Internet Draft L. Yang
Expiration: June 2003 Intel Corp.
File: draft-ietf-forces-framework-04.txt R. Dantu
Working Group: ForCES Univ. of Texas Dallas
T. Anderson
Intel Corp.
December 2002
Forwarding and Control Element Separation (ForCES) Framework
draft-ietf-forces-framework-04.txt
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document defines the architectural framework for the ForCES
(Forwarding and Control Element Separation) network elements, and
identifies the associated entities and the interaction among them.
Table of Contents
1. Definitions.....................................................2
Internet Draft ForCES Framework Dec. 2002
2. Introduction to Forwarding and Control Element Separation
(ForCES)...........................................................4
3. Architecture....................................................7
3.1. Control Elements and Fr Reference Point....................8
3.2. Forwarding Elements and Fi reference point.................9
3.3. CE Managers...............................................11
3.4. FE Managers...............................................11
4. Operational Phases.............................................12
4.1. Pre-association Phase.....................................12
4.1.1. Fl Reference Point...................................12
4.1.2. Ff Reference Point...................................13
4.1.3. Fc Reference Point...................................14
4.2. Post-association Phase and Fp reference point.............14
4.2.1. Proximity and Interconnect between CEs and FEs.......14
4.2.2. Association Establishment............................14
4.2.3. Steady-state Communication...........................15
4.2.4. Data Packets across Fp reference point...............16
4.2.5. Proxy FE.............................................17
4.3. Association Re-establishment..............................17
5. Applicability to RFC1812.......................................18
5.1. General Router Requirements...............................19
5.2. Link Layer................................................20
5.3. Internet Layer Protocols..................................20
5.4. Internet Layer Forwarding.................................21
5.5. Transport Layer...........................................22
5.6. Application Layer -- Routing Protocols....................22
5.7. Application Layer -- Network Management Protocol..........22
6. Summary........................................................23
7. Normative References...........................................23
8. Informative References.........................................23
9. Security Considerations........................................24
10. Acknowledgments...............................................24
11. Authors' Addresses............................................24
12. Intellectual Property Right...................................24
13. Full Copyright Statement......................................24
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in [RFC-2119].
1. Definitions
A set of terminology associated with the ForCES requirements is
defined in [3] and we only include the ones that are most relevant
to this document here.
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.
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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.
ForCES Network Element (NE) - An entity composed of one or more CEs
and one or more FEs. To entities outside a NE, the NE represents a
single point of management. Similarly, a NE usually hides its
internal organization from external entities.
Pre-association Phase - The period of time during which a 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 a 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
another.
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, or
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.
FE Manager - A logical entity that operates in the pre-association
phase and is responsible for determining to which CE(s) a FE should
communicate. This process is called CE discovery and may involve
the FE manager learning the capabilities of available CEs. A FE
manager may use anything from a static configuration to a pre-
association phase protocol (see below) to determine which CE(s) to
use. Being a logical entity, a 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.
Being a logical entity, a CE manager might be physically combined
with any of the other logical entities mentioned in this section.
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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 a FE.
ForCES Protocol Element - A FE or CE.
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 a lot more design choices and flexibility to 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.
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 router (forwarding) blades, all
interconnected into a switch fabric backplane. In such chassis
configuration, the control blades are the CEs while the router
blades are FEs, and the switch fabric backplane provides the
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physical interconnect for all the blades. Control blade A may be
the primary CE while control blade B is 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 interface for messaging between CEs
and FEs. The goal of ForCES is to replace such proprietary
interface with a standard protocol. With a standard protocol like
ForCES implemented on all blades, it becomes possible for control
blades from vendor X and routing blades from vendor Y to work
seamlessly together in one chassis.
------------------------- -------------------------
| Control Blade A | | Control Blade B |
| (CE) | | (CE) |
------------------------- -------------------------
^ | ^ |
| | | |
| V | V
---------------------------------------------------------
| Switch Fabric Backplane |
---------------------------------------------------------
^ | ^ | ^ |
| | | | à | |
| V | V | V
------------ ------------ ------------
|Router | |Router | |Router |
|Blade #1 | |Blade #2 | |Blade #N |
| (FE) | | (FE) | | (FE) |
------------ ------------ ------------
^ | ^ | ^ |
| | | | à | |
| V | V | V
Figure 1. A router configuration example with separate blades.
------- -------
| CE1 | | CE2 |
------- -------
^ ^
| |
V V
============================================ Ethernet
^ ^ à ^
| | |
V V V
------- ------- --------
| FE#1| | FE#2| | FE#n |
------- ------- --------
^ | ^ | ^ |
| | | | | |
| V | V | V
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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 interconnect
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 only. The separate interconnects may also
provide reliability and redundancy benefits for the NE.
-------------------------------------------------
| | | | | | |
|OSPF |RIP |BGP |RSVP |LDP |à |
| | | | | | |
-------------------------------------------------
| ForCES Interface |
-------------------------------------------------
^ ^
ForCES | |data
control | |packets
messages| |(e.g., routing packets)
v v
-------------------------------------------------
| ForCES Interface |
-------------------------------------------------
| | | | | | |
|LPM Fwd|Meter |Shaper |NAT |Classi-|à |
| | | | |fier | |
-------------------------------------------------
| FE resources |
-------------------------------------------------
Figure 3. Examples of CE and FE functions
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 FE include LPM (longest prefix match) forwarder,
classifiers, traffic shaper, meter, NAT, etc. Figure 3 shows a
diagram with examples 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
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originated/destined from/to the control functions in CE (e.g.,
routing packets). Section 4.2.4 provides more detail on this.
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
protocol(s).
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
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external world. The aggregation of these FE interfaces becomes the
NEÆs 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 5.1). 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 4 and 5 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 where 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 where 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
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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
FEs perform per-packet processing and handling 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 its resource on
the packet processing functions.
------- Fr -------
| CE1 | ------| CE2 |
------- -------
| \ / |
| \ / |
| \ / |
| \/Fp |
| /\ |
| / \ |
| / \ |
------- Fi -------
| FE1 |<----->| FE2 |
------- -------
Figure 5. CE redundancy example.
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 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 CEs.
This architecture permits multiple FEs to be present in a 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
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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 protools so that, in keeping with
the "dumb FEÆÆ 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.
-----------------
| 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
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(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.
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 can be queried from the FEs by CEs.
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. 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
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FE managers are responsible for determining to which CE any
particular FE should initially communicate. Like CE managers, no
restrictions are placed on how a FE manager decides to which CE its
FEs should communicate, nor are restrictions placed on how FE
managers are implemented.
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 a 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
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
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
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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.
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 or may not 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
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.
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 a 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.
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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 7 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 a 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 not be designed to support
configurations where the CE and FE are located arbitrarily in the
network. In particular, ForCES is intended for "very close" CE/FE
localities in IP networks, as defined by ForCES Applicability
Statement ([7]). 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.
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). ForCES will make
use of an existing RFC2914 ([2]) compliant L4 protocol with adequate
reliability, security and congestion control (e.g. TCP, SCTP) for
transport purposes.
4.2.2. Association Establishment
As an example, figure 9 shows some of the message exchange that may
happen before the association between the CE and FE is fully
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established. Either the CE or FE can initiate the connection. 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 another separate document. The model
would allow a 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 sends all necessary configuration to
the FE so that the FE can start receiving and processing packets
correctly. For example, the CE might need to send a snapshot of the
current forwarding table to the FE so that the FE can start routing
packets correctly. 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.
FE CE
| |
|(Hello, are you there?)|
1|<----------------------|
| |
|(Yes. let me join the NE please.)
2|---------------------->|
| |
|(What kind of FE are you? -- capability query)
3|<----------------------|
| |
|(Here is my FE functions/state: use model to describe)
4|---------------------->|
| |
|(How are you connected with others? -- topology query)
5|<----------------------|
| |
|(Here is the topology info)
6|---------------------->|
| |
|(Config for FE, e.g. forwarding table)
7|<----------------------|
| |
|(I am ready to go. Shall I?)
8|---------------------->|
| |
|(Go ahead!) |
9|<----------------------|
| |
Figure 9. Example of message exchange between CE and FE
over Fp to establish NE association
4.2.3. Steady-state Communication
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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.)|
1|<----------------------|
| |
|(Successful.) |
2|---------------------->|
| |
| |
|(Query some stats.) |
1|<----------------------|
| |
|(Reply with stats collected.)
2|---------------------->|
| |
| |
|(My port is down, with port #.)
1|---------------------->|
| |
|(Route to this port instead...)
2|<----------------------|
| |
| |
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
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 signaling and
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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 CE.
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 to
transport 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 that.
--------------------- ----------------------
| | | |
| +--------+ | | +--------+ |
| |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.
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 a FE or CE leaves the NE, the association
with the NE is broken. If the leaving party rejoins a NE later, to
re-establish the association, it may or may not need to re-enter the
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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 (see [3]
Section 5, requirement #7).
The example in Figure 5 is used to illustrate what happens when the
association is broken and later re-established again. Section 3.2
already explains what happens if CE1 fails and how CE2 can take
over. If no CE redundancy is provided, at the association
establishment phase FEs need to be told what to do in the case of CE
failure. FEs may be told to stop packet processing all together if
its CE fails. Or, FEs may be told to continue forwarding packets
for a limited time even in the face of CE failure. No matter what
the instruction is, it needs to be part of the configuration when
the association is established.
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. Or, FE1 may stop functioning simply due to unexpected
failure. In former case, CE1 receives a "leave-association request"
from FE1. In the latter, CE1 detects the failure of FE1 by some
other mean. In both cases, CE1 keeps a note of such event from FE1
while continue commanding FE2. When FE1 decides to rejoin again, or
when it is back up 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 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 initialization information. Suppose that FE1 still has its
routing table and other state information from the last association,
instead of sending all the information again from scratch, it can
choose to use more efficient mechanism to re-sync up the state with
its CE. For example, a checksum 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 CE first, it sends information update only
if it is needed.
5. Applicability to RFC1812
[3] Section 5, requirement #9 dictates that "All 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
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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 | | | |
| | | -------------- | | |
| | | | | |
-----+--+----------------+--+----
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| | | |
| | | |
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 11.
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 in general CEs are required to be capable of
these kind of operations while FEs may or may not 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. Timestamp option should be implemented by FEs to
insert the timestamp most accurately. 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
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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
function.
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, FE verifies the IP header and processes
most of the IP options. Some options can't be processed until the
routing decision has been made. 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, FE determines the next hop IP
address by looking up in its forwarding table. FE also determines
the network interface it uses to send the packets. Sometimes 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, FE decrements TTL by 1 and processes any IP
options that cannot be processed before. 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 don't 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
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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 FE.
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. It is possible
that all FEs inside an NE implements only one such protocol entity.
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.
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
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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 interface between CEs and FEs is within
the scope of ForCES. ForCES alone may not be enough to support all
desirablet NE configurations. However, we believe that ForCES 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 and static configuration
without the need for CE/FE managers.
7. Normative References
[1] F. Baker, "Requirements for IP Version 4 Routers", RFC1812, June
1995.
[2] S. Floyd, "Congestion Control Principles", RFC2914, September
2000.
[3] H. Khosravi, et. al., "Requirements for Separation of IP Control
and Forwarding", work in progress, Oct 2002, <draft-ietf-forces-
requirements-07.txt>.
8. Informative References
[4] J. Case, et. al., "A Simple Network Management Protocol (SNMP)",
RFC1157, May 1990.
[5] M. Daniele, et. al., "Agent Extensibility (AgentX) Protocol
Version 1", RFC2741, Jan 2000.
[6] K. Chan, et. al., "COPS Usage for Policy Provisioning (COPS-
PR)", RFC3084, March 2001.
[7] A. Crouch, et. al., "ForCES Applicability Statement", work in
progress, June 2002, <draft-ietf-forces-applicability-00.txt>.
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9. Security Considerations
The security necessary across each reference point except Fp is
discussed throughout the document. In general, the physical
separation of two entities usually requires much stricter security
measurement in place. For example, we pointed out in Section 5.1
that authentication becomes necessary between CE manager and FE
manager, between CE and CE manager, between FE and FE manager in
some configuration. The physical separation of CE and FE also
imposes serious security requirement for ForCES protocol. The
security requirements for reference point Fp (i.e., ForCES protocol)
are discussed in detail in [3] Section 8.
10. Acknowledgments
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.
11. Authors' Addresses
Lily L. Yang
Intel Corp., MS JF3-206,
2111 NE 25th Avenue
Hillsboro, OR 97124 USA
Phone: +1 503 264 8813
Email: lily.l.yang@intel.com
Ram Dantu
University of Texas Dallas
2601 North Flyod Road
Richardson Texas 75082
Phone: +1 972 883 4653
Email: ram.dantu@utdallas.edu
Todd A. Anderson
Intel Corp.
2111 NE 25th Avenue
Hillsboro, OR 97124 USA
Phone: +1 503 712 1760
Email: todd.a.anderson@intel.com
12. Intellectual Property Right
The authors are not aware of any intellectual property right issues
pertaining to this document.
13. Full Copyright Statement
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Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
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The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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