Queueing
請(qǐng)求文件順序的的排序
什么東西？
瀏覽器有線程限制的，發(fā)請(qǐng)求也不能所有的請(qǐng)求同時(shí)發(fā)送，所以，隊(duì)列嘍。
從添加到待處理隊(duì)列
到實(shí)際開始處理的時(shí)間間隔標(biāo)示

Stalled
是瀏覽器得到要發(fā)出這個(gè)請(qǐng)求的指令到請(qǐng)求可以發(fā)出的等待時(shí)間，一般是代理協(xié)商、以及等待可復(fù)用的TCP連接釋放的時(shí)間，不包括DNS查詢、建立TCP連接等時(shí)間等

DNS Lookup
時(shí)間執(zhí)行DNS查找。每個(gè)新域pagerequires DNS查找一個(gè)完整的往返。 DNS查詢的時(shí)間，當(dāng)本地DNS緩存沒有的時(shí)候，這個(gè)時(shí)間可能是有一段長度的，但是比如你一旦在host中設(shè)置了DNS，或者第二次訪問，由于瀏覽器的DNS緩存還在，這個(gè)時(shí)間就為0了。

Initial connection
建立TCP連接的時(shí)間，就相當(dāng)于客戶端從發(fā)請(qǐng)求開始到TCP握手結(jié)束這一段，包括DNS查詢+Proxy時(shí)間+TCP握手時(shí)間。

Request sent
請(qǐng)求第一個(gè)字節(jié)發(fā)出前到最后一個(gè)字節(jié)發(fā)出后的時(shí)間，也就是上傳時(shí)間

Waiting(TTFB)
請(qǐng)求發(fā)出后，到收到響應(yīng)的第一個(gè)字節(jié)所花費(fèi)的時(shí)間(Time To First Byte),發(fā)送請(qǐng)求完畢到接收請(qǐng)求開始的時(shí)間;這個(gè)時(shí)間段就代表服務(wù)器處理和返回?cái)?shù)據(jù)網(wǎng)絡(luò)延時(shí)時(shí)間了。服務(wù)器優(yōu)化的目的就是要讓這個(gè)時(shí)間段盡可能短。

Content Download
收到響應(yīng)的第一個(gè)字節(jié)，到接受完最后一個(gè)字節(jié)的時(shí)間，就是下載時(shí)間

官方解釋

（免于大家翻墻了）

先說請(qǐng)求的生命周期：

The primary phases of the request lifecycle are:

• Redirect
  Immediately begins startTime.
  If a redirect is happening, redirectStart begins as well.
  If a redirect is occurring at the end of this phase then redirectEnd will be taken.
• App Cache
  If it’s application cache fulfilling the request, a fetchStart time will be taken.
• DNS
  domainLookupStart time is taken at the beginning of the DNS request.
  domainLookupEnd time is taken at the end of the DNS request.
• TCP
  connectStart is taken when initially connecting to the server.
  If TLS or SSL are in use then secureConnectionStart will start when the handshake begins for securing the connection.
  connectEnd is taken when the connection to the server is complete.
• Request
  requestStart is taken once the request for a resource has been sent to the server.
• Response
  responseStart is the time when the server initially responds to the request.
  responseEnd is the time when the request ends and the data is retrieved.

然后在理解那張timing圖

• Queuing
  A request being queued indicates that:
  The request was postponed by the rendering engine because it’s considered lower priority than critical resources (such as scripts/styles). This often happens with images.
  The request was put on hold to wait for an unavailable TCP socket that’s about to free up.
  The request was put on hold because the browser only allows six TCP connections per origin on HTTP 1.
  Time spent making disk cache entries (typically very quick.)

  • Stalled/Blocking
    Time the request spent waiting before it could be sent. It can be waiting for any of the reasons described for Queueing. Additionally, this time is inclusive of any time spent in proxy negotiation.
    Proxy Negotiation
    Time spent negotiating with a proxy server connection.
  • DNS Lookup
    Time spent performing the DNS lookup. Every new domain on a page requires a full roundtrip to do the DNS lookup.
    Initial Connection / Connecting
    Time it took to establish a connection, including TCP handshakes/retries and negotiating a SSL.
  • SSL
    Time spent completing a SSL handshake.
    Request Sent / Sending
    Time spent issuing the network request. Typically a fraction of a millisecond.
  • Waiting (TTFB)
    Time spent waiting for the initial response, also known as the Time To First Byte. This time captures the latency of a round trip to the server in addition to the time spent waiting for the server to deliver the response.
    Content Download / Downloading
    Time spent receiving the response data.

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