Internet DRAFT - draft-ietf-tsvwg-sctp-failover
draft-ietf-tsvwg-sctp-failover
Network Working Group Y. Nishida
Internet-Draft GE Global Research
Intended status: Standards Track P. Natarajan
Expires: August 20, 2016 Cisco Systems
A. Caro
BBN Technologies
P. Amer
University of Delaware
K. Nielsen
Ericsson
February 17, 2016
SCTP-PF: Quick Failover Algorithm in SCTP
draft-ietf-tsvwg-sctp-failover-16.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
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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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 20, 2016.
Copyright Notice
Copyright (c) 2016 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
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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 . . . . . . . . . . . . . . . . . . . 15
9. MIB Considerations . . . . . . . . . . . . . . . . . . . . . 16
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
12. Proposed Change of Status (to be Deleted before Publication) 17
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
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13.1. Normative References . . . . . . . . . . . . . . . . . . 17
13.2. 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 the 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 also impacts the
performance of the protocol. With the procedures specified in
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[RFC4960] SCTP will, after a failover from the primary path, switch
back to use the primary path for data transfer as soon as this path
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 switchovers, 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. Also due to the
coupled tuning of the Path.Max.Retrans (PMR) and the
Association.Max.Retrans (AMR) parameter values in [RFC4960], lowering
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of the PMR threshold may result in lowering of the AMR threshold,
which would result in decrease of the fault tolerance of SCTP.
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 negatively impacting 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. It recommends that by default a destination address is
classified as PF at the occurrence of the 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. When this threshold is exceeded the destination
address is classified as PF. The RECOMMENDED value of PFMR is
0. If PFMR is set to be greater than or equal to
Path.Max.Retrans (PMR), the resulting PF threshold will be so
high that the destination address will reach the inactive state
before it can be classified as PF.
2. The error counter of an active destination address is
incremented or cleared as specified in [RFC4960]. This means
that the error counter of the destination address in active
state 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.
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3. 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 use this destination address for
data transmission.
ii As specified in [RFC4960] section 6.4.1, when the sender
retransmits data that has timed out, it should attempt to
pick a new destination address for data retransmission. In
this case, 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 an SCTP sender will follow the guiding principles of
section 6.4.1 of [RFC4960] 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.
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 SHOULD 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.
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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.
The sender MUST NOT change the state and the error counter of
any destination addresses as the result of the selection.
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
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 where the receipt of a SACK of the data
or a T3-rtx timer expiration on the data can provide equivalent
information, such as the case where the data chunk has been
transmitted to a single destination address only. 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. However, there may be
a situation where HEARTBEAT chunks can go through while DATA
chunks cannot. Hence, 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
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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. 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].
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 move 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 clear the error counter
and move a destination address back to active state by
information other than acknowledgments, when it can uniquely
determine which destination, among multiple destination
addresses, the chunk reached. This document makes no reference
to what such information could consist of, nor how such
information could be obtained.
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-a-vis
the congestion control handling or data retransmission operation
towards a destination address which undergoes a transition from
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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 expose the PF state of its destination
addresses to the ULP as well as provide the means to notify the
ULP of state transitions of its destination addresses 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 transitions, thus allowing for retain of the
simpler state transition model of RFC4960 in the ULP. For this
reason it is recommended that an SCTP stack implementing SCTP-PF
also provides the ULP with the means to suppress exposure of the
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
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 decrease the fault tolerance of the
SCTP-PF operation.
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The below prescription for SCTP-PF dormant state handling MUST 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 before 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
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 the limit to be an order of magnitude higher than the
PMR value. A sender MAY choose to deploy other strategies that
the strategy defined 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.
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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 switchover 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 (PSMR). For SCTP-PF
implementations, the PSMR MUST be set greater or equal to the
PFMR value. For [RFC4960] implementations the PSMR MUST be set
greater or equal to the PMR value. Implementations MUST reject
any other values of PSMR.
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. For SCTP-PF, the recommended value of PSMR 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
MUST support the setting of PSMR = PFMR. A SCTP-PF
implementation of Primary Path Switchover MAY support setting of
PSMR > PFMR.
5. For [RFC4960] SCTP, the recommended value of PSMR 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 PSMR = PMR. An [RFC4960] SCTP
implementation of Primary Path Switchover MAY support larger
settings of PSMR > PMR.
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6. It MUST be possible to disable the Primary Path Switchover
operation and obtain the standard switchback operation of
[RFC4960].
The manner of switchover 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 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 applications'
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 recommendation 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.
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
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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 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:
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:
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;
};
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[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:
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.
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
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from 0xffff is used in spt_pathcpthld. For SCTP-PF, the setting
of spt_pathcpthld < spt_pathpfthld should be rejected with errno
set to EINVAL. For [RFC4960] SCTP, the setting of spt_pathcpthld
< spt_pathmaxrxt should be rejected with errno set to EINVAL. A
SCTP-PF implementation may support only setting of spt_pathcpthld
= spt_pathpfthld and spt_pathcpthld = 0xffff and a [RFC4960] SCTP
implementation may support only setting of spt_pathcpthld =
spt_pathmaxrxt and spt_pathcpthld = 0xffff. In these cases SCTP
shall reject setting of other values with errno set to EINVAL.
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:
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.
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 SCTP
messages on the wire. Also, this document does not introduce any new
SCTP messages on the wire 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/
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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 negatively influence 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 threat analysis. This is because on-path 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 PSMR = 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. MIB Considerations
SCTP-PF introduces new SCTP algorithms for failover and switchback
with associated new state parameters. It is recommended that the
SCTP-MIB defined in [RFC3873] is updated to support the management of
the SCTP-PF implementation. This can be done by extending the
sctpAssocRemAddrActive field of the SCTPAssocRemAddrTable to include
information of the PF state of the destination address and by adding
new fields to the SCTPAssocRemAddrTable supporting
PotentiallyFailed.Max.Retrans (PFMR) and
Primary.Switchover.Max.Retrans (PSMR) parameters.
10. IANA Considerations
This document does not create any new registries or modify the rules
for any existing registries managed by IANA.
11. Acknowledgements
The authors wish to thank Michael Tuexen for his many invaluable
comments and for his very substantial support with the making of this
document.
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12. Proposed Change of Status (to be Deleted before Publication)
Initially this work looked to entail some changes of the Congestion
Control (CC) operation of SCTP and for this reason the work was
proposed as Experimental. These intended changes of the CC operation
have since been judged to be irrelevant and are no longer part of the
specification. As the specification entails no other potential
harmful features, consensus exists in the WG to bring the work
forward as PS.
Initially concerns have been expressed about the possibility for the
mechanism to introduce path bouncing with potential harmful network
impacts. These concerns are believed to be unfounded. This issue is
addressed in Appendix B.
It is noted that the feature specified by this document is
implemented by multiple SCTP SW implementations and furthermore that
various variants of the solution have been deployed in telephony
signaling environments for several years with good results.
13. References
13.1. Normative References
[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.
13.2. 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.
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[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.
[RFC3873] Pastor, J. and M. Belinchon, "Stream Control Transmission
Protocol (SCTP) Management Information Base (MIB)", RFC
3873, DOI 10.17487/RFC3873, September 2004,
<http://www.rfc-editor.org/info/rfc3873>.
[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 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
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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
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 decreases 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.
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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.
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
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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
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
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Karen E. E. Nielsen
Ericsson
Kistavaegen 25
Stockholm 164 80
Sweden
Email: karen.nielsen@tieto.com
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