unexpected newReno behavior in 2.6.21.5

[Sushant]

Hi all,
I am currently doing some analysis on the TCP newReno implementation
in the Linux kernel and it looks like the sender behavior is not
expected. Here is what I am observing.

Linux kernel: stable version 2.6.21.5

1) _sometimes_, there is no fast recovery: i.e. after receiving three
DUP acks, the sender is not transmitting new packets in response to
the more DUP acks it is receiving after the first three. It does
retransmit the lost packet after 3 DUP acks though.

2) Delayed fast retransmit: _sometimes_, instead of retransmitting the
lost packet after receiving 3 DUP acks, sender waits for large number
(which is 127 most of the time) of DUP acks before retransmitting the
lost packet. But, it keeps on transmitting a new packet for every one
of 127 DUP acks it is receiving.

Has someone seen this behavior or is this behavior expected under some
scenarios. I am using wireshark (previously ethereal) on sender to
analyze all this. I can provide logs if needed or any other
information that you might need. I have provided my sysctl output for
TCP parameters at the end of my mail.

Please cc the replies to me as I am not subscribed to the list.

TIA
-Sushant


#  /sbin/sysctl -a  | grep tcp
net.ipv4.tcp_timestamps = 1
net.ipv4.tcp_window_scaling = 1
net.ipv4.tcp_sack = 0
net.ipv4.tcp_retrans_collapse = 1
net.ipv4.tcp_syn_retries = 5
net.ipv4.tcp_synack_retries = 5
net.ipv4.tcp_max_orphans = 32768
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 = 3
net.ipv4.tcp_ecn = 0
net.ipv4.tcp_dsack = 1
net.ipv4.tcp_mem = 1048576      1048576 1048576
net.ipv4.tcp_wmem = 1048576     1048576 1048576
net.ipv4.tcp_rmem = 1048576     1048576 1048576
net.ipv4.tcp_app_win = 31
net.ipv4.tcp_adv_win_scale = 3
net.ipv4.tcp_tw_reuse = 0
net.ipv4.tcp_frto = 0
net.ipv4.tcp_low_latency = 0
net.ipv4.tcp_no_metrics_save = 1
net.ipv4.tcp_moderate_rcvbuf = 1
net.ipv4.tcp_tso_win_divisor = 3
net.ipv4.tcp_congestion_control = reno
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 = reno bic cubic
net.ipv4.tcp_allowed_congestion_control = reno
sunrpc.tcp_slot_table_entries = 16
#
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