Network Buffers and Memory Management

by Alan Cox

The Linux operating system implements the industry-standard Berkeley socket API, which has its origins in the BSD Unix developments (4.2/4.3/4.4 BSD). In this article, we will look at the way the memory management and buffering is implemented for network layers and network device drivers under the existing Linux kernel, as well as explain how and why some things have changed over time.

Core Concepts

The networking layer is fairly object-oriented in its design, as indeed is much of the Linux kernel. The core structure of the networking code goes back to the initial networking and socket implementations by Ross Biro and Orest Zborowski respectively. The key objects are:

  • Device or Interface: A network interface is programming code for sending and receiving data packets. Usually an interface is used for a physical device like an Ethernet card; however, some devices are software only, e.g., the loopback device used for sending data to yourself.

  • Protocol: Each protocol is effectively a different networking language. Some protocols exist purely because vendors chose to use proprietary networking schemes, while others are designed for special purposes. Within the Linux kernel each protocol is a separate module of code which provides services to the socket layer.

  • Socket: A socket is a connection in the networking that provides Unix file I/O and exists as a file descriptor to the user program. In the kernel each socket is a pair of structures that represent the high level socket interface and the low level protocol interface.

  • sk_buff: All the buffers used by the networking layers are sk_buffs. The control for these buffers is provided by core low-level library routines that are available to all of the networking system. sk_buffs provide the general buffering and flow control facilities needed by network protocols.

Implementation of sk_buffs

The primary goal of the sk_buff routines is to provide a consistent and efficient buffer-handling method for all of the network layers, and by being consistent to make it possible to provide higher level sk_buff and socket handling facilities to all of the protocols.

A sk_buff is a control structure with a block of memory attached. Two primary sets of functions are provided in the sk_buff library. The first set consists of routines to manipulate doubly linked lists of sk_buffs; the second of functions for controlling the attached memory. The buffers are held on linked lists optimised for the common network operations of append to end and remove from start. As so much of the networking functionality occurs during interrupts these routines are written to use atomic memory. The small extra overhead that results is well worth the pain it saves in bug hunting.

We use the list operations to manage groups of packets as they arrive from the network, and as we send them to the physical interfaces. We use the memory manipulation routines for handling the contents of packets in a standardised and efficient manner.

At its most basic level, a list of buffers is managed using functions like this:

void append_frame(char *buf, int len)
  struct sk_buff *skb=alloc_skb(len, GFP_ATOMIC);
    skb_append(&my_list, skb);
void process_queue(void)
  struct sk_buff *skb;
    kfree_skb(skb, FREE_READ);

These two fairly simplistic pieces of code actually demonstrate the receive packet mechanism quite accurately. The append_frame() function is similar to the code called from an interrupt by a device driver receiving a packet, and process_frame() is similar to the code called to feed data into the protocols. If you look in net/core/dev.c at netif_rx() and net_bh(), you will see that they manage buffers similarly. They are far more complex, as they have to feed packets to the right protocol and manage flow control, but the basic operations are the same. This is just as true if you look at buffers going from the protocol code to a user application.

The example also shows the use of one of the data control functions, skb_put(). Here it is used to reserve space in the buffer for the data we wish to pass down.

Let's look at append_frame(). The alloc_skb() function obtains a buffer of len bytes (Figure 1), which consists of:

  • 0 bytes of room at the head of the buffer

  • 0 bytes of data, and

  • len bytes of room at the end of the data.

The skb_put() function (Figure 4) grows the data area upwards in memory through the free space at the buffer end, and thus reserves space for the memcpy(). Many network operations that send data packets add space to the start of the frame each time a send is executed, so that headers can be added to the packets. For this reason, the skb_push() function (Figure 5) is provided so that the start of the data frame can be moved down through memory, if enough space has been reserved to leave room for completing this operation.

Figure 1 “After alloc_skb”

Figure 2 “After skb_reserve”

Figure 3 “An sk_buff Containing Data”

Figure 4 “After skb_put has been Called on the Buffer”

Figure 5 “After an skb_push has Occurred on the Previous Buffer”

Immediately after a buffer has been allocated, all the available room is at the end. Another function, skb_reserve() (Figure 2), can be called before data is added. This function allows you to specify that some of the space should be at the beginning of the buffer. Thus, many sending routines start with code that looks like:

    skb=alloc_skb(len+headspace, GFP_KERNEL);
    skb_reserve(skb, headspace);

In systems such as BSD Unix, you don't need to know in advance how much space you will need, as it uses chains of small buffers (mbufs) for its network buffers. Linux chooses to use linear buffers and save space in advance (often wasting a few bytes to allow for the worst case), because linear buffers make many other operations much faster.

Linux provides the following functions for manipulating lists:

  • skb_dequeue() takes the first buffer from a list. If the list is empty, a NULL pointer is returned. This function is used to pull buffers off queues. The buffers are added with the routines skb_queue_head() and skb_queue_tail().

  • skb_queue_head() places a buffer at the start of a list. As with all the list operations, it is atomic.

  • skb_queue_tail() places a buffer at the end of a list and is the most commonly used function. Almost all the queues are handled with one set of routines queuing data with this function and another set removing items from the same queues with skb_dequeue().

  • skb_unlink() removes a buffer from whatever list contains it. The buffer is not freed, merely removed from the list. To make some operations easier, you need not know what list the buffer is in, and you can always call skb_unlink() for a buffer which is not in any list. This function enables network code to pull a buffer out of use even when the network protocol has no idea who is currently using the buffer. A separate locking mechanism is provided, so that a buffer currently in use by a device driver can not be removed.

  • Some more complex protocols, like TCP, keep frames in order, and re-order their input as data is received. Two functions, skb_insert() and skb_append(), exist to allow users to place sk_buffs before or after a specific buffer in a list.

  • alloc_skb() creates a new sk_buff and initializes it. The returned buffer is ready to use but assumes you will fill in a few fields to indicate how the buffer should be freed. Normally this is done by skb->free=1. A buffer can be flagged as not freeable by kfree_skb() (see below).

  • kfree_skb() releases a buffer, and if skb->sk is set, it lowers the memory use counts of the socket (sk). It is up to the socket and protocol-level routines to increment these counts and to avoid freeing a socket with outstanding buffers. The memory counts are very important, as the kernel networking layers need to know how much memory is tied up by each connection in order to prevent remote machines or local processes from using too much memory.

  • skb_clone() makes a copy of a sk_buff, but does not copy the data area, which must be considered read only.

  • Sometimes a copy of the data is needed for editing, and skb_copy() provides the same facilities as skb_clone, but also copies the data (and thus has a much higher overhead).

Figure 6 Flow of Packets

Higher Level Support Routines

The semantics of allocating and queuing buffers for sockets also involve flow control rules and for sending a whole list of interactions with signals and optional settings such as non blocking. Two routines are designed to make this easy for most protocols.

The sock_queue_rcv_skb() function is used to handle incoming data flow control and is normally used in the form:


This function uses the socket read queue counters to prevent vast amounts of data from being queued to a socket. After a limit is hit, data is discarded. It is up to the application to read fast enough, or as in TCP, for the protocol to do flow control over the network. TCP actually tells the sending machine to shut up when it can no longer queue data.

On the sending side, sock_alloc_send_skb() handles signal handling, the non-blocking flag and all the semantics of blocking until there is space in the send queue, so that you cannot tie up all of memory with data queued for a slow interface. Many protocol send routines have this function doing almost all the work:

        return -err;
    skb_reserve(skb, headroom);
    memcpy(skb->data, data, len);

Most of this we have met before. The very important line is skb->sk=sk. The sock_alloc_send_skb() has charged the memory for the buffer to the socket. By setting skb->sk, we tell the kernel that whoever does a kfree_skb() on the buffer should credit the memory for the buffer to the socket. Thus, when a device has sent a buffer and freed it, the user is able to send more.

Network Devices

All Linux network devices follow the same interface, but many functions available in that interface are not needed for all devices. An object-oriented mentality is used, and each device is an object with a series of methods that are filled into a structure. Each method is called with the device itself as the first argument, in order to get around the lack of the C++ concept of this within the C language.

The file drivers/net/skeleton.c contains the skeleton of a network device driver. View or print a copy from a recent kernel and follow along throughout the rest of the article.

Basic Structure

Figure 7 Structure of a Linux Network Device

Each network device deals entirely in the transmission of network buffers from the protocols to the physical media, and in receiving and decoding the responses the hardware generates. Incoming frames are turned into network buffers, identified by protocol and delivered to netif_rx(). This function then passes the frames off to the protocol layer for further processing.

Each device provides a set of additional methods for the handling of stopping, starting, control and physical encapsulation of packets. All of the control information is collected together in the device structures that are used to manage each device.


All Linux network devices have a unique name that is not in any way related to the file system names devices may have. Indeed, network devices do not normally have a file system representation, although you can create a device which is tied to the device drivers. Traditionally the name indicates only the type of a device rather than its maker. Multiple devices of the same type are numbered upwards from 0; thus, Ethernet devices are known as “eth0”, “eth1”, “eth3” etc. The naming scheme is important as it allows users to write programs or system configuration in terms of “an Ethernet card” rather than worrying about the manufacturer of the board and forcing reconfiguration if a board is changed.

The following names are currently used for generic devices:

  • ethn Ethernet controllers, both 10 and 100Mbit/second

  • trn Token ring devices

  • sln SLIP devices and AX.25 KISS mode

  • pppn PPP devices both asynchronous and synchronous

  • plipn PLIP units; the number matches the printer port

  • tunln IPIP encapsulated tunnels

  • nrn NetROM virtual devices

  • isdnn ISDN interfaces handled by isdn4linux (*)

  • dummyn Null devices

  • lo The loopback device

(*) At least one ISDN interface is an Ethernet impersonator—the Sonix PC/Volante driver behaves in all aspects as if it was Ethernet rather than ISDN; therefore, it uses an “eth” device name. If possible, a new device should pick a name that reflects existing practice. When you are adding a whole new physical layer type, you should look for other people working on such a project and use a common naming scheme.

Certain physical layers present multiple logical interfaces over one media. Both ATM and Frame Relay have this property, as does multi-drop KISS in the amateur radio environment. Under such circumstances, a driver needs to exist for each active channel. The Linux networking code is structured in such a way as to make this manageable without excessive additional code. Also, the name registration scheme allows you to create and remove interfaces almost at will as channels come into and out of existence. The proposed convention for such names is still under some discussion, as the simple scheme of “sl0a”, “sl0b”, “sl0c” works for basic devices like multidrop KISS, but does not cope with multiple frame relay connections where a virtual channel can be moved across physical boards.

Registering a Device

Each device is created by filling in a struct device object and passing it to the register_netdev(struct device *) call. This links your device structure into the kernel network device tables. As the structure you pass in is used by the kernel, you must not free this until you have unloaded the device with void unregister_netdev(struct device *) calls. These calls are normally done at boot time or at module load/unload.

The kernel will not object if you create multiple devices with the same name, it will break. Therefore, if your driver is a loadable module you should use the struct device *dev_get(const char *name) call to ensure the name is not already in use. If it is in use, you should pick another name or your new driver will fail. If you discover a clash, you must not use unregister_netdev() to unregister the other device using the name!

A typical code sequence for registration is:

int register_my_device(void)
  int i=0;
        return -EIO;
      return 0;
"100 mydevs loaded. Unable to load more.\n");
  return -ENFILE;
The Device Structure

All the generic information and methods for each network device are kept in the device structure. To create a device you need to supply the structure with most of the data discussed below. This section covers how a device should be set up.


First, the name field holds a string pointer to a device name in the formats discussed previously. The name field can also be " " (four spaces), in which case the kernel automatically assigns an ethn name to it. This special feature should not be used. After Linux 2.0, we intend to add a simple support function of the form dev_make_name("eth") for this purpose.

Bus Interface Parameters

The next block of parameters is used to maintain the location of a device within the device address spaces of the architecture. The irq field holds the interrupt (IRQ) the device is using, and is normally set at boot time or by the initialization function. If an interrupt is not used, not currently known or not assigned, the value zero should be used. The interrupt can be set in a variety of fashions. The auto-irq facilities of the kernel can be used to probe for the device interrupt, or the interrupt can be set when loading the network module. Network drivers normally use a global int called irq for this so that users can load the module with insmod mydevice irq=5 style commands. Finally, the IRQ field can be set dynamically using the ifconfig command, which causes a call to your device that will be discussed later on.

The base_addr field is the base I/O space address where the device resides. If the device uses no I/O locations or is running on a system without an I/O space concept, this field should be set to zero. When this address is user settable, it is normally set by a global variable called io. The interface I/O address can also be set with ifconfig.

Two hardware-shared memory ranges are defined for things like ISA bus shared memory Ethernet cards. For current purposes, the rmem_start and rmem_end fields are obsolete and should be loaded with 0. The mem_start and mem_end addresses should be loaded with the start and end of the shared memory block used by this device. If no shared memory block is used, then the value 0 should be stored. Those devices that allow the user to to set the memory base use a global variable called mem, and then set the mem_end address appropriately themselves.

The dma variable holds the DMA channel in use by the device. Linux allows DMA (like interrupts) to be automatically probed. If no DMA channel is used or the DMA channel is not yet set, the value 0 is used. This option may have to change, since the latest PC boards allow ISA bus DMA channel 0 to be used by hardware boards and do not just tie it to memory refresh. If the user can set the DMA channel, the global variable dma is used.

It is important to realise that the physical information is provided for control and user viewing (as well as the driver's internal functions), and does not register these areas to prevent them being reused. Thus, the device driver must also allocate and register the I/O, DMA and interrupt lines it wishes to use, using the same kernel functions as any other device driver. [See the recent Kernel Korner articles on writing a character device driver in issues 23, 24, 25, 26 and 28, or visit the new Linux Kernel Hackers' Guide at, for more information on the necessary functions—ED]

The if_port field holds the physical media type for multi-media devices such as combo Ethernet boards.

Protocol Layer Variables

In order for the network protocol la

yers to perform in a sensible manner, the device has to provide a set of capability flags and variables that are also maintained in the device structure.

The mtu is the largest payload that can be sent over this interface, i.e., the largest packet size not including any bottom layer headers that the device itself will provide. This number is used by the protocol layers such as IP to select suitable packet sizes to send. There are minimums imposed by each protocol. A device is not usable for IPX without a 576 byte frame size or higher. IP needs at least 72 bytes and does not perform sensibly below about 200 bytes. It is up to the protocol layers to decide whether to co-operate with your device.

The family is always set to AF_INET and indicates the protocol family the device is using. Linux allows a device to be using multiple protocol families at once, and maintains this information solely to look more like the standard BSD networking API.

The interface hardware type field is taken from a table of physical media types. The values used by the ARP protocol (see RFC1700) are used by those media that support ARP, and additional values are assigned for other physical layers. New values are added whenever necessary both to the kernel and to net-tools, the package containing programs like ifconfig that need to be able to decode this field. The fields defined as of Linux pre2.0.5 are:

From RFC1700:
ARPHRD_ETHER         10 and 100Mbit/second Ethernet
ARPHRD_EETHER   Experimental Ethernet (not used)
ARPHRD_AX25          AX.25 level 2 interfaces
ARPHRD_PRONET        PROnet token ring (not used)
ARPHRD_CHAOS         ChaosNET (not used)
ARPHRD_IEE802        802.2 networks notably token ring
ARPHRD_ARCNET        ARCnet interfaces
ARPHRD_DLCI          Frame Relay DLCI
Defined by Linux:
ARPHRD_SLIP          Serial Line IP protocol
ARPHRD_CSLIP         SLIP with VJ header compression
ARPHRD_SLIP6         6bit encoded SLIP
ARPHRD_CSLIP6        6bit encoded header compressed SLIP
ARPHRD_ADAPT         SLIP interface in adaptive mode
ARPHRD_PPP      PPP interfaces (async and sync)
ARPHRD_TUNNEL6  IPv6 over IP tunnels
ARPHRD_FRAD          Frame Relay Access Device
ARPHRD_SKIP          SKIP encryption tunnel
ARPHRD_LOOPBACK Loopback device
ARPHRD_LOCALTLK Localtalk apple networking device
ARPHRD_METRICOM Metricom Radio Network

Those interfaces marked unused are defined types but without any current support on the existing net-tools. The Linux kernel provides additional generic support routines for devices using Ethernet and token ring.

The pa_addr field is used to hold the IP address when the interface is up. Interfaces should start down with this variable clear. pa_brdaddr is used to hold the configured broadcast address, pa_dstaddr is the target of a point to point link, and pa_mask is the IP netmask of the interface. All of these can be initialized to zero. The pa_alen field holds the length of an address (in our case an IP address), and should be initialized to 4.

Link Layer Variables

The hard_header_len is the number of bytes the device needs at the start of a network buffer passed to it. This value does not have to equal the number of bytes of physical header that will be added, although this number is usually used. A device can use this value to provide itself with a scratch pad at the start of each buffer.

In the 1.2.x series kernels, the skb->data pointer will point to the buffer start, and you must avoid sending your scratch pad. This also means that for devices with variable length headers you need to allocate max_size+1 bytes and keep a length byte at the start so that you know where the header actually begins (the header should be contiguous with the data). Linux 1.3.x makes life much simpler. It ensures that you have at least as much room as you requested, free at the start of the buffer. It is up to you to use skb_push() appropriately, as we discussed in the section on networking buffers.

The physical media addresses (if any) are maintained in dev_addr and broadcast respectively and are byte arrays. Addresses smaller than the size of the array are stored starting from the left. The addr_len field is used to hold the length of a hardware address. With many media there is no hardware address, and in this case, this field should be set to zero. For some other interfaces, the address must be set by a user program. The ifconfig tool permits the setting of an interface hardware address. In this case it need not be set initially, but the open code should take care not to allow a device to start transmitting before an address has been set.


A set of flags is used to maintain the interface properties. Some of these are “compatibility” items and as such are not directly useful. The flags are:

  • IFF_UP The interface is currently active. In Linux, the IFF_RUNNING and IFF_UP flags are basically handled as a pair, existing as two items for compatibility reasons. When an interface is not marked as IFF_UP, it can be removed. Unlike BSD, an interface that does not have IFF_UP set will never receive packets.

  • IFF_BROADCAST The interface has broadcast capability. There will be a valid IP address for the interface stored in the device addresses.

  • IFF_DEBUG Indicates debugging is desired. Not currently used.

  • IFF_LOOPBACK The loopback interface (lo) is the only interface that has this flag set. Setting it on other interfaces is neither defined nor a very good idea.

  • IFF_POINTOPOINT This interface is a point to point link (such as SLIP or PPP). There is no broadcast capability as such. The remote point to point address in the device structure is valid. Normally, a point to point link has no netmask or broadcast, but it can be enabled if needed.

  • IFF_NOTRAILERS More of a prehistoric than an historic compatibility flag. Not used.


  • IFF_NOARP The interface does not perform ARP queries. Such an interface must have either a static table of address conversions or no need to perform mappings. The NetROM interface is a good example of this. Here all entries are hand configured as the NetROM protocol cannot do ARP queries.

  • IFF_PROMISC If it is possible, the interface will hear all of the packets on the network. This flag is typically used for network monitoring, although it can also be used for bridging. One or two interfaces like the AX.25 interfaces are always in promiscuous mode.

  • IFF_ALLMULTI Receive all multicast packets. An interface, that cannot perform this operation but can receive all packets, will go into promiscuous mode when asked to perform this task.

  • IFF_MULTICAST Indicates that the interface supports multicast IP traffic, which is not the same as supporting a physical multicast. AX.25 for example supports IP multicast using physical broadcast. Point to point protocols such as SLIP generally support IP multicast.

The Packet Queue

Packets are queued for an interface by the kernel protocol code. Within each device, buffs[] is an array of packet queues for each kernel priority level. These are maintained entirely by the kernel code, but must be initialized by the device itself on boot up. The intialization code used is:

int ct=0;

All other fields should be initialized to 0.

The device gets to select the queue length it needs by setting the field dev->tx_queue_len to the maximum number of frames the kernel should queue for the device. Typically this is around 100 for Ethernet and 10 for serial lines. A device can modify this dynamically, although its effect will lag the change slightly.

Network Device Methods

Each network device has to provide a set of actual functions (methods) for the basic low level operations. It should also provide a set of support functions that interface the protocol layer to the protocol requirements of the link layer it is providing.


The init method is called when the device is initialized and registered with the system, in order to perform any low level verification and checking needed. It returns an error code if the device is not present, if areas cannot be registered or if it is otherwise unable to proceed. If the init method returns an error, the register_netdev() call returns the error code, and the device is not created.

Frame Transmission

All devices must provide a transmit function. It is possible for a device to exist that cannot transmit. In this case, the device needs a transmit function that simply frees the buffer passed to it. The dummy device has exactly this functionality on transmit.

The dev->hard_start_xmit() function is called to provide the driver with its own device pointer and network buffer (a sk_buff) for transmitting. If your device is unable to accept the buffer, it should return 1 and set dev->tbusy to a non-zero value. This action will queue the buffer to be retried again later, although there is no guarantee that a retry will occur. If the protocol layer decides to free the buffer that the driver has rejected, then the buffer will not be offered back to the device. If the device knows the buffer cannot be transmitted in the near future, for example due to bad congestion, it can call dev_kfree_skb() to dump the buffer and return 0 indicating the buffer has been processed.

If there is space the buffer should be processed. The buffer handed down already contains all the headers, including link layer headers, necessary and need only be loaded into the hardware for transmission. In addition, the buffer is locked, which means that the device driver has absolute ownership of the buffer until it chooses to relinquish it. The contents of a sk_buff remain read-only, with the exception that you are guaranteed that the next/previous pointers are free, so that you can use the sk_buff list primitives to build internal chains of buffers.

When the buffer has been loaded into the hardware or, in the case of some DMA driven devices, when the hardware has indicated transmission is complete, the driver must release the buffer by calling dev_kfree_skb(skb, FREE_WRITE). As soon as this call is made, the sk_buff in question may spontaneously disappear, and the device driver should not reference it again.

Frame Headers

It is necessary for the high level protocols to append low level headers to each frame before queuing it for transmission. It is also clearly undesirable that the protocol know in advance how to append low level headers for all possible frame types. Thus, the protocol layer calls down to the device with a buffer that has at least dev->hard_header_len bytes free at the start of the buffer. It is then up to the network device to correctly call skb_push() and to put the header on the packet using the dev->hard_header() method. Devices with no link layer header, such as SLIP, may have this method specified as NULL.

The method is invoked by giving the buffer concerned, the device's pointers, its protocol identity, pointers to the source and destination hardware addresses and the length of the packet to be sent. As the routine can be called before the protocol layers are fully assembled, it is vital that the method use the length parameter, not the buffer length.

The source address can be NULL to mean “use the default address of this device”, and the destination can be NULL to mean “unknown”. If as a result of an unknown destination, the header can not be completed, the space should be allocated and any bytes that can be filled in should be filled in. The function must then return the negative of the bytes of header added. This facility is currently only used by IP when ARP processing must take place. If the header is completely built, the function must return the number of bytes of header added to the beginning of the buffer.

When a header cannot be completed the protocol layers will attempt to resolve the necessary address. When this situation occurs, the dev->rebuild_header() method is called with the address at which the header is located, the device in question, the destination IP address and the network buffer pointer. If the device is able to resolve the address by whatever means available (normally ARP), then it fills in the physical address and returns 1. If the header cannot be resolved, it returns 0 and the buffer will be retried the next time the protocol layer has reason to believe resolution will be possible.


There is no receive method in a network device, because it is the device that invokes processing of such events. With a typical device, an interrupt notifies the handler that a completed packet is ready for reception. The device allocates a buffer of suitable size with dev_alloc_skb(), and places the bytes from the hardware into the buffer. Next, the device driver analyses the frame to decide the packet type. The driver sets skb->dev to the device that received the frame. It sets skb->protocol to the protocol the frame represents, so that the frame can be given to the correct protocol layer. The link layer header pointer is stored in skb->mac.raw, and the link layer header removed with skb_pull() so that the protocols need not be aware of it. Finally, to keep the link and protocol isolated, the device driver must set skb->pkt_type to one of the following:

  • PACKET_BROADCAST Link layer broadcast

  • PACKET_MULTICAST Link layer multicast

  • PACKET_SELF Frame to us

  • PACKET_OTHERHOST Frame to another single host

This last type is normally reported as a result of an interface running in promiscuous mode.

Finally, the device driver invokes netif_rx() to pass the buffer up to the protocol layer. The buffer is queued for processing by the networking protocols after the interrupt handler returns. Deferring the processing in this fashion dramatically reduces the time interrupts are disabled and improves overall responsiveness. Once netif_rx() is called, the buffer ceases to be property of the device driver and can not be altered or referred to again.

Flow control on received packets is applied at two levels by the protocols. First, a maximum amount of data can be outstanding for netif_rx() to process. Second, each socket on the system has a queue which limits the amount of pending data. Thus, all flow control is applied by the protocol layers. On the transmit side a per device variable dev->tx_queue_len is used as a queue length limiter. The size of the queue is normally 100 frames, which is large enough that the queue will be kept well filled when sending a lot of data over fast links. On a slow link such as a slip link, the queue is normally set to about 10 frames, as sending even 10 frames is several seconds of queued data.

One piece of magic that is done for reception with most existing devices, and one that you should implement if possible, is to reserve the necessary bytes at the head of the buffer to land the IP header on a long word boundary. The existing Ethernet drivers thus do:

/* then 14 bytes of ethernet hardware header */

to align IP headers on a 16 byte boundary, which is also the start of a cache line and helps give performance improvements. On the SPARC or DEC Alpha these improvements are very noticeable.

Optional Functionality

Each device has the option of providing additional functions and facilities to the protocol layers. Not implementing these functions will cause a degradation in service available via the interface, but will not prevent operation. These operations split into two categories—configuration and activation/shutdown.

Activation and Shutdown

When a device is activated (i.e., the flag IFF_UP is set), the dev->open() method is invoked if the device has provided one. This invocation permits the device to take any action such as enabling the interface that is needed when the interface is to be used. An error return from this function causes the device to stay down and causes the user's activation request to fail with an error returned by dev->open()

The dev->open() function can also be used with any device that is loaded as a module. Here it is necessary to prevent the device from being unloaded while it is open; thus, the MOD_INC_USE_COUNT macro must be used within the open method.

The dev->close() method is invoked when the device is ready to be configured down and should shut off the hardware in such a way as to minimise machine load (e.g., by disabling the interface or its ability to generate interrupts). It can also be used to allow a module device to be unloaded after it is down. The rest of the kernel is structured in such a way that when a device is closed, all references to it by pointer are removed, in order to ensure that the device can be safely unloaded from a running system. The close method is not permitted to fail.

Configuration and Statistics

A set of functions provide the ability to query and to set operating parameters. The first and most basic of these is a get_stats routine which when called returns a struct enet_statistics block for the interface. This block allows user programs such as ifconfig to see the loading of the interface and any logged problem frames. Not providing this block means that no statistics will be available.

The dev->set_mac_address() function is called whenever a superuser process issues an ioctl of type SIOCSIFHWADDR to change the physical address of a device. For many devices this function is not meaningful and for others it is not supported. In these cases, set this function pointer to NULL. Some devices can only perform a physical address change if the interface is taken down. For these devices, check the IFF_UP flag, and if it is set, return -EBUSY.

The dev->set_config() function is called by the SIOCSIFMAP function when a user enters a command like ifconfig eth0 irq 11. It then passes an ifmap structure containing the desired I/O and other interface parameters. For most interfaces this function is not useful, and you can return NULL.

Finally, the dev->do_ioctl() call is invoked whenever an ioctl in the range SIOCDEVPRIVATE to SIOCDEVPRIVATE+15 is used on your interface. All these ioctl calls take a struct ifreq, which is copied into kernel space before your handler is called and copied back at the end. For maximum flexibility any user can make these calls, and it is up to your code to check for superuser status when appropriate. For example, the PLIP driver uses these calls to set parallel port time out speeds in order to allow a user to tune the plip device for his machine.


Certain physical media types, such as Ethernet, support multicast frames at the physical layer. A multicast frame is heard by a group of hosts (not necessarily all) on the network, rather than going from one host to another.

The capabilities of Ethernet cards are fairly variable. Most fall into one of three categories:

  • No multicast filters. The card either receives all multicasts or none of them. Such cards can be a nuisance on a network with a lot of multicast traffic, such as group video conferences.

  • Hash filters. A table is loaded onto the card giving a mask of entries for desired multicasts. This method filters out some of the unwanted multicasts but not all.

  • Perfect filters. Most cards that support perfect filters combine this option with 1 or 2 above, because the perfect filter often has a length limit of 8 or 16 entries.

It is especially important that Ethernet interfaces are programmed to support multicasting. Several Ethernet protocols (notably Appletalk and IP multicast) rely on Ethernet multicasting. Fortunately, most of the work is done by the kernel for you (see net/core/dev_mcast.c).

The kernel support code maintains lists of physical addresses your interface should be allowing for multicast. The device driver may return frames matching more than the requested list of multicasts if it is not able to do perfect filtering.

Whenever the list of multicast addresses changes, the device drivers dev->set_multicast_list() function is invoked. The driver can then reload its physical tables. Typically this looks something like:

else if(dev->flags&IFF_ALLMULTI)

There are a small number of cards that can only do unicast or promiscuous mode. In this case the driver, when presented with a request for multicasts has to go promiscuous. If this is done, the driver must itself set the IFF_PROMISC flag in dev->flags.

In order to aid the driver writer, the multicast list is kept valid at all times. This simplifies many drivers, as a reset from an error condition in a driver often has to reload the multicast address lists.

Ethernet Support Routines

Ethernet is probably the most common physical interface type that can be handled. The kernel provides a set of general purpose Ethernet support routines that such drivers can use.

eth_header() is the standard Ethernet handler for the dev-hard_header routine, and can be used in any Ethernet driver. Combined with eth_rebuild_header() for the rebuild routine it provides all the ARP lookup required to put Ethernet headers on IP packets.

The eth_type_trans() routine expects to be fed a raw Ethernet packet. It analyses the headers and sets skb->pkt_type and skb->mac itself as well as returning the suggested value for skb->protocol. This routine is normally called from the Ethernet driver receive interrupt handler to classify packets.

eth_copy_and_sum(), the final Ethernet support routine is internally quite complex, but offers significant performance improvements for memory mapped cards. It provides the support to copy and checksum data from the card into a sk_buff in a single pass. This single pass through memory almost eliminates the cost of checksum computation when used and improves IP throughput.

Alan Cox has been working on Linux since version 0.95, when he installed it in order to do further work on the AberMUD game. He now manages the Linux Networking, SMP, and Linux/8086 projects and hasn't done any work on AberMUD since November 1993.

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