Let MBEDTLS_SSL_MAX_CONTENT_LEN to be split into outward & inward sizes
For the situation where the mbedTLS device has limited RAM, but the
other end of the connection doesn't support the max_fragment_length
extension. To be spec-compliant, mbedTLS has to keep a 16384 byte
incoming buffer. However the outgoing buffer can be made smaller without
breaking spec compliance, and we save some RAM.
See comments in include/mbedtls/config.h for some more details.
(The lower limit of outgoing buffer size is the buffer size used during
handshake/cert negotiation. As the handshake is half-duplex it might
even be possible to store this data in the "incoming" buffer during the
handshake, which would save even more RAM - but it would also be a lot
hackier and error-prone. I didn't really explore this possibility, but
thought I'd mention it here in case someone sees this later on a mission
to jam mbedTLS into an even tinier RAM footprint.)
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 4513550..886c14c 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -220,7 +220,7 @@
#endif
/*
- * Maxium fragment length in bytes,
+ * Maximum fragment length in bytes,
* determines the size of each of the two internal I/O buffers.
*
* Note: the RFC defines the default size of SSL / TLS messages. If you
@@ -234,6 +234,14 @@
#define MBEDTLS_SSL_MAX_CONTENT_LEN 16384 /**< Size of the input / output buffer */
#endif
+#if !defined(MBEDTLS_SSL_IN_CONTENT_LEN)
+#define MBEDTLS_SSL_IN_CONTENT_LEN MBEDTLS_SSL_MAX_CONTENT_LEN
+#endif
+
+#if !defined(MBEDTLS_SSL_OUT_CONTENT_LEN)
+#define MBEDTLS_SSL_OUT_CONTENT_LEN MBEDTLS_SSL_MAX_CONTENT_LEN
+#endif
+
/* \} name SECTION: Module settings */
/*
@@ -2418,7 +2426,8 @@
#if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
/**
* \brief Set the maximum fragment length to emit and/or negotiate
- * (Default: MBEDTLS_SSL_MAX_CONTENT_LEN, usually 2^14 bytes)
+ * (Default: the smaller of MBEDTLS_SSL_IN_CONTENT_LEN and
+ * MBEDTLS_SSL_OUT_CONTENT_LEN, usually 2^14 bytes)
* (Server: set maximum fragment length to emit,
* usually negotiated by the client during handshake
* (Client: set maximum fragment length to emit *and*