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Telechat Review of draft-ietf-pcp-authentication-13

Request Review of draft-ietf-pcp-authentication
Requested revision No specific revision (document currently at 14)
Type Telechat Review
Team General Area Review Team (Gen-ART) (genart)
Deadline 2015-07-07
Requested 2015-07-02
Authors Margaret Cullen , Sam Hartman , Dacheng Zhang , Tirumaleswar Reddy.K
I-D last updated 2015-07-05
Completed reviews Genart Last Call review of -11 by Paul Kyzivat (diff)
Genart Telechat review of -13 by Paul Kyzivat (diff)
Opsdir Last Call review of -11 by Jouni Korhonen (diff)
Assignment Reviewer Paul Kyzivat
State Completed
Request Telechat review on draft-ietf-pcp-authentication by General Area Review Team (Gen-ART) Assigned
Reviewed revision 13 (document currently at 14)
Result On the Right Track
Completed 2015-07-05
I am the assigned Gen-ART reviewer for this draft. For background on
Gen-ART, please see the FAQ at <‚Äč>

Please wait for direction from your document shepherd or AD before
posting a new version of the draft.

This draft is on the right track but has open issues, described in this

I've opted to organize my comments in document order. I have tagged
different comments as BUG/TECHNICAL/EDITORIAL/NIT/etc.

I had difficultly creating my earlier last call review because in a
number of places I could not discern the intent of the draft. This
revised version has mostly resolved that problem, and allowed me to be
more concrete in my comments. I do still have a number of remaining

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.

     Paul Kyzivat

* Section 3.1:

[EDITORIAL] In the following sentence:

    ... In the
    message, the Session ID and Sequence Number fields of the
    AUTHENTICATION Opcode are set as 0.

those fields are not not part of the *AUTHENTICATION opcode*. I suggest
replacing this with:

    In the opcode-specific information of the message, the Session ID
    and Sequence Number fields are set as 0.


    The PA-Client message SHOULD
    also contain a NONCE option defined in Section 5.3 which consists of
    a random nonce.

Why is this SHOULD rather than MUST? Are there cases where it is
acceptable to omit the nonce? If so, I would like to see the text
explain the reasons for and consequences of omitting it, and give
example(s) of those cases.

[TECHNICAL] The next paragraph calls for the assignment of a *random*
session identifier. Does it need to be random? Isn't the real
requirement simply that it be unique from all other sessions between
this client and server? (I'm guessing that it would be more appropriate
to replace "random" with "unique", but the key is to achieve whatever
properties are required for this protocol.)

[EDITORIAL] This paragraph also refers to fields of the opcode. I
suggest the following replacement:

    ... and
    fill the identifier into the Session ID field of the AUTHENTICATION
    Opcode in the PA-Server message.  The Sequence Number field of the
    AUTHENTICATION Opcode is set as 0.

    ... and fill the identifier into the Session ID field in the
    server-specific information of the PA-Server message.  The Sequence
    Number field of the message is set as 0.

[EDITORIAL] Also, I'm still troubled by:

    From now on, every PCP message within this PA session MUST contain
    this session identifier.

As I noted previously, this is a truism - it is a restatement of the
definition of a PA session. Also, "From now on" seems a bit informal and
vague. I still suggest replacing this sentence with:

    Subsequent PCP messages are included within this PA session by
    attaching an AUTHENTICATION_TAG option containing this session

If you instead mean that once a PCP SA has been established then all PCP
messages must be sent within this PA session, then you need some
different words. I talk about this more below.

[EDITORIAL] In the figure in this section (and also in 3.1.2) it would
be very helpful if you showed the response code for each of the PA
messages. E.g.

      PCP                                                PCP
      client                                            server
        |-- PA-Initiation-------------------------------->|
        |   (Seq=0, Session ID=0, rc=INITIATION)          |
        |                                                 |
        |<-- PA-Server -----------------------------------|
        |    (Seq=0, Session ID=X, EAP Identity request)  |
        |    (rc=AUTHENTICATION_REQUIRED)                 |
        |                                                 |
        |-- PA-Client ----------------------------------->|
        |    (Seq=1, Session ID=X, EAP Identity response) |
        |    (rc=AUTHENTICATION_REPLY)                    |
        |                                                 |
        |<-- PA-Server -----------------------------------|
        |    (Seq=1, Session ID=X, EAP Identity request)  |
        |    (rc=???)                 |

[TECHNICAL] Note that I find no formal specification of what the return
code must be in the PA messages following the EAP Identity request and
its reply. (Hence the ??? above.)

* Section 3.1.2:

[BUG/MAJOR TECHNICAL] (This is my most major concern!!!)

The first sentence is:

    In the scenario where a PCP server receives a common PCP request
    message from a PCP client which needs to be authenticated, the PCP
    server can reply with a PA-Server message to initiate a PA session.

As I read RFC6887, this cannot be viewed as a response to the common PCP
request!!! The PCP client should still be expecting a response to *that*
request, containing the same opcode as the request. And it will retry
until it gets such a response.

Also, if we ignore that, there is no obvious way of matching this
response to the request. The matching rules on 6887 require the opcodes
to match, and for some opcode-specific matching rules to be applied to
the opcode-specific information. (Note that there could be more than one
outstanding, and some requests might require authentication while others
do not.)

The only way I see to make this work with 6887 is:
- send an error response to the common PCP message being challenged.
   (With the same opcode as the message. Probably with
   AUTHENTICATION_REQUIRED as the response code.

- To include the new session identifier in that response, use an
   option. The AUTHENTICATION_TAG option seems to work for this.

- Then the client could initiate authentication via PA messages.
   This could be just like in 3.1.1, or perhaps be optimized in some

If this isn't an acceptable resolution, then it seems necessary for you
to revise the general PCP response processing in section 8.3 of RFC6887.
And that would be ugly.

* Also in section 3.1.2:

[TECHNICAL] In the following:

    The PCP client MUST NOT retry the common PCP request until it
    receives AUTHENTICATION_SUCCEEDED result code from the PCP server.

Why MUST NOT? ISTM the point is that the request will not succeed
outside a PA session. Presumably you could try it again, but would just
get another request to authenticate. Wouldn't it be better to state that
the client may assume that it is fruitless to try that request outside
of a PA session?

* Section 3.1.3:

[TECHNICAL] The following:

    Therefore, after sending out a PA-Server message, the PCP
    server will not send a new PA-Server message until it receives a PA-
    Client message with a proper sequence number from the PCP client, and
    vice versa.

seems overly constrained. AFAICT this only needs to be true within one
PA session. If there happens to be another PA session, then this
wouldn't have to apply to messages there. So perhaps this could be
stated as:

    Therefore, after sending out a PA-Server message, the PCP
    server will not send a new PA-Server message in the same PA session
    until it receives a PA-Client message with a proper sequence number
    from the PCP client, and vice versa.

[TECHNICAL] And then in:

    If a PCP client receives a PA message containing an EAP
    Identity request and cannot generate an EAP Identity response
    immediately ... the PCP device MUST reply with a PA-Acknowledgement
    message ...

This only applies the constraint to an EAP Identity request. IIUC, the
full EAP negotiation could involve exchange of several EAP messages. I
presume this restriction must apply to all the PA Server messages
containing EAP messages, not just the Identity request. If that is so,
then this sentence needs to be expanded.

(In general, I find that the text insufficiently explains that the
number of messages in the exchange is variable and how that all works.)

In the following:

    In this approach, PCP client and a PCP server MUST perform a key-
    generating EAP method in authentication.  Particularly, a PCP
    authentication implementation MUST support EAP-TTLS [RFC5281] and
    SHOULD support TEAP [RFC7170].

[EDITORIAL] IIUC, this implies that a non-key-generating EAP method MUST
NOT be offered, or selected. It would be helpful to point that out.

[TECHNICAL] Then, in the following:

    After the EAP authentication, ... the PCP server MUST generate a PCP
    SA ... the PCP client needs to generate a PCP SA ...

This implies that there is no PCP SA until after the EAP authentication
has completed. But Section 4 says:

    At the beginning of a PA session, a session MUST generate a PA SA to
    maintain its state information during the session.

That says that the PCP SA is generated at the beginning of the PA
session, which is before the EAP authentication has begun.

This inconsistency needs to be resolved. Since the PCP SA state
information listed in section 4 includes the session identifier and
sequence numbers as well as the security-specific stuff, and since that
stuff is needed to maintain the PA session, ISTM that the PCP SA must be
generated as specified in section 4, and so section 3.1.3 needs to be
updated. For instance, it could say that when the EAP authentication
succeeds, then the PCP SA needs to be *updated* with the negotiated keys.

[NIT] Also in that paragraph:

    ... From then on, all the PCP
    messages within the session are secured with the transport key and
    the MAC algorithm specified in the PCP SA, unless a re-authentication
    is performed.

ISTM the exception in case of re-authentication is unnecessary.  The PA
message exchanges used to perform the re-authentication are secured that
way, and once the re-authentication is complete, the PCP SA is updated
with the new keys, and then subsequent messages continue to be secured
with using the info in the PCP SA.

So I think you can simplify this sentence by omitting the part starting
with ", unless".

[TECHNICAL] At the end of that paragraph:

    ... If PCP
    client sends common PCP request without AUTHENTICATION_TAG option
    then the PCP server rejects the request by returning
    AUTHENTICATION_REQUIRED result code in the PA-server message.

This is new since my prior questions. This effectively says that you
have made the design choice that once a PA session has been established
on a particular 5-tuple it MUST be used for all further PCP
communication between them.

If so, then the consequences of that need to be explored. One
consequence that comes to my mind is: what if, after establishing a PA
session, one of the parties restarts and loses its PCP SA state?

If the client lost state, then when it subsequently attempts to send a
PCP message it will receive a response indicating
AUTHENTICATION-REQUIRED. But it is then expected to use the existing PCP
SA session that it has forgotten. Can it attempt to establish a new
session, as in section 3.1.1? I can't tell if that will work.

Conversely, what if it is the PCP server that loses the PCP SA state? To
handle this case I think you need a new response code:
UNKNOWN_SESSION_ID, along with procedures for when to send it and what
to do when receiving it.

Bottom line - there is more to specify here.

[TECHNICAL] The final (new) paragraph in this section:

    It is possible for independent PCP clients on the host to create
    multiple PA sessions with the PCP server.  It is RECOMMENDED that
    once a PCP client on the host authenticates with the PCP server any
    other PCP clients on the host SHOULD be able to reuse the previously
    negotiated transport key for integrity protection.

is also in response to my earlier comments. It still isn't working for
me, for a couple of reasons:

First, I don't know what you mean by "independent PCP clients". IIUC
this is all in the context of a single client and server over a 5-tuple.
The use case I had in mind was for the aspect of the Advanced Threat
Model of 6887: "Any implementation that wants to be more permissive in
authorizing explicit mappings than it is in authorizing implicit
mappings". Notably where implicitly created mappings can be manipulated
without authentication, but where other explicit mappings may be
manipulated with authentication. So it isn't really different clients,
rather it is code for differing purposes within the same client.

Second, this paragraph is in conflict with my prior comment about
requiring an established PCP SA to be used. After the first such client
established a PCP SA, the others wouldn't be able to establish their own.

* Section 3.2:

(This section is much improved. Thanks.)

[EDITORIAL/TECHNICAL] I still have one comment on:

    ... Upon receiving a
    termination-indicating message, the PCP client MUST respond with a
    termination-indicating PA message, and SHOULD then remove the
    associated PA SA.

Why *SHOULD* remove? ISTM this ought to be MUST. Is there any case where
not removing it is a reasonable thing to do?

* Section 3.3:

[TECHNICAL] This section now says:

    ... The result
    code for PA-Sever message carrying EAP Identity request will be set
    to AUTHENTICATION_REQUIRED and PA-Client message carrying EAP
    Identity response will be set to AUTHENTICATION_REPLY.

This only specifies the response codes used for an EAP Identity request
and its EAP response. My comments to section 3.1.3 regarding response
codes for other EAP messages apply here as well.

I also see a case that isn't mentioned here or elsewhere. The session
lifetime is initialized at the successful completion of EAP
authentication or re-authentication. It is always selected by the PA
server, and is *optionally* communicated to the PA client. There is no
specification of what it is set to prior to this, or what it is set to
in the client if the server doesn't send the value. ISTM that there
ought to be a default value that is established at the time the SA
session is established. That then can govern until/unless a new value is
chosen by the server and sent to the client.

[TECHNICAL] This section fails to discuss the "glare" case. (Both client
and server decide to send RE-AUTHENTICATION at the same time.) A
mechanism needs to be specified to recover from this. (The typical
mechanism if for both ends to send some distinct error code, and then a
deterministic way to choose when to retry so that glare will not recur.)

* Section 4:

This section starts with:

    At the beginning of a PA session, a session MUST generate a PA SA to
    maintain its state information during the session.

[EDITORIAL] There is no such term as PA SA. Rather it is PCP SA.

[EDITORIAL] I don't know what it means for a *session* to generate a PA
SA. I think maybe you mean:

    At the beginning of a new PA session, each PCP device must create
    and initialize state information for a new PA Security Association
    (PCP SA) to maintain its state information for the duration of the
    PA session.

* Section 5.1:

[TECHNICAL] My comment on section 3.3 applies here as well.

[EDITORIAL/TECHNICAL] This section makes a change to the PCP Common
Request Packet Format defined in section 7.1 of RFC6887, by using a
portion of the "Reserved" field for an opcode-specific purpose. This
should be harmless in operation because recipients of this format are
required to ignore this field. But there could be a problem if a future
revision of that RFC were to use the same reserved bits for some other
purpose, without recognizing the conflict with this draft.

At my request during the LC review this document now indicates that it
updates RFC6887. But there still is no explicit statement that future
assignment of that field could explicitly impact this document. I would
suggest explicitly providing a revised version of Figure 2 from RFC6887,
such as:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |  Version = 2  |R|   Opcode    |   Reserved    |  Opcode-      |
      |               | |             |               |  specific-info|
      |                 Requested Lifetime (32 bits)                  |
      |                                                               |
      |            PCP Client's IP Address (128 bits)                 |
      |                                                               |
      |                                                               |
      :                                                               :
      :             (optional) Opcode-specific information            :
      :                                                               :
      :                                                               :
      :             (optional) PCP Options                            :
      :                                                               :

* Section 5.5:

[EDITORIAL NIT] For consistency with the others, the section heading for
this section ought to be "PA_AUTHENTICATION_TAG Option".

[EDITORIAL] Also, "because such information is provided in the
AUTHENTICATION Opcode" ought to be: "because such information is
provided in the Opcode-specific information".

* Section 6.1:

The first paragraph says:

    If a PCP SA is generated as the result of a successful EAP
    authentication process, every subsequent PCP message within the
    session MUST carry an authentication tag which contains the digest of
    the PCP message for data origin authentication and integrity

A couple of things about this:

[TECHNICAL] As I commented earlier, it seems that the PCP SA state needs
to be established before the EAP authentication process is complete. So
some other criterion is needed to decide when to start using the PCP SA
to generate digests of messages.

[NIT] Also "within the session" is unclear. It would be clearer to say
"within the PA session".

* Section 6.2:

[TECHNICAL] Earlier I raised a question about what happens if one of the
PCP devices is reset and loses PA SA state. That impacts this section as
well. If the client loses state, then it will discard any messages
containing one of the authentication tag options. What will the server
do in that situation? It isn't evident to me that the two of them will
be able to get into a functional state again. (By doing authentication
over again.) That would be bad.

* Section 7:

Thanks, this section is now more readable.

[EDITORIAL] Another change would improve it further: create separate
subsections for each distinct registry being updated: opcodes, result
codes, options. Identify the table within the corresponding section.
(The existing subsections for each opcode would then fall within the
opcode section.)

This concludes my Gen-ART comments.