DHCPv6 SOL_MAX_RT (82) Option
What is SOL_MAX_RT
RFC 8415 has defined the DHCPv6 SOL_MAX_RT option, and option-code is 82.
During DHCPv6 handshake, if a client includes an Option Request option (ORO) to request the SOL_MAX_RT option and gets a valid SOL_MAX_RT option from the server, this value of SOL_MAX_RT will replace the default value predefined in the client.
Then, if a client sends a solicit message and fails to get a response from the server after some time due to router power failure or outage, the client will retransmit the solicit message after a backoff timer expiry. And the backoff timer of next retransmission will be calculated according to the previous one until it reaches the value of SOL_MAX_RT. After that, the backoff timer will be calculated base on SOL_MAX_RT.
One use for the SOL_MAX_RT option is to set a higher value for SOL_MAX_RT; this reduces the Solicit traffic from a client that has not received a response to its Solicit messages.
How to calculate SOL_MAX_RT
RT for the first message transmission is based on IRT (i.e. 1 sec) and a random number RAND which is chosen with a uniform distribution between -0.1 and +0.1：
RT = IRT + RAND*IRT (The first retransmission will be 1 +/- 0.1)
So the very first RT will be within 0.9 ~ 1.1 sec
RT for each subsequent message transmission is based on the previous value of RT:
RT = 2*RTprev + RAND*RTprev
If the RT reaches MRT(i.e. SOL_MAX_RT), then:
if (RT > MRT)
RT = MRT + RAND*MRT
Verification of SOL_MAX_RT on isc-dhcpv6 server
1、Configure SOL_MAX_RT by setting the option dhcp6.solmax-rt, e.g. 100 so that the backoff time of SOL_MAX_RT will be ranging from 90 to 110 secs.
2、Save the configuration and restart isc-dhcpv6 server by using command: /etc/init.d/dhcpd6 restart
3、Start capturing packets on the WAN side of the DHCPv6 client. There should be DHCPv6 handshake bewteen client and server (i.e. SARR). From the Solicit message sent by client, we can see the ORO list including SOL_MAX_RT(82).
4、Server responds with SOL_MAX_RT(82) which is configured as 100 in the step 1.(equals to 64 in base 16)
5、Stop isc-dhcpv6 server so that it will not reply to any request from client. Eventually, the backoff time of Solicit will be ranging from 90 to 110 secs.
