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| Pregunta | Respuesta |
| Client/server versus peer-to peer | a client process requests a service by sending one or more messages to a server process. The server process implements a service by reading the client request, performing some action (for example, in the case of an HTTP server, finding a Web page), and sending one or more messages in reply (in the case of HTTP, returning the requested object). In a peer-to-peer approach, the two ends of the protocol are equals (as in a telephone call). |
| Two services provided by the Internet’s transport layer: | reliable, congestion- controlled data transfer (TCP) unreliable data transfer (UDP). |
| Is HTTP Client/server or peer-to peer? | Classis client/server. A client (Web browser) makes a request with a GET message, and a Web server provides a reply |
| An application may choose UDP for a transport protocol for one or more of the following reasons: | it has no connection establishment; it has no connection state at servers; and it has less packet header overhead than TCP. DNS is an example of an application protocol that uses UDP. |
| Why, in the Internet, is the network layer said to provide unreliable data transfer? | When the transport layer in the source host passes a segment to the network layer, the network layer does not guar- antee it will deliver the segment to the transport layer in the destination host. The segment could get lost and never arrive at the destination |
| What is flow control? | Because a connection’s receive buffer can hold only a limited amount of data, there is the danger that the buffer overflows if data enters the buffer faster than it is read out of the buffer. Many transport protocols, including TCP, use flow control to prevent the occurrence of buffer overflow. |
| What is rwnd? (Receive Window) | Every time TCP receives a packet, it needs to send an ack message to the sender, acknowledging it received that packet correctly, and with this ack message it sends the value of the current receive window, so the sender knows if it can keep sending data. It is crucial in flow control. |
| Why does TCP use a "sliding window"? | TCP uses a sliding window protocol to control the number of bytes in flight it can have. In other words, the number of bytes that were sent but not yet acked. |
| When and how does the window size of TCP change? | In broad terms: TCP connection starts with a small w and then "probes" for the existence of additional unused link bandwidth at the links on its end-to-end path by increasing w. It continues to increase w until a segment loss occurs (as detected by a timeout or duplicate acknowledgements) |
| How does TCP react to duplicate acknowledgements, and why? | if a TCP sender receives a specified number of acknowledgements which usually is set to three duplicate acknowledgements with the same acknowledge number (that is, a total of four acknowledgements with the same acknowledgement number), the sender will then retransmit the packet that was presumed dropped before waiting for its timeout. This reduces the time a sender waits before retransmitting a lost segment -> Fast retransmit |
| Congestion control vs flow control | Flow Control: Sender will send enough data that can be accommodated at the receiver end. Congestion Control: Sender will reduce the amount of sent packets to avoid overflowing the router's buffer(Queue) |
| What is TCP congestion window? | In TCP, the congestion window is one of the factors that determines the number of bytes that can be outstanding at any time. The congestion window is maintained by the sender. Note that this is not to be confused with the TCP window size which is maintained by the receiver! |
| Is slow start related to congestion control or flow control? | Congestion control! |
| What does "slow start" entail in TCP? | TCP slow start is one of the first steps in the congestion control process. It balances the amount of data a sender can transmit (known as the congestion window) with the amount of data the receiver can accept (known as the receiver window) |
| How does slow start work? | The slow start algorithm can simplified as this: for every acknowledgment received, increase the CWND by one MSS. (Maximum segment size) |
| How is reliability implemented in Ethernet? | with CSMA/CD |
| Which protocol provides "best effort service"? | IP provides a best effort delivery service for packets (while TCP provides guaranteed delivery) |
| Explain CSMA/CD | 1.Is my frame ready for transmission? If yes, it goes on to the next point. 2. Is medium idle? If not, wait until it becomes ready. 3.Start transmitting and monitor for collision during transmission. 4.Did a collision occur? Send Jam signal to allert 5. End frame transmission. Wait for a random time interval -> resend the frame (if no collision the frame is assumed to have been received correctly) |
| what is an ARP table? | Each host and router has an ARP table in its memory, which contains mappings of IP addresses to MAC addresses. |
| What are the mandatory fields of a forwarding table? | -Destination (subnetwork address consisting of prefix and mask) -Gateway IP address) Interface (output link) |
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