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Shortcomings of MPLS


MPLS essentially attempts to overlay connection-oriented concepts onto connectionless technologies. While providing several advantages, in a number of instances this approach reduces the overall flexibility of the IP protocol. Some of the conclusions that led to the research into multilayer routing, such as that routers are too slow or routing tables becoming too large, have been weakened by the appearance of fast and powerful gigabit routers.

The MPLS framework and architecture define a base-level label swapping technology. As discussed earlier, MPLS allows for traffic to be switched under different circumstances (topology-driven, flow-driven etc.), using different LDPs depending on the circumstances. While this implies that MPLS is flexible, it is likely to be applicable only within well-managed networks, where all components are able to provide support for MPLS and the individual distribution protocols in use.


While the label stack concept provides benefits, the idea of having packets carry a number of labels is likely to increase overheads, certainly in terms of making the MPLS header larger.

With topology-driven label assignment (where labels are allocated and distributed without reference to the traffic), a full mesh of labels will be established. The overhead of this approach is large relative to the size of the network, and has the potential to use a vast number of labels. This can be a large overhead in instances when labels are allocated to routes where very little traffic is flowing.


The current MPLS architecture and framework specifications have left the topic of multicast as an area for further study.


In terms of the provision of varying levels of QoS, MPLS poses a number of issues.

Label assignment based on support for traffic flows will require a path to be put in place the moment the flow is detected, therefore implying that there will be some latency prior to a full path being in place. In this instance, the overhead will increase in relation to the number of flows being supported and the duration of the flows. Label assignment in order to support short flows implies a large overhead. When label distribution is included as part of a reservation protocol (e.g., RSVP), the overheads and scalability of such a protocol must also be considered.

The ordered and independent control of labeled paths (described earlier) are said to be compatible approaches to path setup. However, when they interoperate the overall behavior can only be described as independent because, to ensure QoS, ordered control must be used entirely from ingress to egress node.

LDPs must work in a reliable manner given that the loss of a control message in this instance could cause a delay in the establishment of a label path. This constitutes a serious impediment to the support of critical applications. As mentioned earlier, the use of TCP with a number of LDPs offers the necessary reliability. In the case of flow-based label assignment and the use of RSVP, reliable transmission of the LDP information is not guaranteed due to the use of UDP.

Data Link Layer

The ability of MPLS to support a number of link-layer technologies provides a high degree of flexibility. However, in terms of the provision of connections with a level of associated QoS, mechanisms are required to ensure that the QoS specified for an LSP is maintained by the underlying link layer. This may not be possible in some instances (e.g., with a standard Ethernet, DPT, etc) where firm guarantees cannot be made (because of the inherent nature of the technology. Where ATM technology is used with MPLS, in most instances the LDP acts as the ATM signaling protocol. This implies that a low-level control protocol is required which is able to configure connections with defined levels of QoS. While work is progressing in this area within the IETF GSMP Working Group, wide scale support for this type of protocol by major switch/router vendors is not yet evident.

Note that QoS on the LAN/MAN based on standard MAC protocols represents a major challenge, not so much during the predictable processes, but in sharing the connectionless transmission media with other users/routers in a predictable and quantifiable way. As soon as the critical traffic (e.g., voice/video) reaches the IP network, it must compete with electronic mail traffic, database applications, and file transfers.



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