I’m trying to use the TLS Session Resumption feature in TLS 1.2 and 1.3. I first tested this on iOS, but it didn’t work as expected. To investigate via packet capture, I ran the same code on macOS and saw the same issue. Using URLSession to establish a WebSocket connection, I captured packets in Wireshark to check if Session Resumption was working. The behavior differed from what I expected: 1st TLS handshake – Client Hello does not contain the session_ticket extension (required for session resumption per the TLS spec). 2nd TLS handshake – Client Hello does not contain a pre_shared_key. Test apps: https://github.com/sf-jed-kyung/tls-session-resumption-test Test environment: Xcode 16.3, macOS 15.6, OpenSSL 3.5.1 This repo contains: tls-urlsession-macos – WebSocket via URLSession (shows missing extensions). tls-openssl-macos – Manual TLS handshake via OpenSSL (shows both session_ticket and pre_shared_key). To run this, adjust Header Search Paths and Library Search Paths for your local OpenSSL install. URLSession – 1st Client Hello Transport Layer Security TLSv1.3 Record Layer: Handshake Protocol: Client Hello Content Type: Handshake (22) Version: TLS 1.0 (0x0301) Length: 512 Handshake Protocol: Client Hello Handshake Type: Client Hello (1) Length: 508 Version: TLS 1.2 (0x0303) Random: 0502b10cf04223658... Session ID Length: 32 Session ID: e3b276b14f2deaced... Cipher Suites Length: 42 Cipher Suites (21 suites) Compression Methods Length: 1 Compression Methods (1 method) Extensions Length: 393 ... Extension: server_name (len=26) name=echo.websocket.events Extension: extended_master_secret (len=0) Extension: renegotiation_info (len=1) Extension: supported_groups (len=12) Extension: ec_point_formats (len=2) Extension: application_layer_protocol_negotiation (len=11) Extension: status_request (len=5) Extension: signature_algorithms (len=22) Extension: signed_certificate_timestamp (len=0) Extension: key_share (len=43) x25519 Extension: psk_key_exchange_modes (len=2) Extension: supported_versions (len=7) TLS 1.3, TLS 1.2 Extension: compress_certificate (len=3) ... URLSession – 2nd Client Hello Transport Layer Security TLSv1.3 Record Layer: Handshake Protocol: Client Hello Content Type: Handshake (22) Version: TLS 1.0 (0x0301) Length: 512 Handshake Protocol: Client Hello Handshake Type: Client Hello (1) Length: 508 Version: TLS 1.2 (0x0303) Random: 1e485f35ad66c8598... Session ID Length: 32 Session ID: 99d02000c7ed403a5... Cipher Suites Length: 42 Cipher Suites (21 suites) Compression Methods Length: 1 Compression Methods (1 method) Extensions Length: 393 ... Extension: server_name (len=26) name=echo.websocket.events Extension: extended_master_secret (len=0) Extension: renegotiation_info (len=1) Extension: supported_groups (len=12) Extension: ec_point_formats (len=2) Extension: application_layer_protocol_negotiation (len=11) Extension: status_request (len=5) Extension: signature_algorithms (len=22) Extension: signed_certificate_timestamp (len=0) Extension: key_share (len=43) x25519 Extension: psk_key_exchange_modes (len=2) Extension: supported_versions (len=7) TLS 1.3, TLS 1.2 Extension: compress_certificate (len=3) ... OpenSSL – 1st Client Hello Transport Layer Security TLSv1.3 Record Layer: Handshake Protocol: Client Hello Content Type: Handshake (22) Version: TLS 1.0 (0x0301) Length: 1564 Handshake Protocol: Client Hello Handshake Type: Client Hello (1) Length: 1560 Version: TLS 1.2 (0x0303) Random: aec30b0aad542252... Session ID Length: 32 Session ID: f7ee7178cab8716a625... Cipher Suites Length: 60 Cipher Suites (30 suites) Compression Methods Length: 1 Compression Methods (1 method) Extensions Length: 1427 Extension: renegotiation_info (len=1) Extension: server_name (len=26) name=echo.websocket.events Extension: ec_point_formats (len=4) Extension: supported_groups (len=18) Extension: session_ticket (len=0) Extension: application_layer_protocol_negotiation (len=11) Extension: encrypt_then_mac (len=0) Extension: extended_master_secret (len=0) Extension: signature_algorithms (len=54) Extension: supported_versions (len=5) TLS 1.3, TLS 1.2 Extension: psk_key_exchange_modes (len=2) Extension: key_share (len=1258) X25519MLKEM768, x25519 OpenSSL – 2nd Client Hello Transport Layer Security TLSv1.3 Record Layer: Handshake Protocol: Client Hello Content Type: Handshake (22) Version: TLS 1.0 (0x0301) Length: 1716 Handshake Protocol: Client Hello Handshake Type: Client Hello (1) Length: 1712 Version: TLS 1.2 (0x0303) Random: 3fb3938a88166e4eb... Session ID Length: 32 Session ID: 7f13e54a231c17ccff70... Cipher Suites Length: 60 Cipher Suites (30 suites) Compression Methods Length: 1 Compression Methods (1 method) Extensions Length: 1579 Extension: renegotiation_info (len=1) Extension: server_name (len=26) name=echo.websocket.events Extension: ec_point_formats (len=4) Extension: supported_groups (len=18) Extension: session_ticket (len=0) Extension: application_layer_protocol_negotiation (len=11) Extension: encrypt_then_mac (len=0) Extension: extended_master_secret (len=0) Extension: signature_algorithms (len=54) Extension: supported_versions (len=5) TLS 1.3, TLS 1.2 Extension: psk_key_exchange_modes (len=2) Extension: key_share (len=1258) X25519MLKEM768, x25519 Extension: pre_shared_key (len=148) Since the Client Hello is generated by the client, I believe the session_ticket should be included in the first handshake regardless of server support. However, URLSession omits it entirely. Question: How can I enable TLS Session Resumption when using URLSession?

Development environment Xcode 26.0 Beta 6 iOS 26 Simulator macOS 15.6.1 To verify TLS 1.3 session resumption behavior in URLSession, I configured URLSessionConfiguration as follows and sent an HTTP GET request: let config = URLSessionConfiguration.ephemeral config.tlsMinimumSupportedProtocolVersion = .TLSv13 config.tlsMaximumSupportedProtocolVersion = .TLSv13 config.httpMaximumConnectionsPerHost = 1 config.httpAdditionalHeaders = ["Connection": "close"] config.enablesEarlyData = true let session = URLSession(configuration: config, delegate: nil, delegateQueue: nil) let url = URL(string: "https://www.google.com")! var request = URLRequest(url: url) request.assumesHTTP3Capable = true request.httpMethod = "GET" let task = session.dataTask(with: request) { data, response, error in if let error = error { print("Error during URLSession data task: \(error)") return } if let data = data, let responseString = String(data: data, encoding: .utf8) { print("Received data via URLSession: \(responseString)") } else { print("No data received or data is not UTF-8 encoded") } } task.resume() However, after capturing the packets, I found that the ClientHello packet did not include the early_data extension. It seems that enablesEarlyData on URLSessionConfiguration is not being applied. How can I make this work properly?