Re: [mpls] Benjamin Kaduk's Discuss on draft-ietf-mpls-lsp-ping-lag-multipath-06: (with DISCUSS and COMMENT)

Benjamin Kaduk <kaduk@mit.edu> Thu, 14 March 2019 03:47 UTC

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Date: Wed, 13 Mar 2019 22:46:52 -0500
From: Benjamin Kaduk <kaduk@mit.edu>
To: Mach Chen <mach.chen@huawei.com>
Cc: Benjamin Kaduk via Datatracker <noreply@ietf.org>, The IESG <iesg@ietf.org>, "mpls@ietf.org" <mpls@ietf.org>, "draft-ietf-mpls-lsp-ping-lag-multipath@ietf.org" <draft-ietf-mpls-lsp-ping-lag-multipath@ietf.org>, "mpls-chairs@ietf.org" <mpls-chairs@ietf.org>, "loa@pi.nu" <loa@pi.nu>
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Subject: Re: [mpls] Benjamin Kaduk's Discuss on draft-ietf-mpls-lsp-ping-lag-multipath-06: (with DISCUSS and COMMENT)
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On Wed, Mar 13, 2019 at 08:33:59AM +0000, Mach Chen wrote:
> Hi Benjamin,
> 
> Thanks for your thorough review and detailed comments!
> 
> Please see my replies inline...
> 
> 
> > -----Original Message-----
> > From: Benjamin Kaduk via Datatracker [mailto:noreply@ietf.org]
> > Sent: Wednesday, March 13, 2019 4:31 AM
> > To: The IESG <iesg@ietf.org>;
> > Cc: draft-ietf-mpls-lsp-ping-lag-multipath@ietf.org; mpls-chairs@ietf.org;
> > loa@pi.nu; mpls@ietf.org
> > Subject: Benjamin Kaduk's Discuss on draft-ietf-mpls-lsp-ping-lag-multipath-
> > 06: (with DISCUSS and COMMENT)
> > 
> > Benjamin Kaduk has entered the following ballot position for
> > draft-ietf-mpls-lsp-ping-lag-multipath-06: 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/iesg/statement/discuss-criteria.html
> > for more information about IESG DISCUSS and COMMENT positions.
> > 
> > 
> > The document, along with other ballot positions, can be found here:
> > https://datatracker.ietf.org/doc/draft-ietf-mpls-lsp-ping-lag-multipath/
> > 
> > 
> > 
> > ----------------------------------------------------------------------
> > DISCUSS:
> > ----------------------------------------------------------------------
> > 
> > Thanks for this document; it's clearly going to provide value, and it gives a
> > pretty well-readable description of how things are expected to work.  That
> 
> Thanks.
> 
> > said, there's a number of details that need to be worked out before
> > publication...
> 
> OK.
> 
> > 
> > This document has six listed authors; per RFC 7322, documents listing more
> > than five authors are unusual, and six is greater than five.  Let's talk about the
> > author count.
> 
> Please refer to Loa's answer.
> 
> > 
> > "optional TLV" seems to be used in MPLS contexts as a technical term for
> > "comprehension-optional", not in the "optional to send" sense; it's the
> > counterpart of "mandatory TLV", and this terminology is even documented in
> > the MPLS TLVs registry.  It's my understanding that the LSP Capabilities TLV
> > and the Detailed Interface and Label Stack TLV are intended to be
> > comprehension-required (i.e., "mandatory TLVs"), and thus that we must
> > not use the phrase "optional TLV" in their description.
> > This would be made clear if we listed which range of values we intended to
> > allocate a TLV type from, in the IANA considerations, but that remains
> > unspecified at this time.
> 
> Good catch on the LSP Capabilities TLV, it should be specified as assigned from the range 0-16383 as a response is required. The TLV is optional, as noted this range also covers optional TLVs for which a response is required. Per RFC8029, the Downstream Detailed Mapping TLV which is used in this RFC is also optional.

That works for me, though I'm also watching Alvaro's ballot thread.

> > 
> > In a similar vein (the "comprehension-required" behavior), there are a few
> > places (Section 3 (twice), and Section 6; also Section 3 for the LAG Description
> > Indicator flag; see COMMENT) where we state new normative language
> > ("MUST") that is unenforceable, since it would need to apply to MPLS
> > implementations that do not implement this specification.
> > Fortunately, the comprehension-required TLV ranges provide this
> > functionality for us without the need to use normative language.
> 
> Addressed with the COMMENT below.
> 
> > 
> > Section 4.2 has some conflicting "MUST"s about the ordering/presence of
> > sub-TLVs in the DDMAP TLV (see COMMENT, and also the "MANDATORY"
> > langauge in Figure 2).
> 
> Addressed with the COMMENT below.
> 
> > 
> > If I'm reading Section 5.1.2 correctly, the described procedure is only
> > intended to apply when the "G" flag is present (as well as the "I" flag, the
> > requirement for which is explicitly stated), but this is not explicitly stated.  In
> > particular, the text as written says it applies to all responders that
> > "understand the LAG Description Indicator flag"
> > with no mention of runtime check for the presence of that flag.
> 
> How about add a sentence at the beginning of the section:
> 
> "When received an echo request with the "LAG Description Indicator flag" set, a responder LSR that ..."

Just what I was looking for, thanks.

> > 
> > I also worry that Sections 8, 9, and 10 are insufficiently clear about the
> > encoding of the interface index value -- is it an integer in NBO, an opaque
> > bitstring, or something else?

Do I need to clarify this question more?

> 
> 
> > 
> > 
> > ----------------------------------------------------------------------
> > COMMENT:
> > ----------------------------------------------------------------------
> > 
> > Am I reading this correctly that the "LSR Capability TLV" created here is
> > basically only applicable to properties of MPLS echo request/reply exchanges?
> > If so, then perhaps the moniker "LSR Capability" is overly generic.
> 
> It's currently defined for LSP Ping, but may be used in a more generic use. I incline to keep it as is. 

Ok.

> > 
> > Section 1.2
> > 
> >    o  Label switching over some member links of the LAG is successful,
> >       but will be failed over other member links of the LAG.
> > 
> > nit: there's some verb tense inconsistency here; maybe "but fails over other
> > member links" would help.
> 
> OK.
> 
> > 
> > Section 2
> > 
> >    This document defines an optional TLV to discover the capabilities of
> >    a responder LSR and extensions for use with the MPLS LSP Ping and
> > 
> > nit: is it only the *responder* LSR we care about, or are there potentially
> > intermediaries in play?
> 
> Only the responder LSR we care about.
> 
> > 
> >    The solution consists of the MPLS echo request containing a DDMAP TLV
> >    and the optional LSR capability TLV to indicate that separate load
> > 
> > nit: Is this "optional capability TLV" the same "optional TLV" that we are
> > defining in this document?  If so, calling it "the new optional LSR capability"
> > would aid clarity.  (Also, we seem to use both the "capability" and
> > "Capability" capitzliations for describing this TLV.)
> 
> OK, fixed.
>  
> > 
> > Section 3
> > 
> >    Capability TLV in the MPLS echo request message.  When the responder
> >    LSR receives an MPLS echo request message with the LSR Capability TLV
> >    included, if it knows the LSR Capability TLV, then it MUST include
> >    the LSR Capability TLV in the MPLS echo reply message with the LSR
> >    Capability TLV describing features and extensions supported by the
> >    local LSR.  Otherwise, an MPLS echo reply MUST be sent back to the
> >    initiator LSR with the return code set to "One or more of the TLVs
> >    was not understood".  [...]
> > 
> > This last MUST seems like something that this document cannot mandate (as
> > it attempts to apply to non-implementations of this document); it could only
> > work if it was an existing requirement from the core MPLS spec, in which
> > case lower-case "must" is appropriate (possibly with section reference).
> 
> OK, how about below:
> 
> "Otherwise, an MPLS echo reply must be sent back to the initiator LSR with the return code set to "One or more of the TLVs was not understood", according to the rules as defined Section 3 of RFC8029."

Works for me.

> > 
> >    o  If the responder LSR does not understand the "LAG Description
> >       Indicator flag":
> > 
> >       *  Clear both the "Downstream LAG Info Accommodation flag" and the
> >          "Upstream LAG Info Accommodation flag".
> > 
> > Similarly, if an LSR does not understand a flag, it cannot give special
> > treatment to packets containing that flag.  (Why an LSR would not
> > understand a flag defined in the same document that defines the Capabilities
> > TLV is beyond me, but you seem to want to allow for that
> > case.)
> 
> OK, I think it's OK to remove them.  
> 
> > 
> >    If the responder does not know the LSR Capability TLV, an MPLS echo
> >    reply with the return code set to "One or more of the TLVs was not
> >    understood" MUST be sent back to the initiator LSR.
> > 
> > This duplicates the content I quoted at the top of this section's comments.
> 
> OK, remove this duplicate text.
> 
> > 
> > Section 4.1
> > 
> >    Once the initiator LSR knows the capabilities that a responder
> >    supports, then it sends an MPLS echo request carrying a DDMAP with
> >    the "LAG Description Indicator flag" (G) set to the responder LSR.
> > 
> > nit: I think the key point is not that the initiator knows the capabilities of the
> > peer, but rather that the initiator knows that the peer supports this specific
> > mechanism.
> 
> Yes.
> 
> How about:
> "Once the initiator LSR knows that a responder can support this meachanism, then it sends an MPLS echo request carrying a DDMAP TLV with  the "LAG Description Indicator flag" (G) set to the responder LSR."

Sounds good.

> 
> > Also, is this "a DDMAP TLV"?
> 
> Yes.
> 
> > 
> > Section 4.2
> > 
> > Reading this makes it sound like the "LAG Description Indicator flag" is
> > completely orthogonal to regular DDMAP functionality, so that if I set the
> > LAG description indicator flag but do not otherwise request detailed
> > descriptions, only the LAG interfaces are returned.  However, the flag in
> > question has to be inside a DDMAP TLV (and we should really say so, perhaps
> > as "[flag] set *in the DDMAP TLV*"!), so it seems like in practice the LAG
> > information can only be obtained in conjunction with full detailed description
> > information.  (The specific suggestion here is to note clearly that the flag is se
> > in the DDMAP TLV.)
> 
> Fixed.
> 
> > 
> >       *  The responder LSR MUST include a DDMAP TLV when sending the
> >          MPLS echo reply.
> > 
> > I got confused on my first pass through this section; adding "There is a single
> > DDMAP TLV for the LAG interface, with member links described using sub-
> > TLVs" here would have kept me from veering onto the wrong path.
> > (I thought there was going to be one DDMAP TLV per member link, plus one
> > for the LAG aggregate, with lots of duplication.)
> 
> Sure, added.
> 
> > 
> >    Based on the procedures described above, every LAG member link will
> >    have a Local Interface Index Sub-TLV and a Multipath Data Sub-TLV
> >    entries in the DDMAP TLV.  The order of the Sub-TLVs in the DDMAP TLV
> >    for a LAG member link MUST be Local Interface Index Sub-TLV
> >    immediately followed by Multipath Data Sub-TLV.  A LAG member link
> >    MAY also have a corresponding Remote Interface Index Sub-TLV.  When a
> >    Local Interface Index Sub-TLV, a Remote Interface Index-Sub-TLV and a
> >    Multipath Data Sub-TLV are placed in the DDMAP TLV to describe a LAG
> >    member link, they MUST be placed in the order of Local Interface
> >    Index Sub-TLV, Remote Interface Index-Sub-TLV and Multipath Data Sub-
> >    TLV.
> > 
> > This last MUST contradicts the previous MUST.  I suggest some more text to
> > clarify under which conditions each requirement applies.
> 
> OK, added.
> 
> > 
> > I'd also suggest not using the figure for conveying a (apparent?) mandatory
> > requirement, and instead adding some text at the end: "The block of local
> > interface index, (optional remote interface index) and multipath data sub-
> > TLVs for each member link MUST appear adjacent to each other in order of
> > increasing local interface index."
> 
> OK, added.
> 
> > 
> > It's confusing to label the multiplath data sub-TLV as "MANDATORY" when
> > the body text says "MUST add an [sic] Multipath data Sub-TLV [...] if the
> > received DDMAP TLV requested multipath information", i.e., it is not always
> > present, based on the contents of the echo request.
> 
> Since it's just an example here, will remove the MANDATORY and OPTIONAL. 

Ah, good point.

> >    When none of the received multipath information maps to a particular
> >    LAG member link, then the responder LSR MUST still place the Local
> >    Interface Index Sub-TLV and the Multipath Data Sub-TLV for that LAG
> >    member link in the DDMAP TLV, the value of Multipath Length field of
> >    the Multipath Data Sub-TLV is set to zero.
> > 
> > nit: the last comma is a comma splice.
> 
> OK.
> 
> > 
> > Section 5.1.2
> > 
> > It seems like this places a slightly stronger burden on the responder than
> > stock 8029 does, in that there is a MUST-level requirement to act on the 'I'
> > flag in combination with the 'G' flag, whereas for the 'I'
> > flag alone 8029 only has a SHOULD-level requirement to respond accordingly.
> > We may want to call this out as a change in behavior.
> 
> Will change to "SHOULD", then it can align with RFC8029.

I think that will be okay (it wasn't the change I was expecting, but I will
definitely defer to the authors and WG for this sort of thing!); I wasn't
sure if the divergence was a conscious decision or not.

> > 
> > Section 5.1.3
> > 
> >    Expectation is that there's a relationship between the interface
> >    index of the outgoing LAG member link at TTL=n and the interface
> >    index of the incoming LAG member link at TTL=n+1 for all discovered
> >    entropies.  In other words, set of entropies that load balances to
> > 
> > nit: "The expectation"
> 
> Fixed.
> 
> > nit: "discovered entropies" is a strange wording given how the entropy label
> > works; is this more like "all entropies examined"?
> 
> OK, change to "all entropies examined".
> 
> > 
> > 
> >    The initiator LSR sends two MPLS echo request messages to traverse
> >    the two LAG member links at TTL=n+1:
> > 
> > How does the initiator know (which entropy label values?) to get the echo
> > requests to traverse different member links?
> 
> The initiator cannot exactly know which entropy label values are workable. Presumably, the initiator can set different values according to the number of member links, then the responder can use entropy values for hash to traverse different member link. This is left to implementation. 

Maybe add "(Depending on the LAG traffic division algorithm, the messages
may or may not traverse different member algorithms.)" or similar?

> > 
> >    o  Error case:
> > 
> >       *  One MPLS echo request message reaches TTL=n+1 on an LAG member
> >          link 1.
> > 
> >       *  The other MPLS echo request message also reaches TTL=n+1 on an
> >          LAG member link 1.
> > 
> >       One or two MPLS echo request messages sent by the initiator LSR
> >       cannot reach the immediate downstream LSR, or the two MPLS echo
> >       request messages reach at the immediate downstream LSR from the
> >       same LAG member link.
> > 
> > The description paragraph doesn't seem to match up the scenario described
> > in the sub-bullets, which leaves me confused as to what
> > scenario(s) are intended to be considered as the "error case".
> 
> The sub-bullets are not complete, will add a third sub-bullet as below:
> 
> "One or both MPLS echo request messages cannot reach the immediate downstream LSR on whichever link."

That makes it sound like the sub-bullets are distinct options, and the top
level bullet is an "error cases" plural.  My reading with just the two
sub-bullets present in the -06 was that both sub-bullets had to happen in
order to be in the error case.

> > 
> > Section 6
> > 
> >    This document defines a new optional TLV which is referred to as the
> >    "LSR Capability TLV.  [...]
> > 
> > Is this optional to use or comprehension-optional?  (We elsewhere rely on
> > comprehension-mandatory behavior, so in the RFC 8029 terminology this
> > seems to be a "mandatory TLV" even if it is not mandatory to use.)
> 
> It's "mandatory TLV" according to RFC8029.

I think Alvaro is more on top of the subtleties here, so maybe we should
divert this topic to that ballot thread.

> > 
> >    included in the MPLS echo reply message.  Otherwise, if the responder
> >    does not know the LSR Capability TLV, an MPLS echo reply with the
> >    return code set to "One or more of the TLVs was not understood" MUST
> >    be sent back to the initiator LSR.
> > 
> > This duplicates a requirement stated elsewhere (that really ought to just be
> > using existing required behavior from 8029 anyway).
> 
> Remove it.
> 
> > 
> >    LSR Capability TLV Type is TBD1.  Length is 4.  The value field of
> > 
> > In a hypothetical future where we need to allocate a 33rd capability flag,
> > would we rather create a new "extended capabilities" TLV (burning another
> > 32 bits for type+length) or leave flexibility now for 'length'
> > to increase in multiples of 4 with receivers ignoring bits past what they know
> > about?
> 
> IMHO, 32 bit seems reasonable enough for me. I incline to keep the flag field as a fixed length.
> 
> If we want it be extensible, maybe we could add optional sub-TLVs to this TLV.

I agree that 32 bits seem likely to last for a very long time, so it's
hardly an important point.  I just wondered if anyone was going to enforce
that the length is exactly 4, which would prevent any further changes to
this TLV (other than allocating new bits).

> > 
> >       The "LSR Capability Flags" field is 4 octets in length, this
> >       document defines the following flags:
> > 
> >       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
> >      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
> >      |                  Must Be Zero (Reserved)                  |U|D|
> >      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
> > 
> > I'm wary of describing these as "Must Be Zero", since they're just unallocated
> > but not required to be zero other than by the current state of bit allocations.
> > (I guess this is conventional for MPLS, though?  So maybe it's not a problem.)
> > Just "Reserved" would be fine.
> 
> OK. Change to "Reserved".
> 
> > 
> >       This document defines two flags.  The remaining flags MUST be set
> >       to zero when sending and ignored on receipt.  Both the U and the D
> >       flag MUST be cleared in the MPLS echo request message when
> >       sending, and ignored on receipt.  Neither, either or both the U
> >       and the D flag MAY be set in the MPLS echo reply message.
> > 
> > Similarly, I think we want to say "unallocated flags" rather than "the
> > remaining flags".
> 
> Fixed.
> 
> > 
> > It also seems like the semantics being described here are "the TLV value is
> > ignored in the echo request and only has meaning in the response"
> > (i.e., this is a "tell me your capabilities" exchange, not a "here are mine; what
> > are yours?" exchange).  As written, that applies only to the U and D bits, and
> > is not a general property.  That may well be a fine choice if we think we might
> > want the two-way exchange for some future allocated bit, but if we do think
> > it's a general property of the mechanism, I'd suggest rewording the text.
> 
> It's designed as an unidirectional tool, make it a two-way exchange mechanism will require more considerations and substantial changes to draft.  So, I inline to keep it as-is. If we have such use case in the future, we update this mechanism then.

Sure, thanks for thinking about it.

> > 
> > Section 7
> > 
> > (Per IANA, bits 4 and 5 are already assigned but are not reflected in the
> > diagram.)
> 
> Fixed.
> 
> > 
> > Section 10
> > 
> >    The Detailed Interface and Label Stack TLV format is derived from the
> >    Interface and Label Stack TLV format (from [RFC8029]).  Two changes
> >    are introduced.  The first is that the label stack is converted into
> >    a sub-TLV.  The second is that a new sub-TLV is added to describe an
> >    interface index.  These fields of Detailed Interface and Label Stack
> >    TLV have the same use and meaning as in [RFC8029].  A summary of
> >    these fields is as below:
> > 
> > nit: Are "These fields" just "The other fields"?
> 
> 
> Fixed.
> 
> > 
> >          The Address Type indicates if the interface is numbered or
> >          unnumbered.  It also determines the length of the IP Address
> >          and Interface fields.  The resulting total length of the
> >          initial part of the TLV is listed as "K Octets".  The Address
> >          Type is set to one of the following values:
> > 
> > nit: is it better to list the currently allocated values or just refer to the IANA
> > registry?
> 
> Since here it also include the length field. I'd like to keep it both here and the IANA registry.
> 
> > 
> > I seee that this "index assigned to the interface" language for unnumbered
> > address types originates from RFC 8029, but both this document and 8029
> > leave me confused as to the encoding used for this index (for either/both
> > IPv4 or IPv6 address types).
> 
> It's normally an unsigned integer. 

We probably should specify network byte order for it, then.

> > 
> > [My comment about "Must Be Zero" and allocatable bits also applies here]
> 
> Fixed as above.
> 
> > 
> > Section 12
> > 
> > This document also adds a capability "negotiation" mechanism for MPLS echo
> > request/reply exchanges.  As MPLS does not offer integrity protection for its
> > messages, this negotiation is only as trustworthy as the network from which
> > messages are accepted as valid, and initiators have no recourse but to accept
> > faithfully the echo replies received.
> > 
> >    To prevent leakage of vital information to untrusted users, a
> >    responder LSR MUST only accept MPLS echo request messages from
> >    designated trusted sources via filtering source IP address field of
> >    received MPLS echo request messages.  [...]
> > 
> > I'd prefer to see a note added here about how "source IP address filtering
> > provides only a weak form of access control and is not, in general, a reliable
> > security mechanism.  Nonetheless, it is required here in the absence of any
> > more robust mechanism that might be used."
> 
> OK, added.
> 
> 
> > 
> > Section 13
> > 
> > Should we ask IANA to update the "Interface and Label Stack Address Types"
> > registry to also refer to this document, since we are sharing the registry
> > between the Interface and Label Stack TLV and the Detailed version?
> 
> Added.
> 
> > 
> > Section 13.2
> > 
> > The range 4-31743 spans two different allocation procedures (Standards
> > Action and Specification Required - Experimental RFC needed); I assume that
> > we want the 4-16383 range for Standards Action - mandatory TLVs.
> 
> Yes.
> 
> > 
> > Section 13.4
> > 
> > We don't say what range we want this to be allocated from -- I assume
> > 4-16383 since this is a negotiated feature and sending it outside the
> > negotiated scenario should be an error.
> 
> Yes, suggested range added.
> 
> > 
> > Section 13.4.1
> > 
> > (I note that some existing sub-TLV registries have "Experimental RFC
> > required" for the experimental ranges, but that is arguably more stringent
> > than necessary; Specification Required seems more appropriate for this case.)
> 
> OK, change to " Specification Required".
> 
> > 
> > Appendix A
> > 
> > I was kind of hoping for some more guidance on what an initiator could do in
> > these scenarios to try to get better visibility into the switch behavior (and,
> > hopefully, a workaround to get reliable echo responses that cover all LAG
> > member links).  AFAICT there's not a better option than "try a bunch of
> > entropy labels and see what responses you can get back" and that's the
> > same remedy in all the described topologies, but it would be nice to have this
> > stated explicitly for the reader.
> 
> A note will be added.

Thanks.

> > 
> > Section A.1
> > 
> >                     In the worst case, MPLS echo request messages with
> >    specific entropies to exercise every LAG members from R1 to S1 can
> >    all reach R2 via same LAG member.  [...]
> > 
> > I'm confused by the phrase "specific entropies to exercise every LAG
> > members [sic] from R1 to S1" -- is there some deterministic algorithm by
> > which a specific entropy label value will force a specific LAG member link?
> 
> No.
> 
> > My understanding was that the entropy was used as input to an opaque hash
> > function and that trial-and-error was the only way to explore the mapping
> > from entropy value to LAG member link taken.
> 
> Yes.
> 
> Thanks again for your detailed and valuable comments!

You're selcome!

-Benjamin