Network Working Group P. Natarajan
Internet-Draft P. Amer
Intended status: Standards Track E. Yilmaz
Expires: August 28, 2008 University of Delaware
R. Stewart
Cisco Systems, Inc.
J. Iyengar
Connecticut College
February 25, 2008
Non-Renegable Selective Acknowledgements (NR-SACKs) for SCTP
draft-natarajan-tsvwg-sctp-nrsack-01.txt
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Abstract
Stream Control Transmission Protocol (SCTP) [RFC4960] specifies
Selective Acknowledgements (SACKs) to allow a transport layer data
receiver to acknowledge DATA chunks which arrive out-of-order. In
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SCTP, SACK information is advisory because the data receiver is
permitted to renege; that is, later discard a DATA chunk which
previously has been SACKed. Since delivery of a SACKed out-of-order
DATA chunk is not guaranteed, a copy of this DATA chunk MUST be kept
in the data sender's retransmission queue until this DATA chunk is
cumulatively acked.
This document specifies Non-Renegable Selective Acknowledgements (NR-
SACKs), an extension to SCTP's acknowledgment mechanism. NR-SACKs
enable a data receiver to explicitly acknowledge out-of-order DATA
chunks that have been delivered to the receiving application.
(Recall that, in SCTP, out-of-order data sometimes can be delivered.)
NR-SACKs also enable a data receiver to indicate any out-of-order
DATA chunks on which the receiver guarantees never to renege. As
opposed to SACKed DATA chunks, a sender can consider NR-SACKed DATA
chunks as never requiring retransmission, thus freeing space in the
data sender's retransmission queue sooner.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Negotiation . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. The New Chunk Type: Non-Renegable SACK (NR-SACK) . . . . . . . 6
5. An Illustrative Example . . . . . . . . . . . . . . . . . . . 11
6. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1. Sending an NR-SACK chunk . . . . . . . . . . . . . . . . . 15
6.2. Receiving an NR-SACK Chunk . . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8. IANA considerations . . . . . . . . . . . . . . . . . . . . . 18
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1. Normative References . . . . . . . . . . . . . . . . . . . 18
10.2. Informative References . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . . . 21
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1. Introduction
In providing end-to-end reliable transport layer data transfer, SCTP
specifies Cumulative Acknowledgements (ACKs), Selective ACKs (SACKs),
and Duplicate Selective ACKs (D-SACKs). These three types of acks
are carried in the following fields of the SACK chunk, respectively:
Cumulative TSN Ack, Gap Ack Block, and Duplicate TSN. In this
document, we refer to the Cumulative TSN Ack as the "cum-ack", the
selective Gap Ack Blocks as "gap-acks", and the Duplicate TSN
selective acks as "dup-TSN reports".
Gap-acks acknowledge DATA chunks that arrive out-of-order to a
transport layer data receiver. A gap-ack in SCTP is advisory, in
that, while it notifies a data sender about the reception of
indicated DATA chunks, the data receiver is permitted to later
discard DATA chunks that it previously had gap-acked. Discarding a
previously gap-acked DATA chunk is known as "reneging." Because of
the possibility of reneging in SCTP, any gap-acked DATA chunk MUST
NOT be removed from the data sender's retransmission queue until the
DATA chunk is later cum-acked.
Situations exist when a data receiver knows that reneging on a
particular out-of-order DATA chunk will never take place, such as
(but not limited to) after an out-of-order DATA chunk is delivered to
the receiving application. This document describes an extension to
SCTP to allow for Non-Renegable Selective Acknowledgments (NR-SACKs.)
A new NR-SACK chunk type is described that allows this extension to
be implemented.
The NR-SACK chunk is an extension of the existing SACK chunk.
Several fields are identical, including the Cumulative TSN Ack, the
Advertised Receiver Window Credit (a_rwnd), Gap Ack Blocks, and
Duplicate TSNs. These fields have the same semantics as described in
[RFC4960].
NR-SACKs extend SACKs by also identifying out-of-order DATA chunks
that a receiver either: (1) has delivered to its receiving
application, or (2) takes full responsibility to eventually deliver
to its receiving application. These out-of-order DATA chunks are
"non-renegable." Non-Renegable data are reported in the NR Gap Ack
Block field of the NR-SACK chunk as described in Section 4.1. We
refer to non-renegable selective acknowledgements as "nr-gap-acks."
When an out-of-order DATA chunk is nr-gap-acked, the data sender no
longer needs to keep that particular DATA chunk in its retransmission
queue, thus allowing the data sender to free up its buffer space
sooner than if the DATA chunk were only gap-acked. NR-SACKs are
expected to improve throughput, particularly in multi-streamed SCTP
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associations where out-of-order data often can be delivered, at the
trade-off of requiring additional processing of acknowledgement
information at both the data receiver and data sender [Natarajan].
An SCTP message is encapsulated within a single DATA chunk or within
multiple DATA chunks in case of fragmentation. In this document
without loss of generality, each application message maps to a single
transport layer DATA chunk, and delivering a DATA chunk to a
receiving application means delivering the message carried within the
DATA chunk to a receiving application.
SCTP divides an end-to-end association into independent logical data
streams (a.k.a. multistreaming.) A DATA chunk that arrives in-
sequence within a stream can be delivered to the receiving
application even if the DATA chunk is out-of-order relative to the
association's overall flow of data. These out-of-order DATA chunks
are "deliverable." By definition, a DATA chunk marked for unordered
delivery also is "deliverable" to the receiving application
immediately upon reception, regardless of its position within the
overall flow of data.
With current SACKs in SCTP, it is not possible for a data receiver to
inform a data sender if or when a particular out-of-order
"deliverable" DATA chunk has been "delivered" to the receiving
application. Thus the data sender MUST keep a copy of every gap-
acked out-of-order DATA chunk(s) in the data sender's retransmission
queue until the DATA chunk is cum-acked. This use of the data
sender's retransmission queue is wasteful. Given the receiving
application has received the data, the data sender has no reason to
keep this data in its retransmission queue. Yet, the sending
transport layer keeps the data because no mechanism currently exists
to indicate which out-of-order DATA chunks have been delivered.
(Note: once a DATA chunk is delivered to the receiving application,
it is impossible for the data receiver to renege that DATA chunk.)
If NR-SACKs are used, the data receiver MAY include the TSN of a
delivered out-of-order DATA chunk in an NR-SACK to inform the data
sender that the delivery has occurred, allowing the data sender to
remove the copy of the delivered DATA chunk from the data sender's
retransmission queue even before the DATA chunk is cum-acked.
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].
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3. Negotiation
Before sending/receiving NR-SACKs, both peer endpoints MUST agree on
using NR-SACKs. This agreement MUST be negotiated during association
establishment. NR-SACK is an extension to the core SCTP, and SCTP
extensions that an endpoint supports are reported to the peer
endpoint in Supported Extensions Parameter during association
establishment (see Section 4.2.7 of [RFC5061].) The Supported
Extensions Parameter consists of a list of non-standard Chunk Types
that are supported by the sender.
An endpoint supporting the NR-SACK extension MUST list the NR-SACK
chunk in the Supported Extensions Parameter carried in the INIT or
INIT-ACK chunk, depending on whether the endpoint initiates or
responds to the initiation of the association. If the NR-SACK chunk
type ID is listed in the Chunk Types List of the Supported Extensions
Parameter, then the receiving endpoint MUST assume that the NR-SACK
chunk is supported by the sending endpoint.
Both endpoints MUST support NR-SACKs for either endpoint to send an
NR-SACK. If an endpoint which tries to establish an association with
a remote endpoint does not list NR-SACK in the Supported Extensions
Parameter carried in INIT chunk, then both endpoints of the
association MUST NOT use NR-SACKs. After association establishment,
an endpoint MUST NOT renegotiate the use of NR-SACKs.
Once both endpoints indicate during association establishment that
they support the NR-SACK extension, each endpoint SHOULD acknowledge
received DATA chunks with NR-SACK chunks, and not SACK chunks. That
is, throughout an SCTP association, both endpoints SHOULD send either
SACK chunks or NR-SACK chunks, but not a mixture of the two.
4. The New Chunk Type: Non-Renegable SACK (NR-SACK)
Table 1 illustrates a new chunk type that will be used to transfer
NR-SACK information.
Chunk Type Chunk Name
--------------------------------------------------------------
0x10 Non-Renegable Selective Acknowledgment (NR-SACK)
Table 1: NR-SACK Chunk
As the NR-SACK chunk replaces the SACK chunk, many of the SACK chunk
fields are preserved in the NR-SACK chunk. These fields preserved in
the NR-SACK chunk have the same semantics with the corresponding SACK
chunk fields, as defined in [RFC4960], Section 3.3.4. For
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completeness, we describe all fields of the NR-SACK chunk, including
those that are identical in the SACK chunk.
All arithmetic operations discussed in this document are based on
"Serial Number Arithmetic" as described in Section 1.6 of [RFC4960].
Similar to the SACK chunk, the NR-SACK chunk is sent to a peer
endpoint to (1) acknowledge DATA chunks received in-order, (2)
acknowledge DATA chunks received out-of-order, and (3) indicate DATA
chunks received more than once (i.e., duplicate.) In addition, the
NR-SACK chunk also informs the peer endpoint of non-renegable out-of-
order DATA chunks.
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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 = 0x10 | Reserved |A| Chunk Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cumulative TSN Ack |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertised Receiver Window Credit (a_rwnd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Gap Ack Blocks = N |Number of NR Gap Ack Blocks = M|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Duplicate TSNs = X | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Gap Ack Block #1 Start | Gap Ack Block #1 End |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ ... \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Gap Ack Block #N Start | Gap Ack Block #N End |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NR Gap Ack Block #1 Start | NR Gap Ack Block #1 End |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ ... \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NR Gap Ack Block #M Start | NR Gap Ack Block #M End |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Duplicate TSN 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ ... \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Duplicate TSN X |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 8 bits
This field holds the IANA defined chunk type for NR-SACK chunk. The
suggested value of this field for IANA is 0x10.
Reserved: 7 bits [Same as SACK chunk]
Currently not used. It is recommended a sender set all bits to zero
on transmit, and a receiver ignore this field.
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A bit: 1 bit
The (A)ll bit, if set to '1', indicates that all of the out-of-order
data blocks acknowledged in this NR-SACK chunk are non-renegable. If
(A)ll bit is set to '1', then Number of Gap Ack Blocks (N) MUST be
set to '0' (for example, see CASE-3 in Section 5.)
Chunk Length: 16 bits (unsigned integer) [Same as SACK chunk]
This value represents the size of the chunk in bytes including the
Chunk Type, Chunk Flags, Chunk Length, and Chunk Value fields.
Cumulative TSN Ack: 32 bits (unsigned integer) [Same as SACK chunk]
The value of the Cumulative TSN Ack is the last TSN received before a
break in the sequence of received TSNs occurs. The next TSN value
following the Cumulative TSN Ack has not yet been received at the
endpoint sending the NR-SACK.
Advertised Receiver Window Credit (a_rwnd): 32 bits (unsigned
integer) [Same as SACK chunk]
Indicates the updated receive buffer space in bytes of the sender of
this NR-SACK, see Section 6.2.1 of [RFC4960] for details.
Number of Gap Ack Blocks (N): 16 bits (unsigned integer) [Same as
SACK chunk]
Indicates the number of Gap Ack Blocks included in this NR-SACK. If
the (A)ll bit is set to one, meaning that all out-of-order data
blocks are non-renegable, then the Number of NR Gap Ack Blocks MUST
be set to '0'.
Number of NR Gap Ack Blocks (M): 16 bits (unsigned integer)
Indicates the number of Non-Renegable Gap Ack Blocks included in this
NR-SACK.
Number of Duplicate TSNs (X): 16 bits [Same as SACK chunk]
Contains the number of duplicate TSNs the endpoint has received.
Each duplicate TSN is listed following the NR Gap Ack Block list.
Reserved : 16 bits
Currently not used. It is recommended a sender set all bits to zero
on transmit, and a receiver ignore this field.
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Gap Ack Blocks: [Same as SACK chunk]
The NR-SACK contains zero or more Gap Ack Blocks. The Gap Ack Blocks
in an NR-SACK have the same semantics as in the existing SACK chunk.
Each Gap Ack Block acknowledges a subsequence of TSNs received
following a break in the sequence of received TSNs. By definition,
all TSNs acknowledged by Gap Ack Blocks are greater than the value of
the Cumulative TSN Ack.
Gap Ack Blocks are repeated for each Gap Ack Block up to 'N' defined
in the Number of Gap Ack Blocks field. All DATA chunks with TSNs
greater than or equal to (Cumulative TSN Ack + Gap Ack Block Start)
and less than or equal to (Cumulative TSN Ack + Gap Ack Block End) of
each Gap Ack Block are assumed to have been received correctly.
Gap Ack Block Start: 16 bits (unsigned integer) [Same as SACK chunk]
Indicates the Start offset TSN for this Gap Ack Block. This number
is set relative to the cumulative TSN number defined in Cumulative
TSN Ack field. To calculate the actual start TSN number, the
Cumulative TSN Ack is added to this offset number. The calculated
TSN identifies the first TSN in this Gap Ack Block that has been
received.
Gap Ack Block End: 16 bits (unsigned integer) [Same as SACK chunk]
Indicates the End offset TSN for this Gap Ack Block. This number is
set relative to the cumulative TSN number defined in the Cumulative
TSN Ack field. To calculate the actual TSN number, the Cumulative
TSN Ack is added to this offset number. The calculated TSN
identifies the TSN of the last DATA chunk received in this Gap Ack
Block.
N(on)R(enegable) Gap Ack Blocks:
The NR-SACK contains zero or more NR Gap Ack Blocks. Each NR Gap Ack
Block acknowledges a continuous subsequence of non-renegable out-of-
order DATA chunks. Each sequence of TSNs in an NR Gap Ack Block MUST
be a subsequence of one of the Gap Ack Blocks except for the special
case when 'A' bit is set to 1 (for example, see CASE-3 in Section 5.)
Note that there can be more than one NR Gap Ack Block per Gap Ack
Block. If a TSN is nr-gap-acked in any NR-SACK chunk, then all
subsequent NR-SACKs gap-acking that TSN SHOULD also nr-gap-ack that
TSN.
NR Gap Ack Blocks are repeated for each NR Gap Ack Block up to 'M'
defined in the Number of NR Gap Ack Blocks field. All DATA chunks
with TSNs greater than or equal to (Cumulative TSN Ack + NR Gap Ack
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Block Start) and less than or equal to (Cumulative TSN Ack + NR Gap
Ack Block End) of each NR Gap Ack Block are assumed to be Non-
Renegable.
NR Gap Ack Block Start: 16 bits (unsigned integer)
Indicates the Start offset TSN for this NR Gap Ack Block. This
number is set relative to the cumulative TSN number defined in
Cumulative TSN Ack field. To calculate the actual TSN number, the
Cumulative TSN Ack is added to this offset number. The calculated
TSN identifies the first TSN in this NR Gap Ack Block that has been
received.
NR Gap Ack Block End: 16 bits (unsigned integer)
Indicates the End offset TSN for this NR Gap Ack Block. This number
is set relative to the cumulative TSN number defined in Cumulative
TSN Ack field. To calculate the actual TSN number, the Cumulative
TSN Ack is added to this offset number. The calculated TSN
identifies the TSN of the last DATA chunk received in this NR Gap Ack
Block.
Duplicate TSN: 32 bits (unsigned integer) [Same as SACK chunk]
Contain the TSNs received in duplicate since the last NR-SACK was
sent. They are repeated for each duplicate TSN up to 'X' defined in
Number of Duplicate TSNs field.
Each duplicate TSN is listed in this field as many times as the TSN
was received since the previous NR-SACK was sent.
For example, if a receiver were to get the TSN 19 three times, the
receiver would list 19 twice in the outbound NR-SACK. After sending
the NR-SACK if the receiver received yet one more TSN 19, the
receiver would list 19 as a duplicate once in the next outgoing NR-
SACK.
5. An Illustrative Example
Assume the following DATA chunks have arrived at the receiver.
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--------------------------------
| TSN=16| SID=2 | SSN=N/A| U=1 |
--------------------------------
| TSN=15| SID=1 | SSN= 4 | U=0 |
--------------------------------
| TSN=14| SID=0 | SSN= 4 | U=0 |
--------------------------------
| TSN=13| SID=2 | SSN=N/A| U=1 |
--------------------------------
| |
--------------------------------
| TSN=11| SID=0 | SSN= 3 | U=0 |
-------------------------------
| |
--------------------------------
| |
--------------------------------
| TSN=8 | SID=2 | SSN=N/A| U=1 |
--------------------------------
| TSN=7 | SID=1 | SSN= 2 | U=0 |
--------------------------------
| TSN=6 | SID=1 | SSN= 1 | U=0 |
--------------------------------
| TSN=5 | SID=0 | SSN= 1 | U=0 |
--------------------------------
| |
--------------------------------
| TSN=3 | SID=1 | SSN= 0 | U=0 |
--------------------------------
| TSN=2 | SID=0 | SSN= 0 | U=0 |
--------------------------------
The above figure shows the list of DATA chunks at the receiver. TSN
denotes the transmission sequence number of the DATA chunk, SID
denotes the SCTP stream id to which the DATA chunk belongs, SSN
denotes the sequence number of the DATA chunk within its stream, and
U bit denotes whether the DATA chunk requires ordered or unordered
delivery [RFC4960].
This data can be viewed as three separate streams as follows (assume
each stream begins with SSN=0.) Note that in this example, the
application uses stream 2 for unordered data transfer. By
definition, SSN fields of unordered DATA chunks are ignored.
Stream-0:
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SSN: 0 1 2 3 4
TSN: | 2 | 5 | | 11 | 14 |
U-Bit: | 0 | 0 | | 0 | 0 |
Stream-1:
SSN: 0 1 2 3 4
TSN: | 3 | 6 | 7 | | 15 |
U-Bit: | 0 | 0 | 0 | | 0 |
Stream-2:
SSN: N/A N/A N/A
TSN: | 8 | 13 | 16 |
U-Bit: | 1 | 1 | 1 |
The NR-SACK to acknowledge the above data MUST be constructed as
follows for each of the three cases described below (the a_rwnd is
arbitrarily set to 4000):
CASE-1: Minimal Data Receiver Responsibility - no out-of-order
deliverable data yet delivered
None of the deliverable out-of-order DATA chunks have been delivered
and the receiver of the above data does not take responsibility for
any of the received out-of-order DATA chunks. The receiver reserves
the right to renege on any or all of the out-of-order DATA chunks.
+-----------------------------+-----------------------------+
| Type = 0x10 | 0000000|A=0| Chunk Length = 32 |
+-----------------------------+-----------------------------+
| Cumulative TSN Ack = 3 |
+-----------------------------+-----------------------------+
| a_rwnd = 4000 |
+-----------------------------+-----------------------------+
| Num of Gap Ack Blocks = 3 |Num of NR Gap Ack Blocks = 0 |
+-----------------------------+-----------------------------+
| Num of Duplicates = 0 | 0x00 |
+-----------------------------+-----------------------------+
| Gap Ack Block #1 Start = 2 | Gap Ack Block #1 End = 5 |
+-----------------------------+-----------------------------+
| Gap Ack Block #2 Start = 8 | Gap Ack Block #2 End = 8 |
+-----------------------------+-----------------------------+
| Gap Ack Block #3 Start = 10 | Gap Ack Block #3 End = 13 |
+-----------------------------+-----------------------------+
CASE-2: Minimal Data Receiver Responsibility - all out-of-order
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deliverable data delivered
In this case, the NR-SACK chunk is being sent after the data receiver
has delivered all deliverable out-of-order DATA chunks to its
receiving application. The receiver reserves the right to renege on
all undelivered out-of-order DATA chunks.
+------------------------------+------------------------------+
| Type = 0x10 | 0000000|A=0 | Chunk Length = 44 |
+------------------------------+------------------------------+
| Cumulative TSN Ack = 3 |
+------------------------------+------------------------------+
| a_rwnd = 4000 |
+------------------------------+------------------------------+
| Num of Gap Ack Block = 3 | Num of NR Gap Ack Block = 3 |
+------------------------------+------------------------------+
| Num of Duplicates = 0 | 0x00 |
+------------------------------+------------------------------+
| Gap Ack Block #1 Start = 2 | Gap Ack Block #1 End = 5 |
+------------------------------+------------------------------+
| Gap Ack Block #2 Start = 8 | Gap Ack Block #2 End = 8 |
+------------------------------+------------------------------+
| Gap Ack Block #3 Start = 10 | Gap Ack Block #3 End = 13 |
+------------------------------+------------------------------+
|NR Gap Ack Block #1 Start = 2 | NR Gap Ack Block #1 End = 5 |
+------------------------------+------------------------------+
|NR Gap Ack Block #2 Start = 10| NR Gap Ack Block #2 End = 10 |
+------------------------------+------------------------------+
|NR Gap Ack Block #3 Start = 13| NR Gap Ack Block #3 End = 13 |
+------------------------------+------------------------------+
CASE-3: Maximal Data Receiver Responsibility
In this special case, all out-of-order data blocks acknowledged are
non-renegable. This case would occur when the data receiver is
programmed never to renege, and takes responsibility to deliver all
DATA chunks that arrive out-of-order. In this case the (A)ll bit is
set to '1' indicating all out-of-order data blocks are nr-gap-acked.
Only when A = 1 is a sequence of TSNs in an NR Gap Ack Block not a
subsequence of one of the Gap Ack Blocks.
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+--------------------------------+-------------------------------+
| Type = 0x10 | 0000000|A=1| Chunk Length = 32 |
+--------------------------------+-------------------------------+
| Cumulative TSN Ack = 3 |
+--------------------------------+-------------------------------+
| a_rwnd = 4000 |
+--------------------------------+-------------------------------+
| Num of Gap Ack Blocks = 0 | Num of NR Gap Ack Blocks = 3 |
+--------------------------------+-------------------------------+
| Num of Duplicates = 0 | 0x00 |
+--------------------------------+-------------------------------+
| NR Gap Ack Block #1 Start = 2 | NR Gap Ack Block #1 End = 5 |
+--------------------------------+-------------------------------+
| NR Gap Ack Block #2 Start = 8 | NR Gap Ack Block #2 End = 8 |
+--------------------------------+-------------------------------+
| NR Gap Ack Block #3 Start = 10 | NR Gap Ack Block #3 End = 13 |
+--------------------------------+-------------------------------+
6. Procedures
The procedures regarding when to send an NR-SACK chunk are identical
to the procedures regarding when to send a SACK chunk, as outlined in
Section 6.2 of [RFC4960].
6.1. Sending an NR-SACK chunk
All of the NR-SACK chunk fields identical to the SACK chunk MUST be
formed as described in Section 6.2 of [RFC4960].
It is up to the data receiver whether or not to take responsibility
for delivery of each out-of-order DATA chunk. An out-of-order DATA
chunk that has already been delivered, or that the receiver takes
responsibility to deliver (i.e., guarantees not to renege) is Non
Renegable(NR), and SHOULD be included in an NR Gap Ack Block field of
the outgoing NR-SACK.
Consider three cases:
CASE-1: Data receiver takes no responsibility for delivery of any
out-of-order DATA chunks
CASE-2: Data receiver takes responsibility for all out-of-order DATA
chunks that are "deliverable" (i.e., DATA chunks in-sequence
within the stream they belong to, or DATA chunks whose (U)nordered
bit is set to '1')
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CASE-3: Data receiver takes responsibility for delivery of all out-
of-order DATA chunks, whether deliverable or not deliverable
The data receiver SHOULD follow the procedures outlined below for
building the NR-SACK.
CASE-1:
1A) Identify the TSNs received out-of-order.
1B) For these out-of-order TSNs, identify the Gap Ack Blocks. Fill
the Number of Gap Ack Blocks (N) field, Gap Ack Block #i Start,
and Gap Ack Block #i End where i goes from 1 to N.
1C) Set the (A)ll bit to '0'.
1D) Set the Number of NR Gap Acks (M) field to '0'.
CASE-2:
2A) Identify the TSNs received out-of-order.
2B) For these out-of-order TSNs, identify the Gap Ack Blocks. Fill
the Number of Gap Ack Blocks (N) field, Gap Ack Block #i Start,
and Gap Ack Block #i End where i goes from 1 to N.
2C) For the received out-of-order TSNs, check the (U)nordered bit of
each TSN. Tag unordered TSNs as NR.
2D) For each stream, identify the TSNs which are out-of-order
relative to the association's entire flow of data, but in-sequence
within that stream. Tag those in-sequence TSNs as NR.
2E) For those TSNs tagged as NR, identify the NR Blocks. Fill the
Number of NR Gap Ack Blocks(M) field, NR Gap Ack Block #i Start,
and NR Gap Ack Block #i End where i goes from 1 to M.
CASE-3:
3A) Identify the TSNs received out-of-order. All of these TSNs
SHOULD be nr-gap-acked.
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3B) For these out-of-order TSNs, identify the NR Gap Ack Blocks.
According to that information, fill the Number of NR Gap Ack
Blocks (M) field, NR Gap Ack Block #i Start, and NR Gap Ack Block
#i End where i goes from 1 to M.
3C) Set the (A)ll bit to '1'.
3D) Set the Number of Gap Acks (N) field to '0'.
6.2. Receiving an NR-SACK Chunk
When an NR-SACK chunk is received, all of the NR-SACK fields
identical to a SACK chunk SHOULD be processed and handled as in SACK
chunk handling outlined in Section 6.2.1 of [RFC4960].
The NR Gap Ack Block Start(s) and NR Gap Ack Block End(s) are offsets
relative to the cum-ack. To calculate the actual range of nr-gap-
acked TSNs, the cum-ack MUST be added to the Start and End.
For example, assume an incoming NR-SACK chunk's cum-ack is 12 and an
NR Gap Ack Block defines the NR Gap Ack Block Start=5, and the NR Gap
Ack Block End=7. This NR Gap Ack block nr-gap-acks TSNs 17 through
19(inclusive.)
Upon reception of an NR-SACK chunk, all TSNs falling within an NR-Gap
Ack Block SHOULD be removed from the data sender's retransmission
queue as their delivery to the receiving application has either
already occurred, or is guaranteed by the data receiver.
Most of NR-SACK processing at the data sender can be implemented by
using the same routines as in SACK that process cum ack and gap ack,
followed by removal of nr-gap-acked DATA chunks from the
retransmission queue. However, with NR-SACKs, as out-of-order DATA
chunks also can be removed from the retranmission queue, gap ack
processing routine cannot rely on the contents of the data sender's
retransmission queue for correct processing. For example, while
calculating missing reports,the gap ack processing routine cannot
assume that the highest TSN transmitted is always at the tail (right
edge) of the retransmission queue.
7. Security Considerations
There are no security considerations addressed by this memo.
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8. IANA considerations
This document defines a new chunk type to transfer the NR-SACK
information. Table 2 illustrates the new chunk type.
The new chunk type must come from the range of chunk types where the
upper two bits are zero. We recommend 0x10 but any other available
code point with the upper two bits set to zero is acceptable.
Chunk Type Chunk Name
--------------------------------------------------------------
0x10 Non-Renegable Selective Acknowledgment (NR-SACK)
Table 2: NR-SACK Chunk
9. Acknowledgments
This Internet Draft is the result of a great deal of constructive
discussion with several people, notably Phillip Conrad, Nasif Ekiz,
and Jonathan Leighton.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
Kozuka, "Stream Control Transmission Protocol (SCTP)
Dynamic Address Reconfiguration", RFC 5061,
September 2007.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
10.2. Informative References
[Natarajan]
Natarajan, P., Ekiz, N., Yilmaz, E., Amer, P., Iyengar,
J., and R. Stewart, "Performance of Non-Renegable
Selective Acknowledgments (NR-SACKs) for SCTP", Technical
Report 2008/344, Computer and Information Sciences Dept.,
University of Delaware , (in progress).
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Authors' Addresses
Preethi Natarajan
University of Delaware
Computer and Information Sciences Department
Newark, DE 19716
USA
Phone:
Email: nataraja@cis.udel.edu
Paul D. Amer
University of Delaware
Computer and Information Sciences Department
Newark, DE 19716
USA
Phone:
Email: amer@cis.udel.edu
Ertugrul Yilmaz
University of Delaware
Computer and Information Sciences Department
Newark, DE 19716
USA
Phone:
Email: eyilmaz@cis.udel.edu
Randall R. Stewart
Cisco Systems, Inc.
4875 Forest Drive
Suite 200
Columbia, SC 29206
USA
Phone:
Email: rrs@cisco.com
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Janardhan Iyengar
Connecticut College
Computer Science Department
270 Mohegan Avenue
New London, CT 06320-4196
USA
Phone: 860-439-5048
Email: iyengar@conncoll.edu
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