Use correct TLSv1.2 max frame size limits

2 files changed, 14 insertions, 10 deletions

diff --git a/vespalib/src/vespa/vespalib/net/tls/impl/openssl_crypto_codec_impl.cpp b/vespalib/src/vespa/vespalib/net/tls/impl/openssl_crypto_codec_impl.cpp
index 214b61cc14c..53d21e398ba 100644
--- a/vespalib/src/vespa/vespalib/net/tls/impl/openssl_crypto_codec_impl.cpp
+++ b/vespalib/src/vespa/vespalib/net/tls/impl/openssl_crypto_codec_impl.cpp
@@ -290,7 +290,7 @@ EncodeResult OpenSslCryptoCodecImpl::encode(const char* plaintext, size_t plaint
     }
     size_t bytes_consumed = 0;
     if (plaintext_size != 0) {
-        int to_consume = static_cast<int>(std::min(plaintext_size, MaximumFramePayloadSize));
+        int to_consume = static_cast<int>(std::min(plaintext_size, MaximumFramePlaintextSize));
         // SSL_write encodes plaintext to ciphertext and writes to _output_bio
         int consumed = ::SSL_write(_ssl.get(), plaintext, to_consume);
         LOG(spam, "After SSL_write() -> %d, _input_bio pending=%d, _output_bio pending=%d",
diff --git a/vespalib/src/vespa/vespalib/net/tls/impl/openssl_crypto_codec_impl.h b/vespalib/src/vespa/vespalib/net/tls/impl/openssl_crypto_codec_impl.h
index ad9e8575b97..44ca8859596 100644
--- a/vespalib/src/vespa/vespalib/net/tls/impl/openssl_crypto_codec_impl.h
+++ b/vespalib/src/vespa/vespalib/net/tls/impl/openssl_crypto_codec_impl.h
@@ -27,20 +27,24 @@ class OpenSslCryptoCodecImpl : public CryptoCodec {
 public:
     OpenSslCryptoCodecImpl(::SSL_CTX& ctx, Mode mode);
 
-    // These assumptions are cheekily hoisted from gRPC.
-    // As is mentioned there, the max protocol overhead per frame is not available
-    // dynamically, so an educated guess is made.
-    static constexpr size_t MaximumTlsFrameSize = 16*1024;
-    static constexpr size_t MaximumTlsFrameProtocolOverhead = 100;
-    static constexpr size_t MaximumFramePayloadSize = MaximumTlsFrameSize - MaximumTlsFrameProtocolOverhead;
+    /*
+     * From RFC 8449 (Record Size Limit Extension for TLS), section 1:
+     *   "TLS versions 1.2 [RFC5246] and earlier permit senders to
+     *    generate records 16384 octets in size, plus any expansion
+     *    from compression and protection up to 2048 octets (though
+     *    typically this expansion is only 16 octets). TLS 1.3 reduces
+     *    the allowance for expansion to 256 octets."
+     *
+     * We're on TLSv1.2, so make room for the worst case.
+     */
+    static constexpr size_t MaximumTlsFrameSize = 16384 + 2048;
+    static constexpr size_t MaximumFramePlaintextSize = 16384;
 
     size_t min_encode_buffer_size() const noexcept override {
         return MaximumTlsFrameSize;
     }
     size_t min_decode_buffer_size() const noexcept override {
-        // Technically this would be MaximumFramePayloadSize, but we like power
-        // of two numbers for buffer sizes, yes we do.
-        return MaximumTlsFrameSize;
+        return MaximumFramePlaintextSize;
     }
 
     HandshakeResult handshake(const char* from_peer, size_t from_peer_buf_size,