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]] two or more distinct media, e.g. as is done with wired switches, wired hubs, or multi-channel wireless access points. |
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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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* collision-avoidance strategy for messages, (also called a "media access control" (MAC) discipline) |
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* framing protocol, which participating interfaces use to detect message boundaries |
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== Link Locality == |
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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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[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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A set of hosts is considered to be "on the same link", if: |
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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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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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Revision as of 04:11, 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 two or more distinct media, e.g. as is done with wired switches, wired hubs, or multi-channel wireless access points.
Links are characterized by their:
- capacity, latency, jitter, and error characteristics
- collision-avoidance strategy for messages, (also called a "media access control" (MAC) discipline)
- 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