QoS

發表於 2022-06-26

文章字數： 3.4k 所需閱讀時間 ≈ 6 分鐘

QoS

封包的 QoS(Quality of Service) 表示該封包的品質，品質越好，表示有越低的延遲、掉包、…，作法是利用封包 header 的特定欄位值，來表示此封包的優先等級。
此欄位定義在網路架構第三層(Internet Protocol) header 的欄位中，長度為 8 bits，在 IPv4 和 IPv6 的 header 中的位置不同。
- 在 IPv4 Header 此欄位稱為： DSCP / ToS
- 在 IPv6 Header 此欄位稱為： Traffic Class
  intro
在網路實行 QoS 一般分為 Integrated Services (IntServ) 和 Differentiated Services (DiffServ) 兩大方向
IntServ
IntServ 的 QoS 實施方法是預先在訊息即將通過的每一隻 Router 預留網絡資源 (主要是 Bandwidth)，讓訊息封包通過時保證有足夠資源去提供服務。
- 用道路來作比喻，有一輛消防車趕著去救火，如若遇到交通擠塞便不能提供服務了，IntServ 就好比在每一個路段都把其中一條行車線劃為緊急車輛通道，只有像消防車這類緊急車輛可以使用。
- 缺點是就算沒有緊急車輛通過，所有道路使用者也不可使用這條預留路線。
  DiffServ
DiffServ 則不預先設置緊急通道，當道路暢通時，所有車輛可自由使用任何路線。當某路段發生壅塞 (Congestion) 時，該路段才會實施交通管制，包括把道路分為緊急線、快線和慢線等，然後把車輛分門別類，按其緊急性去使用不同的路線
- 例如緊急車輛使用緊急線，私人車用快線，公共交通工具使用慢線。甚至會在路線上選擇一些車輛炸掉 😨 (Packet Drop) 來舒緩 Congestion，通當是慢線車輛遭殃。
DiffServ 巧妙的地方在於每段道路可設立自己的交通管制方法，路段間互不影響，稱為 Per-hop Behavior (PHB)。而且交通管制措施只會在發生 Congestion 時才會實行，不像 IntServ 需長期預留路線。DiffServ 缺點在於設定比較複雜。

ToS (Type of Service)

ToS (Type of Service)，細分成 Precedence 及 ToS 欄位
- | Precedence(3) | ToS(4) | Unused(1) |
- Precedence (RFC-1112) – 值越高，優先權越高
  1
  2
  3
  4
  5
  6
  7
  8
  | 111 | Network Control |
  | 110 | Internetwork Control |
  | 101 | Critic/ECP |
  | 100 | Flash Override |
  | 011 | Flash |
  | 010 | Immediate |
  | 001 | Priority |
  | 000 | Routine |
- ToS (RFC-1349) – 值越大，優先權越高。這些值的組合是沒有意義的，分別獨立。
  1
  2
  3
  4
  5
  | 1000 | Minimize Delay |
  | 0100 | Maximize Throughput |
  | 0010 | Maximize Reliability |
  | 0001 | Minimize Monetary Cost |
  | 0000 | Normal Service(Default) |
  :::warning
- Type of Service 最早定義在 RFC-791。之後的 RFC-2474 則改變了這些區域的代表意義，稱作 DS (Differentiated Services)。
- ToS 因為定義問題，使得支援性比較低，實際網路上都是用 DS，因此在 IPv6 階段，ToS 已經不支援了。
  :::
  DSCP (Differentiated Service Code Point)
RFC-2474，分成 Class Selector、Drop Precedence 及 Explicit Congestion Notification (ECN)欄位
Class selector values (3)

DSCP	Binary	Hex	Decimal	Typical application	Examples
CS0 (Default)	000	0x00	0
CS1	001	0x08	8	Scavenger	YouTube, Gaming, P2P
CS2	010	0x10	16	OAM	SNMP, SSH, Syslog
CS3	011	0x18	24	Signaling	SCCP, SIP, H.323
CS4	100	0x20	32	Realtime	TelePresence
CS5	101	0x28	40	Broadcast video	Cisco IPVS
CS6	110	0x30	48	Network control	EIGRP, OSPF, HSRP, IKE
CS7	111	0x38	56

Drop Precedence value (3)

Drop Precedence Binary Hex Decimal

Undefined 000 0x00 0

Low 010 0x02 2

Medium 100 0x04 4

Heigh 110 0x06 6

:::danger
The EF marking of 46 does NOT follow the drop preference rules of the assured forwarding model. Please do NOT think that the 11 means high drop preference.
:::

Drop Precedence	Binary	Hex	Decimal
Undefined	000	0x00	0
Low	010	0x02	2
Medium	100	0x04	4
Heigh	110	0x06	6
:::danger

ntp

From tests that were performed, indications are that precise time
distribution requires a very low packet delay variation (jitter)
transport.  Therefore, we suggest that the following guidelines for
Network Time Protocol (NTP) be used:

o  When NTP is used for providing high-accuracy timing within an
   administrator's (carrier's) network or to end users/clients, the
   Telephony service class should be used, and NTP packets should be
   marked with EF DSCP value.
o  For applications that require "wall clock" timing accuracy, the
   Standard service class should be used, and packets should be
   marked with DF DSCP.

wall clock time
- Wall-clock time is the time that a clock on the wall (or a stopwatch in hand) would measure as having elapsed between the start of the process and ‘now’.
- The user-cpu time and system-cpu time are pretty much as you said - the amount of time spent in user code and the amount of time spent in kernel code.
- The units are seconds (and subseconds, which might be microseconds or nanoseconds).
- The wall-clock time is not the number of seconds that the process has spent on the CPU; it is the elapsed time, including time spent waiting for its turn on the CPU (while other processes get to run).

Lin Yunwen's Blog

QoS

QoS

intro

IntServ

DiffServ

ToS (Type of Service)

DSCP (Differentiated Service Code Point)

ntp

Reference