Presented by
Amjad
Outline
Routing on the Internet.
 RIP
 OSPF
 BGP
Broadcast and multicast routing.
Routing Information Protocol (RIP)
Routing Information Protocol (RIP) is a dynamic routing protocol that
uses hop count as a routing metric to find the best path between the source
and the destination network.
Features of RIP
1.
2.
3.
4.
Updates of the network are exchanged periodically.
Updates (routing information) are always broadcast.
Full routing tables are sent in updates.
Routers always trust routing information received from neighbor
routers.
RIP: Example
z
w
A
y
x
D
B
C
Routing table in router D
Destination subnet
Next router
# hops to destination
w
y
z
x
A
B
B
--
2
2
7
1
….
….
....
OSPF (Open Shortest Path First)





“open”: publicly available
uses link state algorithm
 LS packet dissemination
 topology map at each node
 route computation using Dijkstra’s algorithm
OSPF advertisement carries one entry per neighbor
advertisements flooded to entire AS
 carried in OSPF messages directly over IP (rather than
TCP or UDP
IS-IS routing protocol: nearly identical to OSPF
Hierarchical OSPF
boundary router
backbone router
backbone
area
border
routers
area 3
internal
routers
area 1
area 2
Internet inter-AS routing: BGP



BGP (Border Gateway Protocol): the de facto inter-domain routing
protocol
 “glue that holds the Internet together”
BGP provides each AS a means to:
 eBGP: obtain subnet reachability information from neighboring
ASs.
 iBGP: propagate reachability information to all AS-internal
routers.
 determine “ good ” routes to other networks based on
reachability information and policy.
allows subnet to advertise its existence to rest of Internet: “I am
here”
BGP basics

BGP session: two BGP routers (“peers”) exchange BGP messages:
 advertising paths to different destination network prefixes (“path vector”
protocol)
 exchanged over semi-permanent TCP connections

when AS3 advertises a prefix to AS1:
 AS3 promises it will forward datagrams towards that prefix
 AS3 can aggregate prefixes in its advertisement
3c
3b
other
networks
3a
BGP
message
AS3
2c
1c
1a
AS1
1d
2a
1b
2b
AS2
other
networks
Network Layer
BGP basics: distributing path information


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.
eBGP session
3b
other
networks
3a
AS3
iBGP session
2c
1c
1a
AS1
1d
2a
1b
2b
AS2
other
networks
Network Layer
Broadcast routing


Deliver packets from source to all other nodes.
Source duplication is inefficient:
duplicate
duplicate
creation/transmission
R1
R1
duplicate
R2
R2
R3
R4
source
duplication

R3
R4
in-network
duplication
Source duplication: how does source determine recipient addresses?
Spanning tree


First construct a spanning tree.
Nodes then forward/make copies only along spanning tree.
A
A
B
B
c
c
D
F
D
E
F
G
(a) broadcast initiated at A
E
G
(b) broadcast initiated at D
Multicast routing: problem statement
Goal: find a tree (or trees) connecting routers having local multicast
group members
legend
 tree: not all paths between routers used
group
member
 shared-tree: same tree used by all group members
not group
 source-based: different tree from each sender to rcvrs
member
router
with a
group
member
router
without
group
member
shared tree
source-based trees
Shortest path tree

Multicast forwarding tree: tree of shortest path routes from
source to all receivers.
 Dijkstra’s algorithm
LEGEND
s: source
R1
1
2
R2
3
router with attached
group member
R4
5
4
R3
R6
R5
6
R7
i
router with no attached
group member
link used for forwarding,
i indicates order link
added by algorithm