Download 3rd Edition: Chapter 4

Network Layer
CS 381
3/29/2017
2-1
Chapter 4: outline
4.1 Introduction
4.2 virtual circuit and datagram networks
4.3 what’s 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
Network Layer 4-2
Hierarchical routing
Our routing study thus far - idealization
 All routers identical
•
•

Execute same routing algorithms
Same hardware specifications
Network “flat”
 No tiers
 No packet priority
… not true in practice
Network Layer 4-3
Hierarchical routing
Scale: with billions of destinations:
 128 items are added to the Internet every second!
• Calculated by CISCO
• 5,529,600 new devices connected by the end of today.

Can’t store all destinations in routing tables!
 Space requirements too high

Routing table exchange would swamp links!
 No bandwidth left for sending data
Network Layer 4-4
Hierarchical routing
Administrative autonomy:

Internet:
 Network of networks
• Think of how WKU controls its LAN (campus network)
– Policies, bandwidth restrictions, authentication, security, etc.


Organizations desire to run specific routing
protocols and hide aspects of it’s network
 Routing protocols must be universal, or at least adhere
to a set of defined functions
Ideally, an organization should be able to:
 Run and administer its network as it wishes
 Able to connect its network to other networks
Network Layer 4-5
Hierarchical routing

“Autonomous Systems” (AS)
 Group of routers under the same admin control
• Same ISP, company, organization

Routers in same AS run same routing
protocol
 Intra-Autonomous System routing protocol
• Ex: LS or DV
 Routers in different AS can run different intra-AS
routing protocol
 Multiple AS need to connect to each other in some way

Gateway Router:
 At “edge” of its own AS
 Has links to gateway routers in other ASs
Network Layer 4-6
Interconnected ASes

Forwarding table configured by both intra- and
inter-AS routing algorithm
 intra-AS sets entries for internal destinations
 inter-AS & intra-AS sets entries for external destinations
3c
3a
3b
2c
2a
AS3
1c
1a
2b
AS2
1b
1d
Intra-AS
Routing
algorithm
AS1
Inter-AS
Routing
algorithm
Forwarding
table
Network Layer 4-7
Inter-AS tasks

Suppose router in AS1 AS1 must:
receives datagram
1. Learn which destinations are
destined outside of
reachable through AS2,
AS1:
which through AS3
 Router should forward 2. Propagate this reachability
packet to gateway
info to all routers in AS1
router, but which one?
Job of inter-AS routing!
3c
3b
other
networks
3a
AS3
2c
1c
1a
AS1
1d
2a
1b
2b
other
networks
AS2
Network Layer 4-8
Inter-AS tasks

AS learns about a destination from a
neighboring AS
 The AS can advertise this information to other routers

Steps for adding an outside destination:
1. Learn from inter-AS communication that destination is
reachable by multiple gateways
2. Use routing info to determine least-cost paths to each
gateway
3. Choose gateway that has the least cost
4. Determine interface that leads to least cost gateway
5. Update forwarding table with this information
Network Layer 4-9
Chapter 4: outline
4.1 Introduction
4.2 virtual circuit and datagram networks
4.3 what’s 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
Network Layer 4-10
Intra-AS Routing

Also known as Interior Gateway protocols
(IGP)
 Protocols used to determine how routing is performed
within an AS.

Most common intra-AS routing protocols:
 RIP: Routing Information Protocol
 OSPF: Open Shortest Path First
 IGRP: Interior Gateway Routing Protocol
 Cisco proprietary
Network Layer 4-11
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)
v
u
A
B
w
x
z
C
D
y
from router A to destination subnets:
subnet hops
u
1
v
2
w
2
x
3
y
3
z
2
Network Layer 4-12
RIP: example
z
w
A
x
y
B
D
C
routing table in router D
destination subnet
next router
# hops to dest
w
y
z
x
A
B
B
--
2
2
7
1
….
….
....
Network Layer 4-13
RIP: example
dest
w
x
z
….
w
A
A-to-D advertisement
next hops
1
1
C
4
… ...
x
z
y
B
D
C
routing table in router D
destination subnet
next router
# hops to dest
w
y
z
x
A
B
A
B
--
2
2
5
7
1
….
….
....
Network Layer 4-14
RIP: link failure, recovery
If no advertisement heard after 180 sec 
neighbor/link declared dead
 Routes via neighbor invalidated
 New advertisements sent to neighbors based on
timeout
 Neighbors in turn send out new advertisements
(if tables changed)
 Link failure info quickly (?) propagates to entire
net
Network Layer 4-15
OSPF (Open Shortest Path First)


Another Intra-AS routing protocol
“Open”
 Publicly available (RFC 2328)

Uses link state algorithm
 Topology map at each node
 Route computation uses Dijkstra’s algorithm

OSPF advertisement:
 Shortest-path tree to all destinations in AS

Advertisements flooded to entire AS
 Carried in OSPF messages directly over IP (rather than
TCP or UDP

Typically deployed in upper-tier ISPs
 RIP deployed in lower-tier ISPs

Conceived as the successor to RIP
Network Layer 4-16
OSPF “advanced” features (not in RIP)

Security:
 All OSPF messages authenticated (to prevent
malicious intrusion)

Multiple same-cost paths allowed:
 Single path to destination not needed when
multiple equal-cost paths exist
• Only one path in RIP
Integrated uni- and multicast support:
 hierarchical OSPF in large domains:

 Ability to structure ASs
Network Layer 4-17
Internet inter-AS routing: BGP

BGP (Border Gateway Protocol):
 “Glue that holds the Internet together”
 Most widely used inter-domain routing protocol

BGP provides each AS a means to:
 External Border Gateway Protocol (eBGP):
• Obtain subnet reachability information from neighboring ASs.
 Internal Border Gateway Protocol (iBGP):
• Propagate reachability information to all AS-internal routers.

Allows subnet to advertise its existence to
rest of Internet: “I am here”
Network Layer 4-18
BGP basics

BGP session: two BGP routers (“peers”) exchange
BGP messages:
 Advertising paths to different destination networks
 Exchanged over semi-permanent TCP connections
• When AS3 advertises a destination network to
AS1:
• AS3 promises it will forward datagrams towards that
network
• AS3 can aggregate destinations in its advertisement
3c
3b
other
networks
3a
BGP
message
AS3
2c
1c
1a
AS1
1d
2a
1b
2b
other
networks
AS2
Network Layer 4-19
How does entry get in forwarding table?
Summary
1.
Router becomes aware of IP subnet prefix
 via BGP route advertisements from other routers
2.
Determine router output port for prefix
 Use OSPF to find best intra-AS route leading to best
inter-AS route
 Use BGP route selection to find best inter-AS route
 Router identifies port/link for that best route
3.
Enter prefix-port entry in forwarding table
Network Layer 4-23
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

Network Layer 4-21
Chapter 4: outline
4.1 Introduction
4.2 virtual circuit and datagram networks
4.3 what’s 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
Network Layer 4-22
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 all recipient
addresses?
Network Layer 4-23
In-network duplication

Flooding: when node receives broadcast packet,
sends copy to all neighbors
 Problems:
• Cycles, broadcast storms

Controlled flooding: node only broadcasts packet
if it hasn’t broadcast same packet before
 Router keeps track of packet IDs already broadcasted
 Or reverse path forwarding (RPF): Router forwards on all links
except the link which received the broadcast packet

Spanning tree:
 No redundant packets received by any node
 Complexity associated with building and maintaining the spanning
tree instead of source duplication
Network Layer 4-24
 Can be accomplished using Dijkstra’s Algorithm
Multicast routing: problem statement

Multicast service:
 Packets from one or more senders delivered to only a
subset of network nodes

Goal:
 Find a tree (or trees) connecting routers that have local
multicast group members

Isolated Tree:
 Not all paths between routers used
 Some multicast members not connected

Shared Tree:
 Same tree used by all group members
Network Layer 4-25
Tunneling of Mcast Data
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 through “tunnel” via
regular IP unicast to receiving Mcast router



Recall IPv6 inside IPv4 tunneling
Receiving Mcast router unencapsulates to get
Mcast datagram
Network Layer 4-26
Network Layer Summary
Network Layer 4-27
Two key network-layer functions

Forwarding:
 Move packets from router’s
input to appropriate router
output

Analogy:

Routing:
 Determine route taken by
packets from source to
destination.

Forwarding: process of
getting through single
router
Routing: process of
planning trip from
source to destination
Network Layer 4-28
Interplay between routing and forwarding
routing algorithm
routing algorithm determines
end-end-path through network
local forwarding table
header value output link
forwarding table determines
local forwarding at this router
0100
0101
0111
1001
3
2
2
1
value in arriving
packet’s header
0111
1
3 2
Network Layer 4-29
Connection, connection-less service
datagram network provides network-layer
connectionless service
 virtual-circuit network provides network-layer
connection service


Analogous to TCP/UDP connection-oriented /
connectionless transport-layer services, but:
 service: host-to-host
 no choice: network provides one or the other
 implementation: in network core
Network Layer 4-30
Datagram networks
no call setup at network layer
 routers: no state about end-to-end
connections

 no network-level concept of “connection”

packets forwarded using destination host
address
application
transport
network 1. send datagrams
data link
physical
application
transport
2. receive datagrams network
data link
physical
Network Layer 4-31
Router architecture overview
Two key router functions:


Run routing algorithms/protocol (RIP, OSPF, BGP)
Forwarding datagrams from incoming to outgoing link
forwarding tables computed,
pushed to input ports
routing
processor
routing, management
control plane (software)
forwarding data
plane (hardware)
high-seed
switching
fabric
router input ports
router output ports
Network Layer 4-32
The Internet network layer
host, router network layer functions:
transport layer: TCP, UDP
IP protocol
routing protocols
network
layer
• addressing conventions
• datagram format
• packet handling conventions
• path selection
• RIP, OSPF, BGP
forwarding
table
ICMP protocol
• error reporting
• router
“signaling”
link layer
physical layer
Network Layer 4-33
IP datagram format
IP protocol version
number
header length
(bytes)
“type” of data
max number
remaining hops
(decremented at
each router)
upper layer protocol
to deliver payload to
how much overhead?
 20 bytes of TCP
 20 bytes of IP
 = 40 bytes + app
layer overhead
32 bits
total datagram
length (bytes)
ver head. type of
len service
length
16-bit identifier
upper
time to
layer
live
fragment
flgs
offset
header
checksum
for
fragmentation/
reassembly
32 bit source IP address
32 bit destination IP address
options (if any)
data
(variable length,
typically a TCP
or UDP segment)
e.g. timestamp,
record route
taken, specify
list of routers
to visit.
Network Layer 4-34
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/“on”)
 support for mobile users who want to join network
(more shortly)
DHCP overview:
 host broadcasts “DHCP discover” msg [optional]
 DHCP server responds with “DHCP offer” msg
[optional]
 host requests IP address: “DHCP request” msg
 DHCP server sends address: “DHCP ACK” msg
Network Layer 4-35
NAT: network address translation
rest of
Internet
local network
(e.g., home network)
10.0.0/24
10.0.0.1
10.0.0.4
10.0.0.2
138.76.29.7
10.0.0.3
all datagrams leaving local
network have same single
source NAT IP address:
138.76.29.7,different source
port numbers
datagrams with source or
destination in this network
have 10.0.0/24 address for
source, destination (as usual)
Network Layer 4-36
A Link-State Routing Algorithm
Dijkstra’s algorithm:

network 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
 Creates minimum spanning tree

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
iterative: after k iterations,
know least cost path to k
destinations
Network Layer 4-37
Distance vector algorithm
Bellman-Ford equation (dynamic programming)
let
dx(y) := cost of least-cost path from x to y
then
dx(y) = min
{c(x,v)
+
d
(y)
}
v
v
cost from neighbor v to destination y
cost to neighbor v
min taken over all neighbors v of x
Network Layer 4-38
Hierarchical routing
our routing study thus far - idealization
 all routers identical
 network “flat”
… not true in practice
scale: with billions of
destinations:


can’t store all destinations
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
Network Layer 4-39
Chapter 4: done!
4.1 Introduction
4.2 virtual circuit and datagram networks
4.3 what’s inside a router
4.4 IP: Internet Protocol
 Understand principles behind
 datagram format
network layer services:
 IPv4 addressing
 ICMP
 Network layer service models,
 IPv6
 forwarding versus routing
4.5 routing algorithms
 how a router works,
 link state
 routing (path selection),
 distance vector
 broadcast, multicast
 hierarchical routing
 Implementation in the Internet
4.6 routing in the Internet
 RIP
 OSPF
 BGP
4.7 broadcast and multicast routing
Network Layer 4-40