Date: Fri, 09 Feb 2001 01:28:36 +0000 From: Tony Finch <dot@dotat.at> To: FreeBSD-gnats-submit@freebsd.org Subject: kern/24962: properly delay acks in half-closed TCP connections Message-ID: <E14R2MW-0002gK-00@hand.dotat.at>
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>Number: 24962
>Category: kern
>Synopsis: properly delay acks in half-closed TCP connections
>Confidential: no
>Severity: non-critical
>Priority: low
>Responsible: freebsd-bugs
>State: open
>Quarter:
>Keywords:
>Date-Required:
>Class: sw-bug
>Submitter-Id: current-users
>Arrival-Date: Thu Feb 08 17:30:03 PST 2001
>Closed-Date:
>Last-Modified:
>Originator: Tony Finch
>Release: FreeBSD 4.2-STABLE i386
>Organization:
Covalent Technologies, Inc.
>Environment:
FreeBSD hand.dotat.at 4.2-STABLE FreeBSD 4.2-STABLE #2: Thu Feb 8 08:45:48 GMT 2001 fanf@hand.dotat.at:/FreeBSD/obj/FreeBSD/releng4/sys/DELL-Latitude-CSx i386
>Description:
On page 47 of TCP/IP Illustrated, Volume 3, Stevens says:
``One question is why does the client immediately ACK the first two of the
three server replies, since they arrive in a short amount of time (44ms)?
The answer is in the TCP_REASS macro (p. 908 in Volume 2), which is invoked
for each segment of data received by the client. Since the client's end
of the connection enters the FIN_WAIT_2 state then segment 4 is processed,
the test in TCP_REASS for whether the state is ESTABLISHED fails, causing
an immediate ACK instead of a delayed ACK. This "feature" is not unique
to T/TCP but can be seen with the Net/3 code whenever one end half-closes
a TCP connection and enters the FIN_WAIT_1 or FIN_WAIT_2 state. From that
point on, every segment of data from the peer is immediately ACKed. The
test for the ESTABLISHED state in the TCP_REASS macro prevents data from
being passed to the application before the three-way handshake completes.
There is no need to immediately ACK in-sequence data when the connection
state is greater than ESTABLISHED (i.e. this test should be changed).''
>How-To-Repeat:
>Fix:
Index: tcp_input.c
===================================================================
RCS file: /home/ncvs/src/sys/netinet/tcp_input.c,v
retrieving revision 1.107.2.4
diff -u -r1.107.2.4 tcp_input.c
--- tcp_input.c 2000/08/16 06:14:23 1.107.2.4
+++ tcp_input.c 2001/02/08 22:58:38
@@ -176,7 +176,7 @@
#define TCP_REASS(tp, th, tlenp, m, so, flags) { \
if ((th)->th_seq == (tp)->rcv_nxt && \
LIST_EMPTY(&(tp)->t_segq) && \
- (tp)->t_state == TCPS_ESTABLISHED) { \
+ (tp)->t_state >= TCPS_ESTABLISHED) { \
if (tcp_delack_enabled) \
callout_reset(tp->tt_delack, tcp_delacktime, \
tcp_timer_delack, tp); \
>Release-Note:
>Audit-Trail:
>Unformatted:
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