SPRING Working Group W. Jiang
Internet-Draft W. Cheng
Intended status: Standards Track China Mobile
Expires: September 7, 2022 C. Lin
Y. Qiu
New H3C Technologies
March 7, 2022
Use Cases for SR Policy Group
draft-jiang-spring-sr-policy-group-use-cases-00
Abstract
Segment Routing is a source routing paradigm that explicitly
indicates the forwarding path for packets at the ingress node. An SR
Policy is associated with one or more candidate paths, and each
candidate path is either dynamic, explicit or composite. This
document illustrates some use cases for SR policy group composite
candidate path in MPLS and IPv6 environment.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. SR Policy Group . . . . . . . . . . . . . . . . . . . . . . . 3
4. Steering into An SR Policy Group . . . . . . . . . . . . . . 4
5. On-demand SR Policy Group . . . . . . . . . . . . . . . . . . 5
6. SR Policy Group Use Cases . . . . . . . . . . . . . . . . . . 5
6.1. SR Policy Group in L3VPN over TE Scenarios . . . . . . . 5
6.2. SR Policy Group in Cloud Backbone Acceleration Scenarios 6
6.3. SR Policy Group in the L2VPN Network Scenarios . . . . . 7
6.4. SR Policy Group in the Application-aware Scenarios . . . 8
6.5. Application of ODN SR Policy Group in Trusted Network
Scenarios . . . . . . . . . . . . . . . . . . . . . . . . 9
6.6. Best-effort Forwarding Scenarios when SR Policy Becomes
Unavailable . . . . . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Segment routing (SR) [RFC8402] is a source routing paradigm that
explicitly indicates the forwarding path for packets at the ingress
node. The ingress node steers packets into a specific path according
to the Segment Routing policy (SR Policy) as defined in
[I-D.ietf-spring-segment-routing-policy]. In order to distribute SR
policies to the headend, [I-D.ietf-idr-segment-routing-te-policy]
specifies a mechanism by using BGP.
An SR policy is associated with one or more candidate paths. A
composite candidate path acts as a container for grouping SR
policies. As described in section 2.2 in
[I-D.ietf-spring-segment-routing-policy], the composite candidate
path construct enables combination of SR policies, each with explicit
candidate paths and/or dynamic candidate paths with potentially
different optimization objectives and constraints, for load-balanced
steering of packet flows over its constituent SR policies. For
service-class based steering, and in the best-effort forwarding
scenario when SR policy becomes unavailable, packets are also
forwarded over the constituent SR policies of composite candidate
path.
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This document illustrates some use cases for SR policy group
composite candidate path in MPLS and IPv6 environment.
2. Terminology
The definitions of the basic terms are identical to those found in
Alternate Marking [RFC8402].
The important new terms that need to be explained are listed below:
SR policy group: An SR policy which contains a group of constituent
SR policies. An SR policy group represents a composite candidate
path.
ODN: On-demand Next-Hop.
ODN SR policy: Preconfigure an ODN template specified color. When
the device receives a BGP route, if the color extended attribute
value of the BGP route is the same as the color value of an ODN
template, the device can automatically create an SR policy.
ODN SR policy group: An SR policy group dynamically created through
ODN.
3. SR Policy Group
An SR policy group is specified as a group of its constituent SR
policies. It is valid when it has at least one valid constituent SR
policy.
As defined in [I-D.ietf-spring-segment-routing-policy], The endpoints
of the constituent SR policies and the parent SR policy MUST be
identical, and the colors of each of the constituent SR policies and
the parent SR policy MUST be different. SR policy group and its
constituent SR policies follow the same criteria:
o The endpoints of the constituent SR policies and its SR policy
group MUST be identical.
o The colors of each of the constituent SR policies and its SR
policy group MUST be different.
o The constituent SR policies MUST NOT contain SR policy groups.
As a special SR policy, SR policy group also has color attribute,
which is identified by <color, endpoint> on the headend.
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An SR policy can be generated by static configuration or a
centralized controller distribution, also can be generated based on
the on-demand SR policy optimization template dynamically.
4. Steering into An SR Policy Group
A headend can steer a packet flow into a valid SR policy group in
various ways:
o Per-flow Steering: Specify the mapping relationship between color
and flow characteristics (such as DSCP) for SR policy group, and
create a policy group that binds a destination IP address to the
SR policy group. Upon receiving a packet with the specified
destination address, the device searches for the SR policy
containing the color value mapped to the flow characteristics of
the packet in the SR policy group. The device will use the
matching SR policy to forward the packet.
The device obtains an SR policy group for traffic steering as
follows:
* Matches the destination IP/IPv6 address in a packet with an SR
policy group.
* Searches for an SR policy group with color and endpoint address
matching the color extended community attribute and next hop in
a BGP route, and recurses the BGP route to the SR policy group.
The Ingress node can match flow characteristics in its ingress
interfaces (upon any field such as Ethernet
destination/source/VLAN/TOS or IP destination/source/DSCP or
transport ports or application attribute etc.) and color them with
an internal per-packet forwarding-class variable. According to
the forwarding-class variable the ingress node selects the
matching SR policy in the SR policy group.
o Policy-based Steering: incoming packets match a routing policy
that directs them on an SR policy group. Parse the flow
characteristics (such as DSCP/802.1p value) from the packet
header, find its corresponding color, and then match it to an SR
policy in the SR policy group, forward the incoming packets
through the matched SR policy.
If an SR policy group has at least one valid constituent SR policy of
specified color, flow load-balance steer over its valid constituent
SR policies with the same color. When all constituent SR policies of
specified color are invalid, packets are forwarded based on a default
SR policy preconfigured.
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5. On-demand SR Policy Group
SR policies are generally generated by manual static configuration or
distributed by centralized controller. Manual configuration may be
troublesome, especially when many SR policies need to be configured.
The controller mode may also not be suitable for operators who need
to make full use of distributed intelligence.
In scenarios that distinguish service forwarding paths based on DSCP
value and 802.1p priority, SR policy groups can be automatically
created through ODN to establish the dynamic mapping between service
types and SR policy groups, which can greatly reduce the workload of
configuration.
Create the ODN template of SR policy group in the headend. When the
device receives a BGP route, if the color extended community
attribute carried by the BGP route is the same as the color value of
the ODN template, the next hop address of the BGP route is used as
the destination endpoint address of the SR policy group, and the
color value of the ODN template is used as the color attribute of the
SR policy group to generate an SR policy group.
After the SR policy group is created by ODN, its constituent SR
policy is usually generated by ODN. ODN SR policy dynamically
generates candidate paths through affinity attributes, flex algo
algorithm or PCE calculation.
6. SR Policy Group Use Cases
The use cases described in this section do not constitute an
exhaustive list of all the possible scenarios: this section only
includes some of the most common envisioned deployment models for SR
policy group.
6.1. SR Policy Group in L3VPN over TE Scenarios
In Figure 1, CE1 and CE2 belong to the same L3VPN and access the
public network through PE1 and PE2 respectively. There are many
kinds of traffic between CE1 and CE2. When the ordinary traffic is
too large, the forwarding of important traffic will be affected.
In order to ensure the forwarding quality of important services, the
steering based on Forwarding class can be configured using SR policy
group. After the steering based on forwarding class is configured,
the traffic of different service levels will be carried by the
specified SR policy tunnel, which can effectively ensure the
forwarding quality of important services with high service levels.
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+----+
+--| P3 |--+
| +----+ |
| |
+-----+ +-----+ +----+ +----+ +-----+ +-----+
| CE1 |---| PE1 |---| P1 |----| P2 |----| PE2 |---| CE2 |
+-----+ +-----+ +----+ +----+ +-----+ +-----+
| |
|<===========================>|
SR Policy Group
Figure 1
It is assumed that in this network, the policy group contains three
constituent policies: Policy-A, Policy-B and Policy-C. Services with
different forwarding class will carry different DSCP values in the
packet. Identify the customer's service through DSCP on PE1. The
voice traffic of VIP customers is forwarded according to the path of
low-delay Policy-A, other traffic of VIP customers is forwarded
according to the path of Policy-B, and all businesses of non VIP
customers are carried by Policy-C.
6.2. SR Policy Group in Cloud Backbone Acceleration Scenarios
As shown in Figure 2, multiple cloud data centers are interconnected
through cloud backbone networks. In the public cloud, there are
different SLA requirements for different service types, such as voice
service and cloud disk. Deploy a static SR policy group on the core
of the cloud backbone network to prevent network congestion. There
are multiple SR policies in the SR policy group.
In order to ensure the service quality of different types of
services, the service types are distinguished by flow classification,
then different services are mapped to different DSCP value, and
finally the traffic of different DSCP is imported into different SR
policies.
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...................................
. Backbone .
. +--------+ .
. | Public | .
. +--------| Cloud |-----------+ .
. | +--------+ | .
+--------+ . | | . +--------+
| Public |---+ . | | . +---| Public |
| Cloud | |_+----+ +----+ +----+ +----+_| | Cloud |
+--------+ | PE |----| P |-----| P |----| PE | +--------+
|-+----+ +----+ +----+ +----+-|
+--------+ | . | | . | +--------+
|Private |---+ . | +----+ | . +---|Private |
| Cloud | . +--| P |--+ . | Cloud |
+--------+ . +----+ . +--------+
. |<=============>| .
. SR Policy group .
...................................
Figure 2
Through the SR policy group, different forwarding paths can be
introduced based on the DSCP value in the IP/IPv6 packet header.
First, create an SR policy group and assign color identification to
the SR policy group.
Then, configure multiple SR policies into one SR policy group in the
headend, specify the mapping relationship between each SR policy and
DSCP value in the SR policy group, and then bind the service type to
the specified SR policy group.
In this way, when the headend receives traffic, it first matches to
the SR policy group according to the next hop and color of the route,
and then finds the mapped SR policy in the corresponding group
according to the DSCP value carried in the IP/IPv6 packet header.
DSCP based steering is suitable for differentiating services at the
source and specifying different DSCP value scenarios.
6.3. SR Policy Group in the L2VPN Network Scenarios
Similar to the DSCP-based steering scenario, in the layer 2 access
network and L2VPN network, the service types are distinguished by the
802.1p priority in the packet header, and the 802.1p priority is
mapped to color in the SR policy group. Different services can be
forwarded into different paths.
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+----+
+---| P1 |---+
| +----+ |
+-----+ +-----+ | | +-----+ +-----+
| CE1 |---| PE1 |---| |---| PE2 |---| CE2 |
+-----+ +-----+ | | +-----+ +-----+
| +----+ |
+---| P2 |---+
| +----+ |
|<========================>|
SR Policy Group
Figure 3
As shown in Figure 3, CE1 and CE2 belong to the same VPLS and are
connected to the MPLS backbone network through PE1 and PE2
respectively. Establish two MPLS-SR policy tunnels Policy-A and
Policy-B between PE1 and PE2 to carry this VPLS service. Policy-A
and Policy-B are the constituent policies of SR policy group. Two SR
policy tunnels correspond to two different priorities. The VPLS
access end classifies the traffic flow, trusts the priority of
802.1p, and introduces the services of VPLS leased line users and
non-leased line users into different SR policy according to different
priorities.
6.4. SR Policy Group in the Application-aware Scenarios
By carrying the application attribute (including APP ID and APP
parameters) through data packets, i.e., the delivery of application-
aware information and ensuring the security and reliability of
application-aware information, the network senses the application
groups' requirements and provides high-quality differentiated
services according to the demand of the applications. And when the
network transmits the data packets, it matches the SR policy
according to the application attribute in the data packets and
selects the corresponding path of constituent SRv6 policy to transmit
the data packets (e.g., low latency path) to meet the SLA
requirements and service chain in order to improve the service
quality.
As shown in Figure 4 below, the policy group contains three
constituent SR policies: Policy-A, Policy-B and Policy-C. The data
packets of APP1 are forwarded by Policy-A, the data packets of APP2
are forwarded by Policy-B, and the data packets of APP3 are forwarded
by Policy-C.
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+------+ +-----------+ +------+
| APP1 | /=====| Policy-A |=====\ | APP1 |
+------+ / +-----------+ \ +------+
+------+ +-------+ +-----------+ +--------+ +------+
| APP2 |---| PE |====| Policy-B |===| PE |---| APP2 |
+------+ +-------+ +-----------+ +--------+ +------+
+------+ \ +-----------+ / +------+
| APP3 | \=====| Policy-C |=====/ | APP3 |
+------+ +-----------+ +------+
|<============================>|
SR Policy Group
Figure 4
6.5. Application of ODN SR Policy Group in Trusted Network Scenarios
Section 3 of [I-D.lin-opsec-trustroute-problem-statement] introduces
the use case of trusted network. By dynamically creating SR policy
through ODN, automatic steering traffic according to security level
can be realized.
From the perspective of security and trustworthiness, the security
levels for users with different security requirements and the
trustworthiness levels of the network transmission devices can be
determined according to their performance and reliability. Different
forwarding paths are provided for packets with different security
levels.
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----------
/ \
| APP Server |
\ /
----------
|
|
+----------+
| |
+----------------| Device-E |---------------+ =====
| | | | ^ S
| +----------+ | | R
| Trustworthiness level =7 | |
| | | | p
| | | | o
+----------+ +----------+ +----------+ | l
| | | | | | | i
| Device-B | | Device-C | | Device-D | | c
| | | | | | | y
+----------+ +----------+ +----------+ |
Trustworthiness level =1 | Trustworthiness level =3 | g
| Trustworthiness level =3 | | r
| | | | o
| +----------+ | | u
+----------------| |---------------+ V p
| Device-A | =====
+---------------| |---------------+
| +----------+ |
| Trustworthiness level =7 |
| | |
| | |
+----------+ +----------+ +----------+
| User-1 | | User-2 | | User-3 |
+----------+ +----------+ +----------+
security level =1 security level =2 security level =3
Figure 5
As shown in Figure 5, the trustworthiness level is configured on each
network transmission device.
Device-E colors the advertised BGP routes through the color extended
community attribute, and different services correspond to different
colors.
When Device-A receives a BGP route with color C1 and endpoint E,
device a will automatically generate an SR policy group (C1, E)
according to the ODN template of color C1.
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The composition SR policy of SR policy group is also generated
according to ODN template. DSCP1 is mapped to color C2. After
creating an SR policy group (C1, E), Device-A generates an ODN SR
policy (C2, E) according to the mapping relationship between DSCP and
color (DSCP1->C2).
Services with different security levels use different DSCPs. When
the user generates a service packet, it carries the corresponding
DSCP value (DSCP1) on the IPv6 packet header, and sends it to the
Device-A. After receiving the service packet, the service packet is
steered according to SR policy (C2, E).
6.6. Best-effort Forwarding Scenarios when SR Policy Becomes
Unavailable
When all the constituent SR policies in the SR policy group are not
valid, or all the selected paths of the SR policy are unavailable,
the service traffic will not be forwarded according to the specified
path. At this time, the best-effort forwarding path can be
configured for the SR policy group, and the endpoints through which
traffic forwarding must pass can be designed in the best-effort
forwarding path.
During network deployment, The best-effort forwarding path can be a
default SR policy or an SR BE forwarding path. Specify an best-
effort forwarding path in the SR policy group. When all specified
candidate paths are invalid, or the mapping relationship
corresponding to their service type is not matched in the SR policy
group, select the default best-effort path forwarding.
7. IANA Considerations
This document has no IANA actions.
8. Security Considerations
This document presents use cases to be considered by the deployment
of SR Policy. It does not introduce any security considerations.
9. References
[I-D.ietf-idr-segment-routing-te-policy]
Previdi, S., Filsfils, C., Talaulikar, Ed., K., Mattes,
P., Jain, D., and S. Lin, "Advertising Segment Routing
Policies in BGP", draft-ietf-idr-segment-routing-te-
policy-14 (work in progress), November 2021.
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[I-D.ietf-spring-segment-routing-policy]
Filsfils, C., Talaulikar, Ed., K., Voyer, D., Bogdanov,
A., and P. Mattes, "Segment Routing Policy Architecture",
draft-ietf-spring-segment-routing-policy-18 (work in
progress), February 2022.
[I-D.lin-opsec-trustroute-problem-statement]
Lin, T., Li, H., Shi, X., Yin, X., and W. Chen, "Problem
Statement and Use Cases of Trustworthiness-based Routing",
draft-lin-opsec-trustroute-problem-statement-01 (work in
progress), December 2021.
[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
Wenying Jiang
China Mobile
Email: jiangwenying@chinamobile.com
Weiqiang Cheng
China Mobile
Email: chengweiqiang@chinamobile.com
Changwang Lin
New H3C Technologies
Email: linchangwang.04414@h3c.com
Yuanxiang Qiu
New H3C Technologies
Email: qiuyuanxiang@h3c.com
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