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Let us now contrast the IP procedure for routing IP packets with the scheme used in ATM networks to switch ATM packets (commonly known as ATM cells). As will be seen in Chapter 3, an ATM cell has a fixed size of 53 bytes. Of those, 5 bytes are used for the header and the remaining 48 for the payload. For a user to transmit traffic to a destination user over an ATM network, user A first has to request the establishment of a connection, as shown in the example in Figure 1.3. User A sends a SETUP message to ATM switch 1 (to which it is directly connected). The switch calculates a path to the destination ATM user, and then decides whether the path has enough free capacity to accept this new connection. If it does, then the switch forwards the SETUP message to the next switch on the path (switch 2), which in turn has to decide whether to accept the connection, based on how much free capacity it has. If it decides that it can accept the new connection, it forwards the SETUP message to the next switch on the path (switch
3), which forwards the SETUP request to user B. The connection is established when user B returns a CONNECT message, which is propagated all the way back to user A.
The decision as to whether a switch can accept a new connection is crucial to the efficient operation of the network. Each ATM switch tracks all of the connections carried through its switch fabric, the amount of traffic transmitted over each connection, and the quality of service (QoS) requested by each connection. The decision to accept a new connection comes down to whether the prospective traffic can be switched according
IP Router 1 IP Router 2
Figure 1.2 Routing IP packets.
EXAMPLES OF CONNECTIONS
ATM switch 1
ATM switch 2
ATM switch 3
Figure 1.3 Successful establishment of an ATM connection.
to the requested QoS, without affecting the QoS of other existing connections. When a connection is accepted, the switch allocates bandwidth on the outgoing link for the connection. It stops accepting new connections when it runs out of bandwidth, or when it reaches a certain percentage of utilization.
The user starts transmitting ATM cells once it receives the CONNECT message. The ATM cells carry two fields in the header - the virtual path identifier (VPI) and the virtual connection identifier (VCI) - which are used to identify the connection. The ATM switch uses the combined VPI/VCI value to pass a cell through its switch fabric. Specifically, as in the case of an IP router, an ATM switch maintains a table that specifies the next hop for each VPI/VCI value. When a cell arrives at a switch, the virtual path and virtual connection identifiers check the table for the next ATM switch. The cell is then switched through the switch fabric to the output port that connects to the next ATM switch. The ATM table is considerably smaller than an IP forwarding routing table, since it only contains the existing ATM connections, rather than an entire set of IP addresses.
When user A completes its transmission to B, it tears down the connection by sending a RELEASE message to ATM switch 1. This message is propagated through the switches along the path, and each switch releases the bandwidth it had allocated to the connection.
As we can see, transmitting packets through the IP network is a lot simpler than transmitting cells through an ATM network, since it is not necessary to establish a connection first. On the other hand, by establishing a connection in an ATM network, the network can provide QoS guarantees that are not possible in an IP network.
1.2.2 An MPLS Connection
MPLS introduces a connection-oriented structure into the otherwise connectionless IP network. An MPLS-ready IP router does not forward IP packets based on the destination address in the header. Rather, it forwards them based on a label that is very similar in functionality to the VPI/VCI value carried in the header of an ATM cell.
Let us consider an MPLS-enabled IP network that runs over Ethernet. In this case, a special MPLS header, sandwiched between the IP header and the LLC header, is used. The MPLS header contains a label that is a short, fixed-length connection identifier. The MPLS-ready IP router, known as a label switched router (LSR), maintains a table of labels. When an IP packet arrives at the LSR, the label carried in the MPLS header is cross-referenced to the table of labels to find the next hop. The IP packet is then switched
to the destination output port of the LSR that connects to the next hop LSR. The table contains labels for only the existing connections, and therefore it is not as large as the forwarding routing table in an IP router.
The procedure is similar to ATM. In order for a user to transmit over an MPLS-enabled IP network, it has to first request the establishment of a connection. This is done using a signaling protocol, such CR-LDP or RSVP-TE. The connection is known in MPLS as a label switched path (LSP). As in the case of ATM, an LSR is aware of all of the connections that pass through its switch fabric; therefore, it can decide whether to accept a new connection or not based on the amount of traffic that will be transmitted and the requested QoS. The LSR allocates a portion of its bandwidth to a new connection, and it stops accepting new connections when it either runs out of bandwidth or reaches a certain percentage of utilization.