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Let MBEDTLS_SSL_MAX_CONTENT_LEN to be split into outward & inward sizes

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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.)
This commit is contained in:
Angus Gratton 2016-05-25 20:56:48 +10:00
parent 2dbecc04cc
commit d8213d00db
7 changed files with 198 additions and 87 deletions
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46
include/mbedtls/config.h
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@ -2896,7 +2896,51 @@
//#define MBEDTLS_SSL_CACHE_DEFAULT_MAX_ENTRIES 50 /**< Maximum entries in cache */
/* SSL options */
//#define MBEDTLS_SSL_MAX_CONTENT_LEN 16384 /**< Maxium fragment length in bytes, determines the size of each of the two internal I/O buffers */
/** \def MBEDTLS_SSL_MAX_CONTENT_LEN
*
* Maximum fragment length in bytes.
*
* Determines the size of both the incoming and outgoing TLS I/O buffers.
*
* Uncommenting MBEDTLS_SSL_IN_CONTENT_LEN and/or MBEDTLS_SSL_OUT_CONTENT_LEN
* will override this length by setting maximum incoming and/or outgoing
* fragment length, respectively.
*/
//#define MBEDTLS_SSL_MAX_CONTENT_LEN 16384
/** \def MBEDTLS_SSL_IN_CONTENT_LEN
*
* Maximum incoming fragment length in bytes.
*
* Uncomment to set the size of the inward TLS buffer independently of the
* outward buffer.
*/
//#define MBEDTLS_SSL_IN_CONTENT_LEN 16384
/** \def MBEDTLS_SSL_OUT_CONTENT_LEN
*
* Maximum outgoing fragment length in bytes.
*
* Uncomment to set the size of the outward TLS buffer independently of the
* inward buffer.
*
* It is possible to save RAM by setting a smaller outward buffer, while keeping
* the default inward 16384 byte buffer to conform to the TLS specification.
*
* The minimum required outward buffer size is determined by the handshake
* protocol's usage. Handshaking will fail if the outward buffer is too small.
* The specific size requirement depends on the configured ciphers and any
* certificate data which is sent during the handshake.
*
* For absolute minimum RAM usage, it's best to enable
* MBEDTLS_SSL_MAX_FRAGMENT_LENGTH and reduce MBEDTLS_SSL_MAX_CONTENT_LEN. This
* reduces both incoming and outgoing buffer sizes. However this is only
* guaranteed if the other end of the connection also supports the TLS
* max_fragment_len extension. Otherwise the connection may fail.
*/
//#define MBEDTLS_SSL_OUT_CONTENT_LEN 16384
//#define MBEDTLS_SSL_DEFAULT_TICKET_LIFETIME 86400 /**< Lifetime of session tickets (if enabled) */
//#define MBEDTLS_PSK_MAX_LEN 32 /**< Max size of TLS pre-shared keys, in bytes (default 256 bits) */
//#define MBEDTLS_SSL_COOKIE_TIMEOUT 60 /**< Default expiration delay of DTLS cookies, in seconds if HAVE_TIME, or in number of cookies issued */