TCP/IP Internetworking

TCP/IP Internetworking
Chapter 8 Updated January 2007 Panko’s Business Data Networks and Telecommunications, 6th edition Copyright 2007 Prentice-Hall May only be used by adopters of the book

Recap Single Networks (Subnets) Internets
Chapters 4 and 5 covered single LANs Chapters 6 and 7 covered residential Internet access and single WANs Internets Connect multiple single networks using routers 70%-80% of internet traffic follows TCP/IP standards These standards are created by the IETF Chapter 10 looks in more detail at TCP/IP management

Figure 2-8: Hybrid TCP/IP-OSI Architecture
Recap General Purpose Layer Specific Purpose Application-application communication Application (5) Application-application interworking Transmission across an internet Transport (4) Host-host communication Internet (3) Packet delivery across an internet Transmission across a single network (LAN or WAN) Data Link (2) Frame delivery across a network Physical (1) Device-device connection TCP/IP standards dominate at the internet and transport layers— transmission across an internet

Figure 2-11: Internet and Transport Layer, Cont.
Recap Transport Layer end-to-end (host-to-host) TCP is connection-oriented, reliable UDP is connectionless and unreliable Server Client PC Internet Layer (usually IP) hop-by-hop (host-router or router-router) connectionless, unreliable Router 1 Router 2 Router 3

Frames and Packets Messages at the data link layer are called frames
Recap Messages at the data link layer are called frames Messages at the internet layer are called packets Within a single network, packets are encapsulated in the data fields of frames Frame Trailer Packet (Data Field) Frame Header

In an internet with hosts separated by N networks, there will be:
Frames and Packets Recap In an internet with hosts separated by N networks, there will be: 2 hosts One packet (going all the way between hosts) One route (between the two hosts) N frames (one in each network)

Figure 2-21: Combining Horizontal and Vertical Communication
Recap App Transmission Control Protocol (TCP) Or User Datagram Protocol (UDP) Trans Trans Internet Protocol (IP) Int Int IP Int Int DL Phy Source Host Switch 1 Switch 2 Router 1 Switch 3 Router 2 Destination Host

Figure 8-1: Major TCP/IP Standards
5 Application User Applications Supervisory Applications HTTP SMTP Many Others DNS Routing Protocols Many Others 4 Transport TCP UDP 3 Internet IP ICMP MPLS ARP 2 Data Link None: Use OSI Standards 1 Physical None: Use OSI Standards Internetworking is done at the internet and transport layers. There are only a few standards at these layers. We will look at the shaded protocols in this chapter.

Figure 8-1: Major TCP/IP Standards, Continued
5 Application User Applications Supervisory Applications HTTP SMTP Many Others DNS Routing Protocols Many Others 4 Transport TCP UDP 3 Internet IP ICMP ARP 2 Data Link None: Use OSI Standards 1 Physical None: Use OSI Standards At the application layer, there are user applications and supervisory applications. We will look at two TCP/IP application layer supervisory applications in this chapter.

Dotted Decimal Notation for Human Reading (e.g., 128.171.17.13)
IP Addresses 32-Bit Strings Dotted Decimal Notation for Human Reading (e.g., )

Figure 8-3: Hierarchical IP Address
IP addresses are not simple 32-bit numbers. They usually have 3 parts. Consider the example

Hierarchical Addressing
Hierarchical Addressing Brings Simplicity Phone System Country code-area code-exchange-subscriber number Long-distance switches near the top of the hierarchy only have to deal with country codes and area codes to set up circuits Similarly, core Internet routers only have to consider network or network and subnet parts of packets

Router Operation

Figure 8-4: Border Router, Intrernal Router, Networks, and Subnets
Border routers connect different Internet networks (In this case, x.x and 60.x.x.x). An “x” indicates anything.

Figure 8-4: Border Router, Internal Router, Networks, and Subnets
Internal routers connect different subnets in a network. In this case, the three subnets are boxed in red: x, x, and x.

Figure 8-5: Multiprotocol Routing
Real routers must handle multiple internet and transport layer architectures— TCP/IP, IPX/SPX, SNA, etc. We will only look at TCP/IP routing

Figure 8-6: Ethernet Switching Versus IP Routing
Destination address is E5-BB D3-56. Ethernet switches are arranged in a hierarchy. So there is only one possible path between hosts. So only one row can match an Ethernet address. Finding this row is very simple and fast. So Ethernet switching is inexpensive per frame handled. One Correct Row

Figure 8-6: Ethernet Switching Versus IP Routing
Matches Host x Because of multiple alternative routes in router meshes, routers may have several rows that match an IP address. Routers must find All matches and then select the BEST ONE. This is slow and therefore expensive compared to switching.

Figure 8-7: The Routing Process
Processing an individual packet and passing it on its way is called routing Router ports are called interfaces Packet arrives in one interface The router sends the packet out another interface

The Routing Table Each router has a routing table that it uses to make routing decisions Routing Table Rows Each row represents a route for a RANGE of IP addresses—often a network or subnet All packets with addresses in this range are routed according to that row Route IP Address Range Governed by the route Metric Next-Hop Router 1 60.3.x.x 9 B

The Routing Table Routing Table Columns Row (route) number: Not in real routing tables IP address range governed by the row Metric for the quality of the route Next-hop router that should get the packet next if the row is selected as the best match Route IP Address Range Metric Next-Hop Router 1 60.3.x.x 9 B 2 x.x 2 B

A Routing Decision The router looks at the destination IP address in an arriving packet (in this case, ). 1. The router determines which rows match (have an IP address range containing the packet’s destination IP address) The router must check ALL rows for possible matches Route IP Address Range Metric Next-Hop Router Arriving Packet 1 60.3.x.x 9 B Match 2 x.x 2 B No Match

A Routing Decision 2. After finding all matches, the router then determines the BEST-MATCH row 2A. Selects the row with the LONGEST MATCH 60.3.x.x has 16 bits of match x has 24 bits of match so is a better match 2B. If two or more rows tie for the longest match, router uses the METRIC column value If cost, lowest metric value is best If speed, highest metric value is best Etc.

A Routing Decision 3. After selecting the best-match row, the router sends the packet on to the next-hop router indicated in the best-match row—Next-Hop Router B in this example. Send Packet out to NHR B Route IP Address Range Metric Next-Hop Router 1 60.3.x.x 9 B Best-Match Row 2 x.x 2 B

A More Detailed Look at Routing Decisions
Box A More Detailed Look at Routing Decisions

Figure 8-8: Detailed Row-Matching Algorithm
Box Routing Table IP Address Range Row Destination Mask … 1 2 3 Actually, the table does not really have an “IP Address Range” column. Instead, it has two columns to indicate the IP address range: Destination (an IP address) and a mask

Box 1. Basic Rule of Masking Information Bit Mask Bit Result Where mask bits are one, the result gives the original IP address bits Where mask bits are zero, the result contains zeros

Box 2. Example Address (partial) Mask Result

Box 3. Common 8-bit Segment Values in Dotted Decimal Notation Segment Decimal Value 4. Example is 24 ones followed by 8 zero is also called /24 in “prefix notation”

Box Row Destination Mask … 1 Example 1: A Destination IP Address that is in the Range Destination IP Address of Arriving Packet Apply the Mask Result of Masking Destination Value Does Destination Value Match the Masking Result? Yes Conclusion Row 1 is a match.

Box Row Destination Mask … 1 Example 2: A Destination IP Address that is NOT in the Range Destination IP Address of Arriving Packet Apply the Mask Result of Masking Destination Value Does Destination Value Match the Masking Result? No Conclusion Row 1 is NOT a match.

Figure 8-9: Interface and Next-Hop Router
Box Switches A switch port connects directly to a single computer or another switch Sending the frame out a port automatically gets it to the correct destination Frame

Box Routers Router ports (interfaces) connect to subnets, which have multiple hosts and that may have multiple routers The packet must be forwarded to a specific host or router on that subnet Host IP Packet Host Subnet on Router Interface Next-Hop Router Next-Hop Router

Box Interface (port) Next-Hop Router Best-match row has both an interface (indicating a subnet) and also a next-hop router value to indicate a host or router on the subnet. (Not just a Next Hop Router Column)

Dynamic Routing Protocols
Routing Table Information

Figure 8-10: Dynamic Routing Protocols
How do routers get their routing table information? Routers constantly exchange routing table information with one another using dynamic routing protocols Note that the term routing is used in two ways In TCP/IP For IP packet forwarding and For the exchange of routing table information through routing protocols Dynamic Routing Protocol Routing Table Information

Autonomous System An organization’s internal network (internet) Exterior Dynamic Routing Protocols Between Autonomous Systems, companies use an exterior dynamic routing protocol The dominant exterior dynamic routing protocol is the Border Gateway Protocol (BGP) Gateway is an obsolete name for router Company is not free to choose whatever exterior routing protocol it wishes

Interior Dynamic Routing Protocols Within an Autonomous System, firms use interior dynamic routing protocols Can select their own interior dynamic routing protocol Routing Information Protocol (RIP) for small internets Open Shortest Path First (OSPF) for larger internets Enhanced Interior Gateway Routing Protocol (EIGRP) Non-TCP/IP proprietary CISCO protocol Can handle multiple protocols, not just TCP/IP

Recap

The Address Resolution Protocol (ARP)

Figure 8-12: Address Resolution Protocol (ARP)
Packet Frame The Situation: The router wishes to pass the packet to the destination host or to a next-hop router. The router knows the destination IP address of the target. The router must learn the target’s MAC layer address in order to be able to send the packet to the target in a frame. The router uses the Address Resolution Protocol (ARP)

1: Router broadcasts ARP Request to all hosts and routers on the subnet.

2: ARP Reply sent by the host with the target IP address. Other hosts ignore it. This is the Destination host

3. Router puts the MAC address in its ARP cache; uses it for subsequent packets to the host

Multiprotocol Label Switching (MPLS)

Figure 8-13: Multiprotocol Label Switching (MPLS)
Routers are Connected in a Mesh Multiple alternative routes make the routing decision for each packet very expensive PSDNs (Chapter 7) also are Arranged in a Mesh However, a best path (virtual circuit) is set up before transmission begins Once a VC is in place, subsequent frames are handled quickly and inexpensively MPLS Does Something Like this for Routers

MPLS Adds a Label Before Each Packet Label sits between the frame header and the IP header Contains an MPLS label number Like a virtual circuit number in a PSDN frame Label-switching router merely looks up the MPLS label number in its MPLS table and sends the packet back out IP Packet MPLS Label Data Link Header

Port 1 3 Advantages of MPLS Router does a simple table lookup. This is fast and therefore inexpensive per packet handled As fast as Ethernet switching! Can use multiple label numbers to give traffic between two sites multiple levels of priority or quality of service guarantees MPLS supports traffic engineering: balancing traffic on an internet 8 2

First router adds the label Last router drops the label

The Domain Name System (DNS)

Figure 8-14: Domain Name System (DNS) Hierarchy
A domain is a group of resources under the control of an organization. The domain name system is a general system for managing names. It is a hierarchical naming system. Queries to a DNS server can get Information about a domain.

The highest level (0) is called the root. There are 13 DNS Root Servers. They point to lower-level servers.

Top-level domains are generic TLDs (.com, .net., .org, etc.) or country TLDs (.ca, .uk, .ie, etc.)

Organizations seek good second- level domain names cnn.com microsoft.com hawaii.edu etc. Firms get them from address registrars

Host names are the bottom of the DNS hierarchy. A DNS request for a host name will return its IP address.

The Internet Control Message Protocol (ICMP)

Figure 8-15: Internet Control Message Protocol (ICMP) for Supervisory Messages
ICMP is the supervisory protocol at the internet layer. ICMP messages are encapsulated in the data fields of IP packets. There are no transport or Application layer headers or messages

When an error occurs, the device noting the error may try to respond with an ICMP error message describing the problem. ICMP error messages often are not sent for security reasons because attackers can use them to learn about a network

To see if another host is active, a host can send the target host an ICMP echo message (called a ping). If the host is active, it will send back an echo response message confirming that it is active.

Dynamic Host Configuration Protocol (DHCP)
From Chapter 1

Figure 8-16: Dynamic Host Configuration Protocol (DHCP)
DHCP Gives Each Client PC at Boot-Up: A temporary IP Address (we saw this in Chapter 1) A subnet mask The IP addresses of local DNS servers Better Than Manual Configuration If subnet mask or DNS IP addresses change, only the DHCP server has to be updated manually Client PCs are automatically updated when they next boot up

The Internet Protocol (IP) Versions 4 and 6

Figure 8-17: IPv4 and IPv6 Packets
Bit 0 IP Version 4 Packet Bit 31 Version (4 bits) Value is 4 (0100) Header Length (4 bits) Diff-Serv (8 bits) Total Length (16 bits) Length in octets Identification (16 bits) Unique value in each original IP packet Flags (3 bits) Fragment Offset (13 bits) Octets from start of original IP fragment’s data field IPv4 is the dominant version of IP today. The version number in its header is 4 (0100). The header length and total length field tell the size of the packet. The Diff-Serv field can be used for quality of service labeling. (But MPLS is being used instead by most carriers) Time to Live (8 bits) Protocol (8 bits) 1=ICMP, 6=TCP, 17=UDP Header Checksum (16 bits)

The second row is used for reassembling fragmented IP packets, but fragmentation is quite rare, so we will not look at these fields. Bit 0 IP Version 4 Packet Bit 31 Version (4 bits) Value is 4 (0100) Header Length (4 bits) Diff-Serv (8 bits) Total Length (16 bits) Length in octets Identification (16 bits) Unique value in each original IP packet Flags (3 bits) Fragment Offset (13 bits) Octets from start of original IP fragment’s data field Time to Live (8 bits) Protocol (8 bits) 1=ICMP, 6=TCP, 17=UDP Header Checksum (16 bits)

The sender sets the time-to-live value (usually 64 to 128). Each router along the way decreases the value by one. A router decreasing the value to zero discards the packet. It may send an ICMP error message. The protocol field describes the message in the data field (1=ICMP, 2=TCP, 3=UDP, etc.) The header checksum is used to find errors in the header. If a packet has an error, the router drops it. There is no retransmission at the internet layer, so the internet layer is still unreliable. Bit 0 IP Version 4 Packet Bit 31 Version (4 bits) Value is 4 (0100) Header Length (4 bits) Diff-Serv (8 bits) Total Length (16 bits) Length in octets Identification (16 bits) Unique value in each original IP packet Flags (3 bits) Fragment Offset (13 bits) Octets from start of original IP fragment’s data field Time to Live (8 bits) Protocol (8 bits) 1=ICMP, 6=TCP, 17=UDP Header Checksum (16 bits)

Bit 0 IP Version 4 Packet Bit 31 Source IP Address (32 bits) Destination IP Address (32 bits) Options (if any) Padding Data Field The source and destination IP addresses Are 32 bits long, as you would expect. Options can be added, but these are rare.

IP Version 6 is the emerging version of the Internet protocol. Has 128 bit addresses for an almost unlimited number of IP addresses. Needed because of rapid growth in Asia. Also needed because of the exploding number of mobile devices Bit 0 IP Version 6 Packet Bit 31 Version (4 bits) Value is 6 (0110) Diff-Serv (8 bits) Flow Label (20 bits) Marks a packet as part of a specific flow Payload Length (16 bits) Next Header (8 bits) Name of next header Hop Limit (8 bits) Source IP Address (128 bits) Destination IP Address (128 bits) Next Header or Payload (Data Field)

The Transmission Control Protocol (TCP)

Figure 8-18: TCP Segment and UDP Datagram
Bit 0 TCP Segment Bit 31 Source Port Number (16 bits) Destination Port Number (16 bits) Sequence Number (32 bits) Acknowledgment Number (32 bits) The source and destination port numbers specify a particular application on the source and destination multitasking computers (Discussed later) Sequence numbers are 32 bits long. So are acknowledgment numbers. Header Length (4 bits) Reserved (6 bits) Flag Fields (6 bits) Window Size (16 bits) TCP Checksum (16 bits) Urgent Pointer (16 bits) Flag fields are one-bit fields. They include SYN, ACK, FIN, and RST.

Flags are one-bit fields. If a flag’s value is 1, it is “set”. If a flag’s value is 0, it is “not set.” TCP has six flags If the TCP Checksum field’s value is correct, The receiving process sends back an acknowledgment. Bit 0 TCP Segment Bit 31 Source Port Number (16 bits) Destination Port Number (16 bits) Sequence Number (32 bits) Acknowledgment Number (32 bits) Header Length (4 bits) Reserved (6 bits) Flag Fields (6 bits) Window Size (16 bits) TCP Checksum (16 bits) Urgent Pointer (16 bits)

For flow control (to tell the other party to slow down), The sender places a small value in the Window Size field. If the Window Size is small, the receiver will have to stop transmitting after a few more segments (unless it gets a new acknowledgment extending the number of segments it may send.) Bit 0 TCP Segment Bit 31 Source Port Number (16 bits) Destination Port Number (16 bits) Sequence Number (32 bits) Acknowledgment Number (32 bits) Header Length (4 bits) Reserved (6 bits) Flag Fields (6 bits) Window Size (16 bits) TCP Checksum (16 bits) Urgent Pointer (16 bits)

Bit 0 TCP Segment Bit 31 Options (if any) Padding Data Field TCP segment headers can end with options. Unlike IPv4 options, TCP options are very common. If an option does not end at a 32-bit boundary, padding must be added.

The User Datagram Protocol (UDP)

Bit 0 UDP Datagram Bit 31 Source Port Number (16 bits) Destination Port Number (16 bits) UDP Length (16 bits) UDP Checksum (16 bits) Data Field UDP messages (datagrams) are very simple. Like TCP, UDP has 16-bit port numbers. The UDP length field allows variable-length application messages. If the UDP checksum is correct, there is no acknowledgment. If the UDP checksum is incorrect, the UDP datagram is dropped.

Figure 8-19: TCP Connection Openings and Closings
TCP is a connection-oriented protocol Each connection has a formal opening process Each connection has a formal closing process During a connection, each TCP segment is acknowledged (Of course, pure acknowledgments are not acknowledged)

Normal Three-Way Opening SYN SYN/ACK ACK A SYN segment is a segment in which the SYN bit is set. One side sends a SYN segment requesting an opening. The other side sends a SYN/acknowledgment segment. Originating side acknowledges the SYN/ACK.

Normal Four-Way Close FIN ACK FIN ACK A FIN segment is a segment in which the FIN bit is set. Like both sides saying “good bye” to end a conversation.

Abrupt Reset RST An RST segment is a segment in which the RST bit is set. A single RST segment breaks a connection. Like hanging up during a phone call. There is no acknowledgment.

Port Numbers and Sockets in TCP and UDP

TCP and UDP Port Numbers
Computers are multitasking devices They run multiple applications at the same time On a server, a port number designates a specific applications HTTP Webserver Application SMTP Applications Port 80 Port 25 Server

Major Applications Have Well-Known Port Numbers 0 to 1023 for both TCP and UDP HTTP is TCP Port 80 SMTP is TCP Port 25 HTTP Webserver Application SMTP Applications Port 80 Port 25 Server

Clients Use Ephemeral Port Numbers 1024 to 4999 for Windows Client PCs A client has a separate port number for each connection to a program on a server Application on Mail Server Webserver Application on Webserver Port 4400 Port 3270 Client

Figure 8-20: Use of TCP (and UDP) Port Numbers
A socket is an IP address, a colon, and a port number. :80 :25 :2849 It represents a specific application (Port number) on a specific server (IP address) Or a specific connection on a client. Client Webserver Port 80 SMTP Server Port 25 Client PC Port 2849

Client Source: :2707 Destination: :80 This shows sockets for a client packet sent to a webserver application on a webserver Webserver Port 80 SMTP Server Port 25

Client Source: :2707 Destination: :80 Source: :80 Destination: :2707 Webserver Port 80 Sockets in two-way transmission SMTP Server Port 25

Client Source: :2707 Destination: :80 Source: :80 Destination: :2707 Webserver Port 80 Source: :4400 Destination: :25 SMTP Server Port 25 Clients use a different ephemeral port number for different connections

Layer 3 Switches

Figure 8-21: Layer 3 Switches and Routers in Site Networks
Usually too expensive to replace workgroup switches. Usually too limited in functionality to replace border routers. Replaces core switches in the middle.

Topics Covered

Topics Covered Internetworking Recap from Earlier Chapters
Internetworking involves the internet and transport layers Packets are encapsulated in frames in single networks. Transport layer is end-to-end Internet layer is hop-by-hop between routers IP, TCP, and UDP are the heart of TCP/IP internetworking

Topics Covered Hierarchical IP Address parts Router Operation
Network, subnet, and host parts Router Operation Border routers connect networks Internal routers connect subnets We focused on TCP/IP routing, but multiprotocol routing is crucial Router meshes give alternative routes, making routing very expensive

Topics Covered Routing of Packets Routing tables
IP address range governed by a row—usually a route to a network or subnet Metric to help select best matches Next-hop router to be sent the packet next Can be a local host on one of the router’s subnets Process Final all possible routes through row matching Select by length of match, then metric if tie Send out to next-hop router in the best-match row

Topics Covered Detailed Look at Routing Decisions IP address range
Box Detailed Look at Routing Decisions IP address range Destination Mask If the masked destination IP address in an arriving packet matches the destination value, the row is a match Next-Hop Router Interface Next-hop router or destination host

Topics Covered Dynamic Routing Protocols Address Resolution Protocol
Interior dynamic routing protocols within an autonomous system RIP, OSPF, EIGRP Exterior dynamic routing protocols between autonomous systems BGP Address Resolution Protocol Router knows the IP address of the next-hop router or destination host Must learn the data link layer address as well

Topics Covered Multiprotocol Label Switching Domain Name System (DNS)
Routing decisions are based on labels rather than destination IP addresses Reduces routing costs Domain Name System (DNS) General hierarchical naming system for the Internet Internet Control Message Protocol (ICMP) General supervisory protocol at the internet layer Error advisements and Pings (echo requests/replies)

Topics Covered The Internet Protocol (IP) Detailed look at key fields
Protocol field lists contents of the data field 32-bit IP addresses IPv4 is the current version IPv6 offers 128-bit IP addresses to allow many more IP addresses to serve the world

Topics Covered The Transmission Control Protocol (TCP)
Sequence and acknowledgement numbers Flag fields that are set or not set Window size field allows flow control Options are common Three-way openings (SYN, SYN/ACK, and ACK) Four-way normal closings (FIN, ACK, FIN, ACK) One-way abrupt closing (RST)

Topics Covered The User Datagram Protocol (UDP)
Simple four-field header Port Numbers and Sockets in TCP and UDP Applications get well-known port numbers on servers Connections get ephemeral port numbers on clients Socket is an IP address, a colon, and a port number This designates a specific application (or connection) on a specific server (or client) Layer 3 Switches Fast, inexpensive, and limited routers

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