COMS3200/7201 Assignment 2 Solution

Part A will involve a few short questions about the QUIC experimental transport layer protocol. To answer these questions you will have to do some research about this protocol, which will include reading sections of the IETF draft for this protocol (https://datatracker.ietf.org/doc/draft-ietf-quic-transport/). You should make sure to read v19 of the draft as there have been changes to the protocol between drafts. Keep in mind that this draft goes into more technical detail than is required for these questions.
• Introduction (section 1)
• Frame types and formats (sections 12 and 19)
• Variable length encoding (section 16)
• Error handling (section 11)
5. If an endpoint receives an invalid CONNECTION CLOSE frame, how should it signal this error to its peer? (1 mark)
Consider the following hexadecimal sequences representing one or more QUIC frames (these are explained in section 19 of the draft). For each sequence, state what QUIC frame(s) the sequence represents (according to the draft) and explain the purpose of the frame type(s). For example, the hex sequence 0x01 represents a PING frame, which is used to check reachability of a peer.
This question requires you to analyse the Wireshark file file-download.pcapng. This packet capture shows a client downloading a large file from a server, using TCP as a transport layer protocol.
You will need to read some of the relevant IETF RFCs to understand some of the following questions. In particular:
• RFC793 is the base TCP spec
• RFC2018 describes the TCP SACK option
• RFC7323 describes the Window Scale option
Answer each of the following questions in the associated quiz on blackboard, following the specified instructions. All questions will be automatically marked. Recall that Wireshark displays TCP sequence numbers as a value relative to the first sequence number. You will need to disable this to answer any questions that ask for a raw sequence number. It is highly recommended that you turn of TCP reassembly to understand the order of packet transmission.
1. What is the raw TCP sequence number of...
(a) The packet initiating the TCP connection? (1 mark)
(b) The acknowledgement from the server for the above packet? (1 mark)
Frame 10 shows a HTTP GET request from the client to the server. Questions 2 and 3 focus on this frame.
3. What is the length of the TCP header (in bytes)? (1 mark)
The following questions relate to window scaling. You should read RFC7323 before attempting these questions.
5. What is the initial value of the window scale shift count indicated by...
(a) The client? (1 mark)
(b) The server? (1 mark)
Around packet 1037, some frames are lost. Questions 8-10 focus on these lost frames.
The RUSH protocol (Reliable UDP Substitute for HTTP) is a HTTP-like stop-and-wait protocol that uses UDP in conjunction with the RDT rules. You have recently been hired by the multinational tech giant COMS3200 Inc, who have identified your deep knowledge in the field of transport-layer protocols. The CEO of COMS3200 Inc, Dan Kim, has asked you to develop a network server capable of sending RUSH messages to a client. It is expected that the RUSH protocol is able to handle packet corruption and loss according to the RDT rules. Your server program must be written in Python, Java, C, or C++.
ASIDE: The RUSH protocol is not a real networking protocol and has been created purely for this assignment
Program Invocation
Your program should be able to be invoked from a UNIX command line as follows. It is expected that any Python programs can run with version 3.6, and any Java programs can run with version 8.
Python python3 assign2.py
C/C++
make ./assign2
Java
make java Assign2
RUSH Packet Structure
A RUSH packet can be expressed as the following structure (|| is concatenation):
IP-header || UDP-header || RUSH-header || payload
The data segment of the packet is a string of ASCII plaintext. Single RUSH packets must be no longer than 1500 bytes, including the IP and UDP headers (i.e. the maximum length of the data section is 1466 bytes). Packets that are smaller than 1500 bytes should be padded with 0 bits up to that size. The following table describes the header structure of an RUSH packet:
Bit 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
0 Sequence Number
16 Acknowledgement Number
32 Flag s Reserved (should be 0)
The following sections describe each header in this packet further.
Sequence and Acknowledgement Numbers
Sequence numbers are independently maintained by the client and server. The first packet sent by either endpoint should have a sequence number of 1, and subsequent packets should have a sequence number of 1 higher than the previous packet (note that unlike TCP, RUSH sequence numbers are based on the number of packets as opposed to the number of bytes). When the ACK flag is set, the acknowledgement number should contain the sequence number of the packet that is being acknowledged. When a packet is retransmitted, it should use the original sequence number of the packet being retransmitted. Any packet that isn’t a retransmission (including NAKs) should increment the sequence number.
Flags
The Flags section of the header is broken down into the following:
Bit 0 1 2 3 4
0 ACK NAK GET DAT FIN
The purpose of these flags is described in the example below.
RUSH Example
The following situation describes a simple RUSH communication session. Square brackets denote the flags that are set in each step (for example [FIN/ACK] denotes the FIN and ACK flags having the value 1 and the rest having the value 0). Note that RUSH, unlike TCP, is not connection-oriented. There is no handshake to initialise the connection, but there is one to close the connection.
1. The client sends a request packet to the server [GET]
• The sequence number of this packet will always be 1
• The data section of this packet will contain the name of a resource (eg. file.txt)
2. The server transmits the requested resource to the client over (possibly) multiple packets [DAT]
• The first packet should have a sequence number of 1
3. The client acknowledges having received each data packet [DAT/ACK]
• The acknowledgement number of this packet should be the sequence number of the packet being acknowledged
4. After receiving the last acknowledgement, the server signals the end of the connection [FIN]
5. The client acknowledges the connection termination [FIN/ACK]
6. The server acknowledges the client’s acknowledgement and terminates the connection [FIN/ACK]
Your Task
• Listen on an unused port for a client’s message
• Successfully close the connection
When invoked, your program should let the OS choose an unused localhost port and your program should listen on that port. It should output that port as a raw base-10 integer to stdout. For example, if Python was used and port 54321 was selected, your program invocation would look like this:
python3 assign2.py
54321
Any lines in stdout after the port number can be used for debugging. For this section, your program does not need to respond to the GET request. Upon hearing from a client, your program can immediately signal the end of the connection (as described in the example). Once the FIN handshake has been completed, your program should terminate.
• Perform all features outlined in the above section
• Successfully transmit a requested file over one or more packets
• Receive (but not handle) ACKs from the client during transmission
• Perform all features outlined in the above sections
• Retransmit any packet on receiving a NAK for that packet
• Retransmit any packet that has not been acknowledged within 3 seconds of being sent
A client will send a DAT/NAK packet should it receive a corrupted packet or a packet with a sequence number it wasn’t expecting (the NAK packet’s acknowledgement number will contain the sequence number it was expecting). In this case your program should retransmit the packet with that sequence number.
If a DAT, FIN, or ACK packet gets lost during transmission your program should retransmit it after 3 seconds without acknowledgement. If a NAK is received the timer should reset. How you choose to handle timeouts is up to you, however it must work on a UNIX machine (moss). Achieving this through multithreading, multiprocessing, or signals is fine provided you only use standard libraries.
• Perform all features outlined in the above sections
• Gracefully ignore corrupt, invalid, or unexpected packets
When a corrupt, invalid, or unexpected packet is received, your program should ignore it and continue to run without error. Any retransmission timer should also not stop or reset. Part of this task is determining what would constitute as an invalid or unexpected packet. You can assume that the only legal flag combinations are GET, DAT, DAT/ACK, DAT/NAK, FIN, and FIN/ACK. You don’t have to worry about checking the UDP checksum only the contents of the RUSH header and data.
Tips for Success
• Revisit the lectures and labs on reliable data transfer and TCP, ensuring you are familiar with the fundamentals
• Frequently test your code on moss
• Ensure your base functionality is working before attempting the more difficult tasks
• Start early - there will be limited help during the midsemester break so any questions will need to be asked in labs beforehand
Library Restrictions
• You can’t use any libraries that aren’t considered standard for your language (i.e. if you have to download a library to use it it would be considered as non-standard)
Submission
Submit all files necessary to run your program. At a minimum, you should submit a file named assign2.py, assign2.c, assign2.cpp or Assign2.java. If you submit a C/C++ or Java program, you should also submit a makefile to compile your code into a binary named assign2 or a .class file named Assign2.class.
IMPORTANT: If you do not adhere to this (eg. submitting a C/C++/Java program without a Makefile, or a .class file instead of a .java file), you will receive 0 for this part of the assignment.
Marking
Academic Misconduct
• Released assignment
• Fixed the example binary number in part B
• Added a note to part B about TCP reassembly
• Clarified what flag combinations are legal