Hi Gunter,
Prefix Attribute Flags Sub-TLV has been defined as an optional Sub-TLV.
The problem you describe is not specific to Locator TLV, same applies to
regular IPv4/v6 prefixes (forget SR MPLS for a while) - if the Prefix
Attribute Flags TLV is not included, one can not tell whether the prefix
has been propagated (L1->L2) or generated as a result of the local
interface attached on the originator. Same applies to redistribution and
R-flag for IPv4 prefix TLVs.
SRv6 Locator TLV has been defined a while back and the Prefix Attribute
Flags Sub-TLV has always been an optional Sub-TLV of it. I'm not sure we
can start to mandate the Prefix Attribute Flags TLV at this point.
Technically I agree with you and if everybody agrees, I'm fine to
enforce the presence of the Prefix Attribute Flags TLV in the Locator TLV.
thanks,
Peter
On 03/05/2021 10:45, Van De Velde, Gunter (Nokia - BE/Antwerp) wrote:
Hi Peter, All,
Could we update to "draft-ietf-lsr-isis-srv6-extensions" that the
prefix-attribute tlv is mandatory when a locator is redistributed?
Why?
*When calculating a LFA for an SRv6 End.SID we better know if the locator has
been redistributed or not for a correct operation.
Reasoning:
* A locator has the D bit. This one is set when we redistribute from L2 to L1.
** So this end-sid will not be used as we know that it is redistributed.
* In the other direction (L1-L2), we only know that a locator is redistributed
from L1 to L2 if the prefix-attribute sub-tlv is advertised.
** This means if the operator does not configure advertisement of the
prefix-attribute tlv, ISIS could potentially use an end-sid which does not
terminate on the expected node.
* Compared to sr-mpls, a prefix-sid has the R flag indicating it is
redistributed.
* We don't have that for locator end-sids.
Relevant snip from " draft-ietf-lsr-isis-srv6-extensions"
7.1. SRv6 Locator TLV Format
The SRv6 Locator 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 |R|R|R|R| MT ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 27
Length: variable.
R bits: reserved for future use. They MUST be
set to zero on transmission and MUST be ignored on receipt.
MT ID: Multitopology Identifier as defined in [RFC5120].
Note that the value 0 is legal.
Followed by one or more locator entries of the form:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Loc Size | Locator (variable)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLV-len | Sub-TLVs (variable) . . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Metric: 4 octets. As described in [RFC5305].
Flags: 1 octet. The following flags are defined
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|D| Reserved |
+-+-+-+-+-+-+-+-+
where:
D-flag: Same as described in section 4.1. of [RFC5305].
G/
-----Original Message-----
From: Lsr <[email protected]> On Behalf Of The IESG
Sent: Thursday, April 22, 2021 10:14 PM
To: IETF-Announce <[email protected]>
Cc: [email protected]; [email protected]; [email protected];
[email protected]; [email protected]
Subject: [Lsr] Last Call: <draft-ietf-lsr-isis-srv6-extensions-14.txt> (IS-IS
Extension to Support Segment Routing over IPv6 Dataplane) to Proposed Standard
The IESG has received a request from the Link State Routing WG (lsr) to
consider the following document: - 'IS-IS Extension to Support Segment Routing
over IPv6 Dataplane'
<draft-ietf-lsr-isis-srv6-extensions-14.txt> as Proposed Standard
The IESG plans to make a decision in the next few weeks, and solicits final
comments on this action. Please send substantive comments to the
[email protected] mailing lists by 2021-05-07. Exceptionally, comments may be
sent to [email protected] instead. In either case, please retain the beginning of
the Subject line to allow automated sorting.
Abstract
The Segment Routing (SR) allows for a flexible definition of end-to-
end paths by encoding paths as sequences of topological sub-paths,
called "segments". Segment routing architecture can be implemented
over an MPLS data plane as well as an IPv6 data plane. This document
describes the IS-IS extensions required to support Segment Routing
over an IPv6 data plane.
This documents updates RFC 7370 by modifying an existing registry.
The file can be obtained via
https://datatracker.ietf.org/doc/draft-ietf-lsr-isis-srv6-extensions/
The following IPR Declarations may be related to this I-D:
https://datatracker.ietf.org/ipr/3796/
https://datatracker.ietf.org/ipr/4486/
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