RE: Encoding unknown values in the extended_switchstructure

From: Peter Phaal <>
Date: 05/23/09
Message-ID: <8AED6DC17E7B4F23918527AE961738C7@PHAALPC>

It would be nice to have a general purpose "unknown" mechanism and the use
of a bitmap to indicate which elements in a structure fits well with XDR

However, your proposal to take the upper two bytes from the data_format
field has some problems. The sFlow version 5 data_format field is fully
defined. Your proposed change would not be backward compatible for sFlow
collectors since they would need to be altered to ignore the bits (or use
them to mask the structure).

The data_format uniquely identifies the format of an opaque structure in
the sFlow specification. A data_format is contructed as follows:
   - The most significant 20 bits correspond to the SMI Private Enterprise
     Code of the entity responsible for the structure definition. A value
     of zero is used to denote standard structures defined by
   - The least significant 12 bits are a structure format number assigned
     by the enterprise that should uniquely identify the the format of the

I just checked IANA and the largest currently assigned enterprise number is
now 33572. Using the upper two bytes of the data_format would overlap with
enterprise codes, breaking vendor extensibility - an important feature of
sFlow version 5.

I think you may have misunderstood the role of the structure number in the
data_format tag. The entire data_format element (32 bits) is treated as a
unique structure id. The allocation scheme is hierarchical, providing each
enterprise publishing structures with a full 12 bits that they can assign as
they choose (i.e. the structure number by itself does not uniquely
distinguish a structure, it must be combined with the assigning entity to
provide a unique id).

Currently, sFlow, version 5 defines unknown fields as follows:
The following values should be used for fields that are
unknown (unless otherwise indicated in the structure
    - Unknown integer value. Use a value of 0 to indicate that
      a value is unknown.
    - Unknown counter. Use the maximum counter value to indicate
      that the counter is not available. Within any given sFlow
      session a particular counter must be always available, or
      always unavailable. An available counter may temporarily
      have the max value just before it rolls to zero. This is
    - Unknown string. Use the zero length empty string. */

Of course there is also the option of omitting a structure all together to
indicate that none of its fields are known to the agent.

These rules work reasonably well, except for the case of the extended_switch
structure. By "otherwise indicating" the unknown values as 0xFFFFFFFF the
proposal is the smallest possible change that should be backward compatible
for existing agents and collectors.

If an sFlow version 6 is ever defined then your suggestion of using a bit
array to provide a general unknown value mechanism would be an attractive

As new structures are defined for sFlow version 5, adopting a convention in
which the first member of the structure is a bit array indicating any
unknown elements in the structure would be something that authors of
structures should consider if there is any possibility that elements of the
structure might not be known.


> -----Original Message-----
> From: Paolo Lucente []
> Sent: Saturday, May 23, 2009 4:59 AM
> To: Peter Phaal
> Cc:
> Subject: Re: [sFlow] Encoding unknown values in the
> extended_switchstructure
> Hi Peter,
> Not really an objection - rather a proposal for your consideration.
> What about bitmapping fields contained in standard elements within,
> say, the upper two bytes of the element tag?
> typedef struct _SFLFlow_sample_element {
> struct _SFLFlow_sample_element *nxt;
> u_int32_t tag; /* SFLFlow_type_tag */ <-----
> u_int32_t length;
> SFLFlow_type flowType;
> } SFLFlow_sample_element;
> ie.
> #define EX_SWITCH_SRC_VLAN 0x00010000
> #define EX_SWITCH_SRC_PRIORITY 0x00020000
> #define EX_SWITCH_DST_VLAN 0x00040000
> #define EX_SWITCH_DST_PRIORITY 0x00080000
> It would have the beauty of being backward compatible, meaning the
> collector can just ignore it and not take benefit from it, and will
> maintain intact the 32 bit field - which is not a requirement today,
> for this specific case, but you might be less lucky tomorrow trying
> to do the same trick elsewhere; so a more coherent approach.
> Companies developing closed extensions and thus making use of the
> upper bytes of the tag field, ie.
> /* enterprise = 4300 (inmon)...*/
> SFLFLOW_EX_PROCESS = (4300 << 12) + 3
> might (or will need to) use a different way to signal some fields
> are not populated within their elements.
> The only con of this approach is that it would put an upper bound
> to the number of fields that can be packed within an element, ie.
> 15 fields if using the upper 2 bytes (keeping out the '0x00000000'
> value), but i see you have never been a big fan of flat structures
> ( :-) ) so i wouldn't see this as a true biggie. Quickly scanning
> through the existing elements, all seem to fit this approach.
> Cheers,
> Paolo
> On Fri, May 22, 2009 at 11:07:55AM -0700, Peter Phaal wrote:
> > There currently isn't a satisfactory way to indicate that one or more of
> the
> > fields in the extended_switch structure is unknown to the sFlow
> exporter.
> >
> > /* Extended Switch Data */
> > /* opaque = flow_data; enterprise = 0; format = 1001 */
> > /* Note: For untagged ingress ports, use the assigned vlan and priority
> > of the port for the src_vlan and src_priority values.
> > For untagged egress ports, use the values for dst_vlan and
> > dst_priority that would have been placed in the 802.Q tag
> > had the egress port been a tagged member of the VLAN instead
> > of an untagged member. */
> >
> > struct extended_switch {
> > unsigned int src_vlan; /* The 802.1Q VLAN id of incoming frame */
> > unsigned int src_priority; /* The 802.1p priority of incoming frame
> */
> > unsigned int dst_vlan; /* The 802.1Q VLAN id of outgoing frame */
> > unsigned int dst_priority; /* The 802.1p priority of outgoing frame
> */
> > }
> >
> > Since 0 is a valid value for both 802.1Q VLAN 802.1p priority it should
> not
> > be used to encode the unknown value. Rather than omit the whole
> structure
> > because one field isn't known, it makes sense to define an unambiguous
> value
> > to indicate that a field is unknown.
> >
> > Fortunately these fields have relatively small maximum values, vlan <=
> 4096
> > and priority <= 7, so an unambiguous choice for an unknown value would
> be
> > 0xffffffff (Note: this same value that sFlow currently uses to indicate
> that
> > a 32 bit counter value is unknown). If there are no objections, I think
> it
> > would make sense to include a note to this effect on Specifications page
> on
> >
> >
> > Peter
Received on Sat May 23 09:14:32 2009

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