NAT traversal for LISP
draft-ermagan-lisp-nat-traversal-13

Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12                        
          13                                                            
Network Working Group                                         V. Ermagan
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Experimental                               D. Farinacci
Expires: March 10, 2018                                      lispers.net
                                                                D. Lewis
                                                              J. Skriver
                                                                F. Maino
                                                     Cisco Systems, Inc.
                                                                C. White
                                                   Logicalelegance, Inc.
                                                       September 6, 2017


                         NAT traversal for LISP
                draft-ermagan-lisp-nat-traversal-13.txt

Abstract

   This document describes a mechanism for IPv4 NAT traversal for LISP
   tunnel routers (xTR) and LISP Mobile Nodes (LISP-MN) behind a NAT
   device.  A LISP device both detects the NAT and initializes its
   state.  Forwarding to the LISP device through a NAT is enabled by the
   LISP Re-encapsulating Tunnel Router (RTR) network element, which acts
   as an anchor point in the data plane, forwarding traffic from
   unmodified LISP devices through the NAT.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   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 March 10, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Definition of Terms . . . . . . . . . . . . . . . . . . . . .   3
   3.  Basic Overview  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  LISP NAT Traversal Overview . . . . . . . . . . . . . . .   6
   4.  LISP RTR Message Details  . . . . . . . . . . . . . . . . . .   7
     4.1.  Info-Request Message  . . . . . . . . . . . . . . . . . .   7
     4.2.  LISP Info-Reply . . . . . . . . . . . . . . . . . . . . .   9
     4.3.  LISP Map-Register Message . . . . . . . . . . . . . . . .  10
     4.4.  LISP Map-Notify . . . . . . . . . . . . . . . . . . . . .  11
     4.5.  LISP Data-Map-Notify Message  . . . . . . . . . . . . . .  12
   5.  Protocol Operations . . . . . . . . . . . . . . . . . . . . .  14
     5.1.  xTR Processing  . . . . . . . . . . . . . . . . . . . . .  14
       5.1.1.  ETR Registration  . . . . . . . . . . . . . . . . . .  15
       5.1.2.  Map-Request and Map-Reply Handling  . . . . . . . . .  16
       5.1.3.  xTR  Sending and Receiving Data . . . . . . . . . . .  17
     5.2.  Map-Server Processing . . . . . . . . . . . . . . . . . .  18
     5.3.  RTR Processing  . . . . . . . . . . . . . . . . . . . . .  19
       5.3.1.  RTR Data Forwarding . . . . . . . . . . . . . . . . .  21
     5.4.  Multi-homed xTRs  . . . . . . . . . . . . . . . . . . . .  21
     5.5.  Example . . . . . . . . . . . . . . . . . . . . . . . . .  22
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  25
     6.1.  Acknowledgments . . . . . . . . . . . . . . . . . . . . .  26
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Introduction

   The Locator/ID Separation Protocol [LISP] defines a set of functions
   for encapsulating routers to exchange information used to map from
   Endpoint Identifiers (EIDs) to routable Routing Locators (RLOCs).
   The assumption that the LISP Tunnel Routers are reachable at their
   RLOC breaks when a LISP device is behind a NAT.  LISP relies on the
   xTR being able to receive traffic at its RLOC on destination port
   4341.  However nodes behind a NAT are only reachable through the
   NAT's public address and in most cases only after the appropriate



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   mapping state is set up in the NAT.  Depending on the type of the NAT
   device, this mapping state may be address and port dependent.  In
   other words, the mapping state in the NAT device may be associated
   with the 5 tuple that forms a specific flow, preventing incoming
   traffic from any LISP router other than the one associated with the 5
   tuple.  A NAT traversal mechanism is needed to make the LISP device
   behind a NAT reachable.

   This document briefly discusses available NAT traversal options, and
   then it introduces in detail a NAT traversal mechanism for LISP.  Two
   new LISP control messages - LISP Info-Request and LISP Info-Reply -
   are introduced in order to detect whether a LISP device is behind a
   NAT, and discover the global IP address and global ephemeral port
   used by the NAT to forward LISP packets sent by the LISP device.  A
   new LISP component, the LISP Re-encapsulating Tunnel Router (RTR),
   acts as a re-encapsulating LISP tunnel router [LISP] to pass traffic
   through the NAT, to and from the LISP device.  A modification to how
   the LISP Map-Register messages are sent allows LISP device to
   initialize NAT state to use the RTR services.  This mechanism
   addresses the scenario where the LISP device is behind the NAT, but
   the associated Map-Server [LISP-MS] is on the public side of the NAT.

2.  Definition of Terms

   LISP Info-Request:  A LISP control message sent by a LISP device to
      its Map-Server.

   LISP Info-Reply:  A LISP control message sent by a Map Server to a
      LISP device in response to an Info-Request control message.

   LISP Re-encapsulating Tunnel Router (RTR):  An RTR is a re-
      encapsulating LISP Router (see section 8 of the main LISP
      specification) [LISP].  One function that an RTR provides is
      enabling a LISP device to traverse NATs.

   LISP Data-Map-Notify:  A LISP Map-Notify message encapsulated in a
      LISP data header.

   LISP xTR-ID  A 128-bit field that, together with a site-ID, can be
      appended at the end of a Map-Register or Map-Notify message.  An
      xTR-ID is used as a unique identifier of the xTR that is sending
      the Map-Register and is especially useful for identifying multiple
      xTRs serving the same site/EID-prefix.  A value of all zeros
      indicate the xTR-ID is unspecified.

   LISP site-ID  A 64-bit field that, together with a xTR-ID, can be
      appended at the end of a Map-Register or Map-Notify message.  A
      site-ID is used as a unique identifier of a group of xTRs



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      belonging to the same site.  A value of 0 indicate the site-ID is
      unspecified.

   NAT:  "Network Address Translation is a method by which IP addresses
      are mapped from one address realm to another, providing
      transparent routing to end hosts".  "Traditional NAT would allow
      hosts within a private network to transparently access hosts in
      the external network, in most cases.  In a traditional NAT,
      sessions are uni-directional, outbound from the private network."
      --RFC 2663 [NAT].  Basic NAT and NAPT are two varieties of
      traditional NAT.

   Basic NAT:  "With Basic NAT, a block of external addresses are set
      aside for translating addresses of hosts in a private domain as
      they originate sessions to the external domain.  For packets
      outbound from the private network, the source IP address and
      related fields such as IP, TCP, UDP and ICMP header checksums are
      translated.  For inbound packets, the destination IP address and
      the checksums as listed above are translated." --RFC 2663[NAT].

   NAPT:  "NAPT extends the notion of translation one step further by
      also translating transport identifier (e.g., TCP and UDP port
      numbers, ICMP query identifiers).  This allows the transport
      identifiers of a number of private hosts to be multiplexed into
      the transport identifiers of a single external address.  NAPT
      allows a set of hosts to share a single external address.  Note
      that NAPT can be combined with Basic NAT so that a pool of
      external addresses are used in conjunction with port translation."
      --RFC 2663[NAT].  Transport identifiers of the destination hosts
      are not modified by the NAPT.

   In this document the general term NAT is used to refer to both Basic
   NAT and NAPT.

   While this document specifies LISP NAT Traversal for LISP tunnel
   routers, a LISP-MN can also use the same procedure for NAT traversal.
   The modifications attributed to a LISP-Device, xTR, ETR, and ITR must
   be supported by a LISP-MN where applicable, in order to achieve NAT
   traversal for such a LISP node.  A NAT traversal mechanism for LISP-
   MN is also proposed in [NAT-MN].

   For definitions of other terms, notably Map-Request, Map-Reply,
   Ingress Tunnel Router (ITR), and Egress Tunnel Router (ETR), please
   consult the LISP specification [LISP].







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3.  Basic Overview

   There are a variety of NAT devices and a variety of network
   topologies utilizing NAT devices in deployments.  Most NAT devices
   deployed today are designed primarily around the client/server
   paradigm, where client machines inside a private network initiate
   connections to public servers with public IP addresses.  As such, any
   protocol requiring a device or host in a private network behind a NAT
   to receive packets or accept sessions from destinations without first
   initiating a session or sending packets towards those destinations,
   will be challenged by deployed NAT devices.

   NAT devices are loosely classified based on how restrictive they are.
   These classifications are essentially identifying the type of mapping
   state that the NAT device is requiring to allow incoming traffic.
   For instance, the mapping state may be end-point independent: once
   device A inside the private network sends traffic to a destination
   outside, a mapping state in the NAT is created that only includes
   information about device A, namely its IP address and perhaps its
   port number.  Once this mapping is established in the NAT device, any
   external device with any IP address could send packets to device A.
   More restrictive NAT devices could include the 5 tuple information of
   the flow as part of the mapping state, in other words, the mapping
   state in the NAT is dependent upon Source IP and Port, as well as
   destination IP and port (symmetric NAT or Endpoint-dependent NAT).
   Such a NAT only allows traffic from the specified destination IP and
   port to reach the specified source device on the specified source
   port.  Traffic with a different 5 tuple signature will not be allowed
   to pass.  In general, in the case of less restrictive NATs it may be
   possible to eventually establish direct peer-to-peer connections, by
   means of various hole punching techniques and initial rendezvous
   servers.  However, in the case of symmetric NATs or NATs with
   endpoint-address-and-port-dependent mappings, direct connection may
   prove impossible.  In such cases a relay device is required that is
   in the public Network and can relay packets between the two
   endpoints.

   Various methods have been designed to address NAT traversal
   challenges, mostly in the context of peer-to-peer applications and
   protocols.  Among these, the Interactive Connectivity Establishment
   (ICE) [ICE] seems the most comprehensive, which defines a protocol
   that leverages other protocols such as Session Traversal Utilities
   for NAT(STUN) [STUN] and Traversal Using Relays around NAT (TURN)
   [TURN], as well as a rendezvous server to identify and exchange a
   list of potential transport (IP and Port) addresses between the two
   endpoints.  All possible pairs of transport addresses are
   exhaustively tested to find the best possible option for
   communication, preferring direct connection to connections using a



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   relay.  In the case of most restrictive NATs, ICE leads to use of
   TURN servers as relay for the traffic.  TURN requires a list of
   allowed peer IP addresses defined as permissions, before allowing a
   peer to use the relay server to reach a TURN client.

   Common NAT traversal techniques such as ICE generally assume bi-
   directional traffic with the same 5 tuple.  LISP, however, requires
   traffic to use destination UDP port 4341, without specifying the
   source port.  As a result, LISP traffic is generally uni-directional.
   This means that, in the case of symmetric or endpoint-address-and-
   port-dependent mapping NATs, even when an outgoing mapping is
   established, still incoming traffic may not match the established
   mapping and will not be allowed to pass.  As a result, while ICE may
   be used to traverse less restrictive NATs, use of standard TURN
   servers as relays to traverse symmetric NATs for LISP protocol is not
   possible.  The rest of this document specifies a NAT traversal
   technique for the LISP protocol that enables LISP protocol to
   traverse multiple types of NATs including symmetric NATs.

3.1.  LISP NAT Traversal Overview

   There are two attributes of a LISP device behind a typical NAT that
   requires special consideration in LISP protocol behavior in order to
   make the device reachable.  First, the RLOC assigned to the device is
   typically not globally unique nor globally routable.  The NAT likely
   has a restrictive translation table and forwarding policy, requiring
   outbound packets to create state before the NAT accepts inbound
   packets.  Second, LISP protocol requires an xTR to receive traffic on
   a specific UDP port 4341, so the random UDP port allocated by the NAT
   on its public side to associate with a xTR behind the NAT can not be
   used by other xTRs to send LISP traffic to.  This section provides an
   overview of the LISP NAT traversal mechanism which deals with these
   conditions.  The following sections specify the mechanism in more
   detail.

   When a LISP device receives a new RLOC and wants to register it with
   the mapping system, it needs to first discover whether it is behind a
   NAT.  To do this, an ETR queries its Map-Server to discover the ETR's
   translated global RLOC and port via the two new LISP messages: Info-
   Request and Info-Reply.  Once an ETR detects that it is behind a NAT,
   it uses a LISP Re-encapsulating Tunnel Router (RTR) entity as an
   anchor point for sending and receiving data plane traffic through the
   NAT device.  The ETR registers the RTR RLOC(s) to its Map-Server
   using the RTR as a proxy for the Map-Register message.  The ETR
   encapsulates the Map-Register message in a LISP ECM header destined
   to the RTR's RLOC.  The RTR strips the LISP ECM header, re-originates
   the Map-Register message, and sends it to the Map-Server.  This
   initializes state in the NAT device so the ETR can receive traffic on



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   port 4341 from the RTR.  The ETR also registers the RTR RLOC as the
   RLOC where the ETR EID prefix is reachable.  As a result, all packets
   destined to the ETR's EID will go to its RTR.  The RTR will then re-
   encapsulate and forward the ETR's traffic via the existing NAT state
   to the ETR.

   Outbound LISP data traffic from the xTR is also encapsulated to the
   RTR, where the RTR de-capsulates the LISP packets, and then re-
   encapsulates them or forwards them natively depending on their
   destination.

   In the next sections these procedures are discussed in more detail.

4.  LISP RTR Message Details

   The main modifications in the LISP protocol to enable LISP NAT
   traversal via an RTR include: (1) two new messages used for NAT
   discovery (Info-Request and Info-Reply), and (2) encapsulation of two
   LISP control messages (Map-Register and Map-Notify) between the xTR
   and the RTR.  Map-Register is encapsulated in an ECM header while
   Map-Notify is encapsulated in a LISP data header (Data-Map-Notify).
   This section describes the message formats and details of the Info-
   Request, Info-Reply, and Data-Map-Notify messages, as well as
   encapsulation details and minor changes to Map-Register and Map-
   Notify messages.

4.1.  Info-Request Message

   An ETR sends an Info-Request message to its Map-Server in order to

   1.  detect whether there is a NAT on the path to its Map-Server

   2.  obtain a list of RTR RLOCs that can be used for LISP data plane
       NAT traversal.

   An Info-Request message is a LISP control message, its source port is
   chosen by the xTR and its destination port is set to 4342.














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       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Type=7 |R|            Reserved                                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Nonce . . .                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      . . . Nonce                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              Key ID           |  Authentication Data Length   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                     Authentication Data                       ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              TTL                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Reserved    | EID mask-len  |        EID-prefix-AFI         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          EID-prefix                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             AFI = 0           |   <Nothing Follows AFI=0>     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     LISP Info-Request Message Format

      Type: 7 (Info-Request)

      R: R bit indicates this is a reply to an Info-Request (Info-
      Reply).  R bit is set to 0 in an Info-Request.  When R bit is set
      to 0, the AFI field (following the EID-prefix field) must be set
      to 0.  When R bit is set to 1, the packet contents follow the
      format for an Info-Reply, as described below.

      Reserved: Must be set to 0 on transmit and must be ignored on
      receipt.

      TTL: The time in minutes the recipient of the Info-Reply will
      store the RTR Information.

      Nonce: An 8-byte random value created by the sender of the Info-
      Request.  This nonce will be returned in the Info-Reply.  The
      nonce SHOULD be generated by a properly seeded pseudo-random (or
      strong random) source.

   Descriptions for other fields can be found in the Map-Register
   section of the main LISP draft [LISP].  Field descriptions for the
   LCAF AFI = 0 can be found in the LISP LCAF draft [LCAF] .





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4.2.  LISP Info-Reply

   When a Map-Server receives an Info-Request message, it responds with
   an Info-Reply message.  The Info-Reply message source port is 4342,
   and destination port is taken from the source port of the triggering
   Info-Request.  Map-Server fills the NAT LCAF (LCAF Type = 7) fields
   according to their description.  The Map-Server uses AFI=0 for the
   Private ETR RLOC Address field in the NAT LCAF.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Type=7 |R|               Reserved                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Nonce . . .                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      . . . Nonce                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Key ID             |  Authentication Data Length   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                     Authentication Data                       ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              TTL                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Reserved    | EID mask-len  |        EID-prefix-AFI         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          EID-prefix                           |
   +->+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  |           AFI = 16387         |    Rsvd1      |     Flags     |
   |  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  |    Type = 7     |     Rsvd2   |             4 + n             |
   |  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   N  |        MS UDP Port Number     |      ETR UDP Port Number      |
   A  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   T  |              AFI = x          | Global ETR RLOC Address  ...  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   L  |              AFI = x          |       MS RLOC Address  ...    |
   C  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   A  |              AFI = x          | Private ETR RLOC Address ...  |
   F  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  |              AFI = x          |      RTR RLOC Address 1 ...   |
   |  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  |              AFI = x          |       RTR RLOC Address n ...  |
   +->+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      LISP Info-Reply Message Format

   Type: 7 , R = 1, (Info-Reply)



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   The format is similar to the Info-Request message.  See Info-Request
   section for field descriptions.  Field descriptions for the NAT LCAF
   section can be found in the LISP LCAF draft [LCAF] .

4.3.  LISP Map-Register Message

   The third bit after the Type field in the Map-Register message is
   allocated as "I" bit.  I bit indicates that a 128 bit xTR-ID and a 64
   bit site-ID field are present at the end of the Map-Register message.
   If an xTR is configured with an xTR-ID or site-ID, it MUST set the I
   bit to 1 and include its xTR-ID and site-ID in the Map-Register
   messages it generates.  If either the xTR-ID or site-ID is not
   configured an unspecified value is encoded for whichever ID that is
   not configured.

   xTR-ID is a 128 bit field at the end of the Map-Register message,
   starting after the final Record in the message.  The xTR-ID is used
   to identify the intended recipient xTR for a Map-Notify message,
   especially in the case where a site has more than one xTR.  A value
   of all zeros indicate that an xTR-ID is not specified, though encoded
   in the message.  This is useful in the case where a site-ID is
   specified, but no xTR-ID is configured.  When a Map-Server receives a
   Map-Register with an xTR-ID specified (I bit set and xTR-ID has a
   non-zero value), it MUST copy the XTR-ID from the Map-Register to the
   associated Map-Notify message.  When a Map-Server is sending an
   unsolicited Map-Notify to an xTR to notify the xTR of a change in
   locators, the Map-Server must include the xTR-ID for the intended
   recipient xTR, if it has one stored locally.

   site-ID is a 64 bit field at the end of the Map-Register message,
   following the xTR-ID. site-ID is used by the Map-Server receiving the
   Map-Register message to identify which xTRs belong to the same site.
   A value of 0 indicate that a site-ID is not specified, though encoded
   in the message.  When a Map-Server receives a Map-Register with a
   site-ID specified (I bit set and site-ID has non-zero value), it must
   copy the site-ID from the Map-Register to the associated Map-Notify
   message.  When a Map-Server is sending an unsolicited Map-Notify to
   an xTR to notify the xTR of a change in locators, the Map-Server must
   include the site-ID for the intended recipient xTR, if it has one
   stored locally.

   A LISP device that sends a Map-Register to an RTR must encapsulate
   the Map-Register message using an Encapsulated Control Message (ECM)
   [LISP].  The 6th bit in the ECM LISP header is allocated as the "R"
   bit.  The R bit indicates that the encapsulated Map-Register is to be
   processed by an RTR.  The 7th bit in the ECM header is allocated as
   the "N" bit.  The N bit indicates that this Map-Register is being




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   relayed by an RTR.  When an RTR relays the ECM-ed Map-Register to a
   Map-Server, the N bit must be set to 1.

   The outer header source RLOC of the ECM is set to the LISP device's
   local RLOC, and the outer header source port is set to 4341.  The
   outer header destination RLOC and port are set to RTR RLOC and 4342
   respectively.  The inner header source RLOC is set to LISP device's
   local RLOC, and the inner source port is picked at random.  The inner
   header destination RLOC is set to the xTR's Map-Server RLOC, and
   inner header destination port is set to 4342.

4.4.  LISP Map-Notify

   The first bit after the Type field in a Map-Notify message is
   allocated as the "I" bit.  I bit indicates that a 128 bit xTR-ID and
   64 bit site-ID field is present at the end of the Map-Notify message,
   following the final Record in the Map-Notify (See Section 4.3 for
   details on xTR-ID and site-ID).  A Map-Server MUST set the I bit in a
   Map-Notify and include the xTR-ID and/or site-ID of the intended
   recipient xTR if the associated Map-Register has an xTR-ID and/or
   site-ID specified, or when the Map-Server has previously cached an
   xTR-ID and/or site-ID for the destination xTR.

   A LISP device that sends a Map-Notify to an RTR must encapsulate the
   Map-Notify message using an ECM.  The 6th bit in the ECM LISP header,
   allocated as the "R" bit, must be set when the encapsulated Map-
   Notify is to be processed by an RTR.  If the S bit is also set in the
   Map-Notify ECM header, it indicates that additional MS-RTR
   authentication data is included after the LISP header in the ECM.  If
   the I bit is also set in the Map-Notify, the xTR-ID and site-ID
   fields are included in the Map-Notify.  If a Map-Server receiving an
   ECM-ed Map-Register has a shared key associated with the sending RTR,
   it must generate a Map-Notify message with the S bit in the ECM
   header set to 1, and with the additional MS-RTR authentication
   related fields described 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    AD Type    |                  Reserved                     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |        MS-RTR Key ID          |  MS-RTR Auth. Data Length     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~               MS-RTR Authentication Data                      ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Changes to LISP Map-Notify Message




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   AD Type: 2 (RTR Authentication Data)

   MS-RTR Key ID: A configured ID to find the configured Message
   Authentication Code (MAC) algorithm and key value used for the
   authentication function.  See [LISP] section 14.4 for code point
   assignments.

   MS-RTR Authentication Data Length: The length in bytes of the MS-RTR
   Authentication Data field that follows this field.  The length of the
   Authentication Data field is dependent on the Message Authentication
   Code (MAC) algorithm used.  The length field allows a device that
   doesn't know the MAC algorithm to correctly parse the packet.

   MS-RTR Authentication Data: The message digest used from the output
   of the Message Authentication Code (MAC) algorithm.  The entire Map-
   Notify payload is authenticated.  After the MAC is computed, it is
   placed in this field.  Implementations of this specification MUST
   support HMAC-SHA-1-96 [RFC2404] and SHOULD support HMAC-SHA-256-128
   [RFC6234].

   For a full description of all fields in the Map-Notify message refer
   to Map-Notify section in the main LISP draft [LISP].

4.5.  LISP Data-Map-Notify Message

   When an RTR receives an ECM-ed Map-Notify message with R bit in the
   ECM header set to 1, it has to relay the Map-Notify payload to the
   registering LISP device.  After removing the ECM header and
   processing the Map-Notify message as described in Section 5.3, the
   RTR encapsulates the Map-Notify in a LISP data header and sends it to
   the associated LISP device.  This Map-Notify inside a LISP data
   header is referred to as a Data-Map-Notify message.



















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        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / |                       IPv4 or IPv6 Header                     |
   OH  |                      (uses RLOC addresses)                    |
     \ |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / |       Source Port = 4342      |       Dest Port = xxxx        |
   UDP +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ |           UDP Length          |        UDP Checksum           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   L   |                           LISP Header  ~                      |
   I \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   S / |                          ~ LISP Header                        |
   P   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / |                       IPv4 or IPv6 Header                     |
   IH  |                  (uses RLOC or EID addresses)                 |
     \ |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / |       Source Port = 4342      |       Dest Port = 4342        |
   UDP +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ |           UDP Length          |        UDP Checksum           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   LCM |                     LISP Map-Notify Message                   ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       LISP Data-Map-Notify Message

   In a Data-Map-Notify, the outer header source RLOC is set to the
   RTR's RLOC that was used in the associated Map-Register.  This is
   previously cached by the RTR.  The outer header source port is set to
   4342.  The outer header destination RLOC and port are filled based on
   the translated global RLOC and port of the registering LISP device
   previously stored locally at the RTR.  The inner header source
   address is Map-Server's RLOC, and inner header source port is 4342.
   The inner header destination address is set to the LISP device's
   local RLOC also previously cached by the RTR (See Section 5.3 for
   details.).  The inner header destination port is 4342.

   Since a Data-Map-Notify is a control message encapsulated in a LISP
   data header, a special Instance ID is used as a signal for the xTR to
   trigger processing of the control packet inside the data header.  The
   Instance ID value 0xFFFFFF is reserved for this purpose.  The
   Instance ID field in a Data-Map-Notify must be set to 0xFFFFFF.







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5.  Protocol Operations

   There are two main steps in the NAT traversal procedure.  First, the
   ETR's translated global RLOC must be discovered.  Second, the NAT
   translation table must be primed to accept incoming connections.  At
   the same time, the Map-Server and the RTR must be informed of the
   ETR's translated global RLOC including the translated ephemeral port
   number(s) at which the Map-Server and RTR can reach the LISP device.

5.1.  xTR Processing

   Upon receiving a new local RLOC, an ETR first has to detect whether
   the new RLOC is behind a NAT device.  For this purpose the ETR sends
   an Info-Request message to its Map-Server in order to discover the
   ETR's translated global RLOC as it is visible to the Map-Server.  The
   ETR uses its new local RLOC as the source RLOC of the message.  The
   Map-Server, after authenticating the message, responds with an Info-
   Reply message.  The Map-Server includes the source RLOC and port from
   the Info-Request message in the Global ETR RLOC Address and ETR UDP
   Port Number fields of the Info-Reply.  The Map Server also includes
   the destination RLOC and port number of the Info-Request message in
   the MS RLOC Address and MS UDP Port Number fields of the Info-Reply.
   In addition, the Map-Server provides a list of RTR RLOCs that the ETR
   may use in case it needs NAT traversal services.  The source port of
   the Info-Reply is set to 4342 and the destination port is copied from
   the source port of the triggering Info-Request message.

   Upon receiving the Info-Reply message, the ETR compares the source
   RLOC and source port used for the Info-Request message with the
   Global ETR RLOC Address and ETR UDP Port Number fields of the Info-
   Reply message.  If the two are not identical, the ETR concludes that
   its new local RLOC is behind a NAT and that it requires an RTR for
   NAT traversal services in order to be reachable at that RLOC.  An ETR
   behind other statefull devices (e.g. statefull firewalls) may also
   use an RTR and the procedure specified here for traversing the
   statefull device.  Detecting existence of such devices are beyond
   scope of this document.

   It is worth noting that a STUN server can also be used to do NAT
   detection and to discover the NAT-translated public IP address and
   port number for the ETR behind NAT.  If a STUN server is used, list
   of RTR devices that can be used by the xTR for NAT traversal must be
   provisioned to the xTR via other means which are outside the scope of
   this document.

   If there is no NAT on the path identified by an info-Request and an
   Info-Reply, the ETR registers the associated RLOC with its Map-Server
   as described in the main LISP draft [LISP].



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5.1.1.  ETR Registration

   Once an ETR has detected that it is behind a NAT, based on local
   policy the ETR selects one (or more) RTR(s) from the RTR RLOCs
   provided in the Info-Reply and initializes state in the NAT device in
   order to receive LISP data traffic on UDP port 4341 from the selected
   RTR.  To do so, the ETR sends a Map-Register encapsulated in an ECM
   header to the selected RTR(s).  The Map-Register message is created
   as specified in [LISP].  More specifically, the source RLOC of the
   Map-Register is set to ETR's local RLOC, while the destination RLOC
   is set to the ETR's Map-Server RLOC, and destination port is set to
   4342.  The ETR sets the M bit (want-Map-Notify) in Map-Register to 1,
   and it includes the selected RTR RLOC(s) as the locators in the Map-
   Register message.  The ETR can also include its local RLOCs as
   locators in the Map-Register, including weight and priorities, while
   setting the R bit to 0 for each local RLOC.  This can be used by the
   RTR for load balancing when forwarding data to a multi-homed xTR
   behind a NAT.  The R bit is set to 1 for all RTR locators included in
   the Map-Register.  The ETR must also set the I bit in the Map-
   Register message to 1 and include its xTR-ID in t he corresponding
   field.  In the ECM header of this Map-Register the source RLOC is set
   to ETR's local RLOC and the source port is set to 4341, while the
   destination RLOC is the RTR's RLOC and the destination port is set to
   LISP control port 4342.  The R bit in the ECM header is also set to
   1, to indicate that this EDCM-ed Map-Register is to be processed by
   an RTR.

   This ECM-ed Map-Register is then sent to the RTR.  The RTR removes
   the ECM header, re-originates the Map-Register message, encapsulates
   the new Map-Register in a new ECM header with R bit set to 0, and
   sends it to the associated Map-Server.  The RTR then encapsulates the
   corresponding Map-Notify message in a LISP data header (Data-Map-
   Notify) and sends it back to the xTR.

   Upon receiving a Data-Map-Notify from the RTR, the ETR must strip the
   outer LISP data header, and process the inner Map-Notify message as
   described in [LISP].  Since outer header destination port in Data-
   Map-Notify is set to LISP data port 4341, the Instance ID 0xFFFFFF in
   the LISP header of the Data-Map-Notify is used by the ETR to detect
   and process the Data-Map-Notify as a control message encapsulated in
   a LISP data header.  While processing the Data-Map-Notify, the xTR
   also stores the RTR RLOC(s) as its data plane proxy for the
   interface/RLOC behind the NAT.

   If the xTR is not multi-homed, or if all its interfaces are behind
   the NAT and will use the same RTR, then the xTR MAY map the EID
   prefix 0/0 to this RTR RLOC(s) in its map-cache.  This results in the
   xTR encapsulating all LISP data plane traffic to this RTR, reducing



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   the state created in the NAT.  Note that not installing the default
   map-cache entry will lead to normal Map-Request and Map-Reply
   messages for EID mapping lookups.  If outgoing traffic is sent
   directly to destinations without passing through the RTR, this will
   result in additional state to be created in the NAT device.

   At this point the registration and state initialization is complete
   and the xTR can use the RTR services.  The state created in the NAT
   device based on the ECM-ed Map-Register and corresponding Data-Map-
   Notify is used by the xTR behind the NAT to send and receive LISP
   control packets to/from the RTR, as well as for receiving LISP data
   packets form the RTR.

   If ETR receives a Data-Map-Notify with a xTR-ID specified, but the
   xTR-ID is not equal to its local xTR-ID, it must log this as an
   error.  The ETR should discard such Data-Map-Notify message.

   The ETR must periodically send ECM-ed Map-Register messages to its
   RTR in order to both refresh its registration to the RTR and the Map-
   Server, and as a keep alive in order to preserve the state in the NAT
   device.  RFC 2663 [NAT] points out that the period for sending the
   keep alives can be set to default value of two minutes, however since
   shorter timeouts may exist in some NAT deployments, the interval for
   sending periodic ECM-ed Map-Registers must be configurable.

5.1.2.  Map-Request and Map-Reply Handling

   The ETR is in control of how to handle the Map-Requests and Map-
   Replies.  If the ETR wants the Map-Server to proxy-reply as described
   in [LISP], it can register its locators, including the RTR RLOC(s),
   via the ECM-ed Map-Register message.  In this case, if the proxy bit
   is set in the Map-Register, the Map-Server will proxy reply to all
   Map-Requests for the ETR.  As a result traffic for the ETR can be
   encapsulated to its RTR(s).

   If the proxy bit in the ECM-ed Map-Register message is not set, and
   the ETR chooses to receive Map-Requests, the ETR must also initiate
   and preserve state in the NAT device to receive LISP control packets
   from its Map-Server.  To do this, the ETR must periodically send
   Info-Request messages to its Map-Server, and receive Info-Reply
   messages from the Map-Server.  As pointed in RFC 2663 [NAT] the
   default assumption of two minute period for session lifetime can be
   used, however since shorter timeouts may exist in some NAT
   deployments, the interval for sending periodic Info-Requests must be
   configurable.  Furthermore, the ETR must also provide its Map-Server
   with the ETR's translated global RLOC and port as visible to the Map-
   Server.  To do this, ETR includes a copy of the NAT LCAF section of
   the Info-Reply message as one of the locators in its Map-Register



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   along with the RTR(s) RLOC(s).  The ETR can set the priorities of RTR
   RLOC(s) in this Map-Register to 255, resulting in the Map Server
   encapsulating Map-Requests to the ETR's translated global RLOC and
   port so it can receive them through the NAT device.

   If an ETR behind a NAT chooses to receive Map-Requests from the Map-
   Server, it must send Map-Replies to requesting ITRs.  Note that this
   configuration will result in excessive state in the NAT device and is
   not recommended.  ETR must include its RTR RLOC(s) as its locator set
   in the Map-Reply in order to receive data through the NAT device.

   When an ITR behind a NAT is encapsulating outbound LISP traffic, it
   can use its RTR RLOC as the locator for all destination EIDs that it
   wishes to send data to.  As such, the ITR does not need to send Map-
   Requests for the purpose of finding EID-to-RLOC mappings.  However,
   the ITR can choose to send Map-Requests, specially if the ITR is
   multihomed, and could use other interfaces not behind the NAT.  It
   should be noted that sending packets directly to destination RLOCs
   through the interface behind the NAT will result in creating
   additional state in the NAT device.

   For RLOC-probing, the periodic ECM-ed Map-Register and Data-Map-
   Notify messages between xTR and RTR can also serve the purpose of
   RLOC probes.  However, if RLOC-probing is used, no changes are
   required to the RLOC-probing specification in [LISP], except that the
   LISP device behind a NAT only needs to probe the RTR's RLOC.

5.1.3.  xTR Sending and Receiving Data

   When a Map-Request for a LISP device behind a NAT is received by its
   Map-Server or the LISP device itself, the Map-Server, or the LISP
   device (ETR), responds with a Map-Reply including RTR's RLOC as the
   locator for the requested EID.  As a result, all LISP data traffic
   destined for the ETR's EID behind the NAT is encapsulated to its RTR.
   The RTR re-encapsulates the LISP data packets to the ETR's translated
   global RLOC and port number so the data can pass through the NAT
   device and reach the ETR.  As a result the ETR receives LISP data
   traffic with outer header destination port set to 4341 as specified
   in [LISP].

   For sending outbound LISP data, an ITR behind a NAT SHOULD use the
   RTR RLOC as the locator for all EIDs that it wishes to send data to
   via the interface behind the NAT.  The ITR then encapsulates the LISP
   traffic in a LISP data header with outer header destination set to
   RTR RLOC and outer header destination port set to 4341.  This may
   create a secondary state in the NAT device.  ITR SHOULD set the outer
   header source port in all egress LISP data packets to a random but




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   static port number in order to avoid creating excessive state in the
   NAT device.

   If the ITR and ETR of a site are not collocated, the RTR RLOC must be
   configured in the ITR via an out-of-band mechanism.  Other procedures
   specified here would still apply.

5.2.  Map-Server Processing

   Upon receiving an Info-Request message a Map-Server first verifies
   the authenticity of the message.  Next the Map-Server creates an
   Info-Reply message and copies the source RLOC and port number of the
   Info-Request message to the Global ETR RLOC Address and ETR UDP Port
   Number fields of the Info-Reply message.  The Map-Server also
   includes a list of RTR RLOCs that the ETR may use for NAT traversal
   services.  The Map-Server sends the Info-Reply message to the ETR, by
   setting the destination RLOC and port of the Info-Reply to the source
   RLOC and port of the triggering Info-Request.  The Map-Server sets
   the source port of the Info-Reply to 4342.

   Upon receiving an ECM-ed Map-Register message with the N bit in the
   ECM header set to 1, the Map-Server removes the ECM header and if the
   M bit in the Map-Register is set, the Map-Server processes the Map-
   Register message and generates the resulting Map-Notify as described
   in [LISP].  The Map-Server encapsulates the Map-Notify in an ECM
   header and sets the R bit in the ECM header to 1.  This indicates
   that the ECM-ed Map-Notify is to be processed by an RTR.  If the Map-
   Server has a shared secret configured with the RTR sending the Map-
   Register, the Map-Server also sets the S bit in the ECM header of the
   Map-Notify and includes the MS-RTR authentication data after the ECM
   LISP header.  See Security Considerations Section for more details.
   If the I bit is set in the Map-Register message, the Map-Server also
   locally stores the xTR-ID from the Map-Register, and sets the I bit
   in the corresponding Map-Notify message and includes the same xTR-ID
   in the Map-Notify.  The ECM-ed Map-Notify is then sent to the RTR
   sending the corresponding Map-Register.

   If a Map-Server is forwarding Map-Requests to an ETR which has
   registered its RLOC in a NAT LCAF, Map-Server must use the ETR Global
   RLOC Address and ETR UDP Port as the destination RLOC and port for
   outer header of the encapsulated Map-Requests.  If more than one NAT
   LCAF is registered for the same EID prefix, the Map-Server must use
   the NAT LCAF corresponding to the RLOC of this Map-Server.








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5.3.  RTR Processing

   Upon receiving an ECM-encapsulated Map-Register with the R bit set in
   the ECM header, the RTR creates a map-cache entry for the EID-prefix
   that was specified in the Map-Register message.  The RTR stores the
   outer header source RLOC and outer header source port, the outer
   header destination RLOC (RTR's own RLOC), the inner header source
   RLOC (xTR's local RLOC), the xTR-ID, the weight and priority
   associated with the xTR's local RLOC that was used to send this Map-
   Register if present, and the nonce field of the Map-Register in this
   local map-cache entry.  The RTR uses the inner header source address
   to identify which xTR local RLOC (R bit =0) was used by the xTR to
   send this Map-Register.  The outer header source RLOC and outer
   header source port is the ETR's translated global RLOC and port
   number visible to the RTR.  Once the registration process is
   complete, this map-cache entry can be used to send LISP data traffic
   to the ETR.  The inner header source RLOC of the Map-Register is the
   ETR's local RLOC behind the NAT, and the outer header destination
   RLOC is the RTR's RLOC used by the ETR.  The RTR can later use these
   fields as the inner header destination RLOC and source RLOC
   correspondingly, for sending data-encapsulated control messages
   (Data-Map-Notify) back to the ETR.  The nonce field is used for
   security purposes and is matched with the nonce field in the
   corresponding Map-Notify message.  This map-cache entry is stored as
   an "unverified" mapping, until the corresponding Map-Notify message
   is received.

   In the cases where the xTR has multiple RLOCs behind the NAT, and
   requires the RTR to load balance the traffic across those interfaces,
   the xTR must include the local RLOCs associated with each interface
   behind the NAT with the R bit in the locator record set to 0 in the
   ECM-ed Map-Register sent to the RTR.  The RTR uses the weight and
   priority policies of the RLOCs with R=0 in the Map-Register to load
   balance the traffic from the RTR to the xTR behind the NAT.  The RTR
   compares the RLOCs with the R bit set to 0 in the Map-Register to the
   inner header source address of the Map-Register to find the matching
   RLOC that the xTR used to send the Map-Register from.  The RTR
   associates the weight and priority policies of this local RLOC with
   the NAT-translated RLOC and xTR-ID for this map-cache entry.  For all
   other local RLOCs included in the Map-Register, that the Map-Register
   is not originating from, the RTR only updates previously cached
   weight and priority policies if it already has those local RLOCs
   previously stored for that EID prefix and xTR-ID.  In other words,
   the RTR only adds new local RLOCs and their weight and priority
   policies to its cache if the Map-Register is actually originating
   from that RLOC.  The TTL for every map-cache is also only updated
   when a Map-Register is originating from the same RLOC.  However, the
   weight and priorities of all previously cached local RLOCs will be



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   updated by every Map-Register, whether it is originating from that
   RLOC or not.  The xTR-ID is used to define the Merge domain for these
   RLOCs.  In other words, a Map-Register originating from a unique xTR-
   ID will always overwrite previously stored policies for that xTR-ID.
   However it does not modify in any way the policies indicated by any
   other xTR-ID serving the same EID prefix.  As a result, in the case
   of a renumbering or xTR reboot, the xTR uses its unique xTR-ID to
   send a new Map-Register, overwriting the previously stored policies
   for that xTR.  Using this method the xTR can immediately remove any
   RLOCs from the RTR cache that are no longer active.  In order to
   implement this, the RTR must compare the list of local RLOCs in the
   Map-Register (R=0) with the ones it has previously cached associated
   with the same xTR-ID.  If there is any RLOC previously cached that
   does not appear in the newly received Map-Register, the RTR must
   remove that RLOC together with the associated translated RLOC and
   associated policies, because removal of a local (behind-the-NAT) RLOC
   also invalidates the NAT-ed address associated with it. .

   After filling the local map-cache entry, the RTR strips the outer
   header and extracts the Map-Register message, re-originates the
   message by rewriting the source RLOC of the Map-Register to RTR's
   RLOC, encapsulated in a new ECM header with the R bit set to 0, and N
   bit set to 1, and sends the ECM-ed Map-Register to destination Map-
   Server.

   Map-Server responds with a ECM-ed Map-Notify message to the RTR.

   Upon receiving an ECM-ed Map-Notify message with R bit set to 1 in
   the ECM header, if the S bit in ECM header is set to 1, RTR uses the
   MS-RTR Key ID to verify the MS-RTR Authentication Data included after
   the ECM header.  If the MS-RTR authentication fails, the RTR must
   drop the packet.  Once the authenticity of the message is verified,
   RTR can confirm that the Map-Register message for the ETR with the
   matching xTR-ID was accepted by the Map-Server.  At this point the
   RTR can change the state of the associated map-cache entry to
   verified for the duration of the Map-Register TTL.

   The RTR then uses the information in the associated map-cache entry
   to create a Data-Map-Notify message according to the following
   procedure: RTR rewrites the inner header destination RLOC of the Map-
   Notify message to ETR's local RLOC.  Inner header destination port is
   4342.  The RTR encapsulates the Map-Notify in a LISP data header,
   where the outer header destination RLOC and port number are set to
   the ETR's translated global RLOC and port number.  If more than one
   ETR translated RLOC and port exists in the map-cache entry for the
   same EID prefix specified in the Map-Notify, the RTR can use the xTR-
   ID from the Map-Notify to identify which ETR is the correct
   destination for the Data-Map-Notify.  The RTR sets the outer header



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   source RLOC to RTR's RLOC from the map-cache entry and the outer
   header source port is set to 4342.  The RTR also sets the Instance ID
   field in the LISP header of the Data-Map-Notify to 0xFFFFFF.  The RTR
   then sends the Data-Map-Notify to the ETR.

   If the S bit is set to 0 in the ECM header of the Map-Notify, and the
   RTR has a shared key configured locally with the sending Map-Server,
   the RTR must drop the packet.  If the S bit is set to 0, and the RTR
   does not have a shared key configured with the associated Map-Server,
   according to local policy, the RTR may drop the packet.  If the Map-
   Notify with S bit set to 0 is processed, the RTR must match the nonce
   field from this Map-Notify with the nonce stored in the local map-
   cache entry with the matching xTR-ID.  If the nonces do not match,
   the RTR must drop the packet.

5.3.1.  RTR Data Forwarding

   For all LISP data packets encapsulated to RTR's RLOC and outer header
   destination port 4341, the RTR first verifies whether the source or
   destination EID is a previously registered EID.  If so, the RTR must
   process the packet according to the following.  If the destination or
   source EID is not a registered EID, the RTR can drop or process the
   packets based on local policy.

   In the case where the destination EID is a previously registered EID,
   the RTR must strip the LISP data header and re-encapsulate the packet
   in a new LISP data header.  The outer header RLOCs and UDP ports are
   then filled based on the matching map-cache entry for the associated
   destination EID prefix.  The RTR uses the RTR RLOC from the map-cache
   entry as the outer header source RLOC.  The outer header source port
   is set to 4342.  The RTR sets the outer header destination RLOC and
   outer header destination port based on the ETR translated global RLOC
   and port stored in the map-cache entry.  Then the RTR forwards the
   LISP data packet.

   In the case where the source EID is a previously registered EID, the
   RTR process the packet as if it is a Proxy ETR (PETR).  The RTR must
   strip the LISP data header, and process the packet based on its inner
   header destination address.  The packet may be forwarded natively, it
   may be LISP encapsulated to the destination ETR, or it may trigger
   the RTR to send a LISP Map-Request.

5.4.  Multi-homed xTRs

   In the case where an xTR has multiple interfaces and RLOCs, info-
   Requests can be sent per each interface and NAT discovery is done per
   each interface.  NAT traversal is accomplished by following state and
   processes described above per each interface/RLOC.  In other words,



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   if multiple interfaces of an xTR are behind a NAT, the ECM-ed Map-
   Register messages should be sent via each xTR interface behind NAT if
   the xTR desires to receive traffic via that interface.  This is
   required to establish the state in the NAT device for that interface.
   The M bit (want Map-Notify) must be set in ECM-ed Map-Register
   messages sent from at least one of xTR interfaces behind the NAT.  If
   additional interfaces behind the NAT are using the same RTR for NAT
   traversal, no Map-Notify processing is required for such interfaces
   and M bit in Map-Register can be set to 0 for these to reduce
   processing on the RTR and the Map-Server.

   The RLOCs included in Map-Register messages when the xTR has multiple
   interfaces SHOULD be the union of the locators (behind NAT or not)
   resulting from the process defined above per each RLOC of the xTR,
   according to the specifics of that interface (whether it is behind
   the NAT or not).

   In cases where some xTR interfaces are behind NAT while others are
   not, ECM-ed Map-Register messages should be sent via interfaces
   behind the NAT through the selected RTRs. xTR can receive traffic via
   both types of interfaces by including the associated RLOCs (as well
   as the RTR RLOCs) in its ECM-ed Map-Register messages.

5.5.  Example

   What follows is an example of an ETR initiating a registration of a
   new RLOC to its Map-Server, when there is a NAT device on the path
   between the ETR and the Map-Server.

   In this example, the ETR (site1-ETR) is configured with the local
   RLOC of 192.168.1.2.  The NAT's global (external) addresses are from
   2.0.0.1/24 prefix.  The Map-Server is at 3.0.0.1.  And one potential
   RTR has an IP address of 1.0.0.1.  The site1-ETR has an EID Prefix of
   128.1.0.0/16.

   An example of the registration process follows:

   1.   The Site1-ETR receives the private IP address, 192.168.1.2 as
        its RLOC via DHCP.

   2.   The Site1-ETR sends an Info-Request message with the destination
        RLOC of the Map-Server, 3.0.0.1, and source RLOC of 192.168.1.2.
        This packet has the destination port set to 4342 and the source
        port is set to (for example) 5001.

   3.   The NAT device translates the source IP from 192.168.1.2 to
        2.0.0.1, and source port to (for example) 20001 global ephemeral
        source port.



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   4.   The Map-Server receives and responds to this Info-Request with
        an Info-Reply message.  This Info-Reply has the destination
        address set to ETR's translated address of 2.0.0.1 and the
        source address is the Map-Server's RLOC, namely 3.0.0.1.  The
        destination port is 20001 and the source port is 4342.  Map-
        Server includes a copy of the source address and port of the
        Info-Request message (2.0.0.1:20001), and a list of RTR RLOCs
        including RTR RLOC 1.0.0.1 in the Info-Reply contents.

   5.   The NAT translates the Info-Reply packet's destination IP from
        2.0.0.1 to 192.168.1.2, and translates the destination port from
        20001 to 5001, and forwards the Info-Reply to site1-ETR at
        192.168.1.2.

   6.   The Site1-ETR detects that it is behind a NAT by comparing its
        local RLOC (192.168.1.2) with the Global ETR RLOC Address in the
        Info-Reply (2.0.0.2) . Then site1-ETR picks the RTR 1.0.0.1 from
        the list of RTR RLOCs in the Info-Reply.  ETR stores the RTR
        RLOC in a default map-cache entry to periodically send ECM-ed
        Map-Registers to.

   7.   The ETR sends an ECM encapsulated Map-Register to RTR at
        1.0.0.1.  The outer header source RLOC of this Map-Register is
        set to 192.168.1.2 and the outer header source port is set to
        4341.  The outer header destination RLOC and port are set to RTR
        RLOC at 1.0.0.1 and 4342 respectively.  The R bit in ECM header
        is set to 1.  The inner header destination RLOC is set to ETR's
        Map-Server 3.0.0.1, and the inner header destination port is set
        to 4342.  The inner header source RLOC is set to ETR's local
        RLOC 192.168.1.2.  In the Map-Register message the RTR RLOC
        1.0.0.1 appears as the locator set for the ETR's EID prefix
        (128.1.0.0/16).  In this example ETR also sets the Proxy bit in
        the Map-Register to 1, and sets I bit to 1, and includes its
        xTR-ID in the Map-Register.

   8.   The NAT translates the source RLOC in the ECM header of the Map-
        Register, by changing it from 192.168.1.2 to 2.0.0.2, and
        translates the source port in the ECM header from 4341 to (for
        example) 20002, and forwards the Map-Register to RTR.

   9.   The RTR receives the Map-Register and creates a map-cache entry
        with the ETR's xTR-ID, EID prefix, and the source RLOC and port
        of the ECM header of the Map-Register as the locator
        (128.1.0.0/16 is mapped to 2.0.0.2:20002).  RTR also caches the
        inner header source RLOC of the Map-Register namely 192.168.1.2,
        and the outer header destination RLOC of the ECM header in the
        Map-Register (this would be RTR's RLOC 1.0.0.1 ) to use for
        sending back a Data-Map-Notify.  RTR then removes the outer



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        header, re-writes the source RLOC of the Map-Register message to
        its own RLOC 1.0.0.1, adds a new ECM header with R=0, and N=1,
        and forwards the Map-Register to the destination Map-Server.

   10.  The Map-Server receives the ECM-ed Map-Register with N bit set
        to 1, removes the ECM header, and processes it according to
        [LISP].  Since Map-Server has a shared secret with the sending
        RTR, after registering the ETR, Map-Server responds with a ECM-
        ed Map-Notify with the R bit and S bit both set to 1 in the ECM
        header and including the MS-RTR authentication data.  Since the
        I bit is set in the Map-Register, the Map-Server also sets the I
        bit in the Map-Notify and copies the xTR-ID from the Map-
        Register to the Map-Notify.  The source address of this Map-
        Notify is set to 3.0.0.1.  The destination is RTR 1.0.0.1, and
        both source and destination ports are set to 4342.

   11.  The RTR receives the ECM-ed Map-Notify and verifies the MS-RTR
        authentication data.  The RTR data-encapsulates the Map-Notify
        and sends the resulting Data-Map-Notify to site1-ETR with a
        matching xTR-ID.  The outer header source RLOC and port of the
        Data-Map-Notify are set to 1.0.0.1:4342.  The outer header
        destination RLOC and port are retrieved from previously cached
        map-cache entry in step 9 namely 2.0.0.2:20002.  RTR also sets
        the inner header destination address to site1-ETR's local
        address namely 192.168.1.2.  RTR sets the Instance ID in the
        LISP header to 0xFFFFFF.  At this point RTR marks ETR's EID
        prefix as "Registered" status and forwards the Data-Map-Notify
        to ETR.

   12.  The NAT device translates the destination RLOC and port of the
        Data-Map-Notify to 192.168.1.2:4341 and forwards the packet to
        ETR.

   13.  The Site1-ETR receives the packet with a destination port 4341,
        and processes the packet as a control packet after observing the
        Instance ID value 0xFFFFFF in the LISP header.  At this point
        ETR's registration to the RTR is complete.

   Assume a requesting ITR in a second LISP (site2-ITR) site has an RLOC
   of 74.0.0.1.  The following is an example process of an EID behind
   site2-ITR sending a data packet to an EID behind the site1-ETR:

   1.  The ITR sends a Map-Request which arrives via the LISP mapping
       system to the ETR's Map Server.

   2.  The Map-Server sends a Map-Reply on behalf of the ETR, using the
       RTR's RLOC (1.0.0.1) in the Map-Reply's Locator Set.




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   3.  The ITR encapsulates a LISP data packet with ITR's local RLOC
       (74.0.0.1) as the source RLOC and the RTR as the destination RLOC
       (1.0.0.1) in the outer header.

   4.  The RTR decapsulates the packet, evaluates the inner header
       against its map-cache and then re-encapsulates the packet.  The
       new outer header's source RLOC is the RTR's RLOC 1.0.0.1 and the
       new outer header's destination RLOC is the Global NAT address
       2.0.0.2.  The destination port of the packet is set to 20002
       (discovered above during the registration phase) and the source
       port is 4342.

   5.  The NAT translates the LISP data packet's destination IP from to
       2.0.0.2 to 192.168.1.2, and translates the destination port from
       20002 to 4341, and forwards the LISP data packet to the ETR at
       192.168.1.2.

   6.  For the reverse path the ITR uses its local map-cache entry with
       the RTR RLOC as the default locator and encapsulates the LISP
       data packets using RTR RLOC, and 4341 as destination RLOC and
       port.  The ITR must pick a random source port to use for all
       outbound LISP data traffic in order to avoid creating excessive
       state in the NAT.

6.  Security Considerations

   By having the RTR relay the ECM-ed Map-Register message from an ETR
   to its Map-Server, the RTR can restrict access to the RTR services,
   only to those ETRs that are registered with a given Map-Server.  To
   do so, the RTR and the Map-Server may be configured with a shared key
   that is used to authenticate the origin and to protect the integrity
   of the Map-Notify messages sent by the Map Server to the RTR.  This
   prevents an on-path attacker from impersonating the Map-Server to the
   RTR, and allows the RTR to cryptographically verify that the ETR is
   properly registered with the Map-Server.

   Having the RTR re-encapsulate traffic only when the source or the
   destination are registered EIDs, protects against the adverse use of
   an RTR for EID spoofing.

   Upon receiving a Data-Map-Notify, an xTR can authenticate the origin
   of the Map-Notify message using the key that the ETR shares with the
   Map-Server.  This enables the ETR to verify that the ECM-ed Map-
   Register was indeed forwarded by the RTR to the Map-Server, and was
   accepted by the Map-Server.






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6.1.  Acknowledgments

   The authors would like to thank Noel Chiappa, Alberto Rodriguez
   Natal, Lorand Jakab, Albert Cabellos, Dominik Klein, Matthias
   Hartmann, and Michael Menth for their previous work, feedback and
   helpful suggestions.

7.  IANA Considerations

   This document does not request any IANA actions.

8.  Normative References

   [ICE]      Rosenberg, J., "Interactive Connectivity Establishment
              (ICE)", RFC rfc5245, October 2008.

   [LCAF]     Farinacci, D., Meyer, D., and J. Snijders, "LISP Canonical
              Address Format (LCAF)",  RFC 8060, December 2015.

   [LISP]     Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
              "Locator/ID Separation Protocol (LISP)",  RFC 6830,
              January 2013.

   [LISP-MS]  Farinacci, D. and V. Fuller, "Locator/ID Separation
              Protocol (LISP) Map-Server Interface",  RFC 6833, January
              2013.

   [NAT]      Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations",  RFC
              2663, August 1999.

   [NAT-MN]   Klein, D., Hartmann, M., and M. Menth, "NAT traversal for
              LISP mobile node, In Proceedings of the Re-Architecting
              the Internet Workshop (ReARCH '10).", 2010.

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
              and E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,
              <https://www.rfc-editor.org/info/rfc1918>.

   [RFC4632]  Fuller, V. and T. Li, "Classless Inter-domain Routing
              (CIDR): The Internet Address Assignment and Aggregation
              Plan", BCP 122, RFC 4632, DOI 10.17487/RFC4632, August
              2006, <https://www.rfc-editor.org/info/rfc4632>.

   [STUN]     Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC rfc5389,
              October 2008.



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   [TURN]     Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
              Relays around NAT (TURN)", RFC rfc5766, April 2010.

Authors' Addresses

   Vina Ermagan
   Cisco Systems, Inc.

   Email: vermagan@cisco.com


   Dino Farinacci
   lispers.net

   Email: farinacci@gmail.com


   Darrel Lewis
   Cisco Systems, Inc.

   Email: darlewis@cisco.com


   Jesper Skriver
   Cisco Systems, Inc.

   Email: jesper@cisco.com


   Fabio Maino
   Cisco Systems, Inc.

   Email: fmaino@cisco.com


   Chris White
   Logicalelegance, Inc.

   Email: chris@logicalelegance.com












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