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UNIT –III
Network Layer
Prof. Nalini Mhetre,
Dept. of Computer Engg.
SKNCOE, Pune
Network Layer
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Introduction
Basics of IPV4, ICMPV4,
IPV6,
ARP, RARP,
Mobile IP,
Link state routing algorithms,
Distance vector routing algorithms
Routing protocols:
– RIP, OSPF, BGP
Introduction
Position of network layer
Network layer
• transport segment from
sending to receiving host
• on sending side
encapsulates segments into
datagrams
• on receiving side, delivers
segments to transport layer
• network layer protocols in
every host, router
• router examines header
fields in all IP datagrams
passing through it
application
transport
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
application
transport
network
data link
physical
Network layer at the source
Network layer at a router
Network layer at the destination
Network layer duties
The challenge is interconnecting different networks
(various LAN technologies, telephone network, satellite
link, ATM networks etc.) and making them look the
same to the transport layer. The transport layer should
not be worried about the underlying physical network !
Network layer duties
Unique addresses are required to define each
host/machine/device/user in the network. We cannot
use the data link layer addresses !! Because these
addresses depend on the technology used in the data
link layer. Remember, network layer is independent of
the data link layer.
Network layer duties
This is a task common to all layers. The Protocol Data
Units (PDU’s) coming from the transport layer must be
placed in network-layer packets and sent to the datalink layer.
Network layer duties
Remember, the network layer must be able to operate
on top of any data-link layer technology (Ethernet, Fast
Ethernet, ATM etc.). All these technologies can handle a
different packet length. The network layer must be able
to fragment transport layer PDUs into smaller units so
that they can be transferred over various data-link layer
technologies.
Network layer duties
Now that you have your network layer packet, where do
you send it ?
Used in computer networks
and (also in modern
telephone networks).
Packets of bits (not lines)
are switched!
Used in telephone networks
for more than 100 years. A
physical link is dedicated
between Source and
Destination. Data can be
sent as a stream of bits
without the need for
packetising.
(Also called
Connection-oriented
networking)
(Also called
Connectionless
networking)
Note:
An IP address is a 32-bit address.
Note:
The IP addresses are unique
and universal.
Dotted-decimal notation
Note:
In classful addressing, the address
space is divided into five classes: A, B,
C, D, and E.
Finding the class in binary notation
If the addresses are given in binary notation , first few bites
can immediately tell us the class of the address.
Finding the address class
One can follow the procedure to systematically check the bits
& find the class.
Finding the class in decimal notation
When the address is given in dotted-decimal notation , then we
need to look only at the first byte (number) to determine the
class of the address. Each class has a specific range of numbers.
Example
Find the class of each address:
a.
227.12.14.87
b.
252.5.15.111
c.
134.11.78.56
Solution
a.
b.
c.
The first byte is 227 (between 224 and 239); the class is D.
The first byte is 252 (between 240 and 255); the class is E.
The first byte is 134 (between 128 and 191); the class is B.
Unicast ,Multicast & Reserved Addresses
•
Addresses in classes A, B & C are for unicast
communication, form one source to one destination. A host
needs to have at least one unicast address to be send or receive
packets.
• Addresses in class D are for Multicast communication, form
one source to group of destination. Multicast address can be
used only as a destination address, but never as a source
address.
• Addresses in class E are reserved. The original idea was to use
them for special purpose. They have been used only in few
cases.
Netid and hostid
In classful addressing, an IP address in classes A,B & C is
divided into netid and hostid .
These parts are of varying lengths, depending on the class of
the address .
Classes D and E are not divided in to netid and hostid.
Note:
In classful addressing, the network
address is the one that is assigned to
the organization.
Note:
A network address is different from a
netid. A network address has both
netid and hostid,
with 0s for the hostid.
Sample internet
Addresses in a network with and without
subnetting
Hierarchy concept in a telephone number
Default masks
Class
In Binary
A
11111111 00000000
00000000
00000000
B
11111111 11111111
00000000
00000000
C
11111111 111111111 11111111
00000000
In DottedDecimal
Using Slash
255.0.0.0
/8
255.255.0.0
/16
255.255.255.0
/24
Note:
The network address can be found
by applying the default mask to any
address in the block (including itself).
It retains the netid of the block and sets
the hostid to 0s.
Subnet mask
Basics of IPV4
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
IP
v4
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



20 bytes of TCP
20 bytes of IP
= 40 bytes + app
layer overhead
32 bits
ver
head. type of
len service
16-bit identifier
upper
time to
layer
live
length
flgs
fragment
offset
header
checksum
total datagram
length (bytes)
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.
Basics of ICMPV4
ICMP: internet control message protocol
• used by hosts & routers to communicate network-level
information
– error reporting: unreachable host, network, port, protocol
– echo request/reply (used by ping)
• network-layer “above” IP:
– ICMP msgs carried in IP datagrams
• ICMP message: type, code plus first 8 bytes of IP datagram
causing error
ICMP Message format
Type 8-bits
Code 8-bits
Checksum 16-bits
Unused
IP HEADER AND ORIGINAL DATAGRAM
Type: type of ICMP msg. Error reporting or query
Code: parameter of msg type
Checksum: checksum of entire ICMP msg
Sample ‘Type and codes’ with their meaning
Type Code description
0
3
3
3
3
3
3
4
0
0
1
2
3
6
7
0
8
9
10
11
12
0
0
0
0
0
echo reply (ping)
dest. network unreachable
dest host unreachable
dest protocol unreachable
dest port unreachable
dest network unknown
dest host unknown
source quench (congestion
control - not used)
echo request (ping)
route advertisement
router discovery
TTL expired
bad IP header
ICMP basics
• IP provides unreliable & connectionless datagram delivery.
• IP has two deficiencies : Lack of error control & Lack of
assistance mechanisms.
• It has no error reporting & error-correcting mechanisms.
• The Internet Control Message Protocol (ICMP) has been
designed to compensate for the above tow deficiencies. It
is a companion to the IP.
• ICMP itself is a network layer protocol.
• Its messages are not passed directly to the data
link layer .
• Instead ,the messages are first encapsulated
inside IP datagram before going to the lower
layer.
• The value of the protocol field in the IP is 1 to
indicate that the IP data are an ICMP message.
ICMP encapsulation
• ICMP messages are divided in to two broad
categories:
1)Error-reporting messages.
2) Query messages.
Error-reporting messages.
• One of the main responsibility of ICMP is to report
error.
• ICMP does not correct errors, it simply reports them.
• Errors correction is left to the higher layer protocol.
• Error reporting messages are always sent to the
original source because the only information available
in the datagram about the route is the source &
destination IP addresses.
• ICMP uses the source IP address to send the error
message to the source of the datagram.
• ICMP always reports error messages to
the original source.
Basics of IPV6
IPv6: motivation
• initial motivation: 32-bit address space soon
to be completely allocated.
• additional motivation:
– header format helps speed processing/forwarding
– header changes to facilitate QoS
IPv6 datagram format:
– fixed-length 40 byte header
– no fragmentation allowed
IPv6 datagram format
Priority/traffic class: identify priority among datagrams in flow
flow Label: identify datagrams in same “flow.”
(concept of “flow” not well defined).
next header: identify upper layer protocol for data
ver
pri
flow label
hop limit
payload len
next hdr
source address
(128 bits)
destination address
(128 bits)
data
32 bits
• Ver: 4 bits for IP protocol version
• Priority/traffic class: priority of packet wrt
congestion
• flow Label: 3 bytes for special handling at routers
• Payload len:16 bits for total len incuding header
• Next header: 8 bits to specify next imm header
after ip header
• Hop limit: like TTL
• Source and destination addr: each of 128 bits
Other changes from IPv4
• checksum: removed entirely to reduce
processing time at each hop
• options: allowed, but outside of header,
indicated by “Next Header” field
• ICMPv6: new version of ICMP
– additional message types, e.g. “Packet Too Big”
– multicast group management functions
IP v6 addresses (128 bits)
Dotted hexadecimal form is used in practice
Abbreviated address
Abbreviated address with consecutive zeros
Comparison of network layers in
version 4 and version 6
Tunneling
Header translation
ARP and RARP
ARP operation
ARP packet
Note:
An ARP request is broadcast; an ARP
reply is unicast.
Mobile IP
Effect of Mobility on Protocol Stack
•
•
•
•
•
Application: new applications and adaptations
Transport: congestion and flow control
Network: addressing and routing
Link: media access and handoff
Physical: transmission errors and interference
Routing and Mobility
• Finding a path from a source to a
destination
• Issues
– Frequent route changes
– Route changes may be related to host
movement
– Low bandwidth links
Routing and Mobility (contd)
• Goal of routing protocols
– decrease routing-related overhead
– find short routes
– find “stable” routes (despite mobility)
Mobile IP (RFC 3344): Motivation
• Traditional routing
– based on IP address; network prefix determines the
subnet
– change of physical subnet implies
• change of IP address (conform to new subnet), or
• special routing table entries to forward packets to new
subnet
Quick Solution
• Changing of IP address
– Use DHCP to have a new IP address when mobile device moves to a
new subnet
• but then the new address may not be known to anyone
– Take help of DNS to update the entry
• DNS updates take long time
– TCP connections break
– security problems
• Changing entries in routing tables
– change routing table entries as the MN moves from one network to
another
• does not scale with the number of mobile hosts and frequent
changes in the location
• security problems
Mobile IP requirements
• Solution requirements
– Compatibility
• The new standard cannot introduce changes to applications and
network protocols in use
• Should be compatible with lower layers
– use same layer 2 protocols
– Transparency
• New protocol should be transparent to higher layer
– For TCP it means the MN should retain same IP address
– Scalability
• Enhancing IP for mobility should not generate too many messages
• Should scale when there are a large number of MNs
– Security
• Management messages should be authenticated
Mobile IP: Terminology
• Mobile Node (MN)
– node that moves across networks without changing
its IP address
• Correspondent Node (CN)
– host with which MN is “corresponding” (TCP)
• Home Agent (HA)
– host in the home network of the MN, typically a
router
– registers the location of the MN, tunnels IP packets
to the COA
Terminology (contd.)
• Foreign Agent (FA)
– host in the current foreign network of the MN,
typically a router
– forwards tunneled packets to the MN, typically the
default router for MN
• Care-of Address (COA)
– address of the current tunnel end-point for the
MN (at FA or MN)
– actual location of the MN from an IP point of view
Mobility: Vocabulary
home network: permanent
“home” of mobile
(e.g., 128.119.40/24)
Permanent address:
address in home
network, can always be
used to reach mobile
home agent: entity that will
perform mobility functions on
behalf of mobile, when mobile
is remote
wide area
network
e.g., 128.119.40.186
Correspondent node:
Source: Jim Kurose’s slides
Mobility: more vocabulary
Permanent address: remains
constant (e.g., 128.119.40.186)
visited network: network
in which mobile currently
resides (e.g., 79.129.13/24)
Care-of-address: address
in visited network.
(e.g., 79,129.13.2)
wide area
network
correspondent: wants
to communicate with
mobile
Foreign agent: entity
in visited network
that performs
mobility functions on
behalf of mobile.
Source: Jim Kurose’s slides
Mobility: approaches
• Let routing handle it: routers advertise permanent
address of mobile-nodes-in-residence via usual routing
table exchange.
– routing tables indicate where each mobile located
– no changes to end-systems
• Let end-systems handle it:
– indirect routing: communication from correspondent
to mobile goes through home agent, then forwarded
to remote
– direct routing: correspondent gets foreign address of
mobile, sends directly to mobile
Source: Jim Kurose’s slides
Mobility: registration
visited network
home network
2
1
wide area
network
foreign agent contacts home
agent home: “this mobile is
resident in my network”
mobile contacts
foreign agent on
entering visited
network
End result:
• Foreign agent knows about mobile
Source: Jim Kurose’s slides
• Home agent knows location of mobile
Mobility via Indirect Routing
foreign agent
receives packets,
forwards to mobile
home agent intercepts
packets, forwards to
foreign agent
home
network
visited
network
3
wide area
network
correspondent
addresses packets
using home address
of mobile
1
2
4
mobile replies
directly to
correspondent
Source: Jim Kurose’s slides
Indirect Routing: comments
• Mobile uses two addresses:
– permanent address: used by correspondent (hence
mobile location is transparent to correspondent)
– care-of-address: used by home agent to forward
datagrams to mobile
• foreign agent functions may be done by mobile itself
• triangle routing: correspondent-home-network-mobile
– inefficient when
correspondent, mobile
are in same network
Forwarding datagrams to remote mobile
foreign-agent-to-mobile packet
packet sent by home agent to foreign
agent: a packet within a packet
dest: 79.129.13.2
dest: 128.119.40.186
dest: 128.119.40.186
Permanent address:
128.119.40.186
dest: 128.119.40.186
packet sent by
correspondent
Care-of address:
79.129.13.2
Source: Jim Kurose’s slides
Indirect Routing: moving between networks
• suppose mobile user moves to another
network
– registers with new foreign agent
– new foreign agent registers with home agent
– home agent update care-of-address for mobile
– packets continue to be forwarded to mobile (but
with new care-of-address)
• Mobility, changing foreign networks
transparent: on going connections can be
maintained!
Source: Jim Kurose’s slides
Mobility via Direct Routing
correspondent forwards
to foreign agent
foreign agent
receives packets,
forwards to mobile
home
network
4
wide area
network
2
correspondent
requests, receives
foreign address of
mobile
visited
network
1
3
4
mobile replies
directly to
correspondent
Source: Jim Kurose’s slides
Mobility via Direct Routing: comments
• overcome triangle routing problem
• non-transparent to correspondent:
correspondent must get care-of-address from
home agent
– What happens if mobile changes networks?
Source: Jim Kurose’s slides
Mobile IP
• RFC 3344
• has many features we’ve seen:
– home agents, foreign agents, foreign-agent
registration, care-of-addresses, encapsulation
(packet-within-a-packet)
• three components to standard:
– agent discovery
– registration with home agent
– indirect routing of datagrams
Source: Jim Kurose’s slides
Routing
• Routing algorithms
– Link state routing algorithms
– Distance vector routing algorithms
• Routing in the internet
• RIP
• OSPF
• BGP
Graph abstraction
5
2
u
2
1
Graph: G = (N,E)
v
x
3
w
3
1
5
z
1
y
2
N = set of routers = { u, v, w, x, y, z }
E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }
Remark: Graph abstraction is useful in other network contexts
Example: P2P, where N is set of peers and E is set of TCP connections
Graph abstraction: costs
5
2
u
v
2
1
x
What factors influence this cost ?
3
w
3
1
z
1
y
Should costs be only on links ?
5
2
Cost of path (x1, x2, x3,…, xp) = c(x1,x2) + c(x2,x3) + … + c(xp-1,xp)
Question: What’s the least-cost path between u and z ?
Routing algorithm: algorithm that finds least-cost path
Routing Algorithm classification
2 main classes:
Centralized
• all routers have complete topology, link cost info
• “link state” algorithms
Distributed:
• Each router knows link costs to neighbor routers only
• “distance vector” algorithms
Link state routing algorithms
A Link-State Routing Algorithm
Dijkstra’s algorithm
• Link costs known to all nodes
• computes least cost paths from one node (‘source”) to all
other nodes
– gives forwarding table for that node
• iterative: after k iterations, know least cost path to k dest.’s
Dijkstra’s Algorithm
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
1 Initialization:
2 N' = {u}
3 for all nodes v
5
4
if v adjacent to u
5
then D(v) = c(u,v)
v
2
6
else D(v) = ∞
u
7
2
8 Loop
1
x
9 find w not in N' s.t. D(w) is a minimum
10 add w to N'
11 update D(v) for all v adjacent to w and not in N' :
12
D(v) = min( D(v), D(w) + c(w,v) )
13 /* new cost to v is either old cost to v or known
14 shortest path cost to w plus cost from w to v */
15 until all nodes in N'
3
w
3
1
5
z
1
y
2
Dijkstra’s algorithm: example
Resulting shortest-path tree from u:
v
w
u
z
x
Resulting forwarding table in u:
destination
link
v
x
(u,v)
(u,x)
y
(u,x)
w
(u,x)
z
(u,x)
y
Distance vector routing
algorithms
Distributed: Distance Vector
 To find D, node S asks each neighbor X




How far X is from D
X asks its neighbors … comes back and says C(X,D)
Node S deduces C(S,D) = C(S,X) + C(X,D)
S chooses neighbor Xi that provides min C(S,D)
 Later, Xj may find better route to D
 Xj advertizes C(Xj,D)
 All nodes update their cost to D if new min found
Distance Vector Algorithm
Bellman-Ford Equation (dynamic programming)
Define
dx(y) := cost of least-cost path from x to y
v1
Then
x
v2
dx(y) = min {c(x,v) + dv(y) }
v
where min is taken over all neighbors v of x
y
Bellman-Ford example
5
2
u
v
2
1
x
3
w
3
1
5
z
1
y
Clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3
2
B-F equation says:
du(z) = min { c(u,v) + dv(z),
c(u,x) + dx(z),
c(u,w) + dw(z) }
= min {2 + 5,
1 + 3,
5 + 3} = 4
Node that achieves minimum is next
hop in shortest path ➜ forwarding table
Distance Vector: link cost changes
Link cost changes:
 if DV changes, notify neighbors
1
x
4
y
50
1
z
At time t0, y detects the link-cost change, updates its DV,
and informs its neighbors.
At time t1, z receives the update from y and updates its table.
It computes a new least cost to x and sends its neighbors its DV.
At time t2, y receives z’s update and updates its distance table.
y’s least costs do not change and hence y does not send any
message to z.
Comparison of LS and DV algorithms
message complexity
robustness: what happens if
router malfunctions?
• LS: with n nodes, E links, O(nE) msgs sent
LS:
• DV: exchange between neighbors only
– node can advertise incorrect
link cost
– each node computes only its
own table
– convergence time varies
speed of convergence
• LS: O(n2) algorithm requires O(nE) msgs
– may have oscillations
• DV: convergence time varies
– may be routing loops
– count-to-infinity problem
DV:
– DV node can advertise
incorrect path cost
– each node’s table used by
others
• error propagate thru
network
Analogy: Routing in Internet
 Similar to international FedEx routing
 FedEx figures out best route within country
• Uses google maps say
• This is link state -- All info available
 USA FedEx does not have international map,
also no permission to operate outside USA
 Gets price quote from Germany FedEx, Japan FedEx
etc. to route to India
 Chooses minimum price and handles package to say
Germany (Distance Vector)
 Germany has country map (link state)
 Germany asks for cost from Egypt, South Africa …
Internet Routing
• Think of each country FedEx as ISPs
– Routing on internet very similar to prior example
• The link state and DV routing protocols used in
internet routing
– RIP (routing information protocol)
– OSPF (Open shortest path first)
– BGP (Border gateway protocol)
• They utilize the concepts of
– Link state
– Distance vector routing
Routing protocols
• RIP
• OSPF
• BGP
Hierarchical routing
our routing study thus far - idealization
 all routers identical
 network “flat”
… not true in practice
scale: with 600 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
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
gateway router:
• at “edge” of its own AS
• has link to router in
another AS
Interconnected ASes
3c
3a
3b
AS3
2a
1c
1a
1d
2c
2b
AS2
1b AS1
Intra-AS
Routing
algorithm
Inter-AS
Routing
algorithm
Forwarding
table
forwarding table
configured by both intraand inter-AS routing
algorithm
• intra-AS sets entries for
internal dests
• inter-AS & intra-AS sets entries
for external dests
Inter-AS tasks
• suppose router in AS1
receives datagram
destined outside of AS1:
– router should forward
packet to gateway
router, but which one?
AS1 must:
1. learn which dests are
reachable through AS2,
which through AS3
2. propagate this
reachability info to all
routers in AS1
job of inter-AS routing!
3c
3b
other
networks
3a
AS3
2c
1c
1a
AS1
1d
2a
1b
2b
AS2
other
networks
Example: setting forwarding table in router 1d
• suppose AS1 learns (via inter-AS protocol) that subnet x
reachable via AS3 (gateway 1c), but not via 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
– installs forwarding table entry (x,I)
x
3c
3b
other
networks
3a
AS3
2c
1c
1a
AS1
1d
2a
1b
2b
AS2
other
networks
Example: choosing among multiple ASes
• now suppose AS1 learns from inter-AS protocol that subnet
x is reachable from AS3 and from AS2.
• to configure forwarding table, router 1d must determine
which gateway it should forward packets towards for dest x
– this is also job of inter-AS routing protocol!
x
3c
3b
other
networks
3a
AS3
2c
1c
1a
AS1
1d
?
2a
1b
2b
AS2
other
networks
Example: choosing among multiple ASes
• now suppose AS1 learns from 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 job of 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
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)
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)
u
v
A
z
C
B
w
x
D
y
from router A to destination subnets:
subnet hops
u
1
v
2
w
2
x
3
y
3
z
2
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
….
….
....
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
….
….
....
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)
RIP table processing
• RIP routing tables managed by application-level
process called route-d (daemon)
• advertisements sent in UDP packets, periodically
repeated
routed
routed
transport
(UDP)
network
(IP)
link
physical
transprt
(UDP)
forwarding
table
forwarding
table
network
(IP)
link
physical
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
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.
Hierarchical OSPF
boundary router
backbone router
backbone
area
border
routers
area 3
internal
routers
area 1
area 2
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 AS’s.
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 ASinternal 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
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
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 nexthop AS. (may be multiple links from current AS to nexthop-AS)
• gateway router receiving route advertisement uses
import policy to accept/decline
– e.g., never route through AS x
– policy-based routing
BGP route selection
• Router may learn about more than 1 route
to destination AS, selects route based on:
1. Local preference value attribute: policy
decision
2. Shortest AS-PATH
3. Closest NEXT-HOP router: hot potato routing
4. Additional criteria
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
BGP routing policy
legend:
B
W
X
A
customer
network:
C
Y



provider
network
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
BGP routing policy (2)
legend:
B
W
provider
network
X
A
customer
network:
C
Y



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!
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