Hi Byzek,
On Jul 30, 2009, at 8:31 PM, byzek wrote:
It's not about performance; a large percentage of the currently-
deployed
hardware can’t do UDP checksum calculations during encapsulation
because it
doesn’t have access to the entire packet. Most hardware is
streamlined to
only provide the first n bytes of a packet to the forwarding engine,
where
typically n < 128.
This is one of the primary reasons, as I understood it, why UDP-Lite
was defined the way it was -- to allow for a transport-layer checksum,
including an IP pseudo-header checksum, in IPv6, without the need to
traverse the entire packet. In the LISP case, all of the bytes that
would need to be "checksummed" for UDP-Lite would be bytes that were
_added to the packet_ by the LISP encapsulating router.
When an IPv4/UDP header is pre-pended by a LISP router, the lisp ETR
needs to calculate the IP header checksum over 20 bytes (the IP header).
If an IPv6/UDP-Lite header were pre-pended by a LISP router, the ETR
would need to calculate an IP header checksum over 48 bytes (the IP
pseudo-header and the UDP header).
While I understand that 48 bytes is larger than 20 bytes, we aren't
really talking about a major difference when the checksum algorithm is
well-optimized and all of the pre-pended header bytes are already in
memory.
Margaret
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