Lectures notes for 2010 - KTH

• on a BSDI system:
• each of the additional (5) fragments caused an ARP request to be generated
– this violates the Host Requirements RFC - which tries to prevent ARP flooding by limiting
the maximum rate to 1 per second
• when the ARP reply is received the last fragment is sent
– Host Requirements RFC says that ARP should save at least one packet and this should
be the latest packet
• unexplained anomaly: the System Vr4 system sent 7 ARP replies back!
• no ICMP “time exceeded during reassembly” message is sent
– BSD derived systems - never generate this error!
It does set the timer internally and discard the fragments, but never sends an ICMP error.
– fragment 0 (which contains the UDP header) was not received - so there is no way to
know which process sent the fragment; thus unless fragment 0 is received - you are not
required to send an ICMP “time exceeded during reassembly” error.
Not just a fluke (i.e., a rare event)
• The same error occurs even if you don’t have fragmentation - simply
sending multiple UDP datagrams rapidly when there is no ARP entry is
sufficient!
• NFS sends UDP datagrams whose length just exceeds 8192 bytes
• NFS will timeout and resend
• however, there will always be this behavior - if the ARP cache has no entry for this
destination!
Maguire Interaction between UDP and ARP 4: 13 of 74
maguire@kth.se 2010.03.21 Internetworking/Internetteknik