Understanding Networked Applications:
A First Course
Chapter 19
by
David G. Messerschmitt
Understanding Networked Applications:
A First Course
Protocol architecture
by
David G. Messerschmitt
Summary
• Network architecture
• Internet protocols
• Some network security issues
Understanding Networked Applications
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A First Course
Key ideas
• Packet encapsulation: one packet can be
encapsulated in another
• Packet fragmentation: one packet can be
split into two or more packets and
encapsulated
• Protocol layering: one service can be
layered on another by fragmentation and
encapsulation
Understanding Networked Applications
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A First Course
Original packet
New
header
Packet encapsulated in another packet
Original packet
(including its header)
is payload of new packet
Encapsulated again
Encapsulated packet
Original packet
Understanding Networked Applications
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A First Course
Header
Payload
Original packet
Divide into packet fragments
New packets
Reassembled original packet
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A First Course
Layer n+1 protocol header
Layer n+1
Layer n+1
Peer-to-peer communication
Layer n
Layer n
protocol header
Understanding Networked Applications
Layer n
Encapsulated layer n+1
packet, including header
7
A First Course
Layered protocol modularity
• Layer n+1 protocol is logically peer-to-peer
• Layer n+1 protocol depends on services of layer n
• Layer n+1 packets are encapsulated in layer n
packets
• Layer n+1 never sees layer n packet headers
• Layer n ignores layer n+1 packet headers (part of
payload)
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A First Course
Key ideas (again)
• Packet encapsulation: one packet can be
encapsulated in another
• Packet fragmentation: one packet can be
split into two or more packets and
encapsulated
• Protocol layering: one service can be
layered on another by fragmentation and
encapsulation
Understanding Networked Applications
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A First Course
IETF/OMG Layers
Layering
Physical
Logical
Application
Application
ORB
ORB
IIOP
IIOP
TCP or UDP
TCP or UDP
IP
Network 1
IP
N2
Network 2
Switch
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Host B
Host A
N1
IP
A First Course
Internet: logical view
Application
TCP or UDP
Application
Peer-to-peer
Host A
TCP or UDP
Host B
The constituent networks are not visible to the
application; TCP/UDP does not reside in packet
switches
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A First Course
Internet: physical view
IP
Network 1
Host A
IP
N1
IP
N2
Network 2
Host B
Switch
IP serves to connect heterogeneous
networks into an internetwork
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A First Course
What IP does do
• Allow packets to traverse multiple networks
• Deliver packet to specified destination host
• Best effort: deliver as reliably and as soon
as it can
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A First Course
What IP Doesn’t Do
• Guarantee latency for packets that are delivered
• Guarantee delivery, or notify source host if packet is not
delivered
• Guarantee order of delivery
• Guarantee integrity of packet payload
• Maintain conversational context (each packet is
independent)
• Specify what process that should receive the packet at
destination host
Understanding Networked Applications
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A First Course
IP header
Version Priority
PayloadLen
FlowLabel
NextHeader
SourceAddress
HopLimit
DestinationAddress
32 bits
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A First Course
Transport services: UDP and TCP
• Direct packet to a particular process
• UDP adds:
– Payload integrity for packets delivered
• TCP adds:
– Reliable delivery of bytestream session
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A First Course
Comparison of services
Host
Process
IP: host-tohost
Host
Process-to-
TCP
process
UDP
Process
IP: Best-effort datagram
UDP: best-effort datagram with payload integrity

byte
byte
byte
byte
byte
TCP: reliable bi-directional bytestream
Understanding Networked Applications
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byte

A First Course
UDP/TCP ports
(publish/subscribe)
Port
Port
Process
Process
IP: host-tohost
Host
Host
SourcePort DestinationPort
(rest of UDP or TCP header)
Encapsulated
in IP packet
32 bits
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A First Course
HTTP Service
• Client can make requests
– GET (pull)
– POST (push)
– (some others)
• Server responds
– HTTP headers
– HTML document
• or JPEG, or GIF, or…
Understanding Networked Applications
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A First Course
URL Structure
• <scheme>://<host>:<port>/<path>
• Scheme
– HTTP, FTP, GOPHER, MAILTO, ...
• Host
– An IP address or DNS name
• Port
– TCP port number
– Optional (defaults to 80 for http)
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A First Course
HTTP example
• When a browser fetches
http://www.si.umich.edu/~presnick/
– http:// says to use HTTP protocol
– Resolve www.si.umich.edu in DNS
• 141.211.203.34
– Make TCP connection
• 141.211.203.34, port 80
– Send the following text string
• GET /~presnick/
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A First Course
Server sends back
HTTP/1.1 200 OK
Date: Mon, 22 Dec 1997 15:12:32 GMT
Server: Apache/1.2.4
Last-Modified: Thu, 04 Dec 1997 16:26:10 GMT
ETag: "5f2f2-33fd-3486d9a2"
Content-Length: 13309
Accept-Ranges: bytes
Connection: close
Content-Type: text/html
<HTML> ….
Understanding Networked Applications
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A First Course
HTML
<H1>
Paul Resnick</H1>
<IMG SRC="RESNICK.gif" ALT="[PHOTO]" HSPACE=10 ALIGN=LEFT>
<BR>Associate Professor
<BR>University of Michigan
<BR>School of Information
<BR>314 West Hall
<BR>550 East University Avenue
<BR>Ann Arbor, MI 48109-1092
<BR>presnick@umich.edu
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A First Course
What Browsers Send to Servers
• Your IP address
• The browser type
• The refer link
– What URL you last looked at
• Cookies (persistent client state for a URL)
– Server response can include a set-cookie header
– Browser saves the cookie
– Browser resends to server next time
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A First Course
Understanding Networked Applications:
A First Course
How TCP works
by
David G. Messerschmitt
Aggregating byte stream
Original bytes are aggregated and….

byte
byte
byte
byte
byte
byte

….encapsulated in TCP packets, with
a sequence number included in the TCP header

byte byte
byte byte byte
byte byte
byte byte byte

The TCP packets are encapsulated in IP packets
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A First Course
TCP connections
• TCP establishes a session with ordered and
bidirectional reliable delivery of bytes
– Establishment:
• Inform receiving port of connection
• Initialize packet sequence number
• Congestion and flow control state
– Disestablishment
• By either peer
• Free state and resources
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A First Course
HTTP uses TCP
• Often have to request another page
– e.g., image
• HTTP/1.0 requires a new TCP session for
each
– Overhead of session establishment
• HTTP/1.1 permits reuse of one TCP session
for multiple requests
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A First Course
Reliable packet delivery:
acknowledgement and resending
Destination
ACK
IP is used to
send TCP
packets and
return ACK
Understanding Networked Applications
Source
Timeout
Source
Destination

IP loses packets
ACK
29
A First Course
Source
Source
Destination
Destination
Task
ACK
Concurrent
tasks for
higher
throughput
ACK
ACK
ACK
Packets can
be reordered
using sequence
number
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ACK
A First Course
TCP Congestion
• If link is congested
– Switch queue for that link fills up
– Drops packets
• Source resends non-ACK’ed packets
– Makes congestion worse
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A First Course
TCP Congestion Control
• Voluntary source-imposed policy
– Source controls the number of non-ACK’ed
packets that have been sent
• Controls the number of concurrent sends, and hence
packet throughput
– Slow start, slowly increase rate
– Monitor non-ACK’s and delay of ACK’s to
estimate congestion
– Quickly decrease if congestion detected
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A First Course
TCP congestion control flaws
• “Fairness criterion”
– Maybe “equal division” of resources is not what is
wanted
• Estimating congestion by retransmission is flawed
for wireless links
• Depends on accurate implementation -- cheating
possible
• Application can avoid congestion control by using
UDP
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A First Course
TCP Flow Control
• Recall that this is to avoid recipient from
being overwhelmed
• Recipient must control source
• Recipient explicitly requests lower send rate
– MaxUnackedPackets is a parameter of ACK’s
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A First Course
Multicasting
• So far, we’ve assumed node A sends to B
• Multicasting: node A sends same message to B, C, and D
– Could set up A-B, A-C, and A-D connections
– But A becomes a bottleneck
• Handling ACKS and resends
– And it’s inefficient
• Some intermediate nodes may receive the message several times
• Multicast protocols try to have A send only once
– Intermediate nodes do more work
B
A
C
D
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A First Course
Understanding Networked Applications:
A First Course
Domain Name System
by
David G. Messerschmitt
Hosts
NS
NS
“root”
eecs.berkeley.edu
NS
NS
sims.berkeley.edu
berkeley.edu
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A First Course
Delegate name search to local
name server
tj.watson.ibm.com
“root”
NS
info.sims.berkeley.edu
berkeley.edu
NS
watson.ibm.com sims.berkeley.edu
Local server caches
recent search results
NS
sims.berkeley.edu
NS
berkeley.edu
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A First Course
Understanding Networked Applications:
A First Course
Supplements
by
David G. Messerschmitt
Understanding Networked Applications:
A First Course
Network security
by
David G. Messerschmitt
Network security
• Some things to worry about:
– Sniffing
– Spoofing
– Security flaws in public servers
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A First Course
Improving security
• Security tools covered earlier
• Firewalls: a place where security policies
can be enforced
–
–
–
–
–
Who gains access
What servers (ports) can be accessed
What hosts can be accessed
What protocols can pass
Other security policies can be enforced
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A First Course
Public hosts
Global Internet
Firewall
Bastion
hosts
Second
firewall
Internal
hosts
Protected enclave
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A First Course
Problems with firewalls
• “Benign internal users” assumption is naïve
• Obstacle to deployment of innovative applications
and services
• Increasingly organizations want to extend extranet
to suppliers and customers
• Solution: resource-based rather than enclave-based
security
– Analogy: border patrol not enough, need secure
buildings and vehicles, guards, police, etc.
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A First Course
Where to use encryption
•
•
•
•
•
Per link (wireless)
Firewall-to-firewall (extranet)
Host-to-host (IPsec)
Process-to-process (TCP-SSL)
Application
Understanding Networked Applications
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What are some
strengths and
weaknesses in
these approaches?
A First Course
Packet structure
Header:
Payload:
Data for application
Information for switches
Ignored by network and
protocol
Serves as protocol message
(Qualification: may also
be encapsulated packet)
Packet length limited by
network policy
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A First Course
Protocol endpoints
Internetworking layer focuses
on getting datagrams from one
host to another
Host
Process
IP: host-tohost
Host
Process-to-
TCP
process
UDP
Process
Transport layer focuses on process-to-process
communication services
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A First Course
Addresses vs. names
Address specifies
topological location of
host to the network
128 bits (Network,host)
info.sims.berkeley.edu
Understanding Networked Applications
Name is easy to remember
or construct and reflects
administrative boundaries
48
A First Course
Issues in congestion control
• Social issue: how do we divide limited
network resources among
users/applications?
• Approaches:
– Voluntary (e.g. UDP)
• “Bad citizen” is rewarded
– Policy driven (e.g. TCP)
– Incentivized (e.g. pricing)
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A First Course
Advantages of pricing-based
congestion control
• Policies can never take into account the
“importance” of traffic
• Users and applications are forced to consider the
common resource implications of their actions
• Users and applications can choose the most
“important” traffic for periods of congestion
– Shift other traffic to off-peak times
• Source of revenue to expand capacity
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A First Course
Technical approaches to
congestion control
• Source-driven throttle algorithm
– Voluntary, policy, or incentive driven
• Network-driven
– Must use fairness criteria
– Network-to-source flow control
– Network access enforcement (policing)
• Traffic priorities allow source to control what traffic
is discarded
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A First Course
Undesirability of fixed pricing
per unit of capacity
Willingness to pay for
one more unit of capacity
In reducing capacity from c to c’
Revenue gained
Revenue lost
D(c)
p
c’ c
Understanding Networked Applications
Capacity
cmax
52
A First Course
Downsides of pricing
• Infrastructure for
–
–
–
–
Usage monitoring
Congestion monitoring
QoS configuration
Billing
• Operational costs
• How do costs compare to the benefits?
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A First Course