Homework 3-3-2006

1.

Network Address Translation簡稱NAT
NAT可轉換區域網路中的IP位址 因此只有一個位址會送出網際網路 真實IP位址不會暴露於網際網路之上 在防火牆之後上網的所有電腦外表看起來像是使用同一組IP一樣


作者猜測至少有兩種可能性 在login時 決定Nat 和 firewall的information
We believe that Skype client uses its
version of STUN [5] protocol to determine the type of NAT or
firewall it is behind.
(第十頁 右邊結論)


ordinary host 可以使用來傳遞聲音訊息
而任何一個有足夠頻寬 CPU記憶體的node 有機會成supernode
skype客戶端無法避免自己成危supernode
supernode

ordinary host進入Skype login server後 連上superode

each Skype client is always connected to a SuperNode (any Skype client can become a SuperNode, the SuperNode is acting as a hub). SuperNodes are always on routable open IP addresses. When a call is set up the established TCP connection with the SuperNode is used to signal that a call is coming. Dependent on the firewall status of the client the data stream is set up either as UDP (if firewall allows) or in worse case as outgoing TCP which is almost always allowed. If both clients are only allowed to do outgoing TCP calls are routed through another peer."

In other words you get to know there is a call for you and you make the outgoing connection to a known supernode to meet the call. Neat.

2.

Scalability 當系統規模改變時 能持續發揮效能 提供服務

如果當使用者越多 系統的performance會隨之調整 使得系統能

維持一定的效能 否則容易造成當機和暫時停擺的現象

每500個node會形成一個supernode 並逐漸增加


參考資料:An Analysis of the Skype Peer-to-Peer Internet Telephony Protocol
How does Skype get through Firewalls and NAT Routers?