diff --git a/registry/registry.go b/registry/registry.go
index 03a2890105..6ba80cbea5 100644
--- a/registry/registry.go
+++ b/registry/registry.go
@@ -112,6 +112,8 @@ func doWithCookies(c *http.Client, req *http.Request) (*http.Response, error) {
 	return c.Do(req)
 }
 
+// Set the user agent field in the header based on the versions provided
+// in NewRegistry() and extra.
 func (r *Registry) setUserAgent(req *http.Request, extra ...VersionChecker) {
 	if len(r.baseVersions)+len(extra) == 0 {
 		return
@@ -582,6 +584,12 @@ func validVersion(version VersionChecker) bool {
 	return true
 }
 
+// Convert versions to a string and append the string to the string base.
+//
+// Each VersionChecker will be converted to a string in the format of
+// "product/version", where the "product" is get from the Name() method, while
+// version is get from the Version() method. Several pieces of verson information
+// will be concatinated and separated by space.
 func appendVersions(base string, versions ...VersionChecker) string {
 	if len(versions) == 0 {
 		return base
diff --git a/server.go b/server.go
index 70bb0bb871..925e4e3386 100644
--- a/server.go
+++ b/server.go
@@ -26,6 +26,11 @@ func (srv *Server) DockerVersion() APIVersion {
 	}
 }
 
+// plainVersionChecker is a simple implementation of
+// the interface VersionChecker, which is used
+// to provide version information for some product,
+// component, etc. It stores the product name and the version
+// in string and returns them on calls to Name() and Version().
 type plainVersionChecker struct {
 	name    string
 	version string
@@ -39,6 +44,10 @@ func (v *plainVersionChecker) Version() string {
 	return v.version
 }
 
+// versionCheckers() returns version informations of:
+// docker, go, git-commit (of the docker) and the host's kernel.
+//
+// Such information will be used on call to NewRegistry().
 func (srv *Server) versionCheckers() []registry.VersionChecker {
 	v := srv.DockerVersion()
 	ret := make([]registry.VersionChecker, 0, 4)