rfc1989.txt
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RFC 1989 PPP Link Quality Monitoring August 1996
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SaveInLQRs |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SaveInPackets |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SaveInDiscards |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SaveInErrors |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SaveInOctets |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Magic-Number
The Magic-Number field is four octets and aids in detecting links
which are in the looped-back condition. Unless modified by a
Configuration Option, the Magic-Number MUST be transmitted as zero
and MUST be ignored on reception. If Magic-Numbers have been
negotiated, incoming LQR packets SHOULD be checked to ensure that
the local end is not seeing its own Magic-Number and thus a
looped-back link. See the Magic-Number Configuration Option for
further explanation.
LastOutLQRs
The LastOutLQRs field is four octets, and is copied from the most
recently received PeerOutLQRs on transmission.
LastOutPackets
The LastOutPackets field is four octets, and is copied from the
most recently received PeerOutPackets on transmission.
LastOutOctets
The LastOutOctets field is four octets, and is copied from the
most recently received PeerOutOctets on transmission.
PeerInLQRs
The PeerInLQRs field is four octets, and is copied from the most
recently received SaveInLQRs on transmission.
Whenever the PeerInLQRs field is discovered to be zero, the
LastOut... fields are indeterminate, and the PeerIn... fields
contain the initial values for the peer.
Simpson Standards Track [Page 9]
RFC 1989 PPP Link Quality Monitoring August 1996
PeerInPackets
The PeerInPackets field is four octets, and is copied from the
most recently received SaveInPackets on transmission.
PeerInDiscards
The PeerInDiscards field is four octets, and is copied from the
most recently received SaveInDiscards on transmission.
PeerInErrors
The PeerInErrors field is four octets, and is copied from the most
recently received SaveInErrors on transmission.
PeerInOctets
The PeerInOctets field is four octets, and is copied from the most
recently received SaveInOctets on transmission.
PeerOutLQRs
The PeerOutLQRs field is four octets, and is copied from OutLQRs
on transmission. This number MUST include this LQR.
PeerOutPackets
The PeerOutPackets field is four octets, and is copied from the
current MIB ifOutUniPackets and ifOutNUniPackets on transmission.
This number MUST include this LQR.
PeerOutOctets
The PeerOutOctets field is four octets, and is copied from the
current MIB ifOutOctets on transmission. This number MUST include
this LQR.
SaveInLQRs
The SaveInLQRs field is four octets, and is copied from InLQRs on
reception. This number MUST include this LQR.
SaveInPackets
The SaveInPackets field is four octets, and is copied from the
current MIB ifInUniPackets and ifInNUniPackets on reception. This
number MUST include this LQR.
Simpson Standards Track [Page 10]
RFC 1989 PPP Link Quality Monitoring August 1996
SaveInDiscards
The SaveInDiscards field is four octets, and is copied from the
current MIB ifInDiscards on reception. This number MUST include
this LQR.
SaveInErrors
The SaveInErrors field is four octets, and is copied from the
current MIB ifInErrors on reception. This number MUST include
this LQR.
SaveInOctets
The SaveInOctets field is four octets, and is copied from the
current InGoodOctets on reception. This number MUST include this
LQR.
Note that InGoodOctets is not the same as the MIB ifInOctets
counter, as InGoodOctets does not include octets for packets which
are discards or errors.
Simpson Standards Track [Page 11]
RFC 1989 PPP Link Quality Monitoring August 1996
2.7. Transmission of Reports
When the PPP Link Control Protocol has reached the Opened state, the
Link Quality Monitoring process MAY commence sending Link-Quality-
Reports. If a Protocol-Reject is received specifying a LQR packet,
the LQM process MUST cease sending LQRs.
Usually, the LQR is transmitted when the LQR timer for the link
expires. If no LQR timer is used, a LQR is generated upon receipt of
an incoming LQR. The negotiation process ensures that at least one
side of the link is using a LQR timer.
In addition, a LQR is generated whenever two successive LQRs are
received which have the same PeerInLQRs value. This may indicate
that a LQR has been missed, or that the implementation is sending at
a significantly slower rate than the peer, or that the peer has
accelerated LQR generation to better quantify errors on the link.
Whenever a LQR is sent, the LQR timer MUST be restarted.
2.8. Calculations
Each time a Link-Quality-Report packet is received from the inbound
link, the Link-Manager can compare the associated fields. The fields
of the previous LQR can be subtracted from the current LQR values to
obtain an absolute "delta", which allows comparision of the changes
seen by each end of the link.
If the received PeerInLQRs field is zero, the LastOut... fields are
indeterminate, and the PeerIn... fields contain the initial values
for the peer. No calculations using these fields can be performed at
this time.
Implementation Note:
The following counters wrap to zero when their maximum value is
reached. Care must be taken to ensure that correct "delta"
calculations are performed at that time.
The LastOutLQRs field may be directly compared with the PeerInLQRs
field to determine how many outbound LQRs have been lost.
The LastOutLQRs field may be directly compared with the OutLQRs
counter to determine how many outbound LQRs are still in the
pipeline.
Simpson Standards Track [Page 12]
RFC 1989 PPP Link Quality Monitoring August 1996
The change in PeerInPackets may be compared with the change in
LastOutPackets to determine the number of lost packets over the
outgoing link.
The change in PeerInOctets may be compared with the change in
LastOutOctets to determine the number of lost octets over the
outgoing link.
The change in SaveInPackets may be compared with the change in
PeerOutPackets to determine the number of lost packets over the
incoming link.
The change in SaveInOctets may be compared with the change in
PeerOutOctets to determine the number of lost octets over the
incoming link.
The change in the PeerInDiscards and PeerInErrors fields may be used
to determine whether packet loss is due to congestion in the peer
rather than physical link failure.
2.9. Failure Detection
When the link is operating well in both directions of the link, the
LQR is superfluous. The maximum time interval for transmitting LQRs
SHOULD be chosen to minimally interfere with active traffic.
When there is a measurable loss of data in either direction, if the
overall throughput is adequate, conditions are not severe enough to
warrant dropping the link. Sending LQRs faster will gain nothing,
except to measure peaks in the loss rate. The time interval MUST be
chosen to be long enough to have a good smoothing effect on the data,
while short enough to ensure fast enough response to complete
failure.
When the link is good incoming, but very bad outgoing, incoming LQRs
indicate a high loss on the outgoing side of the link. Sending LQRs
faster won't help, because they are probably lost on the way to the
peer.
When the link is good outgoing, but very bad incoming, incoming LRQs
will be frequently lost. In this case, LQRs SHOULD be sent at a
faster rate. This primarily relies on the peer to make an informed
policy decision. The peer will also send LQRs in response (due to
the duplicate PeerInLQRs field), and some of those LQRs may
successfully arrive.
Simpson Standards Track [Page 13]
RFC 1989 PPP Link Quality Monitoring August 1996
When a LQR does not arrive within the time expected, or the LQR
received indicates that the links are truly bad, at least one
additional LQR SHOULD be sent. An algorithmic decision requires at
least 2 round trip intervals. The loss rate could be transient, due
to a heavily loaded link, or a lost outgoing LQR.
2.10. Policy Suggestions
Link-Quality-Report packets provide a mechanism to determine the link
quality, but it is up to each implementation to decide when the link
is usable. It is recommended that this policy implement some amount
of hysteresis so that the link does not bounce up and down. One
policy is to use a K out of N algorithm. In such an algorithm, there
must be K successes out of the last N periods for the link to be
considered of good quality.
Procedures for recovery from poor quality links are unspecified and
may vary from implementation to implementation. A suggested approach
is to immediately close all other Network-Layer protocols (i.e.,
cause IPCP to transmit a Terminate-Request), but to continue
transmitting Link-Quality-Reports. Once the link quality again
reaches an acceptable level, Network-Layer protocols can be
reconfigured.
Simpson Standards Track [Page 14]
RFC 1989 PPP Link Quality Monitoring August 1996
Security Considerations
Security issues are not discussed in this memo.
Acknowledgements
Some of the text in this document is taken from RFC 1172, by Drew
Perkins of Carnegie Mellon University, and by Russ Hobby of the
University of California at Davis.
Special thanks to Craig Fox (Network Systems), and Karl Fox (Morning
Star Technologies), for design suggestions based on implementation
experience.
References
[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
51, RFC 1661, Daydreamer, July 1994.
[2] McCloghrie, K., and M. Rose, "Management Information Base for
Network Management of TCP/IP-based internets: MIB-II", STD
17, RFC 1213, March 1991.
[3] Rose, M., and K. McCloghrie, "Structure and Identification of
Management Information for TCP/IP-based Internets", STD 16,
RFC 1155, May 1990.
Simpson Standards Track [Page 15]
RFC 1989 PPP Link Quality Monitoring August 1996
Chair's Address
The working group can be contacted via the current chair:
Karl Fox
Ascend Communications
3518 Riverside Drive, Suite 101
Columbus, Ohio 43221
EMail: karl@ascend.com
Author's Address
Questions about this memo can also be directed to:
William Allen Simpson
Daydreamer
Computer Systems Consulting Services
1384 Fontaine
Madison Heights, Michigan 48071
Bill.Simpson@um.cc.umich.edu
bsimpson@MorningStar.com (prefered)
Simpson Standards Track [Page 16]
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