[Gen-art] Gen-art LC review of draft-ietf-mpls-rfc4379bis-07

[Elwyn Davies <[e-mail]>]

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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.

[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.

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.

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.

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.

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.

Nits/editorial comments:
======================
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.

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

s2.1, para 9: s/embedded in as/embedded in an/

s2.1, para 9: Useful to add a reference to RFC 4291.

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

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

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.

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

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

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')?

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.

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)

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

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.

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.)

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).

s3.2.9: s/sender's PE IPv4 address/Sender's PE IPv4 Address/; s/remote 
PE IPv4 address/Remote PE IPv4 Address/

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.

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?

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

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

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?

s3.6: Should contain a reference to the IANA registry URL.

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.

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.

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.

s5, Security risks of DoS using Errored TLV?

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.