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SPRING Working Group                                             S. Peng
Internet-Draft                                                     Z. Li
Intended status: Informational                                    Huawei
Expires: April 25, 2019                                 October 22, 2018

                 SRv6 Compatibility with Legacy Devices


   When deploying SRv6 on legacy devices, there are some compatibility
   challenges such as the support of SRH processing.  This document
   identifies some of the major challenges, and provides solutions that
   are able to mitigate those challenges and smooth the evolution
   towards SRv6 deployment.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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
   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 April 25, 2019.

Copyright Notice

   Copyright (c) 2018 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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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Compatibility challenges  . . . . . . . . . . . . . . . . . .   3
     2.1.  Fast Reroute (FRR)  . . . . . . . . . . . . . . . . . . .   3
     2.2.  Traffic Engineering (TE)  . . . . . . . . . . . . . . . .   3
     2.3.  Service Function Chaining (SFC) . . . . . . . . . . . . .   4
     2.4.  iOAM  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Solutions . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  TE  . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.1.  Binding SID (BSID)  . . . . . . . . . . . . . . . . .   5
       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
   4.  Summary . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Segment Routing (SR) is a source routing paradigm, which allows a
   headend node to steer the packets 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.filsfils-spring-srv6-network-programming] is the SR instantiated
   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 encoded as an IPv6 address.
   An ordered list of SRv6 SIDs forms an SR Policy, which can be used
   for, for example, Traffic Engineering (TE), Service Function Chaining
   (SFC), and Operations, Administration, and Maintenance (OAM).
   Meanwhile, it will also bring challenges on the legacy devices to
   support SRv6 correspondingly.

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   This document provides solutions that can mitigate the identified
   compatibility challenges and ease the evolution towards SRv6

2.  Compatibility challenges

   By adopting SR Policy, the states in the network can be greatly
   reduced, which will relieve the devices and evolve into stateless
   fabric ultimately.  However, it will also bring compatibility
   challenges on the legacy devices correspondingly.  In particular, the
   legacy devices need to upgrade in order to support the processing of
   SRH.  Furthermore, as the segments in the segment list increase the
   SR Policy incrementally expends, the encapsulation header overhead
   increases, which will also impose high requirements on the
   performance of hardware forwarding (i.e.  the capability of chipset).

   This section identifies the imposed challenges 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.bashandy-rtgwg-segment-routing-ti-lfa] provides a local repair
   mechnism with the ability to activate the data plane switch-over onto
   a loop free backup path irrespective of topologies prior and after
   the sudden failure.

   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 SR header, 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 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 paths are
   explicitly expressed, while the strict option specifies each
   individual hop in the explicit path, e.g. to encode a low-latency
   path from node A to node B.  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.

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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 enviornment [I-D.xuclad-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 mechnisms, e.g.  ICMP
   and active probing.  The iOAM data fields, i.e. a node data list,
   hold the information collected as the packets traversing the iOAM
   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.

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                                                            |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

   The compatibility challenges on the legacy devices are summarised as

   o  The legacy devices need to upgrade in order to support the
      processing of SRH

   o  As the SRH expands, the overhead increases and correspondingly the
      effective payload decreases

   o  As the SRH expands, the hardware forwarding performance reduces
      which requires high capability of chipset

3.  Solutions

   This section provides solutions to mitigate the above-mentioned

3.1.  TE

   With the strict traffic engineering, the resulted 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 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

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   BSID, the node will steer the packet onto the bound policy
   accordingly.  To reduce the long SID list of a strict TE explicit
   path, BSID can be used at the selected 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 states 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

   PCEP FlowSpec [I-D.ietf-pce-pcep-flowspec] provides a trade-off
   solution.  PCEP FlowSpec is that PCEP with a set of extensions is
   able to disseminate Flow Specifications (i.e.  filters and actions)
   to allow indicating 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, states need 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.xuclad-spring-sr-service-programming] and stateful
   SFC [I-D.guichard-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,
   there are two types of services, i.e. SR-aware service and SR-unware
   service.  An SR-aware service is capable of processing the SRH in the
   received packets.  While 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

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   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 actually 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
   still be carried in the packet along the path till a PSP/USP is

   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.

4.  Summary

   The SRH enables a great number of features for SRv6 and opens new
   network programming possilities.  By using SRH, it relieves the
   network devices from states, evolving towards stateless fabric, while

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   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 are able to mitigate these
   challenges and smooth the evolution in operators' networks.

5.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an

6.  Security Considerations


7.  Acknowledgements


8.  Normative References

              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-01 (work in progress), July 2018.

              Bashandy, A., Filsfils, C., Decraene, B., Litkowski, S.,
              Francois, P., daniel.voyer@bell.ca, d., Clad, F., and P.
              Camarillo, "Topology Independent Fast Reroute using
              Segment Routing", draft-bashandy-rtgwg-segment-routing-ti-
              lfa-05 (work in progress), October 2018.

              Filsfils, C., Camarillo, P., Leddy, J.,
              daniel.voyer@bell.ca, d., Matsushima, S., and Z. Li, "SRv6
              Network Programming", draft-filsfils-spring-srv6-network-
              programming-05 (work in progress), July 2018.

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              Guichard, J., Song, H., Tantsura, J., Halpern, J.,
              Henderickx, W., and M. Boucadair, "NSH and Segment Routing
              Integration for Service Function Chaining (SFC)", draft-
              guichard-spring-nsh-sr-00 (work in progress), September

              Filsfils, C., Previdi, S., Leddy, J., Matsushima, S., and
              d. daniel.voyer@bell.ca, "IPv6 Segment Routing Header
              (SRH)", draft-ietf-6man-segment-routing-header-14 (work in
              progress), June 2018.

              Brockners, F., Bhandari, S., Pignataro, C., Gredler, H.,
              Leddy, J., Youell, S., Mizrahi, T., Mozes, D., Lapukhov,
              P., Chang, R., daniel.bernier@bell.ca, d., and J. Lemon,
              "Data Fields for In-situ OAM", draft-ietf-ippm-ioam-
              data-03 (work in progress), June 2018.

              Dhody, D., Farrel, A., and Z. Li, "PCEP Extension for Flow
              Specification", draft-ietf-pce-pcep-flowspec-02 (work in
              progress), October 2018.

              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.

              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-xuclad-spring-sr-service-
              programming-00 (work in progress), July 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8300]  Quinn, P., Ed., Elzur, U., Ed., and C. Pignataro, Ed.,
              "Network Service Header (NSH)", RFC 8300,
              DOI 10.17487/RFC8300, January 2018,

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   [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>.

Authors' Addresses

   Shuping Peng

   Email: pengshuping@huawei.com

   Zhenbin Li

   Email: lizhenbin@huawei.com

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