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Versions: 00 01                                                         
Internet Engineering Task Force                                ForCES WG
INTERNET-DRAFT                                    Alan Crouch/Intel Labs
draft-crouch-forces-applicability-01.txt                Mark Handley/ACIRI
                                                        28 February 2002
                                                    Expires: August 2002

                     ForCES Applicability Statement

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), 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."

The list of current Internet-Drafts can be accessed at


The list of Internet-Draft Shadow Directories can be accessed at

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     The ForCES protocol defines a standard framework and mechanism
     for the interconnection between Control Elements and
     Forwarding Engines in IP routers and similar devices.  In this
     document we describe the applicability of the ForCES model and
     protocol.  We provide example deployment scenarios and
     functionality, as well as document applications that would be
     inappropriate for ForCES.

1.  Overview

The ForCES protocol defines a standard framework and mechanism for the
exchange of information between the logically separate functionality of
the control and data forwarding planes of IP routers and similar
devices.  It focuses on the communication necessary for separation of
control plane functionality such as routing protocols, signaling
protocols, and admission control from data forwarding plane per-packet
activities such as packet forwarding, queuing, and header editing.

This document defines the applicability of the ForCES mechanisms. It
describes types of configurations and settings where ForCES is most
appropriately applied.  This document also describes scenarios and
configurations where ForCES would not be appropriate for use.

2.  Terminology

CE:  Control Element.  The processor or processors providing the control
     plane functionality in an IP router and similar devices.  The CE
     will normally run routing protocols, signaling protocols,
     admission control mechanisms, and similar functionality.

FE:  Forwarding Engine.  A box, card, or processor that forwards IP
     packets.  An FE will comprise one or more network interfaces, and
     typically provide route lookup, packet filtering, classification,
     queuing and other functionality associated with the forwarding or
     discard of packets.

     refers to the specific protocol and associated conventions used for
     communication between the CE and a set of FEs.

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3.  Applicability to IP Networks

The purpose of this section is to list the areas of ForCES applicability
in IP network devices.  Relatively low performance devices may be
implemented on a simple processor which performs both control and packet
forwarding functionality.  ForCES is not applicable for such devices.
Higher performance devices typically distribute work amongst interface
processors, and these devices (FEs) therefore need to communicate with
the control element(s) to perform their job.  ForCES provides a standard
way to do this communication.

The remainder of this section lists the applicable services which ForCES
may support, applicable FE functionality, applicable CE-FE link
scenarios, and applicable topologies in which ForCES may be deployed.

3.1.  Applicable Services

In this section we describe the applicability of ForCES for the
following control-forwarding plane services:

o    Discovery, Capability Information Exchange

o    Topology Information Exchange

o    Configuration

o    Routing Exchange

o    QoS Exchange

o    Security Exchange

o    Filtering Exchange

o    Encapsulation/Tunneling Exchange

o    NAT and Application-level Gateways

o    Measurement and Accounting

o    Diagnostics

o    Redundancy

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3.1.1.  Discovery, Capability Information Exchange

Discovery is the process by which CEs and FEs learn of each other's
existence.  ForCES assumes that CEs and FEs already know sufficient
information to begin communication in a secure manner.
 The ForCES protocol is only applicable after CEs and FEs have found
each other.  ForCES makes no assumption about whether discovery was
performed using a dynamic protocol or merely static configuration.

During the discovery phase, CEs and FEs may exchange capability
information with each other.  For example, the FEs may express the
number of interface ports they provide, as well as the static and
configurable attributes of each port.

In addition to initial configuration, the CEs and FEs may also exchange
dynamic configuration changes using ForCES.  For example, FE's
asynchronously inform the CE of an increase/decrease in available
resources or capabilities on the FE.

3.1.2.  Topology Information Exchange

In this context, topology information relates to how the FEs are
interconnected with each other with respect to packet forwarding.
Whilst topology discovery is outside the scope of the ForCES protocol, a
standard topology discovery protocol may be selected and used to "learn"
the topology, and then the ForCES protocol may be used to transmit the
resulting information to the CE.

3.1.3.  Configuration

ForCES is used to perform FE configuration.  For example, CEs set
configurable FE attributes such as IP addresses.

3.1.4.  Routing Exchange

ForCES may be used to deliver packet forwarding information resulting
from CE routing calculations.  For example, CEs may send forwarding
table updates to the FEs, so that they can make forwarding decisions.
FEs may inform the CE in the event of a forwarding table miss.

3.1.5.  QoS Exchange

ForCES may be used to exchange QoS capabilities between CEs and FEs.
For example, an FE may express QoS capabilities to the CE.  Such

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capabilities might include metering, policing, shaping, and queuing
functions.  The CE may use ForCES to configure these capabilities.

3.1.6.  Security Exchange

ForCES may be used to exchange Security information between CEs and FEs.
For example, the FE may use ForCES to express the types of encryption
that it is capable of using in an IPsec tunnel.  The CE may use ForCES
to configure such a tunnel.

3.1.7.  Filtering Exchange and Firewalls

ForCES may be used to exchange filtering information.  For example, FEs
may use ForCES to express the filtering functions such as classification
and action that they can perform, and the CE may configure these

3.1.8.  Encapsulation, Tunneling Exchange

ForCES may be used to exchange encapsulation capabilities of an FE, such
as tunneling, and the configuration of such capabilities.

3.1.9.  NAT and Application-level Gateways

ForCES may be used to exchange configuration information for Network
Address Translators.  Whilst ForCES is not specifically designed for the
configuration of application-level gateway functionality, this may be in
scope for some types of application-level gateways.

3.1.10.  Measurement and Accounting

ForCES may be used to exchange configuration information regarding
traffic measurement and accounting functionality.  In this area, ForCES
may overlap somewhat with functionality provided by alternative network
management mechanisms such as SNMP.  In some cases ForCES may be used to
convey information to the CE to be reported externally using SNMP.
However, in other cases it may make more sense for the FE to directly
speak SNMP.

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3.1.11.  Diagnostics

ForCES may be used for CE's and FE's to exchange diagnostics
information.  For example, an FE can send diagnostic information like
self-test results to the CE.

3.1.12.  Redundancy

ForCES is a master-slave protocol where FE's are slaves and CE's are
masters.  FE's process messages in the order in which they are received
from their CE.  Concepts such as CE Redundancy, CE Failover, and CE-CE
communication, while not precluded by the ForCES architecture, are
considered outside the scope of ForCES protocol.

3.2.  CE-FE Link Capacity

When using ForCES, the bandwidth of the CE-FE link is a consideration,
and cannot be ignored.  For example, sending a full routing table of
110K routes is reasonable over a 100Mbit Ethernet interconnect, but is
non-trivial over a T1 line (which could occur in a Close Locality (see
3.3.2). ForCES should be sufficiently future-proof to be applicable in
scenarios where routing tables grow to several orders of magnitude
greater than their current size (approximately 100K routes).  However,
we also note that not all IP routers need full routing tables.

3.3.  CE/FE Locality

We do not intend ForCES to be applicable in configurations where the CE
and FE are located arbitrarily in the network.  In particular, ForCES is
intended for environments where one of the following applies:

o    The control interconnect is some form of local bus, switch, or LAN,
     where reliability is high, closely controlled, and not susceptible
     to external disruption that does not also affect the CEs and/or

o    The control interconnect shares fate with the FE's forwarding
     function.  Typically this is because the control connection is also
     the FE's primary packet forwarding connection, and so if that link
     goes down, the FE cannot forward packets anyway.

The key guideline is that the reliability of the device should not be
significantly reduced by the separation of control and forwarding

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Taking this into account, ForCES is applicable in the following CE/FE
localities in IP networks:

o    Very Close Localities.

o    Close Localities

3.3.1.  Very Close Localities

Very Close localities consist of control and forwarding elements which
are either components in the same physical box, or are separated at most
by one local network hop.  An example of a Very Close locality is a
network element with a single control blade, and one or more forwarding
blades, all present in the same chassis and sharing an interconnect such
as Ethernet or PCI.  In Very Close localities, the data traffic being
forwarded typically does not traverse the same links as the ForCES
control traffic.

3.3.2.  Close Localities

Close localities consist of control and forwarding separation for IP
forwarding devices where the control and forwarding elements are in
close proximity.  The definition of "close proximity" is deliberately
ambiguous, but might include devices located in the same room, or
devices separated by only a very small number of IP hops.  Note that to
satisfy the reliability requirements, if these is more than one IP hop
between a CE and an FE, these hops will not normally be dynamically
routed, as in the general case this would not satisfy the constraints

A specific example of a Close locality is an FE that is located remotely
as Customer Premise Equipment (CPE), and a CE located in their Internet
Service Provider's facilities .  This is an extreme example of the
applicability of ForCES.  Note that natural fate- sharing exists between
the CE and FE.  A potentially unreliable link connects the CE and the
FE, but if that link were lost, the FE would stop forwarding to and from
the ISP, irrespective of the location of the CE.  However, if the FE
were also required to forward traffic between subnets at the customer
premises, this would not satisfy the fate-sharing constraint, as local
forwarding would also cease when the link to the ISP fails.

Note that not all ForCES functionality may be possible in Close
localities.  In particular, if the scenario and traffic conditions call
for a large amount of ForCES traffic, the network between the CE and FE
may not have sufficient capacity to handle the control traffic.
Designers considering using ForCES in Close Localities need to take this

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into account, and ensure that such eventualities do not arise.  Also, as
the control traffic may share network links with data traffic, ForCES
traffic will need to be given priority access to that capacity.
Typically this priority needs to be even higher priority than that of
the CE's routing protocol traffic.

4.  Limitations and Out-of-Scope Items

ForCES was designed to enable logical separation of control and
forwarding planes in IP network devices.  However, ForCES is not
intended to be applicable to all services or to all possible CE/FE

The purpose of this section is to list limitations and out-of-scope
items for ForCES.

4.1.  Out of Scope Services

The following control-forwarding plane services are explicitly not
addressed by ForCES:

o    Label Switching

o    Multimedia Gateway Control (MEGACO).

4.1.1.  Label Switching

Label Switching is the purview of the GSMP Working Group in the Sub- IP
Area of the IETF.  GSMP is a general purpose protocol to control a label
switch.  GSMP defines mechanisms to separate the label switch data plane
from the control plane label protocols such as LDP [5]. For more
information on GSMP, see [4].

4.1.2.  Separation of Control and Forwarding in Multimedia Gateways"

MEGACO defines a protocol used between elements of a physically
decomposed multimedia gateway.  Separation of call control channels from
bearer channels is the purview of MEGACO.  For more information on
MEGACO, see [7].

4.2.  Localities

Examples of network localities that are not appropriate for ForCES are:

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o    Localities where there are a large number of hops between CE and
     FE.  Typically three hops might be considered an upper bound.

o    Localities where the hops between the CE and FE are dynamically
     routing using IP routing protocols.

o    Localities where the loss of the CE-FE link is of non-negligible
     probability, and where if the CE were co-located with the FE,
     useful packet forwarding would have been able to continue despite
     the loss of the link.

o    Localities where two or more FEs controlled by the same CE cannot
     communicate, either directly, or indirectly via other FEs
     controlled by the same CE.

5.  Security Considerations

The security of ForCES protocol will be addressed in the Protocol
Specification [2]. For security requirements, see architecture
requirement #5 and protocol requirement #2 in the Requirements Draft
[1].  The ForCES protocol assumes that the CE and FE are in the same
administration, and have shared secrets as a means of administration.
Whilst it might be technically feasible to have the CE and FE
administered independently, we strongly discourage such uses, because
they would require a significantly different trust model from that
ForCES assumes.

6.  Normative References

[1] Anderson, T et. al., "Requirements for Separation of IP Control and
Forwarding", draft-ietf-forces-requirements-01.txt, Intel Labs,
September 2001.

[2] ForCES Protocol Specification (to-be-written)

7.  Informative References

[3] Salim, J et. al., "Netlink as an IP Services Protocol", draft-salim-
netlink-jhsk-01.txt, Znyx Networks, September 2001.

[4] Doria, A, Sundell, K, Hellstrand, F, Worster, T, "General switch
Management Protocol V3," Internet Draft draft-ietf-gsmp-06.txt, July
2000. work in progress

[5] Andersson et. al., "LDP Specification" RFC 3036, January 2001

[6] Bradner, S, "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, Harvard University, March 1997.

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[7] F. Cuervo et al., "Megaco Protocol Version 1.0" RFC 3015, November

8.  Acknowledgments

The authors wish to thank Jamal Hadi Salim, Hormuzd Khosravi, Vip Sharma, and
many others for their invaluable contributions.

9.  Author's Addresses

     Alan Crouch
     Intel Labs
     2111 NE 25th Avenue
     Hillsboro, OR 97124 USA
     Phone: +1 503 264 2196
     Email: alan.crouch@intel.com

     Mark Handley
     1947 Center Street, Suite 600
     Berkeley, CA 94708, USA
     Email:  mjh@icsi.berkeley.edu

Crouch/Handley                                     Section 8.  [Page 10]