1. Internet protocol stack
application: supporting network applications
FTP, SMTP, HTTP
transport: process-process data transfer
TCP, UDP
network: routing of datagrams from source to destination
IP, routing protocols,
link: data transfer between neighboring network elements
PPP, Ethernet
physical: bits “on the wire”
application
transport
network
link
physical

2. Encapsulation source destination application transport network link
message M
application
transport
network
link
physical
segment Ht M Ht
datagram Ht Hn M Hn
frame Ht Hn Hl M
link
physical
switch
destination
network
link
physical Ht Hn M Ht Hn Hl M M
application
transport
network
link
physical Ht Hn M Ht M Ht Hn M
router Ht Hn Hl M

3. MAC Addresses and ARP 32-bit IP address:
network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address:
used to get frame from one interface to another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM

4. LAN Addresses and ARP Each adapter on LAN has unique LAN address
1A-2F-BB AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Broadcast address =
FF-FF-FF-FF-FF-FF
= adapter

5. LAN Address (more) MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
Analogy:
(a) MAC address: like Social Security Number
(b) IP address: like postal address
MAC flat address ➜ portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached

6. 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
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

7. ARP protocol: Same LAN (network)
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)
soft state: information that times out (goes away) unless refreshed
ARP is “plug-and-play”:
nodes create their ARP tables without intervention from net administrator

8. Routing to another LAN walkthrough: send datagram from A to B via R
assume A know’s B IP address
Two ARP tables in router R, one for each IP network (LAN)
In routing table at source Host, find router
In ARP table at source, find MAC address E6-E BB-4B, etc A R B

9. A R B A creates datagram with source A, destination B
A uses ARP to get R’s MAC address for
A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram
A’s adapter sends frame
R’s adapter receives frame
R removes IP datagram from Ethernet frame, sees its destined to B
R uses ARP to get B’s MAC address
R creates frame containing A-to-B IP datagram sends to B A R B

10. ARP Functionality
There are two main functional parts of the address resolution protocol:
Determine the destination’s physical address before sending a packet.
Answer requests that arrive for it’s own Physical-to-IP address binding.
Because of lost/duplicate packets, ARP must handle this to avoid many re-broadcasts.
Bindings in ARP cache (actual cache table) must be removed after a fixed period of time to ensure validity.
When a packet is received, the sender’s IP address is stripped and the local table is updated (ARP cache), then the rest of the packet is processed.
Two types of incoming packets:
Those to be processed (correct destination).
Stray broadcast packets (can be dropped after updating the ARP cache).
Application programs may request the destination address many times before the binding is complete. This must be handled, by discarding enqueued requests, when the correct binding returns.

11. ARP Functionality ARP sets the field "TYPE" for the ID of a frame.
ARP packets DO NOT have a fixed format header, so they can be used with arbitrary physical addresses and arbitrary protocol addresses.
The lengths of physical addresses may vary up to 48-bits.

12. ARP Header Fields
Hardware Type: (16-bits) - the type of interface the sender seeks an answer for.
Protocol Type: (16-bits) - the high-level software address type provided.
HLEN: (8-bits) - length of arbitrary physical address.
PLEN: (8-bits) - length of arbitrary protocol address.
OPERATION: (16-bits) - the specific type of operation requested.
ARP.request (1)
ARP.response (2)
SENDER HA: (6-octets) - the sender’s actual hardware address, scalable up to six bytes.
SENDER IP: (4-octets) - the sender’s IP address, always 32-bits.
TARGET HA: (6-octets) - the destination node’s hardware address, scalable up to six bytes.
TARGET IP: (4-octets) - the destination node’s IP address, always 32-bits.

13. Ethernet “dominant” wired LAN technology: cheap $20 for 100Mbs!
first widely used LAN technology
Simpler, cheaper than token LANs and ATM
Kept up with speed race: 10 Mbps – 10 Gbps
Metcalfe’s Ethernet
sketch

14. Star topology Bus topology popular through mid 90s
Now star topology prevails
Connection choices: hub or switch (more later)
hub or
switch

15. Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble:
7 bytes with pattern followed by one byte with pattern
used to synchronize receiver, sender clock rates

16. Ethernet Frame Structure (more)
Addresses: 6 bytes
if adapter receives frame with matching destination address, or with broadcast address (eg ARP packet), it passes data in frame to net-layer protocol
otherwise, adapter discards frame
Type: indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC: checked at receiver, if error is detected, the frame is simply dropped

17. Ethernet
Ethernet Frame

18. Encapsulating the Packet
The Ethernet protocol defines the frame format.
Adds headers and trailers around the Layer 3 packet.

19. Encapsulating the Packet
The IEEE Ethernet Frame format:
Minimum Size: 64 Bytes
Maximum Size: 1518 Bytes
If the frame is less than the minimum or greater than the maximum, it is considered corrupt and will be dropped.
LENGTH OF FIELD IN BYTES 7 1 6 2
46 – 1500 4
Preamble
Start of Frame Delimiter
Destination MAC Address
Source MAC Address
Length or Type
Data and Pad
FCS
Header
Trailer

20. Encapsulating the Packet
LENGTH OF FIELD IN BYTES 7 1 6 2
46 – 1500 4
Preamble
Start of Frame Delimiter
Destination MAC Address
Source MAC Address
Length or Type
Data and Pad
FCS
Preamble and Start of Frame Delimiter (SFD) – 8 bytes:
Used to synchronize the NIC with the media in preparation for receiving a frame.
Is not considered part of the frame length.
Will not appear in any capture of the frame.

21. Encapsulating the Packet
LENGTH OF FIELD IN BYTES 7 1 6 2
46 – 1500 4
Preamble
Start of Frame Delimiter
Destination MAC Address
Source MAC Address
Length or Type
Data and Pad
FCS
Destination MAC Address – 6 bytes:
Identifies the node that is to receive the frame.
A receiving device compares its MAC address to the contents of this field.
If the addresses match, the frame is accepted.
Also used by switches to determine the interface to be used to forward the frame.

22. Encapsulating the Packet
LENGTH OF FIELD IN BYTES 7 1 6 2
46 – 1500 4
Preamble
Start of Frame Delimiter
Destination MAC Address
Source MAC Address
Length or Type
Data and Pad
FCS
Source MAC Address – 6 bytes:
Identifies the node that originated the frame.
Also used by switches to add addresses to their internal Port / MAC address tables.

23. Encapsulating the Packet
LENGTH OF FIELD IN BYTES 7 1 6 2
46 – 1500 4
Preamble
Start of Frame Delimiter
Destination MAC Address
Source MAC Address
Length or Type
Data and Pad
FCS
Length / Type – 2 bytes:
DIX used this for type, the original IEEE standard used it for length. The later IEEE standard (Ethernet II) allows it to be used for either.
Ethernet II is the frame type used in TCP/IP networks.
If the value is greater than 1518 (0x600), it contains a code identifying the encapsulated upper layer protocol.
Any other value defines the length of the frame.

24. Encapsulating the Packet
LENGTH OF FIELD IN BYTES 7 1 6 2
46 – 1500 4
Preamble
Start of Frame Delimiter
Destination MAC Address
Source MAC Address
Length or Type
Data and Pad
FCS
Data and Pad – 46 to 1500 bytes:
The encapsulated data from Layer 3.
Most commonly an IPv4 packet.
If the total frame length is less than 64 bytes, the field is padded to the right with enough null characters to meet the minimum frame length.

25. Encapsulating the Packet
LENGTH OF FIELD IN BYTES 7 1 6 2
46 – 1500 4
Preamble
Start of Frame Delimiter
Destination MAC Address
Source MAC Address
Length or Type
Data and Pad
FCS
Frame Check Sequence (FCS)– 4 bytes:
Used to detect errors in a frame that may have occurred during transmission along the media.
The result of a Cyclic Redundancy Check (CRC) is placed in the frame by the sending node.
The receiving node performs the same CRC and compares the values….they should be equal.

26. Ethernet MAC Address
LENGTH OF FIELD IN BYTES 7 1 6 2
46 – 1500 4
Preamble
Start of Frame Delimiter
Destination MAC Address
Source MAC Address
Length or Type
Data and Pad
FCS
In order for a transmission to be received properly at the destination computer, there must be a method of uniquely identifying that host.
A unique address is permanently programmed into ROM in each NIC ("burned in“ ) when it is manufactured.
Because of this, the MAC Address is often referred to as the burned in (BIA) address or physical address of a machine.

27. Ethernet MAC Address 48 bits in length.
Expressed as 12 hexadecimal digits.
The first 6 hexadecimal digits, which are administered by the IEEE, identify the manufacturer or vendor and thus comprise the Organizational Unique Identifier (OUI).
The remaining 6 hexadecimal digits comprise the interface serial number, or another value administered by the specific vendor.

28. Ethernet MAC Address
LENGTH OF FIELD IN BYTES 7 1 6 2
46 – 1500 4
Preamble
Start of Frame Delimiter
Destination MAC Address
Source MAC Address
Length or Type
Data and Pad
FCS
When a network device matches the destination address to the address in the NIC, the NIC passes the frame up the OSI layers where the decapsulation process takes place.
The MAC address is essential to communications on a network. It is the only address that guarantees that the message will be accepted by the destination.

29. Unreliable, connectionless service
Connectionless: No handshaking between sending and receiving adapter.
Unreliable: receiving adapter doesn’t send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gaps
gaps will be filled if app is using TCP
otherwise, app will see the gaps

30. 10BaseT and 100BaseT 10/100 Mbps rate; latter called “fast ethernet”
T stands for Twisted Pair
Nodes connect to a hub: “star topology”; 100 m max distance between nodes and hub
twisted pair
hub

31. Hubs Hubs are essentially physical-layer repeaters:
bits coming from one link go out all other links
at the same rate
no frame buffering
no CSMA/CD at hub: adapters detect collisions
provides net management functionality
twisted pair
hub

32. Manchester encoding Used in 10BaseT Each bit has a transition
Allows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized, global clock among nodes!
Hey, this is physical-layer stuff!

33. Gbit Ethernet uses standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode, CSMA/CD is used; short distances between nodes required for efficiency
uses hubs, called here “Buffered Distributors”
Full-Duplex at 1 Gbps for point-to-point links
10 Gbps now !