Internet DRAFT - draft-sin-sdnrg-sdn-approach
draft-sin-sdnrg-sdn-approach
Network Working Group M. Boucadair
Internet-Draft C. Jacquenet
Intended status: Informational France Telecom
Expires: July 10, 2014 January 6, 2014
Software-Defined Networking: A Perspective From Within A Service
Provider
draft-sin-sdnrg-sdn-approach-09
Abstract
Software-Defined Networking (SDN) has been one of the major buzz
words of the networking industry for the past couple of years. And
yet, no clear definition of what SDN actually covers has been broadly
admitted so far. This document aims at contributing to the
clarification of the SDN landscape by providing a perspective on
requirements, issues and other considerations about SDN, as seen from
within a service provider environment.
It is not meant to endlessly discuss what SDN truly means, but rather
to suggest a functional taxonomy of the techniques that can be used
under a SDN umbrella and to elaborate on the various pending issues
the combined activation of such techniques inevitably raises. As
such, a definition of SDN is only mentioned for the sake of
clarification.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on July 10, 2014.
Boucadair & Jacquenet Expires July 10, 2014 [Page 1]
Internet-Draft On SDN January 2014
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introducing Software-Defined Networking . . . . . . . . . . . 4
2.1. A Tautology? . . . . . . . . . . . . . . . . . . . . . . 4
2.2. On Flexibility . . . . . . . . . . . . . . . . . . . . . 4
2.3. A Tentative Definition . . . . . . . . . . . . . . . . . 5
2.4. Functional Meta-Domains . . . . . . . . . . . . . . . . . 6
3. Reality Check . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Remember The Past . . . . . . . . . . . . . . . . . . . . 7
3.2. Be Pragmatic . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Measure Experience Against Expectations . . . . . . . . . 8
3.4. Design Carefully . . . . . . . . . . . . . . . . . . . . 9
3.5. On OpenFlow . . . . . . . . . . . . . . . . . . . . . . . 9
3.6. Non Goals . . . . . . . . . . . . . . . . . . . . . . . . 10
4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1. Implications Of Full Automation . . . . . . . . . . . . . 10
4.2. Bootstrapping An SDN . . . . . . . . . . . . . . . . . . 12
4.3. Operating An SDN . . . . . . . . . . . . . . . . . . . . 14
4.4. The Intelligence Resides In The PDP . . . . . . . . . . . 15
4.5. Simplicity And Adaptability Vs. Complexity . . . . . . . 15
4.6. Performance And Scalability . . . . . . . . . . . . . . . 16
4.7. Risk Assessment . . . . . . . . . . . . . . . . . . . . . 16
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
6. Security Considerations . . . . . . . . . . . . . . . . . . . 16
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
8. Informative References . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
Boucadair & Jacquenet Expires July 10, 2014 [Page 2]
Internet-Draft On SDN January 2014
1. Introduction
The Internet has become the federative network that supports a wide
range of service offerings. The delivery of network services such as
IP VPNs assumes the combined activation of various capabilities that
include (but are not necessarily limited to) forwarding and routing
capabilities (e.g., customer-specific addressing scheme management,
dynamic path computation to reach a set of destination prefixes,
dynamic establishment of tunnels, etc.), quality of service
capabilities (e.g., traffic classification and marking, traffic
conditioning and scheduling), security capabilities (e.g., filters to
protect customer premises from network-originated attacks, to avoid
malformed route announcements, etc.) and management capabilities
(e.g., fault detection and processing).
As these services not only grow in variety but also in complexity,
their design, delivery and operation have become a complex alchemy
that often requires various levels of expertise. This situation is
further aggravated by the wide variety of (network) protocols and
tools, as well as recent Any Time Any-Where Any Device
(ATAWAD)-driven convergence trends that are meant to make sure an
end-user can access the whole range of services he/she has subscribed
to, whatever the access and device technologies, wherever the end-
user is connected to the network, and whether this end-user is in
motion or not.
Yet, most of these services have been deployed for the past decade,
primarily based upon often static service production procedures that
are more and more exposed to the risk of erroneous configuration
commands. In addition, most of these services do not assume any
specific negotiation between the customer and the service provider or
between service providers besides the typical financial terms.
At best, five-year master plans are referred to as the network
planning policy that will be enforced by the service provider, given
the foreseen business development perspectives, manually-computed
traffic forecasts and the market coverage (fixed/mobile, residential/
corporate). This so-called network planning policy may very well
affect the way resources are allocated in a network, but clearly
fails to be adequately responsive to highly dynamic customer
requirements in an "always-on" fashion. The need for improved
service delivery procedures (including the time it takes to deliver
the service once possible negotiation phase is completed) is even
more critical for corporate customers.
In addition, various tools are used for different, sometimes service-
centric, management purposes but their usage is not necessarily
coordinated for the sake of event aggregation, correlation and
Boucadair & Jacquenet Expires July 10, 2014 [Page 3]
Internet-Draft On SDN January 2014
processing. This lack of coordination may come at the cost of extra
complexity and possible customer's Quality of Experience degradation.
Multi-service, multi-protocol, multi-technology convergent and
dynamically-adaptive networking environments of the near future have
therefore become one of the major challenges faced by service
providers.
This document aims at clarifying the SDN landscape by providing a
perspective on the functional taxonomy of the techniques that can be
used in SDN, as seen from within a service provider environment.
2. Introducing Software-Defined Networking
2.1. A Tautology?
The separation of the forwarding and control planes (beyond
implementation considerations) have almost become a gimmick to
promote flexibility as a key feature of the SDN approach.
Technically, most of current router implementations have been
assuming this separation for decades. Routing processes (such as IGP
and BGP route computation) have often been software-based, while
forwarding capabilities are usually implemented in hardware.
As such, the current state-of-the-art tends to confirm the said
separation, which rather falls under a tautology.
But a somewhat centralized, "controller-embedded", control plane for
the sake of optimized route computation before FIB population is
certainly another story.
2.2. On Flexibility
Promoters of SDN have argued that it provides additional flexibility
in how the network is operated. This is undoubtedly one of the key
objectives that must be achieved by service providers. This is
because the ability to dynamically adapt to a wide range of
customer's requests for the sake of flexible network service delivery
is an important competitive advantage. But flexibility is much, much
more than separating the control and forwarding planes to facilitate
forwarding decision-making processes.
For example, the ability to accommodate short duration extra
bandwidth requirements so that end users can stream a video file to
their 4G terminal device is an example of that flexibility that
several mobile operators are currently investigating.
Boucadair & Jacquenet Expires July 10, 2014 [Page 4]
Internet-Draft On SDN January 2014
From this perspective, the ability to predict the network behavior as
a function of the network services to be delivered is of paramount
importance for service providers, so that they can assess the impact
of introducing new services or activating additional network features
or enforcing a given set of (new) policies from both a financial and
technical standpoints. This argues in favor of investigating
advanced network emulation engines, which can be fed with information
that can be derived from [I-D.ietf-idr-ls-distribution], for example.
Given the rather broad scope that the flexibility wording suggests:
o Current SDN-labeled solutions are claimed to be flexible although
the notion is hardly defined. The exact characterization of what
flexibility actually means is yet-to-be-provided. Further work
needs therefore to be conducted so that flexibility can be
precisely defined in light of various criteria such as network
evolution capabilities as a function of the complexity introduced
by the integration of SDN techniques, seamless capabilities (i.e.,
the ability to progressively introduce SDN-enabled devices without
disrupting network and service operation, etc.).
o The exposure of programmable interfaces is not a goal per se,
rather a means to facilitate configuration procedures for the sake
of improved flexibility.
2.3. A Tentative Definition
We define Software-Defined Networking as the set of techniques used
to facilitate the design, the delivery and the operation of network
services in a deterministic, dynamic, and scalable manner. The said
determinism refers to the ability to completely master the various
components of the service delivery chain, so that the service that
has been delivered complies with what has been negotiated and
contractually defined with the customer.
As such, determinism implies the ability to control how network
services are structured, designed and delivered, and where traffic
should be forwarded in the network for the sake of optimized resource
usage. Although not explicitly reminded in the following sections of
the document, determinism lies beneath any action that may be taken
by a service provider once service parameter negotiation is
completed, from configuration tasks to service delivery, fulfillment
and assurance (see Section 2.4 below).
Such a definition assumes the introduction of a high level of
automation in the overall service delivery and operation procedures.
Boucadair & Jacquenet Expires July 10, 2014 [Page 5]
Internet-Draft On SDN January 2014
Because networking is software-driven by nature, the above definition
does not emphasize the claimed "Software-Defined" properties of SDN-
labeled solutions.
2.4. Functional Meta-Domains
SDN techniques can be classified into the following functional meta-
domains:
o Techniques for the dynamic discovery of network topology, devices
and capabilities, along with relevant information and data models
that are meant to precisely document such topology, devices and
their capabilities.
o Techniques for exposing network services and their
characteristics, and for dynamically negotiating the set of
service parameters that will be used to measure the level of
quality associated to the delivery of a given service or a
combination thereof. For example,
[I-D.boucadair-connectivity-provisioning-profile]) .
o Techniques used by service requirements-derived dynamic resource
allocation and policy enforcement schemes, so that networks can be
programmed accordingly. Decisions made to dynamically allocate
resources and enforce policies are typically the result of the
correlation of various inputs, such as the status of available
resources in the network at any given time, the number of customer
service subscription requests that need to be processed over a
given period of time, the traffic forecasts and the possible need
to trigger additional resource provisioning cycles according to a
typical multi-year master plan, etc.
o Dynamic feedback mechanisms that are meant to assess how
efficiently a given policy (or a set thereof) is enforced from a
service fulfillment and assurance perspective.
3. Reality Check
The networking ecosystem has become awfully complex and highly
demanding in terms of robustness, performance, scalability,
flexibility, agility, etc. This means in particular that service
providers and network operators must deal with such complexity and
operate networking infrastructures that can evolve easily, remain
scalable, guarantee robustness and availability, and are resilient
against denial-of-service attacks.
The introduction of new SDN-based networking features should
obviously take into account this context, especially from a cost
impact assessment perspective.
Boucadair & Jacquenet Expires July 10, 2014 [Page 6]
Internet-Draft On SDN January 2014
3.1. Remember The Past
SDN techniques are not the next big thing per se, but rather a kind
of rebranding of proposals that have been investigated for several
years, like Active or Programmable Networks. As a matter of fact,
some of the claimed "new" SDN features have been already implemented
(e.g., NMS (Network Management System), PCE (Path Computation
Element, [RFC4655])), and supported by vendors for quite some time.
Some of these features have also been standardized (e.g., DNS-based
routing [RFC1383] that can be seen as an illustration of separated
control and forwarding planes or ForCES ([RFC5810][RFC5812])).
Also, the Policy-Based Management framework[RFC2753] introduced in
the early 2000's was designed to orchestrate available resources, by
means of a typical Policy Decision Point (PDP) which masters advanced
offline traffic engineering capabilities. As such, this framework
has the ability to interact with in-band software modules embedded in
controlled devices (or not).
The Policy Decision Point (PDP) is where policy decisions are made.
PDPs use a directory service for policy repository purposes. The
policy repository stores the policy information that can be retrieved
and updated by the PDP. The PDP delivers policy rules to the Policy
Enforcement Point (PEP) in the form of policy-provisioning
information that includes configuration information.
The Policy Enforcement Point (PEP) is where policy decisions are
applied. PEPs are embedded in (network) devices, which are
dynamically configured based upon the policy-formatted information
that has been processed by the PEP. PEPs request configuration from
the PDP, store the configuration information in the Policy
Information Base (PIB), and delegate any policy decision to the PDP.
SDN techniques as a whole are an instantiation of the policy-based
network management framework. Within this context, SDN techniques
can be used to activate capabilities on demand, to dynamically invoke
network and storage resources and to operate dynamically-adaptive
networks according to events (e.g., alteration of the network
topology) and triggers (e.g., dynamic notification of a link
failure), etc.
3.2. Be Pragmatic
SDN approaches should be holistic, i.e., global, network-wide. It is
not a matter of configuring devices one by one to enforce a specific
forwarding policy. SDN techniques are about configuring and
operating a whole range of devices at the scale of the network for
Boucadair & Jacquenet Expires July 10, 2014 [Page 7]
Internet-Draft On SDN January 2014
the sake of automated service delivery
([I-D.boucadair-network-automation-requirements]), from service
negotiation and creation (e.g., [I-D.ietf-idr-sla-exchange]) to
assurance and fulfillment.
Because the complexity of activating SDN capabilities is largely
hidden to the end-user and software-handled, a clear understanding of
the overall ecosystem is needed to figure out how to manage this
complexity and to what extent this hidden complexity does not have
side effects on network operation.
As an example, SDN designs that assume a central decision-making
entity must avoid single points of failure. They must not affect
packet forwarding performances either (e.g., transit delays must not
be impacted).
SDN techniques are not necessary to develop new network services per
se. The basic service remains (IP) connectivity that solicits
resources located in the network. SDN techniques can thus be seen as
another means to interact with network service modules and invoke
both connectivity and storage resources accordingly in order to meet
service-specific requirements.
By definition, SDN technique activation and operation remain limited
to what is supported by embedded software and hardware. One cannot
expect SDN techniques to support unlimited customizable features.
3.3. Measure Experience Against Expectations
Because several software modules may be controlled by external
entities (typically a PDP), there is a need for a means to make sure
that what has been delivered complies with what has been negotiated.
Such means belong to the set of SDN techniques.
These typical policy-based techniques should interact with both
Service Structuring engines (that are meant to expose the service
characteristics and to possibly negotiate those characteristics) and
the network to continuously assess whether the experienced network
behavior is compliant with the objectives set by the Service
Structuring engine, and which may have been dynamically negotiated
with the customer (e.g., as captured in a CPP
[I-D.boucadair-connectivity-provisioning-profile],
[I-D.boucadair-connectivity-provisioning-protocol]). This
requirement applies to several regions of a network, including:
1. At the interface between two adjacent IP network providers.
2. At the access interface between a service provider and an IP
network provider.
Boucadair & Jacquenet Expires July 10, 2014 [Page 8]
Internet-Draft On SDN January 2014
3. At the interface between a customer and the IP network provider.
Ideally, a fully automated service delivery procedure from
negotiation and ordering, through order processing, to delivery,
assurance and fulfillment, should be supported, at the cost of
implications that are discussed in Section 4.1. This approach also
assumes widely adopted standard data and information models, let
alone interfaces.
3.4. Design Carefully
Exposing open and programmable interfaces has a cost, from both a
scalability and performance standpoints.
Maintaining hard-coded performance optimization techniques is
encouraged. So is the use of interfaces that allow the direct
control of some engines (e.g., routing, forwarding) without requiring
any in-between adaptation layer (generic objects to vendor-specific
CLI commands for instance). Nevertheless, the use of vendor-specific
access means to some engines could be beneficial from a performance
standpoint, at the cost of increasing the complexity of configuration
tasks.
SDN techniques will have to accommodate vendor-specific components
anyway. Indeed, these vendor-specific features will not cease to
exist mainly because of the harsh competition.
The introduction of new functions or devices that may jeopardize
network flexibility should be avoided, or at least carefully
considered in light of possible performance and scalability impacts.
SDN-enabled devices will have to coexist with legacy systems.
One single SDN, network-wide deployment is therefore very unlikely.
Instead, multiple instantiations of SDN techniques will be
progressively deployed and adapted to various network and service
segments.
3.5. On OpenFlow
Empowering networking with in-band controllable modules may rely upon
the OpenFlow protocol, but also use other protocols to exchange
information between a control plane and a data plane.
There are indeed many other candidate protocols that can be used for
the same or even broader purpose (e.g., resource reservation
purposes). The forwarding of the configuration information can for
example rely upon protocols like PCEP [RFC5440], NETCONF [RFC6241],
Boucadair & Jacquenet Expires July 10, 2014 [Page 9]
Internet-Draft On SDN January 2014
COPS-PR [RFC3084], Routing Policy Specification Language (RPSL,
[RFC2622]), etc.
There is therefore no 1:1 relationship between OpenFlow and SDN.
Rather, OpenFlow is one of the candidate protocols to convey specific
configuration information towards devices. As such, OpenFlow is one
possible component of the global SDN toolkit.
3.6. Non Goals
There are inevitable trade-offs to be found between operating the
current networking ecosystem and introducing some SDN techniques,
possibly at the cost of introducing new technologies. Operators do
not have to choose between the two as both environments will have to
coexist.
In particular, the following considerations cannot justify the
deployment of SDN techniques:
o Fully flexible software implementations, because the claimed
flexibility remains limited by the own software and hardware
limitations, anyway.
o Fully modular implementations are difficult to achieve (because of
the implicit complexity) and may introduce extra effort for
testing, validation and troubleshooting.
o Fully centralized control systems that are likely to raise some
scalability issues. Distributed protocols and their ability to
react to some events (e.g., link failure) in a timely manner
remains a cornerstone of scalable networks. This means that SDN
designs can rely upon a logical representation of centralized
features (an abstraction layer that would support inter-PDP
communications, for example).
4. Discussion
4.1. Implications Of Full Automation
The path towards full automation is paved with numerous challenges
and requirements, including:
o Make sure automation is well implemented so as to facilitate
testing (including validation checks) and troubleshooting.
* This suggests the need for simulation tools that accurately
assess the impact of introducing a high level of automation in
the overall service delivery procedure, so as to avoid a
typical "mad robot" syndrome whose consequences can be serious,
from a control and QoS standpoints among others.
Boucadair & Jacquenet Expires July 10, 2014 [Page 10]
Internet-Draft On SDN January 2014
* This also suggests careful management of human expertise, so
that network operators can use robust, flexible means to
automate repetitive or error-prone tasks, and then build on
automation or stringing together multiple actions to create
increasingly complex tasks that require less human interaction
(guidance, input) to complete.
o Simplify and foster service delivery, assurance and fulfillment,
as well as network failure detection, diagnosis and root cause
analysis, for the sake of cost optimization:
* Such cost optimization relates to improved service delivery
times as well as optimized human expertise (see above) and
global, technology-agnostic, service structuring and delivery
procedures. In particular, the ability to inject new functions
in existing devices should not assume a replacement of the said
devices, but rather allow smart investment capitalization.
* This can be achieved thanks to automation, possibly based upon
a logically centralized view of the network infrastructure (or
a portion thereof), yielding the need for highly automated
topology, device and capabilities discovery means as well as
operational procedures.
* The main intelligence resides in the PDP, which suggests that
an important part of the SDN-related development effort should
focus on a detailed specification of the PDP function,
including algorithms and behavioral state machineries, based
upon a complete set of standardized data and information
models.
* These information models and data need to be carefully
structured for the sakes of efficiency and flexibility. This
probably suggests a set of simplified pseudo-blocks that can be
assembled as per the nature of the service to be delivered.
o Need for abstraction layers: clear interfaces between business
actors, between layers, let alone cross-layer considerations, etc.
* For the sake of various service structuring and packaging.
* Need for IP connectivity service exposure to customers, peers,
applications, content/service providers, etc. (e.g.,
[I-D.boucadair-connectivity-provisioning-profile]).
* Need for solutions that accommodate IP connectivity service
requirements with network engineering objectives.
* Need for dynamically-adaptive decision-making processes, which
can properly operate according to a set of input data and
metrics, such as current resource usage and demand, traffic
forecasts and matrices, etc., all for the sake of highly
responsive dynamic resource allocation and policy enforcement
schemes.
o Better accommodate technologically heterogeneous networking
environments:
Boucadair & Jacquenet Expires July 10, 2014 [Page 11]
Internet-Draft On SDN January 2014
* Need for vendor-independent configuration procedures, based
upon the enforcement of vendor-agnostic generic policies
instead of vendor-specific languages.
* Need for tools to aid manageability and orchestrate resources.
* Avoid proxies and privilege direct interaction with engines
(e.g., routing, forwarding).
4.2. Bootstrapping An SDN
Means to dynamically discover the functional capabilities of the
devices that will be steered by a PDP intelligence for the sake of
automated network service delivery need to be provided. This is
because the acquisition of the information related to what the
network is actually capable of will help structuring the PDP
intelligence so that policy provisioning information can be derived
accordingly.
A typical example would consist in documenting a traffic engineering
policy based upon the dynamic discovery of the various functions
supported by the network devices, as a function of the services to be
delivered, thus yielding the establishment of different routes
towards the same destination depending on the nature of the traffic,
the location of the functions that need to be invoked to forward such
traffic, etc.
Such dynamic discovery capability can rely upon the exchange of
specific information by means of an IGP or BGP between network
devices or between network devices and the PDP in legacy networking
environments. The PDP can also send unsolicited commands towards
network devices to acquire the description of their functional
capabilities in return and derive network and service topologies
accordingly.
Of course, SDN techniques (as introduced in Section 2.4) could be
deployed in an IGP/BGP-free networking environment, but the SDN
bootstrapping procedure in such environment still assumes the support
of the following capabilities:
o Dynamically discover SDN participating nodes (including the PDP)
and their respective capabilities in a resilient manner, assuming
the mutual authentication of the PDP and the participating devices
(Section 6). The integrity of the information exchanged between
the PDP and the participating devices during the discovery phase
must also be preserved,
o Dynamically connect the PDP to the participating nodes and avoid
any forwarding loop,
Boucadair & Jacquenet Expires July 10, 2014 [Page 12]
Internet-Draft On SDN January 2014
o Dynamically enable network services as a function of the device
capabilities and (possibly) what has been dynamically negotiated
between the customer and the service provider,
o Dynamically check connectivity between the PDP and the
participating nodes and between participating nodes for the
delivery of a given network service (or a set thereof)
o Dynamically assess the reachability scope as a function of the
service to be delivered,
o Dynamically detect and diagnose failures, and proceed with
corrective actions accordingly.
Likewise, means to dynamically acquire the descriptive information
(including the base configuration) of any network device that may
participate to the delivery of a given service should be provided so
as to help the PDP structure the services that can be delivered, as a
function of the available resources, their location, etc.
In IGP/BGP-free networking environments, a specific bootstrap
protocol may thus be required to support the aforementioned
capabilities for the sake of proper PDP and SDN-capable device
operation, let alone the possible need for a specific additional
network that would provide discovery and connectivity features.
In particular, SDN design and operation in IGP/BGP-free environments
should provide performances similar to those of legacy environments
that run an IGP and BGP: for example, the underlying network should
remain operational even if connection with the PDP has been lost.
Furthermore, operators should assess the cost of introducing a new,
specific bootstrap protocol compared to the cost of integrating the
aforementioned capabilities in existing IGP/BGP protocol machineries.
Since SDN-related features can be grafted into an existing network
infrastructure, they may not be all enabled at once from a
bootstrapping perspective: a gradual approach can be adopted instead.
A typical deployment example would be to use an SDN decision-making
process as an emulation platform that would help service providers
and operators in making appropriate technical choices before their
actual deployment in the network.
Finally, the completion of the discovery procedure does not
necessarily mean that the network is now fully operational.
Operationality of the network usually assumes a robust design, based
upon resilience and high availability features.
Boucadair & Jacquenet Expires July 10, 2014 [Page 13]
Internet-Draft On SDN January 2014
4.3. Operating An SDN
From an Operations And Management (OAM) standpoint [RFC6291], running
an SDN-capable network raises several issues such as those listed
below:
o How do SDN service and network management blocks interact? For
example, how the results of the dynamic negotiation of service
parameters with a customer or a set thereof over a given period of
time will affect the PDP decision-making process (resource
allocation, path computation, etc.)?
o What should be appropriate OAM tools for SDN network operation
(e.g., to check PDP or PEP reachability)?
o How performance (expressed in terms of service delivery time, for
example) can be optimized when the activation of software modules
is controlled by an external entity (typically a PDP)?
o To what extent an SDN implementation eases networks manageability,
including service and network diagnosis?
o Should the "control and data plane separation" principle be
applied to the whole network or a portion thereof, as a function
of the nature of the services to be delivered or taking into
account the technology that is currently deployed?
o What is the impact on the service provider's testing procedures
and methodologies (that are used during validation and pre-
deployment phases)? Particularly, (1) how test cases will be
defined and executed when the activation of customized modules is
supported? (2) what is the methodology to assess behavior of SDN-
controlled devices?, (3) how test regression will be conducted?,
etc.
o How do SDN techniques impact service fulfillment and assurance?
How the resulting behavior of SDN devices (completion of
configuration tasks, for example) should be assessed against what
has been dynamically negotiated with a customer? How to measure
the efficiency of dynamically-enforced policies as a function of
the service that has been delivered? How to measure that what has
been delivered is compliant with what has been negotiated? What
is the impact of SDN techniques on troubleshooting practice?
o Is there any risk to operate frozen architectures because of
potential interoperability issues between a controlled device and
a SDN controller?
o How does the introduction of SDN techniques affect the lifetime of
legacy systems? is there any risk of (rapidly) obsoleting existing
technologies because of their hardware or software limitations?
The answers to the above questions are very likely to be service
provider-specific, depending on their technological and business
environments.
Boucadair & Jacquenet Expires July 10, 2014 [Page 14]
Internet-Draft On SDN January 2014
4.4. The Intelligence Resides In The PDP
The proposed SDN definition in Section 2.3 assumes an intelligence
that may reside in the control or the management planes (or both).
This intelligence is typically represented by a Policy Decision Point
(PDP), which is one of the key functional components of the Policy-
Based Management framework [RFC2753].
SDN networking therefore relies upon PDP functions that are capable
of processing various input data (traffic forecasts, outcomes of
negotiation between customers and service providers, resource status
(as depicted in appropriate information models instantiated in the
PIB, etc.) to make appropriate decisions.
The design and the operation of such PDP-based intelligence in a
scalable manner remains of the major areas that needs to be
investigated.
To avoid centralized design schemes, inter-PDP communication is
likely to be required, and corresponding issues and solutions should
be considered. Several PDP instances may thus be activated in a
given domain. Because each of these PDP instances may be responsible
for making decisions about the enforcement of a specific policy
(e.g., one PDP for QoS policy enforcement purposes, another one for
security policy enforcement purposes, etc.), an inter-PDP
communication scheme is required for the sake of global PDP
coordination and correlation.
Inter-domain PDP exchanges may also be needed for specific usages.
Examples of such exchanges are: (1) During the network attachment
phase of a node to a visited network, the PDP operated by the visited
network can contact the home PDP to retrieve the policies to be
enforced for that node. (2) Various PDPs can collaborate together in
order to compute inter-domain paths which satisfy a set of traffic
performance guarantees.
4.5. Simplicity And Adaptability Vs. Complexity
The meta functional domains introduced in Section 2.4 assume the
introduction of a high level of automation, from service negotiation
to delivery and operation. Automation is the key to simplicity, but
must not be seen as a magic button that would be hit by a network
administrator whenever a customer request has to be processed or
additional resources need to be allocated.
The need for simplicity and adaptability thanks to automated
procedures generally assumes some complexity that lies beneath
automation.
Boucadair & Jacquenet Expires July 10, 2014 [Page 15]
Internet-Draft On SDN January 2014
4.6. Performance And Scalability
The combination of flexibility with software inevitably raises
performance and scalability issues as a function of the number and
the nature of the services to be delivered and their associated
dynamics.
For example: Networks deployed in Data Centers (DC) and which rely
upon OpenFlow switches are unlikely to raise important FIB
scalability issues. Conversely, DC interconnect designs that aim at
dynamically managing Virtual Machine (VM) mobility, possibly based
upon the dynamic enforcement of specific QoS policies, may raise
scalability issues.
The claimed flexibility of SDN networking in the latter context will
have to be carefully investigated by operators.
4.7. Risk Assessment
Various risks are to be assessed such as:
o Evaluating the risk of depending on a controller technology rather
than a device technology.
o Evaluating the risk of operating frozen architectures because of
potential interoperability issues between a controller and a
controlled device.
o Assessing whether SDN-labeled solutions are likely to obsolete
existing technologies because of hardware limitations: from a
technical standpoint, the ability to dynamically provision
resources as a function of the services to be delivered may be
incompatible with legacy routing systems because of their hardware
limitations, for example. Likewise, from an economical
standpoint, the use of SDN solutions for the sakes of flexibility
and automation may dramatically impact CAPEX (Capital Expenditure)
and OPEX (Operational Expenditure) budgets.
o Etc.
5. IANA Considerations
This document does not require any action from IANA.
6. Security Considerations
Security is an important aspect of any SDN design, because it
conditions the robustness and reliability of the interactions between
network and applications people, for the sake of efficient access
control procedures and optimized protection of SDN resources against
any kind of attack. In particular, SDN security policies should make
Boucadair & Jacquenet Expires July 10, 2014 [Page 16]
Internet-Draft On SDN January 2014
sure that SDN resources are properly safeguarded against actions that
may jeopardize network or application operations
[I-D.hartman-sdnsec-requirements].
In particular, service providers should define procedures to assess
the reliability of software modules embedded in SDN nodes. Such
procedures should include means to also assess the behavior of
software components (under stress conditions), detect any exploitable
vulnerability, reliably proceed with software upgrades, etc. These
security guards should be activated during initial SDN node
deployment and activation, but also during SDN operation that implies
software upgrade procedures.
Although these procedures may not be SDN-specific (e.g., operators
are familiar with firmware updates with or without service
disruption), it is worth challenging existing practice in light of
SDN deployment and operation.
Likewise, PEP-PDP interactions suggest the need to make sure that (1)
a PDP is entitled to solicit PEPs so that they can apply the
decisions made by the said PDP, (2) a PEP is entitled to solicit a
PDP for whatever reason (request for additional configuration
information, notification about the results of a set of configuration
tasks, etc.), (3) a PEP can accept decisions made by a PDP and (4)
communication between PDPs within a domain or between domains is
properly secured (e.g., make sure a pair of PDPs are entitled to
communicate with each other, make sure the confidentiality of the
information exchanged between two PDPs can be preserved, etc.).
7. Acknowledgements
Many thanks to R. Barnes, S. Bryant, S. Dawkins, A. Farrel, S.
Farrell, W. George, J. Halpern, D. King, J. Hadi Salim, and T. Tsou
for their comments. Special thanks to P. Georgatos for the fruitful
discussions on SDNI (SDN Interconnection) in particular.
8. Informative References
[I-D.boucadair-connectivity-provisioning-profile]
Boucadair, M., Jacquenet, C., and N. Wang, "IP/MPLS
Connectivity Provisioning Profile", draft-boucadair-
connectivity-provisioning-profile-02 (work in progress),
September 2012.
Boucadair & Jacquenet Expires July 10, 2014 [Page 17]
Internet-Draft On SDN January 2014
[I-D.boucadair-connectivity-provisioning-protocol]
Boucadair, M. and C. Jacquenet, "Connectivity Provisioning
Negotiation Protocol (CPNP)", draft-boucadair-
connectivity-provisioning-protocol-01 (work in progress),
October 2013.
[I-D.boucadair-network-automation-requirements]
Boucadair, M. and C. Jacquenet, "Requirements for
Automated (Configuration) Management", draft-boucadair-
network-automation-requirements-01 (work in progress),
June 2013.
[I-D.hartman-sdnsec-requirements]
Hartman, S. and D. Zhang, "Security Requirements in the
Software Defined Networking Model", draft-hartman-sdnsec-
requirements-01 (work in progress), April 2013.
[I-D.ietf-idr-ls-distribution]
Gredler, H., Medved, J., Previdi, S., Farrel, A., and S.
Ray, "North-Bound Distribution of Link-State and TE
Information using BGP", draft-ietf-idr-ls-distribution-04
(work in progress), November 2013.
[I-D.ietf-idr-sla-exchange]
Shah, S., Patel, K., Bajaj, S., Tomotaki, L., and M.
Boucadair, "Inter-domain SLA Exchange", draft-ietf-idr-
sla-exchange-02 (work in progress), November 2013.
[RFC1383] Huitema, C., "An Experiment in DNS Based IP Routing", RFC
1383, December 1992.
[RFC2622] Alaettinoglu, C., Villamizar, C., Gerich, E., Kessens, D.,
Meyer, D., Bates, T., Karrenberg, D., and M. Terpstra,
"Routing Policy Specification Language (RPSL)", RFC 2622,
June 1999.
[RFC2753] Yavatkar, R., Pendarakis, D., and R. Guerin, "A Framework
for Policy-based Admission Control", RFC 2753, January
2000.
[RFC3084] Chan, K., Seligson, J., Durham, D., Gai, S., McCloghrie,
K., Herzog, S., Reichmeyer, F., Yavatkar, R., and A.
Smith, "COPS Usage for Policy Provisioning (COPS-PR)", RFC
3084, March 2001.
[RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655, August 2006.
Boucadair & Jacquenet Expires July 10, 2014 [Page 18]
Internet-Draft On SDN January 2014
[RFC5440] Vasseur, JP. and JL. Le Roux, "Path Computation Element
(PCE) Communication Protocol (PCEP)", RFC 5440, March
2009.
[RFC5810] Doria, A., Hadi Salim, J., Haas, R., Khosravi, H., Wang,
W., Dong, L., Gopal, R., and J. Halpern, "Forwarding and
Control Element Separation (ForCES) Protocol
Specification", RFC 5810, March 2010.
[RFC5812] Halpern, J. and J. Hadi Salim, "Forwarding and Control
Element Separation (ForCES) Forwarding Element Model", RFC
5812, March 2010.
[RFC6241] Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
Bierman, "Network Configuration Protocol (NETCONF)", RFC
6241, June 2011.
[RFC6291] Andersson, L., van Helvoort, H., Bonica, R., Romascanu,
D., and S. Mansfield, "Guidelines for the Use of the "OAM"
Acronym in the IETF", BCP 161, RFC 6291, June 2011.
Authors' Addresses
Mohamed Boucadair
France Telecom
Rennes 35000
France
Email: mohamed.boucadair@orange.com
Christian Jacquenet
France Telecom
Rennes
France
Email: christian.jacquenet@orange.com
Boucadair & Jacquenet Expires July 10, 2014 [Page 19]
