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NOTE

7.1 Computer Network

Historical overview of OSI, TCP/IP layers, representative protocols, and the network transmission flow.

Computer NetworksCreated Updated 2 min readhistorical

This is a historical learning note and may contain outdated or incomplete understanding.

1. OSI Seven-Layer Architecture

From bottom to top:

  • Physical layer
    Transmits bit streams over transmission media.

  • Data-link layer
    Transmits frames between adjacent machine nodes.

  • Network layer
    Transmits data from host to host across a network.

  • Transport layer
    Transmits data from process to process on hosts.

  • Session layer
    Establishes, manages, and terminates sessions.

  • Presentation layer
    Performs data conversion, encryption, and compression.

  • Application layer
    Users access network resources through network applications.

2. TCP/IP Four-Layer Protocol Architecture

From bottom to top:

2.1. Seven Layers vs. Four Layers

  • The data-link layer in the four-layer model corresponds to the physical layer + data-link layer.
  • The application layer in the four-layer model corresponds to the session layer + presentation layer + application layer.

2.2. Typical Protocols

3. Network Transmission Flow

3.1. Application Layer

  • The application layer belongs to user space.
  • Running exec 8<> /dev/tcp/www.baidu.com/80 creates a new fd under /proc/$$/fd; fd 8 points to a socket.
  • Then echo -e "GET /HTTP/1.0\n" 1>& 8 can send data to Baidu.
  • Finally, cat 0<& 8 receives Baidu’s data.

3.2. Transport Layer

  • The transport layer and the layers below it belong to kernel space.
  • The transport layer first establishes a connection and needs to disconnect after data transmission. Connection establishment uses a three-way handshake and disconnection uses a four-way teardown. tcpdump -nn -i eth0 port 80 can be used to capture packets.
  • Established connections can be viewed with netstat -antp. From this command it can also be seen that generally one process, or thread, corresponds to one connection. Can one process correspond to multiple connections? Yes, using I/O multiplexing.
  • A socket is composed of (IP1, Port1, IP2, Port2). Then a question arises: if 127.0.0.1:5555 has established a connection with www.baidu.com:80, can 127.0.0.1:5555 also establish a connection with www.google.com:80? Yes.

3.3. Network Layer

  • Network configuration can be performed through vim /etc/sysconfig/network-scripts/ifcfg-eth0. Four important dimensions are:
IP address
Subnet mask
Gateway
DNS
  • These four dimensions can be observed in the routing table printed by route -n. The routing table solves the problem of where the next hop goes.
  • Whether ping succeeds is also determined through the routing table. First, bitwise-AND the destination IP with Genmask; if the result equals Destination, the packet can be sent to Gateway.
  • Packet transmission within a LAN is implemented through the ARP table, which can be printed with arp -a.

4. References

Discussion

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