Sep 17, 2009

Advanced TCP Implementation(HS-TCP vs S-TCP vs BIC-TCP)

自從接觸了廣域網路加速器這個領域,愈來愈覺得自己對TCP的了解實在是只懂得皮毛而已。原來在TCP上的實現有這麼多種改進方式,使得TCP的傳輸表現更加優越。

以下的內容是節錄自Cisco Press Application Acceleration and WAN Optimization Fundamentals中的章節,由於這幾個協定可以說是各家廣域網路加速器都會參考的標準,用以改善原有TCP設計運作上的缺陷,所以我把它們整理出來加以分較,希望對各位有所幫助!

  • HS-TCP(High Speed TCP)
High-Speed TCP is a advanced TCP implementation that was developed primarily to address bandwidth scalability. HS-TCP uses an adaptive cwnd increase that is is based on the current cwnd value of the connection. When the cwnd value is large, HS-TCP uses a larger cwnd increase when a segment is successfully acknowledged. In effect, this helps HS-TCP to more quickly find the available bandwidth, which leads to higher levels of throughput on large networks much more quickly.

HS-TCP also uses an adaptive cwnd decrease based on the current cwnd value. When the cwnd value for a connection is large, HS-TCP uses a very small decrease to the connection's cwnd value when loss of a segment is detected. In this way, HS-TCP allows a connection to remain at very high levels of throughput even in the presence of packet loss but can also lead to longer stabilization of TCP throughput when other, non-HS-TCP connections are contending for available network capacity. The aggressive cwnd handling of HS-TCP can lead to a lack of fairness when non-HS-TCP flows are competing for available network bandwidth. Over time, non-HS-TCP flows can stabilize with HS-TCP flows, but this period of time may be extended due to the aggressive behavior of HS-TCP.

  • S-TCP(Scalable TCP)
Scalable TCP(S-TCP) is similar to HS-TCP in that it uses an adaptive increase to cwnd. S-TCP will increase cwnd by a value of (cwnd x .01) when increasing the congestion windows, which means the increment is large when cwnd is large and the increment is small when cwnd is small.

Rather than use an adaptive decrease in cwnd, S-TCP will decrease cwnd by 12.5%(1/8) upon encountering a loss of a segment. In this way, S-TCP is more TCP friendly than HS-TCP in high-bandwidth environments. Like HS-TCP, S-TCP is not fair among flows where an RTT disparity exists due to the overly aggressive cwnd handling.

  • BIC-TCP(Binary Increase Congestion TCP)
Bindary Increase Congestion TCP(BIC-TCP) is an advanced TCP stack that uses a more adaptive increase than that used by HS-TCP and S-TCP. HS-TCP and S-TCP use a variable increment to cwnd directly based on the value of cwnd. BIC-TCP uses connection loss history to adjust the behavior of congestion avoidance to provide fairness.

BIC-TCP's congestion avoidance algorithm uses two search modes - linear search and binary search - as compared to the single search mode (linear or linear relative to cwnd) provided by standard TCP, HS-TCP, and S-TCP. These two search modes allow BIC-TCP to adequately maintain bandwidth scalability and fairness while also avoiding additional levels of packet lost caused by excessive cwnd aggressiveness.
  1. Linear search: Uses a calculation of the difference between the current cwnd and the previous cwnd prior to the loss event the determine the rate of linear search.
  2. Bindary search: Used as congestion avoidance approaches the previous cwnd value prior to the loss event. This allows BIC-TCP to mitigate additional loss events caused by the connection exceeding available network capacity after a packet loss event.
The linear search provides aggressive handling to ensure a rapid return to previous levels of throughput, while the binary search not only helps to minimize an additional loss even, but also helps to improve fairness of environments with RTT disparity(that is, two nodes exchanging data are closer than two other nodes that are exchanging data) in that it allows convergence of TCP throughput across connections much more fairly and quickly.


Sep 16, 2009

Cisco Networkers 2009 - CCIE Lunch Invitation


等了很久,終於等到這一封邀請信 - CCIE Lunch Invitation

傳聞在Cisco Networker活動中都會有一場聚會,與會人員是來自世界各地的CCIE,主持人通常是Cisco金字塔高層主管,可以想見這個活動一定是意義非凡(如果各位是比較資深的CCIE應該也曾經參加過台灣思科主辦的CCIE Club活動,可惜的是無以後繼,CCIE Club就這樣地悄悄消失於地球的表面,只留下每人一只Cisco Logo紀念錶)

再過一個多星期就要準備踏上朝聖的道路,心中充滿期待與興奮…(只要是出遠門心情就會不自主的亢奮起來)

台灣大買凱擘吃東森 將躍居有線電視龍頭

一直很希望看到台灣網路業者之後能夠站在公平公正公開的平等地位上互相競爭互相抗衡,終於看到了一線曙光~

台灣大哥大可望買下有線電視第二大系統台凱擘,最快今(16)日召開董事會討論,傾向以現金交易,一舉吃下110萬戶的有線電視用戶與東森電視的十個頻道,躍居台灣最大的有線電視系統台與頻道業者,併購金額上看600億元。

至截稿為止,尚無法聯絡上台灣大與凱擘的發言管道。消息來源透露,台灣大一直有意擴張有線電視經營版圖,之前對麥格理經營的台灣寬頻有興趣,經過評估後,則相中在台北市布局甚深的凱擘,要藉此切入消費實力最高、又最難布線的地區。

此外,凱擘還掌握東森電視台的十個頻道,包括新聞台、幼幼台、戲劇台等,這對有心跨足數位內容的富邦金董事長蔡明忠、台灣大董事長蔡明興而言,更是吸引力十足。

不過,雙方之前一直卡在價格問題,台灣大有意以手中持有21%庫藏股和凱擘交換,原本去年底可望交易,但台灣大當時的股價滑落,近期股價逐步回升。在雙方協調過程中,仍傾向於全部以現金併購,或半換股與半現金的交易方式。

以凱擘約110萬用戶推算約500多億元,加上凱雷持有約64%的東森電視台股權,投入約五、六十億元,併購金額上看600億元。台灣大可望今天召開董事會討論此事,台灣大昨天收盤價約51.9元,小跌0.1元。

之前有線電視併購案的用戶數價格屢屢推高,一度高達每戶4.7 萬元。但在金融風暴後,雖然市場景氣回溫,價格要再創新高不易,傳言每戶金額約在4萬至5萬元。

值得注意的是,台灣大有線電視用戶數約60萬戶,居於市場第四大,凱擘達到110萬戶,是市場第二大,兩者合計達到170萬用戶數,居有線電視市場龍頭。不過,政府目前對有線電視用戶數仍有不得高於三分之一的限制,雙方如何以轉投資架構布局,也有待觀察。

台灣大目前擁有600多萬手機用戶,又經營購物頻道momo 台與親子台,希望藉由行動電話、有線電視、頻道內容等全方位的經營,擴大市場版圖。

目前電信市場的龍頭業者是中華電,也是台灣大最大的對手,中華電最近在推動隨選視訊(MOD)上著力甚深,達到70萬用戶,雙方競爭激烈,台灣大有意藉由更廣的布局,深入用戶的客廳,與中華電全面開戰。

【2009/09/16 經濟日報】@ http://udn.com/

Cisco 360 Learning Program for CCIE R&S: Cisco Lab Safe Promotion

Together, the Cisco 360 Learning Program for CCIE Routing and Switching and the Cisco Lab Safe promotion allow qualifying learners who do not pass their first Cisco CCIE® R&S lab exam attempt to retake the CCIE lab exam at no additional cost (a $1,400 US value).

To be eligible for this promotion, individuals must meet these requirements:

  • Obtain their instructor’s recommendation
  • Complete the Cisco 360 Learning Program for CCIE R&S Essentials Package or a CIERS instructor-led workshop
  • Score 80 percent or better on at least one of the performance assessments that are listed below

Only select Cisco Authorized Learning Partners offer the Cisco 360 Learning Program curriculum and Cisco Lab Safe promotion, which combined provide you with added assurance in the quality and value of your training investment.


CCIELabSafe.jpg