Re: [secdir] Secdir review of draft-ietf-cdni-control-triggers-11

"Murray, Rob (Nokia - GB)" <> Sun, 17 April 2016 11:41 UTC

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From: "Murray, Rob (Nokia - GB)" <>
To: EXT Vincent Roca <>
Thread-Topic: Secdir review of draft-ietf-cdni-control-triggers-11
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Subject: Re: [secdir] Secdir review of draft-ietf-cdni-control-triggers-11
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Done now, these mods are included in "-13". Thank you for your help.

From: Rob Murray <<>>
Date: Saturday, 9 April 2016 at 15:22
To: EXT Vincent Roca <<>>
Cc: IESG <<>>, "<>" <<>>, "<>" <<>>, "<>" <<>>
Subject: Re: Secdir review of draft-ietf-cdni-control-triggers-11

Thanks Vincent, I'll make those mods in the next update.

From: EXT Vincent Roca <<>>
Date: Tuesday, 22 March 2016 at 14:57
To: Rob Murray <<>>
Cc: Vincent Roca <<>>, IESG <<>>, "<>" <<>>, "<>" <<>>, "<>" <<>>
Subject: Re: Secdir review of draft-ietf-cdni-control-triggers-11

Hi Rob,

I've just posted a "-12" of the CDNI Triggers draft, hopefully addressing these comments. Responses inline below.

I see you've added section 2.2.1 to clarify many architectural aspects. That's quite useful.

There are several topics I’d like to discuss:

1- Concerning the use of TLS to carry CI/T traffic, I see it is mandatory
to implement, but still optional to use (1st sentence of Section 8.1). Is
it still reasonable in current Internet? At a minimum I would expect a
« MUST support » / « SHOULD use ».

Section 8.1 goes on to say ...

 [ ... description of benefits of HTTPS ... ]

 In an environment where any such protection is required, mutually
 authenticated encrypted transport MUST be used to ensure
 confidentiality of the CI/T information. To that end, TLS MUST be
 used by CI/T, including authentication of the remote end.

... the Internet is clearly an environment where such protection is
required, and it'll often be used for these interfaces - we could state
that explicitly, would that address the concern?

But, CI/T could also be deployed to run over a VPN or private network
between two interconnected CDNs, in such an environment HTTPS transport
would not have the same benefits.

Yes, that’s my point. Say explicitly that Internet is an example of environment where
such a protection is mandatory.

In "-12" I've aligned the text above with the text used in the cdni-metadata draft:

 TLS MUST be used by the server-side (dCDN) and the client-side (uCDN) of
 the CI/T interface, including authentication of the remote end, unless
 alternate methods are used for ensuring the confidentiality of the
 information in the CI/T interface requests and responses (such as setting
 up an IPsec tunnel between the two CDNs or using a physically secured
 internal network between two CDNs that are owned by the same corporate

I think that covers it better (?).

This is much better. Just a minor comment: "... for insuring the security" instead of
"... for ensuring the confidentiality", since confidentiality is only one aspect of security.

2- Section 8.1, please check the paragraph « Note that in a « diamond »
configuration […] ». This paragraph is rather confusing to me. Confusion
comes from:
- the lack of description of the « diamond configuration ». I understand
there is one dCDN on top and multiple uCDN directly connected below, am I

Yes, that's right. The draft says:

 Note that in a "diamond" configuration, where one uCDN's content can
 be acquired via more than one directly-connected uCDN,

Would the following rearrangement help? ...

 A "diamond" configuration is one where a dCDN can acquire a uCDN's
 content via more than one directly-connected uCDN. Note that
 in a diamond configuration […]

Yes, I prefer this version. However, can the same « uCDN’s content » be
available at several uCDNs in this diamond configuration? Said differently,
what about the following text:

A « diamond » configuration is one where a dCDN can acquire a given
content via more than one directly-connected uCDN. Note that […]

Yes, more than one uCDN will have the same content - but the important point is that ultimately they will all have acquired it from the same uCDN, and that uCDN owns the metadata.

I've tried to provide a better intro/explanation in new section 2.2.1 "Multiple interconnected CDNs". Hopefully that makes it a bit clearer?

Yes, as said above, section 2.2.1 is rather useful.
Just a comment for this section 2.2.1, third paragraph, last sentence. You mention
both "intermediate CDN" and just after "intermediate uCDN". This is a bit confusing.

Another detail: in second paragraph you mention ""intermediate" CDN" with extra " "
around intermediate. Please harmonize.

3- With this « diamond configuration », since several uCDN can act upon the
same content, what happens if they behave in a non coordinated manner (i.e.,
in the general case)? For instance let’s imagine one of them asks the dCDN
to delete the content or cancel the initial command. What happens if another
uCDN then asks the dCDN to invalidate this content, providing the URL of the
Trigger Status Resource which (if I understand correctly) is no longer valid?
This « diamond configuration » can be a little bit tricky and no clear
guideline is provided in the document.

A trigger status resource belongs to one uCDN and cannot be accessed or
manipulated by any other CDN, section 3 para 3 says:

 Trigger Status Resources belonging to a uCDN MUST NOT
 be visible to any other CDN. The dCDN could, for example, achieve
 this by offering different collection URLs to each uCDN, and/or by
 filtering the response based on the uCDN with which the HTTP client
 is associated.

I see… However, if multiple uCDNs can act on the same content (what is said),
even if each uCDN uses a different URL, will simultaneous CI/T commands for
the same content be managed correctly? This is not said in section 3
paragraph 3 either and this is a key point.

Sorry, I'd missed your point.

Interactions can be inefficient, but are otherwise harmless because the strongest of "invalidate" or "purge" will win, and content can always be re-acquired if it's still available from any uCDN.

I've given some examples in new section 2.2.1 "Multiple interconnected CDNs", and added a pointer to that from the Security Considerations section.

Once again, the examples of section 2.2.1 answer my comments. Thanks.

Content always originates from one uCDN, but in a diamond configration there
may be multiple routes (via other CDNs) to a given dCDN.

The most common scenario may well be that the uCDN will initiate all of the
CI/T commands related to its content, and those commands will simply be
forwarded by intermediate CDNs.

But, there's nothing to stop an intermediate CDN initiating or modifying
CI/T commands - and some modifications of the command will be necessary
(for example, metadata URLs will need to be changed).

By analysis of log files it will probably be possible to work out which
uCDN an individual cache in the dCDN acquired each cached resource from.
But the cache is not required to keep a live record of that, and it
probably won't. In that case, when it receives a CI/T command to invalidate
or purge content, the dCDN should apply that command to any content it
may have acquired from the uCDN it got the CI/T command from.

So there is scope for a malicious intermediate CDN to cause extra processing
and acquisition by a dCDN (and that's mentioned in section 8). But a
malicious CDN is likely to find itself removed from the CDN federation fairly
quickly - there will necessarily be a level of trust between interconnected

Would it help to spell out some of that in the draft? (The same or very
similar text has been used in several of the CDNI drafts.)

Yes, this paragraph is worth being detailed a little bit.

In your explanations you mention intermediate CDNs while original text only
mentions directly connected uCDNs. The notion of « intermediate CDN » is
only discussed in sections 2.3 and 4.8. Since malicious intermediate CDNs are
part of a valid attack model, it is worth to explain it (or give a reference to another

When a content owner, the origin, delegates delivery to a CDN it is trusting that CDN to deliver the right content on its behalf. A malicious CDN could deliver anything instead, but it would soon lose the trust of the origin.

CDN-interconnection simply extends that trust to the further-downstream CDNs acting on behalf of the original.

So establishment/enforcement of the trust relationship itself, the contract between the origin and the CDN, is outside the scope of CDNI and in the land of commercial agreements.

Sure, I agree.

I've added a note on that in the intro to the Security Considerations section.

That's perfect.

To conclude the document is okay from my point of view.

Thanks for your detailed answers.