The Class Selector PHB and DSCP Values

Classs Selector And Dscp Values

RFC 2475, which defines DiffServ, became an RFC in December 1998. Even today, some QoS features in IOS do not support DiffServ Some QoS features will never support DiffServ, because newer, better tools that can do the same thing may have been introduced. All tools that support Cisco's strategic direction for QoS configuration, using the Modular QoS command-line interface (MQC), support DSCP. However, depending on the tools you need to use, and the IOS revisions you use in your network, you may...

TCP and UDP Reactions to Packet Loss

UDP and TCP behave very differently when packets are lost. UDP, by itself, does not react to packet loss, because UDP does not include any mechanism with which to know whether a packet was lost. TCP senders, however, slow down the rate at which they send after recognizing that a packet was lost. Unlike UDP, TCP includes a field in the TCP header to number each TCP segment (sequence number), and another field used by the receiver to confirm receipt of the packets (acknowledgment number). When a...

DeJitter Buffer Delay

De-jitter buffer delay is the third voice delay component. Jitter happens in data networks. You can control it, and minimize it for jitter-sensitive traffic, but you cannot eliminate it. Buy why talk about jitter in the section on delay Because a key tool in defeating the effects of jitter, the de-jitter buffer (sometimes called the jitter buffer) actually increases delay. The de-jitter buffer collects voice packets and delays playing out the voice to the listener, to have several ms of voice...

Tail Drop Global Synchronization and TCP Starvation

Global Synchronization Graph Example

Tail drop occurs when a packet needs to be added to a queue, but the queue is full. Yes, tail drop is indeed that simple. However, tail drop results in some interesting behavior in real networks, particularly when most traffic is TCP based, but with some UDP traffic. Of course, the Internet today delivers mostly TCP traffic, because web traffic uses HTTP, and HTTP uses TCP. The preceding section described the behavior of a single TCP connection after a single packet loss. Now imagine an...

Committed Access Rate CAR Configuration

CAR has more similarities than differences when compared to CB policing. Both perform policing on all traffic on either an interface or subinterface. Both can classify traffic to police a subset of traffic as well. Both use the same units when configuring policing parameters bits per second for the policing rate, bytes for the normal and Be values, with the configured Be value actually representing Bc + Be. CAR differs from CB policing regarding four main features. The most obvious is that CAR...