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How to calculate the TLS payload?

asked 2023-06-07 11:47:58 +0000

s.user gravatar image


I am pretty new to Wireshark and trying to understand some basic concepts and terminology. I am analyzing a TCP dump file which contains traffic between an embedded device to remote server. I want to calculate the TLS/TCP payload size in this. As per my understanding, the "Conversations" tab shows the total bytes used that includes MAC, TCP and IP header. Now when I check the "Follow TCP Stream", the byte value shown for the "Entire Conversation" is lesser than "Conversations". I came across a similar post here In that it is mentioned to add a "tcp.len field as a custom column" to show the length of the TCP data segment. In my case the difference between "Length" column and "tcp.len" is always 66. But I am not sure how this value is calculated. Also, is the bytes shown in "Follow TCP Stream" the actual payload size? In the end I want to know the bytes consumed by TLS/TCP payload and headers.

Thanks in advance.

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answered 2023-06-08 21:01:10 +0000

André gravatar image

tcp.len is the size of the TCP data c.q. payload. The length of the frame minus tcp.len gives you the TCP header size plus the size of the underlying protocols (IP, Ethernet). So that is the size of the protocol overhead.
For TCP, TLS is just payload.

The payload of TLS is send in "Application Data" records (tls.record.content_type == 23). The size is available as tls.record.length and is the TLS payload plus MAC (Message Authentication Code). The size of the MAC depends on the hash used. For example 32 bytes for SHA256. So the TLS payload size is tls.record.length minus MAC length.

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Asked: 2023-06-07 11:47:58 +0000

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Last updated: Jun 08 '23