Home > Articles > Networking > Routing & Switching

This chapter is from the book

Why Use Label Switching?

Let's look at the reasons label switching is of such keen interest in the industry. We examine the topics of (a) speed and delay, (b) scalability, (c) simplicity, (d) resource consumption, and (e) route control.

Speed and Delay

Traditional IP-based forwarding is too slow to handle the large traffic loads in the Internet or an internet, a topic explained in Appendix A. Even with enhanced techniques, such as a fast-table lookup for certain datagrams, the load on the router is often more than the router can handle. The result may be lost traffic, lost connections, and overall poor performance in an IP-based network.

Label switching, in contrast to IP forwarding, is proving to be an effective solution to the problem. Label switching is much faster because the label value that is placed in an incoming packet header is used to access the forwarding table at the router; that is, the label is used to index into the table. This lookup requires only one access to the table, in contrast to a traditional routing table access that might require several thousand lookups.

The result of this more efficient operation is that the user's traffic in the packet is sent though the network much more quickly than with the traditional IP forwarding operation, reducing the delay and response time to enact a transaction between users.

Jitter.    For computer networks, speed and its nemesis, delay, have another component. It is the variability of the delay of the user traffic, caused by the packet traversing several to many nodes in the network to reach its destination. It is also the accumulation of this variable delay as the packet makes its way from the sender to the receiver. At each node, the destination address in the packet must be examined and compared to a long list of potential destination addresses in the node's (usually a router) routing table.1

As the packet traverses through these nodes, it encounters both delay and variable delay, depending on how long it takes for the table lookup and, of course, on the number of packets that must be processed in a given period. The end result, say, at the receiving node, is jitter, an accumulation of the variable delays encountered at each node between the sender and the receiver.

This situation is onerous to speech packets because it often translates into uneven speech play-out to the person listening to the speech. It may even result in a person's having to wait a few seconds to receive the final words of a sentence as the speech packets make their way through the network.

Once again, the more efficient label switching operation results in the user's traffic being sent through the network much more quickly and with less jitter than with the traditional IP routing operation.

Scalability

Certainly, speed is an important aspect of label switching, and processing the user traffic quickly in an internet is very important. But fast service is not all that label switching provides. It can also provide scalability. Scalability refers to the ability or inability of a system, in this case the Internet, to accommodate a large and growing number of Internet users. Thousands of new users (and supporting nodes, such as routers and servers) are signing on to the Internet each day. Imagine the task of a router if it has to keep track of all these users. Label switching offers solutions to this rapid growth and large networks by allowing a large number of IP addresses to be associated with one or a few labels. This approach reduces further the size of address (actually label) tables and enables a router to support more users.

Simplicity

Another attractive aspect of label switching is that it is basically a forwarding protocol (or set of protocols, as we shall see). It is elegantly simple: forward a packet based on its label. How that label is ascer-tained is quite another matter; that is, how the control mechanisms are implemented to correlate the label to a specific user's traffic is irrelevant to the actual forwarding of the traffic. These control mechanisms are somewhat complex, but they do not affect the efficiency of the user traffic flow.

Why is this concept important? It means that a variety of methods can be employed to establish a label binding (an association) to the user's traffic. But after this binding has been accomplished, then label switching operations to forward the traffic are simple. Label switching operations can be implemented in software, in application-specific integrated circuits (ASICs), or in specialized processors.

Resource Consumption

The control mechanisms to set up the label must not be a burden to the network. They should not consume a lot of resources. If they do, then their benefits are largely negated. Fortunately, label switching networks do not need a lot of the network's resources to execute the control mechanisms to establish label switching paths for users' traffic (if they do consume a lot of resources, they are not designed well). We spend quite a lot of time in this book examining the control mechanisms to support label switching.

Route Control (Control of the Forwarding Path)

With some exceptions, routing in internets is performed with the use of the IP destination address (or in a LAN, with the destination MAC address). Certainly, many products are available that use other information, such as the IP type of service (TOS) field and port numbers, as part of the forwarding decision. But destination-routing (the destination IP address) is the prevalent forwarding method.

Destination-routing is not always an efficient operation. To see why, consider Figure 1–1. Router 1 receives traffic from routers 2 and 3. If the IP destination address in the arriving IP datagram is for an address found at router 6, the routing table at router 1 directs the router to forward traffic to either router 4 or 5. With some exceptions, no other factor is involved.2

Figure 1-1Figure 1–1 Destination-based routing.

Label switching permits the routes through an internet to be subject to better control. For example, a labeled packet emanating from router 2 may be destined for an address at router 6; likewise, the same situation could hold for a labeled packet starting at router 3. However, the packets' different label values can instruct router 1 to send one labeled packet to router 4 and a packet with a different label value to router 5.

This concept provides a tool to engineer the nodes and links to accommodate traffic more efficiently, as well as to give certain classes of traffic (based on QOS needs) different levels of service. Perhaps the links between routers 1 and 4 and routers between 1 and 5 are DS3 and SONET, respectively. If the user application needs more bandwidth, the user's label can be used to instruct the router to place the traffic on the SONET link instead of the DS3 link. This policy-based approach uses label switching to tailor the network to the needs of the traffic classes, a concept called traffic engineering (TE).

Route Control Using IP

As noted, it is possible to use the TOS field and the precedence bits in this field to ameliorate some of the problems associated with destination routing. However, the precedence bits are used in some networks and not used in others. Although standards define the use (the old RFC 791), a router may leave the bits untouched or may alter them. Likewise, a router may examine them or may ignore them. Notwithstanding these comments, routers can be configured to use the precedence bits. Shortly, the use of these bits is described.

For this discussion, it is a good idea to reset these bits for all traffic entering your network from an unknown network. By resetting the precedence values, you guarantee that users who have set these bits to get special treatment (better service) do not receive this service at the expense of your internal network customers.

Policy-Based Routing with IP.    Continuing the discussion on route control, policy-based routing (PBR) is often associated with label switching protocols, such as Frame Relay, ATM, or MPLS. It can also be implemented with IP by using the TOS field as well as port numbers, the IP protocol ID, or the size of the packets.

Using these fields allows the network provider to classify different types of traffic, preferably at the edge of the network; that is, the ingress to the network. Then the core routers can use the precedence bits to decide how to handle the incoming traffic. This "handling" can entail using different queues and different queuing methods.

IP policy-based routing also allows the network manager to execute a form of constraint-based routing, an operation explained in considerable detail in this book. Based on whether the packet meets or does not meet the criteria just discussed, policies can be executed that enable a router to do the following:

  • Set the precedence value in the IP header.

  • Set the next hop to route the packet.

  • Set the output interface for the packet.

We should note that some of the literature in the industry states that IP, without label switching, is not capable of policy-based and constraint-based routing. These claims are not accurate. The problem with the operations described in this section is not that they do not work; indeed they do. But the fact remains that the public Internet consists of many networks and many Internet Service Providers (ISPs), and there is no agreement among these parties on how to use the IP precedence bits.

The same situation potentially holds for label switching. Like IP precedence operations, label switching is only as effective as the agreements among network operators on how it is used.

IP Source Routing.    Another way to control the route in IP-based networks is to use the options field in the IP datagram header to support source routing. The sender of the datagram (the source) places a list of IP addresses in the options field, and these addresses are used (one after the other in the list) to specify the route of the datagram. The problem with IP source routing is that (a) it requires the sender to know the route in the first place (which in many networks is not feasible), (b) the list of 32-bit addresses can translate into considerable overhead, and (c) many networks do not support the source routing option.

Nonetheless, IP source routing can be useful in private internets, so it should not be ignored. It comes in two flavors. The first, called loose source routing, gives the IP nodes the option of using other intermediate hops to reach the addresses obtained in the source list as long as the datagram traverses the nodes listed. Conversely, strict source routing requires that the datagram travel only through the hops whose addresses are indicated in the source list. If the strict source route cannot be followed, the originating host IP is notified with an ICMP error message.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020