10.3. 品質服務

10.3.1. 原則與機制

服務品質 (Quality of Service 或縮寫為 QoS) 係指提供應用時足以保證或改進服務品質的技術。最常見的技術包括分類網路流量、及依照其類別區分流量的作用。這種區分服務的概念是 流量成形，限制部份服務或主機的資料傳輸率，使其不致塞飽可用的頻寬以及壓縮其他的服務。流量成形是 TCP 流量的特定形式，此協定自動調整可用的頻。

也可能改變流量的優先順序，允許與互動服務有關的 (如 ssh 與 telnet) 或祗服務小量資料的封包優先處理。

Debian 核心包括 QoS 必備的功能與相關的模組。這些模組提供多樣服務，多半係供 IP 封包陣列使用；這些陣列行為包括多種可能的需求。

10.3.2. 組態與應用

經由 tc 命令 (由 iproute 套件提供) 設定 QoS 參數。因為其介面較為複雜，建議使用高階的工具。

10.3.2.1. 減少延遲：`wondershaper`

wondershaper (在同名的套件中) 的主要目的是最小化獨立網路負載的延遲。經由限制整體流量的值縮短連結的飽和值。

組態網路介面後，執行 wondershaper interface download_rate upload_rate命令，設定流量控制。可以是 eth0 或 ppp0 介面，兩者的速度均以每秒千位元為單位。wondershaper remove interface 命令可以在特定介面停用流量控制。

For an Ethernet connection, this script is best called right after the interface is configured. This is done by adding up and down directives to the /etc/network/interfaces file allowing declared commands to be run, respectively, after the interface is configured and before it is deconfigured. For example:

範例 10.9. Changes in the /etc/network/interfaces file

iface eth0 inet dhcp
    up /sbin/wondershaper eth0 500 100
    down /sbin/wondershaper remove eth0

In the PPP case, creating a script that calls wondershaper in /etc/ppp/ip-up.d/ will enable traffic control as soon as the connection is up.

10.3.2.2. Standard Configuration

Barring a specific QoS configuration, the Linux kernel uses the pfifo_fast queue scheduler, which provides a few interesting features by itself. The priority of each processed IP packet is based on the ToS field (Type of Service) of this packet; modifying this field is enough to take advantage of the scheduling features. There are five possible values:

Normal-Service (0);
Minimize-Cost (2);
Maximize-Reliability (4);
Maximize-Throughput (8);
Minimize-Delay (16).

The ToS field can be set by applications that generate IP packets, or modified on the fly by netfilter. The following rules are sufficient to increase responsiveness for a server's SSH service:

iptables -t mangle -A PREROUTING -p tcp --sport ssh -j TOS --set-tos Minimize-Delay
iptables -t mangle -A PREROUTING -p tcp --dport ssh -j TOS --set-tos Minimize-Delay