expiration;         ///< the time at which a transaction expires
    ref_block_num       ///< specifies a block num in the last 2^16 blocks.
    ref_block_prefix    ///< specifies the lower 32 bits of the blockid at get_ref_blocknum
    max_net_usage_words /// upper limit on total network bandwidth (in 8 byte words) billed for this transaction
    max_cpu_usage_ms    /// upper limit on the total CPU time billed for this transaction
    delay_sec           /// number of seconds to delay this transaction for during which it may be canceled.

How to get ref_block_num:

     /**
      * @return the absolute block number given the relative ref_block_num
      */
      block_num_type get_ref_blocknum( block_num_type head_blocknum )const {
         return ((head_blocknum/0xffff)*0xffff) + head_blocknum%0xffff;
      }

Source: https://github.com/EOSIO/eos/blob/e87d245da0e458edc66139c3ca59f71861bcc1dd/libraries/chain/include/eosio/chain/transaction.hpp#L31