SCTP Timers

Oracle provides two retransmission modes: RFC and Linear. The SCTP retransmission control feature allows the tailoring of retransmissions to detect a network fault in a timely fashion through these configuration parameters:

• RMODE: Selects desired retransmission mode (RFC or LIN)
• RTIMES: Maximum number of retransmits attempted before the connection is declared lost (3 to 12); the default is 10
• RTO: Time to wait before the current retransmit attempt is declared a failure. This time is dynamic because it is a moving average of the network
• RMAX: Upper bound of calculated RTO (10 ms to 1,000 ms); the default is 800; Oracle suggests 3 * RMIN
• RMIN: Lower bound of calculated RTO (10 ms to 1,000 ms). The default is 120; Oracle suggests the greater of (1.2 * average RTT) or (10 ms + average RTT).
• CWMIN: Minimum Congestion Window Size (1,500 to 192K); the default is 3K

RFC Timer Setting

With an exponential timer setting, the RTO value is doubled for each retransmit attempt. When transmitting a packet, the RTO has to expire before attempting to retransmit. With the second attempt, the last RTO value is doubled (RTO * 2) before retransmitting; with the third attempt, the last RTO value is doubled again (RTO * 4); and so on. This method significantly increases the time to determine that a link is lost.

For example, if data is being transmitted for five retransmits, the time to determine a lost link is:

RTO.min * Path.Max.Retransmits (or 1 + 2 + 4 + 8 + 16 + 32) = 63 sec

Table 5-19 shows RFC timers and their RFC and Oracle-recommended default values.

Table 5-19 SCTP Configuration Data Descriptions for Oracle EAGLE

RFC Name	Description	RFC Recommended Default Value	Oracle Default Value	Oracle Configurable?	Oracle Ranges
RTO.initial	Initial RTO Value	3 seconds	120 ms	Yes Assoc RMIN parameter	1-1000 ms
RTO.max	Upper limit of RTO	60 seconds	800 ms	Yes Assoc RMAX parameter	1-1000 ms
RTO.min	Lower limit of RTO	1 second	120 ms	Yes Assoc RMIN parameter	1-1000 ms
Max.Init. Retransmits	Maximum Initial Retransmit Attempts	8 attempts	10 attempts	Yes Assoc RTIMES parameter.Not configurable independently of Assoc.max. retrans	1-12
Association. max. retrans	Maximum Association Data Retransmit Attempts	10 attempts	10 attempts	Yes Assoc RTIMES parameter	1-12 ms
Path.max. retrans	Maximum Data Retransmit attempts per Destination (used for multi-homing only)	5 attempts	5 attempts	Indirectly ½ of the assoc RTIMES parameter	1-6 ms
Acknowledge- ment timer	SACK Transmit	User Configurable not to exceed 500 ms	½ RTO or 200 ms, whichever is less	Indirectly RTO is bound by the assoc RMIN and RMAX parameters	5-200 ms
T3-rtx	Timer Data Retransmit	RTO (see RTO.initial for initial value )	RTO (see RTO.initial for initial value)	Yes RTO is bounded by the assoc RMIN and RMAX parameters	10-1000 ms
T1-init	Timer Init retransmit timer	Initially 3 seconds RTO thereafter	Initially 1 second, RTO thereafter	No for initial value Indirectly thereafter via RMIN/RMAX bounding of RTO	10-1000 ms
HB.Interval	Heart Beat Interval	30 seconds	RTO+500 ms	Yes RTO + Assoc HBTIMER parameter	RTO+500 ms to RTO+10000 ms
Shutdown timer	Shutdown timer t2	RTO	RTO	Indirectly RTO is bound by the assoc RMIN and RMAX parameters	10-1000 ms
Cookie Timer	Cookie-t1 – Cookie Echo retransmit timer	Initially 3 seconds RTO thereafter	Initially 1 second RTO thereafter	No for initial value Indirectly thereafter via RMIN/RMAX bounding of RTO	10-1000 ms
Cookie life	Cookie Life	60 seconds	5 seconds	No	5 seconds

Note:

SCTP Heart Beats (HBs) are sent after every HB interval on an idle path of the SCTP association. If more than one idle path exists, then HBs are sent alternately on each idle path, after each HB interval.

LIN Timer Setting

Oracle has implemented a more aggressive timer method called Linear (LIN), in which the RTO between attempts is constant. Oracle recommends this setting to detect a failure more quickly than the RFC method.

With the LIN timer setting, the time to declare the association down is at least

RMIN * RTIMES

For very high-throughput associations, RTIMES (and if possible, RMIN) should be lowered and CWMIN increased. CWMIN is the parameter that sets the minimum size of the congestion window, which determines the number of packets that can be sent without having received the corresponding ACK packets.

On the far end, the LIN mode can coexist with RFC mode, but in contrast to the Signaling Gateway, the far-end may experience congestion in the ASP-to-SGP direction because of network impairments.

Jitter Effects

Since the RTO is a moving average of network RTT samples, as the jitter range increases, bounding the lower limit of the RTO at or near the average will cause the amount of unnecessary retransmissions to increase, since for each transmission that takes longer than the current RTO to acknowledge a retransmission will occur, wasting bandwidth..

If the lower limit of the RTO is bounded to the upper end of the jitter range to minimize retransmits, then connection failure detection time is similarly increased.

So, minimizing jitter in the network translates into a small range for network RTT, and the RTO can be bounded to minimize retransmissions while being able to detect a loss of connection in a timely fashion.