Datornätverk A – lektion 13

Datornätverk A – lektion 13
Kapitel 19: Routing. Kapitel 21: Routing Protocols Forts. kapitel 22: UDP och TCP. (Kapitel 23: Congestion control and QoS översiktligt.)

Hosts and Routers The computers in Internet terminology are called hosts. They usually have one NIC (network interface card = network adapter = nätverkskort) Routers are special purpose computers and they have more than one NIC An old name for routers is gateways Forward packets between networks (route and switch) Transform packets as necessary to meet standards for each network A Windows PC can act as a router if it has more than one NIC, and IP forwarding is enabled in the networking settings.

What Does a Router Do? Accepts incoming packets
Checks the destination address in the IP header Look up for destination in the forwarding table Sends packet to the appropriate next hop The packet may be dropped if There is no space in the router’s buffers The TTL=0 There is no matching row in the routing table

Forwarding (Routing) Table
The forwarding table consists of two columns: “Destination network” and “Next hop”. Destination network is some network address and the next hop is the address of the next router. When the router is connected directly to a network, the “Next hop” is labeled as “Direct” meaning “Directly connected”

Figure 19.29 Network-specific routing

Figure 19.31 Default routing

Default Route In order to make the forwarding table shorter (smaller number of rows) the default route is introduced “Default” or “Else” is a row that points to some “Next hop” and is used whenever a destination is not found in the forwarding table. Hosts send all packets out of their network to the default router (or gateway)

Figure 19.32 Example: Subnet mask based routing table

Example 10 Using the table in Figure 19.32, the router receives a packet for destination For each row, the mask is applied to the destination address until a match with the destination address is found. In this example, the router sends the packet through interface m0 (host specific).

Example 11 Using the table in Figure 19.32, the router receives a packet for destination For each row, the mask is applied to the destination address until a match with the next-hop address is found. In this example, the router sends the packet through interface m2 (network specific).

Example 12 Using the table in Figure 19.32, the router receives a packet for destination For each row, the mask is applied to the destination address, but no match is found. In this example, the router sends the packet through the default interface m0.

Example: Unicast Routing
Host with IP address sends a packet to host Router R1 checks its table and sends it to R2 through its interface 2. Router R2 checks its table and sends it to its interface 1 1 /24 R2 2 2 R3 1 R1 3 /24

The Forwarding Table Necessary in every host and the router
On Windows OS it can be seen using the command netstat –rn at the command prompt Entries in the destination column are networks, not hosts Once the interface on the router through which the packet is to be delivered is known, the physical address is used for delivery Contains the columns: Destination (Network destination), Mask (Netmask), Next hop (Gateway), Interface and Metric

Example

How Routers Build the Routing Tables
Preprogrammed or Static Routes The table is manually configured by a human The routes cannot be dynamically changed if something fails Dynamically calculated routes Calculated by the software built in the routers that provide communication among routers Algorithms that calculate shortest path are used Complexity is increased, but the routes change automatically if some part of the network fails

Metric A metric is a cost assigned for passing through a network
The total cost of the path is the sum of the metrics for the networks that are on the path Metrics are assigned in such a way that the “best pat” is the path with the minimum total cost

The “best path” from S to D is
Factors determining the best path Bandwidth Delay Hop-count Load Money Reliability The cost or the metric can involve a single or several of these factors The “best path” from S to D is A C  B S B D A 4 C 1 2

Interior vs. Exterior Protocols
The worldwide Internet is a very large network It needs to be segmented in areas based upon the entity that administrates the networks and routers in the area Autonomous System (AS) is a collection of networks and routers under single administration authority Interior protocols or IGP (Interior Gateway Protocols) Used for routing inside AS Exterior protocols or EGP (Exterior Gateway Protocols) Used for routing between ASs

Figure 21.3 Autonomous systems

Figure 21.5 Initial routing tables in a small autonomous system

Figure 21.6 Final routing tables for Figure 21.5

Interior Routing Protocols
The goal: To choose the best path, among a set of alternatives based on some or a combination of criteria (e. g. minimum delay, maximum throughput etc.) The objectives are to use the network resources (bandwidth and the router’s buffers and processing power) in the best way Two groups of interior protocols Distance Vector protocols Link State protocols

RIP = Routing Information Protocol OSPF = Open Shortest Path First
Figure Popular routing protocols RIP = Routing Information Protocol OSPF = Open Shortest Path First BGP = Boarder Gateway Protocol

Unicast vs. Multicast Unicast: Multicast:
One source to one destination Multicast: One source to many destinations Many sources to many destinations Many sources to one destination Motivation for multicast routing Growing demand (vide/audio conferences, vide streaming etc) Bandwidth need to be saved

Example Router 3 Receiver 1 Router 1 Receiver 2 Sender Router 2 Receiver 3 If unicast routing is used, the links between the sender and the Router 1 will be overloaded (bandwidth required will depend on the number of receivers)

Figure Multicasting

Group Membership vs. Multicast Routing
IGMP (Internet Group Management Protocol) Keeps router up-to-date with group membership of entire LAN A device can join or leave a group at any moment Multicast Routing Protocols MBone – A set of routers on the Internet that are running multicast routing protocols Tunneling (encapsulation of multicast packets into unicast packets) is used in the rest of the network

Note: In multicast routing, the router may forward the received packet through several of its ports.

Note: IGMP is a group management protocol. It helps a multicast router create and update a list of loyal members related to each router interface.

Figure MBONE

PART V Transport Layer

Position of transport layer

Transport layer duties

Chapters Chapter 22 Process-to-Process Delivery Chapter 23 Congestion Control and QoS

Process-to-Process Delivery: UDP and TCP
Chapter 22 Process-to-Process Delivery: UDP and TCP

The transport layer is responsible for process-to-process delivery.
Note: The transport layer is responsible for process-to-process delivery.

Figure 22.1 Types of data deliveries

Virtual Connection at the Transport Layer
Router Router Host Host TCP, UDP IP Application Physical TCP, UDP IP Application Physical IP Physical IP Physical Protocol stack in the host Protocol stack in the host Protocol stack in the router

A socket is a data flow between two processes
Figure Socket address A socket is a data flow between two processes that is identified by its socket address pair, i.e. a unique combination of: Transport protocol (UDP or TCP). Source IP address and port number. Destination IP address and port number.

Figure 22.7 Connection establishment

Figure 22.8 Connection termination

Client-Server Paradigm
Used most often in Internet process-to-process communication, for example, , web, file transfer, etc. The client process initiates the communication. The server process waits for the client to initiate communication, and responds by sending the information required. Example: Web server, server, ftp server, etc. A firewall often stops external clients from accessing internal servers, except certain web Opposite: Peer-to-peer communication, where a program can act both as client (taking initiative) and server (responding to other).

Multiplexing and Demultiplexing
Sender processes Receiver processes Web MP3 Web MP3 TCP UDP TCP UDP IP IP IP datagrams IP datagrams

Connection-oriented vs. Conectionless
A connection-oriented service requires both sender and receiver to create a connection before any data is transferred TCP provides connection oriented service to the applications A connectionless service does not create a connection first but simply sends the data UDP provides connectionless service to the applications

Example: Connection-oriented Service
An analogy to the connection-oriented service is telephone conversation

Example: Connectionless Service
An analogy to connectionless service is the delivery of the mail

Data-link vs. Transport Layer
Data link layer Responsibile for reliability between two directly connected points Transport layer Resposibe for reliability over the internetwork Duties of the data-link layer Network 1 Network 3 Network 2 Internetwork Duties of the data-link layer Duties of the data-link layer Duties of the transport layer

Reliable vs. Unreliable
Transport layer can offer Unreliable service (UDP) No guarantee that the packet will be delivered to the destination Useful especially for transmitting audio and video files where waiting for acknowledgement can be annoying for the user Reliable service (TCP) Connection establishment Connection maintenance Connection termination

User Datagram Protocol (UDP)
No reliability or connection management! Serves solely as a labeling mechanism for demultiplexing at the receiver end Use predominantly by protocols that do no require the strict service guarantees offered by TCP (e.g. real-time multimedia protocols) Additional intelligence built at the application layer if needed

Transmission Control protocol (TCP)
Provides a connection-oriented end-to-end (user-to-user) reliable byte stream service in both directions (full duplex) Divides a byte stream into a sequence of segments and sends them to the destination via IP Uses the destination port, source port to identify the application to which the segment is sent (multiplexing the sessions) Uses sliding window like scheme for flow control and congestion control

Connection Management
Two way handshake protocol is not enough because of potential delays in either A’s request or B’s responce, as shown below. Possibility of confusion exists. A B A sends a connection request t1 A sends connection request again t2 B receives connection request B establishes a connection and sends an acknowledgement t3 A receives the acknowledgement and establishes a connection t4 A and B exchange data and eventually disconnect B receives connection request B establishes a connection and sends an acknowledgement t5 time time

Three-way Handshake Protocol for Connection Establishment
A sends a connection request with seq. no. x t1 A sends connection request again with seq. no. y t2 t3 B sends acknowledgement y+1 and seq. no. z A receives the acknowledgement y+1 and sends acknowledgement z+1 t4 The connection is established t5 B sends acknowledgement x+1 and seq. no. w A does not send an acknowledgement and no connection is established t6 time time

Connection Establishment and Termination
3-way handshake used for connection establishment Randomly chosen sequence number is conveyed to the other end Similar FIN, FIN+ACK exchange used for connection termination Server does passive open Accept connection request Send acceptance Start connection Active open Send connection request SYN SYN+ACK ACK DATA The three-way handshake TCP segments are labeled with SYN. The length of data in the first two is 0

TCP’s Segments TCP treats data as a sequence of bytes to be divided and sent in segments. The size of the segment depends on the underlying physical network and on the number of bytes the sender is allowed to send (window size) Rather than numbering each segment, TCP stores the sequence number of the data byte in the segment The source and the destination each have separate sequence numbers The acknowledgement numbers are equal to the next expected sequence number

Window Management in TCP
Sliding window scheme is used with variable window The window can change depending on the traffic in the network (TCP provides congestion control) The size of the window is expressed in bytes instead of packets The window size depends on the receiver’s capabilites and the congestion in the network

TCP Sliding Window segment 1 100 bytes of data numbered from 1 to 100
100 bytes of data numbered from 701 to 800, ack 101 acknowledge 801 segment 2 acknowledge 101 segment 1 100 bytes of data numbered from 101 to 200, ack 801 acknowledge 901 segment 3 100 bytes of data numbered from 801 to 900, ack 201 acknowledge 201 segment 2

TCP/IP Client-Server Model
The clent request an unassigned port number from TCP for its own connection. It incorrporates randomly chosen port number in the TCP header and the well known port for the particular application. Then it passes the packet to IP IP handels the routing of the datagram using source/destination address and delivers the datagram to the destination network and then to the destination host

TCP/IP Client-Server Model (cont.)
The datagram is processed and delivered to the TCP layer. TCP processes the segment and delivers the data to the server through its port number The server now knows the port number of the client (as it was contained in the TCP header) enabling bidirectional communication

UDP Port Numbers User Datagram Applications

Note: UDP is a connectionless, unreliable protocol that has no flow and error control. It uses port numbers to multiplex data from the application layer.

Table 22.1 Well-known ports used by UDP
Protocol Description 7 Echo Echoes a received datagram back to the sender 9 Discard Discards any datagram that is received 11 Users Active users 13 Daytime Returns the date and the time 17 Quote Returns a quote of the day 19 Chargen Returns a string of characters 53 Nameserver Domain Name Service 67 Bootps Server port to download bootstrap information 68 Bootpc Client port to download bootstrap information 69 TFTP Trivial File Transfer Protocol 111 RPC Remote Procedure Call 123 NTP Network Time Protocol 161 SNMP Simple Network Management Protocol 162 Simple Network Management Protocol (trap)

Figure 22.10 User datagram format

Note: The calculation of checksum and its inclusion in the user datagram are optional.

Note: UDP is a convenient transport-layer protocol for applications that provide flow and error control. It is also used by multimedia applications.

22.3 TCP Port Numbers Services Sequence Numbers Segments Connection
Transition Diagram Flow and Error Control Silly Window Syndrome

Table 22.2 Well-known ports used by TCP
Protocol Description 7 Echo Echoes a received datagram back to the sender 9 Discard Discards any datagram that is received 11 Users Active users 13 Daytime Returns the date and the time 17 Quote Returns a quote of the day 19 Chargen Returns a string of characters 20 FTP, Data File Transfer Protocol (data connection) 21 FTP, Control File Transfer Protocol (control connection) 23 TELNET Terminal Network 25 SMTP Simple Mail Transfer Protocol 53 DNS Domain Name Server 67 BOOTP Bootstrap Protocol 79 Finger 80 HTTP Hypertext Transfer Protocol 111 RPC Remote Procedure Call

Figure 22.11 Stream delivery

Figure 22.12 Sending and receiving buffers

Figure TCP segments

Example 1 Imagine a TCP connection is transferring a file of 6000 bytes. The first byte is numbered What are the sequence numbers for each segment if data are sent in five segments with the first four segments carrying 1000 bytes and the last segment carrying 2000 bytes? Solution The following shows the sequence number for each segment: Segment 1 ==> sequence number: 10,010 (range: 10,010 to 11,009) Segment 2 ==> sequence number: 11,010 (range: 11,010 to 12,009) Segment 3 ==> sequence number: 12,010 (range: 12,010 to 13,009) Segment 4 ==> sequence number: 13,010 (range: 13,010 to 14,009) Segment 5 ==> sequence number: 14,010 (range: 14,010 to 16,009)

Note: The bytes of data being transferred in each connection are numbered by TCP. The numbering starts with a randomly generated number.

Note: The value of the sequence number field in a segment defines the number of the first data byte contained in that segment.

Note: The value of the acknowledgment field in a segment defines the number of the next byte a party expects to receive. The acknowledgment number is cumulative.

Figure 22.14 TCP segment format

Figure Control field

Table 22.3 Description of flags in the control field
URG The value of the urgent pointer field is valid. ACK The value of the acknowledgment field is valid. PSH Push the data. RST The connection must be reset. SYN Synchronize sequence numbers during connection. FIN Terminate the connection.

Figure 22.16 Three-step connection establishment

Figure 22.17 Four-step connection termination

Table 22.4 States for TCP State Description CLOSED
There is no connection. LISTEN The server is waiting for calls from the client. SYN-SENT A connection request is sent; waiting for acknowledgment. SYN-RCVD A connection request is received. ESTABLISHED Connection is established. FIN-WAIT-1 The application has requested the closing of the connection. FIN-WAIT-2 The other side has accepted the closing of the connection. TIME-WAIT Waiting for retransmitted segments to die. CLOSE-WAIT The server is waiting for the application to close. LAST-ACK The server is waiting for the last acknowledgment.

Figure 22.18 State transition diagram

Note: A sliding window is used to make transmission more efficient as well as to control the flow of data so that the destination does not become overwhelmed with data. TCP’s sliding windows are byte-oriented.

Figure Sender buffer

Figure 22.20 Receiver window

Figure 22.21 Sender buffer and sender window

Figure 22.22 Sliding the sender window

Figure 22.23 Expanding the sender window

Figure 22.24 Shrinking the sender window

Note: In TCP, the sender window size is totally controlled by the receiver window value (the number of empty locations in the receiver buffer). However, the actual window size can be smaller if there is congestion in the network.

Some points about TCP’s sliding windows:
Note: Some points about TCP’s sliding windows: The source does not have to send a full window’s worth of data. The size of the window can be increased or decreased by the destination. The destination can send an acknowledgment at any time.

Figure Lost segment

Figure 22.26 Lost acknowledgment

Figure TCP timers

Congestion Control and Quality of Service
Chapter 23 Congestion Control and Quality of Service

Data Traffic Traffic Descriptor Traffic Profiles

Figure 23.1 Traffic descriptors

Figure 23.2 Constant-bit-rate traffic

Figure 23.3 Variable-bit-rate traffic

Figure Bursty traffic

Congestion Network Performance

Figure 23.5 Incoming packet

Figure 23.6 Packet delay and network load

Figure 23.7 Throughput versus network load

Congestion Control Open Loop Open Loop Closed Loop

23.4 Two Examples Congestion Control in TCP
Congestion Control in Frame Relay

Note: TCP assumes that the cause of a lost segment is due to congestion in the network.

Note: If the cause of the lost segment is congestion, retransmission of the segment does not remove the cause—it aggravates it.

Figure 23.8 Multiplicative decrease

Figure BECN

Figure FECN

Figure 23.11 Four cases of congestion

Quality of Service Flow Characteristics Flow Classes

23.6 Techniques to Improve QoS
Scheduling Traffic Shaping Resource Reservation Admission Control

Figure 23.12 Flow characteristics

Figure FIFO queue

Figure 23.14 Priority queuing

Figure 23.15 Weighted fair queuing

Figure Leaky bucket

Figure 23.17 Leaky bucket implementation

Note: A leaky bucket algorithm shapes bursty traffic into fixed-rate traffic by averaging the data rate. It may drop the packets if the bucket is full.

Figure Token bucket

The token bucket allows bursty traffic at a regulated maximum rate.
Note: The token bucket allows bursty traffic at a regulated maximum rate.

23.7 Integrated Services Signaling Flow Specification Admission
Service Classes RSVP

Integrated Services is a flow-based QoS model designed for IP.
Note: Integrated Services is a flow-based QoS model designed for IP.

Figure Path messages

Figure Resv messages

Figure 23.21 Reservation merging

Figure 23.22 Reservation styles

An Alternative to Integrated Services
Differentiated Services An Alternative to Integrated Services

Differentiated Services is a class-based QoS model designed for IP.
Note: Differentiated Services is a class-based QoS model designed for IP.

Figure DS field

Figure 23.24 Traffic conditioner

23.9 QoS in Switched Networks
QoS in Frame Relay QoS in ATM

Figure 23.25 Relationship between traffic control attributes

Figure 23.26 User rate in relation to Bc and Bc + Be

Figure 23.27 Service classes

Figure 23.28 Relationship of service classes to the total capacity

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