TCP Interview Questions & Answers

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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