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Plan a TCP/IP Network Infrastructure Strategy

One of the major objectives on exam 70-293 is Planning a TCP/IP Network Infrastructure Strategy. The following section is dedicated to discussing the important topics that fall under this objective.

Analyze IP Addressing Requirements

Before you can successfully and effectively implement a TCP/IP network infrastructure, you have to identify your IP addressing requirements. Some of the things that you need to consider include

  • Will you require a public or private IP address range on the internal network?
  • How will client computers be assigned IP addresses?
  • Does the network require multiple subnets?

Public/Private IP Address

Three ranges of IP addresses are reserved, meaning they are not valid on the Internet. Therefore, you can use one of these private ranges on your private network. Of course, one of the disadvantages to this is that a proxy server or NAT server is needed for Internet access because the private IP address must be mapped to a public one. In terms of advantages, private IP addressing is more cost effective, can accommodate growth on your network, and can increase security.

If you do decide to implement a private IP address range, you can use IP addresses from any of the following classes:

Class A 10.0.0.0-10.255.255.255

Class B 172.16.0.0-172.31.255.255

Class C 192.16.8.0.0-192.168.255.255

One of the decisions you are faced with when designing a TCP/IP network is whether you want to use a private IP address range or public IP addresses. Of course, there are disadvantages and advantages to both of them. In making your decisions, keep in mind that any computers that have a direct connection to the Internet will require at least one public IP address. However, for those computers with no direct Internet connection, you have the option of using public or private addresses.

IP Address Assignment

IP addresses can be dynamic or static. With static IP addressing, a computer (or other device) always uses the same IP address. With dynamic addressing, the IP address changes.

There are several ways of configuring a host with an IP address. You can do so manually or automatically, using a service such as DHCP or Automatic Private IP Addressing (APIPA).

Manual IP Addressing

For manual IP address assignment, an administrator or similarly delegated person manually enters a static IP address and other information, such as the subnet mask and default gateway, DNS server, WINS server, and so forth.

Dynamic Host Configuration Protocol

DHCP dynamically assigns randomly available IP addresses from available scopes to DHCP clients. This allows administrators to automatically assign IP addresses to clients without actually having to set all the parameters (default gateway, DNS servers, and so on) for each system, as with manual IP address assignment.

The DHCP lease process begins over UDP ports 67 and 68 as a broadcast message from the client system. For DHCP clients to contact DHCP servers on remote networks successfully, the IP routers must be RFC 1542–compliant. These routers support forwarding DHCP broadcasts off the local subnet. If the routers are not compliant, a DHCP Relay Agent must be in use on that subnet.

The DHCP Relay Agent is available through the Routing and Remote Access MMC on Windows Server 2003 (see Figure 3.1). Systems configured in the role of a DHCP server should not be configured as DHCP Relay Agents because both services use UDP ports 67 and 68 and degrade each other's services if they are installed together. On a single subnet, there is no practical need to do this because the DHCP server should simply respond to user requests on the subnet.

Figure 3.1

Figure 3.1 The first step in configuring the DHCP Relay Agent service in the Routing and Remote Access MMC.

Figure 3.2 shows the available routing protocols in the New Routing Protocol dialog box: DHCP Relay Agent, IGMP Router and Proxy, Open Shortest Path First (OSPF), and RIP Version 2 for Internet Protocol.

Figure 3.2

Figure 3.2 You can add routing protocols through the New Routing Protocol dialog box.

When a client first starts, it sends out a DHCPDISCOVER broadcast message to all addresses (255.255.255.255). The message contains the client's hardware (MAC) address and hostname. (The client also sends this message when its original lease has expired and cannot be renewed.) All available DHCP servers configured to respond receive the DHCPDISCOVER broadcast and send a DHCPOFFER broadcast message back with the following information:

  • The client's hardware address
  • An offered IP address
  • Subnet mask
  • Length of the lease
  • A server identifier (the IP address of the offering DHCP server)

The DHCP client selects the IP address from the first offer it receives and responds with a DHCPREQUEST broadcast message that includes the IP address of the server whose offer was accepted. All the other DHCP servers then retract their lease offers and mark those addresses available for the next IP lease request.

The DHCP server whose lease was accepted responds with a DHCPACK broadcast message, which contains the valid lease period for that IP address and other configuration information outlined in the scope, such as router information (default gateway), subnet mask, and so forth. After the DHCP client receives this acknowledgment, TCP/IP is completely initialized, and the client can use the IP address for communication.

Automatic Private IP Addressing

When a DHCP client sends out the DHCPDISCOVER broadcast message, it waits 1 second for an offer. If the client does not receive a response from a DHCP server, it rebroadcasts the request three times at 9-, 13-, and 16-second intervals, with a random offset length of 0ms and 1000ms. If an offer is not received after the four requests, the client retries once every five minutes.

Beginning with Windows 98, DHCP clients can configure themselves by using APIPA and the DHCP client service. After the four attempts to receive an IP address have failed, the DHCP client auto-configures its IP address and subnet mask using the reserved Class B network address 169.254.0.0 and the subnet mask 255.255.0.0. No default gateway is used, so systems that use APIPA are not routable.

The DHCP client tests for an address conflict to make sure that the IP address it has chosen is not already in use on the network. To do this, it broadcasts its selection from the range to the local subnet. If a conflict is found, the client selects another IP address and continues this process up to 10 attempts.

After the DHCP client makes sure that the address it has chosen is not in conflict with another system on the subnet, it configures its network interface with the IP address. The client then continues to check for a response from the DHCP server every five minutes. If a DHCP server becomes available, the client drops its APIPA settings and uses the address the DHCP server offers at that time.

Optimize TCP/IP Performance

So far you have learned about subnetting and configuring network systems in address class ranges in an effort to optimize TCP/IP configuration, but some other points should be mentioned as well. You need to be sure, above all else, that you understand your network configuration and behavior. Although you can take a few steps to fine-tune TCP/IP traffic, network topology plays a big role.

For TCP/IP specifically, there is the TCP/IP Receive Window Size setting, which is the buffer threshold for inbound packets. The default setting for ethernet networks is 17,520 bytes; when this threshold is met, the receiving system sends out an acknowledgement that the data has been received. This process of sending and acknowledging during a data transmission session repeats every 17,520 bytes until all data has been transmitted. As an administrator, you can adjust this acknowledgement setting to optimize transmissions.

Other settings on the network's Physical and Data Link layers are beyond normal administrative control. Maximum Transmission Units (MTUs), for example, are based on the type of network that is installed. For example, 16Mbps token-ring networks have an MTU setting of 17,914 bytes; 4Mbps token-ring networks have an MTU setting of 4,464 bytes. Ethernet deployments are limited to a 1,500 byte MTU setting. As an analogy, think of the MTU as an envelope in which data is carried.

The Maximum Segment Size (MSS) setting determines the largest segment that can be carried in the MTU. (Think of it as the pages of a letter in an envelope.) This setting also varies depending on the framework. Obviously, the MSS for token-ring networks will be larger than the MSS for ethernet networks.

Networks must consider application requirements when implementing certain services and protocols to optimize bandwidth. Quality of service (QoS) can also be implemented on networks to optimize bandwidth. The main issue on most networks is that all the associated networking equipment needs to support the Resource Reservation Protocol (RSVP). Networks also have certain application requirements to consider, such as the following:

  • Routers forward traffic on a best-effort basis as they receive it. Video conferencing and streaming media suffer when available bandwidth is low.
  • QoS Admission Control Service (QoS ACS) handles bandwidth on a subnet-to-subnet basis.
  • Subnet Bandwidth Management (SBM) manages the use of network resources on a subnet.
  • RSVP is a signaling protocol that enables sender and receiver systems to set up a reserved QoS session. RSVP messages carry the reservation request in an effort to maintain the QoS session. This is why each router and switch along the communication path between the sender and receiver needs to support RSVP.
  • Traffic Control uses the packet classifier to separate packets into queues based on their priority. The Packet Scheduler manages the queues set up by the packet classifier.
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