sip totag / grandstream / register

SIP Question: The Grandstream GXP2000 1.1.2.23 sends SIP REGISTER requests with a To tag. Is my proxy wrong in refusing the request?

REGISTER sip:server SIP/2.0
Via: SIP/2.0/UDP 1.2.3.4:5074;branch=z9hG4bK0b90873d634698eb
From: "phone 123" <sip:123@server>;tag=c29eb9104c6a5a86
To: <sip:123@server>;tag=as77984b6
Contact: <sip:123@1.2.3.4:5074;transport=udp>
Supported: path
Authorization: Digest username="123", realm="server", algorithm=MD5, uri="sip:server", nonce="0997652c", response="3b91afb768c11ae0a0405e1bed41bc23"
Call-ID: 3033205eb0b5d203@192.168.4.117
CSeq: 56349 REGISTER
Expires: 3600
User-Agent: Grandstream GXP2000 1.1.2.23
Max-Forwards: 70
Allow: INVITE,ACK,CANCEL,BYE,NOTIFY,REFER,OPTIONS,INFO,SUBSCRIBE,UPDATE,PRACK,MESSAGE
Content-Length: 0

Answer: no, the proxy is right. RFC 3261 says this about it.

8.1.1.2 To

   A request outside of a dialog MUST NOT contain a To tag; the tag in
   the To field of a request identifies the peer of the dialog.  Since
   no dialog is established, no tag is present.

10.2 Constructing the REGISTER Request

   A REGISTER request does not establish a dialog.

Grandstream has already fixed this bug long ago (in version 1.1.2.27, beginning of 2007) as you can see in the release notes on www.grandstreamsucks.com. You should upgrade the firmware.