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Network Layer
 Introduction
 Datagram networks
 IP: Internet Protocol
 Datagram format
 IPv4 addressing
 ICMP
 What’s inside a router
 Routing algorithms
 Link state
 Distance Vector
 Routing in the
Internet



RIP
OSPF
BGP
 Multicast routing
4-1
Routing in the Internet
The Internet uses hierarchical routing
 The Internet is split into Autonomous Systems (AS’s)

Try: http://whois.arin.net/ui/ to query AS #
 Within an AS, the administrator chooses an Interior
Gateway Protocol (IGP)

Examples of IGPs: RIP (rfc 1058), OSPF (rfc 1247).
 Between AS’s, the Internet uses an Exterior
Gateway Protocol

AS’s today use the Border Gateway Protocol, BGP-4 (rfc 1771)
4-2
Routing in the Internet
AS ‘B’
AS ‘A’
BGP
BGP
Interior Gateway
Protocol
Stub AS
AS ‘C’
Interior Gateway
Protocol
Transit AS
e.g. backbone service provider
Interior Gateway
Protocol
Stub AS
4-3
Hierarchical Routing
Our routing study thus far - idealization
 all routers identical
 network “flat”
… not true in practice
scale: with 200 million
destinations:
 can’t store all dest’s in
routing tables!
 routing table exchange
would swamp links!
administrative autonomy
 internet = network of
networks
 each network admin may
want to control routing in its
own network
4-4
Hierarchical Routing
 aggregate routers into
regions, “autonomous
systems” (AS)
 routers in same AS run
same routing protocol


Gateway router
 Direct link to router in
another AS
“intra-AS” routing
protocol
routers in different AS
can run different intraAS routing protocol
4-5
Routing within a Stub AS
 There is only one exit point, so routers
within the AS can use default routing.
Each router knows all Network IDs within AS.
 Packets destined to another AS are sent to the
default router.
 Default router is the border gateway to the
next AS.

 Routing tables in Stub AS’s tend to be
small.
4-6
Interconnected ASes
3c
3a
3b
AS3
1a
2a
1c
1d
1b
Intra-AS
Routing
algorithm
2c
AS2
AS1
Inter-AS
Routing
algorithm
Forwarding
table
2b
 Forwarding table is
configured by both
intra- and inter-AS
routing algorithm


Intra-AS sets entries
for internal dests
Inter-AS & Intra-As
sets entries for
external dests
4-7
Inter-AS tasks
AS1 needs:
1. to learn which dests
are reachable through
AS2 and which
through AS3
2. to propagate this
reachability info to all
routers in AS1
Job of inter-AS routing!
 Suppose router in AS1
receives datagram for
which dest is outside
of AS1

Router should forward
packet towards one of
the gateway routers,
but which one?
3c
3b
3a
AS3
1a
2a
1c
1d
1b
2c
AS2
2b
AS1
4-8
Example: Setting forwarding table
in router 1d
 Suppose AS1 learns from the inter-AS
protocol that subnet x is reachable from
AS3 (gateway 1c) but not from AS2.
 Inter-AS protocol propagates reachability
info to all internal routers.
 Router 1d determines from intra-AS
routing info that its interface I is on the
least cost path to 1c.
 Puts in forwarding table entry (x,I).
4-9
Example: Choosing among multiple ASes
 Now suppose AS1 learns from the inter-AS protocol
that subnet x is reachable from AS3 and from AS2.
 To configure forwarding table, router 1d must
determine towards which gateway it should forward
packets for dest x.
 This is also the job on inter-AS routing protocol!
 Hot potato routing: send packet towards closest of
two routers.
Learn from inter-AS
protocol that subnet
x is reachable via
multiple gateways
Use routing info
from intra-AS
protocol to determine
costs of least-cost
paths to each
of the gateways
Hot potato routing:
Choose the gateway
that has the
smallest least cost
Determine from
forwarding table the
interface I that leads
to least-cost gateway.
Enter (x,I) in
forwarding table
4-10
Network Layer
 Introduction
 Datagram networks
 IP: Internet Protocol
 Datagram format
 IPv4 addressing
 ICMP
 What’s inside a router
 Routing algorithms
 Link state
 Distance Vector
 Routing in the
Internet



RIP
OSPF
BGP
 Multicast routing
4-11
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
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Interior Routing Protocols
 RIP





Uses distance vector (distributed Bellman-Ford algorithm).
Updates sent every 30 seconds. Each
update/advertisement list of up to 25 destination nets
within AS
No authentication.
Originally in BSD UNIX.
Widely used for many years; not used much anymore.
 OSPF





Link-state updates sent (using flooding) as and when
required.
Every router runs Dijkstra’s algorithm.
Authenticated updates.
Autonomous system may be partitioned into “areas”.
Widely used.
4-13
RIP: Link Failure and 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)
4-14
RIP Table processing
 RIP routing tables managed by application-level
process called route-d (daemon)
 advertisements sent in UDP packets, periodically
repeated
routed
routed
Transprt
(UDP)
network
(IP)
link
physical
Transprt
(UDP)
forwarding
table
forwarding
table
network
(IP)
link
physical
4-15
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
router
 Advertisements disseminated to entire AS (via
flooding)

Carried in OSPF messages directly over IP (rather than TCP
or UDP)
4-16
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;
high for real time)
Integrated uni- and multicast support:
 Multicast OSPF (MOSPF) uses same topology data
base as OSPF
Hierarchical OSPF in large domains.
4-17
Hierarchical OSPF
4-18
Hierarchical OSPF
 Two-level hierarchy: local area, backbone.
Link-state advertisements only in local 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.

4-19
Network Layer
 Introduction
 Datagram networks
 IP: Internet Protocol
 Datagram format
 IPv4 addressing
 ICMP
 What’s inside a router
 Routing algorithms
 Link state
 Distance Vector
 Routing in the
Internet



RIP
OSPF
BGP
 Multicast routing
4-20
Exterior Routing Protocols
Problems:
Topology: The Internet is a complex mesh of
different AS’s with very little structure.
 Autonomy of AS’s: Each AS defines link costs in
different ways, so not possible to find lowest
cost paths.
 Trust: Some AS’s can’t trust others to
advertise good routes (e.g. two competing
backbone providers), or to protect the privacy
of their traffic (e.g. two warring nations).
 Policies: Different AS’s have different
objectives (e.g. route over fewest hops; use one
provider rather than another).

4-21
Border Gateway Protocol (BGP-4)
 BGP is not a link-state or distance-vector routing
protocol.

Instead, BGP uses “Path vector”
 BGP advertises complete paths (a list of AS’s).


Also called AS_PATH (this is the path vector)
Example of path advertisement:
“The network 171.64/16 can be reached via the path {AS1, AS5, AS13}”.
 Paths with loops are detected locally and ignored.
 Local policies pick the preferred path among
options.
 When a link/router fails, the path is “withdrawn”.
4-22
BGP-4
 BGP provides each AS a means to:
1. Obtain subnet reachability information from
neighboring ASs.
2. Propagate the reachability information to all
routers internal to the AS.
3. Determine “good” routes to subnets based on
reachability information and policy.
 Allows a subnet to advertise its existence
to rest of the Internet: “I am here”
4-23
BGP basics
 Pairs of routers (BGP peers) exchange routing info over semi-
permanent TCP conctns: BGP sessions
 Note that BGP sessions do not correspond to physical links.
 When AS2 advertises a prefix to AS1, AS2 is promising it will
forward any datagrams destined to that prefix towards the
prefix.

AS2 can aggregate prefixes in its advertisement
3c
3a
3b
AS3
1a
AS1
2a
1c
1d
1b
2c
AS2
2b
eBGP session
iBGP session
4-24
Customers and Providers
provider
provider
customer
IP traffic
customer
Customer pays provider for access to the Internet
Customer may not always need BGP
4-25
Customer-Provider Hierarchy
provider
customer
IP traffic
4-26
The Peering Relationship
peer
provider
traffic
allowed
peer
customer
traffic NOT
allowed
Peers provide transit between
their respective customers
Peers do not provide transit
between peers
Peers (often) do not exchange $$$
4-27
Distributing reachability info
 With eBGP session between 3a and 1c, AS3 sends prefix
reachability info to AS1.
 1c can then use iBGP do distribute this new prefix reach info
to all routers in AS1
 1b can then re-advertise the new reach info to AS2 over the
1b-to-2a eBGP session
 When router learns about a new prefix, it creates an entry
for the prefix in its forwarding table.
3c
3a
3b
AS3
1a
AS1
2a
1c
1d
1b
2c
AS2
2b
eBGP session
iBGP session
4-28
Path attributes & BGP routes
 When advertising a prefix, advert includes BGP
attributes.

prefix + attributes = “route”  BGP announcement
 Attributes include: Next hop, AS Path, local preference,
Multi-exit discriminator, …

Used to select among multiple options for paths
 Two important attributes:
 AS-PATH: contains the ASs through which the advert
for the prefix passed: AS 67 AS 17
 NEXT-HOP: Indicates the specific internal-AS router to
next-hop AS. (There may be multiple links from current
AS to next-hop-AS.) E.g., 3a in the example
 When gateway router receives route advert, uses
import policy to accept/decline.
4-29
BGP route selection
 Router may learn about more than 1 route
to some prefix. Router must select route.
 Elimination rules:
1.
2.
3.
4.
Local preference value attribute: policy
decision
Shortest AS-PATH
Closest NEXT-HOP router: hot potato routing
Additional criteria
4-30
BGP Route Selection Summary
Highest Local Preference
Enforce relationships
E.g. prefer customer routes
over peer routes
Shortest ASPATH
Lowest MED
i-BGP < e-BGP
traffic engineering
Lowest IGP cost
to BGP egress
Lowest router ID
Throw up hands and
break ties
4-31
ASPATH Attribute
135.207.0.0/16
AS Path = 1755 1239 7018 6341
135.207.0.0/16
AS Path = 1239 7018 6341
AS 1239
Sprint
135.207.0.0/16
AS Path = 6341
AS 6341
AT&T Research
135.207.0.0/16
Prefix Originated
AS 1755
Ebone
135.207.0.0/16
AS Path = 7018 6341
AS 7018
AS 1129
Global Access
135.207.0.0/16
AS Path = 1129 1755 1239 7018 6341
Pick shorter
AS path
AS 12654
RIPE NCC
RIS project
135.207.0.0/16
AS Path = 3549 7018 6341
AT&T
135.207.0.0/16
AS Path = 7018 6341
AS 3549
Global Crossing
4-32
Frank’s Choices…
Route learned from customer
preferred over
route learned from peer, preferred
over
route learned from provider
peer
provider
peer
AS 4
customer
local pref = 80
local pref = 90
AS 3
local pref = 100
Set appropriate “local pref”
to reflect preferences:
Higher Local preference values
are preferred
AS 2
AS 1
13.13.0.0/16
4-33
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

4-34
BGP routing policy
legend:
B
W
provider
network
X
A
customer
network:
C
Y
Figure 4.5-BGPnew: a simple BGP scenario
 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

4-35
BGP routing policy (2)
legend:
B
W
provider
network
X
A
customer
network:
C
Y
 A advertises to B the path AW
Figure 4.5-BGPnew: a simple BGP scenario
 B advertises to X the path BAW
 Should B advertise to C the path BAW?
 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!
4-36
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
4-37
Network Layer
 Introduction
 Datagram networks
 IP: Internet Protocol
 Datagram format
 IPv4 addressing
 ICMP
 What’s inside a router
 Routing algorithms
 Link state
 Distance Vector
 Routing in the
Internet



RIP
OSPF
BGP
 Multicast routing
4-38
Spanning Tree
 First construct a spanning tree
 Nodes forward copies only along spanning
tree
A
B
c
F
A
E
B
c
D
F
G
(a) Broadcast initiated at A
E
D
G
(b) Broadcast initiated at D
4-39
Multicast Trees
The basic idea
G Server
G Server
G
G
G
G
G
G
Multiple unicasts
G
G
Single multicast
4-40
Applications that need multicast
 One way, single sender: “one-to-many”
 TV
 Non-interactive learning
 Database update
 Information dispersal (e.g. Pointcast)
 Software updates/patches
 Two way, interactive, multiple sender:
“many-to-many”
 Teleconference

Interactive learning
4-41
Multicast Routing
 A multicast tree is a spanning tree with the
sender at the root, spanning all the
members of the group.
4-42
Multicast Trees
e.g. a teleconference
S1
Sender/Speaker
Multicast Group (S1,G)
S1 Class
D
R
4-43
Multicast Trees and Addressing
 All members of the group share the same
“Class D” Group Address.
 An end station may be the member of multiple
groups.
 An end-station “joins” a multicast group by
(periodically) telling its nearest router that it
wishes to join (uses IGMP – Internet Group
Management Protocol).
 Routers maintain “soft-state” indicating which
end-stations have subscribed to which groups.
4-44
Multicast Trees
Multiple source trees
S2 Class
D
R
S2
Sender/Speaker
Multicast Group (S2,G)
4-45
Multicast Forwarding is
Sender-specific
Group
Src
Src
Dst
Address Address Interface Interface
G
S1
S1
1
2,3
S2
2
1,3
R
G
1
1
2
3
2
S2 G
3
4-46
Distance-vector Multicast
RPB: Reverse-Path Broadcast
 Uses existing unicast shortest path routing
table.

Computed using Distance vector
 If packet arrived through interface that is
the shortest path to the packet’s SA, then
forward packet to all interfaces.
 Else drop packet.
4-47
Distance-vector Multicast
RPB: Reverse-Path Broadcast
Unicast
DV Routing
Table
Address
Port
S1
1
S1
Sender/Speaker
Multicast Group (S1,G)
1
3
Shortest Path to
Source
2
LAN
Q: Is it shortest path
from source?
4-48
Distance-vector Multicast
RPB: Reverse-Path Broadcast
S1
Sender/Speaker
Multicast Group (S1,G)
Designated Parent
Router:
One parent router
picked per LAN (one
“closest” to source).
LAN
4-49
Internet Multicasting Routing: DVMRP
 DVMRP: distance vector multicast routing
protocol, RFC1075
 flood and prune: reverse path broadcast,
source-based tree
RPB tree based on DVMRP’s own routing tables
constructed by communicating DVMRP routers
 no assumptions about underlying unicast
 initial datagram to mcast group flooded
everywhere via RPB
 routers not wanting group: send upstream prune
msgs

4-50
DVMRP: continued…
 soft state: DVMRP router periodically (1 min.)
“forgets” branches are pruned:
mcast data again flows down unpruned branch
 downstream router: reprune or else continue to
receive data

 routers can quickly regraft to tree

following IGMP join at leaf
 odds and ends
 commonly implemented in commercial routers
 Mbone routing done using DVMRP
4-51
Tunneling
Q: How to connect “islands” of multicast
routers in a “sea” of unicast routers?
physical topology
logical topology
 mcast datagram encapsulated inside “normal” (non-multicast-
addressed) datagram
 normal IP datagram sent thru “tunnel” via regular IP unicast to
receiving mcast router
 receiving mcast router unencapsulates to get mcast datagram
4-52
Multicast: Interesting Questions
 How to make multicast reliable?
 How to implement flow-control?
 How to support/provide different rates for
different end users?
 How to secure a multicast conversation?
 Will multicast become widespread?

Several protocols for multicast routing in IP
• But IP multicast is not enabled in routers!
• No one uses IP multicast, really
• End-system based, overlay-based approaches more popular
4-53