Class Notes for 11/17-11/21

 

11/17

ICMP Types (Error & Diagnostic)

 - Type 4 Source Quench

 - Type 11 Time Exceeded

 - Type 3 Destination Unreachable

 - Type 0 Echo Request/Reply (Ping)

 - Type 5 Redirect

        - sent by router

        - goes to a host in local network

        - it is a request for the host to use a different router or to deliver packet directly

 

Example 1:

128.10.3.0

1.) A wants to send a packet to B, but instead of sending it directly, A sends packet to the router.

2.) Router delivers the packet to B

3.) The router sends an ICMP redirect message telling A to send packets to B directly.

 

Example 2:

A wants to send a packet to B.

1.) A sends the packet to R1.

2.) R1 sends the packet to R2.

3.) R1 sends ICMP redirect message telling A that to reach B it has to send packets to R2.

 

- ICMP messages go back to original host.

- There is no ICMP message that is produced by another ICMP message.  This is to prevent a cascade of ICMP messages.

 

Example:  Assume a packet where the source and destination addresses are incorrect. 

 - The router that will deliver the packet sends an ICMP destination unreachable message back to the source because the destination is unreachable.

 - The ICMP packet will not be delivered by another router because the initial source was incorrect. 

 - If we allow ICMP messages caused by ICMP messages, then this packet will bounce infinitely. 

 - There is no ICMP messages about ICMP messages.

 

Other Uses of ICMP

Traceroute

- It is a tool to determine the routers needed to reach a host:  /usr/local/bin/traceroute  www.cnn.com

        R1

        R2

        R3

         |

         |

        www.cnn.com

 - Traceroute works by sending ping with different values of TTL.

 - It starts by sending a ping with TTL = 1.

 In A    > traceroute B

 - R1 decrements TTL to 0 and drops packet and sends ICMP time expired.

 - A sends ping with TTL = 2.

    R2 drops packet and sends ICMP time expired to A.  traceroute program prints R2.

- and so on until B receives packet and sends ICMP echo reply to A and traceroute ends.

 

Path MTU Discovery

Path MTU is the maximum transfer unit in a path of routers where a packet of that size will not be fragmented.

Path MTU = 200

 - A sends a small packet (50 bytes) and sets the no fragment bit to 1.

 - If A receives a reply it increases the size of packet (100 bytes) and tries again.

 - It continues until A receives an ICMP message that packet could not be fragmented.  The size of the previous packet will be the Path MTU.

 

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11/19

 

 

 

 

Describe Transport Service

1.) Type of Service

        Connection oriented    (TCP)

            start, maintain, close

            Reliable    e.g HTTP

        Connection-less        (UDP)

            opposite of connection oriented

            e.g. NetPhone, Video

 

2.) QoS:

    1.) Acceptable error/loss levels

    2.) Desirable delay

    3.) Desirable throughput

    4.) Priority level

    ftp: -high reliability/throughput

    transaction: secure, low delay

 

3.) Data Transfer:

    half duplex A ------------------> B

    full duplex A <------------------  B

 

4.) UI:

    Procedure Calls :

    Pa --> Pb        Pa ---> O ----> Pb        (asynchronous)

 

5.) Connection Management:

    A <-------------> B

        Who terminates the connection?

 

6.) Security:

    (Alice) -------------------- (Mallory) -------------------- (Bob)

    Man-in-the-Middle

    IP - unreliable

 

1.) Ordered Delivery

2.) Retransmission strategy

            erroneous/lost

    A --------------------> B

                ACK