Prof Messer Net+ 1.1 Data Communication

What is a PDU?

Protocol Data Unit

What is the purpose of a PDU

How we get data from one part of network to another
Aka transmission units

What is the PDU for Ethernet, IP, TCP, and UDP?

Ethernet = frames
IP/L3 = packets
TCP = TCP Segment
UDP = UDP Datagram

Go through the layers and draw what the PDUs look like

L7 L6 L5 [App Data]
L4 [TCP Header[App Data]
L3 [IP Header[TCP[App Data]
L2 [Frame Header[IP[TCP[App Data]Frame Trailer]

Explain the flow of encapsulation and decapsulation

Source will encapsulate data from L7 --> L2, data is then sent via L1 signaling, Destination will decapsulate from L2 --> L7

What do headers do?

Let the computer know how data should be processed

Explain headers and flags

The header describes or ID's the payload
TCP header contains important control info
-Includes set of bits called TCP Flags

Flags control the payload
-Ex Syn sych sequence #, PSH push the data to the app w/out buffering, RST reset the connection, FIN last packet from the sender

Flags are a subset of the information found in headers

What is an MTU

Maximum Transmission Unit

What do the flags of an IP Header involve?

Most flags @ IP Header involve fragmentation of data

What is fragmentation?

Data structure of the network may mandate data be fragmented

Fragmentation can slow things down
-Losing a fragment loses an entire packet
-Requires overhead

What happens to throughput as fragmentation decreases?

Throughput will increase

How are MTUs related to Fragmentation?

MTU is maximum size of a packet that a network can transmit
Fragmentation occurs when packet is too large for network's MTU so the packet gets broken into smaller fragments for transmission
Receiving end reassembles these fragments to reconstruct the original packet
MTU is crucial to avoid fragmentation issues in network communication

What is maximum IP packet size?

1500 bytes

What multiples are fragments in when it comes to size and why?

Fragments are always in multiples of 8 because the # of fragmentation offset bits in the IP header

Why is MTU such a serious consideration when it comes to tunnels?

MTUs are a serious concern for tunneled traffic because the tunnel may be smaller than the local ethernet segment

What happens if you send packets with the "Don't Fragment" flag set?

The router may respond back and tell you to fragment, hopefully you will get an ICMP message indicating it wasn't set

What command can you run to ping without fragmentation and with a specified length?
Windows and Linux

Windows: ping -f -l <size> <IP>
Linux: ping -D -s <size> <IP>

How large is an ICMP header?

8 bytes

Why might it be difficult to know the MTU all the way through a path?

Different hops/network segments could be using different MTUs

Automated methods can be inaccurate, especially because ICMP is sometimes filtered

How large are IP Headers?
How large are TCP Headers?
How large are ICMP Headers?

Both IP and TCP are 20 bytes
An ICMP header is 8 bytes

How large can an Ethernet frame be and what is the maximum payload size of that Ethernet frame?

Ethernet frame can be 1518 bytes
But 18 of those bytes are the header, so the maximum payload size is 1500 bytes

What is in an Ethernet packet? Include sizes of sections

Ethernet packet is broken down into the preamble + SFD (start frame delimiter) followed by the ethernet frame
[preamble | SFD total 8 bytes][Ethernet Frame]

Within the Ethernet Frame
[Ethernet Header which is the Destination and Source MACs, each MAC is 6 bytes]

Full ethernet packet
[Preamble | SFD 8 bytes] [Dest MAC 6 bytes | Src MAC 6 bytes] [Type 2 bytes] [Payload 46-1500 Bytes] FCS 4 bytes][Inter Frame Gap (size not relevant)]

Given that an ethernet payload can only be up to 1500 bytes, what size ping would you send to determine the MTU? Include the Windows and Linux commands

Ethernet payload is 1500 bytes, the IP info will be within that payload.
Given that an ICMP header is 8 bytes, IP header is 20 bytes, leaves 1472 maximum MTU to ping on an ethernet network

Windows: ping -f -l 1472 <IP>
-f being do not fragment, -l being length

Linux: ping -D -s 1472 <IP>
-D being do not fragment, -s size

What is a MTU

The largest size of data packet that can be sent in a single network layer transaction.