Network Working Group P. Quinn, Ed.
Internet-Draft Cisco Systems, Inc.
Intended status: Informational A. Beliveau, Ed.
Expires: August 1, 2014 Ericsson
January 28, 2014
Service Function Chaining (SFC) Architecture
draft-quinn-sfc-arch-04.txt
Abstract
This document describes an architecture used for the creation of
Service Function Chains (SFC). It includes architectural concepts,
principles, and components used in the construction of composite
services through deployment of SFCs in a network. This document does
not propose solutions, protocols, or extensions to existing
protocols.
Status of this Memo
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This Internet-Draft will expire on August 1, 2014.
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Definition of Terms . . . . . . . . . . . . . . . . . . . 3
1.3. Service Function Chaining . . . . . . . . . . . . . . . . 4
2. Architectural Concepts . . . . . . . . . . . . . . . . . . . . 6
2.1. Service Function Chains . . . . . . . . . . . . . . . . . 6
2.2. Service Function Chain Symmetry . . . . . . . . . . . . . 7
2.3. Service Function Paths . . . . . . . . . . . . . . . . . . 7
3. Service Function Chaining Architecture . . . . . . . . . . . . 8
3.1. Architecture Principles . . . . . . . . . . . . . . . . . 8
3.2. Fundamental Components . . . . . . . . . . . . . . . . . . 8
4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5. Security Considerations . . . . . . . . . . . . . . . . . . . 13
6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 14
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Normative References . . . . . . . . . . . . . . . . . . . 18
9.2. Informative References . . . . . . . . . . . . . . . . . . 18
Appendix A. Existing Service Deployments . . . . . . . . . . . . 19
Appendix B. Issues with Existing Deployments . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
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1. Introduction
This document describes an architecture used for the creation of
Service Function Chains (SFC). It includes architectural concepts,
principles, and components to provide SFCs in a network.
1.1. Scope
The architecture described herein is assumed to be applicable to a
single network administrative domain. While it is possible for the
principles and architectural components to be applied to inter-domain
SFCs, these are left for future study.
1.2. Definition of Terms
Classification: Locally instantiated policy and customer/network/
service profile matching of traffic flows for identification of
appropriate outbound forwarding actions.
SFC Network Forwarder: SFC network forwarders provide network
connectivity for service functions forwarders and service
functions. SFC network forwarders participate in the network
overlay used for service function chaining as well as in the SFC
encapsulation.
Service Function Forwarder (SFF): A service function forwarder is
responsible for delivering traffic received from the SFC network
forwarder to one or more connected service functions.
Service Function (SF): 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, HTTP Header Enrichment functions, TCP optimizer, etc.
Service Function Identity (SFID): A unique identifier that
represents a service function. SFIDs are unique for each SF
within an SFC domain.
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Service: An offering provided by an operator that is delivered using
one or more service functions. This may also be referred to as a
composite service.
Note: The term "service" is overloaded with varying definitions.
For example, to some a service is an offering composed of several
elements within the operators network whereas for others a
service, or more specifically a network service, is a discrete
element such as a firewall. Traditionally, these network services
host a set of service functions and have a network locator where
the service is hosted.
Service Node (SN): Physical or virtual element that hosts one or
more service functions and has one or more network locators
associated with it for reachability and service delivery.
Service Function Chain (SFC): A service Function chain defines an
ordered set of service functions that must be applied to packets
and/or frames selected as a result of classification. The implied
order may not be a linear progression as the architecture allows
for nodes that 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 SFC in the
network. Packets follow a service function path from a classifier
through the requisite service functions
1.3. Service Function Chaining
Service chaining enables creation of composite services that consist
of an ordered set of Service Functions (SF) that must be applied to
packets and/or frames selected as a result of classification. Each
SF is referenced using an identifier (SFID) that is unique within an
administrative domain. No IANA registry is required to store the
identity of SFs.
Service Function Chaining is a concept that provides for more than
just the application of an ordered set of SFs to selected traffic;
rather, it describes a method for deploying SFs in a way that enables
ordering and topological independence of those SFs.
A basic SFC may utilize an existing overlay technology alongside
service specific forwarding in the network to steer traffic through
the ordered set of SFs. However, additional information that is
shared across a subset of SFs within an SFC may enable value-added
services with a richer set of functionality. For example, shared
information, such as the results of a classification function, may be
passed to downstream SFs to enable the offloading of service function
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processing. As another example, sharing of information derived at
one SF with the rest of the SFs in the SFC would obviate the need to
re-derive the same information and simplify the service.
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2. Architectural Concepts
The following sections describe the foundational concepts of service
function chaining and the SFC architecture.
2.1. Service Function Chains
In most networks services are constructed as a sequence of SFs that
represent an SFC. As previously stated a SF can be a virtual
instance or be embedded in a physical network element, and one or
more SFs may be deployed within the same physical network element.
At a high level, an SFC creates an abstracted view of a service and
specifies the set of required SFs as well as the order in which they
must be executed. Graphs, as illustrated in Figure 1, define each
SFC. SFs can be part of zero, one, or many SFCs. A given SF can
appear one time or multiple times in a given SFC.
SFCs can start from the origination point of the service function
graph (i.e.: node 1 in Figure 1), or from any subsequent SF node in
the graph. SFs may therefore become branching nodes in the graph,
with those SFs selecting edges that move traffic to one or more
branches. SFCs can have more than one terminus.
,-+-. ,---. ,---. ,---.
/ \ / \ / \ / \
( 1 )+--->( 2 )+---->( 6 )+---->( 8 )
\ / \ / \ / \ /
`---' `---' `---' `---'
,-+-. ,---. ,---. ,---. ,---.
/ \ / \ / \ / \ / \
( 1 )+--->( 2 )+---->( 3 )+---->( 7 )+---->( 9 )
\ / \ / \ / \ / \ /
`---' `---' `---' `---' `---'
,-+-. ,---. ,---. ,---. ,---.
/ \ / \ / \ / \ / \
( 1 )+--->( 7 )+---->( 8 )+---->( 4 )+---->( 7 )
\ / \ / \ / \ / \ /
`---' `---' `---' `---' `---'
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Figure 1: Service Function Chain Graphs
The architecture allows for two or more SFs to be co-resident on the
same service node. In these cases, some implementations may choose
to use some form of internal inter-process or inter-VM messaging
(communication behind the virtual switching element) that is
optimized for such an environment. Implementation details of such
mechanisms are considered out-of-scope for this document.
2.2. Service Function Chain Symmetry
SFCs may be unidirectional or bidirectional. A unidirectional SFC
requires that traffic be forwarded through the ordered SFs in one
direction (SF1 -> SF2 -> SF3), whereas a bidirectional SFC requires a
symmetric path (SF1 -> SF2 -> SF3 and SF3 -> SF2 -> SF1). A hybrid
SFC has attributes of both unidirectional and bidirectional SFCs;
that is to say some SFs require symmetric traffic, whereas other SFs
do not process reverse traffic.
SFCs may contain cycles; that is traffic may need to traverse more
than once one or more SFs within an SFC.
2.3. Service Function Paths
When an SFC is instantiated into the network it is necessary to
select the specific instances of SFs that will be used, and to create
the service topology for that SFC using SF's network locator. Thus,
instantiation of the SFC results in the creation of a Service
Function Path (SFP) and is used for forwarding packets through the
SFC. In other words, an SFP is the instantiation of the defined SFC.
This abstraction enables the binding of SFCs to specific instances of
SFs based on a range of policy attributes defined by the operator.
For example, an SFC definition might specify that one of the SF
elements is a firewall. However, on the network, there might exist a
number of instances of the same firewall (that is to say they enforce
the same policy) and only when the SFP is created is one of those
firewall instances selected. The selection can be based on a range
of policy attributes, ranging from simple to more elaborate criteria.
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3. Service Function Chaining Architecture
3.1. Architecture Principles
Service function chaining is predicated on several key architectural
principles:
1. Topological independence: no changes to the underlay network
forwarding topology - implicit, or explicit - are needed to
deploy and invoke SFs or SFCs.
2. Consistent policy identifiers: a common identifier is used for SF
policy selection.
3. Classification: traffic that satisfies classification rules is
forwarded according to a specific SFC. For example,
classification can be as simple as an explicit forwarding entry
that forwards all traffic from one address into the SFC.
Multiple classification points are possible within an SFC (i.e.
forming a service graph) thus enabling changes/update to the SFC
by SFs.
4. SFC Encapsulation: The SFC encapsulation enables the sharing of
metadata/context: the network and SFs no longer exist in separate
silos. Metadata/context data can be shared amongst SF and
classifiers. In addition to metadata, the encapsulation provides
information used to identify the SFP. Transit nodes -- such as
router and switches -- simply forward SFC encapsulated packets
based on the outer (non-SFC) encapsulation.
5. Heterogeneous control/policy points: allowing SFs to use
independent mechanisms (out of scope for this document) like IF-
MAP or Diameter to populate and resolve local policy and (if
needed) local classification criteria.
3.2. Fundamental Components
The following logical components form the basis of the SFC
architecture:
1. SF: the concept of a SF evolves; rather than being viewed as a
bump in the wire, a SF becomes a resource within a specified
administrative domain that is available for consumption. As
such, SFs have one or more network locators and a variable set of
attributes that describe the function offered. The combination
of network locator and attributes are used to construct an SFC.
SF send/receive SFC encapsulated data from one or more Service
Function Forwarders.
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2. Service Function Forwarder (SFF): a service function forwarder
provides service layer forwarding. An SFF receives packets from
a SFC Network Forwarder (see below) and forwards the traffic to
the required associated SF(s).
3. SFC Network Forwarder (SNF): This component is responsible for
forwarding traffic flows along the SFPs they belong to based on
information contained in the SFC encapsulation. Since SFCs
straddle both the service layer (via the SFC encapsulation) and
the network layer (via the network transport), SNFs can provide
service path load distribution and failover functionality. For
example, SNFs might have two network paths between SF1 and SF2
and utilize local metrics for path selection. Similarly, if a
path fails, the SFC can utilize local failover to select
alternate path(s).
+----------------+
|Service Function|
| (SF) |
+-------+--------+
|
SFC Encapsulation
|
+-------+--------+
| SF Forwarder|
| (SFF) |
+-------+--------+
|
SFC Encapsulation
|
+-------+--------+
| SFC Network |
| Forwarder (SNF)|
+----------------+
|
Network Overlay Transport
|
_,....._
,-' `-.
/ `.
| Network |
`. /
`.__ _,-'
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`''''
Figure 2: Service Function Components
Classifier: A component that performs traffic classification.
Classification is the precursor to the start of an SFP: traffic that
matches classification criteria is forwarded along a given SFP to
realize the specifications of an SFC. The granularity of
classification varies based on operator requirements and device
capabilities. While initial classification at a network node starts
an SFP, subsequent classifications may occur along the SFC and
further alter the SFP. This re-classification may also update the
context information (see below).
Overlay Service Topology: A service topology is created to
interconnect the elements used to form the SFP. This overlay
topology is specific to the SFP: it is created for the express
purpose of steering packets or frames through the SFs and optionally
passing context data. The overlay is formed between SNF elements.
The overlay topology can be constructed using any existing transport,
for example IP, MPLS, etc.
Control plane: The SFC control plane is responsible for constructing
the SFPs; translating the SFCs to the forwarding paths and
propagating path information to participating nodes - network and
service - to achieve requisite forwarding behavior to construct the
service overlay. For instance, a SFC construction may be static -
using specific SF instances, or dynamic - choosing service explicit
SF instances at the time of delivering traffic to the SF. In SFC,
SFs are resources; the control plane advertises their capabilities,
availability and location. The control plane is also responsible for
the creation of the context (see below). The control plane may exist
within distributed routing elements as in traditional networks, or in
a centralized configuration.
Shared context data: Sharing context data allows the network to
provide network-derived information to the SFs, SF to SF information
exchange and the sharing of service-derived information to the
network. This component is optional. SFC infrastructure enables the
exchange of this shared context along the SFP. The shared context
serves several possible roles within the SFC architecture:
o Allows elements that typically operate as ships-in-the-night to
exchange information.
o Encodes information about the network and/or data for post-
service forwarding.
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o Creates an identifier used for policy binding by SFs.
o Context information can be derived in several ways:
* External sources
* Network node classification
* Service function classification
o Resource Control: The SFC system may be responsible for managing
all resources necessary for the SFC components to function. This
includes network constraints used to plan and choose the network
path(s) between service nodes, characteristics of the nodes
themselves such as memory, number of virtual interfaces, routes,
etc..., and configuration of the SFs running on the service nodes.
The figure below provides a high level view of the components:
+-------+
+---------->|control|<----------+
| |plane | |
+-------------->| +---+---+ |
| | ^ |
| | | |
v v v v
+----------+ ,---. ,---. ,---. +----------+
|classifier|+---> / \+------->/ \+-------->/ \+-------->|classifier|
| | ( 1 )<-----+( 2 )<------+( 3 )<-------+| |
+----------+ \ / \ / \ / +----------+
`---' `---' `---'
Figure 3: Service Function Chaining Architecture
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4. Summary
Service function chains enable composite services that are
constructed from one or more service functions. This document
provides a standard architecture, including architectural concepts,
principles, and components, for the creation of Service function
chains.
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5. Security Considerations
This document does not define a new protocol and therefore creates no
new security issues.
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6. 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
Kevin Glavin
Riverbed
Email: Kevin.Glavin@riverbed.com
Ken Gray
Cisco Systems, Inc.
Email: kegray@cisco.com
Jim Guichard
Cisco Systems, Inc.
Email: jguichar@cisco.com
Surendra Kumar
Cisco Systems, Inc.
Email: smkumar@cisco.com
Nic Leymann
Deutsche Telekom
Email: n.leymann@telekom.de
Rajeev Manur
Broadcom
Email: rmanur@broadcom.com
Thomas Nadeau
Lucidvision
Email: tnadeau@lucidvision.com
Carlos Pignataro
Cisco Systems, Inc.
Email: cpignata@cisco.com
Michael Smith
Cisco Systems, Inc.
Email: michsmit@cisco.com
Navindra Yadav
Cisco Systems, Inc.
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Email: nyadav@cisco.com
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7. Acknowledgments
The authors would like to thank David Ward, Abhijit Patra, Nagaraj
Bagepalli, Darrel Lewis, Ron Parker and Christian Jacquenet for their
review and comments.
A special thank you goes to Joel Halpern for his thoughtful, detailed
review and guidance.
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8. IANA Considerations
This document creates no new requirements on IANA namespaces
[RFC5226].
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9. References
9.1. Normative References
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
9.2. Informative References
[NSCprob] "Network Service Chaining Problem Statement", <http://
datatracker.ietf.org/doc/
draft-quinn-nsc-problem-statement/>.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[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.
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Appendix A. Existing Service Deployments
Existing service insertion and deployment techniques fail to address
new challenging requirements raised by modern network architectures
and evolving technologies such as multi-tenancy, virtualization,
elasticity, and orchestration. Networks, servers, storage
technologies, and applications, have all undergone significant change
in recent years: virtualization, network overlays, and orchestration
have increasingly become adopted techniques. All of these have
profound effects on network and services design.
As network service functions evolve, operators are faced with an
array of form factors - virtual and physical - as well as with a
range of insertion methods that often vary by vendor and type of
service.
Such existing services are deployed using a range of techniques, most
often associated with topology or forwarding modifications. For
example, firewalls often rely on layer-2 network changes for
deployment: a VLAN is created for the "inside" interface, and another
for the "outside" interface. In other words, a new L2 segment was
created simply to add a service function. In the case of server load
balancers, policy routing is often used to ensure traffic from
server's returns to the load balancer. As with the firewall example,
the policy routing serves only to ensure that the network traffic
ultimately flows to the service function(s).
The network-centric information (e.g. VLAN) is not limited to
insertion; this information is often used as a policy identifier on
the service itself. So, on a firewall, the layer-2 segment
identifies the local policy to be selected. If more granular policy
discrimination is required, more network identifiers must be created
either per-hop, or communicated consistently to all services.
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Appendix B. Issues with Existing Deployments
Due to the tight coupling of network and service function resources
in existing networks, adding or removing service functions is a
complex task that is fraught with risk and is tied to
operationalizing topological changes leading to massively static
configuration procedures for network service delivery or update
purposes. The inflexibility of such deployments limits (and in many
cases precludes) dynamic service scaling (both horizontal and
vertical) and requires hop-by-hop configuration to ensure that the
correct service functions, and sequence of service functions are
traversed.
A non-exhaustive list of existing service deployment and insertion
techniques as well as the issues associated with each may be found in
[NSCprob].
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Authors' Addresses
Paul Quinn (editor)
Cisco Systems, Inc.
Email: paulq@cisco.com
Andre Beliveau (editor)
Ericsson
Email: andre.beliveau@ericsson.com
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