Issue 1618 review sbweeden attachment transport (#3)

* Describe how to determine authenticator attachment from transports

* Accept @equalsJeffH's rewrite of #sctn-attachment-from-transports

Co-authored-by: =JeffH <[email protected]>

Co-authored-by: =JeffH <[email protected]>

Author: emlun

index.bs

@@ -2429,6 +2429,26 @@ To remove the need to parse CBOR at all in many cases, {{AuthenticatorAttestatio
 
 Note: {{AuthenticatorAttestationResponse/getPublicKey()}} and {{AuthenticatorAttestationResponse/getAuthenticatorData()}} were only added in level two of this spec. [=[RPS]=] SHOULD use feature detection before using these functions by testing the value of `'getPublicKey' in AuthenticatorAttestationResponse.prototype`. [=[RPS]=] that require this function to exist may not interoperate with older user-agents.
 
+
+#### Determining Authenticator Attachment from Credential Transports #### {#sctn-attachments-from-transports}
+
+As a part of creating a {{PublicKeyCredential}} object, as the result of a  [=registration ceremony|registration=] or [=authentication ceremony=], the [=client platform=] determines the [=authenticator attachment modality=] and reports it in {{PublicKeyCredential/authenticatorAttachment}}. This section defines how to make that determination.
+
+If the client platform
+<dl class="switch">
+    : cannot determine the {{transports}} used or the transport is not a member of {{AuthenticatorTransport}}, then
+    :: the [=authenticator attachment modality=] is unknown, and the value of {{PublicKeyCredential/authenticatorAttachment}} MUST be set to null.
+
+    : used an {{AuthenticatorTransport/internal}} transport, then
+    :: {{PublicKeyCredential/authenticatorAttachment}} MUST be set to "platform".
+
+    : used a {{transports|transport}} that is a member of {{transports}} other than {{AuthenticatorTransport/internal}},
+    :: {{PublicKeyCredential/authenticatorAttachment}} MUST be set to "cross-platform".
+
+Note: An [=authenticator's=] [=authenticator attachment modality|attachment modality=] could change over time.
+For example, a mobile phone might at one time only support [=platform attachment=]
+but later receive updates to support [=cross-platform attachment=] as well.
+
 ### Web Authentication Assertion (interface <dfn interface>AuthenticatorAssertionResponse</dfn>) ### {#iface-authenticatorassertionresponse}
 
 The {{AuthenticatorAssertionResponse}} interface represents an [=authenticator=]'s response to a client's request for