Coursify

The checksum in IP must be recomputed at every router because of change in ____ fields. (i) TTL, Options, Identification Number, Offset (ii) TTL, Options, Data, Offset (iii) TTL, Options, Datagram Length, Offset (iv) TTL, Header Length, Offset, ToS

The checksum in IP must be recomputed at every router because of change in ____ fields. (i) TTL, Options, Identification Number, Offset (ii) TTL, Options, Data, Offset (iii) TTL, Options, Datagram Length, Offset (iv) TTL, Header Length, Offset, ToS

Verified Sources
Nov 4, 2026

Router forwarding and IPv4 checksum update

  1. 1
    Step 1

    Router accepts an IPv4 datagram for forwarding.

  2. 2
    Step 2

    Router decrements the TTL value by 1.

  3. 3
    Step 3

    Because TTL is part of the IPv4 header, the header checksum is no longer valid.

  4. 4
    Step 4

    Router recomputes the IPv4 header checksum over the updated header.

    Footnotes

    1. RFC 791: Internet Protocol - Specifies IPv4 header format and the header checksum behavior, including that the checksum must be recomputed when header changes occur (notably TTL decrement at routers). ↩

  5. 5
    Step 5

    "Router forwards the packet with the updated TTL and recomputed checksum."

    Common misconceptions (especially about fragmentation fields)

FAQ: Which fields cause hop-by-hop checksum recomputation?

Which fields are hop-by-hop modified (driving checksum recomputation)?

Routers always decrement TTL; other listed fields are not typically modified per hop.

Pro Tip for MCQs

"When an IPv4 question says “checksum must be recomputed at every router,” immediately look for the header field that routers must change during forwarding—TTL. Then map that to the options provided."

Reference and justification from the IPv4 specification

Why TTL is the decisive field

"IPv4 defines that the header checksum is computed for the header and must be recomputed when header fields are changed; routers decrement TTL, so the checksum must be updated."

Practice Quiz: Checking understanding

Footnotes

  1. RFC 791: Internet Protocol - Specifies IPv4 header format and the header checksum behavior, including that the checksum must be recomputed when header changes occur (notably TTL decrement at routers). ↩

Knowledge Check

Question 1 of 3
Q1Single choice

In IPv4, why must the header checksum be recomputed at every router?

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