Internet Engineering Task Force (IETF) A. Wang

Internet Engineering Task Force (IETF) A. Wang
Request for Comments: 8821 China Telecom
Category: Informational B. Khasanov
ISSN: 2070-1721 Yandex LLC
Q. Zhao
Etheric Networks
H. Chen
Futurewei
April 2021

PCE-Based Traffic Engineering (TE) in Native IP Networks

Abstract

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 Centralized Control Dynamic Routing (CCDR) procedures and
identifies needed extensions for the Path Computation Element
Communication Protocol (PCEP).

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8821.

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Table of Contents

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

1. Introduction

[RFC8283], based on an extension of the PCE architecture described in
[RFC4655], introduced a broader use applicability for a PCE as a
central controller. PCEP continues to be used as the protocol
between the PCE and the Path Computation Client (PCC). Building on
that work, this document describes a solution of 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
criteria:

* Same solution for native IPv4 and IPv6 traffic.

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

* Achieve E2E traffic assurance, with determined QoS behavior, for
traffic requiring a service assurance (prioritized traffic).

* No changes in a router's forwarding behavior.

* Based on centralized control through a distributed network control
plane.

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

* Ability to adjust the optimal path dynamically upon the changes of
network status. No need for reserving resources for physical
links in advance.

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

2. Terminology

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

PCE: Path Computation Element

PCEP: PCE Protocol

PCC: Path Computation Client

Other terms are used in this document:

CCDR: Centralized Control Dynamic Routing

E2E: End to End

ECMP: Equal-Cost Multipath

RR: Route Reflector

SDN: Software-Defined Network

3. CCDR Architecture in a 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, and 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
+---+ +-----+-----+ +-----+-----+ +---+
|SW1+---------+(lo11/lo12)+-------------+(lo21/lo22)+-----------+SW2|
+---+ | R1 +-------------+ R2 | +---+
+-----------+ +-----------+

Figure 1: CCDR Architecture in a Simple Topology

In the intra-domain scenario, IGP and BGP combined with a PCE are
deployed between R1 and R2. In the inter-domain scenario, only
native BGP 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:

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

* 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 IGP.

* 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 bidirectional traffic between the PF11
and PF21, and the bidirectional 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 bidirectional 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 bidirectional
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 to support native
IP and multiple BGP sessions set up via different loopback addresses.

4. CCDR Architecture in a 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 autonomous system. 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 sessions 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 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.

+-----+
+----------------+ PCE +------------------+
| +--+--+ |
| | |
| | |
| +--+---+ |
+----------------+R3(RR)+-----------------+
PF12 | +--+---+ | PF22
PF11 | | PF21
+---+ ++-+ +--+ +--+ +-++ +---+
|SW1+-------+R1+----------+R5+----------+R6+---------+R7+-------+SW2|
+---+ ++-+ +--+ +--+ +-++ +---+
| |
| |
| +--+ +--+ |
+------------+R2+----------+R4+-----------+
+--+ +--+

Figure 2: CCDR Architecture in a Large-Scale Network

5. CCDR Multiple BGP Sessions Strategy

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

* Traffic that requires low latency and is not sensitive to packet
loss.

* Traffic that requires low packet loss and can endure higher
latency.

* Traffic that requires low jitter.

These different traffic requirements are summarized in Table 1.

+================+=========+=============+============+
| 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
has E2E under-loaded links; for Prefix Set No.3, we can let traffic
pass over a determined single path, as no ECMP distribution on the
parallel links is desired.

It is almost impossible to provide an 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 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:

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

* 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.

* PCE sends the route information to the routers (R1, R2, R4, and 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 hops will also be learned via IGP, but the route from the
PCEP has the higher preference. Such a design can assure the IGP
path to the BGP next hop can be used to protect the path assigned
by PCE.

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

* The priority traffic may share some links or nodes if the path the
shared links or nodes can meet the requirement of application.
When the priority traffic prefixes are changed, but the total
volume of priority traffic does not exceed the physical capacity
of the previous E2E path, the PCE needs only change the prefixes
advertised via the edge routers (R1 and R7 in Figure 3).

* 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---+ |
+----------------+R3(RR)+-----------------+
PF12 | +------+ | PF22
PF11 | | PF21
+---+ +v-+ +--+ +--+ +-v+ +---+
|SW1+-------+R1+----------+R5+----------+R6+---------+R7+-------+SW2|
+---+ ++-+ +--+ +--+ +-++ +---+
| |
| |
| +--+ +--+ |
+------------+R2+----------+R4+-----------+
+--+ +--+

Figure 3: CCDR Architecture for Multi-BGP Sessions Deployment

6. PCEP Extension for Critical Parameters Delivery

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

* Peer information that is used to build the BGP session.

* Explicit route information for BGP next hop of advertised
prefixes.

* 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
E2E 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 of the above dynamically created states (BGP sessions, explicit
routes, and advertised prefixes) will be cleared on the expiration of
the state timeout interval, which is based on the existing stateful
PCE [RFC8231] and PCE as a Central Controller (PCECC) [RFC8283]
mechanism.

Regarding the BGP session, it is not different from that configured
manually or via Network Configuration Protocol (NETCONF) and YANG.
Different BGP sessions are used mainly for the clarification of the
network prefixes, which can be differentiated via the different BGP
next hop. Based on this strategy, if we manipulate the path to the
BGP next hop, 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 document.

7. Deployment Considerations

7.1. Scalability

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

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 External BGP (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 1,000 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
1,000 policy routes on them.

7.2. High Availability

The CCDR architecture is based on the use of native IP. 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 or to create a hot standby of 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 assure that the delay, jitter, or packet loss
index meets the original path performance aim. The performance
monitoring results should provide feedback to the PCE in order for it
to accomplish the re-optimization process and send the update control
message to the 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. The Security
Considerations found in Section 10 of [RFC5440] and Section 10 of
[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 has no IANA actions.

10. References

10.1. 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,
<https://www.rfc-editor.org/info/rfc4271>.

[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC4272, January 2006,
<https://www.rfc-editor.org/info/rfc4272>.

[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,
<https://www.rfc-editor.org/info/rfc4456>.

[RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440,
DOI 10.17487/RFC5440, March 2009,
<https://www.rfc-editor.org/info/rfc5440>.

[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,
<https://www.rfc-editor.org/info/rfc7752>.

[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,
<https://www.rfc-editor.org/info/rfc8231>.

[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,
<https://www.rfc-editor.org/info/rfc8253>.

[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,
<https://www.rfc-editor.org/info/rfc8283>.

10.2. Informative References

[RFC4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
Computation Element (PCE)-Based Architecture", RFC 4655,
DOI 10.17487/RFC4655, August 2006,
<https://www.rfc-editor.org/info/rfc4655>.

[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,
<https://www.rfc-editor.org/info/rfc8735>.

Acknowledgments

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, Éric Vyncke, Murray Kucherawy, Erik Kline, and Jessica Chen
for their supports and comments on this document.

Authors' Addresses

Aijun Wang
China Telecom
Changping District
Beiqijia Town
Beijing
102209
China

Email: [email protected]

Boris Khasanov
Yandex LLC
Ulitsa Lva Tolstogo 16
Moscow
Russian Federation

Email: [email protected]

Quintin Zhao
Etheric Networks
1009 S Claremont St
San Mateo, CA 94402
United States of America

Email: [email protected]

Huaimo Chen
Futurewei
Boston, MA
United States of America

Email: [email protected]