2 Sockets

Table of Contents

• What are Sockets?

  • Two Types of Internet sockets

  • Network Theory

• Footnotes

What are Sockets?

Sockets are a way to communicate with other programs using standard UNIX file(everything in UNIX is file, so it doesn't mean much?) descriptors.Unix programs, to do any sort of I/O, read or write to a file descriptor¹.\\ */How to get file descriptor for network communication?/*\\ Simply make a call to the socket() system routine, to get socket descriptor. We can communicate through it using =send()=(man send) and =recv()=(man receive).\\ There are multiple kinds of socket for example DARPA internet addresses(internet sockets), path names on a local node/machine(Unix sockets) etc. We will talk about internet sockets.

Two Types of Internet sockets

"Stream Sockets"(SOCK_STEAM) and "Datagram Sockets"(SOCK_DGRAM) or commonly called as "connectionless sockets".\\ Steam Sockets are a two-way connected communication steams(you [give 1,2] you will receive [1,2], error free). Telnet(tcp part of tcp/ip, ip deals with routing and not data integrity) Uses it.\\ How do stream sockets achieve this high level of data transmission quality? They use a protocol called "The Transmission Control Protocol".\\ *Datagram Sockets*(also use IP for routing but not TCP). Used in tftp, dhcpcd(DHCP client), online gaming, video/audio call etc.If you send a datagram, it may arrive. It may arrive out of order. If it arrives, the data within the packet will be error-free.

Network Theory

Data Encapsulation ,basically, it says this: a packet is born, the packet is wrapped ("encapsulated") in a header (and rarely a footer) by the first protocol (say, the TFTP protocol), then the whole thing (TFTP header included) is encapsulated again by the next protocol (say, UDP), then again by the next (IP), then again by the final protocol on the hardware (physical) layer (say, Ethernet).\\

it means, When another computer receives the packet, the hardware strips the Ethernet header, the kernel strips the IP and UDP headers, the TFTP program strips the TFTP header, and it finally has the data.

Now layer network model describes a system of network functionality that has advantages over other models for instance socket programs that work without caring about how data is physically transmitted(because programs on lower level deal with that for us, so kernel maybe?).\\

Layers of the full-blown model. Remember this for network class exams:

• Application User interaction happens here

• Presentation

• Session

• Transport

• Network

• Data Link

• Physical well, the hardware level, your ethernet, serial etc.

A layered model more consistent with Unix might be:

• Application Layer (telnet, ftp, etc.)

• Presentation Layer

• Session layer

• Host-to-Host Transport Layer (TCP, UDP)

• /Internet Layer/(Network Layer) (IP and routing)

• Data Link layer

• /Network Access Layer/(Physical) (Ethernet, wi-fi, or whatever)

Now to build a simple packet(send data out) we simply need to use =send()=(in case of stream) or =sendto()=(in case of datagram) and as mentioned above, kernel does the rest of the work.

Footnotes: