SCTP-PF: Quick Failover Algorithm in SCTP
draft-ietf-tsvwg-sctp-failover-10
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 7829.
|
|
|---|---|---|---|
| Authors | Yoshifumi Nishida , Preethi Natarajan , Armando L. Caro , Paul D. Amer , karen Nielsen | ||
| Last updated | 2015-03-09 | ||
| Replaces | draft-nishida-tsvwg-sctp-failover | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
GENART Last Call review
(of
-14)
by Alexey Melnikov
Ready w/issues
|
||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Waiting for WG Chair Go-Ahead | |
| Document shepherd | Gorry Fairhurst | ||
| IESG | IESG state | Became RFC 7829 (Proposed Standard) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Martin Stiemerling | ||
| Send notices to | tsvwg-chairs@ietf.org, draft-ietf-tsvwg-sctp-failover@ietf.org, "Gorry Fairhurst" <gorry@erg.abdn.ac.uk> |
draft-ietf-tsvwg-sctp-failover-10
Network Working Group Y. Nishida
Internet-Draft GE Global Research
Intended status: Standards Track P. Natarajan
Expires: September 10, 2015 Cisco Systems
A. Caro
BBN Technologies
P. Amer
University of Delaware
K. Nielsen
Ericsson
March 9, 2015
SCTP-PF: Quick Failover Algorithm in SCTP
draft-ietf-tsvwg-sctp-failover-10.txt
Abstract
One of the major advantages of SCTP is the support of multi-homed
communication. A multi-homed SCTP end-point has the ability to
withstand network failures by migrating the traffic from an inactive
network to an active one. However, if the failover operation as
specified in RFC4960 is followed, there can be a significant delay in
the migration to the active destination addresses, thus severely
reducing the effectiveness of the SCTP failover operation.
This document complements RFC4960 by the introduction of a new path
state, the Potentially Failed (PF) path state, and an associated new
failover operation to apply during a network failure. The algorithm
defined is called SCTP Potentially Failed Algorithm, SCTP-PF for
short. In addition, the document complements RFC4960 by introducing
alternative switchover operation modes for the data transfer path
management after the recovery of a failed primary path. These modes
can allow improvements in the performance of the operation in some
network environments. The implementation of the additional
switchover operation modes is an optional part of SCTP-PF.
The procedures defined in the document require only minimal
modifications to the current 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 September 10, 2015 [Page 1]
Internet-Draft SCTP-PF March 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 September 10, 2015.
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. Issues with the SCTP Path Management . . . . . . . . . . . . 4
4. SCTP with Potentially-Failed Destination State (SCTP-PF) . . 5
4.1. SCTP-PF Concept . . . . . . . . . . . . . . . . . . . . . 5
4.2. Specification of the SCTP-PF Algorithm . . . . . . . . . 6
4.2.1. Dormant State Operation . . . . . . . . . . . . . . . 10
4.3. Permanent Failover . . . . . . . . . . . . . . . . . . . 12
4.3.1. Background . . . . . . . . . . . . . . . . . . . . . 12
4.3.2. Permanent Failover Algorithm . . . . . . . . . . . . 12
5. Socket API Considerations . . . . . . . . . . . . . . . . . . 13
5.1. Support for the Potentially Failed Path State . . . . . . 14
5.2. Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) Socket
Option . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.3. Exposing the Potentially Failed Path State
(SCTP_EXPOSE_POTENTIALLY_FAILED_STATE) Socket Option . . 16
6. Security Considerations . . . . . . . . . . . . . . . . . . . 16
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
8. Proposed Change of Status (to be Deleted before Publication) 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.1. Normative References . . . . . . . . . . . . . . . . . . 17
Nishida, et al. Expires September 10, 2015 [Page 2]
Internet-Draft SCTP-PF March 2015
9.2. 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 . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
The Stream Control Transmission Protocol (SCTP) as specified in
[RFC4960] supports multihoming at the transport layer -- an SCTP
endpoint can bind to multiple IP addresses. SCTP's multihoming
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. The sender fails over to an alternate active
destination address only after failure detection. Until detecting
the failover, the sender continues to transmit data on the failed
path, which degrades the SCTP performance. Concurrent Multipath
Transfer (CMT) [IYENGAR06] is an proposed extension to SCTP that
allows the sender to transmit data on multiple paths simultaneously.
Research [NATARAJAN09] shows that the current failure detection
procedure worsens CMT performance during failover and can be
significantly improved by employing a better failover algorithm.
This document specifies an alternative failure detection and failover
procedure, the SCTP Potentially Failed algorithm, that improves the
performance of SCTP multi-homed operation during a failover.
For multi-homed SCTP 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 path, switch back to the primary path for data transfer
as soon as this path becomes available again. From a performance
perspective, as confirmed in research [CARO02], such a switchback of
the data transmission path is not optimal in general. As an optional
alternative to the switchback operation of [RFC4960], this document
specifies the Permanent Failover procedures proposed by [CARO02].
Additional discussion for alternative approaches that do not require
modifications to [RFC4960], as well as discussion of path bouncing
effects that might be caused by frequent switchover, are provided in
the Appendices.
Nishida, et al. Expires September 10, 2015 [Page 3]
Internet-Draft SCTP-PF March 2015
While the Potentially Failed algorithm primarily is motivated for
improvement of the SCTP 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.
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. Issues with the SCTP Path Management
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
Nishida, et al. Expires September 10, 2015 [Page 4]
Internet-Draft SCTP-PF March 2015
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
detection where the sender incorrectly marks a destination address
as Failed (inactive) during periods of temporary congestion. As
Nishida, et al. Expires September 10, 2015 [Page 5]
Internet-Draft SCTP-PF March 2015
[RFC4960] recommends for a coupling of the PMR value and the
protocol parameter Association.Max.Retrans (AMR) value such
spurious failure detection risks to carry over to spurious
association failure detection and closure. Larger PMR values are
preferable to avoid spurious failure detection.
From the above observations it is clear that tuning the PMR value
involves the following trade off -- a lower value improves
performance but increases the chances of spurious failure detection,
whereas a higher value degrades performance and reduces spurious
failure detection in a wide range of path conditions. Thus, tuning
the association's PMR value is an incomplete solution to address the
performance impact during failure.
SCTP-PF defined in this document introduces the new Potentially
Failed (PF) destination address state in SCTP's path management
procedure. The new Potentially Failed (PF) destination address state
applies to SCTP single-homed operation as well as to SCTP multi-homed
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
The SCTP-PF operation is specified as follows:
1. The sender maintains a new tunable parameter called
PotentiallyFailed.Max.Retrans (PFMR). The RECOMMENDED value of
PFMR is 0 when SCTP-PF is used. The PFMR defines a new
intermediate PF threshold on the destination address error
counter at exceed of which the destination address is classified
as PF and related PF state actions are to be taken. By standard
RFC4960 semantics a destination address is classified as
Inactive once the error counter exceeds PMR. Setting PFMR
larger to or equal to PMR does not result in definition of a PF
threshold for the destination address. I.e., PFMR set larger to
or equal to PMR means that the destination address never will be
classified as PF.
Nishida, et al. Expires September 10, 2015 [Page 6]
Internet-Draft SCTP-PF March 2015
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
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 address 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
Nishida, et al. Expires September 10, 2015 [Page 7]
Internet-Draft SCTP-PF March 2015
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
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 PF destinations with same error
count, the sender should let the choice among the multiple
PF destination 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 PF destinations 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. HEARTBEAT chunks MUST be send to PF destination addresses
regardless of whether the Path Heartbeat function (Section 8.3
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
Nishida, et al. Expires September 10, 2015 [Page 8]
Internet-Draft SCTP-PF March 2015
HEARTBEAT chunks are sent to destination addresses in PF state
once per RTO. The HEARTBEAT sending begins upon that 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 back off the RTO value. If
the error counter is less than PMR, the sender transmits another
packet containing the HEARTBEAT 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. 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
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].
7. 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 frequency as
described in Section 8.3 of [RFC4960] (when this function is
enabled for the destination address).
8. 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)
MUST NOT clear the error count for an inactive destination
address and MUST NOT transition a PF destination address back to
active state, since a sender cannot disambiguate whether the ACK
was for the original transmission or the retransmission(s). The
same ambiguity concerns the related congestion window growth.
The bytes of a newly acknowledged chunk which has been
transmitted to multiple destination addresses SHOULD be
considered for contribution to the congestion window growth
towards the destination address where the chunk was last sent.
The contribution of the ACKed bytes to the window growth is
subject to the prescriptions described in Section 7.2 of
[RFC4960] is fulfilled. A SCTP sender MAY apply a different
approach for both the error count handling and the congestion
Nishida, et al. Expires September 10, 2015 [Page 9]
Internet-Draft SCTP-PF March 2015
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
destination address only MUST clear the error counter for the
destination address and MUST transition a PF destination address
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-a-vis
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.
10. 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 state transitions from Active to PF, and vice-versa. When
doing this, such an SCTP stack MUST provide the ULP with the
means to suppress exposure of PF state and associated state
transitions as well.
4.2.1. 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
retransmissions in parallel with RTO intervals when there are
multiple destinations addresses in PF state. This argument pressumes
that RTO << HB.interval of [RFC4960]. One could use higher values of
Nishida, et al. Expires September 10, 2015 [Page 10]
Internet-Draft SCTP-PF March 2015
PMR, which makes the dormant state situations less likely to happen.
The downside of increasing the PMR value is that destination address
failure detections and notifications of such events to ULP is
weakened.
A design goal of SCTP-PF is that it should 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. For this reason, SCTP-PF SHOULD perform the following
operations during dormant state, while this is an implementation
decision in [RFC4960].
a. When the destination addresses are all in inactive state, the
sender MUST choose one destination when data is transmitted. 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.
b. 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
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.
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
Nishida, et al. Expires September 10, 2015 [Page 11]
Internet-Draft SCTP-PF March 2015
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
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 Permanent Failover operation requires only sender side changes.
The details are:
Nishida, et al. Expires September 10, 2015 [Page 12]
Internet-Draft SCTP-PF March 2015
1. The sender maintains a new tunable parameter, called
Primary.Switchover.Max.Retrans (PSMR). The PSMR MUST be set
greater or equal to the PFMR value. Implementations MUST reject
any other values of PSMR.
2. When the path error counter on a set primary path exceeds PSMR,
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. The recommended value of PSMR is PFMR when Permanent Failover is
used. This means that no forced switchback to a previously
failed primary path is performed. An implementation of Permanent
Failover MUST support the setting of PSMR = PFMR. An
implementation of Permanent Failover MAY support setting of PSMR
> PFMR.
5. It MUST be possible to disable the Permanent Failover and obtain
the standard switchback operation of [RFC4960].
To support optimal operation in a wider range of network scenarios,
it it proposed for an SCTP-PF implementation to implement Permanent
Failover operation as an optional feature. The implementation of the
Permanent Failover feature is optional for an SCTP-PF implementation.
For an SCTP implementation that implements Permanent Failover, this
specification RECOMMENDS that the standard RFC4960 switchback
operation is retained as the default operation.
5. 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.
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.
Nishida, et al. Expires September 10, 2015 [Page 13]
Internet-Draft SCTP-PF March 2015
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
PSMR parameters described in Section 4. 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().
5.1. 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:
struct sctp_paddr_change {
uint16_t spc_type;
uint16_t spc_flags;
uint32_t spc_length;
struct 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 September 10, 2015 [Page 14]
Internet-Draft SCTP-PF March 2015
struct sctp_paddrinfo {
sctp_assoc_t spinfo_assoc_id;
struct 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.
5.2. 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 and before the primary
path is changed automatically. 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:
struct sctp_paddrthlds {
sctp_assoc_t spt_assoc_id;
struct 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-to-many 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 September 10, 2015 [Page 15]
Internet-Draft SCTP-PF March 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 0xffff 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 errno set to EINVAL if a value different
from 0xffff is used in spt_pathcpthld. Setting of spt_pathcpthld
< spt_pathpfthld should be rejected with errno set to EINVAL. An
implementation MAY support only setting of spt_pathcpthld =
spt_pathpfthld and spt_pathcpthld = 0xffff. In this case it shall
reject setting of other values with errno set to EINVAL.
5.3. 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 5.1. 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:
struct 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-to-many 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.
6. Security Considerations
Security considerations for the use of SCTP and its APIs are
discussed in [RFC4960] and [RFC6458]. The logic described here is
for sender-side only enabled by configuration and does not have any
Nishida, et al. Expires September 10, 2015 [Page 16]
Internet-Draft SCTP-PF March 2015
impacts on protocol messages on the wire. No new chunk type or new
field parameter is not required in this document.
7. IANA Considerations
This document does not create any new registries or modify the rules
for any existing registries managed by IANA.
8. Proposed Change of Status (to be Deleted before Publication)
Initially this work looked to entail some changes of the Congestion
Control (CC) operation of SCTP and for this reason the work was
proposed as Experimental. These intended changes of the CC operation
have since been judged to be irrelevant and are no longer part of the
specification. As the specification entails no other potential
harmful features, consensus exists in the WG to bring the work
forward as PS.
Initially concerns have been expressed about the possibility for the
mechanism to introduce path bouncing with potential harmful network
impacts. These concerns are believed to be unfounded. This issue is
addressed in Appendix B.
It is noted that the feature specified by this document is
implemented by multiple SCTP SW implementations and furthermore that
various variants of the solution have been deployed in Telco
signaling environments for several years with good results.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC
4960, September 2007.
9.2. 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 Multihoming",
MILCOM 2004 , 11 2004.
Nishida, et al. Expires September 10, 2015 [Page 17]
Internet-Draft SCTP-PF March 2015
[CARO05] Caro Jr., A., "End-to-End Fault Tolerance using Transport
Layer Multihoming", Ph.D Thesis, University of Delaware ,
1 2005.
[FALLON08]
Fallon, S., Jacob, P., Qiao, Y., Murphy, L., Fallon, E.,
and A. Hanley, "SCTP Switchover Performance Issues in WLAN
Environments", IEEE CCNC 2008, 1 2008.
[GRINNEMO04]
Grinnemo, K-J. and A. Brunstrom, "Performance of SCTP-
controlled failovers in M3UA-based SIGTRAN networks",
Advanced Simulation Technologies Conference , 4 2004.
[IYENGAR06]
Iyengar, J., Amer, P., and R. Stewart, "Concurrent
Multipath Transfer using SCTP Multihoming 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, December 2011.
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.
In fact, this is recommended in some research results [JUNGMAIER02]
[GRINNEMO04] [FALLON08]. For example, if when Path.Max.Retrans=0,
SCTP switches to another destination address on a single timeout.
This smaller value for Path.Max.Retrans can results in spurious
failover, which might be a problem.
Nishida, et al. Expires September 10, 2015 [Page 18]
Internet-Draft SCTP-PF March 2015
Unlike SCTP-PF, the interval for heartbeat packets is governed by
'HB.interval' even during failover process. 'HB.interval' is usually
set in the order of seconds (recommended value is 30 seconds). When
the primary path becomes inactive, the next HEARTBEAT can be
transmitted only seconds later. Meanwhile, the primary path may have
recovered. In such situations, post failover, an endpoint is forced
to wait on the order of seconds before the endpoint can resume
transmission on the primary path. However, using smaller value for
'HB.interval' might help this situation, but it will be the waste of
bandwidth in most cases.
In addition, smaller Path.Max.Retrans values also affect
'Association.Max.Retrans' values. When the SCTP association's error
count (sum of error counts on all ACTIVE paths) exceeds
Association.Max.Retrans threshold, the SCTP sender considers the peer
endpoint unreachable and terminates the association. Therefore,
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. To avoid
such inconsistent behavior an SCTP implementation SHOULD reduce
Association.Max.Retrans accordingly whenever it reduces
Path.Max.Retrans. However, smaller Association.Max.Retrans value
increases 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 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
enough knowledge about the network characteristics between end
points. Otherwise, it can introduce adverse side-effects such as
spurious timeouts.
Nishida, et al. Expires September 10, 2015 [Page 19]
Internet-Draft SCTP-PF March 2015
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
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
Nishida, et al. Expires September 10, 2015 [Page 20]
Internet-Draft SCTP-PF March 2015
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
Karen E. E. Nielsen
Ericsson
Kistavaegen 25
Stockholm 164 80
Sweden
Email: karen.nielsen@tieto.com
Nishida, et al. Expires September 10, 2015 [Page 21]