S

Secure Hash Algorithm (SHA-1)

A widely used hash function for use with Digital Signal Standard (DSS). SHA-1 is more secure than MD5.

secure shell (SSH)

A protocol that provides a secured method of logging in to a remote network system.

security association (SA)

An IPSec term that describes an agreement between two parties about what rules to use for authentication and encryption algorithms, key exchange mechanisms, and secure communications.

Security Parameter Index (SPI)

A portion of the IPSec Authentication Header that communicates which security protocols, such as authentication and encryption, are used for each packet in a VPN connection.

segmentation and reassembly (SAR)

Method used in ATM to transform IP packets into ATM cells and cells into IP packets.

Serving GPRS Support Node (SGSN)

Device in the mobile network that requests PDP contexts with a GGSN.

Session Announcement Protocol (SAP)

Used with multicast protocols to handle session conference announcements.

session attribute object

RSVP message object that is used to control the priority, preemption, affinity class, and local rerouting of the LSP.

Session Description Protocol (SDP)

Used with multicast protocols to handle session conference announcements.

shared tree

The multicast forwarding tree established from the RP to the last hop router for a particular group address.

shim header

The name used to describe the location of the MPLS header in a data packet. The JUNOS software always places (shims) the header between the existing Layers 2 and 3 headers.

Short Message Service (SMS)

GSM service that enables short text messages to be sent to and from mobile telephones.

shortest path first (SPF)

An algorithm used by IS-IS and OSPF to make routing decisions based on the state of network links. Also called the Dijkstra algorithm.

shortest-path tree

The multicast forwarding tree established from the first hop router to the last hop router for a particular group address.

show route advertising-protocol

JUNOS software command that displays the routes sent to a neighbor for a particular protocol.

show route receive-protocol

JUNOS software command that displays the routes received from a neighbor for a particular protocol.

signaled path

In traffic engineering, an explicit path; that is, a path determined using RSVP signaling. The ERO carried in the packets contains the explicit path information.

Signaling System 7 (SS7)

Protocol used in telecommunications for delivering calls and services.

Simple Network Management Protocol (SNMP)

Protocol governing network management and the monitoring of network devices and their functions.

simplex interface

An interface that assumes that packets it receives from itself are the result of a software loopback process. The interface does not consider these packets when determining whether the interface is functional.

soft state

A database structure maintained by an RSVP router to store information about a particular label-switched path.

SONET Clock Generator (SCG)

On a M40e or M160 router as well as on a T320 or T640 routing node, the SCG provides Stratum 3 clock signal for the SONET/SDH interfaces. It also provides external clock inputs.

source-based tree

The multicast forwarding tree established from the source of traffic to all interested receivers for a particular group address. It is often seen in a dense-mode forwarding environment.

source-specific multicasting

As part of the IGMPv3 specification, it allows an end host to request multicast traffic for a group address from a specific source of traffic.

sparse mode

A method of operating a multicast domain where sources of traffic and interested receivers meet at a central rendezvous point. A sparse-mode network assumes that there are very few receivers for each group address.

sparse mode

A method of operating a multicast domain where sources of traffic and interested receivers meet at a central rendezvous point. A sparse-mode network assumes that there are very few receivers for each group address.

Split Horizon

Method used in distance-vector networks to avoid routing loops. Each router does not advertise routes back to the neighbor it received them from.

static label-switched path (static LSP)

See static path.

static path

In the context of traffic engineering, a static route that requires hop-by-hop manual configuration. No signaling is used to create or maintain the path. Also called a static LSP.

static route

A configured route that includes a route and a next hop. It is always present in the routing table and doesn't react to topology changes in the network.

static RP

One of three methods of learning the rendezvous point to group address mapping in a multicast network. Each router in the domain must be configured with the required RP information.

strict

In the context of traffic engineering, a route that must go directly to the next address in the path. (Definition from RFC 791, modified to fit LSPs.)

strict hop

Routers in an MPLS named path that are required to be directly connected to the previous router in the configured path.

Strict Priority Queuing (SPQ)

Dequeuing method that provides a special queue that is serviced until it is empty. The traffic sent to this queue tends to maintain a lower latency and more consistent latency numbers than traffic sent to other queues. See also Alternate Priority Queuing (APQ).

stub area

In OSPF, an area through which, or into which, AS external advertisements are not flooded.

subnet mask

The number of bits of the network address used for the network portion of a Class A, Class B, or Class C IP address.

summary-link advertisement

OSPF link-statement advertisement flooded throughout the advertisement's associated areas by area border routers to describe the routes that they know about in other areas.

Switch Interface Board (SIB)

On a T320 or T640 routing node, provides the switching function to the destination Packet Forwarding Engine.

Switching and Forwarding Module (SFM)

On an M40e or M160 router, a component of the Packet Forwarding Engine that provides route lookup, filtering, and switching to FPCs.

Synchronous Digital Hierarchy (SDH)

CCITT variation of SONET standard.

Synchronous Optical Network (SONET)

High-speed synchronous network specification developed by Bellcore and designed to run on optical fiber. STS-1 is the basic building block of SONET. Approved as an international standard in 1988. See also Synchronous Digital Hierarchy (SDH).

Synchronous Transport Module (STM)

CCITT specification for SONET at 155.52Mbps.

Synchronous Transport Signal (STS) Level 1

Basic building block signal of SONET, operating at 51.84Mbps. Faster SONET rates are defined as STS-n, where n is a multiple of 51.84Mbps. See also Synchronous Optical Network (SONET).

sysid

System identifier. A portion of the ISO Network Entity Title (NET) address. The sysid can be any 6 bytes that are unique throughout a domain.

syslog

A method for storing messages to a file for troubleshooting or record-keeping purposes. It can also be used as an action within a firewall filter to store information to the messages file.

System Control Board (SCB)

On an M40 router, the part of the Packet Forwarding Engine that performs route lookups, monitors system components, and controls FPC resets.

System Switching Board (SSB)

On an M20 router, Packet Forwarding Engine component that performs route lookups and component monitoring and monitors FPC operation.