Forwarding and Control Element Separation (ForCES) Framework
draft-ietf-forces-framework-13
The information below is for an old version of the document that is already published as an RFC.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 3746.
|
|
|---|---|---|---|
| Authors | Lily Yang , Todd A. Anderson , Ram Gopal , Ram Dantu | ||
| Last updated | 2018-05-01 (Latest revision 2004-01-15) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Informational | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 3746 (Informational) | |
| Action Holders |
(None)
|
||
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Alex D. Zinin | ||
| Send notices to | <dro@zurich.ibm.com>, <David.Putzolu@intel.com> |
draft-ietf-forces-framework-13
Internet Draft L. Yang
Expiration: July 2004 Intel Corp.
File: draft-ietf-forces-framework-13.txt R. Dantu
Working Group: ForCES Univ. of North Texas
T. Anderson
Intel Corp.
R. Gopal
Nokia
January 2004
Forwarding and Control Element Separation (ForCES) Framework
draft-ietf-forces-framework-13.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2003). 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 interactions among them.
Table of Contents
Internet Draft ForCES Framework January 2004
1. Definitions...................................................3
1.1. Conventions used in this document........................3
1.2. Terminologies............................................3
2. Introduction to Forwarding and Control Element Separation
(ForCES).........................................................5
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.............................................15
3.4. FE 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.....18
4.2.2. Association Establishment..........................18
4.2.3. Steady-state Communication.........................20
4.2.4. Data Packets across Fp reference point.............20
4.2.5. Proxy FE...........................................22
4.3. Association Re-establishment............................22
4.3.1. CE graceful restart................................22
4.3.2. FE restart.........................................24
5. Applicability to RFC1812.....................................25
5.1. General Router Requirements.............................25
5.2. Link Layer..............................................26
5.3. Internet Layer Protocols................................27
5.4. Internet Layer Forwarding...............................28
5.5. Transport Layer.........................................29
5.6. Application Layer -- Routing Protocols..................29
5.7. Application Layer -- Network Management Protocol........29
6. Summary......................................................30
7. Acknowledgements.............................................30
8. Security Considerations......................................30
8.1. Analysis of Potential Threats Introduced by ForCES......31
8.1.1. "Join" or "Remove" Message Flooding on CEs.........31
8.1.2. Impersonation Attack...............................32
8.1.3. Replay Attack......................................32
8.1.4. Attack during Fail Over............................32
8.1.5. Data Integrity.....................................32
8.1.6. Data Confidentiality...............................33
8.1.7. Sharing security parameters........................33
8.1.8. Denial of Service Attack via External Interface....33
8.2. Security Recommendations for ForCES.....................34
8.2.1. Using TLS with ForCES..............................34
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8.2.2. Using IPsec with ForCES............................35
9. Normative References.........................................37
10. Informative References......................................37
11. Authors' Addresses..........................................38
12. Intellectual Property Right.................................39
13. Full Copyright Statement....................................39
1. Definitions
1.1. 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.
1.2. 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.
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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
whether an FE and a CE should be part of the same network element.
It is possible for some elements of the NE to be in pre-association
phase while other elements are in the post-association phase.
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
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, 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
information.
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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
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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) |
------------ ------------ ------------
^ | ^ | ^ |
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| | | | . . . | |
| 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
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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 |
-------------------------------------------------
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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
Protocol.
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
Fp: CE-FE interface
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Fi: FE-FE interface
Fr: CE-CE interface
Fc: Interface between the CE Manager and a CE
Ff: Interface between the FE Manager and an FE
Fl: Interface between the CE Manager and the FE Manager
Fi/f: FE external interface
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 ancillary entities provide configuration to the
corresponding CE or FE in the pre-association phase (see Section
4.1).
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.
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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 ForCES 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
resource change taken from an existing FE (already participating in
ForCES post-association phase), 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.
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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. Unless otherwise configured or
determined by a ForCEs Protocol exchange, each FE will process
authorized incoming commands directed at it as it receives them on
a first come first serve basis.
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 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
CEs.
------- 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
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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.
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-----------
| 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.
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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 whether an FE and a 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 whether an
individual CE and FE, or a set of CEs and FEs should be associated.
Communication across the Fl reference point is optional in this
architecture. No requirements are placed on this reference point.
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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
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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
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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.) |
1|<--------------------->|
| |
|(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|---------------------->|
| |
|(Initial config for FE -- optional)
5|<----------------------|
| |
|(I am ready to go. Shall I?)
6|---------------------->|
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| |
|(Go ahead!) |
7|<----------------------|
| |
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 security handshake should include mutual
authentication and authorization between the CE and FE, but the
exact details depend on the security solution chosen by ForCES
Protocol. Authorization can be as simple as checking against the
list of authorized end points provided by the FE or CE manager
during the pre-association phase. Both authentication and
authorization must be successful before the association can be
established. If either authentication or authorization fails, the
end point must not be allowed to join the NE. After the successful
security handshake, message authentication and confidentiality are
still necessary for the on-going information exchange between the
CE and FE, unless some form of physical security exists. Whenever
a packet fails authentication, it must be dropped and a
notification may be sent to alert the sender of the potential
attack. 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 optionally 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 the necessary initial information is exchanged, the process of
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association is completed. Packet processing and forwarding at the
FE cannot begin until association is established. After the
association is established, 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.)|
1|<----------------------|
| |
|(Successful.) |
2|---------------------->|
| |
| |
|(Query some stats.) |
1|<----------------------|
| |
|(Reply with stats collected.)
2|---------------------->|
| |
| |
|(My port is down, with port #.)
1|---------------------->|
| |
|(Here is a new forwarding table)
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
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--------------------- ----------------------
| | | |
| +--------+ | | +--------+ |
| |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
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 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.
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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 to terminate the Fp reference point instead of
the physical FE. 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. It is also possible that one proxy act on behalf of
multiple physical FEs.
One needs to be aware of the security implication introduced by the
proxy FE. Since the physical FE is not capable of implementing
ForCES itself, the security mechanism of ForCES can only secure the
communication channel between the CE and the proxy FE, but not all
the way to the physical FE. It is recommended that other security
mechanisms (including physical security property) be employed to
ensure the security between the CE and the physical FE.
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 (including both authentication and authorization) exists
before and after the association re-establishment.
When an FE leaves or joins an existing NE that is already in
operation, the CE needs to be aware of the impact on FE topology
and deals with the change accordingly.
4.3.1. CE graceful restart
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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]) 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
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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 CE1 to 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 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
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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
requirement.
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
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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
11.
5.2.Link Layer
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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
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.
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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
FE.
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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." 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 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
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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. Susan Hares pointed out the issue of graceful
restart with ForCES. Russ Housley, Avri Doria, Jamal Hadi Salim
and many others in the ForCES mailing list also provided valuable
feedback.
8. Security Considerations
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 below 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
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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.
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.
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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 as having
come from an FE or CE on the list of the authorized ForCES elements
(provided by the CE or FE Manager in the pre-association phase)
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 has not been established prior
or during the fail-over.
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
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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
compromised.
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
interface.
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 the FE and CE. Rate Limiter SHOULD be configured at
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the 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 [18] 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. In addition, other approaches may be used as well
but are not documented here, including using L2 security mechanisms
for a given L2 interconnect technology, as long as the requirements
can be satisfied.
When ForCES is deployed between CEs and FEs inside a box or a
physically secured room, 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. 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
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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.
8.2.2. 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
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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
manually.
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
9.
1) During Pre-association phase all FEs are configured with
the CEs (including active CE and standby CE), IPsec policy
etc.
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).
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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
communication.
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)
Interface.
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
2000.
[3] Khosravi, H. and Anderson, T., "Requirements for Separation of
IP Control and Forwarding", RFC 3654, November 2003.
10. Informative References
[4] Case, J., et al., "Introduction and Applicability Statements
for Internet Standard Management Framework", RFC 3410, December
2002.
[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.
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[7] Crouch, A. et al., "ForCES Applicability Statement", work in
progress, June 2003, <draft-ietf-forces-applicability-02.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", RFC 3623, November
2003.
[11] Sangli, S. et al., "Graceful Restart Mechanism for BGP", work
in progress, September 2003, < draft-ietf-idr-restart-08.txt>.
[12] Shand, M. and L. Ginsberg, "Restart Signaling for IS-IS", work
in progress, July 2003, <draft-ietf-isis-restart-04.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 2003, <draft-bellovin-useipsec-02.txt>.
[17] Hardjono, T. and Weis, B. "The Multicast Security
Architecture", work in progress, November 2003, <draft-ietf-msec-
arch-04.txt>.
[18] S. Bellovin, J. Schiller, and C. Kaufman, "Security Mechanisms
for the Internet", RFC 3631, December, 2003.
11. Authors' Addresses
L. Lily 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
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Department of Computer Science,
University of North Texas,
Denton, TX 76203, USA
Phone: +1 940 565 2822
Email: rdantu@unt.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
Ram Gopal
Nokia Research Center
5, Wayside Road,
Burlington, MA 01803, USA
Phone: +1 781 993 3685
Email: ram.gopal@nokia.com
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