Network Communications Lecture 3

by Gilbert Keith

How may bits in an IP address?

  • 32

How many bits in an Ethernet address?

  • 48 bits

What does the Ethernet address signify?

  • It is a unique identifer attached to a device which communicates via ethernet.

What does the port number signify?

  • It is an address for a specific channel which can be reserved for use by an application (eg FTP uses 21)

What do the various layers in the simplified TCP/IP protocol stack correspond to in the OSI model?

  • Application/Presentation ==> Application
  • Transpor/Sessiont ==> Transport
  • Network ==> Network layer
  • Data Link/Physical ==> Link layer

Why is the Transport layer called “End to End/Host to Host?”

  • Host to Host ==> only cares about info pertaining to source/destination; does not matter how the info got there; doesn’t care about the minutiae of routing, which the Network and link layers care about.

IP is unreliable, yet TCP uses IP. How does TCP provide reliable service to the application layer?

  • TCP has intelligent handling of packets and does error correction to check that the entire file has been received. If it hasn’t, it sends a request to the server/client to resend the file; this is how TCP ensures integrity.

List two common applications of UDP:

  • DNS services
  • DHCP services

Why is IP considered unreliable?

  • It does not check for errors, it simply passes a packet over to the layer above it.
  • It does not check for out of order delivery.

What does TCP do if the message to be sent is larger than simple datagram’s limits?

  • It breaks up the file into more chunks.

If a 1000 byte message is sent using TFTP, what will the size be for the ethernet packet?

  • Minimum size will be 1050.

What are the minimum and maximum header sizes of an IP packet?

  • minimum- 5 bytes. Maximum- 20 bytes.

What is the purpose of the “Time to Live” field in the IP header?

  • It is an integer which signifies the maximum number of hops a message can take to reach the destination – used to prevent messages from getting caught in infinite loops.

If the IP header is 192 bits long, what will the value of the HLEN field be?

  • 6 ==> 192/32

What is the maximum size of data that can be accommodated in an IP datagram?

  • 65536 bytes.

Answers to Lecture #2 questions –  I must say these came out pretty well! Pretty happy about it. Now I am ~7.5% done with this course. 🙂