I have asked my current L3 MPLS Service Provider to pass my Nortel Passport traffic . They say same will not be possible as their MPLS network does not support cRTP packets.
I am confused. Pls clarify why such a limitation. Thanks in advance.
RTP Header Compression and QoS
RFC 1889 leavingcisco.com specifies the RTP, which manages the audio path transport for Voice over IP (VoIP). RTP provides such services as sequencing to identify lost packets and 32-bit values to identify and distinguish between multiple senders in a multicast stream. Importantly, it does not provide or ensure QoS.
VoIP packets are composed of one or more speech codec samples or frames encapsulated in 40 bytes of IP/UDP/RTP headers. 40 bytes is a relatively large amount of overhead for the typical 20-byte VoIP payloads, particularly over low-speed links. RFC 2508 leavingcisco.com specifies compressed RTP (cRTP), which is designed to reduce the IP/UDP/RTP headers to two bytes for most packets in the case where no UDP checksums are being sent, or four bytes with checksums. The compression algorithm defined in this document draws heavily upon the design of TCP/IP header compression as described in RFC 1144 leavingcisco.com .
RFC 2508 actually specifies two formats of cRTP:
Compressed RTP (CR) - Used when the IP, UDP, and RTP headers remain consistent. All three headers are compressed.
Compressed UDP (CU) - Used when there is a large change in the RTP timestamp or when the RTP payload type changes. The IP and UDP headers are compressed, but the RTP header is not.
you understand that if IPv4+UDP+RTP headers are reduced to two bytes it is difficult for a L3 device in the middle to understand what is the frame it is receiving
Hope to help