The Need for QoS on the LAN 691
For an instant, the buffer of the uplink port can become full, potentially causing packets to drop. In a typical TCP/IP data-networking environment, this is not a concern because the packet is retransmitted. In an environment comprised of real-time applications, such as IP telephony and video conferencing, instantaneous buffer overruns (overflows) can affect the quality of the voice or video streams.
In a Cisco IP telephony environment, a G.729 digital signal processor (DSP) can rebuild up to 30 ms of lost voice. If the Cisco standard 20 ms per packet has been deployed, a single packet can be dropped without degrading voice quality; if two consecutive voice packets are lost, resulting in 40 ms of lost voice conversation, however, the algorithm cannot compensate, and a clip is heard in the conversation. In the event that the Real Time Protocol (RTP) stream carries a fax or modem conversation, a single packet results in a modem retrain, whereas two consecutive packets result in a dropped connection.
By classifying the real-time applications on your LAN and scheduling the desired level of priority for each real-time application, you can avoid these problems; however, you cannot remedy the problem by adding bandwidth. QoS tools are required to manage these buffers to minimize loss, delay, and jitter. You must properly enable and configure QoS to set priority by matching a traffic flow with a desired queue or threshold.
Bandwidth is not a substitute for LAN QoS! LAN QoS is a buffer management issue.
Marking and Classification
As discussed in Chapter 3, “Classification and Marking,” marking at Layer 2 takes place in the 3-bit User Priority field called the Class of Service (CoS), which resides inside an Ethernet header. The CoS field only exists inside Ethernet frames when trunking (either 802.1Q or InterSwitch Link [ISL]) is used. The field can be used to set eight different binary values, which the classification features of other QoS tools can use.
Figure 10-2 shows the general location of the CoS field inside the 802.1q and ISL headers.
As discussed in Chapter 3, Layer 3 marking takes place in the Type of Service (ToS) or Differentiated Services (DS) field in the IP Header. The IP Precedence and Differentiated Services Code Point (DSCP) fields can be marked with any valid binary value of either 3 or 6 bits, respectively. Chapter 2, “QoS Tools and Architectures,” contains detailed discussion of the recommended values used in these two fields. Figure 10-3 outlines the two fields and their positions inside an IP header.
692 Chapter 10: LAN QoS
Figure 10-2 Class of Service Fields
ISL User Field (1 byte)
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ISL Header |
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User |
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Priority |
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802.1q Tag Field (2 bytes) |
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Figure 10-3 IP Precedence and IP DSCP Fields
Precedence TOS CU
IP Header, |
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TOS Byte |
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Byte 1 |
DS Field* |
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DSCP CU
