Subject: RE: [RRG] Fwd: Tunnel MTU

From: "Templin, Fred L" <[email-masked]>
Date: Mon, 24 Sep 2007 14:33:12 -0700

Brian, 

> -----Original Message-----
> From: Brian E Carpenter [mailto:[email-masked]] 
> Sent: Sunday, September 23, 2007 4:35 PM
> To: Templin, Fred L
> Cc: [email-masked]
> Subject: Re: [RRG] Fwd: Tunnel MTU
> 
> I must confess that I don't like loading up tunnel end-points
> with this kind of complexity.

Ideally, the tunnel endpoint (TE) could simply set an MTU
of (64KB - ENCAPS) and admit all packets smaller than that
size. But, that would result in excessive in-the-network
fragmentation and/or silent loss due to path MTU restrictions.
Also, the tunnel must be made capable of passing packets as
large as 1280 bytes (for IPv6) and should be made capable of
passing packets up to 1500 bytes or larger (for all tunnels).

> Architecturally, I much prefer
> the e2e RFC 4821 approach. That isn't exclusive to TCP.

I agree that the e2e RFC 4821 approach is desireable, but
the tunnel cannot control the e2e use of RFC 4821 and
cannot even detect when RFC4821 is being used. So, the
tunnel has to provide consistent handling of packets
of various sizes independent of RFC821.     
 
> In fact there's a fairly simple approach to MTU discovery for
> UDP based protocols that have any kind of feedback from destination
> to source (i.e. start small and increase the packet size until
> the far end detects fragmentation or loss). 4821 does discuss this
> briefly.

RFC4821 also discusses the use of IP fragmentation as a
packetization layer in Section 10.3.
 
> One could also think about a generic non-ICMP UDP echo process
> for PLPMTUD, although that does raise DoS and firewall issues.

AFAICT, a simple mitigation for the DoS issue exists. Also
AFAICT, the firewall issue should not come into play if the
echos are made to look like ordinary tunneled data. Please
let me know if I am missing something.

Thanks - Fred
[email-masked]
 
> Regards
>     Brian Carpenter
>     University of Auckland
> 
> 
> On 2007-09-21 09:50, Templin, Fred L wrote:
> > (Fwd'd from 'int-area'):
> > Please see below for a proposal that addresses the MTU issues
> > for *-in-IPv4 tunnels. It also addresses the multi-mtu subnet
> > issue, since it does not rely on ICMP "packet too big" messages
> > from the last-hop router. Elements of the proposal include:
> > 
> >   - trailing "footer" and data as part of encapsulation
> >   - tiny echo requests wrapped in encapsulation headers
> >     and trailing padding - used as probes to elicit tiny
> >     echo replies
> >   - tunnel endpoint discovers tunnel far end EMTU_R and
> >     reassemby timeout values in initial probes=20
> >   - periodic probing to discover the tunnel path MTU,
> >     plus EMTU_R; reassembly timeout fluctuations
> >   - inner fragmentation to create inner packets no larger
> >     than EMTU_R
> >   - outer fragmentation to create outer packets no larger
> >     than the tunnel path MTU
> >   - coservative use of fragmentation to avoid packet loss
> >     within the tunnel
> >   - TE drops unfragmentable packets larger than EMTU_R
> >     and sends ICMP PTB w/o wasting tunnel resources
> >   - protection against fragment misassociations
> >   - protection against off-path attacks
> >   - protection against wrapped ip_id
> >   - backwards compatibility
> >   - incremental deployment
> >   - NAT traversal
> > 
> > Please review and send comments.
> > 
> > Thanks - Fred
> > [email-masked]
> > 
> > ------_=_NextPart_002_01C7FA2B.91342580
> > Content-Type: text/plain;
> > 	charset="iso-8859-1"
> > Content-Transfer-Encoding: quoted-printable
> > 
> > A New Internet-Draft is available from the on-line 
> Internet-Drafts=20
> > directories.
> > 
> > 
> > 	Title		: Packetization Layer Path MTU Discovery for=20
> >                           IP/*/IPv4 Tunnels
> > 	Author(s)	: F. Templin
> > 	Filename	: draft-templin-inetmtu-00.txt
> > 	Pages		: 20
> > 	Date		: 2007-9-18
> > 
> >    The nominal Maximum Transmission Unit (MTU) MTU of the 
> Internet has
> >    become 1500 bytes, but existing IP/*/IPv4 tunneling 
> mechanisms impose
> >    an encapsulation overhead that can reduce the effective 
> path MTU to
> >    smaller values.  Additionally, existing IP/*/IPv4 tunneling
> >    mechanisms are limited in their ability to discover and utilize
> >    larger MTUs.  This document specifies new mechanisms for 
> conveying
> >    packets over IP/*/IPv4 tunnels that address these issues.
> > 
> > A URL for this Internet-Draft is:
> > http://www.ietf.org/internet-drafts/draft-templin-inetmtu-00.txt
> > 
> > --
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> > 
> 

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