Technical Manual – Signaling Protocols
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Table of Contents
Table of Contents
Chapter 2 SCTP ............................................................................................................................. 3-1
2.1 Overview of SCTP ............................................................................................................. 3-1
2.1.1 Related Terms......................................................................................................... 3-1
2.1.2 Functions of SCTP .................................................................................................. 3-2
2.2 SCTP Messages ................................................................................................................ 3-4
2.2.1 Message Structure .................................................................................................. 3-4
2.2.2 Message Format ..................................................................................................... 3-6
2.3 Basic SCTP Procedures .................................................................................................... 3-8
2.3.1 Establishment of Associations and Transmission of Chunks ................................. 3-8
2.3.2 Shutdown of Associations ..................................................................................... 3-11
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Chapter 2 SCTP
Chapter 2 SCTP
This chapter describes stream control transmission protocol (SCTP) based on IP,
including:
z
Overview of SCTP
z
SCTP messages
z
Basic SCTP procedures
2.1 Overview of SCTP
The SCTP provides a reliable datagram transmission protocol based on IP. It is used in
the IP network to transmit SCN narrowband signaling messages. Some defects of TCP
are corrected in the SCTP, which makes the SCTP more reliable. The SCTP has the
following features in the design:
z
Appropriate congestion control
z
Flooding and spoof attack prevention
z
Better real-time performance and multi-home attribute
The SCTP is a protocol for the transmission layer. Its upper layer is the SCTP user
application layer, and its lower layer is the packet network layer.
2.1.1 Related Terms
This section introduces some terms involved in SCTP.
I. Transport Address
A transport address of the SCTP is defined as combination of an IP address and an
SCTP port number.
The SCTP port number is used for the SCTP to identify users with the same address. It
is a concept identical to the TCP port number. For example, the IP address
"10.105.28.92" and SCTP port number "1024" forms a transport address, whereas
"10.105.28.93" and "1024" forms another transport address. Similarly, "10.105.28.92"
and "1023" forms a different transport addresses.
II. Host and Endpoint
A host is a computer configured with one or more IP addresses. It is a typical physical
entity.
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An endpoint is a basic SCTP concept. It is a logical sender and receiver of SCTP
packets. It is a typical logical entity.
As prescribed in the SCTP, only one association is established between two endpoints,
but a host may have multiple endpoints.
III. Association and Stream
An association is the logical relationship or a channel established between two SCTP
endpoints for data transmission through the four-way handshake mechanism
prescribed in the SCTP.
The SCTP is characterized by streams. In an SCTP association, a stream is a
uni-directional logical channel established between one endpoint and another
associated endpoint. The data to be delivered in sequence must be transmitted in a
stream.
An association may contain multiple streams.
IV. TSN and SSN
A transmission sequence number (TSN) is a 32-bit sequence number used internally
by the SCTP. One TSN is configured to each chunk to allow the receiving SCTP
endpoint to acknowledge the receipt. A TSN is maintained based on association.
A stream sequence number (SSN) is a 16-bit sequence number in each stream of an
association configured for each chunk sent in the stream by the local end to ensure that
the chunk can be transmitted in sequence. A SSN is maintained based on streams.
TSNs are SSNs are assigned separately.
V. CWND and RWND
The SCTP is a sliding window protocol. A congestion window (CWND) is maintained
based on each destination address. It is adjusted according to the network condition.
When the length of an unacknowledged message sent from the destination address
exceeds the value of a CWND, the endpoint stops sending data to this address.
A receiver window (RWND) is used to describe the size of the receiving buffer of an
association in the peer end. During establishment of an association, both ends
exchange their initial RWNDs. RWNDs vary with data acknowledged or sent. The size
of an RWND determines how much data it can send. When an RWND is set to 0, the
SCTP can send a packet so that it knows changes of the buffer in the peer end.
2.1.2 Functions of SCTP
The SCTP has the following functions:
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Establishment and shutdown of associations
z
Sequential message transmission in streams
z
User data segmentation
z
Acknowledgement and congestion avoidance
z
Chunk bundling
z
Packet verification
z
Path management
I. Establishment and Shutdown of Associations
SCTP is an association-oriented transmission protocol. In general, data can be
transmitted only when two endpoints establish their association. Therefore,
establishment and shutdown of associations are preconditions for providing other
services.
II. Sequential Message Transmission in Streams
In the SCTP, streams are used to indicate sequences for the user messages to be sent
to a high layer. In a same stream, messages must be transferred in sequence.
SCTP users can specify the number of streams supported by an association when they
establish an association. User messages are associated through the SSN.
III. User Data Segmentation
When sending messages to a low layer, the SCTP segments these messages to make
them meet requirements of the maximum transmission unit (MTU). The receiving end
needs to combine the segmented messages into complete messages and then sends
them to SCTP users.
IV. Acknowledgement and Congestion Avoidance
The SCTP segments all user messages. If a message is not segmented, it assigns a
TSN for the message. The receiving end is responsible for acknowledging the TSN.
The SCCP sends a message again if it does not receive the acknowledge message
within the specified time.
V. Chunk Bundling
If a large SCTP message header is contained in a short user message, the message
will be transmitted inefficiently. In this case, you can bundle more than one message to
an SCTP packet to improve the utilization ratio of bandwidth.
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VI. Packet Verification
Each SCTP packet header contains a mandatory Verification Tag and a 32-bit
verification field. The value of the Verification Tag is selected by an endpoint of an
association. If the receiver does not receive the expected Verification Tag, it discards
the packet. The verification field is set by the sender of an SCTP packet. The receiver
checks the checksum to make sure whether the user data is damaged.
VII. Path Management
An SCTP user can use a group of transport addresses as the destination of SCTP
packets.
The path management function makes the following operations possible:
z
Select a destination transport address for each SCTP packet to be sent based on
instructions of SCTP users and accessibility of the current destination set.
z
Monitor whether a destination address is accessible through heartbeat messages
and notify SCTP users about changes of the accessibility of destination
addresses.
z
Report the qualified local transport address set to the remote end and send
returned transport address reports to local SCTP users.
2.2 SCTP Messages
This section describes:
z
Message structure
z
Message format
2.2.1 Message Structure
Figure 2-1 shows the structure of an SCTP packet.
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Chapter 2 SCTP
16 bits
16 bits
Destination Port
Number
Source Port
Number
Common
Header
Verification Tag
Checksum
Chunk Type
Chunk Flags
Chunk Length
Chunk #1
Chunk Value
Chunk Type
Chunk Flags
Chunk Length
Chunk
#n
Chunk Value
Figure 2-1 SCTP packet structure
An SCTP packet contains the following:
z
A common header
z
Several chunks.
Each chunk contains control information or user data.
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Note:
Multiple chunks can be bundled in an SCTP packet to meet requirements of the MTU
except the following types of chunks:
z
INIT
z
INIT ACK
z
SHUTDOWN COMPLETE
These chunks may not be bundled with other chunks in a packet. If a user message
cannot be put in an SCTP packet, the message can be divided into several data
chunks.
2.2.2 Message Format
I. Format of Common Header
An SCTP common header contains the following fields:
z
Source Port Number
A Source Port Number identifies the SCTP port number of the sending endpoint.
The receiving end can use Source Port Number, Source IP Address, Destination
Port Number, and Destination IP Address to identify the association of SCTP
packets.
z
Destination Port Number
A Destination Port Number is the SCTP port number of the destination endpoint.
Using a destination port number, the receiving host can multiplex an SCTP packet
to a correct endpoint or an application.
z
Verification Tag
A Verification Tag is a random identifier generated by the local endpoint for an
association. During establishment of an association, the sender and the receiver
exchange Verification Tags. The sender must carry this tag in the common header
for verification.
Functions of a Verification Tag are detailed in subsequent
procedure.
z
Checksum
The SCTP uses the ADLER-32 algorithm for user data to figure out a 32-bit
checksum. This checksum is carried in packets. When a packet is sent, the
receiver carries out the same calculation to obtain a checksum. Users can verify
whether user data is damaged by judging whether this checksum is the same as
the previous one.
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II. Chunk Format
A chunk contains the following fields:
z
Chunk type
A chunk type is the type of a message in terms of chunk value. Table 2-1 lists
major SCTP chunk types.
Table 2-1 SCTP chunk types
Chunk
z
Function
DATA
Payload chunk
INIT
Used to start an association between two SCTP
endpoints.
INIT ACK
Used to verify startup of associations.
SACK
Used to verify the DATA chunks received at the peer end
through TSNs, and inform the peer end of the interval for
receiving a DATA chunk.
HEARTBEAT
Used to check that a specified destination address is
accessible in the current association when an endpoint
sends this type of chunk to the peer end.
HEARTBEAT ACK
Used to respond to a HEARTBEAT chunk sent by an
SCTP endpoint.
ABORT
Used to abort the association between two SCTP
endpoints.
SHUTDOWN
Used to shut down an association normally.
SHUTDOWN ACK
Used to acknowledge the SHUTDOWN chunk received.
ERROR
Used to inform the peer end of some connection errors.
COOKIE ECHO
Used to start an association.
COOKIE ACK
Used to acknowledge the COOKIE ECHO chunk
received.
SHUTDOWN
COMPLETE
Used to acknowledge the SHUTDOWN ACK chunk
received after an association is shut down.
Chunk flags
Chunk flags are determined by chunk types. Unless set to other values, chunk
flags are set to 0 during delivery and the receiving endpoint ignores the flags.
z
Chunk length
Chunk length is the number of binary bytes used to indicate the length of chunk
type, chunk flag, and chunk value.
z
Chunk value
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A chunk value is the information to be sent in a chunk. Its contents are determined
by chunk types. The length of a chunk value is not fixed.
Note:
The total length of a chunk must be integral times of four bytes. If not, the sender fills
zeros in the chunk. These zeros are not accounted in the length of the chunk. The
sender must fill at most three bytes, whereas the receiver ignores the filled bytes.
2.3 Basic SCTP Procedures
The basic SCTP procedures include:
z
Establishment of associations and transmission of trunks
z
Shutdown of associations
2.3.1 Establishment of Associations and Transmission of Chunks
SCTP endpoint A starts to establish an association and sends a chunk to endpoint B. B
then sends two chunks to A. Suppose these messages are not bundled or segmented.
Figure 2-2 shows the SCTP procedure. This procedure includes the following two parts:
z
Establishment of associations
z
Transmission of chunks
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Endpoint A
Endpoint B
(1) INIT
(2) INIT ACK
(3) COOKIE ECHO
(4) COOKIE ACK
(5) DATA
(6) SACK
(7) DATA
(8) DATA
(9) SACK
Figure 2-2 Establishment of an association
I. Establishment of Associations
Establishment of an association is a four-way handshake processing, including
interactions of the following chunks:
z
INIT
z
INIT ACK
z
COOKIE ECHO
z
COOKIE ACK
The procedure is as follows:
1)
A sends an INIT chunk to B.
In the INIT chunk, A must provides its Verification Tag (Tag_A) in the Initiate Tag
field. Tag_A shall be a random number in the range of 1 to 429496729.
After sending the INIT, A starts the INIT timer and enters the COOKIE-WAIT state.
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Note:
The INIT timer sets the duration for waiting for the peer endpoint to return the
ININT ACK chunk. If the INTI ACK chunk is not received after the timer expires,
the local endpoint resends the INIT chunk until the maximum resending times are
reached.
2)
B shall respond immediately with an INIT ACK chunk after receiving the INIT
chunk.
The destination IP address of the INIT ACK chunk must be set to the source IP
address of the INIT to which B is responding.
In the INIT ACK chunk, besides filling in other parameters, B must set the
Verification Tag field to Tag_A, and also provide its own Verification Tag (Tag_B) in
the Initiate Tag field. Moreover, B must generate and send along with the INIT ACK
a State Cookie.
3)
Upon reception of the INIT ACK from B, A shall stop the INIT timer and leave the
COOKIE-WAIT state. A shall then send the State Cookie received in the INIT ACK
chunk in a COOKIE ECHO chunk, starts the COOKIE timer, and enters the
COOKIE-ECHOED state.
4)
Upon reception of the COOKIE ECHO chunk, B will reply with a COOKIE ACK
chunk after moving to the ESTABLISHED state. In addition, B sends the
COMMUNICATION UP primitive to SCTP.
5)
Upon reception of the COOKIE ACK chunk, A will move from the
COOKIE-ECHOED state to the ESTABLISHED state, stopping the COOKIE timer.
A notifies SCTP users about successful establishment of the association with a
COMMUNICATION UP primitive.
II. Transmission of Trunks
The procedure of trunk transmission is as follows:
1)
Endpoint A sends a DATA chunk to endpoint B and starts the T3-RTS timer.
The DATA chunk must contain the following parameters:
2)
z
TSN: Initial TSN of the DATA chunk.
z
Stream Identifier: Identifier of the user data stream. Suppose the identifier is 0.
z
SSN: This field is in the range of 0 to 65535.
z
User Data: Contain payload user data.
After receiving the DATA chunk, B returns a SACK chunk.
The SACK chunk must contain the following parameters:
z
Cumulative TSN Ack: Initial TSN of A.
z
Gap Ack Block. This value is 0.
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3)
After receiving the SACK, A stops the T3-RTX timer.
4)
B sends the first DATA chunk to A.
The DATA chunk must contain the following parameters:
5)
z
TSN: Initial TSN of the DATA chunk sent by B.
z
Stream Identifier: Identifier of the user data stream. Suppose the identifier is 0.
z
SSN: Suppose the SSN is 0.
z
User Data: Contain payload user data.
B sends the second DATA chunk to A.
The DATA chunk must contain the following parameters:
6)
z
TSN: One plus initial TSN of the DATA chunk sent by B.
z
Stream Identifier: Identifier of the user data stream. Suppose the identifier is 0.
z
SSN. The SSN is 1.
z
User Data: Contain payload user data.
After receiving the DATA chunk, A returns a SACK chunk.
The SACK chunk must contain the following parameters:
z
Cumulative TSN Ack: Initial TSN of B.
z
Gap Ack Block. This value is 0.
2.3.2 Shutdown of Associations
When an endpoint exits, its association must be shut down. The following two
procedures can be used to shut down an association:
z
Abortion process
z
Shutdown process
I. Abortion of Associations
An association may be aborted when there are unacknowledged messages. In this
case, both ends of the association discard data and do not send it to the peer end. In
this way, the security of data is not considered. The procedure of aborting an
association is as follows:
1)
The initiating end sends an ABORT chunk to the peer end.
The Verification Tag of the peer endpoint must be filled in the SCTP packet sent.
No DATA chunk is bundled in the ABORT chunk.
2)
After receiving the ABORT chunk, the receiving endpoint checks the Verification
Tag.
If the Verification Tag is the same as that of the local endpoint, the receiving
endpoint removes the association from records and reports it to SCTP users.
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II. Shutdown of Associations
When an endpoint executes the shutdown program, both ends of the association stop
receiving new data from their SCTP users. They send data in packets to SCTP users
when sending or receiving the SHUTDOWN chunk.
Shutdown of an association ensures that data not sent or acknowledged at both ends
can be sent or acknowledged before the association is aborted.
Figure 2-3 shows the shutdown procedure of an association.
Endpoint A
Endpoint B
(1) SHUTDOWN
(2) SHUTDOWN ACK
(3) SHUTDOWN COMPLETE
Figure 2-3 Association shutdown procedure
Upon receiving a SHUTDOWN primitive from SCTP users, endpoint A enters the
SHUTDOWN-PENDING state. It keeps this state until all unacknowledged data is
acknowledged by the peer end. In this case, A does not receive data from a high layer.
The procedure of shutting down an association is as follows:
1)
Endpoint A sends a SHUTDOWN chunk to endpoint B.
Once all unacknowledged data is acknowledged, endpoint A sends the
SHUTDOWN chunk to the peer end. In addition, endpoint A starts the
T2-shutdown timer and enters the SHUTDOWN-SENT state. If the timer expires,
endpoint A must resend the SHUTDOWN chunk.
2)
After receiving the SHUTDOWN chunk, endpoint B performs the following
operations:
z
Enters the SHUTDOWN-RECEIVED state.
z
Stops receiving new data from its SCTP users.
z
Verify that all its outstanding DATA chunks are received by the
SHUTDOWN sender by checking the Cumulative TSN ACK field of the
chunk.
If endpoint B has no more outstanding DATA chunks, it shall send a SHUTDOWN
ACK chunk and start a T2-shutdown timer of its own, entering the
SHUTDOWN-ACK-SENT state.
3)
Upon receiving the SHUTDOWN ACK, endpoint A shall stop the T2-shutdown
timer, send a SHUTDOWN COMPLETE chunk to its peer, and remove all records
of the association.
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After receiving the SHUTDOWN COMPLETE chunk, endpoint B verifies that it is in
the SHUTDOWN-ACK-SENT state.
If it is not in the SHUTDOWN-ACK-SENT state, B discards the chunk. If it is in the
state, B stops the T2-shutdown timer and removes all records of the association.
Thus, the association enters the CLOSED state.
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