Subject: Re: Question on Martini AAL-5 SDU mode

From: Eric Rosen <[email-masked]>
Date: Tue, 20 Aug 2002 10:50:35 -0400

David> The  PW provider  doesn't know  what the  Ethernet, or  FR or  ATM is
David> carrying in its  payload.  It may be protected by  the upper layer or
David> not, but  the expectation is that  it was protected as  though it had
David> been passed on the original layer 2,

I  think by "upper  layer" here  you mean  the endsystem  application.  That
application cannot have  any expectations of the data  link layer, unless it
knows that  the two ends  of the application  are attached by a  single data
link. There  are a  few such  applications: DEC's old  LAT protocol  is one,
NETBEUI is  another.  Happily  these have largely  been replaced  by TCP/IP.
However, it is fair to say  that running these applications over an ethernet
pseudowire  provides reduced integrity  when compared  to running  them over
real ethernet.   Whether this matters in  practice depends on  how many more
errors go undetected.  

Generally applications  that don't provide  their own integrity get  it from
the  transport layer.   In  connectionless networking,  the transport  layer
provides  integrity, but  in connection-oriented  networking,  the transport
layer may get integrity from the  network layer.  If, for example, there are
applications which  are designed to run directly  over a connection-oriented
network  layer, or applications  which run  over a  transport layer  that is
designed  to  run  over  a  connection-oriented network  layer,  then  these
applications cannot be assured the integrity  they expect.  So I think it is
fair to say that the AAL5  emulation, e.g., is sufficient to emulate AAL5 as
a data link  but not as a  network layer. whether this is  a practical issue
depends on how much use there is of ATM as a true network layer, rather than
as a data link. 

Anyway,  this  seems   to  all  be  in  the   area  of  "perfect  emulation"
vs.  "imperfect  emulation  with  applicability  statements  expressing  the
limitations", which we are not going to debate again, I hope.

---

From: David Sinicrope <[email-masked]>
Date: Tue, 20 Aug 2002 12:30:52 -0400

At 10:50 AM 8/20/2002, Eric Rosen wrote:
>Eric> I  think by "upper  layer" here  you mean  the 
>endsystem  application.  That
>Eric> application cannot have  any expectations of the data  link layer, 
>unless it
>Eric> knows that  the two ends  of the application  are attached by 
>a  single data
>Eric> link. There  are a  few such  applications: DEC's old  LAT 
>protocol  is one,
>Eric> NETBEUI is  another.  Happily  these have largely  been replaced  by 
>TCP/IP.

IBM's SNA is another and that too has probably been replaced by TCP/IP or 
runs on top
of TCP/IP (i.e., Data Link Switching).  However, there are still folks that 
run it over LAN
protocols that are bridged over FR or ATM because the service definitions 
of these protocols
guarantee integrity across the VC.  So the expectation is still that the 
data link
provides the integrity even though the expectation shouldn't be there.

The FR or ATM PW provider can't know whether it is transporting these types 
of protocols or not.
As you mention below, the provider can only define the service as "perfect" 
or "imperfect"
emulation and state the limitations.  So it comes down to the user having 
to read the fine
print of their ATM or FR PW service descriptions and know the subtle 
characteristics of the application protocols
they are running.  (I wonder how many providers would define their service 
as "imperfect". :-)

>Eric> However, it is fair to say  that running these applications over an 
>ethernet
>Eric> pseudowire  provides reduced integrity  when compared  to 
>running  them over
>Eric> real ethernet.   Whether this matters in  practice depends on  how 
>many more
>Eric> errors go undetected.

It also depends on how the application protocol handles undetected 
errors.  If it assumes
integrity is provided,  it probably doesn't handle them too well.

>Eric> Generally applications  that don't provide  their own integrity 
>get  it from
>Eric> the  transport layer.   In  connectionless networking,  the 
>transport  layer
>Eric> provides  integrity, but  in connection-oriented  networking,  the 
>transport
>Eric> layer may get integrity from the  network layer.  If, for example, 
>there are
>Eric> applications which  are designed to run directly  over a 
>connection-oriented
>Eric> network  layer, or applications  which run  over a  transport 
>layer  that is
>Eric> designed  to  run  over  a  connection-oriented 
>network  layer,  then  these
>Eric> applications cannot be assured the integrity  they expect.  So I 
>think it is
>Eric> fair to say that the AAL5  emulation, e.g., is sufficient to emulate 
>AAL5 as
>Eric> a data link  but not as a  network layer. whether this is  a 
>practical issue
>Eric> depends on how much use there is of ATM as a true network layer, 
>rather than
>Eric> as a data link.

I guess I'm looking at this from the bottom up.  Since the PW provider 
doesn't know how someone will
use the AAL5 emulation, data link or network layer, all they can do is 
state the service characteristics and
let the user decide.  I think this could get pretty confusing for the 
user.  "Is it ATM AAL5 or not?"

>Eric> 
>Eric> 
>Eric> limitations", which we are not going to debate again, I hope.
>

I hope I didn't start this debate again.  I can see that imperfect 
emulation has a place.  I'm not sure integrity
is one of the things that should be optional.

============================================================
David Sinicrope   +1-919-472-9967  [email-masked]
Ericsson IP Infrastructure               Fax:+1-919-472-9999