Exhausted net.inet.tcp.reass.maxsegments block recovering tcp session
(for this socket and any other socket waiting for retransmited packets).

After exhausted net.inet.tcp.reass.maxsegments allocation new entry in
tcp_reass failed (for this socket and any other socket waiting for
retransmited packets).

        te = uma_zalloc(V_tcp_reass_zone, M_NOWAIT);
        if (te == NULL) {
                TCPSTAT_INC(tcps_rcvmemdrop);
                m_freem(m);
                *tlenp = 0;
                return (0);
        }

That is packet can be reduce reassemble queue dropped.

Fix: 

Workaround: set net.inet.tcp.reass.maxsegments in /boot/loader.conf to big value.
How-To-Repeat: Need high bandwidth-delay channel with high, but rare drop.

After lost incoming TCP segment and before got retranmited this
segment net.inet.tcp.reass.maxsegments exhausted.

Software emulation:

Tuning by http://fasterdata.es.net/fasterdata/host-tuning/freebsd/
sysctl net.inet.tcp.recvspace=1048576
sysctl net.inet.ip.dummynet.pipe_slot_limit=3000
ipfw pipe 1 config delay 300 queue 3000
ipfw add 100 pipe 1 ip from test_hot to me

fetch http://test_host/big_file

After reach 7MB/s: ipfw pipe 1 config delay 300 queue 3000 plr 0.1