Re: [Idr] RFC-compliant use of Extended Length for new BGP attributes

["Vishwas Manral" <vishwas.ietf at gmail.com>]

Hi Marcelo,

RFC4271 obsoletes RFC1771. In my view as the RFC4271 does not mention
the "Extended Length" restriction as defined below in RFC1771, the
restriction no longer applies to RFC4271.

"Extended Length may be used only if the length of the attribute value
is greater than 255 octets."

I agree clarification on the same would have been good though.

Thanks,
Vishwas

On Thu, Mar 13, 2008 at 8:48 AM, Marcelo Schmidt <mschmidt at equinix.com> wrote:
> I  am  interested  in input from the list on loose behavior we've seen
>  from various BGP specs.  The specific example in question is a case we
>  are seeing in the wild where a NEW_AS_PATH attribute has 8 data bytes,
>  but  sets the Extended Length bit and uses a 2-byte length field.  RFC
>  1771 explicitly states "Extended Length may be used only if the length
>  of  the  attribute value  is  greater  than 255 octets."  However, the
>  superceding  RFC  4271 does not have this explicit language, therefore
>  would  imply that  either  of the following cases would technically be
>  compliant:
>
>  A.  Ext-Len is set (1), 2-byte length field is used, regardless of
>  length of attribute
>
>  B.    Ext-Len   is   clear  (0),  1-byte  length  field  is  uses,
>  and attribute is <= 255 bytes
>
>  Furthermore, I've      seen      loose      language,      such     as
>  draft-kumaki-pce-bgp-disco-attribute-00.txt  that  seems  to  imply to
>  always set Ext-Len bit regardless.
>
>  Specifically,   is  anyone  aware  of  any  new  BGP  extensions  that
>  would REQUIRE  Ext-Len  to  be set, even if attribute is <= 255 bytes?
>  Our  thought  is  that if we receive a case A above with length <= 255
>  bytes,  we  are  free to propogate to downstream neighbors as a case B
>  (generically, for any attribute in BGP)."
>
>  --
>  -m
>
>  ====================================================================
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>  ====================================================================
>
>
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