Download The Network Layer

Chapter 5
The Network Layer
2010
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Network Layer Task
•getting packets from the source all the way to the destination
•may require many hops through intermediate routers.
•This contrasts with the data link layer, which just moves
frames from one end of a wire to another.
•it must know about the topology of the communication subnet (
the set of all routers) and choose appropriate paths through it.
•It must take care to choose routers to avoid overloading some of
the lines and routers while leaving others idle.
•When source and destination are in different networks, it has to
deal with the differences.
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Services to transport layer
Goals:
1.The services should be independent of the router technology
2.The transport layer should be shielded from the number, type
and topology of the subnets present.
3.The network addresses made available to the transport layer
should use a uniform numbering plan across LAN’s and WAN’s
The Internet community argues that a subnet is inherently unreliable,
the hosts should do error control and flow control. The service should
thus be connectionless, but as reliable as possible, and most of the
complexity is placed on the hosts.
The telephone companies argue that the subnet should provide a
reliable, connection-oriented service, placing the complexity in their
subnets.
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Implementation of Connectionless Service
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Implementation of Connection-Oriented Service
A route from source to destination is chosen as part of the connection
setup. Such a route is called a virtual circuit (VC). Each router along the
path puts an entry in a table, linking a VC to an outgoing line.
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Comparison of Virtual-Circuit and
Datagram Subnets
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Flooding
A simple static algorithm is flooding, in which every incoming packet
is sent out on every outgoing line except the one it arrived on.
It generates a vast number of duplicate packets, an infinite number
unless some measures are taken to damp the process. E.g. a hop
counter in the header of each packet, which is decremented at each
hop, and the packet is discarded when the counter reaches 0.
In selective flooding the packets are only sent out on those lines that
are going approximately in the right direction.
Flooding might be usable in military applications, large numbers of
routers may be blown to pieces at any instant, as it is very robust.
Also during initialization of routers.
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Shortest Path Routing
•Subnet as an undirected
graph
•node: a router
•arc: a communication link
•labeled with a length.
Dijkstra's (or another) algorithm is used to compute the path with the
shortest length between any two nodes.
In general the labels on the arcs can be computed as a function of
distance, bandwidth, average traffic, communication costs, mean
queue length, measured delay, etc.
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Distance Vector Routing
A routing table in each
router contains for each
router the preferred
outgoing line for that
router and an estimate
for the “cost” to that
destination.
The cost metric might be number of hops, queue length, time delay,
etc. Time delay is measured by periodically sending ECHO packets.
Once every T msec each router sends to its neighbors a list of
estimated
“costs” to each destination.
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Link State Routing
Distance vector routing reacts slowly on bad news, e.g. break down
of a link (count to infinity problem). The core of the problem is
that when X tells Y that it has a path somewhere, Y has no way
of knowing whether it itself is on the path.
Link State Routing: each router sends costs to neighbors to all other
routers.
Each router must:
1. Discover its neighbors, learn their network address.
2. Measure the delay or cost to each of its neighbors.
3. Construct a packet telling all it has just learned.
4. Send this packet to all other routers.
5. Compute the shortest path to every other router.
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Link State Packets
The trickiest part is distributing the link state packages reliably, to
assure that each router has basically the same view of the subnet.
A 32 bit sequence number (sufficient for 137 years, if it is updated
every second) is used.
An age field is decremented every second and at every send and the
packet is discarded if the age reaches 0.
All link state packets are acknowledged.
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Hierarchical Routing
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Congestion
When too many packets are
present, buffers get full,
packets are discarded, more
retransmissions and less
packets delivered.
Congestion thus tends to
feed upon itself and
become worse, leading to
collapse of the system.
The reason congestion and flow control are often confused is that some
congestion control algorithm operate by sending messages back to
various sources, telling them to "slow down". Thus a host can get a
"slow down" message either because the receiver on the direct link
cannot handle the load or because the network cannot handle it.
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Quality of Service Requirements
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Jitter Control
For applications such as audio and video streaming, it does not matter
much if the packets take 20 or 30 msec to be delivered, as long as the
transit time is constant. The jitter should be small.
In some applications, like video on demand, jitter can be compensated
for by buffering at the receiver. For others, like Internet telephony or
videoconferencing, the delay inherent in buffering is not acceptable.
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Quality of Service
1. Constant bit rate (e.g. telephony), attempts to simulate a wire,
providing uniform bandwidth and delay.
2. Variable bit rate (e.g. compressed videoconferencing), images must
arrive in time independent on how much they could be compressed.
3. Non-real-time variable bit rate (e.g. watching a movie over
internet), a lot of buffering at the receiver is allowed.
4. Available bit rate (e.g. file transfer), not sensitive to jitter or delay.
Not present in original Internet, becomes more and more important.
More or less provided by “sufficient bandwidth”
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Fragmentation
Transparent and non-transparent fragmentation.
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The IPv4 Protocol
The IHL field tells how long the header is, in 32 bit words.
The Type of Service field contains a 3 bit Precedence field, used for
the priority from 0 (normal) to 7 (network control packet), and 3 flags
Delay, Throughput and Reliability, to specify what is most important
for the packet. In practice, current routers mostly ignore the TOS field.
The situation is changing.
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Some options for IPv4
The Time to Live field is a counter to limit packet lifetimes, it must be
decremented at each hop. The packet is discarded when TOL hits 0.
The Protocol field tells the receiving host which transport process
(TCP, UDP or other) the packet should be given to.
The Header checksum verifies the header only, useful for detecting
errors by bad memory bytes or corrupted software inside a router. It
must be recomputed at each hop, because the TTL changes.
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IP Addresses
The class A, B resp. C formats allow for 126, 16382 resp. 2 million
networks with 16 million, 64K resp. 254 hosts.
Network addresses were given to organizations, leading to many
unused host numbers.
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Special IP Addresses
IP addresses of the form 10.x.y.z (and other one) are intended for use
within a LAN (company or home nowadays). They are not intended to
go on the public internet.
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CIDR – Classless InterDomain Routing
Class A and B networks were given out, Class C were too small.
A basic idea is to allocate the remaining class C networks (more than 2
million, and later A and B) in variable sized blocks of 254 addresses, a
site needing 8000 addresses then gets 32 contiguous class C networks.
The world was divided up into 4 zones to easy hierarchical routing. A
site outside Europe, that gets a packet destinated for 194... or 195... can
just send it to its standard European gateway.
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NAT – Network Address Translation
Dirty trick!
NAT makes the IP network in fact connection-oriented as it maintains
information on each connection passing through it. A crash of the NAT
box terminates every TCP connection.
Some protocols send IP numbers (and port numbers) in data, to be used
by the other side. They have been adapted or other ways are used.
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Internet Control Message Protocol
When something unexpected occurs in a router or host, this event is
reported by ICMP. The most important messages are in the table.
It is also used by routers to test the internet or to obtain information to
be use in routing decisions.
Each ICMP message is encapsulated in an IP packet.
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ARP– The Address Resolution Protocol
IP addresses must be linked to data link layer addresses, like Ethernet
addresses or other types.
With ARP the host broadcast a frame asking who owns a certain IP
address, like E1 asking for 192.31.65.5. Host E2 alone will answer with
a broadcast frame telling its IP and ethernet number.
Entries in the ARP cache time out to allow for hardware changes.
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Dynamic Host Configuration Protocol
If a computer boots ups, what is it IP address?
It could be a fixed number, which is in the computer. But this requires
administrative procedures, which cost time and are error prone.
DHCP (Dynamic Host Configuration Protocol) assigns IP addresses
dynamically.
Older protocols for this are RARP and BOOTP.
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IPv6
The major goals of the new IPv6 protocol were:
•
Support billions of hosts, even with inefficient address space
allocation
•
Reduce the size of the routing tables
•
Simplify the protocol, to allow routers to process packets faster
•
Provide better security (authentication and privacy)
•
Pay more attention to type of service, particularly for real time data
•
Aid multicasting by allowing scopes to be specified
•
Make it possible for a host to roam without changing its address
•
Allow the protocol to evolve in the future
•
Permit the old and the new protocols to coexist for years
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The Main IPv6 Header
Traffic class, is
used to distinguish
between packets
whose sources can
be flow controlled,
values between 0
and 7, or not, values
between 8 and 15.
The flow label is also still experimental but will be used to allow a source and
destination to set up a pseudo-connection with particular properties and requirements.
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Extension Headers
Extension header
Description
Miscellaneous information for routers
Hop-by-hop options Support for datagrams larger than 64K
(jumbograms)
Routing
Full or partial route to follow
Fragmentation
Management of datagram fragments
Similar to IPv4, but only the sending
host can fragment a packet
Authentication
Verification of the sender's identity
Encrypted payload
Information about the encryption
Destination options
Additional information for the destination
The use of jumbograms is important for supercomputer applications that must transfer
gigabytes efficiently across the Internet.
The routing header list up to 24 routers that must be visited on the way to the
destination. Both strict (the full path is supplied) and loose (only selected routers are
supplied) are available, and they can be combined.
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Addresses
Prefix
Usage
Fraction
0000 0000
Reserved, including IPv4
1/256
0000 001
OSI NSAP addresses
1/128
0000 010
Novell IPX addresses
1/128
010
Provider-based addresses
1/8
100
Geographic-based addresses 1/8
1111 1110 10
Link local use addresses
1/1024
1111 1110 11
Site local use addresses
1/1024
1111 1111
Multicast
1/256
other
unassigned
371/512
In addition to multicast,
also anycast is
supported. The
destination is a group of
addresses, but it is tried
to deliver the packet to
just 1 of them, usually
the nearest one. This can
be used for example to
contact a group of
cooperating file servers.
The 16 byte addresses are written as 8 groups of 4 hexadecimal digits with colons
between the groups, leading 0's can be left out and 1 or more groups of 16 0's can be
replaced by a pair of colons.: 8000::123:4567:89AB:CDEF.
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