Routing Architecture and Protocols

1
Routing Architecture and Protocols

• NETS3303/3603
• Week 6

2
Outline

• Intro concepts
• Issues
• Architecture
• Routing protocols
• Distance-vector
• Link-state
• Autonomous System and Exterior Gateway Protocol
• BGP
• Interior Gateway Protocol
• RIP1 and RIP2
• OSPF

3
Review - Internet Routing

• IP implements datagram forwarding
• Both hosts and routers
• Have an IP module
• Forward datagrams
• IP forwarding is table-driven
• Table known as routing table

4
Elements of Routing

• routing protocols that allow info to be gathered
  and distributed routers communicate with these
  protocols
• routing algorithms compute good routes based on
  gathered data (like Bellman-Ford and Dijkstra)
• routing table database of routes

5
Its Classic problem!
6
How / When Are IP Routing Tables Built?

• Depends on size / complexity of internet
• Static routing
• Fixes routes at boot time
• Useful only for simplest cases
• Dynamic routing
• Table initialized at boot time
• Values inserted / updated by protocols that
  propagate route information
• Necessary in large internets

7
Routing Tables

• Two sources of information
• Initialization (e.g., from disk)
• Update (e.g., from protocols)
• Hosts tend to freeze the routing table after
  initialization
• But, routers use protocols to learn new
  information and update their routing table
  dynamically

8
Original Arpanet Routing Architecture

• Small set of core routers with complete
  information about all destinations
• Other routers know local destinations and use the
  core as central router (default route)
• Disadvantages of original core
• Central bottleneck for all traffic
• No shortcut routes possible
• Does not scale

9
General Idea Better!

• Have a set of core routers know routes to all
  locations
• Devise a mechanism that allows other routers to
  contact the core to learn routes (spread
  necessary routing information automatically)
• Continually update routing information

10
Automatic Route Propagation

• Two basic algorithms used by routing update
  protocols
• Distance-vector
• Link-state
• Many variations in implementation details

11
Distance-Vector Algorithm

• Initialize routing table with one entry for each
  directly connected network
• Periodically run a distance-vector update to
  exchange information with routers that are
  reachable over directly connected networks
• Each router sends list of its routes to another

12
DV algorithm

• examples RIP, BGP
• Its algorithm elements
• send every N seconds out all connected
  interfaces broadcast 2-tuples (to network X,
  hop count Y) ...
• recv if new tuple, add to routing table if
  better tuple, change existing if dead tuple,
  remove
• timeout if no refresh, timeout entry in N Y
  seconds

13
Example Of DV Update

• Router K received an update from router J

• (a) is existing routing table at K
• (b) incoming update (marked items cause change)

14
slow convergence/count to infinity

• DVs big problem!
• changes can be sent when they occur, but must
  recompute a bit so convergence takes time (made
  worse by possible loops)
• count to infinity problem can occur too - routing
  loop until hopcount reaches impossible value

15
Count to infinity

• C crashes, B knows C crashed but hasnt told A,
  but unfortunately A talks to B first B is told by
  A
• I can get to C in two hops (and note it doesnt
  mention to B that the path is thru B)
• B says AHA!, that means I can get to C in three
  hops and reports that to A
• A says AHA!, its now four hops to B and tells B
  etc...

16
split-horizon and poison reverse fixup

• Split-horizon
• A does not tell B that it can reach C (white lie)
• Because its through B
• Poison reverse
• When B loses connection to C, its distance to C
  is changed to infinity
• An immediate update is triggered, without wait
  for regular update
• when link goes away, B will know that there is no
  path to C, and tell A
• Still doesnt work in all cases

17
Link-State Algorithm

• Alternative to distance-vector
• Distributed computation
• Broadcast information
• Allow each router to compute shortest paths
• Avoids problem where one router can damage the
  entire internet by passing incorrect information
• Also called Shortest Path First (SPF)

18
Link-State Update

• Participating routers learn internet topology
• Think of routers as nodes in a graph, and
  networks connecting them as edges or links
• Pairs of directly-connected routers periodically
• Test link between them
• Propagate (broadcast) status of link
• All routers
• Receive link status messages
• Recompute routes from their local copy of
  information

19
1 - Determine link-state
20
2 - Send LS-update
21
3 - Compute shortest path
22
link-state pros/cons

• pros
• converges faster, no count to infinity problem
  router can forward LSP immediately
• more functionality e.g., each router has map of
  net, can make network debugging easier
• cons
• more compute than DV (does this matter?)

23
ROUTING EXTERIOR GATEWAYPROTOCOLS AND
AUTONOMOUSSYSTEMS (BGP)
24
General Principle for Internet Routing

• Although it is desirable for routers to exchange
  routing information, it is impractical for all
  routers in an arbitrarily large internet to
  participate in a single routing update protocol
• Consequence routers must be divided into groups

25
A Practical Limit On Group Size

• Up to a dozen routers to participate in a single
  routing area across a WAN
• approximately five times as many can safely
  participate across a set of LANs

26
Router Outside A Group

• Does not participate directly in groups routing
  information propagation algorithm
• Problems
• Will not choose optimal routes if it uses a
  member of the group for general delivery
• May not know all networks from other groups

27
The Extra HopProblem

• Non-participating router picks one participating
  router to use (e.g., R2)
• Non-participating router routes all packets to R2
  across backbone
• Router R2 routes some packets back across
  backbone to R1
• So, a mechanism is needed that allows
  nonparticipating routers to learn routes from
  participating routers
• so they can choose optimal routes.

28
The Hidden Networks Problem

• Group must learn routes from nonparticipating
  routers
• Example owner of networks 1 and 3 must tell
  group that there is a route to network 4

29
Autonomous System Concept (AS)

• Group of networks under one administrative
  authority
• Free to choose internal routing update mechanism
• Connects to one or more other autonomous systems

30
EGPs Exterior GatewayProtocols

• A protocol for communicating routes between two
  autonomous systems
• Solves two problems
• Allows router outside a group to advertise
  networks hidden in another autonomous system
• Allows router outside a group to learn
  destinations in the group

31
Border Gateway Protocol

• The most popular (virtually the only) EGP in use
  in the Internet
• Current version is BGP-4
• Supports CIDR (mask accompanies each route)
• Each AS designates a border router to speak on
  its behalf
• Two border routers become BGP peers

32
Key Characteristics Of BGP

• Provides inter-autonomous system communication
• Propagates reachability information
• Follows next-hop paradigm
• Provides support for policies
• Sends path information
• Permits incremental updates
• Allows route aggregation
• Allows authentication

33
Additional BGP Facts

• Uses reliable transport (i.e., TCP)
• Unusual most routing update protocols use
  connectionless transport (e.g., UDP)
• Sends keepalive messages so other end knows
  connection is valid (even if no new routing
  information is needed)

34
Four BGP Message Types
35
BGP Message Header

• Each BGP message starts with this header
• Marker is used by peers to indicate message
  boundary gt synchronisation

36
BGP Open Message

• Used to start a connection
• HOLD TIME specifies max time that can elapse
  between BGP messages

37
BGP Update Message

• Sender can advertise new routes or withdraw old
  routes
• Each route entry consists of address and mask
• Entry can be compressed to eliminate zero bytes

38
BGP Must Consider Receivers Perspective

• Two issues are considered policies and optimal
  routes
• Advertise not just destinations but report
  reachability too

39
Path Metric Interpretation

• Each AS use own IGP may be with different metric
  (hop count, delay, policy-based values)
• So, an exterior gateway protocol does not
  communicate or interpret metrics, even if metrics
  are available!
• BGP only propagates reachability information
• a receiver can implement policy constraints
• BUT cannot choose a least cost route.

40
ROUTING INSIDE ANAUTONOMOUS SYSTEM(RIP, OSPF)

• Static routes
• Initialized at startup
• Never change
• Typical for host
• Sometimes used for router
• Dynamic routes
• Initialized at startup
• Updated by route propagation protocols
• Typical for router
• Sometimes used in host

41
Exchanging Routing Information Within An
Autonomous System

• Mechanisms called interior gateway protocols,
  IGPs
• Choice of IGP is made by autonomous system

42
Example Of Two Autonomous SystemsAnd the Routing
Protocols Used
RIP
OSPF
43
Routing Information Protocol (RIP)

• Implemented by UNIX program routed
• Uses hop count metric
• Distance-vector protocol
• Relies on broadcast
• Assumes low-delay LAN
• Uses split horizon and poison reverse techniques
  to solve inconsistencies
• Current standard is RIP2

44
Two Forms Of RIP

• Active
• Form used by routers
• Broadcasts routing updates periodically
• Uses incoming messages to update routes
• Passive
• Form used by hosts
• Uses incoming messages to update routes
• Does not send updates

45
RIP Operation

• Each router sends update every 30 seconds
• Update contains pairs of
• (destination address, distance)
• Distance of 16 is infinity (i.e., no route)
• This limits span of its internet to 16 hops
• Any 2 nodes have at most 15 routers between

46
Illustration Of HostsUsing Passive RIP

• Host routing table initialized to

Destination Route
128.10.0.0 default Direct 128.10.0.200

• Host listens for RIP broadcast and uses data to
  update table
• Eliminates ICMP redirects

47
Changes To RIP In Version 2

• RIP1 does not include subnet mask
• Only suitable for classful or fixed-len subnets
• Update includes subnet mask
• Authentication supported
• Explicit next-hop information
• Messages can be multicast (optional)
• IP multicast address is 224.0.0.9

48
RIP2 Update Format
49
Open Shortest Path First (OSPF)

• Developed by IETF in response to vendors
  proprietary protocols
• Uses link-state algorithm
• More powerful than most predecessors
• Permits hierarchical topology
• More complex to install and manage

50
OSPF Features

• Type of service routing the first to offer
• Load balancing across multiple paths
• Networks partitioned into subsets called areas
• Designated router per area
• Message authentication
• Virtual network topology abstracts away details
• Can import external routing information

51
OSPF Message Header

• Each message starts with same header
• OSPF Message Types
• 1 Hello (used to test reachability)
• 2 Database description (topology)
• 3 Link status request
• 4 Link status update
• 5 Link status acknowledgement

52
OSPF HELLO Message Format

• Used to establish and test reachability

53
OSPF Database Description Message Format
Router Network Summary of nets AS boundary

• Initialises network topology database
• One serve as a Master other slave
• Can be large gt separate into different msgs
• I is 1 for first message, M is 1 for more messages

54
Summary

• Internet is too large for all routers to
  participate in one routing update protocol
• Group of networks and routers under one
  administrative authority is called Autonomous
  System (AS)
• EGP is used to communicate routing information
  between two autonomous systems
• Each AS chooses its own interior routing update
  protocol
• Popular IGPs include
• RIP (distance vector algorithm)
• OSPF (link-state algorithm)