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參數(shù)資料
| 型號： | RTL8305SB |
| 英文描述： | RTL8305SB |
| 中文描述： | RTL8305SB |
| 文件頁數(shù)： | 46/66頁 |
| 文件大小： | 963K |
| 代理商： | RTL8305SB |
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RTL8305SB
2002/04/09
46
Rev.1.0
7.3.8 QoS Function
The RTL8305SB can recognize the QoS priority information of the incoming packets to give a different egress service priority.
The RTL8305SB identifies the packets as high priority based on several types of QoS priority information that based on: 1) Port
based priority; 2) 802.1p/Q VLAN priority tag; 3) TCP/IP's TOS/DiffServ (DS) priority field. These types of QoS are selected by
hardware pins DisPortPri[4:0], DisTagPri and DisDSPri respectively upon reset or internal registers via SMI after reset.
There are 2 priority queues, a high-priority queue and a low-priority queue, supported by the RTL8305SB to buffer high and
low priority frames. The queue service rate is based on the Weighted Round Robin algorithm, the packet based service weight
ratio of high-priority queue and low-priority queue can be set as 4:1, 8:1, 16:1 or "Always high priority first" by hardware pins
QWeight[1:0] upon reset or internal register via SMI after reset.
When Port based priority is applied, any packets received from the high priority port, which is set by DisPortPri[4:0], will be
sent to the high priority queue of the destination port.
When 802.1p VLAN tag priority applied, the RTL8305SB can recognize the 802.1Q VLAN tag frames and extract the 3-bit
User_Priority information from the VLAN tag. The RTL8305SB sets the threshold of User_Priority as 3. Therefore, VLAN
tagged frames with User_Priority value = 4~7 will be treat as high priority frames, other User_Priority values (0~3) as low
priority frames (follows 802.1p standard).
When TCP/IP's TOS/DiffServ(DS) based priority is applied, the RTL8305SB can recognize TCP/IP Differentiated Services
Codepoint (DSCP) priority information from the DS-field defined on RFC2474. The DS field byte for the IPv4 is
Type-of-Service (TOS) octet and for IPv6 is Traffic-Class octet. The recommended DiffServ Codepoint is defined in RFC2597
to classify the traffic into different service classes. The RTL8305SB can extract the codepoint value of DS-field from IPv4 and
IPv6 packets, and identify the priority of the incoming IP packet follows the definition as bellow:
High priority: whose DS-field = (EF ,expected forwarding:) 101110;
(AF, Assured Forwarding:) 001010; 010010; 011010; 100010
(Network Control:) 110000 and 111000.
Low priority: whose DS-field = others values.
The VLAN tagged frame and 6-bit DS-field in IPv4 and IPv6 frame format are shown bellow:
6 bytes
DA
6 byte
SA
2 byte
81-00
3 bits
User-Priority
( 0~3:Low-pri ; 4~7: High-pri )
----
Table 3: 802.1Q VLAN tag frame format
6 bytes
DA
6 byte
SA
4 byte
802.1Q Tag
(optional)
2 byte
08-00
4 bits
Version
IPv4= 0100
4 bit
IHL
6 bits
TOS[0:5]
= DS-field
----
6 bytes
DA
6 byte
SA
4 byte
802.1Q Tag
(optional)
Table 4: IPv4/6 frame format
Note: IPv6 refer to rcf2460
2 byte
08-00
4 bits
Version
IPv6= 0110
6 bits
Traffic Class [0:5]
=DS-field
----
The RTL8305SB can automatically turn off 802.3x flow control and Back pressure flow control for 1~2 sec whenever the port
receives VLAN-tag or TOS/DS high priority frames, the flow control is re-enabled when no priority frame is received during a
1~2 sec duration. The hardware pin DisFCAutoOff upon reset or internal register via SMI after reset can enable this auto-turn
off function.
7.3.9 Insert/Remove VLAN Priority Tag
When QoS function is enabled
, a tag can be inserted or removed on a per output port basis. The RTL8305SB will insert
VLAN priority-tag (VID=0x000) for untagged frames only, or remove a tag for all types tagged frames. Port 4 can use two
strapping pins upon reset or internal registers via SMI after reset to set Insert/Remove function. Other ports can use a serial
EEPROM upon reset or internal registers via SMI after reset to set this function. For example: When P0IRTag[1:0]=10, the
RTL8305SB will insert priority tag for untagged frame from Output High Queue (user priority field=0b111, CFI=0,
VID=0x000) of Port0 and Low Queue of Port0 (user priority field=0b000, CFI=0, VID=0x000). When P0IRTag[1:0]=01, the
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