Hi,
Seems like I did a copy/paste error. Please skip Q3-Q6, the issues/questions
will come later in the review.
Regards,
Christer
On 02/05/2020, 1.44, "last-call on behalf of Christer Holmberg via
Datatracker" <last-call-boun...@ietf.org on behalf of nore...@ietf.org> wrote:
Reviewer: Christer Holmberg
Review result: Ready with Issues
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Document: draft-ietf-httpbis-client-hints-13
Reviewer: Christer Holmberg
Review Date: 2020-05-01
IETF LC End Date: 2020-05-08
IESG Telechat date: Not scheduled for a telechat
Summary: The document is easy to read, and I understand the general concept
of
the mechansim. However, I have a number of questions, some related to the
usage, which I think need to be clarified, and some more editorial.
Q3:
Section 2.1. describes sending of Client Hints, based on Accept-CH, and
Section
3.1. defines the Accept-CH header field.
But, there is no guidance on what a client does BEFORE it receives
Accept-CH. I
assume it does not include support of any features.
Also, there is no guidance on what a client does if it does NOT receive
Accept-CH (because the server does not support it). Will it then send
another
request and include supported features ? What if it is too late, and the
server
has already made choises?
I think some client behavior guidance would be useful.
---
Q4:
Related to Q3, there is not server procedure on when Accept-CH is sent to
the
client.
---
Q5:
Related to Q4, what happens if a server receives hints that it does not
understand, or does not support?
---
Q6:
Section 3.1 says:
“It SHOULD be persisted and bound to the origin to enable delivery of
Client
Hints on subsequent requests to the server's origin.”
…and the subsequent text then gives an example.
First, what is the time scope of “subsequent requests”? A session? An hour?
A
day? For how long does the client need to remember the Accept-CH header
field
value for a given origin server?
Second, the procedure does not seem to take into account that certain
aspects,
e.g., network characteristics, may change between when requests are sent to
an
origin server.
-------
Major issues:
MaQ1:
Section 2.1. describes sending of Client Hints, based on Accept-CH, and
Section
3.1. defines the Accept-CH header field.
First, there is no guidance on what a client does BEFORE it receives
Accept-CH.
I assume it does not include support of any features.
Second, there is no guidance on what a client does if it does NOT receive
Accept-CH (because the server does not support it). Will it then send
another
request and include supported features ? What if it is too late, and the
server
has already made choises?
I think some client behavior guidance would be useful.
---
MaQ2:
Related to Q3, there is not server procedure on when Accept-CH is sent to
the
client. Also, can an Accept-CH with updated information be sent?
---
MaQ3:
Related to MaQ2, what happens if a server receives hints that it does not
understand, or does not support?
---
MaQ4:
Section 3.1 says:
“It SHOULD be persisted and bound to the origin to enable delivery of
Client
Hints on subsequent requests to the server's origin.”
…and the subsequent text then gives an example.
First, what is the time scope of “subsequent requests”? A session? An hour?
A
day? For how long does the client need to remember the Accept-CH header
field
value for a given origin server?
Second, the procedure does not seem to take into account that certain
aspects,
e.g., network characteristics, may change between when requests are sent to
an
origin server.
--------
Minor issues:
MiQ1:
Section 1 described that proactive content negotiation allows servers to
silently fingerprint the user agent.
But, later in the Section it is described that Client Hints also allow a
server
the perform fingerprinting, and the Security Considerations also say that
there
is really no difference.
So, does Section 1 need to talk about fingerprinting at all?
---
MiQ2:
The 4th last paragraph of Section 1 says:
“It also defines guidelines for content negotiation mechanisms that use
it,
colloquially referred to as Client Hints.”
The 2nd last paragraph of Section 1 says:
“This document defines Client Hints, a framework that enables servers
to opt-in to specific proactive content negotiation features,
adapting their content accordingly.”
The 2nd last pargraph also talks about “usage of infrastructure”, which I
don’t
really understand. I assume you mean the Client Hints framework?
First, I think the text in the 4th last paragraph should be replaced by the
text in the 2nd last paragraph.
Second, I think the text introducing the framework should come BEFORE the
text
introducing the Accept-CH header field.
Something like:
"This document defines Client Hints, a framework that enables servers
to opt-in to specific proactive content negotiation features,
adapting their content accordingly. This document also defines a new
response header, Accept-CH, that allows an origin server to explicitly
ask that clients send these headers in requests.
Client Hints mitigate performance concerns by assuring that clients
will only send the request headers when they're actually going to be
used, and privacy concerns of passive fingerprinting by requiring
explicit opt-in and disclosure of required headers by the server
through the use of the Accept-CH response header.
The document does not define specific usages of Client Hints. Such usages
Need to be defined in their respective specifications.
One example of such usage is the User Agent Client Hints [UA-CH]."
-------
Nits/editorial comments:
EdQ1:
The document uses both “client” and “user agent” terminology. Is there a
reason
for that, or could one be picked?
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