Chi-Cheng Lin, Winona State University CS 313 Introduction to Computer Networking & Telecommunication Chapter 6 Transport Layer

2 Topics l Transport Service l Elements of Transport Protocols

3 Transport Service l Services provided to the upper layers l Goal  Provide efficient, reliable, and cost- effective services to its users (application/session layer processes) l Transport entity  Hardware/software within transport layer to do the work

4 Logical Relationship The network, transport, and application layers

5 Transport Services l Connection-oriented  E.g., TCP (Transmission Control Protocol) in TCP/IP l Connectionless  E.g., UDP (User Datagram Protocol) in TCP/IP l Why another layer between network and application/session layers?

Transport Service Primitives The primitives for a simple transport service

7 Transport Service Primitives l TPDU  Transport protocol data unit  Message transmitted between transport entities Nesting of TPDUs, packets, and frames.

8 Transport Service Primitives l Client-server example Client processServer process CONNECTLISTEN ACCEPT SENDRECEIVE RECEIVESEND ::DISCONNECT

9 Elements of Transport Protocols l Addressing l Connect l Disconnect l Flow control and buffering l Multiplexing l Crash recovery

10 Elements of Transport Protocols l Transport protocol  Used to implement transport service  Resembles data link protocol  Error control, sequencing, flow control, etc.  BUT, their environments are different! Environment of data link layerEnvironment of transport layer

11 Elements of Transport Protocols l Differences from data link protocol  Explicit addressing of destination  More complicated initial connection establishment  Storage capacity in subnet  Packet might be "hiding" in subnet  Large and dynamically varying number of connections  buffering and flow control needed in both layers, but different approaches required

12 Addressing l Which remote application process to connect to l Transport address  Which process can listen for connection  TSAP  Transport service access point  Internet: Port  Usually multiple TSAP supported by transport entity  NSAP  Network service access point  Internet: IP address  One or more NSAP (e.g., host w/ more than one connections, router, etc.)

Addressing

14 Establishing a Connection l Problem: delayed duplicates  Congestion, timeout, retransmission, packet hidden in subnet l Solution  Ensure no packet lives longer than some known time  Lifetime control  Restricted subnet design  Putting a hop counter in each packet  Timestamping each packet

Connection Establishment Three protocol scenarios for establishing a connection using a three-way handshake. CR denotes CONNECTION REQUEST. Normal operation.

Connection Establishment Three protocol scenarios for establishing a connection using a three-way handshake. CR denotes CONNECTION REQUEST. Old duplicate CONNECTION REQUEST appearing out of nowhere.

Connection Establishment Three protocol scenarios for establishing a connection using a three-way handshake. CR denotes CONNECTION REQUEST. Duplicate CONNECTION REQUEST and duplicate ACK

Connection Release Abrupt disconnection with loss of data

19 Analogy - Two-Army Problem l Unreliable channel  Three-way, four-way, …, N-way handshake  NONE OF THEM WORKS

20 Releasing a Connection l Three-way handshake + timers l Fails when all DRs from source lost  Half-open connection l Solution:  If no TPDUs have arrived for a certain amount of time disconnect  Timer needed  Dummy TPDU might be needed to keep a connection alive

Connection Release Four protocol scenarios for releasing a connection. (a) Normal case of three-way handshake

Connection Release Four protocol scenarios for releasing a connection. (b) Final ACK lost.

Connection Release Four protocol scenarios for releasing a connection. (c) Response lost

Connection Release Four protocol scenarios for releasing a connection. (d) Response lost and subsequent DRs lost.

25 Flow Control l Flow control  Similar to data link layer  Sliding windows (or some other scheme) needed  Difference  Host: numerous connections Router: a few lines  different buffering strategies