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TCP Read Problem - a Summary

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TCP Read is a problem in itself whether it's a blocking or non-blocking IO. Problem with blocking is well known, therefore for scaling default choice is non-blocking read.

In non-blocking, it's advisable to do read in a while loop till read/recv returns - less than what you asked for or set errno EAGAIN (specially for edge triggered case).

Here one important decision point is how much to read per recv call (recv call's buffer size).

There is one more way of reading data from TCP socket which can be beneficial in both blocking and non-blocking IO, which is knowing the data pending in TCP recvbuffer before making a recv/read call -

size_t buffer_size;
ioctl(fd, FIONREAD, &buffer_size);

from ioctl man page:

FIONREAD int - Get the number of bytes that are immediately available for reading.

and then doing read.

posted Sep 19, 2014 by Sumit Jindal

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TCP/IP means Transmission Control Protocol and Internet Protocol. It is the network model used in the current Internet architecture as well.

Protocols are set of rules which govern every possible communication over a network. These protocols describe the movement of data between the source and destination or the internet. These protocols offer simple naming and addressing schemes.

The TCP/IP Model was developed by Department of Defence's Project Research Agency (ARPA, later DARPA) as a part of a research project of network interconnection to connect remote machines.

The features that stood out during the research, which led to making the TCP/IP reference model were:

  • Support for a flexible architecture. Adding more machines to a network was easy.
  • The network was robust, and connections remained intact untill the source and destination machines were functioning.

The overall idea was to allow one application on one computer to talk to(send data packets) another application running on different computer.

 

TCI/IP model consists of 4 different layers:

  1. Network Access Layer.
  2. Internet Layer.
  3. Transport Layer.
  4. Application Layer.

 

LEVEL 1

Network Access Layer is the first layer of the four layer TCP/IP model. Network Access Layer defines details of how data is physically sent through the network, including how bits are electrically or optically signaled by hardware devices that interface directly with a network medium, such as coaxial cable, optical fiber, or twisted pair copper wire.

The protocols included in Network Access Layer are Ethernet, Token Ring, FDDI, X.25, Frame Relay etc.

The most popular LAN architecture among those listed above is Ethernet. Ethernet uses an Access Method called CSMA/CD (Carrier Sense Multiple Access/Collision Detection) to access the media, when Ethernet operates in a shared media. An Access Method determines how a host will place data on the medium.

LEVEL 2

Internet Layer is the second layer of the four layer TCP/IP model. The position of Internet layer is between Network Access Layer and Transport layer. Internet layer pack data into data packets known as IP datagrams, which contain source and destination address (logical address or IP address) information that is used to forward the datagrams between hosts and across networks. The Internet layer is also responsible for routing of IP datagrams.

Packet switching network depends upon a connectionless internetwork layer. This layer is known as Internet layer. Its job is to allow hosts to insert packets into any network and have them to deliver independently to the destination. At the destination side data packets may appear in a different order than they were sent. It is the job of the higher layers to rearrange them in order to deliver them to proper network applications operating at the Application layer.

The main protocols included at Internet layer are IP (Internet Protocol), ICMP (Internet Control Message Protocol), ARP (Address Resolution Protocol), RARP (Reverse Address Resolution Protocol) and IGMP (Internet Group Management Protocol).

LEVEL 3

Transport Layer is the third layer of the four layer TCP/IP model. The position of the Transport layer is between Application layer and Internet layer. The purpose of Transport layer is to permit devices on the source and destination hosts to carry on a conversation. Transport layer defines the level of service and status of the connection used when transporting data.

The main protocols included at Transport layer are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

LEVEL 4

Application layer is the top most layer of four layer TCP/IP model. Application layer is present on the top of the Transport layer. Application layer defines TCP/IP application protocols and how host programs interface with Transport layer services to use the network.

Application layer includes all the higher-level protocols like DNS (Domain Naming System), HTTP (Hypertext Transfer Protocol), Telnet, SSH, FTP (File Transfer Protocol), TFTP (Trivial File Transfer Protocol), SNMP (Simple Network Management Protocol), SMTP (Simple Mail Transfer Protocol) , DHCP (Dynamic Host Configuration Protocol), X Windows, RDP (Remote Desktop Protocol) etc.

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Internet Protocol, or IP, provides an unreliable packet delivery system--each packet is an individual, and is handled separately. Packets can arrive out of order or not at all. The recipient does not acknowledge them, so the sender does not know that the transmission was successful. There are no provisions for flow control--packets can be received faster than they can be used. And packet size is limited.

Transmission Control Protocol (TCP) is a network protocol designed to address these problems. TCP uses IP, but adds a layer of control on top. TCP packets are lost occasionally, just like IP packets. The difference is that the TCP protocol takes care of requesting retransmits to ensure that all packets reach their destination, and tracks packet sequence numbers to be sure that they are delivered in the correct order. While IP packets are independent, with TCP we can use streams along with the standard Java file I/O mechanism.

Think of TCP as establishing a connection between the two endpoints. Negotiation is performed to establish a "socket", and the socket remains open throughout the duration of the communications. The recipient acknowledges each packet, and packet retransmissions are performed by the protocol if packets are missed or arrive out of order. In this way TCP can allow an application to send as much data as it desires and not be subject to the IP packet size limit..
TCP is responsible for breaking the data into packets, buffering the data, resending lost or out of order packets, acknowledging receipt, and controlling rate of data flow by telling the sender to speed up or slow down so that the application never receives more than it can handle.

There are four distinct elements that make a TCP connection unique:

  • IP address of the server
  • IP address of the client
  • Port number of the server
  • Port number of the client
    Each requested client socket is assigned a unique port number while the server port number is always the same. If any of these numbers is different, the socket is different. A server can thus listen to one and only one port, and talk to multiple clients at the same time.

So a TCP connection is somewhat like a telephone connection; you need to know not only the phone number (IP address), but because the phone may be shared by many people at that location, you also need the name or extension of the user you want to talk to at the other end (port number). The analogy can be taken a little further. If you don't hear what the other person has said, a simple request ("What?") will prompt the other end to resend or repeat the phrase, and the connection remains open until someone hangs up.

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