QoS Techniques: MPLS
CET 2486C – Network Technologies
November 27, 2012
MPLS or Multi Protocol Label Switching is a networking technology that functions between layers 2 and 3 of the OSI model. MPLS constitutes of adding a label (sometimes called “Shim” because of their placement between layer 3 and layer 2 headers.) to the data package, this label contains special addressing and sometimes prioritization information. Because the MPLS label contains all the information necessary for the router to forward the package to the next hop, the router does not have to spend time analyzing the entire package thus improving network latency or bottlenecks. Due to its multi protocol capabilities MPLS can be integrated with different networking technologies from ATM to native IP environments; in addition, this multi protocol capability also provides a way to converge different types of traffic such as data, voice and video onto one network. MPLS technology also provides some other advantageous features like Traffic Engineering (TE), VPN, Any Transport over MPLS (AToM) and Quality of Service (QoS). This paper will help provide an understanding of how MPLS works and the QoS capabilities it can provide.
History of MPLS
In 1996 a group from Ipsilon Networks introduced a “flow management protocol”, this technology only worked with ATM transmissions and did not become very popular in the market. Not long after Cisco Systems proposed a similar technology called “Tag Switching” that was not restricted to ATM transmissions; at first this technology was proprietary to Cisco. Cisco renamed the technology to “Label Switching” and gave it to the Internet Engineering Task Force (IETF) for open standardization. The IETF worked on the technology and in 1999 MPLS was introduced to the world. Introduction to MPLS
Multi Protocol Label Switching (MPLS) is a networking technology that functions between layer 2 (Data Link Layer) and layer 3 (Network Layer) of the OSI model. In some instances MPLS is said to operate at “Layer 2.5”, this is mostly because the “Shim” or MPLS label that is inserted by routers fall between the layer 2 and layer 3 headers of the packet. The MPLS header “Shim” contains one or more labels, these are known as label stack, each of these labels are four bytes in size and contain four fields. Figure 1 – MPLS label structure
Figure 1, above provides a graphical example on how an MPLS label is structure. The “Label” field contains the label value and is 20 bits in size. The Exp field stands for “Experimental Use” is 3 bits in size and it is used to assign the Class of Service (CoS), priority and ECN (Explicit Congestion Notification) for QoS purposes. The S field is 1 bit in size, if this is set it signifies that the current label is the last in a stack. The last field is the TTL it determines the time to live of the packet; this field is 8 bits in size. In simple terms MPLS works by adding an identifier (a label or “Shim”) to the packets as soon as it enters the MPLS network thru a Label Edge Router (LER). LERs are the most advanced routers in an MPLS network due to the fact that they are responsible for the ingress and egress of an MPLS network, the LERs add a label to incoming packets and remove a label of outgoing packets as they enter and exit the MPLS enabled network. The packets now with labels allow the Label Switch Routers (LSR) to determine when and with what degree of priority packets are sent. Before continuing it is important to understand that in an MPLS network LSRs and LERs regularly send each other label and reachability information using a Label Distribution Protocol (LDP) in order to build a complete datagram of the network, that along with the Label Forwarding Information base (LFIB) which is a type of routing table that MPLS routers use provide faster lookup and addressing. Once the packet is received by the LSR the label is...
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