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Understanding Layer 2,
3, and 4 Protocols
hile many of the concepts well known to traditional Layer 2 and Layer 3 net-
working still hold true in content switching applications, the area introduces
new and more complex themes that need to be well understood for any success-
ful implementation. Within the discussion of content networking, we will
replace terms such as packets and frames with sessions and transactions as we move
our attention further up the OSI Seven Layer Model. Before we move into
these new terms, however, let’s look at some standard Layer 2, 3, and 4 network-
ing concepts.
The OSI Seven Layer Model—What Is a Layer?
Established in 1947, the International Organization for Standardization (ISO)
was formed to bring together the standards bodies from countries around the
world. Their definition of the model for Open Systems Interconnection, or
OSI, is used to define modes of interconnection between different components
in a networking system. This means that the physical method of transport can
be designed independently of the protocols and applications running over it. For
example, TCP/IP can be run over both Ethernet and FDDI networks, and
Novell’s IPX and Apple’s AppleTalk protocols can both be run over Token Ring
networks. These are examples of having independence between the physical net-
work type and the upper layer protocols running across them. Consider also, two
TCP/IP-enabled end systems communicating across a multitude of different
W
14 Chapter 2 ● UNDERSTANDING LAYER 2, 3, AND 4 PROTOCOLS
network types, such as Ethernet, Frame Relay, and ATM. Figure 2–1 shows the
OSI Seven Layer Model.
When we talk about Layer 2 and Layer 3 networking, it is these layers that
we’re referring to, and logically the further up the OSI model we move, the
greater intelligence we can use in networking decisions.
Each layer plays its part in moving data from one device to another across a net-
work infrastructure by providing a standard interface to the surrounding layers.
The Application Layer (Layer 7)
The top layer in the stack, the Application layer is where the end-user applica-
tion resides. Think of the Application layer as the browser application or email
client for a user surfing the Web or sending email. Many protocols are defined
for use at the Application layer, such as HTTP, FTP, SMTP, and Telnet.
In content switching terms, Layer 7 refers to the ability to parse information
directly generated by the user or application in decision making, such as the URL
typed by the user in the Web browser. For example, http://www.foocorp.com is an
example of Application layer data.
The Presentation Layer (Layer 6)
The Presentation layer is used to provide a common way for applications (resid-
ing at the Application layer) to translate between data formats or perform
encryption and decryption. Mechanisms to convert between text formats such
as ASCII and Unicode may be considered part of the Presentation layer, along
with compression techniques for image files such as GIF and JPEG.
Figure 2–1 The OSI Seven Layer Model.
7 Application Layer 6 Presentation Layer 5 Session Layer 4 Transport Layer 3 Network Layer 2 Data Link Layer 1 Physical Layer
16 Chapter 2 ● UNDERSTANDING LAYER 2, 3, AND 4 PROTOCOLS
forwarded. Hardware-based Layer 3 switches also use Layer 3 information in
forwarding decisions. Layer 3 routers and switches are not concerned whether
the packets contain HTTP, FTP, or SMTP data, but simply where the packet is
flowing to and from.
The Data Link Layer (Layer 2)
The Data Link layer also defines a lower level addressing structure to be used
between end systems as well as the lower level framing and checksums being
used to transmit onto the physical medium. Ethernet, Token Ring, and Frame
Relay are all examples of Data Link layer or Layer 2 protocols.
Traditional Ethernet switches operate at the Data Link layer and are con-
cerned with forwarding packets based on the Layer 2 addressing scheme. Layer
2 Ethernet switches are not concerned with whether the packet contains IP,
IPX, or AppleTalk, but only with where the MAC address of the recipient end
system resides.
The Physical Layer (Layer 1)
As with all computer systems, networking is ultimately about making, moving,
and storing 1s and 0s. In networking terms, the Physical layer defines how the
user’s browser application data is turned into 1s and 0s to be transmitted onto
the physical medium. The Physical layer defines the physical medium such as
cabling and interface specifications. AUI, 10Base-T, and RJ45 are all examples
of Layer 1 specifications.
Putting All the Layers Together
Let’s take an example of a Web user visiting the Web site of Foocorp, Inc.
Within the browser application, at the Application layer, the user will type in
the URL, typically something like http://www.foocorp.com/. While this is the
only input the user will provide the application, there is much more information
generated by the browser application itself, including:
- The type of browser being used (e.g., Microsoft Internet Explorer,
Netscape)
- The operating system running on the user’s machine
The OSI Seven Layer Model—What Is a Layer? 17
- The version of the HTTP protocol being used by the browser
- The language, or languages, supported by the browser (e.g., English,
Japanese, etc.)
- Any Presentation layer standards that are supported by the browser, such
as compression types, text formats, and file types
In terms of HTTP-based Web browser traffic, these pieces of information
can be thought of as the Application, Presentation, and Session layers of the
OSI model. They provide not only the raw data input by the user in the applica-
tion, but also information needed by the application to ensure successful com-
munication with the end system; in this case, a Web server at Foocorp. HTTP
information for the Web user would look something like:
Hypertext Transfer Protocol
GET / HTTP/1.0\r\n Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg\r\n Accept-Language: en-gb\r\n User-Agent: Mozilla/4.0 (compatible; MSIE 5.01; Windows NT 5.0)\r\n Host: www.foocorp.com\r\n Connection: Keep-Alive\r\n \r\n
Once this application information has been generated, it can be packaged and
passed on to the next layer for transport. HTTP requires a connection-oriented
Transport layer protocol to guarantee the delivery of each packet in the session.
Transmission Control Protocol (TCP) is used in HTTP applications to ensure
this successful packet delivery. Other applications will make use of different
Transport layer protocols. TFTP, for example, uses the User Datagram Protocol
(UDP) as its Layer 4 transport because it does not require the guaranteed deliv-
ery provided by TCP. Routing updates sent between Layer 3 devices can use
OSPF, RIP, or BGP as their Layer 4 transport.
At the Transport layer, information about the port numbers, sequence num-
bers, and checksums are included to provide reliable transport. The Layer 4
headers in our example would look something like:
Transmission Control Protocol Source port: 3347 (3347) Destination port: http (80) Sequence number: 52818332 Next sequence number: 52818709 Acknowledgement number: 3364222344
Switching at Different Layers 19
Source: 00:30:ab:17:0d:1a (00:30:ab:17:0d:1a) Type: IP (0x0800)
Figure 2–2 depicts this process of repackaging each layer with new header
information at the layer below.
Switching at Different Layers
Now that we’ve seen examples of different information available within different
layers of the OSI model, let’s look at how this information can be used to make
intelligent traffic forwarding decisions. Before the development of switching,
Ethernet relied on broadcast or flooding of packets to all end stations within a
network to forward traffic. Ethernet is effectively a shared medium with only
one Ethernet end station able to transmit at any time. Combine this with early
implementation techniques relying on every end station in an Ethernet network
seeing every packet, even if it was not addressed to it, and issues of scalability
quickly surface.
Figure 2–2 Passing data through the seven OSI layers.
http://www.foocorp.com/
...11001001001001101010101011010101000010010011010100011010010101...
User
Layer 4 Headers
Application, Presentation, and Session Layers
Layer 3 Headers
Layer 2 Headers
20 Chapter 2 ● UNDERSTANDING LAYER 2, 3, AND 4 PROTOCOLS
Layer 2 Switching
The first implementation of Ethernet or Layer 2 switching uses information in
the Ethernet headers to make traffic forwarding decisions. Intelligent switches
learn which ports have which end stations attached by recording the Ethernet
MAC addresses of packets ingressing the switch. Using this information along
with the ability to parse the Layer 2 headers of all packets means that a Layer 2
switch need only forward frames out of ports where it knows the end station to
be. For end station addresses that have not yet been learned, frames with
unknown destination MAC addresses are flooded out of every port in the switch
to force the recipient to reply. This will allow the switch to learn the relevant
MAC address, as it will be the source address on the reply frame.
Layer 2 switching is implemented along side Layer 3 routing for local area
networks to facilitate communication between devices in a common IP subnet.
As the information at this layer is relatively limited, the opportunity to configure
Layer 2 switches to interpret address information and act upon it in any way
other than described previously is generally not required. Many Layer 2 switches
will offer the ability to configure intelligent services such as Quality of Service
(QoS), bandwidth shaping, or VLAN membership based on the Layer 2 infor-
mation. Figure 2–3 shows a simplified Layer 2 frame with examples of informa-
tion that might be used to make switching decisions.
Layer 3 Switching and Routing
Traditional protocol routers work by using information in the Layer 3 headers of
Ethernet frames. While routing platforms exist for many different protocols
(e.g., IPX, AppleTalk, and DECNet), in TCP/IP terms a router or routing
device will typically use the destination IP address in the Layer 3 header to
make a forwarding decision. The main advantage of Layer 3 routing in its earli-
est guises was that it gave the network designer the ability to segregate the net-
work into distinct IP networks and carefully control the traffic and reachability
between each.
Figure 2–3 Example Layer 2 headers for switching.
Dst: 00:20:6f:14:58:2f Src: 00:30:ab:17:0d:1a OtherHeaders^ L2 Payload
22 Chapter 2 ● UNDERSTANDING LAYER 2, 3, AND 4 PROTOCOLS
IGMP, and IP itself can all be identified by a unique protocol number in the IP
header (see Figure 2–5).
The list of IP protocol numbers is administered and controlled by the Inter-
net Assigned Numbers Authority (IANA), and a comprehensive list can be
found at www.iana.org/. Table 2–1 lists some of the more common IP protocol
numbers.
Some Layer 4 protocols effectively operate at this layer alone. VRRP, for
example, uses Layer 4 headers to transport all information between a series of
participating routers in an IP subnet and consequently has no need for upper
layer protocol information. Its payload is simply the information contained at
Layer 4. Other routing protocols, such as the Border Gateway Protocol (BGP),
will use the reliable Layer 4 Transport layer protocol with the BGP routing
information and updates carried in the upper layer payloads.
In terms of content switching, the two most commonly understood Layer 4
protocols are TCP and UDP. The majority of the standard Application layer
protocols are carried either within TCP or UDP depending on whether there is
a requirement for a reliable end-to-end connection. Taking a Web user example,
the browser application needs to ensure that all packets are successfully delivered
when presenting the user with the desired Web page. The HTTP protocol will
Table 2–1 Some Examples of Common IP Protocol Numbers
IP PROTOCOL NUMBER LAYER 4 PROTOCOL
1 ICMP—Internet Control Message Protocol
6 TCP—Transmission Control Protocol
17 UDP—User Datagram Protocol
112 VRRP—Virtual Router Redundancy Protocol
Figure 2–5 Different IP protocol numbers identify which Layer 4 protocol is being used.
Src: 192.168.254.201 Dst: 216.239.51.101 IP Proto Payload
1 = ICMP 6 = TCP 17 = UDP
L2 Headers
Transport Control Protocol (TCP) 23
therefore rely on TCP as its Transport layer protocol, to guarantee delivery,
which in turn will use IP as its delivery mechanism.
Transport Control Protocol (TCP)
As the Layer 3 IP protocol is principally a connectionless and best-efforts delivery
mechanism, there is a requirement for many applications to ensure the correctly
sequenced delivery of all packets within a conversation. Consequently, many appli-
cations will use Transport Control Protocol (TCP) at Layer 4 to guarantee success-
ful delivery. TCP has several characteristics built in to ensure this delivery:
- Checksum : The TCP header contains a 16-bit data checksum that is
computed from all other data elements in the TCP header. The receiving
end station uses this checksum to ensure that the packet arrived without
corruption.
- Sequence and acknowledgment numbers : Each octet of data sent and
received by end stations has an associated sequence number associated
with it. These sequence numbers are cumulative, whereby a certain
sequence number inside the TCP header will be used to indicate that all
data up to and including X should have been received. Sequence and
acknowledgment numbering is used to bring the concept of order to
packet delivery over IP.
- Windowing : The TCP windowing technique allows two communicat-
ing end stations to build on the sequencing and acknowledgments above
by removing the need for each sequence of data to be individually
acknowledged. In LANs where packet loss is usually minimal, it is far
more efficient to allow the sender to transmit several frames of data
before an acknowledgment is sent.
Along with these mechanisms, TCP must also be able to uniquely identify
each conversation within an internetwork. We’ve already seen the idea of a TCP
port number that is used, among other things, to identify the application process
to the high OSI layers during the conversation. Within a TCP conversation,
there are in fact two port numbers used: one to identify the sender’s listening
port and the other to identify the receiver’s listening port. Depending on the
direction of each individual frame in the conversation, these ports become either
the source port or the destination port within the Layer 4 headers.
Transport Control Protocol (TCP) 25
TCP ports used by both the client and server, each side also uses its own
sequence numbering to identify where within the session each frame fits.
Once the client has determined these two variables, it is ready to send the first
packet of the session and initiate the connection to the server. Using TCP flags,
the client will indicate to the server that it wants to initiate a connection by set-
ting the SYN or synchronize flag showing that this is the first pack in the ses-
sion. In TCP terms, this element is the first packet in what is commonly
referred to as the “three-way handshake.” This is simply because three packets
are exchanged between the client and server to bring the TCP state into that
which can transport data. Consequently, no Application layer data is transmit-
ted until at least the fourth packet in the session, a concept which we will see has
an important consequence when applied to content switching. Figure 2–6 shows
a simplified representation of the three-way handshake to illustrate which side
sends which of the packets when a new connection is initiated.
Taking this sequence packet by packet, we can see the importance of the port and
sequence numbers in ensuring the reliable transport between the client and server.
The first frame from the client to the server initiates the connection by setting the
client side port and sequence numbers as shown in Figure 2–7. As we can see,
the client chooses a random source port that will be used by the client to identify
this session uniquely in cases where it has concurrent sessions to the same server.
Figure 2–6 The TCP three-way handshake.
User^ Client Initiating a TCP Connection to the Server
Server
SYN, SPort 3086, Dport 80, Seq 713245119, Ack 0
SYN-ACK, SPort 80, DPort 3086, Seq 20072, Ack 713245120 ACK, SPort 3086, Dport 80, Seq 713245120, Ack 20073
26 Chapter 2 ● UNDERSTANDING LAYER 2, 3, AND 4 PROTOCOLS
When the server replies, both the SYN and ACK flags are set in the TCP
headers to indicate that the server acknowledges the client’s connection request.
To ensure that each packet can be accounted for, the server will set an acknowl-
edgment number that is equal to the last byte received from the client, relative to
the starting sequence number, plus one. In our example, the client started with a
sequence number of 713245119 and transmitted no user data, meaning that the
server will use an acknowledgment of 713245120.
It is also important to notice the change in source and destination ports
depending on which way a particular packet is directed. In our example, the cli-
ent sends on port 80 and listens on port 3086, whereas the server sends on port
3086 and listens on port 80. Figure 2–8 shows the return packet from the server
to the client.
The final packet exchanged during this handshake period is an acknowledg-
ment from the client to the server. This allows the client to correctly acknowl-
edge the sequence numbering used by the server in the previous packet and
remove the SYN flag being used to show the start of the session. Once this final
Figure 2–7 The SYN packet sent by the client.
Internet Protocol Headers Version: 4 Time to live: 128 Protocol: TCP (0x06) Header checksum: 0xb926 (correct) Source: 10.10.10. Destination: 20.20.20. Transmission Control Protocol Headers
Source port: 3086 (3086) Destination port: http (80) Sequence number: 713245119
Header length: 28 bytes Flags: 0x0002 (SYN) 0... .... = Congestion Window Reduced (CWR): Not set .0.. .... = ECN-Echo: Not set ..0. .... = Urgent: Not set ...0 .... = Acknowledgment: Not set .... 0... = Push: Not set .... .0.. = Reset: Not set .... ..1. = Syn: Set .... ...0 = Fin: Not set
28 Chapter 2 ● UNDERSTANDING LAYER 2, 3, AND 4 PROTOCOLS
3. Terminating a Session
Unlike the session initiation, the termination of a TCP connection can be initi-
ated from either side. Once one side of the connection decides that it has no
more data to transmit, it will set the FIN flag in the TCP header to indicate to
the other side that it is ready to terminate the connection. In simple terms, the
receiving station will then acknowledge the FIN, by setting the ACK flag, and
set its own FIN flag to show that it too is ready to terminate the connection.
This series of exchanges results in both sides moving through the TIME WAIT
state to the CLOSED state and the connection is closed.
In some instances, when the client receives the FIN it might still have data to
send, in which case it will issue only an ACK back to the closing station. This
allows the client to continue sending data until it is complete and then issue a
FIN to show that the termination of the session can commence. During this
period, the initiator and recipient of the initial FIN are referred to as being in
the FIN WAIT 2 and CLOSE WAIT states, respectively. Some applications, such
Figure 2–9 The final ACK packet of the handshake.
Internet Protocol Headers Version: 4 Time to live: 128 Protocol: TCP (0x06) Header checksum: 0xb92c (correct) Source: 192.168.254.201 (192.168.254.201) Destination: 212.58.226.40 (212.58.226.40) Transmission Control Protocol Headers
Source port: 3086 (3086) Destination port: http (80) Sequence number: 713245120 Acknowledgement number: 20073
Header length: 20 bytes Flags: 0x0010 (ACK) 0... .... = Congestion Window Reduced (CWR): Not set .0.. .... = ECN-Echo: Not set ..0. .... = Urgent: Not set ...1 .... = Acknowledgment: Set .... 0... = Push: Not set .... .0.. = Reset: Not set .... ..0. = Syn: Set .... ...0 = Fin: Not set
User Datagram Protocol (UDP) 29
as Web browsers, will often use this type of exchange to leave the connection in
a type of half-closed state, thereby allowing the connection to be brought back
into use when needed without having to reinitiate the entire connection (see
Figure 2–10).
A more detailed description of TCP can be found in RFC 793.
User Datagram Protocol (UDP)
The User Datagram Protocol, or UDP, is the other most commonly used Trans-
port layer protocol found within the Internet. While TCP is designed to provide
connection-oriented delivery of packets, UDP implements a connectionless or
unguaranteed delivery mechanism that is suitable for a number of upper layer
applications. For some applications, the overhead of TCP, such as handshaking,
is not required and for these, UDP is best suited.
A comparison between TCP and UDP can be drawn from the world of cellu-
lar phones. TCP is similar in nature to a full telephone conversation, whereby
you establish a connection to the receiving station by dialing their number, hold
a conversation with them using verbal interaction and acknowledgments, and
finally terminate the call. UDP is much more akin to SMS or text messaging,
Figure 2–10 Closing a TCP session.
User^ Server terminating TCP session with client Server
FIN, SPort 80, DPort 3086, Seq 20231, Ack 713245462
ACK, SPort 80, Dport 3086, Seq 20072, Ack 713245463
FIN, SPort 3086, Dport 80, Seq 713245462, Ack 20232
Virtual Router Redundancy Protocol (VRRP) 31
This is again a very brief overview of the UDP protocol. A more detailed
description is available in RFC 768, available on the IETF Web site.
Virtual Router Redundancy Protocol (VRRP)
The Virtual Router Redundancy Protocol, or VRRP, is inextricably linked with
the implementation of content switching, not because it is used by user applica-
tions, but because it provides a mechanism to eliminate single points of failure
within content switching topologies. VRRP provides a mechanism to group two
or more IP addresses, typically representing a routed interface, and make them
appear to all surrounding devices as a single logical IP address.
Many of the topologies described later in this book will show how multiple
content switches, and other routers, can be deployed to ensure a resilient and
fault-tolerant implementation. For this reason, we need to examine the concepts
and theory of VRRP in some more detail.
Figure 2–11 A UDP-based DNS query.
Internet Protocol Headers Version: 4 Time to live: 249 Protocol: UDP (0x11) Header checksum: 0xc8de (correct) Source: 20.20.20. Destination: 10.10.10. Transmission Control Protocol Headers
Source port: domain (53) Destination port: 1763 (1763) Length: 276 Checksum: 0x04bc (correct)
Domain Name System (response) Answers www.foo.com: type A, class inet, addr 1.2.3. Name: www.foo.com Type: Host address Class: inet Time to live: 10 minutes Data length: 4 Addr: 1.2.3.
32 Chapter 2 ● UNDERSTANDING LAYER 2, 3, AND 4 PROTOCOLS
Layer 2 and 3 Redundancy
Let’s consider a network as shown in Figure 2–13. To eliminate a single point of
failure for clients on the network accessing the Internet, the network adminis-
trator might consider deploying two Internet facing routers, R1 and R2. The
client PC on the network will have been configured with a default route; for
example, 10.10.10.2 pointing to router R1.
This “hard-coding” of the default gateway IP address into the client’s TCP/IP
settings presents the network administrator with two challenges when consider-
ing resilience:
- Router R1 might fail, leaving the client with a default gateway of an
unreachable IP address.
- The client PC will resolve the IP address of the default gateway to the
Ethernet address of router R1. This means that even if we replace the
hardware of router R1, the client will still not have access to the Internet
until its ARP cache has timed out or has been cleared.
Figure 2–12 The UDP-based DNS response.
Internet Protocol Headers Version: 4 Time to live: 249 Protocol: UDP (0x11) Header checksum: 0xc8de (correct) Source: 20.20.20. Destination: 10.10.10. Transmission Control Protocol Headers
Source port: domain (53) Destination port: 1763 (1763) Length: 276 Checksum: 0x04bc (correct)
Domain Name System (response) Answers www.foo.com: type A, class inet, addr 1.2.3. Name: www.foo.com Type: Host address Class: inet Time to live: 10 minutes Data length: 4 Addr: 1.2.3.
