Anne Reinarz Durham University




The network core

- Mesh of interconnected routers - Packet-switching: hosts break application-layer messages into packets - Forward packets from one router to the next, across links on path from source to destination - Each packet transmitted at full link capacity

Two key network-core functions

1. Routing: determines source-destination route taken by packets
2. Forwarding: move packets from router’s input to appropriate router output


Packet-switching: store-and-forward


Packet Switching: queuing delay and loss

- If arrival rate (in bits) to link exceeds transmission rate of link for a period of time: - Packets will queue, wait to be transmitted on link. - Packets can be dropped (lost) if memory (buffer) fills up
--- # How do loss and delay occur? - Packets queue in router buffers - Packet arrival rate to link (temporarily) exceeds output link capacity - Packets queue, wait for turn </img> --- # Packet loss - Queue (aka buffer) preceding link in buffer has finite capacity - Packet arriving to a full queue dropped (aka lost) - Lost packet may be retransmitted by previous node, by source end system, or not at all </img> --- # Four sources of packet delay </img>
## $d_{nodal} = d_{proc} + d_{queue} + d_{trans} + d_{prop}$
--- # Four sources of packet delay
- $d_{proc}$: nodal processing - Check bit errors - Determine output link - Typically < msec - $d_{queue}$: queueing delay - Time waiting at output link for transmission - Depends on congestion level of router
- $d_{trans}$: transmission - L: packet length (bits) - R: link bandwidth (bps) - $d_{trans}$ = L/R - $d_{prop}$: propagation - d: length of physical link - s: propagation speed (~2x108 meters/sec) - $d_{prop}$ = d/s
--- # A practical example </img> - How long does it take a packet of 1Mb from source to destination? - Assume that the propagation speed in all segments is 2 km/s --- # “Real” Internet delays and routes - What do “real” Internet delay & loss look like? - traceroute program: provides delay measurement from source to router along end-end Internet path towards destination. - For all i: - Sends three packets to router i on path towards destination - Router i will return packets to sender - Sender times interval between transmission and reply --- # “Real” Internet delays and routes </img> --- # Alternative core: circuit switching ### End-end resources allocated to, reserved for “call” between source and destination: - In diagram, each link has four circuits. - Call gets 2nd circuit in top link and 1st circuit in right link. - Dedicated resources: no sharing - Circuit-like (guaranteed) performance - Circuit segment idle if not used by call (no sharing) - Commonly used in traditional telephone networks --- # Alternative core: circuit switching </img> --- # Protocol layers --- # Protocol layers - Protocols determine the format and order of messages between devices - Protocol layering has conceptual and structural advantages ### Protocol stack: protocols of the various layers --- # Protocol layers - Layers: each layer implements a service - Via its own internal-layer actions - Relying on services provided by layer below
</br> ## Why layering? --- # Protocol layers - Explicit structure allows identification, relationship of complex system’s pieces - Modularization eases maintenance, updating of system - Change of implementation of layer’s service transparent to rest of system

</br> --- # Internet protocol stack
### Application: supporting network applications - FTP, SMTP, HTTP
### Transport: process-process data transfer - TCP, UDP
### Network: routing of datagrams - IP, routing protocols
### Link: data transfer between neighboring network elements - Ethernet, 802.11 (WiFi), PPP
### Physical: bits “on the wire”
--- # ISO/OSI reference model
- Presentation: allow applications to interpret meaning of data, e.g., encryption, compression, machine-specific conventions - Session: synchronization, checkpointing, recovery of data exchange - Internet stack “missing” these layers! - These services, if needed, must be implemented in application
--- # Summary - Network core, - Packet-switching - Circuit-switching - Packet loss - Internet delay - Layering, service models