Label Allocation Schemes THE MULTI-PROTOCOL LABEL SWITCHING MPLS ARCHITECTURE

146 THE MULTI-PROTOCOL LABEL SWITCHING MPLS ARCHITECTURE the higher-level peer groups. When a calling user wishes to establish a connection to a destination user, it sends a SETUP message to its ingress ATM switch using the Q.2931 signaling protocol. The ingress ATM switch calculates a path through the ATM network, and then using the PNNI protocol it forwards the SETUP message to the next switch along the path, which forwards it to the next ATM switch on the path, and so on, until the SETUP message reaches the egress ATM switch which serves the called user. The egress switch forwards the SETUP message to the called user using the Q.2931 signaling protocol, and if the called user accepts it, a confirmation is returned back to the calling user. At that time, the calling user can start transmitting data to the called user. In MPLS over ATM, this entire signaling functionality is removed from the ATM switches. Instead, each ATM switch is identified by an IP address and runs IP routing protocols. Such an ATM switch is referred to as an ATM-LSR. As in an IP router, using IP routing protocols an ATM-LSR can learn about its neighbors and about the topology of its IP domain, and it can calculate the next hop ATM-LSR for each IP destination. In MPLS over ATM, an LSP is nothing else but an ATM connection which is set up using MPLS. The MPLS label is carried in the VPIVCI field of the cell. If a label stack is used, then only two labels can be carried. The top label is carried in the VPI field and the bottom one in the VCI field. The advertising of label bindings is done using downstream allocation on demand. That is, when an ATM-LSR identifies a new FEC it allocates a label, but it does not advertise it to its neighbors. An upstream ATM-LSR obtains the label binding by sending a request. A predefined ATM VC connection is used for exchanging label binding information. Let us consider now a network of ATM-LSRs. An IP packet at the ingress ATM-LSR, is first encapsulated into a CPS-PDU using AAL 5. Then, it is segmented into an integer number of 48-byte blocks and each block is carried in a different ATM cell. The label associated with the particular LSP is carried in the VPIVCI field of the cell. When an ATM cell reaches the next hop ATM-LSR, its label is replaced by the outgoing label and the cell is switched to the appropriate output from where it is transmitted out to the next ATM-LSR. This continues until the cell reaches the egress ATM-LSR. There, the cell is assembled with other cells into the original AAL 5 CSC-PDU, and the IP packet is recovered from its payload and is delivered to the IP protocol. Therefore, an IP packet traverses the network of ATM-LSRs in a sequence of ATM cells that are switched through each ATM-LSR without ever having to reconstruct the original IP packet at each intermediary ATM-LSR, except at the egress ATM-LSR. This is very similar to the IP switching scheme.

6.3.1 VC Merging

An interesting problem that arises in label switching over ATM is VC merging. This problem arises when two ATM-LSRs are both connected to the same downstream ATM- LSR. Let us consider the four ATM-LSRs A, B, C, and D in Figure 6.9, and let us assume that the flow of IP packets for a specific FEC is from A to C and then to D, and from B to C and then to D. Assume that D is the edge LSR that serves the hosts associated with the FEC. The allocated labels are shown in Figure 6.9 in bold. Now let us see what happens when A has an IP packet call it packet 1 to transmit that belongs to this FEC. This packet will be encapsulated by AAL 5 and then it will be segmented into an integer number of 48-byte blocks. Each block will then be carried in the payload of an PROBLEMS 147 ATM-LSR A ATM-LSR B ATM-LSR C ATM-LSR D 20 15 15 Figure 6.9 VC merging. ATM cell labeled with the value 15. The cells will be transmitted to C, where they will have their label changed to 20 and then they will be forwarded to the buffer of the output port that connects to D. In this buffer, it is possible that these cells will get interleaved with the cells belonging to an IP packet, call it packet 2, associated with same FEC and transmitted from B. That is, as the cells are queued-up into the buffer, packet 1 cells can find themselves between two successive packet 2 cells. Since all of these cells will be sent to D with the label of 20, D will not be able to identify which of these cells belong to packet 1 or packet 2. Consequently, D will be not be able to reconstruct the original AAL 5 PDUs. A simple solution to this problem is to first collect all of the cells belonging to the same IP packet in the buffer of the output port of C. Once all of the cells have arrived, then they can be transmitted out back-to-back to D. To do this, the switch will need to be able to identify the beginning cell and last cell of an AAL 5 PDU. If the switch is set up with the early-packet and partial-packet discard policies, then the mechanism to do this might be in place. Otherwise, multiple labels can be used, so that the path from A to D is associated with a different set of labels than the path from B to D.

6.3.2 Hybrid ATM Switches

It was mentioned above that in MPLS over ATM, the entire ATM signaling functionality is removed from the ATM switches. Instead, each ATM switch runs the MPLS control plane. That is, the MPLS protocol, a label distribution protocol, and IP routing protocols. However, the existence of MPLS on an ATM switch does not preclude the switch from running the ATM signaling protocol. In fact, both schemes can coexist on the same ATM switch and on the same ATM interface. PROBLEMS 1. Make-up an arbitrary IP header and calculate its checksum. Introduce errors in the bit stream i.e. flip single bits so that the checksum when calculated by the receiver will fail to detect that the IP header has been received in error. 2. Consider the IP address: 152.1.213.156. a What class address is it? b What is the net and host address in binary?