Subject: RE: IPv6 MTU for tunnel pseudo interfaces

From: Jung Cyndi-ACJ099 <[email-masked]>
Date: Thu, 8 Aug 2002 09:48:48 -0500

This sounds like a case where you should apply the old rule "be conservative
with 
what you send and generous with what you are willing to receive" - if you
have 
successfully received the whole packet, please don't drop it.

Cyndi

-----Original Message-----
From: Pekka Savola [mailto:[email-masked]]
Sent: Thursday, August 08, 2002 12:01 AM
To: [email-masked]
Cc: [email-masked]
Subject: Re: IPv6 MTU for tunnel pseudo interfaces 

On Thu, 8 Aug 2002 [email-masked] wrote:
> >> 	i know of no nodes that transmit packet too big messgages, when
> >> 	incoming packets exceed MRU.  
> >No nodes do; but I know of no nodes that drop packets they're able to 
> >receive based on some configured maximum value either.
> >> too big messages are generated
> >> 	when outgoing packets exceed MTU.
> >But if a node X is able to send packet on a medium (in case of tunneling,
> >nothing physical though), and node Y is able to receive it, why should it
> >check MRU?
> 
> 	since there's no standard, some implementation can be picky and
> 	assume MRU == MTU.  maybe RFC2893 should talk more about MRU and
MTU.

I've configured Ethernet MTU to be 1500 on the sender side and 1280 on the
recipient side (MRU is generally unconfigurable); it works without
problems as far as I've noticed.

These issues are not specific to tunnels as far as I can see.

-- 
Pekka Savola                 "Tell me of difficulties surmounted,
Netcore Oy                   not those you stumble over and fall"
Systems. Networks. Security.  -- Robert Jordan: A Crown of Swords

--------------------------------------------------------------------
IETF IPng Working Group Mailing List
IPng Home Page:                      http://playground.sun.com/ipng
FTP archive:                      ftp://playground.sun.com/pub/ipng
Direct all administrative requests to [email-masked]
--------------------------------------------------------------------
--------------------------------------------------------------------
IETF IPng Working Group Mailing List
IPng Home Page:                      http://playground.sun.com/ipng
FTP archive:                      ftp://playground.sun.com/pub/ipng
Direct all administrative requests to [email-masked]
--------------------------------------------------------------------

---

From: "Mohan Parthasarathy" <[email-masked]>
Date: Mon, 12 Aug 2002 09:53:58 -0700

> 
> >At a minimum, implementations should be able to convert the=20 ICMP 
> >errors happening from within the IPv4 tunnel back to ICMPV6 
> error and 
> >send it back to the original sender (IPv6 sender in case of 
> IPv6 over 
> >IPv4 tunnels). Otherwise path mtu discovery will never work assuming 
> >you set the DF bit in the IPv4 header. if you do this, storing the 
> >state is not really complex I guess.
> 
> 	if you don't set DF bit on IPv4 packet (after encapsulation),
> 	it is not mandatory to convert ICMPv4 to ICMPv6 
> (actually, you won't
> 	see ICMPv4 need fragment).  simpler the better.
> 
I am not really sure how much more simpler it would be (it is extra
lines of code). But you are arguing that doing path mtu discovery
is not useful because it would make the code complex which I
don't understand.

-mohan

> itojun