connection_bucket_write_limit reveals used link protocol
Plain text of TLS record can't be more than 16384 bytes. Tor before v4 link protocol used 512 bytes cells. connection_bucket_write_limit allows to flush not more than 32 cells. That was 16K limit exactly. With v4 size of cells changed, now it's 514 bytes, and connection_bucket_write_limit allows to flush 32 cells still that in result more than 16Kbytes. Passive adversary can to detect used protocol, and openssl can to create two records per one flush while second record contains not so much actual bytes.
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