Transport Control Protocol (TCP) and User Datagram Protocol (UDP) are the two most common Layer 4 protocols. TCP is connection oriented. Data is sent bidirectionally once a connection is established. TCP uses sequencing to make sure segments are proccessed in the correct order and are not missed. TCP is reliable and sender and receiver create acknolwedgments back and forth when traffic is received. If a segment is lost they are resent. TCP conducts flow control where a receiver can tell the sender to slow down traffic rates if they are too high.
TCP creates a standardized header for encapsulated data that gets sent over network connections. This includes source and destination ports, header length, code bits and checksums.
TCP utilizes a three-way handshake. When the sender device establishes a connection with a receiving device it issues a SYN (synchronize message) to the receiver. The receiver willl issue a SYN-ACK synchronized acknowledgement back to the sender. Then the sender sends an ACK acknowledgment back to the receiver. Once this initiaion is complete, the connection is established and the hosts can send network traffic over it.
UDP protocols operate as “best effort”. It does not establish handshake connection set up, does not carryout sequencing and is not connection oriented. UDP is considered “not reliable” as devices do not send acknowledgments back and forth. There is no flow rate control. Error detection and recovery is conducted by the upper layers. Applications that operate in real time such as voice and video cannot afford the extra effort of TCP so use best effort UDP.
Common TCP Applications and their Ports:
FTP – 21
SSH – 22
Telnet – 23
HTTP – 80
HTTPS – 443
Common UDP Applications and their Ports:
TFTP – 69
SNMP – 161
DNS – 53
Cisco CCNA 200-301 – The Complete Guide to Getting Certified https://www.udemy.com/course/ccna-complete/
Neil Anderson
Computer Networking Course – Network Engineering [CompTIA Network+ Exam Prep]
https://www.youtube.com/watch?v=qiQR5rTSshw
Brian Ferrel
CCNA Certification Study Guide, Volume 2: Exam 200-301
Todd Lammle
Netstream 360 