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CSEE W4140 Networking Laboratory

CSEE W4140 Networking Laboratory. Lecture 6: TCP and UDP Jong Yul Kim 03.01.2010. Maximum Transmission Unit. Maximum size of IP datagram is 65535, but the data link layer protocol generally imposes a limit that is much smaller For example:

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CSEE W4140 Networking Laboratory

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  1. CSEE W4140Networking Laboratory Lecture 6: TCP and UDP Jong Yul Kim 03.01.2010

  2. Maximum Transmission Unit • Maximum size of IP datagram is 65535, but the data link layer protocol generally imposes a limit that is much smaller • For example: • Ethernet frames have a maximum payload of 1500 bytes IP datagrams encapsulated in Ethernet frame cannot be longer than 1500 bytes • The limit on the maximum IP datagram size, imposed by the data link protocol is called maximum transmission unit (MTU) • MTUs for various data link layers: Ethernet: 1500 FDDI: 4352 802.3: 1492 ATM AAL5: 9180 802.5: 4464 PPP: 296 • What if the size of an IP datagram exceeds the MTU? IP datagram is fragmented into smaller units. • What if the route contains networks with different MTUs?

  3. Maximum Transmission Unit • Maximum size of IP datagram is 65535, but the data link layer protocol generally imposes a limit that is much smaller • Example: • Ethernet frames have a maximum payload of 1500 bytes  IP datagrams encapsulated in Ethernet frame cannot be longer than 1500 bytes • The limit on the maximum IP datagram size, imposed by the data link protocol is called maximum transmission unit (MTU) • MTUs for various data link protocols: Ethernet: 1500 FDDI: 4352 802.3: 1492 ATM AAL5: 9180 802.5: 4464 PPP: negotiated

  4. IP Fragmentation • What if the size of an IP datagram exceeds the MTU? IP datagram is fragmented into smaller units. • What if the route contains networks with different MTUs? • MTUs:FDDI: 4352Ethernet: 1500 • Fragmentation: • IP router splits the datagram into several datagram • Fragments are reassembled at receiver

  5. Where is Fragmentation done? • Fragmentation can be done at the sender or at intermediate routers • The same datagram can be fragmented several times. • Reassembly of original datagram is only done at destination hosts !!

  6. What’s involved in Fragmentation? • The following fields in the IP header are involved: Identification When a datagram is fragmented, the identification is the same in all fragments Flags DF bit is set: Datagram cannot be fragmented and must be discarded if MTU is too small MF bit set: This datagram is part of a fragment and an additional fragment follows this one

  7. What’s involved in Fragmentation? • The following fields in the IP header are involved: Fragment offset Offset of the payload of the current fragment in the original datagram Total length Total length of the current fragment

  8. Example of Fragmentation • A datagram with size 2400 bytes must be fragmented according to an MTU limit of 1000 bytes

  9. Today’s topics • We move one layer up and look at the transport layer.

  10. UDP vs TCP

  11. UDP - User Datagram Protocol • UDP supports unreliable transmissions of datagrams • UDP merely extends the host-to-to-host delivery service of IP datagram to an application-to-application service • The only thing that UDP adds is multiplexing and demultiplexing

  12. UDP Format • Port numbers identify sending and receiving applications (processes). Maximum port number is 216-1= 65,535 • Message Length is at least 8 bytes (I.e., Data field can be empty) and at most 65,535 • Checksum is for header (of UDP and some of the IP header fields)

  13. Port Numbers • UDP (and TCP) use port numbers to identify applications • A globally unique address at the transport layer (for both UDP and TCP) is a tuple <IP address, port number> • There are 65,535 UDP ports per host.

  14. TCP - Transmission Control Protocol • Reliable and in-order byte-stream service • TCP format • Connection establishment • Flow control • Reaction to congestion Next Lecture

  15. TCP Format • TCP segments have a 20 byte header with >= 0 bytes of data.

  16. TCP header fields • Sequence Number (SeqNo): • Sequence number is 32 bits long. • So the range of SeqNo is 0 <= SeqNo <= 232 -1  4.3 Gbyte • Each sequence number identifies a byte in the byte stream • Initial Sequence Number (ISN) of a connection is set during connection establishment Q: What are possible requirements for ISN ?

  17. TCP header fields • Acknowledgement Number (AckNo): • The AckNo contains the next SeqNo that a hosts wants to receiveExample: The acknowledgement for a segment with sequence numbers 0-1500 is AckNo=1501 • A hosts uses the Acknowlegement Number field to send acknowledgements. (If a host sends an AckNo in a segment it sets the “ACK flag”) • Acknowledgements are piggybacked, i.e., a segment from A -> B can contain an acknowledgement for a data sent in the B -> A direction Q: Why is piggybacking good ?

  18. TCP header fields • Header Length ( 4bits): • Length of header in 32-bit words • Note that TCP header has variable length (with minimum 20 bytes)

  19. TCP header fields • Flag bits: • URG: Urgent pointer is valid • If the bit is set, the following bytes contain an urgent message in the range:SeqNo <= urgent message <= SeqNo+urgent pointer • ACK: Acknowledgement Number is valid • PSH: PUSH Flag • Notification from sender to the receiver that the receiver should pass all data that it has to the application. • Normally set by sender when the sender’s buffer is empty

  20. TCP header fields • Flag bits: • RST: Reset the connection • The flag causes the receiver to reset the connection • Receiver of a RST terminates the connection and indicates higher layer application about the reset • SYN: Synchronize sequence numbers • Sent in the first packet when initiating a connection • FIN: Sender is finished with sending • Used for closing a connection • Both sides of a connection must send a FIN

  21. TCP header fields • Window Size: • Each side of the connection advertises the window size • Window size is the maximum number of bytes that a receiver can accept. • Maximum window size is 216-1= 65535 bytes • TCP Checksum: • TCP checksum covers over both TCP header and TCP data (also covers some parts of the IP header) • 16-bit one’s complement • Urgent Pointer: • Only valid if URG flag is set

  22. Three-Way Handshake Why is two-way handshake not enough?

  23. TCP States in “Normal” Connection Lifetime

  24. Resetting Connections • Resetting connections is done by setting the RST flag • When is the RST flag set? • Connection request arrives and no server process is waiting on the destination port • Abort (Terminate) a connection Causes the receiver to throw away buffered data. Receiver does not acknowledge the RST segment

  25. Announcement • Midterm next week • Prelabs 1 ~ 5 • Lectures 1 ~ 6 • Labs 1 ~ 4

  26. Main Points of Lab 5 Parts 1~4 • TCP and UDP • Throughput • IP Fragmentation • TCP Connection Management

  27. Homework • No prelab due this Friday • Lab report 5 part 1 due by next week

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