M05 Network and Transport Layer

• Network and Transport Layer

  •Transport layer◦Layer 4 in the OSI Model◦Links application and network layers◦Responsible for segmentation and reassembly◦Session management◦Responsible for end-to-end delivery of messages•Network layer ◦Layer 3 in the OSI Model◦Responsible for addressing and routing of messages

  
• 1. Linking to the Application Layer


  •TCP or UDP may serve any application layer protocol• Application developer decides based on needs• Ports used to ID application process• 2-byte numbers: 65,535 possible• These are not physical ports; they’re controlled by software• Many server ports follow standards• Common port standards (primarily/originally in 0-1024 range)• HTTP:  TCP port 80 / HTTPS:  TCP port 443• SMTP:  TCP port 25• IMAP:  TCP port 143• POP3:  TCP port 110 (more commonly TCP port 995 secure version)• DNS:  TCP or UDP port 53 (most commonly UDP).
• 1. Linking to Application Layer Services

  
• 2. Segmenting

  •Breaking up large files into smaller segments (and reassembling)• Segments may be passed individually to application layer or after reassembly• How large are the segments?

  Size depends on the network and data link layer protocols

  ◦Maximum segment size (MSS) is negotiated during TCP handshake◦ e.g., if the maximum size of the data in an Ethernet frame is 1,500 bytes and TCP and IP use 20 byte headers, the maximum segment size is 1460 bytes

  
• 3. Session Management


  •A session can be thought of as a conversation between two computers or creating a virtual circuit.• Using a session to send data is also called connection-oriented messaging (TCP).• Sending messages without establishing a session is connectionless messaging (UDP).• TCP connections are opened using a three-way handshake◦ SYN◦ SYN-ACK◦ ACK• Sessions provide reliable end-to-end connections.
• Connection-Oriented Messaging – Stop-and-Wait ARQ 

  
• Connection-Oriented Messaging – Continuous ARQ 


  •With continuous ARQ, the sender does not wait for an acknowledgment after sending a message; it immediately sends the next one. • Although the messages are being transmitted, the sender examines the stream of returning acknowledgments. • If it receives an NAK, the sender retransmits the needed messages. • Continuous ARQ is by definition a full-duplex transmission technique, because the sender is sending messages, and the receiver is sending ACKs and NAKs.
• Transport Layer Protocols – Transmission Control Protocol (TCP)

  •Transmission Control Protocol (TCP)◦Most common transport layer protocol◦PDU called a segment (because it’s one segment of many in data stream)◦Used for reliable transmission of data

  
• Connectionless Messaging 1 of 2


  •Each packet is treated separately and makes its own way through the network, possibly through different routes depending on the type of routing used and the amount of traffic. • Because packets following different routes may travel at different speeds, they may arrive out of sequence at their destination. 
• Connectionless Messaging 2 of 2 


  • The sender’s network layer, therefore, puts a sequence number on each packet, and the network layer must reassemble them.• When connectionless messaging is desired, the TCP segment is replaced with a UDP packet. The UDP header is much smaller than the TCP header (only 8 bytes).
• Transport Layer Protocols - User Datagram Protocol (UDP)

  •PDU called a datagram (or segment)• Used in time-sensitive situations, for control messages, when reliability noncritical or done at application layer• Often used for single small packet (ex: DNS request)

  
• Quality-of-Service (QoS)


  •A special type of connection-oriented messaging in which different connections are assigned different priorities based upon time-dependence • Example: videoconferencing• Protocols:◦ Resource Reservation Protocol (RSVP) Real-Time Streaming Protocol (RTSP) 
• Routing


  •Process of identifying what path to have a packet take through a network from sender to receiver• Routing tables◦ Used to make routing decisions◦ Shows which path to send packets on to reach a given destination◦ Kept by computers making routing decisions• Routers◦ Special purpose devices used to handle routing decisions on the Internet ◦ Maintain their own routing tables
• Router Connections, Interfaces, and Subnets

  
• Sample Routing Tables

  
• Network Layer Function - Routing


  • At the network layer, packets must be moved from the source to the destination across the Internet• Routers: special computers moving packets through Internet◦ Routers have operating systems -- typically Cisco IOS◦ Read destination IP address and forward through one of their physical ports to next “hop” (router or destination)

  
• Network Layer Functions

  •Routing - Identifying what path to have a packet take through a network from sender to receiver◦ Routing Tables◦ Used to make routing decisions◦ Identifies path to use to reach given destination◦ Kept by computers making routing decisions◦ Routers maintain their own routing tables (unless part of SDN)◦
• Routing 1

  
• Routing 2

  
• Routing 3

  
• Routing Types: Centralized Routing


  Centralized Routing (host-based networks, uncommon)◦ Routing decisions made by one computer
• Routing Types: Decentralized


  (i.e. Internet) Decisions made by each node independently◦ Information needs to be exchanged to prepare routing tables◦Static Routing◦ Fixed routing tables◦ Manually configured by network managers. ◦Local adjustments when computers added or removed◦TCP/IP Clients typically use static routing to reach gateway router◦ Dynamic Routing (sometimes called adaptive routing)◦ Routing tables updated periodically◦ Routers exchange data using protocols to update tables◦ TCP/IP Gateway routers typically use dynamic routing to ensure current and expedient paths through network
• Dynamic Routing Algorithms

  •Distance vector: based on number of “hops” between two devices•Link state: based on number of hops, circuit speed, and traffic congestion◦Provides more reliable, up-to-date paths to destinations
• Interior Routing Protocols – ICMP


  •Internet Control Message Protocol (ICMP)◦ The most basic routing protocol; actually an error-reporting protocol that enables computers to report routing errors to message senders• Used by the “Ping” command to check very basic status
• Interior Routing Protocols – RIP 


  •Routing Information Protocol (RIP)◦ Dynamic distance vector protocol used for interior routing◦ Operation◦ Network manager builds the routing table◦ Routing tables broadcast periodically (e.g., every minute or so)◦ When new computers are added, router counts “hops” and selects the shortest route◦ Useful in smaller, less complex networks
• Interior Routing Protocols - OSPF


  •Open Shortest Path First (OSPF)◦ Dynamic link state protocol used for interior routing◦ Most widely used interior routing protocol on large enterprise networks◦ More reliable paths◦ Less burdensome to the network because only updates sent
• Interior Routing Protocols – EIGRP 


  •Enhanced Interior Gateway Routing Protocol (EIGRP)◦ A dynamic link state protocol (developed by Cisco)◦ Records transmission capacity, delay time, reliability and load for all paths◦ Keeps the routing tables for its neighbors and uses this information in its routing decisions as well
• Exterior Routing Protocol – BGP 


  •If each network uses a different protocol internally, how are they able to communicate?• Border Gateway Protocol (BGP)◦ Dynamic distance vector protocol used for exterior routing◦ Far more complex than interior routing protocols◦ Provide routing info only on selected routes (e.g., preferred or best route)
• BGP, OSPF, and RIP In Action


  BGP used to provide routing information between autonomous systems using different interior routing protocols

  
• Multicasting


  •Unicast: one computer to another computer• Broadcast: one computer to all computers in the network• Multicast: one computer to a group of computers (e.g., videoconference)◦ Same data needs to reach multiple receivers and avoid transmitting it once for each receiver.◦ Particularly useful if access link has bandwidth limitations◦ Many implementations at different layers◦ In IP multicast, hosts dynamically join and leave multicast groups using Internet Group Management Protocol (IGMP).
• Anatomy of a Router


  •A network manager can connect to a router and configure and maintain it through: ◦ Console ports◦Network interface ports Auxiliary ports