Title: Routing and Router in Internet
1Routing and Router in Internet
2I. Routing in the Internet
- Intra-Autonomous System Routing
- (routing within an Autonomous Systems by
using the routing tables) - - RIP Routing Information Protocol
- - OSPF Open Shortest Path First
- - IGRP Internal Gateway Routing Protocol
- Inter-Autonomous System Routing
- (routing between Autonomous Systems)
- - BGP Border Gateway Protocol
- Differences between Intra-Autonomous System and
Inter-Autonomous System Routing Protocols
31. Intra-Autonomous System Routing
- RIP Routing Information Protocol
- - A distance vector protocol
- - Hop count as a cost metric
- - Response message (advertisement) containing
that hosts routing - table entries for up to 25 destination
networks exchanged every 30 - seconds between neighbors
41. Intra-Autonomous System Routing
- A simple example of how RIP (Routing Information
Protocol) advertisements work
Routing table in router before Receiving
advertisement
Advertisement from other router A
51. Intra-Autonomous System Routing
- A simple example of how RIP (Routing Information
Protocol) advertisements work
Routing table after receiving advertisement
61. Intra-Autonomous System Routing
- 180 seconds waiting for its neighbor with no
response - - That neighbor considered to be no longer
reachable - - Modifying its local routing table and
propagates this information by - sending advertisements to its neighboring
routers - RIP (Routing Information Protocol) request and
response messages to each other over UDP using
port number 520 in a standard IP packet
71. Intra-Autonomous System Routing
- OSPF Open Shortest Path First
- - Link-state protocol (using flooding of link
state information and a - Dijkstra least cost path algorithm)
- - How it works with OSPF (Open Shortest Path
First) - a. A router constructs a complete
topological map of the entire - autonomous system
- b. The router locally runs Dijkstras
shortest path algorithm to - determine a shortest path tree to all
networks - c. The routing table is obtained from this
shortest path tree - - Individual link costs are configured by the
network administrator
81. Intra-Autonomous System Routing
- Advertising techniques of RIP (Router Information
Protocol) and OSPF (Open Shortest Path First) are
duals of each other - - In a RIP (Router Information Protocol)
advertisement - a. Information about all the networks in
the autonomous system - b. Sending this information to its
neighboring routers - - In a OSPF (Open Shortest Path First)
advertisement - a. Information about the routers neighbors
- b. Sending this information to all other
routers in the autonomous - system
91. Intra-Autonomous System Routing
- Advances embodied in OSPF (Open Shortest Path
First) - - Security
- - Multiple same-cost paths
- - Different cost metrics for different TOS
(Type Of Service) traffic - - Integrated support for unicast and
multicast routing - - Support for hierarchy within a single
routing domain
101. Intra-Autonomous System Routing
- Autonomous system can be configured into area
- - Each area runs its own OSPF (Open Shortest
Path First) link state - routing algorithm, with each router in an
area broadcasting its link - state to all other routers in that area.
- - The internal details of an area thus remain
invisible to all routers - outside the area.
- - Intra-area routing involves only those
routers within the same area
111. Intra-Autonomous System Routing
121. Intra-Autonomous System Routing
- IGRP Internal Gateway Routing Protocol
- - Proprietary routing algorithm by Cisco
Systems, Inc. - - Administrator-defined costs in making route
selection - - A reliable transport protocol to
communicate routing information - a. The use of update messages sent only
when routing table costs - change
- b. The use of a distributed diffusing
update routing algorithm to - quickly compute loop free routing paths
132. Inter-Autonomous System Routing
- BGP Border Gateway Protocol
- - A path vector protocol (instead cost
information, path information) - - The policy for making the actual route
selections among the - interconnected autonomous systems up to the
network - administrator
- - BGP information propagated through the
network by exchanges of - BGP messages (4 types of messages Open /
Update / - Notification / KeepAlive) between peers
142. Inter-Autonomous System Routing
- A simplified description of BGP (Border Gateway
Protocol) work - - The whole internet is a graph of Autonomous
Systems, each - Autonomous Systems identified by an
Autonomous Systems - number
- - Autonomous System X has listed in its BGP
table such a path - X,Y1,Y2,Y3,Z from itself to Z
- - X sends updates to its BGP neighbors, X
actually sends the entire - path information X,Y1,Y2,Y3,Z
- - If W is a neighbor of X, and receives an
advertisement - X,Y1,Y2,Y3,Z then W list a new entry
W,X,Y1,Y2,Y3,Z in its BGP - table
- - If it is a undesirable, for example, loop
in the routing (Y2 W), then - it decide not to create this entry for its
policy decision
152. Inter-Autonomous System Routing
- IBGP Internal BGP (Border Gateway Protocol)
- - Multiple gateway routers in an Autonomous
System - - Used inside an Autonomous Systems as a pipe
to exchange BGP - updates among gateway routers belonging to
the same - Autonomous System
- EBGP External BGP (Border Gateway Protocol)
163. Differences between Intra- and Inter-AS
Routing Protocols
- Differences between the goals of routing within
an Autonomous System and among Autonomous Systems - - Policy
- - Scale
- - Performance
17II. Inside a router
- Input ports
- Switching fabric
- Output ports
- Queueing
181. Input ports
- Line termination function and Data link
processing implement the physical and data link
layers. - Lookup/forwarding function is central to the
switching function of the router.
191. Input ports
- Decentralized switching
- - The lookup/forwarding function of the input
port - a. The router determines the output port to
which an arriving - datagram will be forwarded via the
switching fabric. - b. The choice of the output port is made
using the information - contained in the routing table.
- c. With a Shadow copy of the routing
table, the switching decision - can be made locally, at each input
port, without invoking the - centralized routing processor.
- Routers with limited processing capabilities
taken when a workstation or server serves as a
router.
201. Input ports
- Given the existence of a routing table, we just
search through the routing table, looking for a
destination entry that matches the destination
address of the datagram, or a default route. - The importance of performance requirements for
lookup speed. (backbone routers) - Todays high link speeds require more fast and
reasonable technique to store the routing table
entries. (a tree data structure) - A packet for which the output port has been
determined and forwarded into the switching
fabric may be temporarily blocked from entering
it.
212. Switching fabric
- Switching via memory
- - Routers, traditional computers,
- with switching between input
- and output port being done
- under direct control of the CPU
- - Input and output ports
- functioned as traditional I/O
- devices
- - How it works
- a. An input port with an arriving
- datagram first signaled the
- routing processor via an
- interrupt.
222. Switching fabric
- Switching via memory
- b. The packet then copied from
- the input port intro processor
- memory.
- c. The routing processor then
- extracted the destination
- address from the header,
- looked up the appropriate
- output port in the routing
- table, and copied the packet
- to the output ports buffers.
232. Switching fabric
- Switching via a bus
- - The input ports transfer a
- datagram directly to the output
- port over a shared bus,
- without intervention by the
- routing processor.
- - Only one packet at a time can
- be transferred over the bus at
- a time
242. Switching fabric
- Switching via an interconnection network
- - Way to overcome
- the bandwidth
- limitation of a
- single, shared bus
253. Output ports
- Data link protocol processing and line
termination are the send-side link and physical
layer functionality. - The queuing and buffer management functionality
are needed when the switch fabric delivers
packets to the output port at a rate that exceeds
the output link rate.
264. Queueing
- At time t, a packet has arrived at each of the
incoming input ports, each destined for the
uppermost outgoing port. - Identical line speeds and a switch operating at
three times the line speed.
- The actual location of packet loss will depend on
the traffic load, the relative speed of the
switching fabric and the line speed.
274. Queueing
- One time later, all three original packets have
been transferred to the outgoing port and are
queued awaiting transmission. - In the next time unit, one of these three packets
will have been transmitted over the outgoing
link, two new packets have arrived at the
incoming side of the switch.
284. Queueing
- A packet scheduler at the output port must choose
one packet among those queued for transmission. - FCFS (first-come-first-served)
- WFQ (weighted fair queueing)
- Packet scheduling plays a crucial role in
providing quality of service guarantees.