Internet Draft                                              L. Yang
    Expiration: March April 2004                                  Intel Corp.
    File: draft-ietf-forces-framework-09.txt draft-ietf-forces-framework-10.txt                   R. Dantu
    Working Group: ForCES                          Univ. of North Texas
                                                            T. Anderson
                                                            Intel Corp.
                                                               R. Gopal


                                                         October 2003

        Forwarding and Control Element Separation (ForCES) Framework



 Status of this Memo

    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of RFC2026.  Internet-Drafts are
    working documents of the Internet Engineering Task Force (IETF),
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    Internet-Drafts are draft documents valid for a maximum of six
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 Copyright Notice

    Copyright (C) The Internet Society (2003).  All Rights Reserved.


    This document defines the architectural framework for the ForCES
    (Forwarding and Control Element Separation) network elements, and
    identifies the associated entities and the interactions among them.

 Table of Contents
    1. Definitions...................................................3
       1.1. Conventions used in this document........................3
       1.2. Terminologies............................................3
    2. Introduction to Forwarding and Control Element Separation
    3. Architecture..................................................9
       3.1. Control Elements and Fr Reference Point.................10
       3.2. Forwarding Elements and Fi reference point..............11
       3.3. CE Managers.............................................14
       3.4. FE Managers.............................................14 Managers.............................................15
    4. Operational Phases...........................................15
       4.1. Pre-association Phase...................................15
          4.1.1. Fl Reference Point.................................15
          4.1.2. Ff Reference Point.................................16
          4.1.3. Fc Reference Point.................................17
       4.2. Post-association Phase and Fp reference point...........17
          4.2.1. Proximity and Interconnect between CEs and FEs.....17
          4.2.2. Association Establishment..........................18
          4.2.3. Steady-state Communication.........................19
          4.2.4. Data Packets across Fp reference point.............20
          4.2.5. Proxy FE...........................................21
       4.3. Association Re-establishment............................21
          4.3.1. CE graceful restart................................22
          4.3.2. FE restart.........................................23
    5. Applicability to RFC1812.....................................24
       5.1. General Router Requirements.............................24 Requirements.............................25
       5.2. Link Layer..............................................26
       5.3. Internet Layer Protocols................................26
       5.4. Internet Layer Forwarding...............................27
       5.5. Transport Layer.........................................28
       5.6. Application Layer -- Routing Protocols..................28
       5.7. Application Layer -- Network Management Protocol........28 Protocol........29
    6. Summary......................................................29
    7. Acknowledgements.............................................29
    8. Security Considerations......................................29
       8.1. Analysis of Potential Threats Introduced by ForCES......30
          8.1.1. "Join" or "Remove" Message Flooding on CEs.........30
          8.1.2. Impersonation Attack...............................30
          8.1.3. Replay Attack......................................30 Attack......................................31
          8.1.4. Attack during Fail Over............................31
          8.1.5. Data Integrity.....................................31
          8.1.6. Data Confidentiality...............................31 Confidentiality...............................32
          8.1.7. Sharing security parameters........................32
          8.1.8. Denial of Service Attack via External Interface....32
       8.2. Security Recommendations for ForCES.....................32
          8.2.1. Security Configuration.............................33
          8.2.2. Using TLS with ForCES..............................33 ForCES..............................34
          8.2.3. Using IPsec with ForCES............................34 ForCES............................35
    9. Normative References.........................................36
    10. Informative References......................................36 References......................................37
    11. Authors' Addresses..........................................37
    12. Intellectual Property Right.................................38
    13. Full Copyright Statement....................................38 Statement....................................39

 1. Definitions

 1.1. Conventions used in this document

    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    this document are to be interpreted as described in RFC 2119.

 1. Definitions

 1.2. Terminologies

    A set of terminology associated with the ForCES requirements is
    defined in [3] and we only include the definitions that are most
    relevant to this document here.

    Addressable Entity (AE) - An entity that is directly addressable
    given some interconnect technology.  For example, on IP networks,
    it is a device to which we can communicate using an IP address; on
    a switch fabric, it is a device to which we can communicate using a
    switch fabric port number.

    Physical Forwarding Element (PFE) - An AE that includes hardware
    used to provide per-packet processing and handling.  This hardware
    may consist of (but is not limited to) network processors, ASICs
    (Application-Specific Integrated Circuits), or general purpose
    processors, installed on line cards, daughter boards, mezzanine
    cards, or in stand-alone boxes.

    PFE Partition - A logical partition of a PFE consisting of some
    subset of each of the resources (e.g., ports, memory, forwarding
    table entries) available on the PFE.  This concept is analogous to
    that of the resources assigned to a virtual switching element as
    described in [8].

    Physical Control Element (PCE) - An AE that includes hardware used
    to provide control functionality.  This hardware typically includes
    a general purpose processor.

    PCE Partition - A logical partition of a PCE consisting of some
    subset of each of the resources available on the PCE.

    Forwarding Element (FE) - A logical entity that implements the
    ForCES protocol.  FEs use the underlying hardware to provide per-
    packet processing and handling as directed by a CE via the ForCES
    protocol.  FEs may happen to be a single blade (or PFE), a
    partition of a PFE or multiple PFEs.

    Control Element (CE) - A logical entity that implements the ForCES
    protocol and uses it to instruct one or more FEs how to process
    packets.  CEs handle functionality such as the execution of control
    and signaling protocols.  CEs may consist of PCE partitions or
    whole PCEs.

    ForCES Network Element (NE) - An entity composed of one or more CEs
    and one or more FEs.  To entities outside an NE, the NE represents
    a single point of management.  Similarly, an NE usually hides its
    internal organization from external entities.

    Pre-association Phase - The period of time during which an FE
    Manager (see below) and a CE Manager (see below) are determining
    which FE and CE should be part of the same network element.

    Post-association Phase - The period of time during which an FE does
    know which CE is to control it and vice versa, including the time
    during which the CE and FE are establishing communication with one

    ForCES Protocol - While there may be multiple protocols used within
    the overall ForCES architecture, the term "ForCES protocol" refers
    only to the ForCES post-association phase protocol (see below).

    ForCES Post-Association Phase Protocol - The protocol used for
    post-association phase communication between CEs and FEs.  This
    protocol does not apply to CE-to-CE communication, FE-to-FE
    communication, nor to communication between FE and CE managers.
    The ForCES protocol is a master-slave protocol in which FEs are
    slaves and CEs are masters.   This protocol includes both the
    management of the communication channel (e.g., connection
    establishment, heartbeats) and the control messages themselves.
    This protocol could be a single protocol or could consist of
    multiple protocols working together, and may be unicast based or
    multicast based. A separate protocol document will specify this

    FE Manager - A logical entity that operates in the pre-association
    phase and is responsible for determining to which CE(s) an FE
    should communicate.  This process is called CE discovery and may
    involve the FE manager learning the capabilities of available CEs.
    An FE manager may use anything from a static configuration to a
    pre-association phase protocol (see below) to determine which CE(s)
    to use; however, this is currently out of scope.  Being a logical
    entity, an FE manager might be physically combined with any of the
    other logical entities mentioned in this section.

    CE Manager - A logical entity that operates in the pre-association
    phase and is responsible for determining to which FE(s) a CE should
    communicate.  This process is called FE discovery and may involve
    the CE manager learning the capabilities of available FEs.  A CE
    manager may use anything from a static configuration to a pre-
    association phase protocol (see below) to determine which FE to
    use, however this is currently out of scope.   Being a logical
    entity, a CE manager might be physically combined with any of the
    other logical entities mentioned in this section.

    Pre-association Phase Protocol - A protocol between FE managers and
    CE managers that is used to determine which CEs or FEs to use.  A
    pre-association phase protocol may include a CE and/or FE
    capability discovery mechanism.  Note that this capability
    discovery process is wholly separate from (and does not replace)
    that used within the ForCES protocol.  However, the two capability
    discovery mechanisms may utilize the same FE model.

    FE Model - A model that describes the logical processing functions
    of an FE.

    ForCES Protocol Element - An FE or CE.

    Intra-FE topology -                      - Representation of how a single FE is realized
    by combining possibly multiple logical functional blocks along
    multiple data path. This is defined by the FE model.

    FE Topology -- Representation of how the multiple FEs in a single
    NE are interconnected.  Sometimes it is called inter-FE topology,
    to be distinguished from intra-FE topology used by the FE model.

    Inter-FE topology - see FE Topology.

 2. Introduction to Forwarding and Control Element Separation (ForCES)

    An IP network element (NE) appears to external entities as a
    monolithic piece of network equipment, e.g., a router, NAT,
    firewall, or load balancer.  Internally, however, an IP network
    element (NE) (such as a router) is composed of numerous logically
    separated entities that cooperate to provide a given functionality
    (such as routing).  Two types of network element components exist:
    control element (CE) in control plane and forwarding element (FE)
    in forwarding plane (or data plane).  Forwarding elements typically
    are ASIC, network-processor, or general-purpose processor-based
    devices that handle data path operations for each packet.  Control
    elements are typically based on general-purpose processors that
    provide control functionality like routing and signaling protocols.

    ForCES aims to define a framework and associated protocol(s) to
    standardize information exchange between the control and forwarding
    plane.  Having standard mechanisms allows CEs and FEs to become
    physically separated standard components.  This physical separation
    accrues several benefits to the ForCES architecture.  Separate
    components would allow component vendors to specialize in one
    component without having to become experts in all components.
    Standard protocol also allows the CEs and FEs from different
    component vendors to interoperate with each other and hence it
    becomes possible for system vendors to integrate together the CEs
    and FEs from different component suppliers.  This interoperability
    translates into more design choices and flexibility for the system
    vendors.  Overall, ForCES will enable rapid innovation in both the
    control and forwarding planes while maintaining interoperability.
    Scalability is also easily provided by this architecture in that
    additional forwarding or control capacity can be added to existing
    network elements without the need for forklift upgrades.

         -------------------------       -------------------------
         |  Control Blade A      |       |  Control Blade B      |
         |       (CE)            |       |          (CE)         |
         -------------------------       -------------------------
                 ^   |                           ^    |
                 |   |                           |    |
                 |   V                           |    V
         |               Switch Fabric Backplane                 |
                ^  |            ^  |                   ^  |
                |  |            |  |     . . .         |  |
                |  V            |  V                   |  V
            ------------    ------------           ------------
            |Router    |    |Router    |           |Router    |
            |Blade #1  |    |Blade #2  |           |Blade #N  |
            |   (FE)   |    |   (FE)   |           |   (FE)   |
            ------------    ------------           ------------
                ^  |            ^  |                   ^  |
                |  |            |  |     . . .         |  |
                |  V            |  V                   |  V

         Figure 1. A router configuration example with separate blades.

    One example of such physical separation is at the blade level.
    Figure 1 shows such an example configuration of a router, with two
    control blades and multiple forwarding blades, all interconnected
    into a switch fabric backplane.  In such a chassis configuration,
    the control blades are the CEs while the router blades are FEs, and
    the switch fabric backplane provides the physical interconnect for
    all the blades.  Control blade A may be the primary CE while
    control blade B may be the backup CE providing redundancy.  It is
    also possible to have a redundant switch fabric for high
    availability support.  Routers today with this kind of
    configuration use proprietary interfaces for messaging between CEs
    and FEs.  The goal of ForCES is to replace such proprietary
    interfaces with a standard protocol.  With a standard protocol like
    ForCES implemented on all blades, it becomes possible for control
    blades from vendor X and forwarding blades from vendor Y to work
    seamlessly together in one chassis.

         -------         -------
         | CE1 |         | CE2 |
         -------         -------
            ^               ^
            |               |
            V               V
     ============================================ Ethernet
         ^       ^       . . .   ^
         |       |               |
         V       V               V
      -------  -------         --------
      | FE#1|  | FE#2|         | FE#n |
      -------  -------         --------
        ^  |     ^  |            ^  |
        |  |     |  |            |  |
        |  V     |  V            |  V

         Figure 2. A router configuration example with separate boxes.

    Another level of physical separation between the CEs and FEs can be
    at the box level.  In such configuration, all the CEs and FEs are
    physically separated boxes, interconnected with some kind of high
    speed LAN connection (like Gigabit Ethernet).  These separated CEs
    and FEs are only one hop away from each other within a local area
    network.  The CEs and FEs communicate to each other by running
    ForCES, and the collection of these CEs and FEs together become one
    routing unit to the external world. Figure 2 shows such an example.

    In both examples shown here, the same physical interconnect is used
    for both CE-to-FE and FE-to-FE communication.  However, that does
    not have to be the case.  One reason to use different interconnects
    is that CE-to-FE interconnect does not have to be as fast as the
    FE-to-FE interconnect, so the more expensive fast connections can
    be saved for FE-to-FE.  The separate interconnects may also provide
    reliability and redundancy benefits for the NE.

    Some examples of control functions that can be implemented in the
    CE include routing protocols like RIP, OSPF and BGP, control and
    signaling protocols like RSVP (Resource Reservation Protocol), LDP
    (Label Distribution Protocol) for MPLS, etc.  Examples of
    forwarding functions in the FE include LPM (longest prefix match)
    forwarder, classifiers, traffic shaper, meter, NAT (Network Address
    Translators), etc.  Figure 3 provides example functions in both CE
    and FE.  Any given NE may contain one or many of these CE and FE
    functions in it.  The diagram also shows that ForCES protocol is
    used to transport both the control messages for ForCES itself and
    the data packets that are originated/destined from/to the control
    functions in CE (e.g., routing packets).  Section 4.2.4 provides
    more detail on this.

         |       |       |       |       |       |       |
         |OSPF   |RIP    |BGP    |RSVP   |LDP    |. . .  |
         |       |       |       |       |       |       |
         |               ForCES Interface                |
                                 ^   ^
                         ForCES  |   |data
                         control |   |packets
                         messages|   |(e.g., routing packets)
                                 v   v
         |               ForCES Interface                |
         |       |       |       |       |       |       |
         |LPM Fwd|Meter  |Shaper |NAT    |Classi-|. . .  |
         |       |       |       |       |fier   |       |
         |               FE resources                    |
                 Figure 3. Examples of CE and FE functions

    A set of requirements for control and forwarding separation is
    identified in [3].  This document describes a ForCES architecture
    that satisfies the architectural requirements of that document and
    defines a framework for ForCES network elements and the associated
    entities to facilitate protocol definition.  Whenever necessary,
    this document uses many examples to illustrate the issues and/or
    possible solutions in ForCES.  These examples are intended to be
    just examples, and should not be taken as the only or definite ways
    of doing certain things.  It is expected that separate document
    will be produced by the ForCES working group to specify the ForCES

 3. Architecture

    This section defines the ForCES architectural framework and the
    associated logical components.  This ForCES framework defines
    components of ForCES NEs including several ancillary components.
    These components may be connected in different kinds of topologies
    for flexible packet processing.

                            | ForCES Network Element              |
     --------------   Fc    | --------------      --------------  |
     | CE Manager |---------+-|     CE 1   |------|    CE 2    |  |
     --------------         | |            |  Fr  |            |  |
           |                | --------------      --------------  |
           | Fl             |         |  |    Fp       /          |
           |                |       Fp|  |----------| /           |
           |                |         |             |/            |
           |                |         |             |             |
           |                |         |     Fp     /|----|        |
           |                |         |  /--------/      |        |
     --------------     Ff  | --------------      --------------  |
     | FE Manager |---------+-|     FE 1   |  Fi  |     FE 2   |  |
     --------------         | |            |------|            |  |
                            | --------------      --------------  |
                            |   |  |  |  |          |  |  |  |    |
                                |  |  |  |          |  |  |  |
                                |  |  |  |          |  |  |  |
                                  Fi/f                   Fi/f

                 Figure 4. ForCES Architectural Diagram
    The diagram in Figure 4 shows the logical components of the ForCES
    architecture and their relationships.  There are two kinds of
    components inside a ForCES network element: control element (CE)
    and forwarding element (FE).  The framework allows multiple
    instances of CE and FE inside one NE.  Each FE contains one or more
    physical media interfaces for receiving and transmitting packets
    from/to the external world.  The aggregation of these FE interfaces
    becomes the NE's external interfaces.  In addition to the external
    interfaces, there must also exist some kind of interconnect within
    the NE so that the CE and FE can communicate with each other, and
    one FE can forward packets to another FE.  The diagram also shows
    two entities outside of the ForCES NE: CE Manager and FE Manager.
    These two entities provide configuration to the corresponding CE or
    FE in the pre-association phase (see Section 4.1).  There is no
    defined role for FE Manager and CE Manager in post-association
    phase, thus these logical components are not considered part of the
    ForCES NE.

    For convenience, the logical interactions between these components
    are labeled by reference points Fp, Fc, Ff, Fr, Fl, and Fi, as
    shown in Figure 4.  The FE external interfaces are labeled as Fi/f.
    More detail is provided in Section 3 and 4 for each of these
    reference points.  All these reference points are important in
    understanding the ForCES architecture, however, the ForCES protocol
    is only defined over one reference point -- Fp.

    The interface between two ForCES NEs is identical to the interface
    between two conventional routers and these two NEs exchange the
    protocol packets through the external interfaces at Fi/f.  ForCES
    NEs connect to existing routers transparently.

 3.1. Control Elements and Fr Reference Point

    It is not necessary to define any protocols across the Fr reference
    point to enable control and forwarding separation for simple
    configurations like single CE and multiple FEs.  However, this
    architecture permits multiple CEs to be present in a network
    element.  In cases where an implementation uses multiple CEs, the
    invariant that the CEs and FEs together appear as a single NE must
    be maintained.

    Multiple CEs may be used for redundancy, load sharing, distributed
    control, or other purposes.  Redundancy is the case where one or
    more CEs are prepared to take over should an active CE fail.  Load
    sharing is the case where two or more CEs are concurrently active
    and any request that can be serviced by one of the CEs can also be
    serviced by any of the other CEs.  For both redundancy and load
    sharing, the CEs involved are equivalently capable.  The only
    difference between these two cases is in terms of how many active
    CEs there are.  Distributed control is the case where two or more
    CEs are concurrently active but certain requests can only be
    serviced by certain CEs.

    When multiple CEs are employed in a ForCES NE, their internal
    organization is considered an implementation issue that is beyond
    the scope of ForCES.  CEs are wholly responsible for coordinating
    amongst themselves via the Fr reference point to provide
    consistency and synchronization.  However, ForCES does not define
    the implementation or protocols used between CEs, nor does it
    define how to distribute functionality among CEs.  Nevertheless,
    ForCES will support mechanisms for CE redundancy or fail over, and
    it is expected that vendors will provide redundancy or fail over
    solutions within this framework.

 3.2. Forwarding Elements and Fi reference point

    An FE is a logical entity that implements the ForCES protocol and
    uses the underlying hardware to provide per-packet processing and
    handling as directed by a CE.  It is possible to partition one
    physical FE into multiple logical FEs.  It is also possible for one
    FE to use multiple physical FEs.  The mapping between physical
    FE(s) and the logical FE(s) is beyond the scope of ForCES.  For
    example, a logical partition of a physical FE can be created by
    assigning some portion of each of the resources (e.g., ports,
    memory, forwarding table entries) available on the physical FE to
    each of the logical FEs.  Such concept of FE virtualization is
    analogous to a virtual switching element as described in [8].  If
    FE virtualization occurs only in the pre-association phase, it has
    no impact on ForCES. However, if FE virtualization results in
    dynamic resource change on FE during post-associaiton post-association phase, the FE
    model needs to be able to report such capability and the ForCES
    protocol needs to be able to inform the CE of such change via
    asynchronous messages (see [3], Section 5, requirement #6).

    FEs perform all packet processing functions as directed by CEs.
    FEs have no initiative of their own.  Instead, FEs are slaves and
    only do as they are told.  FEs may communicate with one or more CEs
    concurrently across reference point Fp.  FEs have no notion of CE
    redundancy, load sharing, or distributed control.  Instead, FEs
    accept commands from any CE authorized to control them, and it is
    up to the CEs to coordinate among themselves to achieve redundancy,
    load sharing or distributed control.  The idea is to keep FEs as
    simple and dumb as possible so that FEs can focus their resource on
    the packet processing functions.

    For example, in Figure 5, FE1 and FE2 can be configured to accept
    commands from both the primary CE (CE1) and the backup CE (CE2).
    Upon detection of CE1 failure, perhaps across the Fr or Fp
    reference point, CE2 is configured to take over activities of CE1.
    This is beyond the scope of ForCES and is not discussed further.

    Distributed control can be achieved in the a similar fashion, without
    much intelligence on the part of FEs.  For example, FEs can be
    configured to detect RSVP and BGP protocol packets, and forward
    RSVP packets to one CE and BGP packets to another CE.  Hence, FEs
    may need to do packet filtering for forwarding packets to specific

                 -------   Fr  -------
                 | CE1 | ------| CE2 |
                 -------       -------
                   |   \      /   |
                   |    \    /    |
                   |     \  /     |
                   |      \/Fp    |
                   |      /\      |
                   |     /  \     |
                   |    /    \    |
                 -------  Fi   -------
                 | FE1 |<----->| FE2 |
                 -------       -------

                    Figure 5. CE redundancy example.

    This architecture permits multiple FEs to be present in an NE. [3]
    dictates that the ForCES protocol must be able to scale to at least
    hundreds of FEs (see [3] Section 5, requirement #11).  Each of
    these FEs may potentially have a different set of packet processing
    functions, with different media interfaces.  FEs are responsible
    for basic maintenance of layer-2 connectivity with other FEs and
    with external entities.  Many layer-2 media include sophisticated
    control protocols.  The FORCES protocol (over the Fp reference
    point) will be able to carry messages for such protocols so that,
    in keeping with the dumb FE model, the CE can provide appropriate
    intelligence and control over these media.

    When multiple FEs are present, ForCES requires that packets must be
    able to arrive at the NE by one FE and leave the NE via a different
    FE (See [3], Section 5, Requirement #3).  Packets that enter the NE
    via one FE and leave the NE via a different FE are transferred
    between FEs across the Fi reference point.  Fi reference point
    could be used by FEs to discovery their (inter-FE) topology,
    perhaps during pre-association phase.  The Fi reference point is a
    separate protocol from the Fp reference point and is not currently
    defined by the ForCES architecture.

    FEs could be connected in different kinds of topologies and packet
    processing may spread across several FEs in the topology.  Hence,
    logical packet flow may be different from physical FE topology.
    Figure 6 provides some topology examples.  When it is necessary to
    forward packets between FEs, the CE needs to understand the FE
    topology.  The FE topology may be queried from the FEs by the CEs
    via ForCES protocol, but the FEs are not required to provide that
    information to the CEs. So, the FE topology information may also be
    gathered by other means outside of the ForCES protocol (like inter-
    FE topology discovery protocol).

                         |      CE       |
                          ^      ^      ^
                         /       |       \
                        /        v        \
                       /      -------      \
                      /    +->| FE3 |<-+    \
                     /     |  |     |  |     \
                    v      |  -------  |      v
                  -------  |           |  -------
                  | FE1 |<-+           +->| FE2 |
                  |     |<--------------->|     |
                  -------                 -------
                     ^  |                   ^  |
                     |  |                   |  |
                     |  v                   |  v

                 (a) Full mesh among FE1, FE2 and FE3.

                         |   CE    |
                        ^ ^       ^ ^
                       /  |       |  \
                /------   |       |   ------\
                v         v       v          v
            -------   -------   -------   -------
            | FE1 |<->| FE2 |<->| FE3 |<->| FE4 |
            -------   -------   -------   -------
              ^  |     ^  |       ^  |     ^  |
              |  |     |  |       |  |     |  |
              |  v     |  v       |  v     |  v

                 (b) Multiple FEs in a daisy chain

                            ^ |
                            | v
                         |   FE1   |<-----------------------|
                         -----------                        |
                           ^    ^                           |
                          /      \                          |
                   | ^   /        \   ^ |                   V
                   v |  v          v  | v                ----------
                 ---------        ---------              |        |
                 | FE2   |        |  FE3  |<------------>|   CE   |
                 ---------        ---------              |        |
                     ^  ^          ^                     ----------
                     |   \        /                        ^  ^
                     |    \      /                         |  |
                     |    v     v                          |  |
                     |   -----------                       |  |
                     |   |   FE4   |<----------------------|  |
                     |   -----------                          |
                     |      |  ^                              |
                     |      v  |                              |
                     |                                        |

                 (c) Multiple FEs connected by a ring

                 Figure 6. Some examples of FE topology.

 3.3.CE Managers

    CE managers are responsible for determining which FEs a CE should
    control.  It is legitimate for CE managers to be hard-coded with
    the knowledge of with which FEs its CEs should communicate with.  A
    CE manager may also be physically embedded into a CE and be
    implemented as a simple keypad or other direct configuration
    mechanism on the CE.  Finally, CE managers may be physically and
    logically separate entities that configure the CE with FE
    information via such mechanisms as COPS-PR [6] or SNMP [4].

 3.4. FE Managers

    FE managers are responsible for determining with which CE any
    particular FE should initially communicate.  Like CE managers, no
    restrictions are placed on how an FE manager decides with which CE
    its FEs should communicate, nor are restrictions placed on how FE
    managers are implemented.  Each FE should have one and only one FE
    manager, while different FEs may have the same or different FE
    manager(s).  Each manager can choose to exist and operate
    independently of other manager.

 4. Operational Phases

    Both FEs and CEs require some configuration in place before they
    can start information exchange and function as a coherent network
    element.  Two operational phases are identified in this framework:
    pre-association and post-association.

 4.1.Pre-association Phase

    Pre-association phase is the period of time during which an FE
    Manager and a CE Manager are determining which FE and CE should be
    part of the same network element.  The protocols used during this
    phase may include all or some of the message exchange over Fl, Ff
    and Fc reference points.  However, all these may be optional and
    none of this is within the scope of ForCES protocol.

 4.1.1. Fl Reference Point

    CE managers and FE managers may communicate across the Fl reference
    point in the pre-association phase in order to determine which CEs
    and FEs should communicate with each other.  Communication across
    the Fl reference point is optional in this architecture.  No
    requirements are placed on this reference point.

    CE managers and FE managers may be operated by different entities.
    The operator of the CE manager may not want to divulge, except to
    specified FE managers, any characteristics of the CEs it manages.
    Similarly, the operator of the FE manager may not want to divulge
    FE characteristics, except to authorized entities.  As such, CE
    managers and FE managers may need to authenticate one another.
    Subsequent communication between CE managers and FE managers may
    require other security functions such as privacy, non-repudiation,
    freshness, and integrity.

         FE Manager      FE               CE Manager     CE
          |              |                 |             |
          |              |                 |             |
          |(security exchange)             |             |
         1|<------------------------------>|             |
          |              |                 |             |
          |(a list of CEs and their attributes)          |
         2|<-------------------------------|             |
          |              |                 |             |
          |(a list of FEs and their attributes)          |
         3|------------------------------->|             |
          |              |                 |             |
          |              |                 |             |
          |<----------------Fl------------>|             |

      Figure 7. An example of message exchange over Fl reference point

    Once the necessary security functions have been performed, the CE
    and FE managers communicate to determine which CEs and FEs should
    communicate with each other.  At the very minimum, the CE and FE
    managers need to learn of the existence of available FEs and CEs
    respectively.  This discovery process may entail one or both
    managers learning the capabilities of the discovered ForCES
    protocol elements.  Figure 7 shows an example of possible message
    exchange between CE manager and FE manager over Fl reference point.

 4.1.2. Ff Reference Point

    The Ff reference point is used to inform forwarding elements of the
    association decisions made by the FE manager in pre-association
    phase.  Only authorized entities may instruct an FE with respect to
    which CE should control it.  Therefore, privacy, integrity,
    freshness, and authentication are necessary between the FE manager
    and FEs when the FE manager is remote to the FE.  Once the
    appropriate security has been established, the FE manager instructs
    the FEs across this reference point to join a new NE or to
    disconnect from an existing NE. The FE Manager could also assign
    unique FE identifiers to the FEs using this reference point.  The
    FE identifiers are useful in post association phase to express FE
    topology.  Figure 8 shows example of message exchange over Ff
    reference point.

         FE Manager      FE               CE Manager     CE
          |              |                |             |
          |              |                |             |
          |(security exchange)            |(security exchange)
         1|<------------>|authentication 1|<----------->|authentication
          |              |                |             |
          |(FE ID, attributes)            |(CE ID, attributes)
         2|<-------------|request        2|<----------->|request
          |              |                |             |
         3|------------->|response       3|------------>|response
          |(corresponding CE ID)          |(corresponding FE ID)
          |              |                |             |
          |              |                |             |
          |<-----Ff----->|                |<-----Fc---->|

                     Figure 8. Examples of message exchange
                        over Ff and Fc reference points.

    Note that the FE manager function may be co-located with the FE
    (such as by manual keypad entry of the CE IP address), in which
    case this reference point is reduced to a built-in function.

 4.1.3. Fc Reference Point

    The Fc reference point is used to inform control elements of the
    association decisions made by CE managers in pre-association phase.
    When the CE manager is remote, only authorized entities may
    instruct a CE to control certain FEs.  Privacy, integrity,
    freshness and authentication are also required across this
    reference point in such a configuration.  Once appropriate security
    has been established, the CE manager instructs CEs as to which FEs
    they should control and how they should control them.  Figure 8
    shows example of message exchange over Fc reference point.

    As with the FE manager and FEs, configurations are possible where
    the CE manager and CE are co-located and no protocol is used for
    this function.

 4.2. Post-association Phase and Fp reference point

    Post-association phase is the period of time during which an FE and
    CE have been configured with information necessary to contact each
    other and includes both association establishment and steady-state
    communication.  The communication between CE and FE is performed
    across the Fp ("p" meaning protocol) reference point. ForCES
    protocol is exclusively used for all communication across the Fp
    reference point.

 4.2.1. Proximity and Interconnect between CEs and FEs
    The ForCES Working Group has made a conscious decision that the
    first version of ForCES will be focused on "very close" CE/FE
    localities in IP networks.  Very Close localities consist of
    control and forwarding elements that either are components in the
    same physical box, or are separated at most by one local network
    hop ([7]).  CEs and FEs can be connected by a variety of
    interconnect technologies, including Ethernet connections,
    backplanes, ATM (cell) fabrics, etc.  ForCES should be able to
    support each of these interconnects (see [3] Section 5, requirement
    #1).  When the CEs and FEs are separated beyond a single L3 routing
    hop, the ForCES protocol will make use of an existing RFC2914
    compliant L4 protocol with adequate reliability, security and
    congestion control (e.g. TCP, SCTP) for transport purposes.

 4.2.2. Association Establishment

                 FE                      CE
                 |                       |
                 |(Security exchange.)   |
                 |                       |
                 |(Let me join the NE please.)
                 |                       |
                 |(What kind of FE are you? -- capability query)
                 |                       |
                 |(Here is my FE functions/state: use model to
                 |                       |
                 |(How are you connected with other FEs?)
                 |                       |
                 |(Here is the FE topology info)
                 |                       |
                 |(Initial config for FE -- optional)
                 |                       |
                 |(I am ready to go. Shall I?)
                 |                       |
                 |(Go ahead!)            |
                 |                       |

          Figure 9. Example of message exchange between CE and FE
                    over Fp to establish NE association

    As an example, figure 9 shows some of the message exchange that may
    happen before the association between the CE and FE is fully
    established.  Either the CE or FE can initiate the connection.
    Security handshake is necessary to authenticate the two
    communication endpoints to each other before any further message
    exchange can happen. The exact details of the security handshake
    depend on the security solution chosen by ForCES protocol.  Section
    8 provides more details on the security considerations for ForCES.
    After the successful security handshake, the FE needs to inform the
    CE of its own capability and its topology in relation to other FEs.
    The capability of the FE is represented by the FE model, described
    in a separate document.  The model would allow an FE to describe
    what kind of packet processing functions it contains, in what order
    the processing happens, what kinds of configurable parameters it
    allows, what statistics it collects and what events it might throw,
    etc.  Once such information is available to the CE, the CE may
    choose to send some initial or default configuration to the FE so
    that the FE can start receiving and processing packets correctly.
    Such initialization may not be necessary if the FE already obtains
    the information from its own bootstrap process.  Once FE starts
    accepting packets for processing, we say the association of this FE
    with its CE is now established.  From then on, the CE and FE enter
    steady-state communication.

 4.2.3. Steady-state Communication

    Once an association is established between the CE and FE, the
    ForCES protocol is used by the CE and FE over Fp reference point to
    exchange information to facilitate packet processing.

                 FE                      CE
                 |                       |
                 |(Add these new routes.)|
                 |                       |
                 |(Successful.)          |
                 |                       |
                 |                       |
                 |(Query some stats.)    |
                 |                       |
                 |(Reply with stats collected.)
                 |                       |
                 |                       |
                 |(My port is down, with port #.)
                 |                       |
                 |(Here is a new forwarding table)
                 |                       |
         Figure 10. Examples of message exchange between CE and FE
                 over Fp during steady-state communication

    Based on the information acquired through CEs' control processing,
    CEs will frequently need to manipulate the packet-forwarding
    behaviors of their FE(s) by sending instructions to FEs.  For
    example, Figure 10 shows message exchange examples in which the CE
    sends new routes to the FE so that the FE can add them to its
    forwarding table.  The CE may query the FE for statistics collected
    by the FE and the FE may notify the CE of important events such as
    port failure.

 4.2.4. Data Packets across Fp reference point

         ---------------------           ----------------------
         |                   |           |                    |
         |    +--------+     |           |     +--------+     |
         |    |CE(BGP) |     |           |     |CE(BGP) |     |
         |    +--------+     |           |     +--------+     |
         |        |          |           |          ^         |
         |        |Fp        |           |          |Fp       |
         |        v          |           |          |         |
         |    +--------+     |           |     +--------+     |
         |    |  FE    |     |           |     |   FE   |     |
         |    +--------+     |           |     +--------+     |
         |        |          |           |          ^         |
         | Router |          |           | Router   |         |
         | A      |          |           | B        |         |
         ---------+-----------           -----------+----------
                  v                                 ^
                  |                                 |
                  |                                 |

        Figure 11. Example to show data packet flow between two NEs.

    Control plane protocol packets (such as RIP, OSPF messages)
    addressed to any of NE's interfaces are typically redirected by the
    receiving FE to its CE, and CE may originate packets and have its
    FE deliver them to other NEs.  Therefore, ForCES protocol over Fp
    not only transports the ForCES protocol messages between CEs and
    FEs, but also encapsulates the data packets from control plane
    protocols.  Moreover, one FE may be controlled by multiple CEs for
    distributed control.  In this configuration, the control protocols
    supported by the FORCES NEs may spread across multiple CEs.  For
    example, one CE may support routing protocols like OSPF and BGP,
    while a signaling and admission control protocol like RSVP is
    supported in another CE.  FEs are configured to recognize and
    filter these protocol packets and forward them to the corresponding

    Figure 11 shows one example of how the BGP packets originated by
    router A are passed to router B.  In this example, the ForCES
    protocol is used to transport the packets from the CE to the FE
    inside router A, and then from the FE to the CE inside router B.
    In light of the fact that the ForCES protocol is responsible for
    transporting both the control messages and the data packets between
    the CE and FE over Fp reference point, it is possible to use either
    a single protocol or multiple protocols to achieve this.

 4.2.5. Proxy FE

    In the case where a physical FE cannot implement (e.g., due to the
    lack of a general purpose CPU) the ForCES protocol directly, a
    proxy FE can be used in the middle of Fp reference point.  This
    allows the CE communicate to the physical FE via the proxy by using
    ForCES, while the proxy manipulates the physical FE using some
    intermediary form of communication (e.g., a non-ForCES protocol or
    DMA).  In such an implementation, the combination of the proxy and
    the physical FE becomes one logical FE entity.

 4.3. Association Re-establishment

    FEs and CEs may join and leave NEs dynamically (see [3] Section 5,
    requirements #12).  When an FE or CE leaves the NE, the association
    with the NE is broken.  If the leaving party rejoins an NE later,
    to re-establish the association, it may need to re-enter the pre-
    association phase.  Loss of association can also happen
    unexpectedly due to loss of connection between the CE and the FE.
    Therefore, the framework allows the bi-directional transition
    between these two phases, but the ForCES protocol is only
    applicable for the post-association phase.  However, the protocol
    should provide mechanisms to support association re-establishment.

    This includes the ability for CEs and FEs to determine when there
    is a loss of association between them, ability to restore
    association and efficient state (re)synchronization mechanisms (see
    [3] Section 5, requirement #7). Note that security association and
    state must be also re-established to guarantee the same level of
    security exists before and after the association re-establishment.

    When an FE leaves or joins an existing NE that is already in post-
    association phase, the CE needs to be aware of the impact on FE
    topology and deals with the change accordingly.

 4.3.1. CE graceful restart

    The failure and restart of the CE in a router can potentially cause
    much stress and disruption on the control plane throughout a
    network.  Because when a CE has to restart for any reason, the
    router loses routing adjacencies or sessions with its routing
    neighbors.  Neighbors who detect the lost adjacency normally re-
    compute new routes and then send routing updates to their own
    neighbors to communicate the lost adjacency.  Their neighbors do
    the same thing to propagate throughout the network.  In the
    meantime, the restarting router cannot receive traffic from other
    routers because the neighbors have stopped using the router's
    previously advertised routes. When the restarting router restores
    adjacencies, neighbors must once again re-compute new routes and
    send out additional routing updates. The restarting router is
    unable to forward packets until it has re-established routing
    adjacencies with neighbors, received route updates through these
    adjacencies, and computed new routes.  Until convergence takes
    place throughout the network, packets may be lost in transient
    black holes or forwarding loops.

    A high availability mechanism known as the "graceful restart" has
    been used by the IP routing protocols (OSPF [10], BGP [11], BGP
    [11]) and MPLS label distribution protocol (LDP [9]) to help
    minimize the negative effects on routing throughout an entire
    network caused by a restarting router.  Route flap on neighboring
    routers is avoided, and a restarting router can continue to forward
    packets that would otherwise be dropped.

    While the details differ from protocol to protocol, the general
    idea behind the graceful restart mechanism remains the same.  With
    the graceful restart, a restarting router can inform its neighbors
    when it restarts.  The neighbors may detect the lost adjacency but
    do not recompute new routes or send routing updates to their
    neighbors.  The neighbors also hold on to the routes received from
    the restarting router before restart and assume they are still
    valid for a limited time.  By doing so, the restarting router's FEs
    can also continue to receive and forward traffic from other
    neighbors for a limited time by using the routes they already have.
    The restarting router then re-establishes routing adjacencies,
    downloads updated routes from all its neighbors, recomputes new
    routes and uses them to replace the older routes it was using. It
    then sends these updated routes to its neighbors and signals the
    completion of the graceful restart process.

    Non-stop forwarding is a requirement for graceful restart.  It is
    necessary so a router can continue to forward packets while it is
    downloading routing information and recomputing new routes.  This
    ensures that packets will not be dropped.  As one can see, one of
    the benefits afforded by the separation of CE and FE is exactly the
    ability of non-stop forwarding in the face of the CE failure and
    restart.  The support of dynamic changes to CE/FE association in
    ForCES also makes it compatible with high availability mechanisms
    such as graceful restart.

    ForCES should be able to support CE graceful restart easily.  When
    the association is established the first time, the CE must inform
    the FEs what to do in the case of CE failure.  If graceful restart
    is not supported, the FEs may be told to stop packet processing all
    together if its CE fails.  If graceful restart is supported, the
    FEs should be told to cache and hold on to its FE state including
    the forwarding tables across the restarts.  A timer must be
    included so that the timeout causes such cached state to expire
    eventually.  Those timers should be settable by the CE.

 4.3.2. FE restart

    In the same example in Figure 5, assuming CE1 is the working CE for
    the moment, what would happen if one of the FEs, say FE1, leaves
    the NE temporarily?  FE1 may voluntarily decide to leave the
    association.  Alternatively, FE1 may stop functioning simply due to
    unexpected failure.  In the former case, CE1 receives a "leave-
    association request" from FE1.  In the latter, CE1 detects the
    failure of FE1 by some other means.  In both cases, CE1 must inform
    the routing protocols of such an event, most likely prompting a
    reachability and SPF (Shortest Path First) recalculation and
    associated downloading of new FIBs from CE1 to the other remaining
    FEs (only FE2 in this example).  Such recalculation and FIB update
    will also be propagated from the CE1 to its the NE's neighbors that are
    affected by the connectivity of FE1.

    When FE1 decides to rejoin again, or when it restarts again from
    the failure, FE1 needs to re-discover its master (CE).  This can be
    achieved by several means.  It may re-enter the pre-association
    phase and get that information from its FE manager.  It may
    retrieve the previous CE information from its cache, if it can
    validate the information freshness.  Once it discovers its CE, it
    starts message exchange with the CE to re-establish the association
    just as outlined in Figure 9, with the possible exception that it
    might be able to bypass the transport of the complete initial
    configuration.  Suppose that FE1 still has its routing table and
    other state information from the last association, instead association. Instead of
    sending all the information again from scratch, it may be able to
    use more efficient mechanism to re-sync up the state with its CE if
    such mechanism is supported by the ForCES protocol.  For example,
    CRC-32 of the state might give a quick indication of whether or not
    the state is in-sync with its CE.  By comparing its state with the
    CE first, it sends an information update only if it is needed.
    ForCES protocol may choose to implement similar optimization
    mechanisms, but it may also choose not to, as this is not a

 5. Applicability to RFC1812

    [3] Section 5, requirement #9 dictates "Any proposed ForCES
    architecture must explain how that architecture supports all of the
    router functions as defined in RFC1812."  RFC1812 discusses many
    important requirements for IPv4 routers from the link layer to the
    application layer.  This section addresses the relevant
    requirements in RFC1812 for implementing IPv4 routers based on
    ForCES architecture and explains how ForCES satisfies these
    requirements by providing guidelines on how to separate the
    functionalities required into forwarding plane and control plane.

    In general, the forwarding plane carries out the bulk of the per-
    packet processing that is required at line speed, while the control
    plane carries most of the computationally complex operations that
    are typical of the control and signaling protocols.  However, it is
    impossible to draw a rigid line to divide the processing into CEs
    and FEs cleanly.  Nor should the ForCES architecture limit the
    innovative approaches in control and forwarding plane separation.
    As more and more processing power is available in the FEs, some of
    the control functions that traditionally are performed by CEs may
    now be moved to FEs for better performance and scalability.  Such
    offloaded functions may include part of ICMP or TCP processing, or
    part of routing protocols.  Once off-loaded onto the forwarding
    plane, such CE functions, even though logically belonging to the
    control plane, now become part of the FE functions.  Just like the
    other logical functions performed by FEs, such off-loaded functions
    must be expressed as part of the FE model so that the CEs can
    decide how to best take advantage of these off-loaded functions
    when present on the FEs.

 5.1. General Router Requirements

    Routers have at least two or more logical interfaces.  When CEs and
    FEs are separated by ForCES within a single NE, some additional
    interfaces are needed for intra-NE communications.  Figure 12 shows
    an example to illustrate that.  This NE contains one CE and two
    FEs.  Each FE has four interfaces; two of them are used for
    receiving and transmitting packets to the external world, while the
    other two are for intra-NE connections.  CE has two logical
    interfaces #9 and #10, connected to interfaces #3 and #6 from FE1
    and FE2, respectively.  Interface #4 and #5 are connected for FE1-
    FE2 communication.  Therefore, this router NE provides four
    external interfaces (#1, 2, 7 and 8).

                 |               router NE       |
                 |   -----------   -----------   |
                 |   |   FE1   |   |   FE2   |   |
                 |   -----------   -----------   |
                 |   1| 2| 3| 4|   5| 6| 7| 8|   |
                 |    |  |  |  |    |  |  |  |   |
                 |    |  |  |  +----+  |  |  |   |
                 |    |  |  |          |  |  |   |
                 |    |  | 9|        10|  |  |   |
                 |    |  | -------------- |  |   |
                 |    |  | |    CE      | |  |   |
                 |    |  | -------------- |  |   |
                 |    |  |                |  |   |
                      |  |                |  |
                      |  |                |  |

                 Figure 12. A router NE example with four interfaces.

    IPv4 routers must implement IP to support its packet forwarding
    function, which is driven by its FIB (Forwarding Information Base).
    This Internet layer forwarding (see RFC1812 [1] Section 5)
    functionality naturally belongs to FEs in the ForCES architecture.

    A router may implement transport layer protocols (like TCP and UDP)
    that are required to support application layer protocols (see
    RFC1812 [1] Section 6).  One important class of application
    protocols is routing protocols (see RFC1812 [1] Section 7).  In
    ForCES architecture, routing protocols are naturally implemented by
    CEs.  Routing protocols require routers communicate with each
    other.  This communication between CEs in different routers is
    supported in ForCES by FEs' ability to redirect data packets
    addressed to routers (i.e., NEs) and CEs' ability to originate
    packets and have them delivered by their FEs.  This communication
    occurs across Fp reference point inside each router and between
    neighboring routers' external interfaces, as illustrated in Figure

 5.2.Link Layer

    Since FEs own all the external interfaces for the router, FEs need
    to conform to the link layer requirements in RFC1812.  Arguably,
    ARP support may be implemented in either CEs or FEs.  As we will
    see later, a number of behaviors that RFC1812 mandates fall into
    this category -- they may be performed by the FE and may be
    performed by the CE.  A general guideline is needed to ensure
    interoperability between separated control and forwarding planes.
    The guideline we offer here is that CEs MUST be capable of these
    kind of operations while FEs MAY choose to implement them.  FE
    model should indicate its capabilities in this regard so that CEs
    can decide where these functions are implemented.

    Interface parameters, including MTU, IP address, etc., must be
    configurable by CEs via ForCES.  CEs must be able to determine
    whether a physical interface in an FE is available to send packets
    or not.  FEs must also inform CEs the status change of the
    interfaces (like link up/down) via ForCES.

 5.3.Internet Layer Protocols

    Both FEs and CEs must implement IP protocol and all mandatory
    extensions as RFC1812 specified.  CEs should implement IP options
    like source route and record route while FEs may choose to
    implement those as well.  The timestamp option should be
    implemented by FEs to insert the timestamp most accurately.  The FE
    must interpret the IP options that it understands and preserve the
    rest unchanged for use by CEs.  Both FEs and CEs might choose to
    silently discard packets without sending ICMP errors, but such
    events should be logged and counted.  FEs may report statistics for
    such events to CEs via ForCES.

    When multiple FEs are involved to process packets, the appearance
    of single NE must be strictly maintained.  For example, Time-To-
    Live (TTL) must be decremented only once within a single NE.  For
    example, it can be always decremented by the last FE with egress

    FEs must receive and process normally any packets with a broadcast
    destination address or a multicast destination address that the
    router has asked to receive.  When IP multicast is supported in
    routers, IGMP is implemented in CEs.  CEs are also required of ICMP
    support, while it is optional for FEs to support ICMP.  Such an
    option can be communicated to CEs as part of the FE model.
    Therefore, FEs can always rely upon CEs to send out ICMP error
    messages, but FEs also have the option to generate ICMP error
    messages themselves.

 5.4.Internet Layer Forwarding

    IP forwarding is implemented by FEs.  When the routing table is
    updated at CEs, ForCES is used to send the new route entries from
    CEs to FEs.  Each FE has its own forwarding table and uses this
    table to direct packets to the next hop interface.

    Upon receiving IP packets, the FE verifies the IP header and
    processes most of the IP options.  Some options cannot be processed
    until the routing decision has been made.  The routing decision is
    made after examining the destination IP address.  If the
    destination address belongs to the router itself, the packets are
    filtered and either processed locally or forwarded to CE, depending
    upon the instructions set-up by CE.  Otherwise, the FE determines
    the next hop IP address by looking up in its forwarding table.  The
    FE also determines the network interface it uses to send the
    packets.  Sometimes an FE may need to forward the packets to
    another FE before packets can be forwarded out to the next hop.
    Right before packets are forwarded out to the next hop, the FE
    decrements TTL by 1 and processes any IP options that cannot be
    processed before.  The FE performs any IP fragmentation if
    necessary, determines link layer address (e.g., by ARP), and
    encapsulates the IP datagram (or each of the fragments thereof) in
    an appropriate link layer frame and queues it for output on the
    interface selected.

    Other options mentioned in RFC1812 for IP forwarding may also be
    implemented at FEs, for example, packet filtering.

    FEs typically forward packets destined locally to CEs.  FEs may
    also forward exceptional packets (packets that FEs do not know how
    to handle) to CEs.  CEs are required to handle packets forwarded by
    FEs for whatever different reasons.  It might be necessary for
    ForCES to attach some meta-data with the packets to indicate the
    reasons of forwarding from FEs to CEs.  Upon receiving packets with
    meta-data from FEs, CEs can decide to either process the packets
    themselves, or pass the packets to the upper layer protocols
    including routing and management protocols.  If CEs are to process
    the packets by themselves, CEs may choose to discard the packets,
    or modify and re-send the packets.  CEs may also originate new
    packets and deliver them to FEs for further forwarding.

    Any state change during router operation must also be handled
    correctly according to RFC1812.  For example, when an FE ceases
    forwarding, the entire NE may continue forwarding packets, but it
    needs to stop advertising routes that are affected by the failed

 5.5. Transport Layer

    Transport layer is typically implemented at CEs to support higher
    layer application protocols like routing protocols.  In practice,
    this means that most CEs implement both the Transmission Control
    Protocol (TCP) and the User Datagram Protocol (UDP).

    Both CEs and FEs need to implement ForCES protocol.  If some layer-
    4 transport is used to support ForCES, then both CEs and FEs need
    to implement the L4 transport and ForCES protocols.

 5.6. Application Layer -- Routing Protocols

    Interior and exterior routing protocols are implemented on CEs.
    The routing packets originated by CEs are forwarded to FEs for
    delivery.  The results of such protocols (like forwarding table
    updates) are communicated to FEs via ForCES.

    For performance or scalability reasons, portions of the control
    plane functions that need faster response may be moved from the CEs
    and off-loaded onto the FEs.  For example in OSPF, the Hello
    protocol packets are generated and processed periodically.  When
    done at CEs, the inbound Hello packets have to traverse from the
    external interfaces at the FEs to the CEs via the internal CE-FE
    channel.  Similarly, the outbound Hello packets have to go from the
    CEs to the FEs and to the external interfaces.  Frequent Hello
    updates place heavy processing overhead on the CEs and can
    overwhelm the CE-FE channel as well.  Since typically there are far
    more FEs than CEs in a router, the off-loaded Hello packets are
    processed in a much more distributed and scalable fashion.  By
    expressing such off-loaded functions in the FE model, we can ensure
    interoperability. However, the exact description of the off-loaded
    functionality corresponding to the off-loaded functions expressed
    in the FE model are not part of the model itself and will need to
    be worked out as a separate specification.

 5.7. Application Layer -- Network Management Protocol

    RFC1812 also dictates "Routers MUST be manageable by SNMP."  (see
    [4] Section 8)  In
    general, for post-association phase, most external management tasks
    (including SNMP) should be done through interaction with the CE in
    order to support the appearance of a single functional device.
    Therefore, it is recommended that SNMP
    management agent be implemented by CEs
    and the SNMP messages received by FEs be redirected to their CEs.
    AgentX framework defined in RFC2741 ([5]) may be applied here such
    that CEs act in the role of master agent to process SNMP protocol
    messages while FEs act in the role of subagent to provide access to
    the MIB objects residing on FEs.  AgentX protocol messages between
    the master agent (CE) and the subagent (FE) are encapsulated and
    transported via ForCES, just like data packets from any other
    application layer protocols.

 6. Summary

    This document defines an architectural framework for ForCES.  It
    identifies the relevant components for a ForCES network element,
    including (one or more) FEs, (one or more) CEs, one optional FE
    manager, and one optional CE manager.  It also identifies the
    interaction among these components and discusses all the major
    reference points.  It is important to point out that, among all the
    reference points, only the Fp interface between CEs and FEs is
    within the scope of ForCES.  ForCES alone may not be enough to
    support all desirable NE configurations.  However, we believe that
    ForCES over Fp interface is the most important element in realizing
    physical separation and interoperability of CEs and FEs, and hence
    the first interface that ought to be standardized.  Simple and
    useful configurations can still be implemented with only CE-FE
    interface being standardized, e.g., single CE with full-meshed FEs.

 7. Acknowledgements

    Joel M. Halpern gave us many insightful comments and suggestions
    and pointed out several major issues.  T. Sridhar suggested that
    the AgentX protocol could be used with SNMP to manage the ForCES
    network elements.  Many of our colleagues and people in the ForCES
    mailing list also provided valuable feedback.

 8. Security Considerations
    In general, the physical separation of two entities usually results
    in a potentially insecure link between the two entities and hence
    much stricter security measurements are required.  For example, we
    pointed out in Section 4.1 that authentication becomes necessary
    between CE manager and FE manager, between CE and CE manager,
    between FE and FE manager in some configurations.  The physical
    separation of CE and FE also imposes serious security requirement
    for ForCES protocol over Fp interface.  This section first attempts
    to describe the security threats that may be introduced by the
    physical separation of the FEs and the CEs, and then it provides
    recommendation and guidelines for secure operation and management
    of ForCES protocol over Fp interface based on existing standard
    security solutions.

 8.1. Analysis of Potential Threats Introduced by ForCES

    This section provides the threat analysis for ForCES, with a focus
    on Fp interface.  Each threat is described in details with the
    effects on the ForCES protocol entities or/and the NE as a whole,
    and the required functionalities that need to be in place to defend
    the threat.

 8.1.1. "Join" or "Remove" Message Flooding on CEs

    Threats:  A malicious node could send a stream of false "join NE"
    or "remove from NE" requests on behalf of non-existent or
    unauthorized FE to legitimate CEs at a very rapid rate and thereby
    create unnecessary state in the CEs.

    Effects: If by maintaining state for non-existent or unauthorized
    FEs, a CE may become unavailable for other processing and hence
    suffer from denial of service (DoS) attack similar to the TCP SYN
    DoS.  If multiple CEs are used, the unnecessary state information
    may also be conveyed to multiple CEs via Fr interface (e.g., from
    the active CE to the stand-by CE) and hence subject multiple CEs to
    DoS attack.

    Requirement:  A CE that receives a "join" or "remove" request
    should not create any state information until it has authenticated
    the FE endpoint.

 8.1.2. Impersonation Attack

    Threats: A malicious node can impersonate a CE or FE and send out
    false messages.

    Effects: The whole NE could be compromised.

    Requirement:  The CE or FE must authenticate the message before
    accepting and processing it.

 8.1.3. Replay Attack

    Threat: A malicious node could replay the entire message previously
    sent by an FE or CE entity to get around authentication.

    Effect: The NE could be compromised.

    Requirement:  Replay protection mechanism needs to be part of the
    security solution to defend against this attack.

 8.1.4. Attack during Fail Over

    Threat: A malicious node may exploit the CE fail-over mechanism to
    take over the control of NE. For example, suppose two CEs, say CE-A
    and CE-B, are controlling several FEs. CE-A is active and CE-B is
    stand-by.  When CE-A fails, CE-B is taking over the active CE
    position.  The FEs already had a trusted relationship with CE-A,
    but the FEs may not have the same trusted relationship established
    with CE-B prior to the fail-over.  A malicious node can take over
    as CE-B if such trusted relationship is not established during the

    Effect: The NE may be compromised after such insecure fail-over.

    Requirement:  The level of trust relationship between the stand-by
    CE and the FEs must be as strong as the one between the active CE
    and the FEs.  The security association between the FEs and the
    stand-by CE may be established prior to fail-over.  If not already
    in place, such security association must be re-established before
    the stand-by CE takes over.

 8.1.5.  Data Integrity

    Threats: A malicious node may inject false messages to legitimate
    CE or FE.

    Effect: An FE or CE receives the fabricated packet and performs
    incorrect or catastrophic operation.

    Requirement: Protocol messages require integrity protection.

 8.1.6.  Data Confidentiality

    Threat: When FE and CE are physically separated, a malicious node
    may eavesdrop the messages in transit.  Some of the messages are
    critical to the functioning of the whole network, while others may
    contain confidential business data.  Leaking of such information
    may result in compromise even beyond the immediate CE or FE.

    Effect: Sensitive information might be exposed between CE and FE.

    Requirement: Data confidentiality between FE and CE must be
    available for sensitive information.

 8.1.7. Sharing security parameters

    Threat: Consider a scenario where several FEs communicating to the
    same CE share the same authentication keys for the Fp interface.
    If any FE or the CE is compromised, all other entities are

    Effect: The whole NE is compromised.

    Recommendation: To avoid this side effect, it's better to configure
    different security parameters for each FE-CE communication over Fp

 8.1.8. Denial of Service Attack via External Interface

    Threat: When an FE receives a packet that is destined for its CE,
    the FE forwards the packet over the Fp interface.  Malicious node
    can generate huge message storm like routing protocol packets etc.
    through the external Fi/f interface so that the FE has to process
    and forward all packets to CE through Fp interface.

    Effect: CE encounters resource exhaustion and bandwidth starvation
    on Fp interface due to an overwhelming number of packets from FEs.

    Requirement: Some sort of rate limiting mechanism MUST to be in
    place at both FE and CE.  Examples of such mechanisms include
    explicit rate limiters or congestion control algorithms.  Rate
    Limiter SHOULD be configured at FE for each message type that are
    being received through Fi/F interface.

 8.2. Security Recommendations for ForCES

    The requirements document [3] suggested that ForCES protocol should
    support reliability over Fp interface, but no particular transport
    protocol is yet specified for ForCES.  This framework document does
    not intend to specify the particular transport either, and so we
    only provide recommendations and guidelines based on the existing
    standard security protocols that can work with the common transport
    candidates suitable for ForCES.

    We review two existing security protocol solutions, namely IPsec
    (IP Security) [14] or TLS (Transport Layer Security) [13].  TLS
    works with reliable transports such as TCP or SCTP for unicast,
    while IPsec can be used with any transport (UDP, TCP, SCTP) and
    supports both unicast and multicast.  Both TLS and IPsec can be
    used potentially to satisfy all of the security requirements for
    ForCES protocol.  Other approaches may be used as well but are not
    documented here. here, including using L2 security mechanisms for a given
    L2 interconnect technology, as long as the requirements can be

    When ForCES is deployed between CEs and FEs inside a box,
    authentication, confidentiality and integrity may be provided by
    the physical security of the box and so the security mechanisms may
    be turned off, depending on the networking topology and its
    administration policy.  However, it is important to realize that
    even if the NE is in a single-box, the DoS attacks as described in
    Section 8.1.8 can still be launched through Fi/f interfaces.
    Therefore, it is important to have the corresponding counter-
    measurement in place even for single-box deployment.

 8.2.1. Security Configuration

    The NE administrator has the freedom to determine the exact
    security configuration that is needed for the specific deployment.
    For example, ForCES may be deployed between CEs and FEs connected
    to each other inside a box over a backplane.  In such scenario,
    physical security of the box ensures that most of the attacks such
    as man-in-the-middle, snooping, and impersonation are not possible,
    and hence ForCES architecture may rely on the physical security of
    the box to defend against these attacks and protocol mechanisms may
    be turned off.  However, it is also shown that denial of service
    attack via external interface as described in Section 8.1.8 is
    still a potential threat even for such "all-in-one-box" deployment
    scenario and hence the rate limiting mechanism is still necessary.
    This is just one example to show that it is important to assess the
    security needs of the ForCES-enabled network elements under
    different deployment scenarios.  It should be possible for the
    administrator to configure the level of security needed for the
    ForCES protocol.

 8.2.2. Using TLS with ForCES

    TLS [13] can be used if a reliable unicast transport such as TCP or
    SCTP is used for ForCES over the Fp interface.  The TLS handshake
    protocol is used during association establishment or re-
    establishment phase to negotiate a TLS session between the CE and
    FE.  Once the session is in place, the TLS record protocol is used
    to secure ForCES communication messages between the CE and FE.

    A basic outline of how TLS can be used with ForCES is described
    below.  Steps 1) till 7) complete the security handshake as
    illustrated in Figure 9 while step 8) is for all the further
    communication between the CE and FE, including the rest of messages
    after the security handshake shown in Figure 9 and the steady-state
    communication shown in Figure 10.

         1) During Pre-association phase all FEs are configured with
         the CEs (including both the active CE and the standby CE).
         2) The FE establishes a TLS connection with the CE (master)
         and negotiates a cipher suite.
         3) The FE (slave) gets the CE certificate, validates the
         signature, checks the expiration date, checks if the
         certificate has been revoked.
         4) The CE (master) gets the FE certificate and performs the
         same validation as the FE in step 3).
         5) If any of the check fails in step 3) or step 4), endpoint
         must generate an error message and abort.
         6) After successful mutual authentication, a TLS session is
         established between CE and FE.
         7) The FE sends a "join NE" message to the CE.
         8) The FE and CE use TLS session for further communication.

    Note that there are different ways for the CE and FE to validate a
    received certificate.  One way is to configure the FE Manager or CE
    Manager or other central component as CA, so that the CE or FE can
    query this pre-configured CA to validate that the certificate has
    not been revoked.  Another way is to have the CE and the FE
    configured directly a list of valid certificates in the pre-
    association phase.

    In the case of fail-over, it is the responsibility of the active CE
    and the standby CE to synchronize ForCES states including the TLS
    states to minimize the state reestablishment during fail-over.
    Care must be taken to ensure that the standby CE is also
    authenticated in the same way as the active CE, either before or
    during the fail-over.

 8.2.3. Using IPsec with ForCES

    IPsec [14] can be used with any transport protocol, such as UDP,
    SCTP and TCP over Fp interface for ForCES.  When using IPsec, we
    recommend using ESP in transport mode for ForCES because message
    confidentiality is required for ForCES.

    IPsec can be used with both manual and automated SA and
    cryptographic key management.  But Ipsec's replay protection
    mechanisms are not available if manual key management is used.
    Hence, automatic key management is recommended if replay protection
    is deemed important.  Otherwise, manual key management might be
    sufficient for some deployment scenarios, esp. when the number of
    CEs and FEs is relatively small.  It is recommended that the keys
    be changed periodically even for manual key management.

    IPsec can support both unicast and multicast transport. At the time
    this document was published, MSEC working group is actively working
    on standardizing protocols to provide multicast security [17].
    Multicast-based solutions relying on IPsec should specify how to
    meet the security requirements in [3].

    Unlike TLS, IPsec provides security services between the CE and FE
    at IP level, and so the security handshake as illustrated in Figure
    9 amounts to a "no-op" when manual key management is used.  The
    following outline the steps taken for ForCES in such a case.

         1) During Pre-association phase all FEs are configured with
         the CEs (including active CE and standby CE) and SA parameters
         2) The FE sends a "join NE" message to the CE.  This message
         and all others that follow are afforded security service
         according to the manually configured IPsec SA parameters, but
         replay protection is not available.

    It is up to the administrator to decide whether to share the same
    key across multiple FE-CE communication, but it is recommended that
    different keys be used.  Similarly, it is recommended that
    different keys be used for inbound and outbound traffic.

    If automatic key management is needed, IKE [15] can be used for
    that purpose. Other automatic key distribution techniques such as
    Kerberos may be used as well.   The key exchange process
    constitutes the security handshake as illustrated in Figure 9.  The
    following shows the steps involved in using IKE with IPsec for
    ForCES.  Steps 1) to 6) constitute the security handshake in Figure

         1) During Pre-association phase all FEs are configured with
         the CEs (including active CE and standby CE), IPsec policy
         2) The FE kicks off IKE process and tries to establish an
         IPsec SA with the CE (master).  The FE (Slave) gets the CE
         certificate as part of the IKE negotiation.  The FE validates
         signature, checks the expiration date, checks if the
         certificate has been revoked.
         3) The CE (master) gets the FE certificate and performs the
         same check as the FE in step 2).
         4) If any of the check fails in step 2) or step 3), the
         endpoint must generate an error message and abort.
         5) After successful mutual authentication, IPsec session is
         established between the CE and FE.
         6) The FE sends a "join NE" message to CE.  No SADB entry is
         created in FE yet.
         7) The FE and CE use the IPsec session for further

    FE Manager or CE Manager or other central component can be used as
    CA for validating CE and FE certificates during the IKE process.
    Alternatively, during the pre-association phase, the CE and FE can
    be configured directly with the required information such as
    certificates or passwords etc depending upon the type of
    authentication that administrator wants to configure.

    In the case of fail-over, it is the responsibility of active CE and
    standby CE to synchronize ForCES states and IPsec states to
    minimize the state reestablishment during fail-over.
    Alternatively, the FE needs to establish different IPsec SA during
    the startup operation itself with each CE.  This will minimize the
    periodic state transfer across IPsec layer though Fr (CE-CE)

 9. Normative References

    [1] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812,
    June 1995.

    [2] Floyd, S., "Congestion Control Principles", RFC 2914, September

    [3] Khosravi, H. et al., "Requirements for Separation of IP Control
    and Forwarding", work in progress, May 2003, <draft-ietf-forces-

 10. Informative References

    [4] Case, J., et al., "A Simple Network "Introduction and Applicability Statements
    for Internet Standard Management Protocol
    (SNMP)", Framework", RFC 1157, May 1990. 3410, December

    [5] Daniele, M. et al., "Agent Extensibility (AgentX) Protocol
    Version 1", RFC 2741, January 2000.

    [6] Chan, K. et al., "COPS Usage for Policy Provisioning (COPS-
    PR)", RFC 3084, March 2001.

    [7] Crouch, A. et al., "ForCES Applicability Statement", work in
    progress, June 2002, <draft-ietf-forces-applicability-00.txt>.

    [8] Anderson, T. and J. Buerkle, "Requirements for the Dynamic
    Partitioning of Switching Elements", RFC 3532, May 2003.

    [9] Leelanivas, M. et al., "Graceful Restart Mechanism for Label
    Distribution Protocol", RFC 3478, February 2003.

    [10] Moy, J. et al., "Graceful OSPF Restart", work in progress,
    March 2003, <draft-ietf-ospf-hitless-restart-07.txt>.

    [11] Sangli, S. et al., "Graceful Restart Mechanism for BGP", work
    in progress, January 2003, < draft-ietf-idr-restart-06.txt>.

    [12] Shand, M. and L. Ginsberg, "Restart Signaling for IS-IS", work
    in progress, March 2003, <draft-ietf-isis-restart-03.txt>.

    [13] Dierks, T. and C. Allen, "The TLS Protocol, version 1.0", RFC
    2246, January 1999.

    [14] Kent, S. and R. Atkinson, "Security Architecture for the
    Internet Protocol", RFC 2401, November 1998.

    [15] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE) ",
    RFC 2409, November 1998.

    [16] Bellovin, S., "Guidelines for Mandating the Use of Ipsec",
    work in progress, October 2002, <draft-bellovin-useipsec-00.txt>.

    [17] Hardjono, T. and Weis, B. "The Multicast Security
    Architecture", work in progress, August 2003, <draft-ietf-msec-

 11. Authors' Addresses
    L. Lily Yang
    Intel Corp., MS JF3-206,
    2111 NE 25th Avenue
    Hillsboro, OR 97124, USA
    Phone: +1 503 264 8813

    Ram Dantu
    Department of Computer Science,
    University of North Texas,
    Denton, TX 76203, USA
    Phone: +1 940 565 2822

    Todd A. Anderson
    Intel Corp.
    2111 NE 25th Avenue
    Hillsboro, OR 97124, USA
    Phone: +1 503 712 1760

    Ram Gopal
    Nokia Research Center
    5, Wayside Road,
    Burlington, MA 01803, USA
    Phone: +1 781 993 3685

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