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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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3
include/mbedtls/compat-1.3.h
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@ -1378,7 +1378,8 @@
#define SSL_ANTI_REPLAY_ENABLED MBEDTLS_SSL_ANTI_REPLAY_ENABLED
#define SSL_ARC4_DISABLED MBEDTLS_SSL_ARC4_DISABLED
#define SSL_ARC4_ENABLED MBEDTLS_SSL_ARC4_ENABLED
#define SSL_BUFFER_LEN MBEDTLS_SSL_BUFFER_LEN
#define SSL_BUFFER_LEN ( ( ( MBEDTLS_SSL_IN_BUFFER_LEN ) < ( MBEDTLS_SSL_OUT_BUFFER_LEN ) ) \
? ( MBEDTLS_SSL_IN_BUFFER_LEN ) : ( MBEDTLS_SSL_OUT_BUFFER_LEN ) )
#define SSL_CACHE_DEFAULT_MAX_ENTRIES MBEDTLS_SSL_CACHE_DEFAULT_MAX_ENTRIES
#define SSL_CACHE_DEFAULT_TIMEOUT MBEDTLS_SSL_CACHE_DEFAULT_TIMEOUT
#define SSL_CBC_RECORD_SPLITTING_DISABLED MBEDTLS_SSL_CBC_RECORD_SPLITTING_DISABLED