Solutions to Chapter 2  (Note: solution to Problem 62 to be added)
1.  Explain how the notion of layering and internetworking make the rapid growth of applications such as the World Wide Web possible.
Solution:
Internetworking allows many component networks each with different underlying technology and operation to work together and form one large network. As new network technologies are introduced they can be readily incorporated into the Internet.  This provides the ubiquitous connectivity for applications like WWW.
The layering concept hides the specific underlying network technology from the upper layers and provides a common networking platform. Using the communication service provided by the layers below, new applications can be introduced independently and at a rapid rate.
2.    (a)  What universal set of communication services is provided by TCP/IP?
Solution:
The TCP/IP protocol stack provides two basic types of communications services through its two transport layer protocols: TCP provides reliable connection-oriented transfer of a byte stream; UDP provides for best-effort connectionless transfer of individual messages.  TCP/IP provides with
globally unique logical addressing that enables machines connected to the Internet to access these two services.  The IP addressing scheme is very scalable because of its hierarchical structure.
2.    (b)How is independence from underlying network technologies achieved?
Solution:
The two basic communications services provided by TCP and UDP are built on the connectionless packet transfer service provided by the Internet Protocol (IP). Many network interfaces are defined to
support IP. The salient part of the above figure is that all of the higher layer protocols access the network interfaces through IP. This is what provides the ability to operate over multiple networks.
2.    (c)  What economies of scale result from (a) and (b)?
Solution:
Once a network interface for IP is defined for a given network technology, then hosts connected using the given network technology can connect to the Internet. This allows the reach of the Internet to grow rapidly, leveraging multiple coexisting networks technologies. Thus investment in new network technologies extends the reach of the Internet.
3.  What difference does it make to the network layer if the underlying data link layer provides a connection-oriented service versus a connectionless service?
Solution:
If the data link layer provides a connection-oriented service to the network layer, then the network layer must precede all transfer of information with a connection setup procedure. If the connection-oriented service includes assurances that frames of information are transferred correctly and in
sequence by the data link layer, the network layer can then assume that the packets it sends to its neighbor traverse an error-free pipe.
On the other hand, if the data link layer is connectionless, then each frame is sent independently through the data link, probably in unconfirmed manner (without acknowledgments or
tcpip详解 第二版retransmissions). In this case the network layer cannot make assumptions about the sequencing or correctness of the packets it exchanges with its neighbors.
The Ethernet local area network provides an example of connectionless transfer of data link frames.
The transfer of frames using "Type 2" service in Logical Link Control (discussed in Chapter 6)
provides a connection-oriented data link control example.
4.  Suppose transmission channels become virtually error-free. Is the data link layer still needed?
Solution:
The data link layer is still needed for framing the data and for flow control over the transmission
channel. In a multiple access medium such as a LAN, the data link layer is required to coordinate access to the shared medium among the multiple users.
5.  Why is the transport layer not present inside the network?
Solution:
Some of the functions provided by the transport layer can be provided inside the networks, but other functions cannot.  For example, the transport layer provides functions at the end-system to
compensate for the limitations and impairments of the network layer, in order to meet requirements
(e.g. QoS) of the upper layer. For example in TCP/IP, IP provides only best effort service. To provide
the reliable service required by some applications - that is, to compensate for the shortcomings of best
effort service - TCP establishes connections and implements error control on an end-to-end basis.  One can imagine that a service provider could incorporate this error control function at the edge of its network.  On the other hand, one of the main purposes of the transport layer is to allow multiple processes in the end systems to share a network service.  This cannot be achieved inside the network.
6.  Which OSI layer is responsible for the following?
(a)Determining the best path to route packets.
The network layer is concerned with the selection of paths across the network.
(b)Providing end-to-end communications with reliable service.
The transport layer is concerned with providing reliable service on an end-to-end basis across the network.
(c)Providing node-to-node communications with reliable service.
The data link layer provides for the reliable transfer of information between adjacent nodes in a
network.
7.  Should connection establishment be a confirmed service or an unconfirmed service? What about data transfer in a connection-oriented service? Connection release?
Solution:
In general, the establishment of a connection needs to be confirmed before information transfer can commence across a connection. Therefore connection establishment should be a confirmed service.
A connection-oriented service is usually reliable so confirmation of data delivery is not necessary. In
certain situations, however, it is possible that the transfer across a connection is not reliable; in this case confirmation of correct data transfer may be required.
In general it is desirable that the release of a connection be confirmed by the parties involved. We will see in Chapter 8, section 5, that sometimes it is not easy to confirm that a connection has been
closed. Consequently, many protocols attempt to confirm the closing of a connection several times, and then give up and simply stop transmitting.
8.  Does it make sense for a network to provide a confirmed, connectionless packet transfer service?
Solution:
Yes. Connectionless packet transfer is often unreliable, that is, packets may be lost or discarded inside a network. Certain applications, for example, signaling in connection setup, require
confirmation to acknowledge the receipt of packets.
9.  Explain how the notion of multiplexing can be applied at the data link, network, and transport layers. Draw a
figure that shows the flow of PDUs in each multiplexing scheme.
Solution:
Transport Layer:  Multiple application layers processes can share the service provided by UDP.
When a UDP PDU arrives from the network layer, the destination port number in the PDU is used to deliver the SDU to the appropriate application layer process.  Multiple application layer processes also share the service provided by TCP.  In this case, when a TCP segment arrives, the TCP
connection ID, consisting of (source port #, source IP address, destination port #, destination IP address), is used to determine which application process to deliver the SDU to.
Network Layer:  The packet transfer service provided by IP can be used by all transport layers
operating in a machine.  Each transfer layer passes SDUs to the IP layer which prepares IP packets with appropriate source and destination IP addresses for transfer across the Internet.  Upon receiving an IP packet, a machine examines the protocol type field to determine which transport layer service to deliver the SDU to.  We can also view all transport layer PDUs as sharing the IP packet transfer service between a source machine and a destination machine.
Data Link Layer:  Network layer packets from different protocols (IP, IPX, Appletalk, etc)  can share a data link (such as PPP or Ethernet).  We can also view packet flows that traverse a data link
between two routers as sharing the link.
10.  Give two features that the data link layer and transport layer have in common. Give two features in
which they differ. Hint: Compare what can go wrong to the PDUs that are handled by these layers.
Solution:
Features they have in common:
•Both layers can provide recovery from transmission errors.
•Both layers can provide flow control.
•Both layers can support multiplexing.
Features in which they differ:
•The transport layer is end to end and involves the interaction of peer processes across the network.  The data link layer involves the interaction of peer-to-peer processes that are
connected directly. In general, the time that elapses in traversing a data link is much smaller than the time traversing a network, where packets can become trapped in temporary routing loops.
Consequently, transport layer protocols must be able to deal with out-of-sequence PDUs and a
much larger backlog of PDUs than data link layers.
•The data link layer is concerned with framing and the transport layer is not.
•The data link layer may be concerned with medium access control, the transport layer does not have this concern.
11(a).  Can a connection-oriented, reliable message transfer service be provided across a connectionless packet network? Explain.
Solution:
Yes. To provide a connection-oriented service, the transport layer can establish a logical connection across the connectionless packet network by setting up state information (for example, packet
sequence number) at the end systems. During the connection setup, the message is broken into separate packets, and each packet is assigned a sequence number.
Using the sequence numbers, the end-system transport-layer entities can acknowledge received packets, determine and retransmit lost packets, delete duplicate packets, and rearrange out-of-order packets.  The original message is reassembled as packets arrive at the receiving end.
For example, TCP provides a connection-oriented reliable transfer service over IP, a connectionless packet transfer service.
11b.  Can a connectionless datagram transfer service be provided across a connection-oriented network?
Solution:
Yes. The connectionless datagram transfer service can be implemented by simply setting up a
connection across the network each time a datagram needs to be transferred.  Alternatively, all nodes can have permanent connections to a “connectionless server” that has the function of relaying
datagrams in connectionless fashion.
12.  An internet path between two hosts involves a hop across network A, a packet-switching network, to a router and then another hop across packet-switching network B. Suppose that packet switching network A carries the packet between the first host and the router over a two-hop path involving one intermediate packet switch. Suppose also that the second network is an Ethernet LAN. Sketch the sequence of IP and non-IP packets and frames that are generated as an IP packet goes from host 1 to
host 2.

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