Network Working Group                                        K. Kompella
Internet-Draft                                                  J. Drake
Updates: 3031 (if approved)                             Juniper Networks
Intended status: Standards Track                               S. Amante
Expires: November 6, 2011 May 3, 2012                         Level 3 Communications, LLC
                                                           W. Henderickx
                                                                 L. Yong
                                                              Huawei USA
                                                             May 5,
                                                        October 31, 2011

              The Use of Entropy Labels in MPLS Forwarding


   Load balancing is a powerful tool for engineering traffic across a
   network.  This memo suggests ways of improving load balancing across
   MPLS networks using the concept of "entropy labels".  It defines the
   concept, describes why entropy labels are useful, enumerates
   properties of entropy labels that allow maximal benefit, and shows
   how they can be signaled and used for various applications.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

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

   This Internet-Draft will expire on November 6, 2011. May 3, 2012.

Copyright Notice

   Copyright (c) 2011 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
   ( 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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Conventions used . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Motivation . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Approaches . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Entropy Labels . . . . . . . . . . and Their Structure . . . . . . . . . . . . . .  7
   4.  Data Plane Processing of Entropy Labels  . . . . . . . . . . .  8
     4.1.  Ingress LSR  . . . . . . . . . . . . . . . . . . . . . . .  8
     4.2.  Transit LSR  . . . . . . . . . . . . . . . . . . . . . . .  9
     4.3.  Egress LSR . . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  Signaling for Entropy Labels . . . . . . . . . . . . . . . . .  9 10
     5.1.  LDP Signaling  . . . . . . . . . . . . . . . . . . . . . . 10
     5.2.  BGP Signaling  . . . . . . . . . . . . . . . . . . . . . . 11
     5.3.  RSVP-TE Signaling  . . . . . . . . . . . . . . . . . . . . 12
   6.  Operations, Administration, and Maintenance (OAM) and
       Entropy Labels . . . . . . . . . . . . . . . . . . . . . . . . 13
   7.  MPLS-TP and Entropy Labels . . . . . . . . . . . . . . . . . . 14
   8.  Point-to-Multipoint LSPs and Entropy Labels  . . . . . . . . . 15
   9.  Entropy Labels and Applications  . . . . . . . . . . . . . . . 15
     9.1.  Tunnels  . . . . . . . . . . . . . . . . . . . . . . . . . 15
     9.2.  LDP Pseudowires  . . . . . . . . . . . . . . . . . . . . . 17
     9.3.  BGP Applications . . . . . . . . . . . . . . . . . . . . . 18
       9.3.1.  Inter-AS BGP VPNs  . . . . . . . . . . . . . . . . . . 19
     9.4.  Multiple Applications  . . . . . . . . . . . . . . . . . . 20
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
     11.1. LDP Entropy Label TLV  . . . . . . . . . . . . . . . . . . 22
     11.2. BGP Entropy Label Attribute  . . . . . . . . . . . . . . . 22
     11.3. Attribute Flags for LSP_Attributes Object  . . . . . . . . 22
     11.4. Attributes TLV for LSP_Attributes Object . . . . . . . . . 22
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 23
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     13.2. Informative References . . . . . . . . . . . . . . . . . . 23
   Appendix A.  Applicability of LDP Entropy Label sub-TLV  . . . . . 24
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25

1.  Introduction

   Load balancing, or multi-pathing, is an attempt to balance traffic
   across a network by allowing the traffic to use multiple paths.  Load
   balancing has several benefits: it eases capacity planning; it can
   help absorb traffic surges by spreading them across multiple paths;
   it allows better resilience by offering alternate paths in the event
   of a link or node failure.

   As providers scale their networks, they use several techniques to
   achieve greater bandwidth between nodes.  Two widely used techniques
   are: Link Aggregation Group (LAG) and Equal-Cost Multi-Path (ECMP).
   LAG is used to bond together several physical circuits between two
   adjacent nodes so they appear to higher-layer protocols as a single,
   higher bandwidth 'virtual' pipe.  ECMP is used between two nodes
   separated by one or more hops, to allow load balancing over several
   shortest paths in the network.  This is typically obtained by
   arranging IGP metrics such that there are several equal cost paths
   between source-destination pairs.  Both of these techniques may, and
   often do, co-exist in various parts of a given provider's network,
   depending on various choices made by the provider.

   A very important requirement when load balancing is that packets
   belonging to a given 'flow' must be mapped to the same path, i.e.,
   the same exact sequence of links across the network.  This is to
   avoid jitter, latency and re-ordering issues for the flow.  What
   constitutes a flow varies considerably.  A common example of a flow
   is a TCP session.  Other examples are an L2TP session corresponding
   to a given broadband user, or traffic within an ATM virtual circuit.

   To meet this requirement, a node uses certain fields, termed 'keys',
   within a packet's header as input to a load balancing function
   (typically a hash function) that selects the path for all packets in
   a given flow.  The keys chosen for the load balancing function depend
   on the packet type; a typical set (for IP packets) is the IP source
   and destination addresses, the protocol type, and (for TCP and UDP
   traffic) the source and destination port numbers.  An overly
   conservative choice of fields may lead to many flows mapping to the
   same hash value (and consequently poorer load balancing); an overly
   aggressive choice may map a flow to multiple values, potentially
   violating the above requirement.

   For MPLS networks, most of the same principles (and benefits) apply.
   However, finding useful keys in a packet for the purpose of load
   balancing can be more of a challenge.  In many cases, MPLS
   encapsulation may require fairly deep inspection of packets to find
   these keys at transit LSRs.

   One way to eliminate the need for this deep inspection is to have the
   ingress LSR of an MPLS Label Switched Path extract the appropriate
   keys from a given packet, input them to its load balancing function,
   and place the result in an additional label, termed the 'entropy
   label', as part of the MPLS label stack it pushes onto that packet.

   The packet's MPLS entire label stack can then be used by transit LSRs
   to perform load balancing, as the entropy label introduces the right
   level of "entropy" into the label stack.

   There are four key reasons why this is beneficial:

   1.  at the ingress LSR, MPLS encapsulation hasn't yet occurred, so
       deep inspection is not necessary;

   2.  the ingress LSR has more context and information about incoming
       packets than transit LSRs;

   3.  ingress LSRs usually operate at lower bandwidths than transit
       LSRs, allowing them to do more work per packet, and

   4.  transit LSRs do not need to perform deep packet inspection and
       can load balance effectively using only a packet's MPLS label

   This memo describes why entropy labels are needed and defines the
   properties of entropy labels; in particular how they are generated
   and received, and the expected behavior of transit LSRs.  Finally, it
   describes in general how signaling works and what needs to be
   signaled, as well as specifics for the signaling of entropy labels
   for LDP ([RFC5036]), BGP ([RFC3107], [RFC4364]), and RSVP-TE

1.1.  Conventions used

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   The following acronyms are used:

      LSR: Label Switching Router;

      LER: Label Edge Router;

      PE: Provider Edge router;
      CE: Customer Edge device; and

      FEC: Forwarding Equivalence Class.

   The term ingress (or egress) LSR is used interchangeably with ingress
   (or egress) LER.  The term application throughout the text refers to
   an MPLS application (such as a VPN or VPLS).

   A label stack (say of three labels) is denoted by <L1, L2, L3>, where
   L1 is the "outermost" label and L3 the innermost (closest to the
   payload).  Packet flows are depicted left to right, and signaling is
   shown right to left (unless otherwise indicated).

   The term 'label' is used both for the entire 32-bit label and the 20-
   bit label field within a label.  It should be clear from the context
   which is meant.

1.2.  Motivation

   MPLS is very successful generic forwarding substrate that transports
   several dozen types of protocols, most notably: IP, PWE3, VPLS and IP
   VPNs.  Within each type of protocol, there typically exist several
   variants, each with a different set of load balancing keys, e.g., for
   IP: IPv4, IPv6, IPv6 in IPv4, etc.; for PWE3: Ethernet, ATM, Frame-
   Relay, etc.  There are also several different types of Ethernet over
   PW encapsulation, ATM over PW encapsulation, etc. as well.  Finally,
   given the popularity of MPLS, it is likely that it will continue to
   be extended to transport new protocols.

   Currently, each transit LSR along the path of a given LSP has to try
   to infer the underlying protocol within an MPLS packet in order to
   extract appropriate keys for load balancing.  Unfortunately, if the
   transit LSR is unable to infer the MPLS packet's protocol (as is
   often the case), it will typically use the topmost (or all) MPLS
   labels in the label stack as keys for the load balancing function.
   The result may be an extremely inequitable distribution of traffic
   across equal-cost paths exiting that LSR.  This is because MPLS
   labels are generally fairly coarse-grained forwarding labels that
   typically describe a next-hop, or provide some of demultiplexing
   and/or forwarding function, and do not describe the packet's
   underlying protocol.

   On the other hand, an ingress LSR (e.g., a PE router) has detailed
   knowledge of an packet's contents, typically through a priori
   configuration of the encapsulation(s) that are expected at a given
   PE-CE interface, (e.g., IPv4, IPv6, VPLS, etc.).  They also have more
   flexible forwarding hardware.  PE routers need this information and
   these capabilities to:

      a) apply the required services for the CE;

      b) discern the packet's CoS forwarding treatment;

      c) apply filters to forward or block traffic to/from the CE;

      d) to forward routing/control traffic to an onboard management
      processor; and,

      e) load-balance the traffic on its uplinks to transit LSRs (e.g.,
      P routers).

   By knowing the expected encapsulation types, an ingress LSR router
   can apply a more specific set of payload parsing routines to extract
   the keys appropriate for a given protocol.  This allows for
   significantly improved accuracy in determining the appropriate load
   balancing behavior for each protocol.

   If the ingress LSR were to capture the flow information so gathered
   in a convenient form for downstream transit LSRs, transit LSRs could
   remain completely oblivious to the contents of each MPLS packet, and
   use only the captured flow information to perform load balancing.  In
   particular, there will be no reason to duplicate an ingress LSR's
   complex packet/payload parsing functionality in a transit LSR.  This
   will result in less complex transit LSRs, enabling them to more
   easily scale to higher forwarding rates, larger port density, lower
   power consumption, etc.  The idea in this memo is to capture this
   flow information as a label, the so-called entropy label.

   Ingress LSRs can also adapt more readily to new protocols and extract
   the appropriate keys to use for load balancing packets of those
   protocols.  This means that deploying new protocols or services in
   edge devices requires fewer concommitant changes in the core,
   resulting in higher edge service velocity and at the same time more
   stable core networks.

2.  Approaches

   There are two main approaches to encoding load balancing information
   in the label stack.  The first allocates multiple labels for a
   particular Forwarding Equivalance Class (FEC).  These labels are
   equivalent in terms of forwarding semantics, but having multiple
   labels allows flexibility in assigning labels to flows belonging to
   the same FEC.  This approach has the advantage that the label stack
   has the same depth whether or not one uses label-based load
   balancing; and so, consequently, there is no change to forwarding
   operations on transit and egress LSRs.  However, it has a major
   drawback in that there is a significant increase in both signaling
   and forwarding state.

   The other approach encodes the load balancing information as an
   additional label in the label stack, thus increasing the depth of the
   label stack by one.  With this approach, there is minimal change to
   signaling state for a FEC; also, there is no change in forwarding
   operations in transit LSRs, and no increase of forwarding state in
   any LSR.  The only purpose of the additional label is to increase the
   entropy in the label stack, so this is called an "entropy label".
   This memo focuses solely on this approach.

3.  Entropy Labels and Their Structure

   An entropy label (as used here) is a label:

   1.  that is not used for forwarding;

   2.  that is not signaled; and

   3.  whose only purpose in the label stack is to provide 'entropy' to
       improve load balancing.

   Entropy labels are generated by an ingress LSR, based entirely on
   load balancing information.  However, they MUST NOT have values in
   the reserved label space (0-15).  Entropy labels MUST be at the
   bottom of the label stack, and thus the 'Bottom of Stack' (S) bit
   ([RFC3032]) in the label should be set.  To ensure that they are not used
   inadvertently for forwarding, entropy labels SHOULD have a TTL of 0.
   The CoS field of an entropy label can be set to any value deemed

   Since entropy labels are generated by an ingress LSR, an egress LSR
   MUST be able to tell unambiguously that a given label is an entropy
   label.  If any ambiguity is possible, the label above the entropy
   label MUST be an 'entropy label indicator' (ELI), which indicates
   that the following Label is an entropy label.  An ELI is typically
   signaled by an egress LSR and is added to the MPLS label stack along
   with an entropy label by an ingress LSR.  For many applications, the
   use of entropy labels is unambiguous, and an ELI is not needed.  If
   used, an  An
   ELI MUST have S 'Bottom of Stack' (S) bit = 0 and SHOULD have a ([RFC3032]).  The TTL of 0.
   SHOULD be set to whatever value the label above it in the stack has.
   The CoS field can be set to any value deemed appropriate; typically,
   this will be the value in the label above it in the stack.

   Applications for MPLS entropy labels include pseudowires ([RFC4447]),
   Layer 3 VPNs ([RFC4364]), VPLS ([RFC4761], [RFC4762]) and Tunnel LSPs
   carrying, say, IP traffic.  [I-D.ietf-pwe3-fat-pw] explains how
   entropy labels can be used for RFC 4447-style pseudowires, and thus
   is complementary to this memo, which focuses on several other
   applications of entropy labels.

4.  Data Plane Processing of Entropy Labels

4.1.  Ingress LSR

   Suppose that for a particular application (or service or FEC), an
   ingress LSR X is to push label stack <TL, AL>, where TL is the
   'tunnel label' and AL is the 'application label'.  (Note the use of
   the convention for label stacks described in Section 1.1.  The use of
   a two-label stack is just for illustrative purposes.)  Suppose
   furthermore that the egress LSR Y has told X that it is capable of
   processing entropy labels for this application.  If X can cannot insert
   entropy labels, it may use simply uses a label stack of <TL, AL, EL> for this
   application, where EL is the entropy label.

   When a packet AL> for this application arrives at X,
   application.  If X can insert entropy labels, it does the
   following: following
   for an incoming packet:

   1.  X identifies the application to which the packet belongs,
       identifies the egress LSR as Y, and thereby picks the outgoing
       label stack <TL, AL> to push onto the packet to send to Y; Y.

   2.  X determines which keys that it will use for load balancing; balancing.

   3.  X, having kept state that Y can process entropy labels for this
       application, generates an entropy label EL (based on the output
       of the load balancing function), and function).

   4.  If Y does not need an ELI, X pushes <TL, AL, EL> on to onto the packet
       before forwarding it to the next LSR on its way hop to Y.

   EL is a 'regular' 32-bit label whose S bit MUST be 1 and whose TTL
   field SHOULD be 0.  The load balancing information is encoded in the
   20-bit label field.

   5.  If X is told (via signaling) that it must use Y requires an
   entropy label indicator with label value E, then ELI, X instead pushes <TL, AL, ELI, E, EL> onto the packet, packet
       before forwarding it to the next hop to Y, where ELI E is a label
       whose S bit
   MUST be 0, whose TTL SHOULD be 0, and whose 20-bit label field MUST
   be E. The CoS fields for EL and is the ELI can be that Y signaled, and whose
       other fields are set to any values. as per Section 3.

   Note that ingress LSR X MUST NOT include an entropy label unless the
   egress LSR Y for this application has indicated that it is ready to
   receive entropy labels.  Furthermore, if Y has signaled that an ELI
   is needed, then X MUST include the ELI before the entropy label.

   Note that the signaling and use of entropy labels in one direction
   (signaling from Y to X, and data path from X to Y) has no bearing on
   the behavior in the opposite direction (signaling from X to Y, and
   data path from Y to X).

4.2.  Transit LSR

   Transit LSRs have virtually no change in forwarding behavior.  For
   load balancing, transit LSRs SHOULD use the whole label stack as keys
   for the load balancing function.  Transit LSRs MUST NOT include
   reserved labels as input to its load balancing function.  Transit
   LSRs MAY choose to look beyond the label stack for further keys;
   however, if entropy labels are being used, this may not be very
   useful.  Looking beyond the label stack may be the simplest approach
   in an environment where some ingress LSRs use entropy labels and
   others don't, or for backward compatibility.  Thus, other than using
   the full label stack as input to the load balancing function, transit
   LSRs are almost unaffected by the use of entropy labels.

4.3.  Egress LSR

   If egresss

   Suppose egress LSR Y signals that it is capable of processing entropy
   labels without for a tunnel or an application with label L. There are three
   cases of interest: (a) L is the implicit NULL label, in which case an
   ELI for is mandatory; (b) L is not the implicit NULL label and an application, ELI is
   not required (L's S bit will be used to determine whether or not
   there is an EL); and (c) L is not the implicit NULL label but an ELI
   is required.

   a1) Y receives an unlabeled packet.  There is obviously no EL; Y
       processes the packet as usual.

   a2) Y receives a packet whose top label is the ELI.  Y processes the
       TTL and CoS fields of the ELI label, ensures that the S bit is 0,
       then pops it, and pops the next label as well (which must be the
       EL), then when pops it.  Y processes the remaining payload as usual.

   b)  Y receives a packet with the application label, then top label L, and an ELI is not required.
       Y looks to see processes L as usual; if the L's S bit is set.  If so, Y applies its usual processing rules to the packet,
   including popping 1, the application label. label stack is
       done.  If the L's S bit is not set, Y
   assumes that the label below 0, the application following label is an entropy
   label and pops both the application label and the entropy label. EL.  Y
   SHOULD ensure that pops
       the entropy label has its S bit set. EL.  Y then processes the packet payload as usual.  Implementations may choose the order
   in which they apply these operations, but the net result should be as

   If Y signals that it is capable of processing entropy labels but that
   an ELI is necessary for a given application, then when

   c)  Y receives a packet with the application label, top label L. Y processes the application label L as usual, then pops it.  Y then checks whether the usual; if
       L's S bit on is 1, the
   application label stack is set. done.  If not, L's S bit is 0, Y looks to see if the label below
       checks the application label following label.  If it is the ELI.  If so, ELI label, Y further pops both processes
       the ELI TTL and CoS fields of the label below (which should be the entropy label).  Y SHOULD
   ensure ELI, ensures that the ELI has its S bit unset, is 0,
       pops the ELI label and that the entropy following label
   has its S bit set.  If (which is the EL), and
       processes the remaining payload as usual.

   If there is an ELI with S bit of the application label = 1, there is set, or an error in the label below is not
   stack.  Note that the ELI, Y processes TTL field of the packet as usual
   (there is no entropy label). EL (if present) will be 0; Y
   MUST NOT react to this.

5.  Signaling for Entropy Labels

   An egress LSR Y may signal to ingress LSR(s) its ability to process
   entropy labels on a per-application (or per-FEC) basis.  As part of
   this signaling, Y also signals the ELI to use, if any.

   In cases where an application label is used and must be the
   bottommost label in the label stack, Y MAY signal that no ELI is
   needed for that application.

   In cases where no application label exists, or where the application
   label may not be the bottommost label in the label stack, Y MUST
   signal a valid ELI to be used in conjunction with the entropy label
   for this FEC.  In this case, an ingress LSR will either not add an
   entropy label, or push the ELI before the entropy label.  This makes
   the use or non-use of an entropy label by the ingress LSR
   unambiguous.  Valid ELI label values are strictly greater than 15.

   It should be noted that egress LSR Y may use the same ELI value for
   all applications for which an ELI is needed.  The ELI MUST be a label
   that does not conflict with any other labels that Y has advertised to
   other LSRs for other applications.  Furthermore, it should be noted
   that the ability to process entropy labels (and the corresponding
   ELI) may be asymmetric: an LSR X may be willing to process entropy
   labels, whereas LSR Y may not be willing to process entropy labels.
   The signaling extensions below allow for this asymmetry.

   For an illustration of signaling and forwarding with entropy labels,
   see Figure 9.

5.1.  LDP Signaling

   When using LDP for signaling tunnel labels ([RFC5036]), a Label
   Mapping Message sub-TLV (Entropy Label sub-TLV) is used to signal an
   egress LSR's ability to process entropy labels.

   The presence of the Entropy Label sub-TLV in the Label Mapping
   Message indicates to ingress LSRs that the egress LSR can process an
   entropy label.  In addition, the Entropy Label sub-TLV contains a
   label value for the ELI.  If the ELI is zero, this indicates the
   egress doesn't need an ELI for the signaled application; if not, the
   egress requires the given ELI with entropy labels.  An example where
   an ELI is needed is when the signaled application is an LSP that can
   carry IP traffic.

   The structure of the Entropy Label sub-TLV is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |U|F|        Type (TBD)         |           Length (8)          |
   |               Value                   |     Must Be Zero      |

                      Figure 1: Entropy Label sub-TLV


      U: Unknown bit.  This bit MUST be set to 1.  If the Entropy Label
      sub-TLV is not understood, then the TLV is not known to the
      receiver and MUST be ignored.

      F: Forward bit.  This bit MUST be set be set to 1.  Since this
      sub-TLV is going to be propagated hop-by-hop, the sub-TLV should
      be forwarded even by nodes that may not understand it.

      Type: sub-TLV Type field, as specified by IANA.

      Length: sub-TLV Length field.  This field specifies the total
      length in octets of the Entropy Label sub-TLV.

      Value: value of the Entropy Label Indicator Label.

5.2.  BGP Signaling

   When BGP [RFC4271] is used for distributing Network Layer
   Reachability Information (NLRI) as described in, for example,
   [RFC3107], [RFC4364] and [RFC4761], the BGP UPDATE message may
   include the Entropy Label attribute.  This is an optional, transitive
   BGP attribute of type TBD.  The inclusion of this attribute with an
   NLRI indicates that the advertising BGP router can process entropy
   labels as an egress LSR for that NLRI.  If the attribute length is
   less than three octets, this indicates that the egress doesn't need
   an ELI for the signaled application.  If the attribute length is at
   least three octets, the first three octets encode an ELI label value
   as the high order 20 bits; the egress requires this ELI with entropy
   labels.  An example where an ELI is needed is when the NLRI contains
   unlabeled IP prefixes.

   A BGP speaker S that originates an UPDATE should only include the
   Entropy Label attribute if both of the following are true:

   A1:  S sets the BGP NEXT_HOP attribute to itself; AND

   A2:  S can process entropy labels for the given application.

   If both A1 and A2 are true, and S needs an ELI to recognize entropy
   labels, then S MUST include the ELI label value as part of the
   Entropy Label attribute.  An UPDATE SHOULD contain at most one
   Entropy Label attribute.

   Suppose a BGP speaker T receives an UPDATE U with the Entropy Label
   attribute ELA.  T has two choices.  T can simply re-advertise U with
   the same ELA if either of the following is true:

   B1:  T does not change the NEXT_HOP attribute; OR

   B2:  T simply swaps labels without popping the entire label stack and
        processing the payload below.

   An example of the use of B1 is Route Reflectors; an example of the
   use of B2 is illustrated in Section

   However, if T changes the NEXT_HOP attribute for U and in the data
   plane pops the entire label stack to process the payload, T MUST
   remove ELA.  T MAY include a new Entropy Label attribute ELA' for
   UPDATE U' if both of the following are true:

   C1:  T sets the NEXT_HOP attribute of U' to itself; AND

   C2:  T can process entropy labels for the given application.

   Again, if both C1 and C2 are true, and T needs an ELI to recognize
   entropy labels, then T MUST include the ELI label value as part of
   the Entropy Label attribute.

5.3.  RSVP-TE Signaling

   Entropy Label support is signaled in RSVP-TE [RFC3209] using an
   Entropy Label Attribute TLV (Type TBD) of the LSP_ATTRIBUTES object
   [RFC5420].  The presence of this attribute indicates that the
   signaler (the egress in the downstream direction using Resv messages;
   the ingress in the upstream direction using Path messages) can
   process entropy labels.  The Entropy Label Attribute contains a value
   for the ELI.  If the ELI is zero, this indicates that the signaler
   doesn't need an ELI for this application; if not, then the signaler
   requires the given ELI with entropy labels.  An example where an ELI
   is needed is when the signaled LSP can carry IP traffic.

   The format of the Entropy Label Attribute is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |    Entropy Label Attribute    |           Length (4)          |
   |              ELI Label                |         MBZ           |

   An egress LSR includes the Entropy Label Attribute in a Resv message
   to indicate that it can process entropy labels in the downstream
   direction of the signaled LSP.

   An ingress LSR includes the Entropy Label Attribute in a Path message
   for a bi-directional LSP to indicate that it can process entropy
   labels in the upstream direction of the signaled LSP.  If the
   signaled LSP is not bidirectional, the Entropy Label Attribute SHOULD
   NOT be included in the Path message, and egress LSR(s) SHOULD ignore
   the attribute, if any.

   As described in Section 8, there is also the need to distribute an
   ELI from the ingress (upstream label allocation).  In the case of
   RSVP-TE, this is accomplished using the Upstream ELI Attribute TLV of
   the LSP_ATTRIBUTES object, as shown below:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |    Upstream ELI Attribute     |           Length (4)          |
   |              ELI Label                |         MBZ           |

6.  Operations, Administration, and Maintenance (OAM) and Entropy Labels

   Generally OAM comprises a set of functions operating in the data
   plane to allow a network operator to monitor its network
   infrastructure and to implement mechanisms in order to enhance the
   general behavior and the level of performance of its network, e.g.,
   the efficient and automatic detection, localization, diagnosis and
   handling of defects.

   Currently defined OAM mechanisms for MPLS include LSP Ping/Traceroute
   [RFC4379] and Bidirectional Failure Detection (BFD) for MPLS
   [RFC5884].  The latter provides connectivity verification between the
   endpoints of an LSP, and recommends establishing a separate BFD
   session for every path between the endpoints.

   The LSP traceroute procedures of [RFC4379] allow an ingress LSR to
   obtain label ranges that can be used to send packets on every path to
   the egress LSR.  It works by having ingress LSR sequentially ask the
   transit LSRs along a particular path to a given egress LSR to return
   a label range such that the inclusion of a label in that range in a
   packet will cause the replying transit LSR to send that packet out
   the egress interface for that path.  The ingress provides the label
   range returned by transit LSR N to transit LSR N + 1, which returns a
   label range which is less than or equal in span to the range provided
   to it.  This process iterates until the penultimate transit LSR
   replies to the ingress LSR with a label range that is acceptable to
   it and to all LSRs along path preceding it for forwarding a packet
   along the path.

   However, the LSP traceroute procedures do not specify where in the
   label stack the value from the label range is to be placed, whether
   deep packet inspection is allowed and if so, which keys and key
   values are to be used.

   This memo updates LSP traceroute by specifying that the value from
   the label range is to be placed in the entropy label.  Deep packet
   inspection is thus not necessary, although an LSR may use it,
   provided it do so consistently, i.e., if the label range to go to a
   given downstream LSR is computed with deep packet inspection, then
   the data path should use the same approach and the same keys.

   In order to have a BFD session on a given path, a value from the
   label range for that path should be used as the EL value for BFD
   packets sent on that path.

   As part of the MPLS-TP work, an in-band OAM channel is defined in
   [RFC5586].  Packets sent in this channel are identified with a
   reserved label, the Generic Associated Channel Label (GAL) placed at
   the bottom of the MPLS label stack.  In order to use the inband OAM
   channel with entropy labels, this memo relaxes the restriction that
   the GAL must be at the bottom of the MPLS label stack.  Rather, the
   GAL is placed in the MPLS label stack above the entropy label so that
   it effectively functions as an application label.

7.  MPLS-TP and Entropy Labels

   Since MPLS-TP does not use ECMP, entropy labels are not applicable to
   an MPLS-TP deployment.

8.  Point-to-Multipoint LSPs and Entropy Labels

   Point-to-Multipoint (P2MP) LSPs [RFC4875] typically do not use ECMP
   for load balancing, as the combination of replication and
   multipathing can lead to duplicate traffic delivery.  However, P2MP
   LSPs can traverse Bundled Links [RFC4201] and LAGs.  In both these
   cases, load balancing is useful, and hence entropy labels can be of
   some value for P2MP LSPs.

   There are two potential complications with the use of entropy labels
   in the context of P2MP LSPs, both a consequence of the fact that the
   entire label stack below the P2MP label must be the same for all
   egress LSRs.  First, all egress LSRs must be willing to receive
   entropy labels; if even one egress LSR is not willing, then entropy
   labels MUST NOT be used for this P2MP LSP.  Second, if an ELI is
   required, all egress LSRs must agree to the same value of ELI.  This
   can be achieved by upstream allocation of the ELI; in particular, for
   RSVP-TE P2MP LSPs, the ingress LSR distributes the ELI value using
   the Upstream ELI Attribute TLV of the LSP_ATTRIBUTES object, defined
   in Section 5.3.

   With regard to the first issue, the ingress LSR MUST keep track of
   the ability of each egress LSR to process entropy labels, especially
   since the set of egress LSRs of a given P2MP LSP may change over
   time.  Whenever an existing egress LSR leaves, or a new egress LSR
   joins the P2MP LSP, the ingress MUST re-evaluate whether or not to
   include entropy labels for the P2MP LSP.

   In some cases, it may be feasible to deploy two P2MP LSPs, one to
   entropy label capable egress LSRs, and the other to the remaining
   egress LSRs.  However, this requires more state in the network, more
   bandwidth, and more operational overhead (tracking EL-capable LSRs,
   and provisioning P2MP LSPs accordingly).  Furthermore, this approach
   may not work for some applications (such mVPNs and VPLS) which
   automatically create and/or use P2MP LSPs for their multicast

9.  Entropy Labels and Applications

   This section describes the usage of entropy labels in various
   scenarios with different applications.

9.1.  Tunnels

   Tunnel LSPs, signaled with either LDP or RSVP-TE, typically carry
   other MPLS applications such as VPNs or pseudowires.  This being the
   case, if the egress LSR of a tunnel LSP is willing to process entropy
   labels, it would signal the need for an Entropy Label Indicator to
   distinguish between entropy labels and other application labels.

   In the figures below, the following convention is used to depict
   information signaled between X and Y:

                             X ---------- ... ---------- Y
                        app:   <--- [label L, ELI value]

   This means Y signals to X label L for application app.  The ELI value
   can be one of:

      -: meaning entropy labels are NOT accepted;

      0: meaning entropy labels are accepted, no ELI is needed; or

      E: entropy labels are accepted, ELI label E is required.

   The following illustrates a simple intra-AS tunnel LSP.

                         X -------- A --- ... --- B -------- Y
           tunnel LSP L:   [TL,  E] <---  ...  <--- [TL0, E]

           IP pkt:         push <TL, E, EL> --------------->

                 Figure 2: Tunnel LSPs and Entropy Labels

   Tunnel LSPs may cross Autonomous System (AS) boundaries, usually
   using BGP ([RFC3107]).  In this case, the AS Border Routers (ASBRs)
   MAY simply propagate the egress LSR's ability to process entropy
   labels, or they MAY declare that entropy labels may not be used.  If
   an ASBR (say A2 below) chooses to propagate the egress LSR Y's
   ability to process entropy labels, A2 MUST also propagate Y's choice
   of ELI.

                      X ---- ... ---- A1 ------- A2 ---- ... ---- Y
     intra-AS LSP A2-Y:                             <--- [TL0, E]
     inter-AS LSP A1-A2:                 [AL, E]
     intra-AS LSP X-A1: <--- [TL1, E]

     IP pkt:           push <TL1, E, EL>

   Here, ASBR A2 chooses to propagate Y's ability to process entropy
   labels, by "translating" Y's signaling of entropy label capability
   (say using LDP) to BGP; and A1 translate A2's BGP signaling to (say)
   RSVP-TE.  The end-to-end tunnel (X to Y) will have entropy labels if
   X chooses to insert them.

             Figure 3: Inter-AS Tunnel LSP with Entropy Labels

                      X ---- ... ---- A1 ------- A2 ---- ... ---- Y
     intra-AS LSP A2-Y:                             <--- [TL0, E]
     inter-AS LSP A1-A2:                 [AL, E]
     intra-AS LSP X-A1: <--- [TL1, -]

     IP pkt:            push <TL1> -->

   Here, ASBR A1 decided that entropy labels are not to be used; thus,
   the end-to-end tunnel cannot have entropy labels, even though both X
   and Y may be capable of inserting and processing entropy labels.

           Figure 4: Inter-AS Tunnel LSP with no Entropy Labels

9.2.  LDP Pseudowires

   [I-D.ietf-pwe3-fat-pw] describes the signaling and use of entropy
   labels in the context of RFC 4447 pseudowires, so this will not be
   described further here.

   [RFC4762] specifies the use of LDP for signaling VPLS pseudowires.
   An egress VPLS PE that can process entropy labels can indicate this
   by adding the Entropy Label sub-TLV in the LDP message it sends to
   other PEs.  An ELI is not required.  An ingress PE must maintain
   state per egress PE as to whether it can process entropy labels.

                         X -------- A --- ... --- B -------- Y
           tunnel LSP L:   [TL,  E] <---  ...  <--- [TL0, E]
           VPLS label:     <------------------------ [VL, 0]

           VPLS pkt:       push <TL, VL, EL> -------------->

                  Figure 5: Entropy Labels with LDP VPLS

   Note that although the underlying tunnel LSP signaling indicated the
   need for an ELI, VPLS packets don't need an ELI, and thus the label
   stack pushed by X do not have one.

   [RFC4762] also describes the notion of "hierarchical VPLS" (H-VPLS).
   In H-VPLS, 'hub PEs' remove the label stack and process VPLS packets;
   thus, they must make their own decisions on the use of entropy
   labels, independent of other hub PEs or spoke PEs with which they
   exchange signaling.  In the example below, spoke PEs X and Y and hub
   PE B can process entropy labels, but hub PE A cannot.

                 X ---- ... ---- A ---- ... ---- B ---- ... ---- Y
   spoke PW1:                                      <--- [SL1, 0]
   hub-hub PW:                     <---- [HL, 0]
   spoke PW2:      <--- [SL2, -]

   SPW2 pkt:       push <TL1, SL2>
   H-H PW pkt:                     push <TL2,HL,EL>
   SPW1 pkt:                                       push <TL3,SL1,EL>

                   Figure 6: Entropy Labels with H-VPLS

9.3.  BGP Applications

   Section 9.1 described a BGP application for the creation of inter-AS
   tunnel LSPs.  This section describes two other BGP applications, IP
   VPNs ([RFC4364]) and BGP VPLS ([RFC4761]).  An egress PE for either
   of these applications indicates its ability to process entropy labels
   by adding the Entropy Label attribute to its BGP UPDATE message.
   Again, ingress PEs must maintain per-egress PE state regarding its
   ability to process entropy labels.  In this section, both of these
   applications will be referred to as VPNs.

   In the intra-AS case, PEs signal application labels and entropy label
   capability to each other, either directly, or via Route Reflectors
   (RRs).  If RRs are used, they must not change the BGP NEXT_HOP
   attribute in the UPDATE messages; furthermore, they can simply pass
   on the Entropy Label attribute as is.

                         X -------- A --- ... --- B -------- Y
           tunnel LSP L:   [TL,  E] <---  ...  <--- [TL0, E]
           BGP VPN label:  <------------------------ [VL, 0]

           BGP VPN pkt:    push <TL, VL, EL> -------------->

              Figure 7: Entropy Labels with Intra-AS BGP apps

   For BGP VPLS, the application label is at the bottom of stack, so no
   ELI is needed.  For BGP IP VPNs, the application label is usually at
   the bottom of stack, so again no ELI is needed.  However, in the case
   of Carrier's Carrier (CsC) VPNs, the BGP VPN label may not be at the
   bottom of stack.  In this case, an ELI is necessary for CsC VPN
   packets with entropy labels to distinguish them from nested VPN
   packets.  In the example below, the nested VPN signaling is not
   shown; the egress PE for the nested VPN (not shown) must signal
   whether or not it can process egress labels, and the ingress nested
   VPN PE may insert an entropy label if so.

   Three cases are shown: a plain BGP VPN packet, a CsC VPN packet
   originating from X, and a transit nested VPN packet originating from
   a nested VPN ingress PE (conceptually to the left of X).  It is
   assumed that the nested VPN packet arrives at X with label stack <ZL,
   CVL> where ZL is the tunnel label (to be swapped with <TL, CL>) and
   CVL is the nested VPN label.  Note that Y can use the same ELI for
   the tunnel LSP and the CsC VPN (and any other application that needs
   an ELI).

                         X -------- A --- ... --- B -------- Y
       tunnel LSP L:       [TL,  E] <---  ...  <--- [TL0, E]
       BGP VPN label:      <------------------------ [VL, 0]
       BGP CsC VPN label:  <------------------------ [CL, E]

       BGP VPN pkt:        push <TL, VL, EL> -------------->
       CsC VPN pkt:        push <TL, CL, E, EL> ----------->
       nested VPN pkt:     swap <ZL> with <TL, CL> -------->

                   Figure 8: Entropy Labels with CoC VPN

9.3.1.  Inter-AS BGP VPNs

   There are three commonly used options for inter-AS IP VPNs and BGP
   VPLS, known informally as "Option A", "Option B" and "Option C".
   This section describes how entropy labels can be used in these
   options.  Option A Inter-AS VPNs

   In option A, an ASBR pops the full label stack of a VPN packet
   exiting an AS, processes the payload header (IP or Ethernet), and
   forwards the packet natively (i.e., as IP or Ethernet, but not as
   MPLS) to the peer ASBR.  Thus, entropy label signaling and insertion
   are completely local to each AS.  The inter-AS paths do not use
   entropy labels, as they do not use a label stack.  Option B Inter-AS VPNs

   The ASBRs in option B inter-AS VPNs have a choice (usually determined
   by configuration) of whether to just swap labels (from within the AS
   to the neighbor AS or vice versa), or to pop the full label stack and
   process the packet natively.  This choice occurs at each ASBR in each
   direction.  In the case of native packet processing at an ASBR,
   entropy label signaling and insertion is local to each AS and to the
   inter-AS paths (which, unlike option A, do have labeled packets).

   In the case of simple label swapping at an ASBR, the ASBR can
   propagate received entropy label signaling onward.  That is, if a PE
   signals to its ASBR that it can process entropy labels (via an
   Entropy Label attribute), the ASBR can propagate that attribute to
   its peer ASBR; if a peer ASBR signals that it can process entropy
   labels, the ASBR can propagate that to all PEs within its AS).  Note
   that this is the case even though ASBRs change the BGP NEXT_HOP
   attribute to "self", because of clause B2 in Section 5.2.  Option C Inter-AS VPNs

   In Option C inter-AS VPNs, the ASBRs are not involved in signaling;
   they do not have VPN state; they simply swap labels of inter-AS
   tunnels.  Signaling is PE to PE, usually via Route Reflectors;
   however, if RRs are used, the RRs do not change the BGP NEXT_HOP
   attribute.  Thus, entropy label signaling and insertion are on a PE-
   pair basis, and the intermediate routers, ASBRs and RRs do not play a

9.4.  Multiple Applications

   It has been mentioned earlier that an ingress PE must keep state per
   egress PE with regard to its ability to process entropy labels.  An
   ingress PE must also keep state per application, as entropy label
   processing must be based on the application context in which a packet
   is received (and of course, the corresponding entropy label

   In the example below, an egress LSR Y signals a tunnel LSP L, and is
   prepared to receive entropy labels on L, but requires an ELI.
   Furthermore, Y signals two pseudowires PW1 and PW2 with labels PL1
   and PL2, respectively, and indicates that it can receive entropy
   labels for both pseudowires without the need of an ELI; and finally,
   Y signals a L3 VPN with label VL, but Y does not indicate that it can
   receive entropy labels for the L3 VPN.  Ingress LSR X chooses to send
   native IP packets to Y over L with entropy labels, thus X must
   include the given ELI (yielding a label stack of <TL, ELI, EL>).  X
   chooses to add entropy labels on PW1 packets to Y, with a label stack
   of <TL, PL1, EL>, but chooses not to do so for PW2 packets.  X must
   not send entropy labels on L3 VPN packets to Y, i.e., the label stack
   must be <TL, VL>.

                         X -------- A --- ... --- B -------- Y
           tunnel LSP L:   [TL,  E] <---  ...  <--- [TL0, E]
           PW1 label:      <----------------------- [PL1, 0]
           PW2 label:      <----------------------- [PL2, 0]
           VPN label:      <----------------------- [VL,  -]

           IP pkt:         push <TL, ELI, EL> ------------->
           PW1 pkt:        push <TL, PL1, EL> ------------->
           PW2 pkt:        push <TL, PL2> ----------------->
           VPN pkt:        push <TL, VL> ------------------>

            Figure 9: Entropy Labels for Multiple Applications

10.  Security Considerations

   This document describes advertisement of the capability to support
   receipt of entropy-labels and an Entropy Label Indicator that an
   ingress LSR may apply to MPLS packets in order to allow transit LSRs
   to attain better load-balancing across LAG and/or ECMP paths in the

   This document does not introduce new security vulnerabilities to LDP.
   Please refer to the Security Considerations section of LDP
   ([RFC5036]) for security mechanisms applicable to LDP.

   Given that there is no end-user control over the values used for
   entropy labels, there is little risk of Entropy Label forgery which
   could cause uneven load-balancing in the network.

   If Entropy Label Capability is not signaled from an egress PE to an
   ingress PE, due to, for example, malicious configuration activity on
   the egress PE, then the PE's will fall back to not using entropy
   labels for load-balancing traffic over LAG or ECMP paths which, in
   some cases, in no worse than the behavior observed in current
   production networks.  That said, operators are recommended to monitor
   changes to PE configurations and, more importantly, the fairness of
   load distribution over equal-cost LAG or ECMP paths.  If the fairness
   of load distribution over a set of paths changes that could indicate
   a misconfiguration, bug or other non-optimal behavior on their PE's
   and they should take corrective action.

   Given that most applications already signal an Application Label,
   e.g.: IPVPNs, LDP VPLS, BGP VPLS, whose Bottom of Stack bit is being
   re-used to signal entropy label capability, there is little to no
   additional risk that traffic could be misdirected into an
   inappropriate IPVPN VRF or VPLS VSI at the egress PE.

   In the context of downstream-signaled entropy labels that require the
   use of an Entropy Label Indicator (ELI), there should be little to no
   additional risk because the egress PE is solely responsible for
   allocating an ELI value and ensuring that ELI label value DOES NOT
   conflict with other MPLS labels it has previously allocated.  On the
   other hand, for upstream-signaled entropy labels, e.g.: RSVP-TE
   point-to-point or point-to-multipoint LSP's or Multicast LDP (mLDP)
   point-to-multipoint or multipoint-to-multipoint LSP's, there is a
   risk that the head-end MPLS LER may choose an ELI value that is
   already in use by a downstream LSR or LER.  In this case, it is the
   responsibility of the downstream LSR or LER to ensure that it MUST
   NOT accept signaling for an ELI value that conflicts with MPLS
   label(s) that are already in use.

11.  IANA Considerations

11.1.  LDP Entropy Label TLV

   IANA is requested to allocate the next available value from the IETF
   Consensus range in the LDP TLV Type Name Space Registry as the
   "Entropy Label TLV".

11.2.  BGP Entropy Label Attribute

   IANA is requested to allocate the next available Path Attribute Type
   Code from the "BGP Path Attributes" registry as the "BGP Entropy
   Label Attribute".

11.3.  Attribute Flags for LSP_Attributes Object

   IANA is requested to allocate a new bit from the "Attribute Flags"
   sub-registry of the "RSVP TE Parameters" registry.

   Bit | Name                 | Attribute  | Attribute  | RRO
   No  |                      | Flags Path | Flags Resv |
   TBD   Entropy Label LSP          Yes          Yes       No

11.4.  Attributes TLV for LSP_Attributes Object

   IANA is requested to allocate the next available value from the
   "Attributes TLV" sub-registry of the "RSVP TE Parameters" registry.

12.  Acknowledgments

   We wish to thank Ulrich Drafz for his contributions, as well as the
   entire 'hash label' team for their valuable comments and discussion.

13.  References

13.1.  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.

   [RFC3107]  Rekhter, Y. and E. Rosen, "Carrying Label Information in
              BGP-4", RFC 3107, May 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.

   [RFC5420]  Farrel, A., Papadimitriou, D., Vasseur, JP., and A.
              Ayyangarps, "Encoding of Attributes for MPLS LSP
              Establishment Using Resource Reservation Protocol Traffic
              Engineering (RSVP-TE)", RFC 5420, February 2009.

13.2.  Informative References

              Bryant, S., Filsfils, C., Drafz, U., Kompella, V., Regan,
              J., and S. Amante, "Flow Aware Transport of Pseudowires
              over an MPLS PSN", draft-ietf-pwe3-fat-pw-05 Packet Switched Network",
              draft-ietf-pwe3-fat-pw-07 (work in progress), October 2010. July 2011.

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

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

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

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

   [RFC4447]  Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G.
              Heron, "Pseudowire Setup and Maintenance Using the Label
              Distribution Protocol (LDP)", RFC 4447, April 2006.

   [RFC4761]  Kompella, K. and Y. Rekhter, "Virtual Private LAN Service
              (VPLS) Using BGP for Auto-Discovery and Signaling",
              RFC 4761, January 2007.

   [RFC4762]  Lasserre, M. and V. Kompella, "Virtual Private LAN Service
              (VPLS) Using Label Distribution Protocol (LDP) Signaling",
              RFC 4762, January 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.

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

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

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

Appendix A.  Applicability of LDP Entropy Label sub-TLV

   In the case of unlabeled IPv4 (Internet) traffic, the Best Current
   Practice is for an egress LSR to propagate eBGP learned routes within
   a SP's Autonomous System after resetting the BGP next-hop attribute
   to one of its Loopback IP addresses.  That Loopback IP address is
   injected into the Service Provider's IGP and, concurrently, a label
   assigned to it via LDP.  Thus, when an ingress LSR is performing a
   forwarding lookup for a BGP destination it recursively resolves the
   associated next-hop to a Loopback IP address and associated LDP label
   of the egress LSR.

   Thus, in the context of unlabeled IPv4 traffic, the LDP Entropy Label
   sub-TLV will typically be applied only to the FEC for the Loopback IP
   address of the egress LSR and the egress LSR will not announce an
   entropy label capability for the eBGP learned route.

Authors' Addresses

   Kireeti Kompella
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089


   John Drake
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089


   Shane Amante
   Level 3 Communications, LLC
   1025 Eldorado Blvd
   Broomfield, CO  80021


   Wim Henderickx
   Copernicuslaan 50
   2018 Antwerp


   Lucy Yong
   Huawei USA
   1700 Alma Dr. Suite 500
   Plano, TX  75075