Document: draft-ietf-mext-binding-revocation-10
Reviewer: Ben Campbell
Review Date: 25 Aug 2009
IESG Telechat date: 27 Aug 2009

Note: I was assigned to review revision 08 of this draft for the last call ending 28 Aug, as well as this version (10) for the 27 Aug Telechat. This review serves both purposes.

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. (Perhaps it is required by the underlying technology? If so, that should be mentioned here.) 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. It would be helpful if the Security Considerations section discussed the consequences of security failures, possible attacks, etc.



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?


-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?

-S5, second paragraph: "UDP encapsulation to traverse NATs"

Can you include a reference for UDP encapsulation?

-- 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".

-- 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"?

-- Section 7.3:

RFC3775 already talks about retransmission for Binding Update messages. Does this really need to be specified separately?

-- 2nd paragraph: "SHOULD retransmit"

Can you offer guidance on when an implementation might reasonably not do this? (i.e. why not a MUST?)

-- 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...)

-- Last para: "SHOULD NOT"

Why not MUST NOT?

-- 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.)

-- 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?

-- 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.

-- Bullet 3: "mobile node MUST send"

Even if A bit is not set?

-- 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?

-- Bullet 4: "MUST silently discard the Binding Revocation Indication message"

Even if A bit is set?

-- 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.

-- 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?

-- 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?

-- "initial minimum delay: "The default is 1 second but not less than
0.5 seconds"

The default is a range? Or do you mean to say that the  timer value MUST not be set to less than 0.5 seconds?




Nits/editorial comments:

-- General: 

This draft has some significant organization issues that make it harder to read than it needs to be. In particular, the sections that discuss protocol details keep repeating text that is effectively the same for each type of device. It would be much easier to read if you did things like separate out acknowledgement handling, race condition handling, binding identification, retransmissions,  handovers, etc into their own sections, and have the sections on specific device roles only discuss things specific to those devices. You have some of the common bits separated out, but you still repeat them over and over in the role specific sections. Not only is this hard to read, it is prone to error.

As an example, I found that this structure confused my attempts to understand when an acknowledgement is required. Some sections said "if the Acknowledge bit was set", but other sections that did not mention the A bit also required acknowledgement.

The sentence structure tends to be very long and wordy, with very little punctuation. This is aggravated by the fact that you don't make use of the acronyms and device role names once you establish them. For example, spelling out phrases like Binding Revocation Indication and Local Mobility Anchor every time they are used makes for very long sentences.

Please proof read the draft again. I found lots of missing articles and plural/singular mismatches. I report a few of these below, but I gave up on capturing them part way through. The RFC editor will probably fix any of these that get through, but if you fix it yourself, there is less danger of the meaning being changed by edits.

-- S1, paragraph 2:

Please expand MH on first use.

"notify its local mobility anchor peer with a bulk termination"

Does it really notify "with" a bulk termination, or "about" a bulk termination?

-- S3: "describe the protocol overview"

s/describe/present

-- S3.1, paragraph 1:"the Acknowledge (A) bit is set"

The description seems to mix the two elements--the notification and the request for acknowledgement. It might be easier to read and understand if you separated out a  single section on sending BRA when the A bit is set, rather than repeating it for each scenario.

-- Paragraph 3: "On the other hand, the mobile access gateway usually sends a de- registration message by sending a Proxy Binding Update with a lifetime of zero to indicate to the local mobility anchor of the termination of the PMIPv6 mobile node binding registration."

Sentence logic is inverted. Suggest " ... indicate the termination...to the local mobility anchor" This sentence structure repeats throught the document. While the inverted form is not technically wrong, it's difficult to read (for me, anyway).

-- Last Paragraph: "anchor, mobile access"

I suggest " ...anchor, or mobile access gateway..."

-- S3.2, first sentence
s/Binding.../The Binding...

-- Same paragraph, last sentence:

Do you mean "user" or "mobile node"?

-- S3.3, title:

s/"Multi-Care of"/"Multiple Care-of"

-- 3.4, first paragraph: "...where Binding Revocation mechanism is needed..."

s/Binding/"The Binding"

-- S3.4.1, first paragraph, first sentence:

Unclear sentence: What is doing the "hosting"? The LMA, the MAG, or the BRI message? I think you mean the MAG, but the sentence structure is ambiguous.

-- same paragraph: "In this case, the mobile access gateway could send a Router Advertisement message to the mobile node with the home network prefix valid lifetime set to zero."

In order to accomplish what?

-S 7.1, first paragraph: "node, initiator, constructs"

Confusing sentence structure. Is "initiator" a name for the sending mobility node? If so, it would help to use it later (the next paragraph uses the same structure again.)

-- 2nd paragraph: "In the BRI message, the initiator MUST set the Sequence Number field to the next sequence number available for Binding Revocation"

Does this conflict with the section 6.1 statement that it "could be random"

-- Same Paragraph: "Since sending Binding Revocation Indication message is not done on a regular basis, a 16 bit sequence number field is large enough to allow the initiator to match the Binding Revocation Acknowledgement to the outstanding Binding Revocation Indication with Acknowledge (A) bit set using the sequence number field only."

Sentence is hard to follow. I suggest separating the idea that you can match BRAs to BRIs with a 16 bit sequence number from the idea that you only get a BRA if the BRI had it's A bit set.

-- last paragraph: "A mobility entity MUST secure Binding Revocation Indication and Binding Revocation Acknowledgement messages with the same underlying security association, e.g., IPsec SA, that has been used to secure the mobile node binding registration signaling."

You stated this normatively in section 4. Also, that section said "if IPSec is used"--what if it's not?

-- S7.2, paragraph 2: "Since some mobility entities, e.g., local mobility anchor and mobile access gateway, are allowed to receive and possibly send a Binding Revocation Indication or Binding Revocation Acknowledgement for different cases, therefore, if IPsec is used to secure signaling between the local mobility anchor and mobile access gateway, it prevents any of them from processing a Binding Revocation message that was not constructed by an authorized party."

I have trouble parsing this sentence.

-- Paragraph 3: "Upon receiving a packet carrying a Binding Revocation Indication or Binding Revocation Acknowledgement, the receiving mobility entity verifies that the packet was received protected with the security association that has been used during the binding registration signaling phase, e.g., an IPsec SA."

Normative?

-- paragraph 5: "value that NOT supported"

Normative NOT does not make sense here. I think you meant "... set to a value that is not supported."

-- S 8.1, 2nd bullet: "The Revocation Trigger may be used by the mobile node to take further steps if necessary"

I'm not sure what this means. More specification needed?

-- 4th bullet: "unless an Alternate Care-of Address mobility option is included"

In which case you do what?

-- 1st paragraph after bullets: "terminating its IP connection"

What do you mean by terminating an IP connection?

-- 2nd para after bullets:

Please expand BCE on first use.

-- S9.1.1, third bullet: "The Revocation Trigger may be used by the mobile access gateway to learn the mobile node’s latest movement."

I don't understand what you mean here.

-- 7th bullet:

Please expand HNP on first use.

-- last bullet: "unless an Alternate Care-of Address mobility option is included in the Binding Revocation Indication message."

in which case do what?

-- S 9.1.2, last bullet: "SHOULD examine mobility options"

What will it do with them? Is there more guidance that can be offered, or is it entirely a matter "local policy"?

S 9.2.1, first bullet: "Binding Revocation Indication is formatted as in Section 6.1"

Missing word(s)? Did you mean to say "Validate that..."

-- 2nd bullet: "If the Global (G) bit is set and the Revocation Trigger value is "Per-Peer Policy", the Proxy (P) bit MUST be set and the Binding Revocation Indication SHOULD contain the mobile access gateway ID in the MN-ID option."

What if it's not? Also, the language here sounds more like you are talking about the initiator. The responder validates these are true, but does not set the values, correc

-- Bullet 5:

Why not MUST?

-- S 10.1.1, paragraph 1:  "MUST validate the packet according to Section 7.2 and the following"

Much of "the following" involves things well beyond validation.

-- 4th bullet: "which SHOULD include at least the MN-ID option"

What if it doesn't? (and this seems like a strange place for a normative SHOULD, as we are talking about responder behavior not initiator behavior.)

-- 5th bullet: "The mobile access gateway SHOULD validate that the mobile node is no longer attached to the mobile access gateway before sending a successful Binding Revocation Acknowledgement message to the local mobility anchor. However, if the mobile access gateway verified that the mobile node is still directly attached, the mobile access gateway MUST set the status field in the Binding Revocation Acknowledgement to "Revocation failed - MN is Attached"."

These two sentences seem to contradict each other. I think you mean to check if the node is attached, then do one thing if not and another thing if it is.

-- S 10.1.2, title: "Sending Binding Revocation Acknowledgement"

The previous section said a lot about sending BRAs as well.

-- S 11.1, 5th bullet: "with this home address are being revoked"

s/are/as

-- bullet 6: "has multi Care-of Address bindings"

s/multi/multiple

-- same bullet: "consider all of its registered care-of addresses bindings with this home address are being revoked"

s/are/as

-- IANA considerations: "<IANA-TBD>"

It's worth noting to the RFC editor to please replace this with the actual value assigned by IANA.

"Binding Revocation Message"

Can you include a URL pointing to the table that this is to be inserted into?

"new name space"

Where does the new name space go? Can you offer a URL to the registry for it? (applies to all 3 name spaces)

Also, in the various name spaces, do you want a column indicating what RFC or other document specifies a given value?

-- References:

A later version (-15) exists of draft-ietf-netlmm-pmip6-ipv4-support-14