From 6211377d0ec29fddae268d7eb500d0baf3613dd0 Mon Sep 17 00:00:00 2001
From: Martin Thomson <mt@lowentropy.net>
Date: Fri, 22 Aug 2025 16:59:41 +1000
Subject: [PATCH 1/3] Add notes about how to set these values

---
 api.bs | 34 ++++++++++++++++++++++++++++++++--
 1 file changed, 32 insertions(+), 2 deletions(-)

diff --git a/api.bs b/api.bs
index 78ef894..58c5df0 100644
--- a/api.bs
+++ b/api.bs
@@ -1295,8 +1295,9 @@ given [=site=] |site|,
 returning an [=epoch index=]:
 
 1.  Let |startEpoch| be a [=user agent=]-defined value
-    for the earliest [=epoch index=]
-    that is supported for attribution.
+    for the <dfn>earliest epoch index</a>,
+    which is the first [=epoch index=]
+    that is supported for attribution on |site|.
 
     <p class=note>
     This value is not a constant [=epoch index=].
@@ -1525,6 +1526,8 @@ To <dfn>validate {{AttributionConversionOptions}}</dfn> |options|:
     :: |credit|
 1.  Let |lookback| be |options|.{{AttributionConversionOptions/lookbackDays}} [=days=]
     if it [=map/exists=], the [=implementation-defined=] maximum otherwise.
+1.  Let |lookback| be the [=implementation-defined=] maximum
+    if it is larger than that maximum.
 1.  If |lookback| is 0 [=days=], throw a {{RangeError}}.
 1.  If the [=list/size=] of
     |options|.{{AttributionConversionOptions/matchValues}} is
@@ -1769,6 +1772,33 @@ To <dfn>fill a histogram with last-n-touch attribution</dfn>, given a [=set=] of
 
 </div>
 
+### Setting Implementation-Defined Values ### {#impl-def}
+
+This specification identifies several [=implementation-defined=] values.
+This section includes some recommendations for implementations
+regarding how to best set those values.
+
+An implementation sets [=implementation-defined=] maximum values
+for {{AttributionImpressionOptions/lifetimeDays}} and
+{{AttributionConversionOptions/lookbackDays}}.
+There is no point to having different maximum values for
+these values as the smaller of the two values
+will determine which saved [=impressions=] are available for conversions.
+
+An implementation can also use the value it sets
+for for {{AttributionImpressionOptions/lifetimeDays}} and
+{{AttributionConversionOptions/lookbackDays}}
+to set the [=earliest epoch index=]
+when executing the [=get the starting epoch for attribution=] algorithm.
+Invoking [=get the current epoch=] algorithm for the site,
+passing the current time less the corresponding duration
+ensures that the [=earliest epoch index=]
+includes all possible [=impressions=] that can be queried.
+
+Deciding on a value for differential privacy parameters
+is hard and therefore TBD.
+
+
 ## User Control and Visibility ## {#user-control}
 
 <p class=issue>

From 069423cdfe53831fce92640edd194d1eaee1d56d Mon Sep 17 00:00:00 2001
From: Martin Thomson <mt@lowentropy.net>
Date: Fri, 22 Aug 2025 17:50:13 +1000
Subject: [PATCH 2/3] Guidance on setting implementation-defined values

Closes #231.
---
 api.bs | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/api.bs b/api.bs
index 58c5df0..c4ed36b 100644
--- a/api.bs
+++ b/api.bs
@@ -1289,20 +1289,21 @@ Resetting the budget is necessary to ensure
 that no information about the gap in browsing history
 is exposed.
 
-<div algorithm>
+<div algorithm="get the starting epoch for attribution">
 To <dfn>get the starting epoch for attribution</dfn>
 given [=site=] |site|,
 returning an [=epoch index=]:
 
 1.  Let |startEpoch| be a [=user agent=]-defined value
-    for the <dfn>earliest epoch index</a>,
+    for the <dfn>earliest epoch index</dfn>,
     which is the first [=epoch index=]
     that is supported for attribution on |site|.
 
     <p class=note>
     This value is not a constant [=epoch index=].
     A value is chosen for each site
-    based on [=user agent=] preferences or configuration.
+    based on [=user agent=] preferences or configuration;
+    see [[#impl-def]].
 
 1.  If the [=last browsing history clear=] is set,
     perform the following steps:
@@ -1772,7 +1773,7 @@ To <dfn>fill a histogram with last-n-touch attribution</dfn>, given a [=set=] of
 
 </div>
 
-### Setting Implementation-Defined Values ### {#impl-def}
+## Setting Implementation-Defined Values ## {#impl-def}
 
 This specification identifies several [=implementation-defined=] values.
 This section includes some recommendations for implementations

From f729c383c4f172750119527ccdf4f5fc3b408b21 Mon Sep 17 00:00:00 2001
From: Martin Thomson <mt@lowentropy.net>
Date: Mon, 25 Aug 2025 10:31:38 +1000
Subject: [PATCH 3/3] four

Co-authored-by: Andrew Paseltiner <apaseltiner@google.com>
---
 api.bs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/api.bs b/api.bs
index c4ed36b..752b298 100644
--- a/api.bs
+++ b/api.bs
@@ -1787,7 +1787,7 @@ these values as the smaller of the two values
 will determine which saved [=impressions=] are available for conversions.
 
 An implementation can also use the value it sets
-for for {{AttributionImpressionOptions/lifetimeDays}} and
+for {{AttributionImpressionOptions/lifetimeDays}} and
 {{AttributionConversionOptions/lookbackDays}}
 to set the [=earliest epoch index=]
 when executing the [=get the starting epoch for attribution=] algorithm.