Internet DRAFT - draft-hares-idr-update-attrib-low-bits-fix
draft-hares-idr-update-attrib-low-bits-fix
InterDomain Routing Group (IDR) S. Hares
Internet-Draft Huawei Technologies (USA)
Updates: 4271 (if approved) J. Scudder
Intended status: Standards Track Juniper Networks
Expires: August 29, 2013 February 25, 2013
Update Attribute Flag Low Bits Clarification
draft-hares-idr-update-attrib-low-bits-fix-01
Abstract
This draft provides an update to RFC 4721 to clarify the use of the
lower-order four bits of the Attribute flag in the Update message.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 29, 2013.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Change to RFC 4271 Section 4.3 . . . . . . . . . . . . . . . . 3
3. Known BGP Implementation Habits . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
6. Normative References . . . . . . . . . . . . . . . . . . . . . 4
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 5
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1. Introduction
[RFC4271] specifies in section 4.3 that that the low order four bits
of the the Attribute Flags octet are unused, and MUST be zero when
sent. There is a disagreement on what when sent means. This draft
specifies the meaning.
The issue has been that one school of thought considers that "when
sent" means when originated. Another holds that "when sent" means
when originated or propagated. This draft takes the second approach
of "when sent" being when originated or propagated.
The real issue is that reserved flags are only useful if there is
some hope of someday using them for something. If implementations
reset these flags on propagation, then a future revision to the BGP
specification which introduces a new flag will not be able to
propagate the new attribute flag end to end, since it would be very
likely that some well-meaning intermediate router would zero on it.
The effort to roll out implementations that transited the new flag
would almost certainly be prohibitive.
2. Change to RFC 4271 Section 4.3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attr. Flags |Attr. Type Code|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Original Text:
The lower-order four bits of the Attribute Flags octet are
unused. They MUST be zero when sent and MUST be ignored when
received
Corrected Text:
The lower-order four bits of the Attribute Flags octet are
unused. They MUST be zero when originated or sent.
When received, any MUST be accepted and ignored.
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3. Known BGP Implementation Habits
The following are BGP implementation habits regarding the unused flag
bits
o always ignore bits received, and always send zero (originated or
propagated);
o always ignore bits received, always send zero bits (originated),
and propagate what was received;
o if non-zero bits are received, drop the peering session;
o by special condition (policy) handle set bits or set bits, and
propagate;and
o always sets bits under special conditions, and propagates bits.
The reset of BGP sessions based on non-zero bits has been documented
at:
http://mailman.nanog.org/pipermail/nanog/2012-November/053754.html
Compliance with this draft, as well as [RFC4271], means that routers
should not reset BGP sessions if if non-zero lower bits are received.
4. IANA Considerations
This document includes no request to IANA.
5. Security Considerations
This document has no new security cases.
It clarifies some BGP UPDATE packet flag values and thus may aid in
improving BGP security. In particular, it makes it even clearer that
routers must not reset a session upon receiving unexpected flag
values. Behaving otherwise exposes a router to a denial-of-service
attack since a distant party might be able to inject such flag
values.
6. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
Authors' Addresses
Susan Hares
Huawei Technologies (USA)
2330 Central Expressway
Santa Clara, CA 95050
USA
Email: Susan.Hares@huawei.com
John Scudder
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
USA
Email: jgs@juniper.net
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