Transcript Routing in the Inernet

Routing in the Inernet
• Outcomes:
– What are routing protocols used for Intra-ASs
Routing in the Internet?
– The Working Principle of RIP and OSPF
– What is the routing protocols used for InterASs rouging in the Internet?
– The Working principle of BGP
Routing Information Protocol
• RIP is a distance vector protocol, it is specified in
RFC 1508
• In RIP, routing updates are exchanged between
neighbours approximately every 30 seconds using
a so-called RIP response message.
• Response message sent by a router or host
contains a list of up to 25 destination networks
within the AS, as well as the sender’s distance to
each of those networks. Response messages are
also known as RIP advertisements.
Working Principle of RIP
• RIP routers exchange advertisements approximately every 30
seconds.
• If a router does not hear from its neighbor at least once every
180 seconds, that neighbor is considered to be no longer
reachable. RIP modifies the local forwarding table and then
propagates this information by sending advertisements to its
neighboring routers.
• A router can also request information about its neighbor’s cost
to a given destination using RIP request message.
• Router send RIP request and response message each other over
UDP using port number 520.
Open Shortest Path First
• OSPF (RFC 2328) is a link state protocol
that uses flooding of link state information
and a Dijkstra least-cost path algorithm.
– With OSPF, a router constructs a complete
topological map of the entire AS.
– Run the LS algorithm
– The administrator configure the cost of the
individual link
Advances in OSPF
• Security. All exchanges between OSPF routers (e.x. link
state updates) are authenticated.
• Multiple same-cost paths. When multiple paths to a
destination have the same cost, OSPF allows multiple
paths to be used.
• Integrated support for unicast and multicast routing.
• Support for hierarchy within a single routing domain.
OSPF is able to construct an ASs hierarchically
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Internal routers
Area border routers
Backbone routers
Boundary routers
Inter-AS Routing: BGP
• The Border Gateway Protocol (BGP4 RFC1771), is the de facto
standard interdomain routing protocol in today’s Internet
• BGP uses distance vector protocol.
– It is more appropriately characterized as a path vector protocol.
– Neighboring BGP routers, known as BGP peers, exchanged detailed path
information (such as the list of ASs on a path to a given destionation)
rather than cost information.
• In BGP, an autonomous system is identified by its globally
unique autonomous system number (ASN). AS numbers, like IP
addresses, are assigned by ICANN regional registries.
• At the heart of BGP are route advertisements. An advertisement
consists of a CIDRized destination network address and a set of
attributes associated with the path to that destination network.
Two of the most important attributes are the path attribute (an
explicit list of all ASs on the path to the specified destination
network) and the identity of the next-hop router along that path
to the destination network.
BGP activities
• Receiving and filtering route advertisements from
directly attached neighbour's.
• Route selection. A BGP router may receive several
route advertisements to the same destination AS,
and must choose which route it will use from
among those advertised.
– The destination AS and next hop for the chosen path
must then be installed in the router’s forwarding tables.
– BGP makes a clear distinction between routing
mechanism and routing policy. BGP does not specify
how an AS should choose a path from among those
advertisement. This is a policy decision that is left up to
the AS’s network administrator.
• Sending route advertisements to neighbours.
BGP-Border Gateway Protocol
• BGP peers communicate using the TCP protocol and port
number 179. The BGP protocol defines the four types of
messages:
– OPEN. When a BGP router first wants to establish contact with a
BGP peer, an OPEN message is sent to the peer. If the OPEN is
acceptable to the peer, it will send back a KEEPALIVE message.
– UPDATE. A BGP router uses the UPDATE message to advertise a
path to a given destination to the BGP peer. The UPDATE message
can also be used to withdraw a path that had previously been
advertised.
– KEEPAVLIVE. This BGP message is used to let a peer know that the
sender is alive but that the sender doesn’t have other information to
send
– NOTIFICATION. This BGP message is used to inform a peer that an
error has been detected or that the sender is about to close the BGP
session.