Last Call Review of draft-ietf-mpls-multipath-use-03
I have reviewed this document as part of the operations directorate's
ongoing effort to review all IETF documents being processed by the
IESG. These comments were written primarily for the benefit of the
operations area directors. Document editors and WG chairs should treat
these comments just like any other last call comments.
This draft is on the right track but has open issues, described in the review.
I apologize for this review showing up a couple of days after the end of
IETF Last Call.
This draft discusses considerations for what are effectively MPLS-in-MPLS
tunnels for multipathing - an LSP carries another LSP, with the inner
(client) LSPs being distributed across multiple outer (server) LSPs that
use different paths. This draft is motivated by considerations specific
to MPLS-TP that complicate multipathing when one of the two MPLS instances
I consider the following to be open issues that need attention (each
letter is used to tag the issue in the text that follows - [B] occurs
multiple times, as there are multiple aspect to that issue):
[A] Summary text in Introduction appears to be wrong.
[B] Section 3.2 needs a summary.
[C] Entropy label "sandwich" seems peculiar and at least needs an explanation.
[D] Discussion of multipath impacts on MPLS OAM should be added to Section 4.
--- Comments on draft text
[A] Section 1 (Introduction) 2nd paragraph:
RFC 5654 requirement 33 requires the capability to carry a client
MPLS-TP or MPLS layer over a server MPLS-TP or MPLS layer [RFC5654].
This is possible in all cases with one exception. When an MPLS LSP
exceeds the capacity of any single component link it MAY be carried
by a network using multipath techniques, but MAY NOT be carried by a
single MPLS-TP LSP due to the inherent MPLS-TP capacity limitation
imposed by MPLS-TP Operations, Administration, and Maintenance (OAM)
fate sharing constraints and MPLS-TP LM OAM packet ordering
constraints (see Section 3.1).
Section 3.1 concerns carrying MPLS-TP over MPLS, whereas the above
exception appears to concern the reverse, and Section 4 on carrying MPLS
over MPLS-TP allows for MPLS LSPs that exceed component link capacity.
Was the exception intended to be for an MPLS-TP LSP that exceeds the
capacity of any single component link?
Also, "MAY NOT" is not an RFC 2119 term (for good reason - it's equivalent
to "MAY OR MAY NOT"). I think "cannot" was intended her, but "MUST NOT"
is another possibility.
[B] Section 3.2 contains a lot of useful details, but it could really use
a summary at the end on the options and recommendations for meeting the
four requirements (this summary could be a new Section 3.3). It's a bit
difficult to discern what's going on, as I had to read this section more
than once in order to discern the following:
[C] In Section 3.2, use of an MPLS entropy label to meet requirement MP#1
requires that it be "below the MPLS-TP label" whereas, to meet requirement
MP#2, the MPLS entropy label needs to be "just above the MPLS-TP LSP label".
This combination appears to suggest "sandwiching" the MPLS-TP label between
two entropy labels that effectively carry the same information (albeit,
applied and removed at different places in the network) - was that intended?
This is important, as there seems to be no reasonable alternative
to the entropy label for meeting requirement MP#2, and the draft's
alternative to the entropy label for meeting requirement MP#1 is "some form
of configuration" - the entropy label would seem to be preferred to that
based on my reading of this draft and the comment in RFC 5706 section 2.2
that "Anything that can be configured can be misconfigured."
[B] A summary of requirements for implementation and deployment at the end
of Section 3.2 would have made this much more obvious. That should be
provided in addition to addressing the above question about duplicate use
of the MPLS entropy label.
--- Relevant Q&A based on RFC 5706 Appendix A
I'm omitting management concerns, as both MPLS and MPLS-TP have extensive
management infrastructure, and this draft's notion of stacking LSPs so
that one instance of MPLS (or MPLS-TP) can be a server to (i.e., carry)
another is not novel to this draft.
A.1.1. Has deployment been discussed?
[B] Yes, although Section 3.2 is weak on deployment requirements (some of
which are implementation requirements) and could use a summary.
A.1.2. Has installation and initial setup been discussed?
No, although because this draft reuses existing functionality in different
configurations, this really reduces to the A.1.9 configuration question
below. In practice, quite a bit of operator knowledge and insight is
required to get initial setup right, e.g., as traffic engineering LSPs to
avoid congestion problems (needed to meet requirement MP#3) requires
significant knowledge of network structure and expected traffic flows.
This should be obvious to anyone who works w/MPLS, but it's probably
worth stating for completeness.
A.1.6. Have suggestions for verifying correct operation been discussed?
Yes and no. Section 3.2 discusses OAM, and contains this important
requirement for OAM traffic:
An Entropy Label must be used to insure that all of the
MPLS-TP payload and OAM traffic are carried on the same component,
except during very infrequent transitions due to load balancing.
[D] OTOH, there is no discussion of OAM traffic for the MPLS LSP in
Section 4; that should be added.
A.1.9. Is configuration discussed?
[B] Not much. Configuration is required to meet requirements MP#3 and MP#4,
and is one of the options for MP#1. A summary of what has to be configured
as part of the summary at the end of Section 3.2 would be a good idea.
A.2.* [skipped, see above]
There's quite a bit of operational material contained in this draft; a
separate section on operations and management considerations would not
There is a useful implementation status section which appears to imply
that MPLS-TP over MPLS is not currently deployable due to absence of
entropy label support, and because deployed MPLS equipment does not
generally support multipathing. Nonetheless, data center experience
indicates significant value and widespread use of multipathing based
on link aggregation and ECMP; corresponding benefits can be expected
for use of multipathing in the Internet beyond data centers.
--- Editorial comments/nits:
- Section 3.1, after RFC 5960 quotes:
[RFC5960] paragraph 3 requires that packets within a specific ordered
Insert "Section 3.1.1., before "paragraph 3" and likewise for "paragraph 6"
later in the same paragraph in the draft.
- Section 3.2, requirement MP#3:
MP#3 It SHOULD be possible to insure that MPLS-TP LSPs will not be
"insure" -> "ensure" or "assure". Surely this draft does not envision
operators taking out insurance policies on MPLS TP LSP behavior ;-).
- Section 3.2, last paragraph on p.8:
An MPLS-TP LSP may not traverse multipath links on the path where
MPLS-TP forwarding requirements cannot be met.
"may not" is meaningless - I believe "MUST NOT" was intended here.
- Section 4, middle of 3rd para:
Editorial suggestions for clarity:
For those LSP that are larger than component link
capacity, their capacity are not integer multiples of convenient
capacity increments such as 10Gb/s.
For those LSPs that are larger than a component link
capacity, the LSP capacities need not be (and often are not)
integer multiples of convenient capacity increments such as 10Gb/s.
In particular, "are not" seems wrong, as the "integer multiples"
case is possible, so I changed "are not" to "need not be (and often
are not)" in the suggested new text.
David L. Black, Distinguished Engineer
EMC Corporation, 176 South St., Hopkinton, MA 01748
+1 (508) 293-7953 FAX: +1 (508) 293-7786
david.black at emc.com Mobile: +1 (978) 394-7754