One document matched: draft-nishida-tsvwg-sctp-failover-05.xml
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<front>
<title abbrev="SCTP Quick Failover">
Quick Failover Algorithm in SCTP
</title>
<author initials='Y.N' surname="Nishida" fullname='Yoshifumi Nishida'>
<organization> WIDE Project </organization>
<address><postal> <street>Endo 5322</street>
<city>Fujisawa</city> <region>Kanagawa </region><code>252-8520</code>
<country>Japan</country>
</postal>
<email>nishida@wide.ad.jp</email>
</address>
</author>
<author initials='P.N' surname="Natarajan" fullname='Preethi Natarajan'>
<organization> Cisco Systems </organization>
<address><postal> <street>510 McCarthy Blvd</street>
<city>Milpitas</city> <region>CA </region><code>95035 </code>
<country>USA</country>
</postal>
<email>prenatar@cisco.com</email>
</address>
</author>
<author initials='A.C' surname="Caro" fullname='Armando Caro'>
<organization> BBN Technologies </organization>
<address><postal> <street> 10 Moulton St. </street>
<city>Cambridge</city> <region> MA</region><code> 02138</code>
<country>USA</country>
</postal>
<email>acaro@bbn.com</email>
</address>
</author>
<date/>
<abstract>
<t>
One of the major advantages in SCTP is supporting multi-homing communication.
If a multi-homed end-point has redundant network connections, SCTP sessions can
have a good chance to survive from network failures by migrating inactive
network to active one.
However, if we follow the SCTP standard, there can be significant delay for the
network migration. During this migration period, SCTP cannot transmit much data to the
destination. This issue drastically impairs the usability of SCTP in some situations.
This memo describes the issue of SCTP failover mechanism and discuss its solutions
which require minimal modification to the current standard.
</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>
The Stream Control Transmission Protocol (SCTP) <xref target="RFC4960"/>
natively supports multihoming at the transport layer --
an SCTP association can bind
to multiple IP addresses at each endpoint. SCTP's multihoming features include
failure detection and failover procedures to provide network interface redundancy
and improved end-to-end fault tolerance.
</t>
<t> In SCTP's current failure detection procedure, the sender must
experience Path.Max.Retrans (PMR) number of consecutive
timeouts on a destination before detecting path failure. The sender fails over
to an alternate active destination only after failure detection.
Until failover, the sender
transmits data on the failed path, degrading SCTP performance.
Concurrent Multipath Transfer (CMT) <xref target="IYENGAR06" /> is an extension to SCTP
and allows the sender to transmit data on multiple paths simultaneously. Research
<xref target="NATARAJAN09" /> shows that the current failure detection procedure
worsens CMT performance during failover and
can be significantly improved by employing a better failover algorithm. </t>
<t> This document proposes an alternative failure detection procedure for SCTP (and CMT)
that improves SCTP (CMT) performance during failover.
</t>
</section>
<section title="Conventions and Terminology">
<t>
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 <xref target="RFC2119"/>.
</t>
</section>
<section title="Issue in SCTP Path Management Process">
<t>
SCTP can utilize multiple IP addresses for a single SCTP association.
Each SCTP endpoint exchanges the list of available addresses on the node during initial
negotiation. After this, endpoints select one address from the list and define this
as the primary destination. During normal transmission, SCTP sends all data to the primary destination.
Also, it sends heartbeat packets to other
(non-primary) destinations at a certain interval to check the reachability of the path.
</t><t>
If sender has multiple active destination addresses, it can retransmit data to
secondary destination address when the transmission to the primary times out.
</t><t>
When sender receives the acknowledgment for data or heartbeat packets from one of the
destination addresses, it considers the destination is active. If it fails to receive
acknowledgments, the error count for the address is increased.
If the error counter exceeds the protocol parameter 'Path.Max.Retrans', SCTP endpoint
considers the address is inactive.
</t><t>
The failover process of SCTP is initiated when the primary path becomes inactive
(error counter for the primacy path exceeds Path.Max.Retrans). If the primary path is
marked inactive, SCTP chooses new destination address from one of the active destinations
and start using this address to send data.
If the primary path becomes active again, SCTP uses the primary destination for subsequent
data transmissions and stop using non-primary one.
</t><t>
An issue in this failover process is that it usually takes significant amount of time
before SCTP switches to the new destination. 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 secondary path.
This is because the recommended value for Path.Max.Retrans in the standard is 5,
which requires 6 consecutive timeouts before failover takes place.
Before SCTP switches to the secondary address, SCTP keeps trying to send packets to
the primary and only retransmitted packets are sent to the secondary can be reached
at the receiver.
This slow failover process can cause significant performance degradation and
will not be acceptable in some situations.
</t>
</section>
<section title="Existing Solutions for Smooth Failover">
<t>
The following approach are conceivable for the solutions of this issue.
</t>
<section title="Reduce Path.Max.Retrans">
<t>
If we choose smaller value for Path.Max.Retrans, we can shorten the duration of failover
process. In fact, this is recommended in some research results
<xref target="JUNGMAIER02"/>
<xref target="GRINNEMO04"/>
<xref target="FALLON08"/>.
For example, if we set Path.Max.Retrans to 0, SCTP switches to another destination on a
single timeout. However, smaller value for Path.Max.Retrans might cause spurious failover.
In addition, if we use smaller value for Path.Max.Retrans, we may also need to choose
smaller value for 'Association.Max.Retrans'. The Association.Max.Retrans indicates the
threshold for the total number of consecutive error count for the entire SCTP association.
If the total of the error count for all paths exceeds this value, the endpoint considers
the peer endpoint unreachable and terminates the association.
According to the Section 8.2 in <xref target="RFC4960"/>, we should avoid having the value of
Association.Max.Retrans larger than the summation of the Path.Max.Retrans of
all the destination addresses. Otherwise, even if all the destination addresses become
inactive, the endpoint still considers the peer endpoint reachable.
The behavior in this situation is not defined in the RFC and depends on each implementation.
In order to avoid inconsistent behavior between implementations, we had better use smaller
value for Association.Max.Retrans.
However, if we choose smaller value for Association.Max.Retrans, associations will prone
to be terminated with minor congestion.
</t><t>
Another issue is that the interval of heartbeat packet: 'HB.interval' may not be small.
(recommended value is 30 seconds) This means once failover takes place, an endpoint
might need a certain amount of time to use the primary path again. This can cause
undesirable effects in case of spurious failover. If we choose smaller value for HB.interval,
the traffic used for path probing in a session will be increased.
</t><t>
The advantage of tuning Path.Max.Retrans is that it requires no modification to the current
standard, although it needs to ignore several recommendations.
In addition, some research results indicate path bouncing caused by spurious failover
does not cause serious problems. We discuss the effect of path bouncing
in the section 5.
</t>
</section>
<section title="Adjust RTO related parameters">
<t>
As several research results indicate, we can also shorten the duration of failover process
by adjusting RTO related parameters
<xref target="JUNGMAIER02"/>
<xref target="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.
</t><t>
Similar to reducing Path.Max.Retrans, the advantage of this approach is that it requires no modification
to the current standard, although it needs to ignore several recommendations.
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.
</t>
</section>
</section>
<section title="Proposed Solution: SCTP with Potentially-Failed Destination State (SCTP-PF)">
<section title="SCTP-PF Description">
<t>
Our proposal stems from the following two observations about SCTP's failure detection procedure:
<list style="symbols">
<t>
In order to minimize performance impact during failover, the sender should avoid transmitting data to the failed destination as early as possible. In the current SCTP path management scheme, the sender stops transmitting data to a destination only after the destination is marked Failed. Thus, a smaller PMR value is ideal so that the sender transitions a destination to the Failed state quicker.
</t>
<t>
Smaller PMR values increase the chances of spurious failure detection where the sender incorrectly marks a destination as Failed during periods of temporary congestion. Larger PMR values are preferable to avoid spurious failure detection.
</t>
</list>
</t>
<t>
From the above observations it is clear that tweaking the PMR value involves the following tradeoff -- 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, tweaking the association's PMR value is an incomplete solution to address performance impact during failure.
</t>
<t> We propose a new "Potentially-failed" (PF) destination state
in SCTP's path management procedure. The PF state was originally proposed to improve CMT performance <xref target="NATARAJAN09" />. The PF state is an intermediate state between 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 single timeout on a path,
a sender is unsure, and marks the corresponding destination
as PF. A PF destination is not used for data transmission except in special cases (discussed below). The new failure detection algorithm requires only sender-side changes. Details are:
<list style="numbers">
<t>
The sender maintains
a new tunable parameter called Potentially-failed.Max.Retrans (PFMR).
The recommended value of PFMR = 0 when quick failover is used.
When an association's PFMR >= PMR, quick failover is turned off.
</t>
<t>
Each time the T3-rtx timer expires on an active or idle destination,
the error counter of that destination address will be incremented.
When the value in the error counter exceeds PFMR, the endpoint should
mark the destination transport address as PF. SCTP MUST NOT send any
notification to the upper layer about the active to PF state transition.
</t>
<t>The sender SHOULD avoid data transmission to PF destinations.
When all destinations are in either PF or Inactive state, the sender
MAY either move the destination from PF to active state (and transmit data to
the active destination) or the sender MAY transmit data to a PF destination.
In the former scenario, (i) the sender MUST NOT notify the ULP about the state transition,
and (ii) MUST NOT clear the destination's error counter.
It is recommended that the sender picks the PF destination with least error count
(fewest consecutive timeouts) for data transmission.
In case of a tie (multiple PF destinations with same error count),
the sender MAY choose the last active destination.
</t>
<t>
Only heartbeats MUST be sent to PF destination(s) once per RTO.
This means the sender SHOULD ignore
HB.interval for PF destinations. If an heartbeat is unanswered, the sender
increments the error counter
and exponentially backs off the RTO value. If error counter is
less than PMR, the sender SHOULD transmit
another heartbeat immediately after T3-timer expiration.
</t>
<t>
When the sender receives an heartbeat ACK from a PF destination,
the sender clears the destination's error counter and transitions the PF destination
back to active state. This state transition
MUST NOT be notified to the ULP. This destination's
cwnd is set to 1 MTU (TODO: or 2? Needs more text discussing rationale; can revisit later?)
</t>
<t>
An additional (PMR - PFMR) consecutive timeouts on a PF destination
confirm the path failure, upon which the destination transitions to the
Inactive state. As described in <xref target="RFC4960"/>, the sender (i) SHOULD
notify ULP about this state transition, and (ii) transmit heartbeats
to the Inactive destination at a lower frequency as
described in Section 8.3 of <xref target="RFC4960"/>.
</t>
<t>
When all destinations are in the
Inactive state, the sender picks one of the Inactive destinations for data transmission. This proposal recommends that the
sender picks the Inactive destination with least error count
(fewest consecutive timeouts) for data transmission.
In case of a tie (multiple Inactive destinations with same error count),
the sender MAY choose the last active destination.
</t>
<t>
ACKs for retransmissions do not transition a PF destination back to the active state,
since a sender cannot disambiguate whether the ack was for the original transmission
or the retransmission(s).
</t>
</list>
</t>
</section>
<section title="Effect of Path Bouncing">
<t>
The methods described above can accelerate failover process.
Hence, it might introduce path bouncing effect which keeps changing the data transmission
path frequently. This sounds harmful for data transfer, however several research results
indicate that there is no serious problem with SCTP in terms of path bouncing effect
<xref target="CARO04" />
<xref target="CARO05" />.
</t>
<t>
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 to another path, it starts with minimal cwnd because of
slow-start. Hence, there is little chance for packet reordering or duplicating.
</t>
<t>
Second, even if all communication paths between end-nodes share the same bottleneck,
the proposed method does not make situations worse. In case of congestion, the current
standard tries to transmit data packets to the primary during failover, while the
proposed method tries to explore other destinations.
In any case, the same amount of data packets sent to the same bottleneck.
</t>
</section>
<section title="Permanent Failover">
<t>
When primary path becomes active again after failover, SCTP migrates back to the
primary path. After this, SCTP starts data transfer with minimal cwnd. This is
because SCTP must perform slow-start when it migrates to new path.
However, this might degrade the communication performance in case that the performance
of the alternative path is relatively good.
In order to mitigate this effect of slow-start, permanent failover was proposed in
<xref target="CARO02"/>.
Permanent failover allows SCTP to remain the alternative path even if the primacy path
becomes active again.
This approach can improve performance in some cases, however, it will require more
detail analysis since it might impact on SCTP failover algorithm.
Since we prefer to keep the current behavior of the standard as possible,
we recommend not to take this approach for now.
</t>
</section>
<section title="Handling Error Counter">
<t>
When multiple destinations are in the PF state, the sender may transmit
heartbeats to multiple destinations at the same time. This allows sender to
quickly track and respond to network status change. However, when all PF
destinations become unavailable, this approach increases the total number of
consecutive retransmissions rather aggressively than the current SCTP spec does.
Because of this aggressive increase, an SCTP association may be terminated rather
earlier than the standard <xref target="RFC4960"/>.
</t>
<t>
One way to avoid early termination is to send retransmitted data or
HB to only one PF destination at a time, but this approach may
delay path status tracking. An alternative solution is to exclude
HB timeouts from incrementing the error count. The latter approach
is preferred but requires an update to Section 8.3 of <xref target="RFC4960"/>.
</t>
</section>
</section>
<section title="Socket API Considerations">
<t>This section describes how the socket API defined in
<xref target='I-D.ietf-tsvwg-sctpsocket'/> is extended
to provide a way for the application to control the quick
failover behavior.</t>
<t>Please note that this section is informational only.</t>
<t>A socket API implementation based on
<xref target='I-D.ietf-tsvwg-sctpsocket'/>
is extended by adding a new read/write socket option for the
level IPPROTO_SCTP and the name SCTP_PEER_ADDR_THLDS
as described below. This socket option is used to read/write the value of PFMR
parameter described in Section 5.</t>
<t>Support for the SCTP_PEER_ADDR_THLDS socket option needs
also to be added to the function sctp_opt_info().</t>
<section title="Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) socket option">
<t>Applications can control the quick failover behavior by getting or
setting the number of timeouts before a peer address is considered
potentially failed or unreachable.</t>
<t>The following structure is used to access and modify the thresholds:
<figure>
<artwork>
struct sctp_paddrthlds {
sctp_assoc_t spt_assoc_id;
struct sockaddr_storage spt_address;
uint16_t spt_pathmaxrxt;
uint16_t spt_pathpfthld;
};
</artwork>
</figure>
<list style='hanging'>
<t hangText="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 for this query.
It is an error to use SCTP_{CURRENT|ALL}_ASSOC in spt_assoc_id.</t>
<t hangText="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.</t>
<t hangText="spt_pathmaxrxt:">
Each peer address of interest is considered unreachable,
if its path error counter exceeds spt_pathmaxrxt.</t>
<t hangText="spt_pathpfthld:">
Each peer address of interest is considered potentially failed,
if its path error counter exceeds spt_pathpfthld.</t>
</list></t>
</section>
</section>
<section title="Security Considerations">
<t>
There are no new security considerations introduced in this document.
</t>
</section>
<section title="IANA Considerations">
<t>
This document does not create any new registries or modify the rules
for any existing registries managed by IANA.
</t>
</section>
</middle>
<back>
<references title='Normative References'>
<?rfc include="reference.RFC.2119" ?>
<?rfc include="reference.RFC.4960" ?>
</references>
<references title='Informative References'>
<?rfc include="reference.I-D.ietf-tsvwg-sctpsocket" ?>
<reference anchor="IYENGAR06" target="">
<front>
<title> Concurrent Multipath Transfer using SCTP Multihoming
over Independent End-to-end Paths.</title>
<author initials="J." surname="Iyengar" fullname="">
<organization />
</author>
<author initials="P." surname="Amer" fullname="">
<organization />
</author>
<author initials="R." surname="Stewart" fullname="">
<organization />
</author>
<date month="10" year="2006" />
</front>
<seriesInfo name="IEEE/ACM Trans on Networking" value="14(5)" />
</reference>
<reference anchor="NATARAJAN09" target="">
<front>
<title> Concurrent Multipath Transfer during Path Failure
</title>
<author initials="P." surname="Natarajan" fullname="">
<organization />
</author>
<author initials="N." surname="Ekiz" fullname="">
<organization />
</author>
<author initials="P." surname="Amer" fullname="">
<organization />
</author>
<author initials="R." surname="Stewart" fullname="">
<organization />
</author>
<date month="5" year="2009" />
</front>
<seriesInfo name="Computer Communications" value="" />
</reference>
<reference anchor="JUNGMAIER02" target="">
<front>
<title> On the use of SCTP in failover scenarios
</title>
<author initials="A." surname="Jungmaier" fullname="">
<organization />
</author>
<author initials="E." surname="Rathgeb" fullname="">
<organization />
</author>
<author initials="M." surname="Tuexen" fullname="">
<organization />
</author>
<date month="7" year="2002" />
</front>
<seriesInfo name="World Multiconference on Systemics, Cybernetics and Informatics" value="" />
</reference>
<reference anchor="GRINNEMO04" target="">
<front>
<title> Performance of SCTP-controlled failovers in M3UA-based SIGTRAN networks
</title>
<author initials="K-J" surname="Grinnemo" fullname="">
<organization />
</author>
<author initials="A." surname="Brunstrom" fullname="">
<organization />
</author>
<date month="4" year="2004" />
</front>
<seriesInfo name="Advanced Simulation Technologies Conference" value="" />
</reference>
<reference anchor="FALLON08" target="">
<front>
<title> SCTP Switchover Performance Issues in WLAN Environments
</title>
<author initials="S." surname="Fallon" fullname="">
<organization />
</author>
<author initials="P." surname="Jacob" fullname="">
<organization />
</author>
<author initials="Y." surname="Qiao" fullname="">
<organization />
</author>
<author initials="L." surname="Murphy" fullname="">
<organization />
</author>
<author initials="E." surname="Fallon" fullname="">
<organization />
</author>
<author initials="A." surname="Hanley" fullname="">
<organization />
</author>
<date month="1" year="2008" />
</front>
<seriesInfo name="IEEE CCNC" value="2008" />
</reference>
<reference anchor="CARO04" target="">
<front>
<title> End-to-End Failover Thresholds for Transport Layer Multihoming
</title>
<author initials="A." surname="Caro Jr." fullname="">
<organization />
</author>
<author initials="P." surname="Amer" fullname="">
<organization />
</author>
<author initials="R." surname="Stewart" fullname="">
<organization />
</author>
<date month="11" year="2004" />
</front>
<seriesInfo name="MILCOM 2004" value="" />
</reference>
<reference anchor="CARO05" target="">
<front>
<title> End-to-End Fault Tolerance using Transport Layer Multihoming
</title>
<author initials="A." surname="Caro Jr." fullname="">
<organization />
</author>
<date month="1" year="2005" />
</front>
<seriesInfo name="Ph.D Thesis, University of Delaware" value="" />
</reference>
<reference anchor="CARO02" target="">
<front>
<title> A Two-level Threshold Recovery Mechanism for SCTP
</title>
<author initials="A." surname="Caro Jr." fullname="">
<organization />
</author>
<author initials="J." surname="Iyengar" fullname="">
<organization />
</author>
<author initials="P." surname="Amer" fullname="">
<organization />
</author>
<author initials="G." surname="Heinz" fullname="">
<organization />
</author>
<author initials="R." surname="Stewart" fullname="">
<organization />
</author>
<date month="7" year="2002" />
</front>
<seriesInfo name="Tech report, CIS Dept, University of Delaware" value="" />
</reference>
</references>
</back>
</rfc>
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