Download Gateways - Sistel IMT 2010

Modul 5
Gateway and Routing Protocol
Mata Kuliah
Sistem Telekomunikasi
Semester Genap 2009 - 2010
Outline
•
•
•
•
•
•
•
Gateways, Bridges, and Routers
Gateway Protocols
Routing & Routing Daemons
The IGP and EGP Gateway Protocols
Gateway-to-Gateway Protocol (GGP)
Interior Gateway Protocols (IGP)
The External Gateway Protocol (EGP)
2
Pengenalan
• TCP/IP telah berkembang membentuk jaringan LAN
bahkan internet dengan ribuan server dan jaringan yang
kompleks terhubung satu sama lain (interworks).
• Hal ini dimungkinkan dengan adanya perangkatperangkat berbasis IP seperti Gateway, Bride, Router dll.
• Penyampaian pesan dari satu komputer ke komputer
membutuhkan metode routing tertentu.
• Metode untuk menyampaikan informasi routing dalam
jaringan sangat tergantung role network gateways.
• Terdapat protokol khusus yang dikembangkan untuk
berbagai macam gateway. Protocol ini bekerja
bersama-sama dengan TCP.
3
Gateways, Bridges, and Routers
• Gateway adalah sebuah perangkat yang menjalankan fungsi
routing, biasa perangkat stand-alone yang juga menjalankan
translasi protokol dari satu jaringan ke jaringan lainnya :
– Kemampuan konversi protokol sangat penting biasanya terjadi di layer
rendah (physical, data link, network) namun kadang-kadang termasuk
layer transport.
– Konversi dapat terjadi dalam berbagai bentuk, misalnya ketika paket
berpindah dari format LAN ke Ethernet (terjadi perubahan format paket)
atau dari sebuah file yang mempunyai konvensi proprietary ke bentuk
lainnya.
• Bridge adalah perangkat jaringan yang menghubungkan satu atau
lebih jaringan yang menggunakan protokol yang sama.
• Router adalah sebuah node jaringan yang meneruskan (forward)
datagrams melalui jaringan IP.
4
Devices
Bridge
Kemampuan Bridge antara lain:
• Semua kemampuan repeater
terdapat pada Bridge.
• Menghubungkan dua segmen
dan regenerate signal pada
level paket
• Berfungsi pada Data Link Layer
(melihat sinyal melalui MAC
Addressnya)
• Menghubungkan media fisik
berbeda seperti twisted pair
dengan coaxial ethernet.
• Menghubungkan antar segmen
jaringan berbeda seperti
ethenet dan token ring.
Devices
Hub
Hub melakukan fungsi :
• Sebagai konsentrator
• Pada aktif hub dapat menjadi
multiport repeater
• Bekerja pada layer 1 model OSI
(melihat sinyal pada level bit)
Devices
Switch
Fungsi Switch :
• Sebagai konsentrator
• Sebagai multiport bridge
• Bekerja pada layer 2 OSI
(melihat sinyal melalui MAC
Addressnya)
Devices
Router
Kemampuan router antara lain:
• Membagi segmen jaringan yang
besar menjadi segmen yang
kecil-kecil.
• Memfilter dan mengisolasi
trafik.
• Menghubungkan segman
jaringan yang berbeda topologi
dan metode akses.
• Dapat melalukan routing paket
dengan shorthest path, dari
banyak pilihan jalur.
Broadcasts
Semua hub memforward semua
traffic ke semua perangkat
Broadcasts
Jadi jika Host 1 ingin melakukan ping Host 2,
semua perangkat akan melihat paket ping yang
dikirimkan.
1
2
Semua host akan menerima paket ping request dari host 1,
tapi hanya host 2 yang akan menjawab
Bridge
Untuk mengurangi jumlah traffik, mulai digunakan
bridges untuk memfilter paket berdasar alamat MAC
Bridge
Sekarang, jika Host 1 melakukan ping ke Host 2, maka
hanya semua host dalam satu LAN segment yang melihat
paket ping. Bridges stop the ping.
1
2
Switch
Sebuah switch (multi-port bridge), secara efektif
menggantikan keempat bridge yang digunakan.
Switch
Keungungan lain adalah, setiap LAN segment
akan memperoleh dedicated bandwidth.
10 Mbps
10 Mbps
The Cloud
10 Mbps
10 Mbps
10 Mbps
Switch
Sebuah switch tidak bisa menghentikan paket ping yang
ditujukan pada LAN segment yang berbeda, sehingga
ditujukan ke semua port dari switch.
1
2
Switch
For example, Host 1 pings Host 16. Since Host
16 isPerangkat
on another
LAN
segment,
apa
yang
bisa the switch will
floodmemperbaikinya?
the ping request out all ports.
1
16
Router
Routers memfilter traffic berdasarkan alamat IP, dimana
alamat IP akan memberitahu router segment mana yang
harus dituju oleh paket ping.
1
16
Devices Function At Layers
Jadi, dapat diambil kesimpulan,
bahwa suatu perangkat tidak
hanya bekerja pada layernya
sendiri, tapi juga layer
dibawahnya
Gateway Protocols
Gateway Protocol
• Gateway protocols are used to exchange information
with other gateways in a fast, reliable manner
• The Internet provides two types of gateways: core and
non-core
– All core gateways are administered by the Internet Network
Operations Center (INOC).
– Non-core gateways are not administered by this central authority
but by groups outside the Internet hierarchy
• The origin of core gateways arose from the ARPANET
– ARPANET called them stub gateways,
• Any gateway not under direct control (non-core in
Internet terms) was called a nonrouting gateway.
20
Internet from the start
• First, there was ARPANET
– Routers had complete information about all the possible
destinations – core routers
– GGP (gateway-to-gateway) protocol was used for routing – a
distance vector protocol
R
R
H
R
H
R
H
Nov 04, 2004
CS573: Network Protocols and
Standards
21
Internet from the start
• Then, LANs were connected to ARPANET
ARPANET
R
LAN
Nov 04, 2004
R
LAN
Core Routers
R
LAN
CS573: Network Protocols and
Standards
22
Internet from the start
• Problems with above configuration:
– Routing overhead increased with the number of
connected routers
• Number of routes increased with the number of connected
segments
• Frequency of routing exchanges increased
• Higher likelihood that something went wrong somewhere
requiring updates
– Number of different types of routers increased
– Slow deployment of new versions of routing
algorithms
Nov 04, 2004
CS573: Network Protocols and
Standards
23
Gateway-to-Gateway Protocol (GGP)
• The move to the Internet and its proliferation of gateways
required the implementation of the Gateway-to-Gateway
Protocol (GGP), which was used between core
gateways.
• The GGP was usually used to spread information about
the non-core gateways attached to each core gateway,
enabling routing tables to be built.
24
Interior and Exterior Gateway
• As the Internet grew, it became impossible for any one
gateway to hold a complete map of the entire
internetwork
• If the local network has more than one gateway and they
can talk to each other, they are considered interior
neighbors. (The term interior neighbor is sometimes
applied to the machines within the network, too, not just
the gateways.)
• If the gateways belong to different autonomous systems,
they are exterior gateways.
– when default routes are required, it is up to the exterior gateways
to route messages between autonomous systems.
– Interior gateways are used to transfer messages into an
autonomous system.
25
Interior and Exteriour Gateway Protocol
• the method of transferring routing information between interior
gateways is usually the Routing Information Protocol (RIP) or the
less common HELLO protocol, both of which are Interior Gateway
Protocols (IGPs).
– These protocols are designed specifically for interior neighbors.
• On the Internet, messages between two exterior gateways are
through the Exterior Gateway Protocol (EGP).
• RIP, HELLO, and EGP all rely on a frequent (every thirty seconds)
transfer of information between gateways to update routing tables.
– EGP is used between gateways of autonomous systems,
– whereas the IGPs RIP and HELLO are used within the network itself.
– GGP is used between core gateways.
26
Ilustrasi
27
Routing and Routing Daemon
Routing
•
•
•
•
•
Routing refers to the transmission of a packet of information from one
machine through another.
Each machine that the packet enters analyzes the contents of the packet
header and decides its action based on the information within the header.
If the destination address of the packet matches the machine's address, the
packet should be retained and processed by higher-level protocols.
If the destination address doesn't match the machine's, the packet is
forwarded further around the network.
Forwarding can be to the destination machine itself, or to a gateway or
bridge if the packet is to leave the local network.
29
Routing (cont)
• Routing is a primary contributor to the complexity of
packet-switched networks.
• It is necessary to account for an optimal path from
source to destination machines
• It is necessary to handle problems such as
– a heavy load on an intervening machine or
– the loss of a connection.
• The route details are contained in a routing table
– several sophisticated algorithms work with the routing table to
develop an optimal route for a packet.
30
Routing in the Internet
• Routing Algorithms
– Bellman-Ford
– Dijkstra
• Routing Protocols
– Distance Vector
– Link State
• Routing Hierarchy
– Interior Gateway Protocols (RIP, OSPF, IGRP)
– Exterior Gateway Protocols (EGP, BGP, CIDR, Policy Routing)
– Multicasting (IGMP)
Nov 04, 2004
CS573: Network Protocols and
Standards
31
Routing Daemon
• Routing daemons initialize and dynamically maintain the kernel
routing table by communicating with daemons on other systems to
exchange routing information
– For example, what networks are known by the machine on which the
daemon is running.
• Routing daemon is used to handle the routing tables :
– A daemon for most UNIX systems called routed.
– A few systems run a daemon called gated.
• Both routed and gated can exchange RIP messages with other
machines, updating their route tables as necessary.
• Both routed and gated can be managed by the system administrator
to select favorable routes, or to tag a route as not reliable.
• The gated program can also handle EGP and HELLO messages,
updating tables for the internetwork.
32
Methods of Building A Routing Table
• A fixed table is created with a map of the network, which must be
modified and reread every time there is a physical change anywhere
on the network.
– Less complex but it is inflexible and can't react to changes in the
network topology quickly.
• A fixed central routing table is used that is loaded from the central
repository by the network nodes at regular intervals or when
needed.
– Simpler than a fixed table because it is possible for an administrator to
maintain the single table much more easily than a table on each node.
• A dynamic table is used that evaluates traffic load and
messages from other nodes to refine an internal table.
– It is the best for reacting to changes, although it does require better
control, more complex software, and more network traffic.
– Since the advantages outweigh the disadvantages, a dynamic table is
the method most frequently used on the Internet.
33
Fewest-Hops Routing
• Most networks and gateways to internetworks
work on the assumption that the shortest route
• Each machine that a message passes through is
called a hop, so this routing method is known as
fewest hops.
• Although experimentation has shown that the
fewesthops method is not necessarily the fastest
method
– because it doesn't take into account transmission
speed between machines
• it is one of the easiest routing methods to
implement.
34
Fewest-Hops Routing
the tables of the gateways
through which a message travels to its
destination should have same route
information
Disadvantages :
The fewest-hops method doesn't account
for transfer speed, line failures,
or other factors that could affect the overall
time to travel to the destination
35
Type of Service Routing
• This type of routing depends on the type of routing
service available from gateway to gateway.
– This is called type of service (TOS) routing
– Also more formally called quality of service (QOS) by OSI.
• TOS includes consideration for the speed and reliability
of connections, as well as security and route-specific
factors.
• most systems use dynamic updating of tables that reflect
traffic and link conditions
– dynamic updating occurs at regular but not too frequent intervals
– The IP header's Time to Live (TTL) field is very important to
dynamic gateway routing protocols to prevent datagrams
circulate throughout the network indefinitely.
36
Updating Gateway Routing Information
• Gateway C has a copy of gateway A's routing table, and vice versa.
• Gateways B and D each have copies of the other's routing tables, as
well.
• These copies are transmitted at intervals so the gateways can
maintain an up-to-date picture of the connections available through
the other gateway.
• The gateways use EGP to send the messages. (They would use
GGP if they were core gateways.)
37
Update Routing Table : EGP to GGP
• Core gateways use GGP, and non-core gateways use EGP, so
there must be some method for the two to communicate with each
other to find out about hidden machines and networks that lie
beyond their routing tables.
• Gateway A is a core gateway leading from the internetwork to a
network that has non-core gateways leading to two other networks.
• Another gateway on the internetwork does not have information
about the networks and gateways past the core gateway, unless
specifically
updated
about
them
through
a
request.
38
IP Routing Protocols
Gateway-to-Gateway Protocol
GGP
Nov 04, 2004
CS573: Network Protocols and Standards
39
GGP
• The “old” ARPANET routing protocol
• Defined in RFC 823
• A distance-vector routing protocol
– Only core routers participate in GGP
• GGP messages travel in IP datagrams with
protocol type = 3
• GGP measures distance in router hops. i.e., the
number of hops along a path refers to the
number of routers
Nov 04, 2004
CS573: Network Protocols and
Standards
40
GGP Message Types
• 4 types of GGP messages
– GGP Routing Update message (type 12)
– GGP Acknowledgment message (type 2/10)
– GGP Echo Request or Reply (type 0 or 8)
Nov 04, 2004
CS573: Network Protocols and
Standards
41
GGP Routing Update
• A router sends this message to advertise the
destination networks it knows how to reach
• To keep the size of message small, networks
are grouped by distance
– In the message “Distance” is followed by a list of “Net”
addresses that are at this distance
– Contains a field that tells how many distance groups
are being reported (3 in case below)
• D1 – Net1, Net5, Net11
• D2 – Net4, Net2, Net7, Net16
• D3 – Net6, Net9
Nov 04, 2004
CS573: Network Protocols and
Standards
42
IGP Routing Protocols
Routing Information Protocol
RIP
Nov 04, 2004
CS573: Network Protocols and Standards
43
Routing Information Protocol
•
•
•
•
•
•
•
A distance vector based IGP
Similar to GGP
Designed at UC Berkeley
Based on Xerox XNS
Distributed with 4BSD UNIX (routed)
First RFC was 1058, current RFC is 2453
Started off in small networks and then extended
to larger networks
• See Huitema, Chapter 5
Nov 04, 2004
CS573: Network Protocols and
Standards
44
RIP Details
• Routers are active machines
– Advertise their routes (IP NET, distance) to others
• Hosts are passive machines
– They listen and update their routes but do not
advertise
• RIP uses hop count metric
• RIP messages are transmitted using UDP at port
520
Nov 04, 2004
CS573: Network Protocols and
Standards
45
RIP Route Computation
• There is a cost associated with each link
– Typically cost =1 i.e., number of hops
• Each router receives route advertisements from its
neighbors
– Advertisements show distances to all destinations in the network
• For each destination in the network:
– The router takes each received advertisement and adds to it the
cost to reach that neighbor who sent this advertisement; this
gives the distance to the destination
– The router selects lowest of these as path/cost to that destination
Nov 04, 2004
CS573: Network Protocols and
Standards
46
Algorithm Properties
• Convergence is guaranteed in a finite time given that
topology remains static
• Starting value of distance estimates to each destination
can be any non-negative number
• No assumption is made as to when the updates are sent
or when the distances are computed
– Each router can work based on its own clock and send its
updates asynchronously
• If the network changes, routes converge to a new
equilibrium point
Nov 04, 2004
CS573: Network Protocols and
Standards
47
Example
Advertisement:
Distance to A is 2
Distance to B is 3
Distance to C is 5
Cost = 1
P1
Cost = 3
Router
P3
Advertisement:
Distance to A is 1
Distance to B is 4
Distance to C is 1
P2
Cost = 2
Advertisement:
Distance to A is 2
Distance to B is 1
Distance to C is 3
Distance to
Nov 04, 2004
Through
Destination
Port P1
Port P2
Port P3
A
3
4
4
B
4
3
7
C
6
5
4
CS573: Network Protocols and
Standards
48
Counting to Infinity
Routes to Target:
A: route via B, distance 3
B: route via D, distance 2
C: route via B, distance 3
D: direct, distance 1
1
A
C
1
1
10
Target
To reach target …
B
D
1
1
Assume that B to D link goes down, and B notices.
Fro
m
A
Vi
a
B
Dis
t
3
Vi
a
C
Dis
t
4
Vi
a
C
Dis
t
5
Vi
a
C
Dis
t
6
Vi
a
C
Dis
t
11
Vi
a
C
Dis
t
12
B
x
-
C
4
C
5
C
6
C
11
C
12
C
B
3
A
4
A
5
A
6
A
11
D
11
D
di
1
di
1
di
1
di
1
di
1
di
1
…
x = destination unreachable; di = directly connected
What if the link from C to D also goes down? Counting to Infinity!!!
Nov 04, 2004
CS573: Network Protocols and
Standards
49
Some Solutions
• Split Horizon
– If A reaches a destination through B, it makes no sense for B to
reach the same destination through A
– Instead of broadcasting the same distance vector on all links,
send different versions on each outgoing link by removing the
entries for the destinations that are reachable through that link
• Split Horizon with Poisonous Reverse
– Include all the destinations in advertisements; even those which
were missing in split horizon, but…
– Set those vector distances to infinity that were missing in the
simple version of split horizon
Nov 04, 2004
CS573: Network Protocols and
Standards
50
Triggered Updates
• Split Horizon can work in loops with two
gateways, but not with three or more
– See example in book by Huitema
• Another solution to deal with “count to Infinity”
problem is triggered updates
– A gateway is required to send an immediate update
when any route changes. This reduces the
occurrence of loops
– Flood of triggered updates resolves loops faster when
these happen
Nov 04, 2004
CS573: Network Protocols and
Standards
51
RIPv2 Message Format
8
COMMAND (1-5)
16
24
VERSION (2)
31
AS NUMBER
FFFF
AUTHENTICATION TYPE
AUTHENTICATION HEADER
FAMILY OF NET 1
MUST BE ZERO
ADDRESS OF NET 1
MASK
NEXT HOP
DISTANCE TO NET 1
…………
Nov 04, 2004
CS573: Network Protocols and
Standards
52
Message Format
Command
Nov 04, 2004
Meaning
1
Request for partial or full routing information
2
Response containing network-distance pairs from
sender’s routing table
3
Turn on trace mode (obsolete)
4
Turn off trace mode (obsolete)
5
Reserved for Sun Microsystems Internal Use
CS573: Network Protocols and Standards
53
RIPv2 Message Format
• Address format is not limited to TCP/IP
• RIP can be used with multiple network protocol suites
• Family of net i:
– Identifies the protocol family under which the network address
should be interpreted
– IP addresses are assigned value 2
• Next hop
– The sending router can specify another router’s IP address as
next hop for the network
• Set to 0.0.0.0 for sender itself
• Solves similar problem (extra hop) as ICMP redirect
Nov 04, 2004
CS573: Network Protocols and
Standards
54
RIP Metrics and Updates
• By default, RIP uses hop count as the
distance metric
– Integers 1 through 15
– 16 denotes infinity
• Packets are normally sent every 30sec
• If a route is not refreshed within 180
seconds, distance is set to infinity and
later entry is removed
Nov 04, 2004
CS573: Network Protocols and
Standards
55
Input Processing
• How to process incoming RIP packets?
– Examine entries one by one
– Validation check
•
•
•
•
Address is valid class A, B, or C
Network number is not 127
Host port is not a “broadcast” address
Metric is not larger than infinity (16)
– Incorrect entries are ignored
• And should be reported as errors
Nov 04, 2004
CS573: Network Protocols and
Standards
56
Input Processing
• Metric for entry is increased by link cost
• Routing table is searched for an entry
corresponding to the destination
– If the entry is not present, it is added
– If the entry is present but with a larger metric
• Entry is updated and timer restarted
– Entry is present and next hop router is sender of
response message
• Metric is updated and timer restarted
– For all other cases, entry is ignored
Nov 04, 2004
CS573: Network Protocols and
Standards
57
RIP Responses
• A separate response is prepared for all connected
interfaces/ports
– Information sent on different ports may vary due to
• Split Horizon processing
• Subnet summarization
– For triggered updates: may include only those entries that have
been updated since last transmission
• Maximum message size: 512 bytes (up to 25 entries)
– Multiple messages have to be sent if more than 512 bytes
– Source IP address is that of the interface on which the message
is sent
– Destination IP address is the broadcast address
Nov 04, 2004
CS573: Network Protocols and
Standards
58
IP Routing Protocols
Exterior Gateway Protocol
EGP
Core
• A small set of routers that have consistent & complete
information about all destinations.
• Outlying routers can have partial information provided
they point default routes to the core
– Partial info allows site administrators to make local routing
changes independently.
CORE
S1
S2
...
Sm
Peer Backbones
• Initially NSFNET had only one connection to
ARPANET (router in Pittsburg) => only one route
between the two.
• Addition of multiple interconnections => multiple
possible routes => need for dynamic routing
• Single core replaced by a network of peer
backbones => more scalable
– Today there are over 30 backbones!
• Routing protocol at cores/peers: GGP -> EGP->
BGP-4
Exterior Gateway Protocol (EGP)
• A mechanism that allows non-core routers to
learn routes from core (external routes) routers
so that they can choose optimal backbone routes
• A mechanism for non-core routers to inform core
routers about hidden networks (internal routes)
• Autonomous System (AS) has the responsibility
of advertising reachability info to other ASs.
– One+ routers may be designated per AS.
– Important that reachability info propagates to core
routers
Purpose of EGP
you can reach
net A via me
AS2
EGP
AS1
R3
R2
traffic to A
R1
table at R1:
dest next hop
A
R2
A
R
border router
internal router
Share connectivity information across ASes
EGP Operation
• Neighbor Acquisition: Reliable 2-way handshake
• Neighbor Reachability:
– Hellos: j out of m hellos OK => Neighbor UP
– k out of n hellos NOT OK => Neighbor DOWN
• Updates/Queries:
– EGP is an incremental protocol. New info => send
updates
– Each router can query neighbors as well
– Reachability advertized; metrics ignored
– Requires a tree topology of ASes to avoid loops (eg:
see next slide)
Why EGP Requires a Tree Structure..
EGP weaknesses
• EGP does not interpret the distance metrics in
routing update messages => cannot be compute
shorter of two routes
• As a result it restricts the topology to a tree
structure, with the core as the root
– Rapid growth => many networks may be temporarily
unreachable
– Only one path to destination => no load sharing
• Need new protocol => BGP-4
Today’s Big Picture
Large ISP
Large ISP
Stub
Small ISP
Dial-Up
ISP
Access
Network
Stub
Large number of diverse networks
Stub
Internet AS Map: caida.org
Autonomous System(AS)
• Internet is not a single network
– Collection of networks controlled by different administrations
• An autonomous system is a network under a single
administrative control
• An AS owns an IP prefix
• Every AS has a unique AS number
• ASes need to inter-network themselves to form a single
virtual global network
– Need a common protocol for communication
Intra-AS and Inter-AS routing
C.b
A.a
a
C
Gateways:
B.a
b
d
A
A.c
a
b
c
a
c
B
b
•perform inter-AS
routing amongst
themselves
•perform intra-AS
routers with other
routers in their AS
network layer
inter-AS,
intra-AS
routing in
gateway A.c
link layer
physical layer
Who speaks Inter-AS routing?
AS2
BGP
AS1
R3
R2
R1
R
border router
internal router
 Two types of routers
 Border router(Edge), Internal router(Core)
 Two border routers of different ASes will have a BGP
session
Intra-AS vs Inter-AS
• An AS is a routing domain
• Within an AS:
– Can run a link-state routing protocol
– Trust other routers
– Scale of network is relatively small
• Between ASes:
– Lack of information about other AS’s network (Linkstate not possible)
– Crossing trust boundaries
– Link-state protocol will not scale
– Routing protocol based on route propagation
Autonomous Systems (ASes)


An autonomous system is an autonomous routing domain that has been
assigned an Autonomous System Number (ASN).
All parts within an AS remain connected.
… the administration of an AS appears to other ASes to
have a single coherent interior routing plan and presents a
consistent picture of what networks are reachable through it.
RFC 1930: Guidelines for creation, selection,
and registration of an Autonomous System
IP Address Allocation and Assignment: Internet
Registries
IANA
www.iana.org
ARIN
www.arin.org
RIPE
www.ripe.org
APNIC
www.apnic.org
Allocate to National and local registries and ISPs
Addresses assigned to customers by ISPs
RFC 2050 - Internet Registry IP Allocation Guidelines
RFC 1918 - Address Allocation for Private Internets
RFC 1518 - An Architecture for IP Address Allocation with CIDR
AS Numbers (ASNs)
ASNs are 16 bit values.
64512 through 65535 are “private”
Currently over 11,000 in use.
•
•
•
•
•
•
•
•
Genuity: 1
MIT: 3
Harvard: 11
UC San Diego: 7377
AT&T: 7018, 6341, 5074, …
UUNET: 701, 702, 284, 12199, …
Sprint: 1239, 1240, 6211, 6242, …
…
ASNs represent units of routing policy
Nontransit vs. Transit ASes
ISP 2
ISP 1
Traffic NEVER
flows from ISP 1
through NET A to ISP 2
NET A
Internet Service
providers (ISPs)
have transit
networks
Nontransit AS
might be a corporate
or campus network.
Could be a “content
provider”
Selective Transit
NET B
NET A DOES NOT
provide transit
Between NET D
and NET B
NET C
NET A
NET A provides transit
between NET B and NET C
and between NET D
and NET C
NET D
Most transit ASes allow only selective transit
key impact of commercialization
Customers and Providers
provider
provider
IP traffic
customer
customer
Customer pays provider for access to the Internet
Customer-Provider Hierarchy
provider
customer
IP traffic
The Peering Relationship
peer
provider
peer
customer
Peers provide transit between
their respective customers
Peers do not provide transit
between peers
traffic
allowed
traffic NOT
allowed
Peers (often) do not exchange $$$
Peering Wars
Peer
• Reduces upstream transit
costs
• Can increase end-to-end
performance
• May be the only way to
connect your customers
to some part of the
Internet (“Tier 1”)
Don’t Peer
• You would rather have
customers
• Peers are usually your
competition
• Peering relationships may
require periodic
renegotiation
Peering struggles are by far the most
contentious issues in the ISP world!
Peering agreements are often confidential.
Requirements for Inter-AS Routing
• Should scale for the size of the global Internet.
– Focus on reachability, not optimality
– Use address aggregation techniques to minimize core
routing table sizes and associated control traffic
– At the same time, it should allow flexibility in
topological structure (eg: don’t restrict to trees etc)
• Allow policy-based routing between autonomous systems
– Policy refers to arbitrary preference among a menu of
available routes (based upon routes’ attributes)
– Fully distributed routing (as opposed to a signaled
approach) is the only possibility.
– Extensible to meet the demands for newer policies.
Summary : Distributed Routing
Techniques
Link State
• Topology information is
flooded within the routing
domain
• Best end-to-end paths are
computed locally at each
router.
• Best end-to-end paths
determine next-hops.
• Based on minimizing some
notion of distance
• Works only if policy is shared
and uniform
• Examples: OSPF, IS-IS
Vectoring
• Each router knows little
about network topology
• Only best next-hops are
chosen by each router for
each destination network.
• Best end-to-end paths result
from composition of all nexthop choices
• Does not require any notion
of distance
• Does not require uniform
policies at all routers
• Examples: RIP, BGP
Terima Kasih
References
• Shivkumar Kalyanaraman, “Exterior Gateway Protocols:
EGP, BGP-4, CIDR”, Rensselaer Polytechnic Institute,
http://www.ecse.rpi.edu/Homepages/shivkuma
• Tim Parket, Dean Miller, “Teach Yourself TCP/IP in 14
Days”, Second Edition, Sams Publishing, Indianapolis,
US
85