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Re: [tae] [tsv-area] Transport negotiation

Joe Touch <touch@ISI.EDU> Wed, 10 December 2008 17:56 UTC

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Date: Wed, 10 Dec 2008 09:55:55 -0800
From: Joe Touch <touch@ISI.EDU>
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To: Stuart Cheshire <cheshire@apple.com>
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Subject: Re: [tae] [tsv-area] Transport negotiation
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Stuart Cheshire wrote:
> On 9 Dec, 2008, at 19:01, james woodyatt wrote:
> 
>> From where I stand, i.e. smack in the middle of your transport-aware
>> packet-forwarding plane
> 
> Why do we want a transport-aware packet-forwarding plane?
> 
> The primary reason NAT gateways need to peek into the transport header
> is to get at the ports for receiver demultiplexing. 

The IP header has exactly what a NAT needs for receiver demultiplexing -
the IP address.

> It's purely a
> historical mistake that we forgot to put the demultiplexing identifiers
> into the IP-layer header like other protocols do, which is why every
> IP-based transport layer has to duplicate that functionality, which is
> why NAT gateways need to peek into those transport headers.

Ports are intended for demultiplexing within a host. The fact that a NAT
masks multiple hosts breaks port demultiplexing *by design*; NATs are
intended to hide the fact that there are multiple hosts.

> It's too late to fix the IPv6 header format, but if we regard UDP/IPv6
> as our new combined datagram header, the way Bryan Ford proposes, then a
> whole bunch of problems go away. It's not a deficiency of NAT gateways
> that they need to understand every transport header (and thereby impede
> adoption of transports they don't understand); it's a deficiency of our
> transport headers that they include demultiplexing identifiers that they
> should not.

IMO, NATs need to understand the demultiplexing that is designed to be
internal to a host exactly because of what a NAT is and what it does. It
doesn't make sense to be to bemoan that fact.

Joe
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