Chapter 4: Network Layer

1
Chapter 4 Network Layer

• 4. 1 Introduction
• 4.2 Virtual circuit and datagram networks
• 4.3 Whats inside a router
• 4.4 IP Internet Protocol
• Datagram format
• IPv4 addressing
• ICMP
• IPv6

• 4.5 Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• 4.6 Routing in the Internet
• RIP
• OSPF
• BGP
• 4.7 Broadcast and multicast routing

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Hierarchical Routing

• Our routing study thus far - idealization
• all routers identical
• network flat
• not true in practice

• scale with 200 million destinations
• cant store all dests 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

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Hierarchical Routing

• aggregate routers into regions, autonomous
  systems (AS)
• routers in same AS run same routing protocol
• intra-AS routing protocol
• routers in different AS can run different
  intra-AS routing protocol

• special routers in AS
• run intra-AS routing protocol with all other
  routers in AS
• also responsible for routing to destinations
  outside AS
• run inter-AS routing protocol with other gateway
  routers

4
Intra-AS and Inter-AS routing

• Gateways
• perform inter-AS routing amongst themselves
• perform intra-AS routers with other routers in
  their AS

b
a
a
C
B
d
A
network layer
inter-AS, intra-AS routing in gateway A.c
link layer
physical layer
5
Intra-AS and Inter-AS routing
Host h2
Intra-AS routing within AS B
Intra-AS routing within AS A

• Well examine specific inter-AS and intra-AS
  Internet routing protocols shortly

6
Chapter 4 Network Layer

• 4. 1 Introduction
• 4.2 Virtual circuit and datagram networks
• 4.3 Whats inside a router
• 4.4 IP Internet Protocol
• Datagram format
• IPv4 addressing
• ICMP
• IPv6

• 4.5 Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• 4.6 Routing in the Internet
• RIP
• OSPF
• BGP
• 4.7 Broadcast and multicast routing

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Routing in the Internet

• The Global Internet consists of Autonomous
  Systems (AS) interconnected with each other
• Stub AS small corporation one connection to
  other ASs
• Multihomed AS large corporation (no transit)
  multiple connections to other ASs
• Transit AS provider, hooking many ASs together
• AS Lookup
• https//www.ultratools.com/tools/asnInfo
• Two-level routing
• Intra-AS administrator responsible for choice of
  routing algorithm within network
• Inter-AS unique standard for inter-AS routing
  BGP

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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)

9
Chapter 4 Network Layer

• 4. 1 Introduction
• 4.2 Virtual circuit and datagram networks
• 4.3 Whats inside a router
• 4.4 IP Internet Protocol
• Datagram format
• IPv4 addressing
• ICMP
• IPv6

• 4.5 Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• 4.6 Routing in the Internet
• RIP
• OSPF
• BGP
• 4.7 Broadcast and multicast routing

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RIP ( Routing Information Protocol)

• Distance vector algorithm
• Included in BSD-UNIX Distribution in 1982
• Distance metric of hops (max 15 hops)
• Can you guess why?
• Distance vectors exchanged among neighbors every
  30 sec via Response Message (also called
  advertisement)
• Each advertisement list of up to 25 destination
  nets within AS

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RIP Example
z
w
x
y
A
D
B
C
Destination Network Next Router Num. of
hops to dest. w A 2 y B 2
z B 7 x -- 1 . . ....
Routing table in D
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RIP Example
Dest Next hops w - - x -
- z C 4 . ...
Advertisement from A to D
Destination Network Next Router Num. of
hops to dest. w A 2 y B 2 z B
A 7 5 x -- 1 . . ....
Routing table in D
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RIP Link Failure and Recovery

• If no advertisement heard after 180 sec --gt
  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)
• RIP is based on UDP packets

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Chapter 4 Network Layer

• 4. 1 Introduction
• 4.2 Virtual circuit and datagram networks
• 4.3 Whats inside a router
• 4.4 IP Internet Protocol
• Datagram format
• IPv4 addressing
• ICMP
• IPv6

• 4.5 Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• 4.6 Routing in the Internet
• RIP
• OSPF
• BGP
• 4.7 Broadcast and multicast routing

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OSPF (Open Shortest Path First)

• Successor to RIP
• Uses Link State algorithm
• LS packet dissemination
• Route computation using Dijkstras algorithm
• Advertisements disseminated to entire AS (via
  flooding)
• Carried in OSPF messages directly over IP (no TCP
  or UDP)
• ICMP is directly on IP, too

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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)
• Hierarchical OSPF in large domains.

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Hierarchical OSPF
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Hierarchical OSPF

• 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 ASs.

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Chapter 4 Network Layer

• 4. 1 Introduction
• 4.2 Virtual circuit and datagram networks
• 4.3 Whats inside a router
• 4.4 IP Internet Protocol
• Datagram format
• IPv4 addressing
• ICMP
• IPv6

• 4.5 Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• 4.6 Routing in the Internet
• RIP
• OSPF
• BGP
• 4.7 Broadcast and multicast routing

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Inter-AS routing in the Internet BGP
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Internet inter-AS routing BGP

• BGP (Border Gateway Protocol) the de facto
  standard for inter-AS routing
• BGP glues the whole Internet together
• Uses TCP (service port 179)
• Path Vector protocol
• similar to Distance Vector protocol
• each Border Gateway broadcast to neighbors
  (peers) entire path (i.e., sequence of ASs) to
  destination
• BGP routes to networks (ASs), not individual
  hosts
• E.g., Gateway X may send its path to dest. Z
• Path (X,Z) X,Y1,Y2,Y3,,Z

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Internet inter-AS routing BGP

• Suppose gateway X send its path to peer gateway
  W
• W may or may not select path offered by X
• cost, policy (dont route via competitors AS),
  loop prevention reasons
• If W selects path advertised by X, then
• Path (W,Z) w, Path (X,Z)
• Note X can control incoming traffic by
  controlling its route advertisements to peers
• e.g., dont want to route traffic to Z -gt dont
  advertise any routes to Z

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BGP controlling who routes to you

• 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

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BGP controlling who routes to you

• A advertises to B the path AW
• 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 Bs customers
• B wants to force C to route to w via A
• B wants to route only to/from its customers!

Policy !!
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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

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Why different Intra- and Inter-AS routing ?

• Policy
• Inter-AS admin wants control over how its
  traffic routed, who routes through its net
  (policy).
• 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

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Network Layer summary

• What weve covered
• network layer services
• routing principles link state and distance
  vector
• Hierarchical routing
• IP
• Internet routing protocols RIP, OSPF, BGP
• whats inside a router?
• IPv6

• Next stop
• the Data
• link layer!