11/17/03

ICMP

Types:

4 – Source Quench; sent by router when a packet is dropped because of buffer overrun which is the result of network congestion

2 – Time exceeded

3 – Destination Unreachable

0 – Echo request reply

5 – Redirect; sent by router; goes to host in local network; message says to use a different router or deliver directly

 

• ICMP messages go back to original host
• There is no ICMP message that in response to a request raised by another ICMP; this prevents unlimited cascade

 

 

Path MTU Discovery

 

Path MTU is maximum transfer unit in a path of routes where a packet of that size will not be send fragmented

 

 

 

MTU = 200

 

• A sends a small packet 50 bytes and sets ‘no fragment’ bit to 1
  • If A receives a reply it doubles the size of a packet and tries again
  • A continues until A receives ICMP message that the packet could not be fragmented; in which case the size of previously successful packet will be used to identify the path MTU

 

11/21/03

Internetworking

Two transport protocols

• Transmission Control Protocol (TCP)
• User Datagram Protocol (UDP)

 

UDP

·         Unreliable transfer of data

·         Uses minimal overhead for computation and communication’

·         Favorite for LAN applications

·         Connectionless service paradigm

·         UDP header identifies sender and receiver applications

 

Application

·         Identified by a port number

·         Server uses same typically lower port number

·         Client uses higher port number provided by protocol software

 

TCP

·         Main protocol for Internet

·         Reliable transfer of data

·         Connection-oriented

·         Point2point

·         Full-duplex communication

·         Stream interface

·         Stream divided into segments for transmission

·         Each segment encapsulated in IP datagram

·         Uses protocol ports to identify applications

 

 

TCP Reliability

·         Positive acknowledgement

-          Receiver returns acknowledgement

·         Retransmission

-          Timer is started at send, retransmit occurs as timer expires without acknowledgement