Internet Protocols - Data Communications - Lecture Slides, Slides of Data Communication Systems and Computer Networks

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Chapter 18

Internet Protocols

Protocol Functions

• Small set of functions that form basis of all protocols
• Not all protocols have all functions — Reduce duplication of effort — May have same type of function in protocols at different levels
• Encapsulation
• Fragmentation and reassembly
• Connection control
• Ordered delivery
• Flow control
• Error control
• Addressing
• Multiplexing
• Transmission services

Fragmentation and Reassembly

(Segmentation – OSI)

• Exchange data between two entities
• Characterized as sequence of PDUs of some bounded size — Application level message
• Lower-level protocols may need to break data up into smaller blocks
• Communications network may only accept blocks of up to a certain size — ATM 53 octets — Ethernet 1526 octets
• More efficient error control — Smaller retransmission
• Fairer — Prevent station monopolizing medium
• Smaller buffers
• Provision of checkpoint and restart/recovery operations

Disadvantages of

Fragmentation

• Make PDUs as large as possible because — PDU contains some control information — Smaller block, larger overhead
• PDU arrival generates interrupt — Smaller blocks, more interrupts
• More time processing smaller, more numerous PDUs

PDUS and Fragmentation

(Copied from chapter 2 fig 2.4)

Connection Control

• Connectionless data transfer — Each PDU treated independently — E.g. datagram
• Connection-oriented data transfer — E.g. virtual circuit
• Connection-oriented preferred (even required) for

lengthy exchange of data

• Or if protocol details must be worked out dynamically
• Logical association, or connection, established between

entities

• Three phases occur — Connection establishment — Data transfer — Connection termination — May be interrupt and recovery phases to handle errors Docsity.com

Connection Establishment

• Entities agree to exchange data
• Typically, one station issues connection request — In connectionless fashion
• May involve central authority
• Receiving entity accepts or rejects (simple)
• May include negotiation
• Syntax, semantics, and timing
• Both entities must use same protocol
• May allow optional features
• Must be agreed
• E.g. protocol may specify max PDU size 8000 octets;

one station may wish to restrict to 1000 octets

Data Transfer and Termination

• Both data and control information exchanged

—e.g. flow control, error control

• Data flow and acknowledgements may be in one

or both directions

• One side may send termination request

• Or central authority might terminate

Ordered Delivery

• PDUs may arrive out of order — Different paths through network
• PDU order must be maintained
• Number PDUs sequentially
• Easy to reorder received PDUs
• Finite sequence number field — Numbers repeat modulo maximum number — Maximum sequence number greater than maximum number of PDUs that could be outstanding — In fact, maximum number may need to be twice maximum number of PDUs that could be outstanding - e.g. selective-repeat ARQ

Flow Control

• Performed by receiving entity to limit amount or rate of

data sent

• Stop-and-wait — Each PDU must be acknowledged before next sent
• Credit — Amount of data that can be sent without acknowledgment — E.g. HDLC sliding-window
• Must be implemented in several protocols — Network traffic control — Buffer space — Application overflow - E.g. waiting for disk access

Addressing

• Addressing level

• Addressing scope

• Connection identifiers

• Addressing mode

TCP/IP Concepts

Addressing Scope

• Global address — Global nonambiguity — Identifies unique system — Synonyms permitted — System may have more than one global address — Global applicability — Possible at any global address to identify any other global address, in any system, by means of global address of other system — Enables internet to route data between any two systems
• Need unique address for each device interface on network — MAC address on IEEE 802 network and ATM host address — Enables network to route data units through network and deliver to intended system — Network attachment point address
• Addressing scope only relevant for network-level addresses
• Port or SAP above network level is unique within system — Need not be globally unique — E.g port 80 web server listening port in TCP/IP Docsity.com

Connection Identifiers

• Entity 1 on system A requests connection to entity 2 on system B, using global address B.2.
• B.2 accepts connection
• Connection identifier used by both entities for future transmissions
• Reduced overhead — Generally shorter than global identifiers
• Routing — Fixed route may be defined — Connection identifier identifies route to intermediate systems
• Multiplexing — Entity may wish more than one connection simultaneously — PDUs must be identified by connection identifier
• Use of state information
• Once connection established, end systems can maintain state information about connection — Flow and error control using sequence numbers