→ Static routing vs. Session routing Static routing 和 Session routing 都是使用 Dijkstra's all-pairs SPF algorithm. Static routing 只在模擬開始時執行一次 Dijkstra's all-pairs SPF algorithm. Session routing 除了在模擬開始時執行一次 Dijkstra's all-pairs SPF algorithm 之外, 當在模擬中發生拓樸改變 (ex: 連結斷掉), Session routing 會再執行 Dijkstra's all-pairs SPF algorithm 以重新計算最短路徑. 如此一來, 透過迅速地重新計算就可避免短暫的傳輸中斷. → Dijkstra's all-pairs SPF algorithm 介紹 Dijkstra's all-pairs SPF algorithm 定義 : == Definition == N = 網路中所有點的集合 s = source node T = 已經經由 Dijkstra's all-pairs SPF algorithm 找到最短路徑的點的集合 w( i , j ) = node I 到 node j 的 link cost w( i , i ) = 0 w( i , j ) = ∞ 如果兩個 nodes 沒有直接地連結 w( I , j ) ≧ 0 如果兩個 nodes 有直接地連結 L(n) = cost of the least-cost path from node s to node n that is currently known to the algorithm; at termination, this is the cost of the least-cost path in the graph from s to n. == Step1 - Initialization == T = { s } L(n) = w( i , j ) for n ≠ s == Step2 - Get Next Node == Find x which does not belong to T such that L(x) = min L( j ), j does not belong to T == Step3 - Update Least-Cost Paths == L(n) = min [ L(n), L(x)+w(x,n)] for all n which does not belong to M *Steps 2 and 3 are repeated until T=N. 接下來就以一連串的圖來解釋 Dijkstra's all-pairs SPF algorithm : → Static Routing example 程式碼解說 以下為Static Routing example的程式碼解說. Session Routing example與Static Routing example的寫法類似, 不再進行說明. 程式碼下載 routeStatic.tcl, routeSession.tcl. Static Routing example 的程式碼部分 # 開啟一個模擬物件 set ns [new Simulator] # 開啟nam trace file set nf [open routeStatic.nam w] $ns namtrace-all $nf # 定義結束的行程 proc finish {} { global ns nf $ns flush-trace # 關閉trace file close $nf # 執行nam exec nam routeStatic.nam & exit 0 } # 產生6個nodes for {set i 0} {$i < 6} {incr i} { set n($i) [$ns node] } # 建立node間的link for {set i 0} {$i < 6} {incr i} { $ns duplex-link $n($i) $n([expr ($i+1)%6]) 1Mb 10ms DropTail } $ns duplex-link $n(0) $n(4) 1Mb 10ms DropTail # 設每一條link的link cost $ns cost $n(0) $n(1) 2 $ns cost $n(1) $n(0) 2 $ns cost $n(1) $n(2) 1 $ns cost $n(2) $n(1) 1 $ns cost $n(2) $n(3) 5 $ns cost $n(3) $n(2) 5 $ns cost $n(3) $n(4) 10 $ns cost $n(4) $n(3) 10 $ns cost $n(4) $n(5) 10 $ns cost $n(5) $n(4) 10 $ns cost $n(5) $n(0) 10 $ns cost $n(0) $n(5) 10 $ns cost $n(4) $n(0) 10 $ns cost $n(0) $n(4) 10 # 在node0上建立UPD Agent set udp0 [new Agent/UDP] $ns attach-agent $n(0) $udp0 # 設定flow id為0 $udp0 set fid_ 0 # 設定封包顏色 $ns color 0 darkgreen # 在node1上建立UPD Agent set udp1 [new Agent/UDP] $ns attach-agent $n(1) $udp1 # 設定flow id為1 $udp1 set fid_ 1 # 設定封包顏色 $ns color 1 red # 在node4上建立NULL Agent set null4 [new Agent/Null] $ns attach-agent $n(4) $null4 # 連結UDP Agent & NULL Agent $ns connect $udp0 $null4 $ns connect $udp1 $null4 # 在node0上產生CBR Traffic set cbr0 [new Application/Traffic/CBR] $cbr0 attach-agent $udp0 # 在node1上產生CBR Traffic set cbr1 [new Application/Traffic/CBR] $cbr1 attach-agent $udp1 # 指定使用 Static Routing $ns rtproto Static # 為CBR Traffic排行程 $ns at 0.1 "$cbr0 start" $ns at 0.3 "$cbr1 start" # 0.7s時node0到node4的link斷掉 $ns rtmodel-at 0.7 down $n(0) $n(4) # 1s時node0到node4的link恢復 $ns rtmodel-at 1 up $n(0) $n(4) # 模擬結束 $ns at 1.5 "finish" # 開始跑模擬 $ns run → 執行結果說明 0.1s時 node0 開始送 packet 到 node4 0.3s時 node1 開始送 packet 到 node4 0.7s 時 node0 到 node4 的 link 斷掉. 由於 Static Routing 並不會重新 run Dijkstra's all-pairs SPF algorithm, 所以造成 node0 到 node4 以及 node1 到 node4 的資料傳輸中斷. 1s 時 node0 到 node4 的 link 恢復. node0 到 node4 以及 node1到 node4 的資料傳輸恢復.