Midterm Review
EE 122, Fall 2013
Sylvia Ratnasamy
http://inst.eecs.berkeley.edu/~ee122/
Material thanks to Ion Stoica, Scott Shenker, Jennifer
Rexford, Nick McKeown, and many other colleagues
Logistics

Test is in this classroom starting at 5:40 exactly

Closed book, closed notes, etc.

Single two-sided “cheat sheet”, 8pt minimum

No calculators, electronic devices, etc.


Test does not require any complicated calculation
I will have extra office hours tomorrow, Oct 22.

11am-1pm in 413 Soda Hall
General Guidelines (1)

Test only assumes material covered in lecture & sections


The test doesn’t require you to do complicated calculations
or packet accounting


Use this as a hint to whether you are on the right track
You don’t need to memorize packet headers


Text: only to clarify details and context for the above
We’ll provide the IP header for your reference on the exam sheet
You do need to understand how things work

not for the sake of knowing gory details but to understand pros/cons,
when a solution is applicable/useful/useless, etc.
General Guidelines (2)

Be prepared to:

Weigh design options outside of the context we studied them in
e.g., I had a TCP connection, then BGP went nuts…”

Contemplate new designs we haven’t talked about



e.g., I introduce a new IP address format; how does this affect..”
e.g., I take a little bit of UDP, mix it with some TCP sliding window..”
Don’t let this daunt you. Reason from what you know about the
pros/cons of solutions we did study

e.g., TCP is inefficient when…
General Guidelines (3)

Exam format (tentative!)
Q1) 20 multiple-choice questions

ordered (roughly) from easiest to hardest
Q2) A set of “here’s a scenario, tell me if the following is
true/false”-style questions

ordered (roughly) from easiest to hardest within each scenario
Q3,4,5,6: more traditional questions



(we think) 3 < 4 < 5 < 6 (< implies easier than)
sub-questions within each question ordered easiest to hardest
Pace yourself accordingly.
This Review

Walk through what we expect you to know: key
topics, important aspects of each

Just because I didn’t cover it in review doesn’t
mean you don’t need to know it


But if I covered it today, you should know it
My plan: summarize, not explain

Stop me when you want to discuss something further!
Topics


Architecture: concepts and principles (lecs. 2, 3)
IP Routing (lecs. 4-9)






Transport (lecs. 10-13)




Overall context
Computing routes (DV, LS, path vector)
IP addressing
Inter-domain
Router architecture
Role of the transport layer
UDP vs. TCP
TCP details: reliability, flow and congestion control
Fairness and Router-assisted CC (lec. 13)
Architecture

Concepts you should be familiar with





packet delays
packet vs. circuit switching
statistical multiplexing
layering
end-to-end argument and best-effort service
Properties of Links
bandwidth
delay x bandwidth
Latency

Bandwidth (capacity): “width” of the link


Latency (delay): “length” of the link


number of bits sent (or received) per unit time (bits/sec or bps)
propagation time for data to travel along the link (seconds)
Bandwidth-Delay Product (BDP): “volume” of the link

amount of data that can be “in flight” at any time
Packet Delay
Sending a 100B packet from A to B?
A
B
1Mbps, 1ms
time=0
Time to transmit
800 bits=800x1/106s
100Byte packet
The last bit
reaches B at
TimeDelay =
(800x1/106)+1/103s
Packet
Packet Delay = Transmission Delay + Propagation =Delay
(Packet Size ÷ Link Bandwidth) + Link Latency 1.8ms
Circuit switching
Idea: source reserves network capacity along a path
A
B
(1) Node A sends a reservation request
(2) Interior switches establish a connection -- i.e., “circuit”
(3) A starts sending data
(4) A sends a “teardown circuit” message
Circuit switching: pros and cons

Pros



guaranteed performance
fast transfers (once circuit is established)
Cons



wastes bandwidth if traffic is “bursty”
connection setup adds delay
recovery from failure is slow
Packet Switching

Data is sent as chunks of formatted bits (Packets)

Packets consist of a “header” and “payload”

Switches “forward” packets based on their headers

Each packet travels independently

No link resources are reserved in advance. Instead
packet switching leverages statistical multiplexing
Packet switching: pros and cons

Cons




no guaranteed performance
header overhead per packet
queues and queuing delays
Pros



efficient use of bandwidth (stat. muxing)
no overhead due to connection setup
resilient -- can `route around trouble’
Statistical Multiplexing:
Three Flows with Bursty Traffic
Data Rate 1
Time
Data Rate 2
Capacity
Time
Data Rate 3
Time
When Each Flow Gets 1/3rd of
Capacity
Data Rate 1
Frequent Overloading
Time
Data Rate 2
Time
Data Rate 3
Time
When Flows Share Total Capacity
Time
No Overloading
Time
Time
Statistical Multiplexing

Allows efficient use of network bandwidth

Relies on the assumption that not all flows burst
at the same time


similar assumption and failure case as insurance
Queues absorb transient traffic bursts but also
introduce queuing delay

We will not be asking you to do detailed calculations
of queuing delay on the test
Layering

Layering is a particular form of modularization

System is broken into a vertical hierarchy of logically
distinct entities (layers)

Service provided by one layer is based solely on the
service provided by layer below

Rigid structure: easy reuse, performance suffers
In the context of the Internet
Applications
…built on…
This is
not a typo
Reliable (or unreliable) transport
…built on…
Best-effort global packet delivery
…built on…
Best-effort local packet delivery
…built on…
Physical transfer of bits
L7
Application
L4
Transport
L3
Network
L2
Data link
L1
Physical
Protocols and Layers
L7
L7
Application
Application
L4
Transport
Transport
L4
L3
Network
Network
L3
L2
Data link
Data link
L2
L1
Physical
Physical
L1
Communication between peer layers on
different systems is defined by protocols
Layer Encapsulation
User A
User B
Appl: Get index.html
Trans: Connection ID
Net: Source/Dest
Link: Src/Dest
What gets implemented where?


Lower three layers implemented everywhere
Top two layers implemented only at hosts
Application
Transport
Network
Datalink
Physical
Network
Datalink
Physical
Application
Transport
Network
Datalink
Physical
Host A
Router
Host B
Protocols at different layers
L7
Application
L4
Transport
L3
Network
L2
Data link
L1
Physical
SMTP
HTTP
DNS
TCP
NTP
UDP
IP
Ethernet
optical
copper
FDDI
radio
PPP
PSTN
There is just one network-layer protocol, IP
The “narrow waist” of the Internet hourglass
Implications of Hourglass
Benefits of a single network-layer protocol (IP)
 Allows arbitrary networks to interoperate


Any network that supports IP can exchange packets
Decouples applications from low-level
networking technologies

Applications function on all networks
Benefits of Layering



Modularity allows abstraction and reuse
Narrow waist allows interoperability
Innovation at each layer can proceed in parallel
Drawbacks of Layering

Rigid structure may hurt performance




Duplication of functionality
Information hiding
(Secondary) Headers start to get really big
Layer violations


for performance: e.g., TCP-over-wireless
for security: e.g., firewalls
Summary of End-to-End Principle


Design principle guiding the placement of functionality in a
(networked) system
Builds on observation that some application properties can only be
correctly implemented end-to-end


Reliability, security, etc.
Supporting these properties in the network may be unwise



Doesn’t reduce host implementation complexity
Does increase network complexity
Might impose overhead on apps that don’t need the functionality

However, implementing in the network can improve performance in
some cases

Key argument for why IP offers only “best-effort” delivery

Reliability implemented at the endhost (TCP)
Topics


Architecture: concepts and principles (lecs. 2, 3)
IP Routing (lecs. 4-9)






Transport (lecs. 10-13)




Overall context
Computing routes (DV, LS, path vector)
IP addressing
Inter-domain
Router architecture
Role of the transport layer
UDP vs. TCP
TCP details: reliability, flow and congestion control
Fairness and Router-assisted CC (lec. 13)
“Autonomous System (AS)” or “Domain”
Region of a network under a single administrative entity
Context and Terminology
“End hosts”
“Clients”, “Users”
“End points”
“Border Routers”
“Route” or “Path”
“Interior Routers”
Internet Routing

Internet Routing works at two levels

Each AS runs an intra-domain routing protocol that
establishes routes within its domain



Link State, e.g., Open Shortest Path First (OSPF)
Distance Vector, e.g., Routing Information Protocol (RIP)
ASes participate in an inter-domain routing protocol that
establishes routes between domains

Path Vector, e.g., Border Gateway Protocol (BGP)
Link State Routing






Each node maintains its local “link state” (LS)
Each node floods its local link state
Hence, each node learns the entire network topology
Can use an algorithm such as Dijkstra’s to compute
shortest paths
We won’t test your ability to solve Dijkstra’s under pressure
But you should know the high level properties of LS



every node maintains complete topology
link updates flooded everywhere
may have transient loops while nodes have inconsistent topology
information
Distance Vector Routing

Each router knows the links to its neighbors

Each router has provisional “least cost” estimate to
every other router -- its distance vector (DV)

E.g.: Router A: “A can get to B with cost 11”

Routers exchange this DV with their neighbors

Routers look over the set of options offered by their
neighbors and select the best one

Iterative process converges to set of shortest paths
DV: You should understand

How DV works


The counting to infinity problem


what’s in a DV; how nodes process and update DVs
why it occurs
Poison Reverse

when it does/doesn’t fix counting-to-infinity
60 B
4
1
A
C
50
DV: Link Cost Changes
Stable
state
via
Node A
to
B
C
Node B
Node C
A-B changed
A sends its
DV to B, C
B sends its
DV to A, C
C sends its
DV to A, B
B
B
B
B
C
B
4
51
B 60 51
5This
50
C
C
B 60 51
C
B 60 51
is Cthe61“Counting
to Infinity”
61 50
C 61
50
CProblem
50
C
B 60 51
C 61 50
A
C
A
C
A
C
A
C
A
C
A
4
6
A 60
6
A 60
6
A 60
6
A 60
8
C
9
1
C 65
1
C 110
1
C 110
1
C 110
1
A
B
A
B
A
B
A
B
A
B
A 50
5
A 50
5
A 50
5
A 50
7
A 50
7
B 54
1
B 54
1
B 101
1
B 101
1
B 101
1
Link cost changes here
“bad news travels slowly”
(not yet converged)
DV: Poisoned Reverse
60
4
A
If B routes through C to get to A:
B tells C its (B’s) distance to A is infinite
Stable
state
via
Node A
to
1
50
C
A-B changed
A sends its
DV to B, C
B sends its
DV to A, C
C sends its
DV to A, B
B
B
B
C
C
C
C
B
C
B
B
4
51
B 60 51
B 60 51
B 60 51
B 60 51
C
5
50
C 61 50
C 61 50
C 61 50
C 61 50
A
C
6∞
A
Node B
A
C
A
C
A
C
A 60
C
6∞
A 60
6∞
A 60
6∞
A 60
51
1
∞
C 65
1
C 110
1
C 110
1
C 110
1
A
B
A
B
A 50
5
A 50
5
B 54
1
B 54
1
A
C
Node C
B
4
∞
9
Link cost changes here
A
A 50
∞
B 101
B
A
B
A
5
A 50
61
A 50
B
∞
61
1
∞
B 101
1
∞
B 101
1
Converges after C receives
another update from B
“Count to Infinity”: root cause
A
B
C
• C’s path to A goes via B
• C advertises its distance-to-A to B
• When B’s link to A increases in cost; B thinks
it can get to A via C at a lower cost
“Poison Reverse”: key idea
A
B
C
• C’s path to A goes via B
• C tells B its distance to A is infinity
• When B’s link to A increases in cost; B no longer
thinks it can get to A via C at a lower cost
Topics


Architecture: concepts and principles (lecs. 2, 3)
IP Routing (lecs. 4-9)






Transport (lecs. 10-13)




Overall context
Computing routes (DV, LS, path vector)
IP addressing
Inter-domain
Router architecture
Role of the transport layer
UDP vs. TCP
TCP details: reliability, flow and congestion control
Fairness and Router-assisted CC (lec. 13)
Addressing Goal: Scalable Routing

State: Small forwarding tables at routers


Much less than the number of hosts
Churn: Limited rate of change in routing tables

Traffic, inconsistencies, complexity
Ability to aggregate addresses is crucial for both
(one entry to summarize many addresses)
Hierarchy in IP Addressing


32 bits are partitioned into a prefix and suffix components
Prefix is the network component; suffix is host component
12
34
158
5
00001100 00100010 10011110 00000101
Network (23 bits)


Host (9 bits)
Interdomain routing operates on the network prefix
“slash” notation: 12.34.158.0/23  network with a 23 bit
prefix and 29 host addresses
IP addressing  scalable routing?

Hierarchical address allocation helps routing
scalability if allocation matches topological
hierarchy

Problem: may not be able to aggregate addresses
for “multi-homed” networks


similar problem with mobile hosts
Two competing forces in scalable routing


aggregation reduces number of routing entries
multi-homing increases number of entries
BGP and Inter-Domain Routing



Destinations are IP prefixes (12.0.0.0/8)
Nodes are Autonomous Systems (ASes)
Links represent both physical connections and
business relationships


customer-provider or peer-to-peer
BGP is the protocol for inter-domain routing


path-vector protocol
policy-driven route selection
BGP extends DV

With some important differences



routes selected based on policy, not just shortest path
path vector (useful to avoid loops)
may aggregate routes (aggregating prefixes)
Topology and policy is shaped by the
business relationships between ASes

Three basic kinds of relationships between ASes




AS A can be AS B’s customer
AS A can be AS B’s provider
AS A can be AS B’s peer
Business implications


Customer pays provider
Peers don’t pay each other

Exchange roughly equal traffic
Policy imposed in how routes are
selected and exported
Route export
Route selection
Customer
1
Can reach
128.3/16
blah blah
10
Competitor


5
Selection: Which path to use?
 controls whether/how traffic leaves the network
Export: Which path to advertise?
 controls whether/how traffic enters the network
Typical Export Policy
Destination prefix
advertised by…
Export route to…
Customer
Everyone
(providers, peers,
other customers)
Peer
Customers
Provider
Customers
We’ll refer to these as the “Gao-Rexford” rules
You must
know
this!-- practice!)
(capture common
-- but not
required!
Typical Selection Policy

In decreasing order of priority





make/save money (send to customer > peer > provider)
maximize performance (smallest AS path length)
minimize use of my network bandwidth (“hot potato”)
…
BGP uses route attributes to implement the above

ASPATH, LOCAL_PREF, MED, …
You should know the general idea/goal for each attribute;
we won’t quiz you on the detailed implementation
Policy Dictates Route Selection
Pr
Q
A
B
D
E
traffic allowed


Peer
C
F
traffic not allowed
ASes provide “transit” between their customers
Peers do not provide transit between other
Cu
Peer
20 Bytes of Standard Header, then Options
4-bit
8-bit
4-bit
Version Header Type of Service
Length
(TOS)
3-bit
Flags
16-bit Identification
8-bit Time to
Live (TTL)
16-bit Total Length (Bytes)
8-bit Protocol
13-bit Fragment Offset
16-bit Header Checksum
32-bit Source IP Address
32-bit Destination IP Address
Options (if any)
Payload
Comparison of Design Philosophy
IPv6
IPv4
Version
IHL
Type of Service
Identification
Time to Live
Protocol
Total Length
Flags
Fragment
Offset
Version
Traffic Class
Payload Length
Flow Label
Next
Header
Hop Limit
Header Checksum
Source Address
Source Address
Destination Address
Options
Padding
To Destination and Back (expanded)
Deal with Problems (greatly reduced)
Read Correctly (reduced)
Special Handling (similar)
Destination Address
Topics


Architecture: concepts and principles (lecs. 2, 3)
IP Routing (lecs. 4-9)






Transport (lecs. 10-13)




Overall context
Computing routes (DV, LS, path vector)
IP addressing
Inter-domain
Router architecture
Role of the transport layer
UDP vs. TCP
TCP details: reliability, flow and congestion control
Fairness and Router-assisted CC (lec. 13)
What’s inside a router?
Input and Output for
the same port are on one
physical linecard
Processes packets
on their way in
Route/Control
Processor
Processes packets
Linecards
(output)
before
they leave
Linecards (input)
1
1
2
2
Interconnect
(Switching)
Fabric
N
Transfers packets
from input to
output ports
N
What’s inside a router?
Route/Control
Processor
(1) Implement IGP
and
BGP forwarding
protocols;
(2) Push
compute
tables torouting
the linetables
cards
Linecards (input)
Linecards (output)
1
1
2
2
Interconnect
(Switching)
Fabric
N
N
What’s inside a router?
Constitutes the
control plane
Route/Control
Processor
Constitutes the
data plane
Linecards (input)
Linecards (output)
1
1
2
2
Interconnect
Fabric
N
N
Challenges in Router Design

@ Line cards: destination lookups at high speed

e.g., find the longest prefix match (LPM) in the table that matches
the packet destination address

@ Switch fabric: head-of-line blocking, scheduling the
switch fabric at high speed

@ Route processor: complexity/correctness more a
problem than performance

You should understand why these challenges arise but
we don’t expect you to know how to fix them

e.g., specifics of scheduling algorithms or LPM lookups
Topics


Architecture: concepts and principles (lecs. 2, 3)
IP Routing (lecs. 4-9)






Transport (lecs. 10-13)




Overall context
Computing routes (DV, LS, path vector)
IP addressing
Inter-domain
Router architecture
Role of the transport layer
UDP vs. TCP
TCP details: reliability, flow and congestion control
Fairness and Router-assisted CC (lec. 13)
Role of the Transport Layer

Communication between application processes



Mux and demux from/to application processes
Implemented using ports
[Optionally] Provide common end-to-end services
for app layer


Reliable, in-order data delivery
Well-paced data delivery
UDP vs. TCP

Both UDP and TCP provide
multiplexing/demultiplexing via ports
UDP
Data
abstraction
Service
Applications
TCP
Packets (datagrams)
Stream of bytes of
arbitrary length
Best-effort (same as IP) •Reliability
•In-order delivery
•Congestion control
•Flow control
Video, audio streaming File transfer, chat
59
Reliable Transport:
General Concepts



Checksums (for error detection)
Timers (for loss detection)
Acknowledgments (feedback from receiver)




cumulative: “received everything up to X”
selective: “received X”
Sequence numbers (detect duplicates, accounting)
Sliding Windows
You should know:

for efficiency: to allow multiple
packets
in flight are
• what
these concepts
• why they exist
• how TCP uses them
Things to know about TCP






How TCP achieves reliability
RTT estimation
Connection establishment/teardown
Flow Control
Congestion Control
For each, know how the functionality is implemented
and why it is needed
E.g., RTT Estimation


Why? TCP uses timeouts to retransmit packets
But RTT may vary (significantly!) for different reasons and on
different timescales





due to temporary congestion
due to long-lived congestion
due to a change in routing paths
An incorrect RTT estimate might introduce spurious
retransmissions or overly long delays
RTT estimators should react to change but not too quickly

proposed solutions use EWMA, incorporate deviations
E.g., Reliability

Why? IP is best-effort but many apps. need reliable delivery


Having TCP take care of it simplifies application development
How






checksums and timers (for error and loss detection)
fast retransmit (for faster-than-timeout loss detection)
cumulative ACKs (feedback from receiver -- what’s lost/what’s not)
sliding windows (for efficiency)
buffers at sender (to hold packets while waiting for ACKs)
buffers at receiver (to reorder packets before delivery to app.)
E.g., Connection Establishment

Why?




TCP is a stateful protocol (CWND, buffer space, ISN, etc.)
Need to initialize connection state at both ends
Exchange initial sequence numbers (why?)
How? Three-way handshake




Host A sends a SYN to host B
Host B returns a SYN acknowledgment (SYN ACK)
Host A sends an ACK (+ data) to acknowledge the SYN ACK
Hosts exchange proposed Initial Sequence Numbers at each step
E.g., Flow Control

Why?






TCP offers a reliable in-order byte stream abstraction
Hence, TCP at the receiver must buffer a packet until all packets
before it (in byte-order) have arrived and the receiving application
has consumed available bytes
Hence receiver advances its window when the receiving application
consumes data
But sender advances its window when new data ACK’d
Hence, a risk the sender might overrun the receiver’s buffers
How? “Advertised Window” field in TCP header

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Receiver advertises the “right hand edge” of its window to sender
Sender agrees not to exceed this amount
E.g., Congestion Control
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Why?
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Because a sender shouldn’t overload the network itself
But yet, should make efficient use of available network capacity
While sharing available capacity fairly with other flows
And adapting to changes in available capacity
How?
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Study the TCP state machine diagram from the text
Also posted on piazza (thanks Andrew!)
TCP State Machine (partial)
timeout
slow
start
cwnd > ssthresh
congstn.
avoid.
timeout
new ACK
timeout
new ACK
dupACK=3
dupACK=3
dupACK
fast
recovery
new
ACK
Congestion Control: Details to know
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Slow Start:
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Congestion Avoidance:
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Why: discover available bandwidth from a cold start
How: exponential increase (every ACK, cwnd = cwnd + 1)
Why: adapt to variations in available bandwidth
How: AIMD (every ACK, cwnd = cwnd + 1/cwnd)
Why AIMD? For fairness
Fast Recovery:
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Keeps packets “in flight” after an isolated loss (why?)
How: artificially inflate the CWND on every duplicate ACK (for a
while)
CC and Fairness: More things you
should know
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TCP throughput equation
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Max-Min Fairness
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Definition, simple example of how to calculate
Fair Queuing in routers
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Implications
Pros and cons (router complexity, use in congestion control)
Router assisted congestion control
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pros (better info.) and cons (more complexity in routers)
e.g., explicit rate allocation (e.g., RCP): basic idea + pros/cons
e.g., explicit congestion notification (e.g., ECN) : basic idea only
Final Questions?
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Good luck!