Internet DRAFT - draft-guichard-sfc-nsh-sr
draft-guichard-sfc-nsh-sr
SFC J. Guichard, Ed.
Internet-Draft H. Song
Intended status: Informational Huawei
Expires: December 20, 2018 J. Tantsura
Nuage Networks
J. Halpern
Ericsson
W. Henderickx
Nokia
M. Boucadair
Orange
June 18, 2018
NSH and Segment Routing Integration for Service Function Chaining (SFC)
draft-guichard-sfc-nsh-sr-02
Abstract
This document describes two application scenarios where Network
Service Header (NSH) and Segment Routing (SR) techniques can be
deployed together to support Service Function Chaining (SFC) in an
efficient manner while maintaining separation of the service and
transport planes as originally intended by the SFC architecture.
In the first scenario, an NSH-based SFC is created using SR as the
transport between SFFs. SR in this case is just one of many
encapsulations that could be used to maintain the transport-
independent nature of NSH-based service chains.
In the second scenario, SR is used to represent each service hop of
the NSH-based SFC as a segment within the segment-list. SR and NSH
in this case are integrated.
In both scenarios SR is responsible for steering packets between SFFs
along a given SFP while NSH is responsible for maintaining the
integrity of the service plane, the SFC instance context, and any
associated metadata.
These application scenarios demonstrate that NSH and SR can work
jointly and complement each other leaving the network operator with
the flexibility to use whichever transport technology makes sense in
specific areas of their network infrastructure, and still maintain an
end-to-end service plane using NSH.
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Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. SFC Overview and Rationale . . . . . . . . . . . . . . . 3
1.2. SFC within SR Networks . . . . . . . . . . . . . . . . . 4
2. NSH-based SFC with SR-based transport tunnel . . . . . . . . 5
3. SR-based SFC with Integrated NSH Service Plane . . . . . . . 9
4. Encapsulation Details . . . . . . . . . . . . . . . . . . . . 11
4.1. NSH using MPLS-SR Transport . . . . . . . . . . . . . . . 11
4.2. NSH using SRv6 Transport . . . . . . . . . . . . . . . . 12
5. Security Considerations . . . . . . . . . . . . . . . . . . . 13
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Normative References . . . . . . . . . . . . . . . . . . 13
8.2. Informative References . . . . . . . . . . . . . . . . . 13
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
1.1. SFC Overview and Rationale
The dynamic enforcement of a service-derived, adequate forwarding
policy for packets entering a network that supports advanced Service
Functions (SFs) has become a key challenge for operators and service
providers. Particularly, cascading SFs, for example at the Gi
interface in the context of mobile network infrastructure, have shown
their limits, such as the same redundant classification features must
be supported by many SFs in order to execute their function, some SFs
are receiving traffic that they are not supposed to process (e.g.,
TCP proxies receiving UDP traffic), which inevitably affects their
dimensioning and performance, an increased design complexity related
to the properly ordered invocation of several SFs, etc.
In order to solve those problems and to avoid the adherence with the
underlying physical network topology while allowing for simplified
service delivery, Service Function Chaining (SFC) techniques have
been introduced.
SFC techniques are meant to rationalize the service delivery logic
and master the companion complexity while optimizing service
activation time cycles for operators that need more agile service
delivery procedures to better accommodate ever-demanding customer
requirements. Indeed, SFC allows to dynamically create service
planes that can be used by specific traffic flows. Each service
plane is realized by invoking and chaining the relevant service
functions in the right sequence. [RFC7498] provides an overview of
the SFC problem space and [RFC7665] specifies an SFC architecture.
The SFC architecture has the merit to not make assumptions on how
advanced features (e.g., load-balancing, loose or strict service
paths) have to be enabled with a domain. Various deployment options
are made available to operators with the SFC architecture and this
approach is fundamental to accommodate various and heterogeneous
deployment contexts.
Many approaches can be considered for encoding the information
required for SFC purposes (e.g., communicate a service chain pointer,
encode a list of loose/explicit paths, disseminate a service chain
identifier together with a set of context information, etc.).
Likewise, many approaches can also be considered for the channel to
be used to carry SFC-specific information (e.g., define a new header,
re-use existing fields, define an IPv6 extension header, etc.).
Among all these approaches, the IETF endorsed a transport-independent
SFC encapsulation scheme: NSH [RFC8300]; which is the most mature SFC
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encapsulation solution. This design is pragmatic as it does not
require replicating the same specification effort as a function of
underlying transport encapsulation. Moreover, this design approach
encourages consistent SFC-based service delivery in networks enabling
distinct transport protocols in various segments of the network or
even between SFFs vs SF-SFF hops.
1.2. SFC within SR Networks
As described in [I-D.ietf-spring-segment-routing], Segment Routing
(SR) leverages the source routing technique. Concretely, a node
steers a packet through an SR policy instantiated as an ordered list
of instructions called segments. While initially designed for
policy-based source routing, SR also finds its application in
supporting SFC [I-D.xu-clad-spring-sr-service-chaining]. The two SR
flavors, namely MPLS-SR [I-D.ietf-spring-segment-routing-mpls] and
SRv6 [I-D.ietf-6man-segment-routing-header], can both encode a
Service Function (SF) as a segment so that an SFC can be specified as
a segment list. Nevertheless, and as discussed in [RFC7498], traffic
steering is only a subset of the issues that motivated the design of
the SFC architecture. Further considerations such as simplifying
classification at intermediate SFs and allowing for coordinated
behaviors among SFs by means of supplying context information should
be taken into account when designing an SFC data plane solution.
While each scheme (i.e., NSH-based SFC and SR-based SFC) can work
independently, this document describes how the two can be used
together in concert and complement each other through two
representative application scenarios. Both application scenarios may
be supported using either MPLS-SR or SRv6:
o NSH-based SFC with SR-based transport: in this scenario segment
routing provides the transport encapsulation between SFFs while
NSH is used to convey and trigger SFC polices.
o SR-based SFC with integrated NSH service plane: in this scenario
each service hop of the SFC is represented as a segment of the SR
segment-list. SR is responsible for steering traffic through the
necessary SFFs as part of the segment routing path and NSH is
responsible for maintaining the service plane, and holding the SFC
instance context and associated metadata.
It is of course possible to combine both of these two scenarios so as
to support specific deployment requirements and use cases.
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2. NSH-based SFC with SR-based transport tunnel
Because of the transport-independent nature of NSH-based service
chains, it is expected that the NSH has broad applicability across
different domains of a network. By way of illustration the various
SFs involved in a service chain are available in a single data
center, or spread throughout multiple locations (e.g., data centers,
different POPs), depending upon the operator preference and/or
availability of service resources. Regardless of where the service
resources are deployed it is necessary to provide traffic steering
through a set of SFFs and NSH-based service chains provide the
flexibility for the network operator to choose which particular
transport encapsulation to use between SFFs, which may be different
depending upon which area of the network the SFFs/SFs are currently
deployed. Therefore from an SFC architecture perspective, segment
routing is simply one of multiple available transport encapsulations
that can be used for traffic steering between SFFs. Concretely, NSH
does not require to use a unique transport encapsulation when
traversing a service chain. NSH-based service forwarding relies upon
underlying service node capabilities.
The following three figures provide an example of an SFC established
for flow F that has SF instances located in different data centers,
DC1 and DC2. For the purpose of illustration, let the SFC's Service
Path Identifier (SPI) be 100 and the initial Service Index (SI) be
255.
Referring to Figure 1, packets of flow F in DC1 are classified into
an NSH-based SFC and encapsulated after classification as <Inner
Pkt><NSH: SPI 100, SI 255><Outer-transport> and forwarded to SFF1
(which is the first SFF hop for this service chain).
After removing the outer transport encapsulation, that may or may not
be MPLS-SR or SRv6, SFF1 uses the SPI and SI carried within the NSH
encapsulation to determine that it should forward the packet to SF1.
SF1 applies its service, decrements the SI by 1, and returns the
packet to SFF1. SFF1 therefore has <SPI 100, SI 254> when the packet
comes back from SF1. SFF1 does a lookup on <SPI 100, SI 254> which
results in <next-hop: DC1-GW1> and forwards the packet to DC1-GW1.
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+--------------------------- DC1 ----------------------------+
| +-----+ |
| | SF1 | |
| +--+--+ |
| | |
| | |
| +------------+ | +------------+ |
| | N(100,255) | | | F:Inner Pkt| |
| +------------+ | +------------+ |
| | F:Inner Pkt| | | N(100,254) | |
| +------------+ ^ | | +------------+ |
| (2) | | | (3) |
| | | v |
| (1) | (4) |
|+------------+ ----> +--+---+ ----> +---------+ |
|| | NSH | | NSH | | |
|| Classifier +------------+ SFF1 +--------------+ DC1-GW1 + |
|| | | | | | |
|+------------+ +------+ +---------+ |
| |
| +------------+ +------------+ |
| | N(100,255) | | N(100,254) | |
| +------------+ +------------+ |
| | F:Inner Pkt| | F:Inner Pkt| |
| +------------+ +------------+ |
| |
+------------------------------------------------------------+
Figure 1: SR for inter-DC SFC - Part 1
Referring now to Figure 2, DC1-GW1 performs a lookup on the
information conveyed in the NSH which results in <next-hop: DC2-GW1,
encapsulation: SR>. The SR encapsulation has the SR segment-list to
forward the packet across the inter-DC network to DC2.
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+----------- Inter DC ----------------+
| (5) |
+------+ ----> | +---------+ ----> +---------+ |
| | NSH | | | SR | | |
+ SFF1 +----------|-+ DC1-GW1 +-------------+ DC2-GW1 + |
| | | | | | | |
+------+ | +---------+ +---------+ |
| |
| +------------+ |
| | S(DC2-GW1) | |
| +------------+ |
| | N(100,254) | |
| +------------+ |
| | F:Inner Pkt| |
| +------------+ |
+-------------------------------------+
Figure 2: SR for inter-DC SFC - Part 2
When the packet arrives at DC2, as shown in Figure 3, the SR
encapsulation is removed and DC2-GW1 performs a lookup on the NSH
which results in next-hop: SFF2. The outer transport encapsulation
may be any transport that is able to identify NSH as the next
protocol.
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+------------------------ DC2 ----------------------+
| +-----+ |
| | SF2 | |
| +--+--+ |
| | |
| | |
| +------------+ | +------------+ |
| | N(100,254) | | | F:Inner Pkt| |
| +------------+ | +------------+ |
| | F:Inner Pkt| | | N(100,253) | |
| +------------+ ^ | | +------------+ |
| (7) | | | (8) |
| | | v |
| (6) | (9) |
|+----------+ ----> +--+---+ ----> |
|| | NSH | | IP |
|| DC2-GW1 +------------+ SFF2 | |
|| | | | |
|+----------+ +------+ |
| |
| +------------+ +------------+ |
| | N(100,254) | | F:Inner Pkt| |
| +------------+ +------------+ |
| | F:Inner Pkt| |
| +------------+ |
+---------------------------------------------------+
Figure 3: SR for inter-DC SFC - Part 3
The benefits of this scheme are listed hereafter:
o The network operator is able to take advantage of the transport-
independent nature of the NSH encapsulation.
o The network operator is able to take advantage of the traffic
steering capability of SR where appropriate.
o Light-weight NSH is used in the data center for SFC and avoids
more complex hierarchical SFC schemes between data centers.
o Clear responsibility division and scope between NSH and SR.
Note that this scenario is applicable to any case where multiple
segments of a service chain are distributed into multiple domains or
where traffic-engineered paths are necessary between SFFs (strict
forwarding paths for example).
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3. SR-based SFC with Integrated NSH Service Plane
In this scenario we assume that the SFs are NSH-aware and therefore
it should not be necessary to implement an SFC proxy to achieve
Service Function Chaining. The operation relies upon SR to perform
SFF-SFF transport and NSH to provide the service plane between SFs
thereby maintaining SFC context and metadata.
When a service chain is established, a packet associated with that
chain will first encapsulate an NSH that will be used to maintain the
end-to-end service plane through use of the SFC context. The SFC
context (e.g., the service plane path referenced by the SPI) is used
by an SFF to determine the SR segment list for forwarding the packet
to the next-hop SFFs. The packet is then encapsulated using the
(transport-specific) SR header and forwarded in the SR domain
following normal SR operation.
When a packet has to be forwarded to an SF attached to an SFF, the
SFF strips the SR information of the packet, updates the SR
information, and saves it to a cache indexed by the NSH SPI. This
saved SR information is used to encapsulate and forward the packet(s)
coming back from the SF.
When the SF receives the packet, it processes it as usual and sends
it back to the SFF. Once the SFF receives this packet, it extracts
the SR information using the NSH SPI as the index into the cache.
The SFF then pushes the SR header on top of the NSH header, and
forwards the packet to the next segment in the segment list.
Figure 4 illustrates an example of this scenario.
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+-----+ +-----+
| SF1 | | SF2 |
+--+--+ +--+--+
| |
| |
+-----------+ | +-----------+ +-----------+ | +-----------+
|N(100,255) | | |F:Inner Pkt| |N(100,254) | | |F:Inner Pkt|
+-----------+ | +-----------+ +-----------+ | +-----------+
|F:Inner Pkt| | |N(100,254) | |F:Inner Pkt| | |N(100,253) |
+-----------+ | +-----------+ +-----------+ | +-----------+
(2) ^ | (3) | (5) ^ | (6) |
| | | | | |
| | v | | v
+------------+ (1)--> +-+----+ (4)--> +---+--+ (7)-->IP
| | NSHoSR | | NSHoSR | |
| Classifier +--------+ SFF1 +---------------------+ SFF2 |
| | | | | |
+------------+ +------+ +------+
+------------+ +------------+
| S(SF1) | | S(SF2) |
+------------+ +------------+
| S(SFF2) | | N(100,254) |
+------------+ +------------+
| S(SF2) | | F:Inner Pkt|
+------------+ +------------+
| N(100,255) |
+------------+
| F:Inner Pkt|
+------------+
Figure 4: NSH over SR for SFC
The benefits of this scheme include:
o It is economically sound for SF vendors to only support one
unified SFC solution. The SF is unaware of the SR.
o It simplifies the SFF (i.e., the SR router) by nullifying the
needs for re-classification and SR proxy.
o It provides a unique and standard way to pass metadata to SFs.
Note that currently there is no solution for MPLS-SR to carry
metadata and there is no solution to pass metadata to SR-unaware
SFs.
o SR is also used for forwarding purposes including between SFFs.
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o It takes advantage of SR to eliminate the NSH forwarding state in
SFFs. This applies each time strict or loose SFPs are in use.
o It requires no interworking as would be the case if MPLS-SR based
SFC and NSH-based SFC were deployed as independent mechanisms in
different parts of the network.
4. Encapsulation Details
4.1. NSH using MPLS-SR Transport
MPLS-SR instantiates Segment IDs (SIDs) as MPLS labels and therefore
the segment routing header is a stack of MPLS labels.
When carrying NSH within an MPLS-SR transport, the full encapsulation
headers are as illustrated in Figure 5.
+------------------+
~ MPLS-SR Labels ~
+------------------+
| NSH Base Hdr |
+------------------+
| Service Path Hdr |
+------------------+
~ Metadata ~
+------------------+
Figure 5: NSH using MPLS-SR Transport
As described in [I-D.ietf-spring-segment-routing] the IGP signaling
extension for IGP-Prefix segment includes a flag to indicate whether
directly connected neighbors of the node on which the prefix is
attached should perform the NEXT operation or the CONTINUE operation
when processing the SID. When NSH is carried beneath MPLS-SR it is
necessary to terminate the NSH-based SFC at the tail-end node of the
MPLS-SR label stack. This is the equivalent of MPLS Ultimate Hop
Popping (UHP) and therefore the prefix-SID associated with the tail-
end of the SFC MUST be advertised with the CONTINUE operation so that
the penultimate hop node does not pop the top label of the MPLS-SR
label stack and thereby expose NSH to the wrong SFF. It is
RECOMMENDED that a specific prefix-SID be allocated at each node for
use by the SFC application for this purpose.
At the end of the MPLS-SR path it is necessary to provide an
indication to the tail-end that NSH follows the MPLS-SR label stack.
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There are several ways to achieve this but its specification is
outside the scope of this document.
4.2. NSH using SRv6 Transport
When carrying NSH within an SRv6 transport the full encapsulation is
as illustrated in Figure 6.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len | Routing Type | Segments Left |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Last Entry | Flags | Tag | S
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ e
| | g
| Segment List[0] (128 bits IPv6 address) | m
| | e
| | n
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ t
| |
| | R
~ ... ~ o
| | u
| | t
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ i
| | n
| Segment List[n] (128 bits IPv6 address) | g
| |
| | S
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ R
// // H
// Optional Type Length Value objects (variable) //
// //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver|O|U| TTL | Length |U|U|U|U|MD Type| Next Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ N
| Service Path Identifier | Service Index | S
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ H
| |
~ Variable-Length Context Headers (opt.) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: NSH using SRv6 Transport
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5. Security Considerations
Generic SFC-related security considerations are discussed in
[RFC7665]. NSH-specific security considerations are discussed in
[RFC8300].
6. IANA Considerations
This memo includes no request to IANA.
7. Acknowledgments
TBD.
8. References
8.1. Normative References
[I-D.ietf-spring-segment-routing]
Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing
Architecture", draft-ietf-spring-segment-routing-15 (work
in progress), January 2018.
[I-D.ietf-spring-segment-routing-mpls]
Bashandy, A., Filsfils, C., Previdi, S., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing with MPLS
data plane", draft-ietf-spring-segment-routing-mpls-12
(work in progress), February 2018.
[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>.
8.2. Informative References
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[I-D.ietf-6man-segment-routing-header]
Previdi, S., Filsfils, C., Raza, K., Dukes, D., Leddy, J.,
Field, B., daniel.voyer@bell.ca, d.,
daniel.bernier@bell.ca, d., Matsushima, S., Leung, I.,
Linkova, J., Aries, E., Kosugi, T., Vyncke, E., Lebrun,
D., Steinberg, D., and R. Raszuk, "IPv6 Segment Routing
Header (SRH)", draft-ietf-6man-segment-routing-header-09
(work in progress), March 2018.
[I-D.xu-clad-spring-sr-service-chaining]
Clad, F., Xu, X., Filsfils, C., daniel.bernier@bell.ca,
d., Decraene, B., Yadlapalli, C., Henderickx, W., Salsano,
S., and S. Ma, "Segment Routing for Service Chaining",
draft-xu-clad-spring-sr-service-chaining-00 (work in
progress), December 2017.
[RFC7498] Quinn, P., Ed. and T. Nadeau, Ed., "Problem Statement for
Service Function Chaining", RFC 7498,
DOI 10.17487/RFC7498, April 2015,
<https://www.rfc-editor.org/info/rfc7498>.
Authors' Addresses
James N Guichard (editor)
Huawei
2330 Central Express Way
Santa Clara
USA
Email: james.n.guichard@huawei.com
Haoyu Song
Huawei
2330 Central Express Way
Santa Clara
USA
Email: haoyu.song@huawei.com
Jeff Tantsura
Nuage Networks
USA
Email: jefftant.ietf@gmail.com
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Joel Halpern
Ericsson
USA
Email: joel.halpern@ericsson.com
Wim Henderickx
Nokia
USA
Email: wim.henderickx@nokia.com
Mohamed Boucadair
Orange
USA
Email: mohamed.boucadair@orange.com
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