Internet Engineering Task Force N. Akiya
Internet-Draft M. Binderberger
Intended status: Standards Track Cisco Systems
Expires: October 4, 2012 April 2, 2012
Mandatory interval support in BFD
draft-akiya-bfd-intervals-00
Abstract
This document defines a set of interval values that must be supported
for transmitting BFD control packets and for calculating the
detection time in the receive direction. Implementations are allowed
to define a lower limit for the interval so that only set values
equal or larger than the lower limit needs to be supported.
This solves the problem of finding an interval value that both BFD
speakers can support while allowing a simplified implementation as
seen for hardware-based BFD.
Requirements Language
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 RFC 2119 [RFC2119].
Status of this Memo
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This Internet-Draft will expire on October 4, 2012.
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document authors. All rights reserved.
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Internet-Draft Mandatory interval support in BFD April 2012
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The problem with few supported intervals . . . . . . . . . . . 3
3. Well-defined intervals . . . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
6. Normative References . . . . . . . . . . . . . . . . . . . . . 4
Appendix A. Why some intervals are mandatory . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 5
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1. Introduction
The standard [RFC5880] describes how to calculate the transmission
interval and the detection time. It does not make any statement
though how so solve a situation where one BFD speaker cannot support
the calculated value. In practise this may not been a problem as
long as software-implemented timers have been used and as long as the
granularity of such timers was small compared to the interval values
being supported.
In the meantime requests exist for very fast interval values, down to
3.3msec for MPLS-TP. At the same time the requested scale for the
number of BFD sessions in increasing. Both requirements have driven
vendors to use Network Processors (NP), FPGAs or other hardware-based
solutions to offload the periodic packet transmission and the timeout
detection in the receive direction. A potential problem with this
hardware-based BFD is the granularity of the interval timers.
Depending on the implementation only a few intervals may be
supported, which can cause interoperability problems. This document
proposes a set of interval values that must be supported by all
implementations. Details are laid out in the following sections.
2. The problem with few supported intervals
Lets assume vendor "A" supports 10msec, 100msec and 1sec interval
timers in hardware. Vendor "B" supports every value from 20msec
onward, with a granularity of 1msec. For a BFD session "A" tries to
set up the session with 10msec while "B" uses 20msec as the value for
RequiredMinRxInterval and DesiredMinTxInterval. RFC5880 describes
that the negotiated value for Rx and Tx is 20msec. But system "A" is
not able to support this. Multiple ways exist to resolve the dilemma
but none of them is without problems.
a. Realizing that it cannot support 20msec, system "A" sends out a
new BFD packet, advertising the next larger interval of 100msec
with RequiredMinRxInterval and DesiredMinTxInterval. The new
negotiated interval between "A" and "B" then is 100msec, which is
supported by both systems. The problem though is that we moved
from the 10/20msec range to 100msec, which likely is not fitting
the requirements.
b. System "A" could violate RFC5880 and use the 10msec interval for
the Tx direction. In the receive direction it could use an
adjusted multiplier value M' = 2 * M to match the correct
detection time. Now beside the fact that we explicitly violate
RFC5880 there may be the problem that system "B" drops up to 50%
of the packets; this could be the case when "B" uses an ingress
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rate policer to protect itself and the policer would be
programmed with an expectation of 20msec receive intervals.
The example above could be even worse when we assume that system "B"
can only support a few timer values itself. Lets assume "B" supports
"20msec", "300msec" and "1sec". If both systems would adjust their
advertised intervals then the adjustment ends at 1sec.
3. Well-defined intervals
The problem can be reduced by defining interval values that must be
supported by all implementations. Then the adjustment mechanism
could find a commonly supported interval without deviating too much
from the original request.
The proposed set of mandatory interval values is: 3.3msec, 10msec,
20msec, 50msec, 300msec and 1sec.
What does "mandatory" mean exactly? In the example above, vendor "A"
has a fastest interval of 10msec and thus would be required to
support all intervals in the mandatory set that are equal or larger
than 10msec, i.e. it would support 10msec, 20msec, 50msec, 300msec,
1sec. Vendor "B" has a fastest interval of 20msec and thus would
need to support 20msec, 50msec, 300msec and 1sec. This does not
restrict both vendors from implementing additional values outside of
the mandatory set. As long as requirements are met for the mandatory
set of values, then both vendor "A" and "B" are free to support
additional values outside of the mandatory set.
4. IANA Considerations
No request to IANA.
5. Security Considerations
This document does not introduce any additional security issues and
the security mechanisms defined in [RFC5880] apply in this document.
6. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
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(BFD)", RFC 5880, June 2010.
Appendix A. Why some intervals are mandatory
The list of mandatory interval values is trying to balance various
objectives. The list should not contain too many values as more
timers may increase the implementation costs. On the other hand less
values produces larger gaps and adjustment jumps. Larger values in
the lower interval range may be easier to support, potentially even
in software instead of hardware.
o 3.3msec: required by MPLS-TP
o 10msec and 20msec: both values allow to detect faster than 50msec,
when used with a multiplier of 2 or 3 (for 10msec). A compromise
could be a single interval of 15msec.
o 50msec: this seems an interval often supported by software
implementations, so the assumption here is that for convenience
this value should be supported.
o 300msec: this would support large scale of 3 x 300msec setups used
by customers to have a detection time slightly below 1sec for VoIP
setups.
o 1sec: as mentioned in RFC5880
With that stated, one of the primary intention of this first draft is
to seek feedback on the number of interval values in the mandatory
set as well as each value.
Authors' Addresses
Nobo Akiya
Cisco Systems
Email: nobo@cisco.com
Marc Binderberger
Cisco Systems
Email: mbinderb@cisco.com
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