[AAA-WG]: Reconciling Radius/Diameter SIP application

[Miguel Garcia <[e-mail]>]

Hi:

I am deliberately cross posting to AAA and Radius-ext because I believe 
this topic is of interest for both lists.

Yesterday Wolfgang and me met for a few hours and set the goal of 
reconcile the Diameter SIP application and the RADIUS HTTP/SIP Digest 
drafts. These are the drafts:

http://www.ietf.org/internet-drafts/draft-ietf-aaa-diameter-sip-app-04.txt

http://www.ietf.org/internet-drafts/draft-sterman-aaa-sip-04.txt

These are the main points we discuss. If you have any comments, speak up 
now, otherwise we will do the proposed changes in the relevant drafts:

1- We agreed to modify the Diameter SIP application so that it imports 
the Digest-* AVP from the corresponding RADIUS attributes address space.
This came out of a comment from Jari Arkko, and I think it is important 
to avoid duplication of AVP/attributes.

As a side effect of the above, the SIP-Authentication-Context will 
disappear in Diameter SIP app. This was a grouped AVP that only 
contained a Digest-Entity-Body-Hash AVP. The latter will remain (in the 
RADIUS draft).

2- We noticed a divergence in the Digest-Expected-Response AVP in 
Diameter, which does not exist in RADIUS. RADIUS defines a Digest-HA1 
attribute. The difference: Digest-HA1 contains H(A1), which is computed 
at the Radius server and sent to the Radius client, and it is used to 
compute the expected response. In Diameter, the expected response is 
computed at the Diameter server and sent to the client. This assumes a 
secure connection to avoid eavesdropping, which is not a problem for 
Diameter that mandates IPsec or TLS. I think Diameter can go for the 
Radius approach for compatibility reasons.

Proposal: Diameter will remove Digest-Expected-Response AVP and will 
import Digest-HA1 from Radius.

3- Radius does not define UTF8String data types, so all these attributes 
will remain as String, but the Radius draft will indicate that contain a
UTF8String verbally (this is required by HTTP and SIP).

4- Currently here is a divergence on the definition of Digest-Stale 
attribute: Diameter uses "true" and "false" (as per RFC2617), Radius 
uses 1 and 0. We agreed to use "true" and "false" to avoid stupid 
transcodings.

5- Diameter indicates that the Digest-* AVPs contain the quotes from/to 
the Digest directive, Radius assumes (although does not indicate) no 
quotes. We agreed that the Digest-* attributes do not need to include 
the quotes from HTTP Digest parameters. So, there will be an explicit 
indication that quotes are not part of the attribute value.

6- We run into a problem when multiple SIP proxies are authenticating
the user, because at some point in time the SIP request may contain
several Proxy-Authorization headers. The key here is the realm, it
will be always different. If different Diameter/Radius servers are 
serving different realms, there is not problem. But, if a common 
Diameter/Radius server is serving different realms, then the server is 
not able to determine which credentials should be evaluated.

We propose that the Diameter/Radius client MUST send only one set of 
credentials at a time, those belonging to the served realm. This 
requires to configure the Diameter/Radius client with the realm it is 
serving. We will include some text indicating this case.

7- Some of the attributes (e.g, Digest-Response and
Digest-Response-Auth) had an artificial limit of 32 octets in the
value. While this value is correct for MD5 hashes, if in the future
other hashes are added to HTTP Digest, will run into
trouble. Proposal: don't restrict the length of a value.

8- In the Radius document, scenario 1, we have a question. We suspect
that this scenario does not work if the Radius client generates a
nextnonce. The reason is that in the following authentication, when the
nextnonce becomes a nonce, the Radius server will not recognize the
nonce as locally generated (it was generated by the Radius client), and 
will reject the request with a "state=true". RFC 2617 seems to describe 
the case:

    If the
    nextnonce field is present the client SHOULD use it when constructing
    the Authorization header for its next request. Failure of the client
    to do so may result in a request to re-authenticate from the server
    with the "stale=TRUE".

Does anyone have any comment? Can anyone confirm that the operation in 
Digest is as we indicate? We will add some text indicating the 
limitations of Scenario 1.

9- The Radius draft, in section 2.15, indicates:

          RADIUS
          servers that do not implement a parameter contained in a
          Digest-Auth-Param attribute MUST respond with an Access-Reject
          message.  RADIUS clients that do not implement a parameter
          contained in a Digest-Auth-Param attribute MUST reject the
          original HTTP-style request.

The problem is that the text seems to go against RFC 2617 that says:

    auth-param
      This directive allows for future extensions. Any unrecognized
      directive MUST be ignored.

The proposal is to remove the above text from the RADIUS draft.


10- Similar contradictory text was found in Section 2.16 of the RADIUS 
draft, that says:

   RADIUS servers that do
   not implement AKA Digest MUST response with an Access Reject
   message.

We propose to remove the above text. The motivation is that the 
algorithm already conveys the AKA (AKA extends the algorithm to be 
AKAv1-MD5 and AKAv1-MD5-sess). The server chooses the algorithm, so the 
situation currently described, where the client may include an auts 
attribute, is just an error case, where the client made a mistake and 
included AUTS when not doing AKA. In case the Radius server does not 
implement AKA authentication, it will safely ignore this AVP.

11- We noticed that most of the HTTP Digest directives contain just a 
single token. However, the "qop" directive may contain a comma-separated 
collection of tokens. For instance, qop="auth,auth-int". The question is 
how do encode these tokens in the Digest-Qop attribute. The options are:

a) Treat the whole thing as one token and put it into the attribute value.
b) Each token is an attribute, thus, there might be multiple Digest-Qop 
attributes in a particular Radius/Diameter message.

We think that option b) is cleaner. Particularly it will be easier for 
the Diameter/Radius client to encode different values in different 
attributes.

Any comments to any of the above?

Regards,

           Miguel




-- 
Miguel A. Garcia           tel:[phone]
Nokia Research Center      Helsinki, Finland