1. Chapter 2 Protocols and Architecture
CS408/533 Computer Networks
Chapter 2
Protocols and Architecture

2. Characteristics Direct or indirect Monolithic or structured
Standard or nonstandard

3. Direct or Indirect Direct Systems share a point to point link or
Systems share a multi-point broadcast link
No intervening active agent
a hub or switch possible

4. Direct or Indirect Indirect Switched networks or
Internetworks or internets
Data transfer depend on functioning other entities

5. Monolithic or Structured
single unit for all communication tasks
too complex
a small change effects the whole package
hard to debug
Structured design breaks down problem into smaller units
Layered structure

6. Standard or Nonstandard
Nonstandard protocols
K sources and L receivers leads to K*L protocols and 2*K*L implementations
If common protocol used, K + L implementations needed
Customer demand for standard protocols
not to stick to a specific vendor

7. Use of Standard Protocols

8. Functions Encapsulation Segmentation and reassembly Connection control
Ordered delivery
Flow control
Error control
Addressing
Multiplexing
Transmission services

9. Encapsulation Addition of control information to data
Address information
Error-detecting code
Protocol control

10. Segmentation (Fragmentation)
Application layer data
sequence of blocks of bounded size
continuous stream
Application layer data may be large
Lower level data blocks should be of smaller size
Splitting larger blocks into smaller ones is segmentation (or fragmentation in TCP/IP)
Restoring the original application data is called “reassembly”

11. Segmentation (Fragmentation)
Why fragmentation?
Network limits
ATM blocks (cells) are 53 octets long
Ethernet blocks (frames) are up to 1526 octets long
More efficient error control and recovery
Fair access to network facilities
Smaller buffers needed
Disadvantages
Overheads (each PDU needs a header)
Increased interrupts at receiver
More processing time

12. Connection Control
Connectionless vs. connection oriented data transfer
Connection oriented
Connection Establishment
request/response + negotiation of parameters (e.g. max PDU size)
Data transfer
data and control (flow and error control) info
Connection termination
Maybe connection interruption and recovery
Sequence numbers used for ordered delivery, flow control and error control

13. Connection Oriented Data Transfer

14. Ordered Delivery PDUs may traverse different paths through network
PDUs may arrive out of order
Sequentially number PDUs to allow for ordering

15. Flow Control Done by receiving entity Limit amount or rate of data
Stop and wait
Credit systems
Sliding window

16. Error Control Guard against loss or damage
Two phases: error detection and retransmission
Error detection
Sender inserts error detecting bits
Receiver checks these bits
If OK, acknowledge
If error, discard packet (if cannot be corrected)
Retransmission
If no acknowledge in given time, re-transmit

17. Addressing Addressing level Addressing scope Connection identifiers
Addressing mode

18. Addressing level Level in architecture at which entity is named
Unique address for each end system (computer) and router
Network level address
IP or internet address (TCP/IP)
Network service access point or NSAP (OSI)
Process within the system
Port number (TCP/IP)
Service access point or SAP (OSI)
Network attachment point address
e.g. MAC addresses

19. Address Concepts

20. Addressing Scope Global vs. local addresses Global addresses
e.g. IP addresses
Global nonambiguity
Global address identifies unique system
Local addresses
Port or SAP numbers
local to a system
Network attachment point addresses
local to a network

21. Connection Identifiers
Connection oriented data transfer (virtual circuits)
Allocate a connection name during the transfer phase
Reduced overhead as connection identifiers are shorter than global addresses
Routing may be fixed and identified by connection name
Entities may want multiple connections - multiplexing
State information

22. Addressing Mode Usually an address refers to a single system
Unicast address
Sent to one machine or person
May address all entities within a domain
Broadcast
Sent to all machines or users
May address a subset of the entities in a domain
Multicast
Sent to some machines or a group of users

23. Multiplexing Supporting multiple connections on one machine
Multiple TCP connections terminating in a system, each connection is for different port pair
Mapping of multiple connections at one level to a single connection at another
Upward multiplexing
multiple higher level connections onto a single lower level
necessary where low level connection is limited
Downward multiplexing
single higher level connection is divided among several lower-level connections
useful when application level reliability and performance is important

24. Transmission Services
Priority
some packets should arrive ASAP
e.g. control messages
can be assigned on message or connection based
Quality of service
Minimum acceptable throughput
Maximum acceptable delay
Security

25. OSI - The Model A layer model
Each layer performs a subset of the tasks necessary for communication
Each layer relies on the next lower layer to perform more primitive functions
Each layer provides services to the next higher layer
Changes in one layer should not require changes in other layers
Optimum number of layers

26. The OSI Environment

27. OSI as Framework for Standardization
layer functionalities are described bu ISO, different standards can be developed based on these functionalities

28. Layer Specific Standards

29. Elements of Standardization
Protocol specification
Operates between the same layer on two systems
May involve different operating system
Protocol specification must be precise
Format of data units
Semantics of all fields
allowable sequence of PDUs
Service definition
Functional description of what is provided to the next upper layer
Addressing
Referenced by SAPs

30. OSI Layers (1) Physical Data Link Physical interface between devices
Characteristics
Mechanical - interface specs
Electrical - voltage levels for bits, transmission rate
Data Link
Principle service: error detection and control
Higher layers may assume error free transmission

31. OSI Layers (2) Network Transfer of information through comm. network
Higher layers do not need to know about underlying networking technology
Not needed on direct links
If needed network nodes (relays) should perform switching and routing functions

32. Use of a Relay

33. OSI Layers (3) Transport Session Exchange of data between end systems
In sequence, no losses, no duplicates
Quality of service (e.g. accepatable error rates, max. delay)
Session
Control of dialogues between applications
Dialogue discipline (full-duplex, half-duplex)
Grouping of data (for batch processing)
Checkpointing and recovery

34. OSI Layers (4) Presentation Application Data formats and coding
Data compression
Encryption
Application
Means for applications to access OSI environment

35. TCP/IP Protocol Suite
Most widely used interoperable network protocol architecture
Specified and extensively used before OSI
OSI was slow
Funded by the US Defense Advanced Research Project Agency (DARPA) for its packet switched network (ARPANET)
DoD automatically created an enormous market for TCP/IP
Used by the Internet and WWW

36. TCP/IP Protocol Suite TCP/IP does not have an official layer structure
But protocols imply one
Application layer
Host to host or transport layer
Internet layer
Network access layer
Physical layer

37. Some Protocols in TCP/IP Suite

38. Physical Layer
Physical interface between data transmission device (e.g. computer) and transmission medium or network
Characteristics of transmission medium
Signal levels
Data rates

39. Network Access Layer Exchange of data between end system and network
Destination address must be provided
routing in the network
Invoking services like priority
Different standards (e.g. Ethernet, X.25)

40. Internet Layer Internet Protocol (IP)
Systems may be attached to different networks
Routing functions across multiple networks
Implemented in end systems and routers

41. Transport Layer End-to-end data transfer
Hides detail of underlying network from applications
Transmission Control Protocol (TCP)
connection oriented
Reliable delivery of data
Ordering of delivery
User Datagram Protocol (UDP)
connectionless service

42. Application Layer Support for user applications
e.g. http, SMTP, telnet

43. Operation of TCP and IP

44. PDUs in TCP/IP Dest. Port Sequence number Checksum Dest. Address
Source address
Dest. Network Address
Priority info

45. Some Protocols in TCP/IP Suite

46. Standards Required to allow for interoperability between equipment
Advantages
Ensures a large market for equipment and software
Allows products from different vendors to communicate
Disadvantages
Freeze technology
May be multiple standards for the same thing

47. Standards Organizations
Internet Society
ISO (International Organization for Standardization)
more formal
NGO, but most members are from governments
ITU-T (formally CCITT)
International Telecommunications Union
UN agency
governmental

48. Internet Society Internet development and standardization
3 suborganizations
IAB (Internet Architecture Board)
overall Internet architecture
IETF (Internet Engineering Task Force)
protocol engineering and development
IESG (Internet Engineering Steering Group)
monitors IETF standardization efforts
Voluntary participation in IETF working groups

49. Internet Drafts and RFCs
Draft document
expires in 6 months, if IESG does not approve it as an RFC
can be resubmitted
published online
comments welcome
RFC (Request for Comments)
final version
can obsolete previous RFCs about the same topic
actually an RFC can be of any type of document
not necessarily a standard

50. Internet Standards Track
Steps involve increasing amount of scrutiny and testing
Step 1: Internet Draft
Step 2: Proposed standard
Internet Draft approved as an RFC by IESG
must remain at least six months to advance
Step 3: Draft standard
at least two independent and interoperable implementations
must remain at least 4 months
Step 4: Internet standard
Significant operational experience
key difference between ISOC and others
consensus