Service Function Chaining (SFC) Architecture
draft-merged-sfc-architecture-01
The information below is for an old version of the document.
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|---|---|---|---|
| Authors | Joel M. Halpern , Carlos Pignataro | ||
| Last updated | 2014-08-03 | ||
| Replaces | draft-boucadair-sfc-framework, draft-quinn-sfc-arch | ||
| Replaced by | draft-ietf-sfc-architecture, RFC 7665 | ||
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draft-merged-sfc-architecture-01
Network Working Group J. Halpern, Ed.
Internet-Draft Ericsson
Intended status: Standards Track C. Pignataro, Ed.
Expires: February 4, 2015 Cisco
August 3, 2014
Service Function Chaining (SFC) Architecture
draft-merged-sfc-architecture-01
Abstract
This document describes an architecture for the specification,
creation, and ongoing maintenance of Service Function Chains (SFC) in
a network. It includes architectural concepts, principles, and
components used in the construction of composite services through
deployment of SFCs. This document does not propose solutions,
protocols, or extensions to existing protocols.
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
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and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 4, 2015.
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
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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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 . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Definition of Terms . . . . . . . . . . . . . . . . . . . 4
2. Architectural Concepts . . . . . . . . . . . . . . . . . . . 6
2.1. Service Function Chains . . . . . . . . . . . . . . . . . 6
2.2. Service Function Chain Symmetry . . . . . . . . . . . . . 7
2.3. Service Function Paths . . . . . . . . . . . . . . . . . 8
3. Architecture Principles . . . . . . . . . . . . . . . . . . . 9
4. Core SFC Architecture Components . . . . . . . . . . . . . . 10
4.1. SFC Encapsulation . . . . . . . . . . . . . . . . . . . . 11
4.2. Service Function (SF) . . . . . . . . . . . . . . . . . . 12
4.3. Service Function Forwarder (SFF) . . . . . . . . . . . . 12
4.3.1. Transport Derived SFF . . . . . . . . . . . . . . . . 13
4.4. SFC-Enabled Domain . . . . . . . . . . . . . . . . . . . 14
4.5. Network Components . . . . . . . . . . . . . . . . . . . 14
4.6. SFC Proxy . . . . . . . . . . . . . . . . . . . . . . . . 14
4.7. Classification . . . . . . . . . . . . . . . . . . . . . 15
4.8. Re-Classification and Branching . . . . . . . . . . . . . 16
4.9. Shared Metadata . . . . . . . . . . . . . . . . . . . . . 16
5. Additional Architectural Concepts . . . . . . . . . . . . . . 17
5.1. The Role of Policy . . . . . . . . . . . . . . . . . . . 17
5.2. SFC Control Plane . . . . . . . . . . . . . . . . . . . . 17
5.3. Resource Control . . . . . . . . . . . . . . . . . . . . 18
5.4. Infinite Loop Detection and Avoidance . . . . . . . . . . 19
5.5. Load Balancing Considerations . . . . . . . . . . . . . . 19
5.6. MTU and Fragmentation Considerations . . . . . . . . . . 20
5.7. SFC OAM . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.8. Resilience and Redundancy . . . . . . . . . . . . . . . . 22
6. Security Considerations . . . . . . . . . . . . . . . . . . . 22
7. Contributors and Acknowledgments . . . . . . . . . . . . . . 23
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
9.1. Normative References . . . . . . . . . . . . . . . . . . 24
9.2. Informative References . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
This document describes an architecture used for the creation and
ongoing maintenance of Service Function Chains (SFC) in a network.
It includes architectural concepts, principles, and components.
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An overview of the issues associated with the deployment of end-to-
end service function chains, abstract sets of service functions and
their ordering constraints that create a composite service and the
subsequent "steering" of traffic flows through said service
functions, is described in [I-D.ietf-sfc-problem-statement].
This architecture presents a model addressing the problematic aspects
of existing service deployments, including topological independence
and configuration complexity.
Service function chains enable composite services that are
constructed from one or more service functions.
1.1. Scope
This document defines a framework to enforce Service Function
Chaining (SFC) with minimum requirements on the physical topology of
the network. The proposed solution relies on initial packet
classification. packets are initially classified at the entry point
of an SFC-enabled network, and are then forwarded according to the
ordered set of SF functions that need to be enabled to process these
packets in the SFC-enabled domain.
This document does not make any assumption on the deployment context.
The proposed framework covers both fixed and mobile networks.
The architecture described herein is assumed to be applicable to a
single network administrative domain. While it is possible for the
architectural principles and components to be applied to inter-domain
SFCs, these are left for future study.
1.2. Assumptions
The following assumptions are made:
o Not all SFs can be characterized with a standard definition in
terms of technical description, detailed specification,
configuration, etc.
o There is no global or standard list of SFs enabled in a given
administrative domain. The set of SFs varies as a function of the
service to be provided and according to the networking
environment.
o There is no global or standard SF chaining logic. The ordered set
of SFs that need to be enabled to deliver a given connectivity
service is specific to each administrative entity.
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o The chaining of SFs and the criteria to invoke some of them are
specific to each administrative entity that operates the SF-
enabled network (also called administrative domain).
o SF chaining logic and related policies should not be exposed
outside a given administrative domain.
o Several SF chaining policies can be simultaneously enforced within
an administrative domain to meet various business requirements.
o No assumption is made on how FIBs and RIBs of involved nodes are
populated.
o How to bind the traffic to a given SF chaining is policy-based.
1.3. Definition of Terms
Network 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. The term "service" is used
to denote a "network service" in the context of this document.
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.
SFC Encapsulation: The SFC Encapsulation provides at a minimum SFP
identification, and is used by the SFC-aware functions, such as
the SFF and SFC-aware SFs. The SFC Encapsulation is not used
for network packet forwarding. In addition to SFP
identification, the SFC encapsulation carries dataplane context
information, also referred to as metadata.
Classification: Locally instantiated policy and customer/network/
service profile matching of traffic flows for identification of
appropriate outbound forwarding actions.
Classifier: An element that performs Classification.
Service Function (SF): A function that is responsible for specific
treatment of received packets. A Service Function can act at
various layers of a protocol stack (e.g., at the network layer
or other OSI layers). A Service Function can be a virtual
element or be embedded in a physical network element. One of
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multiple Service Functions can be embedded in the same network
element. Multiple occurrences of the Service Function can be
enabled in the same administrative domain.
One or more Service Functions can be involved in the delivery of
added-value services. A non-exhaustive list of Service
Functions includes: firewalls, WAN and application acceleration,
Deep Packet Inspection (DPI),a LI (Lawful Intercept) module,
server load balancers, NAT44 [RFC3022], NAT64 [RFC6146], NPTv6
[RFC6296], HOST_ID injection, HTTP Header Enrichment functions,
TCP optimizer, etc.
An SF may be SFC encapsulation aware, that is it receives, and
acts on information in the SFC encapsulation, or unaware in
which case data forwarded to the service does not contain 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 via
information carried in the SFC encapsulation.
Service Function Chain (SFC): A service Function chain defines an
abstract set of service functions and their ordering constraints
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, and also allows for cases where there is
flexibility in the order in which services need to be applied.
The term service chain is often used as shorthand for service
function chain.
Service Function Path (SFP): The SFP provides a level of indirection
between the fully abstract notion of service chain as an
abstract sequence of functions to be delivered, and the fully
specified notion of exactly what SFF/SFs the packet will visit
when it actually traverses the network. By allowing the control
components to specify the use of this level of indirection, the
deployment may choose the degree of SFF/SF selection authority
that is delegated to the network.
Rendered Service Path (RSP): The Service Function Path is a
constrained specification of where packets using a certain
service chain must go. While it may be so constrained as to
identify the exact locations, it can be not so specific.
Packets themselves naturally are transmitted from and to
specific places in the network, visiting a specific sequence of
SFFs and SFs. This sequence of actual visits by a packet to
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specific SFFs and SFs in the network is known as the Rendered
Service Path (RSP). This definition is included here for use by
later documents, such as when solutions may need to discuss this
actual sequence of locations the packets visits.
SFC-enabled Domain: A network or region of a network that implements
SFC. An SFC-enabled Domain is limited to a single network
administrative domain.
SFC Proxy: Removes and inserts SFC encapsulation on behalf of an
SFC-unaware service function. SFC proxies are logical elements.
2. Architectural Concepts
The following sections describe the foundational concepts of service
function chaining and the SFC architecture.
Service Function Chaining enables the 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 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
dynamic ordering and topological independence of those SFs as well as
the exchange of metadata between participating entities.
2.1. Service Function Chains
In most networks services are constructed as an abstract sequence of
SFs that represent an SFC. 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 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.
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,-+-. ,---. ,---. ,---.
/ \ / \ / \ / \
( 1 )+--->( 2 )+---->( 6 )+---->( 8 )
\ / \ / \ / \ /
`---' `---' `---' `---'
,-+-. ,---. ,---. ,---. ,---.
/ \ / \ / \ / \ / \
( 1 )+--->( 2 )+---->( 3 )+---->( 7 )+---->( 9 )
\ / \ / \ / \ / \ /
`---' `---' `---' `---' `---'
,-+-. ,---. ,---. ,---. ,---.
/ \ / \ / \ / \ / \
( 1 )+--->( 7 )+---->( 8 )+---->( 4 )+---->( 7 )
\ / \ / \ / \ / \ /
`---' `---' `---' `---' `---'
Figure 1: Service Function Chain Graphs
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. Solutions will need to
ensure suitable disambiguation for such situations.
The architectural allowance that is made for SFPs that delegate
choice to the network for which SFs or SFFs a packet will visit
creates potential issues here. A solution that allows such
delegation needs to also describe how the solution ensures that those
service chains that require service function chain symmetry can
achieve that.
Further, there are state tradeoffs in symmetry. Symmetry may be
realized in several ways depending on the SFF and classifier
functionality. In some cases, "mirrored" classification (S -> D and
D -> S) policy may be deployed, whereas in others shared state
between classifiers may be used to ensure that symmetric flows
correctly identified, then steered along the required SFP. At a
high-level, there are various common cases. In a non-exhaustive way,
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there can be for example: a single classifier (or a small number of
classifiers), in which case both incoming and outgoing flows could be
recognized at a same classifier, so the synchronization would be
feasible by internal mechanism implementation. Another case is the
one of stateful classifiers where several classifiers may be
clustered and share state. Lastly, another case entails fully
distributed classifiers, where synchronization needs to be provided.
This is a non-comprehensive list of common cases.
2.3. Service Function Paths
A service function path (SFP) is a mechanism used by service chaining
to express the result of applying policy and operational constraints
to the abstract requirements of a service chain (SFC). This
architecture does not mandate the degree of specificity of the SFP.
Architecturally, within the same system some SFPs may be fully
specified, selecting exactly which SFF and which SF packets using
that SFP are to visit, while other SFPs may be quite vague, deferring
to the SFF the decisions about the exact sequence steps to be used to
realize the SFC. The specificity may be anywhere in between these
extremes.
As an example of such an intermediate specificity, there may be two
SFPs associated with a single SFC, where one SFP says essentially
that any order of SFF and SF may be used as long as it is within data
center 1, and where the second SFP allows the same latitude, but only
within data center 2.
Thus, the policies and logic of SFP selection or creation (depending
upon the solution) produce what may be thought of as a constrained
version of the original SFC. Since there may be multiple policies
which apply to different traffic that uses the same SFC, it also
follows that there may be multiple SFP associated with a single SFC.
The architecture allows for the same SF to be reachable through
multiple SFFs. In these cases, some SFPs may constrain which SFF are
used to reach which SF, while some SFPs may leave that decision to
the SFF themselves.
Further, the architecture allows for two or more SFs to be attached
to the same SFF, and possibly connected via internal means allowing
more effective communication. In these cases, some solutions or
deployments 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. This must be
coordinated with the SFF handling so that the service function
forwarding can properly perform is job. Implementation details of
such mechanisms are considered out-of-scope for this document, and
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can include a spectrum of methods: for example situations including
all next-hops explicitly, others where a list of possible next-hops
is provided and the selection is local, or cases with just an
indentifier, and all resolution is local.
This architecture also allows the same SF to be part of multiple
SFPs.
3. 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. Plane separation: dynamic realization of SFPs is separated from
packet handling operations (e.g., packet forwarding).
3. Classification: traffic that satisfies classification rules is
forwarded according to a specific SFP. For example,
classification can be as simple as an explicit forwarding entry
that forwards all traffic from one address into the SFP.
Multiple classification points are possible within an SFC (i.e.
forming a service graph) thus enabling changes/updates to the SFC
by SFs.
Classification can occur at varying degrees of granularity; for
example, classification can use a 5-tuple, a transport port or
set of ports, part of the packet payload, or it can come fro
external systems.
4. Shared Metadata: Metadata/context data can be shared amongst SFs
and classifiers, between SFs, and between external systems and
SFs (e.g. orchestration).
Generally speaking, the metadata can be thought of as providing,
and sharing the result of classification (that occurs with the
SFC domain, or external to it) along an SFP. For example, an
external repository might provide user/subscriber information to
a service chain classifier. This classifier in turn imposes that
information in the SFC encapsulation for delivery to the
requisite SFs. The SFs in turn utilize the user/subscriber
information for local policy decisions.
5. Service definition independence: the technical characterization
of each SF is not required to design the SFC architecture and SFC
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data plane operations. Consequently, no IANA registry is
required to store the list of SFs.
6. Service function chain independence: The creation, modification,
or deletion of a service chain have no impact on other service
chains.
7. 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.
4. Core SFC Architecture Components
At a very high level, the logical architecture of an SFC-enabled
Domain comprises:
o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. +--------------+ +------------------~~~
. | Service | SFC | Service +---+ +---+
. |Classification| Encapsulation | Function |sf1|...|sfn|
+---->| Function |+---------------->| Path +---+ +---+
. +--------------+ +------------------~~~
. SFC-enabled Domain
o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 2: Service Function Chain Architecture
The following sub-sections provide details on each logical component
that form the basis of the SFC architecture. A detailed overview of
how each of these architectural components interact is provided in
Figure 3:
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+----------------+ +----------------+
| SFC-aware | | SFC-unaware |
|Service Function| |Service Function|
+-------+--------+ +-------+--------+
| |
SFC Encapsulation No SFC Encapsulation
| SFC |
+----+ +----------------+ Encapsulation +---------+
| SF |-----------------+ \ +------------|SFC Proxy|
+----+ ... ----------+ \ \ / +---------+
\ \ \ /
+-------+--------+
| SF Forwarder |
| (SFF) |
+-------+--------+
|
SFC Encapsulation
|
... SFC-enabled Domain ...
|
Network Overlay Transport
|
_,....._
,-' `-.
/ `.
| Network |
`. /
`.__ __,-'
`''''
Figure 3: Service Function Chain Architecture Components
4.1. SFC Encapsulation
The SFC encapsulation enables service function path selection and the
sharing of metadata/context information.
The SFC encapsulation provides explicit information used to identify
the SFP. However, the SFC encapsulation is not a transport
encapsulation itself: it is not used to forward packets within the
network fabric. The SFC encapsulation therefore, relies on an outer
network transport. Transit forwarders -- such as router and switches
-- simply forward SFC encapsulated packets based on the outer (non-
SFC) encapsulation.
One of the key architecture principles of SFC is that the SFC
encapsulation remain transport independent and as such any network
transport protocol may be used to carry the SFC encapsulation.
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4.2. Service Function (SF)
The concept of an SF evolves; rather than being viewed as a bump in
the wire, an SF becomes a resource within a specified administrative
domain that is available for consumption as part of a composite
service. SFs send/receive data from one or more SFFs. SFC aware SFs
receive this data with the SFC encapsulation.
While the SFC architecture defines a new encapsulation - the SFC
encapsulation - and several logical components for the construction
of SFCs, existing SF implementations may not have the capabilities to
act upon or fully integrate with the new SFC encapsulation. In order
to provide a mechanism for such SFs to participate in the
architecture a logical SFC proxy function is defined. The SFC proxy
acts a gateway between the SFC encapsulation and SFC unaware SFs.
The integration of SFC-unaware service function is discussed in more
detail in the SFC proxy section.
This architecture allows an SF to be part of multiple SFPs and SFCs.
4.3. Service Function Forwarder (SFF)
The SFF is responsible for forwarding packets and/or frames received
from the network to one or more SFs associated with a given SFF using
information conveyed in the SFC encapsulation.
The collection of SFFs and associated SFs creates a service plane
using an overlay in which SFC-aware SFs, as well as SFC-unaware SFs
reside. Within this service plane, the SFF component connects
different SFs that form a service function path.
SFFs maintain the requisite SFP forwarding information. SFP
forwarding information is associated with a service path identifier
that is used to uniquely identify an SFP. The service forwarding
state enables an SFF to identify which SF of a given SFP should be
applied as traffic flows through the associated SFP. While there may
appear to the SFF to be only one available way to deliver the given
SF, there may also be multiple choices allowed by the constraints of
the SFP.
If there are multiple choices, the SFF needs to preserve the property
that all packets of a flow are handled the same way, since the SF may
well be stateful.
The SFF also has the information to allow it to forward packets to
the next SFF after applying local service functions. Again, while
there may be only a single choice available, the architecture allows
for multiple choices for next SFF. As with SF, the solution needs to
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operate such that the behavior flows (see the Rendered Service Path)
is stable. It should be noted that the selection of available SFs
and next SFFs may be interwoven when an SFF support multiple distinct
service functions and the same service function is available at
multiple SFFs. Solutions need to be clear about what is allowed in
these cases.
It should be noted that even when the SFF supports and utilizes
multiple choices, the decision as to whether to use flow-specific
mechanisms or coarser grained means to ensure that the behavior flows
are stable is a matter for specific solutions and specific
implementations.
The SFF component has the following primary responsibilities:
1. SFP forwarding : Traffic arrives at an SFF from the network. The
SFF determines the appropriate SF the traffic should be forwarded
to via information contained in the SFC encapsulation. Post-SF,
the traffic is returned to the SFF, and if needed forwarded to
another SF associated with that SFF. If there is another non-
local (i.e., different SFF) hop in the SFP, the SFF encapsulates
the traffic in the appropriate network transport and delivers it
to the network for delivery to the next SFF along the path.
Related to this forwarding responsibility, an SFF should be able
to interact with metadata.
2. Terminating SFPs : An SFC is completely executed when traffic has
traversed all required SFs in a chain. When traffic arrives at
the SFF after the last SF has finished servicing it, SFF fails to
find the next SF or knows from the service forwarding state that
the SFC is complete. SFF removes the SFC encapsulation and
delivers the packet to the network for forwarding.
3. Maintaining flow state: In some cases, the SFF may be stateful.
It creates flows and stores flow-centric information. This state
information may be used for a range of SFP-related tasks such as
ensuring symmetry or for state-aware SFC Proxy functionality (see
Section 4.8).
4.3.1. Transport Derived SFF
Service function forwarding, as described above, directly depends
upon the use of the service path information contained in the SFC
encapsulation. Existing implementations may not be able to act on
the SFC encapsulation. These platforms may opt to use existing
transport information if it provides explicit service path
information.
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This results in the same architectural behavior and meaning for
service function forwarding and service function paths. It is the
responsibility of the control components to ensure that the transport
path executed in such a case is fully aligned with the path
identified by the information in the service chaining encapsulation.
4.4. SFC-Enabled Domain
Specific features might need to be enforced at the boundaries of an
SFC-enabled domain, for example to avoid leaking SFC information.
Using the term node to refer generically to an entity that is
performing a set of functions, in this context, an SFC Boundary Node
denotes a node that connects one SFC-enabled domain to a node either
located in another SFC-enabled domain or in a domain that is SFC-
unaware.
An SFC Boundary node can act as egress or ingress. An SFC Egress
Node denotes a SFC Boundary Node that handles traffic leaving the
SFC-enabled domain the Egress Node belongs to. Such a node is
required to remove any information specific to the SFC Domain,
typically the SFC Encapsulation. An SFC Ingress Node denotes a SFC
Boundary Node that handles traffic entering the SFC-enabled domain
the ingress Node belongs to. In most solutions and deployments this
will need to include a classifier, and will be responsible for adding
the SFC encapsulation to the packet.
4.5. Network Components
Underneath the SFF, there are components responsible for performing
the overlay encapsulation/de-capsulation and forwarding of packets on
the overlay network. They may consult the SFC encapsulation or the
inner payload of an incoming packet only in the necessary cases to
achieve optimal forwarding in the network.
4.6. SFC Proxy
In order for the SFC architecture to support SFC-unaware SF's (e.g.,
legacy service functions), an optional, logical SFC proxy function
may be used. This proxy removes the SFC encapsulation and then uses
a local attachment circuit to deliver packets to SFC unaware SFs.
Architecturally, the SFC Proxy along with an SFC-unaware Service
Function make up an SF. More specifically:
For traffic received from an SFF, destined to an SF, the SFC proxy:
o Removes the SFC encapsulation from SFC encapsulated packets and/or
frames.
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o Identifies the required SF to be applied based on information
carried in the SFC encapsulation.
o Selects the appropriate outbound local attachment circuit through
which the next SF for this SFP is reachable. This information is
derived from the SFC encapsulation or from local configuration.
Examples of a local attachment circuit include, but are not
limited to, VLANs, IP-in-IP, L2TPv3, GRE, VXLAN.
o Forwards the original payload via a local attachment circuit to
the appropriate SF.
When traffic is returned from the SF:
o Applies the required SFC encapsulation. The determination of the
encapsulation details may be inferred by the local attachment
circuit through which the packet and/or frame was received, or via
packet classification, or other local policy. In some cases,
packet ordering or modification by the SF may necessitate
additional classification in order to re-apply the correct SFC
encapsulation.
o Imposes the appropriate SFC encapsulation based on the
identification of the SFC to be applied.
Alternatively, a service provider may decide to exclude legacy
service functions from an SDC domain.
4.7. Classification
Traffic that satisfies classification criteria is directed into an
SFP and forwarded to the requisite service function(s).
Classification is handled by a logical service classification
function, and initial classification occurs at the edge of the SFC
domain. The granularity of the initial classification is determined
by the capabilities of the classifier and the requirements of the SFC
policy. For instance, classification might be relatively coarse: all
packets from this port are directed into SFP A, or quite granular:
all packets matching this 5-tuple are subject to SFP B.
As a consequence of the classification decision, the appropriate SFC
encapsulation is imposed on the data prior to forwarding along the
SFP. Classification results in attaching the traffic to a specific
SFP.
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4.8. Re-Classification and Branching
The SFC architecture supports re-classification (or non-initial
classification) as well. As packets traverse an SFP, re-
classification may occur - typically performed by a classification
function co-resident with a service function. Reclassification may
result in the selection of a new SFP, an update of the associated
metadata, or both. This is referred to as "branching".
For example, an initial classification results in the selection of
SFP A: DPI_1 --> SLB_8. However, when the DPI service function is
executed "attack" traffic is detected at the application layer.
DPI_1 re-classifies the traffic as "attack" and alters the service
path, to SFP B, to include a firewall for policy enforcement:
dropping the traffic: DPI_1 --> FW_4. In this simple example, the
DPI service function re-classified the traffic based on local
application layer classification capabilities (that were not
available during the initial classification step).
When traffic arrives after being steered through an SFC-unaware SF,
the SFC Proxy must perform re-classification of traffic to determine
the SFP. The SFC Proxy is concerned with re-attaching information
for SFC-unaware SFs, and a state-full SFC Proxy simplifies such
classification to a flow lookup.
4.9. Shared Metadata
Sharing metadata 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. Some SFCs may not
require metadata exchange. SFC infrastructure enables the exchange
of this shared data along the SFP. The shared metadata 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.
o Creates an identifier used for policy binding by SFs.
o Context information can be derived in several ways:
* External sources
* Network node classification
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* Service function classification
5. Additional Architectural Concepts
There are a number of issues which solutions need to address, and
which the architecture informs but does not determine. This section
lays out some of those concepts.
5.1. The Role of Policy
Much of the behavior of service chains is driven by operator and
customer policy. This architecture is structured to isolate the
policy interactions from the data plane and control logic.
Specifically, it is assumed that service chaining control plane
creates the service paths. The service chaining data plane is used
to deliver the classified packets along the service chains to the
intended service functions.
Policy, in contrast interacts with the system in other places.
Policies, and policy engines, may monitor service functions to decide
if additional (or fewer) instances of services are needed. When
applicable, those decisions may in turn result in interactions which
direct the control logic to change the service chain placement or the
packet classification rules.
Similarly, operator service policy, often managed by operational or
business support systems (OSS or BSS), will frequently determine what
service functions are available. Depending upon operator
preferences, these policies may also determine which sequences of
functions are valid and to be used or made available.
The offering of service chains to customers, and the selection of
which service chain a customer wishes to use are driven by a
combination of operator and customer policies using appropriate
portals in conjunction with the OSS and BSS tools. These selections
then drive the service chaining control logic which in turn
establishes the appropriate packet classification rules.
5.2. SFC Control Plane
This is part of the overall architecture but outside the scope of
document.
The SFC control plane is responsible for constructing the SFPs;
translating the SFCs to the forwarding paths and propagating path
information to participating nodes to achieve requisite forwarding
behavior to construct the service overlay. For instance, an SFC
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construction may be static; selecting exactly which SFF and which SF
from those SFF are to be used, or may be dynamic; allowing the
network to perform some or all of the choices of SFF or SF to use to
deliver the selected service chain within the constraints represented
by the service path. In SFC, SFs are resources; the control plane
manages and communicates their capabilities, availability and
location in fashions suitable for the transport and SFC operations in
use. The control plane is also responsible for the creation of the
context (see below). The control plane may be distributed (using new
or existing control plane protocols), or be centralized, or a
combination of the two.
The SFC control plane provides the following functionality:
1. An administrative domain wide view of all available service
function resources as well as the network locator through which
they are reachable.
2. Uses SFC policy to construct service function chains, and
associated service function paths.
3. Selection of specific SFs for a requested SFC, either statically
(using specific SFs) or dynamically (using service explicit SFs
at the time of delivering traffic to them).
4. Provides requisite SFC data plane information to the SFC
architecture components, most notably the SFF.
5. Allocation of metadata associated with a given SFP and
propagation of metadata syntax to relevant SFs and/or SFC
encapsulation-proxies or their respective policy planes.
5.3. 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 function forwarders, network communication paths between
service function forwarders and their attach service functions,
characteristics of the nodes themselves such as memory, number of
virtual interfaces, routes, etc., and configuration of the running
SFs running.
The SFC system will also be required to reflect policy decisions
about resource control, as expressed by other components in the
system.
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It should be noted that while all of these aspects are part of the
overall system, they normal reside outside of the scope of this
architecture.
5.4. Infinite Loop Detection and Avoidance
This SFC architecture is predicated on topological independence from
the underlying forwarding topology. Consequently, a service topology
is created by Service Function Paths or by the local decisions of the
Service Function Forwarders based on the constraints expressed in the
SFP. Due to the overlay constraints, the packet-forwarding path may
need to visit the same SFF multiple times, and in some less common
cases may even need to visit the same SF more than once. The Service
Chaining solution needs to permit these limited and policy-compliant
loops. At the same time, the solutions must ensure that indefinite
and unbounded loops cannot be formed, as such would consume unbounded
resources without delivering any value.
In other words, this architecture prevents infinite Service Function
Loops, even when Service Functions may be invoked multiple times in
the same SFP.
5.5. Load Balancing Considerations
Supporting function elasticity and high-availability shouldn't overly
complicate SFC or lead to unnecessary scalability problems.
In the simplest case, where there is only a single function in the
chain (the next hop is either the destination address of the flow or
the appropriate next hop to that destination), one could argue that
there may be no need for SFC.
In the case where the classifier is separate from the single function
or a function at the terminal address may need sub-prefix or per
subscriber metadata, we would have a single chain (the metadata
changes but the SFC chain does not), regardless of the number of
potential terminal addresses for the flow. This is the case of the
simple load balancer. See Figure 4.
+---+ +---++--->web server
source+-->|sff|+-->|sf1|+--->web server
+---+ +---++--->web server
Figure 4: Simple Load Balancing
By extrapolation, in the case where intermediary functions within a
chain had similar "elastic" behaviors, we do not need separate chains
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to account for this behavior - as long as the traffic coalesces to a
common next-hop after the point of elasticity.
In Figure 5, we have a chain of five service functions between the
traffic source and its destination.
+---+ +---+ +---+ +---+ +---+ +---+
|sf2| |sf2| |sf3| |sf3| |sf4| |sf4|
+---+ +---+ +---+ +---+ +---+ +---+
| | | | | |
+-----+-----+ +-----+-----+
| |
+ +
+---+ +---+ +---+ +---+ +---+
source+-->|sff|+-->|sff|+--->|sff|+--->|sff|+-->|sff|+-->destination
+---+ +---+ +---+ +---+ +---+
+ + +
| | |
+---+ +---+ +---+
|sf1| |sf3| |sf5|
+---+ +---+ +---+
Figure 5: Load Balancing
This would be represented as one service function path:
sf1->sf2->sf3->sf4->sf5. The SFF is a logical element, which may be
made up of one or multiple components. In this architecture, the SFF
handle load distribution based on policy.
It can also be seen in the above that the same service function may
be reachable through multiple SFF, as discussed earlier. The
selection of which SFF to use to reach SF3 may be made by the control
logic in defining the SFP, or may be left to the SFF themselves,
depending upon policy, solution, and deployment constraints.
5.6. MTU and Fragmentation Considerations
This architecture prescribes additional information being added to
packets to represent service function paths and often metadata. It
also envisions adding transport information to carry packets along
service function paths, at least between service function forwarders.
This added information increases the size of the packet to be carried
by service chaining. Such additions could potentially increase the
packet size beyond the MTU supported on some or all of the media used
in the service chaining domain.
Such packet size increases can thus cause operational MTU problems.
Requiring fragmentation and reassembly in SFF would be a major
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processing increase, and may be impossible with some transports.
Expecting service functions to deal with packets fragmented by the
SFC function may be onerous even when such fragmentation is possible.
Thus, at the very least, solutions need to pay attention to the size
cost of their approach. There may be alternative or additional means
available, although any solution needs to consider the impact of
those techniques.
In addition to these considerations applicable to any generic
architecture that increases the header size, there are more specific
MTU considerations: Effects to Path MTU Discovery (PMTUD) as well as
deployment considerations. Deployments within a single
administrateive control or even a single Data Center complex can
afford more flexibility in dealing with larger packets, and deploying
existing mitigations that decrease the likelihood of fragmentation.
5.7. SFC OAM
Operations, Administration, and Maintenance (OAM) tools are an
integral part of the architecture. These serve various purposes,
including fault detection and isolation, and performance management.
For example, there are many advantages of SFP liveness detection,
including status reporting, support for resiliency operations and
policies, and an enhanced ability to load balance.
Service Function Paths create a services topology, and OAM performs
various functions within this service layer. Furthermore, SFC OAM
follows the same architectural principles of SFC in general. For
example, topological independence (including the ability to run OAM
over various overlay technologies) and classification-based policy.
We can subdivide the SFC OAM architecture in two parts:
o In-band: OAM packets run in-band fate sharing with the service
topology. For this, they also follow the architectural principle
of consistent policy identifiers, and use the same path IDs as the
service chain data packets. Load balancing and SFC Encapslation
encapsulation with packet forwarding are particularly important
here.
o Out-of-band: reporting beyond the actual data plane. An
additional layer beyond the data-plane OAM, allows for additional
alerting and measurements.
This architecture prescribes end-to-end SFP OAM functions, which
implies SFF understanding of whether an in-band packet is an OAM or
user packet. However, service function validation is outside of the
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scope of this architecture, and application-level OAM is not what
this architecture prescribes.
Some of the detailed functions performed by SFC OAM include fault
detection and isolation in a Service Function Path or a Service
Function, verification that communication using SFPs is both
effective and directing packets to the intended service functions,
service path tracing, diagnostic and fault isolation, alarm
reporting, performance measurement, locking and testing of service
functions, validation with the control plane (see Section 5.2, and
also allow for vendor-specific as well as experimental functions.
SFC should leverage, and if needed extend relevant existing OAM
mechanisms.
5.8. Resilience and Redundancy
As a practical operational requirement, any service chaining solution
needs to be able to respond effectively, and usually very quickly, to
failure conditions. These failures may be failures of connectivity
in the network between SFF, failures of SFF, or failures of SF. Per-
SF state, as for example stateful-firewall state, is the
responsibility of the SF and not addressed in this section.
There are multiple techniques available to address this issue, and
solutions can describe both what they require and what they allow to
address this. Solutions can make use of the flexibility in the
specificity of service function paths, if the SFF can be given enough
information in a timely fashion to do this. Solutions can also make
use of MAC or IP level redundancy mechanisms such as VRRP. Also,
particularly for SF failures, load balancers co-located with the SFF
or as part of the service function delivery mechanism can provide
such robustness.
Similarly, operational requirements will lead to the need for
resilience in solutions in the face of load changes. While
mechanisms for managing (e.g., monitoring, instantiating, loading
images, providing configuration to service function chaining control,
deleting, etc.) virtual machines are out scope for this architecture,
solutions can and are aided by describing how they can make use of
scaling mechanisms.
6. Security Considerations
This document does not define a new protocol and therefore creates no
new security issues.
Security considerations apply to the realization of this
architecture. Such realization ought to provide means to protect the
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SFC-enabled domain and its borders against various forms of attacks,
including DDoS attacks. Further, SFC OAM Functions need to not
negatively affect the security considerations of an SFC-enabled
domain. Additionally, all entities (software or hardware)
interacting with the service chaining mechanisms need to provide
means of security against malformed, poorly configured (deliberate or
not) protocol constructs and loops.
7. Contributors and Acknowledgments
The editors would like to thank Sam Aldrin, Linda Dunbar, Ken Gray,
Nagendra Kumar, Xiaohu Xu, and L. Yong for a thorough review and
useful comments.
The initial version of this "Service Function Chaining (SFC)
Architecture" document is the result of merging two previous
documents, and this section lists the aggregate of authors, editors,
contributors and acknowledged participants, all who provided
important ideas and text that fed into this architecture.
[I-D.boucadair-sfc-framework]:
Authors:
Mohamed Boucadair
Christian Jacquenet
Ron Parker
Diego R. Lopez
Jim Guichard
Carlos Pignataro
Contributors:
Parviz Yegani
Paul Quinn
Linda Dunbar
Acknowledgements:
Many thanks to D. Abgrall, D. Minodier, Y. Le Goff, D.
Cheng, R. White, and B. Chatras for their review and
comments.
[I-D.quinn-sfc-arch]:
Authors:
Paul Quinn (editor)
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Joel Halpern (editor)
Contributors:
Puneet Agarwal
Andre Beliveau
Kevin Glavin
Ken Gray
Jim Guichard
Surendra Kumar
Darrel Lewis
Nic Leymann
Rajeev Manur
Thomas Nadeau
Carlos Pignataro
Michael Smith
Navindra Yadav
Acknowledgements:
The authors would like to thank David Ward, Abhijit Patra,
Nagaraj Bagepalli, Darrel Lewis, Ron Parker, Lucy Yong and
Christian Jacquenet for their review and comments.
8. IANA Considerations
This document creates no new requirements on IANA namespaces
[RFC5226].
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
[I-D.boucadair-sfc-framework]
Boucadair, M., Jacquenet, C., Parker, R., Lopez, D.,
Guichard, J., and C. Pignataro, "Service Function
Chaining: Framework & Architecture", draft-boucadair-sfc-
framework-02 (work in progress), February 2014.
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[I-D.ietf-sfc-problem-statement]
Quinn, P. and T. Nadeau, "Service Function Chaining
Problem Statement", draft-ietf-sfc-problem-statement-07
(work in progress), June 2014.
[I-D.quinn-sfc-arch]
Quinn, P. and J. Halpern, "Service Function Chaining (SFC)
Architecture", draft-quinn-sfc-arch-05 (work in progress),
May 2014.
[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.
[RFC6296] Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
Translation", RFC 6296, June 2011.
Authors' Addresses
Joel Halpern (editor)
Ericsson
Email: jmh@joelhalpern.com
Carlos Pignataro (editor)
Cisco Systems, Inc.
Email: cpignata@cisco.com
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