Chapter 2
Delivering the data
Adapted from slides provided for:
All material copyright 1996-2010
J.F Kurose and K.W. Ross, All Rights Reserved
Computer Networking:
A Top Down Approach
5th edition.
Jim Kurose, Keith Ross
Addison-Wesley
Network Layer
4-1
Chapter goals:

understand how data moves between layers
and systems on the network
 IP address, subnet
 Routing
Routing table
 Address resolution
 Protocols, ports and sockets

Network Layer
4-2
Interplay between routing, 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
Network Layer
4-3
IP datagram format
IP protocol version
number
header length
(bytes)
“type” of data
max number
remaining hops
(decremented at
each router)
upper layer protocol
to deliver payload to
32 bits
head. type of
length
ver
len service
fragment
16-bit identifier flgs
offset
upper
time to
header
layer
live
checksum
total datagram
length (bytes)
for
fragmentation/
reassembly
32 bit source IP address
32 bit destination IP address
Options (if any)
data
(variable length,
typically a TCP
or UDP segment)
E.g. timestamp,
record route
taken, specify
list of routers
to visit.
Network Layer
4-4
IP Addressing: introduction


IP address: 32-bit
identifier for network
interface
interface: connection
between host/router
and physical link
 router’s typically have
multiple interfaces
 host typically has one
interface
223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4
223.1.1.3
223.1.2.9
223.1.3.27
223.1.2.2
223.1.3.2
223.1.3.1
• Host with multiple
interfaces can acts as a
router
223.1.1.1 = 11011111 00000001 00000001 00000001
 Command:ifconfig
223
1
1
Network Layer
1
4-5
IP Address vs MAC Address

MAC address
 Globally unique
 Statically
configured by
manufacturer
 flat
Discussion: difference between
IP address vs domain name?

IP address
 Not necessarily
unique
 Dynamically
assigned
 Hierarchical: made
up of network part
and host part,
corresponding to
hierarchy in the
Internet
Network Layer
4-6
[zhang@storm ~]$ ifconfig
em1
Link encap:Ethernet HWaddr B4:99:BA:01:3B:F6
inet addr:150.108.68.26 Bcast:150.108.68.255 Mask:255.255.255.0
inet6 addr: fe80::b699:baff:fe01:3bf6/64 Scope:Link
….
IP address
em2
em3
Link encap:Ethernet HWaddr B4:99:BA:01:3B:F8
UP BROADCAST MULTICAST MTU:1500 Metric:1
…
Link encap:Ethernet HWaddr B4:99:BA:01:3B:FA
UP BROADCAST MULTICAST MTU:1500 Metric:1
….
em4
Link encap:Ethernet HWaddr B4:99:BA:01:3B:FC
UP BROADCAST MULTICAST MTU:1500 Metric:1
….
lo
Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
…
virbr0
within
Fordham network
Private IP address used in
CIS dept. network
Link encap:Ethernet HWaddr 52:54:00:F2:86:A6
inet addr:192.168.122.1 Bcast:192.168.122.255 Mask:255.255.255.0
Network Layer
4-7
IP Addresses
“class-full” addressing:
class
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
multicast address
host
192.0.0.0 to
223.255.255.255
224.0.0.0 to
239.255.255.255
32 bits
Network Layer
4-8
Getting a datagram from source to dest.
forwarding table in A
Dest. Net. next router Nhops
223.1.1
223.1.2
223.1.3
IP datagram:
misc source dest
fields IP addr IP addr
data
A
 datagram remains
unchanged, as it travels
source to destination
 addr fields of interest
here
223.1.1.4
223.1.1.4
1
2
2
223.1.1.1
223.1.2.1
B
223.1.1.2
223.1.1.4
223.1.2.9
223.1.2.2
223.1.1.3
223.1.3.1
223.1.3.27
E
223.1.3.2
Network Layer
4-9
Getting a datagram from source to dest.
forwarding table in A
misc
data
fields 223.1.1.1 223.1.1.3
Dest. Net. next router Nhops
223.1.1
223.1.2
223.1.3
Starting at A, send IP datagram
addressed to B:
 look up net. address of B in
forwarding table
 find B is on same net. as A
 link layer will send datagram
directly to B inside link-layer
frame
 B and A are directly
connected
A
223.1.1.4
223.1.1.4
1
2
2
223.1.1.1
223.1.2.1
B
223.1.1.2
223.1.1.4
223.1.2.9
223.1.2.2
223.1.1.3
223.1.3.1
223.1.3.27
E
223.1.3.2
Network Layer 4-10
Getting a datagram from source to dest.
forwarding table in A
misc
data
fields 223.1.1.1 223.1.2.3
Dest. Net. next router Nhops
223.1.1
223.1.2
223.1.3
Starting at A, dest. E:
 look up network address of E





in forwarding table
E on different network
 A, E not directly attached
routing table: next hop
router to E is 223.1.1.4
link layer sends datagram to
router 223.1.1.4 inside linklayer frame
datagram arrives at 223.1.1.4
continued…..
A
223.1.1.4
223.1.1.4
1
2
2
223.1.1.1
223.1.2.1
B
223.1.1.2
223.1.1.4
223.1.2.9
223.1.2.2
223.1.1.3
223.1.3.1
223.1.3.27
E
223.1.3.2
Network Layer
4-11
Getting a datagram from source to dest.
forwarding table in router
misc
data
fields 223.1.1.1 223.1.2.3
Arriving at 223.1.4, destined for
223.1.2.2
 look up network address of E
in router’s forwarding table
 E on same network as router’s
interface 223.1.2.9
 router, E directly attached
 link layer sends datagram to
223.1.2.2 inside link-layer
frame via interface 223.1.2.9
 datagram arrives at
223.1.2.2!!! (hooray!)
Dest. Net router Nhops interface
223.1.1
223.1.2
223.1.3
A
-
1
1
1
223.1.1.4
223.1.2.9
223.1.3.27
223.1.1.1
223.1.2.1
B
223.1.1.2
223.1.1.4
223.1.2.9
223.1.2.2
223.1.1.3
223.1.3.1
223.1.3.27
E
223.1.3.2
Network Layer 4-12
Subnetting
Problem 1: Any network with need for more
than 255 hosts, needed class B addresses,
or get many class C addresses
 Problem 2: Each new network implies
additional entry in forwarding table 
large table
 Solution:

 Share one network number between several
networks.
…Subnetting



Made most sense for large corporations or
campuses
Corporation networks share 1 network number
Number of other networks within the corporation,
using subnet masks
 E.g. a class B address, is shared among 8 networks, by
using a 19-bit “subnet mask” (255.255.224.0 = 11111111
11111111 11100000 00000000)
 I.e. subnet addresses are defined by 1st 19 bits of the IP
address.  Host part now has a “subnet” part in it.

Class B network address continues to be
advertised to the rest of the Internet, subnetting
only used “within campus”
Subnet mask

Introduce another level of hierarchy into
IP address
8 bits are borrowed from the host address field to create
subnet address field.
Subnet mask: 255.255.255.0, i.e., all 1’s in upper 24 bits and
0’s in lower 8 bits
* 24 bits are network number
* 8 bits are host number
Network Layer 4-15
Forwarding Ex. with Subnet Masks
• Routing Table:
SubnetNumber
SubnetMask
NextHop
128.96.170.0
255.255.254.0
Intface 0
128.96.168.0
255.255.254.0
Intface 1
128.96.166.0
255.255.254.0
R2
128.96.164.0
255.255.252.0
R3
Default
R4
D = Dest IP Address
Forwarding
For each table entry (subnetNumber, SubnetMask, NextHop)
pseudocode
If (D & SubnetMask == SubnetNumber)
if NextHop is an interface
forward datagram to the interface
else
deliver datagram to NextHop (a router)
IP addressing: CIDR

Classful addressing:
 inefficient use of address space, address space exhaustion
 e.g., class B net allocated enough addresses for 65K hosts,
even if only 2K hosts in that network

CIDR: Classless InterDomain Routing
 network portion of address of arbitrary length
 address format: a.b.c.d/x, where x is # bits in network
portion of address
network
part
host
part
11001000 00010111 00010000 00000000
200.23.16.0/23
Network Layer 4-17
Special IP address within
subnet

NETWORK ADDRESS
 A network address is an address where all host bits in
the IP address are set to zero (0).
 first and lowest numbered address

BROADCAST ADDRESS
 all host bits in the IP address are set to one (1).
 last address in the range of addresses
 All hosts are to accept and respond to the broadcast
address.
 This makes special services possible.
Network Layer 4-18
Hosts LOOPBACK ADDRESS

127.0.0.0 class 'A' subnet is used for only a
single address, the loopback address
127.0.0.1.
 used to test the local network interface
device's functionality.
 All network interface devices should respond to
this address.
 ping 127.0.0.1 to test network hardware and
software
Network Layer 4-19
Special Use IP addresses

PRIVATE IP ADDRESSES
 RFC 1918 defines a number of IP blocks set
aside by American Registry of Internet
Numbers (ARIN) for use as private
addresses on private networks that are not
directly connected to t Internet.
Class Start
End
A 10.0.0.0
10.255.255.255
B 172.16.0.0 172.31.255.255
C 192.168.0.0 192.168.255.255
Network Layer 4-20
Special Use IP addresses

Multicast IP Addresses
 set aside for special purposes, such as the IP's
used in OSPF, Multicast, and experimental
purposes that cannot be used on the Internet.
Class Start
End
D
224.0.0.0 239.255.255.255
Network Layer 4-21
IP addresses: how to get one?
Q: How does a network get the network part
of IP addr?
A: gets allocated portion of its provider ISP’s
address space
ISP's block
11001000 00010111 00010000 00000000
200.23.16.0/20
Organization 0
Organization 1
Organization 2
...
11001000 00010111 00010000 00000000
11001000 00010111 00010010 00000000
11001000 00010111 00010100 00000000
…..
….
200.23.16.0/23
200.23.18.0/23
200.23.20.0/23
….
Organization 7
11001000 00010111 00011110 00000000
200.23.30.0/23
Network Layer 4-22
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
“Send me anything
with addresses
beginning
199.31.0.0/16”
Network Layer 4-23
Hierarchical addressing: more specific
routes
ISPs-R-Us has a more specific route to Organization 1
Organization 0
200.23.16.0/23
Organization 2
200.23.20.0/23
Organization 7
.
.
.
.
.
.
Fly-By-Night-ISP
Internet
Longest match
200.23.30.0/23
ISPs-R-Us
Organization 1
200.23.18.0/23
“Send me anything
with addresses
beginning
200.23.16.0/20”
“Send me anything
with addresses
beginning 199.31.0.0/16
or 200.23.18.0/23”
Network Layer 4-24
IP addressing: the last word...
Q: How does an ISP get block of addresses?
A: ICANN: Internet Corporation for Assigned
Names and Numbers
 allocates addresses
 manages DNS
 assigns domain names, resolves disputes
Network Layer 4-25
IP addresses: how to get one?
Q: How does a host get IP address?


hard-coded by system admin in a file
 Windows: control-panel->network->configuration>tcp/ip->properties
 UNIX: /etc/rc.config
DHCP: Dynamic Host Configuration Protocol:
dynamically get address from a server
 “plug-and-play”
Network Layer 4-26
DHCP: Dynamic Host Configuration Protocol
Goal: allow host to dynamically obtain its IP address from
network server when it joins network
Can renew its lease on address in use
Allows reuse of addresses (only hold address while connected an
“on”)
Support for mobile users who want to join network (more shortly)
DHCP overview:
 host broadcasts “DHCP discover” msg [optional]
 DHCP server responds with “DHCP offer” msg
[optional]
 host requests IP address: “DHCP request” msg
 DHCP server sends address: “DHCP ack” msg
Network Layer 4-27
DHCP client-server scenario
A
B
223.1.2.1
DHCP
server
223.1.1.1
223.1.1.2
223.1.1.4
223.1.2.9
223.1.2.2
223.1.1.3
223.1.3.1
223.1.3.27
223.1.3.2
E
arriving DHCP
client needs
address in this
network
Network Layer 4-28
DHCP client-server scenario
DHCP server: 223.1.2.5
DHCP discover
arriving
client
src : 0.0.0.0, 68
dest.: 255.255.255.255,67
yiaddr: 0.0.0.0
transaction ID: 654
DHCP offer
src: 223.1.2.5, 67
dest: 255.255.255.255, 68
yiaddrr: 223.1.2.4
transaction ID: 654
Lifetime: 3600 secs
DHCP request
time
src: 0.0.0.0, 68
dest:: 255.255.255.255, 67
yiaddrr: 223.1.2.4
transaction ID: 655
Lifetime: 3600 secs
DHCP ACK
src: 223.1.2.5, 67
dest: 255.255.255.255, 68
yiaddrr: 223.1.2.4
transaction ID: 655
Lifetime: 3600 secs
Network Layer 4-29
DHCP: more than IP address
DHCP can return more than just allocated IP
address on subnet:
 address of first-hop router for client
 name and IP address of DNS sever
 network mask (indicating network versus host
portion of address)
Network Layer 4-30
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
Datagrams with source or
destination in this network
have 10.0.0/24 address for
source, destination (as usual)
Network Layer 4-31
NAT: Network Address Translation

Motivation: local network uses just one IP address as
far as outside world is concerned:
 range of addresses not needed from ISP: just one IP
address for all devices
 can change addresses of devices in local network
without notifying outside world
 can change ISP without changing addresses of
devices in local network
 devices inside local net not explicitly addressable,
visible by outside world (a security plus).
Network Layer 4-32
NAT: Network Address Translation
Implementation: NAT router must:
 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 dest fields of every incoming datagram
with corresponding (source IP address, port #)
stored in NAT table
Network Layer 4-33
NAT: Network Address Translation

16-bit port-number field:
 60,000 simultaneous connections with a single
LAN-side address!

NAT is controversial:
 routers should only process up to layer 3
 violates end-to-end argument
• NAT possibility must be taken into account by app
designers, e.g., P2P applications
 address shortage should instead be solved by
IPv6
Network Layer 4-34
NAT traversal problem

client wants to connect to
server with address 10.0.0.1
 server address 10.0.0.1 local
to LAN (client can’t use it as
destination addr)
 only one externally visible
NATed address: 138.76.29.7

solution 1: statically
configure NAT to forward
incoming connection
requests at given port to
server
Client
10.0.0.1
?
10.0.0.4
138.76.29.7
NAT
router
 e.g., (123.76.29.7, port 2500)
always forwarded to 10.0.0.1
port 25000
Network Layer 4-35
NAT traversal problem

solution 2: Universal Plug and
Play (UPnP) Internet Gateway
Device (IGD) Protocol. Allows
NATed host to:
learn public IP address
(138.76.29.7)
add/remove port mappings
(with lease times)
10.0.0.1
IGD
10.0.0.4
138.76.29.7
NAT
router
i.e., automate static NAT port
map configuration
Network Layer 4-36
NAT traversal problem

solution 3: relaying (used in Skype)
 NATed client establishes connection to relay
 External client connects to relay
 relay bridges packets between to connections
2. connection to
relay initiated
by client
Client
3. relaying
established
1. connection to
relay initiated
by NATed host
138.76.29.7
10.0.0.1
NAT
router
Network Layer 4-37
ICMP: Internet Control Message Protocol



used by hosts & routers to
communicate network-level
information
 error reporting:
unreachable host, network,
port, protocol
 echo request/reply (used
by ping)
network-layer “above” IP:
 ICMP msgs carried in IP
datagrams
ICMP message: type, code plus
first 8 bytes of IP datagram
causing error
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
Network Layer 4-38
Traceroute and ICMP

Source sends series of
UDP segments to dest
 first has TTL =1
 second has TTL=2, etc.
 unlikely port number

When nth datagram arrives
to nth router:
 router discards datagram
 and sends to source an
ICMP message (type 11,
code 0)
 ICMP message includes
name of router & IP
address
when ICMP message
arrives, source calculates
RTT
 traceroute does this 3
times
Stopping criterion
 UDP segment eventually
arrives at destination host
 destination returns ICMP
“port unreachable” packet
(type 3, code 3)
 when source gets this
ICMP, stops.

Network Layer 4-39
Hierarchical Routing
Our routing study thus far - idealization
 all routers identical
 network “flat”
… not true in practice
scale: with 200 million
destinations:


can’t store all dest’s in
routing tables!
routing table exchange
would swamp links!
administrative autonomy


internet = network of
networks
each network admin may
want to control routing in its
own network
Network Layer 4-40
Hierarchical Routing


aggregate routers into
regions, “autonomous
systems” (AS)
routers in same AS run
same routing protocol
gateway router
 at “edge” of its own AS
 has link to router in
another AS
 “intra-AS” routing
protocol
 routers in different AS
can run different intraAS routing protocol
Network Layer 4-41
Interconnected ASes
3c
3a
3b
AS3
1a
2a
1c
1d
1b
Intra-AS
Routing
algorithm
2c
AS2
AS1
Inter-AS
Routing
algorithm
Forwarding
table

2b
forwarding table
configured by both
intra- and inter-AS
routing algorithm
 intra-AS sets entries
for internal dests
 inter-AS & intra-As
sets entries for
external dests
Network Layer 4-42
Intra-AS Routing


also known as Interior Gateway Protocols (IGP)
most common Intra-AS routing protocols:
 RIP: Routing Information Protocol
 OSPF: Open Shortest Path First
 IGRP: Interior Gateway Routing Protocol (Cisco
proprietary)
Network Layer 4-43
RIP ( Routing Information Protocol)


included in BSD-UNIX distribution in 1982
distance vector algorithm
 distance metric: # hops (max = 15 hops), each link has cost 1
 DVs exchanged with neighbors every 30 sec in response
message (aka advertisement)
 each advertisement: list of up to 25 destination subnets (in IP
addressing sense)
u
v
A
z
C
B
D
w
x
y
from router A to destination subnets:
subnet hops
u
1
v
2
w
2
x
3
y
3
z
2
Network Layer 4-44
OSPF (Open Shortest Path First)


“open”: publicly available
uses Link State algorithm
 LS packet dissemination
 topology map at each node
 route computation using Dijkstra’s algorithm


OSPF advertisement carries one entry per neighbor
router
advertisements disseminated to entire AS (via
flooding)
 carried in OSPF messages directly over IP (rather than TCP
or UDP
Network Layer 4-45
UNIX routing Principle
1. Search for a matching host address
 2. Search for a matching network address
 3. Search for a default entry (specified as
a network entry, with network ID of 0

Network Layer 4-46
netstat

Display routing table
[zhang@storm ~]$ netstat -rn
Kernel IP routing table
Destination Gateway
Genmask
Flags MSS Window irtt Iface
0.0.0.0
150.108.68.1 0.0.0.0
UG
00
0 em1
150.108.68.0 0.0.0.0
255.255.255.0 U
00
0 em1
192.168.122.0 0.0.0.0
255.255.255.0 U
00
0 virbr0
Network Layer 4-47
Multiplexing/demultiplexing
Demultiplexing at rcv host:
delivering received segments
to correct socket
= socket
= process
application P3
P1
P1
application
transport
transport
network
network
link
link
physical
physical
host 1
Multiplexing at send host:
gathering data from multiple
sockets, enveloping data with
header (later used for
demultiplexing)
P2
P4
application
transport
network
link
physical
host 2
host 3
Transport Layer 3-48
How demultiplexing works

host receives IP
datagrams
 each datagram has source
IP address, destination IP
address
 each datagram carries 1
transport-layer segment
 each segment has source,
destination port number

host uses IP addresses &
port numbers to direct
segment to appropriate
socket
32 bits
source port #
dest port #
other header fields
application
data
(message)
TCP/UDP segment format
Transport Layer 3-49
Connectionless demultiplexing

recall: create sockets with
host-local port numbers:

 checks destination port
number in segment
 directs UDP segment to
socket with that port
number
DatagramSocket mySocket1 = new
DatagramSocket(12534);
DatagramSocket mySocket2 = new
DatagramSocket(12535);

recall: when creating
datagram to send into UDP
socket, must specify
(dest IP address, dest port number)
when host receives UDP
segment:

IP datagrams with
different source IP
addresses and/or source
port numbers directed
to same socket
Transport Layer 3-50
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428);
P2
SP: 6428
DP: 9157
client
IP: A
P1
P1
P3
SP: 9157
DP: 6428
SP: 6428
DP: 5775
server
IP: C
SP: 5775
DP: 6428
Client
IP:B
SP provides “return address”
Transport Layer 3-51
Connection-oriented demux

TCP socket identified
by 4-tuple:





source IP address
source port number
dest IP address
dest port number
recv host uses all four
values to direct
segment to appropriate
socket

server host may support
many simultaneous TCP
sockets:
 each socket identified by
its own 4-tuple

web servers have
different sockets for
each connecting client
 non-persistent HTTP will
have different socket for
each request
Transport Layer 3-52
Connection-oriented demux
(cont)
P1
P4
P5
P2
P6
P1P3
SP: 5775
DP: 80
S-IP: B
D-IP:C
client
IP: A
SP: 9157
DP: 80
S-IP: A
D-IP:C
server
IP: C
SP: 9157
DP: 80
S-IP: B
D-IP:C
Client
IP:B
Transport Layer 3-53
Connection-oriented demux:
Threaded Web Server
P1
P2
P4
P1P3
SP: 5775
DP: 80
S-IP: B
D-IP:C
client
IP: A
SP: 9157
DP: 80
S-IP: A
D-IP:C
server
IP: C
SP: 9157
DP: 80
S-IP: B
D-IP:C
client
IP:B
Transport Layer 3-54
TCP Connection Management
Recall: TCP sender, receiver


establish “connection”
before exchanging data
segments
initialize TCP variables:
 seq. #s
 buffers, flow control
info (e.g. RcvWindow)
client: connection initiator
Socket clientSocket = new
Socket("hostname","port
number");

server: contacted by client
Socket connectionSocket =
welcomeSocket.accept();
Three way handshake:
Step 1: client host sends TCP
SYN segment to server
 specifies initial seq #
 no data
Step 2: server host receives
SYN, replies with SYNACK
segment
 server allocates buffers
 specifies server initial
seq. #
Step 3: client receives SYNACK,
replies with ACK segment,
which may contain data
Transport Layer 3-55
TCP Connection Management (cont.)
Closing a connection:
client closes socket:
clientSocket.close();
client
close
Step 1: client end system
close
FIN, replies with ACK.
Closes connection, sends
FIN.
timed wait
sends TCP FIN control
segment to server
Step 2: server receives
server
closed
Transport Layer 3-56
TCP Connection Management (cont.)
Step 3: client receives FIN,
replies with ACK.
client
server
closing
 Enters “timed wait” will respond with ACK
to received FINs
closing
Step 4: server, receives
Note: with small
modification, can handle
simultaneous FINs.
timed wait
ACK. Connection closed.
closed
closed
Transport Layer 3-57
TCP Connection Management (cont)
TCP server
lifecycle
TCP client
lifecycle
Transport Layer 3-58
TCP segment structure
URG: urgent data
(generally not used)
ACK: ACK #
valid
PSH: push data now
(generally not used)
RST, SYN, FIN:
connection estab
(setup, teardown
commands)
Internet
checksum
(as in UDP)
32 bits
source port #
dest port #
sequence number
acknowledgement number
head not
len used
UAP R S F Receive window
checksum
Urg data pnter
Options (variable length)
counting
by bytes
of data
(not segments!)
# bytes
rcvr willing
to accept
application
data
(variable length)
Transport Layer 3-59