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Versions: 00                                                            
Internet Engineering Task Force                 Bjorn Chambless
INTERNET DRAFT                                  Portland State University
                                                Jim Binkley
                                                Oregon Graduate Institute
                                                October 27, 1997


                   HARP - "Home Agent Redundancy Protocol"
                   <draft-chambless-mobileip-harp-00.txt>


Status of This Memo

   Distribution of this memo is unlimited.

   This document is an Internet-Draft.  Internet Drafts are working
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   Distribution of this memo is unlimited.


Abstract

   This document presents a protocol called the Home Agent Redundancy
   Protocol or HARP.  HARP is an optional extension to Mobile-IP [RFC-
   2002].  Mobile-IP includes the notion of a Home Agent which is
   a host located on the home IP subnet for Mobile Nodes.  Home Agents
   forward packets to Mobile Nodes that are away from home.  Since Mobile
   Nodes are dependent on the Home Agent for connectivity when away from
   home, the Home Agent represents a possible single source of failure for
   the Mobile IP system.

   HARP is a protocol which allows a pair (or set) of Home Agents
   to cooperate and share Mobile-IP Mobile Node registration
   information.  If one of the HARP peers should fail, the Mobile-IP
   system will not necessarily fail, hence HARP introduces Home
   Agent redundancy into the Mobile-IP system.  Mobile Nodes are
   not aware that HARP exists, so HARP requires no changes to
   Mobile-IP as a protocol.  In this document, we present the HARP
   architecture and protocol.


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Table of Contents

   1. Introduction
      1.1. Design Goals
      1.2. Terminology

   2. Protocol Overview
      2.1  Assumptions
      2.2  Protocol Overview
      2.3  Redundancy Considerations

   3. Mobile-IP Home Link Considerations
      3.1  Non-partitioned Home Subnet
      3.2  Partitioned Home Subnet

   4. HARP Protocol
      4.1. Message Types and Functions
      4.2. Message Formats
         4.2.1. HARP Registration Forward (HARP_FORWARD)
         4.2.2. HARP Ping (HARP_PING)
         4.2.3. HARP Ping Acknowledge (HARP_ACK)
         4.2.4. HARP Registration Dump Request (HARP_DUMP_REQ)
         4.2.5. HARP Registration Dump (HARP_REG_DUMP)

   5. Security Considerations

   References.........................................................
   Contacts...........................................................

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1. Introduction

   Mobile-IP is designed to allow a Mobile Node (MN) to change its
   point of IP subnet attachment in the Internet at the network or
   IP layer.  The MN is always identified by it home IP address
   regardless of its current network location.  Its mobility is
   not limited by conventional IP network boundaries.

   The Mobile-IP system consists of Mobile Nodes, and two kinds of
   agents, known as Home Agents (HA), and Foreign Agents (FA).
   Home Agents remain "home" and when the Mobile Node is not home,
   forward packets sent to the conventional IP subnet of the Mobile
   Node to a possibly distant point of attachment.  The remote
   address is called a Care Of Address (COA), and may be at a Foreign
   Agent or a co-located Mobile Node.  As a Mobile Node travels from one
   IP link to another, it determines possible COAs and uses the
   Mobile-IP registration protocol to inform the Home Agent of its
   current location.  The Home Agent then forwards packets addressed to
   the Mobile Node at its home network to the its current location.

   In Mobile-IP, as currently specified, a single HA services an
   MN.  The MN is reliant on this Home Agent for its connectivity.
   Thus the HA represents the possibility of a single point of
   failure for Mobile-IP.  A Home Agent may be responsible for
   multiple Mobile Nodes on multiple home subnets.  The failure of
   a single HA may then result in the loss of connectivity for
   numerous Mobile-IP Mobile Nodes located throughout the Internet.
   Thus the Home Agent and Mobile Node taken together have a shared
   fate.  A Mobile Node cannot afford the loss of its Home Agent.

   This vulnerability is inconsistent with the fault tolerant nature
   of the Internet.  Additionally redundancy is needed.  We have
   developed the Home Agent Redundancy Protocol (HARP) as an optional
   extension to Mobile-IP to address this problem.

   HARP is a simple protocol based on the notion of one or more
   HARP peers that act as a single shared Home Agent.  Each HARP
   peer is configured with information about its HARP peers and
   forwards any Mobile-IP registration messages it receives to its
   peers.  HARP peers act in parallel to create or delete tunnels
   [RFC 2003] to the Mobile Node's remote COA according to the last
   registration message received.  Although we speak of HARP peers
   as a set, in general, there will probably be only two cooperating
   systems in the HARP sub-system.

   There are three major types of messages, 1.  HARP TCP DUMP, 2.
   HARP UDP FORWARD., and 3. HARP UDP PING.  At boot, a TCP connection
   may optionally be made to a remote HARP peer to exchange mobile
   routing information.  At runtime, HARP UDP PING messages are
   exchanged to determine if remote HARP peers are up.  At runtime,
   HARP UDP FORWARD messages are used to forward Mobile-IP registration
   messages from the receiving HARP agent to its HARP peers.


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   HARP assumes no changes to Mobile-IP proper; i.e., the existence
   of one or more HARP peers is kept hidden from Mobile Nodes.
   Therefore HARP will interoperate with existing Mobile-IP
   implementations.  In routing terms, one may think of the HARP
   peers as advertising the existence of a common IP subnet into
   an interior routing domain.  Externally, a MN's Mobile-IP
   authentication message is routed to the nearest ( according to local
   routing metrics ) HARP peer which, in turn, informs other HARP peers
   about the MN's location via a HARP FORWARD message.  Packets are routed
   to HARP peer Home Agents via conventional routing, and since each HARP
   peer maintains Mobile Node COA information, packets are forwarded to
   the MN.

1.1 Design Goals

   The Home Agent Redundancy Protocol (HARP) aims to remove the
   Home Agent as a single point of failure for Mobile-IP.

   The protocol is implemented entirely through the enhancement of
   Home Agent functionality.  There are no additional responsibilities
   or modifications required of either Mobile Nodes or Foreign
   Agents.  Mobile Nodes and Foreign Agents have no knowledge of
   HARP and Mobile-IP will interoperate with HARP capable Home
   Agents.

   The Home Agent Redundancy Protocol will be made secure, minimally
   with authentication, and optionally with authentication and
   privacy mechanisms.

   Home Agent Redundancy makes no assumptions about the physical
   media utilized by the Mobile-IP environment.  Therefore HARP
   does not limit the physical implementation of Mobile-IP.

   The number of Mobile Nodes is not limited by HARP.

1.2 Terminology

   Home Agent Redundancy Protocol terminology uses and expands on
   the Mobile-IP terminology presented in RFC 2002.  The following
   terms are specific to the Home Agent Redundancy protocol:

        HARP peers -
        co-HAs -
        co-Home Agents - A set  of Home Agents acting in concert
           to provide connectivity to one or more Mobile Nodes.
           These hosts share an IP address on the Home Subnet but
           each has a uniquely identified interface outside of the
           Home Network.  Co-HAs, a priori, know about a small set
           of cooperating Home Agents and exchange registration
           information regarding Mobile Nodes and periodically test
           peer co-HA reachability.  One may assume that there are
           only two Home Agents in a set of co-HAs, but there is no
           inherent limit to the number of peers in a Co-HA set.


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        Primary-HA, Primary - The Home Agent of a co-HA set which
           is currently receiving registration information directly
           from a MN.  The Primary Home Agent shares this information
           with its co-HAs (Secondaries) by forwarding registration
           packets to the peers.  A HA is primary because the IP
           routing infrastructure is currently routing the Mobile-IP
           registration packet to it.

        Secondary-HA, Secondary - A Home Agent of a HARP set which
           is receiving registration information about a given MN
           indirectly through its co-Home Agent which is acting as
           the Primary.

        HARP - Home Agent Redundancy Protocol.

        HARP PORT - The HARP port number which is the same for both
           the TCP and UDP ports.   This port has not yet been allocated
           yet.

        Home Network - Home Subnet - The subnet containing both
           Home Agents and the home addresses of the Mobile Nodes they
           are serving. This subnet may be partitioned in terms of the
           co-HAs or the co-HAs may be physically present on the same
           link.

        Partitioned Subnet - A physically divided Home Subnet.
           Home Agents in a co-HA pair may be thought of as existing
           on a virtual subnet.  Physically divided means that the
           co-HAs cannot use that link to communicate directly.
           Note that this is not a requirement of HARP, but an
           aspect of network design.  We will discuss the network
           design aspects of HARP below.

        AWAY(Mobile-IP state) - The state of a Mobile Node, with
           respect to its Home Agent(s), in which datagrams addressed
           to the MN arrive at its Home-Subnet and are tunneled to
           the MN's Care Of Address by one of the Mobile Node's
           Home Agents.

        AT HOME (Mobile-IP state) - The state of a Mobile Node with
           respect to a Home Agent in which the MN's current point
           of attachment in the Internet is consistent with its IP
           address.  In this state, the Mobile Node will receive
           packets directly.

        At CoHA(Mobile-IP state) - The state of of Mobile Node,
           with respect to a Home Agent, in which the MN's home
           subnet is partitioned and a packet arrives for the MN
           at another link of the partitioned network.  In this
           case, packets addressed to the home subnet may arrive
           on a portion of the home subnet to which the Mobile Node
           has no link layer attachment.  These packets must then
           be forwarded (tunneled) by one HARP peer to another.


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2. Protocol Overview

   This section provides a protocol overview of HARP.  We discuss
   HARP from a routing topological point of view, and provide a
   short discussion of redundancy issues.

2.1  Assumptions

   The following are fundamental assumptions about the design of
   a HARP redundant agent network:

   1.  So that Mobile-IP and Mobile Nodes need not know about the
   HARP sub-system, we assume that the HARP peers share a single
   IP subnet and a single IP network address.

   2.  Due to assumption 1, and because the HARP agents must
   communicate amongst themselves, we assume they are multi-homed
   in the sense that there exist on one node at least two IP
   addresses.  We will call them the "Mobile-IP subnet" address,
   and the "HARP peer" address.  The former is shared.  The latter
   is not shared.  The HARP peer address is unique and is used by
   HARP systems to communicate amongst themselves.  Note that this
   does not rule out an agent with only one physical interface.

   3.  We assume that the HARP peers exist within an interior
   routing domain that runs a IP interior routing protocol such as
   RIPv2 [RFC-1721], or OSPF [RFC-2178].  Thus packets addressed to the
   Mobile-IP subnet, including Mobile-IP authentication packets, are
   routed to the HARP peers according to the local metrics of the
   interior routing protocols.  At any given time, any packet bound for
   a Mobile Host at HOME, will go to exactly one HARP peer.  Ideally,
   if an interior link fails, an interior routing protocol will
   switch Mobile-IP packets to the other HARP system.  This does
   not rule out the possibility of HARP being used with static
   routes in interior routers.  External routes to the Mobile-IP
   subnet may always be changed by hand in the event of HARP agent
   failure.

   Finally, it is NOT assumed that the HARP Mobile-IP subnet is
   partitioned.  Partitioning may add useful redundancy attributes.
   But the subnet need not be partitioned.  How this is handled
   is implementation and site specific and will be discussed more
   in the next section.


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2.2  Protocol Overview


Diagram 1:

                         MN or CH

                            |
                            |  MN/CH path
                            |

                        Internet

                            |
                            |
                            |
                  ------------------------
                 R1    interior path      R2
                 |                        |
                 H1                       H2
                 |                        |
                ----- Mobile-IP subnet ------

   Please refer to diagram 1 for the following discussion.

   Assume we have a Mobile Node that is away from home.  Assume
   that at home we have two routers R1, and R2, which are using a
   local interior routing protocol (for example, OSPF, or static
   routes).  Behind them are two HARP peers which advertise a
   Mobile-IP subnet to the routers and to the network.  Along with
   the Mobile Node on the exterior Internet, there exists a
   Correspondent Host (CH).  The latter is any host interested
   in sending packets to the Mobile Node.

   The Mobile Node is unaware of Home Agent redundancy and sends
   registration information only once to the shared HA address
   located on the Home Subnet. Due to the nature of unicast routing,
   this information may be received by either Home Agent, but is
   likely to be received by only one.  Packets sent from the CH to
   the Mobile Node will likewise be routed to one or the other Home
   Agent (whichever is "closer", according to the metrics of the
   interior routing system).  Note that any of these datagrams
   (Mobile-IP authentication or ordinary datagrams) may at any time
   go to either Home Agent.

   When a Mobile-IP registration packet is received by a co-HA, it
   will encapsulate that registration packet and forward it to
   another co-HA via the HARP UDP port.  Thus the peer co-HA will
   know that the other HA directly received the registration, and
   will also know the current MN Care Of Address; i.e., the
   forwarding address for CH packets.


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   Each HARP peer takes the same internal action whether it receives
   a Mobile-IP registration packet directly from a MN or whether
   it receives it from a HARP peer.  For example, in parallel, the
   Mobile-IP vlist entry is created or refreshed, and tunnels are
   setup (if needed) from the Home Agent to the Care Of Address.
   In Mobile-IP terms, the Home Agents act in parallel.

   Mobile-IP authentication (MN to HA, FA to HA, etc.) is performed
   once at the primary co-HA system.  The secondary is expected to
   perform a separate HARP protocol authentication (HA to HA) on
   packets forwarded to it by the primary via the HARP UDP (or TCP in the
   case of table exchanges) ports.

   If one Home Agent should fail, the interior routing structure
   should notice that it has failed as a router, and normal routing
   convergence will remove that path to the Mobile-IP subnet.
   Convergence may take manual route manipulation in some cases
   where static routes are used.  As a result, the Mobile-IP system
   should be able to survive the loss of a Home Agent as packets
   will be routed to a surviving Home Agent.

   HARP consists of three major packets type sent from one HARP
   peer to other HARP peers.  (Note that some of the types have
   acknowledgements).  We have already mentioned the HARP UDP FORWARD,
   which is a simple repackaging of the Mobile-IP registration
   packet itself.  In addition there are two other kinds of messages
   used in the HARP system.  There is a HARP PING and a HARP boottime
   table exchange.  The former uses UDP and the later uses TCP.

   The UDP HARP PING is used to determine if other HARP peers are
   up.  If a PING fails (say 3 out of 5), a HARP agent knows that
   either its peer has failed or the path to it has been partitioned.
   It may then take implementation-specific actions which should
   include ways to attempt to notify local administrators that
   critical interior links or systems have failed.  Other implementation
   actions may be taken as well if needed.  For example, a dormant
   local link interface might be enabled.

   HARP provides an optional boot protocol which uses TCP to
   exchange all HARP information in one connection.  Thus if one
   HARP system reboots after failure, it can acquire Mobile-IP
   state information from another HARP peer.  At boot, a HARP Home
   Agent will attempt to establish a TCP connections with its co-HAs at
   their respective TCP HARP PORTs.  If successful, these connections
   are used to pass all current Mobile Node registration information from
   a running HA to a booting co-HA.  When all relevant information
   has been passed, and the booting Home Agent is synchronized with
   respect to MN Registrations, the TCP connections are closed.  If
   the peer system is not available, the sending system will
   timeout and proceed as the peer may be unavailable or rebooting.


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2.3 Redundancy Considerations

   There are a number of redundancy considerations regarding HARP
   that have driven its design which we will present in this
   section.

   The failure of one HARP agent, or a network interface on that
   agent, or possibly a path to that agent should not necessarily
   cause the Mobile-IP network to fail.  This is, of course, the
   goal of the protocol itself.  In addition to simple node
   redundancy, redundancy may be possible if a path from the
   MN (CH) in question exists to another HARP agent even when an
   interior (or exterior) router (or associated interface) fails.
   Thus HARP may sometimes be able to take advantage of dynamic
   interior routing.

   On the other hand, the interior path between the HARP agents
   should not be allowed to fail.  If it does, it is possible that
   the Mobile-IP registration packets might go one way and datagram
   packets from a given CH might go another, thus leading to a
   (bizarre) partition.  This is one of the reasons for the HARP
   PING protocol.  Lack of connectivity between the agents should
   lead to a local management alert.  Of course, fundamentally, an
   interior path failure might cut the Mobile Node off from important
   local services and should be taken seriously in any case.

   As part of the overview, we should justify why we have chosen
   UDP for internal HARP forwarding as opposed to say TCP connections
   between HARP agents.  We felt that the HARP protocol should
   internally match the design of Mobile-IP.  Registration for
   Mobile Nodes while away from home is driven by Mobile-IP/UDP
   based packets.  Since the Mobile Node drives the registration,
   we felt that forwarding these packets internally over presumably
   shorter channels (with higher bandwidth) was reasonable.  As a
   result, Mobile-IP registration also drives the HARP sub-system.
   We also felt that given the goal of producing a reliable server
   with reliable software it made more sense to use a simple protocol
   without the enhanced complexity of a TCP state machine to deal
   with possible protocol errors.  Thus agent-side implementation
   should be simpler.  As a final point, even though the number of
   HARP peers in a HARP set is likely to be very small, at some
   future time, one might consider experimentally replacing the
   unicast UDP HARP (re)registration with reliable multicast
   registration.  Of course, TCP lacks multicast capability.

   Redundancy considerations for the Mobile-IP home link itself
   are discussed in the next section.


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3. Mobile-IP Home Link Considerations

   One might claim that the impact of HARP on the actual Mobile-IP
   home subnet link could be regarded as implementation dependent.
   This is because Mobile-IP itself is aimed not so much at dealing
   with how mobile nodes interact at the home subnet, but how they
   can take a home subnet IP address, move to other subnets, and
   retain connectivity.  Thus HARP primarily seeks to address the
   problem of what might happen if the home forwarding agent is
   lost.  Still, the issue of redundancy on the Mobile-IP home link
   exists and there is a small amount of protocol impact on HARP.
   In a very simple sense, having two possible "home" links can
   aid redundancy as well.  If one home fails, a second home might
   be available.  In this section, we will discuss this issue and
   make implementation suggestions.

   In the end local network management must decide what they want,
   according to available local resources and local tradeoffs.
   Therefore we suggest that implementations remain flexible
   where home subnet design is concerned.

   The Home Mobile-IP subnet may be either:

   1. not partitioned; i.e., the HARP Home Agents could reside
   on the same link and would be able to reach each other on that
   link.

   2. partitioned; i.e., the HARP Agents might reside on
   different links, and would NOT be able to reach each other on
   that link.

3.1 Non-Partitioned Home Subnet

   If the link is not-partitioned, the HARP Home Agents can reach
   each other directly.  It is not advisable to have both HARP IP
   interfaces (which by definition share the same IP interface)
   active as confusion with nodes on the shared subnet might result.
   If the systems act as routers only (and not as end systems), one might
   claim that it would not matter, but there is no point in having both
   systems race to answer ARP [RFC-826] requests.  Worse, any node that
   attempted to directly connect to the shared HARP subnet IP address with
   a transport protocol may experience failure due to ARP cache
   overwrites.

   Thus it is reasonable to assume an implementation might provide
   a way to shut down one HARP interior IP interface and dynamically
   bring another up if failure of the other HARP node is detected.
   This can be done by the normal method of designated router
   election.  A set of HARP peers exchanging HARP PING messages


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   may choose the HARP peer with the highest IP address to be the
   only active peer on the interface.  We assume that the PINGS
   are done over the non-Mobile-IP (and non-shared) IP address.
   If PINGS fail from that interface, (say after N failures,
   where N is configured in), the remaining HARP peers would again
   elect a designated router which will take over the "live" ARP
   function.

3.2 Partitioned Home Subnet

   If HARP peers exist on the same link, it is possible that the
   failure of an interior router or router interface could lead to
   the loss of all HARP agents.  Thus one may have replaced the
   Mobile-IP single point of failure mode with a more elaborate
   single failure mode.  We suggest that if practicable (and this
   ultimately depends on per site network resources), it may be
   better to partition the home mobile link.  In other words, the
   internal path to the HARP agents would still be an interior
   routing path, but the agents themselves might best be in widely
   dispersed locations.  For example, HARP peers would be placed
   behind different interior routers, possibly in different buildings.

   Such a partitioned network is not ideal for non-Mobile nodes,
   as IP subnet reachability problems would result if non-Mobile
   nodes were placed on the Mobile subnet.  As a result, one might
   choose to only place Mobile-IP nodes on such partitioned links.

   Where a partitioned scheme is used, it is necessary for the Home
   Agents to install tunnels between themselves.  This mechanism is
   directly analogous to the Mobile-IP tunnel from the Home Agent to
   the Foreign Agent.  A packet sent to a co-HA where the MN is not
   resident, would be forwarded to the other co-HA.  Note that HA
   tunneling is not strictly necessary when the Mobile subnet is
   not partitioned, unless the Home Agents are only speaking one
   at a time to the home link.

   A simple and less elegant solution would be to disable
   Mobile-IP router advertisements for Home Agents where actual
   physical residence is not desired.  A dynamic scheme for election
   here could be used, or this function could be done manually.
   For example, one might choose to have only one Mobile-IP
   home subnet from the interior point of view.  The other home
   might reside on a virtual interface that is not directly accessible
   except in the exterior routing sense.  Mobile Nodes in such a
   system might never be able to directly visit the second home.

   In summary, where interior router resources exist, partitioning
   may provide greater surviveablity.


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4.1 Message Types and Functions

   The Home Agent Redundancy Protocol has five messages:

        HARP_FORWARD - This message consists of an encapsulated
                Mobile Node registration message which is tunneled from
                the receiving Home Agent to its co-HA.  This
                information is used to update the co-HA's registration
                tables.  This message type uses UDP and is sent to
                the HARP UDP PORT.

        HARP_PING -  This message is sent at configurable intervals
                from one co-HA to another to confirm connectivity.
                If the Home Agent receives no response from its
                co-HA peer, the co-HA is assumed to be unreachable.
                This message type uses UDP and is sent to the HARP
                UDP PORT.

        HARP_ACK - Sent in response to a HARP_PING to acknowledge
                that the PING message has been received.  This
                message type uses UDP and is sent to the HARP UDP
                PORT.

        HARP_REG_REQ - A message requesting all Mobile Node
                registration information.  This is the first message
                sent upon establishment of an inter-co-HA TCP
                connection.  This message type utilizes TCP.

        HARP_REG_DUMP - TCP message which contains Mobile Node
                registration information maintained by a Home Agent.
                This message is sent in response to a HARP_REG_REQ.
                Note that more than one DUMP message may be sent,
                but each DUMP message may contain more
                than one Mobile-IP node registration.

4.2. Message Formats

   All HARP messages are structured in a Tag, Length, Value format.

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
      |     Type      |    Length     |    Value ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type and Length occupy one unsigned short and are 16-bit values.
   They are assumed to be in network byte order.  Messages are sent
   to either the HARP UDP or TCP PORT.


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4.2.1. HARP Registration Forward (HARP_FORWARD)

    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=HARP_FORWARD          |     size                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Mobile-IP registration packet ...              |
   |                                                               |

   type: HARP_FORWARD = 1
   size: in bytes of the value field.
   value: Mobile-IP registration packet.

   HARP Registration Forward messages are used to encapsulate and
   forward registration updates received from a Mobile Node.  They
   are sent between co-HAs.  The message consists of two bytes of
   type followed by two bytes indicating the length in bytes of a
   Mobile-IP registration packet.  The Data field is a Mobile-IP
   registration packet of the size indicated by the size field.
   The registration packet has the same format as used with Mobile-IP.
   Optional (TLV) elements may be present and should be processed.
   However it is expected that the receiving Home Agent deals with
   all MN/HA and MN/FA authentication and may remove those elements
   from the registration packet.

4.2.2 HARP Ping (HARP_PING)

    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=HARP_PING             |       size=4                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     HARP peer IP address                                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        type: HARP_PING = 2
        size: 4
        value: (reachable) IP address of HARP sender

   HARP_PING messages are sent between all the members of a HARP
   co-HA set.  They are sent with a configured periodicity and are sent
   within a configured mechanism for determining the loss of an interior
   routing path.  For example, one might send one message a minute, and
   retry N times.  If the sending agent determines that another
   agent is down, it may initiate implementation-dependent mechanisms
   including SNMP alerts, paging a network administrator, and/or
   taking up or down a local interface.   We include the HARP peer
   IP address in order to limit possible problems caused by
   multi-homing.


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4.2.3. HARP Ping Acknowledge (HARP_ACK)

    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=HARP_ACK              |        size=4                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     HARP peer IP address                                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   type: HARP_ACK = 3
   size: 4
   value: HARP peer IP address

   A HARP Ping Acknowledge message is sent in response to a HARP
   Ping.   The peer IP address belongs to the sender of the
   acknowledgement.

4.2.4 HARP Dump Request (HARP_DUMP_REQ)

    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=HARP_DUMP_REQ      |        size=0                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   type: HARP_DUMP_REQ = 4
   size: 0

   The HARP Dump Request message consists of two bytes of type
   followed by two bytes giving the size of the data field, which
   is always zero.

   A HARP_DUMP_REQ is passed from a Home Agent in Initializing State
   to its co-HA through the inter-co-HA TCP connection.  If the receiving
   co-HA does not receive this message, it should shut down the connection
   and log an error.  If the client's connection fails or no DUMP appears
   within a configured timeout interval, the client should disconnect and
   log an error.


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4.2.5 HARP Registration Dump (HARP_REG_DUMP)

    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=HARP_REG_DUMP      |        size=8                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Number of Registrations                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Size of Registrations                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Mobile-IP registration field ...            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Mobile-IP registration field ...            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  etc.                                         |                                                               |
   type: HARP_REG_DUMP = 5
   size: 8
   value:
      Number of Registrations: a 32-bit unsigned integer in network
         byte order
      Size of Registrations: a 32-bit unsigned integer in
         network byte order.  This field contains the size of all
         the Mobile-IP registration sections which must all have
         the same size.
      Some number of Mobile-IP registration requests.

   A HARP Registration Dump message is sent in response to a TCP
   connect and HARP Dump Request.  It contains current registration
   information for all Mobile Nodes serviced by a given Home Agent.  "The
   number of registrations" field tells the receiver how many registration
   messages are being sent.  The size of the registration message gives
   the size of each Mobile-IP registration message which as before, is
   assumed to be the same format as is used with Mobile-IP.  Note
   we assume that all message sub-fields are of the same length.
   Also note that a Home Agent need not write all of the information
   with the same TCP write.  Nor does the responding peer need to
   send all Mobile-IP registration messages in one DUMP message.
   Multiple DUMP messages may be sent over the same connection.

   The channel must be closed by the receiver when it has written
   all of the messages.


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5.  Security Considerations

   In this section, we briefly consider the security of the HARP
   protocol itself.  Security might also be taken to mean
   "survivability" and we will discuss that notion first, and then
   return to HARP protocol network security.

   In terms of survivability, HARP primarily addresses the problem
   that a Mobile-IP system, serving a given number of Mobile Nodes
   may fail due to the loss of a single Home Agent.  Home Agent
   redundancy reduces the odds of a total Mobile-IP system
   outage due to failure of a Home Agent node, a network
   adapter on that node, or possibly an internal routing path to
   that node.  Given that we may have multiple Home Agents that
   cooperate to emulate a single home agent there may also be
   additional security benefits.  For example, a denial of service
   attack against one Home Agent may not apply to the other (perhaps
   if they are not on the same link).  Hence the Mobile-IP network
   might continue to function.

   For protocol security of HARP itself, we require the use of IP
   layer security [RFC 1825] between any given HARP pair in a set
   of HARP peers.  The HARP pair may use a TCP connection
   between them at boot for route synchronization, and will use
   UDP to forward Mobile-IP registration packets.  It is required
   that all of these end to end TCP and UDP channels be protected
   by at least IPSEC authentication [RFC 1826], and optionally by
   authentication and encryption [RFC 1827].  Authentication is a
   requirement.  Thus security will be end to end for the HARP
   protocol between HARP peers.


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References

   [RFC-826]  Plummer, D., "An Ethernet Address Resolution Protocol:
      Or Converting Network Protocol Addresses to 48.bit Ethernet
      Addresses for Transmission on Ethernet Hardware", STD 37,
      November 1982.

   [RFC-1721]  Malkin, G., "RIP Version 2 Protocol Analysis",
      November 1994.

   [RFC-1825]  Atkinson, R., "Security Architecture for the Internet
      Protocol", Naval Research Laboratory, July 1995.

   [RFC-1826]  Atkinson, R., "IP Authentication Header", August 1995.

   [RFC-1827]  Atkinson, R., "IP Encapsulated Security Payload",
       August 1995.

   [RFC-2002]  Perkins, C., "IP Mobility Support", October 1996.

   [RFC-2003]  Perkins, C., "IP Encapsulation within IP",
      October 1996.

   [RFC-2178]  Moy, J., "OSPF Version 2", July 1997.

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Contacts

   Bjorn Chambless
   Computer Science Department
   Portland State University
   Email: bjorn@cs.px.edu

   Jim Binkley
   Computer Science and Engineering Department
   Oregon Graduate Institute
   Email: jrb@cse.ogi.edu

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