Subject: (ION) Question about RFC 2684

From: Michele Fontana <[email-masked]>
Date: Fri, 10 Dec 1999 16:51:39 +0100

Hi  to everybody,

I am a new comer in this mailing list. I have some doubt about RFC 2684,

on Multiprotocol encapsulation over AAL-5.
I apologize if my question was asked and answered already.
My doubt is about the encapsulation of IPv4, IPv6 and MPLS packets:

IPv4:        in RFC2684 is said that IPv4 can be carried in LLC
encapsulation
                as a routed NON ISO protocol.
                The protocol stack is

                                    ATM/AAL-5 + LLC + IPv4

                I think can be either possible to carry an IPv4 packet
in VC-Mux encapsulation.
                Is my assumpion correct?
                I do not see any problem in carrying IPv4 in VC-Mux
encapsulation. The protocol stack
                will be:

                                    ATM/AAL-5 + IPv4

IPv6:        I suppose that IPv6 can be carried in both LLC (maybe as a
routed NON ISO
                protocol) and in VC_Mux encapsulation. The protocol
stacks could be:

                                    ATM/AAL-5 + LLC + IPv6    (LLC case)

                                    ATM/AAL-5 +  IPv4 (VC-Mux case)

MPLS:    In principle i think that a packet with a MPLS shim header can
               can be carried in both LLC (Routed NON ISO protocol?) and

               VC-Mux encapsulation.
                The protocols stacks can be:

                                ATM/AAL-5 + LLC + MPLS        (LLC case)

                                ATM/AAL-5 + MPLS             (VC-Mux
case)

                NOTE: in this case I am not sure if the first label of
the MPLS label
                stack MUST be copied into VPI/VCI bits, or if this is a
simple option:
                for example when the ATM network element is seen as an
ATM-LSR.
                Instead, in case of an ATM PVC between two LSR.  the
                ATM pipe is seen only as a transport layer and I think
that, so there is no need to
                copy the MPLS label into VPI/VCI.

Anyway, i want to ask you if my assumptions are correct and if in
general there
is a rule that says: this protocol must be encapsulated in LLC while
this other protocol
must be encapsulated as VC-Mux. Or if in general each protocol can be
encapsulated
both in LLC and VC-Mux encapsulation.

I thank you in advance for your reply.

Looking forward to hearing from you soon.

Regards

Michele Fontana

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---

From: "Albert Manfredi" <[email-masked]>
Date: Fri, 10 Dec 1999 11:38:41 -0500

Michele, the logic behind when VC multiplexing is okay and when LLC
encapsulation is mandatory is quite simple, really.

Consider the plight of a receiver. Is some transmitter somewhere
likely to set up a VC to that receiver and send unexpected packets
in some unknown format? Or is an existing PVC going to be used to
carry packets of multiple protocol types?

If the answer is no to both questions, e.g. if the _receiver_ always
signals to set up the VC for some particular reason (perhaps a
long-lived SVC to a server), or if a PVC is dedicated always to just
one protocol, then you don't need LLC encapsulation.

The quotes from RFC 2684 that imply this are:

------

5.  LLC Encapsulation

   LLC Encapsulation is needed when more than one
   protocol might be carried over the same VC.

6.  VC Multiplexing

   VC Multiplexing creates a binding between an ATM
   VC and the type of the network protocol carried
   on that VC.  Thus, there is no need for protocol
   identification information to be carried in the
   payload of each AAL5 CPCS-PDU.

------

This holds for any protocol.

Bert
[email-masked]

Michele Fontana wrote:

> (VC-Mux case)
>                                 ATM/AAL-5 + MPLS
>    (VC-Mux