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Documentation of mbedtls_ssl_conf_max_frag_len() inaccurate #2258

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hanno-arm opened this issue Dec 4, 2018 · 1 comment

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commented Dec 4, 2018

The documentation of mbedtls_ssl_conf_max_frag_len() states:

/**
 * \brief          Set the maximum fragment length to emit and/or negotiate
 *                 (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*
 *                 negotiate with the server during handshake)
...
 * \param mfl_code Code for maximum fragment length (allowed values:
 *                 MBEDTLS_SSL_MAX_FRAG_LEN_512,  MBEDTLS_SSL_MAX_FRAG_LEN_1024,
 *                 MBEDTLS_SSL_MAX_FRAG_LEN_2048, MBEDTLS_SSL_MAX_FRAG_LEN_4096)
 *
 * \return         0 if successful or MBEDTLS_ERR_SSL_BAD_INPUT_DATA
 */

This suggests that for, say, MBEDTLS_SSL_IN_CONTENT_LEN = MBEDTLS_SSL_OUT_CONTENT_LEN = 4096, a call

mbedtls_ssl_conf_max_frag_len( conf, MBEDTLS_SSL_MAX_FRAG_LEN_4096 );

would be redundant. In particular, one might think that enabling MBEDTLS_SSL_MAX_FRAGMENT_LENGTH and using a default value for the I/O buffers would lead to the MFL extension being used to communicate the size limitation. However, that's not true: The MFL extension will only be added to the ClientHello if mbedtls_ssl_conf_max_frag_len() has been called. That needs to be clarified in the documentation.

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commented Dec 4, 2018

ARM Internal Ref: IOTSSL-2656

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