What is the largest Safe UDP Packet Size on the Internet

Question

I ve read a number of articles about UDP packet sizes but have been unable to come to a conclusion on whats correct   A number of services restrict the largest UDP packet to 512 bytes  like dns   Given the minimum MTU on the internet is 576   and the size of the IPv4 header is 20 bytes  and the UDP header 8 bytes    This leaves 548 bytes available for user data   Would I be able to use packets up to the size of 548 without packet fragmentation   Or is there something the creators of DNS knew about  and that why they restricted it to 512 bytes   Could I even go higher than 548 bytes safely

User · Answer

I fear i incur upset reactions but nevertheless, to clarify for me if i'm wrong or those seeing this question and being interested in an answer:

my understanding of https://tools.ietf.org/html/rfc1122 whose status is "an official specification" and as such is the reference for the terminology used in this question and which is neither superseded by another RFC nor has errata contradicting the following:

theoretically, ie. based on the written spec., UDP like given by https://tools.ietf.org/html/rfc1122#section-4 has no "packet size". Thus the answer could be "indefinite"

In practice, which is what this questions likely seeked (and which could be updated for current tech in action), this might be different and I don't know.

I apologize if i caused upsetting. https://tools.ietf.org/html/rfc1122#page-8 The "Internet Protocol Suite" and "Architectural Assumptions" don't make clear to me the "assumption" i was on, based on what I heard, that the layers are separate. Ie. the layer UDP is in does not have to concern itself with the layer IP is in (and the IP layer does have things like Reassembly, EMTU_R, Fragmentation and MMS_R (https://tools.ietf.org/html/rfc1122#page-56))

User · Answer

This article describes maximum transmission unit  MTU  http   en wikipedia org wiki Maximum transmission unit   It states that IP hosts must be able to process 576 bytes for an IP packet  However  it notes the minumum is 68  RFC 791   Every internet module must be able to forward a datagram of 68 octets without further fragmentation   This is because an internet header may be up to 60 octets  and the minimum fragment is 8 octets     Thus  safe packet size of 508   576 - 60  IP header  - 8  udp header  is reasonable   As mentioned by user607811  fragmentation by other network layers must be reassembled  https   tools ietf org html rfc1122 page-56 3 3 2  Reassembly          The IP layer MUST implement reassembly of IP datagrams           We designate the largest datagram size that can be reassembled          by EMTU R   Effective MTU to receive    this is sometimes          called the  reassembly buffer size    EMTU R MUST be greater          than or equal to 576

User · Answer

The theoretical limit  on Windows  for the maximum size of a UDP packet is 65507 bytes   This is documented here      The correct maximum UDP message size is 65507  as determined by the following formula    0xffff -  sizeof IP Header    sizeof UDP Header     65535- 20 8    65507   That being said  most protocols limit to a much smaller size - usually either 512 or occasionally 8192   You can often go higher than 548 safely if you are on a reliable network - but if you re broadcasting across the internet at large  the larger you go  the more likely you ll be to run into packet transmission problems and loss

User · Answer

I ve read some good answers here  however  there are some minor mistakes   Some have answered that the Message Length field in the UDP header is a max of 65535  0xFFFF   this is technically true   Some have answered that the actual maximum is  65535 - IPHL - UDPHL   65507    The mistake  is that the Message Length field in the UDP Header includes all payload  Layers 5-7   plus the length of the UDP Header  8 Bytes    What this means is that if the message length field is 200 Bytes  0x00C8   the payload is actually 192 Bytes  0x00C0      What is hard and fast is that the maximum size of an IP datagram is 65535 Bytes   This number is arrived at the sum total of the L3 and L4 headers  plus the Layers 5-7 payload  IP Header   UDP Header   Layers 5-7   65535  Max    The most correct answer for what is the maximum size of a UDP datagam is 65515 Bytes  0xFFEB   as a UDP datagram includes the UDP header  The most correct answer for what is the maximum size of a UDP payload is 65507 Bytes  as a UDP Payload does not include the UDP header

User · Answer

Given that IPV6 has a size of 1500  I would assert that carriers would not provide separate paths for IPV4 and IPV6  they are both IP with different types   forcing them to equipment for ipv4 that would be old  redundant  more costly to maintain and less reliable   It wouldn t make any sense   Besides  doing so might easily be considered providing preferential treatment for some traffic -- a no no under rules they probably don t care much about  unless they get caught    So 1472 should be safe for external use  though that doesn t mean an app like DNS that doesn t know about EDNS will accept it   and if you are talking internal nets  you can more likely know your network layout in which case jumbo packet sizes apply for for non-fragmented packets so for 4096 - 4068 bytes  and for intel s cards with 9014 byte buffers  a package size of     wait   8086 bytes  would be the max   coincidence   snicker      UPDATE      Various answers give maximum values allowed by 1 SW vendor or various answers assuming encapsulation   The user didn t ask for the lowest value possible  like  0  for a safe UDP size   but the largest safe packet size     Encapsulation values for various layers can be included multiple times   Since once you ve encapsulated a stream -- there is nothing prohibiting  say  a VPN layer below that and a complete duplication of encapsulation layers above that   Since the question was about maximum safe values  I m assuming that they are talking about the maximum safe value for a UDP packet that can be received   Since no UDP packet is guaranteed  if you receive a UDP packet  the largest safe size would be 1 packet over IPv4 or 1472 bytes   Note -- if you are using IPv6  the maximum size would be 1452 bytes  as IPv6 s header size is 40 bytes vs  IPv4 s 20 byte size   and either way  one must still allow 8 bytes for the UDP header

User · Answer

It is true that a typical IPv4 header is 20 bytes  and the UDP header is 8 bytes   However it is possible to include IP options which can increase the size of the IP header to as much as 60 bytes   In addition  sometimes it is necessary for intermediate nodes to encapsulate datagrams inside of another protocol such as IPsec  used for VPNs and the like  in order to route the packet to its destination   So if you do not know the MTU on your particular network path  it is best to leave a reasonable margin for other header information that you may not have anticipated   A 512-byte UDP payload is generally considered to do that  although even that does not leave quite enough space for a maximum size IP header

User · Answer

The maximum safe UDP payload is 508 bytes  This is a packet size of 576  the  quot minimum maximum reassembly buffer size quot    minus the maximum 60-byte IP header and the 8-byte UDP header  Any UDP payload this size or smaller is guaranteed to be deliverable over IP  though not guaranteed to be delivered   Anything larger is allowed to be outright dropped by any router for any reason  Except on an IPv6-only route  where the maximum payload is 1 212 bytes  As others have mentioned  additional protocol headers could be added in some circumstances  A more conservative value of around 300-400 bytes may be preferred instead  The maximum possible UDP payload is 67 KB  split into 45 IP packets  adding an additional 900 bytes of overhead  IPv4  MTU 1500  minimal 20-byte IP headers   Any UDP packet may be fragmented  But this isn t too important  because losing a fragment has the same effect as losing an unfragmented packet  the entire packet is dropped  With UDP  this is going to happen either way  IP packets include a fragment offset field  which indicates the byte offset of the UDP fragment in relation to its UDP packet  This field is 13-bit  allowing 8 192 values  which are in 8-byte units  So the range of such offsets an IP packet can refer to is 0   65 528 bytes  Being an offset  we add 1 480 for the last UDP fragment to get 67 008  Minus the UDP header in the first fragment gives us a nice  round 67 KB  Sources  RFC 791  RFC 1122  RFC 2460

User · Answer

UDP is not  quot safe quot   so the question is not great - however -  if you are on a Mac the max size you can send by default is 9216 bytes  if you are on Linux  CentOS RedHat  or Windows 7 the max is 65507 bytes   If you send 9217 or more  mac  or 65508   linux windows   the socket send function returns with an error  The above answers discussing fragmentation and MTU and so on are off topic - that all takes place at a lower level  is  quot invisible quot  to you  and does not affect  quot safety quot  on typical connections to a significant degree  To answer the actual question meaning though - do not use UDP - use raw sockets so you get better control of everything  since you re writing a game  you need to delve into the flags to get priority into your traffic anyhow  so you may as well get rid of UDP issues at the same time

User · Answer

576 is the minimum maximum reassembly buffer size  i e  each implementation must be able to reassemble packets of at least that size  See IETF RFC 1122 for details

User · Answer

512 is your best bet  It s used elsewhere and is a nice even number  half of 1024

User · Answer

IPv4 minimum reassembly buffer size is 576  IPv6 has it at 1500  Subtract header sizes from here  See UNIX Network Programming by W  Richard Stevens

[networking] What is the largest Safe UDP Packet Size on the Internet

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