Category: Network data flow through the linux kernel

We are now through discussing char and block drivers and are ready to move on to the fascinating world of networking. Network interfaces are the third standard class of Linux devices, and this chapter describes how they interact with the rest of the kernel. The role of a network interface within the system is similar to that of a mounted block device. Similarly, a network interface must register itself in specific data structures in order to be invoked when packets are exchanged with the outside world.

There are a few important differences between mounted disks and packet-delivery interfaces. The normal file operations read, write, and so on do not make sense when applied to network interfaces, so it is not possible to apply the Unix "everything is a file" approach to them. Thus, network interfaces exist in their own namespace and export a different set of operations. Although you may object that applications use the read and write system calls when using sockets, those calls act on a software object that is distinct from the interface.

Several hundred sockets can be multiplexed on the same physical interface. But the most important difference between the two is that block drivers operate only in response to requests from the kernel, whereas network drivers receive packets asynchronously from the outside. Thus, while a block driver is asked to send a buffer toward the kernel, the network device asks to push incoming packets toward the kernel.

The kernel interface for network drivers is designed for this different mode of operation. Network drivers also have to be prepared to support a number of administrative tasks, such as setting addresses, modifying transmission parameters, and maintaining traffic and error statistics.

The API for network drivers reflects this need, and thus looks somewhat different from the interfaces we have seen so far.

The network subsystem of the Linux kernel is designed to be completely protocol independent. This applies to both networking protocols IP versus IPX or other protocols and hardware protocols Ethernet versus token ring, etc.

Interaction between a network driver and the kernel proper deals with one network packet at a time; this allows protocol issues to be hidden neatly from the driver and the physical transmission to be hidden from the protocol. This chapter describes how the network interfaces fit in with the rest of the Linux kernel and shows a memory-based modularized network interface, which is called you guessed it snull.

To simplify the discussion, the interface uses the Ethernet hardware protocol and transmits IP packets. The knowledge you acquire from examining snull can be readily applied to protocols other than IP, and writing a non-Ethernet driver is only different in tiny details related to the actual network protocol. This chapter doesn't talk about IP numbering schemes, network protocols, or other general networking concepts.

Such topics are not usually of concern to the driver writer, and it's impossible to offer a satisfactory overview of networking technology in less than a few hundred pages.

The interested reader is urged to refer to other books describing networking issues. The networking subsystem has seen many changes over the years as the kernel developers have striven to provide the best performance possible.

The bulk of this chapter describes network drivers as they are implemented in the 2.Jump to navigation. In June when I discussed basic network configurationone thing I did not talk about then is routing.

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This article provides a very brief introduction to routing for Linux computers, designed for understanding simple environments. This is usually very straightforward because most network environments are very simple and there are only two options for departing packets. All packets are sent either to a device on the local network or to some other, remote network. Our latest Linux articles. Let's be sure to define the "local" network as the logical and usually also the physical network in which the local host resides.

Logically that means the local subnet in which the host is assigned one of the range of the local subnet's IP addresses. Physically that means the host is connected to one or more switches that are also connected to the rest of the local network.

Before we get into routing, it helps to understand a little bit about how packets find their way to the correct host on a network. Each of the layers in the following description of this model is numbered and also contain the names of the data units that are handled by that layer. When you request a web page from a remote web site, a connection request is sent to the web server and the response is sent back to your host at this layer and then your browser displays the web page in its window.

Transport layer: TCP segment. The transport layer provides end-to-end data transport and flow management services that are independent of the data and types of protocols being transported. Internet layer: Packet. Packet routing is performed on the Internet layer. This layer is responsible for routing packets across two or more different networks in order to reach their final destination.

This layer uses IP Addresses and the routing table to determine which device to send the packets to next.

network data flow through the linux kernel

If sent to a router, each router is responsible for sending the data packets only to the next router in the series and not for mapping out the entire route from the local host to the target host. The Internet layer is mostly about routers talking to routers in order to determine the next router in the chain. Data Link layer: Frame. The Link layer manages the direct connections between hardware hosts on a single, local, logical, physical network.

This layer cannot access hosts that are not on the local network. Physical layer: Bits. This is the hardware layer and consists of the NICs and the physical Ethernet cable as well as the hardware level protocols used to transmit individual bits that make up the data frames between any two hosts or other network nodes that are locally connected.

Here is my own made-up description of how data are moved from one network to another. In this example, my computer is sending a request to a remote server for a web page. On the application layer, the browser initiates an HTTP connection request message to the remote host, www.

This is the message, and it includes only the IP Address of the remote web server. The transport layer encapsulates the message containing the web page request in a TCP datagram with the IP address of the remote web server as the destination.

Along with the original request packet, this packet now includes the source port from which the request will originate, usually a very high number random port, so that the return data knows which port the browser is listening on; and the destination port on the remote host, port 80 in this case. The Internet layer encapsulates the TCP datagram in a packet that also contains both the source and destination IP addresses. The default router opens the datagram and determines the destination IP address.

The router uses its own routing table to identify the IP address of the next router that will take the frame onto the next step of its journey.This is the first part of a planned multi-part series on receiving a network packet on a modern Linux kernel. Have you ever wondered what happens when your Linux machine gets a packet from the network? I have, but most of the information I've seen is concerned with the 2.

For my own sake, I decided to take a walk through the Linux networking stack using Linux kernel 2. I've decided to take a bottom up approach and begin with software that interacts with the physical network card, the driver. In general network drivers follow a fairly typical route in processing: the kernel boots up, initializes data structures, sets up some interrupt routines, and tells the network card where to put packets when they are received.

When a packet is actually received, the card signals the kernel causing it to do some processing and then cleans up some resources. I'll talk about the fairly generic routines that network devices share in common and then move to a concrete example with the igb driver.

Network drivers are far from the simplest drivers in the kernel. They push the speed boundaries of modern multi-core processors. Almost every new system comes with a 1 Gigabit per second Gbps network card. Taking into account that the smallest Ethernet frame size is 64 bytes plus 8 bytes for synchronization and a 12 byte inter-frame gapa 1 Gbps network card should by specification be able to handle:.

This leaves us with a processing time of about nanoseconds per frame. A 2 Gigahertz processor can execute about 1, cycles in that time and a single instruction can take multiple cycles. Is a modern operating system able to keep up with that rate? The simple answer is: "no. It still does its best to get as many packets as possible using modern network card resources and the multi-core processors available on the machine. During the boot process the kernel discovers the network card and sets up its data structures.

These data structures include multiple queues for receiving packets while the kernel trying to process them. Each receive queue is independent from the rest so a processor core can work through its backlog safely.

To prevent wasteful context switching, interrupts are disabled while a core works through the packet backlog. The network card also writes packets directly to memory so no processor resources are used to move the packet from one place to another. How does this all happen so quickly? It all depends on some careful coordination between the kernel and the hardware.

Since every piece of hardware is different the kernel creates a standard interface between itself or the user and the underlying hardware through a "driver. The driver uses whatever tricks available to fufill a request. In the figure above, you can see the main components involved in the low level packet reception: main memory, the network card, and the Linux kernel.

On the left side, the network card holds a descriptor ring. An entry in the descriptor ring points to a location in main memory which was set up to be a socket buffer where it will write the packet. Entries can also contain information about the packet or the state of the network card during reception.

network data flow through the linux kernel

On the right side, the Linux kernel maintains a pool of socket buffers. The pool concept works well here because the network card needs quick access to socket buffers, but once used a socket buffer can remain active for a while so the network card can go back to the pool and grab a new one.

There can also be a simple wrapper between the socket buffers and the descriptor ring to decrease overhead when moving around the network stack. Finally, the only real direct communication from the network card to the kernel happens by way of an interrupt.GitHub is home to over 40 million developers working together to host and review code, manage projects, and build software together.

If nothing happens, download GitHub Desktop and try again. If nothing happens, download Xcode and try again. If nothing happens, download the GitHub extension for Visual Studio and try again. Sometimes people are looking for sysctl cargo cult values that bring high throughput and low latency with no trade-off and that works on every occasion.

That's not realistic, although we can say that the newer kernel versions are very well tuned by default. In fact, you might hurt performance if you mess with the defaults. Skip to content.

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Dismiss Join GitHub today GitHub is home to over 40 million developers working together to host and review code, manage projects, and build software together. Sign up. Branch: master. Find file. Sign in Sign up. Go back. Launching Xcode If nothing happens, download Xcode and try again. Latest commit. Latest commit 4b04cfb Apr 22, Feel free to send corrections and suggestions! How: Check command: sysctl net. In one polling cycle interfaces which are registered to polling are probed in a round-robin manner.

Why - each application has different load and need to traffic control and it is used also to fight against bufferbloat How: Check command: sysctl net. If you change this value, you should also change your application to a compatible value i. This is strictly a violation of the TCP specification but required to prevent denial-of-service attacks. You signed in with another tab or window. Reload to refresh your session.

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You signed out in another tab or window. May 22, Apr 22, You are considering upgrading to 10 Gbps Ethernet, but: Does the motherboard and its system bus have the speed to fill a 10 Gbps network? The switch backplane needs bandwidth equal to 2 times the number of ports times the speed.

You can bond multiple Ethernet adapters together and potentially multiply your throughput. You have a choice of several bonding algorithms, see here for details. However, all of them will send all packets of a flow of data between two endpoints through the same interface. Bonding will not increase the throughput between a pair of hosts, as each host will use just one physical interface to send every frame to the other host.

Monitoring and Tuning the Linux Networking Stack: Sending Data

Bonding is only helpful if your data flow topology looks more like a star or a fairly complete mesh. For example, a file server with several concurrent clients.

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If you have the appropriate data flow patterns, then bonding multiple 1 Gbps adapters makes sense compared to the price of upgrading to 10 Gbps hardware. Bonding multiple 10 Gbps adapters gets expensive quickly, and the CPU is already awfully busy working to keep just the one 10 Gbps link filled. The example of protocol overhead came up back on the introductory page.

Every Ethernet frame needs a standard header. Due to design optimization decisions made years ago, the default MTU or maximum Ethernet frame length allows for bytes of payload. The maximum frame length limits latency, the amount of time a host has to wait to transmit their packet, but that limit of bytes of payload plus header and CRC was specified when Ethernet ran at 10 Mbps. With 1, times the speed, the frames could be much longer in bytes and still have far lower latency.

Also, the CPU would be interrupted less often. A jumbo frame is an Ethernet frame with more than bytes of payload. Much modern Ethernet equipment can support frames up to 9, bytes, but make sure to verify that every device on the LAN supports your desired jumbo frame size before making any changes.

Be aware that interface names have changed, it's no longer eth0eth1and so on, but names that encode physical location like enp0s2.

Performance Tuning on Linux — Ethernet

Details on network interface names can be found here. Not every option will supported on a given network interface, and even if its chipset supports something it's possible that the current Linux driver doesn't.Above we read merely about the theory of networking, about the basic ideas, about communication protocols and standards.

Now, let us see, how all of this is being handled by the Linux Kernel 2.

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The main interface between the kernel and userspace is the set of system calls. There are about system calls. Network related system calls include: writes to socket, ….

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Internet archive - Sep. So you can install hardware capable of Ethernet usually a network card or more precisely an Ethernet card on two hostsconnect them with a standardized cable, like a Category 5 cable and communicate with one another over Ethernet as far as your software supports Ethernet Sooner or later the sausage will get to the Ethernet thingy of the network stack, this will prepare the data conforming to the Ethernet standard, then will deliver the frames to the network card drivers and this will make the hardware, the network card, transmit the data.

The NIC on the other side will receive the signal, relay it to the Ethernet thingy of the network stackthis will create one huge data out of the Ethernet frames and relay it to the software. A transmit queue is associated with each device. The driver will then put that packet into the transmit queue of the hardware. User Tools Register Log In. Site Tools Search. Sidebar Welcome to the OpenWrt Project.

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User guide. Developer guide. Submitting patches. Wiki contribution guide. Table of Contents Networking in the Linux Kernel. Network Data Flow through the Linux Kernel. Kernel Flow. Queue No.

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Routers, Switches, Packets and Frames

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I am reading the source code of Linux kernel trying to figure out how the Linux kernel sends the network packets, after many hours, I can only see the flow of network packets walking through TCP Layer, IP Layer and finally Datalink Layer, I can't find out which threads are doing these jobs.

My question is which kernel threads are responsible for sending network packets and where are they? Since we can set a socket as non-blocking and let a user application to send network packets as fast as possible, so when the network is busy there must be some queues to buffer these packets, and there must be some kernel threads running someplace sending these packets.

Good network chips manage a packet queue in hardware. Cheaper chips have a fixed-length queue or no queue at all, and raise an interrupt when a packet has been transmitted; the kernel then enqueues or submits the next packet from its own queue from the interrupt handler. Learn more. Asked 6 years, 4 months ago.

network data flow through the linux kernel

Active 6 years, 4 months ago. Viewed 1k times. Steve Steve 1, 1 1 gold badge 16 16 silver badges 35 35 bronze badges.

Ask on kernelnewbies. Active Oldest Votes. By default, the kernel does not use threads to send network packets. Supposing the send buffer of some socket is full at this moment, and the user process which owns this socket hung up.

network data flow through the linux kernel

Minutes later when the network is free, who will take the packets out of the socket send buffer and resend them again? Isn't there kernel thread moving these packets to the packet queue in the chip? If there isn't, how the TCP layer knows when to resend packets of this socket?

The network chips raises an interrupt "when the network is free", i. Dec 16 '13 at


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