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Routing and Router in Internet

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Title: Routing and Router in Internet


1
Routing and Router in Internet
  • Hongjoon Jeon

2
I. 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

3
1. 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

4
1. 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
5
1. Intra-Autonomous System Routing
  • A simple example of how RIP (Routing Information
    Protocol) advertisements work

Routing table after receiving advertisement
6
1. 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

7
1. 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

8
1. 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

9
1. 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

10
1. 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

11
1. Intra-Autonomous System Routing
12
1. 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

13
2. 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

14
2. 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

15
2. 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)

16
3. 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

17
II. Inside a router
  • Input ports
  • Switching fabric
  • Output ports
  • Queueing

18
1. 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.

19
1. 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.

20
1. 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.

21
2. 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.

22
2. 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.

23
2. 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

24
2. Switching fabric
  • Switching via an interconnection network
  • - Way to overcome
  • the bandwidth
  • limitation of a
  • single, shared bus

25
3. 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.

26
4. 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.

27
4. 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.

28
4. 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.
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