rfc:tls_session_resumption_api

PHP RFC: TLS Session Resumption Support for Streams

Introduction

This RFC proposes adding comprehensive TLS session resumption support to PHP's OpenSSL stream implementation. Session resumption is a critical performance optimization that allows TLS clients and servers to skip the expensive full handshake by reusing cryptographic parameters from previous connections.

Currently, PHP's stream wrapper provides limited control over session management. While OpenSSL's internal session cache works automatically on the server side for some scenarios, developers cannot:

  • Save and restore client sessions across PHP requests
  • Implement custom server-side session storage (Redis, Memcached, databases)
  • Control session cache behavior

This proposal exposes OpenSSL's session management APIs through the existing stream context options, providing fine-grained control while maintaining backward compatibility. 

<?php
// Client: Resume previous session for faster reconnection
$context = stream_context_create([
    'ssl' => [
        'peer_name' => 'api.example.com',
        'session_data' => $_SESSION['tls_session'] ?? null,
        'session_new_cb' => function($stream, $sessionId, $sessionData) {
            $_SESSION['tls_session'] = $sessionData;
        }
    ]
]);
 
$fp = stream_socket_client('tls://api.example.com:443', context: $context);
// Second connection will resume, saving ~100ms of handshake time
?>

Proposal

New Stream Context Options

This proposal adds the following SSL stream context options:

Client-Only Options

  • session_data (string) - Binary session data from a previous connection to resume. Data must be from a session_new_cb callback.
  • session_new_cb (callable) - Callback invoked when a new session is established.
    • Signature: function(resource $stream, string $sessionId, string $sessionData): void
    • $sessionId: Binary session identifier
    • $sessionData: Serialized session (OpenSSL format via i2d_SSL_SESSION)

Server-Only Options

  • session_cache (bool, default: true) - Enable or disable session caching on the server.
  • session_cache_size (int, default: 20480) - Maximum number of sessions in the internal cache. Only applies when using OpenSSL's internal cache.
  • session_timeout (int, default: 300) - Session lifetime in seconds.
  • session_context_id (string) - Server identifier for session binding. Required when using external cache callbacks.
  • session_new_cb (callable) - Callback invoked when a new session is created. Same signature as client version.
  • session_get_cb (callable) - Callback to retrieve a session from external storage. Requires session_new_cb and session_context_id.
    • Signature: function(resource $stream, string $sessionId): ?string
    • Must return session data or null if not found
  • session_remove_cb (callable, optional) - Callback when a session is removed from cache.
    • Signature: function(resource $stream, string $sessionId): void
  • num_tickets (int, default: 2, TLS 1.3 only) - Number of session tickets to issue after handshake.
    • Set to 0 to disable all session resumption
    • Higher values (e.g., 4) support connection parallelization

Behavior

Client Behavior

When a client stream is created: 

1. If ''session_data'' is provided, attempt to resume with that session
2. If ''session_new_cb'' is provided, call it when a new session is established or received
3. Server-only options (''session_get_cb'', ''session_remove_cb'', ''session_cache'', ''num_tickets'', etc.) are ignored

Note: Client-side session resumption requires explicit management via session_data and session_new_cb. PHP does not automatically cache sessions on the client side - each connection uses a fresh SSL context unless session_data is provided.

Server Behavior

When a server stream is created:

Without External Cache (no session_get_cb):

  • Uses OpenSSL's internal session cache
  • session_cache controls whether caching is enabled
  • session_cache_size and session_timeout configure the internal cache
  • session_new_cb can be provided for notifications without external storage

With External Cache (session_get_cb provided):

  • Disables OpenSSL's internal cache (uses SSL_SESS_CACHE_NO_INTERNAL)
  • session_new_cb becomes required (E_WARNING if missing)
  • session_context_id becomes required (E_WARNING if missing)
  • session_remove_cb is optional
  • session_cache_size and session_timeout are ignored (application manages storage)

Examples

Client: Simple Session Caching

<?php
$sessions = [];
 
function create_client_context(string $host): resource {
    global $sessions;
 
    return stream_context_create([
        'ssl' => [
            'peer_name' => $host,
            'session_data' => $sessions[$host] ?? null,
            'session_new_cb' => function($stream, $id, $data) use ($host) {
                global $sessions;
                $sessions[$host] = $data;
                error_log("Saved session for $host");
            }
        ]
    ]);
}
 
// First connection - full handshake
$fp1 = stream_socket_client(
    'tls://api.example.com:443',
    $errno, $errstr, 30,
    STREAM_CLIENT_CONNECT,
    create_client_context('api.example.com')
);
 
// Second connection - resumed! (much faster)
$fp2 = stream_socket_client(
    'tls://api.example.com:443',
    $errno, $errstr, 30,
    STREAM_CLIENT_CONNECT,
    create_client_context('api.example.com')
);
?>

Server: Internal Cache (Default Behavior)

<?php
$context = stream_context_create([
    'ssl' => [
        'local_cert' => '/path/to/cert.pem',
        'local_pk' => '/path/to/key.pem',
        // Session resumption enabled by default
        'session_cache' => true,
        'session_cache_size' => 1024,
        'session_timeout' => 7200,  // 2 hours
    ]
]);
 
$server = stream_socket_server(
    'tls://0.0.0.0:8443',
    $errno, $errstr,
    STREAM_SERVER_BIND | STREAM_SERVER_LISTEN,
    $context
);
 
while ($conn = stream_socket_accept($server)) {
    // Handle connection
    // Client session resumption happens automatically
}
?>

Server: Control Ticket Issuance (TLS 1.3)

<?php
// Example 1: Issue multiple tickets for connection parallelization
$context = stream_context_create([
    'ssl' => [
        'local_cert' => '/path/to/cert.pem',
        'num_tickets' => 4,  // Issue 4 tickets per connection
    ]
]);
 
$server = stream_socket_server('tls://0.0.0.0:8443', context: $context);
 
// Example 2: Disable all resumption (maximum security)
$context = stream_context_create([
    'ssl' => [
        'local_cert' => '/path/to/cert.pem',
        'num_tickets' => 0,  // No tickets = no resumption possible
    ]
]);
 
$server = stream_socket_server('tls://0.0.0.0:8443', context: $context);
?>

Server: External Cache (Redis)

<?php
$redis = new Redis();
$redis->connect('127.0.0.1', 6379);
 
$context = stream_context_create([
    'ssl' => [
        'local_cert' => '/path/to/cert.pem',
        'local_pk' => '/path/to/key.pem',
        'session_context_id' => 'myapp',
 
        'session_new_cb' => function($stream, $id, $data) use ($redis) {
            $key = 'tls_session:' . bin2hex($id);
            $redis->setex($key, 7200, $data);
        },
 
        'session_get_cb' => function($stream, $id) use ($redis) {
            $key = 'tls_session:' . bin2hex($id);
            $result = $redis->get($key);
            return $result !== false ? $result : null;
        },
 
        'session_remove_cb' => function($stream, $id) use ($redis) {
            $key = 'tls_session:' . bin2hex($id);
            $redis->del($key);
        },
    ]
]);
 
$server = stream_socket_server(
    'tls://0.0.0.0:8443',
    $errno, $errstr,
    STREAM_SERVER_BIND | STREAM_SERVER_LISTEN,
    $context
);
 
// Now sessions are shared across PHP-FPM workers via Redis
while ($conn = stream_socket_accept($server)) {
    // Handle connection
}
?>

Server: Disable Session Resumption

<?php
// For high-security scenarios
$context = stream_context_create([
    'ssl' => [
        'local_cert' => '/path/to/cert.pem',
        'session_cache' => false,  // No session resumption
    ]
]);
 
$server = stream_socket_server('tls://0.0.0.0:8443', context: $context);
?>

Edge Case: Invalid Session Data

<?php
// Invalid or expired session data is handled gracefully
$context = stream_context_create([
    'ssl' => [
        'peer_name' => 'example.com',
        'session_data' => 'corrupted_data',  // Invalid
    ]
]);
 
// E_WARNING emitted, falls back to full handshake
$fp = stream_socket_client('tls://example.com:443', context: $context);
?>

Backward Incompatible Changes

This RFC introduces no backward incompatible changes.

  • All new options are optional and have sensible defaults
  • Session resumption is already enabled by default in OpenSSL (this RFC just exposes control)
  • Existing code continues to work without modification
  • New options are ignored in older PHP versions (standard stream context behavior)

Potential Considerations:

  • Applications that rely on every connection being a full handshake (rare) might observe different timing characteristics, but this is already the case with OpenSSL's default behavior
  • The new callbacks use resources passed as parameters - these must not be stored beyond the callback scope (documented limitation)

Proposed PHP Version(s)

Next PHP 8.5

RFC Impact

To SAPIs

No impact. The changes are isolated to the OpenSSL stream wrapper implementation in ext openssl.

To Existing Extensions

No impact to other extensions. Changes are confined to ext openssl's stream transport implementation.

To the Ecosystem

Positive impacts:

  • IDEs/Language Servers: New context options will appear in autocomplete
  • Static Analyzers: Can validate callback signatures and types
  • HTTP Clients: Libraries like Guzzle, Symfony HttpClient can optimize performance with session reuse
  • Async Frameworks: ReactPHP, Amp, Swoole can benefit from persistent session caching across connections
  • Microservices: Service-to-service communication can see significant latency reduction

Documentation needs:

  • PHP manual updates for stream context options
  • Examples showing session resumption patterns
  • Security considerations (forward secrecy implications)

Open Issues

None. All design questions have been resolved.

Future Scope

This RFC lays the groundwork for future TLS 1.3 enhancements:

  • 0-RTT Early Data: Building on session resumption to enable zero round-trip time data transmission
  • PSK (Pre-Shared Key): External PSK support for certificate-free authentication
  • Post-Handshake Authentication: TLS 1.3 client certificate requests after initial handshake
  • Ticket Key Rotation: Callbacks for server-side session ticket key management
  • Session Export: Additional helper functions to inspect session metadata

These features are intentionally excluded from this RFC to maintain focus and allow for iterative development.

Voting Choices

This is a simple yes/no vote requiring a 2/3 majority.

Add TLS session resumption support to streams as described in this RFC?
Real name Yes No Abstain
Final result: 0 0 0
This poll has been closed.

Patches and Tests

Implementation PR: https://github.com/php/php-src/pull/20296

Implementation

After the project is implemented, this section should contain:

  1. the version it was merged into
  2. a link to the git commit(s)
  3. a link to the PHP manual entry for the feature
  4. a link to the language specification section (if any)

References

Rejected Features

SSL_SESS_CACHE_CLIENT and SSL_SESS_CACHE_BOTH constants: These OpenSSL cache modes don't map to PHP's stream architecture where each connection creates a separate SSL_CTX. Client-side resumption is handled explicitly via the session_data option, making cache-mode constants unnecessary. Only boolean session_cache option is provided for servers.

Automatic client-side caching: Considered adding a session_cache => true option for clients that would automatically cache by peer_name, but rejected as it adds magic behavior and state management concerns across requests. Developers can easily implement this pattern with session_new_cb as shown in examples.

Session metadata in callbacks: Discussed passing additional information like protocol version, cipher suite, and expiry time to callbacks. Rejected to keep the API simple - developers can use OpenSSL functions directly if they need to inspect session details.

Changelog

  • 2025-12-22: Initial RFC published
rfc/tls_session_resumption_api.txt · Last modified: by bukka

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