DHCPv6 Packets

DHCPv6 Packet Format

Figure 1 shows the DHCPv6 packet format.

Figure 1 DHCPv6 packet format

**Table 1** Description of each field in a DHCPv6 packet
Field	Length	Description
msg-type	1 byte	Indicates the packet type. The value ranges from 1 to 13. For details, see the DHCPv6 Packet Type.
transaction-ID	3 bytes	Identifies packet transaction between DHCPv6 clients and servers. For example, a DHCPv6 client initiates a Solicit/Advertise transaction or a Request/Reply transaction. Their transaction IDs are different. Transaction IDs have the following characteristics: The transaction ID is randomly generated by a DHCPv6 client. Transaction IDs of request and reply packets must be the same. The transaction ID of a packet initiated by a DHCPv6 server is 0.
Options	Variable	Indicates the option field in a DHCPv6 packet. The option field contains configurations that the DHCPv6 server assigns to IPv6 hosts, such as the IPv6 address of the DNS server.

DHCPv6 Packet Type

DHCPv6 defines 13 types of packets. A DHCPv6 server and a DHCPv6 client communicate by exchanging these types of packets. Table 2 lists DHCPv6 packets and their corresponding DHCPv4 packets and describes the DHCPv6 packets.

**Table 2** Comparisons between DHCPv6 packets and DHCPv4 packets
DHCP Packet Type	DHCPv6 Packet	DHCPv4 Packet	Description
1	SOLICIT	DHCP DISCOVER	A DHCPv6 client sends a Solicit packet to locate DHCPv6 servers.
2	ADVERTISE	DHCP OFFER	A DHCPv6 server sends an Advertise packet in response to a Solicit packet to declare that it can provide DHCPv6 services.
3	REQUEST	DHCP REQUEST	A DHCPv6 client sends a Request packet to request IPv6 addresses and other configuration parameters from a DHCPv6 server.
4	CONFIRM	-	A DHCPv6 client sends a Confirm packet to any available DHCPv6 server to check whether the obtained IPv6 address applies to the link that the DHCPv6 client is connected to.
5	RENEW	DHCP REQUEST	A DHCPv6 client sends a Renew packet to the DHCPv6 server that provides the IPv6 addresses and other configuration parameters to extend the lifetime of the addresses and to update configuration parameters.
6	REBIND	DHCP REQUEST	A DHCPv6 client sends a Rebind packet to any available DHCPv6 server to extend the lifetime of the assigned IPv6 address and to update configuration parameters when the client does not receive a response to its Renew packet.
7	REPLY	DHCP ACK/NAK	A DHCPv6 server sends a Reply packet in the following situations: A DHCPv6 server sends a Reply packet containing IPv6 addresses and configuration parameters in response to a Solicit, Request, Renew or Rebind packet received from a DHCPv6 client. A DHCPv6 server sends a Reply packet containing configuration parameters in response to an Information-Request packet. A DHCPv6 server sends a Reply packet in response to a Confirm, Release, or Decline packet received from a DHCPv6 client.
8	RELEASE	DHCP RELEASE	A DHCPv6 client sends a Release packet to the DHCPv6 server that assigns IPv6 addresses to the DHCPv6 client, indicating that the DHCPv6 client will no longer use the obtained addresses.
9	DECLINE	DHCP DECLINE	A DHCPv6 client sends a Decline packet to a DHCPv6 server, indicating that the IPv6 addresses assigned by the DHCPv6 server are already in use on the link to which the DHCPv6 client is connected.
10	RECONFIGURE	-	A DHCPv6 server sends a Reconfigure packet to a DHCPv6 client, informing the DHCPv6 client that the DHCPv6 server has new addresses or updated configuration parameters.
11	INFORMATION-REQUEST	DHCP INFORM	A DHCPv6 client sends an Information-Request packet to a DHCPv6 server to request configuration parameters except for IPv6 addresses.
12	RELAY-FORW	-	A DHCPv6 relay agent sends a Relay-Forward packet to relay Request packets to DHCPv6 servers.
13	RELAY-REPL	-	A DHCPv6 server sends a Relay-Reply packet to a DHCPv6 relay agent. The Relay-Reply packet carries a packet that the DHCPv6 relay agent needs to deliver to a DHCPv6 client.