1. ECE 4450:427/527 - Computer Networks Spring 2017
Dr. Nghi Tran
Department of Electrical & Computer Engineering
Lecture 6.2: IP
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

2. Internetworking: Discussions
For Internetworking, we shall look at few sub-problems:
Interconnect links of the same type: Switches
We consider an important of class switch: Bridges to interconnect Ethernet segments.
We also look a way to interconnect disparate networks and links: Gateways, or now mostly known as routers. We shall focus on the IP
Once we are able to interconnect a whole lot of links and networks with switches and routers, we will look at a way to find a suitable path, or route through a new working:
Paths that are efficient, loop free, etc.: Routing
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

3. Internetworking What is internetwork
An arbitrary collection of networks interconnected to provide some sort of host-host to packet delivery service
A simple internetwork where H represents hosts and R represents routers
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

4. IP: Internet Protocol What is IP IP stands for Internet Protocol
Key tool used today to build scalable, heterogeneous internetworks
It runs on all the nodes in a collection of networks and defines the infrastructure that allows these nodes and networks to function as a single logical internetwork
A simple internetwork showing the protocol layers
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

5. IP: “Best Effort” Service
IP is a datagram connectionless protocol
Does not provide any type of guarantee about packet delivery
Out of order packet deliveries, duplicate packets, no error correction
If packets are lost, IP does not try to recover or retransmit (though lower or higher layer functionalities may do so)
Global addressing scheme
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

6. IPv4: Packet Format 16-bit Total Length (Bytes) 16-bit Identification
Version
Header
Length
8-bit
Type of Service
(TOS)
16-bit Total Length (Bytes)
16-bit Identification
3-bit
Flags
13-bit Fragment Offset
8-bit Time to
Live (TTL)
8-bit Protocol
16-bit Header Checksum
32-bit Source IP Address
32-bit Destination IP Address
Options (if any)
Payload
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

7. IP Packet Format Version number (4 bits) Header length (4 bits)
Indicates the version of the IP protocol
Necessary to know what other fields to expect
Typically “4” (for IPv4), and sometimes “6” (for IPv6)
Header length (4 bits)
Number of 32-bit words in the header
Typically “5” (for a 20-byte IPv4 header)
Can be more when “IP options” are used
Type-of-Service (8 bits)
Allow packets to be treated differently based on needs
E.g., low delay for audio, high bandwidth for bulk transfer
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

8. IP Packet Format Total length (16 bits)
Number of bytes in the packet
Maximum size is 63,535 bytes (216 -1)
… though underlying links may impose harder limits
Fragmentation information (32 bits)
Packet identifier, flags, and fragment offset
Supports dividing a large IP packet into fragments
… in case a link cannot handle a large IP packet
Time-To-Live (8 bits)
Used to identify packets stuck in forwarding loops
… and eventually discard them from the network
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

9. Time-to-Live (TTL) Potential robustness problem
Forwarding loops can cause packets to cycle forever
Confusing if the packet arrives much later
Time-to-live field in packet header
TTL field decremented by each router on the path
Packet is discarded when TTL field reaches 0…
…and “time exceeded” message is sent to the source
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

10. Protocol Field Protocol (8 bits) Identifies the higher-level protocol
E.g., “6” for the Transmission Control Protocol (TCP)
E.g., “17” for the User Datagram Protocol (UDP)
Important for demultiplexing at receiving host
Indicates what kind of header to expect next
protocol=6
protocol=17
IP header
IP header
TCP header
UDP header
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

11. IP Addresses Field Two IP addresses Destination address Source address
Source IP address (32 bits)
Destination IP address (32 bits)
Destination address
Unique identifier for the receiving host
Allows each node/router to make forwarding decisions
Source address
Unique identifier for the sending host
Recipient can decide whether to accept packet
Enables recipient to send a reply back to source
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

12. IP Fragmentation and Reassembly
Each network has some MTU (Maximum Transmission Unit): largest IP datagram it can carry in a frame
Ethernet (1500 bytes), FDDI (4500 bytes)
Strategy
Fragmentation occurs in a router when it receives a datagram that it wants to forward over a network which has (MTU < received datagram)
Reassembly is done at the receiving host
All the fragments carry the same identifier in the Ident field
Fragments are self-contained datagrams
IP does not recover from missing fragments
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

13. IP Fragmentation and Reassembly
IP datagrams traversing the sequence of physical networks
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

14. IP Fragmentation and Reassembly
Header fields used in IP fragmentation. (a) Unfragmented packet; (b) fragmented packets.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

15. IP Datagram Forwarding
Strategy
every datagram contains destination's address
if directly connected to destination network, then forward to host
if not directly connected to destination network, then forward to some router
forwarding table maps network number into next hop
each host has a default router
each router maintains a forwarding table
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

16. Forwarding Table Dr. Nghi Tran (ECE-University of Akron)

17. IPv4 Addressing
A unique 32-bit number
Identifies an interface (on a host, on a router, …)
Represented in dotted-quad notation 12 34
158 5
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

18. IPv4 Addressing
A host usually has a single link into network: When IP in host wants to send datagram, it does so over the link -> Boundary between host and link: interface.
IP address: technically associated with an interface, rather with the host
How about a router?
Receive datagram on a link and forward to on some other link.
How many interfaces? How many IP addresses?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

19. IPv4: Early Addressing Properties Format Dot notation globally unique
hierarchical: network + host
4 Billion IP address, half are A type, ¼ is B type, and 1/8 is C type
Format
Dot notation
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

20. IP Addressing: CIDR CIDR: Classless InterDomain Routing
subnet portion of address of arbitrary length
address format: a.b.c.d/x, where x is # bits in subnet portion of address
subnet
part
host
part
/23
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

21. Subnets IP address: What’s a subnet ? subnet part (high order bits)
host part (low order bits)
What’s a subnet ?
device interfaces with same subnet part of IP address
can physically reach each other without intervening router
subnet
network consisting of 3 subnets
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

22. Subnet and Subnet Mask Recipe
/24
/24
/24
Recipe
to determine the subnets, detach each interface from its host or router, creating islands of isolated networks
each isolated network is called a subnet.
Subnet mask: /24 Or
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

23. Subnets
How many?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

24. ARP: Address Resolution Protocol
Question: how to determine
MAC address of B
knowing B’s IP address?
Each IP node (host, router) on LAN has ARP table in ARP module
ARP table: IP/MAC address mappings for some LAN nodes
< IP address; MAC address; TTL>
TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)
1A-2F-BB AD
LAN
71-65-F7-2B-08-53
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

25. ARP: Same LAN ARP is “plug-and-play”:
A wants to send datagram to B, and B’s MAC address not in A’s ARP table.
A broadcasts ARP query packet, containing B's IP address
dest MAC address = FF-FF-FF-FF-FF-FF
all machines on LAN receive ARP query
B receives ARP packet, replies to A with its (B's) MAC address
frame sent to A’s MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is “plug-and-play”:
nodes create their ARP tables without intervention from net administrator
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

26. ARP: Packet Format
HardwareType: type of physical network (e.g., Ethernet)
ProtocolType: type of higher layer protocol (e.g., IP)
HLEN & PLEN: length of physical and protocol addresses
Operation: request or response
Source/Target Physical/Protocol addresses
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

27. Addressing: Routing to another LAN
walkthrough: send datagram from A to B via R.
focus on addressing - at both IP (datagram) and MAC layer (frame)
assume A knows B’s IP address
How can A know whether B is in the same subnet/network?
assume A knows B’s MAC address (how?)
assume A knows IP address of first hop router, R (how?)
assume A knows MAC address of first hop router interface (how?) – How many MAC addresses?
49-BD-D2-C7-56-2A
88-B2-2F-54-1A-0F B A R
C-E8-FF-55
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

28. Addressing: Routing to another LAN
A creates IP datagram with IP source A, destination B
A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram
MAC src: C-E8-FF-55
MAC dest: E6-E BB-4B IP
Eth
Phy
IP src:
IP dest:
49-BD-D2-C7-56-2A
88-B2-2F-54-1A-0F B A R
C-E8-FF-55
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

29. Addressing: Routing to another LAN
frame sent from A to R
frame received at R, datagram removed, passed up to IP
MAC src: C-E8-FF-55
MAC dest: E6-E BB-4B IP
Eth
Phy
IP src:
IP dest: IP
Eth
Phy
49-BD-D2-C7-56-2A
88-B2-2F-54-1A-0F B A R
C-E8-FF-55
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

30. Addressing: Routing to another LAN
R forwards datagram with IP source A, destination B
R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagram
MAC src: 1A-23-F9-CD-06-9B
MAC dest: 49-BD-D2-C7-56-2A IP
Eth
Phy
IP src:
IP dest: IP
Eth
Phy
49-BD-D2-C7-56-2A
88-B2-2F-54-1A-0F B A R
C-E8-FF-55
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

31. Addressing: Routing to another LAN
R forwards datagram with IP source A, destination B
R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagram
IP src:
IP dest:
MAC src: 1A-23-F9-CD-06-9B
MAC dest: 49-BD-D2-C7-56-2A IP
Eth
Phy IP
Eth
Phy
49-BD-D2-C7-56-2A
88-B2-2F-54-1A-0F B A R
C-E8-FF-55
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

32. Addressing: Routing to another LAN
R forwards datagram with IP source A, destination B
R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagram
MAC src: 1A-23-F9-CD-06-9B
MAC dest: 49-BD-D2-C7-56-2A
IP src:
IP dest: IP
Eth
Phy
49-BD-D2-C7-56-2A
88-B2-2F-54-1A-0F B A R
C-E8-FF-55
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

33. IP Addresses: How to get one?
MAC address: configured in adapter, globally unique
IP address:
Not only be unique on a given internetwork, but needs to reflect the structure of the internetwork
Not possible to be configured once into host; Hosts might change to another network: IP needs to be reconfigurable
Usually, automatic configuration methods are required: Dynamic Host Configuration Protocol (DHCP): Textbook, 3.2.7
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

34. A day in the life: Connecting to Internet
browser
DNS server
Comcast network
/13
school network
/24
web page
web server
Google’s network
/19
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

35. A day in the life: Connecting to Internet
DHCP
UDP IP
Eth
Phy
DHCP
DHCP
connecting laptop needs to get its own IP address, addr of first-hop router, addr of DNS server: use DHCP
DHCP
DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in Ethernet
DHCP
DHCP
UDP IP
Eth
Phy
DHCP
router
(runs DHCP)
Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server
Ethernet demuxed to IP demuxed, UDP demuxed to DHCP
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

36. A day in the life: Connecting to Internet
DHCP
DHCP
UDP IP
Eth
Phy
DHCP server formulates DHCP ACK containing client’s IP address, IP address of first-hop router for client, name & IP address of DNS server
encapsulation at DHCP server, frame forwarded (switch learning) through LAN, demultiplexing at client
DHCP
UDP IP
Eth
Phy
DHCP
DHCP
router
(runs DHCP)
DHCP client receives DHCP ACK reply
DHCP
Client now has IP address, knows name & addr of DNS
server, IP address of its first-hop router
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

37. A day in the life: ARP (before DNS, HTTP)
before sending HTTP request, need IP address of DNS
DNS
UDP IP
Eth
Phy
DNS
ARP
ARP query
DNS query created, encapsulated in UDP, encapsulated in IP, encapsulated in Eth. In order to send frame to router, need MAC address of router interface: ARP
Eth
Phy
ARP
ARP reply
ARP query broadcast, received by router, which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router, so can now send frame containing DNS query
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

38. NAT All datagrams leaving local Datagrams with source or
rest of
Internet
local network
(e.g., home network)
10.0.0/24
All datagrams leaving local
network have same single source NAT IP address: ,
different source port numbers
Datagrams with source or
destination in this network
have /24 address for
source, destination (as usual)
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

39. NAT 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
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

40. WAN side addr LAN side addr
NAT
NAT translation table
WAN side addr LAN side addr
1: host
sends datagram to
, 80
2: NAT router
changes datagram
source addr from
, 3345 to
, 5001,
updates table
, , 3345
…… ……
S: , 3345
D: , 80 1
S: , 80
D: , 3345 4
S: , 5001
D: , 80 2
S: , 80
D: , 5001 3
4: NAT router
changes datagram
dest addr from
, 5001 to , 3345
3: Reply arrives
dest. address:
, 5001
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

41. NAT: Network Address Translation
NAT is controversial:
routers should only process up to layer 3
violates end-to-end argument: Hosts should be talking directly with each other, without interfereing nodes modifying IP addresses and port numbers
Address shortage should instead be solved by IPv6
But like it or not, NAT becomes an important component of the Internet
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527