- The IP protocol software
must privide
the following
(minimal) services:
- datagram packet delivery
- An IP node must
forward
an IP packet towards
the destination node
- The delivery service is unreliable (datagram)
- An IP node must
forward
an IP packet towards
the destination node
- Fragmentation and
Re-assembly
- Fragmentation:
- One (large) IP packet
can be split up
into
many (smaller) IP packets
(and each smaller IP packet is transmitted separately)
- One (large) IP packet
can be split up
into
many (smaller) IP packets
- Re-assembly:
- The smaller packets that are parts of a larger packet are put back together at the destination node to form the original packet.
- Fragmentation:
- datagram packet delivery
- Recall:
reliable communication
service:
-
A series of messages is
delivered
reliably
if:
- Each message delivered exactly once
- Each message delivered in the order sent
-
A series of messages is
delivered
reliably
if:
- The datagram delivery service:
- is unreliable
I.e.: it can violate both qualities of reliable communication !!!
- What can happen in
unreliable (datagram) delivery:
- A message (IP packet)
can be lost
(i.e., not delivered):
- A message (IP packet)
can even be
delivered multiple times:
- Messages (IP packets) can be delivered
out of order:
- And
any combination of the above
errors:
- In this example, the second packet is lossed, the first packet is delivered multiple times and packets are delivered out of order...
- A message (IP packet)
can be lost
(i.e., not delivered):
- Observation:
- The
datagram delivery service
is a
very simple delivery service
- It is so simple that most networks (even pigeons) can provide this capability
The requirement is set deliberately to such a low level, so that:
- all existing (and possibly future) networks can connected to the Internet
- The
datagram delivery service
is a
very simple delivery service