• Previous networks only considered machines within one network
• seminal paper presents protocol design and philosophy for sharing across many interconnected networks.
• typical network composed of one or more hosts connected by one or more packet switches.

previously networks

• may have distinct ways of addressing receiver
• each network may accept data of different maximum sizes
• success or failure of a transmission and performance in each network is governed by varying delays.
• communication may be disrupted due to unrecoverable mutation of data.

• status, routing, and fault detection are different among networks

• much more convenient to have the same interface on all network devices
• should the interface account for differences between host or process?
• should play a small role, and there should only be one protocol that needs to be implemented on the system.
• interface between networks must play a central role as it is responsible for interconnection of multiple networks.

The Gateway

• connect only two networks?
  • could be implemented as two halves to transmit packets between the networks it interfaces
• gateway needs to understand the source/destination hosts, information must be available in a standard format at the gateway.
• use the Internetwork header
• unless all packets are small enough, gateways the protocol must allow packet fragmenting in the case of a packet being too large.
  • if fragmenting is not allowed, technology could stagnate or features would be difficult to add because not all systems could be upgraded at once.
  • no global coordination required if supporting packet fragmenting
• argue gateway should not perform packet reassembly due to buffering issues

Process-Level Communication

• suppose processes communicate with finite length messages that are served by a Transmission Control Protocol (TCP)
• TCP can break streams into segments and also accept sequence of segments to put together a message.
  • must consider out-of-order packets
  • another case, is that the packet should contain destination process information (a process header) that identifies the destination process.
• to decide if a packet is deliverable to the process, the TCP must determine if data is in the proper sequence - with sequencing and process information, the process identification is immediately available to the TCP for reconstruction.

Address Formats, TCP Addressing, Port Addressing

• address selection is crucial, many network have substantially different address formats and sizes
  • similarly in process addressing
• split address into 8bit/16bit segments, with upper 8 identifying up to 256 networks, and the lower 16 as a TCP identifier. If the network destination isn’t connected to the particular gateway, then route it to the next gateway
• use process “ports” to identify processes within a single machine. Destination host parses the port to determine process “mailbox”

Segment and packet formats

• need End Segment (ES) and End Message (EM) flags on the packet for TCP.
• can re-interpret packets by merely scanning the header flags
• use a windowing method to determine retransmission vs new data
  • left window edge determines the point at which all bytes to the left of the window are received
• flow control

TCP I/O

• new buffer allocated for each TCP arrival stream
• TCP could discard packets for which it doesn’t have buffer space - eventually, they will be retransmitted.
• TCP needs its own header (TCP Control Block, TCB)