diff --git a/lib/protocol/rack/adapter/generic.rb b/lib/protocol/rack/adapter/generic.rb
index 9869ac2..ac2aa48 100644
--- a/lib/protocol/rack/adapter/generic.rb
+++ b/lib/protocol/rack/adapter/generic.rb
@@ -142,6 +142,20 @@ def call(request)
 				def failure_response(exception)
 					Protocol::HTTP::Response.for_exception(exception)
 				end
+				
+				def self.extract_protocol(env, response, headers)
+					if protocol = response.protocol
+						# This is the newer mechanism for protocol upgrade:
+						if env['rack.protocol']
+							headers['rack.protocol'] = protocol
+						
+						# Older mechanism for protocol upgrade:
+						elsif env[CGI::HTTP_UPGRADE]
+							headers['upgrade'] = protocol
+							headers['connection'] = 'upgrade'
+						end
+					end
+				end
 			end
 		end
 	end
diff --git a/lib/protocol/rack/adapter/rack2.rb b/lib/protocol/rack/adapter/rack2.rb
index 5dd26c7..2b43639 100644
--- a/lib/protocol/rack/adapter/rack2.rb
+++ b/lib/protocol/rack/adapter/rack2.rb
@@ -121,9 +121,7 @@ def self.make_response(env, response)
 					# These interfaces should be largely compatible:
 					headers = response.headers.to_h
 					
-					if protocol = response.protocol
-						headers['rack.protocol'] = protocol
-					end
+					self.extract_protocol(env, response, headers)
 					
 					if body = response.body and body.stream?
 						if env[RACK_IS_HIJACK]
diff --git a/lib/protocol/rack/adapter/rack3.rb b/lib/protocol/rack/adapter/rack3.rb
index 6177a25..3f4c322 100644
--- a/lib/protocol/rack/adapter/rack3.rb
+++ b/lib/protocol/rack/adapter/rack3.rb
@@ -92,9 +92,8 @@ def wrap_headers(fields)
 				def self.make_response(env, response)
 					# These interfaces should be largely compatible:
 					headers = response.headers.to_h
-					if protocol = response.protocol
-						headers['rack.protocol'] = protocol
-					end
+					
+					self.extract_protocol(env, response, headers)
 					
 					if body = response.body and body.stream?
 						# Force streaming response: