Subject: Re: Control and Forwarding functions

From: [email-masked]
Date: Tue, 04 Sep 2001 19:55:06 +0000

I don't know why some people will not accept that MPLS 
is mainly a connection oriented technology. The main 
characteristic of a Connection Oriented service is the 
long term association required between two end points. 
The process of how such association is established is 
not important. It is also not important if the 
connection doesn't begin or end at some hosts (why we 
need to use ISDN or X.25 definitions of connections?). 

It is quite obvious that there is very clearly 
established permanent association between two end points 
of LSPs. All LSRs also maintain the necessary states 
about particular paths (similar to circuit switched 
network). 

From all this, MPLS clearly is the connection oriented 
layer at layer 2. Of course, I have also seen comments 
that MPLS doesn't fit in ISO model. Oh Really, !.

Cheers,

--brijesh
Corona Networks
> OK, here we go again....
> 
> To its nature MPLS is not "connection oriented"! I also depends quite a
> bit on the definition of "connection oriented", but if we e.g. let an
> ISDN
> phone call be the model of a connection oriented technology, then you 
> immediately see that MPLS misses the most important part of what a 
> connection oriented technology is all about.
> ISDN is separated into two parts "call control" and "connection
> handling",
> together they form the connection orientation.
> 
> So the "call control" handle e-2-2 compatibility and ability to take the 
> call. This does not exist in MPLS. The call control also OK's the set
> up of the call and is once it is established dependent of the e-2-e 
> connectivity, if the connectivity is broken the call is released.
> Nothing
> of this is present in MPLS.
> 
> Sometimes one hear that MPLS is connection oriented because there is 
> a mapping from node to node over the network, strictly this is not true,
> the only thing that is mapped incoming label to outgoing label and
> interface.
> The rest is routing in the standard IP connections way.
> 
> As for the arguments that "most of the" LSP setup is connection
> oriented,
> usually meaning end-to-end (which really is edge-to-edge, there is not 
> much idea to establish a connection edge-to-edge. But even so
> 
> LDP DU independent liberal is not connection oriented
> LDP DU independent conservative is not connection oriented
> LDP DU ordered liberal is not connection oriented
> LDP DU ordered conservative is not connection oriented
> 
> LDP DoD independent liberal is not connection oriented
> LDP DoD independent conservative is not connection oriented
> LDP DoD ordered liberal is not connection oriented
> LDP DoD ordered conservative could at best emulate part of a connection
>         oriented technology (signaling)
> 
> RSV-TE mostly strictly follows IP routing and is not connection oriented
>        and if there are explicit routes it falls into the same category
>        as LDP DoD ordered conservative
> 
> We are left with a technology where it is required to have an
> MPLS-external
> application that do the call control part of connection orientation. Why
> we would like to do that is beyond me, as we loses all the benefits of
> the
> IP and MPLS combination.
> 
> /Loa
> 
> -- 
> Loa Andersson
> Chief Architect,
> Utfors Research, Architecture and Future Lab (URAX)
> Utfors AB
> Råsundavägen 12
> Box 525, 169 29 Solna
> Office          +46 8 5270 2000
> Office direct   +46 8 5270 5038
> Mobile          +46 70 848 5038
> Email           [email-masked]
> WWW             www.utfors.se

---

From: Loa Andersson <[email-masked]>
Date: Tue, 04 Sep 2001 23:04:16 +0200

So you make your definiton of "connection oriented" then it is
possible
to discuss whether MPLS is connection oriented according to that 
definition. Fair, but changing rules during discussion don't bring 
you anywhere.

According to the OSI model L2 has a common mac, does MPLS?
According to the OSI model L3 as a common addressing scheme, does
MPLS?

/Loa

-------------------

Loa Andersson
Chief Architect,
R?ndav?n 12

---

From: Loa Andersson <[email-masked]>
Date: Tue, 04 Sep 2001 23:22:33 +0200

Further, see comments in line.

/Loa
-------------------
> is mainly a connection oriented technology. 

Basically because we need the dynamics of a connectiion less
technology. There are things in the old style connection 
oriented technologies that we don't want in modern networks.

> The main 

This makes IP a connection oriented technology! 

> need to use ISDN or X.25 definitions of connections?).

If you say that anything that even remotely could be forced into a
e-2-e signalling scheme is connection oriented technology, well 
then the definition loses all content. It is the e-2-e compatibility
negotioation and the dependency on the continous operation
of the connectivity that constitutes the connection orientation. 
y
> of LSPs. 

As a matter of fact an LSP might have any number of upstream
end points,  but these association is not any more pernanent than
an IP route.

So either you have a difference between connection less and conenction
oriented, or you declare everything connection oriented, in which case
I could with equal justification claim that everything is connection
less.

I maintain that there is an important difference, that have impacts on
the
performance of the network. Would be good if we could understand this
difference and use it to the benefit of networking.

> All LSRs also maintain the necessary states 

So does a router
> layer at layer 2. 

So you clain that the link layer supply network wide connection???

Of course, I have also seen comments 

/Loa

Loa Andersson
Chief Architect,

---

From: Eric Gray <[email-masked]>
Date: Tue, 04 Sep 2001 18:07:03 -0400

Brijesh,

    A connection oriented service is one that maintains a persistent
connection between two stations. Connection oriented services
require connected stations to be constantly talking to each other,
regardless of whether or not they have anything worth saying.

    Not all MPLS LSPs are between two stations.  Topology based LSPs
(e.g. - LDP or BGP established) form subsets of the paths from multiple
sources to some definition of a destination.  In this case, LSPs may
capriciously start and stop at any point along such paths and may even
skip around, taking a different path, based on a short term change in
routing.  Also, in these cases, talking between stations required to
maintain LSPs is peer to peer, rather than LSP end to end.

    Also, while MPLS labels may be carried as part of a L2 header,
MPLS is not an L2 mechanism in the sense of the OSI model.  It
exists between the Data Link Layer and the Network Layer.  One
can argue that it 'fits' this model between layers or one can argue
that it doesn't fit - but either position is not very important.  After
all, a model is only a model.  It is - at best - a subset of reality.
Often a model's most useful feature is that it allows us to discuss
some complex notions (e.g. - a network) in a way that is not hopelessly
confusing.

--
Eric Gray

---

From: David Charlap <[email-masked]>
Date: Tue, 04 Sep 2001 18:42:02 -0400

Loa Andersson wrote:
>> 
>> The main characteristic of a Connection Oriented service is the

No.  There is no association between endpoints with _IP_.  However, TCP
does establish such a connection.  TCP is a connection-oriented
protocol, even though it is built over a connectionless network layer.

As for MPLS, it is just as connection-oriented as ATM.  Does the fact
that UNI signaling is point-to-point change the fact that it serves to
establish virtual circuits?  Similarly, does the fact that MPLS
signaling (RSVP-TE, LDP and CR-LDP) act point-to-point change the fact
that the end result is a connection-oriented LSP between two points in
the network.

I am curious, however, why you care.  Does the application of the term
"connection oriented" make the technology any more or less useful?

> end points,  but these association is not any more pernanent than

The existance of an LSP is not the same as deciding what data to
transmit through that LSP.

Will you say that the phone line my modem dials through is not
connection oriented because the computer it's attached to makes
per-packet forwarding decisions from multiple endpoints (the computers
in my home LAN) before sending data over it?

The connection is there, even if connectionless technology is used to
send data over that connection.

-- David

---

From: "Mareline Sheldon" <[email-masked]>
Date: Wed, 5 Sep 2001 09:23:45 +0530

Why doesn't MPLS fit into the ISO 7 layer reference model ?

MPLS doesn't fit into layer 2 (link layer), as it is independent of
a particular link layer technology (it runs over ATM, p2p links,
ethernet, etc). It doesn't form a layer 3 (Network layer) on its
own as its doesn't have its own routing and addressing schemes etc.
Moreover, the ISO reference model assumes that a given link
or a network layer uses a single format for the transport of the
data from the layer above. MPLS clearly violates this
assumption, as over ATM it uses the ATM header to carry label
information, while over ethernet or p2p links it uses the "shim".
Hence MPLS doesn't actually fit into any layer!

Mary
----- Original Message -----
From: <[email-masked]>
To: "Loa Andersson" <[email-masked]>
Cc: "David Allan" <[email-masked]>; "Ding Aijun"
<[email-masked]>; "Hongwei" <[email-masked]>; "David
Escobar" <[email-masked]>; "mpls" <[email-masked]>
Sent: Wednesday, September 05, 2001 1:25 AM
Subject: Re: Control and Forwarding functions

[snip .. ]

> that MPLS doesn't fit in ISO model. Oh Really, !.

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---

From: Eric Osborne <[email-masked]>
Date: Wed, 5 Sep 2001 00:41:09 -0400

> own as its doesn't have its own routing and addressing schemes etc.

True.  

> Hence MPLS doesn't actually fit into any layer!

MPLS over ATM has a given encapsulation.  MPLS over non-ATM has a
different encap.  It's true that these two technologies are
different.  But that in and of itself doesn't mean that there's no
layer they could fit in.  If the concept of MPLS over ATM using
VPI/VCI as label didn't exist, would you say MPLS was a "layer"
because it had a single shim format across all link layers?  If so,
would adding MPLS over ATM suddenly invalidate your previous model?

No, MPLS doesn't fit in the OSI reference model.  But neither does IP
(where's the Presentation and Session layers?  And no, XDR doesn't
count).  

So what?  Does GRE?  Does X25 over TCP?  No.  Now can we drop the
layer bickering and get back to something productive, like arguing
about OAM? :)

eric

---

From: "Sukanta Ganguly" <[email-masked]>
Date: Tue, 04 Sep 2001 22:14:06 -0700

Eric,
  I do agree that it does not completely fit into L2 but even then it does perform the act of maintaining the connection orientedness. The MAC-to-MAC analogy can be applied to the MPLS realm too. If there was no MAC then between routers there would be the MPLS labels which would uniquely identify the ports/interface onto which the packet would have to go. Since we still depend on the MAC-to-MAC at the Ethernet layer, we do overlook this aspect. Thinking more into it, if we slice of the MAC layer underneath and rely on the MPLS labels to be a representation of the ports then do we think it will work? I am sure I can explain its working. 

SG

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