Internet DRAFT - draft-ietf-tsvwg-sctp-ndata
draft-ietf-tsvwg-sctp-ndata
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.
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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
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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
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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
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(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.
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+---+---+---+
| 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.
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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:
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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
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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].
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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.
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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
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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.
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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
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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.
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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
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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
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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.
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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;
};
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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:
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+----------+--------------------------------------------+-----------+
| 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
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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, <https://www.rfc-
editor.org/info/rfc1982>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
editor.org/info/rfc2119>.
[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, <https://www.rfc-
editor.org/info/rfc3758>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<https://www.rfc-editor.org/info/rfc4960>.
[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, <https://www.rfc-
editor.org/info/rfc5061>.
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[RFC6096] Tuexen, M. and R. Stewart, "Stream Control Transmission
Protocol (SCTP) Chunk Flags Registration", RFC 6096,
DOI 10.17487/RFC6096, January 2011, <https://www.rfc-
editor.org/info/rfc6096>.
[RFC6525] Stewart, R., Tuexen, M., and P. Lei, "Stream Control
Transmission Protocol (SCTP) Stream Reconfiguration",
RFC 6525, DOI 10.17487/RFC6525, February 2012,
<https://www.rfc-editor.org/info/rfc6525>.
[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,
<https://www.rfc-editor.org/info/rfc7053>.
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, <https://www.rfc-
editor.org/info/rfc3261>.
[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, <https://www.rfc-
editor.org/info/rfc6458>.
Authors' Addresses
Randall R. Stewart
Netflix, Inc.
Chapin, SC 29036
United States
Email: randall@lakerest.net
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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
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