IP Datagrams

IP transmits data in packets called datagrams. A datagram contains a header and some data, thus:

Note: this diagram shows the datagram format of the "current" version of IP, IPv4.

Datagram Details

The datagram header contains both the source and destination IP addresses as well as various other administrative data. This means that every datagram can be identified, in terms of "where it came from" and "where it's going".
Virtually all "real world" datagrams have a minimal length (20 byte) header. Although various optional (extra) header fields are permitted, these are rarely used in practice.
Each network in the Internet is characterised by a Maximum Transmission Unit (MTU), which defines the largest datagram which can be sent on that network. Whilst the total length of a datagram can (in theory) be up to 64 KB, "real world" MTU limits usually mean datagrams are < 1500 bytes in total length.
The data field contains "higher layer" protocol data--usually a TCP segment^[1] which IP is delivering across the Internet. We say that the TCP segment is encapsulated in an IP datagram. The TCP segment, in turn, usually contains application protocol data in its data field.

^[1] although not always: for example, see UDP, later in this lecture.

IP Routing

There are two levels of complexity involved in IP routing:

Local delivery

when the IP software is presented with a datagram for delivery, it first checks the network part of the destination address to see if matches its own network number. If it does, then the datagram can be delivered locally, and is simply handed to the physical network delivery system (typically an Ethernet device driver) for direct delivery over the local network.

Internet delivery

if the network numbers are different, then the datagram must be forwarded to a router (or IP gateway). In this case, the datagram is forwarded (using direct delivery as above) to the appropriate router for on-delivery

Datagrams cross gateways (routers) from network to network until they reach a network where they can be locally delivered.
The IP software must keep a routing table to know the IP address of an appropriate router. This is commonly done by defining a default route for each network machine.
Note that routers must have (at least) two IP addresses!

IP: Connectionless Datagram Delivery

IP data transfer across an internet is based on three fundamental principles:

Unreliable delivery: delivery of data is not guaranteed. A packet of data may be lost in the network, may be duplicated (ie: delivered twice) or may be delivered out of order. The IP service will not detect such conditions, nor will it notify the sender or receiver if they occur.
Connectionless delivery: each packet is treated entirely indpendently of all others. No information is kept as to which packets have been forwarded, and packets may travel over different routes to the same destination.
Best-Effort delivery: the packet delivery mechanism is engineered to always deliver packets if possible. It will not gratuitously drop packets: unreliability should only occur when underlying resources (eg buffer space) are exhausted.

These specifications allow the IP service to concentrate on its job: delivering packets. As we have seen, higher level protocols (usually TCP) transform the IP service into a reliable, sequenced interprocess communications mechanism.

The Structure of the Internet

The Internet consists of networks (or, more commonly nowadays -- subnets) connected by routers. The traceroute utility shows the path a datagram takes -- the following traceroute is from ironbark to the Victorian Regional Network (VRN).

 
1 r-busbgo.bendigo.latrobe.edu.au (149.144.21.254)  1 ms  2 ms  1 ms
2 r-bgoatm34.bendigo.latrobe.edu.au (149.144.10.250)  1 ms  1 ms  1 ms
3 r-sctech-atm.latrobe.edu.au (131.172.239.3)  5 ms  4 ms  3 ms
4 cisco-ltu-fddi.latrobe.edu.au (131.172.20.12)  5 ms  4 ms  4 ms
5 vic-gw.vrn.EDU.AU (203.21.130.129)  4 ms  5 ms  4 ms

The Transport Layer Revisited: UDP

The User Datagram Protocol provides a connectionless alternative transport service to TCP for applications where reliable stream service is not needed. UDP datagrams can be droppped, duplicated or delivered out of order, exactly as for IP.

The UDP transport service add to IP the ability to deliver a datagram to a specified destination process using a port abstraction, in an analogous way to that used by TCP.

Examples of applications where UDP is used include:

Any application where loss of a datagram is not critical because later datagrams will imply the missing information. Examples include:
- A "timekeeper" host sends the current "wallclock" time to a slave host.
- Routers broadcast copies of their routing tables every 30 seconds.
An application process which performs its own error correction is used.
The reduced overhead of connectionless operation suits some time-critical applications where (occasional) loss of data may be unimportant. An example is voice communications over the Internet.

The Future: IPv6

(Optional material)

The current version of IP is widely acknowledged to have many shortcomings. To address these, the Internet Engineering Task Force (IETF) has defined IP version 6^[1]. Some of its features include:

Addresses are now 128 bits in total length, instead of 32 bits in IPv4. This is a huge increase!
The address structure is hierarchical, but is much more complicated than the simple "address class" used in IPv4.
Compatible with IPv4 for transition purposes.
Support for a variety of "service types", etc.

^[2] or, as it was called during its development, IPng.

This lecture is also available in PostScript format. The tutorial for this lecture is Tutorial #09.
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Phil Scott