MPLS WG                                                      K. Kompella
Internet-Draft                                                 R. Balaji                                          Juniper Networks
Intended status: Standards Track                        Juniper Networks                               R. Balaji
Expires: November 1, 2015 March 11, 2016                           Juniper Networks, Inc.
                                                              G. Swallow
                                                           Cisco Systems
                                                          April 30,
                                                       September 8, 2015

                      Label Distribution Using ARP


   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, which is a key goal of the MPLS Fabric architecture.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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.

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   This Internet-Draft will expire on November 1, 2015. March 11, 2016.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Approach  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Overview of Ethernet ARP  . . . . . . . . . . . . . . . . . .   3
   3.  L-ARP Protocol Operation  . . . . . . . . . . . . . . . . . .   4
     3.1.  Basic Operation  Setup . . . . . . . . . . . . . . . . . . . . .   4 . . . . .   5
     3.2.  Asynchronous operation  Egress Operation  . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Ingress Operation . . . . . . . . . . . . . . . . . . . .   5
   4.  Attributes  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Client-Server Synchronization . . . . . . . . . . . . . .   5
     3.4. . .   6
   6.  Applicability . . . . . . . . . . . . . . . . . . . . . .   6
     3.5. . .   7
   7.  Backward Compatibility  . . . . . . . . . . . . . . . . .   6
   4. . .   7
   8.  For Future Study  . . . . . . . . . . . . . . . . . . . . . .   6
   5.   7
   9.  L-ARP Message Format  . . . . . . . . . . . . . . . . . . . .   7
   6.   8
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  10
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  11
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  Normative  11
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     13.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11  12

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 Centre; Center; here, MPLS tunnels originate from host
   machines.  The host thus needs a mechanism to acquire label bindings
   to participate in the MPLS Fabric, but in a simple, plug-and-play
   manner.  Existing signaling/routing protocols do not always meet this
   need.  Labeled ARP (L-ARP) is a proposal to fill that gap.

   [TODO-MPLS-FABRIC] describes the motivation for using MPLS as the
   fabric technology.

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.

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 5 9
   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

   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
   L-ARP servers.  T  T4 is a member of the MPLS Fabric that may not be an
   L-ARP server.  Within the MPLS Fabric, the 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.

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

                                 Figure 1

3.1.  Basic  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

3.2.  Egress Operation

   A node (say H1) T3) that needs wants an MPLS tunnel to a destination attached node (say H3)
   broadcasts over all its interfaces an L-ARP query with the Target
   Protocol Address set to H3.  A node that has reachability have MPLS
   reachability, allocates a label L3 to H3 (such
   as T1 reach H3, and advertises this
   label into the MPLS Fabric.  This can be triggered by configuration
   on T3, or T2) sends an L-ARP reply via some other protocol.  On receiving a packet with label
   L3, T3 pops the Source Hardware Address
   set 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 locally-allocated 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 label plus reachability to H3 sends an L-ARP reply with the Source
   Hardware Address set to its Ethernet MAC address.

   After 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 one or more this packet, T1 swaps the top label
   with the label(s) for its MPLS tunnel to H3.

   Note that H1 broadcasts its L-ARP replies, request over its attached
   interfaces.  H1 may receive several L-ARP replies; in that case, H1
   can select either T1 or
   T2 any subset of these to send MPLS packets that are destined to H3.
   As described later, the L-ARP response may contain certain parameters
   that enable the client to make an informed choice of the routers.

   As with standard ARP, choice.  If the validity target H3
   belongs to one of the MPLS label obtained using
   L-ARP subnets that H1 participates in, and H3 is time-bound.  The client should periodically resend its
   capable of sending L-ARP
   requests replies, H1 can use H3's response to obtain the latest information, and time out entries send
   MPLS packets to H3.

4.  Attributes

   In addition to carrying a label stack to be used in
   its ARP cache if such an update is not forthcoming.  Once the data plane,
   an L-ARP
   server has advertised a label binding, it MUST NOT change reply carries some attributes that are typically used in the binding
   until expiry
   control plane.  One of the binding's validity time.

   The mechanism defined here these is simplistic; see Section 4.

3.2.  Asynchronous operation

   The preceding sections described a request-response based model.  In
   some cases, metric.  The metric is the distance
   from the L-ARP server may want to asynchronously update its
   clients.  L-ARP uses the gratuitous ARP model [RFC2002] to "push"
   such changes.

   In a pure "push" model, a device may send out updates for all
   prefixes it knows about.  This naive approach will not scale well. destination.  This memo specifies a mode of operation that is somewhere between
   "push" and "pull" model.  An L-ARP server does not advertise any
   binding for a prefix until at least one allows an L-ARP
   client expresses
   interest in that prefix (by initiating an L-ARP query).  As long as 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 has at least one interested client for a prefix, to the
   server sends unsolicited (aka gratuitous, though
   destination; this makes comparing metrics from different servers

   Another attribute, carried in the term 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 less
   appropriate ELC, T3 can include in this context) its signaling to T1 that it is ELC.  In
   that case, if T1's L-ARP replies when reply to H1 consists of a prefix's
   reachability changes.  The server will deem single label, T1
   can set the client's interest ELC bit in
   a prefix to have ceased when the label field of the LST TLV.  This tells H1
   that it does not hear any L-ARP queries for
   some configured timeout period.

3.3. 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.

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 bad
   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

   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 relatively 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.

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.  More generally, L-ARP can be used between
   an "access node" "Access Node" (AN) (e.g., the DSLAM) and its first hop MPLS-enabled MPLS-
   enabled device in the context of Seamless MPLS [reference].
   [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.

   ARP is not a routing protocol.  The use of L-ARP should be limited to
   cases where the an L-ARP client has a small number of one-hop
   connections Ethernet connectivity to its L-ARP
   servers.  The presence of a complex topology
   between the L-ARP client and server suggests the use of a different


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.  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.  Withdrawing a response.

   3.  Dealing with scale.

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

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

   6.  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.

9.  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 2: 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.

   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

   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 3.  Figure 4 describes the format of Label Stack TLV
   carried in L-ARP.  Figure 5 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 3: 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 4: 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.

   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 5: 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).  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.


10.  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.


11.  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.


12.  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


13.  References

13.1.  Normative References

   [RFC0826]  Plummer, D., "Ethernet Address Resolution Protocol: Or
              converting network protocol addresses
              Converting Network Protocol Addresses to 48.bit Ethernet
              Address for transmission Transmission on Ethernet hardware", Hardware", STD 37,
              RFC 826, DOI 10.17487/RFC0826, November 1982. 1982,

   [RFC2002]  Perkins, C., Ed., "IP Mobility Support", RFC 2002,
              DOI 10.17487/RFC2002, October
              1996. 1996,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008. 1997,

   [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. 2012,

13.2.  Informative References

              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


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

   George Swallow
   Cisco Systems
   1414 Massachusetts Ave
   Boxborough, MA  01719