Question

A host launches three IP packets that are 1280 bytes, 1280 bytes and 394 bytes. The MTU of the directly attached network is 1460 bytes, but the next hop has an MTU of 768 bytes. Indicate how the packets will be fragmented and what the values of the Offset field will be for all of the fragments. Please assume that we are presented solely with the payload here and there aren’t any complications with headers.

Answer 1

Fragmentation: When a packet is fragmented, each packet fragment will get a copy of all packet tags, and byte tags will follow the new packet boundaries.

Fragmentation and concatenation

Packets may be fragmented or merged together. For example, to fragment a packet p of 100 bytes into two packets, one containing the first 10 bytes and the other containing the remaining 90, one may call the following code:

Ptr<Packet> fra0 = p->CreateFragment (0, 10);

Ptr<Packet> fra1 = p->CreateFragment (10, 100);

Example XML

<?xml version="1.0" ?>

<FlowMonitor>

<FlowStats>

<Flow flowId="1" timeFirstTxPacket="+0.0ns" timeFirstRxPacket="+20067198.0ns"

,→timeLastTxPacket="+2235764408.0ns" timeLastRxPacket="+2255831606.0ns" delaySum=

,→"+138731526300.0ns" jitterSum="+1849692150.0ns" lastDelay="+20067198.0ns" txBytes=

,→"2149400" rxBytes="2149400" txPackets="3735" rxPackets="3735" lostPackets="0"

,→timesForwarded="7466">

</Flow>

</FlowStats>

<Ipv4FlowClassifier>

<Flow flowId="1" sourceAddress="10.1.3.1" destinationAddress="10.1.2.2" protocol="6

,→" sourcePort="49153" destinationPort="50000" />

</Ipv4FlowClassifier>

<Ipv6FlowClassifier>

</Ipv6FlowClassifier>

<FlowProbes>

<FlowProbe index="0">

<FlowStats flowId="1" packets="3735" bytes="2149400" delayFromFirstProbeSum="+0.

,→0ns" >

</FlowStats>

</FlowProbe>

<FlowProbe index="2">

<FlowStats flowId="1" packets="7466" bytes="2224020" delayFromFirstProbeSum=

,→"+199415389258.0ns" >

</FlowStats>

</FlowProbe>

<FlowProbe index="4">

<FlowStats flowId="1" packets="3735" bytes="2149400" delayFromFirstProbeSum=

,→"+138731526300.0ns" >

</FlowStats>

</FlowProbe>

Example:

4000 byte datagram

MTU =1500 bytes

ð 1480 bytes in data field

ð Offset=1480/8