[connect-tcp] Discuss use of the Capsule Protocol

draft-ietf-httpbis-connect-tcp.md

@@ -145,11 +145,13 @@ Authentication to a templated TCP proxy normally uses ordinary HTTP authenticati
 
 Clients SHOULD assume that all proxy resources generated by a single template share a protection space (i.e., a realm) ({{?RFC9110, Section 11.5}}).  For many authentication schemes, this will allow the client to avoid waiting for a "401 (Unauthorized)" response before each new connection through the proxy.
 
-## Relationship to the Capsule Protocol
+### Use of the Capsule Protocol {#capsule-protocol}
 
-Unlike the datagram-oriented templated HTTP proxying specifications {{CONNECT-UDP}}{{CONNECT-IP}}, this specification does not make use of the Capsule Protocol {{?RFC9297}}.  A future specification could define a procedure for performing TCP proxying using the Capsule Protocol, but no such procedure is defined here.
+In this specification, use of the Capsule Protocol {{?RFC9297}} is OPTIONAL.  Clients MAY request use of the Capsule Protocol by including a "Capsule-Protocol: ?1" header field in the request.
 
-When implementing this specification, clients and servers MUST NOT send a "Capsule-Protocol: ?1" header field.
+Server support for the Capsule Protocol is also OPTIONAL.  If the request includes "Capsule-Protocol: ?1", and the server does not support the Capsule Protocol, the server MUST respond with a 4xx (Client Error) status and a "Capsule-Protocol: ?0" response header field.  Upon receiving such a response, the client SHOULD disable use of the Capsule Protocol with this URI Template for the remainder of the session and retry the request.
+
+When using the Capsule Protocol, TCP payload data is sent in the payload of a new Capsule Type named DATA ({{data-capsule}}).  This capsule's payload carries the same significance that ordinary stream data has when the Capsule Protocol is not in use.  It is applicable whenever use of the Capsule Protocol is optional.
 
 # Additional Connection Setup Behaviors
 
@@ -230,6 +232,14 @@ IF APPROVED, IANA is requested to add the following entry to the "MASQUE URI Suf
 | Path Segment | Description  | Reference       |
 | tcp          | TCP Proxying | (This document) |
 
+## New Capsule Type {#data-capsule}
+
+IF APPROVED, IANA is requested to add the following entry to the "HTTP Capsule Types" registry:
+
+| ----- | ------------ | --------- | ------------------------------------- | ----------------- | ------- |
+| Value | Capsule Type | Status    | Reference                             | Change Controller | Contact |
+| (TBD) | DATA         | permanent | (This document), {{capsule-protocol}} | IETF              | HTTPBIS |
+
 --- back
 
 # Acknowledgments