TCP/IP

Background

Wide Area Networks (WANs)
- Special purpose computer to computer connections (routers)
- Long distances
- Telephone lines or satellite communications
- Modems
- Lower bandwidth (generally)
- Prior to internetworking, incompatible with LANs
- After internetworking, connect LANs into single virtual network (internetwork)
Internet topology
- Network of separate LANs joined by WANs
  - routers
  - backbone
  - mid-level network
  - LANs
  - hosts
TCP/IP plays central role in the Internet

Internet Protocol (IP)

IP is an unreliable, best-effort. connectionless packet delivery system
Three basic functions:
- defines basic unit of transfer (packet or datagram)
- routing
- defines rules of operation (e.g., when to discard packet)
Packet (datagram) format
- header
  - version: 4
  - header length: 4 (# of 32-bit words)
  - service type: 8
  - total length: 16
  - identification: 16
  - flags: 3
  - fragment offset: 13 (# of 8-octets)
  - time to live: 8
  - protocol: 8
  - header checksum: 16
  - source IP address: 32
  - destination IP address: 32
  - IP options: 24
  - padding: 8
- data: <64k
IP Datagram.
Adapted from Comer,IWTCP/IP, Figure 7.3.
Encapsulation
- Ethernet
- datagrams
Fragmentation
- problem: different maximum transfer units (MTUs) for constituent LANs
- router fragments datagram
  - duplicates most of header
  - adds fragment offset
- fragmented datagrams reassembled by destination host
Routing
- performed (usually) by special-purpose computer, called a router (gateway)
- IP address
  - form: X.X.X.X
  - example: 152.2.128.184
- domain name
  - unrelated to IP address
  - form: host.domain, where domain is usually network.type
  - domain names meant for human consumption
  - domain names are mapped to IP addresses by a domain name server (DNS)
- direct routing
  - performed by router or host when destination on same network
  - map IP address into physical (e.g., Ethernet) address
  - encapsulate datagram into network packet (e.g., Ethernet frame)
  - deliver it to destination using network resources (e.g., send over Ethernet wire)
- indirect routing
  - router to router transfers, concluding with a single direct routing transfer
  - router uses IP routing table to determine where to send datagram ("next hop")
    - pairs of IP addresses: (destination, next hop)
    - next hop connected to router's network
    - default next hop, to reduce size of table

Transmission Control Protocol (TCP)

TCP provides a reliable, error-free virtual connection for two-way stream-oriented transfer of data
Five basic features:
- delivers streams of bits with guaranteed delivery in the same order as sent
- virtual circuit connection
- buffered transfer with push semantics
- unstructured data stream
- full duplex connection
Reliability provided through positive acknowledgment with retransmission strategy
Ideal communication
- Sender sends Packet 1
- Receiver receives Packet 1
- Receiver sends acknowledgment (ACK) for Packet 1
- Sender receives ACK 1
- Sender sends Packet 2
- Receiver receives Packet 2
- Receiver sends ACK2
- Sender receives ACK2
- Etc.
Faulty communication
- Sender sends Packet 1
- Sender starts timer
- Receiver does not receive Packet 1
- Sender timer expires
- Sender resends Packet 1
- Sender starts timer
- Receiver receives Packet 1
- Receiver sends ACK1
- Sender receives ACK1
- Sender cancels timer
- Etc.
Sliding Window
- Sender sends Packet 1 and starts Timer 1
- Sender sends Packet 2 and starts Timer 2
- Sender sends Packet N and starts Timer N
- Receiver receives Packet 1 and sends ACK 1
- Receiver receives Packet 2 and sends ACK 2
- Receiver receives Packet N and sends ACK N
- Sender receives ACK 1 before Timer 1 expires
- Sender receives ACK 2 before Timer 2 expires
- Sender receives ACK N before Timer N expires
- Etc.
Connection
- Connection is a basic TCP abstraction defined by a pair of endpoints
- Endpoint is a pair: (host, port)
  - host is the host's IP address
  - port is a TCP port on the host
- Operations using connections described below
TCP Segments
- Data stream transmitted in segments, usually encapsulated within single IP packets
- Concept of sliding window of octets used
  - Sender maintains pointers to octets in sequence sent, those ready to be sent but not sent, and those not ready to be sent
  - Receiver sends acknowledgment of octets received plus advertisement of number it is prepared to receive
- Segment structure
  - Header
    - source port
    - destination port
    - sequence number
    - acknowledgment number
    - header length
    - reserved
    - code bits
      - URG
      - ACK
      - PSH
      - RST
      - SYN
      - FIN
    - window
    - checksum
    - urgent pointer
    - options
    - padding
  - data
TCP Segment.
Adapted from Comer,IWTCP/IP, Figure 13.7.
Establishing TCP connection
- Receiver sends passive open request to operating system, saying it is open for business
- OS assigns Receiver TCP port number, often a well-known number, e.g., port 80
- Time passes . . .
- Sender sends message (TCP segment) requesting connection with SYN bit set and SEQ=some random number; also sends seq=x, where x is random
- Receiver receives message and returns message with SYN and ACK bits set; also saves seq=x, and sends seq=y and x+1(seq. number of first octet it expects to receive)
- Sender receives ACK message and sends its own ACK message back to Receiver; also saves y and sends y+1, confirming receipt of y
- Receiver receives Senders ACK message
- Connection is, thus, established
Closing TCP connection
- Sender closes connection
- Sender sends close connection message with FIN bit set
- Receiver receives close message and returns message with FIN and ACK bits set
- Sender receives ACK message
- Receiver closes connection
- Receiver sends message with FIN and ACK bits set
- Sender receives FIN and ACK message
- Sender sends ACK message
- Receiver receives Senders ACK message
- Connection is, thus, closed

email: jbs@cs.unc.edu

url: http://www.cs.unc.edu/~jbs/

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