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TCP/IP Internetworking. Chapter 8. Recap. Single Networks (Subnets) Chapters 4 and 5 covered single LANs Chapters 6 and 7 covered residential Internet access and single WANs Internets Connect multiple single networks using routers 70%-80% of internet traffic follows TCP/IP standards

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recap
Recap
  • Single Networks (Subnets)
    • Chapters 4 and 5 covered single LANs
    • Chapters 6 and 7 covered residential Internet access and single WANs
  • Internets
    • Connect multiple single networks using routers
    • 70%-80% of internet traffic follows TCP/IP standards
    • These standards are created by the IETF
    • Chapter 10 looks in more detail at TCP/IP management
figure 2 8 hybrid tcp ip osi architecture
Figure 2-8: Hybrid TCP/IP-OSI Architecture

Recap

TCP/IP standards dominate at the

internet and transport layers—

transmission across an internet

figure 2 11 internet and transport layer cont
Figure 2-11: Internet and Transport Layer, Cont.

Recap

Transport Layer

end-to-end (host-to-host)

TCP is connection-oriented, reliable

UDP is connectionless and unreliable

Server

Client PC

Internet Layer

(usually IP)

hop-by-hop (host-router or router-router)

connectionless, unreliable

Router 1

Router 2

Router 3

frames and packets
Frames and Packets

Recap

  • Messages at the data link layer are called frames
  • Messages at the internet layer are called packets
  • Within a single network, packets are encapsulated in the data fields of frames

Frame

Trailer

Packet

(Data Field)

Frame

Header

frames and packets1
Frames and Packets

Recap

  • In an internet with hosts separated by N networks, there will be:
    • 2 hosts
    • One packet (going all the way between hosts)
      • One route (between the two hosts)
    • N frames (one in each network)
figure 2 21 combining horizontal and vertical communication
Figure 2-21: Combining Horizontal and Vertical Communication

Recap

App

Transmission Control Protocol (TCP)

Or User Datagram Protocol (UDP)

Trans

Trans

Internet Protocol

(IP)

Int

Int

Int

Int

IP

DL

Phy

Destination

Host

Source

Host

Switch

2

Router

1

Switch

3

Router

2

Switch

1

figure 8 1 major tcp ip standards
Figure 8-1: Major TCP/IP Standards

5 Application

User Applications

Supervisory Applications

HTTP

SMTP

Many

Others

DNS

Routing

Protocols

Many

Others

4 Transport

TCP

UDP

3 Internet

IP

ICMP

MPLS

ARP

2 Data Link

None: Use OSI Standards

1 Physical

None: Use OSI Standards

Internetworking is done at the internet and transport layers.

There are only a few standards at these layers.

We will look at the shaded protocols in this chapter.

figure 8 1 major tcp ip standards continued
Figure 8-1: Major TCP/IP Standards, Continued

5 Application

User Applications

Supervisory Applications

HTTP

SMTP

Many

Others

DNS

Routing

Protocols

Many

Others

4 Transport

TCP

UDP

3 Internet

IP

ICMP

ARP

2 Data Link

None: Use OSI Standards

1 Physical

None: Use OSI Standards

At the application layer, there are

user applications and supervisory applications.

We will look at two TCP/IP application layer supervisory applications in this chapter.

ip addresses

IP Addresses

32-Bit Strings

Dotted Decimal Notation for Human Reading(e.g., 128.171.17.13)

figure 8 3 hierarchical ip address
Figure 8-3: Hierarchical IP Address

IP addresses are not

simple 32-bit numbers.

They usually have 3 parts.

Consider the example

128.171.17.13

hierarchical addressing
Hierarchical Addressing
  • Hierarchical Addressing Brings Simplicity
    • Phone System
      • Country code-area code-exchange-subscriber number
      • 01-808-555-9889
    • Long-distance switches near the top of the hierarchy only have to deal with country codes and area codes to set up circuits
    • Similarly, core Internet routers only have to consider network or network and subnet parts of packets
figure 8 4 border router intrernal router networks and subnets
Figure 8-4: Border Router, Intrernal Router, Networks, and Subnets

Border routers connect different Internet networks

(In this case, 192.168.x.x and 60.x.x.x).

An “x” indicates anything.

figure 8 4 border router internal router networks and subnets
Figure 8-4: Border Router, Internal Router, Networks, and Subnets

Internal routers connect different subnets in a network.

In this case, the three subnets are boxed in red:

192.168.1.x, 192.168.2.x, and 192.168.3.x.

figure 8 5 multiprotocol routing
Figure 8-5: Multiprotocol Routing

Real routers must handle multiple

internet and transport layer architectures—

TCP/IP, IPX/SPX, SNA, etc.

We will only look at TCP/IP routing

figure 8 6 ethernet switching versus ip routing
Figure 8-6: Ethernet Switching Versus IP Routing

Destination address is E5-BB-47-21-D3-56.

Ethernet switches are arranged in a hierarchy.

So there is only one possible path between hosts.

So only one row can match an Ethernet address.

Finding this row is very simple and fast.

So Ethernet switching is inexpensive per frame handled.

One Correct Row

figure 8 6 ethernet switching versus ip routing1
Figure 8-6: Ethernet Switching Versus IP Routing

Routing

Matches

Host

60.3.47.x

Because of multiple alternative routes in router meshes,

routers may have several rows that match an IP address.

Routers must find All matches and then select the BEST ONE.

This is slow and therefore expensive compared to switching.

figure 8 7 the routing process
Figure 8-7: The Routing Process
  • Routing
    • Processing an individual packet and passing it on its way is called routing
      • Router ports are called interfaces
      • Packet arrives in one interface
      • The router sends the packetout another interface
figure 8 7 the routing process1
Figure 8-7: The Routing Process
  • The Routing Table
    • Each router has a routing table that it uses to make routing decisions
    • Routing Table Rows
      • Each row represents a route for a RANGE of IP addresses—often a network or subnet
      • All packets with addresses in this range are routed according to that row

Route

IP Address RangeGoverned by the route

Metric

Next-Hop

Router

1

60.3.x.x

9

B

figure 8 7 the routing process2
Figure 8-7: The Routing Process
  • The Routing Table
    • Routing Table Columns
      • Row (route) number: Not in real routing tables
      • IP address range governed by the row
      • Metric for the quality of the route
      • Next-hop router that should get the packet next if the row is selected as the best match

Route

IP Address

Range

Metric

Next-Hop

Router

1

60.3.x.x

9

B

2

128.171.x.x

2

B

figure 8 7 the routing process3
Figure 8-7: The Routing Process
  • A Routing Decision
    • The router looks at the destination IP address in an arriving packet (in this case, 60.3.47.12).
    • 1. The router determines which rows match (have an IP address range containing the packet’s destination IP address)
      • The router must check ALL rows for possible matches

Route

IP Address

Range

Metric

Next-Hop

Router

Arriving Packet

60.3.47.12

1

60.3.x.x

9

B

Match

2

128.171.x.x

2

B

No Match

figure 8 7 the routing process4
Figure 8-7: The Routing Process
  • A Routing Decision
    • 2. After finding all matches, the router then determines the BEST-MATCH row
      • 2A. Selects the row with the LONGEST MATCH
        • 60.3.x.x has 16 bits of match
        • 60.3.47.x has 24 bits of match so is a better match
      • 2B. If two or more rows tie for the longest match, router uses the METRIC column value
        • If cost, lowest metric value is best
        • If speed, highest metric value is best
        • Etc.
figure 8 7 the routing process5
Figure 8-7: The Routing Process
  • A Routing Decision
    • 3. After selecting the best-match row, the router sends the packet on to the next-hop router indicated in the best-match row—Next-Hop Router B in this example.

Send Packetout toNHR B

Route

IP Address

Range

Metric

Next-Hop

Router

1

60.3.x.x

9

B

Best-Match Row

2

128.171.x.x

2

B

figure 8 8 detailed row matching algorithm
Figure 8-8: Detailed Row-Matching Algorithm

Box

  • Routing Table

Actually, the table does not really have an “IP Address Range” column.

Instead, it has two columns to indicate the IP address range:

Destination (an IP address) and a mask

figure 8 8 detailed row matching algorithm1
Figure 8-8: Detailed Row-Matching Algorithm

Box

  • 1. Basic Rule of Masking
    • Information Bit 1 0 1 0
    • Mask Bit 1 1 0 0
    • Result 1 0 0 0
  • Where mask bits are one, the result gives the original IP address bits
  • Where mask bits are zero, the result contains zeros
figure 8 8 detailed row matching algorithm2
Figure 8-8: Detailed Row-Matching Algorithm

Box

  • 2. Example
    • Address (partial) 10101010 11001110
    • Mask 11111000 00000000
    • Result 10101000 00000000
figure 8 8 detailed row matching algorithm3
Figure 8-8: Detailed Row-Matching Algorithm

Box

  • 3. Common 8-bit Segment Values in Dotted Decimal Notation
    • Segment Decimal Value

00000000 0

11111111 255

  • 4. Example
    • 255.255.255.0 is 24 ones followed by 8 zero
    • 255.255.255.0 is also called /24 in “prefix notation”
figure 8 8 detailed row matching algorithm4
Figure 8-8: Detailed Row-Matching Algorithm

Box

  • Example 1: A Destination IP Address that is in the Range
  • Destination IP Address of Arriving Packet 10.7.3.47
  • Apply the Mask 255.255.255.0
  • Result of Masking 10.7.3.0
  • Destination Value 10.7.3.0
  • Does Destination Value Match the Masking Result? Yes
  • Conclusion Row 1 is a match.
figure 8 8 detailed row matching algorithm5
Figure 8-8: Detailed Row-Matching Algorithm

Box

  • Example 2: A Destination IP Address that is NOT in the Range
  • Destination IP Address of Arriving Packet 10.7.5.47
  • Apply the Mask 255.255.255.0
  • Result of Masking 10.7.5.0
  • Destination Value 10.7.3.0
  • Does Destination Value Match the Masking Result? No
  • Conclusion Row 1 is NOT a match.
figure 8 9 interface and next hop router
Figure 8-9: Interface and Next-Hop Router

Box

  • Switches
    • A switch port connects directly to a single computer or another switch
    • Sending the frame out a port automatically gets it to the correct destination

Frame

figure 8 9 interface and next hop router1
Figure 8-9: Interface and Next-Hop Router

Box

  • Routers
    • Router ports (interfaces) connect to subnets, which have multiple hosts and that may have multiple routers
    • The packet must be forwarded to a specific host or router on that subnet

Host

IP

Packet

Host

Subnet

on Router

Interface

Next-Hop

Router

Next-Hop

Router

figure 8 9 interface and next hop router2
Figure 8-9: Interface and Next-Hop Router

Next-Hop Router

Box

Interface (port)

Next-Hop Router

Best-match row has both an interface (indicating a subnet)

and also a next-hop router value to indicate a host or router on the subnet.

(Not just a Next Hop Router Column)

dynamic routing protocols

Dynamic Routing Protocols

Dynamic Routing Protocol

Routing Table Information

figure 8 10 dynamic routing protocols
Figure 8-10: Dynamic Routing Protocols
  • Routing
    • How do routers get their routing table information?
    • Routers constantly exchange routing table information with one another using dynamic routing protocols
    • Note that the term routing is used in two ways In TCP/IP
      • For IP packet forwarding and
      • For the exchange of routing table information through routing protocols

Dynamic Routing Protocol

Routing Table Information

figure 8 10 dynamic routing protocols1
Figure 8-10: Dynamic Routing Protocols
  • Autonomous System
    • An organization’s internal network (internet)
  • Exterior Dynamic Routing Protocols
    • Between Autonomous Systems, companies use an exterior dynamic routing protocol
    • The dominant exterior dynamic routing protocol is the Border Gateway Protocol (BGP)
      • Gateway is an obsolete name for router
    • Company is not free to choose whatever exterior routing protocol it wishes
figure 8 10 dynamic routing protocols2
Figure 8-10: Dynamic Routing Protocols
  • Interior Dynamic Routing Protocols
    • Within an Autonomous System, firms use interior dynamic routing protocols
    • Can select their own interior dynamic routing protocol
    • Routing Information Protocol (RIP) for small internets
    • Open Shortest Path First (OSPF) for larger internets
    • Enhanced Interior Gateway Routing Protocol (EIGRP)
      • Non-TCP/IP proprietary CISCO protocol
      • Can handle multiple protocols, not just TCP/IP
figure 8 12 address resolution protocol arp
Figure 8-12: Address Resolution Protocol (ARP)

Packet

Frame

The Situation:

The router wishes to pass the packet to the

destination host or to a next-hop router.

The router knows the destination IP address of the target.

The router must learn the target’s MAC layer address

in order to be able to send the packet to the target in a frame.

The router uses the Address Resolution Protocol (ARP)

figure 8 12 address resolution protocol arp1
Figure 8-12: Address Resolution Protocol (ARP)

1: Router broadcasts ARP Request to all hosts and routers on the subnet.

figure 8 12 address resolution protocol arp2
Figure 8-12: Address Resolution Protocol (ARP)

2: ARP Reply sent by the host with the target IP address.

Other hosts ignore it.

This is the

Destination host

figure 8 12 address resolution protocol arp3
Figure 8-12: Address Resolution Protocol (ARP)

3.

Router puts the MAC address in its ARP cache; uses it for subsequent packets to the host

figure 8 13 multiprotocol label switching mpls
Figure 8-13: Multiprotocol Label Switching (MPLS)
  • Routers are Connected in a Mesh
    • Multiple alternative routes make the routing decision for each packet very expensive
  • PSDNs (Chapter 7) also are Arranged in a Mesh
    • However, a best path (virtual circuit) is set up before transmission begins
    • Once a VC is in place, subsequent frames are handled quickly and inexpensively
  • MPLS Does Something Like this for Routers
figure 8 13 multiprotocol label switching mpls1
Figure 8-13: Multiprotocol Label Switching (MPLS)
  • MPLS Adds a Label Before Each Packet
    • Label sits between the frame header and the IP header
    • Contains an MPLS label number
    • Like a virtual circuit number in a PSDN frame
    • Label-switching router merely looks up the MPLS label number in its MPLS table and sends the packet back out

IP

Packet

MPLS

Label

Data Link

Header

figure 8 13 multiprotocol label switching mpls2
Figure 8-13: Multiprotocol Label Switching (MPLS)

Label

Port

1

3

  • Advantages of MPLS
    • Router does a simple table lookup. This is fast and therefore inexpensive per packet handled
      • As fast as Ethernet switching!
    • Can use multiple label numbers to give traffic between two sites multiple levels of priority or quality of service guarantees
    • MPLS supports traffic engineering: balancing traffic on an internet

8

2

figure 8 13 multiprotocol label switching mpls3
Figure 8-13: Multiprotocol Label Switching (MPLS)

First router

adds the label

Last router

drops the label

figure 8 14 domain name system dns hierarchy
Figure 8-14: Domain Name System (DNS) Hierarchy

A domain is a group of resources

under the control of an organization.

The domain name system is a

general system for managing names.

It is a hierarchical naming system.

Queries to a DNS server can get

Information about a domain.

figure 8 14 domain name system dns hierarchy1
Figure 8-14: Domain Name System (DNS) Hierarchy

The highest level (0) is called the root.

There are 13 DNS Root Servers.

They point to lower-level servers.

figure 8 14 domain name system dns hierarchy2
Figure 8-14: Domain Name System (DNS) Hierarchy

Top-level domains are

generic TLDs (.com, .net., .org, etc.) or

country TLDs (.ca, .uk, .ie, etc.)

figure 8 14 domain name system dns hierarchy3
Figure 8-14: Domain Name System (DNS) Hierarchy

Organizations seek

good second-

level domain

names

cnn.com

microsoft.com

hawaii.edu

etc.

Firms get them from

address registrars

figure 8 14 domain name system dns hierarchy4
Figure 8-14: Domain Name System (DNS) Hierarchy

Host names are the bottom

of the DNS hierarchy.

A DNS request for a host name

will return its IP address.

figure 8 15 internet control message protocol icmp for supervisory messages
Figure 8-15: Internet Control Message Protocol (ICMP) for Supervisory Messages

ICMP is the supervisory protocol

at the internet layer.

ICMP messages are encapsulated in the

data fields of IP packets.

There are no transport or

Application layer headers or messages

figure 8 15 internet control message protocol icmp for supervisory messages1
Figure 8-15: Internet Control Message Protocol (ICMP) for Supervisory Messages

When an error occurs, the device

noting the error may try to respond with an

ICMP error message describing the problem.

ICMP error messages often are not sent

for security reasons because

attackers can use them to learn about a network

figure 8 15 internet control message protocol icmp for supervisory messages2
Figure 8-15: Internet Control Message Protocol (ICMP) for Supervisory Messages

To see if another host is active, a host

can send the target host an ICMP echo

message (called a ping).

If the host is active, it will send back an

echo response message confirming that it is active.

figure 8 16 dynamic host configuration protocol dhcp
Figure 8-16: Dynamic Host Configuration Protocol (DHCP)
  • DHCP Gives Each Client PC at Boot-Up:
    • A temporary IP Address (we saw this in Chapter 1)
    • A subnet mask
    • The IP addresses of local DNS servers
  • Better Than Manual Configuration
    • If subnet mask or DNS IP addresses change, only the DHCP server has to be updated manually
    • Client PCs are automatically updated when they next boot up
figure 8 17 ipv4 and ipv6 packets
Figure 8-17: IPv4 and IPv6 Packets

Bit 0

IP Version 4 Packet

Bit 31

Version

(4 bits)

Value

is 4

(0100)

Header

Length

(4 bits)

Diff-Serv

(8 bits)

Total Length

(16 bits)

Length in octets

Identification (16 bits)

Unique value in each original

IP packet

Flags

(3 bits)

Fragment Offset (13 bits)

Octets from start of

original IP fragment’s

data field

IPv4 is the dominant version of IP today.

The version number in its header is 4 (0100).

The header length and total length field tell the size of the packet.

The Diff-Serv field can be used for quality of service labeling.

(But MPLS is being used instead by most carriers)

Time to Live

(8 bits)

Protocol (8 bits)

1=ICMP, 6=TCP,

17=UDP

Header Checksum

(16 bits)

figure 8 17 ipv4 and ipv6 packets1
Figure 8-17: IPv4 and IPv6 Packets

The second row is used for reassembling fragmented

IP packets, but fragmentation is quite rare,

so we will not look at these fields.

Bit 0

IP Version 4 Packet

Bit 31

Version

(4 bits)

Value

is 4

(0100)

Header

Length

(4 bits)

Diff-Serv

(8 bits)

Total Length

(16 bits)

Length in octets

Identification (16 bits)

Unique value in each original

IP packet

Flags

(3 bits)

Fragment Offset (13 bits)

Octets from start of

original IP fragment’s

data field

Time to Live

(8 bits)

Protocol (8 bits)

1=ICMP, 6=TCP,

17=UDP

Header Checksum

(16 bits)

figure 8 17 ipv4 and ipv6 packets2
Figure 8-17: IPv4 and IPv6 Packets

The sender sets the time-to-live value (usually 64 to 128).

Each router along the way decreases the value by one.

A router decreasing the value to zero discards the packet.

It may send an ICMP error message.

The protocol field describes the message in the data field

(1=ICMP, 2=TCP, 3=UDP, etc.)

The header checksum is used to find errors in the header.

If a packet has an error, the router drops it.

There is no retransmission at the internet layer,

so the internet layer is still unreliable.

Bit 0

IP Version 4 Packet

Bit 31

Version

(4 bits)

Value

is 4

(0100)

Header

Length

(4 bits)

Diff-Serv

(8 bits)

Total Length

(16 bits)

Length in octets

Identification (16 bits)

Unique value in each original

IP packet

Flags

(3 bits)

Fragment Offset (13 bits)

Octets from start of

original IP fragment’s

data field

Time to Live

(8 bits)

Protocol (8 bits)

1=ICMP, 6=TCP,

17=UDP

Header Checksum

(16 bits)

figure 8 17 ipv4 and ipv6 packets3
Figure 8-17: IPv4 and IPv6 Packets

Bit 0

IP Version 4 Packet

Bit 31

Source IP Address (32 bits)

Destination IP Address (32 bits)

Options (if any)

Padding

Data Field

The source and destination IP addresses

Are 32 bits long, as you would expect.

Options can be added, but these are rare.

figure 8 17 ipv4 and ipv6 packets4
Figure 8-17: IPv4 and IPv6 Packets

IP Version 6 is the emerging

version of the Internet protocol.

Has 128 bit addresses for

an almost unlimited number of IP addresses.

Needed because of rapid growth in Asia.

Also needed because of the exploding

number of mobile devices

Bit 0

IP Version 6 Packet

Bit 31

Version

(4 bits)

Value

is 6

(0110)

Diff-Serv

(8 bits)

Flow Label (20 bits)

Marks a packet as part of a specific flow

Payload Length

(16 bits)

Next Header

(8 bits) Name

of next header

Hop Limit

(8 bits)

Source IP Address (128 bits)

Destination IP Address (128 bits)

Next Header or Payload (Data Field)

figure 8 18 tcp segment and udp datagram
Figure 8-18: TCP Segment and UDP Datagram

Bit 0

TCP Segment

Bit 31

Source Port Number (16 bits)

Destination Port Number (16 bits)

Sequence Number (32 bits)

Acknowledgment Number (32 bits)

The source and destination port numbers

specify a particular application on the

source and destination multitasking computers

(Discussed later)

Sequence numbers are 32 bits long.

So are acknowledgment numbers.

Header

Length

(4 bits)

Reserved

(6 bits)

Flag Fields

(6 bits)

Window Size

(16 bits)

TCP Checksum (16 bits)

Urgent Pointer (16 bits)

Flag fields are one-bit fields. They include SYN, ACK, FIN,

and RST.

figure 8 18 tcp segment and udp datagram1
Figure 8-18: TCP Segment and UDP Datagram

Flags are one-bit fields.

If a flag’s value is 1, it is “set”.

If a flag’s value is 0, it is “not set.”

TCP has six flags

If the TCP Checksum field’s value is correct,

The receiving process sends back an acknowledgment.

Bit 0

TCP Segment

Bit 31

Source Port Number (16 bits)

Destination Port Number (16 bits)

Sequence Number (32 bits)

Acknowledgment Number (32 bits)

Header

Length

(4 bits)

Reserved

(6 bits)

Flag Fields

(6 bits)

Window Size

(16 bits)

TCP Checksum (16 bits)

Urgent Pointer (16 bits)

figure 8 18 tcp segment and udp datagram2
Figure 8-18: TCP Segment and UDP Datagram

For flow control (to tell the other party to slow down),

The sender places a small value in the Window Size field.

If the Window Size is small, the receiver will have to stop transmitting

after a few more segments (unless it gets a new acknowledgment

extending the number of segments it may send.)

Bit 0

TCP Segment

Bit 31

Source Port Number (16 bits)

Destination Port Number (16 bits)

Sequence Number (32 bits)

Acknowledgment Number (32 bits)

Header

Length

(4 bits)

Reserved

(6 bits)

Flag Fields

(6 bits)

Window Size

(16 bits)

TCP Checksum (16 bits)

Urgent Pointer (16 bits)

figure 8 18 tcp segment and udp datagram3
Figure 8-18: TCP Segment and UDP Datagram

Bit 0

TCP Segment

Bit 31

Options (if any)

Padding

Data Field

TCP segment headers can end with options.

Unlike IPv4 options,

TCP options are very common.

If an option does not end at a 32-bit boundary,

padding must be added.

figure 8 18 tcp segment and udp datagram4
Figure 8-18: TCP Segment and UDP Datagram

Bit 0

UDP Datagram

Bit 31

Source Port Number (16 bits)

Destination Port Number (16 bits)

UDP Length (16 bits)

UDP Checksum (16 bits)

Data Field

UDP messages (datagrams) are very simple.

Like TCP, UDP has 16-bit port numbers.

The UDP length field allows variable-length application messages.

If the UDP checksum is correct, there is no acknowledgment.

If the UDP checksum is incorrect, the UDP datagram is dropped.

figure 8 19 tcp connection openings and closings
Figure 8-19: TCP Connection Openings and Closings
  • TCP is a connection-oriented protocol
    • Each connection has a formal opening process
    • Each connection has a formal closing process
    • During a connection, each TCP segment is acknowledged
      • (Of course, pure acknowledgments are not acknowledged)
figure 8 19 tcp connection openings and closings1
Figure 8-19: TCP Connection Openings and Closings

Normal Three-Way Opening

SYN

SYN/ACK

ACK

A SYN segment is a segment in which the SYN bit is set.

One side sends a SYN segment requesting an opening.

The other side sends a SYN/acknowledgment segment.

Originating side acknowledges the SYN/ACK.

figure 8 19 tcp connection openings and closings2
Figure 8-19: TCP Connection Openings and Closings

Normal Four-Way Close

FIN

ACK

FIN

ACK

A FIN segment is a segment in which the FIN bit is set.

Like both sides saying “good bye” to end a conversation.

figure 8 19 tcp connection openings and closings3
Figure 8-19: TCP Connection Openings and Closings

Abrupt Reset

RST

An RST segment is a segment in which the RST bit is set.

A single RST segment breaks a connection.

Like hanging up during a phone call.

There is no acknowledgment.

tcp and udp port numbers
TCP and UDP Port Numbers
  • Computers are multitasking devices
    • They run multiple applications at the same time
    • On a server, a port number designates a specific applications

HTTP Webserver

Application

SMTP E-Mail

Applications

Port 25

Port 80

Server

tcp and udp port numbers1
TCP and UDP Port Numbers
  • Major Applications Have Well-Known Port Numbers
    • 0 to 1023 for both TCP and UDP
    • HTTP is TCP Port 80
    • SMTP is TCP Port 25

HTTP Webserver

Application

SMTP E-Mail

Applications

Port 25

Port 80

Server

tcp and udp port numbers2
TCP and UDP Port Numbers
  • Clients Use Ephemeral Port Numbers
    • 1024 to 4999 for Windows Client PCs
    • A client has a separate port number for each connection to a program on a server

E-Mail

Application

on Mail

Server

Webserver

Application

on Webserver

Port 4400

Port 3270

Client

figure 8 20 use of tcp and udp port numbers
Figure 8-20: Use of TCP (and UDP) Port Numbers

A socket is an

IP address, a colon, and a port number.

1.33.17.3:80

123.30.17.120:25

128.171.17.13:2849

It represents a specific application (Port number)

on a specific server (IP address)

Or a specific connection on a client.

Client 60.171.18.22

Webserver

1.33.17.13

Port 80

SMTP Server

123.30.17.120

Port 25

Client PC

128.171.17.13

Port 2849

figure 8 20 use of tcp and udp port numbers1
Figure 8-20: Use of TCP (and UDP) Port Numbers

Client

60.171.18.22

Source: 60.171.18.22:2707

Destination: 1.33.17.13:80

This shows sockets for a client

packet sent to a webserver application

on a webserver

Webserver

1.33.17.13

Port 80

SMTP Server

123.30.17.120

Port 25

figure 8 20 use of tcp and udp port numbers2
Figure 8-20: Use of TCP (and UDP) Port Numbers

Client

60.171.18.22

Source: 60.171.18.22:2707

Destination: 1.33.17.13:80

Source: 1.33.17.13:80

Destination: 60.171.18.22:2707

Webserver

1.33.17.13

Port 80

Sockets in

two-way

transmission

SMTP Server

123.30.17.120

Port 25

figure 8 20 use of tcp and udp port numbers3
Figure 8-20: Use of TCP (and UDP) Port Numbers

Client

60.171.18.22

Source: 60.171.18.22:2707

Destination: 1.33.17.13:80

Source: 1.33.17.13:80

Destination: 60.171.18.22:2707

Webserver

1.33.17.13

Port 80

Source: 60.171.18.22:4400

Destination: 123.30.17.120:25

SMTP Server

123.30.17.120

Port 25

Clients use a different ephemeral

port number for different connections

figure 8 21 layer 3 switches and routers in site networks
Figure 8-21: Layer 3 Switches and Routers in Site Networks

Usually too expensive to replace workgroup switches.

Usually too limited in functionality to replace border routers.

Replaces core switches in the middle.