Plan Ahead
5th week:
 Congestion control, TCP delay modeling
 Network protocols: IPv4, IPv6
6th week:
 network routing, routing in the Internet
7th week:
 Midterm
 Broadcast and multicast routing
Before final:
 Data link layer, Ethernet, switches, wireless networking
5/3/05
1
CS118/Spring05
Congestion Control
Congestion: “too many
sources sending too much
data too fast for network to
handle”
Scenario 1
 2 identical senders, 2
receivers, one router w/infinite
buffer, no retransmission
 when congested:
large delays;
maximum achievable
throughput
5/3/05
2
CS118/Spring05
Congestion: scenario 2
 one router, finite
buffers;
 senders retransmit
when timeout
R/2
R/2
R/4
lout
lout
lout
R/2
lin
lin
l
5/3/05
=
in
lout
R/2
lin
R/2
Data losses leads to
l' in > lout
3
R/2
retransmission of
delayed (not lost)
packet makes l' in
much larger than lout
CS118/Spring05
Congestion:
scenario 3
Host A
lin : original data
lout
l'in : original data, plus
retransmitted data
Q: what happens as l in
and l' in increase?
finite shared output
link buffers
Host B

• Long delays
• superfluous retransmissions
• when a packet is dropped, any
“upstream transmission capacity” used
for that packet was wasted!
5/3/05
4
CS118/Spring05
Approaches towards congestion control
Network-assisted congestion control: routers provide
feedback to end hosts
 Single bit congestion indication
 Explicit rate sender should send at
End-end congestion control: no explicit feedback from
network
 congestion inferred from end-system observed loss, delay

5/3/05
approach taken by TCP
5
CS118/Spring05
TCP Congestion Control
 Add a “congestion control window” congwin on top of flow-control
window
Congwin
recvwin
 Sender limits LastByteSent-LastByteAcked  CongWin
 How to adjust CongWin
 CongWin initialized to 1 mss, increase quickly until loss (= congestion)
 Upon loss: decrease congwin, then begin probing (increasing) again
 two “phases”: (1)slow start, (2)congestion avoidance
• threshold defines the boundary between the two
 How the sender infers congestion: Timeout, or 3 duplicate ACKs
5/3/05
6
CS118/Spring05
Basic idea: learn from observations
 when congwin < threshold, increase
congwin exponentially
 when congwin ≥ threshold, increase
congwin linearly
 if packet lost, have gone too far
 threshold = congwin / 2
 If 3 dup. ACKs: network capable of delivering some
packets, congwin cut in half
 If timeout: slow-start again (congwin = 1 mss)
 Additive Increase, Multiplicative Decrease
(AIMD)
5/3/05
7
CS118/Spring05
TCP SlowStart & Congestion Avoidance
RTT
initialize:
Congwin = 1
threshold = RcvWindow
if (CongWin < threshold)
{ for every segment ACKed
Congwin++
} until (loss event)
/* slowstart is over
*/
{ for every w segments ACKed:
Congwin++
} Until (loss event)
time
/* loss detected */
threshold = Congwin/2
If (3 dup. ACKs)
Congwin = threshold
Else
Congwin = 1 mss
5/3/05
8
CS118/Spring05
TCP sender congestion control
State
5/3/05
Event
TCP Sender Action
Commentary
Slow Start
(SS)
Received
CongWin = CongWin +
MSS,
ACK for
If (CongWin > Threshold)
previously
unacked data
set state to “Congestion
Avoidance”
Resulting in a doubling of
CongWin every RTT
Congestion
Avoidance
(CA)
Received
CongWin = CongWin+MSS *
(MSS/CongWin)
ACK for
previously
unacked data
Additive increase,
resulting in increase of
CongWin by 1 MSS
every RTT
SS or CA
Loss event
detected by 3
duplicate
ACK
Threshold = CongWin/2,
CongWin = Threshold,
Set state to “Congestion
Avoidance”
Fast recovery,
implementing
multiplicative decrease.
CongWin will not drop
below 1 MSS.
SS or CA
Timeout
Threshold = CongWin/2,
CongWin = 1 MSS,
Set state to “Slow Start”
Enter slow start
SS or CA
Duplicate
ACK
Increment duplicate ACK
count for segment being
acked
CongWin and Threshold
not changed
9
CS118/Spring05
Is TCP fair?
Fairness: if N TCP sessions share same bottleneck link, each should
get 1/N of link capacity
Example: 2 competing connections, same RTT
 Additive increase gives slope of 1
 multiplicative decrease decreases throughput proportionally
capacity R
equal bandwidth share
TCP connection 1
TCP conn 2
loss: decrease window by factor of 2
congestion avoidance: additive increase
congestion avoidance: additive increase
loss: decrease window by factor of 2
bottleneck
router
Connection 1 throughput R
5/3/05
10
CS118/Spring05
Fairness (more)
Fairness and parallel TCP
connections
 nothing prevents app from
opening parallel cnctions
 do not want rate throttled by
between 2 hosts.
congestion control
 Web browsers do this
 Instead use UDP:
 Example: link of rate R
 pump audio/video at
supporting 9 cnctions;
Fairness and UDP
 Multimedia apps often do
not use TCP
constant rate, tolerate
packet loss

 Research area: TCP

friendly
5/3/05
11
new app asks for 1 TCP, gets rate
R/10
new app asks for 11 TCPs, gets
R/2 !
CS118/Spring05
Delay modeling
Assumptions:
Q: How long does it take to
receive an object from a
Web server after sending
a request?
Ignoring congestion, delay
is influenced by:
 Assume one link between client
and server of rate R
 no retransmissions (no loss, no
corruption)
Window size:
 First assume: fixed congestion
window, W segments
 Then dynamic window,
modeling slow start
 TCP connection establishment
 data transmission delay
 slow start
5/3/05
12
CS118/Spring05
Fixed congestion window (1)
Notations:
S: #bits in one segment
O: #bits in one object
R: bandwidth
W: window size (# segments)
K: O/WS
Q: # times server idles if O=∞
P = min(Q, K-1)
First case: WS/R > S/R+RTT
ACK for first segment in
window returns before
window’s worth of data sent
delay = 2RTT + O/R
5/3/05
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CS118/Spring05
Fixed congestion window (2)
Second case:
 WS/R < RTT + S/R:
wait for ACK after
sending window’s
worth of data sent
delay = 2RTT + O/R + (K-1)[S/R + RTT - WS/R]
Server's waiting time
5/3/05
14
CS118/Spring05
TCP Delay Modeling: Slow Start (1)
Delay components:
• 2 RTTs for connection
establish and request
• O/R to transmit object
• Server's idle time
initiate TCP
connection
request
object
first window
= S/R
RTT
second window
= 2S/R
Server idles:
P = min{K-1,Q} times
Example:
• O/S = 15 segments
• K = 4 windows
•Q=2
• P = min{K-1,Q} = 2
Server idles P=2 times
5/3/05
third window
= 4S/R
fourth window
= 8S/R
complete
transmission
object
delivered
time at
server
time at
client
15
CS118/Spring05
TCP Delay Modeling: Slow Start (2)
Now suppose window grows according to slow start
The delay for one object is:
P
O
delay   2 RTT   idleTime p
R
p 1
P
O
S
k 1 S
  2 RTT   [  RTT  2
]
R
R
k 1 R
O
S
S
P
  2 RTT  P[ RTT  ]  (2  1)
R
R
R
5/3/05
16
CS118/Spring05
HTTP Modeling
 Assume Web page consists of:
1 base HTML page (of size O bits)
 M images (each of size O bits)
 Non-persistent HTTP:
 M+1 TCP connections in series
 Response time = (M+1)O/R + (M+1)2RTT + sum of idle times
 Persistent HTTP:
 2 RTT to request and receive base HTML file
 1 RTT to request and receive M images
 Response time = (M+1)O/R + 3RTT + sum of idle times
 Non-persistent HTTP with X parallel connections
 Suppose M/X integer.
 1 TCP connection for base file
 M/X sets of parallel connections for images.
 Response time = (M+1)O/R + (M/X + 1)2RTT + sum of idle times

5/3/05
17
CS118/Spring05
HTTP Response time (in seconds)
RTT = 100 msec, O = 5 Kbytes, M=10 and X=5
20
18
16
14
12
10
8
6
4
2
0
non-persistent
persistent
parallel nonpersistent
28
Kbps
100
1
10
Kbps Mbps Mbps
For low bandwidth, connection & response time dominated by
transmission time.
Persistent connections only give minor improvement over parallel
connections.
5/3/05
18
CS118/Spring05
HTTP Response time (in seconds)
RTT =1 sec, O = 5 Kbytes, M=10 and X=5
70
60
50
non-persistent
40
30
persistent
20
parallel nonpersistent
10
0
28
Kbps
100
1
10
Kbps Mbps Mbps
For larger RTT, response time dominated by TCP establishment
& slow start delays. Persistent connections now give important
improvement: particularly in high delay bandwidth networks.
5/3/05
19
CS118/Spring05
Network layer
 transport segment from sending to receiving host
 Source host: encapsulates segments into packets
 Destination host: delivers segments to transport layer
 network layer protocols in every host and router
 Each router examines header fields in all packets passing
through it
Routing: calculate the best path to each destination
 Forwarding: move packets from input to output

To transport protocol
segment
S
segment
Network
protocol
header
5/3/05
R
20
R
D
R
CS118/Spring05
Makeup lectures on Monday June 6
There will be no class on Thursday June 9
To make it up:
 8-9:50am Boelter 5422, or
 6-7:50pm Boelter 5419
Pick the lesser evil one
 Additional office hours on the final exam day:
Saturday June 11: 10:00AM - 1:00PM
And the Final exam is: 3:00 - 6:00PM
5/3/05
21
CS118/Spring05
Always keep the big picture in mind
host
host
HTTP message
HTTP
TCP segment
TCP
router
IP
Ethernet
interface
5/3/05
HTTP
IP packet
Ethernet
interface
IP
TCP
router
IP packet
SONET
interface
SONET
interface
22
IP
IP packet
Ethernet
interface
IP
Ethernet
interface
CS118/Spring05
Network layer:
Connection vs. connection-less service
 Virtual Circuit network provides connection-oriented
service

source-to-dest path works in a way much like telephone circuit
 Datagram network provides connectionless service
 The two services analogous to TCP vs. UDP at transport-
layer, but:
Network delivery service: host-to-host
 No choice: a given network provides one or the other but not
both (as in transport layer)

5/3/05
23
CS118/Spring05
Virtual circuit Network
 Use a signaling protocol to setup connection before data can flow
 every router on source-dest path maintains “state” for each
passing connection
 link, router resources (bandwidth, buffers) allocated to each VC
 each packet carries VC identifier (not destination host address)
 VC number must be changed on each link.

New VC number comes from forwarding table
application
transport
network
data link
physical
5/3/05
5. Data flow begins
6. Receive data
4. Call connected
1. Initiate call
3. Accept call
2. incoming call
24
application
transport
network
data link
physical
CS118/Spring05
Forwarding table
VC number
22
12
1
Forwarding table in
northwest router:
Incoming interface
1
2
3
1
…
3
32
2
interface
number
Incoming VC #
Outgoing interface
12
63
7
97
…
2
1
2
3
…
Outgoing VC #
22
18
17
87
…
Routers maintain connection state information!
5/3/05
25
CS118/Spring05
Internet: A Datagram Network
 hosts are connected to subnets
 subnets are interconnected by IP routers
 All hosts and routers speak IP
 routers also “speak” many different link layer protocols
 IP provides two basic functions
 globally unique address for all connected points
 Best effort datagram delivery from source to destination hosts
• Fragmentation/reassembly of packets whenever needed
H1
IP
IP
ETH
5/3/05
R2
R1
ETH
IP
FDDI
IP
WLAN
FDDI
26
H8
R3
WLAN
IP
ETH
ETH
CS118/Spring05
The Internet Network layer
Host, router network layer functions:
Transport layer: TCP, UDP
Routing protocols
•RIP, OSPF, BGP …
Network
layer
IP protocol
•addressing conventions
•datagram format
•packet handling conventions
forwarding
table
ICMP protocol
•error reporting
•router “signaling”
Link layer
Router
function
5/3/05
physical layer
27
CS118/Spring05
IP datagram format
IP version number
header length
type of
ver head.
len service
Total length
fragment
16-bit identifier flgs
offset
time to
IP header
protocol
live
checksum
source IP address
upper layer protocol
to deliver payload to
how much overhead
for a TCP segment?
 20 bytes of TCP
 20 bytes of IP
 = 40 bytes
5/3/05
destination IP address
E.g. timestamp,
route recording,
Specify list of
routers to visit.
basic header
3 fields used for packet
fragmentation/reassembly
max number
of remaining hops
32 bits
Options (if any)
data
(variable length,
typically a TCP
or UDP segment)
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CS118/Spring05
IP Address structure
•32-bits, uniquely identifies a host or router interface
–interface: connection between host/router and physical link
 IP address space: 2-level hierarchy
173.1.1.1 = 10101101 00000001 00000001 00000001
173
4 byte
Network-ID
1
1
173.1.1.1
host-ID
173.1.2.1
173.1.1.2
173.1.1.4
 What’s a network ? (from IP address
173.1.1.3
perspective)
 device interfaces with same network
part of IP address
 can physically reach each other without
going thru a router
5/3/05
1
173.1.2.9
173.1.3.27
173.1.2.2
LAN
173.1.3.1
29
173.1.3.2
CS118/Spring05
IP Address: how many bits for net-ID
 Original IP design: class-based address
A
0 network
B
10
C
110
D
1110
1.0.0.0 to
127.255.255.255
host
network
128.0.0.0 to
191.255.255.255
host
network
host
multicast address
192.0.0.0 to
223.255.255.255
224.0.0.0 to
239.255.255.255
 Two changes added over the last 25 years
 Subnetting: add a hidden level to address hierarchy
• An organization gets one address block, then split the host
part into two parts: subnet and host parts
Network ID

Host ID
CIDR: Classless InterDomain Routing (today)
• network portion of address of arbitrary length
5/3/05
30
CS118/Spring05
Classless InterDomain Routing
 address format: a.b.c.d/x, x  # bits in network portion
network
part
200.23.16.0/23
host
part
11001000 00010111 00010000 00000000
 Internet Service Providers get blocks of IP addresses from
the Internet address authority
 Internet customers get portion of their ISP’s addr. block
ISP's block
11001000 00010111 00010000 00000000
200.23.16.0/20
Organization 0
11001000 00010111 00010000 00000000
200.23.16.0/23
Organization 1
11001000 00010111 00010010 00000000
200.23.18.0/23
Organization 2
...
11001000 00010111 00010100 00000000
…..
….
200.23.20.0/23
….
Organization 7
11001000 00010111 00011110 00000000
200.23.30.0/23
5/3/05
31
CS118/Spring05
Hierarchical addressing: route aggregation
Hierarchical addressing allows efficient advertisement of routing
information:
Organization 0
200.23.16.0/23
Organization 1
200.23.18.0/23
Organization 2
200.23.20.0/23
Organization 7
.
.
.
.
.
.
Fly-By-Night-ISP
“Send me anything
with addresses
beginning
200.23.16.0/20”
Internet
200.23.30.0/23
ISPs-R-Us
5/3/05
32
“Send me anything
with addresses
beginning
199.31.0.0/16”
CS118/Spring05
Hierarchical addressing: route aggregation
 Route aggregation helps reduce routing table size
 multi-homing defeats address aggregation
 ISPs-R-Us has a more specific route to Org. 7
Organization 0
200.23.16.0/23
Organization 1
“Send me anything with
addresses beginning
200.23.16.0/20”
200.23.18.0/23
Organization 2
200.23.20.0/23
Organization 7
.
.
.
.
.
.
Fly-By-Night-ISP
Internet
200.23.30.0/23
Multi-homing
5/3/05
ISPs-R-Us
33
“Send
me anything
anythingwith
with
“Send me
addresses
beginning
addresses beginning
199.31.0.0/16”
199.31.0.0/16, or
200.23.30.0/23 ”
CS118/Spring05
IP Subnet
 subnet mask: indicates the portion of the address that is
considered as “network ID” by the local site
 subnet
mask does not need to align with a byte boundary
11111111111111111111110000000000
Host ID
Network ID
Viewed from outside
10-bit host ID
Viewed from inside
 Each host must be configured with both an IP address and a
subnet mask
 subnets are invisible outside of the local site
 backbone
routers only know how to forward packets to the
networkID
 Within the organization, routers store: [subnet, mask, next hop]
 Subnet advantages: aggregate local info., keep backbone
routers table size small
5/3/05
34
CS118/Spring05
An example
UCLA CS
Network#
mask
next-hop
131.179.96 255.255.255.0
C
……
………..
Global Internet
Look up IP addr. A
131.179.96.15
B
131.179.96.0
C
Network# next-hop
131.179
B
131.179.96.15
a class-B address
Network ID
host ID
subnet mask(255.255.255.0) 111111111111111111111111 00000000
subnetted address
5/3/05
131 . 179 . 96
35
15
CS118/Spring05
Getting an IP packet from source to dest.
Source host A  destination B:
 Host A: [A’s addr & subnet mask] ═ [B’s addr & subnet mask] ?
 yes: B is on the same net, use link layer to send pkt directly to B
Source host Adestination E:
 [A’s addr & subnet mask] = [E’s
addr & subnet mask] ?
A

yes

No: Send pkt to default router
173.1.1.4
173.1.1.2
173.1.1.4
173.1.1.3
directly connect subnets?
 Yes: send pkt directly to E
 No: forward to another router
according to routing table
5/3/05
173.1.2.1
B
Router: Is E on any of my
173.1.1.1
173.1.3.1
36
173.1.2.9
173.1.3.27
173.1.2.2
E
173.1.3.2
CS118/Spring05
IP Fragmentation & Reassembly
 Different subnets have different
MTUs (Maximum Transmission
Unit)
 Sender host always uses its max MTU
H1 sending an IP packet of
1300 byte data to H2:
MTU=1500B
R1
size
 Routers “fragment” IP packets if the
next link has a smaller MTU
 chop packets to the MTU size of next
link
 further fragmentation down the path
possible
 packet reassembled at dest. host
R2
H1
1300B
reassembly
H2
512B
276B
R3
MTU=532B
5/3/05
37
CS118/Spring05
IP Fragmentation: An example
4
5
TOS
1320
000
0
7394
rest of the IP header
data (1300 bytes)
4
5
TOS
532
001
0
7394
rest of the IP header
H2
5
TOS
532
001
64
7394
rest of the IP header
276B
MTU=532B
At destination:
- identifier: tell all pieces in the
same packet
- the last fragment: MF=0
- the offsets tell whether there
are holes missing in the middle
5
TOS
296
000
128
7394
rest of the IP header
data (276 bytes)
5/3/05
512B
R3
data (512 bytes)
4
1300B
reassembly
data (512 bytes)
4
R2
H1
38
CS118/Spring05
ICMP: Internet Control Message Protocol
 used by hosts & routers to
communicate networklevel information
error reporting: unreachable
host, network, port, protocol
 echo request/reply

 ICMP msgs carried in IP
packets
 ICMP message format
IP header
type
code
checksum
unused (or used by certain ICMP types)
IP header and first 64bits of data
Or
data (according to ICMP types)
5/3/05
39
Type
0
3
3
3
3
3
3
4
Code
0
0
1
2
3
6
7
0
8
9
10
11
12
0
0
0
0
0
description
echo reply (ping)
dest. network unreachable
dest host unreachable
dest protocol unreachable
dest port unreachable
dest network unknown
dest host unknown
source quench (congestion
control - not used)
echo request (ping)
route advertisement
router discovery
TTL expired
bad IP header
CS118/Spring05
NAT: Network Address Translation
rest of
Internet
local network
(e.g., home network)
10.0.0/24
10.0.0.4
10.0.0.1
10.0.0.2
138.76.29.7
10.0.0.3
All datagrams leaving local
network have same single source
NAT IP address: 138.76.29.7,
different source port numbers
5/3/05
Datagrams with source or
destination in this network
have 10.0.0/24 address for
source, destination (as usual)
42
CS118/Spring05
NAT: Network Address Translation
rest of
Internet
local network
(e.g., home network)
10.0.0/24
10.0.0.4
10.0.0.1
10.0.0.2
138.76.29.7
10.0.0.3
All datagrams leaving local
network have same single source
NAT IP address: 138.76.29.7,
different source port numbers
5/3/05
Datagrams with source or
destination in this network
have 10.0.0/24 address for
source, destination (as usual)
43
CS118/Spring05
NAT: Network Address Translation
2: NAT router
changes datagram
source addr from
10.0.0.1, 3345 to
138.76.29.7, 5001,
updates table
2
NAT translation table
WAN side addr
LAN side addr
138.76.29.7, 5001 10.0.0.1, 3345
……
……
S: 10.0.0.1, 3345
D: 128.119.40.186, 80
S: 138.76.29.7, 5001
D: 128.119.40.186, 80
1
10.0.0.4
138.76.29.7
S: 128.119.40.186, 80
D: 138.76.29.7, 5001
S: 128.119.40.186, 80
D: 10.0.0.1, 3345
3
10.0.0.1
10.0.0.2
4
10.0.0.3
4: NAT router
changes datagram
dest addr from
138.76.29.7, 5001 to 10.0.0.1, 3345
3: Reply arrives
dest. address:
138.76.29.7, 5001
5/3/05
1: host 10.0.0.1
sends datagram to
128.119.40, 80
44
CS118/Spring05
NAT implementation
 NAT router must do the following:
 outgoing datagrams: replace (source IP address, port #) of
every outgoing datagram to (NAT IP address, new port #)
• . . . remote clients/servers will respond using (NAT IP address, new port
#) as destination addr.
remember (in NAT translation table) every (source IP address,
port #) to (NAT IP address, new port #) translation pair
 incoming datagrams: replace (NAT IP address, new port #) in
destination fields of every incoming datagram with
corresponding (source IP address, port #) stored in NAT table

 Problems due to NAT
 Increased network complexity, reduced robustness
 Cannot run services from inside a NAT box
 address shortage should instead be solved by IPv6
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IPv6
 Motivation: 32-bit address space exhaustion
 Take the opportunity for some clean-up
 IPv6 datagram format:
 Address length changed from 32 bits to 128 bits
 fragmentation fields moved out of base header
 IP options moved out of base header
• Header Length field eliminated
 Header
Checksum eliminated
 Type of Service field eliminated
 Time to Live Hop Limit, Protocol  Next Header
 Precedence  Priority, added Flow Label field
 Length field excludes IPv6 header
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IPv6 header format
Flow Label
Version Priority
Payload Length
Next Header
Hop Limit
Source Address (16 bytes, 128 bits)
Destination Address (16 bytes)
IPv4
Version Hdr Len Prec
header
Identification
Time to Live
TOS
Total Length
Flags
Protocol
Fragment Offset
Header Checksum
Source Address
Destination Address
Options
Padding
32 bits
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Changes from IPv4
 Priority: identify priority among datagrams in flow
 Flow Label: identify datagrams in same “flow” (concept
of“flow” not well defined).
 Next header: identify upper layer protocol for data
 Options: allowed, but outside of the basic header,
indicated by “Next Header” field
 Checksum: removed entirely to reduce processing time
at each hop
 ICMPv6: new version of ICMP
additional message types, e.g. “Packet Too Big”
 multicast group management functions

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Transition From IPv4 To IPv6
 Not all routers can be upgraded simultaneous
 to allow the Internet operate with mixed IPv4 and IPv6
routers : tunneling
Logical view:
E
F
IPv6
IPv6
A
B
IPv6
IPv6
A
B
C
D
E
F
IPv6
IPv6
IPv4
IPv4
IPv6
IPv6
tunnel
Physical view:
Flow: X
Src: A
Dest: F
data
A-to-B:
IPv6
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Src:B
Dest: E
Src:B
Dest: E
Flow: X
Src: A
Dest: F
Flow: X
Src: A
Dest: F
data
data
B-to-C:
IPv6 inside
IPv4
49
B-to-C:
IPv6 inside
IPv4
Flow: X
Src: A
Dest: F
data
E-to-F:
IPv6
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Interplay between routing and forwarding
routing algorithm
local forwarding table
header value output link
0100
0101
0111
1001
3
2
2
1
value in arriving
packet’s header
0111
1
3 2
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Router Architecture Overview
Two key router functions:
 run routing algorithms/protocol (RIP, OSPF, BGP)
 forwarding datagrams from incoming to outgoing link
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Input Port Functions
Physical layer:
bit-level reception
Data link layer:
e.g., Ethernet
see chapter 5
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Decentralized switching:
 given datagram dest., lookup output port using
forwarding table in input port memory
 goal: complete input port processing at ‘line
speed’
 queuing: if datagrams arrive faster than
forwarding rate into switch fabric
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Output Ports
 Buffering required when datagrams arrive from fabric
faster than the transmission rate
 Scheduling discipline chooses among queued datagrams
for transmission
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