Network Working Group Y. Nishida
Internet-Draft GE Global Research
Intended status: Standards Track P. Natarajan
Expires: March 3, 2016 Cisco Systems
A. Caro
BBN Technologies
P. Amer
University of Delaware
K. Nielsen
Ericsson
August 31, 2015
SCTP-PF: Quick Failover Algorithm in SCTP
draft-ietf-tsvwg-sctp-failover-12.txt
Abstract
SCTP supports multi-homing. However, when the failover operation
specified in RFC4960 is followed, there can be significant delay and
performance degradation in the data transfer path failover. To
overcome this problem this document specifies a quick failover
algorithm (SCTP-PF) based on the introduction of a Potentially Failed
(PF) state in SCTP Path Management.
The document also specifies a dormant state operation of SCTP. This
dormant state operation is required to be followed by an SCTP-PF
implementation, but it may equally well be applied by a standard
RFC4960 SCTP implementation.
Additionally, the document introduces an alternative switchback
operation mode called Primary Path Switchover that will be beneficial
in certain situations. This mode of operation applies to both a
standard RFC4960 SCTP implementation as well as to a SCTP-PF
implementation.
The procedures defined in the document require only minimal
modifications to the RFC4960 specification. The procedures are
sender-side only and do not impact the SCTP receiver.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
Nishida, et al. Expires March 3, 2016 [Page 1]
Internet-Draft SCTP-PF August 2015
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 3, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 4
3. SCTP with Potentially Failed Destination State (SCTP-PF) . . 4
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Specification of the SCTP-PF Procedures . . . . . . . . . 5
4. Dormant State Operation . . . . . . . . . . . . . . . . . . . 9
4.1. SCTP Dormant State Procedure . . . . . . . . . . . . . . 10
5. Primary Path Switchover . . . . . . . . . . . . . . . . . . . 11
6. Suggested SCTP Protocol Parameter Values . . . . . . . . . . 12
7. Socket API Considerations . . . . . . . . . . . . . . . . . . 12
7.1. Support for the Potentially Failed Path State . . . . . . 13
7.2. Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) Socket
Option . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.3. Exposing the Potentially Failed Path State
(SCTP_EXPOSE_POTENTIALLY_FAILED_STATE) Socket Option . . 15
8. Security Considerations . . . . . . . . . . . . . . . . . . . 16
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
11. Proposed Change of Status (to be Deleted before Publication) 17
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
12.1. Normative References . . . . . . . . . . . . . . . . . . 17
Nishida, et al. Expires March 3, 2016 [Page 2]
Internet-Draft SCTP-PF August 2015
12.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. Discussions of Alternative Approaches . . . . . . . 18
A.1. Reduce Path.Max.Retrans (PMR) . . . . . . . . . . . . . . 18
A.2. Adjust RTO related parameters . . . . . . . . . . . . . . 19
Appendix B. Discussions for Path Bouncing Effect . . . . . . . . 20
Appendix C. SCTP-PF for SCTP Single-homed Operation . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
The Stream Control Transmission Protocol (SCTP) specified in
[RFC4960] supports multi-homing at the transport layer. SCTP's
multi-homing features include failure detection and failover
procedures to provide network interface redundancy and improved end-
to-end fault tolerance. In SCTP's current failure detection
procedure, the sender must experience Path.Max.Retrans (PMR) number
of consecutive failed timer-based retransmissions on a destination
address before detecting a path failure. Until detecting the path
failure, the sender continues to transmit data on the failed path.
The prolonged time in which [RFC4960] SCTP continues to use a failed
path severely degrades the performance of the protocol. To address
this problem, this document specifies a quick failover algorithm
(SCTP-PF) based on the introduction of a new Potentially Failed (PF)
path state in SCTP path management. The performance deficiencies of
the [RFC4960] failover operation, and the improvements obtainable
from the introduction of a Potentially Failed state in SCTP, were
proposed and documented in [NATARAJAN09] for Concurrent Multipath
Transfer SCTP [IYENGAR06].
While SCTP-PF can accelerate failover process and improve
performance, the risks that an SCTP endpoint enters in dormant state
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.
The operation after the recovery of a failed path equally well
impacts the performance of the protocol. With the procedures
specified in [RFC4960] SCTP will, after a failover from the primary
Nishida, et al. Expires March 3, 2016 [Page 3]
Internet-Draft SCTP-PF August 2015
path, switch back to use the primary path for data transfer as soon
as this path becomes available again. From a performance perspective
such a forced switchback of the data transmission path can be
suboptimal as the CWND towards the original primary destination
address has to be rebuilt once data transfer resumes, [CARO02]. As
an optional alternative to the switchback operation of [RFC4960],
this document specifies an alternative Primary Path Switchover
procedure which avoid such forced switchbacks of the data transfer
path. The Primary Path Switchover operation was originally proposed
in [CARO02].
While SCTP-PF primarily is motivated by a desire to improve the
multi-homed operation, the feature applies also to SCTP single-homed
operation. Here the algorithm serves to provide increased failure
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.
A brief description of the motivation for the introduction of the
Potentially Failed state including a discussion of alternative
approaches to mitigate the deficiencies of the [RFC4960] failover
operation are given in the Appendices. Discussion of path bouncing
effects that might be caused by frequent switchover, are also
provided there.
2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. SCTP with Potentially Failed Destination State (SCTP-PF)
3.1. Overview
To minimize the performance impact during failover, the sender should
avoid transmitting data to a failed destination address as early as
possible. In the [RFC4960] SCTP path management scheme, the sender
stops transmitting data to a destination address only after the
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.
Nishida, et al. Expires March 3, 2016 [Page 4]
Internet-Draft SCTP-PF August 2015
The solution provided in this document is to extend the SCTP path
management scheme of [RFC4960] by the addition of the Potentially
Failed (PF) state as an intermediate state in between the active and
inactive state of a destination address in the [RFC4960] path
management scheme, and let the failover of data transfer away from a
destination address be driven by the entering of the PF state instead
of by the 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.
The new failure detection algorithm assumes that loss detected by a
timeout implies either severe congestion or network connectivity
failure and it assumes that by default a destination address is
classified as PF already at the occurrence of one first timeout.
3.2. Specification of the SCTP-PF Procedures
The SCTP-PF operation is specified as follows:
1. The sender maintains a new tunable SCTP Protocol Parameter
called PotentiallyFailed.Max.Retrans (PFMR). The PFMR defines
the new intermediate PF threshold on the destination address
error counter at exceed of which the destination address is
classified as PF. The RECOMMENDED value of PFMR is 0, but other
values MAY be used. Setting PFMR larger to or equal to
Path.Max.Retrans (PMR) does not result in definition of a PF
threshold for the destination address. I.e., the destination
address will not be classified as PF prior to reaching inactive
state.
2. The error counter of an active destination address is
incremented as specified in [RFC4960]. This means that the
error counter of the destination address will be incremented
each time the T3-rtx timer expires, or each time a HEARTBEAT
chunk is sent when idle and not acknowledged within an RTO.
When the value in the destination address error counter exceeds
PFMR, the endpoint MUST mark the destination address as in the
PF state.
3. The PFMR threshold defines the point the destination address no
longer is considered a good candidate for data transmission and
a SCTP-PF sender SHOULD NOT send data to destination addresses
Nishida, et al. Expires March 3, 2016 [Page 5]
Internet-Draft SCTP-PF August 2015
in PF state when alternative destination addresses in active
state are available. Specifically this means that:
i When there is outbound data to send and the destination
address presently used for data transmission is in PF state,
the sender SHOULD choose a destination address in active
state, if one exists, and failover to deploy this destination
address for data transmission.
ii When retransmitting data that has timed out and the sender
thus by [RFC4960], section 6.4.1, should attempt to pick a
new destination address for data retransmission, the sender
SHOULD choose an alternate destination transport address in
active state if one exists.
iii When there is outbound data to send and the SCTP user
explicitly requests to send data to a destination address in
PF state, the sender SHOULD send the data to an alternate
destination address in active state if one exists.
When choosing among multiple destination addresses in active
state the following considerations are given:
A. An SCTP sender should comply with [RFC4960], section 6.4.1,
principles of choosing most divergent source-destination
pairs compared with, for i.: the destination address in PF
state that it performs a failover from, and for ii.: the
destination address towards which the data timed out. Rules
for picking the most divergent source-destination pair are
an implementation decision and are not specified within this
document.
B. A SCTP-PF sender MAY choose to send data to a destination
address in PF state, even if destination addresses in active
state exist, have the SCTP-PF sender other means of
information available that disqualifies the destination
address in active state from being preferred. However, the
discussion of such mechanisms is outside of the scope of the
SCTP-PF operation specified in this document.
In all cases, the sender MUST NOT change the state of chosen
destination address, whether this state be active or PF, and it
MUST NOT clear the error counter of the destination address as a
result of choosing the destination address for data
transmission.
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
Nishida, et al. Expires March 3, 2016 [Page 6]
Internet-Draft SCTP-PF August 2015
destination address in PF state and transmit or retransmit data
to this destination address using the following rules:
A. The sender SHOULD choose the destination in PF state with
the lowest error count (fewest consecutive timeouts) for
data transmission and transmit or retransmit data to this
destination.
B. When there are multiple destination addresses in PF state
with same error count, the sender should let the choice
among the multiple destination addresses in PF state with
equal error count be based on the [RFC4960], section 6.4.1,
principles of choosing most divergent source-destination
pairs when executing (potentially consecutive)
retransmission. Rules for picking the most divergent
source-destination pair are an implementation decision and
are not specified within this document.
C. A sender MAY choose to deploy other strategies than the
above when choosing among multiple destinations in PF state
have the SCTP-PF sender other means of information available
that qualifies a particular destination address for being
used. The SCTP-PF protocol operation specified in this
document makes no assumption of the existence of such other
means of information and specifies for the above as the
default operation of an SCTP-PF sender.
The sender MUST NOT change the state and the error counter of
any destination address regardless of whether it has been chosen
for transmission or not.
5. 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
once per RTO. HEARTBEAT chunks SHOULD be sent to destination
addresses in PF state, but the sending of HEARTBEATS MUST honor
whether the Path Heartbeat function (Section 8.3 of [RFC4960])
is enabled for the destination address or not. I.e., if the
Path Heartbeat function is disabled for the destination address
in question, HEARTBEATS MUST NOT be sent. Note that when
Heartbeat function is disabled, it may take longer to transition
a destination address in PF state back to active state.
6. HEARTBEATs are sent when a destination address reaches the PF
state. When a HEARTBEAT chunk is not acknowledged within the
RTO, the sender increments the error counter and exponentially
backs off the RTO value. If the error counter is less than PMR,
the sender transmits another packet containing the HEARTBEAT
Nishida, et al. Expires March 3, 2016 [Page 7]
Internet-Draft SCTP-PF August 2015
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. When the sender receives a HEARTBEAT ACK from a HEARTBEAT sent
to a destination address in PF state, the sender SHOULD clear
the error counter of the destination address and transition the
destination address back to active state. When the sender
resumes data transmission on a destination address after a
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
address in PF state confirm the path failure, upon which the
destination address transitions to the inactive state. As
described in [RFC4960], the sender (i) SHOULD notify the ULP
about this state transition, and (ii) transmit HEARTBEAT chunks
to the inactive destination address at a lower HB.interval
frequency as described in Section 8.3 of [RFC4960] (when the
Path Heartbeat function is enabled for the destination address).
9. Acknowledgments for chunks that have been transmitted to
multiple destinations (i.e., a chunk which has been
retransmitted to a different destination address than the
destination address to which the chunk was first transmitted)
SHOULD NOT clear the error count for an inactive destination
address and SHOULD NOT transition a destination address in PF
state back to active state, since a sender cannot disambiguate
whether the ACK was for the original transmission or the
retransmission(s). A SCTP sender MAY apply a different approach
for the error count 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
Nishida, et al. Expires March 3, 2016 [Page 8]
Internet-Draft SCTP-PF August 2015
such unequivocally information could be obtained. The design of
such an alternative approach is left to implementations.
10. Acknowledgments for data chunks that has been transmitted to one
destination address only MUST clear the error counter for the
destination address and MUST transition a destination address in
PF state back to active state. This situation can happen when
new data is sent to a destination address in the PF state. It
can also happen in situations where the destination address is
in the PF state due to the occurrence of a spurious T3-rtx timer
and acknowledgments start to arrive for data sent prior to
occurrence of the spurious T3-rtx and data has not yet been
retransmitted towards other destinations. This document does
not specify special handling for detection of or reaction to
spurious T3-rtx timeouts, e.g., for special operation Vis-avis
the congestion control handling or data retransmission operation
towards a destination address which undergoes a transition from
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
additional attention, experimentation and specification for
single-homed SCTP as well as for multi-homed SCTP protocol
operation.
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
of state transitions from active to PF, and vice-versa. However
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
destinations and the associated state transition. For this
reason it is 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. Dormant State Operation
In a situation with complete disruption of the communication in
between the SCTP Endpoints, the aggressive HEARTBEAT transmissions of
SCTP-PF on destination addresses in PF state may make the association
enter dormant state faster than a standard [RFC4960] SCTP
implementation given the same setting of Path.Max.Retrans (PMR) and
Association.Max.Retrans (AMR). For example, an SCTP association with
two destination addresses typically would reach dormant state in half
the time of an [RFC4960] SCTP implementation in such situations.
This is because a SCTP PF sender will send HEARTBEATS and data
Nishida, et al. Expires March 3, 2016 [Page 9]
Internet-Draft SCTP-PF August 2015
retransmissions in parallel with RTO intervals when there are
multiple destinations addresses in PF state. This argument presumes
that RTO << HB.interval of [RFC4960]. With the design goal that
SCTP-PF shall provide the same level of disruption tolerance as an
[RFC4960] SCTP implementation with the same Path.Max.Retrans (PMR)
and Association.Max.Retrans (AMR) setting, we prescribe for that an
SCTP-PF implementation SHOULD operate as described below in
Section 4.1 during dormant state.
An SCTP-PF implementation MAY choose a different dormant state
operation than the one described below in Section 4.1 provided that
the solution chosen does not compromise the fault tolerance of the
SCTP-PF operation.
The below prescription 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 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
transmission. When there are multiple destinations with same
error count in inactive state, the sender SHOULD attempt to pick
the most divergent source - destination pair from the last source
- destination pair where failure was observed. Rules for picking
the most divergent source-destination pair are an implementation
decision and are not specified within this document. To support
differentiation of inactive destination addresses based on their
Nishida, et al. Expires March 3, 2016 [Page 10]
Internet-Draft SCTP-PF August 2015
error count SCTP will need to allow for increment of the
destination address error counters up to some reasonable limit
above PMR+1, thus changing the prescriptions of [RFC4960],
section 8.3, in this respect. The exact limit to apply is not
specified in this document but it is considered reasonable to
require for such to be an order of magnitude higher than the PMR
value. A sender MAY choose to deploy other strategies that the
strategy defined by here. The strategy to prioritize the last
active destination address, i.e., the destination address with
the fewest error counts is optimal when some paths are
permanently inactive, but suboptimal when a path instability is
transient.
5. Primary Path Switchover
The objective of the Primary Path Switchover 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 Primary Path Switchover operation requires only sender side
changes. The details are:
1. The sender maintains a new tunable parameter, called
Primary.Switchover.Max.Retrans (PMR). For SCTP-PF
implementations, the PMR MUST be set greater or equal to the PFMR
value. For [RFC4960] implementations the PMR MUST be set greater
or equal to the PMR value. Implementations MUST reject any other
values of PMR.
2. When the path error counter on a set primary path exceeds PMR,
the SCTP implementation MUST autonomously select and set a new
primary path.
3. The primary path selected by the SCTP implementation MUST be the
path which at the given time would be chosen for 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 fails.
4. For SCTP-PF, the recommended value of PMR is PFMR when Primary
Path Switchover operation mode is used. This means that no
forced switchback to a previously failed primary path is
performed. An SCTP-PF implementation of Primary Path Switchover
Nishida, et al. Expires March 3, 2016 [Page 11]
Internet-Draft SCTP-PF August 2015
MUST support the setting of PMR = PFMR. A SCTP-PF implementation
of Primary Path Switchover MAY support setting of PMR > PFMR.
5. For [RFC4960] SCTP, the recommended value of PMR is PMR when
Primary Path Switchover is used. This means that no forced
switchback to a previously failed primary path is performed. A
[RFC4960] SCTP implementation of Primary Path Switchover MUST
support the setting of PMR = PMR. An [RFC4960] SCTP
implementation of Primary Path Switchover MAY support larger
settings of PMR > PMR.
6. It MUST be possible to disable the Primary Path Switchover
operation and obtain the standard switchback operation of
[RFC4960].
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
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, an SCTP
implementation MAY support also, as alternative behavior, the Primary
Path Switchover mode of operation and MAY enable it based on users'
requests.
For an SCTP implementation that implements the Primary Path
Switchover operation, this specification RECOMMENDS that the standard
RFC4960 switchback operation is retained as the default operation.
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
extended to provide a way for the application to control and observe
the SCTP-PF behavior as well as the Primary Path Switchover function.
Nishida, et al. Expires March 3, 2016 [Page 12]
Internet-Draft SCTP-PF August 2015
Please note that this section is informational only.
A socket API implementation based on [RFC6458] is, by means of the
existing SCTP_PEER_ADDR_CHANGE event, extended to provide the event
notification when a peer address enters or leaves the potentially
failed state as well as the socket API implementation is extended to
expose the potentially failed state of a peer address in the existing
SCTP_GET_PEER_ADDR_INFO structure.
Furthermore, two new read/write socket options for the level
IPPROTO_SCTP and the name SCTP_PEER_ADDR_THLDS and
SCTP_EXPOSE_POTENTIALLY_FAILED_STATE are defined as described below.
The first socket option is used to control the values of the PFMR and
PMR parameters described in Section 3 and in Section 5. The second
one controls the exposition of the potentially failed path state.
Support for the SCTP_PEER_ADDR_THLDS and
SCTP_EXPOSE_POTENTIALLY_FAILED_STATE socket options need also to be
added to the function sctp_opt_info().
7.1. Support for the Potentially Failed Path State
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
changes described in [RFC6458], this event is also provided, if a
peer address enters or leaves the potentially failed state. The
notification as defined in [RFC6458] uses the following structure:
strict sctp_paddr_change {
uint16_t spc_type;
uint16_t spc_flags;
uint32_t spc_length;
strict sockaddr_storage spc_aaddr;
uint32_t spc_state;
uint32_t spc_error;
sctp_assoc_t spc_assoc_id;
}
[RFC6458] defines the constants SCTP_ADDR_AVAILABLE,
SCTP_ADDR_UNREACHABLE, SCTP_ADDR_REMOVED, SCTP_ADDR_ADDED, and
SCTP_ADDR_MADE_PRIM to be provided in the spc_state field. This
document defines in addition to that the new constant
SCTP_ADDR_POTENTIALLY_FAILED, which is reported if the affected
address becomes potentially failed.
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
structure:
Nishida, et al. Expires March 3, 2016 [Page 13]
Internet-Draft SCTP-PF August 2015
strict sctp_paddrinfo {
sctp_assoc_t spinfo_assoc_id;
strict sockaddr_storage spinfo_address;
int32_t spinfo_state;
uint32_t spinfo_cwnd;
uint32_t spinfo_srtt;
uint32_t spinfo_rto;
uint32_t spinfo_mtu;
};
[RFC6458] defines the constants SCTP_UNCONFIRMED, SCTP_ACTIVE, and
SCTP_INACTIVE to be provided in the spinfo_state field. This
document defines in addition to that the new constant
SCTP_POTENTIALLY_FAILED, which is reported if the peer address is
potentially failed.
7.2. Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) Socket Option
Applications can control the SCTP-PF behavior by getting or setting
the number of consecutive timeouts before a peer address is
considered potentially failed or unreachable. The same socket option
is used by applications to set and get the number of timeouts before
the primary path is changed automatically by the Primary Path
Switchover 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:
strict sctp_paddrthlds {
sctp_assoc_t spt_assoc_id;
strict sockaddr_storage spt_address;
uint16_t spt_pathmaxrxt;
uint16_t spt_pathpfthld;
uint16_t spt_pathcpthld;
};
spt_assoc_id: This parameter is ignored for one-to-one style
sockets. For One-romany style sockets the application may fill in
an association identifier or SCTP_FUTURE_ASSOC. It is an error to
use SCTP_{CURRENT|ALL}_ASSOC in spt_assoc_id.
spt_address: This specifies which peer address is of interest. If a
wild card address is provided, this socket option applies to all
current and future peer addresses.
spt_pathmaxrxt: Each peer address of interest is considered
unreachable, if its path error counter exceeds spt_pathmaxrxt.
Nishida, et al. Expires March 3, 2016 [Page 14]
Internet-Draft SCTP-PF August 2015
spt_pathpfthld: Each peer address of interest is considered
Potentially Failed, if its path error counter exceeds
spt_pathpfthld.
spt_pathcpthld: Each peer address of interest is not considered the
primary remote address anymore, if its path error counter exceeds
spt_pathcpthld. Using a value of 0off disables the selection of a
new primary peer address. If an implementation does not support
the automatically selection of a new primary address, it should
indicate an error with Erna set to RIVAL if a value different from
0off is used in spt_pathcpthld. For SCTP-PF, the setting of
spt_pathcpthld < spt_pathpfthld should be rejected with Erna set
to RIVAL. For [RFC4960] SCTP, the setting of spt_pathcpthld <
spt_pathmaxrxt should be rejected with Erna set to RIVAL. A SCTP-
PF implementation MAY support only setting of spt_pathcpthld =
spt_pathpfthld and spt_pathcpthld = 0off and a [RFC4960] SCTP
implementation MAY support only setting of spt_pathcpthld =
spt_pathmaxrxt and spt_pathcpthld = 0off. In these cases SCTP
shall reject setting of other values with Erna set to RIVAL.
7.3. Exposing the Potentially Failed Path State
(SCTP_EXPOSE_POTENTIALLY_FAILED_STATE) Socket Option
Applications can control the exposure of the potentially failed path
state in the SCTP_PEER_ADDR_CHANGE event and the
SCTP_GET_PEER_ADDR_INFO as described in Section 7.1. The default
value is implementation specific.
This socket option uses the level IPPROTO_SCTP and the name
SCTP_EXPOSE_POTENTIALLY_FAILED_STATE.
The following structure is used to control the exposition of the
potentially failed path state:
strict sctp_assoc_value {
sctp_assoc_t assoc_id;
uint32_t assoc_value;
};
assoc_id: This parameter is ignored for one-to-one style sockets.
For One-romany style sockets the application may fill in an
association identifier or SCTP_FUTURE_ASSOC. It is an error to
use SCTP_{CURRENT|ALL}_ASSOC in assoc_id.
assoc_value: The potentially failed path state is exposed if and
only if this parameter is non-zero.
Nishida, et al. Expires March 3, 2016 [Page 15]
Internet-Draft SCTP-PF August 2015
8. Security Considerations
Security considerations for the use of SCTP and its APIs are
discussed in [RFC4960] and [RFC6458].
The logic introduced by this document does not impact existing On-
anthe-wire SCTP messages. Also, this document does not introduce any
new On-anthe-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 Primary Path Switchover 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
Primary Path Switchover algorithm. This especially will be the case
when the Primary Path Switchover is used together with SCTP-PF with
the particular setting of PMR = PFMR = 0, as Primary Path Switchover
here happens already at the first RTO timeout experienced. Users of
the Primary Path Switchover 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
for any existing registries managed by IONA.
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.
Nishida, et al. Expires March 3, 2016 [Page 16]
Internet-Draft SCTP-PF August 2015
11. Proposed Change of Status (to be Deleted before Publication)
Initially this work looked to entail some changes of the Congestion
Control (CC) operation of SCTP and for this reason the work was
proposed as Experimental. These intended changes of the CC operation
have since been judged to be irrelevant and are no longer part of the
specification. As the specification entails no other potential
harmful features, consensus exists in the G to bring the work forward
as PS.
Initially concerns have been expressed about the possibility for the
mechanism to introduce path bouncing with potential harmful network
impacts. These concerns are believed to be unfounded. This issue is
addressed in Appendix B.
It is noted that the feature specified by this document is
implemented by multiple SCTP SW implementations and furthermore that
various variants of the solution have been deployed in Tel co
signaling environments for several years with good results.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<http://www.rfc-editor.org/info/rfc4960>.
12.2. Informative References
[CARO02] Caro Jr., A., Iyengar, J., Amer, P., Heinz, G., and R.
Stewart, "A Two-level Threshold Recovery Mechanism for
SCTP", Tech report, CIS Dept, University of Delaware , 7
2002.
[CARO04] Caro Jr., A., Amer, P., and R. Stewart, "End-to-End
Failover Thresholds for Transport Layer Multi homing",
MILCOM 2004 , 11 2004.
[CARO05] Caro Jr., A., "End-to-End Fault Tolerance using Transport
Layer Multi homing", Ph.D Thesis, University of Delaware ,
1 2005.
Nishida, et al. Expires March 3, 2016 [Page 17]
Internet-Draft SCTP-PF August 2015
[FALLON08]
Fallon, S., Jacob, P., Qiao, Y., Murphy, L., Fallon, E.,
and A. Hanley, "SCTP Switchover Performance Issues in ALAN
Environments", IEEE CCNC 2008, 1 2008.
[GRINNEMO04]
Grinnemo, K-J. and A. Brunstrom, "Performance of SCTP-
controlled failover in M3UA-based SIGHT RAN networks",
Advanced Simulation Technologies Conference , 4 2004.
[IYENGAR06]
Iyengar, J., Amer, P., and R. Stewart, "Concurrent
Multipath Transfer using SCTP Multi homing over
Independent End-to-end Paths.", IEEE/ACM Trans on
Networking 14(5), 10 2006.
[JUNGMAIER02]
Jungmaier, A., Rathgeb, E., and M. Tuexen, "On the use of
SCTP in failover scenarios", World Multiconference on
Systemics, Cybernetics and Informatics , 7 2002.
[NATARAJAN09]
Natarajan, P., Ekiz, N., Amer, P., and R. Stewart,
"Concurrent Multipath Transfer during Path Failure",
Computer Communications , 5 2009.
[RFC6458] Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
Yasevich, "Sockets API Extensions for the Stream Control
Transmission Protocol (SCTP)", RFC 6458,
DOI 10.17487/RFC6458, December 2011,
<http://www.rfc-editor.org/info/rfc6458>.
Appendix A. Discussions of Alternative Approaches
This section lists alternative approaches for the issues described in
this document. Although these approaches do not require to update
RFC4960, we do not recommend them from the reasons described below.
A.1. Reduce Path.Max.Retrans (PMR)
Smaller values for Path.Max.Retrans shorten the failover duration and
in fact this is recommended in some research results [JUNGMAIER02]
[GRINNEMO04] [FALLON08]. However to significantly reduce the
failover time it is required to go down (as with PFMR) to
Path.Max.Retrans=0 and with this setting SCTP switches to another
destination address already on a single timeout which may result in
spurious failover. Spurious failover is a problem in [RFC4960] SCTP
as the transmission of HEARTBEATS on the left primary path, unlike in
Nishida, et al. Expires March 3, 2016 [Page 18]
Internet-Draft SCTP-PF August 2015
SCTP-PF, is governed by 'HB.interval' also during the failover
process. 'HB.interval' is usually set in the order of seconds
(recommended value is 30 seconds) and when the primary path becomes
inactive, the next HEARTBEAT may be transmitted only many seconds
later. Indeed as recommended, only 30 secs later. Meanwhile, the
primary path may since long have recovered, if it needed recovery at
all (indeed the failover could be truly spurious). In such
situations, post failover, an endpoint is forced to wait in the order
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
HEARTBEATING 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 the
'Association.Max.Retrans' value. When the SCTP association's error
count exceeds Association.Max.Retrans threshold, the SCTP sender
considers the peer endpoint unreachable and terminates the
association. Section 8.2 in [RFC4960] recommends that
Association.Max.Retrans value should not be larger than the summation
of the Path.Max.Retrans of each of the destination addresses. Else
the SCTP sender considers its peer reachable even when all
destinations are INACTIVE and to avoid this dormant state operation,
[RFC4960] SCTP implementation SHOULD reduce Association.Max.Retrans
accordingly whenever it reduces Path.Max.Retrans. However, smaller
Association.Max.Retrans value compromises the fault tolerance of SCTP
as it increases the chances of association termination during minor
congestion events.
A.2. Adjust RTO related parameters
As several research results indicate, we can also shorten the
duration of failover process by adjusting RTO related parameters
[JUNGMAIER02] [FALLON08]. During failover process, RTO keeps being
doubled. However, if we can choose smaller value for RTO.max, we can
stop the exponential growth of RTO at some point. Also, choosing
smaller values for RTO.initial or RTO.min can contribute to keep the
RTO value small.
Similar to reducing Path.Max.Retrans, the advantage of this approach
is that it requires no modification to the current specification,
although it needs to ignore several recommendations described in the
Section 15 of [RFC4960]. However, this approach requires to have
Nishida, et al. Expires March 3, 2016 [Page 19]
Internet-Draft SCTP-PF August 2015
enough knowledge about the network characteristics between end
points. Otherwise, it can introduce adverse side-effects such as
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
The methods described in the document can accelerate the failover
process. Hence, they might introduce the path bouncing effect where
the sender keeps changing the data transmission path frequently.
This sounds harmful to the data transfer, however several research
results indicate that there is no serious problem with SCTP in terms
of path bouncing effect [CARO04] [CARO05].
There are two main reasons for this. First, SCTP is basically
designed for multipath communication, which means SCTP maintains all
path related parameters (CWND, ssthresh, RTT, error count, etc) per
each destination address. These parameters cannot be affected by
path bouncing. In addition, when SCTP migrates the data transfer to
another path, it starts with the minimal or the initial CWND. Hence,
there is little chance for packet reordering or duplicating.
Second, even if all communication paths between the end-nodes share
the same bottleneck, the SCTP-PF results in a behavior already
allowed by [RFC4960].
Appendix C. SCTP-PF for SCTP Single-homed Operation
For a single-homed SCTP association the only tangible effect of the
activation of SCTP-PF operation is enhanced failure detection in
terms of potential notification of the PF state of the sole
destination address as well as, for idle associations, more rapid
entering, and notification, of inactive state of the destination
address and more rapid end-point failure detection. It is believed
that neither of these effects are harmful, provided adequate dormant
state operation is implemented, and furthermore that they may be
particularly useful for applications that deploys multiple SCTP
associations for load balancing purposes. The early notification of
the PF state may be used for preventive measures as the entering of
the PF state can be used as a warning of potential congestion.
Depending on the PMR value, the aggressive HEARTBEAT transmission in
PF state may speed up the end-point failure detection (exceed of AMR
Nishida, et al. Expires March 3, 2016 [Page 20]
Internet-Draft SCTP-PF August 2015
threshold on the sole path error counter) on idle associations in
case where relatively large HB.interval value compared to RTO (e.g.
30secs) is used.
Authors' Addresses
Yoshifumi Nishida
GE Global Research
2623 Camino Ramon
San Ramon, CA 94583
USA
Email: nishida@wide.ad.jp
Preethi Natarajan
Cisco Systems
510 McCarthy Blvd
Milpitas, CA 95035
USA
Email: prenatar@cisco.com
Armando Caro
BBN Technologies
10 Moulton St.
Cambridge, MA 02138
USA
Email: acaro@bbn.com
Paul D. Amer
University of Delaware
Computer Science Department - 434 Smith Hall
Newark, DE 19716-2586
USA
Email: amer@udel.edu
Nishida, et al. Expires March 3, 2016 [Page 21]
Internet-Draft SCTP-PF August 2015
Karen E. E. Nielsen
Ericsson
Kistavaegen 25
Stockholm 164 80
Sweden
Email: karen.nielsen@tieto.com
Nishida, et al. Expires March 3, 2016 [Page 22]