1. Data Link Layer Data Link Layer Functions
deliver frames over a single link
framing, media access, error checking (error correction), …
Cyclic Redundancy Code for error detection
Local Area Networks (LANs) and MAC Addresses
MAC addresses (vs. IP address)
point-to-point vs. shared access
IP Address Resolution Protocol (ARP) and IP datagram forwarding (revisited!)
CSci4211: Data Link Layer

2. Data Link Layer (cont’d)
Extending LANs & Switched LANs: hubs, bridges, and (layer 2) switches
Bridging self-learning
spanning tree algorithms
Media Access Control
Random Access
Aloha, Slotted Aloha
CSMA, CSMA/CD and Ethernet (802.3)
CSMA/CA and Wireless LAN (covered in Chapter 6)
“Take Turns” Controlled Access
Token Passing and Token Ring (802.5)
Other Link Layer Technology
PPP
ATM/MPLS and Link/Network Virtualization
Readings: Textbook, Chapter 5
CSci4211: Data Link Layer

3. Data Link Layer: Introduction
Some terminology:
hosts and routers are nodes
(bridges and switches too)
communication channels that connect adjacent nodes along communication path are links
wired links
wireless links
LANs (local area networks)
layer 2 PDU (“packet”) referred to as frame, which encapsulates a layer-3 packet, e.g., an IP datagram
CSci4211: Data Link Layer

4. What Does Data Link Layer Do?
Data link layer has responsibility of
transferring frames from one node to adjacent node over a single link
An IP packet from host A to host B may traverses different links using different data link protocols
e.g., Ethernet on first link, PPP on intermediate links, on last link
Each link protocol provides different services
e.g., may or may not provide reliable data delivery
Different link protocols are not inter-operable!
IP packets are encapsulated/decapsulated with appropriate data link protocol header over each link
IP protocol and IP routers glue the links (“physical networks”) together and provide end-to-end data delivery!
CSci4211: Data Link Layer

5. Data Link Layer Functions
Framing
sender (transmitter): encapsulate datagram into frame, adding header, trailer, transmit frame
receiver: detect beginning of frames, receive frame, decapsulate frame, stripping off header, trailer
Link Access (Media Access Control)
determine whether it’s Okay to transmit over the link
particularly important when link shared by many nodes
also an issue over “half-duplex” point-to-point link (why?)
need media access control (MAC)
“physical addresses” identify sender/receiver on a link!
particularly important when link shared by many nodes, while over point-to-point link, not necessary (why?)
“physical addresses” often referred to as “MAC” addresses
different from IP addresses (which are logical & global)!
CSci4211: Data Link Layer

6. Other Data Link Layer Functions
Error Detection (commonly implemented, see Section 5.2)
errors caused by signal attenuation, noise, etc.
sender computes “checksum”, attaches to frame
receiver detects presence of errors by verifying “checksum”
drops corrupted frame, may ask sender for retransmission
Commonly used “checksum”: cyclic redundancy code (CRC)
Reliable delivery between adjacent nodes (optional)
using, e.g., go-back-N or selective repeat protocol
seldom used on low bit error link (fiber, some twisted pair)
wireless links: high error rates
Q: why both link-level and end-end reliability?
Error Correction (optional)
receiver identifies and corrects bit error(s) without resorting to retransmission, using forward error correction (FEC) codes
Flow Control (optional)
negotiating transmission rates between two nodes
CSci4211: Data Link Layer

7. Adaptors Communicating
datagram
link layer protocol
rcving
node
sending
node
frame
frame
adapter
adapter
link layer implemented in “adaptor” (aka NIC), with “transceiver” in it
Ethernet card, dial-up modem, wireless card
sending side:
encapsulates datagram in a frame
adds error checking bits, flow control, reliable data transmission, etc.
receiving side
looks for errors, flow control, reliable data transmission, etc
extracts datagram, passes to receiving node
adapter is semi-autonomous
data link & physical layers are closely coupled!
CSci4211: Data Link Layer

8. Error Detection EDC= Error Detection and Correction bits (redundancy)
D = Data protected by error checking, may include header fields
Error detection not 100% reliable!
protocol may miss some errors, but rarely
larger EDC field yields better detection and correction
Fall 2006
CSci4211: Data Link Layer

9. Parity Checking Two Dimensional Bit Parity: Single Bit Parity:
Detect and correct single bit errors
Single Bit Parity:
Detect single bit errors
CSci4211: Data Link Layer

10. Internet Checksum
Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only)
Sender:
treat segment contents as sequence of 16-bit integers
checksum: addition (1’s complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver:
compute checksum of received segment
check if computed checksum equals checksum field value:
NO - error detected
YES - no error detected. But maybe errors nonetheless? More later ….
CSci4211: Data Link Layer

11. Checksumming: Cyclic Redundancy Check
view data bits, D, as a binary number
choose r+1 bit pattern (generator), G
goal: choose r CRC bits, R, such that
<D,R> exactly divisible by G (modulo 2)
receiver knows G, divides <D,R> by G. If non-zero remainder: error detected!
can detect all burst errors less than r+1 bits
widely used in practice (ATM, HDLC)
CSci4211: Data Link Layer

12. CRC Example Want: equivalently:
D.2r XOR R = nG
equivalently:
D.2r = nG XOR R
if we divide D.2r by G, want remainder R
D.2r G
R = remainder[ ]
CSci4211: Data Link Layer

13. Multiple Access Links and LANs
Two types of “links”:
point-to-point, e.g.,
PPP for dial-up access, or over optical fibers
broadcast (shared wire or medium), e.g.
traditional Ethernet
wireless LAN
CSci4211: Data Link Layer

14. LAN: Issues & Technologies
addressing: physical (or MAC) addresses
media access control (MAC) for broadcast LANs
expanding LANs: connecting multiple LAN segments
hubs, bridges, (layer-2) switches (we’ll explain difference later)
Various commonly used LAN technologies
Ethernet
802.11(WiFi)
PPP
ATM
(not really LAN)
(example of non-broadcast
multi-access network)
CSci4211: Data Link Layer

15. MAC (Physical, or LAN) Addresses
used to get frames from one interface to another physically-connected interface (same physical network, i.e., p2p or LAN)
48 bit MAC address (for most LANs)
fixed for each adaptor, burned in the adapter ROM
MAC address allocation administered by IEEE
1st bit: 0 unicast, 1 multicast.
all 1’s : broadcast
MAC flat address -> portability
can move LAN card from one LAN to another
MAC addressing operations on a LAN:
each adaptor on the LAN “sees” all frames
accept a frame if dest. MAC address matches its own MAC address
accept all broadcast (MAC= all1’s) frames
accept all frames if set in “promiscuous” mode
can configure to accept certain multicast addresses (first bit = 1)
CSci4211: Data Link Layer

16. MAC vs. IP Addresses 32-bit IP address: network-layer address, logical
i.e., not bound to any physical device, can be re-assigned
IP hierarchical address NOT portable
depends on IP network to which an interface is attached
when move to another IP network, IP address re-assigned
used to get IP packets to destination IP network
Recall how IP datagram forwarding is performed
IP network is “virtual,” actually packet delivery done by the underlying physical networks
from source host to destination host, hop-by-hop via IP routers
over each link, different link layer protocol used, with its own frame headers, and source and destination MAC addresses
Underlying physical networks do not understand IP protocol and datagram format!
CSci4211: Data Link Layer

17. Recall: IP Datagram Forwarding
Starting at A, given IP datagram addressed to B:
look up net. address of B, find B on same net. as A
link layer send datagram to B inside link-layer frame A B E
B’s MAC
addr
A’s MAC
A’s IP
B’s IP
IP payload
datagram
frame
frame source,
dest address
datagram source,
CSci4211: Data Link Layer

18. 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; timer>
timer: time after which address mapping will be forgotten (typically 20 min)
try out “arp –a” command
CSci4211: Data Link Layer

19. ARP Protocol
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
A wants to send datagram to B, and A knows B’s IP address.
A looks up B’s MAC address in its ARP table
Suppose B’s MAC address is not in A’s ARP table.
A broadcasts (why?) ARP query packet, containing B's IP address
all machines on LAN receive ARP query
CSci4211: Data Link Layer

20. ARP Messages Hardware Address Type: e.g., Ethernet
Here is the structure of an ARP packet. Whether it’s a request or reply is specified by the operation.
Hardware Address Type: e.g., Ethernet
Protocol address Type: e.g., IP
Operation: ARP request or ARP response
CSci4211: Data Link Layer

21. ARP Request & Response Processing
The requester broadcasts ARP request
The target node unicasts (why?) ARP reply to requester
With its physical address
Adds the requester into its ARP table (why?)
On receiving the response, requester
updates its table, sets timer
Other nodes upon receiving the ARP request
Refresh the requester entry if already there
No action otherwise (why?)
Some questions to think about:
Shall requester buffer IP datagram while performing ARP?
What shall requester do if never receive any ARP response?
We can categorize the nodes in the local network into requester node, target node and all others. Lets see what each of them do in turn.
The target node responds only to the sender with its hardware address. Why not broadcast the reply also? All others may not necessarily be interested in communicating with target node and a broadcast incurs processing overhead at every node in the network. The target node adds the requester’s IP and hardware addresses into its ARP cache (if not already there). Why? It is likely that target node would also send IP packets to the requester node later and so it makes sense to avoid an ARP request broadcast by the target node that time.
The requester node on receiving the response, updates its ARP table.
All other nodes, receive only the request not the reply. They check if the requester has an entry in their caches. If found, they refresh that entry. Otherwise, no action taken. Why not add an entry for the requester? We don’t want to grow the ARP table unnecessarily.
Finally, a lifetime is associated with each entry in the ARP cache and an entry is deleted if it not refreshed within that time.
CSci4211: Data Link Layer

22. ARP Operation Illustration
Here is an illustration of ARP broadcast by W requesting hardware address of Y. As you can see, every node in the local physical network receive this request. Only Y sends the reply and also only to W.
CSci4211: Data Link Layer

23. Forwarding to Another LAN
walkthrough: send datagram from A to B via R
assume A knows 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
CSci4211: Data Link Layer

24. B A R 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 data link layer sends frame
R’s data link layer receives frame
R removes IP datagram from Ethernet frame, sees its destined to B
R uses ARP to get B’s physical layer address
R creates frame containing A-to-B IP datagram sends to B
CSci4211: Data Link Layer

25. Interconnecting LAN Segments & Switched LANs
Hubs
Bridges
(Layer-2) Switches
Remark: switches are essentially multi-port bridges.
What we say about bridges also holds for switches!
CSci4211: Data Link Layer

26. 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
CSci4211: Data Link Layer

27. Interconnecting with Hubs
Backbone hub interconnects LAN segments
Extends max distance between nodes
But individual segment collision domains become one large collision domain
Can’t interconnect 10BaseT & 100BaseT
hub
CSci4211: Data Link Layer

28. Bridge and Layer-2 Switch
Link layer device
stores and forwards Ethernet frames
examines frame header and selectively forwards frame based on MAC destionation address
when frame is to be forwarded on segment, uses CSMA/CD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play, self-learning
switches do not need to be configured
CSci4211: Data Link Layer

29. Bridge/Switch: Traffic Isolation
Bridge/switch installation breaks subnet into LAN segments
Bridge/switch filters packets:
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub
switch
collision domain
collision domain
LAN (IP network)
CSci4211: Data Link Layer

30. Forwarding How do determine onto which LAN segment to forward frame?
hub
switch 1 3 2
How do determine onto which LAN segment to forward frame?
Looks like a routing problem...
CSci4211: Data Link Layer

31. Self Learning A bridge/switch has a forwarding (or switch) table
entry in forwarding table:
(MAC Address, Interface, Time Stamp)
stale entries in table dropped (TTL can be 60 min)
Bridge/switch learns which hosts can be reached through which interfaces
when frame received, switch “learns” location of sender: incoming LAN segment
records sender/location pair in forwarding table
CSci4211: Data Link Layer

32. Filtering/Forwarding
When bridge/switch receives a frame:
index switch table using MAC dest address
if entry found for destination then{
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated }
else flood
forward on all but the interface
on which the frame arrived
CSci4211: Data Link Layer

33. Bridge/Switch Example
Suppose C sends frame to D
hub
switch A B C D E F G H I 1 2 3
address
interface A B E G 1 2 3
Switch receives frame from C
notes in bridge/switch table that C is on interface 1
because D is not in table, switch forwards frame into interfaces 2 and 3
frame received by D
CSci4211: Data Link Layer

34. Bridge/Switch Example
Suppose D replies back with frame to C.
hub
switch B C D E F G H I
address
interface A B E G C 1 2 3 A
Switch receives frame from from D
notes in bridge/switch table that D is on interface 2
because C is in table, switch forwards frame only to interface 1
frame received by C
CSci4211: Data Link Layer

35. Switches: Dedicated Access
Switch with many interfaces
Hosts have direct connection to switch
No collisions; full duplex
Switching: A-to-A’ and B-to-B’ simultaneously, no collisions A C’ B
switch C B’ A’
CSci4211: Data Link Layer

36. More on Switches
cut-through switching: frame forwarded from input to output port without first collecting entire frame
slight reduction in latency
combinations of shared/dedicated, 10/100/1000 Mbps interfaces
CSci4211: Data Link Layer

37. Interconnection without Backbone
Not recommended for two reasons:
- single point of failure at Computer Science hub
- all traffic between EE and SE must path over CS segment
CSci4211: Data Link Layer

38. Backbone Configuration
Recommended !
CSci4211: Data Link Layer

39. Bridges Spanning Tree
for increased reliability, desirable to have redundant, alternative paths from source to dest
with multiple paths, cycles result - bridges may multiply and forward frame forever
solution: organize bridges in a spanning tree by disabling subset of interfaces
Disabled
CSci4211: Data Link Layer

40. Bridge Spanning Tree Algorithm: Algorhyme
I think that I shall never see
A graph more lovely than a tree.
A tree whose crucial property
Is loop-free connectivity.
A tree that must be sure to span
So packets can reach every LAN.
First, the root must be selected.
By ID, it is elected.
Least cost paths from root are traced.
In the tree, these paths are placed.
A mesh is made by folks like me,
Then bridges find a spanning tree
-- Radia Perlman
CSci4211: Data Link Layer

41. Some Bridge/Switch Features
Isolates collision domains resulting in higher total max throughput
limitless number of nodes and geographical coverage
Can connect different Ethernet types
Transparent (“plug-and-play”): no configuration necessary
CSci4211: Data Link Layer

42. Institutional Network
hub
switch
to external
network
router
IP subnet
mail server
web server
CSci4211: Data Link Layer

43. Not an atypical LAN (IP network)
Dedicated
Shared
CSci4211: Data Link Layer

44. Bridges/Switches vs. Routers
both store-and-forward devices
routers: network layer devices (examine network layer headers)
bridges are link layer devices
routers maintain routing tables, implement routing algorithms
bridges maintain bridge forwarding tables, implement filtering, learning and spanning tree algorithms
CSci4211: Data Link Layer

45. Routers vs. Bridges/Switches
Bridges + and -
+ Bridge operation is simpler requiring less packet processing
+ Bridge tables are self learning
- All traffic confined to spanning tree, even when alternative bandwidth is available
- Bridges do not offer protection from broadcast storms
CSci4211: Data Link Layer

46. Routers vs. Bridges/Switches …
Routers + and -
+ arbitrary topologies can be supported, cycling is limited by TTL counters (and good routing protocols)
+ provide protection against broadcast storms
- require IP address configuration (not plug and play)
- require higher packet processing
bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts)
CSci4211: Data Link Layer

47. Summary Comparison
CSci4211: Data Link Layer