RTP fragmentation vs UDP fragmentation - h.264

Question

I don't understand why we bother fragmenting at RTP level if UDP (or IP) layer does the fragmentation.
As I understand it, let's say we are on Ethernet link, the MTU is 1500 bytes.
If I have to send, for example, 3880 bytes, fragmenting at IP layer, would results in 3 packets of respectively 1500, 1500, and 940 bytes (IP header is 20 bytes, so the total overhead results in 60 bytes).
If I do it at UDP layer the overhead will be 84 bytes (3x 28 bytes).
At RTP layer it's 120 bytes of overhead.
At H264/NAL packetization layer, it's 3 more bytes (so 123 bytes final) for FU-A mode.
For such a small packet, it makes a final increase of 3.1% for the initial packet size, while at IP layer, it would only waste 1.5% overall.
Is there any valid reason to bother making such a complex packetization rules at RTP layer knowing it'd always be worse than lower layer fragmentation?

Answer 1

Except for the first fragment, fragmented IP traffic does not contain the source or destination port numbers. Instead it glues packets together using sequence IDs. This makes it impossible for stateless intermediate network devices (switches and routers) that need to re-install QoS (because .1p or DSCP flags were cleared by another device or never existed in the first place.) Unless the device has the resources to manage per-session state, it either has to risk rate-limiting/prioritizing fragments from unrelated streams, or not prioritizing any fragments, some of which can be voice/video.
AFAIK RTP packets never IP-fragment unless the network has MTU mismatches in it. Hence each UDP header has source and destination port numbers, so if you can tame your clients to use known port ranges, you can re-establish QoS markings based on this information, and you can pass IP fragments as vanilla traffic and not worry about dropping voice/video data.

Answer 2

RTP is designed with UDP in mind.
Applications typically run RTP on top of UDP to make use of its
multiplexing and checksum services; both protocols contribute parts of
the transport protocol functionality.
However RTP services that are added to raw UDP such as ability to detect packet reordering, losses and timing require that UDP data consists of RTP payload and also service information.
The Internet, like other packet networks, occasionally loses and
reorders packets and delays them by variable amounts of time. To cope
with these impairments, the RTP header contains timing information
and a sequence number that allow the receivers to reconstruct the
timing produced by the source, so that in this example, chunks of
audio are contiguously played out the speaker every 20 ms. This
timing reconstruction is performed separately for each source of RTP
packets in the conference. The sequence number can also be used by
the receiver to estimate how many packets are being lost.
Then RTP is designed to be extensible, common headers and data specific payload:
RTP is a protocol framework that is deliberately not complete. This document specifies those functions expected to be common across all the applications for which RTP would be appropriate. Unlike conventional protocols in which additional functions might be accommodated by making the protocol more general or by adding an option mechanism that would require
parsing, RTP is intended to be tailored through modifications and/or additions to the headers as needed.
All quotes are from RFC 1889 "RTP: A Transport Protocol for Real-Time Applications".
That is, RTP overhead for H.264 stream is not just a waste of bandwidth. RTP headers and H.264 payload formatting allow, at moderate cost, to handle video data streaming in a more reliable way, and in the same time to leverage specification which is well defined and good for different sorts of data.

Answer 3

I'd like to add that a lot of RTP servers/senders go about sending split datagrams inefficiently.
They use a lot of malloc/free in dynamic buffer contexts.
They also use one syscall per part of the message instead of message-vectors.
To add insult to injury they usually do a lot of time calculation / other handling between sending every part of the datagram.
This causes even more syscalls, sometimes even stretching the packet over a long time because they have no upper bound when the packet should be finished, only that it is finished before sending the next batch of packets.
Inefficient behavior like this gets seriously in the way if you want to scale throughput or on a low power embedded CPU. For bw, network and CPU efficiency reasons, it's usually way better to send the entire datagram in one go to the kernel and let it deal with fragmentation instead of userspace trying to figure it out.

Answer 4

Well, after a lot of thinking about this, there is no reason not to use IP based fragmentation up to 64kB (and this will happen if you have a lot of same timestamp's NAL unit you need to aggregate, via STAP-A for example).
The RFC6184 is clear, you can use up to 64kB of NAL this way since each NAL unit's size of exactly 2 bytes (16 bits) is appended before the actual NAL unit, although staying below the MTU is preferred.
What happen if the "single-time" NAL units cumulated size is larger than 64kB ? The RFC6184 does not say, but I guess you'll have to send all your NAL as separate FU-A packets without increasing the timestamp between them (this is where the only reason why the Start/End bit in the FU-A header is useful, since there is no more 1:1 match between the End bit and the RTP's marker bit).
The RFC states:
An aggregation packet can
carry as many aggregation units as necessary; however, the total
amount of data in an aggregation packet obviously MUST fit into an IP
packet, and the size SHOULD be chosen so that the resulting IP packet
is smaller than the MTU size
When a "single NAL per frame" is larger than the MTU (for example, 1460 bytes with Ethernet), it has to be split with a fragmentation unit packetization (for example, FU-A).
However, nothing in the RFC states that the limit should be 1460 bytes. And it makes sense to have larger than that when doing Ethernet only streaming (as computed above)
If you have a NAL unit larger than 64kB, then you must use FU-A to send it since you can not fit this in a single IP datagram.
The RFC states:
This payload type allows fragmenting a NAL unit into several RTP
packets. Doing so on the application layer instead of relying on
lower-layer fragmentation (e.g., by IP) has the following advantages:
o The payload format is capable of transporting NAL units bigger
than 64 kbytes over an IPv4 network that may be present in pre-
recorded video, particularly in High-Definition formats (there is
a limit of the number of slices per picture, which results in a
limit of NAL units per picture, which may result in big NAL
units).
o The fragmentation mechanism allows fragmenting a single NAL unit
and applying generic forward error correction as described in
Section 12.5.
Which I understand as: "If you NAL unit is less than 64kbytes, and you don't care about FEC, then don't use FU-A, but use a single RTP packet for it"
Another case where FU-A are necessary is when receiving a H264 stream with RTP over RTSP (interleaved mode). The "packet" size must fit in 2 bytes (16bits), so you also must fragment larger NAL unit even if send on a reliable stream socket.