Services
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Network Time Protocol (NTP)
RFC 958 introduced network time protocol. Network time protocol can be utilised to synchronise a set of clocks on network devices on a client to server model. Synchronising time is important on network devices. Time is used for many different features on network devices such as managing time specific passwords, key exchanges, checking the validity…
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Congestion Management: Congestion Avoidance
Congestion avoidance monitors network traffic loads to anticipate and avoid congestion by dropping packets. The default method of dropping packets is tail drop. When the output queue is full, all packets trying to enter the queue are dropped regardless of their priority or classification. Tail drop can be drastic for TCP streams, and can cause…
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Congestion Management: Queuing Algorithms
There are many different types of queuing algorithms, though not all may be suitable in modern networks that can carry many different media types. FIFO – First In, First Out The first packet to be placed on an output interface is the first packet that will leave that interface. All traffic belongs to the same…
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Quality of Service: Congestion Management
Congestion management involves queuing, buffering and scheduling traffic. To queue traffic is to temporary store it on the network device whilst waiting for an outbound interface to clear itself of congestion. Congestion can occur for two reasons, the input interface being faster than the output interface, or the output interface is receiving packets from multiple…
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Policing and Shaping: Types of Policers
Single-Rate Two-Colour Markers and Policers The first policers implemented use a single rate two colour model based on a single token bucket algorithm. For this type of policer, traffic can be either conforming to the committed information rate or exceeding it. Marking down traffic or dropping it can be performed to either of these two…
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Policing and Shaping: Single Token Bucket Algorithm Example
An example of how a single token bucket algorithm works: There is an interface of 1Gbps, with a policer defined with a committed information rate of 120Mbps and committed burst size of 12Mbps. A committed time interval can not be configured in Cisco IOS but can be calculated. Committed time interval = ( Committed Burst…
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Policers and Shapers: Token Bucket Algorithms
Policers and shapers are based on token bucket algorithms in Cisco IOS. Definitions Committed Information Rate The policed traffic rate, in bits per second. Committed Time Interval The time interval in seconds that a committed burst is sent. It can be calculated with the formula Committed Time Interval = (Burst Size / Committed Information Rate)…
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Policing Traffic: Markdown
On a policed interface, when traffic levels are at set are exceeded, a policer can take one of two actions: dropping the traffic, or marking it down with a lower priority. Marking down the traffic involves changing the packets to a lower-priority class value. Excessed traffic marked with AF11 will be marked down to AF12…
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Quality of Service: Where to Place Shapers and Policers in the Network
Policers for incoming network traffic are ideally placed at the edge of the network to prevent bandwidth being wasted throughout the core of the network. Policers for outgoing network traffic are again optimally deployed at the edge of the network or core-facing interfaces on network edge devices. There is a downside to policing that it…
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Quality of Service: Policing and Shaping
Traffic policers and shapers are mechanisms of quality of service that classify and condition traffic. Policers will drop or re-mark incoming or outgoing traffic that goes beyond a desired rate. Shapers buffer and delay egress traffic rates that peak above a desired rate until the egress traffic drops below that desired rate. If it below…