diff --git a/p2p/base/port.cc b/p2p/base/port.cc
index 3bd09dc024..59c344bacd 100644
--- a/p2p/base/port.cc
+++ b/p2p/base/port.cc
@@ -80,13 +80,21 @@ const char* ProtoToString(ProtocolType proto) {
   return PROTO_NAMES[proto];
 }
 
-bool StringToProto(const char* value, ProtocolType* proto) {
+absl::optional<ProtocolType> StringToProto(absl::string_view proto_name) {
   for (size_t i = 0; i <= PROTO_LAST; ++i) {
-    if (absl::EqualsIgnoreCase(PROTO_NAMES[i], value)) {
-      *proto = static_cast<ProtocolType>(i);
-      return true;
+    if (absl::EqualsIgnoreCase(PROTO_NAMES[i], proto_name)) {
+      return static_cast<ProtocolType>(i);
     }
   }
+  return absl::nullopt;
+}
+
+bool StringToProto(const char* value, ProtocolType* proto) {
+  if (absl::optional<ProtocolType> type = StringToProto(value);
+      type.has_value()) {
+    *proto = *type;
+    return true;
+  }
   return false;
 }
 
diff --git a/p2p/base/port.h b/p2p/base/port.h
index b1dab5e92b..65560fe196 100644
--- a/p2p/base/port.h
+++ b/p2p/base/port.h
@@ -18,6 +18,7 @@
 #include <utility>
 #include <vector>
 
+#include "absl/strings/string_view.h"
 #include "absl/types/optional.h"
 #include "api/candidate.h"
 #include "api/field_trials_view.h"
@@ -125,7 +126,10 @@ class CandidateStats {
 typedef std::vector<CandidateStats> CandidateStatsList;
 
 const char* ProtoToString(ProtocolType proto);
-bool StringToProto(const char* value, ProtocolType* proto);
+absl::optional<ProtocolType> StringToProto(absl::string_view proto_name);
+
+// TODO(bugs.webrtc.org/13579): Delete once downstream usage is updated.
+[[deprecated]] bool StringToProto(const char* value, ProtocolType* proto);
 
 struct ProtocolAddress {
   rtc::SocketAddress address;
diff --git a/pc/ice_server_parsing.cc b/pc/ice_server_parsing.cc
index cb4145be1a..73cb8a0f68 100644
--- a/pc/ice_server_parsing.cc
+++ b/pc/ice_server_parsing.cc
@@ -183,12 +183,15 @@ static RTCErrorType ParseIceServerUrl(
       RTC_LOG(LS_WARNING) << "Transport parameter missing value.";
       return RTCErrorType::SYNTAX_ERROR;
     }
-    if (!cricket::StringToProto(tokens[1].c_str(), &turn_transport_type) ||
-        (turn_transport_type != cricket::PROTO_UDP &&
-         turn_transport_type != cricket::PROTO_TCP)) {
+
+    absl::optional<cricket::ProtocolType> proto =
+        cricket::StringToProto(tokens[1]);
+    if (!proto ||
+        (*proto != cricket::PROTO_UDP && *proto != cricket::PROTO_TCP)) {
       RTC_LOG(LS_WARNING) << "Transport parameter should always be udp or tcp.";
       return RTCErrorType::SYNTAX_ERROR;
     }
+    turn_transport_type = *proto;
   }
 
   std::string hoststring;
diff --git a/pc/webrtc_sdp.cc b/pc/webrtc_sdp.cc
index 4ca1636e54..a62b6f7ffa 100644
--- a/pc/webrtc_sdp.cc
+++ b/pc/webrtc_sdp.cc
@@ -1112,12 +1112,13 @@ bool ParseCandidate(absl::string_view message,
   }
   SocketAddress address(connection_address, port);
 
-  cricket::ProtocolType protocol;
-  if (!StringToProto(transport.c_str(), &protocol)) {
+  absl::optional<cricket::ProtocolType> protocol =
+      cricket::StringToProto(transport);
+  if (!protocol) {
     return ParseFailed(first_line, "Unsupported transport type.", error);
   }
   bool tcp_protocol = false;
-  switch (protocol) {
+  switch (*protocol) {
     // Supported protocols.
     case cricket::PROTO_UDP:
       break;
@@ -1225,7 +1226,7 @@ bool ParseCandidate(absl::string_view message,
     }
   }
 
-  *candidate = Candidate(component_id, cricket::ProtoToString(protocol),
+  *candidate = Candidate(component_id, cricket::ProtoToString(*protocol),
                          address, priority, username, password, candidate_type,
                          generation, foundation, network_id, network_cost);
   candidate->set_related_address(related_address);