CS475 – Networks
Lecture 11
Chapter 4 – Advanced Internetworking
Reminders and Assignments
• No class on Tuesday next week (10/3) - instructor out of town
• Reading for Lecture 13: Sections 4.2-4.4
4.1 Global Internet
Routing protocols like those discussed in Chapter 3 do not scale _______________________________.
to the Internet.
Provider and end user networks are administratively independent
To begin discussion of the protocols that are used, we will
entities known as autonomous systems (AS).
initially assume the Internet has the ______________ structure
shown below. (The Internet had this structure in the early 90s.)
In the 90s, end users connected to regional networks (service
providers) that were connected to a common
4.1.1 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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4.1.2 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
___________________. 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.
The modern Internet is more complex than indicated previously.
It consists of (1) Stub ASs (single connection to other Ass), (2)
_____________________ ASs (multiple connections, no transit
traffic) and (3) Transit ASs. It also has multiple backbones.
Peering points are connection points between multiple providers.
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
______________________”. 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.
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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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>.
One important job of BGP is to prevent ____________ 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.
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
Border routers actually communicate using exterior BGP
BGP-4 Update
Packet Format
(eBGP). Routers within an AS can use interior BGP (iBGP) to
choose not to advertise a route at all.
distribute information learned from border routers to routers
There are three common AS relationships.
within the AS.
(1) Provider-Customer, provider advertises all learned routes to
Routers within the AS also use an interior gateway protocol
customer and all routes learned from customer to everyone. (2) (_____), usually RIP or OSPF to distribute information about
Customer-Provider, customer advertises its own prefixes to
how to reach routers within the AS.
provider and learned routes to its customers, but not to other
A stub AS would normally just run an IGP with a default route to
providers. (3) Peer, providers advertise customer routes to each the (one) border router.
other, but not other providers.
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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.
4.1.3 IP Version 6 (IPv6)
The effort that led to IPv6 was known as IP Next Generation
IPv6 is classless, but certain address prefixes have been reserved
(IPng).
as shown in the table below. 99% of the address space is
In addition to moving to a ______________ IP number, IPv6
available for global _____________________ addressing.
was designed to provide better support for real-time services,
Prefix
00...0 (128 bits)
00...1 (128 bits)
1111 1111
1111 1110 10
1111 1110 11
Everything else
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.
Use
Unspecified
Loopback
Multicast
Link local unicast
Site local unicast
Global unicast
An IPv4 compatible address is obtained by zero extending a 32-
47CD:1234::0022:1234:A456:0124
bit IPv4 address to 128 bits. A node that understands only IPv4
IPv6 addresses are allocated in a manner similar to which IPv4 is
can be assigned an IPv6 address by prefixing the IPv4 address
being deployed with CIDR. Service providers will be assigned
with 2 bytes of all 1s and zero extending to 128 bits.
IPv6 address blocks with a certain prefix. The providers can
An IPv6 address is written as:
then give address blocks with longer prefixes to subscribers.
47CD:1234:0000:0000:0022:1234:A456:0124
IPv6 routes are specified with a prefix and slash (like CIDR
Double colons can be used to represent (one set of) contiguous
routes). This method allows for route _____________________
_________________ in the address:
similar to CIDR.
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An IPv6 packet header is shown at the 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 _______________ key
(TCP, UDP, etc).
IPv6 Packet Format
IPv6 Autoconfiguration
Advanced Routing
A simple way to obtain a global IPv6 number for a host is to just One of IPv6's extension headers is a routing header. The routing
prefix the 48 bit Ethernet address with the network address.
header lists a set of IPv6 addresses that the packet should pass
Such a method allows for automatic IP assignment without the
through on its way to its destination. This allows for source-
use of a ___________________ server.
based routing on a packet-by-packet basis.
A router can be configured to periodically broadcast the
A routing header list can contain an ________________ address.
appropriate network prefix. Until the correct prefix is known,
An anycast address represents a set of interfaces (typically
the host can use a link local prefix. (Many hosts may never need interfaces to a particular network). The packet would be routed
a global address and could just always use a link local address.)
through the nearest interface in the list.
In Class Exercises
1) Homework 4
2) Wireshark Project 2
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