Internet DRAFT - draft-quinn-sfc-arch
draft-quinn-sfc-arch
Network Working Group P. Quinn, Ed.
Internet-Draft Cisco Systems, Inc.
Intended status: Informational J. Halpern, Ed.
Expires: November 6, 2014 Ericsson
May 5, 2014
Service Function Chaining (SFC) Architecture
draft-quinn-sfc-arch-05.txt
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
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the Trust Legal Provisions and are provided without warranty as
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Definition of Terms . . . . . . . . . . . . . . . . . . . 3
2. Architectural Concepts . . . . . . . . . . . . . . . . . . . . 5
2.1. Service Function Chains . . . . . . . . . . . . . . . . . 5
2.2. Service Function Chain Symmetry . . . . . . . . . . . . . 6
2.3. Service Function Paths . . . . . . . . . . . . . . . . . . 6
3. Architecture Principles . . . . . . . . . . . . . . . . . . . 7
4. Core SFC Architecture Components . . . . . . . . . . . . . . . 8
4.1. SFC Encapsulation . . . . . . . . . . . . . . . . . . . . 9
4.2. Service Function (SF) . . . . . . . . . . . . . . . . . . 9
4.3. Service Function Forwarder (SFF) . . . . . . . . . . . . . 9
4.3.1. Transport Derived SFF . . . . . . . . . . . . . . . . 11
4.4. Network Forwarder (NF) . . . . . . . . . . . . . . . . . . 11
4.5. Classification/Re-classification . . . . . . . . . . . . . 11
4.6. SFC Control Plane . . . . . . . . . . . . . . . . . . . . 12
4.7. Shared Metadata . . . . . . . . . . . . . . . . . . . . . 13
4.8. Resource Control . . . . . . . . . . . . . . . . . . . . . 13
5. The Role of Policy . . . . . . . . . . . . . . . . . . . . . . 14
6. Load Balancing Considerations . . . . . . . . . . . . . . . . 15
7. SFC Proxy . . . . . . . . . . . . . . . . . . . . . . . . . . 18
8. MTU Considerations . . . . . . . . . . . . . . . . . . . . . . 19
9. SFC OAM . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
10. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
11. Security Considerations . . . . . . . . . . . . . . . . . . . 22
12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 23
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 25
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
15.1. Normative References . . . . . . . . . . . . . . . . . . . 27
15.2. Informative References . . . . . . . . . . . . . . . . . . 27
Appendix A. Existing Service Deployments . . . . . . . . . . . . 28
Appendix B. Issues with Existing Deployments . . . . . . . . . . 29
Appendix C. SFC Encapsulation Requirements . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31
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1. Introduction
This document describes an architecture used for the creation of
Service Function Chains (SFC) in a network. It includes
architectural concepts, principles, and components.
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.
1.1. Scope
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. 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 (NF): SFC network forwarders provide network
connectivity for service function forwarders (SFF) and service
functions (SF).
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 (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.
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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.
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 Identity (SFID): A unique identifier that
represents a service function. SFIDs are unique for each SF
within an SFC domain.
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.
SFC Proxy: Removes and inserts SFC encapsulation on behalf of a SFC-
unaware service function. SFC proxies are logical elements.
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
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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. 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 )
\ / \ / \ / \ / \ /
`---' `---' `---' `---' `---'
Figure 1: Service Function Chain Graphs
The architecture allows for two or more SFs to be co-resident on the
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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,
or set of like 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. 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. 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. 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.
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4. Core SFC Architecture Components
The following sub-sections provide details on each logical component
that form the basis of the SFC architecture. An overview of how each
of these components interact is provided in the following figure.
+----------------+ +----------------+
| SFC-aware | | SFC-unaware |
|Service Function| |Service Function|
+-------+--------+ +-------+--------+
| |
SFC Encapsulation No SFC Encapsulation
| |
| +---------+
+------------------+ +-------------|SFC Proxy|
\ / +---------+
+-------+--------+
| SF Forwarder|
| (SFF) |
+-------+--------+
|
SFC Encapsulation
|
+-------+--------+
| SFC Network |
| Forwarder (NF) |
+----------------+
|
Network Overlay Transport
|
_,....._
,-' `-.
/ `.
| Network |
`. /
`.__ _,-'
`''''
Figure 2: Service Function Chain Architecture Components
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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 nodes -- 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.
4.2. Service Function (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 part of a composite
service. As such, SFs have one or more network locators through
which they are reachable, and a variable set of attributes that
describe the function offered. The combination of network locator
and attributes are used to construct an SFP. SFs send/receive SFC
encapsulated data from one or more SFFs.
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.
4.3. Service Function Forwarder (SFF)
The SFF is responsible for forwarding packets and/or frames received
from an NF to one or more SFs associated with a given SFF using
information conveyed in the SFC encapsulation.
The collection of SFFs 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.
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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 SFC should be
applied as traffic flows through the associated SFP. Each SFF need
only maintain SFC forwarding information that is relevant locally.
The SFC forwarding state at all SFFs collectively represents the SFPs
associated with each SFC in the SFC domain.
+------+----------------------------------+
| SFP | Ordered Service Functions |
|------+----------------------------------+
| ID | order1 | order2 | order3 | ... |
+------+--------+--------+--------+-------+
| SFP1 | SFID1 | SFID5 | SFID20 | |
+------+--------+--------+--------+-------+
| SFP4 | SFID100| SFID3 | SFID4 | SFID9 |
+------+--------+--------+--------+-------+
| ... | | | | |
+------+--------+--------+--------+-------+
Figure 3: SFF Table
Figure 3 depicts a view of the service forwarding state for two SFPs
- SFP1 and SFP4. The SF columns of this table may come from
different SFFs.
The SFF component has the following primary responsibilities:
1. SFP forwarding : Traffic arrives at an SFF from one or more NFs.
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 hop in the SFP, the SFF, encapsulates the traffic in the
appropriate network transport and delivers it to the NF for
delivery to the next SFF along the path.
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 an NF for forwarding.
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3. Maintaining flow state: In some cases, the SFF may be stateful.
It creates flows and stores flow-centric information. When
traffic arrives after being steered through an SFC-unaware SF,
the SFF must perform re-classification of traffic to determine
the SFP. A state-full SFF simplifies such classification to a
flow lookup.
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 a transport
mechanism which carries the service path information from the SFC
encapsulation, and information derived from the SFC encapsulation, to
build transport information.
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. Network Forwarder (NF)
This component is responsible for performing the overlay
encapsulation/de-capsulation and forwarding of packets on the overlay
network. NF forwarding 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.
4.5. Classification/Re-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.
The SFC architecture supports reclassification (or non-initial
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classification) as well. As packets traverse an SFP,
reclassification 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.
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 reclassifies 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 reclassified the traffic based on local
application layer classification capabilities (that were not
available during the initial classification step).
4.6. SFC 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 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 SF instances for a requested SFC, either
statically (using specific SF instances) or dynamically (using
service explicit SF instances at the time of delivering traffic
to the SF).
4. Provides requisite SFC data plane information to the SFC
architecture components, most notably the SFF.
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5. Allocation of metadata associated with a given SFP and
propagation of metadata syntax to relevant SF instances and/or
SFC encapsulation-proxies or their respective policy planes.
4.7. 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. This component is
optional. 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
* Service function classification
4.8. 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.
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5. 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.
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6. 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.
+----+----->web server
source+------>|sf1 +----->web server
|----+----->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
to account for this behavior - as long as the traffic coalesces to a
common next-hop after the point of elasticity.
In the following figure, we have a chain of five service functions
between the traffic source and it's destination.
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+-----+ +-----+
+-->| sf2 +--+ +-->| sf4 +--+
| |-----| | | |-----| |
+------+----+ +-----+ +-+>+-----+----+ +-----+ +-+>+-----+
| sf1 | +-----+ | sf3 | +-----+ | sf5 |
source+----->|------+----+-->| sf2 |+---+>|-----|----+-->| sf4 |+---+>|-----|+----->
| | |-----| | | |-----| | |
+------+----+ +-----+ +-+>+-----+----+ +-----+ +-+>+-----+
| +-----+ | | +-----+ |
+-->| sf2 +--+ +-->| sf4 +--+
|-----| |-----|
+-----+ +-----+
Figure 5: Load Balancing
Either through an imbedded action in sf1 and sf3, or through external
control, the service functions sf2 and sf4 are elastically expanded
and contracted dynamically. This would be represented as one chain:
s1->s2->s3->s4->s5, but with multiple paths (not as a number of
chains equal to the factorial combination of potential end-to-end
paths). The load distribution decision will be localized (in
general, although there might be macro policy controlling that -
which is out of scope for the sake of a simple example). In this
case, the control entity will push to the sf1 nodes, a table of
sorts: sf2 with a series of next hops, and if needed some weighted or
other metrics (these could also be decided locally by some policy,
but sf1 would need to be aware of expand/contract triggers and
actions). sf1 would use local logic -- hash, state table, etc. -- to
distribute the chained packets to sf2.
The addition of high availability should likewise not require a
multitude of new chains.
+-----+-+ +-----+-+
+-->| sf2 |-|+ +-->| sf4 |-|+
+------>|-----| || +------>|-----| ||
+------+|---+ +-----+ |+-->+-----+|---+ +-----+ |+-->+-----+
| sf1 || +-----+ +--->| sf3 || +-----+ +--->| sf5 |
+-->|------||------>| sf2 |+---->|-----||------>| sf4 |+---->|-----|---+
| | || +---->|-----|-+ | || +---->|-----|-+ | | |
| +------+|-|-+ +-----+ |+-->+-----+|-|-+ +-----+ |+-->+-----+ |
source+--+ | | | +-----+ || | | | +-----+ || +----->
| +------++ | +-->| sf2 |-|+ +-----++ | +-->| sf4 |-|+ +-----+ |
| | sf1' | | +-->|-----| +--->| sf3'| | +-->|-----| +--->| sf5'| |
+-->|------|--+ | +-----+----->|-----|--+ | +-----+----->|-----|---+
| | | | | | | |
+------+----+ +-----+----+ +-----+
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Figure 6: Load Balancing and HA
In the figure, sf1, sf3 and sf5 have a redundant counterpart for high
availability purposes (typical of stateful appliance/function
redundancy strategies, these entities may have private connections
for transferring state not shown). Note that the elasticity of sf2
and sf4 provide a separate high availability strategy for those
functions. In the case where sf1', sf3' and sf5' provide transparent
dynamic replacement (they assert the addressing characteristics of
their counterparts via an internal or external trigger), there is
still a single chain (again, not a factorial explosion).
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7. SFC Proxy
In order for the SFC architecture to support SFC-unaware SF's, 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. More specifically:
For traffic received from a NF or SFF, destined to an SF, the SFC
proxy:
o Removes the SFC encapsulation from SFC encapsulated packets and/or
frames.
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, 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.
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8. MTU Considerations
Modern systems are expected to be able to cope gracefully with MTU
issues that may arise from the application of additional headers to a
packet. Adopting the recommendations of other WG's who have recently
tackled this issue (e.g. [RFC6830]), there are several mechanisms
for dealing with packets that are too large to transit the path from
the point of service classification to the last function (SFn) in the
SFC.
In the "stateful" approach, the classifier keeps a per-path record of
the maximum size allowed, and sends an ICMP Too Big message to the
original source when a packet which is too large is seen (where "too
large" implies after the imposition of the appropriate SFC
encapsulation).
In the "stateless" approach, for IPv4, packets without the 'DF' bit
set, too-large packets are fragmented, and then the fragments are
forwarded; all other packets are discarded and an ICMP Too Big
message returned.
A recommendation of a specific mechanism and/or its implementation is
beyond the scope of this document.
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9. 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.
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.
o Out-of-band: reporting beyond the actual dataplane. An additional
layer beyond the data-plane OAM, allows for additional alerting
and measurements.
Some of the detailed functions performed by SFC OAM include fault
detection, continuity checks, connectivity verification, service path
tracing, diagnostic and fault isolation, alarm reporting, performance
measurement, locking and testing of service functions, and also allow
for vendor-specific as well as experimental functions. SFC should
leverage, and if needed extend relevant existing OAM mechanisms.
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10. 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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11. Security Considerations
This document does not define a new protocol and therefore creates no
new security issues.
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12. 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
Andre Beliveau
Ericsson
Email: andre.beliveau@ericsson.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
Darrel Lewis
Cisco Systems, Inc.
Email: darlewis@cisco.com
Nic Leymann
Deutsche Telekom
Email: n.leymann@telekom.de
Rajeev Manur
Broadcom
Email: rmanur@broadcom.com
Thomas Nadeau
Brocade
Email: tnadeau@lucidvision.com
Carlos Pignataro
Cisco Systems, Inc.
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Email: cpignata@cisco.com
Michael Smith
Cisco Systems, Inc.
Email: michsmit@cisco.com
Navindra Yadav
Cisco Systems, Inc.
Email: nyadav@cisco.com
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13. Acknowledgments
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.
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14. IANA Considerations
This document creates no new requirements on IANA namespaces
[RFC5226].
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15. References
15.1. Normative References
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
15.2. Informative References
[NSCprob] "Network Service Chaining Problem Statement", <http://
datatracker.ietf.org/doc/
draft-ietf-sfc-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.
[RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
Locator/ID Separation Protocol (LISP)", RFC 6830,
January 2013.
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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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Appendix C. SFC Encapsulation Requirements
TBD
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Authors' Addresses
Paul Quinn (editor)
Cisco Systems, Inc.
Email: paulq@cisco.com
Joel Halpern (editor)
Ericsson
Email: jmh@joelhalpern.com
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