BGP Extension for SR-MPLS Entropy Label Position
draft-zhou-idr-bgp-srmpls-elp-03
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Liu Yao
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Shaofu Peng
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2021-08-30
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IDR Working Group Yao. Liu
Internet-Draft Shaofu. Peng
Intended status: Standards Track ZTE Corp.
Expires: 3 March 2022 30 August 2021
BGP Extension for SR-MPLS Entropy Label Position
draft-zhou-idr-bgp-srmpls-elp-03
Abstract
This document proposes extensions for BGP to indicate the entropy
label position in the SR-MPLS label stack when delivering SR Policy
via BGP.
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 3 March 2022.
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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions used in this document . . . . . . . . . . . . . . 3
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2.2. Terminology and Acronyms . . . . . . . . . . . . . . . . 3
3. Entropy Label Position in SR-MPLS with the Controller . . . . 3
4. BGP Extensions for ELP in SR Policy . . . . . . . . . . . . . 5
5. Operations . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
Segment Routing (SR) leverages the source routing paradigm. Segment
Routing can be instantiated on MPLS data plane which is referred to
as SR-MPLS [RFC8660]. SR-MPLS leverages the MPLS label stack to
construct the SR path.
Entropy labels (ELs) [RFC6790] are used in the MPLS data plane to
provide entropy for load-balancing. The idea behind the entropy
label is that the ingress router computes a hash based on several
fields from a given packet and places the result in an additional
label named "entropy label". Then, this entropy label can be used as
part of the hash keys used by an LSR. Using the entropy label as
part of the hash keys reduces the need for deep packet inspection in
the LSR while keeping a good level of entropy in the load-balancing.
[RFC8662] proposes to use entropy labels for SR-MPLS networks and
multiple < ELI, EL> pairs may be inserted in the SR-MPLS label stack.
The ingress node may decide the number and position of the ELI/ELs
which need to be inserted into the label stack, that is termed as ELP
(Entropy Label Position) in this document. But in some cases,the the
controller (e.g. PCE) can be used to perform the TE path computation
as well as the Entropy Label Position which is useful for inter-
domain scenarios.
[I-D.ietf-idr-segment-routing-te-policy] specifies the way to use BGP
to distribute one or more of the candidate paths of an SR Policy to
the headend of that policy.
This document proposes extensions for BGP to indicate the ELP in the
segment list when delivering SR Policy via BGP in SR-MPLS networks.
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2. Conventions used in this document
2.1. 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.
2.2. Terminology and Acronyms
EL: Entropy Label
ELI: Entropy Label Indicator
ELC: Entropy Label Capability
ERLD: Entropy Readable Label Depth
ELP: Entropy Label Position
MSD: Maximum SID Depth
3. Entropy Label Position in SR-MPLS with the Controller
As described in [RFC8662] section 7, ELI/EL placement is not an easy
decision, multiple criteria may be taken into account.
First is the Maximum SID Depth (MSD), it defines the maximum number
of labels that a particular node can impose on a packet, and it is a
limit when the ingress node imposing ELI/EL pairs on the SR label
stack.
The Entropy Readable Label Depth(ERLD) value is an important
parameter to consider when inserting an ELI/EL. The ERLD is defined
as the number of labels a router can both read in an MPLS packet
received on its incoming interface(s) and use in its load-balancing
function. An ELI/EL pair must be within the ERLD of the LSR in order
for the LSR to use the EL during load-balancing. It's necessary to
get the ERLD of the nodes along the SR path to achieve efficient
load-balancing.
An implementation MAY try to evaluate if load-balancing is really
expected at a particular node based on the segment type of its label,
which also influences the ELP of a segment list.
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Other criteria includes maximizing number of LSRs that will load-
balance, preference for a part of the path, and etc. Using which
criteria and how to decide the ELP based on the criteria is a matter
of implementation.
As shown in Figure 1, in the inter-domain scenario, a path from A to
Z is required, a centralized controller performs the computation of
the end-to-end path, along which traffic load-balancing is required.
.................... .................... .....................
. . . . . .
.+---+ +---+ . . +---+ +---+ . .+---+ +----+ .
.| A |-------| B |------ | C |------| X |-------| Y |------| Z | .
.+---+ +---+ . . +---+ +---+ . .+---+ +----+ .
. domain 1 . . domain 2 . . domain 3 .
.................... .................... .....................
Figure 1: Figure 1: Entropy Labels in SR-MPLS Inter-Domain Scenario
When the headend node in the first domain can't get the information
of the nodes/SIDs in other domains, e.g, the ERLD of each node or the
type of the SID bounded to a node/link, it's difficult for the
headend node to decide the ELP of the segment list for the path.
Performing the computation of the ELP by the controller is an
alternate, since it's easier for the controller to get the required
information along the segment list prescribed by itself.
For example, the ERLD value can be advertised via IS-
IS[I-D.ietf-isis-mpls-elc] and OSPF[I-D.ietf-ospf-mpls-elc] with the
domain, in each domain, one or more nodes are configured with BGP-LS
so the controller can get the ERLD value of all the nodes through
BGP-LS[RFC9085]. The controller can acquire the MSD of the headend
node or the Binding SID anchor node via BGP-LS[RFC8814] or
PCEP[RFC8664].
Another benefit of utilizing the controller to calculate ELP is that
if the criteria or calculation algorithm is changed, the
corresponding modification only needs to be made on the controller
instead of each headend node in the network.
When the controller performs the computation of the the ELP for a
segment list, the considerations for the placement of ELI/ELs
introduced in [RFC8662] are still applicable. How the controller
computes the ELP is out of scope of the document.
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After the ELP of an SR path is decided, the controller SHOULD inform
the result to the headend node of the path, so the node knows where
to insert the ELI/ELs when needed. Section 4 proposes the detailed
extensions for BGP to carry this information.
4. BGP Extensions for ELP in SR Policy
The Segment Flags for Segment Sub-TLVs are defined in
Section 2.4.4.2.12 of [I-D.ietf-idr-segment-routing-te-policy]. In
this document, the ELP information is transmitted by extending the
flags of Segment Sub-TLVs.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|V|A|S|B|E| |
+-+-+-+-+-+-+-+-+
E-Flag: This flag, when set, indicates that presence of < ELI, EL>
label pairs which are inserted after this segment. E-Flag is
applicable to Segment Types A, C, D, E, F, G and H. If E-Flag
appears with Segment Types B, I, J and K, it MUST be ignored.
5. Operations
Node A receives an SR Policy NLRI with an Segment List sub-TLV from
the controller. The Segment List sub-TLV contains multiple Segment
sub-TLVs, e.g, <S1, S2, S3, S4, S5, S6>, the E-Flags of S3 and S6 are
set, it indicates that if load-balancing is required, two <ELI, EL>
pairs SHOULD be inserted into the label stack of the SR-TE forwarding
entry, respectively after the Label for S3 and Label for S6.
The value of EL is supplemented by the ingress node according to
load-balancing function of the appropriate keys extracted from a
given packet. After inserting ELI/ELs, the label stack on the
ingress node would be <S1, S2, S3, ELI, EL, S4, S5, S6, ELI, EL>.
6. Security Considerations
Procedures and protocol extensions defined in this document do not
introduce any new security considerations beyond those already listed
in [RFC8662] and [I-D.ietf-idr-segment-routing-te-policy].
7. IANA Considerations
This document requests bit 4 for Entropy Label Flag in "SR Policy
Segment Flags" under the "BGP Tunnel Encapsulation" registry.
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Bit Description Reference
------------------------------------------------------------------
4 Entropy Label Position Flag(E-Flag) This document
8. References
8.1. Normative References
[I-D.ietf-idr-segment-routing-te-policy]
Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P.,
Rosen, E., Jain, D., and S. Lin, "Advertising Segment
Routing Policies in BGP", Work in Progress, Internet-
Draft, draft-ietf-idr-segment-routing-te-policy-13, 7 June
2021, <https://www.ietf.org/archive/id/draft-ietf-idr-
segment-routing-te-policy-13.txt>.
[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>.
[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, DOI 10.17487/RFC6790, November 2012,
<https://www.rfc-editor.org/info/rfc6790>.
[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>.
[RFC8662] Kini, S., Kompella, K., Sivabalan, S., Litkowski, S.,
Shakir, R., and J. Tantsura, "Entropy Label for Source
Packet Routing in Networking (SPRING) Tunnels", RFC 8662,
DOI 10.17487/RFC8662, December 2019,
<https://www.rfc-editor.org/info/rfc8662>.
8.2. Informative References
[I-D.ietf-isis-mpls-elc]
Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S.,
and M. Bocci, "Signaling Entropy Label Capability and
Entropy Readable Label Depth Using IS-IS", Work in
Progress, Internet-Draft, draft-ietf-isis-mpls-elc-13, 28
May 2020, <https://www.ietf.org/archive/id/draft-ietf-
isis-mpls-elc-13.txt>.
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[I-D.ietf-ospf-mpls-elc]
Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S.,
and M. Bocci, "Signaling Entropy Label Capability and
Entropy Readable Label Depth Using OSPF", Work in
Progress, Internet-Draft, draft-ietf-ospf-mpls-elc-15, 1
June 2020, <https://www.ietf.org/archive/id/draft-ietf-
ospf-mpls-elc-15.txt>.
[RFC8476] Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak,
"Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476,
DOI 10.17487/RFC8476, December 2018,
<https://www.rfc-editor.org/info/rfc8476>.
[RFC8491] Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
"Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491,
DOI 10.17487/RFC8491, November 2018,
<https://www.rfc-editor.org/info/rfc8491>.
[RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing with the MPLS Data Plane", RFC 8660,
DOI 10.17487/RFC8660, December 2019,
<https://www.rfc-editor.org/info/rfc8660>.
[RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
and J. Hardwick, "Path Computation Element Communication
Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
DOI 10.17487/RFC8664, December 2019,
<https://www.rfc-editor.org/info/rfc8664>.
[RFC8814] Tantsura, J., Chunduri, U., Talaulikar, K., Mirsky, G.,
and N. Triantafillis, "Signaling Maximum SID Depth (MSD)
Using the Border Gateway Protocol - Link State", RFC 8814,
DOI 10.17487/RFC8814, August 2020,
<https://www.rfc-editor.org/info/rfc8814>.
[RFC9085] Previdi, S., Talaulikar, K., Ed., Filsfils, C., Gredler,
H., and M. Chen, "Border Gateway Protocol - Link State
(BGP-LS) Extensions for Segment Routing", RFC 9085,
DOI 10.17487/RFC9085, August 2021,
<https://www.rfc-editor.org/info/rfc9085>.
Authors' Addresses
Liu Yao
ZTE Corp.
Email: liu.yao71@zte.com.cn
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Peng Shaofu
ZTE Corp.
Email: peng.shaofu@zte.com.cn
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