Download Network Layer

Networking
Network Layer
Networking – Network Layer
• The Network Layer is part of the Internet
Protocol stack
• The Network Layer sits between the Transport
Layer and the Link/Physical Layer
• The Network Layer provides communication
services to the physical hosts and devices in
the network
Networking – Network Layer
• The Transport Layer
(TCP/UDP) only ”lives” in
the end hosts – a router
does not know about
TCP/UDP protocols
• The Network Layer (IP)
”lives” in end hosts and
routers
Networking – Network Layer
• Two main categories of Network-Layer
services
– Network Layer connection-oriented service
(virtual circuits)
– Network Layer connectionless service
(datagrams)
• IP (Internet Protocol) offers connectionless
service, which we will focus on
Networking – Network Layer
• Main properties of IP
– A best-effort service – no
guarantees on bandwidth,
delays, order or integrity…
– Data is transferred through
routing – no central entity is
responsible for transferring
data from Sender to Receiver
Networking – Network Layer
• The general problem of routing:
• Given
– A source
– A destination
– A network that contains at least one path from
source to destination
• Find
– The cheapest path from source to destination
Networking – Network Layer
A destination
The source
Networking – Network Layer
A path
(cost = 12)
The source
A destination
Networking – Network Layer
A destination
The source
A path
(cost = 4)
Networking – Network Layer
• How is an optimal path calculated in practice?
• Global routing algorithms – all information
about the available network is known in
advance, by a central unit
• Decentralised routing algorithms – each
”node” in the network only has knowledge
about its own, local costs
Networking – Network Layer
• Global routing in a network is a very wellknown mathematical problem
• Algorithm is called Link State algorithm (aka
Dijkstra’s algorithm)
• Global state could be learned if all nodes
braodcast their state prior to calculation
Networking – Network Layer
• Link State algorithm: calculate the cheapest
path from a source node A to all other nodes
in the network
• Is an iterative algorithm; it extends the set of
known cheapest paths in each step
Networking – Network Layer
• Terminology
– c(i,j): Cost of link from node i to node j
– D(v): Cost of the cheapest path from A to v that is
currently known
– p(v): Previous node to v along the currently known
cheapest path
– N: The set of nodes for which the cheapest path is
definitely known
Networking – Network Layer
// Initialisation
N = {A}
for (all nodes v)
{
if (v is adjacent to A)
D(v) = c(A,v)
else
D(v) = ∞; // infinity
}
Networking – Network Layer
Node
A
B
C
D
E
F
D(v)
0
2
5
1
∞
∞
Networking – Network Layer
// Iteration
Pick w: (D(w) is minimal) AND (w not in N)
Add w to N
for (all nodes v: (adjacent to w) AND (not in N))
{
if ((D(w) + c(w,v)) < D(v))
{
D(v) = D(w) + c(w,v);
}
}
Networking – Network Layer
Node
A
B
C
D
E
F
D(v)
0
2
5
1
∞
∞
Networking – Network Layer
Node
A
B
C
D
E
F
D(v)
0
2
5
1
∞
∞
Networking – Network Layer
Node
A
B
C
D
E
F
D(v)
0
2
4
1
2
∞
Networking – Network Layer
Node
A
B
C
D
E
F
D(v)
0
2
4
1
2
∞
Networking – Network Layer
Node
A
B
C
D
E
F
D(v)
0
2
3
1
2
4
Networking – Network Layer
// Link State Algorithm
Initialisation;
while (still nodes that are not in A)
{
Iteration;
}
Networking – Network Layer
• When we are done, we have
– The cost of the cheapest path from the source to
any destination
– The cheapest path itself from the source to any
destination
• How did we get the path itself…?
Networking – Network Layer
• Link State algorithm is fast,
and is guaranteed to produce the optimal solution…
• …BUT when do we ever
have global information
available…?
• On the Internet as such,
never…
Networking – Network Layer
• In practice, we will often have to rely on
locally available information
• The Distance Vector algorithm is such an
algorithm
• Main features
– Iterative
– Asynchronous
– Distributed
Networking – Network Layer
• Setup for Distance Vector algorithm
• Each node in the network has a number of
direct neighbours DN
• Each node also knows about a number of
destinations DE
• Each node maintains a distance table
– One row for each member of DE
– One column for each member of DN
Networking – Network Layer
DN1
DE1
DE2
DE3
DE4
…
…
DEn
DN2
…
DNi
Networking – Network Layer
X
D (Y,Z)
• This means…
– How much will it cost for node X…
– …to route something to the destination Y…
– …via the direct neighbour Z
Networking – Network Layer
• If the value of DX(Y,Z) is known for all entries in
the distance table for X, then X would always
know where to route data
• How is the distance table built up?
• How is the distance table maintained?
Networking – Network Layer
X
D (Y,Z)
= c(X,Z) + minw
Ahhhrrhhhggg,
MATH ANGST!
Z
(D (Y,w))
Networking – Network Layer
X
D (Y,Z)
= c(X,Z) + minw
Z
(D (Y,w))
• This means…
– How much will it cost for node X…
– …to route something to the destination Y…
– …via the direct neighbour Z
Networking – Network Layer
X
D (Y,Z)
= c(X,Z) + minw
Z
(D (Y,w))
• This means…
– The direct cost of sending data from X to Z
– X knows this, since Z is a direct neighbour of X
Networking – Network Layer
X
D (Y,Z)
= c(X,Z) + minw
Z
(D (Y,w))
• This means…
– How much will it cost for node Z…
– …to route something to the destination Y…
– …via the direct neighbour w
Networking – Network Layer
X
D (Y,Z)
= c(X,Z) + minw
Z
(D (Y,w))
• This means…
– Find the minimal value of the expression in the
brackets, for all the direct neighbours to Z
Networking – Network Layer
• In other words…
• …if a node knows – or can get – the distance
tables for all its neighbours, it can build up its
own distance table
Networking – Network Layer
DN1
12
10
16
DN2
33
28
12
DE4
…
…
16
18
12
DEn
48
22
36
DE1
DE2
DE3
…
DNi
24
45
18
Networking – Network Layer
DN1
12
10
16
DN2
33
28
12
DE4
…
…
16
18
12
DEn
48
22
36
DE1
DE2
DE3
…
DNi
24
45
18
Networking – Network Layer
// The algorithm runs on each node
// Initialisation
for (all nodes v adjacent to myself (X))
{
DX(*,v) = ∞; // * means ”all rows”
DX(v,v) = C(X,v);
}
for (all destinations y)
{
// w: over all X’s neighbours
send minw(D(y,w)) to each neighbour;
}
Networking – Network Layer
// The algorithm runs on each node
// Loop…forever
wait; // until a message is received
if (message to update cost to all destinations via the neighbour
v by the amount d)
{
for (all destinations y) DX(y,v) = DX(y,v) + d;
}
if (message that shortest path from v to some y has changed)
{
DX(y,v) = c(x,v) + newValue;
}
for (all neighbours y) send(new value of minw(D(y,w)));
Networking – Network Layer
• A slightly more complex
algorithm, but still fairly
few lines of code…
• Can we be sure it will
”settle down”?
• Not really, but will provide
a reasonable ”snapshot” of
the total state at any time
Networking – Network Layer
• The algorithm has some weak spots…
• Good news travels fast – if a link cost is
decreased, the information will quickly spread
• Bad news travels slow - if a link cost is
increased, the information spreads slowly (the
count-to-infinity problem)
• Bad news can cause loops – we can get
peculiar routes like A-B-A-D-E
Networking – Network Layer
• There are certain ”tricks”
available to avoid the
problems
• Poisoned reverse: Injecting
false information into the
network (white lies…)
• Total algorithm not trivial…
Networking – Network Layer
• Practical routing problems
• The internet is too big!
There are billions of
possible destinations!
• We can never use a fully
global routing algorithm!
• We can never create a
complete routing table!
Networking – Network Layer
• In practice, routing is hierarchical
• Routers are divided into ”regions” or so-called
autonomous systems (AS)
• An AS could e.g. be
– A company
– A university
– A geographic region
–…
Networking – Network Layer
• Within an AS, all routers
– Know each other
– Run the same routing algorithm
• This is called the intra-AS routing protocol
• Some routers will also be responsible for
exchanging data with other ASs – these are
called gateway routers
Networking – Network Layer
• Since gateway routers talk other gateway
routers in other ASs, they need to use an
inter-AS routing protocol for this purpose
Networking – Network Layer
• Routing from A (in AS X) to B (in AS Y)
– Route from A to gateway router GX in X, using
intra-AS protocol
– Route from gateway router GX in X to gateway
router GY in Y using inter-AS protocol
– Route from gateway router GY in Y to B, using
intra-AS protocol
Networking – Network Layer
X
Y
GX
GY
A
B
Networking – Network Layer
• Note: still only one
routing table…
• …but certain entries
may be populated in
different ways
Networking – Network Layer
• The actual Network Layer protocol used on
the Internet is called IP (Internet Protocol)
• IP implements a best-effort service – no
guarantees on delivery time, order or delivery
at all…
• Two main variants, IPv4 and IPv6
Networking – Network Layer
• More specifically, IP
provides a connectionless
datagram service
• Data is wrapped into an
”envelope” specifying the
destination address, and
reaches the destination
through local routing
Networking – Network Layer
• Recall that hosts in a network
are identified by means of an
IP address
• More precisely, all hosts and
routers have IP addresses
• Even more generally, all
interfaces have IP addresses
Networking – Network Layer
NOTE: Router
has three
interfaces!
Each interface
has its own IP
address
Networking – Network Layer
• IP addresses are hierarchical – first 8/16/24
bits define a network, the rest an interface
within the network
Network class
identification
Networking – Network Layer
• Host/interfaces are uniquely identified by IP
addresses, so IP addresses must be globally
unique!
• How does a host/interface get an IP address?
– Automatic (DHCP)
– Manual
Networking – Network Layer
• DHCP – Dynamic Host Configuration Protocol
• Your computer queries a DHCP server on the
network, asking for a (temporary) IP address
• The DHCP server chooses a vacant IP address,
and returns it to your computer
• No guarantees on durability
Networking – Network Layer
• Manual IP address
• Somebody (network administrator?) chooses
a vacant IP address from a pool of available IP
addresses – you use it!
• Where does the network administrator get IP
addresses from…?
Networking – Network Layer
• Allocation of IP addresses
is (of course) a regulated
process
• Several regional organs
manage this
– RIPE
– ARIN
– APNIC
Networking – Network Layer
• Lets send some data, using IP (IPv4)
Networking – Network Layer
Administrative
stuff
Networking – Network Layer
Packet maximal
lifetime
Networking – Network Layer
TCP, UDP,…
Networking – Network Layer
Error correction
Networking – Network Layer
Where do I
come from…
Networking – Network Layer
Where am I
going…
Networking – Network Layer
Options…
Networking – Network Layer
Data itself…!
Networking – Network Layer
• IPv6
Networking – Network Layer
Label defining a
certain set of
packets…
Networking – Network Layer
Upper-layer
protocol
Networking – Network Layer
Maximal
number of
routings
Networking – Network Layer
Note 128
bits…!
Networking – Network Layer
• Now we will never (?) run out of IP
addresses…
Networking – Network Layer
• Actual routing on the Internet – how…?
• Recall:
– Interfaces divided into autonomous systems (AS)
– Inside an AS, an intra-AS routing protocol is used
– Between ASs, an inter-AS routing protocol is used
• What specific protocols are used in practice?
Networking – Network Layer
• Intra-AS routing protocols
– RIP – Routing Information Protocol
– OSPF – Open Shortest Path First
– IGRP - Internal Gateway Routing Protocol
–…
• RIP and OSPF are the most common choices
Networking – Network Layer
• RIP – Routing Information Protocol
• RIP is a distance vector algorithm
• Cost is defined as one unit per ”hop”, up to a
maximum of 15
• Cost (or routing) tables are exchanged with
neighbours every 30 seconds
• Cost tables can contain 25 entries
Networking – Network Layer
• Note that RIP costs are not sensitive to
”traffic conditions”
• How can a cost ever change…?
– Algorithm needs time to converge
– Adding/removing routers can change cost
– Neighbour is considered unreachable if no
contact in 180 seconds
Networking – Network Layer
Used for all other
destinations
Networking – Network Layer
• Won’t we experience a ”routing table
explosion”, since there are billions of
possible destinations?
• In practice, routers use routing table
aggregation – grouping many destinations
into one entry
• http://www.ripe.net/ripe/docs/ripe-399#3
Networking – Network Layer
•
•
•
•
OSPF – Open Shortest Path First
OSPF is a link-state algorithm
Link costs defined by network administrator
Each router ”broadcasts” its routing table (to
the neighbours) to all other routers
• Each router can then assemble a complete
routing table
Networking – Network Layer
• Other OSPF features
– Security by authetication
– Multiple same-cost paths
– Cost differentiation by type-of-service
– Multicast routing support
– Support for hierarchical routing within an AS
Networking – Network Layer
Networking – Network Layer
• Inter-AS routing protocols
• One protocol called Border Gateway Protocol
(BGP) is the standard for inter-AS routing
• Is a path vector algorithm (not distance…)
• The algorithm propagates path, not costs,
from AS to AS
• Actual choice of path is domain policy…
Networking – Network Layer
Networking – Network Layer
• Why aren’t intra- and inter-AS routing
algorithms the same…?
– Policy – at inter-AS level, some choices may be
impractical, even though they are cheapest
– Scale – size of single AS is controllable, total
number of AS on the Internet is not…
– Performance – ”raw” performance more
important on the intra-AS level
Networking – Network Layer