Set when the expected next acknowledgment number is set for the reverse direction and it’s less than the current acknowledgment number.

    /* ACKED LOST PACKET     * If this segment acks beyond the 'max seq to be acked' in the other direction     * then that means we have missed packets going in the     * other direction.     * It might also indicate we are resuming from a Zero Window,     * where a Probe is just followed by an ACK opening again the window.     * See issue 8404.     *     * We only check this if we have actually seen some seq numbers     * in the other direction.     */    if( tcpd->rev->tcp_analyze_seq_info->maxseqtobeacked    &&  GT_SEQ(ack, tcpd->rev->tcp_analyze_seq_info->maxseqtobeacked)    &&  (flags&(TH_ACK))!=0 ) {        if(!tcpd->ta) {            tcp_analyze_get_acked_struct(pinfo->num, seq, ack, TRUE, tcpd);        }        /* resuming from a Zero Window Probe which re-opens the window,         * mark it as a Window Update         */        if(EQ_SEQ(ack,tcpd->fwd->tcp_analyze_seq_info->lastack+1)        && (seq==tcpd->fwd->tcp_analyze_seq_info->nextseq)        && (tcpd->rev->lastsegmentflags&TCP_A_ZERO_WINDOW_PROBE) ) {            tcpd->rev->tcp_analyze_seq_info->nextseq=ack;            tcpd->rev->tcp_analyze_seq_info->maxseqtobeacked=ack;            tcpd->ta->flags|=TCP_A_WINDOW_UPDATE;        }        /* real ACKED LOST PACKET */        else {            /* We ensure there is no matching packet waiting in the unacked list,             * and take this opportunity to push the tail further than this single packet             */            guint32 tail_le = 0, tail_re = 0;            for(ual=tcpd->rev->tcp_analyze_seq_info->segments; ual; ual=ual->next) {                if(tail_le == tail_re) { /* init edge values */                    tail_le = ual->seq;                    tail_re = ual->nextseq;                }                /* Only look at what happens above the current ACK value,                 * as what happened before is definetely ACKed here and can be                 * safely ignored. */                if(GE_SEQ(ual->seq,ack)) {                    /* if the left edge is contiguous, move the tail leftward */                    if(EQ_SEQ(ual->nextseq,tail_le)) {                        tail_le = ual->seq;                    }                    /* otherwise, we have isolated segments above what is being ACKed here,                     * and we reinit the tails with the current values */                    else {                        tail_le = ual->seq;                        tail_re = ual->nextseq; // move the end tail                    }                }            }            /* a tail was found and we can push the maxseqtobeacked further */            if(EQ_SEQ(ack,tail_le) && GT_SEQ(tail_re, ack)) {                tcpd->rev->tcp_analyze_seq_info->maxseqtobeacked=tail_re;            }            /* otherwise, just take into account the value being ACKed now */            else {                tcpd->rev->tcp_analyze_seq_info->maxseqtobeacked=ack;            }            tcpd->ta->flags|=TCP_A_ACK_LOST_PACKET;        }    }

# cat tcp_acked_unseen_segment.pkt 0   socket(..., SOCK_STREAM, IPPROTO_TCP) = 3+0  setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0+0  bind(3, ..., ...) = 0+0  listen(3, 1) = 0+0 < S 0:0(0) win 16000 <mss 1460>+0 > S. 0:0(0) ack 1 <...>+0.01 < . 1:1(0) ack 1 win 16000+0 accept(3, ..., ...) = 4+0 < P. 1:21(20) ack 1 win 15000+0 < P. 21:41(20) ack 1 win 15000+0 < P. 41:61(20) ack 1 win 15000+0.1 read(4, ..., 60) = 60 #

    /* Store the highest continuous seq number seen so far for 'max seq to be acked',     * so we can detect TCP_A_ACK_LOST_PACKET condition.     * If this ever happens, this boundary value can "jump" further in order to     * avoid duplicating multiple messages for the very same lost packet. See later     * how ACKED LOST PACKET are handled.     * Zero Window Probes are logically left out at this moment, but if their data     * really were to be ack'ed, then it will be done later when analyzing their     * Probe ACK (be it a real Probe ACK, or an ordinary ACK moving the RCV Window).     */    if(EQ_SEQ(seq, tcpd->fwd->tcp_analyze_seq_info->maxseqtobeacked) || !tcpd->fwd->tcp_analyze_seq_info->maxseqtobeacked) {        if( !tcpd->ta || !(tcpd->ta->flags&TCP_A_ZERO_WINDOW_PROBE) ) {            tcpd->fwd->tcp_analyze_seq_info->maxseqtobeacked=tcpd->fwd->tcp_analyze_seq_info->nextseq;        }    }