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.)
This commit is contained in:
Angus Gratton
parent
2dbecc04cc
commit
d8213d00db
7 changed files with 198 additions and 87 deletions
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@ -1378,7 +1378,8 @@
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#define SSL_ANTI_REPLAY_ENABLED MBEDTLS_SSL_ANTI_REPLAY_ENABLED
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#define SSL_ARC4_DISABLED MBEDTLS_SSL_ARC4_DISABLED
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#define SSL_ARC4_ENABLED MBEDTLS_SSL_ARC4_ENABLED
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#define SSL_BUFFER_LEN MBEDTLS_SSL_BUFFER_LEN
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#define SSL_BUFFER_LEN ( ( ( MBEDTLS_SSL_IN_BUFFER_LEN ) < ( MBEDTLS_SSL_OUT_BUFFER_LEN ) ) \
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? ( MBEDTLS_SSL_IN_BUFFER_LEN ) : ( MBEDTLS_SSL_OUT_BUFFER_LEN ) )
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#define SSL_CACHE_DEFAULT_MAX_ENTRIES MBEDTLS_SSL_CACHE_DEFAULT_MAX_ENTRIES
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#define SSL_CACHE_DEFAULT_TIMEOUT MBEDTLS_SSL_CACHE_DEFAULT_TIMEOUT
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#define SSL_CBC_RECORD_SPLITTING_DISABLED MBEDTLS_SSL_CBC_RECORD_SPLITTING_DISABLED
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@ -2896,7 +2896,51 @@
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//#define MBEDTLS_SSL_CACHE_DEFAULT_MAX_ENTRIES 50 /**< Maximum entries in cache */
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/* SSL options */
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//#define MBEDTLS_SSL_MAX_CONTENT_LEN 16384 /**< Maxium fragment length in bytes, determines the size of each of the two internal I/O buffers */
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/** \def MBEDTLS_SSL_MAX_CONTENT_LEN
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*
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* Maximum fragment length in bytes.
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*
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* Determines the size of both the incoming and outgoing TLS I/O buffers.
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*
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* Uncommenting MBEDTLS_SSL_IN_CONTENT_LEN and/or MBEDTLS_SSL_OUT_CONTENT_LEN
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* will override this length by setting maximum incoming and/or outgoing
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* fragment length, respectively.
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*/
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//#define MBEDTLS_SSL_MAX_CONTENT_LEN 16384
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/** \def MBEDTLS_SSL_IN_CONTENT_LEN
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*
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* Maximum incoming fragment length in bytes.
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*
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* Uncomment to set the size of the inward TLS buffer independently of the
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* outward buffer.
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*/
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//#define MBEDTLS_SSL_IN_CONTENT_LEN 16384
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/** \def MBEDTLS_SSL_OUT_CONTENT_LEN
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*
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* Maximum outgoing fragment length in bytes.
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*
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* Uncomment to set the size of the outward TLS buffer independently of the
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* inward buffer.
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*
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* It is possible to save RAM by setting a smaller outward buffer, while keeping
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* the default inward 16384 byte buffer to conform to the TLS specification.
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*
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* The minimum required outward buffer size is determined by the handshake
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* protocol's usage. Handshaking will fail if the outward buffer is too small.
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* The specific size requirement depends on the configured ciphers and any
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* certificate data which is sent during the handshake.
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*
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* For absolute minimum RAM usage, it's best to enable
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* MBEDTLS_SSL_MAX_FRAGMENT_LENGTH and reduce MBEDTLS_SSL_MAX_CONTENT_LEN. This
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* reduces both incoming and outgoing buffer sizes. However this is only
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* guaranteed if the other end of the connection also supports the TLS
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* max_fragment_len extension. Otherwise the connection may fail.
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*/
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//#define MBEDTLS_SSL_OUT_CONTENT_LEN 16384
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//#define MBEDTLS_SSL_DEFAULT_TICKET_LIFETIME 86400 /**< Lifetime of session tickets (if enabled) */
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//#define MBEDTLS_PSK_MAX_LEN 32 /**< Max size of TLS pre-shared keys, in bytes (default 256 bits) */
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//#define MBEDTLS_SSL_COOKIE_TIMEOUT 60 /**< Default expiration delay of DTLS cookies, in seconds if HAVE_TIME, or in number of cookies issued */
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@ -220,7 +220,7 @@
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#endif
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/*
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* Maxium fragment length in bytes,
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* Maximum fragment length in bytes,
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* determines the size of each of the two internal I/O buffers.
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*
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* Note: the RFC defines the default size of SSL / TLS messages. If you
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@ -234,6 +234,14 @@
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#define MBEDTLS_SSL_MAX_CONTENT_LEN 16384 /**< Size of the input / output buffer */
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#endif
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#if !defined(MBEDTLS_SSL_IN_CONTENT_LEN)
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#define MBEDTLS_SSL_IN_CONTENT_LEN MBEDTLS_SSL_MAX_CONTENT_LEN
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#endif
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#if !defined(MBEDTLS_SSL_OUT_CONTENT_LEN)
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#define MBEDTLS_SSL_OUT_CONTENT_LEN MBEDTLS_SSL_MAX_CONTENT_LEN
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#endif
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/* \} name SECTION: Module settings */
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/*
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@ -2418,7 +2426,8 @@ void mbedtls_ssl_conf_cert_req_ca_list( mbedtls_ssl_config *conf,
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#if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
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/**
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* \brief Set the maximum fragment length to emit and/or negotiate
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* (Default: MBEDTLS_SSL_MAX_CONTENT_LEN, usually 2^14 bytes)
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* (Default: the smaller of MBEDTLS_SSL_IN_CONTENT_LEN and
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* MBEDTLS_SSL_OUT_CONTENT_LEN, usually 2^14 bytes)
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* (Server: set maximum fragment length to emit,
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* usually negotiated by the client during handshake
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* (Client: set maximum fragment length to emit *and*
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@ -143,32 +143,73 @@
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#define MBEDTLS_SSL_PADDING_ADD 0
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#endif
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#define MBEDTLS_SSL_PAYLOAD_LEN ( MBEDTLS_SSL_MAX_CONTENT_LEN \
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+ MBEDTLS_SSL_COMPRESSION_ADD \
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+ MBEDTLS_MAX_IV_LENGTH \
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+ MBEDTLS_SSL_MAC_ADD \
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+ MBEDTLS_SSL_PADDING_ADD \
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)
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#define MBEDTLS_SSL_PAYLOAD_OVERHEAD ( MBEDTLS_SSL_COMPRESSION_ADD + \
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MBEDTLS_MAX_IV_LENGTH + \
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MBEDTLS_SSL_MAC_ADD + \
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MBEDTLS_SSL_PADDING_ADD \
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)
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#define MBEDTLS_SSL_IN_PAYLOAD_LEN ( MBEDTLS_SSL_PAYLOAD_OVERHEAD + \
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( MBEDTLS_SSL_IN_CONTENT_LEN ) )
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#define MBEDTLS_SSL_OUT_PAYLOAD_LEN ( MBEDTLS_SSL_PAYLOAD_OVERHEAD + \
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( MBEDTLS_SSL_OUT_CONTENT_LEN ) )
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/* Maximum length we can advertise as our max content length for
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RFC 6066 max_fragment_length extension negotiation purposes
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(the lesser of both sizes, if they are unequal.)
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*/
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#define MBEDTLS_TLS_EXT_ADV_CONTENT_LEN ( \
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(MBEDTLS_SSL_IN_CONTENT_LEN > MBEDTLS_SSL_OUT_CONTENT_LEN) \
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? ( MBEDTLS_SSL_OUT_CONTENT_LEN ) \
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: ( MBEDTLS_SSL_IN_CONTENT_LEN ) \
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)
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/*
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* Check that we obey the standard's message size bounds
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*/
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#if MBEDTLS_SSL_MAX_CONTENT_LEN > 16384
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#error Bad configuration - record content too large.
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#error "Bad configuration - record content too large."
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#endif
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#if MBEDTLS_SSL_PAYLOAD_LEN > 16384 + 2048
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#error Bad configuration - protected record payload too large.
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#if MBEDTLS_SSL_IN_CONTENT_LEN > MBEDTLS_SSL_MAX_CONTENT_LEN
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#error "Bad configuration - incoming record content should not be larger than MBEDTLS_SSL_MAX_CONTENT_LEN."
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#endif
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#if MBEDTLS_SSL_OUT_CONTENT_LEN > MBEDTLS_SSL_MAX_CONTENT_LEN
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#error "Bad configuration - outgoing record content should not be larger than MBEDTLS_SSL_MAX_CONTENT_LEN."
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#endif
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#if MBEDTLS_SSL_IN_PAYLOAD_LEN > MBEDTLS_SSL_MAX_CONTENT_LEN + 2048
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#error "Bad configuration - incoming protected record payload too large."
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#endif
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#if MBEDTLS_SSL_OUT_PAYLOAD_LEN > MBEDTLS_SSL_MAX_CONTENT_LEN + 2048
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#error "Bad configuration - outgoing protected record payload too large."
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#endif
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/* Calculate buffer sizes */
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/* Note: Even though the TLS record header is only 5 bytes
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long, we're internally using 8 bytes to store the
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implicit sequence number. */
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#define MBEDTLS_SSL_HEADER_LEN 13
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#define MBEDTLS_SSL_BUFFER_LEN \
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( ( MBEDTLS_SSL_HEADER_LEN ) + ( MBEDTLS_SSL_PAYLOAD_LEN ) )
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#define MBEDTLS_SSL_IN_BUFFER_LEN \
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( ( MBEDTLS_SSL_HEADER_LEN ) + ( MBEDTLS_SSL_IN_PAYLOAD_LEN ) )
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#define MBEDTLS_SSL_OUT_BUFFER_LEN \
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( ( MBEDTLS_SSL_HEADER_LEN ) + ( MBEDTLS_SSL_OUT_PAYLOAD_LEN ) )
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#ifdef MBEDTLS_ZLIB_SUPPORT
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/* Compression buffer holds both IN and OUT buffers, so should be size of the larger */
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#define MBEDTLS_SSL_COMPRESS_BUFFER_LEN ( \
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( MBEDTLS_SSL_IN_BUFFER_LEN > MBEDTLS_SSL_OUT_BUFFER_LEN ) \
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? MBEDTLS_SSL_IN_BUFFER_LEN \
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: MBEDTLS_SSL_OUT_BUFFER_LEN \
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)
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#endif
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/*
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* TLS extension flags (for extensions with outgoing ServerHello content
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