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Full Discussion: TCP Packet size
Special Forums IP Networking TCP Packet size Post 302523862 by olle on Friday 20th of May 2011 04:21:03 AM
Old 05-20-2011
Quote:
Originally Posted by DGPickett
Maybe it has an MTU of less. Obsolete IP option 12 can send this during connect without the ICMP of PATH MTU Discovery.

The MSS is the total remote buffer, the MTU is the packet max. Disregarding the chance of TCP options, a packet header is 20 IP + 20 TCP, so a packet sends MTU - 40, and the number of packets sent before ack is MSS/(MTU-40) including any partial. Since this is local, the ack round trip should be very short, so packets should not be MSS trimmed unless there is blocking downstream.
Thanks for your support.

Well, I don't find anything in the IP header(s) which looks like IP option 12. Have attached a small wireshark dump, if you would like to have look though.

If it is to any help, the MTU of the interface at my host machine is set to 1500. And regarding the chip, I know it can handle a MTU of 536 + headers, as I can see the Windows application send it packets of this size (from the same machine too).
 

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IPSEC_TNCFG(5)							  [FIXME: manual]						    IPSEC_TNCFG(5)

NAME
ipsec_tncfg - lists IPSEC virtual interfaces attached to real interfaces SYNOPSIS
ipsec tncfg cat/proc/net/ipsec_tncfg OBSOLETE
Note that tncfg is only supported on the classic KLIPS stack. It is not supported on any other stack and will be completely removed in future versions. A replacement command still needs to be designed DESCRIPTION
/proc/net/ipsec_tncfg is a read-only file which lists which IPSEC virtual interfaces are attached to which real interfaces, through which packets will be forwarded once processed by IPSEC. Each line lists one ipsec I/F. A table entry consists of: + an ipsec virtual I/F name + a visual and machine parsable separator '->', separating the virtual I/F and the physical I/F, + a physical I/F name, to which the ipsec virtual I/F is attached or NULL if it is not attached, + the keyword mtu=, + the MTU of the ipsec virtual I/F, + the automatically adjusted effective MTU for PMTU discovery, in brackets, + a visual and machine parsable separator '->', separating the virtual I/F MTU and the physical I/F MTU, + the MTU of the attached physical I/F. EXAMPLES
ipsec2 -> eth3 mtu=16260(1443) -> 1500 shows that virtual device ipsec2 with an MTU of 16260 is connected to physical device eth3 with an MTU of 1500 and that the effective MTU as a result of PMTU discovery has been automatically set to 1443. ipsec0 -> wvlan0 mtu=1400(16260) -> 1500 shows that virtual device ipsec0 with an MTU of 1400 is connected to physical device wvlan0 with an MTU of 1500 and no PMTU packets have gotten far enough to bump down the effective MTU from its default of 16260. ipsec3 -> NULL mtu=0(0) -> 0 shows that virtual device ipsec3 is not connected to any physical device. FILES
/proc/net/ipsec_tncfg, /usr/local/bin/ipsec SEE ALSO
ipsec(8), ipsec_manual(8), ipsec_eroute(5), ipsec_spi(5), ipsec_spigrp(5), ipsec_klipsdebug(5), ipsec_tncfg(8), ipsec_version(5), ipsec_pf_key(5) HISTORY
Written for the Linux FreeS/WAN project <http://www.freeswan.org/> by Richard Guy Briggs. [FIXME: source] 10/06/2010 IPSEC_TNCFG(5)
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