iSCSI a SCSI over TCP mapping a SCSI over TCP mapping iSCSI - - PowerPoint PPT Presentation

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iSCSI a SCSI over TCP mapping a SCSI over TCP mapping iSCSI IETF - - 50 50 IETF Julian Satran IBM Research Lab in Haifa IBM Research Lab in Haifa 03/19/2001 Items to discuss Items to discuss ! CRC32, 64 and other checksums !

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03/19/2001

IBM Research Lab in Haifa IBM Research Lab in Haifa

iSCSI iSCSI – – a SCSI over TCP mapping a SCSI over TCP mapping IETF IETF -

50

Julian Satran

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03/19/2001 iSCSI-IETF50 2

Items to discuss Items to discuss

! CRC32, 64 and other checksums

! What should be our requirements ! Summary of findings

! Security

! Separate presentation

! Formats

! Several alternatives

! Recovery

! What we will attempt to do

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Checksum/CRC requirements Checksum/CRC requirements

! Protect against:

! Protect against sporadic errors (low noise

channels – independent single bit errors)

! Protect against burst errors (as software and

unprotected components can introduce this type

f error too)

! Protect when data is not uniformly distributed

(biased as most real data is)

! Probability of undetected errors in excess of

10^-25 (ideally in excess of 10^-50) for a decent block length

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CRC vs. Checksum CRC vs. Checksum -

performance

performance

low high Data Bias sensitivity 10^-40 10^-35 Puds/8kblock 10^-41 10^-36 Pudb/8kblock 2^31-1 2^19 block (bits) 4 3 distance CRC32C Checksum

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CRC vs. Checksum CRC vs. Checksum -

complexity

complexity

CRC32C Checksum 2^18 bytes

Table-size

0.5 byte

Table lookups

2.75 2/3 Instructions/byte LFSR Adder or Adder+Modulo Hardware

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Checksum/CRC survey Checksum/CRC survey

! Results will be presented in a forthcoming

memo (ID)

! Recommend – CRC32C as mandatory to

implement. Good for all block lengths up to

2^31-1 bits

! CRC32C is:11EDC6F41 ! CRC32Q is:1814141AB

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Formats Formats

! Format alternatives

! Single Header Digest Formats ! Multiple Header Digest Formats

! Format illustration/documentation

! Eliminate common parts ! Always complete

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Single digest Single digest

Digest (if any) AHS fields-optional Type AHS fields-optional Type Data Length-optional Type BHS Length of AHSs or Data Length QL

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Single digest Single digest

! QL (Qualified Length) coding:

! B7- 0 No AHS, 1 – AHS ! B6-4 Reserved ! B3-0 Parity for first 4 bytes (or ECC)

! Type coding

! Bit 7-4 AHS Type

! 0-data ! 1-Bidi ! 2-Extended CDB

! Bit 3-0 Parity for first 4 bytes (or ECC) ! Length not needed always (fixed length AHS)

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Multiple digests Multiple digests

Data Length Type=DL Digest2 AHS fields Type AHS fields Type Digest1 BHS Length QL

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Multiple digests Multiple digests – – Barry Reinhold and Barry Reinhold and Robert Russell Robert Russell

Digest2 Length+Fields Type Length+Fields Type Digest1 BHS Dlength Alength

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Format illustration choices Format illustration choices

! Complete headers (as in pre 04 – including

all BHS fields and length)

! Headers without length (as in in 05) ! Headers with length but in another position? ! Other formats

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Recovery Recovery

! A pseudo-language description of recovery

will be added

! A team is working now on it:

! Mallikarjun C., Somesh Gupta, Steph Bailey,

John Hufferd, Luciano Dalle Ore, Venkat Rangan, Julian Satran

! No recovery is mandatory ! No need to use length that is in doubt ! DataSN is there to ensure sequence.

Recovery is optional

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