Home > Articles > Certification > Cisco Certification > CCNP

  • Print
  • + Share This
This chapter is from the book

Compression Techniques

Various types of compression algorithms are in use in the world today.

For compression, a scope needs to be set ahead of time. There are compression methods for data (entire files), links (data that travels between routers), hard drives (data stored on a hard drive), and so on. This section of the chapter focuses on compression across WAN links (links).

It is also true that too much compression is a bad thing. If data is already compressed when WAN links begin to process it, the ability of the router to further compress that data is affected. Data that is already compressed can actually become larger by recompressing it. The discussion in this chapter focuses on what happens at the WAN interface, regardless of the type of data being transported.

Compression is only one technique for squeezing every possible bit of bandwidth from an existing internetwork deployment. Compression, like queuing, is meant to provide critical time to plan and deploy network upgrades and to reduce overall utilization of a WAN link. However, nothing is free. The execution of the compression algorithm adds a significant number of CPU cycles. Unfortunately, the additional load on the CPU might not be something it can handle.

With compression enabled, CPU utilization of the router increases considerably. On the bright side, the WAN link utilization drops considerably. Thus, compression is a trade-off, because all that has been accomplished is the displacement of utilization from the WAN to the router. Obviously, the effects of compression vary based on the algorithm implemented.

As technology advances, compression will move from a software function to a hardware function. This is already a reality in some router models, with the addition of newly available modules specifically geared toward performing data compression in hardware. Not only is this much faster than software compression, it is less costly for the CPU (generally).

The effects of compression must be taken into account prior to any implementation. If the routers are already running at 80 percent or more CPU utilization (show cpu process command), it is not a good idea to implement compression. Doing so can result in the router literally running out of CPU to do any further processing.

Data compression makes efficient use of bandwidth and increases WAN throughput by reducing the size of the frame being transported. Compression is best utilized on slower WAN links. There comes a point when the router can send the data faster if it is uncompressed than if the router compresses the data, sends it, and then decompresses the data at the other end.

Cisco supports a number of compression types:

  • Link

  • Payload

  • TCP Header

  • Microsoft Point-to-Point

Microsoft Point-to-Point compression, which is an algorithm, is beyond the scope of this book and thus is not discussed further.

Figure 15-8 illustrates where various types of compression make transmission more efficient.

Figure 8Figure 15-8 Compression Methods

Link Compression

Link compression (also known as per-interface compression) compresses the entire frame (Layer 2 through Layer 7) as it leaves the interface. In other words, both the header and the data are compressed. Because the entire frame is compressed, it must be uncompressed immediately upon receipt at the other end of the link so that the header information can be used to further forward the frame. Thus, a link-compressed frame is only compressed for the trip across one link.

Link compression is not dependent on any particular protocol function. Every frame that leaves the interface that is configured for compression is reduced in size—no questions asked, no exceptions to the rule. Cisco supports two algorithms on its router chassis to compress traffic: Stac and Predictor. For HDLC links, Stac is the only available choice.

Because the header and data are unusable after being compressed, link compression should be used only for data transmission over point-to-point dedicated connections.

It is also important to remember that if compression is used on one side of a link, compression must also be enabled on the other side, and the same compression algorithm must be used at both ends. Compression can be compared to a language. If English is spoken on one end, then English must be present on the other end to decipher the communication.

Stac

Stac is based on an algorithm known as Lempel-Ziv (LZ), which searches the data stream for redundant strings and replaces them with a token. The token is an information pointer that is significantly shorter than the string it replaces. If LZ cannot find any duplicated strings in the data, no compression occurs and transmission occurs as if the link had no compression activated. Stac is an open-standard compression algorithm and is used by many different vendors.

There are cases, such as the sending of encrypted data or data that has already been compressed, in which compression actually expands the size of a transmission. In such cases, the original transmission is sent untouched. The Stac compression algorithm tends to be quite CPU intensive and should not be implemented on routers with an already high CPU utilization. Stac might also be a poor selection on smaller routers that do not have much CPU to begin with.

Predictor

The Predictor compression method is rightly named. This Cisco-proprietary algorithm attempts to predict the incoming character sequences by implementing an indexing system that is based on a compression dictionary. It is essentially a code book that is based on possible data sequences. If a character string is found that matches an entry in the dictionary, the string is replaced with the dictionary entry. That entry is a much shorter sequence of characters. At the remote end, the incoming characters are compared to the data dictionary once again to be decoded. The incoming dictionary strings are replaced with the appropriate (and original) information.

The Predictor compression method is like sign language. Rather than spelling out each individual word (no compression), a single hand motion utilizes an entire word or concept (compression). Because both parties understand the hand motions, successful communication occurs. Conversely, when one of the people involved in the communication does not understand sign language, communication does not occur.

Predictor-like algorithms are used in some voice-compression standards. For example, G.729 and G.729a (CSA-CELP) implementations compress a 64-kbps voice stream into an 8-kbps data stream. These implementations are directly based on the code book/dictionary prediction methodology. One big difference between Predictor (link compression) and voice-compression algorithms (payload compression) is that the voice-compression routines compress only the voice data (payload), not the entire frame.

Remember that Stac is CPU intensive. Predictor, on the other hand, tends to be extremely memory intensive. If the router has not been outfitted with a good amount of RAM, Predictor should not be implemented. However, if RAM is plentiful, Predictor is a compression consideration that can be beneficial.

Payload Compression

Payload compression is exactly what its name implies. Also known as per-VC compression, payload compression compresses only the data portion of the transmission. All L2 headers are left intact.

It cannot be assumed that customer WAN links are all dedicated point-to-point connections (PPP or HDLC). For such circuits, link compression can be used because the provider's WAN switches do not examine any portion of the data being transmitted.

However, payload compression is needed if the WAN switches must examine the data sent from a customer location. WAN technologies such as Frame Relay require that the L2 header information be untouched so that the provider's switches can read it and make forwarding decisions based on it. Any implementation of VCs disallows link compression. In these cases, payload compression is appropriate.

TCP Header Compression

RFC 1144 defines the Van Jacobson algorithm. In doing so, it also defines the algorithm for TCP/IP header compression. The 20-byte IP header and the 20-byte TCP header combination (a total of 40 bytes) is compressed to 2 or 4 (typically 4) bytes to reduce overhead across the network. The L2 header remains intact so that it can be utilized by the appropriate L2 transport.

This type of compression is most beneficial when used with implementations that transmit small packets, such as Voice over IP, Telnet, and so forth. This type of compression can be done on just about any WAN implementation (X.25, Frame Relay, ISDN, and so on).

TCP header compression, as suggested in the name, compresses only the TCP and IP headers. If the data payload is 1000 bytes, then the total package (excluding the L2 frame) would be 1000 + 40 (data + TCP + IP) bytes. TCP header compression would bring this down to 1000 + 2 bytes. However, if the payload is 20 bytes, then 20 + 40 becomes 20 + 2 (a big improvement). Table 15-4 summarizes the benefits. This table does not consider the L2 headers.

Table 15-4 TCP Header Compression

Data

TCP Header

IP Header

Total Without Compression

% Overhead

Total With Compression

% Overhead

1000

20

20

1040

3.8%

1002

.2%

20

20

20

60

66.7%

22

9.1%


Note that for packets with larger data portions, the compression provides noticeable improvement (3.8 percent overhead compared to .2 percent). However, for smaller data portions, the improvement is dramatic (66.7 percent overhead compared to 9.1 percent).

As with other forms of compression, TCP header compression must be configured on both ends of the link to ensure a connection. Because the L2 headers are not compressed, TCP header compression can be used on any serial interface, and across WAN clouds that must be able to read the L2 headers during transit.

If any form of compression is used in a Cisco router, the exact same form of compression must be implemented on the other end of the link. Failure to apply the same compression algorithm on each causes all data to fail across the link.

Compression Issues

Compression is not a feature that is simply turned on or off. When selecting the algorithm that is to be utilized for a particular deployment, you should consider the following:

  • Modem compression—Some modems implement compression. Modems that use MNP5 and V.42bis are not compatible. Although each offers 2 and 4 times compression, they cannot communicate with each other. If you use modem compression, make sure that the modems at both ends of the connection are using a common protocol. If compression is being performed by the modem, do not attempt to configure compression at the router level.

***no bullet***

If modem compression is successfully enabled, then data compression (from the router for example) should not be enabled. Remember that compressing a compressed string might increase the size. Conversely, if compression is performed on the router, then the modem should not attempt any further compression.

  • Data encryption—Encryption occurs at the network layer where compression is an L2 function. The main purpose of encryption is security. Encryption removes common patterns in data streams. In other words, when Stac tries to find redundant strings, there are none. When Predictor looks into the dictionary for common patterns, there are none. Therefore, the compression is unsuccessful and can actually expand the traffic it was attempting to compress. In such a case, the traffic is sent uncompressed.

  • CPU and memory—Some algorithms are memory intensive and some are CPU intensive. Thus, before you plan or implement compression, you must know the physical configuration of your router (that is, its RAM and CPU) before ordering additional hardware.

Configuring Compression

To configure compression, there are several commands. Most are technology-specific and fairly intuitive. The compress configuration command is used at the interface level (normally a slow serial interface) to select the link-compression algorithm. Remember to configure the same compression type on both ends of the point-to-point link.

Router(config-if)# compress [predictor | stac | mppc]

For Frame Relay connections, use the frame-relay payload-compress interface-level configuration command to enable Stac compression on an interface or a subinterface (payload compression). There are no additional configuration parameters for use with this command, as shown by the following command structure:

Router(config-if)# frame-relay payload-compress

To enable TCP header compression for a given interface, use the ip tcp header-compression command. The command structure is as follows:

Router(config-if)# ip tcp header-compression [passive]

The passive keyword at the end of the command specifies that compression be performed only if packets received on that interface are compressed on arrival.

  • + Share This
  • 🔖 Save To Your Account

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