sctp_maxspace
#define sctp_sbspace(asoc, sb) ((long) ((sctp_maxspace(sb) > (asoc)->sb_cc) ? (sctp_maxspace(sb) - (asoc)->sb_cc) : 0))
#define sctp_sbspace_failedmsgs(sb) ((long) ((sctp_maxspace(sb) > SCTP_SBAVAIL(sb)) ? (sctp_maxspace(sb) - SCTP_SBAVAIL(sb)) : 0))