Windows SDK PGM相關文章翻譯之三PGM Sender Options

PGM Sender Options

PGM senders are provided with certain default settings that affect the performance of data transmission, and how long data is buffered to account for packet loss and associated PGM client retransmission requests. The following paragraphs describe these default settings.

Window Size and Transmission Rate

The capability to set window size and transmission rate enables applications to control the amount of data the transport buffers for retransmission, and the rate at which the byte-stream is transmitted.

Retransmission data is stored in a file, therefore the maximum window size is limited by disk space usable by the transport. The default window size is 10MB. Although it is possible for a send or message size to exceed the window or buffer size, the data stream remain uninterrupted; the send is pended until the all the data has been sent out.

Note   The maximum buffer space is limited by the maximum number of packets that can be held in the window at any given time, which is equal to 2^31 – 1.

The transmission rate is the combined outflow of original data packets (ODATA), retransmitted data packets (RDATA) and transport-specific bookkeeping packets (SPMs), expressed per second. If the rate limit is set to to 56 kilobits per sec by default. The default window size is 10 megabytes, with a default rate of 56 kilobits per second. Due to the relationship between the three members of the RM_SEND_WINDOW structure, the default window size is therefore 1428 seconds. See RM_SEND_WINDOW for more information.

Window Advance Rate

The window advance rate is set by the RM_SENDER_WINDOW_ADV_RATE socket option. This option enables applications to specify the increment at which the PGM sender's window is advanced, expressed as a non-zero percentage value of the window size. The default value is 15%, and the maximum rate is 50%. If the PGM sender has repair data pending that falls in the space of the increment window, the window is advanced partially as each repair packet in the window is sent out.

Forward Error Correction (FEC)

Forward error correction is set through use of the RM_USE_FEC socket option. This socket option enables the PGM sender to send repair packets as parity packets instead of regular data packets. Doing so minimizes the number of repair packets sent to repair different sequences lost by multiple receivers from within the same data group. Enabling FEC is only set on the PGM sender. PGM receivers automatically follow the policy set by the sender. For a detailed discussion on FEC, refer to the PGM RFC located on the IETF website.

PGM發送選項

PGM 發送方設定特定的預設設定能影響資料轉送的效能，和資料為包丟失重傳做的緩衝時間，和相關用戶端的重傳請求。以下章節將描述3個預設選項。

視窗大小和傳輸速率

應用程式可以通過設定視窗大小和傳輸速率來控製為重新傳輸所準備的緩衝的大小和資料轉送的速率。

重新傳輸的資料是儲存在一個檔案裡面的，因此最大的視窗尺寸被限制為傳輸能使用的最大磁碟空間。預設的視窗尺寸是10MB。儘管發送大小超過視窗尺寸的資料包是可能的，資料流也不會間斷；發送方會掛起直到所有的資料都發送完畢。

注意：在任何時間，緩衝的最大空間受到視窗能夠容納的最大包數量，等同於2^31 – 1。

     每秒鐘傳輸的未經處理資料包（ODATA），重傳資料包（RDATA）和transport-specific bookkeeping packets (SPMs)組合形成傳輸速率。如果速率限制預設設定為56kb/秒。預設的視窗尺寸為1000*1000*10位元組（大約10M），並且預設傳輸速率為56kb/秒。由於RM_SEND_WINDOW結構的三個參數之間是存在關係的，計算出來的預設視窗資料緩衝時間大約為1428秒。請查看RM_SEND_WINDOW的定義瞭解三個參數的關係。

視窗進階速率

視窗進階速率通過RM_SENDER_WINDOW_ADV_RATE socket選項來設定。這個選項允許應用程式指定PGM發送視窗的預先增長方式，表示為視窗尺寸的一個非零比例值。預設值是15%，最大比例不能設定超過50%。如果PGM寄件者在增長的視窗大小範圍內修複丟失掛起的資料包，If the PGM sender has repair data pending that falls in the space of the increment window, the window is advanced partially as each repair packet in the window is sent out.

向前錯誤校正（FEC）

向前錯誤校正通過RM_USE_FEC socket選項來設定。這個選項允許PGM寄件者像發生正常資料包一樣發送補救資料包。Doing so minimizes the number of repair packets sent to repair different sequences lost by multiple receivers from within the same data group.FEC選項僅僅能夠在PGM發送方設定。PGM接收方自動使用PGM發送方的策略。需要瞭解關於FEC的詳細資料，請參考IETF網站上的PGM RFC。

註：紅字部分是不知道怎麼翻譯的，需要再理解下。