Re: [Gen-art] Gen-ART Last Call review of draft-ietf-pcp-authentication-11.txt

["Tirumaleswar Reddy (tireddy)" <[e-mail]>]

Hi Paul,

Thanks for the detailed review. Please see inline

> -----Original Message-----
> From: Paul Kyzivat [mailto:[e-mail]]
> Sent: Saturday, June 27, 2015 2:32 AM
> To: [e-mail]
> Cc: General Area Review Team
> Subject: Gen-ART Last Call review of draft-ietf-pcp-authentication-11.txt
> 
> I am the assigned Gen-ART reviewer for this draft. For background on Gen-ART,
> please see the FAQ at
> <http://wiki.tools.ietf.org/area/gen/trac/wiki/GenArtfaq>.
> 
> Please resolve these comments along with any other comments you may
> receive.
> 
> This draft is on the right track but has open issues, described in this review.
> 
> For the most part my issues are with confusing and/or ambiguous language and
> under-specification. This means some clued in implementers will be able to
> create functional, interoperable, implementations. But others, working just
> from the specification, may build arguably conforming implementations that
> fail to interoperate.
> 
> I've opted not to split major and minor issues out separately, because I thought
> it better to put things in document order. But please take special note of my
> comments on sections 3.1.1 and 3.1.2 that highlight what I believe to be a true
> protocol errors. I did note those points I consider nits.
> 
> Disclaimer: I do not claim to be a security expert. The authors are vastly more
> qualified than I on security matters. So I have not tried to discern if there are
> technical security holes in this specification.
> 
> I’ll start with some general comments, and then follow with specific comments
> on particular parts of the document.
> 
> 	Thanks,
> 	Paul Kyzivat
> 
> * General Comments:
> 
> I assume that a reader of this document is reasonably familiar with the RFCs
> that specify PCP and EAP. (When I began this review, I was not, so I read them
> before starting this review.) I apologize if I have made technical mistakes in my
> understanding of those protocols.
> 
> There is a fundamental inconsistency of approach between PCP and EAP.
> Both are client/server protocols, but for most of the expected usage a PCP
> client will be the EAP server and visa versa. So this draft works hard to reconcile
> this difference. I *think* it has, for the most part, done so *technically*, but not
> so well with the terminology and language. For example, while reading the
> draft it was a constant struggle to understand whether a request is a PCP
> request containing response pertaining to EAP, or a request pertaining to EAP
> contained in a PCP response.
> 
> I have another concern about the big picture of the protocol. There is much
> material about message level details, but not much about how they all play
> together. Some things I had trouble with:
> 
> - Once a PA session is initiated, may common PCP messages that are not part of
> that session continue to be exchanged ?

No, otherwise PCP messages will not get integrity protection.

> 
> - Is it permissible to have more than one concurrent PA session active between
> the same PCP client and server? 

Yes, it is possible but typically PCP authentication only happens once for a host and multiple applications on the host will use same SA.

> These might be multiple sessions created with
> the same credentials, or using different credentials. (E.g.
> to support both a "regular" and "superuser" level of privilege.)

No, typically single session will created for a PCP client. PCP client will be an endpoint and will not have different credentials in the same administrative domain.

> 
> - If multiple sessions are permitted, how to avoid accumulating many active
> sessions ?

PCP server rejects session request from the client, error codes like (USER_EX_QUOTA) are discussed in RFC 6887.

> 
> To address this, I would hope to see some state models and some example
> message sequence diagrams that show how this plays out in practice.
> 
> (Consider: PCP client starts without credentials. At some point it tries a request
> that requires authentication. It does so using "normal"
> credentials. Later, within that PA Session it tries a request that requires
> "superuser" authentication. How would this be indicated? How would the new
> authentication take place? How would the client know what credentials to use?
> How to revert back to "normal" privilege?)

PCP client will typically not be using multiple credentials with the PCP server. PCP server which has PCP authentication enabled will permit MAP/PEER only after authentication is successful.

> 
> Another thing: it appears to me that this document makes a change to the PCP
> protocol that was not anticipated as an extension point in RFC6887.
> Specifically, it uses part of the "Reserved" portion of the PCP request message.
> There should be no technical problem with doing this, but I think this means
> that this document should be marked as updating RFC6887. Ideally 6887 would
> be revised, splitting off the last 8 bits of the Reserved field as a separate field
> that is available for opcode-specific use. That would eliminate the possibility
> that some future revision of 6687 would assign that field for some other use.

This document uses the reserved portion only for the Authentication opcode and will not impact existing opcodes or future opcodes.

> 
> Regarding lifetime: All PCP messages carry a lifetime field. But this seems to
> differ from the "session lifetime" that is passed in an option.
> I find *no* mention of the PCP lifetime in this document. IMO there should be
> *some* statement of what to do with that. 
> ISTM that there are two distinct but
> similar things here:
> 
> - The lifetime of a PA session (validity of the session ID). Presumably it has to
> become valid upon the first PA message from the PA-Client, and it remains
> valid through initial authentication (or until failure of
> that) and through any and all re-authentications. The PA-Server can’t send PA
> messages to the PA-Client without having this session. So it needs to know how
> long it will remain valid. And this lifetime should be bounded so that an
> authentication attempt that never completed doesn’t leave a session forever.
> This is quite analogous to the lifetime of a mapping, and could conceivably be
> managed the same way, using the lifetime field in the PCP messages.
> 
> - The lifetime of a PA SA and associated keys. Presumably this begins at the
> successful completion of the initial EAP negotiation, and is revised upon a
> successful re-authentication. This is needed for authenticating Common PCP
> messages within the PA session.

This document is only discussing lifetime of a PA SA and PCP lifetime discussed in https://tools.ietf.org/html/rfc6887#section-7.2 is the lifetime of the mapping created on PCP-aware firewall and NAT devices using MAP or PEER opcodes. 

Added the following lines to make it clear:
The Requested Lifetime field of PA-Client message and Lifetime field of PA-Server message are both set to 0 on transmission and ignored on reception.

> 
> Currently these two seem to be conflated. Even if you are happy bundling these
> into one concept then still *something* should be said about how the PCP
> lifetime field is used.
> 
> * Section 2:
> 
> Re PCP-Authentication (PA) message:
> 
>     PCP-Authentication (PA) message: A PCP message containing an
>     Authentication Opcode.  Particularly, a PA message sent from a PCP
>     server to a PCP client is referred to as a PA-Server, while a PA
>     message sent from a PCP client to a PCP server is referred to as a
>     PA-Client.  Therefore, a PA-Server is actually a PCP response message
>     specified in [RFC6887], and a PA-Client is a PCP request message.
> 
> This terminology seems odd. It seems to say that the term "PA-Server"
> describes a *message*. But a name like that should seemingly describe a type
> of server, not a message. I suggest not doing this – instead, use "PA-Client
> message" and "PA-Server message" when referring to such messages. Instead,
> define and use "PA-Client" and "PA-Server" 
> analogously to PCP Client and PCP Server.

Updated.

> 
> (Could there be cases where the PCP Client also needs to authenticate the PCP
> Server, and the EAP method used doesn’t support mutual authentication? I
> don’t think the existing text contemplates that possibility, even though
> RFC3748 does. Supporting that would require a lot of changes.)

No, the mandatory to implement EAP method (EAP-TTLS) discussed in the draft does support mutual authentication.

> 
> Re Common PCP message:
> 
>     Common PCP message: A PCP message which does not contain an
>     Authentication Opcode.  This document specifies an Authentication Tag
>     Option to provide integrity protection and message origin
>     authentication for the common PCP messages.
> 
> It isn’t clear to me if this term is intended to cover all PCP messages not
> containing the Authentication opcode, or only messages that contain the
> Authentication Tag Option. The usages in the document seem to make different
> assumptions about this. I’ve assumed that it covers messages both with and
> without that option unless explicitly qualified. It would be helpful to have
> distinct terms for those with and without.

All PCP messages without Authentication opcode will contain the Authentication Tag Option.

> 
> PCP device: This term is not defined in this terminology section, or anywhere
> else I can find, in this document or in RFC6887. But it is used frequently. IIUC it
> means "PCP Client or PCP Server" and is closely related to Session Partner.
> There ought to be a definition of it, or if appropriate, usages could be replaced
> with Session Partner.

Updated terminology.

> 
> * Section 3.1:
> 
>     ... Each PA message is attached with an Authentication Opcode ...
> 
> This language is awkward/confusing. I *think* you mean:
> 
>     ... Each PA message MUST contain an Authentication Opcode

Updated.

> 
> (Actually, all occurrences of "attached with" deserve a look.)
> 
> Then the following:
> 
>     ... The Authentication Opcode consists of two fields: Session ID and
>     Sequence Number. ...
> 
> Again this language seems odd: a PCP opcode doesn’t contain fields like this,
> though it may be *accompanied* by such fields. After reading ahead, I guess
> you mean:
> 
>     ... The opcode-specific information of each PA message contains
>     two fields: Session ID and Sequence Number. ...".

Changed.

> 
> * Section 3.1.1:
> 
> In the following:
> 
>     When a PCP client intends to proactively initiate a PA session with a
>     PCP server, it sends a PA-Initiation message (a PA-Client message
>     with the result code "INITIATION") to the PCP server.
> 
> It seems odd to have a "result code" in a *request*. (I would expect it to be in a
> *response*.) It does make *some* sense when what is being sent is a response
> to a prior EAP request, but that is not the case here. Here it is serving as a sub-
> opcode.
> 
> ISTM that using the PCP result code in PCP requests is confusing, in addition to
> being contrary to the definition of the PCP request message.
> I suggest refining the terminology and language around this. E.g., define
> appropriate new names for these fields and alias them to the PCP fields.
> 
> Then:
> 
>     ... From now on, every PCP message within this session
>     will be attached with this session identifier. ...
> 
> Again the language usage is odd, and apparently non-normative. And what
> does "within this session" mean? This document defines PA Session, so it could
> mean that. Or it could mean PCP Session – except that AFAICT PCP has no
> notion of a PCP session.
> 
> If it means PA Session, then the statement is odd because it is a truism. If that
> is the intent, then I suggest rewording it as:
> 
>     ... Subsequent PCP messages to be included in this PA Session MUST contain
> this session identifier. ...

Yes, updated.

> 
> If this is intended to mean that once a PA Session is established then
> *all* messages must be within it, then more substantial document changes are
> needed.
> 
> Then:
> 
>     ... If the PCP client intends to simplify the
>     authentication process, it MAY append an EAP Identity Response
>     message within the PA-Initiation message so as to inform the PCP
>     server that it would like to perform EAP authentication and skip the
>     step of waiting for the EAP Identity Request.
> 
> I don’t understand how this can work. IIUC the intent is to tunnel EAP within
> PCP, not to change EAP. AFAICT EAP has no provision for sending an EAP
> response message without having first received a corresponding request. The
> response message will have to contain an EAP Identifier, which is normally
> assigned by the Authenticator. In this case that wouldn’t be available, so the
> PCP client would have to make up a value.
> The authenticator won’t be expecting a response for this identifier. (Or worse,
> it may have just happened to send a request using this identifier.)
> 
> If this behavior is desired, then I think an extension to EAP is needed to allow
> an EAP peer to preemptively provide identity. (I apologize if I have
> misunderstood EAP on this point.)

Yes, it is mentioned as "MAY" in case if EAP supports this mechanism in future.

> 
> Then, regarding the message sequence diagram:
> 
> (nit): the message sequence diagram uses "EAP request" and "EAP response"
> while the text uses "EAP identity request" and "EAP identity response".
> Consistent terminology should be used.

Updated.

> 
> * Section 3.1.2:
> 
> I have been studying the message sequence diagram in this section along with
> RFC6887. As best I can understand, this is not a valid sequence.
> 
> I see the PCP client sending a Common PCP request, but I see no matching PCP
> response message for that. (This should trigger retransmission of the request.)
> Instead, I see the PCP server sending a PA-Server message.
> That will have a different opcode, and so can’t be considered a response to the
> Common PCP message. If the PCP client has not previously sent a PA message
> (the case here) it is supposed to ignore this message.

No, PCP allows server to send unsolicited response. For example refer to the usage of ANNOUNCE explained in
https://tools.ietf.org/html/rfc6887#section-14.1.3

> 
> I was *expecting* to see the PCP server send a PCP response to the Common
> PCP message, with result AUTHENTICATION-REQUIRED. Then, the PCP client to
> send a PA-Client message to initiate the PA session. That could be exactly like
> the sequence in 3.1.1, or it might be optimized somehow.
> 
> (I defer commenting on the text in this section pending clarification of these
> issues.)
> 
> * Section 3.1.3:
> 
> IIUC, PCP requires that the PCP client send a PA-client message before the PCP
> server can send any PA-server messages. Once that has been done the PCP
> server is allowed to send an arbitrary number of "responses" to that, enabling
> the exchange of EAP messages. The server messages must "match" the client
> message by opcode and opcode-specific data according to opcode-specific
> rules. (I think the intent for PA messages is that they must match on the session
> ID.)
> 
> If I’m right about this, it would be good to explain that before the content of
> this section. (It took me a lot of reading before I figured this out.)
> 
> Then:
> 
>     ... If a PCP device receives a PA message from its partner and
>     cannot generate an EAP response immediately due to certain reasons
>     (e.g., waiting for human input to construct a EAP message or waiting
>     for the additional PA messages in order to construct a complete EAP
>     message), the PCP device MUST reply with a PA-Acknowledgement message
>     (PA message with a Received Packet Option) ...
> 
> The use of "PCP device" suggests this is intended to apply to both the PCP client
> and server. But the use of "generate an EAP response" implies that this must be
> the PA-Client, since only it sends EAP responses. And that is consistent with
> sending a PA-Acknowledgement (request) message.
> If I’m right about this, then perhaps the following would be a clearer wording
> of the above:
> 
>     ... If a PA-Client receives a PA message containing an EAP request
>     and cannot generate an EAP response immediately due to certain
>     reasons (e.g., waiting for human input to construct a EAP message
>     or waiting for the additional PA messages in order to construct a
>     complete EAP message), the PA-Client MUST send a PA-Acknowledgement
>     message (PA message with a Received Packet Option) ...

Updated.

> 
> Then:
> 
>     In this approach, it is mandated for a PCP client and a PCP server to
>     perform a key-generating EAP method in authentication.  Particularly,
>     a PCP authentication implementation MUST support EAP-TTLS [RFC5281]
>     and SHOULD support TEAP [RFC7170].  Therefore, after a successful
>     authentication procedure, a Master Session Key (MSK) will be
>     generated.  If the PCP client and the PCP server want to generate a
>     transport key using the MSK, they need to agree upon a Pseudo-Random
>     Function (PRF) for the transport key derivation and a MAC algorithm
>     to provide data origin authentication for subsequent PCP messages.
>     In order to do this, the PCP server needs to append a set of PRF
>     Options and MAC Algorithm Options to the initial PA-Server message.
>     Each PRF Option contains a PRF that the PCP server supports, and each
>     MAC Algorithm Option contains a MAC (Message Authentication Code)
>     algorithm that the PCP server supports.  Moreover, in the first PA-
>     Server message, the server MAY also attach an ID Indicator Option
>     defined in Section 5.11 to direct the client to choose correct
>     credentials.  After receiving the options, the PCP client selects the
>     PRF and the MAC algorithm which it would like to use, and then adds
>     the associated PRF and MAC Algorithm Options to the next PA-Client
>     message.
> 
> In the above there appears to be quite a bit of normative behavior that has no
> 2119 language. (E.g., "it is mandated", "will be generated", "the PCP client
> selects ... then adds".) IMO this should be tightened up with
> 2119 language.

Thanks, fixed.

> 
> The last two paragraphs of this section explain, in part, the use of messages
> with result codes AUTHENTICATION-SUCCEEDED, AUTHENTICATION-FAILED,
> and DOWGRADE-ATTACK-DETECTED. Section 3.1.1 explains the use of
> AUTHENTICATION-REQUIRED. Elsewhere is explanation of SESSION-
> TERMINATED. Of those, it appears that only AUTHENTICATION-REQUIRED is
> used with messages carrying EAP messages, and then I only see mention of it
> being used with the *first* EAP message.
> Is AUTHENTICATION-REQUIRED to be used with all PA messages that contain
> EAP messages? 

Yes.
NEW:
The result code for PA-Sever message carrying EAP Identity request will be set to AUTHENTICATION_REQUIRED.

> If so, that should be specified. If not, then what should be used
> with the others?
> 
> Those two paragraphs also say to "terminate the session" in some error cases.
> Then the following section 3.2 talks about Session Termination. I
> *guess* that in the error cases you mean that local session state is dropped
> without sending a SESSION-TERMINATED request. It would be good to be clear
> about that.
> 
> * Section 3.2:
> 
> This says that upon receiving a SESSION-TERMINATED from a partner one must
> also send a SESSION-TERMINATED. Why? That could lead to an infinite
> exchange of messages. Also, once the PA SA is removed, any subsequently
> received PA message won’t match any session and so will presumably be
> dropped. More clarity here would help.

It was added because PCP messages are exchanged over UDP and the PCP device needs to know if it's partner has received the termination-indicating PA message or not.

> 
> * Section 3.3:
> 
>     A session partner may select to perform EAP re-authentication if it
>     would like to update the PCP SA without initiating a new PA session.
>     A re-authentication procedure could be triggered for the following
>     reasons:
> 
>     o  The session lifetime needs to be extended.
> 
>     o  The sequence number is going to reach the maximum value.
>        Specifically, when the sequence number reaches 2**32 - 2**16, the
>        session partner MUST trigger re-authentication.
> 
> (nit): IMO this would read better if you replace "select" with "elect".
> 
> Also, it is unclear (to me) whether the stated reasons are intended to be
> exhaustive, or are simply examples. I *guess* that these are examples, and that
> re-authentication can also be requested at whim.
> Would be good to be more explicit about this.

Okay, mentioned those are only examples.

> 
> Also, what happens if re-authentication is not attempted and the session
> lifetime is reached? Is there a requirement to silently terminate the session ?

Yes, server must attempt re-authentication. This is to handle the case where PCP client could have left the network without terminating the session. In this case re-authentication fails and server has to silently terminate the session.

> 
> Do the PA messages containing RE-AUTHENTICATION contain EAP messages ?

No.

> Or do they trigger sending EAP messages within PA messages having some
> other reason code ?

No, otherwise it will complicate the mechanism.

> 
> What if "glare" occurs? (Both client and server decide to send RE-
> AUTHENTICATION at the same time ?)

I guess then one of the partners can back-off (preferably PCP client) and let PCP server  proceed with re-authentication. 

> 
> * Section 4:
> 
>     At the beginning of a PA session, a session SHOULD generate a PA SA
>     to maintain its state information during the session.  The parameters
>     of a PA SA are listed as follows:
> 
> Why SHOULD rather than MUST? How can any of this work if the SHOULD is
> violated? ISTM the only case would be if the PA-Server refuses the initial PA
> request.

Updated to MUST.

> 
> And I'll restate something I mentioned above: is it permissible for a PCP session
> to have more than one concurrently active PA sessions? If so, each will have its
> own state.
> 
> Included in the listed state are *four* sequence numbers. But there is little
> mention of these elsewhere in the document. Many places simply say things
> like: "A sequence number needs to be incremented", "The sequence number of
> the last received PCP message". (Sections 6.4 and 6.5 are more explicit.) It
> would be helpful for the places that are vague about this to be more explicit.

Please mention the places where you find it is vague.

> 
> * Section 5.1:
> 
> I'll restate that it seems confusing to define "response codes" that are not
> identifying *responses*.
> 
> I guess that because the PA messages containing EAP messages invert the roles
> of client and server (wrt Common PCP messages), it might make sense to have
> response codes in PCP request messages that happen to be EAP response
> messages. But then it is wrong terminology for PCP response messages that
> contain EAP requests.
> 
> I'm not quite sure how to recommend clarifying this. The whole situation is set
> up for confusion of roles, so it does require considerable care to make clear.
> (*I* would be inclined to use separate fields for PCP response codes, EAP
> response codes, and for "sub-opcodes" of PCP opcodes. But you have thought
> about it more than I have.)
> 
> * Section 5.2:
> 
> The section title is inappropriate – the section is not about the Authentication
> Opcode, it is about information that is specific and common to all messages
> (requests and responses) containing the Authentication opcode. I suggest
> retitling this "Opcode-specific information of PCP Auth Messages".

Updated.

> 
> Also, it says:
> 
>        ... A
>        sequence number needs to be incremented on every new (non-
>        retransmission) outgoing message in order to provide an ordering
>        guarantee for PCP messages.
> 
> This could be clearer – stating that the sequence number to use is different for
> requests and responses. (And name which ones.)

It is discussed in https://tools.ietf.org/html/draft-ietf-pcp-authentication-11#section-6.4 

> 
> * Section 5.3:
> 
> Is the following intended to be normative?
> 
>     If the PA-Server message does not carry the correct nonce,
>     the message will be discarded silently.
> 
> If so it should use 2119 language. Perhaps this section isn’t the proper place for
> that normative language, but I don’t find it anywhere else.

Okay, updated. 
NEW:
If the PA-Server message does not carry the correct nonce, the message MUST be discarded silently.

> 
> * Section 5.4:
> 
> As in section 5.2, this would benefit from describing which sequence numbers
> are to be used.

Outgoing and incoming message have their respective sequence numbers and it's usage is explained in https://tools.ietf.org/html/draft-ietf-pcp-authentication-11#section-6.5  

> 
> * Section 5.11:
> 
>        ... The two indicator
>        strings are to be considered equivalent by the client if they are
>        an exact octet-for-octet match.
> 
> Is this intended to mean "if *and only if* they are an exact match"? Or is the
> client being given the option of accepting a looser match if it wishes? Right
> now that seems ambiguous, so it would be good to tighten up the language to
> be explicit about what you mean.

Fixed. NEW:
The two indicator strings are to be considered equivalent by the client if and only if they are an exact octet-for-octet match.

> 
> * Section 6.1:
> 
> In this section there are separate bullets for PA messages and Common PCP
> messages. Both talk about appending an "Authentication Tag Option".
> But there are different options for PA messages and common messages.
> ISTM the text here should reiterate that.

Yup, updated section to make it clear.

> 
> * Section 6.6:
> 
> This says that EAP lower layers indicate to EAP methods the MTU of the lower
> layer. But doesn’t *this* protocol provide the "lower layer" that EAP is running
> over? So isn’t this protocol responsible for determining the MTU of the layer
> *it* runs over, adjust that for the overhead it adds, and providing that to the
> EAP layer ?

No, PCP does not determine the path MTU.

> 
> * Section 7:
> 
> All opcodes, result codes, and options currently registered with IANA have
> names formatted as upper case tokens that are legal C-language identifiers.
> (Using underscore rather than dash.) For consistency the new ones defined in
> this document ought to follow that form. The new response codes are close,
> but use dashes. The others are currently phrases. The new option names read
> more like descriptions. (Those may be appropriate in the Purpose field.) If this
> change is made, then the mnemonics can be used throughout the text. That
> would be clearer.

Yes, updated draft. 

> 
> All of the options registered here are to be allocated in the mandatory-to-
> process range. Yet I *guess* that everything in this draft is optional for PCP. I
> guess that there is a backward compatibility requirement – that
> implementations PCP with this extension interoperate with basic 6887
> implementations. Doesn’t that mean that all the things registered here should
> be optional to process by PCP (though still mandatory to process for those
> implementing this specification.)

No, mandatory-to-process means that PCP server returns UNSUPP_OPTION if the option is unrecognized, unimplemented, or disabled. For more info please refer to https://tools.ietf.org/html/rfc6887#section-7.3.  For PCP authentication to work all the new PCP options introduced in the document are mandatory for PCP client and server to implement.

> 
> It would be good to have numbered subsections for each distinct registry, and
> maybe for each distinct value, so that they show up in the TOC. It would also be
> nice to format these so that it is clear where one element ends and the next
> one starts. (Right now it is very hard to read.)

Sure, added sub-sections.

Cheers,
-Tiru

>