draft-ietf-tsvwg-sctp-failover-16.txt   rfc7829.txt 
Network Working Group Y. Nishida Internet Engineering Task Force (IETF) Y. Nishida
Internet-Draft GE Global Research Request for Comments: 7829 GE Global Research
Intended status: Standards Track P. Natarajan Category: Standards Track P. Natarajan
Expires: August 20, 2016 Cisco Systems ISSN: 2070-1721 Cisco Systems
A. Caro A. Caro
BBN Technologies BBN Technologies
P. Amer P. Amer
University of Delaware University of Delaware
K. Nielsen K. Nielsen
Ericsson Ericsson
February 17, 2016 April 2016
SCTP-PF: Quick Failover Algorithm in SCTP SCTP-PF: A Quick Failover Algorithm for the
draft-ietf-tsvwg-sctp-failover-16.txt Stream Control Transmission Protocol
Abstract Abstract
SCTP supports multi-homing. However, when the failover operation The Stream Control Transmission Protocol (SCTP) supports multihoming.
specified in RFC4960 is followed, there can be significant delay and However, when the failover operation specified in RFC 4960 is
performance degradation in the data transfer path failover. To followed, there can be significant delay and performance degradation
overcome this problem this document specifies a quick failover in the data transfer path failover. This document specifies a quick
algorithm (SCTP-PF) based on the introduction of a Potentially Failed failover algorithm and introduces the SCTP Potentially Failed
(PF) state in SCTP Path Management. (SCTP-PF) destination state in SCTP Path Management.
The document also specifies a dormant state operation of SCTP. This This document also specifies a dormant state operation of SCTP that
dormant state operation is required to be followed by an SCTP-PF is required to be followed by an SCTP-PF implementation, but it may
implementation, but it may equally well be applied by a standard equally well be applied by a standard SCTP implementation, as
RFC4960 SCTP implementation. described in RFC 4960.
Additionally, the document introduces an alternative switchback Additionally, this document introduces an alternative switchback
operation mode called Primary Path Switchover that will be beneficial operation mode called "Primary Path Switchover" that will be
in certain situations. This mode of operation applies to both a beneficial in certain situations. This mode of operation applies to
standard RFC4960 SCTP implementation as well as to a SCTP-PF both a standard SCTP implementation and an SCTP-PF implementation.
implementation.
The procedures defined in the document require only minimal The procedures defined in the document require only minimal
modifications to the RFC4960 specification. The procedures are modifications to the specification in RFC 4960. The procedures are
sender-side only and do not impact the SCTP receiver. sender-side only and do not impact the SCTP receiver.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet Standards is available in Section 2 of RFC 5741.
This Internet-Draft will expire on August 20, 2016. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7829.
Copyright Notice Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 4 2. Conventions and Terminology . . . . . . . . . . . . . . . . . 5
3. SCTP with Potentially Failed Destination State (SCTP-PF) . . 4 3. SCTP with Potentially Failed (SCTP-PF) Destination State . . 5
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Specification of the SCTP-PF Procedures . . . . . . . . . 5 3.2. Specification of the SCTP-PF Procedures . . . . . . . . . 6
4. Dormant State Operation . . . . . . . . . . . . . . . . . . . 9 4. Dormant State Operation . . . . . . . . . . . . . . . . . . . 10
4.1. SCTP Dormant State Procedure . . . . . . . . . . . . . . 10 4.1. SCTP Dormant State Procedure . . . . . . . . . . . . . . 11
5. Primary Path Switchover . . . . . . . . . . . . . . . . . . . 11 5. Primary Path Switchover . . . . . . . . . . . . . . . . . . . 11
6. Suggested SCTP Protocol Parameter Values . . . . . . . . . . 12 6. Suggested SCTP Protocol Parameter Values . . . . . . . . . . 13
7. Socket API Considerations . . . . . . . . . . . . . . . . . . 12 7. Socket API Considerations . . . . . . . . . . . . . . . . . . 13
7.1. Support for the Potentially Failed Path State . . . . . . 13 7.1. Support for the Potentially Failed Path State . . . . . . 14
7.2. Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) Socket 7.2. Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) Socket
Option . . . . . . . . . . . . . . . . . . . . . . . . . 14 Option . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.3. Exposing the Potentially Failed Path State 7.3. Exposing the Potentially Failed Path State
(SCTP_EXPOSE_POTENTIALLY_FAILED_STATE) Socket Option . . 15 (SCTP_EXPOSE_POTENTIALLY_FAILED_STATE) Socket Option . . 16
8. Security Considerations . . . . . . . . . . . . . . . . . . . 15 8. Security Considerations . . . . . . . . . . . . . . . . . . . 16
9. MIB Considerations . . . . . . . . . . . . . . . . . . . . . 16 9. MIB Considerations . . . . . . . . . . . . . . . . . . . . . 17
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16 10.1. Normative References . . . . . . . . . . . . . . . . . . 17
12. Proposed Change of Status (to be Deleted before Publication) 17 10.2. Informative References . . . . . . . . . . . . . . . . . 18
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 Appendix A. Discussion of Alternative Approaches . . . . . . . . 20
13.1. Normative References . . . . . . . . . . . . . . . . . . 17 A.1. Reduce PMR . . . . . . . . . . . . . . . . . . . . . . . 20
13.2. Informative References . . . . . . . . . . . . . . . . . 17 A.2. Adjust RTO-Related Parameters . . . . . . . . . . . . . . 21
Appendix A. Discussions of Alternative Approaches . . . . . . . 18 Appendix B. Discussion of the Path-Bouncing Effect . . . . . . . 21
A.1. Reduce Path.Max.Retrans (PMR) . . . . . . . . . . . . . . 18 Appendix C. SCTP-PF for SCTP Single-Homed Operation . . . . . . 22
A.2. Adjust RTO related parameters . . . . . . . . . . . . . . 19 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 22
Appendix B. Discussions for Path Bouncing Effect . . . . . . . . 20 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
Appendix C. SCTP-PF for SCTP Single-homed Operation . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction 1. Introduction
The Stream Control Transmission Protocol (SCTP) specified in The Stream Control Transmission Protocol (SCTP) specified in
[RFC4960] supports multi-homing at the transport layer. SCTP's [RFC4960] supports multihoming at the transport layer. SCTP's
multi-homing features include failure detection and failover multihoming features include failure detection and failover
procedures to provide network interface redundancy and improved end- procedures to provide network interface redundancy and improved end-
to-end fault tolerance. In SCTP's current failure detection to-end fault tolerance. In SCTP's current failure detection
procedure, the sender must experience Path.Max.Retrans (PMR) number procedure, the sender must experience Path.Max.Retrans (PMR) number
of consecutive failed timer-based retransmissions on a destination of consecutive failed timer-based retransmissions on a destination
address before detecting a path failure. Until detecting the path address before detecting a path failure. Until detecting the path
failure, the sender continues to transmit data on the failed path. failure, the sender continues to transmit data on the failed path.
The prolonged time in which [RFC4960] SCTP continues to use a failed The prolonged time in which SCTP as described in [RFC4960] continues
path severely degrades the performance of the protocol. To address to use a failed path severely degrades the performance of the
this problem, this document specifies a quick failover algorithm protocol. To address this problem, this document specifies a quick
(SCTP-PF) based on the introduction of a new Potentially Failed (PF) failover algorithm called "SCTP-PF" based on the introduction of a
path state in SCTP path management. The performance deficiencies of new Potentially Failed (PF) path state in SCTP path management. The
the [RFC4960] failover operation, and the improvements obtainable performance deficiencies of the failover operation described in RFC
from the introduction of a Potentially Failed state in SCTP, were 4960, and the improvements obtainable from the introduction of a PF
proposed and documented in [NATARAJAN09] for Concurrent Multipath state in SCTP, were proposed and documented in [NATARAJAN09] for
Transfer SCTP [IYENGAR06]. Concurrent Multipath Transfer SCTP [IYENGAR06].
While SCTP-PF can accelerate failover process and improve While SCTP-PF can accelerate the failover process and improve
performance, the risks that an SCTP endpoint enters the dormant state performance, the risk that an SCTP endpoint might enter the dormant
where all destination addresses are inactive can be increased. state where all destination addresses are inactive can be increased.
[RFC4960] leaves the protocol operation during dormant state to [RFC4960] leaves the protocol operation during dormant state to
implementations and encourages to avoid entering the state as much as implementations and encourages avoiding entering the state as much as
possible by careful tuning of the Path.Max.Retrans (PMR) and possible by careful tuning of the PMR and Association.Max.Retrans
Association.Max.Retrans (AMR) parameters. We specify a dormant state (AMR) parameters. We specify a dormant state operation for SCTP-PF,
operation for SCTP-PF which makes SCTP-PF provide the same disruption which makes SCTP-PF provide the same disruption tolerance as
tolerance as [RFC4960] despite that the dormant state may be entered [RFC4960] despite the fact that the dormant state may be entered more
more quickly. The dormant state operation may equally well be quickly. The dormant state operation may equally well be applied by
applied by an [RFC4960] implementation and will here serve to provide an implementation of [RFC4960] and will serve here to provide added
added fault tolerance for situations where the tuning of the fault tolerance for situations where the tuning of the PMR and AMR
Path.Max.Retrans (PMR) and Association.Max.Retrans (AMR) parameters parameters fail to provide adequate prevention of the entering of the
fail to provide adequate prevention of the entering of the dormant dormant state.
state.
The operation after the recovery of a failed path also impacts the The operation after the recovery of a failed path also impacts the
performance of the protocol. With the procedures specified in performance of the protocol. With the procedures specified in
[RFC4960], SCTP will (after a failover from the primary path) switch
[RFC4960] SCTP will, after a failover from the primary path, switch
back to use the primary path for data transfer as soon as this path back to use the primary path for data transfer as soon as this path
becomes available again. From a performance perspective such a becomes available again. From a performance perspective, such a
forced switchback of the data transmission path can be suboptimal as forced switchback of the data transmission path can be suboptimal as
the CWND towards the original primary destination address has to be the Congestion Window (CWND) towards the original primary destination
rebuilt once data transfer resumes, [CARO02]. As an optional address has to be rebuilt once data transfer resumes, [CARO02]. As
alternative to the switchback operation of [RFC4960], this document an optional alternative to the switchback operation of [RFC4960],
specifies an alternative Primary Path Switchover procedure which this document specifies an alternative Primary Path Switchover
avoid such forced switchbacks of the data transfer path. The Primary procedure that avoids such forced switchbacks of the data transfer
Path Switchover operation was originally proposed in [CARO02]. path. The Primary Path Switchover operation was originally proposed
in [CARO02].
While SCTP-PF primarily is motivated by a desire to improve the While SCTP-PF is primarily motivated by a desire to improve the
multi-homed operation, the feature applies also to SCTP single-homed multihomed operation, the feature also applies to SCTP single-homed
operation. Here the algorithm serves to provide increased failure operation. Here the algorithm serves to provide increased failure
detection on idle associations, whereas the failover or switchback detection on idle associations, whereas the failover or switchback
aspects of the algorithm will not be activated. This is discussed in aspects of the algorithm will not be activated. This is discussed in
more detail in Appendix C. more detail in Appendix C.
A brief description of the motivation for the introduction of the A brief description of the motivation for the introduction of the PF
Potentially Failed state including a discussion of alternative state, including a discussion of alternative approaches to mitigate
approaches to mitigate the deficiencies of the [RFC4960] failover the deficiencies of the failover operation in [RFC4960], are given in
operation are given in the Appendices. Discussion of path bouncing the appendices. Discussion of path-bouncing effects that might be
effects that might be caused by frequent switchovers, are also caused by frequent switchovers are also provided there.
provided there.
2. Conventions and Terminology 2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. SCTP with Potentially Failed Destination State (SCTP-PF) 3. SCTP with Potentially Failed (SCTP-PF) Destination State
3.1. Overview 3.1. Overview
To minimize the performance impact during failover, the sender should To minimize the performance impact during failover, the sender should
avoid transmitting data to a failed destination address as early as avoid transmitting data to a failed destination address as early as
possible. In the [RFC4960] SCTP path management scheme, the sender possible. In the SCTP path management scheme described in [RFC4960],
stops transmitting data to a destination address only after the the sender stops transmitting data to a destination address only
destination address is marked inactive. This process takes a after the destination address is marked inactive. This process takes
significant amount of time as it requires the error counter of the a significant amount of time as it requires the error counter of the
destination address to exceed the Path.Max.Retrans (PMR) threshold. destination address to exceed the PMR threshold. The issue cannot
The issue cannot simply be mitigated by lowering of the PMR threshold simply be mitigated by lowering the PMR threshold because this may
because this may result in spurious failure detection and unnecessary result in spurious failure detection and unnecessary prevention of
prevention of the usage of a preferred primary path. Also due to the the usage of a preferred primary path. Also, due to the coupled
coupled tuning of the Path.Max.Retrans (PMR) and the tuning of the PMR and the AMR parameter values in [RFC4960], lowering
Association.Max.Retrans (AMR) parameter values in [RFC4960], lowering the PMR threshold may result in lowering the AMR threshold, which
of the PMR threshold may result in lowering of the AMR threshold, would result in a decrease of the fault tolerance of SCTP.
which would result in decrease of the fault tolerance of SCTP.
The solution provided in this document is to extend the SCTP path The solution provided in this document is to extend the SCTP path
management scheme of [RFC4960] by the addition of the Potentially management scheme of [RFC4960] by the addition of the PF state as an
Failed (PF) state as an intermediate state in between the active and intermediate state in between the active and inactive state of a
inactive state of a destination address in the [RFC4960] path destination address in the path management scheme of [RFC4960], and
management scheme, and let the failover of data transfer away from a let the failover of data transfer away from a destination address be
destination address be driven by the entering of the PF state instead driven by the entering of the PF state instead of by the entering of
of by the entering of the inactive state. Thereby SCTP may perform the inactive state. Thereby, SCTP may perform quick failover without
quick failover without negatively impacting the overall fault negatively impacting the overall fault tolerance of SCTP as described
tolerance of [RFC4960] SCTP. At the same time, RTO-based HEARTBEAT in [RFC4960]. At the same time, HEARTBEAT probing based on
probing is initiated towards a destination address once it enters PF Retransmission Timeout (RTO) is initiated towards a destination
state. Thereby SCTP may quickly ascertain whether network address once it enters PF state. Thereby, SCTP may quickly ascertain
connectivity towards the destination address is broken or whether the whether network connectivity towards the destination address is
failover was spurious. In the case where the failover was spurious broken or whether the failover was spurious. In the case where the
data transfer may quickly resume towards the original destination failover was spurious, data transfer may quickly resume towards the
address. original destination address.
The new failure detection algorithm assumes that loss detected by a The new failure detection algorithm assumes that loss detected by a
timeout implies either severe congestion or network connectivity timeout implies either severe congestion or network connectivity
failure. It recommends that by default a destination address is failure. It recommends that, by default, a destination address be
classified as PF at the occurrence of the first timeout. classified as PF at the occurrence of the first timeout.
3.2. Specification of the SCTP-PF Procedures 3.2. Specification of the SCTP-PF Procedures
The SCTP-PF operation is specified as follows: The SCTP-PF operation is specified as follows:
1. The sender maintains a new tunable SCTP Protocol Parameter 1. The sender maintains a new tunable SCTP Protocol Parameter
called PotentiallyFailed.Max.Retrans (PFMR). The PFMR defines called PotentiallyFailed.Max.Retrans (PFMR). The PFMR defines
the new intermediate PF threshold on the destination address the new intermediate PF threshold on the destination address
error counter. When this threshold is exceeded the destination error counter. When this threshold is exceeded, the destination
address is classified as PF. The RECOMMENDED value of PFMR is address is classified as PF. The RECOMMENDED value of PFMR is
0. If PFMR is set to be greater than or equal to 0. If PFMR is set to be greater than or equal to PMR, the
Path.Max.Retrans (PMR), the resulting PF threshold will be so resulting PF threshold will be so high that the destination
high that the destination address will reach the inactive state address will reach the inactive state before it can be
before it can be classified as PF. classified as PF.
2. The error counter of an active destination address is 2. The error counter of an active destination address is
incremented or cleared as specified in [RFC4960]. This means incremented or cleared as specified in [RFC4960]. This means
that the error counter of the destination address in active that the error counter of the destination address in active
state will be incremented each time the T3-rtx timer expires, or state will be incremented each time the Timer T3 retransmission
each time a HEARTBEAT chunk is sent when idle and not (T3-rtx) timer expires, or each time a HEARTBEAT chunk is sent
acknowledged within an RTO. When the value in the destination when idle and not acknowledged within an RTO. When the value in
address error counter exceeds PFMR, the endpoint MUST mark the the destination address error counter exceeds PFMR, the endpoint
destination address as in the PF state. MUST mark the destination address as in the PF state.
3. A SCTP-PF sender SHOULD NOT send data to destination addresses 3. An SCTP-PF sender SHOULD NOT send data to destination addresses
in PF state when alternative destination addresses in active in PF state when alternative destination addresses in active
state are available. Specifically this means that: state are available. Specifically, this means that:
i When there is outbound data to send and the destination i. When there is outbound data to send and the destination
address presently used for data transmission is in PF state, address presently used for data transmission is in PF
the sender SHOULD choose a destination address in active state, the sender SHOULD choose a destination address in
state, if one exists, and use this destination address for active state, if one exists, and use this destination
data transmission. address for data transmission.
ii As specified in [RFC4960] section 6.4.1, when the sender ii. As specified in Section 6.4.1 of [RFC4960], when the
retransmits data that has timed out, it should attempt to sender retransmits data that has timed out, they should
pick a new destination address for data retransmission. In attempt to pick a new destination address for data
this case, the sender SHOULD choose an alternate destination retransmission. In this case, the sender SHOULD choose
transport address in active state if one exists. an alternate destination transport address in active
state, if one exists.
iii When there is outbound data to send and the SCTP user iii. When there is outbound data to send and the SCTP user
explicitly requests to send data to a destination address in explicitly requests to send data to a destination address
PF state, the sender SHOULD send the data to an alternate in PF state, the sender SHOULD send the data to an
destination address in active state if one exists. alternate destination address in active state if one
exists.
When choosing among multiple destination addresses in active When choosing among multiple destination addresses in active
state an SCTP sender will follow the guiding principles of state, an SCTP sender will follow the guiding principles of
section 6.4.1 of [RFC4960] of choosing most divergent source- Section 6.4.1 of [RFC4960] by choosing the most divergent
destination pairs compared with, for i.: the destination address source-destination pairs compared with, for (the aforementioned
in PF state that it performs a failover from, and for ii.: the points i and ii):
destination address towards which the data timed out. Rules for
picking the most divergent source-destination pair are an i. the destination address in PF state that it performs a
implementation decision and are not specified within this failover from, and
ii. the destination address towards which the data timed out.
Rules for picking the most divergent source-destination pair are
an implementation decision and are not specified within this
document. document.
In all cases, the sender MUST NOT change the state of chosen In all cases, the sender MUST NOT change the state of the chosen
destination address, whether this state be active or PF, and it destination address, whether this state be active or PF, and it
MUST NOT clear the error counter of the destination address as a MUST NOT clear the error counter of the destination address as a
result of choosing the destination address for data result of choosing the destination address for data
transmission. transmission.
4. When the destination addresses are all in PF state or some in PF 4. When the destination addresses are all in PF state, or some are
state and some in inactive state, the sender MUST choose one in PF state and some in inactive state, the sender MUST choose
destination address in PF state and SHOULD transmit or one destination address in PF state and SHOULD transmit or
retransmit data to this destination address using the following retransmit data to this destination address using the following
rules: rules:
A. The sender SHOULD choose the destination in PF state with i. The sender SHOULD choose the destination in PF state with
the lowest error count (fewest consecutive timeouts) for the lowest error count (fewest consecutive timeouts) for
data transmission and transmit or retransmit data to this data transmission and transmit or retransmit data to this
destination. destination.
B. When there are multiple destination addresses in PF state ii. When there are multiple destination addresses in PF state
with same error count, the sender should let the choice with same error count, the sender should let the choice
among the multiple destination addresses in PF state with among the multiple destination addresses in PF state with
equal error count be based on the [RFC4960], section 6.4.1, equal error count be based on the principles of choosing
principles of choosing most divergent source-destination the most divergent source-destination pairs when executing
pairs when executing (potentially consecutive) (potentially consecutive) retransmission outlined in
retransmission. Rules for picking the most divergent Section 6.4.1 of [RFC4960]. Rules for picking the most
source-destination pair are an implementation decision and divergent source-destination pairs are an implementation
are not specified within this document. decision and are not specified within this document.
The sender MUST NOT change the state and the error counter of The sender MUST NOT change the state and the error counter of
any destination addresses as the result of the selection. any destination addresses as the result of the selection.
5. The HB.interval of the Path Heartbeat function of [RFC4960] MUST 5. The HB.Interval of the Path Heartbeat function of [RFC4960] MUST
be ignored for destination addresses in PF state. Instead be ignored for destination addresses in PF state. Instead,
HEARTBEAT chunks are sent to destination addresses in PF state HEARTBEAT chunks are sent to destination addresses in PF state
once per RTO. HEARTBEAT chunks SHOULD be sent to destination once per RTO. HEARTBEAT chunks SHOULD be sent to destination
addresses in PF state, but the sending of HEARTBEATS MUST honor addresses in PF state, but the sending of HEARTBEATs MUST honor
whether the Path Heartbeat function (Section 8.3 of [RFC4960]) whether or not the Path Heartbeat function (Section 8.3 of
is enabled for the destination address or not. I.e., if the [RFC4960]) is enabled for the destination address. That is, if
Path Heartbeat function is disabled for the destination address the Path Heartbeat function is disabled for the destination
in question, HEARTBEATS MUST NOT be sent. Note that when address in question, HEARTBEATs MUST NOT be sent. Note that
Heartbeat function is disabled, it may take longer to transition when the Path Heartbeat function is disabled, it may take longer
a destination address in PF state back to active state. to transition a destination address in PF state back to active
state.
6. HEARTBEATs are sent when a destination address reaches the PF 6. HEARTBEATs are sent when a destination address reaches the PF
state. When a HEARTBEAT chunk is not acknowledged within the state. When a HEARTBEAT chunk is not acknowledged within the
RTO, the sender increments the error counter and exponentially RTO, the sender increments the error counter and exponentially
backs off the RTO value. If the error counter is less than PMR, backs off the RTO value. If the error counter is less than PMR,
the sender transmits another packet containing the HEARTBEAT the sender transmits another packet containing the HEARTBEAT
chunk immediately after timeout expiration on the previous chunk immediately after timeout expiration on the previous
HEARTBEAT. When data is being transmitted to a destination HEARTBEAT. When data is being transmitted to a destination
address in the PF state, the transmission of a HEARTBEAT chunk address in the PF state, the transmission of a HEARTBEAT chunk
MAY be omitted in case where the receipt of a SACK of the data MAY be omitted in the case where the receipt of a Selective
or a T3-rtx timer expiration on the data can provide equivalent Acknowledgment (SACK) of the data or a T3-rtx timer expiration
information, such as the case where the data chunk has been on the data can provide equivalent information, such as the case
transmitted to a single destination address only. Likewise, the where the data chunk has been transmitted to a single
timeout of a HEARTBEAT chunk MAY be ignored if data is destination address only. Likewise, the timeout of a HEARTBEAT
outstanding towards the destination address. chunk MAY be ignored if data is outstanding towards the
destination address.
7. When the sender receives a HEARTBEAT ACK from a HEARTBEAT sent 7. When the sender receives a HEARTBEAT ACK from a HEARTBEAT sent
to a destination address in PF state, the sender SHOULD clear to a destination address in PF state, the sender SHOULD clear
the error counter of the destination address and transition the the error counter of the destination address and transition the
destination address back to active state. However, there may be destination address back to active state. However, there may be
a situation where HEARTBEAT chunks can go through while DATA a situation where HEARTBEAT chunks can go through while DATA
chunks cannot. Hence, in a situation where a HEARTBEAT ACK chunks cannot. Hence, in a situation where a HEARTBEAT ACK
arrives while there is data outstanding towards the destination arrives while there is data outstanding towards the destination
address to which the HEARTBEAT was sent, then an implementation address to which the HEARTBEAT was sent, then an implementation
MAY choose to not have the HEARTBEAT ACK reset the error MAY choose to not have the HEARTBEAT ACK reset the error
counter, but have the error counter reset await the fate of the counter, but have the error counter reset await the fate of the
outstanding data transmission. This situation can happen when outstanding data transmission. This situation can happen when
data is sent to a destination address in PF state. When the data is sent to a destination address in PF state. When the
sender resumes data transmission on a destination address after sender resumes data transmission on a destination address after
a transition of the destination address from PF to active state, a transition of the destination address from PF to active state,
it MUST do this following the prescriptions of Section 7.2 of it MUST do this following the prescriptions of Section 7.2 of
[RFC4960]. [RFC4960].
8. Additional (PMR - PFMR) consecutive timeouts on a destination 8. Additional PMR - PFMR consecutive timeouts on a destination
address in PF state confirm the path failure, upon which the address in PF state confirm the path failure, upon which the
destination address transitions to the inactive state. As destination address transitions to the inactive state. As
described in [RFC4960], the sender (i) SHOULD notify the ULP described in [RFC4960], the sender SHOULD (i) notify the Upper
about this state transition, and (ii) transmit HEARTBEAT chunks Layer Protocol (ULP) about this state transition, and (ii)
to the inactive destination address at a lower HB.interval transmit HEARTBEAT chunks to the inactive destination address at
frequency as described in Section 8.3 of [RFC4960] (when the a lower HB.Interval frequency as described in Section 8.3 of
Path Heartbeat function is enabled for the destination address). [RFC4960] (when the Path Heartbeat function is enabled for the
destination address).
9. Acknowledgments for chunks that have been transmitted to 9. Acknowledgments for chunks that have been transmitted to
multiple destinations (i.e., a chunk which has been multiple destinations (i.e., a chunk that has been retransmitted
retransmitted to a different destination address than the to a different destination address than the destination address
destination address to which the chunk was first transmitted) to which the chunk was first transmitted) SHOULD NOT clear the
SHOULD NOT clear the error count for an inactive destination error count for an inactive destination address and SHOULD NOT
address and SHOULD NOT move a destination address in PF state move a destination address in PF state back to active state,
back to active state, since a sender cannot disambiguate whether since a sender cannot disambiguate whether the ACK was for the
the ACK was for the original transmission or the original transmission or the retransmission(s). An SCTP sender
retransmission(s). A SCTP sender MAY clear the error counter MAY clear the error counter and move a destination address back
and move a destination address back to active state by to active state by information other than acknowledgments, when
information other than acknowledgments, when it can uniquely it can uniquely determine which destination, among multiple
determine which destination, among multiple destination destination addresses, the chunk reached. This document makes
addresses, the chunk reached. This document makes no reference no reference to what such information could consist of, nor how
to what such information could consist of, nor how such such information could be obtained.
information could be obtained.
10. Acknowledgments for data chunks that has been transmitted to one 10. Acknowledgments for data chunks that have been transmitted to
destination address only MUST clear the error counter for the one destination address only MUST clear the error counter for
destination address and MUST transition a destination address in the destination address and MUST transition a destination
PF state back to active state. This situation can happen when address in PF state back to active state. This situation can
new data is sent to a destination address in the PF state. It happen when new data is sent to a destination address in the PF
can also happen in situations where the destination address is state. It can also happen in situations where the destination
in the PF state due to the occurrence of a spurious T3-rtx timer address is in the PF state due to the occurrence of a spurious
and acknowledgments start to arrive for data sent prior to T3-rtx timer and acknowledgments start to arrive for data sent
occurrence of the spurious T3-rtx and data has not yet been prior to occurrence of the spurious T3-rtx and data has not yet
retransmitted towards other destinations. This document does been retransmitted towards other destinations. This document
not specify special handling for detection of or reaction to does not specify special handling for detection of, or reaction
spurious T3-rtx timeouts, e.g., for special operation vis-a-vis to, spurious T3-rtx timeouts, e.g., for special operation vis-
the congestion control handling or data retransmission operation a-vis the congestion control handling or data retransmission
towards a destination address which undergoes a transition from operation towards a destination address that undergoes a
active to PF to active state due to a spurious T3-rtx timeout. transition from active to PF to active state due to a spurious
But it is noted that this is an area which would benefit from T3-rtx timeout. But it is noted that this is an area that would
additional attention, experimentation and specification for benefit from additional attention, experimentation, and
single-homed SCTP as well as for multi-homed SCTP protocol specification for single-homed SCTP as well as for multihomed
operation. SCTP protocol operation.
11. When all destination addresses are in inactive state, and SCTP 11. When all destination addresses are in inactive state, and SCTP
protocol operation thus is said to be in dormant state, the protocol operation thus is said to be in dormant state, the
prescriptions given in Section 4 shall be followed. prescriptions given in Section 4 shall be followed.
12. The SCTP stack SHOULD expose the PF state of its destination 12. The SCTP stack SHOULD expose the PF state of its destination
addresses to the ULP as well as provide the means to notify the addresses to the ULP as well as provide the means to notify the
ULP of state transitions of its destination addresses from ULP of state transitions of its destination addresses from
active to PF, and vice-versa. However it is recommended that an active to PF, and vice versa. However, it is recommended that
SCTP stack implementing SCTP-PF also allows for that the ULP is an SCTP stack implementing SCTP-PF also allows for the ULP to be
kept ignorant of the PF state of its destinations and the kept ignorant of the PF state of its destinations and the
associated state transitions, thus allowing for retain of the associated state transitions, thus allowing for retention of the
simpler state transition model of RFC4960 in the ULP. For this simpler state transition model of [RFC4960] in the ULP. For
reason it is recommended that an SCTP stack implementing SCTP-PF this reason, it is recommended that an SCTP stack implementing
also provides the ULP with the means to suppress exposure of the SCTP-PF also provide the ULP with the means to suppress exposure
PF state and the associated state transitions. of the PF state and the associated state transitions.
4. Dormant State Operation 4. Dormant State Operation
In a situation with complete disruption of the communication in In a situation with complete disruption of the communication in
between the SCTP Endpoints, the aggressive HEARTBEAT transmissions of between the SCTP endpoints, the aggressive HEARTBEAT transmissions of
SCTP-PF on destination addresses in PF state may make the association SCTP-PF on destination addresses in PF state may make the association
enter dormant state faster than a standard [RFC4960] SCTP enter dormant state faster than a standard SCTP implementation of
implementation given the same setting of Path.Max.Retrans (PMR) and [RFC4960] given the same setting of PMR and AMR. For example, an
Association.Max.Retrans (AMR). For example, an SCTP association with SCTP association with two destination addresses would typically reach
two destination addresses typically would reach dormant state in half dormant state in half the time of an SCTP implementation of [RFC4960]
the time of an [RFC4960] SCTP implementation in such situations. in such situations. This is because an SCTP PF sender will send
This is because a SCTP PF sender will send HEARTBEATS and data HEARTBEATs and data retransmissions in parallel with RTO intervals
retransmissions in parallel with RTO intervals when there are when there are multiple destinations addresses in PF state. This
multiple destinations addresses in PF state. This argument presumes argument presumes that RTO << HB.Interval of [RFC4960]. With the
that RTO << HB.interval of [RFC4960]. With the design goal that design goal that SCTP-PF shall provide the same level of disruption
SCTP-PF shall provide the same level of disruption tolerance as an tolerance as a standard SCTP implementation with the same PMR and AMR
[RFC4960] SCTP implementation with the same Path.Max.Retrans (PMR) setting, we prescribe that an SCTP-PF implementation SHOULD operate
and Association.Max.Retrans (AMR) setting, we prescribe for that an as described in Section 4.1 during dormant state.
SCTP-PF implementation SHOULD operate as described below in
Section 4.1 during dormant state.
An SCTP-PF implementation MAY choose a different dormant state An SCTP-PF implementation MAY choose a different dormant state
operation than the one described below in Section 4.1 provided that operation than the one described in Section 4.1 provided that the
the solution chosen does not decrease the fault tolerance of the solution chosen does not decrease the fault tolerance of the SCTP-PF
SCTP-PF operation. operation.
The below prescription for SCTP-PF dormant state handling MUST NOT be The prescription below for SCTP-PF dormant state handling MUST NOT be
coupled to the value of the PFMR, but solely to the activation of coupled to the value of the PFMR, but solely to the activation of
SCTP-PF logic in an SCTP implementation. SCTP-PF logic in an SCTP implementation.
It is noted that the below dormant state operation is considered to It is noted that the below dormant state operation can also provide
provide added disruption tolerance also for an [RFC4960] SCTP enhanced disruption tolerance to a standard SCTP implementation that
implementation, and that it can be sensible for an [RFC4960] SCTP doesn't support SCTP-PF. Thus, it can be sensible for a standard
implementation to follow this mode of operation. For an [RFC4960] SCTP implementation to follow this mode of operation. For a standard
SCTP implementation the continuation of data transmission during SCTP implementation, the continuation of data transmission during
dormant state makes the fault tolerance of SCTP be more robust dormant state makes the fault tolerance of SCTP be more robust
towards situations where some, or all, alternative paths of an SCTP towards situations where some, or all, alternative paths of an SCTP
association approach, or reach, inactive state before the primary association approach, or reach, inactive state before the primary
path used for data transmission observes trouble. path used for data transmission observes trouble.
4.1. SCTP Dormant State Procedure 4.1. SCTP Dormant State Procedure
a. When the destination addresses are all in inactive state and data 1. When the destination addresses are all in inactive state and data
is available for transfer, the sender MUST choose one destination is available for transfer, the sender MUST choose one destination
and transmit data to this destination address. and transmit data to this destination address.
b. The sender MUST NOT change the state of the chosen destination 2. The sender MUST NOT change the state of the chosen destination
address (it remains in inactive state) and it MUST NOT clear the address (it remains in inactive state) and MUST NOT clear the
error counter of the destination address as a result of choosing error counter of the destination address as a result of choosing
the destination address for data transmission. the destination address for data transmission.
c. The sender SHOULD choose the destination in inactive state with 3. The sender SHOULD choose the destination in inactive state with
the lowest error count (fewest consecutive timeouts) for data the lowest error count (fewest consecutive timeouts) for data
transmission. When there are multiple destinations with same transmission. When there are multiple destinations with the same
error count in inactive state, the sender SHOULD attempt to pick error count in inactive state, the sender SHOULD attempt to pick
the most divergent source - destination pair from the last source the most divergent source -- destination pair from the last
- destination pair where failure was observed. Rules for picking source -- destination pair where failure was observed. Rules for
the most divergent source-destination pair are an implementation picking the most divergent source-destination pair are an
decision and are not specified within this document. To support implementation decision and are not specified within this
differentiation of inactive destination addresses based on their document. To support differentiation of inactive destination
error count SCTP will need to allow for increment of the addresses based on their error count, SCTP will need to allow for
destination address error counters up to some reasonable limit incrementing of the destination address error counters up to some
above PMR+1, thus changing the prescriptions of [RFC4960], reasonable limit above PMR+1, thus changing the prescriptions of
section 8.3, in this respect. The exact limit to apply is not Section 8.3 of [RFC4960] in this respect. The exact limit to
specified in this document but it is considered reasonable to apply is not specified in this document, but it is considered
require for the limit to be an order of magnitude higher than the reasonable enough to require that the limit be an order of
PMR value. A sender MAY choose to deploy other strategies that magnitude higher than the PMR value. A sender MAY choose to
the strategy defined here. The strategy to prioritize the last deploy other strategies than the strategy defined here. The
active destination address, i.e., the destination address with strategy to prioritize the last active destination address, i.e.,
the fewest error counts is optimal when some paths are the destination address with the fewest error counts is optimal
permanently inactive, but suboptimal when a path instability is when some paths are permanently inactive, but suboptimal when
transient. path instability is transient.
5. Primary Path Switchover 5. Primary Path Switchover
The objective of the Primary Path Switchover operation is to allow The objective of the Primary Path Switchover operation is to allow
the SCTP sender to continue data transmission on a new working path the SCTP sender to continue data transmission on a new working path
even when the old primary destination address becomes active again. even when the old primary destination address becomes active again.
This is achieved by having SCTP perform a switchover of the primary This is achieved by having SCTP perform a switchover of the primary
path to the new working path if the error counter of the primary path path to the new working path if the error counter of the primary path
exceeds a certain threshold. This mode of operation can be applied exceeds a certain threshold. This mode of operation can be applied
not only to SCTP-PF implementations, but also to [RFC4960] not only to SCTP-PF implementations, but also to implementations of
implementations. [RFC4960].
The Primary Path Switchover operation requires only sender side The Primary Path Switchover operation requires only sender-side
changes. The details are: changes. The details are:
1. The sender maintains a new tunable parameter, called 1. The sender maintains a new tunable parameter, called
Primary.Switchover.Max.Retrans (PSMR). For SCTP-PF Primary.Switchover.Max.Retrans (PSMR). For SCTP-PF
implementations, the PSMR MUST be set greater or equal to the implementations, the PSMR MUST be set greater than or equal to
PFMR value. For [RFC4960] implementations the PSMR MUST be set the PFMR value. For implementations of [RFC4960], the PSMR MUST
greater or equal to the PMR value. Implementations MUST reject be set greater than or equal to the PMR value. Implementations
any other values of PSMR. MUST reject any other values of PSMR.
2. When the path error counter on a set primary path exceeds PSMR, 2. When the path error counter on a set primary path exceeds PSMR,
the SCTP implementation MUST autonomously select and set a new the SCTP implementation MUST autonomously select and set a new
primary path. primary path.
3. The primary path selected by the SCTP implementation MUST be the 3. The primary path selected by the SCTP implementation MUST be the
path which at the given time would be chosen for data transfer. path that, at the given time, would be chosen for data transfer.
A previously failed primary path can be used as data transfer A previously failed primary path can be used as a data transfer
path as per normal path selection when the present data transfer path as per normal path selection when the present data transfer
path fails. path fails.
4. For SCTP-PF, the recommended value of PSMR is PFMR when Primary 4. For SCTP-PF, the recommended value of PSMR is PFMR when Primary
Path Switchover operation mode is used. This means that no Path Switchover operation mode is used. This means that no
forced switchback to a previously failed primary path is forced switchback to a previously failed primary path is
performed. An SCTP-PF implementation of Primary Path Switchover performed. An SCTP-PF implementation of Primary Path Switchover
MUST support the setting of PSMR = PFMR. A SCTP-PF MUST support the setting of PSMR = PFMR. An SCTP-PF
implementation of Primary Path Switchover MAY support setting of implementation of Primary Path Switchover MAY support setting of
PSMR > PFMR. PSMR > PFMR.
5. For [RFC4960] SCTP, the recommended value of PSMR is PMR when 5. For standard SCTP, the recommended value of PSMR is PMR when
Primary Path Switchover is used. This means that no forced Primary Path Switchover is used. This means that no forced
switchback to a previously failed primary path is performed. A switchback to a previously failed primary path is performed. A
[RFC4960] SCTP implementation of Primary Path Switchover MUST standard SCTP implementation of Primary Path Switchover MUST
support the setting of PSMR = PMR. An [RFC4960] SCTP support the setting of PSMR = PMR. A standard SCTP
implementation of Primary Path Switchover MAY support larger implementation of Primary Path Switchover MAY support larger
settings of PSMR > PMR. settings of PSMR > PMR.
6. It MUST be possible to disable the Primary Path Switchover 6. It MUST be possible to disable the Primary Path Switchover
operation and obtain the standard switchback operation of operation and obtain the standard switchback operation of
[RFC4960]. [RFC4960].
The manner of switchover operation that is most optimal in a given The manner of switchover operation that is most optimal in a given
scenario depends on the relative quality of a set primary path versus scenario depends on the relative quality of a set primary path versus
the quality of alternative paths available as well as on the extent the quality of alternative paths available as well as on the extent
to which it is desired for the mode of operation to enforce traffic to which it is desired for the mode of operation to enforce traffic
distribution over a number of network paths. I.e., load distribution distribution over a number of network paths. That is, load
of traffic from multiple SCTP associations may be sought to be distribution of traffic from multiple SCTP associations may be
enforced by distribution of the set primary paths with [RFC4960] enforced by distribution of the set primary paths with the switchback
switchback operation. However as [RFC4960] switchback behavior is operation of [RFC4960]. However, as switchback behavior of [RFC4960]
suboptimal in certain situations, especially in scenarios where a is suboptimal in certain situations, especially in scenarios where a
number of equally good paths are available, an SCTP implementation number of equally good paths are available, an SCTP implementation
MAY support also, as alternative behavior, the Primary Path MAY support also, as alternative behavior, the Primary Path
Switchover mode of operation and MAY enable it based on applications' Switchover mode of operation and MAY enable it based on applications'
requests. requests.
For an SCTP implementation that implements the Primary Path For an SCTP implementation that implements the Primary Path
Switchover operation, this specification RECOMMENDS that the standard Switchover operation, this specification RECOMMENDS that the standard
RFC4960 switchback operation is retained as the default operation. switchback operation of [RFC4960] be retained as the default
operation.
6. Suggested SCTP Protocol Parameter Values 6. Suggested SCTP Protocol Parameter Values
This document does not alter the [RFC4960] value recommendation for This document does not alter the value recommendation for the SCTP
the SCTP Protocol Parameters defined in [RFC4960]. Protocol Parameters defined in [RFC4960].
The following protocol parameter is RECOMMENDED: The following protocol parameter is RECOMMENDED:
PotentiallyFailed.Max.Retrans (PFMR) - 0 PotentiallyFailed.Max.Retrans (PFMR) - 0
7. Socket API Considerations 7. Socket API Considerations
This section describes how the socket API defined in [RFC6458] is This section describes how the socket API defined in [RFC6458] is
extended to provide a way for the application to control and observe extended to provide a way for the application to control and observe
the SCTP-PF behavior as well as the Primary Path Switchover function. the SCTP-PF behavior as well as the Primary Path Switchover function.
Please note that this section is informational only. Please note that this section is informational only.
A socket API implementation based on [RFC6458] is, by means of the A socket API implementation based on [RFC6458] is, by means of the
existing SCTP_PEER_ADDR_CHANGE event, extended to provide the event existing SCTP_PEER_ADDR_CHANGE event, extended to provide the event
notification when a peer address enters or leaves the potentially notification when a peer address enters or leaves the PF state as
failed state as well as the socket API implementation is extended to well as the socket API implementation is extended to expose the PF
expose the potentially failed state of a peer address in the existing state of a peer address in the existing SCTP_GET_PEER_ADDR_INFO
SCTP_GET_PEER_ADDR_INFO structure. structure.
Furthermore, two new read/write socket options for the level Furthermore, two new read/write socket options for the level
IPPROTO_SCTP and the name SCTP_PEER_ADDR_THLDS and IPPROTO_SCTP and the name SCTP_PEER_ADDR_THLDS and
SCTP_EXPOSE_POTENTIALLY_FAILED_STATE are defined as described below. SCTP_EXPOSE_POTENTIALLY_FAILED_STATE are defined as described below.
The first socket option is used to control the values of the PFMR and The first socket option is used to control the values of the PFMR and
PSMR parameters described in Section 3 and in Section 5. The second PSMR parameters described in Sections 3 and 5. The second one
one controls the exposition of the potentially failed path state. controls the exposition of the PF path state.
Support for the SCTP_PEER_ADDR_THLDS and Support for the SCTP_PEER_ADDR_THLDS and
SCTP_EXPOSE_POTENTIALLY_FAILED_STATE socket options need also to be SCTP_EXPOSE_POTENTIALLY_FAILED_STATE socket options also needs to be
added to the function sctp_opt_info(). added to the function sctp_opt_info().
7.1. Support for the Potentially Failed Path State 7.1. Support for the Potentially Failed Path State
As defined in [RFC6458], the SCTP_PEER_ADDR_CHANGE event is provided As defined in [RFC6458], the SCTP_PEER_ADDR_CHANGE event is provided
if the status of a peer address changes. In addition to the state if the status of a peer address changes. In addition to the state
changes described in [RFC6458], this event is also provided, if a changes described in [RFC6458], this event is also provided if a peer
peer address enters or leaves the potentially failed state. The address enters or leaves the PF state. The notification as defined
notification as defined in [RFC6458] uses the following structure: in [RFC6458] uses the following structure:
struct sctp_paddr_change { struct sctp_paddr_change {
uint16_t spc_type; uint16_t spc_type;
uint16_t spc_flags; uint16_t spc_flags;
uint32_t spc_length; uint32_t spc_length;
struct sockaddr_storage spc_aaddr; struct sockaddr_storage spc_aaddr;
uint32_t spc_state; uint32_t spc_state;
uint32_t spc_error; uint32_t spc_error;
sctp_assoc_t spc_assoc_id; sctp_assoc_t spc_assoc_id;
} }
[RFC6458] defines the constants SCTP_ADDR_AVAILABLE, [RFC6458] defines the constants SCTP_ADDR_AVAILABLE,
SCTP_ADDR_UNREACHABLE, SCTP_ADDR_REMOVED, SCTP_ADDR_ADDED, and SCTP_ADDR_UNREACHABLE, SCTP_ADDR_REMOVED, SCTP_ADDR_ADDED, and
SCTP_ADDR_MADE_PRIM to be provided in the spc_state field. This SCTP_ADDR_MADE_PRIM to be provided in the spc_state field. This
document defines in addition to that the new constant document defines the new additional constant
SCTP_ADDR_POTENTIALLY_FAILED, which is reported if the affected SCTP_ADDR_POTENTIALLY_FAILED, which is reported if the affected
address becomes potentially failed. address becomes PF.
The SCTP_GET_PEER_ADDR_INFO socket option defined in [RFC6458] can be The SCTP_GET_PEER_ADDR_INFO socket option defined in [RFC6458] can be
used to query the state of a peer address. It uses the following used to query the state of a peer address. It uses the following
structure: structure:
struct sctp_paddrinfo { struct sctp_paddrinfo {
sctp_assoc_t spinfo_assoc_id; sctp_assoc_t spinfo_assoc_id;
struct sockaddr_storage spinfo_address; struct sockaddr_storage spinfo_address;
int32_t spinfo_state; int32_t spinfo_state;
uint32_t spinfo_cwnd; uint32_t spinfo_cwnd;
uint32_t spinfo_srtt; uint32_t spinfo_srtt;
uint32_t spinfo_rto; uint32_t spinfo_rto;
uint32_t spinfo_mtu; uint32_t spinfo_mtu;
}; };
[RFC6458] defines the constants SCTP_UNCONFIRMED, SCTP_ACTIVE, and [RFC6458] defines the constants SCTP_UNCONFIRMED, SCTP_ACTIVE, and
SCTP_INACTIVE to be provided in the spinfo_state field. This SCTP_INACTIVE to be provided in the spinfo_state field. This
document defines in addition to that the new constant document defines the new additional constant SCTP_POTENTIALLY_FAILED,
SCTP_POTENTIALLY_FAILED, which is reported if the peer address is which is reported if the peer address is PF.
potentially failed.
7.2. Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) Socket Option 7.2. Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) Socket Option
Applications can control the SCTP-PF behavior by getting or setting Applications can control the SCTP-PF behavior by getting or setting
the number of consecutive timeouts before a peer address is the number of consecutive timeouts before a peer address is
considered potentially failed or unreachable. The same socket option considered PF or unreachable. The same socket option is used by
is used by applications to set and get the number of timeouts before applications to set and get the number of timeouts before the primary
the primary path is changed automatically by the Primary Path path is changed automatically by the Primary Path Switchover
Switchover function. This socket option uses the level IPPROTO_SCTP function. This socket option uses the level IPPROTO_SCTP and the
and the name SCTP_PEER_ADDR_THLDS. name SCTP_PEER_ADDR_THLDS.
The following structure is used to access and modify the thresholds: The following structure is used to access and modify the thresholds:
struct sctp_paddrthlds { struct sctp_paddrthlds {
sctp_assoc_t spt_assoc_id; sctp_assoc_t spt_assoc_id;
struct sockaddr_storage spt_address; struct sockaddr_storage spt_address;
uint16_t spt_pathmaxrxt; uint16_t spt_pathmaxrxt;
uint16_t spt_pathpfthld; uint16_t spt_pathpfthld;
uint16_t spt_pathcpthld; uint16_t spt_pathcpthld;
}; };
spt_assoc_id: This parameter is ignored for one-to-one style spt_assoc_id: This parameter is ignored for one-to-one style
sockets. For one-to-many style sockets the application may fill sockets. For one-to-many style sockets, the application may fill
in an association identifier or SCTP_FUTURE_ASSOC. It is an error in an association identifier or SCTP_FUTURE_ASSOC. It is an error
to use SCTP_{CURRENT|ALL}_ASSOC in spt_assoc_id. to use SCTP_{CURRENT|ALL}_ASSOC in spt_assoc_id.
spt_address: This specifies which peer address is of interest. If a spt_address: This specifies which peer address is of interest. If a
wild card address is provided, this socket option applies to all wildcard address is provided, this socket option applies to all
current and future peer addresses. current and future peer addresses.
spt_pathmaxrxt: Each peer address of interest is considered spt_pathmaxrxt: Each peer address of interest is considered
unreachable, if its path error counter exceeds spt_pathmaxrxt. unreachable, if its path error counter exceeds spt_pathmaxrxt.
spt_pathpfthld: Each peer address of interest is considered spt_pathpfthld: Each peer address of interest is considered PF, if
Potentially Failed, if its path error counter exceeds its path error counter exceeds spt_pathpfthld.
spt_pathpfthld.
spt_pathcpthld: Each peer address of interest is not considered the spt_pathcpthld: Each peer address of interest is not considered the
primary remote address anymore, if its path error counter exceeds primary remote address anymore, if its path error counter exceeds
spt_pathcpthld. Using a value of 0xffff disables the selection of spt_pathcpthld. Using a value of 0xffff disables the selection of
a new primary peer address. If an implementation does not support a new primary peer address. If an implementation does not support
the automatically selection of a new primary address, it should the automatic selection of a new primary address, it should
indicate an error with errno set to EINVAL if a value different indicate an error with errno set to EINVAL if a value different
from 0xffff is used in spt_pathcpthld. For SCTP-PF, the setting from 0xffff is used in spt_pathcpthld. For SCTP-PF, the setting
of spt_pathcpthld < spt_pathpfthld should be rejected with errno of spt_pathcpthld < spt_pathpfthld should be rejected with errno
set to EINVAL. For [RFC4960] SCTP, the setting of spt_pathcpthld set to EINVAL. For standard SCTP, the setting of spt_pathcpthld <
< spt_pathmaxrxt should be rejected with errno set to EINVAL. A spt_pathmaxrxt should be rejected with errno set to EINVAL. An
SCTP-PF implementation may support only setting of spt_pathcpthld SCTP-PF implementation may support only setting of spt_pathcpthld
= spt_pathpfthld and spt_pathcpthld = 0xffff and a [RFC4960] SCTP = spt_pathpfthld and spt_pathcpthld = 0xffff and a standard SCTP
implementation may support only setting of spt_pathcpthld = implementation may support only setting of spt_pathcpthld =
spt_pathmaxrxt and spt_pathcpthld = 0xffff. In these cases SCTP spt_pathmaxrxt and spt_pathcpthld = 0xffff. In these cases, SCTP
shall reject setting of other values with errno set to EINVAL. shall reject setting of other values with errno set to EINVAL.
7.3. Exposing the Potentially Failed Path State 7.3. Exposing the Potentially Failed Path State
(SCTP_EXPOSE_POTENTIALLY_FAILED_STATE) Socket Option (SCTP_EXPOSE_POTENTIALLY_FAILED_STATE) Socket Option
Applications can control the exposure of the potentially failed path Applications can control the exposure of the PF path state in the
state in the SCTP_PEER_ADDR_CHANGE event and the SCTP_PEER_ADDR_CHANGE event and the SCTP_GET_PEER_ADDR_INFO as
SCTP_GET_PEER_ADDR_INFO as described in Section 7.1. The default described in Section 7.1. The default value is implementation
value is implementation specific. specific.
This socket option uses the level IPPROTO_SCTP and the name This socket option uses the level IPPROTO_SCTP and the name
SCTP_EXPOSE_POTENTIALLY_FAILED_STATE. SCTP_EXPOSE_POTENTIALLY_FAILED_STATE.
The following structure is used to control the exposition of the The following structure is used to control the exposition of the PF
potentially failed path state: path state:
struct sctp_assoc_value { struct sctp_assoc_value {
sctp_assoc_t assoc_id; sctp_assoc_t assoc_id;
uint32_t assoc_value; uint32_t assoc_value;
}; };
assoc_id: This parameter is ignored for one-to-one style sockets. assoc_id: This parameter is ignored for one-to-one style sockets.
For one-to-many style sockets the application may fill in an For one-to-many style sockets, the application may fill in an
association identifier or SCTP_FUTURE_ASSOC. It is an error to association identifier or SCTP_FUTURE_ASSOC. It is an error to
use SCTP_{CURRENT|ALL}_ASSOC in assoc_id. use SCTP_{CURRENT|ALL}_ASSOC in assoc_id.
assoc_value: The potentially failed path state is exposed if and assoc_value: The PF path state is exposed if, and only if, this
only if this parameter is non-zero. parameter is non-zero.
8. Security Considerations 8. Security Considerations
Security considerations for the use of SCTP and its APIs are Security considerations for the use of SCTP and its APIs are
discussed in [RFC4960] and [RFC6458]. discussed in [RFC4960] and [RFC6458].
The logic introduced by this document does not impact existing SCTP The logic introduced by this document does not impact existing SCTP
messages on the wire. Also, this document does not introduce any new messages on the wire. Also, this document does not introduce any new
SCTP messages on the wire that require new security considerations. SCTP messages on the wire that require new security considerations.
SCTP-PF makes SCTP not only more robust during primary path failure/ SCTP-PF makes SCTP not only more robust during primary path failure/
congestion but also more vulnerable to network connectivity/ congestion, but also more vulnerable to network connectivity/
congestion attacks on the primary path. SCTP-PF makes it easier for congestion attacks on the primary path. SCTP-PF makes it easier for
an attacker to trick SCTP to change data transfer path, since the an attacker to trick SCTP into changing the data transfer path, since
duration of time that an attacker needs to negatively influence the the duration of time that an attacker needs to negatively influence
network connectivity is much shorter than [RFC4960]. However, SCTP- the network connectivity is much shorter than used in [RFC4960].
PF does not constitute a significant change in the duration of time However, SCTP-PF does not constitute a significant change in the
and effort an attacker needs to keep SCTP away from the primary path. duration of time and effort an attacker needs to keep SCTP away from
With the standard switchback operation [RFC4960] SCTP resumes data the primary path. With the standard switchback operation in
transfer on its primary path as soon as the next HEARTBEAT succeeds. [RFC4960], SCTP resumes data transfer on its primary path as soon as
the next HEARTBEAT succeeds.
On the other hand, usage of the Primary Path Switchover mechanism, On the other hand, usage of the Primary Path Switchover mechanism,
does change the threat analysis. This is because on-path attackers does change the threat analysis. This is because on-path attackers
can force a permanent change of the data transfer path by blocking can force a permanent change of the data transfer path by blocking
the primary path until the switchover of the primary path is the primary path until the switchover of the primary path is
triggered by the Primary Path Switchover algorithm. This especially triggered by the Primary Path Switchover algorithm. This will
will be the case when the Primary Path Switchover is used together especially be the case when the Primary Path Switchover is used
with SCTP-PF with the particular setting of PSMR = PFMR = 0, as together with SCTP-PF with the particular setting of PSMR = PFMR = 0,
Primary Path Switchover here happens already at the first RTO timeout as Primary Path Switchover here happens already at the first RTO
experienced. Users of the Primary Path Switchover mechanism should timeout experienced. Users of the Primary Path Switchover mechanism
be aware of this fact. should be aware of this fact.
The event notification of path state transfer from active to The event notification of path state transfer from active to PF state
potentially failed state and vice versa gives attackers an increased and vice versa gives attackers an increased possibility to generate
possibility to generate more local events. However, it is assumed more local events. However, it is assumed that event notifications
that event notifications are rate-limited in the implementation to are rate-limited in the implementation to address this threat.
address this threat.
9. MIB Considerations 9. MIB Considerations
SCTP-PF introduces new SCTP algorithms for failover and switchback SCTP-PF introduces new SCTP algorithms for failover and switchback
with associated new state parameters. It is recommended that the with associated new state parameters. It is recommended that the
SCTP-MIB defined in [RFC3873] is updated to support the management of SCTP-MIB defined in [RFC3873] is updated to support the management of
the SCTP-PF implementation. This can be done by extending the the SCTP-PF implementation. This can be done by extending the
sctpAssocRemAddrActive field of the SCTPAssocRemAddrTable to include sctpAssocRemAddrActive field of the SCTPAssocRemAddrTable to include
information of the PF state of the destination address and by adding information of the PF state of the destination address and by adding
new fields to the SCTPAssocRemAddrTable supporting new fields to the SCTPAssocRemAddrTable supporting
PotentiallyFailed.Max.Retrans (PFMR) and PotentiallyFailed.Max.Retrans (PFMR) and
Primary.Switchover.Max.Retrans (PSMR) parameters. Primary.Switchover.Max.Retrans (PSMR) parameters.
10. IANA Considerations 10. References
This document does not create any new registries or modify the rules
for any existing registries managed by IANA.
11. Acknowledgements
The authors wish to thank Michael Tuexen for his many invaluable
comments and for his very substantial support with the making of this
document.
12. Proposed Change of Status (to be Deleted before Publication)
Initially this work looked to entail some changes of the Congestion
Control (CC) operation of SCTP and for this reason the work was
proposed as Experimental. These intended changes of the CC operation
have since been judged to be irrelevant and are no longer part of the
specification. As the specification entails no other potential
harmful features, consensus exists in the WG to bring the work
forward as PS.
Initially concerns have been expressed about the possibility for the
mechanism to introduce path bouncing with potential harmful network
impacts. These concerns are believed to be unfounded. This issue is
addressed in Appendix B.
It is noted that the feature specified by this document is
implemented by multiple SCTP SW implementations and furthermore that
various variants of the solution have been deployed in telephony
signaling environments for several years with good results.
13. References
13.1. Normative References 10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC [RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
4960, September 2007. RFC 4960, DOI 10.17487/RFC4960, September 2007,
<http://www.rfc-editor.org/info/rfc4960>.
13.2. Informative References 10.2. Informative References
[CARO02] Caro Jr., A., Iyengar, J., Amer, P., Heinz, G., and R. [CARO02] Caro, A., Iyengar, J., Amer, P., Heinz, G., and R.
Stewart, "A Two-level Threshold Recovery Mechanism for Stewart, "A Two-level Threshold Recovery Mechanism for
SCTP", Tech report, CIS Dept, University of Delaware , 7 SCTP", Tech report, CIS Dept., University of Delaware,
2002. July 2002.
[CARO04] Caro Jr., A., Amer, P., and R. Stewart, "End-to-End [CARO04] Caro, A., Amer, P., and R. Stewart, "End-to-End Failover
Failover Thresholds for Transport Layer Multi homing", Thresholds for Transport Layer Multihoming", MILCOM 2004,
MILCOM 2004 , 11 2004. DOI 10.1109/MILCOM.2004.1493253, November 2004.
[CARO05] Caro Jr., A., "End-to-End Fault Tolerance using Transport [CARO05] Caro, A., "End-to-End Fault Tolerance using Transport
Layer Multi homing", Ph.D Thesis, University of Delaware , Layer Multihoming", Ph.D. Thesis, University of Delaware,
1 2005. DOI 10.1007/BF03219970, January 2005.
[FALLON08] [FALLON08]
Fallon, S., Jacob, P., Qiao, Y., Murphy, L., Fallon, E., Fallon, S., Jacob, P., Qiao, Y., Murphy, L., Fallon, E.,
and A. Hanley, "SCTP Switchover Performance Issues in WLAN and A. Hanley, "SCTP Switchover Performance Issues in WLAN
Environments", IEEE CCNC 2008, 1 2008. Environments", IEEE CCNC, DOI 10.1109/ccnc08.2007.131,
January 2008.
[GRINNEMO04] [GRINNEMO04]
Grinnemo, K-J. and A. Brunstrom, "Performance of SCTP- Grinnemo, K-J. and A. Brunstrom, "Performance of SCTP-
controlled failovers in M3UA-based SIGTRAN networks", controlled failovers in M3UA-based SIGTRAN networks",
Advanced Simulation Technologies Conference , 4 2004. Advanced Simulation Technologies Conference, April 2004.
[IYENGAR06] [IYENGAR06]
Iyengar, J., Amer, P., and R. Stewart, "Concurrent Iyengar, J., Amer, P., and R. Stewart, "Concurrent
Multipath Transfer using SCTP Multihoming over Independent Multipath Transfer using SCTP Multihoming over Independent
End-to-end Paths.", IEEE/ACM Trans on Networking 14(5), 10 End-to-end Paths", IEEE/ACM Transactions on Networking,
2006. DOI 10.1109/TNET.2006.882843, October 2006.
[JUNGMAIER02] [JUNGMAIER02]
Jungmaier, A., Rathgeb, E., and M. Tuexen, "On the use of Jungmaier, A., Rathgeb, E., and M. Tuexen, "On the use of
SCTP in failover scenarios", World Multiconference on SCTP in failover scenarios", World Multiconference on
Systemics, Cybernetics and Informatics , 7 2002. Systemics, Cybernetics and Informatics, July 2002.
[NATARAJAN09] [NATARAJAN09]
Natarajan, P., Ekiz, N., Amer, P., and R. Stewart, Natarajan, P., Ekiz, N., Amer, P., and R. Stewart,
"Concurrent Multipath Transfer during Path Failure", "Concurrent Multipath Transfer during Path Failure",
Computer Communications , 5 2009. Computer Communications, DOI 10.1016/j.comcom.2009.05.001,
May 2009.
[RFC3873] Pastor, J. and M. Belinchon, "Stream Control Transmission [RFC3873] Pastor, J. and M. Belinchon, "Stream Control Transmission
Protocol (SCTP) Management Information Base (MIB)", RFC Protocol (SCTP) Management Information Base (MIB)",
3873, DOI 10.17487/RFC3873, September 2004, RFC 3873, DOI 10.17487/RFC3873, September 2004,
<http://www.rfc-editor.org/info/rfc3873>. <http://www.rfc-editor.org/info/rfc3873>.
[RFC6458] Stewart, R., Tuexen, M., Poon, K., Lei, P., and V. [RFC6458] Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
Yasevich, "Sockets API Extensions for the Stream Control Yasevich, "Sockets API Extensions for the Stream Control
Transmission Protocol (SCTP)", RFC 6458, December 2011. Transmission Protocol (SCTP)", RFC 6458,
DOI 10.17487/RFC6458, December 2011,
<http://www.rfc-editor.org/info/rfc6458>.
Appendix A. Discussions of Alternative Approaches Appendix A. Discussion of Alternative Approaches
This section lists alternative approaches for the issues described in This section lists alternative approaches for the issues described in
this document. Although these approaches do not require to update this document. Although these approaches do not require updating RFC
RFC4960, we do not recommend them from the reasons described below. 4960, we do not recommend them for the reasons described below.
A.1. Reduce Path.Max.Retrans (PMR) A.1. Reduce PMR
Smaller values for Path.Max.Retrans shorten the failover duration and Smaller values for Path.Max.Retrans shorten the failover duration and
in fact this is recommended in some research results [JUNGMAIER02] in fact, this is recommended in some research results [JUNGMAIER02],
[GRINNEMO04] [FALLON08]. However to significantly reduce the [GRINNEMO04], and [FALLON08]. However, to significantly reduce the
failover time it is required to go down (as with PFMR) to failover time, it is required to go down (as with PFMR) to
Path.Max.Retrans=0 and with this setting SCTP switches to another Path.Max.Retrans=0 and, with this setting, SCTP switches to another
destination address already on a single timeout which may result in destination address already on a single timeout that may result in
spurious failover. Spurious failover is a problem in [RFC4960] SCTP spurious failover. Spurious failover is a problem in standard SCTP
as the transmission of HEARTBEATS on the left primary path, unlike in as the transmission of HEARTBEATs on the left primary path, unlike in
SCTP-PF, is governed by 'HB.interval' also during the failover SCTP-PF, is governed by HB.Interval also during the failover process.
process. 'HB.interval' is usually set in the order of seconds HB.Interval is usually set in the order of seconds (recommended value
(recommended value is 30 seconds) and when the primary path becomes is 30 seconds) and when the primary path becomes inactive, the next
inactive, the next HEARTBEAT may be transmitted only many seconds HEARTBEAT may be transmitted only many seconds later: as recommended,
later. Indeed as recommended, only 30 secs later. Meanwhile, the only 30 seconds later. Meanwhile, the primary path may have long
primary path may since long have recovered, if it needed recovery at since recovered, if it needed recovery at all (indeed the failover
all (indeed the failover could be truly spurious). In such could be truly spurious). In such situations, post failover, an
situations, post failover, an endpoint is forced to wait in the order endpoint is forced to wait in the order of many seconds before the
of many seconds before the endpoint can resume transmission on the endpoint can resume transmission on the primary path and furthermore,
primary path and furthermore once it returns on the primary path the once it returns on the primary path, the CWND needs to be rebuilt
CWND needs to be rebuild anew - a process which the throughput anew -- a process that the throughput already had to suffer from on
already have had to suffer from on the alternate path. Using a the alternate path. Using a smaller value for HB.Interval might help
smaller value for 'HB.interval' might help this situation, but it this situation, but it would result in a general waste of bandwidth
would result in a general waste of bandwidth as such more frequent as such more frequent HEARTBEATING would take place also when there
HEARTBEATING would take place also when there are no observed are no observed troubles. The bandwidth overhead may be diminished
troubles. The bandwidth overhead may be diminished by having the ULP by having the ULP use a smaller HB.Interval only on the path that, at
use a smaller 'HB.interval' only on the path which at any given time any given time, is set to be the primary path; however, this adds
is set to be the primary path, but this adds complication in the ULP. complication in the ULP.
In addition, smaller Path.Max.Retrans values also affect the In addition, smaller Path.Max.Retrans values also affect the
'Association.Max.Retrans' value. When the SCTP association's error Association.Max.Retrans value. When the SCTP association's error
count exceeds Association.Max.Retrans threshold, the SCTP sender count exceeds Association.Max.Retrans threshold, the SCTP sender
considers the peer endpoint unreachable and terminates the considers the peer endpoint unreachable and terminates the
association. Section 8.2 in [RFC4960] recommends that association. Section 8.2 in [RFC4960] recommends that the
Association.Max.Retrans value should not be larger than the summation Association.Max.Retrans value should not be larger than the summation
of the Path.Max.Retrans of each of the destination addresses. Else of the Path.Max.Retrans of each of the destination addresses.
the SCTP sender considers its peer reachable even when all
destinations are INACTIVE and to avoid this dormant state operation, Otherwise, the SCTP sender considers its peer reachable even when all
[RFC4960] SCTP implementation SHOULD reduce Association.Max.Retrans destinations are INACTIVE. To avoid this dormant state operation,
standard SCTP implementation SHOULD reduce Association.Max.Retrans
accordingly whenever it reduces Path.Max.Retrans. However, smaller accordingly whenever it reduces Path.Max.Retrans. However, smaller
Association.Max.Retrans value decreases the fault tolerance of SCTP Association.Max.Retrans value decreases the fault tolerance of SCTP
as it increases the chances of association termination during minor as it increases the chances of association termination during minor
congestion events. congestion events.
A.2. Adjust RTO related parameters A.2. Adjust RTO-Related Parameters
As several research results indicate, we can also shorten the As several research results indicate, we can also shorten the
duration of failover process by adjusting RTO related parameters duration of the failover process by adjusting the RTO-related
[JUNGMAIER02] [FALLON08]. During failover process, RTO keeps being parameters [JUNGMAIER02] and [FALLON08]. During the failover
doubled. However, if we can choose smaller value for RTO.max, we can process, RTO keeps being doubled. However, if we can choose a
stop the exponential growth of RTO at some point. Also, choosing smaller value for RTO.max, we can stop the exponential growth of RTO
smaller values for RTO.initial or RTO.min can contribute to keep the at some point. Also, choosing smaller values for RTO.initial or
RTO value small. RTO.min can contribute to keeping the RTO value small.
Similar to reducing Path.Max.Retrans, the advantage of this approach Similar to reducing Path.Max.Retrans, the advantage of this approach
is that it requires no modification to the current specification, is that it requires no modification to the current specification,
although it needs to ignore several recommendations described in the although it needs to ignore several recommendations described in
Section 15 of [RFC4960]. However, this approach requires to have Section 15 of [RFC4960]. However, this approach requires having
enough knowledge about the network characteristics between end enough knowledge about the network characteristics between endpoints.
points. Otherwise, it can introduce adverse side-effects such as Otherwise, it can introduce adverse side effects such as spurious
spurious timeouts. timeouts.
The significant issue with this approach, however, is that even if The significant issue with this approach, however, is that even if
the RTO.max is lowered to an optimal low value, then as long as the the RTO.max is lowered to an optimal low value, as long as the
Path.Max.Retrans is kept at the [RFC4960] recommended value, the Path.Max.Retrans is kept at the recommended value from [RFC4960], the
reduction of the RTO.max doesn't reduce the failover time reduction of the RTO.max doesn't reduce the failover time
sufficiently enough to prevent severe performance degradation during sufficiently enough to prevent severe performance degradation during
failover. failover.
Appendix B. Discussions for Path Bouncing Effect Appendix B. Discussion of the Path-Bouncing Effect
The methods described in the document can accelerate the failover The methods described in the document can accelerate the failover
process. Hence, they might introduce the path bouncing effect where process. Hence, they might introduce a path-bouncing effect in which
the sender keeps changing the data transmission path frequently. the sender keeps changing the data transmission path frequently.
This sounds harmful to the data transfer, however several research This sounds harmful to the data transfer; however, several research
results indicate that there is no serious problem with SCTP in terms results indicate that there is no serious problem with SCTP in terms
of path bouncing effect [CARO04] [CARO05]. of the path-bouncing effect (see [CARO04] and [CARO05]).
There are two main reasons for this. First, SCTP is basically There are two main reasons for this. First, SCTP is basically
designed for multipath communication, which means SCTP maintains all designed for multipath communication, which means SCTP maintains all
path related parameters (CWND, ssthresh, RTT, error count, etc) per path-related parameters (CWND, ssthresh, RTT, error count, etc.) per
each destination address. These parameters cannot be affected by each destination address. These parameters cannot be affected by
path bouncing. In addition, when SCTP migrates the data transfer to path bouncing. In addition, when SCTP migrates the data transfer to
another path, it starts with the minimal or the initial CWND. Hence, another path, it starts with the minimal or the initial CWND. Hence,
there is little chance for packet reordering or duplicating. there is little chance for packet reordering or duplicating.
Second, even if all communication paths between the end-nodes share Second, even if all communication paths between the end nodes share
the same bottleneck, the SCTP-PF results in a behavior already the same bottleneck, the SCTP-PF results in a behavior already
allowed by [RFC4960]. allowed by [RFC4960].
Appendix C. SCTP-PF for SCTP Single-homed Operation Appendix C. SCTP-PF for SCTP Single-Homed Operation
For a single-homed SCTP association the only tangible effect of the For a single-homed SCTP association, the only tangible effect of the
activation of SCTP-PF operation is enhanced failure detection in activation of SCTP-PF operation is enhanced failure detection in
terms of potential notification of the PF state of the sole terms of potential notification of the PF state of the sole
destination address as well as, for idle associations, more rapid destination address as well as, for idle associations, more rapid
entering, and notification, of inactive state of the destination entering, and notification, of inactive state of the destination
address and more rapid end-point failure detection. It is believed address and more rapid endpoint failure detection. It is believed
that neither of these effects are harmful, provided adequate dormant that neither of these effects are harmful, provided adequate dormant
state operation is implemented, and furthermore that they may be state operation is implemented. Furthermore, it is believed that
particularly useful for applications that deploys multiple SCTP they may be particularly useful for applications that deploy multiple
associations for load balancing purposes. The early notification of SCTP associations for load-balancing purposes. The early
the PF state may be used for preventive measures as the entering of notification of the PF state may be used for preventive measures as
the PF state can be used as a warning of potential congestion. the entering of the PF state can be used as a warning of potential
Depending on the PMR value, the aggressive HEARTBEAT transmission in congestion. Depending on the PMR value, the aggressive HEARTBEAT
PF state may speed up the end-point failure detection (exceed of AMR transmission in PF state may speed up the endpoint failure detection
threshold on the sole path error counter) on idle associations in (exceed of AMR threshold on the sole path error counter) on idle
case where relatively large HB.interval value compared to RTO (e.g. associations in the case with a relatively large HB.Interval value
30secs) is used. compared to RTO (e.g., 30 seconds) is used.
Acknowledgments
The authors would like to acknowledge members of the IETF Transport
Area Working Group (tsvwg) for continuing discussions on this
document and insightful feedback, and we appreciate continuous
encouragement and suggestions from the Chairs of the tsvwg. We
especially wish to thank Michael Tuexen for his many invaluable
comments and for his substantial supports with the making of the
document.
Authors' Addresses Authors' Addresses
Yoshifumi Nishida Yoshifumi Nishida
GE Global Research GE Global Research
2623 Camino Ramon 2623 Camino Ramon
San Ramon, CA 94583 San Ramon, CA 94583
USA United States
Email: nishida@wide.ad.jp Email: nishida@wide.ad.jp
Preethi Natarajan Preethi Natarajan
Cisco Systems Cisco Systems
510 McCarthy Blvd 510 McCarthy Blvd.
Milpitas, CA 95035 Milpitas, CA 95035
USA United States
Email: prenatar@cisco.com Email: prenatar@cisco.com
Armando Caro Armando Caro
BBN Technologies BBN Technologies
10 Moulton St. 10 Moulton St.
Cambridge, MA 02138 Cambridge, MA 02138
USA United States
Email: acaro@bbn.com Email: acaro@bbn.com
Paul D. Amer Paul D. Amer
University of Delaware University of Delaware
Computer Science Department - 434 Smith Hall Computer Science Department - 434 Smith Hall
Newark, DE 19716-2586 Newark, DE 19716-2586
USA United States
Email: amer@udel.edu Email: amer@udel.edu
Karen E. E. Nielsen Karen E. E. Nielsen
Ericsson Ericsson
Kistavaegen 25 Kistavaegen 25
Stockholm 164 80 Stockholm 164 80
Sweden Sweden
Email: karen.nielsen@tieto.com Email: karen.nielsen@tieto.com
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