Subject: IPv6-Link local address

From: "kernel learner" <[email-masked]>
Date: Thu, 2 Nov 2006 15:54:08 +0530

Hi all,

In IPv6 whenever a NS packet is received,
and if the target address is link local, then second 16 bits from lsb side
are fileed with interface index. and while sending NA as reply those bits in
target address are made zero. Why is it done?

Acc. to rfc, those 16bits are part of 64 bit interface identifier. why some
bits are removed like this? and they are not considered in checksum
calculation also. why?

Thanks n Regards
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From: David Malone <[email-masked]>
Date: Thu, 2 Nov 2006 13:51:10 +0000

> bits are removed like this? and they are not considered in checksum

How are you observing these packets? On some implementations the
interface identifier is written into unused bits of link-local
addresses. However these bits should be cleared before the packet
is transmitted, and so are not present when the checksum is calculated.

If you observe these packets in the IPv6 stack, you may see these
extra bits set. If you observe them after they have left the IPv6
stack (say with tcpdump, which would usually see packets at the
driver level) then you should not see these bits set. I've included
to packets grabbed with tcpdump below.

[This list may not be the best place to ask this question - it sounds
like you are asking about a particular IPv6 implementation and a list
relating to that implementation may be a better choice.]

	David.

13:41:46.897988 IP6 fe80::213:72ff:fe50:32d5 > fe80::20d:56ff:fe22:320c: ICMP6, neighbor solicitation, who has fe80::20d:56ff:fe22:320c, length 32
        0x0000:  6000 0000 0020 3aff fe80 0000 0000 0000  `.....:.........
        0x0010:  0213 72ff fe50 32d5 fe80 0000 0000 0000  ..r..P2.........
        0x0020:  020d 56ff fe22 320c 8700 1e39 0000 0000  ..V.."2....9....
        0x0030:  fe80 0000 0000 0000 020d 56ff fe22 320c  ..........V.."2.
        0x0040:  0101 0013 7250 32d5                      ....rP2.
13:41:46.898077 IP6 fe80::20d:56ff:fe22:320c > fe80::213:72ff:fe50:32d5: ICMP6, neighbor advertisment, tgt is fe80::20d:56ff:fe22:320c, length 24
        0x0000:  6000 0000 0018 3aff fe80 0000 0000 0000  `.....:.........
        0x0010:  020d 56ff fe22 320c fe80 0000 0000 0000  ..V.."2.........
        0x0020:  0213 72ff fe50 32d5 8800 837a 4000 0000  ..r..P2....z@...
        0x0030:  fe80 0000 0000 0000 020d 56ff fe22 320c  ..........V.."2.