Lecture 11: Addressing, Framing, and
Switching in the Link Layer
CS 3035/GZ01: Networked Systems
Kyle Jamieson
Department of Computer Science
University College London
The link layer: Functionality
IP datagram
Link-layer protocol
Sending host
frame
frame
Receiving host
• Enables the exchange of messages (frames) between end hosts
• Functionality:
1. Framing: Determine start and end of bits and frames
2. Error control: Detect and/or correct errors
3. Reliable delivery: Deliver frames exactly once
4. Medium access control: Control hosts’ access to a shared
medium, if applicable (medium access control)
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Today
• We finish the functionality of the link layer, and tie it in to IP
1. Framing and addressing
1. Repeaters, hubs, and switches
2. Bootstrapping a host
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Framing frames
• We have seen how to frame bits on a link
– Ethernet’s Manchester encoding
– Result: An infinite stream of bits on a link
• But, two hosts connected on the same physical medium
need to be able to exchange frames
– Service provided by the link layer
– Implemented by the network adaptor
• Problem: how does the link layer determine where each
frame begins and ends?
(…how hard can that be?)
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Simple approach to framing: count bytes
• Sender includes number of bytes in header
• Receiver extracts this number of bytes of body
53
Body
80
Body
53 bytes of data
21 bytes of data
• But what if the Count field is corrupted?
– L2 will frame the wrong bytes
– This is called a framing error
– With high probability, CRC will detect the framing error and
discard that frame, but:
Bogus count field
61
Body
80
61 bytes of data misdelivered
???
Body
??? bytes of data misdelivered
• This state of persistent framing errors is called desynchronization
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Desynchronization
• Once framing on a link is desynchronized, it can stay that way
• Need a method to resynchronize
• But once we have that method, why use counting?
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Framing with sentinel bytes
• Delineate beginning of frame with special byte (SYN)
• Delineate end of frame with another special byte (ETX)
SYN
Frame contents
ETX
• What if sentinel occurs in data?
– Byte stuffing: insert another special “escape” byte DLE before sentinel
• What if any of the above escape characters occur in data?
– Byte stuffing again: Stuff DLE before DLE occurring in data
• Example:
SYN
DLE, SYN, DLE, DLE, DLE, ETX
ETX
• Can we be more efficient?
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Framing with sentinel bits
• Delineate frame with special bit pattern
– e.g., 01111110  start, 01111111  end
01111110
Frame contents
01111111
• Problem: what if sentinel occurs within frame?
• Solution: bit stuffing
– Sender always inserts a 0 after five 1s in the frame contents
– Receiver always removes a 0 appearing after five 1s
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When receiver sees five 1s…
01111110
Frame content
01111111
• If next bit 0, remove it, and begin counting again
– Because this must be a stuffed bit; we can’t be at
beginning/end of frame (those had six or seven 1s)
• If next bit 1 (i.e., we’ve seen six 1s) then:
– If following bit is 0, this is start of frame
• Because the receiver has seen 01111110
– If following bit is 1, this is end of frame
• Because the receiver has seen 01111111
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Example: sentinel bits
• Original data, including start/end of frame:
01111110011111101111101111100101111111
• Sender rule: five 1s  insert a 0
– After bit stuffing at the sender:
01111110011111010111110011111000101111111
• Receiver rule: five 1s and next bit 0  remove 0
01111110011111101111101111100101111111
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Comparing addressing schemes
• Network layer address (IP address)
– Function: move datagram to destination network
– 32-bit address, dotted quad notation a.b.c.d where each
component is an eight-bit unsigned integer
– Hierarchical address space
• Link layer address (MAC address, Ethernet address):
– Function: move frame from one point to another point on
the same network
– Unique 48-bit address (in most LANs)
– Burned in NIC ROM, also sometimes software settable
– Usually a flat address space
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Ethernet addresses
• 48-bit source and destination addresses
– Receiver’s link layer passes frame up to network-level protocol:
• If destination address matches the adaptor’s
• Or the destination address is the broadcast address (ff:ff:ff:ff:ff:ff)
• Or the card is in a mode of operation that receives all
frames (promiscuous mode)
– Addresses are globally unique
• Assigned by NIC vendors (top three bytes specify vendor)
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Today
• We finish the functionality of the link layer, and tie it in to IP
1. Framing and addressing
1. Repeaters, hubs, and switches
– Comparison
– Self-learning switches
– The Spanning Tree Protocol
2. Bootstrapping a host
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Message, segment, datagram, and frame
host
host
HTTP message
HTTP
TCP segment
TCP
router
IP
HTTP
IP datagram
Ethernet
interface
Ethernet
interface
Ethernet frame
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IP
TCP
router
IP datagram
SONET
interface
SONET
interface
SONET frame
IP
IP datagram
Ethernet
interface
IP
Ethernet
interface
Ethernet frame
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Different devices switch on different information
• Routers: forward IP datagrams
based on network-layer addresses
in the IP header
HHHH
data
Router
IP datagram
Network
Link
Physical
• Switches (Bridges): forward linklayer frames based on link-layer
addresses in the link-layer header
HHHH
data
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data
data
Switch
Link layer frame
Link
Physical
• Repeaters/Hubs: rebroadcast all
bits in the physical-layer frame
HHHH
HH
HHH
data
Hub
Physical-layer frame
Physical
HHHH
data
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Physical Layer: Repeaters
• Distance limitation in local-area networks
– Electrical signal becomes weaker as it travels
– Imposes a limit on the length of a LAN
• In addition to limit imposed by collision detection
• Repeaters join LANs together
– Analog electronic device
– Continuously monitors electrical signals on each LAN
– Transmits an amplified copy
Repeater
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Physical Layer: Hubs
• Joins multiple input lines electrically
– Do not necessarily amplify the signal
• Very similar to repeaters
– Also operate at the physical layer
hub
hub
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hub
hub
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Limitations of repeaters and hubs
• One large place where packets collide (collision domain), since
every bit is sent everywhere
– So, aggregate throughput is limited
– e.g., three departments each get 10 Mbps independently
– … and then if connect via a hub must share 10 Mbps
• Cannot support multiple LAN technologies
– Repeaters/hubs do not buffer or interpret frames
– So, can’t interconnect between different rates or formats
• e.g., no mixing 100 Mbit/s Ethernet and Gigabit Ethernet
• Limitations on maximum nodes and distances
– Does not circumvent limitations of the shared medium
– e.g., still cannot go beyond 2500 m in commercial Ethernet
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Link Layer: Switches
• Switches also connect two or more LANs at the link layer
– Extracts destination address from the frame
– Looks up the destination in a table
– Forwards the frame to the appropriate LAN segment
• Or point-to-point link, for higher-speed Ethernet
• Each port is its own collision domain (if not just a link)
Switch
collision
domain
hub
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Extended LAN
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Switches and concurrent communication
• Host A can talk to C, while B talks to D
B
A
switch
C
D
• If host has (dedicated) point-to-point link to switch:
– Full duplex: each connection can send in both directions
– Completely avoids collisions
 No need for carrier sense, collision detection, and so on
 Change in medium access control, but same framing
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Switches: Advantages over hubs and repeaters
• Only forwards frames as needed
– Filters frames to avoid unnecessary load on segments
– Sends frames only to segments that need to see them
• Extends the geographic span of the network
– Separate collision domains allow longer distances
• Improves privacy by limiting scope of frames
– Hosts can “snoop” the traffic traversing their segment
– … but not all the rest of the traffic
• Applies CSMA/CD in segment (not whole net)
– Smaller collision domain
• Joins segments using different technologies
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Disadvantages over hubs and repeaters
• Higher cost
– More complicated devices that cost more money
• Delay in forwarding frames
– Bridge/switch must receive and parse the frame
– … and perform a look-up to decide where to forward
– Introduces store-and-forward delay
• Can ameliorate using cut-through switching
– Start forwarding after only header received
• Need to learn where to forward frames
– Bridge/switch needs to construct a forwarding table
– Ideally, without intervention from network administrators
– Solution: Self-learning algorithm
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Motivation for self learning
• Benefit if switch forwards frame only on segment(s) that need it
– Allows concurrent use of other links
• Switch forwarding table
– Maps destination link-layer address to outgoing interface
– Goal: construct the switch table automatically
B
A
switch
C
D
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Self learning algorithm: Building the table
• When a frame (e.g., from A to B) arrives at the switch:
– Inspect the source link-layer address
• Associate that address with the incoming switch port
• Store the mapping in the switch table
• Use time-to-live field to eventually forget the mapping an
amount of time later equal to its value
– This is an example of soft state
Switch forwarding table:
B
A  B data
A
Port
1
Time-to-live
2 minutes
2
1
switch
3
4
C
D
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Address
A
Switch just learned
how to reach A.
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Self learning algorithm: Handling misses
• When frame arrives with unfamiliar destination (e.g., B)
– Forward the frame out all ports except for the one on which
the frame arrived
• This is called flooding
– Hopefully, this case won’t happen very often
• When e.g. B replies, switch will learn that node, too
Switch forwarding table:
B
A  B data
A
Address
A
Port
1
Time-to-live
2 minutes
2
1
switch
3
4
C
D
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Self-learning algorithm
When switch receives a frame:
index into the forwarding table using link-layer destination address
if entry found for destination {
if dest on segment from which frame arrived
then drop frame
else forward frame on interface indicated
} else flood the frame
Forward on all ports except the
port on which the frame arrived
Problems?
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Flooding can lead to loops
• Switches sometimes need to flood frames:
– Upon receiving a frame with an unfamiliar destination
– Upon receiving a frame sent to the broadcast address
• Flooding can lead to forwarding loops
– e.g., if the network contains a cycle of switches
• Either accidentally, or by design for higher reliability
How can we revise the bridge learning
• This is catastrophic,
for twoto
reasons:
algorithm
avoid broadcast storms?
1.
2.
Unlike IP, layer 2 has no way of preventing frame looping
Ethernet duplicates frames, leading to an exponential increase, quickly
crashing the extended LAN (this is called a broadcast storm)
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The spanning tree protocol (STP)
• Early 1980s: Digital Equipment Corporation, a key
Ethernet vendor, wanted to leverage the benefits of
loops while avoiding broadcast storms
• Radia Perlman’s idea: Switches agree on a loop-free
and connected spanning tree
– Spanning tree: a sub-graph that touches all vertices but
contains no cycles
Graph with
cycles
Spanning tree
has no cycles
• Once the spanning tree is formed:
– Switches use the switch learning algorithm to forward data
frames over the tree links only
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Spanning Tree Protocol (STP): Overview
• Users connect Ethernet
switches and shared-medium
Ethernet LANs together
4
– Arbitrarily, possibly
creating forwarding loops
• Need a distributed algorithm
so that:
3
1. Switches cooperate to
build the spanning tree
1. Switches adapt
automatically when
failures occur
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1
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STP: Key ingredients of the algorithm
• Switches elect one root switch
from which to build the tree
– Switch identifier = link-layer
address on one port
4
• Switches block some ports from
sending or receiving
frames
of a simplified version of
Let’s
begin
with
Ethernet type IP (or other L3 data)
3
B
the full STP distributed algorithm
• To form tree, switches exchange
configuration messages (R, d, X):
– From switch X
– Proposing switch R (which is d
hops away) as the root
– Configuration messages are
never blocked
Blocked ports
B
2
1
Root switch
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Simplified STP: State at each switch
• Each switch X keeps the following state:
1. Its view of who the root is
– Initially, itself: X
X
Root id: X
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Simplified STP: Startup and calculating the root
• Note: Initially, each switch X periodically sends
(X, 0, X) from all its ports
4
Root id: 4
Root ID rule: Root ID r at switch X is
the minimum of X and root IDs
received at all ports
3
Root id: 3
2
Root id: 2
1
Root id: 1
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Simplified STP: Startup and calculating the root
• Note: Initially, each switch X periodically sends
(X, 0, X) from all its ports
4
Root id: 4
Root ID rule: Root ID r at switch X is
the minimum of X and root IDs
received at all ports
• Switch 2 sends (2, 0, 2); switch 3
sets its root id to 1, switch 1
ignores
3
Root id: 2
 (2, 0, 2)
2
Root id: 2
1
Root id: 1
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Simplified STP: Startup and calculating the root
• Note: Initially, each switch X periodically sends
(X, 0, X) from all its ports
4
Root id: 4
Root ID rule: Root ID r at switch X is
the minimum of X and root IDs
received at all ports
• Switch 1 sends (1, 0, 1); switches 2
and 3 set their root ids to 1
3
Root id: 1
2
Root id: 1
1
Root id: 1
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Simplified STP: Startup and calculating the root
• Note: Initially, each switch X periodically sends
(X, 0, X) from all its ports
4
Root id: 3
Root ID rule: Root ID r at switch X is
the minimum of X and root IDs
received at all ports
• Switch 3 sends (3, 0, 3); switch 4
sets its root id to 3, others ignore
3
Root id: 1
2
Root id: 1
1
Root id: 1
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STP: Startup and calculating the root
• Note: Initially, each switch X periodically sends
(X, 0, X) from all its ports
4
Root id: 3
Root ID rule: Root ID r at switch X is
the minimum of X and root IDs
received at all ports
3
• Switch
4
sends
(4,
0,
4);
switch
3
Not yet agreeing on the identity of the root:Root
let’s
now see
id: 1
ignores
how switches propagate information through the network
2
Root id: 1
1
Root id: 1
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Simplified STP: State at each switch
• Each switch X keeps the following state:
1. Its view of who the root is
– Initially, itself: X
2. Its configuration message to send
– Initially, announcing itself as root with
zero distance to root: (X, 0, X)
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X
Root id: X
Msg: (X, 0, X)
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Simplified STP: Calculating the message
• Switch X finds its distance from the root (d):
1. If X thinks it is the root, d  0
2. Otherwise, d  the minimum distance from
messages received matching X’s root id (call
it r), plus one
Configuration message rule: Switch
X sets its configuration message to
(r, d, X). If configuration message
changes, sends updated message
immediately
4
Root id: 4
Msg: (4, 0, 4)
3
Root id: 3
Msg: (3, 0, 3)
2
Root id: 2
Msg: (2, 0, 2)
1
Root id: 1
Msg: (1, 0, 1)
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Simplified STP: Calculating the message
• Switch X finds its distance from the root (d):
1. If X thinks it is the root, d  0
2. Otherwise, d  the minimum distance from
messages received matching X’s root id (call
it r), plus one
Configuration message rule: Switch
X sets its configuration message to
(r, d, X). If configuration message
changes, sends updated message
immediately
• Switch 1 sends (1, 0, 1), switches 2
and 3 update their root ids and msgs
4
Root id: 3
Msg: (4, 0, 4)
3
Root id: 1
Msg: (1, 1, 3)
2
Root id: 1
Msg: (1, 1, 2)
1
Root id: 1
Msg: (1, 0, 1)
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Simplified STP: Calculating the message
• Switch X finds its distance from the root (d):
1. If X thinks it is the root, d  0
2. Otherwise, d  the minimum distance from
messages received matching X’s root id (call
it r), plus one
Configuration message rule: Switch
X sets its configuration message to
(r, d, X). If configuration message
changes, sends updated message
immediately
• Switch 3 sends (1, 1, 3), switch 4
updates its root id and message
4
Root id: 1
Msg: (1, 2, 4)
3
Root id: 1
Msg: (1, 1, 3)
2
Root id: 1
Msg: (1, 1, 2)
1
Root id: 1
Msg: (1, 0, 1)
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Simplified STP: Calculating the message
• Switch X finds its distance from the root (d):
1. If X thinks it is the root, d  0
2. Otherwise, d  the minimum distance from
messages received matching X’s root id (call
it r), plus one
4
Root id: 1
Msg: (1, 2, 4)
3
Configuration message rule:
Now
agree on the root identifier.RootBut
id: 1how do
SwitchallX switches
sets its configuration
(1, 1, 3) tree?
message
to (r,
d, X) ports to block to form theMsg:
they
decide
which
spanning
2
Root id: 1
Msg: (1, 1, 2)
1
Root id: 1
Msg: (1, 0, 1)
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STP: Port status
4
• All switches connected to a Ethernet LAN (or the
two at the ends of a cable) agree on a single
“designated” port
Designated port: The port on the
shortest path from the LAN or cable to
the root is the designated port (D)
Root id: 1
Msg: (1, 2, 4)
D
3
Root id: 1
Msg: (1, 1, 3)
– The designated port forwards
frames from the LAN to the root
2
Root id: 1
Msg: (1, 1, 2)
– Only designated ports send
configuration messages
D
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D
1
Root id: 1
Msg: (1, 0, 1)
D
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STP: Port status
Root port: Each non-root switch
notes which of its port is on the
shortest path to the root; this port
is the root port (R)
R
4
Root id: 1
Msg: (1, 2, 4)
D
R
3
Root id: 1
Msg: (1, 1, 3)
D
2
Root id: 1
Msg: (1, 1, 2)
R
D
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1
Root id: 1
Msg: (1, 0, 1)
D
43
STP: Port status
R
Blocked port: If neither designated nor
root, a port is a blocked port (B), not
forwarding data traffic.
4
Root id: 1
Msg: (1, 2, 4)
D
R
3
Root id: 1
Msg: (1, 1, 3)
B
B
D
2
Root id: 1
Msg: (1, 1, 2)
R
D
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1
Root id: 1
Msg: (1, 0, 1)
D
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STP: State at each switch
• Each switch X keeps the following state:
1. Its view of who the root is
– Initially, itself: X
2. Its configuration message to send
– Initially, announcing itself as root with
zero distance to root: (X, 0, X)
3.
X
Root id: X
Msg: (X, 0, X)
D: (X, 0, X)
For each of X’s ports:
– Whether designated (D), root (R), or blocking (B) data traffic
• Initially, designated (D)
– “Best” configuration message heard on that port
• Initially, its own configuration message (X, 0, X)
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STP: Designated port rule
• At a switch, for each port p:
– Consider all configuration messages received on port p and
the configuration message the switch would send
– If switch receives a “better” configuration message on a port
p, don’t send configuration messages on port p
– Else, p is designated: send configuration message on p
• Rule for comparing configuration messages:
(R1, d1, X1) better than (R2, d2, X2) if R1 < R2 or
(R1 = R2 and d1 < d2) or
(R1 = R2 and d1 = d2 and X1 < X2)
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STP: Complete example
• All switches begin thinking they are root with
all ports in the designated state
D: (4,0,4)
4
Root id: 4
Msg: (4,0,4)
D: (3,0,3)
D: (3,0,3)
3
Root id: 3
Msg: (3,0,3)
D: (3,0,3)
D: (2,0,2)
D: (2,0,2)
2
Root id: 2
Msg: (2,0,2)
D: (2,0,2)
D: (1,0,1)
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Root id: 1
Msg: (1,0,1)
D: (1,0,1)
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STP: Complete example
• All switches begin thinking they are root with
all ports in the designated state
D: (4,0,4)
• Switch 1 sends (1,0,1), switches 2 and 3
update their root ids, ports, and msgs
Root id: 4
Msg: (4,0,4)
D: (3,0,3)
R: (1,0,1)
3
Root id: 1
Msg: (1,1,3)
D: (3,0,3)
D: (2,0,2) 3
2
B: (1,0,1)
– Each switch’s port remembers the
best configuration message seen
so far
Root id: 1
Msg: (1,1,2)
1 R: (1,0,1)
D: (1,0,1)
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2
1
Root id: 1
Msg: (1,0,1)
D: (1,0,1)
(1, 0, 1) 
– Switch 2 breaks “tie” between the
two copies of (1,0,1) locally by
numbering its ports
4
48
STP: Complete example
– Switch 2, port 3 remains
designated because Switch 2’s
message (1,1,2) is better than
(1,1,3)
(1,1,3) 
• Switch 3 sends (1,1,3) from its
designated ports , switch 4 updates
its root id and message
R: (1,1,3)
4
Root id: 1
Msg: (1,2,4)
D: (3,0,3)
R: (1,0,1)
3
Root id: 1
Msg: (1,1,3)
D: (3,0,3)
D: (1,1,3) 3
2
– Switch 1, port 1 remains
designated because Switch 1’s
message (1,0,1) is better than
(1,1,3)
B: (1,0,1)
Root id: 1
Msg: (1,1,2)
1 R: (1,0,1)
D: (1,0,1)
1
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2
1
Root id: 1
Msg: (1,0,1)
D: (1,0,1)
2
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STP: Complete example
• Switch 2 sends (1,1,2) from port 3 only
R: (1,1,3)
– Switch 3 blocks its port 3 since (1,1,2)
is better than its message (1,1,3)
4
Root id: 1
Msg: (1,2,4)
D: (3,0,3) 2
1
R: (1,0,1)
3
3
Root id: 1
Msg: (1,1,3)
B: (1,1,2)
D: (1,1,3) 3
2
B: (1,0,1)
2
Root id: 1
Msg: (1,1,2)
1 R: (1,0,1)
D: (1,0,1)
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1
Root id: 1
Msg: (1,0,1)
D: (1,0,1)
50
STP: Dynamics
• When do switches send configuration messages?
– If you think you’re the root, send periodically with parameter hello
time (two seconds recommended in 802.1d)
– Other switches send on all designated ports upon receiving root’s message
• How does the algorithm adapt to topology changes?
– State table contains age field, which is updated continuously
– Aging rule: If age reaches a threshold max age (20 sec in 802.1d),
discard that table entry and recalculate using all rules
• What happens if max age is too big? Too small?
– Recalculate when receive better or newer configuration message on
port p (resulting in a table entry being overwritten)
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STP: Handling failures
• Suppose the Ethernet LAN fails
R: (1,1,3)
4
Root id: 1
Msg: (1,2,4)
D: (3,0,3) 2
1
R: (1,0,1)
3
3
Root id: 1
Msg: (1,1,3)
B: (1,1,2)
D: (1,1,3) 3
2
B: (1,0,1)
2
Root id: 1
Msg: (1,1,2)
1 R: (1,0,1)
D: (1,0,1)
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1
Root id: 1
Msg: (1,0,1)
D: (1,0,1)
52
STP: Handling failures
• Suppose the Ethernet LAN fails
R: (1,1,3)
• Switch 3:
– Stops hearing the root’s
messages through port 1, so
it becomes designated
– Port 3 becomes root
– Updates its own message
4
Root id: 1
Msg: (1,2,4)
D: (3,0,3) 2
1
D: (1,2,3)
3
3
Root id: 1
Msg: (1,2,3)
R: (1,1,2)
D: (1,1,3) 3
2
B: (1,0,1)
2
Root id: 1
Msg: (1,1,2)
1 R: (1,0,1)
D: (1,0,1)
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Root id: 1
Msg: (1,0,1)
D: (1,0,1)
53
STP: Handling failures
• Suppose the Ethernet LAN fails
R: (1,2,3)
• Switch 4:
– Updates message heard on
root port
– Updates its own message
Root id: 1
Msg: (1,3,4)
D: (3,0,3) 2
1
D: (1,2,3)
3
3
Root id: 1
Msg: (1,2,3)
• Switch 2:
– Stops hearing the root’s
messages through port 2, so
it becomes designated
R: (1,1,2)
D: (1,1,3) 3
2
D: (1,1,2)
2
Root id: 1
Msg: (1,1,2)
1 R: (1,0,1)
D: (1,0,1)
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1
Root id: 1
Msg: (1,0,1)
D: (1,0,1)
54
STP: Handling topology change
• Suppose we fix the LAN. Now we
have created (temporary)
forwarding loops
– This also happens when
switches are powered-up
R: (1,2,3)
4
Root id: 1
Msg: (1,3,4)
D: (3,0,3) 2
1
D: (1,2,3)
3
3
Root id: 1
Msg: (1,2,3)
R: (1,1,2)
D: (1,1,3) 3
2
D: (1,1,2)
2
Root id: 1
Msg: (1,1,2)
1 R: (1,0,1)
D: (1,0,1)
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Root id: 1
Msg: (1,0,1)
D: (1,0,1)
55
STP: Pre-forwarding port state
• Suppose any of the following
apply to a port:
1. Transition from B  D
2. Any newly-connected port
(detect Ethernet carrier)
3. Any port on a freshlypowered switch
• The port then enters the preforwarding (PF) state, where:
R: (1,2,3)
Networked Systems 3035/GZ01
Root id: 1
Msg: (1,3,4)
D: (3,0,3) 2
1
PF: (1,2,3)
3
3
Root id: 1
Msg: (1,2,3)
R: (1,1,2)
D: (1,1,3) 3
2
PF: (1,1,2)
– It sends configuration
messages and transitions to
blocked and root states as if
designated
– But it does not forward data
frames, so can’t create loops
4
2
Root id: 1
Msg: (1,1,2)
1 R: (1,0,1)
PF: (1,0,1)
1
Root id: 1
Msg: (1,0,1)
D: (1,0,1)
56
STP: Pre-forwarding port state
• Switches 3 returns to old state
R: (1,2,3)
4
Root id: 1
Msg: (1,3,4)
D: (3,0,3) 2
1
R: (1,0,1)
3
3
Root id: 1
Msg: (1,1,3)
R: (1,1,2)
D: (1,1,3) 3
2
PF: (1,1,2)
2
Root id: 1
Msg: (1,1,2)
1 R: (1,0,1)
PF: (1,0,1)
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1
Root id: 1
Msg: (1,0,1)
D: (1,0,1)
57
STP: Pre-forwarding port state
• Switch 3 returns to old state
• Switch 2 returns to old state
R: (1,2,3)
4
Root id: 1
Msg: (1,3,4)
D: (3,0,3) 2
1
R: (1,0,1)
3
3
Root id: 1
Msg: (1,1,3)
R: (1,1,2)
D: (1,1,3) 3
2
B: (1,0,1)
2
Root id: 1
Msg: (1,1,2)
1 R: (1,0,1)
PF: (1,0,1)
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Root id: 1
Msg: (1,0,1)
D: (1,0,1)
58
STP: Pre-forwarding port state
• Switch 3 returns to old state
• Switch 2 returns to old state
• Switch 4 returns to old state
• Now switch 1, port 1 remains in
the pre-forwarding state
R: (1,1,3)
4
Root id: 1
Msg: (1,2,4)
D: (3,0,3) 2
1
R: (1,0,1)
3
3
Root id: 1
Msg: (1,1,3)
R: (1,1,2)
D: (1,1,3) 3
2
B: (1,0,1)
2
Root id: 1
Msg: (1,1,2)
1 R: (1,0,1)
1
PF: (1,0,1)
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1
Root id: 1
Msg: (1,0,1)
2
D: (1,0,1)
59
STP: Leaving the pre-forwarding state
• If still in PF state after some
number of seconds (forwarding
delay parameter) then the port
becomes designated (D)
• How long should forwarding
delay be?
– Long enough for the entire
spanning tree to re-form, i.e.:
R: (1,1,3)
Root id: 1
Msg: (1,2,4)
D: (3,0,3) 2
1
R: (1,0,1)
3
3
Root id: 1
Msg: (1,1,3)
R: (1,1,2)
D: (1,1,3) 3
2
B: (1,0,1)
– Twice the maximum transit
time across the extended LAN
• 30 seconds in 802.1d
2
Root id: 1
Msg: (1,1,2)
1 R: (1,0,1)
1
D: (1,0,1)
Networked Systems 3035/GZ01
4
1
Root id: 1
Msg: (1,0,1)
2
D: (1,0,1)
60
The evolution of Ethernet
• From the coaxial cable shared medium to switches
– Even more capacity, with simultaneous conversations
• From 3 Mbit/s experimental Ethernet to 100 Gbit/s
recent standards
• From electrical signaling to optical
• Changed everything except the frame format
• Lesson: The right interface can accommodate many changes
– Implementation is hidden behind interface
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Today
• We finish the functionality of the link layer, and tie it in to IP
1. Framing and addressing
1. Repeaters, hubs, and switches
2. Bootstrapping a host
– Protocols for bootstrapping: DHCP, ARP
– Communicating over the same, different networks
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What does a host need to know?
• What IP address should the host use?
• What local DNS server to use?
• How to tell which destinations are local?
– How to address them using the local network?
• How to send packets to remote destinations?
??? 1.2.3.7 1.2.3.156
host
host
...
DNS
host
host
...
DNS
5.6.7.0/24
1.2.3.0/23
1.2.3.19
router
Networked Systems 3035/GZ01
router
router
63
Avoiding manual configuration
• Dynamic Host Configuration Protocol (DHCP)
– End host learns how to send packets
– Learn IP address, DNS servers, “gateway,” what’s local
• Address Resolution Protocol (ARP)
– For local destinations, learn the mapping between IP address
and MAC address
1.2.3.48 1.2.3.7 1.2.3.156
host
host
1.2.3.0/23
255.255.254.0
...
DNS
host
host
...
DNS
5.6.7.0/24
1.2.3.19
router
Networked Systems 3035/GZ01
1A-2F-BB-76-09-AD
router
router
64
Key ideas in both protocols
• Broadcasting: when in doubt, shout!
– Broadcast query to all hosts in the local-area-network
– … when you don’t know how to identify the right one
• Caching: remember the past for a while
– Store the information you learn to reduce overhead
– Remember your own address and other host’s addresses
• Soft state: eventually forget the past
– Associate a time-to-live field with the information
– On expiry either refresh or discard the information
– This is key for robustness in the face of unpredictable change
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Bootstrapping problem
• Host doesn’t have an IP address yet
– So, host doesn’t know what source address to use
• Host doesn’t know whom to ask for an IP address
– So, host doesn’t know what destination address to use
host
host
router
Networked Systems 3035/GZ01
router
66
DHCP discovery, from the client
• DHCP Solution: “shout” to discover a server that can help
– Client broadcasts a DHCP discover message (to the broadcast
IP address, 255.255.255.255)
– Two possibilities:
1. Server on same subnet sends a reply offering an address
2. Or: a DHCP relay agent (configured only with DHCP server’s
IP address) unicasts to a DHCP server on another network
• DHCP server replies unicast to relay agent; agent forwards
replies to the new host’s network
host
DHCP server
host
DHCP server
DHCP relay
router
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router
67
Response from the DHCP server
• The server responds with a DHCP offer message
– Contains configuration parameters (including proposed IP
address, mask, gateway router, DNS server)
– Contains lease time (duration the information remains valid)
• Multiple servers may respond
– Multiple servers on the same subnetwork
– Each may respond with an offer
• Accepting one of the offers
– Client sends a DHCP request echoing the parameters
– The DHCP server responds with a DHCP ACK to confirm
– The other servers see they were not chosen
• They can then safely offer those same parameters to other clients
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Dynamic Host Configuration Protocol
Arriving
client
DHCP server
• Why all the broadcasts?
• Discover broadcast: client doesn’t know DHCP server’s identity
• Offer, ACK broadcast: client doesn’t have an IP yet
• Request broadcast: so other servers can see
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Soft state: Refresh or forget
• Why is a lease time necessary?
– Client can release the IP address (DHCP release)
• e.g., clean shutdown of the computer
– But, host might not release the address
• e.g., the host crashes
• e.g., buggy client software
– And you don’t want the address to be allocated forever
• Performance trade-offs
– Short lease time: returns inactive addresses quickly
– Long lease time: avoids overhead of frequent renewals &
lessens frequency of lease being denied
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So, now the host knows things…




IP address
Mask
Gateway router
DNS server
• And can send packets to other IP addresses
• But: how to use the local network to do this?
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Figuring out where to send locally
• Two cases:
1. Destination is on the local network: need to address it directly
2. Destination is not local (remote): need to figure out the first
“hop” on the local network
• Determining if it’s local: use the netmask
– e.g., bitwise-AND the destination IP address with 255.255.254.0
– Is it the same value as when we do the same with ownIP address?
• Yes  destination IP is local; no  destination IP is remote
1.2.3.48 1.2.3.7 1.2.3.156
host
host
1.2.3.0/23
255.255.254.0
Networked Systems 3035/GZ01
...
DNS 1A-2F-BB-76-09-AD
host
host
...
DNS
5.6.7.0/24
1.2.3.19
router
router
router
72
Figuring out where to send locally (2)
• If it’s remote, look up the first hop in a (very small) local routing table
– e.g., by default, route via 1.2.3.19
– Now do the local case but for 1.2.3.19 rather than ultimate
destination IP address
1.2.3.48 1.2.3.7 1.2.3.156
host
host
1.2.3.0/23
255.255.254.0
...
DNS
1A-2F-BB-76-09-AD
host
host
...
DNS
5.6.7.0/24
1.2.3.19
router
router
router
• For the local case, need to determine the destination’s link-layer address
• How does a host translate the next hop IP address to a link-layer address?
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73
Address Resolution Protocol (ARP)
• Every node maintains an ARP table
– (IP address, link-layer address) pairs
• Consult the table when sending a packet
– Map destination IP address to destination MAC address
– Encapsulate and transmit the data packet
• But: what if IP address not in the table?
– Sender broadcasts: “Who has IP address 1.2.3.156?”
– Receiver responds (unicast, to the source of the broadcast):
“link-layer address 58-23-D7-FA-20-B0”
– Sender caches result in its ARP table
• Sender may include its own <IP, link-layer> address mapping in
request, so that receiver can reply back to the sender
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Example: Putting it all together
• How does host A send a datagram to host B?
1. A sends packet to R
2. R sends packet to B
B
A
host
74:29:9c:e8:ff:55
128.16.74.92 netmask 0xfffff000
Network
128.16.64.0/20
e6:e9:00:17:bb:4b
128.16.64.1
Networked Systems 3035/GZ01
49:bd:d2:C7:56:2a host
128.17.0.2
router
R
Network
128.17.64.0/20
1a:23:f9:cd:06:9b
128.17.0.1
75
Host A decides to send through R
• Host A constructs an IP packet to send to B
– IP source 128.16.74.92, IP destination 128.170.0.2
• Host A has a gateway router R
– Used to reach any destination outside of 128.16.64.0/20
– Address 128.16.64.1 for R learned via DHCP
B
A
host
74:29:9c:e8:ff:55
128.16.74.92 netmask 0xfffff000
Network
128.16.64.0/20
e6:e9:00:17:bb:4b
128.16.64.1
Networked Systems 3035/GZ01
49:bd:d2:C7:56:2a host
128.17.0.2
router
R
Network
128.17.64.0/20
1a:23:f9:cd:06:9b
128.17.0.1
76
Host A sends packet through R
• Host A learns the MAC address of R’s interface
– ARP request: broadcast request for 128.16.64.1
– ARP response: R responds with e6:e9:00:17:bb:4b
• Host A encapsulates the packet in a link-layer header and sends to R
B
A
host
74:29:9c:e8:ff:55
128.16.74.92 netmask 0xfffff000
Network
128.16.64.0/20
Network
128.17.64.0/20
To: R A  B data
e6:e9:00:17:bb:4b
128.16.64.1
Networked Systems 3035/GZ01
49:bd:d2:C7:56:2a host
128.17.0.2
router
R
1a:23:f9:cd:06:9b
128.17.0.1
77
R decides how to forward datagram
• Router R’s left interface receives the packet
– R extracts the IP packet from the Ethernet frame
– R sees the IP packet is destined to 128.17.0.2
• Router R consults its forwarding table
– Packet matches 128.17.64.0/20 via right interface
B
A
host
74:29:9c:e8:ff:55
128.16.74.92 netmask 0xfffff000
Network
128.16.64.0/20
A  B data
e6:e9:00:17:bb:4b
128.16.64.1
Networked Systems 3035/GZ01
49:bd:d2:C7:56:2a host
128.17.0.2
router
R
Network
128.17.64.0/20
1a:23:f9:cd:06:9b
128.17.0.1
78
R sends datagram to B
• Router R’s right interface learns the link-layer address of host B
– ARP request: broadcast request for 128.17.0.2
– ARP response: B responds with 49:bd:d2:C7:56:2a
• Router R encapsulates the packet and sends to B
B
A
host
74:29:9c:e8:ff:55
128.16.74.92 netmask 0xfffff000
Network
128.16.64.0/20
To: B A  B data
e6:e9:00:17:bb:4b
128.16.64.1
Networked Systems 3035/GZ01
49:bd:d2:C7:56:2a host
128.17.0.2
router
R
Network
128.17.64.0/20
1a:23:f9:cd:06:9b
128.17.0.1
79
Security analysis of ARP
• Impersonation
– Any node that hears an ARP request can answer…
– …and can say whatever they want
– Actual legit receiver never sees a problem
• Because even though later packets carry its IP address, its
NIC doesn’t capture them since not its link-layer address
• Man-in-the-middle attack
– Imposter updates frames with correct link-layer address and
forwards whatever it receives to the legit destination…
• …but gets to inspect (and maybe alter) it first
• Does the attacker have to “win” a race?
– Maybe not, if sender blindly believes ARP responses
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The problem with extended LANs
• Switched LANs afford greater scalability, but extended LANs do
not isolate traffic
• Three resulting issues:
1. Security: Allows eavesdropping across LANs, just by putting an
interface in promiscuous mode
2. Load: Some LANs are more heavily-used than others, may be
desirable to separate them at times.
3. Broadcast scalability: Broadcast frames traverse the entire
extended LAN; this reduces overall performance
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Virtual LANs (VLANs)
1
2
4
8
9
10
…
Computer Science
15
16
…
Electrical Engineering
• Switch assigns each port a color, an identifier designating the VLAN that
port belongs to
• Traffic isolation: colors = broadcast domains
• Easily reconfigurable port assignments
• Routing between VLANs: layer 3 routing functionality
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VLAN example
• Configure ports on W, X, Y, and Z to
be in appropriate VLANs
– Trunk ports between B1
and B2 configured for both
VLANs
• Bridge inserts VLAN header
containing color between
Ethernet header and payload
Trunk
link
• If a packet contains a VLAN header,
bridges only forward on matchingcolor or trunk ports
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83
Comparing L2 switches and L3 routers
• Advantages of L2 switches over L3 routers
– No human configuration is needed
– Fast filtering and forwarding of frames
• Disadvantages of L2 switches over L3 routers
– Topology restricted to a spanning tree
– Large networks require large ARP tables
– Broadcast storms can cause the network to collapse
– Can’t accommodate non-Ethernet segments (why not?)
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Acknowledgement
Selected parts adapted from lecture material by
Scott Shenker (UC Berkeley) and Kurose and Ross Computer Networking (4/e)
Coursework 2 due Friday 15th November, 4:05 PM
Midterm exam in regular lecture timeslot,
Thursday 14th November
NEXT TIME
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