RTP: Audio and Video for the Internet

RTP header compression ”whether by CRTP or ROHC ”is transparent to the application. When header compression is in use, the compressed link becomes a more efficient conduit for RTP packets, but aside from increased performance, an application should not be able to tell that compression is being used.

Nevertheless, there are ways in which an application can aid the operation of the compressor. The main idea is regularity: An application that sends packets with regular timestamp increments , and with a constant payload type, will produce a stream of RTP packets that compresses well, whereas variation in the payload type or interpacket interval will reduce the compression efficiency. Common causes of variation in the interpacket timing include silence suppression with audio codecs, reverse predicted video frames , and interleaving:

• Audio codecs that suppress packets during silent periods affect header compression in two ways: They cause the marker bit to be set, and they cause a jump in the RTP timestamp. These changes cause CRTP to send a packet containing an updated timestamp delta; ROHC will send a first-order packet containing the marker bit and a new timestamp mapping. Both approaches add at least one octet to the size of the compressed header. Despite this reduction in header compression efficiency, silence suppression almost always results in a net saving in bandwidth because some packets are not sent.

• Reverse predicted video frames ”for example, MPEG B-frames ”have a timestamp less than that of the previous frame. ¹² As a result, CRTP sends multiple timestamp updates, seriously reducing compression efficiency. The effects on ROHC are less severe, although some reduction in compression efficiency occurs there also.

• Interleaving is often implemented within the RTP payload headers, with the format designed so that the RTP timestamp increment is constant. In this case, interleaving does not affect header compression, and it may even be beneficial. For example, CRTP has a loss multiplier effect when operating on high-delay links, which is less of an issue for inter-leaved streams than for noninterleaved streams. In some cases, though, interleaving can result in packets that have RTP timestamps with nonconstant offsets. Thus, interleaving will reduce the compression efficiency and is best avoided.

The use of UDP checksums also affects compression efficiency. When enabled, the UDP checksum must be communicated along with each packet. This adds two octets to the compressed header, which, because fully compressed RTP/UDP/IP headers are two octets for CRTP and one octet for ROHC, is a significant increase.

The implication is that an application intended to improve compression efficiency should disable the checksum, but this may not necessarily be appropriate. Disabling the checksum improves the compression ratio, but it may make the stream susceptible to undetected packet corruption (depending on the link layer; some links include a checksum that makes the UDP checksum redundant). An application designer must decide whether the potential for receiving corrupt packets outweighs the gain due to improved compression. On a wired network it is often safe to turn off the checksum because bit errors are rare. However, wireless networks often have a relatively high bit-error rate, and applications that may be used over a wireless link might want to enable the checksum.

The final factor that may affect the operation of header compression is generation of the IPv4 ID field. Some systems increment the IPv4 ID by one for each packet sent, allowing for efficient compression. Others use a pseudorandom sequence of IPv4 ID values, making the sequence unpredictable in an attempt to avoid certain security problems. The use of unpredictable IPv4 ID values significantly reduces the compression efficiency because it is necessary to convey the two-octet IPv4 ID in every packet, rather than allowing it to be predicted. It is recommended that IP implementations increment the IPv4 ID by one when sending RTP packets, although it is recognized that the IP layer will not typically know the contents of the packets (an implementation might provide a call to inform the system that the IPv4 ID should increment uniformly for a particular socket).