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Spirent TestCenter: What is DNAv4 and how does it work with DHCP?

  • Spirent TestCenter
    • v4.57
    • DHCPv4
DNAv4 (Detecting Network Attachment in IPv4) is defined in RFC 4436.  

The use case is when a node, which has a previously assigned IP address (typically obtained from DHCP), is attaching itself to a network.  The node wants to see if it can use the previously obtained IP address as soon as possible.  But traditionally that node will start in the INIT-REBOOT and send out a DHCPREQUEST which takes "some time".  So the goal of DNAv4 is to optimize the time this node can use an address.

The way it does this is to send a unicast ARP Request out to a target address (MAC and IP) that it has cached from its previously attached network (e.g., the gateway MAC and IP).  This is called a reachability test (see section 2.1 of the RFC).  
  • If there is an ARP Reply then it can assume that it is on the same network and so can and will continue to use the previously assigned IP address.  
    • This is very quick and can be less than a millisecond.  
    • The risk of course that it actually is using a duplicate IP address.  According to the RFC this false negative according the RFC is acceptable.
    • Please see attached "passed reachability test.pcap"
  • If there is no ARP Reply then the node will enter either the DHCP INIT or INIT_REBOOT state and broadcast a DHCPREQUEST.  
    • The argument here is the additional time it takes to perform the reachability test is negligible and so even if it fails, little time is added to the eventual time of entering the INIT or INT_REBOOT states.
    • Please see attached "failed reachability test.pcap"
Attachment Description
RFC 4436 - Detecting Network Attachment in IPv4 (DNAv4)

Attachment Description
passed reachability test.pcap

Attachment Description
failed reachability test.pcap

Product : Access,Spirent TestCenter,DHCP