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RFC 7325 - MPLS Forwarding Compliance and Performance Requiremen


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Internet Engineering Task Force (IETF)                C. Villamizar, Ed.
Request for Comments: 7325                                         OCCNC
Category: Informational                                      K. Kompella
ISSN: 2070-1721                                         Juniper Networks
                                                               S. Amante
                                                              Apple Inc.
                                                                A. Malis
                                                                  Huawei
                                                            C. Pignataro
                                                                   Cisco
                                                             August 2014

        MPLS Forwarding Compliance and Performance Requirements

Abstract

   This document provides guidelines for implementers regarding MPLS
   forwarding and a basis for evaluations of forwarding implementations.
   Guidelines cover many aspects of MPLS forwarding.  Topics are
   highlighted where implementers might otherwise overlook practical
   requirements that are unstated or underemphasized, or that are
   optional for conformance to RFCs but often considered mandatory by
   providers.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see 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/rfc7325.

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

   (http://trustee.ietf.org/license-info) in effect on the date of
   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 and Document Scope . . . . . . . . . . . . . . .   4
     1.1.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Use of Requirements Language  . . . . . . . . . . . . . .   8
     1.3.  Apparent Misconceptions . . . . . . . . . . . . . . . . .   9
     1.4.  Target Audience . . . . . . . . . . . . . . . . . . . . .  10
   2.  Forwarding Issues . . . . . . . . . . . . . . . . . . . . . .  11
     2.1.  Forwarding Basics . . . . . . . . . . . . . . . . . . . .  11
       2.1.1.  MPLS Special-Purpose Labels . . . . . . . . . . . . .  12
       2.1.2.  MPLS Differentiated Services  . . . . . . . . . . . .  13
       2.1.3.  Time Synchronization  . . . . . . . . . . . . . . . .  14
       2.1.4.  Uses of Multiple Label Stack Entries  . . . . . . . .  14
       2.1.5.  MPLS Link Bundling  . . . . . . . . . . . . . . . . .  15
       2.1.6.  MPLS Hierarchy  . . . . . . . . . . . . . . . . . . .  16
       2.1.7.  MPLS Fast Reroute (FRR) . . . . . . . . . . . . . . .  16
       2.1.8.  Pseudowire Encapsulation  . . . . . . . . . . . . . .  17
         2.1.8.1.  Pseudowire Sequence Number  . . . . . . . . . . .  17
       2.1.9.  Layer 2 and Layer 3 VPN . . . . . . . . . . . . . . .  19
     2.2.  MPLS Multicast  . . . . . . . . . . . . . . . . . . . . .  20
     2.3.  Packet Rates  . . . . . . . . . . . . . . . . . . . . . .  21
     2.4.  MPLS Multipath Techniques . . . . . . . . . . . . . . . .  23
       2.4.1.  Pseudowire Control Word . . . . . . . . . . . . . . .  24
       2.4.2.  Large Microflows  . . . . . . . . . . . . . . . . . .  24
       2.4.3.  Pseudowire Flow Label . . . . . . . . . . . . . . . .  25
       2.4.4.  MPLS Entropy Label  . . . . . . . . . . . . . . . . .  25
       2.4.5.  Fields Used for Multipath Load Balance  . . . . . . .  25
         2.4.5.1.  MPLS Fields in Multipath  . . . . . . . . . . . .  26
         2.4.5.2.  IP Fields in Multipath  . . . . . . . . . . . . .  27
         2.4.5.3.  Fields Used in Flow Label . . . . . . . . . . . .  29
         2.4.5.4.  Fields Used in Entropy Label  . . . . . . . . . .  29
     2.5.  MPLS-TP and UHP . . . . . . . . . . . . . . . . . . . . .  30

     2.6.  Local Delivery of Packets . . . . . . . . . . . . . . . .  30
       2.6.1.  DoS Protection  . . . . . . . . . . . . . . . . . . .  31
       2.6.2.  MPLS OAM  . . . . . . . . . . . . . . . . . . . . . .  33
       2.6.3.  Pseudowire OAM  . . . . . . . . . . . . . . . . . . .  34
       2.6.4.  MPLS-TP OAM . . . . . . . . . . . . . . . . . . . . .  34
       2.6.5.  MPLS OAM and Layer 2 OAM Interworking . . . . . . . .  35
       2.6.6.  Extent of OAM Support by Hardware . . . . . . . . . .  36
       2.6.7.  Support for IPFIX in Hardware . . . . . . . . . . . .  37
     2.7.  Number and Size of Flows  . . . . . . . . . . . . . . . .  37
   3.  Questions for Suppliers . . . . . . . . . . . . . . . . . . .  38
     3.1.  Basic Compliance  . . . . . . . . . . . . . . . . . . . .  38
     3.2.  Basic Performance . . . . . . . . . . . . . . . . . . . .  40
     3.3.  Multipath Capabilities and Performance  . . . . . . . . .  41
     3.4.  Pseudowire Capabilities and Performance . . . . . . . . .  41
     3.5.  Entropy Label Support and Performance . . . . . . . . . .  42
     3.6.  DoS Protection  . . . . . . . . . . . . . . . . . . . . .  42
     3.7.  OAM Capabilities and Performance  . . . . . . . . . . . .  42
   4.  Forwarding Compliance and Performance Testing . . . . . . . .  43
     4.1.  Basic Compliance  . . . . . . . . . . . . . . . . . . . .  43
     4.2.  Basic Performance . . . . . . . . . . . . . . . . . . . .  44
     4.3.  Multipath Capabilities and Performance  . . . . . . . . .  45
     4.4.  Pseudowire Capabilities and Performance . . . . . . . . .  46
     4.5.  Entropy Label Support and Performance . . . . . . . . . .  46
     4.6.  DoS Protection  . . . . . . . . . . . . . . . . . . . . .  47
     4.7.  OAM Capabilities and Performance  . . . . . . . . . . . .  47
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  48
   6.  Organization of References Section  . . . . . . . . . . . . .  50
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  50
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  50
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  53
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  59

1.  Introduction and Document Scope

   The initial purpose of this document was to address concerns raised
   on the MPLS WG mailing list about shortcomings in implementations of
   MPLS forwarding.  Documenting existing misconceptions and potential
   pitfalls might potentially avoid repeating past mistakes.  The
   document has grown to address a broad set of forwarding requirements.

   The focus of this document is MPLS forwarding, base pseudowire
   forwarding, and MPLS Operations, Administration, and Maintenance
   (OAM).  The use of pseudowire Control Word and the use of pseudowire
   Sequence Number are discussed.  Specific pseudowire Attachment
   Circuit (AC) and Native Service Processing (NSP) are out of scope.
   Specific pseudowire applications, such as various forms of Virtual
   Private Network (VPN), are out of scope.

   MPLS support for multipath techniques is considered essential by many
   service providers and is useful for other high-capacity networks.  In
   order to obtain sufficient entropy from MPLS, traffic service
   providers and others find it essential for the MPLS implementation to
   interpret the MPLS payload as IPv4 or IPv6 based on the contents of
   the first nibble of payload.  The use of IP addresses, the IP
   protocol field, and UDP and TCP port number fields in multipath load
   balancing are considered within scope.  The use of any other IP
   protocol fields, such as tunneling protocols carried within IP, are
   out of scope.

   Implementation details are a local matter and are out of scope.  Most
   interfaces today operate at 1 Gb/s or greater.  It is assumed that
   all forwarding operations are implemented in specialized forwarding
   hardware rather than on a general-purpose processor.  This is often
   referred to as "fast path" and "slow path" processing.  Some
   recommendations are made regarding implementing control or
   management-plane functionality in specialized hardware or with
   limited assistance from specialized hardware.  This advice is based
   on expected control or management protocol loads and on the need for
   denial of service (DoS) protection.

1.1.  Abbreviations

   The following abbreviations are used.

   AC    Attachment Circuit ([RFC3985])

   ACH   Associated Channel Header (pseudowires)

   ACK   Acknowledgement (TCP flag and type of TCP packet)

   AIS   Alarm Indication Signal (MPLS-TP OAM)

   ATM   Asynchronous Transfer Mode (legacy switched circuits)

   BFD   Bidirectional Forwarding Detection

   BGP   Border Gateway Protocol

   CC-CV Continuity Check and Connectivity Verification

   CE    Customer Edge ([RFC4364])

   CPU   Central Processing Unit (computer or microprocessor)

   CT    Class Type ([RFC4124])

   CW    Control Word ([RFC4385])

   DCCP  Datagram Congestion Control Protocol

   DDoS  Distributed Denial of Service

   DM    Delay Measurement (MPLS-TP OAM)

   DSCP  Differentiated Services Code Point ([RFC2474])

   DWDM  Dense Wave Division Multiplexing

   DoS   Denial of Service

   E-LSP Explicitly TC-encoded-PSC LSP ([RFC5462])

   EBGP  External BGP

   ECMP  Equal-Cost Multipath

   ECN   Explicit Congestion Notification ([RFC3168] and [RFC5129])

   EL    Entropy Label ([RFC6790])

   ELI   Entropy Label Indicator ([RFC6790])

   EXP   Experimental (field in MPLS renamed to "TC" in [RFC5462])

   FEC   Forwarding Equivalence Classes ([RFC3031]); also Forward Error
         Correction in other context

   FR    Frame Relay (legacy switched circuits)

   FRR   Fast Reroute ([RFC4090])

   G-ACh Generic Associated Channel ([RFC5586])

   GAL   Generic Associated Channel Label ([RFC5586])

   GFP   Generic Framing Procedure (used in OTN)

   GMPLS Generalized MPLS ([RFC3471])

   GTSM  Generalized TTL Security Mechanism ([RFC5082])

   Gb/s  Gigabits per second (billion bits per second)

   IANA  Internet Assigned Numbers Authority

   ILM   Incoming Label Map ([RFC3031])

   IP    Internet Protocol

   IPVPN Internet Protocol VPN

   IPv4  Internet Protocol version 4

   IPv6  Internet Protocol version 6

   L-LSP Label-Only-Inferred-PSC LSP ([RFC3270])

   L2VPN Layer 2 VPN

   LDP   Label Distribution Protocol ([RFC5036])

   LER   Label Edge Router ([RFC3031])

   LM    Loss Measurement (MPLS-TP OAM)

   LSP   Label Switched Path ([RFC3031])

   LSR   Label Switching Router ([RFC3031])

   MP2MP Multipoint to Multipoint

   MPLS  Multiprotocol Label Switching ([RFC3031])

   MPLS-TP  MPLS Transport Profile ([RFC5317])

   Mb/s  Megabits per second (million bits per second)

   NSP   Native Service Processing ([RFC3985])

   NTP   Network Time Protocol

   OAM   Operations, Administration, and Maintenance ([RFC6291])

   OOB   Out-of-band (not carried within a data channel)

   OTN   Optical Transport Network

   P     Provider router ([RFC4364])

   P2MP  Point to Multipoint

   PE    Provider Edge router ([RFC4364])

   PHB   Per-Hop Behavior ([RFC2475])

   PHP   Penultimate Hop Popping ([RFC3443])

   POS   PPP over SONET

   PSC   This abbreviation has multiple interpretations.

         1.  Packet Switch Capable ([RFC3471]

         2.  PHB Scheduling Class ([RFC3270])

         3.  Protection State Coordination ([RFC6378])

   PTP   Precision Time Protocol

   PW    Pseudowire

   QoS   Quality of Service

   RA    Router Alert ([RFC3032])

   RDI   Remote Defect Indication (MPLS-TP OAM)

   RSVP-TE  RSVP Traffic Engineering

   RTP   Real-time Transport Protocol

   SCTP  Stream Control Transmission Protocol

   SDH   Synchronous Data Hierarchy (European SONET, a form of TDM)

   SONET Synchronous Optical Network (US SDH, a form of TDM)

   T-LDP Targeted LDP (LDP sessions over more than one hop)

   TC    Traffic Class ([RFC5462])

   TCP   Transmission Control Protocol

   TDM   Time-Division Multiplexing (legacy encapsulations)

   TOS   Type of Service (see [RFC2474])

   TTL   Time-to-live (a field in IP and MPLS headers)

   UDP   User Datagram Protocol

   UHP   Ultimate Hop Popping (opposite of PHP)

   VCCV  Virtual Circuit Connectivity Verification ([RFC5085])

   VLAN  Virtual Local Area Network (Ethernet)

   VOQ   Virtual Output Queuing (switch fabric design)

   VPN   Virtual Private Network

   WG    Working Group

1.2.  Use of Requirements Language

   This document is Informational.  The uppercase [RFC2119] key words
   "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" are used in
   this document in the following cases.

   1.  RFC 2119 keywords are used where requirements stated in this
       document are called for in referenced RFCs.  In most cases, the
       RFC containing the requirement is cited within the statement
       using an RFC 2119 keyword.

   2.  RFC 2119 keywords are used where explicitly noted that the
       keywords indicate that operator experiences indicate a
       requirement, but there are no existing RFC requirements.

   Advice provided by this document may be ignored by implementations.
   Similarly, implementations not claiming conformance to specific RFCs
   may ignore the requirements of those RFCs.  In both cases,
   implementers should consider the risk of doing so.

1.3.  Apparent Misconceptions

   In early generations of forwarding silicon (which might now be behind
   us), there apparently were some misconceptions about MPLS.  The
   following statements provide clarifications.

   1.  There are practical reasons to have more than one or two labels
       in an MPLS label stack.  Under some circumstances, the label
       stack can become quite deep.  See Section 2.1.

   2.  The label stack MUST be considered to be arbitrarily deep.
       Section 3.27.4 ("Hierarchy: LSP Tunnels within LSPs") of RFC 3031
       states "The label stack mechanism allows LSP tunneling to nest to
       any depth" [RFC3031].  If a bottom of the label stack cannot be
       found, but sufficient number of labels exist to forward, an LSR
       MUST forward the packet.  An LSR MUST NOT assume the packet is
       malformed unless the end of packet is found before the bottom of
       the stack.  See Section 2.1.

   3.  In networks where deep label stacks are encountered, they are not
       rare.  Full packet rate performance is required regardless of
       label stack depth, except where multiple pop operations are
       required.  See Section 2.1.

   4.  Research has shown that long bursts of short packets with 40-byte
       or 44-byte IP payload sizes in these bursts are quite common.
       This is due to TCP ACK compression [ACK-compression].  The
       following two sub-bullets constitute advice that reflects very
       common nonnegotiable requirements of providers.  Implementers may
       ignore this advice but should consider the risk of doing so.

       A.  A forwarding engine SHOULD, if practical, be able to sustain
           an arbitrarily long sequence of small packets arriving at
           full interface rate.

       B.  If indefinitely sustained full packet rate for small packets
           is not practical, a forwarding engine MUST be able to buffer
           a long sequence of small packets inbound to the on-chip
           decision engine and sustain full interface rate for some
           reasonable average packet rate.  Absent this small on-chip
           buffering, QoS-agnostic packet drops can occur.

       See Section 2.3.

   5.  The implementations and system designs MUST support pseudowire
       Control Word (CW) if MPLS-TP is supported or if ACH [RFC5586] is
       being used on a pseudowire.  The implementation and system
       designs SHOULD support pseudowire CW even if MPLS-TP and ACH

       [RFC5586] are not used, using instead CW and VCCV Type 1
       [RFC5085] to allow the use of multipath in the underlying network
       topology without impacting the PW traffic.  [RFC7079] does note
       that there are still some deployments where the CW is not always
       used.  It also notes that many service providers do enable the
       CW.  See Section 2.4.1 for more discussion on why deployments
       SHOULD enable the pseudowire CW.

   The following statements provide clarification regarding more recent
   requirements that are often missed.

   1.  The implementer and system designer SHOULD support adding a
       pseudowire Flow Label [RFC6391].  Deployments MAY enable this
       feature for appropriate pseudowire types.  See Section 2.4.3.

   2.  The implementer and system designer SHOULD support adding an MPLS
       Entropy Label [RFC6790].  Deployments MAY enable this feature.
       See Section 2.4.4.

   Non-IETF definitions of MPLS exist, and these should not be used as
   normative texts in place of the relevant IETF RFCs.  [RFC5704]
   documents incompatibilities between the IETF definition of MPLS and
   one such alternative MPLS definition, which led to significant issues
   in the resulting non-IETF specification.

1.4.  Target Audience

   This document is intended for multiple audiences: implementer
   (implementing MPLS forwarding in silicon or in software); systems
   designer (putting together a MPLS forwarding systems); deployer
   (running an MPLS network).  These guidelines are intended to serve
   the following purposes:

   1.  Explain what to do and what not to do when a deep label stack is
       encountered. (audience: implementer)

   2.  Highlight pitfalls to look for when implementing an MPLS
       forwarding chip. (audience: implementer)

   3.  Provide a checklist of features and performance specifications to
       request.  (audience: systems designer, deployer)

   4.  Provide a set of tests to perform.  (audience: systems designer,
       deployer).

   The implementer, systems designer, and deployer have a transitive
   supplier-customer relationship.  It is in the best interest of the
   supplier to review their product against their customer's checklist
   and secondary customer's checklist if applicable.

   This document identifies and explains many details and potential
   pitfalls of MPLS forwarding.  It is likely that the identified set of
   potential pitfalls will later prove to be an incomplete set.

2.  Forwarding Issues

   A brief review of forwarding issues is provided in the subsections
   that follow.  This section provides some background on why some of
   these requirements exist.  The questions to ask of suppliers is
   covered in Section 3.  Some guidelines for testing are provided in
   Section 4.

2.1.  Forwarding Basics

   Basic MPLS architecture and MPLS encapsulation, and therefore packet
   forwarding, are defined in [RFC3031] and [RFC3032].  RFC 3031 and RFC
   3032 are somewhat LDP centric.  RSVP-TE supports traffic engineering
   (TE) and fast reroute, features that LDP lacks.  The base document
   for MPLS RSVP-TE is [RFC3209].

   A few RFCs update RFC 3032.  Those with impact on forwarding include
   the following.

   1.  TTL processing is clarified in [RFC3443].

   2.  The use of MPLS Explicit NULL is modified in [RFC4182].

   3.  Differentiated Services is supported by [RFC3270] and [RFC4124].
       The "EXP" field is renamed to "Traffic Class" in [RFC5462],
       removing any misconception that it was available for
       experimentation or could be ignored.

   4.  ECN is supported by [RFC5129].

   5.  The MPLS G-ACh and GAL are defined in [RFC5586].

   6.  [RFC5332] redefines the two data link layer codepoints for MPLS
       packets.

   Tunneling encapsulations carrying MPLS, such as MPLS in IP [RFC4023],
   MPLS in GRE [RFC4023], MPLS in L2TPv3 [RFC4817], or MPLS in UDP
   [MPLS-IN-UDP], are out of scope.

   Other RFCs have implications to MPLS Forwarding and do not update RFC
   3032 or RFC 3209, including:

   1.  The pseudowire (PW) Associated Channel Header (ACH) is defined by
       [RFC5085] and was later generalized by the MPLS G-ACh [RFC5586].

   2.  The Entropy Label Indicator (ELI) and Entropy Label (EL) are
       defined by [RFC6790].

   A few RFCs update RFC 3209.  Those that are listed as updating RFC
   3209 generally impact only RSVP-TE signaling.  Forwarding is modified
   by major extensions built upon RFC 3209.

   RFCs that impact forwarding are discussed in the following
   subsections.

2.1.1.  MPLS Special-Purpose Labels

   [RFC3032] specifies that label values 0-15 are special-purpose labels
   with special meanings.  [RFC7274] renamed these from the term
   "reserved labels" used in [RFC3032] to "special-purpose labels".
   Three values of NULL label are defined (two of which are later
   updated by [RFC4182]) and a Router Alert Label is defined.  The
   original intent was that special-purpose labels, except the NULL
   labels, could be sent to the routing engine CPU rather than be
   processed in forwarding hardware.  Hardware support is required by
   new RFCs such as those defining Entropy Label and OAM processed as a
   result of receiving a GAL.  For new special-purpose labels, some
   accommodation is needed for LSRs that will send the labels to a
   general-purpose CPU or other highly programmable hardware.  For
   example, ELI will only be sent to LSRs that have signaled support for
   [RFC6790], and a high OAM packet rate must be negotiated among
   endpoints.

   [RFC3429] reserves a label for ITU-T Y.1711; however, Y.1711 does not
   work with multipath and its use is strongly discouraged.

   The current list of special-purpose labels can be found on the
   "Multiprotocol Label Switching Architecture (MPLS) Label Values"
   registry reachable at IANA's pages at <http://www.iana.org>.

   [RFC7274] introduces an IANA "Extended Special-Purpose MPLS Label
   Values" registry and makes use of the "extension" label, label 15, to
   indicate that the next label is an extended special-purpose label and
   requires special handling.  The range of only 16 values for special-
   purpose labels allows a table to be used.  The range of extended
   special-purpose labels with 20 bits available for use may have to be
   handled in some other way in the unlikely event that in the future

   the range of currently reserved values 256-1048575 is used.  If only
   the Standards Action range, 16-239, and the Experimental range,
   240-255, are used, then a table of 256 entries can be used.

   Unknown special-purpose labels and unknown extended special-purpose
   labels are handled the same.  When an unknown special-purpose label
   is encountered or a special purpose label not directly handled in
   forwarding hardware is encountered, the packet should be sent to a
   general-purpose CPU by default.  If this capability is supported,
   there must be an option to either drop or rate limit such packets
   based on the value of each special-purpose label.

2.1.2.  MPLS Differentiated Services

   [RFC2474] deprecates the IP Type of Service (TOS) and IP Precedence
   (Prec) fields and replaces them with the Differentiated Services
   Field more commonly known as the Differentiated Services Code Point
   (DSCP) field.  [RFC2475] defines the Differentiated Services
   architecture, which in other forums, is often called a Quality of
   Service (QoS) architecture.

   MPLS uses the Traffic Class (TC) field to support Differentiated
   Services [RFC5462].  There are two primary documents describing how
   DSCP is mapped into TC.

   1.  [RFC3270] defines E-LSP and L-LSP.  E-LSP uses a static mapping
       of DSCP into TC.  L-LSP uses a per-LSP mapping of DSCP into TC,
       with one PHB Scheduling Class (PSC) per L-LSP.  Each PSC can use
       multiple Per-Hop Behavior (PHB) values.  For example, the Assured
       Forwarding service defines three PSCs, each with three PHB
       [RFC2597].

   2.  [RFC4124] defines assignment of a class-type (CT) to an LSP,
       where a per-CT static mapping of TC to PHB is used.  [RFC4124]
       provides a means to support up to eight E-LSP-like mappings of
       DSCP to TC.

   To meet Differentiated Services requirements specified in [RFC3270],
   the following forwarding requirements must be met.  An ingress LER
   MUST be able to select an LSP and then apply a per-LSP map of DSCP
   into TC.  A midpoint LSR MUST be able to apply a per-LSP map of TC to
   PHB.  The number of mappings supported will be far less than the
   number of LSPs supported.

   To meet Differentiated Services requirements specified in [RFC4124],
   the following forwarding requirements must be met.  An ingress LER
   MUST be able to select an LSP and then apply a per-LSP map of DSCP
   into TC.  A midpoint LSR MUST be able to map LSP number to Class Type

   (CT), then use a per-CT map to map TC to PHB.  Since there are only
   eight allowed values of CT, only eight maps of TC to PHB need to be
   supported.  The LSP label can be used directly to find the TC-to-PHB
   mapping, as is needed to support L-LSPs as defined by [RFC3270].

   While support for [RFC4124] and not [RFC3270] would allow support for
   only eight mappings of TC to PHB, it is common to support both and
   simply state a limit on the number of unique TC-to-PHB mappings that
   can be supported.

2.1.3.  Time Synchronization

   PTP or NTP may be carried over MPLS [TIMING-OVER-MPLS].  Generally,
   NTP will be carried within IP, and IP will be carried in MPLS
   [RFC5905].  Both PTP and NTP benefit from accurate timestamping of
   incoming packets and the ability to insert accurate timestamps in
   outgoing packets.  PTP correction that occurs when forwarding
   requires updating a timestamp compensation field based on the
   difference between packet arrival at an LSR and packet transmit time
   at that same LSR.

   Since the label stack depth may vary, hardware should allow a
   timestamp to be placed in an outgoing packet at any specified byte
   position.  It may be necessary to modify Layer 2 checksums or frame
   check sequences after insertion.  PTP and NTP timestamp formats
   differ in such a way as to require different implementations of the
   timestamp correction.  If NTP or PTP is carried over UDP/IP or
   UDP/IP/MPLS, the UDP checksum will also have to be updated.

   Accurate time synchronization, in addition to being generally useful,
   is required for MPLS-TP Delay Measurement (DM) OAM.  See
   Section 2.6.4.

2.1.4.  Uses of Multiple Label Stack Entries

   MPLS deployments in the early part of the prior decade (circa 2000)
   tended to support either LDP or RSVP-TE.  LDP was favored by some for
   its ability to scale to a very large number of PE devices at the edge
   of the network, without adding deployment complexity.  RSVP-TE was
   favored, generally in the network core, where traffic engineering
   and/or fast reroute were considered important.

   Both LDP and RSVP-TE are used simultaneously within major service
   provider networks using a technique known as "LDP over RSVP-TE
   Tunneling".  This technique allows service providers to carry LDP
   tunnels inside RSVP-TE tunnels.  This makes it possible to take
   advantage of the traffic engineering and fast reroute on more
   expensive intercity and intercontinental transport paths.  The

   ingress RSVP-TE PE places many LDP tunnels on a single RSVP-TE LSP
   and carries it to the egress RSVP-TE PE.  The LDP PEs are situated
   further from the core, for example, within a metro network.  LDP over
   RSVP-TE tunneling requires a minimum of two MPLS labels: one each for
   LDP and RSVP-TE.

   The use of MPLS FRR [RFC4090] might add one more label to MPLS
   traffic but only when FRR protection is in use (active).  If LDP over
   RSVP-TE is in use, and FRR protection is in use, then at least three
   MPLS labels are present on the label stack on the links through which
   the Bypass LSP traverses.  FRR is covered in Section 2.1.7.

   LDP L2VPN, LDP IPVPN, BGP L2VPN, and BGP IPVPN added support for VPN
   services that are deployed by the vast majority of service providers.
   These VPN services added yet another label, bringing the label stack
   depth (when FRR is active) to four.

   Pseudowires and VPN are discussed in further detail in Sections 2.1.8
   and 2.1.9.

   MPLS hierarchy as described in [RFC4206] and updated by [RFC7074] can
   in principle add at least one additional label.  MPLS hierarchy is
   discussed in Section 2.1.6.

   Other features such as Entropy Label (discussed in Section 2.4.4) and
   Flow Label (discussed in Section 2.4.3) can add additional labels to
   the label stack.

   Although theoretical scenarios can easily result in eight or more
   labels, such cases are rare if they occur at all today.  For the
   purpose of forwarding, only the top label needs to be examined if PHP
   is used, and a few more if UHP is used (see Section 2.5).  For deep
   label stacks, quite a few labels may have to be examined for the
   purpose of load balancing across parallel links (see Section 2.4);
   however, this depth can be bounded by a provider through use of
   Entropy Label.

   Other creative uses of MPLS within the IETF, such as the use of MPLS
   label stack in source routing, may result in label stacks that are
   considerably deeper than those encountered today.

2.1.5.  MPLS Link Bundling

   MPLS Link Bundling was the first RFC to address the need for multiple
   parallel links between nodes [RFC4201].  MPLS Link Bundling is
   notable in that it tried not to change MPLS forwarding, except in

   specifying the "all-ones" component link.  MPLS Link Bundling is
   seldom if ever deployed.  Instead, multipath techniques described in
   Section 2.4 are used.

2.1.6.  MPLS Hierarchy

   MPLS hierarchy is defined in [RFC4206] and updated by [RFC7074].
   Although RFC 4206 is considered part of GMPLS, the Packet Switching
   Capable (PSC) portion of the MPLS hierarchy is applicable to MPLS and
   may be supported in an otherwise GMPLS-free implementation.  The MPLS
   PSC hierarchy remains the most likely means of providing further
   scaling in an RSVP-TE MPLS network, particularly where the network is
   designed to provide RSVP-TE connectivity to the edges.  This is the
   case for envisioned MPLS-TP networks.  The use of the MPLS PSC
   hierarchy can add at least one additional label to a label stack,
   though it is likely that only one layer of PSC will be used in the
   near future.

2.1.7.  MPLS Fast Reroute (FRR)

   Fast reroute is defined by [RFC4090].  Two significantly different
   methods are defined in RFC 4090: the "One-to-One Backup" method,
   which uses the "Detour LSP", and the "Facility Backup", which uses a
   "bypass tunnel".  These are commonly referred to as the detour and
   bypass methods, respectively.

   The detour method makes use of a presignaled LSP.  Hardware
   assistance may be needed for detour FRR in order to accomplish local
   repair of a large number of LSPs within the target of tens of
   milliseconds.  For each affected LSP, a swap operation must be
   reprogrammed or otherwise switched over.  The use of detour FRR
   doubles the number of LSPs terminating at any given hop and will
   increase the number of LSPs within a network by a factor dependent on
   the average detour path length.

   The bypass method makes use of a tunnel that is unused when no fault
   exists but may carry many LSPs when a local repair is required.
   There is no presignaling indicating which working LSP will be
   diverted into any specific bypass LSP.  If interface label space is
   used, the bypass LSP MUST extend one hop beyond the merge point,
   except if the merge point is the egress and PHP is used.  If the
   bypass LSPs are not extended in this way, then the merge LSR (egress
   LSR of the bypass LSP) MUST use platform label space (as defined in
   [RFC3031]) so that an LSP working path on any given interface can be
   backed up using a bypass LSP terminating on any other interface.
   Hardware assistance may be needed to accomplish local repair of a
   large number of LSPs within the target of tens of milliseconds.  For
   each affected LSP a swap operation must be reprogrammed or otherwise

   switched over with an additional push of the bypass LSP label.  The
   use of platform label space impacts the size of the LSR ILM for an
   LSR with a very large number of interfaces.

   IP/LDP Fast Reroute (IP/LDP FRR) [RFC5714] is also applicable in MPLS
   networks.  ECMP and Loop-Free Alternates (LFAs) [RFC5286] are well-
   established IP/LDP FRR techniques and were the first methods to be
   widely deployed.  Work on IP/LDP FRR is ongoing within the IETF
   RTGWG.  Two topics actively discussed in RTGWG are microloops and
   partial coverage of the established techniques in some network
   topologies.  [RFC5715] covers the topic of IP/LDP Fast Reroute
   microloops and microloop prevention.  RTGWG has developed additional
   IP/LDP FRR techniques to handle coverage concerns.  RTGWG is
   extending LFA through the use of remote LFA [REMOTE-LFA].  Other
   techniques that require new forwarding paths to be established are
   also under consideration, including the IPFRR "not-via" technique
   defined in [RFC6981] and maximally redundant trees (MRT) [MRT].
   ECMP, LFA (but not remote LFA), and MRT swap the top label to an
   alternate MPLS label.  The other methods operate in a similar manner
   to the facility backup described in RFC 4090 and push an additional
   label.  IP/LDP FRR methods that push more than one label have been
   suggested but are in early discussion.

2.1.8.  Pseudowire Encapsulation

   The pseudowire (PW) architecture is defined in [RFC3985].  A
   pseudowire, when carried over MPLS, adds one or more additional label
   entries to the MPLS label stack.  A PW Control Word is defined in
   [RFC4385] with motivation for defining the Control Word in [RFC4928].
   The PW Associated Channel defined in [RFC4385] is used for OAM in
   [RFC5085].  The PW Flow Label is defined in [RFC6391] and is
   discussed further in this document in Section 2.4.3.

   There are numerous pseudowire encapsulations, supporting emulation of
   services such as Frame Relay, ATM, Ethernet, TDM, and SONET/SDH over
   packet switched networks (PSNs) using IP or MPLS.

   The pseudowire encapsulation is out of scope for this document.
   Pseudowire impact on MPLS forwarding at the midpoint LSR is within
   scope.  The impact on ingress MPLS push and egress MPLS UHP pop are
   within scope.  While pseudowire encapsulation is out of scope, some
   advice is given on Sequence Number support.

2.1.8.1.  Pseudowire Sequence Number

   Pseudowire (PW) Sequence Number support is most important for PW
   payload types with a high expectation of lossless and/or in-order
   delivery.  Identifying lost PW packets and the exact amount of lost

   payload is critical for PW services that maintain bit timing, such as
   Time Division Multiplexing (TDM) services since these services MUST
   compensate lost payload on a bit-for-bit basis.

   With PW services that maintain bit timing, packets that have been
   received out of order also MUST be identified and MAY be either
   reordered or dropped.  Resequencing requires, in addition to sequence
   numbering, a "reorder buffer" in the egress PE, and the ability to
   reorder is limited by the depth of this buffer.  The down side of
   maintaining a large reorder buffer is added end-to-end service delay.

   For PW services that maintain bit timing or any other service where
   jitter must be bounded, a jitter buffer is always necessary.  The
   jitter buffer is needed regardless of whether reordering is done.  In
   order to be effective, a reorder buffer must often be larger than a
   jitter buffer needs to be, thus creating a tradeoff between reducing
   loss and minimizing delay.

   PW services that are not timing critical bit streams in nature are
   cell oriented or frame oriented.  Though resequencing support may be
   beneficial to PW cell- and frame-oriented payloads such as ATM, FR,
   and Ethernet, this support is desirable but not required.
   Requirements to handle out-of-order packets at all vary among
   services and deployments.  For example, for Ethernet PW, occasional
   (very rare) reordering is usually acceptable.  If the Ethernet PW is
   carrying MPLS-TP, then this reordering may be acceptable.

   Reducing jitter is best done by an end-system, given that the
   tradeoff of loss vs. delay varies among services.  For example, with
   interactive real-time services, low delay is preferred, while with
   non-interactive (one-way) real-time services, low loss is preferred.
   The same end-site may be receiving both types of traffic.  Regardless
   of this, bounded jitter is sometimes a requirement for specific
   deployments.

   Packet reordering should be rare except in a small number of
   circumstances, most of which are due to network design or equipment
   design errors:

   1.  The most common case is where reordering is rare, occurring only
       when a network or equipment fault forces traffic on a new path
       with different delay.  The packet loss that accompanies a network
       or equipment fault is generally more disruptive than any
       reordering that may occur.

   2.  A path change can be caused by reasons other than a network or
       equipment fault, such as an administrative routing change.  This
       may result in packet reordering but generally without any packet
       loss.

   3.  If the edge is not using pseudowire Control Word (CW) and the
       core is using multipath, reordering will be far more common.  If
       this is occurring, using CW on the edge will solve the problem.
       Without CW, resequencing is not possible since the Sequence
       Number is contained in the CW.

   4.  Another avoidable case is where some core equipment has multipath
       and for some reason insists on periodically installing a new
       random number as the multipath hash seed.  If supporting MPLS-TP,
       equipment MUST provide a means to disable periodic hash
       reseeding, and deployments MUST disable periodic hash reseeding.
       Operator experience dictates that even if not supporting MPLS-TP,
       equipment SHOULD provide a means to disable periodic hash
       reseeding, and deployments SHOULD disable periodic hash
       reseeding.

   In provider networks that use multipath techniques and that may
   occasionally rebalance traffic or that may change PW paths
   occasionally for other reasons, reordering may be far more common
   than loss.  Where reordering is more common than loss, resequencing
   packets is beneficial, rather than dropping packets at egress when
   out-of-order arrival occurs.  Resequencing is most important for PW
   payload types with a high expectation of lossless delivery since in
   such cases out-of-order delivery within the network results in PW
   loss.

2.1.9.  Layer 2 and Layer 3 VPN

   Layer 2 VPN [RFC4664] and Layer 3 VPN [RFC4110] add one or more label
   entry to the MPLS label stack.  VPN encapsulations are out of scope
   for this document.  Their impact on forwarding at the midpoint LSR
   are within scope.

   Any of these services may be used on an ingress and egress that are
   MPLS Entropy Label enabled (see Section 2.4.4 for discussion of
   Entropy Label); this would add an additional two labels to the MPLS
   label stack.  The need to provide a useful Entropy Label value
   impacts the requirements of the VPN ingress LER but is out of scope
   for this document.

2.2.  MPLS Multicast

   MPLS Multicast encapsulation is clarified in [RFC5332].  MPLS
   Multicast may be signaled using RSVP-TE [RFC4875] or LDP [RFC6388].

   [RFC4875] defines a root-initiated RSVP-TE LSP setup rather than the
   leaf-initiated join used in IP multicast.  [RFC6388] defines a leaf-
   initiated LDP setup.  Both [RFC4875] and [RFC6388] define point-to-
   multipoint (P2MP) LSP setup.  [RFC6388] also defined multipoint-to-
   multipoint (MP2MP) LSP setup.

   The P2MP LSPs have a single source.  An LSR may be a leaf node, an
   intermediate node, or a "bud" node.  A bud serves as both a leaf and
   intermediate.  At a leaf, an MPLS pop is performed.  The payload may
   be an IP multicast packet that requires further replication.  At an
   intermediate node, an MPLS swap operation is performed.  The bud
   requires that both a pop operation and a swap operation be performed
   for the same incoming packet.

   One strategy to support P2MP functionality is to pop at the LSR
   interface serving as ingress to the P2MP traffic and then optionally
   push labels at each LSR interface serving as egress to the P2MP
   traffic at that same LSR.  A given LSR egress chip may support
   multiple egress interfaces, each of which requires a copy, but each
   with a different set of added labels and Layer 2 encapsulation.  Some
   physical interfaces may have multiple sub-interfaces (such as
   Ethernet VLAN or channelized interfaces), each requiring a copy.

   If packet replication is performed at LSR ingress, then the ingress
   interface performance may suffer.  If the packet replication is
   performed within a LSR switching fabric and at LSR egress, congestion
   of egress interfaces cannot make use of backpressure to ingress
   interfaces using techniques such as virtual output queuing (VOQ).  If
   buffering is primarily supported at egress, then the need for
   backpressure is minimized.  There may be no good solution for high
   volumes of multicast traffic if VOQ is used.

   Careful consideration should be given to the performance
   characteristics of high-fanout multicast for equipment that is
   intended to be used in such a role.

   MP2MP LSPs differ in that any branch may provide an input, including
   a leaf.  Packets must be replicated onto all other branches.  This
   forwarding is often implemented as multiple P2MP forwarding trees,
   one for each potential input interface at a given LSR.

2.3.  Packet Rates

   While average packet size of Internet traffic may be large, long
   sequences of small packets have both been predicted in theory and
   observed in practice.  Traffic compression and TCP ACK compression
   can conspire to create long sequences of packets of 40-44 bytes in
   payload length.  If carried over Ethernet, the 64-byte minimum
   payload applies, yielding a packet rate of approximately 150 Mpps
   (million packets per second) for the duration of the burst on a
   nominal 100 Gb/s link.  The peak rate for other encapsulations can be
   as high as 250 Mpps (for example, when IP or MPLS is encapsulated
   using GFP over OTN ODU4).

   It is possible that the packet rates achieved by a specific
   implementation are acceptable for a minimum payload size, such as a
   64-byte (64B) payload for Ethernet, but the achieved rate declines to
   an unacceptable level for other packet sizes, such as a 65B payload.
   There are other packet rates of interest besides TCP ACK.  For
   example, a TCP ACK carried over an Ethernet PW over MPLS over
   Ethernet may occupy 82B or 82B plus an increment of 4B if additional
   MPLS labels are present.

   A graph of packet rate vs. packet size often displays a sawtooth.
   The sawtooth is commonly due to a memory bottleneck and memory
   widths, sometimes an internal cache, but often a very wide external
   buffer memory interface.  In some cases, it may be due to a fabric
   transfer width.  A fine packing, rounding up to the nearest 8B or 16B
   will result in a fine sawtooth with small degradation for 65B, and
   even less for 82B packets.  A coarse packing, rounding up to 64B can
   yield a sharper drop in performance for 65B packets, or perhaps more
   important, a larger drop for 82B packets.

   The loss of some TCP ACK packets are not the primary concern when
   such a burst occurs.  When a burst occurs, any other packets,
   regardless of packet length and packet QoS are dropped once on-chip
   input buffers prior to the decision engine are exceeded.  Buffers in
   front of the packet decision engine are often very small or
   nonexistent (less than one packet of buffer) causing significant QoS-
   agnostic packet drop.

   Internet service providers and content providers at one time
   specified full rate forwarding with 40-byte payload packets as a
   requirement.  Today, this requirement often can be waived if the
   provider can be convinced that when long sequences of short packets
   occur no packets will be dropped.

   Many equipment suppliers have pointed out that the extra cost in
   designing hardware capable of processing the minimum size packets at
   full line rate is significant for very-high-speed interfaces.  If
   hardware is not capable of processing the minimum size packets at
   full line rate, then that hardware MUST be capable of handling large
   bursts of small packets, a condition that is often observed.  This
   level of performance is necessary to meet Differentiated Services
   [RFC2475] requirements; without it, packets are lost prior to
   inspection of the IP DSCP field [RFC2474] or MPLS TC field [RFC5462].

   With adequate on-chip buffers before the packet decision engine, an
   LSR can absorb a long sequence of short packets.  Even if the output
   is slowed to the point where light congestion occurs, the packets,
   having cleared the decision process, can make use of larger VOQ or
   output side buffers and be dealt with according to configured QoS
   treatment, rather than dropped completely at random.

   The buffering before the packet decision engine should be arranged
   such that 1) it can hold a relatively large number of small packets,
   2) it can hold a small number of large packets, and 3) it can hold a
   mix of packets of different sizes.

   These on-chip buffers need not contribute significant delay since
   they are only used when the packet decision engine is unable to keep
   up, not in response to congestion, plus these buffers are quite
   small.  For example, an on-chip buffer capable of handling 4K packets
   of 64 bytes in length, or 256KB, corresponds to 200 microseconds on a
   10 Gb/s link and 20 microseconds on a 100 Gb/s link.  If the packet
   decision engine is capable of handling packets at 90% of the full
   rate for small packets, then the maximum added delay is 20
   microseconds and 2 microseconds, respectively, and this delay only
   applies if a 4K burst of short packets occurs.  When no burst of
   short packets was being processed, no delay is added.  These buffers
   are only needed on high-speed interfaces where it is difficult to
   process small packets at full line rate.

   Packet rate requirements apply regardless of which network tier the
   equipment is deployed in.  Whether deployed in the network core or
   near the network edges, one of the two conditions MUST be met if
   Differentiated Services requirements are to be met:

   1.  Packets must be processed at full line rate with minimum-sized
       packets.  -OR-

   2.  Packets must be processed at a rate well under generally accepted
       average packet sizes, with sufficient buffering prior to the
       packet decision engine to accommodate long bursts of small
       packets.

2.4.  MPLS Multipath Techniques

   In any large provider, service providers, and content providers,
   hash-based multipath techniques are used in the core and in the edge.
   In many of these providers, hash-based multipath is also used in the
   larger metro networks.

   For good reason, the Differentiated Services requirements dictate
   that packets within a common microflow SHOULD NOT be reordered
   [RFC2474].  Service providers generally impose stronger requirements,
   commonly requiring that packets within a microflow MUST NOT be
   reordered except in rare circumstances such as load balancing across
   multiple links, path change for load balancing, or path change for
   other reason.

   The most common multipath techniques are ECMP applied at the IP
   forwarding level, Ethernet Link Aggregation Group (LAG) with
   inspection of the IP payload, and multipath on links carrying both IP
   and MPLS, where the IP header is inspected below the MPLS label
   stack.  In most core networks, the vast majority of traffic is MPLS
   encapsulated.

   In order to support an adequately balanced load distribution across
   multiple links, IP header information must be used.  Common practice
   today is to reinspect the IP headers at each LSR and use the label
   stack and IP header information in a hash performed at each LSR.
   Further details are provided in Section 2.4.5.

   The use of this technique is so ubiquitous in provider networks that
   lack of support for multipath makes any product unsuitable for use in
   large core networks.  This will continue to be the case in the near
   future, even as deployment of the MPLS Entropy Label begins to relax
   the core LSR multipath performance requirements given the existing
   deployed base of edge equipment without the ability to add an Entropy
   Label.

   A generation of edge equipment supporting the ability to add an MPLS
   Entropy Label is needed before the performance requirements for core
   LSRs can be relaxed.  However, it is likely that two generations of
   deployment in the future will allow core LSRs to support full packet
   rate only when a relatively small number of MPLS labels need to be
   inspected before hashing.  For now, don't count on it.

   Common practice today is to reinspect the packet at each LSR and use
   information from the packet combined with a hash seed that is
   selected by each LSR.  Where Flow Labels or Entropy Labels are used,
   a hash seed must be used when creating these labels.

2.4.1.  Pseudowire Control Word

   Within the core of a network, some form of multipath is almost
   certain to be used.  Multipath techniques deployed today are likely
   to be looking beneath the label stack for an opportunity to hash on
   IP addresses.

   A pseudowire encapsulated at a network edge must have a means to
   prevent reordering within the core if the pseudowire will be crossing
   a network core, or any part of a network topology where multipath is
   used (see [RFC4385] and [RFC4928]).

   Not supporting the ability to encapsulate a pseudowire with a Control
   Word may lock a product out from consideration.  A pseudowire
   capability without Control Word support might be sufficient for
   applications that are strictly both intra-metro and low bandwidth.
   However, a provider with other applications will very likely not
   tolerate having equipment that can only support a subset of their
   pseudowire needs.

2.4.2.  Large Microflows

   Where multipath makes use of a simple hash and simple load balance
   such as modulo or other fixed allocation (see Section 2.4), there can
   be the presence of large microflows that each consume 10% of the
   capacity of a component link of a potentially congested composite
   link.  One such microflow can upset the traffic balance, and more
   than one can reduce the effective capacity of the entire composite
   link by more than 10%.

   When even a very small number of large microflows are present, there
   is a significant probability that more than one of these large
   microflows could fall on the same component link.  If the traffic
   contribution from large microflows is small, the probability for
   three or more large microflows on the same component link drops
   significantly.  Therefore, in a network where a significant number of
   parallel 10 Gb/s links exists, even a 1 Gb/s pseudowire or other
   large microflow that could not otherwise be subdivided into smaller
   flows should carry a Flow Label or Entropy Label if possible.

   Active management of the hash space to better accommodate large
   microflows has been implemented and deployed in the past; however,
   such techniques are out of scope for this document.

2.4.3.  Pseudowire Flow Label

   Unlike a pseudowire Control Word, a pseudowire Flow Label [RFC6391]
   is required only for pseudowires that have a relatively large
   capacity.  There are many cases where a pseudowire Flow Label makes
   sense.  Any service such as a VPN that carries IP traffic within a
   pseudowire can make use of a pseudowire Flow Label.

   Any pseudowire carried over MPLS that makes use of the pseudowire
   Control Word and does not carry a Flow Label is in effect a single
   microflow (in the terms defined in [RFC2475]) and may result in the
   types of problems described in Section 2.4.2.

2.4.4.  MPLS Entropy Label

   The MPLS Entropy Label simplifies flow group identification [RFC6790]
   at midpoint LSRs.  Prior to the MPLS Entropy Label, midpoint LSRs
   needed to inspect the entire label stack and often the IP headers to
   provide an adequate distribution of traffic when using multipath
   techniques (see Section 2.4.5).  With the use of the MPLS Entropy
   Label, a hash can be performed closer to network edges, placed in the
   label stack, and used by midpoint LSRs without fully reinspecting the
   label stack and inspecting the payload.

   The MPLS Entropy Label is capable of avoiding full label stack and
   payload inspection within the core where performance levels are most
   difficult to achieve (see Section 2.3).  The label stack inspection
   can be terminated as soon as the first Entropy Label is encountered,
   which is generally after a small number of labels are inspected.

   In order to provide these benefits in the core, an LSR closer to the
   edge must be capable of adding an Entropy Label.  This support may
   not be required in the access tier, the tier closest to the customer,
   but is likely to be required in the edge or the border to the network
   core.  An LSR peering with external networks will also need to be
   able to add an Entropy Label on incoming traffic.

2.4.5.  Fields Used for Multipath Load Balance

   The most common multipath techniques are based on a hash over a set
   of fields.  Regardless of whether a hash is used or some other method
   is used, there is a limited set of fields that can safely be used for
   multipath.

2.4.5.1.  MPLS Fields in Multipath

   If the "outer" or "first" layer of encapsulation is MPLS, then label
   stack entries are used in the hash.  Within a finite amount of time
   (and for small packets arriving at high speed, that time can be quite
   limited), only a finite number of label entries can be inspected.
   Pipelined or parallel architectures improve this, but the limit is
   still finite.

   The following guidelines are provided for use of MPLS fields in
   multipath load balancing.

   1.  Only the 20-bit label field SHOULD be used.  The TTL field SHOULD
       NOT be used.  The S bit MUST NOT be used.  The TC field (formerly
       EXP) MUST NOT be used.  See text following this list for reasons.

   2.  If an ELI label is found, then if the LSR supports Entropy
       Labels, the EL label field in the next label entry (the EL)
       SHOULD be used, label entries below that label SHOULD NOT be
       used, and the MPLS payload SHOULD NOT be used.  See below this
       list for reasons.

   3.  Special-purpose labels (label values 0-15) MUST NOT be used.
       Extended special-purpose labels (any label following label 15)
       MUST NOT be used.  In particular, GAL and RA MUST NOT be used so
       that OAM traffic follows the same path as payload packets with
       the same label stack.

   4.  If a new special-purpose label or extended special-purpose label
       is defined that requires special load-balance processing, then,
       as is the case for the ELI label, a special action may be needed
       rather than skipping the special-purpose label or extended
       special-purpose label.

   5.  The most entropy is generally found in the label stack entries
       near the bottom of the label stack (innermost label, closest to
       S=1 bit).  If the entire label stack cannot be used (or entire
       stack up to an EL), then it is better to use as many labels as
       possible closest to the bottom of stack.

   6.  If no ELI is encountered, and the first nibble of payload
       contains a 4 (IPv4) or 6 (IPv6), an implementation SHOULD support
       the ability to interpret the payload as IPv4 or IPv6 and extract
       and use appropriate fields from the IP headers.  This feature is
       considered a nonnegotiable requirement by many service providers.
       If supported, there MUST be a way to disable it (if, for example,
       PW without CW are used).  This ability to disable this feature is
       considered a nonnegotiable requirement by many service providers.

       Therefore, an implementation has a very strong incentive to
       support both options.

   7.  A label that is popped at egress (UHP pop) SHOULD NOT be used.  A
       label that is popped at the penultimate hop (PHP pop) SHOULD be
       used.

   Apparently, some chips have made use of the TC (formerly EXP) bits as
   a source of entropy.  This is very harmful since it will reorder
   Assured Forwarding (AF) traffic [RFC2597] when a subset does not
   conform to the configured rates and is remarked but not dropped at a
   prior LSR.  Traffic that uses MPLS ECN [RFC5129] can also be
   reordered if TC is used for entropy.  Therefore, as stated in the
   guidelines above, the TC field (formerly EXP) MUST NOT be used in
   multipath load balancing as it violates Differentiated Services
   Ordered Aggregate (OA) requirements in these two instances.

   Use of the MPLS label entry S bit would result in putting OAM traffic
   on a different path if the addition of a GAL at the bottom of stack
   removed the S bit from the prior label.

   If an ELI label is found, then if the LSR supports Entropy Labels,
   the EL label field in the next label entry (the EL) SHOULD be used,
   and the search for additional entropy within the packet SHOULD be
   terminated.  Failure to terminate the search will impact client MPLS-
   TP LSPs carried within server MPLS LSPs.  A network operator has the
   option to use administrative attributes as a means to identify LSRs
   that do not terminate the entropy search at the first EL.
   Administrative attributes are defined in [RFC3209].  Some
   configuration is required to support this.

   If the label removed by a PHP pop is not used, then for any PW for
   which CW is used, there is no basis for multipath load split.  In
   some networks, it is infeasible to put all PW traffic on one
   component link.  Any PW that does not use CW will be improperly
   split, regardless of whether the label removed by a PHP pop is used.
   Therefore, the PHP pop label SHOULD be used as recommended above.

2.4.5.2.  IP Fields in Multipath

   Inspecting the IP payload provides the most entropy in provider
   networks.  The practice of looking past the bottom of stack label for
   an IP payload is well accepted and documented in [RFC4928] and in
   other RFCs.

   Where IP is mentioned in the document, both IPv4 and IPv6 apply.  All
   LSRs MUST fully support IPv6.

   When information in the IP header is used, the following guidelines
   apply:

   1.  Both the IP source address and IP destination address SHOULD be
       used.  There MAY be an option to reverse the order of these
       addresses, improving the ability to provide symmetric paths in
       some cases.  Many service providers require that both addresses
       be used.

   2.  Implementations SHOULD allow inspection of the IP protocol field
       and use of the UDP or TCP port numbers.  For many service
       providers, this feature is considered mandatory, particularly for
       enterprise, data center, or edge equipment.  If this feature is
       provided, it SHOULD be possible to disable use of TCP and UDP
       ports.  Many service providers consider it a nonnegotiable
       requirement that use of UDP and TCP ports can be disabled.
       Therefore, there is a strong incentive for implementations to
       provide both options.

   3.  Equipment suppliers MUST NOT make assumptions that because the IP
       version field is equal to 4 (an IPv4 packet) that the IP protocol
       will either be TCP (IP protocol 6) or UDP (IP protocol 17) and
       blindly fetch the data at the offset where the TCP or UDP ports
       would be found.  With IPv6, TCP and UDP port numbers are not at
       fixed offsets.  With IPv4 packets carrying IP options, TCP and
       UDP port numbers are not at fixed offsets.

   4.  The IPv6 header flow field SHOULD be used.  This is the explicit
       purpose of the IPv6 flow field; however, observed flow fields
       rarely contain a non-zero value.  Some uses of the flow field
       have been defined, such as [RFC6438].  In the absence of MPLS
       encapsulation, the IPv6 flow field can serve a role equivalent to
       the Entropy Label.

   5.  Support for other protocols that share a common Layer 4 header
       such as RTP [RFC3550], UDP-Lite [RFC3828], SCTP [RFC4960], and
       DCCP [RFC4340] SHOULD be provided, particularly for edge or
       access equipment where additional entropy may be needed.
       Equipment SHOULD also use RTP, UDP-lite, SCTP, and DCCP headers
       when creating an Entropy Label.

   6.  The following IP header fields should not or must not be used:

       A.  Similar to avoiding TC in MPLS, the IP DSCP, and ECN bits
           MUST NOT be used.

       B.  The IPv4 TTL or IPv6 Hop Count SHOULD NOT be used.

       C.  Note that the IP TOS field was deprecated.  ([RFC0791] was
           updated by [RFC2474].)  No part of the IP DSCP field can be
           used (formerly IP PREC and IP TOS bits).

   7.  Some IP encapsulations support tunneling, such as IP-in-IP, GRE,
       L2TPv3, and IPsec.  These provide a greater source of entropy
       that some provider networks carrying large amounts of tunneled
       traffic may need, for example, as used in [RFC5640] for GRE and
       L2TPv3.  The use of tunneling header information is out of scope
       for this document.

   This document makes the following recommendations.  These
   recommendations are not required to claim compliance to any existing
   RFC; therefore, implementers are free to ignore them, but due to
   service provider requirements should consider the risk of doing so.
   The use of IP addresses MUST be supported, and TCP and UDP ports
   (conditional on the protocol field and properly located) MUST be
   supported.  The ability to disable use of UDP and TCP ports MUST be
   available.

   Though potentially very useful in some networks, it is uncommon to
   support using payloads of tunneling protocols carried over IP.
   Though the use of tunneling protocol header information is out of
   scope for this document, it is not discouraged.

2.4.5.3.  Fields Used in Flow Label

   The ingress to a pseudowire (PW) can extract information from the
   payload being encapsulated to create a Flow Label.  [RFC6391]
   references IP carried in Ethernet as an example.  The Native Service
   Processing (NSP) function defined in [RFC3985] differs with
   pseudowire type.  It is in the NSP function where information for a
   specific type of PW can be extracted for use in a Flow Label.
   Determining which fields to use for any given PW NSP is out of scope
   for this document.

2.4.5.4.  Fields Used in Entropy Label

   An Entropy Label is added at the ingress to an LSP.  The payload
   being encapsulated is most often MPLS, a PW, or IP.  The payload type
   is identified by the Layer 2 encapsulation (Ethernet, GFP, POS,
   etc.).

   If the payload is MPLS, then the information used to create an
   Entropy Label is the same information used for local load balancing
   (see Section 2.4.5.1).  This information MUST be extracted for use in
   generating an Entropy Label even if the LSR local egress interface is
   not a multipath.

   Of the non-MPLS payload types, only payloads that are forwarded are
   of interest.  For example, payloads using the Address Resolution
   Protocol (ARP) are not forwarded, and payloads using the
   Connectionless-mode Network Protocol (CLNP), which is used only for
   IS-IS, are not forwarded.

   The non-MPLS payload types of greatest interest are IPv4 and IPv6.
   The guidelines in Section 2.4.5.2 apply to fields used to create an
   Entropy Label.

   The IP tunneling protocols mentioned in Section 2.4.5.2 may be more
   applicable to generation of an Entropy Label at the edge or access
   where deep packet inspection is practical due to lower interface
   speeds than in the core where deep packet inspection may be
   impractical.

2.5.  MPLS-TP and UHP

   MPLS-TP introduces forwarding demands that will be extremely
   difficult to meet in a core network.  Most troublesome is the
   requirement for Ultimate Hop Popping (UHP), the opposite of
   Penultimate Hop Popping (PHP).  Using UHP opens the possibility of
   one or more MPLS pop operations plus an MPLS swap operation for each
   packet.  The potential for multiple lookups and multiple counter
   instances per packet exists.

   As networks grow and tunneling of LDP LSPs into RSVP-TE LSPs is used,
   and/or RSVP-TE hierarchy is used, the requirement to perform one or
   more MPLS pop operations plus an MPLS swap operation (and possibly a
   push or two) increases.  If MPLS-TP LM (link monitoring) OAM is
   enabled at each layer, then a packet and byte count MUST be
   maintained for each pop and swap operation so as to offer OAM for
   each layer.

2.6.  Local Delivery of Packets

   There are a number of situations in which packets are destined to a
   local address or where a return packet must be generated.  There is a
   need to mitigate the potential for outage as a result of either
   attacks on network infrastructure, or in some cases unintentional
   misconfiguration resulting in processor overload.  Some hardware
   assistance is needed for all traffic destined to the general-purpose
   CPU that is used in processing of the MPLS control protocol or the
   network management protocol and in most cases to other general-
   purpose CPUs residing on an LSR.  This is due to the ease of
   overwhelming such a processor with traffic arriving on LSR high-speed
   interfaces, whether the traffic is malicious or not.

   Denial of service (DoS) protection is an area requiring hardware
   support that is often overlooked or inadequately considered.
   Hardware assists are also needed for OAM, particularly the more
   demanding MPLS-TP OAM.

2.6.1.  DoS Protection

   Modern equipment supports a number of control-plane and management-
   plane protocols.  Generally, no single means of protecting network
   equipment from DoS attacks is sufficient, particularly for high-speed
   interfaces.  This problem is not specific to MPLS but is a topic that
   cannot be ignored when implementing or evaluating MPLS
   implementations.

   Two types of protections are often cited as the primary means of
   protecting against attacks of all kinds.

   Isolated Control/Management Traffic
       Control and management traffic can be carried out-of-band (OOB),
       meaning not intermixed with payload.  For MPLS, use of G-ACh and
       GAL to carry control and management traffic provides a means of
       isolation from potentially malicious payloads.  Used alone, the
       compromise of a single node, including a small computer at a
       network operations center, could compromise an entire network.
       Implementations that send all G-ACh/GAL traffic directly to a
       routing engine CPU are subject to DoS attack as a result of such
       a compromise.

   Cryptographic Authentication
       Cryptographic authentication can very effectively prevent
       malicious injection of control or management traffic.
       Cryptographic authentication can in some circumstances be subject
       to DoS attack by overwhelming the capacity of the decryption with
       a high volume of malicious traffic.  For very-low-speed
       interfaces, cryptographic authentication can be performed by the
       general-purpose CPU used as a routing engine.  For all other
       cases, cryptographic hardware may be needed.  For very-high-speed
       interfaces, even cryptographic hardware can be overwhelmed.

   Some control and management protocols are often carried with payload
   traffic.  This is commonly the case with BGP, T-LDP, and SNMP.  It is
   often the case with RSVP-TE.  Even when carried over G-ACh/GAL,
   additional measures can reduce the potential for a minor breach to be
   leveraged to a full network attack.

   Some of the additional protections are supported by hardware packet
   filtering.

   GTSM
       [RFC5082] defines a mechanism that uses the IPv4 TTL or IPv6 Hop
       Limit fields to ensure control traffic that can only originate
       from an immediate neighbor is not forged and is not originating
       from a distant source.  GTSM can be applied to many control
       protocols that are routable, for example, LDP [RFC6720].

   IP Filtering
       At the very minimum, packet filtering plus classification and use
       of multiple queues supporting rate limiting is needed for traffic
       that could potentially be sent to a general-purpose CPU used as a
       routing engine.  The first level of filtering only allows
       connections to be initiated from specific IP prefixes to specific
       destination ports and then preferably passes traffic directly to
       a cryptographic engine and/or rate limits.  The second level of
       filtering passes connected traffic, such as TCP connections
       having received at least one authenticated SYN or having been
       locally initiated.  The second level of filtering only passes
       traffic to specific address and port pairs to be checked for
       cryptographic authentication.

   The cryptographic authentication is generally the last resort in DoS
   attack mitigation.  If a packet must be first sent to a general-
   purpose CPU, then sent to a cryptographic engine, a DoS attack is
   possible on high-speed interfaces.  Only where hardware can fully
   process a cryptographic authentication without intervention from a
   general-purpose CPU (to find the authentication field and to identify
   the portion of packet to run the cryptographic algorithm over) is
   cryptographic authentication beneficial in protecting against DoS
   attacks.

   For chips supporting multiple 100 Gb/s interfaces, only a very large
   number of parallel cryptographic engines can provide the processing
   capacity to handle a large-scale DoS or distributed DoS (DDoS)
   attack.  For many forwarding chips, this much processing power
   requires significant chip real estate and power, and therefore
   reduces system space and power density.  For this reason,
   cryptographic authentication is not considered a viable first line of
   defense.

   For some networks, the first line of defense is some means of
   supporting OOB control and management traffic.  In the past, this OOB
   channel might make use of overhead bits in SONET or OTN or a
   dedicated DWDM wavelength.  G-ACh and GAL provide an alternative OOB
   mechanism that is independent of underlying layers.  In other
   networks, including most IP/MPLS networks, perimeter filtering serves
   a similar purpose, though it is less effective without extreme
   vigilance.

   A second line of defense is filtering, including GTSM.  For protocols
   such as EBGP, GTSM and other filtering are often the first line of
   defense.  Cryptographic authentication is usually the last line of
   defense and insufficient by itself to mitigate DoS or DDoS attacks.

2.6.2.  MPLS OAM

   [RFC4377] defines requirements for MPLS OAM that predate MPLS-TP.
   [RFC4379] defines what is commonly referred to as LSP Ping and LSP
   Traceroute.  [RFC4379] is updated by [RFC6424], which supports MPLS
   tunnels and stitched LSP and P2MP LSP.  [RFC4379] is updated by
   [RFC6425], which supports P2MP LSP.  [RFC4379] is updated by
   [RFC6426] to support MPLS-TP connectivity verification (CV) and route
   tracing.

   [RFC4950] extends the ICMP format to support TTL expiration that may
   occur when using IP Traceroute within an MPLS tunnel.  The ICMP
   message generation can be implemented in forwarding hardware, but if
   the ICMP packets are sent to a general-purpose CPU, this packet flow
   must be rate limited to avoid a potential DoS attack.

   [RFC5880] defines Bidirectional Forwarding Detection (BFD), a
   protocol intended to detect faults in the bidirectional path between
   two forwarding engines.  [RFC5884] and [RFC5885] define BFD for MPLS.
   BFD can provide failure detection on any kind of path between
   systems, including direct physical links, virtual circuits, tunnels,
   MPLS Label Switched Paths (LSPs), multihop routed paths, and
   unidirectional links as long as there is some return path.

   The processing requirements for BFD are less than for LSP Ping,
   making BFD somewhat better suited for relatively high-rate proactive
   monitoring.  BFD does not verify that the data plane matches the
   control plane, where LSP Ping does.  LSP Ping is somewhat better
   suited for on-demand monitoring including relatively low-rate
   periodic verification of the data plane and as a diagnostic tool.

   Hardware assistance is often provided for BFD response where BFD
   setup or parameter change is not involved and may be necessary for
   relatively high-rate proactive monitoring.  If both BFD and LSP Ping
   are recognized in filtering prior to passing traffic to a general-
   purpose CPU, appropriate DoS protection can be applied (see
   Section 2.6.1).  Failure to recognize BFD and LSP Ping and at least
   to rate limit creates the potential for misconfiguration to cause
   outages rather than cause errors in the misconfigured OAM.

2.6.3.  Pseudowire OAM

   Pseudowire OAM makes use of the control channel provided by Virtual
   Circuit Connectivity Verification (VCCV) [RFC5085].  VCCV makes use
   of the pseudowire Control Word.  BFD support over VCCV is defined by
   [RFC5885].  [RFC5885] is updated by [RFC6478] in support of static
   pseudowires.  [RFC4379] is updated by [RFC6829] to support LSP Ping
   for Pseudowire FEC advertised over IPv6.

   G-ACh/GAL (defined in [RFC5586]) is the preferred MPLS-TP OAM control
   channel and applies to any MPLS-TP endpoints, including pseudowire.
   See Section 2.6.4 for an overview of MPLS-TP OAM.

2.6.4.  MPLS-TP OAM

   [RFC6669] summarizes the MPLS-TP OAM toolset, the set of protocols
   supporting the MPLS-TP OAM requirements specified in [RFC5860] and
   supported by the MPLS-TP OAM framework defined in [RFC6371].

   The MPLS-TP OAM toolset includes:

   CC-CV
       [RFC6428] defines BFD extensions to support proactive Continuity
       Check and Connectivity Verification (CC-CV) applications.
       [RFC6426] provides LSP Ping extensions that are used to implement
       on-demand connectivity verification.

   RDI
       Remote Defect Indication (RDI) is triggered by failure of
       proactive CC-CV, which is BFD based.  For fast RDI, RDI SHOULD be
       initiated and handled by hardware if BFD is handled in forwarding
       hardware.  [RFC6428] provides an extension for BFD that includes
       the RDI in the BFD format and a specification of how this
       indication is to be used.

   Route Tracing
       [RFC6426] specifies that the LSP Ping enhancements for MPLS-TP
       on-demand connectivity verification include information on the
       use of LSP Ping for route tracing of an MPLS-TP path.

   Alarm Reporting
       [RFC6427] describes the details of a new protocol supporting
       Alarm Indication Signal (AIS), Link Down Indication (LDI), and
       fault management.  Failure to support this functionality in
       forwarding hardware can potentially result in failure to meet
       protection recovery time requirements; therefore, support of this
       functionality is strongly recommended.

   Lock Instruct
       Lock instruct is initiated on demand and therefore need not be
       implemented in forwarding hardware.  [RFC6435] defines a lock
       instruct protocol.

   Lock Reporting
       [RFC6427] covers lock reporting.  Lock reporting need not be
       implemented in forwarding hardware.

   Diagnostic
       [RFC6435] defines protocol support for loopback.  Loopback
       initiation is on demand and therefore need not be implemented in
       forwarding hardware.  Loopback of packet traffic SHOULD be
       implemented in forwarding hardware on high-speed interfaces.

   Packet Loss and Delay Measurement
       [RFC6374] and [RFC6375] define a protocol and profile for Packet
       Loss Measurement (LM) and Delay Measurement (DM).  LM requires a
       very accurate capture and insertion of packet and byte counters
       when a packet is transmitted and capture of packet and byte
       counters when a packet is received.  This capture and insertion
       MUST be implemented in forwarding hardware for LM OAM if high
       accuracy is needed.  DM requires very accurate capture and
       insertion of a timestamp on transmission and capture of timestamp
       when a packet is received.  This timestamp capture and insertion
       MUST be implemented in forwarding hardware for DM OAM if high
       accuracy is needed.

   See Section 2.6.2 for discussion of hardware support necessary for
   BFD and LSP Ping.

   CC-CV and alarm reporting is tied to protection and therefore SHOULD
   be supported in forwarding hardware in order to provide protection
   for a large number of affected LSPs within target response intervals.
   When using MPLS-TP, since CC-CV is supported by BFD, providing
   hardware assistance for BFD processing helps ensure that protection
   recovery time requirements can be met even for faults affecting a
   large number of LSPs.

   MPLS-TP Protection State Coordination (PSC) is defined by [RFC6378]
   and updated by [RFC7324], which corrects some errors in [RFC6378].

2.6.5.  MPLS OAM and Layer 2 OAM Interworking

   [RFC6670] provides the reasons for selecting a single MPLS-TP OAM
   solution and examines the consequences were ITU-T to develop a second
   OAM solution that is based on Ethernet encodings and mechanisms.

   [RFC6310] and [RFC7023] specify the mapping of defect states between
   many types of hardware Attachment Circuits (ACs) and associated
   pseudowires (PWs).  This functionality SHOULD be supported in
   forwarding hardware.

   It is beneficial if an MPLS OAM implementation can interwork with the
   underlying server layer and provide a means to interwork with a
   client layer.  For example, [RFC6427] specifies an inter-layer
   propagation of AIS and LDI from MPLS server layer to client MPLS
   layers.  Where the server layer uses a Layer 2 mechanism, such as
   Ethernet, PPP over SONET/SDH, or GFP over OTN, interwork among layers
   is also beneficial.  For high-speed interfaces, supporting this
   interworking in forwarding hardware helps ensure that protection
   based on this interworking can meet recovery time requirements even
   for faults affecting a large number of LSPs.

2.6.6.  Extent of OAM Support by Hardware

   Where certain requirements must be met, such as relatively high CC-CV
   rates and a large number of interfaces, or strict protection recovery
   time requirements and a moderate number of affected LSPs, some OAM
   functionality must be supported by forwarding hardware.  In other
   cases, such as highly accurate LM and DM OAM or strict protection
   recovery time requirements with a large number of affected LSPs, OAM
   functionality must be entirely implemented in forwarding hardware.

   Where possible, implementation in forwarding hardware should be in
   programmable hardware such that if standards are later changed or
   extended these changes are likely to be accommodated with hardware
   reprogramming rather than replacement.

   For some functionality, there is a strong case for an implementation
   in dedicated forwarding hardware.  Examples include packet and byte
   counters needed for LM OAM as well as needed for management
   protocols.  Similarly, the capture and insertion of packet and byte
   counts or timestamps needed for transmitted LM or DM or time
   synchronization packets MUST be implemented in forwarding hardware if
   high accuracy is required.

   For some functions, there is a strong case to provide limited support
   in forwarding hardware, but an external general-purpose processor may
   be used if performance criteria can be met.  For example, origination
   of RDI triggered by CC-CV, response to RDI, and Protection State
   Coordination (PSC) functionality may be supported by hardware, but
   expansion to a large number of client LSPs and transmission of AIS or
   RDI to the client LSPs may occur in a general-purpose processor.
   Some forwarding hardware supports one or more on-chip general-purpose
   processors that may be well suited for such a role.  [RFC7324], being

   a very recent document that affects a protection state machine that
   requires hardware support, underscores the importance of having a
   degree of programmability in forwarding hardware.

   The customer (system supplier or provider) should not dictate design,
   but should independently validate target functionality and
   performance.  However, it is not uncommon for service providers and
   system implementers to insist on reviewing design details (under a
   non-disclosure agreement) due to past experiences with suppliers and
   to reject suppliers who are unwilling to provide details.

2.6.7.  Support for IPFIX in Hardware

   The IPFIX architecture is defined by [RFC5470].  IPFIX supports per-
   flow statistics.  IPFIX information elements (IEs) are defined in
   [RFC7012] and include IEs for MPLS.

   The forwarding chips used in core routers are not optimized for high-
   touch applications like IPFIX.  Often, support for IPFIX in core
   routers is limited to optional IPFIX metering, which involves a
   1-in-N packet sampling, limited filtering support, and redirection to
   either an internal CPU or an external interface.  The CPU or device
   at the other end of the external interface then implements the full
   IPFIX filtering and IPFIX collector functionality.

   LSRs that are intended to be deployed further from the core may
   support lower-capacity interfaces but support higher-touch
   applications on the forwarding hardware and may provide dedicated
   hardware to support a greater subset of IPFIX functionality before
   handing off to a general-purpose CPU.  In some cases, far from the
   core the entire IPFIX functionality up to and including the collector
   may be implemented in hardware and firmware in the forwarding
   silicon.  It is also worth noting that at lower speeds a general-
   purpose CPU may become adequate to implement IPFIX, particularly if
   metering is used.

2.7.  Number and Size of Flows

   Service provider networks may carry up to hundreds of millions of
   flows on 10 Gb/s links.  Most flows are very short lived, many under
   a second.  A subset of the flows are low capacity and somewhat long
   lived.  When Internet traffic dominates capacity, a very small subset
   of flows are high capacity and/or very long lived.

   Two types of limitations with regard to number and size of flows have
   been observed.

   1.  Some hardware cannot handle some high-capacity flows because of
       internal paths that are limited, such as per-packet backplane
       paths or paths internal or external to chips such as buffer
       memory paths.  Such designs can handle aggregates of smaller
       flows.  Some hardware with acknowledged limitations has been
       successfully deployed but may be increasingly problematic if the
       capacity of large microflows in deployed networks continues to
       grow.

   2.  Some hardware approaches cannot handle a large number of flows,
       or a large number of large flows, due to attempting to count per
       flow, rather than deal with aggregates of flows.  Hash techniques
       scale with regard to number of flows due to a fixed hash size
       with many flows falling into the same hash bucket.  Techniques
       that identify individual flows have been implemented but have
       never successfully deployed for Internet traffic.

3.  Questions for Suppliers

   The following questions should be asked of a supplier.  These
   questions are grouped into broad categories and are intended to be
   open-ended questions to the supplier.  The tests in Section 4 are
   intended to verify whether the supplier disclosed any compliance or
   performance limitations completely and accurately.

3.1.  Basic Compliance

   Q#1   Can the implementation forward packets with an arbitrarily
         large stack depth?  What limitations exist, and under what
         circumstances do further limitations come into play (such as
         high packet rate or specific features enabled or specific types
         of packet processing)?  See Section 2.1.

   Q#2   Is the entire set of basic MPLS functionality described in
         Section 2.1 supported?

   Q#3   Is the set of MPLS special-purpose labels handled correctly and
         with adequate performance?  Are extended special-purpose labels
         handled correctly and with adequate performance?  See
         Section 2.1.1.

   Q#4   Are mappings of label value and TC to PHB handled correctly,
         including L-LSP mappings (RFC 3270) and CT mappings (RFC 4124)
         to PHB?  See Section 2.1.2.

   Q#5   Is time synchronization adequately supported in forwarding
         hardware?

         A.  Are both PTP and NTP formats supported?

         B.  Is the accuracy of timestamp insertion and incoming
             stamping sufficient?

         See Section 2.1.3.

   Q#6   Is link bundling supported?

         A.  Can an LSP be pinned to specific components?

         B.  Is the "all-ones" component link supported?

         See Section 2.1.5.

   Q#7   Is MPLS hierarchy supported?

         A.  Are both PHP and UHP supported?  What limitations exist on
             the number of pop operations with UHP?

         B.  Are the pipe, short-pipe, and uniform models supported?
             Are TTL and TC values updated correctly at egress where
             applicable?

         See Section 2.1.6 regarding MPLS hierarchy.  See [RFC3443]
         regarding PHP, UHP, and pipe, short-pipe, and uniform models.

   Q#8   Is FRR supported?

         A.  Are both "One-to-One Backup" and "Facility Backup"
             supported?

         B.  What forms of IP/LDP FRR are supported?

         C.  How quickly does protection recovery occur?

         D.  Does protection recovery speed increase when a fault
             affects a large number of protected LSPs?  And if so, by
             how much?

         See Section 2.1.7.

   Q#9   Are pseudowire Sequence Numbers handled correctly?  See
         Section 2.1.8.1.

   Q#10  Is VPN LER functionality handled correctly and without
         performance issues?  See Section 2.1.9.

   Q#11  Is MPLS multicast (P2MP and MP2MP) handled correctly?

         A.  Are packets dropped on uncongested outputs if some outputs
             are congested?

         B.  Is performance limited in high-fanout situations?

         See Section 2.2.

3.2.  Basic Performance

   Q#12 Can very small packets be forwarded at full line rate on all
        interfaces indefinitely?  What limitations exist?  And under
        what circumstances do further limitations come into play (such
        as specific features enabled or specific types of packet
        processing)?

   Q#13 Customers must decide whether to relax the prior requirement and
        to what extent.  If the answer to the prior question indicates
        that limitations exist, then:

        A.  What is the smallest packet size where full line rate
            forwarding can be supported?

        B.  What is the longest burst of full-rate small packets that
            can be supported?

        Specify circumstances (such as specific features enabled or
        specific types of packet processing) that often impact these
        rates and burst sizes.

   Q#14 How many pop operations can be supported along with a swap
        operation at full line rate while maintaining per-LSP packet and
        byte counts for each pop and swap?  This requirement is
        particularly relevant for MPLS-TP.

   Q#15 How many label push operations can be supported.  While this
        limitation is rarely an issue, it applies to both PHP and UHP,
        unlike the pop limit that applies to UHP.

   Q#16 For a worst case where all packets arrive on one LSP, what is
        the counter overflow time?  Are any means provided to avoid
        polling all counters at short intervals?  This applies to both
        MPLS and MPLS-TP.

3.3.  Multipath Capabilities and Performance

   Multipath capabilities and performance do not apply to MPLS-TP, but
   they apply to MPLS and apply if MPLS-TP is carried in MPLS.

   Q#17 How are large microflows accommodated?  Is there active
        management of the hash space mapping to output ports?  See
        Section 2.4.2.

   Q#18 How many MPLS labels can be included in a hash based on the MPLS
        label stack?

   Q#19 Is packet rate performance decreased beyond some number of
        labels?

   Q#20 Can the IP header and payload information below the MPLS stack
        be used in the hash?  If so, which IP fields, payload types, and
        payload fields are supported?

   Q#21 At what maximum MPLS label stack depth can Bottom of Stack and
        an IP header appear without impacting packet rate performance?

   Q#22 Are special-purpose labels excluded from the label stack hash?
        Are extended special-purpose labels excluded from the label
        stack hash?  See Section 2.4.5.1.

   Q#23 How is multipath performance affected by high-capacity flows, an
        extremely large number of flows, or very short-lived flows?  See
        Section 2.7.

3.4.  Pseudowire Capabilities and Performance

   Q#24 Is the pseudowire Control Word supported?

   Q#25 What is the maximum rate of pseudowire encapsulation and
        decapsulation?  Apply the same questions as in Section 3.2
        ("Basic Performance") for any packet-based pseudowire, such as
        IP VPN or Ethernet.

   Q#26 Does inclusion of a pseudowire Control Word impact performance?

   Q#27 Are Flow Labels supported?

   Q#28 If so, what fields are hashed on for the Flow Label for
        different types of pseudowires?

   Q#29 Does inclusion of a Flow Label impact performance?

3.5.  Entropy Label Support and Performance

   Q#30 Can an Entropy Label be added when acting as an ingress LER, and
        can it be removed when acting as an egress LER?

   Q#31 If an Entropy Label can be added, what fields are hashed on for
        the Entropy Label?

   Q#32 Does adding or removing an Entropy Label impact packet rate
        performance?

   Q#33 Can an Entropy Label be detected in the label stack, used in the
        hash, and properly terminate the search for further information
        to hash on?

   Q#34 Does using an Entropy Label have any negative impact on
        performance?  It should have no impact or a positive impact.

3.6.  DoS Protection

   Q#35 For each control- and management-plane protocol in use, what
        measures are taken to provide DoS attack hardening?

   Q#36 Have DoS attack tests been performed?

   Q#37 Can compromise of an internal computer on a management subnet be
        leveraged for any form of attack including DoS attack?

3.7.  OAM Capabilities and Performance

   Q#38 What OAM proactive and on-demand mechanisms are supported?

   Q#39 What performance limits exist under high proactive monitoring
        rates?

   Q#40 Can excessively high proactive monitoring rates impact control-
        plane performance or cause control-plane instability?

   Q#41 Ask the prior questions for each of the following.

        A.  MPLS OAM

        B.  Pseudowire OAM

        C.  MPLS-TP OAM

        D.  Layer 2 OAM Interworking

        See Section 2.6.

4.  Forwarding Compliance and Performance Testing

   Packet rate performance of equipment supporting a large number of 10
   Gb/s or 100 Gb/s links is not possible using desktop computers or
   workstations.  The use of high-end workstations as a source of test
   traffic was barely viable 20 years ago but is no longer at all
   viable.  Though custom microcode has been used on specialized router
   forwarding cards to serve the purpose of generating test traffic and
   measuring it, for the most part, performance testing will require
   specialized test equipment.  There are multiple sources of suitable
   equipment.

   The set of tests listed here do not correspond one-to-one to the set
   of questions in Section 3.  The same categorization is used, and
   these tests largely serve to validate answers provided to the prior
   questions.  They can also provide answers where a supplier is
   unwilling to disclose compliance or performance.

   Performance testing is the domain of the IETF Benchmark Methodology
   Working Group (BMWG).  Below are brief descriptions of conformance
   and performance tests.  Some very basic tests, specified in
   [RFC5695], partially cover only the basic performance test T#3.

   The following tests should be performed by the systems designer or
   deployer; or, if it is not practical for the potential customer to
   perform the tests directly, they may be performed by the supplier on
   their behalf.  These tests are grouped into broad categories.

   The tests in Section 4.1 should be repeated under various conditions
   to retest basic performance when critical capabilities are enabled.
   Complete repetition of the performance tests enabling each capability
   and combinations of capabilities would be very time intensive;
   therefore, a reduced set of performance tests can be used to gauge
   the impact of enabling specific capabilities.

4.1.  Basic Compliance

   T#1  Test forwarding at a high rate for packets with varying number
        of label entries.  While packets with more than a dozen label
        entries are unlikely to be used in any practical scenario today,
        it is useful to know if limitations exists.

   T#2  For each of the questions listed under "Basic Compliance" in
        Section 3, verify the claimed compliance.  For any functionality
        considered critical to a deployment, the applicable performance
        using each capability under load should be verified in addition
        to basic compliance.

4.2.  Basic Performance

   T#3  Test packet forwarding at full line rate with small packets.
        See [RFC5695].  The most likely case to fail is the smallest
        packet size.  Also, test with packet sizes in 4-byte increments
        ranging from payload sizes of 40 to 128 bytes.

   T#4  If the prior tests did not succeed for all packet sizes, then
        perform the following tests.

        A.  Increase the packet size by 4 bytes until a size is found
            that can be forwarded at full rate.

        B.  Inject bursts of consecutive small packets into a stream of
            larger packets.  Allow some time for recovery between
            bursts.  Increase the number of packets in the burst until
            packets are dropped.

   T#5  Send test traffic where a swap operation is required.  Also set
        up multiple LSPs carried over other LSPs where the device under
        test (DUT) is the egress of these LSPs.  Create test packets
        such that the swap operation is performed after pop operations,
        increasing the number of pop operations until forwarding of
        small packets at full line rate can no longer be supported.
        Also, check to see how many pop operations can be supported
        before the full set of counters can no longer be maintained.
        This requirement is particularly relevant for MPLS-TP.

   T#6  Send all traffic on one LSP and see if the counters become
        inaccurate.  Often, counters on silicon are much smaller than
        the 64-bit packet and byte counters in various IETF MIBs.
        System developers should consider what counter polling rate is
        necessary to maintain accurate counters and whether those
        polling rates are practical.  Relevant MIBs for MPLS are
        discussed in [RFC4221] and [RFC6639].

   T#7  [RFC6894] provides a good basis for MPLS FRR testing.  Similar
        testing should be performed to determine restoration times;
        however, this testing is far more difficult to perform due to
        the need for a simulated test topology that is capable of
        simulating the signaling used in restoration.  The simulated
        topology should be comparable with the target deployment in the

        number of nodes and links and in resource usage flooding and
        setup delays.  Some commercial test equipment can support this
        type of testing.

4.3.  Multipath Capabilities and Performance

   Multipath capabilities do not apply to MPLS-TP but apply to MPLS and
   apply if MPLS-TP is carried in MPLS.

   T#8  Send traffic at a rate well exceeding the capacity of a single
        multipath component link, and where entropy exists only below
        the top of stack.  If only the top label is used, this test will
        fail immediately.

   T#9  Move the labels with entropy down in the stack until either the
        full forwarding rate can no longer be supported or most or all
        packets try to use the same component link.

   T#10 Repeat the two tests above with the entropy contained in IP
        headers or IP payload fields below the label stack rather than
        in the label stack.  Test with the set of IP headers or IP
        payload fields considered relevant to the deployment or to the
        target market.

   T#11 Determine whether traffic that contains a pseudowire Control
        Word is interpreted as IP traffic.  Information in the payload
        MUST NOT be used in the load balancing if the first nibble of
        the packet is not 4 or 6 (IPv4 or IPv6).

   T#12 Determine whether special-purpose labels and extended special-
        purpose labels are excluded from the label stack hash.  They
        MUST be excluded.

   T#13 Perform testing in the presence of combinations of:

        A.  Very large microflows.

        B.  Relatively short-lived high-capacity flows.

        C.  Extremely large numbers of flows.

        D.  Very short-lived small flows.

4.4.  Pseudowire Capabilities and Performance

   T#14 Ensure that pseudowire can be set up with a pseudowire label and
        pseudowire Control Word added at ingress and the pseudowire
        label and pseudowire Control Word removed at egress.

   T#15 For pseudowire that contains variable-length payload packets,
        repeat performance tests listed under "Basic Performance" for
        pseudowire ingress and egress functions.

   T#16 Repeat pseudowire performance tests with and without a
        pseudowire Control Word.

   T#17 Determine whether pseudowire can be set up with a pseudowire
        label, Flow Label, and pseudowire Control Word added at ingress
        and the pseudowire label, Flow Label, and pseudowire Control
        Word removed at egress.

   T#18 Determine which payload fields are used to create the Flow Label
        and whether the set of fields and algorithm provide sufficient
        entropy for load balancing.

   T#19 Repeat pseudowire performance tests with Flow Labels included.

4.5.  Entropy Label Support and Performance

   T#20 Determine whether Entropy Labels can be added at ingress and
        removed at egress.

   T#21 Determine which fields are used to create an Entropy Label.
        Labels further down in the stack, including Entropy Labels
        further down and IP headers or IP payload fields where
        applicable, should be used.  Determine whether the set of fields
        and algorithm provide sufficient entropy for load balancing.

   T#22 Repeat performance tests under "Basic Performance" when Entropy
        Labels are used, where ingress or egress is the device under
        test (DUT).

   T#23 Determine whether an ELI is detected when acting as a midpoint
        LSR and whether the search for further information on which to
        base the load balancing is used.  Information below the Entropy
        Label SHOULD NOT be used.

   T#24 Ensure that the Entropy Label indicator and Entropy Label (ELI
        and EL) are removed from the label stack during UHP and PHP
        operations.

   T#25 Ensure that operations on the TC field when adding and removing
        Entropy Label are correctly carried out.  If TC is changed
        during a swap operation, the ability to transfer that change
        MUST be provided.  The ability to suppress the transfer of TC
        MUST also be provided.  See the pipe, short-pipe, and uniform
        models in [RFC3443].

   T#26 Repeat performance tests for a midpoint LSR with Entropy Labels
        found at various label stack depths.

4.6.  DoS Protection

   T#27 Actively attack LSRs under high protocol churn load and
        determine control-plane performance impact or successful DoS
        under test conditions.  Specifically, test for the following.

        A.  TCP SYN attack against control-plane and management-plane
            protocols using TCP, including CLI access (typically SSH-
            protected login), NETCONF, etc.

        B.  High traffic volume attack against control-plane and
            management-plane protocols not using TCP.

        C.  Attacks that can be performed from a compromised management
            subnet computer, but not one with authentication keys.

        D.  Attacks that can be performed from a compromised peer within
            the control plane (internal domain and external domain).
            Assume that keys that are per peering and keys that are per
            router ID, rather than network-wide keys, are in use.

        See Section 2.6.1.

4.7.  OAM Capabilities and Performance

   T#28 Determine maximum sustainable rates of BFD traffic.  If BFD
        requires CPU intervention, determine both maximum rates and CPU
        loading when multiple interfaces are active.

   T#29 Verify LSP Ping and LSP Traceroute capability.

   T#30 Determine maximum rates of MPLS-TP CC-CV traffic.  If CC-CV
        requires CPU intervention, determine both maximum rates and CPU
        loading when multiple interfaces are active.

   T#31 Determine MPLS-TP DM precision.

   T#32 Determine MPLS-TP LM accuracy.

   T#33 Verify MPLS-TP AIS/RDI and Protection State Coordination (PSC)
        functionality, protection speed, and AIS/RDI notification speed
        when a large number of Maintenance Entities (MEs) must be
        notified with AIS/RDI.

5.  Security Considerations

   This document reviews forwarding behavior specified elsewhere and
   points out compliance and performance requirements.  As such, it
   introduces no new security requirements or concerns.

   Discussion of hardware support and other equipment hardening against
   DoS attack can be found in Section 2.6.1.  Section 3.6 provides a
   list of questions regarding DoS to be asked of suppliers.
   Section 4.6 suggests types of testing that can provide some assurance
   of the effectiveness of a supplier's claims about DoS hardening.

   Knowledge of potential performance shortcomings may serve to help new
   implementations avoid pitfalls.  It is unlikely that such knowledge
   could be the basis of new denial of service, as these pitfalls are
   already widely known in the service provider community and among
   leading equipment suppliers.  In practice, extreme data and packet
   rates are needed to affect existing equipment and to affect networks
   that may be still vulnerable due to failure to implement adequate
   protection.  The extreme data and packet rates make this type of
   denial of service unlikely and make undetectable denial of service of
   this type impossible.

   Each normative reference contains security considerations.  A brief
   summarization of MPLS security considerations applicable to
   forwarding follows:

   1.   MPLS encapsulation does not support an authentication extension.
        This is reflected in the security section of [RFC3032].
        Documents that clarify MPLS header fields such as TTL [RFC3443],
        the explicit null label [RFC4182], renaming EXP to TC [RFC5462],
        ECN for MPLS [RFC5129], and MPLS Ethernet encapsulation
        [RFC5332] make no changes to security considerations in
        [RFC3032].

   2.   Some cited RFCs are related to Diffserv forwarding.  [RFC3270]
        refers to MPLS and Diffserv security.  [RFC2474] mentions theft
        of service and denial of service due to mismarking.  [RFC2474]
        mentions IPsec interaction, but with MPLS, not being carried by
        IP, the type of interaction in [RFC2474] is not relevant.

   3.   [RFC3209] is cited here due only to make-before-break forwarding
        requirements.  This is related to resource sharing and the
        theft-of-service and denial-of-service concerns in [RFC2474]
        apply.

   4.   [RFC4090] defines FRR, which provides protection but does not
        add security concerns.  RFC 4201 defines link bundling but
        raises no additional security concerns.

   5.   Various OAM control channels are defined in [RFC4385] (PW CW),
        [RFC5085] (VCCV), and [RFC5586] (G-Ach and GAL).  These
        documents describe potential abuse of these OAM control
        channels.

   6.   [RFC4950] defines ICMP extensions when MPLS TTL expires and the
        payload is IP.  This provides MPLS header information that is of
        no use to an IP attacker, but sending this information can be
        suppressed through configuration.

   7.   GTSM [RFC5082] provides a means to improve protection against
        high traffic volume spoofing as a form of DoS attack.

   8.   BFD [RFC5880] [RFC5884] [RFC5885] provides a form of OAM used in
        MPLS and MPLS-TP.  The security considerations related to the
        OAM control channel are relevant.  The BFD payload supports
        authentication.  The MPLS encapsulation, the MPLS control
        channel, or the PW control channel, which BFD may be carried in,
        do not support authentication.  Where an IP return OAM path is
        used, IPsec is suggested as a means of securing the return path.

   9.   Other forms of OAM are supported by [RFC6374] [RFC6375] (Loss
        and Delay Measurement), [RFC6428] (Continuity Check/Verification
        based on BFD), and [RFC6427] (Fault Management).  The security
        considerations related to the OAM control channel are relevant.
        IP return paths, where used, can be secured with IPsec.

   10.  Linear protection is defined by [RFC6378] and updated by
        [RFC7324].  Security concerns related to MPLS encapsulation and
        OAM control channels apply.  Security concerns reiterate
        [RFC5920] as applied to protection switching.

   11.  The PW Flow Label [RFC6391] and MPLS Entropy Label [RFC6790]
        affect multipath load balancing.  Security concerns reiterate
        [RFC5920].  Security impacts would be limited to load
        distribution.

   MPLS security including data-plane security is discussed in greater
   detail in [RFC5920] (MPLS/GMPLS Security Framework).  The MPLS-TP
   security framework [RFC6941] builds upon this, focusing largely on
   the MPLS-TP OAM additions and OAM channels with some attention given
   to using network management in place of control-plane setup.  In both
   security framework documents, MPLS is assumed to run within a
   "trusted zone", defined as being where a single service provider has
   total operational control over that part of the network.

   If control-plane security and management-plane security are
   sufficiently robust, compromise of a single network element may
   result in chaos in the data plane anywhere in the network through
   denial-of-service attacks, but not a Byzantine security failure in
   which other network elements are fully compromised.

   MPLS security, or lack thereof, can affect whether traffic can be
   misrouted and lost, or intercepted, or intercepted and reinserted (a
   man-in-the-middle attack), or spoofed.  End-user applications,
   including control-plane and management-plane protocols used by the
   service provider, are expected to make use of appropriate end-to-end
   authentication and, where appropriate, end-to-end encryption.

6.  Organization of References Section

   The References section is split into Normative and Informative
   subsections.  References that directly specify forwarding
   encapsulations or behaviors are listed as normative.  References that
   describe signaling only, though normative with respect to signaling,
   are listed as informative.  They are informative with respect to MPLS
   forwarding.

7.  References

7.1.  Normative References

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

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, January 2001.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, December 2001.

   [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, May 2002.

   [RFC3443]  Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
              in Multi-Protocol Label Switching (MPLS) Networks", RFC
              3443, January 2003.

   [RFC4090]  Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
              Extensions to RSVP-TE for LSP Tunnels", RFC 4090, May
              2005.

   [RFC4182]  Rosen, E., "Removing a Restriction on the use of MPLS
              Explicit NULL", RFC 4182, September 2005.

   [RFC4201]  Kompella, K., Rekhter, Y., and L. Berger, "Link Bundling
              in MPLS Traffic Engineering (TE)", RFC 4201, October 2005.

   [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, February 2006.

   [RFC4950]  Bonica, R., Gan, D., Tappan, D., and C. Pignataro, "ICMP
              Extensions for Multiprotocol Label Switching", RFC 4950,
              August 2007.

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

   [RFC5085]  Nadeau, T. and C. Pignataro, "Pseudowire Virtual Circuit
              Connectivity Verification (VCCV): A Control Channel for
              Pseudowires", RFC 5085, December 2007.

   [RFC5129]  Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
              Marking in MPLS", RFC 5129, January 2008.

   [RFC5332]  Eckert, T., Rosen, E., Aggarwal, R., and Y. Rekhter, "MPLS
              Multicast Encapsulations", RFC 5332, August 2008.

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

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

   [RFC5884]  Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
              "Bidirectional Forwarding Detection (BFD) for MPLS Label
              Switched Paths (LSPs)", RFC 5884, June 2010.

   [RFC5885]  Nadeau, T. and C. Pignataro, "Bidirectional Forwarding
              Detection (BFD) for the Pseudowire Virtual Circuit
              Connectivity Verification (VCCV)", RFC 5885, June 2010.

   [RFC6374]  Frost, D. and S. Bryant, "Packet Loss and Delay
              Measurement for MPLS Networks", RFC 6374, September 2011.

   [RFC6375]  Frost, D. and S. Bryant, "A Packet Loss and Delay
              Measurement Profile for MPLS-Based Transport Networks",
              RFC 6375, September 2011.

   [RFC6378]  Weingarten, Y., Bryant, S., Osborne, E., Sprecher, N., and
              A. Fulignoli, "MPLS Transport Profile (MPLS-TP) Linear
              Protection", RFC 6378, October 2011.

   [RFC6391]  Bryant, S., Filsfils, C., Drafz, U., Kompella, V., Regan,
              J., and S. Amante, "Flow-Aware Transport of Pseudowires
              over an MPLS Packet Switched Network", RFC 6391, November
              2011.

   [RFC6427]  Swallow, G., Fulignoli, A., Vigoureux, M., Boutros, S.,
              and D. Ward, "MPLS Fault Management Operations,
              Administration, and Maintenance (OAM)", RFC 6427, November
              2011.

   [RFC6428]  Allan, D., Swallow Ed. , G., and J. Drake Ed. , "Proactive
              Connectivity Verification, Continuity Check, and Remote
              Defect Indication for the MPLS Transport Profile", RFC
              6428, November 2011.

   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              RFC 6790, November 2012.

   [RFC7324]  Osborne, E., "Updates to MPLS Transport Profile Linear
              Protection", RFC 7324, July 2014.

7.2.  Informative References

   [ACK-compression]
              Zhang, L., Shenker, S., and D. Clark, "Observations and
              Dynamics of a Congestion Control Algorithm: The Effects of
              Two-Way Traffic", Proc. ACM SIGCOMM, ACM Computer
              Communications Review (CCR) Vol. 21, No. 4, pp. 133-147.,
              1991.

   [MPLS-IN-UDP]
              Xu, X., Sheth, N., Yong, L., Pignataro, C., and F.
              Yongbing, "Encapsulating MPLS in UDP", Work in Progress,
              January 2014.

   [MRT]      Atlas, A., Kebler, R., Bowers, C., Envedi, G., Csaszar,
              A., Tantsura, J., Konstantynowicz, M., and R. White, "An
              Architecture for IP/LDP Fast-Reroute Using Maximally
              Redundant Trees", Work in Progress, July 2014.

   [REMOTE-LFA]
              Bryant, S., Filsfils, C., Previdi, S., Shand, M., and S.
              Ning, "Remote LFA FRR", Work in Progress, May 2014.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
              1981.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474, December
              1998.

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, December 1998.

   [RFC2597]  Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
              "Assured Forwarding PHB Group", RFC 2597, June 1999.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031, January 2001.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP", RFC
              3168, September 2001.

   [RFC3429]  Ohta, H., "Assignment of the 'OAM Alert Label' for
              Multiprotocol Label Switching Architecture (MPLS)
              Operation and Maintenance (OAM) Functions", RFC 3429,
              November 2002.

   [RFC3471]  Berger, L., "Generalized Multi-Protocol Label Switching
              (GMPLS) Signaling Functional Description", RFC 3471,
              January 2003.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3828]  Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., and
              G. Fairhurst, "The Lightweight User Datagram Protocol
              (UDP-Lite)", RFC 3828, July 2004.

   [RFC3985]  Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
              Edge (PWE3) Architecture", RFC 3985, March 2005.

   [RFC4023]  Worster, T., Rekhter, Y., and E. Rosen, "Encapsulating
              MPLS in IP or Generic Routing Encapsulation (GRE)", RFC
              4023, March 2005.

   [RFC4110]  Callon, R. and M. Suzuki, "A Framework for Layer 3
              Provider-Provisioned Virtual Private Networks (PPVPNs)",
              RFC 4110, July 2005.

   [RFC4124]  Le Faucheur, F., "Protocol Extensions for Support of
              Diffserv-aware MPLS Traffic Engineering", RFC 4124, June
              2005.

   [RFC4206]  Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
              Hierarchy with Generalized Multi-Protocol Label Switching
              (GMPLS) Traffic Engineering (TE)", RFC 4206, October 2005.

   [RFC4221]  Nadeau, T., Srinivasan, C., and A. Farrel, "Multiprotocol
              Label Switching (MPLS) Management Overview", RFC 4221,
              November 2005.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340, March 2006.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, February 2006.

   [RFC4377]  Nadeau, T., Morrow, M., Swallow, G., Allan, D., and S.
              Matsushima, "Operations and Management (OAM) Requirements
              for Multi-Protocol Label Switched (MPLS) Networks", RFC
              4377, February 2006.

   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
              Label Switched (MPLS) Data Plane Failures", RFC 4379,
              February 2006.

   [RFC4664]  Andersson, L. and E. Rosen, "Framework for Layer 2 Virtual
              Private Networks (L2VPNs)", RFC 4664, September 2006.

   [RFC4817]  Townsley, M., Pignataro, C., Wainner, S., Seely, T., and
              J. Young, "Encapsulation of MPLS over Layer 2 Tunneling
              Protocol Version 3", RFC 4817, March 2007.

   [RFC4875]  Aggarwal, R., Papadimitriou, D., and S. Yasukawa,
              "Extensions to Resource Reservation Protocol - Traffic
              Engineering (RSVP-TE) for Point-to-Multipoint TE Label
              Switched Paths (LSPs)", RFC 4875, May 2007.

   [RFC4928]  Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal
              Cost Multipath Treatment in MPLS Networks", BCP 128, RFC
              4928, June 2007.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol", RFC
              4960, September 2007.

   [RFC5036]  Andersson, L., Minei, I., and B. Thomas, "LDP
              Specification", RFC 5036, October 2007.

   [RFC5286]  Atlas, A. and A. Zinin, "Basic Specification for IP Fast
              Reroute: Loop-Free Alternates", RFC 5286, September 2008.

   [RFC5317]  Bryant, S. and L. Andersson, "Joint Working Team (JWT)
              Report on MPLS Architectural Considerations for a
              Transport Profile", RFC 5317, February 2009.

   [RFC5462]  Andersson, L. and R. Asati, "Multiprotocol Label Switching
              (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
              Class" Field", RFC 5462, February 2009.

   [RFC5470]  Sadasivan, G., Brownlee, N., Claise, B., and J. Quittek,
              "Architecture for IP Flow Information Export", RFC 5470,
              March 2009.

   [RFC5640]  Filsfils, C., Mohapatra, P., and C. Pignataro, "Load-
              Balancing for Mesh Softwires", RFC 5640, August 2009.

   [RFC5695]  Akhter, A., Asati, R., and C. Pignataro, "MPLS Forwarding
              Benchmarking Methodology for IP Flows", RFC 5695, November
              2009.

   [RFC5704]  Bryant, S., Morrow, M., and IAB, "Uncoordinated Protocol
              Development Considered Harmful", RFC 5704, November 2009.

   [RFC5714]  Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC
              5714, January 2010.

   [RFC5715]  Shand, M. and S. Bryant, "A Framework for Loop-Free
              Convergence", RFC 5715, January 2010.

   [RFC5860]  Vigoureux, M., Ward, D., and M. Betts, "Requirements for
              Operations, Administration, and Maintenance (OAM) in MPLS
              Transport Networks", RFC 5860, May 2010.

   [RFC5905]  Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
              Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, June 2010.

   [RFC5920]  Fang, L., "Security Framework for MPLS and GMPLS
              Networks", RFC 5920, July 2010.

   [RFC6291]  Andersson, L., van Helvoort, H., Bonica, R., Romascanu,
              D., and S. Mansfield, "Guidelines for the Use of the "OAM"
              Acronym in the IETF", BCP 161, RFC 6291, June 2011.

   [RFC6310]  Aissaoui, M., Busschbach, P., Martini, L., Morrow, M.,
              Nadeau, T., and Y(J). Stein, "Pseudowire (PW) Operations,
              Administration, and Maintenance (OAM) Message Mapping",
              RFC 6310, July 2011.

   [RFC6371]  Busi, I. and D. Allan, "Operations, Administration, and
              Maintenance Framework for MPLS-Based Transport Networks",
              RFC 6371, September 2011.

   [RFC6388]  Wijnands, IJ., Minei, I., Kompella, K., and B. Thomas,
              "Label Distribution Protocol Extensions for Point-to-
              Multipoint and Multipoint-to-Multipoint Label Switched
              Paths", RFC 6388, November 2011.

   [RFC6424]  Bahadur, N., Kompella, K., and G. Swallow, "Mechanism for
              Performing Label Switched Path Ping (LSP Ping) over MPLS
              Tunnels", RFC 6424, November 2011.

   [RFC6425]  Saxena, S., Swallow, G., Ali, Z., Farrel, A., Yasukawa,
              S., and T. Nadeau, "Detecting Data-Plane Failures in
              Point-to-Multipoint MPLS - Extensions to LSP Ping", RFC
              6425, November 2011.

   [RFC6426]  Gray, E., Bahadur, N., Boutros, S., and R. Aggarwal, "MPLS
              On-Demand Connectivity Verification and Route Tracing",
              RFC 6426, November 2011.

   [RFC6435]  Boutros, S., Sivabalan, S., Aggarwal, R., Vigoureux, M.,
              and X. Dai, "MPLS Transport Profile Lock Instruct and
              Loopback Functions", RFC 6435, November 2011.

   [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
              for Equal Cost Multipath Routing and Link Aggregation in
              Tunnels", RFC 6438, November 2011.

   [RFC6478]  Martini, L., Swallow, G., Heron, G., and M. Bocci,
              "Pseudowire Status for Static Pseudowires", RFC 6478, May
              2012.

   [RFC6639]  King, D. and M. Venkatesan, "Multiprotocol Label Switching
              Transport Profile (MPLS-TP) MIB-Based Management
              Overview", RFC 6639, June 2012.

   [RFC6669]  Sprecher, N. and L. Fang, "An Overview of the Operations,
              Administration, and Maintenance (OAM) Toolset for MPLS-
              Based Transport Networks", RFC 6669, July 2012.

   [RFC6670]  Sprecher, N. and KY. Hong, "The Reasons for Selecting a
              Single Solution for MPLS Transport Profile (MPLS-TP)
              Operations, Administration, and Maintenance (OAM)", RFC
              6670, July 2012.

   [RFC6720]  Pignataro, C. and R. Asati, "The Generalized TTL Security
              Mechanism (GTSM) for the Label Distribution Protocol
              (LDP)", RFC 6720, August 2012.

   [RFC6829]  Chen, M., Pan, P., Pignataro, C., and R. Asati, "Label
              Switched Path (LSP) Ping for Pseudowire Forwarding
              Equivalence Classes (FECs) Advertised over IPv6", RFC
              6829, January 2013.

   [RFC6894]  Papneja, R., Vapiwala, S., Karthik, J., Poretsky, S., Rao,
              S., and JL. Le Roux, "Methodology for Benchmarking MPLS
              Traffic Engineered (MPLS-TE) Fast Reroute Protection", RFC
              6894, March 2013.

   [RFC6941]  Fang, L., Niven-Jenkins, B., Mansfield, S., and R.
              Graveman, "MPLS Transport Profile (MPLS-TP) Security
              Framework", RFC 6941, April 2013.

   [RFC6981]  Bryant, S., Previdi, S., and M. Shand, "A Framework for IP
              and MPLS Fast Reroute Using Not-Via Addresses", RFC 6981,
              August 2013.

   [RFC7012]  Claise, B. and B. Trammell, "Information Model for IP Flow
              Information Export (IPFIX)", RFC 7012, September 2013.

   [RFC7023]  Mohan, D., Bitar, N., Sajassi, A., DeLord, S., Niger, P.,
              and R. Qiu, "MPLS and Ethernet Operations, Administration,
              and Maintenance (OAM) Interworking", RFC 7023, October
              2013.

   [RFC7074]  Berger, L. and J. Meuric, "Revised Definition of the GMPLS
              Switching Capability and Type Fields", RFC 7074, November
              2013.

   [RFC7079]  Del Regno, N. and A. Malis, "The Pseudowire (PW) and
              Virtual Circuit Connectivity Verification (VCCV)
              Implementation Survey Results", RFC 7079, November 2013.

   [RFC7274]  Kompella, K., Andersson, L., and A. Farrel, "Allocating
              and Retiring Special-Purpose MPLS Labels", RFC 7274, June
              2014.

   [TIMING-OVER-MPLS]
              Davari, S., Oren, A., Bhatia, M., Roberts, P., and L.
              Montini, "Transporting Timing messages over MPLS
              Networks", Work in Progress, April 2014.

Appendix A.  Acknowledgements

   Numerous very useful comments have been received in private email.
   Some of these contributions are acknowledged here, approximately in
   chronologic order.

   Paul Doolan provided a brief review resulting in a number of
   clarifications, most notably regarding on-chip vs. system buffering,
   100 Gb/s link speed assumptions in the 150 Mpps figure, and handling
   of large microflows.  Pablo Frank reminded us of the sawtooth effect
   in PPS vs. packet-size graphs, prompting the addition of a few
   paragraphs on this.  Comments from Lou Berger at IETF 85 prompted the
   addition of Section 2.7.

   Valuable comments were received on the BMWG mailing list.  Jay
   Karthik pointed out testing methodology hints that after discussion
   were deemed out of scope and were removed but may benefit later work
   in BMWG.

   Nabil Bitar pointed out the need to cover QoS (Differentiated
   Services), MPLS multicast (P2MP and MP2MP), and MPLS-TP OAM.  Nabil
   also provided a number of clarifications to the questions and tests
   in Sections 3 and 4.

   Mark Szczesniak provided a thorough review and a number of useful
   comments and suggestions that improved the document.

   Gregory Mirsky and Thomas Beckhaus provided useful comments during
   the review by the MPLS Review Team.

   Tal Mizrahi provided comments that prompted clarifications regarding
   timestamp processing, local delivery of packets, and the need for
   hardware assistance in processing OAM traffic.

   Alexander (Sasha) Vainshtein pointed out errors in Section 2.1.8.1
   and suggested new text that, after lengthy discussion, resulted in
   restating the summarization of requirements from PWE3 RFCs and more
   clearly stating the benefits and drawbacks of packet resequencing
   based on PW Sequence Number.

   Loa Anderson provided useful comments and corrections prior to WGLC.
   Adrian Farrel provided useful comments and corrections prior as part
   of the AD review.

   Discussion with Steve Kent during SecDir review resulted in expansion
   of Section 5, briefly summarizing security considerations related to
   forwarding in normative references.  Tom Petch pointed out some
   editorial errors in private email plus an important math error.  Al

   Morton during OpsDir review prompted clarification in the section
   about the target audience, suggested more clear wording in places,
   and found numerous editorial errors.

   Discussion with Stewart Bryant and Alia Atlas as part of IESG review
   resulted in coverage of IPFIX and improvements to document coverage
   of MPLS FRR, and IP/LDP FRR, plus some corrections to the text
   elsewhere.

Authors' Addresses

   Curtis Villamizar (editor)
   Outer Cape Cod Network Consulting, LLC

   EMail: curtis@occnc.com

   Kireeti Kompella
   Juniper Networks

   EMail: kireeti@juniper.net

   Shane Amante
   Apple Inc.
   1 Infinite Loop
   Cupertino, California  95014

   EMail: amante@apple.com

   Andrew Malis
   Huawei Technologies

   EMail: agmalis@gmail.com

   Carlos Pignataro
   Cisco Systems
   7200-12 Kit Creek Road
   Research Triangle Park, NC  27709
   US

   EMail: cpignata@cisco.com

 

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