[OSPF] AD review of draft-ietf-ospf-segment-routing-extensions-16

Alia Atlas <akatlas@gmail.com> Wed, 31 May 2017 02:05 UTC

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From: Alia Atlas <akatlas@gmail.com>
Date: Tue, 30 May 2017 22:05:18 -0400
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To: OSPF List <ospf@ietf.org>, draft-ietf-ospf-segment-routing-extensions@ietf.org, Alvaro Retana <aretana@cisco.com>, "BRUNGARD, DEBORAH A (ATTSI)" <db3546@att.com>
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Subject: [OSPF] AD review of draft-ietf-ospf-segment-routing-extensions-16
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As is customary, I have done my AD review
of draft-ietf-ospf-segment-routing-extensions-16 once publication has been
requested.  First, I would like to thank the editors & many authors, Peter,
Stefano, Clarence, Hannes, Rob, Wim & Jeff, for the work that they have put
in so far and the remaining work that is greatly needed.

While there are a great many issues to be handled, they fall primarily into
three categories.  The first is simply not going through and tightening up
the details; for example, stating that the length of a TLV is variable
provides no meaning.  The second is that the technical documents from
SPRING that this draft depends on do not adequately describe the use of the
advertised information (SID/Label Binding TLV) or some of the concepts
(e.g. SR Mapping Server).  The third is a more common set of handling error
cases and adding clarity to the intended behavior.  I do not see issues
with the encodings but I do see fragility with the unstated assumptions and
behaviors.  The draft describes encodings, but very little of the handling,
behaviors, or meaning - and the references do not provide adequate detail.

I have spent all day (and evening) doing this review and I am quite
disappointed and concerned about the document.  I would strongly recommend
having sharing the next WGLC with the SPRING working group; perhaps more
eyes will help with the discrepancies.

I have not yet decided what to do about the "early" IANA allocation - which
has now existed for this draft for 3 years.  I do know that there are
but I am currently seeing the failure of this work to successfully complete
as an example of an issue with providing early allocations.


1) This draft has 7 authors.  The limit for authors & editors is 5, as is
clearly stated in RFC 7322 Sec 4.1.1 and has been the case for well over a
decade, unless there are extraordinary circumstances.  Is there a reason to
not simply list the active editor and move the others to contributors?  One
of the authors is already listed there.  I regret that failure to deal
earlier with this long-standing IETF policy will be delaying progressing
the draft.

2) This expired individual
draft(draft-minto-rsvp-lsp-egress-fast-protection-03) is listed as
Informative - but IS ACTUALLY NORMATIVE since it DEFINES the
"M-bit - When the bit is set, the binding represents a mirroring context as
defined in [I-D.minto-rsvp-lsp-egress-fast-protection]."  Unfortunately,
when I look there for the definition of a mirroring context, it doesn't

3) The following Informative references expired several years ago and -
being individual drafts - do not appear to convey the SPRING or TEAS WG
   a)  draft-filsfils-spring-segment-routing-ldp-interop-03 was replaced
with draft-ietf-spring-segment-routing-ldp-interop-07 and there are
considerable differences.
   b) It is unclear what happened
to draft-filsfils-spring-segment-routing-use-cases-01, but I do not see any
successor - or reason for this individual draft to explain the OSPFv2
extensions more than work from the SPRING WG.

4) Sec 3.3: Is it ok to advertise an SRLB TLV without advertising the
SR-Algorithm TLV?  What is the expected behavior and assumptions by the

5) Sec 3.4:  What happens if an SRMS Preference TLV is advertised without
an SR-Algorithm TLV in the same scope?  I see that it says "For the purpose
of the SRMS Preference Sub-TLV advertisement, AS scope flooding is
required." but also provides for area scope flooding.  Some words
clarifying the expected behavior would be useful.

6) Sec 5: "In such case, MPLS EXP bits of the Prefix-SID are not preserved
the final destination (the Prefix-SID being removed)."   I am quite
startled to see an assumption that MPLS Pipe mode is being forced as part
of specifying PHP mode!  This will also break any ECN or 3-color marking
that has affected the MPLS EXP bits.  I would like to see and understand a
clear justification for why short-pipe mode is being required instead of
Uniform (or up to implementation/configuration.).   Basically, this
sentence means that transport considerations are a necessary section -
which is completely inappropriate in an IGP draft.

7) Sec 6: This section defines the SID/Label Binding sub-TLV - which
appears to be a way to advertise an explicit path - and has a SID/Label by
which the path can be entered.   How and what state is set up by the
sending router to create the indicated segment is completely unclear.   I
have hunted
through draft-ietf-spring-segment-routing,
and draft-ietf-spring-segment-routing-ldp-interop, RFC7855,
and draft-ietf-isis-segment-routing-extensions.   As far as I can tell,
NONE of them clearly describe the details of where and why this advertising
is needed.  Obviously, this mechanism does allow the potential shortening
of the MPLS label stack at the cost of advertising multi-hop explicit path
segments across the entire area or AS.  There MUST be a normative
description of what the sending router will do when a packet is received
with the specified label.

8) Sec 4: "The Segment Routing Mapping Server, which is described in
[I-D.filsfils-spring-segment-routing-ldp-interop]"  Where precisely is an
SRMS and its behavior/role actually defined?
 draft-ietf-spring-segment-routing-ldp-interop-07 claims:"SR to LDP
interworking requires a SRMS as defined in
[I-D.ietf-isis-segment-routing-extensions]." but that wouldn't be
appropriate, of course, and it isn't there either!
 draft-ietf-spring-conflict-resolution-04 talks about SRMS, but doesn't
define it.   draft-ietf-spring-segment-routing-11 mentions in Sec 3.5.1
that "A Remote-Binding SID S advertised by the mapping server M" and refers
to the ldp-interop draft for further details - but obviously not about an

Minor Issues:

1) In Sec 3.1, it says: "The SR-Algorithm TLV is optional. It MUST only be
advertised once in the Router Information Opaque LSA.  If the SID/Label
Range TLV, as defined in Section 3.2, is advertised, then the SR-Algorithm
also be advertised."  Please provide a pointer in the text to the behavior
for a receiving router if one or both of these are violated?   For the
requirement to advertise the SR-Algorithm TLV, please clarify that this is
in the same RI LSA as the SID/Label Range TLV was advertised & with the
same scope.  What does it mean, in terms of the receiving router, to
determine that the sending router supports SR or not - given the
possibility of receiving other SR-related TLVS in an RI LSA without getting
an SR-Algorithm TLV?

2) Sec 3.1: The SR-Algorithm TLV simply defines "Length: Variable".  Given
that advertising Algorithm 0 is required, I'm fairly sure that the Length
has to be a minimum of 1 - and, to prevent overrun & weird issues, let's
have a reasonable maximum (for instance, 24) too.  It wouldn't hurt to
remind readers that the length is just that of the value field - though
experienced OSPF implementers will know that.

3) Sec 3.1 & Sec 3.2 & Sec 3.3: "For the purpose of SR-Algorithm TLV
advertisement, area scope flooding is required." and "For the purpose of
SID/Label Range TLV advertisement, area scope flooding is required."  and
"For the purpose of SR Local Block Sub-TLV TLV advertisement, area scope
flooding is required." Please capitalize REQUIRED as per RFC 2119.
Otherwise, please explain behavior when area scope isn't used.

4) Sec 3.2:  The SID/Label Range TLV doesn't indicate that include a
SID/Label sub-TLV is required - but I don't understand how it could be
interpreted otherwise; nor does it indicate what to do if there are
multiple SID/Label sub-TLVs included in a single SID/Label Range TLV. Again
"Length" is just defined as variable.  In this case, it clearly can't be
less than 11 (probably 12, assuming padding to the 32-bit boundary).   It
would be useful to have an upper-bound on length, but at least here I can
see the argument that meaningful flexibility is provided for.

5) SID index is used without introduction in Sec 3.2.  It isn't defined in
the terminology of draft-ietf-spring-segment-routing-11 and the other uses
of it in this document aren't enough to clearly define it.  Please add at
least a description of its meaning before use - in a terminology section,
if necessary.

6) Sec 3.2: "The originating router advertises the following ranges:
         Range 1: [100, 199]
         Range 2: [1000, 1099]
         Range 3: [500, 599]"
Please turn this into the information actually advertised - i.e.
   Range 1: Range Size: 100   SID/Label sub-TLV: 100  => meaning [100, 199]

7) 3.2. SID/Label Range TLV:  Please specify that the sender MUST NOT
advertise overlapping ranges & how to handle the case when it does.  This
is required by draft-ietf-spring-conflict-resolution.

8) Sec 3.3  SR Local Block (SRLB) Sub-TLV: The document doesn't specify
that the SR Local Block TLV MUST include a SID/Label sub-TLV nor indicate
what to do if multiple are included.  The Length, again, isn't specified at
all and clearly has at least a minimum.   I don't see a reference to an SR
Local Block or the need to advertise it
in draft-ietf-spring-segment-routing-11; perhaps I missed where the
requirement and usage are defined?

9) Sec 3.3: "Each time a SID from the SRLB is allocated, it SHOULD also be
   reported to all components..."  Presumably, this is subjected to the
normal OSPF dampening - it'd be nice to note that somewhere - since rapid
sequential allocation may not provide the reporting speed anticipated.

10) Sec 4: "AF: Address family for the prefix. Currently, the only supported
      value is 0 for IPv4 unicast.  The inclusion of address family in
      this TLV allows for future extension."  Could you please clarify if
this is to reuse the same TLV for OSPFv3 so IPv6 can be supported, are you
thinking of extending OSPFv2 for IPv6 prefixes for some cases or something
else? I think the current phrasing is likely to raise questions.
Similarly, please define "Prefix length: Length of the prefix" clearly.  I
really don't understand what the benefit of having a TLV that pretends to
support multiple AFs but can't is versus the clarity of specifying the
prefix lengths.

11) Sec 4:  Again "Length: Variable" - It should have a minimum and
preferable describe a function for how it is computed.  A maximum is
probably unlikely  with sub-TLVs.

12) Sec 4: OSPF Extended Prefix Range TLV:  Does this TLV has any meaning
or action associated with it without including sub-TLVs?  Are there
mandatory sub-TLVs?  What is a receiving router to do with it?

13) Sec 5: "If multiple Prefix-SIDs are advertised for the same prefix, the
  receiving router MUST use the first encoded SID and MAY use
  subsequent SIDs."  What does this even mean?  A receiving router when
making the decision to use a subsequent SID is making a decision to not use
the first encoded SID; it's not like the router is going to stick both
SID/Labels onto the stack.   Please describe this in meaningful normative

14) Sec 5:" When calculating the outgoing label for the prefix, the router
   take into account the E and P flags advertised by the next-hop router
   if that router advertised the SID for the prefix.  This MUST be done
   regardless of whether the next-hop router contributes to the best
   path to the prefix."  First, I assume this is "NP flag" because there is
no P flag.
   Second - please clarify to "take into account, as described below, the E
and NP flags...".  Third, the M flag must also be taken into account -
given the text later in the section.

15) Sec 5: "When a Prefix-SID is advertised in an Extended Prefix Range
TLV, then the value advertised in the Prefix SID Sub-TLV is interpreted as a
   starting SID value."   This appears to contradict "SID/Index/Label:
According to the V and L flags, it contains either:

         A 32-bit index defining the offset in the SID/Label space
         advertised by this router.

         A 24-bit label where the 20 rightmost bits are used for
         encoding the label value."
  I assume that what is meant by the first quote is "...is interpreted, if
the V flag is clear, as a starting SID value, and if the V flag is set, as
a starting Label value."  Otherwise, it looks like the Prefix-SID sub-TLV
couldn't be included in the Extended Prefix Range TLV if a label value
would be used.

It would be helpful for Example 2 to show the label case.

16) Sec 6.1: "aggregate IGP or TE path cost."  Given that this is an OSPF
draft, it'd be helpful to indicate whether there are challenges with
non-comparable OSPF metrics (I'm thinking about AS-external type 2 costs)
or if the path will never include such costs.

17) Sec 6.2: "a domain and hence need to be disambiguated using a
domain-unique Router-ID."  Given that the Prefix-SIDs and sub-TLVs can be
distributed between areas and even redistributed between protocols, please
clearly define what is meant by a "domain" or point to the appropriate

18) Sec 4, 5, 6:  Is it possible to have an OSPF Extended Prefix Range TLV
that includes both a Prefix SID Sub-TLV and a SID/Label Binding Sub-TLV?
What does that mean?

What does it mean if there are multiple prefixes described in the OSPF
Extended Prefix Range TLV that includes a SID/Label Binding Sub-TLV?  Does
the SID/Label sub-sub-TLV indicate a single SID Index or Label that is used
for the single path to all those prefixes?  Is it the start of a list of
SID Indices or Labels?
I see that the SID/Label Binding sub-TLV can be in both the OSPF Extended
Prefx Range TLV as well as the OSPF Extended Prefix TLV - but there is no
text on differences in interpretation.

19) Sec 7.1 & 7.2: Another  couple "Length: Variable."  Please actually
specify the value. I think that, given the padding to 32-bit alignment,
there is a single correct value.

20) Sec 7.1 and 7.2: Given that the Flag bits have exactly the same meaning
- it'd be clearer to have them defined once.

21) Sec 8.1: "An SR Mapping Server MUST use the OSPF Extended Prefix Range
TLV when advertising SIDs for prefixes.  Prefixes of different route-types
can be combined in a single OSPF Extended Prefix Range TLV advertised by an
SR Mapping Server."    So - I can't find a normative definition of an SRMS
to determine why it is always necessary to use an OSPF Extended Prefix
Range TLV instead of an OSPF Extended Prefix TLV.   I don't see how
advertising prefixes from different route-types can work unless the
prefixes are adjacent, which seems likely to be uncommon.  Perhaps what is
meant is "Because the OSPF Extended Prefix Range TLV doesn't include a
Route-Type field, as in the OSPF Extended Prefix TLV, it is possible to
include adjacent prefixes from different Route-Types in the OSPF Extended
Prefix Range TLV."

22) Sec 8.1: "If multiple routers advertise a Prefix-SID for the same
prefix, then
the Prefix-SID MUST be the same.  This is required in order to allow
traffic load-balancing when multiple equal cost paths to the destination
exist in the OSPFv2 routing domain."  How is this enforced?  What are the
consequences of it not being conformed to?  This is NOT a protocol
implementation requirement.  This should really be called out in a
Manageability Considerations with warnings.

23) Sec 8.2:"If no Prefix-SID was advertised for the prefix in the source
      by the router that contributes to the best path to the prefix, the
      originating ABR will use the Prefix-SID advertised by any other
      router when propagating the Prefix-SID for the prefix to other
      areas."  I believe that this depends on the assumption that if a
Prefix-SID is advertised by any router, the Prefix-SID will be the same.
Please be explicit in this assumption, since the requirement on the network
operator should be clear as well as the consequences of not conforming.

24) Sec 10:  The Implementation Status section should indicate that it is
to be removed before publication as an RFC.   Also, the complete
implementation part seems a bit dated - given the draft's technical changes
in the last 2 years.


1) Sec 2.1: s/"SID/Label TLV"/"SID/Label sub-TLV"

2) Sec 3.2:"Initially, the only supported Sub-TLV is the SID/Label TLV as
   in Section 2.1.  The SID/Label advertised in the SID/Label TLV
   represents the first SID/Label in the advertised range."
   replace SID/Label TLV with SID/Label sub-TLV.

3) Sec 3.3 & Sec 3.4: " The SR Local Block (SRLB) Sub-TLV is a top-level
TLV of the Router Information Opaque LSA (defined in [RFC7770])."   Please
correct the descriptions (many) to SR Local Block (SRLB) Sub-TLV to SR
Local Block SRLB TLV.   The same issue exists for "SRMS Preference Sub-TLV".