IP Datagrams

Notes:

• The total length of a datagram can be up to 64 KB, but is usually less than 1500 bytes
• The header contains the source and destination IP addresses as well as various other administrative data, a minimum of 20 bytes in total

IP: Connectionless Datagram Delivery

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.

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 encapsulation and delivery.

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!

The ICMP Protocol

The IP software encapsulates ICMP messages into the data field of an ordinary datagram. For example, if a router is forced, for some reason, to drop a datagram, it sends an ICMP message to the originator of the datagram explaining the reason. It is up to the originator to take appropriate action to ensure that subsequent datagrams are not droppped.

There are several ICMP message types. The echo request and echo reply types are used by the UNIX and Internet ping utility to test reachability. Ping is by far the single most valuable tool available to the network manager, and we return to its use when we discuss network management, later.

The Transport Layer Revisited: UDP

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.