insert barcode in excel 2016 Figure 15-8: Tag-switching architecture in Software

Create Data Matrix ECC200 in Software Figure 15-8: Tag-switching architecture

Figure 15-8: Tag-switching architecture
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Tag switching is lightning fast and largely independent of network protocols Tag routers can be implemented totally in software, avoiding the hassle and expense of a hardware upgrade Therefore, an ATM switch in an Internet backbone topology can participate in tag switching by swapping a tag label for the virtual circuit identifier (VCI) in a cell This results in an ATM switch that supports both conventional ATM and tag switching Tag switching enables QoS within a network by differentiating traffic by class Packets carrying an inelastic application such as VoIP can be assigned tags in one range, packets that are part of an elastic application such as FTP are in another range, and so on When congestion occurs, the network can be configured to drop elastic packets or cells first, and depend on TCP to handle error recovery on a best-effort basis
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Multiprotocol Label Switching (MPLS)
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The IETF is developing Multiprotocol Label Switching (MPLS) as a protocol-independent packetswitching standard In theory, at least, MPLS will be able to work with any media over which layer-3 packets can pass MPLS supports fast label swapping similar to that in tag switching, applying the labels on entry into an MPLS-enabled network, and stripping them off at the point of exit The use of labels means that MPLS need not dissect variable-length headers to obtain routing information making the technology similar to ATM in that devices work with a predictable bit stream when processing forwarding information MPLS builds on early tag-switching technology by providing additional QoS attributes Specifically, the MPLS label not only tells routers where to forward a packet, it also tells them how to send it It does this by storing QoS service class information (priority, service class, and so on) in the forwarding tables to which the label binds in order to receive its next-hop instruction But MPLS is actually more of a traffic engineering protocol than a QoS protocol, because MPLS routing establishes a fixed-bandwidth pipe analogous to the virtual circuits (VCs) of ATM or Frame Relay Perhaps the biggest advantage of MPLS is its scalability The notion of operating committed service levels for multiple classes in an any-to-any network such as the Internet opens the threat of an uncontrollable number of virtual circuits being spawned, along with all their attendant overhead MPLS curtails this by using IP routing information to map a packet's forwarding information ahead of time, and placing it in a table where the label can instruct routers and switches along the entire path In other words, MPLS promises to deliver the benefits of VCs without the overhead required to set up and operate them Thus MPLS potentially may combine the benefits of IP and ATM within a single service For this reason, ISPs always hungry to move high volumes of traffic as fast as possible are particularly interested in seeing MPLS emerge as a viable technology MPLS isn't application controlled (it has no API); and it is "multi-protocol" in that it doesn't depend on any network protocol, not even IP MPLS can work with IP, IPX, PPP, ATM, Frame Relay, and other protocols It can do this because it exists only within routers, its primary mission only to determine which router should be the next hop MPLS uses a layer of abstraction to simplify the routing process As shown in Figure 15-9, an MPLS-enabled router (called a label-switching router, or LSR) uses routing protocol intelligence to compute an MPLS label The LSR attaches the label to the packet The label contains a Forwarding Equivalence Class (FEC) value, a signifier that can be indexed to any other LSR's table to repeat the process Rather than being forwarded based on a routing table, the packet is moved through the MPLS network by repeated application of the FEC in the MPLS label In this sense, the packet is switched rather than routed, a faster and more resource-efficient process The simplifying layer of abstraction is that MPLS just looks at the FEC to compute a next hop, instead of maintaining a routing table that accounts for the entire topology
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Figure 15-9: An MPLS-enabled router The packet is relabeled with a new FEC at each stop along the label-switched path (LSP) The big difference with MPLS is that routing and QoS policy are packaged within the label, or rather in the FEC value encoded into the label The label's 32-bit format carries the FEC as its payload The FEC's "class" never changes per se; it's only adjusted slightly at every stop to indicate the next hop
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MPLS uses a TTL (time to live) in much the same way that an IP packet does When an MPLS label's TTL is decremented to zero, however, the label isn't necessarily discarded as an IP packet would be For a label, the action to take at TTL-zero depends on the FEC value, which (depending on the configuration of the MPLS network) may call for the packet to take a hop back to a predesignated router for redirection As you can see, the MPLS infrastructure is brutally simple, and savvy in the way it computes the initial FEC by using intelligence generated by routing protocols into routing tables The complex part of MPLS involves the distribution and management of labels among MPLS routers For these tasks, the Label Distribution Protocol (LDP) is being developed Other protocols are being proposed, as well, including BGP and RSVP
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As discussed in the first two chapters of this book, technologies are taking form that must be in place to enable QoS in the real world The Lightweight Directory Access Protocol (LDAP) must be pervasively implemented to support the free exchange of information between directories within the Internet This process is well underway, with Microsoft's implementation of LDAP in Active Directory with Windows 2000 This is also true of directory products from Novell, Netscape, and other makers of network software platforms By letting the various directory technologies exchange information freely, LDAP effectively transforms widely distributed and otherwise incompatible directories into a central data store holding valuable information on virtually all network resources In addition to LDAP, QoS needs Directory-Enabled Networking (DEN) DEN itself is not a directory technology or an access protocol Its role is to serve as a data model for designers to extend their directory repositories to finally also include information on network infrastructure network hardware devices, links, media, network protocols, and the like Integrating infrastructure data with extant directory repositories on applications and users will at long last support more-informed
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