Subject: a question about TCP Implications of rfc2292bis

From: JINMEI Tatuya / 神明達哉<[email-masked]>
Date: Sat, 15 Jan 2000 02:49:58 +0900

I've implemented Section 4.1 of rfc2292bis-01 (TCP Implications) and
have a question about receiving options.

The draft has the following text:

   For efficiency reasons it is recommended that a
   TCP implementation not send ancillary data items with every received
   segment but instead try to detect the points in the data stream when
   the requested IPv6 and extension header content changes and only send
   a single ancillary data item at the time of the change.
(in the second paragraph)

My question is "if an extension header disappears in receiving packets,
how can an application know the fact?" For example, suppose that an
application set the IPV6_RECVRTHDR option and the peer of the
application starts sending packet with a routing header. Then the
application will see the routing header as ancillary data when
receiving the 1st packet with the routing header. If the kernel follows
the above text, the application won't see any routing headers unless
the content of the routing header changes. It seems OK, but what if
the peer stops sending a routing header? What should the kernel do in
such a case? Should it send an ancillary data object with the data
length being zero?

Note that such a case is not problematic for UDP or raw socket
applications, since they can detect the lack of routing header by the
fact that no IPV6_RTHDR ancillary data object is contained in the
received cmsg list.

I'm not sure if this has already discussed in the list, but I'd
appreciate any clarification. Thanks,

					JINMEI, Tatuya
					Communication Platform Lab.
					Corporate R&D Center, Toshiba Corp.
					[email-masked]
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From: Erik Nordmark <[email-masked]>
Date: Fri, 14 Jan 2000 19:38:26 -0800 (PST)

> how can an application know the fact?"

Looks like the text isn't clear on that point.
A possibility is to return a zero length ancillary data item of the right type.
> fact that no IPV6_RTHDR ancillary data object is contained in the

Ack.

   Erik