/lib/netlink-socket.h
C Header | 279 lines | 54 code | 21 blank | 204 comment | 0 complexity | 4174e24faa98d8148d66ea219cc68a7e MD5 | raw file
Possible License(s): Apache-2.0, LGPL-2.1
- /*
- * Copyright (c) 2008, 2009, 2010, 2011, 2012, 2013, 2014 Nicira, Inc.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at:
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
- #ifndef NETLINK_SOCKET_H
- #define NETLINK_SOCKET_H 1
- /* Netlink socket definitions.
- *
- * This header file defines functions for working with Netlink sockets. Only
- * Linux natively supports Netlink sockets, but Netlink is well suited as a
- * basis for extensible low-level protocols, so it can make sense to implement
- * a Netlink layer on other systems. This doesn't have to be done in exactly
- * the same way as on Linux, as long as the implementation can support the
- * semantics that are important to Open vSwitch. See "Usage concepts" below
- * for more information.
- *
- * For Netlink protocol definitions, see netlink-protocol.h. For helper
- * functions for working with Netlink messages, see netlink.h.
- *
- *
- * Usage concepts
- * ==============
- *
- * Netlink is a datagram-based network protocol primarily for communication
- * between user processes and the kernel. Netlink is specified in RFC 3549,
- * "Linux Netlink as an IP Services Protocol".
- *
- * Netlink is not suitable for use in physical networks of heterogeneous
- * machines because host byte order is used throughout.
- *
- * The AF_NETLINK socket namespace is subdivided into statically numbered
- * protocols, e.g. NETLINK_ROUTE, NETLINK_NETFILTER, provided as the third
- * argument to the socket() function. Maintaining the assigned numbers became
- * a bit of a problem, so the "Generic Netlink" NETLINK_GENERIC protocol was
- * introduced to map between human-readable names and dynamically assigned
- * numbers. All recently introduced Netlink protocol messages in Linux
- * (including all of the Open vSwitch specific messages) fall under
- * NETLINK_GENERIC. The Netlink library provides the nl_lookup_genl_family()
- * function for translating a Generic Netlink name to a number. On Linux, this
- * queries the kernel Generic Netlink implementation, but on other systems it
- * might be easier to statically assign each of the names used by Open vSwitch
- * and then implement this function entirely in userspace.
- *
- * Each Netlink socket is distinguished by its Netlink PID, a 32-bit integer
- * that is analogous to a TCP or UDP port number. The kernel has PID 0.
- *
- * Most Netlink messages manage a kernel table of some kind, e.g. the kernel
- * routing table, ARP table, etc. Open vSwitch specific messages manage tables
- * of datapaths, ports within datapaths ("vports"), and flows within
- * datapaths. Open vSwitch also has messages related to network packets
- * received on vports, which aren't really a table.
- *
- * Datagram protocols over a physical network are typically unreliable: in UDP,
- * for example, messages can be dropped, delivered more than once, or delivered
- * out of order. In Linux, Netlink does not deliver messages out of order or
- * multiple times. In some cases it can drop messages, but the kernel
- * indicates when a message has been dropped. The description below of each
- * way Open vSwitch uses Netlink also explains how to work around dropped
- * messages.
- *
- * Open vSwitch uses Netlink in four characteristic ways:
- *
- * 1. Transactions. A transaction is analogous to a system call, an ioctl,
- * or an RPC: userspace sends a request to the kernel, which processes
- * the request synchronously and returns a reply to userspace.
- * (Sometimes there is no explicit reply, but even in that case userspace
- * will receive an immediate reply if there is an error.)
- *
- * nl_transact() is the primary interface for transactions over Netlink.
- * This function doesn't take a socket as a parameter because sockets do
- * not have any state related to transactions.
- *
- * Netlink uses 16-bit "length" fields extensively, which effectively
- * limits requests and replies to 64 kB. "Dumps" (see below) are one way
- * to work around this limit for replies.
- *
- * In the Linux implementation of Netlink transactions, replies can
- * sometimes be lost. When this happens, nl_transact() automatically
- * executes the transaction again. This means that it is important that
- * transactions be idempotent, or that the client be prepared to tolerate
- * that a transaction might actually execute more than once.
- *
- * The Linux implementation can execute several transactions at the same
- * time more efficiently than individually. nl_transact_multiple()
- * allows for this. The semantics are no different from executing each
- * of the transactions individually with nl_transact().
- *
- * 2. Dumps. A dump asks the kernel to provide all of the information in a
- * table. It consists of a request and a reply, where the reply consists
- * of an arbitrary number of messages. Each message in the reply is
- * limited to 64 kB, as is the request, but the total size of the reply
- * can be many times larger.
- *
- * The reply to a dump is usually generated piece by piece, not
- * atomically. The reply can represent an inconsistent snapshot of the
- * table. This is especially likely if entries in the table were being
- * added or deleted or changing during the dump.
- *
- * nl_dump_start() begins a dump based on the caller-provided request and
- * initializes a "struct nl_dump" to identify the dump. Subsequent calls
- * to nl_dump_next() then obtain the reply, one message at a time.
- * Usually, each message gives information about some entry in a table,
- * e.g. one flow in the Open vSwitch flow table, or one route in a
- * routing table. nl_dump_done() ends the dump.
- *
- * Linux implements dumps so that messages in a reply do not get lost.
- *
- * 3. Multicast subscriptions. Most kernel Netlink implementations allow a
- * process to monitor changes to its table, by subscribing to a Netlink
- * multicast group dedicated to that table. Whenever the table's content
- * changes (e.g. an entry is added or deleted or modified), the Netlink
- * implementation sends a message to all sockets that subscribe to its
- * multicast group notifying it of details of the change. (This doesn't
- * require much extra work by the Netlink implementer because the message
- * is generally identical to the one sent as a reply to the request that
- * changed the table.)
- *
- * nl_sock_join_mcgroup() subscribes a socket to a multicast group, and
- * nl_sock_recv() reads notifications.
- *
- * If userspace doesn't read messages from a socket subscribed to a
- * multicast group quickly enough, then notification messages can pile up
- * in the socket's receive buffer. If this continues long enough, the
- * receive buffer will fill up and notifications will be lost. In that
- * case, nl_sock_recv() will return ENOBUFS. The client can then use a
- * dump to resynchronize with the table state. (A simple implementation
- * of multicast groups might take advantage of this by simply returning
- * ENOBUFS whenever a table changes, without implementing actual
- * notifications. This would cause lots of extra dumps, so it may not be
- * suitable as a production implementation.)
- *
- * 4. Unicast subscriptions (Open vSwitch specific). Userspace can assign
- * one or more Netlink PIDs to a vport as "upcall PIDs". When a packet
- * received on the vport does not match any flow in its datapath's flow
- * table, the kernel hashes some of the packet's headers, uses the hash
- * to select one of the PIDs, and sends the packet (encapsulated in an
- * Open vSwitch Netlink message) to the socket with the selected PID.
- *
- * nl_sock_recv() reads notifications sent this way.
- *
- * Specifically on Windows platform, the datapath needs to allocate a
- * queue for packets, and it does so only when userspace "subscribe"'s to
- * packets on that netlink socket. Before closing the netlink socket,
- * userspace needs to "unsubscribe" packets on that netlink socket.
- *
- * nl_sock_subscribe_packets() and nl_sock_unsubscribe_packets() are
- * Windows specific.
- *
- * Messages received this way can overflow, just like multicast
- * subscription messages, and they are reported the same way. Because
- * packet notification messages do not report the state of a table, there
- * is no way to recover the dropped packets; they are simply lost.
- *
- * The main reason to support multiple PIDs per vport is to increase
- * fairness, that is, to make it harder for a single high-flow-rate
- * sender to drown out lower rate sources. Multiple PIDs per vport might
- * also improve packet handling latency or flow setup rate, but that is
- * not the main goal.
- *
- * Old versions of the Linux kernel module supported only one PID per
- * vport, and userspace still copes with this, so a simple or early
- * implementation might only support one PID per vport too.
- *
- *
- * Thread-safety
- * =============
- *
- * Most of the netlink functions are not fully thread-safe: Only a single
- * thread may use a given nl_sock or nl_dump at one time. The exceptions are:
- *
- * - nl_sock_recv() is conditionally thread-safe: it may be called from
- * different threads with the same nl_sock, but each caller must provide
- * an independent receive buffer.
- *
- * - nl_dump_next() is conditionally thread-safe: it may be called from
- * different threads with the same nl_dump, but each caller must provide
- * independent buffers.
- */
- #include <stdbool.h>
- #include <stddef.h>
- #include <stdint.h>
- #include "ofpbuf.h"
- #include "ovs-atomic.h"
- #include "ovs-thread.h"
- struct nl_sock;
- #ifndef HAVE_NETLINK
- #ifndef _WIN32
- #error "netlink-socket.h is only for hosts that support Netlink sockets"
- #endif
- #endif
- /* Netlink sockets. */
- int nl_sock_create(int protocol, struct nl_sock **);
- int nl_sock_clone(const struct nl_sock *, struct nl_sock **);
- void nl_sock_destroy(struct nl_sock *);
- int nl_sock_join_mcgroup(struct nl_sock *, unsigned int multicast_group);
- int nl_sock_leave_mcgroup(struct nl_sock *, unsigned int multicast_group);
- #ifdef _WIN32
- int nl_sock_subscribe_packets(struct nl_sock *sock);
- int nl_sock_unsubscribe_packets(struct nl_sock *sock);
- #endif
- int nl_sock_send(struct nl_sock *, const struct ofpbuf *, bool wait);
- int nl_sock_send_seq(struct nl_sock *, const struct ofpbuf *,
- uint32_t nlmsg_seq, bool wait);
- int nl_sock_recv(struct nl_sock *, struct ofpbuf *, bool wait);
- int nl_sock_drain(struct nl_sock *);
- void nl_sock_wait(const struct nl_sock *, short int events);
- int nl_sock_fd(const struct nl_sock *);
- uint32_t nl_sock_pid(const struct nl_sock *);
- /* Batching transactions. */
- struct nl_transaction {
- /* Filled in by client. */
- struct ofpbuf *request; /* Request to send. */
- /* The client must initialize 'reply' to one of:
- *
- * - NULL, if it does not care to examine the reply.
- *
- * - Otherwise, to an ofpbuf with a memory allocation of at least
- * NLMSG_HDRLEN bytes.
- */
- struct ofpbuf *reply; /* Reply (empty if reply was an error code). */
- int error; /* Positive errno value, 0 if no error. */
- };
- /* Transactions without an allocated socket. */
- int nl_transact(int protocol, const struct ofpbuf *request,
- struct ofpbuf **replyp);
- void nl_transact_multiple(int protocol, struct nl_transaction **, size_t n);
- /* Table dumping. */
- #define NL_DUMP_BUFSIZE 4096
- struct nl_dump {
- /* These members are immutable during the lifetime of the nl_dump. */
- struct nl_sock *sock; /* Socket being dumped. */
- uint32_t nl_seq; /* Expected nlmsg_seq for replies. */
- /* 'mutex' protects 'status' and serializes access to 'sock'. */
- struct ovs_mutex mutex; /* Protects 'status', synchronizes recv(). */
- int status OVS_GUARDED; /* 0: dump in progress,
- * positive errno: dump completed with error,
- * EOF: dump completed successfully. */
- };
- void nl_dump_start(struct nl_dump *, int protocol,
- const struct ofpbuf *request);
- bool nl_dump_next(struct nl_dump *, struct ofpbuf *reply, struct ofpbuf *buf);
- int nl_dump_done(struct nl_dump *);
- /* Miscellaneous */
- int nl_lookup_genl_family(const char *name, int *number);
- int nl_lookup_genl_mcgroup(const char *family_name, const char *group_name,
- unsigned int *multicast_group);
- #endif /* netlink-socket.h */