Charles M. Kozierok The TCP-IP Guide

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Host-to-Host Transport Layer (OSI Layer 4) Protocols

The transport layer contains the essential protocols TCP and UDP, as shown in Table 23.

Application Layer (OSI Layer 5/6/7) Protocols

As discussed in the topic on the TCP/IP model, in TCP/IP the single application layer

covers the equivalent of OSI layers 5, 6 and 7. The application protocols covered in this

Guide are shown in Table 24.

Table 23: TCP/IP Protocols: Host-to-Host Transport Layer (OSI Layer 4)

Protocol Name

Protocol

Abbr.

Description

Transmission Control Protocol TCP

The main transport layer protocol for TCP/IP. Establishes

and manages connections between devices and ensures

reliable and flow-controlled delivery of data using IP.

User Datagram Protocol UDP

A transport protocol that can be considered a “severely

stripped-down” version of TCP. It is used to send data in

a simple way between application processes, without the

many reliability and flow management features of TCP,

but often with greater efficiency.

Table 24: TCP/IP Protocols: Application Layer (OSI Layer 5/6/7) (Page 1 of 2)

Protocol Name

Protocol

Abbr.

Description

Domain Name System DNS

Provides the ability to refer to IP devices using names

instead of just numerical IP addresses. Allows machines

to resolve these names into their corresponding IP

addresses.

Network File System NFS

Allows files to be shared seamlessly across TCP/IP

networks.

Bootstrap Protocol BOOTP

Developed to address some of the issues with RARP

and used in a similar manner: to allow the configuration

of a TCP/IP device at startup. Generally superseded by

DHCP.

Dynamic Host

Configuration Protocol

DHCP

A complete protocol for configuring TCP/IP devices and

managing IP addresses. The successor to RARP and

BOOTP, it includes numerous features and capabilities.

Simple Network Management

Protocol

SNMP

A full-featured protocol for remote management of

networks and devices.

Remote Monitoring RMON

A diagnostic “protocol” (really a part of SNMP) used for

remote monitoring of network devices.

File Transfer Protocol, Trivial

File Transfer Protocol

FTP, TFTP

Protocols designed to permit the transfer of all types of

files from one device to another.

RFC 822, Multipurpose Internet

Mail Extensions, Simple Mail

Transfer Protocol, Post Office

Protocol, Internet Message

Access Protocol

RFC 822,

MIME,

SMTP, POP,

IMAP

Protocols that define the formatting, delivery and storage

of electronic mail messages on TCP/IP networks.

Network News Transfer

Protocol

NNTP

Enables the operation of the Usenet online community

by transferring Usenet news messages between hosts.

Hypertext Transfer Protocol HTTP

Transfers hypertext documents between hosts; imple-

ments the World Wide Web.

Gopher Protocol Gopher

An older document retrieval protocol, now largely

replaced by the World Wide Web.

Telnet Protocol Tel net

Allows a user on one machine to establish a remote

terminal session on another.

Berkeley “r” Commands —

Permit commands and operations on one machine to be

performed on another.

Internet Relay Chat IRC Allows real-time chat between TCP/IP users.

Administration and Trouble-

shooting Utilities and Protocols

—

A collection of software tools that allows administrators

to manage, configure and troubleshoot TCP/IP

internetworks.

Table 24: TCP/IP Protocols: Application Layer (OSI Layer 5/6/7) (Page 2 of 2)

Protocol Name

Protocol

Abbr.

Description

TCP/IP Lower-Layer (Interface, Internet and

Transport) Protocols (OSI Layers 2, 3 and 4)

The TCP/IP protocol suite is largely defined in terms of the protocols that constitute it;

several dozen are covered in this Guide. Most of the critical protocols of the suite function at

the lower layers of the OSI Reference Model: layers 2, 3 and 4, which correspond to the

network interface, internet and transport layers in the TCP/IP model architecture. Included

here are the all-important Internet Protocol (IP) at layer 3 and Transmission Control

Protocol (TCP) at layer 4, which combine to give TCP/IP its name.

Due to the importance of these and other TCP/IP protocols at the lower layers, this is the

largest chapter of The TCP/IP Guide. It contains four subsections. The first describes the

two TCP/IP protocols that reside at the network interface layer, layer 2 of the OSI model:

PPP and SLIP. The second describes a couple of “special” protocols that reside architec-

turally between layers 2 and 3: ARP and RARP. The third covers the TCP/IP internet layer

(OSI network layer, layer 3), including IP and several other related and support protocol.

The fourth describes the TCP/IP transport layer protocols TCP and UDP.

Background Information: The high-level section on the TCP/IP protocol suite

describes it in general terms and lays out its architecture and key protocols. If you

have not already read through it, I strongly recommend that you consider doing so

before proceeding here.

TCP/IP Network Interface Layer (OSI Data Link Layer)

Protocols

The lowest layer of the OSI Reference Model is the physical layer, which is responsible for

the "nitty gritty" details of transmitting information from one place to another on a network.

The layer just above the physical layer is the data link layer, called the network interface

layer or just the link layer in the TCP/IP architectural model. Its primary job is to implement

networks at the local level, and to interface between the hardware-oriented physical layer,

and the more abstract, software-oriented functions of the network layer and those above it.

In this section I provide a description of the two protocols that reside at the data link or

network interface layer in the TCP/IP protocol suite. These are the Serial Line Internet

Protocol (SLIP) and the Point-to-Point Protocol (PPP). They are both discussed in a single

subsection below this one. (The reason why this section contains only one subsection is

that the TCP/IP Guide is part of a larger networking reference that contains coverage for

many more technologies and protocols at this layer.)

TCP/IP Serial Line Internet Protocol (SLIP) and Point-to-Point

Protocol (PPP)

The TCP/IP protocol suite is structured around the Internet Protocol (IP). IP operates at

layer three of the OSI Reference Model, and assumes that it will be layered on top of an

existing layer two technology. However, certain types of connections exist that do not

include a layer two protocol over which IP can run. To enable TCP/IP to operate on these

kinds of links, two special TCP/IP data link layer protocols have been created.

In this section I describe the protocols used specifically to implement data link layer connec-

tivity for TCP/IP internetworks. I begin with a brief overview of the two protocols used to

connect devices at layer two and how they fit into the TCP/IP protocol suite as a whole. I

then have a single topic that describes the operation of the rather simple Serial Line Internet

Protocol (SLIP), and a comprehensive section on the much more capable Point-to-Point

Protocol (PPP).

Background Information: I want to emphasize again that IP is really the

foundation of the TCP/IP protocol suite, even though SLIP and PPP are at a lower

architectural layer. For this reason, I strongly recommend at least a basic under-

standing of the Internet Protocol before proceeding with this section. If you are going to

read about both IP and SLIP/PPP, I recommend reading about IP first.

SLIP and PPP Overview and Role In TCP/IP

The TCP/IP protocol suite was generally designed to provide implementation of the

networking stack from the network layer (layer three) and above. The core protocols of

TCP/IP operate at layers three and four of the OSI model, corresponding to the Internet

layer and Host-to-Host Transport layer of the TCP/IP architectural model. Other support

protocols are defined at these two layers, and many application protocols run there too, as

well as at the upper layers of the protocol stack.

However, the TCP/IP architectural model also defines the Network Interface layer, which

corresponds to the data link layer (layer two) in the OSI scheme. In most classical networks,

TCP/IP doesn't define any protocols operating at this layer. TCP/IP assumes that layer two

functionality is provided by a WAN or LAN technology like Ethernet, Token Ring, or IEEE

802.11. These technologies are responsible for the classical layer two functions: physical

layer addressing, media access control, and especially, layer two framing of datagrams

received from layer three.

Note: I'm ignoring the protocols ARP and RARP for now, since they fall into a

“gray area”, which I discuss in the section that covers them.

Why TCP/IP Needs Network Interface Layer Protocols

There's a problem with the assumption that IP can run on top of an existing layer two

protocol: sometimes there isn't one. There are certain technologies that establish only a

basic, low-level connection at the physical layer. The most common example of this is a

simple serial connection established between two devices. Years ago, it was fairly common

for two computers to just be connected using serial ports instead of a full-fledged LAN

protocol. Today, we see this much less, but there's another type of serial connection that is

very popular: serial dial-up networking. When you use a modem to connect to a modem at

your Internet Service Provider, the modems negotiate a connection that architecturally

exists only at the physical layer.

Since the Internet Protocol assumes certain services will be provided at layer two, there is

no way to make it operate directly over a serial line or other physical layer connection. The

most important layer two function that is required at a minimum is some mechanism for

framing the IP datagram for transmission—that is, providing the necessary data packaging

to let datagrams be transmitted over the physical layer network. Without this, IP datagrams

cannot be sent over the link.

TCP/IP Network Interface Layer Protocols: SLIP and PPP

We need something to “fill the gap” between IP at layer three and the physical connection at

layer one. To this end, a pair of special protocols have been defined that operate at layer

two and provide the services that IP requires to function. These are:

☯ Serial Line Internet Protocol (SLIP): A very simple layer two protocol that provides

only basic framing for IP.

☯ Point-to-Point Protocol (PPP): A more complex, full-featured data link layer protocol

that provides framing as well as many additional features that improve security and

performance.

Comparing SLIP and PPP

Both SLIP and PPP are designed for connections that go between just two devices; thus

the name “Point-to-Point” Protocol for PPP. They are used in port topology LAN or WAN

connections, the simplest type. Since there are only two devices, A and B, communication

is straight-forward: A sends to B and B sends to A. Since they only deal with simple two-

device connections, they do not have to worry about complexities like media access control,

or collisions, or unique addressing schemes, the way technologies like Ethernet must. As

mentioned earlier, the primary focus of these protocols is providing framing services to layer

three, as well as extra features as needed.

Why have two protocols? They both get the job done; the difference is in how they do it.

SLIP is extremely simple and easy to implement but lacks the features of PPP, like authen-

tication, compression, error detection and more. PPP is full-featured but much more

complicated.

To draw an analogy, SLIP is a mostly-sturdy, ten-year old compact sedan, while PPP is a

shiny new luxury SUV. Both will get you from here to Grandma's house, but the SUV is

going to be safer, more comfortable and better able to deal with problems that might crop

up on the road. If they cost the same to buy and operate, you'd probably choose the SUV.

Both SLIP and PPP cost about the same, and unlike an SUV, PPP causes no air pollution

and doesn't guzzle gas. For this reason, PPP is the choice of most serial line connections

today, and has all but replaced SLIP.

Key Concept: SLIP and PPP provide layer two connectivity for TCP/IP implementa-

tions that run directly over a physical layer link without a layer two technology. SLIP

is simple and was more commonly used years ago, but today PPP is favored due to

its many features and capabilities.

Are SLIP and PPP “Really” Part of TCP/IP?

Incidentally, I should mention that there are some people who don't even consider SLIP and

PPP to be part of the “true” TCP/IP protocol suite. They argue that TCP/IP really is defined

at layers three and up on the OSI model, and IP itself is the basis of TCP/IP at layer three.

Thus, SLIP and PPP are just “extra” protocols that can be used under TCP/IP. To support

their argument they point to the fact that PPP can be used for protocols other than IP (which

is true). For its part, SLIP is so simple it could carry any layer-three protocol, but I don't

believe it has been implemented for network layer protocols other than IP.

Frankly, I consider these to be “how many angels fit on the head of a pin” type arguments.

As far as I am concerned, SLIP and PPP are part of TCP/IP because they were originally

designed for the specific purpose of letting TCP/IP run on layer one links. They were

defined using the normal Internet RFC process as well. Regardless, whether they are part

of TCP/IP or not, they are used by millions of people every day to enable TCP/IP networks

to operate, and thus are deserving of a place in this Guide.

Serial Line Internet Protocol (SLIP)

The need for a data link layer protocol to let IP operate over serial links was identified very

early on in the development of TCP/IP. Engineers working on the Internet Protocol needed

a way to send IP datagrams over serial connections linking computers together. To solve

the problem they created a very simple protocol that would frame IP datagrams for trans-

mission across the serial line. This protocol is called the Serial Line Internet Protocol, or

SLIP for short.

SLIP Overview and History

SLIP is different from most TCP/IP protocols in that it has never been defined as a

formalized standard. It was created informally in the early 1980s and its use spread as a de

facto standard before it was ever described in an RFC document. Even when it was

eventually published, in 1988, the decision was specifically made that SLIP would not be

designated an official Internet standard. The authors of the paper that describes it, RFC

1055, made sure nobody would miss this point, by naming it A Nonstandard For Trans-

mission Of IP Datagrams Over Serial Lines: SLIP.

On The Web: This is but one of several places where the IETF engineers let their

sense of humor shine through. For another interesting example, see RFC 1313. ☺

Why was SLIP designated as a “nonstandard” instead of a standard? Well, it was

developed as a very rudimentary “stopgap” measure to provide layer two framing when

needed. It's so simple that there really isn't much to standardize. Also, the protocol has so

many deficiencies that the IETF apparently didn't want it given the status of a formalized

standard. RFC 1055 makes specific mention of the problems with SLIP (which we'll see

below) and the fact that work was already underway at that time to define a more capable

successor to SLIP (PPP).

How simple is SLIP? So simple that it is one of the very few technologies in this Guide that

I can describe almost completely without complaining that it's complicated, or resorting to

telling you to “see the defining document for details”. SLIP performs only one function:

framing of data for transmission. It does nothing else.

Key Concept: SLIP provides a layer two framing service for IP datagrams—and no

other features or capabilities.

SLIP Basic Data Framing Method and General Operation

Here's how SLIP framing works. An IP datagram is passed down to SLIP, which breaks it

into bytes and sends them one at a time over the link. After the last byte of the datagram, a

special byte value is sent that tells the receiving device that the datagram has ended. This

is called the SLIP END character, and has a byte value of 192 decimal (C0 hexadecimal,

11000000 binary). And that's basically it: take the whole datagram, send it one byte at a

time, and then send the byte 192 to delimit the end of the datagram.

A minor enhancement to this basic operation is to precede the datagram by an END

character as well. The benefit of this is that it clearly separates the start of the datagram

from anything that preceded it. To see why this might be needed, suppose at a particular

time we have only one datagram to send, datagram #1. So, we send #1, and then send the

END character to delimit it. Now, suppose there is a pause before the next datagram shows

up. During that time we aren't transmitting, but if there is line noise, the other device might

pick up spurious bytes here and there. If we later receive datagram #2 and just start

sending it, the receiving device might think the noise bytes were part of datagram #2.

Starting datagram #2 off with an END character tells the recipient that anything received

between this END character and the previous one is a separate datagram. If that's just

noise, then this “noise datagram” is just gibberish that will be rejected at the IP layer.

Meanwhile, it doesn't corrupt the real datagram we wish to send. If no noise occurred on the

line between datagrams then the recipient will just see the END at the start of datagram #2

right after the one at the end of #1, and will ignore the “null datagram” between the two.

Escaping Special Characters

There is only one other issue SLIP deals with. If the END character is 192 decimal, what

happens if the byte value 192 appears in the datagram itself? Transmitting it “as is” would

fool the recipient into thinking the datagram ended prematurely. To avoid this, a special

Escape character (ESC) is defined, which has a decimal value of 219 (DB in hex, 11011011

in binary). The term “escape” means that this symbol conveys the meaning “this byte and

the next one have a special meaning”. When a value of 192 appears in the datagram, the

sending device replaces it by the ESC character (219 decimal) followed by the value 220

decimal. Thus, a single “192” becomes “219 220” (or “DB DC” in hexadecimal). The

recipient translates back from “219 220” to “192”.

Note: The SLIP ESC character is not the same as the ASCII ESC character. They

both perform an “escaping” operation but are otherwise unrelated.

This leaves one final problem: what happens if the escape character itself is in the original

datagram? That is, what if there's a byte value of 219 in the IP datagram to be sent? This is

handled by a similar substitution: instead of “219” we put “219 221”.

So in summary, this is basically everything SLIP does: