The Hop

One Hop Decrements TTL

A router hop decrements TTL and therefore changes the header bytes.

highlighted = computed this step

One hop subtracts one

At this toy router hop, TTL drops from 64 to 63.

64−1=6364 - 1=63
One hop changes TTLThe before and after IPv4 headers are shown as exact field strips.Before header160 bits / 20 bytes0x4500003c1c4640004006b1e6ac100a63ac100a0cbyte 0byte 1byte 2byte 301000101000000000000000000111100byte 4byte 5byte 6byte 700011100010001100100000000000000byte 8byte 9byte 10byte 1101000000000001101011000111100110byte 12byte 13byte 14byte 1510101100000100000000101001100011byte 16byte 17byte 18byte 1910101100000100000000101000001100version4ihl5dscp-ecn0x00total-length60identification0x1c46fragment-offset0ttl64protocol6->TCPheader-checksum0xb1e6source172.16.10.99destination172.16.10.12reserved: clear df: set mf: clearAfter header160 bits / 20 bytes0x4500003c1c4640003f06b2e6ac100a63ac100a0cbyte 0byte 1byte 2byte 301000101000000000000000000111100byte 4byte 5byte 6byte 700011100010001100100000000000000byte 8byte 9byte 10byte 1100111111000001101011001011100110byte 12byte 13byte 14byte 1510101100000100000000101001100011byte 16byte 17byte 18byte 1910101100000100000000101000001100version4ihl5dscp-ecn0x00total-length60identification0x1c46fragment-offset0ttl63protocol6->TCPheader-checksum0xb2e6source172.16.10.99destination172.16.10.12reserved: clear df: set mf: clear

After header

The after header has TTL 63. The helper recomputes the whole after header from the before header.

after TTL=63\text{after TTL}=63
One hop changes TTLThe before and after IPv4 headers are shown as exact field strips.Before header160 bits / 20 bytes0x4500003c1c4640004006b1e6ac100a63ac100a0cbyte 0byte 1byte 2byte 301000101000000000000000000111100byte 4byte 5byte 6byte 700011100010001100100000000000000byte 8byte 9byte 10byte 1101000000000001101011000111100110byte 12byte 13byte 14byte 1510101100000100000000101001100011byte 16byte 17byte 18byte 1910101100000100000000101000001100version4ihl5dscp-ecn0x00total-length60identification0x1c46fragment-offset0ttl64protocol6->TCPheader-checksum0xb1e6source172.16.10.99destination172.16.10.12reserved: clear df: set mf: clearAfter header160 bits / 20 bytes0x4500003c1c4640003f06b2e6ac100a63ac100a0cbyte 0byte 1byte 2byte 301000101000000000000000000111100byte 4byte 5byte 6byte 700011100010001100100000000000000byte 8byte 9byte 10byte 1100111111000001101011001011100110byte 12byte 13byte 14byte 1510101100000100000000101001100011byte 16byte 17byte 18byte 1910101100000100000000101000001100version4ihl5dscp-ecn0x00total-length60identification0x1c46fragment-offset0ttl63protocol6->TCPheader-checksum0xb2e6source172.16.10.99destination172.16.10.12reserved: clear df: set mf: clear

Checksum changes too

Changing TTL changes a header byte, so the header checksum field also changes from 0xb1e6 to 0xb2e6.

0xb1e6→0xb2e60xb1e6\to0xb2e6
One hop changes TTLThe before and after IPv4 headers are shown as exact field strips.Before header160 bits / 20 bytes0x4500003c1c4640004006b1e6ac100a63ac100a0cbyte 0byte 1byte 2byte 301000101000000000000000000111100byte 4byte 5byte 6byte 700011100010001100100000000000000byte 8byte 9byte 10byte 1101000000000001101011000111100110byte 12byte 13byte 14byte 1510101100000100000000101001100011byte 16byte 17byte 18byte 1910101100000100000000101000001100version4ihl5dscp-ecn0x00total-length60identification0x1c46fragment-offset0ttl64protocol6->TCPheader-checksum0xb1e6source172.16.10.99destination172.16.10.12reserved: clear df: set mf: clearAfter header160 bits / 20 bytes0x4500003c1c4640003f06b2e6ac100a63ac100a0cbyte 0byte 1byte 2byte 301000101000000000000000000111100byte 4byte 5byte 6byte 700011100010001100100000000000000byte 8byte 9byte 10byte 1100111111000001101011001011100110byte 12byte 13byte 14byte 1510101100000100000000101001100011byte 16byte 17byte 18byte 1910101100000100000000101000001100version4ihl5dscp-ecn0x00total-length60identification0x1c46fragment-offset0ttl63protocol6->TCPheader-checksum0xb2e6source172.16.10.99destination172.16.10.12reserved: clear df: set mf: clear

Summary

One hop changes TTL by one and produces a new valid header checksum.

after checksum=0xb2e6\text{after checksum}=0xb2e6
One hop changes TTLThe before and after IPv4 headers are shown as exact field strips.Before header160 bits / 20 bytes0x4500003c1c4640004006b1e6ac100a63ac100a0cbyte 0byte 1byte 2byte 301000101000000000000000000111100byte 4byte 5byte 6byte 700011100010001100100000000000000byte 8byte 9byte 10byte 1101000000000001101011000111100110byte 12byte 13byte 14byte 1510101100000100000000101001100011byte 16byte 17byte 18byte 1910101100000100000000101000001100version4ihl5dscp-ecn0x00total-length60identification0x1c46fragment-offset0ttl64protocol6->TCPheader-checksum0xb1e6source172.16.10.99destination172.16.10.12reserved: clear df: set mf: clearAfter header160 bits / 20 bytes0x4500003c1c4640003f06b2e6ac100a63ac100a0cbyte 0byte 1byte 2byte 301000101000000000000000000111100byte 4byte 5byte 6byte 700011100010001100100000000000000byte 8byte 9byte 10byte 1100111111000001101011001011100110byte 12byte 13byte 14byte 1510101100000100000000101001100011byte 16byte 17byte 18byte 1910101100000100000000101000001100version4ihl5dscp-ecn0x00total-length60identification0x1c46fragment-offset0ttl63protocol6->TCPheader-checksum0xb2e6source172.16.10.99destination172.16.10.12reserved: clear df: set mf: clear