Computer Networks (Graduate level)

1
Computer Networks(Graduate level)
Lecture 5 Internetworking

• University of Tehran
• Dept. of EE and Computer Engineering
• By
• Dr. Nasser Yazdani

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Internetworking

• How to connect existing networks
• TCP/IP protocol
• Assigned Reading
• CK74 A Protocol for Packet Network
  Intercommunication.
• Hin96 IP Next Generation Overview

3
The Problem

• Before Internet
• different packet-switching networks (e.g.,
  ARPANET, ARPA packet radio)
• only nodes on the same physical/link layer
  network could communicate
• want to share room-size computers, storage to
  reduce expense

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The Challenge

• Interconnect existing networks
• but, packet switching networks differ widely
• different services
• e.g., degree of reliability
• different interfaces
• e.g., length of the packet that can be
  transmitted, address format
• different protocols
• e.g., routing protocols

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Key problem Heterogeneity

• Not described as such in paper
• Heterogeneous addressing conventions gt common
  addressing scheme
• Heterogeneous MTU gt fragmentation
• Heterogeneous queuing/transit delays gtadaptive
  timing procedures
• Heterogeneous reliability gt end-to-end recovery
  mechanisms
• Heterogeneous control/status info gt common
  error/coordination mechanism

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Possible solutions

• Reengineer and develop one global packet
  switching network standard
• not economically feasible
• not deployable
• Have every host implement the protocols of any
  network it wants to communicate with
• Complexity/node O(n)
• O(n2) global complexity

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Solution

• Add an extra layer internetworking layer
• hosts
• understand one network protocol
• understand one physical/link protocol
• gateways
• understand one network protocol
• understand the physical/link protocols of the
  networks they gateway
• Complexity to add a node/network O(1) with
  respect to number of existing nodes

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Solution
Gateways
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Challenge 1 Different Address Formats

• Options
• Map one address format to another. Why not?
• Provide one common format
• map lower level addresses to common format
• Format
• Initially 8b network 16b host 24b total
• Before Classless InterDomain Routing (CIDR)
• 7b/24b, 14b/16b, or 21b/8b 32b total
• After CIDR Arbitrary division 32b total
• NAT 32b 16b simultaneously active
• IPv6 128b total

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Address Formats

• 256 networks? What were they thinking?
• Why CIDR?
• What happens if they run out before IPv6?
• Why IPv6?
• Why 128b for IPv6? 248281 trillion.
• Why not variable length addresses?

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Challenge 2 Different Packet Sizes

• Need to define maximum packet size
• Options
• Take the minimum of the maximum packets sizes
  over all networks
• Implement fragmentation/reassembly
• Flexibility to adjust packet sizes as new
  technologies arrive
• IP fragment at routers, reassemble at host
• Why not reassemble at routers?
• Still stuck with 1500B as de facto maximum

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Other Challenges

• Errors ? require end-to-end reliability
• Thought to be rarely invoked, but necessary
• Different (routing) protocols ? coordinate these
  protocols
• Accounting
• Did not envision script kiddies
• Quality of Service
• Not addressed

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Transmission Control Program

• Original TCP/IP (Cerf Kahn)
• no separation between transport (TCP) and network
  (IP) layers
• one common header flow control, but not
  congestion control (why not?)
• fragmentation handled by TCP
• Todays TCP/IP
• separate transport (TCP) and network (IP) layer
  (why?)
• split the common header in TCP and UDP headers
• fragmentation reassembly done by IP
• congestion control

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Devils Advocate

• Who cares about resource sharing?
• 1974 cycles, storage, bandwidth expensive,
  people cheap
• 2002 resources cheap, people expensive
• 1974 Share computer resources
• 2002 Communicate with people, access documents,
  buy, sell
• Does it still make sense to make processes the
  endpoint?
• Where do people and organizations fit into the
  ISO layering model?

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Back to the big picture
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Goals (Clark88)

• Connect existing networks
• initially ARPANET and ARPA packet radio network
• Survivability
• ensure communication service even in the presence
  of network and router failures
• Support multiple types of services
• Must accommodate a variety of networks
• Allow distributed management
• Allow host attachment with a low level of effort
• Be cost effective
• Allow resource accountability

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1. Survivability

• continue to operate even in the presence of
  network failures (e.g., link and router failures)
• failures (excepting network partition) should be
  transparent to endpoints
• maintain state only at end-points (fate-sharing)
• no need to replicate and restore router state
• disadvantages?
• Internet stateless network architecture
• no per-flow state, still have state in address
  allocation, DNS

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2. Types of Services

• Add UDP to TCP to better support other types of
  applications
• e.g., real-time applications
• Probably main reason for separating TCP and IP
• Provide datagram abstraction lower common
  denominator on which other services can be built
• service differentiation considered (ToS header
  bits)
• was not widely deployed (why?)

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Application Assumptions

• Who made them
• Telephone network voice (web, video?)
• Cable broadcast (2-way?)
• X.25 remote terminal access (file transfer?)
• BBS centralized meeting place (web, p2p?)
• NAT client/server model (p2p, IM, IP Telephony?)
• Who didn't Internet
• Caveat best-effort, unicast, fixed location
  (real-time, multicast, mobility?)
• Allows development of unforseen applications
• Web, distributed gaming
• Sometimes too general
• Interdomain, multi-source multicast scales poorly
• Single source multicast scales better Holbrook99

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3. Variety of Networks

• Very successful
• because the minimalist service it requires from
  underlying network only to deliver a packet with
  a reasonable probability of success
• does not require
• reliability, in-order delivery, single delivery,
  QoS guarantees
• IP over everything
• Then ARPANET, X.25, DARPA satellite network..
• Now ATM, SONET, WDM, PPP, USB, 802.11b, GSM,
  GPRS, DSL, cable modems, power lines

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Internet Architecture

• Packet-switched datagram network
• IP is the glue
• Hourglass architecture
• all hosts and routers run IP
• Common Intermediate Representation

TCP
UDP
IP
Satellite
ATM
Ethernet
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Other Goals

• Allow distributed management
• each network can be managed by a different
  organization
• different organizations need to interact only at
  the boundaries
• doesnt work so well for routing, accounting
• Cost effective
• sources of inefficiency
• header overhead
• retransmissions
• routing
• but routers relatively simple to implement
  (especially software side)

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Other Goals (Cont)

• Low cost of attaching a new host
• not a strong point ? higher than other
  architecture because the intelligence is in hosts
  (e.g., telephone vs. computer)
• Moores law made this moot point, both lt100
• bad implementations or malicious users can
  produce considerably harm (remember
  fate-sharing?)
• DDoS possibly biggest threat to Internet
• Accountability
• very little so far

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What About the Future?

• Datagram not the best abstraction for
• resource management,accountability, QoS
• A new abstraction flow?
• Routers require to maintain per-flow state (what
  is the main problem with this raised by Clark?)
• state management
• Solution
• soft-state end-hosts responsible to maintain the
  state

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Summary Minimalist Approach

• Dumb network
• IP provide minimal functionalities to support
  connectivity
• addressing, forwarding, routing
• Smart end system
• transport layer or app does more sophisticated
  functionalities
• flow control, error control, congestion control
• Advantages
• accommodate heterogeneous technologies
• support diverse applications (telnet, ftp, Web, X
  windows)
• decentralized network administration
• Disadvantages
• poor realtime performance
• poor accountability

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How is IP Design Standardized?

• IETF
• Voluntary organization
• Meeting every 4 months
• Working groups and email discussions
• We reject kings, presidents, and voting we
  believe in rough consensus and running code
  (Dave Clark 1992)
• Need 2 independent, interoperable implementations
  for standard

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IP Internet

• Concatenation of Networks or networks of
  Networks.
• R is routers and H is hosts.

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Service Model

• Connectionless (datagram-based)
• Best-effort delivery (unreliable service)
• packets are lost. No recover from lost.
• packets are delivered out of order
• duplicate copies of a packet are delivered
• packets can be delayed for a long time
• Datagram format

Contains all information for routing!
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Packet Headers

• The current Version is 4 or IPv4.
• HLen- the Header Length from 5-15 in 32-bit
  words.
• Length- the total length of the packet including
  headers. Max length is 64K.
• TTL Time To Live is expressed in second. It is
  to prevent packet from permanently circulating in
  a loop.
• Protocol specify the packet application ex. 1
  for ICMP. It is for demultiplexing to higher
  layer protocols.
• Checksum is a 1-complement error checksum for
  the header only.

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Packet Headers (Cont)

• TOS type of Service
• Precedence
• Specify the priority
• Type of Services Specify routing, for instance
  cheapest, fastest and more reliable
• D for Delay
• T for Throughput
• R for Reliability
• C for low cost.
• Note Precedence is only for inside channel
  queuing.
• Replaced by DiffServ.

0 2 3 7 0 2 3 7 0 2 3 7 0 2 3 7 0 2 3 7
Precedence Type of service Type of service Type of service Type of service
Precedence D T R C
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Packet Headers (Cont)

• Options
• If C set, the option will copied to all
  fragments. Otherwise, only to the first one.
• Class 0 for control
• Class 2 for debugging and measurement.
• Options are rarely used in today except for
  loose and strict source routing parameters.
• loose and strict source option sometimes, is
  used for IP encapsulation in another IP or
  Tunneling

C Class Number
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Fragmentation and Reassembly
0 70 70 4 70 7 0 70 70 4 70 7 0 70 70 4 70 7
Identification Flags Fragment Offset

• Flags
• DF Dont Fragment
• MF More Fragment coming
• In fragmentation, IP copy the original header and
  only modify
• The length, which is the new length, and offset.
• Offset is used for reassembly.
• Note Fragmentation may degrade the network
  performance.
• That is why the IP packet should be the same of
  TCP packets
• Modern TCP implement Path MTU discovery.
• It start with large packet and with DF set flag,
  if it passed
• TCP keeps the same packet size, otherwise, it
  reduces it.

0 1 2
0 DF MF
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Fragmentation and Reassembly (cont)

• Each network has a Maximum Transfer Unit size,
  MTU
• Strategy
• fragment when necessary (MTU lt Datagram)
• fragments are self-contained datagrams
• do not recover from lost fragments
• Where to do reassembly?
• End nodes avoids unnecessary work
• Dangerous to do at intermediate nodes
• Buffer space
• Multiple paths through network

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Fragmentation is Harmful

• Uses resources poorly
• Forwarding costs per packet
• Best if we can send large chunks of data
• Worst case packet just bigger than MTU
• Poor end-to-end performance
• Loss of a fragment
• Reassembly is hard
• Buffering constraints

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Path MTU Discovery

• Hosts dynamically discover minimum MTU of path
• Algorithm
• Initialize MTU to MTU for first hop
• Send datagrams with Dont Fragment bit set
• If ICMP pkt too big msg, decrease MTU
• What happens if path changes?
• Periodically (gt5mins, or gt1min after previous
  increase), increase MTU
• Some routers will return proper MTU
• MTU values cached in routing table

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Addressing in IP

• IP addresses are names of interfaces
• Domain Name System (DNS) names are names of hosts
• DNS binds host names to interfaces
• Routing binds interface names to paths

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Addressing Considerations

• Fixed length or variable length?
• Issues
• Flexibility
• Processing costs
• Header size
• Engineering choice IP uses fixed length addresses

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Addressing Considerations

• Structured vs flat
• Issues
• What information would routers need to route to
  Ethernet addresses?
• Need structure for designing scalable binding
  from interface name to route!
• How many levels? Fixed? Variable?

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IP Addresses

• Fixed length 32 bits
• Initial classful structure (1981)
• Total IP address size 4 billion
• Class A 128 networks, 16M hosts
• Class B 16K networks, 64K hosts
• Class C 2M networks, 256 hosts

High Order Bits 0 10 110
Format 7 bits of net, 24 bits of host 14 bits of
net, 16 bits of host 21 bits of net, 8 bits of
host
Class A B C
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IP Address Classes (Some are Obsolete)
Network ID
Host ID
8
16
32
24
Class A
Network ID
Host ID
0
Class B
10
Class C
110
Class D
Multicast Addresses
1110
Class E
Reserved for experiments
1111
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Some Special IP Addresses

• 127.0.0.1 local host (a.k.a. the loopback
  address
• Host bits all set to 0 network address
• Host bits all set to 1 broadcast address

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Subnet Addressing RFC917 (1984)

• For class B C networks
• Very few LANs have close to 64K hosts
• For electrical/LAN limitations, performance or
  administrative reasons
• Need simple way to get multiple networks
• Use bridging, multiple IP networks or split up
  single network address ranges (subnet)
• Must reduce the total number of network addresses
  that are assigned

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Subnetting

• Variable length subnet masks
• Could subnet a class B into several chunks

Network
Host
Network
Host
Subnet
1111..
00000000
..1111
Mask
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Subnetting Example

• Assume an organization was assigned address
  150.100
• Assume lt 100 hosts per subnet
• How many host bits do we need?
• Seven
• What is the network mask?
• 11111111 11111111 11111111 10000000
• 255.255.255.128

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Subnet Addressing Example

• Assume a packet arrives with address
  150.100.12.176
• Step 1 AND address with subnet mask

150.100.12.154
150.100.12.176
H1
H2
150.100.12.128
150.100.12.129
150.100.12.55
150.100.12.24
150.100.0.1
H3
H4
R1
To Internet
150.100.12.4
150.100.12.0
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IP Address Problem (1991)

• Address space depletion
• In danger of running out of classes A and B
• Why?
• Class C too small for most domains
• Very few class A IANA (Internet Assigned
  Numbers Authority) very careful about giving
• Class B greatest problem

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IP Address Utilization (98)
http//www.caida.org/outreach/resources/learn/ipv4
space/
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Some Problems

• Class B sparsely populated
• But people refuse to give it back
• Solution
• Assign multiple class C addresses
• Assign consecutive blocks
• RFC1338 Classless Inter-Domain Routing (CIDR)

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Classless Inter-Domain Routing

• Do not use classes to determine network ID
• Assign any range of addresses to network
• Use common part of address as network number
• e.g., addresses 192.4.16 - 196.4.31 have the
  first 20 bits in common. Thus, we use this as the
  network number
• netmask is /20, /xx is valid for almost any xx
• Enables more efficient usage of address space
  (and router tables)

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Network Address Translation (NAT)

• Sits between your network and the Internet
• Translates local network layer addresses to
  global IP addresses
• Has a pool of global IP addresses (less than
  number of hosts on your network)

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NAT Illustration
Pool of global IP addresses
Destination
Source
P
G
Global Internet
Private Network
NAT
Dg
Sp
Data
Dg
Sg
Data

• Operation Source (S) wants to talk to
  Destination (D)
• Create Sg-Sp mapping
• Replace Sp with Sg for outgoing packets
• Replace Sg with Sp for incoming packets

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Problems with NAT

• What if we only have few (or just one) IP
  address?
• Use NAPT (Network Address Port Translator)
• NAPT translates
• Translates Paddr flow info to Gaddr new flow
  info
• Uses TCP/UDP port numbers
• Potentially thousands of simultaneous connections
  with one global IP address

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Problems with NAT

• Hides the internal network structure
• Some consider this an advantage
• Multiple NAT hops must ensure consistent mappings
• Some protocols carry addresses
• e.g., FTP carries addresses in text
• What is the problem?
• Encryption
• No inbound connections

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IPv6 Changes

• Scale addresses are 128bit
• Header size?
• Simplification
• Removes infrequently used parts of header
• 40byte fixed size vs. 20 byte variable
• IPv6 removes checksum
• Relies on upper layer protocols to provide
  integrity
• IPv6 eliminates fragmentation
• Requires path MTU discovery
• Requires 1280 byte MTU

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IPv6 Header
0
4
16
24
32
12
19
Version
Class
Flow Label
Payload Length
Next Header
Hop Limit
Source Address
Destination Address
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IPv6 Changes

• TOS replaced with traffic class octet
• Flow
• Help soft state systems
• Maps well onto TCP connection or stream of UDP
  packets on host-port pair
• Easy configuration
• Provides auto-configuration using hardware MAC
  address to provide unique base
• Additional requirements
• Support for security
• Support for mobility

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IPv6 Changes

• Protocol field replaced by next header field
• Support for protocol demultiplexing as well as
  option processing
• Option processing
• Options are added using next header field
• Options header does not need to be processed by
  every router
• Large performance improvement
• Makes options practical/useful

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Next-lecture Intra-Domain Routing

• Routing algorithms- two main approaches
• Distance vector protocols
• Link state protocols
• How to make routing adapt to load
• How to make routing scale
• Assigned reading
• KZ89 The revised ARPANET routing metric
• Tsu88 The Landmark Hierarchy A New Hierarchy
  for Routing in Very Large Networks