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3.7 Time Parameters Connected: An Internet Encyclopedia
3.7 Time Parameters

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3.7 Time Parameters

3.7 Time Parameters

There are two time parameters relevant to each element of RSVP path or reservation state in a node: the refresh period R between generation of successive refreshes for the state by the neighbor node, and the local state's lifetime L. Each RSVP Resv or Path message may contain a TIME_VALUES object specifying the R value that was used to generate this (refresh) message. This R value is then used to determine the value for L when the state is received and stored. The values for R and L may vary from hop to hop.

In more detail:

  1. Floyd and Jacobson [FJ94] have shown that periodic messages generated by independent network nodes can become synchronized. This can lead to disruption in network services as the periodic messages contend with other network traffic for link and forwarding resources. Since RSVP sends periodic refresh messages, it must avoid message synchronization and ensure that any synchronization that may occur is not stable.

    For this reason, the refresh timer should be randomly set to a value in the range [0.5R, 1.5R].

  2. To avoid premature loss of state, L must satisfy L >= (K + 0.5)*1.5*R, where K is a small integer. Then in the worst case, K-1 successive messages may be lost without state being deleted. To compute a lifetime L for a collection of state with different R values R0, R1, ..., replace R by max(Ri).

    Currently K = 3 is suggested as the default. However, it may be necessary to set a larger K value for hops with high loss rate. K may be set either by manual configuration per interface, or by some adaptive technique that has not yet been specified.

  3. Each Path or Resv message carries a TIME_VALUES object containing the refresh time R used to generate refreshes. The recipient node uses this R to determine the lifetime L of the stored state created or refreshed by the message.

  4. The refresh time R is chosen locally by each node. If the node does not implement local repair of reservations disrupted by route changes, a smaller R speeds up adaptation to routing changes, while increasing the RSVP overhead. With local repair, a router can be more relaxed about R since the periodic refresh becomes only a backstop robustness mechanism. A node may therefore adjust the effective R dynamically to control the amount of overhead due to refresh messages.

    The current suggested default for R is 30 seconds. However, the default value Rdef should be configurable per interface.

  5. When R is changed dynamically, there is a limit on how fast it may increase. Specifically, the ratio of two successive values R2/R1 must not exceed 1 + Slew.Max.

    Currently, Slew.Max is 0.30. With K = 3, one packet may be lost without state timeout while R is increasing 30 percent per refresh cycle.

  6. To improve robustness, a node may temporarily send refreshes more often than R after a state change (including initial state establishment).

  7. The values of Rdef, K, and Slew.Max used in an implementation should be easily modifiable per interface, as experience may lead to different values. The possibility of dynamically adapting K and/or Slew.Max in response to measured loss rates is for future study.


Next: 3.8 Traffic Policing and Non-Integrated Service Hops

Connected: An Internet Encyclopedia
3.7 Time Parameters

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