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Which statement correctly describes DSCP in the IP header, used in QoS control?

  1. AIt is a 6-bit value indicating how a packet should be treated (its class), and equipment along the path uses it to decide treatment such as putting voice packets in a queue that is sent with priority
  2. BIt is a value giving the maximum number of routers a packet may pass through, decreased by 1 at each router
  3. CIt is a value for checking the header for errors, which each router calculates and compares when it receives the packet
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Answer: A. It is a 6-bit value indicating how a packet should be treated (its class), and equipment along the path uses it to decide treatment such as putting voice packets in a queue that is sent with priority

DSCP uses the upper 6 bits of the ToS field in the IPv4 header (the Traffic Class field in IPv6) to indicate which class of traffic a packet belongs to. Equipment at the edge of the network sets the value according to the type of traffic (marking), and routers and switches along the path vary their choice of queue and the order in which packets are dropped according to the value. For example, voice, which is sensitive to delay and its variation, is often given EF (value 46), which is forwarded with priority. QoS does not add bandwidth; it decides which traffic to favor, and which to delay or drop, when the network is congested.

  • A. Correct. Equipment along the path decides treatment such as priority from the DSCP value.
  • B. This describes the TTL (the hop limit in IPv6).
  • C. This describes the IPv4 header checksum.
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