"Drawback with UDP is that there is no
     guarantee that a datagram will arrive
     at all, and if it arrives, a response
     is not guaranteed."

Isn't DNS name look-up implememted via UDP?

How often does that fail?

Correct me if I'm wrong, but every time you 'click'
on a hypertext link - isn't there a UDP name query
that takes place to get the actual IP address?

(Unless I'm just totally misunderstanding Stevens ...)

How often does that fail over the 'net?

Jim



----- Original Message -----
From: "Byron A Jeff" <byron@CC.GATECH.EDU>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Tuesday, September 04, 2001 6:30 AM
Subject: Re: [PIC]: PIC - TCP/IP / controller / relays / Bit of fun!


> On Tue, Sep 04, 2001 at 03:15:55PM +0800, PeterTiang wrote:
> > > I believe that the appropriate transport protocol is UDP. With its
paper
> > thin
> > > wrapping over the core IP protocol, and its standard and widely
deployed
> > API
> > > interface, it's perfect for a PIC to interact on the Internet.
> >
> >     Drawback with UDP is that there is no guarantee
> >     that a datagram will arrive at all, and if it
> >     arrives, a response is not guaranteed.
> >
> >     You probably need the higher layer apps to
> >     implement retry mechanism on top of UDP/IP stack.
>
> Of course. But it doesn't have to be the extremely complex mechanism that
TCP
> uses for sequencing, syncronization, and acknowledgement.
>
> By building from the bottom up, it is possible to provide only the minimum
> reliability required for the application. Building from the top down with
TCP
> will require a lot more infrastructure in order to meet TCP's minimum
> requirement.
>
> Also the frequency  of many of the potential failures are a function of
the
> underlaying network. Existing UDP protocols such as DNS, TFTP, and NFS are
> generally as reliable as the underlaying networks they operate under.
>
> TCP forces you to code for the absolute worst case. With UDP you can
simplify
> your assumptions based on the fact that the network is generally reliable.
>
> Finally using UDP it is possible to manage the send/ack relationship from
the
> client side application, thereby further simplifying the stack that is
> implemented on the PIC. With TCP it's not possible to create such an
unbalanced
> model.
>
> So between utilizing the improved reliability of the underlaying network,
> a simplifying assumptions, simpler and more reliable PIC based networking
> can be realized. But now you the designer can choose the appropriate
level.
>
> BAJ
>
> --
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>

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