7.5. TCP Analysis
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## 7.5. TCP Analysis

By default, Wireshark’s TCP dissector tracks the state of each TCP session and provides additional information when problems or potential problems are detected. Analysis is done once for each TCP packet when a capture file is first opened. Packets are processed in the order in which they appear in the packet list. You can enable or disable this feature via the “Analyze TCP sequence numbers” TCP dissector preference.

Figure 7.5. “TCP Analysis” packet detail items

TCP Analysis flags are added to the TCP protocol tree under “SEQ/ACK analysis”. Each flag is described below. Terms such as “next expected sequence number” and “next expected acknowledgement number” refer to the following”:

Next expected sequence number
  The last-seen sequence number plus segment length. Set when there are no analysis flags and and for zero window probes. This is initially zero and calculated based on the previous packet in the same TCP flow. Note that this may not be the same as the tcp.nxtseq protocol field.

Next expected acknowledgement number
  The last-seen sequence number for segments. Set when there are no analysis flags and for zero window probes.

Last-seen acknowledgment number
  Always set. Note that this is not the same as the next expected acknowledgment number.

Last-seen acknowledgment number
  Always updated for each packet. Note that this is not the same as the next expected acknowledgment number.

#### TCP ACKed unseen segment

Set when the expected next acknowledgement number is set for the reverse direction and it’s less than the current acknowledgement number.

#### TCP Dup ACK <frame>#<acknowledgement number>

Set when all of the following are true:

- The segment size is zero.
- The window size is non-zero and hasn’t changed.
- The next expected sequence number and last-seen acknowledgment number are non-zero (i.e. the connection has been established).
- SYN, FIN, and RST are not set.

#### TCP Fast Retransmission

Set when all of the following are true:

- This is not a keepalive packet.
- In the forward direction, the segment size is greater than zero or the SYN or FIN is set.
- The next expected sequence number is greater than the current sequence number.
- We have more than two duplicate ACKs in the reverse direction.
- The current sequence number equals the next expected acknowledgement number.
- We saw the last acknowledgement less than 20ms ago.

Supersedes “Out-Of-Order”, “Spurious Retransmission”, and “Retransmission”.

#### TCP Keep-Alive

Set when the segment size is zero or one, the current sequence number is one byte less than the next expected sequence number, and any of SYN, FIN, or RST are set.

Supersedes “Fast Retransmission”, “Out-Of-Order”, “Spurious Retransmission”, and “Retransmission”.

#### TCP Keep-Alive ACK

Set when all of the following are true:

- The segment size is zero.
- The window size is non-zero and hasn’t changed.
- The current sequence number is the same as the next expected sequence number.
- The current acknowledgement number is the same as the last-seen acknowledgement number.
- The most recently seen packet in the reverse direction was a keepalive.
- The packet is not a SYN, FIN, or RST.

Supersedes “Dup ACK” and “ZeroWindowProbeAck”.

#### TCP Out-Of-Order

Set when all of the following are true:

- This is not a keepalive packet.
- In the forward direction, the segment length is greater than zero or the SYN or FIN is set.
- The next expected sequence number is greater than the current sequence number.
- The next expected sequence number and the next sequence number differ.
- The last segment arrived within the calculated RTT (3ms by default).

Supersedes “Spurious Retransmission” and “Retransmission”.

#### TCP Port numbers reused

Set when the SYN flag is set (not SYN+ACK), we have an existing conversation using the same addresses and ports, and the sequencue number is different than the existing conversation’s initial sequence number.

#### TCP Previous segment not captured

Set when the current sequence number is greater than the next expected sequence number.

#### TCP Spurious Retransmission

Checks for a retransmission based on analysis data in the reverse direction. Set when all of the following are true:

- The SYN or FIN flag is set.
- This is not a keepalive packet.
- The segment length is greater than zero.
- Data for this flow has been acknowledged. That is, the last-seen acknowledgement number has been set.
- The next sequence number is less than or equal to the last-seen acknowledgement number.

Supersedes “Retransmission”.

#### TCP Retransmission

Set when all of the following are true:

- This is not a keepalive packet.
- In the forward direction, the segment length is greater than zero or the SYN or FIN flag is set.
- The next expected sequence number is greater than the current sequence number.

#### TCP Window Full

Set when the segment size is non-zero, we know the window size in the reverse direction, and our segment size exceeds the window size in the reverse direction.

#### TCP Window Update

Set when the all of the following are true:

- The segment size is zero.
- The window size is non-zero and not equal to the last-seen window size.
- The sequence number is equal to the next expected sequence number.
- The acknowledgement number is equal to the last-seen acknowledgement number.
- None of SYN, FIN, or RST are set.

#### TCP ZeroWindow

Set when the window size is zero and non of SYN, FIN, or RST are set.

#### TCP ZeroWindowProbe

Set when the sequence number is equal to the next expected sequence number, the segment size is one, and last-seen window size in the reverse direction was zero.

#### TCP ZeroWindowProbeAck

Set when the all of the following are true:

- The segment size is zero.
- The window size is zero.
- The sequence number is equal to the next expected sequence number.
- The acknowledgement number is equal to the last-seen acknowledgement number.
- The last-seen packet in the reverse direction was a zero window probe.

Supersedes “TCP Dup ACK”.

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