Internet DRAFT - draft-ietf-idr-bgp-srmpls-elp
draft-ietf-idr-bgp-srmpls-elp
IDR Y. Liu
Internet-Draft S. Peng
Intended status: Standards Track ZTE
Expires: 8 August 2024 5 February 2024
BGP Extension for SR-MPLS Entropy Label Position
draft-ietf-idr-bgp-srmpls-elp-01
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
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 8 August 2024.
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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. Operation Considerations . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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
controller (e.g. PCE) can be used to perform the SR-TE path
computation as well as the Entropy Label Position which is useful for
inter-domain scenarios.
[I-D.ietf-idr-sr-policy-safi] 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
BMI: Base MPLS Imposition is the number of MPLS labels that can be
imposed inclusive of all service/transport/special labels.
MSD: Maximum SID Depth
BMI-MSD: A type of MSD signals the BMI of a node.
ERLD-MSD: A type of MSD advertises the ERLD of a node.
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 BMI-MSD[RFC8491], 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.
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The Entropy Readable Label Depth(ERLD) [RFC8662] is another 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.
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: 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 can be advertised as ERLD-MSD via IS-
IS[RFC9088] and OSPF[RFC9089] within the domain, in each domain, one
or more nodes are configured with BGP-LS so the controller can get
the ERLD-MSD of all the nodes through BGP-LS[RFC9085]. The
controller can acquire the BMI-MSD of the headend node or the Binding
SID anchor node via BGP-LS[RFC8814] or PCEP[RFC8664].
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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.
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
As defined in [I-D.ietf-idr-sr-policy-safi], the SR Policy encoding
structure is as follows:
SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
Attributes:
Tunnel Encaps Attribute (23)
Tunnel Type: SR Policy
Binding SID
Preference
Priority
Policy Name
Explicit NULL Label Policy (ENLP)
Segment List
Weight
Segment
Segment
...
...
This document defines a new ELP sub-TLV within Segment List sub-TLV.
The ELP 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 | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where,
Type: TBD.
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Length: Specifies the length of the value field (i.e., not including
Type and Length fields) in terms of octets. The value MUST be 2.
RESERVED: 2 octet of reserved bits. MUST be unset on transmission
and MUST be ignored on receipt.
The ELP sub-TLV, when present, indicates that the <ELI, EL> label
pair SHOULD be inserted at this position. It MAY appear multiple
times in the Segment List sub-TLV.
The new SR Policy encoding structure with ELP sub-TLV is expressed as
below:
SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
Attributes:
Tunnel Encaps Attribute (23)
Tunnel Type: SR Policy
Binding SID
Preference
Priority
Policy Name
Explicit NULL Label Policy (ENLP)
Segment List
Weight
Segment
Segment
ELP
Segment
...
Segment
ELP
Segment
Segment
...
...
5. Operation Considerations
After obtaining the topolopy of the network and other necessary
criterias for determining ELP, the controller calculates the path
information of the SR-TE Policy based on these information and builds
the SR-TE Policy from the headend to the endpoint. And then it
delivers the SR path information with the segment list as well as the
ELP included to the headend, leveraging the BGP extension introduced
in this document.
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The <ELI, EL> label pair insertion is indicated at the segment list
level. The following example shows how a headend node operates after
the ELP sub-TLV is introduced.
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 and ELP sub-TLVs, e.g, <S1, S2, S3,ELP, S4, S5, S6, ELP>, 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>.
If the SR Policy is delivered without the ELP sub-TLV, the headend
node MAY still insert ELI/ELs based on its own decision, as the
legacy behavior of ELI/EL insertion defined in [RFC8662], making it
inconvenient for network operation and troubleshooting. So it is
RECOMMENDED that at at least within the same SR policy, the behavior
SHOULD be consistent, that is, either the headend node inserts the
ELI/ELs based on the ELP sub-TLV received or it makes its own
decision regardless of the existence of the ELP sub-TLV.
The Maximum SID Depth defines the maximum number of labels that a
particular node can impose on a packet. When ELI/EL insertion is
required, the number of segments and ELI/ELs to be inserted in the
segment list SHOULD not exceed the BMI-MSD limit. In this case,
depending on whether ELI/EL(s) are required to be inserted and
whether the headend node has sufficient BMI-MSD, the segment list
delivered by the controller to the headend MAY be different. And
with the additional constraints on ELI/EL insertion, the number of
valid paths may be reduced, in some cases there may be no available
path. Whether the ELI/EL insertion should be considered when
computing the SR policy MAY be based on the configuration or local
policy on the controller, or some control plane mechanism between the
headend and the controller MAY be used, the details are out of the
scope of this document.
6. IANA Considerations
This document defines a new sub-TLV in the registry "SR Policy List
Sub-TLVs" [I-D.ietf-idr-sr-policy-safi] to be assigned by IANA:
Codepoint Description Reference
-------------------------------------------------------------
TBD ELP Sub-TLV This document
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7. 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-sr-policy-safi].
8. Acknowledgement
The authors would like to thank Ketan Talaulikar, Jie Dong and Nat
Kao for their helpful review and comments.
9. References
9.1. Normative References
[I-D.ietf-idr-sr-policy-safi]
Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P., and
D. Jain, "Advertising Segment Routing Policies in BGP",
Work in Progress, Internet-Draft, draft-ietf-idr-sr-
policy-safi-00, 4 February 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-sr-
policy-safi-00>.
[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>.
9.2. Informative References
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[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>.
[RFC9088] 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", RFC 9088,
DOI 10.17487/RFC9088, August 2021,
<https://www.rfc-editor.org/info/rfc9088>.
[RFC9089] Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S.,
and M. Bocci, "Signaling Entropy Label Capability and
Entropy Readable Label Depth Using OSPF", RFC 9089,
DOI 10.17487/RFC9089, August 2021,
<https://www.rfc-editor.org/info/rfc9089>.
Authors' Addresses
Yao Liu
ZTE
Nanjing
China
Email: liu.yao71@zte.com.cn
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Shaofu Peng
ZTE
Nanjing
China
Email: peng.shaofu@zte.com.cn
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