lectura teo 09

Question 1

What’s the Internet’s address assignment strategy?

Answer 1

The Internet’s address assignment strategy is known as Classless Interdomain Routing (CIDR—pronounced cider) [RFC 4632]. CIDR generalizes the notion of subnet addressing. As with subnet addressing, the 32-bit IP address is divided into two parts and again has the dotted-decimal form a.b.c.d/x, where x indicates the number of bits in the first part of the address.

Question 2

classful addressing. Pros/contras

Answer 2

Pros: mas simple a primera vista
contra: no se adapta bien a los distintos escenarios. Es menos flexible.

Before CIDR was adopted, the network portions of an IP address were constrained to be 8, 16, or 24 bits in length, an addressing scheme known as classful addressing, since subnets with 8-, 16-, and 24-bit subnet addresses were known as class A, B, and C networks, respectively. The requirement that the subnet portion of an IP address be exactly 1, 2, or 3 bytes long turned out to be problematic for supporting the rapidly growing number of organizations with small and medium-sized subnets. A class C (/24) subnet could accommodate only up to 2 8 2 2 5 254 hosts (two of the 2 8 5 256 addresses are reserved for special use)—too small for many organizations. However, a class B (/16) subnet, which supports up to 65,634 hosts, was too large. Under classful address- ing, an organization with, say, 2,000 hosts was typically allocated a class B (/16) subnet address. This led to a rapid depletion of the class B address space and poor utilization of the assigned address space. For example, the organization that used a class B address for its 2,000 hosts was allocated enough of the address space for up to 65,534 interfaces— leaving more than 63,000 addresses that could not be used by other organizations.

Question 3

Broadcast address.

Answer 3

We would be remiss if we did not mention yet another type of IP address, the IP broadcast address 255.255.255.255. When a host sends a datagram with destination address 255.255.255.255, the message is delivered to all hosts on the same subnet. Routers optionally forward the message into neighboring subnets as well (although they usually don’t).

Question 4

four-step process the DHCP protocol has For a newly arriving host.

Answer 4

• DHCP server discovery
• DHCP server offer(s)
• DHCP request.
• DHCP ACK.

Question 5

four-step process the DHCP protocol has For a newly arriving host: DHCP server discovery

Answer 5

The first task of a newly arriving host is to find a DHCP server with which to interact. This is done using a DHCP discover message, which a client sends within a UDP packet to port 67. The UDP packet is encap- sulated in an IP datagram. But to whom should this datagram be sent? The host doesn’t even know the IP address of the network to which it is attaching, much less the address of a DHCP server for this network. Given this, the DHCP client creates an IP datagram containing its DHCP discover message along with the broadcast destination IP address of 255.255.255.255 and a “this host” source IP address of 0.0.0.0. The DHCP client passes the IP datagram to the link layer, which then broadcasts this frame to all nodes attached to the subnet (we will cover the details of link-layer broadcasting in Section 6.4).

Question 6

four-step process the DHCP protocol has For a newly arriving host: DHCP server offer(s)

Answer 6

A DHCP server receiving a DHCP discover message responds to the client with a DHCP offer message that is broadcast to all nodes on the subnet, again using the IP broadcast address of 255.255.255.255. (You might want to think about why this server reply must also be broadcast). Since several DHCP servers can be present on the subnet, the client may find itself in the enviable position of being able to choose from among several offers. Each server offer message contains the transaction ID of the received discover mes- sage, the proposed IP address for the client, the network mask, and an IP address lease time—the amount of time for which the IP address will be valid. It is com- mon for the server to set the lease time to several hours or days [Droms 2002].

Question 7

four-step process the DHCP protocol has For a newly arriving host: DHCP request.

Answer 7

The newly arriving client will choose from among one or more server offers and respond to its selected offer with a DHCP request message, echoing back the configuration parameters.

Question 8

four-step process the DHCP protocol has For a newly arriving host: DHCP ACK.

Answer 8

The server responds to the DHCP request message with a DHCP ACK message, confirming the requested parameters.

Question 9

NAT tables. Como funcionan?

Answer 9

Network Addressing Table.

Funcionamiento: [ver libro]

Question 10

Contras de NAT.

Answer 10

First, one might argue that, port numbers are meant to be used for addressing processes, not for addressing hosts. This violation can indeed cause prob- lems for servers running on the home network, since, as we have seen in Chapter 2, server processes wait for incoming requests at well-known port numbers and peers in a P2P protocol need to accept incoming connections when acting as servers. Techni- cal solutions to these problems include NAT traversal tools [RFC 5389] and Uni- versal Plug and Play (UPnP), a protocol that allows a host to discover and configure a nearby NAT [UPnP Forum 2016].

More “philosophical” arguments have also been raised against NAT by archi- tectural purists. Here, the concern is that routers are meant to be layer 3 (i.e., net- work-layer) devices, and should process packets only up to the network layer. NAT violates this principle that hosts should be talking directly with each other, without interfering nodes modifying IP addresses, much less port numbers. But like it or not, NAT has not become an important component of the Internet, as have other so-called middleboxes [Sekar 2011] that operate at the network layer but have functions that are quite different from routers. Middleboxes do not perform traditional datagram forwarding, but instead perform functions such as NAT, load balancing of traffic flows, traffic firewalling (see accompanying sidebar), and more. The generalized forwarding paradigm that we’ll study shortly in Section 4.4 allows a number of these middlebox functions, as well as traditional router forwarding, to be accomplished in a common, integrated manner.

Question 11

The most important changes introduced in IPv6 are evident in the datagram format:

Answer 11

• Expanded addressing capabilities
• A streamlined 40-byte header
• Flow labeling