idr Z. Zhang
Internet-Draft Juniper Networks
Intended status: Standards Track July 07, 2021
Expires: January 8, 2022
MPLS Label Stacks in Tunnel Encapsulation Attribute
draft-zzhang-idr-tunnel-encapsulation-label-stack-00
Abstract
[RFC9012] defines an MPLS Label Stack sub-TLV for Tunnel
Encapsulation Attribute, and specifies that it is to be pushed BEFORE
other labels. This document clarifies the use case for that, and
defines a new Tunnel Label Stack sub-TLV for a label stack to be
pushed AFTER other labels (e.g., the label embedded in the NLRI for a
labeled address family, and/or the stack in an MPLS Label Stack sub-
TLV).
Status of This Memo
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1. Introduction
[RFC9012] defines an MPLS Label Stack sub-TLV for Tunnel
Encapsulation Attribute and specifies that:
"If a packet is to be sent through the tunnel identified in a
particular TLV, and if that TLV contains an MPLS Label Stack sub-TLV,
then the label stack appearing in the sub-TLV MUST be pushed onto the
packet before any other labels are pushed onto the packet."
Specifically, the label stack in the sub-TLV is to be pushed BEFORE
any other labels are pushed. This may sound counter-intuitive, in
that if a label stack (e.g. for Segment Routing) is to be used to
steer traffic to the tunnel endpoint, the stack should be pushed
AFTER other labels (e.g. the label embedded in the NLRI).
This document clarifies that it is NOT for steering traffic to but
for steering AFTER the tunnel endpoint. Consider the following use
case:
controller
. .
. .
site1 --- PE1 -------- PE2 --- site2
Two sites are connected to two PEs respectively, and traffic steering
is desired within each site not just among the PEs. While PE2 could
push the label stack used for steering withing site2, there may be
situations where PE2 is not a device capable of pushing a large label
stack so PE1 is tasked with pushing the label stack that is used
after the tunnel end point PE2, within site2.
2. Tunnel Label Stack sub-TLV
Notice that in the above example, the route update that PE1 receives
could be from the controller instead of PE2. The controller may want
to steer traffic both withing site2 and between PE1 and PE2, yet
RFC9012 currently does not specify how to signal the label stack used
to reach the tunnel end point.
To be able to signal that, this document defines a new Tunnel Label
Stack sub-TLV. It has exactly the same syntax as the existing MPLS
Label Stack sub-TLV, but with a semantics that it is pushed AFTER
other labels are pushed.
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3. Security Considerations
This document does not introduce any new security issues besides what
is already discussed in RFC9012.
4. IANA Assignments
IANA is requested to assign a new sub-TLV type for "Tunnel Label
Stack" from "BGP Tunnel Encapsulation Attribute Sub-TLVs" registry,
in the 0~127 range.
5. Acknowledgements
The authors thank Eric Rosen and John Scudder for their valuable
comments and suggestions.
6. Normative 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>.
Author's Address
Zhaohui Zhang
Juniper Networks
Email: zzhang@juniper.net
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