Subject: [dhcwg] Question on RFC2131 / RFC1541

From: Andrew W Elble <[email-masked]>
Date: Mon, 11 Dec 2017 18:43:52 -0500

I'm wondering about the re-use of XID at time T1. RFC1541 had a clear
explanation as to what was expected of XID at renewal. RFC2131 is
lacking that language.

RFC7513 seems to contain a reference to the use of a new TID/XID at
renewal time.

Should clients be re-using the same XID that was used in the initial
DHCPDISCOVER at renewal?

RFC7513 6.4.3.5:
 Event: EVE_DHCP_RENEW - A DHCPv4 Renew or a DHCPv6 Renew
        message is received
   "In such case, a new TID will be used by the client.  The TID field of
   the corresponding entries MUST be set to the new TID.  Note that TID
   check will not be performed on such messages."

RFC2131 4.1:
 Constructing and sending DHCP messages
   "The 'xid' field is used by the client to match incoming DHCP messages
   with pending requests.  A DHCP client MUST choose 'xid's in such a
   way as to minimize the chance of using an 'xid' identical to one used
   by another client. For example, a client may choose a different,
   random initial 'xid' each time the client is rebooted, and
   subsequently use sequential 'xid's until the next reboot.  Selecting
   a new 'xid' for each retransmission is an implementation decision.  A
   client may choose to reuse the same 'xid' or select a new 'xid' for
   each retransmitted message"

RFC1531/RFC1541 4.4.4:
 Reacquisition and expiration
   "At time T1 after the client accepts the lease on its network address,
   the client moves to RENEWING state and sends (via unicast) a
   DHCPREQUEST message to the server to extend its lease.  The client
   generates a random transaction identifier and inserts that identifier
   into the 'xid' field in the DHCPREQUEST. The client records the local
   time at which the DHCPREQUEST message is sent for computation of the
   lease expiration time.  The client MUST NOT include a 'server
   identifier' in the DHCPREQUEST message."

Thanks,

Andy

-- 
Andrew W. Elble
[email-masked]
Infrastructure Engineer, Communications Technical Lead
Rochester Institute of Technology
PGP: BFAD 8461 4CCF DC95 DA2C B0EB 965B 082E 863E C912

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From: Ted Lemon <[email-masked]>
Date: Mon, 11 Dec 2017 19:51:02 -0500

> Should clients be re-using the same XID that was used in the initial

No.