On Aug 26, 2009, at 3:58 AM, Ahmad Muhanna wrote:
Hi Ben,
Thanks for the detailed review and comments.
Please allow me to address your comments in two parts.
1. PART-I: Major and technical issues.
2. PART-II: remaining comments.
Please see answers inline for PART-I.
Regards,
Ahmad
-----Original Message-----
Summary: This draft is on the right track, but there are open
issues.
Additionally, I have a number of editorial comments.
Major issues:
-- I think the security considerations need quite a bit of
work. In particular, there is very little guidance on
authorization for sending BRI messages. This seem to me have
utility for DoS attacks, particularly with the G bit set.
There is mention of reusing existing security associations if
IPSec is used--but no mention of what to do if IPSec is not
used.
[Ahmad]
Binding Revocation is used between two peers to revoke/terminate a
mobility session(s) that have been created using an IPv6 mobility
protocol signaling (Client Mobile IPv6 or Proxy MIP6). RFC3775 and
RFC5213, which are the main protocols targeted by this specification,
specify that "IPsec SHOULD" be used. On the other hand, there is NO
other standard track specification which specify other security
mechanisms to secure the IPv6 mobility signaling. Therefore, Binding
Revocation specification assumes the use of whatever security
mechanism
that currently available to secure the IPv6 mobility signaling.
I think there are still a couple of issues here. First, Since the
underlying RFCs only specify IPSec at SHOULD strength, this draft
needs to discuss the consequences of not using it for BRI. Depending
on those consequences, it might be enough to just warn implementors
that, if you don't use IPSec, certain bad things can happen. OTOH, it
might be that BRI has greater security risks than for 3775/5213, and
you might (for example) need to strengthen the IPSec requirement for
BRI.
I admit to not being an expert on 3375/5213, so it may be true that
BRI is no riskier than the underlying technology--but even if that is
true I'd like to see some discussion to support it.
Second, I think that there is probably more guidance needed on
authorization decisions even if you do use IPSec. For example, do you
assume that any trusted peer can remove any binding? Is a trusted peer
only allowed to remove bindings that it previously established using
the same SA? If an SA is torn down and a new one established, what
authorization gets inherited, if any? Do you assume that a peer that
is trusted to establish bindings is trusted for BRI? Do you need to
provision policies around these, and if so what are the moving parts?
(Perhaps it is required by the underlying technology?
If so, that should be mentioned here.)
[Ahmad]
That is not the intention. Please see above.
You mention that
authorization is required if the G-bit is set, but go on to
say authorization details are out of scope. I think that this
draft needs to either offer much more guidance on
authentication requirements.
[Ahmad]
We could introduce a simple default mechanism inline with what we have
in RFC5213.
It's possible that might help--can you point to the section of 5213
you have in mind? It might also be enough to have more discussion on
what an implementor needs to think about to do authorization
correctly. For example, does it make sense to statically provision
that a trusted peer can remove any binding for "foo.com"? Is
authorization policy dynamically determined by prior actions (i.e. a
peer can revoke all bindings _it_ established for "foo.com", but not
bindings that another device established for "foo.com"?
Probably more than anything, it would help to discuss the sort bad
things that this authorization is intended to prevent.
It would be helpful if the
Security Considerations section discussed the consequences of
security failures, possible attacks, etc.
[Ahmad]
This specification do not introduce any security threats on the top of
what is being discussed in Client MIP6 and Proxy MIP6, RFC3775 and
RFC5213.
That's a little hard to believe without some supporting text. Again,
this could be my lack of knowledge of IPv6 mobility talking. But for
example, do RFC3775 and/or 5213 already have something a mechanism for
one mobility element to tell another to drop bindings in bulk?
Minor issues:
-- S3.4.2, paragraph 1: "responds with the appropriate status
code by sending a Binding Revocation Acknowledgement message"
Always, or just if the A bit is set?
[Ahmad]
This section describes the usecase when Global revocation is being
used
by the MAG; there is no normative text in this section.
Understood (but I had similar questions in the normative sections).
In addition,
section 10.2.1. which talks about the use of the (G) bit by the MAG
and
indicates that whenever the (G) bit is set the (A) bit MUST be set, is
correctly being referenced in this section and mentioned before the
text
quoted above.
But this text talks about how you form a BRA, not how the initiator
formed the BRI. Would you expect a responder to just assume (without
checking) that the A bit was set if it sees a G-bit set?
There's a lot of interaction between these bit settings that make for
some pretty complicated state transitions. As described, it expects
the responder to infer the A bit value based on the G-bit value. It
would be much cleaner to to implement if it were defined so that the
responder always sends a BRA if the A bit is set, and never if it is
not.
As a thought experiment, how would you recommend an implementer to
handle the case where a responder got BRI with the G bit set and the A
bit not set? (I'm not asking for the draft to specify that--it's just
a discussion point.)
-S4, paragraph 2: "verify that the mobile access gateway
sending the binding revocation indication message is
authorized to invoke global revocation"
How does it make such a verification?
[Ahmad]
By checking the identity of the MAG if it is authorized for global
revocation or not. Would a reference to section 9.2.1. makes it
clearer
or you think we need to add more clarification.
Actually, this is really more of a 9.2.1 issue. (I reviewed things
linearly.) I think a reference here would help, but note I had similar
comments about 9.2.1 further down.
-S5, second paragraph: "UDP encapsulation to traverse NATs"
Can you include a reference for UDP encapsulation?
[Ahmad]
No problem.
Thanks!
-- Same Paragraph: "... port numbers ... will also be used"
Should this be normative?
-- Same paragraph, sentence starting with "For example..."
I don't think it's appropriate to include a normative
statement inside an "example". You could fix this by swapping
the descriptive language in the previous sentence with the
normative language in the "example".
[Ahmad]
Agreed. Will fix swap as suggested. Thanks.
Thanks!
-- Section 7.2, last paragraph: "If a mobility node receives
a Binding Revocation Indication message with a Revocation
Trigger value that is NOT in line with the Binding Revocation
Indication message intent, e.g., the Global (G) bit set and
the Revocation Trigger field vale is a per-MN specific, the
receiving mobility node SHOULD reject the Binding Revocation
Indication message by sending a Binding Revocation
Acknowledgement message with the Status field set to
"Revocation Function NOT Supported"."
This paragraph seems to imply some under-specification around
how to tell the Revocation Trigger value is not in line with
the initiator's intent.
Also, do you really mean to send "... not supported"? This
really sounds like more of a "bad request" scenario.
Did you mean to capitalize the final "NOT"?
[Ahmad]
I thought it was a straight forward combination. If the Global (G) bit
is set, the Revocation trigger field value MUST contain one of the
Global revocation triggers. If the (G) bit is cleared, the revocation
trigger MUST contain a per-MN value. Any deviation, means this
functionality is not supported.
The text indicated those as examples. Are they the only scenarios
where the trigger value can be out of line with the "intent"? I guess
part of my problem is that "intent" is a vague term here. The
important thing is to make sure that all legal combinations are
specified. I think they may be later on (again, reviewing linearly),
but they are scattered around the draft.
As far as why having "NOT" in capital letters, some drafts have the
whole cause value in capital letters, but it is also meant to say that
this is a bad request.
-- Section 7.3:
RFC3775 already talks about retransmission for Binding Update
messages. Does this really need to be specified separately?
[Ahmad]
Yes. It is a separate protocol.
Okay.
-- 2nd paragraph: "SHOULD retransmit"
Can you offer guidance on when an implementation might
reasonably not do this? (i.e. why not a MUST?)
[Ahmad]
Since sending a BRI message is NOT a MUST to start with, I do not
believe that the retransmission needs to be mandated as a "MUST". A
strong recommendation using "SHOULD" gives more flexibility to the
initiator to retransmit based on the need and the scenario at hand. In
addition, I did not see anywhere in RFC3775 or RFC5213 where
retransmission is mandated.
A MUST retransmit if you don't get the ack to a BRI does not in any
way imply MUST send a BRI.
In this case, my concern is that you have two ways to decide not to
send a retransmission--one is that the value of BRIMaxRetriesNumber
could be set sufficiently low (zero, I assume) to prevent
retransmissions. The second is that the implementator could choose not
to honor the SHOULD, even if BRIMaxRetriesNumber has a higher value.
If you want to allow the latter, it would help to have some guidance
(or examples) about scenarios where this would make sense, and the
consequences of doing it.
-- S8.1, 3rd para after bullets: "home agent SHOULD handle
this case based on the reason for sending the Binding
Revocation Indication message and its local policy"
Is this entirely local policy? Is there no guidance to offer
about how the "reason for sending" the BRI influences this
decision? If it's really just local policy, then I'm not sure
you need a normative statement (i.e. you SHOULD do whatever
you choose to do...)
[Ahmad]
The intention here is to make sure that the home agent take in
consideration the reason for sending the BRI, i.e., Revocation Trigger
value and NOT to handle all of the BRI cases by applying a single
reaction. For example, if the Revocation Trigger value indicate an
administrative reason, then the HA probably have a lot of options for
handling such a case. On the other hand, an inter-MAG handover
Revocation Trigger value would probably require a more careful
consideration.
A note to that effect would be helpful.
-- Last para: "SHOULD NOT"
Why not MUST NOT?
[Ahmad]
The problem we are trying to avoid here is: if we use "MUST NOT"
then we
need to specify the behavior of the receiving node in case it
receives a
BRI with all of the BID(s) included. Considering such case as an error
scenario is probably not the best way. Allowing it, then it is not
"MUST
NOT" anymore.
On the contrary, it's not necessary to describe what the responder has
to do for every possible violation of MUST level requirements by the
initiator. But it _is_ necessary to do that for violations of SHOULD
level requirements, because that is much more likely to happen. So by
making this a SHOULD you've created more work on the part of the
responder than if it were a MUST.
OTOH, if it really doesn't matter to the responder one way or another,
then I'm not sure you need either.
BTW, It's not necessary for the responder to treat every MUST
violation by the sender as an error--Postel's law should applies here.
I suspect the real requirement is that the _receiver_ MUST ignore any
BIDs if present, right?
-- S 10.1.1, third bullet: "MUST send a Binding Revocation
Acknowledgement"
So the G bit and the revocation trigger field value of
"per-peer policy" is enough to require a BRA? Wouldn't this
only apply when the A bit is set? (I know the initiator may
have been required to set the A bit, but it seems wrong to
expect the responder to infer that.)
[Ahmad]
This case a little similar to the "SHOULD NOT" case above. If the (A)
bit is NOT set, what the receiving node should do. The intention is
for
the MAG (responder) in the case of (G) revocation to always send the
BRA
message.
This goes back to my previous comment. You require the responder to
make complex decisions on whether to send a BRA, based on the A-bit,
the G-bit, the responder role, etc. It would make life much easier
(read: less error prone) for the implementor if you could make this
entirely dependent on a single flag.
-- S 11.1, bullet 2: "SHOULD send a Binding Revocation
Acknowledgement"
Can you document reasons why a responder might not send the
BRA, and the consequences thereof? In particular, are there
scenarios where the initiator might go into retries because
the responder chose not to send a BRA?
[Ahmad]
Sure.
In this bullet it says that if the mobile node receives a BRI message
and the MN has an entry for the binding defined in the received BRI,
then the MN MUST send a BRA. In other words, if the MN successfully
process the BRI and still track this binding and still able to send a
BRA, then it MUST send a BRA. In all other circumstances, e.g., the MN
no longer tracking this binding, the MN received the BRI and before
processing the battery went down and no BRA is sent anyway, etc. The
whole idea is to make sure that the mandate on the mobile node is
reasonable and within the mobile node abilities to send a BRA.
Otherwise, we would like to offer the mobile node a reasonable excuse.
I don't think one needs to worry about scenarios such as "battery
failed" in deciding to make a requirement a MUST or a SHOULD. If we
did, it would not be possible to have _any_ MUSTs.
In this particular case, not sending the BRA appears to do harm, in
that it may induce unnecessary retransmissions on the sender's part.
-- same paragraph: "In all cases, the mobile node MUST follow
Section 11.2"
Do you really mean "in all cases"? This seems to contradict
the SHOULD in the previous sentence, and the "If the A bit is set"
condition in the first sentence in the paragraph.
[Ahmad]
Yes. The bullet correctly reference section 11.2 which says:
"
11.2. Sending Binding Revocation Acknowledgement
When the mobile node receives a Binding Revocation Indication from
its home agent, the mobile node processes the received Binding
Revocation Indication as in Section 11.1. If the mobile node is
required to send a Binding Revocation Acknowledgement message in
response to the received Binding Revocation Indication, the mobile
node sends a packet to its home agent containing a Binding
Revocation
Acknowledgement according to the procedure in Section 7.1 and the
following:
"
The key text is: "If the mobile node is required to send a BRA.." That
requirement is defined in the bullet you reference under section 11.1.
So this is really an editorial issue then. The problem is in the
phrase "in all cases". Putting "all cases" here, then a loophole of
"if required" in the referenced section is confusing. I propose
changing the sentence to say something to the effect of:
"In all cases where the MN sends a BRA, it MUST do so according to
section 11.1"
-- Bullet 3: "mobile node MUST send"
Even if A bit is not set?
[Ahmad]
Please see response to the comment about processing the BRI when the
(G)
bit is set, as described above.
... and please see my response.
-- same bullet: "mobile node SHOULD send a Binding Revocation
Acknowledgement with the status field set to "Binding Does NOT
Exist""
Even if A bit is not set?
[Ahmad]
:)
As you said earlier, it is inferred that is being set but if it is not
being set, we need to specify the behavior of the MN. In this case, it
is very important for the MN to send a BRA message and inform the HA
that since BRI did not have the HoA IPv4 option, the MN can not
identify
the impacted binding. This is to give the HA an indication that it
needs
to resend BRI with the HoA IPv4 option included.
See my previous response about it being better to make explicit
decisions on the A bit rather than inferences due to other bits. In
this particular case, the sender has _already_ violated a MUST if it
didn't set the A-bit. (And I assume that if it did not set the A bit,
it probably isn't waiting for a BRA--otherwise it is doubly broken).
Is it really that important for it to get the BRA under those
circumstances?
-- Bullet 4: "MUST silently discard the Binding Revocation
Indication message"
Even if A bit is set?
[Ahmad]
In other words, this is a fatal error. When the (P) bit is set, the MN
binding is for a proxy MIPv6 binding which SHOULD never be sent to the
MN but to a MAG. In this case, the MN should silently discard the
message.
-- S11.1, last paragraph: "could be used by the mobile node
to define what action"
I think this could use some more guidance, if you expect
consistent behavior across implementations.
[Ahmad]
Thanks. This probably was intentionally left like it is because, it is
probably very difficult to get all implementations to agree on a
common
behavior. However, the text was meant as a reminder to implementations
in order to take that in consideration.
Okay.
-- S 11.2, 2nd bullet: "The mobile node MUST set the Status
field to an appropriate value. The mobile node sets the
Status field to success to reflect that it has received the
Binding Revocation Indication and acknowledge that its IP
connectivity with its home agent has been revoked"
I think this is under-specified. In particular, is the mobile
node allowed to set failure status values?
[Ahmad]
Yes, it could. E.g., if the MN received a BRI with Revocation Trigger
value set to: a non supported value or "8 Possible Out-of Sync BCE
State", the MN could send a BRA with status code set to failure.
Okay--I think adding a sentence or two that at least indicates that it
sends a successful status on success, and sends an appropriate error
status if an error occurs would help.
-- S 12: "BRI Maximum Number of Retries"
Why do you have both a max number of retries _and_ a max
timeout? I gathered from previous sections that retries stop
after the backoff hits max_brack-timeout. Did I read that wrong?
[Ahmad]
May be the name is misleading. What the "Maximum BRA TIMEOUT" means is
the MAX time the initiator waits before it retransmits. Here is the
text, please let me know if you have any suggestions for modification.
"
Maximum BRA TIMEOUT (MAX_BRACK_TIMEOUT)
This variable specifies the maximum delay timeout in seconds
before the revoking mobility entity retransmits a BRI message.
The default is 2 seconds.
"
So it's the maximum interval between any two retransmission, not the
maximum time before retransmissions stop, right? That is, once you
reach MAX_BRACK_TIMEOUT you stop backing-off, but you may continue to
send retransmissions at an interval of MAX_BRACK_TIMEOUT until you
reach max retries? If so, it might be useful to add something to the
effect of:
"Once the interval between retransmissions reaches MAX_BRACK_TIMEOUT,
the exponential back-off will cease. If the total number of
retransmissions has not yet reached BRIMaxRetriesNumber, the sender
will continue to retransmit at intervals of MAX_BRACK_TIMEOUT it does
so."
I will respond to the rest of the comments separately in order to make
it easier for audience to follow.
Many thanks again for the detailed comments.
Regards,
Ahmad
Thanks!
Ben.
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