sctp
- Internet Stream Control Transmission Protocol
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/sctp.h> int
socket (AF_INET SOCK_STREAM IPPROTO_SCTP); int
socket (AF_INET SOCK_SEQPACKET IPPROTO_SCTP);
DESCRIPTION
The
SCTP
protocol provides reliable, flow-controlled, two-way
transmission of data.
It is a message oriented protocol and can
support the
SOCK_STREAM
and
SOCK_SEQPACKET
abstractions.
SCTP
uses the standard
Internet address format and, in addition, provides a per-host
collection of
``port addresses''
Thus, each address is composed of an Internet address specifying
the host and network, with a specific
SCTP
port on the host identifying the peer entity.
There are two models of programming in SCTP.
The first uses the
SOCK_STREAM
abstraction.
In this abstraction sockets utilizing the
SCTP
protocol are either
``active''
or
``passive''
Active sockets initiate connections to passive
sockets.
By default,
SCTP
sockets are created active; to create a
passive socket, the
listen(2)
system call must be used after binding the socket with the
bind(2)
or
sctp_bindx3
system calls.
Only passive sockets may use the
accept(2)
call to accept incoming connections.
Only active sockets may use the
connect(2)
call to initiate connections.
The other abstraction
SOCK_SEQPACKET
provides a
``connectionless''
mode of operation in that the user may send to an address
(using any of the valid send calls that carry a
socket address) and an association will be setup
implicitly by the underlying
SCTP
transport stack.
This abstraction is the only one capable of sending data on the
third leg of the four-way handshake.
A user must still call
listen(2)
to allow the socket to accept connections.
Calling
listen(2)
however does not restrict the user from still initiating
implicit connections to other peers.
The
SCTP
protocol directly supports multi-homing.
So when binding a socket with the
``wildcard''
address
INADDR_ANY
the
SCTP
stack will inform the peer about all of the local addresses
that are deemed in scope of the peer.
The peer will then possibly have multiple paths to reach the local host.
The
SCTP
transport protocol is also multi-streamed.
Multi-streaming refers to the ability to send sub-ordered flows of
messages.
A user performs this by specifying a specific stream in one of the
extended send calls such as the
sctp_send3
function call.
Sending messages on different streams will allow parallel delivery
of data i.e., a message loss in stream 1 will not block the delivery
of messages sent in stream 2.
The
SCTP
transport protocol also provides a unordered service as well.
The unordered service allows a message to be sent and delivered
with no regard to the ordering of any other message.
Extensions
The FreeBSD implementation of
SCTP
also supports the following extensions:
"sctp partial reliability"
This extension allows one to have message be skipped and
not delivered based on some user specified parameters.
"sctp dynamic addressing"
This extension allows addresses to be added and deleted
dynammically from an existing association.
"sctp authentication"
This extension allows the user to authenticate specific
peer chunks (including data) to validate that the peer
who sent the message is in fact the peer who setup the
association.
A shared key option is also provided for
so that two stacks can pre-share keys.
"packet drop"
Some routers support a special satellite protocol that
will report losses due to corruption.
This allows retransmissions without subsequent loss in bandwidth
utilization.
"stream reset"
This extension allows a user on either side to reset the
stream sequence numbers used by any or all streams.
SCTP
supports a number of socket options which can be set with
setsockopt(2)
and tested with
getsockopt(2)
or
sctp_opt_info2:
SCTP_NODELAY
Under most circumstances,
SCTP
sends data when it is presented; when outstanding data has not
yet been acknowledged, it gathers small amounts of output to be
sent in a single packet once an acknowledgement is received.
For some clients, such as window systems that send a stream of
mouse events which receive no replies, this packetization may
cause significant delays.
The boolean option
SCTP_NODELAY
defeats this algorithm.
SCTP_RTOINFO
This option returns specific information about an associations
``Retransmission Time Out''
It can also be used to change the default values.
SCTP_ASSOCINFO
This option returns specific information about the requested
association.
SCTP_INITMSG
This option allows you to get or set the default sending
parameters when an association is implicitly setup.
It allows you to change such things as the maxium number of
streams allowed inbound and the number of streams requested
of the peer.
SCTP_AUTOCLOSE
For the one-to-many model
( SOCK_SEQPACKET
associations are setup implicitly.
This option allows the user to specify a default number of idle
seconds to allow the association be maintained.
After the idle timer (where no user message have been sent or have
been received from the peer) the association will be gracefully
closed.
The default for this value is 0, or unlimited (i.e., no automatic
close).
SCTP_SET_PEER_PRIMARY_ADDR
The dynamic address extension allows a peer to also request a
particular address of its be made into the primary address.
This option allows the caller to make such a request to a peer.
Note that if the peer does not also support the dynamic address
extension, this call will fail.
Note the caller must provide a valid local address that the peer has
been told about during association setup or dynamically.
SCTP_PRIMARY_ADDR
This option allows the setting of the primary address
that the caller wishes to send to.
The caller provides the address of a peer that is to be made primary.
SCTP_ADAPTATION_LAYER
The dynamic address extension also allows a user to
pass a 32 bit opaque value upon association setup.
This option allows a user to set or get this value.
SCTP_DISABLE_FRAGMENTS
By default
SCTP
will fragment user messages into multiple pieces that
will fit on the network and then later, upon reception, reassemble
the pieces into a single user message.
If this option is enabled instead, any send that exceeds the path
maximum transfer unit (P-MTU) will fail and the message will NOT be
sent.
SCTP_PEER_ADDR_PARAMS
This option will allow a user to set or get specific
peer address parameters.
SCTP_DEFAULT_SEND_PARAM
When a user does not use one of the extended send
calls (e.g.,
sctp_sendmsg3)
a set of default values apply to each send.
These values include things like the stream number to send
to as well as the per-protocol id.
This option lets a caller both get and set these values.
If the user changes these default values, then these new values will
be used as the default whenever no information is provided by the
sender (i.e., the non-extended API is used).
SCTP_EVENTS
SCTP
has non-data events that it can communicate
to its application.
By default these are all disabled since they arrive in the data path
with a special flag
MSG_NOTIFICATION
set upon the received message.
This option lets a caller
both get what events are current being received
as well as set different events that they may be interested
in receiving.
SCTP_I_WANT_MAPPED_V4_ADDR
SCTP
supports both IPV4 and IPV6.
An association may span both IPV4 and IPV6 addresses since
SCTP
is multi-homed.
By default, when opening an IPV6 socket, when
data arrives on the socket from a peer's
V4 address the V4 address will be presented with an address family
of AF_INET.
If this is undesireable, then this option
can be enabled which will then convert all V4 addresses
into mapped V6 representations.
SCTP_MAXSEG
By default
SCTP
chooses its message fragmentation point
based upon the smallest P-MTU of the peer.
This option lets the caller set it to a smaller value.
Note that while the user can change this value, if the P-MTU
is smaller than the value set by the user, then the P-MTU
value will override any user setting.
SCTP_DELAYED_ACK_TIME
This option lets the user both set and get the
delayed ack time (in milliseconds) that
SCTP
is using.
The default is 200 milliseconds.
SCTP_PARTIAL_DELIVERY_POINT
SCTP
at times may need to start delivery of a
very large message before the entire message has
arrived.
By default SCTP waits until the incoming
message is larger than one fourth of the receive
buffer.
This option allows the stacks value
to be overridden with a smaller value.
SCTP_FRAGMENT_INTERLEAVE
SCTP
at times will start partial delivery (as mentioned above).
In the normal case successive reads will continue to return
the rest of the message, blocking if needed, until all of
that message is read.
However this means other messages may have arrived and be ready
for delivery and be blocked behind the message being partially
delivered.
If this option is enabled, when a partial delivery
message has no more data to be received, then a subsequent
read may return a different message that is ready for delivery.
By default this option is off since the user must be using the
extended API's to be able to tell the difference between
messages (via the stream and stream sequence number).
SCTP_AUTH_CHUNK
By default only the dynamic addressing chunks are
authenticated.
This option lets a user request an
additional chunk be authenticated as well.
Note that successive calls to this option will work and continue
to add more chunks that require authentication.
Note that this option only effects future associations and
not existing ones.
SCTP_AUTH_KEY
This option allows a user to specify a shared
key that can be later used to authenticate
a peer.
SCTP_HMAC_IDENT
This option will let you get or set the list of
HMAC algorithms used to authenticate peers.
Note that the HMAC values are in priority order where
the first HMAC identifier is the most prefered
and the last is the least prefered.
SCTP_AUTH_ACTIVE_KEY
This option allows you to make a key active for
the generation of authentication information.
Note that the peer must have the same key or else the
data will be discarded.
SCTP_AUTH_DELETE_KEY
This option allows you to delete an old key.
SCTP_USE_EXT_RECVINFO
The sockets api document allows an extended
send/receive information structure to be used.
The extended structure includes additional fields
related to the next message to be received (after the
current receive completes) if such information is known.
By default the system will not pass this information.
This option allows the user to request this information.
SCTP_AUTO_ASCONF
By default when bound to all address and the system administrator has
enables automatic dynamic addresses, the
SCTP
stack will automatically generate address changes into add and
delete requests to any peers by setting this option to
true.
This option allows an endpoint to disable that behavior.
SCTP_MAXBURST
By default
SCTP
implements micro-burst control so that as the congestion window
opens up no large burst of packets can be generated.
The default burst limit is four.
This option lets the user change this value.
SCTP_CONTEXT
Many sctp extended calls have a context field.
The context field is a 32 bit opaque value that will be returned in
send failures.
This option lets the caller set the default
context value to use when none is provided by the user.
SCTP_EXPLICIT_EOR
By default, a single send is a complete message.
SCTP
generates an implied record boundary.
If this option is enabled, then all sends are part of the same message
until the user indicates an end of record with the
special flag
SCTP_EOR
passed in the sctp_sndrcvinfo flags field.
This effectively makes all sends part of the same message
until the user specifices differently.
This means that a caller must NOT change the stream number until
after the
SCTP_EOR
is passed to
SCTP
else an error will be returned.
SCTP_STATUS
This option is a read only option that returns
various status information about the specified association.
SCTP_GET_PEER_ADDR_INFO
This read only option returns information about a peer
address.
SCTP_PEER_AUTH_CHUNKS
This read only option returns a list of the chunks
the peer requires to be authenticated.
SCTP_LOCAL_AUTH_CHUNKS
This read only option returns a list of the locally
required chunks that must be authenticated.
SCTP_RESET_STREAMS
This socket option is used to cause a stream sequence
number or all stream sequence numbers to be reset.
Note that the peer
SCTP
endpoint must also support the stream reset extension
as well.
