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Connection Oriented Networks - Perros H.G

Perros H.G Connection Oriented Networks - John Wiley & Sons, 2005. - 359 p.
ISBN 0-470-02163-2
Download (direct link): connectionorientednetworks2005.pdf
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In Figure 6.6, we show the labels allocated by the LSRs. These labels are similar to the VPI/VCI values in ATM. They have local significance; that is, each label is valid only for
Figure 6.6 Label switched paths.
one link. The sequence of labels 62, 15, 60 form a path known as the label switched path (LSP). This path is analogous to a point-to-point ATM connection, which is defined by a sequence of VPI/VCI values. An ATM connection is associated with two end devices, whereas a label switched path is associated with a FEC. Several label switched paths are typically associated with the same FEC, forming a tree diagram (see Figure 6.6). Each LSP has an ingress LSR and an egress LSR. For instance, in Figure 6.6, LSRs A and E are the ingress and egress LSRs, respectively, for the LSP from the LSR A to LSR E. Likewise, LSRs C and E are ingress and egress LSRs for the LSP from LSR C to LSR E.
Label switching eliminates the CPU-intensive table look-up in the FIB, necessary to determine the next hop router of an IP packet. A table look-up in the LFIB is not as time-consuming since an LFIB is considerably smaller than a FIB. Since the introduction of label switching, however, several CPU-efficient algorithms for carrying out table lookups in the FIB were developed. This did not diminish the importance of label switching since it was seen as a means of introducing QoS in the IP network.
One way that QoS can be introduced in the network is to associate each IP packet with a priority. This priority can be carried in the 3-bit experimental field of the label encapsulation (see Figure 6.3). Priorities can be assigned by an MPLS edge node. Labeled IP packets within an LSR are served according to their priority as in the case of an ATM switch. Recall that, in ATM networks, each VC connection is associated with a QoS category. An ATM switch can determine the QoS of an incoming cell from its VPI/VCI value, and accordingly it can queue the cell into the appropriate QoS queue. An ATM switch maintains different QoS queues for each output interface. These queues are served using a scheduling algorithm, so that VC connections can be served according to their requested QoS. A similar queueing structure can now be introduced in an IP router. IP packets can now be queued at an output interface according to their priority, and they can be transmitted out in an order determined by a scheduler.
6.2.1 Label Allocation Schemes
In the label switching example described above, an LSR binds (i.e., allocates) a label to a FEC and saves this information in its LFIB as the incoming label. It then advertises the binding between the incoming label and the FEC to its neighboring LSRs. An upstream LSR (i.e., an LSR that is upstream of the link as the traffic flows) places the label in the outgoing label field of the entry in its LFIB that is associated with this FEC. A nonupstream LSR can either ignore the label advertisement or store it for future use. Because the LSR, which is downstream of the link with respect to traffic flow, creates the label, and because the label is advertised to its neighbors in an unsolicited manner, the label allocation scheme is known as the unsolicited downstream scheme.
As an example, let us consider LSR B in Figure 6.6. LSR B advertises its incoming label 65 for the FEC (x.0.0.0,y.0.0.0) to its neighboring LSRs A, C, and D. Of these, only LSR A is upstream to LSR B as far as the flow of IP packets towards the destination (x.0.0.0,y.0.0.0) is concerned. In view of this, LSR A will use this label to update its LFIB. LSRs C and D can elect to store this label binding, in case they do become upstream to LSR B as far as the flow of these IP packets is concerned. This can happen if a link or an LSR goes down. For instance, if the link between LSRs C and D is broken, LSR C might have to reroute its traffic through LSR B; in which case, it will become upstream to B. (Given the topology in Figure 6.6, LSR D will never become upstream to LSR B). LSRs C and D can also elect to ignore LSR Bís advertised label binding.
A non-upstream LSR will store or ignore a label binding depending on whether the conservative label retention mode or the liberal retention mode is used. In the conservative retention mode, a label is retained only if the LSR is upstream of the LSR advertising the label binding. In the liberal label retention mode, all labels are retained irrespective of whether the LSR is upstream or not of the LSR advertising the label binding.
MPLS can also use the downstream on demand label allocation. In this case, each LSR binds an incoming label to a FEC and creates an appropriate entry in its LFIB. However, it does not advertise its label binding to its neighbors as in the unsolicited downstream allocation scheme. Instead, an upstream LSR obtains the label information by issuing a request.
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