Network Working Group R. Stewart Internet-Draft Netflix, Inc. Intended status: Standards Track M. Tuexen Expires: March 5, 2018 Muenster Univ. of Appl. Sciences S. Loreto Ericsson R. Seggelmann Metafinanz Informationssysteme GmbH September 1, 2017 Stream Schedulers and User Message Interleaving for the Stream Control Transmission Protocol draft-ietf-tsvwg-sctp-ndata-13.txt Abstract The Stream Control Transmission Protocol (SCTP) is a message oriented transport protocol supporting arbitrarily large user messages. This document adds a new chunk to SCTP for carrying payload data. This allows a sender to interleave different user messages that would otherwise result in head of line blocking at the sender. The interleaving of user messages is required for WebRTC Datachannels. Whenever an SCTP sender is allowed to send user data, it may choose from multiple outgoing SCTP streams. Multiple ways for performing this selection, called stream schedulers, are defined in this document. A stream scheduler can choose to either implement, or not implement, user message interleaving. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on March 5, 2018. Stewart, et al. Expires March 5, 2018 [Page 1] Internet-Draft Stream Schedulers and the I-DATA Chunk September 2017 Copyright Notice Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2. Conventions . . . . . . . . . . . . . . . . . . . . . . . 5 2. User Message Interleaving . . . . . . . . . . . . . . . . . . 5 2.1. The I-DATA Chunk Supporting User Message Interleaving . . 6 2.2. Procedures . . . . . . . . . . . . . . . . . . . . . . . 8 2.2.1. Negotiation . . . . . . . . . . . . . . . . . . . . . 9 2.2.2. Sender Side Considerations . . . . . . . . . . . . . 9 2.2.3. Receiver Side Considerations . . . . . . . . . . . . 10 2.3. Interaction with other SCTP Extensions . . . . . . . . . 10 2.3.1. SCTP Partial Reliability Extension . . . . . . . . . 10 2.3.2. SCTP Stream Reconfiguration Extension . . . . . . . . 12 3. Stream Schedulers . . . . . . . . . . . . . . . . . . . . . . 12 3.1. First Come First Served Scheduler (SCTP_SS_FCFS) . . . . 13 3.2. Round Robin Scheduler (SCTP_SS_RR) . . . . . . . . . . . 13 3.3. Round Robin Scheduler per Packet (SCTP_SS_RR_PKT) . . . . 13 3.4. Priority Based Scheduler (SCTP_SS_PRIO) . . . . . . . . . 13 3.5. Fair Capacity Scheduler (SCTP_SS_FC) . . . . . . . . . . 14 3.6. Weighted Fair Queueing Scheduler (SCTP_SS_WFQ) . . . . . 14 4. Socket API Considerations . . . . . . . . . . . . . . . . . . 14 4.1. Exposure of the Stream Sequence Number (SSN) . . . . . . 14 4.2. SCTP_ASSOC_CHANGE Notification . . . . . . . . . . . . . 15 4.3. Socket Options . . . . . . . . . . . . . . . . . . . . . 15 4.3.1. Enable or Disable the Support of User Message Interleaving (SCTP_INTERLEAVING_SUPPORTED) . . . . . 15 4.3.2. Get or Set the Stream Scheduler (SCTP_STREAM_SCHEDULER) . . . . . . . . . . . . . . . 16 4.3.3. Get or Set the Stream Scheduler Parameter (SCTP_STREAM_SCHEDULER_VALUE) . . . . . . . . . . . . 17 4.4. Explicit EOR Marking . . . . . . . . . . . . . . . . . . 18 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 Stewart, et al. Expires March 5, 2018 [Page 2] Internet-Draft Stream Schedulers and the I-DATA Chunk September 2017 5.1. I-DATA Chunk . . . . . . . . . . . . . . . . . . . . . . 18 5.2. I-FORWARD-TSN Chunk . . . . . . . . . . . . . . . . . . . 19 6. Security Considerations . . . . . . . . . . . . . . . . . . . 19 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 8.1. Normative References . . . . . . . . . . . . . . . . . . 20 8.2. Informative References . . . . . . . . . . . . . . . . . 21 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21 1. Introduction 1.1. Overview When SCTP [RFC4960] was initially designed it was mainly envisioned for the transport of small signaling messages. Late in the design stage it was decided to add support for fragmentation and reassembly of larger messages with the thought that someday Session Initiation Protocol (SIP) [RFC3261] style signaling messages may also need to use SCTP and a single Maximum Transmission Unit (MTU) sized message would be too small. Unfortunately this design decision, though valid at the time, did not account for other applications that might send large messages over SCTP. The sending of such large messages over SCTP as specified in [RFC4960] can result in a form of sender side head of line blocking (e.g., when the transmission of a message is blocked from transmission because the sender has started the transmission of another, possibly large, message). This head of line blocking is caused by the use of the Transmission Sequence Number (TSN) for three different purposes: 1. As an identifier for DATA chunks to provide a reliable transfer. 2. As an identifier for the sequence of fragments to allow reassembly. 3. As a sequence number allowing up to 2**16 - 1 Stream Sequence Numbers (SSNs) outstanding. The protocol requires all fragments of a user message to have consecutive TSNs. This document allows an SCTP sender to interleave different user messages. This document also defines several stream schedulers for general SCTP associations allowing different relative stream treatments. The stream schedulers may behave differently depending on whether user message interleaving has been negotiated for the association or not. Figure 1 illustrates the behaviour of a round robin stream scheduler using DATA chunks when three streams with the Stream Identifiers Stewart, et al. Expires March 5, 2018 [Page 3] Internet-Draft Stream Schedulers and the I-DATA Chunk September 2017 (SIDs) 0, 1, and 2 are used. Each queue for SID 0 and SID 2 contains a single user message requiring three chunks, the queue for SID 1 contains three user messages each requiring a single chunk. It is shown how these user messages are encapsulated in chunk using TSN 0 to TSN 8. Please note that the use of such a scheduler implies late TSN assignment but it can be used with an [RFC4960] compliant implementation that does not support user message interleaving. Late TSN assignment means that the sender generates chunks from user messages and assigns the TSN as late as possible in the process of sending the user messages. +---+---+---+ | 0/0 |-+ +---+---+---+ | | +---+---+---+---+---+---+---+---+---+ +---+---+---+ +->|1/2|1/1|2/0|2/0|2/0|1/0|0/0|0/0|0/0| |1/2|1/1|1/0|--->|---|---|---|---|---|---|---|---|---| +---+---+---+ +->| 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | | +---+---+---+---+---+---+---+---+---+ +---+---+---+ | | 2/0 |-+ +---+---+---+ +-------+ +-------+ |SID/SSN| |SID/SSN| |-------| +-------+ | TSN | +-------+ Figure 1: Round Robin Scheduler without User Message Interleaving This document describes a new chunk carrying payload data called I-DATA. This chunk incorporates the properties of the current SCTP DATA chunk, all the flags and fields except the Stream Sequence Number (SSN), but also adds two new fields in its chunk header, the Fragment Sequence Number (FSN) and the Message Identifier (MID). The FSN is only used for reassembling all fragments having the same MID and ordering property. The TSN is only used for the reliable transfer in combination with Selective Acknowledgment (SACK) chunks. In addition, the MID is also used for ensuring ordered delivery instead of using the stream sequence number (The I-DATA chunk omits a SSN.). Figure 2 illustrates the behaviour of an interleaving round robin stream scheduler using I-DATA chunks. Stewart, et al. Expires March 5, 2018 [Page 4] Internet-Draft Stream Schedulers and the I-DATA Chunk September 2017 +---+---+---+ | 0/0 |-+ +---+---+---+ | | +-----+-----+-----+-----+-----+-----+-----+-----+-----+ +---+---+---+ +->|2/0/2|1/2/0|0/0/2|2/0/1|1/1/0|0/0/1|2/0/0|1/0/0|0/0/0| |1/2|1/1|1/0|--->|-----|-----|-----|-----|-----|-----|-----|-----|-----| +---+---+---+ +->| 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | | +-----+-----+-----+-----+-----+-----+-----+-----+-----+ +---+---+---+ | | 2/0 |-+ +---+---+---+ +-----------+ +-------+ |SID/MID/FSN| |SID/MID| |-----------| +-------+ | TSN | +-----------+ Figure 2: Round Robin Scheduler with User Message Interleaving The support of the I-DATA chunk is negotiated during the association setup using the Supported Extensions Parameter as defined in [RFC5061]. If I-DATA support has been negotiated for an association, I-DATA chunks are used for all user-messages. DATA chunks are not permitted when I-DATA support has been negotiated. It should be noted that an SCTP implementation supporting I-DATA chunks needs to allow the coexistence of associations using DATA chunks and associations using I-DATA chunks. In Section 2 this document specifies the user message interleaving by defining the I-DATA chunk, the procedures to use it and its interactions with other SCTP extensions. Multiple stream schedulers are defined in Section 3 followed in Section 4 by describing an extension to the socket API for using what is specified in this document. 1.2. Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 2. User Message Interleaving The protocol mechanisms described in this document allow the interleaving of user messages sent on different streams. They do not support the interleaving of multiple messages (ordered or unordered) sent on the same stream. Stewart, et al. Expires March 5, 2018 [Page 5] Internet-Draft Stream Schedulers and the I-DATA Chunk September 2017 The interleaving of user messages is required for WebRTC Datachannels as specified in [I-D.ietf-rtcweb-data-channel]. An SCTP implementation supporting user message interleaving is REQUIRED to support the coexistence of associations using DATA chunks and associations using I-DATA chunks. If an SCTP implementation supports user message interleaving and the Partial Reliability extension described in [RFC3758] or the Stream Reconfiguration Extension described in [RFC6525], it is REQUIRED to implement the corresponding changes specified in Section 2.3. 2.1. The I-DATA Chunk Supporting User Message Interleaving The following Figure 3 shows the new I-DATA chunk allowing user message interleaving. 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 = 64 | Res |I|U|B|E| Length = Variable | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TSN | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Stream Identifier | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Payload Protocol Identifier / Fragment Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ \ / User Data / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: I-DATA chunk format The only differences between the I-DATA chunk in Figure 3 and the DATA chunk defined in [RFC4960] and [RFC7053] are the addition of the new Message Identifier (MID) and the new Fragment Sequence Number (FSN) and the removal of the Stream Sequence Number (SSN). The Payload Protocol Identifier (PPID) already defined for DATA chunks in [RFC4960] and the new FSN are stored at the same location of the packet using the B bit to determine which value is stored at the location. The length of the I-DATA chunk header is 20 bytes, which is 4 bytes more than the length of the DATA chunk header defined in [RFC4960] and [RFC7053]. The old fields are: Stewart, et al. Expires March 5, 2018 [Page 6] Internet-Draft Stream Schedulers and the I-DATA Chunk September 2017 Res: 4 bits These bits are reserved. They MUST be set to 0 by the sender and MUST be ignored by the receiver. I bit: 1 bit The (I)mmediate Bit, if set, indicates that the receiver SHOULD NOT delay the sending of the corresponding SACK chunk. Same as the I bit for DATA chunks as specified in [RFC7053]. U bit: 1 bit The (U)nordered bit, if set, indicates the user message is unordered. Same as the U bit for DATA chunks as specified in [RFC4960]. B bit: 1 bit The (B)eginning fragment bit, if set, indicates the first fragment of a user message. Same as the B bit for DATA chunks as specified in [RFC4960]. E bit: 1 bit The (E)nding fragment bit, if set, indicates the last fragment of a user message. Same as the E bit for DATA chunks as specified in [RFC4960]. Length: 16 bits (unsigned integer) This field indicates the length of the DATA chunk in bytes from the beginning of the type field to the end of the User Data field excluding any padding. Similar to the Length for DATA chunks as specified in [RFC4960]. TSN: 32 bits (unsigned integer) This value represents the TSN for this I-DATA chunk. Same as the TSN for DATA chunks as specified in [RFC4960]. Stream Identifier: 16 bits (unsigned integer) Identifies the stream to which the user data belongs. Same as the Stream Identifier for DATA chunks as specified in [RFC4960]. The new fields are: Reserved: 16 bits (unsigned integer) This field is reserved. It MUST be set to 0 by the sender and MUST be ignored by the receiver. Message Identifier (MID): 32 bits (unsigned integer) The MID is the same for all fragments of a user message, it is used to determine which fragments (enumerated by the FSN) belong to the same user message. For ordered user messages, the MID is Stewart, et al. Expires March 5, 2018 [Page 7] Internet-Draft Stream Schedulers and the I-DATA Chunk September 2017 also used by the SCTP receiver to deliver the user messages in the correct order to the upper layer (similar to the SSN of the DATA chunk defined in [RFC4960]). The sender uses for each outgoing stream two counters, one for ordered messages, one for unordered messages. All of these counters are independent and initially 0. They are incremented by 1 for each user message. Please note that the serial number arithmetic defined in [RFC1982] using SERIAL_BITS = 32 applies. Therefore, the sender MUST NOT have more than 2**31 - 1 ordered messages for each outgoing stream in flight and MUST NOT have more than 2**31 - 1 unordered messages for each outgoing stream in flight. A message is considered in flight, if at least on of its I-DATA chunks is not acknowledged in a non-renegable way (i.e. not acknowledged by the cumulative TSN Ack). Please note that the MID is in "network byte order", a.k.a. Big Endian. Payload Protocol Identifier (PPID) / Fragment Sequence Number (FSN): 32 bits (unsigned integer) If the B bit is set, this field contains the PPID of the user message. Note that in this case, this field is not touched by an SCTP implementation; therefore, its byte order is not necessarily in network byte order. The upper layer is responsible for any byte order conversions to this field, similar to the PPID of DATA chunks. In this case the FSN is implicitly considered to be 0. If the B bit is not set, this field contains the FSN. The FSN is used to enumerate all fragments of a single user message, starting from 0 and incremented by 1. The last fragment of a message MUST have the E bit set. Note that the FSN MAY wrap completely multiple times allowing arbitrarily large user messages. For the FSN the serial number arithmetic defined in [RFC1982] applies with SERIAL_BITS = 32. Therefore, a sender MUST NOT have more than 2**31 - 1 fragments of a single user message in flight. A fragment is considered in flight, if it is not acknowledged in a non-renegable way. Please note that the FSN is in "network byte order", a.k.a. Big Endian. 2.2. Procedures This subsection describes how the support of the I-DATA chunk is negotiated and how the I-DATA chunk is used by the sender and receiver. The handling of the I bit for the I-DATA chunk corresponds to the handling of the I bit for the DATA chunk described in [RFC7053]. Stewart, et al. Expires March 5, 2018 [Page 8] Internet-Draft Stream Schedulers and the I-DATA Chunk September 2017 2.2.1. Negotiation An SCTP end point indicates user message interleaving support by listing the I-DATA Chunk within the Supported Extensions Parameter as defined in [RFC5061]. User message interleaving has been negotiated for an association if both end points have indicated I-DATA support. If user message interleaving support has been negotiated for an association, I-DATA chunks MUST be used for all user messages and DATA-chunks MUST NOT be used. If user message interleaving support has not been negotiated for an association, DATA chunks MUST be used for all user messages and I-DATA chunks MUST NOT be used. An end point implementing the socket API specified in [RFC6458] MUST NOT indicate user message interleaving support unless the user has requested its use (e.g. via the socket API, see Section 4.3). This constraint is made since the usage of this chunk requires that the application is capable of handling interleaved messages upon reception within an association. This is not the default choice within the socket API (see the SCTP_FRAGMENT_INTERLEAVE socket option in Section 8.1.20 of [RFC6458]) thus the user MUST indicate to the SCTP implementation its support for receiving completely interleaved messages. Note that stacks that do not implement [RFC6458] may use other methods to indicate interleaved message support and thus indicate the support of user message interleaving. The crucial point is that the SCTP stack MUST know that the application can handle interleaved messages before indicating the I-DATA support. 2.2.2. Sender Side Considerations The sender side usage of the I-DATA chunk is quite simple. Instead of using the TSN for fragmentation purposes, the sender uses the new FSN field to indicate which fragment number is being sent. The first fragment MUST have the B bit set. The last fragment MUST have the E bit set. All other fragments MUST NOT have the B bit or E bit set. All other properties of the existing SCTP DATA chunk also apply to the I-DATA chunk, i.e. congestion control as well as receiver window conditions MUST be observed as defined in [RFC4960]. Note that the usage of this chunk implies the late assignment of the actual TSN to any chunk being sent. Each I-DATA chunk uses a single TSN. This way messages from other streams may be interleaved with the fragmented message. Please note that this is the only form of interleaving support. For example, it is not possible to interleave multiple ordered or unordered user messages from the same stream. Stewart, et al. Expires March 5, 2018 [Page 9] Internet-Draft Stream Schedulers and the I-DATA Chunk September 2017 The sender MUST NOT process (move user data into I-DATA chunks and assign a TSN to it) more than one user message in any given stream at any time. At any time, a sender MAY process multiple user messages, each of them on different streams. The sender MUST assign TSNs to I-DATA chunks in a way that the receiver can make progress. One way to achieve this is to assign a higher TSN to the later fragments of a user message and send out the I-DATA chunks such that the TSNs are in sequence. 2.2.3. Receiver Side Considerations Upon reception of an SCTP packet containing an I-DATA chunk whose user message needs to be reassembled, the receiver MUST first use the SID to identify the stream, consider the U bit to determine if it is part of an ordered or unordered message, find the user message identified by the MID and finally use the FSN for reassembly of the message and not the TSN. The receiver MUST NOT make any assumption about the TSN assignments of the sender. Note that a non-fragmented message is indicated by the fact that both the E and B bits are set. A message (either ordered or unordered) may be identified as being fragmented whose E and B bits are not both set. If I-DATA support has been negotiated for an association, the reception of a DATA chunk is a violation of the above rules and therefore the receiver of the DATA chunk MUST abort the association by sending an ABORT chunk. The ABORT chunk MAY include the 'Protocol Violation' error cause. The same applies if I-DATA support has not been negotiated for an association and an I-DATA chunk is received. 2.3. Interaction with other SCTP Extensions The usage of the I-DATA chunk might interfere with other SCTP extensions. Future SCTP extensions MUST describe if and how they interfere with the usage of I-DATA chunks. For the SCTP extensions already defined when this document was published, the details are given in the following subsections. 2.3.1. SCTP Partial Reliability Extension When the SCTP extension defined in [RFC3758] is used in combination with the user message interleaving extension, the new I-FORWARD-TSN chunk MUST be used instead of the FORWARD-TSN chunk. The difference between the FORWARD-TSN and the I-FORWARD-TSN chunk is that the 16-bit Stream Sequence Number (SSN) has been replaced by the 32-bit Message Identifier (MID) and the largest skipped MID can also be provided for unordered messages. Therefore, the principle applied to Stewart, et al. Expires March 5, 2018 [Page 10] Internet-Draft Stream Schedulers and the I-DATA Chunk September 2017 ordered message when using FORWARD-TSN chunks is applied to ordered and unordered messages when using I-FORWARD-TSN chunks. 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 = 194 | Flags = 0x00 | Length = Variable | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | New Cumulative TSN | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Stream Identifier | Reserved |U| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ \ / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Stream Identifier | Reserved |U| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: I-FORWARD-TSN chunk format The old fields are: Flags: 8-bits (unsigned integer) These bits are reserved. They MUST be set to 0 by the sender and MUST be ignored by the receiver. Same as the Flags for FORWARD TSN chunks as specified in [RFC3758]. Length: 16-bits (unsigned integer) This field holds the length of the chunk. Similar to the Length for FORWARD TSN chunks as specified in [RFC3758]. New Cumulative TSN: 32-bits (unsigned integer) This indicates the new cumulative TSN to the data receiver. Same as the New Cumulative TSN for FORWARD TSN chunks as specified in [RFC3758]. The new fields are: Stream Identifier (SID): 16-bits (unsigned integer) This field holds the stream number this entry refers to. Reserved: 15 bits This field is reserved. It MUST be set to 0 by the sender and MUST be ignored by the receiver. Stewart, et al. Expires March 5, 2018 [Page 11] Internet-Draft Stream Schedulers and the I-DATA Chunk September 2017 U bit: 1 bit The U bit specifies if the Message Identifier of this entry refers to unordered messages (U bit is set) or ordered messages (U bit is not set). Message Identifier (MID): 32 bits (unsigned integer) This field holds the largest Message Identifier for ordered or unordered messages indicated by the U bit that was skipped for the stream specified by the Stream Identifier. For ordered messages this is similar to the FORWARD-TSN chunk, just replacing the 16-bit SSN by the 32-bit MID. Support for the I-FORWARD-TSN chunk is negotiated during the SCTP association setup via the Supported Extensions Parameter as defined in [RFC5061]. Only if both end points indicated their support of user message interleaving and the I-FORWARD-TSN chunk, the partial reliability extension is negotiated and can be used in combination with user message interleaving. The FORWARD-TSN chunk MUST be used in combination with the DATA chunk and MUST NOT be used in combination with the I-DATA chunk. The I- FORWARD-TSN chunk MUST be used in combination with the I-DATA chunk and MUST NOT be used in combination with the DATA chunk. If I-FORWARD-TSN support has been negotiated for an association, the reception of a FORWARD-TSN chunk is a violation of the above rules and therefore the receiver of the FORWARD-TSN chunk MUST abort the association by sending an ABORT chunk. The ABORT chunk MAY include the 'Protocol Violation' error cause. The same applies if I-FORWARD- TSN support has not been negotiated for an association and a FORWARD- TSN chunk is received. 2.3.2. SCTP Stream Reconfiguration Extension When an association resets the SSN using the SCTP extension defined in [RFC6525], the two counters (one for the ordered messages, one for the unordered messages) used for the MIDs MUST be reset to 0. Since most schedulers, especially all schedulers supporting user message interleaving, require late TSN assignment, it should be noted that the implementation of [RFC6525] needs to handle this. 3. Stream Schedulers This section defines several stream schedulers. The stream schedulers may behave differently depending on whether user message interleaving has been negotiated for the association or not. An implementation MAY implement any subset of them. If the Stewart, et al. Expires March 5, 2018 [Page 12] Internet-Draft Stream Schedulers and the I-DATA Chunk September 2017 implementation is used for WebRTC Datachannels as specified in [I-D.ietf-rtcweb-data-channel] it MUST implement the Weighted Fair Queueing Scheduler defined in Section 3.6. The selection of the stream scheduler is done at the sender side. There is no mechanism provided for signalling the stream scheduler being used to the receiver side or even let the receiver side influence the selection of the stream scheduler used at the sender side. 3.1. First Come First Served Scheduler (SCTP_SS_FCFS) The simple first-come, first-served scheduler of user messages is used. It just passes through the messages in the order in which they have been delivered by the application. No modification of the order is done at all. The usage of user message interleaving does not affect the sending of the chunks, except that I-DATA chunks are used instead of DATA chunks. 3.2. Round Robin Scheduler (SCTP_SS_RR) When not using user message interleaving, this scheduler provides a fair scheduling based on the number of user messages by cycling around non-empty stream queues. When using user message interleaving, this scheduler provides a fair scheduling based on the number of I-DATA chunks by cycling around non-empty stream queues. 3.3. Round Robin Scheduler per Packet (SCTP_SS_RR_PKT) This is a round-robin scheduler, which only switches streams when starting to fill a new packet. It bundles only DATA or I-DATA chunks referring to the same stream in a packet. This scheduler minimizes head-of-line blocking when a packet is lost because only a single stream is affected. 3.4. Priority Based Scheduler (SCTP_SS_PRIO) Scheduling of user messages with strict priorities is used. The priority is configurable per outgoing SCTP stream. Streams having a higher priority will be scheduled first and when multiple streams have the same priority, the scheduling between them is implementation dependent. When using user message interleaving, the sending of large lower priority user messages will not delay the sending of higher priority user messages. Stewart, et al. Expires March 5, 2018 [Page 13] Internet-Draft Stream Schedulers and the I-DATA Chunk September 2017 3.5. Fair Capacity Scheduler (SCTP_SS_FC) A fair capacity distribution between the streams is used. This scheduler considers the lengths of the messages of each stream and schedules them in a specific way to maintain an equal capacity for all streams. The details are implementation dependent. Using user message interleaving allows for a better realization of the fair capacity usage. 3.6. Weighted Fair Queueing Scheduler (SCTP_SS_WFQ) A weighted fair queueing scheduler between the streams is used. The weight is configurable per outgoing SCTP stream. This scheduler considers the lengths of the messages of each stream and schedules them in a specific way to use the capacity according to the given weights. If the weight of stream S1 is n times the weight of stream S2, the scheduler should assign to stream S1 n times the capacity it assigns to stream S2. The details are implementation dependent. Using user message interleaving allows for a better realization of the capacity usage according to the given weights. This scheduler in combination with user message interleaving is used for WebRTC Datachannels as specified in [I-D.ietf-rtcweb-data-channel]. 4. Socket API Considerations This section describes how the socket API defined in [RFC6458] is extended to allow applications to use the extension described in this document. Please note that this section is informational only. 4.1. Exposure of the Stream Sequence Number (SSN) The socket API defined in [RFC6458] defines several structures in which the SSN of a received user message is exposed to the application. The list of these structures includes: struct sctp_sndrcvinfo Specified in Section 5.3.2 SCTP Header Information Structure (SCTP_SNDRCV) of [RFC6458] and marked as deprecated. struct sctp_extrcvinfo Specified in Section 5.3.3 Extended SCTP Header Information Structure (SCTP_EXTRCV)of [RFC6458] and marked as deprecated. struct sctp_rcvinfo Stewart, et al. Expires March 5, 2018 [Page 14] Internet-Draft Stream Schedulers and the I-DATA Chunk September 2017 Specified in Section 5.3.5 SCTP Receive Information Structure (SCTP_RCVINFO) of [RFC6458]. If user message interleaving is used, the lower order 16 bits of the MID are used as the SSN when filling out these structures. 4.2. SCTP_ASSOC_CHANGE Notification When an SCTP_ASSOC_CHANGE notification (specified in Section 6.1.1 of [RFC6458]) is delivered indicating a sac_state of SCTP_COMM_UP or SCTP_RESTART for an SCTP association where both peers support the I-DATA chunk, SCTP_ASSOC_SUPPORTS_INTERLEAVING should be listed in the sac_info field. 4.3. Socket Options +-----------------------------+-------------------------+-----+-----+ | option name | data type | get | set | +-----------------------------+-------------------------+-----+-----+ | SCTP_INTERLEAVING_SUPPORTED | struct sctp_assoc_value | X | X | | SCTP_STREAM_SCHEDULER | struct sctp_assoc_value | X | X | | SCTP_STREAM_SCHEDULER_VALUE | struct | X | X | | | sctp_stream_value | | | +-----------------------------+-------------------------+-----+-----+ 4.3.1. Enable or Disable the Support of User Message Interleaving (SCTP_INTERLEAVING_SUPPORTED) This socket option allows the enabling or disabling of the negotiation of user message interleaving support for future associations. For existing associations it allows to query whether user message interleaving support was negotiated or not on a particular association. This socket option uses IPPROTO_SCTP as its level and SCTP_INTERLEAVING_SUPPORTED as its name. It can be used with getsockopt() and setsockopt(). The socket option value uses the following structure defined in [RFC6458]: struct sctp_assoc_value { sctp_assoc_t assoc_id; uint32_t assoc_value; }; assoc_id: This parameter is ignored for one-to-one style sockets. For one-to-many style sockets, this parameter indicates upon which association the user is performing an action. The special Stewart, et al. Expires March 5, 2018 [Page 15] Internet-Draft Stream Schedulers and the I-DATA Chunk September 2017 sctp_assoc_t SCTP_FUTURE_ASSOC can also be used, it is an error to use SCTP_{CURRENT|ALL}_ASSOC in assoc_id. assoc_value: A non-zero value encodes the enabling of user message interleaving whereas a value of 0 encodes the disabling of user message interleaving. sctp_opt_info() needs to be extended to support SCTP_INTERLEAVING_SUPPORTED. An application using user message interleaving should also set the fragment interleave level to 2 by using the SCTP_FRAGMENT_INTERLEAVE socket option specified in Section 8.1.20 of [RFC6458]. This allows the interleaving of user messages from different streams. Please note that it does not allow the interleaving of user messages (ordered or unordered) on the same stream. Failure to set this option can possibly lead to application deadlock. Some implementations might therefore put some restrictions on setting combinations of these values. Setting the interleaving level to at least 2 before enabling the negotiation of user message interleaving should work on all platforms. Since the default fragment interleave level is not 2, user message interleaving is disabled per default. 4.3.2. Get or Set the Stream Scheduler (SCTP_STREAM_SCHEDULER) A stream scheduler can be selected with the SCTP_STREAM_SCHEDULER option for setsockopt(). The struct sctp_assoc_value is used to specify the association for which the scheduler should be changed and the value of the desired algorithm. The definition of struct sctp_assoc_value is the same as in [RFC6458]: struct sctp_assoc_value { sctp_assoc_t assoc_id; uint32_t assoc_value; }; assoc_id: Holds the identifier for the association of which the scheduler should be changed. The special SCTP_{FUTURE|CURRENT|ALL}_ASSOC can also be used. This parameter is ignored for one-to-one style sockets. assoc_value: This specifies which scheduler is used. The following constants can be used: SCTP_SS_DEFAULT: The default scheduler used by the SCTP implementation. Typical values are SCTP_SS_FCFS or SCTP_SS_RR. Stewart, et al. Expires March 5, 2018 [Page 16] Internet-Draft Stream Schedulers and the I-DATA Chunk September 2017 SCTP_SS_FCFS: Use the scheduler specified in Section 3.1. SCTP_SS_RR: Use the scheduler specified in Section 3.2. SCTP_SS_RR_PKT: Use the scheduler specified in Section 3.3. SCTP_SS_PRIO: Use the scheduler specified in Section 3.4. The priority can be assigned with the sctp_stream_value struct. The higher the assigned value, the lower the priority, that is the default value 0 is the highest priority and therefore the default scheduling will be used if no priorities have been assigned. SCTP_SS_FB: Use the scheduler specified in Section 3.5. SCTP_SS_WFQ: Use the scheduler specified in Section 3.6. The weight can be assigned with the sctp_stream_value struct. sctp_opt_info() needs to be extended to support SCTP_STREAM_SCHEDULER. 4.3.3. Get or Set the Stream Scheduler Parameter (SCTP_STREAM_SCHEDULER_VALUE) Some schedulers require additional information to be set for individual streams as shown in the following table: +-----------------+-----------------+ | name | per stream info | +-----------------+-----------------+ | SCTP_SS_DEFAULT | n/a | | SCTP_SS_FCFS | no | | SCTP_SS_RR | no | | SCTP_SS_RR_PKT | no | | SCTP_SS_PRIO | yes | | SCTP_SS_FB | no | | SCTP_SS_WFQ | yes | +-----------------+-----------------+ This is achieved with the SCTP_STREAM_SCHEDULER_VALUE option and the corresponding struct sctp_stream_value. The definition of struct sctp_stream_value is as follows: struct sctp_stream_value { sctp_assoc_t assoc_id; uint16_t stream_id; uint16_t stream_value; }; Stewart, et al. Expires March 5, 2018 [Page 17] Internet-Draft Stream Schedulers and the I-DATA Chunk September 2017 assoc_id: Holds the identifier for the association of which the scheduler should be changed. The special SCTP_{FUTURE|CURRENT|ALL}_ASSOC can also be used. This parameter is ignored for one-to-one style sockets. stream_id: Holds the stream id of the stream for which additional information has to be provided. stream_value: The meaning of this field depends on the scheduler specified. It is ignored when the scheduler does not need additional information. sctp_opt_info() needs to be extended to support SCTP_STREAM_SCHEDULER_VALUE. 4.4. Explicit EOR Marking Using explicit End of Record (EOR) marking for an SCTP association supporting user message interleaving allows the user to interleave the sending of user messages on different streams. 5. IANA Considerations [NOTE to RFC-Editor: "RFCXXXX" is to be replaced by the RFC number you assign this document. ] [NOTE to RFC-Editor: The suggested values for the chunk types and the chunk flags are tentative and to be confirmed by IANA. ] This document (RFCXXXX) is the reference for all registrations described in this section. Two new chunk types have to be assigned by IANA. 5.1. I-DATA Chunk IANA should assign the chunk type for this chunk from the pool of chunks with the upper two bits set to '01'. This requires an additional line in the "Chunk Types" registry for SCTP: Stewart, et al. Expires March 5, 2018 [Page 18] Internet-Draft Stream Schedulers and the I-DATA Chunk September 2017 +----------+--------------------------------------------+-----------+ | ID Value | Chunk Type | Reference | +----------+--------------------------------------------+-----------+ | 64 | Payload Data supporting Interleaving | [RFCXXXX] | | | (I-DATA) | | +----------+--------------------------------------------+-----------+ The registration table as defined in [RFC6096] for the chunk flags of this chunk type is initially given by the following table: +------------------+-----------------+-----------+ | Chunk Flag Value | Chunk Flag Name | Reference | +------------------+-----------------+-----------+ | 0x01 | E bit | [RFCXXXX] | | 0x02 | B bit | [RFCXXXX] | | 0x04 | U bit | [RFCXXXX] | | 0x08 | I bit | [RFCXXXX] | | 0x10 | Unassigned | | | 0x20 | Unassigned | | | 0x40 | Unassigned | | | 0x80 | Unassigned | | +------------------+-----------------+-----------+ 5.2. I-FORWARD-TSN Chunk IANA should assign the chunk type for this chunk from the pool of chunks with the upper two bits set to '11'. This requires an additional line in the "Chunk Types" registry for SCTP: +----------+---------------+-----------+ | ID Value | Chunk Type | Reference | +----------+---------------+-----------+ | 194 | I-FORWARD-TSN | [RFCXXXX] | +----------+---------------+-----------+ The registration table as defined in [RFC6096] for the chunk flags of this chunk type is initially empty. 6. Security Considerations This document does not add any additional security considerations in addition to the ones given in [RFC4960] and [RFC6458]. It should be noted that the application has to consent that it is willing to do the more complex reassembly support required for user message interleaving. When doing so, an application has to provide a reassembly buffer for each incoming stream. It has to protect itself against these buffers taking too many resources. If user message Stewart, et al. Expires March 5, 2018 [Page 19] Internet-Draft Stream Schedulers and the I-DATA Chunk September 2017 interleaving is not used, only a single reassembly buffer needs to be provided for each association. But the application has to protect itself for excessive resource usages there too. 7. Acknowledgments The authors wish to thank Benoit Claise, Julian Cordes, Spencer Dawkins, Gorry Fairhurst, Lennart Grahl, Christer Holmberg, Mirja Kuehlewind, Marcelo Ricardo Leitner, Karen E. Egede Nielsen, Maksim Proshin, Eric Rescorla, Irene Ruengeler, Felix Weinrank, Michael Welzl, Magnus Westerlund, and Lixia Zhang for their invaluable comments. This work has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 644334 (NEAT). The views expressed are solely those of the author(s). 8. References 8.1. Normative References [RFC1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982, DOI 10.17487/RFC1982, August 1996, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC3758] Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P. Conrad, "Stream Control Transmission Protocol (SCTP) Partial Reliability Extension", RFC 3758, DOI 10.17487/RFC3758, May 2004, . [RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol", RFC 4960, DOI 10.17487/RFC4960, September 2007, . [RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M. Kozuka, "Stream Control Transmission Protocol (SCTP) Dynamic Address Reconfiguration", RFC 5061, DOI 10.17487/RFC5061, September 2007, . Stewart, et al. Expires March 5, 2018 [Page 20] Internet-Draft Stream Schedulers and the I-DATA Chunk September 2017 [RFC6096] Tuexen, M. and R. Stewart, "Stream Control Transmission Protocol (SCTP) Chunk Flags Registration", RFC 6096, DOI 10.17487/RFC6096, January 2011, . [RFC6525] Stewart, R., Tuexen, M., and P. Lei, "Stream Control Transmission Protocol (SCTP) Stream Reconfiguration", RFC 6525, DOI 10.17487/RFC6525, February 2012, . [RFC7053] Tuexen, M., Ruengeler, I., and R. Stewart, "SACK- IMMEDIATELY Extension for the Stream Control Transmission Protocol", RFC 7053, DOI 10.17487/RFC7053, November 2013, . 8.2. Informative References [I-D.ietf-rtcweb-data-channel] Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data Channels", draft-ietf-rtcweb-data-channel-13 (work in progress), January 2015. [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, DOI 10.17487/RFC3261, June 2002, . [RFC6458] Stewart, R., Tuexen, M., Poon, K., Lei, P., and V. Yasevich, "Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)", RFC 6458, DOI 10.17487/RFC6458, December 2011, . Authors' Addresses Randall R. Stewart Netflix, Inc. Chapin, SC 29036 United States Email: randall@lakerest.net Stewart, et al. Expires March 5, 2018 [Page 21] Internet-Draft Stream Schedulers and the I-DATA Chunk September 2017 Michael Tuexen Muenster University of Applied Sciences Stegerwaldstrasse 39 48565 Steinfurt Germany Email: tuexen@fh-muenster.de Salvatore Loreto Ericsson Torshamnsgatan 21 164 80 Stockholm Sweden Email: Salvatore.Loreto@ericsson.com Robin Seggelmann Metafinanz Informationssysteme GmbH Leopoldstrasse 146 80804 Muenchen Germany Email: rfc@robin-seggelmann.com Stewart, et al. Expires March 5, 2018 [Page 22]