CS 5565 Network Architecture and Protocols

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CS 5565Network Architecture and Protocols
Lecture 20

• Godmar Back

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Announcements

• Project 2B due in 2 parts
• Apr 29 and May 6
• Extra Credit Opportunities
• Expand simulator (and your implementation) to
  introduce multiple link failures and link
  resurrection
• Additional, requiring reading posted
• Andersen et al SIGCOMM08 Accountable Internet
  Protocol (AIP)
• Casado et al HotNets08 Rethinking Packet
  Forwarding Hardware

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Project 2B

• Simulator Overview

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Project 2B
Simulated link in topology
RPC Outbound/ Inbound over TCP

• High-level View
• Suppose node 2 calls inbound
• Simulator calls outbound to n3

write_msg(/port/2, data)
Sim
read_msg(/from/2, /port/1, data)
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Project 2B Under the hood

• Uses discrete event simulation
• Events are packet received, timer expired, cost
  change, link failure
• Virtual time

cost change on link connecting n2 and n3
n3 receives msg from n2 on port 1
timer on n2 expires,n2 calls write_msg(2,..)
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Routing Algorithms
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Roadmap

• Done
• Discussed forwarding vs routing
• Discussed theory behind two major routing
  algorithms
• Link-state routing
• Distance Vector routing
• Discuss theory behind hierarchical routing
• Discuss application in Internet
• IPv4 addressing
• Next
• Routing in the Internet

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Addressing in IP
223.1.1.2
223.1.1.1
223.1.1.4

• IP address interfaces, not hosts
• Sets of interfaces form subnets
• Subnets share common prefix
• Route to CIDR-ized subnet addresses
• a.b.c.d/x
• Within subnet, reach destination directly

223.1.1.3
223.1.7.1
223.1.9.2
223.1.9.1
223.1.7.2
223.1.8.2
223.1.8.1
223.1.2.6
223.1.3.27
223.1.2.1
223.1.2.2
223.1.3.2
223.1.3.1
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Internet
EthernetLAN 1 60 Machines
191.23.25.193
PPP Link 2
Subnet address 191.23.25.128/26 Default
gateway 191.23.25.129
191.23.25.192/30
191.23.25.197
191.23.25.194
191.23.25.129
PPP Link 1
191.23.25.196/30
EthernetLAN 2 120 Machines
191.23.25.198
191.23.25.1
Subnet address 191.23.25.0/25 Default
gateway 191.23.25.1
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Routing Tables in End Systems

• Typical local subnets default gateway
  (first-hop router)
• Example route print on Windows XP
• 128.173.55.90 FastEthernet
• 192.82.175.230 802.11g wireless

Active Routes Network Destination Netmask
Gateway Interface Metric
0.0.0.0 0.0.0.0 128.173.48.1
128.173.55.90 20 0.0.0.0
0.0.0.0 198.82.174.1 198.82.175.230
25 127.0.0.0 255.0.0.0
127.0.0.1 127.0.0.1 1
128.173.48.0 255.255.248.0 128.173.55.90
128.173.55.90 20 198.82.174.0
255.255.254.0 198.82.175.230 198.82.175.230
25 Default Gateway
128.173.48.1
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ICMP Internet Control Message Protocol

• used by hosts routers to communicate
  network-level information
• error reporting unreachable host, network, port,
  protocol
• echo request/reply (used by ping)
• network-layer above IP
• ICMP msgs carried in IP datagrams
• ICMP message type, code plus first 8 bytes of IP
  datagram causing error

Type Code description 0 0 echo
reply (ping) 3 0 dest. network
unreachable 3 1 dest host
unreachable 3 2 dest protocol
unreachable 3 3 dest port
unreachable 3 6 dest network
unknown 3 7 dest host unknown 4
0 source quench (congestion
control - not used) 8 0
echo request (ping) 9 0 route
advertisement 10 0 router
discovery 11 0 TTL expired 12 0
bad IP header
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Traceroute and ICMP

• Source sends series of UDP segments to dest
• First has TTL 1
• Second has TTL2, etc.
• Unlikely port number
• When nth datagram arrives to nth router
• Router discards datagram
• And sends to source an ICMP message (type 11,
  code 0)
• Message includes name of router IP address

• When ICMP message arrives, source calculates RTT
• Traceroute does this 3 times
• Stopping criterion
• UDP segment eventually arrives at destination
  host
• Destination returns ICMP port unreachable
  packet (type 3, code 3)
• When source gets this ICMP, stops.
• See also Heideman 2008

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IP addresses how to get one?

• Host gets IP address either hardcoded or via DHCP
  (Dynamic Host Configuration Protocol)
• Network gets subnet part of IP address allocated
  from ISPs address space
• ISP gets address space assigned by ICANN
  (Internet Corporation for Assigned Names and
  Numbers)

ISP's block 11001000 00010111 00010000
00000000 200.23.16.0/20 Organization 0
11001000 00010111 00010000 00000000
200.23.16.0/23 Organization 1 11001000
00010111 00010010 00000000 200.23.18.0/23
Organization 2 11001000 00010111 00010100
00000000 200.23.20.0/23 ...
..
. . Organization 7
11001000 00010111 00011110 00000000
200.23.30.0/23
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Hierarchical Addressing Route Aggregation
Hierarchical addressing allows efficient
advertisement of routing information
Organization 0
Organization 1
Send me anything with addresses beginning
200.23.16.0/20
Organization 2
Fly-By-Night-ISP
Internet
Organization 7
Send me anything with addresses beginning
199.31.0.0/16
ISPs-R-Us
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Hierarchical Addressing More Specific Routes
ISPs-R-Us has a more specific route to
Organization 1
Organization 0
Send me anything with addresses beginning
200.23.16.0/20
Organization 2
Fly-By-Night-ISP
Internet
Organization 7
Send me anything with addresses beginning
199.31.0.0/16 or 200.23.18.0/23
ISPs-R-Us
Organization 1
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Intra-AS vs Inter-AS Routing

• In Internet
• Intra-AS known as Interior Gateway Protocols
  (IGP)
• Most common Intra-AS routing protocols
• RIP Routing Information Protocol (original
  protocol, now rarely used)
• OSPF Open Shortest Path First
• IGRP/EIGRP (Enhanced) Interior Gateway Routing
  Protocol
• Inter-AS known as Border Gateway Protocols
• BGP4 Only protocol used

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RIP (Routing Information Protocol)

• Distance vector algorithm
• Included in BSD-UNIX Distribution in 1982
• 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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RIP Example
z
w
x
y
A
D
B
C
Routing table in D
Destination Network Next Router Num.
of hops to dest. w A 2 y B 2
z B 7 x -- 1 . . ....
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RIP Example
Dest Next hops w - - x -
- z C 4 . ...
Advertisement from A to D
Routing table in D
Destination Network Next Router Num.
of hops to dest. w A 2 y B 2 z B
A 7 5 x -- 1 . . ....
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RIP Link Failure and Recovery

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

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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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EIGRP

• Cisco proprietary
• See Cisco Whitepaper, Malhotra 2002
• Distance Vector Protocol with enhancements
• Explicit Signaling (HELLO packets)
• DUAL diffusing update algorithm
• feasible successor concept guarantees loop
  freedom
• Intuition rather than count to infinity, trigger
  route recomputation unless another loop-free path
  is known
• Optimize this by keeping track of all advertised
  routes, not just best one

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OSPF (Open Shortest Path First)

• open publicly available protocol (not
  proprietary)
• Uses Link State algorithm
• LS packet dissemination
• Topology map at each node
• Route computation using Dijkstras algorithm
• OSPF advertisement carries one entry per neighbor
  router
• Advertisements have age field to allow for
  expiration
• Advertisements disseminated to entire AS (via
  flooding)
• Carried in OSPF messages directly over IP (rather
  than TCP or UDP)

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OSPF advanced features (not in RIP)

• Security all OSPF messages authenticated (to
  prevent malicious intrusion)
• Multiple same-cost paths allowed (only one path
  in RIP)
• For each link, multiple cost metrics for
  different TOS (e.g., satellite link cost set
  low for best effort high for real time)
• Integrated uni- and multicast support
• Multicast OSPF (MOSPF) uses same topology data
  base as OSPF
• Hierarchical OSPF in large domains.

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Hierarchical OSPF
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Hierarchical OSPF

• Two-level hierarchy local area, backbone.
• link-state advertisements only in same 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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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 conctns BGP
  sessions
• Note that BGP sessions do not always 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 (like DV routing, except with
  more information!)
• Closest NEXT-HOP router hot potato routing
• Additional criteria

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Path Vector Routing in BGP

• Accomplished via AS-PATH attributes
• Each node is entire AS!

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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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BGP routing policy

• A,B,C are provider networks
• X,W,Y are customer (of provider networks)
• X is dual-homed attached to two networks
• X does not want to route from B via X to C
• .. so X will not advertise to B a route to C

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BGP routing policy (2)

• A advertises to B the path AW
• B advertises to X the path BAW
• Should B advertise to C the path BAW?
• No way! B gets no revenue for routing CBAW
  since neither W nor C are Bs customers
• B wants to force C to route to w via A
• B wants to route only to/from its customers!

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Relationship between OSPFBGP

• OSPF hierarchyis intra-AS
• BGP connectsASs

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Motivation for different Intra/Inter Protocols

• 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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Usage of Routing Protocols
EBGP Sessions IGP IGP IGP IGP
EBGP Sessions OSPF EIGRP RIP Total
Intra- 1,490 9,624 12,741 156 22,521
Inter- 13,830 1,161 1,342 161 2,664

• Sample obtained by reverse-engineering router
  config files
• Source David Maltz et al
• Routing Design in Operational Networks A Look
  from the inside, SIGCOMM 2004

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Summary

• IP
• Addressing, subnets
• ICMP
• RIP
• OSPF
• BGP