Network Working Group R. R. Stewart INTERNET-DRAFT Q. Xie Motorola K. Morneault C. Sharp Cisco H. J. Schwarzbauer Siemens T. Taylor Nortel Networks I. Rytina Ericsson M. Kalla Telcordia L. Zhang UCLA V. Paxson ACIRI expires in six months November 24,1999 Simple Control Transmission Protocol Status of This Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as ``work in progress.'' The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Stewart, et al [Page 1] Internet Draft Simple Control Transmission Protocol November 1999 Abstract This document describes the Simple Control Transmission Protocol (SCTP). SCTP is designed to transport PSTN signalling messages over IP networks, but is capable of broader application. SCTP is an application-level datagram transfer protocol operating on top of an unreliable datagram service such as UDP. It offers the following services to its users: -- acknowledged error-free non-duplicated transfer of user data -- application-level segmentation to conform to discovered MTU size -- sequenced delivery of user datagrams within multiple streams, with an option for order-of-arrival delivery of individual datagrams -- optional multiplexing of user datagrams into SCTP datagrams, subject to MTU size restrictions -- enhanced reliability through support of multi-homing at either or both ends of the association. The design of SCTP includes appropriate congestion avoidance behaviour and resistance to flooding and masquerade attacks. Stewart, et al [Page 2] Internet Draft Simple Control Transmission Protocol November 1999 TABLE OF CONTENTS 1. Introduction.................................................. 5 1.1 Motivation.................................................. 5 1.2 Architectural View of SCTP.................................. 6 1.3 Functional View of SCTP..................................... 6 1.3.1 Association Startup and Takedown........................ 7 1.3.2 Sequenced Delivery within Streams....................... 8 1.3.3 User Data Segmentation.................................. 8 1.3.4 Acknowledgement and Congestion Avoidance................ 8 1.3.5 Chunk Multiplex......................................... 9 1.3.6 Path Management......................................... 9 1.3.7 Message Validation...................................... 9 1.4 Recapitulation of Key Terms.................................10 1.5. Abbreviations..............................................12 2. SCTP Datagram Format..........................................12 2.1 SCTP Common Header Field Descriptions.......................13 2.2 Chunk Field Descriptions....................................14 2.2.1 Optional/Variable-length Parameter Format..............16 2.2.2 Vendor-Specific Extension Parameter Format..............16 2.3 SCTP Chunk Definitions......................................18 2.3.1 Initiation (INIT).......................................18 2.3.1.1 Optional or Variable Length Parameters..............20 2.3.2 Initiation Acknowledgement (INIT ACK)...................23 2.3.2.1 Optional or Variable Length Parameters..............24 2.3.3 Selective Acknowledgement (SACK)........................25 2.3.4 Heartbeat Request (HEARTBEAT)...........................27 2.3.5 Heartbeat Acknowledgment (HEARTBEAT ACK)................28 2.3.6 Abort Association (ABORT)...............................29 2.3.7 Shutdown Association (SHUTDOWN).........................30 2.3.8 Shutdown Acknowledgment (SHUTDOWN ACK)..................30 2.3.9 Operation Error (ERROR).................................31 2.3.10 Encryption Cookie (COOKIE).............................33 2.3.11 Cookie Acknowledgment (COOKIE ACK).....................33 2.3.12 Payload Data (DATA)....................................34 2.4 Vendor-Specific Chunk Extensions............................35 3. SCTP Association State Diagram.................................37 4. Association Initialization.....................................39 4.1 Normal Establishment of an Association......................39 4.1.1 Handle Stream Parameters................................41 4.1.2 Handle Address Parameters...............................41 4.1.3 Generating Responder Cookie.............................41 4.1.4 Cookie Processing.......................................42 4.1.5 Cookie Authentication...................................42 4.1.6 An Example of Normal Association Establishment..........43 4.2 Handle Duplicate INIT, INIT ACK, COOKIE, and COOKIE ACK.....44 4.2.1 Handle Duplicate INIT in COOKIE-WAIT or COOKIE-SENT States...................................45 4.2.2 Handle Duplicate INIT in Other States...................45 4.2.3 Handle Duplicate INIT ACK...............................46 4.2.4 Handle Duplicate COOKIE.................................46 4.2.5 Handle Duplicate COOKIE-ACK.............................47 Stewart, et al [Page 3] Internet Draft Simple Control Transmission Protocol November 1999 4.2.6 Handle Stale COOKIE Error...............................47 4.3 Other Initialization Issues.................................48 4.3.1 Selection of Tag Value..................................48 4.3.2 Initiation from behind a NAT............................48 5. User Data Transfer.............................................48 5.1 Transmission of DATA Chunks.................................49 5.2 Acknowledgment of Reception of DATA Chunks..................51 5.3 Management Retransmission Timer.............................51 5.3.1 RTO Calculation.........................................52 5.3.2 Retransmission Timer Rules..............................53 5.3.3 Handle T3-rxt Expiration................................54 5.4 Multi-homed SCTP Endpoints..................................55 5.4.1 Failover from Inactive Destination Address..............56 5.5 Stream Identifier and Sequence Number.......................56 5.6 Ordered and Un-ordered Delivery.............................56 5.7 Report Gaps in Received DATA TSNs...........................57 5.8 CRC-16 Utilization..........................................58 5.9 Segmentation................................................59 5.10 Bundling and Multiplexing..................................60 6. Congestion Control ..........................................60 6.1 SCTP Differences from TCP Congestion Control................61 6.2 SCTP Slow-Start and Congestion Avoidance....................62 6.2.1 Slow-Start..............................................62 6.2.2 Congestion Avoidance....................................63 6.2.3 Congestion Control......................................63 6.2.4 Fast Retransmit on Gap Reports..........................64 6.3 Path MTU Discovery..........................................64 7. Fault Management..............................................65 7.1 Endpoint Failure Detection..................................65 7.2 Path Failure Detection......................................66 7.3 Path Heartbeat..............................................66 7.4 Verification Tag............................................67 8. Termination of Association.....................................68 8.1 Close of an Association.....................................68 8.2 Shutdown of an Association..................................68 9. Interface with Upper Layer.....................................69 9.1 ULP-to-SCTP.................................................70 9.2 SCTP-to-ULP.................................................77 10. Security Considerations.......................................80 10.1 Security Objectives........................................80 10.2 SCTP Responses To Potential Threats........................80 10.2.1 Countering Insider Attacks.............................80 10.2.2 Protecting against Data Corruption in the Network......80 10.2.3 Protecting Confidentiality.............................81 10.2.4 Protecting against Blind Denial of Service Attacks.....81 10.2.4.1 Flooding...........................................81 10.2.4.2 Masquerade.........................................82 10.2.4.3 Improper Monopolization of Services................83 10.3 Protection against Fraud and Repudiation...................83 11. IANA Consideration............................................84 11.1 IETF-defined Chunk Extension...............................84 Stewart, et al [Page 4] Internet Draft Simple Control Transmission Protocol November 1999 11.2 IETF-defined Chunk Parameter Extension.....................85 11.3 IETF-defined Additional Error Causes.......................85 12. Suggested SCTP Protocol Parameter Values......................86 13. Acknowledgments...............................................87 14. Authors' Addresses............................................87 15. References....................................................88 1. Introduction This section explains the reasoning behind the development of the Simple Control Transmission Protocol (SCTP), the services it offers, and the basic concepts needed to understand the detailed description of the protocol. 1.1 Motivation TCP [RFC 793, "Transmission Control Protocol", Jon Postel ed., September 1981] has performed immense service as the primary means of reliable data transfer in IP networks. However, an increasing number of recent applications have found TCP too limiting, and have incorporated their own reliable data transfer protocol on top of UDP [RFC 768, "User Datagram Protocol", Jon Postel, August 1980]. The limitations which users have wished to bypass relate both to the intrinsic nature of TCP and to its typical implementation. Intrinsic limitations: -- TCP provides both reliable data transfer and strict order- of-transmission delivery of data. Some applications need reliable transfer without sequence maintenance, while others would be satisfied with partial ordering of the data. In both of these cases the head-of-line blocking offered by TCP causes unnecessary delay. -- The stream-oriented nature of TCP is often an inconvenience. Applications must add their own record marking to delineate their messages, and must make explicit use of the push facility to ensure that a complete message is transferred in a reasonable time. -- The limited scope of TCP sockets complicates the task of providing highly-available data transfer capability using multi-homed hosts. Limitations due to implementation: -- TCP is generally implemented at the operating system level. Kernel limitations may constrain the maximum allowable number Stewart, et al [Page 5] Internet Draft Simple Control Transmission Protocol November 1999 of simultaneous TCP connections to a number far below that required for certain applications. -- TCP implementations do not generally allow the application to control the timers which determine how quickly a connection failure is discovered. Some applications are more critically dependent than others on timely initiation of recovery from such failures. Transport of PSTN signalling across the IP network is an application for which all of these limitations of TCP are relevant. While this application directly motivated the development of SCTP, other applications may find SCTP a good match to their requirements. 1.2 Architectural View of SCTP SCTP is viewed as a layer between the SCTP user application ("SCTP user" for short) and an unreliable end-to-end datagram service such as UDP. The basic service offered by SCTP is the reliable transfer of user datagrams between peer SCTP users. It performs this service within the context of an association between two SCTP nodes. Chapter 9 of this document sketches the API which should exist at the boundary between the SCTP and the SCTP user layers. SCTP is connection-oriented in nature, but the SCTP association is a broader concept than the TCP connection. SCTP provides the means for each SCTP endpoint (Section 1.4) to provide the other during association startup with a list of transport addresses (e.g. address/UDP port combinations) through which that endpoint can be reached and from which it will originate messages. The association spans transfers over all of the possible source/destination combinations which may be generated from the two endpoint lists. _____________ _____________ | SCTP User | | SCTP User | | Application | | Application | |-------------| |-------------| | SCTP | | SCTP | | Transport | | Transport | | Service | | Service | |-------------| |-------------| | Unreliable |One or more ---- One or more| Unreliable | | Datagram |port/address \/ port/address| Datagram | | Service |appearances /\ appearances| Service | |_____________| ---- |_____________| SCTP Node A |<-------- Network transport ------->| SCTP Node B Figure 1: An SCTP Association Stewart, et al [Page 6] Internet Draft Simple Control Transmission Protocol November 1999 1.3 Functional View of SCTP The SCTP transport service can be decomposed into a number of functions. These are depicted in Figure 2 and explained in the remainder of this section. SCTP User Application ..----------------------------------------------------- .. _____________ ____________________ | | | Sequenced delivery | | Association | | within streams | | | |____________________| | startup | ..| | ____________________________ | and | | User Data Segmentation | | | |____________________________| | takedown | ..| | ____________________________ | | | Acknowledgement | | | | and | | | | Congestion Avoidance | ..| | |____________________________| | | | | ____________________________ | | | Chunk Multiplex | | | |____________________________| | | | | ________________________________ | | | Path Management | | | |________________________________| | | | | ________________________________ | | | Message Validation | |______________ |________________________________| Figure 2: Functional View of the SCTP Transport Service 1.3.1 Association Startup and Takedown An association is initiated by a request from the SCTP user (see the description of the ASSOCIATE primitive in Chapter 9). A cookie mechanism, taken from that devised by Karn and Simpson in RFC 2522 [6], is employed during the initialization to provide protection against security attacks. The cookie mechanism uses a four-way handshaking, but the last two legs of which are allowed to carry user Stewart, et al [Page 7] Internet Draft Simple Control Transmission Protocol November 1999 data for fast setup. The startup sequence is described in chapter 4 of this document. SCTP provides for graceful takedown of an active association on request from the SCTP user. See the description of the TERMINATE primitive in chapter 9. SCTP also allows ungraceful takedown, either on request from the user (ABORT primitive) or as a result of an error condition detected within the SCTP layer. Chapter 8 describes both the graceful and the ungraceful takedown procedures. 1.3.2 Sequenced Delivery within Streams The term "stream" is used in SCTP to refer to a sequence of datagrams. This is in contrast to its usage in TCP, where it refers to a sequence of bytes. The SCTP user can specify at association startup time the number of streams to be supported by the association. This number is negotiated with the remote end (see section 4.1.1). User datagrams are associated with stream numbers (SEND, RECEIVE primitives, Chapter 9). Internally, SCTP assigns a stream sequence number to each datagram passed to it by the SCTP user. On the receiving side, SCTP ensures that datagrams are delivered to the SCTP user in sequence within a given stream. However, while one stream may be blocked waiting for the next in-sequence user datagram, delivery from other streams may proceed. SCTP provides a mechanism for bypassing the sequenced delivery service. User datagrams sent using this mechanism are delivered to the SCTP user as soon as they are received. 1.3.3 User Data Segmentation SCTP can segment user datagrams to ensure that the SCTP datagram passed to the lower layer conforms to the path MTU. Segments are reassembled into complete datagrams before being passed to the SCTP user. 1.3.4 Acknowledgement and Congestion Avoidance SCTP assigns a Transmission Sequence Number (TSN) to each user data segment or unsegmented datagram. The TSN is independent of any sequence number assigned at the stream level. The receiving end acknowledges all TSNs received, even if there are gaps in the sequence. In this way, reliable delivery is kept functionally separate from sequenced delivery. The Acknowledgement and Congestion Avoidance function is responsible for message retransmission when timely acknowledgement has not been Stewart, et al [Page 8] Internet Draft Simple Control Transmission Protocol November 1999 received. Message retransmission is conditioned by congestion avoidance procedures similar to those used for TCP. See Chapters 5 and 6 for a detailed description of the protocol procedures associated with this function. 1.3.5 Chunk Multiplex As described in Chapter 2, the SCTP datagram as delivered to the lower layer consists of a common header followed by one or more chunks. Each chunk may contain either user data or SCTP control information. The SCTP user has the option to request "bundling", or multiplexing of more than one user datagram into a single SCTP datagram. The chunk multiplex function of SCTP is responsible for assembly of the complete SCTP datagram and its disassembly at the receiving end. 1.3.6 Path Management The sending SCTP user is able to manipulate the set of transport addresses used as destinations for SCTP datagrams, through the primitives described in Chapter 9. The SCTP path management function chooses the destination transport address for each outgoing SCTP datagram based on the SCTP user's instructions and the currently perceived reachability status of the eligible destination set. The path management function monitors reachability through heartbeat messages where other message traffic is inadequate to provide this information, and advises the SCTP user when reachability of any far- end transport address changes. The path management function is also responsible for reporting the eligible set of local transport addresses to the far end during association startup, and for reporting the transport addresses returned from the far end to the SCTP user. At association start-up, a primary destination transport address is defined for each SCTP endpoint, and is used for normal sending of SCTP datagrams. On the receiving end, the path management is responsible for verifying the existence of a valid SCTP association to which the inbound SCTP datagram belongs before passing it for further processing. 1.3.7 Message Validation The common SCTP header includes a validation tag and an optional CRC field. A validation tag value is chosen by each end of the association during association startup. Messages received without the validation tag value expected by the receiver are discarded, as a protection Stewart, et al [Page 9] Internet Draft Simple Control Transmission Protocol November 1999 against blind masquerade attacks and against stale datagrams from a previous association. The CRC may optionally be set by the sender, to provide additional protection against data corruption in the network beyond that provided by lower layers (e.g. the UDP checksum). 1.4 Recapitulation of Key Terms The language used to describe SCTP has been introduced in the previous sections. This section provides a consolidated list of the key terms and their definitions. o SCTP user application (SCTP user): The logical higher-layer application entity which uses the services of SCTP, also called the Upper-layer Protocol (ULP). o User datagram (user message): the unit of data delivery across the interface between SCTP and its user. o User data: the content of user datagrams. o SCTP datagram: the unit of data delivery across the interface between SCTP and the unreliable datagram service (e.g. UDP) which it is using. An SCTP datagram includes the common SCTP header, possible SCTP control chunks, and user data encapsulated within SCTP DATA chunks. o Transport address: an address which serves as a source or destination for the unreliable datagram transport service used by SCTP. In IP networks, a transport address is defined by the combination of an IP address and a UDP port number. o SCTP endpoint: the logical sender/receiver of SCTP datagrams. 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 datagrams can be sent and a set of eligible source transport addresses from which SCTP datagrams can be received. Note, a source or destination transport address can only be included in one unique SCTP endpoint, i.e., it is NOT allowed to have the same SCTP source or destination transport address appear in more than one SCTP endpoint. o SCTP association: a protocol relationship between SCTP endpoints, comprising the two SCTP endpoints and protocol state information including verification tags and the currently active set of Transmission Sequence Numbers (TSNs), etc. Stewart, et al [Page 10] Internet Draft Simple Control Transmission Protocol November 1999 o Chunk: a unit of information within an SCTP datagram, consisting of a chunk header and chunk-specific content. o 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. o Stream: a uni-directional logical channel established from one to another associated SCTP endpoints, within which all user datagrams 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 responsiblity of the SCTP user to create these correlations if they are so desired. o Stream sequence number: a 16-bit sequence number used internally by SCTP to assure sequenced delivery of the user datagrams within a given stream. One stream sequence number is attached to each user datagram. o Bundling: an optional multiplexing operation, whereby more than one user datagram may be carried in the same SCTP datagram. Each user datagram occupies its own DATA chunk. o Outstanding TSN (at an SCTP endpoint): a TSN (and the associated DATA chunk) which have been sent by the endpoint but for which it has not yet received an acknowledgement. o Unacknowledged TSN (at an SCTP endpoint): a TSN (and the associated DATA chunk) which have been received by the endpoint but for which an acknowledgement has not yet been sent. o Receiver Window (rwnd): The most recently advertised receiver window, in number of octets. This gives an indication of the space available in the receiver's inbound buffer. o Congestion Window (cwnd): An SCTP variable that limits the data, in number of octets, a sender can send into the network before receiving an acknowledgment on a particular destination Transport address. o Slow Start Threshold (ssthresh): An SCTP variable. This is the threshold which 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 octets. o Transmission Control Block (TCB): an internal data structure created by an SCTP endpoint for each of its existing SCTP Stewart, et al [Page 11] Internet Draft Simple Control Transmission Protocol November 1999 associations to other SCTP endpoints. TCB contains all the status and operational information for the endpoint to maintain and manage the corresponding association. 1.5. Abbreviations MD5 - MD5 Message-Digest Algorithm [4] NAT - Network Address Translation RTO - Retransmission Time-out RTT - Round-trip Time RTTVAR - Round-trip Time Variation SCTP - Simple Control Transmission Protocol SRTT - Smoothed RTT TCB - Transmission Control Block TLV - Type-Length-Value Coding Format TSN - Transport Sequence Number ULP - Upper-layer Protocol 2. SCTP Datagram Format An SCTP datagram is composed of a common header and chunks. A chunk contains either control information or user data. The SCTP datagram format is 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Common Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Chunk #1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Chunk #n | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Multiple chunks can be multiplexed into one UDP SCTP datagram up to Stewart, et al [Page 12] Internet Draft Simple Control Transmission Protocol November 1999 the MTU size except for the INIT, INIT ACK, and SHUTDOWN ACK chunks. These chunks MUST not be multiplexed with any other chunk in a datagram. See Section 5.10 for more details on chunk multiplexing. If an user data message doesn't fit into one SCTP datagram it can be segmented into multiple chunks using the procedure defined in Section 5.9. All integer fields in SCTP datagrams MUST be transmitted in the network byte order, unless otherwise stated. 2.1 SCTP Common Header Field Descriptions 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Vers | Reserved |C| CRC-16 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Verification Tag | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Version: 4 bits, u_int This field represents the version number of the SCTP protocol, and MUST be set to '0011'. Verification Tag: 32 bit u_int The receiver of this datagram uses the Verification Tag to identify the association. On transmit, the value of this Verification Tag MUST be set to the value of the Initiate Tag received from the peer endpoint during the association initialization. For datagrams carrying the INIT chunk, the transmitter MUST set the Verification Tag to all 0's. If the receiver receives a datagram with an all-zeros Verification Tag field, it checks the Chunk ID immediately following the common header. If the Chunk Type is not INIT or SHUTDOWN ACK, the receiver MUST drop the datagram. For datagrams carrying the SHUTDOWN-ACK chunk, the transmitter SHOULD set the Verification Tag to the Initiate Tag received from the peer endpoint during the association initialization, if known. Otherwise the Verification Tag MUST be set to all 0's. Stewart, et al [Page 13] Internet Draft Simple Control Transmission Protocol November 1999 Reserved: Reserved bits MUST be set to 0 on transmit and should be ignored on reception. C: 1 bit (Octet 2, Bit 8) When the C-bit is set to 1, the CRC-16 field contains the CRC-16 (defined below). When the C-bit is set to 0, the CRC-16 field is not used and MUST be set to 0. CRC-16: (Octets 3 & 4) When the C Bit is set to 1, this field MUST contain a CRC-16. The CRC-16 used is defined in Section 4.2 of ITU Recommendation Q.703 [2]. Section 5.8 defines the use of CRC-16 in SCTP. IMPLEMENTATION NOTE: When the C bit is set to 0, an implementation MAY use the fixed value 0x30000000 as a sanity check on an inbound datagram. If the first long integer is not the fixed value the datagram MAY be discarded with no further processing. 2.2 Chunk Field Descriptions The figure below illustrates the field format for the chunks to be transmitted in the SCTP datagram. Each chunk is formatted with a Chunk ID field, a Chunk-specific flag field, a Length field and a value field. 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 ID |Chunk Flags | Chunk Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ \ / Value / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Chunk ID: 8 bits, u_int This field identifies the type of information contained in the chunk value field. It takes a value from 0x00 to 0xFF. The value of 0xFE is reserved for vendor-specific extensions. The value of 0xFF is Stewart, et al [Page 14] Internet Draft Simple Control Transmission Protocol November 1999 reserved for future use as an extension field. Procedures for extending this field by vendors are defined in Section 2.4. The values of Chunk ID are defined as follows: ID Value Chunk Type ----- ---------- 00000000 - Payload Data (DATA) 00000001 - Initiation (INIT) 00000010 - Initiation Acknowledgment (INIT ACK) 00000011 - Selective Acknowledgment (SACK) 00000100 - Heartbeat Request (HEARTBEAT) 00000101 - Heartbeat Acknowledgment (HEARTBEAT ACK) 00000110 - Abort (ABORT) 00000111 - Shutdown (SHUTDOWN) 00001000 - Shutdown Acknowledgment (SHUTDOWN ACK) 00001001 - Operation Error (ERROR) 00001010 - Responder Cookie (COOKIE) 00001011 - Cookie Acknowledgement (COOKIE ACK) 00001100 to 11111101 - reserved for future IETF usage 11111110 - Vendor-specific chunk extensions 11111111 - IETF-defined Chunk Extension Chunk Flags: 8 bits The usage of these bits depends on the chunk type as given by the Chunk ID. Unless otherwise specified, they are set to zero on transmit and are ignored on receipt. Chunk Length: 16 bits (u_int) This value represents the size of the chunk in octets including the Chunk ID, Flags, Length and Value fields. Therefore, if the Value field is zero-length, the Length field will be set to 0x0004. The Length field does not include any padding. Chunk Value: variable length The Chunk Value field contains the actual information to be transferred in the chunk. The usage and format of this field is dependent on the Chunk ID. The Chunk Value field MUST be aligned on 32-bit boundaries. If the length of the chunk does not align on 32-bit boundaries, it is padded at the end with all zero octets. SCTP defined chunks are described in detail in Section 2.3. The guidelines for Vendor-Specific chunk extensions are discussed in Section 2.4. And the guidelines for IETF-defined chunk extensions can be found in Section 11.1 of this document. Stewart, et al [Page 15] Internet Draft Simple Control Transmission Protocol November 1999 2.2.1 Optional/Variable-length Parameter Format 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 / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Parameter Type: 16 bit u_int The Type field is a 16 bit identifier of the type of parameter. It takes a value of 0x0000 to 0xFFFF. The value of 0xFFFE is reserved for vendor-specific extensions if the specific chunk allows such extensions. The value of 0xFFFF is reserved for IETF-defined extensions. Values other than those defined in specific SCTP chunk description are reserved for use by IETF. Parameter Length: 16 bit u_int The Length field contains the size of the parameter in octets, including the Type, Length, and Value fields. Thus, a parameter with a zero-length Value field would have a Length field of 0x0004. The Length does not include any padding octets. Parameter Value: variable-length. The Value is dependent on the value of the Type field. The value field MUST be aligned on 32-bit boundaries. If the value field is not aligned on 32-bit boundaries it is padded at the end with all zero octets. The value field must be an integer number of octets. The actual SCTP parameters are defined in the specific SCTP chunk section. The guidelines for vendor-specific parameter extensions are discussed in Section 2.2.2. And the rules for IETF-defined parameter extensions are defined in Section 11.2. 2.2.2 Vendor-Specific Extension Parameter Format This is to allow vendors to support their own extended parameters not Stewart, et al [Page 16] Internet Draft Simple Control Transmission Protocol November 1999 defined by the IETF. It MUST not affect the operation of SCTP. Endpoints not equipped to interpret the vendor-specific information sent by a remote endpoint MUST ignore it (although it may be reported). Endpoints that do not receive desired vendor-specific information SHOULD make an attempt to operate without it, although they may do so (and report they are doing so) in a degraded mode. A summary of the Vendor-Specific Extension format is shown below. The fields are transmitted from left to right. 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 = 0xFFFE | Parameter Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Vendor-Id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ \ / Value / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type: 16 bit u_int 0xFFFE for all Vendor-Specific parameters. Length: 16 bit u_int Indicate the size of the parameter in octets, including the Type, Length, Vendor-Id, and Value fields. Vendor-Id: 32 bit u_int The high-order octet is 0 and the low-order 3 octets are the SMI Network Management Private Enterprise Code of the Vendor in network byte order, as defined in the Assigned Numbers (RFC 1700). Value: variable length The Value field is one or more octets. The actual format of the information is site or application specific, and a robust implementation SHOULD support the field as undistinguished octets. The codification of the range of allowed usage of this field is outside the scope of this specification. It SHOULD be encoded as a sequence of vendor type / vendor length Stewart, et al [Page 17] Internet Draft Simple Control Transmission Protocol November 1999 / value fields, as follows. The parameter field is dependent on the vendor's definition of that attribute. An example encoding of the Vendor-Specific attribute using this method follows: 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 = 0xFFFE | Parameter Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Vendor-Id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | VS-Type | VS-Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / VS-Value / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ VS-Type: 16 bit u_int This field identifies the parameter included in the VS-Value field. It is assigned by the vendor. VS-Length: 16 bit u_int This field is the length of the vendor-specific parameter and Includes the VS-Type, VS-Length and VS-Value (if included) fields. VS-Value: Variable Length This field contains the parameter identified by the VS-Type field. It's meaning is identified by the vendor. 2.3 SCTP Chunk Definitions This section defines the format of the different chunk types. 2.3.1 Initiation (INIT) (00000001) This chunk is used to initiate a SCTP association between two endpoints. The format of the INIT message is shown below: Stewart, et al [Page 18] Internet Draft Simple Control Transmission Protocol November 1999 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 1|Chunk Flags | Chunk Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Initiate Tag | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Receiver Window Credit | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Number of Outbound Streams | Number of Inbound Streams | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Initial TSN | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ \ / Optional/Variable-Length Parameters / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The INIT chunk contains the following parameters. Unless otherwise noted, each parameter MUST only be included once in the INIT chunk. Fixed Parameters Status ---------------------------------------------- Initiate Tag Mandatory Receiver Window Credit Mandatory Number of Outbound Streams Mandatory Number of Inbound Streams Mandatory Initial TSN Mandatory Variable Parameters Status Type Value ------------------------------------------------------------- IPv4 Address/Port (Note 1) Optional 0x0005 IPv6 Address/Port (Note 1) Optional 0x0006 Cookie Preservative Optional 0x0009 Note 1: The INIT chunks may contain multiple addresses that may be IPv4 and/or IPv6 in any combination. The sequence of parameters within an INIT may be processed in any order. Vendor-specific parameters are allowed in INIT. However, they MUST be appended to the end of the above INIT chunk. The format of the vendor-specific parameters MUST follow the Type-Length-value format as defined in Section 2.2.2. In case an endpoint does not support the vendor-specific data received, it MUST ignore the additional fields. Initiate Tag: 32 bit u_int The receiver of the INIT (the responding end) records the value of Stewart, et al [Page 19] Internet Draft Simple Control Transmission Protocol November 1999 the Initiate Tag parameter. This value MUST be placed into the Verification Tag field of every SCTP datagram that the responding end transmits within this association. The valid range for Initiate Tag is from 0x1 to 0xffffffff. See Section 4.3.1 for more on selection of the tag value. If the value of the Initiate Tag in a received INIT chunk is found to be 0x0, the receiver MUST treat it as an error and silently discard the datagram. Receiver Window Credit (rwnd): 32 bit u_int This field defines the maximum number of octets of outbound data the receiver of the INIT is allowed to have outstanding (i.e. sent and not acknowledged). Number of Outbound Streams (OS): 16 bit u_int Defines the number of outbound streams the sender of this INIT chunk wishes to create in this association. The value of 0 MUST NOT be used. Number of Inbound Streams (MIS) : 16 bit u_int Defines the maximum number of streams the sender of this INIT chunk allows the peer end to create in this association. The value 0 MUST NOT be used. Initial TSN (I-TSN) : 32 bit u_int Defines the initial TSN that the sender will use. The valid range is from 0x0 to 0xffffffff. This field MAY be set to the value of the Initiate Tag field. The Reserved fields must be set to all 0 by the sender and ignored by the receiver. 2.3.1.1 Optional or Variable Length Parameters The following parameters follow the Type-Length-Value format as defined in Section 2.2.1. The IP address fields MUST come after the fixed-length fields. Any extensions MUST come after the IP address fields. IPv4 Address/Port This parameter contains an IPv4 address/port for use as a destination Stewart, et al [Page 20] Internet Draft Simple Control Transmission Protocol November 1999 transport address by the receiver. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1|0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IPv4 Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Port | Padding = 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ IPv4 Address: 32bit u_int Contains an IPv4 address of this endpoint. It is binary encoded. Port Number: 16 bit u_int Contains the UDP port number which the sender of this INIT wants to use for this address. Padding: 16 bits This field is set to 0x00 on transmit and ignored on receive. IPv6 Address/Port: This parameter contains an IPv6 address/port for use as a destination transport address by the receiver. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0|0 0 0 0 0 0 0 0 0 0 0 1 0 1 1 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | IPv6 Address | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Port | Padding = 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ IPv6 Address: 128 bit u_int Contains an IPv6 address of the sender of this message. It is binary encoded. Port Number: 16 bit u_int Stewart, et al [Page 21] Internet Draft Simple Control Transmission Protocol November 1999 Contains the UDP port number which the sender of this INIT wants to use for this address. Padding: 16 bits This field is set to 0x00 on transmit and ignored on receive. The values passed in the IPv4 and IPv6 Address/Port parameters indicate to the other end of the association which transport addresses this end will support for the association being initiated. Within the association, any one of these addresses may appear in the source address field of a datagram sent from this (the initiating) end, and may be used as a destination of a datagram sent from the other (the responding) end. Note that an endpoint MAY be multi-homed. A multi-homed endpoint may have access to different types of network, thus more than one address type may be present in one INIT chunk, i.e., IPv4 and IPv6 addresses are allowed in the same INIT message. More than one IP Address parameter can be included in an INIT chunk. If the INIT contains a least one IP Address parameter, then only the transport addresses provided within the INIT may be used as destinations by the responding end. If the INIT does not contain any IP Address parameters, the responding end MUST use the source address associated with the received SCTP datagram as its sole destination address for the session. Cookie Preservative The sender of the INIT shall use this parameter to suggest to the receiver of the INIT for a longer life-span of the Responder Cookie. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1|0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Suggested Cookie Life-span Increment (msec.) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Suggested Cookie Life-span Increment: 32bit u_int This parameter indicates to the receiver the need for a cookie that expires in a longer period of time than that of the previous one. It is normally added to the INIT message during the second attempt of establishing an association with a peer after the first attempt failed due to a Stale COOKIE report from the same peer. It is Stewart, et al [Page 22] Internet Draft Simple Control Transmission Protocol November 1999 optional for the receiver to honor the suggested cookie life-span increment based upon its local security requirements. 2.3.2 Initiation Acknowledgement (INIT ACK) (00000010): The INIT ACK chunk is used to acknowledge the initiation of a SCTP association. The parameter part of INIT ACK is formatted similarly to the INIT chunk. It uses two extra variable parameters: The Responder Cookie and the Unrecognized Parameter: The format of the INIT ACK message is 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 1 0| Chunk Flags | Chunk Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Initiate Tag | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Receiver Window Credit | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Number of Outbound Streams | Number of Inbound Streams | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Initial TSN | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ \ / Optional/Variable-Length Parameters / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The INIT ACK contains the following parameters. Unless otherwise noted, each parameter MUST only be included once in the INIT ACK chunk. Fixed Parameters Status ---------------------------------------------- Initiate Tag Mandatory Receiver Window Credit Mandatory Number of Outbound Streams Mandatory Number of Inbound Streams Mandatory Initial TSN Mandatory Stewart, et al [Page 23] Internet Draft Simple Control Transmission Protocol November 1999 Variable Parameters Status Type Value ------------------------------------------------------------- Responder Cookie Mandatory 0x0007 IPv4 Address (Note 1) Optional 0x0005 IPv6 Address (Note 1) Optional 0x0006 Unrecognized Parameters Optional 0x0008 Note 1: The INIT ACK chunks may contain multiple addresses that may be IPv4 and/or IPv6 in any combination. Same as with INIT, more than one IP Address parameter can be included in an INIT ACK chunk. If the INIT ACK contains a least one IP Address parameter, then only The transport addresses provided within the INIT ACK may be used as destinations by the responding end. If the INIT ACK does not contain any IP Address parameters, the responding end MUST use the source address associated with the received SCTP datagram as its sole destination address for the session. The Responder Cookie and Unrecognized Parameters use the Type-Length- Value format as defined in Section 2.2.1 and are described below. The other fields are defined the same as their counterparts in the INIT message. 2.3.2.1 Optional or Variable Length Parameters Responder Cookie: variable size, depending on Size of Cookie This field MUST contain all the necessary state and parameter information required for the sender of this INIT ACK to create the association, along with an MD5 digital signature (128-bit). See Section 4.1.3 for details on Cookie definition. The Cookie MUST be padded with '0' to the next 32-bit word boundary; otherwise, the format of the Cookie is implementation-specific. Unrecognized Parameters: Variable Size. This parameter is returned to the originator of the INIT message if the receiver does not recognize one or more Optional/Variable-length parameters in the INIT chunk. This parameter field will contain the unrecognized parameters copied from the INIT message complete with TLV. Vendor-Specific parameters are allowed in INIT ACK. However, they MUST be defined using the format described in Section 2.2.2, and be appended to the end of the INIT ACK chunk. In case the receiver of the INIT ACK does not support the vendor-specific parameters received, it MUST ignore those fields. Stewart, et al [Page 24] Internet Draft Simple Control Transmission Protocol November 1999 2.3.3 Selective Acknowledgement (SACK) (00000011): This chunk is sent to the remote endpoint to acknowledge received DATA chunks and to inform the remote endpoint of gaps in the received subsequences of DATA chunks as represented by their TSNs. The SACK MUST contain the Cumulative TSN ACK and Receiver Window Credit (rwnd) parameters. By definition, the value of the Cumulative TSN ACK parameter is the last TSN received at the time the Selective ACK is sent, before a break in the sequence of received TSNs occurs; the next TSN value following this one has not yet been received at the reporting end. This parameter therefore acknowledges receipt of all TSNs up to and including the value given. The Selective ACK also contains zero or more fragment reports. Each fragment report acknowledges a subsequence of TSNs received following a break in the sequence of received TSNs. By definition, all TSNs acknowledged by fragment reports are higher than the value of the Cumulative TSN ACK. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 1 1|Chunk Flags | Chunk Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cumulative TSN ACK | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Receiver Window Credit (rwnd) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Number of Fragments = N | (Reserved) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Fragment #1 Start | Fragment #1 End | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / / \ ... \ / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Fragment #N Start | Fragment #N End | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Chunk Flags: Set to all zeros on transmit and ignored on receipt. Cumulative TSN ACK: 32 bit u_int This parameter contains the TSN of the last DATA chunk received in sequence before a gap. Receiver Window Credit (rwnd): 32 bit u_int Stewart, et al [Page 25] Internet Draft Simple Control Transmission Protocol November 1999 This field defines the new maximum number of octets of outbound data the receiver of this SACK is allowed to have outstanding (i.e. sent and not acknowledged). Number of Fragments: 16 bit u_int Indicates the number of TSN fragments included in this Selective ACK. Reserved: 16 bit Must be set to all 0 by the sender and ignored by the receiver. Fragments: These fields contain the ack fragments. They are repeated for each fragment up to the number of fragments defined in the Number of Fragments field. All DATA chunks with TSNs between the (Cumulative TSN ACK + Fragment Start) and (Cumulative TSN ACK + Fragment End) of each fragment are assumed to have been received correctly. Fragment Start: 16 bit u_int Indicates the Start offset TSN for this fragment. To calculate the actual TSN number the Cumulative TSN ACK is added to this offset number to yield the TSN. This calculated TSN identifies the first TSN in this fragment that has been received. Fragment End: 16 bit u_int Indicates the End offset TSN for this fragment. To calculate the actual TSN number the Cumulative TSN ACK is added to this offset number to yield the TSN. This calculated TSN identifies the TSN of the last DATA chunk received in this fragment. For example, assume the receiver has the following datagrams newly arrived at the time when it decides to send a Selective ACK, Stewart, et al [Page 26] Internet Draft Simple Control Transmission Protocol November 1999 ---------- | TSN=17 | ---------- | | <- still missing ---------- | TSN=15 | ---------- | TSN=14 | ---------- | | <- still missing ---------- | TSN=12 | ---------- | TSN=11 | ---------- | TSN=10 | ---------- then, the parameter part of the Selective ACK MUST be constructed as follows (assuming the new rwnd is set to 0x1234 by the sender): +---------------+--------------+ | Cumulative TSN ACK = 12 | ----------------+--------------- | rwnd = 0x1234 | ----------------+--------------- | num of frag=2 | (rev = 0) | ----------------+--------------- |frag #1 strt=2 |frag #1 end=3 | ----------------+--------------- |frag #2 strt=5 |frag #2 end=5 | -------------------------------- 2.3.4 Heartbeat Request (HEARTBEAT) (00000100): An endpoint should send this chunk to its peer endpoint of the current association to probe the reachability of a particular destination transport address defined in the present association. The parameter field contains the Heartbeat Information which is a variable length opeque data structure understood only by the sender. Stewart, et al [Page 27] Internet Draft Simple Control Transmission Protocol November 1999 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 1 0 0| Chunk Flags | Heartbeat Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ \ / Heartbeat Information (Variable-Length) / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Chunk Flags: Set to zero on transmit and ignored on receipt. Heartbeat Length: Set to the size of the chunk in octets, including the chunk header and the Heartbeat Information field. Heartbeat Information: defined as a variable-length parameter using the format described in Section 2.2.1, i.e.: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Heartbeat Info Type=1 | HB Info Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / Sender-specific Heartbeat Info / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Sender-specific Heartbeat Info field should normally include information about the sender's current time when this HEARTBEAT message is sent and the destination transport address to which this HEARTBEAT is sent (see Section 7.3). 2.3.5 Heartbeat Acknowledgment (HEARTBEAT ACK) (00000101): An endpoint should send this chunk to its peer endpoint as a response to a Heartbeat Request (see Section 7.3). The parameter field contains a variable length opeque data structure. Stewart, et al [Page 28] Internet Draft Simple Control Transmission Protocol November 1999 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 1 0 1| Chunk Flags | Heartbeat Ack Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ \ / Heartbeat Information (Variable-Length) / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Chunk Flags: Set to zero on transmit and ignored on receipt. Heartbeat Ack Length: Set to the size of the chunk in octets, including the chunk header and the Heartbeat Information field. Heartbeat Information: The values of this field SHALL be copied from the Heartbeat Information field found in the Heartbeat Request to which this Heartbeat Acknowledgement is responding. 2.3.6 Abort Association (ABORT) (00000110): The ABORT chunk is sent to the peer of an association to terminate the association. The Abort chunk has no parameters. If an endpoint receives an INIT or INIT ACK missing a mandatory parameter, it MUST send an ABORT message to its peer. It SHOULD include a Operational Error chunk with the Abort chunk to specify the reason. If an endpoint receives an ABORT with a format error or for an association that doesn't exist, it drops the chunk and ignores it. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 1 1 0|Chunk Flags |0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Chunk Flags: Set to zero on transmit and ignored on receipt. Stewart, et al [Page 29] Internet Draft Simple Control Transmission Protocol November 1999 2.3.7 SHUTDOWN (00000111): An endpoint in an association MUST use this chunk to initiate a graceful termination of the association with its peer. This chunk has the following 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 1 1 1|Chunk Flags |0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cumulative TSN ACK | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Chunk Flags: Set to zero on transmit and ignored on receipt. Cumulative TSN ACK: 32 bit u_int This parameter contains the TSN of the last chunk received in sequence before any gaps. 2.3.8 Shutdown Acknowledgment (SHUTDOWN ACK) (00001000): This chunk MUST be used to acknowledge the receipt of the SHUTDOWN chunk at the completion of the shutdown process, see Section 8.2 for details. The SHUTDOWN ACK chunk has no parameters. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 1 0 0 0|Chunk Flags |0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Chunk Flags: Set to zero on transmit and ignored on receipt. Note: if the endpoint that receives the SHUTDOWN message does not have a TCB or tag for the sender of the SHUTDOWN, the receiver SHALL still respond. In such cases, the receiver SHALL send back a stand-alone SHUTDOWN ACK chunk in an SCTP datagram with the Verification Tag field of the common header filled with all '0's. Stewart, et al [Page 30] Internet Draft Simple Control Transmission Protocol November 1999 2.3.9 Operation Error (ERROR) (00001001): This chunk is sent to the other endpoint in the association to notify certain error conditions. It contains one or more error causes. It has the following parameters: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 1 0 0 1| Chunk Flags | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ \ / one or more Error Causes / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Chunk Flags: Set to zero on transmit and ignored on receipt. Length: Set to the size of the chunk in octets, including the chunk header and all the Error Cause fields present. Error causes are defined as variable-length parameters using the format described in 2.2.1, i.e.: 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 | Cause Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / Cause-specific Information / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Cause Code: 16 bit u_int Defines the type of error conditions being reported. Cause Length: 16 bit u_int Set to the size of the parameter in octets, including the Cause Code, Cause Length, and Cause-Specific Information fields Cause-specific Information: variable length This field carries the details of the error condition. Stewart, et al [Page 31] Internet Draft Simple Control Transmission Protocol November 1999 Currently SCTP defines the following error causes: Cause of error --------------- Invalid Stream Identifier +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cause Code=1 | Cause Length=8 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Stream Identifier | (Reserved) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Cause of error --------------- Missing Mandatory Parameter +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cause Code=2 | Cause Length=8+N*2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Number of missing params=N | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Missing Param ID #1 | Missing Param ID #2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Missing Param ID #N-1 | Missing Param ID #N | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Each missing mandatory parameter ID should be specified in the message. Cause of error -------------- Stale Cookie Error: indicating the receiving of a valid cookie which is however expired. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cause Code=3 | Cause Length=8 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Measure of Staleness (msec.) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The sender of this error cause MAY choose to report how long past expiration the cookie is, by putting in the Measure of Staleness field the difference, in microseconds, between the current time and the time the cookie expired. If the sender does not wish to provide this information it should set Measure of staleness to 0. Guidelines for IETF-defined Error Cause extensions are discussed in Section 11.3 of this document. Stewart, et al [Page 32] Internet Draft Simple Control Transmission Protocol November 1999 2.3.10 Encryption Cookie (COOKIE) (00001010): This chunk is used only during the initialization of an association. It is sent by the initiator of an association to its peer to complete the initialization process. This chunk MUST precede any DATA chunk sent within the association, but MAY be bundled with one or more DATA chunks in the same datagram. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 1 0 1 0|Chunk Flags | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cookie | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Chunk Flags: 8 bit Set to zero on transmit and ignored on receipt. Length: 16 bit u_int Set to the size of the chunk in octets, including the 4 octets of the chunk header and the size of the Cookie. Cookie: variable size This field must contain the exact cookie received in a previous INIT ACK. 2.3.11 Cookie Acknowledgment (COOKIE ACK) (00001011): This chunk is used only during the initialization of an association. It is used to acknowledge the receipt of a COOKIE chunk. This chunk MUST precede any DATA chunk sent within the association, but MAY be bundled with one or more DATA chunks in the same SCTP datagram. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 1 0 1 1|Chunk Flags |0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Chunk Flags: Set to zero on transmit and ignored on receipt. Stewart, et al [Page 33] Internet Draft Simple Control Transmission Protocol November 1999 2.3.12 Payload Data (DATA) (00000000): The following format MUST be used for the DATA chunk: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0| Reserved|U|B|E| Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TSN | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Stream Identifier S | Sequence Number n | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ \ / User Data (seq n of Stream S) / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Reserved: 5 bits should be set to all '0's and ignored by the receiver. U bit: 1 bit The (U)nordered bit, if set, indicates that this is an unordered data chunk, and there is NO Sequence Number assigned to this DATA chunk. Therefore, the receiver MUST ignore the Sequence Number field. After reassembly (if necessary), unordered data chunks MUST be dispatched to the upper layer by the receiver without any attempt of re-ordering. Note, if an unordered user message is segmented, each segment MUST have its U bit set to 1. B bit: 1 bit The (B)eginning segment bit, if set, indicates the first segment of an SCTP user message. E bit: 1 bit The (E)nding segment bit, if set, indicates the last segment of an SCTP user message. A non-segmented user message shall have both the B and E bits set to 1. Setting both B and E bits to 0 indicates a middle segment of a multi-segment SCTP user message, as summarized in the following table: Stewart, et al [Page 34] Internet Draft Simple Control Transmission Protocol November 1999 B E Description ============================================================ | 1 0 | First piece of a segmented SCTP user message. | +----------------------------------------------------------+ | 0 0 | Middle piece of a segmented user message | +----------------------------------------------------------+ | 0 1 | Last piece of a segmented SCTP user message. | +----------------------------------------------------------+ | 1 1 | Un-segmented Message | ============================================================ Length: 16 bits (16 bit u_int) This field indicates the length of the DATA chunk in octets. It includes the Type field, the Reserved field, the U and B/E bits, the Length field, TSN, the Stream Identifier, the Stream Sequence Number, and the User Data fields. It does not include any padding. TSN : 32 bits (32 bit u_int) This value represents the TSN for this DATA chunk. The valid range of TSN is from 0x0 to 0xffffffff. Stream Identifier S: 16 bit u_int Identifies the stream to which the following user data belongs. Sequence Number n: 16 bit u_int This value presents the sequence number of the following user data within the stream S. Valid range is 0x0 to 0xFFFF. Note, when a user message is segmented by SCTP for transport, the same sequence number MUST be carried in each of the segments of the message. User Data: variable length This is the payload user data. The implementation MUST pad the end of the data to a 32 bit boundary with 0 octets. Any padding should NOT be included in the length field. 2.4 Vendor-Specific Chunk Extensions This Chunk type is available to allow vendors to support their own extended data formats not defined by the IETF. It MUST not affect the operation of SCTP. Endpoints not equipped to interpret the vendor-specific chunk sent by Stewart, et al [Page 35] Internet Draft Simple Control Transmission Protocol November 1999 a remote endpoint MUST ignore it. Endpoints that do not receive desired vendor specific information SHOULD make an attempt to operate without it, although they may do so (and report they are doing so) in a degraded mode. A summary of the Vendor-Specific Chunk format is shown below. The fields are transmitted from left to right. 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 | Flags | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Vendor-Id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ \ / Value / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type: 8 bit u_int 0xFE for all Vendor-Specific chunks. Flags: 8 bit u_int Vendor specific flags. Length: 16 bit u_int Size of this Vendor-Specific chunks in octets, including the Type, Flags, Length, Vendor-Id, and Value fields. Vendor-Id: 32 bit u_int The high-order octet is 0 and the low-order 3 octets are the SMI Network Management Private Enterprise Code of the Vendor in network byte order, as defined in the Assigned Numbers (RFC 1700). Value: Variable length The Value field is one or more octets. The actual format of the information is site or application specific, and a robust implementation SHOULD support the field as undistinguished octets. The codification of the range of allowed usage of this field is outside the scope of this specification. Stewart, et al [Page 36] Internet Draft Simple Control Transmission Protocol November 1999 3. SCTP Association State Diagram During the lifetime of an SCTP association, the SCTP endpoints progress from one state to another in response to various events. The events that may potentially advance an endpoint's state include: o SCTP user primitive calls, e.g., [open], [shutdown], [abort], o reception of INIT, COOKIE, ABORT, SHUTDOWN, etc. control chunks, or o some timeout events. The state diagram in the figures below illustrates state changes, together with the causing events and resulting actions. Note that some of the error conditions are not shown in the state diagram. Full description of all special cases should be found in the text. Note, chunk names are given in all capital letters, while parameter names have the first letter capitalized, e.g., COOKIE chunk type vs. Cookie parameter. ----- -------- (frm any state) / \ / rcv ABORT [abort] rcv INIT | | | ---------- or ---------- --------------- | v v delete TCB snd ABORT generate Cookie \ +---------+ delete TCB snd INIT.ACK ---| CLOSED | +---------+ / \ [open] / \ --------------- | | create TCB | | snd INIT | | strt init timer rcv valid COOKIE | v (1) ---------------- | +------------+ create TCB | | COOKIE_WAIT| (2) snd COOKIE.ACK | +------------+ | | | | rcv INIT.ACK | | ----------------- | | snd COOKIE | | stop init timer | | strt cookie timer | v | +------------+ | | COOKIE_SENT| (3) | +------------+ | | Stewart, et al [Page 37] Internet Draft Simple Control Transmission Protocol November 1999 | | rcv COOKIE.ACK | | ----------------- | | stop cookie timer v v +---------------+ | ESTABLISHED | +---------------+ (from any state except CLOSED) | | /--------+--------\ [shutdown] / \ ----------------- | | check outstanding | | data chunks | | v | +---------+ | |SHUTDOWN | | rcv SHUTDOWN |PENDING | | ---------------- +---------+ | x | | No more outstanding | | ------------------- | | snd SHUTDOWN | | strt shutdown timer | | v v +---------+ +-----------+ (4) |SHUTDOWN | | SHUTDOWN | (5) |SENT | | RECEIVED | +---------+ +-----------+ | | rcv SHUTDOWN.ACK | | x ------------------- | |----------------- stop shutdown timer | | retransmit missing DATA delete TCB | | send SHUTDOWN.ACK | | delete TCB | | \ +---------+ / \-->| CLOSED |<--/ +---------+ Note: (1) If the received COOKIE is invalid (i.e., failed to pass the authentication check), the receiver MUST silently discard the datagram. Or, if the received COOKIE is expired (see Section 4.1.5), the receiver SHALL send an ERROR chunk back. In either case, the receiver SHALL stay in the closed state. Stewart, et al [Page 38] Internet Draft Simple Control Transmission Protocol November 1999 (2) If the init timer expires, the endpoint SHALL retransmit INIT and re-start the init timer without changing state. This SHALL be repeated up to 'Max.Init.Retransmits' times. After that, the endpoint SHALL abort the initialization process and report the error to SCTP user. (3) If the cookie timer expires, the endpoint SHALL retransmit COOKIE and re-start the cookie timer without changing state. This SHALL be repeated up to 'Max.Init.Retransmits' times. After that, the endpoint SHALL abort the initialization process and report the error to SCTP user. (4) In SHUTDOWN-SENT state the endpoint SHALL acknowledge any received DATA chunks without delay (5) In SHUTDOWN-RECEIVED state, the endpoint MUST NOT accept any new send request from its SCTP user. 4. Association Initialization Before the first data transmission can take place from one SCTP endpoint ("A") to another SCTP endpoint ("Z"), the two endpoints must complete an initialization process in order to set up an SCTP association between them. The SCTP user at an endpoint SHOULD use the ASSOCIATE primitive to initialize an SCTP association to another SCTP endpoint. IMPLEMENTATION NOTE: From an SCTP-user's point of view, an association may be implicitly opened, without an ASSOCIATE primitive (see 9.1 B) being invoked, by the initiating endpoint's sending of the first user data to the destination endpoint. The initiating SCTP will assume default values for all mandatory and optional parameters for the INIT/INIT ACK. Once the association is established, unidirectional streams will be open for data transfer on both ends (see Section 4.1.1). 4.1 Normal Establishment of an Association The initialization process consists of the following steps (assuming that SCTP endpoint "A" tries to set up an association with SCTP endpoint "Z" and "Z" accepts the new association): A) "A" shall first send an INIT message to "Z". In the INIT, "A" must provide its security tag "Tag_A" in the Initiate Tag field. Tag_A shall be a random number in the range of 0x1 to 0xffffffff (see Stewart, et al [Page 39] Internet Draft Simple Control Transmission Protocol November 1999 4.3.1 for Tag value selection). After sending the INIT, "A" starts the T1-init timer and enters the COOKIE-WAIT state. B) "Z" shall respond immediately with an INIT ACK message. In the message, besides filling in other parameters, "Z" must set the Verification Tag field to Tag_A, and also provide its own security tag "Tag_Z" in the Initiate Tag field. Moreover, "Z" shall generate and send along with the INIT ACK a responder cookie. See Section 4.1.3 for responder cookie generation. Note: after sending out INIT ACK with the cookie, "Z" should not allocate any resources, nor keep any states for the new association. Otherwise, "Z" will be vulnerable to resource attacks. C) Upon reception of the INIT ACK from "Z", "A" shall stop the T1-init timer and leave COOKIE-WAIT state. "A" shall then send the cookie received in the INIT ACK message in a cookie chunk, restart the T1-init timer, and enter the COOKIE-SENT state. Note, the cookie chunk can be bundled with any pending outbound DATA chunks, but it MUST be the first chunk in the datagram. D) Upon reception of the COOKIE chunk, Endpoint "Z" will reply with a COOKIE ACK chunk after building a TCB and marking itself to the ESTABLISHED state. A COOKIE ACK chunk may be combined with any pending DATA chunks (and/or SACK chunks), but the COOKIE ACK chunk must be the first chunk in the datagram. IMPLEMENTATION NOTE: an implementation may choose to send the Communication Up notification to the SCTP user upon reception of a valid COOKIE. E) Upon reception of the COOKIE ACK, endpoint "A" will move from the COOKIE-SENT state to the ESTABLISHED state, stopping the T1-init timer, and it may also notify its ULP about the successful establishment of the associate with a Communication Up notification (see Section 9). Note: no DATA chunk shall be carried in the INIT or INIT ACK message. Note: if an endpoint receives an INIT, INIT ACK, or COOKIE chunk but decides not to establish the new association due to lack of resources, etc., it shall respond with an ABORT chunk. The Verification Tag field of the common header must be set to equal the Initiate Tag value of the peer. Note: After the reception of the first data chunk in an association the receiver MUST immediately respond with a SACK to acknowledge Stewart, et al [Page 40] Internet Draft Simple Control Transmission Protocol November 1999 the data chunk, subsequent acknowledgements should be done as described in section 5.2. Note: When a SCTP endpoint sends an INIT or INIT ACK it MUST include all of its transport addresses in the parameter section. This is because it may NOT be possible to control the "sending" address that a receiver of a SCTP datagram sees. A receiver thus MUST know every address that may be a source address for a peer SCTP endpoint, this assures that the inbound SCTP datagram can be matched to the proper association. 4.1.1 Handle Stream Parameters In the INIT and INIT ACK messages, the sender of the message shall indicate the number of outbound streams (OS) it wishes to have in the association, as well as the maximal inbound streams (MIS) it will accept from the other endpoint. After receiving these stream configuration information from the other side, each endpoint shall perform the following check: if the peer's MIS is less than the endpoint's OS, meaning that the peer is incapable of supporting all the outbound streams the endpoint wants to configure, the endpoint MUST either settle with MIS outbound streams, or abort the association and report to its upper layer the resources shortage at its peer. After the association is initialized, the valid outbound stream identifier range for either endpoint shall be 0 to min(local OS, remote MIS)-1. 4.1.2 Handle Address Parameters During the association initialization, an endpoint shall use the following rules to discover and collect the destination transport address(es) to its peer. On reception of an INIT or INIT ACK message, the receiver shall record any transport addresses specified as parameters in the INIT or INIT ACK message, and use only these addresses as destination transport addresses when sending subsequent datagrams to its peer. If NO destination transport addresses are specified in the INIT or INIT ACK message, then the source address from which the message arrived should be used as the destination transport address for all datagrams. 4.1.3 Generating Responder Cookie When sending an INIT ACK as a response to an INIT message, the sender Stewart, et al [Page 41] Internet Draft Simple Control Transmission Protocol November 1999 of INIT ACK should create a responder cookie and send it as part of the INIT ACK. Inside this responder cookie, the sender should include a security signature, a time stamp on when the cookie is created, and the lifespan of the cookie, along with all the information necessary for it to establish the association. The following steps SHOULD be taken to generate the cookie: 1) create an association TCB using information from both the received INIT and the outgoing INIT ACK messages, 2) in the TCB, set the creation time to the current time of day, and the lifespan to the protocol parameter 'Valid.Cookie.Life', 3) attach a private security key to the TCB and generate a 128-bit MD5 signature from the key/TCB combination (see [4] for details on MD5), and 4) generate the responder cookie by combining the TCB and the resultant MD5 signature. After sending the INIT ACK with the cookie, the sender SHOULD delete the TCB and any other local resource related to the new association, so as to prevent resource attacks. The private key should be a cryptographic quality random number with a sufficient length. Discussion in RFC 1750 [1] can be helpful in selection of the key. 4.1.4 Cookie Processing When a cookie is received from its peer in an INIT ACK message, the receiver of the INIT ACK MUST immediately send a COOKIE chunk to its peer and MAY piggy-back any pending DATA chunks on the outbound COOKIE chunk. The sender shall also start the T1-init timer after sending out the COOKIE chunk. If the timer expires, the sender shall retransmit the COOKIE chunk and restart the T1-init timer. This is repeated until either a COOKIE ACK is received or the endpoint is marked unreachable. 4.1.5 Cookie Authentication When an endpoint receives a COOKIE chunk from another endpoint with which it has no association, it shall take the following actions: 1) compute an MD5 signature using the TCB data carried in the cookie along with the receiver's private security key, 2) authenticate the cookie by comparing the computed MD5 signature Stewart, et al [Page 42] Internet Draft Simple Control Transmission Protocol November 1999 against the one carried in the cookie. If this comparison fails, the datagram, including the COOKIE and the attached user data, should be silently discarded, 3) compare the creation time stamp in the cookie to the current local time, if the elapsed time is longer than the lifespan carried in the cookie, then the datagram, including the COOKIE and the attached user data, SHOULD be discarded and the endpoint MUST transmit a stale cookie operational error to the sending endpoint, 4) if the cookie is valid, create an association to the sender of the COOKIE message with the information in the TCB data carried in the COOKIE, and enter the ESTABLISHED state, 5) acknowledge any DATA chunk in the datagram following the rules defined in Section 5.2, and, 6) send a COOKIE ACK chunk to the sender acknowledging reception of the cookie. The COOKIE ACK MAY be piggybacked with any outbound DATA chunk or SACK chunk. Note that if a COOKIE is received from an endpoint with which the receiver of the COOKIE has an existing association, the proceedures in section 4.2 should be followed. 4.1.6 An Example of Normal Association Establishment In the following example, "A" initiates the association and then sends a user datagram to "Z", then "Z" sends two user datagrams to "A" later: Endpoint A Endpoint Z {app sets association with Z} (build TCB) INIT [INIT Tag=Tag_A & other info] --------\ (Start T1-init timer) \ (Enter COOKIE-WAIT state) \---> (compose temp TCB and Cookie_Z) /--- INIT ACK [Veri Tag=Tag_A, / INIT Tag=Tag_Z, (Cancel T1-init timer) <------/ Cookie_Z, & other info] (destroy temp TCB) COOKIE [Cookie_Z] -----------\ (Start T1-init timer) \ (Enter COOKIE-SENT state) \---> (build TCB enter ESTABLISHED state) Stewart, et al [Page 43] Internet Draft Simple Control Transmission Protocol November 1999 /---- COOKIE-ACK / (Cancel T1-init timer, <-----/ Enter established state) ... {app sends 1st user data; strm 0} DATA [TSN=initial TSN_A Strm=0,Seq=1 & user data]--\ (Start T3-rxt timer) \ \-> /----- SACK [TSN ACK=init TSN_A,Frag=0] (Cancel T3-rxt timer) <------/ ... ... {app sends 2 datagrams;strm 0} /---- DATA / [TSN=init TSN_Z <--/ Strm=0,Seq=1 & user data 1] SACK [TSN ACK=init TSN_Z, /---- DATA Frag=0] --------\ / [TSN=init TSN_Z +1, \/ Strm=0,Seq=2 & user data 2] <------/\ \ \------> Note that If T1-init timer expires at "A" after the INIT or COOKIE chunks are sent, the same INIT or cookie chunk with the same Initiate Tag (i.e., Tag_A) or cookie shall be retransmitted and the timer restarted. This shall be repeated Max.Init.Retransmits times before "A" considers "Z" unreachable and reports the failure to its upper layer. When retransmitting the INIT, the endpoint SHALL following the rules defined in 5.3 to determine the proper timer value. 4.2 Handle Duplicate INIT, INIT ACK, COOKIE, and COOKIE ACK At any time during the life of an association (in one of the possible states) between an endpoint and its peer, one of the setup chunks may be received from the peer, the receiver shall process such a duplicate has described in this section. The following scenarios can cause duplicated chunks: A) The peer has crashed without being detected, and re-started itself and sent out a new Chunk trying to restore the association, B) Both sides are trying to initialize the association at about the same time, Stewart, et al [Page 44] Internet Draft Simple Control Transmission Protocol November 1999 C) The chunk is a staled datagram that was used to establish the present association or a past association which is no longer in existence, or D) The chunk is a false message generated by an attacker. In case A), the endpoint shall reset the present association and set a new association with its peer. Case B) is unique and is discussed in Section 4.2.1. However, in cases C) and D), the endpoint must retain the present association. The rules in the following sections shall be applied in order to identify and correctly handle these cases. 4.2.1 Handle Duplicate INIT in COOKIE-WAIT or COOKIE-SENT State This usually indicates an initialization collision, i.e., both endpoints are attempting at about the same time to establish an association with the other endpoint. In such a case, each of the two side shall respond to the other side with an INIT ACK, with the Verification Tag field of the common header set to the tag value received from the INIT message, and the Initiate Tag field set to its own tag value (the same tag used in the INIT message sent out by itself). Each responder shall also generate a cookie with the INIT ACK. After that, no other actions shall be taken by either side, i.e., the endpoint shall not change its state, and the T1-init timer shall be let running. The normal procedures for handling cookies will resolve the duplicate INITs to a single association. 4.2.2 Handle Duplicate INIT in Other States Upon reception of the duplicated INIT, the receiver shall follow the normal procedures for handling a INIT message, i.e. generate a INIT ACK with a cookie. In the outbound INIT ACK, the Verification Tag field of the common header shall be set to the peer tag value (from the INIT message), and the Initiate Tag field set to its own tag value (unchanged from the existing association). A cookie should also be included generated with the current time and a updated TCB based upon the INIT message. And no further actions shall be taken. Stewart, et al [Page 45] Internet Draft Simple Control Transmission Protocol November 1999 4.2.3 Handle Duplicate INIT ACK If an INIT ACK is received by an endpoint in any state other than the COOKIE-WAIT state, the endpoint should discard the INIT ACK message. A duplicate INIT ACK usually indicates the processing of a old INIT or duplicated INIT message. 4.2.4 Handle Duplicate Cookie When a duplicated COOKIE chunk is received in any state for an existing association the following rules shall be applied: 1) compute an MD5 signature using the TCB data carried in the cookie along with the receiver's private security key, 2) authenticate the cookie by comparing the computed MD5 signature against the one carried in the cookie. If this comparison fails, the datagram, including the COOKIE and the attached user data, should be silently discarded (this is case C or D above). 3) compare the timestamp in the cookie to the current time, if the cookie is older than the lifespan carried in the cookie, the datagram, including the COOKIE and the attached user data, should be discarded and the endpoint MUST transmit a stale cookie error to the sending endpoint only if the Verification tags of the cookie's TCB does NOT match the current tag values in the association (this is case C or D above). 4) If the cookie proves to be valid, unpack the TCB into a temporary TCB. 5) If the Verification Tags in the Temporary TCB matches the Verification Tags in the existing TCB, the cookie is a duplicate cookie. A cookie ack should be sent to the peer endpoint but NO update should be made to the existing TCB. 6) If the the local Verification Tag in the temporary TCB does not match the local Verification Tag in the existing TCB, then the cookie is a old stale cookie and does not correspond to the existing association (case C above). The datagram should be silently discarded. 7) If the Peers Verification Tag in the temporary TCB does not match the Peer's Verification Tag in the existing TCB then a restart of the peer has occurred (case A above). In such a case, the endpoint should report the restart to its ULP and respond the peer with a COOKIE ACK message. It shall also update the Verification Tag, initial TSN, and the destination Stewart, et al [Page 46] Internet Draft Simple Control Transmission Protocol November 1999 address list of the existing TCB with the information from the temporary TCB. After that the temporary TCB can be discarded. Furthermore, all the congestion control parameters (e.g., cwnd, ssthresh) related to this peer shall be reset to their initial values (see Section 6.2.1). IMPLEMENTATION NOTE: It is an implementation decision on how to handle any pending datagrams. The implementation may elect to either A) send all messages back to its upper layer with the restart report, or B) automatically re-queue any datagrams pending by marking all of them as never-sent and assigning new TSN's at the time of their initial transmissions based upon the updated starting TSN (as defined in section 5.5). 4.2.5 Handle Duplicate COOKIE-ACK. At any state other than COOKIE-SENT, an endpoint may receive a duplicated COOKIE ACK chunk. If so, the chunk should be silently discarded. 4.2.6 Handle Stale COOKIE Error A stale cookie error indicates one of a number of possible events: A) that the association failed to completely setup before the cookie issued by the sender was processed. B) an old cookie was processed after setup completed. C) an old cookie is received from someone that the receiver is not interested in having a association with and the ABORT message was lost. When processing a stale cookie an endpoint should first examine if an association is in the process of being setup, i.e. the association is in the COOKIE-SENT state. In all cases if the association is NOT in the COOKIE-SENT state, the stale cookie message should be silently discarded. If the association is in the COOKIE-SENT state, the endpoint may elect one of the following three alternatives. 1) Send a new INIT message to the endpoint, to generate a new cookie and re-attempt the setup procedure. 2) Discard the TCB and report to the upper layer the inability of setting-up the association. Stewart, et al [Page 47] Internet Draft Simple Control Transmission Protocol November 1999 3) Send a new INIT message to the endpoint, adding a cookie preservative parameter requesting an extentsion on the life time of the cookie. When calculating the time extension, an implementation SHOULD use the RTT information measured based on the previous COOKIE / Stale COOKIE message exchange, and should add no more than 1 second beyond the measured RTT, due to a long cookie life time makes the endpoint more subject to a replay attack. 4.3 Other Initialization Issues 4.3.1 Selection of Tag Value Initiate Tag values should be selected from the range of 0x1 to 0xffffffff. It is very important that the Tag value be randomized to help protect against "man in the middle" and "sequence number" attacks. It is suggested that RFC 1750 [1] be used for the Tag randomization. Moreover, the tag value used by either endpoint in a given association MUST never be changed during the lifetime of the association. However, a new tag value MUST be used each time the endpoint tears-down and then re-establishes the association to the same peer. 4.3.2 Initiation from behind a NAT When a NAT is present between two endpoints, the endpoint that is behind the NAT, i.e., one that does not have a publicly available network address, shall take one of the following options: A) Indicate that only one address can be used by including no transport addresses in the INIT message (Section 2.3.1.1). This will make the endpoint that receives this Initiation message to consider the sender as only having that one address. This method can be used for a dynamic NAT, but any multi-homing configuration at the endpoint that is behind the NAT will not be visible to its peer, and thus not be taken advantage of. B) Indicate all of its networks in the Initiation by specifying all the actual IP addresses and ports that the NAT will substitute for the endpoint. This method requires that the endpoint behind the NAT must have pre-knowledge of all the IP addresses and ports that the NAT will assign. 5. User Data Transfer For transmission efficiency, SCTP defines mechanisms for bundling of small user messages and segmentation of large user messages. Stewart, et al [Page 48] Internet Draft Simple Control Transmission Protocol November 1999 The following diagram depicts the flow of user messages through SCTP. +--------------------------+ | User Messages | +--------------------------+ SCTP user ^ | ==================|==|======================================= | v (1) +------------------+ +--------------------+ | SCTP DATA Chunks | |SCTP Control Chunks | +------------------+ +--------------------+ ^ | ^ | | v (2) | v (2) +--------------------------+ | SCTP datagrams | +--------------------------+ SCTP ^ | ===========================|==|=========================== | v Unreliable datagram service (e.g., UDP) Note: (1) When converting user messages into Data chunks, SCTP sender will segment user messages larger than the current path MTU into multiple data chunks. The segmented message will be reassembled from data chunks before delivery by the SCTP receiver. (2) Multiple data and control chunks may be multiplexed by the sender into a single SCTP datagram for transmission, as long as the final size of the datagram does not exceed the current path MTU. The receiver will de-multiplex the datagram back into the original chunks. The bundling and segmentation mechanisms, as detailed in Sections 5.9 and 5.10, are optional to implement by the data sender, but they MUST be implemented by the data receiver, i.e., a SCTP receiver MUST be prepared to receive and process bundled or segmented data. 5.1 Transmission of DATA Chunks The following general rules SHALL be applied by the sender for transmission and/or retransmission of outbound DATA chunks: A) At any given time, the sender MUST NOT transmit new data onto any destination transport address if it has rwnd or more octets of data outstanding. The outstanding data size is defined as the total size of ALL data chunks outstanding. Stewart, et al [Page 49] Internet Draft Simple Control Transmission Protocol November 1999 However, regardless of the value of rwnd (including if it is 0), the sender can always have ONE data packet in flight to the receiver. This rule allows the sender to probe for a change in rwnd that the sender missed due to the update having been lost in transmission from the receiver to the sender. B) At any given time, the sender MUST NOT transmit new data onto a given transport address if it has cwnd or more octets of data outstanding on that transport address. C) When the time comes for the sender to transmit, before sending new DATA chunks, the sender MUST first transmit any outstanding DATA chunks which are marked for retransmission (limited by the current cwnd). D) Then, the sender can send out as many new DATA chunks as Rule A and Rule B above allow. Note: multiple DATA chunks committed for transmission MAY be bundled in a single packet, unless bundling is explicitly disallowed by ULP of the data sender. Furthermore, DATA chunks being retransmitted MAY be bundled with new DATA chunks, as long as the resulting packet size does not exceed the path MTU. Note: before a sender transmits a data packet, if any received DATA chunks have not been acknowledged (e.g., due to delayed ack), the sender should create a SACK and bundle it with the outbound DATA chunk, as long as the size of the final SCTP datagram does not exceed the current MTU. See Section 5.2. IMPLEMENTATION Note: when the window is full (i.e., transmission is disallowed by Rule A and/or Rule B), the sender MAY still accept send requests from its upper layer, but SHALL transmit no more DATA chunks until some or all of the outstanding DATA chunks are acknowledged and transmission is allowed by Rule A and Rule B again. Whenever a transmission or retransmission is made, if T3-rxt timer is not currently running, the sender MUST start the timer. However, if the timer is already running, the sender SHALL restart the timer ONLY IF the earliest (i.e., lowest TSN) outstanding DATA chunk is being retransmitted. When starting or restarting the T3-rxt timer, the timer value must be adjusted according to the timer rules defined in Sections 5.3.2, and 5.3.3. Stewart, et al [Page 50] Internet Draft Simple Control Transmission Protocol November 1999 5.2 Acknowledgment on Reception of DATA Chunks The SCTP receiver MUST always acknowledge the SCTP sender about the reception of each DATA chunk. The guidelines on delayed acknowledgment algorithm specified in Section 4.2 of RFC 2581 [3] SHOULD be followed. Specifically, an acknowledgement SHOULD be generated for at least every second datagram received, and SHOULD be generated within 200 ms of the arrival of any unacknowledged datagram. IMPLEMENTATION NOTE: the maximal delay for generating an acknowledgement may be configured by the SCTP user, either statically or dynamically, in order to meet the specific timing requirement of the signaling protocol being carried. Acknowledgments MUST be sent in SACK control chunks. A SACK chunk can acknowledge the reception of multiple DATA chunks. See Section 2.3.3 for SACK chunk format. In particular, the SCTP receiver MUST fill in the Cumulative TSN ACK field to indicate the latest cumulative TSN number it has received, and any received segments beyond the Cumulative TSN SHALL also be reported. Upon reception of the SACK, the data sender MUST adjust its total outstanding data count and the outstanding data count on those destination addresses for which one or more data chunks is acknowledged by the SACK. The following example illustrates the use of delayed acknowledgments: Endpoint A Endpoint Z {App sends 3 messages; strm 0} DATA [TSN=7,Strm=0,Seq=3] ------------> (ack delayed) (Start T3-rxt timer) DATA [TSN=8,Strm=0,Seq=4] ------------> (send ack) /------- SACK [TSN ACK=8,Frag=0] (cancel T3-rxt timer) <-----/ ... ... DATA [TSN=9,Strm=0,Seq=5] ------------> (ack delayed) (Start T3-rxt timer) ... {App sends 1 message; strm 1} (bundle SACK with DATA) /----- SACK [TSN Ack=9,Frag=0] \ / DATA [TSN=6,Strm=1,Seq=2] (cancel T3-rxt timer) <------/ (Start T3-rxt timer) Stewart, et al [Page 51] Internet Draft Simple Control Transmission Protocol November 1999 (ack delayed) ... (send ack) SACK [TSN ACK=6,Frag=0] -------------> (cancel T3-rxt timer) 5.3 Management of Retransmission Timer SCTP uses a retransmission timer T3-rxt to ensure data delivery in the absence of any feedback from the remote data receiver. The duration of this timer is referred to as RTO (retransmission timeout). When the receiver endpoint is multi-homed, the data sender endpoint will calculate a separate RTO for each different destination transport addresses of the receiver endpoint. The computation and management of RTO in SCTP follows closely with how TCP manages its retransmission timer. To compute the current RTO, an SCTP sender maintains two state variables per destination transport address: SRTT (smoothed round-trip time) and RTTVAR (round-trip time variation). 5.3.1 RTO Calculation The rules governing the computation of SRTT, RTTVAR, and RTO are as follows: C1) Until an RTT measurement has been made for a packet sent to the given destination transport address, set RTO to the protocol parameter 'RTO.Initial'. C2) When the first RTT measurement R is made, set SRTT <- R, RTTVAR <- R/2, and RTO <- SRTT + 4 * RTTVAR. C3) When a new RTT measurement R' is made, set RTTVAR <- beta * RTTVAR + (1 - beta) * |SRTT - R'| SRTT <- alpha * SRTT + (1 - alpha) * R' (The value of SRTT used in the update to RTTVAR is its value *before* updating SRTT itself using the second assignment.) The above are computed using alpha=1/8 and beta=1/4. After the computation, update RTO <- SRTT + 4 * RTTVAR. C4) When data is in flight and when allowed by rule C5 below, a new RTT measurement MUST be made each round trip. Furthermore, it is RECOMMENDED that new RTT measurements should be made no Stewart, et al [Page 52] Internet Draft Simple Control Transmission Protocol November 1999 more than once per round-trip for a given destination transport address. There are two reasons for this recommendation: first, it appears that measuring more frequently often does not in practice yield any significant benefit [5]; second, if measurements are made more often, then the values of alpha and beta in rule C3 above should be adjusted so that SRTT and RTTVAR still adjust to changes at roughly the same rate (in terms of how many round trips it takes them to reflect new value) as they would if making only one measurement per round-trip and using alpha and beta as given in rule C3. However, the exact nature of these adjustments remains a research issue. C5) Karn's algorithm: RTT measurements MUST NOT be made using packets that were retransmitted (and thus for which it is ambiguous whether the reply was for the first instance of the packet or a later instance). C6) Whenever RTO is computed, if it is less than 1 second then it is rounded up to 1 second. The reason for this rule is that RTOs that do not have a high minimum value are susceptible to unnecessary timeouts [5]. C7) A maximum value may be placed on RTO provided it is at least 60 seconds. There is no requirement for the clock granularity G used for computing RTT measurements and the different state variables, other than G1) Whenever RTTVAR is computed, if RTTVAR = 0, then adjust RTTVAR <- G. Experience has shown that finer clock granularities (<= 100 msec) perform somewhat better than more coarse granularities. 5.3.2 Retransmission Timer Rules The rules for managing the retransmission timer are as follows: R1) Every time a packet containing data is sent (including a retransmission), if the T3-rxt timer is not running, start it running so that it will expire after RTO seconds. The RTO used here is that obtained after any doubling due to previous T3-rxt timer expirations on the coresponding destination address as discussed in rule E2 below. R2) Whenever all outstanding data has been acknowledged, turn off the T3-rxt timer. R3) Whenever a SACK is received that acknowledges new data chunks Stewart, et al [Page 53] Internet Draft Simple Control Transmission Protocol November 1999 including the one with the earliest outstanding TSN (i.e., moving the cumulative ACK point forward), restart T3-rxt timer with the current RTO. The following example shows the use of various timer rules (assuming the receiver uses delayed acks). Endpoint A Endpoint Z {App begins to send} Data [TSN=7,Strm=0,Seq=3] ------------> (ack delayed) (Start T3-rxt timer) {App sends 1 message; strm 1} (bundle ack with data) DATA [TSN=8,Strm=0,Seq=4] ----\ /-- SACK [TSN ACK=7,Frag=0] \ \ / DATA [TSN=6,Strm=1,Seq=2] \ / (Start T3-rxt timer) \ / \ (Re-start T3-rxt timer) <------/ \--> (ack delayed) (ack delayed) ... {send ack} SACK [TSN ACK=6,Frag=0] --------------> (Cancel T3-rxt timer) .. (send ack) (Cancel T3-rxt timer) <-------------- SACK [TSN ACK=8,Frag=0] 5.3.3 Handle T3-rxt Expiration Whenever the retransmission timer T3-rxt expires on a destination address, do the following: E1) On the destination address where the timer expires, adjust its ssthresh with rules defined in Section 6.2.3 and set the cwnd <- MTU. E2) On the destination address where the timer expires, set RTO <- RTO * 2 ("back off the timer"). The maximum value discussed in rule C7 above may be used to provide an upper bound to this doubling operation. E3) Determine how many of the earliest (i.e., lowest TSN) outstanding Data chunks will fit into a single packet, subject to the MTU constraint for the path corresponding to the destination transport address where the retransmission is being sent to (this may be different from the address where the timer expires [see Section 5.4]). Call this value K. Retransmit those K data chunks in a single packet to the address. Stewart, et al [Page 54] Internet Draft Simple Control Transmission Protocol November 1999 E4) Start the retransmission timer on the destination address to where the retransmission is sent, if rule R1 above indicates to do so. Note that after retransmitting, once a new RTT measurement is obtained (which can happen only when new data has been sent and acknowledged, per rule C5, or for a measurement made from a Heartbeat [see Section 7.3]), the computation in rule C3 is performed, including the computation of RTO, which may result in "collapsing" RTO back down after it has been subject to doubling (rule E2). The final rule for managing the retransmission timer concerns failover (see Section 5.4.1): F1) Whenever SCTP switches from the current destination transport address to a different one, the current retransmission timer is left running. As soon as SCTP transmits a packet containing data to the new transport address, restart the timer, using the RTO value for the path to the new address, if rule R1 indicates to do so. 5.4 Multi-homed SCTP Endpoints An SCTP endpoint is considered multi-homed if there are more than one transport addresses that can be used as a destination address to reach that endpoint. Moreover, at the sender side, one of the multiple destination addresses of the multi-homed receiver endpoint shall be selected as the primary destination transport address by the UPL (see Section 9 for details). At association initiation, the initial primary destination transport addresses are: - for the sender of the INIT message, the transport address that the INIT is sent to. - for the sender of the INTI ACK message, any valid transport address obtained from the INIT message. When the SCTP sender is transmitting to the multi-homed receiver, by default the transmission SHOULD always take place on the primary transport address, unless the SCTP user explicitly specifies the destination transport address to use. The acknowledgement SHOULD be transmitted to the same destination transport address from which the DATA or control chunk being acknowledged were received. Stewart, et al [Page 55] Internet Draft Simple Control Transmission Protocol November 1999 However, when acknowledging multiple DATA chunks in a single SACK, the SACK message may be transmitted to one of the destination transport addresses from which the DATA or control chunks being acknowledged were received. Furthermore, when the receiver is multi-homed, the SCTP data sender SHOULD try to retransmit a chunk to an active destination transport address that is different from the last destination address where the data chunk was sent to. Note, retransmissions do not affect the total outstanding data count. However, if the data chunk is retransmitted onto a different destination address, both the outstanding data counts on the new destination address and the old destination address where the data chunk was last sent to shall be adjusted accordingly. 5.4.1 Failover from Inactive Destination Address Some of the destination transport addresses of a multi-homed SCTP data receiver may become inactive due to either the occurrance of certain error conditions (see Section 7.2) or adjustments from SCTP user. When there is outbound data to send and the primary destination transport address becomes inactive (e.g., due to failures), or where the SCTP user explicitly requests to send data to an inactive destination transport address, before reporting an error to its ULP, the SCTP sender should try to send the data to an alternate active destination transport address if one exists. 5.5 Stream Identifier and Sequence Number Every DATA chunk MUST carry a valid stream identifier. If a DATA chunk with an invalid stream identifier is received, the receiver shall respond immediately with an ERROR message with cause set to Invalid Stream Identifier (see Section 2.3.9) and discard the DATA chunk. The stream sequence number in all the streams shall start from 0x0 when the association is established. Also, when the stream sequence number reaches the value 0xffff the next sequence number shall be set to 0x0. 5.6 Ordered and Un-ordered Delivery By default the SCTP receiver shall ensure the DATA chunks within any given stream be delivered to the upper layer according to the order of their stream sequence number. If there are DATA chunks arriving out of order of their stream sequence number, the receiver MUST hold the Stewart, et al [Page 56] Internet Draft Simple Control Transmission Protocol November 1999 received DATA chunks from delivery until they are re-ordered. However, an SCTP sender can indicate that no ordered delivery is required on a particular DATA chunk within the stream by setting the U flag of the DATA chunk to 1. In this case, the receiver must bypass the ordering mechanism and immediately delivery the data to the upper layer (after re-assembly if the user data is segmented by the sender). This provides an effective way of transmitting "out-of-band" data in a given stream. Also, a stream can be used as an "unordered" stream by simply setting the U flag to 1 in all outbound DATA chunks sent through that stream. IMPLEMENTATION NOTE: when sending an unordered DATA chunk, an implementation may choose to place the DATA chunk in an outbound datagram that is at the head of the outbound transmission queue if possible. Note that the 'Sequence Number' field in an un-ordered data chunk has no significance; the sender can fill it with arbitrary value, but the receiver MUST ignore the field. 5.7 Report Gaps in Received DATA TSNs Upon the reception of a new DATA chunk, an SCTP receiver shall examine the continuity of the TSNs received. If the receiver detects that gaps exist in the received DATA chunk sequence, an SACK with fragment reports shall be sent back immediately. Based on the segment reports from the SACK, the data sender can calculate the missing DATA chunks and make decisions on whether to retransmit them (see Section 5.3 for details). Multiple gaps can be reported in one single SACK (see Section 2.3.3). Note that when the data sender is multi-homed, the SCTP receiver SHOULD always try to send the SACK to the same network from where the last DATA chunk was received. Upon the reception of the SACK, the data sender SHALL remove all DATA chunks which have been acknowledged by the SACK. The data sender MUST also treat all the DATA chunks which fall into the gaps between the fragments reported by the SACK as "missing". The number of "missing" reports for each outstanding DATA chunk MUST be recorded by the data sender in order to make retransmission decision, see Section 6.2.4 for details. Stewart, et al [Page 57] Internet Draft Simple Control Transmission Protocol November 1999 The following example shows the use of SACK to report a gap. Endpoint A Endpoint Z {App sends 3 messages; strm 0} DATA [TSN=6,Strm=0,Seq=2] ---------------> (ack delayed) (Start T3-rxt timer) DATA [TSN=7,Strm=0,Seq=3] --------> X (lost) DATA [TSN=8,Strm=0,Seq=4] ---------------> (gap detected, immediately send ack) /----- SACK [TSN ACK=6,Frag=1, / Strt=2,End=2] <-----/ (remove 6 and 8 from out-queue, and strike 7 as "1" missing report) Note: in order to keep the size of the outbound SCTP datagram not to exceed the current path MTU, the maximal number of fragments that can be reported within a single SACK chunk is limited. When a single SACK can not cover all the fragments needed to be reported due to the MTU limitation, the endpoint SHALL send only one SACK, reporting the fragments from the lowest to highest TSNs, within the size limit set by the MTU, and leave the remaining highested TSN fragment numbers unacknowledged. 5.8 CRC-16 Utilization When sending a datagram, the sender can choose to strengthen the data integrity of the transmission by including the CRC-16 value calculated on the datagram, as described below. After the datagram is constructed (containing the SCTP common header and one or more control or DATA chunks), the sender shall: 1) fill in the proper Version number and Verification Tag in the common header, 2) set the C Bit to '1' and fill the 16 bit CRC-16 field with '0', 3) calculate the CRC-16 value of the whole datagram, including the SCTP common header and all t