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Internet Protocols 30-
Internet Protocols
Background
The Internet protocols are the world’s most popular open-system (nonproprietary) protocol suite
because they can be used to communicate across any set of interconnected networks and are equally
well suited for LAN and WAN communications. The Internet protocols consist of a suite of
communication protocols, of which the two best known are the Transmission Control Protocol
(TCP) and the Internet Protocol (IP). The Internet protocol suite not only includes lower-layer
protocols (such as TCP and IP), but it also specifies common applications such as electronic mail,
terminal emulation, and file transfer. This chapter provides a broad introduction to specifications that
comprise the Internet protocols. Discussions include IP addressing and key upper-layer protocols
used in the Internet. Specific routing protocols are addressed individually in Part 6, Routing
Protocols.
Internet protocols were first developed in the mid-1970s, when the Defense Advanced Research
Projects Agency (DARPA) became interested in establishing a packet-switched network that would
facilitate communication between dissimilar computer systems at research institutions. With the
goal of heterogeneous connectivity in mind, DARPA funded research by Stanford University and
Bolt, Beranek, and Newman (BBN). The result of this development effort was the Internet protocol
suite, completed in the late 1970s.
TCP/IP later was included with Berkeley Software Distribution (BSD) UNIX and has since become
the foundation on which the Internet and the World Wide Web (WWW) are based.
Documentation of the Internet protocols (including new or revised protocols) and policies are
specified in technical reports called Request For Comments (RFCs), which are published and then
reviewed and analyzed by the Internet community. Protocol refinements are published in the new
RFCs. To illustrate the scope of the Internet protocols, Figure 30-1 maps many of the protocols of
the Internet protocol suite and their corresponding OSI layers. This chapter addresses the basic
elements and operations of these and other key Internet protocols.
Internet Protocol (IP)
30-2 Internetworking Technology Overview, June 1999
Figure 30-1 Internet protocols span the complete range of OSI model layers.
Internet Protocol (IP)
The Internet Protocol (IP) is a network-layer (Layer 3) protocol that contains addressing information
and some control information that enables packets to be routed. IP is documented in RFC 791 and
is the primary network-layer protocol in the Internet protocol suite. Along with the Transmission
Control Protocol (TCP), IP represents the heart of the Internet protocols. IP has two primary
responsibilities: providing connectionless, best-effort delivery of datagrams through an
internetwork; and providing fragmentation and reassembly of datagrams to support data links with
different maximum-transmission unit (MTU) sizes.
IP Packet Format
An IP packet contains several types of information, as illustrated in Figure 30-2.
Presentation
Application
Network
Transport
Link
Physical
Reference Model^ OSI Internet Protocol Suite
Session
NFS
XDR
RPC
SMTP, SNMPFTP, Telnet,
Not Specified
IP ICMP
TCP, UDP
ith
Routing Protocols ARP, RARP
Internet Protocol (IP)
30-4 Internetworking Technology Overview, June 1999
- Options —Allows IP to support various options, such as security.
- Data —Contains upper-layer information.
IP Addressing
As with any other network-layer protocol, the IP addressing scheme is integral to the process of
routing IP datagrams through an internetwork. Each IP address has specific components and follows
a basic format. These IP addresses can be subdivided and used to create addresses for subnetworks,
as discussed in more detail later in this chapter.
Each host on a TCP/IP network is assigned a unique 32-bit logical address that is divided into two
main parts: the network number and the host number. The network number identifies a network and
must be assigned by the Internet Network Information Center (InterNIC) if the network is to be part
of the Internet. An Internet Service Provider (ISP) can obtain blocks of network addresses from the
InterNIC and can itself assign address space as necessary. The host number identifies a host on a
network and is assigned by the local network administrator.
IP Address Format
The 32-bit IP address is grouped eight bits at a time, separated by dots, and represented in decimal
format (known as dotted decimal notation ). Each bit in the octet has a binary weight (128, 64, 32,
16, 8, 4, 2, 1). The minimum value for an octet is 0, and the maximum value for an octet is 255.
Figure 30-3 illustrates the basic format of an IP address.
Figure 30-3 An IP address consists of 32 bits, grouped into four octets.
IP Address Classes
IP addressing supports five different address classes: A, B,C, D, and E. Only classes A, B, and C are
available for commercial use. The left-most (high-order) bits indicate the network class. Table 30-
provides reference information about the five IP address classes.
32 Bits Network Host
8 Bits
172
DottedDecimal Notation
8 Bits 8 Bits 8 Bits
Internet Protocols 30-
IP Address Classes
Table 30-1 Reference Information About the Five IP Address Classes
Figure 30-4 illustrates the format of the commercial IP address classes. (Note the high-order bits in
each class.)
Figure 30-4 IP address formats A, B, and C are available for commercial use.
The class of address can be determined easily by examining the first octet of the address and
mapping that value to a class range in the following table. In an IP address of 172.31.1.2, for
example, the first octet is 172. Because 172 falls between 128 and 191, 172.31.1.2 is a Class B
address. Figure 30-5 summarizes the range of possible values for the first octet of each address class.
IPAddre ssClass Format Purpose High-OrderBit(s) Address Range No. BitsNetwork/Host Max. Hosts A N.H.H.H 1
1 N = Network number, H = Host number.
Few largeorganizations 0 1.0.0.0 to 126.0.0.0 7/24 16,777, 214(2 24 – 2)^2
2 One address is reserved for the broadcast address, and one address is reserved for the network.
B N.N.H.H Medium-sizeorganizations 1, 0 128.1.0.0 to191.254.0.0 14/16 65, 543 (22)^16 – C N.N.N.H Relatively smallorganizations 1, 1, 0 192.0.1.0 to223.255.254.0 22/8 245 (2^8 – 2) D N/A Multicast groups(RFC 1112) 1, 1, 1, 0 224.0.0.0 to239.255.255.255 N/A (not forcommercial use) N/A E N/A Experimental 1, 1, 1, 1 240.0.0.0 to254.255.255.255 N/A N/A
Class C
Class B
Class A
1 0 Network
1 1 0 Network
No. Bits 7 24
0 Network Host Host Host
Network Host Host
Network Network Host 24143
Internet Protocols 30-
IP Address Classes
Figure 30-6 Bits are borrowed from the host address field to create the subnet addressfield.
Subnet masks use the same format and representation technique as IP addresses. The subnet mask,
however, has binary 1s in all bits specifying the network and subnetwork fields, and binary 0s in all
bits specifying the host field. Figure 30-7 illustrates a sample subnet mask.
Figure 30-7 A sample subnet mask consists of all binary 1s and 0s.
Subnet mask bits should come from the high-order (left-most) bits of the host field, as Figure 30-
illustrates. Details of Class B and C subnet mask types follow. Class A addresses are not discussed
in this chapter because they generally are subnetted on an 8-bit boundary.
Network Host Host
Network Network^ Subnet^ Host
Class B Address: Before Subnetting
Class B Address: After Subnetting
1 0 Network
1 0
Network 11111111
Network 11111111
Subnet 11111111
Host 00000000
255 255 255 0
Binaryrepresentation
Dotted decimalrepresentation 24145
Internet Protocol (IP)
30-8 Internetworking Technology Overview, June 1999
Figure 30-8 Subnet mask bits come from the high-order bits of the host field.
Various types of subnet masks exist for Class B and C subnets.
The default subnet mask for a Class B address that has no subnetting is 255.255.0.0, while the subnet
mask for a Class B address 171.16.0.0 that specifies eight bits of subnetting is 255.255.255.0. The
reason for this is that eight bits of subnetting or 2 8 – 2 (1 for the network address and 1 for the
broadcast address) = 254 subnets possible, with 2 8 – 2 = 254 hosts per subnet.
The subnet mask for a Class C address 192.168.2.0 that specifies five bits of subnetting is
255.255.255.248.With five bits available for subnetting, 2 5 – 2 = 30 subnets possible, with
23 – 2 = 6 hosts per subnet.
The reference charts shown in table 30–2 and table 30–3 can be used when planning Class B and C
networks to determine the required number of subnets and hosts, and the appropriate subnet mask.
Table 30-2 Class B Subnetting Reference Chart Number of Bits Subnet Mask Number of Subnets Number of Hosts 2 255.255.192.0 2 16382 3 255.255.224.0 6 8190 4 255.255.240.0 14 4094 5 255.255.248.0 30 2046 6 255.255.252.0 62 1022 7 255.255.254.0 126 510 8 255.255.255.0 254 254 9 255.255.255.128 510 126 10 255.255.255.192 1022 62 11 255.255.255.224 2046 30 12 255.255.255.240 4094 14
Internet Routing
30-10 Internetworking Technology Overview, June 1999
Figure 30-9 Applying a logical AND the destination IP address and the subnet maskproduces the subnetwork number.
Address Resolution Protocol ( ARP) Overview
For two machines on a given network to communicate, they must know the other machine’s physical
(or MAC) addresses. By broadcasting Address Resolution Protocols (ARPs), a host can dynamically
discover the MAC-layer address corresponding to a particular IP network-layer address.
After receiving a MAC-layer address, IP devices create an ARP cache to store the recently acquired
IP-to-MAC address mapping, thus avoiding having to broadcast ARPS when they want to recontact
a device. If the device does not respond within a specified time frame, the cache entry is flushed.
In addition to the Reverse Address Resolution Protocol (RARP) is used to map MAC-layer addresses
to IP addresses. RARP, which is the logical inverse of ARP, might be used by diskless workstations
that do not know their IP addresses when they boot. RARP relies on the presence of a RARP server
with table entries of MAC-layer-to-IP address mappings.
Internet Routing
Internet routing devices traditionally have been called gateways. In today’s terminology, however,
the term gateway refers specifically to a device that performs application-layer protocol translation
between devices. Interior gateways refer to devices that perform these protocol functions between
machines or networks under the same administrative control or authority, such as a corporation’s
internal network. These are known as autonomous systems. Exterior gateways perform protocol
functions between independent networks.
Routers within the Internet are organized hierarchically. Routers used for information exchange
within autonomous systems are called interior routers, which use a variety of Interior Gateway
Protocols (IGPs) to accomplish this purpose. The Routing Information Protocol (RIP) is an example
of an IGP.
Routers that move information between autonomous systems are called exterior routers. These
routers use an exterior gateway protocol to exchange information between autonomous systems. The
Border Gateway Protocol (BGP) is an example of an exterior gateway protocol.
Note Specific routing protocols, including BGP and RIP, are addressed in individual chapters
presented in Part 6 later in this book.
Network Subnet Host
Destination IPAddress
SubnetMask
16 1 0
24147
Internet Protocols 30-
IP Routing
IP Routing
IP routing protocols are dynamic. Dynamic routing calls for routes to be calculated automatically at
regular intervals by software in routing devices. This contrasts with static routing, where routers are
established by the network administrator and do not change until the network administrator changes
them.
An IP routing table, which consists of destination address/next hop pairs, is used to enable dynamic
routing. An entry in this table, for example, would be interpreted as follows: to get to network
172.31.0.0, send the packet out Ethernet interface 0 (E0).
IP routing specifies that IP datagrams travel through internetworks one hop at a time. The entire route
is not known at the onset of the journey, however. Instead, at each stop, the next destination is
calculated by matching the destination address within the datagram with an entry in the current
node’s routing table.
Each node’s involvement in the routing process is limited to forwarding packets based on internal
information. The nodes do not monitor whether the packets get to their final destination, nor does IP
provide for error reporting back to the source when routing anomalies occur. This task is left to
another Internet protocol, the Internet Control-Message Protocol (ICMP), which is discussed in the
following section.
Internet Control Message Protocol (ICMP)
The Internet Control Message Protocol (ICMP) is a network-layer Internet protocol that provides
message packets to report errors and other information regarding IP packet processing back to the
source. ICMP is documented in RFC 792.
ICMP Messages
ICMPs generate several kinds of useful messages, including Destination Unreachable, Echo Request
and Reply, Redirect, Time Exceeded, and Router Advertisement and Router Solicitation. If an ICMP
message cannot be delivered, no second one is generated. This is to avoid an endless flood of ICMP
messages.
When an ICMP destination-unreachable message is sent by a router, it means that the router is unable
to send the package to its final destination. The router then discards the original packet. Two reasons
exist for why a destination might be unreachable. Most commonly, the source host has specified a
nonexistent address. Less frequently, the router does not have a route to the destination.
Destination-unreachable messages include four basic types: network unreachable, host unreachable,
protocol unreachable, and port unreachable. Network-unreachable messages usually mean that a
failure has occurred in the routing or addressing of a packet. Host-unreachable messages usually
indicates delivery failure, such as a wrong subnet mask. Protocol-unreachable messages generally
mean that the destination does not support the upper-layer protocol specified in the packet.
Port-unreachable messages imply that the TCP socket or port is not available.
An ICMP echo-request message, which is generated by the ping command, is sent by any host to test
node reachability across an internetwork. The ICMP echo-reply message indicates that the node can
be successfully reached.
An ICMP Redirect message is sent by the router to the source host to stimulate more efficient
routing. The router still forwards the original packet to the destination. ICMP redirects allow host
routing tables to remain small because it is necessary to know the address of only one router, even if
that router does not provide the best path. Even after receiving an ICMP Redirect message, some
devices might continue using the less-efficient route.
Internet Protocols 30-
Positive Acknowledgment and Retransmission (PAR)
Each host randomly chooses a sequence number used to track bytes within the stream it is sending
and receiving. Then, the three-way handshake proceeds in the following manner:
The first host (Host A) initiates a connection by sending a packet with the initial sequence number
(X) and SYN bit set to indicate a connection request. The second host (Host B) receives the SYN,
records the sequence number X, and replies by acknowledging the SYN (with an ACK = X + 1).
Host B includes its own initial sequence number (SEQ = Y). An ACK = 20 means the host has
received bytes 0 through 19 and expects byte 20 next. This technique is called forward
acknowledgment. Host A then acknowledges all bytes Host B sent with a forward acknowledgment
indicating the next byte Host A expects to receive (ACK = Y + 1). Data transfer then can begin.
Positive Acknowledgment and Retransmission (PAR)
A simple transport protocol might implement a reliability-and-flow-control technique where the
source sends one packet, starts a timer, and waits for an acknowledgment before sending a new
packet. If the acknowledgment is not received before the timer expires, the source retransmits the
packet. Such a technique is called positive acknowledgment and retransmission (PAR).
By assigning each packet a sequence number, PAR enables hosts to track lost or duplicate packets
caused by network delays that result in premature retransmission. The sequence numbers are sent
back in the acknowledgments so that the acknowledgments can be tracked.
PAR is an inefficient use of bandwidth, however, because a host must wait for an acknowledgment
before sending a new packet, and only one packet can be sent at a time.
TCP Sliding Window
A TCP sliding window provides more efficient use of network bandwidth than PAR because it
enables hosts to send multiple bytes or packets before waiting for an acknowledgment.
In TCP, the receiver specifies the current window size in every packet. Because TCP provides a
byte-stream connection, window sizes are expressed in bytes. This means that a window is the
number of data bytes that the sender is allowed to send before waiting for an acknowledgment. Initial
window sizes are indicated at connection setup, but might vary throughout the data transfer to
provide flow control. A window size of zero, for instance, means “Send no data.”
In a TCP sliding-window operation, for example, the sender might have a sequence of bytes to send
(numbered 1 to 10) to a receiver who has a window size of five. The sender then would place a
window around the first five bytes and transmit them together. It would then wait for an
acknowledgment.
The receiver would respond with an ACK = 6, indicating that it has received bytes 1 to 5 and is
expecting byte 6 next. In the same packet, the receiver would indicate that its window size is 5. The
sender then would move the sliding window five bytes to the right and transmit bytes 6 to 10. The
receiver would respond with an ACK = 11, indicating that it is expecting sequenced byte 11 next. In
this packet, the receiver might indicate that its window size is 0 (because, for example, its internal
buffers are full). At this point, the sender cannot send any more bytes until the receiver sends another
packet with a window size greater than 0.
Transmission Control Protocol (TCP)
30-14 Internetworking Technology Overview, June 1999
TCP Packet Format
Figure 30-10 illustrates the fields and overall format of a TCP packet.
Figure 30-10 Twelve fields comprise a TCP packet.
TCP Packet Field Descriptions
The following descriptions summarize the TCP packet fields illustrated in Figure 30-10:
- Source Port and Destination Port —Identifies points at which upper-layer source and destination
processes receive TCP services.
- Sequence Number —Usually specifies the number assigned to the first byte of data in the current
message. In the connection-establishment phase, this field also can be used to identify an initial
sequence number to be used in an upcoming transmission.
- Acknowledgment Number —Contains the sequence number of the next byte of data the sender of
the packet expects to receive.
- Data Offset —Indicates the number of 32-bit words in the TCP header.
- Reserved —Remains reserved for future use.
- Flags —Carries a variety of control information, including the SYN and ACK bits used for
connection establishment, and the FIN bit used for connection termination.
- Window —Specifies the size of the sender’s receive window (that is, the buffer space available for
incoming data).
- Checksum —Indicates whether the header was damaged in transit.
- Urgent Pointer —Points to the first urgent data byte in the packet.
- Options —Specifies various TCP options.
- Data —Contains upper-layer information.
Sequence number
Options (+ padding)
Checksum
Data (variable)
Destination port
Acknowledgment number
Source port
Data offset Reserved Window Urgent pointer
Flags
S1344a
Internet Protocols Application-Layer Protocols
30-16 Internetworking Technology Overview, June 1999
Table 30-5 Higher-Layer Protocols and Their Applications Application Protocols File transfer FTP Terminal emulation Telnet Electronic mail SMTP Network management SNMP Distributed file services NFS, XDR, RPC, X Windows
