TCP Header – What is Data Offset and Its Uses?

iptcp

I was learning about TCP and came across Data offset. This was the answer I found.

" Data offset specifies the size of the TCP header in 32 bit words. The min. size header is 5 words and the max. is 15 words, thus giving the min. size of 20 bytes and max. of 60 bytes."

What is the word means ? 32 bit words, 5 words, 15 words. How they concluded the size as 20 bytes and 60 bytes.

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The purpose of the data offset is to tell the upper layers where the data starts. As you point out, the TCP header can be anywhere from 5-15 words long. So you need to know where the header ends and the data begins.

Related Solutions

Calculating Exact TCP Overhead Cost

Ethernet packets 1.5k

1500 - 20 B (IPv4) - 20 B (TCP+checksum) = 1460 B DATA (and 40 B Overhead)

Add 40 B + 14 B (Ethernet) + 4 B (FCS) + 12 B (Interframe gap) + 8 B (preamble) = 78 B Overhead

78 / 1460 * 100 = 5.34% overhead

1460 / ( 1460 + 78) * 100 = 94.93% Throughput/Goodput

1,000,000,000(1Gbit) * 94.93% = 949Mbit/s(0.949Gbit/s)

you measured 941Mbit/s that gives (949 - 941) / 949 * 100 = 0.84 % error between theoretical and actual.

Jumbo packets 9k - Theoretical max

(9000-40) / ( 9000 - 40 + 78 ) *100 = 99.14%  (Overhead 0.86%)

1,000,000,000(1Gbit) * 99.14% = 991Mbit/s(0.99Gbit/s)

Tcp – How to determine the throughput of the TCP connection

Yes, this is completely normal. And you use the minimum value of the calculations that you have done above. Think about how long it would take to move a box of paper down the hall - now imagine that you had to do it one page at a time. However, increasing the receive window is not always the answer. Sometimes the client makes small layer 7 requests (SMB does this a lot). So, a 64K TCP window won't help if client is only asking for 4KB blocks waiting for it to arrive then asking for the next one. You may also need to consider TCP slow-start in the equation.

Ok, now imagine you can carry multiple pages down the hall at a time, but you don't want to bring too many at a time. So you start with 3, if you don't drop any then next time you bring , then 8, then 13... Until you drop one, then you reduce by 50% and start ramping up again. That is TCP slow-start. And the count is called the CWND.

But none of that makes any difference because the win/rtt bw=800kb/s but the APP2 consumption rate is only 200 so that is the limiting factor.

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Related Solutions

Calculating Exact TCP Overhead Cost

Tcp – How to determine the throughput of the TCP connection

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