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Function of a Zip Code

What do you think are some of the main functions of a zip code?

Addressing:

Limited geographic area that is efficient to serve by a post office
Address uniquely identifies that area

Mail Routing:

Codes are used within the postal system to forward mail to its next logical facility on the path to its destination

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US Zip Code Zones

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Function of Layer 3

The primary job of Layer 3 is delivering messages between devices – including those on different networks

In most data networks, IP (Internet Protocol) is the primary Layer 3 protocol

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Internet Protocol- Important Functions

Addressing: Mechanism to identify both the network address, as well as a unique host address of a device interface
Routing/Indirect Delivery:

the final destination of a packet is often on a distant network
NIC to NIC – Layer 2 transmission - won’t work in that case
Therefore, the datagram must be delivered indirectly.

Intermediary devices (routers) need to route the packet using IP addresses

Fragmentation and Reassembly:

Layer 1 (Physical) and Layer 2 (Data Link) have limits on maximum frame size
IP includes the ability to fragment IP datagrams into pieces
The receiving device uses the reassembly function to re-create the whole IP datagram.

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IP Protocol Format/Header

So based on the function of IP, what do you think needs to be in the header?

Source and Destination IP Addresses

Fragmentation Info

Next Protocol

Plus some more…

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IP Header

Other Fields of Interest:

Version: IPv4 or v6

Header Length: Multiply by 4 to get byte count

TTL: Time-To-Live…

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Time To Live Field

Why do we need to be concerned about packets roaming forever?

Imagine the Postal Scenario

Letter is sent to an undefined zip code – 00789
Each postal sorting facility decides to send it to their “default” next facility - it could loop through the system forever

The same could happen with an IP packet

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Prevent Packets from Roaming Forever?

TTL in IP Header

Value set when Datagram is created
255 is the max

Deceased by one as it passes each router

If it gets to 0 – router discards

Discarding Router may send an ICMP TTL expired to sender

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TTL in Capture

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Different OS Default TTL

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IP Support Protocols

While IP is the primary Layer 3 protocol:

Some additional protocols operate within Layer 3 to support IP

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�ICMP

ICMP – Internet Control Messaging Protocol

IP Support Protocol
Designed as a health and maintenance protocol
Not intended to carry user data – just system messages

Ping commands typically use ICMP messages

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Internet Control Message Protocol (ICMP)

ICMP contains two message types:

messages used to report errors
messages used to obtain information

Echo Request and Echo Reply obtain information

used by the ping application to test connectivity
When a host receives an echo request message ICMP software on a host or router sends an echo reply that carries the same data as the request

Errors

Time Exceeded and Destination Unreachable report errors when a datagram cannot be delivered successfully
A destination is unreachable if no route exists to the address
A datagram times out if

either the TTL count in the header expires or
fragments of the datagram do not arrive before the timer expires

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Internet Control Message Protocol

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