I'm having trouble doing cvs updates from one of my home machines.  (I 
have a number of them, all behind a (shudder) NAT box.)  netstat shows 
that there's a large unacked send queue:

# netstat -nf inet
Active Internet connections
Proto Recv-Q Send-Q  Local Address          Foreign Address        State
tcp        0  30712  192.168.2.200.60649    204.152.190.16.22      ESTABLISHED

Tcpdump shows retranmissions and no acks:

# tcpdump -lni fxp0 host anoncvs.netbsd.org
tcpdump: listening on fxp0
15:28:46.670821 192.168.2.200.60649 > 204.152.190.16.22: . 3934988374:3934988874
(500) ack 2623415927 win 33580 <nop,nop,timestamp 760 122> [tos 0x8] 
15:29:50.670986 192.168.2.200.60649 > 204.152.190.16.22: . 0:500(500) ack 1 win 
33580 <nop,nop,timestamp 888 122> [tos 0x8] 
15:30:54.671159 192.168.2.200.60649 > 204.152.190.16.22: . 0:500(500) ack 1 win 
33580 <nop,nop,timestamp 1016 122> [tos 0x8] 
15:31:58.671247 192.168.2.200.60649 > 204.152.190.16.22: . 0:500(500) ack 1 win 33580 <nop,nop,timestamp 1144 122> [tos 0x8] 

Note that it's only sending 500 bytes at a time, so it's almost 
certainly not an MTU problem.  Another machine, behind the same NAT, is 
not having any trouble.  Any suggestions?  (The misbehaving machine is 
running 2.0; it's speaking via one port on a 4-port Ethernet card:

ppb1 at pci0 dev 9 function 0: Digital Equipment DECchip 21152 PCI-PCI Bridge (rev. 0x03)
pci2 at ppb1 bus 2
pci2: i/o space, memory space enabled, rd/line, wr/inv ok
fxp0 at pci2 dev 4 function 0: i82559S Ethernet, rev 9
fxp0: interrupting at irq 10
fxp0: Ethernet address 00:20:fc:1e:67:40
inphy0 at fxp0 phy 1: i82555 10/100 media interface, rev. 4
inphy0: 10baseT, 10baseT-FDX, 100baseTX, 100baseTX-FDX, auto
fxp1 at pci2 dev 5 function 0: i82559S Ethernet, rev 9
fxp1: interrupting at irq 3
fxp1: Ethernet address 00:20:fc:1e:67:41
inphy1 at fxp1 phy 1: i82555 10/100 media interface, rev. 4
inphy1: 10baseT, 10baseT-FDX, 100baseTX, 100baseTX-FDX, auto
fxp2 at pci2 dev 6 function 0: i82559S Ethernet, rev 9
fxp2: interrupting at irq 5
fxp2: Ethernet address 00:20:fc:1e:67:42
inphy2 at fxp2 phy 1: i82555 10/100 media interface, rev. 4
inphy2: 10baseT, 10baseT-FDX, 100baseTX, 100baseTX-FDX, auto
fxp3 at pci2 dev 7 function 0: i82559S Ethernet, rev 9
fxp3: interrupting at irq 11
fxp3: Ethernet address 00:20:fc:1e:67:43
inphy3 at fxp3 phy 1: i82555 10/100 media interface, rev. 4
inphy3: 10baseT, 10baseT-FDX, 100baseTX, 100baseTX-FDX, auto

The working machine is running -current, and speaking via a tlp card.

		--Steve Bellovin, http://www.research.att.com/~smb