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Network Working Group R. Stewart
Internet-Draft M. Ramalho
Expires: February 20, 2004 Cisco Systems, Inc.
Q. Xie
Motorola, Inc.
M. Tuexen
Univ. of Applied Sciences Muenster
P. Conrad
Temple University
August 22, 2003
SCTP Partial Reliability Extension
draft-ietf-tsvwg-prsctp-01.txt
Status of this Memo
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This Internet-Draft will expire on February 20, 2004.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This memo describes an extension to the Stream Control Transmission
Protocol (SCTP) RFC2960 [5] that allows an SCTP endpoint to signal to
its peer that it should move the cumulative ack point forward. When
both sides of an SCTP association support this extension, it can be
used by an SCTP implementation to provide partially reliable data
transmission service to an upper layer protocol. This memo describes
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(1) the protocol extensions, which consist of a new parameter for
INIT and INIT ACK, and a new FORWARD TSN chunk type (2) one example
partially reliable service that can be provided to the upper layer
via this mechanism.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Overview of Protocol Extensions . . . . . . . . . . . . . . 3
1.2 Overview of New Services Provided to the Upper Layer . . . . 3
1.3 Benefits of PR-SCTP . . . . . . . . . . . . . . . . . . . . 4
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Protocol Changes to support PR-SCTP . . . . . . . . . . . . 7
3.1 Forward-TSN-Supported Parameter For INIT and INIT ACK . . . 7
3.2 Forward Cumulative TSN Chunk Definition (FORWARD TSN) . . . 7
3.3 Negotiation of Forward-TSN-Supported parameter . . . . . . . 8
3.3.1 Sending Forward-TSN-Supported param in INIT . . . . . . . . 8
3.3.2 Receipt of Forward-TSN-Supported param in INIT or
INIT-ACK . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.3.3 Receipt of Op. Error for Forward-TSN-Supported Param . . . . 9
3.4 Definition of "abandoned" in the context of PR-SCTP . . . . 10
3.5 Sender Side Implementation of PR-SCTP . . . . . . . . . . . 10
3.6 Receiver Side Implementation of PR-SCTP . . . . . . . . . . 13
4. Services provided by PR-SCTP to the upper layer . . . . . . 16
4.1 PR-SCTP Service Definition for "timed reliability" . . . . . 16
4.2 PR-SCTP Association Establishment . . . . . . . . . . . . . 18
4.3 Guidelines for defining other PR-SCTP Services . . . . . . . 19
4.4 Usage Notes . . . . . . . . . . . . . . . . . . . . . . . . 20
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 21
6. Security Considerations . . . . . . . . . . . . . . . . . . 22
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 23
References . . . . . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . 26
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1. Introduction
This memo describes an extension to the Stream Control Transmission
Protocol (SCTP) RFC2960 [5] that allows an SCTP sender to signal to
its peer that it should no longer expect to receive one or more DATA
chunks.
1.1 Overview of Protocol Extensions
The protocol extension described in this document consists of two new
elements:
1. a single new parameter in the INIT/INIT-ACK exchange that
indicates whether the endpoint supports the extension
2. a single new chunk type, FORWARD TSN, that indicates that the
receiver should move its cumulative ack point forward (possibly
skipping past one or more Data chunks that may not yet have been
received and/or acknowledged.)
1.2 Overview of New Services Provided to the Upper Layer
When this extension is supported by both sides of an SCTP
association, it can be used to provide partially reliable transport
service over an SCTP association. We define partially reliable
transport service as a service that allows the user to specify, on a
per message basis, the rules governing how persistent the transport
service should be in attempting to send the message to the receiver.
One example of partially reliable service is specified in this
document, namely a "timed reliability" service. This service allows
the service user to indicate a limit on the duration of time that the
sender should try to transmit/retransmit the message (this is a
natural extension of the "lifetime" parameter already in the base
protocol).
In addition to this example, we will also show that defining the
semantics of a particular partially reliable service involves two
elements, namely:
1. how the service user indicates the level of reliability required
for a particular message, and
2. how the sender side implementation uses that reliability level to
determine when to give up on further retransmissions of that
message.
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Note that other than the fact that the FORWARD-TSN chunk is required,
neither of these two elements impacts the "on-the-wire" protocol;
only the API, and the sender side implementation is affected by the
way in which the service is defined to the upper layer. Therefore,
in principle, it is feasible to implement many varieties of partially
reliable services in a particular SCTP implementation without
changing the on-the-wire protocol. Also, the SCTP receiver does not
necessarily need to know which semantics of partially reliable
service are being used by the sender, since the receiver's only role
is to correctly interpret FORWARD TSN chunks, thereby skipping past
messages that the sender has decided to no longer transmit (or
retransmit).
Nevertheless, it is recommended that a limited number of standard
definitions of partially reliable services be standardized by the
IETF so that that the designers of IETF application layer protocols
can match the requirements of their upper layer protocols to standard
service definitions provided by a particular SCTP implementation.
One such definition, "timed reliability" is included in this
document. Given the extensions proposed in this document, other
definitions may be standardized as the need arises without further
changes to the on-the-wire protocol.
1.3 Benefits of PR-SCTP
Hereafter, we use the notation "PR-SCTP" to refer to the SCTP
protocol extended as defined in this document.
The following are some of the advantages for integrating partially
reliable data service into SCTP, i.e., benefits of PR-SCTP:
1. Some application layer protocols may benefit from being able to
use a single SCTP association to carry both reliable content, --
such as text pages, billing and accounting information, setup
signaling -- and unreliable content, e.g. state that is highly
sensitive to timeliness, where generating a new packet is more
advantageous than transmitting an old one [1].
2. Partially reliable data traffic carried by PR-SCTP will enjoy the
same communication failure detection and protection capabilities
as the normal reliable SCTP data traffic does. This includes the
ability to: - quickly detect a failed destination address; -
fail-over to an alternate destination address, and; - be notified
if the data receiver becomes unreachable.
3. In addition to providing unordered unreliable data transfer as
UDP does, PR-SCTP can provide ordered unreliable data transfer
service.
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4. PR-SCTP employs the same congestion control and congestion
avoidance for all data traffic, whether reliable or partially
reliable - this is very desirable since SCTP enforces
TCP-friendliness (unlike UDP.)
5. Because of the chunk bundling function of SCTP, reliable and
unreliable messages can be multiplexed over a single PR-SCTP
association. Therefore, the number of IP datagrams (and hence
the network overhead) can be reduced versus having to send these
different types of data using separate protocols. Additionally,
this multiplexing allows for port savings versus using different
ports for reliable and unreliable connections.
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2. Conventions
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when
they appear in this document, are to be interpreted as described in
RFC2119 [3].
Comparisons and arithmetic on TSNs are governed by the rules in
Section 1.6 of RFC2960 [5].
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3. Protocol Changes to support PR-SCTP
3.1 Forward-TSN-Supported Parameter For INIT and INIT ACK
The following new OPTIONAL parameter is added to the INIT and INIT
ACK chunks.
Parameter Name Status Type Value
-------------------------------------------------------------
Forward-TSN-Supported OPTIONAL 0xC000
At the initialization of the association, the sender of the INIT or
INIT ACK chunk shall include this OPTIONAL parameter to inform its
peer that it is able to support the Forward TSN chunk. The format of
this parameter is defined as follows:
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 = 0xC000 | Parameter Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 16 bit u_int
0xC000, indicating Forward-TSN-Supported parameter
Length: 16 bit u_int
Indicate the size of the parameter i.e., 4.
3.2 Forward Cumulative TSN Chunk Definition (FORWARD TSN)
The following new chunk type is defined:
Chunk Type Chunk Name
------------------------------------------------------
0xC0 Forward Cumulative TSN (FORWARD TSN)
This chunk shall be used by the data sender to inform the data
receiver to adjust its cumulative received TSN point forward because
some missing TSNs are associated with data chunks that SHOULD NOT be
transmitted or retransmitted by the sender.
Forward Cumulative TSN chunk has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Type = 0xC0 | Flags = 0x00 | Length = Variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| New Cumulative TSN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Stream-1 | Stream Sequence-1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ /
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Stream-N | Stream Sequence-N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Chunk Flags:
Set to all zeros on transmit and ignored on receipt.
New Cumulative TSN: 32 bit u_int
This indicates the new cumulative TSN to the data receiver. Upon
the reception of this value, the data receiver MUST consider
any missing TSNs earlier than or equal to this value as received
and stop reporting them as gaps in any subsequent SACKs.
Stream-N: 16 bit u_int
This field holds a stream number that was skipped by this
FWD-TSN.
Stream Sequence-N: 16 bit u_int
This field holds the sequence number associated with the stream
that was skipped. The stream sequence field holds the largest stream
sequence number in this stream being skipped. The receiver of
the FWD-TSN's can use the Stream-N and Stream Sequence-N fields
to enable delivery of any stranded TSN's that remain on the stream
re-ordering queues. This field MUST NOT report TSN's cooresponding
to DATA chunk that are marked as unordered. For ordered DATA
chunks this field MUST be filled in.
3.3 Negotiation of Forward-TSN-Supported parameter
3.3.1 Sending Forward-TSN-Supported param in INIT
If an SCTP endpoint supports the FORWARD TSN chunk, then any time it
sends an INIT during association establishment, it SHOULD include the
Forward-TSN-supported parameter in the INIT chunk to indicate this
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fact to its peer.
3.3.2 Receipt of Forward-TSN-Supported param in INIT or INIT-ACK
When a receiver of an INIT detects a Forward-TSN-Supported parameter,
and does not support the Forward-TSN chunk type, the receiver SHOULD
treat this parameter as an invalid or unrecognized parameter and
respond to the data sender with an unrecognized parameter in the
INIT-ACK, following the rules defined in Section 3.3.3 of RFC2960
[5].
When a receiver of an INIT-ACK detects a Forward-TSN-Supported
parameter, and does not support the Forward-TSN chunk type, the
receiver SHOULD treat this parameter as an invalid or unrecognized
parameter and respond to the data sender with an unrecognized
parameter error, following the rules defined in Section 3.3.3 of
RFC2960 [4]. This error SHOULD be reported in an ERROR chunk bundled
with the COOKIE-ECHO.
When a receiver of an INIT detects a Forward-TSN-Supported parameter,
and does support the Forward-TSN chunk type, the receiver SHOULD
respond with a Forward-TSN-supported parameter in the INIT-ACK chunk.
When an endpoint that supports the FORWARD TSN chunk receives an INIT
that does not contain the Forward-TSN-Supported Parameter, that
endpoint:
o MAY include the Forward-TSN-Supported parameter in the INIT-ACK,
o SHOULD record the fact that the peer does not support the FORWARD
TSN chunk,
o MUST NOT send a FORWARD TSN chunk at any time during the
associations life,
o SHOULD inform the upper layer, if the upper layer has requested
such notification.
3.3.3 Receipt of Op. Error for Forward-TSN-Supported Param
When an SCTP endpoint that desires to use the FORWARD TSN chunk
feature for partially reliable data transfer receives an operational
error from the remote endpoint (either bundled with the COOKIE or as
a unrecognized parameter in the INIT-ACK), indicating that the remote
endpoint does not recognize the Forward-TSN-Supported parameter, the
local endpoint SHOULD inform its upper layer of the remote endpoint's
inability to support partially reliable data transfer.
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The local endpoint may then choose to either:
1) end the initiation process (in cases where the initiation
process has already ended the endpoint may need to send an ABORT),
in consideration of the peer's inability to supply the requested
features for the new association, or
2) continue the initiation process (in cases where the initiation
process has already completed the endpoint MUST just mark the
association as not supporting partial reliability), but with the
understanding that partially reliable data transmission is not
supported. In this case, the endpoint receiving the operational
error SHOULD note that the FORWARD TSN chunk is not supported, and
MUST NOT transmit a FORWARD TSN chunk at any time during the life
of the association.
3.4 Definition of "abandoned" in the context of PR-SCTP
At some point, a sending PR-SCTP implementation MAY determine that a
particular data chunk SHOULD NOT be transmitted or retransmitted
further, in accordance with the rules governing some particular
PR-SCTP service definition (such as the definition of "timed
reliability" in Section 4.1.) For purposes of this document, we
define the term "abandoned" to refer to any data chunk about which
the SCTP sender has made this determination.
Each PR-SCTP service defines the rules for determining when a TSN is
"abandoned", and accordingly, the rules that govern how, whether, and
when to "abandon" a TSN may vary from one service definition to
another. However, the rules governing the actions taken when a TSN
is "abandoned" do NOT vary between service definitions; these rules
are included in Section 3.5.
3.5 Sender Side Implementation of PR-SCTP
The sender side implementation of PR-SCTP is identical to that of the
base SCTP protocol, except for:
o actions a sending side PR-SCTP implementation must take when a TSN
is "abandoned" (as per the rules of whatever PR-SCTP service
definition is in effect)
o special actions that a PR-SCTP implementation must take upon
receipt of SACK
o rules governing generation of FORWARD TSN chunks.
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In detail, these exceptions are as follows:
A1) The sender maintains an "Advanced.Peer.Ack.Point" for each peer
to track a theoretical cumulative TSN point of the peer (Note,
this is a _new_ protocol variable and its value is NOT necessarily
the same as the SCTP "Cumulative TSN Ack Point" as defined in
Section 1.4 of RFC2960 [5]) and discussed throughout that
document.
A2) From time to time, as governed by the rules of a particular
PR-SCTP service definition (see Section 4), the SCTP data sender
may make a determination that a particular data chunk that has
already been assigned a TSN SHOULD be "abandoned".
When a data chunk is "abandoned", the sender MUST treat the data
chunk as being finally acked and no longer outstanding.
The sender MUST NOT credit an "abandoned" data chunk to the
partial_bytes_acked as defined in Section 7.2.2 of RFC2960 [5],
and MUST NOT advance the cwnd based on this "abandoned" data
chunk.
A3) When a TSN is "abandoned", if it is part of a fragmented message,
all other TSN's within that fragmented message MUST be abandoned
at the same time.
A4) Whenever the data sender receives a SACK from the data receiver,
it MUST first process the SACK using the normal procedures as
defined in Section 6.2.1 of RFC2960 [5].
The data sender MUST then perform the following additional steps :
C1) Let SackCumAck be the Cumulative TSN ACK carried in the
received SACK.
If (Advanced.Peer.Ack.Point < SackCumAck), then update
Advanced.Peer.Ack.Point to be equal to SackCumAck.
C2) Try to further advance the "Advanced.Peer.Ack.Point" locally,
that is, to move "Advanced.Peer.Ack.Point" up as long as the
chunk next in the out-queue space is marked as "abandoned" as
shown in the following example:
Assuming that a SACK arrived with the Cumulative TSN ACK =
102 and the Advanced.Peer.Ack.Point is updated to this
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value:
out-queue at the end of ==> out-queue after Adv.Ack.Point
normal SACK processing local advancement
... ...
Adv.Ack.Pt-> 102 acked 102 acked
103 abandoned 103 abandoned
104 abandoned Adv.Ack.P-> 104 abandoned
105 105
106 acked 106 acked
... ...
In this example, the data sender successfully advanced the
"Advanced.Peer.Ack.Point" from 102 to 104 locally.
C3) If, after step C1 and C2, the "Advanced.Peer.Ack.Point" is
greater than the Cumulative TSN ACK carried in the received
SACK, the data sender MUST send the data receiver a FORWARD TSN
chunk containing the latest value of the
"Advanced.Peer.Ack.Point".
C4) For each "abandoned" TSN the sender of the FORWARD TSN MUST
determine if the chunk has a valid stream and sequence number
(i.e., it was ordered). If the chunk has a valid stream and
sequence number the sender MUST include the stream and sequence
number in the FORWARD TSN. This information will enable the
receiver to easily find any stranded TSN's waiting on stream
reorder queues. Each stream SHOULD only be reported once; this
means that if multiple abandoned messages occur in the same
stream then only the highest abandoned stream sequence number
is reported. If the total size of the FORWARD TSN does NOT fit
in a single MTU then the sender of the FORWARD TSN SHOULD lower
the Advanced.Peer.Ack.Point to the last TSN that will fit in a
single MTU.
C5) If a FORWARD TSN is sent, the sender MUST assure that at least
one T3-rtx timer is running.
A5) Any time the T3-rtx timer expires, on any destination, the sender
SHOULD try to advance the "Advanced.Peer.Ack.Point" by following
the procedures outlined in C1 - C5.
The following additional rules govern the generation of FORWARD TSN
chunks:
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F1) An endpoint MUST NOT use the FORWARD TSN for any purposes other
than circumstances described in this document.
F2) The data sender SHOULD always attempt to bundle an outgoing
FORWARD TSN with outbound DATA chunks for efficiency.
A sender MAY even choose to delay the sending of the FORWARD TSN
in the hope of bundling it with an outbound DATA chunk.
IMPLEMENTATION NOTE: An implementation may wish to limit the
number of duplicate FORWARD TSN chunks it sends by either only
sending a duplicate FORWARD TSN every other SACK or waiting a full
RTT before sending a duplicate FORWARD TSN.
IMPLEMENTATION NOTE: An implementation may allow the maximum delay
for generating a FORWARD TSN to be configured either statically or
dynamically in order to meet the specific timing requirements of
the protocol being carried, but see the next rule:
F3) Any delay applied to the sending of FORWARD TSN chunk SHOULD NOT
exceed 200ms and MUST NOT exceed 500ms. In other words an
implementation MAY lower this value below 500ms but MUST NOT raise
it above 500ms.
NOTE: Delaying the sending of FORWARD TSN chunks may cause delays
in the receiver's ability to deliver other data being held at the
receiver for re-ordering.
F4) The detection criterion for out-of-order SACKs MUST remain the
same as stated in RFC2960, that is, a SACK is only considered
out-of-order if the Cumulative TSN ACK carried in the SACK is
earlier than that of the previous received SACK (i.e., the
comparison MUST NOT be made against "Advanced.Peer.Ack.Point").
F5) If the decision to "abandon" a chunk is made, no matter how such
a decision is made, the appropriate congestion adjustment MUST be
made as specified in RFC2960 if the chunk would have been marked
for retransmission later (e.g. either by T3-Timeout or by Fast
Retransmit).
3.6 Receiver Side Implementation of PR-SCTP
The receiver side implementation of PR-SCTP at an SCTP endpoint A is
capable of supporting any PR-SCTP service definition used by the
sender at endpoint B, even if that service definition is not
supported by the sending side functionality of host A. All that is
necessary is that the receiving side correctly handle the
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Forward-TSN-Supported parameter as specified in Section 3.3, and
correctly handle the receipt of FORWARD TSN chunks as specified
below.
DATA chunk arrival at a PR-SCTP receiver proceeds exactly as for DATA
chunk arrival at a base protocol SCTP receiver---that is, the
receiver MUST perform the same TSN handling including duplicate
detection, gap detection, SACK generation, cumulative TSN
advancement, etc. as defined in RFC2960 [5]---with the following
exceptions and additions.
When a FORWARD TSN chunk arrives, the data receiver MUST first update
its cumulative TSN point to the value carried in the FORWARD TSN
chunk, and then MUST further advance its cumulative TSN point locally
if possible, as shown by the following example:
Assuming that the new cumulative TSN carried in the arrived
FORWARD TSN is 103:
in-queue before processing in-queue after processing the
the FORWARD TSN ==> the FORWARD TSN and further
advancement
cum.TSN.Pt-> 102 received 102 --
103 missing 103 --
104 received 104 --
105 received cum.TSN.Pt-> 105 received
106 missing 106 missing
107 received 107 received
... ...
In this example, the receiver's cumulative TSN point is first
updated to 103 and then further advanced to 105.
After the above processing, the data receiver MUST stop reporting any
missing TSNs earlier than or equal to the new cumulative TSN point.
Note, if the "New Cumulative TSN" value carried in the arrived
FORWARD TSN chunk is found to be behind or at the current cumulative
TSN point, the data receiver MUST treat this FORWARD TSN as
out-of-date and MUST NOT update its Cumulative TSN. The receiver
SHOULD send a SACK to its peer (the sender of the FORWARD TSN) since
such a duplicate may indicate the previous SACK was lost in the
network.
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Any time a FORWARD TSN chunk arrives, for the purposes of sending a
SACK, the receiver MUST follow the same rules as if a DATA chunk had
been received (i.e., follow the delayed sack rules specified in
RFC2960 [5] section 6.2).
Whenever a DATA chunk arrives with the 'U' bit set to '0' (indicating
ordered delivery) and is out of order, the receiver must hold the
chunk for reordering. Since it is possible with PR-SCTP that a DATA
chunk being waited upon will not be retransmitted, special actions
will need to be taken upon the arrival of a FORWARD TSN.
In particular, during processing of a FORWARD TSN, the receiver MUST
use the stream sequence information to examine all of the listed
stream reordering queues, and immediately make available for delivery
stream sequence numbers earlier than or equal to the stream sequence
number listed inside the FORWARD TSN.
After receiving and processing a FORWARD TSN, the data receiver MUST
take cautions in updating its re-assembly queue. The receiver MUST
remove any partially reassembled message which is still missing one
or more TSNs earlier than or equal to the new cumulative TSN point.
In the event that the receiver has invoked the partial delivery API a
notification SHOULD also be generated to inform the upper layer API
that the message being partially delivered will NOT be completed.
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4. Services provided by PR-SCTP to the upper layer
As described in Section 1.2, it is feasible to implement a variety of
partially reliable transport services using the new protocol
mechanisms introduced in Section 3; introducing these new services
requires making changes only at the sending side API, and the sending
side protocol implementation. Thus, there may be a temptation to
standardize only the protocol, and leave the service definition as
"implementation specific" or leave it to be defined in
"informational" documents.
However, for those who may wish to write IETF standards for upper
layer protocols implemented over PR-SCTP, it is important to be able
to refer to a standard definition of services provided. Therefore,
this section provides an example definitions of one such service,
while also providing guidelines for the definition of additional
services as required. Each such service may be proposed as a
separate new standard.
Section 4 is organized as follows:
Section 4.1 provides the definition of one specific PR-SCTP
service: timed reliability.
Section 4.2 describes how a particular PR-SCTP service definition
is requested by the upper layer during association establishment,
and how the upper layer is notified if that request cannot be
satisfied.
Section 4.3 then provides guidelines for the specification of
PR-SCTP services other then the one defined in this memo.
Finally, Section 4.4 describes some additional usage notes that
upper layer protocol designers and implementors may find helpful.
4.1 PR-SCTP Service Definition for "timed reliability"
The "timed reliability" service is a natural extension of the
"lifetime" concept already present in the base SCTP protocol.
When this service is requested for an SCTP association, it changes
the meaning of the lifetime parameter specified in the SEND primitive
(see Section 10.1, part (E) of RFC2960 [5]; note that the parameter
is spelled "life time" in that document.)
In the base SCTP protocol, the lifetime parameter is used to avoid
sending stale data. When a lifetime value is indicated for a
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particular message, SCTP cancels the sending of this message, and
notifies the ULP if the first transmission of the data does not take
place (because of rwnd or cwnd limitations, or for any other reason)
before the lifetime expires. However, in the base protocol, if SCTP
has sent the first transmission before the lifetime expires, then the
message MUST be sent as a normal reliable message. During episodes
of congestion this is particularly unfortunate, as retransmission
wastes bandwidth that could have been used for other (non-lifetime
expired) messages.
When the "timed reliability" service is invoked, this latter
restriction is removed. Specifically, when the "timed reliability"
service is in effect, the following rules govern all messages that
are sent with a lifetime parameter:
TR1) If the lifetime parameter of a message is SCTP_LIFETIME_RELIABLE
(or unspecified) that message is treated as a normal reliable SCTP
message, just as in the base SCTP protocol.
TR2) If the lifetime parameter is not SCTP_LIFETIME_RELIABLE, then
the SCTP sender MUST treat the message just as if it were a normal
reliable SCTP message as long as the lifetime has not yet expired.
TR3) Before assigning a TSN to any message, the SCTP sender MUST
evaluate the lifetime of that message. If it is expired, the SCTP
sender MUST NOT assign a TSN to that message, but instead, SHOULD
issue a notification to the upper layer and abandon the message.
TR4) Before transmitting or retransmitting a message for which a TSN
is already assigned, the SCTP sender MUST evaluate the lifetime of
the message. If the lifetime of the message is expired, the SCTP
sender MUST "abandon" the message, as per the rules specified in
Section 3.5.
TR5) The sending SCTP MAY evaluate the lifetime of messages at
anytime. Expired messages that have not been assigned a TSN MAY be
handled as per rule TR3. Expired messages that HAVE been assigned
a TSN MAY be handled as per rule TR4.
TR6) The sending application MUST NOT change the lifetime parameter
once the message is passed to the sending SCTP.
Implementation Note: Rules TR1 through TR4 are designed in such as
way to avoid requiring the implementer to maintain a separate timer
for each message; instead, the lifetime need only be evaluated at
points in the life of the message where actions are already being
taken, such as TSN assignment, transmission, or expiration of a
retransmission timeout. Rule TR5 is intended to give the SCTP
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implementor flexibility to evaluate lifetime at any other convenient
opportunity, WITHOUT requiring that lifetime be evaluated immediately
at the point in time where it expires.
4.2 PR-SCTP Association Establishment
An upper layer protocol (ULP) that uses PR-SCTP may need to know
whether PR-SCTP can be supported on a given association. Therefore,
the ULP needs to have some indication of whether the FORWARD-TSN
chunk is supported by its peer.
Section 10.1 of RFC2960 [5] describes abstract primitives for the
ULP-to-SCTP interface, while noting that "individual implementations
must define their own exact format, and may provide combinations or
subsets of the basic functions in single calls."
In this section, we describe one additional return value that may be
added to the ASSOCIATE primitive to allow an SCTP service user to
indicate whether the FORWARD-TSN chunk is supported by its peer.
RFC2960 indicates that the associate primitive "allows the upper
layer to initiate an association to a specific peer endpoint". It is
structured as follows:
Format: ASSOCIATE(local SCTP instance name, destination transport addr,
outbound stream count)
-> association id [,destination transport addr list]
[,outbound stream count]
This extension adds one new OPTIONAL return value, such that the new
primitive reads as follows:
Format: ASSOCIATE(local SCTP instance name, destination transport addr,
outbound stream count )
-> association id [,destination transport addr list]
[,outbound stream count] [,forward tsn supported]
NOTE: As per RFC2960, if the ASSOCIATE primitive is implemented as a
non-blocking call, the new OPTIONAL return value shall be passed with
the association parameters using the COMMUNICATION UP notification.
The new OPTIONAL parameter "forward tsn supported" is a boolean flag:
(0) false [default] indicates that FORWARD TSN is not supported by
the peer.
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(1) true indicates that FORWARD TSN is supported by the peer.
4.3 Guidelines for defining other PR-SCTP Services
Other PR-SCTP services may be defined and implemented as dictated by
the needs of upper layer protocols. If such upper layer protocols
are to be standardized and require some particular PR-SCTP service
other than the one defined in this document (i.e., "timed
reliability") then those additional PR-SCTP services should also be
specified and standardized.
It is suggested that any such additional service definitions be
modeled after the contents of Section 4.1 . In particular, the
service definition should provide:
1. A description of how the service user specifies any parameters
that need to be associated with a particular message (and/or any
other communication that takes place between the application and
the SCTP transport sender) that provides the SCTP transport
sender with the information needed to determine when to give up
on transmission of a particular message.
Preferably this description should reference the primitives in
the abstract API provided in Section 10 of RFC2960 [5],
indicating any:
* changes to the interpretation of the existing parameters of
existing primitives,
* additional parameters to be added to existing primitives
(these should be OPTIONAL, and default values should be
indicated),
* additional primitives that may be needed.
2. A description of the rules used by the sender side implementation
to determine when to give up on messages that have not yet been
assigned a TSN. This description should also indicate what
protocol events trigger the evaluation, and what actions to take
(e.g. notifications.)
3. A description of the rules used by the sender side implementation
to determine when to give up on the transmission or
retransmission of messages that have already been assigned a TSN,
and may have been transmitted and possibly retransmitted zero or
more times.
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Items (2) and (3) in the list above should also indicate what
protocol events trigger the evaluation, and what actions to take if
the determination is made that the sender should give up on
transmitting the message (e.g. notifications to the ULP.)
Note that in any PR-SCTP service, the following rule MUST be
specified to avoid a protocol deadlock:
(G1) When the sender side implementation gives up on transmitting a
message that has been assigned a TSN (i.e., when that message is
"abandoned", as defined in Section 3.4) the sender side MUST mark
that TSN as eligible for forward TSN, and the rules in Section 3.4
regarding the sending of FORWARD TSN chunks MUST be followed.
Finally, a PR-SCTP service definition should specify a "canonical
service name" to uniquely identify the service, and distinguish it
from other PR-SCTP services. This name can then be used in upper
layer protocol standards, to indicate which PR-SCTP service
definition is required by that upper layer protocol. It can also be
used in the documentation of APIs of PR-SCTP implementations to
indicate how an upper layer indicates which definition of PR-SCTP
service should apply. The canonical service name for the PR-SCTP
service defined in Section 4.1 is "timed reliability".
4.4 Usage Notes
Detecting missing data in a PR-SCTP stream is useful for some
applications (e.g. Fiber channel or SCSI over IP). With PR-SCTP this
becomes possible - the upper layer simply needs to examine the stream
sequence number of the arrived user messages of that stream to detect
any missing data. Note, this detection only works when all the
messages on that stream are sent in order, i.e., the "U" bit is not
set.
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5. Acknowledgments
The authors would like to thank Brian Bidulock, Scott Bradner, Jon
Berger, Armando L. Caro Jr., John Loughney, Ivan Arias Rodriguez, Ian
Rytina, Chip Sharp, and others for their comments.
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6. Security Considerations
This document does not introduce any new security concerns to SCTP
other than the ones already documented in RFC2960 [5]. In particular
this document shares the same security issues as unordered data
within RFC2960 [5]. An application using the PR-SCTP extension should
not use transport layer security. Further details can be found in
RFC3436 [4].
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7. IANA Considerations
One new chunk type is added to SCTP ('0xC0') by this document.
One new parameter type code is defined by this document to be added
to SCTP ('0xC000').
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References
[1] Clark, D. and D. Tennenhouse, "Architectural Considerations for
a New Generation of Protocols", SIGCOMM 1990 pp. 200-208,
September 1990.
[2] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Jungmaier, A., Rescorla, E. and M. Tuexen, "TLS over SCTP", RFC
3436, December 2002.
[5] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
H., Taylor, T., Rytina, I., Kalla, M., Zhang, L. and V. Paxson,
"Stream Control Transmission Protocol", RFC 2960, October 2000.
Authors' Addresses
Randall R. Stewart
Cisco Systems, Inc.
8725 West Higgins Road
Suite 300
Chicago, IL 60631
USA
Phone: +1-815-477-2127
EMail: [email protected]
Michael A. Ramalho
Cisco Systems, Inc.
1802 Rue de la Porte
Wall Township, NJ 07719-3784
USA
Phone: +1.732.449.5762
EMail: [email protected]
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Qiaobing Xie
Motorola, Inc.
1501 W. Shure Drive, #2309
Arlington Heights, IL 60004
USA
Phone: +1-847-632-3028
EMail: [email protected]
Michael Tuexen
Univ. of Applied Sciences Muenster
Stegerwaldstr. 39
48565 Steinfurt
Germany
EMail: [email protected]
Phillip T. Conrad
Temple University
CIS Department
Room 303, Computer Building (038-24)
1805 N. Broad St.
Philadelphia, PA 19122
US
Phone: +1 215 204 7910
EMail: [email protected]
URI: http://www.cis.temple.edu/~conrad
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