Subject: Re: [RRG] Fwd: Tunnel MTU

From: Brian E Carpenter <[email-masked]>
Date: Mon, 24 Sep 2007 11:35:17 +1200

I must confess that I don't like loading up tunnel end-points
with this kind of complexity. Architecturally, I much prefer
the e2e RFC 4821 approach. That isn't exclusive to TCP.
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.

One could also think about a generic non-ICMP UDP echo process
for PLPMTUD, although that does raise DoS and firewall issues.

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
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> 
> 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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