Internet DRAFT - draft-peng-spring-srv6-compatibility
draft-peng-spring-srv6-compatibility
SPRING Working Group H. Tian
Internet-Draft F. Zhao
Intended status: Informational CAICT
Expires: July 13, 2020 C. Xie
China Telecom
T. Li
J. Ma
China Unicom
S. Peng
Z. Li
Huawei Technologies
J. Guichard
Futurewei Technologies Ltd.
January 10, 2020
SRv6 Compatibility with Legacy Devices
draft-peng-spring-srv6-compatibility-02
Abstract
When deploying SRv6 on legacy devices, there are some compatibility
challenges that must be addressed such as the support for SRH
processing.
This document identifies some of the major challenges, and provides
solutions that can mitigate those challenges and smooth the migration
towards SRv6 deployment.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 13, 2020.
Copyright Notice
Copyright (c) 2020 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Compatibility Challenges . . . . . . . . . . . . . . . . . . 3
2.1. Fast Reroute (FRR) . . . . . . . . . . . . . . . . . . . 3
2.2. Traffic Engineering (TE) . . . . . . . . . . . . . . . . 4
2.3. Service Function Chaining (SFC) . . . . . . . . . . . . . 4
2.4. IOAM . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Traffic Engineering . . . . . . . . . . . . . . . . . . . 6
3.1.1. Binding SID (BSID) . . . . . . . . . . . . . . . . . 6
3.1.2. PCEP FlowSpec . . . . . . . . . . . . . . . . . . . . 6
3.2. SFC . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2.1. Stateless SFC . . . . . . . . . . . . . . . . . . . . 6
3.2.2. Stateful SFC . . . . . . . . . . . . . . . . . . . . 7
3.3. Light Weight IOAM . . . . . . . . . . . . . . . . . . . . 7
3.4. Postcard Telemetry . . . . . . . . . . . . . . . . . . . 8
4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 8
8. Normative References . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
Segment Routing (SR) is a source routing paradigm, which allows a
headend node to steer packets into an SR policy which is instantiated
through an ordered list of instructions, i.e. segments [RFC8402]. A
segment can either be topological or service based. SR over IPv6
(SRv6) [I-D.ietf-spring-srv6-network-programming] is the
instantiation of SR on the IPv6 data plane with a new type of routing
extension header, i.e. SR Header (SRH)
[I-D.ietf-6man-segment-routing-header]. An SRv6 segment, also called
SRv6 SID, is a 128-bit value, represented as LOC:FUNCT:ARGS (ARGS is
optional), and is encoded as an IPv6 address. An ordered list of
SRv6 SIDs forms an SR Policy, which can be used for Traffic
Engineering (TE), Service Function Chaining (SFC), and In-situ
Operations, Administration, and Maintenance (IOAM). Meanwhile, the
deployment of SRv6 will bring challenges for legacy devices that do
not natively support SRv6.
This document provides solutions that can mitigate the identified
compatibility challenges and ease the evolution towards SRv6
deployment.
2. Compatibility Challenges
By adopting SR Policy, state in the network devices can be greatly
reduced, which ultimately evolves the network into a stateless
fabric. However, it also brings compatibility challenges on the
legacy devices. In particular, the legacy devices need to upgrade
software and/or hardware in order to support the processing of SRH.
Furthermore, as the segments in the segment list increase the SR
Policy incrementally expands, the encapsulation header overhead
increases, which imposes high performance requirements on the
performance of hardware forwarding (i.e. the capability of the
chipset).
This section identifies the challenges for legacy devices imposed by
SRv6 in the following SPRING use cases.
2.1. Fast Reroute (FRR)
FRR is deployed to cope with link or node failures by precomputing
backup paths. By relying on SR, Topology Independent Loop-free
Alternate Fast Re-route (TI-LFA)
[I-D.ietf-rtgwg-segment-routing-ti-lfa] provides a local repair
mechanism with the ability to activate the data plane switch-over on
to a loop-free backup path irrespective of topologies prior and after
the failure.
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Using SR, there is no need to create state in the network in order to
enforce FRR behavior. Correspondingly, the Point of Local Repair,
i.e. the protecting router, needs to insert a repair list at the head
of the segment list in the SRH, encoding the explicit post-
convergence path to the destination. This action will increase the
length of the segment list in the SRH as shown in Figure 1.
2.2. Traffic Engineering (TE)
TE enables network operators to control specific traffic flows going
through configured explicit paths. There are loose and strict
options. With the loose option, only a small number of hops along
the path is explicitly expressed, while the strict option specifies
each individual hop in the explicit path, e.g. to encode a low
latency path from one network node to another.
With SRv6, the strict source-routed explicit paths will result in a
long segment list in the SRH as shown in Figure 1, which places high
requirements on the devices.
2.3. Service Function Chaining (SFC)
The SR segments can also encode instructions, called service
segments, for steering packets through services running on physical
service appliances or virtual network functions (VNF) running in a
virtual environment [I-D.ietf-spring-sr-service-programming]. These
service segments can also be integrated in an SR policy along with
node and adjacency segments. This feature of SR will further
increase the length of the segment list in the SRH as shown in
Figure 1.
In terms of SR awareness, there are two types of services, i.e. SR-
aware and SR-unaware services, which both impose new requirements on
the hardware. The SR-aware service needs to be fully capable of
processing SR traffic, while for the SR-unaware services, an SR proxy
function needs to be defined.
If the Network Service Header (NSH) based SFC [RFC8300] has already
been deployed in the network, the compatibility with existing NSH is
required.
2.4. IOAM
IOAM, i.e. "in-situ" Operations, Administration, and Maintenance
(OAM), encodes telemetry and operational information within the data
packets to complement other "out-of-band" OAM mechanisms, e.g. ICMP
and active probing. The IOAM data fields, i.e. a node data list,
hold the information collected as the packets traverse the IOAM
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domain [I-D.ietf-ippm-ioam-data], which is populated iteratively
starting with the last entry of the list.
The IOAM data can be embedded into a variety of transports. To
support the IOAM on the SRv6 data plane, the O-flag in the SRH is
defined [I-D.ali-spring-srv6-oam], which implements the "punt a
timestamped copy and forward" or "forward and punt a timestamped
copy" behavior. The IOAM data fields, i.e. the node data list, are
encapsulated in the IOAM TLV in SRH, which further increases the
length of the SRH as shown in Figure 1.
+-----------+
|IPv6 packet|
+-----------+
/ /
+-----------+ / IOAM Info /
|IPv6 packet| / /
+-----------+ +-----------+ +-----------+
|IPv6 packet| / / / /
+-----------+ +-----------+ / / / /
|IPv6 packet| / / / SF Chain / / SF Chain /
+-----------+ +-----------+ / TE Path / / / / /
|IPv6 packet| /TI-LFA Path/ / / / / / /
+-----------+ +-----------+ +-----------+ +-----------+ +-----------+
|SA,DA | |SA,DA | |SA,DA | |SA,DA | |SA,DA |
+-----------+ +-----------+ +-----------+ +-----------+ +-----------+
SRv6 BE SRv6 BE+ SRv6 TE SRv6 SFC SRv6 SFC+
TI-LFA IOAM
Figure 1. Evolution of SRv6 SRH
Compatibility challenges for legacy devices can be summarized as
follows:
o Legacy devices need to upgrade software and/or hardware in order
to support the processing of SRH
o As the SRH expands, the encapsulation overhead increases and
correspondingly the effective payload decreases
o As the SRH expands, the hardware forwarding performance reduces
which requires higher capabilities of the chipset
3. Solutions
This section provides solutions to mitigate the challenges outlined
in section 2.
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3.1. Traffic Engineering
With strict traffic engineering, the resultant long SID list in the
SRH raises high requirements on the hardware chipset, which can be
mitigated by the following solutions.
3.1.1. Binding SID (BSID)
Binding SID [RFC8402] involves a list of SIDs and is bound to an SR
Policy. The node(s) that imposes the bound policy needs to store the
SID list. When a node receives a packet with its active segment as a
BSID, the node will steer the packet in to the bound policy
accordingly.
To reduce the long SID list of a strict TE explicit path, BSID can be
used at selective nodes, maybe according to the processing capacity
of the hardware chipset. BSID can also be used to impose the repair
list in the TI-LFA as described in Section 2.1.
3.1.2. PCEP FlowSpec
When the SR architecture adopts a centralized model, the SDN
controller (e.g. Path Computation Element (PCE)) only needs to apply
the SR policy at the head-end. There is no state maintained at
midpoints and tail-ends. Eliminating state in the network (midpoints
and tail-points) is a key benefit of utilizing SR. However, it also
leads to a long SID list for expressing a strict TE path.
PCEP FlowSpec [I-D.ietf-pce-pcep-flowspec] provides a trade-off
solution. PCEP FlowSpec is able to disseminate Flow Specifications
(i.e. filters and actions) to indicate how the classified traffic
flows will be treated. In an SR-enabled network, PCEP FlowSpec can
be applied at the midpoints to enforce traffic engineering policies
where it is needed. In that case, state needs to be maintained at
the corresponding midpoints of a TE explicit path, but the SID list
can be shortened.
3.2. SFC
Currently two approaches are proposed to support SFC over SRv6, i.e.
stateless SFC [I-D.ietf-spring-sr-service-programming] and stateful
SFC [I-D.ietf-spring-nsh-sr].
3.2.1. Stateless SFC
A service can also be assigned an SRv6 SID which is integrated into
an SR policy and used to steer traffic to it. In terms of the
capability of processing the SR information in the received packets,
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there are two types of services, i.e. SR-aware service and SR-unware
service. An SR-aware service can process the SRH in the received
packets. An SR-unaware service, i.e. legacy service, is not able to
process the SR information in the traffic it receives, and may drop
the received packets. In order to support such services in an SRv6
domain, the SR proxy is introduced to handle the processing of SRH on
behalf of the SR-unware service. The service SID associated with the
SR-unaware service is instantiated on the SR proxy, which is used to
steer traffic to the service.
The SR proxy intercepts the SR traffic destined for the service via
the locally instantiated service SID, removes the SR information, and
sends the non-SR traffic out on a given interface to the service.
When receiving the traffic coming back from the service, the SR proxy
will restore the SR information and forwards it to the next segment
in the segment list.
3.2.2. Stateful SFC
The NSH and SR can be integrated in order to support SFC in an
efficient and cost-effective manner while maintaining separation of
the service and transport planes.
In this NSH-SR integration solution, NSH and SR work jointly and
complement each other. Specifically, SR is responsible for steering
packets along a given Service Function Path (SFP) while NSH is for
maintaining the SFC instance context, i.e. Service Path Identifier
(SPI), Service Index (SI), and any associated metadata.
When a service chain is established, a packet associated with that
chain will be first encapsulated with an NSH and then an SRH, and
forwarded in the SR domain. When the packet arrives at an SFF and
needs to be forwarded to an SF, the SFF performs a lookup based on
the service SID associated with the SF to retrieve the next-hop
context (a MAC address) between the SFF and SF. Then the SFF strips
the SRH and forwards the packet with NSH carrying metadata to the SF
where the packet will be processed as specified in [RFC8300]. In
this case, the SF is not required to be capable of the SR operation,
neither is the SR proxy. Meanwhile, the stripped SRH will be updated
and stored in a cache in the SFF, indexed by the NSH SPI for the
forwarding of the packet coming back from the SF.
3.3. Light Weight IOAM
In most cases, after the IPv6 Destination Address (DA) is updated
according to the active segment in the SRH, the SID in the SRH will
not be used again. However, the entire SID list in the SRH will
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still be carried in the packet along the path till a PSP/USP is
enforced.
The light weight IOAM method [I-D.li-spring-passive-pm-for-srv6-np]
makes use of the used segments in the SRH to carry the IOAM
information, which saves the extra space in the SRH and mitigate the
requirements on the hardware.
3.4. Postcard Telemetry
Existing in-situ OAM techniques incur encapsulation and header
overhead issues as described in section 2. Postcard-based Telemetry
with Packet Marking for SRv6 on-path
OAM[I-D.song-ippm-postcard-based-telemetry], provides a solution that
avoids the extra overhead for encapsulating telemetry-related
instruction and metadata in SRv6 packets.
4. Summary
The SRH enables a great number of features for SRv6 and opens new
network programming possibilities. By using SRH, it relieves the
network devices from state, evolving towards stateless fabric, while
the complexity in the control plane increases. The corresponding
challenges imposed on the hardware chipset become high as the SRH
expands when supporting the diverse use cases. The trade-off
solutions presented in this document can mitigate these challenges
and smooth the evolution in operators' networks.
5. IANA Considerations
There are no IANA considerations in this document.
Note to RFC Editor: this section may be removed on publication as an
RFC.
6. Security Considerations
TBD
7. Contributors
Hailong Bai
China Unicom
China
Ruizhao Hu
Huawei
China
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Email: huruizhao@huawei.com
Jianwei Mao
Huawei
China
Email: maojianwei@huawei.com
8. Normative References
[I-D.ali-spring-srv6-oam]
Ali, Z., Filsfils, C., Kumar, N., Pignataro, C.,
faiqbal@cisco.com, f., Gandhi, R., Leddy, J., Matsushima,
S., Raszuk, R., daniel.voyer@bell.ca, d., Dawra, G.,
Peirens, B., Chen, M., and G. Naik, "Operations,
Administration, and Maintenance (OAM) in Segment Routing
Networks with IPv6 Data plane (SRv6)", draft-ali-spring-
srv6-oam-02 (work in progress), October 2018.
[I-D.ietf-6man-segment-routing-header]
Filsfils, C., Dukes, D., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", draft-ietf-6man-segment-routing-header-26 (work in
progress), October 2019.
[I-D.ietf-ippm-ioam-data]
Brockners, F., Bhandari, S., Pignataro, C., Gredler, H.,
Leddy, J., Youell, S., Mizrahi, T., Mozes, D., Lapukhov,
P., remy@barefootnetworks.com, r., daniel.bernier@bell.ca,
d., and J. Lemon, "Data Fields for In-situ OAM", draft-
ietf-ippm-ioam-data-08 (work in progress), October 2019.
[I-D.ietf-pce-pcep-flowspec]
Dhody, D., Farrel, A., and Z. Li, "PCEP Extension for Flow
Specification", draft-ietf-pce-pcep-flowspec-07 (work in
progress), January 2020.
[I-D.ietf-rtgwg-segment-routing-ti-lfa]
Litkowski, S., Bashandy, A., Filsfils, C., Decraene, B.,
Francois, P., Voyer, D., Clad, F., and P. Camarillo,
"Topology Independent Fast Reroute using Segment Routing",
draft-ietf-rtgwg-segment-routing-ti-lfa-01 (work in
progress), March 2019.
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[I-D.ietf-spring-nsh-sr]
Guichard, J., Song, H., Tantsura, J., Halpern, J.,
Henderickx, W., Boucadair, M., and S. Hassan, "Network
Service Header (NSH) and Segment Routing Integration for
Service Function Chaining (SFC)", draft-ietf-spring-nsh-
sr-01 (work in progress), October 2019.
[I-D.ietf-spring-sr-service-programming]
Clad, F., Xu, X., Filsfils, C., daniel.bernier@bell.ca,
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-01 (work in progress), November 2019.
[I-D.ietf-spring-srv6-network-programming]
Filsfils, C., Camarillo, P., Leddy, J., Voyer, D.,
Matsushima, S., and Z. Li, "SRv6 Network Programming",
draft-ietf-spring-srv6-network-programming-07 (work in
progress), December 2019.
[I-D.li-spring-passive-pm-for-srv6-np]
Li, C. and M. Chen, "Passive Performance Measurement for
SRv6 Network Programming", draft-li-spring-passive-pm-for-
srv6-np-00 (work in progress), March 2018.
[I-D.song-ippm-postcard-based-telemetry]
Song, H., Zhou, T., Li, Z., Shin, J., and K. Lee,
"Postcard-based On-Path Flow Data Telemetry", draft-song-
ippm-postcard-based-telemetry-06 (work in progress),
October 2019.
[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>.
[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>.
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Authors' Addresses
Hui Tian
CAICT
China
Email: tianhui@caict.ac.cn
Feng Zhao
CAICT
China
Email: zhaofeng@caict.ac.cn
Chongfeng Xie
China Telecom
China
Email: xiechf.bri@chinatelecom.cn
Tong Li
China Unicom
China
Email: litong@chinaunicom.cn
Jichun Ma
China Unicom
China
Email: majc16@chinaunicom.cn
Shuping Peng
Huawei Technologies
China
Email: pengshuping@huawei.com
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Zhenbin Li
Huawei Technologies
China
Email: lizhenbin@huawei.com
James N Guichard
Futurewei Technologies Ltd.
USA
Email: jguichar@futurewei.com
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