Understanding DSCP PHB AF DF

When configuring the ISIS-TE met a pile of mpls dste command, or understand the good DSTE, read a afternoon no clue, this article writes the relationship of DSCP-PHB is quite clear, let's take a look ~~

DSCP (Differentiated Services CodePoint) distinguishes service encoding points

In the IPv4 packet header, DSCP is the first six bits in the TOS domain.

DSCP is used for QoS in DiffServ-based networks. Its function is to select PHB (Per-Hop Behavior), and PHB ensures the QoS (latency, bandwidth, etc.) of IP packets ).

Each DSCP value has one-to-one or multiple-to-one relationship with PHB.

In a DiffServ network, services are classified and traffic is controlled at the network entrance according to service requirements. At the same time, IP packets are configured in the DSCP domain. During route forwarding,

Vro selects PHB Based on the DSCP value (different PHB determines resource allocation, queue scheduling, and Group discard policies), and provides different QoS for IP packets based on the selected PHB.

DiffServ provides class-based QoS with good scalability, but this service model lacks an effective end-to-end QoS control mechanism.

Af phb (Assured Forwarding PHB) ensures Forwarding

Af phb ensures that the packet loss rate is low when the service traffic does not exceed the threshold.

Af phb corresponds to four AF classes

Class 1 Class 2 Class 3 Class 4
+ ------------- + ------------ +
Low Drop Prec | 001010 | 010010 | 011010 | 100010 |
Medium Drop Prec | 001100 | 010100 | 011100 | 100100 |
High Drop Prec | 001110 | 010110 | 011110 | 100110 |
+ ------------ + ------------- +

The default PHB encoding 000000 corresponds to Best-effort traffic.

Explanation of PHB encoding: Taking AF11 (0010100) as an example, the first three bits indicate the IP packet priority, the next two bits indicate the packet loss priority, and the last bit is always 0.
AF11 = 001-01-0

The higher the level of the AF Class, the higher the priority of the corresponding IP packet, and the higher the QoS quality.

Class 1 Class 2 Class 3 Class 4
+ ---------- +
Low Drop Prec | 10 | 18 | 26 | 34 | (decimal)
Medium Drop Prec | 12 | 20 | 28 | 36 |
High Drop Prec | 14 | 22 | 30 | 38 |
+ ---------- +

Ef phb (expedited forwarding) Acceleration forwarding

The DSCP corresponding to ef phb is:, 101-11-0, 46

Ef phb can provide applications with low packet loss rate, low latency, and guaranteed bandwidth. It provides higher QoS than af phb.