Transcript INFO 330 Computer Networking Technology I Chapter 5 The Link Layer & LANs Dr. Jennifer Booker INFO 330 Chapter 5 www.ischool.drexel.edu.

INFO 330
Computer Networking
Technology I
Chapter 5
The Link Layer & LANs
Dr. Jennifer Booker
INFO 330 Chapter 5
1
www.ischool.drexel.edu
The Link Layer
• So, let’s see where we’ve been
– The transport layer provides process to
process communication
– The network layer provides host to
host communication
• Now the Link Layer provides the ability to
send packets across a single … link
– So this layer tells how to send a
packet/segment/datagram from one
router/host to another
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The Link Layer
• There are two types of link layer channels
– Broadcast channels, used in LANs, wireless
LANs, satellite networks, and HFC cable
networks
– Point-to-point communication link, such as
between two routers or between an ISP and
a modem
• We’ll focus on Ethernet and PPP
(Point-to-Point Protocol)
– Wi-Fi (IEEE 802.11 protocols) is in chapter 6
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Link Layer Terms
• A node is a router or host – here we don’t
care which one we’re dealing with!
• Any connection between nodes is a link
– The transmitting node puts the datagram in
a frame, and transmits it into the link
– The receiving node receives the frame, and
extracts the datagram
Datagram
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Link Layer Services
• A link layer protocol moves a datagram
over a (one, individual, eins, uno) link
– It defines the format of packets (frames)
exchanged between nodes at each end of the
link, and the actions the nodes do to send
and receive these packets
– Over a host-to-host route, links may use
several different link-layer protocols – but
only one per link
• Typically, one link layer frame contains
one network layer datagram
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Link Layer Services
• The link layer’s actions can also include
– Framing
– Link access
– Reliable delivery
– Error detection and correction
• Link layer protocols include PPP, Ethernet,
Token Ring, Wi-Fi, and some parts of ATM
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Link Layer Services
• Now elaborate a little on these services
• Framing a datagram into a frame means
we have data (the datagram) and one or
more headers
– Technically, can have header and trailer
fields, but we’ll generically call both headers
– Header format is defined by the protocol
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Link Layer Services
• Link Access uses the Medium Access
Control (MAC) protocol to define how a
frame is transmitted over a link
– MAC negotiates transmission when many
nodes share the same link
• Reliable delivery is provided by high
error- rate links (e.g. wireless) to keep the
transport layer from retransmitting over the
entire route
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Link Layer Services
• Flow control helps keep the sending
node from overwhelming the receiving
node
• Error detection looks for bit errors,
usually more elaborately than in the
transport and network layers
• Error correction – some protocols
(ATM) can also fix errors detected
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Link Layer Services
• Half vs full duplex – with half duplex, a
node can only send or receive at one time;
with full duplex, it can send and receive at
the same time
• Yes, lots of the link layer services are
similar to transport layer services
– But the link layer only provides them between
two nodes, whereas the transport layer does
between hosts
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Adapters
• Most link layer protocols are implemented in
an adapter (since we’re getting really close
to the physical layer!)
– Adapter = network interface card (NIC)
• The adapter is the last connection between
a host and the physical link to the network
– Error checking occurs in the adapter, oblivious to
the host
– Only datagrams which come in cleanly are
passed up the protocol stack to the application
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Adapters
• The main parts of an adapter are the link
interface and the bus interface
– The link interface connects to the physical
network
– The bus interface connects to the “parent”
node’s I/O bus (e.g. PCI, PCI-X, Serial ATA,
IDE, etc.)
• Not much to it!
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Error Detection and Correction
• We can detect, and sometimes correct, bit
errors at the link layer
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Error Detection and Correction
• We add error-detection and correction
(EDC) to the data (D) to be sent across
the link, in addition to other header info
(address, sequence number, etc.)
• At the other end of the link, the data could
be changed (D’) and the EDC info could
be corrupted (EDC’)
• Telling from D’ and EDC’ if the original D
was corrupted isn’t a perfect science!
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Error Detection and Correction
• Hence there could be undetected bit
errors
– The lower the undetected error rate, the
larger the overhead to add to each frame
• Three main methods for detection
– Parity Checks
– Checksum
– Cyclic Redundancy Check (CRC)
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Parity Checks
• A simple error detection scheme, parity
check adds one bit to the data
• That one bit depends on the type of
parity scheme
– For even parity, the parity bit is chosen so that
the total number of 1’s in the frame is … even
– For odd parity, the parity bit is chosen so that
the total number of 1’s in the frame is … odd
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Parity Checks
• If the receiver of an even parity link finds
an odd number of parity, then there must
have been some odd number of bit errors
(1, 3, 5, …)
– Notice that an even number of errors isn’t
detected!
• And yes, it helps if both sides of the link
are using the same parity rules
– Modems used to set even or odd parity
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Parity Checks
• A better approach is to break the data into
a table with i rows and j columns, and
define parity for each row and column
• In this two-dimensional parity check,
there are i+j+1 parity values (bits)
• But by cross-referencing the parity errors,
exactly which bit(s) were in error can be
known, and hence fixed!
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Parity Checks
• If the receiver can detect and fix errors, it’s
forward error correction (FEC)
• Commonly used in audio devices to
compensate for, e.g., scratched CD’s
• In a network, this helps avoid
retransmission, and the associated delays
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Checksum Methods
• Yup, this is just like the approach we saw
before…here we call it an Internet
checksum
– Add the digits of the data
– Take the 1s complement of the result – that’s
the checksum
– Data + checksum = 111111111… if not,
there’s an error somewhere
• See RFC 1071
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Cyclic Redundancy Check
• A Cyclic Redundancy Check (CRC) code
is widely used in the link layer
– Checksums are easy to calculate in software,
so they’re ok for the transport and network
layers, but here we can use hardware to
calculate CRC codes for us
– A.k.a. polynomial codes
• The use of CRC codes provides more
sophisticated error checking
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Cyclic Redundancy Check
• CRC uses modulo-2 arithmetic, a.k.a.
Boolean arithmetic
• It’s equivalent to XOR (exclusive OR):
–
–
–
–
–
A
0
0
1
1
B (A xor B)
0
0
1
1
0
1
1
0
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Cyclic Redundancy Check
• Multiplication by 2^k moves the bits left by
k places
– 1011 * 2^3 = 1011000 (11*8 = 64+16+8=88)
• So much for the math lesson, so what?
• The CRC code defines the ‘r’ CRC bits
with a value of R
• There’s a generator, G, which has some
value starting with 1, and has r+1 bits
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Cyclic Redundancy Check
• Assume our data has ‘d’ bits, and is a
string called D
• The value of R is defined so that D * 2^r
XOR R is equal to some exact integer
multiple of G
– (D * 2^r) XOR R = n*G
– So R = remainder [D*2^r / G]
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Cyclic Redundancy Check
• The value of G is typically predefined by
IEEE standards
– Standard G lengths are 8, 12, 16, and 32 bits
– Hence the corresponding lengths of R are
r = 7, 11, 15, and 31 bits
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Cyclic Redundancy Check
• So how does this mess work?
– Pick a length of G
– Calculate R from the previous slide for each
data frame, D
– Send the frame
– The receiver divides the d+r bits by G
• If the remainder is zero, there are no errors
• If the remainder is not zero, there were errors
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Cyclic Redundancy Check
• So what? Why all this work?
– Errors tend to occur in bursts – not one error
all by itself
– Using CRC codes allows you to catch up to ‘r’
errors in a single frame
• And errors of more than ‘r’ in a frame might be
caught, (1 - 0.5r)*100 percent of the time
• And this will catch any number of odd errors
– So that’s why we use it a lot at the link layer
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Multiple Access Protocols
• Network links can be point-to-point (one
sender and one receiver) or broadcast
links
• For a broadcast link
– A node sends a frame to all of the other
nodes
– Used by wired, wireless, and satellite
networks, plus the occasional cocktail party
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Multiple Access Protocols
• This motivates the multiple access
problem – how do we control transmission
onto a shared broadcast channel
• Frames can arrive at a node (yes,
technically the adapter on that node) at the
same time, producing a collision (both
frames on top of each other, a mess)
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Multiple Access Protocols
• Dozens of multiple access protocols have
been defined, but they fall into three types
– Channel partitioning protocols
– Random access protocols
– Taking-turns protocols
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Multiple Access Protocols
• We want multiple access protocols to
provide
– One node can send data at a rate of R bps
– If M nodes want to transmit, each can transmit
an average of R/M bps
– The protocol should be decentralized, so that
a single point failure doesn’t take down the
system
– It’s cheap to implement and simple
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Channel Partitioning Protocols
• Could use FDM or TDM (frequency or time
division multiplexing) to share a channel’s
bandwidth across some number of slots
– Avoids collisions, which is good
– But each slot only gets a fraction of the
bandwidth, even if no one else is transmitting
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Channel Partitioning Protocols
• Instead use Code Division Multiple Access
(CDMA), which assigns codes to each
node which sends data
– CDMA is also good for avoiding signal
jamming, hence is used by the military
• Is used widely for wireless protocols
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Random Access Protocols
• Here each node transmits as though it has
the full channel bandwidth available
– When a collision occurs, it waits a random
amount of delay time before retransmitting
– Keep retransmitting until the frame gets through
• There are many protocols of this type, e.g.
– Slotted ALOHA
– ALOHA
– CSMA (of which Ethernet is an example)
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Slotted ALOHA
• Suppose
– All frames have size L bits
– Time is divided into slots of duration L/R
seconds (= time to transmit one frame)
– Nodes only transmit at the start of a slot
– Nodes all know when the slots start
– If a collision occurs, the nodes know that
before the end of the slot occurs
– There is a probability, p, between 0 and 1
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Slotted ALOHA
• Slotted ALOHA works like this:
– When a node needs to transmit a frame, it
waits until the next slot starts and transmits it
– If there’s no collision, the node can transmit
the next frame if needed
– If there was a collision, the next time a
random number is greater than p, transmit in
that slot
• So if the random value is less than 1-p, wait for
retransmission
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Slotted ALOHA
• This takes advantage of the link when only
one node is active – it gets the full rate
– If there are multiple active nodes, some slots
will be wasted because nobody is transmitting
• The efficiency is the percent of slots where
a successful transmission occurs
– The efficiency for N active nodes is
N*p*(1-p)^(N-1)
– Bad part is: max efficiency is only 37%
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ALOHA
• What is we ignore the part about
transmitting only at the start of a slot?
– Transmit when you want to
– If there’s a collision, retransmit immediately if
value is >p, otherwise wait one slot duration
and reevaluate retransmitting then
• The icky part is that the efficiency of this is
only half of Slotted ALOHA – the price for
decentralized control
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CSMA
• CSMA (Carrier Sense Multiple Access)
pays attention to whether anyone else is
transmitting, before a node does so
– Like listening for a break in conversation
before jumping in, carrier sensing listens for
a break in link traffic (basic CSMA protocol)
– Collision detection is done by sensing if
another node starts transmitting while you
are (CSMA/CD)
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CSMA
• There are many variations on CSMA &
CSMA/CD
• Collisions can occur because of the time
needed for transmitting frames – the
channel propagation delay
– A problem solved by the binary exponential
backoff algorithm
• CSMA/CD efficiency is 1/(1+5*dprop/dtrans)
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Taking-turns Protocols
• The ALOHA and CSMA protocols both take
advantage of full bandwidth when available,
but neither is good at assuring fair share of
throughput when multiple nodes are active
– To fix the latter, taking-turns protocols have been
made – hundreds of them!
• We’ll focus on two major kinds
– Polling protocols
– Token-passing protocols
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Polling Protocols
• Polling protocols make one node a
master node
– The master node polls each node in turn, and
tells each node it can send some number of
frames
• This eliminates collisions and empty slots
– But it adds a polling delay to notify each node
it’s turn is up, and delays to check nodes
which are inactive
– And it’s really bad if the master node dies!
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Token-passing Protocols
• Token-passing protocols have no master
node, but instead pass a small token
frame among the nodes in a fixed order
– Each node holds the token only if they have
frames to transmit, up to some max number
– Then keep passing the token
• Failure of ANY node crashes the network!
– Or if the token isn’t released, there’s trouble
• FDDI and yes, Token Ring, are examples
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Local Area Networks (LANs)
• Local Area Networks use multiple access
protocols extensively
• Ethernet is the most common random
access protocol
• Token Ring had a slight speed advantage,
so it was popular in the late 1980’s
– A node sends a frame around the network,
and it’s read by the recipient node
– The sender removes it from the network
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Local Area Networks (LANs)
• FDDI (Fiber Distributed Data Interface)
was designed for larger LANs, specifically
Metropolitan Area Networks (MANs)
• Under FDDI, the destination node
removes the frame from the network
– Hence it isn’t a pure broadcast channel, since
nodes downstream will never get the frame
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DOCSIS
• DOCSIS (Data-Over-Cable Service
Interface Specifications) uses a
combination of {FDM, random access
broadcast channels, TDM, and slotted
transmission with collision detection} for
cable access networks
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Link Layer Addressing
• C’mon, we haven’t had an address format
in at least two or three days
• Here we’ll go over MAC and ARP
• As stated earlier, the adapter is the real
location of a link layer address
– The MAC address (a.k.a. LAN address or
physical address) is the link layer address
of an adapter
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MAC Address
• A MAC address usually has 6 bytes, so
there are 2^48 MAC addresses
– 2^48 = 281,474,976,710,656 in case you
wondered
• Each byte is expressed as two
hexadecimal numbers (0-9; A-F for 10-15)
– 01:90:4B:5F:31:13
– Letters are case-insensitive
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MAC Address
• The IEEE makes sure each MAC address
is unique
– The first 24 bits are assigned to the hardware
vendor; the rest are the item identifier
• MAC addresses have no other structure,
and didn’t change for a given adapter
– MAC addresses were supposed to be
permanent, but they can now be changed
via software
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MAC Address
• Like the IP address, the MAC address is
used to verify that the destination host
(adapter) has been reached
• The MAC broadcast address is all F’s,
analogous to the 255.255.255.255 IP
address
– FF:FF:FF:FF:FF:FF
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Address Resolution Protocol
• The Address Resolution Protocol (ARP)
(no, not AARP) translates between IP addresses
and MAC addresses
– RFC 826, and a nice tutorial in RFC 1180
• ARP only works within the local subnet
– Unlike DNS, which resolves addresses anywhere
• Each node (host / router) maintains an ARP
table to map IP addresses & MAC addresses
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Address Resolution Protocol
• ARP also includes a time-to-live, which
is the time before that entry is deleted
– Typically starts at 20 minutes and counts
down
• A special ARP packet is broadcast to all
nodes on the subnet to resolve an
unknown MAC address
• ARP has query and response packets,
both with the same format
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Address Resolution Protocol
• The query is sent in a broadcast frame,
but the response is sent in a standard
frame
• ARP builds itself – if it gets an unknown
address, it works to find the information
• If a node is deleted from the network, its
ARP entries get removed eventually too
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ARP Off Subnet
• To send a frame outside of the local
subnet, first have to use the MAC address
of the interface leading out of the subnet
• Then the frame goes through a router to
the correct subnet, where the interface on
that subnet’s side can resolve the correct
MAC using ARP
INFO 330 Chapter 5
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ARP Off Subnet
•
•
•
•
•
•
•
•
A creates datagram with source A, destination B
A uses ARP to get R’s MAC address for 111.111.111.110
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
INFO 330 Chapter 5
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Ethernet
• Ethernet has been king of wired LANs
since the late 1970’s; why?
– 1) it was the first high speed protocol*
– 2) it’s cheap
* 2.94 Mbps in 1973!
– 3) it has had speed increases to stay
competitive
• The original Ethernet (“thick” and “thin”
Ethernet) used a bus topology
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Ethernet
• Bus topology
Host
A
Host
B
Host
C
Host
D
Host
E
• But now a (hub or) switch is used at the
center of a star topology
hub or
switch
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Ethernet Frame Structure
• Ethernet frames use this structure
• The Preamble is 8 bytes, the first seven of
which are all 10101010, and the 8th is
10101011
– Used to synchronize the clocks between sender
and receiver, since many possible speeds could
be used (10 Mbps to 1000+ Mbps)
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Ethernet Frame Structure
• The Destination Address is the 6-byte
MAC address of the destination
• The Source Address is the sender’s MAC
• The Type field is 2 bytes to explain the
network protocol which created the frame
(IP, IPX, AppleTalk, etc.)
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Ethernet Frame Structure
• The Data field is 46 to 1500 bytes for the IP
datagram, in our case
– Use “stuffing” to pad the Data to 46 B if needed
– 1500 B is the max transfer unit (MTU) for
Ethernet
• Finally, the CRC field is a 4 Byte CRC
code discussed earlier to detect bit errors
in the frame
• So the Ethernet frame has 26 B of headers
plus the data field … psst! WAKE UP!
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Ethernet
• Ethernet is connectionless service, like
IP and UDP – there’s no handshake
• Therefore its service is unreliable
– The CRC check is used, but failed frames
are merely discarded
– A lost frame here means a lost (or
incomplete) segment at the UDP layer
– Ethernet is blissfully unaware if a frame is
new, or retransmitted, or even related to any
other frames
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Ethernet and CSMA/CD
• If a hub is used, Ethernet broadcasts to
all nodes (adapters) on the LAN
• Ethernet uses CSMA/CD
– No slots, just start broadcast when ready
– Use carrier sensing to know when NOT
to broadcast
– Stop transmitting when a collision is detected
– Before retransmitting, wait a short random
time
INFO 330 Chapter 5
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Ethernet and CSMA/CD
• Efficiency can reach 100% in a LAN
• It senses a collision, or the lack of traffic by
monitoring voltage levels on the link
– Pause for an open line is 96 bit times, or 9.6
microsec at 10 Mbps
– If collision is detected, a 48-bit jam signal is
transmitted instead of the frame, to all adapters
– Delay for the nth collision is 512*K bit times
• K is random from {0,1,2,3,…,(2m – 1)} where m=min(n,10)
INFO 330 Chapter 5
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Ethernet and CSMA/CD
• Notice that the more collisions are noted,
the longer the possible delay time
– Called an exponential backoff
• Ethernet efficiency is messy to calculate,
but comes to:
– Efficiency = 1 / (1 + 5*dprop/dtrans)
• For small propagation time and/or large
transmission time, this is about 1
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Ethernet and LANs
• Ethernet is used for most wired LANs
– 100BaseT and 1000BaseT are common
(100 Mbps and 1000 Mbps, respectively)
– 10 Gigabit Ethernet is increasingly common
for servers
• A hub is frequently the center of a simple
star network
– Hubs operate only on physical layer
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Hubs
• Hub are the village idiot of networking
hardware – ok, maybe a handy village idiot
– When a bit arrives on any of its adapters, it
copies it, amplifies it a little, and retransmits it
on all of the other adapters
– They typically have 4-24 adapters, or ports
– Cost is nil for most hubs
– They do nothing for CSMA/CD
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Hubs
• An adapter may malfunction and keep
transmitting (a jabbering adapter), in which
case the hub should detect the problem and
shut off that adapter
• Fancy hubs can collect and report usage
data, collision rates, frame sizes, etc
• Max of 100 meters between hub and hosts
for twisted pair wire – more for optical cables
– The ‘T’ in 100BaseT means twisted
INFO 330 Chapter 5
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Hub Hierarchy
• Hubs can be connected in a multi-tier
hierarchy so that different parts of a
building, or different departments, etc. can
backbone
share resources
hub
hub
hub
INFO 330 Chapter 5
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Hub Hierarchy
• The backbone hub has three LAN
segments attached, each with its own hub
– This extends the max distance covered
• But these are all part of the same
collision domain
– All segments have to share same
Ethernet speed
– Limits throughput across entire network
INFO 330 Chapter 5
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Repeaters
• If we need to extend the range of a wired
network, a repeater can be used
• It’s essentially a 2-port hub, to amplify
(retransmit) the incoming signal
INFO 330 Chapter 5
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Link-layer Switches
• Switches operate on the link layer
– Incoming Ethernet frames are examined
for the layer-2 (link layer) destination (e.g.
MAC address)
– It then forwards the frame to the adapter
leading to that destination (not all of the
adapters)
• If the backbone hub on slide 69 were
replaced by a switch, then each LAN
segment is now its own collision domain
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Link-layer Switches
• Switches can handle multiple network speeds
– Some segments at 10 Mbps, others at 100
Mbps, etc.
– They still allow communication across
the segments
– They can be combined into any size network
• They operate in full duplex (transmit and
receive at once) and provide, um, switching
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Switch Filtering & Forwarding
• Filtering is when a switch can decide to
forward a frame or just drop it
• Forwarding is deciding which interface
a frame needs to go out on, and directing
it there
• A switch table is used for both filtering
and forwarding
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Switch Table
• A switch table has the MAC address of
each node, the corresponding interface
number to get to that node, and the time
the entry was made
• When a frame comes in looking for a given
MAC address
– If the address is from the same interface it
came in on, do nothing (the frame is internal
to that segment); this is filtering the frame
INFO 330 Chapter 5
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Switch Table
– If the address needs to go to another
interface, send it there
– If the address doesn’t exist, see next slide 
• Recall hubs transmit without concern for
existing traffic
• A switch uses CSMA/CD to tell when to
transmit, but its interfaces are not adapters
(they have no MAC addresses)
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Switch Learning
• The switch table is built automatically –
they are self-learning
– The switch table is empty to start
– If a frame arrives with a MAC destination not
in the table, send it to all other interfaces
– Each time a frame is received, record the
interface and address from which it came,
and the current time
• If the aging time expires, remove that address
from the table
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Switches
• Switches are plug-and-play devices,
because they configure the switch table
automatically
• One can have dedicated access to a
switch, with separate connections for
transmitting and receiving data
– This makes collisions impossible for those
hosts
– Dedicated access means point-to-point
connections can be used; no multiple access
protocol needed!
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Switches
• Switches can help a network by:
– Eliminate collisions, if there are no hubs in
the network
– Have links at different speeds and different
media (copper vs fiber), but all the
same protocol
– Shut off misbehaving adapters
– Collect network management data (usage
rates, collision frequency, traffic types, etc.)
INFO 330 Chapter 5
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Switches vs Routers
• Switches are store-and-forward packet
switches that use the layer 2 address
– Routers use the layer 3 address
• Switches can only use the spanning
tree structure
– Routers can use any structure
• Switches are plug-and-play
– Routers need to be maintained manually,
e.g. their IP addresses
INFO 330 Chapter 5
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Switches vs Routers
• A large switched-only network would need
large ARP tables, and provides no protection
against broadcast storms (an errant host
transmitting endlessly)
• Processing time for a switch is typically less
than for a router
• Bottom line – need to use routers to control
larger networks, or when its intelligence is
helpful, use a switch whenever possible
INFO 330 Chapter 5
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Hubs vs. Switches vs. Routers
Device
Layer
Hub
Switch
Router
Physical
Link
Network
Address
used
None
MAC
IP
Device Requires same Isolates collision
link protocol?
domains?
Hub
Yes
No
Switch
Yes
Yes
Router
No
Yes
INFO 330 Chapter 5
Requires inputs have
same speed?
Yes
No
No
Interfaces have
MAC addresses?
No
No
Yes
81
VLANs
• A typical LAN has limitations
– Lack of traffic isolation, unless routers are
used
– Inefficient use of switches for larger LANs
– Hard to manage users who change subnets
• Can solve by using a virtual LAN (VLAN)
• A switch can be configured to have many
VLANS within a physical LAN
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VLANs
• Can have certain switch ports assigned
to each VLAN
– To get data from one VLAN to another, can
use an integrated router
• That gets messy for many VLANS –
instead, use trunking
– A trunk port on the switch belongs to all
VLANS
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VLANs
• An extended Ethernet protocol, 802.1Q,
allows a 4B VLAN tag to be added to
frames to identify its VLAN
– The VLAN tag is only added to the frame
before and after using the trunk ports
• VLANs can also be identified by MAC
addresses, network layer protocols, and
other approaches
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Link Virtualization: MPLS
• Multiprotocol Label Switching (MPLS) is a
packet-switched virtual-circuit network
• MPLS adds a header between the IP and {Ethernet or
PPP} headers
• The header contains a label used for routing instead
of the IP address – similar to the VC identifier – so it
doesn’t have to look up the IP address
• Routers that speak MPLS are called label-switched
routers, and can do traffic engineering to define
routes impossible with IP addressing
• MPLS can also be used to implement VPNs
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Data Center Networking
• Data center networks can host thousands
of hosts to support cloud applications
• Racks contain dozens of blade servers, with a top of
rack (TOR) switch above them
• Hierarchical tiers of switches and then routers help do
load balancing and connect to access and border
routers to go to the Internet (Fig 5-30, p. 491)
• Heavily interconnected switches and routers form a
fully connected topology
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PPPPPPPPPPPPPPPPPPPPPPPP
• The Point-to-Point Protocol (PPP) is the
main protocol used to connect between an
ISP and a customer
– Related, but not addressed here, is the
High-level Data Link Control (HDLC) protocol
• PPP could be used over many types of
connection – a dial-up modem, X.25,
SONET (synchronous optical network),
ISDN, DSL, etc.
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PPP Data Framing
• The PPP data frame steals a little from
HDLC
– A 1-byte Flag of ‘01111110’ starts and ends
each frame
– A 1-byte Address of ‘11111111’ is next
– A 1-byte Control field consists of ‘00000011’
• Pretty boring header, huh?
– Then a 1-2 byte Protocol field tells what
network layer protocol will be used (hex
21=IP, 29=AppleTalk, 27=DECnet)
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PPP Data Framing
– Then comes the Information (data) being
transmitted
• The default max size is 1500 bytes, but that can
be changed
– Then a 2 or 4 byte Checksum using the
format defined by HDLC for a CRC code
– Then the other Flag field ends the frame
• So the headers and trailers total 7 to 10 B
for PPP frames
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Byte Stuffing
• So what if the data includes the Flag
value?
• Add a control escape byte in front of it, to
tell there’s a non-Flag sequence of
‘01111110’
• So the extra byte ‘01111101’ is stuffed in
front of any actual data bytes which
happen to be ‘01111110’
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Summary
• The link layer gets us from one node (host
or router) to another
– All link layer protocols take network layer
datagrams and put them in frames to be sent
over the physical (though not always solid)
medium of the link
– A point-to-point link (PPP) has one sender
and one receiver; multiple access links
(Ethernet, MPLS) can have many of both
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