在VPN內部中，使用QoS-OSPF用於CE和PE路由器之間：路由架構和效能的探討

Abstract

在VPN中搜尋一條有足夠QoS保證的site-to-site路徑，是一個為了支援多媒體在VPN上有相當傳輸品質的需求特性。由於VPN所覆蓋的底層網路是共同分享的，在提供著設備所用來達到上面提及的任務的機制通常被認定為是不可信賴的。因此，不同於網際網路服務提供者的QoS路由問題，Site-to-Site路徑選擇程序應完全依賴嵌進在顧客設備的機制。更特別的，一個監控的機制被採用來偵測VPN的狀態而且一個QoS路由機制被使用來找到一條符合品質的路徑。在這篇論文中，根據採用新提出的MPLS/VPN在ISP的網路中當作其路由和遞送的架構，我們檢查使用OSPF伸展版本來支援QoS(RFC2370)，簡記為QoS-OSPF，作為在顧客設備那端QoS路由機制的可行性。我們認為應該考慮使用其他的LSA type，例如 type 10，為了避免對現存路由機制所需的可能修改，而不是使用原來基本的LSA type，來描述Site-to-Site的路徑狀態。我們也做了一個模擬來比較在QoS-OSPF中所提出二種方法的性能，也就是on-demand和pre-computation.我們知道我們可以藉由降低update-triggering threshold的敏感性或著使用hold-down timer來減少QoS路由的代價，但是這樣將會路由性能。在我們的劇本中，我們在我們的環境中產生二十個點而且每個連接都有45Mbits/sec的頻寬，1 millisecond的傳輸延遲。我們結合各種update-triggering threshold和hold-down timer的使用，在我們的模擬中。我們的模擬顯示針對所有update-triggering threshold的值，on-demand的路徑計算所產生的路由表現優於pre-computation而且他們的效能將會隨著加入hold-down timer而降低。

Searching a site-to-site path with sufficient quality-of-service (QoS) guarantee in a virtual private network (VPN) is a demanding feature to support multimedia transmission over VPN with fair quality. Due to that the underlying network where the VPN overlays is the shared and public, the mechanisms in the provider’s equipment usually assume to be unreliable. Therefore, unlike the problem of QoS routing for Internet service provider, the path searching process shall fully rely on the mechanisms embedded in the customer’s equipments. More specifically, a monitoring mechanism is adopted to detect the status of VPN and a QoS routing mechanism is employed to find a qualified path. In this thesis, by assuming that the newly proposed MPLS/VPN is deployed as the routing and forwarding architecture in the provider’s network, we examine the viability of using the OSPF extended version for supporting QoS (RFC2370), briefed as QoS-OSPF, as the QoS routing mechanism in the customer’s equipment. We argue that rather than the LSA of ordinary types, we shall consider using the other type, e.g., the LSA of type 10, to report the site-to-site path status in order to avoid the possible modification of the existing routing mechanism. We also conduct a simulation to compare the performance of two methods proposed in QoS-OSPF, i.e., on-demand and pre-computation. We know that we can reduce some cost of QoS routing by reducing the sensitivity of the update-triggering threshold or using a hold-down timer, but it will degrade the routing performance. In our scenario, we generate the twenty nodes in our environment and each link is chosen to have a capacity of 45 Mbits/sec and a propagation delay of 1 millisecond. We combined several kinds of update triggering-threshold and hold-down timer in our simulation. Our simulation shows that for all values of update triggering-threshold, on-demand path computation performs better than pre-computation and their performance will degrade with hold-down timer.