Packet capture attached. Is it possible to tell which side is at fault? They are both responding to each other's ACK with another ACK at 400pps. 10:32:28.991111 IP6 2001:8b0:ffea:0:205:b4ff:fe12:530.35545 > 2001:500:b::1.53: . ack 766 win 58 10:32:28.995046 IP6 2001:500:b::1.53 > 2001:8b0:ffea:0:205:b4ff:fe12:530.35545: . ack 1 win 1559 10:32:28.995092 IP6 2001:8b0:ffea:0:205:b4ff:fe12:530.35545 > 2001:500:b::1.53: . ack 766 win 58 10:32:29.009052 IP6 2001:500:b::1.53 > 2001:8b0:ffea:0:205:b4ff:fe12:530.35545: . ack 1 win 1559 10:32:29.009112 IP6 2001:8b0:ffea:0:205:b4ff:fe12:530.35545 > 2001:500:b::1.53: . ack 766 win 58 10:32:29.010046 IP6 2001:500:b::1.53 > 2001:8b0:ffea:0:205:b4ff:fe12:530.35545: . ack 1 win 1559 10:32:29.010093 IP6 2001:8b0:ffea:0:205:b4ff:fe12:530.35545 > 2001:500:b::1.53: . ack 766 win 58 10:32:29.014039 IP6 2001:500:b::1.53 > 2001:8b0:ffea:0:205:b4ff:fe12:530.35545: . ack 1 win 1559 10:32:29.014087 IP6 2001:8b0:ffea:0:205:b4ff:fe12:530.35545 > 2001:500:b::1.53: . ack 766 win 58 10:32:29.018052 IP6 2001:500:b::1.53 > 2001:8b0:ffea:0:205:b4ff:fe12:530.35545: . ack 1 win 1559 10:32:29.018129 IP6 2001:8b0:ffea:0:205:b4ff:fe12:530.35545 > 2001:500:b::1.53: . ack 766 win 58 10:32:29.021053 IP6 2001:500:b::1.53 > 2001:8b0:ffea:0:205:b4ff:fe12:530.35545: . ack 1 win 1559 10:32:29.021097 IP6 2001:8b0:ffea:0:205:b4ff:fe12:530.35545 > 2001:500:b::1.53: . ack 766 win 58 The latency to 2001:500:b::1 is about 45ms (another ACK arrives before the response to the previous ACK could have been received). The trailing 0 bytes on packets going to 2001:8b0:ffea:0:205:b4ff:fe12:530 have been added by my ISP to avoid a Cisco bug. net.ipv4.tcp_timestamps = 1 net.ipv4.tcp_window_scaling = 1 net.ipv4.tcp_sack = 1 net.ipv4.tcp_retrans_collapse = 1 net.ipv4.tcp_syn_retries = 5 net.ipv4.tcp_synack_retries = 5 net.ipv4.tcp_max_orphans = 131072 net.ipv4.tcp_max_tw_buckets = 180000 net.ipv4.tcp_keepalive_time = 7200 net.ipv4.tcp_keepalive_probes = 9 net.ipv4.tcp_keepalive_intvl = 75 net.ipv4.tcp_retries1 = 3 net.ipv4.tcp_retries2 = 15 net.ipv4.tcp_fin_timeout = 60 net.ipv4.tcp_syncookies = 1 net.ipv4.tcp_tw_recycle = 0 net.ipv4.tcp_abort_on_overflow = 0 net.ipv4.tcp_stdurg = 0 net.ipv4.tcp_rfc1337 = 0 net.ipv4.tcp_max_syn_backlog = 1024 net.ipv4.tcp_orphan_retries = 0 net.ipv4.tcp_fack = 1 net.ipv4.tcp_reordering = 50 net.ipv4.tcp_ecn = 1 net.ipv4.tcp_dsack = 1 net.ipv4.tcp_mem = 82752 110336 165504 net.ipv4.tcp_wmem = 262144 1048576 4194304 net.ipv4.tcp_rmem = 262144 1048576 4194304 net.ipv4.tcp_app_win = 31 net.ipv4.tcp_adv_win_scale = 2 net.ipv4.tcp_tw_reuse = 0 net.ipv4.tcp_frto = 2 net.ipv4.tcp_frto_response = 0 net.ipv4.tcp_low_latency = 0 net.ipv4.tcp_no_metrics_save = 0 net.ipv4.tcp_moderate_rcvbuf = 1 net.ipv4.tcp_tso_win_divisor = 3 net.ipv4.tcp_congestion_control = cubic net.ipv4.tcp_abc = 0 net.ipv4.tcp_mtu_probing = 0 net.ipv4.tcp_base_mss = 512 net.ipv4.tcp_workaround_signed_windows = 0 net.ipv4.tcp_slow_start_after_idle = 1 net.ipv4.tcp_available_congestion_control = cubic reno bic westwood highspeed hybla htcp vegas veno scalable lp yeah illinois net.ipv4.tcp_allowed_congestion_control = cubic reno net.ipv4.tcp_max_ssthresh = 0 net.ipv4.tcp_cookie_size = 0 -- Simon Arlott