A SC behind a port restricted NAT exchanged data between SN and some of the

A sc behind a port restricted nat exchanged data

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A SC behind a port-restricted NAT exchanged data between SN, and some of the nodes which responded to its UDP request during login process. The message flow is shown in Figure 4. ‘B’ stands for bytes and ‘N’ stands for node. UDP packets were sent to N1, N2, N3, and N4 during login process and responses were received from them. Message size corresponds to payload size of TCP or UDP packets. A SC behind a port-restricted NAT and UDP-restricted firewall sent the search request over TCP to its SN. SN then performed the search query and informed SC of the search results. Unlike user search by SC on a public IP address, SC did not contact any other nodes. This suggests SC knew that it was behind a UDP-restricted firewall. Figure 3. Message flow for user search when SC has a public IP address. ‘B’ stands for bytes and
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INFOTECH Seminar Advanced Communication Services (ACS), 2005 7 Figure 4 . Message flow for user search when SC is behind a port-restricted NAT. Also the experiment performed by Henning Schulzrinne shows that the SC performs user information caching at intermediate nodes. A conjecture based on the message flows above is that Skype perhaps use Chord [6]-like hash-based methods, combined with traditional blind searching techniques. SN acts like a search proxy for SC and caches searched results. 4.3 NAT and Firewall When connecting a PC to the Internet, it is imperative to safeguard the system from hacker attacks and other unwanted accessibility. A firewall protects the PC by rejecting attacks and illegal data packets, allowing only approved traffic. On a local area network (LAN), where several PCs or other equipment is connected, it is common to have private IP addresses on the LAN and a single common public IP address to the Internet. This is called NAT (Network Address Translation) and is often an integrated part of the firewall. Firewalls and NATs are designed for data traffic that is initiated from the inside of the private network. If instead the data traffic is initiated from the outside, and even worse, must reach a specific user on the private network, serious problems will occur. Normally three workarounds [7] are suggested to solve the Firewall and NAT traversal issues for Voice over IP. The first method is NAT port forwarding, either by manual configuration or via UPnP (Universal Plug and Play) .Second, the STUN protocol enables a client to discover whether it is behind a NAT, and to determine the type of NAT. Third, the ALG (Application Layer Gateway) processes the signaling and media streams so it can modify the signaling to reflect the public IP addresses and ports being used by the signaling and media traffic. In contrast to traditional approaches, schemes used in Skype are natural and elegant, involving little management structure and overhead. As described in Login process, SC uses a variation of typical schemes such as STUN to determine the type of NAT and firewall it is behind. SC refreshes this information periodically.
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