Re: [Tsv-art] Last Call: <draft-ietf-6man-spring-srv6-oam-09.txt> (Operations, Administration, and Maintenance (OAM) in Segment Routing Networks with IPv6 Data plane (SRv6)) to Proposed Standard

["Zafar Ali (zali)" <[e-mail]>]

Hi Magnus,

Many thanks for your comments and detailed review.

We just posted version 10 to address your comments.
https://datatracker.ietf.org/doc/html/draft-ietf-6man-spring-srv6-oam-10

Please see [ZA] in-line on how your comments are addressed.

Thanks

Regards … Zafar

From: Magnus Westerlund <[e-mail]>
Date: Tuesday, March 16, 2021 at 6:42 AM
To: "[e-mail]" <[e-mail]>
Cc: "[e-mail]" <[e-mail]>, "[e-mail]" <[e-mail]>, "[e-mail]" <[e-mail]>, "[e-mail]" <[e-mail]>, "[e-mail]" <[e-mail]>
Subject: RE: Last Call: <draft-ietf-6man-spring-srv6-oam-09.txt> (Operations, Administration, and Maintenance (OAM) in Segment Routing Networks with IPv6 Data plane (SRv6)) to Proposed Standard
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Resent-Date: Tue, 16 Mar 2021 03:42:05 -0700

This document has been reviewed as part of the transport area review team's
ongoing effort to review key IETF documents. These comments were written
primarily for the transport area directors, but are copied to the document's
authors and WG to allow them to address any issues raised and also to the
IETF
discussion list for information.

When done at the time of IETF Last Call, the authors should consider this
review as part of the last-call comments they receive. Please always CC
[e-mail]<mailto:[e-mail]> if you reply to or forward this review.


A) Section 2.1:

I first missed that "SRH.Flags" is intended as reference to the flags field
in the Segment Router Header. As this notation is prevalent through out the
document, could you be more explicit about this notation and also maybe be
clear on what a particular label represents.

[ZA] Updated text in the Section 2.1 to make it clear that the draft defines a flag in the Flags field of SRH [RFC8754].

B) Section 2.1.1:

  When N receives a packet whose IPv6 DA is S and S is a local SID, the
   line S01 of the pseudo-code associated with the SID S, as defined in
   section 4.3.1.1 of [RFC8754], is modified as follows for the O-flag
   processing.
S01.1. IF SRH.Flags.O-flag is set and local configuration permits
             O-flag processing THEN
                a. Make a copy of the packet.
                b. Send the copied packet, along with a timestamp
                to the OAM process for telemetry data collection
                and export.      ;; Ref1

I would note that this  results in the following psudo code as it states
that it modifies line S01.
Did you intended an insert so that S01.1 would be between S01 and SO2?

[ZA] Yes, the S01.1 is inserted after between S01 and S02. Specific text stating the same is added to the Section 2.1.1.

This
Psudeo code just lost its scoping to SRH. I would also note that it doesn't
match the style of what is in RFC 8754. For example a {  } the steps a and b
should be present. Also with the modification in place you also have a
spurious } at the end.

[ZA] Added the RFC8754 style bracket { }. The added text has its own block of brackets { }. With the comment that the line S01.1 is inserted after between S01 and S02, the modified pseudo-code is now well contained in its own block, as follows:

S01. When an SRH is processed {
S01.1.   If O-flag is set and local configuration permits O-flag processing {
            a. Make a copy of the packet.
            b. Send the copied packet, along with a timestamp
               to the OAM process for telemetry data collection
              and export.      ;; Ref1
         } /* Matches S01.1 */
S02.     If Segments Left is equal to zero { … }
…
S26. }  /* Matches S01 */

S01.1. IF SRH.Flags.O-flag is set and local configuration permits
             O-flag processing THEN
                a. Make a copy of the packet.
                b. Send the copied packet, along with a timestamp
                to the OAM process for telemetry data collection
                and export.      ;; Ref1
   S02.   If Segments Left is equal to zero {
   S03.     Proceed to process the next header in the packet,
            whose type is identified by the Next Header field in
            the routing header.
   S04.   }
   S05.   Else {
   S06.     If local configuration requires TLV processing {
   S07.       Perform TLV processing (see TLV Processing)
   S08.     }
   S09.     max_last_entry  =  ( Hdr Ext Len /  2 ) - 1
   S10.     If  ((Last Entry > max_last_entry) or
   S11.          (Segments Left is greater than (Last Entry+1)) {
   S12.       Send an ICMP Parameter Problem, Code 0, message to
              the Source Address, pointing to the Segments Left
              field, and discard the packet.
   S13.     }
   S14.     Else {
   S15.       Decrement Segments Left by 1.
   S16.       Copy Segment List[Segments Left] from the SRH to the
              destination address of the IPv6 header.
   S17.       If the IPv6 Hop Limit is less than or equal to 1 {
   S18.         Send an ICMP Time Exceeded -- Hop Limit Exceeded in
                Transit message to the Source Address and discard
                the packet.
   S19.       }
   S20.       Else {
   S21.         Decrement the Hop Limit by 1
   S22.         Resubmit the packet to the IPv6 module for transmission
                to the new destination.
   S23.       }
   S24.     }
   S25.   }
   S26. }

I would also note that if you wanted more generality you could replace a and
b with

Execute the configured OAM process. E.g. if using IPFix that would be .

C) Section 2.1.1

   The processing node SHOULD rate-limit the number of packets punted to
   the OAM process to avoid hitting any performance impact.

So this is protection against misconfiguration or an actual security breach
isn't it. Because no entity outside of the domain should be able to set this
on packets.

[ZA] Indeed this is the case. We have updated the security section of the draft addressing your comment.

So should this limit be configured by the management system or
baked into the implementation.

[ZA] The rate limit should be configured by the management system. Text has been added to mandate the configurable rate limiting.

And in the later case, isn't the nodes
capability to handle marked packets dependent on the total load across all
packets processed at least in one processing pipe?

[ZA] An implementation may implement rate limit for O-flag independent of any other rate limits applied at the box for any other purposes. The packets are throttled before they reached the telemetry subsystem (or OAM process). Section 2.1.1 text is updated to indicate the rate limit also needed to protect the telemetry subsystem.

And to me it appears that
the limit will be dependent on what OAM Process that has been configured to
be executed. Thus, how can this be robustly implemented so that it doesn't
interfere with the telemetry? Because if there are a mismatch between the O
flag marking entity and the rate limiting, then the telemetry usefulness
could be compromised. Can you clarify the thoughts here?

[ZA] As these are sampled packets anyway (like in the case of IPFIX), the controller is able to work with the subset of the packets reaching it (when the rate limit is effective).

D) Section 3.1 and 3.2: I have the impression that quite a lot hides behind
the "The echo reply message is IP routed." or corresponding. First of all
there need to exist a return route for the source of the PING or Trace
route.
Secondly, as this is routed without SRV the packet can take a
different return route, and in case any firewall or other filtering is in
place the reply may not make it. Is this correctly interpret here? If
correctly understood I am uncertain if any changes are needed.

[ZA] The underlying assumption for ping/ tracing in IP network is that responder has IP reachability back to the initiator. The drafts is relying on the IPv6 ping and traceroute capability. Also, [RFC8754] defines the notion of an SR domain and use of SRH within the SR domain. [RFC8754] security section provides security mechanisms to secure the SR domain. The use of ping/ traceroute defined in this document is restricted to an SR domain. The firewall or other filtering are typically performed outside the trusted SR domain.

Cheers

Magnus