IGP Shortcut Enhancement
draft-cheng-lsr-igp-shortcut-enhancement-00
The information below is for an old version of the document.
| Document | Type |
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|---|---|---|---|
| Authors | Weiqiang Cheng , Liyan Gong , Changwang Lin , Mengxiao Chen | ||
| Last updated | 2023-03-06 | ||
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draft-cheng-lsr-igp-shortcut-enhancement-00
Network Working Group W. Cheng
Internet Draft L. Gong
Intended status: Standards Track China Mobile
Expires: September 6, 2023 C. Lin
M. Chen
New H3C Technologies
March 6, 2023
IGP Shortcut Enhancement
draft-cheng-lsr-igp-shortcut-enhancement-00
Abstract
IGP shortcut mechanism allows calculating routes to forward traffic
over Traffic Engineering tunnels. This document describes the
enhancement of IGP shortcut which can steer routes onto TE-tunnels
based on colors.
Status of this Memo
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Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction...................................................2
1.1. Requirements Language.....................................3
2. Extensions for IGP.............................................3
2.1. IS-IS Color Sub-TLV.......................................3
2.2. OSPF Color Sub-TLV........................................4
3. SPF Computation................................................5
4. Color Values of TE-Tunnels.....................................5
5. Backward Compatibility.........................................6
6. Security Considerations........................................6
7. IANA Considerations............................................6
8. References.....................................................6
8.1. Normative References......................................6
8.2. Informational References..................................7
Authors' Addresses................................................8
1. Introduction
[RFC3906] describes how IGP calculate routes to forward traffic over
Traffic Engineering tunnels. Such mechanism is also referred to as
IGP shortcut.
The granularity of IGP shortcut is based on nodes. If the first-hop
of a node is determined to be a TE-tunnel during the SPF
computation, all routes to IP prefixes advertised by that node will
be over that TE-tunnel. For example, in the following topology, X1
and X2 are IP prefixes advertised by rtrC, and Y1 and Y2 are IP
prefixes advertised by rtrD. Using IGP shortcut, all routes to X1,
X2, Y1 and Y2 will be steered onto T1 since T1 has the lowest cost.
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=== T1(10) ===>
=== T2(15) ===>
rtrA -- rtrB -- rtrC -- rtrD
10 10 | 10 |
X1,X2 Y1,Y2
However, in some scenarios, there may be requirements to steer the
routes to different prefixes of the same node onto different TE-
tunnels. For example, the traffic flows to X1 and Y1 need to be
forwarded over low-cost tunnel T1, but the traffic flows to X2 and
Y2 need to be forwarded over low-delay tunnel T2.
In the BGP-based service, "color" is often used to indicate the
intent of forwarding [RFC9012] [RFC9252]. The Color Extended
Community can be attached to BGP routes, and the associated flows
will be steered into tunnels with the same color.
This document describes the enhancement of IGP shortcut which can
steer routes onto TE-tunnels based on colors. It also defines the
extensions for IGP to advertise colors for prefixes.
In [RFC3906], the term "TE-tunnel" mainly refers to Label Switched
Path, such as MPLS RSVP-TE tunnel. With the development of Segment
Routing (SR) technology, SR Policy [RFC9256] becomes a useful tool
for Traffic Engineering. In the context of this document, SR
Policies are also included as TE-tunnels.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Extensions for IGP
2.1. IS-IS Color Sub-TLV
The IS-IS Color Sub-TLV is defined in this document to advertise
colors for prefixes in IS-IS. The Sub-TLV has the following format:
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Color |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: TBD.
o Length: 1 octet. The length value is variable.
o Flags: 2 octets. No flags are defined in this document. Undefined
flags MUST be set to 0 by the sender, and any unknown flags MUST
be ignored by the receiver.
o Color: 4 octets. Contains color value associated with the prefix.
The IS-IS Color Sub-TLV is applicable to TLVs 27, 135, 235, 236, and
237.
2.2. OSPF Color Sub-TLV
The OSPF Color Sub-TLV is defined in this document to advertise
colors for prefixes in OSPFv2 and OSPFv3. The Sub-TLV has the
following format:
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 | Color |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Color (cont.) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: TBD.
o Length: 1 octet. The length value is variable.
o Flags: 2 octets. No flags are defined in this document.
Undefined flags MUST be set to 0 by the sender, and any unknown
flags MUST be ignored by the receiver.
o Color: 4 octets. Contains color value associated with the prefix.
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The OSPF Color Sub-TLV is applicable to OSPFv2 Extended Prefix TLV
OSPFv3 Inter-Area-Prefix TLV OSPFv3 Intra-Area-Prefix TLV OSPFv3
External-Prefix TLV.
3. SPF Computation
The SPF computation of IGP shortcut is described in Section 2 of
[RFC3906]. The key idea is to determine the first-hop information of
a node with consideration of TE-tunnels:
o Examine the list of tail-end routers directly reachable via a TE-
tunnel. If there is a TE-tunnel to this node, we use the TE-
tunnel as the first-hop.
o If there is no TE-tunnel, and the node is directly connected, we
use the first-hop information from the adjacency database.
o If the node is not directly connected, and is not directly
reachable via a TE-tunnel, we copy the first-hop information from
the parent node(s) to the new node.
This document makes the following changes to the first step of the
above algorithm:
o If there is a TE-tunnel to this node, we add the TE-tunnel into
the first-hop information (without deleting the previous ones).
This document also adds the following steps when calculating next-
hops for prefixes advertised by a node:
o If a prefix is colored, we look up the first-hop information of
the advertiser node for TE-tunnels with the same color.
- If there are eligible TE-tunnels, we compare the costs of
paths over those TE-tunnels, and use the next-hop of the TE-
tunnel with the lowest path cost.
- If there is no eligible TE-tunnel, we use the native
adjacency next-hop.
o If a prefix has no color, we use the next-hop with the lowest
path cost.
4. Color Values of TE-Tunnels
The main idea of this document is to steer the flows to colored
prefixes into tunnels with the same color values.
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Some kinds of TE-tunnels, such as SR Policy [RFC9256], have inherent
color values which can be directly used to match the colors of
prefixes. For the TE-tunnels which have no inherent color, the color
values may be determined by local configurations, which is out of
the scope of this document.
5. Backward Compatibility
If a head-end node does not support the Color Sub-TLV, it will
calculate routes ignoring the colors. As a result, the behavior
would be the same as without this specification.
6. Security Considerations
TBD.
7. IANA Considerations
This document defines the following new Sub-TLV in IS-IS:
TBD - IS-IS Color Sub-TLV
This document defines the following new Sub-TLV in OSPFv2 and
OSPFv3:
TBD - OSPF Color Sub-TLV
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, May 2017
[RFC3906] Shen, N. and H. Smit, "Calculating Interior Gateway
Protocol (IGP) Routes Over Traffic Engineering Tunnels",
RFC 3906, DOI 10.17487/RFC3906, October 2004,
<https://www.rfc-editor.org/info/rfc3906>.
[RFC9256] Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
A., and P. Mattes, "Segment Routing Policy Architecture",
RFC 9256, DOI 10.17487/RFC9256, July 2022,
<https://www.rfc-editor.org/info/rfc9256>
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8.2. Informational References
[RFC9012] Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
"The BGP Tunnel Encapsulation Attribute", RFC 9012, DOI
10.17487/RFC9012, April 2021, <https://www.rfc-
editor.org/info/rfc9012>.
[RFC9252] Dawra, G., Ed., Talaulikar, K., Ed., Raszuk, R., Decraene,
B., Zhuang, S., and J. Rabadan, "BGP Overlay Services
Based on Segment Routing over IPv6 (SRv6)", RFC 9252, DOI
10.17487/RFC9252, July 2022, <https://www.rfc-
editor.org/info/rfc9252>.
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Authors' Addresses
Weiqiang Cheng
China Mobile
China
Email: chengweiqiang@chinamobile.com
Liyan Gong
China Mobile
China
Email: gongliyan@chinamobile.com
Changwang Lin
New H3C Technologies
China
Email: linchangwang.04414@h3c.com
Mengxiao Chen
New H3C Technologies
China
Email: chen.mengxiao@h3c.com
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