rfc9947v1.txt   rfc9947.txt 
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Independent Submission G. Fioccola Independent Submission G. Fioccola
Request for Comments: 9947 T. Zhou Request for Comments: 9947 T. Zhou
Category: Experimental Huawei Category: Experimental Huawei
ISSN: 2070-1721 G. Mishra ISSN: 2070-1721 G. Mishra
Verizon Inc. Verizon Inc.
X. Wang X. Wang
Ruijie Ruijie
G. Zhang G. Zhang
China Mobile China Mobile
M. Cociglio M. Cociglio
February 2026 March 2026
Application of the Alternate-Marking Method to the Segment Routing Application of the Alternate-Marking Method to the Segment Routing
Header Header
Abstract Abstract
This document describes an alternative experimental approach for the This document describes an alternative experimental approach for the
application of the Alternate-Marking Method to Segment Routing for application of the Alternate-Marking Method to Segment Routing for
IPv6 (SRv6). It uses an experimental TLV in the Segment Routing IPv6 (SRv6). It uses an experimental TLV in the Segment Routing
Header (SRH); thus, participation in this experiment should be Header (SRH); thus, participation in this experiment should be
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configured to do so. Both the Destination Options Header before the configured to do so. Both the Destination Options Header before the
SRH and the SRH TLV are processed at the node being indicated in the SRH and the SRH TLV are processed at the node being indicated in the
destination address field of the IPv6 header. destination address field of the IPv6 header.
The distinction between the alternatives is most notable for SRv6 The distinction between the alternatives is most notable for SRv6
packets that traverse a network where the paths between sequential packets that traverse a network where the paths between sequential
segment endpoints include multiple hops. If the Hop-by-Hop Option is segment endpoints include multiple hops. If the Hop-by-Hop Option is
used, then every hop along the path will process the AltMark data. used, then every hop along the path will process the AltMark data.
If the Destination Option positioned before the SRH is used, or the If the Destination Option positioned before the SRH is used, or the
SRH AltMark TLV is used, then only the segment endpoints will process SRH AltMark TLV is used, then only the segment endpoints will process
the AltMark data. the AltMark data, unless the intermediate node has a different
priority rule.
Both [RFC9343] and the approach specified in this document can Both [RFC9343] and the approach specified in this document can
coexist. Indeed, this document does not change or invalidate any coexist. Indeed, this document does not change or invalidate any
procedures defined in [RFC9343]. However, deployment issues may procedures defined in [RFC9343]. However, deployment issues may
arise, as further discussed below. arise, as further discussed below.
The rest of this document is structured as follows: The rest of this document is structured as follows:
* Section 2 covers the application of the Alternate-Marking Method * Section 2 covers the application of the Alternate-Marking Method
to SRv6, to SRv6,
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W bit: Flow direction identification. This flag is used if W bit: Flow direction identification. This flag is used if
backward direction flow monitoring is requested to be set up backward direction flow monitoring is requested to be set up
automatically, so that the egress node is instructed to setup automatically, so that the egress node is instructed to setup
the backward flow monitoring. If W=1, it indicates that the the backward flow monitoring. If W=1, it indicates that the
flow direction is forward. If W=0, it indicates that the flow flow direction is forward. If W=0, it indicates that the flow
direction is backward. direction is backward.
R bit: Reserved. This bit MUST be set to zero and ignored on R bit: Reserved. This bit MUST be set to zero and ignored on
receipt. receipt.
Len: Length. Indicates the length of the extended data fields in Len: Length. Indicates the length of the extended data fields for
bytes for Enhanced Alternate Marking. It includes all of the Enhanced Alternate Marking as multiple of 4 bytes. It includes
fields shown in Figure 2 including any metadata that is present. all of the fields shown in Figure 2 including any metadata that is
present.
Rsvd: Reserved for further use. These bits MUST be set to zero on Rsvd: Reserved for further use. These bits MUST be set to zero on
transmission and ignored on receipt. transmission and ignored on receipt.
MetaInfo: A 16-bit Bitmap to indicate more metadata attached in the MetaInfo: A 16-bit Bitmap to indicate more metadata attached in the
Optional MetaData field for enhanced functions. More than one bit Optional MetaData field for enhanced functions. More than one bit
may be set, in which case the additional metadata is present in may be set, in which case the additional metadata is present in
the order that the bits are set. MetaInfo bits are numbered from the order that the bits are set. MetaInfo bits are numbered from
0 as the most significant bit. Three bits and associated metadata 0 as the most significant bit. Three bits and associated metadata
are defined as follows: are defined as follows:
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Contributors Contributors
The following people provided relevant contributions to this The following people provided relevant contributions to this
document: document:
Fabio Bulgarella Fabio Bulgarella
Telecom Italia Telecom Italia
Email: fabio.bulgarella@guest.telecomitalia.it Email: fabio.bulgarella@guest.telecomitalia.it
Massimo Nilo Massimo Nilo
Telecom Italia FiberCop
Email: massimo.nilo@telecomitalia.it Email: massimo.nilo@fibercop.com
Fabrizio Milan Fabrizio Milan
Telecom Italia FiberCop
Email: fabrizio.milan@telecomitalia.it Email: fabrizio.milan@fibercop.com
Authors' Addresses Authors' Addresses
Giuseppe Fioccola Giuseppe Fioccola
Huawei Huawei
Viale Martesana, 12 Viale Martesana, 12
20055 Vimodrone (Milan) 20055 Vimodrone (Milan)
Italy Italy
Email: giuseppe.fioccola@huawei.com Email: giuseppe.fioccola@huawei.com
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