Hi Guanming,
Thank you for putting this out.
I prefer the original option 1, it's simple and implementation friendly.
About option 2, I was wondering is there any strong usecase besides PSID that
has to carry a 128-bit metadata at last entry? For the In-situ OAM trace data,
RFC 9486 already defines HBH/DOH options for it, and for the custom telemetry
payload, existing IOAM mechanism in RFC9197 also supports carry self-defined
data.
And another concern on option 2 is that, if the last entry is used to carry
different types of metadata, only one of them can be carried in the same SRH,
they can never be used together.
Regards,
Yao
Original
From: zengguanming <[email protected]>
To: SPRING WG List <[email protected]>;
Cc: Cheng Li <[email protected]>;[email protected]
<[email protected]>;DHRUV DHODY <[email protected]>;chengweiqiang
<[email protected]>;[email protected] <[email protected]>;
Date: 2026年01月23日 16:17
Subject: [spring] Seeking WG Consensus on PSID Encoding Options for
draft-ietf-spring-srv6-path-segment
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Dear SPRING WG,
As part of our ongoing effort to finalize the encoding mechanism for the SRv6
Path Segment Identifier (PSID) in
https://datatracker.ietf.org/doc/draft-ietf-spring-srv6-path-segment/, we would
like to present three high-level approaches—along with their sub-options—for
community review and consensus. Thanks to Bruno’s constructive review, comments
and thorough discussion, we finally come up with the following options and
present to the WG:
--------------------------------------------------------------------------------
Option 1: Dedicated P-flag (Current Draft Approach)
Mechanism: Introduce a new SRH flag (e.g., P-flag) solely to indicate that SRH.
SegmentList[Last Entry] carries a PSID.
Pros: Simple, unambiguous, and enables per-packet fast-path processing for
precise OAM (e.g., loss measurement).
Cons: Consumes one of only eight SRH flags for a single function.
Option 2: Generic Metadata Flag (Recommended Evolution)
Mechanism: Define a generic SRH flag (e.g., G-flag) that signals the presence
of a structured 128-bit sid in SegmentList[Last Entry]. The opcode is defined
to distinguish different use cases, for example:
OpCode=0x01: Path Segment ID (PSID)
OpCode=0x02: In-situ OAM trace data
OpCode=0x03: Custom telemetry payload
Pros:
One generic flag supports multiple future extensions, thus addresses
“resource waste” concern by making the flag generically useful.
Maintains high-performance, per-packet processing.
Cons: Slightly more complex: requires defining opcode semantics and
extensibility model.
Option 3: No New Flag
This has three sub-options:
3A: Reuse O-flag
Mechanism: Use the existing OAM flag to signal PSID presence.
Pros:
• No SRH flags consumption.
Cons:
• O-flag implies slow-path, sampled OAM treatment (per RFC 8754), but PSID
often requires fast-path, per-packet handling for accurate end-to-end metrics.
Mismatch in processing model risks under-serving key use cases.
3B: Flag-less (Pure SID Convention)
Mechanism: Rely solely on the END.PSID behavior code (Function = 0x0064); no
flag needed. PSID is placed at SegmentList[n] where n = SRH.LastEntry.
Pros:
Minimalist design; No SRH flags consumption.
Cons:
No visibility for intermediate nodes—limits future telemetry or policy
enforcement.
Functionally restricted to egress-only use cases (e.g., basic path
binding), losing the full programmability advantage of SRv6.
3C: Flag-less with Dedicated PSID Prefix
Mechanism:
Reserve a well-known, non-routable IPv6 prefix (e.g., ::/32) for PSIDs.
Intermediate SR Endpoint nodes inspect SegmentList[n] and recognize PSID by
prefix match.
Pros:
No SRH flag consumption.
Enables intermediate node visibility without a flag.
Cons:
SR nodes on the path needs one more mechanism to read PSID at Segment List[n],
which introduces more complexity
--------------------------------------------------------------------------------
Next Steps
We believe Option 1(Dedicated P-flag) is simple, unambiguous, and enables
per-packet fast-path processing for precise OAM, and Option 2 (Generic Flag)
offers the best long-term balance: it conserves scarce flag space, supports
future extensions (beyond PSID), and maintains performance.
And we kindly ask the WG to share your views on:
Which direction best meets operational and architectural needs?
Any strong objections to the proposed options.
Depending on feedback, we will update the draft accordingly and aim to request
WGLC soon.
Thank you for your engagement!
Best regards,
Guanming Zeng & Cheng Li
Huawei
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