Hi, Marcelo,
I have been selected as the General Area Review Team (Gen-ART)
reviewer for this draft (for background on Gen-ART, please see
http://www.alvestrand.no/ietf/gen/art/gen-art-FAQ.html).
Please resolve these comments along with any other Last Call comments
you may receive.
Document: draft-ietf-shim6-hba-04
Reviewer: Spencer Dawkins
Review Date:
IETF LC End Date: 2007-11-23
Summary: This document is on the right track, but in some places is just not
clear enough for publication yet. I'll call Russ's attention to my comments
about "Perform duplicate address detection if required" and in the security
considerations section (both below). Almost everything else is (nit)s.
Comments:
Abstract
This memo describes a mechanism to provide a secure binding between
the multiple addresses with different prefixes available to a host
within a multihomed site. The main idea is that information about
the multiple prefixes is included within the addresses themselves.
This is achieved by generating the interface identifiers of the
addresses of a host as hashes of the available prefixes and a random
number. Then, the multiple addresses are generated by prepending the
different prefixes to the generated interface identifiers. The
result is a set of addresses, called Hash Based Addresses (HBAs),
that are inherently bound.
Spencer (nit): s/bound/bound to a host/?
3.3. Motivations for the HBA design
Fourth, performance considerations as described in [17] motivated the
usage of a hash based approach as oposed to a public key based
approach based on pure Cryptographic Generated Addresses (CGA), in
order to avoid imposing the performance of public key operations for
every communication in multihomed environments. The HBA appraoch
Spencer (nit): s/appraoch/approach/
presented in this document presents a cheaper alternative that is
attractive to many common usage cases. Note that the HBA approach
and the CGA approaches are not mutually exclusive and that it is
possible to generate addresses that are both CGA and HBA providing
Spencer (nit): "both CGA and HBA" is correct but difficult to parse. Suggest
"that are both valid CGA and HBA addresses" for ease of comprehension.
the benefits of both approaches if needed.
4. Cryptographic Generated Addresses (CGA) compatibility considerations
First, the current Secure Neighbor Discovery specification [3] uses
the CGAs defined in [2] to prove address ownership. If HBAs are not
compatible with CGAs, then nodes using HBAs for multihoming wouldn't
be able to do Secure Neighbor Discovery using the same addresses (at
Spencer (nit): s/Discovery/Discovery (SeND)/
least the parts of SeND that require CGAs). This would imply that
nodes would have to choose between security (from SeND) and fault
tolerance (from shim6). In addition to SeND, there are other
Spencer (nit): shim6 has not been introduced previously in this document -
need at least a reference here, and probably need to spell shim6 out at
least once.
protocols that are considering to benefit from the advantages offered
by the CGA scheme, such as mobility support protocols [13]. Those
protocols would also become incompatible with HBAs if HBAs are not
Spencer (nit): difficult to parse - suggest something like "Those protocols
could not be used with HBAs if HBAs are not compatible with CGAs".
compatible with CGAs.
Even though a CGA compatible approach is adopted, it should be noted
that HBAs and CGAs are different concepts. In particular, the CGA is
inherently bound to a public key, while a HBA is inherently bound to
a prefix set. This means that a public key is not strictly required
Spencer (nit): why "strictly"? the context seems to say "not required", no
adverb needed... unless you mean "a public key is not required to generate
an HBA"?
to generate an HBA. Because of that, we define three different types
of addresses:
- HBA-only addresses: These addresses are bound to a prefix set but
they are not bound to a public key. Because CGA compatibility,
Spencer (nit): I can't parse "Because CGA compatibility". Is the sentence
saying "Because HBAs are compatible with CGA, ..."?
the CGA Parameter Data Structure will be used for their
generation, but a random nonce will be included in the Public Key
field instead of a public key. These addresses can be used for
HBA based multihoming protocols, but they cannot be used for SeND.
6. HBA-Set Generation
1. Multi-Prefix Extension generation. Generate the Multi-Prefix
Extension with the format defined in section 3. Include the
vector of n 64-bit prefixes in the Prefix[1...n] fields. The Ext
Len field value is (n*8 + 4). If a public key is provided, then
the P flag is set. Otherwise, the P flag is reset.
Spencer (nit): "is reset" - I don't actually know what this means. s/is
reset/is cleared/?
6. For i=1 to n do
Spencer (nit) just for clarity, suggest s/n/n (number of prefixes)/?
6.4. Perform duplicate address detection if required. If an
Spencer: not sure why "if required" is here. Can you give explicit guidance
on when duplicate address detection can safely be bypassed? (and I am
concerned about people who take code targeted for environments where DAD can
be bypassed and porting it into environments where DAD is required, but
let's ignore that for now).
address collision is detected, increment the collision count by
one and go back to step (6). However, after three collisions,
stop and report the error.
7.2. Verification that a particular HBA address belongs tto the HBA set
Spencer (nit): s/tto/to/
associated to a given CGA Parameter Data Structure
For multihoming applications, it is also relevant to verify if a
given HBA address belongs to a certain HBA set. An HBA set is
identified by a CGA Parameter Data structure that contains a Multi-
Prefix Extension. So, it is then needed to verify if an HBA belongs
Spencer: "needed to verify" appears twice in two sentences, and I don't
understand what it means. I'm guessing this is saying something like "needed
to verify", but I'm really in the weeds here (I'm not sure who is doing the
verification, either - I can guess, but I can't figure that out from the
text)
to the HBA set defined by a CGA Parameter Data Structure. It should
be noted that it may be needed to verify if an HBA belongs to the HBA
set defined by the CGA Parameter Data Structure of another HBA of the
set. If this is the case, the CGA verification process as defined in
[2] will fail, because the prefix included in the Subnet Prefix field
of the CGA Parameter Data Structure will not match with the one of
the HBA that is being verified. However, this doesn't mean that this
HBA does not belong to the HBA set. In order to address this issue,
Spencer: the last few sentences are difficult to parse, at least for me. I
think the text is saying "HBAs will fail to pass the CGA verification
process defined in [2], because the prefix included in the Subnet Prefix
field of the CGA Parameter Data Structure will not match the prefix of the
HBA that is being verified. To verify if an HBA belongs to an HBA set
associated with another HBA, verify that the HBA prefix is included in the
prefix set defined in the Multi-Prefix Extension, and if this is the case,
then substitute the prefix included in the Subnet Prefix field by the prefix
of the HBA, and then perform the CGA verification process defined in [2]".
If this isn't what you meant, that's a problem, too :-)
2.2. Check that the subnet prefix in the CGA Parameters Data
Structure is equal to the subnet prefix (i.e., the leftmost 64
bits) of the address. The CGA verification fails if the prefix
values differ. [Note: This step is trivially successful
because step 1]
Spencer (nit): s/is trivially successful because/always succeeds because of
the action taken in/
8. Example of HBA application to a multihoming scenario
Host2 in not located in a multihomed site, so there is no need for it
Spencer (nit): s/in/is/
to create HBAs (it must be able to verify them though, in order to
support the shim6 protocol, as we will describe next.)
Eventually, the communication will end and the associated shim6
context information will be discarded.
Spencer: this sentence probably doesn't belong in this spec (it's not about
HBAs, it's about shim6 context management), but if it stays here, it should
probably point to a specific section of a SHIM6 protocol specification...
8.1. Dynamic Address Set Support
In order to verify this new address, CGA capabilities of PA:LA:iidA
are used. Note that the same address is used, only that the
verification mechanism is different. So, if Host1 wants to use PC:
LC:addC to exchange packets in the established communication, it will
use message of the shim6 protocol, conveying the new address, PC:LC:
Spencer: is this "use the Update message of the shim6 protocol"? If so, a
pointer to section 5.10 of the protocol draft would be helpful.
addC, and this message will be signed using the private key
corresponding to the public key contained in CGA_PDS_A. When Host2
receives the message, it will verify the signature using the public
key contained in the CGA Parameter Data Structure associated with the
address used for establishing the communication i.e. CGA_PDS_A and
PA:LA:iidA respectively. Once that the signature is verified, the
new address (PC:LC:addC) can be used in the communication.
11.3. Interaction with IPSec.
In the case that both IPSec and CGA/HBA address are used
simultaneously, it is possible that two public keys are available in
a node, one for IPSec and another one for the CGA/HBA operation. In
this case, an improved security can be achieved by verifying that the
keys are related somehow, (in particular if the same key is used for
Spencer: this is over my head, but did SECDIR go for recommended key sharing
here? If they're good, I'm good...
both purposes). The actual verification procedure is outside the
scope of this specification.
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