Subject: [rddp] Middlebox Taxonomy

From: Caitlin Bestler <[email-masked]>
Date: Mon, 9 Dec 2002 12:53:10 -0600

The WG chair suggested review of RFC 3234 on middlebox
taxonomy. Here's my first pass at the different types of
middleboxes that I believe have DDP implications

NAT

NAT boxes do not interfere with TCP streams. Once the TCP
connection is established each TCP segment is passed
through unmodified for unknown protocols (which all DDP
enabled ULPs would be).

Not all NAT boxes provide port-specific SCTP support (even
though the port fields are in *exactly* the same place).
NAT boxes are therefore an impediment to an SCTP-only
solution.

DDP enabled applications actually have an advantage when
working behind NATs to non DDP enabled applications. Non DDP
applications will sometimes create multiple connections in
order to allow interleaved delivery to multiple buffers
(such as multiple FTP or HTTP connections). However setting
up multiple connections between the same two hosts can be
challenging once NAT middleboxes are involved (especially if
they are present at *both* ends). DDP allows multiple buffer
deliveries to be interleaved over a single DDP stream,
avoiding this potential problem.

Transport Relays / TLGs (Transport Level Gateways) /
TCP Spoofers

The RFC explains that "TLGs are also sometimes used between
two instances of the same transport with signifigantly
different connection characteristics."

When relaying the transport between these two different
environments it is common to re-segment and respace Acks.

These are prime examples of "alignment destroying
middleboxes". They are also the type that could probably be
detected along the lines that David suggested.

TCP Enhancers

TCP enhancers attempt to improve segmentation and/or ACK
pacing. These  are the class of middlebox that I feared
would not "show itself" until ULP work was well in progress.
Any optimiizer will have a threshold of potential
optimization, below which it will not change anything. The
delay in making the change will outweight the optimization.
You could be well into a connection before a worthwhile
optimization presented itself and alignment was finally
destroyed.

Caitlin Bestler
http://asomi.com/CaitlinBestler/
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