QoS

*TCP（四层） VS IP DSCP （三层） 的 拥塞控制 和 拥塞避免 的 同义词

• Congestion Control(SS, SSthrosh, RTT timer, congestion avoidance, FastResend, FastRecovery) VS Soft Queue队列技术 (FIFO / Priority Queue / CBWFQ)

• Congestion Avoidence(linear increase) VS Dropping(Taildrop / WERD)

Class-Based Weighted Fair Queue (CBWFQ)

使用 hold-queue 指令可修改 queue 长度，比如 要修 改 Egress Hardware Queue 长度，设定如下：

R2(config)#int ethernet 1/0

R2(config-if)#hold-queue 1000 out

Soft Queue:

• priority

• bandwidth

• percentage

Weighted Random Early Detection (WRED)

利用show policy-map interface 查看

如要修改，可以 random-detect precedence (IPP/DSCP)

R2(config-pmap-c)#random-detect dscp af11(DSCP值) 25(Minimum Threshold) 30(Probability Denominator) 20(Maximum Threshold)

Minimum Threshold

代表当 Queue 长度达到多少便开始执行 Drop。

Probability Denominator

要 Drop 多少个 Packet 呢？如把 Probability Denominator 设定为 10，意思是把 Queue 中每 10个 Packet 随机选 1 个 Drop 掉。如此类推。

Maximum Threshold

到 Queue 过了 Maximum Threshold，就强制发动Tail Drop 了。

Explict Congestion Notification (ECN)

虽然 Drop Packet 已经可以让 TCP Flow 被动地减 速，但除了 Drop Packet 之外，还有一个方法可让 TCP 主动减速，就是 Explicit Congestion Notification (ECN)。Software Queue 可在 Queue 即将被填满之前告诉 TCP Flow 的 Receiver Congestion 即将发生， Receiver 在收到此讯号後可 在发 TCP ACK 时告知 Sender 减速 (透过调校 Windows Size)，前题是 TCP Flow 的两端 Host 都 ⽀缓 ECN。在介绍 Marking 的章节时说过 ToS 一共 有 8 Bits，DSCP 只用了前 6 Bits，最後两 Bits 就是 当作 ECN，分别是 ECN apable Transport (ECT) 和 Congestion Experienced (CE)。如 Sender 保持 ECN 则会把 ECT 设成 1，Queue 超过 Minimum Threshold 时，如 Software Queue 发现 ECT 是 1，则不 Drop Packet，反⽽会把 CE 设成 1 并传给 Receiver，藉此告知 Receiver 即将发动 Congestion。Receiver 收到 CE=1 後便会告诉 Sender 需要把传送速度降低。如 Host 不减缓 ECN (ECT=0)，Software Queue 则依 WRED 设定 Drop Packet。要设定 ECN 只需在 policy-map 的 class ⾥加入 random-detect ecn。

Double policy-map in sub-interface

policy-map TRAFFIC-POLICING

class class-default

police rate 10000000

service-policy POLICY-INCOMING

When we use shaping we will buffer the traffic to a certain bitrate, policing will drop the traffic when it exceeds a certain bitrate. The difference between the two is that policing will drop the exceeding traffic and shaping will buffer it.

It’s also possible that they shape it down to 10 Mbit but shaping means they have to buffer data while policing means they can just throw it away. The 10 Mbit that we pay for is called the CIR (Commited Information Rate).

There are two reasons why you might want to configure shaping:

• Instead of waiting for the policer of the ISP to drop your traffic, you might want to shape your outgoing traffic towards the ISP so that they don’t drop it.

• To prevent egress blocking. When you go from a high speed interface to a low speed interface you might get packet loss (tail drop) in your outgoing queue. We can use shaping to make sure everything will be sent (until its buffer is full).

To check if traffic matches the traffic contract the policer will measure the cumulative byte-rate of arriving packets and the policer can take one of the following actions:

• Allow the packet to pass.

• Drop the packet.

• Remark the packet with a different DSCP or IP precedence value.

When working with policing there are three categories that we can use to see if a packet conforms the traffic contract or not:

• Conforming

• Exceeding

• Violating

LLQ (Low Latency Queuing) is an extension of CBWFQ (Class Based Weighted Fair Queuing) where we add a priority queue to the scheduler.

x% Bandwidth / Priority

Bandwidth command defines what is the minimum bandwidth reserved for a specific class when the network is congested.

Priority command defines what is the maximum bandwidth reserved for a specific class when the network is congested

DSCP值will pass onto IPSec 包头 if with Tunnel mode

DSCP值will pass into MPLS-VPN 标签 exp 值