ISRO2020-54

Question

Checksum field in TCP header is

• A. ones complement of sum of header and data in bytes
• B. ones complement of sum of header, data and pseudo header in $16$ bit words
• C. dropped from $\textsf{IPv6}$ header format
• D. better than $\textsf{md5}$ or $\textsf{sh1}$ methods

Comments

IPv6 does not have checksum field so c may be answer

---

Option B and C both are true.

---

kpc No, IPv6 has dropped it's own checksum (at Network Layer). It can't drop TCP's checksum (at Transport Layer)

Answer 1

$\underline{\mathbf{Answer:}\Rightarrow}\;\mathbf{b.}$

In the TCP header checksum calculation includes the header, data, and pseudo-header.

All these values are added and stored in one’s complement form.

Answer 2

Option B) is correct

---

Source: Tanenbaum

 

Answer 3

Options B and C both are true.

Explanation:

1. In the TCP header checksum calculation includes the header, data, and pseudo-header. All these values are added and stored in one’s complement form.
2. Higher layer (>=4) protocols like TCP and UDP still have their own checksums. So many lower-layer protocols (<=2) already have their own checksums that having another checksum in IP (layer 3) was seen as unnecessary overhead. Answer is mentioned in stackoverflow .Please refer https://stackoverflow.com/questions/54074898/ipv4-header-included-checksum-but-not-in-ipv6-why

Answer 4

For TCP; the sum of header, data and psuedoheader is taken, in 16-bit or 2 Byte words.

Then it's 1's complement is calculated (1's complement $\equiv$ negation) and stored in the checksum field.

Hence, when we add up all the numbers, we'd get $0$

 

Option B is correct.

---

Bonus

What is Psuedoheader in TCP?