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About the IPv6 Basic Packet Header


This topic identifies the IP version 6 (IPv6) basic packet header fields with their bit lengths and uses.

Header Name

Bit Length




Specifies that IP version 6 is used. The IPv6 version field contains a value of 6 indicating that IPv6 is used, as opposed to 4 for IP version 4.

Traffic Class


Allows source nodes or routers to identify different classes (or priorities for quality of service) for IPv6 packets. (This field replaces the IPv4 Type of Service field.)

Flow Label


Identifies the flow to which the packet belongs. Packets in a flow share a common purpose, or belong to a common category, as interpreted by external devices such as routers or destination hosts.

Note: For IPv6 flow-based packets, Junos OS for SRX Series devices does not use the flow label field.

Payload Length


Specifies the length of the IPv6 packet payload, or contents, expressed in octets.

Next Header


Identifies the type of Internet Protocol for the header that immediately follows the IPv6 header. The Next Header field replaces the IPv4 Protocol field. It is an optional field.

This protocol can be one of two types:

  • An IPv6 extension header. For example, if the device performs IP security on exchanged packets, the Next Header value is probably 50 (ESP extension header) or 51 (AH extension header). Extension headers are optional.

  • An upper-layer Protocol Data Unit (PDU). For example, the Next Header value could be 6 (for TCP), 17 (for UDP), or 58 (for ICMPv6).

The flow module processes these headers sequentially within the context of a packet flow.

If it encounters one of the following extension headers, the software parses it and regards the packet as a corresponding protocol packet.

  • Internet Control Message Protocol version 6 (ICMPv6)

  • Transport Control Protocol (TCP)

    Note: The device checks the TCP header length as part of its sanity checks.

  • UDP

    Note: The device checks the UDP length as part of its sanity checks.

  • Enhanced Security Protocol (ESP) or Authentication Header (AH)

Hop Limit


Specifies the maximum number of hops the packet can make after transmission from the host device. When the Hop Limit value is zero, the device drops the packet and generates an error message. (This field is similar the to Time to Live IPv4 field.)

Source IP Address


Identifies the host device, or interface on a node, that generated the IPv6 packet.

Destination IP Address


Identifies the host device, or interface on a node, to which the IPv6 packet is to be sent.

Note: The destination address can appear twice, the first instance after the hop limit following the source IP address and the second instance after the final extension header.