Lecture 11
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No class on Tuesday (Oct. 3) - instructor out of
town.
Reminder: Homework 4, Wireshark Project 2
due next Thursday.
Questions?
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CS 475 Networks - Lecture 11
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Outline
Chapter 4 - Advanced Internetworking
4.1 The Global Internet
4.2 Multicast
4.3 Multiprotocol label Switching (MPLS)
4.4 Routing among Mobile Devices
4.5 Summary
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The Global Internet
Routing protocols like those discussed in Chapter
3 do not scale to the Internet.
To begin discussion of the protocols that are
used, we will initially assume the Internet has the
tree-like structure shown below. (The Internet
had this structure in the early 90s.)
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The Global Internet
In the 90s, end users connected to regional
networks (service providers) that were connected
to a common backbone.
Provider and end user networks are
administratively independent entities known as
autonomous systems (AS).
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Routing Areas
A LS (OSPF)
protocol allows
partitioning of an
AS into areas.
This topic will not
be discussed
further here.
Refer to section 4.1.1 of the text for details on
routing areas.
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Interdomain Routing (BGP)
As noted previously the Internet is made up of
autonomous systems (AS) or routing domains.
Each AS can choose its own intradomain routing
protocol (RIP or OSPF), but ASs must share
routing information to enable interdomain routing.
An interdomain routing protocol must allow for
routing policies. An AS with connections to AS X
and AS Y may prefer to always route through X
when it can. It can also choose to never be used
to route traffic to either X or Y.
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Interdomain Routing (BGP)
An example network
with two ASs.
Routers R2 and R4 are
border gateways
(routers through which
traffic enters and leaves
the AS).
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Interdomain Routing (BGP)
The modern Internet
is more complex than
indicated previously.
It consists of (1) Stub
ASs (single
connection
to other ASs), (2) Multihomed ASs (multiple
connections, no transit traffic) and (3) Transit ASs.
It also has multiple backbones. Peering points
are connection points between multiple providers.
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Interdomain Routing (BGP)
Two interdomain routing protocols have been
used. The Exterior Gateway Protocol (EGP) did
not extend to the modern structure of the Internet.
EGP was replaced by the Border Gateway
Protocol (BGP-4). BGP requires one node in an
AS to act as the “BGP speaker”. An AS also has
one or more border gateways.
BGP is neither a DV or LS algorithm. BGP
advertises complete paths to a given network as a
list of ASs.
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Interdomain Routing (BGP)
Assume that the
providers are
transit ASs and all
customers are
stubs.
The BGP speaker for AS2 would advertise that
AS2 could be used to reach the networks
assigned to Customers P and Q. AS1 would
advertise those networks could be reached along
the path <AS1, AS2>.
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Interdomain Routing (BGP)
One important job
of BGP is to
prevent looping
paths from
forming.
Loops are prevented by carrying the complete AS
path in the routing messages. When an AS sees
itself in the advertised path, it can conclude that
the path is not useful to it.
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Interdomain Routing (BGP)
Numbers assigned to ASs are
unique. AS numbers are 16 bits.
This allows for 65,000 ASs.
(Stub ASs do not need
numbers).
An AS will only advertise a route
if it satisfies the policies of that
AS. An AS may only advertise a
preferred route or choose not to
advertise a route at all.
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BGP-4 Update
Packet Format
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Interdomain Routing (BGP)
There are three common
AS relationships.
(1) Provider-Customer,
provider advertises all
learned routes to customer and all routes learned
from customer to everyone. (2) CustomerProvider, customer advertises its own prefixes to
provider and learned routes to its customers, but
not to other providers. (3) Peer, providers
advertise customer routes to each other, but not
other providers.
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Interdomain Routing (BGP)
Border routers actually communicate using
exterior BGP (eBGP). Routers within an AS can
use interior BGP (iBGP) to distribute information
learned from border routers to routers within the
AS.
Routers within the AS also use an interior
gateway protocol (IGP), usually RIP or OSPF to
distribute information about how to reach routers
within the AS.
A stub AS would normally just run an IGP with a
default route to the (one) border router.
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Interdomain Routing (BGP)
Routing table for Router B. The BGP table is constructed using iBGP to
communicate with the border routers. Routers A, D, and E also run eBGP to
communicate with routers in neighboring ASs. All routers run the ASs preferred IGP.
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IP Version 6 (IPv6)
The effort that led to IPv6 was known as IP Next
Generation (IPng).
In addition to moving to a 128-bit IP number, IPv6
was designed to provide better support for realtime services, better security, autoconfiguration,
and enhanced routing (including better support for
mobile hosts).
Support for these new features has also been
added to IPv4 in recent years.
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IP Version 6 (IPv6) - Addresses
IPv6 is classless, but certain address prefixes
have been reserved as shown in the table below.
99% of the address space is available for global
unicast addressing.
Prefix
00...0 (128 bits)
00...1 (128 bits)
1111 1111
1111 1110 10
1111 1110 11
Everything else
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Use
Unspecified
Loopback
Multicast
Link local unicast
Site local unicast
Global unicast
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IP Version 6 (IPv6) - Addresses
An IPv4 compatible address is obtained by zero
extending a 32-bit IPv4 address to 128 bits. A
node that understands only IPv4 can be assigned
an IPv6 address by prefixing the IPv4 address with
2 bytes of all 1s and zero extending to 128 bits.
An IPv6 address is written (using hex) as:
47CD:1234:0000:0000:0022:1234:A456:0124
Double colons can be used to represent (one set
of) contiguous zeros in the address:
47CD:1234::0022:1234:A456:0124
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IP Version 6 (IPv6) - Addresses
IPv6 addresses are allocated in a manner similar
to which IPv4 is being deployed with CIDR.
Service providers will be assigned IPv6 address
blocks with a certain prefix. The providers can
then give address blocks with longer prefixes to
subscribers.
IPv6 routes are specified with a prefix and slash
(like CIDR routes). This method allows for route
aggregation similar to CIDR.
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IP Version 6 (IPv6) - Addresses
Routing table for Router B. The BGP table is constructed using iBGP to
communicate with the border routers. Routers A, D, and E also run eBGP to
communicate with routers in neighboring ASs. All routers run the ASs preferred IGP.
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IP Version 6 (IPv6) - Packet Format
An IPv6 packet header is at
right. The Version field is
in the same location as the
Version field of an IPv4
packet.
TrafficClass and
FlowLabel provide for
Quality of Service.
The NextHeader field is an ID for the next header
if there are multiple headers, if not, it is a protocol
demux key (TCP, UDP, etc).
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IP Version 6 (IPv6) Autoconfiguration
A simple way to obtain a global IPv6 number for a
host is to just prefix the 48 bit Ethernet address
with the network address. Such a method allows
for automatic IP assignment without the use of a
DHCP server.
A router can be configured to periodically
broadcast the appropriate network prefix. Until
the correct prefix is known, the host can use a link
local prefix. (Many hosts may never need a
global address and could just always use a link
local address.)
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IP Version 6 (IPv6) Advanced Routing
One of IPv6's extension headers is a routing
header. The routing header lists a set of IPv6
addresses that the packet should pass through on
its way to its destination. This allows for sourcebased routing on a packet-by-packet basis.
A routing header list can contain an anycast
address. An anycast address represents a set of
interfaces (typically interfaces to a particular
network). The packet would be routed through
the nearest interface in the list.
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In-class Exercises
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Homework 4
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Wireshark Project 2
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