Re: questions

[Emmanuel Duros <Emmanuel.Duros@sophia.inria.fr>]

newbie wrote:
> 
> Hi:

Hi -mll,

> I have some questions on
> "draft-ietf-udlr-lltunnel-02.txt".
> 
> 2) can anyone here explain scenario 1 to 4?

The IESG members asked us to make some clarification in the document
(draft-ietf-udlr-lltunnel-03.txt should come out very soon). We have made slight
modifications to section 5, it might be clearer now ?:

5. Emulating a broadcast bidirectional network

   The simplest to use solution is to emulate a broadcast capable link
   layer network. This will allow the immediate deployment of existing
   higher level protocols without change. Though other network
   structures, such as NBMA, could also be emulated, a broadcast network
   is more generally useful. Though a layer 3 network could be emulated,
   a link layer network allows the immediate use of any other network
   layer protocols, and most particularly allows the immediate use of
   ARP.

   A link layer (LL) tunneling mechanism which emulates bidirectional
   connectivity in the presence of a unidirectional link will be
   described in the next section. We first consider the various
   communication scenarios which characterize a broadcast network in
   order to define what functionalities the link layer tunneling
   mechanism has to perform in order to emulate a bidirectional
   broadcast link.

   Here we enumerate the scenarios which would be feasible on a
   broadcast network, i.e if feeds and receivers were connected by a
   bidirectional broadcast link:

   Scenario 1: A receiver can send a packet to a feed (point-to-point
     communication between a receiver and a feed).

   Scenario 2: A receiver can send a broadcast/multicast packet on the
     link to all nodes (point-to-multipoint).

   Scenario 3: A receiver can send a packet to another receiver (point-
     to-point communication between two receivers).

   Scenario 4: A feed can send a packet to a send-only feed (point-to-
     point communication between two feeds).

   Scenario 5: A feed can send a broadcast/multicast packet on the
     unidirectional link to all nodes (point-to-multipoint).

   Scenario 6: A feed can send a packet to receiver or a receive capable
     feed. This is the default communication over a unidirectional link.

   These scenarios are possible on a broadcast network. Scenario 6 is
   already feasible on the unidirectional link. The link layer tunneling
   mechanism should therefore provide the functionality to support
   scenarios 1 to 5.

   Note that regular IP forwarding over such an emulated network (i.e.
   using the emulated network as a transit network) works correctly; the
   next hop address at the receiver will be the unidirectional link
   address of another router (a feed or a receiver) which will then
   relay the packet.



> 2) In section 6:
> 
> A datagram is delivered from the network layer to the
> link layer of the unidirectional interface (see Figure
> 2). It is then encapsulated behind a MAC header
> corresponding to the unidirectional link. This packet
> cannot be sent over the link and is then processed by
> the tunneling mechanism.
> 
> The packet is encapsulated behind an IP header whose
> destination is the IP address of a feed bidirectional
> interface (f1b or f2b), also called the tunnel
> end-point.
> 
> f1b or f2b, according to the figure 1, was the
> interface(s) connecting to bidirectional internet. But
> the text in section 6 seems to say that the tunnel
> goes through the bidirectional internet, instead of
> forming a virtual symmetric link between f1u and x1u.
                                                   ^^^
                                                    ?
Humm, I am not sure I understand what you are explaining. In other words section
6.1 says that: if the IP layer of receiver delivers a packet to the link layer
of the unidirectional interface, then the MAC packet is encapsulated behind a IP
header (with some encapsulation scheme) and sent to a feed via the
"bidirectional network". 

-> A (connectionless) tunnel is set between a receiver and a feed, encapsulated
packets go from a receiver to a feed only. There is no "virtual symmetric link".

Hope this helps !

Emmanuel

> TIA
> 
> -mll
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