1. Internetwork Protocols
Lesson 11
NETS2150/2850
School of Information Technologies

2. Lesson Outcomes
The needs for internetworking and not standalone network!
Design issues in a connection-less model
Understand the intricacies of IP addressing
What’s missing in IPv4?
The importance of ICMP, the companion protocol of IP

3. 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

4. The Internet/Network layer
Host, router network layer functions:
Transport layer: TCP, UDP
IP protocol
addressing conventions
packet format
packet handling conventions
Routing protocols
path selection
RIP, OSPF
Network
layer
routing
table
ICMP protocol
error reporting
router “signalling”
Data link layer
physical layer

5. Architectural Approaches
Mode of Operation:
Connection oriented
Connectionless
application
transport
network
data link
physical
application
transport
network
data link
physical

6. 1: Bridges and routers are examples of ISs.
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.

7. 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

8. 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)

9. 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

10. IP Operation

11. IP Design Issues Routing Datagram lifetime
Fragmentation and re-assembly
Error control
Flow control

12. Routing End systems and routers maintain routing tables Source routing
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

13. 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

14. 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
fragmentation:
in: one large packet
out: 3 smaller packets
reassembly

15. 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!

16. 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

17. 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

18. IP Fragmentation (2) Offset More fragment flag
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

19. IP Fragmentation and ReassemblyID =x
offset =0
Moreflag
length
=4000 =1
=1500
=185
=370
=1040
One large packet becomes
3 smaller packets
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 = ( ) = 370

20. 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

21. 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)

22. IP Checksum – 2 steps
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
Once all the values are added in this manner, invert all the bits in the result - called its “one's complement”

23. Example:
First, we add the 16-bit values 2 at a time:
First 16-bit value
Second 16-bit value
Produced a carry-out, which gets added
+ \ > 1 back into lsb
Third 16-bit value
No carry to swing around (**)
Fourth 16-bit value
Produced a carry-out, which gets added
Our sum
lsb
msb

24. 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:
Our sum
The "one's complement"
So the checksum stored in the header would be

25. 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)

26. Internet Protocol (IP) Version 4
Part of TCP/IP
Specified in RFC 791
Will (eventually) be replaced by IPv6

27. IP Services Send primitive Deliver primitive
Request transmission of data unit onto the network
Deliver primitive
Notify user of arrival of data unit from the network

28. IPv4 Header

29. Header Fields (1) Version (4 bits) Internet header length (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

30. Header Fields (2) Identification (16 bits) Flags (3 bits)
Sequence number
Used with addresses and user protocol to identify packet uniquely
Flags (3 bits)
More bit (1 bit)
Don’t fragment (1 bit)
Fragmentation offset (13 bits)
Time to live (8 bits)
Protocol (8 bits)
Next higher layer to receive data field at destination

31. Header Fields (3) Header checksum (16 bits) Source address (32 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

32. Options Security Label Source routing Route recording
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

33. 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

34. IP Addressing: Introduction
IP address: 32-bit identifier for host, router interface
interface: connection between host/router and physical link
router’s have multiple interfaces
IP addresses associated with each interface
Dotted-decimal notation
Decimal number representing each of the octets, separated by a decimal point =
223 1

35. IP Addressing IP address: 2 parts
network part (high order bits)
host part (low order bits)
What’s a network ? (from IP address perspective)
device interfaces with same network part of IP address
can physically reach each other without intervening router
LAN

36. IP Addresses given notion of “network”, let’s re-examine IP addresses:
“classful” addressing:
class
Address range A to
network
host B to 10
network
host to C
110
network
host to D
1110
multicast address
32 bits
All host ids.

37. Private IP addresses Also called non-routable IP addresses
IP blocks reserved for private networks are:
through
through
through
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!)

38. 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)
network
part
host
/23

39. IP addresses: how to get one?
Q: How does host get IP address?
hard-coded by network admin:
MS Windows: control-panel->network->configuration->tcp/ip->properties
Red-Hat LINUX: /etc/sysconfig
DHCP: Dynamic Host Configuration Protocol: dynamically gets address from a server
Client “plug-and-play”

40. Who assigns IP addresses?
Q: How does network get network part of IP addr?
A: gets allocated portion of its provider ISP’s address space
ISP's block /20
Organization /23
Organization /23
Organization /23
… … ….
Organization /23

41. 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

42. 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

43. 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
echo reply (ping)
dest. network unreachable
dest host unreachable
dest protocol unreachable
dest port unreachable
dest network unknown
dest host unknown
source quench (congestion
control - not used)
echo request (ping)
route advertisement
router discovery
TTL expired
bad IP header IP
ICMP

44. 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

45. 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

46. Ethereal: A Packet Sniffer
Network sniffer or a protocol analyzer: Ethereal
Ethereal: A Packet Sniffer
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

50. 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)!