Internet Draft L. Yang
Expiration: December 2002 Intel Labs
File: draft-yang-forces-model-00.txt J. Halpern
Working Group: ForCES
R. Gopal
Nokia
R. Dantu
Netrake
June 2002
ForCES Forwarding Element Functional Model
draft-yang-forces-model-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are
working documents of the Internet Engineering Task Force (IETF), its
areas, and its working groups. Note that other groups may also
distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as
reference material or to cite them other than as ``work in
progress.''
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
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. Abstract
This document defines a functional model for ForCES forwarding
elements (FEs). This model is used to describe the state of ForCES
forwarding elements within the context of the ForCES protocol, so
that ForCES control elements (CEs) can control the FEs accordingly.
The model is to specify what logical function instances are present
in the FEs and in what order these functions are performed. The
Internet Draft ForCES FE Functional Model June 2002
forwarding element model defined herein is intended to satisfy the
requirements specified in the ForCES requirements draft [FORCES-
REQ]. Using this model, predefined or vendor specific logical
functions can be expressed and configured. However, the definition
of these components are not described and defined in this document.
Definitions
A set of terminology associated with the ForCES requirements is
defined in [FORCES-REQ] and is not copied here. The following list
of terminology is relevant to the FE model defined in this document.
Datapath -- A conceptual path taken by packets within the forwarding
plane, inside an FE. There might exist more than one datapath within
an FE.
Forwarding Element (FE) Block -- An abstraction of the basic packet
processing functions in the datapath. It is the building block of FE
functionality. This concept abstracts away implementation details
from the parameters of interest for configuration, control and
management by CE. The general rule of thumb on the granularity of FE
blocks is that FE blocks should be coarse grained, stateful and
focus on a particular domain. For example, RFC1812 compliant IPv4
forwarder, classifiers, meters, etc..
Forwarding Element (FE) Stage -- Representation of a FE block
instance in a FE's datapath. As a packet flows through an FE along a
datapath, it flows through one or multiple distinct stages, with
each stage implementing an instance of a certain logical function.
There may be one or more combination of such instances in a FE's
datapath. Using NAT as an example, one NAT function is typically
performed before the forwarding stage (packets arriving externally
have their public addresses replaced with private addresses) and one
NAT function is performed after (for packets exiting the domain,
their private addresses are replaced by public ones). So there are
three stages (NAT, forwarding, and NAT again) in this example FE,
with two NAT instances present in two different stages.
2. Motivation and Requirements of FE model
The ForCES architecture allows Forwarding Elements (FEs) of varying
functionality to participate in a ForCES network element (NE). The
implication of this varying functionality is that CEs can make only
minimal assumptions about the functionality provided by its FEs.
Before CEs can configure and control the forwarding behavior of FEs,
CEs need to query and discover the capabilities and state of their
FEs. [FORCES-REQ] mandates that this capability and state
information be expressed in the form of a FE model, and this model
will be used as the basis for CEs to control FEs' capabilities and
manipulate FEs' state via ForCES protocol.
[FORCES-REQ] describes all the requirements placed on the FE model
in detail. We provide a brief summary here to highlight some of the
design issues we face.
Yang, et. al. Expires December 2002 [Page 2]
Internet Draft ForCES FE Functional Model June 2002
. The FE model MUST express what logical functions can be applied
to packets as they pass through a FE.
. The FE model MUST be capable of supporting/allowing variations
in the way logical functions are implemented on a FE.
. The model MUST be capable of describing the order in which
these logical functions are applied in a FE.
. The FE model SHOULD be extendable and should have provision to
express new or vendor specific logical functions. .
. Should be able to support minimal set of logical functions
that are already identified such as port functions,
forwarding functions, QoS functions, filtering functions, high-
touch functions, security functions, and vendor-specific
functions.
Since the motivation of an FE model is to allow CEs later to control
and configure FEs' behavior via ForCES protocol, it becomes
essential to examine and understand what kind of control and
configuration CEs might do to FEs. We believe that there are roughly
three levels of control and configuration that CEs can do to FEs.
The first level of control and configuration is the simplest of all.
It assumes that each FE's capability is already given and remains
static in its lifetime, and CEs can only control its behavior by
manipulating its state. We call this "static FE" control and
configuration. For example, Figure 2 and 3 each shows an FE
configuration example by representing the processing steps in a
directed graph interconnecting all the functional stages that
packets can possibly traverse. If such a configuration remains
static during FE's lifetime, then all CE can control is the
parameters associated with each stage in the graph, for example, the
routing table in the LPM forwarder in Figure 2, or the token bucket
parameters associated with meter1 in Figure 3. But CE cannot
reconfigure the graph topology dynamically, such as adding another
meter or queue onto the FE in Figure 3 on the fly. For this kind of
static FE control and configuration purpose, the useful FE model is
consisted of the state information that FEs allow CEs to manipulate
and the statistics and events that FEs can collect and report back
to CEs. Such a state model doesn't need to describe a lot of other
information, like the packet formats supported between meter1 and
counter1 in Figure 3, for example. Because such information is only
useful when the graph is re-configured dynamically on the fly.
The second level of control and configuration builds on top of the
first that is just described. Using Figure 3 as an example, instead
of presenting the static FE graph to CE, FE can convey its
capabilities to CE by telling CE that "This FE can support one
classifier with up to N filters. This FE can also support M meters,
X queues, etc." We call this dynamic FE control and configuration.
For such control and configuration, a more powerful and flexible FE
capability model is required in addition to the FE state model. For
example, it becomes necessary to model the capability of the
building blocks like classifiers, filters, meters etc. it also needs
Yang, et. al. Expires December 2002 [Page 3]
Internet Draft ForCES FE Functional Model June 2002
to describe the packet formats supported at each input and output of
each block to allow correct reconfiguration of the graph by CE.
The third level of control and configuration is even more powerful
and future looking. In addition to dynamic configuration, CEs might
even be allowed to download a given functionality onto FEs at run
time.
The FE model proposed in this document intends to fully support the
static FE control and configuration. It is also our intention to
allow extension later to support dynamic FE control and
configuration. However, this FE model currently makes no attempt to
address issues beyond the simple static FE control and
configuration.
3. Capability and configuration Model versus State Model
FE Model describes both the static capabilities and the configured
capabilities. At any time after initial configuration the logical
functions associated with an FE model may exhibit different
characteristics and may lead to change in state of FE. The
capability describes set of parameters or attributes of one or more
logical functions of a FE, which may be useful for configuring and
managing a FE. The state model describes the instantaneous values or
operational behaviour of a FE.
Typical capabilities model describes at the coarsest level such
aspects as
- this FE can handle IPv4 and IPv6
- this FE can perform 6-tuple classification (or can only do DA
based processing, ...)
- this FE can perform metering
- this FE can handle multiple queues with multiple priorities
- this FE can add and remove encapsulating headers of types: IPSec,
GRE, L2TP
Where it gets more complicated is coping with the detailed limits.
Issues such has how many classifiers can the FE handle?
How many headers can the FE add and remove? How many queues, and
how many buffer pools can the FE support? How many meters can the
FE provide? How flexibly can these various parts be interconnected?
While one could try to build an object model for representing
capabilities in full, other efforts have found this to be a
significant undertaking. A middle of the road approach is to define
coarse-grained capabilities and simple capacity measures. Then, if
the CE attempts to instruct the FE to set up some specific behavior
it is not capable of, the FE will return an error indicating the
problem.
A state model describes the current state of the FE. It lists
- on a given port the packets are classified using a given
classification filter
- a given classifier results in packets being metered in a certain
way, and then marked in a certain way
Yang, et. al. Expires December 2002 [Page 4]
Internet Draft ForCES FE Functional Model June 2002
- packets coming from specific markers are delivered into a shared
queue for handling, while other packets are delivered to a different
queue
- a specific scheduler with specific behavior and parameters will
service these collected queues.
While the DiffServ and QDDIM models are not designed with the
primary goal of direct machine implementation, we will start with
that model as it is better than no starting point. Alternative
suggestions for a state model which can be implemented more
generally without significant transformation are sought from the
community.
4. FE Model
This section proposes a ForCES FE model to satisfy all the
requirements in [FORCES-REQ] for FE control and configuration. The
approach taken is to first define a set of well-known FE logical
functions (FE blocks) as the basic building blocks, so that any FE
can be modeled by describing the kind of FE blocks or variants of
these blocks it contains, and how the instances of these blocks are
interconnected together. This model also allows new logical
functions be added to accommodate future innovation in forwarding
plane. We believe this approach strikes a good balance between
flexibility and extensibility of the model and ease of use by the
CE.
4.1. FE Blocks
FE blocks are the very basic building blocks from which FEs model
its overall packet processing behavior. The concept of a FE block is
akin to that of an abstract base class in object-oriented
terminology. Different FEs can have different implementation of the
same block, but the packet processing behavior looks the same to the
CE. Sometimes it is difficult to decide the granularity of the FE
blocks. The general rule of thumb is that FE blocks should be coarse
grained, stateful and focus on a particular domain. Basically, if a
logical function doesnÆt have anything to configure, doesnÆt keep
any kind of inter-packet state information (tables, etc), then it
probably is not a FE block. For example, CRC check and IP checksum
are not considered to be FE blocks by themselves, instead should be
part of an FE block (i.e. RFC 1812 IP forwarder).
A well-defined block has a well-defined packet processing behavior,
and a well-defined set of state and parameters that CE can
potentially configure or control via ForCES.
Obviously, a namespace is needed to specify different blocks. The
namespace assigns a unique ID or label to each distinct block type.
For each block, it is necessary to specify the relevant information
and parameters such as:
Yang, et. al. Expires December 2002 [Page 5]
Internet Draft ForCES FE Functional Model June 2002
- how many inputs it takes and what kinds of packets and meta data
it takes for each input;
- how many outputs it produces and what kind of packets and meta
data it emits for each output and;
- the packet processing (such as modification) behavior;
- what information is programmed into it (e.g., LPM list, next hop
list, WRED parameters, etc.) and what parameters among them are
configurable;
- what statistics it keeps (e.g., drop count, CRC error count,
etc.);
- what events it can throw (e.g., table miss, port down, etc.).
CEs later use the information to decide how to configure the
parameters, how to modify the relevant data structures (like
tables), what kind of statistics it can query from FE, and what
actions to take once a certain event happens.
We use the classifier defined in [DS-MODEL] as an example.
"Classifiers are 1:N(fan-out) devices: they take a single traffic
stream as input and generate N logically separate traffic streams as
output. Classifiers are parameterized by filters and output streams.
Packets from the input stream are sorted into various output streams
by filters which match the contents of the packet or possibly match
other attributes associated with the packet." To further define
filters: "A filter consists of a set of conditions on the component
values of a packet's classification key (the header values,
contents, and attributes relevant for classification)." Figure 1
illustrates an example classifier.
Unclassified classified
traffic traffic
+------------+
| |--> match Filter1 --> OutputA
------->| classifier |--> match Filter2 --> OutputB
| |--> no match --> OutputC
+------------+
Figure 1. An Example Classifier
4.2.FE Block Library
We expect a small set of blocks to be defined initially, e.g.,
ingress port, egress port, classifier, forwarder, meter, marker,
shaper, scheduler, queue, encapsulator, decapsulator, encrypter,
decrypter, NAT, mux, demux, header compressor, header decompressor,
etc. Such a set of blocks can be viewed as a FE block library. The
minimum set of FE functions required in [FORCES_REQ] must be part of
this library. It is expected that new FE blocks would be defined and
added into this library over time. It is also expected that the FE
model itself be decoupled from the ForCES protocol so that extension
Yang, et. al. Expires December 2002 [Page 6]
Internet Draft ForCES FE Functional Model June 2002
to basic blocks or changes to the existing blocks should not
impact the protocol itself.
This document only intends to describe the conceptual FE model and
illustrate it with some examples. However, it is not the intention
of this document to define any specific block or the library itself.
Separate document(s) would be written to achieve that.
4.3. FE Stage
As a packet flows through an FE along a datapath, it flows through
one or multiple distinct stages, with each stage instantiating a
certain FE logical function. So an FE stage is simply an instance of
a FE block within an FE's datapath. Each FE allocates a FE-unique
stage ID or label to each of its stages and passes the stage ID or
label along with the corresponding block name as part of the FE
model description. This allows multiple instances of the same block
present in a FE's datapath.
For each stage, the following information must be defined:
- its FE-unique stage ID or label to identify, configure and manage
the logical block of each by CE.;
- the corresponding block name (from the namespace) that this stage
instantiating;
- all the information for this stage as described in Section 5.1;
- the number of downstream stages to which this stage can send
packets;
- for each downstream stage: its stage ID or label.
4.4. Directed Graph of FE Blocks
Once a library of well-understood FE blocks is defined, and a stage
is represented as described in 5.3, any static FE can be modeled by
a directed graph interconnecting all the stages present in the FE.
The static FEs can be defined by identifying all the input stage(s)
and specifying each stage as described in Section 5.3. Figure 2
shows one simple example of such an FE with three stages.
To model a static FE, the following information needs to be
represented:
- number of stages in the FE;
- for each stage: define the stage using the model in 5.3;
- identify the input stages
It is frequently necessary to share state between FE functional
stages in the forwarding plane. Since this model uses blocks as the
basic way of modeling forwarding plane packet processing stages, the
state information is exposed as part of connecting stage. For
example in Figure 2, stage #2 (IPv4 L3 LPM Forwarder) generates some
meta data at its output to carry information on which port the
packets should go to, and #3 (Enet-Egress-port-Manager) uses this
meta data to direct the packets to the right egress port.
Yang, et. al. Expires December 2002 [Page 7]
Internet Draft ForCES FE Functional Model June 2002
+------------+ +------------+ +------------+
input | Ethernet | | | | Ethernet |output
------->| Ingress |-->| IPv4 L3 LPM|-->| Egress |----->
| Port Mgr | | Forwarder | | Port Mgr |
+------------+ +------------+ +------------+
{stage ID=1, {stage ID=2, {stage ID=3,
function= function= function=
Enet-Ing-port, IPv4-L3-LPM-fwd, Enet-Eg-port-Mgr,
#downstream=1, #downstream=1, #downstream=1,
downstream={2} downstream={3} downstream=none
} } }
Figure 2. A very simple example of a static FE.
Queue1
+---+ +--+
| A|------------------->| |--+
+->| | | | |
| | B|--+ +--+ +--+ +--+ |
| +---+ | | | | | |
| Meter1 +->| |-->| | |
| | | | | |
| +--+ +--+ |
| Counter1 Absolute Queue2| +--+
+---+ | Dropper1 +--+ +--->|A |
| A|---+ | |------>|B |
-------->| B|------------------------------>| | +--->|C |------>
| C|---+ +--+ | +->|D |
| X|-+ | | | +--+
+---+ | | +---+ +---+ Queue3| | Scheduler
Classifier1 | | | A|------------>|A | +--+ | |
| +->| | | |->| |--+ |
| | B|--+ +--+ +->|B | | | |
| +---+ | | | | +---+ +--+ |
| Meter2 +->| |-+ Mux1 |
| | | |
| +--+ Queue4 |
| Marker1 +--+ |
+---------------------------->| |----+
| |
+--+
Figure 3. An FE example with multiple datapath.
There might be more than one datapath within an FE, typically due to
blocks that are either 1:N or N:1. Figure 3 shows one such FE block
example (based on an example in [MD-MODEL]). This FE implements QoS
functions via combination of one or multiple instances of logical
functions like classifier, meter, marker, queue, scheduler, etc.
Yang, et. al. Expires December 2002 [Page 8]
Internet Draft ForCES FE Functional Model June 2002
Some of the functions are 1:N (fan-out_ functions. For example, the
stage labeled "Classifier1" is 1:4 function with four downstream
stages, namely, Meter1, Queue2, Meter2, Queue4. A packet entering
this FE can potentially take one of the six distinct datapath.
Note that our stage representation can encode largely arbitrary
topologies of the stages. The only restrictions on topology relate
to the source and sink nature of ingress and egress port functions
respectively. For example, egress port functions must not have nay
downstream stages whereas no other stage may refer to an ingress
port function as one of its downstream stages.
An FE block may contain zero, one or more ingress port stages.
Similarly, an FE block may contain zero, one or more egress port
stages. In another word, not every FE block has to contain any
ingress port or egress port stages. For example, Figure 4 shows two
FE blocks. Block #1 contains one ingress port function but no egress
port function, while block #2 contains one egress port function but
no ingress port function. It is possible to connect these two FE
blocks together to achieve the complete ingress-to-egress packet
processing function. This provides the flexibility to spread the
functions across multiple FEs and interconnect them together later
for certain applications. Figure 4 shows such an example.
-------------------------------------------------------
| +---------+ +------------+ +---------+ |
input| | | | | | output |
---+->| Ingress |-->|Header |-->|IPv4 |---------+--->--+
| | port | |Decompressor| |Forwarder| FE | |
| +---------+ +------------+ +---------+ Block #1| |
------------------------------------------------------| V
|
+-----------------------<-----------------------------+
|
| |-----------------------------------------
V | +------------+ +----------+ |
| input | | | | output |
+->--+->|Header |-->| Egress |---------+-->
| |Compressor | | port | FE |
| +------------+ +----------+ Block #2|
-----------------------------------------|
Figure 4. An example of two different FE blocks connected together.
5. Model Representation
A formal data definition language is needed to represent the FE
model described in this document. The following is a list of some
potential candidates. A suitable candidate needs to be chosen as the
representation of FE model. However, we intend to leave this as an
open issue and much debate is needed in the ForCES WG before a
Yang, et. al. Expires December 2002 [Page 9]
Internet Draft ForCES FE Functional Model June 2002
decision can be made. Therefore, we only provide the candidate list
and some initial discussion here without drawing a conclusion yet.
- XML (Extensible Markup Language) Schema
- ASN.1 (Abstract Syntax Notation One)
- SMI (Structure of Management Information) [RFC1155]
- SPPI (Structure of Policy Provisioning Information) [RFC3159]
- SMIng (Next Generation Structure of Management Information)
- UML (Universal Modeling Language)
XML has the advantage of being human readable with relatively little
effort. However, it is less efficient and requires XML parsing
functions in the CE and FE. Currently XML is not widely deployed and
used in network elements.
ASN.1 format is human readable, and widely used in network
protocols. SMI is based on a subset of ASN.1 and used to define
Management Information Base (MIB) for SNMP. SPPI is the adapted
subset of SMI used to define Policy Information Base (PIB) for COPS.
SMIng is currently being developed by SMIng working group at IETF
and represents a superset of SMIv2 and SPPI. The objective of SMIng
is to replace both SMIv2 and SPPI with a single, updated language as
the data definition language for the monitoring, configuration, and
provisioning of network devices.
6. Security Considerations
The FE model just describes the representation and organization of
data sets and attributes in the forwarding plane. There is no
communication protocol associated defined as part of the FE model
therefore we do not see any security threats in this model.
7. Intellectual Property Right
The authors are not aware of any intellectual property right issues
pertaining to this document.
8. IANA consideration
If we are going to identify and name a new logical components we may
need to standardize those components and the attributes.
9. Normative References
[RFC1812] F. Baker, "Requirements for IP Version 4 Routers",
RFC1812, June 1995.
[RFC1155] M. Rose, et. al., "Structure and Identification of
Management Informationfor TCP/IP-based Internets", May
1990.
Yang, et. al. Expires December 2002 [Page 10]
Internet Draft ForCES FE Functional Model June 2002
[RFC3159] K. McCloghrie, et. al., "Structure of Policy Provisioning
Information (SPPI)", August 2001.
10. Informative References
[FORCES-REQ] T. Anderson, et. al., "Requirements for Separation of
IP Control and Forwarding", work in progress, April 2002,
<draft-ietf-forces-requirements-03.txt>.
[GOPAL-MODEL] R. Gopal, "Forwarding Element Model", work in
progress, February 2002, <draft-gopal-forces-femodel-
00.txt>.
[ANDERSON-MODEL] T. Anderson, "ForCES Architectural Framework and FE
Functional Model", work in progress, November 2001,
<draft-anderson-forces-framework-00.txt>.
[DS-MODEL] Y. Bernet, et. al., "An Informal Management Model for
Diffserv Routers", work in progress, February 2001,
<draft-ietf-diffserv-model-06.txt>.
11. Acknowledgments
This document draws heavily from the concepts presented in [GOPAL-
MODEL], [ANDERSON-MODEL] and [DS-MODEL]. In addition to the authors
of these documents, the authors would also like to thank the
following individuals for their invaluable technical input: David
Putzolu, Hormuzd Khosravi, Eric Johnson, David Durham, Andrzej
Matejko.
12. Authors' Addresses
Lily L. Yang
Intel Labs
2111 NE 25th Avenue
Hillsboro, OR 97124 USA
Phone: +1 503 264 8813
Email: lily.l.yang@intel.com
Joel Halpern
P.O.Box 6049
Leesburg, VA 20178
Phone: +1 703 371 3043
Email: jmh@joelhalpern.com
Ram Gopal
Nokia Research Center
5, Wayside Road,
Yang, et. al. Expires December 2002 [Page 11]
Internet Draft ForCES FE Functional Model June 2002
Burlington, MA 01803
Phone: +1 781 993 3685
Email: ram.gopal@nokia.com
Ram Dantu
Netrake Corporation
3000 Technology Drive
Plano, Texas 75074
Phone: +1 214 291 1111
Email: ramd@netrake.com
1. Abstract........................................................1
2. Motivation and Requirements of FE model.........................2
3. Capability and configuration Model versus State Model...........4
4. FE Model........................................................5
4.1. FE Blocks..................................................5
4.2. FE Block Library...........................................6
4.3. FE Stage...................................................7
4.4. Directed Graph of FE Blocks................................7
5. Model Representation............................................9
6. Security Considerations........................................10
7. Intellectual Property Right....................................10
8. IANA consideration.............................................10
9. Normative References...........................................10
10. Informative References........................................11
11. Acknowledgments...............................................11
12. Authors' Addresses............................................11
Yang, et. al. Expires December 2002 [Page 12]