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MPLS WG                                                      K. Kompella
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                               R. Balaji
Expires: April 22, 2017                           Juniper Networks, Inc.
                                                              G. Swallow
                                                           Cisco Systems
                                                        October 19, 2016


                      Label Distribution Using ARP
                      draft-kompella-mpls-larp-06

Abstract

   This document describes extensions to the Address Resolution Protocol
   to distribute MPLS labels for IPv4 and IPv6 host addresses.
   Distribution of labels via ARP enables simple plug-and-play operation
   of MPLS.

Requirements Language

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

   The term "server" will be used in this document to refer to an ARP/
   L-ARP server; the term "host" will be used to refer to a compute
   server or other device acting as an ARP/L-ARP client.

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 http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on April 22, 2017.






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Copyright Notice

   Copyright (c) 2016 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Approach  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Overview of Ethernet ARP  . . . . . . . . . . . . . . . . . .   4
   3.  L-ARP Protocol Operation  . . . . . . . . . . . . . . . . . .   4
     3.1.  Setup . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Egress Operation  . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Ingress Operation . . . . . . . . . . . . . . . . . . . .   5
   4.  Attributes  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Client-Server Synchronization . . . . . . . . . . . . . . . .   7
     5.1.  Restart Handling  . . . . . . . . . . . . . . . . . . . .   7
       5.1.1.  Server Restart  . . . . . . . . . . . . . . . . . . .   7
       5.1.2.  Client Restart  . . . . . . . . . . . . . . . . . . .   8
     5.2.  Expedited Reachability Determination  . . . . . . . . . .   8
   6.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Backward Compatibility  . . . . . . . . . . . . . . . . . . .   9
   8.  OAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  L-ARP IPv4 FEC  . . . . . . . . . . . . . . . . . . . . .   9
     8.2.  L-ARP IPv6 FEC  . . . . . . . . . . . . . . . . . . . . .   9
   9.  For Future Study  . . . . . . . . . . . . . . . . . . . . . .  10
   10. L-ARP Message Format  . . . . . . . . . . . . . . . . . . . .  11
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  13
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     14.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15







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1.  Introduction

   This document describes extensions to the Address Resolution Protocol
   (ARP) [RFC0826] to advertise label bindings for IP host addresses.
   While there are well-established protocols, such as LDP, RSVP and
   BGP, that provide robust mechanisms for label distribution, these
   protocols tend to be relatively complex, and often require detailed
   configuration for proper operation.  There are situations where a
   simpler protocol may be more suitable from an operational standpoint.

   An example is the case where an MPLS Fabric is the underlay
   technology in a Data Center; here, MPLS tunnels originate from host
   machines.  The host thus needs a mechanism to acquire label bindings
   to participate in the MPLS Fabric.  [TODO-MPLS-FABRIC] describes the
   motivation for using MPLS as the fabric technology.

   Another use-case is Egress Peer Traffic-Engineering (EPE)
   [I-D.gredler-idr-bgplu-epe].  In EPE, if the host makes the decision
   to direct packets towards a specific link using MPLS tunneling
   techniques, there needs to a suitable protocol for the host to
   acquire MPLS labels from the network.

   In both the cases, the mechanism that the host uses to partcipate in
   label exchange with the network needs to be simple, and plug-and-
   play.  Existing signaling/routing protocols do not always meet this
   need.  Labeled ARP (L-ARP) is a proposal to fill that gap.

1.1.  Approach

   ARP is a nearly ubiquitous protocol; every device with an Ethernet
   interface, from hand-helds to hosts, have an implementation of ARP.
   ARP is plug-and-play; ARP clients do not need configuration to use
   ARP.  That suggests that ARP may be a good fit for devices that want
   to source and sink MPLS tunnels, but do so in a zero-config, plug-
   and-play manner, with minimal impact to their code.

   The approach taken here is to create a minor variant of the ARP
   protocol, labeled ARP (L-ARP), which is distinguished by a new
   hardware type, MPLS-over-Ethernet.  Regular (Ethernet) ARP (E-ARP)
   and L-ARP can coexist; a device, as an ARP client, can choose to send
   out an E-ARP or an L-ARP request, depending on whether it needs
   Ethernet or MPLS connectivity.  Another device may choose to function
   as an E-ARP server and/or an L-ARP server, depending on its ability
   to provide an IP-to-Ethernet and/or IP-to-MPLS mapping.







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2.  Overview of Ethernet ARP

   In the most straightforward mode of operation [RFC0826], ARP queries
   are sent to resolve "directly connected" IP addresses.  The ARP query
   is broadcast, with the Target Protocol Address field (see Section 10
   for a description of the fields in an ARP message) carrying the IP
   address of another node in the same subnet.  All the nodes in the LAN
   receive this ARP query.  All the nodes, except the node that owns the
   IP address, ignore the ARP query.  The IP address owner learns the
   MAC address of the sender from the Source Hardware Address field in
   the ARP request, and unicasts an ARP reply to the sender.  The ARP
   reply carries the replying node's MAC address in the Source Hardware
   Address field, thus enabling two-way communication between the two
   nodes.

   A variation of this scheme, known as "proxy ARP" [RFC2002], allows a
   node to respond to an ARP request with its own MAC address, even when
   the responding node does not own the requested IP address.
   Generally, the proxy ARP response is generated by routers to attract
   traffic for prefixes they can forward packets to.  This scheme
   requires the host to send ARP queries for the IP address the host is
   trying to reach, rather than the IP address of the router.  When
   there is more than one router connected to a network, proxy ARP
   enables a host to automatically select an exit router without running
   any routing protocol to determine IP reachability.  Unlike regular
   ARP, a proxy ARP request can elicit multiple responses, e.g., when
   more than one router has connectivity to the address being resolved.
   The sender must be prepared to select one of the responding routers.

   Yet another variation of the ARP protocol, called 'Gratuitous ARP'
   [RFC2002], allows a node to update the ARP cache of other nodes in an
   unsolicited fashion.  Gratuitous ARP is sent as either an ARP request
   or an ARP reply.  In either case, the Source Protocol Address and
   Target Protocol Address contain the sender's address, and the Source
   Hardware Address is set to the sender's hardware address.  In case of
   a gratuitous ARP reply, the Target Hardware Address is also set to
   the sender's address.

3.  L-ARP Protocol Operation

   The L-ARP protocol builds on the proxy ARP model, and also leverages
   gratuitous ARP model for asynchronous updates.

   In this memo, we will refer to L-ARP clients (that make L-ARP
   requests) and L-ARP servers (that send L-ARP responses).  In
   Figure 1, H1, H2 and H3 are L-ARP clients, and T1, T2 and T3 are IP
   routers playing the role of L-ARP server.  T4 is a member of the MPLS
   Fabric that may not be an L-ARP server.  Within the MPLS Fabric, the



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   usual MPLS protocols (IGP, LDP, RSVP-TE) are run.  Say H1, H2 and H3
   want to establish MPLS tunnels to each other (for example, they are
   using BGP MPLS VPNs as the overlay virtual network technology).  H1
   might also want to talk to a member of the MPLS Fabric, say T (not
   depicted in the diagram).

                               . . . . . .
                               .           .
                       H1 --- T1             T4
                          \   .     MPLS      .
                           \  .               .
                            \ .    Fabric     .
                       H2 --- T2             T3 --- H3
                               .            .
                               . . . . . . .

                                 Figure 1

3.1.  Setup

   In Figure 1, the nodes T1-T4, and those in between making up the
   "MPLS Fabric" are assumed to be running some protocol whereby they
   can signal MPLS reachability to themselves and to other nodes (like
   H1-H3).  T1-T3 are L-ARP servers; T4 need not be.  H1-H3 are L-ARP
   clients.

3.2.  Egress Operation

   A node (say T3) that wants an attached node (say H3) to have MPLS
   reachability, allocates a label L3 to reach H3, and advertises this
   label into the MPLS Fabric.  This can be triggered by configuration
   on T3, or via some other protocol.  On receiving a packet with label
   L3, T3 pops the label and send the packet to H3.  This is the usual
   operation of an MPLS Fabric, with the addition of advertising labels
   for nodes outside the fabric.

3.3.  Ingress Operation

   A node (say H1, the L-ARP client) that needs an MPLS tunnel to a node
   (say H3) identified by a host address (either IPv4 or IPv6)
   broadcasts over all its interfaces an L-ARP query with the Target
   Protocol Address set to H3.  A node (say T1, an L-ARP server) that
   has MPLS reachability to H3 sends an L-ARP reply with the Source
   Hardware Address set to its Ethernet MAC address M1, with a new TLV
   containing a label L1.  To send a packet to H3 over an MPLS tunnel,
   H1 pushes L1 onto the packet, sets the destination MAC address to M1
   and sends it to T1.  On receiving this packet, T1 swaps the top label
   with the label(s) for its MPLS tunnel to H3.



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   Note that H1 broadcasts its L-ARP request over its attached
   interfaces.  H1 may receive several L-ARP replies; in that case, H1
   can select any subset of these to send MPLS packets destined to H3.
   As described later, the L-ARP response may contain certain parameters
   that enable the client to make an informed choice.  However, it is
   completely a matter of local policy on H1 which of the many responses
   are used.  Some possibilites include, but not limited to,

   o  Use the first reply that arrives, and ignore the rest

   o  Wait for a certain amount of time, and choose the response
      carrying the least metric

   o  If there is more than one response carrying the least metric,
      perform load-balancing among them

   o  Consult local configuration to select a gateway

   If the target H3 belongs to one of the subnets that H1 participates
   in, and H3 is capable of sending L-ARP replies, H1 can use H3's
   response to send MPLS packets to H3.

4.  Attributes

   In addition to carrying a label stack to be used in the data plane,
   an L-ARP reply carries some attributes that are typically used in the
   control plane.  One of these is a metric.  The metric is the distance
   from the L-ARP server to the destination.  This allows an L-ARP
   client that receives multiple responses to decide which ones to use,
   and whether to load-balance across some of them.  The metric
   typically will be the IGP shortest path distance from server to the
   destination; this makes comparing metrics from different servers
   meaningful.

   Another attribute, carried in the LST TLV, is Entropy Label (EL)
   Capability.  This attribute says whether the destination is EL
   capable (ELC).  In Figure 1, if T3 advertises a label to reach H3 and
   T3 is ELC, T3 can include in its signaling to T1 that it is ELC.  In
   that case, if T1's L-ARP reply to H1 consists of a single label, T1
   can set the ELC bit in the label field of the LST TLV.  This tells H1
   that it may include (below the outermost label) an Entropy Label
   Indicator followed by an Entropy Label.  This will help improve load
   balancing across the MPLS Fabric, and possibly on the last hop to H3.








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5.  Client-Server Synchronization

   In an L-ARP reply, the server communicates several pieces of
   information to the client: its hardware address, the MPLS label,
   Entropy Label capability and metric.  Since ARP is a stateless
   protocol, it is possible that one of these changes without the client
   knowing, which leads to a loss of synchronization between the client
   and the server.  This loss of synchronization can have several
   undesirable effects.

   If the server's hardware address changes or the MPLS label is
   repurposed by the server for a different purpose, then packets may be
   sent to the wrong destination.  The consequences can range from
   suboptimally routed packets to dropped packets to packets being
   delivered to the wrong customer, which may be a security breach.
   This last may be the most troublesome consequence of loss of
   synchronization.

   If a destination transitions from entropy label capable to entropy
   label incapable (an unlikely event) without the client knowing, then
   packets encapsulated with entropy labels will be dropped.  A
   transition in the other direction is benign.

   If the metric changes without the client knowing, packets may be
   suboptimally routed.  This may be the most benign consequence of loss
   of synchronization.

   Standard ARP has similar issues.  These are dealt with in two ways:
   a) ARP bindings are time-bound; and b) an ARP server, recognizing
   that a change has occurred, can send unsolicited ARP messages
   ([RFC2002]).  Both these techniques are used in L-ARP: the validity
   of the MPLS label obtained using L-ARP is time-bound; an L-ARP client
   should periodically resend L-ARP requests to obtain the latest
   information, and time out entries in its ARP cache if such an update
   is not forthcoming.

   Furthermore, an L-ARP server may update an advertised label binding
   by sending an unsolicited L-ARP message if any of the parameters
   mentioned above change.  Likewise, an L-ARP server may withdraw its
   earlier advertisement by sending an unsolicited LARP-NAK message.

5.1.  Restart Handling

5.1.1.  Server Restart

   In order to support graceful restart, the L-ARP server must remember
   the advertised bindings across restarts.  The mechanism that the
   L-ARP server uses to accomplish this is outside the scope of this



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   document.  Some possible mechanisms are, usage of shared memory or
   non-volatile storage to store bindings.  Upon restart, the L-ARP
   server waits until the LSPs advertised in the previous incarnation
   are restored.  Then, it reconciles the L-ARP bindings with the
   current state of the LSPs, updating the clients with unsolicted L-ARP
   replies & NAK for bindings that have undergone changes.

   During the above procedure, the client does not really know that the
   server has restarted.  If there were no changes to the LSPs during
   restart, the client receives no updates.  If there were changes, then
   the client would receive unsolicited updates to the bindings, as it
   would on a normal change.  If the server does not successfully
   restart, the client's periodic refresh will detect the loss of
   connectivity and purge out the bindings.

   If the L-ARP server does not support graceful restart, it SHOULD
   withdraw the advertised bindings before shutting down.  Unplanned
   restarts rely on the slower perioidc refresh mechanism for re-
   synchronization.

5.1.2.  Client Restart

   If the client restarts gracefully, it re-acquires the bindings
   immediately after restart to learn about any changes.

   If the client does not support graceful restart, it leaves the
   bindings to age out.

5.2.  Expedited Reachability Determination

   As with other control protocols, the client and server may use data
   plane liveness detection mechanisms, such as Loss of Signal (LOS)
   and/or BFD, to expedite detection of loss of connectivity.  However,
   usage of these mechanisms are outside the scope of this document.

6.  Applicability

   L-ARP can be used between a host and its Top-of-Rack switch in a Data
   Center.  L-ARP can also be used between a DSLAM and its aggregation
   switch going to the B-RAS.  In seamless MPLS terms, L-ARP can be used
   between an "Access Node" (AN) (e.g., the DSLAM) and its first hop
   MPLS-enabled device in the context of Seamless MPLS
   [I-D.ietf-mpls-seamless-mpls].  The first-hop device is part of the
   MPLS Fabric, as is the Service Node (SN) (e.g., the B-RAS).  L-ARP
   helps create an MPLS tunnel from the AN to the SN, without requiring
   that the AN be part of the MPLS Fabric.  In all these cases, L-ARP
   can handle the presence of multiple connections between the access
   device and its first hop devices.



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   ARP is not a routing protocol.  The use of L-ARP should be limited to
   cases where an L-ARP client has Ethernet connectivity to its L-ARP
   servers.

7.  Backward Compatibility

   Since L-ARP uses a new hardware type, it is backward compatible with
   "regular" ARP.  ARP servers and clients MUST be able to send out,
   receive and process ARP messages based on hardware type.  They MAY
   choose to ignore requests and replies of some hardware types; they
   MAY choose to log errors if they encounter hardware types they do not
   recognize; however, they MUST handle all hardware types gracefully.
   For hardware types that they do understand, ARP servers and clients
   MUST handle operation codes gracefully, processing those they
   understand, and ignoring (and possibly logging) others.

8.  OAM

   L-ARP uses standard MPLS OAM procedures defined in [RFC4379] &
   [RFC6424].  Extending the definitions in section 3.2 of RFC 4379, we
   use a sub-type of [TO-BE-ASSIGNED-BY-IANA-1] to represent L-ARP IPv4
   FEC, and [TO-BE-ASSIGNED-BY-IANA-2] to represent L-ARP IPv6 FEC.  The
   following sub-sections define the format of L-ARP FEC's.

8.1.  L-ARP IPv4 FEC

   The L-ARP IPv4 FEC is defined as follows:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          IPv4 address                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IPv4 address is the tunnel destination address.

                          Figure 2: ARP IPv4 FEC

   The length of the L-ARP IPv4 FEC is 4 bytes.

8.2.  L-ARP IPv6 FEC

   The L-ARP IPv6 FEC is defined as follows:









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        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          IPv6 address                         |
       |                          (16 octets)                          |
       |                                                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IPv6 address is the tunnel destination address.

                          Figure 3: ARP IPv6 FEC

   The length of the L-ARP IPv6 FEC is 16 bytes.

9.  For Future Study

   The L-ARP specification is quite simple, and the goal is to keep it
   that way.  However, inevitably, there will be questions and features
   that will be requested.  Some of these are:

   1.  Keeping L-ARP clients and servers in sync.  In particular,
       dealing with:

       A.  client and/or server control plane restart

       B.  lost packets

       C.  timeouts

   2.  Dealing with scale.

   3.  If there are many servers, which one to pick?

   4.  How can a client make best use of underlying ECMP paths?

   5.  and probably many more.

   In all of these, it is important to realize that, whenever possible,
   a solution that places most of the burden on the server rather than
   on the client is preferable.

   These questions (and others that come up during discussions) will be
   dealt with in future versions of this draft.








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10.  L-ARP Message Format


      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           ar$hrd              |            ar$pro             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     ar$hln    |    ar$pln     |            ar$op              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                     ar$sha (ar$hln octets)                  //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                     ar$spa (ar$pln octets)                  //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                     ar$tha (ar$hln octets)                  //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                     ar$tpa (ar$pln octets)                  //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                     ar$lst (variable...)                    //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                     ar$att (variable...)                    //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 4: L-ARP Packet Format

   ar$hrd  Hardware Type: MPLS-over-Ethernet.  The value of the field
       used here is [HTYPE-MPLS].  To start with, we will use the
       experimental value HW_EXP2 (256)

   ar$pro  Protocol Type: IPv4/IPv6.  The value of the field used here
       is 0x0800 to resolve an IPv4 address and 0x86DD to resolve an
       IPv6 address.

   ar$hln  Hardware Length: 6.

   ar$pln  Protocol Address Length: for an IPv4 address, the value is 4;
       for an IPv6 address, it is 16.

   ar$op   Operation Code: set to 1 for request, 2 for reply, and 10 for
       ARP-NAK.  Other op codes may be used as needed.

   ar$sha  Source Hardware Address: In an L-ARP message, Source Hardware
       Address is the 6 octet sender's MAC address.

   ar$spa  Source Protocol Address: In an L-ARP message, this field
       carries the sender's IP address.






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   ar$tha  Target Hardware Address: In an L-ARP query message, Target
       Hardware Address is the all-ones Broadcast MAC address; in an
       L-ARP reply message, it is the client's MAC address.

   ar$tpa  Target Protocol Address: In an L-ARP message, this field
       carries the IP address for which the client is seeking an MPLS
       label.

   ar$lst  Label Stack: In an L-ARP request, this field is empty.  In an
       L-ARP reply, this field carries the MPLS label stack as an ARP
       TLV in the format below.

   ar$att  Attributes: In an L-ARP request, this field is empty.  In an
       L-ARP reply, this field carries attributes for the MPLS label
       stack as an ARP TLV in the format below.

   This document introduces the notion of ARP TLVs.  These take the form
   as in Figure 5.  Figure 6 describes the format of Label Stack TLV
   carried in L-ARP.  Figure 7 describes the format of Attributes TLV
   carried in L-ARP.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Length    |   Value (Length octets) ...   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type is the type of the TLV; Length is the length of the value field
   in octets; Value is the value field.

                            Figure 5: ARP TLVs

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Length    |   MPLS Label (20 bits)        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       |E|Z|Z|Z|     MPLS Label (20 bits)              |E|Z|Z|Z|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 6: MPLS Label Stack Format

   Label Stack:  Type = TLV-LST; Length = n*3 octets, where n is the
      number of labels.  The Value field contains the MPLS label stack
      for the client to use to get to the target.  Each label is 3
      octets.  This field is valid only in an L-ARP reply message.



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   E-bit:  Entropy Label Capable: this flag indicates whether the
      corresponding label in the label stack can be followd by an
      Entropy Label.  If this flag is set, the client has the option of
      inserting ELI and EL as specified in [RFC6790].  The client can
      choose not to insert ELI/EL pair.  If this flag is clear, the
      client MUST NOT insert ELI/EL after the corresponding label.

   Z  These bits are not used, and SHOULD be set to zero on sending and
      ignored on receipt.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Type     |    Length     |     Metric (4 octets) ...     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  ...  Metric                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 7: Attribute TLV

   Attributes TLV:  Type = TLV-ATT; Length = 4 octets.  The Value field
      contains the metric (typically, IGP distance) from the responder
      to the destination (device with the requested IP address).  If the
      responder is the destination, then the metric value is zero.  This
      field is valid only in an L-ARP reply message.

   If other parameters are deemed useful in the ATT TLV, they will be
   added as needed.

11.  Security Considerations

   There are many possible attacks on ARP: ARP spoofing, ARP cache
   poisoning and ARP poison routing, to name a few.  These attacks use
   gratuitous ARP as the underlying mechanism, a mechanism used by
   L-ARP.  Thus, these types of attacks are applicable to L-ARP.
   Furthermore, ARP does not have built-in security mechanisms; defenses
   rely on means external to the protocol.

   It is well outside the scope of this document to present a general
   solution to the ARP security problem.  One simple answer is to add a
   TLV that contains a digital signature of the contents of the ARP
   message.  This TLV would be defined for use only in L-ARP messages,
   although in principle, other ARP messages could use it as well.  Such
   an approach would, of course, need a review and approval by the
   Security Directorate.  If approved, the type of this TLV and its
   procedures would be defined in this document.  If some other
   technique is suggested, the authors would be happy to include the
   relevant text in this document, and refer to some other document for
   the full solution.



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12.  IANA Considerations

   IANA is requested to allocate a new ARP hardware type (from the
   registry hrd) for HTYPE-MPLS.

   IANA is also requested to create a new registry ARP-TLV ("tlv").
   This is a registry of one octet numbers.  Allocation policies: 0 is
   not to be allocated; the range 1-127 is Standards Action; the values
   128-251 are FCFS; and the values 252-255 are Experimental.

   Finally, IANA is requested to allocate two values in the ARP-TLV
   registry, one for TLV-LST and another for TLV-ATT.

13.  Acknowledgments

   Many thanks to Shane Amante for his detailed comments and
   suggestions.  Many thanks to the team in Juniper prototyping this
   work for their suggestions on making this variant workable in the
   context of existing ARP implementations.  Thanks too to Luyuan Fang,
   Alex Semenyaka and Dmitry Afanasiev for their comments and
   encouragement.

14.  References

14.1.  Normative References

   [RFC0826]  Plummer, D., "Ethernet Address Resolution Protocol: Or
              Converting Network Protocol Addresses to 48.bit Ethernet
              Address for Transmission on Ethernet Hardware", STD 37,
              RFC 826, DOI 10.17487/RFC0826, November 1982,
              <http://www.rfc-editor.org/info/rfc826>.

   [RFC2002]  Perkins, C., Ed., "IP Mobility Support", RFC 2002,
              DOI 10.17487/RFC2002, October 1996,
              <http://www.rfc-editor.org/info/rfc2002>.

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

   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
              Label Switched (MPLS) Data Plane Failures", RFC 4379,
              DOI 10.17487/RFC4379, February 2006,
              <http://www.rfc-editor.org/info/rfc4379>.






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   [RFC6424]  Bahadur, N., Kompella, K., and G. Swallow, "Mechanism for
              Performing Label Switched Path Ping (LSP Ping) over MPLS
              Tunnels", RFC 6424, DOI 10.17487/RFC6424, November 2011,
              <http://www.rfc-editor.org/info/rfc6424>.

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

14.2.  Informative References

   [I-D.gredler-idr-bgplu-epe]
              Gredler, H., Vairavakkalai, K., R, C., Rajagopalan, B.,
              Aries, E., and L. Fang, "Egress Peer Engineering using
              BGP-LU", draft-gredler-idr-bgplu-epe-06 (work in
              progress), June 2016.

   [I-D.ietf-mpls-seamless-mpls]
              Leymann, N., Decraene, B., Filsfils, C., Konstantynowicz,
              M., and D. Steinberg, "Seamless MPLS Architecture", draft-
              ietf-mpls-seamless-mpls-07 (work in progress), June 2014.

Authors' Addresses

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

   Email: kireeti.kompella@gmail.com


   Balaji Rajagopalan
   Juniper Networks, Inc.
   Prestige Electra, Exora Business Park
   Marathahalli - Sarjapur Outer Ring Road
   Bangalore  560103
   India

   Email: balajir@juniper.net









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   George Swallow
   Cisco Systems
   1414 Massachusetts Ave
   Boxborough, MA  01719
   US

   Email: swallow@cisco.com












































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