The MPLS WG Archive

[Giles Heron <giles@goneto.net>]

Hi Jim,

the reason there is a length field in the control word is to handle the
case there one or more of the physical media in the path may have a
minimum frame size.

For example Ethernet has a 64 octet minimum frame size (14 bytes of MAC
header, 46 of data and 4 of FCS.)  If there is less than 46 octets of
data it must be padded to 46 octets.  If, for example, a very small PPP
or FR frame is carried using draft-martini then there will be less than
46 octets of data (even including the MPLS labels and the control
word.)  Since neither MPLS nor Ethernet has a length field the padding
will then be carried on subsequent links in the path (for example if the
next hop is Packet over SONET.)  When the packet arrives at the egress
draft-martini edge device it will need to strip off the padding.  The
only way to do this is using the length field in the control word.

Of course all this isn't a problem with IP over MPLS because IP has a
length field.

Giles

> Jim Loehndorf wrote:
> 
> I have a few questions that I would like some help with.  The
> "draft-martini-l2circuit-encap-mpls-01.txt" specifies that ... "if the
> packet's length is less than 256 bytes, the length field MUST be set
> to the packet's length.  Why?  Is the minimum MPLS packet length fixed
> at 256 bytes?  If not, why would I ever use padding and the length
> field?
> 
> Jim Loehndorf
> 
> ASC

-- 
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Giles Heron      Principal Network Architect      Gone2 Inc.
ph: +44 7880 506185         "if you build it they will yawn"
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