TCP Interview Questions & Answers

TCP Interview Questions

What is TCP?

Transmission Control Protocol is a connection-oriented protocol. This means that before any data transfer can take place, Certain Parameters has to be negotiated in order to establish the connection

 

What is TCP Three Way Handshake process?

TCP Three Way Handshake process is used for a reliable connection. In this process, the transmitting network device first establishes a connection-oriented (reliable) session with its peer system which is called three-way handshake. Data is then transferred. When the Data transfer is finished, connection is terminated and virtual circuit is teared down.

 

What is window size in TCP three way handshake process (3WHS)?

It is 16-bit window field which indicates the number of bytes a sender will send before receiving an acknowledgment from the receiver.

 

What are TCP Flags?

TCP Flags are used to influence the flow of data across a TCP connection. Below are TCP Flags:

  1. PUSH (PSH): It pushes the buffered data to the receiver’s application. If data is to be sent on immediate basis we will push it
  2. Reset (RST): It resets the connection
  3. Finish (FIN): It finishes the session. It means no more data from the sender
  4. Urgent (URG): It is used to set the priority to tell the receiver that this data is important for you
  5. Acknowledgement (ACK): All packets after SYN packet sent by client should have this flag set. ACK=10 means host has received 0 through 9 and is expecting byte 10 next
  6. Synchronize (SYN): It Initiates a connection. It synchronizes the sequence number

 

What is the difference between PUSH and URG flag?

The PUSH flag in the TCP header informs the receiving host that the data should be pushed up to the receiving application immediately. The URG flag is used to inform a receiving station that certain data within a segment is urgent and should be prioritized.

 

What is the importance of Sequence Number and Acknowledgement Number?

Sequence Number is a 32-bit field which indicates the amount of data that is sent during a TCP session. By Sequence Number sender can be assured that the receiver received the data because the receiver uses this sequence number as the acknowledgment number in the next segment it sends to acknowledge the received data.

Acknowledgment number is used to acknowledge the received data and is equal to the received sequence number plus 1.

 

What is encapsulation and decapsulation?

Encapsulation: Preparing and passing he data by any upper layer to the layer below it, is called Encapsulation.

Decapsulation: Decoding data while going upwards from the physical layer till application layer is called decapsulation.

 

What is the difference between TCP and UDP?

Following are differences in TCP and UDP,

  • TCP is connection oriented protocol while UDP is connectionless protocol
  • TCP is more reliable than UDP
  • UDP is faster for data sending than TCP
  • UDP makes error checking but no reporting but TCP checks for errors and performs reporting
  • TCP provides guaranteed Delivery of Data but UDP has no guarantee
  • Header size of TCP is 20 bytes while that of UDP is 8 bytes
  • TCP has acknowledgement segments but UDP has no acknowledgement
  • TCP is used for application that require high reliability but less time critical whereas UDP is used for application that are time sensitive but require less reliability

 

Which one is reliable out of TCP & UDP?

TCP is reliable.

 

What are the Layer 4 well-known port numbers?

Total Port numbers range from 0 to 65535 & it is divided further into three parts:

  • 0-1023: Well-known ports
  • 1024-49151: Registered ports
  • 49152-65535: Dynamic ports (private ports)

Below are famous TCP & UDP Port numbers:

Protocol

TCP/UDP

Port Number

Description

FTP

TCP 20/21 File Transfer Protocol is used for file transfers (RFC 959)

SSH (Secure Shell)

TCP 22

SSH is the primary method used to manage network devices securely at the command level (RFC 4250-4256)

Telnet TCP 23

Telnet is the primary method used to manage network devices at the command level. Unlike SSH which provides a secure connection, Telnet does not, it simply provides a basic unsecured connection (RFC 854)

SMTP

TCP 25 Simple Mail Transfer Protocol is used for two primary functions, it is used to transfer mail (email) from source to destination between mail servers and it is used by end users to send email to a mail system (RFC 5321)
DNS TCP/UDP 53

Domain Name System (RFC 1034-1035)

DHCP

UDP 67/68 Dynamic Host Configuration Protocol (RFC 2131)

TFTP

UDP 69 Trivial File Transfer Protocol is same as FTP above but uses UDP instead of TCP (RFC 1350)

HTTP

TCP 80 Hypertext Transfer Protocol (RFC 2616)
POP3 TCP 110

Post Office Protocol is used for Emails (RFC 1939)

NTP UDP 123

Network Time Protocol (RFC 5905)

NetBIOS

TCP/UDP 137/138/139 NetBIOS (RFC 1001-1002)
IMAP TCP 143

Internet Message Access Protocol (RFC 3501)

SNMP

TCP/UDP 161/162 Simple Network Management Protocol (RFC 1901-1908, 3411-3418)
BGP TCP 179

Border Gateway Protocol (RFC 4271)

LDAP

TCP/UDP 389

Lightweight Directory Access Protocol (RFC 4510)

HTTPS TCP 443

HTTPS is used in conjunction with HTTP to provide the same services but doing it using a secure connection which is provided by either SSL or TLS (RFC 2818)

 

 

 

 

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Ebuete, Abinotami Williams

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