Chapter 07: Transport - TCP, UDP et QUIC

1. Transport Layer Overview

The transport layer provides end-to-end communication services between applications. Two main protocols:

TCP: Reliable, connection-oriented, ordered delivery
UDP: Unreliable, connectionless, low overhead

Diagramme en cours de génération...

2. TCP (Transmission Control Protocol)

2.1 TCP Header

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TCP Flags:

Flag	Bit	Meaning
NS	0	ECN-nonce concealment
CWR	1	Congestion Window Reduced
ECE	2	ECN Echo
URG	3	Urgent pointer
ACK	4	Acknowledgment
PSH	5	Push data
RST	6	Reset connection
SYN	7	Synchronize (connection establishment)
FIN	8	Finish (connection termination)

2.2 TCP 3-Way Handshake

Diagramme en cours de génération...

Sequence numbers:

Initial Sequence Number (ISN) is random
SYN consumes one sequence number
Each byte of data consumes a sequence number
ACK = next expected sequence number

2.3 TCP Connection Termination

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2.4 TCP State Machine

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2.5 TCP Sliding Window

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Window Scaling (RFC 7323):

Window field is 16 bits (max 65535 bytes)
Scaling factor shifts the window value
Scaling factor negotiated during handshake

2.6 TCP Congestion Control

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TCP Reno Phases:

Slow Start: Exponential growth (cwnd doubles per RTT)
Congestion Avoidance: Linear growth (cwnd += 1 MSS per RTT)
Fast Retransmit: 3 duplicate ACKs → retransmit, cwnd = cwnd/2
Fast Recovery: cwnd halved, then linear growth

Diagramme en cours de génération...

BBR (Bottleneck Bandwidth and Round-trip propagation time):

Model-based (not loss-based)
Probes for available bandwidth
Much higher throughput than Cubic
Google's algorithm, used in YouTube, QUIC

CUBIC:

Default Linux TCP congestion control
Cube function for window growth
More aggressive than Reno after window reduction
Better for high-BDP links

2.7 TCP Options

Option	Kind	Description
EOL	0	End of options
NOP	1	No operation (padding)
MSS	2	Maximum Segment Size
WS	3	Window Scale
SACK Permitted	4	Selective ACK allowed
SACK	5	Selective ACK
TS	8	Timestamp
Fast Open	34	TCP Fast Open (TFO)

TCP Fast Open (TFO):

Send data in SYN packet (0-RTT handshake)
Requires TFO cookie
Reduces latency for repeat connections

Selective ACK (SACK):

Acknowledge non-contiguous data
More efficient than cumulative ACK
Especially important for high-BDP links

2.8 TCP Timers

Timer	Purpose	Value
Retransmission (RTO)	Timeout for unacked data	Based on RTT (usually 200ms-1s)
Persist	Avoid deadlock (zero window)	Increasing, up to 60s
Keepalive	Check if peer is alive	2 hours + 9 probes × 75s
TIME_WAIT	2MSL wait	60s (typical)
Delayed ACK	Wait for data piggyback	40-200ms

3. UDP (User Datagram Protocol)

3.1 UDP Header

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Header size: 8 bytes (versus 20-60 for TCP)

3.2 UDP Characteristics

Feature	UDP	TCP
Connection	Connectionless	Connection-oriented
Reliability	None (best effort)	Reliable (retransmission)
Ordering	No ordering	Ordered delivery
Flow control	None	Sliding window
Congestion control	None	Reno, Cubic, BBR
Header size	8 bytes	20-60 bytes
Overhead	Low	High
Use cases	DNS, VoIP, streaming	HTTP, email, file transfer

3.3 UDP Use Cases

Application	Port	Why UDP
DNS	53	Single query/response, fast
DHCP	67/68	Broadcast, no connection
NTP	123	Time sync, precise timing
VoIP (RTP)	16384-32767	Real-time, tolerate loss
Streaming	Dynamic	Real-time, loss tolerant
SNMP	161/162	Simple query/response
QUIC	443 (via UDP)	Multiplexed, reliable over UDP
Syslog	514	Logging, loss tolerant

4. QUIC (Quick UDP Internet Connections)

4.1 QUIC Overview

QUIC (RFC 9000) is a transport protocol developed by Google, used by HTTP/3.

Diagramme en cours de génération...

4.2 QUIC Features

Feature	Description
0-RTT	Send data immediately (repeat connections)
Multiplexing	Multiple streams, no HOL blocking
Connection migration	Survive IP address changes
Built-in encryption	TLS 1.3 mandatory
Monolithic	Combines transport + security
User-space	Implemented in applications, not kernel

4.3 QUIC vs TCP

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4.4 Connection Migration

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5. SCTP (Stream Control Transmission Protocol)

Key features:

Multi-homing (multiple IPs per connection)
Multi-streaming (multiple streams in one connection)
Ordered and unordered delivery
Built-in heartbeat

Used primarily in telecom (SS7 over IP) and WebRTC (data channels).

6. Socket Programming

6.1 TCP Socket Flow

Diagramme en cours de génération...

6.2 Python TCP Example

Server:

import socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind(('0.0.0.0', 8888))
s.listen(5)
while True:
    conn, addr = s.accept()
    data = conn.recv(1024)
    conn.send(data)  # echo
    conn.close()

Client:

import socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(('127.0.0.1', 8888))
s.send(b'Hello')
data = s.recv(1024)
s.close()

7. Summary

Feature	TCP	UDP	QUIC
Transport	TCP	UDP	QUIC over UDP
Reliability	Yes	No	Yes
Encryption	Optional (TLS)	No (DTLS)	Mandatory (TLS 1.3)
Multiplexing	No (HTTP/2 fixes)	No	Yes
0-RTT	No (TFO partial)	N/A	Yes
Connection migration	No	N/A	Yes
Head-of-line blocking	Yes (TCP)	No	No
Kernel	Yes	Yes	User-space

8. Exercises

Exercise 1: TCP Handshake

Given a TCP connection with ISN=1000 (client) and ISN=5000 (server), write the sequence/ack numbers for each step of the 3-way handshake.

Exercise 2: Window Calculation

A TCP connection has Bandwidth=10 Gbps, RTT=50 ms. What is the minimum window size needed to fill the pipe? What window scale factor is needed?

Exercise 3: QUIC vs TCP

List 3 advantages of QUIC over TCP.

Exercise 4: UDP Analysis

An application sends 100-byte messages every 10ms. Compare the overhead (header proportion) between TCP and UDP.

Exercise 5: Congestion Control

Describe how TCP Reno responds to a single packet loss in congestion avoidance.