This is a (draft) community-service posting: The purpose is to unambiguously 
state the specification of "cookie processing wrt public suffixes".

It is somewhat difficult to tease this out of the requisite specification(s) 
and associated documents, e.g., [RFC6265] and the Public Suffix List [PSL], 
and so here it is (corrections/comments welcome).

=JeffH
------
HTTP cookie processing wrt "public suffixes" aka "effective TLDs (eTLDs)"
v0.2

Contents:
   1. Terminology
   2. Summarized Cookie-setting and -returning Algorithms wrt eTLDs
   3. Detailed Cookie-setting and -returning Algorithms wrt eTLDs
   4. Excerpts from [RFC6265]
   5. References


1. Terminology:

Cookie - a unit of HTTP state management data, sent by the server to the UA 
as part of (some) HTTP responses. The UA returns cookie(s) to the server on 
all subsequent requests, modulo various rules (some of which are discussed 
herein, the full story is given by [RFC6265]).

eTLD / effective Top Level Domain - a term closely related to "public 
suffix". It is more general than the term "public suffix" -- i.e., because 
it implies that the domain in question ought to be treated effectively as a 
TLD, but it may actually not be an official TLD, nor might it allow for 
"public" subdomain registrations (as the term "public suffix" specifically 
implies).

Origin - {scheme, host, port} tuple of a webapp [RFC6454], also known as a 
Web Origin.

Public Suffix - A "public suffix" is a DNS domain [RFC1034] under which 
Internet users can directly register names [modulo some policies]. Some 
examples of public suffixes are .com, .co.uk and pvt.k12.ma.us [PSL]. Note 
however, this definition does not adequately address various subtleties in 
practice (hence, in part, the [DBOUND] effort). Also note that the term 
"effective TLD (eTLD)" is closely related, and that the domains listed in 
[PSL] are a superset of actual IANA-registered TLDs [IANA-TLDs].

single-label domain name - A DNS name consisting of a single label, e.g., 
"org", "net", "com". Note that on various networks, often in enterprise 
deployments, existence of "local" single-label domains is not uncommon, 
e.g., "corp", "www". See also eTLD and TLD.

subdomain - a child domain of some given domain [RFC1034]. E.g., 
foo.example.org is a subdomain of example.org.

superdomain - the parent domain of some given domain. E.g., example.org is 
the superdomain of foo.example.org.

TLD / Top-level Domain - TLDs are those single-label DNS domains directly 
under the global public DNS root [IANA-TLDs] [RFC1591]. However, not all 
single-label DNS names are globally-visible TLDs. See also single-label 
domain name.

UA / User Agent - A web browser, or other HTTP client implementation, that 
implements the HTTP State spec [RFC6265], i.e., accepts cookies sent by HTTP 
servers.

webapp - a web application, which has both a server-side and client-side 
instances. The client-side instances, which are emitted by the server-side 
over HTTP, are effectively mobile code (typically composed of 
HTML+JavaScript+CSS+etc), and execute within UAs (as "web pages"), and have 
an origin derived from the server-side's URI. Note that this also 
encompasses "apps" that are not general purpose web browsers but that meet 
the definition of UA.



2. Summarized Cookie-setting and -returning Algorithms wrt eTLDs:

(a) A server-side webapp, whose origin's host component [RFC6454] (aka 
domain name) IS NOT a "public suffix"/eTLD, can "set cookies" (on UAs) for 
its own domain name, or for superdomains -- unless the targeted superdomain 
is a eTLD. In the latter case, the set-cookie attempt is ignored.
(b) The UA will return cookies set for a given host (aka domain) to only 
that host, or, to that host and all subdomains thereof (depending on 
server-specified particulars of the cookie itself).

(c) Conversely, a server-side webapp whose origin's host component IS 
denoted as a "public suffix"/eTLD (i.e., it appears in [PSL] or similar 
compendiums) can set cookies for itself (modulo UA configuration), but its 
subdomains can not set cookies for the eTLD domain.
(d) The UA will not return any of the eTLD's cookies to subdomain servers.


3. Detailed Cookie-setting and -returning Algorithms wrt eTLDs:

Excerpts from [RFC6265] expressing the specifics underlying the 
cookie-setting and -returning algorithms, and in which the effects of 
"public suffixes"/eTLDs are specified, are found in the section after this 
one. They are referred to (using an "S" prefix) in the following 
explanations of the algorithms summarized in the preceding section:

In terms of (a), a server that is NOT a eTLD doing cookie-setting: the 2nd 
portion of S 4.1.2.3 describes the cookie-setting semantics, step 5 of S 5.3 
handles cookie-setting attempts for "public suffixes", step 6 of S 5.3 
defines the determination of the cookie's applicable domain and its notation 
in the cookie's cookie store entry, e.g. setting the host-only-flag. The 
host-only-flag governs whether the cookie will be returned to the origin 
server only, or whether it will be returned to the origin server as well as 
its subdomains.

In terms of (b), a UA returning cookies to non-eTLD servers: S 5.4 governs 
the construction of the Cookie header for the HTTP request, which depends 
directly upon the cookie entry's host-only-flag, and the domain-match 
algorithm (S 5.1.3).


In terms of (c), a "public suffix"/eTLD webapp doing cookie-setting: the 2nd 
portion of S 4.1.2.3 describes again applies, and refers directly to S 5.3 
in this case. S 5.3 steps 5 and 6 apply and the path through them depends on 
whether the UA is config'd to reject "public suffixes" or not. If not, and 
the domain-attribute is empty, then in step 6 the cookie is stored in the 
cookie store with its host-only-flag set to true and its domain set to the 
canonicalized request-host.

In terms of (d), a UA returning cookies to a "public suffix"/eTLD server:
step 1 of S 5.4 will gather only such cookies and return them to ONLY the 
eTLD server, due to the manner that they were recorded in the cookie store.


4. Excerpts from [RFC6265]:

   [Note: I've included the hierarchy of [RFC6265] section titles in
    order to provide context.]

   4. Server Requirements
   4.1. Set-Cookie
   4.1.2. Semantics (Non-Normative)
   4.1.2.3. The Domain Attribute
   https://tools.ietf.org/html/rfc6265#section-4.1.2.3

    The Domain attribute specifies those hosts to which the cookie will
    be sent.  For example, if the value of the Domain attribute is
    "example.com", the user agent will include the cookie in the Cookie
    header when making HTTP requests to example.com, www.example.com, and
    www.corp.example.com. ... If the server omits the Domain attribute,
    the user agent will return the cookie only to the origin server.
    ...
    The user agent will reject cookies unless the Domain attribute
    specifies a scope for the cookie that would include the origin
    server.  For example, the user agent will accept a cookie with a
    Domain attribute of "example.com" or of "foo.example.com" from
    foo.example.com, but the user agent will not accept a cookie with a
    Domain attribute of "bar.example.com" or of "baz.foo.example.com".

    NOTE: For security reasons, many user agents are configured to reject
    Domain attributes that correspond to "public suffixes".  For example,
    some user agents will reject Domain attributes of "com" or "co.uk".
    (See Section 5.3 for more information.)
    ...

   5. User Agent Requirements
   5.1. Subcomponent Algorithms
   5.1.3. Domain Matching
   https://tools.ietf.org/html/rfc6265#section-5.1.3

    A string domain-matches a given domain string if at least one of the
    following conditions hold:

    o  The domain string and the string are identical.  (Note that both
       the domain string and the string will have been canonicalized to
       lower case at this point.)

    o  All of the following conditions hold:

       *  The domain string is a suffix of the string.

       *  The last character of the string that is not included in the
          domain string is a %x2E (".") character.

       *  The string is a host name (i.e., not an IP address).


   5.3. Storage Model
   https://tools.ietf.org/html/rfc6265#section-5.3
   ...
    5.   If the user agent is configured to reject "public suffixes" and
         the domain-attribute is a public suffix:

            If the domain-attribute is identical to the canonicalized
            request-host:

               Let the domain-attribute be the empty string.

            Otherwise:

               Ignore the cookie entirely and abort these steps.

            NOTE: A "public suffix" is a domain that is controlled by a
            public registry, such as "com", "co.uk", and "pvt.k12.wy.us".
            This step is essential for preventing attacker.com from
            disrupting the integrity of example.com by setting a cookie
            with a Domain attribute of "com".  Unfortunately, the set of
            public suffixes (also known as "registry controlled domains")
            changes over time.  If feasible, user agents SHOULD use an
            up-to-date public suffix list, such as the one maintained by
            the Mozilla project at <http://publicsuffix.org/>.

    6.   If the domain-attribute is non-empty:

            If the canonicalized request-host does not domain-match the
            domain-attribute:

               Ignore the cookie entirely and abort these steps.

            Otherwise:

               Set the cookie's host-only-flag to false.

               Set the cookie's domain to the domain-attribute.

         Otherwise:

            Set the cookie's host-only-flag to true.

            Set the cookie's domain to the canonicalized request-host.
    ...

    5.4. The Cookie Header
    https://tools.ietf.org/html/rfc6265#section-5.4

    The user agent includes stored cookies in the Cookie HTTP request
    header.
    ...
    If the user agent does attach a Cookie header field to an HTTP
    request, the user agent MUST send the cookie-string (defined below)
    as the value of the header field.

    The user agent MUST use an algorithm equivalent to the following
    algorithm to compute the "cookie-string" from a cookie store and a
    request-uri:

    1.  Let cookie-list be the set of cookies from the cookie store that
        meets all of the following requirements:

        *  Either:

              The cookie's host-only-flag is true and the canonicalized
              request-host is identical to the cookie's domain.

           Or:

              The cookie's host-only-flag is false and the canonicalized
              request-host domain-matches the cookie's domain.

        *  The request-uri's path path-matches the cookie's path.

    ...

        *  If the cookie's http-only-flag is true, then exclude the
           cookie if the cookie-string is being generated for a "non-
           HTTP" API (as defined by the user agent).
    ...


5. References:

[DBOUND] https://www.ietf.org/mailman/listinfo/dbound

[IANA-TLDs] https://www.iana.org/domains/root/db

[PSL] Public Suffix List
       https://publicsuffix.org/

[RFC1034] DOMAIN NAMES - CONCEPTS AND FACILITIES
           https://tools.ietf.org/html/rfc1034

[RFC1591] Domain Name System Structure and Delegation
           http://tools.ietf.org/html/rfc1591
           see also: https://en.wikipedia.org/wiki/Top-level_domain

[RFC6265] HTTP State Management Mechanism
           https://tools.ietf.org/html/rfc6265

[RFC6454] The Web Origin Concept
           https://tools.ietf.org/html/rfc6454



end