QoS terms defined
 
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Flow
A conversation between a single-source address and a single destination using a unique set of Layer 3 addresses and Layer 4 ports.
Type-of-service (ToS) byte
An eight-bit field in the IP header. IP Precedence, Differentiated Services Code Point and ToS field all use this byte.
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Differentiated Services Code Point (DSCP)
A different use of the ToS byte. Six bits of this byte are being reallocated for use as the DSCP field, where each DSCP specifies a particular per-hop behavior that is applied to a packet. Support for DSCP still is lacking in some network equipment, however (see RFC 2474). DSCP is not compatible with IP Precedence.
IP Precedence field
A three-bit field in the ToS byte of the IP header (see RFC 791). Using IP Precedence, a network administrator can assign values from 0 (the default) to 7 to classify and prioritize types of traffic. IP Precedence is being phased out in favor of DSCP, but is supported by many applications and routers.
ToS field
Lets values from 0 to 15 be assigned to request special handling of traffic (for example, minimize delay, maximize throughput). The ToS field is being phased out in favor of DSCP (see RFC 1349).
Multi-protocol Label Switching (MPLS)
A standard for applying "labels" to traffic for the purposes of routing and special handling. Among other things, MPLS can be used to differentiate and prioritize network traffic. Mostly used in WAN and service provider environments (see RFC 3031).
802.1p
A three-bit value that can be placed inside an 802.1Q frame tag. It serves much the same purpose as IP Precedence, but is done at Layer 2, so it is protocol-independent. Usually converted to IP Precedence or DSCP when the packet reaches the first router.
Packet drop
When a queue reaches its maximum length, packet drops can occur. When a packet drop occurs, connection-based protocols such as TCP slow down their transmission rates in an attempt to let queued packets be serviced, thereby letting the queue empty. This is also known as tail drop because packets at the end of the queue prevent other packets from entering the queue.
Bandwidth carving
Dedicating bandwidth to a specific application or set of applications. Because bandwidth is dedicated, other applications cannot use it, even when there is little congestion on the connection. This usually is done when other QoS methods, such as weighted fair queuing, are not effective in providing the desired handling of priority traffic.
TCP rate shaping
A technique that modifies the TCP window size to control the rate at which hosts will transmit TCP-based traffic. When the host believes that the TCP window is full, it will pause its transmissions.
Priority queuing
Priority queuing supports some number of queues, usually from high to low. Queues are serviced in strict order of queue priority, so the high queue always is serviced first, then the next-lower priority and so on.
If a lower-priority queue is being serviced and a packet enters a higher queue, that queue is serviced immediately. This mechanism is good for important traffic, but can lead to queue starvation.
Weighted fair queuing (WFQ)
WFQ is a flow-based queuing algorithm that schedules low-volume traffic first, while letting high-volume traffic share the remaining bandwidth. This is handled by assigning a weight to each flow, where lower weights are the first to be serviced.
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