diff --git a/pkg/api/types.go b/pkg/api/types.go
index 1d9a41e8051..6916cc24a86 100644
--- a/pkg/api/types.go
+++ b/pkg/api/types.go
@@ -100,6 +100,16 @@ type HostDirectory struct {
 
 type EmptyDirectory struct{}
 
+// Protocol defines network protocols supported for things like conatiner ports.
+type Protocol string
+
+const (
+	// ProtocolTCP is the TCP protocol.
+	ProtocolTCP Protocol = "TCP"
+	// ProtocolUDP is the UDP protocol.
+	ProtocolUDP Protocol = "UDP"
+)
+
 // Port represents a network port in a single container.
 type Port struct {
 	// Optional: If specified, this must be a DNS_LABEL.  Each named port
@@ -109,8 +119,8 @@ type Port struct {
 	HostPort int `yaml:"hostPort,omitempty" json:"hostPort,omitempty"`
 	// Required: This must be a valid port number, 0 < x < 65536.
 	ContainerPort int `yaml:"containerPort" json:"containerPort"`
-	// Optional: Supports "TCP" and "UDP".  Defaults to "TCP".
-	Protocol string `yaml:"protocol,omitempty" json:"protocol,omitempty"`
+	// Optional: Defaults to "TCP".
+	Protocol Protocol `yaml:"protocol,omitempty" json:"protocol,omitempty"`
 	// Optional: What host IP to bind the external port to.
 	HostIP string `yaml:"hostIP,omitempty" json:"hostIP,omitempty"`
 }
@@ -381,8 +391,8 @@ type Service struct {
 
 	// Required.
 	Port int `json:"port" yaml:"port"`
-	// Optional: Supports "TCP" and "UDP".  Defaults to "TCP".
-	Protocol string `yaml:"protocol,omitempty" json:"protocol,omitempty"`
+	// Optional: Defaults to "TCP".
+	Protocol Protocol `yaml:"protocol,omitempty" json:"protocol,omitempty"`
 
 	// This service's labels.
 	Labels map[string]string `json:"labels,omitempty" yaml:"labels,omitempty"`
diff --git a/pkg/api/v1beta1/types.go b/pkg/api/v1beta1/types.go
index 61277c3e801..9225b592e98 100644
--- a/pkg/api/v1beta1/types.go
+++ b/pkg/api/v1beta1/types.go
@@ -100,6 +100,16 @@ type HostDirectory struct {
 
 type EmptyDirectory struct{}
 
+// Protocol defines network protocols supported for things like conatiner ports.
+type Protocol string
+
+const (
+	// ProtocolTCP is the TCP protocol.
+	ProtocolTCP Protocol = "TCP"
+	// ProtocolUDP is the UDP protocol.
+	ProtocolUDP Protocol = "UDP"
+)
+
 // Port represents a network port in a single container.
 type Port struct {
 	// Optional: If specified, this must be a DNS_LABEL.  Each named port
@@ -109,8 +119,8 @@ type Port struct {
 	HostPort int `yaml:"hostPort,omitempty" json:"hostPort,omitempty"`
 	// Required: This must be a valid port number, 0 < x < 65536.
 	ContainerPort int `yaml:"containerPort" json:"containerPort"`
-	// Optional: Supports "TCP" and "UDP".  Defaults to "TCP".
-	Protocol string `yaml:"protocol,omitempty" json:"protocol,omitempty"`
+	// Optional: Defaults to "TCP".
+	Protocol Protocol `yaml:"protocol,omitempty" json:"protocol,omitempty"`
 	// Optional: What host IP to bind the external port to.
 	HostIP string `yaml:"hostIP,omitempty" json:"hostIP,omitempty"`
 }
@@ -394,8 +404,8 @@ type Service struct {
 
 	// Required.
 	Port int `json:"port" yaml:"port"`
-	// Optional: Supports "TCP" and "UDP".  Defaults to "TCP".
-	Protocol string `yaml:"protocol,omitempty" json:"protocol,omitempty"`
+	// Optional: Defaults to "TCP".
+	Protocol Protocol `yaml:"protocol,omitempty" json:"protocol,omitempty"`
 
 	// This service's labels.
 	Labels map[string]string `json:"labels,omitempty" yaml:"labels,omitempty"`
diff --git a/pkg/api/v1beta2/types.go b/pkg/api/v1beta2/types.go
index 61d066dd4f7..44cdfb4be21 100644
--- a/pkg/api/v1beta2/types.go
+++ b/pkg/api/v1beta2/types.go
@@ -100,6 +100,16 @@ type HostDirectory struct {
 
 type EmptyDirectory struct{}
 
+// Protocol defines network protocols supported for things like conatiner ports.
+type Protocol string
+
+const (
+	// ProtocolTCP is the TCP protocol.
+	ProtocolTCP Protocol = "TCP"
+	// ProtocolUDP is the UDP protocol.
+	ProtocolUDP Protocol = "UDP"
+)
+
 // Port represents a network port in a single container.
 type Port struct {
 	// Optional: If specified, this must be a DNS_LABEL.  Each named port
@@ -109,8 +119,8 @@ type Port struct {
 	HostPort int `yaml:"hostPort,omitempty" json:"hostPort,omitempty"`
 	// Required: This must be a valid port number, 0 < x < 65536.
 	ContainerPort int `yaml:"containerPort" json:"containerPort"`
-	// Optional: Supports "TCP" and "UDP".  Defaults to "TCP".
-	Protocol string `yaml:"protocol,omitempty" json:"protocol,omitempty"`
+	// Optional: Defaults to "TCP".
+	Protocol Protocol `yaml:"protocol,omitempty" json:"protocol,omitempty"`
 	// Optional: What host IP to bind the external port to.
 	HostIP string `yaml:"hostIP,omitempty" json:"hostIP,omitempty"`
 }
@@ -391,8 +401,8 @@ type Service struct {
 
 	// Required.
 	Port int `json:"port" yaml:"port"`
-	// Optional: Supports "TCP" and "UDP".  Defaults to "TCP".
-	Protocol string `yaml:"protocol,omitempty" json:"protocol,omitempty"`
+	// Optional: Defaults to "TCP".
+	Protocol Protocol `yaml:"protocol,omitempty" json:"protocol,omitempty"`
 
 	// This service's labels.
 	Labels map[string]string `json:"labels,omitempty" yaml:"labels,omitempty"`
diff --git a/pkg/api/v1beta3/types.go b/pkg/api/v1beta3/types.go
index 661bc855749..382bb1932ea 100644
--- a/pkg/api/v1beta3/types.go
+++ b/pkg/api/v1beta3/types.go
@@ -100,6 +100,16 @@ type HostDirectory struct {
 
 type EmptyDirectory struct{}
 
+// Protocol defines network protocols supported for things like conatiner ports.
+type Protocol string
+
+const (
+	// ProtocolTCP is the TCP protocol.
+	ProtocolTCP Protocol = "TCP"
+	// ProtocolUDP is the UDP protocol.
+	ProtocolUDP Protocol = "UDP"
+)
+
 // Port represents a network port in a single container.
 type Port struct {
 	// Optional: If specified, this must be a DNS_LABEL.  Each named port
@@ -109,8 +119,8 @@ type Port struct {
 	HostPort int `yaml:"hostPort,omitempty" json:"hostPort,omitempty"`
 	// Required: This must be a valid port number, 0 < x < 65536.
 	ContainerPort int `yaml:"containerPort" json:"containerPort"`
-	// Optional: Supports "TCP" and "UDP".  Defaults to "TCP".
-	Protocol string `yaml:"protocol,omitempty" json:"protocol,omitempty"`
+	// Optional: Defaults to "TCP".
+	Protocol Protocol `yaml:"protocol,omitempty" json:"protocol,omitempty"`
 	// Optional: What host IP to bind the external port to.
 	HostIP string `yaml:"hostIP,omitempty" json:"hostIP,omitempty"`
 }
@@ -390,8 +400,8 @@ type Service struct {
 
 	// Required.
 	Port int `json:"port" yaml:"port"`
-	// Optional: Supports "TCP" and "UDP".  Defaults to "TCP".
-	Protocol string `yaml:"protocol,omitempty" json:"protocol,omitempty"`
+	// Optional: Defaults to "TCP".
+	Protocol Protocol `yaml:"protocol,omitempty" json:"protocol,omitempty"`
 
 	// This service's labels.
 	Labels map[string]string `json:"labels,omitempty" yaml:"labels,omitempty"`
diff --git a/pkg/api/validation/validation.go b/pkg/api/validation/validation.go
index 01d96b4c463..d209e3c8790 100644
--- a/pkg/api/validation/validation.go
+++ b/pkg/api/validation/validation.go
@@ -78,7 +78,7 @@ func validateHostDir(hostDir *api.HostDirectory) errs.ErrorList {
 	return allErrs
 }
 
-var supportedPortProtocols = util.NewStringSet("TCP", "UDP")
+var supportedPortProtocols = util.NewStringSet(string(api.ProtocolTCP), string(api.ProtocolUDP))
 
 func validatePorts(ports []api.Port) errs.ErrorList {
 	allErrs := errs.ErrorList{}
@@ -106,7 +106,7 @@ func validatePorts(ports []api.Port) errs.ErrorList {
 		}
 		if len(port.Protocol) == 0 {
 			port.Protocol = "TCP"
-		} else if !supportedPortProtocols.Has(strings.ToUpper(port.Protocol)) {
+		} else if !supportedPortProtocols.Has(strings.ToUpper(string(port.Protocol))) {
 			pErrs = append(pErrs, errs.NewFieldNotSupported("protocol", port.Protocol))
 		}
 		allErrs = append(allErrs, pErrs.PrefixIndex(i)...)
@@ -330,7 +330,7 @@ func ValidateService(service *api.Service) errs.ErrorList {
 	}
 	if len(service.Protocol) == 0 {
 		service.Protocol = "TCP"
-	} else if !supportedPortProtocols.Has(strings.ToUpper(service.Protocol)) {
+	} else if !supportedPortProtocols.Has(strings.ToUpper(string(service.Protocol))) {
 		allErrs = append(allErrs, errs.NewFieldNotSupported("protocol", service.Protocol))
 	}
 	if labels.Set(service.Selector).AsSelector().Empty() {
diff --git a/pkg/kubelet/kubelet.go b/pkg/kubelet/kubelet.go
index bee489f3d55..5ad944dc245 100644
--- a/pkg/kubelet/kubelet.go
+++ b/pkg/kubelet/kubelet.go
@@ -222,7 +222,7 @@ func makePortsAndBindings(container *api.Container) (map[docker.Port]struct{}, m
 		// Some of this port stuff is under-documented voodoo.
 		// See http://stackoverflow.com/questions/20428302/binding-a-port-to-a-host-interface-using-the-rest-api
 		var protocol string
-		switch strings.ToUpper(port.Protocol) {
+		switch strings.ToUpper(string(port.Protocol)) {
 		case "UDP":
 			protocol = "/udp"
 		case "TCP":
diff --git a/pkg/proxy/proxier.go b/pkg/proxy/proxier.go
index ecdf32cf425..339423676fa 100644
--- a/pkg/proxy/proxier.go
+++ b/pkg/proxy/proxier.go
@@ -32,7 +32,7 @@ import (
 
 type serviceInfo struct {
 	port     int
-	protocol string
+	protocol api.Protocol
 	socket   proxySocket
 	timeout  time.Duration
 	mu       sync.Mutex // protects active
@@ -276,8 +276,8 @@ func logTimeout(err error) bool {
 	return false
 }
 
-func newProxySocket(protocol string, host string, port int) (proxySocket, error) {
-	switch strings.ToUpper(protocol) {
+func newProxySocket(protocol api.Protocol, host string, port int) (proxySocket, error) {
+	switch strings.ToUpper(string(protocol)) {
 	case "TCP":
 		listener, err := net.Listen("tcp", net.JoinHostPort(host, strconv.Itoa(port)))
 		if err != nil {
@@ -350,7 +350,7 @@ func (proxier *Proxier) setServiceInfo(service string, info *serviceInfo) {
 // addServiceOnUnusedPort starts listening for a new service, returning the
 // port it's using.  For testing on a system with unknown ports used.  The timeout only applies to UDP
 // connections, for now.
-func (proxier *Proxier) addServiceOnUnusedPort(service, protocol string, timeout time.Duration) (string, error) {
+func (proxier *Proxier) addServiceOnUnusedPort(service string, protocol api.Protocol, timeout time.Duration) (string, error) {
 	sock, err := newProxySocket(protocol, proxier.address, 0)
 	if err != nil {
 		return "", err
diff --git a/pkg/registry/service/rest.go b/pkg/registry/service/rest.go
index 635533ff66e..0394ece8cc5 100644
--- a/pkg/registry/service/rest.go
+++ b/pkg/registry/service/rest.go
@@ -222,9 +222,9 @@ func makeEnvVariableName(str string) string {
 
 func makeLinkVariables(service api.Service, machine string) []api.EnvVar {
 	prefix := makeEnvVariableName(service.ID)
-	protocol := "TCP"
+	protocol := string(api.ProtocolTCP)
 	if service.Protocol != "" {
-		protocol = service.Protocol
+		protocol = string(service.Protocol)
 	}
 	portPrefix := fmt.Sprintf("%s_PORT_%d_%s", prefix, service.Port, strings.ToUpper(protocol))
 	return []api.EnvVar{