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Versions: 00 01 02                                                      
Spring Working Group                                               T. Li
Internet-Draft                                                   X. Zhou
Intended status: Standards Track                               CNIC, CAS
Expires: September 24, 2021                                      Z. Chen
                                                                  Y. Jia
                                                     Huawei Technologies
                                                          March 23, 2021


          Anycast SID for Flexible and Robust Service in SRv6
                 draft-li-spring-anycast-sid-service-02

Abstract

   Segment Routing enables an operator or an application to specify a
   packet processing program.  When Segment Routing is applied to IPv6
   data plane, the list of IPv6 SIDs in SRH can specify a series of
   execution endpoints that hold service functions that process the
   packet.  However, steering traffic dynamically to the different
   execution endpoints requires a specific "re-encapsulating".  This
   procedure may be complex and take time.

   This document proposes A-SID (Anycast-SID) based on SRv6 to achieve
   flexible and robust service provision.  It uses anycast SID to
   identify service functions and locates the service functions based on
   anycast routing.  The proposed solution can stay compatibility with
   the existing SRv6.

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 September 24, 2021.







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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
   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.  Anycast SID (A-SID) . . . . . . . . . . . . . . . . . . . . .   3
   3.  Control Plane . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Data Plane  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Illustration  . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Usecase1 migration of service function  . . . . . . . . .   7
     5.2.  Usecase2 failover of service function . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Segment Routing [RFC8402] enables an operator or an application to
   specify a packet processing program.  SRv6 applies Segment Routing to
   IPv6 data plane.  A new routing header for IPv6, which is called
   Segment Routing Header (SRH) [RFC8754] is defined to carry 128-bit
   SIDs.  The list of IPv6 SIDs in SRH can specify not only a TE path,
   but also a series of execution endpoints that hold service functions
   that process the packet.  In this way, a service function chain
   [RFC7665] is formed based on SRv6.

   However, more and more functions, such as firewall and DPI, are
   deployed in cloud technology and Network Function Virtualization,
   which means that a single function may be deployed on multiple
   execution locations and the function may be migrated to different
   locations frequently.  Steering traffic dynamically to the different




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   execution endpoints requires a specific "re-encapsulating" frequently
   at the ingress router.  This procedure may be complex and take time.

   This document proposes A-SID (Anycast-SID) based on SRv6 to achieve
   flexible and robust service provision.  In addition to the SIDs that
   are used for TE path on the node, specific A-SIDs are used for
   service function chain.  The execution endpoints share a SID Locator
   and the kind of functions is identified by Function and Argument.
   The A-SIDs are advertised by the control plane and the packets are
   forwarded to the execution endpoints based on anycast routing.  The
   proposed solution can stay compatibility with the existing SRv6.

   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, and BCP 14
   [RFC2119]

   The definitions of the SRv6 terms, such as SRv6, SID, SRH, locator
   and function can be found in [RFC8754], [RFC8402], and [RFC8986].
   The definition of Service Function Chain can be found in [RFC7665].
   The definition of anycast can be found in [RFC3513].

   This document introduces the following new terms:

   A-SID: Anycast-based Segment Identifier.

2.  Anycast SID (A-SID)

   The SRv6 SID contains 16 bytes and can be separated into three parts:
   locator, function, and argument.  As for the argument, we consider it
   as the accessory of function.  So this document omits argument for a
   clear description and the SRv6 SID can be mainly separated into two
   parts: locator and function.

   All of the SRv6 endpoints have their own SIDs instantiated in the FIB
   table locally and advertised by the control plane as defined in
   [RFC8986].  In addition to the existing SRv6 SIDs, the execution
   endpoints that hold service functions also have Anycast SIDs.  The
   Locator of Anycast SID is shared by all the execution endpoints in
   the SR domain.  In other words, one specific Locator is allocated to
   the execution endpoints to identify the Anycast SIDs.  The Function
   of Anycast SID identifies the kind of service functions that the
   endpoint node can provide.  That is, if two execution endpoints
   provide the same kind of service functions, they will have the same
   Anycast SID.

   The routing process of A-SID may follow the standard anycast routing.
   As another option, the routing of A-SID may be based on the



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   processing capacity or computing power of endpoint node.  The
   processing capacity or computing power of endpoint can be advertised
   along with the A-SID in the control plane.  In this way, two
   execution endpoints that provide the same kind of service functions
   with the same A-SID can be chosen based on bigger processing capacity
   or computing power.  The detailed design and extensions to control
   plane will be stated in the next version.

          0                                        16 bytes
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |       Locator       |      Function       |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |<---Shared locator-->|
          |<---------------Anycast SID--------------->|

                           Figure 1: Anycast SID

   For example, the shared Locator is A::/64, Service Functions (SFs)1 -
   3 are identified by B1 - B3.  Node 1 provides SF1, node 2 and node 3
   provide SF2, and node 4 provides SF3.  The Anycast SIDs are shown in
   Figure 2.

                 +-+-+         +-+-+         +-+-+
                 |SF1|         |SF2|         |SF3|
                 +-+-+         +-+-+         +-+-+
                   |             | A::B2       |
                   |         +-+-+-+-+         |
                   | +-+-+-+-+node 2 +-+-+-+-+ |
   +-+-+-+-+       | |       +-+-+-+-+       | |       +-+-+-+-+
   |ingress|     +-+-+-+-+               +-+-+-+-+     |egress |
   | node  +-+-+-+node 1 |               |node 4 +-+-+-+ node  |
   |       |     +-+-+-+-+               +-+-+-+-+     |       |
   +-+-+-+-+   A::B1 |       +-+-+-+-+       | A::B3   +-+-+-+-+
                     +-+-+-+-+node 3 +-+-+-+-+
                             +-+-+-+-+
                                 | A::B2
                               +-+-+
                               |SF2|
                               +-+-+

                        Figure 2: the Anycast SIDs

   A::/64 is allocated and shared by node 1 to node 4.








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3.  Control Plane

   The reachability of Anycast SIDs are advertised by control plane.  As
   described in [I-D.draft-ietf-lsr-isis-srv6-extensions], a new flag in
   "Bit Values for Prefix Attribute Flags Sub-TLV" registry [RFC7794] is
   defined to advertise the anycast property.  SRv6 Locator TLV is
   introduced to advertise Locators and End SIDs associated with each
   locator.  This TLV shares the sub-TLV space defined for TLVs 135, 236
   and 237.

   This document adopts the Anycast Flag (A-flag).  In addition, this
   document defines a new flag in SRv6 End SID sub-TLV.  The format is
   as follows:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Flags     |      Endpoint Behavior        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Anycast SID (128 bits) . . .                                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Anycast SID (cont . . .)                                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Anycast SID (cont . . .)                                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Anycast SID (cont . . .)                                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Sub-sub-tlv-len|      sub-sub-TLVs(variable). . .              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 3: Anycast SID Format

   Type: 5 (defined in [I-D.draft-ietf-lsr-isis-srv6-extensions]).

   Length: Variable.

   Flags: 8 bits.

                                0 1 2 3 4 5 6 7
                               +-+-+-+-+-+-+-+-+
                               |A|U|U|U|U|U|U|U|
                               +-+-+-+-+-+-+-+-+

                          Figure 4: Anycast Flag





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   U: Unused and for future use.  Must be 0 on transmission and ignored
   on receipt.

   A: Anycast flag, set when the SRv6 End SID sub-TLV carries the
   Anycast SIDs.

   Endpoint Behavior: 16 bits, code point that identifies the service
   functions.

   Anycast SID: 128 bits.

   Sub-sub-TLV-length: defined in
   [I-D.draft-ietf-lsr-isis-srv6-extensions].

   Sub-sub-TLVs: defined in [I-D.draft-ietf-lsr-isis-srv6-extensions].

   The Anycast SID MUST be a subnet of the associated Locator.  Anycast
   SIDs which are NOT a subnet of the associated locator MUST be
   ignored.

   Multiple Anycast SIDs MAY be associated with the same locator when a
   execution endpoint holds multiple service functions.  In cases where
   the number of SRv6 End SID sub-TLVs exceeds the capacity of a single
   TLV, multiple Locator TLVs for the same locator MAY be advertised.
   Other details are defined in
   [I-D.draft-ietf-lsr-isis-srv6-extensions].

4.  Data Plane

   This document requires no data plane format extensions to SRv6 and
   the Anycast SID has no differences with other SIDs.  Anycast SIDs
   stay together with other SIDs in the SRH and the SID list can not
   only steer the packet along a TE path but also specifies the service
   functions that should process the packet.

   The Anycast SIDs are advertised by control plane and instantiated in
   the local FIB.When a SRv6-capable node receives an IPv6 packet, it
   performs a long-prefix-match lookup on the packets destination
   address.  This lookup may return a FIB entry that represents a
   locally instantiated SID.  If this matched SID is Anycast SID, the
   node should process the packet with the service function identified
   by the Anycast SID.

5.  Illustration







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5.1.  Usecase1 migration of service function

   As illustrated in Figure 5, a SRv6-based service function chain needs
   to go through SF1, SF2 and SF3.  At the beginning, SF1, SF2 and SF3
   are provided on node 1, node 2 and node 4.  Then, SF2 is migrated to
   node 3 and the flow should change the path.  In addition to SRv6 SIDs
   A1::B1, A2::B2, A3::B2 and A4::B3 on the four nodes, they also have
   Anycast SIDs instantiated locally and advertised by the control
   plane.

   If original SRv6 is used, the SRH is (A1::B1, A2::B2, A4::B3) before
   the migration and (A1::B1, A2::B2, A4::B3) after the migration.  The
   ingress node should change the encapsulation strategies under control
   of the controller and re-encapsulate the packets.

   If Anycast SID is used, the SRH is (A::B1, A::B2, A::B3) before and
   after migration.  No changes to the SRH is needed.  Node 2 withdraws
   the route of A::B2 and Node 3 advertises A::B2.  Then the packets are
   forwarded based on the route of A::B2/128.

                 +-+-+         +-+-+         +-+-+
                 |SF1|         |SF2|****     |SF3|
                 +-+-+         +-+-+   *     +-+-+
                   |       A::B2 |     *       |
                   |         +-+-+-+-+ *       |
                   | +-+-+-+-+node 2 +-*-+-+-+ |
   +-+-+-+-+       | |       +-+-+-+-+ *     | |       +-+-+-+-+
   |ingress|     +-+-+-+-+     A2::B2  * +-+-+-+-+     |egress |
   | node  +-+-+-+node 1 |             * |node 4 +-+-+-+ node  |
   |       |     +-+-+-+-+     A3::B2  * +-+-+-+-+     |       |
   +-------+   A::B1 |       +-+-+-+-+ *     | A::B3   +-+-+-+-+
               A1::B1+-+-+-+-+node 3 +-*-+-+-+ A4::B3
                             +-+-^-+-+ *
                                 *     *
                                 *******

               Figure 5: Illustration topology for usecase1

5.2.  Usecase2 failover of service function

   As illustrated in Figure 6, a SRv6-based service function chain needs
   to go through SF1, SF2 and SF3.  At the beginning, SF1, SF2 and SF3
   are provided on node 1, node 2 and node 4.  Suddenly, node 2 is down
   and SF2 should be provided by node 3.  The flow should change the
   path.  In addition to SRv6 SIDs A1::B1, A2::B2, A3::B2 and A4::B3 on
   the four nodes, they also have Anycast SIDs instantiated locally and
   advertised by the control plane.




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   If original SRv6 is used, the SRH is (A1::B1, A2::B2, A4::B3) before
   failover and (A1::B1, A2::B2, A4::B3) after failover.  The ingress
   node should change the encapsulation strategies under control of the
   controller and re-encapsulate the packets.

   If Anycast SID is used, the SRH is (A::B1, A::B2, A::B3) before and
   after failover.  No changes to the SRH is needed.  Node 2 withdraws
   the route of A::B2 and Node 3 advertises A::B2.  The FIB on node 1 is
   updated that the packet destinated to A::B2/128 should be forwarded
   to node 3.  Then the packets are forwarded based on the route of
   A::B2/128.

                 +-+-+         +-+-+         +-+-+
                 |SF1|         |SF2|****     |SF3|
                 +-+-+         +-+-+   *     +-+-+
                   |       A::B2 |     *       |
                   |         +-+-+-+-+ *       |
                   | +-+-+-+-+node 2 +-*-+-+-+ |
   +-+-+-+-+       | |       +-+-+-+-+ *     | |       +-+-+-+-+
   |ingress|     +-+-+-+-+     A2::B2  * +-+-+-+-+     |egress |
   | node  +-+-+-+node 1 |             * |node 4 +-+-+-+ node  |
   |       |     +-+-+-+-+     A3::B2  * +-+-+-+-+     |       |
   +-------+   A::B1 |       +-+-+-+-+ *     | A::B3   +-+-+-+-+
               A1::B1+-+-+-+-+node 3 +-*-+-+-+ A4::B3
                             +-+-+-+-+ *
                           A::B2 |     *
                               +-+-+   *
                               |SF2|<***
                               +-+-+

               Figure 6: Illustration topology for usecase2

6.  Security Considerations

   TBD

7.  IANA Considerations

   IANA is requested to allocated one bit in SRv6 End SID sub-TLV Flags
   to indicate that the sub-TLV carries Anycast SIDs.

8.  References

8.1.  Normative References

   [I-D.draft-ietf-lsr-isis-srv6-extensions]
              Peter, P. and C. Clarence, "IS-IS Extension to Support
              Segment Routing over IPv6 Dataplane", 2021.



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   [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>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [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>.

   [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>.

   [RFC8986]  Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
              D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", RFC 8986,
              DOI 10.17487/RFC8986, February 2021,
              <https://www.rfc-editor.org/info/rfc8986>.

8.2.  Informative References

   [RFC3513]  Hinden, R. and S. Deering, "Internet Protocol Version 6
              (IPv6) Addressing Architecture", RFC 3513,
              DOI 10.17487/RFC3513, April 2003,
              <https://www.rfc-editor.org/info/rfc3513>.

   [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>.

   [RFC7794]  Ginsberg, L., Ed., Decraene, B., Previdi, S., Xu, X., and
              U. Chunduri, "IS-IS Prefix Attributes for Extended IPv4
              and IPv6 Reachability", RFC 7794, DOI 10.17487/RFC7794,
              March 2016, <https://www.rfc-editor.org/info/rfc7794>.





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Authors' Addresses

   Taixin Li
   CNIC, CAS
   No. 4 Zhongguancun South 4th Street
   Beijing  100190
   China

   Email: txli@cnic.cn


   Xu Zhou
   CNIC, CAS
   No. 4 Zhongguancun South 4th Street
   Beijing  100190
   China

   Email: zhouxu@cnic.cn


   Zhe Chen
   Huawei Technologies
   No. 156 Beiqing Rd
   Beijing  100095
   China

   Email: chenzhe17@huawei.com


   Yihao Jia
   Huawei Technologies
   No. 156 Beiqing Rd
   Beijing  100095
   China

   Email: jiayihao@huawei.com















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