Prabhaker Mateti (ack: Many many sources …) TCP/IP Refresher Prabhaker Mateti (ack: Many many sources …)

TCP/IP ? TCP = Transmission Control Protocol IP = Internet Protocol Almost always includes other protocols: UDP, User (Unreliable) Datagram ICMP, Internet Control Message ARP, Address Resolution Mateti, TCP/IP Refresher

What’s a Protocol? An agreed upon convention for communication. Protocols must be formally defined and unambiguous Mateti, TCP/IP Refresher

Layers TCP UDP ICMP other IP layer Physical Physical   UDP ICMP other IP layer Physical   Physical   The relative heights indicate the level of functionality. Mateti, TCP/IP Refresher

Unix is a Layered System Applications Libraries System Calls Kernel Mateti, TCP/IP Refresher

Layers The routines/methods of Layer N will not call Layer N+1. The routines/methods of Layer N typically do call the same layer methods. The routines/methods of Layer N typically do call Layer N-1 methods. Mateti, TCP/IP Refresher

DoD model: Four Layers Network Access Layer: Delivery over physical media in use. Internet Layer: Delivery across different physical networks that connect source and destination machines. Host-to-Host Layer: Connection rendezvous, flow control, retransmission of lost data, etc. TCP and UDP protocols are in this layer. Process Layer: User-level functions, such as SMTP, FTP and rlogin. Mateti, TCP/IP Refresher

OSI Reference Model Seven Layers 7. Application 6. Presentation 5. Session 4. Transport 3. Network 2. Data Link 1. Physical Mateti, TCP/IP Refresher

TCP/IP & OSI In OSI reference model terminology -the TCP/IP protocol suite covers the network and transport layers. TCP/IP can be used on many data-link layers (can support many network hardware implementations). Mateti, TCP/IP Refresher

Process Layer Transport Layer Network Layer Data-Link Layer Process TCP UDP Transport Layer ICMP, ARP & RARP IP Network Layer 802.3 Data-Link Layer Mateti, TCP/IP Refresher

Physical Layer Responsibility: Issues: transmission of raw bits over a communication channel. Issues: mechanical and electrical interfaces time per bit distances Mateti, TCP/IP Refresher

Data Link Layer - Data Link Control Responsibility: provide an error-free communication link Issues: framing (dividing data into chunks) header & trailer bits addressing 10110110101 01100010011 10110000001 Mateti, TCP/IP Refresher

The Data Link Layer - The MAC sub layer Medium Access Control (MAC) - needed by multi-access networks. MAC provides DLC with “virtual wires” on multi-access networks. Mateti, TCP/IP Refresher

Ethernet: A Data-Link Layer IEEE 802.3 Variety of physical layers. Multi-access (shared medium). Interface has a unique 6-byte hardware address. (E.g. 00-D0-09-E8-08-61) The broadcast address is all 1’s. Addresses are assigned to vendors by a central authority. Mateti, TCP/IP Refresher

An Ethernet Frame Preamble Destination Address Source Len CRC DATA 8 bytes 6 6 2 0-1500 4 Preamble is a sequence of alternating 1’s and 0’s used for synchronization. CRC is Cyclic Redundancy Check Mateti, TCP/IP Refresher

Ethernet Addressing Each NIC looks at every frame and inspects the destination address. If the address does not match the hardware address of the interface or the broadcast address, the frame is discarded. Some NICs can be programmed to recognize multicast addresses. Mateti, TCP/IP Refresher

The Network Layer Responsibilities: Issues: path selection between systems (routing). subnet flow control. fragmentation & reassembly translation between different network types. Issues: packet headers virtual circuits Mateti, TCP/IP Refresher

The Transport Layer Responsibilities: Issues: provides virtual end-to-end links between peer processes. end-to-end flow control Issues: headers error detection reliable communication Mateti, TCP/IP Refresher

The Session Layer Responsibilities: establishes, manages, and terminates sessions between applications. service location lookup Many protocol suites do not include a session layer. Mateti, TCP/IP Refresher

The Presentation Layer Responsibilities: data encryption data compression data conversion Many protocol suites do not include a Presentation Layer. Mateti, TCP/IP Refresher

The Application Layer Responsibilities: Issues: anything not provided by any of the other layers Issues: application level protocols appropriate selection of “type of service” Mateti, TCP/IP Refresher

Layering & Headers Each layer needs to add control information. Typically prefixed to the data before passing on to the lower layer. Mateti, TCP/IP Refresher

Headers DATA Process Process Transport H DATA Transport Network H H Data Link H H H DATA Data Link Mateti, TCP/IP Refresher

Example Headers Physical: no header Data Link: address of the receiving endpoints address of the sending endpoint length of the data checksum Mateti, TCP/IP Refresher

Network layer header - examples protocol header checksum source network address destination network address protocol suite version type of service length of the data packet identifier fragment number time to live Mateti, TCP/IP Refresher

Connecting Networks Repeater: physical layer Bridge: data link layer Router: network layer Gateway: network layer and above. Mateti, TCP/IP Refresher

Repeater Copies bits from one network to another Does not look at any bits Allows the extension of a network beyond physical length limitations REPEATER Mateti, TCP/IP Refresher

Bridge Copies frames from one network to another Can operate selectively - does not copy all frames (looks at data-link headers). Extends the network beyond physical length limitations. BRIDGE Mateti, TCP/IP Refresher

Router Copies packets from one network to another. Makes decisions about what route a packet should take (looks at network headers). ROUTER Mateti, TCP/IP Refresher

Gateway Operates as a router Data conversions above the network layer. encapsulation - use an intermediate network translation - connect different application protocols encryption - could be done by a gateway Mateti, TCP/IP Refresher

Encapsulation Example Gateway Gateway Provides service connectivity even though intermediate network does not support protocols. Mateti, TCP/IP Refresher

Translation Translate from green protocol to brown protocol Gateway Mateti, TCP/IP Refresher

Encryption/Decryption Encryption gateway Secure Network Encryption/Decryption Gateways GW ? Insecure Network Mateti, TCP/IP Refresher

Hardware v. Software Repeaters are typically hardware devices. Bridges can be implemented in hardware or software. Routers and gateways are typically implemented in software so that they can be extended to handle new protocols. Many workstations can operate as routers or gateways. Mateti, TCP/IP Refresher

Modes of Service connection-oriented vs. connectionless sequencing error-control flow-control byte stream vs. message based full-duplex vs. half-duplex. Mateti, TCP/IP Refresher

Connection-Oriented Service establishment of a logical connection between two processes. transfer data terminate connection. Mateti, TCP/IP Refresher

Connectionless Service Sends independent messages. Mateti, TCP/IP Refresher

Sequencing Sequencing provides support for an order to communications. A service that includes sequencing requires that messages (or bytes) are received in the same order they are sent. Mateti, TCP/IP Refresher

Error Control Some services require error detection. Checksums provide a simple error detection mechanism. Error control sometimes involves notification and retransmission. Mateti, TCP/IP Refresher

Flow Control Flow control prevents the sending process from overwhelming the receiving process. Flow control can be handled in a variety of ways. Mateti, TCP/IP Refresher

Byte Stream vs. Message Byte stream implies an ordered sequence of bytes with no message boundaries. Message oriented services provide communication service to chunks of data called datagrams. Mateti, TCP/IP Refresher

Full- v. Half-Duplex Full-Duplex services support the transfer of data in both directions. Half-Duplex services support the transfer of data in one direction. Mateti, TCP/IP Refresher

End-to-End v. Hop-to-Hop Service modes, flow control and error control can be Either between endpoints of the communication. Or between consecutive nodes on the path between the endpoints. Mateti, TCP/IP Refresher

End-to-End Process A Process B Mateti, TCP/IP Refresher

Hop-by-Hop Process A Process B Mateti, TCP/IP Refresher

Buffering Buffering can provide more efficient communications. Buffering is most useful for byte stream services. Process A Send Buffer Recv. Buffer Process B Mateti, TCP/IP Refresher

Addresses Physical Layer: no address necessary Data Link Layer: address must be able to select any host on the network. Network Layer: address must be able to provide information to enable routing. Transport Layer: address must identify the destination process. Mateti, TCP/IP Refresher

Broadcasts Broadcast = sending a message from one host to all other hosts on the network. A special address called the “broadcast address” is created. Some popular network services are based on broadcasting (YP/NIS, rup, rusers) Mateti, TCP/IP Refresher

The IP in TCP/IP IP is the network layer packet delivery service (host-to-host). translation between different data-link protocols. Mateti, TCP/IP Refresher

IP Datagrams IP provides connectionless, unreliable delivery of IP datagrams. Connectionless: each datagram is independent of all others. Unreliable: there is no guarantee that datagrams are delivered correctly or at all. Mateti, TCP/IP Refresher

IP Addresses The address must include information about what network the receiving host is on. This makes routing feasible. IP addresses are not the same as the underlying data-link (MAC) addresses. Mateti, TCP/IP Refresher

IP Addresses Includes a network ID and a host ID. A Network ID is assigned to an organization by a global authority (http://www.iana.org/ ) Host IDs are assigned locally by a system administrator. Mateti, TCP/IP Refresher

IP Addresses A single NIC is assigned one IP address. A host may have multiple NICs, and therefore multiple host addresses. Hosts that share a network all have the same IP network address (the network ID). Mateti, TCP/IP Refresher

Subnet Addresses NetID SubnetID HostID An organization can subdivide it’s host address space into groups called subnets. The subnet ID is generally used to group hosts based on the physical network topology. It is possible to have a single wire network with multiple subnets. NetID SubnetID HostID Mateti, TCP/IP Refresher

IP4 Addresses A B C D NetID HostID 10 110 1110 Multicast Address NetID 10 110 1110 Multicast Address HostID A B C D 8 bits Class Mateti, TCP/IP Refresher

IP Addresses An IP broadcast address has a host- ID of all 1’s. An IP address that has a host ID of all 0’s is called a network address and refers to an entire network. Mateti, TCP/IP Refresher

IP Addresses v. MAC Addresses IP Addresses are not recognized by NIC. The process of finding the MAC address of a host given the IP address is called Address Resolution. The process of finding out the IP address of a host given a hardware address is called Reverse Address Resolution. Mateti, TCP/IP Refresher

IPv6 addresses Addresses are scoped Address is 128 bits long (16 bytes) Addresses are written in hexadecimal Addresses can be abbreviated 3FFE:0B00:0000:0000:0000:0000:0000:0001 3FFE:0B00::0001 3FFE:B00::1 There is no broadcast addresses, only multicast. Loopback address is ::1 Addresses are scoped Link-local, site-local, global Mateti, TCP/IP Refresher

IP6 Address 3FFE: 0B00: 1234: 0000: 0001 128 bits 16 bits Mateti, TCP/IP Refresher

IP4-Compatible IP6 Address 80 bits of 0s followed by 16 bits of 0s, followed by a 32 bit IP4 Address: 0000 . . . 0000 0000 IP4 Address 80 bits 16 bits 32 bits Mateti, TCP/IP Refresher

ARP ARP is a broadcast protocol. Each host checks the request against its own host addresses - the matched one responds. Hosts remember the hardware addresses of others. ARP protocol specifies that the receiving host should also remember the IP and hardware addresses of the sending host. Mateti, TCP/IP Refresher

Services provided by IP Connectionless Delivery (each datagram is treated individually). Unreliable (delivery is not guaranteed). Fragmentation / Reassembly (based on hardware MTU). Routing. Error detection. Mateti, TCP/IP Refresher

Destination IP Address IP Datagram VERS HL Fragment Offset Fragment Length Service Datagram ID FLAG TTL Protocol Header Checksum Source IP Address Destination IP Address Options (if any) (TCP) Data 1 byte Mateti, TCP/IP Refresher

IP Datagram Fragmentation Fragmentation can happen when datagrams are forwarded through a network for which they are too big. IP specifies that datagram reassembly is done only at the destination (not on a hop-by-hop basis). If any of the fragments are lost the entire datagram is discarded (and an ICMP message is sent to the sender). Mateti, TCP/IP Refresher

ICMP (Internet Control Message Protocol) ping www.yahoo.com ICMP uses IP to deliver messages. ICMP messages are usually generated and processed by the IP layer, not the user process. Mateti, TCP/IP Refresher

ICMP If packets arrive too fast the receiver discards excessive packets and sends an ICMP message to the sender (SOURCE QUENCH). If an error is found (header checksum problem, say) the packet is discarded and an ICMP message is sent to the sender. Mateti, TCP/IP Refresher

ICMP Message Types Echo Request Echo Response Destination Unreachable Redirect Time Exceeded Redirect (route change) more ... Mateti, TCP/IP Refresher

UDP (User Datagram Protocol) UDP is a transport protocol Uses IP to deliver datagrams Connectionless, Unreliable, Minimal UDP uses ports to provide communication services to individual processes. Mateti, TCP/IP Refresher

Ports Port : an abstract destination point. Ports are identified by a positive 16-bit integer. Operating systems provide some mechanism that processes use to specify a port. Mateti, TCP/IP Refresher

Ports Host A Host B Process Process Process Process Process Process Mateti, TCP/IP Refresher

UDP Datagram Format Source Port Destination Port Length Checksum Data Mateti, TCP/IP Refresher

Sockets Mateti, TCP/IP Refresher

Sockets An active socket is connected to a remote active socket. Closing the connection destroys the active sockets at each endpoint. A passive socket is not connected, but rather awaits an incoming connection, which will spawn a new active socket. Mateti, TCP/IP Refresher

Sockets v. Ports A socket is not a port. A socket is associated with a port. This is a many-to-one relationship. Each port can have a single passive socket, awaiting incoming connections, and multiple active sockets, each corresponding to an open connection on the port. Mateti, TCP/IP Refresher

TCP Connection-oriented Reliable Full-duplex Byte-Stream Transmission Control Protocol : Connection-oriented Reliable Full-duplex Byte-Stream Mateti, TCP/IP Refresher

Connection Four Numbers: Source IP Address, Source Port, Destination IP Address, Destination Port “connection is established”: Operating Systems of both source and destination hosts are maintaining “state information” re the connection. Mateti, TCP/IP Refresher

Connection-Oriented Connection oriented means that a virtual connection is established before any payload data is transferred. If the connection cannot be established the user program is notified. If the connection is ever interrupted the user program is notified. Mateti, TCP/IP Refresher

Connection establishment Connection establishment phase is required Ensures that the receiving process is available and to synchronize sequence numbers, etc. Mateti, TCP/IP Refresher

TCP State Diagram Mateti, TCP/IP Refresher

Reliable Every transmission of data is acknowledged by the receiver. If the sender does not receive ACK within a specified amount of time, the sender retransmits the data. ACK can be piggybacked on data. Mateti, TCP/IP Refresher

Byte Stream Stream means that the connection is treated as a stream of bytes. The user application does not need to package data in individual datagrams (as with UDP). Mateti, TCP/IP Refresher

Buffering TCP is responsible for buffering data and determining when it is time to send a datagram. It is possible for an application to tell TCP to send the data it has buffered without waiting for a buffer to fill up. Mateti, TCP/IP Refresher

Full Duplex TCP provides transport in both directions. To the application program these appear as two unrelated data streams, although TCP can piggyback control and data communication by providing control information (such as an ACK) along with user data. Mateti, TCP/IP Refresher

TCP Ports Interprocess communication via TCP is achieved with the use of ports (just like UDP). UDP ports have no relation to TCP ports (different name spaces). Mateti, TCP/IP Refresher

TCP/UDP Ports Reserved Ports less than 1024: Only root can bind to these ports. Local Port of a process that requested the connection. Usually a random number, 0-65535. Remote Port: What application accepted the connection. Usually a known number. /etc/services. E.g., 80 for HTTP 143 for IMAP 443 for HTTP/SSL Mateti, TCP/IP Refresher

TCP Segments The chunk of data that TCP asks IP to deliver is called a TCP segment. Each segment contains: data bytes from the byte stream control information that identifies the data bytes Mateti, TCP/IP Refresher

TCP Segment Format Destination Port Options (if any) Data 1 byte Source Port Sequence Number Request Number offset Res Control Window Checksum Urgent Pointer Mateti, TCP/IP Refresher

Sequence Number The “positional” number of the first data byte in this segment, except when SYN control flag is 1.  If SYN is 1 the sequence number is the initial sequence number (ISN). 32 bit unsigned integer Mateti, TCP/IP Refresher

Sequence Number Initial Sequence Number (ISN) is randomly generated. What if ISN is not random? You can hijack and kill arbitrary connections! Mateti, TCP/IP Refresher

Acknowledgment Number If the ACK control bit is set, this field contains the value of the next sequence number the sender of the segment is expecting to receive. Once a connection is established this is always included. Mateti, TCP/IP Refresher

Control Bits URG: Urgent Pointer field significant PSH: Push Function ACK: Acknowledgment field significant RST: Reset the connection SYN: Synchronize sequence numbers FIN: No more data from sender Mateti, TCP/IP Refresher

TCP v. UDP Q: Which protocol is better ? A: It depends on the application. TCP provides a connection-oriented, reliable byte stream service (lots of overhead). UDP offers minimal datagram delivery service (as little overhead as possible). Mateti, TCP/IP Refresher

TCP three-way handshake Establishes a connection. A: “I would like to talk to you B.” A sends a SYN packet to B B: “Ok, let's talk.” B sends a SYN-ACK packet to A A: “Thanks for agreeing.” A sends ACK to B Mateti, TCP/IP Refresher

TCP three-way handshake Flags src dst seq ack SYN 1037 80 102723769 0 SYN-ACK 80 1037 1527857206 102723770 ACK 1037 80 102723770 1527857207 Mateti, TCP/IP Refresher

Four-Way Handshake The Four-Way Handshake terminates a previously established connection: A to B: FIN B to A: ACK B to A: FIN A to B: ACK Mateti, TCP/IP Refresher

Connection Resetting Host X sends an RST packet resetting the connection if: Y requested a connection to a non-existent port P on host X, or For whatever reason (idle for a long time, or an abnormal condition, ...), the host X (client or the sever) wishes to close the connection. Resetting is unilateral. Mateti, TCP/IP Refresher