CS6551 COMPUTER NETWORKS

CS6551 COMPUTER NETWORKS
UNIT – III
Dr.A.Kathirvel, Professor, Computer Science and Engg.
M N M Jain Engineering College, Chennai

Unit - III
ROUTING
Routing (RIP, OSPF, metrics) –
Switch basics – Global Internet
(Areas, BGP, IPv6), Multicast –
addresses – multicast routing
(DVMRP, PIM)
 Computer Networks: A Systems Approach, 5e, Larry L. Peterson and Bruce S.
Davie

Intra-AS Routing
also known as interior gateway protocols
(IGP)
most common intra-AS routing protocols:
RIP: Routing Information Protocol
OSPF: Open Shortest Path First
IGRP: Interior Gateway Routing Protocol
(Cisco proprietary)
3

RIP ( Routing Information Protocol)
 included in BSD-UNIX distribution in 1982
 distance vector algorithm
 distance metric: # hops (max = 15 hops), each link has cost 1
 DVs exchanged with neighbors every 30 sec in response message (aka
advertisement)
 each advertisement: list of up to 25 destination subnets (in IP addressing
sense)
DC
BA
u v
w
x
y
z
subnet hops
u 1
v 2
w 2
x 3
y 3
z 2
from router A to destination subnets:
4

RIP: example
destination subnet next router # hops to dest
w A 2
y B 2
z B 7
x -- 1
…. …. ....
routing table in router D
w x y
z
A
C
D B
5

w x y
z
A
C
D B
destination subnet next router # hops to dest
w A 2
y B 2
z B 7
x -- 1
…. …. ....
routing table in router D
A 5
dest next hops
w - 1
x - 1
z C 4
…. … ...
A-to-D advertisement
RIP: example
6

RIP: link failure, recovery
if no advertisement heard after 180 sec -->
neighbor/link declared dead
routes via neighbor invalidated
new advertisements sent to neighbors
neighbors in turn send out new advertisements (if
tables changed)
link failure info quickly (?) propagates to entire net
poison reverse used to prevent ping-pong loops
(infinite distance = 16 hops)
7

RIP table processing
RIP routing tables managed by application-
level process called route-d (daemon)
advertisements sent in UDP packets,
periodically repeated
physical
link
network forwarding
(IP) table
transport
(UDP)
routed
physical
link
network
(IP)
transprt
(UDP)
routed
forwarding
table
8

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
9

OSPF “advanced” features (not in RIP)
security: all OSPF messages authenticated (to prevent
malicious intrusion)
multiple same-cost paths allowed (only one path in RIP)
for each link, multiple cost metrics for different TOS (e.g.,
satellite link cost set “low” for best effort ToS; high for real
time ToS)
integrated uni- and multicast support:
Multicast OSPF (MOSPF) uses same topology data base as OSPF
hierarchical OSPF in large domains.
10

Hierarchical OSPF
boundary router
backbone router
area 1
area 2
area 3
backbone
area
border
routers
internal
routers
11

two-level hierarchy: local area, backbone.
link-state advertisements only in area
each nodes has detailed area topology; only know
direction (shortest path) to nets in other areas.
area border routers: “summarize” distances to
nets in own area, advertise to other Area Border
routers.
backbone routers: run OSPF routing limited to
backbone.
boundary routers: connect to other AS’s.
Hierarchical OSPF
12

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”
13

BGP basics
 when AS3 advertises a prefix to AS1:
 AS3 promises it will forward datagrams towards that prefix
 AS3 can aggregate prefixes in its advertisement
AS3
AS2
3b
3c
3a
AS1
1c
1a
1d
1b
2a
2c
2b
other
networks
other
networks
 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
BGP
message
14

BGP: distributing path information
AS3
AS2
3b
3a
AS1
1c
1a
1d
1b
2a
2c
2b
other
networks
other
networks
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
iBGP session
15

Path attributes and 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., AS 67, AS 17
NEXT-HOP: indicates specific internal-AS router to next-hop
AS. (may be multiple links from current AS to next-hop-AS)
 gateway router receiving route advertisement uses import policy
to accept/decline
e.g., never route through AS x
policy-based routing
16

BGP route selection
router may learn about more than 1 route to
destination AS, selects route based on:
 local preference value attribute: policy
decision
 shortest AS-PATH
 closest NEXT-HOP router: hot potato routing
 additional criteria
17

BGP messages
BGP messages exchanged between peers over TCP
connection
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
18

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
19

BGP routing policy (2)
 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
20

Why different Intra-, 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
21

The Global Internet
The tree structure of the Internet in 1990
22

The Global Internet
A simple multi-provider Internet
23

Interdomain Routing (BGP)
Internet is organized as autonomous systems (AS) each of
which is under the control of a single administrative
entity
Autonomous System (AS)
corresponds to an administrative domain
examples: University, company, backbone network
A corporation’s internal network might be a single AS, as
may the network of a single Internet service provider
24

Interdomain Routing
A network with two
autonomous system
25

Route Propagation
Idea: Provide an additional way to hierarchically
aggregate routing information is a large internet.
Improves scalability
Divide the routing problem in two parts:
Routing within a single autonomous system
Routing between autonomous systems
 Another name for autonomous systems in the
Internet is routing domains
Two-level route propagation hierarchy
Inter-domain routing protocol (Internet-wide standard)
Intra-domain routing protocol (each AS selects its own)
26

EGP and BGP
Inter-domain Routing Protocols
Exterior Gateway Protocol (EGP)
Forced a tree-like topology onto the Internet
Did not allow for the topology to become general
Tree like structure: there is a single backbone and autonomous systems
are connected only as parents and children and not as peers
Border Gateway Protocol (BGP)
Assumes that the Internet is an arbitrarily interconnected set of
ASs.
Today’s Internet consists of an interconnection of multiple
backbone networks (they are usually called service provider
networks, and they are operated by private companies rather
than the government)
Sites are connected to each other in arbitrary ways
27

BGP
Some large corporations connect directly to one
or more of the backbone, while others connect to
smaller, non-backbone service providers.
Many service providers exist mainly to provide
service to “consumers” (individuals with PCs in
their homes), and these providers must connect to
the backbone providers
Often many providers arrange to interconnect
with each other at a single “peering point”
28

BGP-4: Border Gateway Protocol
Assumes the Internet is an arbitrarily interconnected set
of AS's.
Define local traffic as traffic that originates at or
terminates on nodes within an AS, and transit traffic as
traffic that passes through an AS.
We can classify AS's into three types:
 Stub AS: an AS that has only a single connection to one other AS;
such an AS will only carry local traffic (small corporation in the figure of
the previous page).
 Multihomed AS: an AS that has connections to more than one other AS, but
refuses to carry transit traffic (large corporation at the top in the figure of
the previous page).
 Transit AS: an AS that has connections to more than one other AS, and is
designed to carry both transit and local traffic (backbone providers in the
figure of the previous page).
29

The goal of Inter-domain routing is to
find any path to the intended destination
that is loop free
We are concerned with reachability than
optimality
Finding path anywhere close to optimal is
considered to be a great achievement
Why?
BGP
30

Scalability: An Internet backbone router must be able to
forward any packet destined anywhere in the Internet
Having a routing table that will provide a match for any valid IP
address
Autonomous nature of the domains
It is impossible to calculate meaningful path costs for a path that
crosses multiple ASs
A cost of 1000 across one provider might imply a great path but it
might mean an unacceptable bad one from another provid
Issues of trust
Provider A might be unwilling to believe certain advertisements
from provider B
BGP
31

Each AS has:
One BGP speaker that advertises:
local networks
other reachable networks (transit AS only)
gives path information
In addition to the BGP speakers, the AS has one or
more border “gateways” which need not be the same
as the speakers
The border gateways are the routers through which
packets enter and leave the AS
BGP
32

BGP does not belong to either of the two main classes of
routing protocols (distance vectors and link-state
protocols)
BGP advertises complete paths as an enumerated lists of
ASs to reach a particular network
BGP
33
Example

BGP Example
Speaker for AS 2 advertises reachability to P and
Q
Network 128.96, 192.4.153, 192.4.32, and 192.4.3,
can be reached directly from AS 2.
Speaker for backbone network then advertises
Networks 128.96, 192.4.153, 192.4.32, and 192.4.3
can be reached along the path <AS 1, AS 2>.
Speaker can also cancel previously advertised
paths
34

BGP Issues
It should be apparent that the AS
numbers carried in BGP need to be
unique
For example, AS 2 can only recognize
itself in the AS path in the example if no
other AS identifies itself in the same way
AS numbers are 16-bit numbers
assigned by a central authority
35

Integrating Interdomain & Intradomain Routing
All routers run iBGP and an intradomain routing protocol.
Border routers (A, D, E) also run eBGP to other ASs
36

Integrating Interdomain & Intradomain Routing
BGP routing table, IGP routing table, and combined table at router B
37

Routing Areas
A domain divided into area
Backbone area
Area border router
(ABR)
38

Major Features
128-bit addresses
Multicast
Real-time service
Authentication and security
Auto-configuration
End-to-end fragmentation
Enhanced routing functionality, including
support for mobile hosts
40

IPv6 Addresses
Classless addressing/routing (similar to CIDR)
Notation: x:x:x:x:x:x:x:x(x= 16-bit hex number)
contiguous 0s are compressed: 47CD::A456:0124
IPv6 compatible IPv4 address: ::128.42.1.87
Address assignment
provider-based
geographic
41

IPv6 Header
40-byte “base” header
Extension headers (fixed order, mostly fixed
length)
fragmentation
source routing
authentication and
 security
other options
42

Overview
IPv4
class D addresses
demonstrated with MBone
uses tunneling
Integral part of IPv6
problem is making it scale
One-to-many
Radio station broadcast
Transmitting news, stock-price
Software updates to multiple hosts
44

Overview
 Many-to-many
Multimedia teleconferencing
Online multi-player games
Distributed simulations
 Without support for multicast
A source needs to send a separate packet with the identical data
to each member of the group
This redundancy consumes more bandwidth
Redundant traffic is not evenly distributed, concentrated near
the sending host
Source needs to keep track of the IP address of each member in
the group
Group may be dynamic
45

Overview
To support many-to-many and one-to-many IP provides
an IP-level multicast
Basic IP multicast model is many-to-many based
on multicast groups
Each group has its own IP multicast address
Hosts that are members of a group receive copies of
any packets sent to that group’s multicast address
A host can be in multiple groups
A host can join and leave groups
46

Overview
Using IP multicast to send the identical packet to
each member of the group
A host sends a single copy of the packet addressed to
the group’s multicast address
The sending host does not need to know the individual
unicast IP address of each member
Sending host does not send multiple copies of the
packet
IP’s original many-to-many multicast has been
supplemented with support for a form of one-to-
many multicast
47

Overview
 One-to-many multicast
Source specific multicast (SSM)
A receiving host specifies both a multicast group and a specific
sending host
 Many-to-many model
Any source multicast (ASM)
 A host signals its desire to join or leave a multicast group by
communicating with its local router using a special protocol
In IPv4, the protocol is Internet Group Management Protocol
(IGMP)
In IPv6, the protocol is Multicast Listener Discovery (MLD)
 The router has the responsibility for making multicast behave
correctly with regard to the host
48

Multicast Routing
A router’s unicast forwarding tables indicate for any IP
address, which link to use to forward the unicast packet
To support multicast, a router must additionally have
multicast forwarding tables that indicate, based on
multicast address, which links to use to forward the
multicast packet
Unicast forwarding tables collectively specify a set of
paths
Multicast forwarding tables collectively specify a set of
trees
Multicast distribution trees
49

Multicast Routing
To support source specific multicast, the
multicast forwarding tables must indicate
which links to use based on the combination of
multicast address and the unicast IP address of
the source
Multicast routing is the process by which
multicast distribution trees are determined
50

Distance Vector Multicast
Each router already knows that shortest path to
source S goes through router N.
When receive multicast packet from S, forward on
all outgoing links (except the one on which the
packet arrived), iff packet arrived from N.
Eliminate duplicate broadcast packets by only
letting
“parent” for LAN (relative to S) forward
shortest path to S (learn via distance vector)
smallest address to break ties
51

Reverse Path Broadcast (RPB)
Goal: Prune networks that have no hosts in group G
Step 1: Determine of LAN is a leaf with no members in G
leaf if parent is only router on the LAN
determine if any hosts are members of G using IGMP
Step 2: Propagate “no members of G here” information
augment <Destination, Cost> update sent to neighbors with set
of groups for which this network is interested in receiving
multicast packets.
only happens when multicast address becomes active.
Distance Vector Multicast
52

Protocol Independent Multicast (PIM)
Shared Tree
Source specific
tree
53

Protocol Independent Multicast (PIM)
Delivery of a packet along a shared tree. R1 tunnels the
packet to the RP, which forwards it along the shared tree to
R4 and R5.
54

Interdomain Multicast
Multicast Source Discovery Protocol (MSDP)
55

Routing for Mobile Hosts
 Mobile IP
 home agent
Router located on the home network of the mobile hosts
 home address
The permanent IP address of the mobile host.
Has a network number equal to that of the home network and thus of the home
agent
 foreign agent
Router located on a network to which the mobile node attaches itself when it is
away from its home network
56

Problem of delivering a packet to the mobile
node
How does the home agent intercept a packet that is
destined for the mobile node?
Proxy ARP
How does the home agent then deliver the packet to
the foreign agent?
IP tunnel
Care-of-address
How does the foreign agent deliver the packet to the
mobile node?
57

Route optimization in Mobile IP
The route from the sending node to mobile node can be
significantly sub-optimal
One extreme example
The mobile node and the sending node are on the same network, but the
home network for the mobile node is on the far side of the Internet
Triangle Routing Problem
Solution
Let the sending node know the care-of-address of the mobile node. The
sending node can create its own tunnel to the foreign agent
Home agent sends binding update message
The sending node creates an entry in the binding cache
The binding cache may become out-of-date
The mobile node moved to a different network
Foreign agent sends a binding warning message
58

CS6551 COMPUTER NETWORKS

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