Subject: Re: [Tsvwg] Status of TCP ULP Framing

From: Joe Touch <[email-masked]>
Date: Sun, 17 Feb 2002 21:18:50 -0800

Stephen Bailey wrote:

> Joe,
> 
> 
>>A protocol, in this case TCP, is more than a wire specification; 
>>it defines the behavior of the endpoints, as well as the API.
>>
>>At least two of those three are changing.
>>
> 
> To the best of our understanding, the behavior described for a
> framing-aware TCP supporting PDU alignment-mode is not a change from
> what is allowed by the TCP protocol.  In fact, if the application
> knew, or guessed at the EMSS, and tiled PDUs appropriately, some
> current TCPs would be highly likely to behave exactly like a
> framing-aware TCP.  In the cases where the two would differ, the
> framing-aware TCP behavior is a change from current implementation
> practice, but it is not a change from what is specified.

There is absolutely no correlation between ADUs and PDUs in TCP. A given 
TCP implementation is free to 'tile' as it sees fit, including overlapping.

What you really have is a way to embed a new protocol inside TCP, using 
TCP's retransmission/etc., but defining whatever new semantics you 
desire, including alignment. The protocol 'backs off' to TCP when the 
receiver verification fails. This reduces the overhead of connection 
establishment for your protocol when you succeed, at the expense of 
complicating an existing protocol for no other good reason.

Any 'escape' mechanism that attempts to circumvent this characteristic 
is, at least IMO, a new protocol. Embedding it in TCP to accelerate its 
acceptance (vs., e.g., using a new protocol ID) does not make it a 
feature of TCP.

Joe

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From: "Douglas Otis" <[email-masked]>
Date: Mon, 18 Feb 2002 13:51:34 -0800

Joe,

Joe, I agree.  TUF, COWS, FIM, NTD, and COBS are all techniques to enable
ULP based placement within a layer beneath TCP.  The transport serves as the
interface to the application and with inclusion of this layer, additional
parameters must be exchanged.  Modifications of TCP to accommodate placement
steering must be included within drafts to allow review.  TCP specifications
are not limited to structures sent on the wire and placement to buffers
associated by PDU content does not resemble normal TCP behavior from the
perspective of the application.  Creating a generalized format for
synchronization and steering would reduce proliferation of TCP variants.
These drafts only address a desire to synchronize to ULP structures when
received out of sequence.  As a result, placement must accommodate each
application and the impact on TCP remains unrevealed.

Doug

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From: "Brian F. G. Bidulock" <[email-masked]>
Date: Mon, 18 Feb 2002 16:36:05 -0600

Douglas,

With the number of vendors claiming commercial deployments of SCTP
(and SIGTRAN UAs for that matter) I find it hard to believe that
any of these steps are required as intermediary steps to just using
SCTP.  Will TSV next try to shoe-horn multi-homing into TCP?

--brian

-- 
Brian F. G. Bidulock    ¦ The reasonable man adapts himself to the ¦
[email-masked]    ¦ world; the unreasonable one persists in  ¦
http://www.openss7.org/ ¦ trying  to adapt the  world  to himself. ¦
                        ¦ Therefore  all  progress  depends on the ¦
                        ¦ unreasonable man. -- George Bernard Shaw ¦