# &adminguide;

## Overview

This module provides SCTP transport layer for &kamailio;. SCTP is an acronym for Stream Control Transmission Protocol, read more about it at: http://en.wikipedia.org/wiki/Stream_Control_Transmission_Protocol

The module itself implements the callbacks required by the core to receive and send SIP messages over SCTP sockets.

The core Makefile variable SCTP must be set to 1 (which is by default set to 1 in Makefile.defs) and sources compiled with -DUSE_SCTP (automatically set when SCTP=1). In other words, if core Makefiles are not changed and SCTP variable is not overwritten from command line, then the SCTP support in core is enabled.

Note: it is recommended to use recent versions of libsctp (>1.0.10) and kernel (>4.0.0) when enabling SCTP transport.

## Dependencies

### &kamailio; Modules

The following modules must be loaded before this module:

- none.

### External Libraries or Applications

The following libraries or applications must be installed before running &kamailio; with this module loaded:

- libsctp - SCTP user space library available on Linux. To compile the module, libsctp-dev is required as well.

## Parameters

### `sctp_socket_rcvbuf` (int)

Size for the sctp socket receive buffer.

Default value is automatically set based on OS limits.

#### Set `sctp_socket_rcvbuf` parameter

    ...
    modparam("sctp", "sctp_socket_rcvbuf", 14096)
    ...

### `sctp_socket_sndbuf` (int)

Size for the sctp socket send buffer.

Default value is automatically set based on OS limits.

#### Set `sctp_socket_sndbuf` parameter

    ...
    modparam("sctp", "sctp_socket_sndbuf", 14096)
    ...

### `sctp_autoclose` (int)

Number of seconds before autoclosing an idle association. Can be changed at runtime, but it will affect only new associations.

Default value is 180 (seconds).

#### Set `sctp_autoclose` parameter

    ...
    # kamctl rpc cfg.set_now_int sctp autoclose 120
    ...
    modparam("sctp", "sctp_autoclose", 300)
    ...

### `sctp_send_ttl` (int)

Number of milliseconds before an unsent message/chunk is dropped. Can be changed at runtime.

Default value is 32000 (milliseconds - 32 seconds).

#### Set `sctp_send_ttl` parameter

    ...
    # kamctl rpc cfg.set_now_int sctp send_ttl 180000
    ...
    modparam("sctp", "sctp_send_ttl", 10000)
    ...

### `sctp_send_retries` (int)

How many times to attempt re-sending a message on a re-opened association, if the sctp stack did give up sending it (it's not related to sctp protocol level retransmission). Useful to improve reliability with peers that reboot/restart or fail over to another machine.

WARNING: use with care and low values (e.g. 1-3) to avoid "multiplying" traffic to unresponding hosts.

Can be changed at runtime.

Default value is 0.

#### Set `sctp_send_retries` parameter

    ...
    modparam("sctp", "sctp_send_retries", 1)
    ...

### `sctp_assoc_tracking` (int)

Controls whether or not sctp associations are tracked inside &kamailio;. Turning it off would result in less memory being used and slightly better performance, but it will also disable some other features that depend on it (e.g. sctp_assoc_reuse).

Can be changed at runtime (&sercmd; sctp assoc_tracking 0), but changes will be allowed only if all the other features that depend on it are turned off (for example it can be turned off only if first sctp_assoc_reuse was turned off).

Note: turning sctp_assoc_tracking on/off will delete all the tracking information for all the currently tracked associations and might introduce a small temporary delay in the sctp processing if lots of associations were tracked.

Config options depending on sctp_assoc_tracking being on: sctp_assoc_reuse.

Default value is 1 (enabled, 0 - disabled).

#### Set `sctp_assoc_tracking` parameter

    ...
    modparam("sctp", "sctp_assoc_tracking", 0)
    ...

### `sctp_assoc_reuse` (int)

Controls sctp association reuse. For now only association reuse for replies is affected by it. Default is on. Depends on sctp_assoc_tracking being on.

Note that even if turned off, if the port in via corresponds to the source port of the association the request was sent on or if rport is turned on (force_rport() or via containing a rport option), the association will be automatically reused by the sctp stack. Can be changed at runtime (sctp assoc_reuse), but it can be turned on only if sctp_assoc_tracking is on.

Default value is 1 (enabled, 0 - disabled).

#### Set `sctp_assoc_reuse` parameter

    ...
    modparam("sctp", "sctp_assoc_reuse", 0)
    ...

### `sctp_max_assocs` (int)

Maximum number of allowed open sctp associations. -1 means maximum allowed by the OS. Default: -1. Can be changed at runtime (e.g.: kamctl rpc cfg.set_now_int sctp max_assocs 10). When the maximum associations number is exceeded and a new associations is opened by a remote host, the association will be immediately closed. However it is possible that some sip packets get through (especially if they are sent early, as part of the 4-way handshake).

When &kamailio; tries to open a new association and the max_assocs is exceeded the exact behaviour depends on whether or not sctp_assoc_tracking is on. If on, the send triggering the active open will gracefully fail, before actually opening the new association and no packet will be sent. However if sctp_assoc_tracking is off, the association will first be opened and then immediately closed. In general this means that the initial sip packet will be sent (as part of the 4-way handshake).

Default value is -1.

#### Set `sctp_max_assocs` parameter

    ...
    modparam("sctp", "sctp_max_assocs", 10)
    ...

### `sctp_srto_initial` (int)

Initial value of the retransmission timeout (in milliseconds), used in RTO calculations.

Can be changed at runtime (sctp srto_initial) but it will affect only new associations.

Default value is OS specific.

#### Set `sctp_srto_initial` parameter

    ...
    modparam("sctp", "sctp_srto_initial", 1000)
    ...

### `sctp_srto_max` (int)

Maximum value of the retransmission timeout (RTO) in milliseconds.

WARNING: values lower than the sctp sack_delay will cause lots of retransmissions and connection instability (see sctp_srto_min for more details).

Can be changed at runtime (sctp srto_max) but it will affect only new associations.

Default value is OS specific.

#### Set `sctp_srto_max` parameter

    ...
    modparam("sctp", "sctp_srto_max", 2000)
    ...

### `sctp_srto_min` (int)

Minimum value of the retransmission timeout (RTO) in milliseconds.

WARNING: values lower than the sctp sack_delay of any peer might cause retransmissions and possible interoperability problems. According to the standard the sack_delay should be between 200 and 500 ms, so avoid trying values lower than 500 ms unless you control all the possible sctp peers and you do make sure their sack_delay is higher or their sack_freq is 1.

Can be changed at runtime (sctp srto_min) but it will affect only new associations.

Default value is OS specific.

#### Set `sctp_srto_min` parameter

    ...
    modparam("sctp", "sctp_srto_min", 800)
    ...

### `sctp_asocmaxrxt` (int)

Maximum retransmissions attempts per association. It should be set to sctp_pathmaxrxt * no. of expected paths.

Can be changed at runtime (sctp asocmaxrxt) but it will affect only new associations.

Default value is OS specific.

#### Set `sctp_asocmaxrxt` parameter

    ...
    modparam("sctp", "sctp_asocmaxrxt", 5)
    ...

### `sctp_init_max_attempts` (int)

Maximum INIT retransmission attempts.

Can be changed at runtime (sctp init_max_attempts).

Default value is OS specific.

#### Set `sctp_init_max_attempts` parameter

    ...
    modparam("sctp", "sctp_init_max_attempts", 3)
    ...

### `sctp_init_max_timeo` (int)

Maximum INIT retransmission timeout (RTO max for INIT) in milliseconds.

Can be changed at runtime (sctp init_max_timeo).

Default value is OS specific.

#### Set `sctp_init_max_timeo` parameter

    ...
    modparam("sctp", "sctp_init_max_timeo", 1000)
    ...

### `sctp_hbinterval` (int)

SCTP heartbeat interval. Setting it to -1 will disable the heartbeats.

Can be changed at runtime (sctp hbinterval) but it will affect only new associations.

Default value is OS specific.

#### Set `sctp_hbinterval` parameter

    ...
    modparam("sctp", "sctp_hbinterval", 2000)
    ...

### `sctp_pathmaxrxt` (int)

Maximum retransmission attempts per path (see also sctp_asocmaxrxt).

Can be changed at runtime (sctp pathmaxrxt) but it will affect only new associations.

Default value is OS specific.

#### Set `sctp_pathmaxrxt` parameter

    ...
    modparam("sctp", "sctp_pathmaxrxt", 2)
    ...

### `sctp_sack_delay` (int)

Delay until an ACK is generated after receiving a packet (in milliseconds).

WARNING: a value higher than srto_min can cause a lot of retransmissions (and strange problems). A value higher than srto_max will result in very high connections instability. According to the standard the sack_delay value should be between 200 and 500 ms.

Can be changed at runtime (sctp sack_delay) but it will affect only new associations.

Default value is OS specific.

#### Set `sctp_sack_delay` parameter

    ...
    modparam("sctp", "sctp_sack_delay", 400)
    ...

### `sctp_sack_freq` (int)

Number of packets received before an ACK is sent (without waiting for the sack_delay to expire). Default: OS specific.

Note: on linux with lksctp up to and including 1.0.9 is not possible to set this value (having it in the config will produce a warning on startup).

Can be changed at runtime (sctp sack_freq) but it will affect only new associations.

Default value is OS specific.

#### Set `sctp_sack_freq` parameter

    ...
    modparam("sctp", "sctp_sack_freq", 3)
    ...

### `sctp_max_burst` (int)

Maximum burst of packets that can be emitted by an association.

Can be changed at runtime (sctp max_burst) but it will affect only new associations.

Default value is OS specific.

#### Set `sctp_max_burst` parameter

    ...
    modparam("sctp", "sctp_max_burst", 3)
    ...

## RPC Commands

### `sctp.info`

Print information about SCTP transport.

#### Use `sctp.info` with kamctl

    ...
    kamctl rpc sctp.info
    ...

### `sctp.options`

Print the options of SCTP sockets. It can take an optional parameter that specifies the listen address of SCTP socket.

#### Use `sctp.options` with kamctl

    ...
    kamctl rpc sctp.options
    ...