IS-IS Working Group Z. Li
Internet Draft L. Li
Intended status: BEST CURRENT PRACTICE X. Duan
Expires: January 11, 2010 China Mobile
July 12, 2009
Recommendations for Processing Mechanism for Checksum Error LSP in
interoperable Networks using Intermediate System to Intermediate
System (IS-IS)
draft-li-isis-error-lsp-processing-01.txt
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Abstract
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RFC3719 discusses a number of differences between the IS-IS protocol
as described in ISO 10589 and the protocol as it is deployed today.
This document discusses some other differences found in the China
Mobile's backbone network which is constructed with routers from
several manufacturers. The differences include corrupt LSP processing,
zero checksum LSP processing, zero remaining lifetime LSP processing,
and LSP checksum calculation.
1. Introduction
Intermediate System to Intermediate System (IS-IS) Protocol [1] is
one of the Interior Gateway Protocols. It is widely deployed in the
carrier backbone networks.
RFC3719 [2] discusses a number of differences between the IS-IS
protocol as described in ISO 10589 and the protocol as it is deployed
today.
In the China Mobile's backbone network, which is constructed with a
few hundreds of routers from several manufacturers, some other
differences were found. These differences contributed to a severe
network flapping across the whole network.
The differences discussed in this document include corrupt LSP
processing, zero checksum LSP processing, zero remaining lifetime LSP
processing, and LSP checksum calculation.
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 [3].
2. Corrupt LSP Processing
Section 7.3.14.2 e) of [1] states: An Intermediate system receiving a
Link State PDU (LSP) with an incorrect LSP Checksum or with an
invalid PDU syntax shall
1) generate a corruptedLSPReceived circuit event,
2) discard the PDU.
In order to control the processing mechanism of Checksum error LSP,
some equipment manufacturers provide an on-off configuration switch,
such as Cisco's IGNORE-LSP-ERRORS switch and Huawei's IGNORE-LSP-
CHECKSUM-ERROR switch. However, the default state of the switch is
different, thus the processing mechanism of checksum error LSP is not
the same.
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From the carrier's perspective, such on-off configuration switch is
welcome, because the carrier can determine the processing mechanism
through the switch. But the behavior of the switch SHOULD be the same,
as follows.
When the on-off switch is on, the processing mechanism for the
checksum error LSP SHOULD be accordant with what is stated in Section
7.3.14.2 e) of [1]. When the on-off switch is off, the equipment
SHOULD treat the received checksum error LSP in the same way as the
LSP whose remaining lifetime equals 0. The processing mechanism is
specified in Section 7.3.16.4 of [1].
It is RECOMMENDED that the default state of the on-off switch be on.
In this way, the default processing mechanism is in accordance with
Section 7.3.14.2 e) of [1].
3. Zero Checksum LSP Processing
RFC 3719 [2], section 7, suggests an implementation SHOULD treat all
LSPs with a zero checksum and a non-zero remaining lifetime as if
they had as checksum error.
ISO 10589, section 7.3.16.4, note 36, states: A check of the checksum
of a zero Remaining Lifetime LSP succeeds even though the data
portion is not present. Therefore, the LSP with a zero checksum and a
zero remaining lifetime SHOULD be treated as the LSP with correct
checksum.
Consider the above two conditions together, the implementation SHOULD
check the remaining lifetime first, then check the checksum. In
practice, some implementation does the check in the reverse way.
Take the on-off switch into account, equipments with on-off switch
SHOULD treat LSPs with a zero checksum and a non-zero remaining
lifetime in accordance with the processing mechanism mentioned in the
previous section 2. As for the LSPs with a zero checksum and a zero
remaining lifetime, the processing mechanism SHOULD NOT be affected
by the switch state.
4. Zero Remaining Lifetime LSP Processing
ISO 10589, section 7.3.16.4, note 36, states: A check of the checksum
of a zero Remaining Lifetime LSP succeeds even though the data
portion is not present. Therefore, a zero Remaining Lifetime LSP
SHOULD be treated as correct LSP, no matter whether its checksum is
correct or not.
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Some implementations, however, still check the checksum of a zero
Remaining Lifetime LSP.
The processing mechanism SHOULD NOT be affected by the on-off switch
and the details can be seen in section 7.3.16.4.
5. Checksum calculation
Note 36, section 7.3.16.4, ISO 10589 prescribes that examining the
checksum of a zero Remaining Lifetime LSP is always successful. Thus,
some implementations fill in the checksum field with zero in the zero
Remaining Lifetime LSP.
To insure the interoperability and maintain network stability, it is
RECOMMENDED to calculate the checksum of all LSPs correctly,
including zero Remaining Lifetime LSP. The calculation method is
presented in Section 7.3.11, ISO 10589.
6. Security Considerations
The suggestions and clarifications in this document will not cause
extra new security concerns.
7. References
[1] ISO 10589 V2 Telecommunications and information exchange
between systems-Intermediate System to Intermediate System
[2] J. Parker, Ed., "Recommendations for Interoperable Networks
using Intermediate System to Intermediate System (IS-
IS)", RFC 3719, February 2004.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
Author's Addresses
Zhenqiang Li (editor)
China Mobile Research Institute
Gate 2 Dacheng Plaza
No. 28 Xuanwumen West Street
Xuanwu District, Beijing 100053
China
Phone: +86 1391 163 5816
Email: lizhenqiang@chinamobile.com
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Lianyuan Li
China Mobile Research Institute
Gate 2 Dacheng Plaza
No. 28 Xuanwumen West Street
Xuanwu District, Beijing 100053
China
Phone: +86 1391 178 9703
Email: lilianyuan@chinamobile.com
Xiaodong Duan
China Mobile Research Institute
Gate 2 Dacheng Plaza
No. 28 Xuanwumen West Street
Xuanwu District, Beijing 100053
China
Phone: +86 1391 019 1797
Email: duanxiaodong@chinamobile.com
Li January 11, 2010 [Page 5]