Frequently Asked Questions

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1. Getting Started

Q1-1: How do I ask questions?

We use Github Discussions for project-related Q&A.
You can search previously asked questions or open a new discussion.

Q1-2: How do I use Github Discussions?

See https://docs.github.com/en/discussions/quickstart#creating-a-new-discussion

Q1-3: Which category should I use?

There are four categories: Application (for PA1), TCP (for PA2 and PA3), Routing (for PA4), and etc.
You should choose a category based on your assignment.

Q1-4: I need more resources

We also maintain WiKi (https://github.com/ANLAB-KAIST/KENSv3/wiki).

Q1-5: How do I get notified?

Use Watch button on the top.

2. Application Layer

Q2-1: Will the code style be graded?

No.

Q2-2: Is there any extra test cases?

No, passing provided test cases is enough.

Q2-3: Which file do I have to submit?

You only have to submit EchoAssignment.cpp.

Q2-4: Which part do I need to edit?

You only have to edit serverMain and clientMain methods.

KENSv3/app/echo/EchoAssignment.cpp

Lines 24 to 42 in 3098c76

	int EchoAssignment::serverMain(const char *bind_ip, int port,
	const char *server_hello) {
	// Your server code
	// !IMPORTANT: do not use global variables and do not define/use functions
	// !IMPORTANT: for all system calls, when an error happens, your program must
	// return. e.g., if an read() call return -1, return -1 for serverMain.
	return 0;
	}
	int EchoAssignment::clientMain(const char *server_ip, int port,
	const char *command) {
	// Your client code
	// !IMPORTANT: do not use global variables and do not define/use functions
	// !IMPORTANT: for all system calls, when an error happens, your program must
	// return. e.g., if an read() call return -1, return -1 for clientMain.
	return 0;
	}

Q2-5: Can I use fork(), select(), poll(), or epoll()? I want to do I/O multiplexing

No, currently KENS does not support these system calls.

Q2-6: Can I use non-blocking sockets?

No, currently KENS does not support non-blocking sockets.

Q2-7: Can I use getsockopt and setsockopt? I want to set SO_REUSEADDR option

No, currently KENS does not support these functions.

3. Transport Layer

Q3-1. [Handshaking] How much implementation is required in this project?

See https://github.com/ANLAB-KAIST/KENSv3/wiki/Misc:-Statistics

Q3-2. [Handshaking] Can we assume that there are only IPv4 addresses?

Yes

Q3-3. [Handshaking] As far as I know, socket() gets three arguments, domain, type and protocol, while the KENS template(TCPAssignment.cpp) gets only two arguments except for syscallUUID and PID. Do I have to omit one of those arguments in the project?

Domain argument is processed automatically by KENS.

Q3-4. [Handshaking] I don’t understand how simultaneous connect/close work

Please refer to the following material to understand them. https://tools.ietf.org/html/rfc793 (page 32)

Q3-5. [Handshaking] Should we only implement these 4 (accept()/connect()/getpeername()/listen()) or we have to implement packetArrived() or the others as well?

You are going to have to write code in packetArrived() function as blocked system calls are called when they receive appropriate packet from its peer(can be either server or client)

Q3-6. [Handshaking] Is it necessary to check for overlap when implementing implicit bind in connect()?

Yes, you should check the overlap.

Q3-7. [Handshaking] How similar should the pcap files be to the solutions? Should we try to calibrate the sequence number of packets to match the ones in the pcap files so that we get the exact same bytes? or is it enough for the pcap file to show the correct packet send/receive ordering?

You can't have exactly the same pcap files since your sequence numbers will be different. But, we will compare the order of packets, and apart from multi-interface test cases, you are supposed to have the exact same order with the solution pcap files.

Q3-8. [Handshaking] Can multiple processes listen to the same port?

No.

Q3-9. [Handshaking] Do we have to reimplement checksum function from scratch?

No. You can use function NetworkUtil::tcp_sum. See https://github.com/ANLAB-KAIST/KENSv3/wiki/Tip:-Networking-Utilities#tcp-checksum-calculation .

Q3-10. [Handshaking] I had trouble with checksum value. When I tried to send the packet, checksum value sometimes automatically changed to 0xeeee

Comment the line #define UNRELIABLE in testenv.hpp.

Q3-11. [Handshaking] What should a process do after calling listen?

After calling listen, a process waits for incoming connections by calling accept, which returns a descriptor that references a new socket connected to a client. The original socket, s, remains unconnected and ready to receive additional connections. accept returns the address of the foreign system if name points to a valid buffer.
The connection-processing details are handled by the protocol

Q3-12. [Transfer] In the test for sending (e.g. TestTransfer_Accept_Send_Symmetric), I got ACK packets from the receiver after sending all data to the receiver. However, in the solution pcap file, it sends only some packets and gets ACK. Why does this happen?

The solution already has congestion control implemented, so it has features like slow start.

Q3-13. [Transfer] My implementation has no FIN packets, because read() is blocked and it is never unblocked because the remote peer closes the connection and no more sends data, resulting close() is never able to be called. What is the problem in this case?

You must understand EOF behavior. If the remote host sent FIN, this indicates that the remote host declared the end of the file. In the receipt of FIN, the read should not be blocked but return -1 to indicate that the remote host has closed the connection.

Q3-14. [Transfer] Is there any case such that the sender sends 512 bytes but the receiver receives less than 512 bytes?

The incomplete packet error that you mentioned should not happen because both Ethernet and IP layers check the packet length. Sure, it could miraculously happen but the probability would be extremely low.

Q3-15. [Transfer] Can packets engaged in connection setup/teardown also be lost?

Yes, you should handle SYN, SYNACK, or FIN losses.

4. Internet Layer

Q4-1. [Routing] Which protocol do we implement?

RIPv1 and modified version of RIPv1

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