Network2/Concept/Link: Difference between revisions
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A link is a [[Network2/Concept/Network|network]] of nodes in which every pair of nodes can exchange [[Network2/Concept/Protocol|messages]] via [[Network2/Concept/Interface|interfaces]] attached to [[Network2/Concept/Medium|media]]. Frequently, links are made to encompass more nodes by [[Network2/Concept/Bridge|bridging]] or [[Network2/Concept/Router|routing]] between two or more distinct media. |
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When talking about networking, a link is what you get when you associate (connect) a medium with a network interface. In this case a medium is something you can send signals over, such as a network cable or a specific frequency in the electromagnetic spectrum for a wireless networking card. Links become interesting when there are 2 or more interfaces attached to them, which typically happens through [[Network2/Concept/Bridge|bridging]] several mediums. [[Network2/Concept/Bridge|Switches]] and [[Network2/Concept/Bridge|hubs]] serve this role in wired networks, as do Access Points for wireless networks. Links encompass "media access disciplines", called "media access control protocols" (commonly abbreviated as MAC) which operate at a level below that of IP. |
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Links are characterized by their: |
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Ethernet is a common link-layer protocol which commonly carries (fragmented) IPv4 or IPv6 packets, among other things. Ethernet transmits data in frames, which are often smaller than IP packets. When this happens the packets are fragmented, with each ethernet frame having a complete IP header and fragment index. |
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* [[Network2/Concept/Capacity|capacity]], [[Network2/Concept/Latency|latency]], [[Network2/Concept/Jitter|jitter]], and [[Network2/Concept/Error|error]] characteristics |
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So far the links we have been talking about have been built on top of 'physical' mediums; they exist very low in the OSI stack. Links can also be created on tunnels, which present 'media' built from higher level protocols. VPNs are an example of this. |
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* [[Network2/Concept/Unicast|unicast]], [[Network2/Concept/Multicast|multicast]], or [[Network2/Concept/Broadcast|broadcast]] message propagation abilities |
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* media access control (MAC) protocol, which participating interfaces use to avoid message collisions |
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* framing protocol, which participating interfaces use to detect message boundaries |
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* maximum transmission unit (MTU), which is the size of the largest single message that the link can propagate |
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and by what kind of message they carry. |
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== Link Locality == |
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[http://www.ietf.org/rfc/rfc3927.txt RFC 3927] describes Link-Local addressing for IPv4 communication between two hosts on a single link as follows: |
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'''Examples:''' |
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A set of hosts is considered to be "on the same link", if: |
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Important media for data-carrying links include wires and the 2.4 GHz band of the radio spectrum. Important data bridges include Ethernet hubs, switches, and wireless access points. Important data routers include IPv4 and IPv6 routers and IPv4 network address translators (NATs). |
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- when any host A from that set sends a packet to any other host B |
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in that set, using unicast, multicast, or broadcast, the entire |
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Important media for matter-carrying links include roads, rails, airspace, and bodies of water. Important matter bridges include road and rail junctions, bridges, tunnels, and canals. Important matter routers are usually people or organizations working in the field of transit or devices controlled by such people or organizations. |
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link-layer packet payload arrives unmodified, and |
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- a broadcast sent over that link by any host from that set of hosts |
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can be received by every other host in that set |
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The link-layer *header* may be modified, such as in Token Ring Source |
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Routing [802.5], but not the link-layer *payload*. In particular, if |
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any device forwarding a packet modifies any part of the IP header or |
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IP payload then the packet is no longer considered to be on the same |
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link. This means that the packet may pass through devices such as |
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repeaters, bridges, hubs or switches and still be considered to be on |
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the same link for the purpose of this document, but not through a |
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device such as an IP router that decrements the TTL or otherwise |
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modifies the IP header. |
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Latest revision as of 05:33, 23 August 2009
A link is a network of nodes in which every pair of nodes can exchange messages via interfaces attached to media. Frequently, links are made to encompass more nodes by bridging or routing between two or more distinct media.
Links are characterized by their:
- capacity, latency, jitter, and error characteristics
- unicast, multicast, or broadcast message propagation abilities
- media access control (MAC) protocol, which participating interfaces use to avoid message collisions
- framing protocol, which participating interfaces use to detect message boundaries
- maximum transmission unit (MTU), which is the size of the largest single message that the link can propagate
and by what kind of message they carry.
Examples:
Important media for data-carrying links include wires and the 2.4 GHz band of the radio spectrum. Important data bridges include Ethernet hubs, switches, and wireless access points. Important data routers include IPv4 and IPv6 routers and IPv4 network address translators (NATs).
Important media for matter-carrying links include roads, rails, airspace, and bodies of water. Important matter bridges include road and rail junctions, bridges, tunnels, and canals. Important matter routers are usually people or organizations working in the field of transit or devices controlled by such people or organizations.