Telechat Review of draft-ietf-mpls-rfc4379bis-07
review-ietf-mpls-rfc4379bis-07-genart-telechat-davies-2016-10-21-00
| Request | Review of | draft-ietf-mpls-rfc4379bis |
|---|---|---|
| Requested revision | No specific revision (document currently at 09) | |
| Type | Telechat Review | |
| Team | General Area Review Team (Gen-ART) (genart) | |
| Deadline | 2016-10-25 | |
| Requested | 2016-10-13 | |
| Authors | Kireeti Kompella , George Swallow , Carlos Pignataro , Nagendra Kumar Nainar , Sam Aldrin , Mach Chen | |
| I-D last updated | 2016-10-21 | |
| Completed reviews |
Genart Last Call review of -07
by Elwyn B. Davies
(diff)
Genart Telechat review of -07 by Elwyn B. Davies (diff) Secdir Last Call review of -07 by Vincent Roca (diff) Opsdir Last Call review of -07 by Sheng Jiang (diff) Rtgdir Early review of -06 by Daniele Ceccarelli (diff) |
|
| Assignment | Reviewer | Elwyn B. Davies |
| State | Completed | |
| Request | Telechat review on draft-ietf-mpls-rfc4379bis by General Area Review Team (Gen-ART) Assigned | |
| Reviewed revision | 07 (document currently at 09) | |
| Result | Ready w/nits | |
| Completed | 2016-10-21 |
review-ietf-mpls-rfc4379bis-07-genart-telechat-davies-2016-10-21-00
Hi Carlos,
:-)
Thanks for addressing the comments. I have looked through -08 and
there are a couple of extra points that I noticed - the s3.4 issue
was effectively mentioned wrt s4.5 in my previous notes.
Generally things are in good shape but there are some items that
haven't been addressed or there is a quibble.
If there is another version over the weekend I'll do my very best to
check it before Monday.
Regards,
Elwyn
Extra Points:
===========
I Forgot to mention that there is lack of consistency in
capitalisation of the message names: Personally I would go with Echo
Request and Echo Reply throughout to make it clear that these are
message names.
s3,4, para 1:
If the
replying router is the destination (Label Edge Router) of the FEC,
then a Downstream Detailed Mapping TLV SHOULD NOT be included in the
MPLS echo reply. Otherwise, the replying router SHOULD include a
Downstream Detailed Mapping object for each interface over which this
FEC could be forwarded.
I suspect that the SHOULD NOT ought to be MUST NOT. Otherwise it
needs an explanation of the circumstances in which the DDMAP TLV
could be included. Similarly, the SHOULD needs to explain in what
circumstances you wouldn't include one or more DDMAP TLVs.
s6.2.3: The Unassigned row should have a blank reference.
On 28/10/2016 02:29, Carlos Pignataro (cpignata) wrote:
Deal Elwyn,
Many thanks for a great review!
I just finished addressing all your comments: the major
issue (easy to address, editorial fix, but with important
implications), the minors, and all the nits. Surprisingly, I
found a few small additional editorials, which I fixed as
well.
Rev -08 would address all outstanding issues, from this
review, Mirja, and a couple others.
Please see inline for a line-by-line set of responses.
On Oct 20, 2016, at 4:42 PM, Elwyn Davies
<
elwynd at dial.pipex.com
>
wrote:
I am
the assigned Gen-ART reviewer for this draft. The
General Area
Review Team (Gen-ART) reviews all IETF documents
being processed
by the IESG for the IETF Chair. Please treat these
comments just
like any other last call comments.
For more information, please see the FAQ at
<http://wiki.tools.ietf.org/area/gen/trac/wiki/GenArtfaq>
.
Document: draft-ietf-mpls-rfc4379bis-07.txt
Reviewer: Elwyn Davies
Review Date: 2016/10/21
IETF LC End Date: 2016/10/18
IESG Telechat date: (if known) -
Summary: Not ready. There is one major issue
(already notified to authors and agreed as an issue)
and a considerable number of minor and editorial
issues. I have worked through the various RFCs and
errata that are subsumed into the new version and
almost everything has been correctly translated
AFAICS. A couple of minor points are covered in the
comments.
Major issues:
============
s3.4: A number of items that are used in the
normative Downstream Detailed Mapping TLV were
originally defined in s3.3 (Downstream Mapping TLV
format) but have been shifted to Appendix A.2. This
appendix is marked as non-normative. Thus there are
now no normative definitions for the various TLVs
used in s3.4 that are defined in A.2. I fear that
these need to be returned to the normative part of
the specification.
This is an excellent catch. Thank you. The fix is
simple and purely editorial but the implication is
clear.
I finished addressing this, which you will see posted
as the new revision. I am super happy with the outcome.
Looks good to me!
[I
think it would be simplest and least error prone to
swap the text that was in s3.3 of RFC 4379 back out
of A.2 and put backward references to the new s3.4
into A.2 as necessary.]
Minor issues:
============
Sender/receiver terminology: The document can be
somewhat confusing to a naive reader. Sender and
receiver are used in multiple contexts. Where the
context appears to relate to LSP ping, both senders
and receivers are involved in sending LSP ping
packets. In general, sender and receiver appear to
imply sending and receiving of Echo Request messages
with their roles reversed with respect to Echo
Responses, with the receiver stimulated to send an
Echo Response by receiving an Echo Request. It
would help if this terminology and usage was
explicitly set out early in the document.
Additionally, some instances would be made more
comprehensible by making the function explicit in
the text e.g., in the operation of return codes.
Re-reading after fixing all the nits below, which include
some sender clarifications, looks good.
There is one place (s3.1, para 1) where I think it could be made
clearer. Adding a few words to that section will help overall as
well as just in that section.
s1.4/s3/s6.2.3: The R (Global) flag is defined in
RFC 6426. Unfortunately it isn't in the IANA
considerations there as was spotted in RFC Erratum
4012. Might be worth mentioning the erratum
(probably in s1.4?) Alternatively this document can
be made to provide the IANA specification for the R
flag and the erratum closed.
The WG decided to keep the definition of the R Flag
in RFC 6426 and not here — consequently, there’s little
that can really be done as the erratum (which really is
symbolic since the IANA registry is fixed) applies to
RFC 6426 and not to RFC 4379.
OK
s2.1/s6: An update to
http://www.iana.org/assignments/iana-ipv4-special-registry/iana-ipv4-special-registry.xhtml
is needed to replace RFC 4379 with RFC-to-be for
special exceptions to usage rules.
Done.
s3.5,
Clandestine Channel via Pad TLV: As specified the
value part of a Pad TLV can serve as a clandestine
channel since the value field contents are ignored.
Added the following to S5:
The value part of the Pad TLV contains a variable
number of octets.
With the exception of the first octet, these
contents, if any, are
ignored on receipt, and can therefore serve as
a clandestine channel.
Fine.
s3.5,
Usefulness of Pad TLV: Could you explain
circumstances in which a Pad TLV would be needed
please. I can't see any at present.
Sure — when you want to send pings of various sizes
for troubleshooting. I’ve used it in productions :-)
I think a short note in s3.5 about why it might be present would be
useful. e.g.,
The Pad TLV can be added to an Echo Request to create a
message of a specific length
in cases where pings of various sizes are needed for
troubleshooting.
Nits/editorial
comments:
======================
Thank for for these. Unless I make a specific
follow-up inline, the nit is fixed.
There are a couple of places where this doesn't seem to be the case,
especially in a number of places where there are SHOULDs in the text
but the reasons for/consequences of not following the SHOULD are not
spelt out. This can be important for interoperability.
General:
s/i.e. /i.e., / (two instances s3.2, last para;
s4.5.1, para 3)
s1, para 1: s/methods of reliable reply/methods of
providing reliable reply/
s1.4, bullet 4: Need to expand acronym PW on first
use.
s1.4, bullet 4: need to move expansion of FEC
acronym to here from s2.
s1.4, bullet 8: Acronyms DSMAP/DDMAP: When defining
Return Code 14 in s3.1, the text is 'See DDM
TLV...'. DDM is not expanded anywhere although it
is clearly the same as DDMAP. But has by now made
it into the IANA repository and is probably better
to use it for 'Downstream Detailed Mapping', so I
suggest:
OLD:
o Incorporate the updates from RFC 6424, by
deprecating the
Downstream Mapping TLV (DSMAP) and adding the
Downstream Detailed
Mapping TLV (DDMAP), updating two new return
codes, updating the
procedures, IANA section, Security
Considerations, and obsoleting
RFC 6424.
NEW:
o Incorporate the updates from RFC 6424, by
deprecating the
Downstream Mapping TLV (DSM) and adding the
Downstream Detailed
Mapping TLV (DDM), adding two new return
codes, updating the
procedures, IANA section, Security
Considerations, and obsoleting
RFC 6424.
END
Then s/DSMAP/DSM/g, s/DDMAP/DDM/g in the rest of the
document.
This is a good point, where DDM came from RFC 6424.
However, these fields are known as DSMAP and DDMAP.
Consequently, global replacing DDM -> DDMAP
That's OK by me. I wondered whether the DDM in the return code had
been too firmly ensconced to change.
s1.4: Ought to mention the addition of the
motivation (LSP stitching) for the additions in RFC
6424.
s2.1, paras 7 and 8: This contains "the newly
designated IPv4 link local addresses". Given that
RFC 3927 is now over 11 years old, the qualifier is
no longer appropriate, but it might be useful to
provide a ref. Thus:
OLD:
the newly designated IPv4 link local addresses
NEW:
the IPv4 link local addresses [RFC3927]
END
The text in para 8 is also no longer appropriate.
Suggest
OLD:
Furthermore, the IPv4 link local address range
has only recently been
allocated. Many deployed routers would forward a
packet with an
address from that range toward the default route.
NEW:
Older deployed routers may not correctly
implement link local addresses
and would forward a packet with an address from
that range toward the
default route.
END
Yes, many thanks. Updated with a slight change “Older
deployed routers may not (correctly) implement IPv4 link
local addresses …"
s2.1,
para 9: s/embedded in as/embedded in an/
s2.1, para 9: Useful to add a reference to RFC 4291.
This didn't happen. Not essential but useful.
s2.2, para 1: To be clearer about the distinction
between IPv4 and IPv6, suggest:
OLD:
This document requires the use of the Router
Alert Option (RAO) set
in IP header in order to have the transit node
process the MPLS OAM
payload.
NEW:
This document requires that the Router Alert
Option (RAO) is carried
in the IP header in order to have the transit
node process the MPLS OAM
payload. For IPv4 packets the RAO [RFC2111] MUST
be added to the IPv4
header; for IPv6 packets a hop-by-hop RAO
[RFC2711] must be chained to
the IPv6 header.
END
I wanted to keep that paragraph IP version agnostic,
since the specifics for IPv4 and IPv6 come in the next
two paragraphs.
Fair enough.
s3, para 1:
An MPLS echo request is a (possibly labeled) IPv4 or IPv6 UDP packet;
This format applies to both requests and responses
but the response case is not made explicit. Suggest:
OLD:
An MPLS echo request is a (possibly labeled) IPv4 or
IPv6 UDP packet;
NEW:
An MPLS LSP ping message, is a (possibly labeled)
IPv4 or IPv6 UDP packet;
END
That would leave out “traceroute” mode. I’ll add
“request/reply"
OK
s3,
main packet format and associated text: The Sender's
Handle is not the packet sender's handle but the
Echo Request Sender's Handle - it is copied in to
the corresponding Echo Reply. Suggest renaming the
Sender's Handle and Sequence Number to Echo Request
Sender's Handle and Echo Request Sequence Number.
This would affect para 5 of s4.3, para 2 of s4.5 and
para 1 of s4.6 also.
That would be too big of a departure for very
well-known fields.
I just realized that the draft doesn't say that the Sequence Number
is also (I assume) returned unchanged in the Echo Reply. To
emphasize this copying, it would probably be good to use the MUST
word about both request -> reply copies.
s3,
Timestamp format: RFC 5905 allows for 3 different
time formats - the 32 bit basic format is intended:
OLD:
The TimeStamp Sent is the time-of-day (according
to the sender's
clock) in NTP format [RFC5905]
NEW:
The TimeStamp Sent is the time-of-day (according
to the sender's
clock) in 32 bit NTP format [RFC5905]
END
64-bit.
I changed to “64-bit NTP Timestamp format”.
Yes.. oops.
s3,
Global flags: Technically, this doc only defines the
V flag: Also forcing the other bits to be zero
restricts addition of new flags>
OLD:
This document defines three flags, the R, T, and
V bits; the rest
MUST be set to zero when sending and ignored on
receipt.
NEW:
At the time of writing three flags are defined,
the R, T, and V bits; the rest
SHOULD be set to zero when sending and ignored on
receipt.
END
I changed the first part but leave in the MBZ.
OK
s3,
TLV types: The values 4, 6 and 8 for TLV
type and the value 5 for Tthe sub-type of TLV type
1 are specified as 'Not assigned': To be clear
for the future, should these really be marked as
'Reserved' or could they be assigned in future
(and hence s/b marked as 'Available for
assignment')?
They are not assigned. IANA now calls these as Not
Assigned as “Unassigned” — updated..
Fine. I was just wondering.
s3: For
clarity it would be useful to add a sentence to the
end of the section stating:
In Sections 3.2 - 3.4
and their various sub-sections, only the value
section of the TLV is specified.
Sure. But it’s really from 3.2 through 3.9.
True!
As part of this, I also cleaned up all the
“[sub][-]section” citations.
Good.
s3, TLV length calculation: This is shown by
example only. I think it ought to be explained
explicitly in text. I suggest:
The length of a sub-TLV or a TLV whose value is
not a list of sub-TLVs
is the number of significant octets in the value
part of the (sub-)TLV
excluding any final padding. If the value of a
TLV is a list of sub-TLVs,
the length of the TLV is the sum of the overall
lengths of the sub-TLVs
including the sub-TLV header and the length of
the padding, i.e.
4 + ((sub-TLV.length + 4) mod 4)
The examples are clear enough and have been clear
throughout many implementations.
The market may go down as well as up! I'll have to live with this.
s3.1,
para 1: I think this should be interpreted as saying
that the Return Code MUST always be zero in an Echo
Request and the Return Code is set to an appropriate
one of the possible values in an Echo Reply. To be
clear: I take it that it would not be normal for an
Echo Reply to carry a zero Return Code. Assuming
this is right...
OLD:
The Return Code is set to zero by the sender.
The receiver can set
it to one of the values listed below.
NEW:
The Return Code MUST be set to zero in an Echo
Request message.
The responder sets the Return Code in the Echo
Reply message to
an appropriate value other then zero from the
list below.
END
Current text is OK.
Hmm. I found this potentially confusing. This was the main point
at which I thought the use of sender and receiver needed
clarifying. Am I right in thinking that the 'sender' is always the
node sending an Echo Request and the 'receiver' is a node that is
triggered by the Echo Request to send an 'Echo Reply'. The issue
for me is that the receiver also sends messages. How about:
OLD:
The Return Code is set to zero by the sender. The receiver can
set
it to one of the values listed below.
NEW:
The Return Code is set to zero in the Echo Request message by the
(Echo Request) sender.
The ((Echo Request) receiver can set it to one of the values
listed below in the corresponding
Echo Reply that it generates.
END
It is possible that I am misinterpreting sender or receiver.. but if
so I would say that some additional words would help.
s3.1,
Return code 14: Some of the extra text from Section
3.2.1 of RFC 6424 ought to be essential as it
contains 'MUSTS'. Suggest adding this as an extra
note against Return Code 14:
Note 2:
Return Code 14 is used to indicate that an
Echo Reply contains one or more
DDM TLVs (see Section 3.4). In this case
there will be one Return Code and
corresponding <RSC> for each path
described and these are passed in the
DDM TLV(s). This Return Code MUST only be
used in the Echo Reply message
header and MUST NOT be used in the Echo
Request message even if the message
contains a DDM TLV.
Sure, added different text (above is incorrect), but
this is a really good point. Updated also a section
citation to point to this note.
Thew new text is fine.
s3.1: The term IS_EGRESS is used later in the
document to indicate an Echo Reply message with a
Return Code of 3. It should defined here. The
meaning is fairly obvious at its first use in
s3.4(e) but there is not a formal definition.
(AFAICS textual acronyms are not used for any of the
other codes.)
This didn't happen.
s3.2, last but one para: s/previx/prefix/
s3.2.8/s3.2.9/s3.2.11: It would be useful to use the
name of the FEC type from RFC 4447 (PWid FEC) rather
than just its number. (Also in A.1.1).
The names are wildly used, and citations to 4447
exist. I’ll leave it as is.
Wild!
s3.2.9:
s/sender's PE IPv4 address/Sender's PE IPv4
Address/; s/remote PE IPv4 address/Remote PE IPv4
Address/
OK, same for the Appendix and IPv6 PE addresses.
s3.2.9,
para 3: Need to expand PE acronym on first use.
s3.2.10, para 1: The text uses source PE IPv4
address whereas the diagram uses Sender's PE IPv4
Address. Consistency is needed. See also the
previous comment regarding consistency and
capitalization.
This is explained:
Sender's Provider Edge (PE) IPv4 Address (the
source address of the targeted LDP session),
OK. I think I read the text incorrectly here. On reflection it
looks fine as is.
s3.2.10/s3.2.12:
: It would be useful to use the name of the FEC type
from RFC 4447 (Generalized PWid FEC) rather than
just its number.
s3.2.12: The text uses source whereas the diagram
and field name use Sender's... consistency again?
s3.4, DS Flags:
I Interface and Label Stack Object Request
When this flag is set, it indicates that the replying
router SHOULD include an Interface and Label Stack
Object in the echo reply message.
What circumstances would cause the replaying router
not to do this? What should it do otherwise?
This hasn't been addressed AFAICS.
s3.4, Return Code:
The Return Code is set to zero by the sender. The receiver can
set it to one of the values specified in the "Multi-Protocol Label
Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters"
registry, "Return Codes" sub-registry.
a) I suspect that in the basic LSP ping described in
this document, the return codes that ought to be
available are only those specified in s3.1 of this
document except for 14 (which is specifically only
allowed in the header). The registry now contains a
number of other return code values but a basic
implementation wouldn't understand them in general.
b) See the previous comments on meaning of sender
and receiver. Suggest:
OLD:
The Return Subcode is set to zero by the
sender. The receiver can
set it to one of the values specified in the
"Multi-Protocol Label
Switching (MPLS) Label Switched Paths (LSPs)
Ping Parameters"
registry, "Return Codes" sub-registry.
NEW:
The Return Code in the (one) DMM TLV in an
Echo Request message
MUST be set to zero. The responder sets the
Return Code in any
DMM TLV in the Echo Reply message to an
appropriate value other
then zero or 14 ("See DDM TLV for Return Code
and Return Subcode")
taken from the list in Section 3.1.
END
Similar issue with the Subcode (you are mixing RC
with RSC in the OLD/NEW).
Ah! Actually the original text and what I wrote are both wrong!
The text in -08 is arguably not wrong, but is confusing because it
looks like the instructions for Return Code.
VERSION 08 TEXT:
The Return Subcode is set to zero by the sender. The receiver
can
set it to an appropriate value as specified in Section 3.1.
This
field is filled in with the stack-depth for those codes that
specify the stack-depth. For all other codes, the Return
Subcode
MUST be set to zero.
MORE NEWER:
The Return Subcode is set to zero by the sender. The receiver
can [MUST?]
fill this field with the stack-depth for those codes that
specify the stack-depth as indicated in Section 3.1 For all
other codes, the Return Subcode
MUST be set to zero.
s3.4,
Sub-tlv Length: I think that the components of the
DSM are all multiples of 4 octets long so there is
no padding to consider (apart from possibly in FECs
).
OLD:
Total length in bytes of the sub-TLVs
associated with this TLV.
NEW:
Total length in octets of the sub-TLVs
associated with this TLV including the TLV headers
and any padding.
END
Leaving this does not hurt — however, fixed the bytes
-> octets throughout.
Octets: good. I'll live with the rest.
s3.4.1.3,
FEC TLV length: Does this include any trailing
padding and the TLV header?
s3.4.1.3, Operation Rules: Shouldn't these be in s4?
Thinking about this some more, I see these aren't operation rules in
the same sense as s4 uses operation. They are actually ordering
rules:
OLD:
FEC stack change sub-TLV operation rules are as follows:
NEW:
When a DDMAP TLV needs several FEC stack change sub-TLVs to
record the changes
that the LSR makes to the label stack, the following number and
ordering rules MUST
be respected:
END
s3.6: Should contain a reference to the IANA
registry URL.
Sure, why not :-)
s4.1, last para: s/some information how each/some
information as to how each/
s4.2: s/to differentiate whether/to ascertain
whether/
s4.3, para 1: s/MUST be set in IP header/MUST be set
in appropriate IP options/
s4.4, item 1: It would be helpful to remind
implementers how TLVs are marked to be ignored:
OLD:
If there are any TLVs not marked as "Ignore"
NEW:
If there are any TLVs not marked as "Ignore" (i.e.,
if the TLV type is less than 32768, see Section 3)
END
s4.4: s/subsection/Section/g
s4.4, item 3: s/If there is no entry for L {/If
there is no entry for Label-L {/
s4.4, item 4:
OLD:
Set Best-return-code to Return Code
9, "Label switched
but no MPLS forwarding at
stack-depth" and set Best-rtn-
subcode to Label-stack-depth and goto
Send_Reply_Packet.
NEW:
Set Best-return-code to Return Code
9, "Label switched
but no MPLS forwarding at
stack-depth" and set Best-rtn-
subcode to Label-stack-depth and goto
step 7 (Send Reply Packet).
END
s4.4.1, item 5: s/advertise FEC/advertise the FEC/
s4.5:
If the replying router is the destination of the FEC, then Downstream
Detailed Mapping TLVs SHOULD NOT be included in the echo reply.
Under what circumstances might one be included? I
think this is a MUST NOT.
See comment on s3.4 at head of message.
s4.5.2: This section is derived from s4.1.2 of RFC
6424. Whilst the new version appears to contain
sufficient to define the proper normative behaviour,
RFC 6424 contains additional examples of usage.
These look useful to me. I wonder if it might be
useful either to copy the illustrative material to
an appendix or maybe point back to RFC 6424. I am
not sure how the powers-that-be would consider back
pointers to obsoleted documents! Maybe something
like:
[RFC6242] which originally specified the
techniques needed to support tunnel transition
contains some
examples, in Section 4.1.2, of situations where
the technique would be applied.
This was discussed and decided did not want to over
copy when the current text is enough.
OK
s4.6:
If the echo reply contains Downstream Detailed
Mappings, and X wishes
to traceroute further, it SHOULD copy the
Downstream Detailed
Mapping(s) into its next echo request(s) (with
TTL incremented by
one).
Presumably this means one DMM per Echo Request...
might be worth being more explicit.
s5: Security risks of Router Alert. Mention RFC
6398 and maybe copy 2nd para of s6 of RFC 7506.
I believe the RA usage (which is specific and not generic)
is adequately covered.
I was thinking of whether (e.g.) having a RA option set in a packet
other than a MPLS Echo Request/Response could be a way of doing a
DoS attack on an MPLS network since it could overload the
data->control plane link... might wish to filter any packets with
RA set at ingress to MPLS network. Not sure if the UDP port rate
limit would help with this.
s5, Security risks of DoS using Errored TLV?
Injecting MPLS Ping packets with bad TLVs could be a way of creating
a DoS perhaps as it would produce Errored TLV messages.
s6: Given the responses from IANA, a note is needed
to say that entries originated other than from RFC
4379 should remain unaltered in the registry. The
only exception might be the R flag in Global Flags
where it might be sensible to use this document to
fix erratum 4012.
s6.2.5, last line: Remove ']]' which appears to be
spurious.
s8: Several new references are mentioned in these
comments and would need to be added if the
suggestions are actioned.
Very many thanks again for the review!
— Carlos.