One document matched: draft-ietf-tsvwg-sctpsocket-10.txt
Differences from draft-ietf-tsvwg-sctpsocket-09.txt
Network Working Group R. Stewart
Internet-Draft Cisco Systems, Inc.
Expires: August 25, 2005 Q. Xie
Motorola, Inc.
L. Yarroll
TimeSys Corp
J. Wood
DoCoMo USA Labs
K. Poon
Sun Microsystems, Inc.
M. Tuexen
Univ. of Applied Sciences Muenster
February 21, 2005
Sockets API Extensions for Stream Control Transmission Protocol
(SCTP)
draft-ietf-tsvwg-sctpsocket-10.txt
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 3 of RFC 3667. By submitting this Internet-Draft, each
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which he or she become aware will be disclosed, in accordance with
RFC 3668.
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This Internet-Draft will expire on August 25, 2005.
Copyright Notice
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Copyright (C) The Internet Society (2005).
Abstract
This document describes a mapping of the Stream Control Transmission
Protocol SCTP RFC2960 [8] into a sockets API. The benefits of this
mapping include compatibility for TCP applications, access to new
SCTP features and a consolidated error and event notification scheme.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1 Data Types . . . . . . . . . . . . . . . . . . . . . . . . 7
3. one-to-many style Interface . . . . . . . . . . . . . . . . 8
3.1 Basic Operation . . . . . . . . . . . . . . . . . . . . . 8
3.1.1 socket() - one-to-many style socket . . . . . . . . . 9
3.1.2 bind() - one-to-many style socket . . . . . . . . . . 9
3.1.3 listen() - One-to-many style socket . . . . . . . . . 10
3.1.4 sendmsg() and recvmsg() - one-to-many style socket . . 11
3.1.5 close() - one-to-many style socket . . . . . . . . . . 12
3.1.6 connect() - one-to-many style socket . . . . . . . . . 13
3.2 Implicit Association Setup . . . . . . . . . . . . . . . . 13
3.3 Non-blocking mode . . . . . . . . . . . . . . . . . . . . 14
3.4 Special considerations . . . . . . . . . . . . . . . . . . 15
4. one-to-one style Interface . . . . . . . . . . . . . . . . . 17
4.1 Basic Operation . . . . . . . . . . . . . . . . . . . . . 17
4.1.1 socket() - one-to-one style socket . . . . . . . . . . 18
4.1.2 bind() - one-to-one style socket . . . . . . . . . . . 18
4.1.3 listen() - one-to-one style socket . . . . . . . . . . 19
4.1.4 accept() - one-to-one style socket . . . . . . . . . . 20
4.1.5 connect() - one-to-one style socket . . . . . . . . . 20
4.1.6 close() - one-to-one style socket . . . . . . . . . . 21
4.1.7 shutdown() - one-to-one style socket . . . . . . . . . 21
4.1.8 sendmsg() and recvmsg() - one-to-one style socket . . 22
4.1.9 getpeername() . . . . . . . . . . . . . . . . . . . . 23
5. Data Structures . . . . . . . . . . . . . . . . . . . . . . 24
5.1 The msghdr and cmsghdr Structures . . . . . . . . . . . . 24
5.2 SCTP msg_control Structures . . . . . . . . . . . . . . . 25
5.2.1 SCTP Initiation Structure (SCTP_INIT) . . . . . . . . 26
5.2.2 SCTP Header Information Structure (SCTP_SNDRCV) . . . 27
5.3 SCTP Events and Notifications . . . . . . . . . . . . . . 30
5.3.1 SCTP Notification Structure . . . . . . . . . . . . . 30
5.4 Ancillary Data Considerations and Semantics . . . . . . . 40
5.4.1 Multiple Items and Ordering . . . . . . . . . . . . . 40
5.4.2 Accessing and Manipulating Ancillary Data . . . . . . 40
5.4.3 Control Message Buffer Sizing . . . . . . . . . . . . 41
6. Common Operations for Both Styles . . . . . . . . . . . . . 43
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6.1 send(), recv(), sendto(), recvfrom() . . . . . . . . . . . 43
6.2 setsockopt(), getsockopt() . . . . . . . . . . . . . . . . 44
6.3 read() and write() . . . . . . . . . . . . . . . . . . . . 44
6.4 getsockname() . . . . . . . . . . . . . . . . . . . . . . 44
7. Socket Options . . . . . . . . . . . . . . . . . . . . . . . 46
7.1 Read / Write Options . . . . . . . . . . . . . . . . . . . 47
7.1.1 Retransmission Timeout Parameters (SCTP_RTOINFO) . . . 47
7.1.2 Association Parameters (SCTP_ASSOCINFO) . . . . . . . 48
7.1.3 Initialization Parameters (SCTP_INITMSG) . . . . . . . 50
7.1.4 SO_LINGER . . . . . . . . . . . . . . . . . . . . . . 50
7.1.5 SCTP_NODELAY . . . . . . . . . . . . . . . . . . . . . 50
7.1.6 SO_RCVBUF . . . . . . . . . . . . . . . . . . . . . . 51
7.1.7 SO_SNDBUF . . . . . . . . . . . . . . . . . . . . . . 51
7.1.8 Automatic Close of associations (SCTP_AUTOCLOSE) . . . 51
7.1.9 Set Peer Primary Address
(SCTP_SET_PEER_PRIMARY_ADDR) . . . . . . . . . . . . . 51
7.1.10 Set Primary Address (SCTP_PRIMARY_ADDR) . . . . . . 52
7.1.11 Set Adaption Layer Indicator (SCTP_ADAPTION_LAYER) . 52
7.1.12 Enable/Disable message fragmentation
(SCTP_DISABLE_FRAGMENTS) . . . . . . . . . . . . . . 53
7.1.13 Peer Address Parameters (SCTP_PEER_ADDR_PARAMS) . . 53
7.1.14 Set default send parameters
(SCTP_DEFAULT_SEND_PARAM) . . . . . . . . . . . . . 55
7.1.15 Set notification and ancillary events
(SCTP_EVENTS) . . . . . . . . . . . . . . . . . . . 55
7.1.16 Set/clear IPv4 mapped addresses
(SCTP_I_WANT_MAPPED_V4_ADDR) . . . . . . . . . . . . 55
7.1.17 Set the maximum fragmentation size (SCTP_MAXSEG) . . 55
7.1.18 Set/Get the list of chunks that must be
authenticated (SCTP_AUTH_CHUNKS) . . . . . . . . . . 56
7.1.19 Set/Get the current authentication shared secret
(SCTP_AUTH_SECRET) . . . . . . . . . . . . . . . . . 56
7.1.20 Get the list of chunks that peer requires to be
authenticated (SCTP_PEER_AUTH_CHUNKS) . . . . . . . 57
7.2 Read-Only Options . . . . . . . . . . . . . . . . . . . . 57
7.2.1 Association Status (SCTP_STATUS) . . . . . . . . . . . 57
7.2.2 Peer Address Information (SCTP_GET_PEER_ADDR_INFO) . . 58
7.3 Ancillary Data and Notification Interest Options . . . . . 59
8. New Interfaces . . . . . . . . . . . . . . . . . . . . . . . 62
8.1 sctp_bindx() . . . . . . . . . . . . . . . . . . . . . . . 62
8.2 Branched-off Association . . . . . . . . . . . . . . . . . 63
8.3 sctp_getpaddrs() . . . . . . . . . . . . . . . . . . . . . 63
8.4 sctp_freepaddrs() . . . . . . . . . . . . . . . . . . . . 64
8.5 sctp_getladdrs() . . . . . . . . . . . . . . . . . . . . . 64
8.6 sctp_freeladdrs() . . . . . . . . . . . . . . . . . . . . 65
8.7 sctp_sendmsg() . . . . . . . . . . . . . . . . . . . . . . 65
8.8 sctp_recvmsg() . . . . . . . . . . . . . . . . . . . . . . 66
8.9 sctp_connectx() . . . . . . . . . . . . . . . . . . . . . 67
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8.10 sctp_send() . . . . . . . . . . . . . . . . . . . . . . 68
8.11 sctp_sendx() . . . . . . . . . . . . . . . . . . . . . . 68
9. Preprocessor Constants . . . . . . . . . . . . . . . . . . . 70
10. Security Considerations . . . . . . . . . . . . . . . . . . 71
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 72
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 72
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 73
A. one-to-one style Code Example . . . . . . . . . . . . . . . 75
B. one-to-many style Code Example . . . . . . . . . . . . . . . 81
Intellectual Property and Copyright Statements . . . . . . . 83
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1. Introduction
The sockets API has provided a standard mapping of the Internet
Protocol suite to many operating systems. Both TCP RFC793 [1] and
UDP RFC768 [2] have benefited from this standard representation and
access method across many diverse platforms. SCTP is a new protocol
that provides many of the characteristics of TCP but also
incorporates semantics more akin to UDP. This document defines a
method to map the existing sockets API for use with SCTP, providing
both a base for access to new features and compatibility so that most
existing TCP applications can be migrated to SCTP with few (if any)
changes.
There are three basic design objectives:
1) Maintain consistency with existing sockets APIs:
We define a sockets mapping for SCTP that is consistent with other
sockets API protocol mappings (for instance, UDP, TCP, IPv4, and
IPv6).
2) Support a one-to-many style interface
This set of semantics is similar to that defined for
connection-less protocols, such as UDP. A one-to-many style SCTP
socket should be able to control multiple SCTP associations. This
is similar to an UDP socket, which can communicate with many peer
end points. Each of these associations is assigned an association
ID so that an applications can use the ID to differentiate them.
Note that SCTP is connection-oriented in nature, and it does not
support broadcast or multicast communications, as UDP does.
3) Support a one-to-one style interface
This interface supports a similar semantics as sockets for
connection-oriented protocols, such as TCP. A one-to-one style
SCTP socket should only control one SCTP association.
One purpose of defining this interface is to allow existing
applications built on other connection-oriented protocols be
ported to use SCTP with very little effort. And developers
familiar with those semantics can easily adapt to SCTP. Another
purpose is to make sure that existing mechanisms in most OSes to
deal with socket, such as select(), should continue to work with
this style of socket.
Extensions are added to this mapping to provide mechanisms to
exploit new features of SCTP.
Goals 2 and 3 are not compatible, so in this document we define two
modes of mapping, namely the one-to-many style mapping and the
one-to-one style mapping. These two modes share some common data
structures and operations, but will require the use of two different
application programming styles. Note that all new SCTP features can
be used with both styles of socket. The decision on which one to use
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depends mainly on the nature of applications.
A mechanism is defined to extract a one-to-many style SCTP
association into a one-to-one style socket.
Some of the SCTP mechanisms cannot be adequately mapped to existing
socket interface. In some cases, it is more desirable to have new
interface instead of using existing socket calls. Section 8 of this
document describes those new interface.
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2. Conventions
2.1 Data Types
Whenever possible, data types from Draft 6.6 (March 1997) of POSIX
1003.1g are used: uintN_t means an unsigned integer of exactly N bits
(e.g., uint16_t). We also assume the argument data types from
1003.1g when possible (e.g., the final argument to setsockopt() is a
size_t value). Whenever buffer sizes are specified, the POSIX 1003.1
size_t data type is used.
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3. one-to-many style Interface
The one-to-many style interface has the following characteristics:
A) Outbound association setup is implicit.
B) Messages are delivered in complete messages (with one notable
exception).
C) There is a 1 to MANY relationship between socket and association.
3.1 Basic Operation
A typical server in this style uses the following socket calls in
sequence to prepare an endpoint for servicing requests:
1. socket()
2. bind()
3. listen()
4. recvmsg()
5. sendmsg()
6. close()
A typical client uses the following calls in sequence to setup an
association with a server to request services:
1. socket()
2. sendmsg()
3. recvmsg()
4. close()
In this style, by default, all the associations connected to the
endpoint are represented with a single socket. Each associations is
assigned an association ID (type is sctp_assoc_t) so that an
application can use it to differentiate between them. In some
implementations, the peer end point's addresses can also be used for
this purpose. But this is not required for performance reasons. If
an implementation does not support using addresses to differentiate
between different associations, the sendto() call can only be used to
setup an association implicitly. It cannot be used to send data to
an established association as the association ID cannot be specified.
Once as association ID is assigned to an SCTP association, that ID
will not be reused until the application explicitly terminates the
association. The resources belonged to that association will not be
freed until that happens. This is similar to the close() operation
on a normal socket. The only exception is when the SCTP_AUTOCLOSE
option (section 7.1.8) is set. In this case, after the association
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is terminated automatically, the association ID assigned to it can be
reused. All applications using this option should be aware of this
to avoid the possible problem of sending data to an incorrect peer
end point.
If the server or client wishes to branch an existing association off
to a separate socket, it is required to call sctp_peeloff() and in
the parameter specifies the association identification. The
sctp_peeloff() call will return a new socket which can then be used
with recv() and send() functions for message passing. See
Section 8.2 for more on branched-off associations.
Once an association is branched off to a separate socket, it becomes
completely separated from the original socket. All subsequent
control and data operations to that association must be done through
the new socket. For example, the close operation on the original
socket will not terminate any associations that have been branched
off to a different socket.
We will discuss the one-to-many style socket calls in more details in
the following subsections.
3.1.1 socket() - one-to-many style socket
Applications use socket() to create a socket descriptor to represent
an SCTP endpoint.
The syntax is,
sd = socket(PF_INET, SOCK_SEQPACKET, IPPROTO_SCTP);
or,
sd = socket(PF_INET6, SOCK_SEQPACKET, IPPROTO_SCTP);
Here, SOCK_SEQPACKET indicates the creation of a one-to-many style
socket.
The first form creates an endpoint which can use only IPv4 addresses,
while, the second form creates an endpoint which can use both IPv6
and IPv4 addresses.
3.1.2 bind() - one-to-many style socket
Applications use bind() to specify which local address the SCTP
endpoint should associate itself with.
An SCTP endpoint can be associated with multiple addresses. To do
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this, sctp_bindx() is introduced in section Section 8.1 to help
applications do the job of associating multiple addresses.
These addresses associated with a socket are the eligible transport
addresses for the endpoint to send and receive data. The endpoint
will also present these addresses to its peers during the association
initialization process, see RFC2960 [8].
After calling bind(), if the endpoint wishes to accept new
associations on the socket, it must call listen() (see section
Section 3.1.3).
The syntax of bind() is,
ret = bind(int sd, struct sockaddr *addr, socklen_t addrlen);
sd: the socket descriptor returned by socket().
addr: the address structure (struct sockaddr_in or struct
sockaddr_in6 RFC2553 [7]).
addrlen: the size of the address structure.
If sd is an IPv4 socket, the address passed must be an IPv4 address.
If the sd is an IPv6 socket, the address passed can either be an IPv4
or an IPv6 address.
Applications cannot call bind() multiple times to associate multiple
addresses to an endpoint. After the first call to bind(), all
subsequent calls will return an error.
If addr is specified as a wildcard (INADDR_ANY for an IPv4 address,
or as IN6ADDR_ANY_INIT or in6addr_any for an IPv6 address), the
operating system will associate the endpoint with an optimal address
set of the available interfaces.
If a bind() is not called prior to a sendmsg() call that initiates a
new association, the system picks an ephemeral port and will choose
an address set equivalent to binding with a wildcard address. One of
those addresses will be the primary address for the association.
This automatically enables the multi-homing capability of SCTP.
3.1.3 listen() - One-to-many style socket
By default, new associations are not accepted for one-to-many style
sockets. An application uses listen() to mark a socket as being able
to accept new associations. The syntax is,
int listen(int sd, int backlog);
sd - the socket descriptor of the endpoint.
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backlog - if backlog is non-zero, enable listening else
disable listening.
Note that one-to-many style socket consumers do not need to call
accept to retrieve new associations. Calling accept() on a
one-to-many style socket should return EOPNOTSUPP. Rather, new
associations are accepted automatically, and notifications of the new
associations are delivered via recvmsg() with the SCTP_ASSOC_CHANGE
event (if these notifications are enabled). Clients will typically
not call listen(), so that they can be assured that the only
associations on the socket will be ones they actively initiated.
Server or peer-to-peer sockets, on the other hand, will always accept
new associations, so a well-written application using server
one-to-many style sockets must be prepared to handle new associations
from unwanted peers.
Also note that the SCTP_ASSOC_CHANGE event provides the association
ID for a new association, so if applications wish to use the
association ID as input to other socket calls, they should ensure
that the SCTP_ASSOC_CHANGE event is enabled (it is enabled by
default).
3.1.4 sendmsg() and recvmsg() - one-to-many style socket
An application uses sendmsg() and recvmsg() call to transmit data to
and receive data from its peer.
ssize_t sendmsg(int sd, const struct msghdr *message, int flags);
ssize_t recvmsg(int sd, struct msghdr *message, int flags);
sd: the socket descriptor of the endpoint.
message: pointer to the msghdr structure which contains a single user
message and possibly some ancillary data. See Section 5 for
complete description of the data structures.
flags: No new flags are defined for SCTP at this level. See Section
5 for SCTP-specific flags used in the msghdr structure.
As we will see in Section 5, along with the user data, the ancillary
data field is used to carry the sctp_sndrcvinfo and/or the
sctp_initmsg structures to perform various SCTP functions including
specifying options for sending each user message. Those options,
depending on whether sending or receiving, include stream number,
stream sequence number, various flags, context and payload protocol
Id, etc.
When sending user data with sendmsg(), the msg_name field in msghdr
structure will be filled with one of the transport addresses of the
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intended receiver. If there is no association existing between the
sender and the intended receiver, the sender's SCTP stack will set up
a new association and then send the user data (see Section 3.2 for
more on implicit association setup).
If a peer sends a SHUTDOWN, a SCTP_SHUTDOWN_EVENT notification will
be delivered if that notification has been enabled, and no more data
can be sent to that association. Any attempt to send more data will
cause sendmsg() to return with an ESHUTDOWN error. Note that the
socket is still open for reading at this point so it is possible to
retrieve notifications.
When receiving a user message with recvmsg(), the msg_name field in
msghdr structure will be populated with the source transport address
of the user data. The caller of recvmsg() can use this address
information to determine to which association the received user
message belongs. Note that if SCTP_ASSOC_CHANGE events are disabled,
applications must use the peer transport address provided in the
msg_name field by recvmsg() to perform correlation to an association,
since they will not have the association ID.
If all data in a single message has been delivered, MSG_EOR will be
set in the msg_flags field of the msghdr structure (see section
Section 5.1).
If the application does not provide enough buffer space to completely
receive a data message, MSG_EOR will not be set in msg_flags.
Successive reads will consume more of the same message until the
entire message has been delivered, and MSG_EOR will be set.
If the SCTP stack is running low on buffers, it may partially deliver
a message. In this case, MSG_EOR will not be set, and more calls to
recvmsg() will be necessary to completely consume the message. Only
one message at a time can be partially delivered.
Note, if the socket is a branched-off socket that only represents one
association (see Section 3.1), the msg_name field can be used to
override the primary address when sending data.
3.1.5 close() - one-to-many style socket
Applications use close() to perform graceful shutdown (as described
in Section 10.1 of RFC2960 [8]) on ALL the associations currently
represented by a one-to-many style socket.
The syntax is:
ret = close(int sd);
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sd - the socket descriptor of the associations to be closed.
To gracefully shutdown a specific association represented by the
one-to-many style socket, an application should use the sendmsg()
call, and including the MSG_EOF flag. A user may optionally
terminate an association non-gracefully by sending with the MSG_ABORT
flag and possibly passing a user specified abort code in the data
field. Both flags MSG_EOF and MSG_ABORT are passwd with ancillary
data (see Section 5.2.2) in the sendmsg call.
If sd in the close() call is a branched-off socket representing only
one association, the shutdown is performed on that association only.
3.1.6 connect() - one-to-many style socket
An application may use the connect() call in the one-to-many style to
initiate an association without sending data.
The syntax is:
ret = connect(int sd, const struct sockaddr *nam, socklen_t len);
sd: the socket descriptor to have a new association added to.
nam: the address structure (either struct sockaddr_in or struct
sockaddr_in6 defined in RFC2553 [7]).
len: the size of the address.
Multiple connect() calls can be made on the same socket to create
multiple associations. This is different from the semantics of
connect() on a UDP socket.
3.2 Implicit Association Setup
Once the bind() call is complete on a one-to-many style socket, the
application can begin sending and receiving data using the
sendmsg()/recvmsg() or sendto()/recvfrom() calls, without going
through any explicit association setup procedures (i.e., no connect()
calls required).
Whenever sendmsg() or sendto() is called and the SCTP stack at the
sender finds that there is no association existing between the sender
and the intended receiver (identified by the address passed either in
the msg_name field of msghdr structure in the sendmsg() call or the
dest_addr field in the sendto() call), the SCTP stack will
automatically setup an association to the intended receiver.
Upon the successful association setup a SCTP_COMM_UP notification
will be dispatched to the socket at both the sender and receiver
side. This notification can be read by the recvmsg() system call
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(see Section 3.1.3).
Note, if the SCTP stack at the sender side supports bundling, the
first user message may be bundled with the COOKIE ECHO message
RFC2960 [8].
When the SCTP stack sets up a new association implicitly, it first
consults the sctp_initmsg structure, which is passed along within the
ancillary data in the sendmsg() call (see Section 5.2.1 for details
of the data structures), for any special options to be used on the
new association.
If this information is not present in the sendmsg() call, or if the
implicit association setup is triggered by a sendto() call, the
default association initialization parameters will be used. These
default association parameters may be set with respective
setsockopt() calls or be left to the system defaults.
Implicit association setup cannot be initiated by send()/recv()
calls.
3.3 Non-blocking mode
Some SCTP users might want to avoid blocking when they call socket
interface function.
Once all bind() calls are complete on a one-to-many style socket, the
application must set the non-blocking option by a fcntl() (such as
O_NONBLOCK). After which the sendmsg() function returns immediately,
and the success or failure of the data message (and possible
SCTP_INITMSG parameters) will be signaled by the SCTP_ASSOC_CHANGE
event with SCTP_COMM_UP or CANT_START_ASSOC. If user data could not
be sent (due to a CANT_START_ASSOC), the sender will also receive a
SCTP_SEND_FAILED event. Those event(s) can be received by the user
calling of recvmsg(). A server (having called listen()) is also
notified of an association up event by the reception of a
SCTP_ASSOC_CHANGE with SCTP_COMM_UP via the calling of recvmsg() and
possibly the reception of the first data message.
In order to shutdown the association gracefully, the user must call
sendmsg() with no data and with the MSG_EOF flag set. The function
returns immediately, and completion of the graceful shutdown is
indicated by an SCTP_ASSOC_CHANGE notification of type
SHUTDOWN_COMPLETE (see Section 5.3.1.1). Note that this can also be
done using the sctp_send() call described in Section 8.10.
An application is recommended to use caution when using select() (or
poll()) for writing on a one-to-many style socket. The reason being
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that interpretation of select on write is implementation specific.
Generally a positive return on a select on write would only indicate
that one of the associations represented by the one-to-many socket is
writable. An application that writes after the select return may
still block since the association that was writeable is not the
destination association of the write call. Likewise select (or
poll()) for reading from a one-to-many socket will only return an
indication that one of the associations represented by the socket has
data to be read.
An application that wishes to know that a particular association is
ready for reading or writing should either use the one-to-one style
or use the sctp_peelloff() (see Section 8.2) function to seperate the
association of interest from the one-to-many socket.
3.4 Special considerations
The fact that a one-to-many style socket can provide access to many
SCTP associations through a single socket descriptor has important
implications for both application programmers and system programmers
implementing this API. A key issue is how buffer space inside the
sockets layer is managed. Because this implementation detail
directly affects how application programmers must write their code to
ensure correct operation and portability, this section provides some
guidance to both implementors and application programmers.
An important feature that SCTP shares with TCP is flow control:
specifically, a sender may not send data faster than the receiver can
consume it.
For TCP, flow control is typically provided for in the sockets API as
follows. If the reader stops reading, the sender queues messages in
the socket layer until it uses all of its socket buffer space
allocation creating a "stalled connection". Further attempts to
write to the socket will block or return the error EAGAIN or
EWOULDBLOCK for a non-blocking socket. At some point, either the
connection is closed, or the receiver begins to read again freeing
space in the output queue.
For one-to-one style SCTP sockets (this includes sockets descriptors
that were separated from a one-to-many style socket with
sctp_peeloff()) the behavior is identical. For one-to-many style
SCTP sockets, the fact that we have multiple associations on a single
socket makes the situation more complicated. If the implementation
uses a single buffer space allocation shared by all associations, a
single stalled association can prevent the further sending of data on
all associations active on a particular one-to-many style socket.
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For a blocking socket, it should be clear that a single stalled
association can block the entire socket. For this reason,
application programmers may want to use non-blocking one-to-many
style sockets. The application should at least be able to send
messages to the non-stalled associations.
But a non-blocking socket is not sufficient if the API implementor
has chosen a single shared buffer allocation for the socket. A
single stalled association would eventually cause the shared
allocation to fill, and it would become impossible to send even to
non-stalled associations.
The API implementor can solve this problem by providing each
association with its own allocation of outbound buffer space. Each
association should conceptually have as much buffer space as it would
have if it had its own socket. As a bonus, this simplifies the
implementation of sctp_peeloff().
To ensure that a given stalled association will not prevent other
non-stalled associations from being writable, application programmers
should either:
(a) demand that the underlying implementation dedicates independent
buffer space allotments to each association (as suggested above),
or
(b) verify that their application layer protocol does not permit
large amounts of unread data at the receiver (this is true of some
request-response protocols, for example), or
(c) use one-to-one style sockets for association which may
potentially stall (either from the beginning, or by using
sctp_peeloff before sending large amounts of data that may cause a
stalled condition).
An implemenation which dedicates independent buffer space for each
association should define HAVE_SCTP_MULTIBUF to 1.
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4. one-to-one style Interface
The goal of this style is to follow as closely as possible the
current practice of using the sockets interface for a connection
oriented protocol, such as TCP. This style enables existing
applications using connection oriented protocols to be ported to SCTP
with very little effort.
Note that some new SCTP features and some new SCTP socket options can
only be utilized through the use of sendmsg() and recvmsg() calls,
see Section 4.1.8. Also note that some socket interfaces may not be
able to provide data on the third leg of the association set up with
this interface style.
4.1 Basic Operation
A typical server in one-to-one style uses the following system call
sequence to prepare an SCTP endpoint for servicing requests:
1. socket()
2. bind()
3. listen()
4. accept()
The accept() call blocks until a new association is set up. It
returns with a new socket descriptor. The server then uses the new
socket descriptor to communicate with the client, using recv() and
send() calls to get requests and send back responses.
Then it calls
5. close()
to terminate the association.
A typical client uses the following system call sequence to setup an
association with a server to request services:
1. socket()
2. connect()
After returning from connect(), the client uses send() and recv()
calls to send out requests and receive responses from the server.
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The client calls
3. close()
to terminate this association when done.
4.1.1 socket() - one-to-one style socket
Applications calls socket() to create a socket descriptor to
represent an SCTP endpoint.
The syntax is:
int socket(PF_INET, SOCK_STREAM, IPPROTO_SCTP);
or,
int socket(PF_INET6, SOCK_STREAM, IPPROTO_SCTP);
Here, SOCK_STREAM indicates the creation of a one-to-one style
socket.
The first form creates an endpoint which can use only IPv4 addresses,
while the second form creates an endpoint which can use both IPv6 and
IPv4 addresses.
4.1.2 bind() - one-to-one style socket
Applications use bind() to pass an address to be associated with an
SCTP endpoint to the system. bind() allows only either a single
address or a IPv4 or IPv6 wildcard address to be bound. An SCTP
endpoint can be associated with multiple addresses. To do this,
sctp_bindx() is introduced in Section 8.1 to help applications do
the job of associating multiple addresses.
These addresses associated with a socket are the eligible transport
addresses for the endpoint to send and receive data. The endpoint
will also present these addresses to its peers during the association
initialization process, see RFC2960 [8].
The syntax is:
int bind(int sd, struct sockaddr *addr, socklen_t addrlen);
sd: the socket descriptor returned by socket() call.
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addr: the address structure (either struct sockaddr_in or struct
sockaddr_in6 defined in RFC2553 [7]).
addrlen: the size of the address structure.
If sd is an IPv4 socket, the address passed must be an IPv4 address.
Otherwise, i.e., the sd is an IPv6 socket, the address passed can
either be an IPv4 or an IPv6 address.
Applications cannot call bind() multiple times to associate multiple
addresses to the endpoint. After the first call to bind(), all
subsequent calls will return an error.
If addr is specified as a wildcard (INADDR_ANY for an IPv4 address,
or as IN6ADDR_ANY_INIT or in6addr_any for an IPv6 address), the
operating system will associate the endpoint with an optimal address
set of the available interfaces.
If a bind() is not called prior to the connect() call, the system
picks an ephemeral port and will choose an address set equivalent to
binding with a wildcard address. One of those addresses will be the
primary address for the association. This automatically enables the
multi-homing capability of SCTP.
The completion of this bind() process does not ready the SCTP
endpoint to accept inbound SCTP association requests. Until a
listen() system call, described below, is performed on the socket,
the SCTP endpoint will promptly reject an inbound SCTP INIT request
with an SCTP ABORT.
4.1.3 listen() - one-to-one style socket
Applications use listen() to ready the SCTP endpoint for accepting
inbound associations.
The syntax is:
int listen(int sd, int backlog);
sd: the socket descriptor of the SCTP endpoint.
backlog: this specifies the max number of outstanding associations
allowed in the socket's accept queue. These are the associations
that have finished the four-way initiation handshake (see Section
5 of RFC2960 [8]) and are in the ESTABLISHED state. Note, a
backlog of '0' indicates that the caller no longer wishes to
receive new associations.
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4.1.4 accept() - one-to-one style socket
Applications use accept() call to remove an established SCTP
association from the accept queue of the endpoint. A new socket
descriptor will be returned from accept() to represent the newly
formed association.
The syntax is:
new_sd = accept(int sd, struct sockaddr *addr, socklen_t *addrlen);
new_sd: the socket descriptor for the newly formed association.
sd the listening socket descriptor.
addr on return, will contain the primary address of the peer
endpoint.
addrlen on return, will contain the size of addr.
4.1.5 connect() - one-to-one style socket
Applications use connect() to initiate an association to a peer.
The syntax is:
int connect(int sd, const struct sockaddr *addr, socklen_t addrlen);
sd: the socket descriptor of the endpoint.
addr the peer's address.
addrlen the size of the address.
This operation corresponds to the ASSOCIATE primitive described in
section 10.1 of RFC2960 [8].
By default, the new association created has only one outbound stream.
The SCTP_INITMSG option described in Section 7.1.3 should be used
before connecting to change the number of outbound streams.
If a bind() is not called prior to the connect() call, the system
picks an ephemeral port and will choose an address set equivalent to
binding with INADDR_ANY and IN6ADDR_ANY for IPv4 and IPv6 socket
respectively. One of those addresses will be the primary address for
the association. This automatically enables the multi-homing
capability of SCTP.
Note that SCTP allows data exchange, similar to T/TCP RFC1644 [3],
during the association set up phase. If an application wants to do
this, it cannot use connect() call. Instead, it should use sendto()
or sendmsg() to initiate an association. If it uses sendto() and it
wants to change initialization behavior, it needs to use the
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SCTP_INITMSG socket option before calling sendto(). Or it can use
SCTP_INIT type sendmsg() to initiate an association without doing the
setsockopt(). Note that some sockets implementations may not support
the sending of data to initiate an assocation with the one-to-one
style (implementations that do not support T/TCP normally have this
restriction). Implementations which allow sending of data to
initiate an association without calling connect() define the
preprocessor constant HAVE_SCTP_NOCONNECT to 1.
SCTP does not support half close semantics. This means that unlike
T/TCP, MSG_EOF should not be set in the flags parameter when calling
sendto() or sendmsg() when the call is used to initiate a connection.
MSG_EOF is not an acceptable flag with SCTP socket.
4.1.6 close() - one-to-one style socket
Applications use close() to gracefully close down an association.
The syntax is:
int close(int sd);
sd - the socket descriptor of the association to be closed.
After an application calls close() on a socket descriptor, no further
socket operations will succeed on that descriptor.
4.1.7 shutdown() - one-to-one style socket
SCTP differs from TCP in that it does not have half closed semantics.
Hence the shutdown() call for SCTP is an approximation of the TCP
shutdown() call, and solves some different problems. Full
TCP-compatibility is not provided, so developers porting TCP
applications to SCTP may need to recode sections that use shutdown().
(Note that it is possible to achieve the same results as half close
in SCTP using SCTP streams.)
The syntax is:
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int shutdown(int sd, int how);
sd - the socket descriptor of the association to be closed.
how - Specifies the type of shutdown. The values are
as follows:
SHUT_RD
Disables further receive operations. No SCTP
protocol action is taken.
SHUT_WR
Disables further send operations, and initiates
the SCTP shutdown sequence.
SHUT_RDWR
Disables further send and receive operations
and initiates the SCTP shutdown sequence.
The major difference between SCTP and TCP shutdown() is that SCTP
SHUT_WR initiates immediate and full protocol shutdown, whereas TCP
SHUT_WR causes TCP to go into the half closed state. SHUT_RD behaves
the same for SCTP as TCP. The purpose of SCTP SHUT_WR is to close
the SCTP association while still leaving the socket descriptor open,
so that the caller can receive back any data SCTP was unable to
deliver (see Section 5.3.1.4 for more information).
To perform the ABORT operation described in RFC2960 [8] section 10.1,
an application can use the socket option SO_LINGER. It is described
in Section 7.1.4.
4.1.8 sendmsg() and recvmsg() - one-to-one style socket
With a one-to-one style socket, the application can also use
sendmsg() and recvmsg() to transmit data to and receive data from its
peer. The semantics is similar to those used in the one-to-many
style (section Section 3.1.3), with the following differences:
1) When sending, the msg_name field in the msghdr is not used to
specify the intended receiver, rather it is used to indicate a
preferred peer address if the sender wishes to discourage the stack
from sending the message to the primary address of the receiver. If
the transport address given is not part of the current association,
the data will not be sent and a SCTP_SEND_FAILED event will be
delivered to the application if send failure events are enabled.
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4.1.9 getpeername()
Applications use getpeername() to retrieve the primary socket address
of the peer. This call is for TCP compatibility, and is not
multi-homed. It does not work with one-to-many style sockets. See
Section 8.3 for a multi-homed/one-to-many style version of the call
.
The syntax is:
int getpeername(int sd, struct sockaddr *address,
socklen_t *len);
sd - the socket descriptor to be queried.
address - On return, the peer primary address is stored in
this buffer. If the socket is an IPv4 socket, the
address will be IPv4. If the socket is an IPv6 socket,
the address will be either an IPv6 or IPv4
address.
len - The caller should set the length of address here.
On return, this is set to the length of the returned
address.
If the actual length of the address is greater than the length of the
supplied sockaddr structure, the stored address will be truncated.
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5. Data Structures
We discuss in this section important data structures which are
specific to SCTP and are used with sendmsg() and recvmsg() calls to
control SCTP endpoint operations and to access ancillary information
and notifications.
5.1 The msghdr and cmsghdr Structures
The msghdr structure used in the sendmsg() and recvmsg() calls, as
well as the ancillary data carried in the structure, is the key for
the application to set and get various control information from the
SCTP endpoint.
The msghdr and the related cmsghdr structures are defined and
discussed in details in RFC2292 [6]. Here we will cite their
definitions from RFC2292 [6].
The msghdr structure:
struct msghdr {
void *msg_name; /* ptr to socket address structure */
socklen_t msg_namelen; /* size of socket address structure */
struct iovec *msg_iov; /* scatter/gather array */
size_t msg_iovlen; /* # elements in msg_iov */
void *msg_control; /* ancillary data */
socklen_t msg_controllen; /* ancillary data buffer length */
int msg_flags; /* flags on received message */
};
The cmsghdr structure:
struct cmsghdr {
socklen_t cmsg_len; /* #bytes, including this header */
int cmsg_level; /* originating protocol */
int cmsg_type; /* protocol-specific type */
/* followed by unsigned char cmsg_data[]; */
};
In the msghdr structure, the usage of msg_name has been discussed in
previous sections (see Section 3.1.3 and Section 4.1.8).
The scatter/gather buffers, or I/O vectors (pointed to by the msg_iov
field) are treated as a single SCTP data chunk, rather than multiple
chunks, for both sendmsg() and recvmsg().
The msg_flags are not used when sending a message with sendmsg().
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If a notification has arrived, recvmsg() will return the notification
with the MSG_NOTIFICATION flag set in msg_flags. If the
MSG_NOTIFICATION flag is not set, recvmsg() will return data. See
Section 5.3 for more information about notifications.
If all portions of a data frame or notification have been read,
recvmsg() will return with MSG_EOR set in msg_flags.
5.2 SCTP msg_control Structures
A key element of all SCTP-specific socket extensions is the use of
ancillary data to specify and access SCTP-specific data via the
struct msghdr's msg_control member used in sendmsg() and recvmsg().
Fine-grained control over initialization and sending parameters are
handled with ancillary data.
Each ancillary data item is proceeded by a struct cmsghdr (see
Section 5.1), which defines the function and purpose of the data
contained in in the cmsg_data[] member.
There are two kinds of ancillary data used by SCTP: initialization
data, and, header information (SNDRCV). Initialization data
(one-to-many style only) sets protocol parameters for new
associations. Section 5.2.1 provides more details. Header
information can set or report parameters on individual messages in a
stream. See Section 5.2.2 for how to use SNDRCV ancillary data.
By default on a one-to-one style socket, SCTP will pass no ancillary
data; on a one-to-many style socket, SCTP will only pass SCTP_SNDRCV
and SCTP_ASSOC_CHANGE information. Specific ancillary data items can
be enabled with socket options defined for SCTP; see Section 7.3.
Note that all ancillary types are fixed length; see Section 5.4 for
further discussion on this. These data structures use struct
sockaddr_storage (defined in RFC2553 [7]) as a portable, fixed length
address format.
Other protocols may also provide ancillary data to the socket layer
consumer. These ancillary data items from other protocols may
intermingle with SCTP data. For example, the IPv6 socket API
definitions (RFC2292 [6] and RFC2553 [7]) define a number of
ancillary data items. If a socket API consumer enables delivery of
both SCTP and IPv6 ancillary data, they both may appear in the same
msg_control buffer in any order. An application may thus need to
handle other types of ancillary data besides that passed by SCTP.
The sockets application must provide a buffer large enough to
accommodate all ancillary data provided via recvmsg(). If the buffer
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is not large enough, the ancillary data will be truncated and the
msghdr's msg_flags will include MSG_CTRUNC.
5.2.1 SCTP Initiation Structure (SCTP_INIT)
This cmsghdr structure provides information for initializing new SCTP
associations with sendmsg(). The SCTP_INITMSG socket option uses
this same data structure. This structure is not used for recvmsg().
cmsg_level cmsg_type cmsg_data[]
------------ ------------ ----------------------
IPPROTO_SCTP SCTP_INIT struct sctp_initmsg
Here is the definition of the sctp_initmsg structure:
struct sctp_initmsg {
uint16_t sinit_num_ostreams;
uint16_t sinit_max_instreams;
uint16_t sinit_max_attempts;
uint16_t sinit_max_init_timeo;
};
sinit_num_ostreams: 16 bits (unsigned integer)
This is an integer number representing the number of streams that the
application wishes to be able to send to. This number is confirmed
in the SCTP_COMM_UP notification and must be verified since it is a
negotiated number with the remote endpoint. The default value of 0
indicates to use the endpoint default value.
sinit_max_instreams: 16 bits (unsigned integer)
This value represents the maximum number of inbound streams the
application is prepared to support. This value is bounded by the
actual implementation. In other words the user MAY be able to
support more streams than the Operating System. In such a case, the
Operating System limit overrides the value requested by the user.
The default value of 0 indicates to use the endpoint's default value.
sinit_max_attempts: 16 bits (unsigned integer)
This integer specifies how many attempts the SCTP endpoint should
make at resending the INIT. This value overrides the system SCTP
'Max.Init.Retransmits' value. The default value of 0 indicates to
use the endpoint's default value. This is normally set to the
system's default 'Max.Init.Retransmit' value.
sinit_max_init_timeo: 16 bits (unsigned integer)
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This value represents the largest Time-Out or RTO value (in
milliseconds) to use inattempting a INIT. Normally the 'RTO.Max' is
used to limit the doubling of the RTO upon timeout. For the INIT
message this value MAY override 'RTO.Max'. This value MUST NOT
influence 'RTO.Max' during data transmission and is only used to
bound the initial setup time. A default value of 0 indicates to use
the endpoint's default value. This is normally set to the system's
'RTO.Max' value (60 seconds).
5.2.2 SCTP Header Information Structure (SCTP_SNDRCV)
This cmsghdr structure specifies SCTP options for sendmsg() and
describes SCTP header information about a received message through
recvmsg().
cmsg_level cmsg_type cmsg_data[]
------------ ------------ ----------------------
IPPROTO_SCTP SCTP_SNDRCV struct sctp_sndrcvinfo
Here is the definition of sctp_sndrcvinfo:
struct sctp_sndrcvinfo {
uint16_t sinfo_stream;
uint16_t sinfo_ssn;
uint16_t sinfo_flags;
uint32_t sinfo_ppid;
uint32_t sinfo_context;
uint32_t sinfo_timetolive;
uint32_t sinfo_tsn;
uint32_t sinfo_cumtsn;
sctp_assoc_t sinfo_assoc_id;
};
sinfo_stream: 16 bits (unsigned integer)
For recvmsg() the SCTP stack places the message's stream number in
this value. For sendmsg() this value holds the stream number that
the application wishes to send this message to. If a sender
specifies an invalid stream number an error indication is returned
and the call fails.
sinfo_ssn: 16 bits (unsigned integer)
For recvmsg() this value contains the stream sequence number that the
remote endpoint placed in the DATA chunk. For fragmented messages
this is the same number for all deliveries of the message (if more
than one recvmsg() is needed to read the message). The sendmsg()
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call will ignore this parameter.
sinfo_ppid: 32 bits (unsigned integer)
This value in sendmsg() is an opaque unsigned value that is passed to
the remote end in each user message. In recvmsg() this value is the
same information that was passed by the upper layer in the peer
application. Please note that byte order issues are NOT accounted
for and this information is passed opaquely by the SCTP stack from
one end to the other.
sinfo_context: 32 bits (unsigned integer)
This value is an opaque 32 bit context datum that is used in the
sendmsg() function. This value is passed back to the upper layer if
a error occurs on the send of a message and is retrieved with each
undelivered message (Note: if a endpoint has done multiple sends, all
of which fail, multiple different sinfo_context values will be
returned. One with each user data message).
sinfo_flags: 16 bits (unsigned integer)
This field may contain any of the following flags and is composed of
a bitwise OR of these values.
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recvmsg() flags:
MSG_UNORDERED - This flag is present when the message was sent
non-ordered.
sendmsg() flags:
MSG_UNORDERED - This flag requests the un-ordered delivery of the
message. If this flag is clear the datagram is
considered an ordered send.
MSG_ADDR_OVER - This flag, in the one-to-many style, requests the SCTP
stack to override the primary destination address
with the address found with the sendto/sendmsg
call.
MSG_ABORT - Setting this flag causes the specified association
to abort by sending an ABORT message to the peer
(one-to-many style only). The ABORT chunk
will contain an error cause 'User Initiated Abort'
with cause code 12.The cause specific
information of this error cause is provided in msg_iov.
MSG_EOF - Setting this flag invokes the SCTP graceful shutdown
procedures on the specified association. Graceful
shutdown assures that all data enqueued by both
endpoints is successfully transmitted before closing
the association (one-to-many style only).
MSG_SENDALL - This flag, if set, will cause a one-to-many model
socket to send the message to all associations
that are currently established on this socket. For
the one-to-one socket, this flag has no effect.
sinfo_timetolive: 32 bit (unsigned integer)
For the sending side, this field contains the message time to live in
milliseconds. The sending side will expire the message within the
specified time period if the message as not been sent to the peer
within this time period. This value will override any default value
set using any socket option. Also note that the value of 0 is
special in that it indicates no timeout should occur on this message.
sinfo_tsn: 32 bit (unsigned integer)
For the receiving side, this field holds a TSN that was assigned to
one of the SCTP Data Chunks.
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sinfo_cumtsn: 32 bit (unsigned integer)
This field will hold the current cumulative TSN as known by the
underlying SCTP layer. Note this field is ignored when sending and
only valid for a receive operation when sinfo_flags are set to
MSG_UNORDERED.
sinfo_assoc_id: sizeof (sctp_assoc_t)
The association handle field, sinfo_assoc_id, holds the identifier
for the association announced in the SCTP_COMM_UP notification. All
notifications for a given association have the same identifier.
Ignored for one-to-one style sockets.
A sctp_sndrcvinfo item always corresponds to the data in msg_iov.
5.3 SCTP Events and Notifications
An SCTP application may need to understand and process events and
errors that happen on the SCTP stack. These events include network
status changes, association startups, remote operational errors and
undeliverable messages. All of these can be essential for the
application.
When an SCTP application layer does a recvmsg() the message read is
normally a data message from a peer endpoint. If the application
wishes to have the SCTP stack deliver notifications of non-data
events, it sets the appropriate socket option for the notifications
it wants. See Section 7.3 for these socket options. When a
notification arrives, recvmsg() returns the notification in the
application-supplied data buffer via msg_iov, and sets
MSG_NOTIFICATION in msg_flags.
This section details the notification structures. Every notification
structure carries some common fields which provides general
information.
A recvmsg() call will return only one notification at a time. Just
as when reading normal data, it may return part of a notification if
the msg_iov buffer is not large enough. If a single read is not
sufficient, msg_flags will have MSG_EOR clear. The user MUST finish
reading the notification before subsequent data can arrive.
5.3.1 SCTP Notification Structure
The notification structure is defined as the union of all
notification types.
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union sctp_notification {
struct {
uint16_t sn_type; /* Notification type. */
uint16_t sn_flags;
uint32_t sn_length;
} sn_header;
struct sctp_assoc_change sn_assoc_change;
struct sctp_paddr_change sn_paddr_change;
struct sctp_remote_error sn_remote_error;
struct sctp_send_failed sn_send_failed;
struct sctp_shutdown_event sn_shutdown_event;
struct sctp_adaption_event sn_adaption_event;
struct sctp_pdapi_event sn_pdapi_event;
};
sn_type: 16 bits (unsigned integer)
The following list describes the SCTP notification and event types
for the field sn_type.
SCTP_ASSOC_CHANGE: This tag indicates that an association has either
been opened or closed. Refer to Section 5.3.1.1 for details.
SCTP_PEER_ADDR_CHANGE: This tag indicates that an address that is
part of an existing association has experienced a change of state
(e.g. a failure or return to service of the reachability of a
endpoint via a specific transport address). Please see
Section 5.3.1.2 for data structure details.
SCTP_REMOTE_ERROR: The attached error message is an Operational Error
received from the remote peer. It includes the complete TLV sent
by the remote endpoint. See Section 5.3.1.3 for the detailed
format.
SCTP_SEND_FAILED: The attached datagram could not be sent to the
remote endpoint. This structure includes the original
SCTP_SNDRCVINFO that was used in sending this message i.e. this
structure uses the sctp_sndrecvinfo per Section 5.3.1.4.
SCTP_SHUTDOWN_EVENT: The peer has sent a SHUTDOWN. No further data
should be sent on this socket.
SCTP_ADAPTION_INDICATION: This notification holds the peers indicated
adaption layer. Please see Section 5.3.1.6.
SCTP_PARTIAL_DELIVERY_EVENT: This notification is used to tell a
receiver that the partial delivery has been aborted. This may
indicate the association is about to be aborted. Please see
Section 5.3.1.7
All standard values for sn_type are greater than 2^15. Values from
2^15 and down are reserved.
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sn_flags: 16 bits (unsigned integer)
These are notification-specific flags.
sn_length: 32 bits (unsigned integer)
This is the length of the whole sctp_notification structure including
the sn_type, sn_flags, and sn_length fields.
5.3.1.1 SCTP_ASSOC_CHANGE
Communication notifications inform the ULP that an SCTP association
has either begun or ended. The identifier for a new association is
provided by this notification. The notification information has the
following format:
struct sctp_assoc_change {
uint16_t sac_type;
uint16_t sac_flags;
uint32_t sac_length;
uint16_t sac_state;
uint16_t sac_error;
uint16_t sac_outbound_streams;
uint16_t sac_inbound_streams;
sctp_assoc_t sac_assoc_id;
uint8_t sac_info[0];
};
sac_type:
It should be SCTP_ASSOC_CHANGE.
sac_flags: 16 bits (unsigned integer)
Currently unused.
sac_length: 32 bits (unsigned integer)
This field is the total length of the notification data, including
the notification header.
sac_state: 16 bits (signed integer)
This field holds one of a number of values that communicate the event
that happened to the association. They include:
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Event Name Description
---------------- ---------------
SCTP_COMM_UP A new association is now ready
and data may be exchanged with this
peer.
SCTP_COMM_LOST The association has failed. The association
is now in the closed state. If SEND FAILED
notifications are turned on, a SCTP_COMM_LOST
is followed by a series of SCTP_SEND_FAILED
events, one for each outstanding message.
SCTP_RESTART SCTP has detected that the peer has restarted.
SCTP_SHUTDOWN_COMP The association has gracefully closed.
SCTP_CANT_STR_ASSOC The association failed to setup. If non blocking
mode is set and data was sent (in the udp mode),
a SCTP_CANT_STR_ASSOC is followed by a series of
SCTP_SEND_FAILED events, one for each outstanding
message.
sac_error: 16 bits (signed integer)
If the state was reached due to a error condition (e.g.
SCTP_COMM_LOST) any relevant error information is available in this
field. This corresponds to the protocol error codes defined in
RFC2960 [8].
sac_outbound_streams: 16 bits (unsigned integer)
sac_inbound_streams: 16 bits (unsigned integer)
The maximum number of streams allowed in each direction are available
in sac_outbound_streams and sac_inbound streams.
sac_assoc_id: sizeof (sctp_assoc_t)
The association id field, holds the identifier for the association.
All notifications for a given association have the same association
identifier. For one-to-one style socket, this field is ignored.
sac_info: variable
If the sac_state is SCTP_COMM_LOST and an ABORT chunk was received
for this association, sac_info[] contains the complete ABORT chunk as
defined in the SCTP specification RFC2960 [8] section 3.3.7.
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5.3.1.2 SCTP_PEER_ADDR_CHANGE
When a destination address on a multi-homed peer encounters a change
an interface details event is sent. The information has the
following structure:
struct sctp_paddr_change {
uint16_t spc_type;
uint16_t spc_flags;
uint32_t spc_length;
struct sockaddr_storage spc_aaddr;
int spc_state;
int spc_error;
sctp_assoc_t spc_assoc_id;
}
spc_type:
It should be SCTP_PEER_ADDR_CHANGE.
spc_flags: 16 bits (unsigned integer)
Currently unused.
spc_length: 32 bits (unsigned integer)
This field is the total length of the notification data, including
the notification header.
spc_aaddr: sizeof (struct sockaddr_storage)
The affected address field, holds the remote peer's address that is
encountering the change of state.
spc_state: 32 bits (signed integer)
This field holds one of a number of values that communicate the event
that happened to the address. They include:
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Event Name Description
---------------- ---------------
SCTP_ADDR_AVAILABLE This address is now reachable.
SCTP_ADDR_UNREACHABLE The address specified can no
longer be reached. Any data sent
to this address is rerouted to an
alternate until this address becomes
reachable.
SCTP_ADDR_REMOVED The address is no longer part of
the association.
SCTP_ADDR_ADDED The address is now part of the
association.
SCTP_ADDR_MADE_PRIM This address has now been made
to be the primary destination address.
spc_error: 32 bits (signed integer)
If the state was reached due to any error condition (e.g.
SCTP_ADDR_UNREACHABLE) any relevant error information is available in
this field.
spc_assoc_id: sizeof (sctp_assoc_t)
The association id field, holds the identifier for the association.
All notifications for a given association have the same association
identifier. For one-to-one style socket, this field is ignored.
5.3.1.3 SCTP_REMOTE_ERROR
A remote peer may send an Operational Error message to its peer.
This message indicates a variety of error conditions on an
association. The entire ERROR chunk as it appears on the wire is
included in a SCTP_REMOTE_ERROR event. Please refer to the SCTP
specification RFC2960 [8] and any extensions for a list of possible
error formats. SCTP error notifications have the format:
struct sctp_remote_error {
uint16_t sre_type;
uint16_t sre_flags;
uint32_t sre_length;
uint16_t sre_error;
sctp_assoc_t sre_assoc_id;
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uint8_t sre_data[0];
};
sre_type:
It should be SCTP_REMOTE_ERROR.
sre_flags: 16 bits (unsigned integer)
Currently unused.
sre_length: 32 bits (unsigned integer)
This field is the total length of the notification data, including
the notification header and the contents of sre_data.
sre_error: 16 bits (unsigned integer)
This value represents one of the Operational Error causes defined in
the SCTP specification, in network byte order.
sre_assoc_id: sizeof (sctp_assoc_t)
The association id field, holds the identifier for the association.
All notifications for a given association have the same association
identifier. For one-to-one style socket, this field is ignored.
sre_data: variable
This contains the ERROR chunk as defined in the SCTP specification
RFC2960 [8] section 3.3.10.
5.3.1.4 SCTP_SEND_FAILED
If SCTP cannot deliver a message it may return the message as a
notification.
struct sctp_send_failed {
uint16_t ssf_type;
uint16_t ssf_flags;
uint32_t ssf_length;
uint32_t ssf_error;
struct sctp_sndrcvinfo ssf_info;
sctp_assoc_t ssf_assoc_id;
uint8_t ssf_data[0];
};
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ssf_type:
It should be SCTP_SEND_FAILED.
The flag value will take one of the following values
SCTP_DATA_UNSENT - Indicates that the data was never put on
the wire.
SCTP_DATA_SENT - Indicates that the data was put on the wire.
Note that this does not necessarily mean that the
data was (or was not) successfully delivered.
ssf_length: 32 bits (unsigned integer)
This field is the total length of the notification data, including
the notification header and the payload in ssf_data.
ssf_error: 16 bits (unsigned integer)
This value represents the reason why the send failed, and if set,
will be a SCTP protocol error code as defined in RFC2960 [8] section
3.3.10.
ssf_info: sizeof (struct sctp_sndrcvinfo)
The original send information associated with the undelivered
message.
ssf_assoc_id: sizeof (sctp_assoc_t)
The association id field, sf_assoc_id, holds the identifier for the
association. All notifications for a given association have the same
association identifier. For one-to-one style socket, this field is
ignored.
ssf_data: variable length
The undelivered message, exactly as delivered by the caller to the
original send*() call.
5.3.1.5 SCTP_SHUTDOWN_EVENT
When a peer sends a SHUTDOWN, SCTP delivers this notification to
inform the application that it should cease sending data.
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struct sctp_shutdown_event {
uint16_t sse_type;
uint16_t sse_flags;
uint32_t sse_length;
sctp_assoc_t sse_assoc_id;
};
sse_type
It should be SCTP_SHUTDOWN_EVENT
sse_flags: 16 bits (unsigned integer)
Currently unused.
sse_length: 32 bits (unsigned integer)
This field is the total length of the notification data, including
the notification header. It will generally be sizeof (struct
sctp_shutdown_event).
sse_flags: 16 bits (unsigned integer)
Currently unused.
sse_assoc_id: sizeof (sctp_assoc_t)
The association id field, holds the identifier for the association.
All notifications for a given association have the same association
identifier. For one-to-one style socket, this field is ignored.
5.3.1.6 SCTP_ADAPTION_INDICATION
When a peer sends a Adaption Layer Indication parameter , SCTP
delivers this notification to inform the application that of the
peers requested adaption layer.
struct sctp_adaption_event {
uint16_t sai_type;
uint16_t sai_flags;
uint32_t sai_length;
uint32_t sai_adaption_ind;
sctp_assoc_t sai_assoc_id;
};
sai_type
It should be SCTP_ADAPTION_INDICATION
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sai_flags: 16 bits (unsigned integer)
Currently unused.
sai_length: 32 bits (unsigned integer)
This field is the total length of the notification data, including
the notification header. It will generally be sizeof (struct
sctp_adaption_event).
sai_adaption_ind: 32 bits (unsigned integer)
This field holds the bit array sent by the peer in the adaption layer
indication parameter. The bits are in network byte order.
sai_assoc_id: sizeof (sctp_assoc_t)
The association id field, holds the identifier for the association.
All notifications for a given association have the same association
identifier. For one-to-one style socket, this field is ignored.
5.3.1.7 SCTP_PARTIAL_DELIVERY_EVENT
When a receiver is engaged in a partial delivery of a message this
notification will be used to indicate various events.
struct sctp_pdapi_event {
uint16_t pdapi_type;
uint16_t pdapi_flags;
uint32_t pdapi_length;
uint32_t pdapi_indication;
sctp_assoc_t pdapi_assoc_id;
};
pdapi_type
It should be SCTP_PARTIAL_DELIVERY_EVENT
pdapi_flags: 16 bits (unsigned integer)
Currently unused.
pdapi_length: 32 bits (unsigned integer)
This field is the total length of the notification data, including
the notification header. It will generally be sizeof (struct
sctp_pdapi_event).
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pdapi_indication: 32 bits (unsigned integer)
This field holds the indication being sent to the application
possible values include:
SCTP_PARTIAL_DELIVERY_ABORTED
pdapi_assoc_id: sizeof (sctp_assoc_t)
The association id field, holds the identifier for the association.
All notifications for a given association have the same association
identifier. For one-to-one style socket, this field is ignored.
5.4 Ancillary Data Considerations and Semantics
Programming with ancillary socket data contains some subtleties and
pitfalls, which are discussed below.
5.4.1 Multiple Items and Ordering
Multiple ancillary data items may be included in any call to
sendmsg() or recvmsg(); these may include multiple SCTP or non-SCTP
items, or both.
The ordering of ancillary data items (either by SCTP or another
protocol) is not significant and is implementation-dependent, so
applications must not depend on any ordering.
SCTP_SNDRCV items must always correspond to the data in the msghdr's
msg_iov member. There can be only a single SCTP_SNDRCV info for each
sendmsg() or recvmsg() call.
5.4.2 Accessing and Manipulating Ancillary Data
Applications can infer the presence of data or ancillary data by
examining the msg_iovlen and msg_controllen msghdr members,
respectively.
Implementations may have different padding requirements for ancillary
data, so portable applications should make use of the macros
CMSG_FIRSTHDR, CMSG_NXTHDR, CMSG_DATA, CMSG_SPACE, and CMSG_LEN. See
RFC2292 [6] and your SCTP implementation's documentation for more
information. Following is an example, from RFC2292 [6],
demonstrating the use of these macros to access ancillary data:
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struct msghdr msg;
struct cmsghdr *cmsgptr;
/* fill in msg */
/* call recvmsg() */
for (cmsgptr = CMSG_FIRSTHDR(&msg); cmsgptr != NULL;
cmsgptr = CMSG_NXTHDR(&msg, cmsgptr)) {
if (cmsgptr->cmsg_level == ... && cmsgptr->cmsg_type == ... ) {
u_char *ptr;
ptr = CMSG_DATA(cmsgptr);
/* process data pointed to by ptr */
}
}
5.4.3 Control Message Buffer Sizing
The information conveyed via SCTP_SNDRCV events will often be
fundamental to the correct and sane operation of the sockets
application. This is particularly true of the one-to-many semantics,
but also of the one-ton-one semantics. For example, if an
application needs to send and receive data on different SCTP streams,
SCTP_SNDRCV events are indispensable.
Given that some ancillary data is critical, and that multiple
ancillary data items may appear in any order, applications should be
carefully written to always provide a large enough buffer to contain
all possible ancillary data that can be presented by recvmsg(). If
the buffer is too small, and crucial data is truncated, it may pose a
fatal error condition.
Thus it is essential that applications be able to deterministically
calculate the maximum required buffer size to pass to recvmsg(). One
constraint imposed on this specification that makes this possible is
that all ancillary data definitions are of a fixed length. One way
to calculate the maximum required buffer size might be to take the
sum the sizes of all enabled ancillary data item structures, as
calculated by CMSG_SPACE. For example, if we enabled
SCTP_SNDRCV_INFO and IPV6_RECVPKTINFO RFC2292 [6], we would calculate
and allocate the buffer size as follows:
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size_t total;
void *buf;
total = CMSG_SPACE(sizeof (struct sctp_sndrcvinfo)) +
CMSG_SPACE(sizeof (struct in6_pktinfo));
buf = malloc(total);
We could then use this buffer for msg_control on each call to
recvmsg() and be assured that we would not lose any ancillary data to
truncation.
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6. Common Operations for Both Styles
6.1 send(), recv(), sendto(), recvfrom()
Applications can use send() and sendto() to transmit data to the peer
of an SCTP endpoint. recv() and recvfrom() can be used to receive
data from the peer.
The syntax is:
ssize_t send(int sd, const void *msg, size_t len, int flags);
ssize_t sendto(int sd, const void *msg, size_t len, int flags,
const struct sockaddr *to, socklen_t tolen);
ssize_t recv(int sd, void *buf, size_t len, int flags);
ssize_t recvfrom(int sd, void *buf, size_t len, int flags,
struct sockaddr *from, socklen_t *fromlen);
sd - the socket descriptor of an SCTP endpoint.
msg - the message to be sent.
len - the size of the message or the size of buffer.
to - one of the peer addresses of the association to be
used to send the message.
tolen - the size of the address.
buf - the buffer to store a received message.
from - the buffer to store the peer address used to send the
received message.
fromlen - the size of the from address
flags - (described below).
These calls give access to only basic SCTP protocol features. If
either peer in the association uses multiple streams, or sends
unordered data these calls will usually be inadequate, and may
deliver the data in unpredictable ways.
SCTP has the concept of multiple streams in one association. The
above calls do not allow the caller to specify on which stream a
message should be sent. The system uses stream 0 as the default
stream for send() and sendto(). recv() and recvfrom() return data
from any stream, but the caller can not distinguish the different
streams. This may result in data seeming to arrive out of order.
Similarly, if a data chunk is sent unordered, recv() and recvfrom()
provide no indication.
SCTP is message based. The msg buffer above in send() and sendto()
is considered to be a single message. This means that if the caller
wants to send a message which is composed by several buffers, the
caller needs to combine them before calling send() or sendto().
Alternately, the caller can use sendmsg() to do that without
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combining them. recv() and recvfrom() cannot distinguish message
boundaries.
In receiving, if the buffer supplied is not large enough to hold a
complete message, the receive call acts like a stream socket and
returns as much data as will fit in the buffer.
Note, the send() and recv() calls may not be used for a one-to-many
style socket.
Note, if an application calls a send function with no user data and
no ancillary data the SCTP implementation should reject the request
with an appropriate error message. An implementation is NOT allowed
to send a Data chunk with no user data RFC2960 [8].
6.2 setsockopt(), getsockopt()
Applications use setsockopt() and getsockopt() to set or retrieve
socket options. Socket options are used to change the default
behavior of sockets calls. They are described in Section 7
The syntax is:
ret = getsockopt(int sd, int level, int optname, void *optval,
socklen_t *optlen);
ret = setsockopt(int sd, int level, int optname, const void *optval,
socklen_t optlen);
sd - the socket descript.
level - set to IPPROTO_SCTP for all SCTP options.
optname - the option name.
optval - the buffer to store the value of the option.
optlen - the size of the buffer (or the length of the option
returned).
6.3 read() and write()
Applications can use read() and write() to send and receive data to
and from peer. They have the same semantics as send() and recv()
except that the flags parameter cannot be used.
Note, these calls, when used in the one-to-many style, may only be
used with branched off socket descriptors (see Section 8.2).
6.4 getsockname()
Applications use getsockname() to retrieve the locally-bound socket
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address of the specified socket. This is especially useful if the
caller let SCTP chose a local port. This call is for where the
endpoint is not multi-homed. It does not work well with multi-homed
sockets. See Section 8.5 for a multi-homed version of the call.
The syntax is:
int getsockname(int sd, struct sockaddr *address,
socklen_t *len);
sd - the socket descriptor to be queried.
address - On return, one locally bound address (chosen by
the SCTP stack) is stored in this buffer. If the
socket is an IPv4 socket, the address will be IPv4.
If the socket is an IPv6 socket, the address will
be either an IPv6 or IPv4 address.
len - The caller should set the length of address here.
On return, this is set to the length of the returned
address.
If the actual length of the address is greater than the length of the
supplied sockaddr structure, the stored address will be truncated.
If the socket has not been bound to a local name, the value stored in
the object pointed to by address is unspecified.
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7. Socket Options
The following sub-section describes various SCTP level socket options
that are common to both styles. SCTP associations can be
multi-homed. Therefore, certain option parameters include a
sockaddr_storage structure to select which peer address the option
should be applied to.
For the one-to-many style sockets, an sctp_assoc_t structure
(association ID) is used to identify the the association instance
that the operation affects. So it must be set when using this style.
For the one-to-one style sockets and branched off one-to-many style
sockets (see Section 8.2) this association ID parameter is ignored.
Note that socket or IP level options are set or retrieved per socket.
This means that for one-to-many style sockets, those options will be
applied to all associations belonging to the socket. And for
one-to-one style, those options will be applied to all peer addresses
of the association controlled by the socket. Applications should be
very careful in setting those options.
For some IP stacks getsockopt() is read-only; so a new interface will
be needed when information must be passed both in to and out of the
SCTP stack. The syntax for sctp_opt_info() is,
int sctp_opt_info(int sd,
sctp_assoc_t id,
int opt,
void *arg,
socklen_t *size);
The sctp_opt_info() call is a replacement for getsockopt() only and
will not set any options associated with the specified socket. A
setsockopt() must be used to set any writeable option.
For one-to-many style sockets, id specifies the association to query.
For one-to-one style sockets, id is ignored.
opt specifies which SCTP socket option to get. It can get any socket
option currently supported that requests information (either
read/write options or read only) such as:
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SCTP_RTOINFO
SCTP_ASSOCINFO
SCTP_DEFAULT_SEND_PARAM
SCTP_GET_PEER_ADDR_INFO
SCTP_PRIMARY_ADDR
SCTP_PEER_ADDR_PARAMS
SCTP_STATUS
SCTP_AUTH_CHUNKS
SCTP_AUTH_SECRET
arg is an option-specific structure buffer provided by the caller.
See Section 8.5) subsections for more information on these options
and option-specific structures.
sctp_opt_info() returns 0 on success, or on failure returns -1 and
sets errno to the appropriate error code.
All options that support specific settings on an association by
filling in either an association id variable or a sockaddr_storage
SHOULD also support setting of the same value for the entire endpoint
(i.e. future associations). To accomplish this the following logic
is used when setting one of these options:
a) If an address is specified via a sockaddr_storage that is included
in the structure, the address is used to lookup the association
and the settings are applied to the specific address (if
appropriate) or to the entire association.
b) If an association identification is filled in but not a
sockaddr_storage (if present), the association is found using the
association identification and the settings should be applied to
the entire association (since a specific address is not
specified). Note this also applies to options that hold an
association identification in their structure but do not have a
sockaddr_storage field.
c) If neither the sockaddr_storage or association identification is
set i.e. the sockaddr_storage is set to all 0's (INADDR_ANY) and
the association identification is 0, the settings are a default
and to be applied to the endpoint (all future associations).
7.1 Read / Write Options
7.1.1 Retransmission Timeout Parameters (SCTP_RTOINFO)
The protocol parameters used to initialize and bound retransmission
timeout (RTO) are tunable. See RFC2960 [8] for more information on
how these parameters are used in RTO calculation.
The following structure is used to access and modify these
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parameters:
struct sctp_rtoinfo {
sctp_assoc_t srto_assoc_id;
uint32_t srto_initial;
uint32_t srto_max;
uint32_t srto_min;
};
srto_initial - This contains the initial RTO value.
srto_max and srto_min - These contain the maximum and minimum bounds
for all RTOs.
srto_assoc_id - (one-to-many style socket) This is filled in
the application, and identifies the association
for this query. If this parameter is '0'
(on a one-to-many style socket), then the change
effects the entire endpoint.
All parameters are time values, in milliseconds. A value of 0, when
modifying the parameters, indicates that the current value should not
be changed.
To access or modify these parameters, the application should call
getsockopt or setsockopt() respectively with the option name
SCTP_RTOINFO.
7.1.2 Association Parameters (SCTP_ASSOCINFO)
This option is used to both examine and set various association and
endpoint parameters.
See RFC2960 [8] for more information on how this parameter is used.
The peer address parameter is ignored for one-to-one style socket.
The following structure is used to access and modify this parameters:
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struct sctp_assocparams {
sctp_assoc_t sasoc_assoc_id;
uint16_t sasoc_asocmaxrxt;
uint16_t sasoc_number_peer_destinations;
uint32_t sasoc_peer_rwnd;
uint32_t sasoc_local_rwnd;
uint32_t sasoc_cookie_life;
};
sasoc_asocmaxrxt - This contains the maximum retransmission attempts
to make for the association.
sasoc_number_peer_destinations - This is the number of destination
addresses that the peer has.
sasoc_peer_rwnd - This holds the current value of the peers
rwnd (reported in the last SACK) minus any
outstanding data (i.e. data inflight).
sasoc_local_rwnd - This holds the last reported rwnd that was
sent to the peer.
sasoc_cookie_life - This is the associations cookie life value
used when issuing cookies.
sasoc_assoc_id - (one-to-many style socket) This is filled in the
application, and identifies the association
for this query.
This information may be examined for either the endpoint or a
specific association. To examine a endpoints default parameters the
association id (sasoc_assoc_id) should must be set to the value '0'.
The values of the sasoc_peer_rwnd is meaningless when examining
endpoint information.
All parameters are time values, in milliseconds. A value of 0, when
modifying the parameters, indicates that the current value should not
be changed.
The values of the sasoc_asocmaxrxt and sasoc_cookie_life may be set
on either an endpoint or association basis. The rwnd and destination
counts (sasoc_number_peer_destinations,
sasoc_peer_rwnd,sasoc_local_rwnd) are NOT settable and any value
placed in these is ignored.
To access or modify these parameters, the application should call
getsockopt or setsockopt() respectively with the option name
SCTP_ASSOCINFO.
The maximum number of retransmissions before an address is considered
unreachable is also tunable, but is address-specific, so it is
covered in a separate option. If an application attempts to set the
value of the association maximum retransmission parameter to more
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than the sum of all maximum retransmission parameters, setsockopt()
shall return an error. The reason for this, from RFC2960 [8] section
8.2:
Note: When configuring the SCTP endpoint, the user should avoid
having the value of 'Association.Max.Retrans' larger than the
summation of the 'Path.Max.Retrans' of all the destination addresses
for the remote endpoint. Otherwise, all the destination addresses
may become inactive while the endpoint still considers the peer
endpoint reachable.
7.1.3 Initialization Parameters (SCTP_INITMSG)
Applications can specify protocol parameters for the default
association initialization. The structure used to access and modify
these parameters is defined in Section 5.2.1). The option name
argument to setsockopt() and getsockopt() is SCTP_INITMSG.
Setting initialization parameters is effective only on an unconnected
socket (for one-to-many style sockets only future associations are
effected by the change). With one-to-one style sockets, this option
is inherited by sockets derived from a listener socket.
7.1.4 SO_LINGER
An application using the one-to-one style socket can use this option
to perform the SCTP ABORT primitive. The linger option structure is:
struct linger {
int l_onoff; /* option on/off */
int l_linger; /* linger time */
};
To enable the option, set l_onoff to 1. If the l_linger value is set
to 0, calling close() is the same as the ABORT primitive. If the
value is set to a negative value, the setsockopt() call will return
an error. If the value is set to a positive value linger_time, the
close() can be blocked for at most linger_time ms. If the graceful
shutdown phase does not finish during this period, close() will
return but the graceful shutdown phase continues in the system.
Note, this is a socket level option NOT an SCTP level option. So
when setting SO_LINGER you must specify a level of SOL_SOCKET in the
setsockopt() call.
7.1.5 SCTP_NODELAY
Turn on/off any Nagle-like algorithm. This means that packets are
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generally sent as soon as possible and no unnecessary delays are
introduced, at the cost of more packets in the network. Expects an
integer boolean flag.
7.1.6 SO_RCVBUF
Sets receive buffer size in octets. For SCTP one-to-one style
sockets, this controls the receiver window size. For one-to-many
style sockets the meaning depends on the constant HAVE_SCTP_MULTIBUF
(see Section 3.4). If the implementation defines HAVE_SCTP_MULTIBUF
as 1, this controls the receiver window size for each association
bound to the socket descriptor. If the implementation defines
HAVE_SCTP_MULTIBUF as 0, this controls the size of the single receive
buffer for the whole socket. The call expects an integer.
7.1.7 SO_SNDBUF
Sets send buffer size. For SCTP one-to-one style sockets, this
controls the amount of data SCTP may have waiting in internal buffers
to be sent. This option therefore bounds the maximum size of data
that can be sent in a single send call. For one-to-many style
sockets, the effect is the same, except that it applies to one or all
associations (see Section 3.4) bound to the socket descriptor
used in the setsockopt() or getsockopt() call. The option applies to
each association's window size separately. The call expects an
integer.
7.1.8 Automatic Close of associations (SCTP_AUTOCLOSE)
This socket option is applicable to the one-to-many style socket
only. When set it will cause associations that are idle for more
than the specified number of seconds to automatically close. An
association being idle is defined as an association that has NOT sent
or received user data. The special value of '0' indicates that no
automatic close of any associations should be performed, this is the
default value. The option expects an integer defining the number of
seconds of idle time before an association is closed.
An application using this option should enable receiving the
association change notification. This is the only mechanism an
application is informed about the closing of an association. After
an association is closed, the association ID assigned to it can be
reused. An application should be aware of this to avoid the possible
problem of sending data to an incorrect peer end point.
7.1.9 Set Peer Primary Address (SCTP_SET_PEER_PRIMARY_ADDR)
Requests that the peer mark the enclosed address as the association
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primary. The enclosed address must be one of the association's
locally bound addresses. The following structure is used to make a
set primary request:
struct sctp_setpeerprim {
sctp_assoc_t sspp_assoc_id;
struct sockaddr_storage sspp_addr;
};
sspp_addr The address to set as primary
sspp_assoc_id (one-to-many style socket) This is filled in by the
application, and identifies the association
for this request.
This functionality is optional. Implementations that do not support
this functionality should return EOPNOTSUPP.
7.1.10 Set Primary Address (SCTP_PRIMARY_ADDR)
Requests that the local SCTP stack use the enclosed peer address as
the association primary. The enclosed address must be one of the
association peer's addresses. The following structure is used to
make a set peer primary request:
struct sctp_setprim {
sctp_assoc_t ssp_assoc_id;
struct sockaddr_storage ssp_addr;
};
ssp_addr The address to set as primary
ssp_assoc_id (one-to-many style socket) This is filled in by the
application, and identifies the association
for this request.
7.1.11 Set Adaption Layer Indicator (SCTP_ADAPTION_LAYER)
Requests that the local endpoint set the specified Adaption Layer
Indication parameter for all future INIT and INIT-ACK exchanges.
struct sctp_setadaption {
uint32_t ssb_adaption_ind;
};
ssb_adaption_ind The adaption layer indicator that will be included
in any outgoing Adaption Layer Indication
parameter.
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7.1.12 Enable/Disable message fragmentation (SCTP_DISABLE_FRAGMENTS)
This option is a on/off flag and is passed an integer where a
non-zero is on and a zero is off. If enabled no SCTP message
fragmentation will be performed. Instead if a message being sent
exceeds the current PMTU size, the message will NOT be sent and
instead a error will be indicated to the user.
7.1.13 Peer Address Parameters (SCTP_PEER_ADDR_PARAMS)
Applications can enable or disable heartbeats for any peer address of
an association, modify an address's heartbeat interval, force a
heartbeat to be sent immediately, and adjust the address's maximum
number of retransmissions sent before an address is considered
unreachable. The following structure is used to access and modify an
address's parameters:
struct sctp_paddrparams {
sctp_assoc_t spp_assoc_id;
struct sockaddr_storage spp_address;
uint32_t spp_hbinterval;
uint16_t spp_pathmaxrxt;
uint32_t spp_pathmtu;
uint32_t spp_sackdelay;
uint32_t spp_flags;
};
spp_assoc_id - (one-to-many style socket) This is filled in the
application, and identifies the association for
this query.
spp_address - This specifies which address is of interest.
spp_hbinterval - This contains the value of the heartbeat interval,
in milliseconds. If a value of zero
is present in this field then no changes are to
be made to this parameter.
spp_pathmaxrxt - This contains the maximum number of
retransmissions before this address shall be
considered unreachable. If a value of zero
is present in this field then no changes are to
be made to this parameter.
spp_pathmtu - When Path MTU discovery is disabled the value
specified here will be the "fixed" path mtu.
Note that if the spp_address field is empty
then all associations on this address will
have this fixed path mtu set upon them.
spp_sackdelay - When delayed sack is enabled, this value specifies
the number of milliseconds that sacks will be delayed
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for. This value will apply to all addresses of an
association if the spp_address field is empty. Note
also, that if delayed sack is enabled and this
value is set to 0, no change is made to the last
recorded delayed sack timer value.
spp_flags - These flags are used to control various features
on an association. The flag field may contain
zero or more of the following options.
SPP_HB_ENABLED - Enable heartbeats on the
specified address. Note that if the address
field is empty all addresses for the association
have heartbeats enabled upon them.
SPP_HB_DISABLED - Disable heartbeats on the
speicifed address. Note that if the address
field is empty all addresses for the association
will have their heartbeats disabled. Note also
that SPP_HB_ENABLED and SPP_HB_DISABLED are
mutually exclusive, only one of these two should
be specified. Enabling both fields will have
undetermined results.
SPP_PMTUD_ENABLED - This field will enable PMTU
discovery upon the specified address. Note that
if the address feild is empty then all addresses
on the association are effected.
SPP_PMTUD_DISABLED - This field will disable PMTU
discovery upon the specified address. Note that
if the address feild is empty then all addresses
on the association are effected. Not also that
SPP_PMTUD_ENABLE and SPP_PMTUD_DISABLE are mutually
exclusive. Enabling both will have undetermined
results.
SPP_SACKDELAY_ENABLED - Setting this flag turns
on delayed sack. The time specified in spp_sackdelay
is used to specify the sack delay for this address. Note
that if spp_address is empty then all addresses will
enable delayed sack and take on the sack delay
value specified in spp_sackdelay.
SPP_SACKDELAY_DISABLED - Setting this flag turns
off delayed sack. If the spp_address field is blank then
delayed sack is disabled for the entire association. Note
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also that this field is mutually exclusive to
SPP_SACKDELAY_ENABLED, setting both will have undefined
results.
To read or modify these parameters, the application should call
sctp_opt_info() with the SCTP_PEER_ADDR_PARAMS option.
7.1.14 Set default send parameters (SCTP_DEFAULT_SEND_PARAM)
Applications that wish to use the sendto() system call may wish to
specify a default set of parameters that would normally be supplied
through the inclusion of ancillary data. This socket option allows
such an application to set the default sctp_sndrcvinfo structure.
The application that wishes to use this socket option simply passes
in to this call the sctp_sndrcvinfo structure defined in
Section 5.2.2) The input parameters accepted by this call include
sinfo_stream, sinfo_flags, sinfo_ppid, sinfo_context,
sinfo_timetolive. The user must set the sinfo_assoc_id field to
identify the association to affect if the caller is using the
one-to-many style.
7.1.15 Set notification and ancillary events (SCTP_EVENTS)
This socket option is used to specify various notifications and
ancillary data the user wishes to receive. Please see Section 7.3)
for a full description of this option and its usage.
7.1.16 Set/clear IPv4 mapped addresses (SCTP_I_WANT_MAPPED_V4_ADDR)
This socket option is a boolean flag which turns on or off mapped V4
addresses. If this option is turned on and the socket is type
PF_INET6, then IPv4 addresses will be mapped to V6 representation.
If this option is turned off, then no mapping will be done of V4
addresses and a user will receive both PF_INET6 and PF_INET type
addresses on the socket.
By default this option is turned on and expects an integer to be
passed where non-zero turns on the option and zero turns off the
option.
7.1.17 Set the maximum fragmentation size (SCTP_MAXSEG)
This socket option specifies the maximum size to put in any outgoing
SCTP DATA chunk. If a message is larger than this size it will be
fragmented by SCTP into the specified size. Note that the underlying
SCTP implementation may fragment into smaller sized chunks when the
PMTU of the underlying association is smaller than the value set by
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the user. The option expects an integer.
The default value for this option is '0' which indicates the user is
NOT limiting fragmentation and only the PMTU will effect SCTP's
choice of DATA chunk size.
7.1.18 Set/Get the list of chunks that must be authenticated
(SCTP_AUTH_CHUNKS)
This options gets or sets a list of chunks that the user is
requesting to be received only in an authenticated way. Changes to
this list will only effect associations that have not been formed.
struct sctp_authchunks {
uint8_t sauth_chunks[];
};
sauth_chunks - This parameter contains an array of chunks
that the user is requesting to be authenticated.
7.1.19 Set/Get the current authentication shared secret
(SCTP_AUTH_SECRET)
This option will get or set the shared secret to be used with any
authentication parameters.
struct sctp_authsecrets {
sctp_assoc_t sca_assoc_id;
uint8_t sca_secret[];
};
sca_assoc_id - This parameter, if non-zero, indicates what
association that the shared secret is being set
upon. Note that if this element contains zero, then
the secret is set upon the endpoint and all future
associations will use this secret (if not changed by
subsequent calls to SCTP_AUTH_SECRET).
sca_secret - This parameter contains an array of bytes
that is to be used by the endpoint (or association)
as the shared secret.
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7.1.20 Get the list of chunks that peer requires to be authenticated
(SCTP_PEER_AUTH_CHUNKS)
This options gets a list of chunks for a specified association that
the peer requires to be authenticated. The requesting to be received
only in an authenticated way. Changes to this list will only effect
associations that have not been formed.
struct sctp_authchunks {
sctp_assoc_t gpauth_assoc_id;
uint8_t gpauth_chunks[];
};
sca_assoc_id - This parameter, indicates for which association the
user is requesting the list of peer authenticated
chunks.
gputh_chunks - This parameter contains an array of chunks
that the peer is requesting to be authenticated.
7.2 Read-Only Options
7.2.1 Association Status (SCTP_STATUS)
Applications can retrieve current status information about an
association, including association state, peer receiver window size,
number of unacked data chunks, and number of data chunks pending
receipt. This information is read-only. The following structure is
used to access this information:
struct sctp_status {
sctp_assoc_t sstat_assoc_id;
int32_t sstat_state;
uint32_t sstat_rwnd;
uint16_t sstat_unackdata;
uint16_t sstat_penddata;
uint16_t sstat_instrms;
uint16_t sstat_outstrms;
uint32_t sstat_fragmentation_point;
struct sctp_paddrinfo sstat_primary;
};
sstat_state - This contains the association's current state one
of the following values:
SCTP_CLOSED
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SCTP_BOUND
SCTP_LISTEN
SCTP_COOKIE_WAIT
SCTP_COOKIE_ECHOED
SCTP_ESTABLISHED
SCTP_SHUTDOWN_PENDING
SCTP_SHUTDOWN_SENT
SCTP_SHUTDOWN_RECEIVED
SCTP_SHUTDOWN_ACK_SENT
sstat_rwnd - This contains the association peer's current
receiver window size.
sstat_unackdata - This is the number of unacked data chunks.
sstat_penddata - This is the number of data chunks pending receipt.
sstat_primary - This is information on the current primary peer
address.
sstat_assoc_id - (one-to-many style socket) This holds the an
identifier for the association. All
notifications for a given association
have the same association identifier.
sstat_instrms - The number of streams that the peer will
be using inbound.
sstat_outstrms - The number of streams that the endpoint is
allowed to use outbound.
sstat_fragmentation_point - The size at which SCTP fragmentation
will occur.
To access these status values, the application calls getsockopt()
with the option name SCTP_STATUS. The sstat_assoc_id parameter is
ignored for one-to-one style socket.
7.2.2 Peer Address Information (SCTP_GET_PEER_ADDR_INFO)
Applications can retrieve information about a specific peer address
of an association, including its reachability state, congestion
window, and retransmission timer values. This information is
read-only. The following structure is used to access this
information:
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struct sctp_paddrinfo {
sctp_assoc_t spinfo_assoc_id;
struct sockaddr_storage spinfo_address;
int32_t spinfo_state;
uint32_t spinfo_cwnd;
uint32_t spinfo_srtt;
uint32_t spinfo_rto;
uint32_t spinfo_mtu;
};
spinfo_address - This is filled in the application, and contains
the peer address of interest.
On return from getsockopt():
spinfo_state - This contains the peer addresses's state (either
SCTP_ACTIVE or SCTP_INACTIVE and possibly the modifer
SCTP_UNCONFIRMED)
spinfo_cwnd - This contains the peer addresses's current congestion
window.
spinfo_srtt - This contains the peer addresses's current smoothed
round-trip time calculation in milliseconds.
spinfo_rto - This contains the peer addresses's current
retransmission timeout value in milliseconds.
spinfo_mtu - The current P-MTU of this address.
spinfo_assoc_id - (one-to-many style socket) This is filled in
the application, and identifies the
association for this query.
To retrieve this information, use sctp_opt_info() with the
SCTP_GET_PEER_ADDR_INFO options.
7.3 Ancillary Data and Notification Interest Options
Applications can receive per-message ancillary information and
notifications of certain SCTP events with recvmsg().
The following optional information is available to the application:
1. SCTP_SNDRCV (sctp_data_io_event): Per-message information (i.e.
stream number, TSN, SSN, etc. described in Section 5.2.2)
2. SCTP_ASSOC_CHANGE (sctp_association_event): (described in
Section 5.3.1.1)
3. SCTP_PEER_ADDR_CHANGE (sctp_address_event): (described in
Section 5.3.1.2)
4. SCTP_SEND_FAILED (sctp_send_failure_event): (described in
Section 5.3.1.4)
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5. SCTP_REMOTE_ERROR (sctp_peer_error_event): (described in
Section 5.3.1.3)
6. SCTP_SHUTDOWN_EVENT (sctp_shtudown_event): (described in
Section 5.3.1.5)
7. SCTP_PARTIAL_DELIVERY_EVENT (sctp_partial_delivery_event):
(described in Section 5.3.1.7)
8. SCTP_ADAPTION_INDICATION (sctp_adaption_layer_event): (described
in Section 5.3.1.6)
To receive any ancillary data or notifications, first the application
registers it's interest by calling the SCTP_EVENTS setsockopt() with
the following structure.
struct sctp_event_subscribe{
uint8_t sctp_data_io_event;
uint8_t sctp_association_event;
uint8_t sctp_address_event;
uint8_t sctp_send_failure_event;
uint8_t sctp_peer_error_event;
uint8_t sctp_shutdown_event;
uint8_t sctp_partial_delivery_event;
uint8_t sctp_adaption_layer_event;
};
sctp_data_io_event - Setting this flag to 1 will cause the reception
of SCTP_SNDRCV information on a per message basis. The application
will need to use the recvmsg() interface so that it can receive the
event information contained in the msg_control field. Please see
Section 5.2 for further details. Setting the flag to 0 will disable
reception of the message control information.
sctp_association_event - Setting this flag to 1 will enable the
reception of association event notifications. Setting the flag to 0
will disable association event notifications. For more information
on event notifications please see Section 5.3.
sctp_address_event - Setting this flag to 1 will enable the reception
of address event notifications. Setting the flag to 0 will disable
address event notifications. For more information on event
notifications please see Section 5.3.
sctp_send_failure_event - Setting this flag to 1 will enable the
reception of send failure event notifications. Setting the flag to 0
will disable send failure event notifications. For more information
on event notifications please see Section 5.3.
sctp_peer_error_event - Setting this flag to 1 will enable the
reception of peer error event notifications. Setting the flag to 0
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will disable peer error event notifications. For more information on
event notifications please see Section 5.3.
sctp_shutdown_event - Setting this flag to 1 will enable the
reception of shutdown event notifications. Setting the flag to 0
will disable shutdown event notifications. For more information on
event notifications please see Section 5.3.
sctp_partial_delivery_event - Setting this flag to 1 will enable the
reception of partial delivery notifications. Setting the flag to 0
will disable partial delivery event notifications. For more
information on event notifications please see Section 5.3.
sctp_adaption_layer_event - Setting this flag to 1 will enable the
reception of adaption layer notifications. Setting the flag to 0
will disable adaption layer event notifications. For more
information on event notifications please see Section 5.3.
An example where an application would like to receive data io events
and association events but no others would be as follows:
{
struct sctp_event_subscribe event;
memset(&event,0,sizeof(event));
event.sctp_data_io_event = 1;
event.sctp_association_event = 1;
setsockopt(fd, IPPROTO_SCTP, SCTP_EVENT, &event, sizeof(event));
}
Note that for one-to-many style SCTP sockets, the caller of recvmsg()
receives ancillary data and notifications for ALL associations bound
to the file descriptor. For one-to-one style SCTP sockets, the
caller receives ancillary data and notifications for only the single
association bound to the file descriptor.
By default both the one-to-one style and one-to-many style socket has
all options off.
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8. New Interfaces
Depending on the system, the following interface can be implemented
as a system call or library function.
8.1 sctp_bindx()
The syntax of sctp_bindx() is,
int sctp_bindx(int sd, struct sockaddr *addrs, int addrcnt,
int flags);
If sd is an IPv4 socket, the addresses passed must be IPv4 addresses.
If the sd is an IPv6 socket, the addresses passed can either be IPv4
or IPv6 addresses.
A single address may be specified as INADDR_ANY or IN6ADDR_ANY, see
Section 3.1.2 for this usage.
addrs is a pointer to an array of one or more socket addresses. Each
address is contained in its appropriate structure. For an IPv6
socket, an array of sockaddr_in6 would be returned. For a IPv4
socket, an array of sockaddr_in would be returned. The caller
specifies the number of addresses in the array with addrcnt. Note
that the wildcard addresses cannot be used with this function, doing
so will result in an error.
On success, sctp_bindx() returns 0. On failure, sctp_bindx() returns
-1, and sets errno to the appropriate error code.
For SCTP, the port given in each socket address must be the same, or
sctp_bindx() will fail, setting errno to EINVAL.
The flags parameter is formed from the bitwise OR of zero or more of
the following currently defined flags:
SCTP_BINDX_ADD_ADDR
SCTP_BINDX_REM_ADDR
SCTP_BINDX_ADD_ADDR directs SCTP to add the given addresses to the
association, and SCTP_BINDX_REM_ADDR directs SCTP to remove the given
addresses from the association. The two flags are mutually
exclusive; if both are given, sctp_bindx() will fail with EINVAL. A
caller may not remove all addresses from an association; sctp_bindx()
will reject such an attempt with EINVAL.
An application can use sctp_bindx(SCTP_BINDX_ADD_ADDR) to associate
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additional addresses with an endpoint after calling bind(). Or use
sctp_bindx(SCTP_BINDX_REM_ADDR) to remove some addresses a listening
socket is associated with so that no new association accepted will be
associated with those addresses. If the endpoint supports dynamic
address a SCTP_BINDX_REM_ADDR or SCTP_BINDX_ADD_ADDR may cause a
endpoint to send the appropriate message to the peer to change the
peers address lists.
Adding and removing addresses from a connected association is
optional functionality. Implementations that do not support this
functionality should return EOPNOTSUPP.
8.2 Branched-off Association
After an association is established on a one-to-many style socket,
the application may wish to branch off the association into a
separate socket/file descriptor.
This is particularly desirable when, for instance, the application
wishes to have a number of sporadic message senders/receivers remain
under the original one-to-many style socket but branch off those
associations carrying high volume data traffic into their own
separate socket descriptors.
The application uses sctp_peeloff() call to branch off an association
into a separate socket (Note the semantics are somewhat changed from
the traditional one-to-one style accept() call). Note that the new
socket is a one-to-one style socket. Thus it will be confined to
operations allowed for a one-to-one style socket.
The syntax is:
new_sd = sctp_peeloff(int sd, sctp_assoc_t assoc_id);
the new socket descriptor representing the branched-off
association.
the original one-to-many style socket descriptor returned from the
socket() system call (see Section 3.1.1).
the specified identifier of the association that is to be branched
off to a separate file descriptor (Note, in a traditional
one-to-one style accept() call, this would be an out parameter,
but for the one-to-many style call, this is an in parameter).
8.3 sctp_getpaddrs()
sctp_getpaddrs() returns all peer addresses in an association. The
syntax is,
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int sctp_getpaddrs(int sd, sctp_assoc_t id,
struct sockaddr **addrs);
On return, addrs will point to an array dynamically allocated
sockaddr structures of the appropriate type for the socket type. The
caller should use sctp_freepaddrs() to free the memory. Note that
the in/out parameter addrs must not be NULL.
If sd is an IPv4 socket, the addresses returned will be all IPv4
addresses. If sd is an IPv6 socket, the addresses returned can be a
mix of IPv4 or IPv6 addresses.
For one-to-many style sockets, id specifies the association to query.
For one-to-one style sockets, id is ignored.
On success, sctp_getpaddrs() returns the number of peer addresses in
the association. If there is no association on this socket,
sctp_getpaddrs() returns 0, and the value of *addrs is undefined. If
an error occurs, sctp_getpaddrs() returns -1, and the value of *addrs
is undefined.
8.4 sctp_freepaddrs()
sctp_freepaddrs() frees all resources allocated by
sctp_getpaddrs(). Its syntax is,
void sctp_freepaddrs(struct sockaddr *addrs);
addrs is the array of peer addresses returned by sctp_getpaddrs().
8.5 sctp_getladdrs()
sctp_getladdrs() returns all locally bound address(es) on a socket.
The syntax is,
int sctp_getladdrs(int sd, sctp_assoc_t id,
struct sockaddr **ss);
On return, addrs will point to a dynamically allocated array of
sockaddr structures of the appropriate type for the socket type. The
caller should use sctp_freeladdrs() to free the memory. Note that
the in/out parameter addrs must not be NULL.
If sd is an IPv4 socket, the addresses returned will be all IPv4
addresses. If sd is an IPv6 socket, the addresses returned can be a
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mix of IPv4 or IPv6 addresses.
For one-to-many style sockets, id specifies the association to query.
For one-to-one style sockets, id is ignored.
If the id field is set to the value '0' then the locally bound
addresses are returned without regard to any particular association.
On success, sctp_getladdrs() returns the number of local addresses
bound to the socket. If the socket is unbound, sctp_getladdrs()
returns 0, and the value of *addrs is undefined. If an error occurs,
sctp_getladdrs() returns -1, and the value of *addrs is undefined.
8.6 sctp_freeladdrs()
sctp_freeladdrs() frees all resources allocated by
sctp_getladdrs(). Its syntax is,
void sctp_freeladdrs(struct sockaddr *addrs);
addrs is the array of peer addresses returned by sctp_getladdrs().
8.7 sctp_sendmsg()
An implementation may provide a library function (or possibly system
call) to assist the user with the advanced features of SCTP.
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sctp_sendmsg(). Its syntax is,
ssize_t sctp_sendmsg(int sd,
const void *msg,
size_t len,
const struct sockaddr *to,
socklen_t tolen,
uint32_t ppid,
uint32_t flags,
uint16_t stream_no,
uint32_t timetolive,
uint32_t context)
sd - is the socket descriptor
msg - is the message to be sent.
len - is the length of the message.
to - is the destination address of the message.
tolen - is the length of the destination address.
ppid - is the same as sinfo_ppid (see section 5.2.2)
flags - is the same as sinfo_flags (see section 5.2.2)
stream_no - is the same as sinfo_stream (see section 5.2.2)
timetolive - is the same as sinfo_timetolive (see section 5.2.2)
context - is the same as sinfo_context (see section 5.2.2)
8.8 sctp_recvmsg()
An implementation may provide a library function (or possibly system
call) to assist the user with the advanced features of SCTP. Note
that in order for the sctp_sndrcvinfo structure to be filled in by
sctp_recvmsg() the caller must enable the sctp_data_io_events with
the SCTP_EVENTS option.
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sctp_recvmsg(). Its syntax is,
ssize_t sctp_recvmsg(int sd,
void *msg,
size_t len,
struct sockaddr *from,
socklen_t *fromlen
struct sctp_sndrcvinfo *sinfo
int *msg_flags)
sd - is the socket descriptor
msg - is a message buffer to be filled.
len - is the length of the message buffer.
from - is a pointer to a address to be filled with
the sender of this messages address.
fromlen - is the from length.
sinfo - A pointer to a sctp_sndrcvinfo structure
to be filled upon receipt of the message.
msg_flags - A pointer to a integer to be filled with
any message flags (e.g. MSG_NOTIFICATION).
8.9 sctp_connectx()
An implementation may provide a library function (or possibly system
call) to assist the user with associating to an endpoint that is
multi-homed. Much like sctp_bindx() this call allows a caller to
specify multiple addresses at which a peer can be reached. The way
the SCTP stack uses the list of addresses to set up the association
is implementation dependant. This function only specifies that the
stack will try to make use of all the addresses in the list when
needed.
Note that the list of addresses passed in is only used for setting up
the association. It does not necessarily equal the set of addresses
the peer uses for the resulting association. If the caller wants to
find out the set of peer addresses, it must use sctp_getpaddrs() to
retrieve them after the association has been set up.
sctp_connectx(). Its syntax is,
int sctp_connectx(int sd,
struct sockaddr *addrs,
int addrcnt)
sd - is the socket descriptor
addrs - is an array of addresses.
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addrcnt - is the number of addresses in the array.
8.10 sctp_send()
An implementation may provide another alternative function or system
call to assist an application with the sending of data without the
use of the CMSG header structures. The function takes the following
form:
sctp_send(). Its syntax is,
int sctp_send(int sd,
const void *msg,
size_t len,
const struct sctp_sndrcvinfo *sinfo,
int flags);
sd - is the socket descriptor
msg - The message to be sent
len - The length of the message
sinfo - A pointer to a sctp_sndrcvinfo struture used
as described in 5.2.2 for a sendmsg call.
flags - is used in the same format as the sendmsg call
flags (e.g. MSG_DONTROUTE).
This function call may also be used to terminate an association using
an association identification by setting the sinfo.sinfo_flags to
MSG_EOF and the sinfo.sinf_associd to the association that needs to
be terminated. In such a case the len of the message would be zero.
8.11 sctp_sendx()
An implementation may provide another alternative function or system
call to assist an application with the sending of data without the
use of the CMSG header structures that also gives a list of
addresses. The list of addresses is provided for implicit
association setup. In such a case the list of addresses serves the
same purpose as the addresses given in sctp_connectx (see
Section 8.9).
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sctp_sendx(). Its syntax is,
int sctp_sendx(int sd,
const void *msg,
size_t len,
struct sockaddr *addrs,
int addrcnt,
struct sctp_sndrcvinfo *sinfo,
int flags);
sd - is the socket descriptor
msg - The message to be sent
len - The length of the message
addrs - is an array of addresses.
addrcnt - is the number of addresses in the array.
sinfo - A pointer to a sctp_sndrcvinfo struture used
as described in 5.2.2 for a sendmsg call.
flags - is used in the same format as the sendmsg call
flags (e.g. MSG_DONTROUTE).
Note that on return from this call the sinfo structure will have
changed in that the sinfo_assoc_id will be filled in with the new
association id.
This function call may also be used to terminate an association using
an association identification by setting the sinfo.sinfo_flags to
MSG_EOF and the sinfo.sinf_associd to the association that needs to
be terminated. In such a case the len of the message would be zero.
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9. Preprocessor Constants
For application portability it is desireable to define pre-processor
constants for determination if sctp is present and supports various
features. The following pre-processor constants should be defined in
a include file, sctp.h.
HAVE_SCTP - If this constant is defined to 1, then an implementation
of SCTP is available.
HAVE_KERNEL_SCTP - If this constant is defined to 1, then a kernel
SCTP implementation is available through the sockets interface.
HAVE_SCTP_PRSCTP - If this constant is defined to 1, then the SCTP
implementation supports the partial reliablility extension to
SCTP.
HAVE_SCTP_ADDIP - If this constant is defined to 1, then the SCTP
implementation supports the dynamic address extension to SCTP.
HAVE_SCTP_CANSET_PRIMARY - If this constant is defined to 1, then the
SCTP implementation supports the ability to request setting of the
remote primary address.
HAVE_SCTP_SAT_NETWORK_CAPABILITY - If this constant is defined to 1,
then the SCTP implementation supports the satellite network
extension to SCTP.
HAVE_SCTP_MULTIBUF - If this constant is defined to 1, then the SCTP
implementation dedicates separate buffer space to each association
on a one-to-many socket. If this constant is defined to 0, then
the implementation provides a single block of shared buffer space
for a one-to-many socket.
HAVE_SCTP_NOCONNECT - If this constant is defined to 1, then the SCTP
implementation supports initiating an association on a one-to-one
style socket without the use of connect(), as outlined in
Section 4.1.5.
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10. Security Considerations
Many TCP and UDP implementations reserve port numbers below 1024 for
privileged users. If the target platform supports privileged users,
the SCTP implementation SHOULD restrict the ability to call bind() or
sctp_bindx() on these port numbers to privileged users.
Similarly unpriviledged users should not be able to set protocol
parameters which could result in the congestion control algorithm
being more aggressive than permitted on the public Internet. These
parameters are:
struct sctp_rtoinfo
If an unprivileged user inherits a one-to-many style socket with open
associations on a privileged port, it MAY be permitted to accept new
associations, but it SHOULD NOT be permitted to open new
associations. This could be relevant for the r* family of protocols.
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11. Acknowledgments
Special acknowledgment is givne to Ken Fujita who helped extensively
in the early formation of this document.
The authors also wish to thank Kavitha Baratakke, Mike Bartlett, Jon
Berger, Scott Kimble, Renee Revis, and many others on the TSVWG
mailing list for contributing valuable comments.
A special thanks to Phillip Conrad, for his suggested text, quick and
constructive insights, and most of all his persistent fighting to
keep the interface to SCTP usable for the application programmer.
12. References
[1] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[2] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
1980.
[3] Braden, B., "T/TCP -- TCP Extensions for Transactions Functional
Specification", RFC 1644, July 1994.
[4] Bradner, S., "The Internet Standards Process -- Revision 3",
BCP 9, RFC 2026, October 1996.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[6] Stevens, W. and M. Thomas, "Advanced Sockets API for IPv6",
RFC 2292, February 1998.
[7] Gilligan, R., Thomson, S., Bound, J. and W. Stevens, "Basic
Socket Interface Extensions for IPv6", RFC 2553, March 1999.
[8] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
H., Taylor, T., Rytina, I., Kalla, M., Zhang, L. and V. Paxson,
"Stream Control Transmission Protocol", RFC 2960, October 2000.
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Authors' Addresses
Randall R. Stewart
Cisco Systems, Inc.
4875 Forest Drive
Suite 200
Columbia, SC 29206
USA
Phone:
Email: rrs@cisco.com
Qiaobing Xie
Motorola, Inc.
1501 W. Shure Drive, #2309
Arlington Heights, IL 60004
USA
Phone:
Email: qxie1@email.mot.com
La Monte H.P. Yarroll
TimeSys Corp
925 Liberty Ave.
Pittsburgh, PA 15222
USA
Phone:
Email: piggy@acm.org
Jonathan Wood
DoCoMo USA Labs
181 Metro Drive, Suite 300
San Jose, CA 95110
USA
Phone:
Email: jonwood@speakeasy.net
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Kacheong Poon
Sun Microsystems, Inc.
4150 Network Circle
Santa Clara, CA 95054
USA
Phone:
Email: kacheong.poon@sun.com
Michael Tuexen
Univ. of Applied Sciences Muenster
Stegerwaldstr. 39
48565 Steinfurt
Germany
Email: tuexen@fh-muenster.de
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Appendix A. one-to-one style Code Example
The following code is a simple implementation of an echo server over
SCTP. The example shows how to use some features of one-to-one style
IPv4 SCTP sockets, including:
o Opening, binding, and listening for new associations on a socket;
o Enabling ancillary data
o Enabling notifications
o Using ancillary data with sendmsg() and recvmsg()
o Using MSG_EOR to determine if an entire message has been read
o Handling notifications
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdlib.h>
#include <unistd.h>
#include <netinet/sctp.h>
#include <sys/uio.h>
#define BUFLEN 100
static void
handle_event(void *buf)
{
struct sctp_assoc_change *sac;
struct sctp_send_failed *ssf;
struct sctp_paddr_change *spc;
struct sctp_remote_error *sre;
union sctp_notification *snp;
char addrbuf[INET6_ADDRSTRLEN];
const char *ap;
struct sockaddr_in *sin;
struct sockaddr_in6 *sin6;
snp = buf;
switch (snp->sn_header.sn_type) {
case SCTP_ASSOC_CHANGE:
sac = &snp->sn_assoc_change;
printf("^^^ assoc_change: state=%hu, error=%hu, instr=%hu "
"outstr=%hu\n", sac->sac_state, sac->sac_error,
sac->sac_inbound_streams, sac->sac_outbound_streams);
break;
case SCTP_SEND_FAILED:
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ssf = &snp->sn_send_failed;
printf("^^^ sendfailed: len=%hu err=%d\n", ssf->ssf_length,
ssf->ssf_error);
break;
case SCTP_PEER_ADDR_CHANGE:
spc = &snp->sn_paddr_change;
if (spc->spc_aaddr.ss_family == AF_INET) {
sin = (struct sockaddr_in *)&spc->spc_aaddr;
ap = inet_ntop(AF_INET, &sin->sin_addr,
addrbuf, INET6_ADDRSTRLEN);
} else {
sin6 = (struct sockaddr_in6 *)&spc->spc_aaddr;
ap = inet_ntop(AF_INET6, &sin6->sin6_addr,
addrbuf, INET6_ADDRSTRLEN);
}
printf("^^^ intf_change: %s state=%d, error=%d\n", ap,
spc->spc_state, spc->spc_error);
break;
case SCTP_REMOTE_ERROR:
sre = &snp->sn_remote_error;
printf("^^^ remote_error: err=%hu len=%hu\n",
ntohs(sre->sre_error), ntohs(sre->sre_length));
break;
case SCTP_SHUTDOWN_EVENT:
printf("^^^ shutdown event\n");
break;
default:
printf("unknown type: %hu\n", snp->sn_header.sn_type);
break;
}
}
static void *
mysctp_recvmsg(int fd, struct msghdr *msg, void *buf, size_t *buflen,
ssize_t *nrp, size_t cmsglen)
{
ssize_t nr = 0, nnr = 0;
struct iovec iov[1];
*nrp = 0;
iov->iov_base = buf;
iov->iov_len = *buflen;
msg->msg_iov = iov;
msg->msg_iovlen = 1;
for (;;) {
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#ifndef MSG_XPG4_2
#define MSG_XPG4_2 0
#endif
msg->msg_flags = MSG_XPG4_2;
msg->msg_controllen = cmsglen;
nnr = recvmsg(fd, msg, 0);
if (nnr <= 0) {
/* EOF or error */
*nrp = nr;
return (NULL);
}
nr += nnr;
if ((msg->msg_flags & MSG_EOR) != 0) {
*nrp = nr;
return (buf);
}
/* Realloc the buffer? */
if (*buflen == (size_t)nr) {
buf = realloc(buf, *buflen * 2);
if (buf == 0) {
fprintf(stderr, "out of memory\n");
exit(1);
}
*buflen *= 2;
}
/* Set the next read offset */
iov->iov_base = (char *)buf + nr;
iov->iov_len = *buflen - nr;
}
}
static void
echo(int fd, int socketModeone_to_many)
{
ssize_t nr;
struct sctp_sndrcvinfo *sri;
struct msghdr msg[1];
struct cmsghdr *cmsg;
char cbuf[sizeof (*cmsg) + sizeof (*sri)];
char *buf;
size_t buflen;
struct iovec iov[1];
size_t cmsglen = sizeof (*cmsg) + sizeof (*sri);
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/* Allocate the initial data buffer */
buflen = BUFLEN;
if (!(buf = malloc(BUFLEN))) {
fprintf(stderr, "out of memory\n");
exit(1);
}
/* Set up the msghdr structure for receiving */
memset(msg, 0, sizeof (*msg));
msg->msg_control = cbuf;
msg->msg_controllen = cmsglen;
msg->msg_flags = 0;
cmsg = (struct cmsghdr *)cbuf;
sri = (struct sctp_sndrcvinfo *)(cmsg + 1);
/* Wait for something to echo */
while (buf = mysctp_recvmsg(fd, msg, buf, &buflen, &nr, cmsglen)) {
/* Intercept notifications here */
if (msg->msg_flags & MSG_NOTIFICATION) {
handle_event(buf);
continue;
}
iov->iov_base = buf;
iov->iov_len = nr;
msg->msg_iov = iov;
msg->msg_iovlen = 1;
printf("got %u bytes on stream %hu:\n", nr,
sri->sinfo_stream);
write(0, buf, nr);
/* Echo it back */
msg->msg_flags = MSG_XPG4_2;
if (sendmsg(fd, msg, 0) < 0) {
perror("sendmsg");
exit(1);
}
}
if (nr < 0) {
perror("recvmsg");
}
if(socketModeone_to_many == 0)
close(fd);
}
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int main()
{
struct sctp_event_subscribe event;
int lfd, cfd;
int onoff = 1;
struct sockaddr_in sin[1];
if ((lfd = socket(AF_INET, SOCK_STREAM, IPPROTO_SCTP)) == -1) {
perror("socket");
exit(1);
}
sin->sin_family = AF_INET;
sin->sin_port = htons(7);
sin->sin_addr.s_addr = INADDR_ANY;
if (bind(lfd, (struct sockaddr *)sin, sizeof (*sin)) == -1) {
perror("bind");
exit(1);
}
if (listen(lfd, 1) == -1) {
perror("listen");
exit(1);
}
/* Wait for new associations */
for (;;) {
if ((cfd = accept(lfd, NULL, 0)) == -1) {
perror("accept");
exit(1);
}
/* Enable all events */
event.sctp_data_io_event = 1;
event.sctp_association_event = 1;
event.sctp_address_event = 1;
event.sctp_send_failure_event = 1;
event.sctp_peer_error_event = 1;
event.sctp_shutdown_event = 1;
event.sctp_partial_delivery_event = 1;
event.sctp_adaption_layer_event = 1;
if (setsockopt(cfd, IPPROTO_SCTP,
SCTP_EVENTS, &event,
sizeof(event)) != 0) {
perror("setevent failed");
exit(1);
}
/* Echo back any and all data */
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echo(cfd,0);
}
}
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Appendix B. one-to-many style Code Example
The following code is a simple implementation of an echo server over
SCTP. The example shows how to use some features of one-to-many
style IPv4 SCTP sockets, including:
o Opening and binding of a socket;
o Enabling ancillary data
o Enabling notifications
o Using ancillary data with sendmsg() and recvmsg()
o Using MSG_EOR to determine if an entire message has been read
o Handling notifications
Note most functions defined in Appendix A are reused in thi
s example.
int main()
{
int fd;
int idleTime = 2;
struct sockaddr_in sin[1];
struct sctp_event_subscribe event;
if ((fd = socket(AF_INET, SOCK_SEQPACKET, IPPROTO_SCTP)) == -1) {
perror("socket");
exit(1);
}
sin->sin_family = AF_INET;
sin->sin_port = htons(7);
sin->sin_addr.s_addr = INADDR_ANY;
if (bind(fd, (struct sockaddr *)sin, sizeof (*sin)) == -1) {
perror("bind");
exit(1);
}
/* Enable all notifications and events */
event.sctp_data_io_event = 1;
event.sctp_association_event = 1;
event.sctp_address_event = 1;
event.sctp_send_failure_event = 1;
event.sctp_peer_error_event = 1;
event.sctp_shutdown_event = 1;
event.sctp_partial_delivery_event = 1;
event.sctp_adaption_layer_event = 1;
if (setsockopt(fd, IPPROTO_SCTP,
SCTP_EVENTS, &event,
sizeof(event)) != 0) {
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perror("setevent failed");
exit(1);
}
/* Set associations to auto-close in 2 seconds of
* inactivity
*/
if (setsockopt(fd, IPPROTO_SCTP, SCTP_AUTOCLOSE,
&idleTime, 4) < 0) {
perror("setsockopt SCTP_AUTOCLOSE");
exit(1);
}
/* Allow new associations to be accepted */
if (listen(fd, 1) < 0) {
perror("listen");
exit(1);
}
/* Wait for new associations */
while(1){
/* Echo back any and all data */
echo(fd,1);
}
}
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Acknowledgment
Funding for the RFC Editor function is currently provided by the
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