diff --git a/src/address.cpp b/src/address.cpp
index ed98cdca..80d9697b 100644
--- a/src/address.cpp
+++ b/src/address.cpp
@@ -56,33 +56,33 @@ zmq::address_t::address_t (const std::string &protocol_,
 
 zmq::address_t::~address_t ()
 {
-    if (protocol == "tcp") {
+    if (protocol == protocol_name::tcp) {
         if (resolved.tcp_addr) {
             LIBZMQ_DELETE (resolved.tcp_addr);
         }
     }
-    if (protocol == "udp") {
+    if (protocol == protocol_name::udp) {
         if (resolved.udp_addr) {
             LIBZMQ_DELETE (resolved.udp_addr);
         }
     }
 #if !defined ZMQ_HAVE_WINDOWS && !defined ZMQ_HAVE_OPENVMS                     \
   && !defined ZMQ_HAVE_VXWORKS
-    else if (protocol == "ipc") {
+    else if (protocol == protocol_name::ipc) {
         if (resolved.ipc_addr) {
             LIBZMQ_DELETE (resolved.ipc_addr);
         }
     }
 #endif
 #if defined ZMQ_HAVE_TIPC
-    else if (protocol == "tipc") {
+    else if (protocol == protocol_name::tipc) {
         if (resolved.tipc_addr) {
             LIBZMQ_DELETE (resolved.tipc_addr);
         }
     }
 #endif
 #if defined ZMQ_HAVE_VMCI
-    else if (protocol == "vmci") {
+    else if (protocol == protocol_name::vmci) {
         if (resolved.vmci_addr) {
             LIBZMQ_DELETE (resolved.vmci_addr);
         }
@@ -92,29 +92,29 @@ zmq::address_t::~address_t ()
 
 int zmq::address_t::to_string (std::string &addr_) const
 {
-    if (protocol == "tcp") {
+    if (protocol == protocol_name::tcp) {
         if (resolved.tcp_addr)
             return resolved.tcp_addr->to_string (addr_);
     }
-    if (protocol == "udp") {
+    if (protocol == protocol_name::udp) {
         if (resolved.udp_addr)
             return resolved.udp_addr->to_string (addr_);
     }
 #if !defined ZMQ_HAVE_WINDOWS && !defined ZMQ_HAVE_OPENVMS                     \
   && !defined ZMQ_HAVE_VXWORKS
-    else if (protocol == "ipc") {
+    else if (protocol == protocol_name::ipc) {
         if (resolved.ipc_addr)
             return resolved.ipc_addr->to_string (addr_);
     }
 #endif
 #if defined ZMQ_HAVE_TIPC
-    else if (protocol == "tipc") {
+    else if (protocol == protocol_name::tipc) {
         if (resolved.tipc_addr)
             return resolved.tipc_addr->to_string (addr_);
     }
 #endif
 #if defined ZMQ_HAVE_VMCI
-    else if (protocol == "vmci") {
+    else if (protocol == protocol_name::vmci) {
         if (resolved.vmci_addr)
             return resolved.vmci_addr->to_string (addr_);
     }
diff --git a/src/address.hpp b/src/address.hpp
index 61b6d717..bcc5c549 100644
--- a/src/address.hpp
+++ b/src/address.hpp
@@ -46,6 +46,23 @@ class tipc_address_t;
 #if defined ZMQ_HAVE_VMCI
 class vmci_address_t;
 #endif
+
+namespace protocol_name
+{
+static const char tcp[] = "tcp";
+static const char udp[] = "udp";
+#if !defined ZMQ_HAVE_WINDOWS && !defined ZMQ_HAVE_OPENVMS                     \
+  && !defined ZMQ_HAVE_VXWORKS
+static const char ipc[] = "ipc";
+#endif
+#if defined ZMQ_HAVE_TIPC
+static const char tipc[] = "tipc";
+#endif
+#if defined ZMQ_HAVE_VMCI
+static const char vmci[] = "vmci";
+#endif
+}
+
 struct address_t
 {
     address_t (const std::string &protocol_,
diff --git a/src/ipc_connecter.cpp b/src/ipc_connecter.cpp
index 7109caa2..9e472108 100644
--- a/src/ipc_connecter.cpp
+++ b/src/ipc_connecter.cpp
@@ -67,7 +67,7 @@ zmq::ipc_connecter_t::ipc_connecter_t (class io_thread_t *io_thread_,
     current_reconnect_ivl (options.reconnect_ivl)
 {
     zmq_assert (addr);
-    zmq_assert (addr->protocol == "ipc");
+    zmq_assert (addr->protocol == protocol_name::ipc);
     addr->to_string (endpoint);
     socket = session->get_socket ();
 }
diff --git a/src/session_base.cpp b/src/session_base.cpp
index 8137dcb7..db899eab 100644
--- a/src/session_base.cpp
+++ b/src/session_base.cpp
@@ -515,7 +515,7 @@ void zmq::session_base_t::reconnect ()
     //  and reestablish later on
     if (_pipe && options.immediate == 1 && _addr->protocol != "pgm"
         && _addr->protocol != "epgm" && _addr->protocol != "norm"
-        && _addr->protocol != "udp") {
+        && _addr->protocol != protocol_name::udp) {
         _pipe->hiccup ();
         _pipe->terminate (false);
         _terminating_pipes.insert (_pipe);
@@ -557,10 +557,11 @@ void zmq::session_base_t::start_connecting (bool wait_)
 
     //  Create the connecter object.
 
-    if (_addr->protocol == "tcp") {
+    if (_addr->protocol == protocol_name::tcp) {
         if (!options.socks_proxy_address.empty ()) {
             address_t *proxy_address = new (std::nothrow)
-              address_t ("tcp", options.socks_proxy_address, this->get_ctx ());
+              address_t (protocol_name::tcp, options.socks_proxy_address,
+                         this->get_ctx ());
             alloc_assert (proxy_address);
             socks_connecter_t *connecter = new (std::nothrow)
               socks_connecter_t (io_thread, this, options, _addr, proxy_address,
@@ -578,7 +579,7 @@ void zmq::session_base_t::start_connecting (bool wait_)
 
 #if !defined ZMQ_HAVE_WINDOWS && !defined ZMQ_HAVE_OPENVMS                     \
   && !defined ZMQ_HAVE_VXWORKS
-    if (_addr->protocol == "ipc") {
+    if (_addr->protocol == protocol_name::ipc) {
         ipc_connecter_t *connecter = new (std::nothrow)
           ipc_connecter_t (io_thread, this, options, _addr, wait_);
         alloc_assert (connecter);
@@ -587,7 +588,7 @@ void zmq::session_base_t::start_connecting (bool wait_)
     }
 #endif
 #if defined ZMQ_HAVE_TIPC
-    if (_addr->protocol == "tipc") {
+    if (_addr->protocol == protocol_name::tipc) {
         tipc_connecter_t *connecter = new (std::nothrow)
           tipc_connecter_t (io_thread, this, options, _addr, wait_);
         alloc_assert (connecter);
@@ -596,7 +597,7 @@ void zmq::session_base_t::start_connecting (bool wait_)
     }
 #endif
 
-    if (_addr->protocol == "udp") {
+    if (_addr->protocol == protocol_name::udp) {
         zmq_assert (options.type == ZMQ_DISH || options.type == ZMQ_RADIO
                     || options.type == ZMQ_DGRAM);
 
@@ -698,7 +699,7 @@ void zmq::session_base_t::start_connecting (bool wait_)
 #endif // ZMQ_HAVE_NORM
 
 #if defined ZMQ_HAVE_VMCI
-    if (_addr->protocol == "vmci") {
+    if (_addr->protocol == protocol_name::vmci) {
         vmci_connecter_t *connecter = new (std::nothrow)
           vmci_connecter_t (io_thread, this, options, _addr, wait_);
         alloc_assert (connecter);
diff --git a/src/socket_base.cpp b/src/socket_base.cpp
index f8657f33..a0884930 100644
--- a/src/socket_base.cpp
+++ b/src/socket_base.cpp
@@ -295,9 +295,9 @@ int zmq::socket_base_t::check_protocol (const std::string &protocol_)
     if (protocol_ != "inproc"
 #if !defined ZMQ_HAVE_WINDOWS && !defined ZMQ_HAVE_OPENVMS                     \
   && !defined ZMQ_HAVE_VXWORKS
-        && protocol_ != "ipc"
+        && protocol_ != protocol_name::ipc
 #endif
-        && protocol_ != "tcp"
+        && protocol_ != protocol_name::tcp
 #if defined ZMQ_HAVE_OPENPGM
         //  pgm/epgm transports only available if 0MQ is compiled with OpenPGM.
         && protocol_ != "pgm"
@@ -305,15 +305,15 @@ int zmq::socket_base_t::check_protocol (const std::string &protocol_)
 #endif
 #if defined ZMQ_HAVE_TIPC
         // TIPC transport is only available on Linux.
-        && protocol_ != "tipc"
+        && protocol_ != protocol_name::tipc
 #endif
 #if defined ZMQ_HAVE_NORM
         && protocol_ != "norm"
 #endif
 #if defined ZMQ_HAVE_VMCI
-        && protocol_ != "vmci"
+        && protocol_ != protocol_name::vmci
 #endif
-        && protocol_ != "udp") {
+        && protocol_ != protocol_name::udp) {
         errno = EPROTONOSUPPORT;
         return -1;
     }
@@ -330,7 +330,7 @@ int zmq::socket_base_t::check_protocol (const std::string &protocol_)
     }
 #endif
 
-    if (protocol_ == "udp"
+    if (protocol_ == protocol_name::udp
         && (options.type != ZMQ_DISH && options.type != ZMQ_RADIO
             && options.type != ZMQ_DGRAM)) {
         errno = ENOCOMPATPROTO;
@@ -511,7 +511,7 @@ int zmq::socket_base_t::bind (const char *addr_)
         return rc;
     }
 
-    if (protocol == "udp") {
+    if (protocol == protocol_name::udp) {
         if (!(options.type == ZMQ_DGRAM || options.type == ZMQ_DISH)) {
             errno = ENOCOMPATPROTO;
             return -1;
@@ -575,7 +575,7 @@ int zmq::socket_base_t::bind (const char *addr_)
         return -1;
     }
 
-    if (protocol == "tcp") {
+    if (protocol == protocol_name::tcp) {
         tcp_listener_t *listener =
           new (std::nothrow) tcp_listener_t (io_thread, this, options);
         alloc_assert (listener);
@@ -596,7 +596,7 @@ int zmq::socket_base_t::bind (const char *addr_)
 
 #if !defined ZMQ_HAVE_WINDOWS && !defined ZMQ_HAVE_OPENVMS                     \
   && !defined ZMQ_HAVE_VXWORKS
-    if (protocol == "ipc") {
+    if (protocol == protocol_name::ipc) {
         ipc_listener_t *listener =
           new (std::nothrow) ipc_listener_t (io_thread, this, options);
         alloc_assert (listener);
@@ -616,7 +616,7 @@ int zmq::socket_base_t::bind (const char *addr_)
     }
 #endif
 #if defined ZMQ_HAVE_TIPC
-    if (protocol == "tipc") {
+    if (protocol == protocol_name::tipc) {
         tipc_listener_t *listener =
           new (std::nothrow) tipc_listener_t (io_thread, this, options);
         alloc_assert (listener);
@@ -636,7 +636,7 @@ int zmq::socket_base_t::bind (const char *addr_)
     }
 #endif
 #if defined ZMQ_HAVE_VMCI
-    if (protocol == "vmci") {
+    if (protocol == protocol_name::vmci) {
         vmci_listener_t *listener =
           new (std::nothrow) vmci_listener_t (io_thread, this, options);
         alloc_assert (listener);
@@ -809,7 +809,7 @@ int zmq::socket_base_t::connect (const char *addr_)
     alloc_assert (paddr);
 
     //  Resolve address (if needed by the protocol)
-    if (protocol == "tcp") {
+    if (protocol == protocol_name::tcp) {
         //  Do some basic sanity checks on tcp:// address syntax
         //  - hostname starts with digit or letter, with embedded '-' or '.'
         //  - IPv6 address may contain hex chars and colons.
@@ -853,7 +853,7 @@ int zmq::socket_base_t::connect (const char *addr_)
     }
 #if !defined ZMQ_HAVE_WINDOWS && !defined ZMQ_HAVE_OPENVMS                     \
   && !defined ZMQ_HAVE_VXWORKS
-    else if (protocol == "ipc") {
+    else if (protocol == protocol_name::ipc) {
         paddr->resolved.ipc_addr = new (std::nothrow) ipc_address_t ();
         alloc_assert (paddr->resolved.ipc_addr);
         int rc = paddr->resolved.ipc_addr->resolve (address.c_str ());
@@ -864,7 +864,7 @@ int zmq::socket_base_t::connect (const char *addr_)
     }
 #endif
 
-    if (protocol == "udp") {
+    if (protocol == protocol_name::udp) {
         if (options.type != ZMQ_RADIO) {
             errno = ENOCOMPATPROTO;
             LIBZMQ_DELETE (paddr);
@@ -897,7 +897,7 @@ int zmq::socket_base_t::connect (const char *addr_)
     }
 #endif
 #if defined ZMQ_HAVE_TIPC
-    else if (protocol == "tipc") {
+    else if (protocol == protocol_name::tipc) {
         paddr->resolved.tipc_addr = new (std::nothrow) tipc_address_t ();
         alloc_assert (paddr->resolved.tipc_addr);
         int rc = paddr->resolved.tipc_addr->resolve (address.c_str ());
@@ -917,7 +917,7 @@ int zmq::socket_base_t::connect (const char *addr_)
     }
 #endif
 #if defined ZMQ_HAVE_VMCI
-    else if (protocol == "vmci") {
+    else if (protocol == protocol_name::vmci) {
         paddr->resolved.vmci_addr =
           new (std::nothrow) vmci_address_t (this->get_ctx ());
         alloc_assert (paddr->resolved.vmci_addr);
@@ -937,7 +937,8 @@ int zmq::socket_base_t::connect (const char *addr_)
     //  PGM does not support subscription forwarding; ask for all data to be
     //  sent to this pipe. (same for NORM, currently?)
     bool subscribe_to_all = protocol == "pgm" || protocol == "epgm"
-                            || protocol == "norm" || protocol == "udp";
+                            || protocol == "norm"
+                            || protocol == protocol_name::udp;
     pipe_t *newpipe = NULL;
 
     if (options.immediate != 1 || subscribe_to_all) {
@@ -1039,7 +1040,7 @@ int zmq::socket_base_t::term_endpoint (const char *addr_)
     // IPv4-in-IPv6 mapping (EG: tcp://[::ffff:127.0.0.1]:9999), so try to
     // resolve before giving up. Given at this stage we don't know whether a
     // socket is connected or bound, try with both.
-    if (protocol == "tcp") {
+    if (protocol == protocol_name::tcp) {
         if (_endpoints.find (resolved_addr) == _endpoints.end ()) {
             tcp_address_t *tcp_addr = new (std::nothrow) tcp_address_t ();
             alloc_assert (tcp_addr);
diff --git a/src/socks_connecter.cpp b/src/socks_connecter.cpp
index 91bcc423..99cbc3df 100644
--- a/src/socks_connecter.cpp
+++ b/src/socks_connecter.cpp
@@ -72,7 +72,7 @@ zmq::socks_connecter_t::socks_connecter_t (class io_thread_t *io_thread_,
     _current_reconnect_ivl (options.reconnect_ivl)
 {
     zmq_assert (_addr);
-    zmq_assert (_addr->protocol == "tcp");
+    zmq_assert (_addr->protocol == protocol_name::tcp);
     _proxy_addr->to_string (_endpoint);
     _socket = _session->get_socket ();
 }
diff --git a/src/tcp_connecter.cpp b/src/tcp_connecter.cpp
index 8886a3d6..9ab5647f 100644
--- a/src/tcp_connecter.cpp
+++ b/src/tcp_connecter.cpp
@@ -82,7 +82,7 @@ zmq::tcp_connecter_t::tcp_connecter_t (class io_thread_t *io_thread_,
     _socket (_session->get_socket ())
 {
     zmq_assert (_addr);
-    zmq_assert (_addr->protocol == "tcp");
+    zmq_assert (_addr->protocol == protocol_name::tcp);
     _addr->to_string (_endpoint);
     // TODO the return value is unused! what if it fails? if this is impossible
     // or does not matter, change such that endpoint in initialized using an
diff --git a/src/zmq.cpp b/src/zmq.cpp
index 5e765f43..43423549 100644
--- a/src/zmq.cpp
+++ b/src/zmq.cpp
@@ -87,10 +87,10 @@ struct iovec
 #include "msg.hpp"
 #include "fd.hpp"
 #include "metadata.hpp"
-#include "signaler.hpp"
 #include "socket_poller.hpp"
 #include "timers.hpp"
 #include "ip.hpp"
+#include "address.hpp"
 
 #if defined ZMQ_HAVE_OPENPGM
 #define __PGM_WININT_H__
@@ -1489,7 +1489,7 @@ int zmq_device (int /* type */, void *frontend_, void *backend_)
 int zmq_has (const char *capability_)
 {
 #if !defined(ZMQ_HAVE_WINDOWS) && !defined(ZMQ_HAVE_OPENVMS)
-    if (strcmp (capability_, "ipc") == 0)
+    if (strcmp (capability_, zmq::protocol_name::ipc) == 0)
         return true;
 #endif
 #if defined(ZMQ_HAVE_OPENPGM)
@@ -1497,7 +1497,7 @@ int zmq_has (const char *capability_)
         return true;
 #endif
 #if defined(ZMQ_HAVE_TIPC)
-    if (strcmp (capability_, "tipc") == 0)
+    if (strcmp (capability_, zmq::protocol_name::tipc) == 0)
         return true;
 #endif
 #if defined(ZMQ_HAVE_NORM)
@@ -1513,7 +1513,7 @@ int zmq_has (const char *capability_)
         return true;
 #endif
 #if defined(ZMQ_HAVE_VMCI)
-    if (strcmp (capability_, "vmci") == 0)
+    if (strcmp (capability_, zmq::protocol_name::vmci) == 0)
         return true;
 #endif
 #if defined(ZMQ_BUILD_DRAFT_API)