draft-ietf-tsvwg-sctp-failover-10.txt   draft-ietf-tsvwg-sctp-failover-11.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: September 10, 2015 Cisco Systems Expires: January 18, 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
March 9, 2015 July 17, 2015
SCTP-PF: Quick Failover Algorithm in SCTP SCTP-PF: Quick Failover Algorithm in SCTP
draft-ietf-tsvwg-sctp-failover-10.txt draft-ietf-tsvwg-sctp-failover-11.txt
Abstract Abstract
One of the major advantages of SCTP is the support of multi-homed SCTP supports multi-homing. However, when the failover operation
communication. A multi-homed SCTP end-point has the ability to specified in RFC4960 is followed, there can be significant delay and
withstand network failures by migrating the traffic from an inactive performance degradation in the data transfer path failover. To
network to an active one. However, if the failover operation as overcome this problem this document specifies a quick failover
specified in RFC4960 is followed, there can be a significant delay in algorithm (SCTP-PF) based on the introduction of a Potentially Failed
the migration to the active destination addresses, thus severely (PF) state in SCTP Path Management.
reducing the effectiveness of the SCTP failover operation.
This document complements RFC4960 by the introduction of a new path The document also specifies a dormant state operation of SCTP. This
state, the Potentially Failed (PF) path state, and an associated new dormant state operation is required to be followed by an SCTP-PF
failover operation to apply during a network failure. The algorithm implementation, but it may equally well be applied by a standard
defined is called SCTP Potentially Failed Algorithm, SCTP-PF for RFC4960 SCTP implementation.
short. In addition, the document complements RFC4960 by introducing
alternative switchover operation modes for the data transfer path Additionally, the document introduces an alternative switchback mode
management after the recovery of a failed primary path. These modes called Permanent Failover that will be beneficial in some situations.
can allow improvements in the performance of the operation in some This mode of operation applies to both a standard RFC4960 SCTP
network environments. The implementation of the additional implementation as well as to a SCTP-PF implementation.
switchover operation modes is an optional part of SCTP-PF.
The procedures defined in the document require only minimal The procedures defined in the document require only minimal
modifications to the current 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
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provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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Copyright Notice Copyright Notice
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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 . . . . . . . . . . . . . . . . . 4
3. Issues with the SCTP Path Management . . . . . . . . . . . . 4 3. SCTP with Potentially-Failed Destination State (SCTP-PF) . . 4
4. SCTP with Potentially-Failed Destination State (SCTP-PF) . . 5 3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. SCTP-PF Concept . . . . . . . . . . . . . . . . . . . . . 5 3.2. Specification of the SCTP-PF Procedures . . . . . . . . . 5
4.2. Specification of the SCTP-PF Algorithm . . . . . . . . . 6 4. Dormant State Operation . . . . . . . . . . . . . . . . . . . 9
4.2.1. Dormant State Operation . . . . . . . . . . . . . . . 10 4.1. SCTP Dormant State Procedure . . . . . . . . . . . . . . 10
4.3. Permanent Failover . . . . . . . . . . . . . . . . . . . 12 5. Permanent Failover . . . . . . . . . . . . . . . . . . . . . 11
4.3.1. Background . . . . . . . . . . . . . . . . . . . . . 12 6. Suggested SCTP Protocol Parameter Values . . . . . . . . . . 12
4.3.2. Permanent Failover Algorithm . . . . . . . . . . . . 12 7. Socket API Considerations . . . . . . . . . . . . . . . . . . 12
5. Socket API Considerations . . . . . . . . . . . . . . . . . . 13 7.1. Support for the Potentially Failed Path State . . . . . . 13
5.1. Support for the Potentially Failed Path State . . . . . . 14 7.2. Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) Socket
5.2. Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) Socket Option . . . . . . . . . . . . . . . . . . . . . . . . . 14
Option . . . . . . . . . . . . . . . . . . . . . . . . . 15 7.3. Exposing the Potentially Failed Path State
5.3. 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
6. Security Considerations . . . . . . . . . . . . . . . . . . . 16 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
8. Proposed Change of Status (to be Deleted before Publication) 17 11. Proposed Change of Status (to be Deleted before Publication) 16
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.1. Normative References . . . . . . . . . . . . . . . . . . 17 12.1. Normative References . . . . . . . . . . . . . . . . . . 17
9.2. 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 . . . . . . . . 20 Appendix B. Discussions for Path Bouncing Effect . . . . . . . . 19
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 . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction 1. Introduction
The Stream Control Transmission Protocol (SCTP) as specified in The Stream Control Transmission Protocol (SCTP) specified in
[RFC4960] supports multihoming at the transport layer -- an SCTP [RFC4960] supports multi homing at the transport layer. SCTP's multi
endpoint can bind to multiple IP addresses. SCTP's multihoming homing features include failure detection and failover procedures to
features include failure detection and failover procedures to provide provide network interface redundancy and improved end-to-end fault
network interface redundancy and improved end-to-end fault tolerance. tolerance. In SCTP's current failure detection procedure, the sender
must experience Path.Max.Retrans (PMR) number of consecutive failed
In SCTP's current failure detection procedure, the sender must timer-based retransmissions on a destination address before detecting
experience Path.Max.Retrans (PMR) number of consecutive failed timer- a path failure. Until detecting the path failure, the sender
based retransmissions on a destination address before detecting a continues to transmit data on the failed path. The prolonged time in
path failure. The sender fails over to an alternate active which [RFC4960] SCTP continues to use a failed path severely degrades
destination address only after failure detection. Until detecting the performance of the protocol. To address this problem, this
the failover, the sender continues to transmit data on the failed document specifies a quick failover algorithm (SCTP-PF) based on the
path, which degrades the SCTP performance. Concurrent Multipath introduction of a new Potentially Failed path state in SCTP path
Transfer (CMT) [IYENGAR06] is an proposed extension to SCTP that management. The performance deficiencies of the [RFC4960] failover
allows the sender to transmit data on multiple paths simultaneously. operation, and the improvements obtainable from the introduction of a
Research [NATARAJAN09] shows that the current failure detection Potentially Failed state in SCTP, were proposed and documented in
procedure worsens CMT performance during failover and can be [NATARAJAN09] for Concurrent Multipath Transfer SCTP [IYENGAR06].
significantly improved by employing a better failover algorithm.
This document specifies an alternative failure detection and failover While SCTP-PF can accelerate failover process and improve
procedure, the SCTP Potentially Failed algorithm, that improves the performance, the risks that an SCTP endpoint enters in dormant state
performance of SCTP multi-homed operation during a failover. where all destination addresses are inactive can be increased.
[RFC4960] leaves the protocol operation during dormant state to
implementations and encourages to avoid entering the state as much as
possible by careful tuning of the Path.Max.Retrans (PMR) and
Association.Max.Retrans (AMR) parameters. We specify a dormant state
operation for SCTP-PF which makes SCTP-PF provide the same disruption
tolerance as [RFC4960] despite that the dormant state may be entered
more quickly. The dormant state operation may equally well be
applied by an [RFC4960] implementation and will here serve to provide
added fault tolerance for situations where the tuning of the
Path.Max.Retrans (PMR) and Association.Max.Retrans (AMR) parameters
fail to provide adequate prevention of the entering of the dormant
state.
For multi-homed SCTP the operation after the recovery of a failed The operation after the recovery of a failed path equally well
path equally well impacts the performance of the protocol. With the impacts the performance of the protocol. With the procedures
procedures specified in [RFC4960], SCTP will, after a failover from specified in [RFC4960] SCTP will, after a failover from the primary
the primary path, switch back to the primary path for data transfer path, switch back to use the primary path for data transfer as soon
as soon as this path becomes available again. From a performance as this path becomes available again. From a performance perspective
perspective, as confirmed in research [CARO02], such a switchback of such a forced switchback of the data transmission path can be
the data transmission path is not optimal in general. As an optional suboptimal as the CWND towards the original primary destination
alternative to the switchback operation of [RFC4960], this document address has to be rebuilt once data transfer resumes, [CARO02]. As
specifies the Permanent Failover procedures proposed by [CARO02]. an optional alternative to the switchback operation of [RFC4960],
this document specifies an alternative Permanent Failover procedure
which avoid such forced switchbacks of the data transfer path. The
Permanent Failover operation was originally proposed in [CARO02].
Additional discussion for alternative approaches that do not require While SCTP-PF primarily is motivated by a desire to improve the
modifications to [RFC4960], as well as discussion of path bouncing multi-homed operation, the feature applies also to SCTP single-homed
effects that might be caused by frequent switchover, are provided in operation. Here the algorithm serves to provide increased failure
the Appendices. detection on idle associations, whereas the failover or switchback
aspects of the algorithm will not be activated. This is discussed in
more detail in Appendix C.
While the Potentially Failed algorithm primarily is motivated for A brief description of the motivation for the introduction of the
improvement of the SCTP multi-homed operation, the feature applies Potentially Failed state including a discussion of alternative
also to SCTP single-homed operation. Here the algorithm serves to approaches to mitigate the deficiencies of the [RFC4960] failover
provide increased failure detection on idle associations, whereas the operation are given in the Appendices. Discussion of path bouncing
failover or switchback aspects of the algorithm will not be effects that might be caused by frequent switchover, are also
activated. This is discussed in more detail in Appendix C. 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. Issues with the SCTP Path Management 3. SCTP with Potentially-Failed Destination State (SCTP-PF)
This section describes issues in the SCTP as specified in [RFC4960]
to be fixed by the approach described in this document.
An SCTP endpoint can support multiple IP addresses. Each SCTP
endpoint exchanges the list of its usable addresses during the
initial negotiation with its peer. Then the endpoints select one
address from the peer's list and use this as the primary destination
address. During normal transmission, an SCTP endpoint sends all user
data to the primary destination address. Also, it sends packets
containing a HEARTBEAT chunk to all idle destination addresses at a
certain interval to check the reachability of these destination
addresses. Idle destination addresses normally include all non-
primary destination addresses.
If a sender has multiple active destination addresses, it can
retransmit data to an non-primary destination address, if the
transmission to the primary times out.
When a sender receives an acknowledgment for DATA or HEARTBEAT chunks
sent to one of the destination addresses, it considers that
destination address to be active and clears the error counter for the
destination address. If it fails to receive acknowledgments, the
error count for the destination address is increased. If the error
counter exceeds the tunable protocol parameter Path.Max.Retrans
(PMR), the SCTP endpoint considers the destination address to be
inactive.
The failover process of SCTP is initiated when the primary path
becomes inactive (the error counter for the primary path exceeds
Path.Max.Retrans). If the primary path is marked inactive, SCTP
chooses a new destination address from one of the active destinations
and starts using this as the destination address for sending data.
If the primary path becomes active again, SCTP reverts to using the
primary destination address for subsequent data transmissions and
stop using the non-primary one.
One issue with this failover process defined in [RFC4960] is that it
usually takes a significant amount of time before SCTP switches to
the new destination address. Let's say the primary path on a multi-
homed host becomes unavailable and the RTO value for the primary path
at that time is around 1 second, it usually takes over 60 seconds
before SCTP starts to use the non-primary path for initial data
transmission. This is because the recommended value for
Path.Max.Retrans in the [RFC4960] is 5, which requires 6 consecutive
timeouts before the failover takes place. Before SCTP switches to
the non-primary address, SCTP keeps trying to send packets to the
primary address and only retransmitted packets are sent to the non-
primary address and thus can be received by the receiver. This slow
failover process can cause significant performance degradation and is
not acceptable in some situations.
Another issue with RFC4960 failover and switchback operation is that
once the primary path becomes active again, the traffic is
unconditionally switched back to use this path. This is not optimal
in some situations. This is further discussed in Section 4.3.
4. SCTP with Potentially-Failed Destination State (SCTP-PF)
To address the issues described in Section 3, this document extends
SCTP path management scheme by adding the Potentially Failed state
and associated protocol operation. The algorithm is called SCTP
Potentially Failed algorithm. SCTP-PF for short. The resulting SCTP
path management operation is called SCTP Potentially Failed
operation.
4.1. SCTP-PF Concept
The introduction of the Potentially Failed state stems from the
following two observations about SCTP's failure detection procedure:
o To minimize the performance impact during failover, the sender
should avoid transmitting data to the failed destination address
as early as possible. In the current SCTP path management scheme,
the sender stops transmitting data to a destination address only
after the destination address is marked Failed (inactive). Thus,
a smaller PMR value is better because the sender can transition a
destination address to the Failed (inactive) state quicker.
o Smaller PMR values increase the chances of spurious failure 3.1. Overview
detection where the sender incorrectly marks a destination address
as Failed (inactive) during periods of temporary congestion. As
[RFC4960] recommends for a coupling of the PMR value and the To minimize the performance impact during failover, the sender should
protocol parameter Association.Max.Retrans (AMR) value such avoid transmitting data to a failed destination address as early as
spurious failure detection risks to carry over to spurious possible. In the [RFC4960] SCTP path management scheme, the sender
association failure detection and closure. Larger PMR values are stops transmitting data to a destination address only after the
preferable to avoid spurious failure detection. destination address is marked inactive. This process takes a
significant amount of time as it requires the error counter of the
destination address to exceed the Path.Max.Retrans (PMR) threshold.
The issue cannot simply be mitigated by lowering of the PMR threshold
because this may result in spurious failure detection and unnecessary
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
Association.Max.Retrans (AMR) parameter values in [RFC4960], may
result in compromisation of the fault tolerance of SCTP.
From the above observations it is clear that tuning the PMR value The solution provided in this document is to extend the SCTP path
involves the following trade off -- a lower value improves management scheme of [RFC4960] by the addition of the Potentially
performance but increases the chances of spurious failure detection, Failed (PF) state as an intermediate state in between the active and
whereas a higher value degrades performance and reduces spurious inactive state of a destination address in [RFC4960] path management
failure detection in a wide range of path conditions. Thus, tuning scheme, and let the failover of data transfer away from a destination
the association's PMR value is an incomplete solution to address the address be driven by the entering of the PF state instead of by the
performance impact during failure. entering of the inactive state. Thereby SCTP may perform quick
failover without compromising the overall fault tolerance of
[RFC4960] SCTP. At the same time, RTO-based HEARTBEAT probing is
initiated towards a destination address once it enters PF state.
Thereby SCTP may quickly ascertain whether network connectivity
towards the destination address is broken or whether the failover was
spurious. In the case where the failover was spurious data transfer
may quickly resume towards the original destination address.
SCTP-PF defined in this document introduces the new Potentially The new failure detection algorithm assumes that loss detected by a
Failed (PF) destination address state in SCTP's path management timeout implies either severe congestion or network connectivity
procedure. The new Potentially Failed (PF) destination address state failure and it assumes that by default a destination address is
applies to SCTP single-homed operation as well as to SCTP multi-homed classified as PF already at the occurrence of one first timeout.
operation. The PF state was originally proposed to improve CMT
performance [NATARAJAN09]. The PF state is an intermediate state
between the Active and Failed states. SCTP's failure detection
procedure is modified to include the PF state. The new failure
detection algorithm assumes that loss detected by a timeout implies
either severe congestion or failure en-route. After a number of
consecutive timeouts on a path, the sender is unsure, and marks the
corresponding destination address as in the PF state. A PF
destination address is not used for data transmission except when it
is the only destination address available (e.g., for single-homed
SCTP) or in other special cases (discussed below). The new failure
detection algorithm requires only sender-side changes.
4.2. Specification of the SCTP-PF Algorithm 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 parameter called 1. The sender maintains a new tunable SCTP Protocol Parameter
PotentiallyFailed.Max.Retrans (PFMR). The RECOMMENDED value of called PotentiallyFailed.Max.Retrans (PFMR). The PFMR defines
PFMR is 0 when SCTP-PF is used. The PFMR defines a new the new intermediate PF threshold on the destination address
intermediate PF threshold on the destination address error error counter at exceed of which the destination address is
counter at exceed of which the destination address is classified classified as PF. The RECOMMENDED value of PFMR is 0, but other
as PF and related PF state actions are to be taken. By standard values MAY be used. Setting PFMR larger to or equal to
RFC4960 semantics a destination address is classified as Path.Max.Retrans (PMR) does not result in definition of a PF
Inactive once the error counter exceeds PMR. Setting PFMR threshold for the destination address. I.e., the destination
larger to or equal to PMR does not result in definition of a PF address will not be classified as PF prior to reaching inactive
threshold for the destination address. I.e., PFMR set larger to state.
or equal to PMR means that the destination address never will be
classified as PF.
2. The error counter of an active destination address is 2. The error counter of an active destination address is
incremented as specified in [RFC4960]. This means that the incremented as specified in [RFC4960]. This means that the
error counter of the destination address will be incremented error counter of the destination address will be incremented
each time the T3-rtx timer expires, or each time a HEARTBEAT each time the T3-rtx timer expires, or each time a HEARTBEAT
chunk is sent when idle and not acknowledged within an RTO. chunk is sent when idle and not acknowledged within an RTO.
When the value in the destination address error counter exceeds When the value in the destination address error counter exceeds
PFMR, the endpoint MUST mark the destination address as in the PFMR, the endpoint MUST mark the destination address as in the
PF state. PF state.
skipping to change at page 8, line 6 skipping to change at page 6, line 37
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.
B. A SCTP-PF sender MAY choose to send data to a destination B. A SCTP-PF sender MAY choose to send data to a destination
address in PF state, even if destination addresses in active address in PF state, even if destination addresses in active
state exist, have the SCTP-PF sender other means of state exist, have the SCTP-PF sender other means of
information available that disqualifies the destination information available that disqualifies the destination
address in active state from being preferred. However, the address in active state from being preferred. However, the
discussion of such mechanisms is outside of the scope of the discussion of such mechanisms is outside of the scope of the
SCTP_PF operation specified in this document. SCTP-PF operation specified in this document.
In all cases, the sender MUST NOT change the state of chosen In all cases, the sender MUST NOT change the state of 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 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
skipping to change at page 8, line 47 skipping to change at page 7, line 30
qualifies a particular destination address for being used. qualifies a particular destination address for being used.
The SCTP-PF protocol operation specified in this document The SCTP-PF protocol operation specified in this document
makes no assumption of the existence of such other means of makes no assumption of the existence of such other means of
information and specifies for the above as the default information and specifies for the above as the default
operation of an SCTP-PF sender. 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. HEARTBEAT chunks MUST be send to PF destination addresses 5. The HB.interval of the Path Heartbeat function of [RFC4960]
regardless of whether the Path Heartbeat function (Section 8.3 MUST be ignored for destination addresses in PF state. Instead
of [RFC4960]) is enabled for the destination address or not.
The HB.interval of the Path Heartbeat function of [RFC4960] MUST
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. The HEARTBEAT sending begins upon that a once per RTO. HEARTBEAT chunks SHOULD be sent to destination
destination address reaches the PF state. When a HEARTBEAT addresses in PF state, but the sending of HEARTBEATS MUST honor
chunk is not acknowledged within the RTO, the sender increments whether the Path Heartbeat function (Section 8.3 of [RFC4960])
the error counter and exponentially back off the RTO value. If is enabled for the destination address or not. I.e., if the
the error counter is less than PMR, the sender transmits another Path Heartbeat function is disabled for the destination address
packet containing the HEARTBEAT chunk immediately after timeout in question, HEARTBEATS MUST NOT be sent. Note that when
expiration on the previous HEARTBEAT. When data is being Heartbeat function is disabled, it may take longer to transition
transmitted to a destination address in the PF state, the PF destination to ACTIVE.
transmission of a HEARTBEAT chunk MAY be omitted in case receipt
of a SACK of or a T3-rtx timer expiration on the outstanding
data can provide equivalent information. Likewise the timeout
of a HEARTBEAT chunk MAY be ignored if data is outstanding
towards the destination address.
6. When the sender receives a HEARTBEAT ACK from a destination 6. HEARTBEATs are sent when a destination address reaches the PF
address in PF state, the sender MUST clear the error counter of state. When a HEARTBEAT chunk is not acknowledged within the
the destination address and transition the destination address RTO, the sender increments the error counter and exponentially
back to active state. When the sender resumes data transmission backs off the RTO value. If the error counter is less than PMR,
on the destination address, it MUST do this following the the sender transmits another packet containing the HEARTBEAT
prescriptions of Section 7.2 of [RFC4960]. chunk immediately after timeout expiration on the previous
HEARTBEAT. When data is being transmitted to a destination
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
expiration on the outstanding data can provide equivalent
information, such as a case where the data chunk has transmitted
to a single destination. Likewise, the timeout of a HEARTBEAT
chunk MAY be ignored if data is outstanding towards the
destination address.
7. Additional (PMR - PFMR) consecutive timeouts on a destination 7. When the sender receives a HEARTBEAT ACK from a HEARTBEAT sent
to a destination address in PF state, the sender MUST clear the
error counter of the destination address and transition the
destination address back to active state. When the sender
resumes data transmission on the destination address, it MUST do
this following the prescriptions of Section 7.2 of [RFC4960].
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 frequency as to the inactive destination address at a lower HB.interval
described in Section 8.3 of [RFC4960] (when this function is frequency as described in Section 8.3 of [RFC4960] (when the
enabled for the destination address). Path Heartbeat function is enabled for the destination address).
8. 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 MUST NOT clear the error count for an inactive destination
address and MUST NOT transition a PF destination address back to address and MUST NOT transition a destination address in PF
active state, since a sender cannot disambiguate whether the ACK state back to active state, since a sender cannot disambiguate
was for the original transmission or the retransmission(s). The whether the ACK was for the original transmission or the
same ambiguity concerns the related congestion window growth. retransmission(s). A SCTP sender MAY apply a different approach
The bytes of a newly acknowledged chunk which has been for the error count handling based on unequivocally information
transmitted to multiple destination addresses SHOULD be on which destination (including multiple destination addresses)
considered for contribution to the congestion window growth the chunk reached. This document makes no reference to what
towards the destination address where the chunk was last sent. such unequivocally information could consist of, neither how
The contribution of the ACKed bytes to the window growth is such unequivocally information could be obtained. The design of
subject to the prescriptions described in Section 7.2 of such an alternative approach is left to implementations.
[RFC4960] is fulfilled. A SCTP sender MAY apply a different
approach for both the error count handling and the congestion
control growth handling based on unequivocally information on
which destination (including multiple destination addresses) the
chunk reached. This document makes no reference to what such
unequivocally information could consist of, neither how such
unequivocally information could be obtained. The design of such
an alternative approach is left to implementations.
9. Acknowledgments for chunks that has been transmitted to one 10. Acknowledgments for 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 PF destination address destination address and MUST transition a destination address in
back to Active state. This situation can happen when new data PF state back to Active state. This situation can happen when
is sent to a destination address in the PF state. It can also new data is sent to a destination address in the PF state. It
happen in situations where the destination address is in the PF can also happen in situations where the destination address is
state due to the occurrence of a spurious T3-rtx timer and in the PF state due to the occurrence of a spurious T3-rtx timer
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-a-vis
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.
10. The SCTP stack SHOULD provide the ULP with the means to expose 11. When all destination addresses are in inactive state, and SCTP
protocol operation thus is said to be in dormant state, the
prescriptions given in Section 4 shall be followed.
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
the state transitions from Active to PF, and vice-versa. When of state transitions from Active to PF, and vice-versa. However
doing this, such an SCTP stack MUST provide the ULP with the it is recommended that an SCTP stack implementing SCTP-PF also
means to suppress exposure of PF state and associated state allows for that the ULP is kept ignorant of the PF state of its
transitions as well. destinations and the associated state transition. For this
reason is it recommended that an SCTP stack implementing SCTP-PF
also should provide the ULP with the means to suppress exposure
of PF state and the associated state transitions.
4.2.1. 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
Association.Max.Retrans (AMR). For example, an SCTP association with Association.Max.Retrans (AMR). For example, an SCTP association with
two destination addresses typically would reach dormant state in half two destination addresses typically would reach dormant state in half
the time of an [RFC4960] SCTP implementation in such situations. the time of an [RFC4960] SCTP implementation in such situations.
This is because a SCTP PF sender will send HEARTBEATS and data This is because a SCTP PF sender will send HEARTBEATS and data
retransmissions in parallel with RTO intervals when there are retransmissions in parallel with RTO intervals when there are
multiple destinations addresses in PF state. This argument pressumes multiple destinations addresses in PF state. This argument presumes
that RTO << HB.interval of [RFC4960]. One could use higher values of that RTO << HB.interval of [RFC4960]. With the design goal that
PMR, which makes the dormant state situations less likely to happen. SCTP-PF shall provide the same level of disruption tolerance as an
The downside of increasing the PMR value is that destination address [RFC4960] SCTP implementation with the same Path.Max.Retrans (PMR)
failure detections and notifications of such events to ULP is and Association.Max.Retrans (AMR) setting, we prescribe for that an
weakened. SCTP-PF implementation SHOULD operate as described below in
Section 4.1 during dormant state.
A design goal of SCTP-PF is that it should provide the same level of An SCTP-PF implementation MAY choose a different dormant state
disruption tolerance as an [RFC4960] SCTP implementation with the operation than the one described below in Section 4.1 provided that
same Path.Max.Retrans (PMR) and Association.Max.Retrans (AMR) the solution chosen does not compromise the fault tolerance of the
setting. For this reason, SCTP-PF SHOULD perform the following SCTP-PF operation.
operations during dormant state, while this is an implementation
decision in [RFC4960].
a. When the destination addresses are all in inactive state, the The below prescription for SCTP-PF dormant state handling SHOULD NOT
sender MUST choose one destination when data is transmitted. The be coupled to the value of the PFMR, but solely to the activation of
sender MUST NOT change the state and the error counter of any SCTP-PF logic in an SCTP implementation.
destination address regardless of whether it has been chosen for
transmission or not.
b. The sender SHOULD choose the destination in inactive state with It is noted that the below dormant state operation is considered to
provide added disruption tolerance also for an [RFC4960] SCTP
implementation, and that it can be sensible for an [RFC4960] SCTP
implementation to follow this mode of operation. For an [RFC4960]
SCTP implementation the continuation of data transmission during
dormant state makes the fault tolerance of SCTP be more robust
towards situations where some, or all, alternative paths of an SCTP
association approach, or reach, inactive state prior to that the
primary path used for data transmission observes trouble.
4.1. SCTP Dormant State Procedure
a. When the destination addresses are all in inactive state and data
is available for transfer, the sender MUST choose one destination
and transmit data to this destination address.
b. The sender MUST NOT change the state of the chosen destination
address (it remains in inactive state) and it MUST NOT clear the
error counter of the destination address as a result of choosing
the destination address for data transmission.
c. 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 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 source
- destination pair where failure was observed. Rules for picking - destination pair where failure was observed. Rules for picking
the most divergent source-destination pair are an implementation the most divergent source-destination pair are an implementation
decision and are not specified within this document. To support decision and are not specified within this document. To support
differentiation of inactive destination addresses based on their differentiation of inactive destination addresses based on their
error count SCTP will need to allow for increment of the error count SCTP will need to allow for increment of the
destination address error counters up to some reasonable limit destination address error counters up to some reasonable limit
above PMR+1, thus changing the prescriptions of [RFC4960], above PMR+1, thus changing the prescriptions of [RFC4960],
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 the active destination address, i.e., the destination address with
fewest error counts is optimal when some paths are permanently the fewest error counts is optimal when some paths are
inactive, but suboptimal when a path instability is transient. permanently inactive, but suboptimal when a path instability is
transient.
An SCTP-PF implementation MAY keep the operation during dormant state
an implementation decision, but it should be careful not to
compromise the fault tolerance of the SCTP operation.
The above prescriptions for SCTP-PF dormant state handling SHOULD NOT
be coupled to the value of the PFMR, but solely to the activation of
SCTP-PF logic in an SCTP implementation. It is further noted that
also a standard [RFC4960] SCTP implementation can use this mode of
operation to improve the fault tolerance (which some implementations
already do).
4.3. Permanent Failover
This section describes an OPTIONAL switchback feature called
Permanent Failover which is beneficiary to deploy in certain
situations.
4.3.1. Background
In [RFC4960], an SCTP sender migrates the traffic back to the
original primary destination address once this address becomes active
again. As the CWND towards the original primary destination address
has to be rebuilt once data transfer resumes, the switch back to use
the original primary address is not always optimal. Indeed [CARO02]
shows that the switch back to the original primary may degrade SCTP
performance compared to continuing data transmission on the same
path, especially, but not only, in scenarios where this path's
characteristics are better. In order to mitigate this performance
degradation, the Permanent Failover operation was proposed in
[CARO02]. When SCTP changes the destination address due to failover,
Permanent Failover operation allows SCTP sender to continue data
transmission on the new working path even when the old primary
destination address becomes active again. This is achieved by having
SCTP perform a switch over of the primary path to the alternative
working path rather than having SCTP switch back data transfer to the
(previous) primary path.
The manner of switch over operation that is most optimal in a given 5. Permanent Failover
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 extent to which it is desired for the mode of operation to
enforce traffic distribution over a number of network paths. I.e.,
load distribution of traffic from multiple SCTP associations may be
sought to be enforced by distribution of the set primary paths with
[RFC4960] switchback operation. However as [RFC4960] switchback
behavior is suboptimal in certain situations, especially in scenarios
where a number of equally good paths are available, it is recommended
for SCTP to support also, as alternative behavior, the Permanent
Failover switch over modes of operation.
4.3.2. Permanent Failover Algorithm The objective of the Permanent Failover operation is to allow the
SCTP sender to continue data transmission on a new working path even
when the old primary destination address becomes active again. This
is achieved by having SCTP perform a switch over of the primary 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
not only to SCTP-PF implementations, but also to [RFC4960]
implementations.
The Permanent Failover operation requires only sender side changes. The Permanent Failover operation requires only sender side changes.
The details are: 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). The PSMR MUST be set Primary.Switchover.Max.Retrans (PSMR). For SCTP-PF
greater or equal to the PFMR value. Implementations MUST reject implementations, the PSMR MUST be set greater or equal to the
PFMR value. For [RFC4960] implementations the PSMR MUST be set
greater or equal to the PMR value. Implementations MUST reject
any other values of PSMR. 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 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. The recommended value of PSMR is PFMR when Permanent Failover is 4. For SCTP-PF, the recommended value of PSMR is PFMR when Permanent
used. This means that no forced switchback to a previously Failover is used. This means that no forced switchback to a
failed primary path is performed. An implementation of Permanent previously failed primary path is performed. An SCTP-PF
Failover MUST support the setting of PSMR = PFMR. An implementation of Permanent Failover MUST support the setting of
implementation of Permanent Failover MAY support setting of PSMR PSMR = PFMR. A SCTP-PF implementation of Permanent Failover MAY
> PFMR. support setting of PSMR > PFMR.
5. It MUST be possible to disable the Permanent Failover and obtain 5. For [RFC4960] SCTP, the recommended value of PSMR is PMR when
Permanent Failover is used. This means that no forced switchback
to a previously failed primary path is performed. A [RFC4960]
SCTP implementation of Permanent Failover MUST support the
setting of PSMR = PMR An [RFC4960] SCTP implementation of
Permanent Failover MAY support larger settings of PSMR > PMR.
6. It MUST be possible to disable the Permanent Failover and obtain
the standard switchback operation of [RFC4960]. the standard switchback operation of [RFC4960].
To support optimal operation in a wider range of network scenarios, The manner of switch over operation that is most optimal in a given
it it proposed for an SCTP-PF implementation to implement Permanent scenario depends on the relative quality of a set primary path versus
Failover operation as an optional feature. The implementation of the the quality of alternative paths available as well as it depends on
Permanent Failover feature is optional for an SCTP-PF implementation. the extent to which it is desired for the mode of operation to
enforce traffic distribution over a number of network paths. I.e.,
load distribution of traffic from multiple SCTP associations may be
sought to be enforced by distribution of the set primary paths with
[RFC4960] switchback operation. However as [RFC4960] switchback
behavior is suboptimal in certain situations, especially in scenarios
where a number of equally good paths are available, an SCTP
implementation MAY support also, as alternative behavior, the
Permanent Failover mode of operation and MAY enable it based on
users' requests.
For an SCTP implementation that implements Permanent Failover, this For an SCTP implementation that implements Permanent Failover, this
specification RECOMMENDS that the standard RFC4960 switchback specification RECOMMENDS that the standard RFC4960 switchback
operation is retained as the default operation. operation is retained as the default operation.
5. Socket API Considerations 6. Suggested SCTP Protocol Parameter Values
This document does not alter the [RFC4960] value RECOMMENDATIONS for
the SCTP Protocol Parameters defined in [RFC4960].
The following protocol parameter is RECOMMENDED:
PotentiallyFailed.Max.Retrans (PFMR) - 0
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. the SCTP-PF behavior as well as the Permanent Failover 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 4. The second one controls the PSMR parameters described in Section 3 and in Section 5. The second
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().
5.1. 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 { struct sctp_paddr_change {
uint16_t spc_type; uint16_t spc_type;
uint16_t spc_flags; uint16_t spc_flags;
skipping to change at page 15, line 21 skipping to change at page 14, line 5
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.
5.2. 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 and before the primary considered potentially failed or unreachable. The same socket option
path is changed automatically. This socket option uses the level is used by applications to set and get the number of timeouts before
IPPROTO_SCTP and the name SCTP_PEER_ADDR_THLDS. the primary path is changed automatically by the Permanent Failover
function. This socket option uses the level IPPROTO_SCTP 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 { 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;
}; };
skipping to change at page 16, line 6 skipping to change at page 14, line 38
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 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 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 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 errno set to EINVAL if a value different
from 0xffff is used in spt_pathcpthld. Setting of spt_pathcpthld from 0xffff is used in spt_pathcpthld. For SCTP-PF, the setting
< spt_pathpfthld should be rejected with errno set to EINVAL. An of spt_pathcpthld < spt_pathpfthld should be rejected with errno
set to EINVAL. For [RFC4960] SCTP, the setting of spt_pathcpthld
< spt_pathmaxrxt should be rejected with errno set to EINVAL. A
SCTP-PF implementation MAY support only setting of spt_pathcpthld
= spt_pathpfthld and spt_pathcpthld = 0xffff and a [RFC4960] SCTP
implementation MAY support only setting of spt_pathcpthld = implementation MAY support only setting of spt_pathcpthld =
spt_pathpfthld and spt_pathcpthld = 0xffff. In this case it shall spt_pathmaxrxt and spt_pathcpthld = 0xffff. In these cases SCTP
reject setting of other values with errno set to EINVAL. shall reject setting of other values with errno set to EINVAL.
5.3. 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 5.1. 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 { struct sctp_assoc_value {
sctp_assoc_t assoc_id; sctp_assoc_t assoc_id;
skipping to change at page 16, line 48 skipping to change at page 15, line 35
}; };
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 potentially failed path state is exposed if and
only if this parameter is non-zero. only if this parameter is non-zero.
6. 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]. The logic described here is discussed in [RFC4960] and [RFC6458].
for sender-side only enabled by configuration and does not have any
impacts on protocol messages on the wire. No new chunk type or new
field parameter is not required in this document.
7. IANA Considerations The logic introduced by this document does not impact existing on-
the-wire SCTP messages. Also, this document does not introduce any
new on-the-wire SCTP messages that require new security
considerations.
SCTP-PF makes SCTP not only more robust during primary path failure/
congestion but also more vulnerable to network connectivity/
congestion attacks on the primary path. SCTP-PF makes it easier for
an attacker to trick SCTP to change data transfer path, since the
duration of time that an attacker needs to compromise the network
connectivity is much shorter than [RFC4960]. However, SCTP-PF does
not constitute a significant change in the duration of time and
effort an attacker needs to keep SCTP away from the primary path.
With the standard switchback operation [RFC4960] SCTP resumes data
transfer on its primary path as soon as the next HEARTBEAT succeeds.
On the other hand, usage of the Permanent Failover mechanism, does
change the treat analysis. This is because attackers can force a
permanent change of the data transfer path by blocking the primary
path until the switchover of the primary path is triggered by the
Permanent Failover algorithm. This especially will be the case when
Permanent Failover is used together with SCTP-PF with the particular
setting of PSMR = PFMR = 0, as Permanent Failover here happens
already at the first RTO timeout experienced. Users of the Permanent
Failover mechanism should be aware of this fact.
The event notification of path state transfer from active to
potentially failed state and vice versa gives attackers an increased
possibility to generate more local events. However, it is assumed
that event notifications are rate-limited in the implementation to
address this threat.
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 IANA.
8. Proposed Change of Status (to be Deleted before Publication) 10. 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.
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 WG to bring the work
forward as PS. forward 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 Telco
signaling environments for several years with good results. signaling environments for several years with good results.
9. References 12. References
9.1. 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, March 1997.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC [RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC
4960, September 2007. 4960, September 2007.
9.2. 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 Multihoming",
MILCOM 2004 , 11 2004. MILCOM 2004 , 11 2004.
skipping to change at page 18, line 47 skipping to change at page 18, line 22
Transmission Protocol (SCTP)", RFC 6458, December 2011. Transmission Protocol (SCTP)", RFC 6458, December 2011.
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. 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]. For example, if when Path.Max.Retrans=0, [GRINNEMO04] [FALLON08]. However to significantly reduce the
SCTP switches to another destination address on a single timeout. failover time it is required to go down (as with PFMR) to
This smaller value for Path.Max.Retrans can results in spurious Path.Max.Retrans=0 and with this setting SCTP switches to another
failover, which might be a problem. destination address already on a single timeout which may result in
spurious failover. Spurious failover is a problem in [RFC4960] SCTP
Unlike SCTP-PF, the interval for heartbeat packets is governed by as the transmission of HEARTBEATS on the left primary path, unlike in
'HB.interval' even during failover process. 'HB.interval' is usually SCTP-PF, is governed by 'HB.interval' also during the failover
set in the order of seconds (recommended value is 30 seconds). When process. 'HB.interval' is usually set in the order of seconds
the primary path becomes inactive, the next HEARTBEAT can be (recommended value is 30 seconds) and when the primary path becomes
transmitted only seconds later. Meanwhile, the primary path may have inactive, the next HEARTBEAT may be transmitted only many seconds
recovered. In such situations, post failover, an endpoint is forced later. Indeed as recommended, only 30 secs later. Meanwhile, the
to wait on the order of seconds before the endpoint can resume primary path may since long have recovered, if it needed recovery at
transmission on the primary path. However, using smaller value for all (indeed the failover could be truely spurious). In such
'HB.interval' might help this situation, but it will be the waste of situations, post failover, an endpoint is forced to wait in the order
bandwidth in most cases. of many seconds before the endpoint can resume transmission on the
primary path and furthermore once it returns on the primary path the
CWND needs to be rebuild anew - a process which the throughput
already have had to suffer from on the alternate path. Using a
smaller value for 'HB.interval' might help this situation, but it
would result in a general waste of bandwidth as such more frequent
HEARBEATING would take place also when there are no observed
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
is set to be the primary path, but this adds complication in the ULP.
In addition, smaller Path.Max.Retrans values also affect In addition, smaller Path.Max.Retrans values also affect the
'Association.Max.Retrans' values. When the SCTP association's error 'Association.Max.Retrans' value. When the SCTP association's error
count (sum of error counts on all ACTIVE paths) exceeds count exceeds Association.Max.Retrans threshold, the SCTP sender
Association.Max.Retrans threshold, the SCTP sender considers the peer considers the peer endpoint unreachable and terminates the
endpoint unreachable and terminates the association. Therefore, association. Section 8.2 in [RFC4960] recommends that
Section 8.2 in [RFC4960] recommends that Association.Max.Retrans Association.Max.Retrans value should not be larger than the summation
value should not be larger than the summation of the Path.Max.Retrans of the Path.Max.Retrans of each of the destination addresses. Else
of each of the destination addresses, else the SCTP sender considers the SCTP sender considers its peer reachable even when all
its peer reachable even when all destinations are INACTIVE. To avoid destinations are INACTIVE and to avoid this dormant state operation,
such inconsistent behavior an SCTP implementation SHOULD reduce [RFC4960] SCTP implementation SHOULD reduce Association.Max.Retrans
Association.Max.Retrans accordingly whenever it reduces accordingly whenever it reduces Path.Max.Retrans. However, smaller
Path.Max.Retrans. However, smaller Association.Max.Retrans value Association.Max.Retrans value compromizes the fault tolerance of SCTP
increases chances of association termination during minor congestion as it increases the chances of association termination during minor
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
smaller values for RTO.initial or RTO.min can contribute to keep RTO smaller values for RTO.initial or RTO.min can contribute to keep the
value small. 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 the
Section 15 of [RFC4960]. However, this approach requires to have Section 15 of [RFC4960]. However, this approach requires to have
enough knowledge about the network characteristics between end enough knowledge about the network characteristics between end
points. Otherwise, it can introduce adverse side-effects such as points. Otherwise, it can introduce adverse side-effects such as
spurious timeouts. spurious timeouts.
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
Path.Max.Retrans is kept at the [RFC4960] recommended value, the
reduction of the RTO.max doesn't reduce the failover time
sufficiently enough to prevent severe performance degradation during
failover.
Appendix B. Discussions for Path Bouncing Effect Appendix B. Discussions for 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 the path bouncing effect where
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 path bouncing effect [CARO04] [CARO05].
There are two main reasons for this. First, SCTP is basically There are two main reasons for this. First, SCTP is basically
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