TCP

TCP Summary

Transmission Control Protocol is a L4 connection-oriented protocol

1. Connection oriented
   1. Before transmitting data, two hosts establish a connection
2. Reliable communication
   1. Destination host MUST send an acknowledge that it received each TCP Segment
   2. If a segment isn't acknowledged, it is resent
3. Sequencing
   1. Sequence numbers allow hosts orient segments in the correct order
   2. Forward Acknowledgement is used to indicate the sequence number of the next segment the host expects to receive
4. Provides Flow Control
   1. Destination can tell source host to change the flow rate of data.
   2. The TCP header Window Size indicates how many segments can be sent before an ACK is required for them

TCP Header

Source: Wikipedia

1. The Source and Destination ports are 16 bits long, so there are 65536 available port numbers
2. The Sequence number and Acknowledgement number are used to provide sequencing and reliable communication
3. There are a series of flags in the 12th octet that are used (when set to 1, they are active)
   1. Flags used to establish/terminate connections
      1. ACK
      2. SYN
      3. FIN
4. Window Size is used for Flow Control

TCP Connection Creation and Termination

TCP Three-Way Handshake (Creation)

1. HostA initiates with a Segment with the SYN flag set
2. HostB replies with a SYN-ACK flags set
3. HostA responds with the ACK bit set

TCP four-way handshake (Termination)

1. HostB initiates termination with a FIN flag
2. HostA responds with an ACK
3. HostA then send it's own segment with a FIN flag
4. HostB responds with an ACK

TCP Sequencing

TCP uses Forward Acknowledgement to sequence messages

1. Each host tracks its own sequence of messages with the Sequence number
2. Each host indicates the next messages sequence number it expects to receive with the Acknowledgement number
3. Example of a 3-way handshake and future dataflow:
   1. HostA starts communication with a SYN segment and a random sequence number of 27
      1. In practice, the random numbers are much larger and do not increment by 1
   2. HostB responds with a SYN-ACK segment with a random sequence number of 3, and an acknowledgement number of 28
      1. The acknowledgement number is in anticipation of receiving a segment with that sequence number
   3. HostA Responds with an ACK segment with a sequence number of 28, and an acknowledgement number of 4
   4. HostB responds with segment with a sequence number of 4, and an acknowledgement number of 29
   5. etc.

Common Protocols

Port 21, FTP
Port 22, SSH
Port 23, Telnet
Port 53, DNS
Port 80, HTTP
Port 443, HTTPS

Metadata

OSI or TCP/IP Layer

CCNA Exam Topic

#extop-1-5

Contributors

Sources

Transmission Control Protocol - Wikipedia