SCTP 协议
Abstract |
SCTP protocol |
Authors |
Walter Fan |
Status |
WIP as draft |
Updated |
2024-08-21 |
SCTP Overview
Data Channel 背后使用的协议是 SCTP
数据通信通过 TCP/TLS 就足够了， 为什么还要 SCTP, 可能是因为 TCP 是面向流的，始终有序和可靠的传输，而我们还想要一种面向消息的，并且可以控制优先级和可靠性的连接， 乱序或者有点丢失也能接受。
SCTP 是基于 DTLS 之上的， 面向消息的， 支持多流，优先级及可靠性可控的连接协议。
假设我们通过一个连接传送流媒体以及控制命令，如果通过 TCP , 包丢失了就要重传，乱序了也一样。SCTP 就可以不一样，流媒体的包可以丢失，控制命令的包不能丢失
它为用户提供以下服务：
确认用户数据的无错误非重复传输，
数据分段以符合发现的路径 MTU 大小，
在多个流中按顺序传递用户消息，使用单个用户的到达顺序交付选项消息，
可选地将多个用户消息捆绑到单个 SCTP 数据包，和
通过支持多宿主实现网络级容错在关联的一端或两端。
SCTP 的设计包括适当的拥塞避免行为以及对洪水和伪装攻击的抵抗力。
SCTP packets consist of a common header and one or more chunks, each of which serves a specific purpose.
DATA chunk — carries user data
INIT chunk — initiates an association between SCTP endpoints
INIT ACK chunk — acknowledges association establishment
SACK chunk — acknowledges received DATA chunks and informs the peer endpoint of gaps in the received subsequences of DATA chunks
HEARTBEAT chunk — tests the reachability of an SCTP endpoint
HEARTBEAT ACK chunk — acknowledges reception of a HEARTBEAT chunk
ABORT chunk — forces an immediate close of an association
SHUTDOWN chunk — initiates a graceful close of an association
SHUTDOWN ACK chunk — acknowledges reception of a SHUTDOWN chunk
ERROR chunk — reports various error conditions
COOKIE ECHO chunk — used during the association establishment process
COOKIE ACK chunk — acknowledges reception of a COOKIE ECHO chunk
SHUTDOWN COMPLETE chunk — completes a graceful association close
术语
Active destination transport address: A transport address on a peer endpoint that a transmitting endpoint considers available for receiving user messages
Bundling: An optional multiplexing operation, whereby more than one user message may be carried in the same SCTP packet. Each user message occupies its own DATA chunk.
Chunk: A unit of information within an SCTP packet, consisting of a chunk header and chunk-specific content.
Congestion window (cwnd): An SCTP variable that limits the data, in number of bytes, a sender can send to a particular destination transport address before receiving an acknowledgement
Cumulative TSN Ack Point: The TSN of the last DATA chunk acknowledged via the Cumulative TSN Ack field of a SACK.
Idle destination address: An address that has not had user messages sent to it within some length of time, normally the HEARTBEAT interval or greater.
Inactive destination transport address: An address that is considered inactive due to errors and unavailable to transport user messages.
Path: The route taken by the SCTP packets sent by one SCTP endpoint to a specific destination transport address of its peer SCTP endpoint. Sending to different destination transport addresses does not necessarily guarantee getting separate paths.
Primary Path: The primary path is the destination and source address that will be put into a packet outbound to the peer endpoint by default. The definition includes the source address since an implementation MAY wish to specify both destination and source address to better control the return path taken by reply chunks and on which interface the packet is transmitted when the data sender is multi-homed.
Receiver Window (rwnd): An SCTP variable a data sender uses to store the most recently calculated receiver window of its peer, in number of bytes. This gives the sender an indication of the space available in the receiver’s inbound buffer.
SCTP association: A protocol relationship between SCTP endpoints, composed of the two SCTP endpoints and protocol state information including Verification Tags and the currently active set of Transmission Sequence Numbers (TSNs), etc. An association can be uniquely identified by the transport addresses used by the endpoints in the association. Two SCTP endpoints MUST NOT have more than one SCTP association between them at any given time.
SCTP endpoint: The logical sender/receiver of SCTP packets. On a multi-homed host, an SCTP endpoint is represented to its peers as a combination of a set of eligible destination transport addresses to which SCTP packets can be sent and a set of eligible source transport addresses from which SCTP packets can be received. All transport addresses used by an SCTP endpoint must use the same port number, but can use multiple IP addresses. A transport address used by an SCTP endpoint must not be used by another SCTP endpoint. In other words, a transport address is unique to an SCTP endpoint.
SCTP packet (or packet): The unit of data delivery across the interface between SCTP and the connectionless packet network (e.g., IP). An SCTP packet includes the common SCTP header, possible SCTP control chunks, and user data encapsulated within SCTP DATA chunks.
SCTP user application (SCTP user): The logical higher-layer application entity which uses the services of SCTP, also called the Upper-Layer Protocol (ULP).
Slow-Start Threshold (ssthresh): An SCTP variable. This is the threshold that the endpoint will use to determine whether to perform slow start or congestion avoidance on a particular destination transport address. Ssthresh is in number of bytes.
Stream: A unidirectional logical channel established from one to another associated SCTP endpoint, within which all user messages are delivered in sequence except for those submitted to the unordered delivery service. Note: The relationship between stream numbers in opposite directions is strictly a matter of how the applications use them. It is the responsibility of the SCTP user to create and manage these correlations if they are so desired.
Stream Sequence Number: A 16-bit sequence number used internally by SCTP to ensure sequenced delivery of the user messages within a given stream. One Stream Sequence Number is attached to each user message.
Tie-Tags: Two 32-bit random numbers that together make a 64-bit nonce. These tags are used within a State Cookie and TCB so that a newly restarting association can be linked to the original association within the endpoint that did not restart and yet not reveal the true Verification Tags of an existing association. and is not an old or stale packet from a previous association.
Transmission Control Block (TCB): An internal data structure created by an SCTP endpoint for each of its existing SCTP associations to other SCTP endpoints. TCB contains all the status and operational information for the endpoint to maintain and manage the corresponding association.
Transmission Sequence Number (TSN): A 32-bit sequence number used internally by SCTP. One TSN is attached to each chunk containing user data to permit the receiving SCTP endpoint to acknowledge its receipt and detect duplicate deliveries.
Transport address: A transport address is traditionally defined by a network-layer address, a transport-layer protocol, and a transport-layer port number. In the case of SCTP running over IP, a transport address is defined by the combination of an IP address and an SCTP port number (where SCTP is the transport protocol).
Unacknowledged TSN (at an SCTP endpoint): A TSN (and the associated DATA chunk) that has been received by the endpoint but for which an acknowledgement has not yet been sent. Or in the opposite case, for a packet that has been sent but no acknowledgement has been received. Stewart Standards Track [Page 9] RFC 4960 Stream Control Transmission Protocol September 2007
Unordered Message: Unordered messages are “unordered” with respect to any other message; this includes both other unordered messages as well as other ordered messages. An unordered message might be delivered prior to or later than ordered messages sent on the same stream.
User message: The unit of data delivery across the interface between SCTP and its user.
Verification Tag: A 32-bit unsigned integer that is randomly generated. The Verification Tag provides a key that allows a receiver to verify that the SCTP packet belongs to the current association
SCTP Association
SCTP 关联是指 SCTP 端点之间的连接
An SCTP association is uniquely identified by the transport addresses used by the endpoints in the association.
An SCTP association can be represented as a pair of SCTP endpoints, for example, assoc = { [IPv4Addr : PORT1], [IPv4Addr1, IPv4Addr2: PORT2]}.
Only one association can be established between any two SCTP endpoints.
SCTP Endpoint
is a sender or receiver of SCTP packets. An SCTP endpoint may have one or more IP address but it always has one and only one SCTP port number. An SCTP endpoint can be represented as a list of SCTP transport addresses with the same port, for example, endpoint = [IPv6Addr, IPv6Addr: PORT].
An SCTP endpoint may have multiple associations.
SCTP Path
is the route taken by the SCTP packets sent by one SCTP endpoint to a specific destination transport address or its peer SCTP endpoint. Sending to different destination transport addresses does not necessarily guarantee separate routes.
SCTP Primary Path
is the default destination source address, the IPv4 or IPv6 address of the association initiator. For retransmissions however, another active path may be selected, if one is available.
SCTP Stream
is a unidirectional logical channel established between two associated SCTP endpoints. SCTP distinguishes different streams of messages within one SCTP association. SCTP makes no correlation between an inbound and outbound stream.
SCTP Transport Address
is the combination of an SCTP port and an IP address. For the current release, the IP address portion of an SCTP Transport Address must be a routable, unicast IPv4 or IPv6 address.
An SCTP transport address binds to a single SCTP endpoint.
基于消息的多流协议
SCTP applications submit data for transmission in messages (groups of bytes) to the SCTP transport layer. SCTP places messages and control information into separate chunks (data chunks and control chunks), each identified by a chunk header.
The protocol can fragment a message into multiple data chunks, but each data chunk contains data from only one user message. SCTP bundles the chunks into SCTP packets. The SCTP packet, which is submitted to the Internet Protocol, consists of a packet header, SCTP control chunks (when necessary), followed by SCTP data chunks (when available).
SCTP Flow
participant Client as c
participant Server as s
autonumber
c->s: INIT
s-->c: INIT ACK
c->s: COOKIE ECHO
s-->c: COOKIE ACK
SCTP packet format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Common Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Chunk #1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Chunk #n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SCTP Common Header Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port Number | Destination Port Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Verification Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SCTP Chunk fields
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Chunk Type | Chunk Flags | Chunk Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ Chunk Value /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Chunk Type: 8 bits (unsigned integer)
ID Value Chunk Type
----- ----------
0 - Payload Data (DATA)
1 - Initiation (INIT)
2 - Initiation Acknowledgement (INIT ACK)
3 - Selective Acknowledgement (SACK)
4 - Heartbeat Request (HEARTBEAT)
5 - Heartbeat Acknowledgement (HEARTBEAT ACK)
6 - Abort (ABORT)
7 - Shutdown (SHUTDOWN)
8 - Shutdown Acknowledgement (SHUTDOWN ACK)
9 - Operation Error (ERROR)
10 - State Cookie (COOKIE ECHO)
11 - Cookie Acknowledgement (COOKIE ACK)
12 - Reserved for Explicit Congestion Notification Echo (ECNE)
13 - Reserved for Congestion Window Reduced (CWR)
14 - Shutdown Complete (SHUTDOWN COMPLETE)
15 to 62 - available
63 - reserved for IETF-defined Chunk Extensions
64 to 126 - available
127 - reserved for IETF-defined Chunk Extensions
128 to 190 - available
191 - reserved for IETF-defined Chunk Extensions
192 to 254 - available
255 - reserved for IETF-defined Chunk Extensions
Chunk Types are encoded such that the highest-order 2 bits specify the action that must be taken if the processing endpoint does not recognize the Chunk Type.
00 - Stop processing this SCTP packet and discard it, do not
process any further chunks within it.
01 - Stop processing this SCTP packet and discard it, do not
process any further chunks within it, and report the
unrecognized chunk in an 'Unrecognized Chunk Type'.
10 - Skip this chunk and continue processing.
11 - Skip this chunk and continue processing, but report in an
ERROR chunk using the 'Unrecognized Chunk Type' cause of
error.
- Optional/Variable-Length Parameter Format
Chunk values of SCTP control chunks consist of a chunk-type-specific header of required fields, followed by zero or more parameters.
The optional and variable-length parameters contained in a chunk are defined in a Type-Length-Value format as shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameter Type | Parameter Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ Parameter Value /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SCTP Chunk data definition
1. Payload Data (DATA) (0)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0 | Reserved|U|B|E| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TSN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Stream Identifier S | Stream Sequence Number n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Protocol Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ User Data (seq n of Stream S) /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2. Initiation (INIT) (1)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 1 | Chunk Flags | Chunk Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiate Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertised Receiver Window Credit (a_rwnd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Outbound Streams | Number of Inbound Streams |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initial TSN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ Optional/Variable-Length Parameters /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3. Initiation Acknowledgement (INIT ACK) (2)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 2 | Chunk Flags | Chunk Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiate Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertised Receiver Window Credit |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Outbound Streams | Number of Inbound Streams |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initial TSN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ Optional/Variable-Length Parameters /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4. Selective Acknowledgement (SACK) (3)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 3 |Chunk Flags | Chunk Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cumulative TSN Ack |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertised Receiver Window Credit (a_rwnd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Gap Ack Blocks = N | Number of Duplicate TSNs = X |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Gap Ack Block #1 Start | Gap Ack Block #1 End |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ ... \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Gap Ack Block #N Start | Gap Ack Block #N End |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Duplicate TSN 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ ... \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Duplicate TSN X |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5. Heartbeat Request (HEARTBEAT) (4)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 4 | Chunk Flags | Heartbeat Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ Heartbeat Information TLV (Variable-Length) /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6. Heartbeat Acknowledgement (HEARTBEAT ACK) (5)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 5 | Chunk Flags | Heartbeat Ack Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ Heartbeat Information TLV (Variable-Length) /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
7. Abort Association (ABORT) (6)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 6 |Reserved |T| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ zero or more Error Causes /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
8. Shutdown Association (SHUTDOWN) (7)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 7 | Chunk Flags | Length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cumulative TSN Ack |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
9. Shutdown Acknowledgement (SHUTDOWN ACK) (8)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 8 |Chunk Flags | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
10. Operation Error (ERROR) (9)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 9 | Chunk Flags | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ one or more Error Causes /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Cause Code: 16 bits (unsigned integer)
Defines the type of error conditions being reported.
Cause Code
Value Cause Code
--------- ----------------
1 Invalid Stream Identifier
2 Missing Mandatory Parameter
3 Stale Cookie Error
4 Out of Resource
5 Unresolvable Address
6 Unrecognized Chunk Type
7 Invalid Mandatory Parameter
8 Unrecognized Parameters
9 No User Data
10 Cookie Received While Shutting Down
11 Restart of an Association with New Addresses
12 User Initiated Abort
13 Protocol Violation
Restart of an Association with New Addresses (11)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=11 | Cause Length=Variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ New Address TLVs /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
User-Initiated Abort (12)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=12 | Cause Length=Variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Upper Layer Abort Reason /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
13. Shutdown Complete (SHUTDOWN COMPLETE) (14)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 14 |Reserved |T| Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SCTP Association State Diagram
----- -------- (from any state)
/ \ / rcv ABORT [ABORT]
rcv INIT | | | ---------- or ----------
--------------- | v v delete TCB snd ABORT
generate Cookie \ +---------+ delete TCB
snd INIT ACK ---| CLOSED |
+---------+
/ \ [ASSOCIATE]
/ \ ---------------
| | create TCB
| | snd INIT
| | strt init timer
rcv valid | |
COOKIE ECHO | v
(1) ---------------- | +------------+
create TCB | | COOKIE-WAIT| (2)
snd COOKIE ACK | +------------+
| |
| | rcv INIT ACK
| | -----------------
| | snd COOKIE ECHO
| | stop init timer
| | strt cookie timer
| v
| +--------------+
| | COOKIE-ECHOED| (3)
| +--------------+
| |
| | rcv COOKIE ACK
| | -----------------
| | stop cookie timer
v v
+---------------+
| ESTABLISHED |
+---------------+
|
|
/--------+--------\
[SHUTDOWN] / \
-------------------| |
check outstanding | |
DATA chunks | |
v |
+---------+ |
|SHUTDOWN-| | rcv SHUTDOWN
|PENDING | |------------------
+---------+ | check outstanding
| | DATA chunks
No more outstanding | |
---------------------| |
snd SHUTDOWN | |
strt shutdown timer | |
v v
+---------+ +-----------+
(4) |SHUTDOWN-| | SHUTDOWN- | (5,6)
|SENT | | RECEIVED |
+---------+ +-----------+
| \ |
(A) rcv SHUTDOWN ACK | \ |
----------------------| \ |
stop shutdown timer | \rcv:SHUTDOWN |
send SHUTDOWN COMPLETE| \ (B) |
delete TCB | \ |
| \ | No more outstanding
| \ |-----------------
| \ | send SHUTDOWN ACK
(B)rcv SHUTDOWN | \ | strt shutdown timer
----------------------| \ |
send SHUTDOWN ACK | \ |
start shutdown timer | \ |
move to SHUTDOWN- | \ |
ACK-SENT | | |
| v |
| +-----------+
| | SHUTDOWN- | (7)
| | ACK-SENT |
| +----------+-
| | (C)rcv SHUTDOWN COMPLETE
| |-----------------
| | stop shutdown timer
| | delete TCB
| |
| | (D)rcv SHUTDOWN ACK
| |--------------
| | stop shutdown timer
| | send SHUTDOWN COMPLETE
| | delete TCB
| |
\ +---------+ /
\-->| CLOSED |<--/
+---------+
SCTP over DTLS
+----------+
| SCTP |
+----------+
| DTLS |
+----------+
| ICE/UDP |
+----------+
Figure 1: Basic Stack Diagram
This document uses SCTP [RFC4960] with the following restrictions, which are required to reflect that the lower layer is DTLS instead of IPv4 and IPv6 and that SCTP does not deal with the IP addresses or the transport protocol used below DTLS:
o A DTLS connection MUST be established before an SCTP association can be set up.
o Multiple SCTP associations MAY be multiplexed over a single DTLS connection.
The SCTP port numbers are used for multiplexing and demultiplexing the SCTP associations carried over a single DTLS connection.
All SCTP associations are single-homed, because DTLS does not expose any address management to its upper layer.
Therefore, it is RECOMMENDED to set the SCTP parameter path.max.retrans to association.max.retrans.
The INIT and INIT-ACK chunk MUST NOT contain any IPv4 Address or IPv6 Address parameters.
The INIT chunk MUST NOT contain the Supported Address Types parameter.
The implementation MUST NOT rely on processing ICMP or ICMPv6 packets, since the SCTP layer most likely is unable to access the SCTP common header in the plain text of the packet, which triggered the sending of the ICMP or ICMPv6 packet.
This applies in particular to path MTU discovery when performed by SCTP.
- If the SCTP layer is notified about a path change by its lower
layers, SCTP SHOULD retest the path MTU and reset the congestion state to the initial state. The window-based congestion control method specified in [RFC4960] resets the congestion window and slow-start threshold to their initial values.
Why
SCTP over DTLS
DTLS/SCTP supports:
preservation of message boundaries.
a large number of unidirectional and bidirectional streams.
ordered and unordered delivery of SCTP user messages.
the partial reliability extension as defined in [RFC3758].
the dynamic address reconfiguration extension as defined in [RFC5061].
However, the following limitations still apply:
The maximum user message size is 2^14 bytes, which is the DTLS limit.
The DTLS user cannot perform the SCTP-AUTH key management because this is done by the DTLS layer.
How
SDP
m=application 9 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 0.0.0.0
a=ice-ufrag:u8aT
a=ice-pwd:nTH+98fL7o+XacAd//X7uStI
a=ice-options:trickle
a=fingerprint:sha-256 6E:FD:8F:7C:E7:6B:DF:2B:6F:D6:32:B6:A6:00:62:D5:7E:4E:11:91:91:37:95:BE:2C:00:3F:B2:67:6F:DF:3C
a=setup:actpass
a=mid:4
a=sctp-port:5000
a=max-message-size:262144
Example
A simple example based on https://github.com/P1sec/pysctp
server
import socket
import sctp
host = '192.168.1.10'
port = 12345
sock = sctp.sctpsocket_tcp(socket.AF_INET)
sock.bind((host, port))
sock.listen(1)
while True:
# wait for a connection
print ('waiting for a connection')
connection, client_address = sock.accept()
try:
# show who connected to us
print ('connection from', client_address)
print connection
# receive the data in small chunks and print it
while True:
data = connection.recv(999)
if data:
# output received data
print ("Data: %s" % data)
connection.sendall("Got " + str(len(data)) + ", I'm fine, thank you. And you?")
else:
# no more data -- quit the loop
print ("no more data.")
break
finally:
# Clean up the connection
connection.close()
client
import socket
import sctp
sk = sctp.sctpsocket_tcp(socket.AF_INET)
sk.connect(("192.168.1.10", 12345))
sk.sctp_send(msg='how are you')
sk.shutdown(0)
sk.close()
Reference
RFC4960: Stream Control Transmission Protocol
RFC6083: Datagram Transport Layer Security (DTLS) for Stream Control Transmission Protocol (SCTP)
RFC8261: Datagram Transport Layer Security (DTLS) Encapsulation of SCTP Packets
RFC8841: Session Description Protocol (SDP) Offer/Answer Procedures for Stream Control Transmission Protocol (SCTP) over Datagram Transport Layer Security (DTLS) Transport