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Waht is the minimum MSS captured by wireshark?

asked 2022-04-20 19:29:57 +0000

I found out that the default MSS is 536 bytes, is that what the wireshark will capture or can be there be a lower MSS that gets captured?

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answered 2022-04-21 09:21:23 +0000

Jaap gravatar image

The MSS has nothing to do with packet capture. MSS is a setting of the TCP/IP stack to define what the TCP segment size may be, so from 0 up to MSS. And Wireshark will happily dissect TCP packets with segments of any size.

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And note that the "default" MSS is usually not the MSS that's used. An Ethernet packet has a maximum payload of 1500 bytes, an IPv4 header is a minimum of 20 bytes long, and a TCP header is a minimum of 20 bytes long, so the maximum TCP payload is 1500-(20+20) = 1460 bytes. That's the usual MSS that's negotiated.

Wireshark will, by default, capture the entire packet, however big that might be; as Jaap notes, that is not affected by the MSS.

Guy Harris gravatar imageGuy Harris ( 2022-04-22 08:56:45 +0000 )edit

While it looks like a MSS value is been negotiated, it is actually announced (advertised) in each of the two flows individually, instead of being negotiated.

Sidenote: This can lead to problems when routes are assymetrical over links with different MSS adjustments due to different MTU sizes or different amounts of encapsulation of each link.

SYN-bit gravatar imageSYN-bit ( 2022-04-24 15:16:59 +0000 )edit

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Asked: 2022-04-20 19:29:57 +0000

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Last updated: Apr 21 '22