TEAS Working Group                                               A. Wang
Internet-Draft                                             China Telecom
Intended status: Informational                               B. Khasanov
Expires: August 6, 2021                                       Yandex LLC
                                                                 Q. Zhao
                                                        Etheric Networks
                                                                 H. Chen
                                                        February 2, 2021

Path Computation Element (PCE) based Traffic Engineering (TE) in Native
                              IP Networks


   This document defines an architecture for providing traffic
   engineering in a native IP network using multiple BGP sessions and a
   Path Computation Element (PCE)-based central control mechanism.  It
   defines the Central Control Dynamic Routing (CCDR) procedures and
   identifies needed extensions for the Path Computation Element
   Communication Protocol (PCEP).

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on August 6, 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

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   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  CCDR Architecture in Simple Topology  . . . . . . . . . . . .   4
   4.  CCDR Architecture in Large Scale Topology . . . . . . . . . .   5
   5.  CCDR Multiple BGP Sessions Strategy . . . . . . . . . . . . .   6
   6.  PCEP Extension for Critical Parameters Delivery . . . . . . .   8
   7.  Deployment Consideration  . . . . . . . . . . . . . . . . . .   9
     7.1.  Scalability . . . . . . . . . . . . . . . . . . . . . . .   9
     7.2.  High Availability . . . . . . . . . . . . . . . . . . . .  10
     7.3.  Incremental deployment  . . . . . . . . . . . . . . . . .  10
     7.4.  Loop Avoidance  . . . . . . . . . . . . . . . . . . . . .  10
     7.5.  E2E Path Performance Monitoring . . . . . . . . . . . . .  10
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  11
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     11.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   [RFC8283], based on an extension of the Path Computation Element
   (PCE) architecture described in [RFC4655] , introduced a broader use
   applicability for a PCE as a central controller.  PCEP Protocol
   (PCEP) continues to be used as the protocol between PCE and Path
   Computation Client (PCC).  Building on that work, this document
   describes a solution using a PCE for centralized control in a native
   IP network to provide End-to-End (E2E) performance assurance and QoS
   for traffic.  The solution combines the use of distributed routing
   protocols and a centralized controller, referred to as Centralized
   Control Dynamic Routing (CCDR).

   [RFC8735] describes the scenarios and simulation results for traffic
   engineering in a native IP network based on use of a CCDR
   architecture.  Per [RFC8735], the architecture for traffic
   engineering in a native IP network should meet the following

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   o  Same solution for native IPv4 and IPv6 traffic.

   o  Support for intra-domain and inter-domain scenarios.

   o  Achieve End to End traffic assurance, with determined QoS
      behavior, for traffic requiring a service assurance (prioritized

   o  No changes in a router's forwarding behavior.

   o  Based on centralized control through a distributed network control

   o  Support different network requirements such as high traffic volume
      and prefix scaling.

   o  Ability to adjust the optimal path dynamically upon the changes of
      network status.  No need for physical links resources reservations
      to be done in advance.

   Building on the above documents, this document defines an
   architecture meeting these requirements by using a multiple BGP
   session strategy and a PCE as the centralized controller.  The
   architecture depends on the central control (PCE) element to compute
   the optimal path, and utilizes the dynamic routing behavior of IGP/
   BGP protocols for forwarding the traffic.

2.  Terminology

   This document uses the following terms defined in [RFC5440]:

   o  PCE: Path Computation Element

   o  PCEP: PCE Protocol

   o  PCC: Path Computation Client

   Other terms are used in this document:

   o  CCDR: Central Control Dynamic Routing

   o  E2E: End to End

   o  ECMP: Equal-Cost Multipath

   o  RR: Route Reflector

   o  SDN: Software Defined Network

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3.  CCDR Architecture in Simple Topology

   Figure 1 illustrates the CCDR architecture for traffic engineering in
   a simple topology.  The topology is composed of four devices which
   are SW1, SW2, R1, R2.  There are multiple physical links between R1
   and R2.  Traffic between prefix PF11(on SW1) and prefix PF21(on SW2)
   is normal traffic, traffic between prefix PF12(on SW1) and prefix
   PF22(on SW2) is priority traffic that should be treated accordingly.

                    +----------+ PCE +--------+
                    |          +-----+        |
                    |                         |
                    | BGP Session 1(lo11/lo21)|
                    |                         |
                    | BGP Session 2(lo12/lo22)|
PF12                |                         |                    PF22
PF11                |                         |                    PF21
+---+         +-----+-----+             +-----+-----+              +---+
+---+         |    R1     +-------------+    R2     |              +---+
              +-----------+             +-----------+

           Figure 1: CCDR architecture in simple topology

   In the Intra-AS scenario, IGP and BGP combined with a PCE are
   deployed between R1 and R2.  In the inter-AS scenario, only the
   native BGP protocol is deployed.  The traffic between each address
   pair may change in real time and the corresponding source/destination
   addresses of the traffic may also change dynamically.

   The key ideas of the CCDR architecture for this simple topology are
   the following:

   o  Build two BGP sessions between R1 and R2, via the different
      loopback addresses on these routers (lo11 and lo12 are the
      loopback address of R1, lo21 and lo22 are the loopback address of

   o  Using the PCE, set the explicit peer route on R1 and R2 for BGP
      next hop to different physical link addresses between R1 and R2.
      The explicit peer route can be set in the format of a static
      route, which is different from the route learned from the IGP

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   o  Send different prefixes via the established BGP sessions.  For
      example, send PF11/PF21 via the BGP session 1 and PF12/PF22 via
      the BGP session 2.

   After the above actions, the bi-directional traffic between the PF11
   and PF21, and the bi-directional traffic between PF12 and PF22 will
   go through different physical links between R1 and R2.

   If there is more traffic between PF12 and PF22 that needs assured
   transport, one can add more physical links between R1 and R2 to reach
   the next hop for BGP session 2.  In this case, the prefixes that are
   advertised by the BGP peers need not be changed.

   If, for example, there is bi-directional priority traffic from
   another address pair (for example prefix PF13/PF23), and the total
   volume of priority traffic does not exceed the capacity of the
   previously provisioned physical links, one need only advertise the
   newly added source/destination prefixes via the BGP session 2.  The
   bi-directional traffic between PF13/PF23 will go through the same
   assigned dedicated physical links as the traffic between PF12/PF22.

   Such a decoupling philosophy of the IGP/BGP traffic link and the
   physical link achieves a flexible control capability for the network
   traffic, satisfying the needed QoS assurance to meet the
   application's requirement.  The router needs only support native IP
   and multiple BGP sessions setup via different loopback addresses.

4.  CCDR Architecture in Large Scale Topology

   When the priority traffic spans a large-scale network, such as that
   illustrated in Figure 2, the multiple BGP sessions cannot be
   established hop by hop within one AS.  For such a scenario, we
   propose using a Route Reflector (RR) [RFC4456] to achieve a similar
   effect.  Every edge router will establish two BGP sessions with the
   RR via different loopback addresses respectively.  The other steps
   for traffic differentiation are the same as that described in the
   CCDR architecture for the simple topology.

   As shown in Figure 2, if we select R3 as the RR, every edge router(R1
   and R7 in this example) will build two BGP session with the RR.  If
   the PCE selects the dedicated path as R1-R2-R4-R7, then the operator
   should set the explicit peer routes via PCEP protocol on these
   routers respectively, pointing to the BGP next hop (loopback
   addresses of R1 and R7, which are used to send the prefix of the
   priority traffic) to the selected forwarding address.

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               +----------------+ PCE +------------------+
               |                +--+--+                  |
               |                   |                     |
               |                   |                     |
               |                +--+---+                 |
  PF12         |                +--+---+                 |          PF22
  PF11         |                                         |          PF21
  +---+       ++-+          +--+          +--+         +-++        +---+
  +---+       ++-+          +--+          +--+         +-++        +---+
               |                                         |
               |                                         |
               |            +--+          +--+           |
                            +--+          +--+
            Figure 2: CCDR architecture in large-scale network

5.  CCDR Multiple BGP Sessions Strategy

   Generally, different applications may require different QoS criteria,
   which may include:

   o  Traffic that requires low latency and is not sensitive to packet

   o  Traffic that requires low packet loss and can endure higher

   o  Traffic that requires low jitter.

   These different traffic requirements can be summarized in the
   following table:

      | Prefix Set No. |    Latency  |  Packet Loss  |   Jitter        |
      |        1       |    Low      |   Normal      |   Don't care    |
      |        2       |   Normal    |   Low         |   Don't care   |
      |        3       |   Normal    |   Normal      |   Low           |
                 Table 1. Traffic Requirement Criteria

   For Prefix Set No.1, we can select the shortest distance path to
   carry the traffic; for Prefix Set No.2, we can select the path that

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   has end to end under-loaded links; for Prefix Set No.3, we can let
   traffic pass over a determined single path, as no Equal Cost
   Multipath (ECMP) distribution on the parallel links is desired.

   It is almost impossible to provide an End-to-End (E2E) path
   efficiently with latency, jitter, and packet loss constraints to meet
   the above requirements in a large-scale IP-based network only using a
   distributed routing protocol, but these requirements can be met with
   the assistance of PCE, as that described in [RFC4655] and [RFC8283].
   The PCE will have the overall network view, ability to collect the
   real-time network topology, and the network performance information
   about the underlying network.  The PCE can select the appropriate
   path to meet the various network performance requirements for
   different traffic.

   The architecture to implement the CCDR Multiple BGP sessions strategy
   is as follows:

   The PCE will be responsible for the optimal path computation for the
   different priority classes of traffic:

   o  PCE collects topology information via BGP-LS [RFC7752] and link
      utilization information via the existing Network Monitoring System
      (NMS) from the underlying network.

   o  PCE calculates the appropriate path based upon the application's
      requirements, and sends the key parameters to edge/RR routers(R1,
      R7 and R3 in Figure 3) to establish multiple BGP sessions.  The
      loopback addresses used for the BGP sessions should be planned in
      advance and distributed in the domain.

   o  PCE sends the route information to the routers (R1,R2,R4,R7 in
      Figure 3) on the forwarding path via PCEP, to build the path to
      the BGP next-hop of the advertised prefixes.  The path to these
      BGP next-hop will also be learned via the IGP protocol, but the
      route from the PCEP has the higher preference.  Such design can
      assure the IGP path to the BGP next-hop can be used to protect the
      path assigned by PCE.

   o  PCE sends the prefixes information to the PCC(edge routers that
      have established BGP sessions) for advertising different prefixes
      via the specified BGP session.

   o  The priority traffic may share some links or nodes, if path the
      shared links or nodes can meet the requirement of application.
      When the priority traffic prefixes were changed but the total
      volume of priority traffic does not exceed the physical capacity

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      of the previous E2E path, the PCE needs only change the prefixed
      advertised via the edge routers (R1,R7 in Figure 3).

   o  If the volume of priority traffic exceeds the capacity of the
      previous calculated path, the PCE can recalculate and add the
      appropriate paths to accommodate the exceeding traffic.  After
      that, the PCE needs to update the on-path routers to build the
      forwarding path hop by hop.

                            | Application|
               +----------+SDN Controller/PCE+-----------+
               |          +--------^---------+           |
               |                   |                     |
               |                   |                     |
          PCEP |             BGP-LS|PCEP                 | PCEP
               |                   |                     |
               |                +--v---+                 |
   PF12        |                +------+                 |          PF22
   PF11        |                                         |          PF21
  +---+       +v-+          +--+          +--+         +-v+        +---+
  +---+       ++-+          +--+          +--+         +-++        +---+
               |                                         |
               |                                         |
               |            +--+          +--+           |
                            +--+          +--+

        Figure 3: CCDR architecture for Multi-BGP sessions deployment

6.  PCEP Extension for Critical Parameters Delivery

   The PCEP protocol needs to be extended to transfer the following
   critical parameters:

   o  Peer information that is used to build the BGP session

   o  Explicit route information for BGP next hop of advertised prefixes

   o  Advertised prefixes and their associated BGP session.

   Once the router receives such information, it should establish the
   BGP session with the peer appointed in the PCEP message, build the

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   end-to-end dedicated path hop-by-hop, and advertise the prefixes that
   are contained in the corresponding PCEP message.

   The dedicated path is preferred by making sure that the explicit
   route created by PCE has the higher priority (lower route preference)
   than the route information created by other dynamic protocols.

   All above dynamically created states (BGP sessions, Explicit route
   and Prefix advertised prefix) will be cleared on the expiration of
   the state timeout interval which is based on the existing Stateful
   PCE [RFC8231] and PCECC [RFC8283] mechanism.

   Regarding the BGP session, it is not different from that configured
   manually or via NETCONF/YANG.  Different BGP sessions are used mainly
   for the clarification of the network prefixes, which can be
   differentiated via the different BGP nexthop.  Based on this
   strategy, if we manipulate the path to the BGP nexthop, then the path
   to the prefixes that were advertised with the BGP sessions will be
   changed accordingly.  Details of communications between PCEP and BGP
   subsystems in the router's control plane are out of scope of this

7.  Deployment Consideration

7.1.  Scalability

   In the CCDR architecture, only the edge routers that connect with the
   PCE are responsible for the prefixes advertisement via the multiple
   BGP sessions deployment.  The route information for these prefixes
   within the on-path routers is distributed via the BGP protocol.

   For multiple domain deployment, the PCE, or the pool of PCEs
   responsible for these domains, needs only to control the edge router
   to build the multiple EBGP sessions; all other procedures are the
   same as within one domain.

   The on-path router needs only to keep the specific policy routes for
   the BGP next-hop of the differentiated prefixes, not the specific
   routes to the prefixes themselves.  This lessens the burden of the
   table size of policy based routes for the on-path routers; and has
   more expandability compared with BGP flowspec or Openflow solutions.
   For example, if we want to differentiate 1000 prefixes from the
   normal traffic, CCDR needs only one explicit peer route in every on-
   path router, whereas the BGP flowspec or Openflow solutions need 1000
   policy routes on them.

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7.2.  High Availability

   The CCDR architecture is based on the use of the native IP protocol.
   If the PCE fails, the forwarding plane will not be impacted, as the
   BGP sessions between all the devices will not flap and the forwarding
   table remains unchanged.

   If one node on the optimal path fails, the priority traffic will fall
   over to the best-effort forwarding path.  One can even design several
   paths to load balance/hot-standby the priority traffic to meet a path
   failure situation.

   For ensuring high availability of a PCE/SDN-controllers architecture,
   an operator should rely on existing high availability solutions for
   SDN controllers, such as clustering technology and deployment.

7.3.  Incremental deployment

   Not every router within the network needs to support the necessary
   PCEP extension.  For such situations, routers on the edge of a domain
   can be upgraded first, and then the traffic can be prioritized
   between different domains.  Within each domain, the traffic will be
   forwarded along the best-effort path.  A service provider can
   selectively upgrade the routers on each domain in sequence.

7.4.  Loop Avoidance

   A PCE needs to assure calculation of the E2E path based on the status
   of network and the service requirements in real-time.

   The PCE needs to consider the explicit route deployment order (for
   example, from tail router to head router) to eliminate any possible
   transient traffic loop.

7.5.  E2E Path Performance Monitoring

   It is necessary to deploy the corresponding E2E path performance
   monitoring mechanism to keep assure that the delay, jitter or packet
   loss index meet the original path performance aim.  The performance
   monitoring results should feedback to the PCE to let it accomplish
   the re-optimize process, send the update control message to related
   PCC if necessary.  Traditional OAM methods(ping, trace) can be used.

8.  Security Considerations

   The setup of BGP sessions, prefix advertisement, and explicit peer
   route establishment are all controlled by the PCE.  See [RFC4271] and
   [RFC4272] for BGP security considerations.  Security consideration

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   part in [RFC5440] and [RFC8231] should be considered.  To prevent a
   bogus PCE sending harmful messages to the network nodes, the network
   devices should authenticate the validity of the PCE and ensure a
   secure communication channel between them.  Mechanisms described in
   [RFC8253] should be used.

   The CCDR architecture does not require changes to the forwarding
   behavior of the underlay devices.  There are no additional security
   impacts on these devices.

9.  IANA Considerations

   This document does not require any IANA actions.

10.  Acknowledgement

   The author would like to thank Deborah Brungard, Adrian Farrel,
   Vishnu Beeram, Lou Berger, Dhruv Dhody, Raghavendra Mallya , Mike
   Koldychev, Haomian Zheng, Penghui Mi, Shaofu Peng, Donald Eastlake,
   Alvaro Retana, Martin Duke, Magnus Westerlund, Benjamin Kaduk, Roman
   Danyliw, Eric Vyncke, Murray Kucherawy, Erik Kline and Jessica Chen
   for their supports and comments on this draft.

11.  References

11.1.  Normative References

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,

   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
              RFC 4272, DOI 10.17487/RFC4272, January 2006,

   [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route
              Reflection: An Alternative to Full Mesh Internal BGP
              (IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,

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   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,

   [RFC8231]  Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for Stateful PCE", RFC 8231,
              DOI 10.17487/RFC8231, September 2017,

   [RFC8253]  Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody,
              "PCEPS: Usage of TLS to Provide a Secure Transport for the
              Path Computation Element Communication Protocol (PCEP)",
              RFC 8253, DOI 10.17487/RFC8253, October 2017,

   [RFC8283]  Farrel, A., Ed., Zhao, Q., Ed., Li, Z., and C. Zhou, "An
              Architecture for Use of PCE and the PCE Communication
              Protocol (PCEP) in a Network with Central Control",
              RFC 8283, DOI 10.17487/RFC8283, December 2017,

11.2.  Informative References

   [RFC4655]  Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
              Element (PCE)-Based Architecture", RFC 4655,
              DOI 10.17487/RFC4655, August 2006,

   [RFC8735]  Wang, A., Huang, X., Kou, C., Li, Z., and P. Mi,
              "Scenarios and Simulation Results of PCE in a Native IP
              Network", RFC 8735, DOI 10.17487/RFC8735, February 2020,

Authors' Addresses

   Aijun Wang
   China Telecom
   Beiqijia Town, Changping District
   Beijing  102209

   Email: wangaj3@chinatelecom.cn

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   Boris Khasanov
   Yandex LLC
   Ulitsa Lva Tolstogo 16

   Email: bhassanov@yahoo.com

   Quintin Zhao
   Etheric Networks
   SAN MATEO, CA  94402

   Email: qzhao@ethericnetworks.com

   Huaimo Chen
   Boston, MA

   Email: huaimo.chen@futurewei.com

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