Network Working Group P. Natarajan
Internet-Draft Cisco Systems
Intended status: Standards Track P. Amer
Expires: August 19, 2010 E. Yilmaz
University of Delaware
R. Stewart
Researcher
J. Iyengar
Franklin & Marshall College
February 15, 2010
Non-Renegable Selective Acknowledgements (NR-SACKs) for SCTP
draft-natarajan-tsvwg-sctp-nrsack-05.txt
Abstract
Stream Control Transmission Protocol (SCTP) [RFC4960] specifies
Selective Acknowledgements (SACKs) to allow an SCTP data receiver to
acknowledge DATA chunks which arrive out-of-order. In SCTP, SACK
information is advisory -- though SACKs notify a data sender about
the reception of specific out-of-order data, the SCTP data receiver
is permitted to later discard the data, a.k.a reneging. 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.
By definition, data that has been delivered to the application is
non-renegable by the SCTP data receiver. (Recall that, in SCTP, out-
of-order data can sometimes be delivered.) Also, SCTP
implementations can be configured such that the SCTP data receiver is
not allowed to, and therefore, never reneges on out-of-order data.
With SCTP's current SACK mechanism, non-renegable out-of-order data
is selectively acked, and is (wrongly) deemed renegable by the SCTP
data sender.
This document specifies an extension to SCTP's acknowledgment
mechanism called Non-Renegable Selective Acknowledgements (NR-SACKs.)
NR-SACKs enable a data receiver to explicitly inform the data sender
of non-renegable out-of-order data. As opposed to renegable data, a
data sender can consider non-renegable data as never requiring
retransmission, and therefore can remove non-renegable data from the
retransmission queue.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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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 . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Sending an NR-SACK chunk . . . . . . . . . . . . . . . . . 14
6.2. Receiving an NR-SACK Chunk . . . . . . . . . . . . . . . . 16
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8. IANA considerations . . . . . . . . . . . . . . . . . . . . . 17
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1. Normative References . . . . . . . . . . . . . . . . . . . 18
10.2. Informative References . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
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1. Introduction
In providing end-to-end reliable 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), 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 have
been shown to better utilize the data sender's memory and improve
throughput, at the trade-off of generating and processing additional
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acknowledgement information [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 on 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 establishes an association with a remote
endpoint that 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, never 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 SACK chunk fields
are preserved in the NR-SACK chunk. These preserved fields have the
same semantics with the corresponding SACK chunk fields, as defined
in [RFC4960], Section 3.3.4. The Gap Ack fields from RFC4960 have
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been renamed as R Gap Ack to emphasize their renegable nature. Their
semantics are unchanged. For completeness, we describe all fields of
the NR-SACK chunk, including those that are identical in the SACK
chunk.
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) identify DATA
chunks received more than once (i.e., duplicate.) In addition, the
NR-SACK chunk (4) informs the peer endpoint of non-renegable out-of-
order DATA 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 = 0x10 | Chunk Flags | Chunk Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cumulative TSN Ack |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertised Receiver Window Credit (a_rwnd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Number of R Gap Ack Blocks = N |Number of NR Gap Ack Blocks = M|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Duplicate TSNs = X | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| R Gap Ack Block #1 Start | R Gap Ack Block #1 End |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ ... \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| R Gap Ack Block #N Start | R 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 |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
Chunk Flags: 8 bits
Currently not used. It is recommended a sender set all bits to zero
on transmit, and a receiver ignore this field.
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 (R)enegable Gap Ack Blocks (N): 16 bits (unsigned integer)
Indicates the number of Renegable Gap Ack Blocks included in this NR-
SACK.
Number of (N)on(R)enegable 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
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Currently not used. It is recommended a sender set all bits to zero
on transmit, and a receiver ignore this field.
(R)enegable Gap Ack Blocks:
The NR-SACK contains zero or more R Gap Ack Blocks. Each R Gap Ack
Block acknowledges a subsequence of renegable out-of-order TSNs. By
definition, all TSNs acknowledged by R Gap Ack Blocks are "greater
than" the value of the Cumulative TSN Ack.
Because of TSN numbering wraparound, comparisons and all arithmetic
operations discussed in this document are based on "Serial Number
Arithmetic" as described in Section 1.6 of [RFC4960].
R Gap Ack Blocks are repeated for each R Gap Ack Block up to 'N'
defined in the Number of R Gap Ack Blocks field. All DATA chunks
with TSNs >= (Cumulative TSN Ack + R Gap Ack Block Start) and <=
(Cumulative TSN Ack + R Gap Ack Block End) of each R Gap Ack Block
are assumed to have been received correctly, and are renegable.
R Gap Ack Block Start: 16 bits (unsigned integer)
Indicates the Start offset TSN for this R 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 R Gap Ack Block that has been
received.
R Gap Ack Block End: 16 bits (unsigned integer)
Indicates the End offset TSN for this R 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 R 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. If a TSN is nr-gap-acked in any NR-SACK chunk,
then all subsequently transmitted NR-SACKs with a smaller cum-ack
value than 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
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with TSNs >= (Cumulative TSN Ack + NR Gap Ack Block Start) and <=
(Cumulative TSN Ack + NR Gap Ack Block End) of each NR Gap Ack Block
are assumed to be received correctly, and are 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.
Note:
NR Gap Ack Blocks and R Gap Ack Blocks in an NR-SACK chunk SHOULD
acknowledge disjoint sets of TSNs. That is, an out-of-order TSN
SHOULD be listed in either an R Gap Ack Block or an NR Gap Ack Block,
but not the both. R Gap Ack Blocks and NR Gap Ack Blocks together
provide the information as do the Gap Ack Block of a SACK chunk, plus
additional information about non-renegability.
If all out-of-order data acked by an NR-SACK are renegable, then the
Number of NR Gap Ack Blocks MUST be set to 0. If all out-of-order
data acked by an NR-SACK are non-renegable, then the Number of R Gap
Ack Blocks SHOULD be set to 0. TSNs listed in R Gap Ack Block will
be referred as r-gap-acked.
Duplicate TSN: 32 bits (unsigned integer) [Same as SACK chunk]
Indicates a duplicate TSN received since the last NR-SACK was sent.
Exactly 'X' duplicate TSNs SHOULD be reported where 'X' was 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
data receiver were to get the TSN 19 three times, the data receiver
would list 19 twice in the outbound NR-SACK. After sending the NR-
SACK if the receiver received one more TSN 19, the receiver would
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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.
--------------------------------
| 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 stream id to which the DATA chunk belongs, SSN denotes
the sequence number of the DATA chunk within its stream, and the U
bit denotes whether the DATA chunk requires ordered(=0) or
unordered(=1) delivery [RFC4960]. Note that TSNs 4,9,10, and 12 have
not arrived.
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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:
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 SHOULD 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 any or all of the out-of-order DATA chunks.
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+-----------------------------+-----------------------------+
| Type = 0x10 | 00000000 | Chunk Length = 32 |
+-----------------------------+-----------------------------+
| Cumulative TSN Ack = 3 |
+-----------------------------+-----------------------------+
| a_rwnd = 4000 |
+-----------------------------+-----------------------------+
| Num of R Gap Ack Blocks = 3 |Num of NR Gap Ack Blocks = 0 |
+-----------------------------+-----------------------------+
| Num of Duplicates = 0 | 0x00 |
+-----------------------------+-----------------------------+
|R Gap Ack Block #1 Start = 2 | R Gap Ack Block #1 End = 5 |
+-----------------------------+-----------------------------+
|R Gap Ack Block #2 Start = 8 | R Gap Ack Block #2 End = 8 |
+-----------------------------+-----------------------------+
|R Gap Ack Block #3 Start = 10| R Gap Ack Block #3 End = 13 |
+-----------------------------+-----------------------------+
CASE-2: Minimal Data Receiver Responsibility - all out-of-order
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(i.e., TSNs 5,6,7,8,13, and 16.) The receiver
reserves the right to renege on all undelivered out-of-order DATA
chunks(i.e., TSNs 11,14, and 15.)
+------------------------------+------------------------------+
| Type = 0x10 | 0x00 | Chunk Length = 40 |
+------------------------------+------------------------------+
| Cumulative TSN Ack = 3 |
+------------------------------+------------------------------+
| a_rwnd = 4000 |
+------------------------------+------------------------------+
| Num of R Gap Ack Blocks = 2 | Num of NR Gap Ack Blocks = 3 |
+------------------------------+------------------------------+
| Num of Duplicates = 0 | 0x00 |
+------------------------------+------------------------------+
| R Gap Ack Block #1 Start = 8 | R Gap Ack Block #1 End = 8 |
+------------------------------+------------------------------+
| R Gap Ack Block #2 Start = 11| R Gap Ack Block #2 End = 12 |
+------------------------------+------------------------------+
|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 |
+------------------------------+------------------------------+
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|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 Num of R Gap Ack
Blocks is zero indicating all reported out-of-order TSNs are nr-gap-
acked.
+--------------------------------+-------------------------------+
| Type = 0x10 | 0x00 | Chunk Length = 32 |
+--------------------------------+-------------------------------+
| Cumulative TSN Ack = 3 |
+--------------------------------+-------------------------------+
| a_rwnd = 4000 |
+--------------------------------+-------------------------------+
| Num of R 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. All other out-of-order data is (R)enegable,
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and SHOULD be included in R Gap Ack Block field of the outgoing NR-
SACK.
Consider three types of data receiver:
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 1)
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 R Gap Ack Blocks.
Fill the Number of R Gap Ack Blocks (N) field, R Gap Ack Block #i
Start, and R Gap Ack Block #i End where i goes from 1 to N.
1C) Set the Number of NR Gap Ack Blocks (M) field to 0.
CASE-2:
2A) Identify the TSNs received out-of-order.
2B) For the received out-of-order TSNs, check the (U)nordered bit of
each TSN. Tag unordered TSNs as NR.
2C) For each stream, also identify the TSNs received out-of-order
but are in-sequence within that stream. Tag those in-sequence
TSNs as NR.
2D) Tag all out-of-order data that is not NR as (R)enegable.
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2E) For those TSNs tagged as (R)enegable, identify the (R)enegable
Blocks. Fill the Number of R Gap Ack Blocks(N) field, R Gap Ack
Block #i Start, and R Gap Ack Block #i End where i goes from 1 to
N.
2F) 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.
3B) Set the Number of R Gap Ack Blocks (N) field to 0.
3C) For these out-of-order TSNs, identify the NR Gap Ack 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.
RFC4960 states that the SCTP endpoint MUST report as many Gap Ack
Blocks as can fit in a single SACK chunk limited by the current path
MTU. When using NR-SACKs, the SCTP endpoint SHOULD fill as many R
Gap Ack Blocks and NR Gap Ack Blocks starting from the Cumulative TSN
Ack value as can fit in a single NR-SACK chunk limited by the current
path MTU. If space remains, the SCTP endpoint SHOULD fill as many
Duplicate TSNs as possible starting from Cumulative TSN Ack value.
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 listed in either R Gap
Ack Block(s) or NR Gap Ack Block(s) SHOULD be processed as would be
TSNs included in Gap Ack Block(s) of a SACK chunk. All TSNs in all
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NR Gap Ack Blocks 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.
Although R Gap Ack Blocks and NR Gap Ack Blocks SHOULD be disjoint
sets, NR-SACK processing SHOULD work if an NR-SACK chunk has a TSN
listed in both an R Gap Ack Block and an NR Gap Ack Block. In this
case, the TSN SHOULD be treated as Non-Renegable.
Implementation Note:
Most of NR-SACK processing at the data sender can be implemented by
using the same routines as in SACK that process the cum ack and the
gap ack(s), followed by removal of nr-gap-acked DATA chunks from the
retransmission queue. However, with NR-SACKs, as out-of-order DATA
is sometimes removed from the retransmission queue, the gap ack
processing routine should recognize that the data sender's
retransmission queue has some transmitted data removed. 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
This document does not add any security considerations to those
specified in [RFC4960].
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
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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, "Non-Renegable Selective
Acknowledgments (NR-SACKs) for SCTP", International
Conference on Network Protocols (ICNP) 2008, Orlando,
October 2008 .
Authors' Addresses
Preethi Natarajan
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA
Phone:
Email: prenatar@cisco.com
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Paul D. Amer
University of Delaware
Computer and Information Sciences Department
Newark, DE 19716
USA
Phone: 302-831-1944
Email: amer@cis.udel.edu
Ertugrul Yilmaz
University of Delaware
Computer and Information Sciences Department
Newark, DE 19716
USA
Phone:
Email: yilmaz@udel.edu
Randall R. Stewart
Researcher
Chapi, SC 29036
USA
Phone: 202-595-8626
Email: randall@lakerest.net
Janardhan Iyengar
Franklin & Marshall College
Math and Computer Science
PO Box 3003
Lancaster, PA 17604-3003
USA
Phone: 717-358-4774
Email: jiyengar@fandm.edu
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