General Computer Science 320201 GenCS I & II Lecture ... - Kwarc

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4 version IPv4 or IPv6 packet
4 Header Length in multiples 4 bytes (e.g., 5 means 20 bytes)
8 QoS Quality of Service, i.e. priority
16 length of the packet in bytes
16 fragid to help reconstruct the packet from fragments,
3 fragmented DF ˆ= “Don’t fragment”/MF ˆ= “More Fragments”
13 fragment offset to identify fragment position within packet
8 TTL Time to live (router hops until discarded)
8 protocol TCP, UDP, ICMP, etc.
16 Header Checksum used in error detection,
32 Source IP
32 target IP
. . . optional flags according to header length
Note that delivery of IP packets is not guaranteed by the IP protocol.
c○: Michael Kohlhase 343
As the internet protocol only supports addressing, routing, and packaging of packets, we need
another layer to get services like the transporting of files between specific computers. Note that
the IP protocol does not guarantee that packets arrive in the right order or indeed arrive at all,
so the transport layer protocols have to take the necessary measures, like packet re-sending or
handshakes, . . . .
The Transport Layer
Definition 502 The transport layer is responsible for delivering data to the appropriate
application process on the host computers by forming data packets, and adding source and
destination port numbers in the header.
Definition 503 The internet protocol mainly suite uses the Transmission Control Protocol
(TCP) and User Datagram Protocol (UDP) protocols at the transport layer.
TCP is used for communication, UDP for multicasting and broadcasting.
TCP supports virtual circuits, i.e. provide connection oriented communication over an underlying
packet oriented datagram network. (hide/reorder packets)
TCP provides end-to-end reliable communication (error detection & automatic repeat)
c○: Michael Kohlhase 344
We will see that there are quite a lot of services at the network application level. And indeed,
many web-connected computers run a significant subset of them at any given time, which could
lead to problems of determining which packets should be handled by which service. The answer
to this problem is a system of “ports” (think pigeon holes) that support finer-grained addressing
to the various services.
Ports
Definition 504 To separate the services and protocols of the network application layer,
network interfaces assign them specific port, referenced by a number.
Example 505 We have the following ports in common use on the Internet
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