2. Internetwork
- Logical network built out of a collection of physical
networks to provide some host to host packet delivery
service.
Router – the node that interconnects the network.
Router is also called as gateway.
3.
4. Internet Protocol
• Key tool used to build scalable, heterogeneous internetworks.
• Also called as Kahn – Cerf protocol.
Service model,
o Provide host to host services.
o Two parts,
- Addressing Scheme – to identify all the hosts in the internetwork.
- Datagram Model – for data delivery
6. Internet Protocol
Also called as best effort model.
Best effort model,
It makes no guarantee for datagram delivery.
If packets gets lost, corrupted, misdelivered or it fails to reach
intended destination, it does nothing.
Unreliable service.
8. Internet Protocol
• Version (4 bit) – Version - version of the Internet Protocol.
• HLen (4 bit) – Header Length – specifies the length of the header in 32 bit
words. (Normally it contains value 5 i.e., 5 words – 20 Bytes).
• TOS (8 bit) – Type of Service–defines type of service that allows packets to be
treated differently based on the application need. (i.e., priority to packets)
• Length (8 bit) –length of the datagram including the header in bytes.
(Maximum value is 65,525 bytes).
9. Internet Protocol
• Identifier, flag, offset – used in fragmentation.
• Flag:
- DF- do not fragment bit. If this bit is 1, we cannot fragment that packet.
- M – More fragment bit. This bit is one when more fragments to follow.
• TTL (8 bit) –Time to live–number of seconds that the packet would be
allowed to live (default value is 64)
Reserved DF LF or M
10. Internet Protocol
• Protocol (8 bit) –demultiplexing key for higher level layer protocols.
• TCP – 6
• UDP – 17
• Checksum (16 bit) – used to detect errors in header.
- Checksum is calculated by considering the entire header as a sequence of 16 bit
words, adding them and taking the ones compliment of the result
• SourceAddr, DestAddr (32 bit each)–IP address of the source and destination
host.
11. Internet Protocol – Fragmentation and
Reassembly
Fragmentation and Reassembly,
• In a heterogeneous collection of networks, each network has its own
capacity to receive a packet called Maximum Transmission Unit (MTU).
• MTU – largest IP datagram that it can carry in a frame.
• Ethernet can accept up to 1500 bytes long packet. For FDDI, it is 4500
bytes long and for PPP 512 bytes long.
• Fragmentation occurs in a router when it receives a datagram that it wants
to forward over a network which has MTU <IP datagram size
12. Internet Protocol – Fragmentation and
Reassembly
• Reassembly is done at the receiving host
• All the fragments carry the same identifier in the Ident field
• IP does not recover any missing fragments.
13. Internet Protocol – Fragmentation and
Reassembly
Two important points,
• Each fragment is itself a self-contained IP datagram that is transmitted over
a sequence of physical networks, independent of other fragments.
• Each IP datagram is re-encapsulated for each physical network over which
it travels.
15. Internet Protocol – Fragmentation and
Reassembly
In the above mentioned model, fragmentation is occurred in R2 (router 2).
• When the packet (1400 bytes of data) arrives at R2, which has an MTU of 532
bytes, so it has to be fragmented.
• 532 bytes MTU can hold 512 bytes of data + 20 bytes of IP header.
• IP header fields used in fragmentation,
• Flags,
Reserved DF LF or M
lf (last fragment) or M (More fragment) bit – set when more fragments to follow.
16. Internet Protocol – Fragmentation and
Reassembly
• Offset – used to identify the starting byte of the fragment.
- In first fragment it sets to 0.
- Offset field counts 8 bytes.
- In our example, offset field in the second fragment is 64 i.e., 512 / 8.
• Ident (Identifier) – used to identify the packet to which the fragments
are belongs to.
