Capwap Mailing List: Re: Issue #5: Benefits of Using UDP-Lite?

[Abhijit Choudhury (achoudhu) (achoudhu]

Included below is the original email arguing the need for
UDP-lite. Note that in CAPWAP, IP packets are
encapsulated in UDP or UDP-lite in WTPs and ACs, and not at
the source or sink of the data.  TSo, adding the
mandatory requirement for UDP checksum (as needed in IPv6)
imposes an additional burden on the hardware. Note that
for IPv4, typically the UDP checksum will be set to 0.


Thanks,
Abhijit
 
        -----Original Message-----
        From: David Melman [mailto:davidme [at] marvell.com] 
        Sent: Tuesday, March 13, 2007 1:29 AM
        To: capwap [at] frascone.com
        Subject: [Capwap] UDP-Lite as optional transport for CAPWAP

        Hi.

        In supporting CAPWAP-over-UDP-IPv6, I'm concerned about the cost
& performance overhead 
        of supporting a full UDP checksum over the entire UDP payload.

        While UDP over IPv4 allows the UDP checksum to be set 0
indicating the 
        UDP checksum is not calculated, UDP over IPv6 requires the UDP
header to 
        contain a checksum which covers the UDP pseudo header and the
UDP payload 
        (per RFC 2460). Calculating the checksum over the full data
payload adds 
        significant cost and performance overhead, both to the AC switch
(typically 
        implemented in hardware) and the WTP (typically implemented in
software).

        I propose to allow CAPWAP over UDP-Lite (RFC 3828) as an
optional transport 
        to UDP, particularly for the CAPWAP data channel which would be
impacted 
        the most by the requirement to calculate the UDP checksum over
the entire payload. 

        The UDP-Lite header is identical to the UDP header, except for
the UDP Length 
        field which is replaced with the Checksum Coverage field. The
Checksum Coverage 
        field indicates the number of octets, counting from the first
octet of the 
        UDP-Lite header, that are covered by the checksum. To minimize
the performance 
        impact, the UDP Checksum Coverage is set to the minimal value of
8, which limits 
        the UDP checksum to cover just the UDP pseudo-header, excluding
the UDP payload.  
        (Note that the UDP Length field is not required, as it can
easily be calculated 
        by subtracting 8 (UDP-Lite header length) from the IP Length
field.)

        UDP-Lite (RFC2460) is widely available in IPv6 stacks and in all
the common 
        operating systems we surveyed.

        I believe this change would have minimal impact to the CAPWAP
protocol and 
        has significant value to both hardware and software
implementations.

        Regards,
        David
 
        __________________________________________

        David Melman
        Architecture group - Marvell
        Email - DavidMe [at] marvell.com
 

-----Original Message-----
From: Margaret Wasserman [mailto:margaret [at] thingmagic.com] 
Sent: Wednesday, July 18, 2007 7:44 AM
To: capwap
Subject: [Capwap] Issue #5: Benefits of Using UDP-Lite?

Magnus Westerlund raised some questions regarding why we are using
UDP-Lite in order to turn off UDP checksums on most of the CAPWAP
packet.  These questions need to be discussed and answered by the WG.

You have had the discussion. Can someone please recap what arguments was
used to indicate that you would have better performance by not using the
UDP checksum over the payload? I was under the impression that unless
your capwap boxes are running very special hardware the UDP checksuming
was done in hardware on the network controler. Thus there is no cost for
doing the checksum. And it would save the DTLS processing in the case
the packet has a bad checksum.

I am trying to understand your argumentation here, not necessarily
change it.

The issue was also raised that the reason to use UDP-Lite is for
protocols that actually want to receive partial/corrupted packets.  That
doesn't seem to apply to CAPWAP, so what is our reason for using it?
_________________________________________________________________
To unsubscribe or modify your subscription options, please visit:
http://lists.frascone.com/mailman/listinfo/capwap

Archives: http://lists.frascone.com/pipermail/capwap