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Diffstat (limited to 'Documentation/networking/packet_mmap.txt')
| -rw-r--r-- | Documentation/networking/packet_mmap.txt | 140 |
1 files changed, 121 insertions, 19 deletions
diff --git a/Documentation/networking/packet_mmap.txt b/Documentation/networking/packet_mmap.txt index 07c53d59603..a22fd85e379 100644 --- a/Documentation/networking/packet_mmap.txt +++ b/Documentation/networking/packet_mmap.txt @@ -4,16 +4,18 @@ 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 +capture network traffic with utilities like tcpdump or any other that needs +raw access to network interface. +You can find the latest version of this document at: http://pusa.uv.es/~ulisses/packet_mmap/ -Please send me your comments to +Howto can be found at: + http://wiki.gnu-log.net (packet_mmap) +Please send your comments to Ulisses Alonso Camaró <uaca@i.hate.spam.alumni.uv.es> + Johann Baudy <johann.baudy@gnu-log.net> ------------------------------------------------------------------------------- + Why use PACKET_MMAP @@ -25,19 +27,24 @@ 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. +configurable circular buffer mapped in user space that can be used to either +send or receive packets. This way reading packets just needs to wait for them, +most of the time there is no need to issue a single system call. Concerning +transmission, multiple packets can be sent through one system call to get the +highest bandwidth. +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 and +transmission 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. For transmission, check the MTU (Maximum Transmission Unit) used and +supported by devices of your network. -------------------------------------------------------------------------------- -+ How to use CONFIG_PACKET_MMAP ++ How to use CONFIG_PACKET_MMAP to improve capture process -------------------------------------------------------------------------------- From the user standpoint, you should use the higher level libpcap library, which @@ -57,7 +64,7 @@ the low level details or want to improve libpcap by including PACKET_MMAP support. -------------------------------------------------------------------------------- -+ How to use CONFIG_PACKET_MMAP directly ++ How to use CONFIG_PACKET_MMAP directly to improve capture process -------------------------------------------------------------------------------- From the system calls stand point, the use of PACKET_MMAP involves @@ -66,6 +73,7 @@ the following process: [setup] socket() -------> creation of the capture socket setsockopt() ---> allocation of the circular buffer (ring) + option: PACKET_RX_RING mmap() ---------> mapping of the allocated buffer to the user process @@ -97,13 +105,75 @@ also the mapping of the circular buffer in the user process and the use of this buffer. -------------------------------------------------------------------------------- ++ How to use CONFIG_PACKET_MMAP directly to improve transmission process +-------------------------------------------------------------------------------- +Transmission process is similar to capture as shown below. + +[setup] socket() -------> creation of the transmission socket + setsockopt() ---> allocation of the circular buffer (ring) + option: PACKET_TX_RING + bind() ---------> bind transmission socket with a network interface + mmap() ---------> mapping of the allocated buffer to the + user process + +[transmission] poll() ---------> wait for free packets (optional) + send() ---------> send all packets that are set as ready in + the ring + The flag MSG_DONTWAIT can be used to return + before end of transfer. + +[shutdown] close() --------> destruction of the transmission socket and + deallocation of all associated resources. + +Binding the socket to your network interface is mandatory (with zero copy) to +know the header size of frames used in the circular buffer. + +As capture, each frame contains two parts: + + -------------------- +| struct tpacket_hdr | Header. It contains the status of +| | of this frame +|--------------------| +| data buffer | +. . Data that will be sent over the network interface. +. . + -------------------- + + bind() associates the socket to your network interface thanks to + sll_ifindex parameter of struct sockaddr_ll. + + Initialization example: + + struct sockaddr_ll my_addr; + struct ifreq s_ifr; + ... + + strncpy (s_ifr.ifr_name, "eth0", sizeof(s_ifr.ifr_name)); + + /* get interface index of eth0 */ + ioctl(this->socket, SIOCGIFINDEX, &s_ifr); + + /* fill sockaddr_ll struct to prepare binding */ + my_addr.sll_family = AF_PACKET; + my_addr.sll_protocol = ETH_P_ALL; + my_addr.sll_ifindex = s_ifr.ifr_ifindex; + + /* bind socket to eth0 */ + bind(this->socket, (struct sockaddr *)&my_addr, sizeof(struct sockaddr_ll)); + + A complete tutorial is available at: http://wiki.gnu-log.net/ + +-------------------------------------------------------------------------------- + PACKET_MMAP settings -------------------------------------------------------------------------------- To setup PACKET_MMAP from user level code is done with a call like + - Capture process setsockopt(fd, SOL_PACKET, PACKET_RX_RING, (void *) &req, sizeof(req)) + - Transmission process + setsockopt(fd, SOL_PACKET, PACKET_TX_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: @@ -117,11 +187,11 @@ this parameter must to have the following structure: }; 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. +circular buffer (ring) of unswappable memory. 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 +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 @@ -336,6 +406,7 @@ 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: ++++ Capture process: from include/linux/if_packet.h #define TP_STATUS_COPY 2 @@ -391,6 +462,37 @@ packets are in the ring: It doesn't incur in a race condition to first check the status value and then poll for frames. + +++ Transmission process +Those defines are also used for transmission: + + #define TP_STATUS_AVAILABLE 0 // Frame is available + #define TP_STATUS_SEND_REQUEST 1 // Frame will be sent on next send() + #define TP_STATUS_SENDING 2 // Frame is currently in transmission + #define TP_STATUS_WRONG_FORMAT 4 // Frame format is not correct + +First, the kernel initializes all frames to TP_STATUS_AVAILABLE. To send a +packet, the user fills a data buffer of an available frame, sets tp_len to +current data buffer size and sets its status field to TP_STATUS_SEND_REQUEST. +This can be done on multiple frames. Once the user is ready to transmit, it +calls send(). Then all buffers with status equal to TP_STATUS_SEND_REQUEST are +forwarded to the network device. The kernel updates each status of sent +frames with TP_STATUS_SENDING until the end of transfer. +At the end of each transfer, buffer status returns to TP_STATUS_AVAILABLE. + + header->tp_len = in_i_size; + header->tp_status = TP_STATUS_SEND_REQUEST; + retval = send(this->socket, NULL, 0, 0); + +The user can also use poll() to check if a buffer is available: +(status == TP_STATUS_SENDING) + + struct pollfd pfd; + pfd.fd = fd; + pfd.revents = 0; + pfd.events = POLLOUT; + retval = poll(&pfd, 1, timeout); + -------------------------------------------------------------------------------- + THANKS -------------------------------------------------------------------------------- |