Thanks for reviewing! Answered your questions below. Let me know if there are any followup questions.
I'll put together a PR that integrates your comments into the draft, and send it your way for review.
On Sat, May 2, 2020 at 12:44 AM Christer Holmberg via Datatracker <noreply@xxxxxxxx> 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.
Generally, without an opt-in (either before Accept-CH is received or when the server does not support it), clients SHOULD NOT send high-entropy hints, but MAY send low-entropy ones.
---
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?
Servers which wish to receive client information through Client Hints SHOULD add Accept-CH to their responses as early as possible.
Later Accept-CH response headers with updated information will override a previous opt-in.
---
MaQ3:
Related to MaQ2, what happens if a server receives hints that it does not
understand, or does not support?
Servers SHOULD ignore hints they do not understand or do 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?
A session, so similar lifetime properties as session cookies.
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.
It is the server preference that's persisted, not the specific information. Varying client characteristics will result in varying Client Hints request headers at different points in time.
--------
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?
Section 1 describes the fact that traditional (read: pre-Client Hints) content negotiation methods relied on sending information to all servers, which enabled passive fingerprinting, and how Client Hints breaks away from that paradigm, by only sending (high entropy) hints when the server needs them and opts-in to receive them.
---
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]."
Makes sense.
-------
Nits/editorial comments:
EdQ1:
The document uses both “client” and “user agent” terminology. Is there a reason
for that, or could one be picked?
No reason in particular. We can probably stick to "client".
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