Simple Internetworking

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• Simple Internetworking

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internetwork n. v.

• 1 logical network built out of a collection of
  physical networks.
• 2 tr. to interconnect physical networks into a
  single logical networks to build an internet.

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Simple internetwork (example)
Question What are the challenges in connecting
two physical networks into a single logical
network?
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Service model
Host-to-host service
Philosophy Define a model that is so undemanding
that almost any network technology is able to
provide the required service. (IP)
Addressing scheme Datagram
Questions What does best-effort mean? How does
it simplify internetworking?
Best-effort
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Data delivery model

• Every datagram contains enough information for
  the network to forward it from source to
  destination.

Question Why is Version on the top of the packet?
Aligned on 32-bit boundaries.
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Data delivery model

• HLen length of header.
• TOS type of service.
• Length length of datagram.
• Questions What is the maximum length of a
  datagram? Will the underlying physical network
  support it?

Aligned on 32-bit boundaries.
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Data delivery model

• TTL time to live (hop count).
• Protocol identity of upper-level protocol to
  which IP passes this packet.
• Checksum applies to header.
• SourceAddr to allow filtering based on source.
• DestinationAddr used for data delivery.
• Options header length determines if any are set.

Aligned on 32-bit boundaries.
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Fragmentation and Reassembly

• Ethernet max. frame length 1500 B.
• FDDI max. frame length 4500 B.

MTU
MTU The largest IP datagram that a network can
carry in a single frame.
Question How does the underlying network
technology affect IP? How does one choose the
maximum datagram size for IP?
Design options (1) Choose IP datagram max.
length to be compatible with MTU. (2) Fragment IP
datagram into multiple MTUs.
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Fragmentation and Reassembly
Gotcha
MTU
header
trailer
Payload IP datagram
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Fragmentation and Reassembly
1. Each fragment is itself a self-contained IP
datagram that is transmitted over a sequence of
physical networks, independent of other
fragments. 2. Each IP datagram is reencapsulated
for each physical network over which it travels.
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Fragmentation and Reassembly
Some nitty-gritty details
If a fragment is lost, the receiver still tries
to assemble the datagram When this fails, it has
to clean up the memory that was used in the
attempt. If hosts can do path MTU discovery,
then IP fragmentation can be avoided altogether.
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Global addresses

• Question What are the challenges?

• Wish List
• Addresses are unique.
• Address structure makes routing/forwarding as
  easy as possible.

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