| Network Working Group | Y.N. Nishida |
| Internet-Draft | GE Global Research |
| Intended status: Experimental | P.N. Natarajan |
| Expires: April 24, 2014 | Cisco Systems |
| A.C. Caro | |
| BBN Technologies | |
| P.A. Amer | |
| University of Delaware | |
| October 21, 2013 |
Quick Failover Algorithm in SCTP
draft-ietf-tsvwg-sctp-failover-02
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.
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The Stream Control Transmission Protocol (SCTP) [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.
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) [IYENGAR06] is an extension to SCTP and allows the sender to transmit data on multiple paths simultaneously. Research [NATARAJAN09] shows that the current failure detection procedure worsens CMT performance during failover and can be significantly improved by employing a better failover algorithm.
This document proposes an alternative failure detection procedure for SCTP (and CMT) that improves SCTP (CMT) performance during failover.
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].
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.
If sender has multiple active destination addresses, it can retransmit data to secondary destination address when the transmission to the primary times out.
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.
The failover process of SCTP is initiated when the primary path becomes inactive (error counter for the primary 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.
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.
The following approach are conceivable for the solutions of this issue.
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 [JUNGMAIER02] [GRINNEMO04] [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 [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.
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.
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.
As several research results indicate, we can also shorten the duration of failover process by adjusting RTO related parameters [JUNGMAIER02] [FALLON08]. During failover process, RTO keeps being doubled. However, if we can choose smaller value for RTO.max, we can stop the exponential growth of RTO at some point. Also, choosing smaller values for RTO.initial or RTO.min can contribute to keep RTO value small.
Similar to reducing Path.Max.Retrans, the advantage of this approach is that it requires no modification to the current 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.
Our proposal stems from the following two observations about SCTP's failure detection procedure:
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.
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 [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:
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 [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 to another path, it starts with minimal cwnd because of slow-start. Hence, there is little chance for packet reordering or duplicating.
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.
Post failover, an SCTP sender migrates back to the original primary destination once this destination becomes active. The sender sets cwnd to the initial cwnd value and performs slow start. [CARO02] shows that the switch over to the original primary may degrade SCTP performance compared to continuing data transmission on the same path, especially in scenarios where this path's characteristics are better. In order to mitigate this performance degradation, permanent failover was proposed in [CARO02]. Permanent failover allows SCTP to remain the alternative path even if the primary path becomes active again. We recommend that SCTP-PF should stick to the standard RFC4960 behavior, i.e., switch back to the original primary once this destination becomes active again. Permanent failover may be considered in the future based on discussions and consensus within the community.
When multiple destinations are in the PF state, the sender may transmit heartbeats to multiple destinations at the same time. This allows SCTP-PF sender to quickly track and respond to network status change compared to an SCTP sender. However, when all PF destinations become unavailable, an SCTP-PF sender has outstanding HBs on all destinations compared to an SCTP sender and increases the count for the total number of consecutive retransmissions faster than the SCTP sender. SCTP-PF's faster increase in the error count will result in association termination sooner than SCTP. The key difference between SCTP and SCTP-PF with regard to this feature is whether checking path status sequentially or concurrently as the number of packets sent for probing is the same.
For deployments where aggressive failure detection and association termination is not desired, we suggest that AMR be set to the recommended maximum value (sum of PMRs of all paths), to delay assoc termination during SCTP-PF. Another option is to send retransmitted data or HB to only one PF destination at a time, but this approach may delay path status tracking. To exclude HB timeouts from incrementing the error count can also be a solution, however, this requires an update to Section 8.1 and Section 8.3 of [RFC4960], otherwise special logics for error counter need to be implemented for SCTP-PF.
This section describes how the socket API defined in [RFC6458] is extended to provide a way for the application to control the quick failover behavior.
Please note that this section is informational only.
A socket API implementation based on [RFC6458] 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.
Support for the SCTP_PEER_ADDR_THLDS socket option needs also to be added to the function sctp_opt_info().
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.
struct sctp_paddrthlds {
sctp_assoc_t spt_assoc_id;
struct sockaddr_storage spt_address;
uint16_t spt_pathmaxrxt;
uint16_t spt_pathpfthld;
};
The following structure is used to access and modify the thresholds:
There are no new security considerations introduced in this document.
This document does not create any new registries or modify the rules for any existing registries managed by IANA.
| [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. |