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IEG3090 Tutorial 2 IntraDomain Routing
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Routing Protocol 1 Routing Information Protocol (RIP) ... Garbage collection Timer. When a route is marked for deletion, a Garbage-Collection timer is also started. ... – PowerPoint PPT presentation
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Title: IEG3090 Tutorial 2 IntraDomain Routing
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IEG3090 Tutorial 2 Intra-Domain Routing
- Fong Chi Hang, Bosco
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Outline
- Routing
- Routing Protocol 1 Routing Information Protocol
(RIP) - Routing Protocol 2 Open Shortest Path First
(OSPF)
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Goal of routing
- You may be able to send the datagram directly to
the destination, - However, the interesting case is when the
destination is not directly reachable. - In this case, the host or gateway attempts to
send the datagram to a gateway that is nearer the
destination. - The goal of a routing protocol is very simple It
is to supply the information that is needed to do
routing.
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Routing Information Protocol (RIP)
- RIP is a Distance Vector protocol.
- Why we still use it
- RIP is one of the easiest to configure and least
resource-demanding of all the routing protocols. - It was included in the popular Berkeley Standard
Distribution (BSD) of UNIX starting in 1982. In
fact, support for RIP has been built into
operating systems for as long as TCP/IP itself
has existed
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Fundamentals
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Fundamentals
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Comments
- RIP is designed so that a routing entry is only
replaced if information is received about a
shorter route. - Naturally, this same propagation scheme will
occur for all the other networks as well. - This propagation of network routing information
occurs on a regular basis. - Regular period is normally 30 seconds.
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Time out Timer
- A special Timeout timer is started whenever a
route entry is installed in the routing table. - Whenever the router receives another RIP Response
with information about that route, the route is
considered refreshed and its Timeout timer is
reset. - The default time-out value for this timer is 120
seconds.
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Garbage collection Timer
- When a route is marked for deletion, a
Garbage-Collection timer is also started. - When this Timer timeout, the marked invalid route
entry will be deleted, - The default time-out value for this timer is 120
seconds.
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RIP Messages
- Communication between RIP software elements is
done with RIP messages. - These messages are sent using the UDP, with
reserved UDP port 520 for RIP-1 and RIP-2. - even though RIP is considered part of layer 3
like other routing protocols, it behaves more
like an application in terms of how it sends
messages.
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Message Types
- RIP Request
- A message sent by a router to another router
asking it to send back all or part of its routing
table. - RIP Response
- A message sent by a router containing all or part
of its routing table. - Note that despite the name, this message is not
sent just in response to an RIP Request message.
So it's not really a good name oh well.
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Message format
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Count-To-Infinity Problem
- At first everything works fine.
- Later Link (A, B) is broken. Router B observed
it, but in his routing table he sees, that router
C has a route to A with 2 hops.
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Count-To-Infinity Problem (cont.)
- The problem is, router B doesn't know that C
routes to A through itself. - So B increases his cost to A to 3 later C
increases his cost to A to 4. - Now lets count to infinity
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Limitations of RIP
- slow convergence (Triggered Updates)
- routing loops
- counting to infinity (Split Horizon)
- small infinity
- Metric
- Hop Count as a distance metric
- Lack of support for dynamic (real-time) metrics
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Triggered Updates
- Whenever a router changes the metric for a route
it is required to (almost) immediately send out
an RIP Response to tell its immediate neighbor
routers about the change. - Why almost
- waits a random amount of time, from 1 to 5
seconds - Reduce the load on network
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Split Horizon
- Something is non-sense
- a simple solution
- have Router C not mention the route to Router A
in any RIP Response messages it sends to Router
B. - New rule
- when a router sends out an RIP Response on any of
the networks to which it is connected, it omits
any route information that was originally learned
from that network. - Split Horizon
- the router effectively splits its view of the
network, sending different information on certain
interfaces than on others.
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Split Horizon With Poisoned Reverse
- Enhancement
- Previous omitting routes learned from a
particular interface when sending RIP Response
messages on that interface - Now include those routes in the message but set
their metric to RIP infinity---16. - Poisoned reverse
- Poisoning the routes that we want to make sure
routers on that interface don't use. - Router B will see Router C advertise a route to
Router A but with a cost of 16, - Provide more insurance
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Split Horizon With Poisoned Reverse
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Split Horizon Limitation
- Note, however, that split horizon may not always
solve the counting to infinity problem,
especially in the case where multiple routers are
connected indirectly. - Classic example
- three routers configured in a triangle. See the
following.
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Count-To-Infinity Problem (cont.)
- Simple example to show split horizon cannot help
much
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Hold Down
- A timer is started when they first receive
information about a network that is unreachable. - Until the timer expires, the router will discard
any subsequent route messages that indicate the
route is in fact reachable. - A typical hold-down time-out value is 180
seconds. - It provides a period of time for out-of-date
information to be flushed from the system
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Summary
- Split horizon and split horizon with poisoned
reverse prevent having a router send invalid
route information back to the router from which
it originally learned the route. - Triggered updates reduce the slow convergence
problem by causing immediate propagation of
changed route information. - Finally, hold-down may be used to provide
robustness when information about a failed route
is received.
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Open Shortest Path First (OSPF)
- Why we use it
- RIP has a long history and work well in a small
group of routers. - However, poorly-suited to larger AS or those with
specific performance issues. - OSPF allows routes to be selected dynamically
based on the current state of the network, not
just a static picture of how routers are
connected. - Also note that it is a complicated protocol,
which means it is often not used unless it is
really needed.
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Fundamentals
- Link state database
- Each router in an autonomous system maintains a
copy of this database, which contains information
in the form of a directed graph that describes
the current state of the autonomous system. - Each link to a network or another router is
represented by an entry in the database, and each
has an associated cost (or metric).
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Link-State Advertisement (LSA)
- In flat topology
- A router's LSDB represents the topology of the
entire AS, including links between routers that
may be rather distant from it. - the routers communicate as peers using link-state
advertisements (LSAs). - The routers that connect the AS to other AS are
often called boundary routers. (OSPF/BGP)
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Hierarchical Topology
- The autonomous system is divided into constructs
called areas, each of which contains a number of
contiguous routers and networks. - These areas are numbered, and managed
independently by the routers within them. - The areas are interconnected so that routing
information can be shared between areas, across
the entire AS.
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Router roles
- There are three different labels applied to
routers in this configuration - Internal Routers These are routers that are only
connected to other routers or networks within a
single area. - Area Border Routers These are routers that
connect to routers or networks in more than one
area. - Backbone Routers These are routers that are part
of the OSPF backbone.
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Hierarchical Topology
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Route determination
- The key data structure link-state database
(LSDB). - The LSDB contains a representation of the
topology of either the entire AS (in basic
topology) or a single area (in hierarchical
topology). - Route determination
- Taking the information in the LSDB and
transforming the graph into a shortest path first
tree or SPF tree.
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SPF Tree
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Operation and message
- Unlike RIP, OSPF does not send its information
using the User Datagram Protocol (UDP). - Instead, OSPF forms IP datagrams directly,
packaging them using protocol number 89 for the
IP Protocol field. - OSPF defines five different message types, for
various types of communication
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Message types
- Hello
- allow a router to discover other adjacent routers
on its local links and networks. - Database Description
- contain descriptions of the topology of the AS or
area.
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Message types
- Link State Request
- These messages are used by one router to request
updated information about a portion of the LSDB
from another router. - Link State Update
- These messages contain updated information about
the state of certain links on the LSDB. They are
sent in response to a Link State Request message,
and also broadcast or multicast by routers on a
regular basis. - Link State Acknowledgment
- These messages provide reliability to the
link-state exchange process, by explicitly
acknowledging receipt of a Link State Update
message.
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Summary
- Hello messages are used to establish contact
between routers, and Database Description
messages to initialize a routers link-state
database. - Routine LSDB updates are sent using Link State
Update messages, which are acknowledged using
Link State Acknowledgments. - A device may also request a specific update using
a Link State Request. - All of these are highly simplified descriptions
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- Thank you!
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Message Format
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