Internetwork Protocols

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Internetwork Protocols

• Lesson 11
• NETS2150/2850
• http//www.ug.cs.usyd.edu.au/nets2150/

School of Information Technologies
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Lesson Outcomes

• The needs for internetworking and not standalone
  network!
• Design issues in a connection-less model
• Understand the intricacies of IP addressing
• Whats missing in IPv4?
• The importance of ICMP, the companion protocol
  of IP

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Internetworking Terms

• Communications Network
• Facility that provides data transfer service
• An internet
• Collection of communications networks
  interconnected by routers
• The Internet - note upper case I
• The global collection of thousands of individual
  machines and networks
• An intranet
• Corporate internet operating within the
  organization
• Uses Internet (TCP/IP) technology to deliver
  documents and resources
• Can be isolated internet, or can have links to
  the Internet

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The Internet/Network layer

• Host, router network layer functions

Transport layer TCP, UDP
Network layer
Data link layer
physical layer
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Architectural Approaches

• Mode of Operation
• Connection oriented

• Connectionless

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Connection Oriented

• Assume that each network is connection oriented
• Intermediate System (IS1) connect two or more
  networks
• Logical connection set up between ESs
• Concatenation of logical connections across
  networks
• Individual network virtual circuits joined by IS

1 Bridges and routers are examples of ISs.
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Connectionless Operation

• Corresponds to datagram mechanism in packet
  switched network
• Each NPDU treated separately
• Network layer protocol common to all ESs and
  routers
• Known generically as the internet protocol
• Internet Protocol (IP)
• One such internet protocol was developed for
  ARPANET (Advanced Research Projects Agency
  Network )
• RFC 791
• Lower layer protocol needed to access particular
  network

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Connectionless Internetworking

• Pros
• Flexibility
• Robust
• No connection setup overhead
• Cons
• Unreliable, not guaranteed delivery
• Not guaranteed order of delivery
• Packets can take different routes
• Reliability is responsibility of next layer up
  (e.g. TCP)

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Ordered Delivery

• PDUs may arrive out of order
• Different paths through network
• But, PDU order must be maintained
• Number PDUs sequentially
• Easy to reorder received PDUs

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IP Operation
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IP Design Issues

• Routing
• Datagram lifetime
• Fragmentation and re-assembly
• Error control
• Flow control

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Routing

• End systems and routers maintain routing tables
• Indicate next router to which packet should be
  sent, for all possible destination network
• Could be static
• But, may contain alternative routes
• Or Dynamic
• Flexible response to congestion and errors
• Source routing
• Source specifies route as sequential list of
  routers to be followed because of
• Security
• Priority

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Datagram Lifetime

• Datagram could loop indefinitely
• Consumes resources
• Transport protocol may need upper bound on
  datagram life
• Datagram marked with lifetime
• TTL field in IP
• Decrement TTL on passing through each router
• Once lifetime expires, datagram discarded

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Fragmentation and Reassembly

• Network links have MTU (max. transmission unit) -
  largest possible data link-level frame
• Different link types, different MTUs
• ATM ? 53 octets
• Ethernet ? 1518 octets
• Pros
• More efficient error control
• Smaller retransmission
• Fairer
• Prevent monopolising
• Smaller buffers at rcvr

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Disadvantages of Fragmentation

• Make PDUs as large as possible because
• PDU contains some control information
• Smaller block, larger overhead to data ratio
• PDU arrival generates interrupt to CPU
• Waste CPU time
• Smaller blocks, more interrupts!

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Fragmentation and Reassembly

• When to re-assemble??
• At destination?
• Results in packets getting smaller as data
  traverses internet
• Intermediate re-assembly?
• Need large buffers at routers
• Buffers may fill with fragments
• All fragments must go through same router
• Inhibits dynamic routing

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IP Fragmentation (1)

• IP reassembles at destination only
• It uses these fields in header
• Identifies end system originated packet
• Source and destination address
• Protocol layer generating data (e.g. TCP)
• Identification supplied by IP layer
• Total Length
• Length of packet in octets

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IP Fragmentation (2)

• Offset
• Position of fragment of user data in original
  packet
• In multiples of 64-bit (8-octet) units
• More fragment flag
• Indicates that this is not the last fragment
• 0 last or the only packet
• 1 not last

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IP Fragmentation and Reassembly

• Example
• 4000-octet packet (with 20-octet header)
• MTU 1500 octets
• Data in each is 1480 octets
• Fragments ?3980/1480? 3

• Offset in 1st fragment 0, 2nd fragment
  (1480/8) 185 and 3rd fragment (185185) 370

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Dealing with Failure

• Reassembly may fail if some fragments get lost
• So, need to detect failure
• Reassembly time out
• Assigned to first fragment to arrive
• If timeout expires before all fragments arrive,
  discard partial data
• Use packet lifetime (time to live)
• If TTL runs out, kill partial data

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Error Control

• IP do NOT guarantee delivery
• IP uses checksum for error detection
• Router should attempt to inform source if packet
  discarded
• e.g. for TTL expiring or destination unreachable
• But, datagram identification needed
• Handled by ICMP protocol (see later)

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IP Checksum 2 steps

1. Add the 16-bit values up. Each time a carry-out
   (17th bit) is produced, swing that bit around and
   add it back into the lsb
2. Once all the values are added in this manner,
   invert all the bits in the result - called its
   one's complement

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Example
1000 0110 0101 1110 1010 1100 0110
0000 0111 0001 0010 1010 1000 0001 1011
0101 First, we add the 16-bit values 2 at a
time 1000 0110 0101 1110 First
16-bit value 1010 1100 0110 0000
Second 16-bit value -------------------
------ 1 0011 0010 1011 1110 Produced a
carry-out, which gets added
\----------------------gt 1 back into lsb
---------------------------- 0011 0010
1011 1111 0111 0001 0010 1010 Third
16-bit value ----------------------------
0 1010 0011 1110 1001 No carry to swing
around () 1000 0001 1011 0101 Fourth
16-bit value ----------------------------
1 0010 0101 1001 1110 Produced a
carry-out, which gets added
\----------------------gt 1 back into lsb
---------------------------- 0010 0101
1001 1111 Our sum
lsb
msb
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Example (Cont.)
Then we have to take the one's complement of the
sum. We do this by simply inverting all the bits
in the final result from above 0010 0101 1001
1111 Our sum 1101 1010 0110 0000 The "one's
complement" So the checksum stored in the
header would be 1101 1010 0110 0000.
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Flow Control

• Allows routers and/or stations to limit rate of
  incoming data
• Difficult in connectionless systems
• Not done here, left to higher layer (i.e.
  transport)

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Internet Protocol (IP) Version 4

• Part of TCP/IP
• Specified in RFC 791
• Will (eventually) be replaced by IPv6

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IP Services

• Send primitive
• Request transmission of data unit onto the
  network
• Deliver primitive
• Notify user of arrival of data unit from the
  network

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IPv4 Header
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Header Fields (1)

• Version (4 bits)
• Currently 4
• Internet header length (4 bits)
• In 32-bit units
• Including options
• Type of service (before) Differentiated Service
  (now) (8 bits)
• Allows classification of packets
• Total length (16 bits)
• in octets
• Header plus data

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Header Fields (2)

• Identification (16 bits)
• Sequence number
• Used with addresses and user protocol to identify
  packet uniquely
• Flags (3 bits)
• More bit (1 bit)
• Dont fragment (1 bit)
• Fragmentation offset (13 bits)
• Time to live (8 bits)
• Protocol (8 bits)
• Next higher layer to receive data field at
  destination

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Header Fields (3)

• Header checksum (16 bits)
• Reverified and recomputed at each router
• Uses 16-bit ones complement sum
• Source address (32 bits)
• Destination address (32 bits)
• Options (variable)
• See next slide
• Padding (variable)
• To fill to multiple of 32 bits long

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Options

• Security Label
• Allows secured handling of packets
• Source routing
• A list of router addresses specifies a route to
  follow
• Route recording
• Records route taken by a packet
• Stream identification
• Allows special handling of streaming traffic
• Timestamping
• Intermediate routers add timestamp on packet

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Data Field

• Carries user data from next layer up
• Integer multiple of 8 bits long (octet)
• Max length of packet (header plus data) 65,535
  octets

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IP Addressing Introduction

• IP address 32-bit identifier for host, router
  interface
• interface connection between host/router and
  physical link
• routers have multiple interfaces
• IP addresses associated with each interface
• Dotted-decimal notation
• Decimal number representing each of the octets,
  separated by a decimal point

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IP Addressing

• IP address 2 parts
• 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

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IP Addresses

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

classful addressing
class
Address range
0.0.0.0 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
All host ids.
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Private IP addresses

• Also called non-routable IP addresses
• IP blocks reserved for private networks are
• 10.0.0.0 through 10.255.255.255
• 172.16.0.0 through 172.31.255.255
• 192.168.0.0 through 192.168.255.255
• Network Address Translation (NAT) protocol could
  be used to map private IP addresses to external
  IP address space (see RFC 1631)
• Use to hide internal network structure from the
  outside world (Security measure!)

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IP addressing CIDR

• Classful addressing contributed to
• 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
• Solution ? classless addressing
• CIDR Classless InterDomain Routing
• network portion of address is of arbitrary length
• address format a.b.c.d/x, where x is bits in
  network portion of address (Slash notation)

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IP addresses how to get one?

• Q How does host get IP address?
• hard-coded by network admin
• MS Windows control-panel-gtnetwork-gtconfiguration-
  gttcp/ip-gtproperties
• Red-Hat LINUX /etc/sysconfig
• DHCP Dynamic Host Configuration Protocol
  dynamically gets address from a server
• Client plug-and-play

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Who assigns IP addresses?

• Q How does network get network part of IP addr?
• A gets allocated portion of its provider 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
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Subnets and Subnet Masks

• Allow arbitrary complexity of internetworked LANs
  within organisation
• Each LAN segment assigned subnet number
• Host portion of address partitioned into subnet
  number and host number
• With the help of subnet mask

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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

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ICMP Internet Control Message Protocol

• Used by hosts, routers to communicate
  network-level information (RFC 792)
• error reporting unreachable host, network, port,
  protocol
• query echo request/reply (used by ping)
• In network-layer above IP
• ICMP msgs carried in IP packets
• ICMP message type, code plus IP header and first
  8 octets of data 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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IP v6 - Version Number

• IP v 1-3 defined and replaced
• IP v4 - current version
• IP v5 - non-IP real-time streaming protocol
• IP v6 - replacement for IP v4
• During development it was called IPng
• Overall spec in RFC 2460

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Why Change IP?

• Address space exhaustion
• Two level addressing (network and host) wastes
  space
• Network addresses used even if not connected to
  Internet
• Exponential growth of the Internet
• Single address per host
• Requirements for new types of service

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Ethereal A Packet Sniffer
Network sniffer or a protocol analyzer Ethereal

• A basic tool for observing messages exchanges
  between protocol entities
• It captures messages being sent/received from/by
  your computer
• Other packet sniffers are tcpdump, Zx Sniffer
  AnalogX PacketMon

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Summary

• IP enables host-to-host delivery of packets,
  unreliably
• Allows a flexible approach
• Some assistance by ICMP when error
• Who looks at process-to-process delivery??
• Transport layer (next lesson!)
• Read Stallings Chapter 18
• Download RFC 791, a classic (1981)!