Internet Engineering Task Force (IETF) Y. Nishida
Request for Comments: 7829 GE Global Research
Category: Standards Track P. Natarajan
ISSN: 2070-1721 Cisco Systems
A. Caro
BBN Technologies
P. Amer
University of Delaware
K. Nielsen
Ericsson
April 2016
SCTP-PF: A Quick Failover Algorithm for the
Stream Control Transmission Protocol
Abstract
The Stream Control Transmission Protocol (SCTP) supports multihoming.
However, when the failover operation specified in RFC 4960 is
followed, there can be significant delay and performance degradation
in the data transfer path failover. This document specifies a quick
failover algorithm and introduces the SCTP Potentially Failed
(SCTP-PF) destination state in SCTP Path Management.
This document also specifies a dormant state operation of SCTP that
is required to be followed by an SCTP-PF implementation, but it may
equally well be applied by a standard SCTP implementation, as
described in RFC 4960.
Additionally, this 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 SCTP implementation and an SCTP-PF implementation.
The procedures defined in the document require only minimal
modifications to the specification in RFC 4960. The procedures are
sender-side only and do not impact the SCTP receiver.
Nishida, et al. Standards Track [Page 1]
RFC 7829 SCTP-PF April 2016
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7829.
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
(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.
Nishida, et al. Standards Track [Page 2]
RFC 7829 SCTP-PF April 2016
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 5
3. SCTP with Potentially Failed (SCTP-PF) Destination State . . 5
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Specification of the SCTP-PF Procedures . . . . . . . . . 6
4. Dormant State Operation . . . . . . . . . . . . . . . . . . . 10
4.1. SCTP Dormant State Procedure . . . . . . . . . . . . . . 11
5. Primary Path Switchover . . . . . . . . . . . . . . . . . . . 11
6. Suggested SCTP Protocol Parameter Values . . . . . . . . . . 13
7. Socket API Considerations . . . . . . . . . . . . . . . . . . 13
7.1. Support for the Potentially Failed Path State . . . . . . 14
7.2. Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) Socket
Option . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.3. Exposing the Potentially Failed Path State
(SCTP_EXPOSE_POTENTIALLY_FAILED_STATE) Socket Option . . 16
8. Security Considerations . . . . . . . . . . . . . . . . . . . 16
9. MIB Considerations . . . . . . . . . . . . . . . . . . . . . 17
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.1. Normative References . . . . . . . . . . . . . . . . . . 17
10.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Discussion of Alternative Approaches . . . . . . . . 20
A.1. Reduce PMR . . . . . . . . . . . . . . . . . . . . . . . 20
A.2. Adjust RTO-Related Parameters . . . . . . . . . . . . . . 21
Appendix B. Discussion of the Path-Bouncing Effect . . . . . . . 21
Appendix C. SCTP-PF for SCTP Single-Homed Operation . . . . . . 22
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
The Stream Control Transmission Protocol (SCTP) specified in
[RFC4960] supports multihoming at the transport layer. 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. Until detecting the path
failure, the sender continues to transmit data on the failed path.
The prolonged time in which SCTP as described in [RFC4960] continues
to use a failed path severely degrades the performance of the
protocol. To address this problem, this document specifies a quick
failover algorithm called "SCTP-PF" based on the introduction of a
new Potentially Failed (PF) path state in SCTP path management. The
Nishida, et al. Standards Track [Page 3]
RFC 7829 SCTP-PF April 2016
performance deficiencies of the failover operation described in RFC
4960, and the improvements obtainable from the introduction of a PF
state in SCTP, were proposed and documented in [NATARAJAN09] for
Concurrent Multipath Transfer SCTP [IYENGAR06].
While SCTP-PF can accelerate the failover process and improve
performance, the risk that an SCTP endpoint might enter the dormant
state where all destination addresses are inactive can be increased.
[RFC4960] leaves the protocol operation during dormant state to
implementations and encourages avoiding entering the state as much as
possible by careful tuning of the 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 the fact that the dormant state may be entered more
quickly. The dormant state operation may equally well be applied by
an implementation of [RFC4960] and will serve here to provide added
fault tolerance for situations where the tuning of the PMR and 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
[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 Congestion Window (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 that avoids such forced switchbacks of the data transfer
path. The Primary Path Switchover operation was originally proposed
in [CARO02].
While SCTP-PF is primarily motivated by a desire to improve the
multihomed operation, the feature also applies 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 PF
state, including a discussion of alternative approaches to mitigate
the deficiencies of the failover operation in [RFC4960], are given in
the appendices. Discussion of path-bouncing effects that might be
caused by frequent switchovers are also provided there.
Nishida, et al. Standards Track [Page 4]
RFC 7829 SCTP-PF April 2016
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 (SCTP-PF) Destination State
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 SCTP path management scheme described in [RFC4960],
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 PMR threshold. The issue cannot
simply be mitigated by lowering 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 PMR and the AMR parameter values in [RFC4960], lowering
the PMR threshold may result in lowering the AMR threshold, which
would result in a 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 PF state as an
intermediate state in between the active and inactive state of a
destination address in the path management scheme of [RFC4960], 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 SCTP as described
in [RFC4960]. At the same time, HEARTBEAT probing based on
Retransmission Timeout (RTO) 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 be
classified as PF at the occurrence of the first timeout.
Nishida, et al. Standards Track [Page 5]
RFC 7829 SCTP-PF April 2016
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 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 Timer T3 retransmission
(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. An 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 Section 6.4.1 of [RFC4960], when the
sender retransmits data that has timed out, they 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.
Nishida, et al. Standards Track [Page 6]
RFC 7829 SCTP-PF April 2016
When choosing among multiple destination addresses in active
state, an SCTP sender will follow the guiding principles of
Section 6.4.1 of [RFC4960] by choosing the most divergent
source-destination pairs compared with, for (the aforementioned
points i and ii):
i. the destination address in PF state that it performs a
failover from, and
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 the 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 are
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:
i. 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.
ii. 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 principles of choosing
the most divergent source-destination pairs when executing
(potentially consecutive) retransmission outlined in
Section 6.4.1 of [RFC4960]. Rules for picking the most
divergent source-destination pairs 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
Nishida, et al. Standards Track [Page 7]
RFC 7829 SCTP-PF April 2016
once per RTO. HEARTBEAT chunks SHOULD be sent to destination
addresses in PF state, but the sending of HEARTBEATs MUST honor
whether or not the Path Heartbeat function (Section 8.3 of
[RFC4960]) is enabled for the destination address. That is, if
the Path Heartbeat function is disabled for the destination
address in question, HEARTBEATs MUST NOT be sent. Note that
when the Path 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 the case where the receipt of a Selective
Acknowledgment (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
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 SHOULD (i) notify the Upper
Layer Protocol (ULP) about this state transition, and (ii)
Nishida, et al. Standards Track [Page 8]
RFC 7829 SCTP-PF April 2016
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 that 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). An 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 have 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 that 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 that would
benefit from additional attention, experimentation, and
specification for single-homed SCTP as well as for multihomed
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
Nishida, et al. Standards Track [Page 9]
RFC 7829 SCTP-PF April 2016
an SCTP stack implementing SCTP-PF also allows for the ULP to be
kept ignorant of the PF state of its destinations and the
associated state transitions, thus allowing for retention 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 provide 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 SCTP implementation of
[RFC4960] given the same setting of PMR and AMR. For example, an
SCTP association with two destination addresses would typically reach
dormant state in half the time of an SCTP implementation of [RFC4960]
in such situations. This is because an 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 PFMR.
5. For standard 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
standard SCTP implementation of Primary Path Switchover MUST
support the setting of PSMR = PMR. A standard SCTP
implementation of Primary Path Switchover MAY support larger
settings of PSMR > PMR.
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. That is, load
distribution of traffic from multiple SCTP associations may be
enforced by distribution of the set primary paths with the switchback
operation of [RFC4960]. However, as switchback behavior of [RFC4960]
Nishida, et al. Standards Track [Page 12]
RFC 7829 SCTP-PF April 2016
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
switchback operation of [RFC4960] be retained as the default
operation.
6. Suggested SCTP Protocol Parameter Values
This document does not alter the 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 PF state as
well as the socket API implementation is extended to expose the PF
state of a peer address in the existing SCTP_GET_PEER_ADDR_INFO
structure.
Furthermore, two new read/write socket options for the level
IPPROTO_SCTP and the name SCTP_PEER_ADDR_THLDS and
SCTP_EXPOSE_POTENTIALLY_FAILED_STATE are defined as described below.
The first socket option is used to control the values of the PFMR and
PSMR parameters described in Sections 3 and 5. The second one
controls the exposition of the PF path state.
Support for the SCTP_PEER_ADDR_THLDS and
SCTP_EXPOSE_POTENTIALLY_FAILED_STATE socket options also needs to be
added to the function sctp_opt_info().
Nishida, et al. Standards Track [Page 13]
RFC 7829 SCTP-PF April 2016
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 PF 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 the new additional constant
SCTP_ADDR_POTENTIALLY_FAILED, which is reported if the affected
address becomes PF.
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;
};
[RFC6458] defines the constants SCTP_UNCONFIRMED, SCTP_ACTIVE, and
SCTP_INACTIVE to be provided in the spinfo_state field. This
document defines the new additional constant SCTP_POTENTIALLY_FAILED,
which is reported if the peer address is PF.
Nishida, et al. Standards Track [Page 14]
RFC 7829 SCTP-PF April 2016
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 PF 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
wildcard 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 PF, 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 automatic 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. For SCTP-PF, the setting
of spt_pathcpthld .
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,