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FreeRTOS+UDP was removed from the FreeRTOS kernel download from FreeRTOS V10.1.0. See the FreeRTOS+TCP stack, which can be configured for UDP only use, as an alternative.

FreeRTOS+UDP Configuration
The FreeRTOSIPConfig.h header file

FreeRTOS+UDP applications must provide a FreeRTOSIPConfig.h header file - in which the parameters described on this page can be defined.

Hardware Specific Settings


If the microcontroller on which FreeRTOS+UDP is running is big endian then ipconfigBYTE_ORDER must be set to FREERTOS_BIG_ENDIAN. If the microcontroller is little endian then ipconfigBYTE_ORDER must be set to FREERTOS_LITTLE_ENDIAN. The Byte Order and Endian section of the Embedded Networking Basics and Glossary page provides an explanation of byte order considerations in IP networks.

Constants Affecting the Stack Execution Behaviour


The IP stack executes it its own task (although any application task can make use of its services through the published sockets API). ipconfigUDP_TASK_PRIORITY sets the priority of the task that executes the IP stack.

The priority is a standard FreeRTOS task priority so can take any value from 0 (the lowest priority) to (configMAX_PRIORITIES - 1) (the highest priority). configMAX_PRIORITIES is a standard FreeRTOS configuration parameter defined in FreeRTOSConfig.h, not FreeRTOSIPConfig.h.

Consideration needs to be given as to the priority assigned to the task executing the IP stack relative to the priority assigned to tasks that use the IP stack.


The size, in words (not bytes), of the stack allocated to the FreeRTOS+UDP task. FreeRTOS includes optional stack overflow detection.


If ipconfigUSE_NETWORK_EVENT_HOOK is set to 1 then FreeRTOS+UDP will call the network event hook at the appropriate times. If ipconfigUSE_NETWORK_EVENT_HOOK is not set to 1 then the network event hook will never be called.


A FreeRTOS queue is used to send events from application tasks to the IP stack. ipconfigEVENT_QUEUE_LENGTH sets the maximum number of events that can be queued for processing at any one time. The event queue must be a minimum of 5 greater than the total number of network buffers.

Constants Affecting the Sockets API Behaviour


Sockets have a send block time attribute. If FreeRTOS_sendto() is called but a network buffer cannot be obtained then the calling task is held in the Blocked state (so other tasks can continue to executed) until either a network buffer becomes available or the send block time expires. If the send block time expires then the send operation is aborted.

The maximum allowable send block time is capped to the value set by ipconfigMAX_SEND_BLOCK_TIME_TICKS. Capping the maximum allowable send block time prevents prevents a deadlock occurring when all the network buffers are in use and the tasks that process (and subsequently free) the network buffers are themselves blocked waiting for a network buffer.

ipconfigMAX_SEND_BLOCK_TIME_TICKS is specified in RTOS ticks. A time in milliseconds can be converted to a time in ticks by dividing the time in milliseconds by portTICK_PERIOD_MS.


Implementing FreeRTOS_inet_addr() necessitates the use of string handling routines, which are relatively large. To save code space the full FreeRTOS_inet_addr() implementation is made optional, and a smaller and faster alternative called FreeRTOS_inet_addr_quick() is provided. FreeRTOS_inet_addr() takes an IP in decimal dot format (for example, "") as its parameter. FreeRTOS_inet_addr_quick() takes an IP address as four separate numerical octets (for example, 192, 168, 0, 1) as its parameters. If ipconfigINCLUDE_FULL_INET_ADDR is set to 1 then both FreeRTOS_inet_addr() and FreeRTOS_indet_addr_quick() are available. If ipconfigINCLUDE_FULL_INET_ADDR is not set to 1 then only FreeRTOS_indet_addr_quick() is available.


The address of a socket is the combination of its IP address and its port number. FreeRTOS_bind() is used to manually allocate a port number to a socket (to 'bind' the socket to a port), but manual binding is not normally necessary for client sockets (those sockets that initiate outgoing connections rather than wait for incoming connections on a known port number). If ipconfigALLOW_SOCKET_SEND_WITHOUT_BIND is set to 1 then calling FreeRTOS_sendto() on a socket that has not yet been bound will result in the IP stack automatically binding the socket to a port number from the range socketAUTO_PORT_ALLOCATION_START_NUMBER to 0xffff. If ipconfigALLOW_SOCKET_SEND_WITHOUT_BIND is set to 0 then calling FreeRTOS_sendto() on a socket that has not yet been bound will result in the send operation being aborted.

Constants Affecting the ARP Behaviour


The ARP cache is a table that maps IP addresses to MAC addresses.

The IP stack can only send a UDP message to a remove IP address if it knowns the MAC address associated with the IP address, or the MAC address of the router used to contact the remote IP address. When a UDP message is received from a remote IP address the MAC address and IP address are added to the ARP cache. When a UDP message is sent to a remote IP address that does not already appear in the ARP cache then the UDP message is replaced by a ARP message that solicits the required MAC address information.

ipconfigARP_CACHE_ENTRIES defines the maximum number of entries that can exist in the ARP table at any one time.


ARP requests that do not result in an ARP response will be re-transmitted a maximum of ipconfigMAX_ARP_RETRANSMISSIONS times before the ARP request is aborted.


ipconfigMAX_ARP_AGE defines the maximum time between an entry in the ARP table being created or refreshed and the entry being removed because it is stale. New ARP requests are sent for ARP cache entries that are nearing their maximum age.

ipconfigMAX_ARP_AGE is specified in tens of seconds, so a value of 150 is equal to 1500 seconds (or 25 minutes). */

Constants Affecting DHCP and DNS Behaviour


Set ipconfigUSE_DNS to 1 to include a basic DNS client/resolver. DNS is used through the FreeRTOS_gethostbyname() API function.


If ipconfigUSE_DHCP is 1 then FreeRTOS+UDP will attempt to retrieve an IP address, netmask, DNS server address and gateway address from a DHCP server - and revert to using the defined static address if an IP address cannot be obtained.

If ipconfigUSE_DHCP is 0 then FreeRTOS+UDP will not attempt to obtain its address information from a DHCP server, and instead immediately use the defined static address information.


When ipconfigUSE_DHCP is set to 1, DHCP requests will be sent out at increasing time intervals until either a reply is received from a DHCP server and accepted, or the interval between transmissions reaches ipconfigMAXIMUM_DISCOVER_TX_PERIOD. The IP stack will revert to using the static IP address passed as a parameter to FreeRTOS_IPInit() if the re-transmission time interval reaches ipconfigMAXIMUM_DISCOVER_TX_PERIOD without a DHCP reply being received.


ipconfigRAND32() is called by the IP stack to generate a random number that is then used as a DHCP transaction number. Random number generation is performed via this macro to allow applications to use their own random number generation method. For example, it might be possible to generate a random number by sampling noise on an analogue input.

Constants Affecting IP and ICMP Behaviour


Defines the Time To Live (TTL) values used in outgoing UDP packets.


If ipconfigCAN_FRAGMENT_OUTGOING_PACKETS is set to 1 then UDP packets that contain more data than will fit in a single network frame will be fragmented across multiple IP packets. Also see the ipconfigNETWORK_MTU setting. If ipconfigCAN_FRAGMENT_OUTGOING_PACKETS is 1 then (ipconfigNETWORK_MTU - 28) must be divisible by 8. Setting ipconfigCAN_FRAGMENT_OUTGOING_PACKETS to 1 will increase both the code size and execution time.


The MTU is the maximum number of bytes the payload of a network frame can contain. For normal Ethernet V2 frames the maximum MTU is 1500. Setting a lower value can save RAM, depending on the buffer management scheme used. If ipconfigCAN_FRAGMENT_OUTGOING_PACKETS is 1 then (ipconfigNETWORK_MTU - 28) must be divisible by 8.


If ipconfigREPLY_TO_INCOMING_PINGS is set to 1 then the IP stack will generate replies to incoming ICMP echo (ping) requests.


If ipconfigSUPPORT_OUTGOING_PINGS is set to 1 then the FreeRTOS_SendPingRequest() API function is available.

Constants Affecting the Network Driver Behaviour


ipconfigNUM_NETWORK_BUFFERS defines the total number of network buffer that are available to the IP stack. The total number of network buffers is limited to ensure the total amount of RAM that can be consumed by the IP stack is capped to a pre-determinable value. How the storage area is actually allocated to the network buffer structures is not fixed, but part of the portable layer. The simplest scheme simply allocates the exact amount of storage as it is required.


If ipconfigFILTER_OUT_NON_ETHERNET_II_FRAMES is set to 1 then Ethernet frames that are not in Ethernet II format will be dropped. This option is included for potential future IP stack developments.


If ipconfigETHERNET_DRIVER_FILTERS_FRAME_TYPES is set to 1 then it is the responsibility of the Ethernet interface to filter out packets that are of no interest. If the Ethernet interface does not implement this functionality, then set ipconfigETHERNET_DRIVER_FILTERS_FRAME_TYPES to 0 to have the IP stack perform the filtering instead (it is much less efficient for the stack to do it because the packet will already have been passed into the stack). If the Ethernet driver does all the necessary filtering in hardware then software filtering can be removed by using a value other than 1 or 0.

Constants specific to FreeRTOS+Nabto


Set ipconfigFREERTOS_PLUS_NABTO to 1 to support the Nabto protocol, or 0 to exclude support for the Nabto protocol. If ipconfigFREERTOS_PLUS_NABTO is set to one then the project must include the FreeRTOS+Nabto source code (or reference a pre-build FreeRTOS+Nabto library).

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