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10. The Internet Transport Protocols: TCP
UDP is a simple protocol and it does some niche uses, such as clientserver interaction and multimedia, but for most Internet applications,
reliable, sequenced delivery is needed, but UDP cannot provide this, so
another protocol is required. It is called TCP and is the main workhorse
of Internet
TCP (Transport Control Protocol) was specifically designed to
provide a reliable end-to-end byte stream over an unreliable internetwork.
An internetwork differs from a single network because different parts
may have widely different topologies, bandwidths, delays, packet sizes,
and other parameters. TCP was designed to dynamically adapt properties
of the internetwork and to be strong in the face of many kinds of failures.
TCP provides multiplexing, demultiplexing, and error detection in
exactly the same manner as UDP. Nevertheless, TCP and UDP differ in
many ways. The most fundamental difference is that UDP is
connectionless, while TCP is connection-oriented. UDP is
connectionless because it sends data without ever establishing a
connection. TCP is connection-oriented because before one application
process can begin to send data to another, the two processes must first
“handshake” with each other-that is, they must send some preliminary
segments to each other to establish the parameters of the ensuring data
transfer.
Each machine supporting TCP has a TCP transport entity, either a
library procedure, a user process, or part of the kernel. In all cases, it
manages TCP streams and interfaces to the IP layer. A TCP entity accepts
user data stream from local processes, breaks them up into pieces not
exceeding 64 Kb (in practice, often 1460 data bytes in order to fit in a
single Ethernet frame with the IP and TCP headers), and sends each piece
as a separate IP datagram. When datagram containing TCP data arrive at
a machine, they are given in the TCP entity, which reconstructs the
original byte streams. For simplicity, we will sometimes use just “TCP”
to mean the TCP transport entity (a piece of software) or the TCP
protocol (a set of rules). From the context it will be clear which is meant.
The IP layer gives no guarantee that datagrams will be delivered
properly, so it is up to TCP to time out and retransmit them as need be.
Datagrams that do arrive may be will in the wrong order; it is also up to
TCP to reassemble them into messages in the proper sequence. In short,
TCP must furnish the reliability that most users want and that IP does not
provide.
a. The TCP service Model
Both the sender and receiver creating end points, called sockets, and
obtain TCP service. Each sockets has a socket number (address)
consisting of the IP address of the host and a 16-bit number local to that
host, called a port. A port is the TCP name. For TCP service to be
obtained, a connection must be clearly established between a socket on
the sending machine and a socket on the receiving machine. The socket
calls (primitives) are listed in Table 5.
A socket may be multiple connections at the same time. In other
words, two or more connections may terminate at the same socket. The
identifiers at both ends identify connections, which are (socket, socket2).
No virtual circuit numbers or other identifiers are used.
Table 5. The socket primitives for TCP
Primitive
Meaning
SOCKET
Create a new communication and port
BIND
Attach a local address to a socket
LISTEN
Announce willingness to accept connections, give queue
size
ACCEPT
Block the caller until a connection attempt arrives
CONNECT Actively attempt to establish a connection
SEND
Send some data over the connection
RECEOVE Receive some data from the connection
CLOSE
Release the connection
Port numbers below 1023 are called well-known ports and reserved for
standard services. For example, any process wishing to establish a
connection to a host to transfer a file using FTP can connect to the
destination host’s port 21 to contact its FTP daemon. The list of known
ports is listed in Table 6.
Table 6. Some assigned ports
Port
Protocol
21
FTP
23
Telnet
25
SMTP
69
TFTP
79
Finger
80
HTTP
110
POP-3
119
NNTP
Use
File transfer
Remote login
E-mail
Trivial file transfer protocol
Lookup information about a user
World Wide web
Remote e-mail access
USENET news
All TCP connections are full duplex and point-to-point, that means that
each connection has exactly two end points. TCP does support
multicasting or broadcasting.
2. Implementation of the NL Services
Network layer can provide two different possible organizations of
services, depending on type of service offered.
If 1. connectionless service is offered, packets are injected into the
subnet individually and routed independently of each other. No advanced
setup is needed. In this context, the packets are frequently called
datagrams and subnet is called a datagram subnet. See Figure 2.
Let us consider datagram subnet. Suppose that the process P1 in Figure 2
has a long message for P2. It hands the message to the transport layer
with instructions to deliver it to process P2 on host H2. The transport
layer code runs on host H1, typically within the operating system. It
depends on a transport header to the front of the message and hands the
results to the network layer, probably just another procedure within the
operating system.
Figure 2. Routing within a diagram subnet.
Let us assume that the message is four times longer than the maximum
packet size, so the network layer has to break it into four packets, 1, 2, 3,
and 4 and sends each of them in turn to router A using some point-topoint protocol. Every router has an internal table telling it where to send
packets for each possible destination. Each table entry is a pair consisting
of a destination and the outgoing line to use for that destination. Only
directly-connected lines can be used. For example, in Figure 2, A has
only two outgoing lines-to B and C-so every incoming packet must be
sent to one of these routers, even if the final destination is some other
router. A’s initial routing table is shown in the Figure 2 under label
“initial”.
As they arrived at A, packets 1, 2, and 3 were stored briefly (to verify
their checksums). Then each was forwarded to C according to A’s table.
Packet 1 was then forwarded to E and then to F. When it got to F, it was
encapsulated in a DLL and sent to H2 over the LAN. Packet 2 and 3
follow the same route.
However, something different happened to packet 4. When it got to A it
was sent to router B, even though it is also destined for F. For some
reason, A decided to send packet 4 via a different route than that of the
first three. Perhaps it learned of a traffic jam somewhere along the ACE
path and updated its routing table, as shown under label “later”. The
algorithm that manages the tables and makes the routing decisions is
called the routing algorithm.
If 2. connection-oriented service is used, a path from the source router to
the destination router must be established before any data packets can be
sent. This connection is called a Virtual Circuit (VC), and subnet is
called Virtual circuit subnet. See Figure 3.
Figure 3. Routing within a virtual-circuit subnet.
For connection-oriented service, we need a virtual circuit subnet. The
idea behind virtual circuits is to avoid having to choose a new route for
every packet sent, unlike in Figure 2. Instead, when a connection is
established, a route from the source machine to the destination machine is
chosen as part of the connection setup and stored in tables inside the
routers. That route is used for all traffic flowing over the connection,
exactly the same way that the telephone system works. When the
connection is released, the virtual circuit is also terminated.
In Figure 3 host H1 has established connection 1 with host H2. It is
remembered as the first entry in each of the routing tables. The first line
of A’s table says that if a packet manner connection identifier 1 comes in
from H1, it is to be sent to router C and given connection identifier 1.
Similarly, the first entry at C routes packet to E, also with connection
identifier 1.
Let us consider what happens if H3 also wants to establish a connection
to H2. It chooses connection identifier 1 (because it is initiating the
connection and this is its only connection) and tells the subnet to establish
the virtual circuit. This leads to the second row in the tables. Note that we
have a conflict here because although A can easily distinguish connection
1 packets from H1 from connection 1 packets from H3, C cannot do this.
For this reason, A assigns a different connection identifier to the outgoing
traffic for the second connection. Avoiding conflicts of this kind is why
routers need the ability to replace connection identifiers in outgoing
packets. In some context, this is called label switching.
3. Comparison of Virtual Circuit and Datagram Subnets
Inside the subnet, several trade-offs exist between virtual circuit and
datagrams. One trade-off is between router memory space and bandwidth.
Virtual circuits allow packets to contain circuit numbers instead of full
destination address. If the packet tends to be quite short, a full destination
address in every packet may represent a significant amount of overhead
and hence, wasted bandwidth. The price paid for using virtual circuits
initially is the table space within the routers. Depending upon the relative
cost of communication circuits versus router memory, one or the other
may be cheaper.
Another trade-off is setup time versus address parsing time. Using virtual
circuits requires a setup phase, which takes time and consumers
resources. However, figuring out what to do with a data packet in a
virtual-circuit subnet is easy: the router just uses the circuit numbers to
index into a table to find out where the packet does. In a datagram subnet,
a more complicated lookup procedure is required to locate the entry for
the destination.
Comparison of datagram and virtual-circuit subnets is shown in Table 1.
Table 1. Comparison of Virtual-Circuit and Datagram Subnets
5-4
4. Routing algorithm
The main function of the network layer is routing packets from the source
machine to the destination machine. In most subnets, packets will require
multiple hops to make the voyage. The only notable exception is for
broadcast networks. The algorithms that choose the routers and the data
structures that they use are a major area of network layer design.
The routing algorithm is that part of the NL software responsible for
deciding which output line an incoming packet should be transmitted on.
If the subnet uses datagrams internally, this decision must be made for
every arriving data packet since the best route may have changed since
last time. If the subnet uses virtual circuit internally, routing decisions are
made only when a new virtual circuit is being set up. Thereafter, data
packets just follow the previously-established route. The latter case is
sometimes called session routing because a route remains in force for an
entire user session.
It is sometimes useful to make a distinction between routing, which is
making the decision which router to use, and forwarding. One can think
of a router as having two processes inside it. One of them handles each
packet as it arrives, looking up the outgoing line to use for it in the
routing tables. This process is forwarding. The other process is
responsible for filling in and updating the routing tables. That is where
the routing algorithm comes into play.
Main requirements in a routing algorithm are: correctness, simplicity,
robustness, stability, fairness, and optimality. In some particular cases
should be made trade-offs between these requirements.
Routing algorithms can be grouped into two major classes: nonadaptive
and adaptive. Nonadaptive algorithms do not base their routing decision
on measurements or estimates of the current traffic and topology. Instead,
the choice of the route to use to get from I to J (for all I and J) is
computed in advance, off-line, and downloaded to the routers when the
network is booted. This procedure is sometimes called static routing.
Adaptive algorithms, in contrast, change their routing decisions to reflect
changes in the topology, and usually the traffic as well. Adaptive
algorithms differ in where they get their information (locally, from
adjacent routers, or from all routers), when they change the routes (every
T sec., when the load changes, or when the topology changes, and what
metric is used for optimization (distance, numbers of hops, or estimated
transmit time