1 / 25

Computer Networks

Computer Networks. Transport Layer. Transport Layer Objectives. Goals Deals with end host pacing, congestion control Provides internal addresses on host (ports) Upward multiplexing Reliable, end-to-end data delivery Challenges

suchi
Download Presentation

Computer Networks

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Computer Networks Transport Layer CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  2. Transport Layer Objectives • Goals • Deals with end host pacing, congestion control • Provides internal addresses on host (ports) • Upward multiplexing • Reliable, end-to-end data delivery • Challenges • Network reliability – lost, delayed, duplicated, out of order packets • Network delay variation (RTT estimation) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  3. Transport Layer Protocols • UDP (User Datagram Protocol) • TCP (Transmission Control Protocol) • SCTP (Stream Control Transmission Prot) • DCCP (Datagram Congestion Ctl Prot) • RTP (Realtime Transport Protocol) • ATP (AppleTalk Transaction Protocol) • NetBEUI (NetBIOS Extended User Interf.) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  4. User Datagram Protocol • “Passthrough” for IP • Upward multiplexing • No reliability guarantees at all • No congestion control • Fields (8 bytes!) • Src & Dest Port (16 bits each) • Optional checksum (16 bits) • 16-bit length (including header) • See IETFRFC 768 CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  5. Transmission Control Protocol • Much more complex • Upward multiplexing • Reliable, in-order byte stream delivery • Congestion control • No timing guarantees/QoS • Session Semantics • Setup: 3-way handshake (SYN, SYN-ACK,ACK) • Use: sequence #s, ACKs • Teardown: FIN, FIN-ACK • See RFC 793, RFC2581, RFC 3168 CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  6. Stream Control Transmission Protocol • Multiple Stream Service • Upward multiplexing • Reliable, in-order delivery • Congestion control • Message streams • Multihoming support • See RFC 2960 (protocol), RFC 3286 (text) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  7. Datagram Congestion Control Protocol • Intermediate between TCP and UDP • Upward multiplexing • Order, reliability not guaranteed • Congestion control • No timing guarantees/QoS • Session semantics (setup, use, teardown) • See RFC 4340 CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  8. RTP • For realtime traffic • Has ports for upward multiplexing • Marker bit • Payload type • 32-bit Timestamp for delivery jitter • No congestion control • 16-bit Sequence numbers, but no order guarantee • Runs on top of UDP • See RFC 3550 CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  9. AppleTalk Transaction Protocol • Part of AppleTalk self-configuration • AppleTalk addr resolution protocol (AARP) hosts automatically generate own NW addrs • Name Binding Protocol (NBP) like DNS • Proprietary  • Request/Response/ACK exchanges • 1 req pkt could garner up to 8 resp pkts • Requestor sent bitmap selective ACK • At-least-once and exactly-once options CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  10. AppleTalk Data Stream Protocol • Late addition to AppleTalk suite • Proprietary  • TCP-like, except: • Host can reject a connection • No half-open connections • See http://en.wikipedia.org/wiki/AppleTalk CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  11. NetBIOS Extended User Interface • Unrouted network-and-transport layer • Can only be used in a broadcast domain • A.k.a. NetBIOS Frame (NBF) • Uses IEEE 802.2 type 1 for name svc, datagram • Uses IEEE 802.2 type 2 for session svc. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  12. Back to TCP • Fields (20 bytes!) • Src & Dest Port (16 bits each) • 32-bit sequence #, ACK # (byte stream) • 4-bit Header Length (in 32-bit words) • Reserved (6 bits) • Flags (6 bits) • 16-bit Window size (bytes willing to Rx) • Checksum (16 bits) • Urgent pointer • Options (0 or more 32-bit words) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  13. TCP Flags • URG • Urgent pointer valid (offset in current segment) • ACK – ACK field is valid • PSH – deliver immediately (don’t buffer) • RST – Reset connection (abort) • SYN – Synchronize (3-way handshake) • FIN – Finish (normal close connection) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  14. TCP Session Initiation • Three-way Handshake • Client makes connection req • Server responds (half-open • Connection now) • Client confirms • Data transfer begins • Syn Flooding Attack… Host A Host B Syn, X, - Syn, ACK, Y, X+1 ACK, X+1, Y+1 Normal CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  15. TCP Initiation Mishaps Host B Host A Syn, X, - Syn, Y, - Syn, ACK, W, Z+1 Syn, ACK, Y, X+1 Syn, ACK, X, Y+1 Call Collision RST Unknown/Late CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  16. TCP States • CLOSED – no active/pending connection • LISTEN – server waits for incoming call • SYN RCVD – connection req received, wait for ACK • SYN SENT – connection requested, wait for SYN-ACK • ESTAB – normal data transfer state • FIN WAIT 1 – application has said it’s done (active) • FIN WAIT 2 – other side has agreed to release (active) • TIMED WAIT – wait for all packets to die off (active) • CLOSING – both sides closed simultaneously (active) • CLOSE WAIT – other side initiated a release (passive) • LAST ACK – wait for all packets to die off (passive) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  17. TCP State Machine - Open Normal Server Connect/SYN Normal Client CLOSED Unusual Close/- Listen/- Close/- SYN/SYN-ACK LISTEN Send/SYN RST/- SYN SENT SYN RCVD SYN/SYN-ACK Close/FIN ACK/- ESTABLISHED FIN WAIT 1 SYN-ACK/ACK CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  18. TCP State Machine - Close SYN RCVD Close/FIN FIN/ACK ESTABLISHED Close/FIN [Active Close] [Passive Close] FIN/ACK FIN WAIT 1 CLOSING CLOSE WAIT ACK/- ACK/- FIN-ACK/ACK Close/FIN FIN/ACK FIN WAIT 2 TIMED WAIT LAST ACK (Timeout) ACK/- CLOSED CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  19. TCP Transmission Policy • Sender not required to send as soon as application delivers data • Windows not tied directly to ACKs, but also to receive window size • Receiver may ACK when segt received, when receive window changes, etc. CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  20. TCP Transmission Example Receiver’s 4K buffer Appl writes 2 K bytes 0 4K 2K, seq=0 empty ACK=2048, WIN=2048 Appl writes 3 K bytes 2K 2K, seq=2048 ACK=4096, WIN=0 Full Sender blocked Appl reads 2 K bytes ACK=4096, WIN=2048 Sender may send up to 2 K bytes 2K 1K, seq=4096 ACK=5020, WIN=1024 1K 2K CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  21. TCP Telnet Example • Interactive editor – reacts on every keystroke • Sender sends each character on arrival • 41 bytes (1 char + 20 TCP + 20 IP) • Receiver sends 40-byte ACK • Editor reads byte, Rx updates WIN (40 more) • Editor processes character, echoes it • 41 more bytes! • Total of 162 bytes in 4 datagrams per char!! CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  22. TCP Solution • Delayed ACKs can alleviate this • Wait 500 ms before sending ACK • Gives time for read, echo, so only 1 DG sent • Ack timer needed, reverse flow trigger • Also trigger ACK on second segment arrival • Still inefficient (82 bytes in 2 DGs) • Nagle’s Algorithm helps further! CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  23. Nagle’s Algorithm • Sender side solution • When data arrives one byte at a time, send first byte, then buffer rest until ACKed • When ACK arrives, or when MSS can be filled, or if half WIN filled, send all chars in buffer in a single segment • Huge bandwidth savings! Widely used. • Disable if using mouse, avoid jerkiness CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  24. Silly Window Syndrome Receiver’s buffer Full Appl reads 1 byte Sender sends 1 byte 40 bytes ACK room for 1 byte 40 bytes hdrs Full 1 byte of data CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  25. Clark’s Solution • Prevent Rx from sending window update for one byte • Rx forced to wait until WIN >= MSS, or buffer is half empty, whichever is smaller • Sender should not send tiny segments • Wait until full MSS or half receiver’s buffer size, whichever smaller CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

More Related