Summary of Key Points

1. Network Layer

Network Layer Functions

  • Transports data from sending to receiving host
  • Encapsulates data into datagrams on sending side; delivers to transport layer on receiving side
  • Network layer protocols exist in every host and router
  • Additional Insight: The network layer is responsible for logical addressing (IP addresses) and path determination across multiple networks

Two Key Functions

  • Forwarding: Moves datagrams from router’s input to appropriate output (local action)
  • Routing: Determines route taken by datagrams using routing algorithms (RIP, OSPF, BGP) - global action
  • Practical Example: Think of forwarding as driving through each intersection (router), while routing is like planning your entire trip using GPS navigation

Forwarding Tables & Longest Prefix Matching

  • Routers forward packets by examining header and consulting forwarding table
  • Longest prefix matching: When multiple prefixes match, use the longest (most specific) one
  • Example: If table has entries for 192.168.1.0/24 and 192.168.1.128/25, and packet destination is 192.168.1.130, the /25 route is chosen

Network Service Models

  • Internet (Best Effort): No bandwidth, loss, order, or timing guarantees
  • ATM CBR: Constant rate, guarantees loss/order/timing
  • ATM VBR/ABR/UBR: Various levels of bandwidth and delivery guarantees
  • Modern Context: Most Internet applications are designed around best-effort service, implementing reliability at higher layers when needed

2. Virtual Circuits vs Datagrams

Feature Virtual Circuits Datagrams
Connection Setup Required Not required
Addressing VC identifier Destination IP address
State Maintenance Per-connection state No connection state
Resource Allocation Possible Not allocated
Reliability Built-in Provided by higher layers
Examples ATM, MPLS, X.25 Internet IP

Key Insight: The Internet chose datagrams for robustness and simplicity, while telephone networks preferred virtual circuits for predictable service quality.

3. Router Architecture

Components

  • Control Plane: Runs routing algorithms (software) - “the brain”
  • Data Plane: Input ports, switch fabric, output ports (hardware) - “the muscle”

Switching Techniques

  • Bus: Simple but limited by bus bandwidth
  • Memory: Flexible but speed limited by memory bandwidth
  • Crossbar: High performance but complex and expensive
  • Modern Evolution: Many routers now use network processors and programmable ASICs for better performance and flexibility

Software-Defined Networking (SDN)

  • Separates control plane from data plane
  • Enables centralized network management and programmability
  • Real-world Impact: Cloud providers like Google and Amazon use SDN to manage their massive networks efficiently

4. IP Protocol

IPv4 Datagram Format

  • Minimum 20-byte header, maximum 60 bytes with options
  • Critical fields: TTL (prevents infinite loops), Protocol (identifies upper layer), Flags & Fragment Offset (for fragmentation)
  • Header Checksum: Provides error detection only for the header, not data

IPv4 Fragmentation & Reassembly

  • Fragmentation can occur at any router when packet exceeds MTU
  • Reassembly occurs only at final destination
  • MTU Discovery: Modern systems often use Path MTU Discovery to avoid fragmentation

IP Addressing & Subnets

  • Classful Addressing: Obsolete system (Class A, B, C)
  • CIDR: Modern flexible addressing a.b.c.d/x
  • Private Ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16
  • Special Addresses: 127.0.0.1 (loopback), 255.255.255.255 (broadcast)

DHCP (Dynamic Host Configuration Protocol)

  • Four-step process: Discover → Offer → Request → ACK
  • DORA Process: Discover, Offer, Request, Acknowledge
  • Lease Time: Addresses are allocated for limited time, then renewed
  • Relay Agents: Enable DHCP across multiple subnets

NAT (Network Address Translation)

  • Basic NAT: One-to-one IP mapping
  • NAPT (Network Address Port Translation): Most common, maps multiple private IPs to one public IP using ports
  • NAT Traversal: Techniques like STUN, TURN, ICE help P2P applications work through NAT
  • Controversy: NAT breaks the end-to-end principle but enabled Internet scaling

5. IPv6

Key Features

  • 128-bit addresses (3.4×10³⁸ addresses vs IPv4’s 4.3×10⁹)
  • Address Notation: 2001:0db8:85a3:0000:0000:8a2e:0370:7334 or compressed 2001:db8:85a3::8a2e:370:7334
  • Stateless Address Autoconfiguration (SLAAC): Devices can self-configure addresses
  • Built-in IPsec: Mandatory support for security

IPv6 vs IPv4 Comparison

Aspect IPv4 IPv6
Address Size 32 bits 128 bits
Header Size 20-60 bytes Fixed 40 bytes
Fragmentation Routers can fragment Only by source
Checksum Header checksum No checksum
Configuration Manual/DHCP SLAAC/DHCPv6

Transition Mechanisms

  • Dual Stack: Devices run both IPv4 and IPv6
  • Tunneling: IPv6 packets encapsulated in IPv4
  • Translation: NAT64, 6to4, Teredo
  • Current Status: ~45% of Google users access via IPv6 (as of 2024)

6. Transport Layer

Services & Protocols

  • TCP: Reliable, connection-oriented, flow and congestion controlled
  • UDP: Unreliable, connectionless, minimal overhead
  • SCTP: Reliable, message-oriented, multi-homing support

Socket Programming

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# Basic TCP client example
import socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(('hostname', 80))
s.send(b'GET / HTTP/1.0\r\n\r\n')
data = s.recv(1024)
s.close()

Port Categories

  • Well-known: 0-1023 (HTTP:80, HTTPS:443, SSH:22)
  • Registered: 1024-49151
  • Dynamic/Private: 49152-65535

7. UDP (User Datagram Protocol)

Header Structure

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 0      7 8     15 16    23 24    31
+--------+--------+--------+--------+
|     Source      |   Destination   |
|      Port       |      Port       |
+--------+--------+--------+--------+
|     Length      |    Checksum     |
+--------+--------+--------+--------+
|          data octets ...          |
+-----------------------------------+

Common UDP Applications

  • DNS: Domain name resolution
  • DHCP: Dynamic host configuration
  • SNMP: Network management
  • RTP: Real-time media transport
  • QUIC: Modern transport protocol

UDP Advantages in Real-time Applications

  • No retransmission delays
  • No congestion control (can transmit at constant rate)
  • Lower header overhead (8 bytes vs TCP’s 20+ bytes)

8. Reliable Data Transfer (RDT)

Protocol Evolution Summary

Version Channel Model Key Mechanisms
Rdt1.0 Perfect None needed
Rdt2.0 Bit errors Checksums, ACK/NAK
Rdt2.1 Corrupted ACK/NAK Sequence numbers
Rdt2.2 NAK-free ACK with sequence numbers
Rdt3.0 Lossy channel Timers, retransmission

Performance Analysis

  • Stop-and-Wait Efficiency: ( U = \frac{L/R}{RTT + L/R} )
  • Pipelining Improvement: With window size N, efficiency improves by factor of N
  • Example: 1Gbps link, 1500-byte packets, 30ms RTT → Stop-and-wait efficiency ≈ 0.04%

9. TCP (Transmission Control Protocol)

TCP Header Deep Dive

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 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                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Data |           |U|A|P|R|S|F|                               |
| Offset| Reserved  |R|C|S|S|Y|I|            Window             |
|       |           |G|K|H|T|N|N|                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Checksum            |         Urgent Pointer        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Options                    |    Padding    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             data                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

TCP Congestion Control States

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Slow Start → Congestion Avoidance → Fast Recovery
    ↑               ↑                   ↑
    └── Timeout ────┘                   │
    └───────── 3 Duplicate ACKs ────────┘

Advanced TCP Features

  • Selective Acknowledgments (SACK): Acknowledge non-contiguous blocks
  • Window Scaling: Support for windows larger than 64KB
  • Timestamps: Better RTT estimation and protection against wrapped sequences
  • Nagle’s Algorithm: Reduces small packets by buffering data

10. Application Layer

HTTP Protocol Evolution Timeline

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HTTP/0.9 (1991) → HTTP/1.0 (1996) → HTTP/1.1 (1997) → HTTP/2 (2015) → HTTP/3 (2022)

HTTP/2 Key Features

  • Binary Framing: Replaces text-based protocol
  • Multiplexing: Multiple requests/responses interleaved
  • Header Compression: HPACK reduces overhead
  • Server Push: Proactive resource sending
  • Stream Prioritization: Better resource loading order

HTTP/3 and QUIC

  • Transport: UDP instead of TCP
  • Encryption: TLS 1.3 built-in
  • Connection Migration: Seamless IP address changes
  • Independent Streams: No head-of-line blocking

DNS Record Types

  • A: IPv4 address
  • AAAA: IPv6 address
  • CNAME: Canonical name (alias)
  • MX: Mail exchange
  • NS: Name server
  • TXT: Text records
  • SRV: Service location

CDN Architecture Types

  • Push CDNs: Content proactively distributed to edges
  • Pull CDNs: Content fetched from origin on demand
  • Hybrid: Combination of both approaches

11. Queuing Theory

Key Performance Metrics

  • Queue Length ((N_q)): Average number waiting
  • System Length ((N)): Average total in system
  • Waiting Time ((W)): Average time in queue
  • System Time ((T)): Average total time
  • Utilization ((ρ)): Server busy probability

M/M/1 Queue Formulas

  • Stability Condition: ( ρ = λ/μ < 1 )
  • Probability of n jobs: ( P_n = (1-ρ)ρ^n )
  • Average jobs in system: ( N = ρ/(1-ρ) )
  • Average response time: ( T = 1/(μ-λ) )

Real-world Applications

  • Web Servers: Request processing queues
  • Routers: Packet buffering
  • Call Centers: Customer waiting times
  • Manufacturing: Production line optimization

Advanced Queue Models

  • M/M/c: Multiple servers
  • M/G/1: General service distribution
  • G/G/1: General arrival and service
  • Priority Queues: Different customer classes

Network Programmability

  • P4: Programming Protocol-independent Packet Processors
  • eBPF: Extended Berkeley Packet Filter for kernel networking
  • Intent-Based Networking: Declarative network configuration

Security Evolution

  • Zero Trust Architecture: “Never trust, always verify”
  • Encrypted DNS: DoH (DNS over HTTPS), DoT (DNS over TLS)
  • Post-Quantum Cryptography: Preparing for quantum computing threats

Performance Innovations

  • Multipath TCP: Using multiple paths simultaneously
  • BBR Congestion Control: Model-based instead of loss-based
  • Time-Sensitive Networking: Deterministic performance for critical applications

说些什么吧！

giscus