Computer Networks and the Internet

Table Of Contents

Terms

• datacenter network
• TCP – Transmission Control Protocol
• IP – Internet Protocol
• ISP – Internet Service Providers
• Packet: data breaks into chunks
• DSL – digital subscriber line
• delay vs. loss
• ourput buffer == output queue
• FDM – frequency-division multiplexing
• Server
• Client
• traffic intensity
• congest → uncontested = no queuing delay
• Throughput
• bottleneck link
• protocol stack
• message 报文
• payload field
• Encapsulation
• OSI – Open Systems Interconnection

What is Internet

Network of networks

Each ISP is a network of packet switches and communication links

递归结构

Components

• hosts / end-system
  • end-system connects with internet through ISP
  • end systems of a computer network are interconnected by a network of communication links and packet switches
• links: wired or wireless
• switches
  • router
  • link-layer switches

Service Description

infrastructure that provides services to applications

usually distributed applications

Protocol

A protocol defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event.

• rules → to control
• order → to action
• principal protocols is TCP/IP

Network Edge

End system == host

Contain clients and servers

Most servers in datacenter

Wireless vs. Wired

Connection:End-systems: application

1. C/S(Client – server): request exchange
2. p2p(Peer – peer): Skype

Access Networks

End-systems connect with each other

• Home Access (Majority 1 & 2 and both asymmetric不对称)
  1. DSL ⇒ telephone
  2. cable ⇒ cable television
  3. FTTH 光纤到户
  4. PON 无源光纤
  5. HFC is used as well
• Enterprise and Home (company)
  1. Ethernet
  2. WiFi
• Wireless

Physical Media 物理媒介

Guided Media vs. Unguided Media

Physical media brings delay

Network Core

2 key functions: routing and forwarding

Packet Switching

• store-and-forward transmission
  • store firstly and then transmit the entire packet together
  • forwarding table – maps destination addresses, formed by routing algorithms
  • router uses the the packet’s destination address and checks its forwarding table to find the appropriate outbound link
• end to end delay
  • dend−to−end=NLR​
  • the bits in packet cannot arrive at the same time → cause a delay (within one packet)
  • 一个分组内到达有延迟
  
• packet loss
  • queuing delay
  • arrive > service
  • buffer completely full
  • 一整个packet到达一个link时 link正在处理传输另一个packet 就导致queuing delay
  • No free buffer → arriving packets dropped→loss
  

Circuit Switching

• mainly used in telephone (traditional digital telephone networks)
• reserves bandwidth in each link along the path
• unused connection causes “wasting”
• before transmitting data between two end systems, the network establishes a dedicated end-to-end connection between the end systems
• oversea / long telephone networks use packet switching instead of tradition circuit

Comparsion

1. packet switching offers better sharing of transmission capacity
2. packet switching is simpler, more efficient, and cost less
3. packet switching is better to handle bursty data

Network of Networks

• organized in a hierarchy
• Tier-n ISPs provide service to tier-(n+1) ISPs
• access ISPs contain residental ISPs, enterprice ISPs, Unverisity ISPs
• employed IP

Delay, Loss, Throughput in packet-switched

throughput 吞吐量

4 types

nodal processing delay

• microseconds or less

1. examine the packet’s header
2. determine where to direct the packet
3. check for bit-level errors

queuing delay

Main

• depend on the number of earlier-arriving packets
• 0 to very long
• microseconds to milliseconds

transmission delay

LR

• 组内延迟 与传输时间无关
  • L 是packet的长度 单位bits
  • R 是链路传输速率 是推出整个分组的速率
• microseconds to milliseconds

propagation delay

ds

• depends on the physical medium
• 2−3×108​ equal to, or a little less than, the speed of light
• 传播延迟 与组内无关 以整体为单位
  • d​ – distance between router
  • s​ – the propagation speed of the link

Packet vs Circuit

• Assuming
  • Total bits – x
  • Total packets – p
  • Transmission rate – b
  • Total link – k
  • Propagation on each link – d
  • time to establish the connection – s
• Packet: 所有分组的传输时间和 + 最后一个分组的等待时间 + 传播时间

xppb+pb(k−1)+dk

• Circuit:

dk+s+xb

traffic intensity

LaR

• a is in units of packets/sec – average rate
• 1 ⇒ arrive > exceed ⇒ queue increases ⇒ delay infinity
• →1 ⇒ delay→1
• →0 ⇒ delay→0

Traceroute

• sends back to the source a short message when arriving at each router
• 向每台router发一个packet
• 时间是往返时延！
• n 可能比 n+1 大

Throughput

FT

• 接收量 / 时间
• For a router, F=min{Rc,Rs}​ ⇒ 类似“关键活动”
• in > out ⇒ backlog bits ⇒ unexpected
• bottleneck link

Conclusion

• Total delay is all this 4 types combined
• processing delay strongly influences a router’s maximum throughput
• Queuing delay is the most decisive
• the buffer of a router is full ⇒ no place to store packet ⇒ packet dropped ⇒ loss
• assuming network uncongested, dend−end=N(dproc+dtrans+dprop)​

Protocol Layers and Their Service Models

Layered architecture

• Each layer provides its service by:
  1. performing certain actions within that layer
  2. using the services of the layer directly below it
• Advantage
  1. the modularity making it much easier to change the implementation of the service provided by the layer
  2. the ability to change the implementation of a service without affecting other components of the system
• Implement of service ≠​ service
• protocol stack
  • application
    • message
    • network applications and their application-layer protocols
    • HTTP, SMTP, FTP
  • transport
    • segment
    • TCP: connection-oriented, congestion-control
    • UDP: connectionless, no reliability, no flow control, no congestion control
    • process to process
  • network
    • datagram
    • IP
    • host to host
  • link
    • frame
    • Ethernet, WiFi, and the cable access network’s DOCSIS protocol
    • node to node
  • physical

• OSI
  • add 2 layer
    • presentation: allow applications to interpret meaning of data 加密压缩 数据格式转换
    • session: synchronization, checkpointing, recovery of data exchange 同步 数据交换
  

Encapsulation

• header fields and a payload field in each layer ( PDU )
• payload field is from previous layer
• 逐层封装

Internet Architecture

• IP over everything, everything over IP
• Defined by IETF
• Features
  1. Does not imply strict layering 不是完全封闭按流程 Application 可以也直接使用其他协议
  2. An hour-glass shape