IP Address Routing _________________2_IP Routing.pdf

IP Routing
Mekuanint A. (PhD)
memekuanint@gmail.com
Faculty of Computing
Bahir Dar Institute of Technology
Bahir Dar University
Advanced Computer Networks

IPAddressing
• how hosts and routers are connected into the network?
• A host typically has only a single link into the network
• IP in the host wants to send a datagram, it does over this link
• The boundary between the host and the physical link is called an
interface
• router: receives a datagram on one link and forwards the datagram
on some other link
• router necessarily has two or more links to which it is connected

• A router has multiple interfaces, one for each of its links
• Because every host and router is capable of sending and
receiving IP datagrams, IP requires each host and router
interface to have its own IP address
• Thus, an IP address is technically associated with an interface,
rather than with the host or router containing that interface

• Each IP address is 32 bits long
• A total of 2^32 possible IP addresses: 4 billon IP addresses
• typically written in dotted-decimal notation
• Each interface on every host and router in the global Internet
must have an IP address that is globally unique

IPAddressing
• IP address: 32-bit
identifier for host, router
interface
• interface: connection
between host/router and
physical link
– router‟s typically have
multiple interfaces
– host typically has one
interface
– IP addresses associated
with each interface
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2
223.1.3.1
223.1.3.27
223.1.1.1 = 11011111 00000001 00000001 00000001
223 1 1
1

Subnet
• A subnet is also called an IP network or simply a network in
the Internet literature
• IP addressing assigns an address to the subnet: 223.1.1.0/24
• the /24 notation, sometimes known as a subnet mask, indicates
that the leftmost 24 bits of the 32-bit quantity define the subnet
address
• Any additional hosts attached to the 223.1.1.0/24 subnet
would be required to have an address of the form 223.1.1.xxx.

Subnets
• IP address:
– subnet part (high order
bits)
– host part (low order bits)
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2
223.1.3.1
223.1.3.27
network consisting of 3 subnets
subnet

Subnets
223.1.1.0/24
223.1.2.0/24
223.1.3.0/24
• To determine the subnets,
detach each interface from
its host or router, creating
islands of isolated
networks.
• Each isolated network is
called a subnet.
Subnet mask: /24

Subnets
• How many subnets?
 all devices on a given
subnet have the same
subnet address
 different subnets could
have quite different
subnet addresses
223.1.1.1
223.1.1.3
223.1.1.4
223.1.2.2
223.1.2.1
223.1.2.6
223.1.3.2
223.1.3.1
223.1.3.27
223.1.1.2
223.1.7.0
223.1.7.1
223.1.8.0
223.1.8.1
223.1.9.1
223.1.9.2

IP addressing
CIDR: The Internet's address assignment strategy: Classless
Inter-Domain Routing (CIDR)
• Generalizes the notion of subnet addressing
– subnet portion of address of arbitrary length
– address format: a.b.c.d/x, where x is # bits in subnet portion of
address
11001000 00010111 00010000 00000000
subnet
part
host
part
200.23.16.0/23

• The x most significant bits of an address of the form a.b.c.d/x
constitute the network portion of the IP address (prefix or network
prefix of the address)
• Organization: has a range of addresses with a common prefix
• when a router outside the organization forwards a datagram whose
destination address is inside the organization, only the leading x
bits of the address need be considered
• The remaining 32-x bits of an address can be thought of as
distinguishing among the devices within the organization
• These are the bits that will be considered when forwarding
packets at routers within the organization

Obtaining a Block of Addresses
• ISP: provide addresses from a larger block of addresses that had already
been allocated to the ISP
• ISP may itself have been allocated the address block 200.23.16.0/20
• ISP divides its address block into eight equal-sized contiguous address
blocks

DHCP: Dynamic Host Configuration Protocol
 A system administrator will typically manually configure the
IP addresses into the router
 Host addresses can also be configured manually, but more
often this task is now done using DHCP
 DHCP allows a host to obtain (be allocated) an IP address
automatically

DHCP client-server scenario
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2
223.1.3.1
223.1.3.27
A
B
E
DHCP
server
arriving DHCP
client needs
address in this
network
DHCP is a client-server protocol

DHCP: Dynamic Host Configuration Protocol
Goal: allow host to dynamically obtain its IP address from network server when it joins
network
Can renew its lease on address in use
Allows reuse of addresses (only hold address while connected an “on”)
Support for mobile users who want to join network (more shortly)
DHCP overview:
– host broadcasts “DHCP discover” msg
– DHCP server responds with “DHCP offer” msg
– host requests IP address: “DHCP request” msg
– DHCP server sends address: “DHCP ack” msg

DHCP client-server scenario
DHCP server: 223.1.2.5 arriving
client
time
DHCP discover
src : 0.0.0.0, 68
dest.: 255.255.255.255,67
yiaddr: 0.0.0.0
transaction ID: 654
DHCP offer
src: 223.1.2.5, 67
dest: 255.255.255.255, 68
yiaddrr: 223.1.2.4
transaction ID: 654
Lifetime: 3600 secs
DHCP request
src: 0.0.0.0, 68
dest:: 255.255.255.255, 67
yiaddrr: 223.1.2.4
transaction ID: 655
Lifetime: 3600 secs
DHCP ACK
src: 223.1.2.5, 67
dest: 255.255.255.255, 68
yiaddrr: 223.1.2.4
transaction ID: 655
Lifetime: 3600 secs

Forwarding and Routing
• forwarding: move packets
from router‟s input to
appropriate router output
• routing: determine route
taken by packets from source
to dest.
– routing algorithms
analogy:
 routing: process of
planning trip from source
to dest
 forwarding: process of
getting through single
interchange

1
2
3
0111
value in arriving
packet’s header
routing algorithm
local forwarding table
header value output link
0100
0101
0111
1001
3
2
2
1
Routing and forwarding

u
y
x
w
v
z
2
2
1
3
1
1
2
5
3
5
Graph: G = (N,E)
N = set of routers = { u, v, w, x, y, z }
E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }
Graph Abstraction

Graph abstraction: costs
u
y
x
w
v
z
2
2
1
3
1
1
2
5
3
5 • c(x,x‟) = cost of link (x,x‟)
- e.g., c(w,z) = 5
Cost of path (x1, x2, x3,…, xp) = c(x1,x2) + c(x2,x3) + … + c(xp-1,xp)
Question: What‟s the least-cost path between u and z ?
Routing algorithm: algorithm that finds least-cost path

Routing Algorithm classification
Global or decentralized
information?
Global:
• all routers have complete
topology, link cost info
• “link state” algorithms
Decentralized:
• router knows physically-
connected neighbors, link costs
to neighbors
• iterative process of computation,
exchange of info with neighbors
• “distance vector” algorithms
Static or dynamic?
Static:
• routes change slowly over
time
Dynamic:
• routes change more
quickly
– periodic update
– in response to link cost
changes

A Link-State Routing Algorithm
Dijkstra‟s algorithm
• net topology, link costs known to
all nodes
– accomplished via “link state
broadcast”
– all nodes have same info
• computes least cost paths from
one node („source”) to all other
nodes
– gives forwarding table for that
node
• iterative: after k iterations, know
least cost path to k dest.‟s
Notation:
• c(x,y): link cost from node x to
y; = ∞ if not direct neighbors
• D(v): current value of cost of
path from source to dest. v
• p(v): predecessor node along
path from source to v
• N': set of nodes whose least cost
path definitively known

Dijsktra’s Algorithm
1 Initialization:
2 N' = {u}
3 for all nodes v
4 if v adjacent to u
5 then D(v) = c(u,v)
6 else D(v) = ∞
7
8 Loop
9 find w not in N' such that D(w) is a minimum
10 add w to N'
11 update D(v) for all v adjacent to w and not in N' :
12 D(v) = min( D(v), D(w) + c(w,v) )
13 /* new cost to v is either old cost to v or known
14 shortest path cost to w plus cost from w to v */
15 until all nodes in N'

Dijkstra’s algorithm: example
Step
0
1
2
3
4
5
N'
u
ux
uxy
uxyv
uxyvw
uxyvwz
D(v),p(v)
2,u
2,u
2,u
D(w),p(w)
5,u
4,x
3,y
3,y
D(x),p(x)
1,u
D(y),p(y)
∞
2,x
D(z),p(z)
∞
∞
4,y
4,y
4,y
u
y
x
w
v
z
2
2
1
3
1
1
2
5
3
5

Dijkstra’s algorithm
u
y
x
w
v
z
Resulting shortest-path tree from u:
v
x
y
w
z
(u,v)
(u,x)
(u,x)
(u,x)
(u,x)
destination link
Resulting forwarding table in u:

Distance Vector Algorithm
• Distance vector (DV) algorithm is iterative, asynchronous,
and distributed
 Distributed:
• each node receives information from its directly attached
neighbors
• performs a calculation
• distributes the results of its calculation back to its
neighbor
 Iterative:
• continues until no nodes exchange info
• self-terminating: no “signal” to stop
 Asynchronous
• it does not require all of the nodes to operate in lockstep
with each other

Bellman-Ford Equation
Define
dx(y) := cost of least-cost path from x to y
Then
dx(y) = min {c(x,v) + dv(y) }
where min is taken over all neighbors v of x
v

Bellman-Ford Example
u
y
x
w
v
z
2
2
1
3
1
1
2
5
3
5
Clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3
du(z) = min { c(u,v) + dv(z),
c(u,x) + dx(z),
c(u,w) + dw(z) }
= min {2 + 5,
1 + 3,
5 + 3} = 4
Node that achieves minimum is next
hop in shortest path ➜ forwarding table
B-F equation says:

 Dx(y) = estimate of least cost from x to y
 With the DV algorithm, each node x maintains the following
routing information:
• Node x knows cost to each neighbor v: c(x,v)
• Node x maintains distance vector Dx = [Dx(y): y є N ]
• Node x also maintains its neighbors‟ distance vectors
– For each neighbor v, x maintains
Dv = [Dv(y): y є N ]

Distance vector algorithm
Basic idea:
• From time-to-time, each node sends its own distance
vector estimate to neighbors
• When a node x receives new DV estimate from neighbor, it
updates its own DV using B-F equation:
Dx(y) = minv{c(x,v) + Dv(y)} for each node y ∊ N
 Under minor, natural conditions, the estimate Dx(y)
converge to the actual least cost dx(y)

x y z
x
y
z
0 2 7
∞ ∞ ∞
∞ ∞ ∞
from
cost to
from
from
x y z
x
y
z
0
from
cost to
x y z
x
y
z
∞ ∞
∞ ∞ ∞
cost to
x y z
x
y
z
∞ ∞ ∞
7 1 0
cost to
∞
2 0 1
∞ ∞ ∞
2 0 1
7 1 0
time
x z
1
2
7
y
node x table
node y table
node z table
Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)}
= min{2+0 , 7+1} = 2
Dx(z) = min{c(x,y) +
Dy(z), c(x,z) + Dz(z)}
= min{2+1 , 7+0} = 3
3
2

x y z
x
y
z
0 2 7
∞ ∞ ∞
∞ ∞ ∞
from
cost to
from
from
x y z
x
y
z
0 2 3
from
cost to
x y z
x
y
z
0 2 3
from
cost to
x y z
x
y
z
∞ ∞
∞ ∞ ∞
cost to
x y z
x
y
z
0 2 7
from
cost to
x y z
x
y
z
0 2 3
from
cost to
x y z
x
y
z
0 2 3
from
cost to
x y z
x
y
z
0 2 7
from
cost to
x y z
x
y
z
∞ ∞ ∞
7 1 0
cost to
∞
2 0 1
∞ ∞ ∞
2 0 1
7 1 0
2 0 1
7 1 0
2 0 1
3 1 0
2 0 1
3 1 0
2 0 1
3 1 0
2 0 1
3 1 0
time
x z
1
2
7
y
node x table
node y table
node z table
Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)}
= min{2+0 , 7+1} = 2
Dx(z) = min{c(x,y) +
Dy(z), c(x,z) + Dz(z)}
= min{2+1 , 7+0} = 3

Distance Vector: link cost changes
Link cost changes:
 node detects local link cost change
 updates routing info, recalculates
distance vector
 if DV changes, notify neighbors
x z
1
4
50
y
1
At time t0, y detects the link-cost change, updates its DV,
and informs its neighbors.
At time t1, z receives the update from y and updates its table.
It computes a new least cost to x and sends its neighbors its DV.
At time t2, y receives z‟s update and updates its distance table.
y‟s least costs do not change and hence y does not send any
message to z.

Distance Vector: link cost changes
Link cost changes:
 good news travels fast
 bad news travels slow - “count
to infinity” problem!
 44 iterations before algorithm
stabilizes
Poisoned reverse:
 If Z routes through Y to get to
X :
 Z tells Y its (Z‟s) distance to X
is infinite (so Y won‟t route to
X via Z)
 will this completely solve count
to infinity problem?
x z
1
4
50
y
60

Comparison of LS and DV algorithms
Message complexity
• LS: with n nodes, E links, O(nE)
msgs sent
• DV: exchange between neighbors
only
– convergence time varies
Speed of Convergence
• LS: O(n2) algorithm requires
O(nE) msgs
– may have oscillations
• DV: convergence time varies
– may be routing loops
– count-to-infinity problem
Robustness: what happens if
router malfunctions?
LS:
– node can advertise incorrect
link cost
– each node computes only its
own table
DV:
– DV node can advertise
incorrect path cost
– each node‟s table used by
others
• error propagate thru network

Class Work
• Consider the network shown below, and assume that each node
initially knows the cost to each of its neighbors. Consider the
distance-vector algorithm and show the distance table entries at
node E.

Intra-AS Routing
• An intra-AS routing protocol is used to determine how routing
is performed within an autonomous system (AS)
• also known as Interior Gateway Protocols (IGP)
• most common Intra-AS routing protocols of Internet:
– RIP: Routing Information Protocol
– OSPF: Open Shortest Path First
– IGRP: Interior Gateway Routing Protocol

Inter-AS routing
• Different routing protocols on each AS
• Different metrics in each AS
• The AS‟s didn't trust each other
• routes are growing very fast
• The point of Border Gateway Protocol (BGP) is to route traffic
between AS‟s which didn‟t trust each other

Border Gateway Protocol (BGP)
• BGP provides each AS a means to:
1. Obtain subnet reachability information from neighboring
ASs
2. Propagate reachability information to all AS-internal
routers
3. Determine “good” routes to subnets based on reachability
information and policy
• allows subnet to advertise its existence to rest of Internet: “I
am here”
• If it weren't for BGP, each subnet would be isolated-alone and
unknown by the rest of the Internet

BGP basics
 Pairs of routers (BGP peers) exchange routing info over TCP connections: BGP sessions
 BGP session that spans two ASs: external BGP (eBGP) session
 BGP session between routers in the same AS: internal BGP (iBGP) session
 BGP sessions need not correspond to physical links.
 when AS2 advertises a prefix to AS1:
 AS2 promises it will forward datagrams towards that prefix.
 AS2 can aggregate prefixes in its advertisement
3b
1d
3a
1c
2a
AS3
AS1
AS2
1a
2c
2b
1b
3c
eBGP session
iBGP session

Distributing reachability info
• using eBGP session between 3a and 1c, AS3 sends prefix reachability info to
AS1.
– 1c can then use iBGP do distribute new prefix info to all routers in AS1
– 1b can then re-advertise new reachability info to AS2 over 1b-to-2a eBGP
session
• when router learns of new prefix, it creates entry for prefix in its forwarding
table.
3b
1d
3a
1c
2a
AS3
AS1
AS2
1a
2c
2b
1b
3c
eBGP session
iBGP session

Path attributes & BGP routes
• advertised prefix includes BGP attributes.
– prefix + attributes = “route”
• two important attributes:
– AS-PATH: contains ASs through which prefix advertisement
has passed: e.g, AS67, AS17
– NEXT-HOP: indicates specific internal-AS router to next-hop
AS. (may be multiple links from current AS to next-hop-AS)
• when gateway router receives route advertisement, uses import
policy to accept/decline.

BGP route selection
• router may learn about more than one route to some prefix.
Router must select route.
• Elimination rules:
1. local preference value attribute: policy decision
2. shortest AS-PATH
3. closest NEXT-HOP router
4. additional criteria

BGP messages
• BGP messages exchanged using TCP.
• BGP messages:
– OPEN: opens TCP connection to peer and authenticates sender
– UPDATE: advertises new path (or withdraws old)
– KEEPALIVE keeps connection alive in absence of UPDATES;
also ACKs OPEN request
– NOTIFICATION: reports errors in previous msg; also used to
close connection

BGP routing policy
 A,B,C are provider networks
 X,W,Y are customer (of provider networks)
 X is dual-homed: attached to two networks
 X does not want to route from B via X to C
 .. so X will not advertise to B a route to C
A
B
C
W
X
Y
legend:
customer
network:
provider
network

BGP routing policy
• A advertises path AW to B
• B advertises path BAW to X
• Should B advertise path BAW to C?
• No way! B gets no “revenue” for routing CBAW since
neither W nor C are B‟s customers
• B wants to force C to route to w via A
• B wants to route only to/from its customers!
A
B
C
W
X
Y
legend:
customer
network:
provider
network

Why different Intra- and Inter-AS routing ?
Policy:
• Inter-AS: admin wants control over how its traffic routed, who
routes through its net.
• Intra-AS: single admin, so no policy decisions needed
Scale:
• hierarchical routing saves table size, reduced update traffic
Performance:
• Intra-AS: can focus on performance
• Inter-AS: policy may dominate over performance

IP Address Routing _________________2_IP Routing.pdf

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