Internet DRAFT - draft-quinn-sfc-problem-statement
draft-quinn-sfc-problem-statement
Network Working Group P. Quinn, Ed.
Internet-Draft Cisco Systems, Inc.
Intended status: Informational T. Nadeau, Ed.
Expires: June 12, 2014 Juniper Networks
December 9, 2013
Service Function Chaining Problem Statement
draft-quinn-sfc-problem-statement-02.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 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
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This Internet-Draft will expire on June 12, 2014.
Copyright Notice
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Definition of Terms . . . . . . . . . . . . . . . . . . . 3
2. Problem Space . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Service Function Chaining . . . . . . . . . . . . . . . . . . 8
4. Service Function Chaining Use Cases . . . . . . . . . . . . . 10
4.1. Enterprise Data Center Service Chaining . . . . . . . . . 10
4.2. 3GPP Gi Interface Service Function Chaining . . . . . . . 10
5. Related IETF Work . . . . . . . . . . . . . . . . . . . . . . 12
6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 15
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
10. Informative References . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
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1. Introduction
The delivery of end-to-end services often require various Service
Functions (SF) 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, policy integration with service chains, and
policy enforcement within the network infrastructure.
1.1. Definition of Terms
Classification: Locally instantiated customer/network/service policy
used to identify and select traffic flow(s) requiring appropriate
outbound forwarding actions.
Network Overlay: A logical network built, via virtual links or
packet encapsulation, over an existing network (the underlay).
Service Function Chain: A service chain defines an ordered set of
service functions that must be applied to packets and/or frames
selected as a result of classification
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,
WAN and application acceleration, Deep Packet Inspection (DPI),
server load balancers, NAT44 [RFC3022], NAT64 [RFC6146], HOST_ID
injection [RFC6967], HTTP Header Enrichment functions, TCP
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optimizer, etc.
The generic term "L4-L7 services" is often used to describe many
service functions.
Service Node: Physical or virtual element offering one or more
Service Functions.
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.
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2. Problem Space
The following points describe aspects of existing service deployments
that are problematic, and are being addressed by the Service Function
Chaining effort.
1. Topological Dependencies: Network service deployments are often
coupled to network topology. 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.
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. 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 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.
The service itself can contribute to the complexity: it may
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require an intricate combination of very different capabilities,
regardless of the underlying topology. QoS-based, resilient VPN
service offerings are a typical example of such complex service
offerings.
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.
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 functions that are used to
deliver a given service.
5. Service Chain Construction: 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/management planes provide
not only connectivity state, but will also be increasingly
utilized for the creation of network services. Such a control/
management planes could be centralized, or be distributed.
6. 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.
7. 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 transports or for a
transport gateway function to be present.
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8. Elastic Service Delivery: Given 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.
9. 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.
10. 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.
11. 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.
12. Symmetric Traffic Flows: Service function chains may be
unidirectional or bidirectional; unidirectional is one where
traffic is passed through a set of service functions in one
forwarding direction only. Bidirectional is one where traffic
is passed through a set of service functions in both forwarding
directions. 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 forwarding path.
13. 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, SFC will investigate
solutions that address the following elements:
1. Service Overlay: Service function chaining utilizes a service
specific overlay that creates the service topology. The service
overlay is independent of the network topology and allows
operators to use whatever overlay or underlay they prefer to
create a path between Service nodes, 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.
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.
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 is
used to start the service function chain. Subsequent
classification can be used within a given service function chain
to alter the sequence of service functions applied. Symmetric
classification ensures that forward and reverse chains are in
place.
4. Dataplane Metadata: Data plane metadata provides the ability to
exchange information between the network and service functions,
between service functions, and service functions and the network.
Metadata can include the result of antecedent classification,
information from external sources or forwarding related data.
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For example, 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. Service Function Chaining Use Cases
The following sections provide high level overviews of several common
service chaining deployments.
4.1. Enterprise Data Center Service Chaining
TBD
4.2. 3GPP Gi Interface Service Function Chaining
3GPP defines the Gi interface as the reference point between the GGSN
(Gateway GPRS Support Node) and an external PDN (Packet Domain
Network) [RFC6459]. This interface reference point is called SGi in
4G networks (i.e., between the PDN Gateway and an external PDN)
[RFC6459]. Note, there is no standard specification of such
reference points (i.e., Gi and SGi) in terms of functions to be
located in that segment.
In light of current deployments, plenty of Service Functions are
enabled in the Gi Interface (e.g., DPI, billing and charging, TCP
optimization, web optimization, video optimization, header
enrichment, etc.). Some of these Service Functions are co-located on
the same device while others are enabled in standalone boxes. In
order to fulfill new business needs, especially to offer innovative
added-value services, the number of enabled Service Functions in the
Gi Interface is still growing.
Several (S)Gi Interfaces can be deployed within the same PLMN (Public
Land Mobile Network). This depends mainly on the number of PDNs and
other factors. Each of these interfaces may involve a differentiated
set of Service Functions to be involved.
The current model that consists of adding new "boxes" to fulfill new
business guidelines has shown its limit. Concretely, current
deployments suffer from the following problems:
o Complexity to introduce new Service Functions because of the
constraint on the underlying topology.
o Lack of visibility on dependency between Service Functions.
o Lack of automated and flexible means to assess the impact of
withdrawing a Service Function or a feature offered by a Service
Function from the traffic forwarding path.
o The connectivity service logic may be stalled because of the
dependency on the physical topology.
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o Lack of deterministic means to:
* Improve service provisioning and delivery.
* Ease the manageability of the SGi/Gi Interface.
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5. 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 service chaining 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.
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6. Summary
This document highlights problems associated with network service
deployment today and identifies several key areas that will be
addressed by the service chaining working group. Furthermore, this
document identifies four components that are the basis for serice
chaining. These components will form the areas of focus for the
working group.
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7. 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 service chaining working group.
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8. Contributors
The following people are active contributors to this document and
have provided review, content and concepts (listed alphabetically by
surname):
Puneet Agarwal (pagarwal@broadcom.com), Mohamed Boucadair
(mohamed.boucadair@orange.com), Abhishek Chauhan
(Abhishek.Chauhan@citrix.com), Uri Elzur (uri.elzur@intel.com), Kevin
Glavin (Kevin.Glavin@riverbed.com), Ken Gray (kgray@juniper.net), Jim
Guichard (jguichar@cisco.com), Christian Jacquenet
(christian.jacquenet@orange.com), Surendra Kumar (smkumar@cisco.com),
Nic Leymann (n.leymann@telekom.de), Darrel Lewis
(darlewis@cisco.com), Rajeev Manur (rmanur@broadcom.com), Brad
McConnell (bmcconne@rackspace.com), Carlos Pignataro
(cpignata@cisco.com), Michael Smith (michsmit@insiemenetworks.com),
Navindra Yadav (nyadav@insiemenetworks.com).
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9. Acknowledgments
The authors would like to thank David Ward, Rex Fernando and Jim
French for their contributions.
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10. Informative References
[ALTO] "Application-Layer Traffic Optimization (alto)",
<http://datatracker.ietf.org/wg/alto/>.
[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)
Juniper Networks
Email: tnadeau@juniper.net
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