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Transmission Control Protocol (TCP)

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1. Overview and Core Services

The Transmission Control Protocol (TCP) (RFC 793) is the primary connection-oriented, reliable transport protocol of the Internet.

Key Architectural Services:

1. Connection-Oriented: A logical end-to-end connection is established via a Three-Way Handshake before any user data is exchanged.
2. Reliable Byte-Stream Service: Data is treated as an unstructured sequence of continuous bytes. Every transmitted byte is assigned an incremental 32-bit sequence number.
3. Full-Duplex Communication: Data can travel in both directions simultaneously between two established endpoints.
4. End-to-End Flow Control: Prevents a fast sender from overrunning the receiver's memory buffer using a dynamic Advertised Receive Window (rwnd).
5. End-to-End Congestion Control: Dynamically throttles transmission rates to prevent collapse of the intermediate global Internet routers using a Congestion Window (cwnd).

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2. TCP Segment Header Format

The TCP header is between 20 and 60 bytes in length:

Critical Header Fields:

1. Sequence Number (32 bits): Specifies the byte number of the first byte carried in this segment's payload relative to an Initial Sequence Number (ISN).
2. Acknowledgment Number (32 bits): Carries the byte number of the next byte expected from the other party (cumulative acknowledgment).
3. Data Offset / HLEN (4 bits): Specifies the total header size in 4-byte (32-bit) words. Minimum value is $5$ ($5 \times 4 = 20\text{ bytes}$), maximum is $15$ ($15 \times 4 = 60\text{ bytes}$).
4. Control Flags (6 bits):
   • URG: Urgent pointer field is valid (high-priority data).
   • ACK: Acknowledgment field is valid (set on all packets after initial SYN).
   • PSH: Push data immediately to application without waiting to fill buffer.
   • RST: Reset / abort connection due to fatal error or rejected port.
   • SYN: Synchronize sequence numbers during connection establishment.
   • FIN: Finish / graceful connection termination (no more data to send).
5. Window Size (16 bits): Advertises the receiving host's available buffer capacity (rwnd) in bytes for flow control (can be scaled up to 1 GB via Window Scale option).
6. Checksum (16 bits): Covers TCP header, payload, and an IP pseudo-header.

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3. Connection Establishment: The Three-Way Handshake

To prevent stale duplicate connection requests from establishing phantom connections, TCP uses a Three-Way Handshake:

Note: A SYN segment consumes exactly 1 sequence number even though it carries zero payload bytes.

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4. Connection Termination: The Four-Way Handshake

Because TCP is full-duplex, each transmission direction must be shut down independently:

Why the TIME_WAIT State (2 MSL)?

The client must remain in TIME_WAIT for $2 \times \text{MSL}$ (Maximum Segment Lifetime $\approx 2 \text{ minutes}$) to ensure:

1. Its final ACK arrived at the server. If the ACK was lost, the server retransmits FIN, which the client can re-acknowledge.
2. Any delayed duplicate segments wandering through the Internet expire before a new connection reuses the same port numbers.

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5. TCP Congestion Control Algorithms

TCP dynamically adjusts its Congestion Window (cwnd) based on network feedback:

1. Slow Start:
   • Begins with small $cwnd = 1\text{ to }10\text{ MSS}$.
   • For every ACK received, $cwnd$ increases by 1 MSS. The window doubles every Round-Trip Time (RTT) (exponential growth: $1 \rightarrow 2 \rightarrow 4 \rightarrow 8 \dots$).
   • Continues until $cwnd$ reaches the Slow Start Threshold (ssthresh).
2. Congestion Avoidance:
   • Once $cwnd \geq ssthresh$, growth switches to Additive Increase (Linear):
   • $cwnd$ increases by only $+1\text{ MSS}$ per entire RTT, cautiously probing for extra bandwidth without inducing packet loss.
3. Congestion Response:
   • Timeout (Severe Congestion):

(Resets all the way back to Slow Start!)

• Triple duplicate ACKs (Mild Congestion - Fast Retransmit & Fast Recovery):
  • 3 duplicate ACKs indicate 1 packet was dropped, but later packets are still flowing through.
  • The missing packet is immediately retransmitted without waiting for timeout.
  • $ssthresh \leftarrow \frac{cwnd}{2}, \quad cwnd \leftarrow ssthresh + 3\text{ MSS}$ (Fast Recovery skips slow start and resumes linear growth).

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6. Comprehensive Comparison: TCP vs. UDP

Feature	TCP (RFC 793)	UDP (RFC 768)
Connection Type	Connection-Oriented (3-way handshake)	Connectionless (0 setup delay)
Reliability	Guaranteed (ACKs & Retransmissions)	Best-Effort (No ACKs, packets can drop)
Data Flow Unit	Byte Stream (No message boundaries)	Discrete Datagram (Preserves boundaries)
Header Size	$20 - 60\text{ bytes}$	Fixed $8\text{ bytes}$
Flow Control	Yes (Sliding window via rwnd)	None
Congestion Control	Yes (Slow start, AIMD via cwnd)	None (Transmits at application rate)
Transmission Mode	Strictly Unicast (Point-to-Point)	Unicast, Broadcast, and Multicast
Header Checksum	Mandatory	Optional in IPv4, Mandatory in IPv6
Speed / Latency	Higher latency (buffer & retransmission)	Minimal latency
Primary Protocols	HTTP, HTTPS, FTP, SSH, SMTP, BGP	DNS, DHCP, VoIP, TFTP, NTP, SNMP, RTSP

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