When a packet is being sent after the SYN ACK connection has been established between a host and a server, packets can now be sent between the hosts and a server. Sometimes, there’s a window size which is determinable by ACK sent after reception of a segment.

Let’s say the window size is 100, the sender can send 100 segments before it expects to receive an ACK. But what happens is segment 50 gets missing along the line?

I read somewhere that 1-49 & 51-100 gets sent but then the receiver ACKs for 50 and the sender resend 1 segment with segment number 50. I was also reading somewhere that, for example, let’s say sender has a window size of three, and my sender sends segment 1,2,3, receiver ACKS for 4. The next window must be 4,5,6.

Let’s say 4 doesn’t get sent, the receiver ACKS for 4 again, and the whole segment gets sent again, which conflicts with the earlier idea. I was thinking 5-6 gets sent before an ACK for 4 gets sent, then the 4th segment gets sent after 5-6.

Which idea is right?

  • The TCP send window doesn't count segments but bytes.
    – Zac67
    Apr 8, 2021 at 11:18
  • @Zac67 My bad. Kindly ignore the error and address the main question if you can Apr 8, 2021 at 11:23
  • "Let’s say the window size is 100, the sender can send 100 segments before it expects to receive an ACK." No, it is 100 bytes.
    – Ron Maupin
    Apr 8, 2021 at 12:26

1 Answer 1


what happens if segment 50 gets missing along the line?

The receiver can either

  • simply wait for retransmission (when the sender's timer for a unACKed segment expires it is resent automatically)
  • send duplicate ACKs (DUPACK) for 50 (the next expected segment) to prompt a faster retransmission

The receiver can also selectively ACK (SACK) 51-100 to avoid a retransmission of successfully received segments. Without selective ACK, the sender retransmits 50-100.

Note that TCP doesn't have a client/server concept. There are two hosts, each of them able to send and receive. Clients and servers only exist in the application layer.


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