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Chapter 3
TCP/IP Architecture
Advance Features
Computer Networking:
A Top Down Approach
5th edition.
Jim Kurose, Keith Ross
Addison-Wesley, April
2009.
TCP/IP Architecture Advanced Features
1
Chapter 3: Network Layer.
Associated Protocols
 Chapter goals:
 understand congestion control in TCP
 understand advanced principles behind network
layer services:
•
•
•
•
•
address translation
routing (path selection)
network layer error control
IPv4 address dynamic configuration
Advanced topics: IPv6.
TCP/IP Architecture Advanced Features
2
Chapter 2 outline
 3.1 TCP congestion
control
 3.2 Address
Translation: NAT
 3.3 Routing in the
Internet



 3.4 IPv4 error control:
ICMP
 3.5 IPv4 address
dynamic configuration:
DHCP
 3.6 IP version 6
RIP
OSPF
BGP
TCP/IP Architecture Advanced Features
3
Chapter 2 outline
 3.1 TCP congestion
control
 3.2 Address
Translation: NAT
 3.3 Routing in the
Internet



 3.4 IPv4 error control:
ICMP
 3.5 IPv4 address
dynamic configuration:
DHCP
 3.6 IP version 6
RIP
OSPF
BGP
TCP/IP Architecture Advanced Features
4
Principles of Congestion Control
Congestion:
 informally: “too many sources sending too much
data too fast for network to handle”
 different from flow control!
 manifestations:


lost packets (buffer overflow at routers)
long delays (queuing in router buffers)
TCP/IP Architecture Advanced Features
5
Causes/costs of congestion
Host A
 Two tx:



lIN = Sending rate
lOUT = Receiving
rate
C= Router capability
Host B
lout
lin : original data
unlimited shared
output link buffers
 Queue theory
 large delays
when congested
 maximum
achievable
throughput
TCP/IP Architecture Advanced Features
6
Causes/costs of congestion:
 Supposition: Tx retx lost packets
 Now, lIN is not tx -> higher rate lIN’
 Congestion costs:


more work (retx)
unneeded retransmissions: link carries multiple copies of pkt
Host A
Host B
lout
lin : original data
unlimited shared
output link buffers
TCP/IP Architecture Advanced Features
7
Causes/costs of congestion
 Two tx and the same dest
 Supposition: congestion in the router between A and dest
 Another cost of congestion:

when a packet is dropped, any upstream transmission capacity
used for that packet was wasted
Host A
lin : original data
lout
l'in : original data, plus
retransmitted data
finite shared output
link buffers
Host B
TCP/IP Architecture Advanced Features
8
Causes/costs of congestion
 Congestion costs: a summary
 Big delays when the incoming rate to a router is near to the
link capability.
 The tx must retx the lost segments due to the router
overflow.
 Unnecessary BW is used for the retx of duplicated copies of
segments (due to delays).
 Upstream transmission capacity used for a packet is wasted
when it is discarded.
TCP/IP Architecture Advanced Features
9
Approaches towards congestion control
Two broad approaches towards congestion control:
Network-assisted
congestion control:
End-end congestion
control:
 no explicit feedback from
network
 congestion inferred from
end-system observed loss,
delay
 approach taken by TCP
 routers provide feedback
to end systems
 E.g. single bit indicating
congestion
TCP/IP Architecture Advanced Features
10
TCP Congestion Control
Host A
 sender limits
Host B
Host A
Host B
LastByteSent-LastByteAcked 
CongWin
 How does sender
timeout
transmission:
perceive congestion?

X
loss
loss event = timeout or 3
duplicate acks
TCP sender reduces rate
(CongWin) after loss
event
timeout

X
time
lost ACK
scenario
time
Resending a segment
after triple duplicate
ACK
TCP/IP Architecture Advanced Features
11
TCP congestion control:
additive increase,
multiplicative decrease (AIMD)
 Approach: increase transmission rate (window size),
Saw tooth
behavior: probing
for bandwidth
congestion window size
probing for usable bandwidth, until loss occurs
 additive increase: increase CongWin by 1 MSS
every RTT until loss detected
 multiplicative decrease: cut CongWin in half after
loss
congestion
window
24 Kbytes
16 Kbytes
8 Kbytes
time
time
TCP/IP Architecture Advanced Features
12
TCP Congestion Control: details
 three phases:
 Roughly,
rate =

CongWin
Bytes/sec
RTT
 When connection begins,
CongWin = 1 MSS


Example: MSS = 500 bytes & RTT
= 200 ms
initial rate = 20 kbps


slow start (SS)
Congestion avoidance (CA):
e.g. AIMD
Fast recovery (FR)
 First two are compulsory in
TCP, while the last one is
recommendable
TCP/IP Architecture Advanced Features
13
TCP Slow Start
 available bandwidth may
Host B
RTT
be >> MSS/RTT
 When connection begins,
increase rate
exponentially fast until
first loss event
 initial rate is slow but
ramps up exponentially
fast
Host A
time
TCP/IP Architecture Advanced Features
14
Congestion avoidance
 After 3 dup ACKs:
 CongWin is cut in half
 window then grows linearly
 But after timeout event:
 CongWin instead set to 1
MSS;
 window then grows
exponentially
 to a threshold, then grows
linearly
Philosophy:
 3 dup ACKs indicates
network capable of
delivering some segments
 timeout indicates a “more
alarming” congestion scenario
TCP/IP Architecture Advanced Features
15
Summary: TCP Congestion Control

When CongWin is below Threshold, sender in slow-start phase,
window grows exponentially.

When CongWin is above Threshold, sender is in congestion-avoidance
phase, window grows linearly.

When a triple duplicate ACK occurs, Threshold set to CongWin/2 and
CongWin set to Threshold.

When timeout occurs, Threshold set to CongWin/2 and CongWin is
set to 1 MSS.
Note: TCP version varies the way
congestion control is applied. For
example, TCP Tahoe always reduces
Congestion Window to its initial
value after a loss event. Meanwhile,
in TCP Reno, all the congestion
phases take place.
Transport Layer Advanced Features
16
Chapter 2 outline
 3.1 TCP congestion
control
 3.2 Address
Translation: NAT
 3.3 Routing in the
Internet



 3.4 IPv4 error control:
ICMP
 3.5 IPv4 address
dynamic configuration:
DHCP
 3.6 IP version 6
RIP
OSPF
BGP
TCP/IP Architecture Advanced Features
17
Address Translation: NAT
 Problem: limited number of IP addr.
 Solutions
o Subnetting
o Private IP addr
• 10.0.0.0/8
• 172.16.0.0/12
• 192.168.0.0/16
• 169.254.0.0/16
TCP/IP Architecture Advanced Features
18
Address Translation: NAT
 Private IP addr: how can the dest know whose is
the private IP addr?
 Solution: NAT (Network Address Translation)
o Mechanism used to modify the IP addr inside pkts
o Allows private IP addr (inside networks) communication
with public IP addr (outside networks)
o NAT routers must keep a NAT table with translation in
both directions
TCP/IP Architecture Advanced Features
19
NAT basic functioning
NAT Router has one or more IP addr for NAT -> for translating private IP
into public IP
 NAT Router modifies “Source IP addr” field in the IP header & stores
matching public & pvt addr in the NAT Table
 Dest responds to modified addr
 NAT Router searches on its NAT Table & matches public & pvt addr,
sending the pkt to the pvt IP

Source addr
192.168.1.10
Dest addr
150.214.141.20
Source addr
Dest addr
150.214.141.2
150.214.141.20
IP packet
IP packet
Inside network
Outside network
NAT Router
192.168.1.1
150.214.141.1
NAT Table
Host A
192.168.1.10
Source addr
150.214.141.20
Dest addr
Host Z
150.214.141.20
150.214.141.2
IP packet
TCP/IP Architecture Advanced Features
20
Types of NAT
 Dynamic NAT
 Static NAT
 NAPT (Network Address Port Translation)
TCP/IP Architecture Advanced Features
21
Dynamic NAT
 Translation is unidirectional



Traffic comes from the inside network
Translation is temporary
Once public IP addr is not in use, it is deleted from NAT Table
Source addr
192.168.1.10
Dest addr
150.214.141.20
Source addr
Dest addr
150.214.141.2
150.214.141.20
IP packet
IP packet
Inside network
Outside network
NAT Router
192.168.1.1
150.214.141.1
NAT Table
Private IP
192.168.1.10
Public IP
150.214.141.2
Host A
192.168.1.10
Source addr
150.214.141.20
Host Z
150.214.141.20
Dest addr
150.214.141.2
IP packet
Source addr
150.214.141.20
Dest addr
150.214.141.2
IP packet
TCP/IP Architecture Advanced Features
22
Dynamic NAT

Advantage
 Saves public IP addr

Disadvantage
 Traffic always initiated from the inside network -> does not allow servers
Source addr
192.168.1.10
Dest addr
150.214.141.20
Source addr
Dest addr
150.214.141.2
150.214.141.20
IP packet
IP packet
Inside network
Outside network
NAT Router
192.168.1.1
150.214.141.1
NAT Table
Private IP
192.168.1.10
Public IP
150.214.141.2
Host A
192.168.1.10
Source addr
150.214.141.20
Host Z
150.214.141.20
Dest addr
150.214.141.2
IP packet
Source addr
150.214.141.20
Dest addr
150.214.141.2
IP packet
TCP/IP Architecture Advanced Features
23
Static NAT
Translations are in the NAT Table since routers are configured
 Communication may be initiated by both the inside & the outside networks
 Public IP addr must be known (via DNS) by the hosts in the outside network

Source addr
192.168.1.10
Dest addr
150.214.141.20
Source addr
Dest addr
150.214.141.2
150.214.141.20
IP packet
IP packet
Outside network
Inside network
NAT Router
192.168.1.1
150.214.141.1
NAT Table
Private IP
192.168.1.10
Public IP
150.214.141.2
static
Host A
192.168.1.10
Source addr
150.214.141.20
Host Z
150.214.141.20
Dest addr
150.214.141.2
IP packet
Source addr
150.214.141.20
Dest addr
150.214.141.2
IP packet
TCP/IP Architecture Advanced Features
24
Static NAT

Advantage
 Allows servers

Disadvantage
 One public IP addr per pvt IP addr… but both dynamic & static NAT may
be used together
Source addr
192.168.1.10
Dest addr
150.214.141.20
Source addr
Dest addr
150.214.141.2
150.214.141.20
IP packet
IP packet
Outside network
Inside network
NAT Router
192.168.1.1
150.214.141.1
NAT Table
Private IP
192.168.1.10
Public IP
150.214.141.2
static
Host A
192.168.1.10
Source addr
150.214.141.20
Host Z
150.214.141.20
Dest addr
150.214.141.2
IP packet
Source addr
150.214.141.20
Dest addr
150.214.141.2
IP packet
TCP/IP Architecture Advanced Features
25
NAPT
 Transport-Layer Port Identifiers are used
 Several pvt IP addr may be translated into a unique public IP addr
Source addr Source port
Host A
192.168.1.11
192.168.1.11
1576
Dest addr
Dest port
150.214.141.19
80
Source addr Source port
150.214.141.2
IP packet
1576
Dest addr
150.214.141.19
Dest port
Host Y
150.214.141.19
80
IP packet
NAPT Router
192.168.1.1
150.214.141.1
Inside network
Outside network
NAPT Table
Host B
192.168.1.12
Source addr Source port
192.168.1.12
1576
IP packet
Private IP
local 192.168.1.11:1576
local 192.168.1.12:1576
Public IP
global 150.214.141.2:1576
global 150.214.141.2:1577
Dest addr
Source addr Source port
150.214.141.20
Dest port
21
150.214.141.2
1577
Dest addr
150.214.141.20
Host Z
150.214.141.20
Dest port
21
IP packet
TCP/IP Architecture Advanced Features
26
About NAT
 Not every app work properly when going through a
NAT router (e.g, BOOTP)
 Difficult to follow the traffic that goes through
several NAT routers
 NAT increases router processing time
 NAPT is a type of NAT -> there are also dynamic
NAPT & static NAPT. Both may also be used
together
TCP/IP Architecture Advanced Features
27
Chapter 2 outline
 3.1 TCP congestion
control
 3.2 Address
Translation: NAT
 3.3 Routing in the
Internet



 3.4 IPv4 error control:
ICMP
 3.5 IPv4 address
dynamic configuration:
DHCP
 3.6 IP version 6
RIP
OSPF
BGP
TCP/IP Architecture Advanced Features
28
Routing in the internet
 Network layer
functions


Addressing
Routing
 Routing: finding the
BEST route


Which route?
Best route depends on
different criteria: # of
hops, transfer rate,
load of a link, cost,
reliability…
Different routes
• Link breakage
• Slow links
TCP/IP Architecture Advanced Features
29
Routing in the internet
Concept: Autonomous System (AS)
 AS: IP networks with a common routing
management
 Two types of protocols


IGP (Interior Gateway Protocols): define routing inside an AS.
(RIP, OSPF…)
EGP (Exterior Gateway Protocols): define routing between
different AS.
TCP/IP Architecture Advanced Features
30
Routing in the internet
Important concepts about routing:
 Virtual circuit vs datagrama
 Packet flow



Unicast
Broadcast
Multicast
 Unicast routing algorithms


Static
Adaptive
• Centralized
• Isolated
• Distributed
TCP/IP Architecture Advanced Features
31
Routing in the internet
 Two approaches in routing
 Virtual circuit
• Route is established in
connection start

Datagram
• Rcvr addr is inside every packet
Which route?
TCP/IP Architecture Advanced Features
32
Virtual circuit vs. Datagram
 Virtual circuit
 Error & flow control;
order maintenance
 Connection oriented
 Datagram
 Each packet is an
independent unit
• Dest addr in every packet
• Packets may arrive
disordered to dest
• No error or flow control
• Three phases:
establishment, transfer
& termination

Advantages
• Efficiency
• QoS

Advantages
• More simple
• No connection -> better
for short-time tx
• More reliable
• Better for heterogeneous
networks
TCP/IP Architecture Advanced Features
33
Packet flow
 Unicast
 Broadcast
 Multicast
TCP/IP Architecture Advanced Features
34
Unicast
 Individual flows: one tx, one rx
A
4 flows
2 flows
B
C
D
E
F
TCP/IP Architecture Advanced Features
35
Broadcast
 one tx, everyone rx
A
1 flow
1 flow
B
C
D
E
F
TCP/IP Architecture Advanced Features
36
Multicast
 one flow, only desired rx
A
1 flow
1 flow
B
C
D
E
F
TCP/IP Architecture Advanced Features
37
Unicast routing
 Objective
 Carry pkts from sender
to receiver
 Network-layer routing
 Hierarchic addressing:
first finds network & the
host
 Routing algorithm
 Router calculates the way
of routing
 Routing algorithm
features





Correct
Simple
Robust
Fair
Optimum
TCP/IP Architecture Advanced Features
38
Routing algorithms
 Router
 Network device that interconnects networks & implements
routing algorithm
 Routing algorithm decides which interface is correct
Routing
Engine
Routing
Table
Input
gateways
Router
Output
gateways
TCP/IP Architecture Advanced Features
39
Routing algorithms
 Classification
 Static routing
 Adaptive routing
• Centralized
• Isolated
• Distributed: most used on the internet (RIP/OSPF)
TCP/IP Architecture Advanced Features
40
Static Routing
 Does not consider current network condition
 Routes are determined before starting network
service
 Advantages
 Simple
 Good results for constant traffic and network topology
 Disadvantages
 Not suitable for networks with changing topology
 Not suitable for big networks -> not scalable
TCP/IP Architecture Advanced Features
41
Adaptive Routing
 Decisions based on
 Current topology
 Network condition (congested links)
 Better than static routing but more difficult to
implement
 Three subgroups



Centralized Adaptive Routing
Isolated Adaptive Routing
Distributed Adaptive Routing
TCP/IP Architecture Advanced Features
42
Centralized Adaptive Routing
 Routing Control Center (RCC)
 Nodes (routers) send info about their condition to RCC



List of neighbour nodes
Queue length
Use of links
 RCC




Receives the information
Calculates optimum route for every two nodes
Calculates routing table for every node
Distributes tables to nodes
 Disadvantages


RCC & links to RCC -> bottlenecks
Inaccurate calculation
TCP/IP Architecture Advanced Features
43
Isolated Adaptive Routing
 No info interchange between nodes
 Decisions based only on local info -> simple
 Example

Flooding
TCP/IP Architecture Advanced Features
44
Distributed Adaptive Routing
 Used on the internet
 Two subgroups

Distance vector algorithms
• Decisions based on the information received from neighbour nodes
• E.g. RIP (Routing Information Protocol)

Link state algorithms
• All the nodes know the state of the network
• Some time to propagate changes
• E.g. OSPF (Open Shortest Path First)
TCP/IP Architecture Advanced Features
45
RIP: Routing Information Protocol
 Routing Information Protocol
(RIP) – RFC 1058 (RIP), RFC
1723 (RIPv2):
 Routing Protocol (applicationlayer protocol over UDP – port
520 -)
 Used for interior routing
inside an AS
 RIPv2 is identical to RIP, but
with two extensions:


Allows CIDR
Authentication mechanism
 Metric: hop counts
 Maximum number of hops ->
15
 Better for homogeneous
networks
 Routing table distance
vector-based
 Routing table updates:


Periodic updates: via
broadcast (in RIPv2:
multicast to 224.0.0.9)
Updates when network
topology changes
TCP/IP Architecture Advanced Features
46
RIP: Routing Information Protocol
RIP functioning
 Update from neighbour N arrives to a router R:





Networks known by N, and not by R, are included in R’s routing
table.
If N knows a better route for a network known by both, R’s
routing table is updated
Metric (# of hops) is increased by one
Router R publishes information increasing in one unit what it is
introduced in its routing table.
Update messages tx to neighbours. Two ways
• Not using split horizon technique: Updates are sent to all the
neighbours.
• Using split horizon technique. Updates sent to all the neighbours,
except to those who informed about the best route.
TCP/IP Architecture Advanced Features
47
RIP: Routing Information Protocol
RIP timers
 Routing-update timer: 30 seconds – random time
 Route-timeout timer: after timeout -> invalid route
 Route-flush timer: after timeout -> deletes route
from enrouting table
TCP/IP Architecture Advanced Features
48
RIP: Routing Information Protocol
RIP message
 Command


Request (1)
Response (2): most common (updates)
 Version: v1 or v2
 RIP version 2: most important field -> subnet mask ->
allows CIDR (subnetting)
TCP/IP Architecture Advanced Features
49
OSPF: Open Shortest Path First
Open Shortest Path First (OSPF)
 Used for interior routing inside an AS
 Larger networks than RIP (usually)
 Open protocol (RFC 2328)
TCP/IP Architecture Advanced Features
50
OSPF: Basic Foundations
Open Shortest Path First
(OSPF)
Open Shortest Path First
(OSPF)
 Link state routing protocol
 Updates disseminated to entire
 Dijkstra algorithm to find the
BEST route
 A router builds a graph
 Graph weights are configured by
the network administrator



All equal to 1 (similar to RIP)
Inversely proportional to BW
(usual criterium)
Any criteria may be fixed
AS (via flooding)


Changes in topology
Periodically (once every 30 min, at
least)
 carried in OSPF messages
directly over IP (rather than TCP
or UDP) -> protocol field: 89
 Link connectivity is checked by
HELLO msgs for neighbours
TCP/IP Architecture Advanced Features
51
OSPF advanced features (not in RIP)
 security: all OSPF messages authenticated
 For each link, multiple cost metrics for different
TOS
 hierarchical OSPF in large domains.
TCP/IP Architecture Advanced Features
52
OSPF header
 Checksum: error
control
 Authentication
Value
Type
1
HELLO
2
Database description
3
Link state request
4
Link state update
5
Link state ACK
TCP/IP Architecture Advanced Features
53
BGP: Border Gateway Protocol
 Problem: Different IP networks do not have to use
same routing protocol
 Two types of protocols


IGP (Interior Gateway Protocols): define routing inside an
AS. (RIP, OSPF…)
EGP (Exterior Gateway Protocols): define routing between
different AS.
 BGP (Border Gateway Protocol): most common EGP
(RFC 4271)
TCP/IP Architecture Advanced Features
54
BGP: Main features
 BGP provides each AS means to:
1. Obtain subnet reachability information from
neighboring ASs.
2. Propagate reachability information to all ASinternal routers.
 allows subnet to advertise its existence to
rest of Internet
TCP/IP Architecture Advanced Features
55
BGP: Main features
 Every AS: ASN = Autonomous System
Number)
 Inside every AS -> AS routing protocol
 Outside AS -> “border router”: border
routers in different AS exchange their
routing tables
 BGP works over TCP (port 179)
TCP/IP Architecture Advanced Features
56
BGP: Example
TCP/IP Architecture Advanced Features
57
Chapter 2 outline
 3.1 TCP congestion
control
 3.2 Address
Translation: NAT
 3.3 Routing in the
Internet



 3.4 IPv4 error control:
ICMP
 3.5 IPv4 address
dynamic configuration:
DHCP
 3.6 IP version 6
RIP
OSPF
BGP
TCP/IP Architecture Advanced Features
58
ICMP: Internet Control Message Protocol
 used by hosts & routers to
communicate network-level
information
 error reporting:
unreachable host,
network, port, protocol
 warnings from routers
or receivers
 network-layer above IP:
 ICMP msgs carried in IP
datagrams (but ICMP is
not a transport-layer
protocol!)
 all the nodes using IP must
have ICMP implemented
 ICMP msgs are only
created for the first IP
fragment
TCP/IP Architecture Advanced Features
59
ICMP: Internet Control Message
Protocol
 Message format

ICMP msgs carried in IP
datagrams
• Protocol field = 1 in IP
header
• Source IP addr = host
sending ICMP msg
Type
0
3
3
3
3
5
8
11
Code
0
0
1
2
3
0
0
0
description
echo reply (ping)
dest. network unreachable
dest host unreachable
dest protocol unreachable
dest port unreachable
ICMP redirect
echo request (ping)
TTL expired
TCP/IP Architecture Advanced Features
60
ICMP: Internet Control Message
Protocol
 Ping
 Sends data pkt and waits for a response
 Based on Type 8 and Type 0 ICMP messages
 Functions:
• Check host connectivity
• Ping sends pkts with unique seq numbers
– Detection ofduplicate, reordered and eliminated pkts
• Ping use cheksums in every pkt
– Detection of corrupted pkts
• Ping allows RTT (Round Trip Time) calculation
• Ping allows the detection of other ICMP messages.
TCP/IP Architecture Advanced Features
61
ICMP: Internet Control Message
Protocol
 Ping
Echo request and echo reply
1 byte
1 byte
2 bytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
Type
|
Code
|
Checksum
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
Id
|
Sequence number
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 8 (Echo request) or 0 (Echo reply)
Code: 0
Id: number of identificación (optional)
Sequence number (optional)
TCP/IP Architecture Advanced Features
62
ICMP: Internet Control Message
Protocol
 Ping
Syntax
ping [options] destination_host
Options
-w timeout
Timeout in milliseconds to wait for each reply.
-i TTL
Time To Live.
-v TOS
Type Of Service.
-a
Resolve addresses to hostnames.
-n
count Number of echo requests to send.
-t
Ping the destination host until interrupted. To see statistics and continue type
Control-Break; To stop type Control-C.
-l size
Send buffer size. -f Set Don't Fragment flag in packet (IPv4-only).
-r count
Record route for count hops (IPv4-only).
-s count
Timestamp for count hops (IPv4-only).
-j host_list
Loose source route along host_list (IPv4-only).
-k host_list
Strict source route along host_list (IPv4-only). destination_host The name of the
remote host
-R
Use routing header to test reverse route also (IPv6-only).
-S srcaddr
Source address to use (IPv6-only).
-4
Force using IPv4.
-6
Force using IPv6
TCP/IP Architecture Advanced Features
63
ICMP: Internet Control Message
Protocol
 Tracert (Trace route)

Source sends echo
requests to dest

• First has TTL =1
• Second has TTL=2, etc.

When nth datagram
arrives to nth router:
• Router discards datagram
• And sends to source an
ICMP message (type 11,
code 0)
• Message includes name of
router& IP address

Stopping criterion


Echo request eventually
arrives at destination host
Destination returns echo
reply
When source gets this
ICMP, stops.
Tracert does this 3 times
per router
TCP/IP Architecture Advanced Features
64
ICMP: Internet Control Message
Protocol
 Tracert (Trace route)
Echo request
TTL = 3
Echo request
TTL = 2
Echo request
TTL = 1
Echo request
TTL = 2
Echo request
TTL = 1
X
X
Echo request
TTL = 4
Echo request
TTL = 3
ICMP message
TTL exceeded (type 11) ICMP message
ICMP message
TTL exceeded (type 11)
TTL exceeded (type 11)
Echo request
TTL = 1
X
Echo request
TTL = 2
Echo request
TTL = 1
Echo reply
TCP/IP Architecture Advanced Features
65
ICMP: Internet Control Message
Protocol
 Destination unreachable (type 3)

Most usual code values (causes)
• Code 1: host unreachable
• Code 3: port unreachable
• Code 4: fragmentation needed
TCP/IP Architecture Advanced Features
66
ICMP: Internet Control Message
Protocol
 ICMP Redirect (type 5)

useful when there are some possible
gateways
Steps

NETWORK A

Router A
(2)
(1)
Router B

Packet is sent to default
gateway
Default gateway sends
packet to router B
Default gateway sends
ICMP redirect to host
(3)
NETWORK B
TCP/IP Architecture Advanced Features
67
Chapter 2 outline
 3.1 TCP congestion
control
 3.2 Address
Translation: NAT
 3.3 Routing in the
Internet



 3.4 IPv4 error control:
ICMP
 3.5 IPv4 address
dynamic configuration:
DHCP
 3.6 IP version 6
RIP
OSPF
BGP
TCP/IP Architecture Advanced Features
68
Dynamic Address Configuration
 Address configuration
static
 dynamic: automatic and more efficient
 Dynamic Address Configuration Protocols
 RARP: Reverse Address Resolution Protocol
 BootP: Bootstrap Protocol
 DHCP: Dynamic Host Configuration Protocol

TCP/IP Architecture Advanced Features
69
Dynamic Address Configuration
 RARP
RARP: Reverse Address Resolution Protocol
 Given a MAC, an IP address is assigned
 Messages have the same structure as ARP
ones
 RARP is limited and, thus, obsolete.

TCP/IP Architecture Advanced Features
70
Dynamic Address Configuration
 BootP
 BootP Protocol:
Bootstrap Protocol
 Used to obtain IP
automatically (generally
in boot process)
 Not usually utilized for
Dynamic Address
Configuration, as DHCP is
an improved version of
BootP.
 BootP process





Host determines its own
MAC
Host sends its IP to server’s
port 67 (0.0.0.0 if doesn’t
know its IP and
255.255.255.255 if doesn’t’
know the server’s one)
Server searches host’s MAC
in a configuration file
Server fills server’s and
host’s IPs in UDP datagram
and sends it to client’s port
68
Host saves its IP and boots
TCP/IP Architecture Advanced Features
71
Dynamic Address Configuration
 BootP message






Code: BootPRequest & BootPReply
Transaction id
HW address: e.g. MAC
Server host name (server does not
need to be in the same broadcast
domain)
Boot file name
Vendor specific area
• Magic cookie: shows the kind of
optional info below
• A critical field for DHCP
TCP/IP Architecture Advanced Features
72
Dynamic Address Configuration
 DHCP
Dynamic Host Configuration Protocol (DHCP) – RFC 2131:



Capable of automatically assign reusable (IP address leasing)
Based on BootP, standardizing Vendor Specific Area field in BootP PDU (312
bytes).
3 mechanisms for the assignment of IP addresses:
• Automatic assignment:
–
DHCP assigns a permanent IP address to the host.
• Dynamic assigment:
–
DHCP assigns an IP address for a limited period of time -> automatic reuse of nonneccessary IP addresses
• Static assignment:
–
IP address assigned by network administrator
TCP/IP Architecture Advanced Features
73
Dynamic Address Configuration
 DHCP message

Same as BootP message except
options field (312 bytes) instead of
Vendor Specific Area
• First four bytes: magic cookie ->
99.130.83.99 (means DHCP).
• Different options. Most important
below
– 50: requested IP address
– 51: IP address lease time
– 53: type of DHCP message
TCP/IP Architecture Advanced Features
74
Dynamic Address Configuration
 DHCP cycle




Besides:
1. DHCP DISCOVER: tries to find a
DHCP Server.
2. DHCP OFFER: the server/s
offers an IP address
3. DHCP REQUEST: the client
requests some parameters
(generally the ones oferred by the
server)
4. DHCP ACK: server ACK
• DHCP RELEASE: releases IP
address
• DHCP DECLINE. The offered
IP is in use
• DHCP INFORM: requests some
configuration parameters
• DHCP NAK: if server does not
accept request
1
 Note: Basic cycle is executed if an
2
IP address is not available. Other
way, only half the cycle take place.
3
DHCP Client
(port 68)
4
DHCP Server
(port 67)
TCP/IP Architecture Advanced Features
75
Dynamic Address Configuration
 Other DHCP features






A router may be a DHCP server
BootP Relay: when the server is not in the same client’s broadcast
domain
Gratuitous ARP: ARP request by the client of its own IP. Checks if
the assigned IP is busy.
Option 50: client requests a fixed IP
Option 51: IP address lease time
Maximum DHCP message size: 576 bytes
TCP/IP Architecture Advanced Features
76
Chapter 2 outline
 3.1 TCP congestion
control
 3.2 Address
Translation: NAT
 3.3 Routing in the
Internet



 3.4 IPv4 error control:
ICMP
 3.5 IPv4 address
dynamic configuration:
DHCP
 3.6 IP version 6
RIP
OSPF
BGP
TCP/IP Architecture Advanced Features
77
IPv6
 Initial motivation: 32-bit address space
completely allocated.
 Basic changes:

128-bit address space
• E.g: 2002:96d6:8ddc::96dc:6301 (the bits remaining are
zeros)
header format helps speed processing/forwarding
 40-byte header (fixed size)
 Other changes based on previous experience on
IPv4

TCP/IP Architecture Advanced Features
78
IPv6 header
Version: 6
Priority: identify priority among datagrams in flow
Flow Label: identify datagrams in same “flow.” (concept of“flow”
not well defined).
Payload length: data field length
Next header: identify
upper layer protocol for
data
Hop limit: analog to IPv4
TTL field
Src & Dest addr: 128
bytes
TCP/IP Architecture Advanced Features
79
Changes from IPv4
 Checksum: removed entirely to reduce
processing time at each hop
 Fragmentation: removed from IPv6, also to
reduce processing time
 Options: allowed, but outside of header,
indicated by “Next Header” field
 ICMPv6: new version of ICMP
 additional
message types, e.g. “Packet Too Big”
TCP/IP Architecture Advanced Features
80
Transition from IPv4 to IPv6
 Not all routers can be upgraded simultaneous
no D Day
 How does the internet operate with mixed IPv4
and IPv6 routers?

 Tunneling: IPv6 carried as payload in IPv4
datagram among IPv4 routers
TCP/IP Architecture Advanced Features
81