In Stream Control Transmission Protocol (SCTP), the receiver has one buffer (queue) and threeUDP Header data (if any) 16-bit sour ce port number 16-bit destination port number 16-bit UDP length 16-bit UDP checksum 8 b ytes TCP Header 32-bit sequence number 32-R bit acknowledgment number options (if any) data (if any) 16-bit sour ce port number 16-bit destination port number 16-bit TCP checksum 16-bit ur gent pointer 4-bit header length. An example of ROHC compression/decompression of IP/UDP/RTP headers for communication over a radio link 1.User Datagram Protocol (UDP), is a suitable transport protocol for The source port address on the UDP user datagram header defines. This document uses Augmented Packet Header Diagrams AUGMENTED-DIAGRAMS to describe UDP , and is intended to further discussion about the design and implementation of the Augmented Packet Header Diagram language and tooling.Given this purpose, this document is not intended as a contribution to any ongoing or future work on FIGURE 5-3.
2.Optimistic mode (O): the bi-directional optimistic mode is similar to the uni-directional mode, except that a feedback channel is used to send error recovery requests and (optionally) acknowledgments of significant context updates from the decompressor to compressor. This mode therefore makes ROHC usable over links where a return path from decompressor to compressor is unavailable or undesirable. Uni-directional mode (U): in the uni-directional mode of operation, packets are only sent in one direction, from compressor to decompressor. /// as being part of the data portion of the packet. /// Calculate a checksum for a packet built on IPv4. /// Calculate a checksum for a packet built on IPv4.
The ROHC feedback is used only to indicate to the compressor side that there was an error and probably the context is damaged. ROHC does not attempt re-transmission when an error occurs and the erroneous packet is dropped. ROHC always starts header compression using U-mode even if it is used in a bi-directional link. For bi-directional links, O-mode uses positive feedback packets (ACK) and R-mode use positive and negative feedback packets (ACK and NACK). The most important differences are a more intensive usage of the feedback channel and a stricter logic at both the compressor and the decompressor that prevents loss of context synchronization between compressor and decompressor except for very high residual bit error rates.The U-mode is used when the link is uni-directional or when feedback is not possible. 3.Reliable mode (R): the bi-directional reliable mode differs in many ways from the previous two.
Udp Packet Header Full Packet Headers
This means that the compressor uses the same header format for a number of packets. To increase the compression level, an optimistic approach is used for the compressor to ensure that the context has been correctly established at the decompressor side. The ROHC compressor has three compression states as follows : 1.Initialization and Refresh (IR), where the compressor has just been created or reset and full packet headers are sent 2.First Order (FO) where the compressor has detected and stored the static fields such as IP addresses and port numbers on both sides of the connection 3.Second Order (SO) where the compressor is suppressing all dynamic fields such as RTP sequence numbers, and sending only a logical sequence number and partial checksum to cause the other side to generate based on prediction, and verify the headers of the next expected packet.In the U-mode, the feedback channel is not used.
The decompressor has three states as follows: (1) No Context (NC), where there is no context synchronization (2) Static Context (SC), where the dynamic information of the context has been lost and (3) Full Context (FC), when the decompressor has all the information about header fields. To ensure correct decompression, the context should be always synchronized. Both the compressor and the decompressor use a context to store all the information about the header fields. The decompressor works at the receiving end of the link and decompresses the headers based on the header fields’ information of the context.
The decompressor also sends feedback according to the operation mode (see Figure 5-4).The values of the ROHC compression parameters that determine the efficiency and robustness are not defined in ROHC specification and are not negotiated initially, but are stated as implementation dependent. If k CRC failures have occurred, it assumes the context has been corrupted and transitions to an initial state (SC or NC). The decompressor uses the “ k out of n” rule by looking at the last n received packets.
/tcp-headers-f2c0881ea4c94e919794b7c0677ab90a.jpg "udp packet header")
However, it only contains static and random fields (i.e., UDP checksum). In case of version 6 of the Internet Protocol, only the base header is considered (i.e., no extension headers such as routing, hop-by-hop options, and so on are present) UDP:The UDP packet header adds an extra eight bytes of overhead to the IP header. For pure header compression, we can utilize the following streams found in the pcaps directory of the project repository: IP:In the case of IPv4 the header contains every mandatory field, resulting in 20 bytes of header information where the identification number always advances by exactly one for each consecutive packet. Sliding Window Width ( SWW): the compressor, while compressing header fields like Sequence Number (SN) and Timestamp, uses Window-based Least Significant Bit ( W_LSB) encoding that uses a Sliding Window of width equal to SWW.In this section, we apply the previously discussed methods to actual packet headers, where we simulate applied header compression for the reduction of encapsulation overhead with various real-life streams that were captured on the network interface.
We use scapy to load a stream and call the BasicModel's convert_pcap_to_df() method to transform the PCAP data into a processable format. The same is true for the packet type, which randomly changes after packets and the timestamp, which varies with a delta between 150 and 300 every packets.All these streams are contained in Packet Capture (PCAP) format and can be read in Python with the scapy library. Hard RTP:This stream is the same as the easy RTP, except that the marker-bit sets and unsets after randomly determined time intervals with a uniform distribution between the bounds. In turn, the RTP adds extra 12 bytes of overhead. This refers to the omission of optional and variable-sized fields (the Contributing Source (CSRC) identifiers and the extension header). For this evaluation, we only consider a constant size RTP header.
The stream contains a single IPv4 header and a unique (and uninterpretable for Wireshark) payload of 235 bytes. During the recording, the arm performed repeated circle drawing motions. Franka:This stream was captured on a direct link between a Franka Emika robotic arm 1 and its controlling computer. For the creation of the models, we can take the first 100 packets of each stream as the training datasets and evaluate the compression with the following independent 900 packets. Most of the captured streams contain 14 bytes of Ethernet headers. In case of these header streams, we omit the payload to measure the pure header compression.Additional examples provided include real-world application network traffic captures.
Ekiga:This scenario represents a video conferencing session, which was originally generated using the Ekiga open source softphone software. The RTP marker bit is always zero, except in the first two packets. The RTP timestamp and sequence numbers have a constant increase (delta) of 160 and one, respectively. 3 This configuration utilized the GSM 06.10 codec, which is the full-rate audio codec version, which results in 33 bytes of payload.
An example of this can be seen in Listing 17.11.Kostas Pentikousis. 5 This stream capture represents an RTP session again however, the underlying network protocol in this example is IPv6, whereas the logical payload is 320 bytes. VLC:This scenario represents a high-fidelity audio transmission and was generated using the RTP streaming features of the popular VideoLan Client (VLC) open-source software. This stream is the only one that contains no Ethernet header. Radio:A TCP acknowledgment stream of a digital radio station, which, in general, is efficient to compress, as no logical payload is present.
Level-1 application aggregation is far more effective, allowing the testbed to carry twice as many flows (34) as the baseline scenario. ROHC allowed us to inject one additional bidirectional VoIP call with moderate quality (sample loss rate below 5%). The baseline experiment (no ROHC, no aggregation) showed that the testbed can carry 17 bidirectional VoIP flows. We found that ROHC does not provide for significant gains.
If ROHC and Level-2 aggregation is employed, 48 simultaneous VoIP conversations can be sustained without quality degradation. Level-2 application-layer aggregation increases the amount of sustained simultaneous VoIP conversations to 47.
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