state on one side of the connection (perhaps as part of a forwarding
plane reset or failover to a standby node), it is possible that a
large number of packets will be sent in-order, but be wrongly
detected by the peer as out-of-order. This can be generally
characterized for a window size, w, sequence number space, s, and
number of packets lost in transit between L2TP endpoints, p, as
follows:
If s > p > w, then an additional (s - p) packets that were otherwise
received in-order, will be incorrectly classified as out-of-order and
dropped. Thus, for a sequence number space, s = 128, window size, w
= 64, and number of lost packets, p = 70; 128 - 70 = 58 additional
packets would be dropped after the outage until the sequence number
wrapped back to the current expected next sequence number.
To mitigate this additional packet loss, one MUST inspect the
sequence numbers of packets dropped due to being classified as "old"
and reset the expected sequence number accordingly. This may be
accomplished by counting the number of "old" packets dropped that
were in sequence among themselves and, upon reaching a threshold,
resetting the next expected sequence number to that seen in the
arriving data packets. Packet timestamps may also be used as an
indicator to reset the expected sequence number by detecting a period
of time over which "old" packets have been received in-sequence. The
ideal thresholds will vary depending on link speed, sequence number
space, and link tolerance to out-of-order packets, and MUST be
configurable.
Editors’ Addresses
Jed Lau
cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
EMail: jedlau@cisco.com
W. Mark Townsley
cisco Systems
EMail: mark@townsley.net
Ignacio Goyret
Lucent Technologies
EMail: igoyret@lucent.com
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