9/6/2023 0 Comments Wireshark pcap problemThe TCP MSS does not include the IP header or the TCP header. MSS stands for Maximum Segment Size which specifies the largest amount of data that a device can receive in a single TCP segment. Depending on if you included the Ethernet frame or not the standard is 1500 bytes (Wireshark will show 1514 bytes as length since the Ethernet frame is included) for a TCP packet that would be the IP Header (20 bytes) + TCP Header (20 bytes) + TCP segment length (1460 bytes). MTU stands for Maximum transmission unit meaning the size on the largest network layer protocol data unit that can be communicated in a single network transaction. We will also look briefly on Selective Acknowledgement (SACK) which also can be good to know when troubleshooting TCP problems. We will now try to explain how SEQ numbers MSS and MTU is directly related to each other and we will also look at an example from when it works. To make it easier to read we will have the setting enabled in this How To.Īn explanation of Sequence numbers, MSS and MTU: You can read more about the setting at this link. By having it enabled the relative SEQ and ACK numbers will be shown, meaning that all SEQ and ACK numbers always start at 0 for the first packet seen in each conversation. This is totally up to the user and how you prefer to read it, by disabling it you will see the real/absolute SEQ, Next SEQ and ACK numbers and that can be cumbersome to read. There is one thing more I usually change and that is to disable “Relative sequence numbers” in Wireshark for the TCP protocol. I recommend that you have them in the same order as pictures shows since it will be easier to read, you can also rename the columns to something shorter for example SEQ, Next SEQ and ACK to make it even easier to read. Wireshark3.png (24.45 KiB) Viewed 43229 times
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