Part I: Introduction

1
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• ????? ????? ??? ???? ?????
• ???? 9-12
• ???? ????????? 111

2
Last Week

• Basic Routing Schemes
• Link State
• broadcast link information
• Local computation on global topology
• Distance vector
• Exchange distance information with neighbors
• Local updates based on neighbors information
• Hierarchical Routing
• Broadcast and multicast
• Using a tree topology

3
This week

• Hierarchical routing
• IP addresses
• Definition of network
• Network Address Translation (NAT)
• Routing algorithms implementations

4
The Internet Network layer

• Host, router network layer functions

Transport layer TCP, UDP
Network layer
Link layer
physical layer
5
Hierarchical Routing

• Our routing study thus far - idealization
• all routers identical
• network flat
• not true in practice

• scale with 50 million destinations
• cant store all dests in routing tables!
• routing table exchange would swamp links!

• administrative autonomy
• internet network of networks
• each network admin may want to control routing in
  its own network

6
Hierarchical Routing

• aggregate routers into regions, autonomous
  systems (AS)
• routers in same AS run same routing protocol
• intra-AS routing protocol
• routers in different AS can run different
  intra-AS routing protocol

• special routers in AS
• run intra-AS routing protocol with all other
  routers in AS
• also responsible for routing to destinations
  outside AS
• run inter-AS routing protocol with other gateway
  routers

7
Intra-AS and Inter-AS routing

• Gateways
• perform inter-AS routing amongst themselves
• perform intra-AS routers with other routers in
  their AS

b
a
a
C
B
d
A
network layer
inter-AS, intra-AS routing in gateway A.c
link layer
physical layer
8
Intra-AS and Inter-AS routing
Host h2
Intra-AS routing within AS B
Intra-AS routing within AS A

• Well examine specific inter-AS and intra-AS
  Internet routing protocols shortly

9
Routing Example
d
E
d-gta2 I can reach hosts in D my path D
Export to E i-gte I can reach hosts in D path
IBCD
a2
a2-gta1 I can reach hosts in D path D
a1-gti I can reach hosts in D my path AD
a1
No Exportto F
i
F
AS C
choose BCD using i2
i2-gti I can reach hosts in D path BCD
b-gti I can reach hosts in D my path BCD
i2
b
b-gti2 I can reach hosts in D my path BCD
AS I
10
Routing Example
d1
d
E
d2
a2
a1-gti I can reach hosts in D my path AD
i
F
AS C
a1
How to specify?
b
AS I
11
IP Addressing Scheme

• We need an address to uniquely identify each
  destination
• Routing scalability needs flexibility in
  aggregation of destination addresses
• we should be able to aggregate a set of
  destinations as a single routing unit
• Preview the unit of routing in the Internet is a
  network---the destinations in the routing
  protocols are networks

12
IP Addressing introduction
223.1.1.1

• IP address 32-bit identifier for host, router
  interface
• interface connection between host, router and
  physical link
• routers typically have multiple interfaces
• host may have multiple interfaces
• IP addresses associated with interface, not host,
  or router

223.1.2.9
223.1.1.4
223.1.1.3
223.1.1.1 11011111 00000001 00000001 00000001
223
1
1
1
13
IP Addressing
223.1.1.1

• IP address
• network part
• high order bits
• host part
• low order bits
• Whats a network ? (from IP address perspective)
• device interfaces with same network part of IP
  address
• can physically reach each other without
  intervening router

223.1.2.1
223.1.1.2
223.1.2.9
223.1.1.4
223.1.2.2
223.1.1.3
223.1.3.27
LAN
223.1.3.2
223.1.3.1
network consisting of 3 IP networks (for IP
addresses starting with 223, first 24 bits are
network address)
14
IP Addressing
223.1.1.2

• How to find the networks?
• Detach each interface from router, host
• create islands of isolated networks

223.1.1.1
223.1.1.4
223.1.1.3
223.1.7.0
223.1.9.2
223.1.9.1
223.1.7.1
223.1.8.0
223.1.8.1
223.1.2.6
223.1.3.27
Interconnected system consisting of six networks
223.1.2.1
223.1.2.2
223.1.3.2
223.1.3.1
15
IP Addresses

• given notion of network, lets re-examine IP
  addresses

class-full addressing
class
1.0.0.0 to 127.255.255.255
A
network
0
host
128.0.0.0 to 191.255.255.255
B
192.0.0.0 to 223.255.255.255
C
224.0.0.0 to 239.255.255.255
D
32 bits
16
IP addressing CIDR

• classful addressing
• inefficient use of address space, address space
  exhaustion
• e.g., class B net allocated enough addresses for
  65K hosts, even if only 2K hosts in that network
• CIDR Classless InterDomain Routing
• network portion of address of arbitrary length
• address format a.b.c.d/x, where x is bits in
  network portion of address

17
CIDR Address Aggregation
d1
d
a2
130.132.1/24
i-gta1 I can reach 130.132/16 my path I
i
a1
intradomain routing uses /24
130.132.2/24
130.132.3/24
AS I
18
CIDR Address Aggregation
B
x00/24 B
x/22 A
C
A
x01/24 C
x11/24 GF
x10/24 E
E
x11/24 F
x11/24 F
F
19
IP addresses how to get one?

• Hosts (host portion)
• hard-coded by system admin in a file
• DHCP Dynamic Host Configuration Protocol
  dynamically get address plug-and-play
• host broadcasts DHCP discover msg
• DHCP server responds with DHCP offer msg
• host requests IP address DHCP request msg
• DHCP server sends address DHCP ack msg
• The common practice in LAN and home access (why?)

20
IP addresses how to get one?

• Network (network portion)
• get allocated portion of ISPs address space

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
21
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
22
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
23
Network Address Translation Motivation

• A local network uses just one public IP address
  as far as outside world is concerned
• Each device on the local network is assigned a
  private IP address

rest of Internet
local network (e.g., home network) 192.168.1.0/24
192.168.1.2
192.168.1.1
192.168.1.3
138.76.29.7
192.168.1.4
All datagrams leaving local network have same
single source NAT IP address 138.76.29.7, differe
nt source port numbers
Datagrams with source or destination in this
network have 192.168.1/24 address for source,
destination (as usual)
24
NAT Network Address Translation

• Implementation NAT router must
• outgoing datagrams replace (source IP address,
  port ) of every outgoing datagram to (NAT IP
  address, new port )
• . . . remote clients/servers will respond using
  (NAT IP address, new port ) as destination
  addr.
• remember (in NAT translation table) every (source
  IP address, port ) to (NAT IP address, new port
  ) translation pair
• incoming datagrams replace (NAT IP address, new
  port ) in dest fields of every incoming datagram
  with corresponding (source IP address, port )
  stored in NAT table

25
NAT Network Address Translation
NAT translation table WAN side addr LAN
side addr
138.76.29.7, 5001 192.168.1.2, 3345

192.168.1.2
192.168.1.1
192.168.1.3
138.76.29.7
192.168.1.4
4 NAT router changes datagram dest addr
from 138.76.29.7, 5001 to 192.168.1.2, 3345
3 Reply arrives dest. address 138.76.29.7,
5001
26
Network Address Translation Advantages

• No need to be allocated range of addresses from
  ISP - just one public IP address is used for all
  devices
• 16-bit port-number field allows 60,000
  simultaneous connections with a single LAN-side
  address !
• can change ISP without changing addresses of
  devices in local network
• can change addresses of devices in local network
  without notifying outside world
• Devices inside local net not explicitly
  addressable, visible by outside world (a security
  plus)

27
NAT Network Address Translation

• If both hosts are behind different NAT, they will
  have difficulty establishing connection
• NAT is controversial
• routers should process up to only layer 3
• violates end-to-end argument
• NAT possibility must be taken into account by app
  designers, e.g., P2P applications
• address shortage should instead be solved by
  having more addresses --- IPv6 !

28
IP addressing the last word...

• Q How does an ISP get block of addresses?
• A ICANN Internet Corporation for Assigned
• Names and Numbers
• allocates addresses
• manages DNS
• assigns domain names, resolves disputes

29
Getting a datagram from source to dest.
routing table in A

• IP datagram

• datagram remains unchanged, as it travels source
  to destination
• addr fields of interest here
• mainly dest. IP addr

30
Getting a datagram from source to dest.
misc fields
data
223.1.1.1
223.1.1.3

• Starting at A, given IP datagram addressed to B
• look up net. address of B
• find B is on same net. as A
• link layer will send datagram directly to B
  inside link-layer frame
• B and A are directly connected

31
Getting a datagram from source to dest.
misc fields
data
223.1.1.1
223.1.2.2

• Starting at A, dest. E
• look up network address of E
• E on different network
• A, E not directly attached
• routing table next hop router to E is 223.1.1.4
• link layer sends datagram to router 223.1.1.4
  inside link-layer frame
• datagram arrives at 223.1.1.4
• continued..

32
Getting a datagram from source to dest.
misc fields
data
223.1.1.1
223.1.2.2

• Arriving at 223.1.4, destined for 223.1.2.2
• look up network address of E
• E on same network as routers interface 223.1.2.9
  
• router, E directly attached
• link layer sends datagram to 223.1.2.2 inside
  link-layer frame via interface 223.1.2.9
• datagram arrives at 223.1.2.2!!! (hooray!)

33
IP datagram format
IP protocol version number
32 bits
total datagram length (bytes)
header length (bytes)
type of service
head. len
ver
length
for fragmentation/ reassembly
fragment offset
type of data
flgs
16-bit identifier
max number remaining hops (decremented at each
router)
upper layer
time to live
Internet checksum
32 bit source IP address
32 bit destination IP address
upper layer protocol to deliver payload to
E.g. timestamp, record route taken, specify list
of routers to visit.
Options (if any)
data (variable length, typically a TCP or UDP
segment)
34
IP Fragmentation Reassembly

• network links have MTU (max.transfer size) -
  largest possible link-level frame.
• different link types, different MTUs
• large IP datagram divided (fragmented) within
  net
• one datagram becomes several datagrams
• reassembled only at final destination
• IP header bits used to identify, order related
  fragments

fragmentation in one large datagram out 3
smaller datagrams
reassembly
35
IP Fragmentation and Reassembly

• Example
• 4000 byte datagram
• MTU 1500 bytes

1480 bytes in data field
offset 1480/8
36
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

37
Internet AS Hierarchy
Inter-AS border (exterior gateway) routers
Intra-AS interior (gateway) routers
38
Intra-AS Routing

• Also known as Interior Gateway Protocols (IGP)
• Most common IGPs
• RIP Routing Information Protocol
• OSPF Open Shortest Path First
• IGRP Interior Gateway Routing Protocol (Cisco
  propr.)

39
RIP ( Routing Information Protocol)

• Distance vector algorithm
• Included in BSD-UNIX Distribution in 1982
• Distance metric of hops (max 15 hops)
• why?
• Distance vectors exchanged every 30 sec via
  Response Message (also called advertisement)
• Each advertisement route to up to 25 destination
  nets

40
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
41
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)

42
OSPF (Open Shortest Path First)

• open publicly available
• 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 disseminated to entire AS (via
  flooding)

43
OSPF advanced features (not in RIP)

• Security all OSPF messages authenticated (to
  prevent malicious intrusion) TCP connections
  used
• Multiple same-cost paths allowed
• only one path in RIP
• For each link, multiple cost metrics for
  different ToS (eg, 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.

44
Hierarchical OSPF
45
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.

46
IGRP (Interior Gateway Routing Protocol)

• CISCO proprietary successor of RIP (mid 80s)
• Distance Vector, like RIP
• several cost metrics (delay, bandwidth,
  reliability, load etc)
• uses TCP to exchange routing updates
• Loop-free routing via Distributed Updating Alg.
  (DUAL) based on diffused computation

47
Inter-AS routing
48
Internet inter-AS routing BGP

• BGP (Border Gateway Protocol) the de facto
  standard
• Path Vector protocol
• similar to Distance Vector protocol
• each Border Gateway broadcast to neighbors
  (peers) entire path (I.e, sequence of ASs) to
  destination
• E.g., Gateway X may send its path to dest. Z
• Path (X,Z) X,Y1,Y2,Y3,,Z

49
Internet inter-AS routing BGP

• Suppose gateway X send its path to peer gateway
  W
• W may or may not select path offered by X
• cost, policy (dont route via competitors AS),
  loop prevention reasons.
• If W selects path advertised by X, then
• Path (W,Z) W, Path (X,Z)
• Note X can control incoming traffic by
  controlling its route advertisements to peers
• e.g., dont want to route traffic to Z -gt dont
  advertise any routes to Z

50
Internet inter-AS routing BGP

• 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

51
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

52
Extra
53
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
54
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 dest port unreachable
  packet (type 3, code 3)
• When source gets this ICMP, stops.

55
Example tracert www.yahoo.com

• Tracing route to www-real.wa1.b.yahoo.com
  69.147.76.15
• over a maximum of 30 hops
• 1 lt1 ms lt1 ms lt1 ms 132.67.250.1
• 2 lt1 ms 1 ms lt1 ms
  dmz-cc-gw.math.tau.ac.il 132.67.252.2
• 3 lt1 ms lt1 ms lt1 ms
  tel-aviv.tau.ac.il 132.66.4.1
• 4 1 ms lt1 ms lt1 ms
  gp1-tau-ge.ilan.net.il 128.139.191.70
• 5 1 ms 1 ms
  gp0-gp1-te.ilan.net.il 128.139.188.2
• 6 87 ms 86 ms 87 ms
  iucc.rt1.fra.de.geant2.net 62.40.125.121
• 7 87 ms 87 ms 87 ms
  TenGigabitEthernet7-3.ar1.FRA4.gblx.net
  207.138.144.45
• 8 177 ms 177 ms 177 ms 204.245.39.226
• 9 180 ms 177 ms 265 ms
  ae1-p151.msr2.re1.yahoo.com 216.115.108.23
• 10 177 ms 177 ms 177 ms
  te-9-4.bas-a2.re1.yahoo.com 66.196.112.203
• 11 177 ms 177 ms 177 ms
  f1.www.vip.re1.yahoo.com 69.147.76.15
• Trace complete.

56
IPv6

• Initial motivation 32-bit address space soon to
  be completely allocated.
• Additional motivation
• header format helps speed processing/forwarding
• header changes to facilitate QoS
• IPv6 datagram format
• fixed-length 40 byte header
• no fragmentation allowed

57
IPv6 Header (Cont)
Priority identify priority among datagrams in
flow Flow Label identify datagrams in same
flow. (concept offlow
not well defined). Next header identify upper
layer protocol for data
58
Other Changes from IPv4

• Checksum removed entirely to reduce processing
  time at each hop
• Options allowed, but outside of header,
  indicated by Next Header field
• ICMPv6 new version of ICMP
• additional message types, e.g. Packet Too Big
• multicast group management functions

59
Transition From IPv4 To IPv6

• Not all routers can be upgraded simultaneous
• no flag days
• How will the network operate with mixed IPv4 and
  IPv6 routers?
• Tunneling IPv6 carried as payload in IPv4
  datagram among IPv4 routers

60
Tunneling
tunnel
Logical view
IPv6
IPv6
IPv6
IPv6
Physical view
IPv6
IPv6
IPv6
IPv6
IPv4
IPv4
A-to-B IPv6
E-to-F IPv6
B-to-C IPv6 inside IPv4
B-to-C IPv6 inside IPv4
61
IPv6 status report

• Operating systems
• wide support early 2000
• Windows (2000, XP, Vista), BSD, Linux, Apple
• Networking infrastructure
• Cisco
• Deployment
• Slow
• Penetration
• Host - minor (less than 1)
• Used in 2008 in China Olympic games
• Motivation CIDR NAT