Why default mss values silently changed to 1260?

[watanabe.m]

Hi someone,

I login to Cisco1812 using telnet and enter "show tcp" command then mss value displayed below.
"Datagrams (max data segment is 1260 bytes):"

The IOS version is 12.4(15)T9.

My Router was only configured the ip address and line vty, minimum configuration to telnet.

Not configured the "ip tcp path-mtu-discovery", "ip tcp adjust-mss" and "ip tcp mss" commands.

This commands are disabled by default.

When the IOS version is 15.1(1)XB, the mss value was displayed 536.

Does anyone know why my router changes mss value to 1260?

Regards,

Mitz.

[Collin Clark]

I'm not sure what that means, but I don't think it's the MTU.( mine says 516). You can

view the MTUs with a ~

show interfaces | include MTU

[Ganesh Hariharan · Accepted solution]

Hi someone,
I login to Cisco1812 using telnet and enter "show tcp" command then mss value displayed below.
"Datagrams (max data segment is 1260 bytes):"
The IOS version is 12.4(15)T9.
My Router was only configured the ip address and line vty, minimum configuration to telnet.
Not configured the "ip tcp path-mtu-discovery", "ip tcp adjust-mss" and "ip tcp mss" commands.
This commands are disabled by default.
When the IOS version is 15.1(1)XB, the mss value was displayed 536.
Does anyone know why my router changes mss value to 1260?
Regards,
Mitz.

Hi Mitz,

Genrally Max TCP segments  size (MSS) value influences the resulting size of TCP packets. The majority of data packets on a network are TCP. Other than video, suitable UDP applications (such as DNS and NTP) exhibit an average packet size of less than 300 bytes. Use the router to influence or set the TCP MSS for TCP flows so as to reduce the data packet size. The effect is to reduce the impact of serialization delay, where no Layer 2 Fragmentation and Interleaving (LFI/FRF.12) technique exists.

The maximum IP packet size for off-net (hosts not on a directly connected interface) was 1300 bytes. The TCP MSS is the number of bytes following the IP and TCP header, so the default MSS size was 1260 bytes. The IP and TCP header are each 20 bytes, so 1300 minus 40 equals 1260.

The MSS option can appear only in a TCP SYN packet, and each end announces its own MSS. Although not required, it is frequently the same in both directions.

Hope to Help !!

Remember to rate the helpful post

Ganesh.H

[watanabe.m]

Hi Genesh,

Thanks for your advise!

I checked behavior(same subnet or not) in my Lab.

When the router and PC in the same IP subnet then MSS in routers SYNACK was 1460.

Router and PC in the different subnet, then it was 536.

It was same result at both 12.4T and 15.0.

I cannot find why MSS was 1260.

But I understand the router will change MSS when PC is in the different subnet.

I got some logs and packets in my lab.FYI.

Thanks Regards,

Mitz.