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Versions: 00 01

Internet Draft                                         M. S. Corson, UMD
Expiration: February 7, 1999                       S. Papademetriou, UMD
                                                   P. Papadopoulos, ORNL
                                                            V. Park, NRL
                                                        A. Qayyum, INRIA

                                                          August 7, 1999

     An Internet MANET Encapsulation Protocol (IMEP) Specification
                   draft-ietf-manet-imep-spec-01.txt

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

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

   To view the entire list of current Internet-Drafts, please check the
   ``1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
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   munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or
   ftp.isi.edu (US West Coast).

   Distribution of this memo is unlimited.


Abstract

   This memo describes a multipurpose network-layer protocol---named the
   Internet MANET Encapsulation Protocol (IMEP)---designed to support
   the operation of many routing algorithms, network control protocols
   or other Upper Layer Protocols (ULP) (where ``upper" denotes *any*
   layer above IMEP) intended for use in Mobile Ad hoc Networks (MANET).
   The protocol incorporates mechanisms for supporting link status and
   neighbor connectivity sensing, control packet aggregation and
   encapsulation, one-hop neighbor broadcast (or multicast) reliability,
   multipoint relaying, network-layer address resolution and provides
   hooks for interrouter authentication procedures.  Indirectly, the
   IMEP also puts forth a framework for MANET router and interface
   identification and addressing.

1. Introduction



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   The primary purpose of the Internet MANET Encapsulation Protocol
   (IMEP) is to improve overall network performance by reducing the
   *number* of network control packet broadcasts through encapsulation
   and aggregation of multiple MANET control packets (e.g. routing
   protocol packets, acknowledgements, link status sensing packets,
   ``network-level" address resolution, etc.) into larger IMEP messages.
   Usage of the IMEP is desirable because per-message, multiple access
   delay in contention-based schemes such as CSMA/CA, IEEE 802.11, FAMA
   etc. is significant, and thus favors the use of fewer, larger
   messages.  It also may be useful in reservation-based, time-slotted
   access schemes where smaller packets must be aggregated into
   appropriately-sized IP packets for transmission in a given time slot.
   Upper Layer Protocols (ULP) *other than routing* may make use of this
   encapsulation functionality for the same purpose.

   Its secondary purpose concerns the commonality of certain
   functionality in many network-level control algorithms.  Many
   algorithms intended for use in a MANET will require common
   functionality such as link status sensing, security authentication
   with adjacent routers, one-hop neighbor broadcast (or multicast)
   reliability of control packets, etc.. This common functionality can
   be extracted from these individual protocols and put into a unified,
   generic protocol useful to all. MANET control algorithms would also
   benefit from a common approach to router and interface identification
   and addressing, and this protocol supports a framework for unifying
   the protocols under a common architecture.

   The IMEP will run at the network layer (see Figure 1), and will be an
   adjunct to whichever network protocol is using it.  ULP packets will
   be encapsulated in IMEP messages, which will be further encapsulated
   into IP packets.




















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      +------+ +-----+ +-----+     +-----+
      |Telnet| | FTP | | TFTP| ... | ... |
      +------+ +-----+ +-----+     +-----+    +---------+  +-----+
            |   |         |           |       | Routing |  | ULP |
           +-----+     +-----+     +-----+    +---------+  +-----+
           | TCP |     | UDP | ... | ... |            |      /
           +-----+     +-----+     +-----+          +---------+
              |           |           |     <-----  |  IMEP   |
        +---------------------------------+         +---------+
        | Internet Protocol & ICMP & IGMP |              |
        +---------------------------------+         +---------+
                          |                         |   IP    |
             +---------------------------+          +---------+
             |   Local Network Protocol  |
             +---------------------------+

              Protocol Relationships               Encapsulation

                             Figure 1

2.0 Terminology

   This section provides definitions for the terminology used throughout
   this document.  Many of these definitions may be replaced by or
   merged with those of the MANET working group's terminology draft now
   under development.

   MANET router or router:
        A device---identified by a ``unique Router ID" (RID)---that exe-
        cutes a MANET routing protocol and, under the direction of
        which, forwards IP packets.  It may have multiple interfaces,
        each identified by an IP address.  Associated with each inter-
        face is a physical-layer communication device.  These devices
        may employ wireless or hardwired communications, and a router
        may simultaneously employ devices of differing technologies.
        For example, a MANET router may have four interfaces with
        differing communications technologies: two hardwired (Ethernet
        and FDDI) and two wireless (spread spectrum and impulse radio).

   medium:
        A communication channel such as free space, cable or fiber
        through which connections are established.

   communications technology:
        The means employed by two devices to transfer information
        between them.

   connection:



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        A physical-layer connection---which may be through a wired or
        wireless medium---between a device attached to an interface of
        one MANET router and a device utilizing the same communications
        technology attached to an interface on another MANET router.

   link:
        A ``logical connection" consisting of the logical *union* of one
        or more connections between two MANET routers--identified by a
        (RID, RID) pair. Thus a link may consist of a heterogeneous com-
        bination of connections through differing media using different
        communications technologies.

   neighbor:
        From the perspective of a given MANET router, a ``neighbor" is
        any other router to which it has a link.

   adjacency:
        The name given to an ``interface on a neighboring router".  From
        the perspective of a given router, a connection is a (interface,
        adjacency) pair.

   topology:
        A network can be viewed abstractly as a ``graph" whose ``topol-
        ogy" at any point in time is defined by set of ``points" con-
        nected by ``edges".  (This term comes from the branch of
        mathematics bearing the same name that is concerned with those
        properties of geometric configurations (such as point sets)
        which are unaltered by elastic deformations (such as stretching)
        that are homeomorphisms.)

   physical-layer topology:
        A topology consisting of connections (the edges) through the
        *same* communications medium between devices (the points) com-
        municating using the *same* communications technology.   Multi-
        ple physical-layer topologies may exist for a given medium and
        communications technology if adaptive or proactive power con-
        trol, frequency or code division, or other physical-layer
        mechanisms are employed.

   network-layer topology:
        A topology consisting of links (the edges) between MANET routers
        (the points) which is used as the basis for MANET routing. Since
        ``links" are the logical union of physical-layer ``connections",
        it follows that the ``network-layer topology" is the logical
        union of the various ``physical-layer topologies".

   IP routing fabric:
        The heterogeneous mixture of communications media and



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        technologies through which IP packets are forwarded whose topol-
        ogy is defined by the network-layer topology.

   Security Context:
        A security context between two routers defines the manner in
        which two routers choose to mutually authentication each other,
        and indicates an authentication algorithm and mode.

   Mobility Security Association:
        A collection of security contexts, between a pair of routers,
        which may be applied to IMEP protocol messages exchanged between
        them.

   Security Parameter Index (SPI):
        An index identifying a security context between a pair of
        routers among the contexts possible in the Mobility Security
        Association.

3.0 Protocol Overview

   The mechanisms contained in the IMEP are:

        Message Aggregation (AGGR)

        Network-layer Address Resolution (NARP)

        Link/Connection Status Sensing (LCSS)

        Broadcast Reliability (REL)

        Multipoint Relaying (MPR)

        Authentication (AUTH)

   Message aggregation occurs as packets from ULPs become IMEP objects,
   and IMEP packs a number of objects into larger IMEP messages for
   transmission.  NARP--a protocol to determine the *binding* of a RID
   with each of its IP interface address--occurs implicitly in the
   current specification as the router ID of a given router is put in
   the header of each IMEP message.  As each IMEP packet is encapsulated
   in an IP packet, and its header contains the IP address of the
   transmitting interface in the source field of the IP packet, the
   binding can be made on reception of any IMEP packet (more on this
   later).  Usage of the remaining mechanisms is *optional*.  The fol-
   lowing dependency graph shows their relationships.






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                           AGGR---NARP
                             |
                      +------+-------+
                      |      |       |
                     REL    LCCS    MPR
                      |
                 -----+----
                 |        |
               AUTH      MPR

   This simply means that everything uses IMEP's aggregation facility.
   NARP occurs implicitly in every IMEP transmission. Usage of reliabil-
   ity, LCCS, MRP and AUTH are optional.  MRP traffic may be sent reli-
   ably or unreliably.  Authentication, if enabled, occurs reliably.

3.1 Relationship with Upper Layer Protocols

   IMEP is intended to support the operation of many ULPs. ULPs that
   wish to utilize IMEP must dynamically *register* with an IMEP imple-
   mentation prior to using IMEP (more on registration in a moment).

3.1.1 Protocol Type Values

   All ULPs which intend to utilize IMEP must have protocol type value,
   and must give this value to IMEP during registration.  This value is
   used by a receiving IMEP implementation for purposes of demultiplex-
   ing ULP objects within a received IMEP message so that they may be
   passed to the appropriate ULPs. IMEP implementations receiving
   objects with unknown (i.e. unregistered) protocol type values will
   silently discard those objects.  Several protocol types have already
   been assigned well-known values (see the protocol grammar section),
   but a protocol need not have a pre-assigned type value to make use of
   IMEP, nor must the well-known assignments be adhered to.  IMEP
   currently does not specify how protocol type values are assigned or
   used within a given administrative domain.

3.1.2 Protocol Handles

   ULPs registering with IMEP must pass to IMEP a protocol ``handle"
   which IMEP may then use to pass information back to the ULP.  The
   mechanism used to implement the handle is not specified (this is
   implementation dependent)--it could be a pointer to a function with a
   known signature, an object reference in a middleware-based implemen-
   tation, etc..

3.1.3 Protocol Epitaphs




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   ULPs registering with IMEP must specify an ``epitaph" object.  The
   epitaph object specifies a signal to be broadcast reliably to all
   one-hop peer ULPs if the registered ULP fails.  This permits peer
   ULPs (on neighboring routers) to take appropriate action in case of
   peer process failure.  Protocols may re-register with IMEP at any
   time in order to change the epitaph object, or to remove it if
   desired.

   Registration with an ``epitaph" object amounts to creating and main-
   taining a symbiotic relationship between IMEP and a registered ULP.
   There must exist a mechanism (not specified--implementation depen-
   dent) that guarantees ``mutual liveness" to each protocol so that,
   should either protocol fail, the other is reliably informed within
   the time of a BEACON_PERIOD (defined subsequently).

   The principle purpose for epitaph-based registration is *bandwidth
   conservation*. Without such a mechanism, it is not possible for peer
   ULP processes--who have previously exchanged control information and
   remain connected via IMEP--to be assured of mutual vitality without
   exchanging keepalive packets over the communication channel.

3.1.4 IMEP Signalling Support

   ULPs registering with IMEP must indicate the level of IMEP signalling
   support (ISS) they wish to receive from IMEP.  IMEP signalling sup-
   port is only meaningful if LCSS is enabled, and consists of signals
   being generated by IMEP in response to topological change events
   detected by LCSS, and then passed to subscribing ULPs (those ULPs
   requesting ISS).  Three levels of support are possible:


   0) Connection-level:
        All connection-level topological change events are passed to the
        subscribing ULPs.  Connection-level topological change events
        consist of ``connection" activation and failure (recall a con-
        nection consists of an (interface, adjacency) pair).  Thus, all
        physical-layer topology information is passed to the ULPs, per-
        mitting these ULPs to have a complete internal view of the IP
        routing fabric.

   1) Link-level:
        All link-level topological change events are passed to the sub-
        scribing ULPs.  Link-level topological change events consist of
        ``link" activation and failure (recall a link consists of a
        (RID, RID) pair).  Thus, only network-layer topology information
        is passed to the ULPs, permitting these ULPs to have only an
        external view of the IP routing fabric.




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   2) Disabled:
        No topological change events generated by IMEP as a result of
        LCSS are passed to the ULP.  This is the default mode.


3.1.5 ULP Registration

   ULPs must register with IMEP *prior* to usage.  ULP registration con-
   sists of passing IMEP a protocol type value, a *handle* to the ULP
   allowing IMEP to pass received objects to it (handle mechanism not
   specified--implementation dependent), an *epitaph* object (this may
   be null), and a parameter indicating the level of IMEP signaling sup-
   port desired by the ULP.

3.2 Message Aggregation

   MANET routing (and other) control protocols exchange control informa-
   tion and other data in the form of routing control packets or
   ``objects". To minimize the number of channel accesses generated by
   control traffic, the IMEP aggregates and encapsulates these objects
   into larger IMEP ``messages".  The objects are treated as ``opaque"
   objects by the IMEP protocol; i.e. IMEP is not aware of the contents
   of the objects, only of the protocol ``type" of the object block
   (necessary for protocol demultiplexing at a receiver) and the length
   of each object. These ULP object blocks are contained in yet larger
   IMEP messages which are passed to the IP layer for encapsulation and
   forwarding.  A single IMEP message can contain a mixture of reliable
   and unreliable objects.  The details can be found in the IMEP message
   format section.

3.3 Network-level Address Resolution

   IMEP supports a framework or architecture for MANET router and inter-
   face identification and addressing.  IMEP operates simultaneously on
   two different topological levels: the ``logical network" topology
   level---which is concerned with interrouter connectivity---and the
   ``physical" topology level---which is concerned with interface con-
   nectivity.  Router IDs (RID) identify routers in the logical topol-
   ogy, and IP addresses identify interfaces in the physical topology.
   There may be *multiple* IP addresses associated with a given RID.

   The purpose of a Network-level Address Resolution Protocol (NARP) is
   to discover the mapping between RIDs and IP addresses.  This is
   envisioned typically only to be needed when a new connection is
   discovered, as it is necessary to be able to associate an interface
   (an IP address) with a router (an RID).





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                    +----------+
                    |  Router  |                      RID
                    +----------+
                     |        |
         +--------------+   +--------------+
         |  Interface   |   |  Interface   |       IP Address
         +--------------+   +--------------+
                |                   |
         +--------------+   +--------------+
         | Phys Device  |   | Phys Device  |       MAC Address
         +--------------+   +--------------+

                Figure 4: RIDs, IP and MAC addresses


   While it is true that---as currently defined---RIDs are not
   ``addresses" in the strict sense, they do uniquely identify a router
   for purposes of internal routing computations and somewhat resemble a
   logical ``router address".  Thus, the IP address-to-RID mapping is
   similar in spirit to IP address-to-MAC address mapping performed by
   the present ARP protocol.  Each mapping simply associates an IP
   address with another identifier as shown in Figure 4.  As with ARP, a
   ``reverse" mapping is also defined as the Reverse Network-level
   Address Resolution Protocol (RNARP).  However, unlike RARP, a RNARP
   request seeks to discover the *set* of IP addresses associated with a
   given RID.  The two mappings are shown in Figure 5.

      ARP:  IP --> MAC     RARP:  MAC --> IP

      NARP: IP --> RID     RNARP: RID --> {IP1,IP2,...,IPn}

        Figure 5: ARP/RARP and NARP/RNARP

   NARP is currently implemented *implicitly* through inclusion of the
   RID in every IMEP message header.  RNARP is not required in the
   present specification, but may be specified and required in future
   versions if deemed necessary.

3.4 Link/Connection Status Sensing

3.4.1 Definition of Link/Connection Status

   Link/Connection Status Sensing (LCSS) is an optional mode that may be
   enabled in IMEP.  Many control protocols require accurate knowledge
   of the status of links/connections between neighboring routers.
   ``Link status" in the IP routing fabric is determined from the union
   of the status of physical-layer ``connections" between interfaces.




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   The relationship of interfaces, adjacencies, connections and links is
   depicted in Figure 2 from the perspective of router i.  Router i has
   two interfaces f1 and f2, each of which has a physical-layer connec-
   tion with multiple interfaces attached to other routers---these
   interfaces are referred to as adjacencies from router i's perspective
   and are numbered with a's. In this figure, there are two connections
   (f1,a1) and (f2,a2), the logical union of which composes the logical
   link (i,k) between routers i and k.
                +----------+
                | Router i |
                +----------+
    +--------------+    +--------------+
    | Interface f1 |    | Interface f2 |
    +--------------+    +--------------+
            |                   |
            |                   |
            |                   |
            |                   |
            |                   |
            |                   |
    +--------------+    +--------------+
    | Adjacency a1 |    | Adjacency a2 |
    +--------------+    +--------------+
                +----------+
                | Router k |
                +----------+

   Figure 2: Shown from router i's perspective, interfaces f1 and f2 are
   connected  to  adjacencies  a1  and  a2  via  connections (f1,a1) and
   (f1,a2)---the union of which forms link (i,k).

   The status of a connection may be INcoming or OUTgoing (either of
   which meaning it is unidirectional) or BIdirectional.  A unidirec-
   tional link is composed from one or more similarly-directed, uni-
   directional connections.  A BIdirectional link may be composed from
   the union of one or more bidirectional connections, or two or more
   oppositely-directed, unidirectional connections, or some combination
   thereof.  A connection or link which is present or ``active" (i.e.
   which has a non-null status, and is either uni or bidirectional), is
   referred to as ``UP".  A connection or link which is not active (i.e.
   which has a null status) is referred to as ``DOWN".

   The IMEP may be configured to run in the following ``connection
   notification" modes:

   BI-directional:
        This mode requires that physical-layer connectivity between an
        interface and an adjacency be established in *both* IN and OUT



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        directions before a connection is considered UP, and any
        registered ULPs are subsequently notified.

   UNI-directional:
        This mode requires that physical-layer connectivity between an
        interface and an adjacency need only be established in one
        direction (IN or OUT) before a connection is considered UP and
        the registered ULPs are subsequently notified.

   As determined by the connection notification mode, the ULP is noti-
   fied whenever there is a change (addition, modification, deletion) in
   the status of an interface's connections.  This notification is
   implemented via a handle registered via the ULP/IMEP interface.

3.4.2 Link/Connection Status Sensing Packet Exchange Mechanism

   The IMEP uses a combination of BEACON and ECHO packets (and other
   equivalent packets to be described shortly) to ascertain connection
   (and indirectly link) status.  On initialization, an interface under
   the control of IMEP broadcasts a BEACON packet to all adjacencies.
   (Note: The format of a BEACON packet is specified in a later section,
   but it essentially consists of an *empty* IMEP message; i.e.  an IMEP
   message containing only the IMEP message header.).  Recall that adja-
   cencies are interfaces that are only one hop away such as those on
   the same Ethernet subnet, or those within wireless transmission range
   of the broadcasting interface.  (Note:  Usage of the term ``broad-
   cast" here means to transmit a *single* copy of a packet to *all*
   interfaces reachable over one hop.  As is the convention with other
   Internet routing protocols, this is done using IP multicast. An IP
   multicast address ``ALL_IMEP_ROUTERS" will be reserved with IANA, and
   all MANET router interfaces will be configured to listen for this
   address.)  The purpose of a BEACON packet is to alert any adjacencies
   of the existence and identity of the broadcasting interface; an
   interface's identity is its IP address. The interface must ensure
   that a BEACON packet (or *any* other packet, since all packets are
   ``BEACON-equivalent") is transmitted at least once every
   BEACON_PERIOD (BP) time units; i.e. no more than BP time units may
   pass between subsequent transmissions of a BEACON (or ``BEACON-
   equivalent") packet.

   Reception of a BEACON at an interface implies either reconfirmation
   or creation of ``IN" (read ``INcoming") status of a connection at
   that interface, depending on whether or not the connection already
   exists, respectively.  Thus, BEACONs serve to tell a receiving inter-
   face that ``someone else is out there."  Once present, the status
   remains for MAX_BEACON_TIME (MBT) time units, at which time it times
   out if no subsequent BEACONs have been received; i.e. the link is
   declared DOWN and is removed from the data structures.  Creation or



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   loss of IN status may require notification of an upper level proto-
   col, depending on its signalling support mode.

   ECHO (or ``ECHO-equivalent") packets are used to respond to BEACONs.
   The purpose of an ECHO packet is to let a ``BEACONing" router know
   that someone hears its BEACON.  An ECHO packet contains the identity
   (i.e.  IP interface address) of the interface broadcasting the ECHO
   and the identity of the BEACONing interface to which it is respond-
   ing.  An ECHO packet is generated immediately in response to an ini-
   tial BEACON reception.  Subsequently, as long as the interface is
   considered UP (i.e. IN or BI), an ECHO packet must be generated at
   least once every BP time units; i.e. no more than BP time units may
   pass between subsequent generations of an ECHO or ECHO-equivalent
   packet.

   Reception of an ECHO at an interface implies either reconfirmation or
   creation of ``BIdirectional" status of an connection at that inter-
   face, depending on whether or not the connection already exists,
   respectively.  This is because reception of ECHO packet confirms that
   someone hears this interface (i.e. that is has OUTgoing status), and
   simultaneously confirms that it itself can receive them and, hence,
   also has INcoming status for that connection.

   ECHO packets may be broadcast in accordance with one of two ``signal-
   ling" modes, which applies to both ECHO and ACK semantics (more on
   ACKs later):

   Single Interface (SI):
        An interface only sends ECHOs in response to BEACONs it
        receives.  This is the standard mode which permits efficient
        link-layer detection of BI connections.  This mode should be
        enabled if the BI-directional connection notification mode is
        enabled.

   Multiple Interface (MI):
        An interface sends ECHOs in response to BEACONs it receives, and
        IMEP also sends Indirect ECHOs (IECHO) out *all* other inter-
        faces.  An IECHO carries the address of the interface being
        echo'ed (as does an ECHO) but, additionally, carries the address
        of the interface on the echoing router that received the
        transmission being echoed.  This mode is necessary to permit
        ``IMEP-based detection" of BIdirectional links composed of
        oppositely-directed, unidirectional connections between neigh-
        boring routers. Note that by using this Echo mode (i.e. via
        reception of IECHOS at other interfaces), an interface can be
        informed (solely via IMEP) that it has an ``OUTgoing" connection
        without also having ``INcoming" status and, hence, a BIdirec-
        tional link.  This mode should be enabled if the UNI-directional



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        connection notification mode is enabled.


   To make this clear, consider Figure 3.

                      +----------+
                      | Router i |
                      +----------+
          +--------------+    +--------------+
          | Interface f1 |    | Interface f2 |
          +--------------+    +--------------+
                  |             IN    ^
                  |                   |
                  |                   |
                  |                   |
                  |                   |
            IN    V                   |
          +--------------+    +--------------+
          | Adjacency c1 |    | Adjacency c2 |
          +--------------+    +--------------+
                      +----------+
                      | Router k |
                      +----------+

   Figure 3: A bidirectional link consisting of two  oppositely-directed
   connections.

   Assume that SI Echo mode is being used, and the wireless directional
   connectivity is as shown. From router i's perspective, it can only
   receive over interface f2, and thus classifies connection (f2,c2) as
   IN.  It is unaware that its BEACON packets being broadcast from
   interface f1 are being received at interface c1 on router k.  How-
   ever, if MI mode is used, then router k will advertise (via IECHO
   transmissions from c2) the reception of BEACON packets from f1 at c1
   thereby informing router i that connection (f1,c1) should be classi-
   fied as OUT.  Of course, the reverse but same is true from router k's
   perspective.

   The additional functionality provided by the MI mode comes at the
   cost of broadcasting IECHOs out one or more interfaces in addition to
   the ECHO sent over the interface over which the corresponding BEACON
   was received.  This creates more ECHO overhead.  For a given network,
   this cost must be balanced against the frequency of occurrence of the
   situation depicted in figure 3.

   Additional activity at an ULP (involving communication over multiple
   hops) is necessary to detect purely UNIdirectional links (i.e. links
   consisting of one or more unidirectional connections) between



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   adjacent routers.

3.4.3 BEACON and ECHO ``Equivalency"

   BEACON and ECHO packets are necessary for ascertaining current con-
   nection status.  From the perspective of a given router, BEACONs
   announce the presence of a broadcasting interface, and ECHOs simul-
   taneously announce the presence of an adjacency *and* that the adja-
   cency can receive from the broadcasting interface.  However, it
   should be clear that the same information can be gleaned from other
   IMEP packets.  Specifically, all transmissions signal the presence of
   a broadcasting interface and are, in this sense, ``equivalent" to
   BEACON packets.  Similarly, ACKnowledgements both announce the pres-
   ence of an adjacency and, through the process of acknowledgement,
   confirm that the adjacency recently received from the broadcasting
   interface.  Thus, in this sense, ACKs are equivalent to ECHOs.  The
   equivalency is depicted in the Figure 6.

                                 BEACON  -->
                                 ALL/OBJ -->
   +----------+ +-------------+              +-------------+
   | Router i |-| Interface f |  -  -  -  -  | Adjacency c |
   +----------+ +-------------+              +-------------+
                                 <-- ECHO or IECHOS
                                 <-- ACK or IACKS

              Figure 6: BEACON and ECHO Equivalency
   Transmission or reception of a BEACON or ECHO-equivalent packet
   affects the link-status sensing timers as would transmission or
   reception of a BEACON or ECHO, respectively.  Thus, during periods of
   heavy data traffic, it is expected that BEACONs and ECHOs will rarely
   be transmitted as their respective ``equivalent" packets will serve
   their role in link status sensing. During periods of light or no
   traffic, BEACONs or ECHOs will be transmitted as necessary to satisfy
   the aforementioned timing requirements.

   If MI mode is in use, the Indirect ECHOS are being sent out all
   interfaces.  In a corresponding fashion, Indirect ACKS (IACKS) must
   be sent out all interfaces to provided reliability over BIdirection
   links consisting of oppositely-direction, UNIdirectional connections.
   These IACKS are also ``echo equivalent" and must indicate the address
   of the interface they are IACKing, as well as the interface address
   on the IACKing router which received the object being indirectly
   ACKed.

3.4.4 Connection Failure Detection

   Expiration of the MBT timer signals connection failure.  Note that



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   separate timers are used to monitor IN and OUT connection status.
   Thus, a connection may lose its OUT status while still retaining IN
   status and vice versa.  Obviously, a connection satisfying both IN
   and OUT timing requirements is marked as BI.


3.5 Neighbor Broadcast Reliability

   IMEP supports two broadcast delivery modes:

   BROADCAST (IMPLICIT):
        Delivery to the current one-hop neighbor set.

   MULTICAST (EXPLICIT):
        Delivery to a pre-specified subset of the one-hop neighbor set.
        A ULP may specify one, some or all current neighbors.

   Of course, both are delivered using one-hop scoped, multicast
   addressing as is every IMEP message.

   IMEP supports two reliability modes:

   UNRELIABLE:
        Unreliable, unsequenced delivery of either neighbor broadcast or
        neighbor multicast data.

   RELIABLE:
        Reliable, sequenced delivery of either neighbor broadcast or
        neighbor multicast data.

   Thus, delivery may be implicit or explicit, and reliable or unreli-
   able:  all four combinations are possible. These modes are used for
   delivery of opaque protocol objects, where reliable delivery-- i.e.,
   broadcast or multicast --is also guaranteed to be delivery ``in
   order" of transmission. (Note: This should not be confused with
   transport-layer, reliable multicast across an entire multihop net-
   work.)

   IMEP uses a ``point-to-multipoint selective repeat" algorithm to
   guarantee broadcast or multicast reliability and ordered delivery.
   This approach eliminates unnecessary retransmissions of the type com-
   monly associated with ``go back n" algorithms, and is in keeping with
   the greater IMEP goal of minimizing the number of required channel
   accesses.

   To support reliability, each object block is given a SEQUENCE number,
   and is broadcast with that number. To provide in-order delivery, a
   connection protocol is utilized to synchronize receivers with the



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   current broadcast SEQUENCE number.  The connection and transmission
   protocol is designed so that an explicit receiver list does not have
   to be appended to every reliable object block. Instead, an implicit
   list is used by ``coloring" all messages. If a message is received
   with the correct color, then the SEQUENCE number has meaning and its
   objects can be forwarded up the protocol stack. If the color is
   incorrect, the receiver does not forward its objects up the protocol
   stack.  The connection protocol reliably transmits the current group
   color to all members of the group.

   When broadcast, a copy of the object block with a response list (i.e.
   the set of neighbors that are required to acknowledge this block) is
   stored.  A retransmission timer is set to RETRANS_PERIOD (RP) time
   units which, upon expiration, will cause the object to be rebroadcast
   to any neighbors which have not acknowledged the object (this causes
   the retransmission timer to be set again to RP).  The time the packet
   was initially broadcast is also stored.  If the object's response
   list is not empty (i.e it has not been acknowledged by some adjacen-
   cies) after MAX_RETRANS_TIME (MRT) time units, the connections to
   those adjacencies are declared DOWN.

   Acknowledgements (ACKs) are sent in response to object block recep-
   tions when (i) reliable delivery is indicated and (ii) when the
   receiver is contained in the response list (either implicitly or
   explicitly).  Once a neighboring router has ACKed a given block, it
   will be removed from the block's response list so that it will not be
   required to ACK any future retransmissions.

3.5.1 The Reliable Delivery Neighborhood

   Each router keeps track of the neighbors that can be reached reliably
   through regular Beacon-Echo exchanges. For discussion purposes, con-
   sider a single router, termed a ``base-router", B and any number of
   ``neighbor routers", N(i), i=1,2, ..., P, where P is the number of
   routers that can currently hear transmissions from B. Each router
   N(i), will respond with an ECHO packet within the time constraints of
   the BEACON-ECHO protocol outlined previously. If B hears an ECHO
   packet from N(i), then N(i) is a candidate member of B's reliable
   delivery neighborhood (RDN).  For N(i) to become a member of B's
   reliable delivery neighborhood (i.e., connected to B), B must broad-
   cast a group COLOR with an explicit membership list.   This object is
   called a NEWCOLOR and must be acknowledged by every router on the
   explicit membership list before B considers a reliable delivery
   neighborhood to be formed.

   From N(i)'s perspective, the neighborhood rooted at B is has COLOR K.
   N(i) is a member of this neighborhood if the NEWCOLOR object expli-
   citly contains N(i) as a member. A reliable delivery neighborhood



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   rooted at B with COLOR K and current sequence J is specified in the
   triple RDN(B,K,J). The COLOR K is updated by B every time a change to
   its RDN is discovered (either a new router comes in range or an
   existing router moves out of range or becomes hidden). Every router R
   in a MANET network will have a single RDN rooted at R. R can be a
   member of any number of RDN's that are not rooted at R. Every router
   keeps track of its RDN and of the RDN's for which it is a member. If
   a router hears a router R1 but itself is not an explicit member of
   RDN(R1,K,J), then it marks the current COLOR of RDN(R1,K,J) as color-
   less or as RDN(R1,0,J). The format for a NEWCOLOR object is given in
   a later section.

3.5.2  Neighborhood definitions


   RDN(B):
        Reliable delivery neighborhood rooted at MANET router B.


   RDN(B,K):
        Reliable delivery neighborhood rooted at MANET router B, with
        COLOR K.


   RDN(B,K,J):
        Reliable delivery neighborhood rooted at MANET router B, with
        COLOR K, and current broadcast sequence number J.


3.5.3  Reliable, Sequenced Delivery

   Objects passed to IMEP from an ULP may be delivered reliably or
   unreliably, and is specified by the ULP.  This section addresses
   reliable, sequenced delivery of ULP objects by IMEP to all members of
   a RDN.  Every reliable object in IMEP delivered from B to the
   RDN(B,K,J) is colored with COLOR K and sequence number J. A router
   N(i) is an intended receiver of the object if its notion of the COLOR
   K associated with RDN(B) matches exactly the color contained in the
   broadcast object.  Therefore, N(i) may deliver a reliable object to
   its ULP only if the object from B matches the COLOR and SEQUENCE that
   N(i) has recorded for the RDN(B). If an object arrives with the
   correct COLOR but the incorrect SEQUENCE number, then N(i) must
   determine if the object is a duplicate or simply out of sequence.  If
   a duplicate, then N(i) discards the object. If out of sequence, then
   N(i) retains the object until all earlier objects arrive. If an
   object arrives with the incorrect COLOR, then N(i) discards the
   object.




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   From the ULP's perspective, objects are delivered reliably and in
   sequence to *only* those members of the RDN(B) that exists at the
   time when the object was received by IMEP (Note this may not be the
   time when the object was sent to IMEP from the ULP's perspective, due
   possibly to interprocess communication delay between IMEP and the
   local ULP).  This is referred to as an (implicit) ``neighbor broad-
   cast" object.

   If the ULP requires a object to be delivered to a specific subset of
   one-hop neighbors, then it should use ``neighbor multicast" objects
   (see below). This latter delivery semantic frees ULPs from having to
   decide whether or not a object is valid. Every reliable object passed
   to the ULP from IMEP is guaranteed to be intended for the ULP, as
   specified by the sender.

   Reliability is established between *routers*, not interfaces.  Thus,
   the reliability semantics are the same regardless of whether BIdirec-
   tion notification with SI signalling or UNIdirectional notification
   with MI signalling is in use.

3.5.3.1   Sequence Numbers and Associations using Broadcast Semantics

   The coloring of the RDN(B) corresponds to a single sender with a
   number of ``associated" receivers. ECHOs from a router can be viewed
   as a association request. If an association is already established
   from B to N(i), then this request is vacuous. If, however, no associ-
   ation from B to N(i) exists, the ECHO then acts like a association
   request. A NEWCOLOR object with N(i) on the list completes the asso-
   ciation from B to N(i) (from N(i)'s perspective) and N(i)'s ack-
   nowledgement of the NEWCOLOR object completes the association from
   B's perspective.

   The RDN(B) maintains a single sequence number that all members of
   RDN(B) must track. NEWCOLOR objects contain not only a new group
   COLOR, but also the next expected SEQUENCE number. This allows sender
   and receivers to synchronize the sequence numbers to provide in-order
   delivery.

   There are (subtle) consequences of these semantics.

      1) An RDN(B) maintains a *single* sequence number for the neigh-
      borhood.  Hence, every N(i) must acknowledge *every* reliable
      object to ensure that all members of RDN(B) maintain the sequence
      order.  Of course, multiple reliable objects contained in the same
      IMEP message are acknowledged simultaneously with a single ACK.
      If an object is intended for a single recipient, all must ack-
      nowledge (to keep sequence numbers synchronized) and information
      specific to this object must further designate the intended



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      recipient.  This is due to the fact that the current scheme is
      optimized for implicit neighbor broadcast delivery, not explicit
      neighbor multicast.

      2) When RDN(B,K0) is updated to RDN(B,K1) (color changes from K0
      to K1), then all reliable objects must first be retired from B's
      retry queue before the NEWCOLOR object can be transmitted.

      3) The explicit association (via a colored neighborhood) means
      that the first time a reliable object is transmitted, an explicit
      recipient list can be (and is) omitted. This reduces the size of
      objects and allows the receiver to determine if it should forward
      the object up the protocol stack based on only the COLOR and
      SEQUENCE number of the object.  An additional feature of this
      association is that if a single receiver fails to acknowledge an
      object, an explicit recipient list may be appended to the reliable
      object to indicate those routers that should re-ack the object. In
      the case of delivery failure, this reduces the number of a media
      accesses by requiring only those who have not acknowledged a
      object to explicitly respond.

3.6 Multipoint Relaying

   IMEP supports Multipoint Relaying (MR)--an optional mode or mechanism
   designed to minimize the overhead of packet *flooding* throughout a
   MANET by optimizing/reducing the number of duplicate retransmissions.
   As control overhead expenditure is required to support MR, it is
   recommended that this mode be enabled only when sufficient flooding
   traffic exists so that the benefit derived from MR justifies its
   cost.

   Before describing MR in detail, we first give some terminology
   specific to MR:

   MultiPoint Relay (MPR):
        A router which is selected by a one-hop neighbor to forward or
        retransmit that neighbor's packets.

   Multipoint Relay Selector (MPRS):
        Each MPR has one or more neighbors which have selected it as a
        MPR--each such neighbor is referred to as a ``Multipoint Relay
        Selector".  Each MPR keeps a table of RIDs identifying the
        members of its MRS set so that it knows which packets to
        retransmit via MR.

   Source of the Multipoint Relay (SMR):
        Each router which originally transmits a data packet via MR is
        known as the ``Source of the Multipoint Relay" for that packet,



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        and is so identified in the packet.

   Every router has a set of nodes one hop away N1 (its one-hop neighbor
   set) and a set of nodes two hops away N2 (its two-hop neighbor set).
   The objective of a router participating in MR is to select a minimal
   subset M of MPRs from N1 so that their retransmissions cover N2.

   Multipoint relaying proceeds as follows:

   Each MR router periodically broadcasts a Multipoint Relaying Adver-
   tisement (MRA) packet once every Multipoint Relaying Period (MRP)
   containing its RID, the RIDs of all its one-hop neighbors in N1, and
   the subset M of these neighbors it has selected as its MPRs.  This is
   an implicit broadcast to the current one-hop neighbor set N1 which
   may occur reliably or unreliably as desired.  It can easily be seen
   that with each MR router transmitting the identity of its set N1,
   every MR router learns its set N2.

   The algorithm for selection of the set M is not prescribed. It is
   required only that the set M be chosen so as to cover N2.  The aim is
   to select the ``minimum" number of MPRs to do so.

   One possible algorithm is:

      1. Start with an empty set M.
      2. First select as MPRs those routers from F1 which
         provide the ``only path" to reach some routers in N2.
      3. While there still exist some routers in N2 that are not
         covered by M:
         3.1 For each router in N1, calculate the number of routers
             in N2 reachable through this router which are not
             yet covered by M;
         3.2 Select as a MPR that router which reaches the
             maximum number of uncovered routers in R2.

   A ``flood termination" mechanism is also required and is implemented
   simply by including a SMR field and a sequence number in every MR
   object.  This enables routers to maintain a list of recently-received
   MR objects.  MR objects are passed to the appropriate ULP the *first*
   time they are recieved at a router, and are silently discarded
   thereafter.

3.7 Authentication

   Authentication is optional.  If authentication is enabled, MANET
   routers have the choice of implementing multiple authentication
   options ranging from simple to complex.  IMEP messages between MANET
   routers are authenticated with the IMEP Authentication object, which



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   contains the option is use. This object immediately follows all non-
   authentication objects.

4. IMEP Message Format

   The following describes the message format of the proposed protocol.
   An IMEP message format consists of several  fixed,  mandatory  fields
   followed  by  a  self-formatting  byte stream.  The stream is aligned
   along  ``byte"  boundaries---not  32-bit  word  boundaries---to
   save transmission  overhead  at  the cost of extra processing at a
   router.  An IMEP message typically contains at least one of  several
   optional object blocks.  A message containing no objects is a BEACON
   message.  The following ``grammar" describes the syntax of an IMEP
   message.

   <IMEP message>      :  <IMEP_MSGHDR> <IMEP_OBJECTLIST>

   <IMEP_MSGHDR>       :  <IMEP_VERSION> <COLOR> <MESSAGE_LENGTH> <RID>

   <IMEP_OBJECTLIST>   :  <IMEP_OBJECTLIST> <IMEP_OBJECT>
                       |  <IMEP_OBJECT>

   <IMEP_OBJECT>       :  <OBJECT_HDR> <RELIABLE_OBJECT>
                       |  <OBJECT_HDR> <UNRELIABLE_OBJECT>

   <OBJECT_HDR>        :  <OBJTYPE> <SEQUENCE> <OBJECT_LENGTH>

   <RELIABLE_OBJECT>   :  <DATA>
                       |  <DATA> <ACK List>

   <UNRELIABLE_OBJECT> :  <DATA>

   <DATA>              :  <ECHO>
                       |  <BCAST>
                       |  <MCAST> <DELIVERY_LIST>
                       |  <MR>
                       |  <ACK>
                       |  <NEWCOLOR>
                       |  <MRA>
                       |  <AUTH>

   <BCAST>             :  <PROTOCOL> <OBJLEN> <OBJDATA>

   <MCAST>             :  <PROTOCOL> <OBJLEN> <DELIVERY_LIST_LEN>
                          <OBJDATA>

   <MR>                :  <SMRRID> <MRSEQUENCE> <PROTOCOL>
                          <OBJLEN> <OBJDATA>



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4.1 <IMEP_MSGHDR>

      Every IMEP message contains header information. A message with
      no objects is termed a BEACON message. Included in
      every header is the <RID> of the sending IP interface.

      <IMEP_MSGHDR> : <IMEP_VERSION> <COLOR> <MESSAGE_LENGTH> <RID>

       31           24 23           16 15            8 7             0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      (a)       |      (b)     |              (c)              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       31           24 23           16 15            8 7             0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              (d)                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      (a) <IMEP_VERSION> Protocol version (8 bits)

      (b) <COLOR> Group color (8 bits)
          ==  0      - colorless
          otherwise  - reliability sequence numbers are prefixed by
                       this color

      (c) <MESSAGE_LENGTH> Total message length (in bytes) of this
          IMEP packet (16 bits) which lies in the following range:

                3 < IMEP_LENGTH <= MAX_IMEP_LENGTH <= 65535

      (d) <RID> Router Id associated with the sender's IP interface.



















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4.1.1 <OBJECT_HDR>

       31           24 23           16 15            8 7             0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      (a)      |      (b)      |              (c)              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      (a) <OBJTYPE> object type (8 bits)
          0       - reserved
          1-127   - object does not carry reliability information,
                    seq# ignored
          128-255 - object must be delivered reliably, in order,
                    according to color and seq #

      (b) <SEQUENCE> Sequence number for this object (8 bits)

      (c) <OBJECT_LENGTH> Length (in bytes) of this object
          (16 bits). <OBJECT_LENGTH> does not include the length
          of the SUBMESSAGE HEADER, but does include the length of
          the explicit ack list, if any.

             (<OBJECT_LENGTH> <= <MESSAGE_LENGTH> - 4)





























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4.1.2 <OBJTYPE>

   The following object types are defined for this version of IMEP.

   Unreliable Object Types:

        1   - SM_ECHO    :  <ECHO> object
        2   - SM_ACK     :  <ACK> object
        3   - SM_UBCAST  :  <BCAST> object, delivered unreliably
        4   - SM_UMCAST  :  <MCAST> object, delivered unreliably
        5   - SM_UMRA    :  <MRA> object, delivered unreliably
        6   - SM_UMR     :  <MR> object, delivered unreliably
        7   - SM_IECHO   :  <IECHO> object
        8   - SM_IACK    :  <IACK> object
        [65,73]          :  (future) IPV6 Versions of the above
                            objects

   Reliable Object Types:

        128 - SM_NEWCOLOR  :  <NEWCOLOR> object
        129 - SM_BCAST     :  <BCAST> object delivered reliably
        130 - SM_MCAST     :  <MCAST> object delivered reliably
        131 - SM_AUTH      :  <AUTH> object delivered reliably
        132 - SM_MRA       :  <MRA> object, delivered reliably
        133 - SM_MR        :  <MR> object delivered reliably
        [192,197]          :  (future) IPV6 Versions of the above
                              objects

4.2 IMEP objects

   This section describes the ordering of IMEP objects a MANET router
   may include in an IMEP message. This following ordering MUST be fol-
   lowed:

      a) The fixed-length IMEP message header, followed by

      b) If present, any non-authentication objects, followed by

      c) The IMEP Authentication object.

   The authentication in the IMEP messages MUST be checked.  The receiv-
   ing router MUST check for the presence of a valid IMEP Authentication
   object, and perform the indicated authentication.  Exactly one IMEP
   Authentication object MUST be present in the IMEP message, and the
   home agent MUST check the Authenticator value in the object.  If no
   IMEP Authentication object is found, or if more than one IMEP Authen-
   tication object is found, or if the Authenticator is invalid, the
   receiving router MUST discard the IMEP message and SHOULD log the



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   error as a security exception.

4.2.1 <ECHO>

   The <ECHO> block may contain any number (subject  to  message  length
   restrictions) of Addresses

   <ECHO>    : <ECHO_LIST>

   <ECHO_LIST> : <ECHO_LIST> <ECHO_ENTRY>
                | <ECHO_ENTRY>

   <ECHO_ENTRY> : <ECHO_IF>

   A <ECHO_ENTRY> is a 32-bit address that contains the interface  being
   echo'ed.

       31           24 23           16 15            8 7             0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              (a)                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   (a) <ECHO_IF> IPV4 of interface that is being echo'ed (4 bytes)


   The  number  of  addresses  in  this  list  are  inferred  from   the
   <OBJECT_LENGTH> field.
























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4.2.2 <ACK>

   The ACK Block format is:

   <ACK>      :  <Ack List>

   <Ack List> :  <Ack List> <Ack Entry>
              |  <Ack Entry>

   <Ack Entry> : <ACK_IPADDR> <ACK_COLOR> <ACK_SEQUENCE>

   <Ack Entry> is defined as follows: This block may contain any  number
   (up  to total length restrictions) of acknowledgements interfaces and
   sequence #'s

   numAcks = <OBJECT_LENGTH>/6

   ACK Block 6-byte byte block:

      31            24 23           16 15            8 7             0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              (a)                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      15             8 7             0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      (b)      |      (c)      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       (a) <ACK_IPADDR> IPV4 address of interface being ACKed (4 bytes)
       (b) <ACK_COLOR> Group Color (8 bits)
       (c) <ACK_SEQUENCE> object sequence# (8 bits)



















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4.2.3 <IECHO>

   The <IECHO> block may contain any number (subject to  message  length
   restrictions) of <IECHO_ENTRY>s.

   <IECHO>       : <IECHO_LIST>

   <IECHO_LIST>  : <IECHO_LIST> <IECHO_ENTRY>
                 | <IECHO_ENTRY>

   <IECHO_ENTRY> : <ECHO_IF> <RCV_IF>

   A <IECHO_ENTRY> consists of two 32-bit  addresses  that  contain  the
   interface  being  echo'ed  by  the  router  and  the  interface which
   received the BEACON-equivalent, for which this is an *indirect* echo.

       31           24 23           16 15            8 7             0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              (a)                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       31           24 23           16 15            8 7             0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              (b)                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   (a) <ECHO_IF> IPV4 of interface that is being echo'ed (4 bytes)

   (b) <RCV_IF> IPV4 of interface of the receiving interface (4 bytes)

   The  number  of  entries  in  this  list  are   inferred   from   the
   <OBJECT_LENGTH> field.



















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4.2.4 <IACK>

   The <IACK> Block format is:

   <IACK>       :  <IACK_LIST>

   <IACK_LIST>  :  <IACK_LIST> <IACK_ENTRY>
                |  <IACK_ENTRY>

   <IACK_ENTRY> : <ACK_IPADDR> <RCV_IPADDR> <ACK_COLOR> <ACK_SEQUENCE>

   <IACK_ENTRY> is defined as follows: This block may contain any number
   (up to total length restrictions) of indirect acknowledgements.

   numIAcks = <OBJECT_LENGTH>/10

   IACK Block 10-byte byte block:

      31            24 23           16 15            8 7             0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              (a)                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      31            24 23           16 15            8 7             0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              (b)                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      15             8 7             0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      (c)      |      (d)      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       (a) <ACK_IPADDR> IPV4 address of interface being IACKed (4 bytes)
       (b) <RCV_IPADDR> IPV4 address of receiving interface (4 bytes)
       (c) <ACK_COLOR> Group Color (8 bits)
       (d) <ACK_SEQUENCE> object sequence# (8 bits)














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4.2.5 <NEWCOLOR>

   <NEWCOLOR>  : <NEW_COLOR> <NEW_SEQUENCE>

   This contains the information about a new COLOR and  SEQUENCE  for  a
   multicast   group.  The  membership  list  is  done  as  an  explicit
   <ACK_LIST> and is not handled here.

   numMembers = (<OBJECT_LENGTH> - 2)/4

      15             8 7             0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      (a)      |      (b)      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   (a) <NEW_COLOR> New group color (8 bits)

   (b) <NEW_SEQUENCE> Next valid sequence# (8 bits)

4.2.6 <MRA>

   The MRA Block format is:

   <MRA>               :  <MRSRID> <NUM_NBRS> <NUM_MPRFLAGWORDS>
                          <NBR List> <MPRFLAGWORDS List>

   <NBR List>          :  <NBR List> <NBR Entry>
                       |  <NBR Entry>

   <MPRFLAGWORDS List> :  <MPRFLAGWORDS List> <MPRFLAGWORD>
                       |  <MPRFLAGWORD>

   <MRA> is defined as follows: This block contains the RID of the
   advertising MRS, followed by a counter indicating the number of
   neighbors and a counter indicating the number of words required to
   hold the MPR flags indicating which of those neighbors are MPRs. The
   MRA may contain any number (up to total length restrictions) of one-
   hop neighbor RIDs, and associated flags specifying which of these
   neighbors have been selected as MPRs.












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      31            24 23           16 15            8 7             0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              (a)                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      31            24 23           16 15            8 7             0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              (b)              |              (c)              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       (a) <MRSRID> Router ID of advertising MRS (4 bytes)
       (b) <NUM_NBRS> Number of one-hop neighbors (16 bits)
       (c) <NUM_MPRFLAGWORDS> Number of 32-bit words required for
           MPRFLAGS (16 bits)

           NUM_MPRFLAGWORDS = (NUM_NBRS+31)/32

      31            24 23           16 15            8 7             0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              (d)                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       (d) <NBR Entry> Neighbor Router ID (4 bytes)
           One entry per neighbor.

      31            24 23           16 15            8 7             0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              (e)                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       (e) <MPRFLAGWORD> 32-bit word containing 32 1-bit MPR flags
           One word required for 32 neighbors.
           The i-th bit in the j-th word indicates the MPR status
           of the n-th (n = j*32 + i) neighbor in the neighbor list
           where 1 indicates the neighbor is a MPR, and 0 indicates
           otherwise.
















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4.2.7 <BCAST>

   A broadcast object block is used for delivering encapsulated data  to
   an upper-layer protocol (ULP). This block will be received and passed
   to the appropriate ULP by all  receivers.  If  the  <BCAST>  is  sent
   reliably,  then  only those routers with a matching color may forward
   the message to the  appropriate  ULP.    Each  object  block  may  be
   independently- sequenced by virtue of its object header. However, all
   blocks with reliability share the same group color.

   <BCAST> : <PROTOCOL> <OBJLEN> <OBJDATA>

      23            16 15            8 7             0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      (a)      |              (b)              |       (c) ....
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   (a) <PROTOCOL> protocol type (8 bits)
       0     - reserved
       1     - TORA
       2     - AODV
       3-255 - unassigned

   (b) <OBJLEN> block length (in bytes) (16 bits)

   (c) <OBJDATA> This is <OBJLEN> bytes of data encapsulated by IMEP

   <BCAST> blocks are delivered reliably,  and  can  therefore  have  an
   explicit  acknowledgement list. The <OBJLEN> in (b) can be subtracted
   from  the  <OBJECT_LENGTH>  to  determine  the  number  of   explicit
   addresses that should generate acknowledgments.

   numExplicitAcks = (<OBJECT_LENGTH> - (<OBJLEN> + 3))/4


















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4.2.8 <MCAST>

   A multicast (or explicit) object block is very similar to a broadcast
   object in that it is also used for delivering encapsulated data to an
   upper-layer protocol  (ULP).  The  difference  is  that  the  <MCAST>
   contains  an  *explicit* delivery list.  This implies that the object
   data block can be  passed to the appropriate ULP  only  by  receivers
   that  are  members  of  the  <DELIVERY_LIST>.  If the <MCAST> is sent
   reliably, then only those routers with a matching color  may  forward
   the  message  to  the  appropriate  ULP.    Each  object block may be
   independently-sequenced by virtue of its object header. However,  all
   blocks  with  reliability  share  the  same group color. It should be
   noted  that  if  this  block  is  sent  with  reliability,  then  all
   receivers,  not  just  those on the <DELIVERY_LIST>, must ACKnowledge
   receipt of the message.

   <MCAST> : <PROTOCOL> <OBJLEN> <DELIVERY_LIST_LEN> <OBJDATA>

      23            16 15            8 7             0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      (a)      |              (b)              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       15            8 7             0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              (c)              |    (d) ....
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   (a) <PROTOCOL> protocol type (8 bits)
       0     - reserved
       1     - TORA
       2     - AODV
       3     - DSR
       4     - ZRP
       5-255 - unassigned

   (b) <OBJLEN> block length (in bytes) (16 bits)

   (c) <DELIVERY_LIST_LEN> - Length of the explicit delivery list
       (in bytes). (16 bits)

   (d) <OBJDATA> This is <OBJLEN> bytes of data encapsulated by IMEP

   <MCAST> blocks may be delivered reliably, and can therefore  have  an
   explicit   acknowledgement   list.   The  <OBJLEN>  in  (b)  and  the
   <DELIVERY_LIST_LEN> in (c)  can  be  subtracted  from  the  from  the
   <OBJECT_LENGTH>  to  determine  the number of explicit addresses that
   should generate acknowledgments.



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   numExplicitAcks = (<OBJECT_LENGTH> - (<OBJLEN> +  <DELIVERY_LIST_LEN>
   + 3))/4


4.2.9 <MR>

   A multipoint relaying object block is also similar to a broadcast
   object in that it is also used for delivering encapsulated data to an
   upper-layer protocol (ULP). The difference is that the <MR> contains
   an implicit delivery list as determined by the MR algorithm. The
   object data block is only passed to the appropriate ULP the *first*
   time it is received at a router--any subsequently received copies are
   silently discarded. Routers maintain a list of recently-received <MR>
   blocks indexed by SMR and MRSEQUENCE to determine whether a block was
   previously received.

   If the <MR> is sent reliably, then only those routers with a matching
   color may forward the object to the appropriate ULP.   Each object
   block may be independently-sequenced by virtue of its object header.
   However, all blocks with reliability share the same group color. It
   should be noted that if this block is sent with reliability, then all
   receivers, not just the MPRs, must ACKnowledge receipt of the mes-
   sage.

   <MR> :  <SMRRID> <MRSEQUENCE> <OBJLEN> <OBJDATA>

      31            24 23           16 15            8 7             0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              (a)                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      31            24 23           16 15            8 7             0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      (b)      |      (c)      |              (d)              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      31            24 23           16 15            8 7             0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      (e)....
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   (a) <SMRRID> protocol type (32 bits)
       Router ID of Source of the Multipoint Relay packet.

   (b) <MRSEQUENCE> Multipoint Relay packet sequence# (8 bits)

   (c) <PROTOCOL> protocol type (8 bits)



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       0     - reserved
       1     - TORA
       2     - AODV
       3     - DSR
       4     - ZRP
       5-255 - unassigned

   (d) <OBJLEN> block length (in bytes) (16 bits)

   (e) <OBJDATA> This is <OBJLEN> bytes of data encapsulated by IMEP

4.2.10 <ACK List>, <DELIVERY_LIST>

   Lists are arrays of IPV4 addresses. Each entry is a 32-bit address in
   network  byte  order. The length of the list is either stored as part
   of the object information (see <DELIVERY_LIST_LEN>) or inferred  from
   other available lengths (see <OBJECT_LENGTH> and <OBJLEN>).

4.2.11 <AUTH> (The IMEP Authentication object)

   The IMEP Authentication object is used to authenticate all IMEP
   objects.   The types of authentication to be supported will be speci-
   fied in a proposed MANET Authentication Architecture under develop-
   ment.

4.3  ULP/IMEP Interface

   Other than registration, IMEP interacts with ULPs in several funda-
   mental ways.  Here this interaction is specified in a format which
   loosely follows the Object Management Group's (OMG) Interface Defini-
   tion Language (IDL).




















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4.3.1  Registration

   ULPs must register with IMEP prior to use.  Registration consists
   of calling the following register function.

   typedef enum SignallingSupport { CONN, LINK, DISABLED };

   void register (in <PROTOCOL> type,
                     // indicates Protocol type of data object
                     // if not valid, an InvalidProtocolType exception
                     // is thrown.
                  in any ULPhandle,
                     // *implementation-dependent*
                     // a handle is passed to IMEP depending on the
                     // implementation of the ULP/IMEP system that allows
                     // IMEP to pass signals to the ULP.
                     // if not valid (and this is detectable by IMEP),
                     // an InvalidULPhandle exception is thrown.
                  in <OBJLEN> epitaphLength,
                     // indicates length of the epitaph object;
                     // if length = 0, this indicates no epitaph message and
                     // the OBJDATA field is ignored.
                     // if length > MAX_EPITAPH_LENGTH, then
                     // an InvalidByteLength exception is thrown
                  in <OBJDATA> epitaph,
                     // opaque epitaph data object
                  in SignallingSupport mode)
                     // indicates IMEP Signalling Support mode
                     // if incorrect, an InvalidSignallingSupport exception
                     // is thrown
                  raises (InvalidProtocolType,
                          InvalidULPhandle,
                          InvalidByteLength,
                          InvalidSignallingSupport);

















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4.3.2  Encapsulation

   IMEP principally aggregates and encapsulates ULP objects into longer
   IMEP messages.  From a ULP's perspective, these may be delivered
   reliably or unreliably, and either implicitly broadcast to the
   entire one-hop neighbor set, or explicitly multicast to a one-hop
   neighbor subset.  Thus, an object being given to IMEP for transmission
   must come with this additional information.  The following
   specifies the operation ``encapsulate".

   typedef enum Boolean { TRUE, FALSE };
   typedef enum ForwardingMode { BCAST, MCAST, MR };

   void encapsulate (in <PROTOCOL> type,
                        // indicates Protocol type of data object
                        // if not valid, an InvalidProtocolType exception
                        // is thrown.
                     in <OBJLEN> length,
                        // indicates length of data object;
                        // if length > MAX_IMEP_LENGTH, then
                        // an InvalidByteLength exception is thrown
                     in <OBJDATA> data,
                        // data object to be transmitted
                     in ForwardingMode mode,
                        // indicates IMEP forwarding mode
                        // if incorrect, an InvalidForwardingMode exception
                        // is thrown
                     in <DELIVERY_LIST> list,
                        // List of IPv4 addresses to which object
                        // should be explicitly delivered via MCAST.
                        // If one or more addresses are incorrect,
                        // an InvalidInterface exception is thrown
                     in Boolean reliability)
                        // indicates whether reliable delivery is desired
                     raises (InvalidProtocolType,
                             InvalidByteLength,
                             InvalidForwardingMode,
                             InvalidInterface);

5. Security Considerations

   The MANET computing environment is very different from the ordinary
   computing environment.  In many cases, mobile computers will be con-
   nected to the network via wireless links.  Such links are particu-
   larly vulnerable to passive eavesdropping, active replay attacks, and
   other active attacks.  Among its many uses, the networking protocol
   described in this document enables inter-router communication for
   purposes of network control.  This control function could be a



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   significant vulnerability if IMEP messages are not authenticated.

Authors' Addresses:

   M. Scott Corson
   Institute for Systems Research
   A.V. Williams Building (115)
   University of Maryland
   College Park, MD 20742, USA
   (301) 405-6630
   corson@isr.umd.edu

   S. Papademetriou
   Institute for Systems Research
   A.V. Williams Building (115)
   University of Maryland
   College Park, MD 20742, USA
   (301) 405-7933
   spyro@isr.umd.edu

   Philip Papadopoulos
   Computer Science and Mathematics Division
   Oak Ridge National Laboratory
   Oak Ridge, TN 37831-6367, USA
   (423) 241-3972
   papadopoulpm@ornl.gov

   Vincent Park
   Information Technology Division
   Code 5540
   Naval Research Laboratory
   Washington, DC 20375, USA
   (202) 767-5098
   vpark@itd.nrl.navy.mil

   Amir Qayyum
   INRIA
   Sophia-Antipolis, France
   Amir.Qayyum@inria.fr












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