Network Working Group P. Quinn, Ed.
Internet-Draft Cisco Systems, Inc.
Intended status: Informational T. Nadeau, Ed.
Expires: October 19, 2014 Brocade
April 17, 2014
Service Function Chaining Problem Statement
draft-ietf-sfc-problem-statement-05.txt
Abstract
This document provides an overview of the issues associated with the
deployment of service functions (such as firewalls, load balancers)
in large-scale environments. The term service function chaining is
used to describe the definition and instantiation of an ordered set
of instances of such service functions, and the subsequent "steering"
of traffic flows through those service functions.
The set of enabled service function chains reflect operator service
offerings and is designed in conjunction with application delivery
and service and network policy.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 19, 2014.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Definition of Terms . . . . . . . . . . . . . . . . . . . 3
2. Problem Space . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Topological Dependencies . . . . . . . . . . . . . . . . . 5
2.2. Configuration complexity . . . . . . . . . . . . . . . . . 5
2.3. Constrained High Availability . . . . . . . . . . . . . . 6
2.4. Consistent Ordering of Service Functions . . . . . . . . . 6
2.5. Application of Service Policy . . . . . . . . . . . . . . 6
2.6. Transport Dependence . . . . . . . . . . . . . . . . . . . 7
2.7. Elastic Service Delivery . . . . . . . . . . . . . . . . . 7
2.8. Traffic Selection Criteria . . . . . . . . . . . . . . . . 7
2.9. Limited End-to-End Service Visibility . . . . . . . . . . 7
2.10. Per-Service (re)Classification . . . . . . . . . . . . . . 7
2.11. Symmetric Traffic Flows . . . . . . . . . . . . . . . . . 8
2.12. Multi-vendor Service Functions . . . . . . . . . . . . . . 8
3. Service Function Chaining . . . . . . . . . . . . . . . . . . 9
3.1. Service Overlay . . . . . . . . . . . . . . . . . . . . . 9
3.2. Control Plane . . . . . . . . . . . . . . . . . . . . . . 9
3.3. Service Classification . . . . . . . . . . . . . . . . . . 9
3.4. Dataplane Metadata . . . . . . . . . . . . . . . . . . . . 10
4. Related IETF Work . . . . . . . . . . . . . . . . . . . . . . 11
5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 14
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
9. Informative References . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
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1. Introduction
The delivery of end-to-end services often require various service
functions including traditional network service functions (for
example firewalls and server load balancers), as well as application-
specific features. Service functions may be delivered within the
context of an isolated user group, or shared amongst many users/user
groups.
Current service function deployment models are relatively static in
that they are tightly coupled to network topology and physical
resources. The result of that static nature of existing deployments
greatly reduces, and in many cases, limits the ability of an operator
to introduce new services and/or service functions. Furthermore
there is a cascading effect: service changes affect other services.
This document outlines the problems encountered with existing service
deployment models for Service Function Chaining (SFC) (often referred
to simply as service chaining; in this document the terms will be
used interchangeably), as well as the problems of service chain
creation, deletion, modification/update, policy integration with
service chains, and policy enforcement within the network
infrastructure.
1.1. Definition of Terms
Classification: Locally instantiated policy that results in matching
of traffic flows for identification of appropriate outbound
forwarding actions.
Network Overlay: A logical network built, via virtual links or
packet encapsulation, over an existing network (the underlay).
Network Service: An externally visible service offered by a network
operator; a service may consist of a single service function or a
composite built from several service functions executed in one or
more pre-determined sequences and delivered by one or more service
nodes.
Service Function: A function that is responsible for specific
treatment of received packets. A Service Function can act at the
network layer or other OSI layers. A Service Function can be a
virtual instance or be embedded in a physical network element.
One of multiple Service Functions can be embedded in the same
network element. Multiple instances of the Service Function can
be enabled in the same administrative domain.
A non-exhaustive list of Service Functions includes: firewalls,
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WAN and application acceleration, Deep Packet Inspection (DPI),
server load balancers, NAT44 [RFC3022], NAT64 [RFC6146], HOST_ID
injection [RFC6967], HTTP Header Enrichment functions, TCP
optimizer, etc.
The generic term "L4-L7 services" is often used to describe many
service functions.
Service Function Chain (SFC): A service Function chain defines an
ordered set of service functions that must be applied to packets
and/or layer-2 frames selected as a result of classification. The
implied order may not be a linear progression as nodes may copy to
more than one branch. The term service chain is often used as
shorthand for service function chain.
Service Function Path (SFP): The instantiation of a service function
chain in the network. Packets follow a service function path from
a classifier through the required instances of service functions
in the network.
Service Node (SN): Physical or virtual element that hosts one or
more service functions.
Service Overlay: An overlay network created for the purpose of
forwarding data along a service function path.
Service Topology: The service overlay connectivity forms a service
topology.
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2. Problem Space
The following points describe aspects of existing service deployments
that are problematic, and that the Service Function Chaining (SFC)
working group aims to address.
2.1. Topological Dependencies
Network service deployments are often coupled to network topology,
whether it be real or virtualized, or a hybrid of the two. Such
dependency imposes constraints on the service delivery, potentially
inhibiting the network operator from optimally utilizing service
resources, and reduces the flexibility. This limits scale, capacity,
and redundancy across network resources.
These topologies serve only to "insert" the service function (i.e.,
ensure that traffic traverses a service function); they are not
required from a native packet delivery perspective. For example,
firewalls often require an "in" and "out" layer-2 segment and adding
a new firewall requires changing the topology (i.e., adding new
layer-2 segments).
As more service functions are required - often with strict ordering -
topology changes are needed before and after each service function
resulting in complex network changes and device configuration. In
such topologies, all traffic, whether a service function needs to be
applied or not, often passes through the same strict order.
The topological coupling limits placement and selection of service
functions: service functions are "fixed" in place by topology and
therefore placement and service function selection taking into
account network topology information is not viable. Furthermore,
altering the services traversed, or their order, based on flow
direction is not possible.
A common example is web servers using a server load balancer as the
default gateway. When the web service responds to non-load balanced
traffic (e.g., administrative or backup operations) all traffic from
the server must traverse the load balancer forcing network
administrators to create complex routing schemes or create additional
interfaces to provide an alternate topology.
2.2. Configuration complexity
A direct consequence of topological dependencies is the complexity of
the entire configuration, specifically in deploying service function
chains. Simple actions such as changing the order of the service
functions in a service function chain require changes to the
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topology. Changes to the topology are avoided by the network
operator once installed, configured and deployed in production
environments fearing misconfiguration and downtime. All of this
leads to very static service delivery deployments. Furthermore, the
speed at which these topological changes can be made is not rapid or
dynamic enough as it often requires manual intervention, or use of
slow provisioning systems.
2.3. Constrained High Availability
An effect of topological dependency is constrained service function
high availability. Worse, when modified, inadvertent non-high
availability or downtime can result.
Since traffic reaches many service functions based on network
topology, alternate, or redundant service functions must be placed in
the same topology as the primary service.
2.4. Consistent Ordering of Service Functions
Service functions are typically independent; service function_1
(SF1)...service function_n (SFn) are unrelated and there is no notion
at the service layer that SF1 occurs before SF2. However, to an
administrator many service functions have a strict ordering that must
be in place, yet the administrator has no consistent way to impose
and verify the ordering of the service functions that are used to
deliver a given service.
Service function chains today are most typically built through manual
configuration processes. These are slow and error prone. With the
advent of newer service deployment models the control and policy
planes provide not only connectivity state, but will also be
increasingly utilized for the creation of network services. Such
control/management planes could be centralized, or be distributed.
2.5. Application of Service Policy
Service functions rely on topology information such as VLANs or
packet (re) classification to determine service policy selection,
i.e. the service function specific action taken. Topology
information is increasingly less viable due to scaling, tenancy and
complexity reasons. The topological information is often stale,
providing the operator with inaccurate placement that can result in
suboptimal resource utilization. Furthermore topology-centric
information often does not convey adequate information to the service
functions, forcing functions to individually perform more granular
classification.
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2.6. Transport Dependence
Service functions can and will be deployed in networks with a range
of transports, including under and overlays. The coupling of service
functions to topology requires service functions to support many
transport encapsulations or for a transport gateway function to be
present.
2.7. Elastic Service Delivery
Given that the current state of the art for adding/removing service
functions largely centers around VLANs and routing changes, rapid
changes to the service deployment can be hard to realize due to the
risk and complexity of such changes.
2.8. Traffic Selection Criteria
Traffic selection is coarse, that is, all traffic on a particular
segment traverse service functions whether the traffic requires
service enforcement or not. This lack of traffic selection is
largely due to the topological nature of service deployment since the
forwarding topology dictates how (and what) data traverses service
function(s). In some deployments, more granular traffic selection is
achieved using policy routing or access control filtering. This
results in operationally complex configurations and is still
relatively inflexible.
2.9. Limited End-to-End Service Visibility
Troubleshooting service related issues is a complex process that
involve both network-specific and service-specific expertise. This
is especially the case when service function chains span multiple
DCs, or across administrative boundaries. Furthermore, the physical
and virtual environments (network and service), can be highly
divergent in terms of topology and that topological variance adds to
these challenges.
2.10. Per-Service (re)Classification
Classification occurs at each service function independent from
previously applied service functions. More importantly, the
classification functionality often differs per service function and
service functions may not leverage the results from other service
functions.
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2.11. Symmetric Traffic Flows
Service function chains may be unidirectional or bidirectional
depending on the state requirements of the service functions. In a
unidirectional chain traffic is passed through a set of service
functions in one forwarding direction only. Bidirectional chains
require traffic to be passed through a set of service functions in
both forwarding directions. Many common service functions such as
DPI and firewall often require bidirectional chaining in order to
ensure flow state is consistent.
Existing service deployment models provide a static approach to
realizing forward and reverse service function chain association most
often requiring complex configuration of each network device
throughout the SFC.
2.12. Multi-vendor Service Functions
Deploying service functions from multiple vendors often require per-
vendor expertise: insertion models differ, there are limited common
attributes and inter- vendor service functions do not share
information.
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3. Service Function Chaining
Service Function Chaining aims to address the aforementioned problems
associated with service deployment. Concretely, the SFC working
group will investigate solutions that address the following elements:
3.1. Service Overlay
Service function chaining utilizes a service specific overlay that
creates the service topology. The service overlay provides service
function connectivity and is built "on top" of the existing network
topology and allows operators to use whatever overlay or underlay
they prefer to create a path between service functions, and to locate
service functions in the network as needed.
Within the service topology, service functions can be viewed as
resources for consumption and an arbitrary topology constructed to
connect those resources in a required order. Adding new service
functions to the topology is easily accomplished, and no underlying
network changes are required.
Lastly, the service overlay can provide service specific information
needed for troubleshooting service-related issues.
3.2. Control Plane
Service aware control plane(s) provide information about the
available service functions on a network. The information provided
by the control plane includes service network location (for topology
creation), service type (e.g. firewall, load balancer, etc.) and,
optionally, administrative information about the service functions
such as load, capacity and operating status. The service aware
control plane allows for the formulation of service function chains
and exchanges requisite information needed to instantiate the service
function chains in the network.
Furthermore, the service aware control plane may interact with the
topology aware control plane (if separate) to ensure optimal
selection (and possibly placement) of service functions within a
service function path.
3.3. Service Classification
Classification is used to select which traffic enters a service
overlay. The granularity of the classification varies based on
device capabilities, customer requirements, and service offered.
Initial classification determines the service function chain required
to process the traffic. Subsequent classification can be used within
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a given service function chain to alter the sequence of service
functions applied. Symmetric classification ensures that forward and
reverse chains are in place. Similarly, asymmetric -- relative to
required service function -- chains can be achieved via service
classification.
3.4. Dataplane Metadata
Data plane metadata provides the ability to exchange information
between logical classification points and service functions (and vice
versa) and between service functions. As such metadata is not used
as forwarding information to deliver packets along the service
overlay.
Metadata can include the result of antecedent classification and/or
information from external sources. Service functions utilize
metadata, as required, for localized policy decisions.
In addition to sharing of information, the use of metadata addresses
several of the issues raised in section 2, most notably the de-
coupling of policy from the topology, and the need for per-service
classification (and re-classification).
A common approach to service metadata creates a common foundation for
interoperability between service functions, regardless of vendor.
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4. Related IETF Work
The following subsections discuss related IETF work and are provided
for reference. This section is not exhaustive, rather it provides an
overview of the various initiatives and how they relate to network
service chaining.
1. [L3VPN]: The L3VPN working group is responsible for defining,
specifying and extending BGP/MPLS IP VPNs solutions. Although
BGP/MPLS IP VPNs can be used as transport for service chaining
deployments, the SFC WG focuses on the service specific
protocols, not the general case of VPNs. Furthermore, BGP/MPLS
IP VPNs do not address the requirements for service chaining.
2. [LISP]: LISP provides locator and ID separation. LISP can be
used as an L3 overlay to transport service chaining data but does
not address the specific service chaining problems highlighted in
this document.
3. [NVO3]: The NVO3 working group is chartered with creation of
problem statement and requirements documents for multi-tenant
network overlays. NVO3 WG does not address service chaining
protocols.
4. [ALTO]: The Application Layer Traffic Optimization Working Group
is chartered to provide topological information at a higher
abstraction layer, which can be based upon network policy, and
with application-relevant service functions located in it. The
mechanism for ALTO obtaining the topology can vary and policy can
apply to what is provided or abstracted. This work could be
leveraged and extended to address the need for services
discovery.
5. [I2RS]: The Interface to the Routing System Working Group is
chartered to investigate the rapid programming of a device's
routing system, as well as the service of a generalized, multi-
layered network topology. This work could be leveraged and
extended to address some of the needs for service chaining in the
topology and device programming areas.
6. [ForCES]: The ForCES working group has created a framework,
requirements, a solution protocol, a logical function block
library, and other associated documents in support of Forwarding
and Control Element Separation. The work done by ForCES may
provide a basis for both the separation of SFC elements, as well
as provide protocol and design guidance for those elements.
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5. Summary
This document highlights problems associated with network service
deployment today and identifies several key areas that will be
addressed by the SFC working group. Furthermore, this document
identifies four components that are the basis for service function
chaining. These components will form the areas of focus for the
working group.
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6. Security Considerations
Security considerations are not addressed in this problem statement
only document. Given the scope of service chaining, and the
implications on data and control planes, security considerations are
clearly important and will be addressed in the specific protocol and
deployment documents created by the SFC WG.
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7. Contributors
The following people are active contributors to this document and
have provided review, content and concepts (listed alphabetically by
surname):
Puneet Agarwal
Broadcom
Email: pagarwal@broadcom.com
Mohamed Boucadair
France Telecom
Email: mohamed.boucadair@orange.com
Abhishek Chauhan
Citrix
Email: Abhishek.Chauhan@citrix.com
Uri Elzur
Intel
Email: uri.elzur@intel.com
Kevin Glavin
Riverbed
Email: Kevin.Glavin@riverbed.com
Ken Gray
Cisco Systems
Email: kegray@cisco.com
Jim Guichard
Cisco Systems
Email:jguichar@cisco.com
Christian Jacquenet
France Telecom
Email: christian.jacquenet@orange.com
Surendra Kumar
Cisco Systems
Email: smkumar@cisco.com
Nic Leymann
Deutsche Telekom
Email: n.leymann@telekom.de
Darrel Lewis
Cisco Systems
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Email: darlewis@cisco.com
Rajeev Manur
Broadcom
Email:rmanur@broadcom.com
Brad McConnell
Rackspace
Email: bmcconne@rackspace.com
Carlos Pignataro
Cisco Systems
Email: cpignata@cisco.com
Michael Smith
Cisco Systems
Email: michsmit@cisco.com
Navindra Yadav
Cisco Systems
Email: nyadav@cisco.com
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8. Acknowledgments
The authors would like to thank David Ward, Rex Fernando, David
Mcdysan, Jamal Hadi Salim, Charles Perkins, Andre Beliveau, Joel
Halpern and Jim French for their reviews and comments.
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9. Informative References
[ALTO] "Application-Layer Traffic Optimization (alto)",
<http://datatracker.ietf.org/wg/alto/>.
[ForCES] "Forwarding and Control Element Separation (forces)",
<http://datatracker.ietf.org/wg/forces/>.
[I2RS] "Interface to the Routing System (i2rs)",
<http://datatracker.ietf.org/wg/i2rs/>.
[L3VPN] "Layer 3 Virtual Private Networks (l3vpn)",
<http://datatracker.ietf.org/wg/l3vpn/>.
[LISP] "Locator/ID Separation Protocol (lisp)",
<http://datatracker.ietf.org/wg/lisp/>.
[NVO3] "Network Virtualization Overlays (nvo3)",
<http://datatracker.ietf.org/wg/nvo3/>.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
January 2001.
[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, April 2011.
[RFC6967] Boucadair, M., Touch, J., Levis, P., and R. Penno,
"Analysis of Potential Solutions for Revealing a Host
Identifier (HOST_ID) in Shared Address Deployments",
RFC 6967, June 2013.
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Authors' Addresses
Paul Quinn (editor)
Cisco Systems, Inc.
Email: paulq@cisco.com
Thomas Nadeau (editor)
Brocade
Email: tnadeau@lucidvision.com
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