[pim] Re: Ketan Talaulikar's Discuss on draft-ietf-pim-p2m p-policy-ping-18: (with DISCUSS and COMMENT)

Hi Ketan

Thanks for your comments

Inline

Thanks
Hooman


-----Original Message-----
From: Ketan Talaulikar via Datatracker <[email protected]>
Sent: Wednesday, August 20, 2025 7:00 AM
To: The IESG <[email protected]>
Cc: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]
Subject: Ketan Talaulikar's Discuss on draft-ietf-pim-p2mp-policy-ping-18: (with DISCUSS and COMMENT)


CAUTION: This is an external email. Please be very careful when clicking links or opening attachments. See the URL nok.it/ext for additional information.



Ketan Talaulikar has entered the following ballot position for
draft-ietf-pim-p2mp-policy-ping-18: Discuss

When responding, please keep the subject line intact and reply to all email addresses included in the To and CC lines. (Feel free to cut this introductory paragraph, however.)


Please refer to https://www.ietf.org/about/groups/iesg/statements/handling-ballot-positions/
for more information about how to handle DISCUSS and COMMENT positions.


The document, along with other ballot positions, can be found here:
https://datatracker.ietf.org/doc/draft-ietf-pim-p2mp-policy-ping/



----------------------------------------------------------------------
DISCUSS:
----------------------------------------------------------------------

Thanks to the authors and the WG for their work on this document.

I have a few points that I would like to discuss with the authors and the WG.

discuss#1 This one should be easy to fix. Since the document is about MPLS and not SRv6, the correct title for this document would be "Segment Routing MPLS Point-to-Multipoint (P2MP) Policy Ping" ?

HB> sorry I am not following, as per draft this doc only addresses the MPLS encapsulation and not the SRv6. SRv6 will be in future documents.

discuss#2 This is related to a point of discussion that I've also raised on the p2mp policy document. It arises from the lack of clarity on whether the SR P2MP Policy construct is instantiated on the root node or not. There is text in section 3.1.1 which seems to leave this critical aspect to implementations and will result in interoperability problems. The base spec needs to be very clear on this point and then this document updated to reflect it. I believe having the construct instantiated in the root will greatly benefit and simplify OAM operations. And things would then become very similar to RSVP-TE P2MP trees?

Quoting some text from section 3.1.1 that is problematic:

"Only the Root node, where the P2MP SR Policy is programmed, may have visibility  into the leaf nodes."

"In the case of P2MP SR Policies, the Root of the tree may have full visibility into the egress nodes if the P2MP SR Policy is PCC-initiated. If the P2MP SR Policy is PCE-initiated, the Root may or may not have visibility into the egress  nodes, as this depends on the specific implementation and configuration of the PCE. "

Further dependencies like the following seem unnecessary:

"Based on this, a P2MP SR Policy SHOULD follow the recommendations in Section
4.3.1 of [RFC6425], depending on the level of visibility the Root has into the  egress nodes. For example, in a PCC-initiated P2MP SR Policy, the Root can learn egress node identities through Next-Generation MVPN procedures and BGP, as described in [RFC6514]. In contrast, for a PCE-initiated P2MP SR Policy, the PCE may not provide the egress node information to the Root, making this process optional and implementation-specific."

The lack of clarity hurts interoperability and would affect operations in a multi-vendor network. I would like to discuss why all of this cannot be simplified by ensuring that the SR P2MP Policy construct is instantiated on the root node.

HB> Thanks but I think this is clearly been explained in the introduction, as per below paragraph. I clearly says P2MP Policy finest object is a TI which is the actual P2MP tunnel setup from the "Root" to a set of leaves.

"A P2MP Policy can have one or multiple Candidate Paths (CPs). The CP with highest preference is designated as the active CP, while all other CPs are the backup CPs. To enable seamless global optimization a CP may consist of multiple Tree Instances (TIs), allowing for Make-Before-Break (MBB) procedures between an active TI and a newly established, optimized TI. A TI is the actual P2MP tunnel set up from the Root to a set of Leaves via transit routers. A TI is identified on the Root node by the Root-ID which is the Root's node IP address, treeID and TI's instance ID."

discuss#3 My understanding is that the P2MP MPLS trees that are setup by MLDP or RSVP-TE are hop by hop in nature. While in this case, the packet can travel multiple hops from one node to the next intermediate node using that next intermediate node's Prefix SID.

HB> That is not correct, a P2MP MPLS tree for mLDP, P2MP RSVP-TE or treeSID is signalled from the root to the a let of leaves. There is no difference on the datapath between mLDP, P2MP RSVP-TE or treeSID. They all use MPLS switching to forward a multicast stream from the root to a set of leaves.

In this case, how would operation like traceroute (or even errors in the case of ping) work when the packet is exposed at a node that is doing unicast forwarding and has no replication segment context for that specific P2MP Tree? Now, section 3.1.3 is covering this, but talking about it as "unicast SR domains" is very misleading since there is only an SR domain and it is just that the specific P2MP tree context is not required to be instantiated on a transit node. Does this mean that this mechanism works only when the P2MP Tree is setup up hop-by-hop? If so, this should be clearly called out as a caveat upfront and the text in 3.1.3 updated appropriately.

HB> the replication SID is at the bottom of the stack, so a transit node that is capable of treeSID can push adjacency SID or node SID of another treeSID router that is multi-hop away. As such 2 treeSID capable routers can be connected via a unicast SR domain, i.e. treeSID tunnel over Unicast SR tunnel (tunnel over tunnel). Only the treeSID routers will decrement the treeSID TLL and make appropriate decisions if the TLL has expired. This tunnel in tunnel concept is nothing new to treeSID and it is the concept of segment routing which treeSID is part of.

discuss#4 This is an easy one to fix - the following is not a normative reference, please move to informative.

[IANA-AF]
"IANA Assigned Port Numbers, "http://www.iana.org/assignments/address-family-numbers"".

HB> ok thanks done in version 19.

----------------------------------------------------------------------
COMMENT:
----------------------------------------------------------------------

Please also find below some comments provided inline in the idnits format of the v18 of this document. On all editorial and minor comments, I will leave it to the authors discretion. On the major ones, I would appreciate responses and clarifications.

Please look for <EoRv18> at the end of this review and if it is not there, then likely the email has gotten truncated by your client (please refer to the mailing list in that case).

94      1.  Introduction

96         A P2MP Policy can have one or multiple Candidate Paths (CPs).  The CP

<minor> Please align terminologies with the SR P2MP policy draft. Term is SR P2MP Policy, then there is P2MP Tree (Instance), etc. Would be nice to avoid introducing new terms (e.g., TI) in this document related to any of the constructs.

HB> ok thanks, changed to p2mp policy and p2mp tree instance

97         with highest preference is designated as the active CP, while all
98         other CPs are the backup CPs.  To enable seamless global optimization

<minor> The CP preference is only the first tiebreaker in the selection of active CP. Perhaps "One of the CPs (e.g., with highest preference) is designated ..."
HB> this might be implementation specific. In the SR p2mp policy the candidate path with highest preference is the active CP.

131        [draft-ietf-pim-sr-p2mp-policy] section 2, defines terms and concepts
132        specific to SR P2MP Policy including the CP and the TI.

<minor> I couldn't find TI defined in that document. Please introduce in the base.

HB> it is introduced in the introduction
"To enable seamless global optimization a CP may consist of multiple P2MP Tree Instances (TIs), allowing for Make-Before-Break (MBB) procedures between an active TI and a newly established, optimized TI."

142     3.  Motivation

144        A P2MP Policy and its corresponding Replication Segments are
145        typically provisioned via a centralized controller or configured
146        using NETCONF/YANG or CLI.  The root and the leaves are discovered in

<minor> Perhaps you mean that the network topology that includes the root and leaves is discovered?

HB> no, the controller sucks up the topology but the controller initiates the P2MP policy and its replication segments on the root, transit and leaf nodes.

161        This diagnostic process can be initiated from the node responsible
162        for establishing the P2MP Policy, ensuring proactive monitoring and
163        rapid fault detection.

<minor> Is use of "rapid" appropriate here? Rapid as in BFD?

HB> There are cases and implementations, that rapid pings are send for fault detection.

295     3.2.1.1.  P2MP Policy CP FEC Stack Sub-TLVs

297        The P2MP Policy MPLS Candidate Path sub-TLV value field follows the

<major> Please consider changing the name of this TLV since it is not about CP but about the P2MP Tree instance under a CP. Perhaps "SR P2MP Policy Tree FEC Stack sub-TLV" ... or something similar. I was trying to not make it too long by including the CP in there, but that would also be ok.

HB> It is actually detecting fault on a CP. A CP can have single TI or multiple TI. Hence why we choose the CP as part of the name.


298        format specified in Section 2 of [draft-ietf-pim-sr-p2mp-policy].
299        The structure of this sub-TLV is illustrated in the figure below.

<major> Please add text to clarify here that the CP identifiers are not required since the Instance-ID is unique within the SR P2MP Policy context (with a reference to section 2.3 of the p2mp policy draft).

HB> There is no CP identifier in any text or document. Adding this would create more confusion and have the reader searching for what is a CP identifier.

315        *  Address Family: (2 octets) IPv4/IPv6 ADDRESS FAMILY NUMBERS as
316           specified in [IANA-AF] , indicating the address family of the
317           Root.

<major> Are all AFIs allowed? I believe it has to allow only IPv4 or IPv6?

HB> as per text "Address Family: (2 octets) IPv4/IPv6 ADDRESS FAMILY NUMBERS as specified in"

319        *  Address Length: (1 octet) specifying the length of the Root
320           Address in octets (4 octets for IPv4, 16 octets for IPv6).

<major> reserved is missing; also I believe it MBZ
HB> good point thanks!

382     5.  IANA Consideration

384        IANA has assigned a TEMPORARY code point for the "P2MP Policy MPLS
385        Candidate Path" Sub-TLV Name.  This Sub-TLV is assigned from TLV type
386        1 (Target FEC Stack) from the "Multi-Protocol Label Switching (MPLS)
387        Label Switched Paths (LSPs) Ping Parameters" registry group.  The
388        Sub-TLVs for TLV type 1 are listen under "Sub-TLVs for TLV Types 1,

<minor> s/listen/listed

HB> thanks! Updated.

397     6.  Security Considerations

399        Overall, the security needs for P2MP policy ping are the same as
400        [RFC8029].  The P2MP policy ping is susceptible to the same three
401        attack vectors as explained in RFC8029 section 5.  The same
402        procedures and recommendations explained in [RFC8029] section 5
403        should be taken and implemented to mitigate these attack vectors for
404        P2MP policy Ping as well.

<major> Should this not include reference to the security considerations of the SR P2MP policy draft as well?

HB> ok thanks added a text

<EoRv18>



_______________________________________________
pim mailing list -- [email protected]
To unsubscribe send an email to [email protected]