Subject: Re: Ospf Packet checksum

From: Don Goodspeed <[email-masked]>
Date: Thu, 6 Mar 2003 15:09:52 -0500

Ajay,

I'm not a checksum calculation expert (there are others on this
list who are, and can explain this further), but as I understand
it, a Fletcher-computed checksum such as what OSPF uses, cannot
result in a zero value 0x0000.

The only time that OSPF does use a zero checksum value is when
message-digest (MD5) authentication is used.  There's a blurb
on this in the appendix (E, I believe) in RFC2328.

-don

 --- On Thu 03/06, Ajay Virginkar < [email-masked] > wrote:
From: Ajay Virginkar [mailto: [email-masked]]
To: [email-masked]
Date: Thu, 6 Mar 2003 10:47:36 -0800
Subject: Ospf Packet checksum

Hi,
I have an issue where a router (from vendor X) complains of receiving
checksum FFFF and expects a zero. I checked the OSPF mailing list archives
but could not come to any conclusions from the previous discussion regarding
this.
Both sides are configured with no authentication. The question I guess is
that, is it valid to send a checksum of 0 in the OSPF packet header. It
seems that in the IP headers, a checksum of 0 is considered invalid. Does
OSPF permit checksum of value 0?
Thanks
Ajay

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---

From: Ajay Virginkar <[email-masked]>
Date: Thu, 6 Mar 2003 14:03:32 -0800

Fletcher's algo is used for LS Checksum. I think for the ospf packet
checksum, we use the standard IP checksum mechanism.
There is a reference to this special case for checksum values in RFC 1624
which throws some light on this issue.
Ajay

---

From: Acee Lindem <[email-masked]>
Date: Thu, 6 Mar 2003 17:20:41 -0500

Hi Don,

One correction.

> result in a zero value 0x0000.

While LSAs use a fletcher checksum, OSPF packets use the internet
checksum originally described in RFC 1071. I don't remember enough about
the properties of the inet checksum to say for sure that it couldn't
result in a value of 0x0000. Perhaps someone else could comment.

--
Acee