Transcript Lecture 18: Layer 3 -

CS 453
Computer Networks
Lecture 18
Introduction to Layer 3
Network Layer
Network Layer
So far, our communications has assumed
that everything is point to point…
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We have a message at A and we send it to B
We had, in layers 1 and 2, channels that
gave us a “connection” between A and B
Network Layer
Granted, we discussed a lot of media and
protocols to make these connections…
…and we had hubs and switches in the
middle…
But the intervening media (and hubs and
switches) to took care of path…
And made it seem like a “wire”
Network Layer
Well, as you probably have guessed, its
usually not that simple
Once we push our networking out past a
LAN we have a new realm of
complications
Internetworking LANs or even stations is
the domain of the network layer
Network Layer
Stations connect to networks, networks
connect to other network, which connect to
other networks,….
Networks take on complex topologies
Fundamentally, the job of the network
layer is to take messages from the
transport layer and create packets for the
data link layer
It must do this in the context of the
topology of the network
Network Layer
Application
Layer
Transport
Layer
Network Layer
Data Link
Layer
From: http://en.wikipedia.org/wiki/Tcp/ip
Network Layer
From: http://en.wikipedia.org/wiki/Tcp/ip
Network Layer
Network layer services
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Independent of router technology
Network topology, technology should be
transparent to the Transport layer
Addressing scheme should be uniform
Two classes
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Connectionless Service
Connection-oriented Service
Network Layer
Connectionless Service
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Strongly advocated by the Internet community
Underlying principle of the Internet
That the Network layer should
Send Packets
Receive Packets
And that’s about it
Network Layer
Connectionless Service
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Packets are fed into the network
Network must route packets to destination
Packets are routed to destination
independently
Packets transmitted in connectionless context
are often referred to as datagrams
A connectionless network or portion thereof,
might be called a datagram subnet
Network Layer
Connectionless Service
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Think of a datagram like a postal letter
You drop it in the mailbox and it arrives at its
destination
It could have taken any of several destinations to get
there,…
But figuring that out is not your problem
Imagine sending several letters to the same
destination
They all arrive, but…
They may have taken different routes to get there
Network Layer
Connectionless Service
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Imagine that the transport layer (a process) in host H1
has a message to send to host H2, and,…
… it is a big message – 4 times the max packet size
The network layer takes the message and breaks into
4 packets, and sends it…
Network Layer
Connectionless Service
From: Tanenbaum, 2003, pg. 346
Network Layer
Connectionless Service
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Note: each packet can take a different route
Route decisions are made by the routers…
On packet by packet basis
Route decisions are made based on route tables
located in each route
Route table does not tell exact how to get to the
destination, but which direction (which router port, or
connection link)
Network Layer
Connectionless Service
From: Tanenbaum, 2003, pg. 346
Each router has its own
table
Each table entry says for a
given destination address –
send the packet out on this
port
So, in router A, a packet with
a destination of F is sent to
port C (like to router C) .. At
least initially
Router C gets the packet,
see that it is destined for F
sends it out on port E
Notice that router A’s route
table changes so that
packets headed for F get
sent to B (rather than C)
Network Layer
Connectionless Service
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Routers can change their route tables
Routers’s job to maximize the flow of packets
May change route tables because
Link down
Congested link
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Router decisions and route table updates are
Routing Algorithms
Routing Algorithms - complex topic
We will look at Routing Algorithms in more detail
Network Layer
Connectionless Service
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By the way, this is the way IP routing works
Network Layer
Connection-Oriented Service
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Route through network established before packets
flow
Fixed route
Once defined all packets flow through the same route
Referred to as a Virtual Circuit
When a sending host wants to send a packet…
The network first sends a setup packet
The setup packet finds a path and …
Make forwarding table entries along the path
Network Layer
Connection-Oriented Service
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Virtual Circuit has a connection ID
Once a packet is assigned a connection ID (a label) it
no longer needs a destination address
Packet follows the virtual circuit, …
…not routed by destination address
Sort of like a phone call – once you are talking the
other person, you don’t need their phone number to
continue the conversation
When the packet stream is finished, the connection is
released, and ….
The virtual circuit is torn-down
Network Layer
Connection-Oriented Service
From: Tanenbaum (2003), pg 348
Network Layer
Connection-Oriented Service
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Once the circuit is know the virtual circuit label is
added to the packet
Virtual circuit labels are per hop
…that is, the label in a packet can be changed by the
routers along the route
Network Layer
Connection-Oriented Service
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In the previous diagram, H1, then H3 initiate the flow
of packets.
H1 establishes VC 1, so it VC label is 1
H3 starts a flow of packets,
…and since it is coorinating with H1, it assigns the VC
with label 1…
But Router A recognizes that VC 1 belongs to another
packet flow,…so
It renames the VC as 2,
From A to the destination then that VC is VC 2…
Network Layer
Connection-Oriented Service
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So, at A the router removes the VC label from the
connection Id field of the packet header, and…
…replaces it the new VC number (2, in this example)
Replacing a label in route is called “label switch”
We will talk about this more later
Network Layer
Network Layer
So, how do they compare
 Connectless vs. Connection-Oriented Service
 Datagrams vs. Virtual Circuits
Datagrams must carry full source and
destination addresses
VC packets only carry the VC Label
Datagram addresses must be parse and
translated at each hop
VC packet label simply used as a lookup
Network Layer
So, how do they compare
 Connectionless service requires no setup
Send a datagram and the routers will help it find its
way
No pretransmission overhead
Overhead per packet
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Connection-Oriented Services requires the
VC to be setup before packet flow
Setup packet sent first to establish VC
For short transactions – a packet or two –
connection-oriented service very expensive
Network Layer
So, how do they compare
Resilency to failure - router failure, line failure
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Connectionless – good, basically doesn’t care
Not a problem – routers find a different route
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Connection Oriented – big problem
VC torn down, packet flow lost
New VC must be established be transmission restarted
Network Layer
So, how do they compare
Quality of Service
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Connectionless Service
None, difficult, maybe
Connectionless – best available
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Connection-Oriented Service
Good, easy to establish
Resources can be reserved when VC is setup
Network Layer
So, how do they compare
Traffic load, congestion handling
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Connectionless Service
Not much, sort of
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Connection-Oriented
Easy, reliable
Reserve resources (bandwidth) at VC setup