Internet DRAFT - draft-proshin-tsvwg-sctp-rtx-bit
draft-proshin-tsvwg-sctp-rtx-bit
Internet Engineering Task Force M. Proshin
Internet-Draft Ericsson
Updates: 4960 (if approved) June 01, 2020
Intended status: Standards Track
Expires: December 3, 2020
Retransmit bit for SCTP DATA, I-DATA and SACK
draft-proshin-tsvwg-sctp-rtx-bit-03
Abstract
This document defines a method which helps an SCTP sender to
understand when a received SACK acknowledges the original
transmission of a TSN or its retransmission. It is done by
specifying a new bit, called Retransmit bit (R-bit), in the header of
DATA, I-DATA and SACK chunks. The bit is used when a TSN is
retransmitted and returned back in the acknowledgement. This
document updates [RFC4960] if approved.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Updates in SCTP Chunks Header . . . . . . . . . . . . . . . . 3
3.1. R-bit in DATA Chunk Header . . . . . . . . . . . . . . . 3
3.2. R-bit in I-DATA Chunk Header . . . . . . . . . . . . . . 4
3.3. R-bit in SACK Chunk Header . . . . . . . . . . . . . . . 4
4. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Negotiation . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Sender Side Considerations . . . . . . . . . . . . . . . 6
4.3. Receiver Side Considerations . . . . . . . . . . . . . . 7
4.4. Processing of SACK with and without R-bit . . . . . . . . 7
5. R-bit vs Duplicate TSN for Detection of Spurious
Retransmission . . . . . . . . . . . . . . . . . . . . . . . 8
6. Interoperability Considerations . . . . . . . . . . . . . . . 9
7. Socket API Considerations . . . . . . . . . . . . . . . . . . 9
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
10. Security Considerations . . . . . . . . . . . . . . . . . . . 11
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
11.1. Normative References . . . . . . . . . . . . . . . . . . 11
11.2. Informative References . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
SCTP which is defined in [RFC4960] is a reliable message-oriented
protocol. The SCTP sender splits user messages to DATA chunks and
sends them to the receiver. The SCTP receiver uses the SACK chunk to
acknowledge incoming data. The reliability in SCTP is achieved by
the retransmission of DATA chunks which were not acknowledged.
If a DATA chunk has been retransmitted at least once, at SACK
reception SCTP cannot understand if the SACK was sent in response to
the originally sent DATA or retransmitted one. Thus, due to that
ambiguity, [RFC4960] prohibits making RTT measurements. Some other
SCTP mechanisms such as loss recovery and congestion control are not
accurate in that case either.
This document describes a simple extension of the DATA and SACK
chunks by a new bit, so called Retransmit bit (R-bit). The sender
sets the R-bit in the DATA chunk header when it retransmits a DATA
and the receiver sets it in the SACK chunk header when a DATA with
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R-bit is acknowledged. The sender can now distinguish when a SACK
acknowledges the originally sent DATA or retransmitted one. The
extension requires support by the sender and the receiver.
The mechanism described in this document is equally relevant for
I-DATA chunk which is introduced in [RFC8260].
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [[RFC8174]] when, and only when, they appear in all
capitals, as shown here.
3. Updates in SCTP Chunks Header
3.1. R-bit in DATA Chunk Header
Figure 1 describes the extended DATA chunk header.
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 = 0 | Res |R|I|U|B|E| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TSN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Stream Identifier | Stream Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Protocol Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ User Data /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Extended DATA chunk
The only difference between the DATA chunk in Figure 1 and the DATA
chunk defined in [RFC4960] is the addition of the R-bit in the flags
field of the DATA chunk header. [RFC4960] specified that bit as
Reserved and that it should be set to 0 by the sender and ignored by
the receiver.
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3.2. R-bit in I-DATA Chunk Header
Figure 2 describes the extended DATA chunk header.
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 |R|I|U|B|E| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TSN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Stream Identifier | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Protocol Identifier / Fragment Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ User Data /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Extended I-DATA chunk
The only difference between the I-DATA chunk in Figure 2 and the
I-DATA chunk defined in [RFC8260] is the addition of the R-bit in the
flags field of the I-DATA chunk header. [RFC8260] specified that bit
as Reserved and that it should be set to 0 by the sender and ignored
by the receiver.
3.3. R-bit in SACK Chunk Header
Figure 3 describes the extended SACK chunk header.
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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 = 3 | Reserved |R| Chunk Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cumulative TSN Ack |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertised Receiver Window Credit (a_rwnd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Gap Ack Blocks = N | Number of Duplicate TSNs = X |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Gap Ack Block #1 Start | Gap Ack Block #1 End |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ ... \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Gap Ack Block #N Start | Gap Ack Block #N End |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Duplicate TSN 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ ... \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Duplicate TSN X |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Extended SACK chunk
The only difference between the SACK chunk in Figure 3 and the SACK
chunk defined in [RFC4960] is the addition of the R-bit in the flags
field of the SACK chunk header. [RFC4960] specified that bit as
Reserved and that it should be set to 0 by the sender and ignored by
the receiver.
4. Procedures
4.1. Negotiation
R-bit MUST NOT be used unless both SCTP peers negotiated its support.
The following new optional parameter is added to the INIT and INIT
ACK chunks to negotiate R-bit support during association setup:
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+----------------+-------------------------------------------+
| Parameter Type | Parameter Name |
+----------------+-------------------------------------------+
| 0x8100 | Retransmit Bit Supported (RBIT-SUPPORTED) |
+----------------+-------------------------------------------+
Table 1
The parameter format is the following:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameter Type = 0x8100 | Parameter Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Format of RBIT-SUPPORTED
Parameter Type: 2 bytes (unsigned integer)
This value MUST be set to 0x8100 (33024).
Parameter Length: 2 bytes (unsigned integer)
This value MUST be set to 4.
The RBIT-SUPPORTED parameter MAY be included once in the INIT or INIT
ACK chunk if the sender wants to inform its peer that it supports
R-bit.
The new parameter type is encoded so that it requires the receiver to
skip it and continue processing if the parameter is not recognized
according to [RFC4960].
4.2. Sender Side Considerations
SCTP MUST NOT set the R-bit when it sends a DATA or I-DATA chunk
first time.
If R-bit support is negotiated as described in Section 4.1, SCTP
SHOULD set the R-bit every time it retransmits a DATA or I-DATA
chunk. This is regardless of if the chunk is retransmitted on the
same path or on an alternative one.
Note that it is possible that the same SCTP packet includes DATA or
I-DATA chunks with and without the R-bit set in case when SCTP
bundles chunks which are marked for retransmission with chunks which
are sent first time. This is aligned with [RFC4960] which allows
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bundling of DATA chunks marked for retransmission with new DATA
chunks.
IMPLEMENTATION NOTE: According to [RFC4960] new DATA chunks always
follow DATA chunks marked for retransmission when bundled in one
packet.
4.3. Receiver Side Considerations
SCTP MUST NOT set the R-bit when it sends a SACK which acknowledges a
DATA or I-DATA chunk without the R-bit set. The delay for a SACK
without the R-bit set is defined according to [RFC4960].
When SCTP receives a packet with DATA or I-DATA chunk(s) with the
R-bit set, it MUST immediately respond with a SACK with the R-bit set
acknowledging only DATA or I-DATA chunks where the R-bit was set. If
the packet also contains DATA or I-DATA chunk(s) without the R-bit
set, SCTP MUST NOT acknowledge them in the same SACK chunk.
TBD: SACK with the R-bit bundled with SACK without the R-bit? It may
be useful.
4.4. Processing of SACK with and without R-bit
If a DATA or I-DATA was retransmitted and the corresponding SACK is
received, SCTP can distinguish if the SACK acknowledges the original
transmission or retransmission by checking the R-bit in the SACK.
SCTP mechanisms which can be improved by that information include,
but are not limited to, the following:
o RTO Calculation: [RFC4960] refers to Karn's algorithm and
prohibits SCTP to make RTT measurements using packets that were
retransmitted and for which it is ambiguous whether the reply was
for the original transmission or retransmission(s).
o Path Failure Detection: [RFC4960] specifies that the sender may
choose not to clear the path error counter if there is undesirable
ambiguity when a DATA is retransmitted on an alternative path.
o SCTP-PF Operation in [RFC7829]: additionally to the path error
counter case described in the previous bullet [RFC7829] also does
not recommend to move a destination address in PF state back to
the active state in case of ambiguity.
o Detection of spurious retransmissions: using R-bit SCTP can detect
spurious retransmissions. Namely, if a DATA was retransmitted and
SACK acknowledging it does not include R-bit, it means that the
retransmission was spurious. Note that this is valid even if a
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DATA was retransmitted multiple times which makes this method more
effective than detecting of spurious retransmissions based on
DSACK. When a spurious retransmission is detected, SCTP
implementation may:
* Choose to revert the congestion control state.
* Choose to adjust RTO settings such as the RTO.Min value to
mitigate further spurious retransmissions.
* Indicate the SCTP user.
o SCTP latency of retransmitted data: If the original DATA is lost,
the SCTP receiver will immediately acknowledge the retransmitted
DATA.
o Calculation of Maximum Ack Delay: SCTP implementations can support
a technique for calculating of Maximum Ack Delay in run-time which
is impossible to do properly in case of retransmissions. With
R-bit SCTP can distinguish if the SACK acknowledges the original
transmission or retransmission and can measure the delay even for
a retransmitted DATA.
o Measurement of packet loss: R-bit can be used for passive loss
rate calculation.
TBD: dup TSN but without R-bit: SACK loss or reordering: Can be used
somehow?
Note that this document does not solve the problem when the same DATA
or I-DATA chunk is retransmitted multiple times. In that case, when
SCTP receives a SACK without the R-bit set, it can ensure that the
SACK acknowledges the original transmission but when SCTP receives a
SACK with the R-bit set, it cannot distinguish which retransmission
is actually acknowledged. Such limitation is not considered as
severe because multiple retransmissions of the same DATA or I-DATA is
a corner case and, if it happens, SCTP transmission is anyway
inefficient.
5. R-bit vs Duplicate TSN for Detection of Spurious Retransmission
The SACK chunk according to [RFC4960] contains the Duplicate TSN
field which is used by the receiver to indicate TSNs received
multiple times. This could happen due to spurious retransmissions or
if packets were duplicated in the network between endpoints. The
Duplicate TSN field in the SACK chunk can also be used by the sender
to detect spurious retransmissions in some cases. However, the
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mechanism based on the Duplicate TSN field would have serious
limitations compared to the mechanism based on R-bit:
o With R-bit the SCTP sender has an exclusive match between DATA and
SACK while in case of the Duplicate TSN it is not guaranteed.
Thus, if the original or retranmitted DATA is lost or one of the
SACKs is lost or the packets were retransmitted, the SCTP sender
cannot rely on the Duplicate TSN field.
o Even in those cases where the sender could rely on the Duplicate
TSN, it would need to wait the second SACK to detect the spurious
retransmission, while with R-bit, the sender can detect it as soon
as the first SACK is received.
o In case of the Duplicate TSN the SCTP sender needs to keep
information about the retransmitted TSN until the second SACK is
received or during some time period which impacts memory usage and
SCTP performance and complicates implementation.
6. Interoperability Considerations
This document does not introduce any interoperability issues.
Section 4.1 requires both ends to negotiate R-bit support before its
usage. [RFC4960] requires the receiver of a DATA or SACK chunk with
the R-bit set to ignore the bit if it is not recognized. [RFC8260]
requires the receiver of an I-DATA chunk with the R-bit set to ignore
the bit if it is not recognized.
7. Socket API Considerations
This document does not address any changes to the socket API defined
in [RFC6458].
8. Acknowledgements
TBD
9. IANA Considerations
[NOTE to RFC-Editor:
"RFCXXXX" is to be replaced by the RFC number you assign this
document.
]
IANA should assign 33024 (0x8100) as a new parameter type to SCTP.
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Following the chunk flag registration procedure defined in [RFC6096],
IANA should register a new bit, the R-bit, for the DATA chunk. The
suggested value is 0x10 and the reference should be RFCXXXX.
This requires an update of the "DATA Chunk Flags" registry for SCTP:
+------------------+-----------------+-----------+
| Chunk Flag Value | Chunk Flag Name | Reference |
+------------------+-----------------+-----------+
| 0x01 | E bit | [RFC4960] |
| 0x02 | B bit | [RFC4960] |
| 0x04 | U bit | [RFC4960] |
| 0x08 | I bit | [RFC7053] |
| 0x10 | R bit | RFCXXXX |
| 0x20 | Unassigned | |
| 0x40 | Unassigned | |
| 0x80 | Unassigned | |
+------------------+-----------------+-----------+
Table 2
Following the chunk flag registration procedure defined in [RFC6096],
IANA should register a new bit, the R-bit, for the SACK chunk. The
suggested value is 0x01 and the reference should be RFCXXXX.
This requires an update of the "SACK Chunk Flags" registry for SCTP:
+------------------+-----------------+-----------+
| Chunk Flag Value | Chunk Flag Name | Reference |
+------------------+-----------------+-----------+
| 0x01 | R bit | RFCXXXX |
| 0x02 | Unassigned | |
| 0x04 | Unassigned | |
| 0x08 | Unassigned | |
| 0x10 | Unassigned | |
| 0x20 | Unassigned | |
| 0x40 | Unassigned | |
| 0x80 | Unassigned | |
+------------------+-----------------+-----------+
Table 3
Following the chunk flag registration procedure defined in [RFC6096],
IANA should register a new bit, the R-bit, for the I-DATA chunk. The
suggested value is 0x10 and the reference should be RFCXXXX.
This requires an update of the "I-DATA Chunk Flags" registry for
SCTP:
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+------------------+-----------------+-----------+
| Chunk Flag Value | Chunk Flag Name | Reference |
+------------------+-----------------+-----------+
| 0x01 | E bit | [RFC8260] |
| 0x02 | B bit | [RFC8260] |
| 0x04 | U bit | [RFC8260] |
| 0x08 | I bit | [RFC8260] |
| 0x10 | R bit | RFCXXXX |
| 0x20 | Unassigned | |
| 0x40 | Unassigned | |
| 0x80 | Unassigned | |
+------------------+-----------------+-----------+
Table 4
10. Security Considerations
This document does not introduce any additional security
considerations in addition to the ones described in [RFC4960] and
[RFC8260].
11. References
11.1. Normative References
[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>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<https://www.rfc-editor.org/info/rfc4960>.
[RFC8260] Stewart, R., Tuexen, M., Loreto, S., and R. Seggelmann,
"Stream Schedulers and User Message Interleaving for the
Stream Control Transmission Protocol", RFC 8260,
DOI 10.17487/RFC8260, November 2017,
<https://www.rfc-editor.org/info/rfc8260>.
11.2. Informative References
[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>.
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[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>.
[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>.
[RFC7829] Nishida, Y., Natarajan, P., Caro, A., Amer, P., and K.
Nielsen, "SCTP-PF: A Quick Failover Algorithm for the
Stream Control Transmission Protocol", RFC 7829,
DOI 10.17487/RFC7829, April 2016,
<https://www.rfc-editor.org/info/rfc7829>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
Author's Address
Maksim Proshin
Ericsson
Kistavaegen 25
Stockholm 164 80
Sweden
Email: mproshin@tieto.mera.ru
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