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According to the DiffServ architecture RFC-2475, which two major components are necessary to deliver end-to-end QoS? (Choose two.)
A. Layer 2 transport QoS mechanism such as FRF.12
B. packet marking with 6 bits to classify packets
C. ToS or IP precedence with 3 bits
D. PHB
E. RSVP
Correct Answer: BD
Explanation/Reference:
Explanation:
In order to deliver end-to-end QoS, this architecture (RFC-2475) has two major components-packet marking using the IPv4 ToS byte and PHBs. Unlike the IPprecedence solution, the ToS byte is completely redefined [Figure3]. Six bits are now used to classify packets. The field is now called the Differentiated Services (DS) field, with two of the bits unused. Reference: https://www.cisco.com/en/US/technologies/tk543/tk766/technologies_white_paper09186a00800a3e2f.html
DSCP uses the six bits in the IPv4 header to specify class of service for each IP packet
(IETF RFC 2474). The 6 leftmost bits from the ToS byte in an IPv4 header form the
DiffServ (DS) field. With 6 bits, DSCP has up to 64 DSCP values (0 to 63) that are
assigned to various classes of traffic. The Internet Engineering Task Force (IETF) recommends
selective DSCP values to maintain relative levels of priority. These selective values
are called per-hop behaviors (PHB) and determine how packets are treated at each hop
along the path from the source to the destination.