draft-ietf-tsvwg-sctp-failover-11.txt   draft-ietf-tsvwg-sctp-failover-12.txt 
Network Working Group Y. Nishida Network Working Group Y. Nishida
Internet-Draft GE Global Research Internet-Draft GE Global Research
Intended status: Standards Track P. Natarajan Intended status: Standards Track P. Natarajan
Expires: January 18, 2016 Cisco Systems Expires: March 3, 2016 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
July 17, 2015 August 31, 2015
SCTP-PF: Quick Failover Algorithm in SCTP SCTP-PF: Quick Failover Algorithm in SCTP
draft-ietf-tsvwg-sctp-failover-11.txt draft-ietf-tsvwg-sctp-failover-12.txt
Abstract Abstract
SCTP supports multi-homing. However, when the failover operation SCTP supports multi-homing. However, when the failover operation
specified in RFC4960 is followed, there can be significant delay and specified in RFC4960 is followed, there can be significant delay and
performance degradation in the data transfer path failover. To performance degradation in the data transfer path failover. To
overcome this problem this document specifies a quick failover overcome this problem this document specifies a quick failover
algorithm (SCTP-PF) based on the introduction of a Potentially Failed algorithm (SCTP-PF) based on the introduction of a Potentially Failed
(PF) state in SCTP Path Management. (PF) state in SCTP Path Management.
The document also specifies a dormant state operation of SCTP. This The document also specifies a dormant state operation of SCTP. This
dormant state operation is required to be followed by an SCTP-PF dormant state operation is required to be followed by an SCTP-PF
implementation, but it may equally well be applied by a standard implementation, but it may equally well be applied by a standard
RFC4960 SCTP implementation. RFC4960 SCTP implementation.
Additionally, the document introduces an alternative switchback mode Additionally, the document introduces an alternative switchback
called Permanent Failover that will be beneficial in some situations. operation mode called Primary Path Switchover that will be beneficial
This mode of operation applies to both a standard RFC4960 SCTP in certain situations. This mode of operation applies to both a
implementation as well as to a SCTP-PF implementation. standard RFC4960 SCTP implementation as well as to a SCTP-PF
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 RFC4960 specification. 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 Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on January 18, 2016. This Internet-Draft will expire on March 3, 2016.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 4 2. Conventions and Terminology . . . . . . . . . . . . . . . . . 4
3. SCTP with Potentially-Failed Destination State (SCTP-PF) . . 4 3. SCTP with Potentially Failed Destination State (SCTP-PF) . . 4
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Specification of the SCTP-PF Procedures . . . . . . . . . 5 3.2. Specification of the SCTP-PF Procedures . . . . . . . . . 5
4. Dormant State Operation . . . . . . . . . . . . . . . . . . . 9 4. Dormant State Operation . . . . . . . . . . . . . . . . . . . 9
4.1. SCTP Dormant State Procedure . . . . . . . . . . . . . . 10 4.1. SCTP Dormant State Procedure . . . . . . . . . . . . . . 10
5. Permanent Failover . . . . . . . . . . . . . . . . . . . . . 11 5. Primary Path Switchover . . . . . . . . . . . . . . . . . . . 11
6. Suggested SCTP Protocol Parameter Values . . . . . . . . . . 12 6. Suggested SCTP Protocol Parameter Values . . . . . . . . . . 12
7. Socket API Considerations . . . . . . . . . . . . . . . . . . 12 7. Socket API Considerations . . . . . . . . . . . . . . . . . . 12
7.1. Support for the Potentially Failed Path State . . . . . . 13 7.1. Support for the Potentially Failed Path State . . . . . . 13
7.2. Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) Socket 7.2. Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) Socket
Option . . . . . . . . . . . . . . . . . . . . . . . . . 14 Option . . . . . . . . . . . . . . . . . . . . . . . . . 14
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 . . 15
8. Security Considerations . . . . . . . . . . . . . . . . . . . 15 8. Security Considerations . . . . . . . . . . . . . . . . . . . 16
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
11. Proposed Change of Status (to be Deleted before Publication) 16 11. Proposed Change of Status (to be Deleted before Publication) 17
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
12.1. Normative References . . . . . . . . . . . . . . . . . . 17 12.1. Normative References . . . . . . . . . . . . . . . . . . 17
12.2. Informative References . . . . . . . . . . . . . . . . . 17 12.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. Discussions of Alternative Approaches . . . . . . . 18 Appendix A. Discussions of Alternative Approaches . . . . . . . 18
A.1. Reduce Path.Max.Retrans (PMR) . . . . . . . . . . . . . . 18 A.1. Reduce Path.Max.Retrans (PMR) . . . . . . . . . . . . . . 18
A.2. Adjust RTO related parameters . . . . . . . . . . . . . . 19 A.2. Adjust RTO related parameters . . . . . . . . . . . . . . 19
Appendix B. Discussions for Path Bouncing Effect . . . . . . . . 19 Appendix B. Discussions for Path Bouncing Effect . . . . . . . . 20
Appendix C. SCTP-PF for SCTP Single-homed Operation . . . . . . 20 Appendix C. SCTP-PF for SCTP Single-homed Operation . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 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 multi [RFC4960] supports multi-homing at the transport layer. SCTP's
homing features include failure detection and failover procedures to multi-homing features include failure detection and failover
provide network interface redundancy and improved end-to-end fault procedures to provide network interface redundancy and improved end-
tolerance. In SCTP's current failure detection procedure, the sender to-end fault tolerance. In SCTP's current failure detection
must experience Path.Max.Retrans (PMR) number of consecutive failed procedure, the sender must experience Path.Max.Retrans (PMR) number
timer-based retransmissions on a destination address before detecting of consecutive failed timer-based retransmissions on a destination
a path failure. Until detecting the path failure, the sender address before detecting a path failure. Until detecting the path
continues to transmit data on the failed path. The prolonged time in failure, the sender continues to transmit data on the failed path.
which [RFC4960] SCTP continues to use a failed path severely degrades The prolonged time in which [RFC4960] SCTP continues to use a failed
the performance of the protocol. To address this problem, this path severely degrades the performance of the protocol. To address
document specifies a quick failover algorithm (SCTP-PF) based on the this problem, this document specifies a quick failover algorithm
introduction of a new Potentially Failed path state in SCTP path (SCTP-PF) based on the introduction of a new Potentially Failed (PF)
management. The performance deficiencies of the [RFC4960] failover path state in SCTP path management. The performance deficiencies of
operation, and the improvements obtainable from the introduction of a the [RFC4960] failover operation, and the improvements obtainable
Potentially Failed state in SCTP, were proposed and documented in from the introduction of a Potentially Failed state in SCTP, were
[NATARAJAN09] for Concurrent Multipath Transfer SCTP [IYENGAR06]. proposed and documented in [NATARAJAN09] for Concurrent Multipath
Transfer SCTP [IYENGAR06].
While SCTP-PF can accelerate failover process and improve While SCTP-PF can accelerate failover process and improve
performance, the risks that an SCTP endpoint enters in dormant state performance, the risks that an SCTP endpoint enters in dormant state
where all destination addresses are inactive can be increased. 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 to avoid entering the state as much as
possible by careful tuning of the Path.Max.Retrans (PMR) and possible by careful tuning of the Path.Max.Retrans (PMR) and
Association.Max.Retrans (AMR) parameters. We specify a dormant state Association.Max.Retrans (AMR) parameters. We specify a dormant state
operation for SCTP-PF which makes SCTP-PF provide the same disruption operation for SCTP-PF which makes SCTP-PF provide the same disruption
tolerance as [RFC4960] despite that the dormant state may be entered tolerance as [RFC4960] despite that the dormant state may be entered
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The operation after the recovery of a failed path equally well The operation after the recovery of a failed path equally well
impacts the performance of the protocol. With the procedures impacts the performance of the protocol. With the procedures
specified in [RFC4960] SCTP will, after a failover from the primary specified in [RFC4960] SCTP will, after a failover from the primary
path, switch back to use the primary path for data transfer as soon path, switch back to use the primary path for data transfer as soon
as this path becomes available again. From a performance perspective as this path becomes available again. From a performance perspective
such a forced switchback of the data transmission path can be such a forced switchback of the data transmission path can be
suboptimal as the CWND towards the original primary destination suboptimal as the CWND towards the original primary destination
address has to be rebuilt once data transfer resumes, [CARO02]. As address has to be rebuilt once data transfer resumes, [CARO02]. As
an optional alternative to the switchback operation of [RFC4960], an optional alternative to the switchback operation of [RFC4960],
this document specifies an alternative Permanent Failover procedure this document specifies an alternative Primary Path Switchover
which avoid such forced switchbacks of the data transfer path. The procedure which avoid such forced switchbacks of the data transfer
Permanent Failover 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 primarily is motivated by a desire to improve the
multi-homed operation, the feature applies also to SCTP single-homed multi-homed operation, the feature applies also 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
Potentially Failed state including a discussion of alternative Potentially Failed state including a discussion of alternative
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operation are given in the Appendices. Discussion of path bouncing operation are given in the Appendices. Discussion of path bouncing
effects that might be caused by frequent switchover, are also effects that might be caused by frequent switchover, 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 Destination State (SCTP-PF)
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 [RFC4960] SCTP path management scheme, the sender
stops transmitting data to a destination address only after the stops transmitting data to a destination address only after the
destination address is marked inactive. This process takes a destination address is marked inactive. This process takes a
significant amount of time as it requires the error counter of the 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 Path.Max.Retrans (PMR) threshold.
The issue cannot simply be mitigated by lowering of the PMR threshold The issue cannot simply be mitigated by lowering of the PMR threshold
because this may result in spurious failure detection and unnecessary because this may result in spurious failure detection and unnecessary
prevention of the usage of a preferred primary path as well as it, prevention of the usage of a preferred primary path as well as it,
due to the coupled tuning of the Path.Max.Retrans (PMR) and the due to the coupled tuning of the Path.Max.Retrans (PMR) and the
Association.Max.Retrans (AMR) parameter values in [RFC4960], may Association.Max.Retrans (AMR) parameter values in [RFC4960], may
result in compromisation of the fault tolerance of SCTP. result in compromisation 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 Potentially
Failed (PF) state as an intermediate state in between the active and Failed (PF) state as an intermediate state in between the active and
inactive state of a destination address in [RFC4960] path management inactive state of a destination address in the [RFC4960] path
scheme, and let the failover of data transfer away from a destination management scheme, and let the failover of data transfer away from a
address be driven by the entering of the PF state instead of by the destination address be driven by the entering of the PF state instead
entering of the inactive state. Thereby SCTP may perform quick of by the entering of the inactive state. Thereby SCTP may perform
failover without compromising the overall fault tolerance of quick failover without compromising the overall fault tolerance of
[RFC4960] SCTP. At the same time, RTO-based HEARTBEAT probing is [RFC4960] SCTP. At the same time, RTO-based HEARTBEAT probing is
initiated towards a destination address once it enters PF state. initiated towards a destination address once it enters PF state.
Thereby SCTP may quickly ascertain whether network connectivity Thereby SCTP may quickly ascertain whether network connectivity
towards the destination address is broken or whether the failover was towards the destination address is broken or whether the failover was
spurious. In the case where the failover was spurious data transfer spurious. In the case where the failover was spurious data transfer
may quickly resume towards the original destination address. may quickly resume towards the 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 and it assumes that by default a destination address is failure and it assumes that by default a destination address is
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thus by [RFC4960], section 6.4.1, should attempt to pick a thus by [RFC4960], section 6.4.1, should attempt to pick a
new destination address for data retransmission, the sender new destination address for data retransmission, the sender
SHOULD choose an alternate destination transport address in SHOULD choose an alternate destination transport address in
active state if one exists. 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 in
PF state, the sender SHOULD send the data to an alternate PF state, the sender SHOULD send the data to an alternate
destination address in active state if one exists. destination address in active state if one exists.
When choosing among multiple destination address in active state When choosing among multiple destination addresses in active
the following considerations are given: state the following considerations are given:
A. An SCTP sender should comply with [RFC4960], section 6.4.1, A. An SCTP sender should comply with [RFC4960], section 6.4.1,
principles of choosing most divergent source-destination principles of choosing most divergent source-destination
pairs compared with, for i.: the destination address in PF pairs compared with, for i.: the destination address in PF
state that it performs a failover from, and for ii.: the state that it performs a failover from, and for ii.: the
destination address towards which the data timed out. Rules destination address towards which the data timed out. Rules
for picking the most divergent source-destination pair are for picking the most divergent source-destination pair are
an implementation decision and are not specified within this an implementation decision and are not specified within this
document. document.
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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 in PF
state and some in inactive state, the sender MUST choose one state and some in inactive state, the sender MUST choose one
destination address in PF state and transmit or retransmit data destination address in PF state and transmit or retransmit data
to this destination address using the following rules: to this destination address using the following rules:
A. The sender SHOULD choose the destination in PF state with A. 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 PF destinations with same error B. When there are multiple destination addresses in PF state
count, the sender should let the choice among the multiple with same error count, the sender should let the choice
PF destination with equal error count be based on the among the multiple destination addresses in PF state with
[RFC4960], section 6.4.1, principles of choosing most equal error count be based on the [RFC4960], section 6.4.1,
divergent source-destination pairs when executing principles of choosing most divergent source-destination
(potentially consecutive) retransmission. Rules for picking pairs when executing (potentially consecutive)
the most divergent source-destination pair are an retransmission. Rules for picking the most divergent
implementation decision and are not specified within this source-destination pair are an implementation decision and
document. are not specified within this document.
C. A sender MAY choose to deploy other strategies than the C. A sender MAY choose to deploy other strategies than the
above when choosing among multiple PF destinations have the above when choosing among multiple destinations in PF state
SCTP-PF sender other means of information available that have the SCTP-PF sender other means of information available
qualifies a particular destination address for being used. that qualifies a particular destination address for being
The SCTP-PF protocol operation specified in this document used. The SCTP-PF protocol operation specified in this
makes no assumption of the existence of such other means of document makes no assumption of the existence of such other
information and specifies for the above as the default means of information and specifies for the above as the
operation of an SCTP-PF sender. default operation of an SCTP-PF sender.
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 address regardless of whether it has been chosen any destination address regardless of whether it has been chosen
for transmission or not. for transmission or not.
5. The HB.interval of the Path Heartbeat function of [RFC4960] 5. The HB.interval of the Path Heartbeat function of [RFC4960] MUST
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 the Path Heartbeat function (Section 8.3 of [RFC4960])
is enabled for the destination address or not. I.e., if the is enabled for the destination address or not. I.e., if the
Path Heartbeat function is disabled for the destination address Path Heartbeat function is disabled for the destination address
in question, HEARTBEATS MUST NOT be sent. Note that when in question, HEARTBEATS MUST NOT be sent. Note that when
Heartbeat function is disabled, it may take longer to transition Heartbeat function is disabled, it may take longer to transition
PF destination to ACTIVE. 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 receipt of a SACK of or a T3-rtx timer MAY be omitted in case receipt of a SACK of or a T3-rtx timer
expiration on the outstanding data can provide equivalent expiration on the outstanding data can provide equivalent
information, such as a case where the data chunk has transmitted information, such as a case where the data chunk has transmitted
to a single destination. Likewise, the timeout of a HEARTBEAT to a single destination. Likewise, the timeout of a HEARTBEAT
chunk MAY be ignored if data is outstanding towards the chunk MAY be ignored if data is outstanding towards the
destination address. 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 MUST clear the to a destination address in PF state, the sender SHOULD clear
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. When the sender destination address back to active state. When the sender
resumes data transmission on the destination address, it MUST do resumes data transmission on a destination address after a
this following the prescriptions of Section 7.2 of [RFC4960]. transition of the destination address from PF to active state,
it MUST do this following the prescriptions of Section 7.2 of
[RFC4960]. In a situation where a HEARTBEAT ACK arrives while
there is data outstanding towards the destination address to
which the HEARTBEAT was sent, then an implementation MAY choose
to not have the HEARTBEAT ACK reset the error counter, but have
the error counter reset await the fate of the outstanding data
transmission. This situation can happen when data is sent to a
destination address in PF state.
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 (i) SHOULD notify the ULP
about this state transition, and (ii) transmit HEARTBEAT chunks about this state transition, and (ii) transmit HEARTBEAT chunks
to the inactive destination address at a lower HB.interval to the inactive destination address at a lower HB.interval
frequency as described in Section 8.3 of [RFC4960] (when the frequency as described in Section 8.3 of [RFC4960] (when the
Path Heartbeat function is enabled for the destination address). 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 which has been
retransmitted to a different destination address than the retransmitted to a different destination address than the
destination address to which the chunk was first transmitted) destination address to which the chunk was first transmitted)
MUST NOT clear the error count for an inactive destination SHOULD NOT clear the error count for an inactive destination
address and MUST NOT transition a destination address in PF address and SHOULD NOT transition a destination address in PF
state back to active state, since a sender cannot disambiguate state back to active state, since a sender cannot disambiguate
whether the ACK was for the original transmission or the whether the ACK was for the original transmission or the
retransmission(s). A SCTP sender MAY apply a different approach retransmission(s). A SCTP sender MAY apply a different approach
for the error count handling based on unequivocally information for the error count handling based on unequivocally information
on which destination (including multiple destination addresses) on which destination (including multiple destination addresses)
the chunk reached. This document makes no reference to what the chunk reached. This document makes no reference to what
such unequivocally information could consist of, neither how such unequivocally information could consist of, neither how
such unequivocally information could be obtained. The design of such unequivocally information could be obtained. The design of
such an alternative approach is left to implementations. such an alternative approach is left to implementations.
10. Acknowledgments for chunks that has been transmitted to one 10. Acknowledgments for data chunks that has been transmitted to one
destination address only MUST clear the error counter for the destination address only MUST clear the error counter for the
destination address and MUST transition a destination address in destination address and MUST transition a destination address in
PF state back to Active state. This situation can happen when PF state back to active state. This situation can happen when
new data is sent to a destination address in the PF state. It new data is sent to a destination address in the PF state. It
can also happen in situations where the destination address is can also happen in situations where the destination address is
in the PF state due to the occurrence of a spurious T3-rtx timer in the PF state due to the occurrence of a spurious T3-rtx timer
and Acknowledgments start to arrive for data sent prior to and acknowledgments start to arrive for data sent prior to
occurrence of the spurious T3-rtx and data has not yet been occurrence of the spurious T3-rtx and data has not yet been
retransmitted towards other destinations. This document does retransmitted towards other destinations. This document does
not specify special handling for detection of or reaction to not specify special handling for detection of or reaction to
spurious T3-rtx timeouts, e.g., for special operation vis-a-vis spurious T3-rtx timeouts, e.g., for special operation Vis-avis
the congestion control handling or data retransmission operation the congestion control handling or data retransmission operation
towards a destination address which undergoes a transition from towards a destination address which undergoes a transition from
active to PF to active state due to a spurious T3-rtx timeout. active to PF to active state due to a spurious T3-rtx timeout.
But it is noted that this is an area which would benefit from But it is noted that this is an area which would benefit from
additional attention, experimentation and specification for additional attention, experimentation and specification for
Single Homed SCTP as well as for Multi Homed SCTP protocol single-homed SCTP as well as for multi-homed SCTP protocol
operation. 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 provide the ULP with the means to expose 12. The SCTP stack SHOULD provide the ULP with the means to expose
the PF state of its destinations as well as the means to notify the PF state of its destinations as well as the means to notify
of state transitions from Active to PF, and vice-versa. However of state transitions from active to PF, and vice-versa. However
it is recommended that an SCTP stack implementing SCTP-PF also it is recommended that an SCTP stack implementing SCTP-PF also
allows for that the ULP is kept ignorant of the PF state of its allows for that the ULP is kept ignorant of the PF state of its
destinations and the associated state transition. For this destinations and the associated state transition. For this
reason is it recommended that an SCTP stack implementing SCTP-PF reason it is recommended that an SCTP stack implementing SCTP-PF
also should provide the ULP with the means to suppress exposure also should provide the ULP with the means to suppress exposure
of PF state and the associated state transitions. of 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 [RFC4960] SCTP
implementation given the same setting of Path.Max.Retrans (PMR) and implementation given the same setting of Path.Max.Retrans (PMR) and
skipping to change at page 11, line 5 skipping to change at page 11, line 17
section 8.3, in this respect. The exact limit to apply is not section 8.3, in this respect. The exact limit to apply is not
specified in this document but it is considered reasonable to specified in this document but it is considered reasonable to
require for such to be an order of magnitude higher than the PMR require for such to be an order of magnitude higher than the PMR
value. A sender MAY choose to deploy other strategies that the value. A sender MAY choose to deploy other strategies that the
strategy defined by here. The strategy to prioritize the last strategy defined by here. The strategy to prioritize the last
active destination address, i.e., the destination address with active destination address, i.e., the destination address with
the fewest error counts is optimal when some paths are the fewest error counts is optimal when some paths are
permanently inactive, but suboptimal when a path instability is permanently inactive, but suboptimal when a path instability is
transient. transient.
5. Permanent Failover 5. Primary Path Switchover
The objective of the Permanent Failover operation is to allow the The objective of the Primary Path Switchover operation is to allow
SCTP sender to continue data transmission on a new working path even the SCTP sender to continue data transmission on a new working path
when the old primary destination address becomes active again. This even when the old primary destination address becomes active again.
is achieved by having SCTP perform a switch over of the primary path This is achieved by having SCTP perform a switch over of the primary
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 [RFC4960]
implementations. implementations.
The Permanent Failover operation requires only sender side changes. The Primary Path Switchover operation requires only sender side
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 (PMR). For SCTP-PF
implementations, the PSMR MUST be set greater or equal to the implementations, the PMR MUST be set greater or equal to the PFMR
PFMR value. For [RFC4960] implementations the PSMR MUST be set value. For [RFC4960] implementations the PMR MUST be set greater
greater or equal to the PMR value. Implementations MUST reject or equal to the PMR value. Implementations MUST reject any other
any other values of PSMR. values of PMR.
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 PMR,
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 which 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 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 Permanent 4. For SCTP-PF, the recommended value of PMR is PFMR when Primary
Failover is used. This means that no forced switchback to a Path Switchover operation mode is used. This means that no
previously failed primary path is performed. An SCTP-PF forced switchback to a previously failed primary path is
implementation of Permanent Failover MUST support the setting of performed. An SCTP-PF implementation of Primary Path Switchover
PSMR = PFMR. A SCTP-PF implementation of Permanent Failover MAY MUST support the setting of PMR = PFMR. A SCTP-PF implementation
support setting of PSMR > PFMR. of Primary Path Switchover MAY support setting of PMR > PFMR.
5. For [RFC4960] SCTP, the recommended value of PSMR is PMR when 5. For [RFC4960] SCTP, the recommended value of PMR is PMR when
Permanent Failover is used. This means that no forced switchback Primary Path Switchover is used. This means that no forced
to a previously failed primary path is performed. A [RFC4960] switchback to a previously failed primary path is performed. A
SCTP implementation of Permanent Failover MUST support the [RFC4960] SCTP implementation of Primary Path Switchover MUST
setting of PSMR = PMR An [RFC4960] SCTP implementation of support the setting of PMR = PMR. An [RFC4960] SCTP
Permanent Failover MAY support larger settings of PSMR > PMR. implementation of Primary Path Switchover MAY support larger
settings of PMR > PMR.
6. It MUST be possible to disable the Permanent Failover and obtain 6. It MUST be possible to disable the Primary Path Switchover
the standard switchback operation of [RFC4960]. operation and obtain the standard switchback operation of
[RFC4960].
The manner of switch over operation that is most optimal in a given The manner of switch over 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 it depends on the quality of alternative paths available as well as it depends on
the extent to which it is desired for the mode of operation to the extent to which it is desired for the mode of operation to
enforce traffic distribution over a number of network paths. I.e., enforce traffic distribution over a number of network paths. I.e.,
load distribution of traffic from multiple SCTP associations may be load distribution of traffic from multiple SCTP associations may be
sought to be enforced by distribution of the set primary paths with sought to be enforced by distribution of the set primary paths with
[RFC4960] switchback operation. However as [RFC4960] switchback [RFC4960] switchback operation. However as [RFC4960] switchback
behavior is suboptimal in certain situations, especially in scenarios behavior is suboptimal in certain situations, especially in scenarios
where a number of equally good paths are available, an SCTP where a number of equally good paths are available, an SCTP
implementation MAY support also, as alternative behavior, the implementation MAY support also, as alternative behavior, the Primary
Permanent Failover mode of operation and MAY enable it based on Path Switchover mode of operation and MAY enable it based on users'
users' requests. requests.
For an SCTP implementation that implements Permanent Failover, this For an SCTP implementation that implements the Primary Path
specification RECOMMENDS that the standard RFC4960 switchback Switchover operation, this specification RECOMMENDS that the standard
operation is retained as the default operation. RFC4960 switchback operation is 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 RECOMMENDATIONS for This document does not alter the [RFC4960] value RECOMMENDATIONS for
the SCTP Protocol Parameters defined in [RFC4960]. the SCTP 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 Permanent Failover 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 potentially
failed state as well as the socket API implementation is extended to failed state as well as the socket API implementation is extended to
expose the potentially failed state of a peer address in the existing expose the potentially failed state of a peer address in the existing
SCTP_GET_PEER_ADDR_INFO structure. SCTP_GET_PEER_ADDR_INFO 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 PMR parameters described in Section 3 and in Section 5. The second
one controls the exposition of the potentially failed path state. one controls the exposition of the potentially failed 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 need also 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 address enters or leaves the potentially failed state. The peer address enters or leaves the potentially failed state. The
notification as defined in [RFC6458] uses the following structure: notification as defined in [RFC6458] uses the following structure:
struct sctp_paddr_change { strict 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; strict 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 in addition to that the new 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 potentially failed.
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 { strict sctp_paddrinfo {
sctp_assoc_t spinfo_assoc_id; sctp_assoc_t spinfo_assoc_id;
struct sockaddr_storage spinfo_address; strict 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 in addition to that the new constant
SCTP_POTENTIALLY_FAILED, which is reported if the peer address is SCTP_POTENTIALLY_FAILED, which is reported if the peer address is
potentially failed. 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 potentially failed or unreachable. The same socket option
is used by applications to set and get the number of timeouts before is used by applications to set and get the number of timeouts before
the primary path is changed automatically by the Permanent Failover the primary path is changed automatically by the Primary Path
function. This socket option uses the level IPPROTO_SCTP and the Switchover function. This socket option uses the level IPPROTO_SCTP
name SCTP_PEER_ADDR_THLDS. and the 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 { strict sctp_paddrthlds {
sctp_assoc_t spt_assoc_id; sctp_assoc_t spt_assoc_id;
struct sockaddr_storage spt_address; strict 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-romany style sockets the application may fill in
in an association identifier or SCTP_FUTURE_ASSOC. It is an error an association identifier or SCTP_FUTURE_ASSOC. It is an error to
to use SCTP_{CURRENT|ALL}_ASSOC in spt_assoc_id. 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 wild card 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
Potentially Failed, if its path error counter exceeds Potentially Failed, if 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 0off disables the selection of a
a new primary peer address. If an implementation does not support new primary peer address. If an implementation does not support
the automatically selection of a new primary address, it should the automatically selection of a new primary address, it should
indicate an error with errno set to EINVAL if a value different indicate an error with Erna set to RIVAL if a value different from
from 0xffff is used in spt_pathcpthld. For SCTP-PF, the setting 0off is used in spt_pathcpthld. For SCTP-PF, the setting of
of spt_pathcpthld < spt_pathpfthld should be rejected with errno spt_pathcpthld < spt_pathpfthld should be rejected with Erna set
set to EINVAL. For [RFC4960] SCTP, the setting of spt_pathcpthld to RIVAL. For [RFC4960] SCTP, the setting of spt_pathcpthld <
< spt_pathmaxrxt should be rejected with errno set to EINVAL. A spt_pathmaxrxt should be rejected with Erna set to RIVAL. A SCTP-
SCTP-PF implementation MAY support only setting of spt_pathcpthld PF implementation MAY support only setting of spt_pathcpthld =
= spt_pathpfthld and spt_pathcpthld = 0xffff and a [RFC4960] SCTP spt_pathpfthld and spt_pathcpthld = 0off and a [RFC4960] 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 = 0off. In these cases SCTP
shall reject setting of other values with errno set to EINVAL. shall reject setting of other values with Erna set to RIVAL.
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 potentially failed path
state in the SCTP_PEER_ADDR_CHANGE event and the state in the SCTP_PEER_ADDR_CHANGE event and the
SCTP_GET_PEER_ADDR_INFO as described in Section 7.1. The default SCTP_GET_PEER_ADDR_INFO as described in Section 7.1. The default
value is implementation specific. value is implementation 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
potentially failed path state: potentially failed path state:
struct sctp_assoc_value { strict 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-romany 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 potentially failed path state is exposed if and
only if this parameter is non-zero. only if this 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 on- The logic introduced by this document does not impact existing On-
the-wire SCTP messages. Also, this document does not introduce any anthe-wire SCTP messages. Also, this document does not introduce any
new on-the-wire SCTP messages that require new security new On-anthe-wire SCTP messages that require new security
considerations. 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 to change data transfer path, since the
duration of time that an attacker needs to compromise the network duration of time that an attacker needs to compromise the network
connectivity is much shorter than [RFC4960]. However, SCTP-PF does connectivity is much shorter than [RFC4960]. However, SCTP-PF does
not constitute a significant change in the duration of time and not constitute a significant change in the duration of time and
effort an attacker needs to keep SCTP away from the primary path. effort an attacker needs to keep SCTP away from the primary path.
With the standard switchback operation [RFC4960] SCTP resumes data With the standard switchback operation [RFC4960] SCTP resumes data
transfer on its primary path as soon as the next HEARTBEAT succeeds. transfer on its primary path as soon as the next HEARTBEAT succeeds.
On the other hand, usage of the Permanent Failover mechanism, does On the other hand, usage of the Primary Path Switchover mechanism,
change the treat analysis. This is because attackers can force a does change the treat analysis. This is because attackers can force
permanent change of the data transfer path by blocking the primary a permanent change of the data transfer path by blocking the primary
path until the switchover of the primary path is triggered by the path until the switchover of the primary path is triggered by the
Permanent Failover algorithm. This especially will be the case when Primary Path Switchover algorithm. This especially will be the case
Permanent Failover is used together with SCTP-PF with the particular when the Primary Path Switchover is used together with SCTP-PF with
setting of PSMR = PFMR = 0, as Permanent Failover here happens the particular setting of PMR = PFMR = 0, as Primary Path Switchover
already at the first RTO timeout experienced. Users of the Permanent here happens already at the first RTO timeout experienced. Users of
Failover mechanism should be aware of this fact. the Primary Path Switchover mechanism 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
potentially failed state and vice versa gives attackers an increased potentially failed state and vice versa gives attackers an increased
possibility to generate more local events. However, it is assumed possibility to generate more local events. However, it is assumed
that event notifications are rate-limited in the implementation to that event notifications are rate-limited in the implementation to
address this threat. address this threat.
9. IANA Considerations 9. IANA Considerations
This document does not create any new registries or modify the rules This document does not create any new registries or modify the rules
for any existing registries managed by IANA. for any existing registries managed by IONA.
10. Acknowledgements 10. Acknowledgements
The authors wish to thank Michael Tuexen for his many invaluable The authors wish to thank Michael Tuexen for his many invaluable
comments and for his very substantial support with the making of this comments and for his very substantial support with the making of this
document. document.
11. Proposed Change of Status (to be Deleted before Publication) 11. Proposed Change of Status (to be Deleted before Publication)
Initially this work looked to entail some changes of the Congestion Initially this work looked to entail some changes of the Congestion
Control (CC) operation of SCTP and for this reason the work was Control (CC) operation of SCTP and for this reason the work was
proposed as Experimental. These intended changes of the CC operation proposed as Experimental. These intended changes of the CC operation
have since been judged to be irrelevant and are no longer part of the have since been judged to be irrelevant and are no longer part of the
specification. As the specification entails no other potential specification. As the specification entails no other potential
harmful features, consensus exists in the WG to bring the work harmful features, consensus exists in the G to bring the work forward
forward as PS. as PS.
Initially concerns have been expressed about the possibility for the Initially concerns have been expressed about the possibility for the
mechanism to introduce path bouncing with potential harmful network mechanism to introduce path bouncing with potential harmful network
impacts. These concerns are believed to be unfounded. This issue is impacts. These concerns are believed to be unfounded. This issue is
addressed in Appendix B. addressed in Appendix B.
It is noted that the feature specified by this document is It is noted that the feature specified by this document is
implemented by multiple SCTP SW implementations and furthermore that implemented by multiple SCTP SW implementations and furthermore that
various variants of the solution have been deployed in Telco various variants of the solution have been deployed in Tel co
signaling environments for several years with good results. signaling environments for several years with good results.
12. References 12. References
12.1. Normative References 12.1. 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>.
12.2. Informative References 12.2. Informative References
[CARO02] Caro Jr., A., Iyengar, J., Amer, P., Heinz, G., and R. [CARO02] Caro Jr., 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 , 7
2002. 2002.
[CARO04] Caro Jr., A., Amer, P., and R. Stewart, "End-to-End [CARO04] Caro Jr., A., Amer, P., and R. Stewart, "End-to-End
Failover Thresholds for Transport Layer Multihoming", Failover Thresholds for Transport Layer Multi homing",
MILCOM 2004 , 11 2004. MILCOM 2004 , 11 2004.
[CARO05] Caro Jr., A., "End-to-End Fault Tolerance using Transport [CARO05] Caro Jr., A., "End-to-End Fault Tolerance using Transport
Layer Multihoming", Ph.D Thesis, University of Delaware , Layer Multi homing", Ph.D Thesis, University of Delaware ,
1 2005. 1 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 ALAN
Environments", IEEE CCNC 2008, 1 2008. Environments", IEEE CCNC 2008, 1 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 failover in M3UA-based SIGHT RAN networks",
Advanced Simulation Technologies Conference , 4 2004. Advanced Simulation Technologies Conference , 4 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 Multi homing over
End-to-end Paths.", IEEE/ACM Trans on Networking 14(5), 10 Independent End-to-end Paths.", IEEE/ACM Trans on
2006. Networking 14(5), 10 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 , 7 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 , 5 2009.
[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. Discussions 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 to update
RFC4960, we do not recommend them from the reasons described below. RFC4960, we do not recommend them from the reasons described below.
A.1. Reduce Path.Max.Retrans (PMR) A.1. Reduce Path.Max.Retrans (PMR)
Smaller values for Path.Max.Retrans shorten the failover duration and Smaller values for Path.Max.Retrans shorten the failover duration and
skipping to change at page 18, line 36 skipping to change at page 19, line 10
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 which may result in
spurious failover. Spurious failover is a problem in [RFC4960] SCTP spurious failover. Spurious failover is a problem in [RFC4960] 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. 'HB.interval' is usually set in the order of seconds process. 'HB.interval' is usually set in the order of seconds
(recommended value is 30 seconds) and when the primary path becomes (recommended value is 30 seconds) and when the primary path becomes
inactive, the next HEARTBEAT may be transmitted only many seconds inactive, the next HEARTBEAT may be transmitted only many seconds
later. Indeed as recommended, only 30 secs later. Meanwhile, the later. Indeed as recommended, only 30 secs later. Meanwhile, the
primary path may since long have recovered, if it needed recovery at primary path may since long have recovered, if it needed recovery at
all (indeed the failover could be truely spurious). In such all (indeed the failover could be truly spurious). In such
situations, post failover, an endpoint is forced to wait in the order situations, post failover, an endpoint is forced to wait in the order
of many seconds before the endpoint can resume transmission on the of many seconds before the endpoint can resume transmission on the
primary path and furthermore once it returns on the primary path the primary path and furthermore once it returns on the primary path the
CWND needs to be rebuild anew - a process which the throughput CWND needs to be rebuild anew - a process which the throughput
already have had to suffer from on the alternate path. Using a already have had to suffer from on the alternate path. Using a
smaller value for 'HB.interval' might help this situation, but it smaller value for 'HB.interval' might help this situation, but it
would result in a general waste of bandwidth as such more frequent would result in a general waste of bandwidth as such more frequent
HEARBEATING would take place also when there are no observed HEARTBEATING would take place also when there are no observed
troubles. The bandwidth overhead may be diminished by having the ULP troubles. The bandwidth overhead may be diminished by having the ULP
use a smaller 'HB.interval' only on the path which at any given time use a smaller 'HB.interval' only on the path which at any given time
is set to be the primary path, but this adds complication in the ULP. is set to be the primary path, but this adds 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
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. Else
the SCTP sender considers its peer reachable even when all the SCTP sender considers its peer reachable even when all
destinations are INACTIVE and to avoid this dormant state operation, destinations are INACTIVE and to avoid this dormant state operation,
[RFC4960] SCTP implementation SHOULD reduce Association.Max.Retrans [RFC4960] 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 compromizes the fault tolerance of SCTP Association.Max.Retrans value compromises 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 failover process by adjusting RTO related parameters
[JUNGMAIER02] [FALLON08]. During failover process, RTO keeps being [JUNGMAIER02] [FALLON08]. During failover process, RTO keeps being
doubled. However, if we can choose smaller value for RTO.max, we can doubled. However, if we can choose smaller value for RTO.max, we can
stop the exponential growth of RTO at some point. Also, choosing stop the exponential growth of RTO at some point. Also, choosing
 End of changes. 74 change blocks. 
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