TCP Windows size | Max Segment Size

[Ibrahim Jamil]

Hi

would you pls help me to understand the below  in red?

Thanks

Ibrahim

[Raju Sekharan]

Hi Ibrahim

TCP Widnow size is the amount of unacknowledged data that can be in transit at any given time and , and is communicated via a 16-bit field in the TCP header.

The maximum segment size (MSS) is a parameter of the TCP protocol that specifies the largest amount of data, specified in octets, that a computer or communications device can receive in a single TCP segment, and therefore in a single IP datagram. It does not count the TCP header or the IP header

Both these values will be communicated to TCP neighbor during negotiation and it will use that for communication

Raju

[Raju Sekharan]

Hi Ibrahim,

Let me also explain with the values you have it in your config. The numbers here are in bytes

MSS= 8960.

Tcp Window = 56K=56000

If the other side accepts the TCP negotiation session with these values and if there is a requirement for a large  data transfer, the other side will do like this

Each packet size will be 9000 ( 8960 + 20 Byts IP header + 20 bytes TCP header)

At a time it will send 6 packets of 9000+ 1 packet of 2000 ( 9000X6+ 2000 = 56000)

Then it wiill wait for the acknowledment. once it receives ACK, it can send again 56k Bytes as explained above

Thanks

Raju

[Ibrahim Jamil]

thanks rajs

[Joseph W. Doherty]

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Posting

To add to what the other posters have provided.  Normally, to avoid IP fragmentation, you don't want MSS larger than what a MTU L3 packet can contain (discounting IP and TCP options - MTU - 40).

Ideally you configure receive window in a multiple even number of MSS.

Newer TCP implementations support a scale option for receive window, so it can be (much) larger than what a 16 bit field supports.  (This is needed to support larger BDPs.)

[Ibrahim Jamil]

thanks joseph