Re: [tram] Roman Danyliw's Discuss on draft-ietf-tram-turnbis-27: (with DISCUSS and COMMENT)

Hi Tiru!

> -----Original Message-----
> From: iesg [mailto:iesg-bounces@ietf.org] On Behalf Of Konda, Tirumaleswar
> Reddy
> Sent: Thursday, July 11, 2019 5:07 AM
> To: Roman Danyliw <rdd@cert.org>; The IESG <iesg@ietf.org>
> Cc: tram-chairs@ietf.org; draft-ietf-tram-turnbis@ietf.org; tram@ietf.org;
> brandon.williams@akamai.com
> Subject: RE: [tram] Roman Danyliw's Discuss on draft-ietf-tram-turnbis-27:
> (with DISCUSS and COMMENT)
> 
> Hi Roman,
> 
> Thanks for the review. Please see inline
> 
> > -----Original Message-----
> > From: tram <tram-bounces@ietf.org> On Behalf Of Roman Danyliw via
> > Datatracker
> > Sent: Thursday, July 11, 2019 4:20 AM
> > To: The IESG <iesg@ietf.org>
> > Cc: tram-chairs@ietf.org; draft-ietf-tram-turnbis@ietf.org;
> > tram@ietf.org; brandon.williams@akamai.com
> > Subject: [tram] Roman Danyliw's Discuss on draft-ietf-tram-turnbis-27:
> > (with DISCUSS and COMMENT)
> >
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> > Roman Danyliw has entered the following ballot position for
> > draft-ietf-tram-turnbis-27: Discuss
> >
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> > for more information about IESG DISCUSS and COMMENT positions.
> >
> >
> > The document, along with other ballot positions, can be found here:
> > https://datatracker.ietf.org/doc/draft-ietf-tram-turnbis/
> >
> >
> >
> > ----------------------------------------------------------------------
> > DISCUSS:
> > ----------------------------------------------------------------------
> >
> > (1) Section 12.1.6.  (Per the back-and-forth on Chris Wood’s SECDIR
> > review – thank you Chris!) Per “Confidentiality is only a secondary
> > concern, as TURN control messages do not include information that is
> > particularly sensitive”, wouldn’t the USERNAME and REALM potentially
> > be privacy sensitive?  If they aren’t sensitive in all cases (e.g.,
> > usernames might be ephemeral), this should be noted and cited.
> 
> Agreed, added the following paragraph:
> 
>    If the TURN client and server use the STUN Extension for Third-Party
>    Authorization [RFC7635] (for example it is used in WebRTC), the
>    username does not reveal the real user's identity, the USERNAME
>    attribute carries an ephemeral and unique key identifier.  If the
>    TURN client and server use the STUN long-term credential mechanism
>    and the username reveals the real user's identity, the client needs
>    to use (D)TLS transport between the client and the server or use the
>    USERHASH attribute instead of the USERNAME attribute to anonynmize
>    the username.
> 
>    If the TURN client and server use the STUN long-term credential
>    mechanism and realm information is privacy sensitive, TURN can be run
>    over (D)TLS.  As a reminder, STUN Extension for Third-Party
>    Authorization does not use realm.

Thank you.  This text would address my concern.

> > ----------------------------------------------------------------------
> > COMMENT:
> > ----------------------------------------------------------------------
> >
> > (2) This draft relies on the draft-ietf-tram-stunbis’s STUN Password
> > Algo Registry which has MD5 and SHA-256.  Section 16.1.1 of that draft
> > already discusses the limitation of SHA-256 (which it might be useful to
> reference).
> > Nevertheless, are there cases where MD5 should be used over SHA-256 if
> > there is a choice?  Is there a reason not to recommend that
> > implementations “SHOULD NOT use MD5”?
> 
> The only reason draft-ietf-tram-stunbis retained MD5 is for backward-
> compatibility (client using older version of STUN (RFC5389)).

Understood.  Could we say something on the order of "if a client supports both MD5 and SHA-256, the latter SHOULD BE used"?

> >
> > (3) Section 5. Per “The client SHOULD include the SOFTWARE …” and “The
> > client and the server MAY include the FINGERPRINT attribute …”, why is
> > the sending of SOFTWARE not a “MAY” too?
> 
> This same behavior is defined in RFC5766 and turnbis does not modify the
> behavior, and to address the comment from Chris Wood (Secdir review)
> added the following line to Security Considerations section:
> 
> SOFTWARE attribute can reveal the specific software version of the TURN
> client and server to eavesdropper and it might possibly allow attacks against
> vulnerable software that is known to contain security holes. If it is important
> to prevent an eavesdropper from learning the software version, TURN can
> be run over (D)TLS.

Understood.  I can live with an argument that turnbis shouldn't modify the behavior in this case given this additional (helpful) language.  Thanks.

> >
> > (4) Section 5.  (Per the back-and-forth on Chris Wood’s SECDIR review)
> > Recommend citing Section 6.3.1 of [draft-ietf-tram-stunbis] as source
> > of 40 second request buffer timeout
> 
> Done.

Thanks.

> >
> > (5) Section 21.4.  Per “It is RECOMMENDED that TURN servers not accept
> > allocation or channel binding requests from addresses known to be
> > tunneled”, I concur with the advice.  How would one recognize that the
> > address is being tunneled?
> 
> By managing drop-list of tunnel IP addresses, it is the same technique used
> by several websites to block VPN access (see
> https://www.howtogeek.com/403771/why-do-some-websites-block-vpns/)

Right.  I didn't know if there was something fancier I was missing.

I appreciate these changes and explanations!

Roman

> Cheers,
> -Tiru
> 
> >
> >
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