PACKET_MMAP's documentation

From: uaca
Date: Sun Mar 28 2004 - 09:41:30 EST


If you agree with it I will send two small patches to modify
kernel's configure help.



This file documents the CONFIG_PACKET_MMAP option available with the PACKET
socket interface on 2.4 and 2.6 kernels. This type of sockets is used for
capture network traffic with utilities like tcpdump or any other that uses
the libpcap library.

You can find the latest version of this document at

Please send me your comments to

Ulisses Alonso Camaró <uaca@xxxxxxxxxxxxxxxxxxxxxxxx>


In Linux 2.4/2.6 if PACKET_MMAP is not enabled, the capture process is very
inefficient. It uses very limited buffers and requires one system call
to capture each packet, it requires two if you want to get packet's
timestamp (like libpcap always does).

In the other hand PACKET_MMAP is very efficient. PACKET_MMAP provides a size
configurable circular buffer mapped in user space. This way reading packets just
needs to wait for them, most of the time there is no need to issue a single
system call. By using a shared buffer between the kernel and the user
also has the benefit of minimizing packet copies.

It's fine to use PACKET_MMAP to improve the performance of the capture process,
but it isn't everything. At least, if you are capturing at high speeds (this
is relative to the cpu speed), you should check if the device driver of your
network interface card supports some sort of interrupt load mitigation or
(even better) if it supports NAPI, also make sure it is enabled.


>From the user standpoint, you should use the higher level libpcap library, wich
is a de facto standard, portable across nearly all operating systems
including Win32.

Said that, at time of this writing, official libpcap 0.8.1 is out and doesn't include
support for PACKET_MMAP, and also probably the libpcap included in your distribution.

I'm aware of two implementations of PACKET_MMAP in libpcap: (by Simon Patarin, based on libpcap 0.6.2) (by Phil Wood, based on lastest libpcap)

The rest of this document is intended for people who want to understand
the low level details or want to improve libpcap by including PACKET_MMAP

+ How to use CONFIG_PACKET_MMAP directly

>From the system calls stand point, the use of PACKET_MMAP involves
the following process:

[setup] socket() -------> creation of the capture socket
setsockopt() ---> allocation of the circular buffer (ring)
mmap() ---------> maping of the allocated buffer to the
user process

[capture] poll() ---------> to wait for incoming packets

[shutdown] close() --------> destruction of the capture socket and
deallocation of all associated

socket creation and destruction is straight forward, and is done
the same way with or without PACKET_MMAP:

int fd;

fd= socket(PF_PACKET, mode, htons(ETH_P_ALL))

where mode is SOCK_RAW for the raw interface were link level
information can be captured or SOCK_DGRAM for the cooked
interface where link level information capture is not
supported and a link level pseudo-header is provided
by the kernel.

The destruction of the socket and all associated resources
is done by a simple call to close(fd).

Next I will describe PACKET_MMAP settings and it's constraints,
also the maping of the circular buffer in the user process and
the use of this buffer.

+ PACKET_MMAP settings

To setup PACKET_MMAP from user level code is done with a call like

setsockopt(fd, SOL_PACKET, PACKET_RX_RING, (void *) &req, sizeof(req))

The most significant argument in the previous call is the req parameter,
this parameter must to have the following structure:

struct tpacket_req
unsigned int tp_block_size; /* Minimal size of contiguous block */
unsigned int tp_block_nr; /* Number of blocks */
unsigned int tp_frame_size; /* Size of frame */
unsigned int tp_frame_nr; /* Total number of frames */

This structure is defined in /usr/include/linux/if_packet.h and establishes a
circular buffer (ring) of unswappable memory mapped in the capture process.
Being mapped in the capture process allows reading the captured frames and
related meta-information like timestamps without requiring a system call.

Captured frames are grouped in blocks. Each block is a physically contiguous
region of memory and holds tp_block_size/tp_frame_size frames. The total number
of blocks is tp_block_nr. Note that tp_frame_nr is a redundant parameter because

frames_per_block = tp_block_size/tp_frame_size

indeed, packet_set_ring checks that the following condition is true

frames_per_block * tp_block_nr == tp_frame_nr

Lets see an example, with the following values:

tp_block_size= 4096
tp_frame_size= 2048
tp_block_nr = 4
tp_frame_nr = 8

we will get the following buffer structure:

block #1 block #2
+---------+---------+ +---------+---------+
| frame 1 | frame 2 | | frame 3 | frame 4 |
+---------+---------+ +---------+---------+

block #3 block #4
+---------+---------+ +---------+---------+
| frame 5 | frame 6 | | frame 7 | frame 8 |
+---------+---------+ +---------+---------+

A frame can be of any size with the only condition it can fit in a block. A block
can only hold an integer number of frames, or in other words, a frame cannot
be spawn accross two blocks so there are some datails you have to take into
account when choosing the frame_size. See "Maping and use of the circular
buffer (ring)".

+ PACKET_MMAP setting constraints

In kernel versions prior to 2.4.26 (for the 2.4 branch) and 2.6.5 (2.6 branch),
the PACKET_MMAP buffer could hold only 32768 frames in a 32 bit architecture or
16384 in a 64 bit architecture. For information on these kernel versions

Block size limit

As stated earlier, each block is a contiguous physical region of memory. These
memory regions are allocated with calls to the __get_free_pages() function. As
the name indicates, this function allocates pages of memory, and the second
argument is "order" or a power of two number of pages, that is
(for PAGE_SIZE == 4096) order=0 ==> 4096 bytes, order=1 ==> 8192 bytes,
order=2 ==> 16384 bytes, etc. The maximum size of a
region allocated by __get_free_pages is determined by the MAX_ORDER macro. More
precisely the limit can be calculated as:


In a i386 architecture PAGE_SIZE is 4096 bytes
In a 2.4/i386 kernel MAX_ORDER is 10
In a 2.6/i386 kernel MAX_ORDER is 11

So get_free_pages can allocate as much as 4MB or 8MB in a 2.4/2.6 kernel
respectively, with an i386 architecture.

User space programs can include /usr/include/sys/user.h and
/usr/include/linux/mmzone.h to get PAGE_SIZE MAX_ORDER declarations.

The pagesize can also be determined dynamically with the getpagesize (2)
system call.

Block number limit

To understand the constraints of PACKET_MMAP, we have to see the structure
used to hold the pointers to each block.

Currently, this structure is a dynamically allocated vector with kmalloc
called pg_vec, its size limits the number of blocks that can be allocated.

| x | x | x | x |
| | | |
| | | v
| | v block #4
| v block #3
v block #2
block #1

kmalloc allocates any number of bytes of phisically contiguous memory from
a pool of pre-determined sizes. This pool of memory is mantained by the slab
allocator wich is at the end the responsible for doing the allocation and
hence wich imposes the maximum memory that kmalloc can allocate.

In a 2.4/2.6 kernel and the i386 architecture, the limit is 131072 bytes. The
predetermined sizes that kmalloc uses can be checked in the "size-<bytes>"
entries of /proc/slabinfo

In a 32 bit architecture, pointers are 4 bytes long, so the total number of
pointers to blocks is

131072/4 = 32768 blocks

PACKET_MMAP buffer size calculator


<size-max> : is the maximum size of allocable with kmalloc (see /proc/slabinfo)
<pointer size>: depends on the architecture -- sizeof(void *)
<page size> : depends on the architecture -- PAGE_SIZE or getpagesize (2)
<max-order> : is the value defined with MAX_ORDER
<frame size> : it's an upper bound of frame's capture size (more on this later)

from these definitions we will derive

<block number> = <size-max>/<pointer size>
<block size> = <pagesize> << <max-order>

so, the max buffer size is

<block number> * <block size>

and, the number of frames be

<block number> * <block size> / <frame size>

Suposse the following parameters, wich apply for 2.6 kernel and an
i386 architecture:

<size-max> = 131072 bytes
<pointer size> = 4 bytes
<pagesize> = 4096 bytes
<max-order> = 11

and a value for <frame size> of 2048 byteas. These parameters will yield

<block number> = 131072/4 = 32768 blocks
<block size> = 4096 << 11 = 8 MiB.

and hence the buffer will have a 262144 MiB size. So it can hold
262144 MiB / 2048 bytes = 134217728 frames

Actually, this buffer size is not possible with an i386 architecture.
Remember that the memory is allocated in kernel space, in the case of
an i386 kernel's memory size is limited to 1GiB.

All memory allocations are not freed until the socket is closed. The memory
allocations are done with GFP_KERNEL priority, this basically means that
the allocation can wait and swap other process' memory in order to allocate
the nececessary memory, so normally limits can be reached.

Other constraints

If you check the source code you will see that what I draw here as a frame
is not only the link level frame. At the begining of each frame there is a
header called struct tpacket_hdr used in PACKET_MMAP to hold link level's frame
meta information like timestamp. So what we draw here a frame it's really
the following (from include/linux/if_packet.h):

Frame structure:

- Start. Frame must be aligned to TPACKET_ALIGNMENT=16
- struct tpacket_hdr
- struct sockaddr_ll
- Gap, chosen so that packet data (Start+tp_net) alignes to
- Start+tp_mac: [ Optional MAC header ]
- Start+tp_net: Packet data, aligned to TPACKET_ALIGNMENT=16.
- Pad to align to TPACKET_ALIGNMENT=16

The following are conditions that are checked in packet_set_ring

tp_block_size must be a multiple of PAGE_SIZE (1)
tp_frame_size must be greater than TPACKET_HDRLEN (obvious)
tp_frame_size must be a multiple of TPACKET_ALIGNMENT
tp_frame_nr must be exactly frames_per_block*tp_block_nr

Note that tp_block_size should be choosed to be a power of two or there will
be a waste of memory.

+ Maping and use of the circular buffer (ring)

The maping of the buffer in the user process is done with the conventional
mmap function. Even the circular buffer is compound of several physically
discontiguous blocks of memory, they are contiguous to the user space, hence
just one call to mmap is needed:

mmap(0, size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);

If tp_frame_size is a divisor of tp_block_size frames will be
contiguosly spaced by tp_frame_size bytes. If not, each
tp_block_size/tp_frame_size frames there will be a gap between
the frames. This is because a frame cannot be spawn across two

At the beginning of each frame there is an status field (see
struct tpacket_hdr). If this field is 0 means that the frame is ready
to be used for the kernel, If not, there is a frame the user can read
and the following flags apply:

from include/linux/if_packet.h

#define TP_STATUS_COPY 2

TP_STATUS_COPY : This flag indicates that the frame (and associated
meta information) has been truncated because it's
larger than tp_frame_size. This packet can be
read entirely with recvfrom().

In order to make this work it must to be
enabled previously with setsockopt() and

The number of frames than can be buffered to
be read with recvfrom is limited like a normal socket.
See the SO_RCVBUF option in the socket (7) man page.

TP_STATUS_LOSING : indicates there were packet drops from last time
statistics where checked with getsockopt() and

TP_STATUS_CSUMNOTREADY: currently it's used for outgoing IP packets wich
it's checksum will be done in hardware. So while
reading the packet we should not try to check the

for convenience there are also the following defines:

#define TP_STATUS_USER 1

The kernel initializes all frames to TP_STATUS_KERNEL, when the kernel
receives a packet it puts in the buffer and updates the status with
at least the TP_STATUS_USER flag. Then the user can read the packet,
once the packet is read the user must zero the status field, so the kernel
can use again that frame buffer.

The user can use poll (any other variant should apply too) to check if new
packets are in the ring:

struct pollfd pfd;

pfd.fd = fd;

if (status == TP_STATUS_KERNEL)
retval = poll(&pfd, 1, timeout);

It doesn't incur in a race condition to first check the status value and
then poll for frames.


Jesse Brandeburg, for fixing my grammathical/spelling errors

>>> EOF
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