Routing Protocol

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Routing Protocol
2
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Transport layer TCP, UDP
Network layer
Link layer
physical layer
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Why is a routing protocol needed?

• Early requirements to exchanges data between
  computers over interconnected networks.
• Routing entities had to make a judgement on which
  path to route traffic to destination.

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Routing

• Routing ?????????,??????????????
• ?? A, B, C, D, E, F?router,??????????????????????
  ????????????cost??
• Good Path ????cost???????,???????????????????????
  

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Routing Algorithm classification

• Global or decentralized information?
• Global
• all routers have complete topology, link cost
  info
• link state algorithms
• Decentralized
• router knows physically-connected neighbors, link
  costs to neighbors
• iterative process of computation, exchange of
  info with neighbors
• distance vector algorithms

• Static or dynamic?
• Static
• routes change slowly over time
• Dynamic
• routes change more quickly
• periodic update
• in response to link cost changes

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Routing in the Internet

• The Global Internet consists of Autonomous
  Systems (AS) interconnected with each other
• Stub AS small corporation
• Multihomed AS large corporation (no transit)
• Transit AS provider
• Two-level routing
• Intra-AS administrator is responsible for choice
• Inter-AS unique standard

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Internet AS Hierarchy
Intra-AS border (exterior gateway) routers
Inter-AS interior (gateway) routers
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Intra-AS and Inter-AS routing
Host h2
Intra-AS routing within AS B
Intra-AS routing within AS A
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Intra-AS Routing

• Also known as Interior Gateway Protocols (IGP)
• Most common Intra-AS routing protocols
• RIP Routing Information Protocol
• OSPF Open Shortest Path First
• IGRP Interior Gateway Routing Protocol (Cisco
  proprietary)

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Background to RIP

• RIP dates back to 1969, the early networking days
  and ARPNET when Xerox and Berkleys Unix
  implemented it broadly similar protocols.
• RFC 1058(version 1), RFC 1723(version 2)
• Included in BSD-UNIX Distribution in 1982
• RIP uses a single class of routing algorithm
  known as distance vector - based on a simple hop
  count algorithm (derived from Bellmans
  equation).
• Although superseded by more complex algorithms,
  its simplicity means is still found widely in
  smaller autonomous systems.

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RIP advertisements

• Distance metric of hops (max 15 hops)
• Distance vectors exchanged among neighbors every
  30 sec via Response Message (also called
  advertisement)
• Each advertisement list of up to 25 destination
  nets within AS

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Routing metrics

• Routing entities keep a database (look up table)
  of basic information based on numeric result s
  (metric) of an algorithm to forward a datagram
  onward to its next destination.
• Each entity participating in routing decisions
  sends update messages to its neighbour.
• In order to provide complete network routing
  information every router within the AS must
  participate in the protocol.
• Each router has a lookup table which contains one
  entry for every destination that is reachable.

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How does a metric work?

• Metrics are the result of a formula based on a
  choice of measurement criteria.
• Example, travel cost by taxi
• 10 to go by taxi from Edinburgh to Livingston.
  (P1)
• 25 to go from Livingston to Glasgow (P2)
• 15 to go from Edinburgh to Falkirk (P3)
• 30 to go from Falkirk to Glasgow (P4)
• Cost (Edinburgh, Glasgow) P1P2 35
• also/or P3P4 45

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What is in a RIP routing table?

• Address - IP address of host or network
  destination.
• Router - First router along the route to
  destination.
• Interface - The physical network which must be
  used to reach the next router.
• Metric - A number indicating the distance to the
  destination. This number is the sum of the
  costs that have to be transversed to get to the
  destination.
• Timers - Time since entry was last updated and
  others.
• Flags - Various flags to indicate status of
  various adjacent routers (for example).

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RIP (Routing Information Protocol)
z
w
x
y
A
D
B
C
Destination Network Next Router Num. of
hops to dest. w A 2 y B 2
z B 7 x -- 1 . . ....
Routing table in D
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????

• ?????(Triggered Update)
• ???????,???????30?????????????????????????????,???
  ????????????????????,???????????,???????????,?????
  ?????15??
• ?????(Spilt Horizons)
• ???????,?????????????,??????????????????????????,?
  ??????????????,????,????????????????????,?????????
  ,??????????????????
• ????(Poison Reverse)
• ?????????????????,???????????,????????????????????
  ???,?????????16???

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RIP Table processing

• RIP routing tables managed by application-level
  process called route-d (daemon)
• advertisements sent in UDP packets, periodically
  repeated

Transprt (UDP)
Transprt (UDP)
network forwarding (IP) table
network (IP)
forwarding table
link
link
physical
physical
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RIP Table???
Router giroflee.eurocom.fr
Destination Gateway
Flags Ref Use Interface
-------------------- -------------------- -----
----- ------ --------- 127.0.0.1
127.0.0.1 UH 0 26492 lo0
192.168.2. 192.168.2.5 U
2 13 fa0 193.55.114.
193.55.114.6 U 3 58503 le0
192.168.3. 192.168.3.5 U
2 25 qaa0 224.0.0.0
193.55.114.6 U 3 0 le0
default 193.55.114.129 UG
0 143454

• ????Loop-Back?
• ??????192.168.2.5?193.55.114.6?192.168.3.5?
• ????IP multicast?
• ??????????????IP???193.55.114.129?

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RIP Link Failure and Recovery

• If no advertisement heard after 180 sec --gt
  neighbor/link declared dead
• routes via neighbor invalidated
• new advertisements sent to neighbors
• neighbors in turn send out new advertisements (if
  tables changed)
• link failure info quickly propagates to entire
  net
• poison reverse used to prevent ping-pong loops
  (infinite distance 16 hops)

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OSPF (Open Shortest Path First)

• OSPF RFC 2178 (Version 2)
• OSPF???Link-State Algorithm,??router?????AS???(Top
  ology)???
• Flooding??router?????????neighbor??????????AS?
• Security??router???????????(authenticated),??????
  ???????routing table?
• Multiple same-cost paths ?????????????(??cost)???
  ??

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Dijkstra???

• ?????(???)?????
• ????????0,?????????,????????????
• ??????????????
• ???????????,?????????
• ??????????????????
• ???????????????????
• ?????????????
• ????3?5?????????????

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Dijkstra???
8
0
D
A
10
15
5
10
5
7
F
N5
N1
C
N3
N2
14
B
E
5
9
N4
11
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Hierarchical OSPF

• Two-level hierarchy local area, backbone.
• Link-state advertisements only in area
• each nodes has detailed area topology only know
  direction (shortest path) to nets in other areas.
• Area border routers summarize distances to
  nets in own area, advertise to other Area Border
  routers.
• Backbone routers run OSPF routing limited to
  backbone.
• Boundary routers connect to other ASs.

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Hierarchical OSPF
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Inter-AS routing
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Intra-AS and Inter-AS routing
Host h2
Intra-AS routing within AS B
Intra-AS routing within AS A
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Internet inter-AS routing BGP

• BGP (Border Gateway Protocol) the de facto
  standard
• BGP provides each AS a means to
• Obtain subnet reachability information from
  neighboring ASs.
• Propagate the reachability information to all
  routers internal to the AS.
• Determine good routes to subnets based on
  reachability information and policy.
• Allows a subnet to advertise its existence to
  rest of the Internet I am here

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BGP basics

• Pairs of routers (BGP peers) exchange routing
  info over semi-permanent TCP connects BGP
  sessions
• Note that BGP sessions do not correspond to
  physical links.
• When AS2 advertises a prefix to AS1, AS2 is
  promising it will forward any datagrams destined
  to that prefix towards the prefix.
• AS2 can aggregate prefixes in its advertisement

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Distributing reachability info

• With eBGP session between 3a and 1c, AS3 sends
  prefix reachability info to AS1.
• 1c can then use iBGP do distribute this new
  prefix reach info to all routers in AS1
• 1b can then re-advertise the new reach info to
  AS2 over the 1b-to-2a eBGP session
• When router learns about a new prefix, it creates
  an entry for the prefix in its forwarding table.

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Path attributes BGP routes

• When advertising a prefix, advert includes BGP
  attributes.
• prefix attributes route
• Two important attributes
• AS-PATH contains the ASs through which the
  advert for the prefix passed AS 67 AS 17
• NEXT-HOP Indicates the specific internal-AS
  router to next-hop AS. (There may be multiple
  links from current AS to next-hop-AS.)
• When gateway router receives route advert, uses
  import policy to accept/decline.

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BGP route selection

• Router may learn about more than 1 route to some
  prefix. Router must select route.
• Elimination rules
• Local preference value attribute policy decision
• Shortest AS-PATH
• Closest NEXT-HOP router hot potato routing
• Additional criteria

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BGP messages

• BGP messages exchanged using TCP.
• BGP messages
• OPEN opens TCP connection to peer and
  authenticates sender
• UPDATE advertises new path (or withdraws old)
• KEEPALIVE keeps connection alive in absence of
  UPDATES also ACKs OPEN request
• NOTIFICATION reports errors in previous msg
  also used to close connection

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Why different Intra- and Inter-AS routing ?

• Policy
• Inter-AS admin wants control over how its
  traffic routed, who routes through its net.
• Intra-AS single admin, so no policy decisions
  needed
• Scale
• hierarchical routing saves table size, reduced
  update traffic
• Performance
• Intra-AS can focus on performance
• Inter-AS policy may dominate over performance

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Internet inter-AS routing BGP

• BGP Border Gateway Protocol.
• Path Vector Protocol
• ?Distance Vector???
• Mechanism router??neighbor????????path (sequence
  of ASs)?
• Policy??????????path??routing table?

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Internet inter-AS routing BGP

• ??X??Z?path??W?????
• W???????????X??????
• Cost???
• ????????AS?
• ????Loop?
• ??W???X??????
• Path (W,Z) W, Path (X,Z)
• X?????????advertisement????????????
• X?????????X??Z???????Z?Path?

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Internet inter-AS routing BGP

• BGP???TCP??????
• BGP message???
• OPEN ?????TCP??????Sender?
• UPDATE ????path,??????path?
• KEEPALIVE ?????TCP??(????????UPDATE??),?????OPEN?
  ACKs?
• NOTIFICATION????????????????????