dpif: Document.

Signed-off-by: Ben Pfaff <[email protected]>

lib/dpif.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2008, 2009, 2010, 2011, 2012 Nicira, Inc.
+ * Copyright (c) 2008, 2009, 2010, 2011, 2012, 2013 Nicira, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -14,7 +14,310 @@
  * limitations under the License.
  */
 
-
+/*
+ * dpif, the DataPath InterFace.
+ *
+ * In Open vSwitch terminology, a "datapath" is a flow-based software switch.
+ * A datapath has no intelligence of its own.  Rather, it relies entirely on
+ * its client to set up flows.  The datapath layer is core to the Open vSwitch
+ * software switch: one could say, without much exaggeration, that everything
+ * in ovs-vswitchd above dpif exists only to make the correct decisions
+ * interacting with dpif.
+ *
+ * Typically, the client of a datapath is the software switch module in
+ * "ovs-vswitchd", but other clients can be written.  The "ovs-dpctl" utility
+ * is also a (simple) client.
+ *
+ *
+ * Overview
+ * ========
+ *
+ * The terms written in quotes below are defined in later sections.
+ *
+ * When a datapath "port" receives a packet, it extracts the headers (the
+ * "flow").  If the datapath's "flow table" contains a "flow entry" whose flow
+ * is the same as the packet's, then it executes the "actions" in the flow
+ * entry and increments the flow's statistics.  If there is no matching flow
+ * entry, the datapath instead appends the packet to an "upcall" queue.
+ *
+ *
+ * Ports
+ * =====
+ *
+ * A datapath has a set of ports that are analogous to the ports on an Ethernet
+ * switch.  At the datapath level, each port has the following information
+ * associated with it:
+ *
+ *    - A name, a short string that must be unique within the host.  This is
+ *      typically a name that would be familiar to the system administrator,
+ *      e.g. "eth0" or "vif1.1", but it is otherwise arbitrary.
+ *
+ *    - A 32-bit port number that must be unique within the datapath but is
+ *      otherwise arbitrary.  The port number is the most important identifier
+ *      for a port in the datapath interface.
+ *
+ *    - A type, a short string that identifies the kind of port.  On a Linux
+ *      host, typical types are "system" (for a network device such as eth0),
+ *      "internal" (for a simulated port used to connect to the TCP/IP stack),
+ *      and "gre" (for a GRE tunnel).
+ *
+ *    - A Netlink PID (see "Upcall Queuing and Ordering" below).
+ *
+ * The dpif interface has functions for adding and deleting ports.  When a
+ * datapath implements these (e.g. as the Linux and netdev datapaths do), then
+ * Open vSwitch's ovs-vswitchd daemon can directly control what ports are used
+ * for switching.  Some datapaths might not implement them, or implement them
+ * with restrictions on the types of ports that can be added or removed
+ * (e.g. on ESX), on systems where port membership can only be changed by some
+ * external entity.
+ *
+ * Each datapath must have a port, sometimes called the "local port", whose
+ * name is the same as the datapath itself, with port number 0.  The local port
+ * cannot be deleted.
+ *
+ * Ports are available as "struct netdev"s.  To obtain a "struct netdev *" for
+ * a port named 'name' with type 'port_type', in a datapath of type
+ * 'datapath_type', call netdev_open(name, dpif_port_open_type(datapath_type,
+ * port_type).  The netdev can be used to get and set important data related to
+ * the port, such as:
+ *
+ *    - MTU (netdev_get_mtu(), netdev_set_mtu()).
+ *
+ *    - Ethernet address (netdev_get_etheraddr(), netdev_set_etheraddr()).
+ *
+ *    - Statistics such as the number of packets and bytes transmitted and
+ *      received (netdev_get_stats()).
+ *
+ *    - Carrier status (netdev_get_carrier()).
+ *
+ *    - Speed (netdev_get_features()).
+ *
+ *    - QoS queue configuration (netdev_get_queue(), netdev_set_queue() and
+ *      related functions.)
+ *
+ *    - Arbitrary port-specific configuration parameters (netdev_get_config(),
+ *      netdev_set_config()).  An example of such a parameter is the IP
+ *      endpoint for a GRE tunnel.
+ *
+ *
+ * Flow Table
+ * ==========
+ *
+ * The flow table is a hash table of "flow entries".  Each flow entry contains:
+ *
+ *    - A "flow", that is, a summary of the headers in an Ethernet packet.  The
+ *      flow is the hash key and thus must be unique within the flow table.
+ *      Flows are fine-grained entities that include L2, L3, and L4 headers.  A
+ *      single TCP connection consists of two flows, one in each direction.
+ *
+ *      In Open vSwitch userspace, "struct flow" is the typical way to describe
+ *      a flow, but the datapath interface uses a different data format to
+ *      allow ABI forward- and backward-compatibility.  datapath/README
+ *      describes the rationale and design.  Refer to OVS_KEY_ATTR_* and
+ *      "struct ovs_key_*" in include/linux/openvswitch.h for details.
+ *      lib/odp-util.h defines several functions for working with these flows.
+ *
+ *      (In case you are familiar with OpenFlow, datapath flows are analogous
+ *      to OpenFlow flow matches.  The most important difference is that
+ *      OpenFlow allows fields to be wildcarded and prioritized, whereas a
+ *      datapath's flow table is a hash table so every flow must be
+ *      exact-match, thus without priorities.)
+ *
+ *    - A list of "actions" that tell the datapath what to do with packets
+ *      within a flow.  Some examples of actions are OVS_ACTION_ATTR_OUTPUT,
+ *      which transmits the packet out a port, and OVS_ACTION_ATTR_SET, which
+ *      modifies packet headers.  Refer to OVS_ACTION_ATTR_* and "struct
+ *      ovs_action_*" in include/linux/openvswitch.h for details.
+ *      lib/odp-util.h defines several functions for working with datapath
+ *      actions.
+ *
+ *      The actions list may be empty.  This indicates that nothing should be
+ *      done to matching packets, that is, they should be dropped.
+ *
+ *      (In case you are familiar with OpenFlow, datapath actions are analogous
+ *      to OpenFlow actions.)
+ *
+ *    - Statistics: the number of packets and bytes that the flow has
+ *      processed, the last time that the flow processed a packet, and the
+ *      union of all the TCP flags in packets processed by the flow.  (The
+ *      latter is 0 if the flow is not a TCP flow.)
+ *
+ * The datapath's client manages the flow table, primarily in reaction to
+ * "upcalls" (see below).
+ *
+ *
+ * Upcalls
+ * =======
+ *
+ * A datapath sometimes needs to notify its client that a packet was received.
+ * The datapath mechanism to do this is called an "upcall".
+ *
+ * Upcalls are used in two situations:
+ *
+ *    - When a packet is received, but there is no matching flow entry in its
+ *      flow table (a flow table "miss"), this causes an upcall of type
+ *      DPIF_UC_MISS.  These are called "miss" upcalls.
+ *
+ *    - A datapath action of type OVS_ACTION_ATTR_USERSPACE causes an upcall of
+ *      type DPIF_UC_ACTION.  These are called "action" upcalls.
+ *
+ * An upcall contains an entire packet.  There is no attempt to, e.g., copy
+ * only as much of the packet as normally needed to make a forwarding decision.
+ * Such an optimization is doable, but experimental prototypes showed it to be
+ * of little benefit because an upcall typically contains the first packet of a
+ * flow, which is usually short (e.g. a TCP SYN).  Also, the entire packet can
+ * sometimes really be needed.
+ *
+ * After a client reads a given upcall, the datapath is finished with it, that
+ * is, the datapath doesn't maintain any lingering state past that point.
+ *
+ * The latency from the time that a packet arrives at a port to the time that
+ * it is received from dpif_recv() is critical in some benchmarks.  For
+ * example, if this latency is 1 ms, then a netperf TCP_CRR test, which opens
+ * and closes TCP connections one at a time as quickly as it can, cannot
+ * possibly achieve more than 500 transactions per second, since every
+ * connection consists of two flows with 1-ms latency to set up each one.
+ *
+ * To receive upcalls, a client has to enable them with dpif_recv_set().  A
+ * datapath should generally support multiple clients at once (e.g. so that one
+ * may run "ovs-dpctl show" or "ovs-dpctl dump-flows" while "ovs-vswitchd" is
+ * also running) but need not support multiple clients enabling upcalls at
+ * once.
+ *
+ *
+ * Upcall Queuing and Ordering
+ * ---------------------------
+ *
+ * The datapath's client reads upcalls one at a time by calling dpif_recv().
+ * When more than one upcall is pending, the order in which the datapath
+ * presents upcalls to its client is important.  The datapath's client does not
+ * directly control this order, so the datapath implementer must take care
+ * during design.
+ *
+ * The minimal behavior, suitable for initial testing of a datapath
+ * implementation, is that all upcalls are appended to a single queue, which is
+ * delivered to the client in order.
+ *
+ * The datapath should ensure that a high rate of upcalls from one particular
+ * port cannot cause upcalls from other sources to be dropped or unreasonably
+ * delayed.  Otherwise, one port conducting a port scan or otherwise initiating
+ * high-rate traffic spanning many flows could suppress other traffic.
+ * Ideally, the datapath should present upcalls from each port in a "round
+ * robin" manner, to ensure fairness.
+ *
+ * The client has no control over "miss" upcalls and no insight into the
+ * datapath's implementation, so the datapath is entirely responsible for
+ * queuing and delivering them.  On the other hand, the datapath has
+ * considerable freedom of implementation.  One good approach is to maintain a
+ * separate queue for each port, to prevent any given port's upcalls from
+ * interfering with other ports' upcalls.  If this is impractical, then another
+ * reasonable choice is to maintain some fixed number of queues and assign each
+ * port to one of them.  Ports assigned to the same queue can then interfere
+ * with each other, but not with ports assigned to different queues.  Other
+ * approaches are also possible.
+ *
+ * The client has some control over "action" upcalls: it can specify a 32-bit
+ * "Netlink PID" as part of the action.  This terminology comes from the Linux
+ * datapath implementation, which uses a protocol called Netlink in which a PID
+ * designates a particular socket and the upcall data is delivered to the
+ * socket's receive queue.  Generically, though, a Netlink PID identifies a
+ * queue for upcalls.  The basic requirements on the datapath are:
+ *
+ *    - The datapath must provide a Netlink PID associated with each port.  The
+ *      client can retrieve the PID with dpif_port_get_pid().
+ *
+ *    - The datapath must provide a "special" Netlink PID not associated with
+ *      any port.  dpif_port_get_pid() also provides this PID.  (ovs-vswitchd
+ *      uses this PID to queue special packets that must not be lost even if a
+ *      port is otherwise busy, such as packets used for tunnel monitoring.)
+ *
+ * The minimal behavior of dpif_port_get_pid() and the treatment of the Netlink
+ * PID in "action" upcalls is that dpif_port_get_pid() returns a constant value
+ * and all upcalls are appended to a single queue.
+ *
+ * The ideal behavior is:
+ *
+ *    - Each port has a PID that identifies the queue used for "miss" upcalls
+ *      on that port.  (Thus, if each port has its own queue for "miss"
+ *      upcalls, then each port has a different Netlink PID.)
+ *
+ *    - "miss" upcalls for a given port and "action" upcalls that specify that
+ *      port's Netlink PID add their upcalls to the same queue.  The upcalls
+ *      are delivered to the datapath's client in the order that the packets
+ *      were received, regardless of whether the upcalls are "miss" or "action"
+ *      upcalls.
+ *
+ *    - Upcalls that specify the "special" Netlink PID are queued separately.
+ *
+ *
+ * Packet Format
+ * =============
+ *
+ * The datapath interface works with packets in a particular form.  This is the
+ * form taken by packets received via upcalls (i.e. by dpif_recv()).  Packets
+ * supplied to the datapath for processing (i.e. to dpif_execute()) also take
+ * this form.
+ *
+ * A VLAN tag is represented by an 802.1Q header.  If the layer below the
+ * datapath interface uses another representation, then the datapath interface
+ * must perform conversion.
+ *
+ * The datapath interface requires all packets to fit within the MTU.  Some
+ * operating systems internally process packets larger than MTU, with features
+ * such as TSO and UFO.  When such a packet passes through the datapath
+ * interface, it must be broken into multiple MTU or smaller sized packets for
+ * presentation as upcalls.  (This does not happen often, because an upcall
+ * typically contains the first packet of a flow, which is usually short.)
+ *
+ * Some operating system TCP/IP stacks maintain packets in an unchecksummed or
+ * partially checksummed state until transmission.  The datapath interface
+ * requires all host-generated packets to be fully checksummed (e.g. IP and TCP
+ * checksums must be correct).  On such an OS, the datapath interface must fill
+ * in these checksums.
+ *
+ * Packets passed through the datapath interface must be at least 14 bytes
+ * long, that is, they must have a complete Ethernet header.  They are not
+ * required to be padded to the minimum Ethernet length.
+ *
+ *
+ * Typical Usage
+ * =============
+ *
+ * Typically, the client of a datapath begins by configuring the datapath with
+ * a set of ports.  Afterward, the client runs in a loop polling for upcalls to
+ * arrive.
+ *
+ * For each upcall received, the client examines the enclosed packet and
+ * figures out what should be done with it.  For example, if the client
+ * implements a MAC-learning switch, then it searches the forwarding database
+ * for the packet's destination MAC and VLAN and determines the set of ports to
+ * which it should be sent.  In any case, the client composes a set of datapath
+ * actions to properly dispatch the packet and then directs the datapath to
+ * execute those actions on the packet (e.g. with dpif_execute()).
+ *
+ * Most of the time, the actions that the client executed on the packet apply
+ * to every packet with the same flow.  For example, the flow includes both
+ * destination MAC and VLAN ID (and much more), so this is true for the
+ * MAC-learning switch example above.  In such a case, the client can also
+ * direct the datapath to treat any further packets in the flow in the same
+ * way, using dpif_flow_put() to add a new flow entry.
+ *
+ * Other tasks the client might need to perform, in addition to reacting to
+ * upcalls, include:
+ *
+ *    - Periodically polling flow statistics, perhaps to supply to its own
+ *      clients.
+ *
+ *    - Deleting flow entries from the datapath that haven't been used
+ *      recently, to save memory.
+ *
+ *    - Updating flow entries whose actions should change.  For example, if a
+ *      MAC learning switch learns that a MAC has moved, then it must update
+ *      the actions of flow entries that sent packets to the MAC at its old
+ *      location.
+ *
+ *    - Adding and removing ports to achieve a new configuration.
+ */
 #ifndef DPIF_H
 #define DPIF_H 1