What Happens When TCP Handshake or Teardown Packets Are Lost?

TCP Three‑Way Handshake Packet Loss

First handshake (SYN) loss

When a client initiates a TCP connection it sends a SYN packet and enters the SYN_SENT state. If the SYN is lost, the client starts a timeout‑retransmission timer. The SYN is retransmitted with the same sequence number. Retransmission intervals double (1 s, 2 s, 4 s, 8 s, 16 s, 32 s) and the total wait time is about 63 seconds.

# cat /proc/sys/net/ipv4/tcp_syn_retries
5

The maximum number of SYN retransmissions is controlled by the tcp_syn_retries kernel parameter (default 5). After the final timeout the client aborts the connection.

If tcp_syn_retries is set to 3, the client will retransmit the SYN three times (1 s, 2 s, 4 s) and then wait an additional 8 s before giving up.

Second handshake (SYN‑ACK) loss

After the server receives the SYN it replies with a SYN‑ACK and moves to SYN_RCVD. If this SYN‑ACK is lost, the client assumes its SYN was not acknowledged and retransmits the SYN according to tcp_syn_retries. Simultaneously the server retransmits the SYN‑ACK according to tcp_synack_retries (default 5).

# cat /proc/sys/net/ipv4/tcp_synack_retries
5

Client retransmits SYN up to tcp_syn_retries times.

Server retransmits SYN‑ACK up to tcp_synack_retries times.

Third handshake (ACK) loss

When the client receives the SYN‑ACK it sends an ACK and enters ESTABLISHED. The ACK itself is not retransmitted; instead the server will retransmit its SYN‑ACK if the ACK is not received, using the same tcp_synack_retries limit.

If tcp_synack_retries is 2, the server will resend the SYN‑ACK twice before aborting.

TCP Four‑Way Termination Packet Loss

First FIN loss (client initiates close)

The client sends a FIN and enters FIN_WAIT_1. If the FIN is lost, the client retransmits it according to tcp_orphan_retries (default 0, often set to a small value). After exceeding the limit the client stops sending FIN and eventually closes the socket.

Second FIN/ACK loss (server acknowledges first FIN)

The server replies with an ACK and moves to CLOSE_WAIT. If this ACK is lost, the client will retransmit its FIN (again governed by tcp_orphan_retries) until the server finally receives it.

Third FIN loss (server initiates its own FIN)

After the server calls close it sends a FIN and enters LAST_ACK. If this FIN is lost, the server retransmits it according to tcp_orphan_retries. The client, after receiving the server’s FIN, sends an ACK and enters TIME_WAIT. If the client does not receive the server’s FIN, it will keep waiting up to tcp_fin_timeout (default 60 s) before aborting.

Fourth ACK loss (client acknowledges server’s FIN)

The client’s ACK to the server’s FIN puts the client into TIME_WAIT. If this ACK is lost, the server will retransmit its FIN (still limited by tcp_orphan_retries) until the client finally acknowledges or the retry limit is reached.

Both sides use tcp_orphan_retries to limit FIN retransmissions.

The client’s TIME_WAIT state lasts for 2 MSL (maximum segment lifetime) before the socket is fully closed.