CCIE network engineer technical analysis
RSVP resource reservation protocol
There are a large number of intermediate nodes in the Internet. If the user uses a connectionless protocol to transmit the data stream, each datagram of the data stream may cause two problems when forwarded through the intermediate node. One is that the forwarding path of each datagram is different, and it does not reach the destination in order. Some data The report may be delayed; the second is that when the datagram is queued for forwarding at the intermediate node, the queuing time is uncertain, and when the intermediate node is congested due to lack of resources, a packet loss strategy will be adopted to channel the traffic. For end-to-end communication, it means transmission delay and delay jitter. These are disadvantageous for multimedia communication, which seriously affects the service quality of end-to-end multimedia communication. The basic method to solve this problem is that the end point and the intermediate node should cooperate closely. Based on the connectionless protocol, a fixed transmission path is established for a specific data stream, and system resources are reserved for it. The transmission delay is limited to a specified range, thereby ensuring Quality of service for end-to-end multimedia communications. RSVP (Resource Reservation Protocol) proposed by IETF is based on the above method.
Usually RSVP request will cause resource reservation on each node’s data path.
RSVP only requests resources in one direction, so although the same application may act as both the sender and the receiver, RSVP is logically different between the sender and the receiver. RSVP runs on the upper layer of IPV4 or IPV6 and occupies the space of the transmission protocol in the protocol stack. RSVP does not transmit application data, but supports Internet control protocols such as ICMP, IGMP, or routing protocols. Just like the implementation of routing and management protocols, the operation of RSVP is also performed in the background, not on the data forwarding path.
RSVP is not essentially a routing protocol. The design goal of the RSVP protocol is to run simultaneously with current and future unicast and multicast routing protocols. The RSVP process refers to the local routing database to obtain the transmission path. Taking multicast as an example, the host sends IGMP information to join the multicast group, and then sends RSVP information along the multicast group transmission path to reserve resources. The routing protocol determines where the data packets are forwarded. RSVP only considers the QOS of the forwarded packets based on routing. In order to effectively adapt to the needs of large groups, dynamic group members, and receivers of different models, through RSVP, the receiver can request a specific QOS.
The QOS request is transmitted from the receiving host application to the local RSVP process, and then the RSVP protocol transmits the request to all nodes (routers and hosts) along the opposite data path, but only reaches the receiving data path to join the multicast distribution The router in the tree. Therefore, the RSVP reservation overhead is logarithmic and non-linear with the number of receivers.
Because RSVP packets must propagate upstream, pass all intermediate routers, and finally reach all sending hosts. However, the routing protocol lacks reverse routing information, so RSVP introduced path messages. All hosts participating in the multicast group as senders must send path messages and transmit them to all multicast destinations via the distribution tree.
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RSVP protocol resource reservation process
1. The source end sending data determines the bandwidth, delay and delay jitter required to send the data stream, and includes it in the PATH packet to send to the receiving end.
2. When a router in the network receives the PATH packet, it stores the path status information in the PATH packet. The path status information describes the upper-level source address on the PATH packet (that is, the upper One-hop router address).
3. When the receiving end receives the PATH packet, it follows the RESV packet in a direction opposite to the source path obtained in the PATH packet. The RESV packet contains QoS information such as flow and performance expectations that need to be described for resource reservation for the data flow.
4. When a router receives a RESV packet, it determines whether there are enough resources to satisfy the QoS request through admission control. If so, reserve bandwidth and buffer space, and store some specific information related to the data stream, and then forward the RESV packet to the next router; if the router must reject the request, it returns an error to the receiving end information.
The RSVP resource reservation message is initiated by the receiver and transmitted upstream at a time. The upstream here is the direction from the receiver to the sender. At each node of the route, the resource reservation request triggers the following two actions:
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1. Resource reservation on the link
The RSVP process at each node will pass the message requesting resource reservation to the admission control component (Admission Control) and policy control component (Policy Control). As long as any one of these two components fails to detect the admissibility, the resource reservation request will be rejected; meanwhile, the RSVP process generates an error message and sends it to the receiver. If both can succeed, the node will set the packet flow classifier at the same time, so that in the actual data flow transmission, this reserved data packet can be selected from all the packets entering the router, and then the It provides QoS guarantee.
2. Forward resource reservation requests to upstream nodes
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