[dhcwg] recommendation on DHCP6 source port numbers

[Tomoyuki Sahara <[e-mail]>]

Hi, DHC wg members:

Can we make recommendations on source port numbers of DHCP6 messages
in rfc8415bis?

DHCP6 specification says that DHCP6 clients and servers listen on UDP
port 546 and 547 respectively, in RFC8415 section 7.2.  It implies
that DHCP6 clients MUST send messages to UDP port 547 (server port) and
servers MUST send messages to UDP port 546 (client port) to work with
their counterpart correctly (though restrictions can be relaxed with
RFC8357 for relays).
  
But it says nothing about source port numbers.  Without any
restrictions, some implementations use ephemeral source port
(e.g. 12345) to send their messages.  DHCP6 conversations look like:

  1. client send Solicit    fe80::2#49876    -> ff02::1:2#547
  2. server send Advertise  fe80::1#547      -> fe80::2#546 (!)
  3. client send Request    fe80::2#49877(?) -> ff02::1:2#547
  4. server send Confirm    fe80::1#547      -> fe80::2#546

This behavior is not prohibited by the specification but makes
confusions for DHCP6 implementer and network/firewall operators (*1).
Most Internet protocols nowadays assume that servers send response
messages from the port number they received on.
(*1 e.g. https://bugzilla.redhat.com/show_bug.cgi?id=952126 )

In my humble opinion, it is too late to require that DHCP6 client and
server MUST send messages from the fixed port number (546/547) because
there are too many DHCP6 implementations in the wild.  But making a
recommendation is helpful for new implementations/deployments of DHCP6.

An idea to make such recommendation is adding a text in rfc8415bis:

  OLD:
    7.2. UDP Ports
      Clients listen for DHCP messages on UDP port 546.  Servers and
      relay agents listen for DHCP messages on UDP port 547.

  NEW:
    7.2. UDP Ports
      Clients listen for DHCP messages on UDP port 546.  Servers and
      relay agents listen for DHCP messages on UDP port 547.

      Clients are RECOMMENDED to send DHCP messages from UDP port 546.
      Servers and relay agents are RECOMMENDED to send DHCP messages
      from UDP port 547 (unless relay agent includes Relay Source Port
      Option for DHCP6 [RFC8357]).

I know WGLC has been concluded but I believe the recommendations above
encourage new implementations to use the standard DHCP6 port numbers
on UDP source port.


Best regards,
Tomoyuki Sahara