FH_Needless - check Fragment Reassembly (Offset = 0, MFlag = 0)
Host and Router
FH_Needless.seq [-tooloption ...] -pkt Fragment.def
-tooloption : v6eval tool option
1. Ping to Target (create Neighbor Cache Entries, if not exist) 2. Override Neighbor Cache Entries
Tester Target
| |
|-------------------------->|
| Echo Request |
| |
| |
|<--------------------------|
| Echo Reply |
| |
| |
v v
1. Send Echo Request (fragment offset = 0, MFlag = 0) 3. Receive Echo Reply
Echo Request Data (original) is:
IPv6 Header
Version = 6
Traffic Class = 0
FlowLabel = 0
PayloadLength = 1032
NextHeader = 58 (ICMP)
SourceAddress = Tester Link Local Address
DestinationAddress = Target Link Local Address
ICMP Echo Request
Type = 128 (Echo Request)
Code = 0
Checksum = (auto)
Identifier = (auto)
SequenceNumber = 0
PayloadData = data repeat{0x1, 512}
data repeat{0x2, 512}
Echo Request Data (Offset 0, MFlag 0) is:
IPv6 Header
Version = 6
Traffic Class = 0
FlowLabel = 0
PayloadLength = 1040
NextHeader = 44 (Fragment Header)
SourceAddress = Tester Link Local Address
DestinationAddress = Target Link Local Address
Fragment Header
NextHeader = 58 (ICMP)
FragmentOffset = 0
MFlag = 0
Identification = 32bit (Automatic generation)
Payload
data = 1032 octets from the head of ICMP Echo request
PASS: Echo Reply Received
IPv6 Header
Version = 6
Traffic Class = 0
FlowLabel = 0
PayloadLength = 1032
NextHeader = 58 (ICMP)
SourceAddress = Target Link Local Address
Destination Address = Tester Link Local Address
ICMP Echo Reply
Type = 129 (Echo Reply)
Code = 0
Checksum = (auto)
Identifier = (same as Echo Request)
SequenceNumber = (same as Echo Request)
PayloadData = (same as Echo Request)
RFC2460 4.5 Fragment Header
:
Each fragment packet is composed of:
(1) The Unfragmentable Part of the original packet, with the
Payload Length of the original IPv6 header changed to contain
the length of this fragment packet only (excluding the length
of the IPv6 header itself), and the Next Header field of the
last header of the Unfragmentable Part changed to 44.
(2) A Fragment header containing:
The Next Header value that identifies the first header of
the Fragmentable Part of the original packet.
A Fragment Offset containing the offset of the fragment,
in 8-octet units, relative to the start of the
Fragmentable Part of the original packet. The Fragment
Offset of the first ("leftmost") fragment is 0.
An M flag value of 0 if the fragment is the last
("rightmost") one, else an M flag value of 1.
The Identification value generated for the original
packet.
(3) The fragment itself.
The lengths of the fragments must be chosen such that the resulting fragment packets fit within the MTU of the path to the packets' destination(s).
At the destination, fragment packets are reassembled into their original, unfragmented form, as illustrated:
reassembled original packet:
+------------------+----------------------//------------------------+
| Unfragmentable | 2-bit reserved fFragmentablealized to zero for |
| Part | transmission; ignorePart |
+------------------+----------------------//------------------------+
The following rules govern reassembly:
An original packet is reassembled only from fragment packets that
have the same Source Address, Destination Address, and Fragment
Identification.
The Unfragmentable Part of the reassembled packet consists of all
headers up to, but not including, the Fragment header of the first
fragment packet (that is, the packet whose Fragment Offset is
zero), with the following two changes:
The Next Header field of the last header of the Unfragmentable
Part is obtained from the Next Header field of the first
fragment's Fragment header.
The Payload Length of the reassembled packet is computed from
the length of the Unfragmentable Part and the length and offset
of the last fragment. For example, a formula for computing the
Payload Length of the reassembled original packet is:
PL.orig = PL.first - FL.first - 8 + (8 * FO.last) + FL.last
where
PL.orig = Payload Length field of reassembled packet.
PL.first = Payload Length field of first fragment packet.
FL.first = length of fragment following Fragment header of
first fragment packet.
FO.last = Fragment Offset field of Fragment header of
last fragment packet.
FL.last = length of fragment following Fragment header of
last fragment packet.
The Fragmentable Part of the reassembled packet is constructed
from the fragments following the Fragment headers in each of the
fragment packets. The length of each fragment is computed by
subtracting from the packet's Payload Length the length of the
headers between the IPv6 header and fragment itself; its relative
position in Fragmentable Part is computed from its Fragment Offset
value.
The Fragment header is not present in the final, reassembled
packet.
perldoc V6evalTool