In: Categories » Computers and technology » Networks » Multiprotocol Label Switching and Asynchronous Transfer Mode Architecture
Asynchronous Transfer Mode is a connection-oriented protocol that the ITU-T developed. It is connection-oriented because virtual circuits are signaled that carry the Asynchronous Transfer Mode traffic. The Asynchronous Transfer Mode traffic consists of fixedsized cells of 53 bytes. Of those 53 bytes, 5 are the cell header and 48 are the cell data. The success of Asynchronous Transfer Mode was predominantly in the WAN network. Many vendors built Asynchronous Transfer Mode switches that could set up virtual circuits in the WAN network. The advantages of Asynchronous Transfer Mode are the following:
■ A fixed packet size, resulting in a transmission with low jitter
■ Guaranteed quality of service (QoS)
■ Great flexibility
The success of Asynchronous Transfer Mode was limited to its use in the WAN network. As IP became the de facto standard networking protocol that almost everyone used, much effort was spent on getting IP traffic across the Asynchronous Transfer Mode core network. Several schemes were devised:
■ Encapsulation according to RFC 1483
■ Lane Emulation (LANE)
■ Multiprotocol over Asynchronous Transfer Mode (MPOA)
RFC 1483 (made obsolete by RFC 2684) specified how to encapsulate multiple routed and bridged protocols over Asynchronous Transfer Mode adaptation layer (AAL) 5. LANE specified how to carry Ethernet frames across the Asynchronous Transfer Mode cloud. MPOA provided a tight integration of IP over Asynchronous Transfer Mode, but it was a complex solution. None of these solutions was perfect in providing a better fit between IP and Asynchronous Transfer Mode. One of the driving reasons for Multiprotocol Label Switching was just that: a better integration between IP and Asynchronous Transfer Mode. With Multiprotocol Label Switching, the Asynchronous Transfer Mode switches would need to run an IP routing protocol and a label distribution protocol to exchange IP prefixes and labels between themselves and to the routers.
The result would be that the overlay model of IP over Asynchronous Transfer Mode would no longer be needed. With Multiprotocol Label Switching, it became a peer model. The GFC field provides local functions for the Asynchronous Transfer Mode cell. Local means that it is not end to end, and the intermediate switches override the field. Local functions might mean flow control and identification of multiple stations on a single Asynchronous Transfer Mode interface. The VPI and VCI fields are used together and identify the next destination of the Asynchronous Transfer Mode cell.
The three bits of the PT field are defined as follows:
■ The first bit indicates whether the cell contains user data or control data.
■ The second bit indicates whether congestion is present.
■ The third bit indicates whether the cell is the last cell of an AAL5 frame (PDU).
Asynchronous Transfer Mode can have statically defined PVCs, or private network-network interface (PNNI) can assign the virtual circuits dynamically. PNNI is a hierarchical link-state routing protocol that lays out the virtual circuits throughout the Asynchronous Transfer Mode network. For the cells to be interpreted correctly and used by upper layer protocols, ITU-T specified a layer between the Asynchronous Transfer Mode layer and the upper layer protocols. This layer is called AAL, and it has five categories. AAL1 is connection-oriented and used for delay-sensitive services and circuit emulation. AAL2 is also connection-oriented, but it is used for variable rate services. AAL3/4 is connectionless and used mainly for the older SMDS. AAL5 can be connection-oriented or connectionless and is used for varying bit rate demands. It is used mostly for IP and LANE. To carry IP traffic across the Asynchronous Transfer Mode cloud, the routers on the edge of the Asynchronous Transfer Mode WAN cloud are interconnected across Asynchronous Transfer Mode PVCs. To connect the routers in the most efficient way, you need to connect them directly to each other across PVCs. This is needed so that the IP traffic does not cross the Asynchronous Transfer Mode cloud twice.
Therefore, the routers need to be interconnected in a fully meshed way. This is called the overlay model because all the routers have an Interior Gateway Protocol (IGP) adjacency (a peering) with each other across the Asynchronous Transfer Mode cloud. For the traffic to be forwarded correctly through the Asynchronous Transfer Mode Label Switch Routers, the traffic must be Multiprotocol Label Switching encapsulated, and the Multiprotocol Label Switching label value must be mapped to VPI/VCI values. That is because the Asynchronous Transfer Mode switches are still switching Asynchronous Transfer Mode cells on virtual circuits. Because the Asynchronous Transfer Mode switches need to be able to map the Multiprotocol Label Switching label value to a VC, they must first learn those label values. Hence, the Asynchronous Transfer Mode switches must run a label distribution protocol. An Asynchronous Transfer Mode Label Switch Router consists of the following:
■ A routing protocol in the control plane
■ A label distribution protocol in the control plane
■ Switching Asynchronous Transfer Mode cells in the data plane
The Cisco Asynchronous Transfer Mode switches support Open Shortest Path First (OSPF) as the routing protocol and Label Distribution Protocol as the label distribution protocol. The Cisco Asynchronous Transfer Mode Label Switch Routers distribute the routes in OSPF and the label bindings associated with the routes with Label Distribution Protocol. The incoming and outgoing labels are mapped to incoming and outgoing VPI/VCI pairs. The result is that in the data plane, the Asynchronous Transfer Mode switch just needs to switch cells from the incoming virtual circuit to the outgoing virtual circuit, just like regular Asynchronous Transfer Mode forwarding. The Asynchronous Transfer Mode switch never forwards IP packets. If this is needed, the Asynchronous Transfer Mode switch would need to reassemble all incoming Asynchronous Transfer Mode cells into frames first. Every Asynchronous Transfer Mode switch along the path would need to do this. This is undesirable for performance reasons.
Label Encoding
Asynchronous Transfer Mode switches that are running Multiprotocol Label Switching are still switching Asynchronous Transfer Mode cells. As such, they cannot forward labeled frames. Because the Multiprotocol Label Switching labels are mapped to VCs in the Asynchronous Transfer Mode cloud, the Multiprotocol Label Switching label value is mapped to the VPI/VCI pair. If the labeled packet has a label stack with more than one label, only the value of the top label is mapped to the VPI/VCI fields. When the edge Asynchronous Transfer Mode Label Switch Router receives a frame, the frame is chopped up into cells. Only the top label value is encoded as a VPI/VCI value. The rest of the labels in the label stack are not needed to forward the cells. Nevertheless, the complete label stack is present in the frame (now chopped up).
These labels will be needed again when the Asynchronous Transfer Mode cells are reassembled into a frame and the frame needs further Multiprotocol Label Switching forwarding outside the Asynchronous Transfer Mode network. The label value of the top label is encoded in the VPI/VCI field and changes at every Asynchronous Transfer Mode Label Switch Router, and the label value of the top label in the label stack is set to 0. The label is kept, however, for the three other fields: TTL, EXP, and End-of-Stack bit. The TTL sets the outgoing TTL when the packet is reassembled on the egress edge Asynchronous Transfer Mode Label Switch Router. The EXP bits set the QoS of the packet on the egress edge Asynchronous Transfer Mode Label Switch Router. Even if the label stack consists of only one label, it is still carried across the Asynchronous Transfer Mode cloud in the first cell. This enables the egress Asynchronous Transfer Mode Label Switch Router to figure out whether the packet actually had a label stack or not. Because the VCI value is 16 bits, there can be 216 or 65,536 labels. Considering that the number of VCs is limited on the Asynchronous Transfer Mode switch, this value alone should be enough for all the labels needed on one interface. The VPI value is 12 bits, so there can be 212 or 4096 labels there.
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