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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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6.1.4 IP Version 6 (IPv6)
Due to the rapid growth of the Internet, it was felt that the address space of the current IP will soon be inadequate to cope with the demand for new IP addresses. This consideration coupled with the need to provide new mechanisms for delivering real-time traffic, such as audio and video, led to the development of a new IP, known as IPv6.
IPv6 retains many of the basic concepts from IPv4. The new features are 128-bit addresses, new header format, extension headers, support for audio and video, and extensible protocol.
MPLS is an IETF standard based on Cisco’s tag switching. The original intention was to be used in conjunction with different networking protocols, such as IPv4, IPv6, IPX and AppleTalk. However, MPLS has been developed exclusively for IP networks, which makes the name of the protocol more general than it is in reality.
In order to understand the basic concept behind MPLS, we need to take a look at how an IP router works. An IP router implements both control and forwarding components. The control component consists of routing protocols, such as the open shortest path
first (OSPF), the border gateway protocol (BGP), and the protocol independent multicast (PIM), used to construct routes and exchange routing information among IP routers. This information is used by the IP routers to construct the forwarding routing table, referred to as the forwarding information base (FIB). The forwarding component consists of procedures that a router uses to make a forwarding decision on an IP packet. For instance, in unicast forwarding, the router uses the destination IP address to find an entry in the FIB, using the longest match algorithm. The result of this table look-up is an interface number, which is the output port connecting the router to the next hop router, to which the IP packet should be sent.
A router forwards an IP packet according to its prefix. In a given router, the set of all addresses that have the same prefix, is referred to as the forwarding equivalent class (FEC, pronounced as fek). IP packets belonging to the same FEC have the same output interface. In MPLS, each FEC is associated with a different label. This label is used to determine the output interface of an IP packet without having to look-up its address in the FIB. A label is a short fixed-length identifier that has local significance. That is, it is valid on a single hop interconnecting two routers. A label is similar in functionality to the VPI/VCI value associated with an ATM cell.
In IPv6, the label can be carried in the flow label field. In IPv4, however, there is no space for such a label in the IP header. If the IP network runs on top of an ATM network, then the label is carried in the VPI/VCI field of an ATM cell. If it is running over frame relay, the label is carried in the DLCI field. For Ethernet, token ring, and point-to-point connections that run a link layer protocol (e.g. PPP), the label is encapsulated and inserted between the LLC header and the IP header (see Figure 6.3). (Note that in tag switching, the encapsulated label was referred to as a shim header.) The first field of the label encapsulation is a 20-bit field used to carry the label. The second field is a 3-bit field used for experimental purposes. It can for instance carry a class-of-service (CoS) indication, which can be used to determine the order in which IP packets will be transmitted out of an interface. The S field is used in conjunction with the label stack, which will be discussed in detail later on in this chapter. Finally, the time-to-live (TTL) field is similar to the TTL field in the IP header.
An MPLS network consists of label switching routers (LSR) and MPLS nodes. An LSR is an IP router that runs the MPLS protocol. It can bind labels to FECs, forward IP packets based on their labels, and carry the customary IP forwarding decision by carrying out a table look-up in the FIB using a prefix. An MPLS node is an LSR, except that it does not necessarily have the capability to forward IP packets based on prefixes.
header encapsulation header header
Label Exp S TTL
(20 bits) (3 bits) (1 bit) (6 bits)
Figure 6.3 Label encapsulation.
Figure 6.4 MPLS domains, LSRs, and MPLS nodes.
Figure 6.5 An example of multi-protocol label switching.
A contiguous set of MPLS nodes that are in the same routing or administrative domain forms an MPLS domain. Within an MPLS domain, IP packets are switched using their MPLS label. An MPLS domain can be connected to a node outside the domain, which might belong to an MPLS or a non-MPLS IP domain (that is, an IP domain where the routers use the customary forwarding decision based on prefixes). As shown in Figure 6.4, the MPLS domain B consists of five routers, two of which are LSRs (LSR 1 and LSR 2); the remaining three might be either LSRs or MPLS nodes. MPLS domain B is connected to the MPLS domain A via LSR 1, and is connected to the non-MPLS IP domain C via LSR 2. LSRs 1 and 2 are referred to as MPLS edge nodes. For simplicity, we will assume that all nodes within an MPLS domain are LSRs.
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