David,
Most of these responses look fine. I do think additional text
should be added on the topics of:
- warnings about when not to use unordered delivery
- explanation of when UDP use is appropriate
More details inline ...
Some responses also in line. I have deleted the stuff where we have agreed.
> Section 6.1:
>
> There is an additional service provided by SCTP and useful in
> conjunction with PR-SCTP: unordered delivery. This also works
on a
> per-message basis by declaring that a given message should be
> delivered at the receiver as soon as it is received rather than
kept
> in sequence; however, it should be noted that unless explicitly
> requested by the sender even messages sent partially reliably
will
> still be delivered in order.
>
> [1] Isn't this likely to cause problems when messages are
> processed out of order? If three messages are sent containing
> absolute counts of 45, 87, and 138, out of order processing
> combined with a delay to the first message could result in the
> count being 45 at the recipient after the three messages are
> processed, which seems rather wrong. Whose responsibility
> is it to prevent this? In contrast, partial reliability skips
> updates that would be generated by processing the dropped
> messages, and hence seems ok.
I propose adding the following text at the end of the paragraph (to
recommend not using unordered delivery when order may matter):
Unordered delivery SHOULD NOT be used when the order of IPFIX Messages
may matter: e.g., a Template or Options Template.
I think some more explanation is in order about why ordering matters
for templates. I would also say that unordered delivery SHOULD
NOT be used when Total (absolute) Counters are used, as reordering
could result in the counter value decreasing at the Collecting Process,
and even being left with a stale value if the last message processed
is stale.
ok, so the proposed text on unordered delivery is the following:
There is an additional service provided by SCTP and useful in
conjunction with PR-SCTP: unordered delivery. This also works on a
per-message basis by declaring that a given message should be delivered
to the receiver as soon as it is queued rather than kept in sequence;
however, it should be noted that unless explicitly requested by the
sender even messages sent partially reliably will still be delivered in
order.
Unordered delivery SHOULD NOT be used when the order of IPFIX Messages
may matter: e.g., a Template or Options Template. Unordered delivery
SHOULD NOT be used when Total (absolute) Counters are used, as
reordering could result in the counter value decreasing at the
Collecting Process, and even being left with a stale value if the last
message processed is stale.
(note that I've appended the sentence you proposed)
> Section 6.2: UDP
>
> [2] I don't see any discussion of congestion control in here.
> Something needs to be done to avoid a high rate IPFIX protocol
> session over UDP worsening a congestion situation because not
> only is the IPFIX flow non-responsive to congestion, but
> congested conditions may increase the volume of IPFIX data
> to be reported. At the very least, this draft should repeat
> and reinforce the discussion in Section 10.3.1 of
> draft-ietf-ipfix-protocol-24.txt, which says that UDP should
> not be used over congestion sensitive network paths - I'm not
> thrilled about that approach, but the ipfix-protocol draft
> is already in the RFC Editor Queue. A general recommendation
> against UDP when TCP or SCTP is possible may be appropriate,
> and the first item in Section 10.1 is related to this concern,
> as it requires availability of a congestion-controlled protocol.
UDP is not the recommended protocol for IPFIX and is intended for use
in
cases in which IPFIX is replacing an existing NetFlow infrastructure,
with the following properties:
1. a dedicated network
2. within a single administrative domain
3. where SCTP is not available due to implementation constraints
4. and the collector is as close as possible to the exporter.
Would you like to see this text in the document or is the current text
at the beginning of 6.2 enough:
[ ... snip ... ]
Please put the above text into the document and explain "dedicated
network" in more detail - that is crucial to ensuring the additional
IPFIX traffic generated as a consequence of high traffic situations
does not make the situation worse from a congestion standpoint. In
writing this text, please review draft-ietf-tsvwg-udp-guidelines-02.txt,
although I suspect that much of it will not be applicable to IPFIX
(it is not necessary to respond to the guidelines in that draft
point-by-point, but it would be good to mention any that do apply).
ok, so the (proposed) text at beginning of 6.2 now is:
UDP is useful in simple systems where an SCTP stack is not available,
and where there is insufficient memory for TCP buffering.
However, UDP is not a reliable transport protocol, and IPFIX messages
sent over UDP might be lost as with partially-reliable SCTP streams.
UDP is not the recommended protocol for IPFIX and is intended for use
in cases in which IPFIX is replacing an existing NetFlow
infrastructure, with the following properties:
o A dedicated network,
o Within a single administrative domain,
o Where SCTP is not available due to implementation constraints,
o And the collector is as close as possible to the exporter.
Note that because UDP itself provides no congestion control mechanisms,
it is up to the applications that use UDP to incorporate congestion
avoidance mechanisms. [RFC2309] discusses the dangers of
congestion-unresponsive flows and [draft-ietf-tsvwg-udp-guidelines-02]
provides guidelines for the designers of such applications.
> The possibility of Exporter vs. Collector misconfiguration
> on template resend interval vs. template expiry time is
> unfortunate and (as described) can cause interoperability
> issues. Can IPFIX define a template for an Exporter to
> report its resend interval? That would enable a Collector
> to determine an appropriate expiry time for each Exporter,
> and might obviate the need for the packet-count-based
> resend mechanism discussion.
Do you plan to do anything about this?
Sorry, I missed this one.
Not in this document, because this feature is currently not present in
IPFIX (there's no appropriate Information Element for this)
There was an additional comment I didn't answer in my previous email:
Section 4.2:
Although it is not explicitly mentioned in the protocol draft the
order of Information Elements within the Template MAY be changed by
Exporting or Collecting Processes for example for alignment
purposes.
It looks like this allows a single Collecting implementation
(multiple Collecting Processes) to receive information from
multiple Exporting Processes using the same template but with
different ordering of information sent by each Exporting Process.
No it doesn't... This section is essentially a restatement of the
fact that ordering of multiple identical IEs is important. I'm afraid
that its potential for confusing people exceeds its usefulness...
After reading it again I wonder whether we should keep this section at
all. The importance of keeping order for IEs of the same type is already
mentioned in 3.4.
I propose to remove section 4.2.
If that is correct, it would be good to advise implementers of
this possibility and suggest that a single normalized order of
stored information for each template be used in a Collecting
implementation to avoid confusion if/when this happens -
somewhere in Section 5 would be an appropriate location
for this advice.
thanks,
Elisa
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