Subject: [Capwap] Issue 161: NAT Inconsistencies

From: "Pat Calhoun (pacalhou)" <[email-masked]>
Date: Tue, 29 Jul 2008 08:45:22 -0700

Pasi's Comment was:

   There's some inconsistency about NAT detection: Sections 4.6.12,
4.6.13, and 
   4.6.15 say it's done using "CAPWAP Local IPv4/6 address"
   message elements; Sections 4.6.44, 4.6.45, and 6.1 say it's using
"WTP
   IPv4/6 address" message elements.

Pasi's comment is spot on. The document was unfortunately very
inconsistent. There were two pairs of message elements that was doing
basically the same thing, and to make things worse, there was no clear
text that explained which pair to use. So I have removed the "WTP
IPv4/v6 Address" message elements, since we need a way for both the WTP
and the AC to communicate their address.

Most of the text below contains changes only, except a new paragraph in
the NAT Considerations section.

<text>
4.6.12.  CAPWAP Local IPv4 Address

   The CAPWAP Local IPv4 Address message element is sent by either the
   WTP, in the Join Request, or by the AC, in the Join Response.  The
   CAPWAP Local IPv4 Address message element is used to communicate the
   IP Address of the transmitter.  The receiver uses this to determine
   whether a middlebox exists between the two peers, by comparing the
   source IP address of the packet against the value of the message
   element.
[...]

4.6.13.  CAPWAP Local IPv6 Address

   The CAPWAP Local IPv6 Address message element is sent by either the
   WTP, in the Join Request, or by the AC, in the Join Response.  The
   CAPWAP Local IPv6 Address message element is used to communicate the
   IP Address of the transmitter.  The receiver uses this to determine
   whether a middlebox exists between the two peers, by comparing the
   source IP address of the packet against the value of the message
   element.
[...]

4.6.15.  CAPWAP Transport Protocol

   When CAPWAP is run over IPv6, the UDP-Lite or UDP transports MAY be
   used (see Section 3).  The CAPWAP IPv6 Transport Protocol message
   element is used by either the WTP or the AC to signal which transport
   protocol is to be used for the CAPWAP data channel.

   Upon receiving the Join Request, the AC MAY set the CAPWAP Transport
   Protocol to UDP-Lite in the Join Response message if the CAPWAP
   message was received over IPv6, and the CAPWAP Local IPv6 Address
   message element (see Section 4.6.13) is present and no middlebox was
   detected (see Section 11).

   Upon receiving the Join Response, the WTP MAY set the CAPWAP
   Transport Protocol to UDP-Lite in the Configuration Status Request or
   Image Data Request message if the AC advertised support for UDP-Lite,
   the message was received over IPv6, the CAPWAP Local IPv6 Address
   message element (see Section 4.6.13) and no middlebox was detected
   (see Section 11).  Upon receiving either the Configuration Status
   Request or the Image Data Request, the AC MUST observe the preference
   indicated by the WTP in the CAPWAP Transport Protocol, as long as it
   is consistent with what the AC advertised in the Join Response.
[...]

6.1.  Join Request
[...]
   At least one of the following message element MUST be included in the
   Join Request message.

   o  CAPWAP Local IPv4 Address, see Section 4.6.12

   o  CAPWAP Local IPv6 Address, see Section 4.6.13

6.2.  Join Response
[...]
   One of the following message elements MUST be included in the Join
   Response Message:

   o  CAPWAP Local IPv4 Address, see Section 4.6.12

   o  CAPWAP Local IPv6 Address, see Section 4.6.13

11.  NAT Considerations
[...]
   In order for a CAPWAP WTP or AC to detect whether a middlebox is
   present, both the Join Request (see Section 6.1) and the Join
   Response (see Section 6.2) include either the CAPWAP Local IPv4
   Address (see Section 4.6.12), or the CAPWAP Local IPv6 Address (see
   Section 4.6.13) message element.  Upon receiving one of these
   messages, if the packet's source IP address differs from the address
   found in either one of these message elements, it indicates that a
   middlebox is present.
</text>

PatC
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