Transport Layer
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Transport Layer



The Transport Layer

The Transport Layer



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    Transport Layer Transport Layer Presentation Transcript

    • TCP,UDP, ICMP and the transport layer Lecture February 4,2001 Professor Tom Mavroidis
    • Protocols
      • IP is a connectionless, datagram-oritented, and packet forwarding protocol
      • IP sends a packet based on the destination IP address and routing information held internally within the protocol
      • Packets may pass through several gateways or hops before reaching their destination
    • IP - Internet Protocol
      • Unreliable protocol
      • relies on best-effort delivery
      • packets are not acknowledged
      • Designed to be simple, efficient, and straightforward to implement
    • ICMP - Internet Control Message Protocol
      • Located in the Network Layer
      • Purpose is to provide error control to IP
      • Two basic types of messages:
      • error control and querying
    • Common error control ICMP messages
      • 3 Destination Unreachable
      • 4 Source Quench
      • 5 Redirection
      • 11 Time Exceeded
      • 12 Parameter Problem
    • ICMP Messages for Querying
      • 0 Echo Reply
      • 8 Echo Request
      • 9 Router Advertisement
      • 10 Router Solicitation
      • 13 Timestamp Request
      • 14 Timestamp Reply
      • 17 Address Map Request
      • 18 Address Mask Reply
    • UDP - User Datagram Protocol
      • Transport-layer Protocol
      • Used mainly for one way, short communications
      • datagrams don’t contain information about other packets
      • Each packet independent of each other
      • Messages can be no larger than the max UDP packet size
    • UDP - continued
      • Lack of error control
      • Checksum is only check, packet is dropped if incorrect
      • Relies on ICMP for error control
      • Good alternative for short messages without large communications overhead used by TCP
    • TCP - Transmission Control Protocol
      • Used where reliable communications is required
      • Provisions for error control, streaming data, and two way communications
      • Connections are managed both ways using flags in the TCP packet header
    • Three way handshake TCP
      • SYN(synchronize) - sent at the beginning of a connection from source host. Contains initialization information.
      • ACK(acknowledgement) - sent when packet has been received by the destination host
      • SYN - then sent by destination host
      • ACK - sent by source host
    • TCP error control
      • Implemented through the ACK flag
      • Each packet numbered and a timer is started on transmission
      • Packet is retransmitted if ACK not received before timeout
    • IP Packet Format - Header Length field
      • Specifies length of header
      • Is in 32-bit works
      • Maximum header size of 60 bytes
      • Normally length is 5
    • Version Field
      • Carries the current version of IP
      • Currently 4 IPv4
      • Next version will be 6 IPv6
      • Also called Ipng - next generation
    • Service Type Field
      • Used by some routers when deciding how to forward the datagram
      • Choices are quickest, cheapest, most reliable, or highest bandwidth
      • Datagram can be sent with no Service Type flag set
    • Total Length Field
      • Overall size of the datagram in bytes including the header
      • Maximum datagram size of 65,536 bytes or 64kb
    • Identification, Flags, and Fragment Offset fields
      • Used to control fragmentation and reassembly of datagrams if necessary
    • Time to live field
      • Counter that is decremented every time the packet crosses a gateway.
      • When counter reaches 0 packet is discarded
    • Protocol Field
      • Indicates which transport protocol is to receive the packet at the destination
      • All IP protocols (TCP,UDP,ICMP) carry checksums of their own headers and data
      • The header checksum only covers the IP header to avoid redundancy
      • Corrupt IP layer datagrams are discarded at the same layer at the receiving end
    • IP Options
      • Can be used to cover the routing
      • strict- or loose-source routing
      • security and handling restrictions
      • can record the addresses of routers the datagram passes through and at what time
      • some routers ignore these options
    • IP Checksum
      • Adds considerable overhead
      • IPv6 is removing this checksum
    • Type of Service (TOS)
      • Allows a packet to be routed :
        • minimal delay
        • maximal throughput
        • maximal reliability
        • minimal cost
    • Fragmentation and Reassembly
      • Transport layer passes data to IP
      • IP organizes data into datagram
      • If datagram is larger than Maximum Transmission Unit (MTU), IP will split data into smaller packets called fragments
      • Fragments are encapsulated in individual IP packets, sent to destination host, and then reassembled.
    • Fragmentation and Reassembly
      • Packets could be routed via different routes
      • Therefore reassembly only takes place at the destination host
      • MTU’s may vary from network to network
      • If datagram containing a fragment encounters a network with a smaller MTU, the fragment is fragmented further
    • Fragmentation and Reassembly
      • You can specify to the Transport layer that a datagram not be fragmented.
      • If the packet finds a network with a smaller MTU, it will be discarded and ICMP will cause an error message to be sent back
    • Record Route Option
      • The route that the packet took is inserted into the IP Header in the sequence that it passed through all routers
      • The IP addresses of the routers is recorded
      • There is a limited amount of space to record this information, and data will be truncated
      • Instead use a utility like Traceroute
    • Timestamp Option
      • Extension to Record Route
      • Time and date packet passed through router is recorded with its IP address
      • Same space limitations as Record Route
      • Times on routers may not be synchronized giving misleading information
      • Not a popular method and many routers don’t support it
    • Source Routing
      • Specifies which route a packet follows
      • Two Methods
        • Loose-source routing (specifies a list of routers the packet must pass through, may also pass through routers not listed)
        • Strict-source routing (explicitly specifies which routers the packet must pass through, and may not travel via any that are not listed
    • Source Routing
      • Many routers refuse to accept source-routed packets
      • Routing is usually controlled via the use of policy-based dynamic routing protocols
    • Error Handling - ICMP
      • IP has no built in error handling
      • ICMP adds error reporting to IP
      • resides at the Network Layer
      • uses IP as its carrier
      • used for flow control as well -source quench
      • can request info i.e. timestamp, echo to ping
      • also sent in response to broadcast datagrams
    • ICMP Header Format
      • ICMP messages are encapsulated in IP datagrams
      • Message has 4 parts
        • type of message (15 different types)
        • message code (subtype, 16 different types)
        • checksum (covers both header and data)
        • data (will differ depending on type and code)
    • ICMP Message types
        • 8 echo request (sent by ping)
        • 0 echo reply (received by ping)
        • 3 destination unreachable
        • 4 source quench
        • 5 redirect
        • 9 router advertisement
        • 10 router solicitation
        • 11 Time exceeded
        • 12 Parameter problem
        • 13 Timestamp request
        • 14 Timestamp reply
        • 15 Information request (obsolete)
        • 16 Information reply (obsolete)
        • 17 Address mask request
        • 18 Address mask reply
    • TCP and the Transport Layer
      • Lives above the IP Layer
      • adds the missing pieces to IP
      • offers
        • a way to talk to a specific application or host
        • reliable data transfer mechanism
        • way to send a continuous data stream
        • flow control of data
    • The Transport Layer
      • Directs data to and from applications across networks
      • Two types of protocols are
        • connectionless (no guarantee of delivery to destination)
        • connection-oriented (establish connection, transfer data, close connection)
    • Connection Oriented
      • Two-way connection
      • Similar to that of a telephone conversation
      • Reliable
      • guaranteed delivery of data
    • TCP Frame Format
    • Frame Format
      • TCP supports full duplex, byte-stream-oriented communication between processes
      • The data stream must be broken into segments that can be encapsulated in IP datagrams for transmissions
      • “ Source Port” and “Destination Port” identify the sending and receiving applications
    • Sequence Number
      • Indicates the position of this segment in the data stream
    • Acknowledgment Number
      • Indication of how much of the data stream has been successfully received
    • HLEN
      • 4-bits , gives the length of TCP in 32-bit words-max size 60 bytes-norm size 20 bytes
    • Code Bits field
      • Flags for reset,sync,finish,etc. reside here
    • Window Field
      • Indicates how many bytes the receiver can accept
    • Checksum
      • Covers data in the segment, its header and the source and destination IP addresses and protocol value from the IP address
    • Urgent Pointer
      • Mechanism for sending urgent data along with normal data
    • Options
      • Similar to IP, options can be added to the header
    • TCP Handshaking
      • Six flags
        • SYN Sync seq #’s to initiate a connection
        • ACK Acknowledgment frame
        • RST Reset the connection
        • URG Urgent incoming data
        • PSH Pass data onto the app ASAP
    • TCP Flow Control
      • Indicates how many bytes of data can safely be received at any time, called the window
      • Window size is the max # of bytes that can be sent without receiving an ACK.
      • Receiver stores up segments regardless of the order they arrive in.
      • If duplicate segments are received they are discarded
    • Ports and Sockets
      • A method for piping a data flow to and from a particular application
      • A socket is a pair of data queues for an application process (one send one receive)
      • A port is an address on a socket
      • Incoming data is bound via its port address to a socket
    • Port Numbers
      • Servers listen on specific ports waiting for clients (RFC 1700)
      • Ports 1 to 1023 are managed by IANA (InterNet Assigned Numbers Authority)
        • reserved for well-known services
        • known as privileged ports