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Internet Engineering Task Force R. Stewart
INTERNET DRAFT Cisco Systems
L. Ong
Ciena Systems
I. Arias-Rodriguez
Nokia
K. Poon
Sun Microsystems
Armando L Caro Jr.
University Of Delaware
expires in six months May 12 2002
Stream Control Transmission Protocol (SCTP) Implementers Guide
<draft-ietf-tsvwg-sctpimpguide-05.txt>
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document contains a compilation of all defects found up until
January 2002 for the Stream Control Transmission Protocol (SCTP)
[RFC2960]. These defects may be of an editorial or technical nature.
This document may be thought of as a companion document to be used in
the implementation of SCTP to clarify errors in the original SCTP
document.
Stewart et.al. [Page 1]
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This document updates RFC2960 and text within this document
supersedes the text found in RFC2960.
Table of Contents
1. Introduction......................................... 2
1.1 Conventions........................................ 3
2. Corrections to RFC2960............................... 3
2.1 Incorrect error type during chunk processing....... 3
2.2 Parameter processing issue......................... 3
2.3 Padding issues..................................... 4
2.4 Parameter types across all chunk types............. 5
2.5 Stream parameter clarification..................... 7
2.6 Restarting association security issue.............. 8
2.7 Implicit ability to exceed cwnd by PMTU-1 bytes....12
2.8 Issues with Fast Retransmit........................13
2.9 Missing statement about partial_bytes_acked update.16
2.10 Issues with Heartbeating and failure detection....17
2.11 Security interactions with firewalls..............20
2.12 Shutdown ambiguity................................21
2.13 Inconsistency in ABORT processing.................23
2.14 Cwnd gated by its full use........................24
2.15 Window probes in SCTP.............................26
2.16 Fragmentation and Path MTU issues.................28
2.17 Initial value of the cumulative TSN ACK...........29
2.18 Handling of address parameters within the INIT or
INIT-ACK..........................................30
2.19 Handling of stream shortages......................31
2.20 Indefinite postponement...........................32
3. Acknowledgments......................................33
4. Authors' Addresses...................................34
5. References...........................................35
1. Introduction
This document contains a compilation of all defects found up until
January 2002 for the Stream Control Transmission Protocol (SCTP)
[RFC2960]. These defects may be of an editorial or technical nature.
This document may be thought of as a companion document to be used in
the implementation of SCTP to clarify errors in the original SCTP
document.
This document updates RFC2960 and text within this document, where
noted, supersedes the text found in RFC2960. Each error will be
detailed within this document in the form of:
- The problem description,
- The text quoted from RFC2960,
- The replacement text,
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- A description of the solution.
1.1 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].
2. Corrections to RFC2960
2.1 Incorrect error type during chunk processing.
2.1.1 Description of the problem
A typo was discovered in [RFC2960] that incorrectly specifies an
action to be taken when processing chunks of unknown identity.
2.1.2 Text changes to the document
---------
Old text: (Section 3.2)
---------
01 - Stop processing this SCTP packet and discard it, do not process
any further chunks within it, and report the unrecognized
parameter in an 'Unrecognized Parameter Type' (in either an
ERROR or in the INIT ACK).
---------
New text: (Section 3.2)
---------
01 - Stop processing this SCTP packet and discard it, do not process
any further chunks within it, and report the unrecognized
chunk in an 'Unrecognized Chunk Type'.
2.1.3 Solution description
The receiver of an unrecognized Chunk should not send a 'parameter'
error but instead the appropriate chunk error as described above.
2.2 Parameter processing issue
2.2.1 Description of the problem
A typographical error was introduced through an improper cut
and paste in the use of the upper two bits to describe proper
handling of unknown parameters.
2.2.2 Text changes to the document
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---------
Old text: (Section 3.2.1)
---------
00 - Stop processing this SCTP packet and discard it, do not process
any further chunks within it.
01 - Stop processing this SCTP packet and discard it, do not process
any further chunks within it, and report the unrecognized
parameter in an 'Unrecognized Parameter Type' (in either an
ERROR or in the INIT ACK).
---------
New text: (Section 3.2.1)
---------
00 - Stop processing this SCTP chunk and discard it, do not process
any further parameters within this chunk.
01 - Stop processing this SCTP chunk and discard it, do not process
any further parameters within this chunk, and report the
unrecognized parameter in an 'Unrecognized Parameter Type' (in
either an ERROR or in the INIT ACK).
2.2.3 Solution description
It was always the intent to stop processing at the level one was at
in an unknown chunk or parameter with the upper bit set to 0. Thus if
you are processing a chunk, you should drop the packet. If you are
processing a parameter, you should drop the chunk.
2.3 Padding issues
2.3.1 Description of the problem
A problem was found in that when a Chunk terminated in a TLV
parameter. If this last TLV was not on a 32 bit boundary (as
required), there was confusion as to if the last padding was included
in the chunk length.
2.3.2 Text changes to the document
---------
Old text: (Section 3.2)
---------
Chunk Length: 16 bits (unsigned integer)
This value represents the size of the chunk in bytes including the
Chunk Type, Chunk Flags, Chunk Length, and Chunk Value fields.
Therefore, if the Chunk Value field is zero-length, the Length
field will be set to 4. The Chunk Length field does not count any
padding.
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Chunk Value: variable length
The Chunk Value field contains the actual information to be
transferred in the chunk. The usage and format of this field is
dependent on the Chunk Type.
The total length of a chunk (including Type, Length and Value fields)
MUST be a multiple of 4 bytes. If the length of the chunk is not a
multiple of 4 bytes, the sender MUST pad the chunk with all zero
bytes and this padding is not included in the chunk length field.
The sender should never pad with more than 3 bytes. The receiver
MUST ignore the padding bytes.
---------
New text: (Section 3.2)
---------
Chunk Length: 16 bits (unsigned integer)
This value represents the size of the chunk in bytes including the
Chunk Type, Chunk Flags, Chunk Length, and Chunk Value fields.
Therefore, if the Chunk Value field is zero-length, the Length
field will be set to 4. The Chunk Length field does not count any
chunk padding.
Chunks (including Type, Length and Value fields) are padded out by
the sender with all zero bytes to be a multiple of 4 bytes long.
This padding MUST NOT be more than 3 bytes in total. The Chunk
Length value does not include terminating padding of the Chunk.
However, it does include padding of any variable length parameter
except the last parameter in the Chunk. The receiver MUST ignore
the padding.
Note: A robust implementation should accept the Chunk whether
or not the final padding has been included in the Chunk Length.
Chunk Value: variable length
The Chunk Value field contains the actual information to be
transferred in the chunk. The usage and format of this field is
dependent on the Chunk Type.
2.3.3 Solution description
The above text makes clear that the padding of the last parameter is
not included in the Chunk Length field. It also clarifies that the
padding of parameters that are not the last one must be counted in
the Chunk Length field.
2.4 Parameter types across all chunk types
2.4.1 Description of the problem
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A problem was noted when multiple errors are needed to be sent
regarding unknown or unrecognized parameters. Since often times the
error type does not hold the chunk type field, it may become
difficult to tell which error was associated with which chunk.
2.4.2 Text changes to the document
---------
Old text: (Section 3.2.1)
---------
The actual SCTP parameters are defined in the specific SCTP chunk
sections. The rules for IETF-defined parameter extensions are
defined in Section 13.2.
---------
New text: (Section 3.2.1)
---------
The actual SCTP parameters are defined in the specific SCTP chunk
sections. The rules for IETF-defined parameter extensions are
defined in Section 13.2. Note that a parameter type MUST be unique
across all chunks. For example, the parameter type '5' is used to
represent an IPv4 address (see section 3.3.2). The value '5' then is
reserved across all chunks to represent an IPv4 address and MUST NOT
be reused with a different meaning in any other chunk.
---------
Old text: (Section 13.2)
---------
13.2 IETF-defined Chunk Parameter Extension
The assignment of new chunk parameter type codes is done through an
IETF Consensus action as defined in [RFC2434]. Documentation of the
chunk parameter MUST contain the following information:
a) Name of the parameter type.
b) Detailed description of the structure of the parameter field.
This structure MUST conform to the general type-length-value
format described in Section 3.2.1.
c) Detailed definition of each component of the parameter type.
d) Detailed description of the intended use of this parameter type,
and an indication of whether and under what circumstances multiple
instances of this parameter type may be found within the same
chunk.
---------
New text: (Section 13.2)
---------
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13.2 IETF-defined Chunk Parameter Extension
The assignment of new chunk parameter type codes is done through an
IETF Consensus action as defined in [RFC2434]. Documentation of the
chunk parameter MUST contain the following information:
a) Name of the parameter type.
b) Detailed description of the structure of the parameter field. This
structure MUST conform to the general type-length-value format
described in Section 3.2.1.
c) Detailed definition of each component of the parameter type.
d) Detailed description of the intended use of this parameter type,
and an indication of whether and under what circumstances multiple
instances of this parameter type may be found within the same
chunk.
e) Each parameter type MUST be unique across all chunks.
2.4.3 Solution description
By having all parameters unique across all chunk assignments (the
current assignment policy) no ambiguity exists as to what a parameter
means based on context. The trade off for this is a smaller parameter
space i.e. 65,536 parameters versus 65,536 * Number-of-chunks.
2.5 Stream parameter clarification
2.5.1 Description of the problem
A problem was found where the specification is unclear on the
legality of an endpoint asking for more stream resources than were
allowed in the MIS value of the INIT. In particular the value in the
INIT ACK requested in its OS value was larger than the MIS value
received in the INIT chunk. This behavior is illegal yet it was
unspecified in [RFC2960].
2.5.2 Text changes to the document
---------
Old text: (Section 3.3.3)
---------
Number of Outbound Streams (OS): 16 bits (unsigned integer)
Defines the number of outbound streams the sender of this INIT ACK
chunk wishes to create in this association. The value of 0 MUST
NOT be used.
Note: A receiver of an INIT ACK with the OS value set to 0 SHOULD
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destroy the association discarding its TCB.
---------
New text: (Section 3.3.3)
---------
Number of Outbound Streams (OS): 16 bits (unsigned integer)
Defines the number of outbound streams the sender of this INIT ACK
chunk wishes to create in this association. The value of 0 MUST
NOT be used and the value MUST NOT be greater than the MIS value
sent in the INIT chunk.
Note: A receiver of an INIT ACK with the OS value set to 0 SHOULD
destroy the association discarding its TCB.
2.5.3 Solution description
The change in wording, above, changes it so that a responder to an
INIT chunk does not specify more streams in its OS value than was
represented to it in the MIS value i.e. its maximum.
2.6 Restarting association security issue
2.6.1 Description of the problem
A security problem was found when a restart occurs. It is possible
for an intruder to send an INIT to an endpoint of an existing
association. In the INIT the intruder would list one or more of
the current addresses of an association and its own. The normal restart
procedures would then occur and the intruder would have hi-jacked an
association.
2.6.2 Text changes to the document
---------
Old text: (Section 3.3.10)
---------
Cause Code
Value Cause Code
--------- ----------------
1 Invalid Stream Identifier
2 Missing Mandatory Parameter
3 Stale Cookie Error
4 Out of Resource
5 Unresolvable Address
6 Unrecognized Chunk Type
7 Invalid Mandatory Parameter
8 Unrecognized Parameters
9 No User Data
10 Cookie Received While Shutting Down
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Cause Length: 16 bits (unsigned integer)
Set to the size of the parameter in bytes, including the Cause
Code, Cause Length, and Cause-Specific Information fields
Cause-specific Information: variable length
This field carries the details of the error condition.
Sections 3.3.10.1 - 3.3.10.10 define error causes for SCTP.
Guidelines for the IETF to define new error cause values are
discussed in Section 13.3.
---------
New text: (Section 3.3.10)
---------
Cause Code
Value Cause Code
--------- ----------------
1 Invalid Stream Identifier
2 Missing Mandatory Parameter
3 Stale Cookie Error
4 Out of Resource
5 Unresolvable Address
6 Unrecognized Chunk Type
7 Invalid Mandatory Parameter
8 Unrecognized Parameters
9 No User Data
10 Cookie Received While Shutting Down
11 Restart of an association with new addresses
Cause Length: 16 bits (unsigned integer)
Set to the size of the parameter in bytes, including the Cause
Code, Cause Length, and Cause-Specific Information fields
Cause-specific Information: variable length
This field carries the details of the error condition.
Sections 3.3.10.1 - 3.3.10.11 define error causes for SCTP.
Guidelines for the IETF to define new error cause values are
discussed in Section 13.3.
---------
New text: (Note no old text, new error added in section 3.3.10)
---------
3.3.10.11 Restart of an association with new addresses (11)
Cause of error
--------------
Restart of an association with new addresses: An INIT was received
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on an existing association. But the INIT added addresses to the
association that were previously NOT part of the association. The
New addresses are listed in the error code. This ERROR is normally
sent as part of an ABORT refusing the INIT (see section 5.2).
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=11 | Cause Length=Variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ New Address TLVs /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
---------
Old text: (Section 5.2.1)
---------
Upon receipt of an INIT in the COOKIE-WAIT or COOKIE-ECHOED state, an
endpoint MUST respond with an INIT ACK using the same parameters it
sent in its original INIT chunk (including its Initiation Tag,
unchanged). These original parameters are combined with those from
the newly received INIT chunk. The endpoint shall also generate a
State Cookie with the INIT ACK. The endpoint uses the parameters
sent in its INIT to calculate the State Cookie.
---------
New text: (Section 5.2.1)
---------
Upon receipt of an INIT in the COOKIE-WAIT state, an endpoint MUST
respond with an INIT ACK using the same parameters it sent in its
original INIT chunk (including its Initiation Tag, unchanged). When
responding the endpoint MUST send the INIT ACK back to the same
address that the original INIT (sent by this endpoint) was sent to.
Upon receipt of an INIT in the COOKIE-ECHOED state, an endpoint MUST
respond with an INIT ACK using the same parameters it sent in its
original INIT chunk (including its Initiation Tag, unchanged)
provided that no NEW address have been added to the forming
association. If the INIT message indicates that a new address(es)
have been added to the association, then the entire INIT MUST be
discarded and NO changes should be made to the existing association.
An ABORT MUST be sent in response that SHOULD include the error
'Restart of an association with new addresses'. The error SHOULD list
the addresses that were added to the restarting association.
When responding in either state (COOKIE-WAIT or COOKIE-ECHOED) with
an INIT ACK the original parameters are combined with those from the
newly received INIT chunk. The endpoint shall also generate a State
Cookie with the INIT ACK. The endpoint uses the parameters sent in
its INIT to calculate the State Cookie.
---------
Old text: (Section 5.2.2)
---------
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5.2.2 Unexpected INIT in States Other than CLOSED, COOKIE-ECHOED,
COOKIE-WAIT and SHUTDOWN-ACK-SENT
Unless otherwise stated, upon reception of an unexpected INIT for
this association, the endpoint shall generate an INIT ACK with a
State Cookie. In the outbound INIT ACK the endpoint MUST copy its
current Verification Tag and peer's Verification Tag into a reserved
place within the state cookie. We shall refer to these locations as
the Peer's-Tie-Tag and the Local-Tie-Tag. The outbound SCTP packet
containing this INIT ACK MUST carry a Verification Tag value equal to
the Initiation Tag found in the unexpected INIT. And the INIT ACK
MUST contain a new Initiation Tag (randomly generated see Section
5.3.1). Other parameters for the endpoint SHOULD be copied from the
existing parameters of the association (e.g. number of outbound
streams) into the INIT ACK and cookie.
After sending out the INIT ACK, the endpoint shall take no further
actions, i.e., the existing association, including its current state,
and the corresponding TCB MUST NOT be changed.
Note: Only when a TCB exists and the association is not in a COOKIE-
WAIT state are the Tie-Tags populated. For a normal association INIT
(i.e. the endpoint is in a COOKIE-WAIT state), the Tie-Tags MUST be
set to 0 (indicating that no previous TCB existed). The INIT ACK and
State Cookie are populated as specified in section 5.2.1.
---------
New text: (Section 5.2.2)
---------
5.2.2 Unexpected INIT in States Other than CLOSED, COOKIE-ECHOED,
COOKIE-WAIT and SHUTDOWN-ACK-SENT
Unless otherwise stated, upon reception of an unexpected INIT for
this association, the endpoint shall generate an INIT ACK with a
State Cookie. Before responding the endpoint MUST check to see if the
unexpected INIT adds new addresses to the association. If new
addresses are added to the association, the endpoint MUST respond
with an ABORT copying the 'Initiation Tag' of the unexpected INIT
into the 'Verification Tag' of the outbound packet carrying the ABORT.
In the ABORT response the cause of error SHOULD be set to 'restart
of an association with new addresses'. The error SHOULD list the
addresses that were added to the restarting association.
If no new addresses are added, when responding to the INIT in the
outbound INIT ACK the endpoint MUST copy its current Verification Tag
and peer's Verification Tag into a reserved place within the state
cookie. We shall refer to these locations as the Peer's-Tie-Tag and
the Local-Tie-Tag. The outbound SCTP packet containing this INIT ACK
MUST carry a Verification Tag value equal to the Initiation Tag found
in the unexpected INIT. And the INIT ACK MUST contain a new
Initiation Tag (randomly generated see Section 5.3.1). Other
parameters for the endpoint SHOULD be copied from the existing
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parameters of the association (e.g. number of outbound streams) into
the INIT ACK and cookie.
After sending out the INIT ACK or ABORT, the endpoint shall take no
further actions, i.e., the existing association, including its
current state, and the corresponding TCB MUST NOT be changed.
Note: Only when a TCB exists and the association is not in a COOKIE-
WAIT, COOKIE-ECHOED or SHUTDOWN-ACK-SENT state are the Tie-Tags
populated with a value other than 0. For a normal association INIT
(i.e. the endpoint is in the CLOSED state), the Tie-Tags MUST be set
to 0 (indicating that no previous TCB existed).
2.6.3 Solution description
A new error code is being added and specific instructions to send
back an ABORT to a new association in a restart case or collision
case, where new addresses have been added. The error code can be used
by a legitimate restart to inform the endpoint that it has made a
software error in adding a new address. The endpoint then can choose
to wait until the OOTB ABORT tears down the old association, or
restart without the new address.
Also the Note at the end of section 5.2.2 explaining the use of the
Tie-Tags was modified to properly explain the states in which the
Tie-Tags should be set to a value different than 0.
2.7 Implicit ability to exceed cwnd by PMTU-1 bytes
2.7.1 Description of the problem
Some implementations were having difficulty growing their cwnd. This
was due to an improper enforcement of the congestion control rules.
The rules, as written, provided for a slop over of the cwnd value.
Without this slop over the sender would appear to NOT be using its
full cwnd value and thus never increase it.
2.7.2 Text changes to the document
---------
Old text: (Section 6.1)
---------
B) At any given time, the sender MUST NOT transmit new data to a
given transport address if it has cwnd or more bytes of data
outstanding to that transport address.
---------
New text: (Section 6.1)
---------
B) At any given time, the sender MUST NOT transmit new data to a
given transport address if it has cwnd or more bytes of data
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outstanding to that transport address. The sender may exceed cwnd
by up to (PMTU-1) bytes on a new transmission if the cwnd is not
currently exceeded.
2.7.3 Solution description
The text changes make clear the ability to go over the cwnd value by
no more than (PMTU-1) bytes.
2.8 Issues with Fast Retransmit
2.8.1 Description of the problem
A problem was found in the current specification of fast retransmit.
In particular in a high bandwidth * delay network. The current
wording did not require GAP ACK blocks to be sent, even though they
are essential to the workings of SCTP's congestion control. Also the
specification left unclear how to handle the fast retransmit cycle,
having the implementation to wait on the cwnd to retransmit a TSN
that was marked for fast retransmit. Also no limit was placed on how
many times a TSN could be fast retransmitted.
2.8.2 Text changes to the document
---------
Old text: (Section 6.2)
---------
Acknowledgments MUST be sent in SACK chunks unless shutdown was
requested by the ULP in which case an endpoint MAY send an
acknowledgment in the SHUTDOWN chunk. A SACK chunk can acknowledge
the reception of multiple DATA chunks. See Section 3.3.4 for SACK
chunk format. In particular, the SCTP endpoint MUST fill in the
Cumulative TSN Ack field to indicate the latest sequential TSN (of a
valid DATA chunk) it has received. Any received DATA chunks with TSN
greater than the value in the Cumulative TSN Ack field SHOULD also be
reported in the Gap Ack Block fields.
---------
New text: (Section 6.2)
---------
Acknowledgments MUST be sent in SACK chunks unless shutdown was
requested by the ULP in which case an endpoint MAY send an
acknowledgment in the SHUTDOWN chunk. A SACK chunk can acknowledge
the reception of multiple DATA chunks. See Section 3.3.4 for SACK
chunk format. In particular, the SCTP endpoint MUST fill in the
Cumulative TSN Ack field to indicate the latest sequential TSN (of a
valid DATA chunk) it has received. Any received DATA chunks with TSN
greater than the value in the Cumulative TSN Ack field MUST also be
reported in the Gap Ack Block fields.
---------
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Old text: (Section 7.2.4)
---------
When the TSN(s) is reported as missing in the fourth consecutive
SACK, the data sender shall:
1) Mark the missing DATA chunk(s) for retransmission,
2) Adjust the ssthresh and cwnd of the destination address(es) to
which the missing DATA chunks were last sent, according to the
formula described in Section 7.2.3.
3) Determine how many of the earliest (i.e., lowest TSN) DATA chunks
marked for retransmission will fit into a single packet, subject
to constraint of the path MTU of the destination transport address
to which the packet is being sent. Call this value K. Retransmit
those K DATA chunks in a single packet.
4) Restart T3-rtx timer only if the last SACK acknowledged the lowest
outstanding TSN number sent to that address, or the endpoint is
retransmitting the first outstanding DATA chunk sent to that
address.
Note: Before the above adjustments, if the received SACK also
acknowledges new DATA chunks and advances the Cumulative TSN Ack
Point, the cwnd adjustment rules defined in Sections 7.2.1 and 7.2.2
must be applied first.
A straightforward implementation of the above keeps a counter for
each TSN hole reported by a SACK. The counter increments for each
consecutive SACK reporting the TSN hole. After reaching 4 and
starting the fast retransmit procedure, the counter resets to 0.
Because cwnd in SCTP indirectly bounds the number of outstanding
TSN's, the effect of TCP fast-recovery is achieved automatically with
no adjustment to the congestion control window size.
---------
New text: (Section 7.2.4)
---------
When the TSN(s) is reported as missing in the fourth consecutive
SACK, the data sender shall:
1) Mark the missing DATA chunk(s) for retransmission as described
below in M1-M3,
2) Adjust the ssthresh and cwnd of the destination address(es) to
which the missing DATA chunks were last sent, according to the
formula described in Section 7.2.3.
3) Determine how many of the earliest (i.e., lowest TSN) DATA chunks
marked for retransmission will fit into a single packet, subject
to constraint of the path MTU of the destination transport address
to which the packet is being sent. Call this value K. Retransmit
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those K DATA chunks in a single packet. When a Fast Retransmit is
being performed the sender SHOULD ignore the value of cwnd and
SHOULD NOT delay retransmission.
4) Restart T3-rtx timer only if the last SACK acknowledged the lowest
outstanding TSN number sent to that address, or the endpoint is
retransmitting the first outstanding DATA chunk sent to that
address.
5) Mark the DATA chunk(s) as being fast retransmitted and thus
ineligible for a subsequent fast retransmit. Those TSNs marked
for retransmission due to the Fast Retransmit algorithm that
did not fit in the sent datagram carrying K other TSNs are also
marked as ineligible for a subsequent fast retransmit. However,
as they are marked for retransmission they will be retransmitted
later on as soon as cwnd allows.
Note: Before the above adjustments, if the received SACK also
acknowledges new DATA chunks and advances the Cumulative TSN Ack
Point, the cwnd adjustment rules defined in Sections 7.2.1 and 7.2.2
must be applied first.
A straightforward implementation of the above is as follows:
M1) Each time a new DATA chunk is transmitted set the
'TSN.Missing.Report' count for that TSN to 0. The
'TSN.Missing.Report' count will be used to determine missing
chunks and when to fast retransmit.
M2) Each time a SACK arrives reporting 'Stray DATA chunk(s)' record
the highest TSN reported as newly acknowledged, call this
value 'HighestTSNinSack'. A newly acknowledged DATA chunk is one
not previously acknowledged in a SACK.
When the SCTP sender of data receives a SACK chunk that
acknowledges, for the first time, the receipt of a DATA chunk,
all the still unacknowledged DATA chunks whose TSN is older than
that newly acknowledged DATA chunk, are qualified as
'Stray DATA chunks'.
M3) Examine all 'Unacknowledged TSN's', if the TSN number of an
'Unacknowledged TSN' is smaller than the 'HighestTSNinSack'
value, increment the 'TSN.Missing.Report' count on that chunk if
it has NOT been fast retransmitted or marked for fast retransmit
already.
M4) If any DATA chunk is found to have a 'TSN.Missing.Report' value
larger than or equal to 4, mark that chunk for retransmission and
start the fast retransmit procedure (steps 2-5 above).
M5) If a T3-rtx timer expires, the 'TSN.Missing.Report' of all
affected TSNs is set to 0.
Because cwnd in SCTP indirectly bounds the number of outstanding
Stewart et.al. [Page 15]
Internet Draft SCTP Implementers Guide May 2002
TSN's, the effect of TCP fast-recovery is achieved automatically with
no adjustment to the congestion control window size.
2.8.3 Solution description
The effect of the above wording changes are as follows:
- It requires with a MUST the sending of GAP Ack blocks instead of
the current [RFC2960] SHOULD.
- It allows a TSN being Fast Retransmitted (FR) to be sent only once
via FR.
- It ends the delay in awaiting for the flight size to drop when a
TSN is identified ready to FR.
- It changes the way chunks are marked during fast retransmit, so
that only new reports are counted (using M1-M4 above).
These changes will effectively allow SCTP to follow a similar model
as TCP+SACK in the handling of Fast Retransmit.
2.9 Missing statement about partial_bytes_acked update
2.9.1 Description of the problem
SCTP uses four control variables to regulate its transmission rate:
rwnd, cwnd, ssthresh and partial_bytes_acked. Upon detection of
packet losses from SACK or when the T3-rtx timer expires on an
address cwnd and ssthresh should be updated as stated in section
7.2.3. However, that section should also clarify that
partial_bytes_acked must be updated as well, having to be reset to 0.
2.9.2 Text changes to the document
---------
Old text: (Section 7.2.3)
---------
7.2.3 Congestion Control
Upon detection of packet losses from SACK (see Section 7.2.4), An
endpoint should do the following:
ssthresh = max(cwnd/2, 2*MTU)
cwnd = ssthresh
Basically, a packet loss causes cwnd to be cut in half.
When the T3-rtx timer expires on an address, SCTP should perform slow
start by:
ssthresh = max(cwnd/2, 2*MTU)
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cwnd = 1*MTU
---------
New text: (Section 7.2.3)
---------
7.2.3 Congestion Control
Upon detection of packet losses from SACK (see Section 7.2.4), an
endpoint should do the following:
ssthresh = max(cwnd/2, 2*MTU)
cwnd = ssthresh
partial_bytes_acked = 0
Basically, a packet loss causes cwnd to be cut in half.
When the T3-rtx timer expires on an address, SCTP should perform slow
start by:
ssthresh = max(cwnd/2, 2*MTU)
cwnd = 1*MTU
partial_bytes_acked = 0
2.9.3 Solution description
The missing text added solves the doubts about what to do with
partial_bytes_acked in the situations stated in section 7.2.3, making
clear that along with ssthresh and cwnd, partial_bytes_acked should
also be updated, having to be reset to 0.
2.10 Issues with Heartbeating and failure detection
2.10.1 Description of the problem
Five basic problems have been discovered with the current heartbeat
procedures:
- The current specification does not specify that you should count a
failed heartbeat as an error against the overall association.
- The current specification is un-specific as to when you start
sending heartbeats and when you should stop.
- The current specification is un-specific as to when you should
respond to heartbeats.
- When responding to a Heartbeat it is unclear what to do if more
than a single TLV is present.
- The jitter applied to a heartbeat was meant to be a small
variance of the RTO and is currently a wide variance due to the
default delay time and incorrect wording within the RFC.
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2.10.2 Text changes to the document
---------
Old text: (Section 8.1)
---------
8.1 Endpoint Failure Detection
An endpoint shall keep a counter on the total number of consecutive
retransmissions to its peer (including retransmissions to all the
destination transport addresses of the peer if it is multi-homed).
If the value of this counter exceeds the limit indicated in the
protocol parameter 'Association.Max.Retrans', the endpoint shall
consider the peer endpoint unreachable and shall stop transmitting
any more data to it (and thus the association enters the CLOSED
state). In addition, the endpoint shall report the failure to the
upper layer, and optionally report back all outstanding user data
remaining in its outbound queue. The association is automatically
closed when the peer endpoint becomes unreachable.
The counter shall be reset each time a DATA chunk sent to that peer
endpoint is acknowledged (by the reception of a SACK), or a
HEARTBEAT-ACK is received from the peer endpoint.
---------
New text: (Section 8.1)
---------
8.1 Endpoint Failure Detection
An endpoint shall keep a counter on the total number of consecutive
retransmissions to its peer (this includes retransmissions to all the
destination transport addresses of the peer if it is multi-homed),
including unacknowledged HEARTBEAT Chunks. If the value of this
counter exceeds the limit indicated in the protocol parameter
'Association.Max.Retrans', the endpoint shall consider the peer
endpoint unreachable and shall stop transmitting any more data to it
(and thus the association enters the CLOSED state). In addition, the
endpoint shall report the failure to the upper layer, and optionally
report back all outstanding user data remaining in its outbound
queue. The association is automatically closed when the peer
endpoint becomes unreachable.
The counter shall be reset each time a DATA chunk sent to that peer
endpoint is acknowledged (by the reception of a SACK), or a
HEARTBEAT-ACK is received from the peer endpoint.
---------
Old text: (Section 8.3)
---------
8.3 Path Heartbeat
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By default, an SCTP endpoint shall monitor the reachability of the
idle destination transport address(es) of its peer by sending a
HEARTBEAT chunk periodically to the destination transport
address(es).
---------
New text: (Section 8.3)
---------
8.3 Path Heartbeat
By default, an SCTP endpoint shall monitor the reachability of the
idle destination transport address(es) of its peer by sending a
HEARTBEAT chunk periodically to the destination transport
address(es). HEARTBEAT sending MAY begin upon reaching the
ESTABLISHED state, and is discontinued after sending either SHUTDOWN
or SHUTDOWN-ACK. A receiver of a HEARTBEAT MUST respond to a
HEARTBEAT with a HEARTBEAT-ACK after entering the COOKIE-ECHOED state
(INIT sender) or the ESTABLISHED state (INIT receiver), up until
reaching the SHUTDOWN-SENT state (SHUTDOWN sender) or the
SHUTDOWN-ACK-SENT state (SHUTDOWN receiver).
---------
Old text: (Section 8.3)
---------
The receiver of the HEARTBEAT should immediately respond with a
HEARTBEAT ACK that contains the Heartbeat Information field copied
from the received HEARTBEAT chunk.
---------
New text: (Section 8.3)
---------
The receiver of the HEARTBEAT should immediately respond with a
HEARTBEAT ACK that contains the Heartbeat Information TLV, together
with any other received TLVs, copied unchanged from the received
HEARTBEAT chunk.
---------
Old text: (Section 8.3)
---------
On an idle destination address that is allowed to heartbeat, a
HEARTBEAT chunk is RECOMMENDED to be sent once per RTO of that
destination address plus the protocol parameter 'HB.interval' , with
jittering of +/- 50%, and exponential back-off of the RTO if the
previous HEARTBEAT is unanswered.
---------
New text: (Section 8.3)
---------
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Internet Draft SCTP Implementers Guide May 2002
On an idle destination address that is allowed to heartbeat, a
HEARTBEAT chunk is RECOMMENDED to be sent once per RTO of that
destination address plus the protocol parameter 'HB.interval' , with
jittering of +/- 50% of the RTO value, and exponential back-off
of the RTO if the previous HEARTBEAT is unanswered.
2.10.3 Solution description
The above text provides guidance as to how to respond to the five
issues mentioned in 2.10.1. In particular the wording changes provide
guidance as to when to start and stop heartbeating, how to respond to
a heartbeat with extra parameters, and clarifies the error counting
procedures for the association.
2.11 Security interactions with firewalls
2.11.1 Description of the problem
When dealing with firewalls it is advantageous to the firewall to be
able to properly determine the initial startup sequence of a reliable
transport protocol. With this in mind the following text is to be
added to SCTP's security section.
2.11.2 Text changes to the document
---------
New text: (no old text, new section added)
---------
11.4 SCTP interactions with firewalls
Per [RFC1858], it is helpful for some firewalls if they can inspect
just the first fragment of a fragmented SCTP packet and unambiguously
determine whether it corresponds to an INIT chunk. Accordingly, we
stress the requirements stated in 3.1 that (1) an INIT chunk MUST NOT
be bundled with any other chunk in a packet, and (2) a packet
containing an INIT chunk MUST have a zero Verification Tag.
Furthermore, we require that the receiver of an INIT chunk MUST
enforce these rules by silently discarding an arriving packet with an
INIT chunk that is bundled with other chunks.
---------
Old text: (Section 17)
---------
17. References
[RFC768] Postel, J. (ed.), "User Datagram Protocol", STD 6, RFC
768, August 1980.
[RFC793] Postel, J. (ed.), "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
Stewart et.al. [Page 20]
Internet Draft SCTP Implementers Guide May 2002
[RFC1123] Braden, R., "Requirements for Internet hosts - application
and support", STD 3, RFC 1123, October 1989.
[RFC1191] Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191,
November 1990.
[RFC1700] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC
1700, October 1994.
[RFC1981] McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
---------
New text: (Section 17)
---------
17. References
[RFC768] Postel, J. (ed.), "User Datagram Protocol", STD 6, RFC
768, August 1980.
[RFC793] Postel, J. (ed.), "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1123] Braden, R., "Requirements for Internet hosts - application
and support", STD 3, RFC 1123, October 1989.
[RFC1191] Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191,
November 1990.
[RFC1700] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC
1700, October 1994.
[RFC1858] Ziemba, G., Reed, D. and Traina P., "Security
Considerations for IP Fragment Filtering", RFC 1858,
October 1995.
[RFC1981] McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
2.11.3 Solution description
The above text adding a new subsection to the Security Considerations
section of RFC 2960 makes clear that, to make easier the interaction
with firewalls, an INIT chunk must not be bundled in any case with
any other chunk, being this rule enforced by the packet receiver,
that will silently discard the packets that do not follow this rule.
2.12 Shutdown ambiguity
2.12.1 Description of the problem
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Currently there is an ambiguity between the statements in section 6.2
and section 9.2. Section 6.2 allows the sending of a SHUTDOWN chunk
in place of a SACK when the sender is in the process of shutting
down, while section 9.2 requires both a SHUTDOWN chunk and a SACK
chunk to be sent.
Along with this ambiguity there is a problem where in an errant
SHUTDOWN receiver may fail to stop accepting user data.
2.12.2 Text changes to the document
---------
Old text: (Section 9.2)
---------
If there are still outstanding DATA chunks left, the SHUTDOWN
receiver shall continue to follow normal data transmission procedures
defined in Section 6 until all outstanding DATA chunks are
acknowledged; however, the SHUTDOWN receiver MUST NOT accept new data
from its SCTP user.
While in SHUTDOWN-SENT state, the SHUTDOWN sender MUST immediately
respond to each received packet containing one or more DATA chunk(s)
with a SACK, a SHUTDOWN chunk, and restart the T2-shutdown timer. If
it has no more outstanding DATA chunks, the SHUTDOWN receiver shall
send a SHUTDOWN ACK and start a T2-shutdown timer of its own,
entering the SHUTDOWN-ACK-SENT state. If the timer expires, the
endpoint must re-send the SHUTDOWN ACK.
---------
New text: (Section 9.2)
---------
If there are still outstanding DATA chunks left, the SHUTDOWN
receiver shall continue to follow normal data transmission procedures
defined in Section 6 until all outstanding DATA chunks are
acknowledged; however, the SHUTDOWN receiver MUST NOT accept new data
from its SCTP user.
While in SHUTDOWN-SENT state, the SHUTDOWN sender MUST immediately
respond to each received packet containing one or more DATA chunk(s)
with a SHUTDOWN chunk, and restart the T2-shutdown timer. If a
SHUTDOWN chunk by itself cannot acknowledge all of the received DATA
chunks (i.e. there are TSN's that can be acknowledged that are larger
than the cumulative TSN and thus gaps exist in the TSN sequence) then
a SACK chunk MUST also be sent.
The sender of the SHUTDOWN MAY also start an overall guard timer
'T5-shutdown-guard' to bound the overall time for shutdown sequence.
At the expiration of this timer the sender SHOULD abort the
association by sending an ABORT chunk. If the 'T5-shutdown-guard'
timer is used, it SHOULD be set to the recommended value of 5 times
'RTO.Max'.
Stewart et.al. [Page 22]
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If the receiver of the SHUTDOWN has no more outstanding DATA chunks,
the SHUTDOWN receiver shall send a SHUTDOWN ACK and start a
T2-shutdown timer of its own, entering the SHUTDOWN-ACK-SENT state.
If the timer expires, the endpoint must re-send the SHUTDOWN ACK.
2.12.3 Solution description
The above text clarifies the use of a SACK in conjunction with a
SHUTDOWN chunk. It also adds a guard timer to the SCTP shutdown
sequence to protect against errant receivers of SHUTDOWN chunks.
2.13 Inconsistency in ABORT processing
2.13.1 Description of the problem
It was noted that the wording in section 8.5.1 did not give proper
directions in the use of the 'T bit' with the verification tags.
2.13.2 Text changes to the document
---------
Old text: (Section 8.5.1)
---------
B) Rules for packet carrying ABORT:
- The endpoint shall always fill in the Verification Tag field of
the outbound packet with the destination endpoint's tag value
if it is known.
- If the ABORT is sent in response to an OOTB packet, the
endpoint MUST follow the procedure described in Section 8.4.
- The receiver MUST accept the packet if the Verification Tag
matches either its own tag, OR the tag of its peer. Otherwise,
the receiver MUST silently discard the packet and take no
further action.
---------
New text: (Section 8.5.1)
---------
B) Rules for packet carrying ABORT:
- The endpoint shall always fill in the Verification Tag field of
the outbound packet with the destination endpoint's tag value
if it is known.
- If the ABORT is sent in response to an OOTB packet, the
endpoint MUST follow the procedure described in Section 8.4.
- The receiver of a ABORT shall accept the packet if the
Verification Tag field of the packet matches its own tag OR it
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is set to its peer's tag and the T bit is set in the Chunk
Flags. Otherwise, the receiver MUST silently discard the packet
and take no further action.
2.13.3 Solution description
The above text change clarifies that the T bit must be set before an
implementation looks for the peers tag.
2.14 Cwnd gated by its full use
2.14.1 Description of the problem
A problem was found with the current specification of the growth and
decay of cwnd. The cwnd should only be increased if it is being fully
utilized, and after periods of under utilization, the cwnd should be
decreased. In some sections, the current wording is weak and is not
clearly defined. Also, the current specification unnecessarily
introduces the need for special case code to ensure cwnd degradation.
2.14.2 Text changes to the document
---------
Old text: (Section 6.1)
---------
D) Then, the sender can send out as many new DATA chunks as Rule A
and Rule B above allow.
---------
New text: (Section 6.1)
---------
D) When the time comes for the sender to transmit new DATA chunks, the
protocol parameter Max.Burst MUST first be applied to limit how many
new DATA chunks may be sent. The limit is applied by adjusting cwnd
as follows:
if((flightsize + Max.Burst*MTU) < cwnd)
cwnd = flightsize + Max.Burst*MTU
E) Then, the sender can send out as many new DATA chunks as Rule A
and Rule B above allow.
---------
Old text: (Section 7.2.1)
---------
o When cwnd is less than or equal to ssthresh an SCTP endpoint MUST
use the slow start algorithm to increase cwnd (assuming the
current congestion window is being fully utilized). If an
incoming SACK advances the Cumulative TSN Ack Point, cwnd MUST be
increased by at most the lesser of 1) the total size of the
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previously outstanding DATA chunk(s) acknowledged, and 2) the
destination's path MTU. This protects against the ACK-Splitting
attack outlined in [SAVAGE99].
---------
New text: (Section 7.2.1)
---------
o When cwnd is less than or equal to ssthresh an SCTP endpoint MUST
use the slow start algorithm to increase cwnd only if the
current congestion window is being fully utilized and an
incoming SACK advances the Cumulative TSN Ack Point. Only when
these two conditions are met can the cwnd be increased otherwise
the cwnd MUST not be increased. If these conditions are met then
cwnd MUST be increased by at most the lesser of 1) the total
size of the previously outstanding DATA chunk(s) acknowledged,
and 2) the destination's path MTU. This protects against the
ACK-Splitting attack outlined in [SAVAGE99].
---------
Old text: (Section 7.2.1)
---------
o When the endpoint does not transmit data on a given transport
address, the cwnd of the transport address should be adjusted to
max(cwnd/2, 2*MTU) per RTO.
---------
New text: (Section 7.2.1)
---------
o When the association does not transmit data on a given transport address
within an RTO, the cwnd of the transport address MUST be adjusted to
2*MTU.
---------
Old text: (Section 7.2.2)
---------
o Same as in the slow start, when the sender does not transmit DATA
on a given transport address, the cwnd of the transport address
should be adjusted to max(cwnd / 2, 2*MTU) per RTO.
---------
New text: (Section 7.2.2)
---------
o Same as in the slow start, when the sender does not transmit DATA on
a given transport address within an RTO, the cwnd of the transport
address should be adjusted to 2*MTU.
---------
Old text: (Section 14)
---------
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14. Suggested SCTP Protocol Parameter Values
The following protocol parameters are RECOMMENDED:
RTO.Initial - 3 seconds
RTO.Min - 1 second
RTO.Max - 60 seconds
RTO.Alpha - 1/8
RTO.Beta - 1/4
Valid.Cookie.Life - 60 seconds
Association.Max.Retrans - 10 attempts
Path.Max.Retrans - 5 attempts (per destination address)
Max.Init.Retransmits - 8 attempts
HB.interval - 30 seconds
---------
New text: (Section 14)
---------
14. Suggested SCTP Protocol Parameter Values
The following protocol parameters are RECOMMENDED:
RTO.Initial - 3 seconds
RTO.Min - 1 second
RTO.Max - 60 seconds
Max.Burst - 4
RTO.Alpha - 1/8
RTO.Beta - 1/4
Valid.Cookie.Life - 60 seconds
Association.Max.Retrans - 10 attempts
Path.Max.Retrans - 5 attempts (per destination address)
Max.Init.Retransmits - 8 attempts
HB.Interval - 30 seconds
2.14.3 Solution description
The above changes strengthens the rules and makes it much more apparent
as to the need to block cwnd growth when the full cwnd is not being
utilized. The changes also applies cwnd degradation without introducing
the need for complex special case code.
2.15 Window probes in SCTP
2.15.1 Description of the problem
When a receiver clamps its rwnd to 0 to flow control the peer,
the specification implies that one must continue to accept
data from the remote peer. This is incorrect and needs
clarification.
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2.15.2 Text changes to the document
---------
Old text: (Section 6.2)
---------
The SCTP endpoint MUST always acknowledge the reception of each valid
DATA chunk.
---------
New text: (Section 6.2)
---------
The SCTP endpoint MUST always acknowledge the reception of each
valid DATA chunk when the DATA chunk received is inside its receive
window.
When the receiver's advertised window is 0, the receiver MUST drop
all new incoming DATA chunk and immediately send back a SACK with
the current receive window showing only DATA chunks received and
accepted so far. The dropped DATA chunk MUST NOT be included in the
SACK as they were not accepted. The receiver MUST also have an
algorithm for advertising its receive window to avoid receiver silly
window syndrome (SWS) as described in RFC 813. The algorithm can be
similar to the one described in Section 4.2.3.3 of RFC 1122.
Because of receiver SWS avoidance, even when the receiver's internal
buffer is not full anymore, as long as the advertised window is
still 0, the receiver MUST still drop all new incoming DATA chunk.
---------
Old text: (Section 6.1)
---------
A) At any given time, the data sender MUST NOT transmit new data to
any destination transport address if its peer's rwnd indicates
that the peer has no buffer space (i.e. rwnd is 0, see Section
6.2.1). However, regardless of the value of rwnd (including if it
is 0), the data sender can always have one DATA chunk in flight to
the receiver if allowed by cwnd (see rule B below). This rule
allows the sender to probe for a change in rwnd that the sender
missed due to the SACK having been lost in transit from the data
receiver to the data sender.
---------
New text: (Section 6.1)
---------
A) At any given time, the data sender MUST NOT transmit new data to
any destination transport address if its peer's rwnd indicates
that the peer has no buffer space (i.e. rwnd is 0, see Section
6.2.1). However, regardless of the value of rwnd (including if it
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is 0), the data sender can always have one DATA chunk in flight to
the receiver if allowed by cwnd (see rule B below). This rule
allows the sender to probe for a change in rwnd that the sender
missed due to the SACK having been lost in transit from the data
receiver to the data sender.
When the receiver's advertised window is zero, this probe is called
a zero window probe. Note that zero window probe SHOULD only be sent
when all outstanding DATA chunks have been cumulatively acknowledged
and no DATA chunk(s) are in flight. Zero window probing MUST
be supported.
When a sender is doing zero window probing, it should not time
out the association if it continues to receive new packets from
the receiver. The reason is that the receiver MAY keep its window
closed for an indefinite time. Refer to Section 6.2 on the receiver
behavior when it advertises a zero window. The sender SHOULD
send the first zero window probe after 1 RTO when it detects that
the receiver has closed its window, and SHOULD increase the probe
interval exponentially afterwards. Also note that the cwnd SHOULD
be adjusted according to Section 7.2.1. Zero window probing does
not affect the calculation of cwnd.
The sender MUST also have algorithm in sending new DATA chunks to
avoid silly window syndrome (SWS) as described in RFC 813. The
algorithm can be similar to the one described in Section 4.2.3.4
of RFC 1122.
2.15.3 Solution description
The above allows a receiver to drop new data that arrives
and yet still requires the receiver to send a SACK showing
the conditions unchanged (with the possible exception of
a new a_rwnd) and the dropped chunk as missing. This will
allow the association to continue until the rwnd condition
clears.
2.16 Fragmentation and Path MTU issues
2.16.1 Description of the problem
The current wording of the Fragmentation and Reassembly forces
an implementation that supports fragmentation to always
fragment. This prohibits an implementation from offering its
users an option to disable sends that exceed the SCTP
fragmentation point.
The restriction in [RFC2960] section 6.9 was never meant
to restrict an implementations API from this behavior.
2.16.2 Text changes to the document
---------
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Old text: (Section 6.1)
---------
6.9 Fragmentation and Reassembly
An endpoint MAY support fragmentation when sending DATA chunks, but
MUST support reassembly when receiving DATA chunks. If an endpoint
supports fragmentation, it MUST fragment a user message if the size
of the user message to be sent causes the outbound SCTP packet size
to exceed the current MTU. If an implementation does not support
fragmentation of outbound user messages, the endpoint must return an
error to its upper layer and not attempt to send the user message.
IMPLEMENTATION NOTE: In this error case, the Send primitive
discussed in Section 10.1 would need to return an error to the upper
layer.
---------
New text: (Section 6.1)
---------
6.9 Fragmentation and Reassembly
An endpoint MAY support fragmentation when sending DATA chunks, but
MUST support reassembly when receiving DATA chunks. If an endpoint
supports fragmentation, it MUST fragment a user message if the size
of the user message to be sent causes the outbound SCTP packet size
to exceed the current MTU. If an implementation does not support
fragmentation of outbound user messages, the endpoint must return an
error to its upper layer and not attempt to send the user message.
Note: If an implementation that supports fragmentation makes
available to its upper layer a mechanism to turn off fragmentation
it may do so. However in so doing, it MUST react just like an
implementation that does NOT support fragmentation i.e. it MUST
reject sends that exceed the current P-MTU.
IMPLEMENTATION NOTE: In this error case, the Send primitive
discussed in Section 10.1 would need to return an error to the upper
layer.
2.16.3 Solution description
The above wording will allow an implementation to offer the
option of rejecting sends that exceed the P-MTU size even
when the implementation supports fragmentation.
2.17 Initial value of the cumulative TSN Ack
2.17.1 Description of the problem
The current description of the SACK chunk within the RFC does not
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clearly state the value that would be put within a
SACK when no DATA chunk has been received.
2.17.2 Text changes to the document
---------
Old text: (Section 3.3.4)
---------
Cumulative TSN Ack: 32 bits (unsigned integer)
This parameter contains the TSN of the last DATA chunk received in
sequence before a gap.
---------
New text: (Section 3.3.4)
---------
Cumulative TSN Ack: 32 bits (unsigned integer)
This parameter contains the TSN of the last DATA chunk received in
sequence before a gap. In the case where no DATA chunk has
been received, this value is set to the peers Initial TSN minus
one.
2.17.3 Solution description
This change clearly states what the initial value will be for a
SACK sender.
2.18 Handling of address parameters within the INIT or INIT-ACK
2.18.1 Description of the problem
The current description on handling address parameters contained
within the INIT and INIT-ACK do not fully describe a requirement
for their handling.
2.18.2 Text changes to the document
---------
Old text: (Section 5.1.2)
---------
C) If there are only IPv4/IPv6 addresses present in the received INIT
or INIT ACK chunk, the receiver shall derive and record all the
transport address(es) from the received chunk AND the source IP
address that sent the INIT or INIT ACK. The transport address(es)
are derived by the combination of SCTP source port (from the
common header) and the IP address parameter(s) carried in the INIT
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or INIT ACK chunk and the source IP address of the IP datagram.
The receiver should use only these transport addresses as
destination transport addresses when sending subsequent packets to
its peer.
---------
New text: (Section 5.1.2)
---------
C) If there are only IPv4/IPv6 addresses present in the received INIT
or INIT ACK chunk, the receiver shall derive and record all the
transport address(es) from the received chunk AND the source IP
address that sent the INIT or INIT ACK. The transport address(es)
are derived by the combination of SCTP source port (from the
common header) and the IP address parameter(s) carried in the INIT
or INIT ACK chunk and the source IP address of the IP datagram.
The receiver should use only these transport addresses as
destination transport addresses when sending subsequent packets to
its peer.
D) When searching for a matching TCB upon reception of an INIT
or INIT-ACK chunk the receiver SHOULD use not only the
source address of the packet (containing the INIT or
INIT-ACK) but the receiver SHOULD also use all valid
address parameters contained within the chunk.
2.18.3 Solution description
This new text clearly specifies to an implementor the need
to look within the INIT or INIT-ACK. Any implementation that
does not do this, may not be able to establish associations
in certain circumstances.
2.19 Handling of stream shortages
2.19.1 Description of the problem
The current wording in the RFC places the choice of sending
an ABORT upon the SCTP stack when a stream shortage occurs.
This decision should really be made by the upper layer
not the SCTP stack.
2.19.2 Text changes to the document
---------
Old text:
---------
5.1.1 Handle Stream Parameters
In the INIT and INIT ACK chunks, the sender of the chunk shall
indicate the number of outbound streams (OS) it wishes to have in the
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association, as well as the maximum inbound streams (MIS) it will
accept from the other endpoint.
After receiving the stream configuration information from the other
side, each endpoint shall perform the following check: If the peer's
MIS is less than the endpoint's OS, meaning that the peer is
incapable of supporting all the outbound streams the endpoint wants
to configure, the endpoint MUST either use MIS outbound streams, or
abort the association and report to its upper layer the resources
shortage at its peer.
---------
New text: (Section 5.1.2)
---------
5.1.1 Handle Stream Parameters
In the INIT and INIT ACK chunks, the sender of the chunk shall
indicate the number of outbound streams (OS) it wishes to have in the
association, as well as the maximum inbound streams (MIS) it will
accept from the other endpoint.
After receiving the stream configuration information from the other
side, each endpoint shall perform the following check: If the peer's
MIS is less than the endpoint's OS, meaning that the peer is
incapable of supporting all the outbound streams the endpoint wants
to configure, the endpoint MUST use MIS outbound streams and MAY
report any shortage to the upper layer. The upper layer can then
choose to abort the association if the resource shortage
is unacceptable.
2.19.3 Solution description
The above changes take the decision to ABORT out of the
realm of the SCTP stack and places it into the users
hands.
2.20 Indefinite postponement
2.20.1 Description of the problem
The current RFC does not provide any guidance on the assignment
of TSN sequence numbers to outbound message nor reception of
these message. This could lead to a possible indefinite postponement.
2.20.2 Text changes to the document
---------
Old text: (Section 6.1)
---------
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Internet Draft SCTP Implementers Guide May 2002
Note: The data sender SHOULD NOT use a TSN that is more than 2**31 -
1 above the beginning TSN of the current send window.
6.2 Acknowledgment on Reception of DATA Chunks
---------
New text: (Section 6.1)
---------
Note: The data sender SHOULD NOT use a TSN that is more than 2**31 -
1 above the beginning TSN of the current send window.
The algorithm by which an implementation assigns sequential TSNs to
messages on a particular association MUST ensure that no user
message that has been accepted by SCTP is indefinitely postponed
from being assigned a TSN. Acceptable algorithms for assigning TSNs
include
(a) assigning TSNs in round-robin order over all streams with
pending data
(b) preserving the linear order in which the user messages were
submitted to the SCTP association.
When an upper layer requests to read data on an SCTP association,
the SCTP receiver SHOULD choose the message with the lowest TSN from
among all deliverable messages. In SCTP implementations that allow a
user to request data on a specific stream, this operation SHOULD NOT
block if data is not available, since this can lead to a deadlock
under certain conditions.
6.2 Acknowledgment on Reception of DATA Chunks
2.20.3 Solution description
The above wording clarifies how TSNs SHOULD be assigned by
the sender.
3. Acknowledgments
The authors would like to thank the following people that have
provided comments and input for this document:
A special thanks to Mark Allman, who should actually be a co-author
for his work on the max-burst, but managed to wiggle out due to
a technicality.
For their comments on the list, Atsushi Fukumoto, David Lehmann.
For their participation in the RTP Bakeoff number 2 and all of their
input, Heinz Prantner, Jan Rovins, Renee Revis, Steven Furniss, Manoj
Solanki, Mike Turner, Jonathan Lee, Peter Butler, Laurent Glaude, Jon
Berger, Jon Grim, Dan Harrison, Sabina Torrente, Tomas Orti Martin,
Jeff Waskow, Robby Benedyk, Steve Dimig, Joe Keller, Ben Robinson,
Stewart et.al. [Page 33]
Internet Draft SCTP Implementers Guide May 2002
David Lehmann, John Hebert, Sanjay Rao, Kausar Hassan, Melissa Campbell,
Sujith Radhakrishnan, Michael Tuexen, Andreas Jungmaier, Mitch Miers,
Fred Hasle, Oliver Mayor, Cliff Thomas, Jonathan Wood, Kacheong Poon,
Sverre Slotte, Wang Xiaopeng, John Townsend, Harsh Bhondwe, Sandeep
Mahajan, RCMonee, Ken FUJITA, Yuji SUZUKI, Mutsuya IRIE, Sandeep
Balani, Biren Patel, Qiaobing Xie, Karl Knutson, La Monte Yarroll,
Gareth Keily, Ian Periam, Nathalie Mouellic, and Stan McClellan.
For their comments on the list and his detailed analysis and
simulations of SCTP, Rob Brennan and Thomas Curran.
4. Authors' Addresses
Randall R. Stewart
Cisco Systems Inc.
24 Burning Bush Trail.
Crystal Lake, IL 60012
USA
EMail: [email protected]
Lyndon Ong
Ciena Systems
10480 Ridgeview Ct
Cupertino, CA 95014
USA
EMail: [email protected]
Ivan Arias-Rodriguez
Nokia Research Center
PO Box 407
FIN-00045 Nokia Group
Finland
EMail: [email protected]
Kacheong Poon
Sun Microsystems, Inc.
901 San Antonio Road
Palo Alto, CA 94303
USA
Email: [email protected]
Armando L. Caro Jr.
Department of Computer & Information Sciences
University of Delaware
103 Smith Hall
Newark, DE 19716, USA
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Internet Draft SCTP Implementers Guide May 2002
Email: [email protected]
5. References
[RFC1858] Ziemba, G., Reed, D. and Traina P., "Security
Considerations for IP Fragment Filtering", RFC 1858,
October 1995.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC2960] R. R. Stewart, Q. Xie, K. Morneault, C. Sharp, H. J.
Schwarzbauer, T. Taylor, I. Rytina, M. Kalla, L. Zhang,
and V. Paxson, "Stream Control Transmission Protocol,"
RFC 2960, October 2000.
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Stewart et.al. [Page 35]
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Acknowledgment
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Stewart et.al. [Page 36]
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