[lisp] Mirja Kühlewind's Discuss on draft-ietf-lisp-rfc6833bis-13: (with DISCUSS and COMMENT)

Mirja Kühlewind <ietf@kuehlewind.net> Tue, 11 September 2018 15:02 UTC

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Mirja Kühlewind has entered the following ballot position for
draft-ietf-lisp-rfc6833bis-13: Discuss

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DISCUSS:
----------------------------------------------------------------------

1) Versioning and backward compatibility

Section 5.2 says: "Support for requesting multiple EIDs in a single Map-Request
      message will be specified in a future version of the protocol."
However, there is no versioning mechanism for this protocol specified. How is
versioning supposed to work?

Further given there is no new version, I wonder if the changes as outlined in
section 10 are all backward-compatible? Especially for the introduction of the
Message-Notify-Ack message, I guess there is no problem if a server sends it,
however, as the sender of the Message-Notify message might not know if the
other end supports sending of the Message-Notify-Ack it can't rely on it. This
should be further discussed in the doc! Or is there another strategy to achieve
backward compatibility?

2) Size and MTU

As outlined in the TSV-ART review (Thanks Colin!) this document does not
discuss fragmentation or Path MTU discovery. RFC8085 recommends to either
perform Path MTU discovery or limit the message to 576 bytes for IPv4 or 1280
bytes for IPv6 (minus any static header). As this seems to be an appropriate
size for LISP messages, I would recommend this approach. Relying on IP
fragmentation (as indicated in the reply to the TSV-ART review) is not
recommended by RFC8085 as this would lead to IP packet without a UDP header, in
the case of LISP, which can cause problem and loss when NATs are involved. In
any case the chosen approach needs to be further discussed in the doc.

3) Rate-limiting and congestion control

Sec 5.3: "Map-Requests MUST be rate-limited.  It is RECOMMENDED that a Map-
   Request for the same EID-Prefix be sent no more than once per second."
As already noted by the TSV-ART review (Thanks Colin!), RFC8085 actually
recommends to not send more the one packet per 3 seconds, and that is a
restriction for all traffic not on a per-receiver base, or implement congestion
control. This limit is meant to not only protect the receiver but also the
network from overloading. Why do you use a smaller interval here? Also if
(appropriate) rate limiting is used, this should either be a MUST or more
explanation when it is okay to use a smaller rate limit should be provided.
However, after all, I don't think you those the right approach here for rate
limiting. A Map-Request is always expected to be followed by some reply. For
these kind of communication pattern, RFC8085 recommends to limit the number of
outstanding requests to 1 (see sec 3.1.1 of RFC8085 recommending one packet per
RTT), also for all traffic and not only per receiver. However, this would also
require to implement some simple mechanism to detect a message as lost (see
also further below in point 4).

Similarly I'm not sure about the intent of this requirement in section 5.5:
"Map-Replies SHOULD be sent for an EID-Prefix no more often than once
   per second to the same requesting router. "
My understanding is that Replies are only sent when a request is received. Why
is this additional rate limit needed? Again if used it should be 3 seconds for
all traffic to be inline with RFC8085. Also again, why is that not a MUST?
Further recommendation are needed here.

Further section 6.1 say "Both the SMR sender and the Map-Request responder MUST
rate-limit
   these messages.  Rate-limiting can be implemented as a global rate-
   limiter or one rate-limiter per SMR destination."
This seems to be the same rate limit as mention above, or not...? It would
probably make sense to rate limit the SMR even further. Please clarify and
provide more guidance, e.g. what should the value of a potential additional
rate limit for SMR be?

Respectively the following sentence in section 6.1 is also unclear:
"The remote ITR MUST rate-limit the Map-Request until it gets a Map-Reply"
Why is the rate-limit as currently proposed depend on the fact if a Map-Reply
is received? Is the ITR supposed to retransmit the Map-Request...?

And finally the Map-Register, Map-Notify and Map-Notify-Ack messages does not
seem to have any rate-limits. Recommendations inline with RFC8085 should be
provided for the total traffic and not only for a few message types. Again,
Map-Notify and Map-Notify-Ack messages should be send only once per RTT as
there is a feedback mechanism.

For Map-Register sec 8.2 say: "Map-Register messages are sent periodically from
an ETR to a Map-
   Server with a suggested interval between messages of one minute."
However, this a rather a low bound than an upper bound. A required (MUST) rate
limit is still needed.

4) Loss detection and retransmission

As also mention by the TSV-ART review (Once more thanks to Colin!), this spec
has an ACK mechanism for Map-Requests and now also for Map-Notify, however, it
does not specify what to do if the ACK is not received (loss detection and
retransmission scheduling). This makes the spec incomplete and needs to be
further specified in the doc (and also has a relation to the point 3 above of
course).


----------------------------------------------------------------------
COMMENT:
----------------------------------------------------------------------

Further comments:

1) The example given in 5.5 should probably used IPv6 addresses and use the IP
address space that is reserved for documentation purposes.

2) I find the security requirements in this doc very unsatisfying. Most
important the doc requires the support of authentication mechanism but not the
use of it. I would like to see more clear MUST requirements here. Further,
today and at this stage of the protocol (moving from exp to PS) I find it not
acceptable anymore to have certain security feature as optional and outsourced
into a different work-in-process draft. However, I leave further discussion to
the SEC ADs.

3) Given the following statement:
"Note that while this document assumes a LISP-ALT database mapping
   infrastructure to illustrate certain aspects of Map-Server and Map-
   Resolver operation..."
it seems that RFC6836 should be a normative reference, as it might not be
possible to understand all details explained in this doc with knowing ALT.

4) Further I would also think that I-D.ietf-lisp-mn and I-D.ietf-lisp-pubsub
should be normative references as the meaning of the respective bits it not
further specified in this doc. Or can these bits just be ignored if
I-D.ietf-lisp-mn and I-D.ietf-lisp-pubsub are not implemented? If so that
should be stated.

Clarification questions:
1) Sec 5.3.:
"For the initial case, the destination IP
   address used for the Map-Request is the data packet's destination
   address (i.e., the destination EID) that had a mapping cache lookup
   failure."
Does that mean that the Map-Request needs to use the IPv4 or IPv6 depending on
the IP version used by the initial message from the EID. Is that always the
case or just for this initial message? I would assume that for all other cases
this is actually independent...? Because otherwise there would be a constraint
on what needs to be requested. I would like t see further clarification about
this in the doc.

2) In section 5.3: "The ITR MAY include all locally
   configured Locators in this list or just provide one locator address
   from each address family it supports."
Would it make sense to include a SHOULD requirement to at least the address
family that is used to send the Request is included (to increase chance to
enable a communication/get a reply)...?

3) Sec 5.4: "If all Weights for a Locator-Set are equal,
      the receiver of the Map-Reply will decide how to load-split the
      traffic. "
Shouldn't the receiver in this case split the traffic equally? Otherwise how
would you signal that the traffic should be split equally? Maybe use all zero
instead to let the receiver decide...?

4) sec 6.1: "When an ITR receives an SMR-based Map-Request for which it does not
   have a cached mapping for the EID in the SMR message, it may not send
   an SMR-invoked Map-Request."
I guess this should be normative and probably also a MUST NOT or at least
SHOULD NOT.

5) Section 7 seems to imply that if it is detected that no route is available,
the ITR should basically do nothing and just drop any incoming packets for that
ETR. Would it make sense for incremental deployability, to just forward the
packet to the IP address of the EID instead...? This way the source host would
not benefit in mobility cases but still gets connectivity otherwise. Or is that
anyway not the implication? If that is the case, that should be further
clarified in the doc.

6) Section 8.2 says: "Note that the Map-Notify message
   is sent to UDP destination port 4342, not to the source port
   specified in the original Map-Register message."
Actually why is that?

Some minor editorial comments:

1) First sentence in intro: the pointer to ietf-lisp-introduction as currently
introduced, makes this reference look very normative: "The Locator/ID
Separation Protocol [I-D.ietf-lisp-introduction] and [I-D.ietf-lisp-rfc6830bis]
specifies..." I would recommend the following wording: "The Locator/ID
Separation Protocol [I-D.ietf-lisp-rfc6830bis] (see also
[I-D.ietf-lisp-introduction]) specifies..."

2) Also in intro: Given that 6830bis is a normative reference "LISP RFC
6830bis" should be replaced with the new RFC number in the text. This should be
noted to the RFC editor; probably this is more obvious if RFCXXX is used
instead.

3) Sec 5.4: "...for another way the R-bit MAY be used."
This looks like a lower case may would be more appropriate.