[[Networking]] [[TCP Flow, Error and Congestion Control]] [[TCP Options]]

Overview

• Layer 4 protocol used to transport data
• One of the most common protocols
• Connection oriented
• Sending host numbers packets and sets a timer at send time
• Receiving host reorders and accounts for all of the packets
• Every time a packet is sent there is an acknowledgement (ACK)
• If a packets timer expires before the ACK is complete, it is reset

TCP Header Makeup

• 20-60 Bytes in length (20 = default)
• Source port - 16 bits
• Destination port - 16 bits
• Sequence number - 32 bits
• Acknowledgement number - 32 bits
• Header Length - 4 bits
• Reserved, not used - 6 bits
• FLAGS there is alot of them - 6 bits
• Window size - 16 bits
• Checksum -16 bits
• Urgent pointer - 16 bits

Control Field

• A set amount of bits within the TCP header that control what happens with it
• HLEN - Reserved bits - Flags

TCP Flags

• URG: Urgent pointer is valid
• ACK: Acknowledgement is valid
• PSH: Request for push up to next layer
• RST: Restart connection
• SYN: Initiates connection and sentimentalizes numbers
• FIN: Connection is complete

3-Way Handshake

• SYN: Cannot carry data
• SYN + ACK: cannot carry data but consumes sequence number
• ACK: Can carry data; if no data, consumes no sequence number
• In order for a TCP connection, there must be a session acknowledgement
• Essentially two one way connections communicating together
• Basic Steps
  • Initiator sends an empty packet (SYN = 1)
  • Recipient responds with SYN and ACK set to 1
  • Initiator responds with ACK set to 1
  • The connection is now made
• A bit more in depth (Ex: Host A and Host B)
  • When handshake begins, Host A SYN is given a sequence number
  • When Host B receives Host A’s packet, Host B sends a SYN/ACK with a new SEQ# and an ACK number that is an increment of the original SYN
  • Host A then sends back an ACK with an ACK# of the incremented ACK# from Host B
  • Each host basically acknowledges each others “unique” values. Each SEQ# is tied to the other sides ACK#

Window Size & Sliding Window

• Window size is just the amount of data that can be processed at a time
• Both sides of the handshake have their own window size, however the window size will only change when data is received by a host
• Host advertises its window size in TCP header
  • This tells the other host the max buffer space available
• When data transfer starts host updates other host on available space that can be taken
• Processed data sits in buffer and waits until it can be pushed up the stack
• When the window is full, the recipient uses PSH flags to move packets up a layer
• Once window is empty, more data can be received and process.
• In order to not become overwhelmed, window size will be set to 0 when buffer reaches capacity

Teardown Sequence

• When communication is complete, the connection needs to be terminated in the same way that it was initiated.
• Initiator send a FIN flag
• Receiver replies with an ACK and a FIN back
• Initiator ACK’s the receivers FIN and the connection is closed