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Internet Protocols-1.5 protocol layers, service models

分类: 文章 • 2025-04-23 19:57:34

1.5 protocol layers, service models

Protocol “layers”

  • hosts
  • routers
  • links of various media
  • applications
  • protocols
  • hardware, software

Layers: each layer implements a service
▪ via its own internal-layer actions
▪ relying on services provided by layer below

分层的结果：每层实现一个服务。服务通过内层的行动来实现，同时依赖着下一层提供的服务。

1.5.1 Why layering?

dealing with complex systems:
▪ explicit structure allows identification, relationship of complex system’s pieces

  • layered reference model for discussion

▪ modularization eases maintenance, updating of system

  • change of implementation of layer’s service transparent to rest of system
  • e.g., change in gate procedure doesn’t affect rest of system

▪ layering considered harmful?

1.5.2 TCP/IP model (internet model)

TCP/IP传输协议，即传输控制/网络协议，它是在网络的使用中的最基本的通信协议。TCP/IP传输协议对互联网中各部分进行通信的标准和方法进行了规定。并且，TCP/IP传输协议是保证网络数据信息及时、完整传输的两个重要的协议。

Internet Protocols-1.5 protocol layers, service models

layer 层及功能
application 应用层: supporting network applications ( FTP, SMTP, HTTP)
transport 传输层: process-process data transfer (TCP, UDP)
network 网络层: routing of datagrams from source to destination ( IP, routing protocols)
link 链路层: data transfer between neighboring network elements ( Ethernet, 802.111 (WiFi), PPP)
physical 物理层: bits “on the wire”

1.5.3 ISO/OSI reference model

ISO = International Standards Office
OSI = Open Systems Interconnection
Internet Protocols-1.5 protocol layers, service models
多了两层:
presentation: allow applications to interpret meaning of data, e.g., encryption, compression, machine- specific conventions
session: synchronization, checkpointing, recovery of data exchange

1.5.4 Layering and headers

Most layers of the TCP/IP model are associated with a particular type of “header” or sometimes a “header and trailer”.

What is a header?

  • Information separate from the data being sent that says things about that data.

  • Which computer is it being sent to?

  • Which program on that computer must receive it?

  • How long is this data?

    For example layer 3 (network) has a “network address” which identifies the host that should receive the data.
    Layer 4 (transport) has a port that identifies which program should receive it.
    At each lower layer a new header is added incorporating the headers underneath. A layer 2 packet has a layer 2 header but includes headers from layer 3 and 4

关于封装

Internet Protocols-1.5 protocol layers, service models

Internet Protocols-1.5 protocol layers, service models

1.5.5 TCP/IP layers

  • Layer 7 – Application layer – this is the data for programs you use on your computer
    HTTP (www) data, SMTP (sent emails), FTP (file transfer) and specific formats for games, torrent etc.

  • Layer 6 – Presentation layer
    Related to character sets and presentation of data (unused in TCP/IP or real Internet)

  • Layer 5 – Session layer
    Related to whole lifetime of connection – is the connection real time (unused in TCP/IP or real Internet)

  • Layer 4 – Transport layer – this is for the end-to-end connection
    between machines.
    • Information related to reliability
    • Information related to which program on machine sent/receives data.

  • Layer 3 – Network (Internet) layer – this is to get the data the
    whole journey from its start computer to its end computer (Internet, between networks)
    • Address of computer on internet (IP address)
    • Checksum to see if data is corrupted

  • Layer 2 – Data link layer – this is to get the data to nearby computers (on same local network)
    • Media Access Control (MAC address) specific to individual computer (see later lectures)

  • Layer 1 – Physical layer (how bits 1s and 0s are actually transmitted)
    • Think of cables in the ground or radio waves in the air

1.5.5 Devices for layers

Router
• This is a layer 3 device – it reads a layer 3 address and works out which direction a packet should go.
• It is typically more complex and adaptive than a switch.

Switch
• This is a layer 2 device – it reads a layer 2 address and works out which nearby computer should get a message.
• Typically simpler than a router.

Repeater
• This is a layer 1 device – it strengthens or reconstructs a
corrupted signal and carries on sending it.

1.5.6 ISO/OSI (left) vs TCP/IP (right)

Why two models of layers?

  • International Standards Office/Open Systems Interconnection model planned by committee.
    • Planning took a long time.
    • Model is idealized.

  • Transmission Control Protocol/Internet Protocol built by engineers
    • Built up over time to “get things working”.
    • New applications and changes to protocols through experience.

When ISO/OSI design completed TCP/IP already “too big to change”.
• Would it be better if we had ISO/OSI?
• Session layer presentation layer useful but don’t exist today.
• Perhaps but we can’t get there from where we are.

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