Rekhter, Ed., "Virtual Private
              LAN Service (VPLS) Using BGP for Auto-Discovery and
              Signaling", RFC 4761, DOI 10.17487/RFC4761, January 2007,
              <https://www.rfc-editor.org/info/rfc4761>.

   [RFC7432]  Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
              Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
              Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
              2015, <https://www.rfc-editor.org/info/rfc7432>.

   [RFC4446]  Martini, L., "IANA Allocations for Pseudowire Edge to Edge
              Emulation (PWE3)", BCP 116, RFC 4446,
              DOI 10.17487/RFC4446, April 2006,
              <https://www.rfc-editor.org/info/rfc4446>.

   [RFC4762]  Lasserre, M., Ed. and V. Kompella, Ed., "Virtual Private
              LAN Service (VPLS) Using Label Distribution Protocol (LDP)
              Signaling", RFC 4762, DOI 10.17487/RFC4762, January 2007,
              <https://www.rfc-editor.org/info/rfc4762>.

Authors' Addresses

   Kireeti Kompella
   Juniper Networks
   1133 Innovation Way
   Sunnyvale, 94089
   United States of America
   Email: kireeti.ietf@gmail.com

   Stewart Bryant
   University of Surrey 5GIC
   Email: sb@stewartbryant.com

   Matthew Bocci
   Nokia
   Email: matthew.bocci@nokia.com

   Greg Mirsky (editor)
   Ericsson
   Email: gregimirsky@gmail.com

   Loa Andersson
   Huawei Technologies
   Email: loa@pi.nu

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   Jie Dong
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing, 100095
   China
   Email: jie.dong@huawei.com

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