Computer Networks Test 5

In the slow start phase

The size of the congestion window increases exponentially until it reaches a threshold.

In congestion avoidance 

The size of the congestion window increases additively until it reaches a threshold.

In congestion detection

the size of the threshold is dropped to half (multiplicative decrease). It may vary based on the type of congestion is severe or mild

SMTP stands for Simple Mail Transfer Protocol. SMTP is used when email is delivered from an email client, such as Outlook Express, to an email server or when email is delivered from one email server to another. SMTP uses port 25.

POP3 stands for Post Office Protocol. POP3 allows an email client to download an email from an email server. The POP3 protocol is simple and does not offer many features except for download. Its design assumes that the email client downloads all available email from the server, deletes them from the server and then disconnects. POP3 normally uses port 110.

IMAP stands for Internet Message Access Protocol. IMAP shares many similar features with POP3. It, too, is a protocol that an email client can use to download email from an email server. However, IMAP includes many more features than POP3. The IMAP protocol is designed to let users keep their email on the server. IMAP requires more disk space on the server and more CPU resources than POP3, as all emails are stored on the server. IMAP normally uses port 143.

Concept:

In a sliding window protocol,

window size of sender = W_s = 2⁶ bits = 64

window size of sender = window size of receiver

Calculation:

∴ Ws +Wr   =  n

Ws = Wr

∴ 2Ws    =  n 

∴ n = 2 × 64  =  128

1st frame → 0 (sequence number)

128th frame → 127

129th frame → 0

180th frame → 52

4 Gbps → 1 second

\(1b \to \frac{1}{{4 \times {{10}^9}\;}}\;seconds\)

\(\frac{1}{8}\;B \to \frac{1}{{500 \times {{10}^6}\;}}\;seconds\)

\(1 \; B \rightarrow \frac{8}{{4 \times {{10}^9}\;}}\;seconds\)

Sequence number = 32 bits

\({2^{32}}\;B \rightarrow \frac{8 \times 2^{32}}{{4 \times {{10}^9}\;}}\;seconds\)

\({2^{32}}B \to 8.5899\;seconds\)

The minimum time before this sequence number can be used again is 8.5899  ≈ 9 seconds

The client sends the first segment, an SYN segment, in which only the SYN flag is set. This segment is for synchronization of sequence numbers. This segment does not contain an acknowledgment number. It does not define the window size either.

The server sends the second segment, an SYN + ACK segment with two flag bits set: SYN and ACK. An SYN + ACK segment cannot carry data but does consume one sequence number.

The client sends the third segment, ACK segment. It acknowledges the receipt of the second segment with the ACK flag and acknowledgment number field. An ACK segment, if carrying no data, consumes no sequence number.

Data:

Propagation delay = 5 ms

Round Trip Time RTT = 2 × 5 = 10 ms

Size of receiver window = 256 KB

Slow start threshold = 64 KB

Maximum Segment size MSS = 4 KB

At t = 10, there are no timeouts.

Threshold = 32 KB

MSS = 4 KB

Explanation:

The state of congestion window changes as below

At  10 ms: 1 MSS

Now, for every RTT, MSS will grow exponentially until the slow start threshold/2 is reached.

The size of the congestion window size is 80 KB.

Data size = 2000 bytes 

Size of 6 segments + Last segment = 2000 bytes

Size of 6 segments = 1500 bytes

Since each segment length is equal

Size of 1 segment = \(\frac{1500}{6}= 250 \;bytes\)

First segment: 12345 to 12594  \\ (12345+250-1=  12594)

Second segment: 12595 to 12844

Third segment: 12845 to 13094

Fourth segment: 13095 to 13344

Fifth segment: 13345 to 13594

Sixth segment: 13595 to 13844

Seventh segment: 13845 to 14344

Sum =  13844 + 14344 = 28188

Tips and Tricks:

Seventh segment last byte: 12345 + 2000 - 1 = 14344

Sixth segment last byte: 12345 + 250×6 - 1 = 13844

Bind() is used to associate a socket with a port on the local machine. The port number is used by the kernel to match an incoming packet to a process.

Listen() waits for incoming connections. It converts an unconnected active TCP socket into a passive socket. Bind() must be called before Listen().

accept() gets the pending connection on the port user is listen() ing on.

C = capacity of bucket = 9 GB

M = maximum output rate = 250 MBps

e = Input rate​

S = minimum time required to transmit the data

\(S = \frac{C}{(M-e)}\)

\(50 = \frac{(9 ×10^{9})}{(250×10^{6}-e)}\)

e = \(70 \times 10^6\)Bps

e = 70 MBps

x = e = 70

Notes:

Bandwidth is always taken in power of 10, that is,

1KB = 10³ B

1MB = 10⁶ B

1GB = 10⁹ B