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Re: [HOKEY] WGLC: draft-ietf-hokey-preauth-ps-02
Yoshi,
Thanks for addressing the comments. Few answers inline
regards,
_Subir
>
>
>>
>> This mechanism allows mobile devices to pre-
>> authenticate using EAP to one or more candidate authenticators over
>> the wired medium, by way of the serving authenticator. IEEE 802.11r
>> [802.11r] defines Fast BSS transition mechanisms involving a
>> definition of key management hierarchy and setup of session keys
>> before the re-association to the target AP. These mechanisms, as
>> indicated before, are defined for IEEE 802.11 technologies and are
>> only applicable within a certain access domain and fall short when it
>> comes to inter-access technology handovers. They also require L2
>> (e.g., Ethernet) connectivity for transfer of encapsulated signaling
>>
>> SD> What ? encapsulating signaling??
>>
>
> We can revise the sentence to:
>
> "They also require L2 (e.g., Ethernet) connectivity for transfer of key
> management signaling ..."
>
SD> Much better.
>
>
>> Such use of EAP pre-authentication would enable the
>> mobile device to authenticate and setup keys prior to connecting to
>> one of the candidate authenticators.
>>
>> This framework has general applicability to various deployment
>> scenarios in which proactive signaling can take effect. In other
>> words, applicability of EAP pre-authentication is limited to the
>> scenarios where candidate authenticators can be easily discovered, an
>> accurate prediction of movement can be easily made.
>>
>> Also the
>> effectiveness of EAP pre-authentication may be less significant for
>> particular inter-technology handover scenarios where simultaneous use
>> of multiple technologies is not a major concern or where there is
>> sufficient radio-coverage overlap among different technologies.
>>
>> SD> The above paragraph is not very clear. Is it the case of using multiple radio interfaces? If the overlapping radio coverage is not there how the seamless handover would be possible? Does EAP pre-authentication assume that it is only applicable where overlapping coverage is not available?
>>
>
> I think we should remove "or where there is sufficient radio-coverage
> overlap among different technologies" as some overlapping coverage
> would be still needed for seamless handover.
>
SD> Good.
>> A mobile node is attached to the serving access network. Before the
>> mobile node performs handover from the serving access network to a
>> candidate access network, it performs EAP pre-authentication with a
>> candidate authenticator, an authenticator in the candidate access
>> network, via the serving access network. The mobile node may perform
>> EAP pre-authentication with one or more candidate authenticators. It
>> is assumed that each authenticator has an IP address when
>> authenticators are on different IP links.
>>
>> SD> Does it mean that authenticators will not have an IP address when they are on the same IP link. Does it mean to say that authenticators have separate IP address when they are on different IP links?
>>
>
> It means that each authenticator has an IP address. We will revise
> the sentence accordingly.
>
SD> Good.
>
>> It is assumed that there
>> is at least one candidate authenticator in each candidate access
>> network while the serving access network may or may not have a
>> serving authenticator.
>>
>> SD> Is it true that serving network may not have a serving authenticator?
>> If it is true, how come in the first case it enters into the network? Is serving network is an open network? If so, is it a valid handover scenario and is EAP pre-authentication applicable there? Also use case scenario does not cover it.
>>
>
> The serving network could be an open access L2 network and may be
> using http authentication. Also there may be no roaming agreement
> between the serving and target networks. EAP pre-authentication is
> applicable to such scenarios.
>
SD> This scenario is possible and the target network may require EAP
authentication and can be done
via pre-authentication but I was thinking how much it is relevant in
terms of seamless handover.
For example, if the two networks does not have roaming agreement, can a
seamless handover
possible in real case? Also if it is a managed network, is it not fair
to assume that both serving and
target networks will have EAP authentication? Just a point to think.
>
>>
>> 4.1. Direct Pre-authentication
>>
>> Direct pre-authentication signaling is shown in Figure 2.
>>
>> Mobile Serving Candidate AAA
>> Node Authenticator Authenticator Server
>> (MN) (SA) (CA)
>> | | | |
>> | | | |
>> | MN-CA Signaling (L3) | AAA |
>> |<------------------+------------------->|<----------------->|
>> | | | |
>> | | | |
>>
>> Figure 2: Direct Pre-authentication
>>
>> SD> Does the MN-CA Signaling always needs to happen via L3? It may be worth mentioning the underlying assumption here.
>>
>
> The assumption is that there is no direct L2 connectivity between MN
> and CA. We can add the assumption.
>
SD> Good.
>
>> 4.2. Indirect Pre-authentication
>>
>> Indirect pre-authentication signaling is shown in Figure 3.
>>
>> Mobile Serving Candidate AAA
>> Node Authenticator Authenticator Server
>> (MN) (SA) (CA)
>> | | | |
>> | | | |
>> | MN-SA Signaling | SA-CA Signaling | AAA |
>> | (L2 or L3) | (L3) | |
>> |<----------------->|<------------------>|<----------------->|
>> | | | |
>> | | | |
>>
>> Figure 3: Indirect Pre-authentication
>>
>> With indirect pre-authentication, the serving authenticator is
>> involved in EAP pre-authentication signaling. Indirect pre-
>> authentication is needed if the MN cannot discover the CA's IP
>> address or if IP communication is not allowed between the candidate
>> authenticator and unauthorized nodes for security reasons.
>>
>> SD> IP communication may not even be possible even if mobile node is authorized, say, because of topology.
>>
>
> We can add the topology reason here.
>
SD> Good.
>
>> 5. Architectural Considerations
>>
>> There are two architectural issues relating to pre-authentication,
>> i.e., authenticator discovery and context binding.
>>
>> SD> Is context binding an architectural issue or a general issue? Location of target authenticator may have some architectural implications. How about transport?
>>
>
> The context binding issue is specific to L3 pre-authentication, and
> not a general issue. Transport (direct or indirect) does not have
> architectural implications in terms of context binding.
>
SD> I did not mean to say that transport has architectural relationship
with context binding. May be I
should have been a little bit clear. Three questions: i) If context
binding is a general issue, does it fit under
architectural issue section? ii) Does transport of pre-auth parameters
has any architectural impacts?, and
iii) Does location of target authenticator has any architectural impacts?
>
>
>> When pre-authentication is used for inter-
>> technology or inter-subnet handover, a candidate authenticator needs
>> to have a global IP address and a mechanism for discovering the
>> candidate authenticators IP address is needed.
>>
>> SD> Does it always require to have a global IP address even if serving and candidate authenticators are within one operator?s domain
>>
>
> A global address may not be needed for intra-domain handover. We can
> revise the text to:
>
> "
> When pre-authentication is used for inter-technology or inter-subnet
> handover, a candidate authenticator needs to have an IP address and a
> mechanism for discovering the candidate authenticators IP address is
> needed. For inter-domain handover, the IP address of a candidate
> authenticator needs to be global unique.
>
SD> May we should say "For both intra-domain and inter-domain handover,
the IP address of a candidate
authenticator must be reachable by the mobile node that is performing
the pre-authntication " something
like that...
> "
>
>
>> For example, IEEE
>> 802.21 Information Service (IS) [802.21] provides a link-layer
>> independent mechanism for obtaining neighboring network information
>> by defining a set of Information Elements (IEs), where one of the IEs
>> is defined to contain an IP address of a point of attachment. IEEE
>> 802.21 IS queries for such an IE may be used as a method for
>> authenticator discovery.
>>
>>
>> An authenticator discovery mechanism requires a database on the
>> neighboring network information. Provisioning of a server with such
>> a database is another issue.
>>
>> SD> The above is a solution and does it require to have in the problem statement document?
>>
>
> The above paragraph was added based on Ajay Rajkumar's review. I
> think this is a good to have, while I agree with you that it is not
> mandatory to have in the problem statement document.
>
SD> Ok. May be we can say in preamble, if 802.21-based or similar
mechanism is used.....
>> 5.2. Context Binding
>>
>> When a candidate authenticator uses different EAP transport protocols
>> for normal authentication and pre-authentication, a mechanisms is
>> needed to bind link-layer independent context carried over pre-
>> authentication signaling to the link-layer specific context of the
>> link to be established between the mobile node and the candidate
>> authenticator.
>>
>> SD> Is it true that this context binding needs to happen after the handover?
>> If so it may be appropriate to reflect that in the last sentence of the above paragraph.
>>
>
> Context binding can happen before handover.
>
SD> Does this mean it can happen after handover too. If so, may be it
should clarify both options.
>
>> The link-layer independent context includes the
>> identities of the peer and authenticator as well as the MSK. The
>> link-layer specific context includes link-layer addresses of the
>> mobile node and the candidate authenticator.
>>
>> There are two possible approaches to address the context binding
>> issue. The first approach is based on communicating the lower-layer
>> context as opaque data via pre-authentication signaling and perform
>> the link-layer specific secure association procedure after handover.
>> The second approach is based on running EAP over the link-layer of
>> the candidate authenticator after handover using short-term
>> credentials generated via pre-authentication, followed by the link-
>> layer specific secure association procedure. In this case, the
>> short-term credentials are shared between the mobile node and the
>> candidate authenticator, and hence the EAP server for the post-
>> handover EAP resides in the candidate authenticator.
>>
>> SD> Last part of the above sentence is not clear.
>>
>
> The last part means that post-handover EAP terminates between MN and
> CA, and does not require AAA signaling.
>
SD> Does this mean the target network does not have a AAA server? If
that is the case, it may be good
to clarify it.
>
>> In both
>> approaches, the binding needs to be securely made between the peer
>> and the candidate authenticator using a security association
>> established via pre-authentication.
>>
>>
>> 6. AAA Issues
>>
>> Most of the AAA documentations today do not distinguish between a
>> full authentication and a pre-authentication and this creates a set
>> of open issues:
>>
>> Pre-authentication authorization: Many users may not be allowed to
>> have more than one logon session at the time. This means, when
>> such users actively engage in an active session (as a result of a
>> previously valid authentication), they will not be able to perform
>> pre-authentication. The AAA server currently has no way of
>> distinguishing between a full authentication request and a pre-
>> authentication request.
>>
>>
>> SD> The assumption here seems to me that mobile node has performed the network access authentication in the first place. Therefore, the earlier statement saying that serving network may or may not have an authenticator is not correct. Pl. refer to earlier comment on this as well.
>>
>
> I don't think this contradicts with the earlier statement. If there
> is no authenticator in the serving network, then the mobile node will
> be able to perform pre-authentication to the target network even if
> the AAA server does not allow more than one logon session. In other
> words, there is no logon session before pre-authentication in this
> case.
>
SD> Does that mean if the serving network does not have an authenticator
that interacts with AAA,
multiple logons is a non-issue? If that is the case and we allow such a
scenario (e..g, serving network
does not have an authenticator), this AAA issue is not a general issue.
>
>
>> Completion of network attachment: Once the peer has successfully
>> attached to the new point of attachment, it needs to convert its
>> authentication state from pre-authenticated to fully attached and
>> authorized. There may need to be a mechanism within the AAA
>> protocol to provide this indication to the AAA server.
>>
>> Session Resumption: In case the peer ping pongs between a network
>> N1 with which it has a full authentication state to another
>> network N2 and then back to N1, it should be possible to simply
>> convert the full authentication state to a pre-authenticated
>> state. The problems around handling session life time and keying
>> material caching needs to be dealt with.
>>
>> Multiple candidate authenticators: There may be situations where
>> the mobile node may need to make a selection between a number of
>> candidate attachment points. In such cases, it is desirable for
>> the mobile to perform pre-authentication with multiple
>> authenticators. In such cases the AAA server may need to be aware
>> of the situation.
>>
>> Roaming support: In case the pre-authentication is being performed
>> through a serving network that is foreign to the MN's home AAA
>> server, the AAA server needs to obtain the information about the
>> serving network in addition to the information about the candidate
>> network, so that the AAA server can make authorization decisions
>>
>> SD> Is not correct that this information will be available to the home AAA when MN first attaches a foreign network in the first place?
>>
>
> This is a good point. I think you are right. Perhaps we can remove
> this bullet.
>
SD> Yes, that would be better.
>
>> accordingly, e.g., depending on the authorization policy, the home
>> AAA server may not allow pre-authentication via a particular
>> serving network.
>>
>> Inter-technology support: Current specifications on pre-
>> authentication mostly deal with homogeneous 802.11 networks. The
>> AAA attributes such as Calling-Station-ID [I-D.aboba-radext-wlan]
>> may need to be expanded to cover other access technologies.
>> Furthermore, heterogeneous handovers may require a change of the
>> MN identifier as part of the handover. Investigation on the best
>> type of identifiers for MNs that support multiple access
>> technologies is required.
>>
>> Network controlled handovers: It is becoming quite common for the
>> network operators to maintain the control over the handovers for
>> various reasons including load balancing and performance. Hence
>> the network may need to direct the MN to perform pre-
>> authentication to a set of candidate authenticators in an
>> anticipation for a handover. The AAA protocol extensions for
>> carrying out such procedures need to be provided.
>>
>> SD> Not clear how the above is related to AAA protocol extensions. This could be handover policy and may be obtained via different means.
>>
>
> I agree that network controlled handover is more general than
> pre-authentication. We can remove this bullet.
>
SD> Good.
>
>> 7. Security Considerations
>>
>> Since pre-authentication described in this document needs to work
>> across multiple authenticators, any solution for this problem needs
>> considerations on the following security threats.
>>
>> First, a possible resource consumption denial of service attack where
>> an attacker that is not on the same IP link as the mobile node or the
>> candidate authenticator may send unprotected pre-authentication
>> messages to the mobile node or the candidate authenticator to let the
>> legitimate mobile node and candidate authenticator spend their
>> computational and bandwidth resources. This attack is possible for
>> both direct and indirect pre-authentication scenarios. To mitigate
>> this attack, the candidate network or authenticator should apply non-
>> cryptograhpic packet filtering so that pre-authentication messages
>> received from only a specific set of serving networks or
>> authenticators are processed. In addition, a simple solution for the
>> peer side would be to let the peer always initiate EAP pre-
>> authentication and not allow EAP pre-authentication initiation from
>> authenticator side.
>>
>> SD> Again the last part provides a solution not an issue?
>>
>
> This part is describing a guidance (solution) to mitigate an attack.
> As far as I understand, Security Considerations section can contain a
> guidance.
>
SD> Ok.
>
>> Second, consideration for the Channel Binding problem described in
>> [I-D.ietf-eap-keying] is needed as lack of Channel Binding may enable
>> an authenticator to impersonate another authenticator or communicate
>> incorrect information via out-of-band mechanisms (such as via a AAA
>> or lower layer protocol) [RFC3748]. It should be noted that, when
>> normal authentication uses link-layer EAP transport,
>>
>> SD> The statement ?when normal authentication uses link-layer EAP transport,?
>> may be redundant.
>>
>
> I agree, we can remove the statement.
>
SD> Good
>
>> it would be
>> easier to launch such an impersonation attack for pre-authentication
>> than normal authentication because an attacker does not need to be
>> physically on the same link as the legitimate peer to send a pre-
>> authentication trigger to the peer.
>>
>>
>>
>
> Thanks,
> Yoshihiro Ohba
>
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