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 client session can be copied using stream_socket_enable_crypto session_stream parameter, developers cannot:

  • Save and restore client sessions across PHP requests
  • Implement custom server-side session storage (Redis, Memcached, databases)
  • Control session cache behavior and use internal session cache (due to not re-using SSL_CTX)

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
$previousSession = $_SESSION['tls_session'] ?? null;
 
$context = stream_context_create([
    'ssl' => [
        'peer_name' => 'api.example.com',
        'session_data' => $previousSession ? OpenSSLSession::import($previousSession) : null,
        'session_new_cb' => function($stream, OpenSSLSession $session) {
            $_SESSION['tls_session'] = $session->export();
        }
    ]
]);
 
$fp = stream_socket_client('tls://api.example.com:443', context: $context);
// Second connection will resume, saving ~100ms of handshake time
?>

Proposal

OpenSSLSession class

The proposal introduces a new class representing an SSL session that is used by new stream context options and callbacks. The class wraps an underlying SSL_SESSION and provides methods for inspection, serialization, and import/export.

Class Definition

final class OpenSSLSession
{
    /** @var string Binary session ID assigned by the server */
    public readonly string $id;
 
    /**
     * Export the session to a portable format.
     *
     * @param int $format One of existing OPENSSL_ENCODING_PEM (default) or OPENSSL_ENCODING_DER
     * @return string Encoded session data
     * @throws \Exception on export failure
     */
    public function export(int $format = OPENSSL_ENCODING_PEM): string {}
 
    /**
     * Import a session from previously exported data.
     *
     * @param string $data Encoded session data (DER or PEM)
     * @param int $format One of exiting OPENSSL_ENCODING_PEM (default) or OPENSSL_ENCODING_DER
     * @return OpenSSLSession
     * @throws \Exception on import failure
     */
    public static function import(string $data, int $format = OPENSSL_ENCODING_PEM): OpenSSLSession {}
 
    /** Whether this session can be used for resumption */
    public function isResumable(): bool {}
 
    /** Session lifetime in seconds as configured by the server */
    public function getTimeout(): int {}
 
    /** Unix timestamp of when the session was created */
    public function getCreatedAt(): int {}
 
    /** TLS protocol version string (e.g. "TLSv1.3") or null if unknown */
    public function getProtocol(): ?string {}
 
    /** Cipher suite name (e.g. "TLS_AES_256_GCM_SHA384") or null */
    public function getCipher(): ?string {}
 
    /** Whether the session contains a session ticket */
    public function hasTicket(): bool {}
 
    /** Server-suggested ticket lifetime in seconds, or null if no ticket */
    public function getTicketLifetimeHint(): ?int {}
 
    /** @internal Serializes to PEM for use with serialize()/unserialize() */
    public function __serialize(): array {}
 
    /** @internal Restores from PEM data */
    public function __unserialize(array $data): void {}
}

Construction

OpenSSLSession cannot be directly instantiated. Instances are obtained through:

  • The session_new_cb callback (receives an OpenSSLSession for a newly established session)
  • OpenSSLSession::import() (restores a session from previously exported data)
  • PHP's unserialize() (restores a session serialized with serialize())

Export/Import Formats

Two encoding formats are supported via the existing OPENSSL_ENCODING_DER and OPENSSL_ENCODING_PEM constants:

  • PEM (default): Base64-encoded text format, human-readable and suitable for configuration files, text-based storage, and debugging
  • DER: Compact binary format suitable for storage in databases, Redis, or binary-safe caches
<?php
// Export as PEM (default, text-based)
$pem = $session->export(); // or $session->export(OPENSSL_ENCODING_PEM)
$restored = OpenSSLSession::import($pem);
 
// Export as DER (compact, binary)
$der = $session->export(OPENSSL_ENCODING_DER);
$restored = OpenSSLSession::import($der, OPENSSL_ENCODING_DER);
 
// PHP serialization also works (uses PEM internally)
$serialized = serialize($session);
$restored = unserialize($serialized);
?>

Introspection

The class provides read-only access to session metadata for logging, debugging, and cache management: 

<?php
// Inside session_new_cb
$newCb = function($stream, OpenSSLSession $session) {
    echo "Session ID: " . bin2hex($session->id) . "\n";
    echo "Protocol: " . $session->getProtocol() . "\n";       // "TLSv1.3"
    echo "Cipher: " . $session->getCipher() . "\n";           // "TLS_AES_256_GCM_SHA384"
    echo "Resumable: " . ($session->isResumable() ? "yes" : "no") . "\n";
    echo "Has ticket: " . ($session->hasTicket() ? "yes" : "no") . "\n";
    echo "Timeout: " . $session->getTimeout() . "s\n";
    echo "Created: " . date('c', $session->getCreatedAt()) . "\n";
 
    if ($session->hasTicket()) {
        echo "Ticket lifetime hint: " . $session->getTicketLifetimeHint() . "s\n";
    }
};
?>

New Stream Context Options

This proposal adds the following SSL stream context options:

Client-Only Options

  • session_data (?OpenSSLSession) - Instance of OpenSSLSession from a previous connection to resume. Data is typically obtained from a session_new_cb callback and restored via OpenSSLSession::import(). The null value is ignored.
  • session_new_cb (callable) - Callback invoked when a new session is established.
    • Signature: function(resource $stream, OpenSSLSession $session): void
    • $stream: The SSL stream resource
    • $session: The OpenSSLSession object representing the new session

Server-Only Options

  • session_cache (bool, default: true) - Enable or disable internal 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_id_context (string) - Server identifier for session binding. Required when session_new_cb is set with peer verification enabled, and for the internal session cache with peer verification.
  • 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.
    • Signature: function(resource $stream, string $sessionId): ?OpenSSLSession
    • Must return an OpenSSLSession instance 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
  • no_ticket (bool) - Disable session ticket mechanism (TLS 1.2 Session Tickets / TLS 1.3 NewSessionTicket).

Behavior

Client Behavior

When a client stream is created: 

1. If ''session_data'' is provided as an ''OpenSSLSession'' instance, attempt to resume with that session
2. If ''session_new_cb'' is provided, call it with the stream and an ''OpenSSLSession'' object when a new session is established or a new session ticket is received
3. If ''session_stream'' is provided (existing option), the SSL context and session are copied from the provided stream
4. When ''session_new_cb'' or ''session_data'' is set, client-side external cache mode is enabled (''SSL_SESS_CACHE_CLIENT | SSL_SESS_CACHE_NO_INTERNAL'')

Note: Client-side session resumption requires either explicit management via session_data and session_new_cb or setting session_stream which copies the session from the provided stream.

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 (ValueError if missing)
  • Session tickets are disabled (SSL_OP_NO_TICKET is set); attempting to explicitly enable tickets raises ValueError
  • session_remove_cb is optional

Session Context ID Requirements:

  • session_id_context is required when peer verification is enabled (SSL_VERIFY_PEER) and either session_new_cb or the internal session cache is used
  • It binds sessions to a specific server context, preventing session reuse across different virtual hosts or applications

Error Handling

The following validation errors are raised:

  • TypeError if session_new_cb, session_get_cb, or session_remove_cb is not a valid callable
  • TypeError if session_data is not an OpenSSLSession instance
  • TypeError if session_get_cb returns a value that is not OpenSSLSession or null
  • ValueError if session_get_cb is provided without session_new_cb
  • ValueError if session_id_context is missing when required
  • ValueError if session_cache_size or session_timeout is not positive
  • ValueError if no_ticket is set to false when session_get_cb is set
  • E_WARNING if callbacks are used with persistent streams (not supported)
  • E_WARNING if session_data contains an OpenSSLSession with an invalid or expired session (falls back to full handshake)

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, OpenSSLSession $session) use ($host) {
                global $sessions;
                $sessions[$host] = $session;
            }
        ]
    ]);
}
 
// 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')
);
?>

Client: Cross-Request Persistence

<?php
// Save session to file/cache for use in subsequent HTTP requests
$sessionFile = '/tmp/tls_session_api.pem';
 
$previousSession = null;
if (file_exists($sessionFile)) {
    $previousSession = OpenSSLSession::import(file_get_contents($sessionFile));
    if (!$previousSession->isResumable()) {
        $previousSession = null;
    }
}
 
$context = stream_context_create([
    'ssl' => [
        'peer_name' => 'api.example.com',
        'session_data' => $previousSession,
        'session_new_cb' => function($stream, OpenSSLSession $session) use ($sessionFile) {
            file_put_contents($sessionFile, $session->export());
        }
    ]
]);
 
$fp = stream_socket_client('tls://api.example.com:443', context: $context);
?>

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_id_context' => 'example.com',
        '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_id_context' => 'myapp',
 
        'session_new_cb' => function($stream, OpenSSLSession $session) use ($redis) {
            $key = 'tls_session:' . bin2hex($session->id);
            $redis->setex($key, 7200, $session->export());
        },
 
        'session_get_cb' => function($stream, string $id) use ($redis): ?OpenSSLSession {
            $key = 'tls_session:' . bin2hex($id);
            $result = $redis->get($key);
            return $result !== false ? OpenSSLSession::import($result) : null;
        },
 
        'session_remove_cb' => function($stream, string $id) use ($redis): void {
            $key = 'tls_session:' . bin2hex($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);
?>

Session mode differences

When session is enabled there are some differences in behaviour that is good to keep in mind.

  • Applications that rely on every connection being a full handshake (rare) might observe different timing characteristics
  • The SSL_CTX is shared which means that the server certificates are loaded earlier and are not reloaded on subsequent acceptence (this is only if session is used however)

Backward Incompatible Changes

This RFC introduces no backward incompatible changes

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:

  • 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

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

Changelog

  • 0.2 2026-02-13: Updated callback signatures to use OpenSSLSession, session_data and session_get_cb accept only OpenSSLSession and other changes
  • 0.1 2025-12-22: Initial RFC published
rfc/tls_session_resumption_api.txt · Last modified: by bukka

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