Subject: Additional implementation scenario for PMTU RFC 1981

From: Rajesh <[email-masked]>
Date: Fri, 19 Oct 2007 17:26:44 +0800

In PMTU RFC 1981 Most of the implementation scenarios have been considered.

This RFC suggests that the IP layer stores PMTU information, so that PMTU
information can be shared across different packetization layer.

If IP layer discovers PMTU information due to the packet sent by one of the
packetization layer (Assume TCP), when other packetization layer (UDP/RAWIP)

uses this PMTU information, we must notify about this PMTU information to
Application layer which is using UDP/RAWIP as packetization layer.

This implementation scenario is missing in RFC 1981.

TCP application sends a packet to destination with Link MTU as PMTU, PMTU
discovery happens IP layer stores PMTU information corresponding to this
destination address and inform TCP layer about the same.

When UDP/RAWIP application send packets to the same destination, IP layer
make use of learned PMTU information and fragments the packet. But UDP/RAWIP
application must be notified of this PMTU value.

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From: [email-masked] (Jun-ichiro itojun Hagino)
Date: Fri, 19 Oct 2007 18:43:20 +0900 (JST)

> This implementation scenario is missing in RFC 1981.

	are you talking about API, or any other protocol interactions?
	please clarify.

	in either case.
	- with multicast, it is very difficult to perform effective PMTUD,
	  so applications are implemented using minimum MTU (1280).
	- with unicast, applications either use minimum MTU (1280), or
	  rely on kernel layer fragmentation mechanism.

itojun