A Primer on the Development of MPLS
draft-bryant-mpls-dev-primer-00

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Author Stewart Bryant 
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MPLS                                                           S. Bryant
Internet-Draft                                              Futurewei US
Intended status: Informational                            March 31, 2021
Expires: October 2, 2021

                  A Primer on the Development of MPLS
                    draft-bryant-mpls-dev-primer-00

Abstract

   There has been significant recent interest in developing MPLS to
   address new needs.  This memo collects together various documents
   that together describe the key aspects of the MPLS architecture
   together with the development proposals that the author is aware of.

   The purpose of this document is to bring everyone up to speed on the
   rational for the existing design and to alert them to the new
   proposals.

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
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 2, 2021.

Copyright Notice

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Background Documents  . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Multiprotocol Label Switching Architecture (RFC 3031) . .   3
     2.2.  MPLS Label Stack Encoding (RFC 3032)  . . . . . . . . . .   4
     2.3.  Control Word for Use over an MPLS PSN (RFC5586) . . . . .   4
     2.4.  Avoiding Equal Cost Multipath (ECMP) Treatment in MPLS
           Network . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.5.  Synonymous Flow Labels  . . . . . . . . . . . . . . . . .   5
     2.6.  Residence Time Measurement in MPLS Networks . . . . . . .   5
   3.  New Proposals . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  MPLS Extension Header Architecture  . . . . . . . . . . .   5
     3.2.  MPLS Label Operations in MPLS EH capable networks . . . .   6
     3.3.  Encapsulation For MPLS Performance Measurement with
           Alternate Marking . . . . . . . . . . . . . . . . . . . .   7
     3.4.  MPLS Data Plane Encapsulation for In-situ OAM Data  . . .   7
     3.5.  Multi-purpose Special Purpose Label for Forwarding
           Actions . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.6.  No Further Fast Reroute . . . . . . . . . . . . . . . . .   8
     3.7.  Carrying Virtual Transport Network Identifier in MPLS
           Packet  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     3.8.  Segment Routed Time Sensitive Networking  . . . . . . . .   8
     3.9.  Options for MPLS Extension Header Indicator . . . . . . .   8
     3.10. MPLS Extension Header . . . . . . . . . . . . . . . . . .   9
     3.11. MPLS Payload Protocol Identifier  . . . . . . . . . . . .   9
     3.12. Generic Transport Functions . . . . . . . . . . . . . . .  10
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   There has been significant recent interest in developing the MPLS
   data plane to address new needs.  This memo collects together various
   documents that together describe the key aspects of the MPLS
   architecture together with the development proposals that the author
   is aware of.

   The intent of this work is to bring everyone up to speed on the
   rational for the existing design and to alert them to the new

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   proposals.  If I have missed any proposals, then please accept my
   apologies for the oversight, let me know and I will include it in the
   list.

   I do not anticipate that this memo will progress to become an RFC.

   The interest in this work to develop MPLS was noted by the Chairs of
   the DETNET, MPLS, PALS, and SPRING IETF working groups who called a
   joint meeting at IETF 110.  The agenda, slides notes etc of the
   meeting are currently at https://datatracker.ietf.org/meeting/agenda/

   Editor's note the above URL will change when the meeting is included
   in the IETF Proceedings.

   A video recording of the meeting is to be found at https://youtu.be/
   rt_vQTToO1s

2.  Background Documents

2.1.  Multiprotocol Label Switching Architecture (RFC 3031)

   [RFC3031] describes the base architecture of MPLS.  This is updated
   by:

   o  [RFC6178] which describes label edge router forwarding of IPv4
      option packets and is not relevant to this discussion.

   o  [RFC6790] which describes the use of entropy labels in MPLS
      forwarding.  The entropy label is a two stack entry tuple that
      consists of a special purpose label (SPL) followed by a label
      stack entry (LSE) who's label is pure entropy to be applied to the
      selection of a path from the Equal Cost Multi-Path (ECMP) set.
      Its use in choosing the ECMP member is optional.  This was the
      first formal introduction of the concept of an MPLS forwarder
      needing to parse below the top of the label stack.

   Prior to the introduction of RFC 6790 it was already common practice
   for LSRs to scan the label stack to the bottom of stack (a simple
   task requiring the checking of a single bit in each LSE) to
   heuristically check that the packet was IP and if so to hash the five
   tuple to determine the ECMP path to use.

   This approach worked well until Ethernet addresses were
   (legitimately) deployed that confused these forwarders into thinking
   that Ethernet packets were IP packets ((RFC8469}}.

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2.2.  MPLS Label Stack Encoding (RFC 3032)

   [RFC3032] Specifies the encoding of an MPLS LSE.  This has been
   updated by:

   o  [RFC3443] describes the two models of TTL handling in MPLS.  In
      addition that it should be noted that some LSR decrement the TTL
      of the TOP label _before_ inspecting the label in the top LSE.

   o  [RFC3270] describes the application of diffserv to MPLS and the
      pipe vs uniform models.  It may apply to this work if we are
      popping xSPLs.

   EDITOR'S NOTE need to think [RFC3270] some more.

   o  [RFC5129] describes Explicit Congestion Marking in MPLS.  It is
      not clear how widely this is deployed.  This is something that
      needs to be thought through when re-purposing the TC bits as has
      been proposed in some of the drafts listed below.

   o  [RFC5462] renames the EXP field to TC field.  This is a naming
      convention and not a technology change.

   o  [RFC5586] defines the Generic Associated Channel (see later).

   o  [RFC7274] defines the process for allocating and retiring special
      purpose labels (SPLs) (formerly known as Reserved Labels).  It
      also creates the extended special purpose labels (eSPLs).  eSPLs
      are another instance of a twin label in the stack, with the first
      label (15) from the SPL range followed by another label that
      specifies the function of the twin labels.

2.3.  Control Word for Use over an MPLS PSN (RFC5586)

   [RFC5586] describes the first use of a type of meta-data between the
   bottom of the MPLS label stack and the packet payload.  The
   pseudowire (PW) control word (CW) is used to carry additional
   information that the egress Provider Edge (PE) LSR needs to construct
   the outgoing packet.  It also tells the forwarder that the packet is
   not an IP packet and so should not be subjected to ECMP.

   [RFC8469] is an update to the Ethernet PW specification [RFC4448] to
   strongly encourage the use of the CW for Ethernet PWs.  All other PWs
   that have been defined require the use of the CW.

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2.4.  Avoiding Equal Cost Multipath (ECMP) Treatment in MPLS Network

   [RFC4928] describes the issue of accidental ECMP of packets not carry
   IP.  This arises because some forwarders accidentally mistake a non-
   IP packet for an IP packet.  They use a heuristic method of
   determining that the packet is IP.  They sometimes get this wrong
   causing a misordered the delivery of some of the packets.  [RFC4928]
   formally introduces the concept of using the first nibble of 0b0000
   to avoid this.  Note that the technique is actually older than this
   having been introduced in [RFC4385].

2.5.  Synonymous Flow Labels

   [RFC8957] describes a technique whereby additional labels are
   introduced to an MPLS network that mimic the behavior of other MPLS
   labels but also introduce new properties to the FEC.  The main use is
   to trigger OAM actions, but the method can be used to trigger other
   agreed actions.

2.6.  Residence Time Measurement in MPLS Networks

   [RFC8169] describes a method of measuring the dwell or residence time
   of a packet in a router.  The time is accumulated in a G-ACh packet
   carried below the label stack.  Note that the G-ACh uses first nibble
   = 0b0001.  This is the first instance of a G-ACh packet being
   specified for operation on a user data packet.

   The data packet is carried over an RSVP-TE path and thus the top of
   stack label indicates to the forwarder both the next-hop and outgoing
   label, but also indicates the presence of the G-ACh and the need to
   perform the residence time accumulation.

   The RFC predates segment routing (SR) and does not mention Software
   Defined Networks (SDNS), but the method could be used in those
   environments.

3.  New Proposals

   This section catalogues new proposals for how metadata is carried and
   how its presence is indicated to the forwarder.

3.1.  MPLS Extension Header Architecture

   [I-D.andersson-mpls-eh-architecture] specifies an architecture for
   the extension of MPLS to include Extension Headers (EH).  The
   proposal is for the EHs to carry information on in-network services
   and functions in an MPLS network.  The extension headers are carried

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   after the MPLS Label Stack, and the presence of EHs are indicated in
   the label stack by an Extension Header Indicator (EHI).

   Proposed use cases are:

   o  In-situ OAM

   o  Network Telemetry and Measurement

   o  Network Security

   o  Segment Routing

   o  Network Programming

   The draft calls two types of EH:

   o  "hop-by-hop" (HBH)

   o  "End to end" (E2E).

   The draft proposes to indicate the presence of the EH via the FEC.
   The ability of a router on the LSP to process a packet correctly is
   advertised in the routing protocol.

3.2.  MPLS Label Operations in MPLS EH capable networks

   [I-D.andersson-mpls-eh-label-stack-operations] provides the operating
   procedures for EH-capable and non-EH-capable LSRs where MPLS
   Extension Headers (EH) are carried below the MPLS label stack.
   Further this describes how MPLS EHs can be gradually introduced into
   an existing MPLS network.  The capability to handle EHs is announced
   throughout the MPLS network, and LSRs that don't understand this
   information simply ignore it.

   The extension headers are carried after the MPLS Label Stack, and the
   presence of EHs are indicate in the label stack by a Extended Special
   Purpose label called Extension Header Indicator (EHI) in the label
   stack.

   The EH(s) are carried over a G-ACh.  Three ACHs are suggested E2E,
   HBH, Both.  A number of EHs can be accommodated with the number being
   indicated by a parameter in the ACH.

   The draft considers the stack structure in a number of cases such as
   VPN (and presumably PW) and non-VPN (native IP payload) cases.

   The draft shows how RSVP-TE signaling would work.

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3.3.  Encapsulation For MPLS Performance Measurement with Alternate
      Marking

   [I-D.cheng-mpls-inband-pm-encapsulation] shows how a flow ID can be
   carried in a packet.  The application is Alternate Marking (AM) and
   includes an indicator of the measurement type.

   In an early version it was proposed that an SPL was used, but in the
   latest version an eSPL is proposed, in both cases at the bottom of
   stack.

   The draft discusses IP and VPN, but it is not clear if this what
   happens in the PW case.

3.4.  MPLS Data Plane Encapsulation for In-situ OAM Data

   [I-D.gandhi-mpls-ioam-sr] shows how the IOAM data fields defined in
   [I-D.ietf-ippm-ioam-data] could be carried in MPLS.  It carries the
   OAM data in an G-Ach and specifies both hop-by-hop and end-to-end
   versions.

   The OAM present/type indicator is an e(SPL) at the bottom of the MPLS
   label stack requiring a P-router to scan the stack to find the label.

   The draft proposes the stacking of G-Ach blocks at the bottom of
   stack with the IOAM G-Ach first and any subsequent G-Ach located
   through the use of a length field in the IOAM G-Ach.

3.5.  Multi-purpose Special Purpose Label for Forwarding Actions

   [I-D.kompella-mpls-mspl4f] notes that the forwarder does not need to
   use the TC, or TTL fields in an LSE that does not become top of
   stack.  It proposes to exploit these fields as indicators of
   forwarding actions, by modifying the semantics of these fields.

   There are a number of key proposals in the draft:

   o  Using the "spare bits" as forwarding indicator flags to specify
      actions or in some cases inactions

   o  Using the method to multi-purpose SPLs and thus expand the number
      of single label SPLs available to the IETF.

   o  Reusing the Entropy Label fields to carry additional data needed
      by the forwarder.  This latter point could be adopted by any eSPL.
      One use for this additional data that was proposed (certainly in
      discussion but I cannot see it in the draft) was the use of this
      facility to carry a network slice identifier.

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3.6.  No Further Fast Reroute

   [I-D.kompella-mpls-nffrr] proposes the use of an SPL (note not an
   eSPL) to indicate that a fast re-route action is not to be undertaken
   on the packet.

   Uses an SPL for this single purpose

3.7.  Carrying Virtual Transport Network Identifier in MPLS Packet

   [I-D.li-mpls-enhanced-vpn-vtn-id] is a method of carrying a virtual
   network identifier in an MPLS packet.  It does this by carrying meta-
   data below the MPLS label stack.  It does not use the G-ACh but
   instead a new design with a first nibble value of 0b0011.

   Note that when we define new first nibbles we are technically taking
   IP versions away from the IETF Internet Area.  When PWE3 first
   proposed this we agreed with the IETF of the day that we would only
   take 0b0000 and 0b0001.  I am looking to see if this agreement was
   documented.

   The presence of the VTN is indicted by an SPL (note not an eSPL)
   somewhere in the label stack.  The draft discusses how multiple VTNs
   can be placed in the packet, but not how multiple types of meta-data
   are to be carried.

3.8.  Segment Routed Time Sensitive Networking

   [I-D.stein-srtsn] describes how information can be encoded in the
   MPLS label stack to inform the forwarder when a time sensitive packet
   should be sent.  Each LSE becomes 64 bits, the first 32 bits a
   conventional MPLS label and the second part contains dispatch time
   information.

   Note that as far as I can see there is no provision for an S bit
   making label stack scanning for other information liable to make a
   mistake.

   There is no information that I can see stating how the LSR knows that
   the LSE is in this format and so I assume that it knows from the FEC.

3.9.  Options for MPLS Extension Header Indicator

   [I-D.song-mpls-extension-header] provides a catalogue of methods of
   identifying the presence of presence of an extension header after the
   label stack.  The methods could of course be used for identifying the
   presence of some other structure after the label stack.

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   The methods listed are:

   o  A special purpose label

   o  An extended special purpose label pair

   o  A GAL and an associated channel header

   o  A GAL followed by a structure with a different first nibble value

   o  The use of a new FEC

3.10.  MPLS Extension Header

   [I-D.song-mpls-extension-header] describes a design for an MPLS
   extension header to be placed after the MPLS label stack.  The Header
   of Extension Headers (HEH) specifies the number of extension headers
   that follow.  The HEH has the four bit ECMP defeat nibble, a count of
   number of extension headers, the length of the set of extension
   headers and the type of the following extension header.  An Extension
   Header (EH) starts with the type of the header that follows this EH,
   the length of this EH followed by the EH data/payload.

   Two generic types of EH as specified, End to End and Hop by Hop.

3.11.  MPLS Payload Protocol Identifier

   [I-D.xu-mpls-payload-protocol-identifier] describes a method of
   adding a protocol identifier (PID) to an MPLS packet.

   A 16 bit PID is carried in a 32 bit structure following the label
   stack.  The structure just has an ECMP defeat nibble 0b000 and the
   PID.

   Presence of the PID is indicated by an SPL or an eSPL at the bottom
   of stack.

   An alternative method of indicating the PIL is also proposed by using
   a first nibble of 0b1111.  Note that this might be defeated by an
   MPLS payload other than IP.  For reasons discussed in [RFC8469] in
   this arrangement the PIL could not be used at a mid-point, but would
   be safe at an endpoint.  The first nibble comments in {#VTN} also
   apply to this proposal.

   No provision is made for carrying other data beyond the bottom of
   stack, and there is no discussion on how this works with VPNs and
   PWs.

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3.12.  Generic Transport Functions

   [I-D.zzhang-tsvwg-generic-transport-functions] describes a method of
   adding fragmentation to a number of protocols including MPLS.  The
   fragmentation header follows the end of stack.  It does not take any
   ECMP precautions through the first nibble.  Some mitigation could be
   achieved by the use of the ELI/EL where the P routers can support
   this.

   Indication of the fragmentation header is indicated by the FEC.

   Note that the draft referenced pseudowires (PWs) and that PWs have a
   fragmentation method [RFC4623].  However this feature is not thought
   to be widely implemented.

4.  Security Considerations

   Any changes to the MPLS security model as a result of a change will
   need to be considered within the proposals themselves.  This document
   is a catalog of existing RFCs and design proposals and does not
   itself modify the security of MPLS networks.

5.  IANA Considerations

   This document has no IANA requests.

6.  References

6.1.  Normative References

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <https://www.rfc-editor.org/info/rfc3031>.

   [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>.

6.2.  Informative References

   [I-D.andersson-mpls-eh-architecture]
              Andersson, L., Guichard, J., Song, H., and S. Bryant,
              "MPLS Extension Header Architecture", draft-andersson-
              mpls-eh-architecture-00 (work in progress), February 2019.

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   [I-D.andersson-mpls-eh-label-stack-operations]
              Andersson, L., Guichard, J., Song, H., and S. Bryant,
              "MPLS Label Operations in MPLS EH capable networks",
              draft-andersson-mpls-eh-label-stack-operations-00 (work in
              progress), February 2019.

   [I-D.cheng-mpls-inband-pm-encapsulation]
              Cheng, W., Min, X., Zhou, T., Dong, X., and Y. Peleg,
              "Encapsulation For MPLS Performance Measurement with
              Alternate Marking Method", draft-cheng-mpls-inband-pm-
              encapsulation-04 (work in progress), September 2020.

   [I-D.gandhi-mpls-ioam-sr]
              Gandhi, R., Ali, Z., Filsfils, C., Brockners, F., Wen, B.,
              and V. Kozak, "MPLS Data Plane Encapsulation for In-situ
              OAM Data", draft-gandhi-mpls-ioam-sr-05 (work in
              progress), January 2021.

   [I-D.ietf-ippm-ioam-data]
              Brockners, F., Bhandari, S., and T. Mizrahi, "Data Fields
              for In-situ OAM", draft-ietf-ippm-ioam-data-11 (work in
              progress), November 2020.

   [I-D.kompella-mpls-nffrr]
              Kompella, K. and W. Lin, "No Further Fast Reroute", draft-
              kompella-mpls-nffrr-01 (work in progress), November 2020.

   [I-D.song-mpls-extension-header]
              Song, H., Li, Z., Zhou, T., and L. Andersson, "MPLS
              Extension Header", draft-song-mpls-extension-header-02
              (work in progress), February 2019.

   [I-D.xu-mpls-payload-protocol-identifier]
              Xu, X., Assarpour, H., Ma, S., and F. Clad, "MPLS Payload
              Protocol Identifier", draft-xu-mpls-payload-protocol-
              identifier-08 (work in progress), December 2020.

   [I-D.zzhang-tsvwg-generic-transport-functions]
              Zhang, Z., Bonica, R., and K. Kompella, "Generic Transport
              Functions", draft-zzhang-tsvwg-generic-transport-
              functions-00 (work in progress), November 2020.

   [RFC3270]  Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
              P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
              Protocol Label Switching (MPLS) Support of Differentiated
              Services", RFC 3270, DOI 10.17487/RFC3270, May 2002,
              <https://www.rfc-editor.org/info/rfc3270>.

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   [RFC3443]  Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
              in Multi-Protocol Label Switching (MPLS) Networks",
              RFC 3443, DOI 10.17487/RFC3443, January 2003,
              <https://www.rfc-editor.org/info/rfc3443>.

   [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>.

   [RFC4448]  Martini, L., Ed., Rosen, E., El-Aawar, N., and G. Heron,
              "Encapsulation Methods for Transport of Ethernet over MPLS
              Networks", RFC 4448, DOI 10.17487/RFC4448, April 2006,
              <https://www.rfc-editor.org/info/rfc4448>.

   [RFC4623]  Malis, A. and M. Townsley, "Pseudowire Emulation Edge-to-
              Edge (PWE3) Fragmentation and Reassembly", RFC 4623,
              DOI 10.17487/RFC4623, August 2006,
              <https://www.rfc-editor.org/info/rfc4623>.

   [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>.

   [RFC5129]  Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
              Marking in MPLS", RFC 5129, DOI 10.17487/RFC5129, January
              2008, <https://www.rfc-editor.org/info/rfc5129>.

   [RFC5462]  Andersson, L. and R. Asati, "Multiprotocol Label Switching
              (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
              Class" Field", RFC 5462, DOI 10.17487/RFC5462, February
              2009, <https://www.rfc-editor.org/info/rfc5462>.

   [RFC5586]  Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
              "MPLS Generic Associated Channel", RFC 5586,
              DOI 10.17487/RFC5586, June 2009,
              <https://www.rfc-editor.org/info/rfc5586>.

   [RFC6178]  Smith, D., Mullooly, J., Jaeger, W., and T. Scholl, "Label
              Edge Router Forwarding of IPv4 Option Packets", RFC 6178,
              DOI 10.17487/RFC6178, March 2011,
              <https://www.rfc-editor.org/info/rfc6178>.

   [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>.

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Internet-Draft               MPLS-Dev-Primer                  March 2021

   [RFC7274]  Kompella, K., Andersson, L., and A. Farrel, "Allocating
              and Retiring Special-Purpose MPLS Labels", RFC 7274,
              DOI 10.17487/RFC7274, June 2014,
              <https://www.rfc-editor.org/info/rfc7274>.

   [RFC8169]  Mirsky, G., Ruffini, S., Gray, E., Drake, J., Bryant, S.,
              and A. Vainshtein, "Residence Time Measurement in MPLS
              Networks", RFC 8169, DOI 10.17487/RFC8169, May 2017,
              <https://www.rfc-editor.org/info/rfc8169>.

   [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>.

   [RFC8957]  Bryant, S., Chen, M., Swallow, G., Sivabalan, S., and G.
              Mirsky, "Synonymous Flow Label Framework", RFC 8957,
              DOI 10.17487/RFC8957, January 2021,
              <https://www.rfc-editor.org/info/rfc8957>.

Author's Address

   Stewart Bryant
   Futurewei US

   Email: sb@stewartbryant.com

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