Fixed Fields

Version and Flow Label

The first fixed-header fields say this is IPv6 and pin a toy flow label.

highlighted = computed this step

The first nibble names the version

The packet starts with IP version 6. That nibble is checked from the first header byte.

version=6\text{version}=6
Version and flow labelThe fixed header fields are decoded from the pinned packet bytes.Version and flow label320 bits / 40 bytes0x600123450004114020010db800000000000000000000000120010db8000000000000000000000002byte 0byte 1byte 2byte 301100000000000010010001101000101byte 4byte 5byte 6byte 700000000000001000001000101000000byte 8byte 9byte 10byte 1100100000000000010000110110111000byte 12byte 13byte 14byte 1500000000000000000000000000000000byte 16byte 17byte 18byte 1900000000000000000000000000000000byte 20byte 21byte 22byte 2300000000000000000000000000000001byte 24byte 25byte 26byte 2700100000000000010000110110111000byte 28byte 29byte 30byte 3100000000000000000000000000000000byte 32byte 33byte 34byte 3500000000000000000000000000000000byte 36byte 37byte 38byte 3900000000000000000000000000000010version6traffic-class0flow-label0x12345payload-length4next-header17->UDPhop-limit64source2001:db8::1(cont.)(cont.)(cont.)destination2001:db8::2(cont.)(cont.)(cont.)

Traffic class is pinned

The traffic class field is 0 in this toy packet.

traffic class=0\text{traffic class}=0
Version and flow labelThe fixed header fields are decoded from the pinned packet bytes.Version and flow label320 bits / 40 bytes0x600123450004114020010db800000000000000000000000120010db8000000000000000000000002byte 0byte 1byte 2byte 301100000000000010010001101000101byte 4byte 5byte 6byte 700000000000001000001000101000000byte 8byte 9byte 10byte 1100100000000000010000110110111000byte 12byte 13byte 14byte 1500000000000000000000000000000000byte 16byte 17byte 18byte 1900000000000000000000000000000000byte 20byte 21byte 22byte 2300000000000000000000000000000001byte 24byte 25byte 26byte 2700100000000000010000110110111000byte 28byte 29byte 30byte 3100000000000000000000000000000000byte 32byte 33byte 34byte 3500000000000000000000000000000000byte 36byte 37byte 38byte 3900000000000000000000000000000010version6traffic-class0flow-label0x12345payload-length4next-header17->UDPhop-limit64source2001:db8::1(cont.)(cont.)(cont.)destination2001:db8::2(cont.)(cont.)(cont.)

Flow label is the next wide field

The flow label value is 74565. It is still just pinned bits here.

flow label=74565\text{flow label}=74565
Version and flow labelThe fixed header fields are decoded from the pinned packet bytes.Version and flow label320 bits / 40 bytes0x600123450004114020010db800000000000000000000000120010db8000000000000000000000002byte 0byte 1byte 2byte 301100000000000010010001101000101byte 4byte 5byte 6byte 700000000000001000001000101000000byte 8byte 9byte 10byte 1100100000000000010000110110111000byte 12byte 13byte 14byte 1500000000000000000000000000000000byte 16byte 17byte 18byte 1900000000000000000000000000000000byte 20byte 21byte 22byte 2300000000000000000000000000000001byte 24byte 25byte 26byte 2700100000000000010000110110111000byte 28byte 29byte 30byte 3100000000000000000000000000000000byte 32byte 33byte 34byte 3500000000000000000000000000000000byte 36byte 37byte 38byte 3900000000000000000000000000000010version6traffic-class0flow-label0x12345payload-length4next-header17->UDPhop-limit64source2001:db8::1(cont.)(cont.)(cont.)destination2001:db8::2(cont.)(cont.)(cont.)

Summary

Version, traffic class, and flow label are read before any address bytes.

flow label=74565\text{flow label}=74565
Version and flow labelThe fixed header fields are decoded from the pinned packet bytes.Version and flow label320 bits / 40 bytes0x600123450004114020010db800000000000000000000000120010db8000000000000000000000002byte 0byte 1byte 2byte 301100000000000010010001101000101byte 4byte 5byte 6byte 700000000000001000001000101000000byte 8byte 9byte 10byte 1100100000000000010000110110111000byte 12byte 13byte 14byte 1500000000000000000000000000000000byte 16byte 17byte 18byte 1900000000000000000000000000000000byte 20byte 21byte 22byte 2300000000000000000000000000000001byte 24byte 25byte 26byte 2700100000000000010000110110111000byte 28byte 29byte 30byte 3100000000000000000000000000000000byte 32byte 33byte 34byte 3500000000000000000000000000000000byte 36byte 37byte 38byte 3900000000000000000000000000000010version6traffic-class0flow-label0x12345payload-length4next-header17->UDPhop-limit64source2001:db8::1(cont.)(cont.)(cont.)destination2001:db8::2(cont.)(cont.)(cont.)