may 16
DESCRIPTION
TRANSCRIPT
CS335 Networking &
Network Administration
Tuesday, November 18, 2008
Internet routing
Propagation of routing tables Routing table updates
Static vs. dynamic routing
Static routes do not change Routes are loaded when the system starts
Dynamic routes – table information changes over time Routes are also loaded when the system starts System also starts route propagation software or
routing software Routing software on one computer interacts with
routing software on others to learn about optimal routes. Tables are then updated.
Static routing
Does not require extra routing software Does not consume bandwidth No CPU cycles used to propagate routing
info
Default route
Static routes
Most hosts use static routing Host’s routing table contains two entries One for the network to which the host
attaches A default entry that directs all other traffic to a
specific router
Dynamic routing
Each router runs routing software that learns about destinations other routers can reach and informs other routers about destinations that it can reach. Routing tables are updated continuously.
Routing in the global internet
To limit routing traffic, the Internet uses a two-level routing hierarchy.
Routers and networks are divided into groups All routers in a group exchange routing
information. At least one router in each group summarizes
information before passing it on to other groups
Autonomous system concept
The concept of groups of routers Designers left the concept flexible to
accommodate many possibilities of routing groups
Each group is an autonomous system Each group shares routing information The group’s information is summarized
before being sent to other groups
Internet routing protocols Interior gateway protocols (IGP’s)
Routers within an autonomous system use this to exchange routing information
Exterior gateway protocols (EGP’s) A router from one system uses these protocols to exchange
routing information with another autonomous system
Optimal routes
Optimal route is hard to define Depends on the application For an interactive login app a path with least
delay is optimal For a browser downloading a large graphic
file a path with maximum throughput is optimal
For an audio webcast path with least jitter is best
Routing metrics The measure of the path that routing software uses
when choosing a route Bandwidth – data capacity of a link Delay – length of time required to move a packet along
each link from source to destination Load – amount of activity on a network resource such as a
router or a link Reliability – a reference to the error rate of each network
link Ticks – the delay on a data link using IBM PC clock ticks
(approximately 55 milliseconds or 1/18 second) Cost – an arbitrary value, usually based on bandwidth,
monetary expense, or other measurement, that is assigned by a network administrator
Routing metrics
IGPs use routing metrics to find optimal routes
EGPs merely finds a path because it cannot compare routing metrics from multiple autonomous systems
Routes and data traffic
A response to a routing advertisement is data Data traffic for a given destination flows in
exactly the opposite direction of routing traffic
Border gateway protocol (BGP) Routing among autonomous systems – BGP provides routing
information at the autonomous system level Provision for policies – BGP allows senders and receivers to
enforce policies, a manager can restrict which routes BGP advertises to outsiders
Facilities for transit routing – BGP classifies each autonomous system as a transit system if it agrees to pass traffic to another autonomous system or as a stub system if it does not
Reliable transport – BGP uses TCP for all communication for the reliability of data transfer that TCP provides
ISPs use BGP to exchange routing information among autonomous systems in the global Internet
BGP resources
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/bgp.htm
http://www.academ.com/nanog/feb1997/BGPTutorial/sld001.htm
http://www.bgp4.as/
RIP Routing information protocol
Routing within an autonomous system – RIP is designed as an IGP used to pass information among routers within an autonomous system
Hop count metric – RIP measures distance in network hops where each network between source and destination is a single hop. Origin-one counting where a directly connected network is 1 hop away, not zero
Unreliable transport – uses UDP Broadcast of multicast delivery – intended for use
over LANs that support broadcast
RIP Routing information protocol
Support for default route propagation – allows a router to advertise a default route. An organization can use RIP to install a default route in each router such that the default routes all forward traffic to the ISP
Uses distance vector algorithm - http://www.freesoft.org/CIE/RFC/1058/6.htm
Passive version for hosts – although only a router can propagate routing information, RIP allows a host to listen passively and update its routing table
RIP
Chief advantage is simplicity Needs little configuration The routers broadcast messages to each
other After a short time all routers in an
organization will have all routes to all destinations
Also handles propagation of the default route
RIP packet
A RIP update message
RIP
RIP is an Interior Gateway protocol (IGP) Uses distance vector algorithm to propagate
routing information A router running RIP advertises the
destinations it can reach along with a distance to each destination
Adjacent routers receive the information and update their routing tables
RIP Routing information protocol
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/rip.htm
http://www.faqs.org/rfcs/rfc1058.html http://www.networksorcery.com/enp/protocol/
rip.htm
Open shortest path first protocol (OSPF)OSPF Graph
Open shortest path first protocol (OSPF) Designed to scale better for large organizations OSPF is an IGP to route within an autonomous system Full CIDR support – includes the full 32 bit address mask with
each address Authenticated message exchange – OSPF can ensure that
messages are only accepted from trusted sources Imported routes – OSPF allows a router to introduce routes
learned form other means (e.g., From BGP) Uses link-state algorithm
http://www.answers.com/topic/link-state-protocol Has support for metrics
OSPF areas
Can scale to handle a larger number of routers than other IGPs
Does hierarchical routing Allows a manger to divide the routers and networks
in an autonomous system into subsets called areas OSPF allows communication between areas http://www.cisco.com/warp/public/104/1.html http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito
_doc/ospf.htm
Multicast routing
IP multicast allows applications to: Join multicast group at any time Leave the group at any time Membership in a multicast group is dynamic
IGMP Internet Group Multicast Protocol
The protocol uses a host to inform a router when it joins or leaves an Internet multicast group
IGMP is only used on the local network Routers must use another multicast routing
protocol to inform other routers of group membership
Forwarding and multicast techniques
Flood and prune Configuration and tunneling Core-based discovery
Multicast protocols Distance vector multicast routing protocol (DVMRP) Core based trees (CBT) Protocol independent multicast – sparse mode (
PIM-SM) Protocol independent multicast – dense mode (PIM-
DM) Multicast extensions to the open shortest path first
protocol (MOSPF) The dynamics characteristics of Internet multicast
make the problem of multicast route propagation difficult. Internet does not have an Internet-wide multicasting facility
Routing summary
Both hosts and routers contain an IP routing table. Hosts use a static table
Internet is divided into autonomous systems EGPs and IGPs exchange routing information Multicast route propagation is difficult Many routing protocols