Service Function Chaining (sfc) H. Li
Internet-Draft Q. Wu
Intended status: Informational O. Huang
Expires: December 4, 2015 Huawei
M. Boucadair, Ed.
C. Jacquenet
France Telecom
W. Haeffner
Vodafone
S. Lee
ETRI
R. Parker
Affirmed Networks
L. Dunbar
A. Malis
Huawei Technologies
J. Halpern
Ericsson
T. Reddy
P. Patil
Cisco
June 2, 2015
Service Function Chaining (SFC) Control Plane Components & Requirements
draft-ww-sfc-control-plane-05
Abstract
This document describes requirements for conveying information
between Service Function Chaining (SFC) control elements (including
management components) and SFC functional elements. Also, this
document identifies a set of control interfaces to interact with SFC-
aware elements to establish, maintain or recover service function
chains. This document does not specify protocols nor extensions to
existing protocols.
This document exclusively focuses on SFC deployments that are under
the responsibility of a single administrative entity. Inter-domain
considerations are out of scope.
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
Task Force (IETF). Note that other groups may also distribute
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working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on December 4, 2015.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 5
2. Generic Considerations . . . . . . . . . . . . . . . . . . . 6
2.1. Generic Requirements . . . . . . . . . . . . . . . . . . 6
2.2. SFC Control Plane Bootstrapping . . . . . . . . . . . . . 6
2.3. Coherent Setup of an SFC-enabled Domain . . . . . . . . . 7
3. SFC Control Plane Components & Interfaces . . . . . . . . . . 8
3.1. Reference Architecture . . . . . . . . . . . . . . . . . 8
3.2. Centralized vs. Distributed . . . . . . . . . . . . . . . 9
3.3. Interface Reference Points . . . . . . . . . . . . . . . 10
3.3.1. C1: Interface between SFC Control Plane & SFC
Classifier . . . . . . . . . . . . . . . . . . . . . 10
3.3.2. C2: Interface between SFC Control Plane & SFF . . . . 12
3.3.3. C3: Interface between SFC Control Plane & SFC-aware
SFs . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.3.4. C4: Interface between SFC Control Plane & SFC Proxy . 13
4. Additional Considerations . . . . . . . . . . . . . . . . . . 14
4.1. Discovery of the SFC Control Element . . . . . . . . . . 14
4.2. SF Symmetry . . . . . . . . . . . . . . . . . . . . . . . 14
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4.3. Pre-deploying SFCs . . . . . . . . . . . . . . . . . . . 14
4.4. Withraw a Service Function (SF) . . . . . . . . . . . . . 14
4.5. SFC/SFP Operations . . . . . . . . . . . . . . . . . . . 15
4.6. Unsolicited (Notification) Messages . . . . . . . . . . . 15
4.7. SF Liveness Detection . . . . . . . . . . . . . . . . . . 15
4.8. Monitoring & Counters . . . . . . . . . . . . . . . . . . 16
4.9. SFC/SFP Diagnosis . . . . . . . . . . . . . . . . . . . . 16
4.10. Considerations Specific to the Centralized Path
Computation Model . . . . . . . . . . . . . . . . . . . . 17
4.10.1. Service Function Path Adjustment . . . . . . . . . . 17
4.10.2. Head End Initiated SFP Establishment . . . . . . . . 18
4.10.3. (Regional) Restoration of Service Functions . . . . 18
5. Security Considerations . . . . . . . . . . . . . . . . . . . 19
5.1. Secure Communications . . . . . . . . . . . . . . . . . . 19
5.2. Pervasive Monitoring . . . . . . . . . . . . . . . . . . 19
5.3. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.4. Denial-of-Service (DoS) . . . . . . . . . . . . . . . . . 20
5.5. Illegitimate Discovery of SFs and SFC Control Elements . 20
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
8.1. Normative References . . . . . . . . . . . . . . . . . . 20
8.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. RSP-related Considerations . . . . . . . . . . . . . 23
A.1. Encoding the Exact SFF-SF-sequence in Data Packets . . . 23
A.2. Fully Controlled SFF-SF-Sequence for a SFP . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
1. Introduction
The dynamic enforcement of a service-derived forwarding policy for
packets entering a network that supports advanced Service Functions
(SFs) has become a key challenge for operators. Typically, many
advanced Service Functions (e.g., Performance Enhancement Proxies
([RFC3135]), NATs [RFC3022][RFC6333][RFC6146], firewalls
[I-D.ietf-opsawg-firewalls], etc.) are solicited for the delivery of
value-added services, particularly to meet various service objectives
such as IP address sharing, avoiding covert channels, detecting and
protecting against ever increasing Denial-of-Service (DoS) attacks,
etc.
Because of the proliferation of such advanced service functions
together with complex service deployment constraints that demand more
agile service delivery procedures, operators need to rationalize
their service delivery logics and master their complexity while
optimising service activation time cycles. The overall problem space
is described in [RFC7498]. A more in-depth discussion on use cases
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can be found in [I-D.ietf-sfc-use-case-mobility] and
[I-D.ietf-sfc-dc-use-cases].
[I-D.ietf-sfc-architecture] presents a model addressing the
problematic aspects of existing service deployments, including
topological dependence and configuration complexity. It also
describes an architecture for the specification, creation, and
ongoing maintenance of Service Function Chains (SFC) within a
network. That is, how to define an ordered set of Service Functions
and ordering constraints that must be applied to packets and/or
frames and/or flows selected as a result of classification.
1.1. Scope
While [I-D.ietf-sfc-architecture] focuses on data plane
considerations, this document describes requirements for conveying
information between SFC control elements (including management
components) and SFC functional elements. Also, this document
identifies a set of control interfaces to interact with SFC-aware
elements to establish, maintain or recover service function chains.
Both distributed and centralized control plane schemes to install
SFC-related state and influence forwarding policies are discussed.
This document does not make any assumption on the deployment use
cases. In particular, the document implicitly covers fixed, mobile,
data center networks and any combination thereof.
This document does not make any assumption about which control
protocol to use, whether one or multiple control protocols are
required, or whether the same or distinct control protocols will be
invoked for each of the control interfaces. It is out of scope of
this document to specify a profile for an existing protocol, to
define protocol extensions, or to select a protocol.
Considerations related to the chaining of Service Functions that span
domains owned by multiple administrative entities are out of scope.
It is out of scope of this document to discuss SF-specific control
and policy enforcement schemes; only SFC considerations are
elaborated, regardless of the various connectivity services that may
be supported in the SFC domain. Likewise, only the control of SFC-
aware elements is discussed.
Service catalogue (including guidelines for deriving service function
chains) is out of scope.
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1.2. Terminology
The reader should be familiar with the terms defined in [RFC7498] and
[I-D.ietf-sfc-architecture].
The document makes use of the following terms:
o SFC data plane functional element: Refers to SFC-aware Service
Function, Service Function Forwarder (SFF), SFC Proxy, or SFC
Classifier as defined in the SFC data plane architecture
[I-D.ietf-sfc-architecture].
o SFC Control Element: A logical entity that instructs one or more
SFC data plane functional elements on how to process packets
within an SFC-enabled domain.
o SFC Classification entry: Refers to an entry maintained by an SFC
Classifier that reflects the policies for binding an incoming
flow/packet to a given SFC. Actions are associated with matching
criteria. For example, packets can be marked with the appropriate
SFC-related information to differentiate flows so that subsequent
SFFs can forward the flows to a sequence of SFs in a given order.
The set of classification entries maintained by a Classifier are
referred to as in the classification policy table.
o SFC Forwarding Policy Table: this table reflects the SFC-specific
traffic forwarding policy enforced by SFF components for every
relevant incoming packet that is associated to one of the existing
SFCs.
[[Note: The question of whether the data plane operates just in
terms of SFP IDs or needs SFC IDs, as described in this version
of the draft, is still under discussion among the authors.]]
1.3. Assumptions
This document adheres to the assumptions listed in Section 1.2 of
[I-D.ietf-sfc-architecture].
This document does not make any assumptions about the co-location of
SFC data plane functional elements; this is deployment-specific.
This document can accommodate a variety of deployment contexts such
as (but not limited to):
o A Service Function Forwarder (SFF) can connect instances of the
same or distinct SFs.
o A SF instance can be serviced by one or multiple SFFs.
o One or multiple SFs can be co-located with a SFF.
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o A boundary 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) can act as an egress node and an ingress node for
the same flow.
o Distinct ingress and egress nodes may be crossed by a packet when
forwarded in an SFC-enabled domain.
o Distinct ingress nodes may be solicited for each traffic direction
(e.g., upstream and downstream).
o An ingress node can embed a Classifier.
o An ingress node may not embed a Classifier, but it can be
responsible for dispatching flows among a set of Classifiers.
o The same boundary node may act as an ingress node, an egress node,
and also embed a Classifier.
o A Classifier can be hosted in a node that embeds one or more SFs.
o Many network elements within an SFC-enabled domain may behave as
egress/ingress nodes.
Furthermore, the following assumptions are made:
o A Control Element can be co-located with a Classifier, SFF or SF.
o One or multiple Control Elements can be deployed in an SFC-enabled
domain.
o State synchronization between Control Elements is out of scope.
2. Generic Considerations
2.1. Generic Requirements
For deployments that would require so, SFC forwarding must be allowed
even if no control protocols are enabled. Static configuration must
be allowed.
A permanent association between an SFC data plane element with a
Control Element must not be required; specifically, the SFC-enabled
domain must keep on processing incoming packets according to the SFC
instructions even during unavailability events of control plane
components. SFC implementations that do not meet this requirement
will suffer from another flavor of the constrained high availability
issue, discussed in Section 2.3 of [RFC7498], supposed to be solved
by SFC designs.
2.2. SFC Control Plane Bootstrapping
The interface that is used to feed the SFC control plane with service
objectives and guidelines is not part of the SFC control plane
itself. Therefore, this document assumes the SFC control plane is
provided with a set of information that is required for proper SFC
operation with no specific assumption about how this information is
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collected/provisioned, nor about the structure of such information.
The following information that is likely to be provided to the SFC
control plane at bootstrapping includes (non-exhaustive list):
o Locators for Classifiers/SFF/SFs/Proxies, etc.
o SFs serviced by each SFF.
o A list of service function chains, including how they are
structured and unambiguously identified.
o Status of each SFC: active/pre-deployment phase/etc. A SFC can be
defined at the management level and instantiated in an SFC-enabled
domain for pre-deployment purposes (e.g., testing). Actions to
activate, modify or withdraw an SFC are triggered by the control
plane. Nevertheless, this document does not make any assumption
about how an operator instructs the control plane.
o A list of classification guidelines and/or rules to bind flows to
SFCs/SFPs.
o Optionally, (traffic/CPU/memory) load balancing objectives at the
SFC level or on a per node (e.g., per-SF/SFF/Proxy) basis.
o Security credentials.
o Context information that needs to be shared on a per SFC basis.
Also, the SFC control plane may gather the following information from
an SFC-enabled domain at bootstrapping (non-exhaustive list). How
this information is collected is left unspecified in this document:
o The list of active SFC-aware SFs (including their locators).
o The list of SFFs and the SFs that are attached to.
o The list of enabled SFC Proxies, and the list of SFC-unware SFs
attached to.
o The list of active SFCs/SFPs as enabled in an SFC-enabled domain.
o The list of Classifiers and their locators, so as to retrieve the
classification policy table for each Classifier, in particular.
o The SFC forwarding policy tables maintained by SFFs.
During the bootstrapping phase, a Control Element may detect a
conflict between the running configuration in an SFC data plane
element and the information maintained by the control plane.
Consequently, the control plane undertakes appropriate actions to fix
those conflicts. This is typically achieved by invoking one of the
interfaces defined in Section 3.3.
2.3. Coherent Setup of an SFC-enabled Domain
Various transport encapsulation schemes and/or variations of SFC
header implementations may be supported by one or several nodes of an
SFC-enabled domain. For the sake of coherent configuration, the SFC
control plane is responsible for instructing all the involved SFC
data plane functional elements about the behavior to adopt to select
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the transport encapsulation scheme(s), the version of the SFC header
to enable, etc.
3. SFC Control Plane Components & Interfaces
3.1. Reference Architecture
The SFC control plane is responsible for the following:
o Build and monitor the service-aware topology. For example, this
can be achieved by means of dynamic SF discovery techniques.
Those means are out of scope of this document.
o Maintain a repository of service function chains, SFC matching
criteria to bind flows to a given service function chain, and
mapping between service function chains and SFPs.
o Guarantee the coherency of the configuration and the operation of
an SFC-enabled domain.
o Dynamically compute a service-aware forwarding path (distributed
model, see Section 3.2)
o Determine a forwarding path in the context of a centralized
deployment model (see Section 3.2).
o Update service function chains or adjust SFPs (e.g., for
restoration purposes) based on various inputs (e.g., external
policy context, path alteration, SF unavailability, SF withdrawal,
service decommissioning, etc.).
o Populate SFC forwarding policy tables of involved SFC data plane
elements and provides Classifiers with traffic classification
rules.
Figure 1 shows the overall SFC control plane architecture, including
interface reference points.
This document does not elaborate on the internal decomposition of the
SFC Control & Management Plane functional blocks. The components
within the SFC Control & Management Planes and their interactions are
out of scope.
As discussed in Section 3.2, the SFC control plane can be implemented
in a (logically) centralized or distributed fashion.
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+----------------------------------------------+
| |
| SFC Control & Management Planes |
+-------| |
| | |
C1 +------^-----------^-------------^-------------+
+---------------------|C3---------|-------------|-------------+
| | +----+ | | |
| | | SF | |C2 |C2 |
| | +----+ | | |
| +----V--- --+ | | | |
| | SFC | +----+ +-|--+ +----+ |
| |Classifier |---->|SFF |----->|SFF |------->|SFF | |
| | Node |<----| |<-----| |<-------| | |
| +-----------+ +----+ +----+ +----+ |
| | | | |
| |C2 ------- | |
| | | | +-----------+ C4 |
| V +----+ +----+ | SFC Proxy |--> |
| | SF | |SF | +-----------+ |
| +----+ +----+ |
| |C3 |C3 |
| SFC Data Plane Components V V |
+-------------------------------------------------------------+
Figure 1: SFC Control Plane: Overview
3.2. Centralized vs. Distributed
The SFC control plane can be (logically) centralized, distributed or
a combination thereof. Whether one or multiple SFC Control Elements
are enabled is deployment-specific. Nevertheless, the following
comments can be made:
SFC management (including SFC monitoring and supervision): is likely
to be centralized.
SFC Mapping Rules: i.e., service instructions to bind a flow to a
service function chain are likely to be managed by a central SFC
Control Element, but the resulting policies can be shared among
several Control Elements. Note, these policies can be
complemented with local information (e.g., an IPv4 address/IPv6
prefix assigned to a customer) because such information may not be
available to the central entity but known only during network
attachment phase.
Path computation: can be either distributed or centralized.
Distributed path computation means that the selection of the exact
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sequence of SF functions that a packet needs to invoke (along with
instances and/or SFF locator information) is a result of a
distributed path selection algorithm executed by involved nodes.
For some traffic engineering proposes, the SFP may be constrained
by the control plane; as such, some SFPs can be fully specified
(i.e., list all the SFF/SFs that need to be solicited) or
partially specified (e.g., exclude some nodes, explicitly select
which instance of a given SF needs to be invoked, etc.).
SFC Resiliency (including restoration) refers to mechanisms to
ensure high available service function chains. It includes means
to detect node/link/path failures. Both centralized and
distributed mechanism to ensure SFC resiliency can be envisaged.
Implementing a (logically) centralized path computation engine
requires information to be dynamically communicated to the central
SFC Control Element, such as the list of available SF instances, SFF
locators, load status, SFP availability, etc.
3.3. Interface Reference Points
The following sub-sections describe the interfaces between the SFC
Control & Management Planes, as well as various SFC data plane
elements
3.3.1. C1: Interface between SFC Control Plane & SFC Classifier
As a reminder, a Classifier is a function that is responsible for
classifying traffic based on (pre-defined) rules.
This interface is used to install SFC classification rules in
Classifiers. Once classification rules are populated, SFC
Classifiers are responsible for binding incoming traffic to service
function chains according to these classification rules. Note, the
SFC control plane must not make any assumption on how the traffic is
to be bound to a given SFC. In other words, classification rules are
deployment-specific. For instance, classification can rely on a
subset of the information carried in a received packet such as
5-tuple classification, be subscriber-aware, be driven by traffic
engineering considerations, or any combination thereof.
The SFC control plane should be responsible for removing invalid (and
stale) mappings from the classification tables maintained by the
classifiers. Also, local sanity checks mechanisms may be supported
locally by the Classifiers, but those are out of scope.
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Classification rules may be updated, deleted or disabled by the
control plane. Criteria that would trigger those operations are
deployment-specific.
Given that service function chaining solutions may be applied to very
large sets of traffic, any control solution should take scaling
issues into consideration as part of the design.
Below are listed some functional objectives for this interface:
o Rationalize the management of classification rules.
o Maintain a global view of instantiated rules in all Classifiers in
an SFC-enabled domain.
o Check the consistency of instantiated classification rules within
the same Classifier or among multiple Classifier.
o Assess the impact of removing or modifying a classification entry
on packets entering an SFC-enabled domain.
o Aggregate classification rules for the sake of performance
optimization (mainly reduce lookup delays).
o Adjust classification rules when rules are based on volatile
identifiers (e.g., an IPv4 address, IPv6 prefix).
o Allow to rapidly restore SFC states during failure events that
occurred at a Classifier (or a Control Element).
The control plane must instruct the Classifier whether it can trust
an existing SFC marking of an incoming packet or whether it must be
ignored.
For bidirectional packet processing purposes (e.g., full or partial
path symmetry), the control plane invokes this interface to configure
the appropriate classification entries.
A Classifier can send unsolicited messages through this interface to
notify the SFC Control & Management Planes about specific events.
When re-classification is allowed in an SFC-enabled domain, this
interface can be used to control Classifiers co-resident with SFC-
aware SFs, SFC Proxies, or SFFs to manage re-classification rules .
SFC Classification policy entry should be bound to one single service
function chain (or one single SFP); when an incoming packet matches
more than one classification entry, tie-breaking criteria should be
specified (e.g., priority). Such tie-breaking criteria should be
instructed by the control plane.
The identification of instantiated SFCs/SFPs is local to each
administrative domain; it is policy-based and deployment-specific.
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3.3.2. C2: Interface between SFC Control Plane & SFF
SFFs make traffic forwarding decisions according to the entries
maintained in their SFC forwarding policy table. Such table is
populated by the SFC control plane through the C2 interface.
This interface is also used to instruct a SFF about the SFC-aware SFs
that it can service. Local means may be enabled between the SFC-
aware SFs and SFFs to allow for the dynamic attachment of SFs to a
SFF but those means are unspecified in this document.
The C2 interface is also used for collecting states of attributes
(e.g., availability, workload, latency), for example, to dynamically
adjust Service Function Paths.
3.3.3. C3: Interface between SFC Control Plane & SFC-aware SFs
The SFC control plane uses this interface to interact with SFC-aware
SFs.
SFs may need to output some processing results of packets to the SFC
control plane. This information can be used by the SFC control plane
to update the SFC classification rules and the SFC forwarding policy
table entries.
This Interface is used to collect such kind of feedback information
from SFs. For example, the following information can be exchanged
between a SF and the SFC control plane:
o SF execution status: Some SFs may need to send information to the
control plane to fine tune SFPs. For example, a threat-detecting
SF can periodically send the threat characteristics via this
interface, such as high probability of threat with packet size
equal to 47. The control plane can then add "packet-size=47"
matching criteria to SFF to steer traffic with packet size equal
to 47 to a scrubbing center.
o SF Liveliness update: When SFs are under stress that yielded the
crossing of some performance thresholds, the SFC control plane
needs to be notified to adjust SFPs accordingly (especially when
the centralized path computation mode is enabled).
The SFC control needs the above status information for various tasks
it undertakes, but this information may be acquired directly from SFs
or indirectly from other management and control systems in the
operational environment.
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This interface is also used to instruct an SFC-aware SF about any
context information it needs to supply in the context of a given SFC.
Also, this interface informs the SFC-aware SF about the semantics of
a context information, which would otherwise have opaque meaning.
Several attributes may be associated with a context information such
as (but not limited to) the "scope" (e.g., per-packet, per-flow or
per host), whether it is "mandatory" or "optional" to process flows
bound to a given chain, etc. Note that a context may be mandatory
for "chain 1", but optional for "chain 2".
The control plane may indicate, for a given service function chain,
an order for consuming a set of contexts supplied in a packet.
A SFC-aware SF can also be instructed about the behavior is should
adopt after consuming a context information that was supplied in the
SFC header. For example, the context can be maintained or stripped.
The SFC-aware SF can be instructed to inject a new context header
into the SFC header.
Multiple SFs may be located within the same physical node, and no SFF
is enabled in that same node, means to unambiguously forward the
traffic to the appropriate SF must be supported.
An SF can be instructed to strip the SFC information for the chains
it terminates.
3.3.4. C4: Interface between SFC Control Plane & SFC Proxy
The SFC control plane uses this interface to interact with an SFC
Proxy.
The SFC proxy can be instructed about authorized SFC-unware SFs it
can service. A SFC Proxy can be instructed about the behavior it
should adopt to process the context information that was supplied in
the SFC header on behalf of a SFC-unware SF, e.g., the context can be
maintained or stripped.
The SFC proxy is also instructed about the semantics of a context
information, which would otherwise have opaque meaning. Several
attributes may be associated with a context information such as (but
not limited to) the "scope" (e.g., per-packet, per-flow or per host),
whether it is "mandatory" or "optional" to process flows bound to a
given chain, etc.
The SFC Proxy can also be instructed to add SF some new context
information into the SFC header on behalf of a SFC-unaware SF.
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The C4 interface is also used for collecting attribute states (e.g.,
availability, workload, latency), for example, to dynamically adjust
Service Function Paths.
4. Additional Considerations
4.1. Discovery of the SFC Control Element
SFC data plane functional elements need to be provisioned with the
locators of the Control Elements. This can be achieved using a
variety if mechanisms such as static configuration or the activation
of a service discovery mechanism. The exact specification of how
this provisioning is achieved is out of scope.
4.2. SF Symmetry
Some SFs require both directions of a flow to traverse. Some service
function chains require full symmetry. If a SF (e.g., stateful
firewall or NAT) needs both direction of a flow, it is the SF
instantiation that needs both direction of a flow to traverse, not
the abstract SF (which can have many instantiations spread across the
network).
4.3. Pre-deploying SFCs
Enabling service function chains should preserve some deployment
practices adopted by Operators. Particularly, installing a service
function chain (and its associated SFPs) should allow for pre-
deployment testing and validation purposes (that is a restricted and
controlled usage of such service function chain (and associated
SFPs)).
4.4. Withraw a Service Function (SF)
During the lifetime of a SFC, a given SF can be decommissioned. To
accommodate such context and any other case where a SF is to be
withdrawn, the control plane should instruct the SFC data plane
functional element about the behavior to adopt. Particularly:
1. a first approach would be to update the service function chains
(and associated SFPs) where that SF is present by removing any
reference to that SF. Doing so avoids to induce service failures
for end users.
2. a second approach would be to delete/deactivate any service
function chain (and its associated SFPs) that involves that SF
but install new service function chains.
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4.5. SFC/SFP Operations
Various actions can be executed on a service function chain (and
associated SFPs) that is structured by the SFC Control & Management
planes. Indeed, a service function chain (and associated SFPs) can
be enabled, disabled, its structure modified by adding a new SF hop
or remove an SF from the sequence of SFs to be invoked, its
classification rules modified, etc.
A modification of a service function chain can trigger control
messages with the appropriate SFC-aware nodes accordingly.
4.6. Unsolicited (Notification) Messages
Involved SFC data plane functional element must be instructed to send
unsolicited notifications when loops are detected, a problem in the
structure of a service function chain is encountered, a long
unavailable forwarding path time is observed, etc.
Specific criteria to send unsolicited notifications to a Control
Element should be fine tuned by the control plane using the interface
defined in Section 3.3.
4.7. SF Liveness Detection
The control plane must allow to detect the liveliness of SFs of an
SFC-enabled domain. In particular, it must allow to dynamically
detect that a SF instance is out of service and notify the relevant
Control Element elements accordingly. The liveness information may
be acquired directly from SFs or indirectly from other management and
control systems in the operational environment.
Liveness status records for all SF instances, and service function
chains (including the SFPs bound to a given chain) are maintained by
the SFC Control & Management.
The Classifier may be notified by the control plane or be part of the
liveness detection procedure.
The ability of a SFC Control Element to check the liveness of each SF
present in service function chain has several advantages, including:
o Enhanced status reporting by the control & management planes
(i.e., an operational status for any given service chain derived
from liveness state of its SFs).
o Ability to support various resiliency policies (i.e., bypass a
node embedding an SF, use alternate node, use alternate chain,
drop traffic, etc.) .
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o Ability to support load balancing capabilities to solicit multiple
SF instances that provide equivalent functions.
Because a node embedding a SF can be responsive from a reachability
standpoint (e.g., IP level) while the function its provides may be
broken (e.g., a NAT module may be down), additional means to assess
whether an SF is up and running are required. These means may be
service-specific.
4.8. Monitoring & Counters
SFC-specific counters and statistics must be provided using of the
interfaces defined in Section 3.3. These data include (but not
limited to):
o Number of flows ever and currently assigned to a given service
function chain and a given SFP.
o Number of flows, packets, bytes dropped due to policy.
o Number of packets and bytes in/out per service function chain.
o Number of flows, packets, bytes dropped due to unknown service
function chain (this is valid in particular for a SF node).
4.9. SFC/SFP Diagnosis
[[Note: This section is expected to be removed once the working
group adopts a document on OAM.]]
The Control & Management planes should allow for the following:
o Assess the status of the serviceability of a SF (i.e., the SF
provides the service(s) it is configured for). Obviously, this
assessment must not rely only on IP reachability to decide whether
a SF is up and running.
o Diagnose the availability of a SFC (including the availability of
a particular SFP bound to a given SFC).
o Retrieve the set of service function chains that are enabled
within a domain.
o Assess whether an SFC-enabled domain is appropriately configured
(including, check the configured chains are matching what should
be configured in that domain, and ensure coherent classification
rules are installed in and enforced by all the Classifiers of the
SFC-enabled domain).
o Correlate classification policies with observed forwarding actions
(including, assess the output of the classification rule applied
on a packet presented to a Classifier of an SFC-enabled domain).
o Support the correlation between a service function chain and the
actual forwarding path followed by a packet matching that service
function chain.
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o Notify the SFC Control Element whenever some (critical) events
occur (for example, a malfunctioning SF instance).
o Re-use SF built-in diagnostic procedures specific to each SF.
The SFC control plane must be able to invoke SFC OAM mechanisms, and
to determine the results of OAM operations.
4.10. Considerations Specific to the Centralized Path Computation Model
This section focuses on issues that are specific to the centralized
deployment model (Section 3.2).
4.10.1. Service Function Path Adjustment
A SFP is determined by composing SF instances and overlay links among
SFFs. Thus, the status of a SFP depends on the states or attributes
(e.g., availability, topological location, latency, workload, etc.)
of its components. For example, failure of a single SF instance
results in failure of the whole SFP. Since these states or
attributes of SFP components may vary in time, their changes should
monitored and SFPs should be dynamically adjusted.
Examples of use cases for SFP adjustment are listed below:
SFP fail-over: re-construct a SFP with replacing the failed SF
instance with another instance of the same SF.
SFP with better latency experience: re-construct a SFP with a low
path stretch considering the changes in topological locations of
SF instances and the latency induced by the (overlay) connectivity
among SFFs.
Traffic engineered SFC: re-construct SFPs to localize the traffic in
the network considering various TE goals such as bypass a node,
bypass a link, etc. These techniques may be used for planned
maintenance operations on a SFC-enabled domain.
SF/SFC Load balancing: re-construct SFPs to distribute the workload
among various SF instances.
For more details about the use cases, refer to
[I-D.lee-nfvrg-resource-management-service-chain].
The procedures for SFP adjustment may be handled by the SFC control
plane as follows:
o Collect and monitor states and attributes of SF instances and
overlay links via the C2 interface (Section 3.3.2) and the C3
interface (Section 3.3.3).
o Evaluate SF instances and overlay links based on the monitoring
results.
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o Select SF instances to re-determine a SFP according to the
evaluation results.
o Replace target SF instances (e.g., in a failure or overladed) with
newly selected ones.
o Enforce the updated SFP for upcoming SFC traversal to SFFs via the
C1 interface (Section 3.3.1) or the C2 interface (Section 3.3.2).
4.10.2. Head End Initiated SFP Establishment
In some scenarios where a SFC Control Element is not connected to all
SFFs in a SFC-enabled domain, the SFC control plane can send the
explicit SFF-SF-sequence or SF-sequence to the SFC head-end, e.g.,
the SFC Classifier via the C1 interface (Section 3.3.1). SFC head-
end can use a signaling protocol to establish the SFF-SF-sequence
based on the SF-sequence.
4.10.3. (Regional) Restoration of Service Functions
There are situations that it might not be feasible for the Classifier
to be notified of the changes of SFF-sequence or SFF-SF-Sequence for
a given SFP because of the time taken for the notification and the
limited capability of the Classifiers.
If a SF has a large number of instantiations, it scales better if the
Classifier doesn't need to be notified with status of visible
instantiations of SFs on a SFP.
It might not be always feasible for the Classifier to be aware of the
exact SF instances selected for a given SFP due to too many instances
for each SF, notifications not being promptly sent to the Classifier,
or other reasons. This is about multiple instances of the same SF
attached to one SFF node; those instances can be handled by the SFF
via local load balancing schemes.
Regional restoration can take the similar approach as the global
restoration: choosing a regional ingress node that can take over the
responsibility of installing the new steering policies to the
involved SFFs or network nodes. Typically, the regional ingress node
should be:
o on the data path of the flow of the given SFC;
o in front of the relevant SFFs or network nodes that are impacted
by the change of the SFP;
o capable of encoding the detailed SFP to the Service Chain Header
of data packets of the identified flow; and
o capable of removing the detailed SFP encoding in data packets
after all the impacted SFFs and network nodes completed the policy
installation.
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5. Security Considerations
5.1. Secure Communications
The SFC Control Elements and the participating SFC data plane
elements must mutually authenticate. SFC data plane elements must
ignore instructions received from unauthenticated SFC Control
Elements. The credentials details used during authentication can be
used by the SFC control plane to decide whether specific
authorization may be granted to a Service Function with regards to
some specific operations (e.g., authorize a given SF to access
specific context information).
In case multiple SFC data plane elements are embedded in the same
node, the authentication mechanism may be executed as a whole; not
for each instance.
A SFC data plane element must be able to send authenticated
unsolicited notifications to a SFC Control Element.
The communication between a Control Element and SFC data plane
elements must provide integrity and replay protection.
An SFC Control Element may instruct a Service Function to include
specific security token(s) that may be used to decrypt traffic
upstream. The security token may be supplied by the SFC control
plane or by an authorized Service Function (e.g., TLS proxy). The
exact details on how authorization is granted to a specific SF,
including via a control plane interface, should be specified.
A Service Function must by default discard any action from a SFC
Control Element that requires specific right privileges (e.g., access
to a legal intercept log, mirror the traffic, etc.).
5.2. Pervasive Monitoring
The authentication mechanism should be immune to pervasive monitoring
[RFC7258]. An attacker can intercept traffic by installing
classification rules that would lead to redirect all or part of the
traffic to an illegitimate network node. Means to protect against
attacks that would lead to install, remove, or modify classification
rules must be supported.
5.3. Privacy
The SFC control plane must be able to control the information that is
leaked outside an SFC-enabled domain. Particularly, the SFC control
plane must support means to preserve privacy [RFC6973]. Context
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headers may indeed reveal privacy information (e.g., IMSI, user name,
user profile, location, etc.). Those headers must not be exposed
outside the operator's domain. Also, means to protect context
headers from eavesdroppers should be enforced.
5.4. Denial-of-Service (DoS)
In order to protect against denial of service that would be caused by
a misbehaving trusted SFC Control Element, SFC data plane elements
should rate limit the messages received from an SFC Control Element.
5.5. Illegitimate Discovery of SFs and SFC Control Elements
Means to defend against soliciting illegitimate SFs/SFFs that do not
belong to the SFC-enabled domain must be enabled. Such means must be
defined in service function discovery and SFC Control Element
discovery specification documents.
6. IANA Considerations
This document does not require any IANA actions.
7. Acknowledgements
This document is the result of merging with
[I-D.lee-sfc-dynamic-instantiation].
The authors would like to thank Shibi Huang for providing input and
LAC Chidung for his review and comments that helped improve this
document.
The text about the semantic of a context information is provided by
Dave Dolson.
8. References
8.1. Normative References
[I-D.ietf-sfc-architecture]
Halpern, J. and C. Pignataro, "Service Function Chaining
(SFC) Architecture", draft-ietf-sfc-architecture-08 (work
in progress), May 2015.
8.2. Informative References
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[I-D.ietf-opsawg-firewalls]
Baker, F. and P. Hoffman, "On Firewalls in Internet
Security", draft-ietf-opsawg-firewalls-01 (work in
progress), October 2012.
[I-D.ietf-sfc-dc-use-cases]
Surendra, S., Tufail, M., Majee, S., Captari, C., and S.
Homma, "Service Function Chaining Use Cases In Data
Centers", draft-ietf-sfc-dc-use-cases-02 (work in
progress), January 2015.
[I-D.ietf-sfc-use-case-mobility]
Haeffner, W., Napper, J., Stiemerling, M., Lopez, D., and
J. Uttaro, "Service Function Chaining Use Cases in Mobile
Networks", draft-ietf-sfc-use-case-mobility-03 (work in
progress), January 2015.
[I-D.lee-nfvrg-resource-management-service-chain]
Lee, S., Pack, S., Shin, M., and E. Paik, "Resource
Management in Service Chaining", draft-lee-nfvrg-resource-
management-service-chain-01 (work in progress), March
2015.
[I-D.lee-sfc-dynamic-instantiation]
Lee, S., Pack, S., Shin, M., and E. Paik, "SFC dynamic
instantiation", draft-lee-sfc-dynamic-instantiation-01
(work in progress), October 2014.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, January
2001.
[RFC3135] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
Shelby, "Performance Enhancing Proxies Intended to
Mitigate Link-Related Degradations", RFC 3135, June 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.
[RFC6333] Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
Stack Lite Broadband Deployments Following IPv4
Exhaustion", RFC 6333, August 2011.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973, July
2013.
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[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, May 2014.
[RFC7498] Quinn, P. and T. Nadeau, "Problem Statement for Service
Function Chaining", RFC 7498, April 2015.
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Appendix A. RSP-related Considerations
This section records some contributions proposed by L. Dunbar and A.
Malis, but have not been discussed yet among authors.
A.1. Encoding the Exact SFF-SF-sequence in Data Packets
Encoding the exact RSP in every packet has the benefit and the issues
associated with source routing. This approach may not be optimal
when the SFP doesn't change very frequently, as in minutes or hours.
There are contexts that it might not be feasible for the head end
Classifier to be notified of the changes of SFF-sequence or SFF-SF-
Sequence for a given SFP because of the time taken for the
notification and the limited capability of the Classifier nodes.
A.2. Fully Controlled SFF-SF-Sequence for a SFP
This section describes the information that can be exchanged over C2
interface (Section 3.3.2) when the SFC Control Element explicitly
passes the steering policies to all SFFs for the SFF-SF-Sequence of a
given SFC. In this model, each SFF doesn't need to signal other SFFs
for the SFP.
Suppose the SFC ID for this SFP is "yellow", an example of policy to
"sff-a" is depicted in Figure 2 (for illustration proposes)
Matching | Action
----------------------------------------+-------------------------
SFC ID = "yellow" & ingress = sffx-port | next-hop: "sf2" & VID
SFC ID = "yellow" & ingress = sf2-port | next-hop: "sf3" & VID
SFC ID = "yellow" & ingress = sf3-port | next-hop: sff-b
Figure 2: Example of Traffic Steering Policy to a SFF node
The SFF nodes may not be directly adjacent to each other. They can
be interconnected by tunnels, such as GRE, VxLAN, etc. SFs are
attached to a SFF node or SFC Proxy node via Ethernet link or other
link types. Therefore, the steering policies to a SFF node for
service function chain depends on if the packet comes from previous
SFF or comes from a specific SF, i.e., the SFC Forwarding Policy
Table entries have to be ingress port specific. There are multiple
different steering policies for one flow within one SFF and each set
of steering policies is specific for an ingress port.
The semantics of traffic steering rules can be "Match" and "Action",
similar to the "route" described in [I-D.ietf-i2rs-rib-info-model].
The "match" and "action" for distinct ports can be different. The
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matching criteria for SFF can be more sophisticated. For example,
the matching criteria could be any fields in the data packets:
o Ingress port
o Destination MAC address
o Source MAC address
o VLAN_id,
o Destination IP address
o Source IP address
o Source port number
o Destination port number
o DSCP
o Packet size, etc., or any combination thereof.
A SFF node may not support some of the matching criteria listed
above. It is important that SFC control plane can retrieve the
supported matching criteria by SFF nodes. The "Actions" for traffic
steering could be to steer traffic to the attached service function
or SF instantiations via a specific port.
The "Actions" to SFC Proxy may include a method to map the SFC
Identifier carried in the packet header to a locally significant link
identifier, e.g., VLAN-ID, and a method to construct and encapsulate
the SFC header back to the packets when they come back from the
attached SFs.
This approach does not require using an end-to-end signaling protocol
among Classier nodes and SFF nodes. However, there may be problems
encountered if SFF nodes are not updated in the proper order or not
at the same time. For example, if the SFF "A" and SFF "C" get flow
steering policies at slightly different times, some packets might not
be directed to some service functions on a chain.
Authors' Addresses
Hongyu Li
Huawei
Huawei Industrial Base,Bantian,Longgang
Shenzhen
China
EMail: hongyu.li@huawei.com
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Qin Wu
Huawei
101 Software Avenue, Yuhua District
Nanjing, Jiangsu 210012
China
EMail: bill.wu@huawei.com
Yong(Oliver) Huang
Huawei
Huawei Industrial Base,Bantian,Longgang
Shenzhen
China
EMail: oliver.huang@huawei.com
Mohamed Boucadair (editor)
France Telecom
Rennes 35000
France
EMail: mohamed.boucadair@orange.com
Christian Jacquenet
France Telecom
Rennes 35000
France
EMail: christian.jacquenet@orange.com
Walter Haeffner
Vodafone D2 GmbH
Ferdinand-Braun-Platz 1
Duesseldorf 40549
DE
EMail: walter.haeffner@vodafone.com
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Seungik Lee
ETRI
218 Gajeong-ro Yuseung-Gu
Daejeon 305-700
Korea
Phone: +82 42 860 1483
EMail: seungiklee@etri.re.kr
Ron Parker
Affirmed Networks
Acton
MA 01720
USA
EMail: ron_parker@affirmednetworks.com
Linda Dunbar
Huawei Technologies
USA
EMail: ldunbar@huawei.com
Andrew Malis
Huawei Technologies
USA
EMail: agmalis@gmail.com
Joel M. Halpern
Ericsson
EMail: joel.halpern@ericsson.com
Tirumaleswar Reddy
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
India
EMail: tireddy@cisco.com
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Prashanth Patil
Cisco Systems, Inc.
Bangalore
India
EMail: praspati@cisco.com
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