This toy IPv6 fixed header has no IPv4-style header checksum field.
There is no header checksum field here
This fixed header has no IP version 4-style header checksum field.
header checksum field = 0 \text{header checksum field}=0 header checksum field = 0 Fixed-header boundary The source and destination addresses are decoded from the fixed header. Fixed-header boundary 320 bits / 40 bytes 0x600123450004114020010db800000000000000000000000120010db8000000000000000000000002 byte 0 byte 1 byte 2 byte 3 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 1 1 0 1 0 0 0 1 0 1 byte 4 byte 5 byte 6 byte 7 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 1 0 1 0 0 0 0 0 0 byte 8 byte 9 byte 10 byte 11 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1 1 0 1 1 0 1 1 1 0 0 0 byte 12 byte 13 byte 14 byte 15 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 byte 16 byte 17 byte 18 byte 19 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 byte 20 byte 21 byte 22 byte 23 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 byte 24 byte 25 byte 26 byte 27 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1 1 0 1 1 0 1 1 1 0 0 0 byte 28 byte 29 byte 30 byte 31 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 byte 32 byte 33 byte 34 byte 35 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 byte 36 byte 37 byte 38 byte 39 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 version 6 traffic-class 0 flow-label 0x12345 payload-length 4 next-header 17->UDP hop-limit 64 source 2001:db8::1 (cont.) (cont.) (cont.) destination 2001:db8::2 (cont.) (cont.) (cont.)
The fixed part is forty bytes
The fixed header is 40 bytes, then the payload marker follows.
fixed header bytes = 40 \text{fixed header bytes}=40 fixed header bytes = 40 Fixed-header boundary The source and destination addresses are decoded from the fixed header. Fixed-header boundary 320 bits / 40 bytes 0x600123450004114020010db800000000000000000000000120010db8000000000000000000000002 byte 0 byte 1 byte 2 byte 3 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 1 1 0 1 0 0 0 1 0 1 byte 4 byte 5 byte 6 byte 7 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 1 0 1 0 0 0 0 0 0 byte 8 byte 9 byte 10 byte 11 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1 1 0 1 1 0 1 1 1 0 0 0 byte 12 byte 13 byte 14 byte 15 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 byte 16 byte 17 byte 18 byte 19 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 byte 20 byte 21 byte 22 byte 23 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 byte 24 byte 25 byte 26 byte 27 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1 1 0 1 1 0 1 1 1 0 0 0 byte 28 byte 29 byte 30 byte 31 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 byte 32 byte 33 byte 34 byte 35 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 byte 36 byte 37 byte 38 byte 39 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 version 6 traffic-class 0 flow-label 0x12345 payload-length 4 next-header 17->UDP hop-limit 64 source 2001:db8::1 (cont.) (cont.) (cont.) destination 2001:db8::2 (cont.) (cont.) (cont.)
What this does not model
NOTE: toy IP version 6 fixed-header example only; not extension headers, neighbor discovery, SLAAC, fragmentation headers, UDP checksum details, routing policy, or real capture behavior.
payload length = 4 \text{payload length}=4 payload length = 4 Fixed-header boundary The source and destination addresses are decoded from the fixed header. Fixed-header boundary 320 bits / 40 bytes 0x600123450004114020010db800000000000000000000000120010db8000000000000000000000002 byte 0 byte 1 byte 2 byte 3 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 1 1 0 1 0 0 0 1 0 1 byte 4 byte 5 byte 6 byte 7 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 1 0 1 0 0 0 0 0 0 byte 8 byte 9 byte 10 byte 11 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1 1 0 1 1 0 1 1 1 0 0 0 byte 12 byte 13 byte 14 byte 15 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 byte 16 byte 17 byte 18 byte 19 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 byte 20 byte 21 byte 22 byte 23 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 byte 24 byte 25 byte 26 byte 27 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1 1 0 1 1 0 1 1 1 0 0 0 byte 28 byte 29 byte 30 byte 31 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 byte 32 byte 33 byte 34 byte 35 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 byte 36 byte 37 byte 38 byte 39 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 version 6 traffic-class 0 flow-label 0x12345 payload-length 4 next-header 17->UDP hop-limit 64 source 2001:db8::1 (cont.) (cont.) (cont.) destination 2001:db8::2 (cont.) (cont.) (cont.)
Summary
The example shows the fixed header fields and stops before extension headers or transport checksums.
fixed header bytes = 40 \text{fixed header bytes}=40 fixed header bytes = 40 Fixed-header boundary The source and destination addresses are decoded from the fixed header. Fixed-header boundary 320 bits / 40 bytes 0x600123450004114020010db800000000000000000000000120010db8000000000000000000000002 byte 0 byte 1 byte 2 byte 3 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 1 1 0 1 0 0 0 1 0 1 byte 4 byte 5 byte 6 byte 7 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 1 0 1 0 0 0 0 0 0 byte 8 byte 9 byte 10 byte 11 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1 1 0 1 1 0 1 1 1 0 0 0 byte 12 byte 13 byte 14 byte 15 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 byte 16 byte 17 byte 18 byte 19 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 byte 20 byte 21 byte 22 byte 23 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 byte 24 byte 25 byte 26 byte 27 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1 1 0 1 1 0 1 1 1 0 0 0 byte 28 byte 29 byte 30 byte 31 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 byte 32 byte 33 byte 34 byte 35 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 byte 36 byte 37 byte 38 byte 39 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 version 6 traffic-class 0 flow-label 0x12345 payload-length 4 next-header 17->UDP hop-limit 64 source 2001:db8::1 (cont.) (cont.) (cont.) destination 2001:db8::2 (cont.) (cont.) (cont.)