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tags: Computer Network
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# NCCUCS NET22 Lab02

## 4-1
**What is the IP address and TCP port number used by your client computer (source) to transfer the file to gaia.cs.umass.edu?**

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IP Address: 10.232.204.251
TCP Port Number: 61488
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## 4-2
**What is the sequence number of the TCP SYN segment that is used to initiate the TCP connection between the client computer and gaia.cs.umass.edu? What is it in the segment that identifies the segment as a SYN segment?**

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Sequence Number: 3200817078

The SYN Flag in the Flags section is set to 1 which indicates that this segment is a SYN segment.
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## 4-3
**What is the sequence number of the SYNACK segment sent by gaia.cs.umass.edu to the client computer in reply to the SYN?
What is the value of the Acknowledgement field in the SYNACK segment?
How did gaia.cs.umass.edu determine that value?
What is it in the segment that identifies the segment as a SYNACK segment?**

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According to the above figure, the sequence number of the SYNACK segment sent by gaia.cs.umass.edu to the client computer in reply to the SYN is 0.

The value of the acknowledgement field in the SYNACK segment is 1. The value of the ACK field in the SYNACK segment is determined by the server gaia.cs.umass.edu.

The server adds 1 to the initial sequence number of SYN segment form the client computer.

A segment will be identified as a SYNACK segment if both SYN Flag and Acknowledgement in the segment are set to 1.
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## 4-4
**What is the sequence number of the TCP segment containing the HTTP POST command?
Note that in order to find the POST command, you’ll need to dig into the packet content field at the bottom of the Wireshark window, looking for a segment with a “POST” within its DATA field.**

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## 4-5-1
**Consider the TCP segment containing the HTTP POST as the first segment in the TCP connection.
What are the sequence numbers of the first six segments in the TCP connection (including the segment containing the HTTP POST)?**

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Sequence Numbers: 1, 714, 2088, 3462, 4836, 6210
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## 4-5-2
**At what time was each segment sent?
When was the ACK for each segment received?
What is the RTT value for each of the six segments?**

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## 4-5-3
**What is the Estimated RTT value after the receipt of each ACK?**

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According to the formula: EstimatedRTT = 0.875 * EstimatedRTT + 0.125 * SampleRTT

EstimatedRTT after the receipt of the ACK of segment 1:
EstimatedRTT = RTT for Segment 1 = 0.095674 s

EstimatedRTT after the receipt of the ACK of segment 2:
EstimatedRTT = 0.875 * 0.095674 + 0.125 * 0.095864= 0.09569775 s

EstimatedRTT after the receipt of the ACK of segment 3:
EstimatedRTT = 0.875 * 0.09569775 + 0.125 * 0.09682= 0.09583803125 s

EstimatedRTT after the receipt of the ACK of segment 4:
EstimatedRTT = 0.875 * 0.09583803125 + 0.125 * 0.098154= 0.09612752734 s

EstimatedRTT after the receipt of the ACK of segment 5:
EstimatedRTT = 0.875 * 0.09612752734 + 0.125 * 0.112884= 0.09822208642 s

EstimatedRTT after the receipt of the ACK of segment 6:
EstimatedRTT = 0.875 *0.09822208642 + 0.125 * 0.113781= 0.10016695061 s
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## 4-6
**What is the length of each of the first six TCP segments?**

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Length of the first six segments: 713, 1374, 1374, 1374, 1374, 1374
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## 4-7
**What is the minimum amount of available buffer space advertised at the received for the entire trace?
Does the lack of receiver buffer space ever throttle the sender?**

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28960 -> 278144
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## 4-8
**Are there any retransmitted segments in the trace file? What did you check for (in the trace) in order to answer this question?**

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Yes.
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## 4-9
**Can you identify cases where the receiver is ACKing every other received segment.**

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## 4-10
**What is the throughput (bytes transferred per unit time) for the TCP connection? Explain how you calculated this value.**

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153034 / (51.698177 - 50.814763) = 153034 / 0.883414 = 173230.218 (B/s)
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## 5-1
**Select one UDP packet from your trace. From this packet, determine how many fields there are in the UDP header. (You shouldn’t look in the textbook! Answer these questions directly from what you observe in the packet trace.) Name these fields.**

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UDP header contains 4 fields:

1. source port
2. destination port
3. length
4. checksum
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## 5-2
**By consulting the displayed information in Wireshark’s packet content field for this packet,determine the length (in bytes) of each of the UDP header fields.**

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The UDP header has a fixed length of 8 bytes. Each of these 4 header fields is 2 bytes long.
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## 5-3
**The value in the Length field is the length of what? (You can consult the text for this answer). Verify your claim with your captured UDP packet.**

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The length field specifies the number of bytes in the UDP segment (header plus data). An explicit length value is needed since the size of the data field may differ from one UDP segment to the next.
The length of UDP payload for selected packet is 32 bytes. 40 bytes - 8 bytes = 32 bytes.
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## 5-4
**What is the maximum number of bytes that can be included in a UDP payload? (Hint: the answer to this question can be determined by your answer to 2. above)**

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65535 - 8 = 65527 (B)
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## 5-5
**What is the largest possible source port number? (Hint: see the hint in 4.)**

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The largest possible source port number is 65536 - 1 = 65535.
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## 5-6
**What is the protocol number for UDP? Give your answer in both hexadecimal and decimal notation. To answer this question, you’ll need to look into the Protocol field of the IP datagram containing this UDP segment (see Figure 4.13 in the text, and the discussion of IP header fields)**

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0x11 in hex, and 17 in decimal value.
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## 5-7
**Examine a pair of UDP packets in which your host sends the first UDP packet and the second UDP packet is a reply to this first UDP packet. (Hint: for a second packet to be sent in response to a first packet, the sender of the first packet should be the destination of the second packet). Describe the relationship between the port numbers in the two packets.**

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The source port of the UDP packet sent by the host is the same as the destination port of the reply packet, and conversely the destination port of the UDP packet sent by the host is the same as the source port of the reply packet.

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