diff --git a/lib/protocol/rack/adapter/generic.rb b/lib/protocol/rack/adapter/generic.rb
index 61392f4..0bdb5ef 100644
--- a/lib/protocol/rack/adapter/generic.rb
+++ b/lib/protocol/rack/adapter/generic.rb
@@ -68,6 +68,11 @@ def unwrap_request(request, env)
 					
 					self.unwrap_headers(request.headers, env)
 					
+					# For the sake of compatibility, we set the `HTTP_UPGRADE` header to the requested protocol.
+					if protocol = request.protocol and request.version.start_with?('http/1')
+						env[CGI::HTTP_UPGRADE] = Array(protocol).join(",")
+					end
+					
 					# HTTP/2 prefers `:authority` over `host`, so we do this for backwards compatibility.
 					env[CGI::HTTP_HOST] ||= request.authority
 								
diff --git a/lib/protocol/rack/adapter/rack2.rb b/lib/protocol/rack/adapter/rack2.rb
index cf5b16d..de95506 100644
--- a/lib/protocol/rack/adapter/rack2.rb
+++ b/lib/protocol/rack/adapter/rack2.rb
@@ -124,7 +124,6 @@ def self.make_response(env, response)
 					
 					if protocol = response.protocol
 						headers['rack.protocol'] = protocol
-						# headers['upgrade'] = protocol
 					end
 					
 					if body = response.body and body.stream?
diff --git a/lib/protocol/rack/constants.rb b/lib/protocol/rack/constants.rb
index c8a2a7c..b55c075 100644
--- a/lib/protocol/rack/constants.rb
+++ b/lib/protocol/rack/constants.rb
@@ -11,6 +11,7 @@ module Rack
 		# CGI keys <https://tools.ietf.org/html/rfc3875#section-4.1>:
 		module CGI
 			HTTP_HOST = 'HTTP_HOST'
+			HTTP_UPGRADE = "HTTP_UPGRADE"
 			PATH_INFO = 'PATH_INFO'
 			REQUEST_METHOD = 'REQUEST_METHOD'
 			REQUEST_PATH = 'REQUEST_PATH'
diff --git a/lib/protocol/rack/request.rb b/lib/protocol/rack/request.rb
index cd4ab72..f92ed28 100644
--- a/lib/protocol/rack/request.rb
+++ b/lib/protocol/rack/request.rb
@@ -6,6 +6,7 @@
 require 'protocol/http/request'
 require 'protocol/http/headers'
 
+require_relative 'constants'
 require_relative 'body/input_wrapper'
 
 module Protocol
@@ -30,12 +31,10 @@ def initialize(env)
 				)
 			end
 
-			HTTP_UPGRADE = 'HTTP_UPGRADE'
-
 			def self.protocol(env)
 				if protocols = env['rack.protocol']
 					return Array(protocols)
-				elsif protocols = env[HTTP_UPGRADE]
+				elsif protocols = env[CGI::HTTP_UPGRADE]
 					return protocols.split(/\s*,\s*/)
 				end
 			end