User’s Manual of FGSW-2620VM / FGSW-2620PVM

4.9.3 TOS/DSCP

TOS/DSCP priority is obtained through a 6-bitType-of-Service (TOS) or Differentiated Service Code Point (DSCP) to 3-bit priority mapping.

The Type of Service (TOS) octet in the IPv4 header is divided into three parts; Precedence (3 bits), TOS (4 bits), and MBZ (1 bit). The Precedence bits indicate the importance of a packet, whereas the TOS bits indicate how the network should make tradeoffs between throughput, delay, reliability, and cost (as defined in RFC 1394). The MBZ bit (for “must be zero”) is currently unused and is either set to zero or just ignored.

0

 

1

 

2

3

4

 

5

6

7

 

Precedence

 

 

 

TOS

 

MBZ

 

 

 

 

 

 

 

 

 

 

 

Pv4 Packet Header Type of Service Octet

The four TOS bits provide 15 different priority values, however only five values have a defined meaning.

DiffServ Code Point (DSCP) － is the traffic prioritization bits within an IP header that are encoded by certain applications and/or devices to indicate the level of service required by the packet across a network. DSCP are defined in RFC2597 for classifying traffic into different service classes. The Managed Switch extracts the codepoint value of the DS field from IPv4 packets and identifies the priority of the incoming IP packets based on the configured priority.

 

 

 

 

4 bit

 

4 bit

6 bit

 

 

2 bit

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

VER=0100

Header Size

DiffServ

 

 

RES

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Destination

Source

VLAN

 

Ethernet Type

 

 

 

 

 

 

 

 

 

 

Preamble

TAG

 

 

 

 

Data

 

FCS

 

 

Address

Address

 

(0800)

 

 

 

 

 

 

 

(Optional)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

6 bytes

6 bytes

4 bytes

 

2 bytes

2 bytes

46-1517 bytes

4 bytes

 

 

 

Figure 4-9-4:IPv4 frame format

 

 

 

 

 

 

 

 

 

The DSCP is six bits wide, allowing coding for up to 64 different forwarding behaviors. The DSCP retains backward compatibility with the three precedence bits so that non-DSCP compliant, TOS-enabled devices, will not conflict with the DSCP mapping. Based on network policies, different kinds of traffic can be marked for different kinds of forwarding.

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