A Framework for Constructing Service Function Chaining Systems Based on Segment Routing
draft-li-spring-sr-sfc-control-plane-framework-04
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Cheng Li
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Ahmed El Sawaf
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Ruizhao Hu
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Zhenbin Li
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2021-05-05
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SPRING Working Group C. Li
Internet-Draft Huawei Technologies
Intended status: Informational A. Sawaf
Expires: November 7, 2021 Saudi Telecom Company
R. Hu
Z. Li
Huawei Technologies
May 6, 2021
A Framework for Constructing Service Function Chaining Systems Based on
Segment Routing
draft-li-spring-sr-sfc-control-plane-framework-04
Abstract
Segment Routing (SR) allows for a flexible definition of end-to-end
paths by encoding paths as sequences of topological sub-paths, called
"segments". Segment routing architecture can be implemented over an
MPLS data plane as well as an IPv6 data plane.
Service Function Chaining (SFC) provides support for the creation of
composite services that consist of an ordered set of Service
Functions (SF) that are to be applied to packets and/or frames
selected as a result of classification.
SFC can be implemented based on several technologies, such as Network
Service Header (NSH) and SR. This document describes a framework for
constructing SFC based on Segment Routing. The document reviews the
control plane solutions for route distribution of service function
instance and service function path, and steering packets into a
service function chain.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://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
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on November 7, 2021.
Copyright Notice
Copyright (c) 2021 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
(https://trustee.ietf.org/license-info) in effect on the date 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. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview of SR Based SFC Control Plane . . . . . . . . . . . 4
3. Stateless SR Based SFC . . . . . . . . . . . . . . . . . . . 6
3.1. Service Function Instance Route Distribution . . . . . . 6
3.2. Service Function Path Route Distribution . . . . . . . . 7
3.3. Steer Packets into SFC . . . . . . . . . . . . . . . . . 7
4. Stateful SR Based SFC . . . . . . . . . . . . . . . . . . . . 8
4.1. Service Function Route Distribution . . . . . . . . . . . 8
4.2. Service Function Path Route Distribution . . . . . . . . 8
4.3. Steer Packets into SFC . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
Segment routing (SR) [RFC8402] is a source routing paradigm that
explicitly indicates the forwarding path for packets at the ingress
node by inserting an ordered list of instructions, called segments.
When segment routing is deployed on MPLS data plane, it is called SR-
MPLS [RFC8660]. When segment routing is deployed on IPv6 data plane,
it is called SRv6 [RFC8754].
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Service Function Chaining (SFC) [RFC7665] provides an architecture
that supports the creation of composite service that consist of an
ordered set of Service Functions (SF) that are to be applied to
packets and/or frames selected as a result of classification.
SFC can be implemented based on Network Service Header [RFC8300]. In
NSH-based SFC, per-SFC state, such as a mapping between Service Path
Identifier (SPI) and Service Index (SI) to next-hop forwarding, needs
to be maintained on nodes along the Service Function Path(SFP), and
it can therefore, be termed as "stateful SFC".
[I-D.ietf-bess-nsh-bgp-control-plane] defines the use of BGP as a
control plane for networks that support SFC based on NSH and MPLS.
The document introduces a new BGP address family called the SFC AFI/
SAFI with two route types: Service Function Instance Route (SFIR) and
Service Function Path Route (SFPR). An NSH or MPLS based SFC can be
constructed based on the information of SFIR and SFPR.
SFC can also be instantiated based on SR. In SR, the forwarding path
is explicitly encoded into the packets on the SR source node. In SR-
based SFC, an SFC can be represented by a SID list explicitly
indicated by the source SR node. The SID in SID list may need to be
associated with service information in order to indicate network
service, such as Deep Packet Inspection (DPI). Therefore, no per-SFC
state needs to be maintained along with the SFP, and it can therefore
be termed "stateless SFC".
In order to construct SR-based SFC, several mechanisms are proposed,
including the mechanisms of SFIR and SFPR distribution, as well as
the mechanism of steering packets into an SFP. This document reviews
these solutions to describe a framework for the construction of an
SFC system based on Segment Routing.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Terminology
MPLS: Multiprotocol Label Switching.
SID: Segment Identifier.
SR: Segment Routing.
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SR-MPLS: Segment Routing with MPLS data plane.
SRH: Segment Routing Header.
SFIR: Service Function Instance Route
SFPR: Service Function Path Route
Further, this document makes use of the terms defined in [RFC7665]
and [I-D.ietf-spring-sr-service-programming].
2. Overview of SR Based SFC Control Plane
As per [RFC7665], the architecture of SFC consists of classifiers,
Service Function Forwarders (SFFs), Service Functions (SFs) and SFC
Proxies. This is illustrated in Figure 1.
+-----+ +-----+ +-----+
| | | SFC | | |
| SF1 | |Proxy|---| SF2 |
+-----+ +-----+ +-----+
| |
+--------------+ | |
| Service | SFC +------+ +------+
|Classification| Encapsulation | SFF1 | | SFF2 |
--->| Function |+---------------->| |--------| |------->
| | | | | |
+--------------+ +------+ +------+
SFC-enabled Domain
Figure 1. SFC Architecture
In order to construct an SFC, SFIR and SFPR should be distributed to
classifiers and SFFs. Also, the rules of steering packets into
specific SFPs should be configured at the classifier.
[I-D.ietf-bess-nsh-bgp-control-plane].
In SR, a source node can explicitly indicate the forwarding path for
packets by inserting an ordered list of instructions. These packet
steering policies, known as SR policy, can be installed by a central
controller via BGP [I-D.ietf-idr-segment-routing-te-policy] or other
mechanisms.
When SFC is constructed based on SR, SFPR and packet steering rules
can be installed by SR policy at the ingress node, which plays the
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role of classifier in the SFC architecture. In other words, SFPR
does not need to be distributed to all the nodes along the SFP. The
architecture of SR based SFC is illustrated in Figure 2.
+-----+ +-----+ +-----+ +-----+
| | | | | SR | | |
|SR-C | | SF1 | |Proxy|---| SF2 |
+-----+ +-----+ +-----+ +-----+
| | |
| | |
+--------------+ +------+ +------+
| | SFC Encap/SR | SFF1/| | SFF2/|
--->|CF/SR ingress |+---------------->| SR |--------| SR |------->
| | | | | |
+--------------+ +------+ +------+
SFC-enabled Domain
Figure 2. SR based SFC architecture.
o CF/SR ingress: an SR ingress node plays the role of Classifier in
the SFC architecture, and it connects to an SR controller, where
the SR policies originate.
o SR-C: SR Controller (SR-C) is connected to the SR ingress node,
and may be attached to any node in the network. SR-C is capable
of discovering topology, and calculating constrained paths for
SFCs.
o SFF/SR nodes: the SFF component in SFC architecture, which enables
SR to steer packets to SFs.
o SFn: Service Functions, can be SR-aware or SR-unaware. If an SF
is SR-unaware then SR proxy is needed.
o SR proxy: A proxy between SFF/SR nodes and SR-unaware SF.
There are two solutions to encode SFC in the SR data plane.
[I-D.ietf-spring-sr-service-programming] defines data plane
functionality required to implement service segments and achieve
service programming in SR-enabled MPLS and IP networks. It can be
termed "Stateless SFC" since no per-SFC state is maintained on the SR
nodes along the SFP.
The second solution can be termed "Stateful SFC"
[I-D.ietf-spring-nsh-sr], since it still maintains per-SFC state on
nodes. [I-D.ietf-spring-nsh-sr]describes two modes of operation:
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o NSH-based SFC with SR-based transport tunnel: SR is used as the
transport tunnel to route packets between classifier and SFF or
SFFs. Service plane routing relies on NSH.
o SR-based SFC with Integrated NSH Service Plane: The SFP is encoded
within the SR segment-list, while the NSH only maintains the
service plane context information, which will be used at NSH-aware
SFs, and at SFFs as a pointer to cache SR segment-lists.
In order to support these data plane encodings, control plane
mechanisms are required. The existing control plane mechanisms are
shown in table 1.
+------------------------------------------------------------+
| SR based SFC | SFIR | SFPR | Steering policy|
+-------------------+-----------+-----------+----------------+
| | BGP | BGP | BGP |
|Stateless | BGP-LS | PCEP | PCEP |
| | IGP | | |
+-------------------+-----------+-----------+----------------+
|NSH-based SFC | BGP | BGP | BGP |
|with SR-based | | PCEP | |
|transport tunnel | | | |
| | | | |
| | | | |
+-------------------+-----------+-----------+----------------+
|SR-based SFC | BGP | BGP | BGP |
|with Integrated | BGP-LS | PCEP | PCEP |
|NSH Service Plane | IGP | | |
| | | | |
+-------------------+-----------+-----------+----------------+
Table 1. SR based SFC Control Plane Solutions
3. Stateless SR Based SFC
As describe in [I-D.ietf-spring-sr-service-programming], service
instances are associated with a segment, called a service SID. These
service SIDs are leveraged as part of a SID-list to steer packets
through the corresponding services
3.1. Service Function Instance Route Distribution
To associate a segment with a service, service information, such as
Service Function Type (SFT), should be included in segment
distribution. [I-D.dawra-idr-bgp-ls-sr-service-segments] specifies
the extensions to BGP-LS for discovery and advertisement of service
segments to enable setup of service programming paths using Segment
Routing. [I-D.dawra-idr-bgp-ls-sr-service-segments] extends SRv6
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Node SID TLV [I-D.ietf-idr-bgpls-srv6-ext] and SR-MPLS SID/ Label TLV
[I-D.ietf-idr-bgp-ls-segment-routing-ext] to associate the Service
SID Value with Service-related Information using Service Chaining
Sub-TLV. The Service Chaining Sub-TLV contains information of
Service SID value, Function Identifier (Static Proxy, Dynamic Proxy,
Shared Memory Proxy, Masquerading Proxy, SR Aware Service Etc.),
Service Type (DPI, Firewall, Classifier, LB etc.), Traffic Type (IPv4
OR IPv6 OR Ethernet) and Opaque Data (such as brand and version,
other extra information). This extension works for both SR- MPLS and
SRv6.
[I-D.ietf-bess-nsh-bgp-control-plane] proposes a BGP-based SFC
control plane solution, and it works for SR-MPLS as well. Service
function instance route distribution can use SFIR in SFC AFI/SAFI.
SFPR and steering rules for the classifier can be distributed by SR
policy, which is defined in [I-D.ietf-idr-segment-routing-te-policy].
BGP control plane of SRv6-based SFC still needs to be defined.
IGP extensions are proposed by [I-D.xu-isis-service-function-adv] and
[I-D.xu-ospf-service-function-adv]. In IS-IS solution, SFFs within
the SFC domain need to advertise each SF they are offering by using a
new sub-TLV of the IS-IS Router CAPABILITY TLV [RFC4971]. This new
sub-TLV is called Service Function sub-TLV, and it can appear
multiple times within a given IS-IS Router CAPABILITY TLV or when
more than one SF needs to be advertised. OSPF extensions are
similar, and use the OSPF Router Information (RI) Opaque LSA
[RFC4970] to carry Service Function sub-TLV.
However, due to IGP flooding issues, IGP extensions are not very
appropriate, and the drafts have expired for a long time.
3.2. Service Function Path Route Distribution
With SR, the SFPR does not need to be distributed to nodes along the
SFP but only to the ingress node. SFPR and steering rules for the
classifier can be distributed by SR policy. The BGP extension is
defined in [I-D.ietf-idr-segment-routing-te-policy]. The PCEP
extension is defined in [I-D.ietf-pce-segment-routing-policy-cp].
3.3. Steer Packets into SFC
In SR, packet steering rules are learned through SR policy. Thus,
there is no need to install other rules in the classifier, which is
the SR source node.
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4. Stateful SR Based SFC
"Stateful SFC" [I-D.ietf-spring-nsh-sr] proposes two modes of SR
based SFC:
o NSH-based SFC with SR-based transport tunnel
o SR-based SFC with Integrated NSH Service Plane
4.1. Service Function Route Distribution
For NSH-based SFC with SR-based transport tunnel, service information
is maintained by NSH while SR is only used for transport between
SFFs, so [I-D.ietf-bess-nsh-bgp-control-plane] can be used for this
mode.
To indicate NSH, an SFF label [I-D.ietf-mpls-sfc-encapsulation]
should be inserted as the last label in the label stack in SR-MPLS.
The control plane of SFF is also described in
[I-D.ietf-bess-nsh-bgp-control-plane]. For choosing/configuring SR
as the transport tunnel, BGP route of SFF's BGP Tunnel Encapsulation
Attribute Type should be "SR TE Policy Type"
[I-D.ietf-idr-segment-routing-te-policy]. For SR-based SFC with
Integrated NSH Service Plane, there is no control plane solution as
yet defined.
4.2. Service Function Path Route Distribution
Same as SFIR distribution, SFPR BGP distribution in NSH-based SFC
with SR-based transport tunnel is identical to the mechanism defined
in [I-D.ietf-bess-nsh-bgp-control-plane]. PCEP extension for SFPR
distribution can reuse the NSH based SFC extension defined in
[I-D.wu-pce-traffic-steering-sfc]. For SR-based SFC with Integrated
NSH Service Plane, control plane solution is to be added in other
documents.
4.3. Steer Packets into SFC
For NSH-based SFC with SR-based transport tunnel, it is the same with
the NSH based SFC. The Classifier is responsible for determining to
which packet flow a packet belongs (usually by inspecting the packet
header), imposing an NSH, and initializing the NSH with the SPI of
the selected SFP and the SI of its first hop
[I-D.ietf-bess-nsh-bgp-control-plane]. For SR-based SFC with
Integrated NSH Service Plane, control plane solution is to be added
in other document.
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5. IANA Considerations
This document does not require any IANA actions.
6. Security Considerations
This document does not introduce additional security requirements and
mechanisms.
7. Acknowledgements
TBA
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
8.2. Informative References
[I-D.dawra-idr-bgp-ls-sr-service-segments]
Dawra, G., Filsfils, C., Talaulikar, K., Clad, F.,
Bernier, D., Uttaro, J., Decraene, B., Elmalky, H., Xu,
X., Guichard, J., and C. Li, "BGP-LS Advertisement of
Segment Routing Service Segments", draft-dawra-idr-bgp-ls-
sr-service-segments-05 (work in progress), February 2021.
[I-D.ietf-bess-nsh-bgp-control-plane]
Farrel, A., Drake, J., Rosen, E., Uttaro, J., and L.
Jalil, "BGP Control Plane for the Network Service Header
in Service Function Chaining", draft-ietf-bess-nsh-bgp-
control-plane-18 (work in progress), August 2020.
[I-D.ietf-idr-bgp-ls-segment-routing-ext]
Previdi, S., Talaulikar, K., Filsfils, C., Gredler, H.,
and M. Chen, "BGP Link-State extensions for Segment
Routing", draft-ietf-idr-bgp-ls-segment-routing-ext-18
(work in progress), April 2021.
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[I-D.ietf-idr-bgpls-srv6-ext]
Dawra, G., Filsfils, C., Talaulikar, K., Chen, M.,
Bernier, D., and B. Decraene, "BGP Link State Extensions
for SRv6", draft-ietf-idr-bgpls-srv6-ext-07 (work in
progress), March 2021.
[I-D.ietf-idr-segment-routing-te-policy]
Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P.,
Rosen, E., Jain, D., and S. Lin, "Advertising Segment
Routing Policies in BGP", draft-ietf-idr-segment-routing-
te-policy-11 (work in progress), November 2020.
[I-D.ietf-mpls-sfc-encapsulation]
Malis, A. G., Bryant, S., Halpern, J. M., and W.
Henderickx, "MPLS Transport Encapsulation for the Service
Function Chaining (SFC) Network Service Header (NSH)",
draft-ietf-mpls-sfc-encapsulation-04 (work in progress),
March 2019.
[I-D.ietf-pce-segment-routing-policy-cp]
Koldychev, M., Sivabalan, S., Barth, C., Peng, S., and H.
Bidgoli, "PCEP extension to support Segment Routing Policy
Candidate Paths", draft-ietf-pce-segment-routing-policy-
cp-04 (work in progress), March 2021.
[I-D.ietf-spring-nsh-sr]
Guichard, J. N. and J. Tantsura, "Integration of Network
Service Header (NSH) and Segment Routing for Service
Function Chaining (SFC)", draft-ietf-spring-nsh-sr-04
(work in progress), December 2020.
[I-D.ietf-spring-sr-service-programming]
Clad, F., Xu, X., Filsfils, C., Bernier, D., Li, C.,
Decraene, B., Ma, S., Yadlapalli, C., Henderickx, W., and
S. Salsano, "Service Programming with Segment Routing",
draft-ietf-spring-sr-service-programming-04 (work in
progress), March 2021.
[I-D.wu-pce-traffic-steering-sfc]
Wu, Q., Dhody, D., Boucadair, M., Jacquenet, C., and J.
Tantsura, "PCEP Extensions for Service Function Chaining
(SFC)", draft-wu-pce-traffic-steering-sfc-12 (work in
progress), June 2017.
[I-D.xu-isis-service-function-adv]
Xu, X., Wu, N., Shah, H., and L. M. Contreras,
"Advertising Service Functions Using IS-IS", draft-xu-
isis-service-function-adv-05 (work in progress), May 2017.
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[I-D.xu-ospf-service-function-adv]
Xu, X., Wu, N., Shah, H., and L. M. Contreras,
"Advertising Service Functions Using OSPF", draft-xu-ospf-
service-function-adv-02 (work in progress), June 2014.
[RFC4970] Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
S. Shaffer, "Extensions to OSPF for Advertising Optional
Router Capabilities", RFC 4970, DOI 10.17487/RFC4970, July
2007, <https://www.rfc-editor.org/info/rfc4970>.
[RFC4971] Vasseur, JP., Ed., Shen, N., Ed., and R. Aggarwal, Ed.,
"Intermediate System to Intermediate System (IS-IS)
Extensions for Advertising Router Information", RFC 4971,
DOI 10.17487/RFC4971, July 2007,
<https://www.rfc-editor.org/info/rfc4971>.
[RFC7665] Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
Chaining (SFC) Architecture", RFC 7665,
DOI 10.17487/RFC7665, October 2015,
<https://www.rfc-editor.org/info/rfc7665>.
[RFC8300] Quinn, P., Ed., Elzur, U., Ed., and C. Pignataro, Ed.,
"Network Service Header (NSH)", RFC 8300,
DOI 10.17487/RFC8300, January 2018,
<https://www.rfc-editor.org/info/rfc8300>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing with the MPLS Data Plane", RFC 8660,
DOI 10.17487/RFC8660, December 2019,
<https://www.rfc-editor.org/info/rfc8660>.
[RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/info/rfc8754>.
Authors' Addresses
Cheng Li
Huawei Technologies
Email: c.l@huawei.com
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Ahmed El Sawaf
Saudi Telecom Company
Riyadh
Saudi Arabia
Email: aelsawaf.c@stc.com.sa
Ruizhao Hu
Huawei Technologies
Huawei Campus, No. 156 Beiqing Rd.
Beijing 100095
China
Email: huruizhao@huawei.com
Zhenbin Li
Huawei Technologies
Huawei Campus, No. 156 Beiqing Rd.
Beijing 100095
China
Email: lizhenbin@huawei.com
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