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RFC 3954, "Cisco Systems NetFlow Services Export Version 9", October 2004

Source of RFC: INDEPENDENT

Errata ID: 2096
Status: Verified
Type: Technical

Reported By: Paul Aitken
Date Reported: 2010-03-25
Verifier Name: Nevil Brownlee
Date Verified: 2010-04-03

Section 5.3 and 6.2. says:

   Padding
         The Exporter SHOULD insert some padding bytes so that the
         subsequent FlowSet starts at a 4-byte aligned boundary.  It is
         important to note that the Length field includes the padding
         bytes.  Padding SHOULD be using zeros.

It should say:

   Padding
         The Exporter SHOULD insert some padding bytes so that the
         subsequent FlowSet starts at a 4-byte aligned boundary.  It is
         important to note that the Length field includes the padding
         bytes.  The padding length MUST be shorter than any allowable
         record in the Set.  Padding SHOULD be using zeros.

Notes:

Addition of "The padding length MUST be shorter than any allowable record in the Set."

With small field sizes, such that the record size <= 3, it's not possible to distinguish padding from further data records (s 5.3) or options data records (s 6.2).

eg, with a record length of 3, three records will consume 9 octets. Three octets of padding will be added to this, giving a total length of 12 octets. The 12 octets now look like *four* records. In this case, padding is NOT appropriate.

NB1 the same paragraph in section 6.1 is NOT affected, because the fixed size of the other fields dictates that the only possibility is padding of 2 octets.

NB2 this situation is anticipated in IPFIX (RFC 5101), from which the additional text is taken.

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