4. Transport Layer

Module 4· 5 min read· 10 Questions·In Progress

TCP, UDP, flow control, congestion control, and ports

Layer 4 - End-to-end reliable data transfer

What You'll Learn#

Understand TCP vs UDP differences
Learn TCP connection management
Master flow and congestion control
Know common port numbers

1. Transport Layer Functions#

Function	Description
Segmentation	Break data into segments
Multiplexing	Multiple apps share network
Port Addressing	Identify applications
Flow Control	Prevent receiver overflow
Error Control	Reliable delivery (TCP)

2. TCP vs UDP#

Comparison#

Feature	TCP	UDP
Connection	Connection-oriented	Connectionless
Reliability	Reliable (ACKs, retransmit)	Unreliable
Ordering	Guaranteed order	No ordering
Speed	Slower (overhead)	Faster
Header	20-60 bytes	8 bytes
Use Case	Web, Email, File transfer	Streaming, DNS, Gaming

When to Use#

Diagram / Text

TCP: When data integrity matters
- Web browsing (HTTP)
- Email (SMTP)
- File transfer (FTP)
- Database transactions

UDP: When speed matters
- Video streaming
- Online gaming
- DNS queries
- VoIP calls

3. TCP Header#

Diagram / Text

0                   1                   2                   3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
┌───────────────────────────────┬───────────────────────────────┐
│         Source Port           │       Destination Port        │
├───────────────────────────────┴───────────────────────────────┤
│                      Sequence Number                          │
├───────────────────────────────────────────────────────────────┤
│                   Acknowledgment Number                       │
├───────┬───────┬─┬─┬─┬─┬─┬─┬───────────────────────────────────┤
│ Offset│Reserved│U│A│P│R│S│F│           Window Size            │
├───────┴───────┴─┴─┴─┴─┴─┴─┴───────────────────────────────────┤
│           Checksum            │        Urgent Pointer         │
└───────────────────────────────┴───────────────────────────────┘

Flags: URG, ACK, PSH, RST, SYN, FIN

TCP Flags#

Flag	Name	Purpose
SYN	Synchronize	Connection establishment
ACK	Acknowledge	Acknowledge received data
FIN	Finish	Connection termination
RST	Reset	Abort connection
PSH	Push	Send immediately
URG	Urgent	Priority data

4. TCP Connection Management#

Three-Way Handshake (Connection Establishment)#

Diagram / Text

Client                     Server
   |                          |
   |------- SYN -------->     |  (seq=x)
   |                          |
   |<---- SYN + ACK ----      |  (seq=y, ack=x+1)
   |                          |
   |------- ACK -------->     |  (ack=y+1)
   |                          |
   |   CONNECTION OPEN        |

Four-Way Handshake (Connection Termination)#

Diagram / Text

Client                     Server
   |                          |
   |------- FIN -------->     |  Client wants to close
   |                          |
   |<------ ACK -------       |  Server acknowledges
   |                          |
   |<------ FIN -------       |  Server ready to close
   |                          |
   |------- ACK -------->     |  Client acknowledges
   |                          |
   |   CONNECTION CLOSED      |

5. Flow Control#

Prevent sender from overwhelming receiver

Sliding Window#

Diagram / Text

Sender Window = min(Receiver Window, Congestion Window)

Receiver advertises its buffer space in Window Size field
Sender adjusts sending rate accordingly

Example:
Receiver says: "Window = 4000 bytes"
Sender can send 4000 bytes before waiting for ACK

6. Congestion Control#

Prevent sender from overwhelming network

TCP Congestion Algorithms#

Diagram / Text

1. Slow Start
   - Start with cwnd = 1 MSS
   - Double cwnd each RTT (exponential)
   - Until threshold (ssthresh)

2. Congestion Avoidance
   - After threshold, increase cwnd by 1/RTT (linear)
   - Gentle growth to probe capacity

3. Fast Retransmit
   - 3 duplicate ACKs = packet loss
   - Retransmit without waiting for timeout

4. Fast Recovery
   - After fast retransmit
   - cwnd = ssthresh + 3 MSS
   - Avoid slow start

Congestion Window Growth#

Diagram / Text

cwnd                     
  |                   ***
  |              ****   |  congestion avoidance
  |         ****        |  (linear)
  |    ****-------------|---- ssthresh
  |  **                 |
  | *                   |
  |*  slow start        |
  |   (exponential)     |
  +---------------------+---> Time

7. Port Numbers#

Well-Known Ports (0-1023)#

Port	Protocol	Service
20	TCP	FTP Data
21	TCP	FTP Control
22	TCP	SSH
23	TCP	Telnet
25	TCP	SMTP
53	TCP/UDP	DNS
80	TCP	HTTP
110	TCP	POP3
143	TCP	IMAP
443	TCP	HTTPS

Port Ranges#

Diagram / Text

0-1023:     Well-known (system)
1024-49151: Registered (applications)
49152-65535: Dynamic/Private (ephemeral)

8. UDP Header#

Diagram / Text

0                   1                   2                   3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
┌───────────────────────────────┬───────────────────────────────┐
│         Source Port           │       Destination Port        │
├───────────────────────────────┼───────────────────────────────┤
│            Length             │           Checksum            │
└───────────────────────────────┴───────────────────────────────┘

Only 8 bytes! (vs TCP's 20+ bytes)

Key Takeaways#

Concept	Key Point
TCP	Reliable, connection-oriented, slower
UDP	Fast, connectionless, unreliable
3-way handshake	SYN → SYN-ACK → ACK
4-way handshake	FIN → ACK → FIN → ACK
Flow Control	Receiver's window size
Congestion Control	Slow start → congestion avoidance
HTTP Port	80 (HTTP), 443 (HTTPS)

Practice Quiz

Test your understanding with step-by-step solutions

Practice Quiz

10 questions · 90s per question

Each question has a 90-second time limit. Unanswered questions will be auto-submitted when time runs out.

Module 4· 5 min read· 10 Questions·In Progress