Bob,
Thanks for the comments - see below for responses.
On 4/4/2019 7:56 PM, Bob Briscoe via Datatracker wrote:
Reviewer: Bob Briscoe
Review result: On the Right Track
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==1. Introduction==
It would be useful to describe the use-case where the modem does not implement
active queue management but the router does, so the modem can use flow control
to push the queue back into the router, where it can be more intelligently
controlled.
I guess I'll have to talk to the Shepherd on this one to see how much
he/the WG would want to add on this at this point. Perhaps giving a
reference to the earlier PPPOE (rfc5578) would suffice, what do you think?
===Scope within Intro===
Please extend the single sentence about scope (end of 2nd para of Intro) to
explain that pause control only applies to data in the router to modem
direction.
Please also mention that the scope of pause-based flow control is limited to
one hop and to a point-to-point link between router and modem, not multipoint.
The modem does not track the source of the data in its queue, so it cannot
focus pause messages onto particular sending stations on a multipoint link, or
onto particular ingress ports of the router.
Done.
Why is the scope limited to DLEP radio links? I would have thought this
protocol is generally useful between a and modem and router.
Because this is a DLEP extension. Other mechanisms exist for other
technologies, e.g., RFC5578 or Ethernet PAUSE/PFC.
==3. Extension Data Items==
Pls define the direction of the messages:
s/The xxx Data Item is used by a modem to.../
/A modem sends the xxx Data Item to its peer router to.../
okay (paraphrase a bit)
===Unsafe design?===
Is it not good practice to make the data plane robust to unexpected control
plane failures (e.g. key expiry, incorrect or mis-timed change of address,
etc.) and vice versa? So, would it not be more robust for the router to
time-out a pause if no restart appears? Also, if the last message received
before shutting down or suspending was a pause, should the router restart or
resume in the paused state? What if the router enters a power-saving state
after it is paused and misses a restart message?
I generally agree that a control plane fault should not result in a data
plane loss -- in some environments, I'd say this is a must. This said,
your comment goes to a design principle in DLEP RFC8175 where control
plane error result in session resets and data plane impacts. I think
changing this basic behavior of DLEP is beyond the scope of this
extension. I think a general modification of base DLEP to support such
would be a fine idea.
==Queue size in bytes for informational purposes?==
Why? This strikes me as like one of those Government forms you have to fill in
with an ill-formed question that is mandatory to answer, even though sometimes
there is no answer, and you cannot proceed until you've answered, even though
the answer is not needed anyway. For instance, if there is a shared physical
buffer, a size cannot be straightforwardly given for each logical buffer. So,
if buffer size info is not used by the protocol, do not include it in the
protocol.
It is largely for reporting to a user/operator for "informational
purposes". If an implementation chooses, it can put in zero or
infinity. In most cases I understand this will be a straightforward
value that can be reported to the router and its operators.
On the other hand, how is the threshold at which the modem sends a pause
configured. Is that out of scope? If so, where is it specified? Wherever this
is specified, it should be possible to specify the threshold in time units
(queue delay at the current service rate of the queue), not just in bytes. This
is important for queues in a hierarchy where the service rate varies, e.g.
dependent on traffic in another queue with priority over it. Or simply where
the modem can operate at different rates.
I think a service rate / queue delay property would be very interesting
- but this didn't come up in the WG, so I don't think it is appropriate
to add it here. There is also nothing preventing such information being
added in a future extension.
==3.3 Restart==
"A router which receives the Restart Data Item SHOULD resume
transmission of the identified traffic to the modem."
Why only SHOULD? Under what conditions would it not?
if it has no data to send. I don't object changing this to a MUST if you
think important.
==4. Security Considerations==
"The extension does not inherently
introduce any additional vulnerabilities above those documented in
[RFC8175]."
Er, no... What about the ability for an off-path attacker to stop the router
sending data!?
the same attacker can subvert the dlep session an cause a session reset
and take down all traffic. So how is this case different?
The last part about TLS needs to be worded differently. Because of the above
additional vulnerability, the router MUST verify that all 3 types of message
are authenticated by the modem. This requires client authentication mode of
TLS, which is not mentioned in RFC8175, so it needs to be mentioned here. Or
is the TLS session set up by the router (so the modem is the authenticated
server)? Also this raises the question of key management, if every modem has to
be authenticated by its router.
"but this is not a
substantively different attack by such a compromised modem simply
dropping all traffic destined to, or sent by a router."
Er, no... The modem does not need to be compromised for a 3rd party to send
spoof messages purporting to be from the modem.
Fair point - how about:
Note that this extension does allow a compromised or impersonating
modem to suppress transmission by the router. Similar attacks are
generally possible base DLEP, for example an impersonating modem may
cause a session reset or a compromised modem simply can
drop all traffic destined to, or sent by a router. <xref
target="RFC8175"/> defines the use of TLS to protect against the
impersonating attacker.
==Nits==
1. Intro
"DLEP peers are comprised of a modem and a router" is incorrect English for
what you mean (it means that each peer consists of a modem and a router).
Better to do away with this sentence completely, and alter to the previous
sentence to "It provides the exchange of link related control information
between a modem and its DLEP peer router."
sure.
3.1
s/with Queue Index/
/with a Queue Index/
okay
Scale:
s/An 4-bit/
/A 4-bit/
okay
In general, I think the term "queue size" is wrongly being used where "buffer
size" should be used (the queue size is the varying size of the queue within
the buffer at any one time).
Also, pls consistently use the term 'paused' not 'suppressed', which has the
potentially ambiguous meaning of 'discarded'.
will clarify the intent.
Delete gratuitous 'is':
"The motivating use case [is] for this
data item is when a modem's internal queue length exceeds a
particular threshold."
yes,
CURRENT:
"e.g., when there
a non queue related congestion points within a modem, but such are
not explicitly described in this document."
SUGGESTED:
"e.g., when there
are non queue related congestion points within a modem. Such use-cases are
not explicitly described in this document."
Done!
Thank you for the comments.
Lou
PS the working document has been updated, if interested see
https://github.com/louberger/dlep-extensions/tree/master/pause
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