pvxs/documentation/netconfig.rst

PVA Network Configuration

PVA network configuration is conventionally expressed through environment variables. New API users are suggested to start with pvxs::client::Context::fromEnv or pvxs::server::Config::fromEnv.

Environment variables

This table lists all of the $EPICS_PVA* environment variables understood by PVXS. See Client clientconf and Server serverconf for detailed explanations.

Many variables come in pairs of $EPICS_PVA_* and $EPICS_PVAS_*. A Client will look at only $EPICS_PVA_*. A server will prefer $EPICS_PVAS_* if set, or fallback to use the associated $EPICS_PVA_* if set.

Variable	Client	Server
EPICS_PVA_ADDR_LIST	x	x
EPICS_PVAS_BEACON_ADDR_LIST		x
EPICS_PVA_AUTO_ADDR_LIST	x	x
EPICS_PVAS_AUTO_BEACON_ADDR_LIST		x
EPICS_PVAS_INTF_ADDR_LIST		x
EPICS_PVA_SERVER_PORT	x	x
EPICS_PVAS_SERVER_PORT		x
EPICS_PVA_BROADCAST_PORT	x	x
EPICS_PVAS_BROADCAST_PORT		x
EPICS_PVAS_IGNORE_ADDR_LIST		x
EPICS_PVA_CONN_TMO	x	x
EPICS_PVA_NAME_SERVERS	x

Address Spec.

Space separated entries in EPICS_PVA*_ADDR_LIST variables must be in one of the following forms:

• <ip4-or-host>[:<port#>][,TTL#][@ifacename]
• "["<ip6-or-host>"]"[:<port#>][,TTL#][@ifacename]

Examples include:

myhost

Lookup hostname at startup, use default port number. Use OS routing table.

10.1.1.1:5076

Explicit IPv4 address and port number. Use OS routing table.

[2600:1234::42]

Explicit IPv6 address with default port. Use OS routing table.

224.0.2.3,255@192.168.1.1

IPv4 multicast address, with Time To Live set to 255. Send via the network interface with address 192.168.1.1. Use default port number.

[ff02::42:1],1@br0

IPv6 multicast address, with Time To Live set to 1 (roughly equivalent to IPv4 broadcast). Send via the network interface named br0. Use default port number.

PV Search Process

A PV Access network protocol operation proceeds in two phases: PV name resolution, and data transfer. Name resolution is the process is determining which PVA server claims to provide each PV name. Once this is known, a TCP connection is open to that server, and the operation(s) are executed.

The PVA Name resolution process is similar to Channel Access protocol.

When a name needs to be resolved, a PVA client will begin sending UDP search messages to any addresses listed in $EPICS_PVA_ADDR_LIST and also via TCP to any servers listed in $EPICS_PVA_NAME_SERVERS which can be reached.

UDP searches are by default sent to port 5076, subject to $EPICS_PVA_BROADCAST_PORT and port numbers explicitly given in $EPICS_PVA_ADDR_LIST.

The addresses in $EPICS_PVA_ADDR_LIST may include IPv4/6 unicast, multicast, and/or broadcast addresses. By default (cf. $EPICS_PVA_AUTO_ADDR_LIST) the address list is automatically populated with the IPv4 broadcast addresses of all local network interfaces.

Searches will be repeated periodically in perpetuity until a positive response is received, or the operation is cancelled.

In order to reduce the number of broadcast packets, which every PVA host must process, the time between searches will initially be short, then gradually increase as time passes without a positive response.

Server beacon destinations are by default configured using the client configuration. This may be overridden with $EPICS_PVAS_BEACON_ADDR_LIST and $EPICS_PVAS_AUTO_BEACON_ADDR_LIST.